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A MANUAL N^H^r

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OF THE

ANATOMY OF INVERTEBRATED

ANIMALS.

BY

THOMAS H. HUXLEY, LL. D., F. R. S.

NEW YOEK:
D. APPLETON AND COMPANY,

1, S, AND 5 BOND STREET.

1885.

PREFACE.

The ]3resent volume on the Anatomy of Invert ebrated
Animals fulfills an nndertaking to produce a treatise on
comparative anatomy for students, into whicli I entered
two-and-twenty years ago. A considerable installment of
the work, relating- wholly to the Invertebrata, appeared in
the Medical Times and Gazette for the years 1856 and
1857, under the title of " Lectures on General Natural
History." But a variety of circumstances having con-
spired, about that time, to compel me to direct my atten-
tion more particularly to the Vertebrata, I was led to in-
terrupt the publication of the " Lectures " and to com-
plete the Vertebrate half of the proposed work first. This
appeared in 1871, as a " Manual of the Anatomy of Yerte-
brated Animals."

A period of incapacity for any serious toil prevented
me from attempting, before 1874, to grapple with the im-
mense mass of new and important information respecting
the structure, and especially the development, of Inverte-
brated animals, which the activity of a host of investiga-
tors has accumulated of late years.

That my progress has been slow will not surprise any
one who is acquainted with the growth of the literature
of animal morphology, or with the expenditure of time
involved in the attempt to verify for one's self even the
cardinal facts of that science ; but I have endeavored, in

4 PREFACE.

the last chapter, to supply the most important recent ad-
ditions to om- knowledge, respecting the groups treated of
in those which have long been printed.

^^hen I commenced this work, it was mv intention to
continue the plan adopted in the " Manual of the Anatomy
of Tertehrated Animals,'' of giving a summary account
of what appeared to me to be ascertained morphokgical
facts, without referrincr to mr sources of information. I
soon found, however, that it would be inconvenient to
carrv out this scheme consistentlv : and some of mv pao^es
are, I am afi'aid, somewhat bm'dened with notes and ref-
erences.

I am the more careful to mention this circumstance as,
had it been my pui^ose to give any adequate Bibliograj^hy,
the conspicuous absence of the titles of many important
books and memoirs might appear unaccountable and in-
deed blame worth V.

My object, in writing the book, has been to make it
useful to those who wish to become acquainted with the
broad outlines of what is at present known of the moiiDhol-
02T of the Invertehraia : though I have not avoided the
incidental mention of facts connected with their physiol-
osTT and their distribution. On the other hand, I have ab-
stained from discussing questions of aetiology, not because
I underestimate their importance, or am insensible to the
interest of the great problem of Evolution ; but because,
to my mind, the growing tendency to mix up ^etiological
speculations with morphological generalizations will, if
unchecked, throw Biology into confusion.

For the student, that which is essential is a knowledge
of the facts of morphology ; and he should recollect that
generalizations are empty formulas, unless there is some-
thing in his personal experience which gives reality and
substance to the terms of the propositions in which these
generalizations are expressed.

PREFACE. 5

The dissection of a single representative of eacli of the
principal divisions of the Inveriebrata ^\SS. give the student
a more real acquaintance with their comparative anatomy
than any amount of readiug of this, or any other book.
And I have endeavored to facilitate practical study by
supplying a somewhat full description of individual forms,
in the case of the more complicated tj-pes.

That the power of repeating a " Classification of Ani-
mals," with all the appropriate definitions, has anything
to do with genuine knowledge is one of the commonest
and most mischievous delusions of both students and their
examiners.

The real business of the learner is to gain a true and
vivid conception of the characteristics of what may be
termed the natural orders of animals. The mode of ar-
rangement, or classification, of these into larger groups is
a matter of altogether secondary importance. As such, I
have relegated this subject to a subordinate place in the
last chapter ; and I have thought it unnecessary^, either to
discuss the systems proposed by others, or to give reasons
for passing over, in silence, my own former attempts in
this direction.

Of the manifold imperfections in the execution of the
task which I have set myself, few will be more sensible
than I am ; but I trust that the book, such as it is, may
be of use to the beginner.

Those who desire to pursue the study of the Inverte-
hrata further will do well to consult the excellent treatises
of Yon Siebold,' Gegenbaur,' and Glaus;' and the elabo-

* " Lelirbuch der vergleichenden Anatomie der wirbellosen Thiere," 1818.
One of the best books on the subject ever written, and still indispensable.

2 " Grundzuge der vergleichenden Anatomie," 1870 ; and " Grundriss der
vergleichenden Anatomie," 1874.

3 " Grundzuge der Zoologie." 3tte Auflage, 1876.

6 PREFACE.

rate works of Milne-Edwards* and Bronn,*" in wliicli a
very full Bibliography will be met with. Dr. Eolleston's
valuable " Types of Animal Life," and tlie "Elementary
Instruction in Practical Biology," by myself and Dr.
Martin, will prove useful adjuncts to the appliances of the
practical worker.

* " Legons sur la Physiologie et I'Anatomie comparee de I'Homme et des
Animaux." Tomes i.-xii. (incomplete).

2 " Die Klassen und Ordnungen des Tkierreiclis." Bde. i.-vi. (incomplete).

London, June^ 1877.

CONTENTS.

T±CTS

Preface, 3

Introduction : The General Principles of Biology, . 9

Chap. I. — The Distinctive Characters of Animals, . . . .44

11. — The Protozoa, 73

III. — The Porifera and the Coelenterata, 102

lY. — The Turbellaria, the Rotifera, the Trematoda, and the

Cestoidea, 15Y

V. — The Hirudinea, the Oligoch^ta, the Polych^ta, the

Gephyrea, 189

YI. — The Arthropod a, 219

YII. — The Air-breathing Arthropoda, 320

YIII. — The Polyzoa, the Brachiopoda, and the Mollusca, . . 389

IX. — The Echinodermata, 466

X. — The Tunicata or Ascidioida, 510

XI. — The Peripatidea, the Myzostomata, the Enteropneusta,
the Ch^tognatha, the Nematoidea, the Physemaria,

THE ACANTHOCEPHALA, AND THE DiCYEMIDA, . . . 534

XII. — The Taxonomy of Invertebrated Animals, . . . .561
Index, 589

THE ANATOMY

OF

IISrVERTEBEATED AITIMALS.

INTRODUCTION.

I. — THE GENERAL PRINCIPLES OP BIOLOGY.

The biolof?ical sciences are those which deal with the
phenomena manifested by living matter; and though it is
customary and convenient to group apart such of these phe-
nomena as are termed mental, ar^d such of them as are ex-
hibited by men in society, under the heads of Psychology
and Sociology, yet it must be allowed that no natural boun-
dary separates the subject-matter of the latter sciences from
that of Biology. Psychology is inseparably linked with
Physiology ; and the phases of social life exhibited by ani-
mals other than man, which sometimes curiously foreshadow
human policy, fall strictly within the province of the biolo-
gist.

On the other hand, the biological sciences are sharply
marked off from the abiological, or those which treat of the
phenomena manifested by not-living matter, in so far as the
properties of living matter distinguish it absolutely from all
other kinds of things, and as (the present state of knowledge
furnishes us with no link between the living and the not-
living.)

These distinctive properties of living matter are —

1. Its chemical C07n2)0siti07i — containing, as it invariably
does, one or more forms of a complex compound of carbon,
hydrogen, oxygen, and nitrogen, the so-called protein (which
has never yet been obtained except as a product of living
bodies) united with a large proportion of water, and forming

10 THE ANATOMY OF INVERTEBRATED ANIMALS.

the chief constituent of a substance which, in its primary un-
modified state, is known as protoplasm.

2. Its unwersal disintegration and waste hy oxidation y
and its concomitant reintegration hy the hitussusception of
new matter.

A process of waste resulting from the decomposition of
the molecules of the protoplasm, in virtue of which they
break up into more highly-oxidated products, which cease to
form any part of the living body, is a constant concomitant
of life. There is reason to believe that carbonic acid is al-
ways one of these waste products, while the others contain
the remainder of the carbon, the nitrogen, the hydrogen, and
the other elements which may enter into the composition of
the protoplasm.

The new matter taken in to make good this constant loss
is either a ready-formed protoplasmic material, supplied by
some other living being, or it consists of the elements of
protoplasm, united together in simpler combinations, which
consequently have to be built up into protoplasm by the
agency of the living matter itself. In either case, the addi-
tion of molecules to those which already existed takes place,
not at the surface of the living mass, but by interposition
between the existing molecules of the latter. If the processes
of disintegration and of reconstruction which characterize
life balance one another, the size of the mass of living matter
remains stationary, while, if the reconstructive process is the
more rapid, the living bodj^ grotcs. But the increase of size
which constitutes growth is the result of a process of molec-
ular intussusception, and therefore differs altogether from the
process of growth by accretion, which may be observed in
crystals and is effected purely by the external addition of
new matter — so that, in the M^ell-known aphorism of Linnaeus,*
the word "grow," as applied to stones, signifies a totally dif-
ferent process from what is called "growth" in plants and
animals.

3. Its tendency to undergo cyclical changes.

In the ordinary course of Nature, all living matter proceeds
from preexisting living matter, a portion of the latter being
detached and acquiring an independent existence. The new
form takes on the characters of that from which it arose ; ex-
hibits the same power of propagating itself by means of an
offshoot ; and, sooner or later, like its predecessor, ceases to

1 " Lapides crescunt: vegetdbiUa crescunt et vivunt: animalia crescunt, vi-
vunt et sentiunt."

CHARACTERS OF LIVING MATTER. n

live, and is resolved into more higlily-oxidated compounds of
its elements.

Thus an individual living body is not only constantly
changing its substance, but its size and form are undergoing
continual modifications, the end of which is the death and
decay of that individual ; the continuation of the kind being
secured by the detachment of portions which tend to run
through the same cycle of forms as the parent. No forms of
matter which are either not living, or have not been derived
from living matter, exhibit these three properties, nor any
approach to the remarkable phenomena defined under the sec-
ond and third heads. But, in addition to these distinctive
characters, living matter has some other peculiarities, the
chief of which are the dependence of all its activities upon
moisture and upon heat, within a limited range of tempera-
ture, together with the fact that it usually possesses a certain
structure, or organization.

As has been said, a large proportion of water enters into
the composition of all living matter ; a certain amount of dry-
ing arrests vital activity, and the complete abstraction of this
water is absolutely incompatible with either actual or poten-
tial life. But many of the simpler forms of life may undergo
desiccation to such an extent as to arrest their vital manifes-
tations and convert them into the semblance of not-living
matter, and yet remain potentially alive ; that is to say, on
being duly moistened they return to life again. And this
revivification may take place after months, or even years, of
arrested life.

The properties of living matter are intimately related to
temperature. Not only does exposure to heat sufficient to
decompose protein matter destroy life, by demolishing the
molecular structure upon which life depends ; but all vital
activity, all phenomena of nutritive growth, movement, and
reproduction, are possible only between certain limits of tem-
perature. As the temperature approaches these limits the
manifestations of life vanish, though they may be recovered
by return to the normal conditions ; but, if it pass far beyond
these limits, death takes place.

This much is clear ; but it is not easy to say exactly what
the limits of temperature are, as they appear to vary in part
with the kind of living matter, and in part with the con-
ditions of moisture which obtain along with the temperature.
The conditions of life are so complex in the higher organisms,
that the experimental investigation of this question can be

12 THE ANATOMY OF INTERTEBRATED ANIMALS.

satisfactorily attempted only in the lowest and simplest
forms. It appears that, in the dry state, these are able to
bear far greater extremes both of heat and cold than in the
moist condition. Thus Pasteur found that the spores of fungi,
when dry, could be exposed without destruction to a tem-
perature of 120^-125° C. (248^-257° Fahr.), while the same
spores, when moist, were all killed by exposure to 100° C.
(212° Fahr.). On the other hand, Cagniard de la Tour found
that dry yeast might be exposed to the extremely low tem-
perature of solid carbonic acid ( — 60° Cor —76° Fahr.) with-
out baing killed. In the moist state he found that it might
be frozen and cooled to —5° C. (23° Fahr.), but that it was
killed by lower temperatures. However, it is very desirable
that these experiments should be repeated, for Cohn's careful
observations on Hxcteria show that, though they fall into a
state of torpidity, and, like yeast, lose all their powers of ex-
citing fermentation at, or near, the freezing-point of water,
they are not killed by exposure for five hours to a tempera-
ture below —10° C. (14:° Fahr.), and, for some time, sinking
to —18° C. (— 0°.4 Fahr.). Specimens of Spirillum voiutans^
which had been cooled to this extent, be2'an to move about
some little time after the ice containing them thawed. But
Cohn remarks that Euglenos.f which were frozen along with
them, were all killed and disorganized, and that the same fate
had befallen the higher Infusoria and Itotifera, -with the ex-
ception of some encysted Vorticellce, in which the rhythmical
movements of the contractile vesicle showed that life was
preserved.

Thus it would app3ar that the resistance of living matter
to cold depends greatly on the special form of that matter,
and that the limit of the Euglena^ simple organism as it is, is
much higher than that of the Bacterium.

Considerations of this kind throw some light upon the
apparently anomalous conditions under which many of the
lower plants, such as Protococcns and the Diatomacece^ and
some of th3 lower animals, such as the Radiolaria^ are ob-
served to flourish. Protococcus has been found not only on
the snows of great heights in temperate latitudes, but cover-
ing extensive areas of ice and snow in the Arctic regions,
where it must be exposed to extremely low temperatures —
in the latter case for manv months tosrether : while the Arctic
and Antarctic seas swarm with Diatomacece and Radiolaria.
It is on the Diatomace':e^ as Hooker has well shown, that all
surface-life in these regions ultimately depends ; and their enor-

RESISTANCE TO HEAT AND COLD. 13

mous multitudes prove that their rate of multiplication is ade-
quate to meet the demands made upon them, and is not seri-
ously impeded by the low temperature of the waters, never
much above the freezing-point, in which they habitually live.

The maximum limit of heat which living matter can resist
is no less variable than its minimum limit. Kiihne found
that marine Amcebce were killed when the temperature
reached 35° C. (95° Fahr.), while this was not the case with
iresh-^vaiier j^7)ioebce, which survived a heat of 5°, or even 10°,
C. higher. Actlnophrys JElchhornii was not killed until the
temperature rose to 44° or 45° C. Didymiian serpula is killed
at 35° C. ; while another Myxomycete^ ^thallum septiciun^
succumbs only at 40° C.

Colin (" Untersuchungen liber Bacterien," JBeitrdye zur
l^iologie der JPftanzen^ Heft 2, 1872) has given the results of
a series of experiments conducted with the view of ascertain-
ing the temperature at which bacteria are destroyed when
living in a fluid of definite chemical composition, and free
from all such complications as must arise from the inequalities
of physical condition when solid particles other than the j5ac-
teria coexist with them. The fluid employed contained 0.1
gramme potassium phosphate, 0.1 gr. crystallized magnesium
sulphate, 0.1 gr. tribasic calcium phosphate, and 0.2 gr. am-
monium tartrate, dissolved in 20 cubic centimetres of distilled
water. If to a certain quantity of this " normal fluid " a sn.all
proportion of water containing JBacteria was added, the mul •
tiplication of the Sacteria went on with rapidity, whether the
mouth of the containing flask was open or hermetically closed.
Hermetically-sealed flasks, containing portions of the normal
fluid infected with J^acterla^ were submerged in water heated
to various temperatures, the flask being carefully shaken, with-
out being raised out of the water, during its submergence.

The result was, that in those flasks which were thus sub-
jected, for an hour, to a heat of 60°-62° C. (140°-143° Fahr.),
the Bacteria underwent no development, and the fluid re-
mained perfectly clear. On the other hand, in similar experi-
ments in which the flasks were heated only to 40° or 50° C
{104°-122° Fahr.), the fluid became turbid, in consequence of
the multiplication of the Bacteria^ in the course of from two
to three days.

I am in the habit of demonstrating annually, that Pasteur's
solution and hay-infusion, after five minutes' boilino^ in a flask
properly stopped with cotton-wool, remain perfectly clear of
living organisms, however long they may be kept. The same

14: THE ANATOMY OF INYEKTEBRATED ANIMALS.

holds good for a solution analogous to Cohn's, but in which
all the saline ingredients are ammonia salts ; ^ and in which
Bacteria flourish luxuriantly. Prof. Tyndall's large series
of experiments give the same results for fluids of the most
diverse composition. The cases of milk and some other fluids
in which Bacteria are said to appear, after they have been
heated above the boiling-point, require renewed investigation.

Both in Kuhne's and in Cohn's experiments, w^iich last have
lately been confirmed and extended by Dr. Roberts, of Man-
chester, it was noted that long exposure lo a lower temper-
ature than that which brings about immediate destruction of
life produces the same eflect as short exposure to the latter
temperature. Thus, though all the Bacteria were killed, with
certainty, in the normal fluid, by short exposure to temper-
atures at or above 60° C. (140° Fahr.), Cohn observed that,
when a flask containing infected normal fluid was heated to
50°-52° C. (122°-125° Fahr.) for only an hour, the conse-
quent multiplication of the Bacteria was manifested much
earlier than in one which had been exposed for two hours to
the same temperature.

It appears to be very generally held that the simpler vege-
table organisms are deprived of life at temperatures as high
as 60° C. (140° Fahr.) ; but it is affirmed by competent ob-
servers that Algoi have been found living in hot springs at
much higher temperatures, nam.ely, from 168° to 208° Fahr.,
for which latter surprising fact we have the high authority of
Descloiseaux. It is no explanation of these phenomena, but
only another mode of stating them, to say that these organ-
isms have become " accustomed " to such temperatures. If
this degree of heat were absolutely incompatible with the
activity of living matter, the plants could no more resist it
than they could become " accustomed " to be being made red-
hot. (Habit may modify subsidiary, but cannot aifect funda-
mental, conditions.1

Recent investigations point to the conclusion that the im-
mediate cause of the arrest of vitality, in the first place, and
of its destruction, in the second, is the coagulation of certain
substances in the protoplasm, and that the latter contains
various coagulable matters, which solidify at diff"erent temper-
atures. And it remains to be seen how far the death of any
form of living matter, at a given temperature, depends on the

1 These were as pure as I could obtain them. It is possible the fluid may
have contained an infinitesimal proportion of fixed mineral matter.

RESISTANCE TO HEAT AND COLD. 15

destruction of its fundamental substance at that beat, and
how far death is brought about by the coagulation of merely
accessory compounds.

It may be safely said of all those living things which are
laro-e enough to enable us to trust the evidence of micro-
scopes/ that they are heterogeneous optically, and that their
different parts, and especially the surface layer, as contrasted
with the interior, differ physically and chemically ; while, in
most living things, mere heterogeneity is exchanged for a
definite structure, whereby the body is distinguished into
visibly diverse parts, which possess different powers or func-
tions. Living things which present this visible structure are
said to be organized ; and so widely does organization obtain
among living beings, that organized and living are not unfre-
quently used as if they were terms of coextensive applicabil-
ity. This, however, is not exactly accurate, if it be thereby
implied that all living things have a visible organization, as
there are numerous forms of living matter of which it cannot
properly be said that they possess either a definite visible
structure or permanently specialized organs : though doubt-
less the simplest particle of living matter must possess a
highly-complex molecular structure, which is far beyond the
reach of vision.

The broad distinctions which, as a matter of fact, exist
between every known form of living substance and every other
component of the material world, justify the separation of
the biological sciences from all others. But it must not be '
supposed that the differences between living and not-living
matter are such as to bear out the assumption that the forces j
at work in the one are different from those which are to be/ e
met with in the other. [Considered apart from the phenomena ''■"' "^
of consciousness, the plienomena of life are all dependent
upon the working of the same physical and chemical forces
as those which are active in the rest of the world. It may'
be convenient to use the terms " vitality " and " vital force " to
denote the causes of certain great groups of natural opera-

1 In considering the question of the complication of molecular stracture
which even the smallest and simplest of living beings may possess, it is "O'cll
to recollect that an organic particle totoo of an inch in diameter, in -nhich our
best microscopes may be incompetent to reveal the slightest differentiation of
parts, may be made up of 1,000,000 particles rsnhoor, of an inch in diameter,
while the molecules of matter are probably much less than too^otto of an inch in
diameter. Hence in such a body there is ample scope for any amount of com-
plexity of molecular structure.

16 THE ANATOMY OF INVERTEBRATED ANIMALS^,

I tions, as we employ the names of " electricity " and " electrical
\ force " to denote others ; but it ceases to be proper to do so, if
; such a name implies the absurd assumption that either " elec-
. tricity " or "vitality" is an entity playing the part of an effi-
i cient cause of electrical or vital phenomena. A mass of living
' protoplasm is simply a molecular machine of great complexity,
\ the total results of the working of which, or its vital phenom-
i ena, depend, on the one hand, upon its construction, and, on
I the other, upon the energy supplied to it ; and to speak of
1 " vitality " as anything but the name of a series of operations
I is as if one should talk of the " horologity " of a clocE]

Living matter, or protoplasm and the products of its meta-
morphosis, may be regarded under four aspects :

(1.) It has a certain external and internal form, the laiter
being more usually called structure ;

(2.) It occupies a certain position in space and in time ;

(3.) It is the subject of the operation of certain forces, in
virtue of which it undergoes internal changes, modifies exter-
nal objects, and is modified by them ; and —

(4.) Its form, place, and powers, are the effects of certain
causes.

In correspondence with these four aspects of its subject,
Biology is divisible into four chief subdivisions — I. Moephol-
ogt; II. DiSTEiBUTiox ; III. Physiology; IV. JStiology.

I. MOEFHOLOGY.

So far as living beings have a form and structure, they
fall within the province of Anatomy Siud Histology, the latter
being merely a name for that ultimate optical analysis of
living structure which can be carried out only by the aid of
the microscope.

And, in so far as the form and structure of any living
being- are not constant during the whole of its existence, but
undergo a series of changes from the commencement of that
existence to its end, living beings have a Development. The
history of development is an accuont of the anatomy of a liv-
ing being at the successive periods of its existence, and of the
manner in which one anatomical stage passes into the next.

Finally, the systematic statement and generalization of
the facts of Morphology, in such a manner as to arrange liv-
ing beings in groups, according to their degrees of likeness,
is Taxonomy.

HISTOLOGY. 17

The study of Anatomy and Development has brought to
light certain generalizations of wide applicability and great
importance.

1. It has been said that the great majority of living beings
present a very definite structure. Unassisted vision and or-
dinary dissection suffice to separate the body of any of the
higher animals, or plants, into fabrics of different sorts, which
always present the same general arrangement in the same
organism, but are combined in different ways in different
organisms. The discrimination of these comparatively few
fabrics, or tissues, of which organisms are composed, was the
first step toward that ultimate analysis of visible structure
which has become possible only by the recent perfection of
microscopes and of methods of preparation.

Histology, which embodies the results of this analysis,
shows that every tissue of a plant is composed of more or less
modified structural elements, each of which is termed a cell /
which cell, in its simplest condition, is merely a spheroidal
mass of protoplasm, surrounded by a coat or sac — the cell-
wall — which contains cellulose. In the various tissues, these
cells may undergo innumerable modifications of form — the
protoplasm may become differentiated into a nucleus with its
nucleolus, a primordial utricle, and a cavity filled with a wa-
tery fluid, and the cell-wall may be variously altered in com-
position or in structure, or may coalesce with others. But,
however extensive these changes may be, the fact that the
tissues are made up of morphologically distinct units — the
cells — remains patent. And, if any doubt could exist on the
subject, it would be removed by the study of development,
which proves that every plant commences its existence as a
simple cell, identical in its fundamental characters with the less
modified of those cells of which the whole body is composed.

But it is not necessary to the morphological unit of the
plant that it should be always provided with a cell-wall. Cer-
tain plants, such as Protococcus, spend longer or shorter peri-
ods of their existence in the condition of a mere spheroid of
protoplasm, devoid of any cellulose wall, while, at other times,
the protoplasmic body becomes inclosed within a cell-wall, fab-
ricated by its superficial layer.

Therefore, just as the nucleus, the primordial utricle, and
the central fluid, are no essential constituents of the morpho-
logical unit of the plant, but represent results of its meta-
morphosis, so the cell-wall is equally unessential ; and either
the term " cell " must acquire a merely technical significance

18 THE ANATOMY OF INVERTEBRATED ANIMALS.

as the equivalent of morphological unit, or some new term
must be invented to describe the latter. On the whole, it is
probably least inconvenient to modify the sense of the word
"cell."

The histological analysis of animal tissues has led to sim-
ilar results, and to difficulties of terminology of precisely the
same character. In the higher animals, however, the modifi-
cations which the cells undergo are so extensive that the fact
that the tissues are, as in plants, resolvable into an aggrega-
tion of morphological units, could never have been established
without the aid of the study of development, which proves
that the animal, no less than the plant, commences its exist-
ence as a simple cell, fundamentally identical with the less
modified cells which are found in the tissues of the adult.

Though the nucleus is very constant among animal cells,
it is not universally present ; and, among the lowest forms of
animal life, the protoplasmic mass which represents the mor-
phological unit may be, as in the lowest plants, devoid of a
nucleus. In the animal the cell- wall never has the character
of a shut sac containing cellulose ; and it is not a little diffi-
cult, in many cases, to say how much of the so-called " cell-
wall " of the animal cell answers to the " primordial utricle "
and how much to the proper " cellulose cell-wall " of the vege-
table cell. Bat it is certain that in the animal, as in the
plant, neither cell-wall nor nucleus is an essential constituent
of the cell, inasmuch as bodies which are unquestionably the
equivalents of cells — true morphological units — may be mere
masses of protoplasm, devoid alike of cell-wall and nucleus.

For the whole living world, then, it results : that the mor-
phological unit — the primary and fundamental form of life —
is merely an individual mass of protoplasm, in which no fur-
ther structure is discernible ; that independent living forms
may present but little advance on this structure ; and that all
the higher forms of life are aggregates of such morphological
units or cells variously modified.

Moreover, all that is at present known tends to the conclu-
sion that, in the complex aggregates of such units of which
ail the higher animals and plants consist, no cell has arisen
otherwise than by becoming separated from the protoplasm
of a preexisting cell ; whence the aphorism, " Omnis cellula e
celluldy

It may further be added, as a general truth applicable to
nucleated cells, that the nucleus rarely undergoes any consid-
erable modification, the structures characteristic of the tis-

DEVELOPMENT. X9

sues being formed at the expense of the more superficial pro-
toplasm of the cells ; and that, when nucleated cells divide,
the division of the nucleus, as a rule, precedes that of the
whole cell.

2. In the course of its development every cell proceeds,
from a condition in which it closely resembles every other
cell, through a series of stages of gradually-increasing diver-
gence, until it reaches that condition in which it presents the
characteristic features of the elements of a special tissue.
The development of the cell is, therefore, a gradual progress
from the general to the special state.

The like holds good of the development of the body as a
whole. However complicated one of the higher animals or
plants may be, it begins its separate existence under the
form of a nucleated cell. This, by division, becomes con-
verted into an aggregate of nucleated cells — the parts of this
aggregate, following different laws of grow^th and multiplica-
tion, give rise to the rudiments of the organs ; and the parts
of these rudiments again take on those modes of growth, mul-
tiplication, and metamorphosis, which are needful to convert
the rudiment into the perfect structure.

The development of the organism as a whole, therefore,
repeats in principle the development of the cell. It is a prog-
ress from a general to a special form, resulting from the grad-
ual differentiation of the primitively similar morphological
units of which the body is composed.

Moreover, when the stages of development of two animals
are compared, the number of these stages which are similar
to one another is, as a general rule, proportional to the close-
ness of the resemblance of the adult forms ; whence it fol-
lows that the more closely any two animals are allied in adult
structure, the later are their embrj^onic conditions distinguish-
able. And this general rule holds for plants no less than for
animals.

The broad principle, that the form in which the more com-
plex living things commence their development is always the
same, was first expressed by Harvey in his famous aphorism,
" Omne vivum ex ovo^^'^ which was intended simply as a mor-
phological generalization, and in no wise implied the rejection
of spontaneous generation, as it is commonly supposed to do.
Moreover, Harvey's study of the development of the chick led
him to promulgate that theory of "epigenesis," in which the
doctrine that development is a progress from the general to
the special is implicitly contained.

20 THE ANATOMY OF INVERTEBRATED ANIMALS.

Caspar F. Wolff furnished further, and indeed conclusive,
proof of the truth of the theory of epigenesis ; but, unfortu-
nately, the authority of Haller and the speculations of Bonnet
led science astray, and it was reserved for Von Baer to put the
nature of the process of development in its true light, and to
formulate it in his famous law.

3. Development, then, is a process of differentiation by
which the primitively similar parts of the living body become
more and more unlike one another.

This process of differentiation may be effected in several
ways:

(1.) The protoplasm of the germ may not undergo divi-
sion and conversion into a cell aggregate ; but various parts
of its outer and inner substance may be metamorphosed di-
rectly into those physically and chemically different materials
which constitute the body of the adult. This occurs in such
animals as the Infusoria, and in such plants as the unicellular
A.lgce and Fungi,

(2.) The germ may undergo division, and be converted
into an aggregate of division masses, or llastomeres, which
become cells, and give rise to the tissues by undergoing a
metamorphosis of the same kind as that to which the whole
body is subjected in the preceding case.

The body, formed in either of these ways, may, as a whole,
undergo metamorphosis by differentiation of its parts ; and
this differentiation may take place without reference to any
axis of symmetry, or it may have reference to such an axis.
In the latter case, the parts of the body which become dis-
tinguishable may correspond on the two sides of the axis (bi-
lateral symmetry), or may correspond along several lines paral-
lel with the axis (radial symmetry).

The bilateral or radial symmetry of the body may be fur-
ther complicated by its segmentation, or separation by divi-
sions transverse to the axis, into parts, each of which corre-
sponds with its predecessor or successor in the series.

In the segmented body, the segments may or may not give
rise to symmetrically or asymmetrically disposed processes,
which are appendages, using that word in its most general
sense.

And the highest degree of complication of structure, in
both animals and plants, is attained by the body when it be-
comes divided into segments provided with appendages ; when
the segments not only become very different from one another,
but some coalesce and lose their primitive distinctness ; and

DIFFERENTIATION OF STRUCTURE. 21

when the appendages and the segments into which they are
subdivided similarly become diflferentiated and coalesce.

It is in virtue of such processes that the flowers of plants,
and the heads and limbs of the Arthropoda and of the Ver-
tehrata^ among animals, attain their extraordinary diversity
and complication of structure. A flower-bud is a segmented
body or axis, with a certain number of whorls of appendages ;
and the perfect flower is the result of the gradual difi'erentia-
tion and confluence of these primitively similar segments and
their appendages. The head of an insect or of a crustacean
is, in like manner, composed of a number of segments, each
with its pair of appendages, which by diff"erentiation and con-
fluence are converted into the feelers and variously modified
oral appendages of the adult.

In some complex organisms, the process of difi"erentiation
by which they pass from the condition of aggregated embryo
cells to the adult, can be traced back to the laws of growth
of the two or more cells into which the embryo cell is divided,
each of these cells giving rise to a particular portion of the
adult organism. Thus the fertilized embryo cell in thearche-
gonium of a fern divides into four cells, one of which gives
rise to the rhizome of the young fern, another to its first root-
let, while the other two are converted into a placenta-like
mass which remains imbedded in the prothallus.

The structure of the stem of Chara depends upon the dif-
ferent properties of the cells, which are successively derived
by transverse division from the apical cell. An internodal
cell, which elongates greatly, and does not divide, is suc-
ceeded by a nodal cell, which elongates but little, and becomes
greatly subdivided ; this by another internodal cell, and so
on in regular alternation. In the same way the structure of
the stem, in all the higher plants, depends upon the laws
which govern the manner of division and of metamorphosis
of the apical cells, and of their continuation in the camhiuni
layer.

In all animals which consist of cell-aggregates, the cells
of which the embryo is at first composed arrange themselves
by the splitting, or by a process of invagination, of the blas-
toderm into two layers, the epihlast and the hypohlast^ be-
tween which a third intermediate layer, the mesoblast^ ap-
pears ; and each layer gives rise to a definite group of organs
in the adult. Thus, in the Vertehrata^ the epiblast gives rise
to the cerebro-spinal axis, and to the epidermis and its deriva-
tives ; the hypoblast, to the epithelium of the alimentary

22 THE ANATOMY OF INVERTEBRATED ANIMALS.

canal and its derivatives ; and the mesoblast, to intermediate
structures. The tendency of recent inquiry is to prove that
the several layers of the germ evolve analogous organs in in-
vertebrate animals, and to indicate the possibility of tracing
the several germ-layers back to the blastomeres of the yelk,
from the subdivision of which they proceed.

It is conceivable that all the forms of life should have pre-
sented about the same differentiation of structure, and should
have differed from one another by superficial characters, each
form passing by insensible gradations into those most like it.
In this case Taxonomy^ or the classification of morphological
facts, would have had to confine itself to the formation of a
serial arrangement, representing the serial gradation of these
forms in Nature.

It is conceivable, again, that living beings should have dif-
fered as widely in structure as they actually do, but that the
interval between any two extreme forms should have been
filled up by an unbroken series of gradations ; in which case,
again, classification could only affect the formation of series —
the strict definition of groups would be as impossible as in the
former case.

As a matter of fact, living beings differ enormously, not
only in differentiation of structure, but in the modes in which
that differentiation is brought about ; and the intervals be-
tween extreme forms are not filled up, in the existing world,
by complete series of gradations. Hence it arises that living
beings are, to a great extent, susceptible of classification into
groups, the members of each group resembling one another,
and differing from all the rest, by certain definite peculiarities.

No two living beings are exactly alike, but it is a matter
of observation that, among the endless diversities of living
things, some constantly resemble one another so closely that
it is impossible to draw any line of demarkation between them,
while they differ only in such characters as are associated
with sex. Such as thus closely resemble one another consti-
tute a niorphologianl species ; while different morphological
species are defined by constant characters which are not
merely sexual.

The comparison of these lowest groups, or morphological
species, with one another, shows that more or fewer of them
possess some character or characters in common — some feat-
ure in which they resemble one another and differ from all
other species — and the group or higher order thus formed is

MORPHOLOGICAL GROUPS. 23

a genus. The generic groups thus constituted are susceptible
of being arranged in a similar manner into groups of succes-
sively higher order, which are known as families^ orders,
classes, and the like.

The method pursued in the classification of living forms is,
in fact, exactly the same as that followed by the maker of an
index in working out the heads indexed. In an alphabetical
arrangement, the classification may be truly termed a mor-
phological one, the object being to put into close relation all
those leading words which resemble one another in the
arrangement of their letters, that is, in their form, and to keep
apart those which difi'er in structure. Headings which begin
with the same word, but difi'er otherwise, might be compared
to genera with their species ; the groups of words with the
same first two syllables, to families ; those with identical first
syllables, to orders ; and those with the same initial letter, to
classes. But there is this diff'erence between the index and
the Taxonomic arrangement of living forms, that in the for-
mer there is nothing but an arbitrary relation between the
various classes, while in the latter the classes are similarly
capable of coordination into larger and larger groups, until
all are comprehended under the common definition of living
beings.

The differences between " artificial " and " natural " clas-
sifications are diff'erences in degree, and not in kind. In each
case the classification depends upon likeness ; but in an artifi-
cial classification some prominent and easilj^-observed feature
is taken as the mark of resemblance or dissemblance ; while, in
a natural classification, the things classified are arranged ac-
cording to the totality of their morphological resemblances,
and the features which are taken as the marks of groups are
those which have been ascertained by observation to be the
indications of many likenesses or unlikenesses. And thus a
natural classification is a great deal more than a mere index.
It is a statement of the marks of similarity of organization ;
of the kinds of structure which, as a matter of experience, are
found universally associated together ; and, as such, it fur-
nishes the whole foundation for those indications by which
conclusions as to the nature of the whole of an animal are
drawn from a knowledge of some part of it.

When a paleontologist argues from the characters of a
bone or a shell to the nature of the animal to which that bone
or shell belonged, he is guided by the empirical morphologi-
cal laws established by wide observation, that such a kind of

24 THE ANATOMY OF INVERTEBRATED ANIMALS.

bone or shell is associated with such and such structural feat-
ures in the rest of the body, and no others. And it is these
empirical laws which are embodied and expressed in a natural
classification.

II. DiSTEIBUTIOX.

Living beings occupy certain portions of the surface of
the earth, inhabiting either the dry land, or the fresh or salt
waters; or being competent to maintain their existence in
either. In any given locality, it is found that these different
media are inhabited by different kinds of living beings ; and
that the same medium, at different heights in the air and at
different depths in the water, has different living inhabitants.

Moreover, t!ie living populations of localities which differ
considerably in latitude, and hence in climate, always present
considerable differences. But the converse proposition is not
true — that is to say, localities which differ in longitude, even
if they resemble one another in climate, often have very dis-
similar Faunae and Florm.

It has been discovered, by careful comparison of local fau-
nae and floras, that certain areas of the earth's surface are
inhabited by groups of animals and plants which are not found
elsewhere, and which thus characterize each of these areas.
Such areas are termed Provinces of Distribution. There is
no parit}'- between these provinces in extent, nor in the phys-
ical configuration of their boundaries ; and, in reference to
existing conditions, nothing can appear to be more arbitrary
and capricious than the distribution of living beings.

The study of distribution is not confined to the present
order of Nature ; but, by the help of geology, the naturalist is
enabled to obtain clear, though too fragmentary, evidence of the
characters of the faunae and florae of antecedent epochs. The re-
mains of organisms which are contained in the stratified rocks
prove that, in any given part of the earth's surface, the living
population of earlier epochs was different from that which now
exists in the locality ; and that, on the whole, the difference
becomes greater the farther we go back in time. The organic
remains which are found in the later Cainozoic deposits of any
district are always closely allied to those now found in the
province of distribution in which that locality is included ;
while in the older Cainozoic the resemblance is less ; and in
the Mesozoic, and the Palaeozoic strata, the fossils may be
similar to creatures at present living in some other province,
or may be altogether unlike any which now exist.

DISTRIBUTION IN TIME. 25

In any given locality, the succession of liHng forms may
appear to be interrupted by numerous breaks — the associated
species in each fossiliferous bed being quite distinct from
those above and those below them. But the tendency of all
palaeontological investigation is to show that these breaks are
only apparent, and arise from the incompleteness of the series
of remains which happens to have been preserved in any given
locality. As the area over which accurate geological investi-
gations have been carried on extends, and as the fossiliferous
rocks found in one locality fill up the gaps left in another, so
do the abrupt demarkations between the faunas and floras of
successive epochs disappear — a certain proportion of the gen-
era and even of the species of every period, great or small,
being found to be continued for a longer or shorter time into
the next succeeding period. It is evident, in fact, that the
changes in the living population of the globe which have taken
place during its history have been eflected, not by the sud-
den replacement of one set of living beings by another, but
by a process of slow and gradual introduction of new species,
accompanied by the extinction of the older forms.

It is a remarkable circumstance that, in all parts of the
globe in which fossiliferous rocks have yet been examined,
the successive terms of the series of li\4ng forms which have
thus succeeded one another are analogous. The life of the
Mesozoic epoch is everywhere characterized by the abundance
of some groups of species of which no trace is to be found in
either earlier or later formations ; and the like is true of the
Palaeozoic epoch. Hence it follows, not only that there has
been a succession of species, but that the general nature of
that succession has been the same all over the globe ; and it
is on this ground that fossils are so important to the geologist
as marks of the relative age of rocks.

The determination of the morphological relations of the
species which have thus succeeded one another, is a problem
of profound importance and difficulty, the solution of which,
however, is already clearly indicated. For, in several cases,
it is possible to show that, in the same geographical area, a
form iV, which existed during a certain geological epoch, has
been replaced by another form B, at a later period ; and that
this form B has been replaced, still later, by a third form C.
When these forms. A, B, and C, are comparcl together they
are found to be organized upon the same plan, and to be
very similar even in most of the details of their structure;
but B differs from A by a sli2:ht modification of some of its
2

26 THE ANATOMY OF INYERTEBRATED ANIMALS.

parts, which icodification is carried to a still greater extent
in C.

In ether words, A, B, and C, differ from one another in
the same fashion as the earlier and later stages of the em-
bryo of the same animals differ ; and, in successive epochs,
we have the group presenting that progressive specialization
which characterizes the development of the individual. Clear
evidence that this progressive specialization of structure has
actually occurred has as yet been obtained in only a few cases
(e. g., Equidce^ Crocodilia)^ and these are confined to the
highest and most complicated forms of life ; while it is de-
monstrable that, even as reckoned by geological time, the pro-
cess must have been exceedingly slow.

Among the lower and less complicated forms, the evidence
of progressive modification, furnished by ccmiparison of the
oldest with the latest forms, is slight, or absent ; and some
of these have certainly persisted, wdth very little change,
from extremely ancient times to the present day. It is as
important to recognize the fact that certain forms of life have
thus persisted, as it is to admit that others have undergone
progressive modification.

It has been said that the successive terms in the series of
living forms are analogous in all parts of the globe. But the
species which constitute the corresponding or homotaxic terms
in the series, in different localities, are not identical. And,
though the imperfection of our knowledge at present pre-
cludes positive assertion, there is every reason to believe that
geographical provinces have existed throughout the period
durino: w^hich oro^anic remains furnish us with evidence of the
existence of life. The wide distribution of certain Palaeozoic
forms does not militate against this view ; for the recent in-
vestigations into tlie nature of the deep-sea fauna have shown
that numerous Crustacea, £chi7iodermo.ta, and other inver-
tebrate animals, have as wide a distribution now as their ana-
logues possessed in the Silurian epoch.

III. Physiologt.

Tlius far, living beings have been regarded mereh' as
definite forms of matter, and biology has presented no con-
siderations of a different order from those whicli meet the
student of mineralogy. But living tilings are not only natural
bodies, having a definite form and mode of structure, growth,
and development. They arc machines in action ; and, under

FUNCTIONS AND ORGANS. 27

this aspect, the phenomena which they present have no par-
allel in the mineral world.

The actions of living matter are termed its functions ;
and these functions, varied as they are, may be reduced to
three categories. They are either — (1), functions which affect
the material composition of the body, and determine its mass,
which is the balance of the proces^ses of waste on the one
hand and those of assimilation on the other ; or (2), they are
functions which subserve the process of reproduction, which
is essentially the detachment of a part endowed with the pow-
er of developing into an independent whole ; or (3), they are
functions in virtue of which one part of the body is able to
exert a direct influence on another, and the body, by its parts
or as a whole, becomes a source of molar motion. The first
may be termed sustentative^ the second generative, and the
third correlative functions.

Of these three classes of functions the first two only can
be said to be invariably present in living beings, all of which
are nourished, grow, and multiply. But there are some forms
of life, such as many Fungi^ which are not known to possess
any powers of changing their form ; in which the protoplasm
exhibits no movements, and reacts upon no stimulus ; and in
which any influence which the different parts of the body ex-
ert upon one another must be transmitted indirectly from
molecule to molecule of the common mass. In most of the
lowest plants, however, and in all animals yet known, the
body either constantly or temporarily changes its form, either
with or without the application of a special stimulus, and
thereby modifies the relations of its parts to one another, and
of the whole to surrounding bodies ; while, in all the higher
animals, the different parts of the body are able to affect, and be
affected by one another, by means of a special tissue, termed
nerve. Molar motion is effected on a large scale by means of
another special tissue, muscle / and the organism is brought
into relation with surrounding bodies by means of a third
kind of special tissue — that of the sensory organs — by means
of which the forces exerted by surrounding bodies are trans-
muted into affections of nerve.

In the lo^vest forms of life, the functions which have been
enumerated are seen in their simplest forms, and they are ex-
erted indifferently, or nearly so, by all parts of the proto-
plasmic body ; and the like is true of the functions of the
body of even the highest organisms, so long as they are in
the condition of the nucleated cell, which constitutes the

28 THE ANATOMY OP INVERTEBRATED ANIMALS.

starting-point of their development. But the first process in
that development is the division of the germ into a number
of morphological units or blastomeres, which, eventually, give
rise to cells ; and, as each of these possesses the same physio-
logical functions as the germ itself, it follows that each mor-
phological unit is also a physiological unit, and the multicellu-
lar mass is strictly a compound organism, made up of a mul-
titude of physiologically independent cells. The physiologi-
cal activities manifested by the complex whole represent the
sum, or rather the resultant, of the separate and independent
physiological activities resident in each of the simpler con-
stituents of that whole.

The morphological changes which the cells undergo in
the course of the further development of the organism do
not affect their individuality ; and, notwithstanding the modi-
fication and confluence of its constituent cells, the adult or-
ganism, however complex, is still an aggregate of morphologi-
cal units. Nor is it less an aggregate of physiological units,
each of which retains its fundamental independence, though
that independence becomes restricted in various ways.

Each cell, or that element of a tissue which proceeds from
the modification of a cell, must needs retain its sustentative
functions so long as it grows or maintains a condition of
equilibrium ; but the most completely metamorphosed cells
show no trace of the generative function, and many exhibit
no correlative functions. Contrariwise, those cells of the adult
organism which are the unmetamorphosed derivatives of the
germ exhibit all the primary functions, not only nourishing
themselves and growing, but multiplying, and frequently
showing more or less marked movements.

Organs are parts of the body which perform particular
functions. In strictness, perhaps, it is not quite right to
speak of organs of sustentation or generation, each of these
functions being necessarily performed by the morphological
unit which is nourished or reproduced. What are called the
organs of these functions are the apparatuses by which cer-
tain operations, subsidiary to sustentation and generation, are
carried on.

Thus, in the case of the sustentative functions, all those
organs mny be said to contribute to these functions wliich are
concerned in brinains: nutriment within the reach of the ulti-
mate cells, or in removing waste matter from them ; while in
the case of the generative function, all those organs contribute
to the function whicii produce the cells from which germs are

MUSCLE AND NERYE. 29

given off; or help in the evacution, or fertilization, or develop-
ment, of these germs.

On the other hand, the correlative functions, so long as
they are exerted by a simple undifferentiated morphological
unit or cell, are of the simplest character, consisting of those
modifications of position which can be effected by mere
changes in the form or arrangement of the parts of the pro-
toplasm, or of those prolongations of the protoplasm which
are called pseudopodia or cilia. But, in the higher animals
and plants, the movements of the organism and of its parts
are brought about by the change of the form of certain tis-
sues, the property of which is to shorten in one direction
when exposed to certain stimuli. Such tissues are termed
contractile ; and, in their most fully developed condition,
muscular. The stimulus by which this contraction is natu-
rally brought about is a molecular change, either in the sub-
stance of the contractile tissue itself, or in some other part
of the body ; in which latter case, the motion which is set up
in that part of the body must be propagated to the contractile
tissue through the intermediate substance of the body. In
plants, there seems to be no question that parts which retain
a hardly modified cellular structure may serve as channels for
the transmission of this molecular motion ; wdiether the same
is true of animals is not certain. But, in all the more com-
plex animals, a peculiar fibrous tissue — nerve — serves as the
agent by w4iich contractile tissue is affected by changes oc-
curring elsewhere, and by which contractions thus initiated
are coordinated and brought into harmonious combination.
While the sustentative functions in the higher forms of life
are still, as in the lower, fundamentally dependent upon the
powers inherent in all the physiological units which make up
the body, the correlative functions are, in the former, deputed
to two sets of specially modified units, which constitute the
muscular and the nervous tissues.

When the different forms of life are compared together as
physiological machines, tliey are found to differ as machines
of human construction do. In the lower forms, the mechan-
ism, though perfectly well adapted to do the work for which
it is required, is rough, simple, and weak ; while, in the
higher, it is finished, complicated, and powerful. Considered
as machines, there is the same sort of difference between a
polyp and a horse as there is between a distaff and a spin-
ning-jenny. In the progress from the lower to the higher
organism, there is a gradual differentiation of organs and of

30 THE ANATOMY OF INTERTEBRATED ANIMALS.

functions. Each function is separated into many parts, wbich
are severally intrusted to distinct organs. To use the strik-
ing phrase of Milne-Edwards, in passing from low to high
organisms, there is a division of physiological labor. And
exactly the same process is observable in the development of
any of the higher organisms ; so that, physiologically as well
as morphologically, development is a progress from the gen-
eral to the special.

Thus far, the physiological activities of living matter have
been considered in themselves, and without reference to an}^-
thing that may affect them in the world outside the living
body. But living matter acts on, and is powerfully affected
by, the bodies which surround it; and the study of the in-
fluence of the " conditions of existence " thus determined
constitutes a most important part of physiolog^^

The sustentative functions, for example, can only be ex-
erted under certain conditions of temperature, pressure, and
light, in certain media, and with supplies of particular kinds
of nutritive matter ; the sufficiency of which supplies, again,
is greatly influenced by the competition of other organisms,
which, striving to satisfy the same needs, give rise to the
passive " struggle for existence." The exercise of the correl-
ative functions is influenced by similar conditions, and by the
direct conflict with other organisms, which constitutes the ac-
tive struggle for existence. And, finally, the generative func-
tions are subject to extensive modifications, dependent partly
upon what are commonly called external conditions, and part-
ly upon wholly unknown agencies.

In the lowest forms of life, the only mode of generation
at present known is the division of the bodj" into two or more
parts, each of which then grows to the size and assumes the
form of its parent, and repeats the process of multiplication.
This method of multiplication by fission is properly called
generation, because the parts which are separated are sev-
erally competent to give rise to individual organisms of the
same nature as that from which they arose.

In many of the lowest organisms the process is modified
so far that, instead of the parent dividing into two equal
parts, only a small portion of its substance is detached, as a
bud, which develops into the likeness of its parent. This
is generation by gemination. Generation by fission and by
gemmation is not confined to the simplest forms of life,
however. On the contrary, both modes of multiplication are

AGAMOGENESIS. 31

common not only among plants, but among animals of con-
siderable comp]exit3^

The multiplication of flowering plants by bulbs, tliat of
annelids by fission, and that of polyps by budding, are well-
known examples of these modes of reproduction. In all
these cases, the bud or the segment consists of a multitude
of more or less metamorphosed cells. But, in other in-
stances, a single cell detached from a mass of such undiffer-
entiated cells contained in the parental organism is the foun-
dation of the new organism, and it is hard to say whether such
a detached cell may be more fitly called a bud or a segment
— whether the process is more akin to fission or to gemma-
tion.

In all these cases the development of the new being from
the detached germ takes place without the influence of other
living matter. Common as the process is in plants and in
the lower animals, it becomes rare among the higher animals.
In these, the reproduction of the whole organism from a part,
in the way indicated above, ceases. At most we find that
the cells at the end of an amputated portion of the organism
are capable of reproducing the lost part ; in the very highest
animals, even this power vanishes in the adult ; and, in most
parts of the body, though the undifferentiated cells are
capable of multiplication, their progeny grow, not into whole
organisms like that of which they form a part, but into ele-
ments of the tissues.

Throughout almost the whole series of living beings, how-
ever, we find concurrently with the process of agamog&nesis,
or asexual generation, another method of generation, in which
the development of the germ into an organism resembling
the parent depends on an influence exerted b}' living matter
different from the germ. This is gamogenesis or sexual gen-
eration. Looking at the facts broadly, and without reference
to many exceptions in detail, it may be said that there is an
inverse relation between agamogenetic and gamogenetic re-
production. In the lowest organisms gamogenesis has not
yet been observed, while in the highest agamogenesis is ab-
sent. In many of the lower forms of life agamogenesis is the
common and predominant mode of reproduction, while gamo-
genesis is exceptional ; on the contrary, in many of the high-
er, while gamogenesis is the rule, agamogenesis takes place
exceptionally.

In its simplest condition, which is termed ^^ conjugation^'*
sexual generation consists in the coalescence of two similar

32 THE ANATOMY OF IXTERTEBRATED ANIMALS.

masses of protoplasmic matter, derived from dijGferent parts
of the same organism, or from two organisms of the same
species, and the single mass which results from the fusion
develops into a new organism.

In the majority of cases, however, there is a marked mor-
phological ditference between the two factors in the process,
and then one is called the raale, and the other the female,
element. The female element is relatively large, and under-
goes but little change of form. In all the higher plants and
animals it is a nucleated coll, to which a greater or less
amount of nutritive material, constituting a food-yelk, may
be added.

The male element, on the other hand, is relatively small.
It may be conveyed to the female element by an outgrowth
of the wall of its cell, which is short in many AlgcB and Fungi,
but becomes an immensely elongated tubular filament, in the
case of the pollen-cell of flowering plants. But, more com-
monly, the protoplasm of the male cell becomes converted
into rods or filaments, which usually are in active vibratile
movement, and sometimes are propelled by num.erous cilia.
Occasionally, however, as in manv JSTeinatoidea and Arthro-
poclci, they are devoid of mobility.

The manner in which the contents of the pollen-tube
affect the embryo cell in flowering plants is unknown, as no
perforation through, which the contents of the pollen-tube
may pass, so as actually to mix with the substance of the em-
bryo cell, has been discovered ; and there is the same diffi-
culty with respect to the conjugative processes of some of the
Cryptogennia. But in the great majority of plants, and in
all animals, there can be no doubt that the substance of the
male element actually mixes with that of the female, so
that, in all these cases, the sexual process remains one of con-
jugation ; and impregnation is the physical admixture of pro-
toplasmic matter derived from two sources, which may be
either different parts of the same organism, or different organ-
isms.

The effect of impregnation appears in all cases to be that
the impregnated protoplasm tends to divide into portions
(blcistomeres), which may remain united as a single cell-aggre-
gate, or some or all of which may become separate organ-
isms. A longer or shorter period of rest, in many cases,
intervenes between the act of impregnation and the com-
mencement of the process of division.

As a general rule, the female cell, which directly receives

GAMOGENESIS. 33

the influence of the male, is that which undergoes division
and eventual development into independent germs ; but there
are some plants, such as the FloridecE, in which this is not
the case. In these, the protoplasmic body of the trichogyne,
which unites with the spermatozooids, does not undergo
division itself, but transmits some influence to adjacent cells,
in virtue of which they become subdivided into independent
germs or spores.

There is still much obscurity respecting the reproductive
processes of the Infusoria ; but, in theVorticellidce, it would
appear that conjugation merely determines a condition of the
whole organism, which gives rise to the division of the endo-
plast or so-called nucleus, by which germs are thrown ofl";
and, if this be the case, the process would have some analogy
to what takes place in the FloridecB.

On the other hand, the process of conjugation by which
two distinct Dlporpce combine into that extraordinary double
organism, the Diplozoon paradoxum, does not directly give
rise to germs, but determines the development of the sexual
organs in each of the conjugated individuals; and the same
process takes place in a large number of the Infusoria,, if
what are supposed to be male sexual elements in them are
really such.

The process of impregnation in the Floridem is remark-
ably interesting, from its bearing upon the changes which
fecundation is known to produce upon parts of the parental
organism other than the ovum, even in the highest animals
and plants.

The nature of the influence exerted by the male element
upon the female is wholly unknown. No morphological dis-
tinction can be drawn between those cells which are capable
of reproducing the whole organism without impregnation
and those which need it, as is obvious from what happens in
insects, where eggs which ordinarily require impregnation,
exceptionally, as in many moths, or regularh^, as in the case
of the drones among bees, develop without impregnation.
Even in the higher animals, such as the fowl, the earlier
stages of division of the germ may take place without im-
pregnation.

In fact, generation may be regarded as a particular case
of cell-multiplication, and impregnation simply as one of tlie
many conditions which may determine or affect that process.
In the lowest organisms the simple protoplasmic ma^s divides,
and each part retains all the physiological properties of the

34 THE ANATOMY OF INVERTEBRATED ANIMALS.

whole, and consequently constitutes a germ whence the whole
body can be reproduced. In more advanced organisms each of
the multitude of cells into which the embryo cell is converted
at first, probably retains all, or nearly all, the physiological
capabilities of the whole, and is capable of serving as a re-
productive germ ; but, as division goes on, and many of the
cells which result from division acquire special morphological
and phj'siological properties, it seems not improbable that they,
in proportion, lose their more general characters. In propor-
tion, for example, as the tendency of a giv^en cell to become a
muscle-cell or a cartilage-cell is more marked and definite, it
is readily conceivable that its primitive capacity to reproduce
the whole organism should be reduced, though it might not be
altogether abolished. If this view is well based, the power of
reproducing the whole organism would be limited to those
cells which had acquired no special tendencies, and conse-
quently had retained all the powers of the primitive cell in
which the organism commenced its existence. The more ex-
tensively diffused such cells were, the more generalh' might
multiplication by budding or fission take place ; the more lo-
calized, the more limited would be the parts of the org-anism
in which such a process would take place. And, even w^here
such cells occurred, their development or non-development
might be connected with conditions of nutrition. It depends
en the nutriment supplied to the female larva of a bee wheth-
er it shall become a neuter or a sexually perfect female ; and
the sexual perfection of a large proportion of the internal
parasites is similarly dependent upon their food, and perhaps
on other conditions, such as the temperature of the medium
in which they live. Thus the gradual disappearance of aga-
mo2:enesis in the hig-her animals would be related witli that
increasing specialization of function which is their essential
characteristic ; and, when it ceases to occur altogether, it
may be supposed that no cells are left which retain unmodified
the powers of the primitive embryo cell. The organism is
like a society in which every one is so engrossed by his spe-
cial business that he has neither time nor inclination to marrv.
Even the female elements in the highest organisms, little
as they differ to all appearance from undifferentiated cells,
and though they are directly derived from epithelial cells
which have undergone very little modification from the condi-
tion of blastomeres, are incapable of full development unless
they are subjected to the influence of the male element, which
may, as Caspar Wolff suggested, be compared to a kind of

THE ALTERNATION OF GENERATIONS. 35

nutriment. But it is a living nutriment, in some respects
comparable to that which would be supplied to an animal
kept alive by transfusion, and its molecules transfer to the
impregnated embryo cell all the special characters of the or-
ganism to w^hich it belonged.

The tendency of the germ to reproduce the characters of
its immediate parents, combined, in the case of sexual genera-
tion, with the tendency to reproduce the characters of the
male, is the source of the singular phenomena of hereditary
transmission. No structural modification is so slight, and no
functional peculiarity is so insignificant in either parent, that
it may not make its appearance in the offspring. But the
transmission of parental peculiarities depends greatly upon
the manner in w^hich they have been acquired. Such as have
arisen naturally, and have been hereditary through many an-
tecedent generations, tend to appear in the progeny with
great force ; while artificial modifications — such, for example,
as result from mutilation — are rarely, if ever, transmitted.
Circumcision through innumerable ancestral generations does
not appear to have reduced that rite to a mere formality, as
it should have done if the abbreviated prepuce had become
hereditary in the descendants of Abraham ; while modern
lambs are born with long tails, notwithstanding the long-con-
tinued practice of cutting those of every generation short.
And it remains to be seen whether the supposed hereditary
transmission of the habit of retrieving among dogs is really
what it seems at first sight to be ; on the other side, Brown-
Sequard's case of the transmission of artificially-induced epi-
lepsy in Guinea-pigs is undoubtedly very weighty.

Although the germ always tends to reproduce, directly or
indirectly, the organism from which it is derived, the result
of its development differs somewhat from the parent. Usually
the amount of variation is insignificant ; but it may be con-
siderable, as in the so-called " sports ; " and such variations,
whether useful or useless, may be transmitted with great te-
nacity to the offspring of the subjects of them.

In many plants and animals which multiply both asexually
and sexually there is no definite relation between the aga-
mogenetic and the gamogenetic phenomena. The organism
may multiply asexually before, or after, or concurrently with,
the occurrence of sexual generation.

But in a great many of the lower organisms, both animal
and vegetable, the organism (A) which results from the im-
pregnated germ produces offspring only agamogenetically.

36 THE ANATOMY OF INVERTEBRATED ANIMALS.

It thus gives rise to a series of independent organisms (B,
B, B, . . .), which are more or less different from A, and
which sooner or later acquire generative organs. From their
impregnated germs A is reproduced. The process tlius de-
scribed is what has been termed the " alternation of genera-
tions " under its simplest form — for example, as it is exhibited
by the ScdpcB, In more complicated cases the independent
organisms which correspond with B may give rise agamo-
^eaetically to others (BJ, and these to others (B^), and so
on (e. g., Aphis). Bat, however long the series, a tinal term
appears which develops sexual organs, and reproduces A.
The " alternation of generations " is, therefore, in strictness,
an alternation of asexual with sexual generation, in which
the products of the one process differ from those of the
other.

The Hi/drozoa offer a complete series of gradations be-
tween those cases in which the term B is represented by a
free, self-nourishing organism (e. g., CyanfEO), through those
in which it is free but unable to feed itself ( (Jalycophoridce),
to those in which the sexual elements are developed in bodies
which resemble free zooids, but are never detached, and are
mere generative organs of the body on which they are devel-
oped ( (7i> rdi/loph ra) .

In the last case the " individual " is the total product of
the development of the impregnated embryo, all the parts of
which remain in material continuity with one another. The
multiplication of mouths and stomachs in a Cordylophora no
more makes it an aggregation of different individuals than
the multiplication of segments and legs in a centipede con-
verts that Arthropod into a compound animal. The Cordy-
lophora is a differentiation of a whole into many parts, and
the use of any terminology which implies that it results from
the coalescence of many parts into a whole is to be depre-
cated.

In Cordylophora the generative organs are incapable of
maintaining a separate existence ; but in nearly-allied Hydro-
zoa the unquestionable homologues of these organs become
free zooids, in many cases capable of feeding and growing,
and developing the sexual elements only after they have un-
dergone considerable changes of form. Morphologically, the
swarm of 3fedusrje thus set free from a Hydrozoon are as
much organs of the latter as the multitudinous pinnules of a
Comatida, with their genital glands, are organs of the Echi-
noderm. Morphologically, therefore, the equivalent of the

CAUSES OF THE PflENOMEXA OF LIFE. 37

individual Comatula is the Hydrozoic stock plus all the 3Ie-
dusce which proceed from it.

No doubt it sounds paradoxical to speak of a million of
A.phides^ for example, as parts of one morphological individ-
ual ; but beyond the momentary shock of the paradox no
harm is done. On the other hand, if the asexual Aphides
are held to be individuals, it follows, as a logical consequence,
not only that all the polyps on a Cordylophora tree are
" feeding individuals," and all the genital sacs " generative
individuals," while the stem must be a " stump individual,"
but that the eyes and legs of a lobster are "ocular" and
" locouiotive individuals." And this conception is not only
somewhat more paradoxical than the other, but suggests a
conception of the origin of the complexity 01 animal struct-
ure which is wholly inconsistent with fact.

IV. JEtiologt.

Morphology, distribution, and physiology, investigate and
determine the facts of biology. ^Etiology has for its object
the ascertainment of the causes of these facts, and the ex-
planation of biological phenomena, by showing that they con-
stitute particular cases of general physical laws. It is hardly
needful to say that aetiology, as thus conceived, is in its in-
fancy, and that the seething controversies, to which the
attempt to found this branch of science made in the " Origin
of Species " has given rise, cannot be dealt with in this place.
At most, the general nature of the problems to be solved, and
the course of inquiry needful for their solution, may be indi-
cated.

In any investigation into the causes of the phenomena of
life, the first question which arises is. Whether we have any
knowledge, and if so, what knowledge, of the origin of living
matter ?

In the case of all conspicuous and easily-studied organ-
isms, it has been obvious, since the study of Nature began,
that living beings arise by generation from living beings of
a like kind ; but, before the latter part of the seventeenth cen-
tury, learned and unlearned alike shared the conviction that
this rule was not of universal application, and that multitudes
of the smaller and more obscure organisms were produced by
the fermentation of not-living, and especially of putrefying
dead matter, by what was then termed generatio cequlvoca
or spontanea^ and is now called ahiogenesis. Redi showed

38 THE ANATOMY OF INVERTEBRATED ANIMALS.

that the general belief was erroneous in a multitude of in-
stances ; Spallanzani added largely to the list ; while the in-
vestigations of the scientific helminthologists of the present
century have eliminated a further category of cases in which
it was possible to doubt the applicability of the rule " omne
viviun e vivo'''* to the more complex organisms which consti-
tute the present fauna and flora of the earth. Even the most
extravagant supporters of abiogenesis at the present day do
not pretend that organisms of higher rank than the lowest
Fungi and Protozoa are produced otherwise than by genera-
tion from preexisting organisms. But it is pretended that
Bacteria, Torula^, certain Fungi, and *' Monads," are de-
veloped under conditions which render it impossible that
these organisms should have proceeded directly from living
matter.

The experimental evidence adduced in favor of this prop-
osition is always of one kind, and the reasoning on which
the conclusion that abiogenesis occurs is based may be stated
in the following form :

All living matter is killed by being heated to n degrees.

The contents of a vessel, the entry of germs from without
into which is prevented, have been heated to n degrees.

Therefore, all living matter which may have existed there-
in has been killed.

But living Bacteria, etc., have appeared in these contents
subsequently to their being heated.

Therefore, they have been formed abiogenetically.

No objection can be taken to the logical form of this rea-
soning, but it is obvious that its applicability to any particu-
lar case depends entirely upon the validity, in that case, of
the first and second propositions.

Suppose a fluid to be full of Bacteria in active motion,
what evidence have we that they are killed when that fluid
is heated to n degrees ? There is but one kind of conclusive
evidence, namely, that from that time forth no living Bacteria
make their appearance in the liquid, supposing it to be prop-
erly protected from the intrusion of fresh Bacteria. The
only other evidence, that, for example, which may be fur-
nished by the cessation of the motion of the Bacteria, and
such slight changes as our microscopes permit us to observe
in their optical characters, is simply presumptive evidence of
death, and no more conclusive than the stillness and paleness
of a man in a swoon are proof that he is dead. And the
caution is the more necessary in the case of Bacteria, since

ABIOGENESIS. 39

tnany of them naturally pass a considerable part of their ex-
istence in a condition in which the)^ show no marks of life
whatever save growth and multiplication.

If indeed it could be proved that, in cases which are not
open to doubt, living matter is always and invariably killed
at precisely the same temperature, there might be some
ground for the assumption that, in those which are obscure,
death must take place under the same circumstances. But
what are the facts? It has already been pointed out that,
leaving Bacteria aside, the range of high temperatures be-
tween the lowest, at which some living things are certainly
killed, and the highest, at which others certainly live, is rather
more than 100° Fahr., that is to say, between 104° Fahr. and
208° Fahr. It makes no sort of difference to the argument
how living beings have come to be able to bear such a tem-
perature as the last mentioned ; the fact that they do so is
sufficient to prove that, under certain conditions, such a tem-
perature is not sufficient to destroy life.*

Thus it appears that there is no ground for the assumption
that all living matter is killed at some given temperature be-
tween 104° and 208° Fahr.

No experimental evidence that a liquid may be heated to
n degrees, and yet subsequently give rise to living organisms,
is of the smallest value as proof that abiogenesis has taken
place, and for two reasons : Firstly, there is no proof that
organisms of the kind in question are dead, except their per-
manent incapacity to grow and reproduce their kind ; and,
secondly, since we know that conditions may largely modify
the power of resistance of such organisms to heat, it is far
more probable that such conditions existed in the experiment
in question, than that the organisms were generated afresh
out of dead matter.

Not only is the kind of evidence adduced in favor of
abiogenesis logically insufficient to furnish proof of its occur-
rence, but it may be stated, as a well-based induction, that
the more careful the investigator, and the more complete his
mastery over the endless practical difficulties which surround
experimentation on this subject, the more certain are his ex-
periments to give a negative result ; while positive results
are no less sure to crown the efforts of the clumsy and the
careless.

1 Messrs. Dallinger and Drysdale have recently sho\%Ti ffood grounds for
belie v'ing that the germs of some Monads are not destroyed dj exposure to a
temperature of 280° Fahr. or even 300° Fahr,

40 THE ANATOMY OF INVERTEBKATED ANIMALS.

It is argued that a belief in abiogenesis is a necessary
corollary from the doctrine of Evolution. This may be true
of the occurrence of abiogenesis at some time; but if the
present day, or any recorded epoch of geological time, be in
question, the exact contrary holds good. If _all living beings
have been evolved from preexisting forms of life, it is enough^
that a single particle of living protoplasm should once have
appeared on the globe, as the result of no matter what agency.
In the eyes of a consistent evolutionist, any further indepen-
dent formation of protoplasm would be sheer waste.

The production of living matter since the time of its first
appearance, only by way of biogenesis, implies that the spe-
cific forms of the lower kinds of life have undergone but little
change in the course of geological time, and this is said to be
inconsistent with the doctrine of evolution. But, in the first
place, the fact is not inconsistent with the doctrine of evolu-
tion properly understood, that doctrine being perfectly con-
sistent with either the progression, the retrogression, or the
stationary condition, of any particular species for indefinite
periods of time ; and, secondl}^ if it were, it would be so much
the worse for the doctrine of evolution, inasmuch as it is un-
questionably true that certain, even highly-organized, forms
of life have persisted without any sensible change for very
long periods. The Terehratula psittacea of the present day,
for example, is not distinguishable from that of the Cretaceous
epoch, while the highly-organized Teleostean fish, JBeryx^ of
the Chalk, differed only in minute specific characters from
that which now lives. Is it seriously suggested that the ex-
isting lerehratulcE and ISeryces are not the lineal descendants
of their Cretaceous ancestors, but that their modern repre-
sentatives have been independently developed from primordial
germs in the interval ? But if this is too fantastic a sugges-
tion for grave consideration, why are we to believe that tbe
Glohigerince of the present day are not lineally descended
from the Cretaceous forms ? And, if their unchanged genera-
tions have succeeded one another for all the enormous time
represented by the deposition of the Chalk and that of the
Tertiary and Quaternary deposits, what difficulty is there in
supposing that they may not have persisted unchanged for a
greatly longer period ?

The fact is, that at the present moment there is not a
shadow of trustworthy direct evidence that abiogenesis does
take place, or has taken place, within the period during
which the existence of life on the globe is recorded. But it

ORIGIN OF SPECIES. 41

need hardly be pointed out that the fact does not in the,
slightest degree interfere with any conclusion that may be i^ nr,
arrived at, deductively, from other considerations that, at
some time or other, abiogenesis must have taken place.^

If the hypothesis of evolution is true, living matter must
have arisen from not-living matter ; for, by the hypothesis,
the condition of the globe was at one time such that living
matter could not have existed in it,^ life being entirely in-
compatible with the gaseous state. But, living matter once
originated, there is no necessity for another origination, since
the hypothesis postulates the unlimited, though perhaps not
indefinite, modifiability of such matter.

Of the causes which have led to flie origination of living
matter, then, it may be said that we know absolutely nothing.
But postulating the existence of living matter endowed with
that power of hereditary transmission, and with that tendency
to vary which is found in all such matter, Mr. Darwin has
shown good reasons for believing that the interaction betw^een
living matter and surrounding conditions, which results in
the survival of the fittest, is sufficient to account for the
gradual evolution of plants and animals from their simplest
to their most complicated forms, and for the known phe-
nomena of Morphology, Physiology, and Distribution.

Mr. Darw^in has further endeavored to give a physical
explanation of hereditary transmission by his hypothesis
of Pangenesis ; while he seeks for the principal, if not the
only cause of variation in the influence of changing condi-
tions.

It is on this point that the chief divergence exists among
those who accept the doctrine of evolution in its general
outlines. Three views may be taken of the causes of varia-
tion :

a. In virtue of its molecular structure, the organism may
tend to vary. This variability may either be indefinite, or
may be limited to certain directions by intrinsic conditions.
In the former case, the result of the struggle for existence
would be the survival of the fittest among an indefinite
number of varieties ; in the latter case, it would be the
survival of the fittest among a certain set of varieties, the

1 It makes no difference if we adopt Sir TV. Thomson's hvpothesis, and
suppose that the germs of living things have been transported to our globe
from some other, seeing that there is as much reason for supposing that all
stellar and planetary components of the universe are or have been gaseous, as
that the earth has passed through this stage.

42 THE ANATOMY OF INVERTEBRATED ANIMALS.

nature and number of which would be predetermined by the
molecular structure of the organism.

h. The organism may have no intrinsic tendency to vary,
but variation may be brought about by the influence of con-
ditions external to it. And in this case, also, the variability
induced may be either indefinite or defined by intrinsic limi-
tation.

c. The two former cases may be combined, and variation
may to some extent depend upon intrinsic, and to some ex-
tent upon extrinsic, conditions.

At present it can hardly be said that such evidence as
would justify the positive adoption of any one of these views
exists.

If all living beings have come into existence by the gradual
modification, through a long series of generations, of a pri-
mordial living matter, the phenomena of embryonic develop-
ment ought to be explicable as particular cases of the general
law of hereditary transmission. On this view, a tadpole is
first a fish, and then a tailed amphibian, provided with both
gills and lungs, before it becomes a frog, because tlie frog
was the last term in a series of modifications whereby some
ancient fish became a urodele amphibian; and the urodele
amphibian became an anurous amphibian. In fact, the de-
velopment of the embryo is a recapitulation of the ancestral
history of the species.

If this be so, it follows that the development of any
organism should furnish the key to its ancestral history ; and
the attempt to decipher the full pedigree of organisms from
so much of the family history as is recorded in their develop-
ment has given rise to a special branch of biological specula-
tion, termed pliylogeny.

In practice, however, the reconstruction of the pedigree of
a group from the developmental history of its existing mem-
bers is fraught with difficulties. It is highly probable that
the series of developmental stages of the individual organism
never presents more than an abbreviated and condensed sum-
mary of ancestral conditions ; while this summary is often
strangely modified by variation and adaptation to conditions ;
and it must be confessed that, in most cases, we can do little
better than guess what is genuine recapitulation of ancestral
forms, and what is the effect of comparatively late adapta-
tion.

The only perfectly safe foundation for the doctrine of evolu-
tion lies in the historical, or rather archagological, evidence

PHYLOGENY. 43

that particular organisms have arisen by the gradual modifi-
cation of their predecessors, which is furnished by fossil
remains. That evidence is daily increasing in amount and in
weight ; and it is to be hoped that the comparison of the
actual pedigree of these organisms with the phenomena of
tbeir development may furnish some criterion by which the
validity of phylogenetic conclusions, deduced from the facts
of embryology alone, may be satisfactorily tested.

* " ' t

CHAPTER I.

I. THE DISTINCTIVE CHAEACTERS OF ANIMALS.

The more complicated forms of the living things, the
general characters of which have now been discussed, appear
to be readily distinguishable into widely-separated groups,
animals, and plants. The latter have no power of locomo-
tion, and only rarely exhibit any distinct movement of their
parts when these are irritated, mechanically or otherwise.
They are devoid of any digestive cavity; and the matters
which serve as their nutriment are absorbed in the gaseous
and fluid state. Ordinary animals, on the contrary, not only
possess conspicuous locomotive activit}^, but their parts
readily alter their form or position when irritated. Their
nutriment, consisting of other animals or of plants, is taken
in the solid form into a digestive cavity.

But even without descending to the very lowest forms of
animals and plants, we meet with facts which weaken the
force of these apparently broad distinctions. Among animals,
a coral or an oyster is as incapable of locomotion as an oak;
arid a tape-worm feeds by imbibition and not by the ingestion
of solid matter. On the other hand, the Sensitive-Plant and
the Sundew exhibit movements on irritation, and the recent
observations of Mr. Darwin and others leave little doubt that
the so-called " insectivorous plants " really digest and assimi-
late the nutritive matters contained in the living animals
which they catch and destroy. All the higher animals are
dependent for the protein compounds which they contain
upon other animals or upon plants. They are unable to man-
ufacture protein out of simpler substances ; and, although
positive proof is wanting that this incapacity extends to all
animals, it may safely be assumed to exist in all those forms
of animal life which take in solid nutriment, or which live
parasitically on other animals or plants, in situations in which
they are provided with abundant supphes of protein in a
dissolved state.

THE DISTINCTIVE CHARACTERS OF ANIMALS. 45

The great majority of the higher plants, on the contrary,
are able to manufacture protein when supplied with carbonic
acid, ammoniacal salts, water, and sundry mineral phosphates
and sulphates, obtaining the carbon which they require by
the decomposition of the carbonic acid, the oxygen of which
is disengaged. One essential factor in the performance of
this remarkable chemical process is the chlorophyll which
these plants contain, and another is the sun's light.

Certain animals {^Infusoria, Coelenterata, TarhellaricL)
possess chlorophyll, but there is no evidence to show what
part it plays in their economy. Some of the higher plants
when parasitic, and a great group of the lower plants, the
Fungi (which may be parasitic or not), are, however, devoid
of chlorophyll, and are consequently totally unable to derive
the carbon which they need from carbonic acid. Nevertheless
they are sharply distinguished from animals, inasmuch as they
are still, for the most part, manufacturers of protein. Thus
such a Fungus as Penicillium is able to fabricate all the con-
stituents of its body out of ammonium tartrate, sulphate, and
phosphate, dissolved in water (see siqyra, p. 14, note) ; and
the yeast-plant flourishes and multiplies with exceeding rapid-
ity in water containing sugar, ammonium tartrate, potassium
phosphate, calcium phosphate, and magnesium sulphate.

Nevertheless, the experiments of Mayer have shown that
when peptones are substituted for the ammonium tartrate,
the nutrition of the yeast-plant is favored instead of being
impeded. So that it would seem that the yeast-plant is able
to take in protein compounds and assimilate them, as if it
were an animal ; and there can be no reasonable doubt that
many parasitic Fungi, such as the Botrytis Bassiana of the
silk-worm caterpillar, the Empusa of the house-fly, and, very
probably, the Peronospora of the potato-plant, directly as-
similate the protein substances contained in the bodies of the
plants and animals which they infest ; nor is it clear that
these Fungi are able to maintain themselves upon less fully
elaborated nutriment.

Cellulose, amyloid, and saccharine compounds were former-
ly supposed to be characteristically vegetable products ; but
cellulose is found in the tests of Ascidians; and amyloid and
saccharine matters are of very wide, if not universal, occur-
rence in animals.

And on taking a comprehensive survey of the whole ani-
mal and vegetable w^orlds, the test of locomotion breaks down
as completely as does that of nutrition. For it is the rule

46 THE ANATOMY OF INVERTEBRATED ANIMALS.

rather than the exception among the lowest plants, that at
one stage or other of their existence they should be actively
locomotive, their motor organs being usually cilla^ altogether
similar in character and function to the motor organs of the
lowest animals. Moreover, the protoplasmic substance of the
body in many of these plants exhibits rhythmically pulsating
spaces or contractile vacuoles of the same nature as those
characteristic of so many animals.

No better illustration of the impossibility of drawing any
sharply-defined distinction between animals and plants can be
found than that which is supplied by the history of what are
commonly termed '"Monads,"

The name of "Monad"* has been commonly applied to
minute free or fixed, rounded or oval bodies, provided with
one or more long cilia {flagella), and usually provided with
a nucleus and a contractile vacuole. Of such bodies, all of
which would properly come under the old group of Motiadi-
dcBy the history of a few has been completely worked out ;
and the result is that, while some (e. g., Chlamydomonas,
zoospores of Peronospora and Coleochcete) are locomotive
conditions of indubitable plants, others {Hadiolaria, Nocti-
lucci) are embryonic conditions of as indubitable animals.
Yet others (zoospores of 3Iyxomyc€tes) are embryonic forms
of organisms which appear to be as much animals as plants ;
inasmuch as in one condition they take in solid nutriment,
and in another have the special morphological, if not physio-
logical peculiarities of plants; while, lastly, in the case of
such monads as those recently so carefully studied by Messrs.
Dallinger and Drysdale, the morphological characters of which
are on the whole animal, while their mode of nutrition is un-
known, it is impossible to say whether they should be regarded
as animals or as plants.

Thus, traced down to their lowest terms, the series of
plant forms gradually lose more and more of their distinctive
vegetable features, while the series of animal forms part with
more and more of their distinctive animal characters, and the
two series converge to a common term. Thfi most character-
istic morphological peculiarity of the plant is the investment
of each of its component cells by a sac, the walls of which
contain cellulose, or some closely analogous compound ; and

- 0. F. Muller, " Historia Vermium," 1773. " Vermis inconspicuus, sim-
plicissimus, pellucidus, punctiformis."

MORPHOLOGICAL DIFFERENTIATION. 47

the most characteristic physiological peculiarity of the plant
is its power of manufacturing protein from chemical com- i
pounds of a less complex nature.

The most characteristic morphological peculiarity of the i
animal is the absence of any such cellulose investment/ The '
most characteristic physiological peculiarity of the animal is
its want of power to manufacture protein out of simpler
compounds.

The great majority of living things are at once referable
to one of the two categories thus defined ; but there are some
in which the presence of one or other characteristic mark
cannot be ascertained, and others which appear at different
periods of their existence to belong to different categories.

n. THE MOEPHOLOGICAL DIFFEEEXTIATION OF A:NIMALS.

The simplest form of animal life imaginable would be a
protoplasmic body, devoid of motility, maintaining itself by
the ingestion of such proteinaceous, fatty, amyloid, and min-
eral matters as might be brought into contact with it by ex-
ternal agencies ; and increasing by simple extension of its
mass. But no animal of this degree of simplicity is known
to exist. The very humblest animals with which we are ac-
quainted exhibit contractility, and not only increase in size,
but, as they grow, divide, and thus undergo multiplication.
In the simplest known animals — the Protozoa — the proto-
plasmic substance of the body does not become differentiated
into discrete nucleated masses or cells, which by their meta-
morphosis give rise to the different tissues of which the adult
body is composed. And, in the low^est of the Protozoa^ the
body has neither a constant form nor any further distinction
of parts than a greater density of the peripheral, as com-
pared with the central, part of the protoplasm. The first
steps in complication are the appearance of one or more
rhythmically contractile vacuoles, such as are found in some
of the lower plants ; and the segregation of part of the in-

1 No analysis of the substance composing the cysts in -which so many of the
Protozoa inclose themselves temporarily has yet been made. But it is not im-
probable that it may be analogous to cliitin /"and, if so, it is 's\-ortby of remark
that, though cbitin is a nitrogenous body, it readily yields a substance ajipar-
ently identical with cellulose when heated with the double hyposulphite of
copper and ammonia. It is possible, therefore, that the difference between
the chitinous investment of an animal and the cellulose investment of a plant
may depend upon the proportion of nitrogenous matter which is present in
each case in addition to the chitiu.

48 THE ANATOMY OF INVERTEBRATED ANIMALS.

terlor protoplasm as a rounded mass, the " endoplast " or
" nucleus." Other Protozoa advance further and acquire
permanent locomotive organs. These may be developed
only on one part of the surface of the body, which may be
modified into a special organ for their support. In some, a
pedicle of attachment is formed, and the body may acquire a
dense envelope {Infusoria), or secrete an internal skeleton of
calcareous or silicious matter {Forarninifera, Itadiolaria), or
fabricate such a skeleton by gluing together extraneous par-
ticles [Forambiifera).

A mouth and gullet, with an anal aperture, may be formed,
and the permeable soft central portion of the protoplasm may
be so limited as to give rise to a virtual alimentary tract be-
tween these two apertures. The contractile vacuole may be
developed into a complicated system of canals (Paramceci-
um)y and the endoplast may take on more and more definite-
ly the characters of a reproductive organ, that is, may be the
focus of origin of germs capable of reproducing the individ-
ual ( Vorticella), In fact, rudiments of all the chief system
of organs of the higher animals, with the exception, more or
less doubtful, of the nervous, are thus sketched out in the
Protozoa, just as the organs of the higher plants are sketched
out in Cauler]}a,

In the Metazoa, which constitute the rest of the animal
kingdom, the animal, in its earliest condition, is a protoplas-
mic mass with a nucleus — is, in short, a Protozoon. But it
never acquires the morphological complexity of its adult state
by the direct metamorphosis of the protoplasmic matter of
this nucleated body — the ovum — into the different tissues.
On the contrary, the first step in the development of all the
Metazoa is the conversion of the single nucleated body into
an a <?2:re oration of such bodies of smaller size — the Morula —
by a process of division, which usually takes place with great
regularity, the ovum dividing first into two segments, which
then subdivide, giving rise to four, eight, sixteen, etc.,
portions, which are the so-called division masses or blasto-
meres.

A similar process takes place in sundry Protozoa and gives
rise to a protozoic aggregate, which is strictly comparable to
the Morxda. But the members of the protozoic aggregate
become separate, or at any rate independent existences.
What distiniruishes the metazoic asrarreorate is that, thoucfh its
component blastomeres also retain a certain degree of physi-
ological independence, they remain united into one morpho-

MORPHOLOGICAL DIFFERENTIATION. 49

logical whole, and their several metamorphoses are so ordered
and related to one another that they constitute members of a
mutually dependent commonalty.

The Metazoa are the only animals which fall under com-
mon observation, and have therefore been known from the
earliest times. All the higher languages possess general
names equivalent to our beast, bird, reptile, fish, insect, and
worm ; and this shows the very early perception of the fact
that, notwithstanding the wonderful diversity of animal forms,
they are modeled upon comparatively few great types.

In the middle of the last century the founder of modern
Taxonomy, Linnaeus, distinguished animals into Mammalia^
Aves, Amphibia, Pisces, Insecta, and Vermes, that is to say,
he converted common-sense into science by defining and giv-
ing precision to the rough distinctions arrived at by ordinary
observation.

At the end of the century, Lamarck made a most impor-
tant advance in general morphologj'', by pointing out that
mammals, birds, reptiles, and fishes, are formed upon one type
or comm^on p)lan, the essential character of which is the pos-
session of a spinal column, interposed between a cerebro-spi-
nal and a visceral cavit}' ; and that in no other animals is the
same plan of construction to be discerned. Hence he drew a
broad distinction between the former and the latter, as the
Vertebrata and the I]st^ertebrata. But the advance of
knowledge respecting the structure of invertebrated animals,
due chiefly to Swammerdam, Trembley, Reaumur, Peyssonel,
Goeze, Roesel, Ellis, Fabricius, O. F. Miiller, Lyonet, Pallas,
and Cuvier, speedily proved that the Invertebrata are not
framed upon one fundamental plan, but upon several ; and,
in 1795, Cuvier ^ showed that, at fewest, three morphological
types, as distinct from one another as they are from that of
the vertebrated animals, are distinguishable among the I?i-
vertebrata. These he named — I. Mollusques ; II. Insectes et
Vers ; III. Zoophytes. In the " Rescue animal " (1816), those
terms are Latinized, Animalia Mollusca, Articulata, and Ma-
diata. Thus, says Cuvier : " It will be found that there ex-
ist four principal forms, four general plans, if it may thus be
expressed, on which all animals appear to have been modeled ;
and the ulterior divisions of which, under whatever title natu-
ralists may have designated them, are merely slight modifica-
tions, founded on the development or addition of certain parts.

» Tableau ^lemeutaire de I'Histoire des Animaux, An vi.

3

50 THE ANATOMY OF INVERTEBRATED ANIMALS.

These four common plans are those of the Vertehrata, the MdU
lusca, the Articulata, and the Hadiata.^''

I For extent, variety, and exactness of knowledge, Cuvier
was, beyond all comparison, the greatest anatomist who has
ever lived ; but the absence of two conditions rendered if
1 impossible that his survey of the animal kingdom should be
' exhaustive, grand and comprehensive as it was.

Up to the time of Cuvier's death in 1832, microscopic in-
vestigation was in its infancy, and hence the great majority
of the lowest forms were either unknown or little understood;
and it was only in the third decade of the present century
that Rathke, Dollinger, and Von Baer, commenced that won-
derful series of exact researches into embryology which Von
Baer organized into a special branch of morphology, develop-
ing all its most important consequences and raising it to its
proper position, as the criterion of morphological theories.

Upon embryological grounds Von Baer arrived at the
same conclusion as Cuvier, that there are four ccmmon plans
of animal structure.

In the course of the last half-century the activity of anat-
omists and embryologists has been prodigious, and it may
be reasonably doubted whether any form of animal life re-
mains to be discovered which will not be found to accord
with one or other of the common plans now known. But at
the same time this increase of knowledge has abolished the
broad lines of demarkation which formerly appeared to sepa-
rate one common plan from another.

Even the hiatus between the Vertehrata and Ihe Tnver-
tehrata is partly, if not wholly, bridged over; and though
among the Invertehrata there is no difficulty in distinguish-
ing the more completely differentiated representatives of
such types or common plans as those of the Arthropoda^ the
A7ineUda, the 3Iollusca, the Tu7iicata, the Ecliinodermata^
the Coelenterata^ and the Porifera^ yet everj- year brings
forth fresh evidence to the effect that, just as the plan of the
plant is not absolutely distinct from that of the animal, so
that of the Vertebrate has its points of community with that
of certain of the Invertebrates ; that the Arthropod, the Mol-
lusk, and the Ecliinoderm plans are united by that of the
lower worms; and that the plan of the latter is separated by
no very great differences from that of the Coelenterate and
that of the Sponge.

Whatever speculative views may be held or rejected as to
ithe origin of the diversities of animal form, the facts of anat-

ANNULOSE DIFFERENTIATION. 51

omy and development compel the morphologist to regard
the whole of the Metazoa as modifications of one actual or \
ideal primitive type, which is a sac with a double cellular 1
wall, inclosing a central cavity and open at one end. This is )
what Haeckel terms a Gastrcea. The inner wall of the sac is
the hypoblast {endoderm of the adult), the outer the epiblast
.(ectoderm). Between the two, in all but the very lowest
Metazoa, a third layer, the mesoblast {mesoderm of the adult),
makes its appearance.

In the Porifera, the terminal aperture of the gastrsea
becomes the egestive opening of the adult animal, and the
ingestive apertures are numerous secondary pore-like aper-
tures formed by the separation of adjacent cells of the ec-
toderm and endoderm. The body may become variously
branched, a fibrous or spicular endoskeleton is usually de-
veloped in the ectoderm, and no perivisceral cavity is de-
veloped. There are no appendages for locomotion or pre-
hension ; no nervous system nor sensory organs are known to
exist ; nor are there any circulatory, respiratory, renal, or
generative organs.

In the Coeletiterata, the terminal aperture of the gastraea
becomes the mouth, and, if pores perforate the body-walls,
they do not subserve the ingestion of food. There is no sep-
arate perivisceral cavity, but, in man}'-, an enterocoele or sys-
tem of cavities, continuous with, but more or less separate
from, the digestive cavity, extends through the body. Pre-
hensile appendages, tefitacula, are developed in great variety.
A chitinous exoskeleton appears in some, a calcareous or chit-
iiious endoskeleton in others. There are no circulatory, re-
spiratory, or renal organs (though it is possible that certain
cells in the PorpitCG, e. g., may have a uropoietic function);
but special genital organs make their appearance, as do a
definitely-arranged nervous system and organs of sense.

The lowest Turhellaria are on nearly the same grade of
organization as the lower Coelenterata, but the thick meso-
derm is traversed by canals which constitute a loater-vascular
system. In the adult state these canals open, on the one side,
into the interstices of the mesodermal tissues, and, on the
other, communicate with the exterior. Their analogy to the
contractile vacuoles of the Infusoria on the one hand, and to
the segmental organs of the Annelids on the other, lead me
to think that they are formed by a splitting of the mesoblast,
and that they thus represent that form of perivisceral cavity
which I have termed a schizocoele, A nervous system, cou^

52 THE ANATOMY OF IXVERTEBRATED ANIMALS.

sisting of a single or double gangliou with two principal lon-
gitudinal nerve-cords, is found in many ; and there may be
eyes and auditory sacs.

Upon this foundation a gradual complication of form is
based, brought about by —

1. The elongation of the bilaterally symmetrical body and
the formation of a chitinous exoskeleton.

2. The development of a secondary aperture near the an-
terior end of the body, which becomes the permanent mouth.

3. The division of the mesoblast into successive segments
{somites).

4. The development of two nervous ganglia in each somite.

5. The outgrowth of a pair of appendages from each so-
mite, and their segmentation.

6. The gradual specialization of the somites into cephalic,
thoracic and abdominal groups ; and that of their appendages
into sense organs, jaws, locomotive limbs, and respiratory or-
gans.

7. The conversion of the schizocoele into a spacious peri-
visceral cavity containing blood ; the reduction of the water-
vascular system, and the appearance of pseudo-haemal vessels ;
and the replacement of these, in the higher forms, by a heart,
arteries, and veins, which contain blood.

8. The conversion of the simple inner sac of the gastra^a
into a hig-hly-complex alimentary canal, with special glandu-
lar appendages, representing the liver and the kidneys.

9. A similar differentiation of the genital apparatus.

10. A gradual complication of the eye, which, in its most
perfect form, presents a series of crystal-clear conical rods,
disposed perpendicularly to the transparent corneal region
of the chitinous exoskeleton, and connected by their inner
ends with the optic nerves of the pr^-oesophageal ganglia.

By such modifications as these the plan of the simple
Turbellarian gradually passes into that of the highest Ar-
thropod.

Starting from the same point, if the mesoblast does not
become distinctly segmented i if few, probably not more
than three, pairs of ganglia are formed ; if there are no seg-
mented appendages, but the chief locomotive organ is a mus-
cular foot developed in the neural aspect of the bod}^; if, in
the place of the chitinous exoskeleton, a shell is secreted by
a specially modified part of the haemal wall termed the man-
tle ; if the schizocoele is converted into a blood-cavity, which
communicates with the exterior by an organ of Bojanus, which

THE PLAN OF THE . ECHINODERMS. 53

appears to represent the water-vascular system and the seg-
mental organs ; and if, along with these changes, the aliment-
ary, circulatory, respiratory, genital, and sensory organs take
on special characters, we arrive at the complete Molluscan
plan.

From the Turbellarian to the Tunicate, or Ascidian, the
passage is indicated, if not effected, by J^alanoglofsus, which,
in its larval state, is comparable to an Appendicular ia with-
out its caudal appendage. On the other hand, the large
pharynx of the Tunicata and the circle of tentacula around
the oral aperture, with the single ganglion, approximate them
to the Polyzoa. In the perforation of the pharynx by lateral
apertures, which communicate with the exterior, either di-
rectly or by the intermediation of an atrial cavity, the Tuni-
cata resemble only Balanoglossus and tlie Vertebrata. The
axial skeleton of the caudal appendage has no parallel except
in the vertebrate notochord. In the structure of the heart
and the regular reversal of the direction of its contractions,
the Tunicata stand alone. The general presence of a test
solidified by cellulose is a marked peculiarity, but in esti-
mating its apparent singularity the existence of cellulose as
a constituent of chitin must be remembered. Finally, the
tadpole-like larvae of many Ascidians are comparable o\\\y to
the Cercarioe of Trematodes, on the one hand, and to ver-
tebrate larval forms on the other.

Yet another apparently very distinct type is met with in
the extensive group of the Echinodermata.

In all the other Metazoa, except the Porifera and Coelen-
terata, the plan of the body is, obviously, bilaterally sym-
metrical, the halves of the body on each side of a median ver-
tical plane being similar. Any disturbance of this symmetrv,
such as is found in some Artliropoda and in many Mollusca,
arises from the predominant development of one half. But,
in a Sea-urchin or Starfish, five or more similar sets of parts
are disposed around a longitudinal axis, which has the mouth
at one end and the anus at the other ; there is a radial sym-
metry, as in a sea-anemone or a Ctenophoran. Nevertheless,
close observation shows that, as is also the case in the Actinia
or Ctenophoran, this radial symmetry is never perfect, and
that the body is really bilaterally symmetrical in relation to
a median plane which traverses the centre of length of one
of the radiating metameres.

Another marked peculiarity of the Echinoderm type is

54 THE ANATOMY OF INVERTEBRATED ANIMALS.

the general, if not universal, presence of a system of " am-
bulacral vessels" consistino- of a circular canal around the
mouth, whence canals usually arise and follow the middle line
of each of the ambulacral metameres. And, in the typical
Echinoderm, these canals give off prolongations which enter
certain diverticula of the body-wall, the pedicels or suckers.

All Echinoderms have a calcareous endoskeleton.

In the chapter allotted to these animals, it will be shown
that they are modifications of the Turbellarian type, brought
about by a singular series of changes undergone by the endo-
derm and mesoderm of the larva or Echinoposdium.

III. — THE PHYSIOLOGICAL DIFFEEENTIATI0:N^ OF AI^IMALS, AND
THE MOKPHOLOGICAL DIFFERENTIATION OF THEIR ORGANS.

Regarded as machines for doing certain kinds of work,
animals differ from one another in the extent to which this
work is subdivided. Each subordinate group of actions or
functions is allotted to a particular portion of the body, which
thus becomes the organ of those functions ; and the extent
to which this division of physiological labor is carried differs
in degree within the limits of each common plan, and is the
chief cause of the diversity in the working out of the common
plan of a group exhibited by its members. Moreover, there
are certain types which never attain the same degree of physi-
ological differentiation as others do.

Thus, some of the Protozoa attain a grade of physiological
complexity as high as that which is reached by the lower Me-
tazoa. And, notwithstanding the multiplicity of its parts, no
Echinoderm is so highly differentiated a physiological ma-
chine as is a snail.

A mill with ten pairs of millstones need not be a more
complicated machine than a mill with one pair ; but if a mill
have two pairs of millstones, one for coarse and one for fine
grinding, so arranged that the substance ground passes from
one to the other, then it is a more complicated machine — a
machine of higher order — than that with ten pairs of similar
grindstones. In other words, it is not mere multiplication of
organs which constitutes physiological differentiation ; but
the multiplication of organs for different functions in the first
place, and the degree in which they are coordinated, so as to
work to a common end, in the second place. Thus, a lobster
is a higher animal, from a physiological point of view, than a

THE TEGUMENTARY SYSTEM. 55

Cyclops^ not because it has more distinguishable organs, but
because these organs are so modified as to perform a much
greater variety of functions, while they are all coordinated
toward the maintenance of the animal, by its well -developed
nervous system and sense-organs. But it is impossible to say
that, e. g., the Arthropoda, as a whole, are physiologically
higher than the 3Iollusca, inasmuch as the simplest embodi-
ments of the common plan of the Arthropoda are less differ-
entiated physiologically than the great majoritj'^ of Mollusks.
I may now rapidly indicate the mode in which physiologi-
cal differentiation is effected in the different groups of organs
of the body among the Metazoa.

Integumentary Organs. — In the lowest JTe^a^o (7, the integ-
ument and the ectoderm are identical, but, so soon as a mes-
oderm is developed, the layer of the mesoderm which is in
contact with the octoderm becomes virtually part of the in-
tegument, and in all the higher animals is distinguished as
the dermis {enderon^^ while the ectodermal cells constitute
the epiderm^is {ecdero7i). The connective tissue and muscles
of the integument are exclusively developed in the enderon ;
while, from the epidermis, all cuticular and cellular exoskele-
tal parts, and all the integumentary glands, are developed.
The latter are always involutions of the epidermis. The hard
protective skeletons in all invertebrate Metazoa, except the
JPorifera, the Actinozoa, the Echinodermata, and the Tuni-
cata, are cuticular structures, which may be variously impreg-
nated with calcareous salts formed on the outer surface of the
epidermic cells.

In the Porifera, the calcareous or silicfous deposit takes
place within the ectoderm itself, and probably the same pro-
cess occurs, to a greater or less extent, in the Actinozoa. In
those Tunicata which possess a test, it appears to be a struct-
ure sui ge7ieris, consisting of a gelatinous basis excreted by
the ectoderm, in which cells detached from the ectoderm
divide, multiply, and give rise to a deposit of cellulose. The
test may take on the structure of cartilage or even of connec-
tive tissue. In the Vertebrata alone do we find hard exo-
skeletal parts formed by the cornification and cohesion of epi-
dermic cells.

In the Actinozoa and the Echinodermata, the hard skele-
ton is, in the main, though perhaps not whoUj^ the result of
calcification of elements of the mesoderm. In some Mollusks
portions of the mesoderm are converted into true cartilage,

56 THE ANATOMY OF INYERTEBRATED ANIMALS.

while the enderon of the integument often becomes the seat
of calcareous deposit. The endoskeleton and the dermal exo-
skeleton of the Vertehrata are cellular (cartilage, notochord)
or fibrous (connective tissue) modifications of the mesoderm,
which may become calcified (bone, dentine). Recent investi-
gations tend to show that the enamel of the teeth is derived
from the ectoderm.

The Alimentary Ai^j-ciratus. — From the simple sac of the
Hydra or aproctous Turbellarian, we pass to the tubular ali-
mentary tract of the proctuchous Turhellaria, In the Itoti-
fera and Polyzoa there is a marked distinction into buccal
cavity, pharynx, oesophagus, stomach, and intestines ; while
distinct salivary, hepatic, and renal glands, are found in the
majority of the higher invertebrates, and, not unfrequently,
glands secreting an odorous or colored fluid appear in the
region of the termination of the alimentary canal.

The oral and gastric regions are armed with cuticular
teeth in many Invertehrata / but teeth formed by the calcifi-
cation of papillary elevations of the enderon of the lining of
the mouth are confined to the Vertehrata y unless, as seems
probable, the teeth of the EcMnidea have a similar origin.

The lining membrane of the oral cavity is capable of being
everted, as a proboscis, in many Invertehrata. The margins
of the mouth may be raised into folds, armed with cuticular
plates. In the Vertehrata^ the jaws are such folds, supported
by endoskeletal cartilages, belonging to the system of the
visceral arches, or by bones developed in and around them ;
but, in the Arthropoda^ what are usually termed jaws are
modified limbs.

The Blood and Circidatory Apjoaratus. — In the Coelen-
terata, the somatic cavity, or enterocoele, is in free commu-
nication with the digestive cavity, and not unfrequently
communicates with the exterior by other apertures. The fluid
which it contains represents blood ; it is moved by the con-
tractions of the body, and generally by cilia developed on the
endodermal lining of the enterocoele. In the Turhellaria^
Trematoda, and (Jestoidea^ the lacunae of the mesoderm and
the interstitial fluid of its tissues are the only representatives
of a blood-vascular system. It is probable that these com-
municate directly with the terminal ramifications of the water-
vascular system. In the Rotifer a., a spacious perivisceral
cavity separates the mesoderm into two layers, the splanch-

THE BLOOD -SYSTEM. 57

nopleure^ which forms the enderon of the alimentary canal,
and the somatopleure^ which constitutes the enderon of the
integument. The terminations of the water- vessels open into
this cavity. In Annelids, there is a similar perivisceral cavity
communicating in the same way with the segmental organs ;
but, in most, there is, in addition, a system of canals with
contractile walls, which, in some, communicate freely with
the perivisceral cavity, but, in the majority, are shut off from
it. These canals are filled by a clear, usually non-corpuscu-
lated fluid, which may be red or green, and constitute the
pseud-hf^mal system. The fluid which occupies the perivis-
ceral cavity contains nucleated corpuscles, and has the
characters of ordinary blood. It seems probable that the
fluid of the pseud-h^mal vessels, as it contains a substance
resembling hsemoglobin, represents a sort of respiratory
blood.

In the Arthropoda^ no segmental organs or pseud-haemal
vessels are known. In the lowest forms, the perivisceral
cavity and the interstices of the tissues represent the whole
blood-svstem, and colorless blood- cells float in their fluid con-
tents. In the higher forms, a valvular heart, with arteries
and capillaries, appears, but the venous system remains more
or less lacunar. In the Mollusca, the same gradual differen-
tiation of the blood- vascular system is observable. In very-
many, if not all, the blood-cavities communicate directly with
the exterior by the " organs of Bojanus " — which resemble
very simple segmental organs, and appear to be always asso-
ciated with the renal apparatus.

In the Vertebrata^ Amphioxus has a system of blood-ves-
sels, with contractile walls, and no distinct heart. In all
the other Vertebrates there is a heart with at fewest three
chambers {sinus veuosuSy atrhwi, ventricle), arteries, capil-
laries, and veins, and a system of lymphatic vessels connected
with the veins. The lymphatic fluid consists of a colorless
plasma, with equally colorless nucleated corpuscles ; the blood-
plasma contains, in addition, red corpuscles, which are nucle-
ated in Ichthyopsida and /Scmropsida, but have no nucleus
in the 3Iamnialia. The lymphatic vessels always communi-
cate with the interstitial lacunae of the tissues, and in the
lower Vertebrates are themselves, to a great extent, irregular
sinuses. The venous system presents many large sinuses in
the lower Vertebrates; while, in the higher forms, these
sinuses are for the most part replaced by definite vessels with
muscular walls. But the *' serous cavities " remain as vast

58 THE ANATOMY OF IXVERTEB RATED ANIMALS.

lymphatic lacunas. Valves make their appearance in the lym-
phatics and in tlie veins, and the heart becomes subdivided in
such a manner as to bring about a more and more complete
separation of the systemic circulatory apparatus from that
which supplies the respiratory organs.

The Respiratory System. — In the lower Metazoa respira-
tion is effected by the general surface of the body. In the
Annelids, processes of the integument, which are sometimes
branched and usually are abundantly ciliated and supplied
with pseud-haemal vessels, give rise to hranchice. Branchiae
abundantly supplied with blood-vessels, but never ciliated,
attain a great development in the Crustacea. The access of
fresh water to them is secured by their attachment to some
of the limbs ; and, in the higher Crustaceans, one of the ap-
pendages, the second maxilla, serves as an accessory organ
of respiration. Although especially adapted for aquatic res-
piration, they are converted into air-breathing organs in the
land-crabs, being protected and kept moist in a large cham-
ber formed by the carapace.

In some moUusks (e. g., Pteropoda^^ the delicate lining
membrane of the pallial cavity serves as the respiratory
organ ; but, in most, branched or laminated processes of the
body give rise to distinct branchiae. The mantle becomes an
accessory organ of respiration, being so modified as to direct,
or to cause, the flow of currents of water over the branchi?e
contained in its cavity. In many adult urodele Amphibia
{Perennihranchiata)^ and in the embryonic condition of all
Amphibia and of many fishes, branchiae of a similar character,
abundantly supplied with blood-vessels, are attached to more
or fewer of the visceral arches.

In all these cases the branchiae are external, and are de-
veloped from the integument. In Crustaceans and Mollusks
the blood with which they are supplied is returning to the
heart ; while, in the Vertebrata mentioned, it is flowing from
the heart ; and it will be observed that the gradual per-
fectioning of the respiratory machinery consists, first, in the
outgrowth of parts of the integument specially adapted to
subserve the interchange between the gases contained in the
blood and those in the surrounding medium; secondly, in the
increase of the surface of the branchiae, so as to enable them
to do their work more rapidly; thirdly, in the development
of accessory organs, by which the flow of water over the
branchiae is rendered definite and constant, and may be in-

THE RESPIRATORY SYSTEM. 59

creased or diminished in accordance with the needs of the
economy.

It is probable that the water-vascular system and the seg-
mental organs of Turbellarians and Annelids, the cloacal
tubes of the Gephyrea and of some Holothxiridea^ the ambu-
lacral vesicles of the Echinoderms, and the large pharyngeal
cavity of the Polyzoa^ to a greater or less extent, subserve
respiration, and constitute internal respiratory organs.

In Myriapoda and Insecta, the tracliece — tubes which
open on the surface of the body and contain air, and are
curiously similar in their distribution to the water-vessels of
the worms — constitute a y^Tj complete internal aerial respira-
tory apparatus.

In Arachnida^ trachese may exist alone, or be accom-
panied by folded pulmonary sacs, or the latter may exist
alone, as in the Scorpion. In this case, these lungs are sup-
plied by blood which is returning from the heart.

In these animals, the flow of air into and out of the air-
cavities is governed by the contractions of muscles of the
body, disposed so as to alter its vertical and longitudinal
dimensions. In the higher forms, the entrance and exit of
air is regulated by valves, placed at the external openings
(stigmata) of the tracheae, and provided with muscles, by
which they can be shut.

In the Enteropneusta and the Tunicata a new form of
internal aquatic respiratory apparatus appears. The large
pharynx is perforated by lateral apertures, which place its
cavity in communication with the exterior ; and water, taken
in by the mouth, is driven through these branchial clefts and
aerates the blood which circulates in their interspaces.

The respiratory apparatus of Amphioxt/s, of all adult
fishes, and of the tadpoles of the higher anurous Amp)hihia,
in a certain stage of their existence, is of an essentially simi-
lar character. The accessory respiratory apparatus for the
maintenance and the regulation of the currents of water over
the gills is furnished by the visceral arches and their mus-
cles ; and the respiratory blood flows from the heart.

In MoUusks which live on land {Pidmog aster o]?oda), the
lining wall of the mantle cavity becomes folded and highly
vascular, and subserves the aeration of the venous blood,
which flows through it on its way to the heart. The lung is
here a modification of the integument, and might be termed
an external lung. The lungs of the air-breathing J^ertehrata,
on the contrary, are diverticula of the alimentary canal, pos-

60 THE ANATOMY OF LWERTEBRATED ANIMALS.

terior to the binderraost of the visceral arches. They receive
their blood from the hindermost aortic arch. It therefore
flows from the heart. The gradual improvement of these
lungs as respiratory machines is effected, first, by the increase
of the surface over which the venous blood brought to the
lungs is distributed; secondly, by changes in the walls of
the cavity in which the lungs are contained, by which that
cavity gradually becomes shut off from the peritoneal cham-
ber, and divided from it by a muscular partition. Concur-
rently with these modifications, a series of alterations takes
place in the accessory apparatus of respiration, whereby the
machinery of inspiration, which, in the lower Vertebrata, is a
buccal force-pump, which drives air into the lungs, in the same
way as water is driven through the branchiae, is replaced by
a thoracic suction-pump, which draws air into the lungs by
dilatation of the walls of the closed cavity in which they are
contained. Alongr with these chansres, modifications of the
heart take place, in virtue of which one-half of its total
mechanical power becomes more and more exclusively ap-
propriated to the task of driving the blood through the lungs.
The term " double circulation " applied to the course of the
blood in the highest Vertebrata is, however, a misnomer. In
the highest, as in the lowest, of these animals, the blood com-
pletes but one circle, and the respiratory organ is in the
course of the outward current.

Many animals are truly amphibious, combining aquatic
and aerial respiratory organs.

Thus, among Mollusks, Ampullarla and Onchidum com-
bine branchias with pulmonary organs ; many Teleostean fishes
have the lining membrane of the enlarged branchial chamber
vascular and competent to subserve aerial respiration. And
in the Ganoids and Teleostei the presence of an air-bladder,
which is both functionally and morphologically of the same
nature as a lung, is yqtw common. But, in the majority of
the Teleostei, the air-bladder is turned aside from its pulmo-
nary function to subserve mechanical purposes, in affecting
the specific gravity of the body. On the other hand, in the
Ganoids and Dipnoi, the whole series of modifications by
which the air-bladder passes into the lung are patent. In
such lower Amphibia as Proteus and Menobranchus, bran-
chial respiration is predominant, and the lungs are subsidi-
ary ; but, in the higher, the lungs acquire greater importance,
while the branchije diminish, and eventually disappear.

THE UROPOIETIC SYSTEM. 61

The TIropoietic System. — Uropoietic organs, distinct from
the alimentary canal, are probably represented by the water-
vascular system and segmental organs of the worms. The
*' organs of Bojanus " of MoUusks are sacs or tubes opening,
on the one side, on the exterior of the body, and, on the
other, into some part of the blood-vascular system. So far,
as Gegenbaur has shown, they resemble the segmental organs
of Annelids. In the majority of the 3Iollusca, some part of
the wall of the organ of Bojanus is in close relation with the
venous system near the heart, and the nitrogenous waste of
the body is here eliminated from the venous blood. In the
Vertebrata, the renal apparatus is constructed en the same
principle. If for simplicity's sake we reduce a mammalian
kidnev to a ureter with a sino-le uriniferous tubule, it cor-
responds with an organ of Bojanus, so far as it contains a
cavity' communicating with the exterior at one end, and hav-
ing a vascular plexus — the Malpighian body — in intimate
contact with the opposite end. In the adult mammal there is
no direct communication between the urinary duct and the
blood-vascular system. But, inasmuch as recent researches
have proved that the ureter is formed by subdivision of the
Wolffian duct, and that the Wolffian duct is primitively a di-
verticulum of the peritoneal cavity, and remains for a longer
or shorter time (permanently, in some of the lower Verte-
bratn, as 3Iyxine) in communication therewith ; and since it
has further been shown that the peritoneal cavity communi-
cates directly with the lymphatics, and therefore indirectly
with the veins ; it follows that the vertebrate kidney is an
extreme modification of an organ, the primitive type of which
is to be found in the organ of Bojanus of the Mollusk, and in
the segmental organ of the Annelid ; and, to go still lower,
in tlie water-vascular system of the Turbellarian. And this, in
its lowest form, is so similar to the more complex conditions
of the contractile vacuole of a Protozoon, that it is hardly
straining analogy too far to regard the latter as the primary
form of uropoietic as well as of internal respiratory apparatus.

The j^ervous System. — In its essential nature, a nerve is
a definite tract of living substance, through w^hich the molec-
ular changes which occur in any one part of the organism
are conveyed to and affect some other part. Thus, if, in the
simple protoplasmic body of a Protozoon, a stimulus applied
to one part of the body were more readily transmitted to
some other part, along a particular tract of the protoplasm,

62 THE ANATOMY OF IxWERTEBRATED ANIMALS.

that tract would be a virtual nerve, altliougli it might have
no optical or chemical characters which should enable us to
distinguish it from the rest of the protoplasm.

It is important to have this definition of nerve clearly
before us in considering the question whether the lowest
animals possess nerves or not. Assuredly nothing of the
kind is discernible, b}' such means of investigation as we at
present possess, in Protozoa or Porifera y but an}' one who
has attentively watched the ways of a Colpodo, or still more
of a Vorticella^ will probably hesitate to deny that they
possess some apparatus by which external agencies give
rise to localized and coordinated movements. And when we
reflect that the essential elements of the highest nervous
system — the fibrils into which the axis-fibres break up — are
filaments of the extremest tenuity, devoid of any definite
structural or other characters, and that the nervous system
of animals only becomes conspicuous by the gathering to-
gether of these filaments into nerve-fibres and nerves, it will
be obvious that there are as strong morphological, as there
are physiological, grounds for suspecting that a nervous sys-
tem may exist very low down in the animal scale, and possi-
bly even in plants.

The researches of Kleinenberg, which may be readily veri-
fied, have shown that, in the common Hydra, the inner ends
of the cells of the ectoderm are prolonged into delicate pro-
cesses, which are eventually continued into very fine longi-
tudinal filaments, forming a layer between the ectoderm and
the endoderm.

Kleinenberg terms these neuro-muscular elements, and
thinks that they represent both nerve and muscle in their
undifferentiated state. But it appears to me that while the
assumed contractility of these fibres might account for the
shortening of the body of the Polyp, they can have nothing
to do with its lengthening. As the latter movements are at
least as vigorous as the former, we are therefore obliged to
assume sufficient contractility in the general constituents of
the body to account for them. And if so, what ground is
there for supposing that this contractility can be exerted by
only one tissue when the body shortens? To my mind, it is
more probable that " Kleinenherg's fibres " are solely inter-
nuncial in function, and therefore the primary form of nerve.
The prolongations of the ectodermal cells have indeed a
strangely close resemblance to those of the cells of the olfac-
tory and other sense-organs in the Vertebrata / and it seems

THE NERVOUS SYSTEM. 63

probable that they are the channels by which impulses affect-
ing any of the cells of the ectoderm are conveyed to other
cells and excite their contraction.

The researches of Eimer * upon the nervous system of the
Ctenophora are in perfect accordance with this view. The
mesoderm is traversed in all directions by very fine fibrils,
varying in diameter from ^qIqq to tj^qT ^^ ^^ inch. These
fibrils present numerous minute varicosities, and, at intervals,
larger swellings which contain nuclei, each with a large and
strongly refracting nucleolus. These fibrils take a straight
course, branch dichotomously, and end in still finer filaments,
which also divide, but become no smaller. They terminate
partly in ganglionic cells, partly in muscular fibres, partly in
the cells of the ectoderm and endoderm. Manv of the nerve-
fibrils take a longitudinal course beneath the centre of each
series of paddles, and these are accompanied by ganglionic
cells, which become particularly abundant toward the aboral
end of each series. The eight bands meet in a central tract
at the aboral pole of the body; but Eimer doubts the nervous
nature of the cellular mass which lies beneath the lithocyst
and supports the eye-spots.

The nervous system of the Ctenophoran is, therefore, just
such as would arise in Sjdra^ if the development of a thick
mesoderm gave rise to the separation and elongation of
Kleinenberg's fibres, and if special bands of such fibres,
developed in relation with the chief organs of locomotion,
united in a central tract directly connected with the higher
sensory organs. We have here, in short, virtual, though in-
completely differentiated, brain and nerves.

AH recent investigation tends more and more completely
to establish the following conclusions : firstly, that the central
ganglia of the nervous system in all animals are derived from
the ectoderm; secondly, that all the nerves of the sensory
organs terminate in cells of the ectoderm ; thirdly, that all
motor nerves end in the substance of the muscular fibres to
which they are distributed. So that, in the highest animals,
the nervous system is essentially similar to that of the lowest;
the difference consisting, in part, in the proportional size of
the nerve-centres, and, in part, in the gathering together of
the internuncial filaments into bundles, having a definite
arrangement, which are the nerves, in the ordinary anatomical
sense of the term.

» " Zoologiscln Studien auf Capri," Leipsic, 1873.

64 THE ANATOMY OF INVERTEBRATED ANIMALS.

And as respects the ectodermal cells which constitute the
fundamental part of the organs of the special senses, it is
becoming clear that the more perfect the sensory apparatus,
the more completely do these sensigenous cells take on the
form of delicate rods or filaments. Whether we consider the
organs of the lateral line in fishes and amphibia, the gusta-
tory bulbs, the olfactory cells, the auditory cells, or the
elements of the retina, this rule holds good.

Every one of the organs of the higher senses makes its
appearance in the animal series as a part of the ectoderm,
the cells of which have undergone a slight modification. In
the case of the eye, accessory structures, consisting of vari-
ously-colored masses of pigment, which surround the visual
cells, and of a transparent refracting cuticular or cellular
structure which lies superficially to them — a rudimentary
choroid and cornea — are next added. The highest form of
compound Arthropod eye differs from this only in the differ-
entiation of the layer of sensigenous cells into the crystalline
cones and their appendages, and it has not been clearly made
out that the simple eyes of most other Invertebrata have
undergone any further change.

But in JSfautiliis the nerve-cells and choroid line the walls
of a deep cup open externally ; which, though its development
has not been traced, may be safely assumed to result from
the involution of the retinal ectoderm. It may be compared
to an arthropod compound eye become concave instead of
convex.

In the higher Cephalopoda, the margins of the ocular
pouch nnite and give rise to a true cornea, which, however,
frequently remains perforated, and a crystalline lens is de-
veloped. In the higher Vertebrata the retina is still a modi-
fied portion of the ectoderm. For, inasmuch as the anterior
cerebral vesicle is formed by involution of the epiblast, and
the optic vesicle is a diverticulum of the anterior cerebral
vesicle, it necessarily follows that the outer wall of the optic
vesicle is really part of the ectoderm, its inner face being,
morphologically, a portion of the surface of the body. The
rods and cones of the vertebrate eye, therefore, exactly corre-
spond with the crj'stalline cones, etc., of the Arthropod eye;
and the reversal of the ends which are turned toward the
light in the Vertebrata is a necessary result of the extraor-
dinary change of position which the retinal surface undergoes
in them.

In the part of the ectoderm which takes on the auditory

REPRODUCTIVE ORGANS. 65

function, two kinds of accessory organs, solid particles sus-
pended in a fluid and fine hair-like filaments, are developed
in close relation with the nerve-endings. In the Crustacea
both are combined, and an involution of the sensory region
takes place, which usually remains open throughout life, and
represents the most rudimentary form of auditory labyrinth.
■^ The Crustacean ear is the parallel of the JVaiitihcs eye. In
the Vertehrata the membranous labyrinth is similarly an in-
volution of the integument, which remains open throughout
life in many fishes, but becomes shut off and surrounded by
thick mesoblastic structures in all the higher Vertebrata.
The tympanum and the ossicula auditus are additional
accessory structures, formed at the expense of the hyoman-
dibular cleft and its boundary-walls.

The Reproductive System. — The relation of the reproduc-
tive elements to the primitive layers of the germ is as yet
uncertain. E. van Beneden has brought forward very strong
evidence to the effect that in Hydractinia the spermatozoa
are modified cells of the ectoderm, and the ova of those of the
endoderm ; but, whether it can be safely concluded that this
rule holds good for animals generally, is a question that can
only be settled by much and difficult investigation. The fact
that, in the Vertehrata, the ova and spermatozoa are products
of the epithelial lining of the peritoneal cavity, and therefore
proceed from the mesoblast, appears at first sight directly to
negative any such generalization. But it must be remem-
bered that the origin of the mesoblast itself is yet uncertain,
and that it is quite possible that one portion of that layer may
originate in the ectoderm and another in the endoderm.

There is some reason to suspect that hermaphrodism was
the primitive condition of the sexual apparatus, and that uni-
sexuality is the result of the abortion of the organs of the other
sex, in males and females respectively.

Very low down in the animal series, among the Turhella-
ria^ the accessory organs of generation acquire a great com-
plexity. In the lower Turhellaria the excretory duct is a
mere short, wide passage. But, in the higher Turhellaria and
Trematoda, the female apparatus presents a germarium, in
which the ova are developed ; vitellarian glands, which give
rise to a supplemental or food yelk ; an oviduct ; a uterus and
vagina; and a spermatheca, in which the semen is stored up.
The male apparatus presents a testis, a vas deferens, and a
penis. The function of the vitellarian gland may be taken on

66 THE ANATOMY OF INVERTEBRATED ANIMALS.

by cells of the ovary, or oviduct ; or accessory yelk-substance
may be formed within the primitive ovum itself, in the Arthro-
poda and in most Mollusca ; but the reproductive organs in
all these animals are reducible to the Turbellarian type.

In the Annelids ( OUgochceta and Polychcjeta), the ovaria
and testes often have no special ducts, and their products
make their way out of the body by canals which appear to be
modified segmental organs.

In the Cephalopoda^ again, the ovaria and testes part with
their contents by dehiscence into chambers connected with the
water-cavities, which are prolongations of the organs of Boja-
nus. And they are conveyed away from these chambers by
ducts, the oviducts or vasa deferentia, which commence by
open mouths in them.

In the Vertehrata^ the reproductive organs either dehisce
and pour their contents into the peritoneal cavity, whence
they are conveyed outward by abdominal pores {Marsipo-
branchii^ many Teleostei), or they are continued into ducts
which open behind the anus separately from the renal open-
ing in the females, but in common with it in the males (most
Teleosteans) ; or their ducts are derived from portions of the
primitive renal apparatus which, as we have seen, is a struct-
ure of the same order as the organs of Bojanus and the seg-
mental organs. The testis is usually converted into a mass
of tubuli, which eventually open directly into the ducts (e^n-
didymis, vas deferens) derived from the renal organs. The
ovary, on the other hand, becomes an aggregation of sacs —
the Graafian follicles — and the oviducts open into the perito-
neal cavity.

Develojyment. — The embryo either passes through all
stages from the morula to a condition differing from the adult
only in size, proportions, and sexual characters, or it leaves the
e^g^ in a condition more or less remote from the adult state,
and sometimes exceedingly different from it. In the latter
case, the animal is said to undergo a metamorphosis. Each of
these modes of development occurs in members of the same
group, and often in closely allied forms : as, for example, the
former in the crayfish (Astacus), and the latter in the lobster
{Ilomarus).

When metamorphosis occurs, the larva may live under
conditions totally different from those under which the adult
passes its existence, and its structure may be variously modi-
fied in relation to these conditions. Thus the larva of an

DEVELOPMENT. 67

animal wbich is fixed in the adult state may be provided with
largely-developed locomotive organs ; while that of an adult
which" feeds by suction may be provided with powerful appa-
ratus for the seizure and manducation of vegetable and ani-
mal prey.

The larva of a free adult may be parasitic, or that of a
parasitic adult free and actively locomotive. Moreover, the
whole course of development maj' take place outside the body
of the parent, or more or less extensively within it ; whence
the distinction of ovijyarous, ovoviviparous^ and vivix>arous^
animals.

Finally, when development takes place within the body of
the parent, the foetus may receive nourishment from the latter
by means of an apparatus termed a placenta, by which an
exchange between the parental and foetal blood is readily
effected. Examples of placentas are found not only in the
higher mammals, but in some Plagiostome fishes and among
the Tunicata,

In many insects and in the higher Vertebrates, the em-
bryo acquires a special protective envelope, the amnion^
which is thrown off at birth ; while, in many Vertebrates,
another foetal appendage, the allantois, subserves the respi-
ration and nutrition of the foetus.

The strange phenomena included under the head of the
"Alternation of Generations," and which result from the di-
vision, by budding or otherwise, of the embryo which leaves
the egg, into a succession of independent zooids, only the last
of which acquires sexual organs, have already been gener-
ally discussed.

IV. — THE DISTEIBUTIOK^ OF ANIMALS.

The distribution of animals has to be considered under
two points of view : first, in respect of the present condi-
tion of Nature ; and secondly, in respect of past conditions.
The first is commonly termed Geo<^raphical, the second
Geological, or JPaleo7itological, Distribution. A little con-

* As eggs capable of development are alive, this terminology is etymologi-
cally bad ; and ovovivi parous is particularly objectionable, as all animals bring
forth live eggs, or that which proceeds from them. But, as understood to ap-
ply to animals which lay esrgs, to those in which the eggs are hatched within
the interior of the body without any special fcetal nutritive apparatus, and to
those in which the young are provided with such an apparatus, it has a certdn
convenience.

68 THE ANATOMY OF INVERTEBKATED ANIMALS.

sideration, however, will show that this classification of the
facts of distribution is essentially faulty, inasmuch as many
of the phenomena included under the second head are of the
same order as those comprehended under the first. Zoological
Distiibution comprehends all the facts which relate to the
occurrence of animals upon the earth's surface throughout
the time during which animal life has existed on the globe.
Therefore it embraces :

First, Zoological Chronology^ or the duration and order of
succession of living forms in time ; and —

Secondly, Zoological Geography^ or the distribution of life
on the earth's surface at any given epoch.

What is commonly termed Geographical Distribution is
simply that distribution which obtains at the present epoch ;
but it is obvious that, at any given moment in their past his-
tory, animals must have had some sort of geographical distri-
bution ; and considerable acquaintance with the nature of that
distribution has now been obtained for all the epochs, the
nature of the living population of which has been revealed by
fossil remains. I do not propose to deal at length with either
branch of distribution in this place, but a few broad truths
which have been established may be mentioned.

Geographical Distribution at the Present Epoch. — The
fauna of the deep sea (below five hundred fathoms) has been
shown, by the investigations of Wyville Thomson and his
associates of the Challenger, to present a striking general uni-
formity (in all parts of the world hitherto explored, in corre-
spondence with the general uniformity) of conditions at such
depths.

With respect to the surface of the sea, the observations of
the same naturalists tend to establish a like uniformity of the
great types of foraminiferal life throughout the tropical and
temperate zones — with a diminution in the abundance of that
life toward the arctic and antarctic regions, where it appears
to be replaced by Hadiolaria and Diatomaceous plants.

With regard to higher organisms, the oceanic Ilydrozoa
and the Ctenophora are undoubtedly very widely spread. It
is probable that they attain their maximum development in
warm seas, though the know^n facts are insufficient for the
definite conclusion. Sagitta and Appenclicularia, with many
genera of Copepoda^ Crustacea, and Pteropoda, are of world-
wide distribution ; and it is at present doubtful whether any
well-marked provinces of the ocean can be defined by the oc-

MARINE DISTRIBUTION. 69

currence of purely pelagic animals. On the other hand, shal-
low-water marine animals fall into assemblages characteristic
of definite areas or provinces of distribution — that is to say,
though many species have a world-wide distribution, others
occur only in particular localities, and certain geog'raphical
areas are marked by the existence in them of a number of
such peculiar species. The basins of the Pacific, the Indian
Ocean, the Atlantic, the Mediterranean, and the Arctic seas,
are thus especially characterized ; and even limited areas of
these great geographical divisions, such as the Celtic, (he
Lusitanian, and the Australian, have their peculiar features.

But, though the shallow-water marine faunae thus follow
the broad features of physical geography, and though, within
each great province of distribution thus marked out, temper-
ature and other physical conditions have an obvious influence
in determining the range of species ; yet, on comparing any
two great areas together, difierences in climatal conditions
are at once seen to be inadequate to account for the diifer-
ences between the faunae of the two areas. Climate in no
way enables us to understand why the Trigonia^ the pearly
N^autilus^ the Cestracion, the eared seals, and the penguins,
are found in the Pacific and not in the Atlantic area ;^ nor
why the Cetacea of the arctic and antarctic regions should be
as difl'erent as they are. When we turn to the distribution
of land-animals, the boundaries of the provinces of distribu-
tion correspond neither with physical features nor with cli-
matic conditions. Mammals, birds, reptiles, and amphibians,
are so distributed at the present day as to mark out four great
areas or provinces of distribution of very unequal extent, in
each of which a number of characteristic types, not found
elsewhere, occur. These are : 1. The Arctogceal, including
North America, Europe, Africa, and Asia as far as Wallace's
line, or the boundary between the Indian and the Papuan
divisions of the Indian Archipelago ; 2. The Austrocohim-
bian, comprising all the American Continent south of Mexico;
3. The Australian^ from Wallace's line to Tasmania ; 4. The
Kovozelanian^ including the islands of New Zealand.'^

* Penguins are found at the Cape of Good Hope and at the Falkland Islands,
"but not in the northern parts of tlae west coast of Africa, nor of the east coast
of South America. In the Pacific they stretch north to the Papuan and Peru-
vian coasts.

2 On the classification and distribution of the AlectoromorpJia, and Hetero-
morplM : Proceedings of the Zoological Society, 1868. Sclater on the " Gop-
graphical Distribution of Birds," Ibid., vol. ii. Pucheran, " Eevue et Magasin
de Zoologie," 1865. Murray, " The Geographical Distribution of Mammals,"

70 THE ANATOMY OF INVERTEBRATED ANIMALS.

There is no doubt that provinces of distribution, closely
corresponding with these, existed at the time of the Qua-
ternary and later Tertiary rocks. In Europe, North America,
and Asia, the Arctogseal province was as distinctly charac-
terized in the Miocene, and probably in the Eocene epoch, as
it is at present. What may have been the case in Austroco-
lumbia, Australasia, and JNovozelania, we have no means of
being certain, in the absence of sufficient knowledge of the
Miocene and Eocene deposits of those regions.

Our present knowledge of the geographical distribution
which obtained in the older periods does not enable us to
speak with any confidence as to the limits of the provinces of
distribution in the past. But this much is certain, that as far
back as the epoch of the Trias — at the dawn of the Secondary
period — the Meptilia and Amphibia of Europe, India, and
South Africa, and probably North America, presented the
same kind of resemblance as the mammals and birds of the
corresponding Arctogasal fauna do now. But then there is
no information respecting the reptiles and amphibians of the
corresponding epoch in Austrocolumbia and Australia, so that
it is impossible to say whether, in Triassic times, the Arcto-
gaeal province was limited as it is now.

Outside the limits of the Arctogasal province, the mate-
rials for forming a judgment of the distribution of animals
are altogether insufficient to enable us to draw any conclu-
sion as to the existence, and still less as to the boundaries, of
definite provinces of distribution in Palaeozoic times. No
remains of land-animals have yet been discovered. The
fresh-water fauna consists of Amphibians and Fishes, and we
know nothing, or next to nothing, of these in any^art of the
world except the Arctogaeal province.

A good deal is known of the older Silurian fauna outside
the boundaries of the present Arctogaeal province, and within
those of both the Austrocolumbian and Australasian prov-
inces. With a generally similar fades, the faunae of these
regions present clear differences. And, considering that the
groups of animals which are represented are chiefly deep-sea
and pelagic forms, it is not wonderful that this similarity of
facies should exist. The investigations of the Challenger
expedition show that such forms present a like similarity of
facies at the present day.

One of the most important facts which have been estab-
lished under the head of Zoological Chronology is, that in all
parts of the world the fauna of the later part of the Tertiary

THE OLDEST KNOWN FAUNA. 71

period, in any province of distribution, was made up of forms
either identical with, or very similar to, those now living in
that area.

For example, the elephants, tigers, bears, bisons, and hip-
popotamuses of the later tertiary deposits of England are all
closely allied to members of the existing Arctogaeal fauna ;
the great armadillos, anteaters, and platyrrhine apes of the
caves of South America, are as closely related to the existing
Austrocolumbian fauna ; and the fossil kangaroos, wombats
and phalangers of the Australian tertiaries to those which
now live in the Australasian province.

No remains of elephants occur in Australia, nor kangaroos
in Austrocolumbia ; nor anteaters and armadillos in Europe
in Tertiary deposits.

But, as we go back in time from the Tertiary to the Sec-
ondary, this law no longer holds good. Most of the few ter-
restrial mammals of secondary age which have been dis-
covered belong to Australasian and not to Arctogfeal types,
and the marine fauna resembles that of the existing Pacific
more than it does that of the Atlantic area, but differs from
both in the presence of numerous wholly extinct groups. It
looks as if, in the latter part of the Cretaceous epoch, a
great change in the limits of the then existing distributional
area had taken place, and the types now characteristic of
the Arctogaeal province had invaded regions from which
they had before been shut out. And the assumption of a
process of a similar character appears to me to be the only
rational explanation of the rapid advent of types absent in
the Palaeozoic deposits known to us, in the earlier Secondary
rocks.

Yet other results of first-rate importance have come out
of the study of the chronological relations of fossil remains.
Cuvier's investigations proved that the hiatuses between
existing groups of ungulate mammals tend to be filled up by
extinct forms. Later investigations have not only confirmed
this conclusion, but have shown that, in several cases, an
existing much-modified form can be shown to have been pre-
ceded in time, in the same distributional area, by exactly
such forms as it is necessar}^ should have existed, if the much-
modified existing animal had proceeded by way of evolution
from a simpler form.

For certain groups of animals, then, there is as much and
as good evidence of their having been evolved by successive
modification of a primitive form as the nature of the case per-

72 THE AXATOMY OF INYERTEBRATED ANIMALS.

mits us to expect. But the groups in which there is evi-
dence of such modifications during geologically recorded
time, all belong to the most differentiated members of their
classes. Lower forms, coextensive in duration, exhibit no
sign of having undergone any notable modification. While
the former are mutable^ the latter diie persistent types in rela-
tion to oreolocjical time.

Leaving the debatable question of the nature of Eozoon
aside, the oldest fossiliferous rocks are the Cambrian. The
scanty fauna therein preserved consists of forms which are
neither Protozoa nor Porifera^ nor even appertain to the
lowest groups of their respective classes. There is no reason
to believe that it gives a just notion of the contemporaneous
fa ana, nor is there any valid reason for the supposition that
it represents the forms of animal life which were the first to
make their appearance on our planet.

CHAPTER IL

THE PROTOZOA.

In its feeblest manifestations, the contractility of animals
results in mere changes of the form of the body, as in the
adult Gregarinoe ; but, from the sluggish shortenings and
lengthenings of the different diameters of the body which
these creatures exhibit, all gradations are traceable, through
those animals which push out and retract broad lobular pro-
cesses, to those in which the contractile prolongations take
the form of Ions: and slender filaments. Whether thick or
filamentous, such contractile processes are called "pseudo-
podia," when their movements are slow, irregular, and in-
definite ; " cilia " or " flagella," when they are rapid and occur
rhythmically in a definite direction ; but the two kinds of or-
gans are essentially of the same nature. It will be convenient
to distinguish those Protozoa which possess pseudopodia, as
myxopods^ and those which are provided with cilia or flagella,
as mastigopocls.

The Protozoa are divisible into a lower and a higher
group. In the former — the Monera — no definite structure is
discernible in the protoplasm of the body ; in the latter — the
ExDOPLASTicA — a Certain portion of this substance (the so-
called nucleus) is distinguishable from the rest;^ and, very
common!}', one or more " contractile vacuoles " are present.
The name of contractile vacuoles is given to spaces in the pro-
toplasm, which slowly become filled with a clear, watery fluid,
and, when they have attained a certain size, are suddenly
obliterated by the coming to2:ether, on all sides, of the proto-
plasm in which they lie. This systolic and diastolic move-
ment usually occurs at a fixed point in the protoplasm, at regu-
lar intervals, or rhythmically. But the vacuole has no proper

* I adopt this distinction as a matter of temporary convenience, though
I entertain great doubt whether it will stand the test of further investigation.

74 THE ANATOMY OF INYERTEBRATED ANIMALS.

wall, nor, in most cases, is any trace of it discernible at the
end of the systole. Occasionally, the vacuole certainly com-
municates with the exterior, and there is some reason to
think that such a communication mav always exist. The
function of these organs is entirely unknown, though it is an
obvious conjecture that it may be respiratory or excretory.

The "nucleus" is a structure which is often wonderful]}''
similar to the nucleus of an histological cell ; but, as its iden-
tity with this is not fully made out, it may better be termed
" endoplast." It is, usually, a rounded or oval body imbed-
ded in the protoplasm, and but slightly different therefrom
in either its optical or chemical characters. Generally it be-
comes more deeply stained by such coloring-matters as haema-
toxylin or carmine, and resists the action of acetic acid better
than the surrounding protoplasm.

In a few Protozoa there are many endoplasts in the sub-
stance of the body, and the protoplasm shows some tendency
to become partially differentiated into cells. But where, as
in the higher Infusoria^ the body presents a definite organi-
zation, with permanently differentiated constituents, which
may be properly termed tissues, these tissues do not result
from the metamorphosis of cells, but originate from the pro-
toplasm directly by changes of its physical and chemical char-
acters.

Conjugation, followed by the development of germs, which
are set free and assume the form of the parent, has been ob-
served in several groups of tlie Protozoa, but it is not yet
quite certain how far sexual distinctions are established among
these animals.

I. — THE MOXERA.

In these lowest forms of animals the entire living body
consists of a particle of gelatinous protoplasm, in which
no nucleus, contractile vacuole, or other definite structure,
is visible ; and which, at most, presents a separation into
an outer, more clear, and denser layer, the ectosarc ; and
an inner, more granular and fluid matter, the endosarc. The
outer layer is the seat of active changes of form, whereby
it is produced into pseudopodia, which attain a certain
length, and are then retracted, or are effaced by the devel-
opment of others from adjacent parts of the body. These
pseudopodia are sometimes broad, short lobes ; at others, elon-
gated filaments. When lobate, the pseudopodia remain dis-

THE MONERA. 75

tinct from one another, their margins are clear and transpar-
ent, and the granules which they may contain plainly flow
into their interior from the more fluid central part of the
body. But, when they are filiform, they are very apt to run
into one another, and give rise to networks, the constituent
filaments of which, however, readily separate and regain their
previous form ; and, whether they do this or not, the surfaces
of these pseudopodia are often beset by minute granules,
which are in incessant motion — like those which are observ-
able on the reticulations of the protoplasm of the cells in a
Trachscantia hair.

The rayxopod thus described moves about by means of its
contractile pseudopodia, and takes the solid matters which
serve as its food into all parts of its body by their aid ; w^hile
the undigested exuvia of the food are rejected from all parts
of the body in the same indiscriminate way. It is an organ-
ism which is devoid of any visible organs except pseudopodia ;
and, so far as is known at present, it multiplies by simple di-
vision.

The Protamoeha (with lobate pseudopodia) and Protoge-
nes (with filamentous pseudopodia), of Haeckel, are Monera
of this extremaly simple character. In Myxodictyiun (Haeck-
el) the pseudopodia of a number of such Monera run togeth-
er, and give rise to a complex network, or common plasmo-
dlum.

It is open to doubt, however, whether either Protamoeha,
Protogenes, or Myxodlctyum, is anything but one stage of a
cycle of forms, which are more completely, though perhaps
not yet wholly, represented by some other very interesting
Monera^ also described by Haeckel.

Thus, the genus 'Vampyrella is a myxcpod with filanien-
tous pseudopodia, a species of which infests one of the stalked
Diatomace:^, Gomplionema, feeding upon the soft parts of the
frustules of its host, by inserting some of its pseudopodia
through the raphe of the frustule, which it envelops, and
absorbing the contained protoplasm. Having thus provided
itself with abundant nourishment, by creeping from frustule
to frustule of the Gomphonema, it thrusts aside the last
evacuated frustule from its peduncle, and, taking its place,
draws in its pseudopodia, becomes spherical, and surrounds
itself with a structureless cvst, inclosed in which it remains
perched upon the peduncle of the Gomplionema. Soon its
protoj^lasm undergoes division into four equal masses, and
each of these, becoming converted into a young Vampyrella^

76

THE ANATOMY OF INVERTEBRATED ANIMALS.

escapes from the cyst, and recommences the predatory life of
its parent. In this case the myxopod becomes encysted, and

Fig, 1. — Trotomyx.a aurardiaca (Haeckel").— c, the Ptill condition snrronnded hy a
Btructurcless cyst ; h. encysted form, the protoplasm of which is dividinc:; c, the
cy?t burstini^and pivin;jf exit to the bodies into which the protoplasm breaks np.
These are at first "monads,'' d, earh beinp: provided with a flajrelliform cilinm,
by means of which it propels itself (cT). After a time each monad retracts its
cilinm. and resumes an Amoeba like form (e> ; many of these coalesce and form
a single plasmodinm. which crows and feeds nnder the form /. The specimen
fieared contains a Peridinhim. (above"), three J)ictyocyi(t(v. (below), and two I)<th'
mi'v (Diatomaceoup plants). In the centre. (Hacckel^ " Stadicn uber Mouereu,"
1870.)

THE MOXERA. 77

then undergoes fission into bodies, each of which passes direct-
ly into the form of the parent.

In another genus [3fb'xastrum) an additional complication
is introduced ; the myxopod becomes encysted, and then di-
vides into many portions ; each of these elongates, and sur-
rounds itself with a delicate, fusiform, silicious case. Thus
inclosed, the germs are set free by the bursting of the cyst ;
and, after a while, the contents of the silicious cases emerge,
and pass at once into the myxopod state.

In other genera, not only does the myxopod become en-
cysted before it undergoes fissive multiplication, but the forms
tlius produced differ from the myxopod in being free-swim-
ming organisms, propelled by a long vibratile filament or fla-
gellum, like those flagellate Infusoria which are termed "mo-
nads." After swimming about for a while, these mastigopods
draw in their flagella, and become creeping myxopod s. This
cycle of forms is exhibited by the genus Protomonas of
Haeckel. Lastl}^, in Protorayxa (Fig. 1) (Haeckel), there is
an alternation of a mastigopod (c/) with a myxopod form (e), as
in Protomonas. But each myxopod does not usually become
encysted alone. On the contrary, a certain number of the
myxopods unite together, and become fused into an active
Plasmodium ( /"), which exhibits no trace of their primitive
separation. The plasmodium becoming quiescent and sphe-
roidal, surrounds itself with a structureless cyst (a), divides
into numerous portions (^), which are converted into flagellate
mastigopods, and these finally return to the myxopod condi-
tion (c, c7, e). The cycle of life is here singularly similar to
that presented by the Myxomycetes, which have hitherto been
usually regarded as plants.

There is no means of knowing whether the cycle of forms
presented by Protomonas and Protomyxa is complete, or
whether some term of the series is still wanting ; and, con-
sidering how low down among plants the sexual process oc-
curs, it seems quite possible that some corresponding sexual
process yet waits to be discovered among the Monera. It is
posible that the fusion of separate Myxodictya and Proto-
myx(B into a plasmodium may be a process of sexual conjuga-
tion. On the other hand, it may well be that these extremely
simple organisms have not yet reached the stage of sexual
differentiation.

The Foramixifera. — Doubtless many 3Ionera remain to
be discovered, but they will probably be minute and inconspic-

78

THE AXATOMY OF INYERTEBRATED ANIMALS.

uous organisms like the rnajority of those already described.
The I orainimfera^ on the other hand, are Monera of the
Protogenes type, which, nevertheless, play and have played an
important part in the history of the globe, by reason of their
power of fabricating skeletons or shells, which may be com-
posed of horny (chitinous?) matter, or of carbonate of lime,
secreted from the water in which they live, or may be fabri-
Lated by sticking together extraneous matter, such as par-
ticles of sand.

The first step from such an organism as Protogenes to the
Forciraimfera is seen in Lieherkuhnici of Claparede, where
the pseudopodia are given off from only a small part of the
surface of the bodv, the rest remainino- naked and flexible.

In Gromia there is a similar restriction of the area from

Fig. 2.— a Eotalia, with extended pseudopodia : with an enbrged sectional view of the

chambered skeleton (after Schulze).

which pseudopodia proceed, but the rest of the body is in-
vested by a case of a membranous substance. Let this case
become hardened bv the attachment of foreiern bodies — as
particles of sand, or fragments of shelly matter, as in the so-
called arenaceous Foraminifera — or let a deposit of calca-
reous salts take place in it, and the Gromia would be con-
verted into a Foraminifer.

The infinitely diversified characters of the skeleton of the
Foraminifera depend — firsth^, upon the structure of the skele-
tal substance itself ; and, secondly, upon the form of the pro-
toplasmic body, which last, again, is largely dependent upon
the manner in which successive buds of protoplasm are devel-
oped from the parent mass, which, to begin with, is always
simple in form and commonly globular.

The substance of the calcareous skeleton itself, whatever

THE FORAMINIFERA.

79

be its form, is either perforated or imperforate. In the Im-
perforata ( Gromidce, Lituitidm^ MilioUdcB) the pseudopodia
are protruded from only one end of the body, the rest of
which is cut off from the exterior by the skeleton. In the
Perforata the substance of the shell is traversed by more or
less delicate canals filled with the protoplasm, which thus

Fig, 3.— Diagrams of Foraminifera.—A^ monothalamian ; B, (7, polj'thalamian ; i?,
horizontal ; i'and F, vertical sections of helicoid form. In E^ ihe chambers of
each turn of the spiral overlap their predecessors and conceal them, as in the
genus Numinuiites.

reaches the surface and gives off pseudopodia all over the
body. Hence, while the hard parts of the Imperforata form
a sort of exoskeleton, those of the Perforata have rather
the nature of an endoskeleton.

The simplest skeletons are spherical or flask-shaped, and
single-chambered. But complication arises by the addition
of new chambers, which may form a linear series, or be coiled
upon one another in various ways, or be irregularly aggre-
gated. Moreover, the new chambers may overlap those al-
ready formed in different degrees, and the interspaces between
the walls of the chambers may be variously filled up by sec-
ondary deposition until such large and apparently compli-
cated bodies as the Nummulites are built up.

The Foraniinifera are almost all marine animals, living
in the sea, from the surface to great depths, sometimes free,
and sometimes attached to other bodies.

The investigations of Major Owen, confirmed and extend'
ed by the recent work of H. M. S. Challenger, have proved
that such forms as Glohigerina^ Puli^inularia^ and OrhuUna^

80 THE ANATOMY OF IXVERTEBRATED ANIMALS.

constantly occur at the surface of all temperate and tropical
seas, and, together with the HacUolaria and the diatoma-
ceous plants which accompany them, form an important in-
gredient in the food of pelagic animals, such as the Salpcp..

It is no less certain that, at all depths down to 2,400 fath-
oms or thereabouts, GlobigeriiicB i\\ all stages of growth, and
containing more or less protoplasmic matter, are found at the
bottom mixed with the cases of the surface Diatoms and the
skeletons of Itadiolaria. The proportion of Glohigerincp,
OrbuUnce, and Pulvinularice, in the deep-sea mud increases
with the depth until, at depths beyond 1,000 fathoms, the
sea-bottom is composed of a fine, chalky ooze made up of
little more than the remains of these Foraminifera and their
associated Diatoms and JRadiolaria.

It may be regarded as certain, therefore, that some of the
chalky ooze arises from the precipitation to the bottom of the
skeletons of dead Glohigerinoe^ Pulmnularia?^ and Orbulince^
and it may be that the whole has this origin. On the other
hand, it may be that a greater or smaller proportion of these
Foraminifera really live at the bottom, as their congeners
are known to do at less depths.

It has been said that the condition of the surface-waters
and sea-bottom which has just been described obtains in all
temperate and hot seas ; or, speaking roughly, for 55° on
either side of the equator. Toward the northern and south-
ern limits of this zone the Foraininifera diminish, while Fui-
diolaria remain and Diatomaccce increase in proportion, so
that, in the circumpolar areas north and south of G0° in each
hemisphere, tlie surface-organisms are chiefly such as have
silicious skeletons. In accordance with this condition of the
surface-life, the ooze covering the sea-bottom in these regions
is no longer calcareous but silicious, being composed of the
cases of Diatoms and the skeletons of Fadiolaria often
largely mixed with ice, drifted mud, stones, gravel, and bowl-
ders.

If we suppose the globe to be uniformly covered with an
ocean 1,000 fathoms deep, the solid land forming its bottom
would be out of reach of rain, waves, and other agents of
degradation, and no sedimentary deposits would be formed.
But if Foraminifera and Diatoms, following the same huvs
of distribution as at present obtain, were introduced into
this ocean, the fine rain of their silicious and calcareous hard
parts would commence, and a circumpolar cap of silicious
deposit would begin to make its appearance in the north and

PROTOZOA AS ROCK-BUILDERS. 81

in the south ; while the intermediate zone would be covered
with Globijerina ooze, containing a comparatively small pro-
portion of silicious matter. The thickness of the calcareo-
silicious and silicious beds thus formed would be limited only
by time and the depth of the ocean. These strata, once ac-
cumulated, would be liable to all those influences of percolat-
ins: moisture and subterranean heat which are known to suf-
fice to convert silicious matters into opal, or quartzite, and
calcareous matters into the various forms of limestone and
marble. And such metamorphic agencies might more or less
completely obliterate the traces of their primitive structure.

But yet other changes might be effected. At the present
day, in the Gulf of Mexico, off the Agulhas Bank and else-
where, at no great depths (100 to 300 fathoms) the Fora-
miniferal mud is undergoing a metamorphosis of another
character. The chambers of the For a mi nif era become filled
by a green silicate of iron and alumina, which penetrates into
even their finest tubuli, and takes exquisite and almost in-
destructible casts of their interior. The calcareous matter is
then dissolved away, and the casts are left, constituting a
fine dark sand, which, when crushed, leaves a greenish mark,
and is known as " green-sand."

Moreover, the researches of the Challenger have shown
that in great areas of the Atlantic and Pacific Oceans over
which the sea has a depth exceeding 2,4:00 fathoms — areas in
some cases of many thousand square miles in superficies — the
bottom is covered not by Glohigerina ooze, but by a fine red
clay, which is also a silicate of iron and alumina. In this clay
no remains of Glohigerina or other calcareous organisms are
found ; but, where these great depths gradually pass into shal-
lower water, they make their appearance in a fragmentary
condition — gradually becoming more and more perfect as the
depth diminishes to 2,4:00 fathoms or thereabouts.

Xevertheless the Glohigerinm and other Foraminifera
abound at the surface over these areas as they do elsewhere,
and their remains must be rained down upon it. Why they
disappear, and what relation the red-clay mud has to them, is
a problem not yet satisfactorily solved. It has been suffo^ested
that they are dissolved and that the red clay is merely the
insoluble residue, left after the calcareous portion of their
skeletons has disappeared. In this case the red clay, like the
Glohigerina ooze, the silicious mud, and the green-sand, will
be an indirect product of living action.

Metamorphic processes operating upon clay, however, may

82 THE AXATOMY OF INVERTEBRATED ANIMALS.

convert it into slate ; and thus, all the fundamental minerals
of which rock-masses are composed may have fcrmed part of
living organisms, though no trace of their origin may be dis-
cernible in them in their final state.

Paleontology lends niuch support to the view that what
is here suggested as a theoretically possible origin of much
of the superficial crust of the globe may have been its actual
origin.

The nummulitic limestones of the Eocene epoch cover an
enormous area of Central and Southern Europe, North Africa,
West Asia, and India. And their chief mass is made up of
the more or less metamorphosed remains of Foraminifera.

The beds of chalk which underlie the nummulitic lime-
stones, and occupy a still greater area, are essentially iden-
tical with the Glohigerina ooze, the species of Globigerina
found in it being indistinguishable from those now living.
The remains of Ji oraminifera have been detected in the lime-
stones of all epochs as far as the Silurian, and Ehrenberg dis-
covered that an old Silurian green-sand, near St. Petersburg,
is composed of casts of Foraminifera just such as are now
being formed in the Gulf of Mexico. And if the JEozoon C ana-
dense be, as it appears to be, nothing but an incrusting form
of Foraminifer, the existence of these oganisms is carried back
to an epoch far beyond that at which any other evidence of
life has yet been found. So that it is possible that, as Wy-
ville Thomson has suggested, the enormously thick " azoic "
slaty and other rocks, which constitute the Laurentian and
Cambrian formations, may be to a great extent the metamor-
phosed products of Foraminiferal life.

Hence the words of Linnaeus may be literally true :

" Petrefaota non a calce, sed calx a petrefactis. Sic lapirlcs ah animalibus,
nee vice versa. Sic rupes saxei non primsevi, sed temporis filise."

And there may be no part of the common rocks, which enter
into the earth's crust, which has not passed through a living
organism at one time or another.

II. — THE e:n^doplastica.

1. The Radiolaria. — Most species of the genus Actino-
phrys or " sun-animalculo," which is common in ponds, are
simply free-swimming mvxopods with stiffish pseudopodia,
which radiate from all sides of the globular body. The sub-
stance of the latter presents one or more " contractile spaces "

THE HADIOLARIA.

83

or "vacuoles," which, rhythmically, become distended with
water, and are then obliterated by the contraction of the sur-
rounding protoplasm. But in the Actinophrys (or more
properly Actinosphceriiim) Eichornii (Fig. 4), the central
part of the protoplasm is distinguished from the rest by con-
taining a number of endoplasts. It thus leads to the RacUola-
ria [Polycistina of Ehrenberg), the simplest forms of which

zi — :? E*

Fig. A.-AcUnosphrBrium Elchhornii (after Hertwig and Lesser, " Ueber Rhi^opo-
den," Schulze's Archiv, 1876).

I.--The entire animal; c, c, contractile vacuoles. . v. x-* • i i.

II.— Part of the periphery much magnified; a, a, a, pseudopodia with stiff axial sub-
stance; n, nuclei or endoplasts. , . J i J ^;„

ni.— A very young Actinosphoerium, with only two nuclei and two pseudopodia,
much magnified.

consist essentially of a myxopod provided with filamentous,
radiating, and often anastomosing, pseudopodia. The centre
of the body is occupied bv a capsule filled with protoplasm ;

84 THE ANATOMY OF INVERTEBRATED ANIMALS.

this sometimes contains only an oil-globule, at others cells, or
nuclei, and crystalline bodies. In the layer of protoplasm

Fig. b.—Sphcerozoum pundatum.—A, a mass of the natural size ; B, two of the oval
central sacs with the colored vesicles and spicula which lie iu the investing pro-
toplasm, magnified.

Fig. Q.—Sphcerozoum ovodimare (after Haeckel), magnified.

from which the pseudopodia proceed, cell^eform bodies of a
bright-yellow color, which have been found to contain starch,
are usually developed,^ and this layer also gives rise to a skele-
ton of a horny, or, more usually, silicious character, which

» Even after the death of the Radiolarian, these yellow cells are said bv Cien-
kowsky to thrive and multiply, and the possibility that they may be parasites
must be borne in mind.

THE RADIOLARIA. 85

may have the form of detached spicula, or of coarticulated
rods, or of networks, or of plates of silicious matter, often of
the most exquisite delicacy and beauty. Most of the Hadi-
olaria are simple, solitary, and microscopical in size ; but
some, such as CollosphcBra and Splicerozoum (Figs. 5 and 6),
are formed of aggregates of such simple forms, and float, as
visible gelatinous masses, at the surface of the sea, which is
the habitation of the great majority of the Radiolaria.

The manner of multiplication and the development of the
Madiolaria have not yet been thoroughly worked out. Cien-
kowsky, however, has observed, in Collosphmra^ that the
protoplasm contained in the central capsule breaks up into
numerous rounded masses. The several capsules which are
associated together in the compound Radiolarian then be-
come isolated, by the dissolution of the protoplasm w^iich
invested and connected them, and finally burst, giving exit
to the rounded bodies ; which, while yet within the capsules,
were observed to be in active motion. The germs (for such
they appear to be) thus set free are 0.008 mm. long, ovate,
and carry two flagelliform cilia at their narrow ends ; so that
they are " monads." Each has in its interior a crystalline rod
and a few minute oil-globules. The further development of
these mastigopods has not yet been traced ; but, if, as is
probable, they pass into young Madiolaria (which, according
to Haeckel, possess no capsule, but resemble Actinosphm-
ria)^ the Radiolaria^ as members of the Endoplastica^ would
typify Protomonas among the Monera. Neither conjugation
nor fission has been observed amono; the ordinarv Radio-
laria, but both these processes take place in Actvnosphm-
rium j and, considering the resemblance of the young Radio-
laria to Actinosphmrium^ it seems probable that conjugation
and fission will yet be discovered among them.

ActinosphcBrium has been observed to undergo multipli-
cation, by division of its central substance into a certain
number of spheroids, and every spheroid becomes inclosed in
a silicious case. After a period of rest, a young Actinosphm-
riwn emerges from each of these cysts.

The marine Radiolaria all inhabit the superficial stratum
of the sea, and must fabricate their skeletons at the expense
of the infinitesimally small proportion of silex which is dis-
solved in sea-water; but, when they die, these skeletons sink
to the bottom, and there accumulate, together with the Fora-
'ininifera, in warm and temperate regions ; and with the
cases of the diatomaceous plants, which abound at the sur-

86 THE ANATOMY OF INVERTEBRATED ANIMALS.

face, along with the JRadiolaria^ all over the globe {see p. 80).
The late investigations of Archer and others have demon-
strated the existence of a considerable number of fresh-water
Radiolaria,

Extensive masses of tertiary rock, such as that which is
found at Oran, and that w^hich occurs at Bissex Hill, in Bar-
badoes, are very largely made up of exquisitely preserved
skeletons of Madiolaria. But, though there can be little
doubt that Madiolaria abounded in the Cretaceous sea, none
are found in the chalk, their silicious skeletons having prob-
ably been dissolved and redeposited as flint.

2. The Peotoplasta. — The proper Amoehce have broad
and ovate pseudopodia, and resemble JProtamoeha (p. 75) very
closely; but they present an advance upon its structure, by
exhibiting a distinct endoplast (nucleus) and a contractile
vacuole. In Arcella, there are many such nuclei. They thus
stand in somewhat the same relation to JProtamoeha as A.cti-
nophrys does to Protogenes.

Moreover, there are Amoehce in which the power of throw-
ing out pseudopodia is confined to one region of the body ;
and others, as Arcella, in w^hich a shell is formed over the
rest of the body. In other Amoehce, as A. radiosa^ the pseu-
dopodia are few, narrow, and but little mobile. But the
Amoehce present no such diversity of skeletal development as
the Forami7iifera do. They multiply by division, and in
some cases — e. g.. A, sphoerococcus of Haeckel — become en-
cysted before they divide.

Amoehce (the " proteus animalcules " of the older writers)
are not uncommon, and sometimes are very abundant, in
fresh waters ; they also occur in damp earth and in the sea,
but there is much doubt whether many of them are to be
regarded as independent organisms, or -whether they are not
rather stages in the development of other animals or even
of plants, such as Myxomycetes. Leaving out the contractile
vacuole, the resemblance of an Amoeha in its structure, man-
ner of moving, and even of feeding, to a colorless corpuscle
of the blood of one of the higher animals is particularly note-
worthy.^

3. The Gregari:^d>^ are very closely allied to the Amoe-
hce, but, in the cycle of forms through which they pass, they
curiously resemble 3Iyxastrum. In form they are spheroidal

1 Contractile vacuoles have been observed in the colorless blood-corpus-
cles of Amphibia under certain conditions.

THE GREGARIXIDyE.

87

or elongated oval bodies, sometimes divided by constrictions
into segments. Occasionally, one end of the body is pro-
duced into a sort of rostrum, which may be armed with re-
curved horny spines.

In the ordinary GregarinoPy the body presents a denser
cortical layer (ectosarc) and a more fluid inner substance
(endosarc), in which last the endoplast (nucleus) is imbed-
ded. The presence of contractility is manifested merely by
slow changes of form, and nutrition appears to be effected by
the imbibition of the fluid nutriment, prepared by the organs
of the animals in which the Gregarince are parasitic. There
is no contractile vacuole.

The Greg ar mm have a peculiar mode of multiplication,
sometimes preceded by a process which resembles conju-
gation. A single Gregarbia (or two which have become
applied together) surrounds itself with a structureless cyst.

Fio. 7.— .4, aregarina of the earthworm (after Lieberkuhn) ; B. encvsted ; C. D,
contents divided into peeudo-navicellae ; E, F, free pseudo-navicel!», G, H, free
amcebiform contents of the latter.

The nucleus disappears, and the protoplasm breaks up (in a
manner very similar to that in which the protoplasm of a

88 THE ANATOMY OF INVERTEBRATED ANIMALS,

sporangium of Mucor divides into spores) into small bodies,
each of which acquires a spindle-shaped case, and is known
as a 'pseudo-nanicella. On the bursting of the cyst these
bodies are set free, and, when placed in favorable circum-
stances, the contained protoplasm escapes as a small active
body like a Protamoeha. M. E. van Beneden has recently dis-
covered a very large Gregarina (G. gigantea), which inhab-
its the intestine of the lobster, and his careful investigation
of its structure and development has yielded very interesting
results.

Gregarina gigantea attains a length of two-thirds of an
inch. It is long and slender, and tapers at one extremity,
while the other is obtuse, rounded, and separated by a slight
constriction from the rest of the body, which is cylindroidal.
The outer investment of the body is a thin structureless cu-
ticle ; beneath this lies a thick cortical layer (ectosarc), dis-
tinguished by its clearness and firmness from the semifluid
central substance (endosarc), which contains many strongly-
refracting granules. In the centre of the body, the ellipsoid
"nucleus," with its "nucleolus," fills up the whole cavity of
the cortical layer, and thus divides the medullary substance
into two portions. The body of this Gregarina may present
longitudinal striations, arising from elevations of the inner
surface of the cortical layer, which fit into depressions of the
medullary substance ; but these are inconstant. On the other
hand, there are transverse striations which are constant, and
which arise from a layer of what are apparently muscular
fibrilloe, developed in a peripheral part of the cortical layer,
immediately below the cuticle. The fibrillas themselves are
formed of elongated corpuscles joined end to end. A trans-
verse partition separates the ceplialic enlargement from the
body, and the layer of muscular fibres only extends into the
posterior part of the enlargement.

The embryos of Gregarina gigantea^ when they leave
their pseudo-navicellse, are minute masses of protoplasm simi-
lar to ProtamoebcB, and like them devoid of nucleus and con-
tractile vacuole. They soon cease to show any change of
form, and acquire a globular shape, the peripheral region of
the body at the same time becoming clear. Next, two long
processes bud out from this body; one is actively mobile, the
other still. The former, detaching itself, assumes the appear-
ance and exhibits the motions of a minute thread-worm,
whence M. van Beneden terms it a pseudo-filaria. The en-
largement at one end becomes apparent, the pseudo-filaria

THE INFUSORIA. 89

passes into a quiescent state, and the "nucleolus " makes its
appearance in its interior. Around tliis a clear layer is differ-
entiated, giving rise to the " nucleus," and the pseudo-fllaria
passes into the condition of the adult Gregarina gigantea.

4. The Catallacta of Haeckel, represented by the genus
Magosphoera^ are, in one stage, myxopcds with long pseudo-
podia, which, broad and lobe-like at the base, break up into
fine filaments at their ends, and may therefore be said to be
intermediate between those of JProtogenes and those of Prot-
aniceha. The myxopod is provided with a distinct endoplast
and a well-marked contractile space. When fully fed, it se-
cretes a cyst and divides into a number of masses, each of
which is converted into a conical body, with its base turned
outward and its apex inward. These conical bodies are im-
bedded in gelatinous matter, and thus cohere into a ball, from
the centre of which they radiate. Each develops cilia around
its base, and contains an endoplast and a contractile vacuole.
After the complex globe thus formed has burst its envelope,
it swims about for a while, like a Volvox. The several cilia-
ted animalcules feed by taking in solid particles through the
disk. They then separate, and, finally, retracting their cilia,
become myxopods such as those with which the series started.
MagosjyJic^ra is thus very nearly an endoplastic repetition of
the moneran Protomonas — the mastigopod being provided
with many small cilia, instead of with a couple of large fla-
gella. On the other hand, the Catallacta are closely allied
to the next group, and, I am disposed to think, might well be
included in it.

5. The Ixfusoeta. — Excluding from the miscellaneous as-
semblage of heterogeneous forms, which have passed under
this name, the Pesmidicp^ Piatomaccce^ Volvochiece, and

V^ibrionidce, which are true plants, on the one hand ; and the
comparatively highly-organized Potifera^ on the other ; there
remain three assemblages of minute organisms, which may be
conveniently comprehended under the general title of Infu-
soria. These are — («) the so-called " ]\Ionads," or Pnfusoria
Jlagellata ; (b) the Acinetae^ or Pnfusoria tentacuUfera ^ and
(c) the Pnfusoria ciliata.

(a.) The Flagellata. — These are characterized by pos-
sessing only one or two long, whip-like cilia, sometimes (when
more than one are present) situated at the same end of the
body, sometimes far apart. The body very generally exhib-
its an endoplast and a contractile vacuole. There is no per-
manently open oral aperture, but there is an oral region, into

90 THE ANATOMY OF INVERTEBRATED ANIMALS.

which the food is forced, and, passing into the endosarc, re-
mains for some time smrounded by a globule of contempo-
raneously ingested water — a so-called " food- vacuole." Prof.
H. James Clark, who has recently carefully studied the Fla-
gellata, points out that, in Bicosoeca and Codonoeca, a fixed
monadiform body is inclosed within a structureless and trans-
parent calyx. In Codosiga a similar transparent substance
rises up round the base of the flagellum, like a collar. Jn
Salimigceca the collar around the base of the flagellum is
combined w ith a calycine investment for the whole animal.
In A^ithophysa, there are two motor organs — the one a stout
and comparatively stiff flagellum, which moves by occasional
jerks, and the other a very delicate cilium, which is in con-
stant vibratory motion.

The discrepancy between the two kinds of locomotive
organs attains its maximum in Aniso7ie7na.f which presents
interesting points of resemblance to Noctiluca.

Multiplication by longitudinal fission was observed in
Codosiga and Anthophy s a, find ^roh^hly occurs in the other
genera. In Codosiga the flagellum is retracted before fission
takes place, but the body does not become encysted ; in An-
thophysa the body assumes a spheroidal form, and is sur-
rounded by a structureless cyst, before division occurs.

Conjugation has not been directly observed among most
of the I)tfusoria flagellata^ nor do any of them exhibit a
structure analogous to the endoplastule of the Ciliata,

Messrs. Dallinger and Drysdale have recently worked out
the life-history of several flagellate " Monads," which occur
in putrefying infusions of fish. They show that these Ha-
gellata not only present various modes of agamic multiplica-
tion by fission, preceded or not by encyst ment, but that they
conjugate, and that the compound body which results (the
equivalent of the zygospore in ])lants) becomes encysted.
Sooner or later, the contents of the cyst become divided
either into comparatively large or excessively minute bod-
ies, which enlarge and gradually take on the form of the
parent.

The careful investigations of these authors lead them to
conclude that, while the adult forms are destroyed at from
61°-80° C, the excessively minute sporules which have been
mentioned, and which may have a diameter of less than
i^QQQQQ of an inch, may be heated to 148° C. without the
destruction of their vitality.

In Euglena viridis (which, however, may be a plant),

THE FLAGELLATA. 91

Stein ^ has observed a division of the "nucleus" to take place,
whereby it becomes converted into separate masses, some of
which acquire an ovate or fusiform shape, surrounding them-
selves with a dense coat, while others become thin-walled
sacs, full of minute granules, each of which is provided with
a single cilium. The ultimate fate of these bodies has not
been traced.

A careful study of the singular genus N'octiluca led me,
in 1855, to assign it a place among the Infusoria^ and recent
investigations have conclusively proved that it is one of the
Flagellata.

The spheroidal body of N^octiluca oniliGrls (Fig. 8) is
about one-eightieth of an inch in diameter, and, like a peach,
presents a meridional groove, at one end of which the mouth
is situated. A long and slender, transversely striated ten-
tacle overhangs the mouth, on one side of which a hard-
toothed ridge projects. Close to one end of this is a vibratile
cilium. A funnel-shaped depression leads into a central
mass of protoplasm, connected by fine radiating bands with
a layer of the same substance which lines the cuticular enve-
lope of the body. There is no contractile vacuole, but an
oval endoplast lies in the central protoplasm. Bodies which
are ingested are lodged in vacuoles of the latter until they
are digested.

According to the observations of Cienkowsky,'^ if a JS'oc-
tiluca be injured, the body bursts and collapses, but the pro-
toplasmic and other contents, together with the tentacle, form
an irregular mass, the periphery of which eventually becomes
vacuolated, enlarges, and secretes a new investment. But
even a small portion of the protoplasm of a mutilated Nocti-
luca is able to become a perfect animal. Under some condi-
tions, the tentacle of a Noctiluca may be retracted into the
body, and, at all times of the year, spheroidal N'octilucce,
devoid of flagellum, tooth, or meridional groove, but other-
wise normal, are to be found. These last are probably to be
regarded as encj^sted forms. Multiplication may take place
in at least two ways. Fission may occur in the spheroidal
forms, as well as in those possessed of a tentacle ; it is in-
augurated by the enlargement, constriction, and division, of
the endoplast. This process takes place more especially in
the latter part of the year.

1 " Oreranismus der Infusionsthlere," ii., 5fi.

» "Ueber Noctiluca miliaris." (Schulze's " Arcliiv fiir mikroskop. Anato-
mie," 1872.)

92 THE ANATOMY OF IXYERTEBRATED ANIMALS.

Another mode of a sexual multiplication, which has a sin-
gular resemblance to the process of partial yelk division,

Fig. 8. — XoctUiica miliai^.—e, gastric vacuole ; g, radiating filaments ; /, anal

aperture (.?).

occurs only in the spheroidal JS^octihicce. The endoplast dis-
appears, and the protoplasm, accumulating on the inner side
of one region of the cuticle, divides first into two, then four,
eight, sixteen, thirty-two, or more masses ; the division of the
protoplasm being accompanied by the elevation of the cuticle
into protuberances, which, at first, corresjDond in number and
dimensions with these division masses. When the division
masses have become very numerous, each protrudes upon the
surface, and is converted into a free monadiform gernr., pro-
vided with an endoplast, a beak, and a long tentacle, which
is hardly to be distinguished from a flagelliform cilium.

The process of conjugation has been directly observed.
Two JVoctilucce, applying themselves by their oral surfaces,
adhere closely together, and a bridge of protoplasm connect-
ing the endoplasts of the two becomes apparent. The ten-
tacula are thrown off, the two bodies gradually coalesce, and
the endoplasts fuse into one. The whole process occupies
five or six hours. Spheroidal or encysted JVoctllucce may
conjugate in a similar manner. In this case, the regions
nearest the endoplasts are those which become applied to-
gether. Whether this process is of a sexual nature, or not,
is not clearly made out. Cienkowsky admits that it may

THE FLAGELLATA, 93

hasten the process of multiplication by monadiform germs
described above.

Ifoctiluca is extremely abundant in the superficial waters
of the ocean, and is one of the most usual causes of the phos-
phorescence of the sea. The light is given out by the pe-
ripheral layer of protoplasm which lines the cuticle.

The PeridlnecB (see Fig. 1, /) form another aberrant
group of the Flagellata, which lead to the Ciliata. Tiie
body is inclosed in a hard case (sometimes produced into
rays), which, at one part, presents a groove-like interruption,
laying bare the contained protoplasm, in which lies an endo-
plast, and in some cases a contractile vacuole. One or more
flagelliform cilia, and usually a wreath of short cilia, are pro-
truded from the protoplasm, and serve as locomotive organs.
The mouth is a depression, whence, in some cases, an oeso-
phageal canal is continued and terminates abruptly in tlie
semi-fluid central substance of the body, the food-particles
being lodged in vacuoles formed at its extremity, as in the
Ciliata. No other mode of multiplication than that by fission
has vet been observed in the Peridinece ; but this fission is
sometimes preceded by the inclosure of the animal in an
elongated, crescent-shaped cyst.

(b.) The TEN-XACULiFER-i. — The Acinetce (Fig. 9, D, E,
F^ G) have no oral aperture of the ordinary kind, but filiform
processes or tentacula, which are usually slender, simple, and
more or less ri2:id, radiate from the surface cf the bodv gen-
erally, or from one or more regions of that surface. At first
sight, these tentacula resemble the radiating pseudopodia of
A.oti)iophrys, but, on closer inspection, they are seen to have
a different character. Eich, in fact, is a delicate tube, pre-
senting a structureless external wall, with a semi-fluid granu-
lar axis, and usually ends in a slight enlargement or knob. It
may be slowly pushed out or retracted, or diversely bent.
But, instead of playing the part of mere prehensile organs,
these tentacles act, in addition, as suckers; the Acineta ap-
plying one or more of these organs to the body of its prey ^ —

» Stein ("Der Organismus der Infusionsthiere," i., VC) thus describes the
method by which an Acineta seizes its prey : " If an Infusorium swims within
reach of the Acineta^ the nearest tentacles are swiftly thrown toward it, and, at
the same time, often become much elongated, bent, or irregularly twisted about.
The knob-like ends of these tentacle's, which come into immediate contact
with the surface of the entangled prey, spread out into disks, and adhere iixedly
to it. When many of the tentacles' have thus attached themselves, the im-
prisoned animal is no longer able to escape, its movements become slower, and
at length cease. Those tentacles which have fixed themselves most firmly
shorten and thicken, and draw the prev nearer to the body. . . . Suddenly, as

94

THE ANATOMY OF IXVERTEBRATED ANIMALS.

usually some other species of Infusorium — when the substance
of the latter travels along the interior of the sucker into the

Fio. 9.—^, Yorticella, active ; B, C, encysted ; Z>, E, F, G, Acinetoe (after Stein).

body of the Acineta. Solid food is not ingested through these
tentacles, so that the Acinetce cannot be fed with indigo or
carmine. In the interior of the body there is an endoplast *
with one or more contractile vacuoles, and it may be either
fixed by a stalk or free.

The Acinetce multiply by several methods. One of these
is simple longitudinal fission, which appears to be rare among
them. Another method consists in the development of ciliated
embryos in the interior of the body. These embr^^os result
from a separation of a portion of the endoplast, and its con-

soon as the suc'kinor (5isk has bored throncrli the cuticula of the prey, a very-
rapid stream, indicated by the fattv particles which it carries, sets along the
axis of the tentacle, and, at its base, pours into the rxcighboring part, of the
body of the Acineta. . . . The cause of the movement is unknown. It is not,
accompanied bv any discernible movement of the walls of the tentacle."

' No endoplastule, such as exists in other Infusoria^ has been observed as
yet in the Acivdoe. Under some circumstances, tlie Acinetm draw in their
radiating processes, and surround themselves with a structureless cyst; but
this process does not appear to have any relation to either mode of multiplica-
tion.

In Acineta mystacina and Porlopliryafixa, a peculiar mode of multiplication
bv division occurs. At the free end of the body a portion becomes constricted
off, together with part of the endoplast, from the remaining stalked part. The
tentacula are drawn in, and the segment becoming elongated, develops cilia
over its whole surface and swims away.

THE INFUSORIA. 95

version into a globular or oval germ, which, in some species,
is wholly covered with vibratile cilia, while, in others, the cilia
are confined to a zone around the middle of the embryo.
The germ makes its escape by bursting through the body-wall
of its parent. After a short existence (sometimes limited to
a few minutes) in the condition of a free-swimming animal-
cule, provided with an endoplast and a contractile vacuole,
but devoid of a mouth, the characteristic knobbed radiating-
processes make their appearance, the cilia vanish, and the ani-
mal passes into the Acineta state.

The Acinetce have frequently been observed to conju-
gate, the separate individuals becoming completely fused into
one and their endoplasts coalescing into the single endoplast
of the resultant Achieta y but it is not certainly made out
whether this process has, or has not, anything to do with the
process of the development of ciliated embryos just described.

(c.) The Ciliata. — The characteristic feature of the Ciliata
is, that the outer surface of the body is provided with numer-
ous vibratile cilia, which are the organs of prehension and loco-
motion. According to the distribution of the cilia, Stein has
divided them into the Holotricha^ in which the cilia are scat-
tered over the whole body, and are of one kind ; the Hetero-
tricha, in v/hich the widely-diffused cilia are of different kinds,
some larger and some smaller ; the Ilypotricha^ in which the
cilia are confined to the under or oral side of the body; and the
JPeritricha^ in which they form a zone round the body. The
great majority of these anim.als are asymmetrical.

In the simplest and smallest Ciliata^ the body resembles
that of one of the Flagellata in being differentiated merely
into an ectosarc and endosarc, with an endoplast and a con-
tractile vacuole. In most, if not all cases, however, there
is not only an oral region, through which the ingestion of
food takes place, but an oesophageal depression leads from
this into the endosarc ; and it m.ay be doubted whether, even
in the simplest Ciliata, there is not an anal area through
which the undigested parts of the food are thrown out.

The genus Colpoda, which is very common in infusions of
hay, is a good example of this low form of ciliated Infuso-
rium. It has somewhat the form of a bean flattened on one
side, and moves actively about by means of numerous cilia,
the longest of which are situated at the interior end of the
body. At i\\Q posterior end is the contractile vacuole, while
a large endoplast lies in the middle, as Stein originally dis-
covered. Colpodm frequently become quiescent, retract their

96 THE ANATOMY OF INVERTEBRATED ANIMALS.

cilia, and surround themselves with a structureless cyst. Each
encysted Colpoda then divides into two, four, or more por-
tions, ^yhich assume the adult form and escape from the cysts
to resume an active existence.

Allman has described the encystment of a Vorticellidan,
followed by division of the nucleus into many germs, with-
out any antecedent process of conjugation ; and Everts has
observed that the progeny of an encysted Yorticella take on
the form of Trichodina grandinella. The Trichodi7ice mul-
tiply by transverse divisions, and then grow into Vb'rti-
cellce.^

Encystment, whether followed or not by division, is very
common among all the Cillata^ and a species of Amphilep-
tus has been seen to swallow — or rather envelop — a stalked
bell-animalcule (Vorticella), and then become encysted upon
the stalk of its prey, just as Vam^yi/rella becomes perched
upon the stalk of the devoured Gomphonema.

In the higher Clliata^ the protoplasm of the body becomes
directly differentiated into various structures, in the same
way as has already been seen to be the case in Gregarina
gigantea^ but to a much greater degree.

Thus, in the Peritricha, of which the bell-animalcules, or
Vortieellce (Fig. 9, A^ B, (7), are the commonest examples,
the oral region presents a depression, the vestibule (Fig. 9, a)
from which a permanent oesophageal canal leads into the soft
and semi-fluid endosarc, where it terminates abruptly ; and
immediately beneath the mouth, in the vestibule, there is an
anal region which gives exit to the refuse of digestion, but
presents an opening only when fecal matters are passing
out. Except where the ciliated circlet, or rather spiral, is
situated, the outer wall of the body gives rise to a relatively
dense cuiicula^ and not unfrequently secretes a transparent
cup or case, foreshadowing the theca of hydrozoal polyps.
Moreover, in the permanently fixed Vorticelkr, the stalk of
attachment may present a central muscular fibre (Fig. 9,/"),
by the sudden contraction of which the body is retracted,
the stalk being at the same time throwm into a spiral. In
the holotrichous Paramcecium (Fig. 10) beneath the thin su-
perficial transparent cuticle from which the cilia proceed,
there is a very distinct cortical layer, fibrillated in a direc-
tion perpendicular to the surface, and, in some species of this
or other genera, as StromhkUuni and Polykricos (Biitschli),
beset with minute rod-like bodies similarly disposed, w^hich,

Allman, " Presidential Address to the Linnsean Societv," 1875,

THE INFUSORIA.

97

under some circumstances, shoot out into long filaments,
and have been termed trlchocysts. In I*, bursariay minute

WWW

Fi». 10. — Paramecium bursaria (after Stein).— ^, the animal viewed from the dorsal
side : a, cortical layer of the body ; b, endoplast ; c, contractile space ; d d', mat-
ters taken in as food ; e, chlorophyl granules.

JB, the animal viewed from the ventral side: a, depression leading to 5, mouth ;
c, gullet ; d, euJoplast; rf', endoplastule ; e. central protoplasm. In both these
figures the arrowi? indicate the direction of the circulation.

C, ParamcRcimn dividing trausvere-ly : a a\ contractile spaces ; b b, endoplast divid-
ing ; c c\ endoplastules.

green granules of chlorophyl are dispersed tlirough this layer,
and Cohn demonstrated, in 1851, that these yield the same
reactions as the chlorophyl grains of the Algae. In Salanti-
dium^ N^yctotherus^ Spirostomum^ and many others, the cor-
tical layer is divided bv linear markino^s into bands, which
there is reason to believe are rudimentary muscular fibres.

In many ClUata, the endosarc appears to be almost fluid.
The food, which is driven into the mouth and down the oesoph-
agus by the constant action of the cilia, accumulates at the
bottom of the oesophagus ; and then, with the water which
surrounds it, is passed, at intervals, with a sort of jerk, into
the endosarc, where it lies close to the end of the oesophagus,
as a food-vacuole, for a short time. But it soon begins to
move, and, along with other such vacuoles formed before and
after it, circulates in a definite course up one side of the body
and down the other, between the cortical layer and the endo-
plast. This movement is particularly free and unrestricted in
Salantidmmfi • in Parcnnoechim, the tract through which the
food- vacuoles move is more definitely limited,* while in iVyc-

1 In Faramczcium hursaria Colin observed that the circulation was completed
in Isf to 2 minutes, which gives a rate of rotation of ^oW to fj^oo oi' an inch in
a second.

98 THE ANATOMY OF INYERTEBRATED ANIMALS.

totherus it appears to be confined to a part of the body be-
tween the end of the gullet and the anal region, which in
this Infusorium is seated at one end of the body. In fact, the
finely granular endosarc of Nyctotherus so limits the passage
of the food-vacuoles that the tract along which they pass
might properly be described as a rudimentary intestinal canal.

The oral cavity is usually ciliated : sometimes, as in ChilO'
don, it has a chitinous armature, which becomes somewhat
complicated in Ermlia {Dysteria ^) and the Didinium de-
scribed by Balbiani.

Torquatella (Lankester) has a plicated membrane around
the mouth in the place of cilia.

The contractile vacuoles attain their greatest complexity
in the Paramoecia, in which there are two — one toward each
end of the body. They are lodged in the cortical layer, and,
in diastole, a portion of their outer periphery is bounded only
by the cuticle, through which it is very probable that they
communicate with the exterior. When the systole takes
place, a number of fine canals, which radiate from each vac-
uole, are seen to become distended with clear, watery fiuid.
These canals are constant in their position, and some of
them may be traced nearly as far as the mouth ; so that the
canals and vacuoles form a permanent water-vascular system.
The endoplast is finely granular, like the substance of the
endosarc. It is frequently said to be enveloped in a distinct
membrane, but I am disposed to think that this is always a
product of reagents. Attached to one part of it there is very
generally (but not in the Vorticellce) a small oval or rounded
body, the so-called "nucleolus" or e?z^op?as^w?e. The endo-
plast is commonly said to be imbededd in the cortical layer,
but this is certainly not the case in Colpoda, Paramoecium^
Palantidium, or Kyctotherus,

The outermost, or cuticular, layer of a large portion of the
body becomes hardened and forms a sort of shell, in many of
the free Infusoria. In the free marine Pictyocystida and
Codonellida of Haeckel, the body has a bell-shaped enve-
lope, which in the Pictyocystida {see Fig. 1) is strengthened
by a siliceous skeleton like that of a Radiolarian. In both
genera the circular lip which surrounds the oral end is pro-
vided with numerous long flagelliform cilia."

Most of the Ciliata, while in full activity, multiply by di-

^ Huxley, "On Dysteria." ( Quarterly Journal of Microscopical Science^ 1857.)
3 Haeckel, " Zur Morphologic der lufusorien," 1873.

THE INFUSORIA. 99

vision ; this is generally effected by the formation of a more
or less transverse constriction, whereby the body becomes
divided into two parts, which separate, each developing those
structures which are needed for its completion. The endo-
plast, however, always elongates and divides, one portion
going along with each product of fission. Neither budding
nor longitudinal fission occurs among the free Infusora, the
appearances which have been regarded as evidence of these
processes being due to the opposite operation of conjugation.
M. Balbiani,' its discoverer, thus describes the process of conju-
gation in Paramoecium hursaria :

*' The Paramoecia assemble in great numbers either tow-
ard the bottom or on the sides of the vessel in which they
are contained. They then conjugate in pairs, their anterior
ends being closely united ; and they remain in this state for
five or six days or more. During this period the nucleus and
nucleolus become transformed into sexual organs.

" The nucleolus is changed into an oval capsule, marked
superficially by longitudinal striag. Sooner or later, it usually
becomes divided into two or four portions, which grow inde-
pendently, and form many separate capsules. About the time
of separation, each of these is found to be a capsule containing
a bundle of curved rods {baguettes), enlarged in the middle,
and thinner at the ends.

" The nucleus also becomes enlarged, and gives rise — in a
manner not clearly explained — to small spherical bodies anal-
ogous to ovules.

" It is usually about the fifth or sixth day after conjuga-
tion that the first germs appear, as little rounded bodies formed
of a membrane which is rendered visible by acetic acid, and
of grayish pale homogeneous or almost imperceptibly granu-
lar contents, in which, as yet, neither nucleus nor contractile
vacuole is distinguishable. It is only later that these organs
appear. The observations of Stein and of F. Cohn have
shown how these embryos leave the body of the mother un-
der the form of Acinetce, provided with knobbed tentacles and
true suckers, by means of which they remain for some time
adherent to her, and nourish themselves from her substance.
But their investigations have not disclosed the ultimate fate
of the vouno;.

" I have been able to follow them for a long period after

» Balbiani, " Note relative a I'Existence d'une Generation Sexuelle chez
les Infusoires." {Journal de la Physiologic^ tome i., 1858.)

100 THE ANATOMY OF IXVERTEBRATED ANIMALS.

their detachment from the maternal organism ; and I have
been able to assure myself that, after having lost their ten-
tacles, becoming clothed with vibratile cilia, and acquiring a
mouth, which makes its appearance as a longitudinal groove,
they return definitely to the parental form, developing in
their interior the green granules which are characteristic of
this Pararaoecium^ without undergoing any more extensive
metamorphosis."

In Figs. 19-22 of Plate IV., which accompanies his paper,
Balbiani figures all the stages by which the acinetiform em-
brj'o becomes a Faramoecium.

So far as the fact of conjugation, the changes in the " nu-
cleolus," and the development of filaments in it, with the
subsequent detachment, by division, of masses from the " nu-
cleus," are concerned, these statements have not been modi-
fied by M. Balbiani, while they are fully confirmed by the ob-
servations made by himself, Claparede and Lachmann, Stein,
Kolliker, and others, in Paramoecium bursaria, P. aurelia,
and other ciliated Pnfusoria.

In the closely allied Paramoeciuni aiirelia, the occurrence
of the various stages of conjugation, conversion of the " nu-
cleolus " into bundles of spermatozoa, and subsequent division
of the " nucleus," is also established by the coincident testi-
mony of Balbiani and Stein. Balbiani affirms that, in this spe-
cies, the clear globular bodies which result from the division
of the " nucleus " pass out of the body without undergoing
any further modification, and he considers them to be ovules.
Stein also admits that he has never seen acinetiform embryos
in this species.

But, as it would seem, on the strength of these negative
observations in Paramoechnn aurelia^ Balbiani, in his later
publications, asserts that the " acinetiform embryos " observed
not only in Paramoecium^ but in Styloi^ychia, Stentor, and
many other ciliated Infusoria^ are not embryos at all, but
parasitic Acinetm / and he makes this assertion without ex-
plicitly withdrawing the statement given above of his own ob-
servation of the passage of the acinetiform embryo of Para-
moecium hursaria into the parental form. Engelmann and
Stein, on the other hand, hold by Balbiani's original doctrine,
and ffive stronfj reasons for so doing". Amonof- the most for-
cible analogical arguments are those afforded by the process of
sexual reproduction observed by Stein in the peritrichous In-
fusoria.

In the Perltrlcha ( VorticelUdce^ Ophrydidce^ TrlchodidcB)

THE INFUSORIA. lOl

conjugation takes place by the complete and permanent
fusion of two individuals, which are sometimes of equal
dimensions ; though, in other cases, one is much smaller than
the other, and, while it is in course of absorption, looks like a
bud, and was formerly taken for such (Fig. 9, A, g^ h). Ihe
small individuals usually take their origin from a group of
small stalked VorticellGe^ which are produced by the repeat-
ed longitudinal division of a Vorticella of the ordinary size.
The result of the conjugative act is that the " nuclei " of the
two individuals, either before or after their coalescence,
break up into a number of segments. The segments may
remain separate, or coalesce into a single mass, called by
^tQm. i^lacenta. In the former case, some of the segments
become germ-masses, while the others reunite to form a new
"nucleus ;" in the latter, the placenta throws out a number
of germ-masses, and then assumes the form of an ordinary
" nucleus." The germ-masses give oft' portions of their sub-
stance, including part of their " nucleus," and these become
converted into ciliated embrj^os, which escajDe by a special
opening. Knobbed tentacles, like those of the Acinetcp,
have not been observed in the embryos of the Peritricha^
nor has their development been traced out.

If the bodies regarded as acinetiform embryos of the
Ciliata are really such, they may be taken to represent the
myxopod stage of the Catallacta, and the relations of the
Acinitm to the Ciliata would appear to be that they arc
modifications of a common type, differing from the Catal-
lacta in having tentacula instead of ordinary pseudopodia.
In the Acinetce^ the tentaculate stage is the more permanent,
the ciliated stage transitory ; while, in the Ciliata, the cili-
ated stage is the more permanent, and the tentaculate stage
transitory.

CHAPTER III.

THE PORIFERA A]S"D THE CCELENTEKATA.

1. The Porifera or Spoxgida. — It has been seen that,
in the Protozoa^ the germ undergoes no process of division
analogous to the " yelk division " of the higher animals, and
to the corresponding process by which the embryo cell of
every plant but the very lowest becomes converted into a
cellular embryo. Consequently, there is no blastoderm ; the
body of the adult Protozoon is not resolvable into morpho-
logical units, or cells, more or less modified ; and the aliment-
ary cavity, when it exists, has no special lining. Moreover,
the occurrence of sexual reproduction in most of the Proto-
zoa is doubtful, and there is, at present, no evidence of the
existence of male elements, in the form of filamentous sper-
matozoa, in any group but the Infusoria / and even here the
real nature of these bodies is still a matter of doubt.

In all the Metazoa^ the germ has the form of a nucleated
cell. The first step in the process of development is the
production of a blastoderm by the subdivision of that cell
and the cells of the blastoderm give rise to the histoloo-ical
elements of the adult body. With the exception of certain
parasites, and the extremely modified males of a few species,
all these animals possess a permanent alimentary cavity,
lined by a special layer of cells. Sexual reproduction always
occurs ; and, very generally, though by no means invariably,
the male element has the form of filiform spermatozoa.

The lowest term in the series of the Metazoa is un-
doubtedly represented by the Porifera or Sponges, which,
after oscillating between the vegetable and the animal king-
doms, have, in recent times, been recognized as animals by
all who have sufficiently studied their structure and the
manner in which their functions are performed.

But the place in the Animal Kingdom which is to be as-
signed to the sponges has been, and still is, a matter of de-

THE PORIFERA. 103

bale. It is certain tliat an ordinary sponge is made up of an
aggregation of corpuscles, some of which have all the charac-
ters of ji)noebce, while others are no less similar to Monads ;
and therefore, taking adult structure only into account, the
comparison of a sponge to a sort of compound Protozoon is
perfectly admissible, and, in the absence of other evidence,
would justify the location of the sponges among the Protozoa,

But, within the last few years, the development of the
sponges has been carefully investigated ; and, as in so many
other cases, a knowledge of that process necessitates a recon-
sideration of the views suggested by adult structure.

The impregnated ovum undergoes regular division ; a blas-
toderm is formed, consisting of two layers of cells — an epiblast
and a hypoblast — and the young animal has the form of a
deep cup, the w^all of which is composed of two layers, an ec-
toderm and an endoderm^ which proceed respectively from the
epiblast and hypoblast. The embryo sponge is, in fact, simi-
lar to the corresponding stage of a hj'drozcon, and is totally
unlike any known condition of a protozoon.

Beyond this early stage, however, the sponge-embryo
takes a line of its own, and its subsequent condition differs
altogether from anj^thing known among the Ccelenterata j all
of which, on the other hand, present close and intimate resem-
blances in their future development, as in their adult structure.

It is not long since the only sponge of the structure and
development of w^hich we were accurately informed was the
Spongilla jfuviatilis^ or fresh-water sponge, the subject of the
elaborate researches of Lieberkuhn and Cai'ter. But, recently,
a flood of light has been thrown upon the morphology and phys-
iology of the marine sponges, particularly of those sponges
with calcareous skeletons, which are termed Ccdcispongic^^
bv Lieberkuhn, Oscar Schmidt, and especially Haeckel. It
has become clear that Spongilla is a somewhat aberrant
form, and that the fundamental type of Poriferal organization
is to be sought among the Calcispongice. In the least com-
plicated of the calcareous sponges, the body has the form of
a cup, and is attached bv its closed extremity. The open ex-
tremity is the osculum^ and leads directly into the spacious
ventricidus^ or cavity of the cup. The comparatively thin
wall of the cup is composed of two layers, readily distinguish-
able by their structure — the outer is the ectoderm^ the in-
ner the endodertn. The ectoderm is a transparent, slightly
granular, gelatinous mass in which the nuclei are scattered, but
which, in the unaltered state, shows no trace of the primitive

104 THE iiNATOMT OF IXVERTEBRATED AmifALS.

Fio. 'il.—Ascfiffa vrimorcUalis (after Ilaecke]).

I. A mature Asceffa, Tpart of one side of the body of which i? removed: o, the exhal-
ei)t aperture ; p. inhnleut pore? in the wall of the body ; i, endoderm ; <?, ecto-
derm ; (7, ova. The triradiate spicula are seen imbedded in the ectoderm.

n. A portion of the endoderm. with two pores (p); i. endodermal cells— those round
the marjrins of the pores have their cilia directed inward ; e, ectodermal syncy-
tium: (7, ova ; z, sperm-cells.

ITT. A monadiform endodermal coll.

IV. An endodermal cell, with retracted cilium, and having the characters of an
Ammbn.

V. The ciliated emhryo of A'cetfa mirrrbUh.

VI. The same embryo in optical longitudinal section: e, epiblast : i, hypoblast ; «?,
blastocoele.

THE POiaFERA. 105

distinctness of the cells which contain these nuclei, and is
therefore termed by Haeckel a syncytium. It is elastic and
contractile, and sometimes exhibits an approach to fibrillation.

The endoderm, on the contrary, is composed of a layer
of very distinct cells, each of which contains a nucleus and
one or more contractile vacuoles, and is produced at its free
extremity into a long solitary cilium or flagellum. Around
the base of this, the transparent outer portion of the proto-
plasm of the cell is produced into an upstanding ridge like a
collar, so that each cell has a wonderful resemblance to some
forms of flagellate Infusoria. Microscopic apertures — the
pores — scattered over the outer surface of the cup, lead into
short passages which perforate the ectoderm and endoderm,
and thus place the ventriculus in communication with the ex-
terior. The workinof of the flaarella of the endodermic cells
causes the water contained in the gastric cavity to flow out
of the osculum ; to make good this outflow, minute streams
set in by the pores, which have consequently been called in-
halent^ while the osculum has been termed the exhalent aper-
ture. It is said, however, that the direction of these currents
is not invariable ; and it is certain that the pores are not
constant, but that they may be temporarily or permanently
closed, and new ones formed in other positions.

The skeleton of the calcareous sponges always consists of
a multitude of separate spicula, composed of an animal sub-
stance, more or less strongly impregnated with carbonate of
lime, which is deposited in concentric layers around a central
axis, formed by the animal basis. This skeleton is devel-
oped exclusively in the ectoderm, and is not supported by
any framework of fibrous animal matter.

The calcareous sponges are frequently, if not always,
hermaphrodite. The reproductive elements are ova and
spermatozoa. There is some reason for assuming that the
latter originate in metamorphosed cells of the endoderm, as
they are found scattered between ordinary cells of the latter.
The ova, on the other hand, occur sometimes between the
cells of the endoderm, sometimes imbedded in the syncytium
itself. But the question of the origin of the sexual elements
in these and other animals needs much further investigation.
The spermatozoa are very delicate, and have minute, rod-like
heads, with long flagella. The ova present the normal ger-
minal vesicle and spot, but exhibit active amoeboid move-
ments.

Impregnation is effected, and the first stages of develop-

106 THE ANATOMY OF INVERTEBRATED ANIMALS.

ment take place, while the ova are still imbedded in the body
of the sponge.

Metschnikoff ^ has recently described the development of
Sycoti cillatuni. The ovum, after impregnation, becomes a
morula, with a central cleavage cavity or blastocoele. But
the blastomeres of the two halves of the morula take on dif-
ferent characters — those of the one half elongating and
acquiring flagelliform cilia, while those of the opposite half
remain globular and develop no cilia. The latter now coa-
lesce into a syncytium, and develope spicula, while the layer
of ciliated cells becomes invaginated within the syncytium.
More usually, however, it appears that a gastrula is formed
by invagination of the morula, the ectoderm of which has the
structure of the endoderm of the adult, while the cells of the
endoderm, or lining membrane of the gastric cavity, are de-
void of cilia. The embryo quits the parent, propelled by the
flagelliform cilia which cover the outer surface of the ecto-
derm. After a time, it fixes itself by the closed end ; the
fiagella of the cells of the ectoderm are retracted, the cells
themselves become flattened and coalesce so completely that
their boundaries cease to be distinguishable, and the ectoderm
passes into the condition of a syncytium. At the same time,
the cells of the endoderm multiply, elongate, and take on the
form which characterizes them in the adult. In this state
the young sponge is termed an Asciila. The transition to
the final condition is effected by the development of the spic-
ula in the syncytium and the separation of some of the con-
stituent cells of the syncytium to form the inhalent pores.

In the simplest Calcis]Jongice, forming the family to
which Haeckel applies the name oi Ascones^ the wall of the
ventriculus is thin, and the pores open directly into the ven-
tricular cavity ; but in another family, the Leuco7ies, the syn-
cytium becomes greatly thickened, and the pores are conse-
quently prolonged into canals (which may be ramified and
anastomose), connecting the ventriculus with the exterior.
The endodermic cells, which in these, as in the Asco7ies, at
first form a continuous layer, are eventually restricted to the

1 " Zar Entwickelungs-cfoschichte der Kalkschwammc." {ZoiUdiriftfur
wissenschqftliche Zoologic^ Bd. xxiv. ) F. E. Schuke, so far as I follow Hae-
ckel's account of his recent onservations ('•'■ Die Gastrula und die Eifurcbung
der Thiere,'' p. 158), agrees with Metschnikoff as to t'ne first sta'^es of develop-
ment, but differs in regard to subsequent stances. Haeckel withdraws his ear-
lier account of the formation of the gastrula by delamination, or splitting of the
walls of an oval shut^?aww/a-3ac into two layers, and the subsequent opening
of the planula at one end.

THE PORIFERA. 107

canals, or even to local dilatations of these canals — the so-
called " ciliated chambers."

The same relative disproportion of the ectoderm, with the
consequent development of passages which traverse the mass
of the sponge, and are provided at intervals with ciliated
chambers, is found in the silicious sponges, in which the
spicula, if they possess any, are formed by a deposit of silex ;
and in which, as a rule, the sponge-corpuscles are supported
by a more or less complete skeleton of a tough animal sub-
stance, termed Jceratose.

Malisarca^ however, is devoid both of skeleton and spicula,
and the minute structure of the curious boring-sponges — the
Clionm — has yet to be elucidated.

Ilaliphysema and Gastrophysema, of Haeckel, appear to
be sponges which get no further than the Gastrula condi-
tion, and thus form a connecting link between the Sponges
and the Hydrozoa,

The fresh-water sponge {^SpongilUi) has been studied with
extreme care by Lieberkiihn, and the following account,
based upon the investigations of that author, is given for the
use of the student to whom Spongilla flumalis is likely to
be the most readily accessible of the sponges.

The fresh-water sponge grows on the banks of docks,
canals, rivers, and on floating timber, in the form of thick
incrusting masses, which usually have a green color, and
require a constant supply of fresh water for their healthy
maintenance. The surface presents irregular conical emi-
nences perforated at their summit like small volcanic craters,
and from these exhalent funnels, which answer to the oscnla
of the Calcispongioe^ currents of the water are continually
flowing. Careful examination of the surface of the Spongilla
between the exhalent craters, shows that it is formed by a
delicate membranous expansion, separating which from the
deeper substance of the &pongilla are a number of irregular
cavities. In some case?, these run into one great water-
chamber. The superficial chambers (or chamber) communi-
cate w4th the exterior by pores, which perforate the mem-
branous expansion, are similar to those in the outer surface of
the ventricular wall of a simple calcareous sponge, and sub-
serve the same inhalent function. On their inner face, or
floor, the superficial chambers exhibit the apertures of in-
numerable canals, which traverse the deep substance of the
Spongilla in all directions, and, sooner or later, unite into
passages which lead directly into the cavities of the exhalent

103 THE AXATOMY OF INVERTEBRATSD AXIMALS.

craters. Dilatations of the canals occur at intervals, and are
lined by the characteristic monadiform endodermic cells,
which are restricted to the walls of these ciliated chambers.
It is by the working of the cilia of these cells that currents
of water are made continually to enter by the inhalent pores
and to pass out by the exhalent craters. The whole fabric
is supported and strengthened by a skeleton, which consists,
in the first place, of bands and filaments of keratose, and,
secondly, of silicious spicula, the majority of which resemble
needles pointed at each end, and contain a fine central canal
filled with an unsilicified substance. The individuality of
these animals is so little marked that two Sjyongillm, when
brought into contact, before long fuse into one; while they may
divide spontaneously, or be separated artificially into different
portions each of which will maintain an independent existence.

A process analogous to the formation of cysts, which is so
common among the Protozoa^ takes place in the deeper sub-
stance of the body, especially in the autumn. A number of
adjacent sponge-corpuscies, losing their granular appearance,
become filled with clear, strongly refracting granules, the nu-
cleus ceasing to be visible. The sponge-corpuscles which
surround these become closBly applied together, and secrete
coats of keratose, which fuse with those of the adjacent cor-
puscles. In the interior of each a singular silicious spiculum
is formed, consisting of two toothed disks, like cogged wheels,
united by an axis. As this " amjyhidlscus " enlarges, the proto-
plasm of the corpuscle disappears, and at length nothing is left
but the envelope of keratose, with the imbedded amphidisks,
disposed perpendicularly to its surface. At one point of the
spheroidal envelope a small opening is left, and the so-called
"seed " of the SponglUa is complete. It remains throughout
the winter unchanged; but, with the return of warmth, the
sponge-corpuscles inclosed within the coat of the " seed," or
more properly cyst, slowly escape through the pore, become
perforated with inhalent and exhalent apertures and canals, and
develop the characteristic spicula of a young Spongilla.

This process of encystment, which may be regarded as a
kind of budding, akin to propagation by bulbs among plants,
has not been observed among marine sponges.

Sexual propagation takes place in the same way as in the
Calcispongice^ and the embryo passes through morula and
planula stages. But the ciliated cells which form the outer
wall of the latter, and constitute its locomotive apparatus,
seem to vanish when the embryo fixes itself, and the body of

THE PORLFERA. 109

the young Flhrospongia appears to be developed out of the
inner cells, which, in the mean while, have become spiculiger-
ous. However, the details of the mode of development of the
Fibrospongim require further elucidation.

In both the marine and fresh-water sponges the ingestion
of solid matters — such as carmine and indigo — by the mo-
nadiform endodermic cells has been seen by several observ-
ers. According to Haeckel, the solid particles, which usually
are taken in between the flagellum and the collar, may also be
ingested at other parts of the surface of the endodermic cell.
In the course of such experiments, also, granules of the pig-
ment may be found in the ectoderm, but, whether they enter
it directly or secondarily from the endoderm, is unknown.
Sponges absorb oxygen, and give oif carbonic acid with great
ra{)iditv ; and the manner in which they render the water in
which they live impure, and injurious to other organisms, sug-
gests the elimination of nitrogenous waste matter.

The syncytium may contract as a whole, and is liable to
local contractions, as when the oscula or the pores shut or
open. The contours of the cells of which it is composed are
invisible in the fresh state, and hence it appears as a mere
"sarcode" or transparent gelatinous conlraciile substance,
in which nuclei and granules are imbedded here and there.
But Lieberkiihn has shown that, when the water in which
Spong'dla lives is heated to the point at which thermic coagu-
lation of the protoplasm of the cells occurs, their boundaries
at once become defined, and the cells commonly detach them-
selves from one another. The syncytium is therefore formed
by the -close union, and not by the actual fusion, of the cells
of the body.

It is a very interesting fact that thread-cells, similar to
those which are so abundant in the Coelenterata, are said to
occur in some sponges. Eimer ^ finds these structures in
species of the Renierincie. The thread-cells are scattered
through both endoderm and ectoderm, and abound on the
free surface of the former, where it limits the canals of the
sponge, but do not occur on the outer surface of the ectoderm.
The same observer states that he found partly digested re-
mains of small crustaceans in the ventricular cavities and
passages of both silicious and calcareous sponges.

The Porlfera present three principal modifications — the
Myxospongi(j>^ the Calcisponguje^ and the Fibrospongloe — the

1 " Ncsseizellen und Saamen bei See-Schwdmincn." (ArcJtiv far Mikro-
skovische Anatomies viii., 1872.)

110 THE ANATOMY OF IXYERTEBRATED ANIMALS.

Myxospongim being altogether devoid of skeleton ; the Cal-
cispongice possessing calcareous spicula, but no fibrous kera-
tose skeleton ; and the Fihrospongim having a fibrous skele-
ton, and (usually) spicula of a silicious nature. To these it is
probable that the Clionid(J& must be added, as a fourth type,
devoid of a fibrous skeleton, but possessing silicious spicula
of a very peculiar kind, by the help of which they are able
to burrow parasitically in the shells of mollusks. Finally,
Haliphysema and Gastrophyseina appear to be even simpler
than the Myxosponglm.

The division of the Myxospongice contains only the ge-
latinous Halisarca, The Calcispo?igiCB, in addition to the two
families of Asco7tes and Leiicones^ already referred to, include
a third — the Sycones^ which are essentially composite As-
coyies. The I^ibrospongice present a great diversity of form
and structure. They may have the form of flattened or glob-
ular masses, arborescent, tree-like growths, flagellate expan-
sions, or wide or deep cups. The sponge of commerce de-
rives its value from the fact that its richly-developed fibrous
skeleton is devoid of spicula. On the other hand, in such
sponges as Hyalonema and Euplectella^ the silicious spicula
attain a marvelous development and complexit}^ of arrange-
ment. In the latter genus, they form a fibrous network with
regular polygonal meshes. These appear to be the repre-
sentatives of the Ventriculites^ which were so common in
the seas of the Cretaceous epoch.

Sponges abound in the waters of all seas, but Spongilla
is the sole fresh-water form. Clionidm existed in the Silu-
rian epoch, but the most plentiful remains of sponges have
been yielded by the chalk.

The CoELE^'TERATA. — This group of the Metazoa contains
those animals which are commonly known as Polyps, Jelly-
fishes, or Medusce^ Sea-anemones, and Corals. They exhibit
two well-marked series of modifications, termed the Hydrozoa
and the Actmozoa,

The Hydrozoa. — The fundamental element in the^ struct-
ure of this group is the Hydranth., or Polypite. ^ This is es-
sentially a sac having at one end an ingestive or oral open-
ing, which leads into a digestive cavity. The wall of the sac
is composed of two cellular membranes, the outer of which is
termed the ectoderm^ and the inner the endoderm^ the former
having the morphological value of the epidermis of the higher

THE PORIFERA.

Ill

'"''m^m^^'"'

Fig. 12. — A. Hypothetical section of a SpongiTIa: a. superficial layer; b, fnhnlent
apertures ; c, ciliated chambers ; d, an esh^lent aperture ; e, deeper substance
of the sponge. The arrows indicate the direction of t'le currents. B. Kf>^msiWSpon-
qilla with a single exhalent aperture, seen from above (after Lieberkiihn) : a. in-
nalent apertures ; c, ciliated chambers ; d, exhalent aperture, C. A ciliated
chamber. D. A free-swimming ciliated embryo.

112 THE ANATOMY OF INVERTEBnATED ANIMALS.

animals, and the latter that of the epithelium of the aliment-
ary canal/ Between these two layers, a third layer — the

Fis. 13. — Diasrramp illustrative of the mutnal relations! of the HijrTrozon :

1. Hydra. 2. Sertularian. 3. Calvcophoridati. 4. Physuphoriclan. 5. Lucernariati.

a. Ectoderm, b. Endoderm. c. The digestive and somatic cavity.

P. Tentacles. N. Nectoialyx. T. Coeno?arc. B. Hydrophyllium. C. Hydrothoca. S.

Hydranth. Q Gonophore, A. Air- Vesicle contained in F. Pneumatophore. c,

Digective and somatic cavity.
I., II., III., I v\, represent the successive stages of development of a Medusiform

gonophore.

mesoderm — which represents the structures which lie between

* " The body of every Hydrozoon is essentially a sac composed of two mem-
branes, an external and an internal, which have been conveniently denomi-
nated by the terms ectoderm and endoderm. The cavity of the sac, which will
be called the somatic cavity ^ contains a fluid, charged with nutritive matter in

THE HYDROZOA. 113

the epidermis and the epithelium in more complex animals,
may be developed, and sometimes attains a great thickness,

solution, and sometimes, if not always, with suspended solid particles, which
perform the functions of the blood in animals of higher organization, and may
he termedihe S07natic fluid. . . . Notwithstanding the extreme variety of form
exhibited by the Hydrozoa, and the multiplicity and complexity of the organs
which some of them possess, they never lose the traces of this primitive sun-
plicity of organization ; and it is but rarely that it is even disguised to any con-
siderable extent. . . . This important and obvious structural peculiarity could
hardly escape notice, and I find it to have been observed by Trembley, Baker
and Laurent, Corda and Ecker in Hydra ; by Eathke, in Coryne ; by Frey and
Leuckart, in Lucernaria ; and it is given as a character of the hydroid po-
lyps in general {Hydrce^ CorynidcB^ and ISertularidce)^ in the second edition of
Ouvier's ' Legons.' I pointed it out as the general law of structure of the hy-
droid polyps, Biphydoi and PhysopJioridcc^ in a paper ^ sent to the Linna^an So-
ciety, from Australia, in 1847, but not read before tbat body till January, 1849 ;
and I extended the generalization to the whole of the Hydrozoa^ in a ' Memoir
on the Anatomy and Affinities of the Meduscx,^ read before the Koyal Society
in June, 1849.

" Prof. Allman, in his valuable memoir ' On Cordylophora ' ('Philosophical
Transactions,' 1855), has adopted and confirmed this moi-phological law, intro-
ducing the convenient terms ' ectoderm ' and ' endoderm,' to denote the imier
and outer membranes ; and Gegenbaur (' Beitriige zur niiheren Kenntniss der
Schwimmpolypen; 1854, p. 42) has partially noticed its exemplification in
Apolemia and Bkizophysa; but it seems singularly enough to have failed to
attract the attention of other excellent German observers, to whose late im-
portation investigations I shall so often have occasion to advert. The pecu-
liarity in the structure of the body walls of the Hydrozoa^ to which 1 have just
referred, possesses a singular interest in its bearing upon the truth (for, with
due limitation, it is a great truth) that there is a certain similarity between the
adult states of the lower animals and the embryonic conditions of those of
higher organization.

" For it is well known that, in a very early state, the germ, even of the
highest animals, is a more or less complete sac, whose thin wall is divisible into
two membranes, an inner and an outer ; the latter turned toward the external
world ; the former, in relation with the nutritive liquid, the yelk. The inner
layer, as Eemak has more particularly shown, undergoes but little histological
change, and throughout life remains "more particularly devoted to the functions
of alimentation, T\^iile the outer gives rise, by manifold differentiations of its
tissue, to those complex structures which we know as integument, bones, mus-
cles, nerves, and sensory apparatus, and which especially subserve the func-
tions of relation. At the same time, the various orofans are produced by a process
of budding from one or other, or both, of these primary layers of the germ.

" Just so in the Hydrozoon : the ectoderm gives rise to the hard tegument-
ary tissues, to the more important masses of muscular fibres, and to those
organs which we have every reason to believe are sensory, while the endoderm
undergoes but very little modification. And every organ of a Hydrozoon is
produced by budding from one, or other, or both, of these primitive membranes ;
the ordinary case being that the new part commences its existence as a papillary
process of both membranes, including, of coui-se, a diverticulum of the somatic
cavity.

" Thus there is a very real and genuine analogy between the adult Hydro-
zoon and the embryonic vertebrate animal ; but I need hardly say it \)\ no
means justifies the' assumption that the Hydrozoa are in any sense 'arrested
developments ' of higher organisms. All thiit can justly be affirmed is, that the

» "Observations upon the Anatomy of the Diphydje and the UnUy of Orgnnizi-
tion of the DipJiydse and Physophoridfp." An abstract of this essay "was published
in the '• Proceedings of the Linnsean Society " for 1849.

U4

THE ANATOMY OF INYERTEBRATED ANIMALS.

but it is a secondary and, in the lower Sydrozoa^ inconspicu-
ous production.

All the Hydrozoa are provided Avith tentactda' These
are elongated and sometimes filiform organs of prehension,
which are generally diverticula of both ectoderm and endo-
derm, but may be outgrowths of only one of them.

Thread-cells, or nematocysts, are very generally distributed
through the tissues of the (Joelenterata. In its most perfect
form, a nematocyst is an elastic, thick-walled sac, coiled up in
the interior of which is a long filament, often serrated or pro-
vided with spines. The filament is hollow, and is continuous
with the wall of the sac at its thicker or basal end, while its
other pointed end is free. Very slight pressure causes the

Fig. 14— Sacculus of a tent-icle with nematocvsta of Athorybia: J., peduncle or
stalk, and 5, involucrum of the sacculus C; A filaments; </, ectoderm; e, eado-
derm ; /, neinatocysts; 1, small nematocysts of the filaments and involucrum;
2, 3, larger nematocysts of the sac ; 4, largest nematocyats.

thread to be swiftly protruded, apparently by a process of
evagination, and the nematocyst now appears as an empty

Hydrozoon travels for a certain distance aloasr the same ?reat highway of de-
velopment as the hiofher animal, before it turns off to follow the road which
leads to its special destination."

In this passage of my work on the " Oceanic Hydrozoa " (1859), I expanded
the idea enunciated in the memoir on the Medusfe here referred to, that " the
outer and inner membranes appear to bear the same physiological relation to
one another as do the serous and mucous layers of the germ." The diagram
(Fig. 13), exhibiting the relations of the different groups of the Hydrozoa, was
published in the Medical Times and Gazeiti in June, 1856.

THE HYDROZOA. 115

sac, to one end of which a long filament, often provided with
two or three spines near its base, is attached. Many of the
Coelenterata^ and notably the Fhysalla, give rise to violent
urtication when their tentacles come in contact with, the hu-
man skin, whence it may be concluded that the nematocysts
produce a like injurious effect upon the bodies of those ani-
mals which are seized and swallowed by the Polyps and Jelly-
fishes.

As regards the existence of a nervous system in the Hy-
drozoa^ very diverse opinions have been entertained, and it
may be doubted if the problem has even yet received its final
solution, I have already discussed Kleinenberg's suggestion
that the branched prolongations of the inner ends of the cells
of the ectoderm in Hyclra^ which end in the longitudinal fibres
which lie between the ectoderm and the endoderm, may be
nerves in their earliest stage of differentiation. Haeckel de-
scribes a nervous system in Glossocodon and Carmarina. It
consists of a circular band which lies on the inner side of the
circular canal of the bell-shaped swimming-organ of these
Medusae^ and presents a ganglionic enlargement at the base of
each of the lithocysts. Of these eight ganglia, the four which
correspond to the openings of the four radial canals into the
circular canal are the larger. Each of these gives off four
branches, one of which follows the course of the radial canal
to the central polypite or manubrium ; two others go to the
adjacent tentacles, and the last to the lithocyst.^

There can be little doubt that the lithocysts, or sacs con-
taining mineral particles, which are so frequently found in the
Medusas, are of the nature of auditory organs ; while the mass-
es of pigment, -with imbedded refracting bodies, which often
occur associated with the lithocysts, are doubtless rudimentary
ej'es.

The sexual reproductive elements are ova and spermato-
zoa — the ova being very often devoid of a vitelline membrane.
The fully-formed generative elements lie between the ecto-
derm and the endoderm of that part of the body -wall in which
they make their appearance. In Hydractinia, as has already
been pointed out, the ova appear to be modified cells of the
endoderm, and spermatozoa modified cells of the ectoderm;

I Haeckel, "Beitrasre zur Naturgesclnchte der Hydromednsen." The ana-
tomical disposition of this nervous apparatus accords very well with the recent
important observations of Mr. Romanes on the " Locomotor System of Medu-
sae." (" Proceedings of the Eoyal Society," December, 1S75.)

116 THE ANATOMY OF IXTERTEBRATED ANIMALS.

but it remains to be seen how far this rule is of general appli-
cation.

Usually the region of the body in which the generative
organs are produced undergoes a special modification before
the reproductive elements make their appearance in it, giving
rise to a peculiar organ, the gonophore. In its simplest con-
dition the gonophore is a mere sac-like diverticulum, or out-
ward process of the body-wall. But, from this state, the
gonophore presents every degree of complication, until it ac-
quires the form of a bell-shaped body called from its resem-
blance to a Medusa or jelly-hsh a mediisoid.^

In its most complete form, the medusoid consists of a disk
having the form of a shallow or deep cup [nectoclyx), from the
centre of the concavity of which projects a sac termed the nin-
nuhrimn. The cavity of the sac is continued into that of
sundry symmetrically disposed canals, most commonly four in
number, which radiate from the centre of the disk to its cir-
cumference, where they open into a circular marginal canaL
A membranous fold, the veliun^ which contains muscular fibres
arranged concentrically to its free margin, is attached to
the inner circumference of the mouth of the bell, and pro-
jects, like a shelf, into its interior. Lithocysts are usually
developed on the margins of the bell, wd]ich may also give
rise to tentacles. The manubrium, opening at its free end,
may become functionally, as well as structurally, a hydranth,
and may serve to feed the medusoid when it is detached from
the hydrosomn, or body of the hydrozoon. However com-
plex its structure may be, the medusoid commences as a sim-
ple bud-like outgrowth, which thickens at its free end ; the
central part of this thickening becomes the manubrium,
"while its periphery, splitting away from the manubrium, is
converted into the disk (Fig. 13). A single prolongation of
the somatic cavity is continued into the manubrium, w^hile
several, usually four, symmetrically arranged diverticula ex-
tend into the nectocalyx and become its radiating canals.
The distal ends of these subsequently throw out lateral
branches, which unite and give rise to the circular canal.

The lithocysts are usually, but not always, free and promi-

1 From the imperfection of our knowledcre respectino^ the origin of many
of the medusifonn Hydrozoa^ it is ditilcult to employ any terminology with
strict consistency. If "medusoid" is restricted to what are known to be
gonophorcs developed by gemmation, " medusa '' may be employed, in a gen-
eral sense, as the equivalent of the somewhat inconvenient vernacular term
"jelly-fish."

THE HYDROZOA. 117

nent, and the one or many solid mineral bodies which they
contain are inclosed in special envelopes. Their structure
appears to be more complicated in the Geryonidce than in
other Medusae. (Haeckel, loc. cit.)

In some of those medusoid gonophores, the reproductive
elements are developed while the gonophore is still attached
to the hydrosoma, and then they always make their appear-
ance in the wall of the manubrium. But, in other cases, the
medusoid becomes detached before the development of the
reproductive elements, and, feeding itself, increases largely
in size before the ova or spermatozoa appear. Sooner or
later, however, the reproductive organs are developed, either
in the walls of the manubrial hydranth, or in those of the
canals of the nectocalyx of the medusoid.

In an early stage of its existence, every hydrozoon is
represented by a single hydranth, but, in the great majority
of the Hydrozoa^ new hydranths are developed from that
first formed, by a process of gemmation or of fission. In
the former case the bud is almost always an outgrowth or
diverticulum of the ectoderm and endoderm, into which a
prolongation of the cavity of the body extends. Sometimes
the hydranth formed by gemmation becomes detached from
the body ; but, in many cases, the buds developed from the
primary hydranth remain connected together by a common
stem or coenosarc^ and thus give rise to a compound body, or
k}/d)'Osoma.

In many Hydrozoa^ the ectoderm gives rise to a hard cu-
ticular coating, and in some of these (Gamjjanidarida?^ Ser-
tularidce^ Fig. 13, 2), this cuticular investment, on the hy-
dranth, takes the shape of a case or " cell " — the hydrotheca
— into which the hydranth may be more or less completely
retracted. In other Hydrozoa, protective coverings are af-
forded to the hydranths by the development of processes of
the body-wall, which become thick, variously-shaped, glassy
lainella3. These appendages are termed hydrophylUa (Fig.

Again, certain groups (the CcdycojjhoridcB and most Phy-
sojjhoridce) are provided with bell-shaped organs of propul-
sion, produced by the metamorphosis of lateral buds of the
hydrosoma. These nectocalyces ha.ve the structure of a med-
usoid, devoid of a manubrium. In others {Physophoridce),
one extremity of the hydrosoma is dilated, contains air in-
closed within a sac formed by an involution of the ectoderm,
and constitutes a float or piieumatophore ; while in yet others

118 THE ANATOMY OF INVERTEBRATED ANIMALS.

(Discophora) the aboral end of the hydranth is dilated into
a disk or umbrella^ which is susceptible of rhythmical con-
tractile movements, by which the body is propelled through
the water. Thus, notwithstanding its different mode of de-
velopment, it has a close resemblance to a medusoid. Ac-
cording to the existence or absence of these various append-
ages, and the manner in which they are disposed, the Hy-
drozoa are distinguishable into three groups — 1, the Sydro-
phora ; 2, the Discophora ; 3, the Siphonophora,

1. The Hydeophoka are, in all cases but that of Hydra^
fixed ramified hydrosomes, on which many hydranths and
gonophores are developed. The somatic cavity contained in
the hydrosoma always retains a free communication with the
gastric cavities of the hydranths. In other words, it is an
enterocoele. The tentacula are either scattered over the hy-
dranths (Cory7ie), or are arranged in one circle round the
mouth [Sertularia) ; or in two circles, one close to the mouth,
and one near the aboral end { Tubular ia). Very generally —
e. g., in all Sertularidcp, GamiJanidaridm and Tuhidaridce —
there is a hard, chitinous, cuticular skeleton (perisarc of All-
man), which frequently gives rise to hydrothec^e, into which
the hydranths can be retracted (Fig. 13, 2).

The gonophores present every variety, from simple sac-
cular diverticula of the hydrosoma to free-swimming medu-
soids. The inner margin of the bell in these medusoids is
always produced into a velum, and otolithic sacs and eye-
spots are very generally disposed at regular intervals around
the circumference of the bell. The great majority of what
were formerly termed the naked-eyed MedusiB {Gymnoph-
thalmata) are merely the free-swimming gonophores of the
Hydrophora. Thus the medusoids known as Sarsiadce are
the free gonophores of the Corynidm ; the JSougainmllem
and Lizzice of the Eudendridoe ; many Oceanidm proceed
from Tubular id(B ; Thaumaiitidce and ^quoridcB from (7am-
panularidce.

In some Hydrophora (e. g., Calycella) the margins of the
hydrotheca are prolonged into triangular processes, which
serve as an operculum.

Certain PlumularidcB are provided with prominences of
the hydrosoma surrounded by a chitinous investment, which
is open at the extremity. The inclosed soft ectoderm usual-
ly contains many thread-cells, and has the power of throw-
ing out contractile pseudopodial processes. These have been

THE HYDROPHORA.

119

termed nematophores by Mr. Busk.* In Ophiodes (Hincks)
they are tentaculiform.

It frequently happens that the gonophores are developed
upon special stalks, each of which has essentially the struct-

PiG. 15.— CVzmpawwZcria (after Gesccbaur).— -4, Hydranth : e, its peduncle; «', hy-
drotlieca ; 0, mouih; <e, tentacles; ^', dii^estive cavity, continuous with the bo-
matic cavity k, contained in the peduncle and in the creeping stem, S. B, gonan-
giura containing two medusiform zc5ids or gonophores it' / the somatic cavit;

t" is in connection with that of the creeping stem. C, Bud.

ure of a mouthless hydranth. This is termed a blastostyle.
In some blastostyles (Fig. 15), during the development of the
buds of the gonophores, the ectoderm splits into two layers —
an inner, which invests the central axis formed by the endoderm
with the contained prolongation of the somatic cavity ; and
an outer, chiefly, if not wholly, chitinous layer. Into the in-
terspace between these two, the budding gonophores project,
and may emerge from the summit of the gonaiigiwn., thus
formed, either to develop the reproductive elements, and shed
them while still attached, or to be set at liberty as free medu-
soids (Fig. 16).

Allman * has shown that, in Dicoryne conferta^ the gono-

_ 1 They are described under the name of" clavate ororans," and compared
with the tentacles oi Diphydoe in rav memoir on the "Affinities of the Medu-
sae." ("Philosophical Transactions," 1849.)

2 'I Monograph of the Gymnoblastic, or Tubularian Hydroids," 1871, p. 31.
In this beautifully illustrated and elaborate work, the student will find, not

120 THE ANITOMY OF INVERTEBRATED ANIMALS.

phore contained in a gonangiuni, somewhat like that of Ziao-
medea, is set free as a ciliated bitentaculate body, on the cen-
tral axis of which the ova and spermatozoa are developed.

Fig. 16 — Medusiform zoilid of Ca7npam«?ana (after Gegenbaur) : J, nectocalyx ; te^
tentacles ; a. lithocysts; J.', velum ; k\ manubrium, inclosing the digestive cavity ;
o, mouth ; k'\ radial canals.

In the genus Aglaophenia {PlumularidcB)^ groups of
gonangia are inclosed in a common receptacle {corhula,
Allman), formed by the development and union of lateral
processes (comparable in some respects to the hydrophyllia
of the Cahjcophoridoe) from that region of the hydrosoma
which bears the gonophores.

Some medusoids, such as Sarsia prolifera and WiUsia, the
hydroid stages of which are not at present certainly known,
but which are probably coryniform, produce medusoids simi-
lar to themselves by budding. The buds may be developed
either from the manubrium, or from the maro-inal canal of the
nectocalyx, or from the bases of the tentacula, or even from
their whole length.

In August, 1849, while in the North Pacific, off the Loui-
siade Archipelago, 1 took a species of W^illsia (Fig. 17), in
which stolons were developed at the bifurcation of each of
the four principal radiating canals of the nectocalyx. Each
stolon was terminated by a knobbed extremity containing
many nematoc3^sts ( C, g), and gave rise, on one side, to a
series of buds, of which those nearest the free end of the
stolon had acquired the form of complete medusoids. They
had four uabranched radiating canals and four tentacles ; but
it is probable that they would assume the form of the parent
stock after detachment.

only a full account of the ori?anizatlon of the grouo of which it treats, but
much inforuiation respecting the Hijdrozoa in general.

THE DISCOPHORA.

121

In striking contrast with the complexity of these repro-
ductive processes, the gonophore is represented, in Hydra^

■fi^

Fig. n.—WiUsia, sp. ; A, the mednsa, with buddins stolons. B. one of the buds
developed on a stolon ; h, radial canal of the nectocalvx: e, manubrium. C, a
stolon: g, its free end beset with nematocysts ; b, c, d, budding medusoids ;
/, medusoid nearly ready to be detached; «, its manubrium; d, its nectocalyx
h, a radial canal.

by a mere enlargement of the body-wall, situated close to the
bases of the tentacula, in the case of the testes, and nearer
the attached end of the body in that of the ovary. The ovary
develops a single ovum, which, as Kleinenberg has shown,
undergoes division and invests itself with a chitinous coat
while still attached to the body of the parent. This chiti-
nous investment is more or less spinose, and is often con-
founded with an egg-shell. It obviously answers to the
perisarc of a Tubularian, and its presence in the embryo of
the Hydra., in which no perisarc is developed by the adult,
suggests that Sydra may not represent the simplest primary
condition of a Hydrophoran, but may be a reduced modifica-
tion of a Tubularian.

2. The Discophora. — These " Medusa? " resemble the
more perfect free medusoid gonophores of the Ilydrophora^
in so far as they consist of a hydranth or polypi te attached
to the centre of a gelatinous contractile swimming disk. But
they differ from the medusoids of the Hydrophora, inasmuch
as they are developed either directly from the impregnated
ovum ; or by gemmation from a Medusa which arises in this
6

123

THE ANATOMY OF INVERTEBEATED ANIMALS.

way ; or by the transverse fission of the hydriform product
of the development of the impregnated ovum.

In some of these (e. g., Carniarina, Polyxenia, ^gmopsis^
Trachynema) ^ the disk is similar to the nectocalyx of one of
the medusoicls of the Sydropliora y and, like it, is provided
with a velum. But in the rest (Lucer)iarici^ and the Stega-
nophtJialmata) the disk is either devoid of a velum, or pos-
sesses only a rudiment of that structure, and is termed an
umbrella. The edges of the umbrella are divided into lobes
by marginal notches in which the lithocysts are lodged.
Moreover, in these, the mineral particles of the lithocysts are
numerous, and not inclosed in seperate sacs. The lithocysts
are often covered by hood-like processes of the umbrella,
whence they have been termed " covered-ej^ed " or Stega-
noplithalmata,

Lucernaria is fixed by the aboral side of its umbrella
(Fig. 13, 5), by means of a longer or shorter peduncle. The
umbrella is divided into eight lobes, at the extremities of
each of which there is a group of short tentacles. The

Fig. 18.— I. Aurelia aurita: i, the prolonged angles of the mouth ; G, genital cham-
bers : m. lithocysts. x r ^v «„,i;„f5,,^

II. Under view of a segment of the disk, to show the arrangement of the radiatmg
canals: the aperture of a genital chamber and the plaited genital niembrane
showing through its ventral wall; and a lithocyst with its protective hooa (in).

hydranth stands up in the centre of the umbrella, and its
cavity communicates with a central chamber, whence four
wide chambers pass into the lobes. These chambers are
separated by septa, the free central edges of which are beset
with slender tentacles. The reproductive organs are double

THE DISCOPHORA. 123

radiating series of thickenings of the oral wall of each cham-
ber/

All the other Dlscophora^ which are what are commonly
known as " Jelly-fish," are free, and some attain a very large
size. In the adult (Fig. 18) the umbrella is thick and divided
by small marginal notches into as many (usually eight) lobes.
At the bottom of each notch, often protected by special lob-
ules, is an oval lithocyst, supported by a cylindrical pedun-
cle, the cavity of which is in direct communication with
one of the radiating canals of the umbrella (Fig. 28, IV.).
This canal communicates with the exterior on the aboral side
of the base of the peduncle.'' The thick mesoderm of which
the great mass of the umbrella consists is composed of a ge-
latinous connective tissue, in the meshes of which is a watery
fluid, containing numerous nucleated cells which exhibit amoe-
boid movements. On the oral face there is a broad zone of
striped muscle, made up of fusiform fibres placed side by
side. In Aurelia aurita, the angles of the four-sided hy-
dranth are produced into long foliaceous lips, the margins
of which are beset with minute solid tentacula (Fig. 18).
The gastric cavity contained in the hydranths terminates, be-
neath the centre of the umbrella, in a somatic cavity which
passes into four radially-disposed, wide offshoots, or genital
sinuses, the oral walls of which constitute the roof of the gen-
ital chambers (Fig. 18, II.). From their margins the narrow
branching radial canals are given off. The peripheral ends
of these unite when they reach the margin.

Each genital chamber is a recess, surrounded by a thick
wall of the oral face of the umbrella, in the centre of which
only a small aperture is left (Fig. 18, I., G). The roof of
this cavity is the floor of the genital sinus ; it is much plaited
and folded, and the genital elements are developed in it. Its
inner or endodermal wall is beset with small tentacular fila-

* The relations of Lucernaria with the Discophora were shown in my lect-
ures, Medical Times and Gazette^ 1856, Keferstein, " Untersuehungen liber
niedere Seethiere" (1862), in his monograph on the genus, fully confirms
this view, aad Prof H. J. Clark arrived independently at the same conclusion :
'-'- Lucernaria the Coenotype of the AcalephcB^'' ("Proceedings of the Boston
Society of Natural History," 1862). The Lucernaria {CarduelU, Allman)
cijathiforrnis of Sars diifers mucn from the ordinary Lucernariae^ especially in
the position of the genital organs as lonGritudinal thickeninsrs in the walls of
the gastric cavity. See Allman, " On the Structure of Carduella cyathiformu^^
("Transactions of the Microscopical Society," viii.).

''The circular canal of the nectocalyx 'communicates with the exterior by
apertures on the summits of papillose elevations in some medusoids.

124

THE ANATOMY OF INVERTEBRATED ANIMALS.

ments (Fig. 28, III.). The ova or the spermatozoa pass out
of the apertures of the genital chambers, and the ova are re-

Fig. 19. — Cephea ocdlata (?).— The entire animal : a, the umbrella ; &, the ramifications
of the brachia ; c, the tentacles which terminate them; o. the pillars which sns-
pend the bracliiferouB disk which forms the floor of the sub-umbrellar cavity ; I,
short ciavate tentacles between the oral pores.

ceived into small pouches or folds of the lips, and there under-
go the preliminary stages of their development.

In the Mhizostomidce (as was originally suggested by Von
Baer and has been proved by L. Agassiz and A. Brandt * )
the margins of the lips of the hydranth unite, leaving only a
multitude of small apertures for the ingestion of food on the
long arms, which represent prolongations of the lips of the
hydranth (Figs. 19, 20, 21). The polystomatous condition
thus brought about, by the subdivision of a primitively sim-
ple oral cavity, is obviously quite different in its nature from
that which occurs in the JPorifera.

In most of the Mhizosto'inidce, not only do the edges of
the lips unite, but the opposite walls of the hydranth beneath
the umbrella are, as it were, pushed in, so as to form four

» "Memoires de PAcademie do St.-Petersbourg," xvi., 1870.

THE RHIZOSTOMID^.

125

chambers, the walls of which unite, become perforated, and
thus give rise to a sub-umbrellar cavity with a roof formed

Fig. %Q.—Cephea ocellata (?).— ^, part of the umbrella, viewed from below, to show
the plaited genital membrane (/) and the divided attachment of one of the pillars ;
d^ place of one of the liihocysts. B^ one of the oral pores (m) surrounded by ten-
tacula (n) ; g, one of the clavate tentacles interspersed between the oral pores. C,
one of the pedunculated lithocysts {i) in its notch {d) seen from below, with the
oval plate from which muscular fibres (h) take their origin ; e, the radiating canal
with its caecal lateral branches, g.

by the umbrella and a floor, the hrachiferous disJc^ suspended
by four pillars. In the roof the plaited genital membranes

A i

Fig. 2\.— Cephea ocellata (?).— ^, lithocyst enlarged with its hood (A;) and the aboral
pore of the canal {c) ; d, the notch of the margin of the umbrella. B, the hrachifer-
ous disk with the origins of the arms ; /, eudoderm ; o, ectoderm. C, tentaculate
lip of an oral pore enlarged ; m, oral cavity; n, nematocysts.

are developed. The floor (Fig. 21, B) gives off the subdivided
arms, the free margins of which bear the oral pores, and

126 THE ANATOMY OF INVERTEBRATED ANIMALS.

Fig. 22.— a, Biphyes appendiculata.—a, hj-dranths and hydropliyllia on the hydrosoma ;
6, proximal nectocalyx ; c, aperture of distal nectocalyx; rf, somatocyst; c, pro-
longation of the distal nectocalyx, by which it is attached to the hy<lro8oma;
/, point of attacbment of the hydrosoma in the cavity, or hydrcecium, of the proxi-
mal nectocalyx. B, the distal nectocalyx with the canal (through which the bris-
tle a is passed), which is traversed by the hydrosoma in A. C\ extremity of the
distal nectocalyx, with its muscular velum.

which are traversed by canals which unite, pass through the
pillars, and open into the central cavity of the umbrella.*

Fig 23.— >4, 5, DiphyzoOid (Sphe7ioides),\SLtGT^\ and front views. ^, DiphvzoCid of
Abyla {Cuboides). cr, e. gonoijhore or reproductive oriran ; b, hvdranth; c, phyl-
locyst or cavity of hydrophyllium, with its process {d). D, free gonophore, its
manubrium (a) containing ova.

* The species of Cephea., the anatomy of which is here given, was obtained
in the South Pacific, near the LouLsiade Archipelago, on the 11th of July, 1849.

THE SIPHONOPHORA.

127

3. The Siphonophora. — In this group the hydrosoma is
always free and flexible, the ectoderm developing no hard
chitinous exoskeleton, save in the case of the pneumatophores
of some species. In most, the hydranths are of equal size ;
but in Velella and JPorpita, the hydranth situated in the
centre of the discoidal body is very much larger than the
rest, which occupy a circumferential zone around it ; and the

Fig. 2i.~A(horybia rosacea.— A, lateral view ; B. from above; C, 2), detached hydro-
phyllia ; a, polypites ; 6, tentacles ; c, sacculi of the tentacles ; d, hydropbyilia ;
/, pneumatophoie.

principal function of which is to develop the gonophores
from their pedicles. In these two genera the tentacula are
separate from the hydranths, and form the outermost circle
of appendages.

The hydranths of the Siphonophora (Fig. 25, A) never
possess a circlet of tentacula round the mouth, which, when
expanded, is trumpet-shaperl. The endoderm of the hydranth
is ciliated, and villus-like prominences project into its cavity.

The aboral surface of the umbrella was of a brownish-srrav color, variesrated
with oval white spots ; the oral surface, liorht brown with eisrht bluish-green
lines radiating toward the lithocvsts ; the brachia, gray with brown dpts.^ The
brachia divide into two at their origin, and then subdivide into an infinity of
small branches. The general color of the smaller branches is light brown, the
small interspersed clavate tentacles being white. The long tentacles which
terminate each brachium are blue and cylindrical at their origin, but become
trigonal farther on, where they are shaded with brown and green. Is it identi-
calwith the Cephea ocellata of Peron and Lesueur? The individual figured
was a young male.

128 THE ANATOMY OF INVERTEBRATED ANIMALS.

The interior of these frequently contains vacuolar spaces
(Fig. 24, B^ C). A valvular "pylorus" separates the gastric
from the somatic cavity in the Calycophoridm. Long tenta-
cles, frequently provided with unilateral series of branches,
are developed, either one from the base of each hydranth, or,
independently of the hydranths, from the coenosarc.

In the Calycophoridm and many Fhysophoridce, complex

Tig. 25.— AfhoryMa rosacea.— A. a hydranth with villi (a). B, one of the rilli in its
elongated state, enlargeii, C, a small retracted villus, still more magnified, with
its vacuolar epaces and ciliated surface.

organs, containing a sort of battery of thread-cells, terminate
each lateral branch of a tentacle (Figs. 24 and 26). Each
consists of an elongated saccidvs, terminated by two fila-
mentous appendages, and capable of being spirally coiled up.
In this state it is invested by an involucrum, which surrounds
its base. Tlie somatic cavity is continued through the branch,
v/hich constitutes the peduncle of this organ, into the saccu-
lus and its terminal filaments. In the latter it is narrow, and
their thick walls contain numerous small spherical nemato-
cysts. In the sacculus the cavity is wider. One wall is very
thick, and multitudes of elongated nematocysts, the lateral
series of which are sometimes larger than the rest, are dis-
posed parallel with one another, and perpendicular to the
surface of the sac. Like the other organs, each of these
tentacular appendages commences as a simple diverticulum
of the ectoderm and endoderm, and passes through the stages
represented in Fig. 26.

In Physalia the tentacula may be several feet long. They
have no lateral branches, but the large nematocysts are situ-

THE SIPflONOPHORA.

129

ated in transverse reniform thickenings of the wall of the ten-
tacle, which occur at regular intervals.

FiQ. 26. — Athorybia rosacea.— The ends of the tentacnlar branches in varioas stages
of developmeut. A, lateral branch, commeucin^ as a bud from the tentacle. In
B, terminal papillaa, the rudiments of ihe filaments, are developed at the extremi-
ty of the branch ; and, in (7, the sacculus is beginning to be marked off, and thread-
cells have appeared in its walls ; in Z>, the division into involucrum and sacculus
is apparent; in £", the involucrum has inve!*ted the sacculus, the extremity of
which is straight, while the lateral processes have curled round it.

Hydrophyllia are generally present, and, like the tentacu-
la, are developed either from the pedicle of a hydranth, in
which case they inclose the hydranth with its tentacle and a
group of gonophores (GalycopJioridoe)^ or, independently of
the hydranths, from the coenosarc (many Physophoridce).

The hydrophyllia are transparent, and often present very
beautifully defined forms, so that they resemble pieces of cut
glass. They are composed chiefly of the ectoderm (and meso-
derm), but contain a prolongation of the endoderm, with a
corresponding diverticulum of the somatic cavity. They are,
in fact, developed as ccecal processes of the endoderm and
ectoderm ; but the latter, with the mesodermal layer, rapidly
predominates.

The gonophores of the Slphonophora present every varie-
ty, from a simple form, in which the medusoid remains in a
state of incomplete development, to free medusoids of the
Gymnophthalmatous type. As an example of the former

130

THE ANATOMY OF INVERTEBRATED ANIMALS.

condition the gonophores of Atkoryhia may be cited (Fig*.
27) ; of the latter, the gonophores of Physalla^ Porpita^ and
Velella.

In Athoryhia, groups of gonophores, together with pyri-
form sacs, which resemble incompletely develoj^ed hydranths
(hydrocysts — Fig. 27, -4, a), are borne upon a common stem,
and constitute a gonohlastidium, (Fig. 27, A), The groups
of male and female gonophores (Fig. 27, A^ h^ c) are borne
upon separate branches of the gonohlastidium (androphores

Fig. Vl.—Athoryhia rosacea.— A, gonoblastidium bearincr three hydrocysts, a ; gyno-
phore, b ; and two androphores, c. B, female gonophores on their common i<tem
or gynophore, showinir the inchidod ovum, a, and the radical canals, b. O, X>,
female gonophores enlarged ; a, germinal vehicle ; b, vifellua; c, radial canals of
the imperfect nectocalyx ; d, canals ol the manubrial cavity. E, male gouophore,

and gyjiojihores). Each female gonophore contains only a
single ovum, which projects into the cavity of the imperfectly

THE STPHOXOPHORA. 131

differentiated manubrium, and narrowing its cavity at differ-
ent points gives rise to the irregular canals (Fig. 27, D^ d).
In the male gonophore the nectocal3'^x is more distinct from
the manubrium, and its extremity has a rounded aperture
(Fig. 27, E).

In the Calycoplioridoe^ as in the elongated Physophoridoe,^
the development of new hydranths and their appendages,
which is constantly occurring, takes place at that end of the
hydrosoma which corresponds to the fixed extremity of one
of the Hydrophora / and, if we consider this to be the proxi-
mal end, new buds are developed on the proximal side of
those already formed. Moreover, these buds are formed on
one side only of the hydrosoma. Hence the appendages are
strictly unilateral, though they may change their position so
as eventually to appear bilateral or even whorled. In the
Calycopjhoridct^^ the saccular proximal end of the coenosarc
(Fig. 22, A^ d) is inclosed within the anterior nectocalyx, at
the posterior end of which is a chamber, the hydroechctn
(Fig. ^'i^ A, c). The second, or posterior, nectocalyx is at-
tached in such a way that its anterior end is inclosed within
the hydroecium of the anterior nectocalyx, while its contrac-
tile chamber lies on the opposite side of the axis to that on
which the anterior nectocalyx is placed (Fig. 22, A). Sets
of appendages (Fig. 22, A, a ; Fig. 23), each consisting of a
hydrophyllium, a hydranth with its tentacle, and gonophores,
which last bud out from the pedicle of the hydranth — are
developed at regular intervals on the coenosarc, and the long
chain trails behind as the animal swims with a darting mo-
tion, caused by the simultaneous rhythmical contraction of
its nectocalyces, through the water (Fig. 22).

From what has been said, it follows that the distal set of
appendages is the oldest, and, as they attain their full de-
velopment, each set becomes detached, as a free-swimming,
complex Dlphyzooid (Fig. 23). In this condition they grow
and alter their form and size so much, that they were for-
merly regarded as distinct genera of what were termed mono-
gastric Diphydce. The gonophores, with which these are
provided, in their turn become detached, increase in size,
become modified in form, and are set free as a third series
of independent zooids (Fig. 23, D), But their manubrium
does not develop a mouth and become a functional hydranth ;
on the contrary, the generative elements are developed in
its wall, and are set free by its dehiscence.

In the Physophoridce, ithe proximal end of the hydrosoma

133 THE ANATOMY OF INVERTEBRATED ANIMALS.

is provided with a pneumatophore. This is a dilatation, into
which the ectoderm is invaginated, so as to form a receptacle,
which becomes filled with air and sometimes has a terminal
opening, through which the air can be expelled (Fig. 13, 4).
Tt is sometimes small, relatively to the hydrosoma (Agalma,
Physojyhora) ; sometimes so large {Athorybia, Fig. 24 ; Phy-
salia, Forpita, Velella), that the whole hydrosoma becomes
the investment of the pyriform or discoidal air-sac ; while the
latter is sometimes converted into a sort of harl inner shell,
its cavity being subdivided by septa into numerous chambers
(Porpita, Velella).

Nectocalyces may be present or absent in the Fhysojyho-
ridce. When present, their number varies, but they are con-
fined to the region of the hydrosoma which lies nearest to the
pneumatophore.

In the great majority of the Hydrozoa, the ovum under-
goes cleavage and conversion into a morula, and subsequently
into a planula, possessing a central cavity inclosed in a double
cellular wall, the inner layer of which constitutes the hypo-
blast, and the outer the epiblast.

In most S'jdrophora the ciliated, looomodve, planula be-
com3s elongated and fixed by its aboral pole. At the oppo-
site end, the mouth appears and the embryo passes into the
gastrula stage. Tentacles next bud out round the mouth,
and to this larval condition, common to all the Sydrophora,
AUman has sriven the name of Actinida.

Generally, the embryo fixes itself by its aboral extremity
at the end of tlie planula stage ; but, in certain Tubularidce^
while the embryo is still free, a circlet of tentacles is devel-
oped close to the aboral end ; and this form of larva differs
but very slightly from that w^iich is observed in the Disco-
'pJiora.

In the genus Pelagia, for example, the tentacles are de-
veloped from the circumference of the embryo, midway be-
tween the oral and aboral poles ; but it neither fixes itself
nor elongates into the ordinary actinula-form. On the con-
trary, it remains a free-swimming organism, and, by degrees,
' that moiety of the body which lies on the aboral side of the
tentacular circlet widens and is converted into the umbrella,
the other moiety becoming the hydranth, or " stomach," of
the Medusa.

In Lucernaria^ it is probable that the larva fixes itself be-
fore or during the development of the umbrella, and passes

THE DEVELOPMENT OF THE HYDROZOA. 133

directly into the adult condition. But, in most Discopliora^
tlie embryo becomes a fixed actinula (the so-called Hydra
tuba OT jScyj^histoma, Fig. 28, 1.), multiplies agamogenetically
by budding, and gives rise to permanent colonies of Hydri-
form polyps. At certain seasons of the year, some of these
enlarge and undergo a farther agamogenetic multiplication
by fission (Fig. 28, II.). In fact, each divides transversely
into a number of eight-iobed discoidal medusoids i^'' Ephyr(B "
or '■' 3Ieduscje bijidce,''^ Fig. 28, II. and III.), and thus passes
into what has been termed the Strohlla stage. The Ephyrm^
becoming detached from one another and from the stalk of
the Strobila, are set free, and, undergoing a great increase
in size, take on the form of the adult Discophore, and acquire
reproductive organs. The base of the Strobila may develop
tentacles (Fig. 28, II.) and resume the Scyphlstoma condition.

Metschnikofi"^ has recently traced out the development of
Geryonia ( Carmarina)^ Polyxenia^^glnopsis^ and other i>?5-
cophora^ which diflfer from the foregoing in possessing a velum ;
and in these, as in the Trachynema ciliatum^ observed by
Gegenbaur,' the process appears to be of essentially the same
nature as in Pelagia, The Scyi^histoma of Aurelia, Cyanoea^
and their allies, is probably to be regarded, like the larva of
Pelagia^ as a Discophore with a rudimentary disk ; in which
case the reproduction of the Ephyra-ioiWi^ of young Disco-
phora will not be comparable to the development of medusoid
gonophores among the Hydrophora^ but will merely be a pro-
cess of multiplication, by transverse fission, of a true, though
undeveloped, Discophore.

In the Siphonoijhora^ the result of yelk division Is the
formation of a ciliated body consisting of a small-celled
ectoderm investing a solid mass of large blastomeres, which
eventually pass into the cells of the endoderm. This body
does not take the form of an actinula. On the contrary, it
appears to be the rule that buds from w^hich a hydroph3dlium,
a nectocalyx, a tentacle, or pneumatophore, or even all of
them, will be developed, take their origin antecedently to the
formation of the first polypite and of the gastric cavity.

As Metschnikoff well remarks, the m.ode of development
of the Siplionopliora is wholly inconsistent with the doctrine
that the various appendages of the hydrosoma in these ani-

^ " Studien uber die Entwickelung der Medusen und Siplionoplioren."
{Zeitschrift fur wiss. Zool., xxiv.)

a " Zur Lehre der Generationsvrechsel." 1854.

3 See especially tlie late observations of AletschnikofF, he. cit.

134 THE ANATOMY OF INVERTEBEATED ANIMALS.

li liiKij i i li^if iiniil I I i\ \

' '. ' '.\ : ' ' ■ ! ' ? ?•• • ! !: ■ :'■ ;: ; •' •

ii!\iiiiin iinUniHlnlin J

riiiill nniirin \\\\\ I
iiiliUililiihUy lii \ il

i UU \iUn i \ liu uM \ I

Fro. 28.— I. and II.— Cyanoea capillata (after Van Beneden^).

I. Two Hydrce tubce (Scyphistoma stage), exhibiting their ordinary character?, and
between them two (a, b) which are undergoing fission (Sii^obila ?tage).

n. The two StrobUm, a and ft, three day? later. In a. tentacles are developed be-
neath the lowest of the Ephyrae, from the stalk of the Strobila, which will persist
as a Hydra tuba.

III. Hair the disk of an Ephyra of Aurelia aurita, seen from the oral face. The
Bmall tentacles which lie between the mouth and the band of circular muscular
fibres are inside the somatic cavity, whence sixteen short and wide radial canala
extend to the periphery, where they are united by transverse branches. Eight
of the radial canals enter the corresponding lobes, and finally divide into three
branches: one which enters the peduncle of the lithocyst. and two lateral caeca.
Radiating bands of muscular fibres accompany these canals.

rV". Side view of one of the lithocysts with its peduncle. The arrow indicates the
direction in which the cilia of the exterior work.

' "Eecherches sur la Faune littorale de Belgique. Polypes." 1866.

THE DEVELOPMENT OF THE HYDROZOA. I35

mals represent individuals. The Hydrozoa are not properly-
compound organisms, if this phrase implies a coalescence of
separate individualities ; but they are organisms, the organs
of which tend more or less completely to become independent
existences or zooids. A medusoid, though it feeds and main-
tains itself, is, in a morphological sense, simply the detached
independent generative organ of the hydrosoma on which it
was developed ; and what is termed the " alternation of gen-
erations," in these and like cases, is the result of the dissocia-
tion of those parts of the organism on which the generative
function devolves, from the rest/

In certain Z>/5ca;9^ora belonging to the group of Trachy-
Jiemata, a method of multiplication by gemmation has been
observed, which is unknown among the other Hydrozoa. It
mav be termed entogastrio gemmation, the bud growing out
from the wall of the gastric cavit}^, into which it eventually
passes on its way outward ; while, in all other cases, gemma-
tion takes place by the formation of a diverticulum of the
whole wall of the gastro-vascular cavity which projects on to
the free surface of the body, and is detached thence (if it be-
come detached), at once, into the circumjacent water. The de-
tails of this process of entogastric gemmation have been traced
by Haeckel ^ in Gannirlna hastata, one of the Geryonidm,
As in other members of that family, a conical process of the
mesoderm, covered by the endoderm, projects from the roof
of the gastric cavity and hangs freely down into its interior.
Upon the surface of this, minute elevations of -g-Jifth of an
inch in diameter make their appearance. The cells of w^hich
these outgrowths are composed next become differentiated
into two layers — an external clear and transparent layer,
which is in contact with the cone, and invests the sides of the
elevation ; and an inner darker mass. The external layer is the
ectoderm of the young medusoid, the inner its endoderm. A
cavity, which is the commencement of the gastric cavity, ap-
pears in the endodermal mass, and opens outward on the free
side of the bud. The latter, now ^tq-*^ ^^ ^^ ^"^^^^ ^" diameter,
has assumed the form of a plano-convex disk, fixed by its flat
side to the cone, and having the oral aperture in the centre of
its convex free side. The disk next increasing in height, the

» I have seen no reason to depart from the opinions on the subject of
'Animal individuality' enunciated in my lecture published in the Annals and
Magazine of Natural History for June, 1852.

2 " Beitrage zur Naturgeschi elite der Hy drome dusen," 1865.

136 THE ANATOMY OF IXVERTEBRATED ANIMALS.

body acquires the form of a flask with a wide neck. The belly
of the flask is the commencement of the umbrella of the bud-
ding medusoid ; the neck is its gastric division. The belly of
the flask, in fact, continues to widen out until it has the form
of a flat cup, from the centre of which the relatively small
gastric neck projects, and the bud is converted into an unmis-
takable medusoid, attached to the cone by the centre of the
aboral face of its umbrella. In the mean while, the gelatinous
transparent mesoderm has appeared, and, in the umbrella, has
acquired a great relative thickness. Into this, eight prolonga-
tions of the gastric cavity extend, and give rise to the radial
canals, which become united into a circular canal at the cir-
cumference of the disk. The velum, tentacula, and lithocysts
are developed, and the bud becomes detached as a free swim-
ming medusoid. But this medusoid is very different from the
Garmarina from which it has budded. For example, it has
eight radial canals, while the Cannarina has only six ; it has
solid tentacles, while the adult Cannarina has tubular tenta-
cles ; it has no gastric cone, and has differently disposed lith-
ocysts. Haeckel, in fact, identifies it with Cunina rhodo-
dactyla, a form which had hitherto been considered to be not
only specifically and generically different from Carmarina^
but to be a member of a distinct family — that of the uS^ginidm,

What makes this process of asexual multiplication more
remarkable is, that it takes place in Carmarince which have
already attained sexual maturity, and in males as well as in
females.

There is reason to believe that a similar process of ento-
gastric proliferation occurs in several other species of uSiJgi'
nidm — ^gineta prolifera (Gegenbaur), Eury stoma ruhigi-
nosum (KoUiker), and Cunina Kollikeri (F. Miiller) ; but,
in all these cases, the medusoids which result from the gem-
mative process closely resemble the stock from which they
are produced.

As might be expected, the Hydrozoa are extremely rare
in the fossil state, and probably the last animal the discovery
of fossil remains of which could be anticipated is a jelly-fish.
Nevertheless, some impressions of Medusae, in the Solenhofen
slates, are sufficiently well preserved to allow of their deter-
mination as members of the group of Rhizostomidm^ The

* Haeckel, " Ueher zwei neiie fossile Meclnsen aus der Fainilie der BH-
zostomiden." (" Jahrbuch fiir Mineralogie," 1866.)

THE ACTINOZOA. 137

apparent absence of the remains of Hydrophora in the meso-
zoic and newer palaeozoic rocks is very remarkable. Some
singular organisms, termed Graptolites^ which abound in the
Silurian rocks, may possibly be Hydrozoa, though they
present points of resemblance with the Polyzoa. They are
simple or branched stems, sometimes slender, sometimes ex-
panded or foliaceous ; occasionally the branches are connected
at their origin by a membranous expansion. The stems are
tubular, and beset on one or both sides with minute cup-
shaped prolongations, like the thecse of a Sertularian. A solid
thickening of the skeleton may have the appearance of an
independent axis. Allman has suggested that the theciform
projections of the Graptolite stem may correspond with the
mematophores of Sertularians, and that the branches may
have been terminated by hydranths. Appendages which ap-
pear to be analogous to the gonophores of the Hydrophora
have been described in some Graptolites.^

With a very few exceptions {Hydra, CordylopJiora) the
Hydrozoa are marine animals ; and a considerable number,
like the CalycoplioridcQ and Physoplioridae, are entireh' pe-
lagic in their habits.

The Actixozoa. — The essential distinctions between the
Actinozoa and the Hydrozoa are two. In the first place, the
oral aperture of an Actinozoon leads into a sac, which, with-
out prejudice to the question of its exact function, may be
termed " gastric," and which is not, like the hydranth of the
Hydrozoon, free and projecting, but is sunk within the body.
From the walls of the latter it is separated by a cavity, the
sides of which are divided- by partitions, the tnesenteries,
which radiate from the wall of the gastric sac to that of the
body, and divide the somatic cavity into a corresponding num-
ber of interinesenteric chambers. As the gastric sac is open
at its inner end, however, its cavity is in free communication
with that of the central space which communicates with the
intermesenteric chambers ; and the central space, together
with the chambers, which are often collectively termed the
*' body cavity " or " perivisceral cavity," are, in reality, one
with the digestive cavity, and, as in the Hydrozoa, consti-
stute an enter ocode. Thus an Actinozoon might be com-
pared to a Lucernarla, or still better to a Carduella, in which
the outer face of the hydranth is united with the inner face

1 Hall, " Graptolltes of the Quebec Series of North America," I860. Nichol-
son, " Monograph of the British Graptolitidiis," 1872.

138 THE ANATOMY OF INYERTEBRATED ANIMALS.

of the umbrella ; under these circumstances the canals of the
umbrella in the Hydrozoon would answer to the intermesen-
teric chambers in the Actinozoon.

Secondly, in the Actmozoa^ the reproductive elements
are developed in the walls of the chambers or canals of the en-
terocoele, just as they so commonly are in the walls of the
gastro-vascular canals of the Hjdrozoa^ but the generative
organs thus constituted do not project outwardly, nor dis-
charge their contents directly outward. On the contrary, the
ova and spermatozoa are shed into the enterocoele, and event-
ually make their way out by the mouth. In this respect,
again, the Actinozoon is comparable to a Lucernaria modi-
fied by the union of the hydranth with the ventral face of the
umbrella ; under which circumstances the reproductive ele-
ments, which in all Hydrozoa are developed, either in the
walls of the hydranth or in those of the oral face of the um-
brella, would be precluded from making their exit by any
other route than through the gastro-vascular canals and the
mouth.

In the fundam3ntal composition of the body of an ecto-
derm and endoderm, with a more or less largely developed
mesoderm, and in the abundance of thread-ceils, the Actino-
zoa agree with the Hydrozoa.

In most of the Actinozoa, the simple polyp, into which
the embryo is converted, gives rise by budding to many
zooids which form a coherent whole, termed by Lacaze-Du-
thiers a zoanthodeme.

The C0RA.LLIGEXA. — The Actinozoa comprehend two
groups — the Goralligena and the Ctenophora — which are
widely different in appearance though fund amen tall}^ similar
in structure. In the former, the mouth is always surrounded
by one or more circlets of tentacles, which may be slender
and conical, or short, broad, and fimbriated. The mouth is
usually elongated in one direction, and, at the extremities of
the long diameter, presents folds which are continued into
the gastric cavity. The arrangement of the parts of the body
is therefore not so completely radiate as it appears to be.
The enterocoele is divided into six, eight, or more wide inter-
mesenteric chambers, which communicate with the cavities of
the tentacles, and sometimes directly with the exterior, by
apertures in the parietes of the body. The mesenteries which
separate these wide chambers are thin and membranous. Two
of them, at opposite ends of a transverse diameter of the Ac-

THE CORALLIGENA.

139

tinozo(3n, are often different from the rest. Each mesentery
ends, at its aboral extremity, in a free edge, often provided

Fig. 29.— Perpendicular section of Actinia holsatica (after Frey and Leuckart).— a,
mouth; 6, j^astric cavity; c, common cavity, iuto wliich the gastric cavity and
the intermesenteric chambers open; c?, intermesenteric chambers; e, thickened
free margin, containing thread-cells of, /, a mesentery; g^ reproductive organ ; A,
tentacle.

with a thickened and folded margin ; and these free edges
look toward the centre of an axial cavity,^ into which the gas-
tric sac and all the intermesenteric chambers open.

In the Coralligena., the outer wall of the body is not pro-
vided with bands of large paddle-like cilia. Most of them are
fixed temporarily or permanently, and many give rise by
gemmation to turf-like, or arborescent, zoanthodemes. The
great majority possess a hard skeleton, composed principally
of carbonate of lime, which may be deposited in permanently
disconnected spicula in the walls of the body ; or the spicula
may run into one another, and form solid networks, or dense
plates, of calcareous matter. When the latter is the case, the
calcareous deposit may invade the base and lateral walls of
the body of the Actinozoon, thus giving rise to a simple cup,
or theca. The skeleton thus formed, freed of its soft parts, is
a " cup-coral," and receives the name of a corallite.

In a zoanthodeme, the various polyps (anthozooids)
formed by gemmation may be distinct, or their several enter-
ocoeles may communicate ; in which last case, the common
connecting mass of the body, or ccenosarc, may be traversed
by a regular S3'stem of canals. And, when such compound

» Partially-digested substances are often found in this axial space, and it is
not improbable that it may functionally represent the stomach or the com-
mencement of the intestine in higher animals.

140 THE ANATOMY OF INVEKTEBRATED ANIMALS.

Actinozoa develop skeletons, tLe corallites may be distinct,
and connected only by a substance formed by the calcifica-
tion of the coenosarc, which is termed coenenchyma j or the
thecas may be imperfectly developed, and the septa of adja-
cent corallites run into one another. There are cases, again,
in which the calcareous deposit in the several polyps of a
compound Actinozoon, and in the superficial parts of the coe-
nenchyma, remains loose and spicular, wiiile the axial por-
tion of the coenosarc is converted into a dense chitincus cr cal-
cified mass — the so-called sclerohase.

The mesoderm contains abundantly developed muscular
fibres. The question whether the Coralligena possess a ner-
vous system and organs of sense, hardly admits of a definite
answer at present. It is only in the Actinidce that the ex-
istence of such organs has been asserted ; and the nervous
circlet of Actinia^ described by Spix, has been seen by no
later investigator, and may be safely assumed to be non-exist-
ent. Prof. P. M. Duncan, F. R. S.,^ however, has recently
described a nervous apparatus, consisting of fusiform gan-
glionic cells, united by nerve-fibres, which resemble the sym-
pathetic nerve-fibrils of the Yertehrata^ and form a plexus,
which appears to extend throughout the pedal disk, and
very probably into other parts of the body. In some of the
ActmidcB (e. g.-, Actinia mesembryantliemwni)^ brightly-col-
ored bead-like bodies are situated in the oral disk outside
the tentacles. The structure of these "chromatcphores," or
" bourses calicinales," has been carefully investigated by
Schneider and Rotteken, and by Prof. Duncan. They are
diverticula of the body wall, the surface of which is com-
posed of close-set " bacilli," beneath w^iich lies a layer of
strongly-refracting spherules, followed by another layer of
no less strongly-refracting cones. Subjacent to these, Prof.
Duncan finds ganglion cells and nervous plexuses. It would
seem, therefore, that these bodies are rudimentary eyes.

The sexes are united or distinct, and the ovum is ordina-
rily, if not always, provided with a vitelline membrane. The
impregnated ovum gives rise to a ciliated morula, which may
either be discharged or undergo further development within
the somatic cavity of the parent. The morula becomes a gas-
trula, but whether by true invagination or by delamination,
as in most of the Ilydrozoa^ is not quite clear. The gastrula
usually fixes itself by its closed end, while tentacles are de-

» " On the Nervous System of Actinia." (" Proceedings of the Eoyal Socie-
ty," October 9, 1873.)

THE DEVELOPMENT OF THE CORALLIGENA, 141

veloped from its oral end. It can hardly be doubted that the
intermesenterio chambers are diverticula of the primitive en-
terocoele ; but the exact mode of their origin needs further
elucidation.

Lacaze-Duthiers ^ has recently thrown a new light upon
the development of the Coralligena^ and particularly of the
Acthiim {Actinia, Sagartia, Bimodes). These animals are
generally hermaphrodite, testes and ovaria being usually found
in the same animal, and even in the same mesenteries ; but
it may happen that the organs of one or the other sex are, at
any given time, exclusively developed. The ova undergo the
early stages of their development within the body of the
parent. The process of yelk division was not observed, and
in the earliest condition described the embryo was an oval
planula-like body, composed of an inner colored substance
and an outer colorless layer. The outer layer (epiblast = ec-
toderm) soon becomes ciliated. An oval depression appears
at one end, and becomes the mouth ^ and gastric sac, while, at
the opposite extremity, the cilia elongate into a tuft. The
ectoderm extends into and lines the gastric sac, while the in-
terior of the colored hypoblast becomes excavated by a cav-
ity, the enterocoele, which communicates with the gastric sac.
In this condition the embryo swims about with its oral pole
directed backward.

The oral aperture changes its form and becomes elongated
in one direction, which may be termed the oral axis. The
mesenteries are paired processes of the transparent outer
layer (probably of that part which constitutes the mesoderm)
which mark off corresponding segments of the enterocoele.
The first which make their appearance are directed nearly at
right angles to the oral axis near, but not exactly in, the
centre of its length. Hence they divide the enterocoele into
two primitive chambers, a smaller (A) at one end of the oral
axis, and a larger (A') at the other. This condition may be
represented by A-4-A'; the dots indicating the position of
the primitive mesenteries, and the hyphen that of the oral
axis. It is interesting to remark that, in this state, the em-

1 " Developpement, des CoralUaircs." {ArcMves de Zoologie experim^ntale^
1872.)^

2 Kowalewsky describes the formation of a ffastrula by invagination in a spe-
cies of Actinia and in Cereanthus^ the aperture of invacfination becoming the
mouth (llofmann and Schwalbe, " Jahresbericht," Bd. II., p. 269). In other
species of Actinia and in Alcijonium^ the planuhx seems to delamiuate. Ordi-
nary yelk division occurs in some Anthozoa^ while in others {Alcyonium) the
process rather resembles that which occurs in most Arthropods.

142 THE ANATOMY OF INYERTEBRATED ANIMALS.

bryo is a bilaterally symmetrical cylindrical body, with a cen-
tral canal, the future gastric sac ; and, communicating there-
with, a bilobed enterocoele, which separates the central canal
from the body- wall. In fact, in principle, it resembles the
early condition of the embryo of a Ctenophore, a Brachiopod,
or a Sag lit a.

Another pair of mesenteric processes now makes its ap-
pearance in the larger chamber A', and cuts off two lateral
chambers, B, B, which lie between these secondary mesenteries
and the primary ones. In this state the enterocoele or somat-
ic cavity is four-chambered f A-f-r? AM. Next a third pair

of mesenteries appear in the smaller chamber (A), and divide
it into three portions, one at the end of the oral axis (A),
and two lateral (C, C). In this stage there are therefore six

A p-^-T) A' ) ; but almost immediately the number

is increased to eight, by the development of a fourth pair of
mesenteries in the chambers B, B, which thus give rise to the
chambers D, D, between the primitive mesenteries and them-
selves. The embryo remains in the eight-chambered condition

(a p/-T-yA -D AM for some time, until all the chambers and their

dividing mesenteries become equal. Then a fifth and a sixth
pair of mesenteries are formed in the chambers C, C, and D, D ;
two pairs of new chambers, E and F, are produced, and thus the

Actinia acquires twelve chambers (A p p^^T^ t) "r -^ /> ^^^

of which result from the subdivision of the smaller primary
chamber, and seven from that of the larger primary chamber.
The various chambers now acquire equal dimensions, and the
tentacles begin to bud out from each. The appearance of
the tentacles, however, is not simultaneous. That which pro-
ceeds from the chamber A' is earliest to appear, and for some
time is largest, and, at first, eight of the tentacles are larger
than the other four.

The coiled marginal ends of the mesenteries appear at
first upon the edges of the two primary mesenteries ; then
upon the edge of the fourth pair, and afterward upon those
of the other pairs.

For tb.e further chan2:es of the young Actioiia, I must
refer to the work cited. Sufficient has been said to show that
the development of the ActinicB follows a law of bilateral
symmetry, and to bring out the important fact that, in the

THE OCTOCORALLA. 143

course of its development, the finally hexamerous Antho-
zoon passes through a tetramerous and an octomerous stage.

Phenomena analogous to the " alternation of generations,"
which is so common among the Hydrozoa^ are unknown
among the great majority of the Actinozoa. But Semper ^
has recently described a process of agamogenesis in two spe-
cies of Fungice, which he ranks under this head. The Fungim
bud out from a branched stem, and then become detached
and free, as is the habit of the genus. To make the parallel
with the production of a medusoid from a hydroid polyp
complete, however, the stem should be nourished by a sexless
anthozooid of a different character from the forms of Fimgice
which are produced by gemmation. And this does not appear
to be the case.

In one division of the Coralligena — the Octocoralla —
eight enterocoele chambers are developed, and as many ten-
tacles. Moreover, these tentacles are relatively broad, flat-
tened, and serrated at the edges, or even pinnatifid. The
Actinozoun developed from the e^g wi^^y remain simple
(Haimea^ Milne-Edwards), but usually gives rise to a zoan-
thodeme.

The coenosarc of the zoanthodeme in the Octocoralla is a
substance of fleshy consistence, which is formed chiefly of a
peculiar kind of connective tissue, containing many muscular
fibres developed in the thickened mesoderm. The axial cavity
of each anthozooid is in communication with a system of
large canals. In Alcyonixtm^ a single large canal descends
from each anthozooid into the interior of the zoanthodeme,
and the eight mesenteries are continued as so man}' ridges
throughout its entire length,'' so that these tubes have been
compared to the thecal canals of the Millepores. In the red
coral of commerce ( Corallium ruhruin^ Fig. 30), the large
canals run parallel with the axial skeleton. A delicate net-
work, which traverses the rest of the substance of the coeno-
sarc, appears to be sometimes solid and sometimes to form a
system of fine canals opening into the larger ones. The
anthozooids possess numerous muscles by which their move-
ments are effected. The fibres are delicate, pale, and not
striated. Nerves have not been certainly made out.

It is in these Octocoralla that the form of skeleton which
is termed a sclerobase, which is formed by cornification or

1 " Ueber Generations-Wechsel bei Steinkorallen." Leipsic, 1872.

2 Pouchet and Myevre, " Contribution a I'Auatomie des Alcyonaires."
{Journal d' Anatomie et de la Physiologie^ 1870.)

144 THE ANATOMY OF INVERTEBRATED ANIMALS.

Fig. 30. — Corallium rubrum (after Lacaze-Duthiers ^).

I. The end of a branch with A, B, C, three anthozoSids in different desrees of ex-
pansion ; k, the mouth ; a, that part of the coenosarc which rises into a cup
around the base of each anthozoOid.

II. Portion of a branch, the coenosarc of which has been divided longitudinallj' and
partially removed; B, B\ B'\ anthozooids in section; B^ anthozoOid with ex-
panded tentacles; k, mouth ; m, gastric sac ; e, its inferior edge; j^ mesenteries.

J?', anthozoOid retracted, with the tentacles {d) drawn back into the intermesenteric
chambers; c, oriftcea of the cavities of the invaginated tentacles ; e, circum-oral
cavity ; 6, the part of the body which forms the projecting tube when the antho-
zoOid is expanded ; a, festooned edges of the cup.

B" ^ antliozoOid, showing the transverse sections of the mesenteries.

-4, A, coenosarc, with its deep longitudinal canals (/'), and superficial, irregular,
reticulated canals (A). P, the hard axis of the coral, with longitudinal grooves
fj7) answering to the longitudinal vessels.

III., IV. Free ciliated embryos.

1 " Ilistoire Naturelle du Corail," 1864.

THE ACTiNOZOA. 145

calcification of the axial connective tissue of the zoantho-
deme, occurs. It is an unattached simple rod in Pennatula
and Veretilluin^ but fixed, tree-like, branched, and even retic-
ulated, in the Gorgonim and the red coral of commerce ( Co-
rallium). In the Alci/o?iia, or " Dead-men's-fingers," of our
own shores, there is no sclerobase, nor is there any in Tuhi-
pova^ the organ-coral. But, whereas in all the other Octoco-
ralla the bodies of the polyps and the coenosarc are beset with
loose spicula of carbonate of lime, Tuhipora is provided with
solid tubiform thecse, in which, however, there are no septa.

Dimorphism has been observed by Kolliker to occur exten-
sively among the Pennatulidce. Each zoanthodeme presents
at least two different sets of zooids, some being fully devel-
oped, and provided with sexual organs, while the others have
neither tentacles nor generative organs, and exhibit some
other peculiarities.* These abortive zooids are either scat-
tered irregularly among the others (e. g., S>arcophyton^ Fere-
tilliwi)^ or may occupy a definite position (e. g., Virgularia),

In the other chief division of the Coralligena — the Hexa-
eoralla — the fundamental number of enterocoele chambers and
of tentacles is six," and the tentacles are, as a rule, rounded
and conical, or filiform.

The Actinozoon developed from the ^gg in some of the
Hexacoralla remains simple, and attains a considerable size.
Of these — the Actinidce — many are to some extent locomo-
tive, and some {Minyas) float freely by the help of their
contractile pedal region. The most remarkable form of this
group is the genus Cereanthiis, which has two circlets, each
composed of numerous tentacles, one immediately around the
oral aperture, the other at the margin of the disk. The foot
is elongated, subcorneal, and generally presents a pore at its
apex. Of the diametral folds of the oral aperture, one pair is
much longer than the other, and is produced as far as the
pedal pore. The larva is curiously like a young hydrozoon
with four tentacles, and, at one time, possesses four mesen-
teries.

The ZoanthidcB differ from the Actmidce in little more
than their multiplication by buds, which remain adherent,
either by a common connecting expansion or by stolons ; and
in the possession of a rudimentary, spicular skeleton. In the
Antlpathidm there is a sclerobasic skeleton. The proper

1 " Abhandlungen der Senkenberffischen naturforschenden Gesellscliaft,"
isd. vu., vui.

2 That is to say, in the adult, they are either six or some multiple of six.

7

146 THE ANATOMY OF INVERTEBRATED ANIMALS.

stone-corals are essentially Actinice, which become converted
into zoanthodemes by gemmation or fission, and develop a
continuous skeleton.

The skeletal parts ^ of all the Acti7iozoa, consist either of
a substance of a horny character ; or of an organic basis im-
pregnated with earthy salts (chiefly of lime and magnesia),
but which can be isolated by the action of dilute acids ; or,
finally, of calcareous salts in an almost crystalline state, form-
ing rods or corpuscles, which, when treated with acids, leave
only an inappreciable and structureless film of organic matter.
The hard parts of all the Aporosa, Perforata^ and Tcibulata
of Milne-Edwards are in the last-mentioned condition ; while,
in the Octocoralla^ except Tuhipora, and in the Antipathidc^^
andZoa^z^A/f^tf, among the Hexacoralla, the skeleton is either
horny ; or consists, at any rate, to begin with, of definitely
formed spicula, which contain an organic basis, and frequently
present a laminated structure. In the organ-coral [Tuhi2:)ora),
the skeleton has the character of that of the ordinary stone-
corals, except that it is perforated by numerous minute canals.

The skeleton appears, in all cases, to be deposited within
the mesoderm, and in the intercellular substance of that layer
of the body. Even the definitely shaped spicula of the Octo-
coralla seem not to result from the metamorphosis of cells.
In the simple aporose corals the calcification of the base and
side walls of the body gives rise to the cup or theca / from
the base the calcification extends upward in lamelhe, which
correspond with the interspaces between the mesenteries, and
gives rise to as many vertical septa^ the spaces between which
are termed loculi ; while, in the centre, either by union of the
septa or independently, a column, the columella^ grows up.
Small separate pillars between the columella and the septa are
termed p)aluli. From the sides of adjacent septa scattered
processes of calcified substance, or synapticnlw, may grow
out toward one another, as in the Fimgidm ; or the interrup-
tion of the cavities of the loculi may be more complete in
consequence of the formation of shelves stretching from sep-
tum to septum, but lying at different heights in adjacent
loculi. These are interseptal dissephnents. Finally, in the
Tahidata, horizontal plates, which stretch completely across
tlie cavity of the thcca, are formed one above the other and
constitute tahidar dissejnments.

1 See Kolliker, " Icones ITistolosricoe," 1860.

2 Lacaze-Duthiers's investifxations on Astr(xa cali/cularis i>TO\'e that the septa
begin to be fonnecl before the theca.

THE "TABULATA." 147

In the Aj^orosa the theca and septa are almost invariably
imperforate; but, in the Perforata^ they present apertures,
and, in some Madrepores, the whole skeleton is reduced to
a mere network of dense calcareous substance. When the
Hexacoralla multiply by gemmation or fission, and thus give
rise to compound massive or arborescent aggregations, each
newly-formed coral polyp develops a skeleton of its own, which
is either confluent with that of the others, or is united with
them by calcification of the connecting substance of the com-
mon body. Tliis intermediate skeletal layer is then termed
coenenchyma.

The septa in the adult Hexacoralla are often very numer-
ous and of different lengths, some approaching the centre
more closely than others do. Those of the same lengths are
members of one " cycle ; " and the cycles are numbered ac-
cording to the lengths of the septa, the longest being counted
as the first. In the young, six equal septa constitute the first
cycle. As the coral grows, another cycle of six septa arises
by the development of a new septum between each pair of
the first cycle ; and then a third cycle of twelve septa^ di-
vides the previously existing twelve interseptal chambers into
twenty-four. If we mark the septa of the first cycle A, those
of the second B, and those of the third C, then the space be-
tween any two septa (A A) of the first cycle will be thus rep-
resented when the third cycle is formed — A C B C A.

When additional septa are developed, the fourth and fol-
lowing cycles do not consist of more than twelve septa each ;
hence the septa of each new cycle appear in twelve of the
previously existing interseptal spaces, and not in all of them;
and the order of their appearance follows a definite law, which
has been worked out by Milne-Edwards and Haime. Thus,
the septa of the fourth cycle of twelve (d) bisect the inter-
septal space A C ; and those of the fifth cycle (e) the inter-
septal space B C ; the septa of the sixth cycle (f), A d and
d A ; those of thes eventh cycle (g), e B and B e ; those of the
eighth cycle (h), d C and C d; and those of the ninth cycle
(i), C e and e C.

Hence, after the formation of nine cycles, the septa added
between every pair of primary septa (A, A) will be thus ar-
ranged — A fdhCiegBgeiChdf A.^

The stone-corals ordinaril}'^ known as Mlllepores are char-

1 That the order of oceurreTice of the septa of various lensfths, at the differ-
ent stagfes of growth of a corallite, is that indicated, seems to be clear, whatever
may be the exact mode of development of the septa in each cycle.

148 THE ANATOMY OF INVERTEBRATED ANIMALS.

acterized by being traversed by numerous tubular cavities,
which open at the surface, and the deeper parts of which are
divided by numerous close-set transverse partitions, or tabular
cUsse2nments, while vertical septa are rudimentary or alto-
gether absent. These were regarded as Anthozoa, and
classed together in the division of Tabidata^ until the elder
Agassiz * published his observations on the living Millepora
alcicornis, which led him to the conclusion that the Tabulata
are Hydrozoa allied to Hydractinia, and that the extinct Mu-
gosa were probably of the same nature.

The evidence adduced by Agassiz, however, was insuffi-
cient to prove his conclusions ; and the subsequent discovery
by Verrill that another tabulate coral, Pocillopora, is a true
Hexacorallan, while Moseley ^ has proved that Heliopora
coeridea is an Octocorallan, gave further justification to those
who hesitated to accept Agassiz's views.

The recent very thorough and careful investigation of a
species of Millepora occurring at Tahiti,^ by Mr. Moseley,
although it still leaves us in ignorance of one important
point, namely, the characters of the reproductive organs, yet
permits no doubt that Millepora is a true Hydrozoon allied to
Hydractinia^ as Agassiz maintained. The surface of the
living Millepora presents short, broad hydranths, the mouth
of which is surrounded by four short tentacles. Around each
of these alimentary zooids is disposed a zone of from five to
twenty or more, much longer, mouthless zooids, over the bod-
ies of which numerous short tentacles are scattered. Each
of these zooids expands at its base into a dilatation, whence
tubular processes proceed, which ramify and anastomose, giv-
ing rise to a thin expanded hydrosoma. The calcareous mat-
ter (composed as usual of carbonate, with a small proportion
of phosphate of lime) forms a dense continuous crust upon
the ectoderm of the ramifications of the hydrosoma, that part
of it which underlies the dilatations of the zooids constituting
the septa. As the first formed hydrosomal expansion is com-
pleted, another is formed on its outer surface, and it dies.
The " thecal " canals of the coral arise from the correspond-
ence in position of the dilatations of the zooids of successive
hydrosomal layers, and the tabulte are their supporting plates.

Thus the grou|) of the Tabulata ceases to exist, and its

» " Natural History of the United States," vols. iii. and iv., 1860-'62.
2 Moseley, " The Structure and Relations of the Alcyonaruxn, Heliopora,
carulea^'' etc. (" Proceedinijs of the Roval Society," November, 1875.)
8 " Proceedin^rs of the Pvoval Societv," 1876.

THE REEF-BUILDIXG CORALS. 149

members must be grouped either with the Scxacoralla^ the
Octocoralla, or the Hydrozoa.

The Hugosa constitute a group of extinct and mainl^r
Palaeozoic stone-corals, the thecae of which are provided with
tabular dissepiments, and generally have the septa less de-
veloped than those of the ordinary stone-corals. The arrange-
ment of the parts of the adult Hugosa in fours, and the
bilateral symmetry which they sometimes exhibit, are inter-
esting peculiarities when taken in connection with the te-
tramerous and asymmetrical states of the embryonic Hexaco-
ralla. On the other hand, some of the Hugosa possess oper-
cula, which are comparable to the skeletal appendages of the
Alcyonarian Prlmnoa observed by LindstrOm, and the te-
tramerous arrangement of their parts suggests affinity with
the Octocoralla. It seems not improbable tiiat these ancient
corals represent an intercalary type between the Hexccoralla
and the Octocoralla.

All the Actbiozoa are marine animals. The Actinice,
among the Hexacoralla^ and various forms of Gctocoralla^
have an exceedingly wide distribution, while the latter are
found at very great depths.

The stone-corals, again, have a wide range, both as respects
depth and temperature, but they are most abundant in hot
seas, and many are confined to such regions. Some of these
stone-corals are solitary in habit, while others are social, grow-
ing together in great fields, and forming what are called
" coral reefs." The latter are restricted within that compara-
tively narow zone of the earth's surface which lies between
the isotherms of 60°, or, in other words, they do not extend
for more than about 30° on either side of the equator. It is
not conditions of temperature alone, however, which limit
their distribution ; for, within this zone, the reef-builders are
not found alive at a greater depth than from fifteen to twenty
fathoms, while at the equator, an average temperature of 68°
is not reached within a depth of 100 fathoms.

Not only heat, then, but light, and probably rapid and
effectual aeration, are essential conditions for the activity of
the reef-building Actinozoa. But, even within the coral zone,
the distribution of the reef-builders appears to be singularly
capricious. None are found on the west coast of Africa, very
few on the east coast of South America, none on the west
coast of North America ; while in the Indian Ocean, the Pa-
cific, and the Caribbean Sea, they cover thousands of square

150 THE ANATOMY OF IXVERTEBRATED ANIMALS.

miles. It is by no means certain, however, that any one
species of West India reef-coral is identical with any East
Indian species, and the corals of the central Paciiic differ very
considerably from those of the Indian Ocean.

Different species of corals exhibit great differences as to
the rapidity of their growth, and the depth at which they
flourish best ; and no one must be taken as evidence for anoth-
er in these respects. Certain species of Perforata {Madre-
poridce and Foritidce) appear to be at once the fastest grow-
ers, and those which delight in the shallowest waters. The
AstrceidcB among the Aporosa, and Seriatopora among the
Tahulata^ live at greater depths, and are probably slower of
increase.

Under the peculiar conditions of existence winch have
just been described, it would seem easy enough to compre-
hend, a priori^ the necessary arrangement of coral-reefs. As
the reef-building Actinozoa cannot live at greater depths than »
twenty fathoms, or thereabouts, it is clear that no reef can
be originally formed at a greater depth below the surface, and
such a depih usually implies no very great distance from land.
Furthermore, we should expect that the growth of the coral
would fill up all the space between the shore and this farthest
limit of its growth ; so that the shores of coral seas would
be fringed by a sort of flat terrace of coral, covered, at most,
by a very few feet of water ; that this terrace would extend
out until the shelving land upon which it had grown descended
to a depth of some twenty fathoms ; and that then it would
suddenly end in a steep wall, the summit and upper parts of
which would be crowned with overhann^inof ledges of livina:
coral, while its base would be hidden by a talus of dead
fragments, torn off and accumulated by the waves. Such a
"fringing reef" as this, in fact, surrounds the island of
Mauritius. The beach here does not gradually shelve down
into the depths of the sea, but passes into a flat, irregular
bank, covered by a few feet of water, and terminating at a
greater or less distance from the shore in a ridge, over which
the sea constantly breaks, and the seaward face of which
slopes at once sheer down into fifteen or twenty fathoms of
water.

The structure of a frino-ingr reef varies at different dis-
tances from the land, and at different depths in its seaward
face. The edge beaten by the surf is composed of living
masses of Porites, and of the coral-like plant, the Nulllpore ;
deeper than this is a zone of Aporosa (Astrceidce), and of

FRINGING REEFS.— ATOLLS. 151

Millepores {Seriato2^ora) ; while, deeper still, all living coral
ceases ; the lead bringing up either dead branches, or show-
ing the existence of a flat, gently-sloping floor, the true sea-
bottom, covered with fine coral sand and mud. Passing from
the edge of the reef landward, the Poritldm cease, and are
replaced by a ridge of agglomerated dead branches and sand,
coated with Nullipore ; the floor of th3 shallow basin, or
" lagoon," inclosed between the reef and the land, is formed
by a conglomerate, composed of fragments of coral cemented
by mud ; and, on this, Meandrmce and FungicB rest and
flourish, exhibiting the most gaudy coloration, and sometimes
attaining a great size. During storms, masses of coral are
hurled on to the floor of the lagoon, and there gradually in-
crease the accumulation of rocky conglomerate ; but in no
other way can a frin-jino: reef, which has once attained its
limit in depth, increase in size, unless, indeed, the talus ac-
cumulatins: at the foot of its outer wall should ever rise sufti-
ciently high to aff'ord a footing for the corals within their pre-
scribed limits of depth.

Such is the structure of a fringing reef ; but the great
majority of reefs in the Pacific are very different in their
character. Along the northeastern coasts of New Holland,
for instance, a vast aggregation of reefs lies at a distance
from the shore which varies from a hundred to ten miles ;
forming a mighty wall or barrier against the waves of the
Pacific. At a few hundred yards outside this " barrier reef "
no bottom can be obtained with a sounding-line of a thousand
fathoms ; between the reef and the mainland, on the con-
trary, the sea is hardly ever more than thirty fathoms deep.
Many of the islands of the Pacific, again, are encircled with
reefs corresponding exactly in their character with the barrier
reef ; separated, that is, by a relatively shallow channel from
the land, but facing the sea with an almost perpendicular wall
which rises from a very great depth.

Finally, in many cases, especially among the single reefs,
which taken together constitute the great Australian barrier,
there is no trace of any central island ; but a circular reef,
usually having an opening on its leeward side, stands out in
the midst of the sea. These reefs, apparently unconnected
with other land, are what are called " Atolls."

How have these barrier reefs, encircling reefs, and atolls,
been formed ? It is certain that the fabricators of these reefs
cannot live at a greater depth than in the fringing reefs.
How can they have grown up, then, from a thousand fathoms

152 THE ANATOMY OF INYERTEBRATED ANIMALS.

or more ? Why do they take so generally tbe circular form ?
What is the connection, finally, between fringing reefs and
atolls ? The only thoroughly satisfactory answer to these
questions has been given by Mr. Darwin, from whose beauti-
ful work on " Coral Reefs " I have borrowed most of the fore-
going details. Consider for a moment what would be the
effect of a slow and gradual submergence of the island of
Mauritius — a submergence, perhaps, of a few feet in a century
(at any rate, not greater than the rate of upward growth of
coral), continued for age after age. As the edge of the fring-
ing reef sank, new coral would grow up from it to the sur-
face ; and, as the most active and important of the reef- build-
ers flourish best in the very surf of the breakers, so the margin
of the reef would grow faster than its inner portion, and the
discrepancy would increase as the latter, sinking deeper and
deeper, became farther removed from the region of active
growth. Nevertheless, the sea-bottom within the reef w^ould
constantly tend to be raised by the accumulation of frag-
ments, and by the deposit of fine mud, in its sheltered and
comparatively calm waters. On the other hand, on the sea-
ward face of the reef, no possible extension could take place
by direct growth; and that by accumulation must be exceed-
ingly slow, the incessant wash of tides, waves, and currents,
tending incessantly to spread any talus over a wider and
wider area.

Thus, then, the edge of the reef unceasingly compensates
itself for the depression which it undergoes, while, inside the
reef, only a partial compensation takes place, and, outside,
hardly any at all. Continue the sinking process until its
highest peak was but a few hundred feet above the surface,
and all that would be left of Mauritius would be an island
surrounded by an encircling reef ; carry on the depression
further still, and a circular reef, or atoll, alone would remain.
But the region of the coral-reefs is, for the most part, that of
constant winds. During the whole process of growth of the
reef, therefore, one of its sides — that to windward — has been
exposed to more surf than that to leeward. Not only will
the greater quantity of debris, therefore, have been heaped
up by storms upon the windward side, but the coral-builders
themselves will here have been better fed, better aerated, and
consequently more active. Hence it is that, other things
being alike, there is a probability that the leeward side of
the reef will grow more slowly, and repair any damages less
easily, than the windward side ; and hence, again, as a result,

ANCIENT REEFS. 153

the known fact that the practicable channels of entrance into
encircling reefs or atolls are usually to leeward.

The winds and waves are singularly aided in grinding
down the corals into mud and fragments by the Scari and
Holothurim which haunt the reefs ; the former browsing
upon the living polyps, with their hard and parrot-like jaws,
and passing a fine calcareous mud in their excrements ; the
latter, more probably, swallowing only the smaller fragments
and mud, and, having extracted from them such nourishment
as they may contain, casting out a similar product. It is
curious to reflect upon the similarity of action of these worm-
like Holothurlw upon the sea-meadows of coral, to that
which the Earthworms, as Darwin has shown, exert upon our
land-meadows !

In the Palaeozoic period reefs like those which have just
been described appear to have abounded in our own latitudes ;
and there is the most striking superficial resemblance be-
tween the ancient beds of calcareous rock which record their
existence, and the masses of coral limestone, hard enough to
clink with a hammer, which are now being formed in the
Pacific, by the processes of accumulation of coral mud and
fragments, and their consolidation by percolating water.
Closer examination, however, shows an important difi'erence
in the nature of the corals which compose the two reefs. The
modern limestones are made up of Perforata, Millepores,
and Aporos'i. The ancient ones contain Millepores, but usu-
ally neither Perforata nor Aporosa — both these groups being
replaced by the Hiigosa, none of whose members (with some
doubtful exceptions) have survived the Palaeozoic period.
On the other hand, Palmocyclus and Pleurodictyon are the
only genera belonging to the Aporosa or Perforata, which
have yet been discovered in strata of greater than mesozoic
age.

The Ctexophora.' — These are freely-swimming marine
animals, which never give rise by gemmation to compound
organisms, and are always of a soft and gelatinous consist-
ence, their chief bulk being made up by the greatly -devel-
oped mesoderm. Many are oval or rounded {Beroe, Pleuro-

1 Allman (" Monograpli of the Tubularian HydrolcU," 1871, pagre 3) consid-
ers that the CUnophora are more properly arranfjed amonsj the Hydrozoa. I
confess, however, that I see no reason to depart irom the conclusion to which
I was led by the study of the structure of Pleurobrachia^ many years ago, that
the Ctenophora are peculiarly modified Actinozoa.

154 THE ANATOMY OF INYERTEBRATED ANIMALS.

hrachia^ Fig. 31), while in others the body is produced into
lobes {Callianh'a), or may even be ribbon-shaped (Cestwn) ;
but, whatever their form, they present a distinct bilateral
symmetry, similar parts being disposed upon opposite sides
of a median plane, which is traversed by the axis of the
body. The mouth is situated at one end of this axis, which
may be termed the oral pole. At the opposite, or aboral
pole, there is no median aperture, but usually, if not inva-
riably, a pair of apertures a short distance apart. The faces
of the halves of the body present four longitudinal bands of
long and strong cilia, disposed in transverse rows, like so
many paddles ; these constitute the chief organs of locomo-
tion. Each half is also often provided with a long retractile
tentacle ; and lobed processes of the body, or non-retractile
tentacula, may be developed on its oral face. The mouth
leads into a wide, but flattened, gastric sac, the aboral end of
which is perforated, and leads into a chamber termed the
infundibuluin. From the aboral face of this, a canal which
bifurcates, or two canals, lead to the aboral apertures. On
opposite sides of the infundibulum a canal is given off toward
the middle of each half of the body, which sooner or later
divides into two, and these two again subdivide, so that four
canals, which diverge and radiate toward the inner faces of
the rows of paddles, are eventually formed. Having reached
the surface, each radiating canal enters a longitudinal canal,
which underlies the row of paddles, and may give off branches,
or unite with the other longitudinal canals in a circular canal
at the aboral end of the body. In addition, two other canals,
which run parallel with each flat face of the gastric sac, open
into the infundibulum. And, w^hen retractile tentacula are
present, their cavities also communicate with the same cham-
ber.

The entire system of canals is in free communication with
the gastric cavity, and corresponds Avith the enterocoele of
an JLctinia. Indeed, an Actinia with only eight mesenter-
ies, and these exceedingly thick, whereby the intermesenteric
chambers would be reduced to canals ; with two aboral pores
instead of the one pore, which exists in Cereanthus j and
with eight bands of cilia corresponding with the reduced
intermesenteric chambers, would have all the essential pecu-
liarities of a Ctenophoran.

The question whether the Ctenopliora possess a nervous
system or not is still under debate. Between the aboral aper-
tures there is a rounded cellular body, on which there is

THE CTENOPHORA.

155

seated, in many cases, a sac containing solid particles, like
one of the lithocysts of the medusiform Hydrozoa, I see no
reason to doubt that the rounded body is a ganglion and the
sac a rudimentary auditory organ. Bands which radiate
from the ganglion to the rows of paddles may be regarded
as nerves ; though they may contain other than nervous
structures.^

The ova and spermatozoa are developed in the lateral
walls of the longitudinal canals, which correspond with the
faces of the mesenteries in the CoralUgena^ and the sexes
are usually united in the same individual.

Fig. 31.— Dia;r'am of Ple>jrdbrachia.—a. month ; 6, stomach ; r, inftindlbulam ; d,
hoilzontal canal; «, one of its branches dividini; ajjain at / into two branches
which open Into the longitudinal canals, g g, parallel with which the ciliated
area runs ; h, sac of the tentacle, 2, with one of its branches, Jc ; I, canal run-
n ins; by the side of the stomach; m, tentaculigeroas canal; n n, canals opening
at tbe'aboral apertures, o, on each side of j3, the ganglion and lithocyst.

1 Grant originally described a nervous ganglionated ring, whence longitu-
dinal cords proceeded in Cydippe {PleurobracJiia)^ but Ms observation has not
been veritied by subsequent investigators. According to Milne-Edwards, fol-
lowed by others (among whom I must include myself), the nervous system
consists of a ganglion, situated at the aboral pole of the body, "whence nerves
radiate, the most conspicuous of which are eight cords which run down the
corresponding series of paddles ; and a sensor}' organ, having the characters
of an otolithic sac, is seated upon the ganglion. Agassiz and Kolliker, on the
other hand, have denied that the appearances described (though they really
exist) are justly interpreted. And again, though the body, described as an
otolithic sac, undoubtedly exists in the position indicated in all or most of the
Cterwphora^ the question has been raised whether it is an auditory or visual
organ.

These problems have been recently reinvestigated with great care, and by
the aid of the refined methods of modern histology, by Dr. Eimer, whose de-
scription of the nervous system has already been quoted {supra, p. 63).

156 THE ANATOMY OF INYERTEBRATED ANIMALS.

The development of the Ctenophora has recently been
thoroughly investigated by Kowalewsky and by A. Agassiz
(" Memoirs of the American Academy of Arts and Sciences,"
1874).

The laid egg is contained in a spacious capsule, and con-
sists of an external thin layer of protoplasm, which, in some
cases, is contractile, investing an inner vesicular substance.
After fecundation, the vitellus thus constituted divides into
two, four, and finally eight masses ; on one face of each of
these the protoplasmic layer accumulates, and is divided oif
as a blastomere of much smaller size than that from which it
arises. By repeated division, each of these gives rise to still
smaller blastomeres, which become distinctly nucleated wdien
the}'' have reached the number of thirty-two, and form a
layer of cells, which gradually spreads round the large blas-
tomeres, and invests them in a complete blastodermic sac.
At the pole of this sac, on the face opposite to that on which
these blastoderm-cells begin to make their appearance, an
ingrowth or involution of the blastoderm takes place, which,
extending: throiio'h the middle of the laro-e velk-masses tow-
ard the opposite pole, gives rise to the alimentary canal.
This, at first, ends by a rounded blind termination ; but from
it, at} a later period, prolongations are given off which be-
come the canals of the enterocoele.

At the opposite pole, in the centre of the region corre-
spondino: with that in which the cells of the blastoderm first
make their appearance, the nervous ganglion is developed by
metamorphosis of some of these cells.

The invaginated portion of the blastoderm, which gives
rise to the alimentary canal, appears to answer to the hypo-
blast, while the rest corresponds w^th the epiblast. The
large blastomeres which become inclosed between the epi-
blast and hypoblast in the manner described seem to serve
the purpose of a food-yelk ; and the space which they origi-
nally occupied is eventually filled by a gelatinous connective
tissue, which possibly derives its origin from wandering cells
of the epiblast.

In those Ctenophora the bodies of which depart widely
from the globular form in the adult state, the young undergo
a sort of metamorphosis after they leave the Qgg, and have
acquired all the essential characters of the group to which
they belong.

As might be expected from their extreme softness and
perishable nature, no fossil Ctenophora are known.

CHAPTER IV.

THE TURBELLAEIA, THE ROTIFERA, THE TREMATODA, AND THE

CESTOIDEA.

The TtTRBELLAKiA. — The animals which constitute this
group inhabit fresh and salt water and damp localities on
land. The smallest are not larger than some of the Infusoria,
which they approach very closely in appearance, while the
largest may attain a length of many feet. Some are broad,
flattened, and discoidal, while others are extremely elongated
and relatively narrow. None are divided into distinct seg-
ments, except the genus Alaurina, in which there are four ;
and the ectoderm, which constitutes the outer surface of the
body, is everywhere beset wi1?h vibratile cilia. Rod-like
bodies, similiar to those met with in some Infusoria and in
many Annelida, are often imbedded in its substance, and in
some genera (e. g., Microstomum, Thysanozoon) true thread-
cells occur. Stiff setae project from the ectoderm in some
species.

The aperture of the mouth is sometimes situated at the
anterior end of the body, sometimes in the middle, or toward
the posterior end, of its ventral face. In many, the oral
aperture is surrounded by a flexible muscular lip, which some-
times takes on the form of a protrusible proboscis.

A definite digestive cavity can hardly be said to exist in
the lowest Tarbellaria (e. g., Convoluta) in which the endo-
dermal cells are not arranged in such a manner as to bound a
central alimentary cavity, and the food finds its way through
the interstices of an endodermal parenchyma. In the higher
forms, the alimentary cavity, which may be simple or rami-
fied, provided with an anal aperture or without one, is lined
by the endoderm, between which and the ectoderm is an in-
terspace more or less completely occupied by the connective
and muscular tissues of the mesoderm. Hence there is no
definite perivisceral cavity.

153 THE ANATOMY OF INVERTEBRATED ANIMALS.

The Turhellaria possess vessels of two kinds : 1. Water-
vessels, which open externally by one or more pores, and are
ciliated. When these vessels are present, there are usually
two chief lateral trunks, from which many branches are given
off. It is probable that the ultimate ends of these branches
open into lacunar interspaces between the elements of the
tissues of the mesoderm. 2. JPsend-hcemal vessels, which ap-
pear to form a closed system, usually consisting of one median
dorsal and two lateral trunks, which anastomose anteriorly
and posteriorly. The walls of these vessels are contractile
and not ciliated, and their contents are clear, and may be
colored. These two systems of vessels have been shown by
Schulze to coexist in Tetrastemma, The nervous system con-
sists of two ganglia placed in the anterior end of the body,
from which, in addition to other branches, a longitudinal cord
extends backward on each side of the body. In some cases,
these lateral trunks exhibit ganglionic enlargements, from
which nerves are given off; and they may become approxi-
mated on the ventral side of the body, thereby showing a
tendency to the formation of the double ganglionated chain
characteristic of higher worms. Most possess eyes, and some
have auditory sacs. The Turhellaria are both monoecious
and dioecious, and the reproductive organs vary from the
utmost simplicity of structure to considerable complexity.
In most, the embryo passes by insensible gradations into the
form of the adult, but some undergo a remarkable metamor-
phosis.

The Turhellaria are divisible into two groups. In the one,
the Aprocta, the digestive cavity is caecal, having no anal
aperture ; in the other, the Proctucha, it is provided with an
anal opening. The two groups form parallel series, in each
of which organization advances, from forms w^iich are little
more than gastrulse provided with reproductive organs, to
animals of relatively high organization. In the simplest of
the Aprocta, such as Macrostomum^ the oral opening is
devoid of any protrusible muscular proboscis, and the aliment-
ary sac is a simple straight bag. The male and female gen-
erative organs are united in the same individual, and each
consists of an aggregation of cells; w^hich, in the former case,
gradually enlarge, fill with yelk-granules, and become ova;
while, in the latter, thc}^ are converted into spermatozoa.
The generative cells are contained within a sac, which opens

1 E. Van Beneden, " Eecherclies sur la Composition ct la Signification de
rCEuf," 1S70, p. 64.

THE TURBELLARIA.

159

externally by a median pore on the oral face of the body, the
male aperture being posterior to the female. The margins of
the male aperture are produced into a curved prominence, the
penis.

Those Tarhellaria which resemble 3facrostommn in having
a straight, simple digestive cavity, are termed Ukabdocoela.
They, for the most part, possess a buccal proboscis, which is
capable of being protruded from, or retracted into a chamber

§^^^

s-^:

Fig. S2.—Opisthomum (aftp.r Schulze).— a, central nerrous system ; ramifications of
the water- vessels are seen close to it; b, mouth; c, proboscis; d, testes; e, vasa
deferentia; /, vesicula seminalis ; g, penis ; A, sexual aperture ; i, vatrina ; k, sper-
raatheca ; L germarium; m, vitellarium ; n, uterus with two ova inclosed within^
their hard shells.

formed by the walls of the circum-oral region of the body
(Fig. 32, c).

In some (e. g., Prostomum) the anterior end of the body is

160 THi: ANATOMY OF INVERTEBRATED ANIMALS.

provided with a second hollow muscular proboscidiform organ,
which may be termed the frontal proboscis.

In all the higher rhabdocoelous Turhellaria^ the female
generative apparatus becomes complicated by the presence
of a special gland, the vitellariuin (Fig. 32, m), in which an
accessory vitelline substance is formed. There is a single or
double germariuin (Fig. 32, I), having nearly the same struct-
ure as the ovary of Macrostonium^ and the ova are formed
in it in the same w^ay. When detached, how^ever, they con-
tain no vitelline granules ; but the two vitellaria, which are
long and simple or branched tubes, open into the oviduct ;
and the vitelline matter which they secrete envelops the
proper ovum, and becomes more or less fused w^ith it, as it
passes into the uterine continuation of the oviduct connected
with the outer, or vaginal, end of the uterus. There is usually
a spermatheca, or receptacle for the seminal fluid (Fig. 32, k),
and the eggs, after impregnation, are inclosed within a hard
shell (Fig. 32, oi). The testes and vasa deferentia (Fig. 32,
d, e) generally have the form of two long tubes. The penis
is often eversible and covered with spines (Fig. 32, g).

In some genera a difference is observed between the eggs
produced in summer, which have a soft vitelline membrane,
and those produced later. These so-called winter ova have
hard shells.

The water-vascular system consists of lateral trunks,
which open hy a terminal pore, or by many pores, and give
off numerous ramifications. They are not contractile, but
their inner surface is ciliated.

Many of the Ilhabdocoela multiply by transverse fission ;
and, in the genus Catenula, the incompletely separated ani-
mals produced in this way swim about in long chains.

The vitellus of the impregnated ovum undergoes complete
yelk-divison, and the embryos pass directly into the form of
the parent ; but the precise nature of the steps of the devel-
opmental process requires further investigation. How^ever,
there seems little reason to doubt that the ectoderm and en-
doderm are formed by delamination.

In the remaining Ajyrocta^ termed Dendroccela^ the diges-
tive cavity gives off many csecal, frequently branched, pro-
cesses into the mesoderm, one of which is always median and
anterior (Fig. 33) ; and the mouth is always provided with a
proboscis. Some {Procotyla) have a frontal proboscis, and
others {JBdellura) a posterior sucker. The animals commonly

THE DENDKOCCELA.

161

known as Planarice belong to this division. Some are ma-
rine, some fresh-water, and some terrestrial.

In the fresh-water forms, the female reproductive appa-
ratus has a distinct vitellarium, as in the higher Hhahdocoela,
and there is only one common genital aperture. But, in the
marine Planar ioe (Fig. 33), there is no vitellarium ; the ova-
ries and testes are numerous, and scattered through the meso-
derm, being connected with the exterior by ramifications of
the oviducts and of the vasa deferentia. A ramified gland,
which secretes a viscid albumen or envelope for the eggs,

Fig. Zi.—Polycdis (Leptoplana) IcBvigata (after Quatrcfast^p)-— o, mouth; ft, buccal
cavity; c, (Esophageal orifice ; d, stomach ; e, ramifications of gastric caeca; f,
ganglia ; g, testes ; h, vesiculiB seminales ; i, male genital canal and peuis ; k, ovi-
dacts ; /, spermathecal dilatation at their junction ; m, vulva.

opens into the vagina, and the female is distinct from the
male aperture. Planaria dioica is unisexual.

In some of the Planarice there are distinct water-vascular

162 THE ANATOMY OF INYERTEBRATED ANIMALS.

canals of the ordinary kind ; but in the land Planarians ^ two
nearly simple canals, occupied by a spongy tissue, and the
connection of which with the exterior has not been observed,
occupy the place of the water-vessels.

The fresh-water Planarke, like the Rkahdocoela^ undergo
no metamorphosis in the course of their development ; and
the like is true of some of the marine Dendroccela. Kefer-
stein ' has carefully worked out the development of Lepto-
plana [Polycells). The vitellus undergoes division first into
two and then into four equal blastomeres ; next, from one
surface of these four blastomeres, four small segments are, as
it were, pinched off. These divide rapidly, and form a blas-
toderm, which grows over the more slowly dividing large seg-
ments, and eventually incloses them. So far, the process is
very similar to that which has been described in the Cteno-
phora. But though Keferstem describes and figures the
various stages by which the globular ciliated embryo attains
the form of the adult, neither his description nor the figures
enable one to say whether the alimentary cavity arises by de-
lamination or by invagination, nor to trace the mode of origi-
nation of the buccal proboscisough, th this organ is one of
the first to make its appearance, and its aperture becomes the
future mouth.

In some of the marine Planarim, however, the embryo,
wlien it leaves the Qg^, differs very widely from the adult.
Johannes Miiller described such a larva, in which the body is
provided with eight lobes or processes, one ventral and median
in front of the mouth, three lateral, and one dorso-median.
The edges of these processes are fringed by a continuous
series of cilia, which pass from one process on to another, so
as to form a complete circlet round the body. The successive
working of the cilia forming this lobed transverse girdle of the
body produces the appearance of a rotating wheel, as in the
RotAfera. The eyes are situated on the aboral face of the
embryo, in front of the ciliated circlet, while the mouth opens
immediately behind it. As development proceeds, tlie lobes
disappear, and the body takes on the ordinary Planarian
character.

As will be seen, some of the Proctucha have larvae simi-
larly provided with a pra3-oral ciliated zone ; and larvae of

* Moseley, " On the Anatomy and Histology of the Land Planarians of Cey-
lon." (" Philosophical Transactions," 1873.)

"^ "Beitrilge zur Anatomie und Entwickelungsgeschichte ciniger See-Plana-
rien," 1868.

THE PROCTUCnA.

1G3

the same fundamental type abound among tlie polycbastous
Annelida, the Uchinodermata, and the Mollusca,

mm'

Fig. 34.—^, young Tetrastemma.—aa, central ^nglia of the nervous system; 55, cil-
iated fossae ; c, aperture through which the proboscis is protrude!; d, anterior
portion of proboscis ; e, posterior muscular part, fixed to ttie parietes at/; g, in-
testine; h, anal aperture: i, water- vessels; k, rhythmically contracting vessels.
(After Schulze.) B, anterior extremity of the everted proboscis of letrastemmay
exhibiting the principal and the reserve stilets. (After Schulze.)

The lowest Proctucha^ such as 3Iicrostomiim^ have no
frontal proboscis (whence they are termed Arhynchid)^ and
they differ very little from the lowest Rhahdocoela., save in the
fact that there is an anus, and that the sexes are distinct.
But all the other Proctucha [Rhynchoccela., or Nemerteans)
are provided with a frontal proboscis, which sometimes oc-
cupies the greater part of the length of the body (Fig. 34).
It has special retractor muscles, and its internal surface is
either merely papillose, or may possess a peculiar armature,

164 THE ANATOMY OF INVERTEBRATED ANIMALS.

consisting of a sharp cLitinous style (Fig. 34, J3). There is
no buccal proboscis, but the mouth leads into a long, straight
intestine, with short, lateral, caecal dilatations.^

The Proctucha usually present only the pseud-htemal ves-
sels, though, as has been mentioned above, Schulze found water-
vessels coexisting with them in Tetrastemma (Fig. 34).

The nervous system of the Proctucha is like that of the
Aprocta ; but, in correspondence with the often extreme elon-
gation of the body, the backwardly- prolonged cords are very
stout. Moreover, the ganglia are united by an additional
commissure over the proboscis, which thus traverses a ner-
vous ring. In some, the lateral cords approach one another
on the ventral aspect of the body, and ganglionic enlarge-
ments appear where the nerves are given otr, thus present-
ing an approximation to the double ganglionated chain of
higher forms.

In addition to eyes, almost all the Proctucha possess two
ciliated foss?e, one on each side of the head (Fig. 34, bh)^
which receive nerves from the ganglia. Occasionally two
otolithic vesicles are attached to the cerebral ganglia.

The Proctucha are almost alw^ays dioecious. The simple
reproductive glands are lodged in the intervals between the
saccular dilatations of the intestine, and the ova and sper-
matozoa usually make their way out by the dehiscence of
the integument. In some, however, the embryos are devel-
oped in the ovarian sacs, or in the cavity of the body. In
most of the Proctucha, the egg, after passing through the
morula stage, acquires an alimentary cavity, apparently by
delamination, and passes, without other metamorphosis than
the shedding of a ciliated outer investment, into the form of
the adult.

Prof. A. Agassiz'' has described a free-swimming larva,
the broad anterior end of the body of which is surrounded
by a zone of cilia, immediately behind which the mouth opens ;
while around the anal aperture, at the narrow posterior end,
is a second circlet of cilia. This larva exactly resembles
those forms of polj'chffitous Annelidan larvre which are called
Telotrocha. As in these Annelids, the region of the body
which lies between the two ciliated rings elongates and be-
comes segmented, while a pair of eyes and two short tenta-

1 For the orsranization of the Rhynchocoele Tvrhdlaria, or Nemerteans, see
Dr. C. Mcintosh's elaborate monof?raph lately published by the Ray Society.

3 " On the Young Stages of a few Annelids." (Annals of the Lyceum of
New York, 1864.)

THE PROCTUCHA.

165

cles are developed on the head in front of the prae-oral ciliated
band. But, as development advances, the segmentation be-
comes obliterated, the ciliated bands and the feelers vanish,
and the worm assumes the characters of a Nemertean/

Fig. 35.

Fig. 37.

Fig. 36.

Fig. ZS-Zl.—Pllidhim gyrans (after Leuckart and Pagenstecher).

85. YoTing PUidium : a, alimentary canal; 6, radimeut of the Nemertean.

36. Pilidium with a more advanced Nemertean.

37. Newly -freed Nemertean.

In species of the genus Lineus^ the ciliated embr^'O which
leaves the egg is speedily converted into a body like a helmet
with ear-lappets, and having a tuft of cilia in place of a plume

1 It is very probable, however, that this larva belongs to the genus Poh/qor-
dius, which appears to be an annectent form between the Turbellaria and other
groups. See Schneider, " Ueber Bau und Entwickelung von Polygordius."
C' Archiv fur Anatomie und Physiologic," 1808.)

166 THE ANATOMY OF INVERTEBRATED AMMALS.

(Fig. 35). The lappets are fringed with long cilia, and be-
tween them, where the head would fit into a helmet, is the
aperture of a mouth, which leads into a csecal pouch-like ali-
mentary cavity. This larv^a w'as named by MuUer, w^ho dis-
covered it, Pllidmin gyra7is. On each side of the ventral
face of the Pllidimn^ two involutions of the integument take
place. Aggregations of cells in relation with these, and
probably forming part of the mesoblast, appear, eventually in-
close the alimentary canal of the Pllidium, and give rise to
an elongated vermiform body, in which the characteristic feat-
ures of a Nemertean soon become discernible (Fig, 36). The
worm thus developed becomes detached (Fig. 37) and falls to
the bottom, carrying with it the alimentary canal of the Pi-
lidliDn, and leaving the ciliated integument to perish.

In this remarkable process of development the formation
of the Nemertean body may be compared, on the one hand,
to that of the segmented mesoblast in Aniielida and Arthro-
poda^ and, on the other, to that of an Echinoderm (especially
Echinus), within its larva.

Tpie Rotifera. — The " wheel-animalcules," as they were
termed by the older observers, on account of the appearance
of rotation produced, as in many Annelid larvae, by the work-
ing of the vibratile cilia with which the oral end of the body
is provided, were formerly included among the Infusoria,
However, they are true Metazoa^ as their vitellus undergoes
division into blastomeres, and the tissues of the body are pro-
duced by the metamorphosis of the cells into which the blas-
tomeres are converted. They are free or adherent, but never
absolutely fixed animals, and they do not multiply by gem-
mation or fission. The oral end of the body is usually broader
than the opposite extremity, and presents the form of a disk,
sometimes produced into tentacle-like prolongations (Fig. 39),
The edges of this trochcd disk are fringed with long cilia, but
the general surface of the body, instead of being ciliated,
as in the Turhellaria^ is formed by a dense, generally chiti-
nous, cuticular layer, which is sometimes converted into a kind
of shell and variously sculptured. Transverse constrictions,
which are slight in the anterior part of the body, but may
become more marked toward its posterior end, give rise to an
imperfect segmentation. The segments do not appear to ex-
ceed six, and the divisions are less marked in the tubicolous
than in the free Motifera. The mouth is a funnel-shaped
cavity, situated in the middle, or on one side, of the trochal

THE ROTIFERA. 167

disk. The walls of this cavitv are abundantly ciliated, and
at the bottom is a muscular pharynx, or mastax^ provided
with a peculiar armature. Sometimes, as in Stephanoceros, a
large crop-like cavity lies between the mouth and the mastax,
and the aperture of communication between this crop and
the mouth is guarded by a valve formed by two broad mem-
branous folds which project into the cavity of the crop. The
armature of the mastax generally consists of four pieces — two
lateral, the mallei^ and two central, constituting the mens.
The contraction of the muscular masses, to which the mallei
are attached, causes the free ends of the latter to work back-
ward and forward upon the incus, and crush the prey which
is taken into the mouth.*

A short oesophagus, provided with cilia or vibratile mem-
branes, leads into a digestive cavity bounded by the endo-
derm. The anterior or gastric part of this cavity is usually
dilated, and gives ofif a large caecum on each side. The pos-
terior, narrower, intestinal part usually opens externally by a
cloacal chamber ; but, in some Rotifers (e. g., Kotommatd)^
the alimentary cavity is a blind sac, devoid of intestine or
anus ; and in the males, so far as they are known, the whole
alimentary canal is aborted and represented by a solid cord.

A spacious perivisceral cavity occupies the interval be-
tween the walls of the alimentary canal and the parietes of
the body. The latter contains circular and longitudinal mus-
cular fibres, which may be smooth or striated.

Opening into the cloaca there is usually a large thin-walled
vesicle with rhythmically contractile walls ; and, in connection
with this, are two delicate water-vessels, which pass forward,
often giving off short lateral branches, and eventually break
up into numerous ramifications in the trochal disk. The
branches are open at the ends, wherel^y the cavities of the
water-vessels are in communication with the perivisceral cav-
ity on the one side, and with the surrounding water on the
other. Here and there, in the course of the main trunks and
at the ends of the branches, long cilia, which, by their con-
stant undulation, give rise to a flickering motion, are situated.

The nervous system is represented by a relatively large
single ganglion placed on one side of the body, near the tro-
chal disk. One or more eye-spots are sometimes seated on
the ganglion, and there are other organs which appear to be

1 i've^, for the various forms of this apparatus, Gosse, "On the Structure,
Functions, and Tlomolosrues of the Manducating Apparatus in the Botifera,^'*
(rhilosophical Transactions, 1855.)

168

THE ANATOMY OF IXVERTEBRATED ANIMALS.

sensory. Such are the ciliated pit and the spur-like process
(calcar) or processes, provided at the end with a tuft of setae,
which occur in many Rotifers, and are more or less closely
connected with the ganglion. In some there is a sac filled
with calcareous matter (otocyst ?) attached to the ganglion.

Fig. 2S.—n ydalina senta (after Cohn).— ^, female : a, anas ; b, contractile vesicle ;
c, water-vessels ; g. ovary ; f. <,'anii:lion. B, male : a, peuis ; 6, coutractile vesicle ;
c, testis ; /, ganglion ; g, setigerous pit.

The ovarium and the testis are simple glands which open
into the cloaca, and are always placed in distinct individuals.
All the males at present known differ from the females in be-
ing much smaller, and in their digestive canal being arrested
in its development. The males copulate with the females,
and the eggs are sometimes attached to, and carried about by,
the latter — e.g., Brachionus.

In some Rotifers, the eggs are distinguishable, as in cer-
tain Turhellaria, into summer and winter ova. The latter
are inclosed in a peculiar shell. In Lacimilaria, it appeared
to me that the winter ova were segregated portions of the
ovarium, and that they were probably developed without im-
pregnation. Cohn, on the contrary, has given reasons for be-

THE ROTIFERA. 169

lieving that the summer ova are occasionally, if not alwaj-s,
developed without fecundation, and that it is the winter ova
which are fecundated.

The egg undergoes complete yelk-division, and the em-
brj'O gradually passes into the adult form. The blastomeres
are soon of unequal sizes, and the smaller, as an epiblast, in-
vest the larger, which form the hypoblast.

Salensky's ^ recent observations on jB}'achionus urceolaris
show that a depression arises on one face of the epiblast and
that the antero-lateral parts of this depression are converted
into the trochal disk, while its median posterior part grows
out into the "foot; " and he points out the resemblance of
the embryo in its early stages to that of some Gasteropods.

An involution of the epiblast at the bottom of the depres-
sion gives rise not only to the oral chamber, but also to the
mastax ; eventually communicating with the gastro-intestinal
division, w^hich is developed out of the hypoblast. The gan-
glion is a product of the epiblast.

Some of the modifications of the general structure thus
described, which occur in the different groups of the Hotife-
ra, are of considerable interest.

Thus, in the tubicolous forms, the body is elongated and
terminated posteriorly by a discoidal surface of adhesion.
The animals (of which a number are often associated together),
fixed by this disk, inclose themselves in cases, the foundation
of which is a gelatinous secretion. The intestine is bent
upon itself {Lacinularia, Fig. 39, II.), and opens upon the
face of the body opposite to that upon which the ganglion is
placed. The peduncle of attachment is therefore a process of
the neural face of the body. In these Jiotifera the trochal
disk is sometimes produced into long ciliated tentacula,
which surround the mouth symmetrically i^Stephanoceros^
Fig. 39, v.), or its edges may be provided with two circlets of
cilia, one in front of, and the other behind, the oral aperture ;
and it may be bilobed or horseshoe-shaped, as in Melicerta^
and Lacinularia ' (Fig. 39, I., II.).

In the free Rotifers, the body may be rounded, sac-like,
and devoid of appendages, as in the genus Asplanchiia, which
has neither anus nor intestine. In Albertia and Lindia^ on
the other hand, the body is elongated and vermiform. Most
of the free JRotifera (Fig. 38) are provided with a segmented

1 ZdUchHft fTir wiss. Zoologie^ 18T2.

2 Huxley, Lacinutaria socialis. (Transactions of the Microscopical Society,

8

170 THE ANATOMY OF INYERTEBRATED ANIMALS.

and sometimes telescopically-jointed " foot," usually termi-
nated by two styles, which can be approximated or divari-

FiG. 39.— Diagrams showing the arrangement of the cilia of the trochal diek in the
jRotifera. I. Larval Lacinularia. If. Adult Lacinularia. III. Philodina. IV.
Brachionus. V. StephanocerOH. M, mouth ; G, gan^ilion ; J., anus.

cated like pincers, and serve to anchor the body. This foot
is a median process of that face of the body which is opposite
to that on which the ganglion is placed, so that it is not the
homologue of the peduncle of the tubicolous forms.

Polyarthra and Iriarthra possess long, symmetrically ar-
ranged, movably articulated setas ; and Pedalion has median
appendages proceeding from both the neural and the opposite
faces of the body, as well as lateral appendages.

In most of the free Rotifers the trochal disk is large ; it
may be bilobed or folded upon itself (Fig. 39, III.), or its sur-
face may give rise to ciliated processes (Fig. 39, IV.). In
Alhertia and Notomynata tardigrada^ however, the trochal
disk is reduced to a small ciliated lip around the oral aper-
ture ; and there is no trochal disk in Apsilus, Lindia, Ta-
phrocampa, and Balatro. Some few Rotifera are parasitic.
Thus Alhertia is an entoparasite, and Bcdatro an ectopara-
site, upon oligochaetous Annelids.

Under the name of Gasterotricha^ Metschnikoff and Cla-
parede * include the curious aquatic genera ChcBtonotus, Ich-
thydiwin^ Choetura^ Cephalidium^ Dasydltis, Turhanella^ and
Ilernidasys^ the last of which alone is marine. These animals
have been united with the Motifera, but they differ from them
in the absence of a mastax and in the disposition of the cilia,
which are restricted to the ventral surface of the body. It

* Olaparede and Metschnilcoff, "Beitriige zur Kenntniss der Entwickelungs-
geschichte der Chaetopoden," 1868.

THE TREMATODA. 171

appears probable that tliey form an annectent group between
the Rotifera and the Turbellaria, which last approach the Eo-
tlfera by such forms as Dinophilus,

The free Rotifers present marked resemblances to the
telotrochous larvae of Annelids. The young Lacinularia, for
example, has a circular prae-oral disk provided with two eye-
spots and a second circle of cilia behind the mouth, and is
wonderfully like an Annelid larva (Fig. 39, I.). The append-
ages of Triarthra and Polyarthra may be compared to the
lateral bundles of long setae of the larvae of Spio and Nerine,
and the pharyngeal armature is essentially Annelidan. On
the other hand, in the sessile tubicolous Rotifera, the trcchal
disk assumes the characters of the lophophore in the Polyzoa,
and of the tentacular circlet of the Gephyrean Phoronis,
Many years ago I drew attention to the points of resem-
blance between the Rotifera and the larvae of Echinoderms
C' Oa Zctcmularia socialis,'" I. c). Of any such close and
direct relations with the Crustacea, I see no evidence ; but
Pedalion,^ with its jointed setose appendages and curious
likeness to some J^aupUus conditions of the lower Crustacea,
suggests that connecting links in this direction may be found.*
In fact, the Rotifera, as low 3Ietazoa with nascent segmenta-
tion, naturally present resemblances to all those groups which,
in their simpler forms, converge toward the lower Metazoa.

The TREiiATODA. — These are all parasitic, either upon the
exterior (ectoparasites) or in the internal organs (endopara-
sites) of other animals. Many are microscopic, and none
attain a length of more than an inch or two. Most have a
broad and flattened form, one face being ventral and tlie
other dorsal, and the hody is never segmented.

In the adult, the ectoderm is not ciliated, but its outer-
most layer is a chitinous cuticula. In most Trematoda, one
or more suckers are developed upon the ventral surface of the
body, behind the mouth. These are sometimes armed with
chitinous spines or hooks ; and setae of the same character

1 Hudson, " On a New Rotifer." {Montlily Microscopical Journal^ 1871.)
3 The singular marine genus EcTiinoderes (Dujardin) is perhaps such a link.
These are minute worm like animals, with a rounded head, followed by a num-
ber (ten or eleven) of distinct ses^raents, the last of which is bifurcated. There
are no litnbs, but the head is provided with recurved hooks, and the body seg-
ments with paired setae. The nervous system appears to be represented by a
single ganglion, which lies in the head and presents eye-spots. The develop-
ment or Echiiioderes is unknown. (See Greef, " Arcbiv flir Naturgeschichte,"
1869.)

172 THE ANATOMY OF INVERTEBRATED ANIMALS.

ma}' be developed in other parts of the body, especially in the
region of the head.

The mouth is usually terminal, but is sometimes ventral
and sub-central ; it is ordinarily placed in the centre of a
muscular sucker, rarely [)roboscidiform. The alimentary canal
is never provided with an anus. Sometimes a simple sac, it
is often bifurcated, and occasionally branched, like that of the
dendrocoele Turbellaria, Sometimes (Amphilina, Amphipty-
ches) the alimentary canal is absent ; and, according to Van
Beneden, it becomes aborted in the adult Distoma filicolle.
The interval between the endoderm and the ectoderm is oc-
cupied by a cellular or reticulated mesoderm, in which abun-
dant muscular fibres are developed. The peripheral muscular
fibres form an external circular and an internal longitudinal
layer.

The water-vascular system is well developed, and may
consist of — (1) a contractile sac, which opens externally and
communicates with (2) longitudinal vessels with contractile
non-cihated walls, from which proceed (3) non-contractile and
ciliated branches which ramify through the bodj'', and the
ultimate ramifications of which probably end by open mouths,
as in the Rotifera.

There is no pseud-hcemal system. The nervous S3"stem has
not been discovered in all ; but, when it exists, it has the
same arrangement as in the aproctous Turbellaria. Eye-
spots have been observed, but no other sense-organs. With
rare exceptions, the Trematoda are hermaphrodite, and the
reproductive organs are constructed upon the same type as
in the rhabdocoele Turbellaria , a large vitellarium being al-
ways present. The accessory vitellus is included, in the
form of numerous pellets, along with the primitive ovum, and
is absorbed pari passu with the development of the embryo.

Asjndogaster concliioola (Fig. 40) inhabits the pericardial
cavity of the fresh-water muscle ; it is a very convenient sub-
ject for examination on account of its small size, and the ease
with which it can be rendered sufficiently transparent for the
displa}^ of the arrangement of its internal organs, by the
judicious use of the compressorium. The flat oval bod}'",
rounded posteriorly, is produced in front into a truncated
cone, on the face of which the mouth opens. The ventral
sucker is very large, and its surface is subdivided into rectan-
gular areas. There is no perivisceral cavitj'-, its place being
occupied by a mass of spongy cellular tissue. The oral cavity
leads into an oval, thick-walled, muscular pharyngeal bulb.

ASPIDOGASTER COXCHICOLA.

173

"whence an elongated pyriform sac, which constitutes the rest
of the alimentary canal, is continued. This occupies a great
part of the body, and extends nearly to its posterior end ; but
there is no anus. A contractile vacuole placed at the hinder
extremity of the body opens outward by a small pore (Fig.
41, a), and gives off two lateral contractile non-ciliated canals
(b), which pass to the anterior end of the central sucker and
there end blindly ; but before reaching this termination each
gives off a non-contractile ciliated vessel (Fig. 41, c), which,
on arriving at the pharynx, turns backward and ramifies
through the body. The cilia diminish toward the extremi-
ties of these vessels, the terminations of the corresponding
canals in the Motife^'a being, on the contrary, richly ciliated.
No nerves have as yet been found in Asjndogaster,

Fio. ^(i.—Aspidogaster concMcola.—A, arransement of the alimentary and reproduc-
tive organs ; profile of the animal in outline : a, mouth ; 6. mufecular pharynx ; c,
stomach; rf, germarium ; e, internal vas deferens;/, common vitellarian duct ;
<7, vitellarium ; ;^,one of itp ducts ; i, k, oviduct ; L uterns; m, testis ; o. vagina;
p, penis, continuous posteriorly with the external vas deferens ; B, one of the
lateral contractile vessels ; (7, ramifications of the ciliated vessels.

As in most Trematoda, the genitalia (Figs. 40 and 42)
form a large part of the viscera, and the structure of the com-
plex hermaphrodite apparatus is in some respects so peculiar
that it is needful to describe it in detail. It consists of —
1. The germarium. 2. The vitellarium. 3. The oviduct.
4. The uterus and vagina. 5. The common vestibule. 6. The
testi?. 7. The vasa deferentia, internal and external. 8. The
penis and its sac. The ovary {cT) is the anterior of two round-

174 THE ANATOMY OF INVERTEBRATED ANIMALS.

ed masses lying* in the sucker. At first sight it appears to be
oval, but it is, in fact, pyriform, the larger end being anterior,
while the posterior narrower extremity is bent backward be-

FiG. 41. — A, water- vascular system of Aspidogaster conchicola: a, terminal pore;
6, lateral contractile vessels ; 6% lateral ciliated trunks, that of the left side shaded :
d, dilatation of this trunk ; B, one of larger, and C, one of the smaller, ciliated
vessels.

neath the anterior end. Before it reaches the anterior ex-
tremity of the mass, however, it is bent sharply back again,
parallel with itself, and so passes into the oviduct (Fig. 40, ^).
The ovary is surrounded by a delicate, but strong coat, inclos-
ing a mass of transparent protoplasm. At the anterior end
of the ovary minute granules are scattered through this sub-
stance, and are occasionally surrounded by a faint, clear area
(Fig. 43, A 1). These are the rudimentary germinal spots
and vesicles of the future ova, the course of whose develop-
ment may be readily traced by working from the anterior to
the posterior extremity of the ovary. The germinal spots
become larger, and gradually assume the appearance of vesic-
ular nuclei ; while the clear area around them in like manner
becomes larger, and acquires more and more the appearance
of a cavity. While this cavity is small, it has no distinct
wall, but, as it enlarges, the contour of the wall becomes dis-
tinctly marked (Fig. 43, A 2, 3, 4). On examining the ovary
close to the commencement of the oviduct, a division of the
homogeneous protoplasmic basis or matrix of the ovary into
areas surrounding each germinal vesicle becomes obvious. On
the application of pressure, the matrix breaks up into masses
corresponding with these areas in size, which are very flexible,
but when left to themselves assume a rounded or oval form,
and have all the appearance of perfect ova, except that they
possess no vitelline membrane, and that the yelk, instead of
being granular, is clear, and comparatively small. These

ASPIDOGASTER CONCHICOLA.

175

'primary ova^ as they may be termed, become detached, and
pass into the oviduct. Here they are fecundated, and, be-
coming surrounded by a great mass of accessory yelk, and a
shell, gradually acquire the appearance of the complete ova.

The accessory yelk is the product of the vitellarium — a
large double gland consisting of a number of oval, pyriform,
or irregular granular masses placed on each side, at the junc-
tion of the sucker with the body (Fig. 40, g).

These masses appear to be quite independent of one an-
other; nor do they at first present any obvious communication
with the genitalia ; but if the oviduct, just after it becomes
free from tlie ovarium, be examined, it will be found to re-
ceive a short duct (Fig. 42,/"), filled with strongly retracting
granules of the same nature as those in the vitellarium. This
duct is enlarged posteriorly, and then divides into two ducts
filled with the same matter, which take a direction toward the
vitellarium, but can be traced no further than they contain
granules (Fig. 42). By the careful application of pressure,
however, the granules may be forced from the vitellarium,
through an anterior and posterior branch upon each side, into
these ducts.

Fig. 42. — Aspidogaster concMcnla. — Reproductive onrans on a larger scale. Letters
as in Fis:. 40. The commeucemeut of the external vas deferena'ia seen behind the
vitellarian ducts.

The oviduct (Fig. 42, %) is richly ciliated internally ; it is
at first applied to the under surface of the ovarium, and when
it becomes free it receives a canal (e), which may be traced

176 THE ANATOMY OF IXYERTEBRATED ANIMALS.

back to the testis, and which would appear to correspond
with the internal vas deferens of other Trematoda described
by Von Siebold/ This canal, however, presents no dilatation,
or internal vesicula seminalis. The oviduct next receives the
duct of the vitellarium, and then becoming much convoluted
(y^), and rapidly widening, passes into the uterus (/), a wide
tube, which runs forward, disposed in many undulating curves
(Fig. 40, ^), to terminate on the left side of the anterior part
of the body, close to the male organs. Posteriorly, the walls
of the uterus are thin ; but in its anterior, or vaginal, part
they become thick and muscular. The genital vestibule into
which the vagina opens is very small.

The testis (m) is an oval body of the same size as the
ovarium, and situated just behind it. Minute water-vessels
ramify upon it, as upon the ovarium ; and it contains a gran-
ular and cellular mass, but no spermatozoa. The external
vas deferens (Figs. 40 and 42) is a delicate duct, which
passes forward and comes into contact with the ovarium,
without, however, so far as I could observe, communicating
u'ith it or with the oviduct ; it then bends backward and up-
ward, passing between the anterior vitellarian masses into
the fore part of the body. Here it suddenly becomes about
twice as wide as before, and runs forward, as an undulating
thick tube, to the penis (Fig. 40, jo), a short and conical body,
occupying the bottom of a large pyriform sac, which opens
in common with the uterus. The spermatozoa are linear.

The development of the ova presents many very interest-
ing peculiarities (Fig. 43). Above the junction of the duct
of the vitellarium with the oviduct the contents of the latter
were pale and clear, and presented no formed particles beside
the primary ova which had just been detached from the ova-
rium (Fig, 43, C). Below the insertion of the vitellarian
duct, however, the oviduct was full of granules like those in
the vitellarium, mixed up with ova in a more advanced state.
In the smallest of these (Fig. 43, D), the shell of the ovum
had commenced, but was incomplete at one end. At the op-
posite extremity, it inclosed a mass of irregularly aggregated
vitelline granules, which covered almost one-half of a round
pale mass, not larger than one of the primary ova ; in which,
however, three nuclei (two of which were very close together,

^ The connection of this duct with the testis \\\ the Trematoda has recently
been denied by Stieda (" Miiller's Arcliiv," 1871). I had no doubt of its exist-
ence in Aspidogaster^ but I have had no opportunity of reexamining this ani-
mal since the publication of Stieda' s paper.

THE DEVELOPMENT OF ASPIDOGASTER.

177

as if they had just divided) were to be distinguished. In
more advanced ova the shell was complete, but either color-
less or of a very pale-brown hue. In some of these the pri-
mary ova contained many nuclei and were imbedded in and
surrounded by a confused mass of accessory yelk-granules ;
while in others these granules were aggregated into a num-
ber of regular spheroidal masses (Fig. 43, £).

As dev:elopment proceeds, the accessory j'elk-masses grad-
ually disappear ; the primitive ovum, now become the homo-
losrue of the blastodermic disk or vesicle in other animals, to
all appearance increasing at their expense. At the same
time, clear rounded vacuoles in various numbers appear in its
substance ; but the nuclei of the germ, though very minute,
can, with proper care, be readily detected between these. In
the final stages the shell becomes browner, the vacuoles and
granules disappear, and the substance of the embryo appears
homogeneous. Bat, if carefully examined, the minute nuclei
become visible, especially if water be allowed to act on the

¥iG. i^.—A^pidogaster concMcola.—A, section of the ovary: 1, its anterior end: 2,
germinal spot surrounded by a distinct wall ; 3. 4, a complete germinal vesicle
and spot ; C, a primary ovum ; Z), voune: state of a complete ovum ; the primary
ovum partially surrounded by yelk-granules and a shell ; B, complete ovum, with
the accessory yelk aggregated into spheroids ; E, vacuolated embryonic mass ; F,
embryo.

tissue, and, if the shell be burst, and its contents poured out,
they readily break up into small but well-marked cells, each
with its nucleus. ^ At the same time, the embryo takes on a
form not very distantly resembling that possessed by the

178 THE ANATOMY OF INVERTEBRATED ANIMALS.

adult; into which it eventually passes without any metamor-
phosis.^

Thus it appears that, in Aspidogaster, the ovarium gives
rise to primary ova, which pass down the oviduct and become
fecundated, either by the spermatozoa conveyed by the inter-
nal vas deferens, or by those received by the vagina when
copulation with another individual, or, possibly, self-impreg-
nation, occurs ; that, next, the essential part of the process of
"yelk-division" takes place, the germinal spot dividing and
subdividing, and the primary ovum becoming in this way con-
verted into the spheroidal blastoderm ; that, contemporane-
ously, the blastoderm becomes invested by the accessory yelk-
granules poured in by the vitellarian duct, and by a shell ;
that the accessory yelk arranges itself into spheroidal masses,
which probably supply the blastoderm with the means of its
constant enlargement ; and that, finally, the accessory yelk
disappears, and the blastoderm becomes converted into the
embryo.

The modifications exhibited by other Trematoda concern
the number of the suckers, of which there are usually several
in the ectoparasites, but not more than one in the endopara-
sites ; their support on a chitinous framework, or the addition
to them of spines or booklets, similar to those of Cestoidea
or Acanthocephala : the bifurcation of the intestinal canal,
and the ramification of its branches, so that the forms of the
alimentary apparatus repeat the two extremes observed in
the aproctous Turhellaria ; the existence of two nervous
ganglia with a single transverse commissure in many ; and
the occasional presence of sensory organs (ej^e-spots). The
non-contractile canals of some genera are destitute of cilia,
except at their inner terminations.

The variations of the reproductive organs are rather of
position than of structure. Dioecious Trematodes are very
rare, the most important being the formidable Silha/rzia, the
male of which is the larger and retains the female in a gynm-
cophore^ or canal, which is formed by the infolding of the
margins of the concave side of the body. Bllharzia has
neither intromittent organ nor seminal pouch, and the history
of its development has not been traced beyond the escape of

1 The substance of this account of the structure and development of A'>pido-
aaster, with the illustrative fisrures, was published in IBS') in The Medical
'Times and (razeite. M. E. Van Beneden has recently thrown much light on the
mode in which the ova of the Trematoda are formed and developed, in his
"Eechcrches surla Composition et la Signification de l'(Euf."

THE DEVELOPMENT OF THE TREMATODA.

179

a ciliated embryo from the ovum. This parasite is found in
the blood-vessels of man, chiefly in those of the urinary or-
gans, the ova escaping from the body through the ulcerated
surfaces to which the parent gives rise. In the ectoparasites,

Fig. 44.— a. B, Mnnostomnm mntabile.—A. the ciliated emhryo (a) inclosinsr the
zooid, {b.) represented free in B (after Siebold) : C, Redia, or kine's yellow worm
of Distnma pacificum, containine grerms of other Redice ; D. Redia containine
CercaricE (a): E, Cercaria; F, Distoma, which results from the metamorphosis
of the Cercaria. (After Steenstrup.)

the embryo passes into a form identical with or closely resem-
bling that of the parent while still within the egg, as in As-
pidogaster. When this happens (e. g., Distoma variegatiim,
D. tereticolle), the one end of the embryo is often provided
with spines, and it is capable of slow creeping movements.
But, in most of the endoparasites, the embryo leaves the
parent as a morula, which is usually ciliated. Thus, in Disto-
ma lanceolatum^ D. hepaticimi, and 3Io7iosto7ni(m mutabiley
the embryo which escapes from the egg has a ciliated invCwSt-
ment, which propels it rapidly through the water, and may
be provided with eyespots and water-vessels (Fig. 44, A).
On becoming attached to the animal upon which it is parasit-
ic, the embryo of 3fonost07nu7n gives exit to a larva, having
the form of a cylindrical sac with two lateral prolongations
and a tapering tail. The Jiedia, as this form is called (Fig.
44, j&, (7), has a mouth and a simple c^ecal intestine, but no
other organs. In its cavity a process of internal gemmation
takes place, giving rise to bodies resembling the parent in
shape, but destitute of reproductive organs, and furnished

180 THE ANATOMY OF INYERTEBRATED ANIMALS.

with long tails, by which they are propelled. These creatures,
called Cercarice (Fig. 44, E)^ escape by bursting through the
Hedia, and, after a free-swimraing existence, penetrate the
body of some other animal, their tails dropping oif. They
then become encysted, and, under suitable conditions, assume
the adult form, and develop reproductive organs (Fig. 44, F).
The cycle of forms through which Distoma milltare passes
has been nearly completely traced, and may be briefly stated
as follows : 1. The parent form, whose habitat is the in-
testines of water-birds, bears on its anterior extremity two
alternating circles of larger and smaller booklets, and a few
others, irregularly disposed. Rings of papillie give the cen-
tre of the body an annulated aspect. The mouth, almost
terminal, leads into the lono*, straight diofestive csecum. The
generative organs are similar to those of Asjndogaster / the
testes are, however, double, and lack the internal vas deferens.
The ova are few, eight or ten in number. 2. From each
ovum issues a ciliated larva, showing the rudiments of — 3. A
Media, but the mode of development of the latter has not
been fully traced. The perfect Media is found attached to
the body of a water-snail (Maludi?ia), the ciliated investment
having disappeared. It consists of a sac, within which is
suspended a tubular bag, containing colored masses, probably
alimentary. Anteriorly, the head is represented by a kind of
crown, in which no oesophagus exists as yet, and not far from
the posterior extremity the two lateral projections, character-
istic of Distomatous Mediae, appear. Daring the rapid growth
of the zooid, the head becomes marked off by a constriction,
and a mouth and gullet, with a pharyngeal dilatation, admit
aliment to the digestive sac. In the body cavity, external to
this sac, vesicles appear, rapidly increase, and take the form
of Cercarice ; the Media bursts, and these new zooids are
set free. 4. The Cercaria has a long tail with lateral mem-
branous expansions, by means of which it swims after the
fashion of a tadpole. The pharyngeal bulb is followed by an
oesophagus, which, opposite the ventral sucker, divides ; the
two branches ending in a csecum on either side of the <;on-
tractile vacuoles of the water-vascular sj'stem. These are
median, the terminal quadrate chamber opening into an an-
terior circular one, whence are given off the two main canals
which traverse the body longitudinally, and are then lost. 5.
After swimming about freely for a while, the Cercaria fixes
itself upon, or bores its way into, a Paludina ; the tail drop-
ping off, and the body coating itself with a structureless cyst,

THE DEVELOPMENT OF THE TREMATODA.

181

in which it remains quiescent, but undergoes some further
advances in development, the coronal hooklets making their
appearance. 6. When a Paludina, thus infested, is swal-
lowed by a w^ater-bird and digested, the cj'sts are set free in
the alimentary canal of the bird ; sexual organs appear within
the included Distoma / the body elongates and narrows an-
teriorly ; the sucker moves nearer the head, and the coronal
circlets reach their full development. The Distoma gradually
assumes the form of the parent, attaches itself by its hooklets
to the intestinal walls, and acquires complete sexual organs.*
Thus the developmental stages of Distoma militare may be
summed up, as : 1. Ciliated larva. 2. liedia. 3. Cercaria,

4. Cercaria^ tailless and encysted, or incomplete Distoma,

5. Perfect Distoma,

The stages of transition vary in different genera. Thus,
several generations of Hedioe may intervene between the

Fig. 45.— Bucephalus polymorphus of the fresh-water muscle.— J, ramified sporocyst ;
^, portion of the same more man^nified: a, outer coar, 6, inner; c, <?, germ-
masses in course of development ; C, one of the germ-masses more highly mag-
nified ; i?, Bucephalus ; a, b, suckers ; c, clear cavity ; d, caudal appendages.

third and fourth stages ; or the mature animal may appear at
the close of this stage, having undergone no Cercarian meta-
morphosis.

In Bucephalus polym^orphus^ a parasite of the fresh-
water muscle (Fig. 45), two caudal appendages, which seem
to correspond with the tail of the ordinary Cercarice^ become

1 Van Beneden, " Memoire sur les Vers Intestinaux.'

182 THE ANATOMY OF INVERTEBRATED ANIMALS.

enormously elongated. They are converted into ramified
tubes called sporocysts^ "which sometimes occupy all the inter-
spaces of the viscera of the muscle. These develop new
l^ucejyhali by internal gemmation. The Trematode condition
appears to be the genus Gasterostomum^ which inhabits fresh-
water fishes.

The Sporocysts, Redias, and Cercariae, free or encysted,
are found almost exclusively in invertebrated animals, while
the corresponding adult Trematodes are met with in the verte-
brated animals which prey upon these Invertehrata.

The singular double-bodied Diplozoon ijaradoxum has
been shown by Von Siebold to result from a sort of conjuga-
tion between two individuals of a Trematode, which, in the
separate state, has been named Diporpa. The DiporpxB^
when they leave the eg^^ are ciliated and provided with two
eye-spots, with a small ventral sucker and a dorsal papilla.
After a time the Diporpm approach, each applies its ventral
sucker to the dorsal papilla of the other, and the coadapted
parts of their bodies coalesce. They acquire fully developed
sexual organs only this after union. ^

Gyrodactylus multiplies agamically by the development
of a young Trematode within the body, as a sort of internal
bud. A second generation appears within the first, and even
a third within the second, before the young Gyrodactylus is
born.

The Cestoidea. — The Tape-worms are all endoparasites,
and, in their adult condition, infest the intestines of verte-
brated animals.

The simplest form known is Caryophyllaeus^ found in
fishes of the Carp tribe. It has a slightly elongated body,
dilated and lobed at one end, so as to resemble a clove,
whence the name of the genus. In structure it resembles a
Trematode, devoid of any trace of an alimentary canal, but
provided with the characteristic water-vascular system and
with a single set of hermaphrodite reproductive organs.

In lAqida^ the body is much elongated, and, at the head-
end, exhibits two lateral depressions. It is not divided into
segments, but there are numerous sets of sexual organs ar-

1 Zeller, " TJntersuchun^en ilber die Entwickelung des Diplozoon paradox-
um." {Zeitschrift fiirwiss. Zoologie, 1872.)

2 See the " Meraoire surles Vers tntestinaux," 1858, by M. P. J. Van Beneden,
to which I am much indebted for information respecting this and other genera
of Cestoidea which have not fallen under my own observation. Also Leuckart,
*' Die menschlichen Parasiten," 1863 ; andCobbold, *' Entozoa."

THE CESTOIDEA.

183

ranged in longitudinal series. The openings of the genital
glands are situated in the middle line of the body. These
parasites inhabit fishes and amphibians, as well as water-
birds, but they attain their sexual state only in the latter.

Fig. 46 —Diagram of the structure of a cestoid worm, with only one joint. The posi-
tion of the liooks of a Tainia and of one of the proboscides of a letrarhynchua
is inoicated. -4, head and neck; 5, segment of the body corresponding with a
proglottis: a. rosteUum; 6, rostella spines {ToBnia); c, </, </', spinose eversible
proboscis {Titrarhynchus) ; rf, sucker; e, ganglion (?); /, lateral, and g, circular
water-ves;?el ; A, ramifications of the water-vessels ; k, anastomosing trunk ; *,
contractile vacuole ; /, genital vestibule ; m, penis and vas deferens ; «, vagina ;
o, common cavity ani vesictiCa seminalis interior ; p, ovary; q, uterus ; r, vitel-
larian duct.

In the more typical Cestoidea the body is elongated, and
presents, at one end, a head provided with suckers, and very
generally with chitinous hooks, either disposed circularly
around the summit of the head, or upon proboscidiform ten-
tacles, which can be retracted into, or protruded from, the
head. Sometimes the head is produced into lobes ; and very
generally, when lobes or tentacles exist, they are four in
number, and are disposed symmetricall}^ round the head. A
short distance bej^ond the latter, the slender body widens and
becomes transversely grooved, so as to be marked out into
segments. Longitudinal water-vessels run parallel with one
another through the body, and are connected by transverse
trunks in each segment, and by a circular vessel in the head.
In Bothriocephalus latus, the principal trunks are occupied
by a spongy reticulated tissue.

In most of the tape-worms, innumerable, solid, strongly-

184

THE ANATOMY OF INVERTEBRATED ANIMALS.

refracting corpuscles are scattered through the substance of
the body (Fig. 48, A). It is probable that these are more or
less calcified connective-tissue corpuscles. Similar bodies
which occur in some Trematoda were found by Claparede to
be lodged in dilated ends of the water-vessels, but it would
appear that they are not so situated in the Gestoidea.^

The distance between these transverse grooves, and their
depth, increase toward the hinder end of the body ; and each
segment is eventually found to contain a set of male and
female organs. The genital organs are constructed upon the
same general plan as those of the Trematoda, but the uterus,
as it fills with ova, usually takes the form of a ramified sac.
At the extreme end of the body, the segments become de-
tached, and may for some time retain an independent vitality.
In this condition each segment is termed a proglottis ^ and
its uterus is full of ova.

The embryo is developed in these ova in the same way as
in the Trematoda ; and, as in the latter group, it may either
be ciliated (as in JBothriocephalus) or non-ciliated, which last
is the more usual case. The embryo is a solid morula, on one
face of which four or six chitinous hooks, disposed symmet-
rically on either side of a median line, are developed.

Fig. 47.— Diagrams illustrative of the relation hetween Tcvnia, Cysticercus, Coe,nnrus^
and Echinocoecus.—A, J5, youn? Tcenim in the Scolex stajje, the latter with an
en\?^xze'^receptactilum SGoUcis,\ntowh.\ch the head and neck are withdrawn in
C. Cusficercus ; 7), C(»nurus ; E. hypothetical condition of Echinococcm, in which
" Trpnia heads" are developed only on the inner surface of the primary cysts; S^t
Echinococcus with secondary cysts; G, embryo Taenia (after Stein).

If the egg is placed in appropriate conditions, the hooked
embryo emerges from the shell, and rapidly increases in size.

» Sommer and Landois, " Ueber den Bau der ^eschlechtsreifen Glieder
von Bothriocfiphalys latus." (Zeitschrift fur wiss. Zoolocfie. 1872). Leuckart,
however, maintains the contrary opinion, " Die menschlichen Parasiten," p.
175.

THE CESTOIDEA.

185

After a time, a cavity appears in the midst of the cells of
which the morula is composed, and a chitinous cuticula is
developed upon the outer surface of the embryo. Ramified
water-vessels make their appearance in the wall of the sphe-
roidal sac thus formed, and in some cases open by an external
pore. There is, therefore, a very close resemblance between
this cestoid embryo and the sporocyst of a Trematode.

When the saccular embryo has attained a certain size, a
thickening and invagination take place, usually at one {I'm-
Olid), sometimes at many {Coenurus, Echinococcus) points of
its wall. The invagination of tlie wall elongates inward, and
becomes a caecum, the cavity of which opens outward. At
the bottom of the interior of this caecum, and therefore on
what is morphologically its external surface, the hooks of
those species which possess them are developed, while, upon

7<i^^.-..

IF

Fig. 4S.—Echinococcifs veterinoritm.—A, "Taenia head," or Scolex : a, hooks; ft,
puckers; c, cilia in water-vessels; d, oval, stronsrly refracting particles; B, single
hooks; C, portion of the elastic cyst, a; with the inner membranous primary
cyst, b; c and e, ScoUces developing from its inner surface; d, a secondary cyst.

the side-walls, elevations arise, which become converted into
suckers. The ca3cum is next evaginated or turned inside

186 THE ANATOMY OF INYERTEBRATED ANIMALS.

out, and tlie embryo has the form of a phial, of which the
evaginated caecum forms the neck. Round its apex are the
hooks, and below these the suckers, forming a complete ces-
toid head ; while the sac answers to the body of the phial.
The original hooks of the embryo are cast off in the course
of the process.

If the eggs of the Tape-worm have passed into the aliment-
ary canal of an animal in which the worm is unable to attain
its sexual condition, the hooked embryo, as soon as it is
hatched, bores its way through the walls of the alimentary
canal, and eventually becomes lodged in the connective tissue
batween the muscles, or in the liver, or in the brain or eye.
Here it oroes throusrh the chanofes which have been described,
and, generally, the sac undergoes very great dilatation. The
rearion of the wall of the sac to which the cestoid head is at-

CD

tached becomes invaginated, and thus is inclosed within a
chamber, the parietes of which are really constituted by the
outside of its own body. In this condition, the animal is
what is termed a Cystic worm, or bladder-worm ; and when
there is only one head it is a Cystlcercus, In the genera
Goenurus and EMnococcus the cystic worm has many heads;
and, in Eshmococcus, the structure of the cystic worm is still
further complicated by its proliferation, the result of which is
the formation of many bladder-worms inclosed one within
the other, and contained in a strong laminated sac or cyst, ap-
parently of a chitinous nature, secreted by the parasite (Fig. '

In the cystic condition, the Tape-worms never acquire
sexual organs ; but, if transported into the alimentary canal
of their appropriate hosts, the heads become detached from
the cysts, and, rapidly growing, give rise to segments, which
become sexual proglottides. The Tape-worms are rarely met
with in both the cystic and cestoid conditions in the same
animal ; but the cystic form is found in some creature which
serves as prey to the animal in which the cestoid form occurs.
Thus :

Cystic Form. Cestoid Form.

Cystkercus cellulosm. Tarnia solium.

(Muscles of the Pig) (Man)

Cysticercm ? Taenia mediocanellata.

(Muscles of the Ox) (Man)

Cysticerms plnformis. Tmnia serrata.

(Liver of the Rabbit) (Dog, Fox)

THE DEVELOPMEXT OF THE CESTOIDEA. 187

Cystic Form:. Cestoid Form.

Cysticercus fasciolaris. Tcenia crassicollis,

(Liver of Rats and Mice) (Cat)

Canurus cerebi'alis. Tcania ccenurus.

(Sheep's brain) (Dog)

JEchinococcus veterinorum. Tcenia Echinococcus.

(Liver of Man and of (Dog)
domestic Ungulata)

The embryo of loenia cuciimeri7ia passes, in the body of
the Dog-louse {Trichodectes cmiis), into a Cysticercoid^ or
minute unjointed and sexless Tsenia, without any terminal
dilatation. The dog devours the louse and the Cysticercoid
becomes a Tcenia cucumerlna in his intestine. The eggs of
the Tcenia^ contained in fieces adherent to the hair of the
dog, are in turn devoured by the louse, and thus the " vicious
circle" of parasitism is maintained.

The cystic Tetraphyllidea frequent osseous fishes, their
sexual maturity being attained in the bodies of Plagiostomes.
The head is provided with four suckers or lobes, which may
be stalked and unarmed, as in Echeneibothrium^ or furnished
with hooklets as in Acanthohothrhim ; while, in Tetrarhyn-
chiis^ four proboscidiform tentacles, thickly set with booklets,
are retracted into sheaths alongside of the suckers (Fig. 46).

The Diphyllidea have two suctorial disks, two armed
rostellar prominences, and a collar of hooklets on the neck.

The migrations of the JPseudophyllidea are chiefly from
fishes and amphibians to water-birds, one genus {Bothrio-
cephalus) containing species which enter the human body, prob-
ably in the flesh of fresh-water fishes. The head has neither
suckers nor lobes, but is deeply grooved on either side. In
JBothriocephalus the genital apertures are in the middle of
each segment. The embryo is ciliated, and swims actively in
water. Recent experiments tend to show that the develop-
ment of the embryo in this genus may take place directly, or
without the intervention of a Cysticercus stage.

It is obvious that the Cestoidea are very closely related to
the Trematoda. In fact, inasmuch as some of the latter are
anenterous, and some of the former are not segmented, it is
impossible to draw anv absolute line of demarkation between
the two groups. It would appear that the Cestoidea are
either Trematodes which have undergone retrogressive met-
amorphosis and have lost the alimentary canal which they
primitively possessed, or that they are modifications of a

183 THE ANATOMY OF INVERTEBRATED AXIMALS.

Trematode type, in which the endoderm has got no further
than the spongy condition which it exhibits in Convoluta
among the Turbellaria, and in which no oral aperture has
been formed ; or, lastly, it is possible that the central cavity
of the body of the embryo Taenia simply represents a blas-
tocoele.

If the Cestoidea are essentially Trematodes, modified by
the loss of their digestive organs, some trace of the digestive
apparatus ought to be discoverable in the embryo tape-worm.
Nevertheless, nothing of the kind is discernible, unless the
cavity of the saccular embryo is an enterocoele. And if this
cavit}'' is a blastoccele, and not an enterocoele, it may become
a question whether the tape-worms are anything but gigantic
morul®, so to speak, which have never passed through the
gastrula stage.

CHAPTER V.

THE HIRTJDIXEA, THE OLIGOCH^TA, THE POLTCH^TA, THE

GEPHYKEA.

The Hirudinea. — -The Leeches are aquatic or terrestrial,
more or less distinctly segmented, vermiform animals, most
of which suck blood, though some devour their prey. The
ectoderm is a cellular layer, covered externally by a chitinous
cuticula, and, except in Malacohdella^ devoid of cilia. Very
commonly it is marked by transverse constrictions into rings,
■which are more numerous than the true somites as indicated
by the ganglia and the segmental organs ; and simple glands
may open upon its surface. One or more suckers, which
serve as organs of adhesion, are developed upon it. In some
{Acanthohdelli) bundhs of setae are present ; in others [Brayi-
chelllon) the sides of the body are produced into lobe-like
appendages ; but none have true limbs, unless the lateral ap-
pendages of Histriohdella are to be considered as such ; nor
are the anterior segments of the body so modified as to give
rise to a distinct head.

The mouth is generally situated at the anterior end of the
body ; the anus at the opposite extremity, on the dorsal side
of the terminal sucker. The buccal cavity may be armed
with several serrated chitinous plates, as in the Medicinal
Leech, where there are three such teeth. By their aid the
Leech incises the skin and gives rise to the well-known tri-
ardiate mark of a leech-bite. The buccal cavity usually
opens into a muscular, sometimes protrusible, pharynx, from
which a narrow oesophagus leads into a stomach, which is fre-
quently produced into lateral coeca. In the Medicinal Leech
(Fig. 49), for example, there are eleven pairs of such creca,
increasing in length and capacity from before backward.
From the stomach a narrow intestine leads to the anus. In

190

THE ANATOMY OF INVERTEBRATED ANIMALS.

ft.

r-^

'm

t-M

\

<N^?

3falacohdella the alimentary canal is a sim-
ple tube bent several times upon itself.
The alimentary canal is lined by the cells
of the endoderm, and the space between
them and the ectoderm is occupied by the
mesoderm, which contains abundant con-
nective and muscular elements, and is ex-
cavated by the blood-channels, which some-
times have the form of wide sinuses, but
in other cases are comparatively narrow
vessels with definite walls.

In the lower Hirudinea^ as Clepsine,
the sinuses and vessels appear to form one
continuous system of cavities containing a
fluid which must be regarded as blood. But
in the Leech a distinct pseud-haemal vascu-
lar system has attained a great degree of
definition and complexity : it consists of
(1) a median dorsal trunk ; (2) a median
ventral trunk, in which the ganglionic nerve-
chain lies ; (3, 4) two wide lateral longitu-
dinal trunks (Fig. 50). These anastomose
with one another, and give off numerous
branches, which open into a rich capillary
network, situated in the muscular layer of
the mesoderm, and on the segmental and
reproductive organs. The fluid contained
within these vessels has a red color, and
contains no corpuscles.

More or fewer of the segments of the
body are provided with what are termed
segmental organs. These are tubes which
open externally on the ventral wall of the
body, while at their other extremities they
either open into the sinuses by ciliated
mouths (Clepsine)^ or form a closed and
more or less reticulated non-ciliated coil
{Hirudo). These obviously answer to the
ciliated water-vessels of the Turhellaria
and Trematoda.

The nervous system consists of a cerebral mass in front of
the mouth, proceeding from which, on each side, is a commis-

1 " Die mensclilichen Parasiten," 1863.

THE HIRUDINEA.

191

sure connecting it with a ventral cord on which ganglia, cor-
responding in number with the somites of the body, are de-

~" * ^

TrV-— =

Fig. 50.— a diasrammatic view of the arransrement of the principal vessele of the
leech {Hirudo medicinalia). after Gratiolet. The inner surface of a portion of
one-half of the body is depicted: a, a, the ventral trunk; e, e', c", the lateral
trunk and its branches ; /, /', the dorsal trunk and its branches ; g, the slender
transverse trunks which branch out at each end; A, i, the transverse ventrai
branches of the lateral trunk ; k, I, the branch to the testis (e). and the segmental
organ (d) ; m, branch from the dilatation on the testis to the parietal plexuses;
b, b, vas deferens.

veloped. In Malacohdella, these cords are lateral and wide
apart, but, in all the other Hirudinea^ they come close to-
gether behind the mouth, and occupy the middle line of the
ventral face of the body. In the Leech, according to Leuck-
art, there are originally thirty pairs of post-oral ganglia, but
the seven posterior and the three anterior pairs coalesce, so
that only twenty-three pairs are distinguishable in the adult.
Nerves are given off to the pharynx and intestines, and the
former develop special ganglia.

Simple eyes are usually present on the anterior or oral
segment, and receive nerves from the supraoesophaireal gan-
glia. In the Leech these eyes are situated in the first three
segments. Cup-shaped depressions of the integument of the
anterior segments of the body, lined by peculiar glassy cells

192 THE ANATOMY OF INVERTEBRATED ANIMALS.

and in relation with nerves which terminate in fine filaments,
have been discovered by Leydig in several of the Slrudinea,^

The elongated spindle-shaped muscle-cells of thel)ody are
abundant, and are disposed in a superficial circular and deep
longitudinal layer, while dorso-ventral bands pass from the
dorsal to the opposite body-wall.

Malacohdella and Histriohdella are dioecious, but the other
Ulrudlnea are hermaphrodite. The male organs consist of
numerous testicular sacs, situated on each side of the body,
and connected by a vas deferens, which usually opens into
a sac, terminating in an eversible penis. The spermatozoa
are often inclosed in a case or sperrnatophore. The female
organs, much smaller than the male, consist of ovaries, with
oviducts opening into a vagina. The vaginal orifice is behind
that of the penis. In the Leech the eggs are inclosed in a
sort of cocoon, formed by a viscid secretion of the integu-
ment.

The observations of Rathke and Leuckart on the develop-
ment of N^ephelis^ Clepsbie^ and Hlrudo show that, after the
division of the vitellus into a few equal-sized large blasto-
meres, small blastomeres are separated from the large ones (as
in the Ctenophora and Polycelis)^ and the rapidly-multiply-
ing small blastomeres form an investment to the slowly-divid-
ing large ones. This investment is the epiblast, and becomes
the ectoderm, while the included larger blastomeres are event-
ually converted into the cells of the endoderm. At one end
of the body the oral aperture appears, in some cases (e. g.,
Nephelis) surrounded by a raised lip, as in the embryo Pla-
narian ; and the embryo passes into the Gastrula stage. The
body now elongates, and, on the ventral face, the mesoblast
makes its appearance as a layer of cells, sometimes divided
into two longitudinal bands, separated by a median interval.
Three pairs of segmental organs, which have only a tempo-
rary existence and have been regarded as primordial kidneys,
are developed at the posterior end of the body. The meso-
blast next becomes divided transversely into the number of
somites of which the body is eventually composed, the divis-
ion first making its appearance on the ventral face of the
body. A pair of ganglia, probably derived from the epiblast,
is developed in each segment.

Thus, in the Leeches, the segmentation of the body is the
result of the segmentation of the mesoblast, which becomes

i " Arcliiv fiir Anatomie und Physiologie," 1861.

THE OLIGOCHJETA. I93

the mesoderm of the adult. And it is this seg^mentation of
the mesoblast, and consequently of the mesoderm, which con-
stitutes the most important difference between the Leech on
the one hand and the Turbellarian and Trematode on the
other.

On the other hand, in the development of a mesoblast
which undergoes division into segments, the Leeches exhibit
the fundamental character of all such seo^mented Invertebrates
as the chsetophorous A?inelkla and the Arthropoda,

The Oligoch^ta. — The earthworm (Lumhriciis) and
fresh-water worms (JVais, Tubifex^ Chcetogaster)^ which are
included under this name, are closely allied w^ith the Leeches
in the essential points of their structure and development,
much as they differ from them in habit and appearance.

They have elongated, rounded, segmented bodies, often
divided by many superficial transverse constrictions into
rings, which, as in the leeches, may be more numerous than
the proper somites. There are no limbs, but each segment
is usually provided with two or four sets of longer or shorter
chitinous seta?, which are developed and lodged in integument-
ary sacs. The outermost layer of the ectoderm is a non-cili-
ated chitinous cuticle.

The mouth is situated close to the anterior end of the
body, but a " cephalic lobe " not unfrequently projects be-
yond it on the dorsal side. The anus is at the opposite ex-
tremity of the body, and the straight alimentary tract which
connects the two and is lined by the endoderm is usually
divided into a pharyngeal, oesophageal, and gastro-intestinal
portion, the latter often being produced laterally into short
cjBca. The mesoderm presents well-developed transverse,
longitudinal, and dorso-ventral muscular fibres, as in the
Leeches. It is excavated by a spacious perivisceral cavity,
which contains a colorless corpusculated fluid, and is divided
by thin but muscular mesenteries, which stretch from the in-
testine to the parietes, and thus break up the perivisceral
cavity into partially separate chambers. In addition, there
is a system of pseud-hremal vessels, like those of the Leeches,
provided with contractile walls, and containing a red non-
corpusculated fluid. No communication has been ascertained
to exist between these vessels and the perivisceral cavity ;
but there can be little doubt that, as in the case of the
Leeches, they must be regarded as a specially differentiated
part of the general system of the perivisceral cavity.

y

194 THE AXATOMY OF INVERTEBRATED ANIMALS.

In the majority of the segments there are, as in the Si-
rudinea^ paired segmental organs ; these are ciliated and
their inner ends open into the perivisceral chamber.

The nervous system consists of prae-oral or cerebral gan-
glia, continued backward, on the ventral aspect of the body,
by commissures on each side of the oesophagus into a double
chain of closely united post-oral ganglia.

Large tubular fibres are imbedded in the neurilemma of
the ganglionic chain on its dorsal face. In the earthworm
there are three of these — one median and two lateral — ex-
tending along the whole length of the ventral end, but not
into the oesophageal commissures.^ The nature of these
structures is unknown.

These animals are hermaphrodite. The generative organs
are situated in the front part of the body, the male organs
being anterior to the female. In the aquatic Oligochceta
(N^ais, Tnbifex) the genital glands have no proper ducts, but
the segmental organs of the segments in which they are con-
tained convey the generative products outward. In the ter-
ricolous forms {Lumhricus) the vasa deferentia are continuous
with the testes, which are very large. The ovaries, on the
other hand, are minute solid bodies attached to one of the
mesenteries, and the oviducts are separate tubes with funnel-
shaped mouths, w^hich open into the cavity of the segment.

In iVa?5 and Chcjetogaster^ agamic multiplication occurs
by the development of posterior segments of the body into
zo5ids, Avhich may remain associated in chains for some time,
but eventually become detached and assume the parental
form. Schulze has observed that when a JSTais has divided
into an anterior and posterior zoOid, the last somite of the
former gradually enlarges, and becomes divided into new
somites, the anterior of which give rise to a head. A new
zouid is thus developed between the previously existing ones.
This process is repeated in what was the penultimate, but is
now the ultimate somite of the anterior zooid ; and again in
the penultimate somite when it has, in tlie same way, become
terminal.

As the Earthworm is a very accessible subject, it may be
useful to the student to be furnished with an account of some
of the chief points of its organization more in detail.

The exterior of the body of an Earthworm (Lumhricus
terrestriSf rubellus, or communis) shows a number of close-set

» Claparede, " Histologische Untersuchungen uber den Kegenwurm," 1869.

THE STRUCTURE OF THE EARTHWORM. 195

transverse grooves which divide its body into numerous nar-
row rings or segments/ The most anterior segment is small
and conical, and presents, on its under surface, a depression
which is the oral aperture. The anus is at the opposite end
of the body. Behind the mouth, the successive segments rap-
idly attain their average size ; but, in a full-grown worm, a
part of the bod}'', into which more or fewer of the segments
between the twenty-fourth and thirty-sixth inclusively (29 ?-
36, L. terrestris ; 24-29 ?, L. ruhellus ; 26-32, L. communis)
enter, is swollen, of a different color from the rest, provided
with abundant cutaneous glands, and receives the name of
cingulum or clitellum.

In the dorsal median line there is a series of small aper-
tures or pores, one for each segment except the most an-
terior, which lead into the perivisceral cavity ; while upon
the ventral surface of the anterior part of the body the eight
apertures of the organs of generation are situated. Of these,
four, situated two on each side, between the ninth and tenth,
and the tenth and eleventh segments, are the openings of the
receptacula seminis. The openings of the two oviducts are
on the fourteenth segment ; those of the two vasa deferentia
on the fifteenth. Besides these, all the segments, except
some of the most anterior, exhibit a pair of minute openings
appertaining to the segmental organs ; and they are further
perforated by the four longitudinal double rows of setas,
which project slightly beyond the surface of the integument,
and offer a certain resistance when the worm is drawn from
tail to head througrh the fino^ers.

The body is invested in a thin and transparent but dense
cuticula, perforated by excessively minute vertical canals.
Within this lies a thicker layer, consisting of a reticulated
nucleated protoplasm, the meshes of which are filled with
a transparent gelatinous substance. This layer probably rep-
resents both the dermis and epidermis^ and has been termed
the hypodermis. Internal to it lies a thick layer of circular
muscular bands, in the interstices of which pigment-granules
occur ; and, still more internallj'", is a much thicker coat of
muscular fibres, which are disposed longitudinall3\

The cavity circumscribed by this longitudinally fibrous
muscular layer is lined by a kind of connective tissue. Cor-
responding with the divisions between every pair of segments

^ The question how far all these segments represent somites may be left
open. The history of the development of the Earthworm is in favor of their
beino: true somites.

X96 THE ANATOMY OP INVERTEBRATED ANIMALS.

(except in the most anterior part of the body), this connective
tissue is continued transversely toward the axis of the bod\^,
and passes into that which forms the wall of the intestine ;
while, on the ventral side, it forms an arch over the ventral
nervous cord and the vessels which accompany it. In the
interior of each of these mesenteric septa, radiating and circu-
lar muscular fibres are abundantly developed, and the former
are connected externally with the superficial layer of trans-
verse muscles.

The perivisceral cavity is thus divided into nearly as many
short chambers as there are segments; each chamber com-
municates with the exterior, directly by the dorsal pore and
indirectly through the segmental organs, while fluid may pass
from one to the other by the supra-neural archways.

The short and curved setae project much farther into the
interior of the body than they do on to its exterior. The free
apices of each pair are situated close together, while their
inner ends diverge from one another. Each is inclosed in a
sac in which it is developed, and to which the muscles, by
which it is protruded, are attached. There are eight setae to
each somite, one pair not far from the ventral median line on
each side ; and the other pair placed in the same transverse
line, but further outward.

The mouth leads into a muscular pharynx, with a com-
paratively small internal cavity, which reaches as far back
as the seventh segment. From this a narrow oesophagus is
continued as far back as the fifteenth or sixteenth segment ;
and presents three pairs of lateral glandular diverticula, which
contain a calcareous matter,^ in the region of the twelfth and
thirteenth segments. Posteriorly, the gullet opens into a
crop, which is succeeded, about the eighteenth segment, by a
thickened and muscular gizzard.

Upon this follows the intestine, wliich has the appearance
of a simple tube; but is in reality complicated by the invo-
lution of its wall, along the dorsal median line, into a thick
fold, which projects into the interior of the intestinal cavity,
and is the so-called typhlosole. The exterior of the intestine
and the cavity of the typhlosole present a coating of yellow-
ish-brown cells.

The segmental organs are greatly convoluted tubes, situ-

1 The nature of this substance has recently been rUscnssed by M. E. Perrier,
*'!^tude sur un trenre nouveau des Loinbriciens." ("Archives de Zoologie ex-
p^rimentale," 1873.)

THE STRUCTURE OF THE EARTHWORM. 197

ated one on each side of every segment except the first, and
attached to the posterior mesenteric septum of the segment.
Each canal communicates internally, by a wide funnel-shaped
ciliated aperture, with the perivisceral cavity, while extern al-
I3' it opens by a minute pore, which is usually close to the in-
ternal pair of setie/

A colorless fluid, containing colorless corpuscles, and an-
swering to the blood of other invertebrated animals, occu-
pies the perivisceral cavity ; but, in addition to this, there is
a deep-red fluid, devoid of corpuscles, which fills a very large-
ly developed system of pseud-hicmal vessels. These consist
of longitudinal and transverse principal trunks, and of very
numerous branches which proceed from them and ramify in
all parts of the body, except the cuticle and hypodernus.

The longitudinal trunks are three : one siqyra-intesthial,
which lies along the dorsal aspect of the alimentary canal;
one sicb-i?itestinal, which corresponds with this on the ven-
tral aspect of that canal ; and one suh-neiiral, which lies be-
neath the ganglionic cord.

The supra-intestinal and sub-intestinal vessels are con-
nected in the greater number of the segments by pairs of com-
missural transverse trunks, which embrace the intestine, and
give off numerous branches to it. The supra-intestinal and
sub-neural vessels give off transverse trunks into the mesenter-
ic septa, which branch out into the muscular layers, and some
of which anastomose so as to form a second set of transverse
communications. Moreover, the sub-neural trunk and the
sub-intestinal trunk respectively send branches to each seg-
mental organ, upon which they are distributed, and, anastomo-
sing, give rise to another series of communications between
the longitudinal trunks.

In the seven most anterior segments, the longitudinal
vessels break up into a network, and there are no distinct
transverse commissural vessels. Behind these, and in the
region of the generative apparatus, the commissural vessels
are greatly dilated, and form from five to eight pairs of so-
called hearts which are attached to the anterior faces of as
many mesenteries. These contract from the dorsal toward
the ventral side.

The nervous system consists of two cerebral ganglia
lodged above the pharynx in the third segment, and united

1 Ge^enhaur, " Ueberdie sogenannten Eespiratioasorganc des Eegenwurms."
{Zeitschrift fur wiss. Zoologie^ 1852.)

198 THE ANITOMY OF INVERTEBRATED ANIMALS.

by commissural cords with the anterior ganglia of the chain,
which extends through the whole length of the body on the
ventral wall of the perivisceral cavity.

There are no eyes, nor are any other organs of special
sense known.

The Earthworm is hermaphrodite. The testes are two
pairs of large sacs, each of the anterior pair being bilobed.
The testes of opposite sides are united in a common median
reservoir, situated in the tenth and eleventh segments, from
which, on each side, ducts take their origin. The two ducts
of the testes of the same side unite into a single vas deferens,
and these two vasa deferentia open externally on the ventral
aspect of the fifteenth segment. The ovaries are two minute
solid bodies, not more than y^g- of an inch long, attached to
the posterior face of the mesenteric septum which separates
the twelfth and thirteenth segments. They therefore lie in
the cavity of the latter. The oviducts are quite distinct from
the ovaries, and open internally by wide, funnel-shaped aper-
tures, situated in the cavity of the thirteenth segment. From
these funnel-shaped ends the oviducts are continued, as
slender tubes, through the mesenteric septum which separates
the thirteenth from the fourteenth segment, and open on the
ventral face of the latter.

Four globular spermathecce, or receptacles of the sper-
matozoa, are situated, two on each side, in the tenth and
eleventh segments, and open on the ventral face between
the ninth and tenth and the tenth and eleventh seo^ments
respectively. These are filled when copulation takes place,
during which process the two worms are said to be bound
together by a tough secretion of their clitella.

The development of the Ollgochceta has recently been
carefully investigated by Kowalewsky. The eggs of the
Earthworm are laid in chitinous cocoons cr cases, which are
probably secreted by the clitella. In addition to the eggs,
the cocoons inclose an albuminous fluid, and packets of sper-
matozoa. The vitellus is invested by a membrane, and con-
tains a germinal vesicle and spot. Complete yelk-division
takes place, and eventually the blastocoele becomes reduced
to a mere cleft. The blastomeres are disposed in two layers
— one consisting of small and the other of large blastomeres.
The embryo thus formed becomes concave on the side formed
by the large blastomeres, until it assumes the form of a sac,
ciliated externally, with an opening, the future mouth, at one
end ; the cavity of the sac being the primitive alimentary

THE POLYCH^TA. 199

canal, and the layer of large blastomeres, the hypoblast. Be-
tween the two, a mesoblastic layer appears, but the exact
manner of its origin is not known. On one face of the sac-
cular embryo the mesoblast becomes divided into a series of
quadrate masses, like the protovertebras of a vertebrate em-
bryo, disposed symmetrically on each side of a median line,
which corresponds with the future ventral median line of the
body. Along this line, the epiblast becomes thickened in-
wardj and the thickening is converted into the ganglionic
chain. At the same time, each quadrate mass of the meso-
blast is excavated by the development of a cavity in its in-
terior, whereby it becomes converted into a sort of sac. The
adjacent anterior and posterior walls of successive sacs unite,
and give rise to the mesenteric septa, while their cavities
beconie the chambers of the perivisceral cavity. The seg-
mental organs commence as cellular outgrowths from the
posterior face of each septum thus formed, and only subse-
quently become excavated and communicate with the exte-
rior.

The development of the Earthvrorm, therefore, closely re-
sembles that of the Hlrudiiiea^ and more especially that of
the Medicinal Leech, in which the digestive cavity of the
embryo would seem to be formed, as in the Earthworm, by a
process which is, in a sense, invagination. It would appear
that the first-formed aperture is the mouth ; while the anus
is a secondary perforation; and the segmentation of the body
commences in the mesoblast.

In the fresh-water Oligoehceta^ Euaxes and Tuhifex^ the
vitellus also becomes divided into large and small blastomeres.
The latter extend over the larger blastomeres, and form the
epiblast (= ectoderm). A mesoblast (= mesoderm), divided
into two broad longitudinal bands, is developed, and the oral
cavity is said to be formed by invagination of the epiblast
between the anterior ends of the two bands of the mesoblast.
In this case, the mouth in these genera is a secondary forma-
tion. The innermost layer of large blastomeres becomes the
hypoblast (= endoderm).*

The PoLYcn.ETA. — Except that the Polychrpta are almost
invariably dioecious and marine, while the OVgochcEta are
monoecious, and inhabitants either of land or fresh water, it

1 Kowalewskv, " Embrjologisclie Studien." (" Momoires de 1' Acaddmie de
St. P^tersbourg," 1S61.)

200 THE ANATOMY OF INVERTEBRATED ANIMALS.

is hard to say what absolute characters separate these two
groups. The lowest forms of the Polycha^ta^ such as Capi-
tella and Polyophthahnus^ might be regarded as marine dioe-
cious KaidcB. But, in the higher Folychoeta^ each segment
of the body develops lateral processes — the parapodia^ or
rudimentary limbs, which are usually provided with abundant
strong set« ; a distinct cephalic segment, the p>rcestomii(m^
appears in front of and above the mouth, and bears eyes and
tentacles ; while those parapodia which lie in the vicinity of
the mouth may be specially modified in form and direction,
foreshadowing the jaws of the Arthropoda. Ciliated, som,e-
times plumose, processes of the dorsal walls of more or fewer
of the segments may perform the office of external hrancldoe j
and, occasionally, the dorsal surface gives rise to flat shield-
like processes, the so-called elytra.

The following detailed description of a very com.mon
species of Polynoe will give a fair conception of a pol3'ch8e-
tous Annelid, in which the highest degree of comj^lexity of
organization known in the group is attained :

Polynoe sqiiamata is an elongated vermiform animal,
about an inch long, the body of which is divided into a suc-
cession of portions, for the most part similar and equivalent
to one another, but presenting peculiar modifications at the
anterior and posterior extremities. Each such portion is
properly termed a somite j while the term "segment" may
be retained to indicate generally a portion of the body, with-
out implying its precise equivalency to one somite or to
many. Thus, then, the body of the Polynoe is composed of
a series of twenty-six "somites," terminated anteriorly by a
"segment," the prcestoynium ("Kopf-lappen," Grube), and
posteriorly by another, the pygidium^ which may or may not
represent single somites.

If one of the somites from the middle of thp body (Fig.
51, <7, Z>) be examined separatel}^ it will be found to be
transversely elongated, so as to be about three times as broad
as it is long, and to be slightly convex above and below,
presenting a deep, median, longitudinal groove inferiorly.
Laterally the somite is produced into two thick processes,
the '•'' parapodia, ^"^

Each parapodium divides at its extremity into two por-
tions, a superior and an inferior, which may be denominated
respectively the notopodium (Fig. 51, ^) and the neuropodiiim
(Jc)^ the one occupying the " ha?mal " or dorsal, the other the
"neural" or ventral aspect. The latter is, in this species

POLYNOE SQUAMATA.

201

so much the larger, that the notopodium appears like a mere
tubercle projecting from its upper surface. In other A?i?ie'
lida, however, and in the young state of Polynoe^ the notopo-

FiQ. hl.—Polynde squamata.

A. Viewed from above and enlarged : a. b, c, etc., as in Fi^. 53, 5; e, elytra ; /, space
left between the two posterior elytra ; g, sets and fimbriag oftlie elytra.

B. Po!?terior extremity, inferior view: (?, pygidial cirri; h, inferior tubercle; c,c^,
notopodial and neuropodial cirri.

O. Section of half a somite with elytron: i, notopodium; k, ncuropodium.
I). Section of half a somite with cirrus.

dium is as large as the neuropodium. Both divisions of the
parapodia are armed with peculiar stiff, hair-like appendages
{g)y composed of chitin, and developed within diverticula of
the integment, or trichophores^ in which their bases always
remain inclosed. These can be protruded and retracted by
muscles attached to their sacs, and they vary exceedingly in
form. Three distinct kinds are observable in Polynoe alone.
The notopodium and the neuropodium carry each a single,
sharp, style-like aciculum, the greater part of the length of
which is imbedded in the parapodium and its divisions, while
the point just projects at about the centre of the latter. The
neuropodial is very much longer than the notopodial aciculum.

202

THE ANATOMY OF IXVERTEBIUTED ANIMALS.

Superiorly, the notopodium carries two transverse rows of
more slender organs of a similar nature, the setoe : the proxi-
mal set are much shorter than the distal, but even the latter
do not attain a length of more than y^g of an inch (Fig. 52,

The proximal set are somewhat knife-like in shape if viewed
in profile, consisting of a comparativeh" short, straight "han-
dle," by which they are imbedded in their sacs, and of a thick,
rounded, curved blade, tapering to a fine point at its extrem-
ity. Close-set transverse ridges, finely serrated at their edges,
and inclined obliquely to the surface of the blade, traverse
its convex anterior circumference, leaving the back free. The
distal setae (Fig. 52, G) have a very similar structure, but they
are much elongated and very slender. The handle is longer;
and the blade, little curved and simply set on an angle with

f'?^3>^^

Fig. 5Z.—Po!ynoe sqnamata.

A, elvtron viewed from above. B. a tooth. C. D, nenropodial setae. F, F, part? of

the blade of the same, more highly ma;?nified. O, free extremity of a uotopodial

peta.

the handle, is produced at the end into a long and delicate
filament. The base of the blade {£J) is beset with incomplete

polynOe SQUAMATA. 203

ridges, like those of the short setse, but toward the middle
(I^) these ridges appear to encircle the blade completely, as-
suming the aspect of so many closely-imbricated concentric
scales, before finally becoming obsolete upon the extremity
of the seta.

The neuropodial aciculum needs no special notice, except
that the extremity of its trichophore projects as a sort of
papilla, less obvious in full-grown specimens, which divides
the neuropodium into an upper and a lower portion, the for-
mer containing about half as many setas as the latter. The
apertures of the trichophores are placed between lobe-like
prolongations of the neuropodium, to which the special term
of labia (Grube) may be applied. In this species they pre-
sent no remarkable peculiarity bej^ond their inequality.

The neuropodial set^e (Fig. 52, C, jD), although at first
sight very different from the notopodial setfB, are, in truth,
constructed on essentially the same plan, the blade being
short, while the handle is proportionally elongated. The
blade is subcylindrical at its base, pointed and slightly curved.
Eight or nine transverse ridges extend around about two-thirds
of the circumference of its proximal half ; the basal ridges
are narrow, and merely serrated, but toward the apex the
ridges become deeper, and the serrations pass into strong
teeth ; at the same time, one side of the ridge is elongated
into a strong point.

Attached to the under surface of the parapodium by a
somewhat enlarged base, with which it is articulated, is a
smooth, conical, very flexible filament — the neuropodial ciV'
rus (Fig. 51, c') ; it hardly reaches to the end of the neuro-
podium. Again, springing from the neural surface of the
somite, close to the parapodium, there is a small pyriform
tubercle (A), divided by longitudinal grooves into about eight
segments. This is possibly connected with the reproductive
function.

The appendage of the notopodium, or rather of the noto-
podial side of the parapodium and somite, varies according
to the particular somite which may be examined. In some
somites this appendage is a cirrus (Fig. 51, 7), c) similar to
the neuropodial cirrus, but much larger, equaling the semi-
diameter of the body in length, and presenting an enlarged
pigmented bulb of attachment to which the filament of the
cirrus, which is cylindrical for about two-thirds of its length,
and then becomes enlarged and suddenly tapers to its extrem-
ity, is articulated.

304 THE ANATOMY OF INVERTEBRATED ANIMALS.

la the other somites the notopodial appendage is a large,
thin, oval plate — the elytron (Fig. 51, O, c). It is attached
by a thick peduncle, and has its long axis directed obliquely
outward and backward. The surface of the elytron (Fig. 52,
A) is covered with an ornamentation of larger or smaller
tubercular prominences, granulated and ridged upon their
surface. A part of the inner and anterior edge of each ely-
tron overlaps or is overlapped by its fellows for a certain ex-
tent of its circumference, which is so far smooth, but in the
rest of its extent it is fringed with coarse brownish filaments
ov Jimbrice, which arise from the upper surface just within the
edge, and are obviously outgrowths of the same order as the
tubercles.

Such is the structure of one of the middle somites of
Polynoe squamata. The anterior and posterior somites, with
the exception of the first and second, present only minor dif-
ferences, as in the proportion of the setse, or in the figure of
the elytra. The first somite, which contains the mouth, is the
peristomiwn (" Mund-Ssgment " of Grube). The parapodia
of this somite are narrow and elongated (Fig. 53, J3, (7, m) ;
they are obscurely divided at their extremity into a rudimen-
tary neuropodium and notopodium, and give attachment to a
pair of large peristomial cirri (c' c) (" cirrhes tentaculaires,"
Audouin and Milne-Edwards ; " Fiihler-cirren," Grube), of
the same structure as the notopodial cirri, which stretch for-
ward by the sides of the mouth.

Tiie apex of a single small aciculum issues rather above
the point of division of the peristomial parapodium, and two
minute curved setae accompany it. These have been generally
overlooked ; ^ but they seem to demonstrate, in a very inter-
esting manner, the nature of the appendages of the peristo-
mial searment.

The second somite differs from the rest only in the great
elongation of its neuropodial cirrus, which is directed forward
and applied against the mouth.

The peristomium and the prsestomium together are ordi*
narily confounded under the common term of " head." The
latter (Fig. 53, ^, C, I) is an oval segment flattened superior-
ly, placed altogether in front of and above the mouth, pre-
senting on its post ero -lateral edges four dark spots, the eyes,
and possessing five cirriform appendages, two pairs and a

* At least, in the descriptions of the adult Poli/noe. They are particularly
mentioned, however, by Max Miiller in his valuable paper, " Ueber die Ent-
wickelung und Metamorphose der Polynoen." {MuUer^s Archiv^ 1841.)

POLYNOE SQUAMATA.

205

single median one. The latter (a), or the 2:)rce^tomial tentacle
(" antenne mediane," Milne-Edwards), is similar in structure
to an ordinary cirrus. Of the other appendages, the upper
one upon each side (supero-lateral prsestomial cirrus, " an-
tenne mitoyenne ") also resembles an ordinary cirrus (b) ; but
the lower (infero-lateral prsestomial cirrus, " antenne ex-
terne ") (b') is much larger, and is capable of extreme elon-

FiG. 5S.—Pol7jnoe sqnamafa.

A. Posterior extremity from above : c, notopodial cirrus of last somite; d, pygidial
cirri ; x, anus.

B. Anterior extremity from above : a, prsestomial tentacle ; b, superior and b'
.inferior prasstomial cirrus ; c, c', notopodial and neuropudial cirri ; e, peduncle
of first elytron ; L prsestomium ; /«, parapodium of pcrlstomium. C. Inferior
view of anierior extremity, letters as before.

gation and contraction,* while the ordinary cirri are merely
flexible. Altliough at first sight probable, yet it would ap-
pear, from Max Muller's account of the development of Poly-
noe^ that these two appendages do not, like the two peristo-
mial cirri which they essentially resemble, correspond with
the notopodial and neuropodial cirri of a single parapodium,
inasmuch as they arise from perfectly distinct portions of the
pra^stomium. It is very possible that each represents the
appendage of a somite, and in this case the praestomium
w^ould be composed of at least two somites. Whether the
prsestomial tentacle indicates another, or w'hether it is merely

* I have never o"b*;er\-ed any invagination such as is stated to occur bv
Audouin and Milne-Edwards, 1834. (" Histoire Naturelle du Littoral de la
France," p. 10.)

206 THE ANATOMY OF IXVERTEBRATED ANIMALS.

an appendage of such a nature as the labrum or the rostrnm
of a Crustacean, there is no evidence at present to show.

It is highly interesting to remark that thus, in the Poly-
noe, as in the Arthropoda^ the *'head " results from the modi-
fication of a number of somites, some of which lie in front of,
and others behind, the mouth. The movements and evident
extreme sensitiveness of the inferior praestomial cirri during
life indicate that they perform the functions, as well as occupy
the position, of antennae.

The hindermost segment of the body, or pygidimn (Fig.
51, -S, Fig. 53, A)^ is narrow, and divided at the end into two
supports for the pygidial (d) cirri which are as long as the
three last somites, and resemble the notopodial cirri in form
and structure. They extend directly backward, almost paral-
lel with one another, and with the notopodial cirri of the last
somite, which are thrown backward and downward (Fig. 53,
A^ c). It seems probable that the pygidium represents only
a siuQ-le somite.

The anus is not terminal, as in many Annelids, but is
seated in the middle of a strongly-raised papilla (Fig. 53,
A^ cc), which projects from the dorsal surface of the penulti-
mate somite ; its sides are produced into about fourteen folds.
The two last elytra have their edges excavated, so as to leave
a space over the anus (Fig. 51, A^f).

The notopodial cirri and the elytra do not coexist upon
the same somites ; and the order of arrangement of the ely-
trigerous and cirrigerous somites is very curious. The 1st or
peristomial somite is cerrigerous, and so are the 3d, 6th, 8th,
10th, 12th, 14th, 16th, 18th, 20th, 22d, 24th, 25th, and 26th ;
while the 2d, 4th, 5th, 7th, 9th, 11th, 13th, loth, 17th, 19lh,
21st, and 23d, somites bear elytra, making twelve pairs in all.

In no polychaetous Annelid is the structure of a somite
more complex than in Polynoe / and there are but very few
parts not found in Polynoe to be met with in other Annelida,
The careful study of this species, therefore, furnishes us with
an almost complete nomenclature for the external organs of
the whole group ; and it will be found that the other forms
of Annelida differ mainly in the greater or less development
and modification of the organs which have just been de-
scribed. A large proportion of the Polychata are like Poly-
noe, free and actively locomotive animals, which rarely fabri-
cate tubular habitations, and are therefore termed Errantia ^
they possess a praestomium, usually provided with eyes and
feelers, and have many parapodia, which are not confined to

THE POLYCHJ^TA. 207

the anterior region of the bod}-. They very generally have a
proboscis, provided with chitinous teeth.

The singular genus, Tomopteris, is a transparent pelagic
Annelid, with numerous parapodia, each terminated by two
lobes representing the neuropodium and notopodium, but
with setae, two of w^hich are very long, only in the cephalic
region.

The sedentary Annelids [Tuhicola) fabricate tubes, either
by gluing together particles of sand and shells, or by secret-
ing a chitinous or calcified shelly substance, in which they
remain (e. g., Protula^ Fig. 54). The praestomium is small or
wanting ; none have a proboscis ; there are no cirri ; and the
parapodia are short or rudimentary. The branchiae are devel-
oped only on the anterior somites, and the latter are often
markedly diflferent from those which constitute the posterior
part of the body.

In some (/Serpuliclce) a tentacle is enlarged and its end
secretes a shelly plate which serves as an operculum, and
shuts down over the mouth of the calcareous tube inhabited
by the animal, when it is retracted. The dilated end of the
opercular tentacle sometimes serves as a chamber in which
the young undergo their development (species of Spirorhis),

The alimentary canal of the polychaetous An7ielida rarely
presents any marked distinction between stomach and intes-
tine, and is almost always of the same length as the body, ex-
tending, without folds or convolutions, from its anterior to
its posterior extremity; but in Siphonostomum (Chloroemd)^
Pectlnaria and others, it is more or less convoluted. It is
attached by membranous bands, or more complete mesenteries,
to the walls of each somite, and very commonly presents a dila-
tation between every pair of mesenteries. In most Polychceta,
the intestine acquires in this wav merely a moniliform appear-
ance, but in Polynoe^ Aphrodite^ Sigaliony and their allies,
long cpeca are given off upon each side of the alimentary
canal, and, sometimes becoming more or less convoluted, ter-
minate at the upper part of each segment (Fig. 51, D) close
beneath, or in the branchiae, w^iere such organs exist.

The anterior portion of the alimentary canal is, in a great
number of the Polychceta^ in fact in all the typical Errantia,,
so modified as to constitute a distinct muscular pharynx, the
anterior portion of the wall of which can be everted like the
finger of a glove, from the aperture of the mouth, and the
posterior portion protruded, so as to form a proboscis. In
Polynoe squamata, the proboscis is one-fourth as long as the

208 THE ANATOMY OF INVERTEBRATED AKIMALS.

Fig. bi—Protida Thjsteri. A, the Bexual. mature animal, extracted from its calca-
reous tube : a, branchial plumes ; b, hood-like expansion of the anterior end of the
bofly : c, the mouth ; c?, the stomach ; e, the anu^ ;/, the testos ; g^ the ova. B^
a Protula in course of proliferation ; &, the branchiae of the zo5id,

bod}', and its walls are very thick and muscular. At its an-
terior extremity it is surrounded with a circle of small papil-
lae, immediately behind which are four strong, pointed and
curved horny teeth, implanted in the muscular wall (Fig. 52,

THE POLYCHiETA. 209

S). Each tooth has a little projection upon its convex edge,
which is connected by a short strong ligament with the cor-
responding projection of another tooth ; and the one pair of
teeth, thus connected, works vertically against the opposite
pair. In JSTereis, there are two powerful teeth which work
horizontally, besides minute accessory denticles. In Syllis,
the chitinous lining of the pharj'nx is produced into a circle
of sharp teeth anteriorly, and there is, in addition, a much
stronger triangular median tooth. In Glycera, which pos-
sesses a pair of teeth, the extremity of the protruded pro-
boscis is covered with very remarkable papillae. The most
complex arrangement of teeth, however, is that presented by
the EunicidiB. In Eunice^ there are altogether nine distinct
pieces : two large, flat, more or less calcified portions united
together below, and three cutting and tearing teeth on the
right side working against four on the left. As has has been
already stated, the tubicolar Annelids possess neither probos-
cis nor teeth.

No special hepatic gland appears to exist in the Annelida^
unless the intestinal ca?ca perform that function, and the
secretion of the bile is doubtless effected by the glandular
tract, which extends for a greater or less distance in the walls
of the alimentary canal. A pair of glandular CtBca, the func-
tion of which is not known, is appended to the base of the
proboscis in Nereis.

The general cavity of the body, or perivisceral cavity,
which is included between the parietes of the alimentary
canal and those of the body, is filled with a fluid which con-
tains corpuscles, which are usually, as in the Invertebrata in
general, colorless. They are red, however, in Glycera, and
in a species of Apneumea (De Quatrefages). The parapodia,
the cirri, the branchiae, and all the other important appendages
of the PolycJiceta^ contain a cavity continuous with the peri-
visceral cavity, and are therefore equally filled with the blood.
The circulation of this fluid is effected partly by the contrac-
tion of the body and its appendages, partly by the vibratile
cilia, with which a greater or less extent of the walls of the
perivisceral cavity is covered.

In a great number of the FolychfMa no part of the body
is specially adapted to perform the function of respiration,
the aeration of the blood probably taking place w^ierever
the inteofument is sufficientlv thin ; and, even when distinct
branchiae ordinarily exist, members of the same family may
be deprived of them. In Polynoe squamata, ciliated spots

210 THE ANATOMY OF INVERTEBRATED ANIMALS.

which appear to represent branchiae, may be discovered on
the dorsal side of the bases of the parapodia, at any rate, in
youno- specimens. In some species of Pohjnoe the parapodia
ffive °ise, at corresponding points, to large, richly cihated,
malleiform tubercles, in which the caeca of the ahraentary
canal terminate. In Slgalion, a filiform, cihated brancbia
depends from the upper part of the somite, beneath the ely-
tron • and, besides this, curious little ciliated palettes are
arrano-ed upon the dorsal surface of the parapodia, and upon
th- sfd-s of the anterior somites. But the best-developed
brinchiie among these Annelids are possessed by the Anqylii-
nomldx, and the JEimicld'B among the Errant la; the 'Jere-
hdUd%, and the Sirpulid:^ among the Tuhicola. In the
three former families the branchiae are ciliated branched
plumes, or tufts, attached to the dorsal surface of more or
fewer of the somites. In the last (Fig. 54) they are exclu-
sively attached to the anterior segment of the body, and
present the form of two large plumes, each consisting of a
principal stem, with many lateral branches. The stem is
supported by a kind of internal skeleton, of cartilaginous
consistence, which sends off processes into the lateral branches.

I have been unable to find any pseud-haemal vessels in
Polynoe squam'-xta, and, as Claparede ' could discover none in
the transparent P. litmdata, it is safe to assume their non-
existence. Claparede, in fact, denies them to the whole of
the Aohrodltldoe.

When it is present, the pseud-haemal system varies very
much in the arrangement of its great trunks ; but they com-
monly consist of one or two principal longitudinal dorsal and
ventral vessels, which are connected in each somite by trans-
verse branches. Where branchise exist, loops or processes of
one or other of the great trunks enter them. The dorsal and
the ventral trunks are usually rhythmically contractile, and
contractile dilatations at the bases of the branchiae (Eunice),
in portions of the lateral trunks {Are?iicola), or in those
which supply the proboscis (Eunice, Nereis), have received
the name of " hearts." The direction of the contractions is
usually such that the blood is propelled from behind forward
in the dorsal vessel, and in the opposite direction in the ven-
tral vessel ; but the course which it pursues in the lateral
trunks is probably very irregular. In Chlorcema, in which
even the smallest ramifications of the vessels are contractile, I

» "Annelides Ch^topodes du Golfe de Naples," 1868, p. 65.

THE POLYCH^TA. 211

have observed c.'ecal branches depending into the perivisceral
cavity in which the contained fluid underwent merely an alter-
nate flux and reflux. Ramified caeca of a similar kind appear
to exist in the oligochsetous genera, Euaxes and Lumbriculus.
The principal trunks give otf a great number of branches,
which ramify very minutely in some Annelids [Euyiice) and
may give rise to retia mirabilia {Nereis) ; but in many (e. g.,
Protula) there are hardly any branches and no minute capil-
lary ramifications.

In many Folychmta no segmental organs have yet been
discovered, and in others they appear to be represented by
mere openings in the parietes of the body. I have observed
short ciliated canals opening externally upon the ventral sur-
face at the bases of the parapodia in Phyllodoce viridis, and
there are indications of the existence of similar organs in
Syllis vittata. True segmental organs have, however, been
found by Ehlers and Claparede in many JPolychceta. In some
cases their walls are thick and glandular, and they probably
have a renal function. In addition, they frequently play the
part of oviducts and spermiducts. "Whether the ciliated canal
extending along the ventral surface of the intestine, which I
have described in Protula, is a structure of the same order or
not. I am not prepared to say.

The nervous system of the Polychceta usually consists of
a chain of ganglia — one pair for each somite — connected
together by "longitudinal and transverse commissural bands,
which diverge between the cerebral ganglia and the succeed-
ing pair, to allow of the passage of the oesophagus. The most
important diff'erences presented by the nervous systems of the
Polychosta result from the varying len<rth of the transverse
commissures. In Vermilia, Serpula, Sahella, these commis-
sures are very long, so that two distinct and distant series of
ganglia appear to run through the body, while, in JVepthys,
the two series of ganglia are fused into a sina^le cord enlarged
at intervals. Every transitional condition between these is
observable in Terehella^ Aonia, Glycera^ Phyllodoce^ and
Aphrodite. In most Polychrpta a very extensive series of
visceral nerves supplies the alimentary canal.

The recognizable organs of sense in the ArineUda are eyes
and auditory vesicles. The former are usually very simple,
consisting of an expansion of the extremity of the optic nerve,
imbedded in pigment, and provided occasionally, but not in-
invariably, with transparent spheroids or cones. Alciope and
Torrea have very well-developed and large eyes. The eyes

212 THE ANATOMY OF IXVERTEBRATED AXIMALS.

are usually confined to the anterior extremity of the body, and
to the prc^stomium where it exists ; but, iiQ the remarkable
genus Polyophthalmus, De Quatrefages discovered, besides

—c

Fig. 55. — A, anterior end of the nerrous system of Tclynde sq-uamcfa (aftr-r De Qna-
trefagest : o, cerebral yanfrlia ; b, oesi)phai;eal cominissures ; e. !oi,<riiuaiual com-
missures of the venrral ganglia.

B, anterior end of the nervous system of SaMla fiabellcta (after De Quatrefages) : o,
cerebral gan^rlia; b, oesophageal commissures ; c.loniritr.dinal commissures of the
venrral giinglia. Those of opposite sides are united by long transverse commie-
Eures.

the ordinary cephalic eyes, a double series of additional visual
organs, one pair being allotted to each somite. In 13ran-
chiotnma, eyes are situated at the ends of the branchial
plumes. Ehrenberg has described two caudal eyes in Ay/iphi-
cora, and De Quatrefages has shown that similarly placed
eyes exist in three other species of PolychcpAa^ two of which
are closely allied to Amphicora, while the other is an errant
form, related to Lvmhrinereis. These curious worms are said
to swim about with the caudal extremity forward.

Auditory sacs, containing m,any otoliths, have been ob-
served upon each side of the oesophageal ring in Arenicola^
and similar organs have been noticed in other Tiibicola ; but
hitherto their existence has not been certain]}'" determined in
the Kr rant la.

The crenitalia of the polychaetous Annelida are excessively
simple in their structure ; indeed, special reproductive organs
can hardly be said to exist in most, the generative products

THE DEVELOPMENT OF THE POLYCHJETA. 213

being merely developed from some part of the walls of the
perivisceral cavity, in which thev eventually freely float, mak-
ing their way out in a manner which is not quite understood
at present ; probably, however, through temporary or perma-
nent apertures at the bases of the parapodia. In many, the
segmental organs appear to serve as excretory ducts. As a
rule, the polychaetous Annelids are dioecious ; but some (e. g.,
Protula^ Fig. 54) are hermaphrodite. The ova undergo their
development within the body of the parent in some species
of EiUiice y in pouches attached to the body in Exogone / in
masses of gelatinous matter which adhere to the tubes of the
vermidom in Protula j beneath the elytra in Polynoe cir-
rata / in the cavity of the opercular tentacle in some Spii'-
orhes / while, in other cases, they appear to become, almost
immediately, free ciliated embryos.

The vitellus undergoes division, and is converted, as in
the case of the Oligochceta and Ilirudmea, into blastomeres
of two kinds. This contrast between the two components of
the embryo commences with the division of the vitellus into
two, inasmuch as the first fissure is usually so directed as to
divide the yelk into unequal portions. Both subdivide, but
the smaller much faster than the larger ; so that the former
becomes converted into very small blastomeres, which grad-
ually envelop the larger blastomeres resulting from the sub-
division of the latter. The larger included blastomeres are
destined to form the alimentary tract ; the smaller peripheral
ones, on the other hand, give rise to the ectoderm, and to the
nervous ganglia.' As in the Ollgochmta and Ulrudlnea,
again, the mesoblast forms a thick band on each side of the
median ventral line, and its transverse division originates the
segmentation of the body. But, generally, the development
of the 2:>rotosomUes, as these segments might be called, does
not occur until some time after the embryo has been hatched.
The somites increase in number by the addition of new ones
between the last and the penultimate somite.

The embryos of the Polychoeta differ from those of the Oli-
goch(Bta and Hlrudinea in being ciliated. In some cases, the
cilia form a broad zone which encircles the body, leaving at
each end an area, which is either devoid of cilia, or, as is fre-
quently the case, has a tuft of long cilia at the cephalic end.
Such larvre are termed Atrocha.

In other embryos the cilia are arranged in one or more

1 Claparede and raetschnikofF, " Beitrage zur Kenntniss der Entwickelungs-
geschiclite der Chaetopoden," 18G8.

214 THE ANATOMY OF INVERTEBRATED ANIMALS.

narrow bands, which surround the body. A very common
arrangement is one in which a band of cilia encircles the body
immediately in front of the mouth, the region in front of the
band bearing eyes, and becoming the praestomium of the adult
(e. g., Polynoe). In such embryos, there is very commonly a
second band of cilia around the anal end of the embryo, and
a tuft of cilia is attached to the centre of the praestomium.
These larvae are called Telotrocha. In other cases, one or
many bands of cilia surround the middle of the body, between
the mouth and the hinder extremity. These are Mesotrocha.

In the telotrochous larva of JPhyllodoce, a shield-shaped,
mantle-like elevation of the integument covers the dorsal
region of the body behind the prae-oral ciliated ring. In the
larvae of the Serjndidce a process of the integument grows
out behind the mouth, and surrounds the anterior part of the
body of the larva like a turned-back collar. It persists, as a
kind of hood, in the adult.

Some larvae are provided with setse of a different charac-
ter from those which are possessed by the adult, and which
are cast off as development advances.

Many Polychmta multiply by a process of zooid develop-
ment, which, in some cases, appears to be a combination of
fission with gemmation ; in others, to approach very nearly
to pure fission or pure gemmation. The result is, not infre-
quently, the formation of long chains of connected zooids.

The method of multiplication which De Quatrefages ob-
served in SylUs prollfera., is nearly siniple fission, the animal
dividing near its middle, and the posterior division acquiring
a new head.

In Myrianida., Milne-Edwards has described the occur-
rence of a sort of continuous budding between the ultimate
and penultimate segments, in which region new segments are
formed until the zooid has attained its full length.

Frey and Leuckart and Krohn have shown that Autolytus
prolifer multiplies in a somewhat similar manner ; but, in-
stead of each new zooid being formed at the expense of an
entire somite, it is developed from only a portion of one.
Finally, I found in Protula Dysteri that, when the Protida
had attained a certain lensrth, all the somites behind the six-
teenth became eventually separated as a new zooid ; but the
development of the latter is not mere fission, inasmuch as one
of the earliest steps in the process is the enlargement of the
seventeenth somite, and its conversion into the head and

AGAMOGENESIS AMONG POLYCH^TA. 215

thorax of the bud (Fig. 54, B). Sars has described a similar
mode of multiplication in his iilograna implexa^ a very close-
ly allied form.

In SylUs and in Protula, the producing and the produced
zooids alike develop generative products, but, in Autolyhis^
Krohn has shown that the primary producing zouid remains
sexless, the secondary produced zooids having a somewhat
different form, and alone giving rise to ova and spermatozoa.

In some species of the genus JVereiSy the worm, after the
development of its genital organs has taken place, takes on
the characters of what was formerly considered a distinct
genus, ITeterotiereis ; and the males and the females of the
same species of Nereis have even been regarded as different
species of Setero nereis.^

The series of forms represented by the Tiirhellaria^ the
Jlirudinea, the Oligochceta, and the Polychata^ illustrates
the manner in which a type of organization, which, in its
simplest condition, exhibits but little advance upon a mere
Gastrula, passes into one in which the body is divided into
manv segments, each provided with a pair of appendages or
rudimentary limbs.

The segmentation, or serial repetition of homologous
somites, extends to the nervous system, and, more or less, to
the vascular and reproductive organs, in the higher forms of
these "Annulose" animals; from which a further extension
of the same process of segmentation, with a fuller develop-
ment of the appendages and a more complete appropriation
of some of them to manducatory purposes, leads us to the
Arthropoda.

The Gephyrea. — These are marine vermiform animals
without distinct external segmentation or parapodial append-
ages. The ectoderm has a chitinous cuticle, and is often
provided with tubercles, hooks, or set^e, of chitin i^EcMurus^
Sternaspis). No calcareous skeleton is found in any of the
Gephyrea. The integument frequently contains numerous
simple glands, the apertures of wliich perforate the cuticle.
In one genus i^Sternaspis)^ two shield-shaped plates, fringed
with setae, are developed upon the hinder part of the ventral
surface bf the body. There are external circular and internal
longitudinal muscular fibres beneath the ectoderm. An inner

1 Ehlers, " Die Gattung Heteronereisy (" GGttlngen Nachrichten," ISCr.)

216 THE ANATOMY OF INVERTEBRATED ANIMALS.

layer of circularly disposed muscular fibres may be added.
The oral end of the body may have the form of a retractile
proboscis (I^riapiilus), or be provided with tentacular append-
ages. These may be arranged in a circle round the mouth,
and short (Slpicncidus, Fig. 56, L, T), or long (Phorojiis)^ or
there may be a single long, sometimes bifurcated and ciliated,
tentacular appendage {£onellia). Filamentous appendages,
which are probably branchiae, are given off at the hinder end
of the body in Sternaspis and JPriapulus. The endoderm is
usually ciliated throughout. The intestine is straight in most
genera, but is coiled and bent upon itself, so as to terminate
in the middle of the body, in Sipunculus (Fig. 56, I.). In
Phoronis the anus is close to the mouth. The anal aperture
is always situated upon the dorsal aspect of the body. There
is a spacious perivisceral cavity, undivided by mesenteries,
which in some cases (Priapulus^ Sipunculus) opens externally
by a terminal pore. In Echiuriis^ JBonellia, Thalassema^ a
pair of tubular, sometimes branched organs, which are ciliated
internally, and communicate by ciliated apertures with the
perivisceral cavity, open into the rectum. These appear to
represent the water-vessels of the Itotifera and the respira-
torv tubes of the Holothurioe.

A pseud-hsemal system exists in most [SijDunculus^ Sternas-
pis, PoneUla, JEJjhiurus, and Phoro7iis), and, when fully devel-
oped, consists of two longitudinal trunks — one dorsal, or su-
pra-intestinal, the other ventral, with their terminal and lateral
communications. The pseud-hremal fluid is colorless, or may
have a pale reddish tinge, in most. In Phoro7iis it is said to
contain red corpuscles. In Sipunculus, the cavities of the
tentacles communicate with a circular vessel provided with
cascal appendages ; and this circular vessel is said to open
into the pseud-htemal vessels.

The nervous system presents a collar, which surrounds the
oesophagus, and from which a simple or ganglionated cord
proceeds backward in the ventral median line, giving off lat-
eral branches. The ventral cord contains a central canal, and
the collar usually presents a cerebral ganglionic enlartrement.
Rudimentary eyes are sometimes connected with the cerebral
ganglion.

The sexes are distinct, and the reproductive elements are
developed either from the parietes of the perivisceral cavity
or in simple ccecal glands. In Sipunculus, the ova and sper-
matozoa float freely in the perivisceral cavity.

The actively locomotive embryo of Sipunculus (Fig. 56, II.)

THE GEPHYREA.

217

is surrounded by a circular band of cilia placed immediately
behind the mouth ( W] TF), and resembles a Rotifer or a meso-
trochal Annelidan larva. As developuient advances it loses

Fig. 5Q.—Sipunculus nudus (after Keferstein and Ehlers).^

I. The animal laid open longitudinally — | n. s. T, tentacles; r, the four retractor
muscles of the proboscis; r, the points at which they were*attached to the walls
of the body ; te, oesophagus ; i. intestine; a, anu:? ; X J\ loops of the intestine ;
«, y, appendages of the rectum; 2, fusiform muscle; w, ciliated groove on the
inner side of the intestine ; g, anal muscles ; s, csecal glands ; t. cseca which open
on each side of the nervous cord, and are gonerally considered to be testes ; p,
pore at the hinder end of the body; ??, nervous cord, which ends in a Inbed gan-
glionic mass, close to the mouth, and presents an enlargement, g\ at its poste-
rior end ; wi, m', m". muscles associated with the nervous cords.

II, A larval Sipunculus about rV of an inch long: 0, mouth ; ce, gnllet ; 8, csecal
gland; t. intestine with masses of fatty cells ; a, anus ; w, ciliated groove of the
intestine ; g. brain with two pairs of red eye-spots ; », nervous cord, p, pore;
t, t, so-called testes ; TF, TF, circlet of cilia.

this apparatus, and passes gradually into the adult form. In
PJioronis, the embryo is also mesotrochal, but it has two
ciliated bands, one circular, round the anus, and the other im-
mediately behind the mouth. The post-oral band of cilia is
produced into numerous tentaculiform lobes, and fringes the
free edge of a broad concave lobe of the dorsal side of the
body, vvhich arches over the mouth. In this state the embrj^o

1 "

Zoologische Beitrage," 1861.

10

218 THE ANATOMY OF INVERTEBRATED ANIMALS.

is the so-called Actinotrocha.^ An invagination of the ven-
tral integument of the larva connects itself with the middle
of the intestine, and then, becoming evaginated, pulls the in-
testine, in the form of a loop, into the ventral process thus
formed, which gives rise to the body of the Fhoronis^ while
the tentacles of the larva grow into those of the adult.
Schneider has suggested that the bell-shaped larva, with long
seta3, termed Mitraria by Miiller, is the embryo of Sternaspis.
The affinities of the Gephyrea with the Turhellaria^ with
the Annelida^ and with the JRotifera^ are unmistakable. In
fact, it may be doubted whether Sternaspis should not be
associated with the Polychoeta, and £o7iellia is in many re^
spects comparable to a colossal Rotifer. Their usually as-
sumed connection with the Echinodermata is more question-
able. The circular canal which communicates with the cavi-
ties of the tentacles in Sipunculiis has been compared to the
ambulacral system of the Echinoderms, but the manner of
its development is not yet sufficiently understood to justify
the expression of an opinion on this subject. Krohn has de-
scribed a bilobed organ on the ventral face of the gullet of the
larva of Sipunculus, which opens externally in front of the
ciliated band by a narrow ciliated duct ^ (Fig. 56, II., S). It
has a striking similarity to the " water-vessel " of the larva
of £alanoglossus, which, however, lies on the opposite side
of the body.

1" Schneider, "TJeber die Metamorpliose der Aciinotrocha Iranchiata.''^
(" Archiv fur Anatomie," 1862.)

" " Ueber die Larve des Si^vnculus nvdusy (" Archiv fiir Anatomic,"
1851.)

CHAPTER VI.

IHE AKTHROPODA.

The segmentation of the body, that is, its division into
a series of somites, each provided with a pair of lateral ap-
pendages, which is so characteristic a feature of the higher
Annelids, is exhibited in a still more marked degree by the
Arthropoda. In these animals, moreover, the appendages,
themselves are usually divided into segments, while one or
more pairs of the appendages in the neighborhood of the
mouth are modified in form and position to subserve man-
ducation. Segmental organs, at least in their Annelidan
form, are wanting in the Arthropoda, and neither in the em-
bryonic nor the adult condition do they ever possess cilia.

The process of yelk-division may be complete or incom-
plete, but no known Arthropod ovum gives rise to a vesicular
morula, nor is the alimentary cavity ordinarily formed by in-
vagination/ The precise mode of origin of the mesoblast
has yet to be worked out, but the perivisceral cavity appears
always to be developed by its splitting. In other words, it is
a schizocoele.

As with Annelids, the segmentation of the body results
from the subdivision of the mesoblast by transverse constric-
tions into 2yrotosomites ; and there is every reason to believe
that the ganglionated nervous chain arises from an involution
of the epiblast.

The neural face of the embryo is fashioned first, and its
anterior end terminates in two rounded expansions — the j^ro-
cephcdie lobes — which are converted into the sides and front
of the head. The appendages are developed as paired out-

» The recent observations of Bobretzky on the development of Oniscus and
Astacm (Hofmann and Schwalbe, " Jahresberichte," Bd. ii., 1875), however,
tend to show that the hypoblast arises by a sort of modified invasrination of
the primitive blastoderm. And in other Arthropoda there are indications of a
similar process.

220 THE ANATOMY OF INVERTEBRATED ANIMALS.

growths from the neural aspect of each somite, and, whatever
their ultimate form, they are, at first, simple bud-like pro-
cesses. Very generally, a broad median prolongation of the
sternum of the somite which lies in front of the mouth gives
rise to a labrum ; while a corresponding, but often bifid me-
dian elevation, behind the mouth, becomes a metastoma.

In many Arthropods, the haemal or tergal face of the body
grows out into lateral processes, which may either be fixed,
or more or less movable. The lateral prolongations of the
carapace in the Critstacea and the wings of J?isecta are
structures of this order.

In a number of Insects belonging to different orders of
the class, an amnionic investment is developed from the
extra-neural part of the blastoderm by a method similar to
that which gives rise to the amnion in the higher Vertebrata,

In all the higher Arthropods, a certain number of the
somites which constitute the anterior end of the body coa-
lesce and form a head, distinct from the rest of the body ;
and the appendages belonging to these confluent somites un-
dergo remarkable modifications, whereby they are converted
into organs of the higher senses and into jaws. In many
cases, the somites of the middle and posterior parts of the
body become similarly difi'erentiated into groups of poly-
somitic segments, which then receive the name of thorax and
abdomen. The somites entering into each of these groups
may remain distinct or may coalesce. The tergal expansions
of the somites of the head, or of both head and thorax, may
take the shape of a broad shield, or carapace. This may con-
stitute a continuous whole (e. g., Apus, Astacns) ; or its two
halves may be movably connected by a median hinge, like a
bivalve shell [Cypris^ Lhnnadia) ; or, finally, the tergal pro-
cesses of each side may remain distinct from one another and
freely movable on their respective somites (wings of In-
sects).

Limbs, or appendages capable of effectino;* locomotion, are
always attached either to the head or to the thorax,^ or to
both. They may be present or absent in the abdominal re-
gion. In adnlt Arachnida and Insecta^ there are no abdomi-
nal limbs, unless the accessory organs of generation, the stings
of some insects, and the peculiar appendages of the abdomen
in the Thysaniira and CoUembola, be such.

The alimentary apparatus presents very wide diversities

1 The extinct Trilohites possibly form an exception to this rule.

THE ARTHROPODA. 221

in form and structure, and in the number and nature of its
glands. The anus, which is very rarely absent, is situated in
the hindermost somite.

In like manner, the blood-vascular system varies from a
mere perivisceral cavity without any heart ( Ostracoda, Cirri-
pedicC) up to a complete, usually many-chambered heart with
well-developed arterial vessels. The venous channels, how-
ever, always have the nature of more or less definite lacunae.
The blood-corpuscles are colorless, nucleated cells.

Special respiratory organs may be absent, or they may
take one of the following forms :

1. Branc1ii<x, Externally projecting processes of the
body or limbs, supplied with venous blood, which is thus
brought into contact wdth the air dissolved in water.

2. Tracheae. Tubes w^hich traverse the body and gen-
erally open upon its exterior by apertures termed stigmata, u«x,^
and thus bring air into contact with the blood and the tissues
generally. Saccular reservoirs of air are often foim.ed by
dilatations of these tubes.

The so-called Tracheo-hranchim of some aquatic Insect
larvae are usually laterally projecting processes of more or
fewer of the thoracic or abdominal somites, containing abun-
dant tracheae, which communicate with those which traverse
the body [^Ephemeridce, Perlaridaf). They are in no sense
branchiae, but simply take the place cf stigmata. The ex-
changee of constituents between the air contained in the
tracheae of these animals and that of the surrounding medium
is effected indirectly, by diffusion through the walls of the
tracheo-branchia?, instead of directly, through the stigmata,
as in other cases.

In the aquatic larvre of many Dragon-flies (Zibelhdidc^),
the function of the tracheo-branchiae is performed by folds of
the lining membrane of the rectum, which contain abundant
tracheae. Water is drawn into, and expelled from, the cavity
of the rectum by rhythmical contractions of its walls, so as
to secure the exchanjre of g-aseous constituents between the
air which it contains and that v.'hich fills the tracheae.

3. Pulmoiiary sacs. These are met with only in some
Arachnida. They are involutions of the integument, the
walls of w^hich are folded in such a manner as to expose a
large surface to the air, which is alternately taken into, and
expelled from, their apertures. The blood is brought to these
sacs by venous channels.

The exact mode by which the separation of the nitro-

222 THE ANATOMY OF IXVERTEBRATED ANIMALS.

genous products of the waste of the tissues from the blood
is effected in Arthropods requires further elucidation. In
many, however, such products, notably uric acid, have been
found to abound in the co'rpus adiposum — a cellular mass
which lies in the walls of, and more or less fills, the peri-
visceral cavity — and in the Malpighian glands. In the latter
case, thev are conveyed out of the body by the intestine.

The nervous system consists primitively of a pair of gan-
glia for each somite, but the number of ganglia discoverable
in the adult depends on the extent to which these primitive
ganglia coalesce. There is usually, if not always, a well-
developed system of ganglionated visceral nerves, connected
with the cerebral ganglia and distributed to the gullet and
stomach.

Eyes are usually present ; and, when they exist, they are
almost always situated in the head and are connected with
the cerebral ganglia. Among the Crustacea, however, JEu-
2)hausia has eyes in some of the thoracic limbs, and in some
abdominal somites. The eyes may be simple or compound.
In the latter case there are, in correspondence with the num-
ber of parts into which the transparent corneal continuation
of the chitinous cuticula over the eye is divided, a number
of elongated bodies which lie between the outer surface of
the ganglionic expansion of the optic nerve and the inner
face of the cornea. These bodies consist of two parts: an
external transparent crystalline cone and an internal pris-
matic rod. The broad end of the cone is external, and is ap-
plied to the inner surface of the corneal facet ; its narrow
end is continuous with the outer extremity of the prismatic
rod, which, by its inner end, is connected with the ultimate
ramifications of the optic nerve. Each of these crystalline
cones and prismatic rods is separated from the rest by a pig-
mented sheath.^

Distinct auditory organs have been observed in Crus-
taceans and Insects. They are not exclusively confined to
the head. In the opossum shrimp (My sis), for example, they
are placed in the appendages of the last somite of the ab-
domen. And, in Insects, the only organs to which the audi-
tory function can be certainly assigned are situated in the
thorax or in the legs.

» Leydi^, "Das AuEje der Gliederthiere," 1864. Schulze, "Untersuch-
ungen," 1868. Mr. E. T. Newton has given a very good account of the struct-
ure of the eye of the lobster, accompanied by full references to the literature
of the subject, in the Quarterly Journal of Microscopical Science for 1875.

THE ARTHROPODA. 223

There is some reason to think that the antennae of Insects
are the seat of the olfactory function, but no certain infor-
mation of this head has been obtained. The very fine sette
to the bases of which nerves can be traced, which abound on
the antennary organs of Insecta and Crustacea, but are found
in other regions of the body, are probably partly tactile and
partly auditory organs.

As a general rule, all the muscles of the Arthropoda, even
those of the alimentary canal, are striated. Those of the
body and limbs are often attached by chitinized tendons to
the parts which they have to move. As the hard skeleton is
hollow and the muscles are inside it, it follows that the body,
or a limb, is bent toward that side of its axis which is oppo-
site to that on which a contracting muscle is situated.

Sounds are produced by many Insects ; but in most cases
they cannot be properly referred to a voice, in the sense in
which that term is applied to the sounds produced in the
higher animals, by the vibrations of the atmosphere arising
from the impact of a current of air upon the free edges of
membranes bounding the aperture of exit of the current.
The chirping and humming of Insects often arise from the
friction of their hard parts against one another, or from the
rapid vibration of their wnngs : in some instances, however,
recent investigations render it probable that they are pro-
duced by the action of expiratory currents on tense mem-
branes which bound the stigmata.

Agamogenesis is very common among some groups of the
Arthropoda, such as the Crustacea and the Insecta, but has
not yet been observed in the Myriapoda or the Arachnida,
It may be effected in one of two ways:

1. Either individuals which are, by their structure, inca-
pable of being impregnated and are therefore physiologically
sexless, though it may happen that they more or less approxi-
mate females morphologically, give rise to offspring ( Cecido-
myia larvae. Aphis) ;

2. Or individuals which are capable of being impregnated,
and are thus both morphologically and physiologically true
females, give rise to eggs which develop without impreg-
nation. (The queen-bee, so far as the production of drones is
concerned ; many Lepidoptera^.

The cases of Apus, Daphnia, and Cypris, would belong
to the latter category, if it were certain that the very same
females which, for a certain period, produce young agamo-

224: THE ANATOMY OF IXVERTEBRATED ANIMALS.

genetically, at another time undergo fecundation. Multipli-
cation by lission or external gemmation is not known to take
place in any Arthropod. Hermaphrodisrn occurs as a rule in
some few Arthropods (e. g., the CirrijKclia and Tardigradd)^
and as an abnormal "sport" in sundry Crustacea and in
many Insecta.

In absolute number of species, the ArtJiropoda far ex-
ceed all the rest of the animal kingdom put together. Thus
Gerstaecker/ while allowing 50,000 species for the latter,
estimates the number of species of Arthropoda as rather
above than below 200,000 ; by far the larger proportion of
these, probably more than 150,000, being Insects.

The Arthropoda are commonlj- divided into the Crustacea,
the Arachnida^ the Myriapoda. and the Insecta ; and though
it is impracticable to give a definition which shall absolutely
separate the first two groups, it is perhaps not worth while
to disturb an arrangement which has much practical con-
venience. But, for purely morphological purposes, it may be
instructive to regard them from another point of view.

The Arthrojjoda may, in fact, be divided into two series.
One of these consists almost whollv of air-breathing forms,
which, if they possess special respiratory organs, have either
pulmonary sacs or trachete, or both combined ; while the
other includes a corresponding predominance of water-breath-
ing animals, which, if they possess respiratory organs, have
branchiie. Tlie latter series contains the Crustacea ; the
former comprises the Arachnida, Myriapoda^ and Insecta.

In the course of the development of the higher Arthro-
poda, there is a stage in which the body begins to be seg-
mented, but the appendages are not developed. This is
followed by a stage in which appendages make their appear-
ance, but the antennary and manducatory appendages [gna-
thites) are like the other limbs: and, finally, there is a stage
in which the gnathites are completely converted into jaws.
Now, among the water-breathing Arthr o jyoda, r\o trace of
limbs has yet been certainly discovered among the Trilo-
hita ; in the JSIerostomata \Eurypterida and Xiphosura)
the gnathites are completely pediform ; while, in the Ejito-
rnostraca and Malacostraca, more or fewer of the gnathites
are so modified as to subserve manducation and no other
function.

» Bronu's "Klassen und Ordnungen des TliierreiciLS," vol. v., p. 273. 1868.

THE GROUPS OF THE ARTHROPODA. 225

In the air-breatliing series no completely apodal forms are
known. The Tardigrada and the Pentastomida appear to
have no jaws ; but the presence of oral stilets in the former,
and the position of the hooks which represent the limbs in
the latter, throw some doubt upon this point.

In the Arachnlda and the Peripatidea the gnathites are
completely pediform. But in the Myriapoda^ and still more
in the Insecta^ the gnathites lose the character of legs, and
are completely converted into manducatory organs. Thus
we arrive at the following arrangement of the Arthropoda :

Aetheopoda.

/. WitJiout Gnathites.
Teilobita. Taedigeada (?) Pextastomida (?)

//. With Pediform Gnathites.
Meeostoi>iata. Aeachnida. Peeipatidea.

///. With Maxilliforni Gnathites.
Entomosteaca. Mteiapoda.

Malacosteaca. Insecta.

Water-breathers. Air-breathers.

For the most part.

Of the four great groups, the Cnista^iea are those which
present the greatest and the most instructive variations upon
the fundamental type of structure; while the modifications
of the Insecta, Arachnlda, and JSIyriapoda, are less exten-
sive, and may be regarded as of secondary morphological im-
portance. The Crustacea will, therefore, be treated of at
some length, while the other groups will be passed over more
lightly.

THE CEUSTACEA.

The Trilobita. — These ancient Arthropods, which have
been extinct since the latter part of the PalfBozoic epoch, oc-
cur in the fossil state in great numbers, and in conditions
very favorable for their preservation ; but, up to this time, no
certain indications of the existence of appendages, nor even
of any hard, sternal body-wall, have been discovered, though

2'26 THE AXATOMY OF INVERTEBRATED AXIMALS.

a shield-shaped labrum, which lies in front of the mouth, has
been preserved in some specimens. Tlie body consists of a
cephalic shield (Fig. 57, A) ', oi a, variable number of mov-
ably-articulated thoracic somites (Fig. 57, £) ; and of a,py-
gidium^ composed of a variable number of the somites which
succeed the thorax, united together (Fig. 57, C).

Each thoracic somite presents a median portion, convex
from side to side, termed the axis or tergnin, and two flat-
tened lateral portions, the pleura. The former overlap one
another largely when the body is extended, the latter when
it is flexed, and the freedom of motion permitted by this ar-
rangement is so great that many Trilobites were able to roll
themselves up like wood-lice, and are found fossilized in that
condition. At the lateral edge of each pleuron, the cuticular
substance of which it is composed folds inward, and can be
traced on the ventral or sternal side for some distance. But
in the middle of the ventral region no indication of a sternum
is discoverable. It may, therefore, be concluded that the
sternal region of the somite was of a soft and perishable na-
ture ; and that the thoracic somite of a Trilobite resembled
one of the abdominal somites of a crab in this and in some
other respects.

The glabellum (Fig. 57, 4), or central raised ridge of the
cephalic shield, is a continuation of the thoracic axis^ the lo-
cation of its sides perhaps referring to the number of primi-
tive somites it represents. The limb, or lateral area on either
side, answers to a thoracic pleuron ; its thickened margin
(Fig. 57, 1) is produced into two longer or shorter posterior
angles {ff) ; inferiorly, the marginal band is reflected inward
for a short distance, as the subfrontal fold, the remaining
sternal area being incomplete. A median movable plate
answers to the labrum of Apiis and Limulus. On the occip-
ital or lateral margin of the limb a suture (Fig. 57, 5) com-
mences, and, passing between the eye and the glabellum,
meets that of the opposite side either in front of the latter,
or on the margin of the limb, or on the subfrontal fold, and
is connected with the labral suture by one or two sutures.
The limb is thus divided into two parts — one fixed (the fixed
gena, Fig. 57, «), attached to the glabellum ; the other sep-
arable (the movable gena, Fig. 57, h), on which the eye is
placed. The eves, are absent in some genera. In others they
occur as isolated ocelli ; or in groups, their interspaces being
occupied by the common integument ; or they may resemble
the compound eyes of other Arthropods,

THE TRILOBITA.

227

M. Barrande ^ has succeeded in tracing out the develop-
ment of some species of Trilobites. He finds that the small-

FiG. 57.— Diacrram of Dalmanites (after Pictet).— ^, head ; l,Tiiarcrinal band ; 2, mar-
ginal groove, internal to the band ; 3, occipital segment ; 4, glabellum ; 5, great
suture ; 6. eyes ; a, fixed gena ; ^>, separable gena ; g, genal angle ; B, thorax ; 7,
axis or terguni ; 8, pleuron ; C, pygidium ; 9, tergal ; 10, pleural portions of the
pygidium.

est, and therefore the youngest, forms are discoidal bodies,
without any clear evidence of segmentation. The division
into somites takes place by degrees, the number increasing
up to the adult condition. It is possible that still younger
conditions may have escaped fossilization, but the analogy
of Liinulus suggests that these small discoidal forms really
represent the condition in which the Trilobite left the e^g.

The Mekostomata.' — The only existing representative^ of
this division of the Crustacea is the genus Lirtxulus (the King
Crabs or Horseshoe Crabs), the various species of which are

^ " Systeme Silurien du centre de Boheme." tome i. Trilobites. 1852.
» H. Wood-n-ard, " A Monosrraph of the British Fossil Crustacea belonging
to the Order Merostomata," 1866.

228

THE ANATOMY OF INVERTEBRATED ANIMALS.

found in America and in the Moluccas. They are usually-
classed as a distinct order of the Crustacea^ termed ^ipho-
sura or Poecilopoda.

The body of Lhnulus (Fig. 58) is naturally divided into
three parts, which are movably articulated together. The
most anterior is a shield-shaped portion, curiously similar in
form to the head of a Trilobite. Its convex dorsal surface is
similarly divided into a median and two lateral regions; its
edges are thickened, and its posterior and external angles are
produced backward. At the anterior end of the median re-
gion two simple eyes are situated, and at its sides are two
large compound eyes. The sternal surface presents, ante-
riorly, a flattened subfrontal area^ behind which it is deeply
excavated, so that the labrum and the appendages are hidden
in a deep cavity formed by its shelving walls. The middle
division of the body of Lhnulus exhibits markings which in-
dicate that it is composed of, at fewest, six coalesced somites;
its margins are spinose, and its excavated sternal face lodges
the appendages of this region.

Fio. 58.—^, Linwlus moluccanus (dorsal view). 5. L. rotundlcauda (ventral view)
(after Milne-Edwards): a. anterior; b, mid 11? division of the body ; c, telson , d\
Bubfrontal area; e, anteunules ; f, antennae ; g, operculum; A, branchiferoua ap-
pendages.

The terminal division is a long, pointed, and laterally ser-
rated spine, which is termed the telson.

THE MEROSTOMATA. 229

The mouth is placed in the centre of the sternal surface of
the anterior division ; the anus opens on the same surface, at
the junction between the middle division and the telson. A
movable, escutcheon-shaped labrum projects backward in the
middle line, immediately behind the subfrontal area (d) ; and
on each side of it is a three-jointed appendage, the second joint
of which is prolonged in such a manner as to form with the
third a pincer or chela. The attachment of this appendage is
completely in front of the labrum, which separates it from the
mouth.

In each of the next five pairs of appendages, the basal
joint is enlarged ; and, in the anterior four, its inner edge is
beset with numerous movable spines. The attachment of the
basal joint of the foremost of these appendages (the second
of tlie whole series) is in front of the mouth ; but its pro-
longed, spinose, posterior and internal angle may be made to
project a little into the oral cavity. The basal joints of the
following three appendages are articulated at the sides of the
mouth, and the inner angle of each is provided with a spinose
process which projects into the oral cavity. The second,
third, fourth, and fifth appendages in the females are chelate ;
in the males of most species the second, and sometimes the
third, are not chelate. The large basal joint of the sixth ap-
pendage is almost devoid of spines, and bears a curved, spat-
ulate process, which is directed backward between the ante-
rior and middle divisions of the body. The fifth joint of this
limb carries four oval lamellae. The appendages which form
the seventh pair, very unlike the rest, are short, stout, and
single-jointed.

The eighth pair of appendages, again, are of a totally dif-
ferent character from those which precede them. They are
united in the middle line into a single broad plate, which
forms a sort of cover, or operculum, over the succeeding ap-
pendages, when the animal is viewed from the sternal side.
On the dorsal face of this plate are seated the two apertures
of the reproductive organs.

From the inner face of the anterior, or sternal, wall of
each half of the operculum a strong process arises, and passes
upw^ard to be attached to a corresponding process of the ter-
gal wall of the anterior division of the body. By far the
greater part of the large levator muscle of the appendage
arises from the tergal wall of the anterior division of the
body, and the nerve which supplies the limb is derived direct-
ly from the posterior part of the multiganglionate cord which

230 THE ANATOMY OF IIs^YERTEBRATED ANIMALS.

surrounds the gullet and supplies the appendages which lie
in front of the operculum.

The five pairs of appendages which remain resemble the
operculum in their general form, and have ascending process-
es, which are connected with inward prolongations of the ter-
gal wall of the middle division of the body. Their nerves are
derived from the ganglia which lie in this region of the body.

Thus there are altogether thirteen pairs of appendages,
eight of which are connected with the anterior, and five with
the middle division of the body ; and the appendages in the
region of the mouth are essentially ordinary limbs, the basal
joints of some of which are so modified as to subserve man-
ducation.

The determination of the homologies of the parts hither-
to spoken of as the anterior and middle divisions of the body,
and of their appendages, is a matter of some difficulty ; but,
on comparing the disposition of the limbs and their nervous
supply with what obtains in the higher Crustacea^ it seems
hardly doubtful that the first pair of appendages answer to
the antennules ; the second, to the antennae ; the third, to
the mandibles ; the fourth and fifth, to the maxilla? ; and the
sixth, seventh, and eighth, to the maxillipedes of Astacus or
Soinarus y and, in this case, the anterior division is a ceph-
alo-thorax. If the position of the genital openings marks
the hinder boundary of the thorax, the middle division of the
body represents an abdomen, composed cf five somites. But,
on the other hand, it may be that the genital organs open in
front of the hinder extremity of the thorax, as in female
Pod ophthalmia^ and that the five somites which form the
middle division correspond with the remaining five somites
of the thorax of a Podophthalmian. In this case, the region
which corresponds with the abdomen in the higher ciusta-
ceans is undeveloped.

The alimentary canal of Limiihis is very peculiarly ar-
ranged. The gullet passes directly forward and upward,
and gradually widens into the stomach, the walls of which
are provided with many longitudinal folds. The pylorus is
prolonged into a narrow tube which projects into the intes-
tine. The two biliary ducts on each side are far apart, and
branch out into minute tubules, which form a mass occupying
the greater part of the cavity of the body. The rectum, a
slender canal with plaited walls, and very short, opens into a
sort of cloaca situated between the telson and the sternal wall
of the abdomen.

THE MEROSTOMATA. 231

The heart, in Limidus polyphemus, is an elongated mus-
cular tube, divided into eight chambers, and having as many
pairs of lateral valvular apertures. It lies in a large peri-
cardial sinus, which, in its abdominal portion, presents on
each side five apertures, the terminations of the branchial
veins. The branchiae consist of numerous delicate semicir-
cular lamellae, attached transversely to the posterior faces of
the five post-opercular appendages, and superimposed upon
one another like the leaves of a book.

The nervous system appears, at first sight, to be very con-
centrated, its principal substance being disposed in a ring,
embracing the oesophagus ; but, on closer inspection, it is
found to consist of an anterior mass, representing the prin-
cipal part of the cerebral ganglia in most other Crustacea^
and of two ganglionic cords which proceed from the outer
and posterior angles of that mass, and extend as far as the
interval between the last and penultimate pairs of append-
ages. These cords are thick, and lie on each side of the
oesophagus, around which they converge, so as to come into
close union and almost confluence, immediately behind it.
In front of this point, however, they are connected by three
or four transverse commissures, which curve round the poste-
rior wall of the oesophagus, and become gradually shorter
from before backward.

The first of these commissures unites the tw^o cords oppo-
site the origin of the nerves to the third pair of appendages,
which I reg^ard as the homologues of the mandibles. In front
of this point, the cerebral ganglia give ofi', from their ante-
rior edges, the nerves to the ocelli, eyes, and frontal region ;
and, from their posterior and under surfaces, those to the an-
tennules. The nerves to the antennte arise from the cord
close to the outer and posterior angles of the cerebral gan-
glia, and some distance in front of those to the mandibles.
Close behind the latter arise the large nerves to the fifth and
sixth cephalo-thoracic appendages.

The nerves to the rudimentary seventh pair of append-
ages are slender, and arise rather from the under part of
the post-oesophageal ganglia ; those which suppl}" the eighth
pair of appendages, constituting the operculum, are also
slender, and seem to come off from the two longitudinal com-
missural cords, which connect the post-oesophngeal ganglia
with those which are situated in the second division of the
body, though they are, in truth, only united in one sheath
with them for a short distance, and can be readily traced to

232 THE ANATOMY OF INVERTEBRATED ANIMALS.

the post-oesophageal ganglia, internal to the nerves of the
seventh pair of appendages. The longitudinal commissures
are very long, and are inclosed in a continuation of the same
sheath ; they pass back into the second division of the body,
and there present four ganglionic enlargements, whence the
nerves of the post-opercular appendages proceed. The last
of these ganglia is much larger than the others, and appears
to consist of several confluent masses. The nerves diverge
from it in such a manner as to resemble a Cauda equina.

The reproductive organs of both sexes consist of a mass
of glandular ceeca, which ramify through the body amid the
hepatic tubules, and eventually open on papiilie situated on
the posterior face of the operculum. The males are much
smaller than the females, and present, in many species, an
external sexual distinction in the peculiarity of their second
and third appendages already referred to.

The young of Limidus acquires all its cliaracteristic
features while still within the Qgg. The interesting obser-
vations of A. Dohrn ^ have shown that, in an early stage, the
embryo is provided with the nine anterior pairs of append-
ages, and is marked out into fourteen somites by transverse
grooves upon its sternal face. The body has the form of a
thick rounded disk, divided into an anterior shield composed
of six somites, and a posterior, likewise shield-shaped region,
formed by the union of eight somites. The telson has not
made its appearance. In this condition, its resemblance,
apart from the limbs, to such a Trilobite as Trinucleiis is, as
Dohrn points out, most remarkable.

The JCiphosura were represented in the Carboniferous
epoch [Belllnurus),

Tlie Eicrypterida (Fig. 59) are extinct Crusiacea of Pa-
laeozoic (Silurian) age, which sometimes attain a very large
size and in many respects resemble Limidus^ while, in others,
they present approximations to other Crustacea,, especially
the Copepoda. An anterior, eye-bearing, shield-shaped di-
vision of the body is succeeded by a number (12 or more)
of free somites, and the body is ended by a broad, or narrow
and spine-like, telson. Five pairs, at most, of limbs, pro-

^ " Tlntcrsnchungen tiber Bau und Entwiclvclun;^ der Artliropoden." (Jena-
iscJie Zeifsclirift^ Bd. vi.) See also the observations of Lockwood and Packard,
American Naturalift, vol. iv., 1871, vol. vii., 1873, and " Memoirs of the Boston
Society of Natural History," 1872 ; with the discussion of the systematic place
of Zimulus by E. Van Beneden, Journal de Zoologie^ 1872.

THE MERCSTOMATA.

233

vided with toothed basal joints, are attached to the sternal
surface of the shield, and the mouth is covered, behind them,
by a large oval plate which appears to represent a meta-
stoma (Fig-. 59, -S, g). Some of the anterior limbs are fre-
quently chelate [Fterygotus) ; the terminal joints of the most

Fig. m.—Emimterm rsmims (after Nieszkowpki).i— J, do-sal aspect. B, ventral
aspect. Cth, t'le cepTialo-thoracic shield bearing a, the eyes, and 0, c. d, e,/, the
locomotive limhs; T, telson ; g, the metastoma; h, the sternal plates of the an-
terior free somites.

posterior pair are generally expanded and paddle-like. The
integument often presents a peculiar sculpture, simulating
minute scales. The sternal surface of one or more of the
anterior free somites is occupied by a broad plate, with a
median lobe, and two laterally-expanded side-lobes (Fig. 59,

1 " Der Enrypterm remipes^ au3 den obersilurisclien Schicliten der Insel
Oesel." 1859.

234 THE AXATOMY OF INVERTEBRATED ANIMALS.

3, A), having a remote resemblance to the operculum of
Limiilus,

The Extomostraca. — All the remaining Crustacea have
completely specialized jaws; and as many as six pairs of
appendages may be converted into gnathites.

In the Entomostraca, if the body possesses an abdomen
(reckoning as such the somites which lie behind the genital
aperture), its somites are devoid of appendages. Moreover,
the somites, counting that which bears the eyes as the first,
are more or fewer than twenty. There are never more than
three pairs of gnathites. The embryo almost always leaves
the eg^ in the condition of a Nauplius y that is, an oval
body, provided with two or three pairs of appendages, which
become converted into antennary organs and gnathites in the
adult. The division of the JEntomostraca comprises the
Copepoda^ the Epizoa^ the Branchiopoda^ the Ostracoda^
and the Pectostraca,

The Copepoda. — Tn these Entomostraca, which come
nearest to the Eurypterida^ the cephalic shield, which is dis-
coidal and not folded longitudinally, is succeeded by a certain
number of free thoracic and abdominal somites. The anten-
nules and antennae are large, and, as in the Eurypterida^ are
organs o