ELEMENTS

OF

COMPAEATIYE ANATOMY.

ELEMENTS

OF

COMPARATIVE ANATOMY.

BY

CARL GEGENBAUR,

Professoii op Anatomt and Dibector op the Ahatomical Institute at Heidelbero.

TRANSLATED BY

E. JEFFREY BELL, B.A.,

Magdalen College, Oxpord.

THE TRANSLATION REVISED AND A PREFACE BRITTEN BY

E. RAY LANKESTER, ALA., F.R.S.,

Fellow op Exeter College, Oxford, and Professor op Zoology and Comparative Anatomy
IN University College, London.

LONDON :

MACMILLAN AND CO,

1878.

CHARLES DICKERS AND EVANS,
CRYSTAL PALACE PRESS.

PREFACE TO THE SECOND EDITION.

OuE knowledge lias been so inucli increased in extent and exactness
in almost every department of Comparative Anatomy since the
time when I converted my Grundzuge into the first edition
of this smaller manual — the Grnndriss — that the publication

of a second edition hardly seemed an easy task. Nevertheless^ I
gladly undertook it, for I had observed so much new evidence of
the importance of the doctrine of development in anatomical
enquiry. The road along which science may travel forward success-
fully seems indeed to be growing easier, yet the distance which
we have made is but short in comparison with that which lies
in front of us, and far beyond our view. Every question solved
leads again to fresh problems, and renders unstable even what
seemed to have taken a definite form. There are, therefore, great
difiiculties in giving such a comprehensive presentation of the
subject as a text-book ought to supply. I have tried as much
as possible to evade these difiiculties where I have been unable
to overcome them. Much remains unaltered, because recent in-
vestigations appear to demand fundamental changes, the concrete
expression of which cannot be immediately taken in hand.

I have somewhat modified the arrangement of the matter. I
can hardly be blamed for separating the Brachiopoda from the
Mollusca, and treating them as forming an independent phylum.
Nor indeed is the change a real one, for even in my Grundzuge
I drew especial attention to the great difference that obtained
between them and the other Mollusca.^ ^ The Tunicata have

VI

PKEFACE.

been treated in tlie same way, but this does not require any
apology at tbe present date.

By treating tbe subject more concisely I bave been able to
increase tbe real matter to a certain extent, without enlarging
tbe size of tbe book. I bave, of course, only dealt with wbat
bas seemed to me to be of capital importance ; many, and even
important, details bave been omitted, owing to tbe limits imposed
by tbe aim of tbe book.

I bave endeavoured to correct some previous mistakes and to
suj)ply omissions. If any sucb bave been retained, or bave newly
crept in, I shall be fairly judged, I know, by anatomists, who
will remember tbe vast extent of our science and tbe object of
this work. I hope that I bave satisfied them, and if I bave my
toil is well repaid.

Heidelberg, November, 1877.

C. Gegenbaur.

PREFACE TO THE ENGLISH TRANSLATION.

It is a great pleasure to me to be able to place in the hands of my pupils
in Oxford and London an English translation of Professor Gegenbaur’s
Grundriss der Yergleichenden Anatomic.” I have to thank the energy and
industry of Mr. Jeffrey Bell, of Magdalen College, Oxford (now one of
the staff of the British Museum), for the translation which he undertook and
carried through at my request, Avhen I found that my time Avas too fully occu-
pied Avith other Avork to alloAv of my completing it myself Avithin a suffi-
ciently short period from the date of publication of the German Avork.

My share of the present Avork has therefore consisted in a careful
revision of the MS. and proof-sheets, Avhich has been by no means a mere
formality, but enables me to give the assurance that the original Avork is
faithfully rendered in the translation. The chapter on the Tunicata I took
occasion to translate myself.

That Professor Gegenbaur’s Avork Avill be of great service to those
English students Avho do not already read German cannot be doubted. We
have some excellent treatises in the English language on animal morpho-
logy, notably the Manuals of the Anatomy of Vertebrate and Invertebrate
Animals, by Professor Huxley. But we do not possess any modern Avork
on Comparative Anatomy, properly so-called ; that is to say, a Avork in
AAdiich the comparative method is put prominently forAvard as the guiding
principle in the treatment of the results of anatomical investigation. The
present Avork therefore appears to me to form a most important supplement
to our existing treatises on the structure and classification of animals. It
has, over and above this, a distinctive and Aveighty recommendation in that
throughout and A\dthout reserve the Doctrine of Evolution appears as the
living, moving investment of the dry bones of anatomical fact. Hot only
is the student thus taught to retain and accumulate his facts in relation to
definite problems AAdiich are actually exercising the ingenuity of investigators,

Yin

PEEFACE.

but lie is encouraged, and to a certain extent trained, in the healthy use of
his speculative faculties ; in fact the one great method by which new knoAv-
ledge is attained, Avhether of little things or of big things — the method of
observation (or experiment), directed by speculation — becomes the con-
scious and distinctive characteristic of his mental activity. Thus avc may
claim for the study of Comparative Anatomy, as set forth in the present
Avork, the poAA^er of developing Avhat is called “common sense” into the
more precisely fixed “ scientific habit ” of mind.

I liaA^e made no notes nor additions of any kind to the original text,
Avith the exception of a feAV references to English Avorks likely to be nseful
to the English student. These additions are indicated by brackets.

AYhilst the Avork is thus presented to the reader precisely as its author
designed that it should be, there can be no objection to the introduction
in this place of a feAv remarks suggested by the fact that this English
translation is intended for the use of English students, and that it is
therefore desirable, in order to prevent confusion and perplexity, to point
out certain statements of fact, or of interpretation of fact, in Avhich Professor
Gegenbaur differs Avidely from authorities usually foUoAved in this country.
I shall, moreover, refer to some recent additions to knoAAdedge published
since this Avork left Professor Gegenbaur’s hands. It Avill be understood
that the foUoAving paragraphs are intended as a supplement necessitated
by the special objects of this translation, and are by no means to bo
regarded as conceived in the spirit of criticism or discussion, Avhich Avould
assuredly ill befit a Avriter Avho is making knoAvn to a noAv audience the
teachings of a master to aaEoiii he is deeply indebted.

Xudei of Cells. — In the first place, it seems necessary to notice
that, Avhilst the last German edition of this work Avas in the press, very
important additions to our knoAvledge of the nucleus of organic cells or
plastids Avere being made. Though these hwestigations are not yet complete
they tend to modify AAEat is said concerning the nucleus on j^ages 15 and
16. The student is referred to an article by Mr. Priestley in the Quart.
Journ. Microsc. Science, vol. xvi. (1876), for an account of the obserA^ations
of Auerbach, Strasburger, Hertavig, and Yah Beneden, and to part iii.
of the same Journal, a^oI xviii. (1878), for original observations on the same
subject by Dr. Klein.

Eeproduction of Infusoria. — A most important modification in the
current AueAVS as to the reproduction of the Infusoria has resulted from
the same line of study as that just mentioned, Avhen carried into the
domain of unicellular organisms. 0. Butschli and Engelaiann have shoAvn
that AA^e are not at present in a position to assert that the process of con-
jugation in the Infusoria is folloAved by a production of spores (see § 70).
It results from their investigations that conjugation in the Infusoria is
attended by a definite breaking-up of the nucleus and so-called nucleolus
(paranucleus) of the conjugating individuals ; but that the conjugating

PREFACE.

ix

individuals separate, and after expelling portions of the hroken-up nuclear
structures (probably as effete products), proceed to re-form the nucleus,
or nucleus and nucleolus characteristic of the species. The so-called Acineti-
forin embryos appear to be parasites, the rod-like bodies occasionally observed
in the nucleus are also parasites, Avhilst the striated structure and spindle-
shape exhibited by the nucleolus or paranucleus in such forms as Para-
nioecium and Stylonicbia at the period of conjugation, are simply due to
changes in this body which are exactly paralleled in the nuclei of egg-cells and
other tissue-elements of multicellular organisms, when those cells are about
to divide by transverse fission. The process of conjugation in the Infusoria
may be, and probably is, attended by an exchange of nuclear material
between the conjugating mdividuals, and is so far comparable to sexual
congress, but it results in a simple rejuvenescence ” of the conjugating
individuals and not in a production of spores. Reproduction by fission and
l)y the modification of fission, known as gemmation, has been accurately
observed in Infusoria, but of the formation of “ spores ” in this group
we are at present ignorant, in spite of all that has been written on the
subject.

Origin of Male and Female Reproductive Elements from
different Germ-layers. — In § 95 Professor Gegenbaur has described
the observations of Ed. Van Beneden on the development of the sexual
products in Hydractinia, and has adopted his generalisation, so far at
least as it applies to the Hydromedusoe. From more recent observa-
tions (CiAMiciAN, Zeitschr. fiir wiss. Zoologie, vol. xxx. p. 501, 1878)
it appears that in other genera of hydroid polyps the same arrangement
does not obtain. In Eudendrium ramosum the ova appear to develop from
the ectoderm, and the sperm from the endoderm ; in Tubularia meseni-
bryanthemum both ova and sperm are ectodermal in origin according to
CiAMiciAN ; Van Beneden found the ova to be endodermal and the sperm
ectodermal in Hydractinia, whilst Kleinenberg ascribes both to the ecto-
derm in Hydra.

Nervous System and Sensory Organs of Medusae. — During the
past year a considerable addition has been made to knowledge on these
points, by the researches of the two Hertwigs (“ Das Nervensystem und die
Sinnesorgane der Medusen.” Leipzig, 1877). It is no longer possible to
deny the existence of differentiated nervous tissue in the Medusae — the
central organ having the form of a ring situated along the line of insertion
of the velum in the Craspedota, and of a series of isolated ganglia, usually
eight in number, placed on the edge of the disc in the Acraspeda. (See for
an abstract of recent researches on this subject. Quart. Journal of Microsc.
Science, vol. xviii. p. 340.)

Cirri and Elytra of Aphroditaceae. — The statement in § 105, that
the elytra of the chaetopodous Worms, allied to Aphrodite, are formed by the
metamorphosis of the dorsal cirri of the parapodia, appears to be contradicted

X

PREFACE.

by tlie fact, that in Sigalion the elytra and dorsal cirri exist side by side on
the same segment.

Homologies of the Rami of the Appendages in Astacns. — The
view taken by Professor Gegenbaur, as to the homologies of the parts of the
appendages immediately following the month in Astacns, differs somewhat
from that which is current in this country. In Fig. 122, p. 239, the
mandible, two maxilhe, and three maxillipedes of the right side of Astacns
flnviatilis are figured. This woodcut was kindly re-drawn for the English
edition by the author, at my re(][nest, and gives a more complete outline of
the parts in question, than does the older cut of the German edition.
Throughout the series of appendages, three divisions are distinguished by
the letters a, c, d. Taking the lowest figure first (the third maxillipede) we
find the endopodite marked a, the exopodite marked c, and the letter d
placed with the single epipodite (podobranchia, Huxley) to its inner side,
whilst the double arthrobranchia (Huxley) not forming part of the appendage
proper, but a distinct respiratory development, is seen on its outer side. In
the next figure (the second maxiUipede), a indicates endopodite, c exopodite,
and d, is placed close to the double arthrobranchia on its outer side, whilst
the modified epipodite is seen to the inner side again, of this. In the figure
of the first maxillipede, a is placed near the foliaceous endopodite, which
has a detached outstanding segment, c near the filamentous exopodite, and
d near the broad epipodite. The same explanation of the lettering holds
good for the next appendage, the second maxilla. In the next appendage —
the first maxilla — the absence of the letters c and d, indicates that the
author regards the whole appendage as reduced to the representative of
the foliaceous endopodite a of the two inferior appendages — a view with
which few will disagree. In the case of the mandible, hoAvever, Professor
Gegenbaur marks the “ j^alp ” with the letter c — considering, therefore, the
basal piece of this appendage to represent the endopodite, and the palp to
represent the exopodite. The more usual opinion on this matter is that the
mandible, together with its palp, corresponds to the simple foliaceous fimt
maxilla. The jointed palp, mounted on its solid basal biting-piece, cor-
responds to the jointed endopodite a of the last maxillipede.

The question of the presence or the absence of a representative of the
exopodite in the Decapod’s mandible, is a matter of considerable importance
in reference to possible comparisons between the gnathites of Crustacea and
Tracheata. The actual development of the parts in question from the
nauplius-form of appendage, must be the ultimate test of the homologies of
their rami in the Crustacea.

Blood-corpuscles of the Mollusca. — The statement on p. 375, that
the form-elements of the blood are always colourless ” in the Mollusca, is
one which I may be allowed to correct, since I have published an account of
the blood-corpuscles of Solen legumen (Proc. Royal Society, Ho. 140,
1873), which, besides colourless amoeboid forms, comprise a vast number of

PREFACE.

XI

oval ones, deeply stained by lireinoglobin. The nninber of these corpuscles
is so considerable as to give the blood of Solen leguinen a bright blood-
red colour.

I may add here that I have observed similar thougli larger corpuscles
impregnated with hfemogiobin in the blood of species of Area.

Homologies of the Arms of the Cephalopoda. — The view that
tlie sucker-bearing arms of the cuttlefish are to be regarded as appendages
of the head homologous with the tentacles on tlie head of Gasteropods
(p. 326), is one which, it will be well for the student to remember, is not
that usually taught. He should make himself acquainted with the older
and the newer view, and the grounds on which they are based. Without
entering into a discussion of the arguments which may be adduced in favour
of this or of rival interpretations of the parts, it must suffice here briefly to
mention that the arms of the Cephalopod (the development of which had
been made known by Kolliker), were shown by Professor Huxley,
five-and-twenty years ago, to correspond to the fore-part of the foot of
the Gasteropoda, and the ganglion, from which they receive their nerve
supply, was then considered as corresponding to the pedal (Morphology of
the Cephalous Mollusca, Phil. Trans. 1853). This view was maintained in
the earlier editions of Gegenbaur’s work. It has been abandoned in the
present edition, in deference to the statements of Mr. Jhering (“ Yergleichende
Anatomie des Hervensystems und Phylogenie der Molluscen, Leipzig,” 1877).
The whole of tliat author’s work, both statement of fact and speculative
superstructure, appears to me to call for very cautious treatment, involving
the rejection of some of his principal conclusions.

Origin of the Limbs of Vertebrates. — Professor Gegenbaur is
inclined to regard the skeleton of the limbs and limb-girdles of Vertebrata as
derived from gill-arches and their branchial rays (§ 357). The student is
reminded that another possible derivation of these organs is from primitively
continuous lateral fins — supported by cartilaginous rays, and comparable to
the primitively continuous dorsal median fin. The specialisation and con-
centration of the lateral fin on each side in two regions, thoracic and pelvic,
would be competent to give rise to the two pairs of fins, such as we find in
the Elasmobranchs. Mr. Balfour (“ Development of Elasmobranch Fishes,”
1878) is led to adopt this view by the observation, that in the embryo dog-
fish the lateral fins have precisely the same mode of origin as has the
dorsal median fin, arising as special developments of a continuous ridge
on each side, precisely like the ridges of epiblast, which form the rudiments
of the unpaired fins.” This view of the nature of the vertebrate limbs has
been independently worked out with great care from the point of view of
comparative anatomy, by Mr. J. K. Thacher (Median and Paired Eins,
Transactions Connecticut Academy, vol. iii. 1877). In the important
memoir just cited, Mr. Thacher shows very plausibly how the Elasmobranch
fin, and not only the fin, but the su2:>porting limb-girdle also, may have

XU

PKEFACE.

been derived from the gradual shifting, atrophy, hypertroph}', and con-
crescence of primitively similar cartilaginous rods, which formed a series on
each side of the body, identical in character with the primitive median
dorsal series. According to this view, the archipterygium ” of Professor
Gegenbaur is not antecedent to, but is derived from the type of fin found
in Elasmobranchs. (See also on this subject, Huxley, On Ceratodiis, Proc.
Zool. Soc. vol. 1876, p. 24.)

Pelation of the Malleus and Incus to the Mandibiilar and
Hyoid Arches. — Investigations directed to the development of the skull
led Professor Huxley some years since to adopt the conclusion of Peichert
and of Goodsir, that the small bones of the Mammals’ tympanic cavity were
derived from the upper ends of the anterior visceral arches. At first it
appeared probable that the malleus and incus were both derived from the
upper end of the cartilaginous mandibular arch, the lower part forming
Meckel’s cartilage. This led to the suggestion that the malleus corresponds
to the articulare of the lower jaw of other Vertebrata, whilst the incus was
considered to be the representative of the quadratum, since it articulates
with the malleus just as the quadratum does with the articulare (Croonian
Lecture ‘‘ On the Theory of the Vertebrate Skull,” Proc. Eoyal Society,
vol. ix. p. 398).

Eurther investigation led Professor Huxley to a modification of his
views. The embryological evidence is not quite complete, but the relations
of the parts in question in the developing Erog, in certain Lizards, and in Mam-
malia, have led him to the conclusion (“Manual of Vertebrate Anatomy,”
p. 85, 1871) that whilst the malleus is formed from the uppermost extremity
of the mandibular arch, and therefore represents, not articulare, but quad-
ratum, the incus is developed from the uppermost extremity of the second or
hyoid arch, and corresponds to the hjmmandibidar of fishes. The stapes is
also developed from the upper portion of the hyoid arch, just below the
incus. The incus may therefore be spoken of as the siipra-stapedial portion
of the hyoid arch, and in certain Vertebrata it exists as a mere cartilaginous
supra-stapedial rudiment.

These views in their later form have not been adopted by Professor
Gegenbaur. He observes (§ 402) that the homologies of the ossicula
audit us of the various classes of Vertebrata have not yet been satisfactorily
determined. In § 352 he maintains the earlier determination of the homo-
logy of the mammalian malleus with the articulare of other Vertebrates.
Concerning the homologies of the incus and the stapes, he considers it
advisable, in the present state of knowledge, to make no statement.

The student is advised of these differences of interpretation of structural
fact, in order that he may the more carefully make himself acquainted from
original sources with the details of development, relation to nerves, and other
features of the parts under discussion.

Nomenclature of the Lobes of the Brain in Fishes. — In the
earlier editions of the present work, Professor Gegenbaur, led by the result

PEEFAOE.

xiii

of investigations carried out by bis pupil Miklucho-Maclay (“ Vergleicli.
Aeurologie der Wirbeltbiere,” 1870), modified the current nomenclature of the
lobes of the Fish’s brain, so that the large bispherical part, which was usually
considered as the mesencephalon in the Teleostei and Selachii, was assigned
to the thalamencephalon — or second of the five cerebral segments — whilst
the unpaired large projecting lobe, usually considered as the metencephalon
(cerebellum, fourth segment), was identified with the mesencephalon of higher
Vertebrates, and the cerebellum was considered as being represented by a
small transverse plate, often overlapped by the folded mesencephalon, and
usually of no larger size than the piece similarly identified in the frog. In
the present edition Professor Gegenbaur has modified this system of nomen-
clature, and has returned to the older and usually accepted method of
naming the parts of the Fish’s brain. Thus in Fig. 281, d marks the two
spherical masses which were in former editions assigned to thalamencephalon,
and are now, as is usual with other anatomists, designated mesencephalon,
the expansion between them and g being the reduced area of the thalamen-
cephalon. The letter h is now referred to as metencephalon (cerebellum) :
this was previously referred to as mesencephalon ; the myelencephalon
prosencephalon, and rhinencephala retain their names, which had not been
affected by Maclay’s system.

Wliilst Professor Gegenbaur has returned to the usual system of naming
these parts, he still considers that the facts on which Maclay’s nomenclature
was based possibly point to homologies other than those indicated by the
names ; so that the Fish’s cerebellum does not necessarily agree with that of
higher Vertebrata. He remarks ; The mesencephalon is usually considered
as being confluent with the thalamencephalon in Selachians ; and a part
which really represents it, so far at least as relations of position are con-
cerned, is customarily called by the name ‘ cerebellum.’ ”

In translating the German terms, Vorderhirn, Zwischenhirn, Mittelhirn,
Ilinterhirn, and Hachirn, I have adopted Professor Huxley’s equivalents,
namely Prosencephalon, Thalamencephalon, Mesencephalon, Metencephalon,
and Myelencephalon. In the edition of Quain and Sharpe y’s Anatomy,
published in 1867, a similar but not identical series of terms was suggested.
For the ‘‘primitivenHirnschlitz,” — the early strongly-marked sinking in of the
cerebral roof which separates the prosencephalon from the thalamencej)halon
— we have no special term in use ; primitive cerebral cleft ” is the transla-
tion which has been adopted.

It is worth while pointing out to the student, in connection with this
subject, and in fact in relation to the whole of the chapter on the Vertebrata,
that Professor Gegenbaur assumes some small amount of familiarity on the
part of the reader with descriptive human anatomy ; reference to a manual
treating of this subject, on the part of the student who has not previously
mastered it, is indispensable.

FTomenclature of the Parts of the Digestive Tract. — The transla-
tion in the present work of the simple word Darm,” and its compounds

XIV

PREFACE.

Vorderdariii, ^Jitteldariii, Hinterdarm, Ko})fdarm, lias caused me some
perplexity. It has been variously rendered in the translation by “gut/*’
‘‘enteron/’ “enteric tube,” “alimentary canal,” “digestive tract.” The
fact is that, whilst we have no definite nomenclature at present in use
in English which recognises the true morphology of the canal which com-
mences with the mouth and ends with the anus, the nomenclature in use in
Germany is of very doubtful advantage, since it has not a sound morpho-
logical basis, but is altogether superficial. “ Darin,” for which our readiest
ecpiivalent is “ gut,” is used indifferently for the whole or for any part of
the physiological entity which reaches from oral to anal aperture. Put the
English Avord “ gut ” is associated rather Avith the hinder than Avith the fore-
most portion of this tract. It Avill probably be foimd most convenient to
speak of the physiological Avhole as the “alimentary canal,” or “digestive
tube and these terms I have endeavoured consistently to make use of in
tliis sense, though sometimes the term “enteric tube” has been similarly
applied.

The division of this tube or canal into pharynx, oesophagus, stomach,
and intestine ; or, again, into fore-gut, mid-gut, and hind-gut (Vorderdarm,
^Mitteldarm, Hinterdarm, p. 48), is one based upon superficial adaptations
of form, and does not admit of a comparison of the parts so designated in
the various phyla of the Animal Kingdom. The pharynx and the oeso-
phagus of the Vertebrata are develoxied from the endoderm of the embiyo ;
the parts Avhich receive the same names in the Mollusca and the Arthropoda
are developed from the ectoderm. The hind-gut of the Vertebrate is endo-
dermal in origin, ectodermal in the Arthropod, and partly endodernial
partly ectodermal in the Mollusca. In fact there is no attempt to recog-
nise the facts of embryology in the terminology applied to the alimentary
canal.

Under these circumstances I have proposed (Quarterly Journ. ^licrosc.
Science, April, 1876, and “ Xotes on Embryology and Classification,” London,
1877, p. 11), to distinguish the primitive digestive space Avhich develops
from the endoderm (in fact the gastrula-stomach) as the “ enteron.”
The anterior passage leading into this from the mouth, and formed by
an ingroAvth of ectoderm, I have termed the “ stomodreum,” and the
corresponding passage leading from the anus I similarly propose to call
the “proctodeum.” These three primary factors of the alimentary tract
are most equally developed in the Arthropoda and some ^Mollusca. In
Vertebrata the stomodeum is exceedingly small, if indeed its true homo-
logue exists at all (excepting in the Tunicata). The jn'octodreum is also in
them evanescent. The middle portion of the alimentary tract formed from
the primitive enteron (archenteron), AAdiich does not entirely coincide Avith that
part to Avhich the term “ IMitteldarm ” is applied, does not in all the various
animal phyla take up the AAdiole of the primitive enteron. This, in fact,
only occurs in some of the Cadenterata, Avhich may therefore l)e said to
possess in the adult condition an archenteron. In other groups the

PREFACE.

XV

primitive enteric sae gives off the foundations for a variety of other
structures, so tliat what is left of it as the central element of the alimentary
canal is a changed and hroken-up enteron, which maybe called “metenteron”
as opposed to the unchanged archenteron.”

It is to these three morphological factors then, the metenteron, the
stomodseum, and the proctodreum, that Ave are called upon to assign the
various adaptational swellings, constrictions, and outgrowths of tire alimen-
tary tract of higher animals.

These distinctions are not recognised in Professor Gegenbaur’s Avork.
It Avill be sufficient here to point out that the exact limit of stomodmum
and of proctoda3um in any particular case, can only be ascertained by
direct observation of the process of development. The metenteron is that
part of the alimentary canal Avith Avhich the most important digestive
glands are connected, such as the liver, and from its walls they are
formed as outgroAvths. The stomodaeum gives rise to salivary glands,
and usually to masticatory sacs (gizzards), but these latter may form also
in the metenteron.

The proctodaeum forms the cloacal chamber, Avhere such exists, and
alAvays receives the openings of glands (such as the Malpighian filaments of
insects) Avliich are excretory rather than accessory to digestion.

These explanations Avill be sufficient to make clear to the reader the
sense in Avhich the Avords “ enteron ” and ‘‘ enteric ” have occasionally been
employed in the translation.

Classification. — At the present day, naturalists have learnt to recog-
nise in their efforts after Avhat Avas vaguely called the natural ” system
of classification, an unconscious attempt to construct the pedigree of the
animal Avorld. The attempt has noAV become a conscious one. Xecessarily
classifications Avhich aim at exhibiting the pedigree, vary from year to year
Avith the increase in our knoAvledge. They also vary according to the
importance attached by their authors to one or another class of facts as
demonstrating blood-relationships. Probably no two zoologists of the
present day Avould agree, Avithin Avide limits, as to the classification Avhich
comes nearest to expressing the pedigree. Accordingly it is by no means
desirable that students should be taught to accept any one scheme of clas-
sification as finite. They should be taught to look upon these schemes as
the condensed expression of an author’s vieAvs — as the epitome of his
teaching, facilitating the recollection and the comparison of conflicting
solutions of the vast series of unsolved problems of morphology.

I propose here, for the convenience of the student, to place side by side
the general outlines of the schemes of classification adopted by Professor
Huxley in 1869 (No. I.), that adopted by Professor Gegenbaur in the
present Amlume (No. II.), and that AAdiich I have made use of in my lectures
during the j^ast year (No. III.).

I have taken the older classification adopted by Professor Huxley rather
than that more recently put forAA^ard by him, because it is one Avith Avhich

XVI

PEEFACE.

my experience as teacher and examiner lias shown me that Engdish students
are thoroughly familiarised.

I.

SUB-KINGDOMS.

II.

PHYLA.

III.

PHYLA.

Protozoa.

Protozoa.*

(Rhizopoda, Gregarinida,
Radiolaria, Spongida.)
Infusoria.

ClELENTERATA.

COELENTER-ATA.

(Hydrozoa, Aclinozoa.)

VERMES.f

Annuloida.

(Scolecida, Echinoderma.)

Annulosa.

Echinoderma.

(Crustacea, Arachnida,

Myriapoda, Insecta, Chte-
tognatha, Annelida.)

Brachiopoda.^

Molluscoida.

(Polyzoa, Brachiopoda,

Tunicata.)

Arthropoda.

Mollusca.

(Lamellibranchiata, Bran-

Mollusca.

chiogastropoda, Pulmo-

gastropoda, Pteropoda,

Cephalopoda.)

Tunicat.a.§

Vertebrata.

(Pisces, Amphibia, Reptilia,
Aves, Mammalia.)

Vertebrata.

Protozoa.* * * §

PORIFERA.

Ivematoritoka.

Platyhelmia.II

Gepiiyrea.II

Echinoderma.

ExtEROPNEUSTA. II

NeMATOIDEA. II

Ch.etognatha.||

ApPEXDICL'LATA.*!|’

MoLLL’SCA.**

VERTEBRATA.ff

Seeing that one of my chief objects in superintending the translation of
the treatise to Avliich these few pages are introductory, has been to be able

* The Protozoa in Nos. II. and III. include the same organisms as in No. I.,
excepting that the Infusoria are included in that phylum in Nos. II. and III., and that
the Sponges are excluded, being in No. II. placed under the Coelenterata, and in
No. III. forming the phylum Porifera under the “ grade ” Coclentera, as shown in the
genealogical tree on the adjacent page.

f The Vermes of No. II. include all the Annulo'ida of No. I. excepting the
Echinoderma, which are raised to the rank of an independent jDhylum. They also
include the Annelida (Chajtopoda, Hirudinea, and Gephyrea) from amongst the
Annulosa of No. I. and the Polyzoa from amongst the Molluscoida of the same series.

The Brachiopoda, raised to the position of a distinct phylum in No. II., are
placed among the Molluscoida in No. I. and amongst the Mollusca in No. III.

§ The Tunicata, considered as an independent phylum in No. II., are found
amongst the Molluscoida in No. I. and form a section of the Vertebrata in No. III.

II The Platyhelmia, Gephyrea, Enteropneusta, Nematoidea, and Chmtognatha
form in No. III. a number of independent phyla. Together with the Polyzoa
(included in No. III. under the Mollusca), the Ilotifera, and the Chgetopeda, included
under the Appendiculata, they constitute the series of phyla which are in No. 11.
massed together as “Vermes.”

•fT The Appendiculata include animals with lateral locomotive appendages, and
usually a segmented body. The group is, excepting that it has the addition of the
Ilotifera, nearly co-extensive with the Annulosa of No. I.

** The phylum Mollusca in No. III. includes the Polyzoa and Brachiopoda,
excluded from it in both No. I. and No. II.

ft The Vertebrate phylum in No. III. includes the Tunicata, which it will be seen
by reference to page 70 are already placed on the Vertebrate stem by Professor
Gegenbaur.

PEEFACE. xvli

to place the work in the hands of the students of iny own classes, I need
not apologise for adding here further details of the classification which I
find it most convenient to adopt in teaching. I have arranged the chief
phyla first of all in the form of a genealogical tree, and secondly in a
series exhibiting their subdivisions into classes, etc. This classification is
of course to a largo extent only a modification and adaptation of systems
already put forAvard by other naturalists.

o

ra

Ph

o

I

CS

s

C3

(U

u

rS=i

Ph

o

O

P

o

p

p^

o

<D

P

p

o

P^

P

Q

<4

P

Grade II. C

H

O

iH

(ELOMATA.

Grade I. CCELENTEEA.

Grade II. ENTEROZOA.

Grade I. PLASTIDOZOA.

A

xyiii

PREFACE.

rnoTOzoA.

GRADE A. GYMNOMYXA.

Class 1. Gymuomyxa.

GRADE B. CORTICA TA .

Class 1. Lipostoma (Gregarinre).

2. Suctoria.

3. Ciliata.

4. Flagellata.

5. Proboscidea (N’octiliica).

PORIFERA.

Class 1. Calcispongioe.

2. Fibrospongiae.

3. Myxospongice.

NEMATOPHORA.

Class 1. Hydromedusas.

2. Scypbomedusa?.

3. Antbozoa.

4. Ctenopbora.

ECHINODERMA

BRANCH. AMBULACRATA.

Class 1. Holotburidea.

2. Ecbinoidea.

3. Asteroidea.

BRANCH. TENTACULATA.

Class 1. Crinoidea.

2. Blastoidea.

3. Cystidea.

LATYHELMTA.

BRANCH. CILIATA.

Class 1. Planariae.

2. Nemertina.

BRANCH. SUCTORIA.

Class 1. Trematoidea.

2. Cestoidea.

3. Hirudinea.

GEPHYREA.

Class 1. Ecbiuridao.

2. Priapulida?.

3. Sipunculidao.

4. Pboronida?.

PREFACE.

XIX

VERTEBRATA.

BRANCH. UROCHORDA (TUNICATA).

Class 1. Larvalia.

2. Saccata.

BRANCH. CEPHALOCHORDA.

Class. Leptocardia.

BRANCH. CRANIATA.

GRADE A. CYCLOSTOMA.

Class. Cyclostoma.

GRADE B. GNATIIOSTOMA.

Grade a. Heterodactyla hrunchiatu.

Class 1. Pisces.

2. Dipnoi.

Grade j3. Pentadactyla hranchiata.
Class. Amphibia.

Grade y. Pentadactyla li2:)ohranchia.

Class 1. Reptilia.

2. Aves.

3. Mammalia.

= Branch. Monocondyla.
= Branch. Amphicondyla.

APPENDICtJLATA.

BRANCH. CH^TOPOHA.

Class li Oligochaotai
Class 2. Polychmta.

branch. ROTIFERA.

Class. (Orders only*)

BRANCH. GNATHOPODA (ARTHROPODA);

GRADE A. MALACOrODA.

Class. Peripatideai

GRA DE B. COND YLOPODA .

Class 1. Crustacea.

2. Hexapoda.

3. Myriapoda.

4. Arachnida.*

* Following Prof. Ed. Van Beneden, I include Limulus, the Eurypterina, and
I'rilobites under the Arachnida as Branchiopulmonata.

XX

PEEFAOE.

MOLLUSCA.

BRANCH. EUCEPHALA.

GRADE A. LIPOGLOSSA.

Class. Scolecomorpha (Neomenia).

GRADE B. ECIIINOGLOSSA.

Class 1. Gastropoda,*

2. Cephalopoda.f

3. Scaphopoda.

BRANCH. LIPOCEPHALA.

Class 1. Tentaculibranchia (Polyzoa).

2. Spirobrancbia (Bracbiopoda).

3. Lamellibrancbia.

The phyla EiiteropneiLsta, Cha3tognatha, and Xematoidea, containing
respectively the genus Balanoglossus, the genus Sagitta, and the various
families of thread-worms, do not admit of subdivision into classes or orders.

* Includes the Chitons as a separate archaic grade “ Amphomoea,” the remaining
Gastropoda, all of which are asymmetrical, being placed in a higher grade as
“ Cochlides.”

t Includes Siphonopoda (Cuttles and Nautilus) and Pteropoda.

E. BAY LAXKESTEK,

Exeter College, Oxford.

September, 1878.

TABLE OF CONTENTS,

Paragraph Page

I. 2. Introduction

3 — 10. Scope of Comparative Anatomy 1

General Part.

II. structure of the animal body ....... 13

Organs and organism 13

12. Differentiation ........... IT

13 — 15. Earliest stage of the animal organism ..... 15

The cell 15

16. Differentiation of the animal organism .... 18

17. Origin of the tissues 20

18. 19. A. Vegetative tissues 21

Epithelium .......... 21

20 — 23. Connective substances 23

24. Form-elements of the nutrient fluid 29

25. B. Animal tissues ...... .... 30

26. Muscular tissue . . . . . . . . .31

27. Nervous tissue .......... 32

28. 29. Origin of the organs ......... 34

30. Systems of organs ......... 37

a) Integument . . . . . . . . . .37

31. b) Skeleton .......... 38

32. 33. c) Muscles 39

34. 35. d) Nervous system 40

36 — 38. e) Sensory organs ......... 42

39. f) Kespiratory organs of the integument (Dermal gills) . . 45

40. g) Excretory organs ......... 46

41. h) Alimentary Canal ........ 47

42. Kespiratory organs of the enteron ..... 49

43. 44. i) Vascular system ......... 50

45. 46. k) Reproductive organs ........ 52

47. Metamorphosis of the organs 51

Development and degeneration ....... 54

48. Correlation of the organs ........ 67

xxii CONTENTS.

riinigrapli Pago

49. 50. Fuudameutal forms of tlic animal body .... 58

51. 52. Metamerism of tbe body ........ 61

53 — 55. Compai’ison of the organs ......... 63

56 — 58. Classification of the Animal Kingdom ..... 61

59. Bibliographical aids in Comparative Anatomy . . . . .75

Special Part.

First Section. ProtOZOa.

60. General review of the group . . . . . . . .75

Bibliography 77

61 — 70. Organisation of the Protozoa 77

Second Section. Coslenterata (Zoophyta).

71. General review of the group 89

Bibliography 90

72 — 78. Form of the body . 91

79. Appendages ........... 101

80. Integument 103

81. 82. Skeleton 105

83. Muscular system 108

81. Nervous system 108

85. Sensory organs 109

86 — 93. Alimentary canal .......... Ill

94—98. Generative organs 119

Third Section. VermeS.

99. General re^’iew of the group 125

Bibliography ........... 127

100 — 103. Form of the body 128

104. 105. Appendages . . . . * 132

106. External gills 135

107 — 111. Integument . 136

112. Skeleton 142

113. 114. Muscular system .......... 142

115 — 121. Nervous system . . . . . . . . . . 145

122. 123. Sensory organs . . .152

Tactile organs 152

124. 125. Visual organs . . . . . . . . . .153

126. Auditory organs . . . ’ 156

127 — 132. Alimentary canal .156

133. Enteric branchim 163

134. 135. Accessory organs of the alimentary canal 164

136. Coelom . . . ’ . ‘ . . . . . * . . . 165

CONTENTS.

xxiii

Paragraph Page

137 — 141. Vascular system .......... 166

142 — 145. Excretory organs .......... 172

146 — 156. Generative organs 178

Fourth Section. Echinodernia.

157. General review of the group 192

Bibliography ........... 194

158 — 161. Form of the body 194

162. Appendages 199

163 — 167. Integument and dermal skeleton ....... 200

168. Muscular system .......... 207

169. Nervous system 208

170. Sensory organs .......... 210

171 — 173. Alimentary canal 211

174. Organs appended to the alimentary canal ..... 215

175. Coelom 216

176. Vascular system .......... 217

Blood-vessels .......... 217

177. 178. Water-vessels 219

179. Excretory organs .......... 224

180, 181, Generative organs 224

Fifth Section. Arthropoda,

182.

General review of the group

. 228

Bibliography .....

. 232

183.

Form of the body ....

. 234

184.

Appendages

. 237

185.

Appendages of the Branchiata .

9

. 238

186. 187.

Branchia) ....

. 240

188—190.

Appendages of the Tracheata .

. 243

191—193.

Integument

. 248

194.

Muscular system ....

. 251

195—200.

Nervous system ....

. 252

201.

Sensory organs ....

. 260

Tactile organs ....

. 260

202. 203.

Auditory organs

. 261

204—206.

Visual organs ....

. 263

207—211.

Alimentary canal ....

. 267

212.

Organs appended to the alimentary canal .

. 273

1) Appendages of the fore-gut

. 273

213.

2) j, ,, mid-gut

. 274

214.

3) ,, „ hind-gut

. 276

215.

Coelom ......

. 277

216—220.

Vascular system ....

. 279

221.

Excretory organs ....

. 285

222—225.

Tracheae ......

. 286

226—237.

Generative organs ....

. 291

XXIV

CONTENTS.

Sixth Section. Brachiopoda.

Paragraph Page

238. General review of the group 306

Bibliography ........... 307

239. Form of the body .......... 307

210. Integument, shell, and arms ........ 308

211. Muscular system .......... 309

242. Nervous system and sensory organs 309

213. Alimentary canal . . . . . . . . . .310

241. Coelom and circulatory organs. ....... 310

245. Excretory organs 312

216. Generative organs 314

Seventh Section. Mollusca.

217. General review of the group 315

Bibliography 317

248 — 252. Form of the body 318

253. 254. Appendages 325

255. 256. Integument 328

257—259. Shell 329

260—263. Gills 335

264. Internal skeleton 341

265. Muscular system 342

266 — 269. Nervous system 343

Central organs and nerves of the body 343

270. Visceral nerves . . . . . . . . . .351

271. Sensory organs 351

Tactile and olfactory organs ....... 351

272. 273. Visual organs . 353

274. Auditory organs .356

275 — 279. Alimentary canal . 358

280. Organs appended to the alimentaxy canal ..... 363

1) Appendages of the fore-gut 363

281. 2) Appendages of the mid -gut 364

282. 3) Appendages of the hind-gnt 366

283. Coelom 367

281 — 288. Vascular system 368

289 — 292. Excretory organs . . 375

293 — 298. Generative organs 380

Eighth Section. Tunicata.

299. General review of the group . . 388

Bibliography 389

300. 301 . Form of the body .......... 390

302. Integument 393

303. Skeleton 394

304. Muscular system 394

305. 306. Nervous system 395

CONTENTS.

XXV

Paragraph Page

307. ' ■ Sensory organs 397

308. ' ' Alimentary canal . ' . . . » . . . . . . 398

309 — 311. ' Respiratory antechamber (Branchial cavity) .... 399

312. ' Enteron . . - 403

313. Vascular system 404

314. Generative organs . . . . • 406

Ninth Section. Vertebrata.

316. General review of the group ........ 408

Bibliography 412

316. Form of the body . . . ... . . . . 413

317. 318. Appendages 414

319. 320. Integument . . . . . . . . . . . 417

321 — 323. Epidermal structures . . . . , . . . . . 419

324 — 326. Dermal skeleton . . . . 422

327. , Internal skeleton . . , . ^ . . . . . . . 426

328 — 334. Vertebral column . . . , . . . . . . 428

335—337. Ribs 438

338. Sternum ........... 442

339. 340. Cephalic skeleton ......... 444

341—352. Skull 447

353 — 356. Branchial skeleton ......... 468

357. Skeleton of the appendages ....... 472

358 — 360. Anterior appendages ........ 474

Shoulder-girdle . . . . . . . . .474

361 — 365. Anterior extremity ......... 477

366. Posterior appendages 484

Pelvic-girdle .......... 484

867 — 369. Posterior extremity ......... 487

370. Muscular system .......... 491

871. Dermal muscles .......... 492

372 — 377. Musculature of the skeleton ....... 493

378. Electric organs .......... 499

379. Nervous system 501

380. 381. A. Central organs of the nervous system ..... 503

a) Brain .......... 503

382. 383. b) Spinal chord ......... 511

384. c) Investments of the central nervous system . . . 512

885. B. Peripheral nervous system ....... 513

386. a) Spinal nerves ......... 514

887 — 392. b) Cerebral nerves ........ 515

393. c) Visceral nerves 522

894. 396. Sensory organs 523

396. Olfactory organs .......... 525

897 — 399. Visual organs , . 527

400 — 403. Auditory organs 533

404. Alimentary canal .......... 539

405. Respiratory antechamber (Cephalic enteron) .... 540

406 — 409. Branchias ........... 541

XXVI

COIvTENTS.

Paragraph Page

410. Brancliial clefts and palate of the Amniota .... 545

411. 412. Nasal cavity 547

413. Buccal cavity 548

414 — 416. Organs of the buccal cavity ....... 549

417. Alimentary canal proper (Enteron of the trunk) .... 555

418. Fore-gut 556

419. Mid-gut 559

420. Hind-gut 561

421. Organs appended to the mid-gut ....... 563

422. Mesentery 565

423. Pneumatic organs of the enteric tube 566

424. a) Air-bladder 567

425 — 427. b) Lungs 569

428. Coelom 574

429. 430. Vascular system 575

431 — 436. Heart and arterial system 577

437 — 442. Venous system 589

443 — 446. Lymphatic system ........ . 597

447 — 449. Excretory organs 601

460 — 458. Generative organs 608

CORRIGENDA.

t

Page 79, six lines from top, for finer” read “firmer.”

„ 87, three lines from top, for “generation” read “gemmation.”

„ 88, two lines from top, for “ spiral ” read “ special.”

,, 88, in the explanation of Pig. 30, for “broad” read “brood.”

Introduction.

The Scope of Comparative Anatomy.

§ 1-

The department of science which has organic nature for the object
of its investigations^, breaks up into two great divisions, Botany and
Zoology, corresponding to the two kingdoms of organised nature.
The two disciplines together form the science of living nature —
Biology. They are closely connected with one another, in so far as
the phenomena seen in both the animal and vegetable kingdoms
rest on the same fundamental laws, and in so far as, notwithstand-
ing the differences of their special arrangements, animals and plants
have common beginnings, and are, in the economy of nature, closely
interdependent.

In both of these disciplines several kinds of investigation are
possible, and from these new disciplines arise. Putting aside the
realm of Botany, let us follow out that of Zoology into its further
subdivisions. The investigation of the functions of the animal
body, or of its parts, the reduction of these functions to elementary
processes, and the explanation of them by general laws, is the busi-
ness of Physiology. The investigation of the material substratum
of those functions, and accordingly of the phsenomena of form of
the body and its parts, as well as the explanation of the pheenomena
of form by reference to function, is the business of Morphology.
Physiology and Morphology have thus different duties, and their
methods also are different ; but for each it is necessary, although in
different ways, to keep in view the other, as well as the common
aim, which is indicated in the term Biology.

Morphology again is divided into Anatomy and Embryology;
while the former has for the object of its investigations the adult
organism, the latter has the growing organism as the object of its
study.

B .

2

COMPAEATIYE AYATOMY.

Anatomy may be divided into general and special Anatomy.
General Anatomy has to do with the fundamental forms of animal
organisms (Promorphology), and the morphological phaenomena
which arise from them. Special Anatomy takes for its object the
organological composition of the animal body. Histology, or
microscopic Anatomy, forms one of its branches, being the study
of the elementary organs of the animal body. Embryology
explains, by tracing their gradual development^ the complications
of the external and internal organisation, and, in fact, deduces
them from simpler conditions. The changes in organisation can be
followed out in the embryonic life of the individual, and also in the
continuous series of organisms. The discipline ordinarily known
as Embryology deals with the former; and as Ontogeny (or the
development of the individual) is contrasted with Phytogeny (or
the development of the phylum). As the latter includes the earlier
and no longer existent conditions of animals, it also embraces
Palaeozoology. It is the history of the development of the series
of organisms in their geological succession.

§ 2.

Since Anatomy has for its object the composition of organisms,
it may be considered as the doctrine of structure, and is divided,
according to the different points of view from which structure
itself may be regarded, into several divisions. When the com-
position of the body itself, its forms, and the relations of the separate
organs are taken as its scope, it is known as descriptive Anatomy,
for it describes the objects examined, without drawing any further
conclusions from them.. Anatomical fact is the aim of the investiga-
tion, and empiricism satisfies this aim. Owing to its relations to
medicine, and so to practical requirements, the descriptive Anatomy
of the human organism, so far as it is restricted to a special series
of facts, has become developed into a special branch, which, as
Anthropotomy, is put side by side with the similarly descriptive
Zootomy. The two differ only in their subject-matter, and not in
their treatment of it, for both are analytical. In proportion as
either abstains from drawing conclusions from its series of facts,
and giving these the value of abstractions, is it wanting in the
character of a science ; for a science is constituted neither by an
extensive range of observations, nor by the complication of the
methods by means of which such observations are made. A critical
appreciation of the scientific import of any branch of study has,
therefore, little to do with the mechanical apparatus of investiga-

INTEODUCTIOK

3

tiorij whicli has its value only in facilitating the discovery or demon-
stration of facts.

Anatomy assumes a very different character so soon as the
knowledge of facts is only its means, and its aim the conclusions
which can be drawn from an assemblage of such facts ; the facts of
individual phaenomena being regarded not by themselves exclusively,
but being brought into relation with one another. This happens
when what is alike in the organisation of different organisms is
made the object of search, and when the facts thus acquired are
compared. Anatomy thus arrives at scientific results, and shapes
the results of inductive inquiry into deductive conclusions. Thus it
becomes Comparative Anatomy. Its method is synthetical. The
analyses of Descriptive Anatomy (Anthropotomy as well as Zootomy)
provide the basis for it ; they are consequently not only not excluded
from Comparative Anatomy, but are most closely embraced and
logically permeated by it. The more careful the sifting of facts,
the surer the basis of comparison. Empiricism is thus the first
requisite, and abstraction is the second. Abstraction has no basis
without pre-existing empiricism; and empiricism by itself is, from
the scientific point of view, a mere stepping-stone to real knowledge.

§ 3.

The task of Comparative Anatomy is the morphological ex-
planation of the phasnomena of form met with in the organisation of
the animal body. Comparison is the method which serves for the
performance of this task. It shows the way which scientific investi-
gation has to go, and which it is necessary to know in order to avoid
disjointed and fruitless labour. The comparative method seeks
to test, in series of organisms, the morphological results of the
observation of the organs of the body, places together similar
characters, and separates the dissimilar from them. Thus it takes
into consideration everything which can in any way be looked at
as the result of anatomical observation : relation to other parts of
the body, form, number, extent, structure, and texture. It thus
collects series of stages for the several organs, in which the extremes
may be so far different from one another as not to be recognised,
but which are united to one another by numerous intermediate steps.

It is clear, in the first place, from the existence of various forms
of one and the same organ, that the physiological value of an organ
in different stages is not by any means the same, but that an organ,
as its anatomical characters are modified, may come to have very
different functions. The exclusive consideration of their physio-

B 2

4

CO^IPAEATIYE AXATO:\rY.

logical duties may thus bring organs which are morphologically
connected into very different categories. Thence results the sub-
ordinate importance which we must assign to the physiological
duties of an organ when we are engaged in an investigation in
Comparative Anatomy. Physiological value is only to be regarded
at all, and then in the second place only, when we are trying to
make out the relations to the entire organism of those modifica-
tions which an organ may have undergone as compared with some
other condition of the organ.

By this examination of anatomical facts, by means of com-
parison, Comparative Anatomy demonstrates the connection of entire
series of organs. Within these series we find changes of the
most varied range, sometimes slightly, sometimes widely extended ;
modifications which affect the size, number, form, and even the
texture, of the parts of an organ, and which may even lead to
changes in its situation. The review of such a series teaches us
then to recognise a progress presented in those several successive
stages, which the modifications of one and the same organ in different
animals exhibit to us.

§ 4.

We ascribe the existence of a certain amount of similarity in
the organisation of certain larger or smaller divisions of the animal
kingdom to Transmission — a phsenomenon which is exhibited in
the passing on of its organisation by a given organism to its posterity.
The descendants repeat the organisation of the parental organisms.
This is an indubitable fact. Nevertheless now and again objections
are raised either to the existence of Transmission or to its signifi-
cance. The similarity of the organisation of the descendants and
their ancestors is then ascribed, not to Transmission, but to certain
physical forces acting during embryonic life. In reply, we may
ask, how does it happen that in ancestor and descendant these
forces are the same — viz. all those forces of tension, of pressure,
and so on, from which it is sought to deduce the building up of the
embryo ? If, for example, a joint gets its ontogenetic development
by the movement of the parts of the skeleton by means of muscular
activity, a certain arrangement of the muscles is presupposed, and
a perfectly definite structure of the muscles; and for these again,
a perfectly definite number and arrangement of the morphological
elements which make up a muscle. This being so we must ask,
whence comes the definite arrangement of these parts ? whence arises
the similarity of arrangements in the ancestors and the descendants ?

We find, in fact, that we must give full recognition to the exist-

INTEODUCTIOK

5

ence of the transmission of properties, and recognise in it a
phsenomenon of general prevalence, which may indeed present
modifications of, hut never exceptions to, certain definite laws. We
may deduce it from the conditions involved in propagation, and thus
explain it to a certain degree; for it is clear that portions of an
organism, if they give rise to a new organism, will carry on to it
the peculiarities which the primitive organism possessed. This is
clearest in the lower organisms, which are propagated by mere
division. Each portion forms at once an organism like the first.
But from this there extends a continuous series of methods of pro-
pagation, up to those in which generative products come into
action, which are quantitatively very different, although in all cases
derived from the division of the parent organism.

The new organism in this case also represents in actual sub-
stance the continuation of the ancestral, and will therefore possess
qualities which agree with those of the latter.

The amount of similarity or agreement in the organisation of
animals is very various. We recognise animals which differ
from one another by slight points only ; then those which are
separated by considerable differences ; and again others which, in
external or internal organisation, present the greatest differences.
Thus agreement, as well as variation, is found in interminable
gradations. We call things which are more or less like to one
another, related ; and in like manner, when organisms exhibit
likeness, we use that word to denote the reciprocal connection, but
in this case we give to it its full meaning of blood-relationship.

We recognise similar organisms as related to one another,
when we can explain the similarity of the organisation by common
inheritance. But the degree of this similarity measures the degree
of relationship which we can deduce from it. Relationship can be
regarded as close when the differences are slight ; while when the
differences are great it must be regarded as more distant. We thus
substitute for the conception of the agreement, or likeness, of the
organisation, that of relationship, for we regard the agreements
which obtain in the organisation of a collection of organisms as
inherited peculiarities.

The doctrine of the Blood-relationship of Organisms or Phy-
togeny is based on the law of inheritance. Comparative anatomy
thus reveals the relations of affinity within the various divisions of
the Animal Kingdom by pointing out what is alike and what unlike.
[A full account of this most important law of inheritance and
its phaenomena is to be found in Hackel^s luminous essay on the
subject (Generelle Morphologic, vol. ii. p. 170).]

6

COMPAEATIVE ANATOJ^IY.

§ 5.

By means of inheritance, characters are passed on to the
organism, which are afterwards matured in the course of its indi-
vidual development (Ontogeny). There is no such development in
the simplest forms, inasmuch as the young, which arise by division of
the maternal organism, only need increase in size to make them like
the maternal organism. In this case, development is the same thing
as growth, which is completely coextensive with it. The farther
an organism is from a primitively simple condition, or the greater
the sum of characters which have been inherited from its ancestors
and transmitted to their descendants, the less simple is its ontogeny ;
for during it a part at least of the characters which have been
inherited from its ancestors are repeated, and are presented by the
developing body in several successive stages. Ontogeny thus
represents, to a certain degree, palaeontological development, ab-
breviated or epitomised. The stages which are passed through by
higher organisms in their ontogeny, correspond to stages which
are maintained in others as the definitive organisation. These
embryonic stages may accordingly be explained by comparing them
with the mature stages of lower organisms, since we regard them as
forms inherited from ancestors belonging to such lower stages.
Kegarded from this point of view, many of the so-called larval-
stages, with their provisional organs — so named because they are
transitory, and limited to the earlier stages of life only — are seen
to be forms of great importance, and full of significance. Such
organs, besides having physiological relations to the organism
which possesses them, in consequence of which they are preserved
as practical arrangements, and become heritable, can be recog-
nised in lower grades of the existing series of animal forms, and
thus reveal the phytogeny of the animal that possesses them. The
stadium larvatum^^ then, notwithstanding its name, often points
out with great clearness the blood-relationship of an organism. At
times these larval organs are not so well explained by transmission
as by adaptation, and thus the estimation of their true meaning is
made more difficult. The significance of these arrangements is more
obvious in organisms which do not enter immediately into the
struggle for existence in the external world, but are developed for
a certain time within the coverings of the ovum, and so are less
exposed to the moulding influences of the outer world. In these
cases they are provisional arrangements, and may be with greater
certainty regarded as having been transmitted, and consequently as
repetitions of lower stages. The branchial clefts which appear in

INTEODUCTION.

7

tlie embryos of tbe higher Vertebrata^ but by-and-by disappear,
are structures of this kind. Regarded alone they are inexplicable,
for they neither lead to the formation of gills at any time, nor are
they converted (with the exception of the anterior) into definitive
organs of any other kind. Comparison shows us, however, that in a
large division of lower Yertebrata these branchial clefts are important
organs of respiration; and as we also know Yertebrata (Amphibia),
in which the clefts function only for a time as respiratory organs,
and close up later on, we are able to comprehend the branchial
clefts of reptiles, birds, and mammals, as arrangements obtained by
transmission from lower stages, which have lost their primitive
function, and remain for a short time only — during foetal life.

§ 6.

In the sum of the characters of the organisation, which inherit-
ance passes on to an organism, we find, in consequence of what has
been already pointed out, a greater or smaller number of arrange-
ments, which pass on into the permanent adult stage of the
organism without having any recognisable function in it. Such
]3arts are, as a rule, seen in a more or less atrophied and rudimentary
condition, which they often do not acquire until the ontogeny has
run its course. In the early stages of the ontogeny they generally
agree in completeness with the other arrangements which obtained
in the ancestral form from which they are derived. These rudi-
mentary organs commence to atrophy the earlier in proportion as
they were inherited earlier, in a palseontological sense ; and, as a rule,
disappear late when their origin is a relatively late one. The fully-
developed form of the rudimentary organs is consequently to be
found, in the former case, in widely separated species ; in the latter,
on the other hand, in species more closely allied. These organs
are valuable objects, since phylogenetic relations can be very
generally recognised by their aid. They show, too, how little
physiological significance ought to be regarded in a morphological
discussion ; for in most of them a function is not to be made out at
all, or, if it can be made out, is found to be quite different to the
primitive one.

§ 7.

Comparative Anatomy forms part of Ontogeny, inasmuch as it
treats of the phsenomena of the organisation which appear in the
course of the individual development of the animal ; not only in
relation to the complete stage of the organism, but in relation also
to the definitive arrangements of other organisms. Comparative

8

COMPAEATIYE Ai^ATOMY.

Anatomy explains the phenomena of Ontogeny. Ontogeny
by itself does not rise above the level of a descriptive discipline^ and
in proportion to the exactness of its investigation possesses a value
as so much objective material. At the same time Ontogeny
gains scientific importance by its connection with Comparative
Anatomy. Its facts^ which by themselves are incomprehensible, or
are only teleologically explicable in a metaphysical sense, because
restricted to the later events in the history of an organisation,
are, by Comparative Anatomy, put in connection with the known
phsenomena of other organisms, and are thereby rendered explicable
phylogenetically. The necessity of an exact knowledge of Compara-
tive Anatomy for Ontogeny is suflS.ciently obvious. Just as little
can the former separate itself from the latter : since from Ontogeny
Comparative Anatomy gains an insight into the lower stages of
organisation. To the same extent, and in the same way as On-
togeny helps to form the basis of Phylogeny, does it render
indispensable service to Comparative Anatomy.

A Comparative Embryology has sometimes been put in
contrast with Comparative Anatomy ; of course merely as a theo-
retical division of the scope of study. A comparative^^ Ontogeny
of this kind must, just as much as every individual ontogeny,
have regard to the organisation of the fully-developed stage ; and,
in fact, without Comparative Anatomy it cannot lead to any
scientific results.

§ 8.

The relations of every organism to the outer world in which it
lives, from which it obtains material, and to which again it gives it
up, cause the outer world to have an influence on the organism.
This influence is practically seen in the changes of the organism,
which depend further on a Variability which is inherent in it.

Variability comes under our observation as the capacity for
adaptation, and in effect operates as a modifying and even metamor-
phosing agent upon the inherited organisation.

The organism is altered according to the conditions which in-
fluence it. The consequent Adaptations are to be regarded as
gradual, but steadily progressive, changes in the organisation, which
are striven after during the individual life of the organism, pre-
served by transmission in a series of generations, and further deve-
loped by means of natural selection. What has been gained by the
ancestor becomes the heritage of the descendant. Adaptation and
Transmission are thus alternately effective, the former representing
the modifying, the latter the conservative principle. The endless
variation of the phsenomena of organisation is, we see, consequently

INTEODUCTION.

9

due to Adaptation, just as we have seen that similarity is due to
Transmission.

§ 9.

Adaptation is commenced by a change in the function of organs, so
that the physiological relations of organs play the most important
part in it. Since adaptation is merely the material expression of
this change of function, the modification of the function as much as
its expression is to be regarded as a gradual process. As a rule,
therefore. Adaptation can be perceived by its results only in a long
series of generations, while transmission can be recognised in every
generation. Although Adaptation as a process cannot be directly
observed, it is nevertheless possible to infer it with certainty by
comparison. When, for example, we find a simple structure of the
stomach in the Carnivorous Mammalia, and, on the other hand, a
more complicated one in the Herbivora, and especially in those
which take in large quantities of food, as, for example, the Eumi-
nantia, we are entitled to consider the complication in the structure
of these stomachs as a change caused by the food — as an Adaptation
to the mode of nutrition ; and when, further, the ontogeny of the
Ruminantia shows us a form of stomach which is simple in the early
stages of development, and is gradually converted into the more
complicated condition, ontogeny confirms the supposition we have
already gained by comparison. In many cases the influence of
Adaptation on the organisation can be observed also directly ; as, for
example, in many Amphibia, where the branchise which are de-
veloped during the larval stage are retained in function for an
extended period, if the opportunity of escaping from the water does
not arrive ; and in others, again, where the branchiae undergo
atrophy, as soon as their aquatic life has been exchanged for one on
land, although their nearest allies live in the water, and always
retain the branchiae. In the one case we see development, and in
the other atrophy, as phaenomena resulting from Adaptation.

In Adaptation, the closest connection between the function and
the structure of an organ is thus indicated. Physiological functions
govern, in a certain sense, structure ; and so far what is morpho-
logical is subordinated to what is physiological. This dependence
of the form of an organ on its activity is seen in the most elementary
w^ay in the matter of size. When the function is increased, there is
an increase in the size of the organ. The muscular system shows
to what extent increase of activity affects size. Without exercise
the muscles undergo degeneration, till they completely disappear.
If they are kept in exercise, and if the demands on them are
increased, they develop to a considerable size. The amount of

10

COMPAEATIVE AA^ATOMY.

development is in the closest connection witE tEe amount of activity.
But siuce^ wEen a function ceases or diminisEes, atropEy commences^
we obtain^ as a result of tlie process, rudimentary organs. TEey
owe tEeir origin to atropEy. PEysiology alone, tEeu, can give us tEe
explanation of tEe origin of tliese organs ; and tlius again we are
led to observe tEe great influence wEicE it exerts on tlie study of
MorpEology.

§ 10.

An organ can be so mucE cEanged by tEe gradual modiflcation
of its function tEat it becomes, from the physiological point of view,
a new one, and is then placed in quite another physiological category
of organs. This fact is of considerable importance, for it helps to
explain the appearance of new organs, and obviates the difficulty
raised by the doctrine of evolution — viz. that a new organ cannot at
once appear with its function completely developed ; that it there-
fore cannot serve the organism in its first stages whilst it is gradually
appearing ; and that consequently the cause for its development can
never come into operation. Every organ for which this objection has
the appearance of justice can be shown to have made its first ap-
pearance with a significance differing from its later function. Thus,
for example, the lungs of the Vertebrata did not arise simply as a
respiratory organ, but had a predecessor among fishes breathing by
gills, in the swim-bladder, which at first had no relation to respiration.
Even where the lungs first assume the functions of a respiratory
organ (Dipnoi, many Amphibia) they are not the sole organ of
the kind, but share this function with the gills. The organ is there-
fore here caught, as it were, in the stage of conversion into a
respiratory organ, and connects the exclusively respiratory lungs
with the swim-bladder, which arose as an outgrowth of the enteric
tube and was adapted to a hydrostatic function.

The earlier function of an organ which by adaptation is converted
to new uses is generally a lower one, and less important for the
organism, in comparison with the new function which is taken on,
so that the organ thus rises to a higher grade. In other cases the
value of the primary function is less, because it is shared by other
similar organs. It is then quantitatively lower, for a share is taken
by the other similar organs in discharging the total amount of the
function necessary for the full activity of the organism. The
atrophy of some of the organs which are of equal value raises the
value of those that remain by causing their higher development.
To these facts, as to their change of functions, the difference in the
classification of organs, accordingly as we make use of a physio-
logical or a morphological method, is due.

GENERAL PART.

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THE ORGANISM.

13

Structure of the Animal Body.

The Organs and the Organism.

§ 11.

In the living body we observe a number of activities of its
material substratum, by which the series of phaenomena spoken of as
life are conditioned. Underlying these, there are chemico-physical
processes, which are accompanied by a continual degradation of the
material, and consequent metastasis, or change in the arrangement
of chemical elements. The body nourishes itself by replacing the
material used up in metastasis by fresh matter, which is received
from without; and this it assimilates, or makes like to the substances
of which it is itself composed. The substances, partly taken in with
the nutrient matter, partly produced by metastasis, which are of no
further use in the organism, are passed out. Hence results the
excretory function. If the quantity of matter assimilated is greater
than that which is expelled, there is an increase in the size of the
body: it grows. Thus it fulfils the first condition for the production
of that material from which a new organism, like to itself, arises :
and so reproduction is closely connected with nutrition.

The body is, in the first place, in relation to the exterior by its
surface ; this puts it in connection with the surrounding medium.
Changes in the form of the surface of the body result in movements,
and give rise to locomotion. The surface also is the medium by
which it perceives the outer world, or has sensations.

The parts of the body which preside over these processes are the
instruments by which life is carried on — organs. In virtue of
their existence the body is an organism, and when we also include
under the term organisms certain bodies in which no organs can
be individually separated, it is because the virtual existence of
organs in them is to be assumed from the mere fact that life is
carried on in them. The term organism is therefore employed
in this instance, not in an anatomical, but in a physiological sense.
In the simplest condition of the organism, the vital phaenomena
take place in the homogeneous substance which forms the body,
and which is the seat of all these processes equally. The body

14

COMPAEATIYE AXATOMY.

in this case represents, potentially, a collection of organs, which
only appear, in fact, when the different functions are no longer
performed by every part of the body. The condition which the
lower organisms permanently exhibit in regard to this matter is
possessed for a short time only by the more complicated.

Differentiation^

§ 12.

The complication of the organism arises from a process of
division which transfers to separate parts the physiological acti-
vities of the primitively homogeneous body. What was previously
accomplished by the whole body, is, subsequently to that process,
carried out by particular portions. The work is then done either by
a large number of parts, which are distinct from but similar to one
another, or the separate parts acquire dissimilar shapes, and become
different from one another. In the first case the division of labour
is quantitative, in the latter it is qualitative, and the separation
of the different parts is in correspondence with a difference in func-
tion. According to the degree in which the separation or division
originally set up in the primitively indifferent body is repeated in the
organs derived from it, further complications arise, which present
for our observation a step-by-step progression in development.
Hence there arises a difference in the value of the organs, and it
becomes necessary to distinguish their higher and lower conditions.

The distribution of work amongst a number of different organs
leads to the perfecting of the operations of such organs. Each
organ is enabled to develop in a definite direction, in correspond-
ence with the particular function which is undertaken by it. The
organism thus becomes more highly developed, as well as com-
plicated. Division of labour leads to a perfecting of the whole
organism. According as the division of labour involves only a
few or many organs, a greater or less part of the organism is
brought under the operation of this perfecting influence. The
greater the importance, for the whole organism, of the organs
affected, the more considerable is the perfecting accomplished in
it through their modification. The functions which attach them-
selves to definite parts of the body bring about a difference in the
development of those parts proportionate to their own difference ; and
thus it is that new parts and new organs arise, which are different
from those already existing. The division of functions leads to the
establishment of a difference, that is to a differentiation, of the
parts. A part of the body which was formerly like the rest, and con-
sequently not different from it — that is was indifferent — passes into
the condition of being separate, becomes distinguishable, or different
from the rest. And as this differentiation is connected with the
division of labour, inasmuch as it is conditioned by it, it may be
regarded as the product of it. Every physiological function can be
again divided qualitatively into various sub-functions, by the locali-

THE CELL.

15

sation of wLicli fresli organs again arise. Tims the principle of the
division of labour is the cause of very great variation in the organi-
sation, and all morphological phaBnomena are more or less closely
connected with it, and with the differentiation which is due to it.

First Stage of the Animal Organism.

The Cell.

§ 13.

Living matter appears in its simplest form as an albuminous sub-
stance, known as Plasma, or Protoplasm, which, by the aid of our
present optical instruments, seems to be homogeneous throughout.
This substance occurs in the form of small lumps, in which condition
we find the simplest organisms. In those simplest forms, where the
protoplasm is homogeneous, and in which only a few granules at
most are present as heterogeneous elements, there is no limitation of
the lump to the exterior by distinct enveloping structures ; but in
organisms of little higher grade we find an envelope produced by a
chemico -physical change in the most external layer of the proto-
plasm. Thus the protoplasm, which is endowed with all the
phaBnomena of life, and even of movement, is enclosed by a more or
less firm envelope, which forbids alterations in form, and is the
cause of a definite shape being maintained. Such structures may
be combined to form complex organisms, as is the case in many of
the lower plants. This kind of form-element, or morphological
unit, is known as the cy tod, and is rightly distinguished from another
more highly-developed form.

In this higher form there arises in the protoplasm a sharply
marked-off dense structure, which is called the nucleus. It is the
product of the first process of differentiation of the protoplasm, which
no longer alone represents the living substance. In the nucleus a
small body, the nucleolus, generally appears. The nucleus, unlike the
protoplasm, is not contractile, or at any rate has not a large share of
contractility; but it not only takes a part in most of the vital phasno-
mena of the surrounding protoplasm, but frequently gives evidence
of being their regulator. Such corpuscles of protoplasm as are pro-
vided with a nucleus are called cells (cellulaB). These structures,
like the cytods, may form independent organisms, which are then
called unicellular.'’^ When the cells form a complex by multi-
plication, we have a multicellular organism. The smallest parts of
multicellular organisms, no longer separable into constituent pieces
like to one another, are cells; and are therefore the form -elements
of these organisms. The same remark applies to the cytods, or the
simpler condition. While these, however, are rarely present, we find
cells widely distributed in the Vegetable Kingdom, and as the sole
form-elements in the Animal Kingdom.

16

COMPAEATIVE AXAT0:MY.

§ 14.

In tEe indifferent condition, that is as long as changes in a
definite direction do not arise, leading to the formation of definite
new structures, the cells of all animal organisms appear to have
essentially the same character. In them we distinguish according
to what has been remarked above : first, the protoplasm, which
forms the principal mass of the body of the cell ; and secondly, the
cell-nucleus, surrounded by, and different from, the protoplasm, than
which it is usually more dense. The share which this nucleus takes
in many varied pheenomena of the life of the cell, compels us to
regard it as by no means a subordinate part of the cell-body. In
addition to these parts of the cell, some persons recognise (formerly
everyone did so) a membrane which differs from the protoplasm or
contents of the cell, and envelopes it : from its presence arose the
notion of the vesicular form of the cell and its name.

Although it cannot be denied that in many cells there are
envelopes differentiated from the protoplasm, yet this condition is
never found in the earliest life of the cell, but is always the re-
sult of an advanced change, and of the passage of the cell into a
differentiated form.

Automatic movements of the protoplasm of the cell are
such common manifestations of their life, that they are always
definitely apparent as a property of all cells which are not highly
differentiated, that is of cells in which the protoplasm is not meta-
morphosed. In free cells, and such as are not enveloped by firm
membranes, this phaenomenon of movement produces locomotion.
Even in cells that are not free movement may be observed, consisting
partly in a change in the form of the surface, partly in a change
in the position of the granules in the protoplasm. That there are
also properties resident in protoplasm which we may attribute to
sensibility of a very low grade results from many experiments
and observations.

We may further observe nutrition in the cell. At times, indeed,
we can see the protoplasm ingesting food; in all cases the growth
of the cell is an express indication of its nutrition. This phaeno-
menon of growth, which may be seen in any indifferent cell, is
expressed by the increase in size of the protoplasmic body, owing to
the assimilation of matter from without. The growth of the cell
may be quite regular, increase in size obtaining in all directions, as
is the case with all cells in their earliest stages ; as long as this
lasts the cell completely retains its spherical form, unless its move-
ments or external influences modify it. Or growth may be unequal,
and the cell become elongated by increase in size along one axis ;
or, again, it may become stellate by growing along several axes.
Irregular increase of this kind is ordinarily accompanied by differen-
tiation of the cell-substance, and is therefore the commencement of
the conversion of the cells into tissue.

THE CELL.

17

§15.

Anotlier pliaenomenon — that of reproduction — is a result of, and
is indissolubly connected with, the growth of the cell ; for multi-
plication is merely the extension of growth beyond the limits of indi-
vidual cohesion. There are various modes of cell-multiplication ;
the simplest of these is a direct result of growth. A bud is formed
by the cell-body growing out on one side. This gradually increases
in size, and breaks off from the mother-cell, when it becomes a new
free cell. The number of young cells which are budded off from
a single cell is not always the same, also the part taken by the
nucleus of the mother-cell in the process, varies. This mode of
multiplication by gemmation passes imperceptibly into the more
common mode of multiplication by fission. Gemmation is charac-
terised by the difference in size which obtains at first between
the cells that are formed and their mother-cell. If they break
off at once they do not nearly equal it in size ; if they delay their
separation from the mother-cell they gradually get to equal it,
and then the products of division are almost or altogether equal to
one another, so that there is no possibility of distinguishing mother
from daughter. It is evident that in proportion to the extent to
which the products of division differ from one another in size,
does division become more and more like gemmation; the whole
difference therefore between fission and gemmation lies in the
amount of protoplasm which is given over by the parent- cell to the
one which arises from it. The difference is a quantitative one
merely. Division commences by an enlargement of the nucleus ; in
some cases by a formation of fresh nuclei.

No form of reproduction other than multiplication by fission oi'
by gemmation has been certainly observed in the animal-cell ; a
large number of the modes of cell-multiplication, which have been
stated to obtain, such as the so-called endogenous cell-formation and
similar processes, are merely forms of fission. As to free or spon-
taneous cell-formation, so much at least is certain, that it is not as
common as was once supposed.

When the nucleus divides and the cell goes on growing without
the protoplasm becoming marked off into separate portions corre-
sponding to the nuclei, the structure which is formed cannot be
any longer regarded as a single cell. But it is not a compound
of cells either, for this would presuppose the existence of a number
of separate cells. This condition has therefore been very rightly
regarded as a special one, and called a Syncytium. Structures
of this kind are found in nearly all groups of animals. The same
result is obtained by the Concrescence of a number of separate
cells, the protoplasm of which runs together into a continuous
mass, in which there are of course a number of nuclei.

While the protoplasm in the above-mentioned series of phseno-
mena undergoes no perceptible changes in constitution, a change

c

18

COMPAEATIYE ANATOMY.

in tlie protoplasm is essential to another kind of pkaBnomena, to
which we now pass. The substances contained in the protoplasm
become separated from it_, that is^ are secreted from it. This
process of secretion varies in character; it sometimes occurs within
the protoplasmic body, and then substances differing in their chemico-
physical properties from the protoplasm are formed within the cell.
These substances may be distinguished by their constitution, such as
fat, colouring matter, and so on; or by form, such as granules, drops,
crystals. Sometimes secretion affects the surface of the protoplasm;
and then the secreted substance may be fluid, in which case it will
get separated and removed from the protoplasm; or it may be
solid, in which case it will remain more or less intimately connected
to the rest of the unaltered protoplasm. Substances which are
different from the rest of the protoplasm, of a cell arise by chemico-
physical changes of the whole surface, or of a part of it. We
may regard these changes in the protoplasm as differentiations,
for they are differentiated from matter which was previously in
an indifferent condition within the protoplasm. In this way the
structure which has been already alluded to as the cell-membrane,
is formed at the periphery of the cell. But this same process leads
to other arrangements also, which we shall have to examine more
closely hereafter.

The series of vital processes exhibited by a single cell essentially
agrees with those exhibited by any and every other organism, so
that the cell itself is virtually an organism (Elementary organism).

Differentiation of the Animal Organism.

§ 16.

The simplest and lowest stage of the animal organism is repre-
sented by the earliest stage of its development, in which it is
known as the egg. Except in some exceptional cases, which only
prove the rule, and which need not be mentioned here, this egg
is nothing more nor less than a cell. The
egg-cell does not differ from other cells in
any essential points; it may increase in
size, and special particles — yolk granules —
may appear in its protoplasm. Although tbe
presence of these latter alters the character
of egg-cells as indifferent cells, yet it
does not destroy their character as cells,
which is just as little affected by this as in
other cells by the differentiation within their
bodies of substances, such as chlorophyll
granules, starch, pigment granules, &c. This
condition of the egg- cell, which on the whole

Fig. 1. Diagram of
an Ovum, a Granular
protoplasm, h Nucleus
(Germinal vesicle) . c Nu-
cleolus (Germinal spot).

DIFFEEENTIATION OF THE OEGAHISM.

19

is a simple one, agrees in character, althougli it may be for a time
only, with many lower unicellular organisms (Protoplasta).

The egg-cell undergoes changes, which ordinarily commence
after impregnation, and which are accompanied by changes in the
nucleus (the so-called germinal vesicle). In its place, and in part
from the material which formed it, two new nuclei arise, and

Figs. 2-5. Various stages of the so-called cleavage process (Division of the egg).

the egg-cell now begins to divide. Two cells thus arise, which
are either like one another, or differ from one another, in
size or in constitution. In both cases something fresh has arisen
from the egg-cell, and in both there is a differentiation, for
two parts have arisen from it. Four, eight, sixteen cells, and
so on, are formed by continued division, although of course
not always quite regularly, until at last a number of cells are
formed. This process of the division of the egg- cell is known as
the ^‘’segmentation of the yolk,^^ and is a constant phaenomenon,
although it may present various modifications, which are always
due to adaptation, and which may be so explained.

This is the first course of differentiation in the organism ; in
place of a single cell, a number of cells, similar to, or different from
one another, arise. The functions of the organism, which were
all performed previously by the egg- cell, are now performed by
the separate cells. The division of the egg- cell must therefore be
considered as leading to a division of its functions, although
indeed this division is merely a quantitative one.

The various stages of this process of division have other relations
also, for they appear to agree in character with the mature stage
of many lower organisms (Protista), as for example the Volvo cine^
and Catallacta, in the developmental history of which there is at one
time an organism composed of a number of equi-formal cells. The
animal organism, therefore, even in the commencement of
its ontogeny, passes through several morphological stages,
which are permanent among the Protista, and the process
of segmentation of the ovum may be explained as a sur-
vival transmitted from early ancestors. Accordingly the
teleological halo, with which it would necessarily be sur-
rounded, were we limited to seeking its explanation exclusively
in connection with the future organism which is to arise from
this segmentation, is cleared away. The organism does not, how-
ever, get a specifically animal character from this formation of

0 2

20

COMPAEATIVE ANATOMY.

a number of cells ; that character first makes its appearance in the
course of further processes of differentiation.

These processes of differentiation consist in the more or less
similar morphological elements (cells) which represent the organism,
acquiring, in larger or smaller groups, distinct characters : in their
being differentiated, and forming the rudiments (first stages) of
organs, by taking a definite order and arrangement. These organs
then are made up of cells, which form their tissues. We thus
arrive at the essence of the architecture of organisms ; we have
tissues, which make up organs, and are themselves composed of
form -elements — the cells.

Origin of the Tissues.

§ 17-

The cell, then, in those organisms which we regard as animals,
constitutes the whole of the organism only for a time ; that is, so
long as it is an egg- cell. By division a multitude of cells is formed
out of the egg- cell, and these form the rudiments of the animal. In
later stages a part only of the material formed from the ovum
retains the primitive character of the cells ; the form and substance of
most of the cells are altered, and therefore their physiological
properties are altered ; that is, new relations are established. The
new cell-complexes formed from aggregates of similarly altered
cells, and their derivatives, are the tissues. The process which leads
to the formation of these tissues is essentially a differentiation.
This, again, affords us an example of division of labour, for each
differentiated aggregate of cells has to perform a definite function
for the organism, which function was not the duty of a definite set
of cells when the cells were indifferent, and indeed were performed
in common with all others by one cell only (the egg- cell), in the
earliest condition of the individual organism.

In all cases histological differentiation commences in the proto-
plasm of the primitive cell ; the nucleus is less strikingly affected,
but numerous changes may be seen to occur in it. When the chief
part is played by a substance differentiated from the protoplasm, the
nucleus becomes of but slight importance. According to the
characters of their form-elements the tissues are divided into several
large groups : these I call Epithelial tissues, tissues of the
Connective Substance, Muscular and Nervous tissue. The
first two form a lower group, which, as Vegetative tissues we may
distinguish from the other two, which are the Animal tissues.
The difference between the two groups lies in the quality of their
differentiation ; the products of the differentiation of the former
having a more passive relation to the organism, while the products
of the differentiation of the latter exhibit an independent activity in
the carrying on of the life of the organism. The vegetative group,
or tissues analogous to them, are, moreover, most widely distributed in

THE TISSUES.

21

the Vegetable Kingdom ; whilst in that kingdom the animal tissues,
which are the source of the arrangements characteristic of animals,
are wanting. All other tissues, though often distinguished by name,
are either not independent tissues at all, but only much more
complicated structures formed of a variety of tissues, or are forms of
tissue which should be ranged under one of the above-mentioned
categories, or may be merely component parts of the tissues already
named. We overlook the true conception of what a tissue is if we call
structures which are made up of several tissues compound tissues."’^

A. Vegetative Tissues.

Epithelium.

§ 18.

Cells placed side by side, and forming one or more layers which
invest the surface of the body or the walls of internal spaces, are
called epithelial. Epithelial tissue, then, consists simply of cells.
It is distinguished from other tissues by the fact that the cells, at
least so far as their arrangement is concerned, retain their primitive
characters. Epithelium represents phylogenetically, and therefore,
also, ontogenetically, the oldest form of tissue. The germinal
layers which are the earliest organological products of the differ-
entiation of the masses of cells which arise from the segmentation
of the egg-cell, are layers of epithelial cells. Epithelial cells vary
greatly in form, and are the starting-point of various
organs. The protoplasm of epithelial cells very often
loses its homogeneous character, owing to the differen-
tiation of its outermost layer into a thickened membrane.

In stratified epithelium this is best seen in the superficial
layers, the absence of a membrane in the cells of the
deeper layers being an indication of their younger con-
dition.

Another differentiation is the development by the
superficial layer of cells on the surface exposed to the ex-
terior, or lining an internal cavity of the body, of fine pro-
cesses, which are capable of movement ; these processes,
which vibrate during the life of the cell, are known as
cilia. The hairs on these ciliated cells are sometimes
in the form of a single flagellum, or occur in a group
of many as cilia. In the former case the cell runs out
into a fine process, and forms a flagellate cell; these
are most common in the lower animals. Cilia are shown
to be differentiated products, since their movements are
not simply effected by the contractility which is already
inherent in the protoplasm. In many of the lower or-
ganisms cilia are formed for a time and are again drawn in and
their substance fused with the protoplasm. Tins shows that they

1) a,

Fig. 6.
Flagellate
cells, a of
a Hydroid
Polyp, b of
a Sponge
(collar
cell).

22

COMPAEATIVE AIS^ATOMY.

are differentiations of tEe protoplasm^ and tEat tEeir movements are
due to tEe same cause as tEe movements of tEe protoplasm. TEis
indication of tEeir identity witE protoplasm cannot be seen in tEe
more differentiated forms of cilia.

AnotEer differentiation may be seen on tEe outer surfaces of
many epitEelial cells. A membrane^ instead of being formed by a
change of tEe wEole periphery of the superficial layer of protoplasm,
may be formed on a definite portion of it only : in this case it is more
highly developed, and may lead to a partial thickening of the outer-
most layer of protoplasm. In short, a layer of varying thickness
of a substance different from, but as a rule still connected with,
the protoplasm, forms on the outer face of each cell. Homogeneous
membranes — cuticles — are formed by the further differentiation of
the substance thus secreted in a layer from the protoplasm of the cells ;
that is by the part formed from each cell becoming more intimately
connected with the layers formed by the cells around it than with
its own cell. Where these layers are laid down irregularly and
gradually undergo other changes, by means of which each fresh
addition can be distinguished from the preceding one, they become
laminated. The more the substance of which these cuticular struc-
tures are composed differs from the protoplasm of the cells which
have deposited it, the more difficult is it to make out any passage of
the protoplasm into it, and the more distinctly is the formation of
cuticles seen to be a process of secretion. When the cuticle is not
formed regularly on the surface of the separate cells, protoplasmic
processes project from the secreting cell-layer into the secreted layer,
which are then traversed by corresponding canals (pore- canals) :
these are ordinarily very fine. These cuticles dififer greatly in con-
sistency, and present every intermediate step between softness and
extreme hardness. They are often converted into organs of support,
when they are very firm ; in which case they ordinarily consist
of a substance known as chitin.^^ These chitinised cuticles are
very common in the Invertebrata.

§ 19.

The secreting activity of the cells of large epithelial layers may
give rise to liquid, or even to gaseous bodies. The epithelia there-
upon enter into new relations to the economy of the organism; they ,
no longer produce substances destined to build up the organism,
but they present an intermediate step towards that condition of
epithelial structures in which parts of the epithelium enter into the
formation of a tissue of definite function— glandular tissue. As
there is always a direct connection between the aggregation of
cells which form the secreting organs, or glands, and the epithelium,
which either persists permanently, as in the majority of glands, or
which is at any rate present when they are first formed, this
glandular tissue is seen to be nothing more than a modifi-

THE TISSUES.

23

cation of the epithelial tissue, due to its differentiation.
Like it, glandular tissue always consists of cells. In tbe simplest
stage individual cells in a layer of epithelium become secreting- cells,
and function as gland-cells, by forming and secreting a substance
such as is not produced by the other cells. In this way uni-
cellular glands arise. They either retain their
original position between the other cells of the
or sink beneath the level of the epithelium,
open between the other cells by a fine duct
formed by the membrane of the cell (Uig. 7).

If the secreting surface be increased without the
general epithelium taking any share in it, the
sunken epithelium must increase in size, and so
give rise to structures which are more or less
separated off from the epithelium, such as pits,
sacculi, or cmcal tubes ; and these may be again
by fresh growths. The tissue lying
primitive epithelial layer forms en-
these pits as they grow; but it coll-
ide the same relation to them, however
complicated in form the ramifications and similar
proliferations of the epithelium may be, as it pre-
viously had to the simple even layer of epithelium.

Thus the gland in its simplest form appears
as a depression of the epithelium into the sub-
jacent tissue. In the more distinct forms of glands ^°TiSerior

there is a further differentiation of the cells which falivarj glands of
form the gland. The constituent cells of the the ant (after Stein),
gland become separated into those which secrete
and so represent true gland-cells, and those which connect the
secreting portion of the gland with the still indifferent epithelial
layer. These, in contradistinction to the secreting portion of the
gland, form the epithelium of the duct.

Connective Substances.

§ 20.

The phsenomenon which in epithelial tissue leads to the formation
of homogeneous membranes may, by being extended over the whole
periphery of every cell as well as by continued repetition, become of
greater importance. Even in the epithelial tissue we often meet
with a fine intermediate layer, the cement-substance. As the sub-
stance which is differentiated from the protoplasm of a number of
cells gradually increases between the cells containing unaltered
protoplasm, the cells become separated from one another, and a
distinction is made between the cells which form and the inter-
cellular substance which is formed. A number of very different

complicated
beneath the
velopes for
tinues to ha^

layer

and

24

compaeatrt: AXAio:^rY.

tissues present tEis common character in their more intimate structure.
They are called connective substances, as the majority of these
tissues serve to unite other tissues to organs, or systems of organs.

The differences in these tissues are due partly to the character
of their cells, partly to their relations with the intercellular sub-
stance, and partly to the chemico-physical constitution of the
intercellular substance; but all these points are not equally well
marked in every part of them. Whilst this latter circumstance
allows us to recognise the passage of one of these tissues into the
others, the fact that such passages do periodically take place under
our observation, affords a more weighty reason for uniting them
than the fact that we can detect common characters in their
structure, although often hidden by various differences. The
various tissues which belong to this group are : 1) cellular con-

nective tissue, 2) gelatinous tissue, 3) fibrous connective tissue,
4) cartilaginous tissue, 5) osseous tissue.

§ 21.

Connective tissue is divided into the following varieties:

1) Cellular connective tissue (vesicular connective tissue) is

Fig. 8. From the gelatinous substance
of the disc of Aurelia aurita, treated
with iodised serum (after M. Schultze).
X 500. a Branched fibres in which no
cells can be made out. & Cells in the
homogeneous gelatinous substance : the
processes are largely retracted in this
specimen.

the simplest form. It is formed
of rounded or elongated cells,
which are separated by a small
quantity of intercellular sub-
stance only. There are often
vacuolated spaces in the cells,
which are filled with a fluid. The
intercellular substance often has
the form of cell membranes, which
serve to unite the juxtaposed
cells to one another, and are
common to neighbouring cells.
In other cases again it is more
largely present, without prepon-
derating in quantity over the
cells. The differentiation of the
protoplasm of the intercellular
substance varies in degree. This
tissue is most widely found in
the Ai'thropoda and Mollusca.
In the Vertebrata it forms the
chorda dorsalis, or notochord.

2) Gelatinous tissue
(mucous tissue) is distinguished
by the soft gelatinous character
of the intercellular substance ;
it is ordinarily hyaline, and in it

THE TISSUES.

25

are placed either rounded and completely separated or filiform and
branched cells, which are united to one another by their processes.
Chords or tracts of cells also occur. In this way a fine network is
formed, which traverses the gelatinous portion of the structure, the
trabeculm of which may become firmer by further differentiation,
and may become broken up into fibrillae. A similar fibrillation may
affect the intercellular substance, in which case fibrous bands, in
which there are no cells, can be made out. This tissue is found
in many of the lower animals; in the umbrella of the Medusse
(Fig. 8), the integument of the Heteropoda, &c.

3) Fibrous connective tissue maybe regarded as a further
development of gelatinous tissue. Its morphological elements are
elongated or branched cells, embedded in an intercellular substance
formed of fibrous tracts and bundles. This substance is largely
due to a differentiation of the walls of the cells, as is clear from
its development. Development also reveals the fact that part of
the protoplasm which sends off processes, is directly differentiated
into fibrils and fibrous bundles ; these are therefore distinct from
the earlier formed, and more or less homogeneous intercellular
substance. The thickness of the fibres and the direction they take
vary greatly. The fibres, which are generally curved and undu-
lating, sometimes run parallel to one another, sometimes anastomose;
the cells and the cell-processes are, in their earlier stages, arranged
in a manner corresponding to the subsequent arrangement of the
fibres.

Fibrous connective tissue is distinguished as loose, or firm,
according to the characters of its intercellular substance; the
firmer sort is also known as tendinous tissue,^ ^ the fibrous bands
of which are placed parallel to one another. In addition to the
fibrillae, which swell up when treated with acids and alkalies,
there is another form of fibre, which is seen in the intercellular
substance of fibrous connective tissue; this resists these agents
more completely, and is called elastic tissue,^^ on account of its
elasticity. It is, as may be seen from its relation to the inter-
cellular substance, not an independent form of tissue, but merely
a modification of connective tissue.

Inasmuch as a portion of the intercellular substance arises by
subsequent differentiation of the protoplasm of the original cells,
as was remarked above, the morphological elements which are
present in fully-developed connective tissue represent the remains
only of the primitive cells. According to the quantity of pro-
toplasm used, and converted into fibrous structures, and so in-
corporated into the intercellular substance, the nucleus of the
connective-tissne cells is surrounded by more or by less protoplasm,
or the whole protoplasm disappears ; the presence of isolated nuclei
in the fibrous bands of connective tissue is an indication of this.
Where the protoplasm still remains around its nucleus — where,
that is to say, a cell, according to the conception given above, is
present, this cell may undergo fresh changes, which are of so many

26

COMPAEATIYE ANATOMY.

kinds tkat connective tissue is ricker tkan any otker in tke various
pksenomena of differentiation.

§ 22.

4) Cartilaginous tissue is ckaracterised by cells lying in a
firmer intercellular substance. Its cells do not^ except in a few cases,
possess distinct processes, or processes wkick can be easily made
out ; but are very nearly circular in form, or else oval or fusi-
form. Tke amount of intercellular substance varies in amount.
It is distinguisked from tkose forms of connective tissue wkick are
formed of simple cells placed in a komogeneous intercellular sub-
stance, by its greater rigidity. Cartilaginous tissue is well adapted by
tke possession of tkis ckaracter to function as an organ of support.
Wken tke cells predominate, and tkere is but little intercellular
substance, and wken wkat tkere is is in tke form of fine membranes,
cartilage is seen to be directly allied to vesicular connective tissue.
Tke protoplasm of tkese cells often takes on a definite arrangement,
and forms bands wkick extend from tke nucleus to tke peripkery,
and unite togetker tkere. Tkey are separated from one anotker by
spaces wkick contain fluid (Fig. 9). In proportion as its intercellular
substance is diminisked, does tkis tissue differ more and more from

ordinary cartilaginous tissue. In tke
protoplasm of cells of tkis kind, wkick
are found forming a sort of skeleton
in tke Medusm, tke pksenomenon of
streaming of tke protoplasm may be
seen.

If tke intercellular substance in-
Kg. 9. Cartilage cells from the Creases, it either remains homogoiieous
tentacle of a Medusa (Cunina). (hy aim e car t llag e), or it undergoes

further differentiations like tkose of
connective tissue ; but tkese differentiations do not muck affect its
relations to its cells. Wken tke intercellular substance breaks up
into fibres, we get fibrous cartilage; wken elastic nets appear
in it we get elastic cartilage. By gradual changes of tke inter-
cellular substance, as well as of tke cells, cartilaginous tissue passes
into fibrous connective tissue, and thus indicates its close connection
with tkat form of tissue. Tke cells also become more specially modified
in some cases by being elongated or producing radiating processes,
wkick unite with tkose near them : as, for example, in many Selackii,
or more developed still in many Cephalopoda. Tke intercellular
substance then appears to be traversed by tke processes from tke
cells (Fig. 10). The pksenomenon, wkick in tke cases just cited is
greatly exaggerated, obtains also in ordinary hyaline cartilage,
where tke cells are apparently sharply marked off from one anotker ;
for tke intercellular substance may be seen to be traversed by
processes, although tkese are, of course, extremely fine.

Tke intercellular substance of cartilaginous tissue is always

THE TISSUES.

27

distinct from tHe protoplasm of the cartilage cells which lie in its
cavities ; but as it is differentiated from the protoplasm^ it must^
nevertheless^ be regarded as a secreted product of • the protoplasm —
a layer secreted by a cell : often an intercellular substance may be
seen in hyaline cartilage surrounding the cell like a capsule. This
was formerly regarded as a cell-
membrane belonging to the cell.

As these capsules can often
be shown to enclose groups of
cells consisting of several gene-
rations, which have resulted
from the fission of a single
cell, the enclosed cells were
looked upon as mother and
daughter cells, &c., and the
phsenomenon itself was regarded
as a case of endogenous cell-
formation. As a fact, these
capsular systems are merely
the expression of secretions,
not become homogeneous, and
formed by several generations
of cells which arose from one
another. The perfectly gradual
passage of cartilaginous tissue
in which such capsules may be
seen, into tissues where the intercellular substances is completely
homogeneous, shows that we have here to do with different stages
in the differentiation of one and the same secreted substance, which
has arisen in the former case by an interrupted, and in the latter
case by a regular, secreting activity of the cells. In virtue of the
anastomoses of the processes of cartilage- cells, cartilaginous tissue
comes very close to the next form of tissue, and it is only distin-
guished from it by the characters of its intercellular substance.

§ 23.

5) Osseous tissue. This, the firmest form of the connective
substances, consists of an organic intercellular substance combined
with lime- salts, in which there are cells with fine anastomosing
processes ; or it presents a ground-substance like that just men-
tioned, in which, however, there are no cells, but only cell-processes.
These processes traverse it as fine canaliculi. There are therefore
two structural phases of osseous tissue to be distinguished.
Cells enter into the composition of the one, but in the other they
simply send out fine processes into the pore-canals of its solid
ground-substance.

The tissue containing bone-cells is the most common; it is

Fig. 10. Cartilage from a Cephalopod.
a Simple, b Dividing cells, c Canaliculi.
d An empty cartilage capsule with its
pores, e Transverse section of canaliculi
(after M. Fiirbringer).

28

COMPAEATIYE ANATOMY.

found in tlie skeletal organs of all classes of tke Yertebrata ; whilst
that form of osseous tissue with canaliculi only is found in the
skeleton of many fishes^ and as a general rule in the dental organs
of all Yertebrata (dentine).

The development of osseous tissue explains the relations of the

intercellular substance to
the cells. That form of it
which contains cells may
arise in two ways : either
by the ossification of con-
nective tissue^ the cells in
which become converted
into bone-cells by the ossi-
fication of the intercellular
substance, which becomes
impregnated with calca-
reous salts, while the cells
themselves become con-
nected with one another
by their processes, which
traverse the pore- canals in
the intercellular substance;
or the same tissue is formed
by apparently indifferent
cells, which secrete a scle-
rogenous substance. This
substance is laid down in stratified lamellae, into which the secreting
cells send fine protoplasmic processes (Fig. 11, o). The secretion
of this substance is preceded by a change of part of the protoplasm
of the cell. As soon as this is differentiated it does not belong
any longer to the cell, and is therefore secreted from it. If some
of the secreting cells (o'c'') cease to be active, while the cells near
them do not cease to be so, the former gradually get to lie in a
layer of intercellular substance, which finally surrounds them, and
so converts them into bone-cells (o'"). The cells of the secreting
layer (osteoblasts) are continuously connected by fine processes with
those which are already enclosed (bone-cells). Thus each of the
former is rendered capable of becoming a bone-cell.

The other form of osseous tissue is developed in a perfectly
analogous manner, so far as its history is accurately known, through
the development of dentine. In this case also a layer of cells
secretes a substance, which hardens or is sclerogenous, and into this
the cells at the same time send processes, which traverse pore-
canals. But the cells (odontoblasts), instead of gradually sinking
into this extra-cellular substance, always remain outside it, and are
connected with it by their processes only. The secreting substance
is thus traversed by fine parallel canaliculi (the dentinal canals,
so-called, since they were first made out in the dentine). This form of
osseous tissue, notwithstanding its different appearance in later stages.

Fig. 11. Transverse section of the feninr of
Eana. o Osteoblast layer, o' ©"Cells becoming
bone-cells. o'" A bone-cell. p Periosteum,
m Medullary cavity.

THE TISSUES.

29

is very closely allied to tlie former kind, for its intercellular substance
also is secreted from cells — arises, that is, by the differentiation of a
part of the protoplasm. The connection is stilLcloser if we regard
the earliest stage in the process. In both cases a homogeneous sub-
stance is secreted, which is hardened by calcareous compounds, and
into this the cells, which form it, send their processes. If this
process goes on in the same way as it began, so that a complete cell
never passes into the secreted layers, it leads to the formation of
that form of osseous tissue which is traversed by fine pore-canaliculi
only, arranged for the most part in parallel lines. If some of the
secreting cells gradually pass into the secreted substance, that
substance becomes an intercellular substance containing bone-cells.

Morphological Elements of the Nutrient Fluid.

§ 24.

The cells, which are suspended in the nutrient fluid of the
body, and which are its form-elements, are closely connected in
origin with the connective tissue. If it is allowable to regard this
fluid as an intercellular substance, then the whole of the nutrient
fluid might be compared to a tissue, which would not differ from
the other tissues of the connective series in any essential point
other than its fluid condition. Even if we admit it to possess
another function in consequence of this fluid condition, yet this
function must be held as falling well within the category of vege-
tative functions. Apart from the importance of these considerations,
the form- elements in question must be enumerated in the present
place, for they take their earliest origin from the tissue which
forms the walls for the vessels of the nutrient fluid. As far as its
characters are known, a portion of the cells which form the mesoderm
during the processes of division do not become connected with the
rest, but remain isolated in the fluid which fills these canals or
spaces, which fluid is known as hlood. These form-elements then
are the blood-cells. In the Invertebrata they appear, as a rule, in
the form of completely indifferent cells,
consisting of a nucleus (Fig. 12, n) and
protoplasm, which latter exhibits amoeboid
movements. Among the Yertebrata these
morphological elements persist as lymph-
cells in the Craniata, while in the blood-
fluid proper there are elements which are
derived from these forms, but are much
altered. These latter have lost their amoe-
boid character during differentiation, and
have the form of rounded or oval discs,
the nucleus of which disappears in the Mammalia, though present
in the rest of the Yertebrata.

Fig. 12. Blood corpuscles
of a Crustacean (Maja
Squinado) witli protoplas-
mic processes, n Nucleus.

30

COMPAEATIYE ANATOMY.

B. The Animal Tissues.

§ 25.

Ill tlie epitTelialj as well as in connective tissues, the product
of the differentiation of the protoplasm gives rise to phenomena
which are limited to the sphere of vegetative operations. AYhen
a more highly contractile substance arises as a product of the
division of the protoplasm, a new tissue is formed, which is known
as contractile or muscular tissue. Its contractility, however, is
not automatic, but dependent on stimuli, which come from the form-
elements of the nervous system. The contractile form-elements
of the muscular tissue differ therefore essentially from the
indifferent cell formed of protoplasm, although the latter also
is contractile. They presuppose the existence of another,
or nervous tissue, just as it on the other hand determines
the existence of the muscular tissue. These intimate re-
lations explain the causal relationship which these two tissues have
to one another phylogenetically. The two kinds of elements are
differentiated from a single neuro-mu scular cell, which is in
many Coelenterata the representative of the two tissues. (Fig. 13.)
This kind of cell corresponds to an indifferent stage of the animal

tissues, in which they have not yet be-
come distinct tissues. The tissue which
forms the starting-point of the differ-
entiation is not a new structure. It is
the outermost layer of the body, and
consists of cells, which form an epithe-
lium. The neuro-muscular tissue
is therefore a differentiation from the
epithelial tissue, and is thus connected
with a more simple condition. Cells
which are hardly at all different from other epithelial cells give
off a band-like process at their base, which becomes connected to
a layer of longitudinal fibres underlying the epithelium. While
the epithelial cells of the outer layer of the body unite, when in
their indifferent condition, a low grade of sensibility with a low
grade of contractility, the sensibility, when the cells become more
highly specialised, remains with them, and the contractility becomes
assigned to a differentiated process of the protoplasm, which now
appears as a distinct appendage of the cell. Thus commences
that arrangement which, in the more highly differen-
tiated stages, is expressed by the connection between
ganglion-cells, nerve-fibres, and muscle-fibres. By sup-
posing that the fibres, which in the earlier case appear to be
merely processes of the cells, retain their nuclei, and that the
products of the division of the nuclei of the cell gradually become
fibres, and that further the neuro-muscular cell is no longer con-

Fig. 13. Neiiro-mnscular cells
of Hydra. n Processes of the
cells, w Contractile fibres (after
Kleinenberg) .

THE TISSUES.

31

nected directly, but by means of a separate process with the
fibre, which has at the same time itself also become independent,
we can see how the more differentiated stage has been brought
about. Nerves and muscles seem from this point of view to be
the products of the separation of one and the same layer of tissue,
which tissue we shall learn later on to know as the ectoderm.^^
And at the same time a physiological postulate is thus satisfied :
for clearly it is impossible to imagine that nerve or muscle once
came into existence with their elements totally distinct from one
another, and that the connection between them, on which their
functions depend, was the result of a later union.

Muscular Tissue.

§ 26.

The morphological elements of the muscular tissue are, so far
as their more special characters are concerned, divisible into
two groups. One consists of cells simple in form, the other of
fibres derived from cell-aggregates, or from syncytia; the latter
is indicated by the presence of numerous cell-nuclei. In either case
the amount of protoplasm, which retains its indifferent character,
is slight, and subordinate in importance as far as the function of
the form-elements in question is concerned. Further differentiation
of the contractile substance may in either case lead to the higher
development of the fibre.

1) The so-called smooth muscular fibres, or contractile
fibre-cells, constitute the first form. They are spindle-shaped
cells, which are often greatly elongated, and then are band-like in
form ; in these cells either none of the indifferent protoplasm at all
persists, or what does is to be found in the long axis, or at the
periphery of the cell only. In all cases such remaining protoplasm
surrounds the nucleus. The contractile substance is homogeneous
and limited externally by a membrane, which is often difficult to
demonstrate. The reaction of these muscle-fibres to nerve stimu-
lation is slow.

Owing to differentiation of the contractile substance into singly
and doubly refractive particles, the fibres gain the appearance of
transverse striation ; such is the origin of that variety of the
tissue, which is known as transversely-striated muscular fibre.
There are various intermediate forms between this kind of striated
tissue, which consists of fibres derived each from a simple cell, and
the other more homogeneous kind of fibrous muscular tissue.

2) The elementary parts of the other form of muscular tissue
are formed by cell-aggregates (syncytia). They generally arise, as
it seems, from the growth of one cell, the nucleus of which multiplies,
so that they may be regarded as arising from the continuous but
imperfect division of one cell. Their contractile substance either
has a cylindrical shape, is limited externally by a homogeneous

32

COMPAEATWE AIS^ATOMY.

membrane (tbe sarcolemma), and contains several nuclei^ witb
remnants of protoplasm along its axis ; or tbe contractile substance
forms a solid cylinder^ on tbe surface of wbicb, and immediately
below tbe sarcolemma^ are tbe nuclei witb tbe remains of tbe proto-
plasm.

Further, there are two varieties of this form of muscle-tissue
in which tbe contractile substance is respectively more homogeneous
or more heterogeneous. If more homogeneous tbe fibres resemble
tbe so-called smooth fibrous cells, from wbicb indeed they differ only
by tbe fact that they do not correspond to a single cell, but to a
multiple of cells, as is clear from tbe number of nuclei appertaining
to tbe fibre. In tbe other condition, owing to tbe differentiation
of tbe contractile substance, they resemble tbe second form of
simple muscular fibres, and, like them, are transversely striped.
These also correspond to a number of cells although they are
derived from a single cell, and owe their elongation to its growth.
Tbe reaction to stimulus is, in transversely-striped fibres, rapid.

Nervous Tissue.

§ 27.

Nervous tissue appears (as has been already explained) at tbe
same time as muscular tissue in tbe Animal Kingdom, and is
distinguished by its functions, even in its lower conditions, from
other tissues. It receives and passes on stimuli, converts them into
sensations, and produces voluntary excitations. Two conditions are
to be distinguished in tbe morphological characters of tbe elementary
parts, nerve- fibres and nerve-cells. Tbe former are mostly present
in tbe peripheral portion of tbe nervous system, and are tbe con-
ducting organs, while tbe latter form tbe central elements.

1) Nerve -fibres have not always tbe same relations, and their
different conditions are to be regarded as stages of differentiation.

a) In their simplest form they are elongated homogeneous
band-like bundles composed of fibres wbicb are so slightly separated
from one another that they appear to be merely striated. For tbe
majority of Invertebrata tbe relation of nerve-trunks of this kind
and their branches to tbe histological form- elements is not thoroughly
made out ; and even tbe question whether tbe numerous striations
of tbe nerve-trunks are to be regarded as tbe indication of their
being composed of separate fibres, is an open one. Tbe presence of
nuclei in their structures is tbe sole fact which points to their
relation to cells. In other cases fibres united into bundles may be
distinguished as individual elements of structure. Tbe fibre consists
of an apparently homogeneous substance wbicb is limited super-
ficially by a fine membrane, beneath wbicb are tbe nuclei. Remains
of protoplasm may be at times made out around tbe nucleus, wbicb
shows that tbe rest of tbe fibre is a differentiated substance. Tbe

THE TISSUES.

33

structure of these nerve-fibres is therefore histologically of a similar
grade to that of muscular-fibre^ and the only difference between
the two is in the quality of the differentiated protoplasm, which in
one case gives rise to muscle, and in the other to nerve-substance.
Such fibres are to be seen in the Invertebrata as well as in
Amphioxus and the Oyclostomi. In the higher Vertebrata they are
present only in the sympathetic nervous system.

b) Further differentiation gives rise to a second stage of the
nerve-fibre. The nerve-substance, which lies beneath a more or less
delicate envelope, is differentiated into a chord which traverses the
axis of the fibre — the axis-cylinder — and into a fatty substance
which surrounds it. The latter, known as the medullary cylinder
(medullary sheath), gives a highly refractive contour to the nerve-
fibre, and can be separated from the axis-cylinder only by artificial
means. The homogeneous sheath which surrounds the medullary
cylinder — the neurilemma — contains the nuclei which are the remains
of the cells from which the fibre was formed. So far as is yet
known this form obtains in the Grnathostomous Vertebrata only.

2) The other form-element of nervous tissue is represented by
cells, which are called ganglion-cells, as they are principally
present in the swellings (ganglia) of the nervous system. They
form the central apparatus. Their substance is generally finely
granular in character, with many other peculiarities which cannot
be entered into more closely here. The nucleus, which as a rule
is provided with distinct nucleoli, lies in the middle of the granular
substance ; this latter is often limited by an external membranous
and firmer layer. A very complicated structure is ascribed to these
cells, and is explained by every observer in essentially different
ways, so that the questions involved appear to be still far from
being settled.

The ganglion-cells possess processes by which they are con-
nected partly to one another, and partly to nerve-fibres. They
form therefore the points of origin of the nerve-fibres. It is not
yet settled how ganglion-cells, which are devoid of processes, and
therefore completely isolated, can be of any service. The fact is,
that the belief in their existence grows less and less every day.
The processes of the nerve-cells vary greatly in number, as well
as in their relation to the fibres ; the only point to be noted about
them is that in the differentiated fibres it is the axis-cylinder which
is continued into the substance of the cell, while the medullary
cylinder ceases at some distance from it, or, rather, is no longer
differentiated. The relations of the axis-cylinder to the substance
of the cell appear to vary greatly, and are in many points a problem
still.*

* SoLBRiG, A., Ueb. d. fein. Structur der Nerronelemente der Gasteropoden,
Leipzig, 1872.

D

34

COMPAEATIYE AI4ATOMY.

Origin of the Organs.

§ 28.

In section 13, the title of organs was given to those parts of
the body which were entrusted with a definite function for the
purposes of the organism, and which had a form in correspondence
with this function. In this general sense every form-element is
an organ, just as much as the parts, which are made up of form-
elements, and have a definite function, are organs. The conception
of an organ is therefore a relative one. We must accordingly
separate organs into those of a lower and those of a higher order.
The former are represented by the morphological units or form-
elements — elementary organs — while the organs of a higher order
are those which are made up of a number of elementary organs —
cells, and their derivatives (tissues) — and which are set apart for
a single function. There are but few of these organs of a higher
order in the lowest stages of animal organisation, owing to the
simplicity of the organism. But these few organs form the ground-
work on which the gradual complication of the organism is raised
up by continued differentiation, and in accordance with the principle
of the division of labour. We may therefore call those simple
organs of a higher order, from which complex organs are developed
by differentiation, primitive organs."’

When we examine these primitive organs more closely, we find
it convenient to associate them with the earliest processes of dif-
ferentiation which take place in the organism, for they can be
derived from them. A collection of smaller cells arises from the
division of the egg-cell, and these have not all the same position.
Some occupy the inner part of the organism, and others form a
layer which surrounds it, and at the same time
forms the external boundary of the body
(Fig. 14). If in this stage of development
the taking in of food into the body com-
mences, then the inner cell mass becomes con-
verted into the limiting layer of the digestive
cavity, and forms a primitive gut (enteron).
In many observations the process of division
into two layers is described as due to the
invagination of a one-layered vesicle. In
other cases it is represented as taking place
differently, so that it is impossible to make out
whether there is any phaenomenon common
to all cases, and, if so, how far it is common.
Let us therefore turn to the results of the
process, without making any generalisation about it. We now
have an organism made up of two layers of cells. An outer one,
or ectoderm, which forms the primitive integument, and an inner

Fig. 14. Separation of
the mass of cells formed
by change of the yolk
into a peripheral (c)
and a central (d) por-
tion.

THE OEGANS.

35

one_, or en do derm, whicli surrounds a primitive enteric cavity.
The two layers pass into one another at the oral opening which
leads into the cavity. The two cell-layers, which form the body
of such an organism, furnish the conditions under which it is
possible for it to lead an independent animal existence. The outer
one is the organ of support, and may be converted into an organ
of locomotion if it gives rise to cilia, and may be the seat of
respiratory functions also. In so far as it per-
ceives the state of the surrounding medium it
is an organ of sensation too. The inner layer
is nutritive in function, produces changes in
the food which is taken in, and allows what
can be assimilated to pass into its cells ; and
' these in their turn feed the outer layer of
cells. What is useless is passed out again by
the same opening as that by which it entered.

As the functions of the two layers are dif-
ferent the special characters of the morpho-
logical elements which compose them are
different also ; we need only call attention
now to the much greater size in most cases
of the cells of the endoderm, as compared with
the cells of the ectoderm.

This grade of organisation is to be seen
in some of the lower divisions of the Animal
Kingdom (Coelenterata and Yermes), where it
represents a lowly stage of development.

Indications of it are to be seen even in the
higher divisions. This form has been called
account of the dominant development of the
from the hypothesis that forms agreeing with a G-astrula in all
essential points were the precursors of all the higher forms of
animal organisation, a Gastraea-form resembling the Gastrula has
been regarded as the primitive ancestral form of all animals. This
Gastraea theory is based, first, on the existence of independent
animal forms which resemble the Gastraea; secondly, on the fact
that the embryonic body which commences with a Gastrula, does
not, in the lower divisions, rise very much above it, so that
even apparently considerable complications of the organism can
be traced back to the existence of these two layers of the body ;
thirdly, the presence of these two layers of cells, forming the
ectoderm and endoderm, as a general, constant, and therefore regular
phaenomenon, even in the higher divisions of the Animal Kingdom,
as well as their constant relation to the same functions, is a fact of
the greatest importance for the hypothesis in question ; indeed the
occurrence of these layers as the so-called germinal layers,
which make up the embryonic body, cannot be rightly understood
without a reference to a hypothetical Gastrasa-form. This hypo-
thesis may therefore be regarded as justified.

D 2

a.

I

c

d

Fig. 15. Diagram to
represent the first dif-
ferentiation of the or-
ganism into ectoderm
and endoderm, and the
formation of a digestive
cavity, a Mouth, h En-
teric cavity, c Endo-
derm. d Ectoderm. (In
transverse section.)

the Gastrula, on
enteron. Starting

36

COMPARATIVE ANATOMY.

We recognise then the Gastrsea as a fundamental form, and
discover in the differentiation of two layers corresponding respec-
tively to endoderm and ectoderm, which are present even in the
highest grades of the Animal Kingdom, facts which point to such
a Gastrma stage, and are due to it. But it must not be at all sup-
posed that we have advanced farther than just on to the threshold
of a knowledge of these relationships. The definitive explana-
tion of many of the points which have considerable importance in
this matter is still far distant, and but little light has yet fallen on
even such apparently simple points as the origin of the Gastrula
and its two layers. It is a question whether the form which pre-
cedes the Gastrula is a one-layered vesicle — that is, whether the
two layers of the body are due to the endoderm being formed by
invagination ; or whether the endoderm is developed from a primitive
internal cell-layer delamination. And again, whether the two con-
ditions that have been observed are independent of or derivable from
one another. Further investigations will have to settle all this, and
our judgment must therefore be proportionately reserved until such
investigations have been made.

§ 29. •

The two layers of which the body of the lower animals is made
up during their early stages, and which are, in the higher divisions,
represented by the germ -layers — that is, the ectoderm and
endoderm — give rise to an intermediate layer or mesoderm, in the
formation of which the other two apparently take an equal share.
But it is not yet definitely known what share each takes, since the
earliest processes of the differentiation of the rudiments of the body
still require much careful investigation, and, moreover, they do not
present the same characters in all cases. These three layers appear
directly after the segmentation of the ovum in the higher animal
organisms, and are coincident with the first traces of histological dif-
ferentiation. They represent the outline of the organism in the condi-
tion of a germ, and from this the whole organism by differentiation
evolves itself.

These rudiments of the body exhibit great modifications in the
higher divisions of the Animal Kingdom, and the stage which is
represented by the Gastrula form is more difficult to make out in
proportion as the differentiations through which the organism has
to pass are more considerable; but the principal features can be
easily recognised as identical in all cases. The outer germinal layer
(deric layer or ectoderm) forms the outer limiting layer of the body,
and the inner (lower) germinal layer (enteric layer, glandular layer,
or endoderm) the foundation of the gut or enteron. The middle layer
(mesoderm) afterwards arises between them.

As the ectoderm and endoderm are the first organs marked off
in the course of development, the germinal layers are to be regarded as
primitive organs, which have been transmitted from the earliest

THE OHGAHS.

37

stages in the differentiation of the animal organism to later, and there-
fore higher stages, and which, following the law of the division of
labour, give rise to series of new organs. We do not yet know enough
of the details of the organological differentiation of the germinal
layers to be able to give the history of every organ. However, the
facts which are clearly established with regard to, at any rate, some
divisions of the Animal Kingdom enable us to follow out the first
steps in the process of differentiation. The organs which put the
organism into relation with the outer world, such as organs of
defence, of support, and of sensation, are principally derived from
the ectoderm (hence called the sensory layer), also those of move-
ment ; while the endoderm principally provides the organs for the
preservation of the individual and of the species (nutritive layer).
As the origin of the mesoderm, out of which important organs
are formed, is still very obscure, the relations of these organs to one
or other of the two primitive germinal layers must be left an open
question.

The primitive character of the organism more or less disappears
as the rudiments of the body are formed out of the germinal layers,
and as fresh organs which render the organism more complicated
arise in it. Organs differentiated from the germ layers which act
the part of primitive organs are reckoned as secondary organs.
From these, tertiary organs are formed, and so on. The separate
organs differentiated out of a primitive organ remain connected
together, owing to the fact of these processes of separation being due
to the division of a function, and of the separate functions being
subordinated to the primary function, from the breaking up of which
they took their origin. Combinations of organs are therefore
formed, which are known as organic systems, on account of their
morphological and physiological connection.

This connection does not always persist in the adult condition ;
and, in fact, organs primitively connected often become separated.
This obtains chiefly in those organs which serve several purposes,
for when the functions become independent the organs become so
too. But even in these cases ontogeny indicates what was the
primitive condition.

Systems of Organs,

a) Integument.

§ 30.

The ectoderm, as the outermost layer of the body, forms the
simplest condition of the integument of animal organisms. In
the lowest organisms (Protista) there is either no integument at all,
the protoplasm which forms the body being protracted into ever-
changing processes (pseudopodia), or the integument is represented

38

COMPAEATIYE A^-ATOMY.

by the outermost layer of tbe protoplasm of a single cell, in wbich
case we bave the first example of a denser stratum of tbe cell becoming
separable as a distinct envelope and covering for tbe rest of tbe
organism. Tbe ectoderm bas tbe function of an organ of defence
wben its cells secrete a substance wbicb invests, more or less per-
fectly, tbe surface of tbe body. This substance may harden and
give rise to tests or shells, or form a continuous covering for the
body, like tbe carapace of the Arthropoda.

When a mesoderm is formed, that part of it wbicb becomes con-
nected with the ectoderm takes on, in various ways, the functions
of an organ of support. Tbe calcareous deposits in tbe complicated
integument of tbe Ecbinodermata are examples of this.

Tbe activity of tbe ectoderm in producing firm organs wbicb
protect tbe body is seen also in tbe Yertebrata, where numerous
and varied parts, wbicb function as investing and protecting organs,
are produced by it.

b) Skeleton.

§31.

In proportion as tbe various protective organs wbicb are formed
from tbe ectoderm increase in size or in strength, and at tbe same
time become connected with internal organs, they attain tbe function
of organs of support also. Such organs we designate as the skeleton.
Tbe combination of inorganic substances (chiefly calcareous salts)
with an organic base plays an important part here. Tbe supporting
function of tbe integument gives rise to numerous adaptations.
Tbe combination of tbe functions of both protection and support
is clearly a lower stage as compared with tbe formation of internal
skeletons, wbicb indicate a higher functional differentiation, and
function exclusively as organs of support. Here, too, we meet
with tbe most various conditions. Tbe lowest forms, tbe first
beginning of such internal skeletal organs, are solid deposits in
tbe tissues, tbe separate pieces of wbicb bave no connection with
one another. Tbe growth and union of these deposits give rise
to skeletal formations, wbicb may be also regarded as excretions.
Examples of them are found even in tbe Coelenterata. Wben a
definite tissue, tbe properties of which specially fit it for tbe
function of support, is brought into use, tbe internal skeleton
assumes a higher degree of development. Tbe differentiation of
cartilage from tbe indifferent connective tissue is tbe first ex-
pression of this pbgenomenon. As low down as tbe Medusae,
among tbe Yermes and among tbe Mollusca, the employment of
cartilaginous tissue for organs of support is commenced, and in tbe
Yertebrata it attains to greater importance, until it is pushed aside
by a second and more perfect skeletal tissue — tbe osseous.

THE ORGANS.

39

c) Muscles.

§ 32.

The locomotion of the body exhibits itself in its simplest phase
as a change in the form of the body due to the contractility of
its protoplasm. When these changes in form follow one another
rapidly, and have all the same direction, the body either elongating
or sending out processes which attaching themselves to some fixed
point, are followed gradually by the rest of the semi-fluid body
(Rhizopoda), locomotion is effected. The difference between this
mode of locomotion and undefined change of form is seen to be
merely one of degree. The contractility of protoplasm may pro-
duce changes in position even when it is invested by a differentiated,
though soft, integument. This layer of integument will follow the
movements of the body it invests. In such cases, and they are very
common among the Protista, special organs of locomotion cannot be
said to exist, for the cilia have other functions to perform for the
organism in addition to locomotive ones ; such, for example, as that
of aiding in the ingestion of food.

Specific organs of locomotion make their first appearance when
the contractile morphological elements known as muscle-fibres are
differentiated; these, in the simplest case, form a muscular layer
lying beneath the ectoderm.

The genesis of this earliest musculature of the body is due to a
differentiation of the ectoderm (Hy droid polyps), the cells of which
give off flattened processes, which form a continuous layer of
contractile fibres.

Each individual ectoderm- cell concerned in the formation of this
layer of fibres represents accordingly a sensory apparatus, which
stands in direct continuity with a contractile apparatus. The cell is
indeed replaced, when the musculature is differentiated, by groups of
muscles which work so as to balance one another, and completely
harmonise in their action (compare Sect. 31). We cannot yet say how
far this process, which gives so deep an insight into the mode of
differentiation of the tissues as well as of the organs, is repeated in
the ontogeny of the higher forms of animals. In all divisions above
the Coelenterata we always find the separation between ectoderm
and muscle complete. It may therefore be doubted whether a
process of the kind described in the Hydroid polyps always accom-
panies the origin of the muscular system. But yet it is very
probable that something of the kind does occur. Even though the
process of differentiation in the higher organisms does not enable
us to recognise these processes in their case, yet it is not to be
assumed without further reason that the mode of origin of the
muscular tissue was in them primitively different, for Ontogeny
very seldom repeats phylogenetic processes in every detail.

40

COMPARATIVE AT4 ATOMY.

§ 33.

The earliest musculature of the body is closely related to the
integument_, from which it can with difficulty be separated. Since
this is the case not in the Coelenterata alone^ we have here an
instance in favour of an essentially equivalent origin for this part of
the musculature in all cases. Together with the integument, it
forms, on the appearance of a body-cavity, a dermo-muscular
tube,*^ which encloses the other organs. The arrangement of
the muscular fibre seldom presents much regularity till the body
becomes jointed into separate parts, placed one behind the other
(metameres) ; and with the development of organs of support the
muscles become differentiated into separate groups. Collections
of fibres form bundles, and these again make up larger complexes,
muscles. The segmentation of the muscular system corresponds
therefore to the segmentation of the body, and the separate
segments differ in proportion to the difference in function of the
metameres. The various kinds of movement which are produced
by the crossing of the fibres of the dermo-muscular layer in
different strata, are, where the muscular system is more highly
differentiated, effected by groups of muscles acting in opposition
to one another, and completely balancing one another in their
action.

Locomotion by movement of the whole body is brought about by
the dermo-muscular tube and the differentiations which arise from
it ; the whole integument, in the first instance, takes a share in this
activity. A further differentiation arises from this state of things
when special appendages are formed, as limbs, on certain parts of
the body. When the animal changes its place these act as the arms
of a lever. They have the form of simple soft processes of the
dermo-muscular tube (Ringed worms), or of jointed organs, which
are supported by the integument (Arthropoda), or by means of internal
skeletal structures (Vertebrata). The complication of the muscular
system is in close connection with the development of supporting
organs ; and the two form a single locomotor system in which the
skeleton plays the passive part.

d) Nervous System.

§ 34.

In the lowest conditions of animal organisation the protoplasm of
the cells is the seat of sensation, as well as of movement ; and this
is permanently the case in the lowest organisms. As the muscular
layer of the body becomes differentiated, the ectoderm becomes the
principal organ of sensation. The differentiation of a nervous
system is due to the further development of a portion of this layer

THE OEGAKS.

41

as a sensory organ^ for wliicli reason sncTi an organ mnst be at first
placed superficially. This superficial position of the earliest rudi-
ments of the nerve-centre has been already made out in so many
forms that it may be regarded as a general phaenomenon. As the
sensory organ becomes differentiated from the ectoderm it sinks
down into the body. The developing central organ is thus
gradually covered over by other layers of the body. This
arrangement, which is most peculiar, and by itself most unin-
telligible, is explained if we regard it as inherited from a more
primitive stage, in which the nervous system was but slightly
differentiated, and was represented by the whole cell-layer of the
ectoderm, or by part of it. We must consider its gradual attain-
ment of an internal position to be a process due to its continued
differentiation, and consequent higher potentiality ; the organ, which
has become of greater value to the organism, gets hidden within the
body.

With regard to the structural characters of the differentiated
nervous system, the central organ, which is chiefly composed of
ganglion-cells, is to be distinguished in the first place from the
nerves, which pass to the terminal apparatus, and consist of fibrous
elements (peripheral nervous system).

§ 35.

The earliest complications are due to the appearance of several
parts (ganglia), in which are central form- elements connected with
one another : the further development of these parts is very various.
The ganglionic mass, which forms the central organ, is primitively
dorsal, owing to the earliest separation of the central organs taking
place from the dorsal ectoderm, as we have already seen. This
dorsal nervous mass, which generally lies near the entrance to the
alimentary canal, is differentiated into several parts, which are con-
nected together by commissures; their fibres form an oesophageal

In the animals built on a radiate plan the number of the ganglia is
increased in correspondence with the radii ; the peripheral distribution
of the nerves also follows these general structural relations exactly.
The nervous system in bilaterally- symmetrical animals follows the
bilateral arrangement. The more primitive form is represented by
a superior ganglionic mass (cerebral ganglion). Other ganglia do
not seem to be formed until the metameres are formed. We then
are able to distinguish dorsal and ventral ganglia ; the latter may form
ganglionic masses along a continuous longitudinal trunk, or a single
suboesophageal ganglion. The variations in size of these oesophageal
ganglia are in the closest connection with the nerves which pass off
from them. When sensory organs are developed, the ganglion
which sends off their nerves becomes of considerable size, while it
seems to degenerate when they grow less. The supra-oesophageal
ganglia are the most important in this relation, for it is from

42

COMPAEATIVE AiSTATOMY.

them that the nerves of the higher sensory organs arise, which in
position and direction have one widely- distributed arrangement.

From this form another is directly derived, determined apparently
by the well-marked metamerism of the body. Whilst in unseg-
mented animals possessing an oesophageal ring, the ventral parts of
the body are supplied by nerves which arise from the suboesophageal
ganglia, we find that the number of ventral ganglia is increased when
the body is broken up into parts lying one behind the other (meta-
meres). A ventrally-placed series of ganglia is formed by the
development of a separate pair of ganglia for each segment ; and
these, being united to each other by longitudinal commissures, form
a ganglionic chain. The Ringed worms and the Arthropoda present
us with this form. Further differentiation gives rise to all kinds of
variations of this type.

In the first place, the size of the ganglia varies with the size of
those parts of the body that have to be innervated; and in the
second place, the ganglia of several segments of the ventral cord
fuse into larger ganglionic masses.

Even when the central nervous system is entirely dorsal, as in the
Yertebrata, it undergoes differentiations of this kind. When the
most anterior part of the body is developed into a head, the most
anterior part of the central nervous system is developed into a special
region, the brain, which is marked off from the remainder of the
medullary tube, or spinal cord, which remains more equal in
size throughout. As differentiation advances, variously developed
regions appear in the brain.

e) Sensory Organs.

§ 36.

The sensory organs inform the organism of the condition of
the outer world. Protoplasm, in its indifferent condition, charac-
teristic of the lowest organisms, reacts to various stimuli from with-
out, and appears to be the seat of the lowest kind of sensation.
When the surface of the body is not completely marked off from the
inner portion of the organism (Rhizopoda) it is used as an organ of
perception, of course of the very lowest grade ; it functions therefore
as a sensory organ of the lowest order. When the surface is more
distinctly marked off, and a distinct outer layer of the body is
established (Infusoria, Gregarinse), we get a differentiation of great
importance for sensory perception.

Although, indeed, particular parts of the surface in the Infusoria
specially acquire the function of sensory organs, yet there is no
ground for speaking of sensory organs in an anatomical sense, in
this case, any more than there is in the still lower stages. Sensory
organs only appear when a nervous system is marked off, for sen-
sory organs are the end-organs of the sensitive nerves.

THE OEGAHS.

43

Their presence therefore presupposes that form of differentiation
which we treated of above, when speaking of the nervous system.
Since ontogenetic facts point to the primitive segregation of the
nervous system from the ectoderm, as being most probably a funda-
mental process, this same outermost layer of the body becomes also
of the greatest importance in studying the origin of the sensory
organs. Almost all the sensory organs are derived, directly or
indirectly, from it ; whence arises the permanent or temporary
connection of these organs with the integument.

It is very unsafe to assert what are the functions of many of the
sensory organs of the lower animals. This applies to all those organs
which are not comprised amongst those which fall within the domain
of our own judgment, on account of our possessing them or their
hornologues, in which case only is it possible that the connection
between their structure and specific function can be estimated.
Such outstanding organs have been classed together as organs of a
sixth sense.

§ 37.

The sensory organs are divided into lower and higher. The
former are commonly distributed over the integument, and are simple
in structure. Compared with the higher they represent a more
indifferent condition. Modified cells of the integument, which
generally belong to the epidermis, connected on the one hand with
a nerve fibre, and on the other provided with a process of varying
shape, which is directed towards the surface of the body, are the
most common examples of the lower sort. They are regarded as
the organs of general tactile perception; but the physiological
function of these organs, especially in aquatic animals, has not been
definitely determined, and it is reasonable to suppose that many of
them are the media of specific sensations, in which case they would
resemble the higher organs of sense.

The significance of these arrangements is somewhat more certain
when they are connected with special organs, such as movable
processes of the integument and the like ; they then appear to be
tactile organs. It is still a question whether structures of this
kind, especially in the lower divisions of the Animal Kingdom, are
of use for perceptions other than tactile.

The higher sensory organs present themselves to us as special
elaborations, with one special function and capable of response only
to stimuli of one special kind; they are to be regarded as developed
from the lower kind of sensory organs, and oftentimes still possess
the essential structure of that lower kind. Organs of taste and
smell can only be certainly distinguished in the higher divisions of
the Animal Kingdom, and the function of the latter is certain only in
those Verteb rata which live in the air; in the lower divisions it is
still doubtful. Even in the case of the organs of taste the greatest
caution is necessary as to their real import. The value of a sensory
organ to the organism determines its being protected against

44

COMPARATIVE AAE4TOMY.

external influences. This explains the foldings-in of the portion of
the integument which is about to be differentiated into a sensory
organ. It is for this reason that the higher sensory organs gradually
sink beneath the level of the ectoderm as they are developing, and
attain a favourable position for further development.

Vesicles filled with a fluid, on the walls of which a nerve ends,
are regarded as auditory organs (otocysts). In its simplest form
the vesicle is directly connected to the central nervous system, or the
nerve passes from it to the vesicle. These vesicles almost always
contain firm concretions or crystalline structures ; and very often
crystals of calcic carbonate. There are often hair-like prolongations
of the end-organs in addition to them, which project into the lumen
of the vesicle. This form of auditory organ, which obtains in the
Invertebrata, is complicated in the Vertebrata by diverticula and
outgrowths which form a labyrinth. New arrangements are produced
in the form of organs for carrying and increasing the sound, which
become attached to the auditory organ, although they primitively
presided over other functions. Inasmuch as the labyrinth- vesicles
of the Vertebrata are developed from the integument, the terminal
organs of the auditory nerve which are differentiated in its walls
are genetically connected with the terminal organs of the tactile
nerves, which lie in the integument ; they may therefore be
regarded as a specific development of a lower sensory organ. The
genetic relations of the simpler otocysts of most Invertebrata are
as yet unknown, but all the more exact results point to the supposi-
tion that they arise by a differentiation of the ectoderm.

The optic organ also has a simple mode of origin. We
exclude the pigment spots, which used to be often called eyes, and
only recognise an eye where a nerve-ending of definite form can be
detected, either under or on the surface of the body, acting as an
organ for the perception of light. By the light-absorbing property
of the pigment it is possible that indefinite sensations of light and
shade may be produced, or other sensations altogether unlike that
which we call “ sight may possibly be produced by the heat-rays
alone of the light.

The function of pigment in the way just noticed is doubtful, but
when it surrounds a part only of a rod-hke nerve-ending, and that
in such a way as to leave the outermost end free, and exposed alone
to the influence of light, it has, clearly enough, a definite function.
Optic organs of various degrees of complexity are formed by the
union of a few or of many nerve-endings; the elements which
are the medium of light-perception (rods) forming a convex or
concave layer. Another complication is due to the addition of
organs to refract the light (lenses) ; these, too, may have all kinds
of relations, but they are always, either directly or indirectly,
derived from the integument. In eyes in which the surface of the

THE OEGAXS.

45

layer of rods is convex there are, as a rule, as many lenses as there
are perceptive nerve-endings ; when the layer of rods is concave,
there is one lens only. By the addition of other arrangements to
the nervous apparatus of the eye, by which its functional capacity
is modified or increased, this organ becomes one of the most com-
plicated of the animal economy. In most of the lower divisions the
optic organ, even when fully developed, still retains its primitive
relation to the ectoderm. In the higher divisions it is separated
from it, and gets to lie, together with its perceptive apparatus, be-
neath the integument, or the perceptive apparatus is derived from
the embryonic foundations of the nerve-centre.

The phaenomena of differentiation may be seen even in what
relates to the position of the optic organ, for the parts of the body
which carry the eyes, as well as the number of the eyes, varies
greatly in the lower divisions of Animals. Connected with this is the
occurrence of a great number of eyes on the anterior part of the
body, which goes to form the head, until, finally, the number of
eyes on the part in question becomes limited to two. The different
position occupied by visual organs forbids us to suppose that they
have had a common hereditary origin, and is in favour of these
heterotopic organs having been independently differentiated from
an indifferent apparatus. On the other hand, that eyes which are
connected to the cerebral ganglion, or the dorsal nerve-centre, have
.a common genetic relation, is not to be disputed.

f) Kespiratory Organs of the Integument.

(Dermal Branchise.)

§ 39.

An important part is played by the integument, and therefore by
the ectoderm, in the formation of respiratory organs. Before
they appear the gas exchange is carried on probably by the whole
surface of the body, and this mode of respiration obtains in many
of the lower aquatic animals. A change of the surrounding medium
is effected, partly by the movements of the body, and partly by
special organs, for example cilia ; thus fresh quantities of it are con-
tinually brought into contact with the respiratory surface. This is
not, however, the only method of respiration in the lower animals, for
the introduction of water into the interior of the body, in fact
the bathing of the alimentary canal by water, is certainly not
without significance in this direction, while it is of great im-
portance as being the beginning of a long series of differentiations.
Certain limited portions of the surface become more developed in this
direction as the function becomes localised, and, in compensation
for this limitation, acquire the form of blood-carrying processes,
which are called branchiae. In many cases they are £fferentiated

46

COMPAEATIYE ANATOMY.

from the appendages (Vermes, Crustacea). An increase of the surface,
which is brought about in various ways, is the mode in which the
further complication of branchiae takes place ; it is very frequently
accompanied by a reduction in the number of separate branchial
organs.

The importance of branchiae to the body calls into existence
various kinds of supporting arrangements for these organs,
which, in their lowest condition, project freely from the surface of
the body. Neighbouring parts of the integument being raised up
into covering lamellae, the branchiae become hidden in cavities
(branchial cavities), and the same tegumentary folds give rise to
afferent and efferent canals for the water, which serves for respira-
tion (Mollusca, higher Crustacea). In this way the development of
respiratory organs may affect other parts of the integument, the
direct relation of which to respiration had been lost for a very
long time.

g) Excretory Organs.

§ 40.

Just as the gaseous excretory matters are eliniinated from the
organism by the respiratory organs, so too are there arrangements
for eliminating the solid or fluid matters which have become useless
to the organism. The whole surface of the ectoderm performs this
function in the lower organisms ; in the higher forms of life, on the
contrary, there are special organs, dermal glands, which have this
function. Of those general arrangements which function as organs
of secretion we have to do here with those special ones only which
eliminate the excretory matters, and which are distinguished as
excretory organs from those glands which secrete matters
which are of use to the organism ; these latter are either indepen-
dent, or are united to deflnite systems of organs, of which they are,
in that case, specialised parts.

The excretory nature of the products of secretion of those
secreting organs, which are formed by the ectoderm, is least open to
doubt, for the products are removed at once from the organism by
the emptying of the gland.

Of the various kinds of organs which open on the surface of the
body one sort attains to general importance. These are the kidney-
like excreting organs, which eliminate the nitrogenous excreta
from the body. These organs are distinctly derived from dermal
glands, notwithstanding that in the Vermes, where they seem to
have their most simple form, they penetrate deeply into the body ;
nor does the fact that in many cases (Annelida, Mollusca) the organ,
which in other points also is much modifled, opens into the body-
cavity, and so connects it with the surrounding medium, and even
serves in many groups (Mollusca) as a means for introducing water.

THE OEGAHS.

47

affect the question of its origin. In other forms (Annulata) these
organs, having a tubular form, assist in the generative functions, by
serving as ducts for the generative products. The recurrence of this
function for a portion of the primitive excretory apparatus (primi-
tive kidney, archinephron) of the Vertebrata might be explained as
an inheritance from a lower stage. How far such a view is justified
is still uncertain. In any case a genetic point of contact between
the primordial kidney of Vertebrata and the renal tubes of lower
organisms, can only be looked for where the apparatus is, as in the
Vertebrata, single on each side of the body.

h) Alimentary Canal.

§ 41.

The ingestion of nutrient matter into the body is, in some of the
lowest organisms, effected by endosmotic processes, in which the
surface of the body takes the principal share. In others solid nutri-
ment is ingested, the soft protoplasm sending out pseudopodia, and
embracing the nutrient matter which happens to come into the
neighbourhood (Hhizopoda). The formation of a definite part of
the surface of the body, serving for the ingestion of nutriment, is
really a step towards organ ological differentiation (Infusoria) ; but
this does not constitute an alimentary canal, which does not appear
as a separate organ till the body is differentiated into cell-layers.
The cell-layers, when they do appear — an inner and an outer — pass
into one another at the margin of the orifice of entrance.

The inner layer, or endoderm, lining a space open to the
exterior, forms the wall of a digestive cavity. In the simplest
form, represented by the Gastrula, the endoderm is the sole wall of
the primitive enteric cavity. The formation of a mesoderm gives
rise to other layers external to this one. Of these the most im-
portant is a muscular layer, for by it the intestine is enabled to per-
form independent movements. The opening which leads into the
enteric tube serves as a mouth for the ingestion of nutrient matters,
as well as for an opening for the rejection of the undigested remains of
the food (Coelenterata, many Vermes). The appearance of an anal
orifice produces a further separation of functions, and converts the
blindly- ending gut or enteron into a tube open at both ends, the
separate portions of which take on various functions, and so undergo
different adaptations. The first portion, which is connected with the
mouth, forms an oesophagus, which serves for the introduction
of food ; then follows the true digestive cavity, which is generally
widened, or provided with caecal sacs, and is generally called the
stomach, though this name is not always applied to equivalent
parts. The terminal part of the whole system serves for further
alteration of the food, as well as for the excretion of the remnants

48

COMPAEATIYE ANATOMY.

by the anus. This differentiation of the digestive tube into
several unequal parts is the most important complication which
it undergoes ; any further differentiations are subordinate to this.
Three tracts are accordingly henceforward distinguished^ as fore-
gut^ mid-gut_, and hind-gut.

In addition to the varying and numerous changes in size which
the different portions of the canal undergo,, other arrangements^ due
either to special new functions, or mere expressions of further
division of labour, arise in it. Organs for seizing and comminuting
the food become attached to the mouth, or mark off a portion of the
oesophagus (masticatory organs). In the stomach also there
are sometimes masticatory organs of this kind. When they occur at
the commencement of the oesophagus, just behind the mouth, this
part, which is frequently distinguished by its larger supply of
muscles, is known as the pharynx.

The size of the cavity of the canal is increased by dilatations, or
csecal diverticula. Crops are formed in the course of the oesophagus,
caecal sacs on the stomach and on the rest of the intestine, which
are variously complicated in number and arrangement. When the
length of the alimentary canal is greater than that of the body, it
is arranged in ascending and descending loops, or in coils, and so
adapted to the size of the cavity in which it is contained. Both the
quantity and quality of the food ingested is of the greatest import-
ance as affecting all these relations of parts; and nowhere is the
adaptation of the organ to its function — which results from the
mode of life of the animal — more clearly seen than in the arrange-
ments of the alimentary canal.

Secretory organs are generally connected with the alimentary
canal, to aid in the process of digestion ; their products dissolve,
and act on the nutrient matter by chemical change. Glands of
this kind are sometimes distributed over the whole canal, and some-
times distinguish certain portions of it only. In their simplest form
they are not differentiated from the enteric wall, and in that case
are not distinctly marked-off parts. Those marked off from the wall
of the enteron are separated into two chief divisions. One of them
comprises the glands which open in the buccal cavity, or its neigh-
bourhood, and are distinguished as salivary glands. Another
group is formed in the portion which serves for digestion, and is
regarded as a bile-producing organ, a liver. It is right to note that
the distinguishing of these organs by names which are applied to
organs of higher organisms, physiologically better understood, is
merely hypothetical, for nothing is known of the physiology of
most of the organs of the lower animals. This holds especially for
the epithelium of the gut, which generally appears coloured, and
which is often called the liver. This organ appears under the form
of an epithelium, lining a part of the digestive cavity in the Coelen-
terata, in many Yermes, and even in Insects, till at last it becomes
limited to definite caecal appendages of the alimentary canal, and so
attains to the lowest grade of independence. The liver presents

THE ORGAiS^S.

49

itself from tMs point onwards either in tlie form of numerous
follicles, wRicli beset a large portion of tlie canal, or it forms a large
group of glands, whicli open separately or together into the
alimentary canal. The differentiation of the liver leads to a gradual
separation of that organ from the digestive tube, so that finally it is
merely connected to the canal by its ducts (higher Mollusca, V erte-
brata).

Respiratory Organs of the Enteron,

§ 42.

The general differentiations of the primitive gut (archenteron),
formed by the endoderm, which have been already mentioned, give
rise, in obedience to the principle of division of 4abour, to organs
which serve for the ingestion and digestion of nutrient matters;
these do not confer any essentially new functions on the gut. But
such a new function does appear when the gut acquires relations to
respiration. It is not certain whether this function obtains in the
primitive gut, although this is probable, for the endoderm is bathed
by the surrounding medium, like the outer layer of the body, while
the water which is taken in with the food may serve for respiration.
The relation becomes much more definite when we note the regular
streaming in of water into the rectum, which obtains in many
Vermes and Mollusca. This phaenomenon is an indication of the
respiratory function of the gut, but has no bearing upon the forma-
tion of distinct respiratory organs, which are differentiated from the
digestive tube.

Such a respiratory organ is brought into existence in the most
anterior portion of the gut, by its walls being broken through by
lateral pores; by special relations of these pores to the vascular
system it acquires a respiratory significance. This arrangement,
which already makes its appearance in the lower divisions, occurs
again in the Vertebrata. Processes, known as branchiae, in which
the respiratory vascular network is distributed, arise on the walls of
the clefts of this cavity. A region of the primitive digestive tube
is thus converted into a special portion, a branchial cavity, at the
hinder end of which the tube which serves exclusively for nutrition
commences.

Another form of respiratory organ is developed from the wall
of the gut, in the form of a diverticular outgrowth of the anterior
portion of that organ. This appendage of the gut is filled with air,
and in fishes has merely a hydrostatic function. As the relations
of the circulation become changed it is gradually converted into a
respiratory organ, and becomes the lungs; in the higher divisions
of the Vertebrata new organs, namely, those for the production of
a voice, are developed on the passages leading into it.

50

COMPAEATIVE ANATOMY.

i) Vascular System.

§43.

The substances prepared by the digestive process for the
nourishment of the body are^ in the lowest organisms which take
in solid food, merely distributed from the digestive spaces into the
protoplasm of the body. When a distinct digestive tube is formed,
nutriment passes through its walls straight into the parenchyma of
the body, so that the mesoderm and ectoderm, with the organs
differentiated from them, are nourished by the endoderm. This is
characteristic of the Coelenterata and some groups of Vermes. In
many others a dividing of the mesoderm occurs, which takes the
form either of canalicular cavities, or of a complete splitting of the
mesoderm into an outer plate attached to the ectoderm, and an
inner one attached to the endoderm. Between these dermal and
gastric layers of the mesoderm is the body- cavity, or perienteric-
cavity (coelom), in which a fluid, to be regarded as the nutrient
fluid, is collected. When morphological elements are found in this
fluid, they are derived from the cells of the mesoderm. This fluid
is not at first exclusively nutrient ; it also subserves locomotion, by
swelling out different parts of the body at the will of the animal.
An important function of this kind is also played by the water, which
in many cases is taken into the coelom from the exterior.

The movement of the fluid in the general cavity of the coelom is
at first effected by the movements of the body. Contractions and
expansions of the body-wall cause the fluid which is shut in by the
dermo -muscular tube to continually change its position ; this may
be regarded as the lowest form of a circulation of the blood.
In this case the passages have not special walls, nor are there any
special arrangements for regulating the circulation.

This simple condition persists in many divisions in which the
coelom is developed (Bryozoa); in others canalicular cavities arise,
which are arranged regularly, and have the form of vessels, and
may undergo further complications. Their contents form the haemal
fluid or blood (Nemer tines). When in addition to these vessels
a perienteric-cavity is formed, the vascular system, which is partly
enclosed in it, is either completely shut off from it (many Annelida),
or is placed freely in communication with it at one or more points
(Mollusca, Arthropoda, Vertebrata). In the latter case the
vascular cavities must have arisen as portions of the body-cavity,
while in the former case the body- cavity was not formed until after
the vessels. The formation of the body-cavity is therefore, in the
case exemplified by the Annelids, to be regarded as a secondary
process ; and the formation of a hollow space in the mesoderm has
accordingly led to two different results successively; on the first
occasion to the formation of blood vessels, on the second occasion to
the formation of a body-cavity.

THE OEGANS.

51

§ 44.

Certain portions of the hollow cavitary system, which forms the
haemal passages, are converted into contractile vessels by the
development of muscles in their walls. The earliest circulatory
system arises by these producing by rhythmical action the regular
in-and-out flow of the blood. But the direction of the stream of
blood is not yet constant; it can be driven first to one side and then
to the other. The portions of the vascular system which are dis-
tinguished by their greater contractility are sometimes extended
over a large surface, and sometimes limited to shorter parts. They
are the beginning of the formation of a heart.

The heart is therefore an organ differentiated from the blood-
vascular passages, and in its simplest form is a portion of the vessels
which is able to move its contents in two directions. It is only
when valves appear at the ostia of the cardiac tube, that the
direction of the flow is defined ; the structure of the heart thus be-
comes complicated, and is further elaborated by being divided
internally into separate portions (ventricles and auricles). Contractile
organs of this kind often appear as the only differentiated parts of
the blood- vascular system, formed from the cavity of the coelom. The
blood passes directly from the heart into lacunar portions of the
coelom, between the different organs, and from thence back again
to the heart (Arthropoda), or there are definite vessels going off
from the heart, which sometimes traverse the body in the place of
the hollow cavity, or only partially replace the lacunar passage,
in that they do not on their way back to the heart reach it as
vessels, but into lacunar spaces. In this- case the cavity of
the coelom shows itself as a portion of the blood passage, which is
only partly represented by true vessels (Mollusca). Where the
vessels are completely developed and the heart differentiated, the
vascular system is divided into three parts. That which leads from
the heart and distributes the blood in the body is called the arterial,
and its vessels arteries. The passage which takes the blood back
to the centre of the circulation is formed by the veins, and the
part of the passage which lies between the afferent and efferent vessels,
forms a network of extremely fine canaliculi (capillaries). This
intermediate portion is very frequently replaced by a lacunar system,
in which case the greater number of the venous passages also have
no special walls.

It is very often difficult to say what should be regarded as a
vessel and what as a lacuna, and the distinction often depends on
very unimportant points. It is not sufficient to say that the essen-
tial character of a vessel is the investment of a cavity by flattened
elements derived from modified connective tissue, for these elements
might just as fairly be regarded as the covering of the other organs
which wall in these spaces ; it is therefore questionable to call wide
internal cavities, invested by such cells, ^Wessels."’^ This cannot

E 2

52

COMPAEATIYE AXAT02*IY.

indeed be regarded as tlie sole criterion^ and it should only have
weight when considered together with the greater or less regularity
of the lumen. But in examining this question we must bear in
mind one thing ; namely^ that in all these cases we have to do with
spaces which are walled in by connective substances, and that
vessels are differentiations of these spaces, and therefore presuppose
an indifferent condition. Between the two stages, the differentiated
and the undifferentiated, there are all kinds of intermediate steps.

k) Reproductive Organs.

§45.

The phaenomenon of the multiplication of the individual is
primitively closely connected with nutrition. Not only is nutrition
the cause of the growth and consequent increase in size of the body,
but it gives rise to a condition in which the organism converts the
excess of nutrient material brought to it into the means for pro-
ducing a new individual. In the lower forms, as in elementary
organisms, a process beginning’ with gemmation, and leading on
to fission, is a very common phaenomenon. The manner in which
multiplication is effected varies with the amount of material which
is used by an organism in producing a new organism.

The phaenomena of multiplication by gemmation and spore-for-
mation, which are so common in the lower divisions of the Inverte-
brata, have some relations to sexual differentiation, which indeed
does occur among the Protista. It is derived from a stage in which
two similar germ-cells fuse to form a new organism (Conjuga-
tion). As the two uniting cells become gradually dissimilar they
become distinguished into egg-cells and sperm-cells ; these are
the morphological elements of the sexual reproductive matter
throughout the whole Animal Kingdom, notwithstanding their
numerous modifications, which are seen most markedly in the
seminal cells. While the ovum retains its most essential characters,
and can be recognised as such in every division, the seminal cell
very early undergoes considerable changes. Like other cells it gets
a fiagellate process, which may be greatly developed, while the cell-
body and its nucleus are so reduced that they ordinarily form a
structure of no great size. In this way filamentous structures —
spermatozoa — are formed from the seminal cell. Sexual Repro-
duction, then, does not exhibit a real contrast to the asexual.

§ 4G.

We do not exactly know in all cases what are the relations
between the place where the reproductive matters are formed and
the early rudiments of the body, but from what has been observed

THE OEGANS.

53

in certain Coelenterata and Mollusca we may suppose that primi-
tively the relations of the two are very different, for in these forms
the ova are derived from the endoderm, and the sperm from the
ectoderm. The endoderm is, therefore, the female, and the ectoderm
the male germinal layer. But it is not yet known how far these
relations obtain among the higher animals. As yet there are only
uncertain indications, but these speak to a general agreement with
the results already obtained.

The parts of the body which are set apart for the formation
of the sexual products gradually take on the form of glands. This
is a further step in differentiation, and is connected with the local-
isation of the function.

In the simplest cases the two kinds of generative products are
formed in special parts of the body, which function as sexual organs
(reproductive glands); but these parts are not at first distinguished
by any special characters. The organs which produce the semen are
called testes, and those which produce ova, ovaries. Going a
step further, we find the reproductive glands still further differen-
tiated. In their simplest condition the products of these organs
merely break away from the spot where they are formed, and pass
into the digestive sac, or into the body-cavity, or even directly to
the exterior. Gradually, however, ducts, which are often very com-
plicated in character, are added on : it is probable that these ducts
are not primitively connected with the germinal glands. Where
these ducts can be seen to have any relations to other organs, these
appear to be excretory organs (§ 40) which have entered into the
service of the genital organs, and have been altered so as to
correspond to this function. It becomes a great question whether
the excretory ducts of the reproductive matter are not in all cases
excretory organs. Beceptacles which serve for the collection of
the sperm are formed on the outlet-tubes (seminal ducts) of the
organs which produce the sperm ; from the wall of these canals
glands are differentiated, which secrete a fluid to be mixed with the
sperm ; finally, there are arrangements for passing the sperm into
the system of the other sex (copulatory organs). The differen-
tiations of the egg-forming organ are no less varied; the duct
(oviduct) of the ovary is provided with dilatations, in which the
ova get special envelopes, or are further developed. These portions
of the oviduct are called the uterus. Special glands. Yolk
glands, are formed from the ovary, and secrete a substance which
is either taken up by the ovum or which merely forms an envelope
for it. Appended organs receive the semen which is passed in
copulation, and are known as receptacula seminis; lastly, other
parts serve for the reception of the co23ulatory organ, or for the
deposition or preservation of the ova.

The relation of the egg-forming and sperm-forming organs to
one another varies greatly, and must be considered from the stand-
point of differentiation. In the lower divisions organs of both
kinds are united with one another, sometimes in such a way that one

54

COMPAEATIVE ANATOMY.

and the same gland produces both semen and ova (hermaphrodite
gland). The ducts^ also, are often more or less common to them
both. But in other forms the genital organ is divided, the products
of its two parts being different ; testes and ovaries, that is, are
present as separate organs, the excretory organs of which only are
united more or less extensively; or each of them may have its
separate- orifice. All those animals which unite in themselves both
kinds of reproductive organs are known as Hermaphrodites.
A separation of sexes is apparently foreshadowed in various forms,
by the alternating activity of the organs, at one time the egg-
forming and at another time the sperm-forming organ exercising its
function.

The hermaphrodite stage is the lower, and the condition of dis-
tinct sexes has been derived from it. This change is due to the
decrease in size of one or the other organ, so that hermaphroditism
is the precursor of sexual differentiation. This differentiation, by
the reduction of one kind of sexual apparatus, takes place at very
different stages in the development of the organism, and often when
the sexual organs have attained a very high degree of differentiation.
In these cases ontogeny exhibits the two kinds of organs primitively
united, and so causes the individual to be hermaphrodite at a certain
stage in development.

The separation of the sexes affects the whole of the organism,
for it produces a series of changes in each sex, which affect organs
that had primitively little to do with the sexual function. Sexual
differentiation is completed when the two kinds of organs are
given over to different individuals. Thenceforward for reproduction,
not only two different substances, semen and ova, and two different
organs for producing them, are necessary, but also two individuals ;
these are distinguished as male and female.

Changes in the Organs.

Development and Degeneration.

§ 47.

The result of the continued differentiation of a given organ is a
complication by which the organ is removed proportionately further
from its primitive condition. As the primitive condition is the lower
differentiation, it entails a perfecting corresponding to a higher con-
dition. This is clear on the principle of division of labour, which is
the cause of all differentiation (cf. § 12). In obedience to this law
a function can be the more perfectly carried out, the more exclu-
sively the organ is related to that function. The more an organ is
exercised for one function only, the more suitable are the conditions
for its development in one direction, for there is no competition with

DEVELOPMENT AND DEGENEEATION.

55

other directions of development. A limb which is a gill too, that is
which has both locomotive and respiratory functions, is of a lower
grade than an arrangement resulting from a division of the two
functions, in which a part separated off from the appendage repre-
sents a gill, and the rest an organ of locomotion. When the functions
are united, locomotion is necessary for respiration, but when they
are separated, they are independent of one another, and respiration
is effected without the aid of locomotion, by the development of
special organs for changing the water, these organs so far taking
the place of locomotion. In both organs the independence which is
necessary for their further development in one direction is gained.

The organs of the body are not always developed to the same
extent. One or another often remains in a lower condition, and so
retains its more lowly character in an otherwise highly differentiated
organism. It is not therefore wise to draw any conclusions as to
the extent of the differentiation of single organs from that of the
organism itself; it is better to judge organs by comparing them
with equivalent organs in other organisms.

The real factor in the development of an organ by differentiation
must be sought for in the increased or modified function of the organ
in the struggle for existence, that is in its adaptations to the ex-
ternal conditions of life. It is hence that transmission acquires its
importance, since it not only causes a perpetuation of inherited
characters, but is enabled to effect an elevation in those characters.

Degeneration or reduction is another constant phsenomenon
which is dependent on differentiation, inasmuch as it presupposes it.
Its result is, in itself, the exact opposite to that of differentiation. For
while differentiation is the cause of complications, reduction is the
cause of simplifications of the organism, and is therefore the cause
of organs or of organisms passing to a relatively lower stage. With
regard, however, to the general organism, and its relation to other
organisms, it produces the same effect as differentiation, for it leads
to variety in form.

Eeduction, like differentiation, varies in degree; it may affect
separate portions of the body, or groups of organs, or finally the
whole of the body. It is different, again, according as it affects
the individual, the species, or the genus. In one case it may be
seen to be a definite process, in another a condition, which can only
be assigned its place as one of the several stages of such a process
by the aid of a comparative series of allied forms. It may affect
organs in two different ways. The affected organ may be in-
dependent of the general arrangements which obtain in the de-
veloped organism to which it belongs, and then reduction has but
a transitory or provisional signification. Reductions of this kind
during the course of the development produce simplifications, but as
the differentiation which is going on in other parts may be producing
new and higher organs, this reduction does not hold the organism
back, but is a cause rather of the progress of differentiation in
another direction. The reduction of parts which belong to certain

56

COMPAEATIVE ANATOMY.

developmental stages in the individual are examples of this kind of
reduction (Larval organs). (Cf. § 5.)

The other kind of reduction affects organs which belong to the
developed organism or its rudiments. It may affect the fully
formed and completely functional organ as well as one just laid
down^ and in the primary state of differentiation. The process of
reduction is seen therefore in various degrees of intensity. The
process is often difficult to perceive amid the various other processes
of differentiation which are affecting the rest of the organism when
the organ affected is only just making its appearance : the further_,
however^ differentiation has gone^ the more striking must the
process be.

The reduction of an organ is necessarily connected with its
function, a change in which must be regarded as the cause of the
reduction. Loss of function produces retrograde changes in an
organ, but of course neither process is a sudden one.

Although reduction is, on the whole, the cause of the simplifica-
tion of an organ, and therefore of the organism also, it is not a
phenomenon which makes the organism absolutely lower in degree.
Reduction may rather lead to a higher differentiation, as it does when
larval organs are removed ; it may give rise to higher forms even in
whole series of organisms derived from one another, by facilitating
the higher- development of those not affected by it. In this case
again reduction precedes differentiation. This is strikingly seen in
the numerical relations of parts, which become individually more
perfect as they dnninish in number.

As reduction is a gradual process, the organs which are affected
by it may be met with in various stages. These rudimentary
organs are most significant indications of genetic relations, while
they at the same time show us how an organ which has lost its
primitive function, and which may even have no intelligible signifi-
cation as regards the purposes of the organism, may persist for
a very long time before it completely disappears. ‘(Cf. supra, § 6.)

Reduction may affect every organic system and be observable in
every part of it. It is expressed in the form as well as in the size
and number of the parts, and even in their histological characters.
Its conditions are to be sought for, first of all, in the relations
which alter the organism. According to the number of organs
affected, reduction is more or less manifest in the organism as a
whole.

Inasmuch as comparison everywhere reveals to us evidence of
either progressive or of retrogressive change, we may regard the
organism as a thing caught in the act of moving’, as arrested in
the midst of a career through the most diverse ranges of form.
The changes of the various organs, and the phaenomena which are
observed in the elementary structure in the cell, are what make np
this movement.

COKKELATIO:^'.

57

Correlation of Organs.

§ 48.

The changes in the organism which are due to differentiation
and reduction are the cause of a fresh series of phaenomena in the
factors which gave rise to them. From the conception that life is
the harmonious expression of a collection of phaenomena regularly
conditioned^ it follows that the activity of an organ cannot be
regarded as really existing for itself alone.

Every kind of arrangement presupposes a series of other arrange-
ments ; every organ^ therefore, must have intimate relations with
the rest, and be more or less dependent on others. Every move-
ment in a muscle presupposes the existence of a nerve ; and both
of these organs presuppose the existence of a nutrient system.
In this way one function has an intimate connection with other
apparently dissimilar functions. This relation, which was first defi-
nitely pointed out by Cuvier, and which is known as Correlation,
shows us the road by which we can attain to a correct appreciation
of animal organisation. By far the most important point is the
conception of the organism as an individual whole, which is as much
conditioned by its parts, as one part is conditioned by others.
Correlation is a necessary result of this conception.

Not only the general arrangements of the organisation, but also
its apparently more subordinate features, exhibit intimate relations
with one another, and a change which affects one system of organs,
simultaneously produces modifications in some of the other organs.
These are adaptations to changes, which themselves are due to
adaptations. They are, however, of a secondary character, while
those which are of a primary character have their origin in the
outer world.

Correlation may be divided into the more and the less remote ;
where less remote it is expressed in one system of organs or in
other systems functionally connected with this ; when • more
remote, in organs which are functionally less related to it. Physio-
logical principles are essential in the investigation of correlation,
and it is necessary, therefore, to know what are the functions of
separate organs, or at least what their value is in the animal
economy, in order to be able to recognise it. So, too, it is of im-
portance to know what are the habits of the animal, for the original
forces, on which the various relations of the organs depend, are due
to them.

As the forces which cause changes in the organism either lie
without the organism, or, as most of them, are to be sought for
without it, they do not come within the scope of our work. Com-
parative Anatomy, therefore, is limited all round by a wide but
uncultivated region, in which rich harvests may be gathered for
biological science, whenever its treatment is taken in hand.

58

COMPAEATIVE AjS"ATO:\IY.

Fundamental Forms of the Animal Body.

§ 49.

Owing to tlie infinite variety of tlie external characters of animal
organisms, it is necessary to seek for fundamental forms to wliicli
this variety may he referred. must also ascertain the con-

ditions which influence and give rise to the most important modifi-
cations of these forms. These results may he obtained in different
ways. We will choose the shortest hy commencing with the lowest
stage of the animal organism.

This is the stage which the Gastrula form presents to us ; hy its
wide distribution this form will provide us with the characters which
are best adapted for our purpose. An organism at this stage is some-
what spherical or oval in shape, and the mouth
will he found at a point on the surface.

If we imagine an axis (Fig. 16, AB) drawn
straight through the digestive cavity, the pole
corresponding to the opening of the mouth
represents the oral, and the opposite the ahoral
pole. This axis {A B) is the primary axis
of the body. In a body of a regular cylin-
drical or spheroidal shape we can imagine as
many lines as we please drawn through the
body perpendicular to this axis. (Secondary
axes, ah, c d.) In this instance they are all
equivalent. The secondary axes are in this
case indifferent to one another, and are cha-
racteristic of a lower condition. The organism,
either when moving freely in the water, or
when fixed (by the aboral pole, of course), as
it afterwards is, is differentiated by the develop-
ment of a certain number of secondary axes,
their development having relation to the main-
tenance of the balance of the body. We here,
then, have to do with a statical cause. The
development of the organism along its secon-
dary axes takes place through the development
of external appendages, tentacles and the like,
or through differentiation of the enteric cavity,
or through the laying down of other organs
(such as the generative glands) in the direction
of those axes. In consequence all the conceivable secondary axes
are no longer equivalent. Those along which organs are differentiated
are distinguished from the rest. They have passed, in fact, from the
previously indifferent condition to a differentiated one. The so-
called radiate fundamental-form of the body which is commonly
found in the Ccelenterata, arises in this way, as may be seen by

6

Fig. 16. Diagram of
the axes of the body.
A B Primary axis, a h,
c cl Secondary axes. The
lower figure is a trans-
verse section of the
upper one, showdug its
two secondary axes.

FUNDAMENTAL FOEMS.

59

studying the relations of the axes to one another^ as explained
above (cf. Fig. 17). The importance of the month to the organism
causes the differen-

tiations which obtain
around it to have a
special value. These
differentiations are de-
veloped as tentacles
of various form^ and
cause the parts around
the mouth to be much
more varied in cha-
racter than those at
the aboral pole.

If the body grows
in the direction of its
primary axis^ without
becoming attached to
the ground, the axes
may acquire modified
importance if locomo-
tion in the direction of
the animaFs length be
established. The pri-
mary axis will remain
as before, but the
secondary axes will
necessarily differ ac-
cording to the signifi-
cance of the surfaces
which they connect.
When one and the same
surface always touches
the supporting object,
it becomes the ventral

ce

Fig. 17. Radiate fun-
damental form ; letters
as in Fig. 16. The an-
terior surface of the
body is seen in the lower
figure, and shows the
appendages (tentacles)
which are differentiated
along the two transverse
axes.

A

Fig. 18. Diagram to show
the differentiation of the
secondary axes. In the
upper figure a cephalic
portion is indicated by
the development of a pair
of dorsal tentacles. The
lower is a transverse sec-
tion of the upper figure,
and the secondary axes
are consequently seen in
it.

surface, and the oppo-
site one becomes the dorsal. These two surfaces, the dorsal and
the ventral, are placed under different conditions, and must therefore
be differentiated in different ways, while the two sides, or — when
the body is perfectly flattened out — the two lateral edges necessarily
come to differ in character from the dorsal and ventral surfaces.

Such cases are instances of the development of only two
inequivalent secondary axes. One connects the ventral and dorsal
surfaces, and is the dorso-ventral axis (Fig. 18, a h), the other
connects the sides (c d) of the body, and is the transverse axis. The
surfaces which correspond to the poles of the first or dorso-ventral
axis are not, while those which correspond to the poles of the
transverse axis are, equivalent. A primitive condition which has
disappeared in the dorso-ventral axis in consequence of the differen-

GO

COMPAEATIYE ANATOMY.

tiation of its surfaces^ is retained in tlie case of the transverse axis.
This, the second form, which can be derived from the Gastrnla, and
is ordinarily known as that of bilateral symmetry, first appears in
the Vermes, and prevails in all the divisions above them.

When the secondary axes of the body retain their primitive
indifferent character, we may imagine that there are as many similar
pieces in the architectural composition of the body as there are
possible secondary axes. But when the secondary axes become
differentiated, the divisions of the body take on a definite numerical
relation. They are known as antimeres. If two secondary axes
become differentiated, and are like in character, we have four
antimeres, for we can divide the body into four similar parts, along
these secondary axes. But when two unlike secondary axes are
differentiated the body is only made up of two antimeres ; two
halves of the body, distinguished as right and left, are the parts
corresponding to one another. In this way the eudipleural funda-
mental form is developed.

§ 50.

The differentiation which marks off the oral from the aboralpole,
and which has been already mentioned, gives a higher significance to
the former region of the body. This differentiation asserts itself in
other forms, as in the primary radiate form, and in very various ways.
It is not only the presence of the mouth, which favours the
differentiation of organs around it, as organs for aiding in the
prehension or ingestion of food, but the greater significance of the
anterior end of the body in locomotion is also a cause of differen-
tiation. This part takes the initiative. It has to show the way to
the rest of the body, and often indeed, to lead it ; it meets with a
thousand foreign objects, which it has to examine, to follow, or to
avoid. It is therefore exposed to external influences other than
those which act on the opposite end of the body. The dignity of
the relations of this region explains how it is that the mouth is not
by any means always at the anterior end of the body, and that it
much more frequently is placed close to or even altogether on the
ventral surface, without the anterior end of the body being less highly
developed. The high specialization of the anterior region is caused
principally by the development of various kinds of sensory organs,
and therefore of organs which put the organism into relation with the
outer world ; the region moreover often has various organs of defence
connected with it, and with it is closely connected the development
of the central nervous system. The whole region thus gets a higher
value in comparison with the general organism, for it shelters and
carries the organs which elevate and rule the latter. This anterior
region of the body is therefore called the head, or chief portion.
The differentiation of a head seems to depend primarily on the
position of the mouth. This dhects the course of movement, and
to this, as a secondary cause, the anterior part of the body owes its

METAMEEIC SEOMEETATION.

G1

various distinctions. The appearance of a head is at the same time
a result which affects the whole body^ for the body can now be
divided into two portions at leasts which differ in character.

Metamerism of the Body.

§ 51.

The planning out of the individual organism as a single struc-
tural entity is only characteristic of lower conditions of development,
whether permanent, as in nearly all Coelenterata and in the lower
classes of Worms, or transitory, as in the higher divisions of the
Animal Kingdom. Simultaneously with the growth of the body to a
considerable length, we observe the beginning of the division of
the organism into separate segments, following one on another,
noticeable externally through the occurrence of separating constric-
tions, or through the regular distribution of appendicular structures
or processes of the body, internally represented by the arrangement
of the organs in the distinct successive compartments of the body.
We term this segmentation of the body Metamerism ; the separate
segments are me tamer es. The metamerism which thus breaks
up the body is only a further example of differentiation. From the
primitive homogeneous indifferent body a heterogeneous, diversified
body is developed, and the separate metameres differ from one
another ; not only are they something new in comparison with the
earlier condition, but they are also — notwithstanding their resem-
blance one to another — different from one another, owing to the
position which each occupies.

Metamerism is not in all cases, where it is perceptible, exhibited
with equal clearness. Sometimes it is apparent in this or that
organ, or system of organs, more than in another, and, again, in
other organs may be altogether wanting. It is easy to recognise
very various conditions of the commencement and of the incomplete
carrying out of the process. Where we find it in fullest develop-
ment it dominates the whole organism, and is exhibited in all organs ;
so that each metamere possesses its individual system of organs,
and particular systems of organs common to all metameres present
a special differentiation of their structure in each metamere (ventral
ganglion chain) . In this manner the organism becomes built up of
many component parts. And hereupon we have to take note of
conditions in which independent importance is acquired by the
metameres. Little by little a metamere, in virtue of the elaboration
of its own set of organs, ceases to be dependent on the total
organism, emancipates itself from the commonwealth, and gains the
capability of leading a free existence. To this many phoenomena are
traceable, which are usually called gemmation (Worms).

G2

COMPARATIVE ANATOMY.

§ 52.

An efficient cause for metamerism may be sought^ as has been
above indicated^ in the pbasnomena of growth. We can imagine a
repetition of local outgrowths, resulting in practical advantage to the
organism, taking place in particular systems of organs simultaneously
with the elongation of the body. In this way the external meta-
merism may be brought into connection with the movement of the
body, which was perhaps the earliest cause of this pliEenomenon.
Many facts point to its being so. In any case there are numerous
examples of the gradual elaboration of metamerism without all
systems of organs being at once affected by it. Metamerism has,
however, a less doubtful origin in its connection with gemmation,
which is itself a kind of growth. It seems, indeed, in many cases, as
if gemmation led to metamerism, in such a way that the metameres
represent buds, which remain connected with the organism, and only
in some cases attain to a higher stage of individual existence.
Numerous instances of incomplete metamerism prevent us, however,
from attributing a general significance to this process, and it cannot
in any sense be regarded as the sole cause of metamerism.

Metamerism leads to perfection of the organism. By it
the organism is enabled to get a larger number of organs, although,
indeed, they are at first mere repetitions of one and the same
arrangement. As the separate segments become more independent
their action becomes more free, till at last the differentiation of a
larger number of separate organs gives a larger scope for action.
Differentiation, then, gains ground in every part, and alters the
organs of the separate metameres in different ways, according as
their functions become more various. By the development and
reduction of metameric organs the metameres get to differ in value,
and become differentiated themselves ; this differentiation is expressed
externally by the difference in their size and form. This leads to
the disappearance of the primitive similarity of the metameres. The
amount, too, of their independence may be lessened, and a number
of primitively separate metameres may gradually fuse into larger
divisions. This gives rise to complexes of metameres, in which the
fact of their being composed of separate units of the body is only
suggested, and that often faintly; sometimes a large, sometiuies a
small number of segments undergo concrescence. This, too, is on
the whole a cause of differentiation of the organism, as the body
consists in consequence of some independent and of some fused
metameres. Finally, metamerism may altogether disappear, and
the presence of separate organs alone indicate, and that often
obscurely, the phenomenon which obtained in the primitive state.
Every stage in metamerism is therefore a source of variation in the
external and internal organisation of the organism.

HOMOLOGIES.

63

Comparison of Organs.

§ 53.

The variations in organisation among the various larger and
smaller divisions of the Animal Kingdom are such as to lead us, at
first sight, to perceive the points of difference rather than those of
agreement. And this is more marked in proportion to the diver-
gence between the particular divisions compared. It is, however,
the business of Comparative Anatomy to follow out the changes in
the organisation, and to discover what is similar in the changed
and metamorphosed forms, however deeply hid it may be. An
organ may be similar to another in one of two ways. Either
in its functional relations, that is from a physiological point of
view ; or in its genetic and therefore anatomical relations, that is
from a morphological point of view. These two relations of an
organ must be kept well apart. The change of function in one and
the same organ, as well as the similarity in arrangement of organs
which are morphologically very different, compels us to ascribe a
subordinate value to physiological relations, when we are comparing
organs. The gills of a Fish, of a Crab, and of a Cephalopod, are
organs of respiration, and have many structural points in common ;
yet they are very different organs morphologically, as the relations of
each of the three to the whole organism shows. By insisting on
similarity of function, we bring together organs which are morpho-
logically different, and so turn aside from the object of Comparative
Anatomy. We distinguish, accordingly, physiological likeness, or
Analogy, from morphological likeness, or Homology, and only
consider the proof of the latter as our task.

The smaller the division to which the objects of comparison
belong, the more obvious is the homology. Homology therefore
corresponds to the hypothetical genetic relationship. In the more
or the less clear homology, we have the expression of the more or
less intimate degree of relationship. Blood-relationship becomes
dubious exactly in proportion as the proof of homologies is uncertain.
It is impossible therefore to say exactly how far homology extends
throughout the Animal Kingdom. As a matter of fact, numerous
investigations have discovered a larger number of homologous
arrangements even in otherwise divergent groups, and have thereby
extended the boundaries of homology further than was formerly
thought possible.

In consequence of the existence of various possible modes of
morphological agreement, homology is divided into two primary
groups: General and Special Homology.

G4

COMPAPvATIYE AXATOMY.

§ 54.

I. General Homology is under consideration wlien an organ
is referred to a category of organs, or when a single organ com-
pared with another is taken merely as the representative of snch a
category. These categories always consist of several organs or
parts present in the body. When we compare the body-segments
of an Annelid, the vertebrae, or the appendages of an animal with
one another, we lay the foundations of general homology. This
again consists of several subdivisions, according to the kind of
category which is made use of in the comparison.

1) Homotypy has reference to organs which are fellows to one
another, snch as the organs of the two sides of the body ; the right
kidney is homotypical with the left, and the right eye with the left
eye, and so on. Whilst these examples may not show the necessity
for the formation of this division, it should be noticed in addition
that homctypical organs have not always the same characters. They
are often so changed that their homotypy cannot be recognised,
and has to be worked out.

2) H o m o d y n a m y (equivalent to the general homology of Owen,
and partly also to his serial homology) subsists between parts of
the body which are affected by a general morphological phseno-
menon serially expressed in the organism. Homodynamy is dis-
tinguished from the next subdivision by the fact that the parts
in question are arranged along the long axis of the organism and
define its type. The metameres therefore are homodynamons parts ;
as are the segments of the Arthropoda, the primitive vertebrae of
the Vertebrates, etc.

3) Homonomy. This describes the relation to one another
of those parts which are arranged along a transverse axis of the
body, or in one segment only of its long axis. The rays of the
pectoral and pelvic fins of fishes, the individual fingers and toes
of the higher Vertebrata are homonomons structures.

Besides these there are other subdivisions of general homology
distinguishable, which are however of very subordinate importance.

§ 55.

II. Special Homology, Homology in the restricted
sense. This is the name we give to the relations which obtain
between two organs which have had a common origin, and which
accordingly have also a common embryonic history. As exact
proofs of genetic relations are necessary for the investigation of
special homologies, this mode of comparison is generally limited
in the lower divisions of the Animal Kingdom to systems of organs;
it is only in the Vertebrata that it is possible to extend this method
to more minute features. Thus among the Vermes or the Mollusca
we can hardly indicate, with any certainty, particular parts of

ilOMOLOGlES.

G5

tlie alimentary canal as lioniologous ; while in the Vertebrata we
can confidently assert that even such unimportant structures as
the CEeca of the intestine from the Amphibia onwards are homo-
logous. The homologies of the parts of the skeleton^ which are the
organs that have been investigated with the greatest exactness, are
those which can be most definitely recognised. It is a large part
of the main task of Comparative Anatomy to prove special homo-
logies.

Special homology must be again separated into subdivisions,
according as the organs dealt with are essentially unchanged in
their morphological characters, or are altered by the addition or
removal of parts. I therefore distinguish —

1) Complete Homology, when the organ referred to is
unchanged in position and connection, and is still perfect however
much modified in form, size, and various other points. This kind
of homology is generally found within the limits of small divisions,
less often in larger ones. For example, the bones of the upper
arm from the Amphibia to the Mammalia, the heart of the Amphibia
and Reptilia, etc., exhibit complete homology.

2) Incomplete Homology. This consists herein, that an
organ which is otherwise completely homologous with another, has
other parts which are wantiug in the latter added to it, or conversely,
when an organ is wanting in some essential part in comparison with
another organ. The heart of the Vertebrata may serve as an
example. The organ is homologous throughout the division from
the Cyclostomi onwards, but- the homology is incomplete; for in
Fishes a part, the venous sinus, which in the higher divisions is
taken into the heart, and which in the Mammalia is absorbed into
the right auricle, lies outside the heart. The homology between the
heart of the Fish and of the Mammal is consequently incomplete
owing to addition. In another case it may be incomplete owing
to diminution. The reverse of the previous case may serve as an
example, were it allowable to regard the heart of the fish as a
reduced one. An example is presented by the pectoral fins of
fishes. The skeleton of this organ in the Ganoidei or Teleostei
is, owing to reduction, incompletely homologous with that of the
Selachi. Parts have in this case disappeared which did primitively
belong to the organ, just as in the former case parts, which although
they were primitively present did not belong to the organ, were
added to it.

Systematic Olassilication of the Animal Kingdom.

§ 56.

In the general organisation of every animal we recognise a
number of arrangements which it has in common with a greater or
less number of other animals. These relations are partly of a more

F

G6

COMPAKATIVE ANATOMY.

general nature^ affecting tlie position or arrangement of tlie most
important systems of organs, and partly tliey affect tlie special
development of individual organs; they extend to agreement in
form, size, and number. The classifying spirit of man has formed
definite conceptions of these relations of organisms to one another.
All those individuals which agree in essential points he has called
a species, and has united into a genus those species which resemble
one another in a number of points ; these again he has united into
larger divisions, families, orders, and classes. Thus arose the
zoological system, which, in so far as it unites what agree, and
separates what differ, has come to be the expression of our general
knowledge of the Animal Kingdom.

In this way the wEele Animal Kingdom can be broken up into
several large divisions, each of which differs from the rest by a
number of special characteristics. The essential character may be
recognised in all the subdivisions, and even under great individual
variations. This has been called the ‘^‘'type.^^ The type then
means a collection of characteristics which are expressed in the
organism, and which are predominant in a large division of the
Animal Kingdom, and which are evident in the course of develop-
ment as well as in the adult condition. Such larger divisions which
differ from others in certain fundamental points of organisation are
themselves called types.^^

Within each type we note a variation in the characters of the
divisions which make it up, and this often to such an extent that
what is characteristic of the type appears to be lost in some forms.
In this case it is always individual development which enables us
to recognise the connection of these forms with the type.'’^

If we admit that similarity of organisation in different individuals
is explicable by the fact that they have a common ancestor, and that
therefore these similarities are due to affinity, we must regard less
close similarities as due to a less close relationship. We therefore
regard the individuals which belong to one species as more closely
allied than are the representatives of different species, and within
the limits of one species we shall again derive from common ancestors
those individuals which are distinguished by special characters, and
which we unite into a sub-species.

ISTo one has any hesitation in recognising within the limits of
small groups of individuals the phgenomenon of the continuation of the
peculiarities of a given organism into other individuals by means of
transmission ; indeed it is often possible to perceive, by direct ob-
servation, that descendants are like their ancestors. By extending
this conception of affinity to a wider circle, and regarding what is
common in organisation as due to a common descent, and what is
divergent as due to adaptations, we take our stand on the theory
of Descent (cf. §§ 4 and 5). We consequently regard the large
divisions known as ^‘'types,^^ as phyla, or leading branches of the
genealogical tree, and by so doing point to the cause which has
determined their existence.

CLASSIFICATIOX.

G7

WitHin one pliykmi a form of animal organisation is evolved
along the most varied lines, which gradually lead from the simple to
the more complex, and from the lower to the higher. The categories
which we distinguish as species, genera, families, orders, and
classes are due to continued differentiation. These subdivisions
correspond to the ramifications of the branch, and in them the
divergence of character is expressed.

The differences between the classes, orders, and so on, are so
great that they do not seem to have any connecting links at all, but
we must take into consideration the fact that in living forms we
have before us only the final offshoots of developmental series of
organisms, which have been ramified in very various ways, and
which lived in earlier and often very far-distant periods, and which
have gradually disappeared. The palaeontological record proves
this partly, though it may be but very slightly. In the strata of the
earth remnants of forms which have disappeared, and which were
the predecessors, and, in fact, the direct ancestors, of later living
organisms, are preserved. Inasmuch as the living forms are but a
small portion of the whole world of organisms, which has existed in
the course of geological periods of development, we cannot expect
that far- distant connections should be always evident, the inter-
mediate steps determinable, and the genealogical connection made
clear and indubitable. It is necessary to try and put the whole
together out of fragments and to find lost traces of continuity.
The most important part of the business of Comparative Anatomy is
to find indications of genetic connection in the organisation of the
Animal body.

Following out this conception we have to represent to ourselves
a developmental series of organisms arising in each phylum from a
primitive form, which has been, during geological development,
differentiated into many branches and twigs, most of which have
disappeared at different periods, while some, greatly changed
though they may have been, have lived on until to-day. The
general character which has remained in these various stages of
differentiation, and has been transmitted, with modifications, from
the stem-form, is what is typical in the organisation.

§ 57.

It is not always possible to prove, to the same extent, in all of
the large divisions which are regarded as types, the common ancestry
of the forms which belong to it. It is in fact very probable that
several divisions have had a polyphyletic origin, in which case the
organisms which belong to them must be united together for reasons
other than genealogical. Such divisions cannot be regarded as phyla.

We meet with such relations in the lowest forms, in the
boundary-territory between Animals and Plants. It is difficult to
find a boundary line, for there are organisms which seem to belong to
one as much as to the other Kingdom according to the phoenomena

68

CO:\rPAEATIYE AXATOMY.

to wliicli they give rise. The idea of a boundary line presu]3poses a
rigidly-defined conception of Animal and Plant. The characteristic
of the animal organism may be taken to be that differentiation affects
the whole organism. This differentiation consists in its division into
two layers, which have been already (§ 28) called ectoderm and endo-
derm, and from which the germinal layers of the higher divisions are
derived. But the exclusion of all the lower organisms, which do
not undergo this division from the Animal Kingdom, w^ould put out
of our scope many pheenomena which are of great importance, if
we would understand animal organisation. Although it might be
best to regard this world of lower and very varied organisms as a
special Kingdom placed between the Animal and the Vegetable, and
containing the beginnings of both, that of the Protista, yet we,
as our work embraces the connections between animals and these
lowest organisms, must enter into a consideration of them. We
therefore unite a number of those divisions of the Protista, which are .
more nearly related to animals than plants, as the Protozoa. As
their genetic relations to one another are altogether unknown, the
division which is formed by these organisms cannot be regarded as
a phylum."’^ Nor is there a type common to them all. We there-
fore regard them as lower organisms, which are the nearest of the
Protista to Animals, and we must compare them, not with the
separate divisions of the higher animal organism, but with them
all toR'ether. This compels us to unite the latter into a single

group.

which has been called the Metazoa.

The Protozoa and Metazoa are not so very sharply marked
off from one another. Not a few of the Protozoa are known to be
composed of a number of cells. It is the arrangement of cells in
layers of definite physiological value which characterises the metazoic
organism. This seems to take place very gradually, and at first
the layers are incomplete. We find representatives of this in the
pamsitic Dicyemidte, which live in the so-called veinous appendages
of the Cephalopoda, and deserve to be specially mentioned. A

germinal cell gives rise to a number of cells
by division, among which a single large
cell becomes surrounded by a number of
smaller cells, which form a continuous
layer.

The central cell represents the endo-
derm, and is covered by the peripheral
layer, which represents the ectoderm at all
but one small spot. (Fig. 19.) The endo-
dermal cell elongates considerably, and
its protoplasm becomes differentiated in
various ways. It forms the groundwork
of the elongated body, and remains covered
by the ectodermal cells, which also grow,
without however multiplying very much ; these give off fine cilia,
and form the protective and locomotor organs of the body, while

Fig. 19.
Gastrula
stage of
Dicyema
typus.

Fig. 20. Vermi-
form embryo of
Dicyema typus
(after E. van
Beneclen).

THE DICYEMIDzE.

69

the endodermal cell in the axis of the body becomes an organ of
nutrition^ and takes on the function of repro-
duction, for the germs of young forms are found
in it of two different types.

The organism of Dicyema is thus seen to be
two-layered, and the two layers have different
functions ; the inner one is morphologically least
differentiated, for it consists of a single cell.

It is not quite certain whether this corresponds
to a primitive condition or no, for the parasitic
life of the Dicyemidae may have been the cause
of the reduction of a multicellular endoderm.

But their development is quite easy to make
out, and in it there is never more than one
endodermal cell, so that the fact becomes more
significant. Just as among the Protozoa the
multicellular forms are allied by intermediate
steps to the unicellular, from which they have
been developed, so among the Metazoa does
Dicyema exhibit to us the commencement of the
separation of the body into cell-layers, and
although the arrangement is not as perfect as it
is in the rest, yet it is in the same direction as
that which in them arrives at full expression.

Van Beneden, Ed., Eecherches sur les Dicyemidse. Bull.

Acad. Belg. xli., xlii., 1876.

o K o' Eig. 21. Vermiform

s embryo of Dicyema

. 1 -T-v • T T • 1 typus. n Nucleus of

Passing over the Dicyemida3, 1 recognise the the endodermal cell

following divisions of the Metazoa : D^ter E^ van Bene-

1. Coelenterata.

2. Vermes.

3. Echinoderma.

4. Arthropoda.

5. Brachiopoda.

6. Mollusca.

7. Tunicata.

8. Vertebrata.

These divisions represent in a general way separate branches of
the pedigree of animals, and each of them contains higher and lower
forms in various proportion. But the degree and extent to which
their organisation is developed is different in each of them. The
divergence of organisation expressed in each division is indicated by
their relations to one another, and it shows us how the lower forms
of the higher phyla may have started from the lower phyla. These

70

COMPAEATIVE ANATOMY.

large divisions are therefore arranged in genealogical connection.
The extent to which each division is separated from its fellows
varies in each case. The relation of the various large divisions to
one another is seen in the subjoined tree.

Yertebrata

Ccelenterata

Protozoa

The more exact limitation of the separate divisions, as well as the
reasons for the genealogical relations here merely indicated, will be
found in the special chapters.

BIBLIOGEAPHY.

71

Bibliographical Aids in Comparative Anatomy.

§ 50.

The principal work which is to be recommended as a scientific guide to
the whole of Morphology, and especially to the questions which I have, in
the foregoing paragraphs, treated very concisely, and which should be care-
fully studied, is :

Hackel, E., Generelle Morphologie der Organismen. Allgemeine Grundziige der Fonnenwissen-
schaft, mechanisch begriindet dnroh die von Ch. Daewin reformirte Descendenztheorie. 2 Bde.
Berlin, 1866.

The following books also treat of Morphology philosophically:

Lexjckabt, R., Die Morphologie nnd die Verwandtschaftsverhaltnisse der wirbellosen Thiore.
Braunschweig, 1848.

Carijs, V., System der thierischen Morphologie. 1853.

Beonk, Morphologische Studien liber die Gestaltungsgesetze der NaturkOrper. Leipzig und
Heidelberg, 1858.

a. Compreliensive Works on tke whole subject of Comparative

Anatomy :

Cuvier, G., LeQons d’anatomie compar(5e recueillics et publiees par Dum^ril et Duveexot. 5 vols.
Paris, 1798-1805.

— Leeons, etc., recueillies et publiees par Dumeril. Seconde Edition. 8 vols. Paris, 1835-46.

Meckel, J. F,, System der vergleich. Anatomie. 6 Bde. Halle, 1821-33 (incomplete— sexual
organs wanting).

Milxe-Edwaeds, H., Lemons sur la physiologie et I’anatomie comparee de I’homme et des animaux.
T. I— XII. Paris, 1857-76. Unfinished.

Letdio, F., Vom Bau des thierischen Korpers. I. Band. 1 Hiilfte. Tiibingen, 1864.

b. Text-books and Manuals of Comparative Anatomy :

Carus, C. G., Lehrbuch der Zootomie. Leipzig, 1818. Second edition, under the title of Lehrbuch
der vergl. Zootomie. 2 Bde. Leipzig, 1834.

Wagner, R., Handbuch der vergleichenden Anatomie. 2 Bde. Leipzig, 1834. New edition, under
the title of Lehrbuch der Zootomie. 2 Bde. Leipzig, 1843-48. (The second volume, containing
the Anatomy of the Invertebrata, is by H. Frey and R. Leuckart.)

V. SiEBOLD and Stannius, Lehrbuch der vergleichenden Anatomie. 2 Bde. Berlin, 1845. Second
edition under the title of Lehrbuch der Zootomie. Band I. Heft 1-2, containing the Anatomy
of Fishes and Amphibia, is all that has as yet appeared. It will be continued.

Bergmanx, C., and Leuckart, R., Anatomisch-physiologische Uebersicht des Thierreiches. Stutt-
gart, 1852.

Schmidt, O., Handbuch der vergl. Anatomie. 7e Auflage. Jena, 1876.

OwEX, R., Lectm’es on the comparative anatomy and physiology of the Invertebrate Animals.
Second edition. London, 1855. — Of the Vertebrate Animals. Pt. I. Fishes. London, 1846.

Jokes, Rymee, General Outline of the Organisation of the Animal Kingdom, and Manual of
Comparative Anatomy. Foui'th edition. London, 1871.

Hartixg, P., Leerboek van de Grondbeginselen der Dierkunde in harcn gcheelen Omvang. Deel
I — III. Tiel, 1864-74 (deals also with Comparative Anatomy).

St. George Mivaet, Lessons in Elementary Anatomy. London, 1873. (Introduction to human
anatomy.)

c. Figures of the Structure of Animals :

Carus, C. G., and Otto, Erlauterungstafeln zur vergleichenden Anatomie. 8 Hefte.
1826-52.

Wagner, R., leones zootomicse, Handatlas zur vergl. Anatomie. Leipzig, 1841.

Schmidt, O., Handatlas der vergl. Anatomie. Jena, 1852.

Carus, V., leones zootomie®. Leipzig, 1857. First half (Invertebrata).

Leydig, F., Tafeln zur vergl. Anatomie. Erstes Heft. Tiibingen, 1864.

Leipzig

72

COMPAIJATIYE AXATOMY.

d. Comparative Histology :

Leydig, F,, Lelirbucli tier Histologie ties Mensclien imd tier Thiere. Frankfort, 1857.

e. Ontogeny;

Fostek,M., anti Balfoue.T.M., The Elements of Embryology. Fart I. Macmillan and Co. London,
1874.

Kollikek, a., Entwickelungsgescliichte ties Menschen u. tier hoheren Tliiere. 2^ Auflage. 1 Hulfte.
Leipzig, 1876.

In addition to these works, numerous monographs, treatises and essays,
contained in the transactions of academies, and other learned societies, or
Journals of Natural History, Zoology, and Anatomy, should be consulted.

[The English student should consult especially the “Journal of Anatomy”
(Macmillan), dating from 1867,. and the “Quarterly Journal of Microscopical
Science ” (Churchill), dating from 1853.— E. R. L.]

SPECIAL PART.

PROTOZOA.

First Section.

Protozoa.

General Review of the Group.

§ 60.

I RECKON among these certain divisions of those organisms which,
owing to the simplicity of their organisation, are examples of
the lowest grades of living forms. The most essential character
appears to be the absence of organs differentiated for the most im-
portant functions. The imperfect limitation of this division is due
to this negative character ; in it, nothing that is typicaF^ of all its
members, either in the relation of the body to its morphological
elements, or in its organisation, can be made out. The want of any
histological differentiation is a reason for considering the organisms
enumerated in this group, in company with others, which we are
wont to regard as lowly plants, as forms of life which stand between
the Animal and Vegetable Kingdoms. It is on this consideration that
the plan of uniting all the lower organisms which cannot be regarded
as Animals or Plants into the Kingdom of the Protista is based. If
we recognise this conception, it appears unnecessary to form a division
of the Protozoa.^^ But the knowledge of the stages of organisation,
which obtain in the Protista, are of such high value for the compre-
hension of animal organisms, that to pass them over completely
would not be in accordance with the aim of this book. I have,
therefore, retained in this place the division of the Protozoa, and
place in it a number of forms which, by the simple conditions of
their organisation, and by the low grade of their differentiation, are
well adapted for giving an idea of the group.

First of all I exclude from it those forms which have not a
nucleus, that is, which do not pass beyond the cytod stage. The
development of a nucleus in the otherwise simple protoplasmic body

76

CO.AIPAIJATIYE AXATU^EY.

of the orgauism is^ when compared with the cytod stage^ the ex-
pression of a considerable advance^ and prevents ns from uniting
the forms that do possess it with those that do not, whatever other
points of agreement there may be in their protoplasm ; even though
it is quite clear that in these cytod forms (Monera) we have the
beginnings of the higher stages. These beginnings vary greatly,
correspond to separate divisions of more developed forms, and
make it probable that the latter have had a polyphyletic origin.

Of the groups of the Protista I regard the Khizopoda, Gre-
garinee, and Infusoria as Protozoa.

There is no permanent limitation of the protoplasmic body in
the Rhizopoda; their protoplasm sends out varying processes.
The lowest division is that of the AmoobideD, the organism of which
is of the grade of a cell. The body is formed by protoplasm and
a nucleus ; it is ordinarily naked, but it can temporarily surround
itself with a capsule, or the capsule may be a permanent covering,
open at one or two points. The organism communicates with the
outer world by this opening, and by it can extend itself over its
shell. If there are several nuclei present the organism forms a
syncytium. The Foraminifera form the second division. It is
very probable that they all have a nucleus ; and these organisms,
therefore, are also similar to a cell. But the formation of a covering,
provided with numerous pores, and often considerably complicated,
is an indication of a tendency to higher development. The Heliozoa
form a small group more nearly allied to the next division. The
Radiolaria, finally, are distinguished from all other Rhizopoda by
the possession of a central capsule within the body. The central
capsule contains a number of nuclear structures; these, indeed,
render the Radiolaria similar to cells, but their body cannot be re-
garded as a cell. Another course of differentiation seems to have
affected them. Further, in some the entire capsular protoplasm
contains separate cells, which are regarded by many as structures
not belonging to the organism (yellow cells). The development of
various kinds of supporting organs gives a peculiar character to the
Radiolaria. W e can make out a large number of axes in the body,
by the aid of these skeletons.

The second division of Protozoa is formed by the Gregarinm.
An outer limiting portion surrounding the nucleus, that is a body
of the grade of a cell, is wanting in the earliest stages only. They
pass, therefore, through the Cytod-stage. In the mature organism
an envelope, differentiated from the protoplasm within, can be made
out ; and they give indications of higher differentiations of the
subjacent layer of protoplasm. An anterior portion is in many
separated by a constriction from the cylindrical or band-like body.

The Infusoria form the third large division. I do not, as do
many, include the Flagellata among them. The body, formed of
protoplasm, has its external form defined by the differentiation of a
cortical layer. This cortical layer surrounds the more indifferent
protoplasm, which, may in many cases be seen to be rotating, and so

PEOTOZOA.

77

calls to mind tlie streaming of tlie protoplasm in certain vegetable
cells. A nucleus,, wliicli may be various in form^ sliows that the
body of tlie Infusorian is equivalent to a cell. In some tliere are
several nuclei. A liigher degree of potentiality is expressed by the
differentiation of various liistological structures in tlie cortical layer.
But tliis does not affect the character of the Infusoria as unicellular
organisms^ so long as they have only one nucleus^ if we may suppose
that the cell is no longer in its indifferent condition. In many forms
there is a small nuclear structure^, the nucleolus, in addition to the
nucleus. The Infusoria are divided into the Suctoria (Acinita)
and the Ciliata. The former have definitely arranged fine pro-
cesses, capable of a small amount of movement, which serve in
the ingestion of food. The Ciliata are distinguished by an invest-
ment of cilia ; and subdivisions are formed according to the way in
which these cilia are distributed over the body.

Bibliography.

Rhizopoda : Auerbach, C., Zeitsclu’. f. wiss. Zool. Bel. VII.— DcjardijS- in Ann. sc. I. III. IV. —
ScHULTzE, M., Ueber elen Organismus cler Polythalamien. Leipzig,' 1854. — Carpenter, W.,
Researches on the Foraminifera. Phil. Tr. 1856, 1859. — The same, Introduction to the study of
the Foraminifera. London, 1862. (R. S.) — Huxley, Th. H., On Thalassicolla. Ann. nat. hist.

1851.— Muller, J., Abhandl. der Berliner Acad. 1858. — Hackel, E., Die Radiolarien. Eino
Monographie. Berhn, 1862.— Schulze, F.E.,Rhizopodenstudien. Arch. f. mikr. anat. Bd. X — XII.
— Hertwig, R., Arch. f. mikr. anat. Bd. X. Suppl. — The same, Zm-. Histolog. der Radiolarien.
Leipzig, 1876.

Gregarinse : Steih, Ueber die Natur der Gregarinen. Arch. f. Anat. u. Phyl. 1848. — Kolliker,
Beitr. z. Kenntniss niederer Thiere. Zeits. f. Zool. I. — Lieberkuhh, Uvolut. des Gregarines.
Acad. Roy. de Belgique. Mem. des Soc. etrangeres. T. XXVI. Ed. van Beheden, Rech. sur
I’evolut. des Gregarhaes. Bull, de I’Acad. royale de Belgique. 2me Ser. T. XXXI. Sur la Struct,
des Greg. Ibidem. T. XXXIII.

Infusoria: Ehrenberg, C. G., Die Infusiousthiere als vollkommene Organismen. Leipzig, 1838.—
Dujardih, Hist. nat. des Infusoires. Paris, 1811. — Steih, Fr., Die Infusionsthiere auf ihre
Entwickelung uiitersucht. Leipzig, 1854. — The same, Der Organismus der Infusionsthiere. I. II.
Leipzig, 1859-66. — Cl.aparede, E., et Lachmanx, Etudes sur les Lifusoires et les Rliizopodes.
Geneve, 1858-61. — Engelmahn, Th. W., Zur Naturgeschichte der Infusionsthiere. Leipzig,
Zeitschr. f. Zool. XI. — Morphol. Jahrb. Bd. I. — Hackel, Z. Morphol. d. Infusor. Jen. Zeit-
schrift VII. — Butschli, Archiv. f. mikr. Anat. IX.— Zeitschr. f. w. Zool. XXVIII. — Herxwig,
R., Ueber Podophrya gemmipora. Morph. Jahrb. I.

§ Gl.

As the body of the lowest organisms is formed of contractile
protoplasm, which changes continually in form, there is no definite
boundary to the body, nor any kind of differentiated integument.
We see the bodies of most of those Protista, which are not provided
with an envelope, alter in contour, just like the indifferent cells of
higher organisms; processes of the protoplasm are extended from
different points at different times, and the rest of the body flows
after them. Thus, as the body moves, there is always a change in
its surface, so that a particle of its substance, which at one moment
is found in the interior, may at another moment enter into the
formation of a process. The processes. Pseudopodia, have some-

78

COMPARATIVE ANATOMY.

Fig. 22. An Amoeba figured at two
different moments during move-
ment. n Nucleus, i Ingested food.
Some vacuoles may also be noted.

times the appearance of broad lobate prolongations (Fig. 22)
separated from one another by shallow depressions ; sometimes they

flow out as delicate^ sometimes as
wedge-shapedj currents, which divide
in all kinds of ways at their periphery,
and so form ramified processes. These
changes are always within definite
limits of form in the various divisions,
so that the form of the pseudopodia
is the expression of the first differen-
tiation of a definite morphological
character of the protoplasm. The
pseudopodia characterise the Rhizo-
poda, the superficial protoplasm of
which is able to emit them in every
form of this group (Fig. 23). Neigh-
bouring pseudopodia can unite with
one another at any point and in various numbers (Fig. 23, x), or
may even become connected in a retiform manner. This character

of the protoplasm is
\\ V \"\ ' ’ / . :■ / not affected by in-

. . W\ \\ : ; • ; •; / ; / // tcmal differentiation

C' (skeletal organs, etc.).

It is the expression of
a stage in the lowest
living material when
it is not differentiated
at its periphery.

When the outer-
most layer of the
body is hardened,
pseudopodia cannot
be formed at every
point. The chemico-
physical changes in
the peripheral parts
lead to the formation
of something unlike
the rest of the proto-
plasm, whichretains its
indifferent character,

and indeed still gives signs of its power of movement, but is
restrained by the firmer cortical layer from any considerable exten-
sion of itself. This stage is found in the (Iregarinm; characters
which obtain in many of the Amoebas exhibit intermediate steps
towards it. A firm homogeneous membrane sometimes delicately
striated, extends in these forms over the whole body, which is
formed by a single cell. It passes directly into the soft protoplasm,
and appears to be a cuticle differentiated from it. Like all cuticles, it

Fig. 23. A Foraminifer (Rotalia) with extended
pseudopodia, which pass through the pores of the
multiloculate shell. At oj, several pseudopodia have
united together.

PEOTOZOA.

79

lias no contractility ; but it is extensible and elastic, and is thus
able to follow tbe contractions and expansions of the protoplasm.

In addition to this separation of the

finer than the richly-granulated protoplasm ;
this is also found in the Infusoria.

§ 62.

With the separation of the body into an
external cortical layer, and an internal par-
enchymatous substance, further metamor-
phoses of the cortical layer are connected.

Of these the first that should be mentioned
are the cilia which are widely distributed
in the Infusoria. They appear to be direct
but actively motile prolongations of the in-
tegument : if combined with a cuticle they
traverse it. They either beset a limited part
of the body only, as the so-called oral open-
ing, or they are extended over larger tracts, or over the whole body
often, with great regularity.

According to the definite distribution and arrangement of these
cilia, the Infusoria have been subdivided into Holotricha, Hetero-
tricha, Hypotricha, and Peritricha. It is clear that they are differ-
entiations of the protoplasm, from what happens in some other
groups of the Protista, where they form temporary structures only,
and, like the pseudopodia, can be again withdrawn into the proto-
plasm of the rest of the body.

The flagella, as well as the undulating rnembranes, which are
often formed in the region of the mouth of many Infusoria, are
modifications of the cilia. The cilia ap^Dear sometimes to be modified
in a peculiar way to form stiff processes, movable only at their point
of attachment to the body (Stylonychia) ; sometimes, indeed, they
are flattened and broadened.

The cilia, as well as the style-shaped processes, serve as locomotor
organs, and show us that locomotion is connected with the integu-
ment, just as it was connected with the temporary external layer of
the body, where pseudopodia were formed.

Another structure observed in the cutis of many Infusoria (e.g.
Parammcium), are firm rod-like bodies (Trichocysts), which, under
certain influences, emit a fine stiff filament. These structures lie
close to one another in the cortical layer, and in a direction per-
pendicular to the long axis of the body. They call to mmd the
stinging cells of the Coelenterata, but they are not to be regarded as
the same things, for they are not formed from cells.

cuticular layer there is also, in the Gre
garinas, a cortical layer separated off from
the internal parts, which appears to bo

Fig. 24. Gregarinas from
the enteric canal of Opatrum
sabulosum ; a is the younger
stage, provided with a “ pro-
boscidiform” process, a An-
terior portion, h Posterior
portion of the body, c Nu-
cleus.

80

CO:\rPARATIVE AXATOMY.

§ 03.

In tlio cortical layer of tlie body of tire Gregarin^e^ and of many
Infusoria_, tliere are indications of bands, or fibres, resembling
muscles. In the Gregarinas these structures are arranged cir-
cularly, or spirally, and form a layer just below the cuticle; it
extends over a short part only of the head,^^ which, as a rule, is
separated from the body by a constriction, but it never passes into
the wall of partition which separates this part from the body.

Among the Infusoria these contractile bands are principally
known in the larger species of some genera (Spirostomum, Stentor,
Prorodon, etc.). In others they are absent. They sometimes run
spirally, sometimes longitudinally. They are also present in the
V orticellinaB, where they form spiral coils towards that end of the
body which passes into the stalk. It is clear that these structures
do not form the sole contractile system of the body, for those
Infusoria which do not possess them are capable of executing
powerful contractions. But that they are really contractile is shown
by Spirostomum, in which the contractions of the body are not
effected along its long axis but in the direction of the striated band,
which describes several spiral turns. The contractile band, which
runs in the interior of the stalk of the Vorticellinae, is a differ-
entiation from the protoplasm of the same kind ; in Zoothammium it
branches with the colony, in Carchesium it exists separately in
each individual of the colony. Although these structures in many
points resemble muscular fibres, and are physiologically the same,
they cannot be compared with these histological elements from
a morphological point of view, for neither cells nor the products
of cells take any share in forming them. They are differentiations
from the protoplasm of the organism, whilst in the tissues of the
Metazoa they are formed by the differentiation of a whole lot of
morphological elements. The whole contractile apparatus cor-
responds therefore to a muscular system in function only. The
separate bands or stripes are merely analogous to muscles
(myophana) .

§ 04.

The function of supporting organs of the body is performed
in the Protozoa by firm structures, which either traverse the soft
substance of the body as a framework, or invest it in the form of
shells and tests. The latter serve as organs of defence in proportion
to their size and firmness. All the structures here to be enumerated
are either directly or indirectly differentiations of the protoplasm,
which are formed either on the surface or in the parenchyma of the
body. The more completely these secretions cover the body in the
form of tests, the more do they interfere with its freedom of
movement, unless there are compensating arrangements. Shells and
internal supports are widely distributed in all divisions of the lower

PEOTOZOA.

81

organisms ; they vary greatly in complexity^ and this complexity is
sometimes in inverse proportion to that of the body. Simple shells,
generally oval in form, and
provided with an orifice,
obtain in one division of
the Amoebas (Difflugia,

Arcella). The shell is
sometimes soft ; but some-
times it is more firm, and
this firmness is increased
by the addition of foreign
bodies. They sometimes
seem to be internal shells,
owing to the extension of
the protoplasm over them.

The shells of the Foramini-
fera are more complicated
in form, owing to the for-
mation of new parts around
a simple rounded test, which
then form separate cham-
bers communicating with
one another by orifices, and
with the exterior by pores
(Figs. 23, 25). These many- chambered shells become very firm by
the addition of chalk, or, though more rarely, of silica (Polymorphina
Nonionina) ; owing to the variations in the relative position, size,
and mode of connection of the chambers, these structures vie in
wealth of form with the more lightly constructed internal supports
of the E-adiolaria.

When the chambers are ranged along a straight line, rod-shaped
shells, often swollen into beads, are formed ; the separate portions
of which, known as chambers,^'’ may be all of the same size, or may
increase in size from one end to the other (Nodosaridm). A spiral
arrangement of the chambers which lie in the same, or in different
planes, gives rise to structures like those of the shells of the Nautilus
(Fig. 23). Special modifications are due to the superposition of the
spiral coils, the elongation or abbreviation of the spiral axis, and so
on. The planorbis-like shells of the Milliolidse, in which partial
constrictions give the first sign of the formation of chambers, repre-
sent the simplest condition of these forms. By the unequal addition
of fresh chambers the spiral form is completely effaced (Acervulinae),
and can only be seen in the earliest-formed chambers. These tests
are usually confounded with external shells. But this only holds for
a few. The shell seems rather to be an internal support, in those
cases especially where the partition-walls of the so-called chambers are
frequently broken through, and the pore-canals at the same time pass
through the shell to the exterior, so that the protoplasm of the pseudo-
podia is able to cover the outside of the shell. When the partitions

Fig. 25. Transverse section of a Foraminifer
( Alveolina Quoii) ; the arrangement of the separate
chambers in relation to one another can be seen
(after W. Carpenter) .

82

COMPAEATIYE A^-ATOMY.

are merely represented by several separate columns,, or lamellse,
which have wide openings between them (Fig. 25), and the cavity of
the chamber itself is less than the various connections between two
chambers, and where, finally, all the neighbouring chamber-spaces
communicate with one another, and the whole test is thereby
traversed by a cavitary system which communicates in all directions,
then the character of an external shell is completely lost. And so,
since the protoplasm is in all cases able to draw itself over the outer
surface of the shell, the shells in the Foraminifera ought to be
regarded as internal ones, and grouped accordingly with the skeletons
of the Radiolaria.

§ 65.

The “central capsule” must be noted as an organ of support
common to all the Eadiolaria, although it is not very ap|)arent.
It is a capsular-closed organ, various in form, placed in the middle
of the body, and formed of a membrane which closely resembles
chitin. In addition to fat-globnles and structures which are re-
garded as nuclei, it always contains a quantity of protoplasm, which
is apparently continuous with the extra- capsular protoplasm by means
of fine pore^canals. In most Radiolaria (not in Thalassicolla,
Thalassolampe, and Oollozoum) there is, in addition to this capsule, a

skeleton, ordinarily formed
of silica, which traverses the
capsule when most fully de-
veloped (Fig. 26). Several
spicules then radiate from a
common centre, and these
may be connected with one
another by means of a con-
centric highly fenestrated
framework (Fig. 26). In
some (Acanthometridse) the
organic base of the frame-
work predominates, and is
but slowly replaced by silica.

Separate spicular bits
of silica, lying freely in
the protoplasm outside the
central capsule, are the
earliest indications of this*
firm skeleton in the Collid^e
and Polyzoa. In some they
become arranged in a radiate
manner,without being firmly
connected together. Circular skeletons, forming an open network,
are formed by the radial spicula being connected together at equal
distances by rods placed perpendicularly to them. When a very
fine supporting reticulum is arranged around the radial spicula in

Fig. 26. Skeleton of a Radiolarian (Actinomma
asteracanthion). Two concentrically-arranged
fenestrated shells are broken through at one
point, to show a third one (after E. Hiickel).

PEOTOZOA.

83

a more irregular manner, tlie skeleton is sjoonge-like. The infinite
variety of form is increased by discoidal and basket-like skeletons ;
or these skeletons may have a spiral arrangement. In this way a
supporting organ of great complexity is formed, in which the soft
parts of the body are embedded, and the separate pieces of which
are developed in the protoplasm.

§

Compared with these internal organs of support in the Ehizopoda
the tests of the Infusoria form a distinct series of arrangements,
for they are only secretions from the surface of the body : they
resemble the tests of the Arcellac, mentioned above. The secreting
matrix is in this case an anatomically distinct part of the body. But
this need not be regarded as a higher stage, for it is closely allied to
the lowest, i.e. to the formation of a cell-membrane. Tests are
principally developed in the fixed Infusoria. They are formed by
the secretion of a substance, which is primitively soft, and which
gradually hardens ; this surrounds the body of the animal like a cup
or an urn, except at one point, where it allows of communication
with the exterior. These tests are distinguished from the merely
cuticular structure, which tends to form a carapace, in that the
differentiated layers attain greater firmness, and become separated
from the greater part of their matrix surface. But the genesis of
both structures is the same. It is identical with the formation
of cysts, a process which is very common among the Infusoria, and
by means of which the organism shuts itself off from the exterior
for a time, so as to withstand unfavourable conditions (loss of water,
and so on). The immovable stalk of Epistylis, and the external
layer of the contractile stalk of the Yorticellinse and Carchesinm
must be regarded as cuticular differentiations of this kind. The
tests maybe soft or hard, and membranous. Some are distinguished
by the agglutination of foreign bodies — cemented grains of sand,
and so on. The genera Vaginicola, Tintinnus, etc., have shells.
Stentor has one in certain cases. Fenestrated shells have also
been observed (Dictyocyrta) . A carapace is formed from the firm
hyaline cuticle in Stylonychia, Euplotes, Aspidisca, Spirochona,
Coleps, etc.

' §67.

Organs for the prehension and alteration of food are
wanting in the lowest organisms. In the Grregarinae food is taken
in by endosmotic processes at the surface, and solid nutritive
matters do not reach the interior of the body. Where the body
is not peripherally differentiated there is, on the other hand, a
direct taking in of food, which may go on at any part of the
body. This is the case in the Ehizopoda. In this case the nutritive
matter is surrounded by the soft substance of the body, or it is
embraced by the processes of the body — the pseudopodia. In both

G 2

CO:\IPAKATIVE ANATOMY.

«4

cases tliere is really one and tlie same plimnomenon. Any place in
the protoplasm can act as a digestive cavity by enveloping
and absorbing nutritive matter, and at any neighbouring part
of the surface the undigested substances can be again expelled.
Even in Actinosphmrium solid food can be taken into the body ; but
in it the pseudopodia are not the direct agents, for they draw the
prey to the body, and cause it to pass into^ the yielding parenchyma

of the cortical layer at some suitable
point (Fig. 27) ; thence it passes to
the central substance of the body.
In comparison with other forms Acti-
nosphserium is characterised by taking
its food directly into the more differ-
entiated parts of the body, and not
surrounding it by the amorphous
protoplasm of the pseudopodia.

In the Infusoria the arrangement
is more definite. They take in food
in two different ways. In the Suc-
toria (Acinetinm) there is no mouth ;
the radiate pseudopodia-like processes
pass through the envelope of the
body, and act as suckers (Fig. 30).
They attach themselves by their
sucker-like enlargements to the prey
which has come within their reach
(which consists of other Infusoria, etc.), and cause it to flow, as
through a tube, into their body, where it fills the parenchyma in
the form of drops. The presence of similar processes in the
embryos of other Infusoria shows that this mode of nutrition is
a very common one. A higher grade is represented in the other
forms ; in the Ciliata there are not only definitely organised parts
for the reception of food, but also definite parts for the ejection
of what is useless. An enteric tube is, however, wanting in all
of them, and these differentiations are limited to the cortical layer
of the body, so that the food passes beneath it into soft paren-
chyma, i.e. into the undifferentiated protoplasmic part of the body,
in which there are no passages with special walls. Temporary
spaces, which act as digestive cavities for the spherical food masses,
are formed in it ; these cavities are not permanent ones, as may be
seen from their frequent disappearance when the protoplasm is in
movement. In this point, therefore, they resemble the Ehizopoda ;
for a part of the digestive apparatus, that is those parts in which
the food is digested, have no organological differentiation.

AVhen the Ciliata have a month, it is either in the form of a
simple cleft, which is in many cases only apparent when food is
taken in ; or it is not directly on the surface of the body, but at the
bottom of a depression (vestibule), the form of which is very varied,
and which at times contains the orifice of egestion also ; the sur-

Fig. 27. Actiiiosphterium. a A
morsel wliicli has been taken in as
food, and just pushed into the soft
cortical layer 6, by the animal,
c Central parenchyma of the bod}^
d Some balls of food in it. e Psen-
dopodia of the cortical layer.

PliOTOZOA.

85

rounding region (peristoma) has often also a special form. A
tubular portion, or pharynx (Fig. 28, h) often passes from the mouth
into the parenchyma of the body, and from
it the ingested morsel finds its way into the
soft substance of the latter.

The position and form of the mouth varies
greatly. In many cases it can only be made
out during the ingestion of food (as in Amphi-
leptus, Loxophyllum), and disappears as soon
as the morsel has passed into the parenchyma.

There is sometimes an investment of cilia on
the tubular pharynx (Paramaecium aurelia
and bursaria) ; or an undulating membrane
(Bursaria fiava) ; or a covering of rod-shaped
denticles, or fine longitudinal ridges.

Porodon, Chilodon, Nassula, etc., have an
investment of small rods in the pharynx,
arranged in eel-pot form. A regular thicken-
ing of its walls has been observed in Ervilia
and Liosiphon.

The general presence of an anal opening
is not by any means established. There is
only in some few cases a permanent and dis-
tinctly-marked opening; it can generally be distinguished during
the expulsion of undigested food only. This anal spot is as a
rule at the posterior end of the body, but is, on the whole, very
changeable. It may even appear at the anterior end of the body ;
thus, in Stentor it lies near the mouth, and in the Vorticellinm
and Ophrydise in the vestibule. Taken on the whole it appears
to consist more in the localisation of a function than in the develop-
ment of an organ. The products of excretion pass through the
differentiated cortical layer of the body at a certain spot, without
there being any special organisation of that spot.

§ 08.

Ill all Protozoa the outermost layer of the body has a respira-
tory significance, for it is by it alone that an exchange of gases
with the surrounding medium can be effected. This relation must
also be borne in mind in reference to the increase of surface, which
is due to the pseudopodia. The cilia of the Infusoria are of import-
ance in changing the water.

More definite respiratory arrangements are seen when, as in
many Protozoa, water is taken into the body. Cavities, which are
filled with a fluid, and which gradually contract and completely
empty themselves, after having reached their maximum of distension,
appear within the protoplasm ; when empty they seem to disappear.
These vacuoles, like the vacuoles in the cells of certain tissues, are
partly variable structures, now appearing and now disappearing, and

Fig. 28. Diagram of
the digestive cavity of
Paramsecium. a Body-
space filled with soft pro-
toplasm, into which the
food is taken, h Month,
c Anns. d Contractile
vesicles (after Lach-
manu) .

86

COMPAEATIYE AXATOMY.

partly constant. AYlien they are constant their function is in-
creased, and they often expand and contract regularly and rhyth-
mically, like the cardiac systole and diastole. Contractile vesicles
of this kind are often seen in the AmoebiB (Difflugia and Arcella),
and are very common among the Infusoria. They are also known
as vacuoles. The fluid which collects in the vesicles is drawn from
the parenchyma of the body and is returned to it, or passed out to
the exterior on the contraction of the vesicle. Fine communications
with the exterior have been made out, so that the latter course is the
probable one ; but we need on this account conclude that water does
not enter by the same passage.

In the Inf iisoria the vesicles lie in the cortical layer (Fig. 28, d(/),
generally just under the delicate cuticle, and at definite points. If
only one vesicle is present, it lies either anteriorly or posteriorly :
if two, there is one near each end of the body. Trachelius ovum is
remarkable for a large number of small vesicles. No special mem-
branes can be made out on the wall of the vesicle nor in the canals
which pass off from it. Like the vesicle the canals can only be
made out while they are filling. The vesicle and canals contract
alternately. In Paramaecium the canals enlarge at the commence-
ment of the systole, and approach one another as the vesicle diminishes
in size, so that they form a stellate figure at the moment when its
systole is most complete and the vesicle has disappeared. While the
vesicle is filling the canals look like small diverticula on it, and are
not again fully distended until the diastole is complete. The number
of canals, which is limited in P. aurelia to eight or ten, is increased
to thirty in Bursaria flava, and is much higher in Cyrtostomum
leucas. In these forms the canals have a wave-like course, and ramify
at their extremities. Canalicular tracts are formed by the fusion of
several spaces filled with water into longer tracts, as in Stylonychia
(St. mytilus), and they empty themselves into the contractile vesicle,
by definite passages. The long canals of Spirostomum ambiguum,
which also are visible for a time only, but which are longer
than these, are like them, so that we can make out a continuous
series from the first appearance of an apparently indifferent cavity to
a definitely arranged system of tubes.

Another arrangement can be put beside this formation of in-
different vacuoles. When such spaces in the protoplasm increase
in number they run together, and so give to the protoplasm the
appearance of a network which traverses the interior of the body,
which is filled with fluid (Trachelius ovum). These hollow spaces
have then become perfectly different organs from the pulsating
vacuoles, and they may both exist at the same time.

§

In correspondence with their low grade of organisation the
Protozoa have no sexual organs, and give but the faintest indica-

PEOTOZOA.

87

tions of sexual differentiation. They always propagate, therefore,
by modes wbicli are called asexual, among whicli tlie chief part is
played by fission and generation. The nucleus appears to be of
great importance in all their modes of multiplication.

Spores have been observed to be formed within the organism in
Ehizopoda. A larger or smaller part of the protoplasm of the body
is used in forming them ; when a larger portion is used this mode of
multiplication is allied to that mode which is so common among the
Protista, in which the whole body breaks up into a number of
spores, and so multiplies by division. In the Eadiolaria the contents
of the central capsule are active in reproduction. The nuclei in
it become surrounded with protoplasm, and form flagellate swarm -
spores.

The mode of reproduction is most exactly known in the Gre-
garinas. As a rule multiplication commences by the concrescence of
two individuals ; this generally occurs very early, so that the two
individuals, which form one body, the anterior end of one being
attached to the posterior end of the other (Fig. 29),
go on growing for some time ; or conjugation
may only take place later, when the forms are
mature. After this comes a condition of rest,
accompanied by ency station, in which the two
individuals form a rounded body, with a partition
between them. Then the partition disappears,
and the substance of the body, and also the
nucleus, breaks up into an amorphous mass, from
which numerous vesicles gradually arise. From
these latter a number of germ corpuscles, called
Pseudonavicellae,^^ on account of their shape,
are formed. These gradually fill the whole cyst,
and each gives rise to a single very small organism,
consisting of protoplasm solely, and this, being
without a nucleus, corresponds to a cytod. Each
of these structures moves about in an amcBboid
manner, and is gradually differentiated into a
young Gregarina, after which a nucleus is differentiated in its interior,
and it becomes limited externally by a cortical layer.

Although conjugation has no exclusive signification in bringing
about these processes, as separate Gregarinse are also able to pass
through these reproductive processes in just the same way, yet it is
not the less important. It points, at least in the cases where it
exists, to the necessity of two individuals to bring about reproduc-
tion. It is, consequently, a phsenomenon preliminary to sexual
differentiation.

Fig. 29. a h T^yo con-
jugated individuals of
Gregarina sacnuridis.
c Their nuclei.

§ 70.

Conjugation also plays a part in the reproductive proceedings of
Infusoria, for it is the first step in their multiplication. In this the

88

CO^rPAEATIVE AXATOjMY.

niicleu

s is of considerable importance;^ it (Fig. 30, u) is

a firm

structure, sometimes provided with a spiral envelope, very various

in form. It lies in the cortical
substance of the body, or^ is
surrounded by a continuation
of this substance, if it is more
deeply sunk in the interior. It
is sometimes oval or round, or
it is flattened and curved (Vor-
ticellinm), or it is even greatly
elongated and regularly con-
stricted (Spirostomum). The
importance of the nucleolus,
which differs from the nucleus
in nothing but its smaller size,
is more obscure. The act of
reproduction commences as a
rule with the complete or partial
fusion of two individuals, which
may be of the same or of dif-
ferent sizes ; this fact led to
the mistaking of conjugation
for stages of fission or gemma-
tion. This concrescence gives
the stimulus to changes in the
appropriate parts. The nucleus
becomes divided into a certain
number of parts, around which the protoplasm is disposed. In
this way a number of spores are formed, each of which becomes a
new individual while within the mother-cell; and then gets an
investment of cilia, and escapes to the exterior.

It is still a question as to the share which the nucleolus takes in
this process ; and the statement that in one group of the Ciliata it
has the function of a sperm-forming organ, while the nucleus has
the function of an ovary, requires to be confirmed. In any case this
differentiation of a male organ is not a common phenomenon, but is
limited to a very narrow circle. The nucleus, therefore, and the
nucleus alone, is certainly known to take an active share in repro-
duction, and this share is of just the same character as that which it
was seen above to have in spore formation, and as it has in gemma-
tion, in which the nucleus of the bud has often been observed to
arise from the previous gemmation of the nucleus of the mother-cell
(Podophrya). Finally, multiplication by fission is very common,
although conjugation was often confounded with this process at one
time.

Fig. 30. — An A cine t a with part of its
stalk, p Pseudopoclia-like, hnt stiff, ten-
tacles. V Vacuoles, n Kucleus. e Aciliated
young form lying in the so-called broad
cavity.

Second Section.

Ccelenterata (Zoophyta).

General Review.

§ 71.

This division is the fii\st of the Metazoa, or organisms which are
undoubtedly animals. The embryonic body separates into two
cell-layers — ectoderm and endoderm ; which in many Sponges alone
form the permanent body, though in many a mesoderm is developed.
In the lower Acalephae the formation of a mesoderm is incomplete ;
that is, the mesoderm is not an independent tissue as it is in all the
higher Acalephae. The most essential character of the animals which
make up this division is the arrangement of the nutritive apparatus, a
cavity sunk into the parenchyma of the body, and either divided into
canals or extended into wider spaces. This digestive cavity, with its
appended spaces, is invested by the endoderm, and in the lower
forms is the sole representative of hollow organs in the body.
When several individuals are united to form a colony, the canal
system, which arises from the digestive cavity, is common to all of
them, and is continued into the common substance of the colony or
coenenchyma. The primary axis alone can be made out in the body,
and the secondary axes are indilferent, or if present appear to be
equivalent.

I. Spongiae.

Gastrgeades.*

Haliphysema, Gastrophysema.

Porifera.

Myxospongiae.

Halisarca.

^ The Gastraeades represent stages which are not permanent in the rest of the
Spongiae.

90

COIMPAEATIYE AXATOISIY.

FibrospongiaD,

Ceraspongise.

Euspongia, Spongelia, Poterium.
Halichondrise.

Axinella, Spongilla.

Corticata.

Thetya,

Hyalospongiae.

Euplectella.

Calcispongiao.

Ascon, Leucon, Sycon.

II. Acalephse.

1. Hyclromedusee.

Ilyclrif ormes.

Hydra Cordylophora Hydractinia ;

— Coryne, Syncorj^ie, Eudendrhim ; —
Tubuiaria, Corymorpha ; — Campanit-
laria, Sertalaria, Plumularia.

Siphonophora.

Velella, Porpita Diphyes, Abyla j— Athorybia,

2. Calycozoa.

Lucernaria.

Medusif ormos.

Sarsia, Bougainvillea, Lizzia, Oceania ;
— Eacope, Thaumantias Tracliynema ;
— ^gina, Cunina Liriope, Geryonia,

^quorea.

Agalma, Physophora, Physalia.

8. ThecomedusoD.

Stephanoscy phus .

4. Medusce (Discopliora).

Charybdea, Pelagia, Aurelia, Rliizostoraa, Cassiopeia.

5. Antbozoa.

Tetractinia.

Cereantlius, Cyatliophyllum.

Hexactinia.

Antipatlies, Fungia, Madrepora, Astraea, Oculina, Caryophyllia.
Octactinia (Alcyonaria).

Alcyonium, Pennatula, Virgularia, Yeretillum, Renilla, Goi’gonia, Isis,
Corallium, Tubipora.

0. Ctenopbora.

Beroe, Cydippe, Cestum, Eurharapluea, Mnemia, Eucharis,

Bibliography.

Spongiee : Grant, R. E., Observ. on the struct, and funct, of Sponges. Edinb. New Pliil,
Journal, 1826-32. — Lieberkuhn, Beitr. z. EutAv. der Spongillen. Arch. f. Anat. u. Physiol.
1856. Zur Anat. d. Spongillen, ibid.— The same, Z. Anat. d. Spongien, ibid. 1857, 1859, 1863.
— ScHULTZE, M., Die Hyalonemen. Bonn, 1860. — Schmidt, O., Die Spongien des adriat. Meeres.
Leipzig, 1662. Supidement, 1864. Zweites Supplement, 1867. Drittes Supplement, 1868.— Claus,
Ueber Euplectella Aspergillum. Marb. u. Leipzig, 1868. — Haeting, P., Sur le genre Poterium,
Natum-kund. Verhandelingen. P. II. St. 2. Utrecht, 1870. — Hackel, Die KaUcschwamme, eine
Monographie. 3 Bde. Berlin, 1872.— The same : Die Physemarien. Jen. Zeitschr. Bd. X.—
Schulze, F. E., Ueber Bau u. EnDv. v. Sycandra raphanes. Zeitschr. f. w. Zool. Bd. XV. — The
same, Untersuch. liber d. Bau u. d. Entw. der Spongien (Halisarca), ibid. Bd. XXVIII.

Acalephee : Catolini, Memorie per scrvdre alia storia dei polipi marini. Napoli, 1755. (German
by Sprengel. Numberg, 1813.) — Eschscholtz, System der Acalephen. Berlin, 1829. — Lesson,
Zoophytes acal^phes. Paris, 1813. (Suite iX Buffon).— Saes, Fauna littoralis Norvegia? 1. 1840.

C(ELENTEEATA.

01

' — Frey u. Leuckart, Beitrligc zur nalieren Kcnntniss wirbelloser Thiere. Braunscliweig, 1817.

— Hujcley, On the anatomy and affinities of the family of the Medusae. Phil. Tr. 18^19. —
Agassiz, L., Contributions to the nat. hist, of the Acalephae of N. Am. (Mem. of the Arner.
Acad, of Arts and Sc. Cambridge, 1850). — The same, Contrib. to the nat. hist, of the United
States of North America. Vol. III. IV. 1860-02.

Hydromedusae: Van Beneden, P., Mem. sur les Campanulaires de la cote d’Ostende. (Nouv.
M^m. de I’Acad. royale de Bruxelles. T. XVII.) Rccherches sur Pembryogenie des Tubulaires
(ibid.). — Kollikee, Die Schwimmpolypen von Messina. Leipzig, 1853.— Leuckaet, R., Zur
naheren ICenntniss der Siphonophoren von Nizza. Arch. f. Nat. 1851. — Gegenbaue, Beitr. zur
naheren Kenntniss der Siphonophoren. Zeitschr. f. wiss. Zoologie. Bd. V. — Vogt, C., Sur les.
Siphonophores de la iner de Nice. Mdm. de I’inst. Genevois, 1851. — Claus, Ueber Physophora
hydrostatica. Zeitschr. fixr w. Zoolog. Bd. X. NeueBeobachtungen. Ibid. Bd. XIII. — Hackel,
E., Zur Bntwickelungsgesch. der Siphonophoren. Natuurkund. Verhandelingen. P. I. St. G,
Utrecht, 1869. — Huxley, Oceanic Hydi’ozoa. London, 1859. (R.S.) — Foebes, Ed., A monograph of
the British naked-eyed Medusae. London, 1818. (R.S.) — Hackel, Die Familie der RUsselquallen .
Jenaische Zeitschrift. Bd. I. II. (Also under the title : Beitr. zur Naturgesch, d. Hydromedusen.
I. 1865). — Schulze, F. E., Ueber den Bau und die Entwickelung der Cordylophora lacustris.
Leipzig, 1871.— The same, Ueber d. Bau v. Syncoryne, etc. Leipzig, 1873. — Kleinenbeeg, N.,
Hydra. Leipzig, 1872. — Allman, G. J., Monograph of the Gymnoblastic or tubularian Hydroids.
P. I. and II. London, 1871-72. (R.S.)— The same, On the structure and develop, of Myriothela.

Phil. Trans. Vol. 165.

Calycozoa: Clark, H., Prodromus of the History, etc., of the order Lucernaria. Jounial of Bost.
Soc. of Nat. Hist. 1863.

Thecomedusae: Allman, G. J., On the Structui-e and systemat. position of Stephanoscyphus
mirabilis. Trans. Linn. Soc. Sec. ser. Vol. I. Zool.

Discophora: Ehrenberg, Ueber Acalephcn dcs rothen Meeres und d. Organismus der Medusen
der Ostsec. Abhandl. der Berl. Acad. 1835. — Milne-Edwaeds, Ann. sc. nat. HI. xvi. — Wagner,
R., Ueber den Bau der Pelagia noctiluca und iiber die Organisation der Medusen. Leipzig, 1841,
—Hackel, E., Ueber die Crambessiden. Zeitschr. f. wiss. Zoolog. Bd. XIX. — Brandt, A., Ueber
Rhizostoma Cuvieri. Mdm. Acad. Imp. des Sc. de St. Petersb. VII. Ser. T. XVI.— Grenachee,
H., and Nolc, F. 0., Beitr. z. Anatomic u. Systematik der Rhizostomeen. Abh. d. Senckenb.
Gesellsch. Bd. X.

Anthozoa: Ehrenberg, Die Corallenthiei’e des rothen Meeres. (Abh. d. Bcrl. Acad., 1832). —
Hollaed, Monographie anatomique du genre actinia. Ann. sc. nat. III. xv. — Haime, J., Mem.
sur le genre Cereanthus. Ann. sc. nat. IV. i. — Lacaze-Duxhiees, Hist. nat. du corail. Paris,
1864. — The same, Memoires sur les Antipathaircs. Ann. sc. nat. V. ii. iv. Ddveloppement des
Coralliaires. Archives de Zoolog. exp. Vol. I. II. — Kollikee, Die Pennatuliden. Abh. d.
Senckenb. Gesellsch. Bd. VII. — Eisen, G., Bidr. tid kiinnedomen om Renilla. Kongl. Svensk.
Vet. Handl. Bd. XIII. — v. Kocn, G., Anat. d. OrgelcoraUe. Jena, 1874. — Moseley, H. N.,
On the structure and relations of the Alcyonarian Heliopora cserulea, etc. Phil. Transac.
Vol. 166, Pt. I.

Ctenophora: Will, Horse tergestinse. Leipzig, 1844. — Milne-Edwards, Ann. sc. nat. Sdr. IV.
Vol. VII. — Kowalevskat, A., Entw. des Rippenquallen. Mdm. de I’Acad. Imp. de St. Pdtersb.
VII. Sdr. Tom. X. — Fol., H., Beitr. z. anatom. Entvvickl. einiger Rippenquallen. Berlin, 1869,
— Eimee, Th., Zoologische Studien auf Capri I. Ueber Beroe ovatus, 1873.

Form of the Body,

§ 72.

The forms of body of the two great divisions which make up
the Coelenterata are alike in the lowest stage ; in that stage, namely,
which has been already (§28) called the Gastrula,^^ on account
of the development of an enteric cavity. This form represents a
larval stage, in which an investment of cilia function as a locomotor
organ, and which may fairly be regarded as the common funda-
mental form of the two chief divisions of the Zoophyta. In this
form only one axis, the primary, can be made out ; it extends
from the oral to the aboral pole. The secondary axes are indif-
ferent, for all the transverse axes drawn perpendicularly through the
primary, at any angle whatsoever, are completely equivalent one
with the other. This stage is permanent in all the Spongiae ; in
the Acalephm it passes on into a condition which is characterised by
differentiation of transverse axes.

Among the Spongiae the Gastrula attains, when attached by its

92

co:\rPAiLmyE axatomy.

aboral pole, to a definite cliaracter of tlie most simple form in the
Pliysemari^e, and in Olyntlius among the Ascones. In other Calci-
spongia), also, this simple form of body is retained, but more
considerable changes in its internal characters obtain.

The most important changes in the form of the body are due to
the formation of colonies. Colonies of the most varied form (cormi) are
formed by budding or by incomplete division, the separate animals
(persons) of which are connected wdth one another in very various
ways, and may even partly or completely fuse with one another in
different w^ays. When these stocks are fused they often have the
appearance of single animals, and in proportion to the simplification
of their external form is the complication of their internal organi-
sation. The modification of the mouths of the colony affects
their external form just as much as does this concrescence ; the
mouths may be collected into groups, or united as one, or they
may completely disappear.

The great variety of form in this division, which is due to the
causes here only indicated, may be still further modified by numerous
adaptations, due to their position in space. Nowhere in the Animal
Kingdom does the form of the body appear to be so changeable as
in the Spongiae, so that it is impossible to separate them into largo
divisions, to say nothing of species.

§ 73.

In almost all the divisions of the Acalephm the body developed
from the Gastrioa-form is adapted to a sessile or fixed condition ;
the stomachal cavity which is formed when this development com-
mences causes the organism to have, in all essential points, the same
simple character as it has in the corresponding stage in the life-
history of the Spongise. Processes, knowm as tentacles, are developed
on the anterior region of the body- wall, which encloses the stomachal
cavity ; they present to us the earliest indication of a differentia-
tion of the secondary axes, and therefore establish a well-marked
distinction between these forms and the Spongite.

The Ilydroida or Hydroid-Polyps (Ilydriformes) are the lowest
of the Hydromedusae.

Tentacles are placed in many cases irregularly on the parts of
the body which surround the mouth (Coryne, Syncoryne, Cordy-
lophora), or their number may be indefinite, even when the structures
are limited to definite zones of the body, and encircle the mouth in
the anterior region (Hydractinia, Eudendrium, Campanularia) . As
the number of tentacles varies we cannot suppose that the secondary
axes are definitely differentiated. It is only in a few cases that they
are definitely expressed by the position of the tentacles (Stauridium).

fihe free portion of the body, with its tentacles, becomes more
independent of the rest of the body, which forms a stalk, while the
aboral pole grows out into a stalk-like part, which carries the head,
and is distinguished as the capitellum or 'niydranth.'"

FOEM OF BODY OF CaXENTEEATA.

93

Colonies (cormi) are formed from single animals by gemmation.
This may either occur at any point of the surface of the body (Hydra),
and end by the bud breaking off, or it may take place in the stalk-
like part. The creeping cormi of the Syncorynidgo, Hydractinim, etc.,
are formed by processes of the basal part, which give off now animals,
attached here and there to it. When gemmation occurs in the free
part of the stalk we get free, branched colonies, which become compli-
cated in very various ways (Eudendrium, Campanularia), or become
regularly branched (Sertularia, Plumularia).

The formation of colonies is almost always accompanied by the
formation of a tubular investment, which is a secretion from the
surface of the body, and which serves as a support for the whole
trunk, as well as for its branches ; it is also continued, in various
degrees, on to the persons of the colony.

§ 74.

The process of gemmation in the Hydroid Polypes produces, in
addition to the growth of the colony by freshly-formed similar
individuals (persons), structures of quite a different kind, the most
differentiated forms of which are developed into Medusae.

The body of these buds is of a bell-shaped or discoid form
(Fig. 32, m), and by its internal organisation, as well as by the
tentacles, which arise from the edge of the bell, or disc, we are able
to make out secondary axes, generally two in number, which cross
the primary axis at right angles to one another, and are completely
equivalent one with the other. A higher grade than that of the
Hydroid-Polyps is expressed in this
organisation. The animals move by
contractions of the bell, the edge of
which is produced into a membrane,
the velum, which is also contractile.

These Medusa-gemmm always carry
the organs of reproduction ; from their
ova Hydroid Polypes again arise.

(Alternation of Generation.)

While gemmation of Medusae in-
tended for a free life distinguishes
some Hydroid-Polyps (Fig. 31, a-e,

Fig. 32, a-e), in others the process
stops at the formation of a Medusa-
bud, the organisation of whicb does
not quite attain that stage of develop-
ment which conditions the free mode
of life, and it remains therefore con-
nected with the colony. Nevertheless,
the normal development of the sexual organs proceeds, and in fact
these rudimentary Medusm form generative buds (Gonophores),

Fig. 31. Syncoryne with a mim-
ber of budding Medusa3 on it at
different stages (cu-e) of develop-
ment (after Desor).

94

CO^IPAEATIVE ANATOMY.

the products of ’o'hicli are developed in them hi the same manner
as are those of the free Medusae.

AYith these are connected still simpler forms of buds, and the
series ends with buds the structure of which has scarcely anything
in common with a Medusa. But the series which leads to these
is perfect^ owing to numerous intermediate forms, so that external
buds, merely containing generative products, and Medusae of a rela-
tively high organisation, which only become sexually mature sonie

time after leaving the Hy droid
stock, must be regarded as the
widely-separateterminalpoints
of one series.

This jDhaenomenon is ex-
plained by the conception of a
division of labour, in which
the function of feeding the
stock falls to the share of the
individuals which remain ses-
sile, while others which are
broken off take on the duty
of sexual reproduction. Those
buds which wall become free
have a higher organisation,
which has been gradually de-
veloped from the lower forms,
and primitively resembled
those that remain sessile. The
separation from the stock may
therefore be regarded as the
primary cause of the differen-
tiation of the sexual individuals
in the medusoid direction,
while the permanence of the
sessile habit of the medusoid
buds, in other cases, is accom-
panied by a degeneration of
their medusoid organisation.
But if this organisation, as we supposed above, has been obtained
by a primitive freedom of life, the medusoid buds must necessarily
be regarded not as arrested in an onward development, but rather
as Medusa-buds in course of degeneration. A definite conclusion
on the subject is not possible, on account of the fact that the several
stages of degeneration might be precisely similar to those of deve-
lopment, and retrogressive metamorphoses have not been directly
observed.

The gemmation of generative individuals, for such must the
medusiform buds and theff modifications be considered to be^ takes
place at different points. As the formation of a stock is a secondary
process, the production of buds on the body of the single animalmnstbe

Fig. 32. A portion of a colony of a llydroid-
Polyp (Eudcndrium ramosum) -svith bud-
diug Medusa), p p p Nutritive persons.
ah c cl e f Different stages in the differen-
tiation of tlie budding Medusa?, m m Free
Medusa? in different positions.

rOEM OF BODY OF CCELENTEEATA.

95

the primitive mode. It is found on this region in all divisions of the
Hydroid-Polyps. The stocks of the Corynidee have buds distributed
over the hydranth. They are frequently placed between the
tentacles. In Pennaria they are found within the circlet of tentacles ;
and on the same place in the Tubularige, where they are always placed
in some quantity on a common stalky forming groups like grapes^ or
ears of corn. Oemmation on the body of the Hydroida is, in many
cases, accompanied by a degeneration. In many Campanulariae,
Hydractinise, and others, the proliferating individual gives up its
share in the duty of feeding the stock, as is expressed by the
diminished size of its tentacles and stomachal cavity. The animal
stock is therefore composed of nutritive and proliferating persons,
the latter again bearing the buds or generative persons. This is
the way in which the Dimorphism of Persons arises in these
stocks, and this passes into Polymorphism, in consequence of a
number of the nutritive persons undergoing still further changes
(HydractiniaB) .

The proliferating persons of a colony present various degrees of
degeneration. In the most extreme case a portion only of the
individual which bore the buds remains after they are developed
(e.g. in many Camp anul arise) . The complete degeneration of the
proliferating person causes the gemmse to arise from any part of the
common stock, without any relation to a Hydroid person.

In the higher divisions of the Medusiformes, relations to the
Hydroida are lost. Although there are many considerable complica-
tions in the mode of reproduction (see below. Generative Organs),
yet, so far as is yet known, there is among the Trachynemidse,
Hjginidae, and Geryonidse no return to the hydroid form; and it
is doubtful whether any such relation does obtain.

§ 75.

The division of labour, which in Hydroid-Polyps is essentially
limited to the nutritive and generative functions of the persons
united into a colony, is extended over a larger series in the Sipho-
nophora ; and we met therefore with great variety in the form of
the component parts of the colony. Division of labour thus leads
to Polymorphism of the persons. They all follow the medusiform
type, which is developed to a greater or less extent. When it
is distinctly developed, the fundamental form which shows itself
in the medusa-buds of the Hydroid-Polyps predominates, so that
it is clear that both groups have had a common origin. The
Siphonophora, therefore, appear to be swimming Hydroid colonies,
all the persons of which have passed into the medusa form ; a change
which is complete in the generative persons only of the Hydroid-
Polyps. The separate persons of the colony of the Siphonophora
are developed on a common contractile stem, which in most forms
represents the axis of the stock, and around which the persons
which function as organs of the whole colony appear to be arranged.

96

COMPARATIVE AXATOMY.

These are —

1) Locomotive Persons (Nectocaljces) : these conform most
completely to the Medusa-type^ and are united together by twos

Fi«<. 33. Some colonics of Siphonoplioi’a. ui D ix^hy es campaniil ata. 5 A group
of appcncliigcs of the stem of the same Diphyes. C Physophora hydrostatica.
A separate nectocalyx of it. E Cluster of female generative buds of Agalma Sarsii.
1) a Triiuk or axis of the colony, d Air-bladder. m Nectocalyx. c Cavity in
nectocalyx, invested by a contractile membrane, v Canals in the walls of this cavity,
o Opening of the nectocalyx. t Bracts (in c represented by tentacles), n Stomach,
t Fisliing-lincs. g Generative organs.

(Dipliyidto), or, by a larger number, to form a swimming column
(Physophoridm), which occupies the end of the stem (Fig. 33,
A G, m 71), and which, therefore, takes the lead when the colony is
moving, and is directed anteriorly.

2) Nutritive Persons are present in the second division of

FOKM OF BODY OF CCELENTEEATA.

97

the stem, where they have the form of stomachal tubes (stomachs,
sucking tubes) {B C n.) In some cases some of them are not fully
developed, but form tubes closed at the end, which function as
tentacles/^

3) Protective Persons (Hydrophyllia) : in these one can often
make out the Medusa-type quite clearly, but in other cases it is
much less distinct, and they then have the appearance of hyaline,
lamellar pieces, overlapping the persons described under 2, 4, and 5.

4) Tentacular Persons: these form simple or elongated
filaments (grappling-lines), which are arranged in tufts, are capable
of great extension, and are provided with special urticating organs
(Urticating batteries). The primitive Medusa form can be made out
in a few only of these organs, and that faintly.

5) Generative Persons : as in the Hydroid-Polyps, these may
be seen in various stages of development. Although it is in a very
few cases only that they are metamorphosed into Medusae that
become free (Velella — Chrysosmitra), the medusiform type is
very commonly well marked among them. They are generally found
in racemose bunches, just as in the Tubulariae.

The arrangement of these very variously differentiated persons of
the stock of the Siphonophora differs in the different divisions, while
the locomotor and the protecting persons are completely wanting
in many genera. In general the arrangement or distribution of the
polymorphous persons of the stock is observed to be very constant
in genera and species ; gemmation from the stock takes place on
one side only, the arrangement of the buds all round the stock being
due to its spiral twisting. This is the cause of the arrangement of
the nectocalyces in two or more rows, as well as of the grouping of
the other organs. Nutritive, generative, and tentacular individuals
are generally placed together in groups, in such a way that there is
one bract to a group. While in most Physophoridee, these groups
are very close to one another, they are set at greater distances
from one another in the Diphyidm (Fig. 33, A B), and each group is
composed of a certain number of persons which, by breaking off
from the stock, may become distinct individuals (Eudoxiae).

The anterior end of the stem, which is distinguished by the
presence of locomotor persons, becomes in many divisions par-
ticularly perfect, owing to the development of an air sac. This has
the functions of a hydrostatic organ, and causes the anterior end of
the body to be always directed upwards while the stock is at rest
(Physophoridse) . (c d.) It has an opening to the exterior, which
can be closed, and by which air has been observed to escape. The
greater development of these bladders, which in most Physophoridm
are rather small, appears to cause a degeneration of the locomotor
buds of the stock; and thus there seems to be a kind of compensat-
ing arrangement, through which, however, the power of the colony
to move actively is diminished. Instead of swimming, it now is
driven through the water. The locomotor persons are, for example,
absent in Rhizophysa, in which the air sac is increased in size. By

H

98

CO:\IPAEATiyE AAEiTOMY.

Ijecomiug greatly increased into a wide cavity, the air sac occupies the
greatest part of the stem, and thus forms the largest portion of the
colony, the separate pieces of which look like appendages placed on
one side of the bladder. This character is greatly developed in the
Pliysalidoo, and is accompanied by the shortening of the stem.
Another condition obtains in the Velellida3, where the air sac is
placed on the end of the greatly shortened trunk, and is developed
by lateral extension into a disc, the cartilaginous firm walls of which
divide the internal cavity into numerous chambers by forming walls
of partition. In the earliest stages of development the air sac is
simple in these forms also. In Porpita, the disc remains flat and
circular; in Velella it is produced into a diagonal vertical crest, into
which the air spaces of the disc are not continued. The concentri-
cally-arranged chamber-spaces of the air sac in Yelella are connected
by a])ertures. They open to the exterior by a number of holes
placed on the surface. In Porpita, fine air passages, in the form of
canals, pass off from the inferior surface of the air sac, and enter
into and branch in the portion of the stem, which cames the
nutritive individuals.

§ 76.

The Thecomedusae are polypoid Coelenterata provided with a
test, and are allied to the Hydriformes, although in organisation
they resemble Medusse; they are indeed, intermediate between these
two groups, for they are representatives of forms which are closely

allied to the larvge of the
Discophora. This larval
form (Scyphostoma) seems
to be more highly organised
than most of the Hydroid-
Polyps ; it presents, indeed,
points of connection with
only a few of them (Cory-
morpha). It is developed,
just like the Hydroid-Polyps,
from a planula, which is at
first free, and which after-
wards becomes fixed. But
the fundamental form of
the body resembles not
only that of many Hydi’oid-
Polyps, but their Medusa
stage also, for two equiva-
lent secondary axes cross
the primary one. The organs
are arranged by fours, so
that four antimeres can be distinguished in the body. Medusae are
budded off from this polyp-form, but gemmation does not, as in the
Hydroid-Polyps, take place at the side, but at the end. The terminal

Fig. 3A. Young stages of Aurelia aurita.
1 Planula. form, attaching itself. 2, 3 Passage
into the Polyp-fonn. !• Commencement of the
formation of metamcres. 5 Continued forma-
tion of metameres (Strobila) and the differen-
tiation of them (after M. Sars).

rOEM OF BODY OF CO^LENTEEATA,

99

moutE-b earing portion of Scypbostoma is gradually nipped off from
the rest of the body (Fig. 34^ 4). As the body grows the new por-
tions^ which are formed towards the aboral pole, become separated
metamerically (Strobila, Fig. 34, 5), and all are developed on a
Medusa type. The polyp-body is thus divided into a number, often
a large number, of Medusae, which gradually break off (Ephyra
form), and when they are free become more developed.

This process, which has been observed in Cephaea, Aurelia, and
Cassiopeia, does not obtain in Pelagia, the ova of which are con-
verted into swimming larvae, which become young Medusae, without
passing through the polyp stage. The development of Pelagia is
therefore compressed into a few stages, while in the others it is
extended over a large series of forms, and is a more complete
repetition of the palaeontological development. The polypoid must
be regarded as the initial stage, and was followed by the
gradual metamorphosis of the polyp into a free Medusa. On
this hypothesis, the strobilation of Scyphostoma and the consequent
development of a number of Medusae, appears to be a secondary
process, which could only come about gradually, and after the whole
polyp-body had ceased to be converted into a Medusa. It is clear,
from the growth of the polyp, while it is passing into the Strobila,
that an important part must be played by the nutritive relations of
the Scyphostoma stage, in giving rise to the Strobila-form, or, in
other words, in producing the Medusae by gemmation ; the whole
phaenomenon, therefore, appears to be causally related to the nutri-
tion of the Scyphostoma. By the gemmation of Ephyrae, i.e. of
young Discophora, from the body of the Strobila, an asexual mode
of multiplication is interposed in the developmental process of tho
Medusae ; and thus we have brought about one form of the so-called
alternation of generation.

By the Scyphostoma form the Medusae are closely related to
the Calycozoa, which appear to be derived from them. The body,
which is attached by a short stalk, is widened out like an umbrella,
and agrees, as to its axes, with the Scyphostomae, and their
descendants. In many points they also present relations with the
Anthozoa. The Calycozoa, therefore, present us with a very
important intermediate form, which has been continued on, with
relatively few modifications, from the ancestral form common to
several large divisions of the Acalephae.

§ 77.

In the Anthozoa the primitive form of the body is exactly the
same as that of the other Coelenterata ; and even the earliest stages
of the fixed planula present no essential differences. The appear-
ance of tentacles and the subsequent internal differentiation give
rise to various differences, the first of which affects the fundamental
number of the secondary axes of the body. In some only four
tentacles appear (Tetractinia), in others six (Hexactinia), and finally,

H 2

100

COMPAEATIVE A^TATOMY.

in others, eight (Octactinia). In the first two divisions this number
is not permanent^ but the tentacles are soon increased in number,
and there is a corresponding change in the internal organisation.
A larger number of transverse axes can then be made out in the
organism, but their fundamental number in most cases remains
the same as at first. In the Octactinia, however, the first four
transverse axes persist.

The body of the young animal is generally cylindrical, but this
form is retained in a few divisions only (Cereanthus, Actinia). In
the other forms colonies are built up, and this causes the external
appearance to vary greatly. The stocks (Polypariae) are formed
either by incomplete division, or by gemmation, or by both com-
bined.

Longitudinal division aids in the formation of the colony to a
varying extent. In many cases it is merely indicated by transverse
growth, and does not lead to any division of the organism, as in
many Fungia3. In others, division affects the oral surface only, and.
the internal parts remain continuous. When this process goes on
for some time, colonies with a large number of orifices are formed,
which are arranged in variously-curved rows, beset at their edges
with tentacles (Mfeandrina). Whilst fiattened or extended racemose
colonies are formed in this way, branched stocks are formed when
division is combined with a considerable growth of the persons in
a longitudinal direction ; and these stocks may not only vary in size
but also be branched in various ways. In the same way gemma-
tion may be the cause of the formation of complicated colonies. In
either case there is a portion of the body (coenosarc, coenenchyma)
common to all the persons, and belonging to the common stock.
The basal portion of the stocks of those Octactinige, which are not
fixed but set loosely in the mud or sand, are developed from this
coenosarc and form a solid stalk-like portion of the stock, in which
gemmation does not occur (Pennatulidae).

§ 78.

In the Ctenophora, which is the division differing most from the
other Acalepha), the permanent form of the body is developed from
a larva, which in all essential points is similar to that of the others.
In the Ctenophora there are four secondary axes, perpendicular
to the primary, and the most important organs are arranged
conformably to these. The body therefore follows, generally, the
radiate type, which is best developed in the Eeroidge. This eight-
rayed form is, however, derived from a four-rayed form, each radius
having been split up into two. Each of the two radii which arise
from a primitive radius are equivalent to the opposite radii of the
same transverse axes. The development of the body follows the
poles of one of the two transverse axes. The differentiation which
arises in this way is clearly seen in the Cydippidge; it is more

APPENDAGES OF CCELENTEEATA.

101

distinct in the Mnemidae^ owing to the presence of lobate processes
directed towards the oral pole, and most distinct in Cestum, where
the form of the body has become that of a band, from its having
grown in the direction of two similar interradii,

Appendages.

§ 79.

I comprise under the head of appendages those processes of the
body which are known as tentacles; they are either altogether
absent or are only just indicated in the Spongiae, but in the
AcalephaB they are widely distributed, and largely affect the external
form of these organisms, in addition to which they are of great
physiological importance to its general economy. Most of them
are, like the wall of the body, contractile, but there are stiff forms
which are not capable of much movement (Trachynemidae) . The
tentacles are the seat of a large amount of sensibility, and function
as sensory organs ; in many cases they are organs of prehension ;
and, finally, they serve as organs of offence by means of the
urticating cells which are attached to them.

The Hydroid-Polyps present the lowest condition; in many
divisions of them the tentacles are scattered over the surface of the
most anterior portion (or portion lying nearest the oral pole) of the
body. In many they may be seen to be arranged more regularly,
and in others they form a circlet of tentacles (Hydractinia,
Eudendrium, Campanularia). This is generally placed at some
distance from the mouth, and gives a higher importance to this
part, which appears to be analogous to a head ; and, indeed, the
tentacular portion of the body (hydranth) of the Hydroida is called
a capitellum.^^

The development, in the Tubularia, of a second circlet of
tentacles, which directly surrounds the mouth, is correlated with the
higher differentiation of the whole body. The outer circlet of
tentacles is moved to the edge of the hydranth, as this portion
becomes flattened out into a disc. Oral and marginal tentacles
can then be made out. The latter are greatly developed among the
Hydromedus80 as well as among the Medusse.

The marginal tentacles, or marginal filaments, which are
generally greatly elongated filamentous appendages of the edge of
the bell or disc in the Hydromedusas, are always arranged in corre-
spondence with the radii of the body. Where interradial tentacles
are present, they generally follow the radial ones, even when there
is a large number of them. Sometimes they are arranged in tufts
(Lizzia), or are branched (Cladonema). In opposition to the increase
in the number of tentacles, until it surpasses that of the rays, is the

102

COMPAEATIVE AXATOMY.

diminution of these structures. Saphenia has only two tentacles :
in some forms, only one is developed (Stenstrnpia). In the Trachy-
nemidm also, the tentacles are arranged radially, and many, as the
/Eginidse, have interradial ones in addition. The attachment of the
tentacles to the body is peculiar, as the tissue which supports them
often sends a considerable process into the body. Eeduction occurs
here also. yEginopsis has only two tentacles. In the Geryonidae a
change of tentacles takes place, the young animal having filaments
which are not permanent (larval tentacles) and which are different
in structure from the permanent ones.

The oral tentacles distributed among the Hydromedusae like-
wise correspond in number to the radii of the body. They are
sometimes simple, sometimes branched. They are not, however,
always present, and are frequently replaced by outward growths of
the edge of the mouth. They are generally wanting in the Trachy-
nemidrn and .^ginid^.

Among the Siphonophora all the medusiform persons want the
marginal filaments, which seem to be indicated merely as rudiments ;
as, for example, in the nematophorous enlargements of the protective
persons. This want of an organ important in the economy of the
colony is compensated for by the tentacles and the grappling-
liues,^^ which can be shown to be modifications of medusiform
persons (§ 75).

The marginal filaments are wanting in the divisions of the
Ehizostomidac and Cyanege among the Discophora; they have four
large groups of tentacles which arise from the lower surface of the
umbrella, and which can be considered either as marginal filaments,
or as oral tentacles. In others there are marginal filaments present,
corresponding in number to the radii, and sometimes even interradial
ones are present. Even in the Charabdeidae, Charabdea has four
tentacles carried by the arrow-shaped processes of the bell ; in
Tamoya (T. quadrumana) these are represented by the same number
of tufts. The filaments are more numerous in the Pelagiae, whdst
the Aureliae are distinguished by a very large number of fine marginal
ones. Oral tentacles are developed as fine fringing processes on
the edges of the arms which surround the mouth. In the Ehizo-
stomidm they are distributed along the numerous grooves which
carry the oral pores.

Two kinds of marginal filaments may be observed in the Lucern-
arirn ; in one division (L. cyathiformis) the filaments which beset the
edge of the cup-shaped body are just like those which are found in
the Medusa), but they may be seen to be broken up into eight
groups ; in the other (L. auricula) they form eight tufts placed on
the ends of the four processes which project from the body.

I ho tentacles of the Anthozoa are different in the larg*e groups of
that class. Eiglit lamellar tentacles, indented or feathered in appear-
ance, surround the moutli of the Octactinia. There is a much larger
number of cylindrical tentacles in the Hexactinia. They surround

INTEGUMENT OF Ca^ENTEEATA.

103

tEe oral surface of tlie body, or are scattered over it ; they are
sometimes produced on to lobate processes also.

In the Ctenopbora, processes of no great size are also occasionally
present on the edge of the mouth in some families (Calymnidse,
CallianiridoB) ; and there are large elevated lobate extensions of the
body, which we may regard as tentacular organs, although morpho-
logically they are different structures. Besides these, some genera
(Cydippidge) have grappling-lines,^^ which resemble the marginal
filaments in the Medusae, and correspond in position with the poles
of an interradial transverse axis ; sometimes they are provided with
secondary appendages.

Integument.

§80.

The most primitive characters of the integument of the Coelen-
terata are seen in the Spongige, where it is composed of the ecto-
derm, which is but slightly differentiated, and follows the various
changes of form in the endoderm, which limits the nutritive cavity.
The special characters consequent on this relation are referred to
below (§ 87). In the Physemarige the cells of the ectoderm form a
syncytium. In the Porifera they may be sometimes seen to form a
thin layer (Halisarcina, Sycon).

Among the Acalephse, the ectoderm undergoes differentiation
.very early, so that the most external layer of cells, or epidermis,
which is distributed over the whole body, represents in most cases a
portion only of the primitive ectodermal layer. The investment
of cilia, which in the Spongiae is limited to the earlier stages of
development, not only persists in the Acalephaa during the so-
called larval stages, when it has a locomotor function, but is
frequently continued on into the later stages, when it is generally
limited to separate parts, e.g. the tentacles.

As the body increases in size, the importance of the cilia, as
locomotor organs, disappears. In one class only — the Ctenophora —
do they retain this function, and they are then increased in size. In
the place of the general investment, as seen in the larva, structures
resembling cilia are disposed in longitudinal rows, and by increasing
in length and breadth become converted into movable sv^imming
or rowing plates. The plates are attached to the body by their
broader base ; it is at this point only that contractility, dependent
on the voluntary influence of the animal, is manifested; the rest and
larger portion of the plate seems to be rigid. There are generally
eight rows of these plates, which act as steering organs ; but in many
Ctenophora there are only four rows (Cestum). The urticating
capsules (nemocysts) are special differentiations of the epithelial

104

co:mpaeatiye anatomy.

elements^ wHcli are found in all tlie Acalepliae^ altliougli they
are not confined to tliem. They are firm capsules (Fig. 35, B),

which are formed in the protoplasm of the
cell, and in which an elastic, spirally- coiled
thread {A) is found ; this is generally emitted
in the form of a stiff body, when the cap-
sule is touched. These stinging organs
are sometimes solitary, sometimes in groups,
and at times they are very regularly arranged.
They often become greatly comphcated, as
in the stinging knots of the Siphonophora,
where they are often arranged in spiral
bands. These stinging batteries develop
on the surface, but are often provided with
a special investment formed by a fold of
the integument.

Although these structures are scattered
over the whole surface of the body, and are
not absent even from the endoderm and its
products, yet many parts of the body are
specially characterised by them ; above all,
the tentacles, or other processes of the body.
The urticating capsules vary greatly in form,
as does the filament in structure ; and these
differences are characteristic of the different
divisions.

Further, the ectoderm has a secreting
activity, by which tests, which more or
less invest the body, are formed. They are
very common among the Hydroid-Polyps,
where they are formed of a firm substance
allied to chitin, and are often provided with
various sculpturings, flutings, spines, ridges,
and so on. These tubular tests are especially
found among the colonial Hydroid-Polyps ;
they are sometimes limited to the fixed
portion of the common stock (Hydractinia),
sometimes continued on to the branches
of the stock (Tubularia, Eudendrium, Pennaria), and sometimes
they are found even on the separate persons (Campanularia,
Sertularia). This provides the soft polyp-stock with an organ of
support, by which it is enabled to raise itself from the ground, as
well as to attach itself.

Fig. 35. Different forms of
urticating capsules, a Cap-
sule of Coiynactis ; 1 With
the filament spirally coiled ;
2 Extended. B C Capsules
of Siphonophora : the fila-
ments extended and partly
provided with hooks. D Ur-
ticating cells of Medusoe;
filaments still rolled up ; in
one not yet differentiated,
and the nucleus of the cell
still visible.

SKELETON OF CCELENTEEATA.

105

Skeleton.

§ 81.

In addition to tlie organs of support, which are formed by the
above-mentioned tests, the Coelenterata have various other forms of
skeletons, which are differentiations of the mesoderm.

There are none in the Physemarige, which make up for the
absence by taking up foreign bodies into their ectoderm; in the
Porifera, some of which have no firm structures (Halisarcina), organs

Fig. 36. A portion of the surface of the body of a Calcareous Sponge (Sycaltis
perforata) to show the triradiate spicules, o Dermal ostia, each of them surrounded by
a circlet of spicules (after Hackel) .

of support are formed by firm needles (spicula), or softer fibres,
which are placed in the mesoderm. The former are formed of chalk,
or of silica ; the sponges are known accordingly as Calcareous or
Siliceous sponges. The spicules of the Calcispongiae are simpler in
character, for they are either acicular tri- or quadri-radiate ; they
present great regularity of distribution and arrangement, together
with numerous modifications in individual characters. The above
figure gives a representation of the spicula of a Calcareous Sponge.
The hard structures, v/hen formed of silica, are much more varied in
form. In addition to the acicular structures, which are combined
together in various ways to form multiradiate stars, there are
various other kinds of firm parts, as, for example, the amphidiscs

lOG

COMPAEATIVE ANAT0:MY.

Fig. 37. 1 Cell -svitli
a siliceous spicule of
Spongilla. 2 Ves-
icle with an amphi-
disc of Spongilla
(after N. Lieber-
kiihn) .

(Fio-. 37^ 2). The siliceous spicules are often greatly elongated,
and form excessively delicate skeletons (Euplectella), or they form
bulky structures which project as tufts of fila-
ments far beyond the body (Hyalonema). Lastly,
in the Fibrospongio3, the skeleton of the body
is formed by fibres united into a network, which
consist of a substance allied to chitin.

In the Acalephge also the deposition of in-
organic substances in the mesoderm leads to the
formation of various kinds of skeletons. In the
Anthozoa they generally have the form of colonies,
and the hard substance is almost always formed
of calcareous salts. These give rise to deposits
(Fig. 45) of definite form (Fig. 38), which are
scattered in the soft parts of the body; or to
connected masses, which vary in form according
to their mode of development. The calcareous
bodies (spicula) always lie in the connective-tissue
of the parenchyma, and are very varied in form. They have an
organic basis, which retains the form of the spicula after the lime

is removed. The connected skeletal
parts are formed either by the union
of spicula, which are connected to-
gether by a hard organic substance,
as in Corallium, or by the direct cal-
cification of a secreted horny sub-
stance, which lies in the axes of the
coenenchyma, and does not possess
spicula. "When the organic sub-
stances predominate the skeleton is horny, as it is in the Gor-
gonidee and Antipathidte. These axial skeletons are sometimes
limited to the trunk of the colony, as in the Pennatulidae, where
they lie in the shaft of the stock, or they may be continued into
all the branches of the stock. There is another form of skeleton,
which resembles the axial ; it is formed by the gradual calcification
of the parenchyma of the body, in which process spicula sometimes
take part. In this way the aboral portion of the whole body is
more or less completely hardened. A proportionate forward growth
of the body at its oral pole occurs at the same time, and the parts
which arc completely calcified represent the dead base. Skeletons
of this kind form the calcareous supports of the Fungiee, Astrseida3,
]\radreporcs, and of Tubipora. The skeleton thus formed must be
regarded as a continuation and development of the skeletons which
are found in the Spongiee.

§ 82.

Another kind of supporting organ is formed in the interior of
the body by cuticular structures, or by differentiation of more resistent
connective substances. The simplest example is again found in the

Fig. 38. Calcareous Spicules of
Alcyonium.

SKELETON OF CCELENTERATA.

107

Hydroid-Polyps^ in wliicli a homogeneous lamella appears between
the ectoderm and endoderm ; this functions as a supporting
lamella to the softer tissues, which are attached to it. Owing to
the formation of external tests, this structure has less importance as
an organ of support in certain parts of the Hydroida : it is very
thin in the parts where the tests are found, but it is much stronger
in the free parts of the body, which are not sheltered in the test.
We find a strong layer of supporting tissue in the wall of the body
of the Tubularia, which belongs to the free portion or hydranth
of the animal. This tissue consists of a homogeneous substance
traversed by fibres, and embedded between the ectoderm and
endoderm. This appears to be the first step in the formation of
an arrangement, which is highly developed in the Medusae, the
so-called gelatinous disc, but in many of them (Medusae of
Clavatella, and Eleutheria) is also only slightly developed.

The gelatinous disc is in the Hydromedusae either completely
homogeneous or is traversed by fibres, which extend from the
ectoderm to the endoderm. It forms a disc, which is attached to
the aboral surface of the body, and which determines the form of
the body (Fig. 39, 1) ; it
may become modified into
the form of a bell. The
organs derived from the
endoderm, which consist
principally of the gastric
apparatus, lie on the oral
surface of the disc.

Although the gela-
tinous umbrella of the
Discophora agrees in its
external characters with
that of the Hydrome-
dusae, it differs from it in
some not unimportant
characters. Its substance
contains various morpho-
logical elements, which resemble those of gelatinous connective tissue,
and it is continued on the oral surface over the so-called stomachal
stalk. It surrounds, therefore, the greater part of the gastrovascular
system.

Of less moment are the characters presented by the supporting
structures of the tentacles of many Hydromedusae. In both the
Hydriformes and Medusae (Trachynemidae, .^ginidae) the axis of
the tentacle is formed of a series of cells, the elements of which
appear to be encapsuled by a more or less homogeneous mem-
branous layer (cf. Fig. 9). The rows, of cells are thus to a certain
extent rigid. A ring of much the same structure (annular cartilage)
is found on the edge of the disc of the Geryonidae.

Fig’. 39. Diagram of a vertical section through a
growing Cunina rhododactyla ; on the right side
through a radial, on the left through an interradial
vertical plane, h Marginal vesicle, c Radial canal.
g Generative products, h Mantlefold, k Stomach.
I Gelatinous disc, r Radial pouch. 1 1 Tentacle.
t w Base of tentacle, v Velum (after E. Hiickel).

108

COMPAEATIVE ANATOMY.

Muscular System.

§ 83.

Form- elements, referable to muscles, are not certainly known to
exist among the Spongiae ; in tbe more exactly known Calcispongiae
they are certainly absent, and the protoplasm of the ecto- and
endoderm performs all the movements of the animal.

A muscular layer is first marked ofi in the Hydromedusae
(Hydriformes), where the cells of the ectoderm possess band-like
processes, which form a connected stratum beneath that layer of
cells (cf. supra, § 25). This layer is also continued on to the ten-
tacles, but is wanting in those parts of the colony which are sur-
rounded by a test. In some parts, as in the stem of the colony in
the Siphonophora, it is greatly developed. In the Medusae it is
limited to the surface which carries the gastric apparatus, where it
forms the sub-umbrella.'’^ From the edge of the bell or disc it
passes into a more or less broad membranous process, the velum,
which consists essentially of muscular fibres ; it is also continued on
to the tentacular organs. The muscular system is more complicated
in the Discophora, many of which also are provided with a velum
(Aurelia). In all the Medusa3 the form-elements of the muscles
are finely striped transversely, but this is not the case in the same
parts of the Hydriformes.

In the Ctenophora muscular bands have been observed running
along the ciliated ctenophores and there are muscular fibres in
the interior of the gelatinous tissue of the body.

The muscular system appears to be best developed in the
Anthozoa. Thus, in the Actinise, the attached base of the body is
distinctly formed by muscles, and circular and longitudinal fibrous
layers can be made out on the rest of the body, which are continued
into the tentacles. In those Anthozoa that form stocks, the bodies
of the separate animals appear to have circular and longitudinal
muscles, whilst the soft coenenchyma is also contractile, the network
of canals of the gastrovascular system, which traverses it, being
accompanied by muscular fibres.

Nervous System.

§ 84.

The Spongim are placed in the lowest grade of animal differentia-
tion, owing to the absence in them of any arrangements which can
be regarded as special organs of sensation. The Acalephae are
not far above them, for their lower forms likewise give no signs
of distinct organs of this kind. Thus the cellular layer of the

NEEVOUS SYSTEM OF CGELENTEEATA.

109

ectoderm^ in tlie Hydroid-Polyps, is as yet an indifferentiated
organ of sensation. Irritations of it produce movements of tbe
fibres of tbe muscular layer connected with the cells (§ 25) ; and
it is only in the Medusae that distinct parts can be recognised as
belonging to a nervous system. These form a ring, which runs
round the edge of the disc, and which is formed of a fibrous
tissue, on which ganglionic swellings, formed of cellular elements,
are placed at regular intervals. The ganglia correspond in position
to the marginal bodies, which are to be considered as sensory
organs, and send off fibres which pass partly to the tentacles, and
partly accompany the radial canals. This nerve-ring, which is
most accurately known in the Geryonidse, is supported on the
annular cartilage, and lies between it and the circular canal at the
edge of the disc. The swellings of the nerve-ring represent central
organs, which are connected with one another by the fibrous portions.
From experiments also in which the edge of the disc was divided,
it seems clear that there is a central nervous system in it.

The nervous system of the Ctenophora is as yet not well known.
As to the rest of the A.calephse, no organs of this kind are known
with any certainty.

Sensory Organs.

§ 85.

Owing to the imperfection of our knowledge of the nervous
system of the Coelenterata, no definite opinion can be given as to the
parts which are to be regarded as sensory organs. This remark
refers as much to the arrangements which we regard as subservmg
the sense of touch as to higher sensory organs. Special processes
of the body appear to serve for the general tactile sense which is
present in the integument, and these we have already spoken of as
tentacles (§ 79). Whether there are, on the other hand, special
organs, must for the present remain undecided ; although the
presence of stiff setge on the tentacles, and around the mouth,
leads us to admit the existence of distinct organs of touch.

More differentiated organs, adapted for sensory perceptions,
are found in the so-called Marginal bodies,^’ which are attached
to the edge of the umbrella in the free Medusse, and which are of
two distinct kinds. The first have the appearance of vesicular
structures, the second are collections of pigment provided with a
transparent refracting body, similar to those organs which, in the
higher animals, are seen to be the terminal organs of the optic
nerves. The former, or marginal vesicles, are either embedded
in the substance of the disc, or project freely at its edge. They
consist of a homogeneous capsule, covered with epithehum, and
enclose one or more concentrically striated concretions, or small
crystals. The concretions are in close relation with the wall of
the vesicle, being encased in a spherical outgrowth of it. As

no

CO.AIPAEATIVE AXATOMY.

they do not lie in the free cavity of the vesicle they cannot be
definitely regarded as similar to the auditory vesicles of other low
animals ; at the same time, it is not possible to give them exactly
any other signification. It is clear that they represent sensory
organs from their intimate connection with the nerve-ring,' for a
double fibrous band arises from the ganglion, which is placed below
each marginal vesicle, and surrounds the vesicle ; after uniting
with it, the fibres pass into the spherical mass of cells, which
contain the concretion (Geryonidae) . This marginal vesicle is
most common among the Eucopidte, Trachynemidae, Geryonidae,
and ^ginidae.

In Cunina crystals are present, so that its marginal vesicles form
an intermediate step towards the similar structures in the Discophora.
The marginal vesicle in the Discophora is always stalked (Fig. 40,
A B h), and lies in a fissure, or a niche-like depression of the edge
of the disc, covered with a lamellar umbrella-like process of it.
A cavity (ampulla) forms a large part of the marginal body (cZ),
and is connected with the gastrovascular system by means of a
canal, which passes into the stalk (c) ; attached to this ampulla, and
occupying the free edge of this marginal body, is a vesicle (e)
filled with crystals, and resembling the similar one in the EEginidae
(Cunina). The most important difference between the two is
therefore the absence in Cunina of the ampulla formed by the
gastrovascular system.

Organs of another kind also are found in the Hydromedusse.
They appear to be in a relation of mutual exclusion to the marginal
vesicles, as they appear in those families only (Oceanidse) which
have no vesicle. The first indication of them is the appearance
of pigment spots on the base of the tentacle, which, as a rule, have
110 refractive media ; in other cases, on the contrary, they are pro-
vided with structures which call
to infiid the crystalline cones
of other lower animals. In the
Discophora these ocelli are com-
bined with the already-men-
tioned marginal bodies ; they
sometimes consist of pigment
only, while in other forms the
pigment appears as the invest-
ment of a highly refractive body
(Fig. 40,7 ■

In the

are special sensory organs. The
most important is a vesicular
structure, which is attached to
the aboral pole of the body, and
contains solid concretions, like
the otoliths in the auditory vesicles of other lower animals. The
functional importance of this organ is, however, not yet exactly

5

Ctenophora, also, there

Fig. 40. Marginal body of Aci-aspedota}
Medusa): A Of Pelagia noctiluca; B
Of Charybdca marsnpialis. aThefree
part of the marginal body placed between
the marginal notches in the disc, h Stalk,
c Canal in it. d Ampulla, e Crystalline
saccule. /Pigment, Lens-liko body.

ALIMENTARY CANAL OF CGELENTERATA.

Ill

known; just as uncertain is that of the two ciliated surfaces, on
either side of this vesicle — the polar areas, which are surrounded
by short fringe-like processes.

Alimentary Canal.

§ 86.

With the separation of the body into an ectodermal and an
endodermal layer, we get the lowest condition of the organs of
nutrition; the endoderm investing a space which is open to the
exterior, and which is the earliest distinct digestive cavity (stomach,
enteron) — (cf. supra, § 28). This condition is simplest in the Gastrula
form, and undergoes various differentiations in the two chief divisions
of the Coelenterata. The stomachal cavity, that is, does not remain a
mere simple space, but grows out into various kinds of cavities,
canals, pouches, and so on, which are either distributed irregularly
in the organism, or are arranged in a definite way. As a rule,
division of labour takes place at the same time, and only one
definite part, or several such parts, functions as a digestive cavity,
while the other spaces a,re used to distribute the nutrient fluid
(chyme). But this gastric system has other functions too. There
is no doubt that it also has a respiratory function, by distributing
through the body the water that was taken in with the food ; and
for this purpose it has, especially in the Spongiae, a much more
extended surface than has the outside of the body. Finally, it has
important relations to reproduction, for the generative elements
are formed in its walls.

§ 87.

Among the Spongiae the simplest form is limited to the early
stages of development,- or is per-
manent as in the Gastraeades.

Almost the only complication
in the Gastraeades is the de-
velopment of an arrangement
for producing a current at the
entrance of the simple enteric
tube. In the Porifera there
are various new complications.

Temporary spaces appear in the
endodermal layer, which break
through to the exterior, so that
in addition to its mouth (Fig. 41,
o), the enteric cavity is con-
nected with the exterior by
pores at various points, which

open and again close. The number of these pore-canals (dermo

Fig. 41. An Ascon-stock of nine persons
(individuals). Diagram. e Ectoderm.
i Endoderm. o Mouth, g Enteric ca^uty
(after E. Hackel).

112

COMPAEATIYE AXATOMY.

gastric pores), wliicli have consequently a dermal and gastric orifice,
is generally very great ; their number is dependent on that of the
spaces which are bounded by the rays of the spicula (cf. Fig. 36, o).
These characters are very distinct in the lowest forms of the Calci-
spongia3, the Ascones (Olynthus).

The development of diverticula of the enteric cavity gives rise to
a second form; the diverticula are continued into the correspondingly
thickened ectoderm, where they form more or less branched canals ;
from these, again, fine canals, which are also branched, open into the
dermal pores. The enteric cavity, as it becomes more and more
divided into branched canals, loses its importance as a stomach, and
at the same time its endodermal investment, which is now limited to
the branched canals. But the endodermal layer does not extend
over all of them, but is finally restricted to their diverticula, which

are thus converted into
the so-called ciliated
chambers. Thus the
function of the enteric
tube passes more and
more from its primitive
locality into the addi-
tional spaces, which
are gradually developed
from it.

The subjoined figure
(Fig. 42) represents the
last stage in which the
endoderm invests the
ciliated chambers only
(w). Modifications of
this form which obtains
in the group of the Leu-
cones, among the Cal-
cispongije, are formed
by the uuion of the
branched canals and of
the ciliated chambers
one with another,
wdience arise retiform
canal systems. The
Siliceous and fibrous
Sponges conform to
this type.

A third form arises
by the formation of
closely adjoining canals,
directed radially to the
stomachal cavity, which in their characters correspond to the* simple
Ascon form, but which generally communicate with the exterior

Fifj. 12. Diagram of the giistrlc sj'stem of a Leucou
(Dyssycus auauas), -where branched canals are
developed, o Mouth, g Enteric cavity, p Dermal
canals, w Ciliated chambers. The difference between
the ectoderm and endoderm is represented in the same
way as in the previous ligurc (after E. Iliickel).

ALIMENTAEY CANAL OF CCELENTEEATA.

113

by dermal pores only. The primitive enteric cavity in them, as in
the Leucones, where it loses its layer of flagellate cells (endoderm),
also loses its nutritive functions, which are confined to the radial
tubes. These latter seldom remain free, but generally unite either,
in part or completely, by their walls with an important layer which
surrounds the primary enteric cavity. A system of canals, which
are only invested by ectoderm, is formed out of the space between
the radial tubes when these only fuse in part. This form may be
seen in the Sycones, among the Calcareous Sponges.

Innumerable modifications, including individual varieties, are
present within the range of one type-form. The primary enteric
cavity is altered in character by the formation of diverticula, as well
as of septa, or trabeculas, and may even be completely atrophied,
while the canal system arising from it is developed ; this phaenomenon
(lipogastria) is not uncommon in the Fibrous and Siliceous Sponges.
A similar atrophy may even affect the mouth (lipostomia) without
affecting the stomach; in such a case the dermal pores take on
the function of ingestive canals; or numerous small spaces, as in
Euplectella, arise in the place of the mouth.

§ 88.

The form of the gastric system is greatly affected by the forma-
tion of stocks, a process due partly to the concrescence of free persons,
and partly to budding. The union, according to the degree of its
development, may then simply cause communication between the
stomachal cavities, which persist for each person (Fig. 41), or lead to
a complete union of the cavities; in which case the mouths too
may undergo reduction, or become reduced to one, which likewise
may disappear.

A special system of cavities (inter-canal system) also arises from
the formation of stocks. This system is formed from the spaces
which persist between the unconnected parts of the persons, or the
anastomosing branches, of the body ; this, like the system mentioned
above in the Sycones, is bounded by the ectoderm only, and is thus
essentially distinguished from the gastric system. It is remarkable
for considerable irregularities in its arrangement, and forms also
wider spaces, which deceptively resemble a stomachal cavity in that
they possess a mouth.

From all these arrangements a significant change of function
in different parts is seen to accompany the change of form in
the Spongim. The physiological activity of the digestive cavity
is not only shared by the secondary canals which arise from it,
but even passes away from it altogether, or is limited to separate
portions of it, when consequently the stomach sinks, physiologically
speaking, to a lower grade. On the other hand, an important func-
tion becomes localised by this change of the primitively subordinate
portions of the canal system, and even the original surface of the
body of the sponges gets a higher significance, in virtue of its
serving as the lining of the inter- canal system. Everything distinctly

I

114

COMPAEATIYE ANATOMY.

shows that the organisation of the Spongias is not only very vari-
able, but also that to understand it, it is absolutely necessary to
distinguish clearly between the physiological and the morphological
value of an organ.

§ 89.

In its earliest characters the formation of the enteric cavity
of the Acalephoc agrees with that of the Spongige, but in the matured
condition there are peculiarities in the Acaleplne, owing to the
greater regularity in the arrangement of the system, which is
developed out of a simple cavity. The mouth, the extent of which
is often increased by the development of accessory parts in its
neighbourhood, leads into the digestive cavity, and serves also as an
opening for the excretion of undigested matters. The principal
cavity seldom remains single, but grows out into secondary cavities,
which have the character of pouches, or of canals, and which also,
as a rule, correspond to physiological differences, since by them the
chyme which is contained in them is distributed through the body
of the person, and of the stock. These accessory spaces of the
digestive cavity, included with the latter under the designation
‘‘ gastrovascular system,^'’ undertake the function of a circulatory
system, without being, morphologically, anything else than the dif-
ferentiations of a primitive enteric cavity. The gastric system of
the Acalephge agrees therefore genetically with that of the Spongiee,
but is distinguished from it by the exhibition of a higher differentia-
tion. This is seen in the difference between the accessory spaces
and the central primary one, which forms the stomach, to which its
functions are ordinarily limited, and which are not, as in the Spongdae,
handed over to the secondary spaces.

§ 90.

The simplest form of the gastrovascular system is found in the
Hydroida. In Hydra it forms a space traversing the long axis of
the body, which commences with the mouth, in the middle of the
circlet of tentacles, and is continued from the next portion, the
stomach, which is capable of great extension, into the thinner
portion of the body, where it is narrower. This space is also con-
tinued into the tentacles. In the Hydroid-Polyps which form
colonies, the canal wdiich arises from the stomach runs through the
whole stock, and makes the gastrovascular system common to all the
persons. In the stocks of the Siphonophora, some persons only are
set apart for the ingestion of nutriment. Each corresponds in
structure to the stomachal tube of a Medusa, and forms a tube
capable of great extension, which is connected at its base with the
general cavitary system of the stock. In this case, then, we must sup-
pose tliat this sort of individuals has lost all the arrangements found
in tlie body of a Medusa, with the exception of the stomach (§ 75).

The gastric system of the Medusse (Hydromedusae as well as
Hiscophora) presents numerous variations. It always occupies the

ALIMEXTAEY CANAL OF CCELEXTEEATA.

115

Fig. 43. A Thaiimantias.
A From the lower surface.
B Seen in section. In the
middle of the body is the
stomach, from which the
radial canals pass to the
circular canal.

concave portion of tlie gelatinous disc, and consists of a stomach
placed in the middle of this cavity and of hollow spaces which
proceed from it. The stomach either lies
directly beneath this surface^ or is placed on
a special stalk, which arises from it, and is
often of considerable size. This free projec-
tion of an organ, which in other animals is
hidden within the body, is explained by the
differentiation of the stomach of Hydrome-
dusae from the most anterior portion of the
body of the Hydroid-Polyps, so that it does
not represent a single organ, but a complete
portion of the body. The mouth is generally
surrounded by tentacular organs, or pointed
prolongations of the wall of the stomach ;
it seldom opens into a narrow portion resem-
bling an oesophagus. In most Hydromedusae
the stomach is separated from the space that
lies behind it by a ring which is developed at
its base ; by the contraction of this ring, the
stomachal cavity can be shut off from the
rest of the gastro vascular system. The stomach varies greatly in
form and size. It projects far beyond the edge of the bell-shaped
umbrella in the Sarsiadee. The
hollow spaces which are distributed
in the sub-umbrella arise from the
base of the stomach, or from the
space which lies behind it, and have
the form of narrow canals, or of
wide pouch-like diverticula. The
narrow canals take a radial course
(Fig. 43) to the edge of the um-
brella ; they are simple or regularly
branched, and they open into the
circular canal, which often sends
processes into the marginal ten-
tacles also. On them way to the
margin, the radial canals may form
diverticula, which are functionally
connected with the
apparatus (cf. § 9C).

In the.^ginid80 and Discophora
the gastric cavity passes directly
into the radial enlargements ;
these latter are derived from
simpler canals. Narrower canals
sometimes, indeed, alternate with
wider spaces. The canals are
branched (Fig. 44, gv), or form, as in

the Khizostomidae, a peripheral network. As the gelatinous substance

generative

Fig. 44. Aurelia aurita. Half of the
lower surface is seen, a Marginal bodies.
h Oral arms, v Gastric ca^^ty. gv Canals
of the gastrovascular system, which
branch towards the edge and unite into
a circular canal, ov Ovaries.

116

COMPARATIVE A^IATOMY.

of the umbrella is continued into the wall of the stomach in the
Discophora, the stomach is not very sharply marked off from the rest
of the gastric system. Its wall is always continued into arm-like
appendages, which, as a rule, project into folded membranes (oral
arms) ; the mouth is placed between these. Division of these arms
leads to further modifications, which give rise to greatly ramified
appendages. In this case numerous grooves, which gradually unite,
lead to the mouth, in correspondence with the form of the arms.
In' the Rhizostomidm the mouth remains open during an early period
of development only, and afterwards becomes closed by the gradual
union of the arms,^^ which limit it, and in which the grooves form
branched canals, which open at the ends of the ramifications of the
arms by numerous fine pores (polystomia).

In the Lucernarim the structural conditions of the gastro-
vascular system closely resemble those of the Medusae. A stomachal
tube, projecting from the concave surface of the umbrella, and pro-
duced into four angles, leads into a wide space, which is continued
into four pouches, and may be elongated into four canals, which pass
into the stalk. The four pouches correspond to widened radial
canals, and are, as in the Medusae, connected with one another at
the edge of the umbrella, and so form a circular canal. In others
this character is modified in such a way that the stomach is continued
into the body, in a tubular form; and at its end, which projects into
the stalk, gives rise to radial canals, which whilst becoming enlarged

run outwards towards
the margin of the disc.
The gastrovascular
system in the larvae of
the Discophora and in
Scyphostoma is very
similar in character.

§ 91.

The gastric system
of the Anthozoa ex-
tends by means of an
oesophagus from the
centre of the tentacle-
bearing surface of the

body into the interior,
where it opens into
the digestive cavity.
From this part canals
pass upwards along the
sides of the oesopha-
gus into the tentacles.

Owing to the width of the canals connected with the stomach,
the intermediate tissue is reduced to a mere partition (s), which
extends in rays from the wall of the body to the wall of the

Fig. 45. Transverse section througli a ]iortion of the
stock of Alcyoninni, in -which two individuals, A A, are
cut through just below their junction with the coenen-
chyma, and a third, B, somewhat lower, v Wall of
oesophagus, c Eadial canals (chambers of the body,
cavity). 6' Septa, o Ova. Part of the coenenchjTna
traversed by canals is seen to contain calcareous bodies.

ALIMENTARY CANAL OF CaXENTERATA.

117

oesopLagus. The canals thus appear to be chambers (c) attached to
the oesophagus, which unite into a common central space the digestive
cavity, or stomach (B), and so communicate with the oesophagus. The
number of these chambers is eight in the Octactiniae, and varies
in the other Anthozoa, but is arranged according to the same law
of numbers, as is expressed in the other characters of their organisa-
tion, as for instance in the number of the tentacles. The septa of
the gastrovascular system are usually continued for some distance
along the wall of the digestive cavity, and terminate as elongated
bands or pads. When, therefore, the stock is calcified interradial
lamellae are formed, passing inwards from the wall between the
gastric lamellae.

In the colonial Anthozoa, the central cavity is connected in each
person by means of a canal system which traverses the coenenchyma
(Fig. 45), and thus every individual is directly connected with the
rest. This canal system forms a network of tubes of various widths
which distribute the nutritive fluid in the stock. At one point of
the common trunk, in the stacks of the Octactiniae, several canals
are united into a wider space, from which an orifice leads to the
exterior; this, probably, serves as a means of regulating the ingress
and egress of the water which flows through the gastrovascular
system (Pennatula, Renilla). A similar opening has been
observed in Cereanthus ; it corresponds to the pore of the Hydrae,
and like it is placed at the aboral end of the body. These arrange-
ments, which give to the gastric system the significance of a water
vascular system, have, in many Anthozoa (Corals), the form of fine
pores scattered over the surface of the
body; they can only be perceived at the
moment they are in function — that is, when
expelling water. Similar pores are also
found on the tips of the tentacles in many
Actiniae, etc. All these arrangements call
to mind the dermal pores of the Spongiae.

In the Pennatulidae and Alcyonidae (Sar-
cophyton) some, and at times many, per-
sons in a colony are less well-developed, and
seem to have lost the function of ingesting
food. It is not known whether they have
any share in the taking in of water.

§ 02.

In the Ctenophora, the nutrient cavitary gastrovascular

system differs in details only. A stomacli, Lateral ’’month
which is very wide in the Bero’idae, and nar- turned upwards. B Seen
rower in the rest, is sunk in the body along trom the oral pole,
its longitudinal axis ; it passes into a space

which is known as the ^Munnel,^^ by means of a narrow canal, which
can be closed by muscles. Radial canals (Fig. 46) pass out from the
funnel and run along the ciliated ribs or ^^ctenophores.^^ The radial

118

COMPAEATIVE ANATOMY.

canals in tlie Beroidaa and CallianiridaB pass into a circular canal at
the oral pole. In the latter, two canals, which run along the sides of
the wall of the stomach, and which come from the funnel, also enter
the circular canal. In the Cydippidas, these are very wide, and
appear to form a common space around the stomach. Finally, two
shorter canals, which do not pass directly from the funnel, but
from the canals’ derived from it, run outwards and open by pores,
which can be closed, at the sides of the polar areas (cf. p. III).
They are placed diagonally, and provide the gastrovascular system
■with a second means of communication with the surrounding
water.

The form of the body leads to various modifications in the
arrangement of this system of canals. The various groups of canals
may be branched. Thus, in the Beroidae the radial canals form
lateral branched diverticula, which are present, though they are
not so large, in the other forms, and are in connection with the
generative apparatus.

§ 93.

In some divisions of the Acalephae there are filamentous structures,
which project into the central cavity of the gastrovascular system ;
they are called Gastric filaments (and, though less appropriately,
mesenteric filaments) . They are found for example in the Lucernaridae
and Discophora. In the latter they form tufts of filaments, placed in
diverticula of this cavity, and execute vermiform movements. They
have similar characters in the Lucernaridae, but are different in the
Anthozoa. In the Anthozoa pad-like processes, richly provided with
stinging cells, run along the free edge of each septum, turned towards
the gastric cavity ; they seldom become filamentous, and are some-
times limited to two of the septa (Tubipora). Nothing is known
as to the function of these organs, which are differentiated very
early.

Although glandular organs do not seem to be differentiated in
the digestive cavity of the Ccelenterata, yet there is an arrangement
which should be noted here ; it may be regarded as an indication of
a secreting system, perhaps analogous to the liver of higher animals.
It is this, namely, that the epithelial investment of the stomach,
which is present in many Ccelenterata, is distinguished by its peculiar
colour. The pigmented cells are set longitudinally, and are generally
placed on the projecting folds of the wall of the stomack in the
Anthozoa; they are also developed in the Hydromedusm, even in
the polyp forms (e.g. Tubulariaj) ; in the Siphonophora they form
distinct pad-liko longitudinal rows at the base of the digestive
cavity of the nutrient individuals. A network of hepatic canals,^^
attached to the single large stomach of the Velellid^e, appears to
be specially differentiated; it is found on the under surface of the
disc.

SEXUAL ORGAXS OF CCELENTEEATA.

119

Sexual Organs.

§ 94.

Sexual differentiation is not the sole factor in reproduction
among the Coelenterata, for various forms of asexual multiplication
(cf. supra, §§ 73-77) obtain among them. Sexual products have
been observed in most of them, but they are not formed in organs
set apart ; the function seems rather to be one which is being
gradually localised. In the Spongiae the endoderm is said to be the
place where these products are formed, but in those. Porif era which
have a mesoderm, the differentiation appears to take place in it.
The history of the ova is best known ; they arise from cells in the
mesoderm, but they are perhaps endodermal cells which have passed
into it. In addition to what has been directly observed in this
group we must bear in mind the characters which obtain in the
Hydroid-Polyps (see below). The male elements have been less
widely observed. The endoderm has been said to be the place where
the seminal cells are formed, but masses of sperm have been observed
in the mesoderm of Halisarca, together with a sexual differentiation
of the stocks.

§ 95.

The place where the generative matters are formed — as a rule in
the walls of the digestive cavity, or the spaces leading from it — is
most exactly known in the
Hydroida among the Acale-
phac. The material of the
two kinds of generative pro-
ducts is, however, provided
by different layers of the
body : this fact deserves to be
exactly described on account
of its fundamental import-
ance. The first, or indifferent
stage, is represented by di-
verticula of the wall of the
body, which have the form
of buds, surrounding a pro-
longation of the gastric cavity,
and formed by the ectoderm
and endoderm. A number of
the cells of the ectoderm {a)
of the growing bud (Fig. 47, A B) enlarge and become distinguished
by their size from the other endodermal cells, which bound the
gastric cavity {g). These enlarged cells are pushed out towards the

Fig. 47. Two female generative buds of II y-
dractinia echiuata. a Ectoderm, h Eudo-
derm. g Gastric cavity, o Ovarian germs.
In A the ectoderm has begun to be pushed into
the endoderm. In B the invaginated portion
has been constricted off from the rest of the
ectoderm (after Ed. van Beneden).

120

CO:\[PAr.ATIYE AXATO:\IY.

ectoderm, and are the ova (o). They gradually form a layer of cells
placed apparently between the ectoderm and endoderm, and give to
the whole bud the appearance of an ovary. While these processes
of differentiation are going on in the endoderm, a growth of cells
from the ectoderm at the tip of the bud is extending inwards (A) ;
as these cells become separated off from the ectoderm (J5), they form
a thin lamella, which grows around the ovarian layer, but which has
no further function except in another kind of bud.

In the male bud, in fact, the ectoderm has the same characters,
but the endoderm does not undergo any change, and simply forms a
layer of cells, investing the gastric cavity without being differen-
tiated into ova. The depressed portion of the ectoderm being
developed to a great size, forms by constriction a layer between the
ectoderm and endoderm (Fig. 48, A B C), the cells of which give
rise, later on, to the morphological elements of the sperm. In this

B

Fig. 48. Three male generative buds of Hyclrac tin ia echinata. a Testes. Other
letters as in Fig. 47 (after Ed. van Beneden).

way the male products of generation arise from the ectoderm, just as
the female products are formed from the endoderm. The fact that
even in the female buds the ectoderm is depressed, leads us to sup-
pose that the buds were primitively hermaphrodite. It is not yet
known how far the generative products have separate origins in
the re.st of the Acalepha?. The possibility of cellular elements haviug
passed from one layer to another at a very early period of develop-
ment may account for the fact that the endoderm appears to be
the layer in which the products of both sexes are formed. Hydra
appears to form an exception, for in it the generative products
are formed in external bud-like organs, which are differentiations
of the ectoderm. Among the Hydromeduste we not unfrequently
meet with a separation of the sexes, not o-nly into different persons,
but even into different colonies ; in the Siphouophora hermaphrodite
colonies only are found as the rule, but there are exceptions to this.
The generative products give rise to more or less considerable swell-

SEXUAL ORGANS OF CCELEXTERATA.

121

ings in tlie parts of tlie body where they are formed, but as they are
only present when the generative matters are being formed, they
may be regarded as temporary organs/^

There are great peculiarities in the structural relations of the
parts which enclose the generative products, but they are connected
by a large number of intermediate stages. In those Hydroid
colonies that give rise to free Medusae (cf. § 74), the Medusae carry
the generative organs ; the Medusae form the generative animals of
their proper Hydroid- Polyps, and elaborate the semen or ova in the
walls of their stomach, or in the radial canals, or, lastly, sometimes in
the circular canal. In some this production does not take place until
a long time after they have broken away from the Hydroid colony ;
in others it happens earlier. The latter bring us to those in which
the reproductive matters are formed while the IMedusae are still
attached to the Hydroid stock. Next, then, comes that stage in
which the Medusa is not only not broken off, but is not completely
developed. All the organs which are of use in the full and indepen-
dent mode of life — mouth, gastric cavity, tentacles, swimming-bell,
&c. — appear in a state of atrophy. We have in fact medusoid
buds, in which the sexual products arise. In others the medusoid
form is completely lost, and quite simple structures appear on
the Hydroid stock in the form of generative capsules, into which, at
the most, a process of the gastrovascular system still projects. Such
are the structures described above in Hydractinia. These generative
buds arise, like the medusoid buds, and the Medusae themselves,
sometimes on the common stock, sometimes on the body of the
Polyps, and often only at certain parts of the latter, as, for example,
between the outer and inner circlet of tentacles in the Tubularia.
Where the proliferating Polyps are provided with a sheath, the
generative buds are always enclosed by the same test as the Polyps
themselves. Thus the phaenomenon of the budding of Medusae can
be followed back to a stage in which the bud has the appearance
of a mere generative organ of the hydroid stock.

The Siphonophora resemble the Hydroid-Polyps. The formation
in them of sexual animals of the Medusa-type, with the simul-
taneous formation of other medusiform persons, helps to explain
the phaenomenon known in the Hydroid-Polyps as alternation of
generation, as being a division of labour. In some of the Siphono-
phora the generative animals become free Medusae, in the walls of
whose stomachs the generative products are formed (Velella, Chry-
somitra). Most of them have only medusiform buds, which are
found in very various stages of degeneration (cf. Fig. 33, B g E).
The stomach of the Medusa becomes gradually represented by the
generative organs only, and the Medusa-bell degenerates into a
mere covering for them. Thus they occur arranged either singly
(Diphyidfe), or grouped into racemose tufts (Physophoridae), which
are placed on the stem of the stock, or, it may be, on definite
persons belonging to it.

Ed. van Bexeden. De la distinction originelle du testicule ot de I’ovaire. Bull.
Acad. Belg. 2'”® Ser. T. xxxvii. 5. — G. Koch, Morph. Jahrb. Bd. ii. p. 83.

122

co:\rPAEATI^^: anatoisiy.

§ 06.

In those Medusae wliich have no longer any relation to Hydroids,
the generative matters are formed in the wall of the gastro-
vascular system, just as in the Medusae of the Hydroid-Polyps,
and the Siphonophora. These matters are generally formed in the
radial canals (^quorida3), or in the pouch-like diverticula of the
stomach (^ginidfe). When the canals are narrow the genitals form

freely projecting diverti-
cula, which, when much de-
veloped, may have the fonn
of ruff-like folds. The
radial canals form lamellar
enlargements intheGeryo-
nidae, when the generative
matters are developed. In
all forms the lower wall
of the canal, or that placed
away from the umbrella,
forms the genital region
(Fig. 35, (j). The germinal
matters in some cases reach
the exterior through the
stomach, and in other cases
by a rupture of the tissue.

In the Discophora the
generative organs have
always much the same re-
lations, and differ but little
in position and form. They
consist of four or eight
frills, curved in a semi-
lunar form, and arranged
in a rosette on the inner
surface of the umbrella

Fig. 49. Diagram o£ a radial vertical section
through a sexually mature Geryonid (Carmarina
hastata); on the right it is taken througli the
•whole length of a radial canal, and on the left
througli the lateral -wings of a genital lamella, at
an interradial plane, b Marginal vesicles, c Cir-
cular vessel, g Generative products, li Clasp of
the mantle, li Stomach. I Gelatinous umbrella,
p Stomachal stalk, r Eadial canal, s I Its inner ;
r .s Its outer wall, u Ic Cartilaginous ring, v Ve-
lum. Z Tongue-like process (after E. Hiickel).

(cf. supra. Fig. 44, ov) ;
the frills are formed by diverticula of the gastrovascular system.
They are placed in depressions on the lower surface of the disc,
or hang freely down from it, often in numerous folds.

The Lucernarite have the generative organs in the form of eight
radially-arranged longitudinal ridges, placed on that part of the
body which corresponds to the sub-umbrella of the ]\Ieduste, where
they form projections into the pouches of the gastrovascular system.
They represent, therefore, an intermediate form between the Hydro-
medusce and Discophora.

SEXUAL OPvGANS OF CCELEXTEPATA.

123

§ 07.

The generative organs o£ the Anthozoa are very similar in cha-
racter. They are connected with the gastric cavity, so that the gene-
rative matters pass to the exterior through the gullet. The septa
of the gastric spaces, or their ridges, which project into the central
stomachal cavity, generally function as organs of this kind. In
the Alcyonarians the sexual products are formed at the free edge
of these processes, either in the stomach or at some distance from
it, at the base of the gastric space; two septa are sterile. They
are distinguished by the presence of the ridges mentioned above,
(§ 93), which project some way forwards. The other ridges do not
always carry sexual products, for in many Alcyonarians they are
found on four only, or only on two of them. In the Actinia3 the
generative products are formed within the gastric ridges. The same
thing happens in the Antipatharia (Gerardia). The Madreporinm,
also, may be mentioned here, for in them the generative products
are developed in the ridges, which project far into the base of
the gastric cavity, and they there form a special process on each of
the two surfaces of the ridges (Astroides calycularis) .

The sexes are usually separated, and in different persons, but
there may be hermaphrodites also (Cerianthus). In the colonial
forms both dioecious and monoecious conditions have been observed ;
in others these characters vary very greatly (Corallium rubrum).
When the persons of a colony are dimorphic, those which are the
more developed are at the same time those which are functionally
sexual, while the others are sterile. But in some Pennatulidm the
persons without tentacles are
the only ones in which there
are generative organs (Virgu-
laria mirabilis).

d cf

§ 98.

In the Ctenophora the
peripheral portion of the gas-
tric system represents the
genital region. Ca3cal diver-
ticula are develoj^ed from the
sides of the canals which run
parallel with the rows of nata-
tory lamelloo ; in these semen
or ova are formed. One side
of a canal is beset with
ovarian follicles, and the other
with testicular lobules. Hermaphroditism repeats itself therefore
in each of the radial segments of the body. The gastric system
serves to carry the generative products to the exterior. This

Fig. 50. Generative organs of Beroe rufes-
cens ; showing their relation to a tract of the
radial canal, a Stripes running along the
canal (Muscles ?) 6 Semen-producing side,
c Ovarian side, with eggs (after Will).

124

COMPARATR^ AXATO]MY.

arrangement therefore is exactly the same as in some of the
Anthozoa, and if we compare the body- substance between two
radial canals with a septum of the Anthozoa, we find that the
genital regions of both sexes are an’anged in just the same way as
in the hermaphrodite Anthozoa.

As a rule the ova of the Coelenterata have no special coverings,
and in many Spongioe and Hydroida (e.g. Hydra) they appear to
change in form in consequence of amoeboid movements. The
seminal elements in the Acalephfe are formed by a small head with
a movable appendage.

Third Section.

Vermes.

General Review.

§ 99.

In this division a large number of animal forms, which are more
or less allied to one another, are put together ; transverse axes are
differentiated, whilst the longitudinal axis of the body is elabo-
rated. In consequence of this an anterior and a posterior end can
be made out, in addition to a dorsal and ventral surface. They
differ markedly from the Coelenterata in having two antimeres.
The body is, or is not, divided into metameres ; in the more simple
forms the metameres are simple in character, in the higher divisions
they undergo differentiation.

It is not certain whether this division does or does not represent
a single phylum. The existence of a large number of small groups,
represented merely by single forms, shows a considerable amount of
divergence within the division ; and this is still further exhibited by
the fact that almost all the higher animal phyla can be brought into
more or less close connection with forms of Vermes.

I arrange the various divisions of the Vermes in the following
order. They might be considerably increased by the introduction
of a large number of isolated genera ; but a complete classification
of such a kind is not part of our purpose here.

I. Platyhelminthes.

Turbellaria.

Rliabdocoela.

Monocelis, Vortex, Mesostomum, Prostomum.

Dendrocoela.

rianaria, Leptoplana.

12G

COMPAEATIVE ANAT0:MY.

Trematoda.

Distoma, Monostomum, Tristoma, Polystomum, Aspidogaster, Diplo-
zoon, Gyrodactylus.

Cestoda.*

Caryophyllseus, Ligula, Bothryoccptialas, Ta3uia, Tctrarbynchus.

Nemertina (EhyncEocoela) .

Pelagouemertes, Ncmertes, Folia, Borlasia.

11. Nemathelmintlies.

Nematodes.

Rhabditis, Dorylaimus, Strongylus, Ascaris.

Gordiacea.

Gordius, Mcnnis.

III. Chastognathi.t

Sagitta.

IV. Acautliocephali.

Ecliinorhynclius.

V. Bryozoa.J

Phylactoleoma.

Cristatella, Alcyouella, Lophopus, Plumatella.

Gymnolsema.

Crisia, llomera, Alcyonidium, Flustra, Eschara.

VI. Kotatoria.

Hydatina, Notommata, Braclaouus, Meliccrta, Flosculai’ia.

Vll. Enteropneusti.

Balauoglossus.

Vlll. Gepliyrea.§

Inermes.

Sipunculus, Fliascaloooma, Priapulus.

CEsetiferi.

Ecliiui’us, BoncUia,

IX. Anuulata.||

♦ The Cestoda arc derived from a form common to them and to the Ti’ematoda.
Tlie difference in organisation is due to their different kind of parasitic habits. There
arc several forms of which it is doubtful whether they belong to one or the other
division (Amphiptyches).

f The Chactoguathi nuist not bo regarded as allied to the Nemathelminthes because
they arc put next to them ; the same remark holds good for the Acauthocephali.

VedicelUna and Loxosoma are genera allied to the Bryozoa, and they might well
bo united with them into one division, but they must not be subordinated to them.

§ In both divisions of the Gephyrea there is a large number of very divergent
forms.

II Tomopteris, Myzostoma, and Polijgordius are special fonns, allied to, but very
divergent from, the Annulata. The last mentioned unites the characters of the
Nemertina and Nematodes with those of the Auuelides.

VEEMES.

127

Hirudinea.*

Ilaemopid, Sangaisuga, Ncphclis, Clcpsiuo.

Anuclides.

01igoclia3ta.

Scolcina.

Lumbricus, CIia3togastcr, Naia.

Haliscolecina.

Polyoplithalmus, CapitcUa.

Cliaetopoda.

Vagantia.

Siphonostoma, Arcnicola, Glyccra, Ncphthys, Phyllodocc, Alciopa,
Syllis, Nereis, Eunice, Amphinome, Aphrodite, Polynoe.

Tubicola3.

Amphitrite, Hennella, Terebella, Sabclla, Scrpula, Branchiomma.

The position of the genera Neoinenia and Chaetoderma is
not yet settled ; hut they must not be passed over, on account of the
great importance of many points which have been made out in their
organisation. Although they differ in not unimportant points from
one another they are closely allied, and may be reckoned with the
other divisions of the Vermes. I therefore unite them into a division,
which I call that of the Solenogastres. It will not be possible
to form any safe opinion as to their position till they are known
more exactly; and for this knowledge, especially as regards their
development, we must wait.

Bibliography.

V. Babb, Beitriige zui* Kenntniss der niederen Thiere. N. A. Acad. Loop. Carol. XIII. 1826.—
Dujardin, Histoii’C nat. des Helminthes. Paris, 1815. — Van Bbneden, Mdmoire sui’ les vers
intestmaux. Paris, 1861. — Leuckaet, R., Die menschlichen Parasiten. Leipzig uud Heidel-
berg. I. II. 1863-70.— Clapaeede, BeobaclitUBgen uber Anatomio und Eutwickelmigsgescliichto
wii’belloser Thiere. Leipzig, 1803.

ON THE SEPARATE CLASSES.

Platyhelminthes : Duges, Recherches sur I’organisation ct les moeurs des Planaires. Aun. sc.
nat. S6r. I. T. XV. ; also Isis, 1830.— Noedsiann, A. v., Micrographische Beitriige zur Naturgc-
schichte der wirbeUoscn Thiere. Erstes Heft. Berlin, 1832.— Quatrefages, A. do, Mdmoiro
sur quelques Planaires marines. Ann. sc. nat. S^r. III. T. IV. — The same, Sur la famille des
N^mertines. Ib. T. VI. — Schiiidt, O., Die rhabdocolen Strudelwiirmer. Jena, 1818.— The
same, Neue Beitriige zur Naturgeschichte der Wiirmer. Jena, 1818. — The same, Ueber Rhab-
docolen. Wiener Sitzungsbericht. Math. Naturw. Classe. Bd. IX. S. 23. — The same,
Ueber Dendrocolen. Zcitschr. f. w. Zoologie. X. XI. — Van Bbneden, Les vers cestoides.
M^moires de 1’ Academic de Bruxelles. XXV. 1850. — The same, Recherches sur la faune littorale
• de Belgique, TurbeUarids. Ib. XXII. 1860. — Leuckaet, R., Mesostomum Ehrenbergii. Arch,
fiir Nat. 1852. S. 234. — Schultzb, M., Beitriige zui- Naturgeschichte der Turbellarien Greifswaldc.
1851.— The same, Ueber die Microstomeen. Arch. f. Nat., 1849. S. 280. — Wageneb, G., Die Ent-
wickelung der Cestoden. N. A. L. C. T. XXIV. Suppl. 1854.— The same, Beitrage aur Ent-
wickelungsgeschichte der Eingeweidewurmer. Haarlem, 1857. — Stieda, Beitr. z. Anat. v. Both-
ryocephalus. Ai-ch. f. Anat. u. Phys. 1864. — Sommee and Landois, Beitriige z. Anat. d. Platt-
wui'mer. Zeitschr. f. 'Viss. Zool. 1872. — Schneider, A., Ueber Platyhclminthen. Giessen, 1873

* I would place the genus Branchiobdella, which is counted as one of the
Hirudinea with the Scoleina among the Annelides. There is nothing leech-like in the
organisation of this worm except its sucker and jaws; and these parts are really
structures which are due merely to its parasitic mode of life.

128

COMPAEATIYE AXATOMY.

— Zelleh, E., Ueber Polyst. integer, z. f. w. Zool. XXII. p. 1. XXVII. p. 233.— The same, Ueber
Leucochlorid. paradox. Ib. XXIV. p. 5(>4.— Sommek, Ueber de Ban v. Teenia mediocanellata.
Z f w z. XXIV. p. 499.— Moseley, H. N., On the Anat. and Histol. of the Land Planarians of
Ceyion'. Trans. Royal Society. London, 1874.— M'Ixtosh, C., Structure of British Nemertcans.
Eciinb. Roy. Soc. XXV. II. p. 305. — Hcbeecht, Enters iib. Nemertinen. Niederl. Arch. f.
Zool. *Bd. li.— Geaef, L., Zur Kenntniss der Turbellarien. Z. f. wiss. Zool. Bd. XXIV.

Nemathelmintlies : Cloquet, Anatomic des vers intestinaux. Paris, 1824.— Ebekth, Unter-
suchungen tiber Nematoden. Leipzig, 1863. — Schxeideh, Monographie der Nematoden.
Berlin, 1866.— Bastiax, Monograph on the Anguillulidae. Trans. Linn. Soc. vol. XXV. p. II.
1865.— Grenacheb, Zur Anat. dor Gattung Gordius. Z. f. w. Z. XVIII. p. 322.— Claus, Uber
Leptodera appendiculata. Marburg and Leipzig, 1869.— Butschli, Beitr. z. Kenntn. der freile-
benden Nematoden. N. A. L. C. XXXVI.— The same, Z. f. w. z. XXIV. p. 361.— The same,
Abhandl. d. Senkenb. Ges. IX.

Ch0Btognatha : Krohx, Anatomisch-physiol. Beobachtungen iiber die Sagitta bipunctata.
Hamburg, 18^. — The same, Nachtragliche Bemerkungeu dazu. Arch, fur Naturgesch. 1853.—
Wilms, Observationes de Sagitta. Diss. Berol, 1840.

Bryozoa : Van Beneden, Recherches sur I’anatomie, la physiologic et I’cmbiyog^nie des Bryo-
zoaires. M4moires de I’Acad. Royale de Belgique. 1845 et seq.— The same, Recherches sur les
Bryozoaires fluviatiles de Belgique. Ib. 1847. — The same and Dumortier, Histoire naturelle des
Polypes composes d’eau douce. Ib. 1850.— Allman, A Monograph of the Fresh-water Polyzoa.
London, 1856. (R.S.)— H. Nitsche, Beitrage zur Anat. und Entwickelungsgeschichte der phy-
lactolamen Siisswasserbryozoen. Arch. f. Anat. u. Phys. 1808. p. 465.— The same, Beit. z.
Kenntn. d. Bryozoen. Zeitschr. f. wiss. Zool. XX. XXI. XXIV.— Vogt, C., Sur le Loxosoma.
Arch, de Zool. Exp. Vol. V.

Rotatoria : Ehhenberg, Die Infusionthierchen, etc. — Leydig, Zm- Anat. u. Entwickelungsges-
chichte der Lacinularia socialis. Zeitschr. f. w. Zool. III. p. 452. — The same, Uber Bau und
systemat. SteUung der Riiderthiere. Ib. VI. p. 1. — Huxley, Quart. Joum. of Microsc. Sc.
1852.— Cohn, F., Zeitschr. f. w. Zool. VIII. p. 431. IX. p. 284. XII. p. 197.

Enteropneusti : Kowalevsky, Memoires de I’Academie de St. Petersbourg. Ser. 7. T. X. No. 3.—
Agassiz, A., M^m. Amer. Acad. IX.

Gephyrea : Grube, Versuch einer Anatomie des Sipunculus nudus. Arch. f. A. u. Phys. 1837.
p. 237.— Krohn, Ueber Thalassema. Arch. f. Anat. u. Phys. 1842. — Quatrefages, A. de,
Memoire sur I’Echiure. Ann. sc. nat. 3 Ser. T. VII. — Muller, M., Observationes anatomicae de
vermibus quibusdam maritimis. Berol. 1852. — Schmarda, Zur Naturgeschichte der Adria.
Wien. Denkschi’ift math. Naturw. Cl. Bd. 3. 1852. — Lacaze-Duthiers, H., Recherches sur la
Bonellia. Ann. sc. nat. 4 Sdr. T. X. — Theel, H., Rech. sur les Phascolion. K. Vet. Acad.
HancU. XIV.

Annulata : Audouin and Milne-Edwards, Classification des Annelides et description des celles
qui habitent les cotes de la France. Ann. sc. nat. T. XXVII— XXX. 1832-33.— Milne-Edwards’
Article : Annelides, in Todd’s Cyclopaedia, I. 1835. — Grube, De Pleione carunculata. Regiomonti,
1837.— The same, Zur Anatomie und Physiologie der KiememA-urmer. Konigsberg, 1838.— The
same. Die Familien der Annehden. Arch, fiir Naturgesch. 1850. — Quatrefages, Etudes sur les
types inferieures de I’embranchment des aunties. Ann. sc. nat. Ser. 3. Tomes X. XII. XIII.
XIV. XVIII. 1828-52. (The results are given in the “ Histoire nat. des Ann^l<5s ” of the same
Author.) — Leydig, Zur Anatomie von Pisicola geometrica. Zeitscher. fiir. w. Zoolog. I. —
Buchholz, Beitrage zur Anat. der Gattung Enchytraeus. Konigsberger Physikal.-CEkonomische
Schriften. III. 1862. — Claparede, Recherches anatomiques sur les Annelides, etc. Geneve,
1861. — The same, Rech. anat. sur les Oligochetes. Geneve, 1862. — The same, Glanures zooto-
miques parmi les Annehdes. Geneve, 1804. — The same, Les Annelides Ch^topodes du Golfe de
Naples. Geneve et Bale, 1868. Supplement, 1870. — The same, Histol. Untersuchungen fiber d.
Regenwurm. Zeitsch. f. wiss. Zool. XIX. — Perrier, E., Etudes sur I’organis. des Lombriciens
tcrrestres. Arch, de Zool. III. — Greeff, A., Unters. fiber die Alciopiden. N. Acta. Ac. Leop.
Carol. XXXIX.

Solenogastres ; Tullberg, P., Neomenia, a new genus of invertebrate animals. Bihang till
K. Svenskavet. acad. Handlinger III.— Graff, L., Anat. v. Cha?toderma. nitid. Zeitsch. f. w.
Zool. Bd. XXVI.— The same, Ueber Neomenia u. Chastoderma. Ib. Bd. XXVIII.

Form of the Body.

§ 100.

The radiate form of body^ which obtains in most Coelenterata, is
never developed in the Vermes. It is replaced by the eudipleural
form^ which is generally known as that of bilateral symmetry
(cf. supra, p. 59). It predominates in all higher divisions of the
Animal Kingdom. Although in some stages, as for example in the
scolex-form of many Cestoda, this differentiation of the secondary

FOKM OF THE BODY OF VEBMES.

129

axes is not expressed, so that similar characters to those which
obtain in the Coelenterata can be made out, yet I do not regard this
condition as one that has been inherited by the Cestoda, for they can
only be derived from forms, which like the rest of the Flatyhelminthes,
possessed the original eudipleural form. Their condition, which
depends on the equal development of the secondary axes, is at once
explained by their loss of the power of locomotion, and by the
attachment of the body by a point, which corresponds to one pole
of the primary axis.

A head, which has a mouth placed as a rule somewhere on its
ventral surface, can generally be distinguished at the oral pole. In
most Flatyhelminthes the mouth is some distance from the head ; in
the Turbellaria, indeed, it is generally some way back on the ventral
surface of the body. The aboral end of the body carries the anus ;
this, when present, has ordinarily a dorsal position.

In the fixed Vermes the form of the body undergoes considerable
modifications. It is greatly influenced by the development of a
covering, as in the Bryozoa. The aboral end of the body, by which
the animal is attached, can no longer carry the anus, which is
accordingly placed nearer to the anterior portion of the body, which
is not enclosed by the cell.

§ 101.

Another phaenomenon which is first seen among Vermes is the
segmentation of the body. Already in the
Eotatoria the hinder portion of the body 1. 2.

is adapted to locomotion by being broken
up into a number of segments. In this wo
see an indication of a condition which
becomes very important in the higher divi-
sions. In the Cestoda it is further de-
veloped. A differentiation is occasioned
by the growth of the body in the direction
of its primary axis. The anterior and pos-
terior parts of the body no longer enclose
the same organs. Thus in the Caryophyl-
Imi the hinder portion of the body alone
contains the generative organs. In Ligula
this hinder portion of the body is consider-
ably developed by the great repetition of
the generative organs. In the Tmniadm
a very large series of these generative
organs are developed in the hinder end of
the body, and each corresponding area
forms a joint, which is gradually marked
off on the outer surface, and has the rela-
tions of a metamere to the other joints

(Fig. 51). In this way the Taenia-chain is formed, the last meta-
meres of which (the so-called proglottids) break off at a certain

(Tct-

form

Fig. 51. 1. Tacuia

rarhyuclius) ; asexual
(nurse). 2. The same in the
joint-forming stage (strobila),
in which the last joints (pro-
glotticls) are breaking ofF, one
by one (after P. J. van Ben(y.len) .

130

COMPARATIVE AXATOMY.

stage of development, and form more or less independent individuals.
This process represents therefore a process of gemmation, the pro-
duct of which is the Taenia-chain ; each separate joint is a metamere
as compared with the general organism of the chain, but it is also
to be regarded as a separate person, since it is capable of an indepen-
dent existence, the slight duration of which is explained by its struc-
ture, which is adapted to a parasitic habit. The metamerism of the
body, seen in the Cestoda, may be derived from a process of gem-
mation, and gemmation itself is correlated with the elongation of
the body. It is an intermediate stage between the two phaenomena;
and there is, therefore, no well-defined antagonism between them.
Where metamerism is faintly expressed, it becomes more and more
nearly a case of simple elongation.

In many divisions we may find examples of this incomplete
metamerism. It is indicated in various systems of organs in the
Nemertina. In the Gephyrea also, metamerism is not a general
phaenomenou, for several systems of organs are not affected by it.
On the other hand, it obtains generally in the Annulata, and gives to
the organism a multifid appearance. In them it is not unfrequently
associated with a process of distinct gemmation. In the embryonic
body there are generally fewer metameres than in the adult. The
freshly-developed segments are formed in front of the last one. The
elaboration of particular metameres gives rise to a large number
of modifications. Such also result when a number of metameres
undergo concrescence, and the primitive arrangement is only indicated
by certain systems of organs ; this gives rise to conditions which
it is difficult to distinguish from those in which metamerism is
just commencing. When metameres are developed the organism
becomes one of a higher grade of organisation, although indeed
this is not the only path towards such a stage, for we meet with
differentiations of other kinds which lead to higher conditions. The
more definite differentiation of the ventral surface owing to the
development of a groove, as in the Solenogastres (in Chsetoderma
this is found in the posterior region of the body only) is an example
of this ; it represents the first stage in the formation of that pedal
surface of the body which is seen in the lowest Mollusca.

§ 102.

There are various other modifications in certain smaller divisions,
which are to be attributed to adaptations to changed external
conditions of life; this is especially the case in the entoparasitic
Platyhelminthes. The cystic form must be regarded as the
most important of these modifications ; this, which is intercalated
into the developmental history of the Cestoda, is, in its phylogenetic
history, just as certainly due to the organism having entered into
relations which at first w’ere strange and abnormal to it, as is the
general parasitism itself referable to habits, which w'ere only
secondarily acquired. The ])hylogcnetic history of the cystic form

FOEM OF THE BODY OF YEEMES.

131

Fig. 52. Yonng
Tasnia, with head
pushed in. a Head.
h Envelope, c Tlie
six embryonic
hooks, remaining
at one point of the
envelope (after V.

Siebold).

The

same Taenia, with
head protruded.
Letters as in Fig.
52 (after Y. Sic-
bold).

is based on the notion that certain abnormal external conditions of
life gradually became normal, in consequence of tlie adaptation of
the organism to them, and that it did not arise by a simple
antagonism to tbe worm^s primitive ontogenesis, wliicb now includes
this cystic form as a normal part of
its cycle. Wliat has happened is
this — that the process of adaptation
has seized upon and exaggerated
a normal inherited phase of the
worm^s ontogeny, and in virtue
of the continuation of conditions
favourable to the appearance of this
exaggerated phase, its appearance
has become a normal phasnomenon.

The variations of the cystic form
are all readily deducible from the
first developmental stage of the
Cestoda. The embryo is generally
provided with three pairs of hooks,
and a cestoid head may be observed

to be differentiated within it (Fig. 52, a) ; when fully developed
this is pushed out, so that the envelope, which at first was external,
becomes the portion of the body below the head (Fig. 53, b). In
the Cysticercus-form the embryo grows into a vesicle filled with
fiuid, from the walls of which the head is budded
out. When the head is protracted, the vesicle
forms a terminal appendage of the body (Fig. 54).

When a number of buds are formed on the wall
of the vesicle, in which protractile heads are dif-
ferentiated, we have the Coenurus-form. When the
buds break off into the interior of the vesicle, and
there form new vesicles, on the walls of which the
same budding process goes on, leading to the for-
mation of systems of vesicles, placed one within the
other, and when the youngest of these can again
bud off tasnia-heads on its inner wall, we get the
Echinococcus-form.

Notwithstanding the various characters of their
final products, these processes of gemmation may
be derived from a common ground-form. They are
by no means unparalleled among thePlatyhelminthes,
for in not a few an asexual multiplication occurs, which
very similar to these in many points. It is very common among the
Trematoda, where the embryo gives rise to an asexual stage known
as the sporocyst."’^ The body-parenchyma of this sporocyst
becomes di&rentiated, generally into similar structures, in which in
their turn the larvas known as Cercarice are produced, and these
are developed into the sexually mature form. The variation in the
forms of the separate generations seems to be due, in a general

Cyst i
ccllu

Fig. 51.
corcus
losoc ; head pro
traded (nat. size)
a The caudal ves
icle, filled with
fluid, c The an-
terior part of the
body, d The head
(after Y. Siebold).

IS

132

COMPAEATIVE AXATOMY.

wtiy, to the degenerations correlated with their parasitic habit,
and in special points to their relations to their different hosts.
Parasitism, in fact, determines the whole phenomenon in question,
which is spoken of as ‘^alternation of generations,^^ but by no means
('.xplained by that phrase.

§ 103.

Gemmation is a common process among the Bryozoa also, where
it leads to the formation of colonies. As in other Vermes and
Coclenterata, the buds are developed from the wall of the body.
Accordingly as the bud remains at the side of, and on the same
level with, its mother, or grows at one end and raise§ itself from the
ground, flattened or upwardly-growing ramifled cormi are formed.
At the edge of the flattened colony the youngest buds often form the
rudiments of several individuals (persons), which are by-and-by
separated from one another. We observe in the case of develop-
ment by gemmation as also in development from the ovum, that the
anterior portion of the body, which carries the crown of tentacles,
develops inside the hinder portion of the body, which forms the
cell round the animal. The proposal has therefore, though without
reason, been made to regard the two portions as separate “indi-
viduals.^^ All the persons of a Bryozoan colony are not equally well
developed. In many, the parts belonging to the cells and muscles
only are developed; these give rise to the so-called Avicularia,
which function as prehensile organs for the colony. The Vibra-
cularia, which are long spike-shaped structures, continually moving,
are parts further modifled. Lastly, some persons may serve only
for the reception of ova, and form the so-called marsupial cap-
sules. Thus we meet here again with a polymorphism, which is due
to a division of the physiological work of the colony.

Appendages.

§ 104.

The appendages have the form of actively mobile processes of
the body, which may be used for the most varied purposes, accord-
ing to their relation to it, and their more special line of development.
As low down as the Turbellaria processes are found on the portion
of the body which represents the head. In many Planarians lateral
lobate processes are developed as tentacles, or feelers; in others
the dorsal surface of the body is distinguished by similar processes
(Thysanozoon).

While the ])arasitic mode of life of the Trematoda, Cestoda, and
many Nemathelminthes causes structures of this kind to disappear
from them altogether, such structures are largely developed in the
free-living Annulata, and prove to us the great influence of the outer

APPENDAGES OF ^T.P^fES.

133

world on the organism. They are especially developed in tlie Chasto-
poda^ the cephalic region of which is provided with contractile pro-
cesses^ either at the sides or in the middle line (Fig. 55, t t'). These
processes are simple, or are further differentiated by segmentation,
or even distinguished by the possession of secondary processes. By
adaptation to the most varied conditions of life they are converted
into very various structures, and serve for all kinds of functions.

In the tubicolous Cheetopoda, where the cephalic region is that por-
tion of the body which comes into closest relation with the surround-
ing medium, the tentacles are converted into an important apparatus.
They form tufts of contractile filaments on the cephalic lobes, where
they are arranged
in one or more
rows (Terebella
[cf. infra, Fig. 70,
f], Hermella) ; or
they may be con-
verted into strong
plume-like struc-
tures (branchial
tentacles) by the
development of an
internal support
(cartilage), and be
beset with secon-
dary branches j
these, in addition
to their respira-
tory function, may
also aid in the
movement of the
whole apparatus

\^en seizing food Head of Nereis Dumerilii. aa Tentacles.

(Serpulaceac). In V" Feelers, p Parapodia. p/i Pharynx,

some, these bran- w Jaws, i (Esophagus. Glands (after Claparode).
chial tentacles are

arranged in two groups, and resemble an open fan. In Siphonos-
toma they form short simple filaments, with two longer delicate
feelers. In others the base of the two halves of the tuft, which are
separated on the dorsal line, is drawn out into a spirally-coiled
ridge, on which the separate filaments are arranged (Sabellidm).
When optic organs are formed on the separate filaments of the
branchial tufts, the tentacles acquire new and important relations
(Branchiomma) .

Some of the branchial filaments undergo other kinds of changes.
In some Sabellidm one or two of the primitively similar branchial
tentacles (Protula) lose their respiratory function ; in others they are
converted into club-shaped organs, one of which is largely developed,
and serves as an operculum to close the tube in which the animal

134

COMPAEATIYE ANATOMY.

lives. In Filigrana tEe stalk of the operculum retains some of its
primitive characters by being feathered. But this feathered arrange-
ment may be lost (Serpula), and then the operculum, during its
development, passes through stages which are permanent in other
forms. A calcified layer is often secreted in this apparatus, which
owes its origin to adaptation ; it covers the free flattened end like a
disc. In some cases the widened opercular stalk takes up the ova,
and functions as a brooding pouch (Spirorbis spirillum). Thus we
find one and the same organ passing through a series of the most
varied relations, far removed from its original significance, and
caused by certain external relations. In addition to the feelers
there are special tentacles in the Chgetopoda, which are shorter,
but contractile (Fig. 55, a).

The tentacles of the Bryozoa are structures of this kind ; they
have the form of filamentous ciliated and contractile processes of a
discoid or lobate extension of the oral end of the integument (lopho-
phore). The discoid form of lophophore, in which the mouth is
placed in the centre, is the most common. In the other case, the
lophophore is drawn out into two processes, so as to have a horse-
shoe shape (Fig. 60, B hr).

In Pedicellina and Loxosoma, the tentacles, which beset the edge
of a discoid surface, which carries both mouth and anus, are simpler
in character ; they are not hollow internally, like the tentacles of the
other Bryozoa.

§ 106.

Another group of appendages is represented by the locomotor
processes developed in Chsetopoda, which are lateral processes of
the metameres of the body, the foot- stumps or parapodia (Figs.
55, 56, p). They are always arranged in pairs, of which there may be
one or two on each segment. When there are two, one pair occupies
the dorsal, and the other the ventral portion of the side of the body.
They carry setae, and often also filamentous appendages (cirri), which
vary greatly in form, and may be larger than the parapodia, or may
even take the place of these appendages, when the latter are atrophied.
The dorsal and ventral appendages of either side are sometimes
closely approximated; there are all kinds of intermediate steps
between this stage, and that in which they are completely fused
(Syllidm) . Such a fused appendage occupies the side of the body, and
carries the secondary appendages (setse and cirri), which, in others,
are distributed to the dorsal and ventral parapodia. The cirri
appear to be atrophied in the Tubicolm, where they cannot have any
physiological significance, ovfing to the body occupying a tube,
which has sometimes the form of a shell.

The parapodia are developed in very various degrees, and are com-
plicated by their relation to groups of setae. A metamorphosis is
effected by a widening of the ends of the separate, or of fused
parapodia, or rather of their cirri, to form swimming-plates (Phyllo-
docidae). The elytra are special appendages of the parapodia

EXTEEI^AL BEAXCHI^ OF VEEMES.

135

formed by tbe metamorphosis of their dorsal cirri ; they are scale-
like lamell^^ which lie on one another on the dorsal surface, and
alternate with short processes (Aphroditidse) . Although the
parapodia of the Annelida, which function as locomotor organs,
appear to be the rudiments of the appendages, which are more per-
fectly developed in the Arthropoda, they are not independent, for
they have no special muscular apparatus of their own, like the
appendages of the Arthropoda, and they are principally set in action
by the general movement of the metameres to which they belong.

External Branchiae.

§ 106.

The appendages on the head, as well as those on the metameres,
of the Chaetopoda undergo various changes in adaptation to the
respiratory function. Although in most Vermes this is per-
formed by the general surface of the body, in the Cbmtopoda it is
confined to definite parts, which are thereby converted into
branchi^, as may be seen from their relation to the vascular system,
and from other points in their structure.

The cephalic tentacles are the first to enter into these
respiratory relations (§ 104)'. In some (Pectinaria, Terebella) these
structures contain a perienteric fluid, and are not distinctly
branchiae. They are more definitely branchial in the Pheruseidae
(Siphonostoma). In the Sabellidae they are still further differen-
tiated in the manner described above, and the separate gill-filaments
are beset with secondary pinnules, by which their surface is further
increased in size.

Just as gills are formed by the special development of the
cephalic tentacles, so, too, gills are formed from the appendages
of the separate segments of the body, by the modifica-
tions of the cirri which are attached to the parapodia, or by the
formation of special appendages. When simplest the cirri are not
altered in character, but enclose a continuation of the coelom, so
that the perienteric fluid can enter into them. The presence of
cilia on the cirri is also of importance for their respiratory
functions. The exchange of gases is promoted by the walls of the
cirri becoming considerably thinner at certain points. As a rule
it is the dorsal appendages which are developed in this way. The
so-called elytra of the Aphroditidae also belong to this series of
processes. They communicate freely with the coelom. Cirri become
more definitely related to the respiratory function by the continuation
of the blood-vascular system into them. They then form bran chim.
These either retain the condition of simple processes — sometimes they
are lamellar in form — or are variously branched. In Cirratulus
they are greatly elongated single filaments. The branched form
includes the more delicate branchim, which are either comb-like

13G

CO:\rPAEATIYE AXATO:\IY.

(Eanicidoo, Fig. 5G, A B), or are arborescent (Fig. 82, hr) (e. g. in
the Ampliinoineao). As a dorsal cirrus is frequently present in ad-
dition to these branchiae, they appear to be independent organs;

Fig. 5G. Diagrams of vertical sections of Annulata, showing the appendages. A Sec-
tion of Eunice. B of Myrianida. p Kemopodium. p' Notopodium. hr Branchiae.

hr' Cirri.

this is the more probable since they frequently are separated from
the parapodia, and arise directly from the dorsal surface.

They are distributed in varying number over the body. They are
sometimes found on all the segments of it, but they are gene-
rally less abundant at the tail (Eunice sanguinea, Amphinome).
Sometimes they are limited to a number of segments and become-
gradually rudimentary (Arenicola, Hermella). In the tubicolous
forms, the mode of life leads to the development of the anterior,
and the disappearance of the posterior gills. On the three anterior
segments of the Terebellidm there are branched branchial tufts
(Fig. 79, hr), in Pectinaria two comb-like branchite, and in
Eranchiosabella and Sabellides simple filiform appendages at the
same point.

In other divisions, also, of the Vermes the respiratory function is
assigned to processes of the body. This is true of the tcutacles
of the Bryozoa. In the Gephyrea there are special developments as
respiratory processes ; in Sternaspis the hinder end of the body
carries vascular appendages. Finally, even in the Hirudinea there
are lamellar extensions of the integument arranged metamerically
( Branch ellion).

Integument.

§ 107.

The integument of the Vermes, which is separated off from the
ectoderm, is closely united to the muscular system, by which it is
continued into the parenchyma of the body when a coelom is wanting.
This obtains in most of the Platyhelminths (Flat-worms) and Iliru-
dinea. Where the cadom is ])resent, the integument, with the
muscles, forms a dernio-niuscular tube, as in the Acanthocephali,
Gephyrea, and most Annulata.

If we separate the dermo-muscular tube into its two constituen t

137

parts, we find tlie muscular portion to be, as a rule, tlie larger ; and
the layer which corresponds to the true integument to bo propor-
tionately feebly developed.

The proper integument is formed, as a rule, of a layer of cells, the
elements of which are often so slightly separated that they form a
syncytium. This layer corresponds to an epidermis. In the
Turbellaria it is everywhere provided with cilia. In many the
cilia are placed on an apparently homogeneous layer, which resembles
a cuticle. But the cilia must be regarded as processes of the cells.
Even in those forms, such as the Oestoda, in which there are no
cilia in the adult, there is an investment of cilia during the embryonic
stages. The embryos of the Trematoda also have it. In many
Annelids there are ciliated spots at various parts of the body, or
large tracts may be clothed with cilia.

The part that this investment of cilia plays in locomotion is best
seen in the smallest forms. In the young state it is generally the sole
organ of locomotion. By the growth of processes of the body the
cilia-bearing surface is increased, and so the cilia become of greater
imjDortance in locomotion. This is their character in the larvm of
the Gephyrea and of most Annelids. The cilia are arranged on
ridge-like processes, which surround certain tracts of the surface of
the body, as lines or circlets of cilia; the arrangement of these
is generally characteristic of the various divisions. One or more
circlets of cilia surround the body ; and by these the larva) of the
Chgetopoda are divided into mesotrochal, telotrochal, and poly-
trochal forms. Even if the surface of the body bears other cilia also,
those of the circlets are more powerfully developed, and their
action essentially aids in more rapid locomotion. Of these circlets of
cilia (Fig. hi jG B v) one is more remarkable than the rest, it appears

Fig. 57. Arrangement of tlie ciliated bands in the larvae of the Echinoderma,

A B, and of the Vermes, C D. v Anterior; iv Posterior circlet of cilia.

0 Mouth, i Enteric canal, a Anus.

in the very earliest stage, and divides the body into an anterior and
a posterior portion. The former represents the upper part of the
future head of the worm, while from the other portion the whole of
the rest of the body is developed. The primitive circlet of cilia

w

138

COMPARATIVE ANATOMY.

persists in one division of the Vermes^ the Rotatoria. MOiile the
posterior portion is differentiated into a more or less jointed body,
the anterior part, which carries long cilia on a discoid thickening, is
developed into a special organ, which is characteristic of this division.
This wheel-organ — so-called from the movement of its cilia — varies
greatly in character. It may be either permanently simple, and
retain its primitive state, or it may be broadened out into lobate
processes (Tubicolaria), or form tentacular prolongations (Stephano-
ceros), which frequently have a locomotor function in the larval
stages only, and in the later fixed mode of life are used to bring food
to the animal by means of the currents produced by the action of
the cilia. In the Bryozoa, also, a circlet of cilia precedes the
development of the tentacles, which are budded out internally to it.
Owing to the position of the mouth, this circlet of tentacles does not
resemble the more common form ; but it nevertheless has close
relations with the arrangements in some divisions, e.g. the Gephyrea,
the larvae of which have a circlet of cilia surrounding the oral
region . F urther, in Polygordius, which except in this point resembles
the Nematodes, there is a circlet of cilia; so that we recognise in
this circlet of cilia an arrangement which may have been transmitted

from an ancestral form
common to many divisions
of the Vermes.

§ 108.

When cilia are absent the
epidermic layer is covered
by a cuticle, which varies
greatly in character, and is
a product of the secretion
of the epidermic cells. This
cuticle is a thin or even a
soft layer in the Trematoda
and Cestoda among the Pla-
tyhelminthes. It has the
same character in the An-
nelida, but in them it may
be very greatly developed
(Fig. 58, c). It is also pre-
sent in the Acanthocephali.
When this layer is thickened
pore-canals may be seen in
it. In the class of the Nemat-
helminthes it is very greatly
developed, and is thicker than the subjacent matrix. Very often
several layers, differing from one another, can be made out in it ;
tliey are formed of a substance which appears to be closely allied to
chitin. When separate portions of the cuticular investment are very
firm, a kind of dermal skeleton is formed in the Annulata, which is

Fig. 58. Vertical section through the integu-
ment of an Annelid (Spha3rodorum). c Thick
cuticular layer with wide pore-canals, m Mus-
cular layer. m' ^Muscles for the tuft of seta?,
s, which occupies the ventral parapodium,
p, while the dorsal one, d, is represented by a
swelling, which contains glandular tubes.

INTEGU]\rEXT OF yEE:MES.

139

morphologically the same as the chitinous carapace of the Arthro-
poda, although it is not so hard.

The dermal carapace of the Rotatoria resembles completely the
chitinous skeleton of the Arthropoda. Although it may not become
as strong, yet the rigidity of the most anterior segment, as well as of
the succeeding ones, which are connected together by softer inter-
mediate pieces, gives to it the character of a true skeleton, which
serves for the origin of muscles.

The cells of the Bryozoa are also cuticular structures ; they are
sometimes gelatinous (Lophopus crystallinus), soft and flexible ;
sometimes, owing to calcareous deposits, they are much harder.
The latter kind are found in most of the Gymnolscmata. They differ
from the tubes of many Rotatoria, and of tubicolous Annelids, by
their close connection with the body; whereas in Rotatoria and
Annelids the tubes are formed by a secretion which is detached from
the surface of the body. But the fact that in many Rotatoria the
body-wall loses its connection with the hinder portion of the tube,
shows that there is no well-defined boundary between these struc-
tures. We see, in fact, that there are intermediate steps between
typical cuticular structures and other secretions, which are ordinarily,
though wrongly, grouped in contrast with them.

The firm cell of the Bryozoon is not developed over the whole
body. It only surrounds the hinder portion, and is continued into a
more delicate chitinous layer, which invests the anterior tentacle -
bearing portion, but is frequently wanting. This difference in the
differentiation of the integument leads to a difference in the power of
movement of these two regions of the body, and allows the anterior
portion to be retracted, and to hide itself and its crown of tentacles
in the hinder or cell-bearing portion. There are various differentia-
tions of the cell, which, more or less, bring this relation of the parts
to a state of perfection.

§ 109.

Those special structures, the aciculi, seta8, hooks, and so on,
which often play an important part in the economy of the animal,
must be regarded as differentiations of the integument, of the class
of cuticular formations. The structures in question are most extra-
ordinarily varied in character, and may be divided into two groups,
according to their relations to the surface of the body. In the first
group they have the character of simple elevations of the integument.
A thicker cuticular layer is formed on papilliform processes, which
may take the form of a wart, or when elongated, of a hair or seta.
Even when it is very firm, it is still only apparently an indepen-
dent structure, for it is nothing more than a modification of the
cuticle, into which it passes at its base. The firm papillos and
aciculi which are found in the skin of many Trematoda, and some-
times variously extended over the anterior region of the body, are
organs of this kind. So too are the fine and closely approximated
aciculi, which cover the body of the Solenogastres as far as the

140

CO^rPARATIVE AXATO^IY.

Fig. 59. Head of
Taenia coennrus
(vesicle-form : Cocnu-
rus cerebralis seen
from in front). The
four suckers and the
crown of hooks in
the midst of them
can be seen.

Fig. 60. ttbcde Dif-
ferent hooks from the
crown of hooks of the
same form. Repre-
senting stages of de-
velopment (after V.

Siebold) .

ventral groove ; the acicnli of the Echinorhynchi, and lastly, the
hooks of the Cestoda, which are in many arranged in a crown
(Figs. 59, GO), or placed in the wall of four protractile tubes

(Tetrarhynchus). These be-
gin as thickenings of the
cuticle, but they form an in-
termediate step towards the
second group, for as they be-
come chitinised they sink
down into the matrix, and
still deeper.

In the second group the
setae or aciculi no longer arise
on the surface, but in special
depressions, which may be
very well compared with
glands. The secretion is formed from one or more cells, and gets
a definite form as it becomes gradually chitinised ; varying in
different regions of the body. As a rule setae are first formed
when metameres are. These structures vary greatly in size and
form, and are very different in the various genera and species.
AVitli the exception of the Hirudin ea they are found in all the
Annulata. They are almost always arranged in tufts (cf. Fig. 58, a),
two or four of which are connected with the parapodia of each
metamere. They function partly as locomotor organs, working
like oars in the swimming forms (Vagantes) ; when they are meta-
morphosed into hooks they may serve as seizing or clenching
organs (Tubicolge). They are best developed in the Aphroditidte,
where some of the finer setrn form a felted layer, which covers the
back and elytra.

The rod-like bodies in the integument of the Turbellaria are
special structures, as are the similar structures in the Annelides ; in
many cases they call to mind the ^Airticating capsules of the
Acalephae."’^

§ 110.

An organ, the function of which is still somewhat uncertain,
belongs to the category of differentiations of the integument ;
this is the so-called proboscis of the Nemertina. It forms a
tube, which is enclosed in a special sheath, placed above the enteron,
and is often coiled ; this tube opens in the anterior part of the body
above the mouth, whence it can be protruded by eversion. Several
divisions can be made out in this tube, one of which has stylets
at its base — generally a larger stylet in the middle, and at
each side several smaller ones in special pouches, which are
sometimes regarded as reserve stylets, and sometimes as structures
of a supernumerary character. The portion of the tube behind the
stylets is glandular in character, and is provided with an excretory
duct, which is placed close to the stylet. A muscle which arises

IXTEGUMEXT OF VERMES.

141

from the body- wall is attached to the bliud end of the tube;
it is to be regarded as a retractor. In many Ncmertina (Linens,
Nemertes, etc.) the stylets are absent.

In some the tube is small (Folia invohita), and so far resembles
the structures in other Platyhelminthes, which may perhaps be
regarded as the first stage towards the highly differentiated pro-
boscis of the Nemertina. Such are the stylets ])rcsent at the
anterior end of the body of the Cercarim, which serve for boring, and
are placed either on the surface, or at the bottom of a deep folli-
cular depression. Lateral stylets having the same relation to the
median one as in the proboscis of the Nemertina are often observed ;
so that we may conclude that primitively there was the same
organisation in this respect in a large division of the Platyhelminthcs.
Even in certain Nemathelminthes we find similar arrangements, so
that we have here to do with a wide distribution of similar characters.
In some this arrangement is only found in the young stages, and
disappears in the adult organism (Trematoda) ; in others it not only
persists, but is connected with important differentiations (Nemertiua).

§ 111.

The integument of the Vermes attains to a higher position
through the differentiation of glands as special organs of secretion.
Organs of this kind have been recognised in nearly all the divisions
of the Vermes, and are very common among the Annulata. In most
cases they appear to be unicellular, and sometimes lie immediately
beneath the integument, and sometimes, when a distinct coelom is
wanting, in the deeper parts of the body.

Among the Platyhelminthes, unicellular dermal glands are known
in the Trematoda. They are generally jilaced in groups in the
anterior part of the body, and in the hinder part also, where they are
connected with the suckers. The glands are greatly developed in
the Hirudinea, and especially in the Blood-leeches, where they are
scattered in the parenchyma of the body, and open on to the skin by
long ducts. They appear to be developed in relation to the gene-
rative function. Unicellular glands are also present in the integu-
ment of the Scoleina, where they are placed between the cells of the
matrix. In many cases the glands take up a deeper position, and
their ducts, only, pass between the epidermic cells.

In the Gepliyrea tubular glands also are connected with the
integument, and tubes are also found in the Annelides (Fig. 58, d). A
glandular layer is developed on one portion of the l)ody of the
Lumbricidae, as a clitellus; but this organ does not appear to be so
simple in structure, for the tubes are invested by a special epithelium,
and are sometimes lobate in form. Glandular tubes, containing
masses of rod-shaped bodies, are very common among the Chsetopoda
(Spio, Aricia), In the Xemertina there are also glands which secrete
a viscous fluid. In many cases the secretion of the dermal glands is
used to form an investment for the ova.

142

COMPAEATIYE AXATOMY.

Skeleton.

§ 112.

When somewhat firmer than usual the integument in many divi-
sions of the Vermes plays an important part as an organ of support;
these relations have been already referred to. Organs which possess
this function, without any subsidiary relations, are more worthy of
consideration. Supporting organs of this kind are seen in the
cartilaginous pieces in the cephalic segment of a number of tubi-
colous Annelides ; from these pieces processes pass out and ramify,
as fine bands, in the feather-like plates. This is to be regarded
as the formation of an internal skeleton, but it presents analogies
only to other similar arrangements.

This also holds for the branchial skeleton of the Enteropneusti,
which is made up of a lattice-work of homogeneous rods (cuticular
structures). Its arrangement and development call to mind the
branchial skeleton of the lowest Yertebrata (Amphioxus), but it
cannot be said to have any very close relations to it.

Muscular System.

§ 113.

The muscular system of the Vermes is connected with the integu-
ment, and forms in most of them the largest part of the covering of
the internal organs. In some it is only slightly developed. The
general arrangement of the fibres follows one of several types, which
may be thus characterised :

1) Circular, longitudinal, and radial fibres form a connected mass
of muscle, in which the two former are separated into layers, and are
traversed by the radial fibres. The circular fibres form an outer and
an inner layer, between which the longitudinal fibres are placed.
The radial fibres run from the interior of the body to the surface.
At the lateral edges of the body they pass directly from the dorsal
to the ventral surface. This arrangement of the muscles is found in
the Platyhelininthes and Hirudinea. In addition to these muscles
there are fibres which run obliquely ; but they are not present in the
Nemathelminthcs nor in the Turbellaria rhabdocoela.

2) The longitudinal fibrous layer is alone present. This is the
case in the Nematodes, Chmtognathi, and in Polygordius. The
longitudinal muscles are distributed in various ways. The muscular
fibres either pass directly below the epidermal layer (matrix of the
cuticle) in the form of flat bands, the broad sides of which are
approximated, or they have their edges approximated, and their

MUSCLES OF VERMES.

143

surfaces therefore directed inwards and outwards. In either case
the muscles are grouped in particular ways. They are separated
into two lateral masses by a dorsal and median line, formed by
other tissues ; and these masses consist of fibres lying directly on
one another (Gordius, Trichoccphalus). In the majority of
Nemathelminthes there is a further differentiation, duo to the inter-
position of other organs at each side of the dermo-muscular tube.
This lateral line (Fig. 61, A r) is, in very many Nematodes,

Fig. 61. Transverse section of Ascaris lumbricoides, A, and of Ilirudo, B
c Cuticular layer, m Muscular layer, r Lateral line -vrith tho excretory organ,
p Upper and lower median lino, p' Oblique fibres, v Enteron. cZ Dorsal. Z Lateinl
vascular trunk, s Vesicle of the excretory organ, n Ventral nerve-chord.

enlarged into a lateral tract, which is more or less developed; it
is present in the Chaetognathi also.

3) The muscular system of the body consists of a layer of
external circular, and internal longitudinal fibres. Neither are
separated into distinct tracts in the Gephyrea, and Acanthocephali,
although in the former the separate longitudinal or transverse
muscular bands are frequently placed at some distance from one
another. On the other hand, the Annelides, owing to the arrange-
ment of the longitudinal fibres into two dorsal and two ventral
layers, have a distinct lateral field or groove ; the longitudinal layer
is the thicker. A layer of transverse fibres, generally represented
by distinct bundles, passes from the ventral median line to tho
lateral grooves.

In addition to these muscles, which are present throughout tho
body, there are also separate muscles for special organs. We need
only mention here the muscles which move the bundles of setao, and
which are probably nothing more than fibres separated from the
muscular mass, which extends over the whole body.

The suckers found in the Trematoda, Cestoda, and Hirudinea,
are special differentiations of the dermo-muscular tube, which agree
with one another in all the essential points of their structure.

144

COMPAEATIVE AXATOMY.

§ 114.

The muscular system of the Bryozoa consists of an external
layer of circular^ and an internal layer of longitudinal fibres
(Phylactolaema). The circular layer is frequently separated into
distinct bands. The muscles which connect the protractile portion
of the body with the cell are the best developed. When the walls
of the cell are very strong the circular bands are separated (Flustra),
and form bundles which pass from the side walls of the cell to its
superior free edge. Some of these are inserted into the portion of
the cell, which functions as an operculum. When longitudinal
muscles are present, some of the muscular fibres are separated off
behind the invaginated portion of the body, and pass inwards to the
duplicature of the body-wall, whence most of them are produced on
to the base of the tentacles. They form the retractors of the
anterior part of the body (parieto-vaginal muscles).

The Vermes differ considerably from one another in the structure
of the form-elements of their muscular system. The muscular
fibres are more or less elongated structures, which as a rule are the
product of a single cell, even where they are very long, as may be
inferred from the presence of a single nucleus. The lower forms of
the Platyhelminthes have pale fibres often difficult to make out, which
may be branched. In the higher Platyhelminthes they form tubes,
the contractile substance forming a hollow cylinder, which contains
indifferent protoplasm and the nucleus. The contractile portion
of the fibres sometimes presents a fibrillar striation. This is seen in
the Hirudinea, Acanthocephali, and Gephyrea. In the last two of
these divisions the fibres of each layer form a network.

Among the Nemathelminthes the simplest condition is seen in
Gordius. The muscular fibres are broad thin bands, with their
surfaces applied to one another. In others, special differentiations
of the fibres may be seen forming rhomboidal plates, which
are frequently continued into elongated fibres. The contractile
substance is fibrillated and striated, and lies on the outer side of the
fibres, while the portion of the fibre directed towards the coelom is
formed of protoplasm, which remains indifferent, and encloses a
nucleus. With this are allied the special metamorphoses of the
fibres into canalicular, or'fiattened cylindrical forms. Each fibre
has a very deep groove ; this it either retains for’ its whole length,
or it becomes cylindrical towards its ends ; its open part being always
directed towards the body-cavity. The walls consist of contractile
substance, broken up into fibrilla3. Protoplasm fills the small space
of the groove, and a delicate membrane is produced from the edges
into a ponch-shaped organ, which projects from each muscular fibre
into the body- cavity, the greater part of which is filled up by these
pouch-like appendages of the mnscular fibres (ikscaris lum-
bricoides. Fig. Gl, A). From the pouches, oblique fibres run to the
median lines ; they often have a fibrillar character, and have been

NEEVOUS SYSTEM OF VEEMES.

145

regarded as nerves. In some parts they exist as distinctly muscular
fibrillae. If the jDouch is not developed^ these fibres are attached to
processes of the muscular fibres, which often become converted into
tubes flattened at the sides. Both these conditions are found not
only in the same genera, but even in the same individual, where they
gradually pass into one another. In the last-mentioned form of the
muscle-cells, a large number of fibres are generally placed closely side
by side in the muscle-tube. The muscular fibres of the Chmtognathi
are distinctly striated transversely ; there are traces of such striation
in many other Vermes.

Nervous System.

§ 115.

The close relation between the nervous system and the general
organisation is shown by the general arrangement of this system.
The centres and peripheral parts are simple when the body is not
divided into metameres ; while, when the body is segmented, meta-
merism is exhibited with the greatest regularity in the central organs
of the nervous system. In all worms the most important central
organs of the nervous system are placed in the
anterior part of the body, and generally near the
commencement of the alimentary canal. A de-
velopment of the nervous tissue from the ecto-
derm has been made out in several divisions at
least. The central organ above the fore-
gut is the most primitive portion of the
nervous system, whatever modifications
it may present. When a head is separated
off it lies in it, and always innervates the
sensory organs that are developed in the
head; it varies in the degree of its de-
velopment with these organs. Nerve -
trunks, radiating thence to the periphery
of the body, appear in various degrees of
elaboration, in proportion to the extent
of the area of their distribution. Two
different conditions may be developed from this
arrangement. The first consists in the ventral
connection of the superior central organs. This
gives rise to an oesophageal nerve-ring. The
second is distinguished by the development of two
longitudinal trunks, which approach one another
on the ventral surface, and have centrak elements placed in them.

The primitive form of the nervous system is retained in most
of the Platyhelminthes, since they possess two large ganglionic
masses, connected by a transverse commissure in the anterior region
of the body. These cerebral ganglia (Fig. G2, (j), witli the two

L

Fig. G2. Anterior
portion of the body
of M e s o s t o m u m
Ebrenbergii.
g Cerebral ganglia.
n Lateral nerves.
n' Nerves to the ante-
rior end of the body.
cZEntei'on. o Mouth sur-
rounded by a sucker
(after L. Graff).

146

COMPAEATIVE ANATOMY.

longitudinal nerve- trunks (n) wliich pass out from tkein^ form tlie
principal portion of the nervous system ; from it finer branclies pass
off to the sensory organs of the integument [n'), to the dermo-
muscular tube, and to the internal organs. The longitudinal trunks
pass along the lateral edges of the body, and are placed closer
together, or are more widely separated from one another, according
to the breadth of the body. In the dendrocoelous Turbellaria, as well
as in many Trematoda, these lateral longitudinal trunks are only
slightly developed, so that it is difficult to separate them from the
other nerves, which arise from the cerebral ganglia, although they
are not unfrequently distinguished from the other nerves by their
larger size.

The Eotatoria come nearest to the Platyhelminthes. The central
organ is a ganglionic mass lying on, but never surrounding, the
oesophagus. In some it is distinctly separated into two lateral
halves. The peripheral nerves arise from this cerebrum ; and as
they are not collected into longitudinal trunks, the simplest form,
which is most like to that of the Turbellaria, obtains in this group.

The nervous system of Pedicellina appears to be of this low grade,
for it is placed on the stomach, and does not form an oesophageal
ring. It is not quite certain whether or no the ganglionic masses,
which lie on the oesophagus in Echinoderes, are separated from one
another. If there is a dorsal commissure the arrangement would
be similar to that in the lower Platyhelminthes.

The nervous system of the Bryozoa is more highly developed ;
its single central mass is a simple ganglionic swelling, lying between
the mouth and anus, and sending out, in addition to large branches
for the tentacles, two nerves which form a ring around the com-
mencement of the oesophagus. Where the nervous system is most
exactly known, as in Alcyonella, there is no doubt about the
oesophageal ring. From the lateral part of the central nerve-mass
a lobate process goes to the lophophore, and, like the, rest of the
oesophageal ring, gives off nerves to the tentacles.

In addition to this nervous system in each individual, a colonial
nervous system has been recognised in the stock, but it is not
quite certain that this system does exist.

§ IIG.

The nervous system of the Nemathelminthes appears to be ar-
ranged in a special manner, so far as the facts about it are agreed
upon. It consists of a central organ placed on the oesophagus, and
surrounding it as a ring, from which nerves radiate forwards, as well
as backwards. This distribution of the nerves corresponds to the
arrangement of the ganglionic cells of the oesophageal ring. The
nerves which run forward from it may be separated into six fibrous
bundles. Two run in the middle of the lateral tracts, and four in
the direction of the secondary median lines. Ganglionic cells lie
both on their origin and their course. The nerves which pass back-

NERVOUS SYSTEM OF VERMES.

147

wards consist of a dorsal and a ventral trunk, wliicli pass along the
corresponding median lines. In addition to these, two chords arise
from the ventral portion of the oesophageal ring, and, converging
posteriorly, unite to form a mass of ganglionic cells (G. cephali-
cum). The median nerves run along the whole length of the body.
Both send fibres into the matrix of the integument. It is clear that
this arrangement is a modification, speaking generally, of the simple
conditions of the nervous system of other Vermes; but it is so
peculiar that any special comparison is altogether impossible.

The same holds for the nervous system of the Acanthocephali.
A small ganglion placed at the base of the sheath of the pro-
boscis gives off branches an-
teriorly as well as posteriorly.

Its relation to the dorsal cen-
tral organ of other Vermes is
obscure, as it is placed between
the bundles of the ventral re-
tractors of the sheath of the
proboscis.

§ 117.

In the second form of the
nervous system two longitu-
dinal trunks are predominant ;
these arise from the cerebral
ganglia, and pass backwards.

This arrangement is first seen
in the Nemertina, and is di-
rectly related to what obtains
in the' Turbellaria, in which
there are often two greatly
developed longitudinal nerves
passing backwards. The size
of these two peripheral longi-
tudinal trunks is dependent
on the length of the body.

As there are ganglionic cells
in them, they are not exclu-
sively peripheral organs. The
cerebrum, too, in the Nemer-
tina is more largely developed,
for several large segments can
be made out in each of the
two ganglia. The commis-
sure between the two halves
traversed by the oro’an

Fig. G3. Head of a Nemertiue (Omma-
toplea alba). g Central nervous system.
n Lateral trunks, o Eye-spot, pp'p" Pro-
boscis. %>s Its sheath, i Enteron. e Lateral
organ, d Dorsal vascular trunk. I Lateral
vascular trunk (after Carm. M'Intosh).

which we have already de-
scribed above as the proboscis. Although in most of them the
longitudinal trunks (Fig. 63, n) run exactly along the lateral edge of

IS

148

CO^IPAEATIVE AXATOMY.

tlie body (embedded witbiu tlie muscular layers), in others (CErstedia)
they approach one another ventrally, and are distinguished by swell-
ings at the points where neiwe-branches are given off. This is an
anticipation of the future development of ventral ganglia,
the elements of which are already present in the longitudinal trunks.
The ventral approximation of the longitudinal trunks shows ns
how the central system got its ventral position, which becomes
further developed by the formation of ganglia. But the ventral
approximation of the longitudinal trunks, by surrounding the oeso-
phagus, leads to the formation of an oesophageal nerve-ring,
when the ventral longitudinal trunks meet. It is an open question
wdiether the oesophageal ring of the Bryozoa, and of the Nemathel-
niinthes arose in such a way as this, and we would only just remark
that even if they had a similar origin it does not folloAv that there
is any connection between them and the oesophageal nerve-ring of
other worms. For in the Nemertina we see that the origin of this
arrangement is due to the two peripheral longitudinal trunks, which
are not present in the other case.

In the Nemertina the oesophageal ring is not closed ; in the
Annulata it is closed by transverse connections between the primi-
tive longitudinal trunks. These have gained a central significa-
tion, owing to the large number of ganglion cells in them ; the
longitudinal trunks now appear to commence as commissures,
w'hich connect the primitive dorsal nerve-centre (cerebrum) above
the oesophagus with the ventral one, which is formed from the
Ion gitudinal trunks .

§ 118.

The cerebral ganglionic mass is not always present in the
Gephyrea. It is w’ell marked in Sipunculus and Steruaspis, and in
the former it is divided into two parts. In Bonellia and Priapulus,
however, only fibrous elements surround the oesophagus, so that in
comparison with the other two a change has occurred, in which the
central elements must be supposed to have become degenerated, or
to have taken on a ventral position. The consequent arrangement
corresponds to a great development of the commissure, which
was formerly present between the two halves of the superior ganglia.
Instead of the two ventral longitudinal trunks there is a single
nerve-chord, in which the fusion of two trunks is only a matter of
inference. This ventral chord generally lies within the coelom, but
in some it is placed outside the muscular layer, just below the
integument (Priapulus).

As a rule, there are no collections of ganglion cells into special
swellings, or expressions of metamerism ; it is only in Echiurus that
they are present, and in it they are but feebly developed ; in other
cases (Sipunculus, Steruaspis) there is a terminal thickening, of the
chord which gives off fine filaments.

The ventral chord gives off filaments on either side, which are
frequently irregular in origin; they are the peripheral nerves.

NERVOUS SYSTEM OF VERMES.

149

Similar nerves are given oil from tlie oesopliageal ring to the
alimentary canal.

The concrescence of two separate structures in the ventral chord
of the Gephyrea is the reverse of tlie permanent separation of the
two halves of the ventral chords Avhich obtains in other divisions of
the Annulata. It would not_, however, be safe to regard these stages
as lower ones, until observation shall have shown, which it has not done
yet, whether or no they are preceded by an earlier stage, like that
in the Gephyrea. The connection of two separate ventral chords by
means of transverse commissures would be more easily explicable if
the ventral chord were previously single.

The nervous system of Sagitta has a peculiar character. Lateral
commissures from the cerebral ganglion in the head, pass backwards
and downwards to the ventral surface of the body, and pass into a
large ventral ganglion, which lies just below the
integument*, and gives off peripheral nerves to all
sides.

§ 119.

A higher grade of differentiation is seen in the
nervous system of the Hirudinea and Annelides.

The cerebral ganglia are connected by commissures
with a ventral chord, and so far these groups re-
semble the Gephyrea. In many Annelids the two
halves of the ventral chord are homogeneous, and
only indicate their metameric character by giving
off nerves. In most, however, there are central-
form-elements regularly distributed along it. The
ventral chord then appears to be broken up into
separate ganglia, which are connected with one
another by longitudinal commissures. Each gan-
glion, again, is broken up more or less regularly
into two halves, which are connected together by
transverse commissures. The two ventral chords
then form a chain of ventral ganglia (Fig. 64).

In many Hirudinea the longitudinal chords of the
ventral medulla are, during the early stages, sepa-
rated from one another. Later on they are placed
very close to one another, and almost seem to be
a single chord. In this case, therefore, the separa-
tion of the chords must be regarded as the more
primitive condition. The longitudinal chords are
still closer in the Scoleina, and in the Nereidge,

Ampliinomidse, and Eunicem among the Chaeto-
poda; but in all these cases there is not a real fusion, but only
a close approximation, which appears to be still closer on account
of the connective tissue investing the two nerve-chords.

In the tubicolous Annelids the ganglia-bearing longitudinal
trunks are separate ; in the Serpulidae especially, the lateral portions

Capitella capi-
lata. g Cerebral
ganglion. o Optic
nerves, c CEsopha-
geal commissure.
h Ventral chord,
with two ganglia.
n Nerves passing off
from them (after
Claparede).

150

COMPAEATIYE AXATOMY.

of the gaiigliouic chain are widely separated from each other an-
teriorly. In the Sabellidae and Hermellidm the chords are closer
together, and indeed in the anterior portion of the ventral nerve-
chord the transverse commissures are much shorter than in the
230sterior. Finally, the Terebellidoe close the series, for in them the
transverse commissures between the ganglia are distinct in the
posterior portion only, while 'in the anterior portion the ganglia of
either side are completely fused.

With regard to the ganglia, the perfection and greater develop-
ment of the cerebral ganglia in Hirudinea and Annelides must be
mentioned as contrasting with the lower Vermes. The two halves
are very seldom united into a single mass; in Enchytraeus,
this condition appears to be due to degeneration. A breaking-np
into distinct lobate segments, of which the Nemertina present a
simple case, may give rise to a great diversity in form. The lobes
frequently have the form of rounded projections, at times almost
pedunculated. The cerebral ganglia are then complexes of smaller
ganglia.

In the ganglia of the ventral chord also, remarkable differen-
tiations may arise, partly from an increase in size, and partly from
concrescence. In the Hirudinea, the first ganglion is generally very
large, and always larger than the rest; it corresponds to a large
number of separate ganglia united with one another, as may be seen
from the segments which compose it, as well as from the nerve-
branches which arise from it. A similar condition is found at
the end of the ventral chord, where the larger ganglion 23resent,
and innervating the sucker, is produced by the concrescence of
several primitive ganglia (seven in Clepsine), which corresponds
to as many metameres as form the sucker. This phaBuomenon of the
approximation (by shortening of the longitudinal commissures) of
separate ganglia obtains also in the Scoleina, but here the inde-
pendence of the parts is often clearly recognisable, owing to the
presence of separate transverse commissures. The Hermellidm
among the Chaetopoda are an example of this, for in them the first
seven ganglia on each side arc in direct contact with one another.
The length of the commissures, as well as the number of ganglia, is
directly connected with metamerism. In the Lumbricidic with
small rings they are very close together, so that the whole ventral
chord presents a compact series of swellings and constrictions. The
ganglia in Clymene and Cirratulus are still closer together.

Owing to this close association of the ganglionic structures of
the ventral chord, it has been supposed to be analogous to the spinal
chord of the Vertebrata. And the ventral ganglionic chain has there-
fore been called the “ ventral medulla.^^ Although we may allow
the analogy, there is no reason at all for supposing that there is any
homology. Position, development, and structure forbid such a
supposition. As to structure, let it be here noted that the ganglion
cells in the ventral chord are in the periphery of the ganglia,
while the inner part is essentially occupied by fibrous bands.

KERYOITS SYSTEM OF VERMES.

151

§ 120.

The cerebral ganglia give off, chiefly, the nerves of the higher
senses, and are developed in different degrees, according to the’
perfection of these latter. The tentacular nerves, and those of the
organs of sight, deserve to be particularly noticed (Fig. G l, o).

The nerves which rise from the ventral chain go off, as a rule,
from the ganglionic swellings; in many divisions, indeed, there is
an apparent origin from the longitudinal commissures, but the
nerve may then be always referred to the nearest anterior ganglion.
This happens in the Scoleina, in the Siphonostomas, in Aphro-
dite, as well as in the Nereidae, and others. Very often the
lateral branches of the ventral medulla form small ganglia, which
are generally placed at the base of the parapodia, and from
which finer nerve-twigs take their origin (e.g. in the Nereidao).
These ganglia arc often connected together by longitudinal com-
missures, and thus there arises a separate part of the nervous system
which is co-ordinated with the ventral nerve-chord (Pleione).

The visceral nerves are differentiated in the same way. In
the lower divisions of the Vermes nerves pass from the superior
single ganglion to the alimentary canal. This has been observed
in the Turbellaria as well as in the Trematoda. In the Annelides
not only are these nerves more developed, but they become in-
dependent to a certain degree, owing to the deposition of ganglia
in them. We divide the apparatus, which has in this way become
a special system of visceral nerves, into an anterior and a posterior
portion. The former is distributed over the oral region, and is
specially developed in the CheDtopoda which are provided with a
protractile proboscis (Phyllodoce, Griycera, etc.). The posterior
portion, which is less developed, passes on to the enteric tube.
In the Hirudinea there is an azygos enteric nerve ; in the Lumbri-
cidae a nerve is continued from the oesophageal commissure, on either
side, to the ganglia placed on the enteron ; these ganglia have been
observed to vary in number. These two portions of the visceral
nervous system, notwithstanding their distribution in parts which
are physiologically connected, must be kept apart, for the anterior
portion is distributed in parts, which are movable at will, while the
latter alone correspond to a true enteric nervous system, and in
view of their physiological relation can be called a sympathetic
nervous system.

Leydig, Ueber d. Nerveusystem der Anneliden, Arch. f. Anat. Ph. 18G2.

Hermann, E., Des Centraluervensystem von Hirudo mcdicinalis. Munich, 1875.

§ 121.

The nervous system in the Solenogastres differs in several
points from the forms which have been already mentioned as ob-
taining in the Vermes. The cerebral ganglion, which in Cheetoderma

152

COMPARATIYE ANATOMY.

is composed of four lobes, gives oif four nerve-trunks, wbicli
pass backwards. Two of them have a central, and two a lateral
course. They unite in a ganglion near the end of the body. In
Neomenia there is a considerable complication. The cerebrum gives
off a commissure, which surrounds the oesophagus, and of these also
a commissural chord on either side, each of which passes to a ganglion
at the side of the oesophagus ; from each ganglia a lateral nerve-trunk
is given off. The lateral nerves unite in a terminal ganglion
(branchial ganglion). A commissure passes from each of the
lateral ganglia to a ventral ganglion, which gives off a ventral nerve-
trunk, which is connected with its fellow of the opposite side by a
number of transverse commissures. If the lateral and inferior pair of
ganglia be regarded as parts separated off from the cerebrum, this
form would be seen to approximate very closely to Cha3todorma,
and the only difference would lie iu the oesophageal commissure,
in the transverse commissures of the ventral trunks, and in the
exclusion of the latter from any share in the terminal ganglion.
In any case we have in Neomenia a further development of the
simple characters of Chmtoderma. This is not the place to indicate
further points of comparison, for as yet we are only beginning to
obtain any exact knowledge as to the structure of these animals.

Sensory Organs.

Tactile Organs.

§ 122.

The sensory organs of the Vermes are of a high grade of
differentiation. The organs of tactile sensation appear in
the form of fine modifications in the structure of the integument
with which the peripheral nervous system enters into connection. Of
this kind are the true tactile organs, while the coarser arrangements,
such as the processes of the integument, are only bearers of them.
The essential part of these organs consists in the connection between
the sensitive nerve-fibres and the modified cells of the integument ;
these cells, as a rule, project beyond the surface of the integument,
as stiff setiform processes (tactile setaB, or rods). These arrange-
ments are most exactly known in the Rotatoria and Annelida, but
they have been recognised in other divisions.

Tactile setie are widely distributed among the Turbellaria and
Nemertina, where they are sometimes found over the whole body,
and sometimes are richly developed on the head. Tliey are found on
the tentacles of the Rryozoa ; and are widely distributed on the
cephalic segment in the Lumbrickhe, and in the Chaetopoda. They
appear in the Chmtopoda on the true tentacles and anteunaa, as also

VISUAL OEGAXS OF VERMES.

153

on tRose appendages of tlie parapodia, w.liicli are known as cirri,
as well as on the structures which are formed from modifications
of these cirri (cf. § 106). The appendages just mentioned are pro-
vided with a large number of end-organs of sensitive nerves, and
thus become complicated tactile organs, which are of a somewhat
high grade, on account of their power of movement.

A special complication of the tactile rods obtains in some Hiru-
dinea, where groups of these structures are embedded in the base of
cup -shaped organs. There is a large number of such organs in
the head, and they are scattered over the hinder rings of the body.
The arrangement of the sensory parts in depressions of the surface
of the body justifies us in supposing that we have here to do, not with
a special tactile organ, but with a sensory organ of general character.

The tactile papilla3 are less differentiated than the tactile rods
or setae. They are developed in places where the body is covered by
a stronger cuticular layer, and are conical or wart-shaped elevations
of the cuticular layer, which are'^ traversed by a pore-canal. We
find these tactile papillae in the Nematodes, where they are grouped
in a regular manner, some near the oral, and some round the genital
orifice.

§ 123.

Very little is certainly known as to their function, but organs
which may be regarded as sensory are formed by parts of the body
which either carry cilia, or have their epithelium distinguished by
some other peculiarity; such are the cephalic pits of many
Nemertina, and the similar parts in Polygordius. The clefts at
the side of the head lead into a narrow ciliated canal, which is con-
nected, either directly or by means of a fibrous chord, with the
cerebral ganglion. Perhaps the apparatus presented by the pro-
boscis of Balanoglossus may be regarded as an organ of this kind.
It is uncertain whether these organs servo for the perception of the
conditions of the surrounding medium, and possess a function
analogous to that of olfactory organs.

Visual Organs.

§ 124.

The visual organs of the Vermes offer numerous examples of
the gradual evolution of an organ from an indifferent condition. In
many of the lower Vermes, Turbellaria, Trematoda, Nemertina, and
Rotatoria we often find, at the place where other forms have dis-
tinctly developed eyes, pigment -spots only, which are arranged
symmetrically, and either placed directly on the brain, or close to
it. Nothing is known as to the mode of termination of nerves in
these organs, so that it is uncertain whether such eye-spots
should be regarded as organs for the perception of light.

154

COMPAEATIYE AXATO]\IY.

Wo can bo more certain about function when tbe pigmeiit-spot is
merely a covering for the special end-organs of the sensitve nerve.
These end-organs have the form of specially modified cells, which
traverse the pigment either singly or in groups ; judging from what
obtains in cases where the optic organs are more exactly known, we
7uay confidently say that these structures are in direct connection
with nerves. They are the so-called crystalline rods, or crys-
talline cones.

Eyes of this kind are common enough in the Turbellaria, among
the Platyhelminthes (species of Mesostomum and Vortex) ; they
are as a rule found in pairs on the upper surface of the head.
Many marine Planaria have a larger number of regularly-ar-
ranged, well-defined, pigment-spots on this part, some of which
surround a crystalline body. These eyes frequently appear in the
early stages of the embryo as pigment - spots ; this is also their
condition in many Trematode larvae, although, in many, distinct
crystalline bodies may also be made out (Amphistoma subclavatum,
Monostomum mutabile). In the entoparasitic forms of this division
the visual organs disappear, while they are persistent ui many of
the ectoparasitic forms (Dactylogyrus). They are also persistent
in Polystomum. They are absent in all stages of the Cestoda,
unless, indeed, we are to regard the red pigment-spots, which in
some lie behind the suckers, as rudiments of such organs.

In the ISTemertina, where eye-spots are frequently present, true
eyes have been observed in but few cases (Polia coronata, Nemertes
antonina). Eye-spots and true eyes of simple form are found on
their oesophageal ring, in the free living hiematodes (Enoplns), while
they are absent in nearly all the parasitic forms ; here, therefore,
degeneration of sensory organs goes hand in hand with parasitic
habit.

The visual organs in the Eotatoria are placed immediately on the
cerebrum. There is one crystalline rod to each of two connected
pigment-spots; or there is only a single visual organ with one
crystalline rod. Others have a pigment-spot and nothing else.

The complex pair of eyes in Sagitta is distinguished by a large
number of radially- arranged crystalline cones, and with these are
found characters which remind us of what obtains in the Annulata.

§ 125.

Among the Annulata the optic organs of the Hirndinea occupy
the lowest position. The eyes, which are present in many, lie, as in
the Platyhelminthes, on the surface of the cephalic portion of the
body, and are, as in them, generally arranged symmetrically and in
large numbers. In their structure they agree in so remarkable a
manner with the cup-shaped structures mentioned in speaking of the
tactile organs, that in them a condition appears to exist, in which
a specific sensory organ is evolved from the indifferent
organs of sensation, wliich arc found in the integument.

VISUAL OliGANS OF VEiaiES.

155

Fig. 65. Head and
most anterior segments
of a Myrianida.
a Eyes, h Tentacles.
c Unpaired cephalic
tentacle, d Cirri.

Among the Aiiiiolidos wo find tlio eyes of tlic Climtopoda gene-
rally liiddeii beneatli’ the integument, and placed on the cerebral
ganglion in one or two pairs : a single eye is
seldom present. Generally one pair is consider-
ably developed, and the second often reduced
to a pigment-spot. Where these visual organs
are specially developed they stand out on the
surface of the integument (Syllidtc, Nereidte,

Fig. 65, a) ; and may attain to a highly compli-
cated structure. This is the case in the Alciopm,
the pelagic mode of life in which is in con-
nection with the high grade of development of
this sensory organ. This influence of the mode
of life is also seen in their nearest allies, the
Phyllodoceidm, which live at the bottom of the
sea, and have rudimentary or very simple eyes.

The spherical bulb (Fig. 66) presents this, the
highest degree of development in the Alciopidm
only. The integument (c) covers the anterior, strongly-curved sef>*-
ment, immediately behind which there is a spherical lens (/). The
hinder segment, the innermost layer of which forms the layer of
rods (5), surrounds a homogeneous vitreous body [li). A layer of
pigment {j)) separates the layer of rods from the parts of the
retina which lie more to the
exterior; outside all these is
the expansion of fibres of the
optic nerve (o'). While in the
simpler forms of eye the ter-
minal organs of the nerve lie
in the integument, they are
here pressed together into a
concave layer. Influential in the
development of this arrange-
ment is the multiplication of
the perceptive elements, and
the formation of refracting
media. Just as the eyes are
completely wanting in the ma-
jority of the Scoleina which
live in the dark, so also these
organs undergo degeneration
in the Tubicola among the
Chaetopoda. The eyes which
are present in the larvm, and
even in later stages, disappear,
or are represented by mere

pigment-spots, when they enter upon the fixed mode of life.

The development of visual organs on the branchial tufts of the
liead is an adaptation of another kind, which is seen in certain

Fig. 66. Eye of an Alciopicl (Neophanta
celox) (after Greeft). i Integument, cover-
ing the anterior segment of the l)ull), c.
I Lens, li Yiti'eous body, o Optic nerve,
o' Expansion of the optic nerve, p Layer of
pigment. Z) Layer of rods.

156

COMPARATIVE ANATOMY.

SabellidsD (Branchiomma) ; in them the eyes are either placed
in large numbers on the pinnate branches of the branchial filaments,
or at their ends only. In other Annelides there is a similar change
in position as compared with the primitive one. In many there are
eyes at the posterior end of the body, as well as on the cephalic
segment ; and finally, in the genus Polyophthalmus there is a pair of
eyes on each metamere, in addition to those on the head. We here
find an arrangement which is not only of importance as bearing on
the estimation of the metameres, but is also a proof that visual
organs may be developfed at points which in other forms only carry
sensory organs of a lower kind.

Auditory Organs.

§ 126.

We consider as auditory organs in the Vermes organs which, as
in the Coclenterata, consist- of a vesicular capsule, in which there is a
firm, large concretion, or a number of smaller ones. The wall of the
capsule is frequently invested with cilia, as may be seen from the
trembling movements of the auditory stones (otoliths). The difficulty
of making out the nerve-branches in the lower Vermes — in which,
indeed, these organs are most largely distributed — has generally
caused the connection of the auditory vesicles with the nervous
system to be missed.

These auditory vesicles are generally unpaired in the Turbellaria,
in species of Monocelis, Convoluta, Proporus, Derostomum. They
generally lie close to the cerebral ganglia, and are found as a rule in
those genera which are devoid of eyes or eye-spots. In the Nemer-
tina they have only been observed in some cases (OErstedia). In
the rest of the Platyhelminthes these auditory vesicles are not,
apparently, present, and they are also wanting in the Nematodes.

Only in the Annelida do they appear again, where they are
paired, and as a rule placed at the sides of the brain (Alciopidae
Arenicola, Fabricia, Amphiglena, etc.).

Alimentary Canal.

§ 127.

The alimentary canal of the Vermes forms a tube, which is
either embedded in the parenchyma of the body, or, when a coelom
is present, in it ; it has a general adaptation to the form of the
body. The mouth lies, as a rule, at the anterior end of the body, and
is always placed on the ventral surface. Where an anus is present.

ALIMENTAEY CANAL OF VERMES.

157

it is placed, as a rule, at the hinder part of the body, and is some-
times ventral and sometimes dorsal.

A differentiation of the enteric tube into several functionally
different portions can always be made out ; accessory organs for the
prehension of food are also often present at
the entrance into the digestive cavity. The
three portions, which are here present for the
first time, are distinguished as fore-, mid-,
and hind-gut; the last is absent when there
is no anus.

The primitive form of enteron agrees with
the characters which are seen in the Gastrula-
form (§28). It makes its appearance in all in
the embryonic commencement of the organism
as a C80cal cavity, with but slight complica-
tions, which opens on the surface at one point
only; it persists in this form in the lower
Vermes. This opening serves for the inges-
tion of food, and also for the ejection of its
undigested remains ; it is mouth and anus at
the same time. This arrangement is very
common among the Platyhelminthes, being
the only condition of the alimentary canal
among the Trematoda, and the dominant one
among the Turbellaria. In the rhabdocoelous
Turbellaria the alimentary canal is distinctly
marked in its anterior portion only, and has
the form of a simple blind tube extending
through the body. The simple mouth varies
in position ; it may be in the anterior portion
of the body, or towards the middle of the
ventral surface, and lastly, even in the pos-
terior portion; it leads into a muscular
pharynx, which is seldom absent (Schizos-
tomeae), and which is, in many cases, pro-
tractile. This is the portion of the alimentary
tract, which, under many modifications, can
be most clearly traced in most divisions of the
Vermes.

§ 128.

Fig. 67. Prorhynclius
fluviatilis. o Mouth,
oe CEsophagus, protrac-
tile like a proboscis.
i Enteron. gl Glands
opening into the enteron.
c Ciliated pits, x Spike
in the organ above the
oesophagus, which ends
caecally at 1/. or Ovary,
in which there are, in
the anterior parts, ova
at various stages of
development.

In the dendrocoelous Turbellaria the gut
is adapted to the broad form of the body.

The mouth is (Fig. 68, o) placed ventrally, and often near the
middle. The muscular pharynx (p) is often metamorphosed into
a proboscidiform organ, which is capable of great enlargement in
size, and is cylindrical or drawn out into lobes. It leads into an
enteric cavity (p), which occupies the middle of the flat body, and
which is broken up into numerous branches, which pass towards

158

COMPAEATR^ ANATOMY.

Fig. 68. Digestive apparatus
of Eurylepta saugui no-
lent a. 0 Mouth. 2? Pharynx.
V Stomach. gv Ramifica-
tions of the digestive cavity.
n E’erve ganglion (brain) (after
Quatrefages) .

tlie edge of tlie body ; an elegant mesli-work may be formed by
the connections of these with one another (Thysanozoon). Owing

to the free communication that these
branches have with the central cavity,
the chyme is distributed in the body, and
so the enteric canal takes on the function
of a vascular system. The land Plana-
rians are remarkable, inasmuch as their
enteric tube consists anteriorly of a median
canal, while it is divided posteriorly. Nu-
merous and regular transverse processes
pass off from both divisions of it.

The enteric tube is branched in many
of the Trematoda. The gut commences
by a mouth, which is generally placed
in the anterior region of the body, and,
as a rule, has the surrounding parts meta-
morphosed into a sucker (Fig. 69, s) ; this
is followed by a muscular pharynx (Z>),
from which the enter on proper is given
off. This is, when most simple, a csecal
sac (Aspidogaster, Gasterostomum), and
corresponds to a low grade of develop-
ment; this is very common among the
Trematoda at certain stages of their de-
velopment (Redia-f orm) . When more differentiated, the enteron
divides into two branches, which pass backwards, and either give

off greatly ramified branches into the body
(Distoma hepaticum), or form simple
ceecal sacs (c) (Distoma flavescens, D.
lanceolatum) . The two branches may

unite again and form an arrangement like
that which obtains in some Planaria3. It
is clear, from the homogeneity of its struc-
ture, as well as from its contents, that
even in the Trematoda this branching of
the gut is merely an eulargement of the
tract in the body, and not the formation
of heteronomous segments. The texture
of the wall is in correspondence with the
low stage of this form of enteron, for
only the epithelial investment is indepen-
dent, being bounded, exteriorly, by the
tissue of the parenchyma of the body —
connective tissue.

Complete degeneration of the gut is
clearly due to adaptations to definite
modes of life, in which the food passes through the integument
by endosmosis. This phgenomenon, brought about by parasitism,

Fig. 69. Alimentary canal
of Distoma flavescens.
0 Mouth surrounded by a
sucker, s. s' Ventral sucker.
h Muscular portion of the oeso-
phagus or pharynx, c Bifur-
cated enteric tube.

ALIMENTAEY CANAL OF VEEMES.

159

attains its Ligliest development in tLe sporocjst forms of the Trema-
toda. Finally^ the absence of an enteric canal is the rule among
the Cestoda^ where the enteron is not present for a time even.
The enteron is altogether wanting in the Acantho cephalic and for
the same reason — namely, parasitism.

Among the Platyhelminthes there are forms which are distin-
guished by the possession of an anus, and which may be contrasted
with those which indicate their lower condition by not possessing
one. Such are the Microstomege among the Tur-
bellaria rhabdocoela, and the Nemertina, the enteric
tube of which has pretty much the same form
throughout, and which begins by an elongated
ventral mouth, which lies behind the central nervous
system. In Malacobdella the mouth is placed at
the anterior end of the body. A muscular, but
generally feebly-developed, pharynx, leads into the
intestinal tube, which is provided with a large
number of lateral diverticula. This fills the greater
part of the body- cavity, to the walls of which it is
attached by muscular fibres. The lateral diverticula
of the enteric tube are sometimes regularly arranged,
and are the first indications of metamerism. This
is best seen in Pelagonemertes ; and so far this
form calls to mind the dendrocoelous Turbellaria.

§ 129.

In the Nemathelminthes all three portions of the
alimentary tube are generally present. In corre-
spondence with the form of the body, it forms a long
tube, which traverses the body, beginning by a
mouth in the centre of the anterior end of the
body, and ending by a ventrally-placed anus, which
is more or less near the caudal end. The most
anterior portion (oesophagus) forms a narrov/ canal,
the walls of which pass gradually backwards into a . ^
thick-walled pharynx (Pig. 70).^ This_ is distinctly Siy
marked otf from the rest, and is distinguished by Nematode (Dia-
a musculature, which enables it to act as a sucking gram),

organ. The layer of chitin, vdiich invests the tract
from the mouth to this portion, not unfrequently forms ridges or
tooth-like organs. The mid-gut (chyle-stomach), which succeeds
the pharynx, is, as a rule, the largest part ; it has simple walls,
often formed by a single layer of cells, v»^hich in some (Heterakis
vesicularis, Oxyuris vermicnlaris) is provided in places with a
muscular covering of annular fibre. A cuticular layer generally
lies outside the epithelium ; an internal cuticle, which is traversed
by pore-canals, appears to be present also. In many, the mid-gut
forms a csecal diverticulum in its anterior portion. This portion

160

COMPAEATIVE A^^TATOMY.

of tlie intestine is attached by laterally-directed fibrous cliords to
the body-wall^ as a rule^ along tlie lateral lines. The bind-gut,
wbicli arises from tlie mid-gut, is tbe shortest portion of the whole
canal, and is distinguished from the part in front of it by its
diminished breadth.

In tbe Gordiacea the enteric canal is present in the entoparasitic

larval stages only, and undergoes
retrogressive metamorphosis when
the sexual organs are developed.
In Gordius even the mouth dis-
appears. The organism, when it is
free, uses up the material which it
ingested by its enteron during the
earlier stages, in forming genera-
tive products, after it has given up
its parasitic habit, and the ingestion
of food.

The enteric canal of the Chm-
tognathi resembles in many points
that of the Nemathelminthes, but
the enteron is connected to the
body-wall in a different way, namely
along its dorsal and ventral median
lines. Setiform hooks, arranged in
rows at the sides of the mouth,
serve as organs of prehension.

§130.

Although the digestive organs
of the Bryozoa are sharply marked
off into the three primitive di-
visions, they are exceedingly simple
in character. The mouth, which is
surrounded by the tentacles, or
placed in the centre of the lobate
process which carries them, is in
one division (Phyiactolaemata) over-
hung by a movable process — the
epistom. Thence it passes straight
back to an oesophageal portion
(Fig. 71, ve)j which in some is
widened out or even converted into
a gizzard by the development of
denticular processes in one part of
The second portion is separated
from the fore-gut, which is invested with cilia, by a constriction
(?;), and forms the mid-gut. It functions as a stomach, and forms a
cmcum, which generally descends some way down into the coelom.

Fig. VI. Organisation of Bryozoa. A
Palndicella Ebrenbergii. B Pin-
matclla fruticosa. hr Tentacular
branchias. oe CEsophagus (fore-gut).
V Stomach, r Hind-gut. a Anus, i
Covering of the body (cell), x Posterior;
a; 'Anterior chord, at the insertion of
which into the body the generative
products are developed. t Testes.
0 Ovary, m Rcti’actor muscles of the
anterior portion of the cell, mr Prin-
cipal retractor muscle (after Allman).

it (Bowerbankia, Vesicularia).

ALIMENTAEY CANAL OF VEEMES.

IGl

The hind-gut follows a constriction of the somewhat more deeply-
placed pyloric portion, and, ascending by the side of the fore-gut, is
continued into an anus {B a), which is placed close to the mouth, but
always below and outside the circlet of tentacles. The hind-gut is
sometimes widened (Flustra).

The ciliated tentacles function as accessory organs of nutrition,
food being brought to the fixed animals with the changing water.

In the Pedicellinge the same parts can be made out as in the
true Bryozoa, but the stomach has no
csecum.

The enteric canal of the Eotatoria
exhibits sometimes agreement with lower
conditions, for it may consist of fore-
and mid-gut only, the hind-gut being
absent (species of Notommata), while on
the other hand it is more highly de-
veloped, owing to the differentiation of
masticatory organs in the most anterior
portion. These are formed by chitinous
structures, placed at the sides and oppo-
site to one another ; they are provided
with tooth-like processes, etc. (Fig. 43,
m). The fore-gut commences with the
mouth, which lies below the ciliated
velum, and is distinguished by its lesser
width from the mid-gut (ordinarily called
the stomach Where a hind-gut is
continued on from the mid-gut, it turns
to the dorsal surface of the body, to
open into a cavity common to the open-
ings of the excretory and sexual systems
— the cloaca — a peculiarity not found in
other divisions of Vermes.

§ 131.

wliole of it cannot be seen.
2D Front of the proboscis.
ss ' Groove of the proboscis, ii
Enteric canal, m Mesenteric
filaments : they are only drawn
in the anterior region, g Ex-
cretory organs, c Cloaca, u Ute-
rus (after Lacaze-Duthiers).

In the Gephyrea the three divisions
of the enteric canal are, as a rule, distinct
in the earlier stages only ; in some, how-
ever, for a longer time (Priapulus) ; while in others, as the enteric
tube elongates, the separation becomes less noticeable. The enteron
then forms a tube, which is generally a good deal longer than the
body, and which does not vary much in diameter. It is either dis-
posed in several partially-coiled longitudinal loops, when the anus
is on the dorsal surface of the animal (Sipunculus, Phascolosoma) ;
or the enteron (Fig. 72, i) passes to the posterior end of the body,
in numerous shorter coils, without forming large longitudinal loops,
and ends in the anus, which is placed there (Echiurus, Bonellia).
The latter agree with the majority of the other Vermes in the aboral

M

162

COMPAEATIYE AXATOMY.

7

position of tlie anus, wliile tlie Sipunculid^ seem to differ more
from tliem. But this position of the anus is really only a further

development of that dorsal posi-
tion which it has in many of the
Vermes, and does not in any
way affect the homology of their
enteron with that of the other
members of this group.

§ 132.

The metamerism of the body
in the Annulata affects the en-
teric tube; but there are various
other differentiations in it, which
are due to adaptations to special
modes of life. Here again the
enteric canal begins as a csecal
invagination. The aproctous
condition, which persists in most
of the Platyhelminthes,is passed
through by these forms at an
early stage in development. The
entrance to the fore-gut is most
variously differentiated in the
Hirudinea. In some the pro-
tractile oesophagus is greatly
complicated, in others its en-
trance is armed with chitinous ridges, which are the first signs of
jaws. But in most the mid-gut is beset by pouch-like diverticula
(Fig. 73), which are branched in Clepsine; the last two of these
diverticula sometimes form longer c^ecal tubes (c)
on the narrow hind-gut, which extends to the
end of the body (Clepsine, Hcumopis). These are
the only cceca of the gut of Aulacostomum. In
others the cseca are merely indicated by constric-
tions. In all cases these arrangements correspond
to the metamerism expressed also in the nerve-
chord.

In nearly all Annelids the fore-gut is separated
into several, often very different, portions. A
median portion is distinguishable by its more
powerful muscular investment, and is separated
from the mid-gut by a tract of varying length.
Among the Scoleina, this portion, which is known
as the muscular stomach, is very greatly de-
veloped (Lumbricus). It forms the end of the fore-gut. It is placed
more towards the middle of the latter in most Cha3topoda, and is

Fig. V3. En-
teric canal of
S angnisuga.
0 (Esophagus,
c Posterior pair
of ca3ca. a
Aims.

Fig. 74. Enteric canal
of Aphrodite, o An-
terior portion. 6 Middle
(muscular) poi’tion of
the fore-gut. c Branched
caecal appendages of
the mid-gut. a Anus.

Fig. 75. Maxillary
apparatus of a Euni-
cea (Lysidice). a-e
Pairs of jaws (after
Milne-Edwards) .

frequently provided with denticles, which work on one another like

ENTERIC BRANCHING OF VERMES,

163

jaws. There is sometimes only one pair of these jaws (Fig. 55^ m),
and sometimes there are several, which differ from one another in
particular characters, and form a complicated apparatus (Fig. 75).
This portion is very greatly developed in the Aphroditeidae. It
can be protruded, as in many other voracious Annelids (Phyllodoce,
Glycera, etc.), so that the anterior portion is everted and forms the
outer surface of the proboscis.^^ This protrusible portion is some-
times very long.

The whole arrangement undergoes atrophy in the Tubicolae, to
which Arenicola is an intermediate step. The third division of the
fore-gut is feebly developed in the Scoleina, and more so in the
Chastopoda, where it is often seen to be provided with a pair of
caeca (Syllis, Arenicola).

The mid-gut forms the largest, and also the most uniform, portion
of the whole enteric tube. It generally has a perfectly straight
course, and is seldom disposed in coils or loops. It is not only
attached by the muscular lamella from the body-wall, or by the
separate fibres from the edges of the metameres, but is also divided
into separate, and often diverticulate, portions, which correspond to
the metameres. In the family of the Aphroditeidaa, as in the
Hirudinea, diverticula of this kind are developed into larger appen-
dages, which may exhibit numerous branches (Fig. 74, c).

The hind-gut is generally a short portion, and is of some size in
the Tubicolae and in Arenicola only ; it seldom, has a median en-
largement, and generally extends to the anal opening from the
mid-gut, without being sharply marked off from it.

Myzostoma agrees with the Annelides in the character of its
intestinal tube. The fore-gut is represented by a long protractile
proboscis, which leads into a widened mid-gut, from which a narrower
hind-gut leads to the anus. Branched caeca are distributed through
the body from either side of the mid-gut.

Enteric Branchise.

§ 133.

The development of the respiratory function of the enteric tube
leads to special arrangements in it, which are greatly developed in
Balanoglossus. The anterior portion of its enteric tube is separated
by two lateral processes into two semi-canals, which lie one above the
other, and communicate freely by means of the longitudinal opening
which lies between the two laterally outstandingh3rocesses. The semi-
canal, which I regard as the lower one, leads directly to the commence-
ment of that portion of the enteron which functions exclusively as a
nutritive canal. The investment of cilia drives the particles of food
into it; it has a nutritive function. The semi-canal, however, which
is placed dorsally, has a respiratory function. There is a branchial
support in its walls in the form of a delicate framework of chitinous

2

164

COMPAEATIVE ANATOMY.

lamellae, covered with epithelium. There are clefts between the
branchial arches and the several lamellae which go to form them ;
these lead, on each side, to a series of spiracles, by which they open
on to the surface of the body. A network of vessels is spread over
the branchial framework. The water taken in by the mouth streams
through the superior respiratory semi-canal into this branchial
apparatus, and passes again to the exterior by the rows of spiracles.

The hind-gut of many Annelids may be seen to take in water,
and this may be connected with a respiratory function of this division
of the intestine. No special organs of respiration have been ob-
served to be developed in this portion. It is not yet certain
whether the structures which are found in the hind-gut of Neomenia
are really branchise. More exact anatomical knowledge of the pro-
tractile branchiae of Chaetoderma is necessary before we can say
what is their morphological signification.

Accessory Organs of the Alimentary Canal.

§ 134.

The enteric canal of the Vermes has glandular organs of various
kinds connected with it, which are to be regarded as differentia-
tions of the enteric wall, that is to say of the endoderm. Single
cells, or groups of cells, acquire a different character from their
neighbours, and so give rise to special organs, which are of different
degrees of individuality, according as their position is on the
enteric wall, or without it ; in the latter case they are connected
with the lumen of the intestine by ducts. They may further be
more exactly classified, according to their relation to the separate
divisions of the enteron.

In the fore-gut, close behind the muscular pharynx, small gi’oups
of unicellular glands open in the rhabdocoelous Turbellaria. In the
Trematoda similar groups of cells are placed at- the anterior end of
the body, opening near the mouth, and are regarded as pharyngeal
glands. Glandular organs have been observed in the so-called
pharynx of the Nematodes, and distinct glandular cells in the oral
region.

Among the Annulata the histology of the Hirudinea, especially,
has been carefully investigated ; in them, a large number of uni-
cellular glands open into the proboscis, when there is one, or into
the jaws, when they are present. In the Annelides a pair of lobate
glandular tubes are found in the last division of the fore-gut, just
behind its muscular portion in the Nereids and others provided with
pharyngeal jaws (cf. Fig. 55, gl) ; they are modifications of the
simpler tubes of the Syllidm. The Rotatoria are provided with
glandular appendages at the same spot. It is usual to call these
glands, which have however various kinds of functional relations,
salivary glands.

COELOM OF VERMES.

165

§ 135.

The glandular organs connected with the mid-gut are ordinarily
regarded as hepatic, or as a liver. We must be careful not to attri-
bute anything more than the value of a convenient distinction to these
names. Separate glands are almost always absent from the mid-gut
of the Vermes, but the epithelium is generally found to be different
from the epithelia of the other divisions of the enteron, so that a
secretory function is not improbable. This is indicated by the
granular character of the cells in many cases, and by the difference in
the coloration of the cell- contents. The latter point is probably more
important than the former, for the former may be due to the absorbing
function of the enteric epithelium. The mid-gut of the Bryozoa is
distinguished by this character, and even in the Rotatoria the epithelial
layer of this region may be seen to be differentiated. This character
is more highly developed in the Platyhelminthes (Planarias, many
Trematoda), in which the stomachal branches (Fig. 68) are the
chief seat of this peculiarity ; these branches, therefore, may be re-
garded as secreting appendages. Independent glands are still more
distinct in the lateral appendages of the mid-gut of Aphrodite
(Fig. 74), which are developed by the gradual narrowing and
lengthening of the simpler enteric appendages found in the allies of
this genus. Finally, we must mention here the tubular enteric
appendages of Balanoglossus, which beset the whole dorsal surface
of the enteric canal beyond the respiratory segment, and are
grouped in agreement with the segments of the body.

Coelom.

§ 136.

The Coelom of the Vermes is the earliest differentiation of a
hollow cavitary system placed between the enteric tube and the
integument, and leading to the formation of a vascular system, and
takes its origin in a cleavage of the mesoderm. The food taken in
by the enteron is no longer distributed in the organism, as it is in
the Coelenterata, by continual imbibition into the tissues from the
wall of the enteron, but the nutrient fluid is collected into a
perienteric space, where it may enter into relation with organs
differentiated from the enteric canal, and from the integument.

In a large number of Vermes this perienteric space (Coelom) is
either altogether absent, or only rudimentarily present. This is the
case in most of the Platyhelminthes and Nemathelminthes, as well
as in some others, such as Pedicellina. In the land Planarians two
cavities traversed by a reticulum of connective tissue extend along
the body ; they are largely broken up anteriorly. They are to be
regarded as indications of a coelom of this kind. The coelom is

1G6

COMPAEATIYE AXATO:\IY.

well developed in tlie Eotatoria and in most Annulata. It forms a
continuous, and generally a very wide, cavity in tHe Bryozoa and in
the Gephyrea. The coelom of the Annulata is arranged in correla-
tion with the metamerism of the body. Partitions (dissepiments)
extend from the wall of the body to the enteric tube, and form
a series of separate chambers, each of which corresponds to "one
segment of the gut, etc. ; these characters have been already
pointed out in the Nemertina. AVhen the dissepiments are
reduced to single chords the chambers are more or less com-
pletely fused. In many cases this causes the separate chambers
to disappear, either along certain tracts, which are generally placed
in the anterior region, or along the whole length of the body ; and
a single body-space is formed, which is, generally, still traversed by
remnants of the dissepiments, in the form of filaments or fibrous
bands. These fibres keep the enteron in its place, esjoecially when
it is coiled (cf. Bonellia, Fig. 72, m).

The perienteric fluid is generally quite clear, and in most
Yermes contains form- elements, of which there are sometimes a
large quantity. When there is communication between the vascular
system and the coelom, the contents of the two spaces are similar in
character. The movement of the fluid is dependent on the action
of the body-wall, so that in many, locomotion of the body produces
at the same time a circulation of the nutrient fluid : and thus is
established the lowest form of circulation.

The coelom communicates with the surrounding medium, the
water, by means of various arrangements. The excretory apparatus,
with its internal orifices (cf. § 142), is one, but special openings are
also known. For example, there is an opening of this kind in the
Bryozoa, which serves also as the passage for the generative pro-
ducts, and in the Rotatoria, where the orifice is generally drawn oat
into a tube (siphon : cf. Fig. 81, s). Similar orifices have been
observed to be present in the Aiinelides.

Vascular System,

§137.

The differentiation of hollow spaces in the mesoderm is the first
beginning of the formation of a complicated system of canals ; which
gradually acquire special walls and become blood-vessels. The
earliest main trunks form long canals, which are first visible in the
Nemertina. Two (Fig. 7G, //) of the three chief trunks take a lateral
course; the third is dorsal and median. In the cephalic region the
lateral vessel forms several coils, which surround the cerebrum, and
are connected with the dorsal vessel as well as with one another
more anteriorly. At the posterior end of the body all three vessels
are more simply connected. In some genera other vessels are con-
nected with these three ; five transverse vessels connect the dorsal

VASCULAR SYSTEM OE VERMES.

1G7

and the lateral ones at regular distances. In this way the whole
arrangement is in a way segmented^ and corresponds to the meta-
merism indicated in other organs.

The canal system which ramifies through the integument of the
Acantho cephalic and is also united with the canals of the lemnisci
(p. 174), must not be regarded as a similar system
of vessels. It is uncertain to what morphological
group of organs they belong.

§138.

The vascular system of the Annulata resembles
that of the Nemertina in all essential points. In
almost all there are longitudinal trunks, which
have a dorsal and ventral, or even a lateral,
course; these are connected with one another by
transverse anastomoses, and pass into one another
in the anterior as well as in the posterior region.

The dorsal longitudinal vessel, which runs above
the enteron, is the most constant in character ;
it is always contractile, and the current of blood
in it is driven from behind forwards. It cor-
responds to the dorsal median vessel of the Nemer-
tina, while the two lateral vessels of the latter
may correspond to the ventral vessel of the Annu-
lata. These vessels are not closed in all Annulata,
but are connected with wider spaces, which repre-
sent a body-cavity. The vascular system is not
completely differentiated in these forms. The
body-cavity remains in direct connection with
the vascular system in the Hirudinea, owing to
the fact that organs which formerly lay in the
coelom are now enclosed in heemal spaces. There
are usually three such sinuses. A median one,
representing the principal portion of the coelom, Fig. 76. Diagram of
embraces, in Clepsine and Piscicola, the alimentary the vascular system
canal and the ventral medulla, and perhaps also of tiie Nemertma^.
a portion oi the dorsal vessel, except where this trunk. il Lateral
has, as in Piscicola, a special sinus for itself, vessels. The arrows
Two pulsating lateral vessels (Pig. 61, B 1) partly the^ ^^trca^
communicate with the median sinus, and are blood. '

partly connected to each other by transverse
anastomoses. In Hirudo and its allies, the median sinus is, at first,
only present in the region of the head, where it surrounds the oeso-
phageal ring. It has only a ventral development in the rest of
the body, where it encloses the ventral medulla (Fig. 61, B n).
This disappearance of the great sinus is due to the development
of a fine vascular network in its place, and, like it, is connected
with the transverse vessels, which unite the longitudinal vessels with

1G8

COMPAEATIYE ANATOMY.

one anotlier. New longitudinal trunks are formed from tlie vessels
distributed to tlie enteron. While in these forms a complex system
is produced by the combination of the primitive median trunks with
a system of canals formed from lacunm of the coelom^ the whole
vascular system may be made simpler by the disappearance of these
median trunks. This is the case in Nephelis, where there is a wide
median sinus, and two lateral vessels.

This form of vascular apparatus, developed
out of a lacunar system, is limited to the Hiru-
dinea, for in the Annelides the vascular system
is almost always shut olf from the coelom. Where
it is not so, we have to do, not with further
development, as is the case in the differentia-
tion of the coelom of the Hirudinea, but with
degeneration.

The dorsal vessel lies, as a rule, immediately
upon the enteric canal, and often appears to be
embedded in the same glandular layer. In addi-
tion to anterior and posterior connecting vessels,
there are lateral vessels which correspond vdth
the metameres. They are divided into those which
directly surround the enteron, and form a capil-
lary network, often a highly-ramified one, in its
walls (visceral vessels), and into those which pro-
ject into the coelom, and run either along its
walls or its appendages (parietal vessels). In the
Scoleina the arrangement is generally the same
throughout the whole body. In many cases the
transverse vessels, as well as the dorsal longitu-
dinal trunks, are pulsatile, and one or more pairs

portion of the blood • t t i t t /t^- \ t •

vascular system of Considerably widened (Iig. / c). In this

Soenuris varie- differentiation of a portion of the vascular system
gata. d Dorsal ves- ggg beginning of the development of a

c Heart-like enlarge, central Organ 01 the Circulation — a heart. I he
ment of a transverse ventral vessel is very seldom contractile. Fresh
anastomosis. The complications in structure are due to the develop-
direotion of^^th^e cur^ meut of fine vascular networks, such as, in Luni-
rcnt of blood. bricus for example, are distributed as capillaries
through the body. Branchiobdella is allied in
the characters of its vascular system to the simpler conditions found
in the Scoleina.

§ 139.

Tlie develoinnent of the respiratory organs is of influence in
producing changes in the distribution and differentiation of the
blood vascular system. In the Scoleina there are no distinct organs
of this kind, and either the whole surface of the body, or, by the
introduction of water into it, the coelom, acquires a respiratory
function. Consequently we do not see any great variations of the

YASCULAE SYSTEM OF VEEMES.

169

vascular system in various portions of tlie body^ and it is only in
some wbicli live in the mud of fresh water, and in which the hinder
part of the body takes a special share in respiration, that the parietal
vascular coils present any very great development (Lumbriculus) .

Among the Chaetopoda also these simpler relations obtain, but
the greater differentiation
of the head, and of the
fore-gut, is followed by
some changes in the
vascular system. When
branchiae are present the
parietal vascular system
is continued into them ;
in the simplest case a loop
of the vessel passes into
the appendage, which has
the function of a gill.

Here we have the com-
mencement of the gradual
separation of an arterial
and a venous portion.

This arrangement is re-
peated when the branchiae are distributed over a large number of
metameres, as in Eunice and Arenicola. The dorsal trunk gives
off, in addition to the vessels for the enter on, others which pass
to the laterally-placed branch!^ ; from each of these a vessel passes
to the ventral trunk (Fig. 78). The same characters obtain in the
Hermellidae, where the branchiae have but a single central cavity,
and where, therefore, there can be no anatomical separation of the
efferent and afferent blood. In Arenicola these characters obtain in
the hinder half only of the body. In the anterior half one branchial
vessel passes to the chief ventral trunk, and the other to a visceral
ventral vessel.

When the respiratory appendages are limited to a smaller portion
of the body, as is the case in the Tubicolse, there is a corresponding
increase in the difference between the development of various
vascular 'regions. Thus in the Terebellidge (Fig. 79) the dorsal
vessel (vd), above the muscular pharynx, is widened out into a large
tube, which supplies the branchige ipr), and so functions as a
branchial heart. Afferent vessels pass from the branchiae to
the ventral vessel. In many the function of a central organ is taken
on by transverse anastomoses (Scoleina). A vessel of this kind,
which passes from the ventral to the dorsal vessel, is found even in
the Terebellidae, where it functions as a part of the cardiac division
of the dorsal vessel. In Arenicola this vessel is continuous with two
very wide transverse vessels, which pass to the ventral trunk.

The arrangement of blood-vessels is constant enough when they
are not richly distributed ; but this is not the case in those divisions
in which the enteric-wall and the body-wall are largely provided with

Fig. 78. Diagrammatic transverse section through,
the hinder half of the body of Arenicola, to show
the arrangement of the vessels. D Dorsal; V
Ventral side, n Ventral medulla. i Enteric
cavity, hr Branchiae, v Ventral vascular trunk.
ah Branchial vessels, d Dorsal vascular trunk.
h Branch surrounding the enteric canal, v ‘ Visceral
ventral vessel.

170

co:mpaeative axatomy.

vascular ramifications. A breaking-up such as affects the parietal
transverse anastomoses in the gills, may affect also the longitudinal

trunks, which then take
the form of a continuous
network of vessels, from
which new passages are
developed. The j^hteno-
inena, which give rise to
a collateral circulation,
must also furnish us with
the explanation of these
relations. Thus in Poly-
ophthalmus the median
trunk is broken up along
the course of the mid-gut.
Two dorsal and two ven-

tral trunks arise from
the anterior and posterior
simple median vessels
in the Hermellidse ; in
Eunice the ventral, and
in Nephthys the dorsal,
pair are present.

In some the vascular
system is atrophied (Gly-
cera, Capitella).

A combination of the
types of the vascular sys-
tem, which obtain in the
Annelides and the Ke-
mertines, can be made
out in Balanoglossus.
This consists in the pre-
sence of both median
and lateral longitudinal
trunks, but their visceral
branches partly, function
as branchial vessels, and
so give rise to an arrangement which is very different from that of
the majority of Vermes.

Fig. 79. Vasculax' system of Tei’ebella nebxx-
losa ; opened from tbe dorsal surface, t Tentacles
(not fully drawn), hr Three pairs of branchiae.
ph Muscular portion of the foi’e-gut (pharynx).
V Enteron. vd Doi'sal vessel, vv Ventral vessel
(after Milne-Edwards).

§ 140.

The vascular system of the Gephyrea presents characters which,
not only in their relations to the circulatory system of other Vermes,
but even when wo compare their various conditions with ono
another, are by no means easy to understand ; at the same time
there are many important lacuna) in our knowledge of the ana-
tomical facts. The most important of these is tlie question whether
there is any connection between the cavities of the vascular system

VASCULAE SYSTEM OF VEEIMES.

171

and the coelom ; the only character whicli points to the existence of
one is the nature of the perienteric fluid.

The essential arrangement of the course of the circulation is deter-
mined by the presence of two longitudinal trunks, which correspond
to the principal trunks already noted in the Annelides. The ventral
one runs along the wall of the body, while the dorsal one is attached
to the enteric canal, and accompanies it along its coils and loops.
The blood-current has the same direction as in the dorsal and
ventral vessels of the Annelides.

The two vessels are at their simplest in the young stages of the
Sipunculidm. They appear to be connected with one another
around the mouth, where they also communicate with the cavities of
the tentacles. At the posterior end of the body a number of actively
contractile ceeca are connected with the dorsal vessel. In Sternaspis
these c^ca have a special significance, for they are divided into two
tufts which project to the exterior and serve as branchiae. In the
Sipunculidae similar, but internal, appendages are distributed along
the whole dorsal vessel. The dorsal vessel is coiled in Sternaspis,
Bonellia, and Echiurus. Where the tentacles are absent it is
continued into the ventral vessel by a vascular loop which surrounds
the mouth, and which is sometimes broken up into finer vessels.
Owing to the formation in the Bonelliae of a large proboscis out of
the .greatly elongated upper lip, the anterior portion of the vascular
system is very much increased in length. The dorsal vessel is con-
tinued to the end of the proboscis, and divided into two branches,
which pass along its edges and again meet in the body below the
mouth. In Echiurus this arrangement is wanting, as is the pro-
boscis. The ventral vessel formed from the union of the two
vascular loops in Echiurus and Sternaspis, passes backwards, giving
off numerous lateral branches as it does so. In Bonellia it divides
shortly after it becomes a single vessel behind the mouth ; but later
on it again becomes single. It gives off visceral vessels in Echiurus
and in Bonellia ; these, of which there is a large number in Echiurus,
run in the mesentery. In Echiurus the most anterior of these
vessels forms a considerable enlargement on the enteron, whence a
ventral enteric vessel, and two anastomoses, which embrace the
enteric tube, are given off to the dorsal vessel. This is clearly a mode
of connection of the dorsal and ventral vessels similar to that which
is repeated several times in the segments of an Annelid. In these
forms the arrangement is limited to, or greatly developed at, one
point. The variations from the Annelid type are caused by the
distance of the alimentary canal from the ventral median line, in
consequence of which the anastomosis is not paired, but arises as a
single vessel from the ventral trunk. In Bonellia further changes
are noticeable. The transverse anastomosis to the dorsal trunk
running along the intestine is developed, on either side, into a
large tube, from which the dorsal vessel appears to arise anteriorly,
since its posterior portion is either wanting, or has become very
small in comparison with the widened anterior part. In this

172

COMPAEATIVE AXATOMY.

character also distant relations to what obtains in the Annelides are
expressed. The most important difference consists in the limitation
of the transverse anastomoses, which surround the intestines, to one
only, which is consequently very much modified. Here, then, again
the characters exhibited are in correspondence with a rudimentary
metamerism. More or less extended portions of the vessels serve as
organs for the movement of the blood, and are very different in
various genera.

§ 141.

The nutritive fluid forms the contents of the coelom, as well as
of the vascular system; its form-elements are generally slightly
differentiated cells. When the vascular system is marked off from
the coelom its contents are known as blood. The form-elements in
many Annelids are colourless. In many Nemertina the blood-cells
have a red colour (Borlasia) ; the fluid is coloured in many Annelids,
and this colour is occasionally green, but more frequently red. In
several cases the form-elements contain the colouring matter. The
plasma also may be definitely coloured, as, for example, in the
Lumbricidm. When the vascular system is separate, the contents
of the body-cavity are generally indifferent'; we then find a peri-
visceral fluid (known also as chyle) in addition to the blood ; this is
always uncoloured. When the vascular system is atrophied, the
fluid filling the coelom is frequently of a red colour (Glycerido3), like
the blood of other forms.

Excretory Organs.

§ 142.

It is quite uncertain what is the functional significance of a very
large number of organs comprised under this head ; of others, how-
ever, it is certain that their secretion is essentially similar to that of
the renal organs of higher animals. But they all have a number
of common relations to the organism, which are of weight, although
the organs are so differently connected that it is impossible to
completely prove their homology.

Ill its more developed forms the excretory apparatus is a system
oE simple or branched canals, which opens to the exterior on the
surface of the body, and is provided Avith internal orifices Avhen a
caflom is distinctly differentiated ; Avhen it is not, the ends of the
tubes, or the finest branches of the canals, are closed. When the
body is not segmented there is a single pair of these organs ; when
metameres are formed this system is segmented. The indifferent
condition of the excretory organs is represented by a pair of
ca3cal tubes, differentiated from the integument, and therefore
derived from the ectoderm. Organs of this kind, which open behind
the head, are knoAvn in the Nemertina, but the canal system said to

EXCEETOEY OEGAXS OF VEEMES.

173

be continued from them requires to be more closely examined. The
relations of tbe canals, which are known as the water vascular
system, are more exactly known in most Platyhelminthcs. Tliey
have not been observed in the land Planarians. In the Trematoda,
and many Turbellaria, two ' excretory canals, one on each side,
ramify in the body ; the chief trunks giving off fine branches
which traverse its parenchyma (Fig. 80, A B). Long cilia are

Fig. 80. Diagram of the excretory organs of Platyhelminthes, showing the
different forms, which may be derived from one another.

distributed on the walls of the fine canals. The principal trunks,
which are as a rule somewhat enlarged, still open in many
forms in the anterior region of the body (Fig. 80, A) (Tristoma
papillosum). The orifice (porus excretorius) is most commonly
placed towards the posterior region (D), where the two vascular
trunks approximate and unite at a common orifice. In this way a
terminal tract, common to both canals, is formed. This tract, which
is generally enlarged, has the form of a contractile vesicle {E).
Vesicles of this kind may be formed even on trunks which open
separately. They form a third division of the apparatus.

In the Cestoda, the fusion of the excretory canals into a single
porus excretorius, placed at the end of the scolex-body, which in the
other Platyhelminthes is only an acquired character, has apparently
become typical. A contractile vesicle generally forms the point of
union. There are generally a larger number of principal trunks — that
is four, six, or eight — which either unite with one another by loops
in the head, or merely curve round it, and passing backwards branch
again ; in this case their more special characters are similar to what
are found in other Platyhelminthes. When the scolex-form is
divided into metameres, the terminal portion of this canal system is
apportioned to the oldest proglottid, and the succeeding proglottides
contain portions of the canals. At the termination of the metameres
the longitudinal trunks are, in many forms, united by a circular
canal. As the proglottides break off, a new porus excretorius must
be formed every time.

That part of these organs, which is formed of the finest canals,
contains a clear fluid only. In the Tape-Worms, on the other
hand, there are calcareous concretions at the enlarged points, which

174

compaeati\t: axatomy.

appear to be excretory products. In tbe Trematoda these con-
cretions are collected into tbe large trunks, wbicb contract and
drive tliem into the terminal vesicle, whence they are evacuated by
the porus excretorius.

An anastomosis of the finest ramifications of the canals may be
often made out in the Cestoda, as well as in the Trematoda (Distoma
dimorphum), and may even affect the larger vessels, which either
become connected in a simple manner (into a ring in Distoma
rhachiaeum, and into regularly succeeding transverse canals in many
Cestoda) ; or become converted into a rich meshwork, in which the
chief trunks disappear.

The excretory organs in the Nemathelminthes, which again may
be derived from a cmcal tube, are simpler in character. They form
tubes or canals, which pass along the body embedded in the lateral
area) (Fig. Gl, A ?•). Near the fore-gut the canals of either side bend
towards one another, and unite into a common portion of varying
length, which opens to the exterior by a pore in the ventral line.
Sometimes these canals are coiled, and they vary greatly in the
way in which they are connected with the pore. In the Gordiacea

this apparatus appears to be rudimen-
tary, for in Mermis it is simply repre-
sented by a row of cells, and Gordius,
in which there are no lateral tracts, has
no distinct organ of this kind.

It is doubtful whether the organs
found in the anterior region of the body
in the Acanthocephali, which are known
as ‘‘ lemnisci,^^ belong to the excretory
system or not. They form two longisli
lamella), without a lumen, which are
processes of the body-wall, and like it
arc provided with branches of cauals,
between which dark granular masses
occur.

§ 14:J.

When a coelom is formed, the cha-
racters of the excretory organs are so
far altered that the canals communi-
cate with it by internal ciliated orifices.
This new condition must be regarded
as nothing more than a modification of
the blindly-ending canal system, as it
has been already described in the Pla-
tyhelminthes. Internal openings have
been observed in the larva) of Trema-
toda. They characterise also the ex-
cretory canal system of the Kotatoria, which is disposed in just the
same way as in the Trematoda. The canal system, which lies in the

Fi". 81. Organi.sation of a
B r ac h i on u g. a Ciliated cephalic
disc, s Siiihon. in Masticatory
organs, e Glandular layer in the
stomach. o Ovary, u Uterus,
containing an egg. o' Kggs,
attached to the root of the tail.
c Excretoiy canals, v Contractile
terminal vesicle.

EXCEETOEY OEGAXS OF VEEMES.

175

coelom^ or projects into it from the wall of the body, is composed of
two trunks (Fig. 81, c), which in many forms open by lateral branches
into the coelom (species of Notommata). The two principal canals,
which are often much coiled, either unite at the cloaca, and open by
it to the exterior, or they pass into a contractile vesicle (^’), which
must be regarded as a differentiation of the common terminal portion
of the two canals. The internal openings, as well as the lumen of
the two chief trunks, are here and there provided with flagelliform
cilia, which produce a trembling movement. The walls themselves
may be seen to be glandular in character, either along the whole
length of the canal, or in certain portions only of it. In this last
point a further development of the simpler characters found in
the Platyhelminthes ought very probably to be recognised, which
points to a closer relationship with the Annelida.

Echinoderes also possesses two coiled excretory tubes, but they
appear to open separately in the anterior region of the body.

§ 144.

In the Gephyrea we have to distinguish two kinds of excretory
organs.

The first of these is an indication of the connection between the
Gephyrea, and lower stages, for it is
correlated with the undeveloped — or if
developed, developed only externally —
metamerism of. the body. These organs
are formed by tubes which open into
the termination of the enteric canal
(Fig. 72, g), and are provided — at any
rate in the cases where they are most
exactly known (Bonellia) — with nu-
merous ciliated funnels, which open
into the coelom (Fig. 82, u). In other
cases the branches with internal open-
ings seem to be wanting (Echiurus),
and in others, again, the degeneration
is complete. As similar characters may
be seen in the Echinoderma, the form
of excretory organ, which obtains in
the Gephyrea, appears to be common
to a large circle, and referable to a
single typical form, whence it has been
transmitted to the Echinoderma, as well I’ortion of a branch of

as to^ the Gephyrea. Variety^ m the yii^dis. a cniated openings
function of this organ may be inferred (after Lacaze-Duthiers).

from its structure. The excretory

function is certain only in Bonellia, where the walls of its branches
have a glandular character.

The other form consists of paired tubes, oiDening on the ventral

176

co:MPAEATm: axatomy.

surface, which may be derived from the paired organs of similar
form in the Annelides. There is either a single pair (Sipunculus),
or a few pairs (Thalassema, Stemaspis, Echiurus), in correspon-
dence with the slight development of metamerism. Internal open-
ings into the body-cavity lie close to the insertion of the tubes
into the body-wall, and in several are of service in the generative
function, acting as the ducts for the generative products. The
greater part of the tube, that is the caecal portion, which lies
behind the internal orifice, appears to have an excretory function in
the Sipunculidae, and as a rule, is distinguished by its brown colour. In
others the whole tube serves merely as a duct for the generative
system. In most Gephyrea these organs have each and all similar
functions, but in some cases there is a division of labour (Sternaspis),
the posterior pair of tubes being in connection with the generative,
and the anterior with the excretory, function ; so that the difference
of function which appears in different genera, is here expressed in
the same individual.

145.

So far as the structure of the excretory organs is concerned, few
fresh characters appear in the Annulata. The organs correspond
to the metamerism of the body, for they are regularly distributed on
cither side of almost every one of its segments. They have therefore,
though with but little reason, been called “segmental organs a
name which is just as suitable for many other organs. Each of
them consists of a closely-coiled or loop-like canal (looped canals),

which has an internal opening, often peculiar
in form and always ciliated, and which at
the other end opens on to the surface of
the body. This canal is sometimes similar
in character throughout its whole length,
or but slightly differentiated ; frequently
several segments may be made out in it,
which generally correspond with those
already described in the Platyhelminthes
and Eotatoria. The innermost portion,
on which the opening into the coelom is
placed, is ordinarily the longest ; it is dis-
tinguished by its funnel-like or rosette-
shaped orifice (Fig. 83). In the next por-
tion the walls may be seen to be glandular
in structure. The last portion, which is
frequently widened, is provided with a
layer of muscle ; it almost always opens
on to the ventral surface. These organs
are no more purely excretory in function than they are in other
Vermes, for we not unfrequently find them entrusted with other
functions.

Fij?. 83. Internal opening of
a looped canal of Bra n-
chiobdolla.

EXCEETOEY OEGAXS OF VEEMES.

177

In tlie Hirudinea these organs are preceded^ in the embryonic
stage^ by three pairs of looped canals^ which are not connected witli
those formed at a later stage; they are found in the posterior half of
the ventral surface. In structure they are similar to, but simpler than
the permanent canals, and disappear after these are developed. This
most important fact shows that the looped canals of the Annulata
cannot be regarded as the direct homologues of the excretory organs
of the lower Vermes ; at the same time arises the question : Are the
looped canals of those Annulata, which show no signs of primitive
organs of this kind, comparable to the permanent looped canals of
the Hirudinea, or only to the primitive ones ?

In their more special characters there is great variety among the
Hirudinea, the canals in one division having no inner opening.
Instead of this they begin with a closed portion, of the form of a
loop, which consists of numerous canals, united with one another into
a labyrinth (Hirudo). From these looped organs a single canal is
given off, which opens by a vesicular enlargement on the surface of the
body (v. supra. Fig. 01, B). In others (Clepsine, Nephelis) the
labyrinthine portion is present, but it has an internal opening’, which
projects into the lateral blood sinuses of the body.

The division of the Limicolge is remarkable among the Scoleina,
because of the two different conditions of its looped canals. Usually
the canal is much-coiled, and the greater part of it is embedded
in a common mass of cells, and has much the same calibre throughout
its course. The canals always break through the dissepiment in front
of them by the end which carries the internal orifice, so that each pair
of looped canals is connected with two segments of the body. The
portion which leads to the exterior lies in one, the internal orifice in the
other. This form, which is distributed in much the same way through
the greater number of the segments, is wanting in the tracts
occupied by the generative system. In place of the simple looped
canals we there find organs of greater complexity, and proportionately
much more developed ; in structure they resemble the others, but
they function as organs for the excretion of the sperm ; looped
canals are converted into seminal ducts. The same thing happens
in Branchiobdella.

In the Lumbricidse there are none of these physiological changes.
But the apparatus is greatly complicated by the marked development
of its separate divisions, as well as by the arrangement of the loops.
Each canal forms several ascending and descending loops, closely
connected with one another, and surrounded by a rich network of
blood-vessels. The different portions have different significations.
The innermost portion carries the funnel-like opening (Fig. 84, a),
and is followed by a part {h h h) which has transparent walls, and is
provided with cilia at various points. After forming several loo]3s,
this portion passes, by a change in its walls, into another portion,
the lumen of which is widened (d), and surrounded by cells with
finely granular contents. This portion also has a looped course {d'),
and passes into a wider part, which is provided with muscular

N

178

COMPAEATIVE AXATOMY.

walls (e) ; this, after a simple course, passes to the body- wall (c '),
where it finds its opening.

In the Chtetopoda simple forms of looped canals are most

common, the separate canals in them
forming sometimes coiled bodies,
and sometimes presenting a very
few coils. The funnel-like internal
opening, which has been recognised
in many, has in some (Alciopa) just
the same relations to the septa of
the coelom as it has in the Scoleina.
In many of them the relation to the
generative system can be similarly
recognised.

In addition to the more secon-
dary relations which the looped
canals of the Annelides have to the
generative system, either at certain
points only, or for a greater dis-
tance, their relation to the excre-
tion, as well as to the introduction or
expulsion, of water must be borne in
mind. That these organs have a close
relation to the function of excretion
is shown by the glandular investment
of their walls, and the glands which
open directly into them. They, in
fact, resemble the chief trunks of the
excretory organs of the Trematoda.
A relation between the perienteric
fluid and the surrounding medium,
either by the outflow of the former,
or the entrance of the latter, is
rendered possible by the internal
opening of the looped canal. From
the direction of the ciliary move-
ment in the canals, and at their
internal orifices, which is in nearly
all cases towards the exterior, it is probable that solids also may
bo moved in this direction. Further investigation, however, is
necessary in order to confirm this supposition.

Fig. 8t. A looped canal of Liim-
bricus — not too highly magnified.
a, Internal opening. hhh Clear
portion of the canal, arranged in two
double loops, cc NaiTOwer portion
with glandular walls, d Widened
portion, which becomes narrower at
d', and at d" is continued into the
muscular portion, e. e' External
opening.

Generative Organs.

§ 11(3.

AVe meet with a larger number of intermediate steps in the
sexual differentiation of the Vermes than in that of any other
division. The lowest stages are hermaphrodite, but this arrange-

GENEEATIVE OEGANS OF VEEMES.

179

ment is not unfrequently connected with great complications in
comparison with which the arrangements seen in dioecious Vermes
are very simple.

The simplest state is seen in the Bryozoa, the generative pro-
ducts of which are developed either on the inner face of the body-
wall from simple aggregations of cells^ which give rise to seminal
elements or to ova ; or they arise on a chord which extends from the
enteric canal to the inner wall of the body (funiculus) (Fig. 71^ n).
The mature generative products pass into the coelom, and are
thence passed out into the surrounding water by the orifice of
communication mentioned above. The two sexes are ordinarily
united in the same individual^ the male and female germinal glands
being separate.

In all phylactoloematous fresh-water Bryozoa, special bodies,
(statoblasts) formed of an aggregation of cells, are developed in the
body-wall, at the points where the ova are formed j these break off,
just like the ova, and form free-living buds. Various differentiations
give rise to complicated shell-structures around them.

§ 147.

Hermaphroditism obtains also in the Platyhelmiuthes generally
(Turbellaria, Trematoda, Cestoda). The two groups of sexual
organs are, as a rule, united at a common orifice, being other-
wise separated from one another, and embedded in the paren-
chyma of the body. The secreting glands (testis and ovary) are
generally small and simple in character. The excretory duct
and the glandular organs connected with them, as well as the
diverticula, or pouch-like appendages of the former, which act as
places for the development of the fertilised ovum, or as receptacles
for the semen, take by far the greatest share in the complication of
the apparatus.

As to the male organs, the testes vary in number, and are
generally indistinctly marked-off spots for the formation of the
semen, which reaches the common duct by narrow seminal ducts.
A widened portion of the former has the function of a seminal
vesicle, and its end is converted into a protractile organ, which
serves as a penis.

The ovary forms the most important part of the female organs.
An organ, generally widely branched, is connected with its ducts — the
yelk -gland : in the lobules of this gland cells are produced. The cells
of this gland are used in building up the embryo, a number of them
together with the egg- cell forming the egg. The origin of the yelk-
glahd is probably to be found in the division of labour of a primi-
tively very large ovary, a portion only of which has continued as
ovary, while the cells of the other parts have ceased to be ovarian
germs, but becoming surrounded by the products of the fission of
the egg-cells, are taken into the future body of the embryo. The
oviducts and the ducts of the yelk-gland unite in a canal of varying
length, which, according to the number of eggs to be developed, is

N 2

180

CO:\IPAEATIVE AXATO:\IY.

either extraordinarily long, or short and simple, or provided with
diverticula. These cavities are known as uteri ; for in them the egg
is not only enclosed in its shell, but as a rule passes through the
early stages of embryonic development. A diverticulum of the
female excretory duct, which has generally the form of a stalked
vesicle, receives the sperm'‘during copulation. In some cases there
is a second diverticulum also ; it serves apparently for the reception
of the male organ (Bursa copulatrix).

The most important complications of this system are seen in the
parasitic Platyhelminthes. The preservation of the species is here
subject to innumerable difficulties, owing to the animal living in
different hosts at different stages of development, and to the
wanderings which this mode of life entails ; consequently a large
number of ova have to be produced, and the certainty of fecundation
insured.

§ 148.

The more special characters of this generative system exhibit
extraordinary variation. The male portion
has, in most Turbellaria rhabdocoela, the
form of two elongated testicular tubes, from
each of vdiich a vas deferens is given off
(Fig. 85, t). In the Trematoda, also, the
testicles are, as a rule, but few, and rounded
or lobate ; these are represented in the Tur-
bellaria dendrocoola, as well as in several
rhabdocoela (Macrostoma), and Cestoda by
a number, and often a very large number,
of small follicles, scattered in the parenchyma
of the body (Fig. 86, f) ; these are connected
together by long efferent ducts. They may
form a single row on either side (land Pla-
narians). The excretory ducts either form a
common vas deferens, or each passes sepa-
rately to a terminal portion, which is con-
tinued into tlie copulatory organ. The
common excretory duct forms the seminal
vesicle, or, as happens in a few cases, it is
formed by enlargements of the separate vasa
deferentia. The copulatory organ (Fig. 85,

; Fig’. 87, p’) is generally large and mus-
cular, and the seminal vesicle often appears
to bo an appendage of it. It lies in a special
cavity leading to the genital pore (penial
sheath in Planaria, bag of the cirrus in
Cestoda [Fig. 86, c7] and Trematoda) ; glands
are sometimes connected with it (Planaria).
The copulatory organ is, as a rule, protractile, or can be everted,
whereupon certain spines or hooks which lie on its inner surface.

Fig. 85. Generative
system of Vortex
viridis. t i Testes, vd
Vasa deferentia. vs Se-
minal vesicle, p Pro-
tractile organ of copula-
tion. 00 Ovaries. gv
Yolk-glands. rs Rccep-
taculum seminis. v Va-
gina. u Uterus (after
M. S chill tzo).

GENERATIVE ORGANS OF VERMES.

181

in its retracted state come to lie on its outer surface. Most Platy-
lielminthes, except the PlanarieC; Rave tlie penis thus armed; it
appears to be connected
witR a more intimate copu-
lation.

§ 1^49.

There are greater varia-
tions in the female appa-
ratus. The ovaries are, as
a rule, one or two elongated
tubes of no great size (Fig.-
85, 0 ; 87, ov)j in which the
ovarian germs are formed.

Where a single oviduct is
present it becomes con-
nected with accessory parts
as it passes to the genera-

tive pore, and varies in
Several such may
and form a

length.

unite

together

common oviduct. In most
Rhabdocoela, as in the Ces-
toda (Fig. 87, od) and Tre-
matoda, the duct is single,
though the ovaries are
double. It is shortest in

Fig. 86. Male apparatus, with parts of the
female, of Bothryacephalus latus (after
Landois and Sommer), a Testicular follicles :
part only are represented, ve Their excretory
ducts, vd Vas deferens, c Cirrus, cl Bag of
cirrus. Other letters as in Fig. 87.

the Rhabdocoela, where, as in most of the Cestoda, it has an enlarged
portion, which is clearly a receptaculum seminis. This organ
appears as a unilateral diverticulum of the oviduct, and gradually
becomes distinct. It is still more well-marked when it is attached

to the base, or along the course of the oviduct (Fig. 85, r s), in the
form of a stalked appendage. The Planarians have a double ovi-
duct ; as a rule, a short portion only is common to both ducts, and
functions as uterus or vagina. The oviducts are of some length in
the land Planarians, the ovaries of which lie in the most anterior
parts of the body. They may be provided with short lateral branches
along their course, which open into lacunar spaces of the coelom
(Bipalium). This peculiar character raises the question as to
whether these ciliated oviducts are parts of another system of
organs ; for there is no reason for supposing that ovarian tubes
have degenerated so as to form these back wardly- directed lateral
branches. Their open mouths forbid us to suppose that there
has been any such process. These mouths, indeed, point to an excre-
tory organ having here entered into the service of the generative
function.

When yelk -glands are connected with the ovary they appear
as two or more arborescent, ramified, or lobate organs (Fig. 78, gv)j
and are often widely spread out through the parenchyma of the

182

Co:\IPAliATiyE AXATOMY.

body (Fig. 87, d). Their excretory ducts come together from all
sides and form with the oviduct a single common portion ((/').

Special portions of the oviduct function as a Uterus, by which
name parts, very different morphologically, are known. In general

three different kinds of such
uterine organs can be made
out in connection with the
oviduct. In the first the
oviduct itself is used for this
purpose ; and then it is not
only widened, but also greatly
elongated, so that it has the
form of a coiled tube, which
traverses the body several
times. This arrangement is
found in the Trematoda, and
alsoamongtheCestoda (Trim-
nophorus, Ligula, Bothryo-
cephalus) (Fig. 87, n). A
second form is represented
by lateral diverticula, or
pouch-like appendages on
the course of the oviduct ;
this is found in a few Ehab-
docoola and, in a more com-
plicated form, in most Tape-
Worms. In the Tmniadm a
tube passes from the duct,
near the opening of the
yelk-gland, along the middle
line of a sexually-mature proglottis, and forms, according to the
cpiantity of ova in it, a number of arborescent branches. Finally,
a third kind is formed by appendages, which are found only on the
end of the oviduct, or rather in the vestibule common to the organs
of both sexes, and close to the genital pore. This occurs in most
Turbellaria (Fig. 85, ?f) ; in the Bhabdocoela there are, as a rule,
two such uterine pouches, which are considerably distended, and
which may be branched, if they have to serve for the reception of
a large number of ova. In the Dendrococla there is either only one
such uterus, opening into the vestibule, which in them is greatly
distended ; or it is altogether wanting, when the two oviducts take
on its function (Leptoplana). '^Phe size and the number of the ova,
which become mature and get their envelope at one time, is always
in close connection with the condition of the organ which acts as
uterus.

A terminal portion of the oviduct is likewise frequently differ-
entiated into a special canal, known as the vagina ; in some
cases this is further provided with an appendage which has the
function of a bursa copulatrix.^^

Fig. 87. Generative organs of B o t h r y o
ceplialus latus (after Landois and Sommer).
Female portion of the system, v Vaginal canal.
v' Its mouth, u Uterus (with ova), n' Its
mouth, ov Ovar}'. od Oviduct, gl Shell-
glands. d Yelk-glands (part only is shown).
d' Duct of yelk-gland, e Vascular trunks.

GE^IEEATIVE OEGANS OF VEEMES.

183

A large number of unicellular glands are attached to the point
where the ducts of the yelk-gland and the oviduct unite, in Trema-
toda (Distoma, Polystomum, Amphistoma) and Cestoda (Bothryo-
cephalus, Tsenia). This group of glands is known as the shell-gland,
and its secretion serves to form the investment of the ova (Fig. 87, gl).
In addition there is in the Bothryocephali and many Trematoda a
special canal at this point, which opens either into the sinus genitalis,
as in the former, or on the dorsal surface of the body, as in Distoma
hepaticum : it evidently functions as a vagina, as it has been found
full of sperm (Fig. 87, v). This second communication between the
female organs and the exterior makes it possible for impregnation
to take place without interfering with the gradual evacuation and
deposition of the ova. In this double opening of the female organs
we may perceive an indication of the primitive double nature of
the whole system.

The influence of a change in the external conditions of life upon
the genital apparatus in the case of Polystomum (P. integerrimum)
furnishes an instructive example of the capacity for adaptation of
an organ in full function, and therefore presumably mature.
The change is one of abode, and results in an increased pro-
duction of generative matters, and the correlative appearance of new
parts in the generative apparatus.

§ 150.

We know as yet very little as to the way in which the herma-
phrodite apparatus acts in copulation. In many cases there are
arrangements favourable to self-impregnation.

The genital pores differ in position in the various divisions of
the Platyhelminthes. In most cases the generative organs open in
the ventral median line, sometimes very far forwards, just behind
the oral sucker, as in many Trematoda (Distoma, Gyrodactylus, etc.),
and sometimes nearer the hinder end of the body (Turbellaria), or
even quite at that end (Distoma macrostomum). Among the Cestoda
also the ventral position is not uncommon (Ligula, Bothryo-
cephalus) ; in most cases the genital pore is a flattened depression
on the lateral edge of the proglottides, placed alternately on one
and the other lateral edge. The fact, that in some Cestoda (TaBuia
elliptica, T. cucumerina) there are two symmetrically-disposed gene-
rative systems in each proglottid, is important as bearing on the
meaning of this want of symmetry, which obtains even in some Trema-
toda (Tristoma). The unilateral condition may be regarded as the
remains of a primitively double arrangement; it was, then, only
gradually that the system of one side got to predominate over the
other, and led to that relation of parts which is now the most widely
distributed — the unilateral- development of the generative system.

There is, with but few exceptions, only a single genital pore
in the rhabdocoelous Turbellaria, the male and female organs
both leading to it. In the dendrocoelous forms the orifices become

ISi

CO:\IPAEATIYE AXAT03IY.

separated, owing to tlie development of a vestibule. In most
of the marine Planaria they are completely separated, and there
is a double genital pore, the male one lying in front of the female.
In most of the Trematoda, too, the openings of the genital organs
are distinct, although placed close to one another. The same
arrangement is seen in the Cestoda. Even in those cases where the
bag of the cirrus and the vagina open into a genital pore, this latter is
only a flat pit, walled in by the integument. In other cases the two
pores open directly on to the surface, though close together. Or
wo have the noteworthy provision of a second female opening, with a
vaginal duct, as already described. Finally, the two apertures may be
still more separated, the male organ opening on the lateral edge, and
the female on the surface of the proglottid.

The two kinds of organs are sometimes unequally developed in one
and the same individual ; in the Rhabdocoela especially the sexes are
separated, the two organs being unequal in different individuals; in
one the female, and in another the male organs are most developed,
while the organs of the other sex remain rudimentary (Convoluta).
These important examples show us how, by the continued atrophy of
one organ, dioecious forms are derived from hermaphrodite organisms,
d’he process here observed in statu nascenti is complete in other

Turbellaria. The Microstomese have the
sexes separate, as have also some Planarim
and Trematoda. The generative system
is simplified in the Nemertina, which are
almost always dioecious. The various
divisions of the excretory ducts and of
the accessory organs are absent. The
testes and ovaries are the only parts which
are distinctly recognisable. In some (Pro-
rhynchus) these organs occur singly in each
individual (Fig. 67, or), and so call to mind
the rhabdocoelous Turbellaria. In others,
however, there is a large number of fol-
licles on either side of the enteric canal ;
they have no direct connection with one
another, and being arranged regularly in
pairs along the body are evidence of meta-
merism.

§ 151.

In the Nematodes hermaphroditism is
a rare exception. As a rule the sexes are
Fig. 88. Female generative separate. Both kinds of organs consist of

coicles. orOvarie.s. do Ovi. embedded m the coelom and open-

duct. K uterus, r Vagina. ing on to the surface. The organs of the

female s}’stem are very generally paired.
This is less common in the case of the male organs. The double
opening observed in some few cases also speaks to the primitively

GENERATIVE ORGANS OF VERMES.

185

coiled^ according to their length. The terminal

I

f

portion is to be regarded as an ovary (Fig. 88, or),
from which a portion which is ordinarily wider leads
into a canal known as the uterus (^t), and this opens
by a narrow vagina. The female generative pore
is always ventral in position, in front of the anus,
and generally about the middle of the long axis of
the body. The increase in number of the female
generative tubes to five, with the atrophy of one
of the two primitive ones, gives rise to changes in
the form of the apparatus : this is more marked
when particular regions are specially differentiated.

The same thing happens to the male organs. In
some cases the terminal portion of the ovary func-
tions as a yelk-gland (Leptodera).

Mermis, at any rate, among the Gordiacea, re-
sembles the other Nematodes in the characters of
its generative organs. In Gordius the excretory
ducts of the paired germ- glands are, in both sexes,
united with the hind-gut. This happens in male
Nematodes only.

In the Chsetognathi (Sagitta) the arrangement
is somewhat different ; owing to their hermaphro-
ditism, and the way in which the organs are dis-
posed, we cannot compare them very closely with other divisions.
The male and female generative glands lie at the sides of the pos-
terior ends of the body ; the testes lie behind the ovaries, and

double character of these parts. But this would also be the result if
the two tubes were separated from one another in adaptation to the
elongated form of the body. The blind terminal
portion of the generative tubes serves as an ovary
or testis, the remaining portion as an efferent organ,
adapted to different arrangements in different parts,
and variously differentiated.

The male generative tube is a simple one, open-
ing on the ventral face of the hind-gut ; in the larger
species it is several times coiled. It is only by its
epithelial investment that the terminal portion,
which is generally of some length, is distinguished
from the excretory duct, and seen to be a testis ; a
widened portion, or seminal vesicle of the ductus
ejaculatorius, is sometimes added to the duct. Two
fine, and sometimes very long, chitinous rods
(spicula) are developed in the cloacal portion of the
hind-gut, and serve as copulatory organs.

The female generative tubes are, as a rule,
double, and are either separate, except at their
orifice, or have their terminal portions united into
a common tract. The tubes are more or less

Fig. 89. A Sa.
gitta with its
generative organs.

0 Ovary, t Testes,
s Seminal vesicles.

1 Enteron. p Fins.

186

COMPAEAXm: AXATOMY.

occupy tlie extreme end of tlie body. Each opens into a short
excretory duct^ which is directed forwards, and is continued for
some distance on the surface of the body ; it is
often found distended with spermatozoa, and so
functions as a seminal vesicle. According to
the extent to which their contents are developed
the ovaries project more or less into the coelom.
X'hey pass from before backwards, and open to
the exterior by a short and projecting tube, with
which a receptaculum seminis, placed beside the
ovary, is connected.

§ 152.

The generative organs of the Acanthocephali
are very peculiar, a higher stage of development
being implied by the separation in them of the
sexes. A chord (ligamentum suspensorium) tra-
versing the anenterous coelom carries seminal
organs in the male, and ovarian organs in the
female. The testes are two rounded glands,
which lie one above the other; a vas deferens
passes off from each of them to the posterior end
of the body, where it opens, in company with a
number of tubular glands, into the copulatory
organ. This consists of a sucker-like structure,
in the centre of which lies a conical process —
the true joenis. This apparatus can be protracted
and retracted. In copulation it embraces the
similarly- shaped posterior end of the female^s
body. The ova are developed in an ovary (o)
which accompanies the chord-like axis (Fig. 90, 5),
and is either placed on, or is partly enclosed by,
it. The ova escape into the coelom, and are
taken up by the mouth of a wide bell-shaped
organ {g), which projects inwards from the hinder
end of the body, and leads into the short uterus,
which opens externally by a narrow vagina.

Echinorhynclius.

0 Ovary, s Ligameu-
turn suspensorium.
<j Bell-shaped organ.

1 Funnel. Terminal
portion of the ovi-
duct. The arrows
indicato tho course
taken by the ova, as
they pass to tho ex-
terior (after Greeff).

153.

In the arrangement of their generative organs
the Hirudinea closely resemble the Platyhelmin-
thes, and especially the Trematoda and Turbel-
laria dendrocoela. This does not apply only to
their hermaphroditism, but to the double cha-
racter of their germ-glands, which are ordinarily arranged symmet-
rically ; and also to tho position of the orifice of the whole system,
which is placed in the ventral median line. The position of the
male genital pore in front of tho female is a repetition of the

GENEEATIVE OEGANS OE VEEMES.

187

arrangement wEicE obtains in tEe marine Planarians. TEere is
always a larger number of germ-glands (5-12 pairs : t) for tEe male
organ (Fig. 91) ; tEey correspond to a number of metameres, are
rounded bodies, and are arranged serially on eitEer
side. A duct leads from eacE to a laterally-placed
vas deferens {vd)j wEicE widens internally, and be-
comes mucE coiled in front of tEe first pair of
testes [vs). From tEis coiled portion a terminal
piece, accompanied by its fellow of tEe opposite
side, passes to tEe genital pore. A large number
of glandular tubes [g) join tEe united efferent ducts,
and not unfrequently form, as in tEe Planarim, a
large mass of acini (Clepsine). EitEer tlie two
terminal tracts of tEe vas deferens function as copu-
latory organs, and, togetEer witE a portion of tEe
gland wEicE surrounds tEem, project from tEe body
in tEe form of a vesicle (Clepsine, Piscicola) ; or
tliere is a special copulatory organ, wEicE receives
tEe ends of tEe seminal vesicle. In tEis case (San-
guisuga, Heemopis, etc.) tEe portion forined by tEe
union of tEe two seminal ducts is developed into
a strong muscular organ (p), tEe tEinner end of
wEicE forms a sEort penis. As in tEe Planariae
and Trematoda, tEis lies in a penial poucE, wEicE
opens at tEe genital pore, and from wEicE it can
be protruded during copulation.

TEe female organs, also, of tEe Hirudinea re-
semble in many points tEe organs of several Pla-
tylielmintEes (marine Planarise) . TEe ovaries, wEicE
in tEe latter are scattered tErougE tEe body, are
Eere formed by two rounded, tubular, or lobate organs (o), wEicE
lie near tEe middle line of tEe body, just beEind tEe male efferent
organs. In some tEey open, merely by a sEort oviduct, at tEe
female genital pore (LeecEes witE a proboscis). In others the
efferent ducts are separated. TEe narrow oviducts give rise to a
longer common tract (Hirudo). TEe common duct, the several coils
of which are held together by a glandular layer, then widens into
a vagina at the terminal portion of the efferent duct [u).

,9

commune, 'ys Coiled
portion of tlie semi-
nal duct, analogous
to a seminal vesicle,
p Penis, g Glands.
0 Ovaries, u Ya-
gina.

§ 1^4.

In the Scoleina the organs lie in the anterior metameres, generally
occupying the tract between the eighth and fifteenth. Two different
types of the sexual apparatus are to be distinguished. One is well
marked in the Terricolse, and is characterised by the independence
of the efferent organs. TEe male part of the system in the Lum-
bricidm is formed by two pairs of testes, which are connected with
wide sacs, in which the elements of the semen are further developed.

188

COMPAEATIVE AXATO^^IY.

Eacli pail* of testes lias a seminal vesicle of this kind (Fig. 92, 6-'s"),
which is placed across the middle line, and is further provided with

lateral diverticula.
In each of these,
two funnel-shaped
organs are placed,
which are continued
laterally into the
seminal ducts. The
two seminal ducts
of either side unite
into a common duct
(rd), which extends
backwards, and
opens laterally on
the ventral surface.
In the same meta-
mere there are two
protractile copula-
tory organs, de-
veloped from mo-
difications of the
setigerous follicles.
The ovaries (o) are the smallest organs of the female portion of the
generative system. They lie behind the second pair of testes and
on each side of the ventral nerve-chord. Behind them two ovi-
ducts [ad) are attached to a dissepiment, and begin by two wide
abdominal ostia; they pass to the exterior by a short canal placed
in the segment in front of the openings of the male organs. Several
pairs (generally two) of seminal pouches (receptacula semiuis) [rs)
are also present. They are large, rounded organs, which lie near
the testes, and open by a short duct, without having any close rela-
tions to the male apparatus. The paired character of the genital
pores, the position of the female one in front of the male, and,
finally, the connection between the testes of either side, form an
arrangement which, so far as is yet known, has not its like among
living allied forms.

Even in the Limicola3 the organisation is different. The two
kinds of generative organs, which are here also present in one
individual, have no proper efferent ducts. We may suppose that
the system of oviducts, seminal ducts, and seminal vesicles, which is
found in the Lumbricidm is not developed in them, so that there
are only ovaries, testes, and receptacula seminis. So far as we can
see at present, some of the looped canals (cf. p. 177), which in the
Lumbricidm have nothing to do with the generative system, here
form the efferent ducts for the reproductive matters, and undergo
changes in correspondence with this function. Parts of the dis-
sepiments function as germ-glands, on which the generative
products when undeveloped generally form paired saccular diver-

Fig. 92. Generative organs of the Earth-Worm. The
portion of the body which contains these organs is opened
from above, and the walls are laid out on either side.
Segments VIII.-XV. are figured, n Ventral ganglionic
chain, s s' s" Diverticula of the testes, vd Their efferent
ducts. 0 Ovary, ad Ovarian ducts, rs Keceptaculum
seminis (after liering).

GENEEATIVE OEGANS OE VEEMES.

189

ticula which project far into the cavity of the coelom^ and often
extend through several segments. As a rule there are several
(as many as four) testes in different metameres. There is generally
only one pair of ovaries. As these laterally-placed organs, just
like the testes, force their products, when a large quantity is
developed, through several metameres, they seem to surround the
unpaired testes (Tubifex). When the generative matters break off
from their point of origin they float in the coelom. In some
(Enchytrasus) a number of ovarian germs break off together in a
mass, of which one ovum only comes to maturity.

The efferent ducts for the semen are formed from the already-
mentioned looped canals, a pair of which are, as a rule, converted to
this purpose : these generally undergo modiflcations in size. The
funnel-like internal oriflce, just as in the case of ordinary looped
canals, lies in the next preceding segment. The canal continued from
it, which is generally distinguished by its large supply of cilia, passes,
after several coils, to the terminal portion, which opens to the
exterior ; a large lobed glandular organ is attached to it. The
terminal portion anteriorly to its orifice forms an ampulla, into
which the tube is invaginated for some distance, and from which
it may be protracted, so that it serves as a copulatory organ. The
excretory ducts of the ova are either special oviducts, similarly derived
from modified looped canals, or they are functionally connected with
the seminal ducts. In this case the enlarged
terminal portion of the latter consists of a
double tube, the internal portion is the con-
tinuation of the seminal duct, and the external
portion which surrounds it, functions as an
oviduct. Branchiob della also conforms to this
type.

§ 155.

The generative system of the Chaetopoda
is very much like that of the lower division of
the Scoleina. Only in a few, however, does
hermaphroditism persist, and sexual separation
has become the rule coincidently with a more
free habit of life. The generative products
arise in the walls of the body-cavity, as is also
the case in the Gephyrea. As a rule, the
places at which the ova or the sperm is de-
veloped are merely distinguished by the pre-
sence of these products (Eig. 93, o) ; they have dorsal para-

no special characters, and are therefore dis- podia of other Annelids,
tinguishable only at the time when they are ® Ovary: a collection of
lunctionally active. They are developed at the are developed,

same points in the same genera or species;

thus, for example, in Eunice they lie at the sides of the ventral
nerve-chord. In a few cases they are limited to a small number of

of the integument, which
arise from two processes,
homologous with the

190

COMPAEATIVE AlSWi’OMY.

segments^ as in tEe Scoleina. Tlie generative products formed on
the wall of the body break oif when they become mature ; or they
become free whilst still unripe^ and float in the coelom (Fig. 93), in
which they further develop. The looped canals here also seiwe
as efferent ducts for both male and female generative matters ; but
this matter needs more careful investigation. Even in the Gephyrea,
as has been remarked above (§ 1 11), the organs which in them are
the homolognes of the looped canals aid in the generative function,
and present still more remarkable modifications ; which require to
bo more closely examined.

A special place must be assigned to the generative organs of the
Rotatoria. They have nothing in common with those of Chmtopoda
save their dioecious character, and they are distinguished from
them, as from all Annulata, by the fact that they are not meta-
merically repeated. The sexes differ not only in the characters of
their reproductive organs, but also by the rest of their organisation.
The males are not only smaller in size, but several of their systems of
organs are atrophied, especially the enteric canal. The testis consists
of a single tube, which opens at the hinder end of the body ; accessory
glandular tubes are sometimes connected with it. In the female, the
flattened ovary is ventral in position, and opens into the cloaca by
means of a short oviduct. Parts of the duct are widened and serve
as receptacles for the eggs; it thus forms a uterus, and in some
species the eggs are developed into embryos before they leave it.

§ 150.

The generative products have the same form in most
divisions of the Vermes. The egg is represented by a more or less
modified cell. In the Nemathelminthes, the ova are formed in a
peculiar manner; they are budded off from a common nucleated
chord of protoplasm, which forms the contents of the tubular ovary.
A large number of eggs are produced at one time, while the rest of
the protoplasm forms the axis of the chord (the rhachis), and is
surrounded by wedge-shaped ovarian buds. The same thing
happens in the Hirudinea; the ovaries of Haemopis contain a
coiled filament, which corresponds to this rhachis ; and on this
the ovarian germs are budded. The ova are connected with the
filamont by a thin envelope, which is drawn out into a stalk.
In Nephelis there is no chord, and the ovarian germs form groups
of cells. In all forms which have a yelk-gland it is not the egg- cell
only which forms the material for the development of the embryo,
but this is supplemented by the products of the yelk-gland — yelk-
cells (cf. § 147). The structure, therefore, which appears to be the
egg consists in them of a complex of cells, one only of which has
retained the value of an egg-cell. In almost all cases they arc
surrounded by an investment, which varies greatly in character. In
some it is a layer of albumen only, in others this layer is surrounded
by a hard shell.

GENEEATIVE OEGANS OF VEEMES.

191

The form-elements of the sperm are generally rounded or
elongated bodies^ from which a fine movable flagellum projects.
The Nematodes again are peculiar in this matter ; their seminal
elements, like the ova, being budded off from a rhachis. The cells
thus formed increase in size, and form cell-like bodies, which
perform amoeboid movements, but do not develop a flagellum.

In many Annulata the seminal filaments are united into masses
of definite form (spermatophores) in special parts of the male
efferent ducts ; these are passed as such into the female apparatus.
Many Scoleina have spermatophores of this kind, which are merely
formed by agglutinated seminal filaments (Tubifex). In the Hiru-
dinea the spermatophores are provided with a special covering.

Fourth Section.

Echinoderma.

General Review.

§ 157.

The Echinoderma form a strictly -defined and independent
group, owing to the well-marked development of a special type.
It is distinguished from the Ccelenterata by the differentiation
of the enteric canal and formation of a perienteric cavity (coelom).
The calcification of the integumentary layer (perisome), which
encloses this body-cavity, and the radiate arrangement of the
body, which consists of more than two antimeres, affords a fairly
sharp boundary between it and the higher divisions. This
difference between the fully-developed form of the Echinoderma
and that of the other types does not obtain in the larval stages,
so that in these we can make out genetic relations with the other
types. These are the more to be insisted on, as the actinoid type
of the Echinoderma was regarded as a reason for uniting them and
the Ccelenterata into a large group — that of the Radiata ; a union
which on close examination cannot be justified. Such a union is
inconsistent with the close relationship^ between them and the
Vermes, especially the Ann elides and Gephyrea. The observation
that the internal as well as the external organisation of the Echino-
derma gives evidence of metamerism, has further justified their
separation from the Ccelenterata, and has given rise to the hypo-
thesis propounded by Hiickel, which regards the Echinoderma as
derived from colonies of worm-like organisms.

The larval form of the Echinoderma agrees exactly with the larva
of the Vermes. Here, as in many of the latter, a new organism is built
up within the body of the larva. But radial gemmation takes place,
and a number of individuals are produced, and thereupon the pheno-
menon enters upon the well-known series of changes. Gradually the

ECHDn'ODEEMA.

193

radially disposed buds separate from one auotlier to a certain ex-
tent, without however ever becoming completely separated ; so that
a number of organs, or separate portions of organic systems, are
common to them all. In this way the budding individuals remain
connected as a single organism, and losing their individuality,
sink to the level of mere parts only of a body (antimera).

This hypothesis of the derivation of the Echinoderma from the
Vermes leads us to classify the former above the latter, for we
suppose that the Vermes gave the starting-point for the development
of the Echinoderma.

The separate divisions of the Echinoderma (Star-fishes) are thus
arranged :

I. Asteroida (Sea-stars).*

As terida.

Asteracanthion, Solaster, Astropecten, Luidia.

Brisingida.

Brisinga.

Ophiurida.

Ophioderma, Opliiolepis, Opliiotlirix, Ophiocoma.

Euryalida.

Astrophyton.

II. Crinoida (Lily-stars).

Brachiata.

Pentacrinus, Comatula.

III. Echino'ida (Sea-urchins).

Demosticha.

Echinothurida.t

Calveria, Phormosoma.

Cidarida.

Cidaris.

Echinida.

Echinus, Ecliinometra.

Petalosticha.

Spatangida.

Spatangus.

Clypeastrida.

Clypeaster, Lagaimm, Scutella.

IV. Holothuroida (Sea-cucumbers).

Eupodia.

Holothuria, Molpadia, Pentacta, Psolus, Cuvieria,

Apodia.

Synapta, Chii’odota.

The Asteroida must be put first, as including the oldest Echinoderma, and as
being the nearest in organisation to the hypothetical stem form. Among the fossil
forms of this group we find conditions which are allied to those of the next class
(Crinoida).

■f This most important family presents points of connection with the Asterida ;
notably in the possession of a movable dermal skeleton.

0

194

COMPAEATIYE AXATOMY.

Bibliography.

Tiedemanx, Anatomie der Rohrenliolothurie, des pomeranzenfarbigen Scestemes mxd Steiuseeigels.
Landshut, 1816.— Shaepey, Art. Echinodennata in Todd’s Cyclopaedia. II.— Mulleh, J., Anato-
mische Studien iiber die Ecbinoderma. Arch. f. Anat. u. Pbys. 1850. — The same, Die Erzeugung
von Schnecken in Holothurien. Berlio, 1852. — The same, Ueber den Ban der Echinodermen.
Abhandl. d. Berl. Acad. 1853. —The same, Sieben Abhandluogen ubcr die Larven und Metamor-
phosen der Echinodermen. Abhandl. d. Berl. Acad. 1848-55. — Sabs, M., Oversigt of Norges
Echinodermer. Christiania, 1861. — Geeeff, Sitzungsber, d. Gesellsch. z. Beford. d. Naturw. zu
Marburg, 1871-76. — Teuscheb, R., Beitr. z. Anat. d. Echinodermen. Jen. Zeitschr. Bd. X.

Asteroi'da: Foebes, Ed., A Historv- of British Starfishes. London, 1841. — Mulleb, J., and
Teoschbl, System der Asteriden. Braunschweig, 1842. — Agassiz, A., Embryology of the Star-
fish. Contrib. to the nat. hist, of the U. S. Cambridge, 1864.— Hoffmann, C. K., z. Anat. d.
Asteriden. Niederl. Arch. f. Zoolog. Bd. II. — Sabs, G. O., On some remarkable.forms of Animal
Life, II. Christiania, 1875. — Lange, Wilh., z. Anat. d. Aster, u. Ophiur. Morph. Jahrb. Bd. II.

Crinoida : Mulleb, J., Ueber den Ban des Pentacrinus caput medusa?. Abhandl. d. Berl. Acad.,
1844.— Thomson, AV., On the Embiyogeny of Antedon rosaceus. Phil. Trans. 1865, II,—
Cabpenteb, AV. B., Researches on structure, etc., of Antedon rosaceus. Phil. Trans. 1866.—
Sabs, M., M^moire pom* servir a la connaissance des Crinoides vivants. Christiania, 1868. —
Pkebieb, Ed., Rech. s. I’Anat. etc. des bras de la Comatula rosacea. Arch, de Zoolog. ex-
])^riment. T. II. — Gotte, A’ergl. Entwickelungsgesch. d. Comatula rosacea. Arch. f. mikr. Anat.
Bd. Xn.— Ludwig, H., Morph. Stud. (Crinoideen). Zeitschr. f. wiss. Zool. Bd. XXVIII.

Echinoida ; Valentin, Anatomie du genre Echinus, 1841. — Hoffmann, C. K., Anat. d. Echiueu
u. Spatangen. Niederl. Arch. f. Zool. Bd. I.— Agassiz, A., Revision of the Echini, P. 1-4.
Cambridge, U.'S. 1872-73. — Loven, S., Etudes sur les Echinoid^es. Kougl. SvenskaA'et. Acaxl.
Handl. Bd. XI.— AVyville Thomson, On the Echinodea, etc. Phil. Trans. 1874, P. II.

Holothuroida : Quatbefages, Anatomie der Synapta Duvema'a. Ann. sc. nat. II. xviii. —
Danielsen, C., og Koeen, j.. Bidrag til Holothuriarun udviklings-historie in Fauna Httoralis
Norvegiae. II. Bergen, 1866. — Baub, Beitrage zur Naturgeschichte der Synapta digitate. N. A.
L. C. 3&XI. — Sempeb, C., Reisen in den Philippinen. Bd. II. 1. Holothurien. — Selenka, Z. Eutw.
d. Holothur. Zeitchr. f. w. Zool. Bd. XXA’^II.

The morphological relations of the various clmsions of the
Echinoderma to one another, and to the loAA'er forms, are best
understood by a reference to their development. The larva, which
arises from the ovum, has only tAvo antimera (bilateral symmetry)

Fit?, 9 J. Larval forms seen from the side. A Larva of a llolotlmrian. JJ Larva of
a Starfish (Bipiuuaria-type). C D Vermiau larva?, o Mouth, i Stomach, a Anus.
V Pracoral band of cilia, Avhich is independent in B, C, and D, but in A surrounds an

oral tract.

and agrees in all essential points Avith the larva of the A nnelides. A
ciliated band either surrounds the oral surface, Avhich carries the
mouth (cf. Fig. 94, A), or is divided into tAA'o circlets, one of Asdiich
bounds the prajoral, and the other the postoral region {B), The

General Form of the Body.

§ 158.

FOEM OF THE BODY OF ECHIHODEEMA.

195

latter larval form is seen in the Asteroida^ the former in the
Holothuroida. These forms also obtain in the larv80 of other
Echinoderma_, but in the Ophiurida and Echino'ida a number
of processes are developed^ on to which the band of cilia is continued.
In a few cases (where development takes place within the maternal
organism) the larval form is passed ovei% and the Echinoderm
appears without this intermediate stage. The resemblance which
exists between the larval form of even very different divisions points
to the common origin of the phylum ; which origin, indeed, was
from forms which were not of the radiate type. This important
point is ignored by those who try to derive the Echinoderma from
the Coelenterata ; they regard the Echinoderma as forming an
exception, which they are unable to
explain.

The rudiments of the Echino-
derm body appear round the enteron
of the larva. In the Asteroida five
or more parts are budded off from
a common rudiment ; these are the
future ^^arms^^ or ‘^‘'rays^^ of the
starfish (Fig. 95, A). The free end
of the ray appears at first to be in-
dependent, and the other end remains
in connection with the general mass.

This corresponds to the anterior
part, and the free end of the ray to
the posterior part of a worm^s body.

As the rudiment of each arm grows,
joints (metameres) appear in it be-
tween the base and the tip. There
is a certain amount of independent
organisation in each arm of a star-
fish ; its organs, such as the intestine,
nervous, and vascular systems, and
also the sexual organs, have exactly the same position as the homo-
logous organs of an Annulate worm. If, then, we compare each of the
budding arms with a worm-like organism, we must regard the star-
fish developed by this process of gemmation as corresponding to a
multiple of such organisms ; and, further, we must recognise in this
phenomenon the same process of gemmation as that which takes
place in other lower animals ; for example, in the compound Ascidians.
It is a process in which several separate animals are simultaneously
budded off ; the process does not go on till these animals are com-
pletely separated, but stops in such a way as to keep them connected
together as an individual of a higher order.

It is not hard to see how, owing to the incompleteness of their
separation, the products of gemmation are not only connected ex-
ternally, but have a certain number of internal organs connected,
and so permanently common to the whole organism.

Fig. 95. Larva of an Ophiurid
(Pluteus form), a Eudiment of the
Echinoderm with budding arms.
d d' e' Processes_ of the larval body
with their framework of lattice-like
rods (after J. Midler).

196

COMPAEATIVE AXAT0:MY.

§ 159

Thus, then, an organism is developed, the antimeres of which
present themselves as the radially-arranged arms each of these
arms has primitively the value of one person. By the concrescence of
these persons an individual of a higher order, an animal colony, is
formed. In the Astero'ida the number of arms is not definitely
fixed; in some there is a large (14 in Solaster), in others (Astera-
canthion) a smaller number (4). In most the number is limited to
five, and the typical number, which predominates in other Echino-
derma, is fixed at this. The number of arms in Brisinga varies from
9-12. The point at which all the arms are connected forms the
common body in the Astero'ida ; it carries the mouth. This lies on the
ventral surface, Avhich is oral, and has an aboral surface opposed to it.
In the arms the ventral surface is distinguished by the possession of
rows of expansive and movable processes — ambulacral feet ; these
form an ambulacrum in each arm, and are attached to a depression,
which runs along the arm (ambulacral groove). They correspond in
character with the metameric arrangement of the arms, which is ex-
pressed in other parts. There are four rows in Asteracanthion, and
two in most of the other forms. It is uncertain whether these
structures are allied to the parapodia of Yermes or not. The ventral
surface is also known as the ambulacral. There is no difference in
the extent of the ambulacral and antambulacral (dorsal) surfaces.

The radii or arms vary greatly in the extent to which they are
independent of the common body ; in not a few forms they show a
tendency to shorten so as to form a body disc, and so, at the same
time, the indication of the primitively individual character of the

Fig. 9fi. Three forms of Starfishes, .4 B C, iu which coucrcscence aud consequeut
loss of individuality in the arms is being gradually effected. All three are
figured from the oral, or ambulacral, surface of the body. The ambulacra are
represented by rows of dots, o Mouth, r Radii (Arms), ir Interradii.

arms is lost (Oreaster, Pteraster, Asteriscus, etc.). A comparison of
the three forms of Sea-stars (Fig. 96, A B G) here figured will
make this quite clear.

r

rOEM OF THE BODY OF ECHIHODEEMA.

197

Fig. 97. Diagram of the body-
form of an Ophiurid. o Mouth,
s Body disc, r Arms.

§ 160.

The arrangements in other Echinoderma are derived from the
form which obtains in the Astero’ida ; and
that in two divergent lines. In both,
although in different ways, it leads to a
greater centralisation of the organism.

Along one line the arms are more largely
developed, and at the same time gradually
lose their relations to the internal organs.

Along the other the arms pass completely
into the common body. The number of
rays appears to be always five. The first
arrangement is seen in Brisinga and the
Ophiurida, where the body is separated
into a discoid central portion (Fig. 97, .«?),
and the projecting, but sharply sepa-
rated, arms (v). The arms take but a
small share in the formation of the
body-cavity, whichis almost exclusively
limited to the body disc. The ambu-
1 acral groove is wanting in the Ophi-
urida, but the ambulacra still extend
along the arms.

The arms of the Euryalida are
greatly developed, being repeatedly
divided in a dichotomous fashion. A
shallow groove is continued into the
divisions. The Crinoida, which in
earlier periods were widely distributed,
and very rich in forms, but which are
now represented by few genera, have
lost the power of free locomotion, and
passed into the fixed condition. In
the division Brachiata, which includes
the extant forms, a stalk, often of
great size, jointed, and complicated
by branchings and appended struc-
tures (Fig. 98), is developed on the
antambulacral portion of the cup-like
body. This serves as an organ of
attachment. The arms are not always
limited to five, but there is often a
larger number of them ; they are con-
siderably developed, being divided, or
provided with secondary appendages.

These appendages, which are attached
to the arms and have the. form of
feathery plates, are known as pinnulEe.

Fig. 98. Ehizocrinus lofoten-
sis. A A young specimen. B Cup
(k), with seven arms, of a larger
specimen, id Stalk, w Roots. R
Arms, t Tentacles (after M. Sars).

The ambulacral groove

198

COMPARATIVE AXATOMY.

extends on to tlie arms, and gives passage to the tentacula-like feet.
In some the fixed condition obtains in the young only, and later
on in life the arm-bearing body breaks off from its stalk (Antedon,
Comatula).

§ 161.

The other series of modifications of the form of the body leads to
the Echinoida. The arms have altogether disappeared as indepen-
dent parts. In the more or less cone-shaped body of the true Sea-
Urchins (Desmosticha), the ambulacra extend over the greater part
of the surface. The ambulacral tracts form five bands, which extend
from the oral (Fig. 99, A o) to the opposite pole {B a) ; these are
separated by as many tracts devoid of suckers (Interambulacra).

The aboral polar area (apical
pole) is occupied by the
greatly-diminished antambu-
lacral surface. The distribu-
tion of ambulacral (oral) and
antambulacral (aboral) sur-
faces of the body, which is
pretty equal in the Asteroi’da,
is thus completely altered in
the Sea-Urchins, for in them
the former is much larger
than the latter. If we imagine
a Starfish-form, in which
the arms have coinpletel}"
jDassed into the general body
(cf. Fig. 96, c), then the decrease in size of the antambulacral surface,
and the consequent increase in that of the ambulacral, will give us
the Urchin-form.

This arrangement is modified in the Petalosticha, partly by a
change in the relations of the mouth and anus, partly by modifica-
tion of the ambulacral tracts. The diminution in size of these tracts
is of importance. They form a five-leaved rosette on the dorsal
surface, and from these leaf-ends signs of a continuation of the tracts
as far as the mouth can be still followed out in the Clypeastridm.

The indications of the development of the Echinoderm body from
a multiple of individuals are less apparent in the Holothuroi'da
than in the Echinoida. But the sausage-shaped body can be derived
from the arrangement seen in most of the regular Sea-Urchius, by
imagining the body to be pulled out. The oral and aboral poles in
each correspond, the former being distinguished by the oral, the latter
by the anal orifice. The antambulacral surface has altogether disap-
peared. In the true Ilolothuroida (Eiipodia) the ambulacral and
interambulacral tracts extend alternately from the mouth to the anus.
Some of the ambulacral tracts may however undergo an increase,
and the others a decrease in size, according to the varying necessities
of function. Thus the three ambulacral tracts on that surface which

Jj

a

Fig. 99. Diagrammatic figure of a Sea-Urchin.
A From the oral surface. B A lateral view.
The ambulacra are represented by rows of
dots, r Radii, ir Interradii, o Mouth, a Anus.
The latter is surrounded by the antambulacral
surface.

APPENDAGES OE ECHINODEEMA.

199

functions as ventral^ are retained in Psolus, while the other two
which belong to the portion of the surface of the body^ which func-
tions as the dorsal, are atrophied. In Cuvieria this modification of
parts is extended so as to result in the disappearance of the three
ventral ambulacra.

A complete degeneration of the ambulacra takes place in the
Synaptas, and the radiate organisation, which is implied by the
division into ambulacra, is thus lost; they have followed out the
line of modification indicated in those Astero’ida, where the rays
begin to lose the organs belonging to them, as if in preparation for
a centralised organisation.

Appendages.

§ 162.

The organs appended to the integument, which may be regarded
as appendages, are not so various as in the Vermes. Of such parts
the sucking or ambulacral feet must be placed foremost, for
they form the most common arrangement, and belong to the typical
Echinoderm organisation, and are evidently derived from an ancestor
common to the whole group. They are tubular, and generally
cylindrical processes of the body- wall, which not only agree in being
arranged in rows (in accordance with the metamerism of the rays),
but also in their most essential points of structure, with the parapodia
of the Annelides : on the whole they are more simple in character.

'p

Fig. 100. Diagram of tlie cross section of an arm, J., of Asteracantliion rubens;
B, of Opliiura texturata. p Ambulacral feet, p' Ampullae. t Dermal tentacles.
n Nerve-chords, w Ambulacral plates, m Muscles, a Ambulacral vein, h Ventral
plate, c Lateral plates, d Dorsal plate. Tc Calcified portion of the integument

(after Wilh. Lange).

(Fig. 100). As they have always much the same structure they
vary but little in function.

The free end of these tubular structures {p) is either flattened
out, or provided with a sucker-like termination (Echino'ida) ; or it
has a conical tip, or is rounded off (many Asteroida) ; sometimes it is
provided with a rounded head. Others are provided with lateral
indentations or secondary processes (Ophiurida and Crinoida) ; these

200

COMPAKATIVE AXAT0:MY.

are intermediate towards those forms which have lost their locomotor
function, and have the form of ambulacral branchiaB or tentacles
(tentacular organs) .

When they are filled with fluid the feet swell, and are thereby
erected and capable of protrusion. The extent to which they are
protruded depends on the length of the firm appendages of the
integument, so that the sucker feet are longest in the Echinoida with
their long spicules. The end becomes fixed when the sucker is pro-
truded, so that the foot is now able, when it contracts, to draw the
body of the animal towards the point of attachment ; this mode of
locomotion is often effected with great rapidity by the Echinoida. A
whole group of suckers takes part in the movement, and by working
together display considerable energy. The distribution of these
structures has been referred to in the previous paragraphs ; their
relation to the vascular system will be spoken of when that system
is dealt with.

In the Crinoida the circumoral suckers acquire the function of
tentacles ; this is in many other cases combined with the locomotor
functions. But we get also independent tentacular structures,
allied in origin to these, namely, the tentacles in the neighbourhood
of the oral aperture of Holothuro'ida (cf. Fig. 113, T). They are
sometimes pinnate, sometimes branched, and, as a rule, can be
completely drawn in. In many Synaptae they carry suckers (S.
Duvernaea). Their internal cavity is in communication with the
same vascular system, as are the ambulacral feet.

The so-called dermal branchiae, tentacles which are dis-
tributed over the antambulacral (dorsal) surface of the body in
the Starfishes (Fig. 100, f), differ from these; in the Echinoida
they form five pairs of contractile tree-like organs near the mouth.
They communicate with the body-cavity, and are merely protrusions
of the integument.

Integument and Dermal Skeleton.

§ 1C3.

In the Echinoderma the same dermo-muscular layer is present as
in the Vermes, but the integument is more sharply marked off from
the musculature. 3diis latter forms, for the most part, a layer which
limits the body-cavity, and is covered externally by the integument.
Its special characters are due to the fact that the locomotor power of
the body is more or less determined by the deposition of chalk
in that layer, which goes to form, in conj unction with the muscular
layer, the perisome.^^

The same structure exists in the larva; but in it this exoskeleton
is never of any great size, but is merely a firm support formed b}"
a large number of rod-like processes. The ciliated bands extend on
to the processes, which they fringe, and form, in a more or less com-
plicated manner, the locomotor apparatus of tlie larva (vide Fig. 95,

DEKMAL SKELETON OE EOHINODEEMA.

201

d d' e). The whole body is covered with cilia at a period before
that when they are arranged on the ridge-like projections from the
ciliated band ; but this general ciliation is only found during the
most indifferent condition of the larva.

The cilia are retained even later on, on many parts of the soft
dermal layer which invests the calcareous skeleton : as, for example,
in the ciliated tracts, which reach to the mouth in the Spatangidge
(semitae). On other parts, such as the dermal branchiae (cf. supra),
ciliation appears to be correlated with the respiratory function
of the integument, in which the ambulacral feet may also have
a share.

The extent to which the integument is calcified varies greatly.
Sometimes the calcareous particles are united with one another into
larger pieces, and form plates which are movably or immovably
connected together : this arrangement either extends over the whole
body, or is confined to definite tracts of its surface. In other cases
the calcareous particles are scattered, and allow of great variations
in the form of the body. In this case a large number of the
characteristic features of the Echinoderm disappear from other parts
of its organisation ; so that the disappearance of a calcified integu-
ment is a departure from the type, and the general pha0nomenon of
a scanty deposit of calcareous matters is not to be regarded as an
early, but as a final stage in the series of forms.

Calcification converts the integument into an organ of support
for the body, or dermal skeleton;
in many cases this sends out pro-
cesses into the interior of the body.

These give rise to calcified struc-
tures, which form an internal skele-
ton and combine with the external.

The whole thickness of the peri-
some is not affected by the process
of calcification. A thin non-calci-
fied layer of tissue is always found
on the inner, as well as on the outer
surface ; on the latter, however, this
layer disappears at an early period
from some parts, so that the calcified
parts are exposed ; this happens,
for instance, on the spine-like struc-
tures, as well as on other processes
of the calcareous skeleton.

The lime-salts are always de-
posited in a regular manner in the
integumentary layer. They form
delicate frameworks or retiform
structures (Fig. 101), in the spaces
of which soft organic substance persists. The most solid skeletal
parts are thus traversed by soft structures ; when the calcareous

S

Fig. 101. View of a calcareous net-
work of a plate of the dermal skeleton
of an Echinid (Cidaris). b Trabecula?
cut through. These were directly per.
pendicular to the horizontal network
(somewhat highly magnified).

202

COMPARATIVE ANATOMY.

skeleton is merely represented by separate and microscopic deposits,
tkese are generally definite in shape, and characteristic of genera
and species.

The calcareous skeleton of the larva forms an organ of support,
which is generally made up by a framework of delicately attached,
and often perforated rods. They are ordinarily found in the larval
Echinoida and Ophiurida; there are also calcareous bodies in the
larvm of the Holothuroida. The presence of a calcareous skeleton in
the larvo3 is clearly an instance of an arrangement which is common
to the group ; but it must not be forgotten that this larval skeleton
corresponds to the form of the larva, and not to that of the adult
Echinoderm ; none of it passes permanently into the adult form.
There is, in fact, a repeated change of the calcareous skeleton in the
Holothuroida.

§ ]6I..

As regards the special characters of the dermal skeleton, the
presence of pieces, movably connected with one another, on the

ambulacral surface of the arms,
is characteristic of the Aste-
roida. Transversely - placed
pairs of calcareous pieces,
which gradually diminish in
size, are found from the mouth
as far as the tip of the arm
(Fig. 100, A w) ; they form
the floor of a groove — the
tentacular groove. The sepa-
rate pieces form a jointed
series by their articular at-
tachments, and the suckers
pass out between the solid
joints (p). These calcareous
pieces are consequently known
as ambulacral plates. But
as special soft parts (ambii-
lacral canal and nerves) are
also embedded in this gi’oove,
the jointed segments do not
appear to be purely dermo-
skeletal parts. The ambu-
lacral groove is covered by the
integument, which is con-
tinuetl laterally on to the ani-
bnlacral plates. It consists
largely of a layer of long
cylindrical cells, covered by a
cuticle. At the side it passes into a layer of cells, which is placed
much deeper. At the lateral edges of the groove the skeleton is

Fig. 102. Body disc of au Ophiurid
(() p li i o t h r i X f r a g i I i s) ; seen fi’om the
oral surface ; the bases of the arms be-
set -witli spicules may be seen (magnified).
(' Body disc. B Arms, t Calcareous plates,
■which cover the canal -which corresponds
to the tentacular groove of the Asteroida.
g Genital clefts, d Masticatory plates.

DEEMAL SKELETON OF ECHINODEEMA.

203

continuous with the dermal skeleton, which covers the back o£ the
arms ; at this point there are frequently found one or more longi-
tudinal rows of plates or scutes. These structures may be replaced
by knobs, and are sometimes continued on to the integument of
the antambulacral surface of the body ; or the integument is dis-
tinguished by retiform deposits of calcareous matter and smaller
tubercles, separated by the non-calcified parts of the perisome.
Brisinga resembles the Astero’ida in the structure of its arms ; that
is, they have an ambulacral groove.

Larger flat plates, marginal plates, form the edge of the arms ;
these are often distinguished by spicules and other processes.

The structure of the integument of the Ophiurida resembles
that of the Asteroida. There is seldom any great development of
calcareous plates on the antambulacral surface ; they are, as a rule,
found near the base of the arm only in these forms. The ambulacral
or ventral integument is also provided with plates around the mouth
(Fig. 102), But several parts of the Arm skeleton of the arms are
different to those of the same parts in the Asteroida. The pieces
homologous with the ambulacral plates of the latter form a closely-
set series (vertebral pieces. Fig. 100, B w), which almost com-
pletely fill up the arm, and only leave a narrow canal on the dorsal,
and a groove for the nerves and other org^^ns on the ventral surface.
The coelom is therefore continued into the arms as a narrow canal
only. Instead of the soft covering of the ambulacral groove, which
the Asteroida possess, there is a series of .firm calcareous scutes
(Fig. 100, B h) in the Ophiurida; other lateral processes of various
kinds are added on to them.

In the Euryalida also the leathery investment of the body
covers in a skeletal structure, formed of vertebral calcareous plates
attached to one another. This, as in the Ophiurida and Asteroida,
belongs to the oral surface of the body ; the plates are continued
from the edges along' the radii, and to the finest ramifications of
them. In them, too, this skeleton forms the floor' of the ambulacral
groove. On the aboral surface the body-disc is enclosed in a dermis,
merely impregnated with calcareous granules, which passes on to the
arms, and covers them as far as the edge of the ventral groove.

There is a larger number of knob-like and spicular processes in
the integument, which may vary very greatly in character. A
special form is very common among the Asteroida ; namely, bundles
of movable spicules attached to a common stalk (paxillae). The
pedicellarise are described in § 166.

§ 165.

This dermal skeleton is modified in the Crinoida. The dorsal
integument is drawn out into a stalk, to the end of which the
animal is attached. The skeleton of the stalk is formed of cal-
careous plates, which lie regularly one over the other, and are con-
nected with flattened basal pieces, to which other calcareous plates.

f^OMPARATIVE ANATOMY.

20 i

whicli form the boundaries of the body, are attached. In the young
stages of the Comatulm, a simple knob-like piece (centro-dorsal)
unites the skeleton of the stalk with the body. Radial jointed
pieces, which are continued into the joints of the arms, are attached
to the central piece. The ambulacral groove extends along the
dichotomous branches of the arms (Pentacrinus), as well as along
the lateral appendages (pinnulae of Comatula) which are set alter-
nately on either side of the arms. The groove becomes united with
the groove of the next arm, and passes along the ventral surface of
the cup-shaped body as far as the mouth. Deposits of calcareous
plates are embedded in all parts of the portion of the integument
which remains soft and covers over the skeleton.

§ 106.

The differences in the dermal skeleton of the Echinoida, and the
consequent changes in the form of their body, as compared with the
Asteroida, are chiefly due to the calcification of the oral (ventral)
perisome, that is of the portion which covers the ambulacral groove
and the soft parts which lie in it, and which is permanently soft
in the Asteroida. In the place of the articulated joints, there are
plates which are calcified externally, and are connected with the
body in various ways.

In the Desmosticha the portion which corresponds to the dorsal,

or aboral pole of the Starfish is a
small surface, marked off by small
loosely-articulated calcareous plates,
placed excentrically to the anus (Fig.
103, x). This surface, which occupies
the centre of the so-called apical pole
of the Sea-Urchin, is surrounded by
larger calcareous plates, which carry
the orifices of the genital organs, the
genital plates ((/) ; one of these is the
madreporic plate (m). Five pieces
(intergenital plates, fi/) are attached
to, and partly intercalated between
these, and from the former five rows
of paired plates extend to the oral
surface. -These are traversed by fine
p’ores, by means of wdiich the suckers
communicate with the interior. These
are the ambulacral plates (u), which
make up the ambulacral arese. The
ambulacral rows of the calcified peri-
some of the Sea-Urchins are homo-
logous with the permanently soft
perisome of the Starfishes, which covers over the ambulacral groove
on the ventral surface of the arms. The rows of non -perforated

Fig. 103. Apical pole of an
F c h i n u s. a Ambulacral area',
i Interambulacral area', g Genital
plates. Vj; Intergenital plates. mA
genital ])late ^vitll the characters
of a madreporic plate. x Anus
])laced in the apical area surrounded
by the genital jdates. The knobs
of the plates are figuretl in oidy one
ambulacral and one interambulacral
area ; in the former the pores also,
■which are omitted from the other
four, are indicated.

DEEMAL SKELETO^^- OF ECHIXODEEMA.

205

plates^ wliich lie between tbe ambulacral tracts (interambulacral
arese, i)j are homologous with the marginal plates of the arms in
the Astero’ida. There are two rows of interambulacral^ as well as
of ambulacral plates. The Echino'ida of earlier periods had a larger
number of these rows — forms with three^ five, and even seven rows
in one interambulacral area are known.

These pieces are connected with one another in different ways.
In many Echinoida, as in the Asteroida, the calcareous plates of the
perisome are movably connected together, and so allow of changes in
the form of the body. In the family of the Echinothurida the
plates of the perisome are movably connected with one another,
so that the body is able to change its form. The ambulacral as well
as the interambulacral plates project over one another like the tiles of
a roof in the middle of each area, and the interambulacral ones are
separated laterally from one another by a narrow intermediate space.
Owing to the thinness of these plates the soft parts of the perisome are
of more importance than they are in other families of the Echino'ida.
These plates are also continued with slight modifications on to the
area around the mouth, while in the rest of the Desmosticha this
portion is more sharply marked off from the rest. So far the
Echinothurida approximate to indifferent conditions, and form an
intermediate step between the other Desmosticha, and the hypo-
thetical forms which can be derived from the Astero'ida. This point
of view is further justified by the fact that a firm fascia extends
along each ambulacral area on the inner face of the shell, and sepa-
rates the parts (nerves, vessels, ampullae) which lie on the ambulacrum
from the coelom. It forms a process, which is attached to either
side of an ambulacral groove, and projects some way into the coelom ;
this process is, moreover, fenestrated by fine pores. This arrange-
ment corresponds to the skeleton of the ambulacral groove in the
Asterida, which is calcified in this portion in them ; while the same
portion in these forms, in which the perisome corresponding to the
ambulacral groove of the Asterida is formed by calcareous plates,
remains soft.

Several important modifications of the regular form of the dermal
skeleton of the Echino'ida, which are not directly comparable with
the arrangements found in the Astero'ida, are developed ; these are
accompanied by the disappearance of the rest of the primitive dorsal
perisome, and by the passage of the radiate into other forms. The
ambulacral areae no longer extend regularly from the mouth to the
back; in the Spatangid^e and Clypeastrid^ they are limited to a
five-leaved rosette (ambulacra petalo'i'dea) placed on the dorsal sur-
face. At the same time the number of plates, which is very large
in the regular Echino'ida, is diminished ; so that there is a smaller
number of larger plates.

The internal skeleton, which in the Astero'ida is formed by the
skeleton of the ambulacral groove, is represented in the Echino'ida by
processes of the ambulacral plates. These processes, which are well
developed in Cidaris, for example, embrace the nerves as well as the

206

COMPAEATIVE ANATOMY.

iimbulacral canal, and so point to an affinity witli this skeleton.
The skeleton of the masticatory apparatus in the Echinoida and
Clypeastrida is to be regarded as an independent development ; it
surrounds the commencement of the enteron, and consists of a
number of calcareous blocks connected together like a scaffold.

Spine-like processes are connected with the integument in the
Echinoida, as well as in the Asteroida, but they
are more independent owing to their power of
movement. They are articulated to protuber-
ances of the calcareous plates, and are provided
with a special system of muscles. The spines
vary greatly in form and size ; sometimes they
are as fine as hairs (Spatangidae) ; sometimes
club-like structures ( Acrocladia) ; sometimes they
are long rods (Cidaris).

The Pedicel lari m are also dermal organs
of a peculiar character, which are found in the
Asteroida, as well as in the Echinoida. They
consist of a stalk-like muscular process of the
integument, which is supported at its end by a
fine calcareous skeleton; it terminates in t\vo or three pincer-liko
valves, which are movable on one another. These too are provided
with a calcareous skeleton. In the Echinoida the three-valved,
and in the Asterida the two-valved forms predominate. Brisinga
resembles the Asterida in this point. They arc scattered over the
whole body ; but in the Asterida they are principally found at the
base of the spicules, and in the Echinoida in the perisome around
the mouth.

These bodies may be regarded as spines modified in such a way
that the incompletely calcified stalk of the pedicellaria corresponds
to the stalk of the paxilla of the Asterida; the tuft of spinules
on the latter being represented by the arms
of the pedicellaria, which are moved by
muscles, just as are the spines of the Echr-
noida. The four-valved pedicellaria} of Cal-
veria fenestrata are intermediate between
the more common pedicellaria) and the paxilla),
for each of the valves, which is provided with
a long stalk, is continued into a broad lamella,
bent over at its edges.

§ 167.

In the Holothuroida the integument has
no longer any dermo-skeletal significance.
The calcareous plates of the other Echinoderma are represented by
disconnected deposits of lime in the firm dermal layer.

The calcareous deposits of the skin are definite, and often very
regular in form ; and these forms are characteristic both in Synaptm

Fig. 105. A Calcareous
anchor. B Calcareous
plate, ■which serves to
attach the former ; from
the integument of S y-
uapta lappa (after J.

Miiller).

Fig. 101. Pedicellarisc
of Echinus saxa-
t i 1 i s. A A Pedicel-
laria with its pincer-
arms open ; B With
them closed (after
Erdl).

MUSCULAR SYSTEM OF ECHINODERMA.

207

and Holotliuriee. They sometimes form larger firm pieces^ such as
the scutiform structures, which cover the dorsal or antambulacral
surface in Ouvieria, and which though much smaller are widely
distributed in the dermis of Bchinocucumis.

In the Holothuri^ the leathery layer of connective tissue becomes
very strong indeed, but it is quite soft in the Synaptae. In these
latter, however, there are calcareous bodies deposited, which are
frequently definite in form ; such as the calcareous rotulse of the
Chirodotse, or the fenestrated plates (Fig. 105, B) into which the
bases of the anchor-shaped hooks (/I) are inserted. These latter
project from the integument, and give to the skin of the Synaptas
its great power of attachment.

The Holothuroida also possess an internal skeleton proceeding
from their dermal one. It consists of a calcareous ring surrounding
the gullet, which serves as a point of insertion for the muscles of the
body, and as a support for the other organs. In the Holothurias it
consists of 10, and in the Synaptae of 12-15 separate pieces. In
the former five larger pieces alternate with as many smaller ones,
and are more or less movably connected with one another. They
are homologous with the processes, which project inwards from the
oral edge of the shell in the Echinoi'da. Like those processes,
they are provided in the Synaptae with pores, which give passage to
the nerves and ambulacral canals; in the Holothuriae these organs
pass out through forked processes.

Muscular System.

§ 168.

In the Echinoderma, as in the Vermes, the inuscnlar system is
connected with the integument, and the structures derived from it.
The arrangement of it is essentially dependent on the development
of the dermal skeleton, so that it is only developed into a system of
body muscles in those cases, where the body is able, thanks either
to the articulations of its separate firm parts (Asteroida and
Crino'ida), or to the presence of disconnected calcareous deposits in
its integument (Holothuroida), to change its form.

In the Asteroida and Crinoida the musculature of the arms, like
the arms themselves, is jointed; it fills up the interspaces between
the solid parts. In the Crinoida, where the skeletal parts of the
arms are connected together by elastic tissue, these muscles lie on
the ambulacral or ventral surface of the body, and serve principally
to bend them, while the elastic tissue between the joints straightens
them again. The same' arrangement obtains in the pinnulee of the
Crinoida.

This system of muscles is rudimentary in the Echinoida, where
the perisome is hardened into a firm ‘"^shelU'’ consisting of immovably

208

COMPARATIVE AXATOMY.

connected pieces; in them the only separate muscles on the shell
are those which move the spicules or spicular processes ; those too
which are found in the interior of the body serve only to move
definite organs, as for example the muscles of the masticatory appa-
ratus of the Sea-Urchin. In the Spatangida, however, the shell is
movable at one point.

Very different relations obtain in the Holothuroida, where the
absence of large skeletal pieces leads to a proportionate development
of the muscular system. Its connection with the integument is very
definite. There is a circular layer of muscle beneath the connective
tissue of the skin ; within this are five longitudinal bands of muscle,
which are sometimes divided (Fig. 113, m) and are separated by
intermediate spaces of varying breadth ; these bands are inserted
anteriorly into the already described calcareous ring (1\). They are
connected through the five pieces which are pierced so as to give
passage to the nerves and ambulacral vessels. The circular layer is
continuous only in the Synaptm ; in the Holothuria? it is broken
through at the radii, so that it really consists of interradial transverse
fibrous tracts only.

Nervous System.

§ 169.

The principal parts of the nervous system of the Echinoderma
consist of a number of trunks, corresponding with the antimeres of
the body; they are placed ventrally and have a radial direction, and
are also connected by commissures around the oesophagus. These
commissures are formed by each of the nerve-trunks, which ac-
companies the ambulacral vessels, being divided into two halves
near the mouth ; these pass to each side, and become connected
with the corresponding halves of the neighbouring nerve-trunks,
which go to meet them. In this way a ring is formed, which sur-
rounds the gullet, which, however, on account of its mode of
formation, must not be compared with the oesophageal ring of the
Vermes. Each of the radial trunks corresponds rather to the
ventral ganglionic chain, or ventral medulla of the Annulata ; and
the commissures therefore between several such trunks are connec-
tions between the ventral medulla?, which owe their origin to the
concrescence of several incompletely separated persons.

As to their more special characters ; the position of the radial
nerves immediately below the well-developed epithelial layer of the
ambulacral groove (Fig. 100, A n) in the Asteroida and Comatula? is
an important point ; for it indicates that the relations between the
nervous system and the ectoderm are almost direct. Perhaps this
position is due to the mode of development of the radial nerves, and
possibly we here meet with a very low condition, in which differentia-
tion is not completed. The fact that processes of the epithelial

IS^EEVOUS SYSTEM OF ECHINODEEMA.

209

form-elements enter into tlie formation of these nervous tracts^ and
have the function of a supporting tissue, points to this conclusion.
In the Asterida each radial nerve consists of two bands thickened
in the middle, in which cellular and fibrous elements are equally
distributed. At the ends of the arms the radial nerves form a largo
swelling, which is connected with the optic organs placed there.

In Comatula this nervous band has the same characters. It is
accompanied by a blood-vessel, which is placed in the middle of it,
and, being pushed into it from above, divides it into two halves.
Branches are given off in regular order to the pinnulaa. In the
Ophiurida the radial nerve-trunks (Fig. 100, B n) are placed in a
space, covered over by the ventral plates (5), and supported by a
layer which, by its continuation into the ambulacral feet, is
shown to belong to the integument. In many, however (Ophiura
texturata), the nerves themselves are considerably differentiated.
They each consist of two nerve-chords, in which masses of ganglionic
cells are deposited; these correspond to the metameres of the arms.
The longitudinal trunks are connected by transverse commissures
with these ganglia, from which peripheral nerves are given off.
Each radial nerve therefore represents a ventral ganglionic chain.

The connection between the nervous system and the integument,
although at first sight this is implied only by the position of one on
the other, is an important aid to the
comprehension of the relations of the
skeleton. For when this arrangement
obtains the ambulacral groove cannot be
calcified ; this can only happen when
the nervous system, becomes more inde-
pendent.

In the Echinoida with a

masticatory apparatus, the nerve-pen-
tagon is intimately connected with it.

In Echinus (Fig. 106) the nerve-pentagon
lies above the floor of the oral cavity, be-
tween the oesophagus and the tips of the
ossicles of the masticatory apparatus,
and is attached by five pairs of bands
in this position. The nerve-trunks (c)
pass from the angles of the pentagon
to the spaces between the pieces of the
pyramid; and extend thence over the
oral integument to the ambulacral arese.

They are greatly widened in the middle
of their course, and divided into two
lateral halves by a median groove. The lateral branches from the
principal trunks accompany the branches of the ambulacral vessels.
The nervous system has the same arrangement in the Spatangidec,
but in them the oral ring forms a pentagon with unequal angles.

The nervous ring of the Holothuroida lies just in front of and a

Fig\ 106. Nervous system of
Echinus lividus; the masti-
catory apparatus is removed.
a CEsophagus cut through trans-
versely. h The commissures
of the nerve-trunks, forming
a pentagonal oesophageal ring,
c The nerve-trunks passing to
the radii, d Bands which hold
together the tips of the pyramids
of the masticatory apparatus
(after Krohn).

210

co:\rpAEATiyE anatomy.

little inside tlie calcareous ring ; anteriorly it is limited by tbe oral
integument (Fig. 113, n). As it is larger than any of the five nerves
(n) which it gives off, which is not the case in the nervous ring of
the Asteroida and Echinoida, it has more distinctly the significance
of a central organ ; in this point some analogies may be made out
between it and the ganglionic oesophageal ring of other animals.
But it is clear that it has no real homology whatever with any ring
of this kind in consequence of the origin of the oesophageal ring in
the Echinoderma, already described when we were treating of the
Asteroida. The peripheral nerve-trunks pass out by openings in
the five larger pieces of the calcareous ring, and extend, becoming
broader as they go along the outer side of the bands of longitudinal
muscles to the hinder end of the body ; near the cloaca they again
diminish in breadth ; they give off fine branches aloilg their course.
Each radial nerve-trunk may be divided into two layers, which arc
separated from one another by a layer of connective tissue. A
vessel accompanies the radial nerves ; this is separated by a wall
of partition from the ambulacral vessels, which lie still more to
the interior. The oral ring gives off tentacular nerves, in addition
to these radial trunks.

Sensory Organs.

§ 170.

Definite portions of the integument have, in this group also, a
special significance as tactile parts. The tentacles, as well as the
sucker connected with the water-vascular system, may be reckoned
as tactile organs, and the former become greatly developed, and
so of greater importance, when the ambulacral system is reduced, as
it is in the Holothurotda (Apodia).

Five pairs of vesicles, which lie on the roots of the radial trunks
in the Synaptidm, are said to be auditory organs, but their sensory
function is as doubtful as is that of the so-called eye specks in the
same genus.

Visual organs are exactly known in the Asterida only ; in all
other Echinoderma mere collections of pigment are regarded as eyes
or eye-spots.^^ The eyes of the sea-stars are placed at the tip of
each arm, which is ordinarily bent up, and so turned towards the
light; they occupy a pad-like elevation of the end of the ambulacral
groove, the epithelial layer of which is formed of long cylindrical
cells, and is very thick at this point. The rod-shaped cells contain
pigment. The eyes lie on separate points of the optic-pad. A
funnel-like cavity covered by the cuticle has its walls bounded by
rod-like cells, which are inclined from the periphery to the
funnel ; in this way their ends form the wall of the funnel. A
transparent body projects from the pigmented part of the cells into
the cavity of the funnel, and so fills up the greater part of its lumen.

ALIMENTAEY CANAL OF ECHINODERMA.

211

As tills apparatus lies on tRe terminal ganglionic swelling of the
radial nerves^ and the cells give oh fine processes to this ganglion,
the two parts may be regarded as connected at this point (Astera-
canthion rubens). Each eye which consists of a complex of cells
is a differentiation of the epithelial layer, and resembles therefore
the optic organs of other Invertebrata.

Alimentary Canal.

§ 171.

The alimentary canal, which varies greatly in character in the
adult Echinoderma, has a simpler predecessor in the primitive
enteron of the larval form, which is similar in all Echinoderma. Of
course this does not refer to those forms in which there is no larval
stage, and where the develo|)ment is compressed.

The first rudiment of the enteron is formed by the in-growth of
the cell-layer, which invests the body of the young larva. This gives
rise to a cascal tube, which is pushed down into the body, and the wall
of this tube forms the endoderm, while the outer cell-layer represents
the ectoderm. The organism is in fact a Gastrula. The entrance
into the rudimentary enteron is regarded as the primitive mouth.
A second invagination soon grows from the other side of the body
towards the blind end of the enteron ; this unites with the enteron,
becomes hollow, and so forms a continuous tube with the part first
formed. The parts formed last are the mouth and the oesophagus,
which is connected with it, and the part formed first is the mid- and
the hind-gut. The orifice which becomes later the anus, and the
portion of the enteron connected with it, are consequently the parts
first formed.

The larval intestinal canal is formed of three portions (cf. Fig.
94, A B). A wide oral opening leads into a contractile tube lying’
in the long axis of the body; this is the j)harynx or oesophagus.
Then follows a wider part, the mid-gut or stomach, which is con-
tinued into a narrow and retort-shaped tube, which is the hind-gut
and leads to the anus. These three portions correspond exactly to
the primitive divisions of the canal, which are distinguishable in
nearly all Vermes. The mouth and anus are at first on different
surfaces of the body of the larva. As the body is differentiated,
especially by the development of the ciliated band, they apparently
come to lie on one and the same surface, the so-called anterior side.
It is, however, quite clear that the ciliated band distinctly divides
two surfaces of the body; a decreased oral, and an increased anal
surface turned towards the former.

But before the enteron is fully developed by becoming connected
with the fore-gut, a portion of it, which forms a closed vesicle, is
constricted off. Two pieces are then separated off from this vesicle.

212

CO:^rPAEATIVE AXATOMY.

or two new vesicles are formed from tlie sides of tlie enteric caecal
tube. In this way three different bodies are differentiated from the
enteron. The two -paired vesicles^ which lie at the sides of the enteron,
represent the commencement of the coelom; the third vesicle
becomes connected with the dorsal ectoderm and opens onto it ; this
is the commencement of the water- vascular system. This apparatus,
like the lining cell-layer of the coelom, takes its origin therefore
from the enteron, and from that portion of it, which is without
doubt its hinder part, although it appears first of all and grows
inwards, from what is later on the anus. This arrangement appears to
indicate that there are arrangements in the water-vascular system, as
well as in the coelom (for the two are connected together), which are
phylogenetically connected with the terminal division of the enteron ;
in this case this tract of the enteron is not homologous with the
enteron of a Gastrula, but corresponds a priori to a hind-gut, the
early development of which is clearly due to the complicated
character of the organs about to be differentiated from it. These
organs are those which are especially necessary to the organism. I
consider therefore that the first formed rudiment of the enteron is
not a Gastraea-enteron, and that its orifice is not the primitive mouth,
but that they are respectively the true hind-gut and anus. The
median division of the enteron which is divided from the hind-gut
is morphologically a part of it. The differentiation of the above-
mentioned organs out of the hind-gut points to stages in which
organs were connected with the hind-gut in much the same way as
they are in many Gephyrea. But as yet it is impossible to prove
directly that such structures have been passed on to the Echino-
derma; and it is better to regard these remarkable processes as
presenting us with a problem which has still to be solved.

AYhen the body of the Echinoderm is formed in and partly
from the larva, the enteron of the larva does not completely pass
into it. The perisome, when formed, first grows round its middle
part, and in the sea-stars takes up this only with the hind-gut. In
the Echinoida the anus also appears to be formed anew. The larval
enteron is retained most completely in the mature stage of the
Holothuroida.

The fully-developed enteron is found to hang, in the mature
Echinoderm, in a coelom, which is often vdde, and undergoes
various changes during its differentiation, which arc generally corre-
lated with the characters of the perisome. As a rule the mouth
always retains its position in the middle of the ventral surface of the
body.

§ 172.

The mouth in the sea-stars has a radiate form, owing to the
projection into it of interradial processes ; hard papilla3 and spicules
are formed by the perisome, and function as masticatory organs.
They are specially developed in the Ophiurida, where they generally

ALIMENTARY CANAL OF ECHINODERMA.

213

form several rows one above tbe other (Fig. 102, d). In these
therefore the dermal skeleton forms the organs for the comminution
of the food. A short, wide oesophagus follows the month ; this is
continued into a wide mid-gut (stomach), which occupies the middle
of the body. In the Ophinrida and many Asterida (Astropecten,

Fig. 107. Transverse section through the arm and disc of Solasterondeca.
The radial and the interradial portions are figured on opposite sides. o Mouth.
V Stomachal cavity, c Eadial caeca, g Genital gland, rn Madreporic plate, s Stone-
canal with its so-called heart, p Ambulacral feet (after G. 0. Sars).

Lnidia) the stomach is always a blind sac, as it is also in Brisinga.
But in all Asteroi'da it is provided with diverticula or csecal
saccular appendages, which are indicated in the Ophinrida by
radial constrictions. The gastric caeca of the Asterida extend in
pairs into the arms ; they
spring from the stomach, and
have the form of thin-walled
tubes, closely beset with
lateral appendages (Figs.

107, c; 108, h)j which as a
rule are united by pairs into
one canal before they open
into the stomach. This tract
represents an unpaired por-
tion of the enteron belonging
to each antimere (arm) of the
Asteroi'da, while the caecal
tubes form a paired por-
tion. In Astropecten auran-
tiacus these tubes arise sepa-
rately from the stomach. The
unpaired portion in each arm
has therefore disappeared in
this form, and with it the
primitive condition. In most
of the Asterida the short hind-gut is continued from the stomach
to the anus, which is placed on the dorsal surface.

The enteric tube of the Crinoida(Comatula)is modified; it describes
a spiral coil, and its narrower short terminal portion passes into a
tubular and projecting anus, which is placed interradially near the
mouth. This coiled arrangement, which is apparently very anomalous^

Fig. 108. Asteriscns verruculatus:
opened on tlie dorsal surface. a Anus.
i Eosette-shaped enlarged enteron (stomach).
h Tubular radial appendages of the enteron.
g Genital glands.

214

CO:\IPAEATR"E AXATOMY.

may be also seen in young Asteroida ; in them it is only seen for a
time during tlieir development^ but in tbe Crinoida it is continued
as a permanent condition.

Tbe enteron is attached to the body-wall by radial fibres. The
radial C80ca of the Asteroida are attached to their body-wall by a
special peritoneal fold, which extends along each cfecuin,

§ 173.

In the Echinoida the mouth is similarly provided with Mastica-
tory Organs, but they are removed from the outer surface and placed
in the coelom. They there form an apparatus, which, in the Clypea-
strida, consists of five pairs of triangular calcareous pieces, but in the
Echinothurida, Cidarida, and Echinida is much more complicated.
Five pieces directed towards one another carry a tooth-like point,
and arc united with' several others into a complex organ known as
the Lantern of Aristotle ; the oesophagus traverses it. The
enteric tube always describes several coils. The narrow fore-gut
passes into a wider portion, which forms the longest part of the canal.
It has sometimes faintly-indicated diverticula (Echinida), sometimes
veritable coeca (Clypeastrida), which (as in Laganum) project into
the cavity of body which is marked olf by the supporting pillars
of the calcareous shell. In these forms ^Anesenteric fibres extend
to the body-wall for the whole length of the coiled intestine.

In the Holothuroida the enteric tube, which is longer than the
body, forms a double loop, while in the SynajDtaa (with the exception
of the Chirodotas) it extends straight through the body-cavity, and
is provided with numerous diverticula. A muscular portion of the
enteron Vvhich succeeds the oesophagus is to be regarded as a special
differentiation; it appears to function as a muscular stomach
(Synapta)). This character is also seen in the Asteroida, where the
oesophagus has in the same way a stronger muscular wall than the
rest of the intestine. The portion of the intestine behind the
muscular part in the Holothuroida may thus correspond to the
stomach of the Asteroida. The end of the canal is widened out in
the Holothuroida ; but this only corresponds to the hind-gut of the
Asteroida, although it is called a cloaca; it has two or more arbores-
cent organs opening into it.

A sieve-like fenestrated lamella fastens the canal to the body-
wall. This mesentery is simpler in the Synaptas which have a
straight canal, while in Chirodota it is separated into three parts, in
correspondence with the extent of the enteric loop ; each part is
connected Avith an interradial portion of the body-wall.

ALIMENTAKY CAE^AL OF ECHINODEEMA.

215

Appendages of the Alimentary Canal,

§ 174.

The above-mentioned radial caeca o£ the Asteroida might be
regarded as organs differentiated from the primitive enteron^ were
it not that they must be regarded differently from a phylogenetic
point of view. I consider
that only certain other
interradial caeca are to
be regarded as appen-
dages of this kind ; they
present very various de-
grees of development. In
the aproctous Asterida
they are absent_, or are re-
duced to two (Astropec-
ten)^ while in the others
they are often very greatly
developed. Archaster has
five such caecal sacs^divided
at their ends_, and in Cul-
cita this division is carried
still further^ so that each
branch forms a racemose
tube. In this way these
appendages acquire the
form of glands^ and ex-
hibit relationship with a
structure which is very
common in the Holothu-
roi’da.

The structure in ques-
tion is connected with the
terminal portion of the
alimentary canal, known
as the cloaca/^ and
as a rule consists of two
chief trunks, with short
branches, which extend

forwards throughout the whole length of the body-cavity (Fig. 109,
r), and are provided with a large number of ramified csecal tubes.
Although the function of these organs, which were formerly known
as ^^lungs,^'’ and considered to be internal respiratory organs, is
different from that of the interradial C83cal tubes of the enteron of
the Asterida, yet they are exactly the same morphologically, and
are a further development of the simpler tubes of the Asterida.

Fig. 109. Enteric canal and tree-like organs of
aHolotliurian. o Mouth, i Enteric tube.
d Cloaca. a Anus. c Branched stone-canal.
p Polian vesicle, rr Tree-like organs, r' Connec-
tion between them, at the opening into the cloaca.
m Longitudinal muscular layer of the body.

21G

COMPAEATIVE ANATOMY.

The faiiction of these organs is not at all certain. The view
that they are respiratory organs is opposed to the facfc that only one
of them has any connection with the network of blood-vessels^
while the other is merely attached to the body-wall, and projects into
the body-cavity. However, the fact that water is taken up by these
organs, and is again expelled, chiefly by the aid of the strong
muscular wall of the hind-gut, is of importance.

In some Apodia (Molpadia borealis) they are only provided here
and there with branched caeca, while in others the number of cceca is
increased. Thus in M. chilensis not only is one of the trees divided,
but the rectum also bears a number of smaller trees. The organ
is divided five times in some Lisarmatida). They are simpler in
character in Echinocucumis (E. typicus), where they form long fine
tubes, provided with one short branch only.

The tree-like organs of the Holotliuriae are absent in the
Synapta3, but there is an arrangement, which as yet is only veiy
incompletely understood ; this consists of canals, which are placed
along the insertion of the mesentery and open into the coelom by
funnel-like ciliated orifices (Chirodota pellucida).

Glandular organs are also present on the rectum of many
Holothurise in addition to the tree-like organs. These — the Cuvierian
organs — either form unbranched ccecal tubes, which are inserted
singly or in thick tufts (Bohadschia, etc.), or they form racemose
organs (Molpadia), or, finally, filamentous canals, beset with lobate
tufts of glands, and arranged in a whorl (Pentacta and Muelleria).
They secrete a substance which forms fine sticky filaments, which
may serve as organs of defence.

Ccelom.

§ 176.

The development of the coelom from a vesicular structure cut
off from the earliest rudiments of the enteron (§ 171) gives to this
cavity a different signification to that which it has in other divisions,
where the coelom is not formed from any part of the rudiments of
the enteron. Tlie importance of this point must not be overlooked.
But it may wmll be supposed that the w\ater-vascular system, which
is developed in just the same wmy, formed an apparatus which
primitively formed part of the coelom, and w^as connected wfith
the hind-gut.

Tlie two coelomatic tubes nipped off from the enteron gradually
increase in size, and by becoming attached in part to the enteron,
and in part to the body-wall, form the more or less spacious cavity
of the coelom. The mesenteric filaments or bands wdiich pass
from the perisome to the enteron are to bo regarded as the remains
of the w'alls of these primitive structures.

As the radiate Echinoderm body becomes developed, the coelom

VASCULAE SYSTEM OF -ECIimODEEMA.

217

passes into the rays. Thus in the Asterida and in Brisinga it
extends through the arms. The same thing happens in the
Crinoi'da, but in them the canals are narrower. In each arm it can
be divided into three parts^ which are connected with special
divisions of the ccelom of the disc. This latter portion is separated
into several divisions by connective bands, which here and there
form membranous tracts; they communicate with one another at
certain points, and at other points pass into the canals. In the
Echinoida and Holothuroida, where the organism is more concen-
trated, the coelom is more simple. In the former, however, the
mesenteric filaments, and still more the calcified pillars and columns,
which traverse the coelom of the Clypeastridse, remind us of divisions
into separate parts ; several such spaces are also marked off in the
coelom of the Holothuroi'da. In the Asterida and Echinida, as well
as in the Holothuroida, the parietal and visceral tracts of the coelom
have been observed to be provided with cilia. The contents of
the coelom appear to be the same in character as the blood, so that
in it we have to recognise a portion of the blood-cavity. In some
cases communications with the exterior have been definitely observed
(Crinoida) ; as also communications with the water- vascular system
(Crinoida, Holothuroida). The first set are due to numerous
canaliculi, which traverse the interradii of the perisome, and open by
the so-called calycine pores.

Vascular System.

Blood-V essels.

§ 176.

The nutrient fluid in the Echinoderma is a clear, or slightly
opalescent fluid, which is seldom thick or even coloured, and which
is very probably mixed with the water which is taken in from the
exterior. The form- elements in this fluid are simple cells.

The blood- cavity is formed in the flrst place by a special system of
canals, but also by the coelom, which is probably connected with a
third cavitary system, the system of so-called water- vessels. Owing
to the uncertainty of our knowledge of this Vascular System, that
is, of its mutual relations and connections, it is as yet impossible to
make any generalisation which will hold for all the divisions ;
although indeed remarkable progress has lately been made in our
knowledge of this part of the anatomy of the Echinoderma. But
from the similarity of construction of these canals and spaces we
may suppose that a connection between them does really exist.

The close association of the hmmal system and the nerve-tracts
may, however, be regarded as a general arrangement. A blood-
vascular trunk accompanies each radial nerve-trunk, and is continued

218

CO^IPAEATIYE AXATOMY.

into a circular canal^ wliicli surrounds tlie mouth. The radial
vascular trunk corresponds to the ventral vessel of the Vermes^
which has a similar relation to the ventral medulla. A tube
which passes from the oral ring to the stone-canal (for which see
below) was formerly regarded as the heart, but this organ cannot be
regarded as such. The same remark applies to the similar structure
in the Echinoida. We have therefore still to search for a heart as
the central organ of the blood-vascular system. The enteric vessels
form a second division of the blood- vascular system.

In the Echinoida the nerves lie within the radial blood- vascular
trunks ; in the Crinoida and Holothuroida they lie on their outer
side, and the same is the case in the Asterida and Ophiurida. The
circular vessel surrounding’ the mouth in the Asterida and Crinoida
and in the Spatangidoc among the Echinoida, where it has the form
of a wide sinus, is described as having the same relations to the nerve-
tract ; although in Echinus there is said to be a blood-vessel placed
farther from the mouth, and above the masticatory apparatus which
surrounds the oesophagus. It is probable that this separation of the
blood-vessel from the nerve-ring is due to the development of the
masticatory apparatus. In the Holothuroida the adoral blood-
vascular ring is connected with the nerve-ring, but is placed inside
it, and 'nearer the mouth. It may break up into a plexus. The
aboral vascular ring found in the Asterida and Echinida does not
appear to have so much morphological importance, as it is confined
to a few divisions. Other vessels, which surround the generative
glands, and there form wide sinus-like spaces, pass into it, in addition
to the vessels from the perisome. In Comatula also a vessel,
wdiich forms a covering around the genital chord, is continued into
the arms and pinnula3. In the Asteroida and Crinoida the vessels
of the enteric canal are not independent. In Comatula they form a
network, with wide meshes, in the coelom ; this is connected with
the oral vascular ring. A bundle of vessels passes from this net-
work, along the axis of the cup to the centrodorsal plate, forming
a special organ widened out into five chambers, the importance
of which is not known.

The enteric vessels in the Echinoida and Holothuroida are more
independent. A dorsal and a ventral vessel can be distinguished,
which have just the same characters, as the same vessels in the
Vermes (cf. § 138). In Echinus the dorsal vessel is double, for in
addition to the one which runs directly on the enteron there is one a
little way from it, which gives off branches to the former one, as
well as to the enteron. In the Spatangida3 the ventral vessel has
been observed to communicate with the water- vascular ring. The
enteric vascular trunks of the Holothuroida are enlarged in the
middle of their course, and the dorsal vessel passes into retia
mirabilia.

WATEE-YESSELS OE ECHINODEEMA.

219

Water-Yessels.

§ 177.

In describing tlie ambulacra (§ 160)^ mention was made of a
water-vascular system/’’ wbich took in water from tlie exterior,
and carried it to the ambulacral organs, which it put into the condition
of erection. Other organs in addition to the structures which take
part in locomotion are filled by this system of canals ; and these we
have already spoken of as modifications of the ambulacral feet. The
probability of this system of canals being a portion of the blood-
vascular system has been already pointed out. Communications
have been noticed at several points; and in some cases openings
into the coelom also have been distinctly observed. It is, however,
not yet certain how far these vessels have been formed from other
organs. In any case we must still regard the water- vascular system
as being independent, especially as its development shows that it
is so, and as an important division of
the system (stone-canal, etc.) arises as
a structure, which is primitively
quite indejoendent of the circula-
tory system.

In the larvas of the Echinoderma the
water-vascular system is formed by a
differentiation from the earliest rudiment
of the enteron ; as it gets nipped off, it
forms a transparent tube, ciliated in-
ternally, and connected with the integu-
ment on the back of the larva, where it
soon opens by a pore. When in this
condition the organ has a close resem-
blance to the excretory organs in the
larv33 of many Yermes (Sipunculidae),
so that from this point of view it does
not seem improbable that the water-
vascular system has been differentiated
from a primitive excretory apparatus.

This tube, with the other rudiments
of the Echinoderm (Fig. 110, A), becomes
gradually surrounded by the perisome;
it then changes its form by becoming
metamorphosed into a five-leaved rosette
{i). The portion which still continues
to open to the exterior by the dorsal pore gradually changes its
position, and gets to lie on the ventral surface of the Echinoderm ;
each leaf of the rosette is now developed into an elongated canal
with lateral diverticula ; it is like a pinnate leaf, and forms the
rudiments of the ambulacral portion of the water- vessels. In the

Fig. 110. Larva of an Asterid
(B ip inn aria) with budding
Echinoderm. e e‘ d‘ g g' Pro-
cesses of the body, h Mouth.
0 Anus of the larva. A Body
of the embryonic Echinoderm.
h Ciliated tube, i Ambulacral
rosette (Rudimenis of the water-
vessels) (after J. Muller).

220

COMPAEATIVE AXATO^IY.

Holotliuroida tEe similarly rosette-sEaped rudiment forms tlie oral
tentacles, wEicE Eave tEerefore an indubitable relation to tEe ambn-
lacral system (§ 162). The succeeding processes which are of any
importance affect the central portion of the rosette, in which the
canals of the five leaves Eave their common orifice. This is con-
verted into a circular canal, which continues to form the central
portion of the apparatus ; the canals in the leaves of the rosette grow
out radially, and extend whilst the number of their lateral branches
increases, over the ambulacra, which get larger at the same time.

The adult stage may be directly derived from these arrangements,
formed during the development of the Echinoderm body. A branched
vascular system (Fig. Ill) has finally developed from the primitive
tube, and has its ends directly connected with the suckers (p) and
other such processes. The radial trunks of this system communi-
cate with the circular canal (c), and this again is in connection with

Fig. 111. Diagrammatic represcutatiou
of the water- vascular system of a
Starfish, c Circular canal, ap Polian
vesicles, m Madreporic plate, Stone-
canal. r Padially-arrangccl princii)al
f ranks (Ambulacral canals). r‘ Lateral
branches, p Suckers, a Their ampulla3
(part only of the ambulacral canals and
their appendages arc figured).

the surrounding medium. We
have already mentioned the fact
that in Spatangus the water- vas-
cular ring around the mouth is
connected with an enteric vessel ;
and inasmuch as the contents of
the two systems of canals are
similar, it is very probable that
they do not only communicate,
but are also to be regarded as
parts of one structure.

The connection with the ex-
terior is of a special nature, and
is effected in various ways. As the
Echinoderm is being differentiated
in the larva, that portion of the
rudimentary water-vascular system
which is taken into the body of
the Echinoderm remains connected
with the perisome at one spot,
where a porous calcareous jilate —
the madreporic plate {m) — is de-
veloped ; this plate communicates
with the lumen of that portion of
the canal which is connected with
it. The duct {m') leading from the
madreporic plate to the circular
canal, which also is a portion of the

primitive w'ater-vascular system,
has ordinarily calcareous substances deposited in its walls, and
is therefore called the stone -canal; its walls form a complicated
cavitary system. Water passes into the stone-canal by the cribriform
madreporic plate, and thence to the circular vessel. It has also
connections with the coelom.

WATEE-VESSELS OE ECHINODEEMA.

221

Tlie portion corresponding to tlie stone-canal is not always con-
nected with the perisome. In the Holothuroida the connection is
broken close to the dorsal pore of the larva ; the latter disappears,
and the stone-canal hangs freely in the body-cavity, whence it takes
up water by a very complicated and porous terminal apparatus.

There are further complications of the water-vascular system,
due to the formation of contractile diverticula of the water-canals
projecting into the body-cavity ; these must be mentioned in addi-
tion to the arrangement, just sketched. These diverticula vary
greatly in character ; on the circular canal they form large pear-
shaped vesicles (Polian vesicles) (a^^) ; where the ambulacral canals
pass into the sucking feet they form small ampullge (u), which
always project into the body-cavity, and which may be regarded
as enlargements or diverticula of the branches of the ambulacral
canals. They are cavernous in structure. Both these kinds of
organs serve as receptacles for the fluid passing into the canals, and
owe their structure to their adaptation to the function of this
vascular system ; that is to say, when the suckers are drawn in,
their ampullge are always filled, and when the suckers are pro-
truded the contents of the ampullse swell them out. What the
ampullEe are for the separate suckers, the Polian vesicles of the
circular canal are for the whole system of canals ; that is, they
allow of a much more rapid action of the ambulacral structures,
whether these are pushed out or drawn in, than would be possible if
the quantity of fluid needed for the erection of each separate sucker
must be first taken in either by the stone-canal or the madreporic
plate. This activity of the ampullge of the suckers and of the
Polian vesicles of the circular canal is due to the contractility of
their walls, in which a muscular layer has been made out. The dis-
tribution of the fluid is also regulated by muscular fibres, which
here and there enclose the canals. In addition to this the ciliated
epithelium which is found throughout the water- vascular system
serves to distribute and continually change the water, and so without
a doubt render it efficient as an organ
of respiration.

§ 178.

The arrangement already sketched
in a general way applies most com-
pletely to the Asteroida. In them
the stone-canal is always inserted
into a madreporic plate, which, as a
rule, is placed interradially on the
dorsal surface of the body. In some
cases there are several (2-5) madre-
poric plates, and the number of stone-
canals is then also proportionately increased; this condition, how-
ever, does not remain constant in the species of the same genus.
It is to be regarded as the more primitive one ; it is important

Fig. 112. TrausYerse section through
the Stone-canal of Astropecten
aurantiacus (after E. Teuscher).

222

co:\rpARATiyE axato]my.

tlierefore to know tlie earliest rudiments of tliis arrangement. The
stone-canal always runs close to the heart-like tube. Calcareous
bodies are deposited in it and form a tine network ; they do not
differ from those found in the perisome. They are arranged in
rings ; internally to them there is a longitudinal ridge from which
arise two coiled and thinner lamella), similarly calcified. The cavities
which commence at the fine pores of the madreporic plate pass
between these lamella). The ambulacral canals (Fig. 100, A a) extend
along the skeleton of the arms, embedded in the ambulacral groove,
and give off branches to the feet which arise between the lateral
processes of the segments of the ambulacral skeleton; the ampulla) of
the feet pass inwards through the clefts between the calcified
segments, and so come to lie within the arms (ap). At the points
where the ampulhe are connected with the ambulacra! feet there
are valves, which shut when the ampulla) contract (Asteracanthion
rubens) . The number of Polian vesicles varies ; they are sometimes
increased in number, and form racemose tufts (Astropecten auran-
tiacus), or they may be altogether wanting.

In the Ophiurida the stone-canal is inserted into a plate sur-
rounding the mouth ; but this plate is not formed in the same way
as the madreporic plate, but so that the stone-canal takes up fluid from
the body-cavity only. At the circular canal the stone-canal widens
out into an ampulla, and is attached to an interradial portion.
Polian vessels are not always present. The suckers have no ampulla).

In the Crinoida the ambulacral water- vascular trunk runs below
the radial blood-vessel, and sends branches into the tentacles of the
arms, as well as of the pinnulae (Fig. 115, lu). The radial trunks
meet in a circular oral canal, which sends off short canaliculi, with
open mouths, into the coelom. They take the place of the stone-
canal, which is not present. As there are no ampulla) or Polian
vesicles either, the water-vascular system is in the Crinoida of a lower
grade than in the other divisions.

The Echinoida are allied to the Asteroida. The madreporic
plate always lies at the aboral pole ; it is either formed by one of the
genital plates (Fig. 103, m), or by several of these, or an interradial
plate is converted into the madreporic plate, or it is formed by a
special plate (Clypeastrida)). The stone-canal is sometimes soft
(Echinus) and sometimes provided with firm walls (Cidaris). The
circular canal, provided Avith five Polian vesicles (these are absent in
the Spatangida)), lies in the Echiuida at the base of the masticatory
apparatus, and gives off its ambulacral canals downwards, whence
they radiate out to the ambulacra. On the inner side of the shell,
and running along each of the ambulacral area), are the branches of
the ambulacral canals, Avhich are distributed to the pores of the
calcareous plates, and supply the suckers or their equivalents which
arise at this point ; and give origin to transversely-placed ampullar
enlargements (Fig. IIO, a).

In the Ilolothiiroida, owing to the separation of the connecting
piece, Avhich later on functions as the stone-canal, from the perisome
of the larva, which passes into the substance of the Echiuoderm,

WATEE- VESSELS OF ECHINODEEMA.

223

tliese parts liave a different relation to that which they have in the
rest of the Echinoderma. The walls of the stone-canal_, which
hangs freely into the coelom, are more or less calcified; when they
are more so they form a firm capsule. The porous parts of the
canal are usually distinguished by calcification, and so repeat within
the body the arrangements of a madreporic plate. The ends of
each branch carry a porous
piece when the stone-canal
is broken up into branches ;
this repetition of parts leads
to the formation of race-
mose structures, which are
only functionally similar to
a number of madreporic
plates grouped around the
stone- canal. The stone-
canals vary in number, as
well as in arrangement.

Often only one is present ;
in other cases, and notably
in the Synapt^, there are
several arranged around
the circular canal. The
number too of the Polian

hundred.

The canals from the
circular canal {G) run for-
wards inside the calca-
reous ring (R), and give off
branches to the oral tenta-
cles (T) ; a C90cal elongated
tube, corresponding to the
ampullae of the suckers, is
connected with each of them. These tubes are of some size in
the Holothmada, and lie on the outer side of the calcareous ring ;
they are only feebly developed in the Synaptidee. The radial trunks
going to the ambulacra are placed, in Holothuria, in the bundles
of longitudinal muscles, which are thus divided into two halves. In
Cucumaria they are placed on the outer side of these muscles. The
branches of these canals, as in other forms, go to the feet. When
the feet are atrophied the vascular branches, vfhich go to them, are
atrophied also ; but the principal trunks appear to persist even in
the Apodia, for they have been observed in Synapta, although,
indeed, diminished in size.

vesicles (Fig. 113, y>), which
are present in these forms,
varies; in Holothuria and
Molpadia there is one, in
Synapta Beselii about fifty,
and in Cladolabes about a

Fig. 113 Longitudinal section through the
anterior part of the body of Synapta digitata.
B B' Calcareous ring, r Muscles passing from it
to the oesophagus, o Mouth. 1) Enteric tube.
C Circular canal, t Canals to the tentacles T.
p Polian vesicles, n Nerve-ring. n‘ Eadial
nerve-trunk, passing through the calcareous ring
B'. w Bands of longitudinal muscles. (7 Ducts
of the generative organs (after Baur).

224

COMPARATIVE ANATOMY.

Excretory Organs,

§ 179.

The arraugements commonly found among the Annulata
(looped canals, or nephridia) are not found in the Echinoderma, but
there are signs that these organs, or organs of the same type at any
rate, are not entirely foreign to the organisation of the Echinoderma.
For in the Holothuroida two canals have been observed to run in the
wall of the body; and these are beset with infundibular organs,
which open into the coelom (Chirodota pellucida). In the Synaptm
also there are organs which correspond to the internal mouths of the
looped canals of Vermes, but these are not connected with canals.
Finally, in the Crinoida ciliated organs have been made out in the
dorsal canal of the arms, which is a continuation of the coelom. It
cannot be definitely asserted that these structures are all of the same
kind, but from the characters of those first mentioned it is probable
that they have relations to an excretory apparatus. We can as
yet only suggest that a fundamental relation of the same kind holds
for the ambulacral water-vascular system. Anyhow the arrangement
of this system in the body does not justify us in regarding inquiries in
this direction as barren ones, for the excretory organ of many
Mollusca (Nudibranchiata) has the form of a richly-branched system
of canals; and the communication of the Tvater- vascular system
with the exterior, as well as with the blood-vessels (or, what is
the same thing, with the coelom), can hardly point to its being
anything else than part of an excretory apparatus.

Generative Organs.

,§ 180.

The asexual methods of reproduction so common among Vermes
do not obtain in the Echinoderma, except in so far that the animal
itself is the product of gemmation. An indication of this mode of
reproduction is, however, retained in the Asteroida — in the regener-
ation of lost antimeres (arms).

Almost all Echinoderma — there are but few exceptions — have
the sexes separate, and their organs arranged conformably with
the radiate type. The male and female organs are both very
simple in character, ami can only be distinguished easily when the
generative products are mature, tlie ovaries being generally dis-
tinguished by the brighter coloration of the eggs, which arc
yellow or red, whilst the testicular tubes arc almost always
white. The form-elements of the sperm are very generally fila-
mentous structures provided with a small head. The generative

GENERATIVE ORGANS OF ECHINODERMA.

225

Fig. 114. Generative organs of an
Ophiurid (Ophioderma longicauda).
The dorsal integument and the
digestive organs have been removed.
T Arms, cj Ovarian acini.

system is simple in structure,, tlie excretory ducts arc not com-
plicated, and tliere is no intromittent
organ, so tliat tlie surrounding water
is the medium of communication in
impregnation. On the whole there
is a great resemblance to the corre-
sponding structures found in Vermes.

The lowest stages, both as to num-
ber, arrangement, and as to the more
special characters of the organs, are
seen in the Asteroida. The testes or
ovaries are tubular or lobate glan-
dular canals, which in some forms are
arranged in two rows, and disposed
conformably with the metamerism of
the arms (Ophidiaster, Archaster).

In others tliere are only two groups
in each arm, which may extend along
the whole cavity of the arm (this is
the case even in Brisinga), or may
be limited to the interradial space
merely (Fig. 108, cj). A comparison
of these characters shows us, that there
is a gradual reduction in the number
of the generative glands, which corre-
sponds with the gradual centralisation
of the organism, which we have
already seen to occur in the Asteroida.

In the aproctous forms the tubes have
no excretory orifices, and the genera-
tive products are passed into the body-
cavity. We do not yet know how
they reach the exterior. In other
Asteroida the generative glands open
to the exterior by special plates, dis-
tinguished by their fine pores (cribri-
form plates), and placed in the dorsal
interradii ; or they have a single duct
with a narrow orifice (Pteraster) .

Each organ is surrounded by a blood
sinus, which envelopes the separate
lobes and lobules. The generative
products pass into this sinus and are
not evacuated directly.

The structure and arrangement of
the generative organs of the Ophiurida
is the same as in the Asteroida. There is only one example of
hermaphroditism (Ophiura squamata). The generative glands (Fig.
1 14, (j) are arranged by pairs in each interradial space, and on the

Q

Fig. 115. Transverse section
through the pinnula of a sexually
mature Comatula (Antedon Esch-
richti). The dorsal surface is
turned upwards, and the ventral
downwards, p Tentacle, g Lumen
of the genital chord, w Water-
vessel, with lateral branches into
the tentacles, n Nerve - chord.
h Blood-vessel on it. eg Canal
around the genital chord, c d Dorsal,
cv Ventral canal. All three com-
municating with the coelom (after
H. Ludwig).

226

COMPAEATIYE ANATOMY.

dorsal side ; their products appear to pass into the coelom^ whence
they reach the exterior by the narrow pores placed in the interradii
of the ventral surface (Fig. 102, cj). In the ovoviviparous Ophiurida
this cleft may be seen to adapt itself to the necessary size. A chord-
like structure placed in each arm, and ramifying into the pinnulse,
forms the generative organs of Comatula. This genital chord is
surrounded by the blood-vessels just as are the branched tubes of
the Asterida. In the arms it remains sterile, and its products are
developed within the pinnulge from the walls of the chord. The
sperm passes out by the pores already described.

§ 181.

The generative glands, which in the Asteroi’da are arranged by
pairs in each radius, are converted into unpaired structures in the

Echinoi'da ; this implies
further centralisation.
Their relation to the
primitive condition can
only be made out from
I their interradial arrange-
ment, so that each organ
may be regarded as
formed of two radial
ones. They form greatly
ramified glands, which
generally project far into
the coelom (Fig. 116, ^/),
and open at the genital
plates (Fig. 103, g). In
the Spatangidm one of
the five genital glands,
which are the typical set
for the Echinoida, is
atrophied, and at the
same time the genital
plate, which was the madreporic plate also, becomes the madreporic
plate only.

In the Flolothuroida they are still more reduced. The testis or
ovary forms a tuft of richly-branched tubes, which unite at a
common duct (Fig. 113, G) ] this opens near the mouth, and
ordinarily between the tentacles. This indicates a relation to the
radii; the organs, which in other forms are broken up, are here united
into one, and the higher grade of organisation, already seen in the
Echinoida, is still present, as is shown by the presence of an
excretory duct.

In the Synaptac the organs are hermaphrodite, and formed
on the Holothurian type. The various tubular glands are united at
a common excretory duct, which opens to the exterior above the

Fig. 116. Generative organs of an Echinus.
Eather more than the ventral half of the shell has
been removed, a Ampnllse of the ambulacra.
i Hind-gut. g Ovarian acini.

GENEEATIVE OEGAXS OF ECHIXODEEMA.

227

calcareous ring. In each tube (in S. digitata) tbe sperm is developed
on tbe inner surface, while tbe ova are formed beneath it, and when
fully developed form a longitudinal band which projects into the
lumen of the tube. There is a common duct for both sets of
products. If this condition must necessarily be regarded as a lower
one, from which the ordinary dioecious arrangements have been
derived, we find an interesting phenomenon in the Synapte ; for
while their generative gland has retained its primitive structure and
primitive function, great changes have taken place, for the glands
have been reduced in number and degree of complication, and have
only one excretory duct for the whole apparatus.

Q 2

Fifth Section.

Arthropoda.

General Review of the Group.

§ 182.

Ill this division of the Animal Kingdom the body consists of a
number of metameres^ and this number is generally definite for the
various groups. As a rule the metameres are not all differentiated
to the same extent ; this is not implied merely by variations in
external form and size_, but also in the characters of the internal
organs of the body. A number of metameres may be united to
form larger segments, in which the separate metameres lose their
individuality. These larger segments sometimes retain indications
of this kind of composition ; but these indications are sometimes
lost, or only apparent in the early stages of development. This
state of things results in a desegmentation of the body.

The movable appendages of the body form another characteristic
of the whole group ; these appendages are almost always segmented.
In this point therefore, as in the metameric character, they have
some resemblance to the Annulata among the Vermes. We do not
know what are the forms that ally the two groups, and it is not cer-
tain that the two chief groups of the Arthropoda form a common
phylum. There are many reasons for regarding the Branchiata and
Tracheata as being distinct stem-forms. As in the Annulata the
nervous system consists of an oesophageal ring connected with a
ventral ganglionic chain ; while the central organ of the circulatory
apparatus is here, also, dorsal in position. In the Vermes organs are
repeated in each segment, but in the Arthropoda they are common to
the whole of the body ; and even when the metameres are similar
externally, the internal organisation often gives signs of the more
confined dominion over the whole body now possessed by meta-

CLASSinCATIOX OF AETHEOPODA.

229

merism, as compared, that is, with that which it exercised in the
Annulata.

I give the following review of the Classification of the Ar-
thropoda :

A. Branchiata.

I. Crustacea* * * § **).

a) Entomostraca.

1. Cirripedia.

Balanus, Coronula, Lepas.

Rhizocephala.

Sacculina, Peltogaster.

2. Copepoda.

Cyclops, Cyclopsina, Corycnpus, Sapphirina.

Siplionostomat).

Callous, Ergasilus, Dichelestium, Chondracanthns,

Achtheres, Lernaea, Lernreocera, Penella.

3. Ostracodat).

Cypris, Cypridina.

4. Branchiopoda§).

Cladocera.

Daphnia, Sida, Polyphemus, Evadne.

Ph yll op oda.

Branchipus, Apus, Limnadia.

h) Malacostraca||).

1. Thoracostraca (Podophthalma).

Schizopoda.

Mysis, Euphausia, Thysanopus.

Caridaf).

Crangon, Alpheus, Palremon, Hippolyte, Peneus.

Decapoda.

Macrura.

Astacus, Palinurus, Galathea, Pagvirus.

Brachyura.

Carcinus, Maja, Hyas, Dromia, Dorippe.

C u m a c e a “*) .

Cuma.

* The appendages are complete in each segment of the body, although they often
undergo adaptive modifications. They either function directly as respiratory organs,
or the latter are very closely connected with them.

f A large number of families are brought together into this special subdivision,
owing to the more or less complete parasitism they exhibit ; they may be distin-
guished from free-living Copepoda, but must be considered as derived from them.
The Rhizocephala bear the same relation to the Cirripedia.

;{] They show signs of affinity with the developmental stages of the Cirripedia, by
the possession of a mantle-fold, which forms the two-valved shell.

§ This division seems to be_ the most direct continuation of the Nauplius-form,
from which stage it is derived by the mere formation of metameres, while the appen-
dages themselves undergo only slight modifications.

II These forms, which are allied to the preceding division by the Nauplius stage,
which is seen in Peneus and Euphausia, represent on the whole a further development
of the Crustacean organisation.

^ Connect the Schizopoda and Decapoda : sometimes they are classified with the
latter.

** They correspond to the lower developmental stages of the Decapoda, for they
closely resemble the Schizopoda in their form of body. The eyes have no movable
stalk, and so far they approximate to the Arthrostraca.

230

COMPAEATIVE ANATOMY.

b) Malacostraca {continued),

Stomapoda.

Squilla.

Tanaida-).

Tanais.

2. Arthrostraca (Hedrioplithalma).

Amphipoda.

Gammarus, Orchestia, Hyperia, Phronyma.
LEemodipoda.

Caprella, Cyamus.

^ Isopoda.

Bopyi’us, Cymothoa, Spha?roma, Oniscus, Asellus* * * §
Idothea.

II. Poecilop odaf).

Limulus.

B. Protracheatat).

C. Tracheata.

Peripatus.

I. Araclinida.

Antaraclin8e§).

Arthrogastres.

Galeodea.

Galeodes.

Scorpionea.

Scorpio.

Phry nida.

Telyiihonus, Plirynus.

Pseudosc orpionea.

Chelifer.

Opilionea.

Phalangium, Opilio.

Aranea.

Salticus, Thomisus, Argyroneta, Tegenaria, My gale.
Acarinall).

Aoanis, Argus, Ixodes, Gatnasus, Atax, Thrombidium.

* These form a division which is allied partly to the Thoracostraca, and partly to
the Arthrostraca, and which seems to have remained more like the primitive form of
the Malacostraca.

f These are connected with the, palaeontologically, very old, and completely lost
division of the Trilobita, through the fossil Belinuridae. There are many characters
in their structure and development which point to their being distinct from the
Crustacea.

X More exact investigations into the organisation of Peripatus show that this
animal, which as yet has been generally placed with the Vermes, is the representative
of a special class of Arthropoda, which must be placed before the Tracheata ; a much
lower stage is seen in it than in any of the larger divisions of the Tracheata. A form
seems to be here retained, which separated from the Tracheate phylum, before it
broke up into its several branches.

§ The true Arachnida vary considerably in the characters of their body-segments,
and in the larger sections formed by the fusion of a number of segments. We
regard those forms in which there are several such sections, in which it is still possible
to see that they are composed of metameres, as the least altered and most similar to
the primitive form. The Cyphophthalmidae (Gibocellum) and the Chernetidae form
small divisions, which belong to the Arthrogastres ; the former are allied to the
Opilionea, and the latter to the Pseudoscorpionea.

II There seems to be no doubt that degeneration is present in these, and is indi.
cated by the parasitism which obtains in most of the families ; it leads in the family
of the Linguatulida to a very remarkable change in the form of the body.

CLASSIFICATION OF AETHKOPODA.

231

Autaraclin80 {continued),

Linguafculina.

Peutastomum.

Pseudaraclinae*).

Tarcligrada.

Macrobiotus.

Pycnogonicla.

Pycnogonum, Nymplion.

II. Myriapoda.

Chilopoda.

Scolopendra, Lithobius.

Ohilognatha.

Polydesmus, Juliis, Glomeris,

III. Iiisecta (Hexapoda).

]. Apteraf).

Collembola.

Symnthurus, Podura.

Thysanura.

Campodea, Lepisma, Macliilis.

2. Pterygota.

Pseudoneuroptera.

Ampliibiotica.

Ephemera, Chloe, Perla, Libellula, Agrion, ^schna.
Corrodentia.

Psocina.

Psocus, Troctes.

E mb id a.

Embia.

Thysanopoda.

Thi-ips.

Termitida.

, Termes.

N eu roptera.

Planipennia.

Panorpina.

Panorpa, Bittacus.

Sialida.

Rhaphidia, Sialis.

ITemerobida.

Hemerobius, Chrysopn., Myrmeleon.
Trichoptera.

Phryganida.

Phryganea, Limnophilus.

Strepsiptera.

Stylops, Xenos.

* Very divergent forms are found in both divisions of the Pseudarachnm, and
have really very little in common except their divergence from the Autarachuse. It is
not quite certain that the Tardigrada have any relations to the Tracheata.

f The two groups united in the division of the Aptera are removed from all
other Insects, owing to the difference in their organisation ; we cannot therefore place
them with any one order. Though they have much in common with the Pseudo-
neuroptera, it is because the latter are so low. Their want of wings must be re-
garded as a primitive condition, as compared with the adaptative wingless condition,
examples of which may be seen in nearly all orders of the Pterygota.

232

COMPAEATIVE ANATOMY.

2. Pteryg’ota {continued).

Orthoptera,

Ulonata.

Cursoria.

Blatta, Mantis.

Saltatoria.

Grjllus, Giyllotalpa, Acridium, Locnsta.

Labidura.

Forficula.

Coleoptera.

Carabns, Hydropbilus, Silpha, Lucanus, Melolontha, Scaraboeus,
Tenebrio, Meloe, Cbiysomela, Coccionella, Lampyris, Elater,
Bostrichus, Curculio.

Hy nienoptera.

Formica, Bombus, Apis, Yespa, Spbex, Sirex, Tentbredo, Icbneu-
mon, Cynips.

Hemiptera.

Homoptera.

Cicadina.

Tettigonia, Cercopis, Fuigora, Cicada.
Pbytbopbtbires.

Apbis, Cbermes, Coccus.

Heteroptera.

Notonecta, Xepa, Hydrometra, Beditvius, Cimex, Capsus,
Lygaeus, Pentatoma.

Pediculina*).

Pediculus, Pbtbirius.

Diptera.

X emocera.

Tipula, Simulia, Cbironoinus, Coretbra, Culex.
Bracbycera.

(Estrus, Musca, Tacbina, Syrpbus, Bombylius, Tabanus.
Pupiparat).

Melopbagus, Hippobosca.

ApbanipteraJ).

Pulex.

Lepidoptera.

Heterocera.

Pteropborus, Tinea, Toidrix, Geometra, Psycbe, Xoctua,
Cossus, Bombyx, Spbinx, Smerynthus, Zygfena.

Rbopalocera.

Hesperia, Pieris, Vanessa, Colias, Papilio.

Bibliography.

Branchiata.

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t These too show the influence of the parasitic habit.

X They have undergone modification o^Ting to a parasitic habit.

AKTHEOPODA.

233

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Tracheata.

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Swammerdam, Bibel der Natur. 1752.— Lyonet, Traitd anatomique de la Chenille qui ronge
le bois de saule. La Haye, 1762. — Steauss-Durckheim, Considerations sur Tanatomie
comparee des animaux articules, auxquelles on a joint Tanatomie descriptive du Melolontha
vulgaris. 1828. — Bukmeistee, Handbuch der Entomologie. Bd. I. Berlin, 1833. — Newport,
“Insecta” in the Cyclopaedia of anatomy and physiology. Vol. II, 1839. — Dufoue, L.,
Recherches auatomiques et physiologiques sur les Hemiptires. M^in. Acad, des sc. (Sav.
etrangers) IV. 1833. — The same", Sur les Orthoptbres, les Hymenoptbres et les Neuropteres.
Ibid. VII. 1841. — The same, Sur les Dipteres. Ibid. XI. 1851. — Pictet, Recherches pour servir
a Thist. et a Tanatomie des Phryganides. Genbve, 1834. — Leuckaet, Die Fortpflanzung u. Ent-
wick. der Pupiparen. Halle, 1858.— Brauer, Fr., Z. Anat. d. Neuropteren in Schrift. d. zool. -hot.
Vereins z. Wien. — Works by Loew in various entomological joiii’nals.-r-LEYDia’s numerous
researches into the more intimate structure of Insects.— Weissman, Die Entwickelung der
Dipteren. Leipzig, 1864.— Landois, L., Anat. d. Hundeflohes. N. Acta Acad. L. C. Vol.
XXXIII. — The same. Anatomic der Bettwanze. Zeitschr. fiir w. Zool. Bd. XVIII. XIX. —
Lowne, B. Th., Anat. and Phys. of the Blow-Fly. London, 1870. — Muller, Fe., Z, n.
Kenntniss d. Termiten. Jen. Zeitschr. Bd. X. — Kowalea’-sky, A., Embryol, Studien an
Wiirmern und Arthropoden, Mem. Acad. St. Petersbourg. I. XVI. No, 12. — Tullberg, T.,
Sveriges Podurider. Kongl. Vet. Acad. Handl. T. X. — Lubbock, J,, Monogr. of the Collem-
bola and Thysanura. London, 1873. Ray Soc.— Meyer, P., Anat. v. Pyrrhocoris apterus.
Arch. f. Anat. u. Phys. 1875. — The same, Ueber Ontogenie u. Phylogenie der Insecten. Jen.
Zeitschr. Bd. X.

234

COMPAEATIVE AEATOMY.

Fig. 117. Nauplius of a
CopepocI (Cyclops), a h c
Appendages.

Form of the Body.

§ 183.

Among the Crustacea the simplest stage of the Arthropod body
is seen in the Nauplius -form (Fig. 117).
The unsegmented body carries several pairs
of appendages. The body only becomes
segmented by a gradual process of gem-
matioig which has many points of resem-
blance to the process which brings about
metamerism in most of the Annulata. The
most anterior portion of the body of the
Nauplius which carries the first appendages
forms the cephalic segment; the posterior
part becomes the last metamere ; new meta-
meres are formed between these two, on
which appendages similarly sprout. In this
way an organism composed of a larger
number of metameres is gradually formed (Fig. 118); the com-
plication of which, so far as it is due to metamerism, is the product

of a gradual process.
This development of the
form of the body pre-
dominates ill the Ento-
mostraca; it corresponds,
probably enough, to the
phytogeny of these Crus-
tacea, which therefore
may be referred back to
an unsegmented condi-
tion . In the Malacos traca
this process of the forma-
tion of new metameres
can be made out in a few
cases only, and the rudi-
mentary body ordinarily
consists at the very first
of a larger number of
metameres. The con-
secutive formation of
metameres is here com-
pressed ; and the same
happens in the Poecilo-
poda and in most Tra-
cheata. Although this might seem to remove any doubts as to
the common origin of the Arthropoda, it is not of so much weight

Fig. 118. Larva of Brancliipns staguais.
(second stage), ah c Appendages, mx Rudiments
of the maxillEe. F' Caudal fork, o Eye (after C. Claus).

FOKM OF THE BODY OF AETHEOPODA.

235

as are the arguments against sucli a supposition, wMcE are afforded
us hj tlie differences presented Ijy them in many points of their
organisation. At present therefore we can only with safety regard
the Crustacea as having a monophyletic origin; this is indicated
by the common Nauplius stage. This stage, which is seen in all
the Entomostraca, obtains in some cases only among the Mala-
costraca, whence we may conclude that this latter division of the
Crustacea has a later point of origin than have the Entomostraca.

The body of the Arthropoda, which is formed into metameres
either by gradual gemmation, or by the immediate differentiation
of the embryo, gradually loses, to a greater or less extent, the primi-
tive similarity of its segments. Great changes in external form are
brought about by the development of some, and the degeneration of
other, or, lastly, by the concrescence of a number of, metameres.
As a general rule the metameres are similar in the early larval stages,
and so indicate relationship to forms in which the metameres are
still indifferent. The more compact portions of the body, which are
formed by a fusion of the metameres, bear indications of their mode
of origin in the appendages which they carry.

The most anterior metameres are those which most completely
undergo concrescence. In this way a portion which carries the
mouth and the higher sensory organs, and especially the eyes and
tentacles, is formed; this is the head. In the Myriapoda, many
Crustacea, and Insect-larvae, it is the only portion which is formed by
the concrescence of a number of metameres. Owing to this con-
crescence of the metameres the appendages are approximated to the
mouth, where they aid in taking up food, and are converted into
mouth-organs. Other differentiations play various parts in the
different divisions. In the Crustacea a number of metameres behind
the head unite with it to form a cephalothorax. The other meta-
meres are again often divided into two groups, inasmuch as those
behind the cephalothorax sometimes differ from the most posterior
ones. They thus form an abdomen and a post-abdomen. The
segments of the abdomen fuse in the Poecilopoda, where the post-
abdomen is represented by the caudal spine.

Special arrangements for the protection of the appendages
are formed by the folding and extension of the integument. Thus,
in the Decap oda, the dermal skeleton of the cephalothorax grows
out at the sides, covers the gills, and forms on either side a
special cavity — the branchial cavity — which communicates with the
surrounding medium. (Cf. § 187.)

Such developments from the dermal skeleton, belonging to
several primitive body-segments, may extend over the other divisions
of the body, and form a shell for them. The earliest form of
this is seen in the shield-like, enlarged, cephalothorax of the Phyllo-
poda (Apus) among the Branchiopoda. When the two halves of
this structure are further developed we get a bivalve shell (Fig.
124, d) (Limnadia). In the Cladocera also a portion of the dorsal
integument is converted into a shell, which covers the whole of the

236

CO^rPAEATIVE AISAVTO:SIY.

posterior portion of tlie body, and in tlie Ostracoda, as in many
Pliyllopoda, tlie two halves of this structure are movably connected
with one another on the dorsal surface. In them the valves of the
shell extend also over the anterior portion of the body, and so enclose
the whole animal.

The very peculiar modifications of the integument in the Cirri-
pedia are structures of this kind. The fold, which in the Ostracoda
is formed into a bivalve shell, is seen in an early stage in the Cirri-
pedia. When the animal becomes attached by its antennge, the
dorsal portion of the integument is converted into a wide sack or

mantle (Fig. 119, d ef) which
encloses the body; the sack and
the enclosed body are united
to one another in the cephalic
region only. The portion of
this sack, which carries the
primitive point of attachment,
either remains soft and grows
out into a stalk -like organ
(Lepadidoe), or is converted
into a broad basal surface
(Balanidae). In many Cirri-
pedes the whole mantle re-
mains soft (Alepas). But
most of them acquire firm
calcified shell - pieces, which
are developed in the outer
lamella of the mantle. The
rest of the body, with the
post-abdomen, which is beset with tendril-like feet, is enveloped in this
mantle, which has the form of a shell, and is in connection with the
surrounding medium by means of a cleft, which can be closed at will.

In the Ehizocephala this mantle-like envelope forms a tube, which
is smooth externally, or a disc, which is marked off into symmetrical
lobes. A narrow orifice, like that which leads into the mantle cavity
of the Cirripedia, leads into a space which corresponds to this mantle
cavity, and functions as a marsupial pouch. In the Cirripedia part
of the typical crustacean body, with its appendages, is enveloped by
the mantle, and permanently retains its structure ; but in the Rhizo-
cephala the whole surface of the jointed body becomes converted
into the mantle.

Another phenomenon, due to the mode of parasitism, is corre-
lated with this atrophy of the body ; numerous tubules are developed
by the part of the head which is sunk into the body of the host ;
these tubules, which partly anastomose into retiform plexuses, extend
to the enteric canal of the host, and surround long tracts of it. Thus
an apparatus is formed which extracts nutrient fluid directly from
the enteron of the host, and carries it to the parasite. Many other
examples of the degenerating influence of the parasitic habit may be
observed ; as, for example, the varied forms of the Siphonostoma.

e

V ' cl y a h

Fig. 119. Transverse section of a Balanns.
a Month of the animal. h h' Append-
ages converted into tendril-like structures,
c Head of the animal, d Mantle-like en-
velope. e e Movable valves which close
the shell, f f External shells, m Muscles
(after Darwin).

APPENDAGES OF APTHEOPODA.

237

Peripatus lias a simple form of body very similar to that of tbe
Aunulata.

Among the Tracheata the most indifferent condition is found in
the Myriapoda_, where the metameres are similar and separate. The
body of the Arachnida is very variously differentiated. The
Galeodea possess the largest number of segments. A head is sepa-
rated by three thoracic metameres from the succeeding and separate
abdomen^ which is made up of separate metameres. In the Scorpions,
however, the cephalic and thoracic metameres are united into one
portion, and a post-abdomen is differentiated from the abdomen. The
abdomen is more sharply marked off from the cephalothorax in the
Phrynida ; the same happens in the Aranea, but they differ in the
fact that the abdominal segments have undergone more complete
concrescence. In the Acarina the metameres have altogether lost
their independence.

The more richly segmented body of the Insecta presents less
variety in the distribution of the metameres in different portions.
In addition to the head, which is formed of several (3) meta-
meres, there are ordinarily three thoracic segments (Pro- Meso-
and Meta-thorax), which are either indifferent, as in the Thysanura
and many Pseudoneuroptera, and are only distinguished by the
organs appended to them; or they all form a portion which is as
sharply marked off from the head as from the abdomen (Neuroptera,
Ilymenoptera, Diptera, Lepidoptera) ; or the first thoracic segment
only is specially modified, while the second and third are closely
attached to the abdomen ; this arrangement is indicated in the
Orthoptera (Saltatoria), and well marked in the Coleoptera.

The characters of the abdomen are affected by its relations to the
thorax already noted. Its segments are always independent, and
the terminal ones, several of which are converted into parts of the
generative apparatus, are often atrophied.

Appendages.

§ 184.

The appendages of the Arthr(
tures, which are attached to the
metameres, and can be separated
into dorsal and ventral appendages.
These structures are foreshadowed
by the parapodia which ^re found
in the higher Annulata. In the
Arthropoda these processes are more
highly differentiated, for they be-
come jointed (Fig. 120,p), and differ
greatly in form, in correspondence
with their different functions ; their
lower stage of similarity to
seen in their earliest rudimen

poda are paired, jointed struc-

Fig. 120. Transverse section of a
Wood-louse, p A pair of feet, Ab-
dominal appendages, which form a
thoracic cavity (after Lereboullet) .

3iie another is only to be

s.

238

COMPARATIVE AVATOMY.

TRe serial similarity^ which accompanies the lower grade of
structure of tlie parapodia in Annelids, is also seen in the lower
types of the Arthropoda, as for example in Peripatus, the Myria-
poda, and in i^iany Crustacea (Phyllopoda, etc.). In Peripatus the
appendages retain their lowest condition, and resemble the parapodia
of Vermes; their resemblance indeed to the appendages of the
Tracheata is merely due to the possession of a movable terminal
segment, which carries a pair of claws. Two phenomena, which may
be recognised in these appendages of the Arthropoda, tend to convert
the multifid Annelid-like organism into a more compact one.

The first of these is the metamorphosis of the appendages
into a series of different structures having the most varied functions.
As the appendages come to differ in function they change in form,
and become adapted to their new duties.

The second phaenomenon is the limitation of the number
of the appendages, which obtains in the higher divisions, con-
currently with the greater development of heteronomous metameres,
or with the formation of larger divisions of the body by the fusion
of its separate groups of metameres.

Appendages of the Branchiata.

§ 185.

The appendages are seen at their simplest in the Nauplius form
of the Crustacea. At first two, and afterwards three pairs of
jointed appendages appear in the unsegmented body. They all
function as locomotor organs (swimming-feet), and are beset with

set^, which are often arranged in large
bundles. The first pair of these appen-
dages (Fig. 121, u) is simple, the second
and third pairs forked, and this forked
character obtains in all the succeed-
ing appendages of the Crustacea. The
first two pairs are distinguished from the
third pair, and from those which appear
later on, by their connection with the nerves,
which arise from the supraoesophageal gan-
glion, while the third and all succeeding
pairs are supplied by the lower ganglia.
With this is connected a difference in
function, for the two anterior pairs are ordinarily developed into
antennae. In many Copepoda they still function as locomotor
organs ; they do so most completely in the Ostracoda. In the
Cladocera also the second antenna is still developed as a swimming
organ, and this stage is retained by the Phyllopoda during a very
long period of development. This justifies us in reckoning the
dorsal outgrowths (the first two pairs) as strictly belonging to the

Fig*. 121. Naiiplius of a
Copepod (Cyclops), a h c
Appendages.

APPENDAGES OF AliTHEOPODA.

239

category of appendages even on functional grounds. In tlie Mala-
costraca tlie two pairs of antemiBB Pave no relation to locomotion,
as may indeed be always seen from tlieir form. The hinder pair
(Fig. 123, at') is ordinarily placed beside, and often is larger than,
the anterior pair [at). (Of. also Fig. 128, a' d' .)

All the other appendages are ventral in position. When the
metameres begin to be formed they make their appearance following
on the first pair of swimming-feet, found in the
Nauplius ; one pair is distributed to each seg-
ment. Like this swimming-foot, and like the
second pair of antennse they have two ter-
minal branches; these are not as a rule dif-
ferentiated to an equal extent, for one branch is
greatly developed and forms the principal part
of the appendage, while the other forms an
appendage to it. It sometimes, however, has
relations to the respiratory function, and is then
largely developed. The appendages have very
different functions, and are so metamorphosed
as to adapt themselves to these functions.

Such of these more anterior ventral appen-
dages as lie near the mouth are converted into
mouth-organs, and either form jaws ^and jaws
only, or foot -jaws. The relation between this
arrangement and the concrescence seen in the
cephalothorax, has been already referred to. A
few pairs only are converted into gnathites in
the Branchiopoda, and the remaining appendages,
of which there is a very large number in most
of the Phyllopoda, have very much the same
characters as swimming-feet. The same thing
happens in the Ostracoda, Copepoda and Cirri-
pedia. In the last-mentioned group the posterior
appendages are converted into the characteristic
cirri (Fig. 119, hb'). The metamorphosis of the
appendages is seen most completely in the Mala- ^ilis. md Mandible,
costraca ; let us examine more closely the arrange- First ; Second
ment seen in one of the Decapoda. We here find maxilla, m.p''

six pairs of appendages converted into gnathites, podit^TEx
in the more anterior of which the form of the a Epipodite.
Phyllopod foot is retained almost unmodified. A
pair of strong jaws (Fig. 122, mcl), are succeeded by two pairs of
maxillas {mx mx'), and these by three pairs of maxillipedes (mjj>
mp' The latter gradually pass into the locomotor appen-

dages ; of which there are five pairs (Fig. 123, P' — P% which are
attached to the cephalothorax, and together with the maxillipedes
and maxillm indicate the number of metameres of which it is built up.
The terminal joint of most of the ambulatory feet forms a scissor-
shaped termination, owing to the great size of the penultimate

240

COMPAEATIVE ANATOMY.

joiiit; wliicE projects over tlie

Fig. 123. Appendages of Astacus
fiuviatilis, seen from tlie ventral
surface, at Anterior; at' Posterior
antenna, md' Mandibular piece, mp"
Third maxilliped, which covers over
the other gnathites. Ambu-

latory.feet. p- — Swimming-feet

of the abdomen, p^ Telson. a Anus.
0 Orifice of the o\uduct in the basal-
joint of the third ambulatory foot.

ultimate one; tliis arrangement is
generally most marked on tlie first
pair of feet, wliere it serves as an
organ of offence. The ambulatory
feet, as well as the maxillipedes,
have branchial tufts appended to
them.

A number of feet on the abdo-
men are converted into delicate
swimming-feet ; the first of them
functions in the male as a copu-
latory organ ; in the female it is
reduced in size, while the other four
( ]f — p^) carry the ova. The last
pair of appendages are the most
modified, for they (j/) form together
with the last segment of the body
a jDowerful caudal fin, of which they
are the sides.

The other divisions of the
Malacostraca present more or less
marked deviations from this ar-
rangement in the number of their
gnathites or of the appendages,
which are converted into locomotor
organs and adapted to these func-
tions. Thus, for example, in the
wood-louse four appendages are-
converted into gnathites, the next
eight are ambulatory, and the last
four form respiratory plates.

The connection between respi-
ration and locomotion, as implied
by the conversion of appendages
into branchial lamellae, or in the
differentiation of gills of various
form on the appendages, is a
most important phaenomenon (see
Branchiae) .

Branchia).

§ 186.

The tendency to longitudinal division which affects the appen-
dages of the Crustacea adapts these structures to the respiratory as
well as to the locomotor function, by spreading out their joints.
When the integument at certain points gets thinner it gives rise to

BEAKCHIJ^ OF AETHEOPODA.

241

arrangements by wbicli tbe exchange of gases between the blood
circulating within the appendages, and the surrounding medium
is more easily effected, and causes either the whole appendage
or a fork of it to become a respiratory organ.

A further differentiation on the same lines, leads to an increase
in the number of the respiratory lamellae of an appendage, or to
the formation of filamentous structures; the reason for all these
changes is a necessity for increasing the surface. These organs are
branchiae. The connection between the branchiae and the appen-
dages as seen in the Vermes is clearly a foreshadowing of the still
more developed arrangement in the Crustacea, which has here
become typical. But it is, of course, more than doubtful whether it
has been directly derived from the Vermes.

The gradual development of gills may be followed out step by
step through the Crustacea; the functions of respiration and of
locomotion are often so closely united that it is difficult to say
whether certain forms of these appendages should be regarded as
gills, or as feet, or as both com-
bined. The conversion of locomotor
into respiratory organs can not un-
frequently be made out in the suc-
cessive appendages of one and the
same individual. The branchiferous
metameres vary greatly, so that we
may say that the appendages of
each segment can form gills, or
supports for them by developing
branchial organs from one of their
two primitive branches. The num-
ber and special structure, as well as
the position, of these respiratory
organs varies.

Where the feet themselves be-
come gills, they have the form of
broad thin lamellae (Fig. 124, A hr),
the surfaces of which are adapted
to effect the exchange between the
blood that is circulating in them
and the surrounding water. Organs
of this kind are common among the
Branchiopoda, in which, as a rule,
a large number of feet become gills,
while, in addition to these, peculiar
230uch-like appendages may be seen
to be specially entrusted with the
respiratory function. The abdominal feet of the Isopoda form
branchial lamellae. In the Amphipoda the gills are tubular
appendages of the thoracic segments, which are ordinarily attached
to the basal-joints of the feet. In the Stomapoda the original form

Fig. 124. Sections of Crustacea.
^ of a Phyllopod (Limnetis)
(after Grube). B of Squilla (after
Milne-Edwards). c Heart, i Enteron.
n Ventral medulla. hr Branchim.
d Fold of the dorsal integument,
which forms a shell in A.

242

COMPAEATIVE ANATOMY.

gives rise to another arrangement — the five pairs of abdominal
swimming-feet bear at their base a mesially- directed tuft of branched
branchial filaments [B hr).

From the Schizopoda to the Decapoda we meet with a con-
tinuous series leading from the simplest relations to the most com-
plicated ones. Distinct gills are not unfrequently wanting in the
former (Mysidae), or the gills have the form of ramified additions
to the appendages of the cephalo-thorax fioating freely outwards
(Thysanopoda). A fold is gradually developed, from the dermal
skeleton of the cephalo-thorax which forms a plate covering in a
lateral space above the thoracic feet (p. 235). In this space the gills
are placed; it becomes a laterally-placed closed branchial cavity (Deca-
poda), which is in connection with the surrounding medium by means
of a cleft left between the free edge of this lamella, and the basal
portion of the feet. As the covering lamella of the branchial cavity
becomes more closely approximated to the ventral surface of the
body, the primitively simple longitudinal cleft, which gave entrance
to the water, becomes divided into two portions, and so gives rise to
a larger posterior, and a smaller more anteriorly placed opening, by
which the water which has entered by the larger opening, passes
back to the exterior, after it has bathed the gills. The gills may
separate themselves somewhat from the base of the feet, and arise
from the wall of the branchial cavity, but even then they often
correspond in number to the appendages. In most of the Decapoda,
however, the number of gills is greatly increased, the most anterior
of the ambulatory feet being provided with several gills, while some
of the maxillipedes also share in this arrangement. The respiratory
appendages are more distinctly differentiated in the Poecilopoda,
where the anterior appendages have no appended organs, wFile the
five pairs of feet attached to the abdomen are converted into broad
plates, and carry a large number of branchial lamellas.

§ 187.

A more rapid exchange of water around the branchial organs is
effected in various ways. Most simply, when the appendages them-
selves function as gills, or when the gills, notwithstanding their
being special organs, are attached to the swimming-feet. The action
of the appendages produces a continual change of water around
the organs, and puts respiration and locomotion into direct relation
with one another. The appendages of the Branchiopoda and the
swimming-feet of the Stomapoda may be cited as examples of this
arrangement. In others the exchange of water is effected by a special
covering-plate of the branchise which is formed from the modified
abdominal feet ; this is the case in the Poecilopoda and the Isopoda.
The water can be renovated, even when the animal is at rest, by the
continual movement of this covering-plate.

The formation of a branchial cavity leads to the differentiation of
new arrangements, by which the exchange of water is effected. In

APPENDAGES OF AKTHEOPODA.

243

the Decapoda provided with branchial cavities there are special
churning organs (flagella) on either side (Fig. 125,/)^ which reach
over the whole of the gills in the form of thin flat processes^ and are

attached to the base of a maxil-
liped, by which they are kept
constantly moving (Brachyura).

The lamellm of the integu-
mentj which in many Entomo-
straca carry shells_, must be re-
garded as having a respiratory
significance. This relation to
respiration is intelligible when
we note that a considerable
amount of blood passes through
these lamellm of the mantle, and
that the thinness of the walls of
these organs presents a condition
well adapted for the exchange of
the gases ; and that, further, the
movements of the appendages
effect a considerable exchange of
water within the mantle-chamber.
When the pallial lamellae become
more extended (Limnadiaceae)
they become of greater import-
ance for respiration, and this
importance must increase in pro-
portion to the reduction in num-
ber of the appendages, which
lose their respiratory significance
as less blood passes through them
(Ostracoda, Daphnida).

In these cases, however, the

Fig. 125. Branchiao of a Bracliyurons
Decapod. The dorsal integument has
been removed from the greater part of
the cephalothorax. In the middle is seen
the coelom with the intestine arising from
the masticatory stomach v ; the branchial
cavities are placed at the sides and laid
open ; on the right are the branchia3 ar-
ranged in six lamellar rows ; on the left
four of them have been cut away, as well
as the flagellum /, so as to display the
churning apparatus/'/", below the gills.
0 Eyes, d Antennae, ar A single gill
cut short at

mantle does not become specially organised into a branchial organ,
as it does in the Cirripedia. In the Balanidse folded lamellas, which
have been regarded as gills, appear on the inner surface of the
mantle- cavity, between its side- wall and base.

Appendages of the Tracheata.

§188.

The appendages of the Tracheata are distinguished from those of
the Crustacea by the absence of the terminal bifurcation, so that
they are composed of a single series of joints. In Peripatus these
joints are but feebly differentiated. The terminal portion only
which carries a claw has any large amount of independence.

R 2

244

COMPAKATIVE ANATOMY.

All the TracEeata have a single pair of antennas; in this point
the Poccilopoda among the Branchiata resemble them. In the
Poccilopoda and in the Arachnida these antennm are converted into
mouth-organs; in the Scorpions and Spiders they are known as
chelicerge. Notwithstanding these relations these structures are
homologous with the antennse of the Myriapoda and Insecta^ for,
like them, they are innervated from the snpra-oesophageal ganglion.
They are developed in very various ways among the Insecta, in
adaptation to the various functions of the sensory organs connected
with them.

The ventral appendages are disposed symmetrically, and in
Peripatus all but two pairs retain this relation ; in the Myriapoda
most of the appendages are similar in character, but in all the other
forms these appendages, so far as they persist, take on various forms
in accordance with their variations in function. The appendages
of the anterior metameres become month-organs; those of
the posterior metameres are converted into feet, and those on the last
are often completely atrophied, and often are not even formed in rudi-
ment. On the whole the number of appendages is much less than in
the Crustacea ; within the several classes they are always definite in
number, and the number of gnathites as well as of feet is constant.
In Peripatus the first two pairs form mouth-organs; the first of
these is surrounded by the sides of the wall of the month, and the
second pair only lie close to the mouth. In the Arachnida there is
only a single pair of these mouth-organs. In the Arauea it forms
the base of a many- jointed palp, which correspond to the chelae of
the Scorpionea, and to the powerful hooked chelas of the Phrynida.
In the Mites the pieces of either side are united into a grooved
lower lip, in which the stylet-shaped jaw-parts are placed. The
four remaining pairs of appendages persist in all Arachnida as feet ;
the first of these is flagellate in form in the Phrynida.

There are three pairs of gnathites in the Myriapoda ; the first
pair is generally developed into a strong jaw ; in the Chilognatha
the second and third pairs are converted into a kind of lower lip,
and are said to be represented by a single pair in the embryo ; in
which case this order has only got two pairs of gnathites. In the
Chilopoda, however, the second and third pairs are much more inde-
pendent, and the first pair of feet is also associated with the mouth-
organs. The other appendages of the body have all pretty much the
same form ; in the Chilognatha there are two pairs on each meta-
mere. The last pair frequently loses its locomotor function, and
forms an appendage which we shall again meet with in a modified
form in the Insecta.

The appendages are not therefore separated into mouth-organs
and locomotor appendages in the same way in all Tracheata ; they
vary, but not so much as they do in the Crustacea. The develop-
ment of the gnathites, or rather their differentiation from locomotor
appendages, is correlated with the development of the head, that is
to say, the head is formed as a result of this differentiation.

APPETOAGES OE THE APTHEOPODA.

245

§ 189.

Three pairs of the primitively similar ventral appendages are in
the Insecta converted into mouth-organs ; the same number are
formed into feet. The former, arranged round the mouth, may at
first have well served to seize and
hold food, just as we see the maxillm
of the Crab do at the present time.

In such a stage as this the food is
seized as well as comminuted. The
first pair form the mandibles, and
become parts of the mouth, in the
form of a single joint. The second
and third pairs are many- jointed.

But the basal joint only, or a few
of those succeeding it, which are
the nearest to the mouth, serve to
comminute the food ; these parts are
correspondingly metamorphosed.

They form the maxillge, and the
remaining portion of the appen-
dage looks like a jointed addition
to it, and functions as a tactile
organ (palp) ; in this way two
organs, which work in different

from

one

Fig. 12G. Developmental stages of
Hydrophiliis pice us. A An earlier ;
B A later stage. Is Upper lip (labrum) .
at Antennae and first pair of gnatbites
(Mandibles), mco Second pair (Max-
illa). li Third pair (Labium), p"

p”' Feet (after Kowalevsky) .

ways, are differentiated
appendage.

The most indifferent form of
gnathite is found in the Aptera ;
in the Collembola they are sunk
into the buccal cavity, and in the Thysanura they are but feebly
developed. In the former when the parts of the mouth are in active
use they can be protracted and drawn in again ; and thus the mouth
is adapted both to biting and sucking, though, of course, to a
very small extent. This indifferent condition of the organisation is
developed along two distinct lines in the Pterygota.

When the mandibles are well developed they have the form of
cutting organs, which work on one another; the two pairs of
maxillae also become cutting organs, and carry palps at the same
time. This condition is permanent in the Pseudoneuroptera, Neu-
roptera, and Orthoptera, although indeed points of similarity to the
more indifferent stage may be seen in the first of these ; and also
the second pair of maxillae begins to show signs of fusion. When
these gnathites are fused in the middle line the so-called labium
is formed ; its palps then become articulated to it as labial palps,
and indicate the more primitive condition of the organ. The
mouth-organs of the Coleoptera are thus metamorphosed.

These parts undergo more remarkable modifications when they

246

COMPAEATIYE ANATOMY.

are adapted to another mode of ingesting food,, namely, by sucking.
In the Hymenoptera, in which these organs can function either as
sucking or as cutting organs, the parts are of pretty much the same
form as in the mandibulate Insecta, but the maxillae are consider-
ably elongated, as is the labium and its palps. A process, the
tongue (lingua), is developed on the surface of the labium turned
towards the mouth, and this has two lateral appendages, or secondary
tongues (paraglossae), at its base. In many these appendages are as
much developed as the tongue itself.

The mouth parts can be derived from cutting organs even in
those insects that are purely suctorial. The Hemiptera and the
Diptera have the mandibles and maxillae converted into setae; in
many Diptera the maxillary setae are rudimentary. The labium
forms a sheath for these setfe, which is firm and jointed in the
Hemiptera, and soft in most Diptera ; this sheath still carries the
labial palps, or rudiments of them. There is a rudimentary tongue
on the short upper lip (labrum), which is wanting in the Hemiptera.
The mouth-organs of the Lepidoptera are differentiated in a
different way. In them the maxillae are grooved, and united into a
tube, which forms a spirally-coiled proboscis, which is ordinarily
of some length ; there are small maxillary palps at its base, which
are covered by the palps eff the rudimentary
labium, which are generally large.

While gnathites are found on the metameres
which fuse to form the head, the succeeding
appendages form feet, or locomotor organs for
the next three, or thoracic, metameres. The
jointing observable in them is of the same cha-
racter in all, and shows that they have had a
common origin ; more marked differences can
only be made out in their terminal segments,
which are more accessible to adaptation. Certain
peculiarities in them are due to their varied
modes of adaptation to modified requirements.

Although the number of three is constant for
the feet, in many Insects a larger number can
be made ont in the embryo; whence we may
conclude that they have been derived from forms
with a larger number of feet. In the Thysanura
the rudiments of these appendages are retained
(Fig. 127, jj), even on the abdominal metameres
(Campodea). The locomotor processes in many
Insect larvge (Lepidoptera and Tenthredinge) may
be derived from these rudimentary appendages.
The paired processes also of the terminal meta-
meres in the Thysanura, Pseudoneuroptera, etc. are derived from
appendages.

Fig. 127. Anterior
half of the body of
Campodea fragi-
lis. a Antennae, p
Feet, p' Rudiments
of feet, s Stigma
(after J. A. Palmen).

APPE1!^DAGES OF AETHEOPODA.

247

§ 190.

Besides antennae^ dorsal appendages occur in no TracEeata except
tEe Insects. TEey are alfcogetEer absent in tEe TEysanura and Col-
lembola. As tEey are only present on tEe post-cepEalic metameres
tEey are innervated, like all tEe ventral appendages, from tEe ventral
cEord. TEey cannot be said to Eave any relations to tEe brancEise
of tEe Crustacea, or to be derived from tEe dorsal parapodia of tEe
Annelides, and tEey may tEerefore be fairly regarded as independent
structures.

TEe dorsal appendages Eave tEe form of lamellge or filamentous
processes of tEe metameres, wEicE are sometimes grouped into
tufts in tEe aquatic larvse of tlie EpEemerida, Perlida, PErygauida,
etc. TEese appendages Eave a respiratory function ; and as tracEea3
pass into tEem, tEey are known as tracEeal gills. TEey are
widely distributed over the body, not only on the dorsal, but also on
the ventral surface ; and are therefore examples of processes in an
indifferent condition, of which the dorsal ones placed at definite
points acquire a typical significance. TEe lamellar widely- distributed
forms move in such a way as to be of great importance in changing
the water, just like the respiratory appendages of the PEyllopoda ;
but they cannot be said to Eave any locomotor functions.

TEe wings must be regarded as homologous with the lamellar
tracEeal gills, for they do not only agree with them in origin, but
also in their connection with the body, and in structure. In being
limited to the second and third thoracic segments they point to
a reduction in the number of the tracEeal gills. It is quite clear
that we must suppose that the wings did not arise as such, but
were developed from organs which had another function, such as
the tracEeal gills ; I mean to say that such a supposition is neces-
sary, for we cannot imagine that the wings functioned as such in the
lower stages of their development, and that they could Eave been
developed by having such a function.

But if the real cause of the development of these dorsal
appendages into wings cannot be found in the locomotor function, it
must be sought for in another. And this is that of respiration,
which would be well served by increase of surface. Every increase
of surface increases the respiratory value of the organ, and so leads
towards its future function. It is no objection to this hypothesis to
say that ontogenetically the wings are formed and developed later
than the tracEeal gills of other metameres, for these modified
tracEeal gills can only function when the unmodified respiratory
ones Eave lost their function.

In many cases the appendicular character of the wings is in-
dicated by segmentation, but this can only be regarded as a
secondary adaptation. It obtains in the movable second pair of
wings in the Coleoptera and Forficulidse, where the first pair have
been converted into elytra.

24S

COMPAEATIVE AE-ATOMY.

In tlie Psendoneuroptera both pairs of wings are very similar in
character. In the other fonr- winged orders they have undergone
considerable differentiations. In addition to the differences in size,
which in the Hymenoptera and Lepidoptera are generally due to the
increased size of the first pair, there are differences of structure,
which imply a difference in function. In the Orthoptera the first
pair is often merely a covering for the second pair; this is very
distinct in the Beetles where the second pair is often rudimentary.
The elytra have then become organs of defence for the subjacent
abdomen. In the Ilemiptera there is a similar differentiation.
The Diptera have the anterior pair only, the hinder pair being
rudimentary, and represented by the so-called balancers (halteres).
On the other hand, the Strepsiptera possess the hinder pair of
wings, or those attached to the third thoracic segment, only.

Integument.

§ 191.

The integument of the Arthropoda is more independent and less
connected with the muscular system than in the preceding divisions ;
it can always be divided into two layers. The cuticular layer,
secreted by a layer of cells which is often much modified, extends
over the whole surface of the body just as it does in many Vermes;
it is also continued into the openings of the internal organs. Owing
to its strength it forms the most important part of the integument,
but it varies greatly in thickness and firmness. It is soft and flexible
between the segments of the body, where these parts are movably
connected with one another, but it is generally stronger on the
metameres themselves, and on the appendages. Its physical cha-
racters vary widely ; there are all kinds of transitions between the
soft investment of the body of the Spiders and most Insect larvae,
and the firm carapace which covers the body of most of the
Crustacea, the Myriapoda, Scorpions, and such Insects as the Cock-
roach. The variations in its firmness depend not merely on the
thickness of the cuticle, but on the chitinisation of its layers. Even
thick layers, when freshly formed, are soft, and only become resistent
when chemical change occurs. To increase the firmness of this
chitinous carapace calcareous salts are deposited in it, in many
Crustacea, and even in the Myriapoda. When this cuticle has be-
come firm it limits the growth of the body ; in these cases therefore
so long as growth is going on the cuticle is periodically cast
(Ecdysis).

In consequence of its mode of origin the cuticular layer is every-
where distinctly laminated. As a rule it is traversed by pore-canals,
into which processes of the subjacent matrix are continued. The
relatively thin matrix of the cuticular layer is homologous with

lOTEGUMEOT OF AETHEOPODA.

249

the epidermis of other animals. Although in many cases it contains
pigment (Crustacea)^ it is as a rule colourless, the coloration of the
Arthropoda being generally due to the deposition of pigment in the
outer chitinous covering. Below this epithelial layer, which is also
distinguished as the hypoderm,^^ there is a layer of connective
tissue, but this is, as a rule, slightly developed in comparison with
the cuticular layer, and the matrix.

§ 192.

When the secreted chitinous layers are very firm they take on a
new function, and form a dermal . skeleton, which is not only an
organ of protection for the organs placed in the body-cavity, but is
also an organ of support, and provides points of origin and
insertion for the muscles of the body. This relation holds too for
the appendages of the body, the integument of which likewise serves
as a skeleton for them.

The formation of large unequal regions of the body produces
changes in the dermal skeleton by giving rise to differentiations in
it. These are formed by internal processes and continuations of the
dermal skeleton, which are principally found in those segments which
carry oral or locomotor organs. These processes are very greatly
developed on the cephalothorax of the higher Crustacea. But they
are not wanting in the other classes. They are found chiefly in the
head and thorax in many orders of the Insecta (Coleoptera, Hymen-
optera, Orthoptera), where they form a complicated structure
known as the endothorax.'’^ These parts often form an organ of
support for the nervous system. They are also important as
increasing the internal surface of that portion of the dermal skeleton
which gives origin to the muscles, and, in more individualised
structures, they are closely connected with the differentiation of
the musculature.

The shells, too, which are formed from the chitinous investment
of the mantle-folds of many Branchiopoda and Ostracoda, are of
importance as skeletal structures ; so too are the shells of the
Cirripedia. However different they may be in form or size they
have always the same arrangement. Two pairs of ridges or plates
surround the entrance into the mantle-cavity and form a movable
operculum. The shell-pieces developed in the Balanidae, but which
are rudimentary only in the Lepadidge, gives rise in the former to a
continuous rigid cell (Fig. 119,//), the only movable part of which
is the operculum (e) which covers over the entrance into the mantle-
cavity.

§ 193.

Continuations or processes of the integument take on all kinds of
forms, such as spines, setse, or hair-like structures ; these are modi-
fied in innumerable ways in the Crustacea, Arachnida, and Insecta ;
they are sometimes attached closely and immovably to the chitinous

250

COMPAEATIYE ANATOMY.

carapace^ of wMcli tliey form outgrowths, as, for example, the
setee in certain parts of the body of the Crustacea, the hairs of the
Spiders, Caterpillars, etc. ; sometimes they are more developed and
only loosely attached to the body, like the scales of Butterflies, which
are also present in other divisions, e. g. in the Thysanura. In any
case the chitinous investment of the process is continuous with the
rest of the integument. In movable appendages of this kind the
chitinous layer is softer at the point of attachment, while the cuticle
does not change in character on the stiff processes. Integumentary
structures, such as denticles and ridges, are also converted into
organs of voice in many Insecta (Grrasshopper, Cicadas).

There are glandnlar organs in the integument, which are
derived from modiflcations of the epidermal layer. They are not so
common in the Crustacea as in the Insecta. The secreting portion
of the gland consists of one cell merely, or of a few ; and its duct
is principally formed by the pore-canals of the cuticular layer
(of. Pig. 7, p. 28).

The dermal glands are largely developed in certain parts of the
body in those Insects that form wax. In the Aphides, and still
more in some Hymenoptera, groups of dermal glands are converted
into wax-secreting organs. The spinning* glands of Spiders are
further differentiations of dermal glands. Glands which lie in the
abdomen and open by several pairs of papillae placed behind the
anus (spinning papillae) produce a secretion, which hardens into a
chitinous filament when exposed to the air, and so forms the thread
of the Spider^s web. An apparatus, which is only functionally similar
to this is found in Peripatus. Two groups of ramified tubes pass
into an excretory duct on either side ; this duct, which is locally
widened out, opens at the base of the second post-oral appendage.
The secretion is of a sticky character and rapidly hardens. Morpho-
logically, these organs appear to lead to those which are found in
the larvae of many Insecta, which, therefore, may be regarded as
having had a common origin with them. In the larvae of the Lepi-
doptera, and many Coleoptera and Hymenoptera, there is a pair of
long and frequently coiled glandular tubes placed beside the enteron,
the fine ducts of which have a common opening on the labium.
Their secretion forms the silk-threads of the web of these larvae.
These spinning vessels are most highly developed before they
pass into the quiescent pupa stage (Serictariae) ; after the web is
made they become atrophied.

Lastly, other glands seem from their secretion to be poison
glands, e.g. those which open on the claws of the spiders, and on
the caudal spine of the Scorpion ; these add to the variety of
differentiations which the glandular system of the integument may
undergo.

MUSCLES OF AETHEOPODA.

251

Muscular System.

§ 194.

The muscular system of the Arthropoda is not homogeneous in
character, like the circular or longitudinal layers in the dermo-
muscular tube of the Vermes. It is more differentiated, and we
find separate bundles composed of a varying number of trans-
versely-striped muscular fibres. Peripatus is the only exception to
this, and its muscular system, owing to the absence of transverse
strise from the muscular elements, rather resembles that of the
Vermes. The dermo-muscular tube is however generally converted
into a complex of separate muscles, which together form a muscular
system. As the skeleton of the Arthropoda is an external one,
the muscles arise from and are inserted into the inner face of the
hollow cylinder, or portions of a cylinder, which are formed by the
segments of the body as well as by those of the appendages. This
development of a dermal skeleton is to be regarded also as an
important factor in the differentiation of the muscular system, in so
far as separate muscles can only be formed when they have a firm
point of origin and of insertion. In the number as well as in the
varied arrangement of the separate muscles, the muscular system
presents a high grade of development, which always corresponds to
the varying significance and development of the metameres. It
differs from the muscular system of the Annelides in corre-
spondence with the difference expressed by the homononiy of
their metameres, as compared with the heteronomous ones of the
Arthropoda. .

When the metameres are similar the muscles of the metameres
are also similar, and when the separate metameres are unequally
developed, either by the fusion of a small, or a larger number into
larger divisions of the body, or by atrophy, there is a corresponding
want of similarity in the arrangement of the muscles of those parts.
The development of the appendages greatly influences the develop-
ment of the muscular system, and when the metameres which carry
appendages are greatly enlarged in comparison with the rest, the
muscular system takes a large share in the increase.

The numerical relation and arrangement of the muscles often
undergo considerable alterations in those Arthropoda that undergo
metamorphosis. This applies as much to progressive as to retro-
grade metamorphosis. In the former the change leads to a differ-
entiation into unequal groups ; in the latter to an atrophy of very
large parts of the system ; this obtains in the parasitic Crustacea
and in the fixed forms.

252

COMPAEATIYE AXATO^^IY.

Nervous System,

§ 195.

The nervous system of the Arthropoda resembles that of the
Annelides_, with which it completely agrees in its fundamental
characters. A ganglion placed above the oesophagus is the cephalic
ganglion or cerebrum; two commissures from it embrace the
oesophagus^ and form a nervous oesophageal ring, by being
connected with a ventral ganglion. From this inferior ganglion a
series of ganglia, connected by long commissures, extend along the
ventral internal surface of the body, forming the ventral gangli-
onic chain. The greater size of the cephalic ganglion, compared
with that of the ventral ganglia, has been already seen in many of
the Annulata ; in the Arthropoda it is ordinarily still more distinct ;
this condition may be partly explained by its relations to the more
highly developed organs of sense, if we recognise in the dorsal
oesophageal ganglion something similar to the brain of the Verte-
brata. Led by an idea of this kind, some have compared even the
ventral ganglia, or ventral medulla, with the dorsal medulla of the
Vertebrata, and have striven to carry the comparison still further;
these attempts ignore the complete difference between the type of
structure of the Arthropoda and of the Vertebrata.

The increase in size of the cerebrum is, as has been above
indicated, in direct connection with the development of the
higher sensory organs, especially that of sight ; and its modifications
are in great part dependent on these. The ventral chain of
ganglia also undergoes essential, modifications, although it is not
always possible to make out in them a regular dependence on
the metameric condition of the body. The repetition of similar
metameres gires rise to similarity of character in the ganglia of the
ventral chord, and to a regular series of them. Where some meta-
meres are especially developed, their ganglia also are considerably
increased in size, while where there is concrescence of the metameres
an approximation of several groups of ganglia can also be noted ;
this not unfrequently leads to a complete fusion into several larger
ganglia, or the formation of a single large ventral ganglionic mass.

The ganglia of the ventral ganglionic chain are primitively
paired, and connected by a transverse commissure, as in the
Annulata. By the shortening of these transverse commissures, an
approximation, and finally a more complete fusion, takes place.

The peripheral nervous system arises from the ganglia of the
central system, that is, of the cerebrum and ventral chain, which are
distinguished by the possession of ganglionic cells. The nerves either
arise directly from the ganglionic portions, or run for some distance
along the longitudinal commissures, and then pass ofi from them.

The higher sensory nerves arise, as a rule, from the cerebral gan-
glion. This holds especially for the nerves of the eyes and antennte.

NERVOUS SYSTEM OF ARTHROPODA.

253

111 addition to tRe nerves^ set apart for tlie muscular system and
tlie integument, there are also nerves for the viscera ; of these the
enteric nerves are most exactly known. They partly agree with the
characters which obtain in the An-
nelides. As separate ganglia are
embedded in their course, they con-
stitute to a certain extent an inde-
pendent nervous system, which is
called the stomato -gastric nervous
system.'’^ Another special system,
of nerves, which obtains especially
in the Insecta, arises from the gan-
glia of the ventral medulla, and is
known as the sympathetic nervous
system.

§ 196.

The nervous system of the
Crustacea presents us with a large
number of examples of the phe-
nomena described in the foregoing
paragraph. The relation between
the size of the cerebrum and . the
development of the optic organs is
seen in the Thoracostraca, and in
the large-eyed Hyperida (Phronima)
among the Arthrostraca, where the
optic nerves arise from distinct
lobes ; these lobes are also distinct
in the Wood-lice. The separation
of tbe cerebral mass into various
groups of ganglia generally implies
a higher differentiation. With this
ought to be compared those de-
generations which affect the cere-
brum when the optic organs are
reduced or altogether lost ; when
this happens the antennae also gene-
rally disappear. Conditions of this
kind are found in the parasitic Cope-
poda, and in the Cirripedia (Fig.

129, B gs), and in consequence of it
the cerebrum is sometimes repre-
sented by nothing but a commissure.

The most anterior of the ventral ganglia is connected to the
cerebrum by a shorter or longer commissure. The length of this
chord appears to depend on the position of the mouth with regard to
the cerebral ganglia (that is really to the eyes and antennae). In the
Malacostraca the length is very considerable (Fig. 128, c; Fig. 129, A),

joints, p owing- feet ; the last pair
of the foot-like appendages contribute
to form a caudal-fin. m Muscles.
gs Supra-oesophageal ganglion. c
Commissural chords. g> Thoracic
ganglia, g" g'" Abdominal ganglia.

254

COMPAEATIVE AE’ATOMY.

as also in many lower Crustacea, e. g. tlie Cirripedia (Fig. 129, B c);
in others, again, it is so much shortened that the cerebral and ventral
ganglia form a single nervous mass, pierced by the oesophagus
(e. g. in Corycaeid^).

The ganglia of the ventral chain seem to be most regularly
distributed to the separate metameres in the Phyllopoda, which, in
this respect, retain most completely the primitive relations of parts.

The ventral chord is in them
composed of a large number
of pairs of ganglia (about 60
in Apus), which follow one
another, gradually losing their
transverse as well as the longi-
tudinal commissures ; in the
Daphnida the ganglia have
the same characters, although
they are less in number, in cor-
respondence with the smaller
number of the metameres. '
Among the Thoracostraca,
the ganglia of the ventral
chord also are, for the most
part, distinct, but in corre-
spondence with the concre-
scence of the anterior meta-
meres into a more or less
extended cephalothorax, the
anterior ganglionic masses are
fused ; this is either more or is
less distinctly expressed. Thus
in the Stomapoda (Fig. 128),
the ganglia which innervate
the anterior buccal, as well
as the prehensile feet (p),
form a larger complex (g^) ; a
series of independent ganglia,
extending as far as the caudal
segment, is connected with
this g^^^ g^^ )> In the Deca-
poda Macrura, likewise, con-
crescence of the 6 cephalo-
thoracic ganglia seems to be very common; while the 6 smaller
ganglia of the abdomen still correspond exactly to the metameres.
Further fusion is seen in the thoracic ganglia of some Macrura
(Palinurus) ; in Pagurus the ganglia of the abdomen are represented
by one only, in correlation with the shortening of this region. So
too in the Brachyura all the ganglia of the ventral chain are fused
into a single one (Fig. 129, A gi).

Eeductions of this kind obtain, also, in other divisions of the

Fig. 129. A Nervous system of a Crab
(Carcinus niaenas). gs Cerebral ganglia,
o Optic; a Antennary nerve. C ffisophageal
commissure. i Transverse connection of
the commissure, gi Fused ventral medulla
(after Milne-Edwards). B Nervous system
of a Cirripede (Coronula diadem a).
gs c gi as in A. a Antennary nerves which
are distidbuted in the mantle. Between
them is the “ optic ganglion,” connected with
the cerebrum, m Nerve to the stomach, s
Visceral nerve, which unites with a second
visceral nerve from the oesophageal ring to
form a plexus s" (after Darwin).

KEEVOUS SYSTEM OE AETHROPODA.

255

Crustacea, and these also are often explicable as being adaptations to
changes in the form of the body. We find this concentration in the
Copepoda, where the Calanidae possess a ventral chain, formed of
ganglia, whilst in the Corycaeidse this is concentrated into a mass
fused with the cerebrum. So too, among the Cirripedia, the
Lepadidse have a series of 4-5 ganglia in their ventral chord ; in the
Balanidge this is represented by a single ganglionic mass (Fig. 129,
B, gi). Similar phaenomena are seen in the Arthrostraca, but as a
rule they have a large number of ganglia (10-12 in the Amphipoda,
7-13 in the Isopoda).

§ 197.

In the Protracheata the nervous system remains in a lower con-
dition. A highly developed and closely united pair of cerebral
ganglia surround the mouth, and give otf lateral nerve-chords down-
wards. They are closely approximated below the oesophagus, and
then pass ventrally to the hinder end of the body, diverging some-
what in their course. These nerve-chords are united at their ends.
Along their whole length they are connected by fine transverse
commissures (in Peripatus Edwardsii) ; the most anterior of these
are the most distinct. There are no swellings on the ventral chord,
and the ganglionic cells are equally distributed in it. This
arrangement is that of the more indifferent condition of the ventral
ganglionic chain ; the chain is formed by the separation of the
ganglionic cells in the longitudinal trunks into separate parts,
corresponding with the inetameres.

Since in the Branchiata the ventral ganglia are always differen-
tiated, the arrangement in Peripatus is a lower one, and so far
justifies us in regarding the Tracheata as an independent phylum.

§ 198.

The nervous system of the Myriapoda indicates a well-marked
step forward, for there is a ventral chord which traverses the whole
length of the body with scarcely any variation in character, provided
with ganglia so arranged as to correspond exactly with the inetameres.
The first ganglion which supplies the gnathites sometimes indicates
distinctly that it is composed of a number of ganglia. The suc-
ceeding ones differ in size according to the extent to which their
appendages are developed ; they are arranged at regular distances
from one another, and in theDiplopoda succeed one another in pairs.
When the longitudinal commissures are shortened they form closely
applied swellings (Julidse). An approximation of this kind tending
to concrescence is ordinarily seen in the last ganglia, even when
the others are distinctly separated. These ganglia correspond in
number to the inetameres, so that there may be even as many as
140 (Geophilus). In these arrangements we see a condition which
is most like the typical form of the higher Tracheata.

256

COMPAEATIVE ANATOMY.

Among the Arachnida the ventral ganglia are very often reduced
and fused. They are all characterised by the close connection
between the cerebral ganglia, and the ventral chord, owing to the
extreme shortness of the commissures.

The nervous system is most richly segmented in the Scorpions.

The feebly developed cephalic ganglion
gives off two short commissures to the
ventral chain, which consists of 8 ganglia.
The first of these is remarkable for its
size, and appears to be homologous with
the single large ganglion in the cepha-
lothorax of the Spiders. As in them it
is the point of origin of the pedal nerves,
and must therefore be considered as com-
posed of several ganglia. The three suc-
ceeding ganglia are also placed in the
cephalothorax, and the four last, which
are widely separated from one another,
are found in the segments of the tail.

In the Galeodea and Phrynida, as in
the Aranea, the ganglionic chain is re-
placed by a large ventral ganglion, which
(Fig. 130, i) is, especially in the Spiders,
of a radiate form, and gives off the nerves
for the ventral appendages, and also two
which run into the abdomen ; in the
Galeodea branched nerve-trunks are sent
to the segments of the abdomen.

In all these divisions the cerebral gang-
lion, which is generally distinctly paired,
and in the Galeodea (Fig. 130, s) especially
large, gives off nerves for the eyes ; close
by the optic nerves those for the chelicerae
arise in the Spider, and thus we see that
these organs are metamorphosed antennge.

A complete concentration of the central portions of the nervous
system is seen in the Acarina, where the cerebral ganglia, wdiich
are often feebly developed, may be replaced by a commissure. The
ventral medulla, which is large and forms a single mass, exhibits
numerous traces of segmentation in the arrangement of the gang-
lionic cells and fibrous elements ; it gives off nerves all round.

The simple character of the nervous system in the Pycnogonida
is due to the decrease in number of their ganglia, owing to a reduc-
tion of the segments of the body ; their cerebrum is connected by
short commissures with the ventral medulla, which is formed of four
pairs of ganglia.

Fig. 130. Nervous system of
Thelyphoniis caudatus.
s Cerebral ganglion, i Ventral
ganglion. o Eyes, p Palpi.

Feet. tr Lungs, c
Tail-like appendage of tbe
body (after Blanchard).

NEKVOUS SYSTEM OF ARTHROPODA.

257

§ 199.

In tlie Iiisecta_, we find a form which corresponds to the primi-
tively homonomoiis segmentation of the body at the commencement
of the process of development; and all succeeding arrangements
of the nervous system are developed from it. The ventral
chord traverses, as a rule, the whole length of the body, the
ganglia being separated by equal distances from one another,
so that the last ganglion lies in the last segment of the body.
This character corresponds to the equivalence of the metameres,
which obtains at these stages, and points do its transmission from
a lower stage, such as that which we find permanently in the

Fig. 131. Nervous system of Insecta. A Of Termes (after Lespes). B Of a Beetle
(Dytiscus). C C3f a Fly (after Blanchard). gs Supra- oesophageal ganglion
(Cerebral ganglion), gi Sub -oesophageal ganglion, gr g^ g^ Fused ganglia of the

ventral chord, o Eyes.

Myriapoda. It is only when the Insect passes out of its larval
condition to the perfect one that changes appear. The development
of some metameres, the intimate fusion of others to form larger
portions of the body, the greater development of the appendages,
which persist on some metameres only, with the consequent increase
of the muscular supply, as well as many subordinate arrangements,
must be considered as affecting the changes which take place in the
nervous system. The decrease in number of the ganglia, by the
shortening of the longitudinal commissures, and the consequent
fusion of separate ganglia, produces a shortening of the whole
ventral chord. Owing to the independent character of the head of the
Insecta, in comparison with the other regions, the first ganglion of

s

258

COMPARATIVE ANATOMY.

tlie ventral cliord_, wliicli is primitively composed of three, remains
embedded in the head (sub-oesophageal ganglion), and takes no
share in the concrescence which affects the other ganglia ; it is only
in a few cases — in Insecta degraded by parasitism — that a union of
this ganglion with the rest of the ventral chord takes place.

The cerebral ganglion (Fig. 131, A B G, gs) is almost always
distinctly divided into two halves, each of which is again composed
of several smaller masses of ganglia, which are often complicated in
structure. The ganglia of the ventral chord are primitively paired,
and often become closely united. On the other hand, the longitu-
dinal commissures remain double, even when they are closely applied
to one another. There is also a separation of the ventral chord into a
superior and an inferior portion which corresponds to a physiological
differentiation.

The first ganglion of the ventral chord (G. infraoesophagenm)
sends off fibres for the organs of the month. The three succeeding
thoracic ganglia principally give off nerves to the appendages, feet,
and wings ; they are consequently of some size. On the other hand,
the succeeding ganglia are, as a rule, small, the last alone being
an exception to this, for it is of a larger size in correspondence
with its relations to the generative system.

Even in the Aptera there is a fair amount of variation, for
11 ventral ganglia (Lepisma) can be made out in the Thysanura,
while there are only 3-4 in the Collembola. The last portion of the
ventral chord seems to form a complex of ganglia in many (Orchesella,
Achorutes) .

As to the Pterygota, the chief point is that of all the orders the
least amount of metamorphosis is seen in the Pseudoneuroptera.
In them the ventral medulla traverses the whole length of the body,
and there are 5-9 abdominal ganglia in addition to the three thoracic
(Fig. 131, A). The Orthoptera, which have 5-7 abdominal ganglia,
resemble them in this.

There are great variations in the Coleoptera. In some the
ventral chord extends as far as the end of the abdomen, sometimes
possessing 8 separate ganglia (e.g. in Cerambycidm, Carabidm,
etc.); in others again the 3 ganglia of the thoracic portion are
only represented by two, the second and third being fused, whilst
the abdominal ganglia are also connected into one mass, which im-
mediately succeeds the preceding ganglion (Curculionida and Lamel-
licornise). In other families there are connecting links of various
kinds between these conditions which represent the extremes. In
the Hymenoptera we generally find the thoracic ganglia reduced to
two, while the abdominal part of the ventral chord has frequently 5 or
6 separate ganglia. These are in many, however, reduced to 4 or 3,
and even to one. The abdominal part of the ventral chord is, in the
Hemiptera, placed in the thorax, and is represented by a ganglionic
mass connected with the thoracic ganglia, which are also simple, by
a commissure of varying length. The nerves for the abdomen con-
sequently take a longer course, and form two longitudinal trunks

^s'ERVOUS SYSTE:^[ OF AETHKOPODA.

>59

whicE arise from tEe last ganglion. A similar difference in tlie
number of tEe ganglia of tlie ventral cEord obtains in tlie Diptera,
wEere tlie most primitive cEaracters are seen in Ptilex : 3 tlioracic
and 7-8 abdominal ganglia. In others there is generally a consider-
able reduction by the fusion of the thoracic, or of the abdominal
ganglia, or of both (Pig. 131, C). With this is connected the com-
plete fusion of the ventral chord into one somewhat long knot, in
the parasitic Pupipara. We find the same cEaracters in the Strep-
siptera. As to the Lepidoptera there is more uniformity in them,
a constant number of ganglia being found
in the larva, while when it is metamor-
phosed into the butterfly the mode of fusion
appears to be essentially the same in all.

§ 200.

The visceral nervous system of the
Arthropoda shows signs of some common
characters, together with great variation in
particular points. Among the Crustacea
nerve-filaments pass from the oesophageal
commissure to the enteron, or a nerve passes
to the enteric canal from the ventral chord.

(In Astacus from the last ganglion also.)

Even in the Arachnida nerves are given
off partly from the cerebrum, and partly
from the ventral ganglion to the enteron ;
in the Opilionea the posterior ones are pro-
vided with a large number of ganglia.

In the Insecta and Myriapoda the break-
ing up of the visceral nervous system into
several portions has been more generally
made out; we will therefore examine the
arrangement of it more closely. One part
forms the so-called paired system, which
consists of two branches running back-
wards from the cerebral ganglion to the
sides of the oesophagus ; these form a simple
chain of ganglia (Fig. 132, s' s") on either side,
these ganglia varies, and it is often difficult, on account of their
plexus-like connection with the unpaired system, to determine to
which system they belong. The unpaired system (^’ r^) arises in a
ganglion which lies in front of the cerebrum, and is more or less
connected with it. From this ganglion a thicker nerve (r) passes
backwards over the oesophagus to the stomach, and forms a
plexus with the branches of the paired system ; from this plexus
the neighbouring parts, especially those of the digestive system, are
innervated. In many Insects this nerve (N. recurrens) forms a single
ganglion (Coleoptera and Orthoptera), in others several (Lepidoptera)

s 2

Fig. 132. Supra-oesophageal
ganglion and visceral ner-
vous system of one of the
Lepidoptera (Bombyx
Mori), (j s Snpra-cesopha-
geal ganglion (Cerebrum).
a Antennary nerve, o Optic
nerve, r Azygos trunk of
visceral nervous system.
r' Its roots arising from the
snpra-oesophageal ganglion.
s Paired nerve with its
ganglionic enlargements s's"
(after Brandt) .

The number of

2G0

CO]\IPAEATIVE ANATOMY.

There is yet another system of nerve-branches in connection with
this plexus ; it is principally applied to the large branches of the
tracheae, and the muscles of the stigmata. This arrangement is
brought about by a nerve-filament, which runs on the surface of the
ventral chain, and is divided into two fork-like branches in front of
each ganglion (Nervi transversi accessorii). The branches receive
nerve-twigs from the upper chord of the ventral chain, and pass
partly outwards to the branches of the tracheae and the muscles of
the stigmata, and partly backwards, where they unite in the middle
line, and at the next ganglion repeat this arrangement.

Sensory Organs.

Tactile Organs.

§ 201.

The sensory organs of the Arthropoda are, for the most part,
allied to those of the Vermes. All but a few indicate a connection of

this kind, and these few are to be re-
garded as arrangements which are
developed in this division only. The
carapace-like covering of the body
of most Arthropoda requires special
organs to produce the sensation of
touch ; the form- elements of these are
connected with ganglionic cells, and
form rod-like nerve-endings. These
ganglionic cells are generally struc-
tures which are derived from the ecto-
derm, and the whole apparatus not
unfrequently retains its primitive
position.

These end organs, which are found
in the most different parts of the body,
form indifferent sensory organs, which
in certain parts take on the form of
tactile organs (cf. Fig. 133). Organs
of this kind are mostly found on the
appendages, where they present rod-
shaped projecting ends.

In the division of the Crustacea
these tactile rods have been recog-
nised in many forms, and that not
only on the antennae, especially in the lower Crustacea, but even on
other appendages of the body. In the Myifiapoda and Insecta
there are tactile rods on the antennae, and in the latter they are
also found on the tarsal joints of the feet.

Fig. 133. Nerve - ending with
tactile rods, from the proboscis
of a Fly (Musca). n Nerve.
g Ganglionic swolling. s Tactile
rods, c Fine hairs of the cuticle
(after Ley dig).

SEJN'SOEY OEGANS OF AETHEOPODA.

2G1

In addition to these tactile rods special organs resembling them
are found on the antennee of the Crustacea and Insecta ; they are
sometimes of considerable size^, and are innervated in the same way
as are the tactile rods. In the Crustacea they are formed only on
the inner (anterior) pair of antennae. In the Insecta they are much
shorter^ and are conical in form. Their position, in addition to the
circumstance that they are less long than the indifferent bristles, or
are placed in depressions, makes it probable that these organs have
another function, and it is very easy to suppose that they are organs
of smell, or at least of a sensation very much like it. In this case
the antennae, by the differentiation of special nerve-endings, have
more than one function, and do not merely preside over the sense of
touch.

Auditory Organs.

§ 202.

Auditory organs are not widely known in the Arthropoda, no
sign of them having been seen in the Myriapoda and Arachnid a ;
on the other hand, in some divisions of the Crustacea and Insecta,
organs may be made out, which appear to be adapted for the sensation
of sound.

There are two principal forms of the organ, which are exactly
correlated with the medium in which the animal lives. One form is
found in the Crustacea, and consists of a saccular space, formed by
an inpushing of the integument ; it sometimes remains open and is
sometimes closed. These auditory vesicles lie, in most of the
higher Crustacea, in the basal joint of the internal anteunm. Thus
in Leucifer, Sergestes, and other Malacostraca, as also in the
Arthrostraca (Hyperida), a pair of these organs may be found in front
of the cerebrum. As secondary structures they may also be found
on other parts of the body. Thus in the Mysidse, they lie in the two
inner lamellae of the fan of the tail. There are firm structures,
otoliths, in the auditory vesicles, which, when the vesicles are closed
(in Mysis and Hippolyta), consist of a concretion, which is held fast
by fine, regularly-arranged hairs. When they are open, as they
are very commonly among the Decapoda, and also in Tanais, the
orifice is greatly complicated. The place of the otoliths is here taken
by grains of sand brought in from the exterior ; these are regularly
attached by special hairs, which arise from the wall of the auditory
vesicle. They are like the other hairs of the integument, but are
distinguished f:»’om them by not having their shaft directly con-
nected with the floor of the vesicle, most of them standing on a fine
membranous process, to which endings of nerves pass. In this they
agree with the rod-like processes which carry the otoliths in the
Mysidas, to which nerves likewise pass. The auditory nerve in these
forms, in which the auditory vesicle is embedded in the internal

202

COMPAEATIYE AXATOMY.

antenuB3^ is a brancli of tlie internal antennary nerve. Both struc-
tures thus represent the end organs of nerves_, which are set in
vibration by the shaking of the firm body (Otolith) which they
carry, and thus produce an excitation of the nerve.

The general character of this remarkable system shows us how
the auditory organs arise from a differentiation of an indifferent
sensory organ connected with the integument. The auditory hairs
are only modifications of other ‘"Oiairs of the integument which
contain nerve-endings, and just like those which may appear on the
free parts of the body (tactile rods). The formation of the unclosed
auditory vesicle, or auditory pit, represents therefore a second stage
in this differentiation ; and the change into a closed vesicle is a
further stage of this phsenomenon.

Hensen, Zeitschr. f. wiss. Zool. XIII.

§ 203.

The other form of auditory organ is found in the Insecta. It is
principally in the Orthoptera, which are also provided with vocal
organs, that an organ for receiving the waves of sound can be made
out. The ordinary arrangement is a tympanic membrane, stretched
on a firm chitinous ring, one surface being directed to the exterior,
the other to the interior. On its inner side there is a tracheal vesicle,
and on this, or between it and the ‘‘ tympanum, there is a
ganglionic nervous enlargement, from which specially modified
nerve-endings, having the form of small club-shaped rods, arise
between fine filaments. The tympanum, as well as the tracheal
vesicles, serves as an organ for conducting sound. The organs of
perception are represented by the nerve-endings, which are regularly
arranged. In the Acridida the organ lies in the metathorax, just
above the base of the third pair of legs, and receives its nerve from
the third thoracic ganglion. The Locustida and the Achetida have
the organ embedded in the tibia of the two anterior legs. In the
former a tympanum lies on either side of this leg, either super-
ficially, or at the bottom of a cavity, which opens anteriorly by a
single orifice. Two tracheal branches occupy the space between the
two tympana, one of which carries the ridge-shaped nervous end-
organ. This auditory ridge is, in Locusta, formed by a series of cells
which grow smaller towards one end; each of these contains a
“ rod of proportional size. The tympanum in the Achetida lies on
the outer side of the tibia of the anterior leg.

Other organs, the nature of which is less definitely settled, are
allied to these, as by their general structure they represent auditory
organs ; the presence of the same pencil-shaped body in the termi-
nations of nerves justifies us in at least ranking these organs with
the auditory, while, further, such a relationship is implied by the
ganglionic outspreading of the proper nerve along a tracheal branch.
The ends of the nerves are directed towards the integument, the

VISUAL ORGANS OU AETHEOPODA.

263

cliitiiious layer of wliicli is always provided with a number of closely-
applied groups of pore- canals instead of a tympanum. Organs of
this kind have now been recognised at the root of the posterior wing
of the Coleoptera.^ as well as on the base of the halteres of the
Diptera.

The two forms of auditory organs in the Arthropoda are indeed
very different from one another in the details of their arrangements,
but there is, nevertheless, a connection, for in both cases the chitino-
genous cellular layer gives rise to parts which carry the special end-
organs ; in the Crustacea these are connected with processes of the
integument, the auditory hairs; while in the Insecta they are con-
verted into the small pencils, and are consequently differentiated in
another direction ; they remain within the dermal skeleton, and have
no relations to the processes of it. No homology can be made out
between these organs, owing to the diversity of their position, and
from the fact that more complicated organs are derived from an
elementary arrangement, which is distributed generally in the
integument.

Leydig, Arch. f. Anat. u. Phys. 1855. — Graber, V., Die tympanalen Sinnesap-
parate der Orthopteren. Denkschr. d. Wiener Acad. M. N. Cl. Bd. XXXVI.

Visual Organs.

§ 204.

Ill the visual organs of the Arthropoda we meet with points of
resemblance to certain forms of eye found in the Vermes ; to those,
namely, in which a number of end- organs of the optic nerves are
placed directly beneath the integument (Sagitta, Hirudinea, etc.).
But they have no close affinity to the more developed eyes of the
Annelides, which are distinguished by the possession of a separate lens
(§ 125). In Arthropoda, as in other forms, the integument is the spot
at which the eye is differentiated;
its mode of composition out of the
elements of the integument will be
understood by a reference to the sub-
jacent diagram; although, of course,
this does not represent the simplest
condition. The cuticular layer of
the integument forms a biconvex
thickening over the eye (/) ; this
forms a refracting but also a defen-
sive organ, and functions therefore
as a cornealens. The eye, which is
formed from the hypodermic layer
(Ji), lies behind this lens. Around it
the hypodermic cells elongate and change their position; they become
pigment cells (p). The optic cup, into which project transparent

Fig. 134. Section throngli the simple
eye of a young Dytisens larva
(after Grenacher).

264

COMPAEATIYE AKATOMY.

cells {cj), wliicli at first are placed beside tbe pigment cells, comes
next. These cells represent a vitreous body. Lastly, there are the
cells (r) which form a kind of retina ; these are connected with the
optic nerve (c), but converge towards the exterior, and so towards
the posterior surface of the lens, where they undergo various kinds
of differentiation. The vitreous body, pigment cells, and ^A’etina
are therefore clearly continuous with the ectodermal layer (hypo-
derm), and are differentiations of it, just as the cornea-lens was
formed from the cuticular layer, 'which again can be derived from
the hypoderm. The elements which compose the eye undergo
various differentiations. A special structure, the rod,'’^ is generally
differentiated in the anterior end of the retinal cells. When a
number of these cells are united into a single apparatus the rods also
become united, and form a special structure, the rhabdom,^^ in
the long axis of a group of combined retinal cells. The retinal cells,
which give rise to the rhabdom, constitute a retinula.^^ The cells
in the vitreous body in front of the retinal cells may also undergo
great modifications. Each group separates off a transparent highly
refractive substance, which forms the so-called ^^crystalline cone;^^
the apex of this is turned towards the rhabdom, and its base towards
the integument, i.e. to the cornea-lens.

Owing to the different development of the various parts, and to
the various ways in which they are combined, the optic organ
of Arthropoda becomes very varied in character. Muscular fibres
sometimes enter into the composition of the eye, and appear to
form a focussing apparatus.

These eyes belong to the head. The optic nerve arises from
the cerebral ganglion. In all divisions the eyes may undergo
degeneration, and even completely disappear. Eyes are but seldom
developed on other parts of the body, as they often are in the
Annelides, so that the presence of eye-like organs on the thorax and
abdomen of the Schizopod genus Euphausia is a rare exception.

§ 205.

The simplest eyes, although indeed their structure is not yet
exactly known, are found in the Entomostraca. Each eye appears
to possess one crystalline cone only, which is sunk into a mass of
pigment, and is generally separated from the integument. Two such
eyes, which are generally placed immediately on the cerebrum, are
characteristic of the Nauplius form of the Entomostraca. There are
two eyes, connected in the middle line, placed close to one another,
and fused into one organ by the connecting pigment ; when they
are not placed on the cerebrum itself, they are carried on a median
process of it. The Cirripedia and Khizocephala have eyes during
their larval condition ; the latter lose them later on. In many free-
living Copepoda the eye is more or less distinctly divided into two.
In that case there is another and larger eye on each side, in addition
to the larval eye. Each of these is generally provided with a

VISUAL OEGANS OF AETHROPODA.

2G5

crystalline cone of some size, in front of wliich a corresponding por-
tion of tlie cuticnlar layer of the integument forms a lens-like organ
(Corycaeidae). The presence of several crystalline cones in each eye
forms an intermediate step to the more complicated form of eye.
As the integument, which is found over the simple pair of eyes,
becomes thickened into two facets corresponding with the crystal-
line cones, these structures resemble in their mode of formation the
cornea-lenses.

In addition to the median eye, which is sometimes represented by
a mere speck of pigment, the Cladocera and Phyllopoda possess two
compound eyes, from which it may be concluded that the median
one, which corresponds to the eye of the Nauplius, is a special
structure, which does not become developed into the permanent
eye. This larval eye is probably a transmitted arrangement.

By their power of movement, and their position immediately
below the chitinous carapace, the eyes of the Branchiopoda form an
intermediate step towards those in which the chitinous carapace takes
a more direct share in forming the optic organ. Further, the position
of the eye, on a stalk-like process (Artemia and Branchipus), presents
a point of affinity to the podophtlialmate Malacostraca.

Two types of eye are derived from these conditions, which are
very common in the higher divisions of the Crustacea, and in the
Tracheata. According as the elements of the retina, which form
the perceptive apparatus, unite into a compact simple organ, or as
these organs are themselves part of a more complicated structure, the
optic organs are distinguished as simple eyes (Stemmata, Ocelli),
or as compound eyes. The cuticnlar layer of the integument
takes a greater or less share in the formation of this apparatus.

The simple eye (Fig. 134) is commonly found in the larval
forms of the Insecta, and there are generally a number of them on
either side of the head. In the Thysanura this form appears to
persist. The chitinous layer forms a cornea-lens over each eye. In
many Insects we find these simple eyes in company with the compound
ones, between which they are placed, generally in twos or threes,
and on the frontal surface. They are distinguished from the
compound eyes by being composed "of a large number of retinal
elements, which are covered by a simple cornea-lens.

In the Myriapoda the eyes, which are placed in one or two rows,
vary in number (4-8). It seems as if we had the larval stage in the
eye of insects permanently represented, but we have no exact
knowledge on the subject.

In the Arachnida the arrangement is much the same. There
are numerous peculiarities both in the arrangement and number
of their eyes. Two large eyes are, in the Scorpions, approximated
to one another, and on each side there is a group (2-5) of smaller
eyes. In the Spiders and Phrynida there are, as a rule, 8, more
seldom 6, symmetrically distributed on the anterior part of the
cephalothorax ; they generally vary in size ; in the Opilionida there
are only 3 or 4 in the same place, the largest of which are placed on

266

COMPAEATIVE AA^ATOMY.

au elevation of the cephalothorax. In the Pjcnogonida 4 eyes occupy
a similar position. On the other hand they are reduced to 2 in many
Mites, and so also in the Tardigrada ; in many parasitic Mites they
have completely disappeared. The principal point in their structure
is the presence of a cornea-lens, which is ordinarily very large in each
eye ; behind this is a layer of cells, which represents the vitreous
body, and to this the retina is attached. In the Aranea the retina is
formed in two ways, the eyes directed anteriorly differing in structure
from those which are turned upwards. That is to say, the retinal
cells of the former surround a small longitudinally bisected rod at
their anterior end (Epeira).

§ 20G.

The compound eye is characterised by the above-mentioned
fusion (§ 204) of a number (7-4) of retinal cells into a structure
which surrounds the rhabdoni — the ^^retinula^^ 135, C r).

The eye is made up of these retinulm, each of which is enveloped
in pigment. The multifid crystalline cone lies in front of the
retinnla. Two of these structures are represented in Fig. C. The
crystalline cones may be made out in front of the retinulje, and be-
hind the cornea-lenses (c). The whole arrangement is easy to under-
stand, when we derive it from the simple eye. A reduction of the
retinal elements of the simple eye gives rise to the retinnla, and a
compound eye is formed by the gradual concrescence of a number
of simple eyes. Most of the Crustacea have eyes of this hind. In
the Cladocera the movable eye (Fig. 136, oc) lies in a cavity roofed
over by the integument. In the Lasmodipoda also the cuticular
layer of the integument seems to take no part in forming the eye.

On the other hand in the Phyl-
lopoda we meet with a faceting
of the inner surface of the cuticle
covering the eye, the facets corre-
sponding to the crystalline cones.
In the Isopoda the compound eye
still consists of a number of simple
eyes, distinctly separated from one
another. When a number of these
structures, which form the end-
organs of an optic nerve, are closely
connected together, a process, con-
vextowards the outerface,is formed,
the size of which is dependent on
the number of the ^^retinulm^"’ (Fig.
135). The chitinous covering of the
whole eye is either smooth on its
surface, and only forms on its inner
face curves, which correspond to each of the ciystalline cones, or it
forms convexities for each of the separate crystalline cones, or even

Fig. 135. A Diagrammatic section
throngli a compound Arthropod-
cye. « Optic nerve, g Its ganglionic
swellings, r Ketinulm. c Faceted
cuticular layer, each facet of wLich
forms a cornea-lens. B A few cuticular
facets, seen from the surface. C Tavo
retinnla?, r, Avitli their cornea-lenses, c.

alime:n"taey cayal of aetheopoda.

267

marks off each separate area (B). (Faceted eye of the higher
Crustacea and of the Insecta.)

The number of the elements which make up one of these eyes_, as
well as their size^ and the form taken by the several parts, may
undergo various modifications. The crystalline cones are generally
present in this kind of eye in the Crustacea, but in many Insecta
the cells (crystalline cells), which in other eyes are differentiated
into crystalline cones, persist without forming crystalline cones.
Lastly, in many, the primitive condition of the retinula, in which the
separate cells, each with its rod, may still be made out, persists
(Tipulidse). The projecting character of the eye, owing to its curva-
ture, may lead to a stage in which the eye is stalked. When still
more developed this stalk may be movable (Podophthalmata).

Grenacher, H., Untersnchungen iiber das Arthropodenauge. Beilagehcft zu
den klin, Monatsbl. fur Augenbeilkunde. XY. Jahrgang.

Alimentary Canal.

§ 207.

The enteric canal of the Arthropoda is differentiated in much the
same way as in Vermes. The endoderm encloses what yolk-material
is not used up in the early stages of differentiation, and this is
absorbed during the gradual processes of further development. The
mouth and anus, and the connected parts of the enteron, are not
developed according to any general rule. When the enteric walls
are completely differentiated the nutrient canal forms a tube, which
traverses the whole length of the coelom, and is but seldom adapted
to the metameres of the body ; it begins by a mouth, which is placed
on the ventral surface, and extends to an anus, which, as a rule, is
placed in the last metamere. The external chitinous covering of the
body extends into the fore- and hind-gut ; in the mid-gut, which is
formed from the endoderm, it is replaced by a soft cuticle. The
appendages (§ 189), converted into masticatory and other organs, are
grouped round the mouth, and a process, which is formed from the
integument, joins them as an upper lip.

§ 208.

The enteric canal of the Crustacea is distinguished by the
straightness of its course, and the slight extent to which its divisions
are complicated. The mouth, which is ventral, is often placed some
way back, so that the fore-gut, which arises from it, runs forward
at first, and then turns backwards at a sharp angle. The terminal
portion of the ordinarily narrow fore-gut is known as the pharynx
or oesophagus; it is generally widened and distinctly marked off
from the succeeding mid-gut ; in many forms a wedge-like process

268

COJ^IPAEATIYE AXATO:\lY.

of this pharynx projects into the mid-gut. The walls of this portion
are^ as a rule, stronger, and its inner surface is often distinguished
by the possession of a firm chitinous framework, which is pro-
vided with tooth-like processes turned towards one another, and
moved by muscles ; these ridges, spicules, or setas, are derived
from the chitinous membrane, which invests this portion. They
form an apparatus which serves for the comminution of the
ingesta ; and hence this portion is known as the masticatory
stomach. As a rule it is of considerable size, and is regular in form,
owing to its firm framework. It is most largely developed in the
Decapoda (Fig. 143, v). In the Entomostraca it is small, or not
developed at all, while in the Isopoda among the Arthrostraca the
small masticatory stomach is provided with a fairly complex frame-
work, of which there are indications also in the Amphipoda
(Gammarus).

The mid-gut (Fig. 13G i) forms by far the longest portion of the

Fig’. 136. Orgaui satiou of a Daphnia. a Tactile antenna), Cerebrum, oc E}’e
i Enteric canal (mid-gut). h Cascal tubes at its commencement, g Shell gland,
c Heart. I Labrum. ou Ovary, o An Egg in the brood-space (o') formed between
the body and the mantle (after Leydig).

enteric tube, and varies greatly in width, and in the formation of
caecal diverticula. In many cases it has the same calibre all along ;
in others it is wfidened somewhat anteriorly, or mesially ( Chyle-
stomach^^); or the widened portion extends over the whole of the
mid-gut (“Chyle-intestine^^ of the Isopoda).

There are ctecal diverticula at the commencement of the mid-
gut, in all orders of the Crustacea. They arise as paired, and
seldom as unpaired, cmca. In the Copepoda they are found
only in a few genera ; in the Branchiopoda they are more common,
and form either a single pair of short ciecal tubes (Fig. 136, A)
(Daphnida), or are more richly branched (Argulus, Hedessa), or
arise from the enteron in greater numbers, and are differentiated
at their ends into glandular organs (Apus). We find that the same
lfiia3nomenon of metamorphosis of the enteric cmca into secreting
organs obtains in the Malacostraca, and at exactly the same spot.

ALIMENTARY CANAL OF ARTHROPODA.

269

They become organs which will be considered
to speak of the appendages of the mid-
gut.

The hind-gut forms the sh.ortest_, and
generally a narrower portion of the enteric
tract. It is seldom widened mesially^ and
in a few only is it provided with caccal
appendages.

The function of the enteric canal is not,
in all Crustacea, limited to digestion. In
some (Astacus, Limnadia, Daphnia) an
almost rhythmical taking-in and driving-
out of water may be observed in the hind-
gut, so that this portion appears to have a
respiratory function also.

In many lower Crustacea the enteric
canal undergoes degeneration. It disap-
pears in the degenerate males of the para-
sitic Copepoda, and in some Cirripedia
and most Rhizocephala, where nutrition is
effected by other means (cf. supra, p. 236).

when we come

§ 209.

Fig. 137. Digestive organs
of a Spider, oe CEsophagiis.
c Supraoesophageal ganglion
(Cerebrum). v Stomach.
v' Its lateral processes.
v" Appendages directed up-
vs^ards. i Mid-gut. r Cloacal
widened end of the enteron.
hh Openings from the liver
into the enteron. e Urinary
canals (after D ages).

into a larger
The narrow

The three divisions of the enteric tube
are distinctly marked off in the Protra-
cheata ; the mid-gut, which is distinguished
by its width, forms the largest part of it.

The enteric tube of the Arachnida is divided
number of segments, except in the degenerate forms,
fore-gut (Fig. 137, oe) leads into a
mid-gut, which is generally elongated,
and has its more anterior portion (v)
frayed out into lateral caccal sacs.

These are absent in the Phrynida
and Scorpionea. In the Aranea there
are five pairs of them which

extend to the base of the legs and
palps. ’ In the Galeodea four pairs,
the last two of which are bifurcated,
extend into the appendages (feet,
daws, and palps) ; in the Pycnogo-
nida these casca extend almost to the
end of the appendages (Fig. 138, h).

The presence of these parts greatly
increases the internal cavity of the
stomach.

In the Acarina these cmcal sacs are confined to the body ; there
are generally eight of them, ^ but any diminution in number is

Fig. 133. Digestive organs of
Ammotboe pycnogonoicles.
a Stomach. 5 Caeca (after Quatre,
fages).

270

CO:\IPAEATIYE AN-ATOMY.

compensated for by the branching of the cseca. The Opilionida are
provided with a much greater number (about thirty), and a median
pair is provided with secondary appendages.

The portion of the mid-gut behind the stomach varies in length ;
when it is long it is widened out towards its end, and is separated

Fig. 111. Its imago, i Ileacl. 2, .3, 4 Thoracic segments.
5-13 Abdominal segments. V Fore-gut. M Mid-gut.
E Ilind-gut. gs Cerebral ganglion, gi Sub-oesophageal
ganglion, u Ventral ganglion, vm Malpighian vessels.
C Heart. G Generative organs (after NeAvport).

otf from the hind-gut by a constriction ; this
latter is almost always widened, it is pretty
long in the Scorpionca, but shorter in Galeodes,
where it has a cmcal sac. In the Aranea also,
and in the Acarina, the hind-gut (Fig. 137, r)
is of a considerable width.

Fig. 139. Larva of a

Lepidopterous Insect ^ 210.

(Sphinx ligustri) ; seen

from the side. The Myriapoda and Insecta agree in the

mode of arrangement of the chief tracts of
their digestive system ; which at the same time closely reseipbles
what obtains in Peripatus. Of the three divisions of the digestive
tube the fore-gut only prepares the food, and the mid-gut has the
principal share in digesting it* As a rule it forms the longest

ALIMENTAEY CANAL OF AETPIKOPODA.

271

portion, and at the same time undergoes the largest amount of
differentiation.

When most simple the enteric tube traverses the coelom in a
straight line ; in this point the Myriapoda resemble the lower stages
of the Insecta. In the Myriapoda the hind-gut is seldom very long ;
when it is, it is looped. A portion of the mid-gut still more rarely
takes part in forming this loop (Glomeris).

The simple character found in most larvm does not persist in
most divisions of the Insecta; in most the separate portions get to
vary greatly in character, and these changes are, as a rule, correlated
with the appearance of the imago-stage. They are really due to the
great differences in' the relations of the animal to external conditions,
which now commences.

The mode of life seems to be of the greatest influence on the
general configuration of the alimentary canal, and there is often a
greater length of tube in the phytophagous insects — as, indeed,
frequently happens elsewhere in the Animal Kingdom — than in
those which live on animal matters. The character of the food is
also another cause which comes into consideration ; we find, that
is, simpler characters in the intestines of those insects which live on
fluids, while there is greater complication of structure in those
which subsist on solid food.

These characters are seen most strikingly on comparing the
digestive tube of an insect larva with that of a developed insect ;
we see, for example, a caterpillar (Fig. 139) provided with a wide
tube running* straight through the body. This arrangement is
adapted to the enormous amount of food which is taken every
day ; while the butterfly, which takes in only a little food, and
that fluid, has a canal which is longer indeed, but much more
delicate (Fig. 141).

Further, the difference between the enteric canal of the perfect
insect and that of its larva depends on a change in the relations of the
various divisions of it ; while, in the larval stage, the mid-gut is
ordinarily the largest portion, it becomes gradually shorter, and the
hind-gut is proportionately increased in length. Thus the enteric
tube ceases to be a straight one. The increase in length of the
separate portions produces curvatures of the tube, which is now
longer than the whole body-cavity, and these may lead to the
formation of various coils in it. These affect the mid- and hind-
p’uts, while the fore-ffut most completely retains its primitive course
(cf. Figs. 139-141).

Fresh differentiations in the various portions are connected with
these, so that the boundaries between them often disappear. The
mid- is distinguished from the fore-gut by its glandular character,
and where the latter has appendages, or diverticula, they serve for
the reception and the further comminution of the food ; in the latter
case they have the characters of a gizzard. Finally, the hind-gut is
characterised by the Malpighian vessels which open into it.

Plateau, F., Kech. sur la plienom. de la digest, et sur la structure de I’appareil
digest. cFez les Myriapodes. Mem. Acad. Belg. XLII.

272

COMPAEATIVE ANATOMY.

§ 211.

The gut in the Thysanura, and in most Psendoneuroptera and
Neuroptera, is most simple in character, and least different from
that of the larval form ; some of these (Panorpa) have an enlarge-
ment at the end of the fore-gut; this is the masticatory
stomach. The Orthoptera are distinguished by the possession of
a stomach of this kind (Fig. 142, A r), which has long rows of
chitinous plates on its inner surface. It is also found in the
Coleoptera (Carabidfe, Cicindelm, Dytiscidae, etc.), where it carries
setae and ridge-like projections. Many Hymenoptera (Formica,
Cynips) also possess it, as do the larvm of the Dipt era.

Another differentiation seen in
the fore-gut of many insects (Hemip-
tera), which is always short, is an
enlargement which may be present
all round, or on one side only. When
it surrounds the oesophagus it serves
as a crop (ingluvies) (i) ; this is the
case in many Coleoptera and in the
Orthoptera. This dilatation of the
fore-gut is found in the Hymenop-
tera (Wasps and Bees), but in them
it functions as a sucking organ, and
leads towards the formation of the
Sucking stomach, which is found
in other insects. It also forms a
vesicular thin -walled appendage,
which is attached on the course of,
or to the end of the fore-gut; in
the Lepidoptera this opens directly
(Fig. 141, F ), and in the Diptera
by means of a stalk of varying
length (Fig. 142, B, v s). In the
Hymenoptera also an independent and stalked sucking stomach is
formed (Crabro). In the Hemiptera it appears to be replaced by
an enlargement of the fore-gut, which is frequently multi-sacculate
(Bugs).

The mid- gut (^^ chyle-stomach^^) is no less varied in character.
In many Coleoptera it is provided with short tubes, either along its
v/hole length or in separate segments ; these are known as glands."’^
At its commencement we sometimes, and especially in the Orthoptera,
find C£Bcal j)rolongations ; as we do also in some families of the
Diptera. In the latter it is ordinarily disposed in coils (Fig. 142, B v),
in correspondence wnth its great length. The same occurs also in
the long mid-gut of some Coleoptera (e.g. Melolontha), of Bees and
Wasps, and of many Hemiptera ; in these forms further divisions
are differentiated in it.

Fi<?. 142. A Digestive canal of the
Field-cricket. B Of a Fly. oe CEso-
phagus. i Its crop-like enlargement.
V Stomach, c Its appendages, r Hind-
gnt. vm Malpighian canals.

DIGESTIVE CANAL OF ARTIIROPODA.

273

In some cases tlie mid-gut ends blindly and lias no connection
with a liind-gut. This happens in the larva of Bees and Wasps^ of
the Ichneumonida^ and of many Diptera, etc.

The hind-gut in the Insecta generally forms the shortest portion
of the straight alimentary canal. It is often divided into two por-
tions, the second of which is widened out (^Gectum^^) (Fig. 142, A i? r).
In the Coleoptera (as in Dytiscus) its narrower and anterior por-
tion appears to be of considerable length, as in many Orthoptera,
where a larger number of divisions, varying in width, can be made
out ; it is longest in the Cicadm : in all of these forms it is disposed
in coils. As in many the Malpighian vessels, which belong to the
hind-gut, open very far forwards, it appears as if part of the last
division of the hind-gut had passed into the mid-gut, although,
indeed, this rather seems to indicate that the mid-gut has been
reduced in length.

The widened terminal portion of this part of the enteric canal is
distinguished in a large number of Insects by papilliform ridges,
which project into it : in these we find many tracheal branches. This
portion in many aquatic larvae of the Libellnlidas has numerous
lamellee arranged in longitudinal series, and provided with closely-
packed tracheal ramifications. The lamellsc act as organs of respira-
tion during the in-and-out flow of water, which is caused by the
opening and closing of the anus (cf. Fig. 150, B 0). There are
many intermediate forms between these tracheal gills, and the
papilliform processes of the hind-gut (in the larvm of the Phry-
ganeidm), so that these organs can be seen to be homologous.

Plateau, F., Eech. sur les Pheuomenes de la digest, chez les Insectes. Mem.

Acad. Belg. XLI.

Organs appended to the Digestive Canal.

1) Appendages of the Fore-Gut.

§ 212.

. Glandular organs are differentiated in the various portions of
the alimentary canal of the Arthropoda. The salivary glands,
which open into the fore-gnt, are but slightly developed in the
Crustacea. Groups of unicellular glands have been made out in
some divisions. On the other hand, we find them more widely
distributed in the Tracheata, where they may have various functions ;
but as yet we have but very little exact information as to how they
open. The great point which is undetermined is, how far they open
into the mouth itself, or are connected with the fore-gut. But the
glands which open into the gnathites of Peripatus (§ 193) must be
mentioned here. Among the Arachnida, the Scorpions have two

T

274

COMPARATIVE ANATOMY.

pairs of lobed glands ; in the Galeodea they are partly represented
by coiled tubes, and in the Aranea, likewise, these organs do not
appear to be wanting. The salivary glands are highly developed in
Mites, where there are several pairs of them, which vary in structure ;
the secretion of them is probably partly used as poison.

In the Myriapoda, there are simple tubular (Julus), or lobed
(Lithobius), or even racemose glands (Scolopendra), which are
probably salivary.

In the Insecta the salivary glands are very variously developed
as to number, form, and histological structure. It is therefore
certain that they also vary greatly in function. It is in a few only,
as the Ephemerida, Libellulida, and Aphides, that they appear to be
completely wanting*, or only feebly developed, as in the Myrmeleonida
and Sialida. In others, they are long coiled tubes, or lobed or
greatly-branched organs, which frequently accompany the enteric
canal for a certain distance. There are often two, and not un-
frequently three pairs of them, which vary greatly in structure. As
to their external form and their distribution in the different groups
of the Insecta — they have the form of a pair of long tubes in the
Coleoptera, Diptera, and Lepidoptera; in the order of the Hemiptera
and Orthoptera branched, racemose, or lobed forms predominate,
as also in many Coleoptera.

2) Appendages of the Mid-gut.

§ 213.

Another group of glandular organs is differentiated from the mid-
gut. They are regarded as a liver. According to their points of
connection with the enteron, two different organs must be here dis-
tinguished from one another. One of them is connected with the
most anterior portion, and has the form of simple or branched tubes,
which, when more largely developed, form a compound glandular
apparatus (cf. § 208). The ends of these tubes seem to be secretory
organs, but their ducts, owing to their wide lumen, form cavities of
the enteron. The organ is, therefore, not yet completely differenti-
ated from the enteron. This arrangement is seen in the Branchio-
poda, and, among them, most markedly in the Phyllopoda ; some
have a simple or branched csecal tube on either side (Fig. lo6, h) ; in
others it is converted into a liver (Limnadia, Apus), the greater part
of which is placed in the cephalic shield. The Cirripedia have similar
organs. In the Arthrostraca these caecal tubes (Fig. 143, A h) are
long organs, which pass backwards and vary in number. They are
not branched, but this is compensated for by their increased length.
They have the same form in the Schizopoda, but in most of the
Thoracostraca, as in all the Decapoda, they are formed by a paired
mass of glandular matter, which fills up the cephalothorax and is

DIGESTIVE CANAL OF AETHEOPODA.

275

Fig’. 143. Enteric canal and liver of Crustacea.
A Of Onisciis. J5 Of a larval Palinnrus (Phyl.
losoma). V Masticatory stomacli. i Chyle-stomach.
a Anns, h Hepatic tubes.

grouped in tufts. InasmucE as in the larv^ of the Decapoda they
are mere diverticula of the wall of the enteron, it is clear that
they are only more de-
veloped stages of those v/ ~ ^

simpler tubes^ which are
found in many Entomos-
traca.

A second form of this
hepatic organ is distin-
guished from the former
by the larger number of
its separate glands, and
by the more backward
position of its opening
into the mid-gut. Indi-
cations of this are even
found in the Copepoda
in the form of several
succeeding diverticula of
the mid-gut. They are
more developed in some
Isopoda (Bopyrus), where
they beset the whole of the mid-gut in the form of paired and
branched tufts of glands. In the Stomapoda also we find a larger
number (10 pairs) of lobate tufts of glands placed all along the
mid-gut.

The two forms cannot be directly derived from one another, for
those parts which carry glands in the former groups do not
do so in the others. It is possible that both kinds of organs were
united in a common stem-form. If so, we can imagine the whole
mid-gut beset with caeca, from which two sets of glands were de-
veloped ; in one the most anterior pair were developed, and in the
other suppressed, whilst the hinder pair was more or less largely
developed. These hinder glands distinguish the mid-gut of the
Poecilopoda, where they have the form of two pairs of branched
tufts.

Among the Tracheata we find similar diflterentiations of the wall
of the enteron in the Arachnida ; in which division we must regard
them as acquired organs only. The anterior pair are not, however,
always developed into glandular organs, but persist as more or less
wide pouches or tubes ; these have been already fully described as
stomachal caeca (§ 209). In the Opilionida they are exclusively
glandular in character. In the Scorpionea and Aranea separate tufts
of glands open into the hinder portion of the mid-gut. The Aranea
have two or three (Fig. 137, 7i), the Scorpionea five pairs.

These appendages are not found in the Myriapoda or Insecta,
and the absence of diverticula from this portion of the enteron during
development shows that any diverticula, which appear in it, have
merely a secondary significance.

276

CO^rPARATIVE A^7ATOMY.

8) Appendages of the Hind-gut. .

§ 214.

The glands differentiated off with the hind-gut^ which is itself
generally short, do not secrete anything, which is of importance in
digestion or absorption. Their secretions are rather of the excretory
series. And as we have chemical evidence to show that these matters
resemble the urinary excretions of the Vertebrata, we may regard
these bodies as being excretory organs, without thereby pre-
judicing any relations, which they may have, in some cases, to other
functions.

In the Crustacea we sometimes meet with cmcal organs on the
hind-gut, as for example in the larvae of the Copepoda, but we
cannot safely form any opinion as to their significance. It is pro-
bable that the concretions found in the walls of their entera are of
an excretory character.

Excretory glandular organs are very generally found in the
Tracheata; they arise as diverticula of the enteron, and have the
form of long canals, which may be simple or branched, and which
are often arranged in several coils or loops on the enteric canal ; they
open into the terminal widened portion of the enteric canal, and
almost always behind the mid-gut. They are known as Malpi-
ghian vessels, or, from their function, as urinary canals. As they
are formed at the same time as that portion of the hind-gut, which
in the embryo is developed from the ectoderm, it is not improbable
that they primitively opened on to the surface of the body, or were
derived from organs which did so. In all divisions there are two
chief canals, as is often seen at the point where a large number of
canals open and unite ; this number may therefore be regarded as a
primitive character.

Among the Arachnida they are, in the Scorpionea, simple canals,
which run between the lobes of the liver ; one pair of them is
branched. The urinary canals of the Aranea are much branched,
and united into a plexus ; they unite into two common excretory
ducts (Fig. 137, c), by which they open into the wide hind-gut, or its
cmcal sac. In the Opilionida they form two long highly-coiled
canals ; in the Acarina, where they are sometimes provided with
branches, they are of a similar form.

An equally small number of simple urinary vessels is found in
the Myriapoda; the Julida have one and the Scolopendrida two
pairs. They are allied, not only by their number and simplicity of
structure, but also by their arrangement on the alimentary canal, to
the corresponding organs of many Insect larvm.

The greatest variation in number, arrangement, and special
structure obtains in the urinary vessels of the Insecta. They are
wanting in the Collembola, among the Aptera, and in many Thysanura

DIGESTIVE CANAL OF AETIIEOFODA.

277

(Campodea), but four are present in Lepisma. The function of the
urinary canals is notably increased in the Insecta, which have a
perfect inetaniorphosis_, during their larval stage, as is shown not
only by the great development of these organs (Fig. 139, -urn), but
also by the quantity of urine which is collected in the hind-gut
during their pupal stage. This phgenomenon corresponds exactly
to that period in which the most intense plastic activity is going on
in the organism, in connection with the
development of the perfect body. It is
clear that the function of the Malpighian
canals of the Ifisecta is not to be found
exclusively in the excretion of urine ; but
that an older hypothesis, according to which
they were regarded as organs for the excre-
tion of bile also, is not altogether without
justification, is seen from the fact that por-
tions of this canal have a different struc-
ture, while the secretion in these portions is
different also.

The brownish-yellow, or light-yellow
coloration of the urinary canals is due to
the substances deposited in the cells of the
canal-wall, and is more intense when secre-
tion is more active. Most of the Diptera
(Fig. 142, B vm) and Hemiptera have two
urinary canals, which are connected with
one another; there are six canals in the Fig- IIL Portion of a Mai-

Lepidoptera, in many Nocturnal flies, and Vomitoria. tr Tracheai.
in several Pseudoneuroptera (Termites) ; I Lumen. 1 Nucleus,
there are four to six in the Coleoptera;

the Hymenoptera are distinguished by a large number of short
urinary canals, and indeed hundreds have been found in them, and
in many Orthoptera (Fig. 142, A vm). As a rule they are seldom
branched; but we often meet with loop-like connections between
their ends. They open at apparently very different points, accord-
ing to the length of the hind-gut. They open very far forwards in
the Cicadm, Diptera, and Lepidoptera. In the Hymenoptera they
open just behind the mid-gut.

Where the canals unite to form a common excretory duct there
is a circular layer of muscles (Gryllotalpa) . Separate circular fibres
are very seldom found in the secretory canals (Brachinus).

Coelom.

§215.

As the embryonic body is differentiated, a cavity, the coelom, is
formed in the endoderm between the enteron and the wall of the

278

COMPAEATIVE A:^ATO]\rY.

body, just as in tlie liiglier Vermes; it is found in all tlie Artliro-
poda. There are no signs in the Arthropoda of the dissepiments
found in the Annulata in correlation with the metamerism of their
bodies. This shows that their relationship to the Annulata is, to say
the least, a very distant one. In all cases the coelom forms a portion
of the blood-vascular system, so that the perienteric fluid found in
many Vermes as a fluid different from the blood is represented in the
Arthropoda by the blood itself.

In most Arthropoda some of the form-elements of the mesoderm,
which are not applied to the ectoderm — to form the wall of the
body — or to the endoderm to form the wall of the enteron — persist as
a number of cells, which are not apportioned to any definite organ.
These masses of cells remain in various parts of the coelom, and are
often found, just like other connective substance in the Arthropoda,
between the separate organs embedded in the coelom.

Sometimes all of these cells remain indifferent, and form, by
uniting with one another, cords or networks. As a rule, however,
they are differentiated. Drops of fat are formed in them, which
either fill up the cells in a regular manner, or run together into larger
drops ; in consequence of this these cells are comprised under the
name of fat-bodies. This fat is sometimes variously coloured
(yellow or red). Cells of this kind, containing fat-drops, have been
observed in the Crustacea, and especially in the Entomostraca,
where they are sometimes very large in comparison with the size of
the animaks body, in which they are distrilDuted in a constant and
regular manner. This justifies ns in supposing that they have some
hydrostatic significance.

Deposits of this kind are most largely developed in the Insecta,
where the fat-body, especially in the larval stages, consists of
large cells, connected to one another by processes, and filling up a
large part of the coelom. We must not suppose, however, that
these cells contain nothing but fat. This tissue is that in which the
material, which is partly used up in the pupa stage, is deposited ; in
consequence of this not much of it is found in the adult Insect. The
cells vary greatly in their mode of connection. This may be close,
so that the fat-body forms lamellae, or connected lobes, which are
connected with branches of the tracheal system ; or the cells are
loosely connected together. In the most extreme case the cells may
lie separately in the coelom, where they must not be confounded
with the much smaller and more indifferent blood-cells.

The cells of the fat-body in the Tracheata also serve for the
deposition of excretory matters, which are known to be urates.
These form crystalline concretions, or larger spheres of the form of
small granules, which call to mind the renal concretions of the
Mollusca. Among the Arachnida they are found in the Mites ; they
are found in the Myriapoda (Julus, Polydesmus, Glomeris), and are
very common in the Insecta. They do not seem to be altogether
absent from the Crustacea, for similar concretions have been observed
in Asellus.

VASCULAE SYSTEM OF AETHEOPOBA.

270

The luminous organs of the Lampyridm are special modifica-
tions of the fat-body. They are formed of plates of cells, to which
a large number of tracheal and nerve branches are sent. On their
inner side they are covered by other cells, which are not luminous,
but which are loaded with a large number of urinary concretions.
The superficial position of the luminous plates shows that they
belong to the epidermal layer (hypoderm).

The regularity of the coelom, along the long axis of the body, is
modified by the muscular system. Where this is greatly developed
(as in the cephalothorax of the Crustacea and Arachnida, and in
the thoracic metameres of the Insecta) only a small space is left for
the coelom. The processes, too, of the chitinous skeleton are the
cause of variations in it ; chiefly by forming smaller cavities, as they
especially do in the Crustacea. In the Insecta a subneural cavity is
formed by muscles, which in some of them are inserted into the
chain of ventral ganglia. In others similar muscles run horizontally
from one side to the other of the abdomen, and so likewise mark off
a portion of the coelom.

Vascular System.

§ 216.

This system of organs, which in the Vermes had become highly
developed, seems to be less so in many of the Arthropoda, chiefly
because the coelom generally forms a portion of the vascular system.
There is, therefore, no difcrence between the blood and a peri-
enteric fluid.

As a rule, a dorsally-placed vascular trunk is alone de^
veloped to any great extent ; this functions as a heart, and seems to
be homologous with the dorsal vascular trunk of the Vermes, parts
of which may also function as hearts. But it seems to differ by not
being connected with the enteron. The blood is either driven to
the anterior, or to both ends of the body, by the cardiac tube. This
dorsal cardiac tube of the Arthropoda is not, however, provided
with afferent vessels, and the blood which passes into it does so by
narrow venous ostia; so that, although a peripheral system of
vessels may in some divisions be formed from continuations or rami-
fications of the arterial vessels, or by the differentiation of vascular
canals from portions of the coelom, yet close to the heart a sinus is
formed from a portion of this latter. This pericardial sinus
appears, therefore, to be a portion of the coelom ; and the slight
development of the vessels which obtains in many Arthropoda is not
to be regarded as a degeneration from a more perfect stage, but as
'a low stage, which is correlated with a less amount of development.
It is still an open question whether, and how, this simpler form of
the vascular system is connected with the arrangements which are
found in Vermes.

280

COMPARATIVE ANATOMY.

The localisation of the respiratory function leads to complication
of the vessels : even where the vessels are not provided with separate
w^alls the blood always flows in a definite and constant direction.

The blood- fluid of the Arthropoda is, as a rule, colourless; in
a few Insects only it has a greenish, or red colour, owing to the
coloration of the plasma. The formed constituents of the blood
are indifferent colourless cells, very variable in form and size. They
are wanting in many (lower Crustacea). The blood-cells of Insects
are often distinguished by the large number of fine fat-molecules in
them ; but these must not be confounded with the cells of the fatty
bodies, which are often also free.

§ 217.

The simplest form of a circulatory apparatus in the Branchiata
is that of a short tubular heart (cf. Fig. 136, c, of Daphnia), which is
placed above the enteric canal in the anterior part of the body, takes
in the blood by two lateral orifices, and drives it by a short anterior
vessel to the cephalic organs, and especially to the cerebral ganglia.
The blood is distributed in regular currents throughout the body,
and, passing by the parts which serve chiefly as respiratory organs,
again reaches the heart, which it enters by the slit-like orifices in it.
This form of circulatory organ characterises the Copepoda and
Cladocera; but it is also seen in the larval stages of the higher
orders, and even in the developmental stages of the Decapoda,
where it is but slightly modified. The simple forms cannot there-
fore be regarded as being degenerated from more complicated forms.
The circulation is purely lacunar, and there are no vessels of any
kind except those added on to an anterior artery, which is seldom
much branched. In many Copepoda (CorycEeidao), and in the
Cirripedia, there are no circulatory organs.

The heart of the Phyllopoda is more developed. It has the
form of a longer tube, made up by several such simple hearts as
those of the Daphnida, and possessing a number of venous ostia
(as many as 20 pairs in Artemia). The cardiac tube, that is, is
divided into separate chambers, which do not, however, exactly
correspond to the metameres, of which there are more than oue to
each chamber. This segmentation seems, therefore, to be an inde-
pendent one, and is, perhaps, to be regarded as a later arrangement.
An arterial trunk is given off from the most anterior end only. This
passes the blood on to the lacunar tract of the coelom.

The heart of the Arthrostraca extends longitudinally along a
large portion of the body in the Amphipoda and Isopoda; in the
former it is placed in the metameres behind the head, and in the
latter some way farther back. There may be an anterior vessel only,
or it may give off a posterior one also. The former only is branched,
and the branches are confined to the cephalic region. The number
of the ostia varies greatly in the Amphipoda (Phronima has 3,
Caprella o, Gammarus 7 pairs).

VASCULAR SYSTEM OF ARTHROPODA.

281

The larvas of the Thoracostraca have a simple cardiac tube with
two lateral ostia only, which so far resembles the arrangement
described above. A more complicated form is gradually derived
from this, which is developed along two lines. One of these is seen
in the Stomapoda, where the heart is elongated, and an anterior and
a posterior arterial trunk alone given off, although the number of
venous ostia is increased. As the anterior artery alone is branched,
and the posterior has a widely- open mouth, the arrangement which
obtains in the Arthrostraca is again repeated in these forms. Later
on, not only do the anterior and posterior arteries form a larger
number of branches, but a larger number of lateral arterial trunks
are given off from the heart itself.

The second type is seen in the Schizopoda and Decapoda.
Although the heart is provided with several pair of ostia it is more
concentrated in form, and the lumen can no longer be seen to be
divided into' successive chambers. The primitive segmentation
has given way to a more compact arrangement. This cha-
racter is seen even in the arrangement of its several clefts, for they do
not now follow by regular pairs, but are grouped in a different manuer.
The heart of the larvm, however, arises as a thin-walled tube with
only one pair of clefts, and is continued forwards and backwards into
a simple vascular trunk. The anterior one divides into three
branches, which arise directly from the heart when the trunk
shortens ; the posterior trunk remains single. The heart is elon-
gated for a time only, or passes at once into a more compact form.
In both the Schizopoda and Decapoda it is placed in the posterior
portion of the cephalo thorax.

New portions are formed in the arterial system, while the whole
venous portion is represented by lacunae. The vascular system of
the Schizopoda (Mysis) remains at this stage; the Decapoda pass
through the different Schizopod-stages during their development.
In the mature forms of the Macrourous Decapoda we find the mus-
cular cardiac tube (Fig. 145, c) surrounded by a well-developed
pericardial sinus {pc)j from which the blood passes into the heart
by three pairs of symmetrically-arranged clefts. Three anterior
arterial trunks, and one posterior trunk arise from the heart. The
anterior median one (av) runs to the cerebrum and eyes (o), without
branching much; the two lateral trunks (aa) give off branches to
the generative organs, liver, and antennm. The arterial trunk from
the hinder end of the heart divides into two branches, which lie one
over the other, and which may even arise separately from the heart.
The dorsal one {ajy), which is bifurcated in the Brachyura, supplies the
muscles of the back and tail. The ventral branch (a) takes a down-
ward course at once, and divides into an anterior and a posterior
trunk, both of which give off most of their branches to the appen-
dages. There may be two smaller trunks in addition to the posterior
median one. The highly- developed capillary system gradually
passes into afferent canals (veins), which are at first collected into
several trunks on the ventral side, and are then united (u) into a

283

COMPAEATIVE ANATOMY.

large ventral sinus, placed at the base of the gills (in the so-called
sternal canal). Each gill {hr) receives a vessel from this sinus

(branchial artery) . From
the gills the blood passes
into the branchial veins
(vhr), six or seven of which
arise on either side of the
pericardial sinus, into which
they often open by funnel-
shaped mouths.

The valves of the venous
ostia are to be regarded as
special dilferentiations of
the heart, which aid in
dividing it into separate
chambers wdien it is elon-
gated.

The circulatory system
of the Poecilopoda combines
several of these different
stages ; their elongated
heart lies in a pericardial
sinus, from which it re-
ceives blood by seven pairs
of ostia; it does not only
give off arterial trunks an-
teriorly and posteriorly, but
from the sides also, as do
the Stomapoda.

§ 218.

The circulatory organs
of the Tracheata very
much resemble the elon-
gated multi - camerate
hearts found in some Crus-
tacea ; and the differences
are due rather to the de-
gree to which the vascular
system which passes from
the heart is developed. This
again is affected by its
relations to the respiratory
organs, for when they are
limited to a small space the
blood-vessels are more perfectly developed,, while when the respira-
tory organs are distributed over the whole body the arteries are less
well developed. The Protracheata appear to resemble them in this.

aa Hepatic artery, ap Posterior artery of the
body. aTrnukof the ventral artery, or Anterior
ventral artery, v Ventral venous sinus, v hr
Branchial veins. The arrows indicate the
direction of the current of blood.

VASCULAE SYSTEM OF AETIIKOPODA.

283

So far as is yet known tlie circulatory system of Peripatns is
represented by a dorsal vessel/^ just as in tbe Insecta, so that
here we find the simplest characters as compared with the other
Tracheata. In the middle line of the ventral surface of the dorsal
vessel there is a row of clefts^ and it appears to agree with that of the
Myriapoda in extending along the whole body^ while in the Insecta
it is limited to the abdomen. In Insecta it is attached to the wall of
the body, and sometimes even to the tracheae (in the larvm of the
Mnscidae) by the ^^alm cordis 146, in). In the larva) it is
divided into separate chambers, which are often not very distinct on
the outside, and which have, owing partly to the arrangement of
these muscles, and partly to the position of the cleft-like venous
ostia, a metameric signification. The variation
in the number of the^e chambers is not very
great ; in most there are eight of them ; there
are very seldom more, more frequently less than
this. But these numbers still require a mnch
more exact examination. The blood which is
taken into the cardiac tube by the ostia is driven
forwards by the systole of the chambers, and so
passes from chamber to chamber, and from the
most anterior of these into the aorta, where the
pouch-like folds of the edges of the ostia function
as valves, and prevent it from returning to the
heart.

The aorta (Fig. 146, a) is a direct continua-
tion of the heart, which, as compared with the
Myriapoda, has disappeared from the thoracic
metameres. It runs straight forwards to the
cerebrum, but its more intimate relations after
this are not exactly known. It is uncertain
whether the branching of its anterior end, which
is seen in some Insects, is a general phenomenon.

In any case the blood very soon passes through
a lacunar passage between the separate organs
into regular currents ; this may be easily observed in transparent
insect larve; it is again collected into the venous ostia, which lie
near the entrance into the heart. The separate passages in this
tract are sometimes so sharply marked off, that in the appendages,
for instance, vascular spaces appear to be formed.

As the al^ cordis do not end directly on the wall of the heart
but in special cells on it, and at the same time unite to form a
network surrounding the heart, a cavity resembling a pericardial
sinus is formed below them.

§ 219.

The regular extension of the heart of the Myriapoda through-
out the whole length of the body, and the considerable increase in
the number of its chambers, shows that there is in them a closer

Fig. 146. Heart of
Melolontlia. a Ar-
tery arising from the
most anterior cham-
ber. on Aim cordis
(after Burmeister) .

284

COMPAEATIYE AXATO:\IY.

connectiou between tlie external segmentation of tbe body_, and its
internal organisation. Herein we may recognise a lower stage. The
chambers (Fig. 147^ K) are again separated from one another by

valves^ placed at each venous ostium
(o), and are attached by large aim
cordis {m). Paired arterial truuks,
which are especially well developed
in the Scolopendridae, are sent off
from each chamber to its proper
metameres. As compared with the
Insecta^ a higher degree of develop-
ment is implied by this arrangement.
These arteries arise at about the
level of the venous ostia. In the
Julidm they are double^ for each
chamber is composed of two, which
were primitively separate. Three
trunks are given off from the most
anterior chamber; the median one
(c) supplies the cephalic segments,
and the two lateral ones (5) surround
the oesophagus. Where they unite,
a larger trunk is formed, which lies
on the ventral nerve- chord; just as in
the Scorpionea,this runs as far as the
last ganglion of the ventral chain,
and gives off numerous branches.

Fig. 117. Head and two segments of
the body of S col open dr a, with the
most anterior portion of the blood-
vascular system. C Head. G Supra-
oesophageal ganglion (Cerebimm).
0 Eyes. M Mandibles. A Antennm.
K Chambers of the Heart, m Aim
cordis. 0 Venous ostia, a Lateral
arteries, b Arterial arches, c Cephalic
artery (after Newport).

§ 220.

AVe find that the Scorpionea
among the Arachnida are provided
with the most complicated circulatory apparatus. The heart, sur-
rounded by a pericardial sinus, is considerably elongated in correla-
tion with the form of their body ; it is divided into 8 chambers, which
are held fast by lateral muscles (aim cordis). A pair of dorsal clefts
(venous ostia) leads into each chamber; these clefts may be closed
by valves which project into the interior. Arterial vessels are given
off from the anterior as well as from the posterior end of the heart,
of which they are direct prolongations; the anterior one, the aorta,
enters the cephalothorax, while the hinder one runs to the tail. In
addition to these a number of lateral arteries are given off close to
the venous ostia, and are distributed to the neighbouring

Two of the numerous branches given off by the aorta form a vascular
ring around the oesophagus, whence an artery runs back (arteria
supraspinalis) on, and as far as the end of, the ventral nerve-chord; this
artery gives off a large number of branches. The venous blood is
collected into a receptacle which lies directly on the ventral surface,
just as in the higher Crustacea; from this it is carried to the

VxVSCULAE SY8TE:\I of artheopoda.

285

respiratory organs. Before the blood from them gets to the heart,
it passes into the pericardial sinus.

In the other Arachnida the many-chambered cardiac tube is
reduced, and re-
sembles in cha-
racter that of the
Insecta. It always
lies in the abdo-
men; intheAranca
and Opilionida it
is provided with
three pairs of ostia,
by which it is di-
vided into cham-
bers. From the
most anterior of
these an artery is
continued into the
cephalothorax : in
Lycosa this is di-
vided into two
trunks (Fig. 148),
each of which gives

ov

off branches for the
eyes and appen-
dages. The hin-
dermost chamber

Fig. 148. Circulatory organs of Lycosa. A Seen from
above ; B From the side, o Eyes. 1,2,3,4, 5, 6 Appendages.
P Lungs. C Heart, ov Venous ostia of the heart. The
arrows indicate the direction of the blood (after Claparede) .

opens at the end

of the abdomen, and the blood- current from it corresponds to that
which is distributed by the caudal artery in the Scorpionea. Since
there is no pericardial sinus the blood passes to the respiratory
organs, and from them to the heart, by lacunar passages only.

Among the Pycnogonida this apparatus is limited to a three-
chambered heart, into which two pairs of ostia open ; in the Acarina
no heart at all seems to be developed.

Excretory Organs.

§ 221.

The apparatus, which in the Vermes is formed of looped canals,
is found in a modified form in the Crustacea. One of the organs,
which represent it, consists of a coiled tube placed below the integu-
ment of the head, and opening at the base of the second (outer)
pair of antennge. In the Entomostraca, this organ is confined to the
larva, in which it has been made out in most divisions. It may
perhaps be retained in the Cirripedia as the so-called cement
glands,'^ which lie in the stalk of the Lepadidm, and open at its

286

COMPAEATIVE AXATOMY.

lower end ; aud wliiclij in tEe Balanidee, are converted into a special
complex of glands. This organ persists in the Thoracostraca, and
is known as the green gland in the Crayfish.

A second organ of this kind is also found in the Entomostraca,
hut is absent in the higher Crustacea. It lies in the mantle-like
fold of the integument, where it forms a transparent looped canal,
which opens below the mantle (cf. Fig. 136, cj). Owing to its
position below the shell it is known as the shell-gland. Its inner
end is blind.

There are, therefore, two kinds of excretory glandular organs in
the Crustacea, but it is doubtful whether they are homodynamous.
The second organ may be homologous with the loop-like excretory
organ of the Vermes, and be therefore derived from a common stem-
form, while it has lost its metameric signification.

These organs — the functional relations of which cannot as yet be
definitely adjudged, and of which the green gland alone is distinctly
similar to a renal excretory organ — are not found in the Tracheata.
In them the function of excretion is performed by organs, which
have been described anatomically among the appendages of the
hind-gut (§ 214), under the name of urinary canals, or Malpighian
vessels.

Tracheae.

§ 222.

The coelom of the Protracheata and Tracheata is traversed by
an aerating system of tubes, which, so far as is yet known, is derived
from tegumentary organs. The characters of these organs, in
Peripatus, is of the greatest significance as to this point ; irregularly
distributed tufts of fine tubes, filled with air, are distributed on
the inner surface of the body-wall, and also on the oviducts and on
the fore- and hind-gut.

The arrangements in the Tracheata are different from this, for, in
them, the trachem are regularly arranged and symmetrically dis-
tributed. They consist of an outer layer of connective tissue
(Fig. 149, a), the interior of which is covered by a chitinous layer
continuous with the external integument. Its elasticity is almost
altogether due to its chitinous layer, and when the trachea is more
elastic it is because this layer is thicker ; in this case it has the
form of a spiral filament, projecting into the lumen of the trachea.
At certain points the tracheae form saccular enlargements, and at
them this spiral arrangement of the thickenings disappears ; that is
to say, it is only deposited at certain disconnected parts. This
chitinous layer and its spiral ridges do not form a specific arrange-
ment, for the ducts of many glands have a very similar structure in
the Tracheata.

The external orifices (stigmata) of the tracheae are regularly
arranged in pairs on either side of the body ; there is not always the

TKACHE/E OF ARTHROPODA.

287

same nainber of tliem_, and in some cases they are founa on every
segment of the body. Each stigma is an ellipse-shaped cleft, sur-
rounded by a circular thickening
of the external chitinous skeleton,
which can be opened or closed by
valves. Special muscles close these
clefts. Bach trunk from the stigma
is lost, sooner or later, in a tuft of
smaller branches, from which finer
branches arise, which surround the
organs. The way in which these
branch, as well as the length and
strength of the branches, varies
greatly. Separate tracheal trunks
may unite with one another, and
form a system of tubes which passes
longitudinally along, or transversely
across the body, and from which
finer ramifications are given off.

Owing to the distribution of
these tracheae throughout the body
there is a great difference between
the respiratory characters of the
Tracheata and of the Branchiata.

The medium which is to be respired
is distributed through the whole
organism ; it is not only the blood-
fluid which everywhere bathes the
tracheae that can exchange its gases,

but in the tissues themselves respiration can be effected, for the
tracheae are distributed to them, and, indeed, may even come into
relation with their form-elements (cf. Fig. 144, tr). But this does
not apply to all cases, for, when the tracheae are reduced, the
respiratory regions are diminished in number and extent ; in this
way the diffuse respiration is localised. In these cases, as when
there are branchiae, the blood has to seek out the respiratory organs.
In this way the characters of the trachese influence the circulation.
In addition to their respiratory function, the system of tubes filled
with air serves to diminish the specific gravity of the body, and is
just as important in this relation to Insects during their aquatic
stages, as to those Insects which rejoice in wings, and which are
able by special arrangements to increase or diminish the amount
of air in their tracheal system.

Fig. 149. Piece of a trachea of a
Caterpillar, with its branches, B C D.
aEpithelial-like cellular layer, h Nuclei.

§ 223.

The arrangement of the tracheal system varies considerably, but
all its forms may be derived from that simpler one mentioned above,
in which there is a pair of tufted branched trachete in each metamere.

288

CO:\[PAIiATIYE AXATO^EY.

These organs appear to be distributed in a inetameric fashion even
in the cephalic segments^ for the rudiments of tracheae formed by the
ectoderm have been observed in many Insects in these metameres
during their development. None of these rudiments are retained in
any living Tracheate^ owing to the development of the head. In the
succeeding metameres^ also_, the number of tracheae may be diminished,
for in some cases at any rate the tracheal trunks have been observed
to atrophy.

In the Myriapoda the tracheae are ordinarily similar in character
throughout the whole body, however different they may be in the
various orders. The stigmata, which are either placed on the
ventral surface, or more to the sides, and in some indeed on the
dorsal surface (Scutigera), lead into tracheal trunks, which are dis-
tributed in correspondence with the number of metameres. They
are most simply arranged in Julus. A tuft of tracheae passes from
each stigma to the viscera, without branching at all. In Glomeris,
however, the tracheae do branch; and in the Chilopoda they form
longitudinal and transverse anastomoses, so that they get to be
arranged in very much the same way as in many Insects.

Among the Insecta some Aptera appear to have lost their tracheae.
They are almost altogether wanting in the Collembola, two pro-
thoracic tracheae only having been observed in Smynthurus. Among
the Thysanura, Campodea has as many as three pairs (Fig. 150),
belonging to the mesothoracic, metathoracic, and
first abdominal segment respectively. The absence
of anastomoses shows that they are of a lower
grade of development than that found perma-
nently in other insects. There are generally 10
pairs of stigmata. This is the highest number
known in the imago stage of the Pterygota, but
in some larvae there are 11 stigmata, the first
abdominal metamere being sometimes provided
with a stigma, although as a rule it is altogether
absent. There are never any stigmata in the last
tw’o metameres. The number of these stigmata,
and of the tracheal trunks which arise from them,
is not, however, always complete. The number
varies greatly, pairs of stigmata undergoing de-
generation at one point or another, so that only
2 or 3 of them remain open. In the imago they
generally lie in the softer membrane, which con-
nects the segments of the body, and they are
sometimes placed so much on the dorsal surface
of the abdomen that they are covered by the
wings (Coleoptera). The number and arrange-
ment of the tracheae in the imago stage is not the same as in the
pupae or larvae. The differences in the external conditions which
obtain in the two stages define the arrangements of this system of
respiratory tubes. The development of transverse and longitudinal

Fio’, 150. Anterior
lialf of Campodea
fragilis. s Stig-
mata (after Palmen).

TEACHE^E OF AETHROPODA.

280

anastomoses provides for tlie equal distribution of tlie respiratory
medium. When tlie number of stigmata is reduced tbe longitudinal
trunks become of great physiological importance, for they give ofE
tracheal branches to those portions of the body in which there are
no stigmata. The above-mentioned tracheal vesicles may be
developed on the principal trunks, as well as on their branches and
twigs ; their development is correlated with the development of the
power of flight. A very large number of them may be found in the
Coleoptera (Lamellicornes) ; in the Lepidoptera, Hymen optera, and
Diptera they are not so numerous, but are larger ; in the Diptera
they are sometimes represented by a large pair of vesicles, which
almost fills up the abdomen.

As the tracheal system is developed in correlation with aerial
respiration, and therefore with a n on-aquatic habitat, the modifica-
tions which are caused by the aquatic habitat of the larval or adult
stages of many Insects, must be regarded as secondary arrange-
ments. They are adapted to their altered mode of life. Thus in
the larvae of many Diptera there is but a single pair of stigmata,
which are placed in the hinder end of the body (Corethra). A still
further adaptation in the aquatic Hemiptera (Nepa, Ranatra) is tbe
respiratory tube, which projects from the abdomen.

§ 224.

When Insects are most completely adapted to an aquatic habitat,
all the stigmata, and the tracheal trunks from them, are atrophied.
This leads to the formation of the closed tracheal system, which
distinguishes the larvae of the Pseudoneuroptera. The longitudinal
trunks, which are also present in the open tracheal system, form the
chief part of the apparatus. They give off branches to the viscera
(enteron), as well as to the wall of the body. In both parts they
give rise to the development of organs, in which the exchange of
gases is effected. The relations between this closed tracheal system
and the open one are explained by the presence of chords, which
connect the longitudinal trunks with the body-wall, and are inserted
at the very points where stigmata are found later on. The chords
therefore appear to be obliterated tracheal trunks. And this view
is confirmed by the fact that when the larvm undergoes ecdysis, the
intima of a portion of the tracheal system is also cast off by means
of some of these chords and is found in the exuviee (Ephemerida,
Perlida). Part of these chords are cast off again in the last moult,
and form an open tracheal system by giving rise to a stigma at their
point of junction with the skin.

The tracheae, which branch in the integument, allow of a dermal
respiration during the closed condition of the apparatus (many
Perlida). This determines the development of superficial growths,
which lead to the formations of processes, in which a large number
of tracheae ramify (Tracheal gills, cf. § 190). These organs

u

290

co:\iPARATiYE axato:my.

form tufts, or lamellae, wliicli beset tbc abdomen in tlie Epbemerida
and Peril da (Fig. 151, A), or even form a tuft on tbe tliorax (Perlida).
Vague dermal respiration is here localised in definite organs. There is
a larger surface in the tufted form, but this is compensated for, in the
lamellar form, by the movements of the lamellae, and the consequent

increase of rapidity
^ in exchanging the

water. When tra-
cheal branches are
developed in the hind
gut, this region be-
comes respiratory in
function. Respiration
seems to be partly
effected in the same
region in the larva?
of the Ephemerida
and Perlida, although
there are no special
organs developed ; for
they have been ob-
served to take water
into the hind - gut.
This function is more
evident when the sur-
face is increased in
extent, as it is in the
larvae of the Libellu-

!Fig. 151. A Hinder portion of the body of the larva hdcC, by the develop-
of Ephemera vulgata. a Longitudinal tracheal meilt of a large
trunks, h Enteric canal, c Tracheal gills, d Feathered number of lamellm
appendage to the tail. H Larva of -^schna grandis. -i • -i ^

a Superior longitudinal tracheal trunks, h Their an- aiiaugecl in lOllgltU-
terior end. c Posterior portion, branching on the dilial rOWS. TwO
hind-gut. o Eyes. The middle figure represents the longitudinal trunks
enteric canal of the same larva, seen from the side. /p* i? \

d Inferior lateral tracheal trunk, e Communication \ j dJ Cl) glV6

with the upper trunk, a & c as in H (after Suckow). off branches at their

hinder end (c) to the

hind-gut, and form a close plexus of tracheae in its lamellce. These
internal tracheal gills are constantly bathed with water, owing to
the movements of a valvular arrangement at the anus. In these
forms therefore the hind-gut functions as a respiratory organ, just
as it does in many other divisions.

Palmen, J. a., Zur Morphologic des Tracheeusystems. Helsingfors, 1877.

§ 225.

Among the Arachnida the tracheal system of the Galeodea most
nearly resembles that of the Insecta, the separate tracheae being

GENEEATIVE OEGANS OF AETHEOPODA.

291

united by lateral longitudinal trunks. As however they have only
three pairs of stigmata they indicate their affinity to the other
divisions of the Arachnida. A remarkable peculiarity is found in
their tracheal system; a tracheal trunk arises from a stigma and
speedily breaks up into a large number of short lamella-like flattened
branches^ which lie on one another like the leaves of a book ; owing
to this arrangement the whole organ is confined to a small space.
These leaf-like tracheae are known as Lungs.^^ Four pairs of
them open on to the ventral surface of the abdomen in the Scorpionea.
There are two pairs in the Phrynida and Mygalida. In the rest of
the Aranea one pair only is developed, the stigmata of which lie in
the anterior portion, and on the ventral face of, the abdomen. In
some Aranea a second pair of stigmata, placed just behind the first,
lead into tracheae which end by two chief tubes, at the end of which
are extremely fine tubules (Argyroneta, Dysdera, Segestria). In
others this pair of stigmata is fused, and lies in front of the spinning-
warts. Four tubes generally pass from the stigmatic cavity; these
are either branched (Thomisus),,or end simply (Tegeneria, Clubiona,
Lycosa, Epeira). In the absence of branches and of anastomoses,
the leaf-like tracheae resemble the lowest stage of the trachem,
and represent a special development of them.

The Opilicnida, the tracheae of which are distinguished by the
large number of their branches, have only one pair of stigmata.
The number is also reduced in the Acarina, in many of which (e.g.
Sarcoptes) there is no tracheal system at all, as is the case also in
the Linguatulida and Pycnogonida.

Generative Organs.

§ 226.

Keproduction in the Arthropoda is effected solely by the genera-
tive system ; all the modes of reproduction in them, which are called
asexual (phasnomena of Parthenogenesis, and of alternation of
generation), are due to sexual differentiation.

The disposition of the generative organs in different individuals,
which obtains in some divisions only of the Vermes, is the rule in
the Arthropoda; in a few only is the hermaphrodite structure
retained. In many, sexual differentiation extends even to the outer
parts of the body, to its size and form.

The germ-glands are always distinct organs, which are never
distributed in a metameric manner ; they are either single, or there is
but one pair of them. W e cannot yet say whether this is due to
the generative system having been transmitted from animals in
which it was single.

In the general arrangement of the organs there are many constant
and very remarkable points. The typical form of the apparatus is

202

COMPARATIVE ANATOMY.

Diagrams of the characters
generative system in the
Arthropoda. a Germ-glands. &cEffe-
rent duct.

two efferent ducts (Insecta).

evidently a compact germ-gland (Fig. 152, a), from wliicli an
efferent duct {h) passes off on eacli side. W e find this arrangement

of the germ-gland in the Branchiata,
and in some of the Tracheata (Ar-
achnida). In almost all the Crus-
tacea the efferent duct is completely
double as far as its orifice (A). The
same arrangement obtains in the
Myifiapoda, among the Tracheata.
The germ-gland begins to be di-
vided among the Crustacea. The
organ is then divided between its
The approximation of the orifices
leads to the formation of a common orifice, and this to the forma-
tion of an unpaired ]3ortion of the duct (c). In many Arachnida
this azygos tract is connected wfith a circular part, more or less
of which is formed by the germ-gland (G). The ring is then formed
by an inherited (primary) stage — the single germ- gland — and by an
acquired (secondary) stage — the fused part of the efferent ducts.
While the generative glands of the Crustacea form the lowest grade
of this series, the Insecta appear to form the highest, for, owing to
the bilateral separation of the germ-gland, and the terminal fusion of
the efferent ducts, and the formation of a common unpaired portion,
they are the farthest removed from the lowest stage (B). The
germ-glands, as well as the efferent ducts, may undergo various
differentiations : especially the ducts, of which sometimes the paired
and sometimes the unpaired portion is affected. Except in tlie
fixed Cirripedia, impregnation is effected by copulation. In corre-
spondence with this there is a space formed near to, or at some
distance from, the terminal portion of the female efferent duct,
from a diverticulum of a division of it ; this (receptaculum seminis)

serves to take up the sperm, and may be converted into appendices
of a more independent character. Lastly, there is a bursa copulatrix
for the reception of the penis.

The organs which serve to protect the eggs after they have
passed out of the body are of many kinds. Some of the appendages,
in the Crustacea especially, are often metamorphosed in this direc-
tion. Even whole regions of the body may be converted into
marsupial pouches. Much of the difference between the male and
female individuals is due to these relations to the care of the young.
Finally, the number of eggs produced is an important element in
modifying all parts of the female apparatus; for not only the enlarge-
ment of the efferent spaces, but the various changes of all the accessory
organs, and, again, the increased size of the female are all due to the
production of a large number of eggs.

The organs necessary for copulation, as well as the differentiations
which affect the efferent ducts, lead to complications of the male
apparatus. When the protractile end of the efferent ducts does not
serve in copulation, there are special copulatory organs, which are

GENEEATIVE OEGAKS OE AETHEOPODA.

293

partly formed from tlie appendages (Crustacea), or by them and
whole metameres (Insecta). The appendages have further many
other relations to the generative apparatus, for they serve as organs
for seizing and holding the female, and are modified accordingly.
The generative system is here seen to be so correlated with other
parts, as to be of great importance in determining the form of the
whole organism.

§ 227.

Among the Crustacea we meet with hermaphroditism in some of
the Cirripedia. The testes and ovaries are greatly ramified tubes,
which can only be distinguished externally by their position in the
body. In the Lepadidae the ovaries are placed in the stalk
formed by a diverticulum of the mantle ; they give off an oviduct to
the mantle- cavity on either side. In the Balanidse they are em-
bedded in the mantle. In both families the male reproductive
glands are disposed around the intestinal tract, and unite at each
side into a vas deferens, which
runs alongside the hind-gut,
and opens with its fellow of
the other side at the end of
the postabdomen.

In the other, or dioecious
Crustacea, the organs of both
sexes are arranged in very
much the same manner. There
are two different forms of the
generative apparatus accord-
ing as the germ-gland is paired
or unpaired. But these are
connected with one another by forms in which the two germ -glands
are united into an organ which is externally single.

We meet with unpaired germ-glands in the free-living
Copepoda. The ovary or testis (Fig. 153, t) lies in the middle line
above the mid-gut {v). The ovary gives off an oviduct on either
side, which either takes a simple course backwards, or forms in its
terminal portion several coils, which function as a uterus (parasitic
Copepoda) ; or it may be beset along its whole course by a large
number of diverticula (Fig. 154, B), which hold the eggs (Corycseidae) .
The short terminal portion has either glandular walls, or is provided
with a special cement gland. An enlargement of the terminal
portion functions as a receptaculum seminis, and in many cases, as
for example in the Siphonostoma, may form a special tract, pro-
viding a special orifice for the reception of the sperm. In many
Siphonostoma the ovary is double ; but the two ovaries are often
placed close together. We find the same in the male Copepoda,
when the free-living forms have a simple testis, and the Corycieidae
have one divided into two halves, which pass on either side into a
special vas deferens. In many families the right seminal duct is

Fig. 153. Gut and male generative appa-
ratus of Pleuroma. Side view, oe Fore-
gut. V Mid-gut. h Unpaired caecal-sac.
i Hind-gut. c Heart, t Testis, vd Coiled
vas deferens (after Claus).

294

COMPAEATIVE ANATOMY.

atrophied. The many-coiled end of the duct serves as a seminal
vesicle (Fig. 153^ vd), in which the sperinatophores are formed.

In the Branchiopoda the germ-glands are separate tubes
which lie beside the enteric canal. In the Cladocera, where they
are directly continuous with the scarcely separable efferent duct^
they are simple ; the duct of both male and female organs opens
near the end of the body. The Phyllopoda resemble them in this.
Sometimes the testes or ovaries occupy the hinder part only of the
coelom, while the duct is given off from their anterior end (Artemia,
Branchipus) and bends backwards, or they begin farther forwards,
and give off the excretory duct at, or near, their posterior end
(Holopedium). In the former an enlargement of the oviduct serves
as the uterus, while a swelling on the seminal duct forms the seminal
vesicle. This simpler form of the generative organs is modified in
the Phyllopoda, owing to the enlargement of the germinal glands.
The ovary of Limnadia is beset with short pouch-like diverticula,
which form a lobate gland of a larger size in Apus, owing to the
larger number of branches in it. This organ also serves as a uterus
for the ripe eggs. The testis has the same morphological characters.

Among the Ai’thro-
straca the generative
organs are generally
double, each side pro-
vided with a separate
orifice. In the Amphi-
poda the female organs
consist of simple tubes
which open as a rule at
the base of the fifth
thoracic segment. In
the Isopoda (Fig. 154,
C) the tube ends
blindly in front and
behind, and the effer-
ent duct arises on the
course of it. The ends
of the tubes are to be regarded as true germ-glands, while the rest
or largest portion resembles an oviduct or uterus. The male organs
are similar, but in the Isopoda they are peculiar in character, several
testicular tubes (Fig. 155, B) uniting into a special portion, from
which a narrower and much-coiled excretory duct arises. This has
either an orifice of its own, or is united with its fellow of the other
side in front of the orifice.

§ 228.

Among the Thoracostraca the Schizopoda (Mysis) are provided
with the simpler kind of generative organs. The female organs
(Fig. 154, A) consist of an unpaired germ-gland (o), to the sides of
which oviducts are attached, and these are widened out anteriorly into

Fig. 154. Female generative organs in Crustacea.
A Of Mysis. B Of Sapphirina. C Of Oniscus.
0 Ovary, od Oviduct, u Uterus.

GENERATIVE ORGANS OE ARTHROPODA.

2U5

a c^cal uterus which is continuous with the duct. At their hinder end
they send off a short duct [od) to the generative pore. The organs
of either side are united in the same way in the testis. This is
formed of a double row of glandular follicles, which unite into a
coiled canal, which forms the simple excretory duct and opens at the
base of the last pair of feet.

The generative organs of the Decapoda resemble those of Mysis,
by being similarly connected in the middle line ; they appear to be
further developed by various
differentiations. The female
organs are formed by two
long tubes which run for-
wards and backwards, and
are united transversely with
one another ; these tubes
function partly as germinal
glands, but chiefly as ovi-
duct and uterus. In the
Crayflsh the two anterior
divisions have the form of
short lobes, while the two
hinder ones are fused into
an unpaired piece. On each
side a short duct passes to
the genital pore, which in
the Caridina has the same
position as in the Schizo-
poda; in the Macrura it is
placed on the basal joints
of the third pair of feet,
but in the Brachyura on
the segment of the body,
which carries this pair. The Brachyura are also distinguished by a
pouch-like enlargement of the oviduct (seminal pouch). In the
male apparatus the testes are formed by two much- coiled tubes,
which are transversely connected with one another in front, and
which, like the female organs, lie for the most part in the cephalo-
thorax; in Pagurus only are they placed in the abdomen. In the
latter they give off two long and closely-coiled but gradually-widen-
ing ducts. Herein they agree with most of the Decapoda, but they
are distinguished from them partly by the increased size of the
lobes formed by the coils of the seminal canal, and partly also by
the formation of the unpaired piece, which unites the glands of
either side. The germinal glands are more completely united in
Astacus. A vas deferens with long coils passes on each side to the
outer generative orifice, which is placed, as a rule, on the basal joint
of the last pair of feet ; in the Brachyura, however, it is found at
the end of a penis, which is formed by a metamorphosed appen-
dage. The opening of the male apparatus only is then the same

Fig. 155. Male generative organs. A Of
Homarus. B Of Oniscns. tt Testes, vd Vas
deferens, vs Seminal vesicle, o Its orifice,
p Copulatory organs.

296

COMPAEATIVE ANATOMY.

as iu tlie ScEizopoda, wliile the female orifice is placed farther
forwards.

In the generative apparatus of the Stomapoda the testis com-
mences as a fine unpaired tube in the middle line of the caudal fin ;
it is continued forwards into a paired tract, from which a much-
coiled vas deferens arises. Each of these passes to a penis, which
springs from the coxa of the last thoracic foot. An unpaired gland,
which begins in the cephalothorax, opens at the same point. The
ovary passes backwards as an azygos, and afterwards as a paired,
gland, as far as the cephalothorax. Each gives off an oviduct in the
third thoracic segment, which opens at the base of a pouch placed in
the middle line. The Decapod-type prevails in them, though
modified in the female by the approximation of the orifices.

In the Poecilopoda we see a combination of the two forms repre-
sented in the Crustacea. One form is followed in the median con-
nection of the organs of either side, and the other in the large
number of germ-sacs formed by the fine terminal branches of the
network, which makes up the generative organs. The wider tracts
serve as efferent passages, which are considerably widened in the
female so as to collect the eggs, and are continued into the efferent
duct on each side.

§ 229.

A lower stage is seen in the female apparatus of the Protracheata.
The ovary is a body divided into two halves by a septum, and
sends off a paired oviduct ; this passes forwards as a coiled tube,
and then bends round to a widened portion, which functions as a
uterus. These canals are continued backwards, and only unite to
form a common short vagina near the generative pore.

In the male apparatus the testes are completely separated from
one another ; each of them is provided with a glandular appendage,
and is continued into a long looped vas deferens. A common ductus
ejaculatorius, which also opens at the hinder end of the body, is
formed by the union of the two efferent ducts.

§ 230.

The two kinds of generative glands in the Arachnid a are, as a
rule, unpaired; when paired they are connected transversely, and
open either by one or two ducts anteriorly, and on the ventral
surface. In addition to accessory glandular organs, or special
enlargements of the excretory ducts serving for the storage and
reception of the sperm-masses or ova, there are external organs
which carry the sexual products outwards, and which are called
penes or vaginae according to the sex. The male organs repeat with
slight variations the type of the female. The union of the genital
glands of either side and the azygos portion of the apparatus which

GENEP.ATIVE OEGANS OE APTHPOPODA.

297

is formed in conseqnence of it, calls to mind the similar relations of
parts in the Branchiata, and notably in the Poecilopoda.

The ovaries in the Scorpionea are formed of three long tubes
which bend towards, and pass into, one another at their hinder ends,
while they are also connected with one another by four transverse
anastomoses ; in the walls of these tubes, which often form tubular
diverticula, the ova are formed. The segmented character of the
organ is implied by the transverse connections, which form four
wide meshes on either side, for these segments have exactly the
same position as those of the abdomen. Spindle-shaped and
widened oviducts are continued on from the two outer longitudinal
tubes, which function as receptacula seminis for the sperm which
they receive ; they open at the base of the abdomen.

The testes also of the Scorpionea are a pair of winding canals,
united by transverse commissures. Their double character is implied
by the presence of two tubes on either side. The vas deferens of
each testis opens to the exterior, after nniting with its fellow of the
opposite side at just the same point as that occupied by the genera-
tive orifice of the female. In addition to the vas deferens there are
accessory organs on either side, which as a rule have the form of
two pairs of csecal tubes, which vary in length and function partly
as glands, and partly as seminal vesicles.

The separation of the germinal glands of either side is complete
in the Galeodea and male Aranea. The ovaries are two tubes,
on the outer surface of which the ova are developed; in the
Spiders they are developed on stalked processes. In some (Segestria,
Oletera) the ovaries are represented by a closed ring. A vaginal
canal, which is sometimes widened out (Galeodes), is formed from the
union of the two ovarian tubes, which serve to carry the ova out-
wards; this canal has one or two seminal vesicles at its termination.
These are found also in the Aranea, where they often open inde-
pendently in front of the orifice of the vagina. The male organs
in the Galeodea may be derived from those of the Scorpionea, by
supposing that the transverse anastomoses between the longitudinal
trunks have disappeared. Finally, in the Aranea, these longitudinal
tubes are reduced to two.

Bertkau, Ueber cl. Generat.-Apparatus. Araneiden. Arch. f. Nat. 1875.

§ 231.

In the Opilionida and Acarina the circular form of germinal
gland is the dominant and general arrangement ; it is derived from
the transverse connection of the ovaries, which is seen in the
Scorpionea. The unpaired stage of the germinal gland, which is to
be regarded as the more primitive one, is implied by this arrange-
ment. This circular form is most perfect in the Opilionida (Fig.
156, B o). Just as in the Aranea and Scorpionea the ova are formed
in stalked diverticula on the surface of the ring, whence they pass

298

COMPAEATIVE ANATOMY.

into the cavity of the ovarian tubes^ and from it to the efferent
dnct^ which is provided with a considerable enlargement (u) (uterus).
A narrow coiled continuation of this leads to the protractile ovipositor

(o/y). In the male there is a
circular canal^ a portion only
of which forms the testis,
instead of the ovarian ring
(Fig. 156, a t) ; the two
ends of the testis pass into
the efferent duct {vd) which
completes the ring. These
unite into a closely-coiled
portion, from which a
widened canal or seminal
vesicle arises — an organ
similar to the ovipositor,
and, like it, protractile ;
the penis is attached to it.
Two large tufts of acces-
sory glands (gi) are con-
nected with the ends of the
penis.

The circular form of
germinal gland is still retained, in its completeness, in many of
the Acarina. In the female apparatus the greater part of the ring
is converted into the efferent organ, owing to the limitation of
the ova-producing part to a small division of it. This is most
marked in Pentastomum, where the ovary is attached to a circular
canal. The ovary is here differentiated from the canal. The part
of the ring, which forms the efferent ducts and passes into the
single portion, is often widened out into a uterus ; or the uterus
is formed by the unpaired portion alone. This is the casein Pentas-
tomum, the uterus of which forms a coiled canal of some length.
The unpaired portion of the efferent duct is generally much shortened
in the male, and the two parts of the rings connected with it are
widened out into seminal vesicles. Appended glands are connected
with the unpaired portion in both sexes. The great differences in the
distribution of the functions of parts of this canal lead to the separa-
tion of the ring into two genital tubes, the middle of the germ-
producing portion of the ring becoming sterile. The two halves of
the ring are then distributed to the sides, although in some cases
they are still connected by a canal, or by indifferent tissue; this
gives rise to organs which are only united at their orifices, or along
an unpaired portion connected with them (Ixodes).

The hermaphrodite generative organs of the Tardigrada are
altogether unlike these arrangements. They consist of an unpaired
ovary and two testes which lie beside the enteron : their efferent
ducts pass into a receptaculnin seminis, and open, generally provided
with special glands, into the cloaca.

Fig. 156. Generative organs of Phalangium
opilio. A Male organs. t Testes, vd Vas
deferens, p Penis, m Eetractors of penis.
r/i Appended glands (after Krohn). B Female
organs, o Ovary, u Uterus, op Ovipositor,
m Eetractors of ovipositor.

GENERATIVE ORGANS OF ARTHROPOEA.

2'JD

The arrangements in the Pycnogonida are just as peculiar; their
generative products are formed on the wall of the coelom^ and are
passed out by special orifices (which are sometimes found on all^ and
sometimes on only one pair_, of the feet). This character reminds us
of the lower arrangements seen in the Annulata.

The conversion of appendages into copulatory organs, which
obtains in the Crustacea, is seen in the Aranea only from among
the Arachnida; in the males of this order the palpi are organs
of a complicated structure, which convey the sperm to the female
generative orifice.

§ 232.

The generative organs of the Myriapoda are, in their form
and arrangements, most similar to those of the Arachnida, and,
as in these forms, they sometimes open far forwards on the body,
namely, on the third segment of it. The genera-
tive organs of the Scolopendridm are placed in
the hinder end of the body. In the females the
generative glands are either simple externally and
form an elongated tube, on the inner surface of
which the ova form projections (Julidm, Scolopen-
dridm, and Glomeridee), or they are double (Cras-
pedosoma), in which case they are united at their
anterior ends, while the oviducts open separately.

In the Scolopendridae the simple ovarian tube
is, as a rule, continued on by a simple oviduct ;
but the double character of these organs is
implied in the development of ova on both sides
of the ovarian tube.

The accessory organs are formed by two pairs
of bodies, which sometimes open into the ovi-
duct, but ordinarily directly into the genital
orifice ; they partly form cement glands, and
partly receptacula seminis.

The male organs also are often double in their efferent ducts and
accessory parts only. However, many Glomeridas and Julidie are
provided with a double testicular tube, which passes into a common
vas deferens, and seems to be united into a single organ owing to
the large number of its transverse connections (Fig. 157). When
there is only one testicular tube, it is beset with separate follicles.
The vas deferens is occasionally single (some Scolopendridse) ; but
as a rule it is divided into two branches, which either open on a
short papilla (Julidm, Glomeridm), or are connected together and
continued into a short penis placed at the hinder end of the body
(Scolopendridae). The last division of the efferent duct is provided
with enlargements or diverticula, which serve to collect the sperm.
Several pairs of glands are inserted into it just in front of its
orifice. As to the general character of the generative organs, they
are unmistakably approximated to the Crustacea by the possession

Fig. 157. Male ge-
nerative organs of
Julus. t Testicular
follicles, e Efferent
duct (after Stein).

300 COMPAEATIVE ANATOMY.

of separate orifices^ and resemble tbe Aracbnida in forming an annular
portion.

Stein, F., Be Myriapodum part, geuital. Berol. 1841.

§ 233.

Notwithstanding the great variations of more subordinate cha-
racters, the generative organs of the Insecta present on the whole
a well-marked uniformity of structure. The organs and their
accessory parts almost always lie in the abdomen, and generally
open below, or in front of, the anus. The eighth abdominal
segment generally seems to carry the genital orifice. In the
Strepsiptera only is the female generative orifice placed some
way forwards. The germinal glands are, as a rule, disposed
in pairs, and retain this condition, although there are indications
of the primitively single arrangement, or of a connection between
the germ-glands of either side, as in the Arachnida and Myria-
poda. Each germ-gland is composed of a varying number of
equal parts, which are generally tubular in form, grouped into
tufts, and united at an efferent duct. The ducts of the two germ-
glands seldom have separate orifices. They are almost always
united for a certain distance, and receive, before they unite, acces-
sory organs, formed by the differentiation of a portion of their
walls. In the females these organs appended to the ducts are
sometimes pouch-like, or vesicular portions, which either serve for
the reception of the male copulatory organ (bursa copulatrix), or
as glandular organs of various kinds (cement glands), and also as a
store-house for the sperm (receptaculum seminis). In the male, the
paired-glands of the efferent ducts are greatly developed. In
addition to them there are other parts which function as seminal
vesicles (vesiculm seminales).

External organs, which are generally formed by the metamor-
phosis of the terminal metameres and their appendages, are con-
nected with the end of the genital duct ; in the males these form
copulatory organs, and in the female vary in form (as ovipositors).

§ 234

In the female apparatus the complex of ovarian tubes, which is
generally regarded as an ovary, undergoes the most considerable
modifications.

The relations which these tubes have to the formation of ova is
somewhat different to those which they have in other Arthropoda.
Each separate ovarian tube (Fig. 158) gradually widens at one end,
where it is inserted into the oviduct; the opposite end is generally
slender, and is often, indeed, continued into a fine filamentous process.
When there are a large number of ovarian tubes, the free ends are
directed towards a centre and connected together. The ova are

GENERATIVE ORGANS OF ARTIIROPOOA.

301

formed in tliese terminal filaments, the cell-masses of which represent
ovarian germs; these, while undergoing continual differentiations,
gradually make their way out
of the ovarian tube. The ovum
is a true cell at the place where
it is formed, but on its way
through the ovarian tube it in-
creases in size, so that we find
the largest eggs farthest from
the germinal region, and nearest
the oviduct, while behind them
there is a continual series of
smaller and younger formations
up to the above-mentioned blind
end of the ovarian tube. The
separate eggs cause the ovarian
tube to appear to be divided
into segments or chambers. The
gradual descent of the egg is not
only correlated with its growth,
but with various changes also in
the substance of the yolk ; each
egg is provided, especially in the
last segment of the tube, with
an external cuticular investment,
the so-called chorion ; this is
formed by the epithelial layer of
the ovarian tube.

As each egg passes into the
so-called oviduct, a portion of the
ovarian tube is degenerated, and
so the egg next in front is
close to the oviduct.

egg is

accompanied by the growth of
the thin end of the ovarian tube,
which is made up for by its
shortening at the other end. In
many Insects a group of cells is

differentiated with each egg, in addition to the epithelial layer sur-
rounding it; this vitellogenous layer occupies the portion (b) of
the chamber (a) behind the egg-cell (Fig. 158, B u), but is gradually
used up by the latter, as it grows. An ovarian tube, or a collec-
tion of such tubes, does not therefore correspond merely to a germ-
producing reproductive gland, but is an organ entrusted with a
much larger series of functions, and its blind end only is analogous
to an ovary.

The length of the ovarian tube depends on the number of eggs
in it. The smallest number of chambers is found in most of the

brought
The differentiation of the

Fig. 158. A Ovarian tube of the Flea.
n Ovum. [I Germinal vesicle. B Ovarian
tube of a Beetle (Carabns violac ens).
0 Ovarian segment, divided into two por-
tions, of which the ovarian cell is marked
a, and the vitellogenous layer h. The
ovum of the last segment has been ex-
pelled ; the walls of the ovarian tube are
collapsed (after Lubbock).

3U2

COMPAEATIVE AAtATOMY.

Diptera, where not uufrequently there is only one (Fig. 160^ o),
though more commonly two or three. In many Coleoptera and
Hemiptera also the number of chambers is small. The ovarian
tubes are longer in most of the Hemiptera and Hymenoptera ; the
largest number of chambers obtains in the Neuroptera and Orthop-
tera, and lastly in the Lepidoptera, where there are four ovarian tubes
made up of a large number of chambers, which look like a string of
pearls.

The arrangement of the ovarian tubes on -the so-called oviduct
also varies very greatly. They are sometimes united into tufts,
sometimes broken up into groups, and sometimes arranged in rows.

The so-called pseudo va have been distinguished from the eggs
(ova); these structures are partly characterised by the absence of the
germinal spot, like the products of the female generative glands in
certain generations of the Aphides and Coccidge. As the organs
resemble those in which true egg-cells are formed, and as the same
individual is able to produce pseudova and ova at different times, it
is best not to regard the gulf between these two products of the
ovary as a very wide one. These structures are links in a chain of
pha3nomena, which are very common among Insects ; the chain
begins with the arrangement known as parthenogenesis, and
extends to an apparent alternation of generations. The whole
ph agnomen on depends on the emancipation of the ovum from the
influence of the male reproductive elements. The simplest case is
that in which there is no anatomical difference between the eggs,
some of which are developed without previous fertilisation, while
the rest require to be impregnated. The parthenogenesis of Bees,
^Vasps, and many other Insects is of this kind. The arrangement in
Avhich the same individual no longer produces these eggs at one and
the same time is a further differentiation; the emancipated ovarian
products are then, as a rule, differently formed (pseudova). Still
more peculiar is the formation of these eggs in different individuals,
when whole generations can do without the influence of the semen on
the reproductive elements, and at the same time fall to a lower grade
of organisation (Aphides). These structures, finally, may be formed
in an earlier stage in the development of the animal, and from the
still indifferent germinal gland; this arrangement, just like the rest,
with which it is directly allied, is derivable from sexual differen-
tiation (Cecidomyia).

§ 235.

The two, ordinarily short, oviducts seldom open separately into
a depression of the integument (Ephemerida). As a rule this
depression is further developed into a common efferent duct
(Fig. 159, or), the vagina; with this accessory organs, receptaculum
seminis (Fig. 159, rs) and bursa copulatrix (be), are connected. The
seminal receptacle is seldom absent; it is formed of a stalked and
sometimes much-coiled vesicle. The receptacle is often a proper-

GENEKATIVE ORGANS OF ARTHROPODA.

303

Fig. 159. Female generative organs of Hydrobius
fuscipes. 0 Ovarian tubes, ov Oviduct, beset with
glandular appendages, gl Tubular glands, r Yagina.
he Bursa v^opulatrix. rs Receptaculum seminis (after
Stein).

tionately wider and coiled csecal tube, wbicli is sometimes provided
witli an appended gland.

The bursa copnlatrix is another organ, which is directly con-
nected with the vagina; it is a wide caecal-sac (Fig. 159, he), which
looks like a diverticu-
lum of the wall of the
vagina. This organ is
found in some orders
only, and even in them
it is not generally pre-
sent. The bursa copn-
latrix of the Coleop-
tera appears to be the
most independent, and
not nnfreqnently is of
a considerable size ; in
them it is generally
connected to the va-
gina by a canal. In
the Lepidoptera also
it opens into the va-
gina by a narrow duct ;
but it is remarkable
from the fact that it
has another efferent

duct in addition to this one, which it sends off below the female
generative pore, where it opens separately. In the Lepidoptera
fertilisation is effected by means of this canal, the spermatozoa pass-
ing into the receptaculum seminis from the bursa copnlatrix by the
above-mentioned duct, which connects it with the vagina. The open-
ings of the two parts into the vagina are opposite to one another.

The accessory glandular organs
of the vagina either consist of a
pair of simple canals which gene-
rally form long loops (Fig. 160, gl)

(Lepidoptera, many Diptera), or
of short C80cal tubes (Bugs). In
others they are greatly ramified
(Ichneumonidas and Tenthredinidm).

The secretion of these cement-
glands serve to attach the eggs
when laid, and at times to unite
them into masses.

As a rule some portions of the integument, which have the form
of valves, are connected with the female genital pore ; the markings
on these valves are always exactly adapted to the male copulatory
organ ; sometimes they are arranged like nippers, and consist of
23rocesses which work laterally on one another.

Fig. 160. Female generative organs
of Mallopbagus. o Ovarian tube, u
Uterus, gl Glands (after R. Leuckart).

304

COMPAKATIVE ANATOMY.

§ 23G.

The male generative organs of Insects very often repeat in
their development the forms of the female organs, so that even
the separate divisions of both sets of glands not unfreqnently
correspond. The testes, which are always paired, and seldom fused
into one organ, are composed of ca3cal tubes, just like the ovaries ;
they also vary in number and size, and are connected with one
another in all kinds of ways (Figs. 161, 162, t). The testes of either
side are often united in the Lepidoptera. The Diptera and Strep-
siptera, as well as many Neuroptera, have two simple, long, and
always separate testicular tubes. In many Coleoptera, also, each
testis is a long, closely-coiled ceecal tube, surrounded by a special
membrane. The testes of most insects are made up of a number
of tubes. Thus each testis, in most of the Hemiptera, consists

Fig. 161. Testes and efferent ducts Fig. 162. Male generative organs of !Melo.
of Acheta campestris. t Testes. lontha vulgaris, t Testes, vd Yas defe-

V Yas deferens, g Seminal vesicle. reus, vs Its widened portion, gl Coiled

either of several tubes connected together to form a fan-like
organ, or of a large number of separate tubes; this form of testis
is also found in a large number of the Coleoptera. The testes
of most Orthoptera consist of closely-applied tubes, which thus
form a single mass, or of rounded vesicles grouped in a racemose
fashion ; similar structures are also found in the Hymenoptera.

The efferent ducts of the separate testicular tubes are united
into seminal ducts, and these, on each side, into a vas deferens
(Fig. 161, -p; Fig. 162, vd), which, when the tubes are closely
united, passes directly from them. The two seminal ducts are, as
a rule, not very long, but in some cases they are considerably
elongated, and the widened portions of the coiled canals then function
as seminal reservoirs (Fig. 162, vs). A common efferent duct
(ductus ejaculatorius) is given off from their point of union ; this,
too, varies greatly in length, and also serves in part as a reservoir
for the sperm.

The accessory glandular organs are as a rule paired, and like those
of the female apparatus are either long coiled canals (Fig. 162, p/).

appended gland.

GENERATIVE ORGANS OF ARTHROPODA.

305

or shorter and grouped into tufts, or branched tubes; they are
attached to the efferent ducts at various points.

The male copulatory organs of the Insecta resemble the female
ones, and are made up of chitinised ridges and valve-like arrange-
ments, which vary greatly in form, and surround the generative
orifice. They are divided into those which serve only for external
copulation, and into others which are comparable to a penis, and are
capable of intromission. The latter are formed by a tube, which is
either attached externally, oris protrusible ; the ductus ejaculatorius
is continuous with it, and it often carries pincer-like organs at its end.
In the Coleoptera this copulatory organ is enclosed by a thick-walled
chitinous capsule placed in the abdomen, which is often of consider-
able size, and is provided with a special muscular apparatus to
protract it, and draw it in again.

§ 237.

The seminal elements of the Crustacea, though very variable in
form, always agree in being motionless; the seminal filaments of
the Cirripedia are an exception to this. Although the seminal
elements are filamentous in the Isopoda, Amphipoda, and Ostracoda,
they are incapable of movement ; in the last-mentioned group they
are extraordinarily long. Among the Schizopoda, in Mysis, at least,
they are filamentous, and are bent at one end so as to form a hook.
Cell-like bodies are the most common forms ; owing to the presence
of processes they present various peculiarities, the most notable of
which is the radiate form of the semen of the Decapoda radiate
cells'’^). The seminal filaments also of many Arachnida and Myria-
poda appear to be incapable of movement, although in the former
they are motile, when within the female generative organs.

In the Insecta the form-elements of the sperm are movable
filaments, which are generally drawn out into a process at either end.
The union of these filaments into tufts is an arrangement peculiar to
them, as is also the union of the cells in two rows to form a rod-like
structure which resembles in character a spermatophore (Orthoptera).

X

Sixth Section.

Brachiopoda.

General Review of the Group.

§ 238.

The Brachiopoda, which by most authors were formerly placed with
the Mollusca, with which they have little in common save a shell,
which is quite different from the one found in the Mollusca, form a
small and well-defined division, which is derived from the phylum of
the Vermes. Among the Vermes the Chgetopoda, which are highly
differentiated forms, appear to have several points of affinity with
them; but nothing more, for there are such striking peculiarities
even in the most important systems of organs, that it would be going
too far to base any definite phylogenetic assertion on these relations.
In any case the whole organism of the Brachiopoda, as compared
with the Chaetopoda and Annelides, is completely metamorphosed,
and genetic relations can only be made out in a few rudiments.

This eminently isolated position of the Brachiopoda at the
present time corresponds to the slight variations seen in the extant
forms, as well as to the fact that we have here to do with a group
of animals which was richly developed in earlier periods. Some
genera are found as early as the Silurian epoch. But as the palae-
ontological evidence is not sufficient to justify us in associating them
with the Vermes, it is better to treat them separately than to unite
them with those forms.*

We distinguish two orders only.

1) Ecardines.

Lingula, Orbicula, Crania.

2) Testicard ines.

Tei’ebratula, Argiope, Waldheimia, Thecidium.

I recognised their affinity to the Vermes in my “ Grundzuge,” IT. Aufl.

FOKM OF BODY OF BIUCHIOPODA.

.307

Bibliography.

Owen-, R., On the anatomy of the Brachiopoda, Transact. Zoolog. Soc. Vol. I. 1835. — Vogt, C.,
Anatomie der Lingula anatina. Denkschr. der schweiz. Gesellsch. fiir die gcsammte Naturwis-
sensch. Bd. VII. 184.2. — Huxley, Ann. and Mag. Nat. Hist. 1854. — Gratiolet, Journal de Con-
chiliologie, 1857-60. — Hancock, A., Phil. Transact. 1858. — Lacaze-Duthiees, Sur la Thecidie.
Ann. sc. nat. IV. xv. — Morse, B., On :the systematic position of the Brachiopoda. Proceed, of
Boston Soc. of Nat. Hist. Vol. XV. — The same, Embryology of Terebratulina. Mem. of Bost.
Soc. Vol. II.— Kowaleysky, Beobacht. uber die Ent-svickelung der Brachiopoden. Moskau, 1874.
(Russian.)

Form of the Body.

§ 239.

It is necessary to go back to embryonic stages to understand
the characteristics which distinguish the form of the body in the
Brachiopoda. In it, and at an early period, we meet with a stage
in which the previously undivided body is separated into three (in
Thecidium four) metameres ; this discloses the Annulate type. A
terminal circlet of cilia is predominant among the cilia formed all over
the body in Terebratula, as is the case also in many Annelid larvae.
Bundles of setae (Fig. 163, d) appear on the middle segment, which
can be moved just as in the Chaetopoda,
while the first metamere (cephalic segment)
is converted into an umbrella-like enlarge-
ment over the mouth ; this is surrounded
by long cilia (Argiope). In this point also
we can make out an affinity with Vermian
larvae (Actinotrocha) .

While the larva becomes attached by the
last metamere, two elevations are formed
from the middle one, which enclose the first,
and take on the form of two mantle-folds.

The two shells, which can be distinguished
as dorsal and ventral, are developed from
this, and reach to the stalk formed from the
last metamere. Owing to their position on
the body, the shells are clearly quite unlike
those of the Mollusca ; this formation of the
shells is a peculiarity, which marks off the
Brachiopoda. And further this formation
is probably the real cause of the cessation
of the development of metameres, which
is also connected with the fixed position of the animal. Another
peculiarity, the development of arms, is explained by the mode
of life.

Fig. 163. Larva of Argiope.
m Mantle. h Bundles of
seta). d Enteron (after
Kowalevsky).

308

COMPAKATIVE ANATOMY.

A small number of ciliated tentacular processes are developed in
tbe larvae at the sides of tbe moutli. In tbe mature stage of tbe
animal they are generally arranged on a stalk, which can be rolled
up in a spiral fashion ; there is a large number of them, and they are
placed on either side of the mouth. When these arms are rolled
up they are placed in the anterior portion of the mantle-cavity
(Fig. 166, /) ; they appear to be erected by injection of blood.
Owing to the great development of these arms, and of the mantle-
folds, the rest of the body is reduced to a small size ; and even the
organs which, in other cases, lie in the coelom, may be embedded in
the folds of the mantle (pallial cavity). The mantle acquires a
respiratory significance when the inner lamellae of its folds are
increased in surface by the development of ridges ; it then functions
as a gill (Lingula).

The tentacular processes around the mouth call to mind the
tentacles of the Bryozoa, which may also be arranged on arm-
like structures (lophophore) ; but they can no more be com-
pared with these structures than with the branchial tufts of the
Tubicolac.

Finally, the stalk is, in the older forms (Lingula), a long portion
of the body, which passes out between the two shells, and is
movable, while in the Testicardines it is short and largely chitinised.

Integument, Shell, and Arms.

§ 240.

As the two shells cover all the body except the stalk, the parts
of the surface of the body are only free within the pallial cavit}q and
are only exposed when the shell is open. From the fact that muscles
are connected with the integument, we may suppose that there is a
dermo-muscular tube. Calcareous spicules, which distinguish the
integument, are found in the mantle, as well as in the arms. They
are sometimes branched, but may be stellate or form a network.
The setae, which beset the edge of the mantle in various ways
in various genera, are of more significance. Like the setm of
the Chaetopoda they are formed in glandular depressions of the
integument, and, like them, are cuticular formations. They are
generally simple structures, which terminate by a fine point, and
indicate by their transverse striation that they have been gradually
secreted.

In the Ecardines the two valves of the shell have pretty much
the same form. But in the Testicardines the dorsal and ventral
valves are distinctly differentiated. Towards the stalk they are
united by a kind of clasp. The ventral valve also is drawn out into
a beak-like process, the punctured end of which serves as a passage
for the stalk. A skeleton, which projects uiwards, is developed

NERVOUS SYSTEM OF BRACHlOPODA.

309

from the dorsal valve (Fig. 164, c). It serves as a support for the
arms.

When first differentiated the shell is a soft chitinous layer, which
later on becomes calcified. The valves are traversed by pore-canals,
which are filled up by villous processes of the mantle. Between these
the firm substance of the shell is seen to be composed of prismatic
bodies, which can be made out even when the shell is first laid down ;
they are set obliquely to the edge of the shell.

Owing to the great increase in surface of the spirally- coiled
arms of the Brachiopoda, thanks to their investment of tentacles,
they form organs well adapted for the respiratory function. In the
first place the tentacular filaments are suitably arranged for respira-
tion. They are in communication with the blood-sinuses which pass
along the arms. In their functional relation they may therefore be
regarded as gills. The two arms are connected with one another at
their bases, which are directed towards the middle line. A fold
above the mouth extends on either side on to the arms, and aids in
marking off a groove, which extends from the arms to the mouth.
The tentacles or cirri, which are arranged in two rows, and closely
approximated, rise up on the other edge of this
groove they extend to the end of the arms.

Muscular System.

§ 241.

Besides the muscles of the dermo-muscular
tube, such as those of the mouth and of the
arms, we find in the Brachiopoda a number of
independent muscles, which traverse the coelom,
and serve to open and close, as well as to turn
the shell (cf. Fig. 164). According to their
function they traverse the coelom in different
directions, and arise from, as well as insert
themselves into, the valves of the shell, so that they may be regarded
as differentiations of the dermo-muscular tube, which were formed
when these valves were formed.

Fig. 164. Muscular
system of Ter eb rat ula.
a h The two halves of
the shell, c Support for
the arms, d Stalk, e/
g h Muscles for opening
and closing the shell
(after Owen).

Nervous System and Sensory Organs.

§ 242.

The nervous system is very peculiar ; it alone would justify us
iii giving the Brachiopoda an independent position. It is made up

3iO

CO^IPAEATIYE AXATO.MY.

of masses of gaiiglia_, wliicli lie near tlie oesophagus (Fig. 165^ d).
A larger ganglion lies transversely across and below the oesophagus,
or rather, owing to the downward bend of the oesophagus, behind it
(in the Terebratulida) . Two nerve-trunks pass off from it towards the
posterior region, and break up into nerves for the stalk ; they are pro-
vided with swellings [n). The nerves for the ventral lamella of the

mantle are given off from
the swellings on these
trunks. From the large
ganglion, however, a nerve-
trunk is given off, on each
side, to the dorsal lamella
of the mantle, as well as a
nerve to the arms. Two
fine filaments surround the
oesophagus and pass into a
small ganglion, placed in
front of, and therefore on
the dorsal side of, the oeso-
phagus; this is connected
with the other ganglion by a
commissure. In this way an
oesophageal ring is formed,
and the only question is
whether the small upper
ganglia represent central
ganglia or not. If so, then
there is this peculiarity, that
the nerves for the arms
arise from the ventral gan-
glia, and we can hardly re-
gard the arms themselves as
homologous with the ten-
tacles of Vermes, even if it
could be shown that the gan-
glionic parts in the oesopha-
geal ring had altered their
position. The ventral gan-
glionic mass must appa-
rently be compared with a

Fig. 165. Xervons system of Waldheiinia,
from the dorsal surface. The dorsal valve has
been removed, as -well as the left half of the
dorsal mantle, D. V Left half of the ventral
lamella of the mantle. P Stalk, d ffisophagns,
cut through. (A pair of ganglia, -which lie in
front of the oesophagus, and which are connected
with the ganglion (?i) by fine filaments, are
not figured.) n Anterior, n ' Posterior oesopha-
geal ganglion. pr/ Generative organs. mOcclusar-
muscle, mf Divaricator. m" Ventral adjustor.

Ill" Accessory Divaricator (after A. Hancock).

shortened ventral gangli-
onic chain ; but to make a safe comparison we must have more exact
information as to the facts.

The slight development of the superior ganglia is correlated
with the absence of higher sensory organs, and this absence
appears to be an acquired condition, for the four pigment spots,
found on the first segment in the larva, point to the existence
of visual organs (Fig. 163), and lead us to suppose that there were
eyes in the ancestral forms. The two vesicles found in another larval
form similarly point to the previous existence of auditory organs.

Ca:LOM OF BEACHIOPODA.

311

Alimentary Canal.

§ 243.

Ill the Brachiopoda the enteric tube commences by the mouth,
which is placed in the mantle-cavity between the two arms ; thence,
and without any accessory organs, it passes, generally in the form of
a short canal, into the widened mid-gut (Fig. 166, d')) this is
generally known as the stomach. The portion which arises from
this passes into an enteric loop in Lingula, which turns towards the
right side, and opens at the anus into the mantle-cavity. This last
portion of the enteron is rudimentary in the Testicardines, and
generally terminates by a ceecal-sac which turns towards the
ventral valve of the shell ; from this a solid chord, which is perhaps
the obliterated remnant of the enteron, is sometimes continued on.
The end is sometimes widened out into a bulb.

A special peculiarity in the Brachiopoda is the way in which the
enteron is attached. A lamella extends from the mid-gut to the
wall of the body ; this is the gastro-parietal band, which thus forms
a kind of partition in the coelom. It might be regarded as a dis-
sepiment, formed in connection with the already noted metamerism
of the body. This view of its meaning is, when we compare it with
the arrangements seen in the Annelides, confirmed by its relations to
the excretory organs. A second lamella, the ileoparietal band, is
similarly attached to the hind-gut.

All the structures worthy of note which are differentiated from
the enteric wall are to be found in the mid-gut. They have the
form of branched tubes, which in many open into, or behind, the en-
largement of the enteron, already spoken of as the stomach, by several
pores (Crania), and in others are united into several (four) efferent
ducts (Lingula) . They are more largely developed in the Testicardines,
where they are arranged in two lateral groups of glands, which
surround the stomach, and are generally connected with it by a
number of ducts on either side (Fig. 166, li).

Coelom and Circulatory Organs.

§ 244.

The coelom is divided by the organs which are embedded in it
and by the muscles which traverse iL into several continuous spaces
which are connected with the vascular system, and so form haemal
passages. These are also continued^ as sinuses, into the lamella3 of
the mantle, and into the arms ; in the former they break up peri=
pherally, and so come to be regularly arranged. The vascular
apparatus ratnifies in these spaces. The chief point as to their

312

COMPAEATIVE AA'ATOMY.

general arrangement is that tEe large trunks run dorsally along the
enteron ; in this they may he seen to resemble the arrangements
which are found in the Yermes. But the special points in this
system of organs require further investigation.

A saccular organ lying above the stomach is regarded as the heart ;
this receives a vascular trunk which runs from in front above the
oesophagus, and gives off lateral branches. The former is regarded
as an afferent vessel (vein). It seems to collect the blood from
lacunae, which are placed around the enteric canal. Two lateral
vessels given off from the heart are united in the Testicardines
(Waldheimia) for a short distance. In the Ecardines (Lingula) they
are not given off till later from a median longitudinal trunk which
passes backwards on the enteron. Two arterial trunks, which have
been called aortae, soon divide into two branches, one of which passes
forwards and the other backwards. The anterior one represents
the dorsal pallial artery, which divides into a median and a lateral
branch, and supplies the mantle and the organs which lie in it.
Smaller arteries are given off from the lateral branch to the
lacunm of the mantle; they pass to the edge of it, and then
open after having divided several times. The hinder branch of
the aorta also divides into two arteries. One runs along the middle
line, and forms, with its fellow of the other side, an arterial trunk
which passes to the stalk. The other artery turns forwards, and
again divides into two branches in the ventral lobes of the mantle
where it ramifies, in just the same way as the dorsal pallial artery.
On the two pairs of pallial arteries there is a pouch-like appendage,
or accessory heart. The blood seems to pass from the ends of the
arteries into wider lacunae which are placed in the mantle, as well as
between the viscera, and around the muscles ; and these are con-
nected with a complicated system of canals, which traverses the
arms, and is divided into an efferent and an afferent portion.

As the mantle is a secondary structure, its blood-vessels may be
regarded as being so also. The pallial arteries are, therefore, of little
importance, and the large trunks which accompany the enteron
become of greater morphological significance. The heart appears to
be a unilateral enlargement of the longitudinal trunk, as are also
the accessory hearts of the pallial arteries.

Excretory Organs.

§ 245.

The excretory organs found in the Yermes, and adapted to the
presence of a coelom, are also found in the Brachiopoda, where they
have essentially the same characters. Like the looped canals of the
Annelides, these organs have an internal and an external orifice, so
that I have no hesitation in regarding these structures as homologous.

EXCEETOEY OEGAXS OE BEACHIOPODA.

313

evou thougli tlieir f anction be modified. There are either one or two
pairs of them. When there are four, two of them belong to the
so-called dorsal, and two to the ventral half (Ehynchonella) ; this
points to the presence of two metameres, which have disappeared in
this portion of the body. The dorsal ones are absent in Lingula
and the Terebratulida. The canals, which generally open to the
exterior near the base of the arms, open into the co3lom, after having
taken a bent course, by funnel-like enlargements (Fig. 160, 7*),
which are distinguished by their radially-arranged folds. This
orifice passes through the ileoparietal band, and is thus directed

D m i!

Fig. 166. Lateral view of the organisation of Waldheimia australis. 17 Dorsal,
F Ventral surface. P Stalk, ll Spirally-coiled arms, hr Branchial filaments, c An-
terior wall of the perivisceral cavity, d Oesophagus, d' Mid-gut. h Liver, h' Its
openings into the mid-gut. r Internal orifice of the right oviduct (some fold s only
of the left oviduct can be seen), e Brachial canal, m m' m" Muscles to move
the valves of the shell (after A. Hancock).

towards the pericardial cavity. The ileoparietal band resembles
therefore, in its relation to the internal orifice, a dissepiment of the
Vermes (cf. supra, § 243).

Although the walls of these canals appear to be glandular in
character, owing to the possession of jirojections, villous processes,
or folds, we know nothing of their function, save that they have a
relation to the generative organs ; so that they appear to form an
oviduct, and have indeed been hitherto regarded as being such. And
as the looped-canals serve as parts of the generative apparatus in
the Gephyrea and Annelides, it is not to be wondered at that they
have the same relations in the Brachiopoda; but this does not
exclude the possibility of their having an excretory function also.

COMPAEATIVE AXATOMY.

3U

Generative Organs.

§ 246.

In some of tlie Brachiopoda the arrangement of the generative
organs is hermaphrodite, so that the separation of the sexes seems
to be an exception (Thecidium). The organs merely consist of
germinal glands, in which the sperm and ova are formed. In the
hermaphrodite form there are four, and in Thecidium two, glandular
masses. In the Ecardines they lie in the coelom, partly surround-
ing the enteron and the muscles ; in the Testicardines they form
rounded masses in the cavity of the two lamellae of the mantle (con-
tinuations of the coelom) (Fig. 165, g)-, in either case they call to
mind the character of the generative organs of the Gephyrea, and of
the Annelides. In the dioecious forms they are ovaries in one, and
testes in the other. It is not known what relation there is between
the ovarian and seminal regions in the monoecious forms. The
generative products escape into the coelom.

The excretory organs act as the efferent ducts of the generative
glands, so that here too a primitively unconnected apparatus func-
tions as an oviduct, or as a seminal duct, according to the sex.

Seventh Section.

Mollusca.

General Review of the Group.

§ 247.

The general characters of the body, and of its various systems of
organs, distinctly define the phylum of the Mollusca. Owing to the
absence of any distinctly-marked external metamerism, the body
appears to be more compact than in the Arthropoda or Annulata
among Vermes; indications of metamerism may, however, be made
out in various organs. The supraoesophageal position of the
central nervous system, and its connection with lower-lying ganglia,
or with commissures surrounding the pharynx, when taken in con-
nection with the position of the heart, which is always dorsal, are the
definitely typical characters of this division ; to which we may add
that in most forms shells are developed from the dorsal surface.

The complete disappearance of their primitive metamerism, and
the gaps that there are between the classes here united together, are
completely explained by the early appearance (palaeontologically
speaking) of most of the classes of the Mollusca ; while the forms
still living are seen to be an exceedingly small part of the phylum,
which was rich in forms, but which has been continued on in a
relatively small number of divisions. As yet we know very little of
the phylogeny of the Mollusca, but their metameric arrangements,
as indicated by their internal organisation, point to their affinity to
segmented organisms, the nearest allies of which were some of the
Vermes.

Although we can classify the various orders as higher and lower,
yet all the systems of organs have not been developed to the same
extent, so that we are able to find distinct j^roofs of the affinity
between every single division and lower forms.

316

COMPAEATIYE A]SY4T0MY.

I submit tlie following sketcli of tbe classification of tbe group,
and would remark that many of the older views took note of varia-
tions, which would still further separate the divisions, and especially
the smaller ones, of the group.

I. Placophora.

, Chiton, Cryptochiton.

II. Coiichifera.^^

Lamellibranchiata.

Asiphonia.

Ostrea, Anoniia, Pecten, Mytilus, Ai'ca, Anoilouta, Unio.

Siphoniata.

Chama, Cardium, Cyclas, Venus, Tellina, Mactra, Solen, Pholas,
Teredo.

Scaphopoda.f

Dentalium.

Gastropoda. J

Prosobranchiata.

Chiastoneura,

Zeugobranchia.

Fissurella, Haliotis.

Anisobranchia.

Patella, Trochus, Littorina, Cyclostoma, Rissoa, Paludina,
TuiTitella.

Orthoneura.

Nerita, Janthina, Valvata, Sigaretus, Marsenia, Cypraea,
Cerithium, Strombus, Pterocera, Dolium, Cassis, Tri-
toniuin, Voluta, Harpa, Buccinum, Nassa, Purpura,
Murex.

Heteropoda.§

Atlanta, Carinaria, Pterotrachea.

Opisthobranchiata.

Tectibranchiata.

Bulla, Aplysia, Pleurobranchus.

Nudibranchiata.

Tritonia, Polycera., Aeolidia, Phyllirhoi?, Doris, Phyllidia,
Pleurophyllidia.

. Sacoglossa.

Elysia, Limapontia, Placobranchus.

Pulmonata.il

^ What led me most to unite all the Mollusca, vith the exception of the Chitonidae,
into one great division, to which I have given the name Conchifera, was the considera-
tion that we must recognise the great significance of the shell as affecting the whole
organisation of these animals. But although the Placophora are thereby sharply
marked off from the rest, I do not see that there is any sufficient reason for removing
them altogether from the Molluscan phylum, for it is possible to make out in them
many points in which they agree wdth, and are consequently allied to, the Conchifera.
I regard the Placophora as the remnant of a division, the forms of which were allied
to the Solenogastres (p. 127) on the one hand, and on the other were the predecessors
of the Conchifera.

t The Scaphopoda form a division which is allied to the Lamellibranchiata as well
as to the Gastropoda ; but they must not be regarded as a mere intermediate link.

J The Zeugobranchia are, in many points, the oldest of the Gastropoda.

§ I regard the Heteropoda as an order which has bi'anched off from the Proso-
branchiata, and is closely allied to the Orthoneui’a; but which has developed special
characters which are not equal in value to those of the Orthoneura.

li The organisation of the two divisions of the Pulmonata does not seem to me to
be so markedly divergent as to make them of equal value with the two other orders of
the Gastropoda. We cannot as yet form a definite opinion as to many of the genera,
e.g. Onchidium, of the Nephropneusta.

MOLLUSCA,

317

Gastropoda {contlimod) ,

Branchiopneusta.

Lymnseus, Planorljisi, Auricula.
Nephropneusta.

Helix, Bulimus, Clausilia, Limax, Arion.

Pteropoda.*

Tliecosomata.

Hyalea, Clooilora, Clireseis, Cymbulia.
Gymnosoinata.

Clio, Pneuinoclermon.

Ceplialopoda.t

Tetrabranchiata.

Nautilus.

Dibranchiata.

Decapoda.

Spirilla, Sepia, Sepiola, Loligo.

Octopoda.

Octopus, Tremoctopus, Eledone, Argonauta.

Bibliography.

CuviEB, M(imoires pour servir a I’histoire et a I’auatomie des Molluscpies. Paris, 1817. — Van
BENErEN, Exercices zootorniques. Ease. I. II. Bruxelles, 1839. — Quoy and Gaimabd, Voyage
de I’Astrolabe. Zoologie.— Delle Chia.te, Descrizione e notomia degli animali inyertebrati della
Sicilia citeriore. Napoli, 1841-44. — Souleyet, Voyage de laBonite. Zoolog. T. II. Paris, 1852. —
Leuckaht, R., Zoolog. Untersueb. III. Giessen, 1854.— Gegenbaur, C., Unters. iib. Pteropoden
u. Heteropoden. Leipzig, 1855. — Krohn, A., Beitr. z. Entw. d. Pteropoden u. Heteropoden.
Leipzig, 1860. — Lankesteb, Ray, Contrib. to the develop, of the Mollusca. Philosoph. Transac.
1875. — V. Jhebing, Vergleichende Anatomie des Nervensy stems u. Phylogenie der Mollusken.
Leipzig, 1877.

Placophora : .. V. Middendobfe, Anat. v. Chiton. M<im. Acad, de St. Petersbourg. VI. vi. 1849.—
Loven, S., Ofvers. K. Vet. Acad. Forhand. Stockholm, 1S55. (Arch. f. Nat. 1856.)

Lamellibranchiata : Poei, Testacea utriusque Siciliae eorumque historia et anatome. III. Tom
1791-1795.— Bojaxus, Ueber die Athem- und Kreislaufwerkzeuge der zweischaligen Muscheln.
Isis, 1819, 1820, 1827. — Deshayes, Art. Conchifera in Todd’s Encyclopaedia, Vol. 1. 1836.— Garxer,
On the anatomy of the lamellibranchiate Conchifera. Transact. Zoolog. Soc. London. Vol. II.
1841.— Quatrefages, Anatomie von Teredo. Ann. des sc. nat. III. xi.— Lovix, S., Bidrag till
kiinnedomen om utvecklingen of Moll, acephala. Kongl. Vetensk. Acad. Handl. Stockholm,
1850. — Keber, Beitriige zur Anatomie und Physiologic der Weichthiere. 1851. — Davaixe, C., Sur
la general, des Huitres. Paris, 1853. — V. Hesslixg, Die Perhnuscheln. Leipzig, 1859. — Lacaze-
Duxhiers, Anatomie von Anomia. Ann. sc. nat. IV. ii. — L. Vaillaxt, Sur la fam. de Tridac-
nides. Ann. sc. nat. V. iv.— Sabatier, A., Etudes sur la moule commune. M^m. de I’Acad. des
Sc. de Montpellier. 1877.

Scapliopoda : Lacaze-Dpthiers, Hist. nat. organis. developpement, etc. du Dentale. Paris, 1858.

Gastropoda: Nobdmaxx, ^Monographic des Tergipes Bdwardsii. Mdm. de I’Acad. Impt^riale de
St. Petersbourg. IV. 1843. — Alder and Hancock, Monograph of the British Nudibranchiate
Mollusca. Ray Soc. I. — VII. 1845-55. — Hancock and Embletox, On the Anatomy of Eolis.
Ann. and Mag. of Nat. His. XV. 1845. — The same, On the Anatomy of Doris. Phil. Trans. 1852.
Pt. II.— Hancock, Anatomy of Doridopsis. Trans. Linn. Soc. XXV. — Leydig, Ueber Paludina
vivipara. Zeitschr. f. wiss. Zool. II. — Huxley, On the morphology of cephalous Mollusca.
Phil. Trans. 1853.— Bergh, Bidrag til en Monograph! of Marseniademe. Kongl. dansk.
Vidensk. Selsk. Skrifter. 1853. — The same, Anatomisk Undcrsogelse of Fiona atlantica. Vidensk.
Meddelelser for 1857. — The same, Anatomisk Bidrag til Kundskab om Aeolidierne. Danske
Videnskab, Selskabs. Skrifter. 1864. — The same. Bidrag til en Monograph! of Plem-ophyllidierne.
Naturhist. Tidsskrift. 3 Rlikke. 4 Bind. 1866. — The same. Bidrag til en Monograph! of Phylli-
dierne. ebend. 6 Bind. 1869. — The same, Malacolog. Untersuch. Heft. I. — X. Wiesbad. 1870-76.
— Claparede, Anatomie und Entwickelungsgesch. der Neritina fluviatilis. Arch. f. Anat. 1857.
— The same, Beitrage zur Anat. des Cyclostoma elegans, ibid. 1858. — Lacaze-Duthiers, Anatomie
du Pleurobranche. Ann. nat. sc. IV. xi. — The same, Anat. et I’Embryogtinie des Vermets.
Ann. sc. nat. IV. xiii. — Lawson, Anat. etc. of Limax maximus. Quart. Journal of Micr. Sc.
1863.— Fol, H., Sur le ddveloppement des Heteropodes et des Gastdropodes pulmon^es. Comptes-
rendus. T. LXXXI. Nos. 11 et 13. Ai-cliives de zoologie. T. V. .

* In many points of tlieir organisation the Pteropoda indicate a relationship to the
Cephalopoda, but this can only be regarded as a very distant one.

f Most of the oldest fossil forms probably belonged to the Tetrabranchiata, which
are represented by only one extant genus ; at the same time these fossil forms varied
greatly in character.

318

COMPAEATIYE ANATOMY.

Pteropoda : Eschbichx, Ueber d. Clione borealis. Kopenhagen, 1838.— Fol, H., Sui* le developpe-
ment des Pteropodes. Archives de zoologie. T. IV,

Cephalopoda : Gbant, On Loligopsis. Trans. Zool. Soc. 1835.— Owex, Memoir on the Pearly
Nautilus. London, 1832. — The same, Ai’ticle on Cephalopoda in Todd’s Cyclopaedia. I. 1836. —
VALENCiENifEs, Nouvelles recherches sur le Nautile flambe. Archives du Museum. 1841. —
Petebs, Anatomie der Sepiola. Arch. f. Anat. 1842. — Kollikeb, Entwickelungsgesch. der
Cephalopoden. Zurich, 1844. — Van deb Hoeven, Bijdragen tot de Ontleedkrundige Kennis aan-
gaande Nautilus pompihus, Ameterdam, 1856. — Gbenacheb, Zur Entwick. d. Cephalopod.
Zeitschrift f. wise. Zoolog. Bd. XXIV. p. 419. — Fol, H., Note s. 1. developpement des Mollusques
pteropodes et cephalopodes. Arch, de zool. T. III. — Bobbetsky, Untersuch. iiber die Entwick-
elung der Cephalopoden. Nachr. d. k. Gesellsch. der Fi-eunde d. Naturkenntniss etc. zu Moskau.
Bd. XXIV. (Russian.) [Ray Lankesteb, Development of Cephalopoda, Quart. Journ, Mic. Sci.
1875.]

Form of the Body.

§ 248.

The general form of the body of the Mollusca must be regarded
as one so much altered by the relative positions of many organs^
owing to the formation of shells, that it has only been possible to
recognise a ground-form, which shall represent the common origin,
by comparing the earlier larval stages with several mature forms.
The Placophora have a worm-like larva, and a similar kind of
external metamerism is indicated by the number of circlets of cilia
seen in the Gymnosomatous Pteropoda. The relations thus implied
are retained by the Placophora in their mature condition, at least in
the dorsal portion of the body. This is separated from the ventral
portion by a groove, and so defines two regions, which are found
also, under the form of ‘^mantle and ‘Moot,^^ although much
changed, in the Conchifera. The differentiation of a gutter-like
ventral surface in the Solenogastres (cf. p. 130), as has been already
explained, points to the Mollusca having genetic relations to these
worms; this supposition is confirmed by the characters of the
nervous system.

The Lamellibranchiata and Gastropoda, as well as the thecosoma-
tous Pteropoda, develop a well-marked circlet of cilia in the region,
which, later on, corresponds to the head ; this circlet is afterwards
carried on a special, symmetrical, and lobed process — theYelum.
The primitive significance of this circlet is clearly shown by its
presence in otherwise divergent divisions, and is even still more
important from the fact that we can recognise in this organ the
circlet of cilia which surrounds the same part of the body in many
Vermes (cf. § 107). The velum of the Mollusca may therefore be
regarded as an organ inherited from a lower stage.

Below the velum the rudiment of the opening into the enteric
cavity is formed. As in the Placophora, the formation of a dorsal
shell in the Lamellibranchiata does not prevent the enteric tube
from being continued to the aboral pole of the body ; for in the
Placophora the shell, as well as the mantle which carries it, is adapted
to the whole body, and in the latter it is principally developed at the
sides. We are able, therefore, to distinguish a primary axis, which
extends from the oral to the anal pole, and which is crossed by two

FOEM OF BODY OF MOLLUSCA.

319

secondaiy and variously-differentiated axes — tlie dorso-ventral and
the transverse. The body is therefore of the original endiplenral
form, which is the dominant one in the Vermes and Arthropoda.

These relations are different in the Gastropoda, where the dorsal
cup-like shell gradually encloses the greater part of the body, and
leaves a small portion only of the surface of the body exposed in
addition to the head and foot. So that while in the previous case
the shell was adapted to the body, in this case the soft parts of the
body are adapted to the single shell. The body, therefore, becomes
asymmetrical, and the aboral pole no longer carries the anus, which
becomes lateral in position in consequence of the flexure of the
enter on ; this flexure is due to the formation of the shell. All of
the many variations from the symmetrical ground-form, which are
seen in the Gastropod-body, may be regarded as due to this.

The primitive similarity in the form of the body, due to the
possession of a shell, undergoes great modifications even among the
Gastropoda; the Veliger stage is not always developed, and has
never yet been observed in the Cephalopoda. But even in this class
the form of the body, and the disposition of its viscera, may be seen
to be, in all forms, due to the possession of a shell.

§ 249.

The velum has different functions in different divisions. In the
Lamellibranchiata, where it functions for some time as a locomotor
organ, but v/here it is never independent and soon atrophied, its
function is not very great. This may, perhaps, be correlated with
the rudimentary character of the future head, and this, again, with
the rapid disappearance of the free mode of life in this division
( Acephala) .

Two folds, however, which are given off laterally from the dorsal
surface, become considerably developed and form a mantle; they
surround the body, and excrete

the shell, which corresponds
with the lamellae of the mantle
in form and size.

A space, which functions
as the respiratory cavity, is
developed between the edges
of the mantle ; branchiae are
developed from the body- wall,
and project into it (Fig. 167,
A hr). In a few Lamelli-
branchs (Asiphonia) this en-
trance into the mantle-cavity
is a cleft of some size, by

Fig. 167. Diagram of the relations of the
foot and mantle, as seen in transverse sec-
tion. A In Lamellibranchiata. B In Cepha-
lophora. m Mantle, p Foot. Ir Branchiae.

which water

passes

in and out,

and so carries in nutriment and removes excreta. In most, the
two edges of the mantle grow together, and so shut off, more
or less completely, the cavity which surrounds the gills, and cause

320

COMPARATIVE ANATOMY.

tlie two streams of water to enter and escape from it with greater
regularity.

The least amount of concrescence which is observed, gives rise to an
anterior larger, and a posterior smaller, orifice (Mytilidae) . The former
serves as an outlet for the foot, and as the orifice of entrance for the
food, while the latter, in correspondence with its position, is the orifice
of exit for the foecal matters, and of the water that has served for respi-
ration. In the Chamacefe there are also two large openings behind
the anterior and larger cleft, through which the foot is protruded, and
which serve respectively for the entrance and exit of water ; this is
an arrangement which attains a higher , grade of development in a
large division of the Lamellibranchiata (Siphoniata). That part of
the mantle which surrounds these orifices forms an elongated tube
(siphon), which undergoes other modifications in addition to its con-
crescence. The respiratory tubes may sometimes be formed by

Fig. 168. Lateral view of the mantle-cavity of a Mactra; the right mantle-lamella
has been removed. hr 1/ Branchiae, t Tentacle, ta tr Siphons, via Anterior,
vij) Posterior adductor, p Foot, c Umbo.

separate portions of the mantle ; or there may be a respiratory tube,
which is single externally, and is only divided internally into two
canals by a partition ; or the two conditions may be combined
(Fig. 1C8, tr ta) ; or, finally, two completely separate tubes may be
developed : an upper one, the inner orifice of which is opposite the

anus, and serves for the exit of the water,
and a lower one, by which the water passes
in. The investment of cilia causes the
two streams to pass regularly in and out.

Through these forms we are led up
to those in which the respiratory cavity
is most completely closed, and the pallial
tubes most developed. This is accom-
panied by a diminution in the size of the
cleft in the mantle through which the
foot is protruded. This has become much
narrower, and is placed some way from
the respiratory tubes, so that the greater
part of the edges of the mantle have grown together, in consequence
of which the body of the animal is sac-like (Boring Mollusca). The
orifice of passage for the foot is now placed at the anterior end, and the

Fig. 169. The same animal
with its foot and siphons re-
tracted. ms Siphonal muscle.

FOKM OF BODY OF MOLLU8CA.

321

two respiratory tubes in tbe opposite region of tbe body. They are
continued into special divisions of tbe man tie -cavity, owing to tbe
division of tbe latter into an upper smaller, and a lower larger
cavity, wbicb are separated by a partition. Tbe water is brought to
tbe lower one by tbe efferent tube, and passes through tbe gills ;
streaming through tbe orifices in them, into tbe branchial plates
or tbe intrabrancbial cavity, and so into tbe upper division of tbe
mantle- cavity, into wbicb the anus also opens.

The edge of tbe mantle is often tbe seat of special differentia-
tions, which are generally of tbe form of tentacular processes, and
are sometimes of a fair size.

The second differentiation in tbe body of tbe Lamellibrancbiata
affects tbe ventral surface, wbicb is differentiated even in tbe
Placopbora ; it becomes flattened, and serves as tbe organ by which
tbe animal creeps along. It is formed by tbe development of a
muscular foot, wbicb is more or less separated from tbe rest of the
body (Fig. 167, Ap), and wbicb can be protruded from tbe cleft in
tbe mantle, sometimes to a considerable extent. It is then hatchet
or club-shaped, and functions as a locomotor organ. The two lateral
surfaces of tbe foot are ordinarily produced into a median edge, but
in some it is flat and sole-shaped, as in Chitons.

Many Lamellibrancbs live under conditions in wbicb this organ is
not required, and it is then atrophied, as in the fixed Oysters and
Anomiae, and Scallops ; in tbe latter, locomotion is effected by tbe
action of tbe mantle and its shells.

The Scapbopoda are forms allied to tbe Lamellibrancbiata, but
intermediate between them and the Gastropoda. Tbe body, which
is enclosed by a shell, is provided with a mantle-cavity, from which a
trifid foot can be protruded. A part wbicb carries tbe mouth is head-
like in form, but is really more of a proboscis, for it does not contain
the nerve-centres, and is, moreover, enclosed in tbe mantle-cavity.

§ 250.

Tbe velum is largest in the' Gastropoda and tbe thecosomatous
Pteropoda, and is absent in those only in which tbe earliest larval
stages are not free (Land Snails). It has tbe form of a large, and
frequently symmetrically lobate, organ (Fig. 170, A 5 G v)j which in
some is retained for a longer time, and so enables the body to continue
swimming about (Macgillivraya) . Tbe development of this organ,
wbicb in its lower stages is merely represented by a circlet of cilia,
appears to be correlated with tbe development of a shell, for when
this is developed tbe cilia are less widely distributed. The cephalic
portion of tbe body is alone free; and it compensates for tbe absence
of other locomotor organs by the great development of its cilia, and
of its ciliated margin. Tbe velum increases in size, and undergoes
great complications of form, in proportion to the increase in tbe
weight of tbe body due to its shell.

Y

322

COMPAEATIVE ANATOMY.

The size of this velum is correlated with the differentiation of
the head, from the upper surface of which it is developed ; it is in
some Pteropoda only that the head, once formed, undergoes any
considerable atrophy.

Just as in the Lamellibranchiata, the mantle rises up in the
form of a fold of the body- wall, which covers over the dorsal surface
and forms the shell on its outer side. As this dorsal area of the
body — which is surrounded by the mantle-fold, and the shell,
which is being developed into its house — continues to bulge out, it
gradually forms a blind sac, which soon contains the greater part of
the viscera (visceral sac) ; in this way the viscera come under the
direct protection of the shell. As development goes on, the mantle-
fold becomes less intimately connected with the body, and gives
rise, interiorly, to a wider cavity, in which the growing gills are
contained, and which is homologous with the branchial cavity of the
Lamellibranchiata (Fig. 167, A B). This development of a fold of
the integument into the mantle, and the consequent appearance of a

subjacent space, the branchial cavity — which looks like an invagi-
nation from the exterior — undergoes modifications, which are largely
due to the formation of the shell. In consequence of the mantle
growing unequally on either side, and not equally, as in the Lamelli-
branchiata, and from the fact that it is principally developed at one
point in connection with the development of the shell, the branchial
cavity comes to be a single cavity, placed in the same region. This
region is either beneath the hinder portion of the mantle, as in the
Pteropoda (Fig. 170, C), or beneath the anterior portion, as in most
of the Grastropoda {B) . The want of symmetry, which is due to the
coiling of the shell, causes the branchial cavity of most G-astropoda
to lie on one side ; this is an adaptation to the larger amount of
space which is afforded by the lateral portion of the shell. The
production of the unilateral and asymmetrical branchial cavity from
a paired and symmetrical space is proved by numerous facts ; so
that we are led to think that the asymmetry of the shell is probably
a secondary arrangement.

A number of degenerate and more perfect arrangements have

FOEM OF BODY OF MOLLUSCA.

323

their common origin in this disposition o£ parts. The latter are
principally seen as differentiations of the edge of the mantle^ and
are connected with the function of the branchial cavity. Part of the
edge of the mantle is produced into a groove, which serves to bring
in water, and which may be converted into a tube by the folding
over of its two edges. We meet with a siphon of this kind, though
in all stages of gradual differentiation, in a large number of aquatic
Gastropoda (Buccinum, Dolium, Harpa, Tritonium, Murex, etc.). A
second siphou, formed in the same way, but smaller in size, is
generally found at the opposite end of the branchial cavity ; it serves
to carry the water out from it. Various other kinds of processes, or
tentacular appendages, lead to fresh complications in its structure
(e.g. in Strombus, Pterocera).

When the shell undergoes atrophy the mantle generally does so
too. This is mostly the case in the division of the Opisthobranchiata,
some of which have a more or less rudimentary shell ; while in others,
v/hen adult, there is no shell at all. As all these forms had a shelled
larval stage, the atrophy of the shell must have been brought about
during their ontogenetic development ; and it follows that those
Opisthobranchiata, which are naked in their later stages, were derived
from forms that had shells. The larval shell and its accompanying
mantle-fold, even though feebly developed, are therefore rudimentary
organs, which prove that the naked Opisthobranchiata had the same
origin as the rest of the Gastropoda. Where these rudimentary shells
are retained by the adult animal they must even then be regarded
as degenerate parts, and not as developing shells ; for here again a
comparison with the larval forms shows that the shell had a much
greater significance than have the rudimentary structures found in
the adult stage of these organisms. It is of great importance also
as explaining the position of the anus and of the genital orifice,
which can be due to nothing but a former greater development
of the shell.

Within smaller divisions also we meet with series of degene-
ratiug parts, as for example in the Heteropoda, where Atlanta has
a well- developed shell and mantle, while in Carinaria they are both
rudimentary, and in Pterotrachea completely lost. A similar series
is observable in the Nephropneusta.

§ 251.

The varying extent to which the Foot is developed is of im-
portance as affecting the form of the body. In the larvae of the
Pteropoda and Gastropoda it has always very much the form of a
short, conical, somewhat-flattened process, placed below the mouth
(Fig. 170, Ay)). On the hinder, or dorsal surface, a shelly secretion
is formed, which serves as an operculum for the orifice of the shell.
Owing to its increase in size, especially in the aboral direction, the
foot of most Gastropoda comes to have a broad lower face, which is

Y 2

324

COMPAKATIYE AXATOISIY.

tlie reason of its being called a foot (Fig. 171, B). Sometimes,
however, it is elongated, and, at others, discoid in form. In most
of the Gastropoda the foot is only sharply marked off along its
plantar margin. In many of the lower Prosobranchiata (Haliotis) the

surface of the body above
the foot is drawn out into an
encircling edge (epipodium),
which is distinguished from
the mantle by surrounding
the head. The foot of the
Heteropoda is differentiated
into a more independent organ,
which springs from the ventral
surface of the animal, and-
forms a vertical fin. The body
is continued in front of, as
well as behind, the foot. This
arrangement is very different
from the primitive one; the body has no longer a flat surface,
although the end of the foot in Atlanta still carries an operculum.
The structure of the muscular sole of the Gastropod foot is retained
in rudiment as a sucker-like organ, which in the Pterotracheae is
found in the males only. And we are reminded by this that even

when fully developed the foot of the
Gastropod functions as a sucker, for the
animal is able to attach itself by it.

The modifications undergone by the
foot in the Pteropoda are still more
significant. The foot, which, in the ear-
liest larval stages, is formed in just the
same way as in the Gastropoda, gives
rise, in the Cymbulidae and Hyaleidae, to
a median and two lateral pieces (cf. Fig.
170, G In the Hyaleidm the median
portion is feebly developed, while the
lateral lobes become large fins, which
embrace the rudimentary head, just like
wings ; in the Cymbulidae the median
piece is also well developed. It either
fuses at its base only (Cymbulia), or along
its whole length (Tiedemannia), with the two lateral lobes ; in this
way the large fins of these animals are produced.

§ 252.

The greater development of the head in the Cephalopoda is an
important peculiarity as affecting the form of the body, while the
mantle acquires the same relations as it has in the Thecosomatous
Pteropoda, from which therefore they may be derived. The cavity.

^ A

Fig. 172. Diagram of the rela-
tions of the mantle. A In
Pteropoda. B In Cephalo-
poda. p Foot, hr Branchiae.
i Tentacles.

Fig. 171. Diagram of the relations of
the mantle and foot ; vertical section.
A In Lamellibranchiata. B In Cephalo-
pora. m Mantle, p Foot, hr Branchiae.

APPENDAGES OF MOLLUSCA.

325

arclied over by a fold of tbe mantle, occupies the hinder part of the
back, and so forms that region which is ordinarily known as the
ventral surface. To make these relations clear we must imagine
the animal placed in such a position that the aboral end points up-
wards, and the head forwards and downwards (cf. Fig. 172). All
the body above the head would then correspond to the dorsum of
the Gastropoda. The mantle is sometimes separated from the head
by a circular groove (Sepia) ; sometimes this fold of the mantle is
directly continuous with the integument of the head at the sides of
the neck (Octopus), so that the mantle forms a fold above the
branchial cavity only. Lateral processes of this mantle function as
locomotor organs (fins); in the Sepiadm they are generally small,
and extended along its whole length; in the Loliginidse they are
broader, but are limited to the aboral end of the body.

The formation of the mantle-cavity and the position of the anus
lead us to the conclusion that this arrangement is due to the
primitive possession of a shell which covered the whole mantle ; and,
indeed, the shelled Cephalopoda are by far the older forms, while
the remarkable variations seen in the characters of their shells lead
us to think that this structure had a very ancient origin.

An organ which has the same position as the foot of the
Gymnosomatous Pteropoda — the funnel — corresponds to the foot of
the Gastropoda. In Nautilus it is formed of two lamellse, which
arise from the ventral surface below the head, and which form a
tube by being rolled over one another; this tube projects from the
mantle-cavity (Fig. 175, i). In the Dibranchiata this organ cannot
be seen to be composed of two lateral parts, except in the embryo;
they take their origin in the space between the mantle and the
rudiments of the arms. By growing together and gradually fusing
they form a tube which is similar to the one formed in Nautilus,
except that it is closed. The mantle, which is also muscular, is
attached to the periphery of the funnel; this effects powerful con-
tractions, and so drives out the water which has entered the
mantle-cavity between the funnel and the edge of the mantle ; and
the animal is driven in an aboral direction by the force produced by
the expulsion of this stream. The organ retains, therefore, its
primitive locomotor function.

Appendages.

§ 263.

In the Mollusca the development of a cephalic region is
closely connected with the differentiation of processes, which I
regard as appendages, inasmuch as they are homologous with the
antennae and tentacles of Arthropoda and Vermes, and when more
highly differentiated are able to undertake the duties of appendages.

326

COMPAEATIYE ANATOMY.

These structures, which are known as tentacles, are absent in the
Placophora and in the Scaphopoda ; the processes which are found
around the mouth in the latter group being structures of a special
kind, and not appendages as here limited.

In the Lamellibranchiata, lobate appendages (Fig. 168, t) (the so-
called labial palps) are attached to the altogether rudimentaiy head ;
they may be homologous with the more highly- developed tentacles,
which distinguish the cephalic region of the Gastropoda. As in
many Platyhehninthes they are, when simplest, short processes of the
body, but they undergo great differentiations. In the Proso-
branchiata they are • generally limited to two, and are formed from
the surface which is surrounded by the velum (cf. Fig. 170, B t).
In many forms the eye is placed at the base of the tentacle, whicli
may be developed into a special process. The same happens in
other forms, where the optic organ is placed on an optic stalk dif-
ferentiated from the tentacles, and which, when more independent,
may give rise to four tentacles, as in Helix, Limax, etc. These are
invaginated when they are retracted, and are so far more highly
developed. Many Opisthobranchiata are distinguished by the pos-
session of a pair of tentacles, which are greatly developed (Fig. 177,
tt), but in addition to them there are other tentacular cephalic
appendages, which characterise the various subdivisions merely,
according to the way in which they are arranged, and according to
the number present.

They have undergone degeneration in the Thecosomatous Ptero-
poda, for in these forms the tentacles are either completely absent or
are rudimentary (Chreseis). The development of the parts of the
foot which in them are converted into fins, does away with the
necessity for the development of the cephalic tentacles, and explains
why they are absent, just as, on the other hand, the distance of the
fins from the head in the Gymnosomatous forms is the cause of the
development of their tentacles. In these latter they have all kinds
of forms, and one or more pairs of processes (Cephaloconi) arc
present in addition to the superior tentacles ; these lead up to the
tentacles of the Cephalopoda. In Pneumodermon, indeed, two of
these bodies are beset with suckers.

§ 254.

In the Cephalopoda the large number of tentacles, which are
arranged in rows on either side, and spring from lobate processes,
distinguish the head of the Tetrabranchiata. In the Dibranchiata,
where they form arms, there is a smaller number of them, but they
are larger. The Loliginidae, Sepiadae, and Spirulidse have ten arms.
Two of them, which are longer, and in other points different from the
rest, are placed outside the circle around the mouth, which is formed by
the other eight; and as they spring from pouches which are arranged
in pits at the side of the head, they must be distinguished from the
inner series ; so that these inner ones are always eight in number in

APPENDAGES OF MOLLUSCA.

327

all Dibran cliiata. The arms of tbe Octopoda, like tbe similar ones in
tlie Decapoda_, are connected together at their bases by a web, ex-
cepting the pair which are nearest to the sides of the funnel. This
connecting membrane extends farther in some Octopoda ; sometimes
over a few of the arms only (four in Tremoctopus), or over them all
(Histioteuthis, more completely in Cirrotenthis), and is continued
right up to the tips of the arms.

The suckers are special structures found on the arms of the
Cephalopoda ; they generally beset the
oral surface of the arms in two rows
(one in Eledone), and not unfrequently
they are carried on stalks. Their free
edge has often a cuticular thickening
which has the form of a chitinous ring,
and is sometimes toothed. Where a
particular tooth is largely developed the
sucker disappears, and is replaced by a
hook (Onychoteuthis) .

In many Cephalopoda certain of these
arms are peculiarly altered by function-
ing as copulatory organs ; even in
N^;utilus the tentacles perform this func-
tion. It is not always the same arm
that is thus metamorphosed; as a rule
it is one of those which belong to the
so-called ventral side of the animal.

The mode of metamorphosis varies
greatly in the different divisions ; it may
merely consist in the alteration of a part
of the base of the arm (Sepia), or a
more or less large number of the
suckers may be altered, or the tip of
the affected arm may be provided with

a spoon-like hollow process (Octopus, Tremoc-

A . ^ \ r ^ topus caren80. Superior;

Eiedone) . ^ ^ ^ ^2 second pair of arms. P Third

The highest grade of this adaptive left arm. P Inferior pair of
metamorphosis is seen when the arm arms, h Hectocotylus. a; Its

becomes greatly increased in size, as tons appendage, set free from
well as different in organisation inter- the terminal vesicle, i Funnel,
nally (Argonauta, Tremoctopus) . This
hectocotylised arm^^ is not developed, as are the others, by
a process of free gemmation, but it is formed in a vesicle, from
which it is not set loose till it is mature. The greatly- coiled
flagelliform end of the arm (Fig. 173, y)j which is not set free till
the time of copulation, has a similar covering. This appendage,
with its investing membrane {x), corresponds to the modified end
of the arm in Eledone and Octopus. The more highly differen-
tiated copulatory arms may continue to live within the mantle-
cavity of the female for some time after they are broken off ; this

328

COMPAEATIVE AXATOMY.

is tlie reason why these separate arms were formerly regarded as
parasitic organisms (Hectocotylus).

Steenstrup, J. J., Hectocotyldannelsen. Kongl. Dansk. Yid. Selsk. Skrifter. Y. R.
4 Bd.

Integument.

§ 255.

The body of the Mollusca is covered by a soft dermal layer,
which is, as a rule, so closely interwoven with the subjacent muscles
as to form a kind of dermo-muscular tube, just as in the Vermes.
The locomotor organs are formed by the great development of the
musculature in certain regions of the body, and by the consequent
differentiation of some parts of the dermo-muscular tube.

In most divisions of the Mollusca there is an investment of
cilia during the larval stages, which later on extends over the whole,
or some parts of the body. The cilia in the velar circlet (§ 248) are
those that are most markedly developed. The rest are chiefly
found in the respiratory organs. In the Cephalopoda even, nearly
the whole surface of the germinal disc (save the gills) is covered with
cilia during development; later on a ciliated epithelium may be
found on the yolk-sac also.

The integument is easily distinguishable into an epidermis and
cutis. In many Heteropoda (Carinaria, Pterotrachea) the latter is
specially modified ; a strong transparent layer of connective tissue
preventing any great amount of change in the form of the body. In
the rest of the Mollusca the body is generally prevented by the
shell, which is developed from the integument, from undergoing any
great changes in form.

The coloration of the body is due to the deposits of pigment in
the integument. The most remarkable structures concerned in
coloration are those which are found in many Pteropoda, and in all
Cephalopoda — the chromatophores.^^ They are rounded cells,

which are placed at various depths in the integument; they are
filled with granular pigment, and are provided with radial muscular
fibres at their periphery ; when these fibres contract the cell broadens
out, and the pigmented contents are thereby distributed in such a
way that they become easily visible to the eye, as large, stellate,
and often branched spots. Plate-like elements are found deposited
in a layer, which is sometimes differentiated, and these give a
silvery appearance to many parts of the body (spangled layer). The
varying character of these several layers produces that play of colour
which we admire in the skin of the living Cuttlefish.

Other deposits are found in the integument, such as those formed
of carbonate of calcium, which are common in the Gastropoda ; these
are either simple granules, or larger rounded concretions, or they
may be rod-shaped, denticulate, or even branched ; there is often a

INTEGUMENT OF MOLLUSCxV.

329

large number of them, so that they may form a veritable calcareous
network, as in Doris, Polycera, etc., the various species of which are
distinguished by the special manner in which the various calcareous
rods are grouped or arranged, as well as by the way in which they
are formed.

§ 256.

The glands are differentiations of the epidermis; they partly
resemble the structures found in Vermes (unicellular glands). When
simplest, these organs are modifications of epidermal cells, which
are placed between other cells, but are distinguished from them by
having finely-granulated contents and a mouth (goblet-cells). They
are found in the Lamellibranchiata as well as in the Gastropoda. In
the Cephalopoda they are more commonly arranged in groups, and
their blind ends extend below the level of the epidermis. In the Gas-
tropoda— and especially in the terrestrial Pulmonata — they are found
to be placed still deeper in the integument. These glands are variously
modified in different parts of the body. Those found at the edge of
the mantle in the shelled Gastropoda are examples ; they secrete
a fluid in which calcareous salts are dissolved, while others secrete’
colouring matters.

In Aplysia the dermal glands secrete a dark-red fluid. In
Murex and Purpura a layer of epithelium, which is placed between
the gills and the hind-gut, and in the mantle-cavity, functions as a
gland; this layer is formed by large superficially-ciliated cells.
Their secretion gives rise to the substance known as Tyrian purple.

Some Opisthobranchiata (^olidim) are characterised by tlie
possession of urticating cells, which are placed on the ends of
their dorsal papillm.

The By ssus -gland of the Lamellibranchiata is a more inde-
pendent glandular organ of the integument ; when it is formed the
foot undergoes certain modifications ; it becomes, that is, a tongue-
shaped process grooved on its ventral surface. The groove passes to
a depression at the base of the foot, at the bottom of which there is a
gland which secretes the so-called byssus.'’^ Pecten, Lima, Area,
Tridacna, Malleus, Avicula, Mytilus have an organ of this kind;
but it may be considered as generally present, for it is found for a
time in the embryos of the Naiades and of Cyclas. Some divisions
of the Gastropoda (Helicinm, Limacinm) have also a gland in their
foot, which opens anteriorly and below the mouth. A large number
of other kinds of glandular organs are also developed from the
integument.

Shells.

§ 257.

The tegumentary investment is of special importance, for it
secretes firm substances, which are laid down in layers, and which

330

COMPAEATIYE ANATOMY.

produce the varied forms of test and shell which characterise the
Molluscan phylum. The hard structures, therefore, of this division
differ essentially from those which are found in other classes of
animals, by the way in which they are developed. They are
products secreted from the body, and deposited on its exterior, and
are of great importance as organs for the support and defence of
the organisms to which they belong. They, just as much as other
differentiations from the integument, imply that the outer dermal
layer has a secretory activity. Although the outer layers of these
structures often appear to be — as is especially the case in large
shells — distinct from the organism, the shells always do form a part
of it, and are at many points directly and closely connected with
it; as, for example, at the insertion of the muscles into the shells.

The presence of calcified spicules in the integument of the
Placophora calls to mind the relations which obtain in the Soleno-
gastres (p. 139). The spicules arise in follicles, and do not reach
the surface till they are larger in size, when they form slender and
closely-approximated fine processes, or thicker bodies, which are
distributed over the mantle. Bight large calcified plates are also
arranged transversely on it, and form a series of skeletal parts, the
arrangement of which implies that the body is arranged in a meta-
meric fashion. As in Cryptochiton they are covered by the mantle,
there is some reason for supposing that they were developed within
it, and that they resemble the spicules. The plates would then be
structures of the same kind, which had been greatly developed,
while the spicules would be parts which had not enlarged laterally,
but only vertically. This relation between the presence of a mantle
and the formation of firm organs, which, when largely developed,
form shells, is typical of all the other Mollusca ; and the two kinds
of organs are always intimately connected. Instead, however, of the
dorsal plates being developed, as in Chiton, the formation is con-
tinuous, so that it gives rise to a single shell. The shell, therefore,
just as much as the mantle — which we have seen to be homologous
throughout the series of Mollusca — must be regarded as an organ,
which is widely-distributed because inherited, great as may be the
adaptive modifications which it has undergone.

The multifid shell was not replaced by the undivided one by any
new process, but by the development of one part, for we cannot
imagine that the shell, which, as an organ investing a large part of
the body, is one of so great functional importance, appeared all at
once. But, if the shell was at first an inconsiderable organ, it could
not have attained to that perfection of function which would have
been the cause of its being transmitted as a useful arrangement.
We must therefore suppose that the structure, which later on formed
the shell, was primitively one of several similar organs, and that it
gradually got the better of the rest. This gradual development of
the shell is the only mode which is intelligible, while at the same
time it connects together the multifid shell of the Placophora and
the single shell of the Conchifera.

THE SHELL OF MOLLUSCA.

331

§ 258.

The earliest rudiment of the shell appears at the aboral end of
the embryo, at a spot which is distinguished by the growth of its
ectoderm. A viscid substance is secreted in the gland-like invagi-
nation which appears at this point (Fig. 174, s). This substance
gradually fills up the invagination and
reaches the surface, where it is hardened
as soon as it comes in contact with the
water (.<?'). When the invagination dis-
appears its edges remain as a raised ridge,
and so form the rudiment of the mantle,
which is therefore very closely connected
with the formation of the shell. This ar-
rangement, which has been made out in
the larger divisions of the Conchifera,
points to the common origin of this group,
while it also affords an explanation of the
cause of the different ways in which their
shells are formed. When this invagina-
tion disappears the shell becomes external,
and then the edge of the mantle either remains below it or more or
less covers it. The latter arrangement shows how the external shells
are connected with the internal ones, which are formed when the
invagination does not disappear, but becomes still further developed
in the manner already indicated. The shell is then secreted from the
inner face of the walls of this organ, and it varies greatly in character
in various divisions, just as do the external shells.

When simplest, all the lamellae of the shell are similar in
character ; in many, when in their lowest conditions, it is perforated
by pore-canals. The simple condition is complicated by the
appearance of layers of prisms set obliquely or perpendicularly to
the lamellae.

The shell increases in surface at its free edge, the deposits
occurring in layers, and at the side of the mantle ; superficially they
have the appearance of concentric rings. The shell becomes thicker
internally by being supplied from the outer surface of the mantle.
These varying modes of formation give rise to variations in the
structure of the formed shell, the inner portion of which often
consists of a large number of superjacent folded layers, the presence
of which is the cause of the nacreous appearance of the shell (mother-
of-pearl). These layers are covered by the external more compli-
cated and compound layers, which are formed by the edge of the
mantle. The horny covering (periostracum) of many shells is due
to the same part.

The shell of the Lamellibranchiata, like the mantle, is developed
on either side of the body, but it is not calcified in the middle line,
so that it forms two valves, which are connected with one another

Fig. 174. Embryo of a Hete-
ropocl, transverse section.
0 Mouth. V Velum, g Enteric
cavity, p Foot, s Shell-gland
(after H. Fol).

332

coj\[parative anatomy.

along tlie median line by means of tlie uncalcified portion of tbe
sliell. The hinge is formed at the point where the two valves
pass into one another ; the non-calcified chitinous substance which
connects the two shells forms the ligament. Its layers pass into
those of the shells ; and the two valves are seen to be merely parts
of a structure which is rudimentarily single, and later on becomes
so again, and is homologous with the shells of the rest of the
Mollusca. Near the ligament the valves form alternating and inter-
locking processes (cardinal teeth) which serve to close the shell more
perfectly.

The shells of the Gastropoda are most markedly distinguished
from those of the Lamellibranchiata by the absence of any uncalcified
portion. They are not unfrequently internal.

This internal position is generally found in those Tectibranchiata
that have a rudimentary shell, and in some Pulmonata. In the latter
(Helicinm) the shell very soon becomes external, while in others
(Limacinae) it remains rudimentary, and placed within the mantle.
Sometimes it merely consists of a few calcareous concretions.

The various stages of atrophy are also to be seen in the shells of
some other divisions, as, for example, in the Heteropoda, in which
the rudimentary shell of Carinaria is intermediate between the shell
of Atlanta, which covers in the whole body, and that of the Ptero-
trachea), where it is altogether absent. But these latter have a tem-
porary shell during their larval life, which covers in the whole body,
just as it does in the Opisthobranchiata, which are also shell-less in
later life. Its general presence points to its being a common
heritage of all the Gastropoda, some divisions of which lose it early.
The Thecosomatous Pteropoda resemble the Gastropoda in forming a
shell.

The animal does not always occupy the whole of the shell. Many
Gastropoda withdraw themselves from the end of their shell as
growth proceeds ; and the end is then shut off by a layer of shell-
substance. The same thing happens in some Pteropoda (Chreseis),
and indicates the commencement of that arrangement which is so
much more distinctly marked in the Cephalopoda.

The substance of the shell, which is a product of excretion from
the mantle, varies very greatly, from the soft structures of some to the
firm solid parts which form the shells of most Prosobranchiata. The
soft form of shell consists of organic substance merely. Shells
become firmer and horny in character when impregnated with cal-
careous salts ; and when the inorganic substance forms the greater
part of the shell we get strong coverings.

The simple condition of the cup-shaped embryonic shells persists
in some Gastropoda, and by growing regularly gets to have a more or
less flattened or conical shape (e.g. Patella) ; in most, however, it
becomes spiral by growing out unequally, and these spiral forms may
again undergo all kinds of modifications. As the embryonic shells
serve to shelter the whole body, even in those which lose them later
on, we must look for the typical form in them ; from it all the other

THE SHELL OF MOLLUSCA.

333

forms of sliell have branched off. Derived from these we find on
the one hand those which are more highly develop edj and on
the other those rudimentary forms which have the character of
degenerate shells.

§ 259.

The simpler shells of the Cephalopoda must also be regarded as
rudimentary structures,, and not as early forms ; as derived, in fact,
from the more complicated and perfect forms, even if their geological
succession did not indicate that the shell has undergone gradual

Fig. 175. Nautilus; median section of a sliell. Funnel, t Tentacles, v Cephalic
lobes. 0 Eye. 6 Dorsal lobes of the mantle, ll Connections between the shell and
the mantle, s Part of the shell still connected with the right pallial muscle, a Mantle.
s Siphon, s' Siphonal canal of the shell (after Owen).

reduction. Their structural characters, as well as their relations to
the body, that is, to that portion of the dorsal integument which
represents the ^^mantle,^"’ are further instances of the arrangements
which we have already described. We either meet with straight
(in extinct families only) or coiled shells, which are formed by the
mantle, and either completely enclose the animal, or are rudimentary
and contained within the mantle; these latter have lost their signifi-
cance as shells and only form internal organs of support.

The well-developed shells of the Cephalopoda, as seen in the
fossil Ammonites and Orthoceratites, and in the extant Nautilus,

334

COMPAEATIVE ANATOMY.

differ somewliat in structure from those of the Gastropoda and
Pteropoda. They are divided into successive chambers^ the most
anterior of which is alone occupied by the animal^ although the
hinder ones are closely connected with it by means of a tabular
prolongation (siphon) , which is given off by the animal, and traverses
the partitions between the chambers. The animal (cf. Fig. 175)
occupies therefore the last-formed or youngest chamber only. The
separate chambers correspond to an equal number of stages in its
growth ; as each segment of the shell was formed the animal left the
one before occupied ; and as a partition was formed a new chamber
was developed. The arrangement, which was merely indicated, and
rarely seen, in the Gastropoda and Pteropoda, has become typical in
the Cephalopoda. The straight shells of the fossil Orthoceratites,
and those of the Ammonites, which are coiled in one plane, as also
the shells of the Nautilidae, are formed on this type. In these last
a lobe of the mantle (Fig. 175, h) extends from the dorsal side of
the animal over a portion of the shell, the greater thickness of which
appears to be partly due to it. The shell of Spirula is completely
covered in by the mantle, and is similar in character to that of
Nautilus, except that its coils are not in contact with one another;
the shells of the fossil Belemnites are intermediate between those
perfect ones which are only enclosed by the mantle and those
which are placed within it. On account of this the remnants
of shells, which were in all probability largely internal, are of
great morphological value. In them the chambers are found in
a small conical portion only — the so-called phragmocone. The
separate chambers, which form the parts of the phragmocone,
look like horizontal sections of a cone superimposed on one another ;
they, too, are connected with one another by means of a siphon.
The whole phragmocone is surrounded by thickened layers ; but
these are not distributed equally over it, but form a strong solid
process (rostrum) behind its apex. The broadened lamella-like
portion of the thickened layers, which extends forwards over the
base of the phragmocone, is known as the pro-ostracum.-’^ The
phragmocone is the homologue of the chambered shell of the other
Cephalopoda, while the projecting lamella — the so-called pro-ostra-
cum — is a continuation of the wall of the most anterior chamber,
and the massive rostrum, which is generally the best preserved
portion of the whole shell, is derived from the simple thickened
layers, which are formed from that part of the mantle which is
turned over the shell.

The so-called os sepias of the Sepiadae is a flattened shell which
is completely hidden in the mantle ; its posterior tip, however, often
projects, and so calls to mind the shell of the Belemnites. It consists
of several layers, which are rich in organic substances, and are
separated from one another by layers of calcareous deposits in such
a way that it appears to be made up of superimposed lamellaB. The
outermost lamella, which is turned towards the dorsal surface of
the animal, is pretty firm ; it passes directly into the posterior end

THE SHELLS OF MOLLUSCA.

335

of tlie sliellj and forms tlie groundwork for tlie lamellar deposits,,
wliicli often become very thick on the inner face of the shell, the
curve of which is slight. These shells may be directly derived from
those of the Belemnites, especially if we take into consideration those
shells which have a free projecting tip, like the shell of S. Orbig-
niana. The solid tip corresponds to the rostrum of the Belemnites,
while the alveolar cavities of these latter, and the pro^ostracnm, which
is continued on from their dorsal surface, is homologous with the rest
of the shell of the Sepiadm. The partitions which form the chambers
of the phragmocone in the alveoli of the Belemnites are represented
by the flat or slightly concave lamellae of the shell of the Sepiadm.
The layers succeed one another directly, instead of forming separate
chambers. In this way the complicated shell of the Belemnites,
when reduced, may be easily seen to be represented in a lower con-
dition of the shells of the Sepiadae. The shell of the Loliginidm is
still more reduced; it is merely formed by an elongated flexible
horny-blade (calamus), which is placed in the dorsal region of the
mantle. An outwardly projecting carina extends along the middle
line. This rudimentary shell corresponds to the external, curved, and
more highly organic portion of the shell of the Sepiadae, and is there-
fore homologous with the horny-blade of a Belemnite shell. Finally,
in the genus Octopus, where the mantle is not separated from the head
in the region of the neck, we find a pair of thin plates, embedded in
the dorsal integument ; these are the last traces of a shell formed
by the mantle, and are in all respects comparable to those described
as existing in the Gastropoda. Inasmuch as the shell is, even in the
Cephalopoda, formed in the earliest stage by an invagination of the
mantle (Sepia), the internal and external shells are closely allied ; and
at the same time we may see that they are connected with the shells
of other Mollusca.

The shell of Argonauta is to be regarded as altogether different
from all these shells, which are intelligible when closely compared; it is
not secreted by the mantle, but by a pair of arms, which do not lay
down lamellar deposits. In the Gastropoda we met with a special
arrangement by which the so-called operculum ^Lwas formed; we
found this on the dorsal surface of the end of the foot in many
Prosobranchiata, where it served to shut in the animal, when it was
retracted into its shell. The question now arises, may not this
structure be also derived from one plate of the Placophora ?

[E. Ray Lankester, Developt. of the Pond Snail. Quarterly Journ. Microsc. Sci.

1874.]

Branchiae.

§ 260.

The kind of respiratory organs — branchiae — which obtains in the
Mollusca is in correlation with their aquatic habitat ; these are always
differentiations of the integument, and have consequently a

336

COMPAKATIYE ANATOMY.

primitively superficial position, altliougli tliey become covered over
by folds of other regions of the integument (mantle), and so come to
be placed in a special cavity — the branchial cavity.

The function of respiration is part of the duty of the integu-
ment, but it does not seem to be always localised in homologous
regions, so that we cannot regard all the organs which appear to be
gills as morphologically identical. As a rule the gills of the Mollusca
are processes which are placed at the sides of the body, and when
least metamorphosed arise between the foot and the mantle (cf.
Fig. 167, ui B hr). They vary very greatly, not only as to the
extent of the body which they occupy, but also in the way in which
they are connected with different parts. In the Placophora they
merely form a series of folds or lamellae, which encircle the body

between the foot and the mantle,
and appear to be formed from the
epipodiurn (epipodial gills).

In the Lamellibranchiata they
form lamellar organs, which project
between the mantle and the visceral
sac, which ends with the foot, into
the cavity enclosed on either side by
the mantle (Fig. 176, hr hr). Their
free edge is directed towards the
ventral surface.

Almost all the Lamellibranchiata
have two pairs of these gills on
either side, an inner pair, which
are placed mediad, and an outer
pair at the sides of these. The
former are often the larger. Ex-
cept in Anomia, where there are
a large number of other adaptive
modifications, the gills are arranged
symmetrically. Each gill-lamella
is developed from a row of pro-
cesses which bud out close to one another ; in many forms these pro-
cesses remain separate from one another, and form separate branchial
filaments, parallel with each other (Mytilus, Avicula, Area, Pectun-
culus, Pecten, Spondylus). In most, however, the gills lose this
embryonic condition, owing to the concrescence of the gill-filaments.
Owing to this union of the flattened filaments or lamellm, which have
their surfaces directed towards one another, a gill -plate is formed ;
this is either effected by the mere adhesion of the filaments, or by con-
crescence; in the latter case pad-like projections appear on each of
the gill-filaments at regular distances from one another, and these
unite together. As fine clefts are left between these junctions, by
which the water passes through the gills, each plate forms a kind of
lattice-work. These filaments are not simple prolongations, but loops,
so that they enclose a space (intrabranchial space); when the gill-

Fig. 176. Vertical section through an
Anodonta. m Mantle, hr Outer;
fc/ Inner gill-lamella. /Foot. rVen-
tricle. a Auricle, 'pp' Pericardial
cavitv. i Enteric canal.

BEAXCHLE OF MOLLUSC A.

337

filaments grow together this space traverses the whole of the gill-
plate, and communicates with the exterior by means of the clefts
between the filaments. The water which enters by these clefts is
collected into a canal at the point where the plate is attached ; and
is carried by it to the hinder end of the body.

There are chitinous rods in each of the gill-filaments, which form
a special organ of support.

The surface of the whole gill is covered by ciliated epithelium.
Rows of large cilia extend along the ridge-like projections of the
gills; between these there are finer closely-packed cilia, and the
two together keep up a continual stream of water. There is a
groove at the free edge of each gill-plate ; this is formed by depres-
sions on each of the gill-lamellm and is invested by longer cilia ; it
leads to the mouth, and so produces a current of water, which is
well adapted to bring in nutriment.

Great modifications of this system are brought about by the
fusion of the gills of the two sides; this, which obtains posteriorly
to the foot, is either effected by the direct junction of the free edges,
or by the development of a special membrane, which unites the
gills of either side together. This fusion is best seen in the falci-
form curved gill -plates of Anomia, where the whole branchial
apparatus is separated from the greatly-reduced visceral sac, and is
no longer lateral in position.

[R. Holman Peck, The structure of the Lamellibranch gill. Quart. Journal
Microsc. Sci. 1876.]

B ONNET, R., Der Bau u. die Circulationsverhiiltnisse der Acephalenkieme.
Morphol. Jahrb. III.

§261.

The branchial apparatus of the Gastropoda, though greatly
varied in details, is arranged in very much the same way as in the
Lamellibranchiata ; that is to say, it is typically made up by lamellae,
or by more distinctly cylindrical processes, which are arranged
parallel to one another. These project from the surface of the body,
and are, therefore, bathed by the surrounding medium, the water,
while a current of blood passes along them internally. This simi-
larity is still more marked by their position relatively to the mantle,
for they have just the same relation to it as have the gills of the
Lamellibranchiata. As compared with these latter they are less
numerous and more confined ; their structure is comparatively much
simpler. The epipodial gill is arranged circularly in the Placophora,
as it is also in Patella ; but in other Patellidae (Lottia) the two pinnate
branchiae are more distinctly dorsal, so that they seem to be different
from the epipodial gills. Fissurella and Emarginula also have their
two gills placed in the anterior region and below the mantle. In
Haliotis also, they are distinctly paired, but they are placed more to
the left hand of the animal. They are also characteristically modified
in the Zeugobranchia. In the Anisobranchia the left gill is smaller,
and the right one more largely developed ; this arrangement, which is

338

COMPAEATIVE AXATOMY.

distinctly allied to the arrangement in tlie ZeugobrancEia_, is adapted
to the asymmetry of the branchial cavity, which, again, is dependent
on the characters of the shell. The smaller gill is generally approxi-
mated to the other, and becomes asymmetrical in position; in
some Prosobranchiata it disappears altogether (Janthina, Neritacea3,
Heteropoda).

The right gill is generally developed on one side only, so that
it is semi-pinnate, owing to the disappearance of the second row of
lamellae. Although, as a general rule, we find the lamellar structure
to be the most common, a few (Calyptrma, Crepidula) have fila-
mentous gills, and so call to mind the primitive form of the Lamelli-
branch gill.

The gills become modified, and may disappear altogether when
the mantle and the branchial cavity are atrophied. This happens
in various divisions ; thus, in the Heteropoda, among the Proso-
branchiata, the gill of Carinaria is not covered over by the mantle ;
in Pterotrachea, where there is no mantle at all, the gill is quite
free, while in Eiroloi'des, the gill, as well as the mantle, disappears.

Among the Opisthobranchiata the characters of the gills are
equally dependent on the condition of the mantle. There is a gill
on either side, between the mantle and the foot (Pleurophyllidia), or
there is only a single gill in the gill- chamber, or, finally, the gill is
only partly covered over by the mantle (Tectibranchiata) . When
the shell and mantle disappear, gill-like struc-
tures may be developed on the dorsal surface of
the body, as in some of the Nudibranchiata.

Lamellar, or tufted and branched appen-
dages, are sometimes developed in the anal
region (Doris), sometimes in rows over the
whole body (Tritonia, Scyllma). If we are
right in regarding the possession of a shell by
the larvm of all the Opisthobranchiata as a fact,
which proves conclusively that these Gastro-
poda are derived from shelled forms, and if we
must suppose that the primitive position of the
gills was within the mantle -cavity, then we
must regard the arrangement of the gills in
the Dorididae as having been inherited in its
essential features from this condition, for we
must remember that the anus also is placed in
the mantle-cavity. There are many inter-
mediate steps between this and the more
general distribution of gills over the back of
the body ; and further, these gills, howsoever
modified, and howsoever peculiar their form,
are never anything more than mere processes
of the integument. Their varied external form
is due to their superficial position, which is due to the loss of the
mantle which invested them ; and it is because of its absence that they

Fig> 177. Ancula
(Polycera) cristata;
dorsal view. a Anus.
hr Branchiae. t Ten-
tacles (after Alder and
Hancock).

BEANCHIiE OF MOLLUSCA.

339

lose tlieir apparently specific structure^ and get to be more and more
like tlie surrounding integument, of wliicli they form processes varying
greatly in character. Their relations to the circulatory apparatus are
of great significance as bearing on this view, for they so far agree
completely in character with the true gills. Lastly, when most
differentiated, the gills are seen to be distributed over the whole
of the dorsal region of the body, where they form one or more rows
of papillge, or villous processes on either side ; these again may be
branched (^olidiee). The loss of the shell is the cause of the
wider distribution of the gills, just as, on the other hand, the forma-
tion and development of this organ of defence was the cause of the
gills being more limited in extent.

These gills are atrophied in many Opisthobranchiata, when the
whole of the integument takes on the respiratory function (Phyllirhoe,
Elysia, Pontolimax).

§ 262.

Another arrangement of the respiratory apparatus, which is a
modification of the one first described, is due to the development of
the respiratory canal-system in the walls of the mantle -cavity. In
many of the branchiate Gastropoda this network of canals extends
beyond the gills into the neighbouring parts of the branchial cavity,
which are thereby enabled to take part in the respiratory function.
In this way the mantle- cavity is adapted to taking in air, and
becomes a lung. An organ of this kind — ^which is not at all adapted
for those Molluscs that are so organised as to be fit for an aquatic life
— is found in various forms, which belong to very different divisions,
and it is to be regarded as due to a change in tlieir mode of life. A
lung is present, in addition to a gill, in Ampullaria ; in this animal
it forms a sac, which is placed by the side of the gill, and is pro-
vided with a contractile orifice. In the terrestrial genus, Cyclostoma,
the gill has disappeared altogether.

There is a lung in Onchidium, but it is also a renal organ. A
similar cavity is found close to, and has the same orifice as, the
renal organ in the Helicinae and Limacinge ; this functions as a lung.
In the LymnaBidae and Planorbidae, however, the mantle-cavity itself
is adapted to the reception of air. But in these forms the abran-
chiate mantle-cavity also serves as a water-breathing organ, for
many Lymnaeidae are known to live always in deep water.

§ 263.

In the Gymnosomatous Pteropoda either the whole of the integu-
ment (Clio) serves as an organ of respiration, or processes are
developed from its surface which function as gills (Pneumodermon) .
In the thecosomatous forms only do we meet with plaited folds
(Hyalea), which are placed in the mantle-cavity (Fig. 171, A hr), and
so far resemble the arrangements which obtain in the rest of the
Mollusca ; their position is the same as that of the gills in the

z 2

340

COMPAEATIYE ANATOMY.

Ceplialopoda. In these latter the gills arise in just the same way —
between the mantle and the foot (Fig. 171, B hr) — as they do per-
manently in many Gastropoda. When, however, the mantle is
developed they sink downwards, and are then placed in a mantle-
cavity, which, as compared with the Gastropoda, appears to open on
the hinder surface. The gills are arranged symmetrically in all of

Fig. 178. Mantle -cavity and funnel of Sepia officinalis. The mantle-cavity has
been opened by an incision along the middle line. In it is seen the visceral sac
projecting, while posteriorly to it two muscular branches are given off (?n) to the
funnel and head. J5r Branchios. r fer Branchial vein, -y br' Its bulbous enlargement.
t Ink-bag. r Orifice of the excretory organ, opened on the right side, and displaying
at R the venous appendage, g Genital papilla, a Anus. J Funnel, opened by an in-
cision along the middle line, i Tongue-shaped organ, c Depression for the process at
the edge of the mantle (pallial hinge) c'. C Head. P Fins.

them ; there are two pairs in the Nautilus, but only one in all the
other extant Cephalopoda.

As a rule, each gill is pyramidal in form, with the apex directed
towards the side, and its base towards the middle line (Fig. 178, Br).
It either consists of closely-approximated lamellae, which gradually
increase in number at the tip (Nautilus, and most Loliginidae), or
of several much-coiled dermal folds, which arise between the two
branchial vessels which pass to the edge of the mantle (Octopoda).

miERN-AL SKELETON OE MOLLUSCA.

341

In this group the mechanism of respiration is combined with the
locomotion of the animal. Each time that the muscles of the edge
of the mantle relax, water passes into the branchial cavity by its
orifice, that is, at each side of the funnel; after it has bathed
the gills it is driven out again by the contractions of the mantle.
At this moment the cleft of the branchial cavity is closed, so that
the water cannot get out except by the funnel, and this serves not
only as the passage by which the water reaches the exterior, but
also takes an active share in driving it out.

Internal Skeleton.

§ 264.

In most Mollusca the absence of an internal skeleton is compen-
sated by the shells and tests described in § 258; for these serve
as supports for the
internal parts.

Independent in-
ternal organs of sup-
port are, however,
found in the Gastro-
poda. Two, or some-
times four, small
plates of cartilage are
found in the head of
these animals ; they
are surrounded by the
muscles of the pha-
rynx, and are more or
less closely connected
^Yith one another.

They form the sup-
porting apparatus of
the radula and the
parts connected with
it, and also afford
points to which some
of the pharyngeal
muscles, and especi-
ally those of the ra-
dula, are inserted.

Cartilaginous or-
gans of support are
much more highly de-
veloped in the Cepha-
lopoda. The most important one lies in the head, where it serves
as an investment of the nerve-centres, a support for the optic

Fig. 179. Section through the head of Sepia offici-
nalis. IT if' Cephalic cartilages. G Cerebrum, Gan-
glion of the optic nerve, w White body. I Lens, ci Ciliary
body, e Cornea, p Eyelid. P Buccal mass, m External,
u Internal labial membrane. e/Jaws. rEadula. oe CEso-
phagus. t Arms.

342

COMPAKATIVE A^^ATOMY.

and auditory organs, as well as tlie point of origin of a largo
number of muscles. In Nautilus this cephalic cartilage is formed
of two pieces, united along the middle line, and drawn out into
anterior as well as posterior processes ; these surround the com-
mencement of the oesophagus. In the Dibranchiata the cephalic
cartilage is much better developed. It consists of a median portion,
which is traversed by the oesophagus (Fig. 179, K), and of two
lateral processes, which are sometimes mere flat enlargements, in
which case accessory cartilaginous plates are added on to them to
form the orbits ; at other times they are more highly developed,
are then continuous with superior processes [K'), and completely
enclose the orbits. The central nervous system (6r) is placed on
that portion of the cephalic cartilage which is traversed by the
oesophagus.

The Dibranchiata are provided with additional cartilaginous
skeletal pieces. A dorsal cartilage is the most common. In the
Sepiadae this forms a semi-lunar piece, which lies in the anterior
dorsal region of the mantle, and is continued into two small lateral
cornua ; in Octopus, where there is no median pieces, we And the
cornua only.

There is a cartilaginous plate in the neck also, in addition to
two cartilages at the base of the funnel — the hinge -cartilages.
These are less constantly present than those which lie at the base of
the flns, and which are found in all Dibranchiata provided with fins,
for they serve as the point of attachment for the muscles of these
organs.

Muscular System.

§ 265.

We can understand how it is that separate groups of muscles
are so feebly developed, if we bear in mind that there is a dermo^
muscular tube united with the integument and external organs of
support^ and that these^ notwithstanding the great modiflcations
which they undergo, have very much the same character in all cases.
With this may be correlated the absence of internal organs of
support in the lower divisions, and their relatively slight develop-
ment in the higher ones. The muscular system is made up of band-
like flbres, which, not unfrequently, give indications of their greater
differentiation by the possession of transverse striae.

In the Lamellibranchiata the adductor muscles, which pass either
transversely or obliquely through the body from one valve to the
other, are those that are best developed. There are either two of
them, which form bundles separated by some distance from one
another, one anterior (Fig. 167, ma), and the other posterior {m p),
as in Unio or Anodonta; or there is but one muscle, which corre-
sponds to the hinder one of the Dimyaria, and occupies the middle of

.MUSCLES OF MOLLUSCA.

3i3

the shell (Pecteiij Ostrea). Special muscles, which are interwoven
with the integument, serve as retractors of the foot; these arise from
the dorsal portion of the shell, and are sometimes broken up into
several pairs. These retractors are also found again in the shelled
Grastropoda. They generally form a single, but sometimes a double
muscle, which arises from the base of the shell, and which passes to
the anterior regions of the body, increasing in size as it does so.
It supplies the foot as well as the head, and the anterior region
of the digestive tube (pharynx); while further it gives off special
bundles to the other protractile regions, that is, to the tentacles and
copulatory organ. The muscle which arises from the columella of the
shell, and accompanies it, is known as the columella muscle. In the
Heteropoda it has a wide origin in the carinate foot. In the Ptero-
poda it spreads out into the fins given off from the foot. In addition
to these muscles other bundles are given off to the viscera.

The muscular system of the Cephalopoda is much more differen-
tiated, in correlation with the formation of an internal skeleton.
Two powerful retractors are attached to the cephalic cartilage in
Nautilus ; these arise from the sides of the shell-chamber occupied by
the animal. In those Decapoda that have an internal shell these
muscles take their origin from the wall of the outer wall of the shell ;
and in the Octopoda, from a cartilage found at that spot. Two
branches are given off from these two muscles, which pass to the
funnel. Another and much larger pair of muscles arises in the neck
of the animal, and broadens out towards the ventral surface, where
they pass into the funnel. The muscles in the mantle are also
arrange'd in separate layers, as are also the fin-muscles. Lastly,
there is the greatly-developed muscular system of the arms, which
partly arises from the cephalic cartilage, and surrounds a canal
which passes along the axis of the arm.

Nervous System.

Central Organs and Nerves of the Body.

§ 266.

This system of organs also has points in which it resembles that
^of the Vermes. The whole of the central apparatus, that is, is divided
into a superior ganglionic mass, which lies above the commencement
of the oesophagus, the supra-oesophageal or cerebral ganglia, and a
ventral mass which is connected with the other by commissures,
and forms the inferior or pedal ganglia. They are both paired.
The earliest rudiment of the cerebral ganglia is seen as a different
tiation of the ectoderm, the form-elements of which grow inwards,
and are accompanied by the rudiments of the eye (Gastropoda).
The relations between the cerebral ganglia, and the higher sensory

3i4

COMPARATIVE AKATOMY.

organs whicli are placed in tRe Read, prove tliat tRese ganglia
are Romologous witli tlie cerebral ganglia of Vermes (and of
Artliropoda). TRe pedal ganglia may also be derived from a more
simple condition, for in many of tRe lower Mollusca we find tliem
replaced by an arrangement whicR corresponds to tRe ventral cRord
of tRe Annnlata. Longitudinal trunks are given off from tlie pedal
ganglia, and are distributed along tRe foot; since tRey are con-
nected togetRer by transverse chords, they are arranged in tRe same
manner as a ventral nerve-cRain.

AltRougR there may be nothing really fresh in this arrangement
of the nervous system, inasmuch as the two ventral (or pedal) ganglia
must be regarded as a concentrated nervous mass, which is broken
up in lower forms, and constitutes a ventral ganglionic chain; yet the
greatly- developed visceral ganglia form an arrangement which is
nothing like so well marked in the Vermes as it is here. In the
Mollusca the visceral ganglia are not only of importance as forming
a part of the general nervous system, but they may also fuse with the
cerebral ganglia, owing to the gradual shortening of their commissures.
New, and primitively peripherally-placed parts, are thereby added on
to these central organs ; and it becomes a matter of doubt whether
or no these ganglia, wdiich formerly belonged to the visceral nervous
system, should still be regarded as belonging to it. These parts
of the nervous system which supply the viscera (heart, branchial
apparatus, and generative organs) are the cause of great complica-
tions of the whole system ; ownng to the way in which they vary in
number in different divisions, they make comparison very difficult,

as indeed also do the great modifica-
tions in position undergone by the
primitive ganglia, in consequence of
the abbreviation or elongation of their
commissures.

The nervous system of the Placo-
phora is one of the lowest found. A
nervous .band formed of two chords
(Fig. 180, C) surrounds the oesophagus,
but there are no superior enlargements
on it; this is probablydue to the absence
of eyes and tentacles. The inner of the
two chords is continued separately
below the oesophagus ; part meets its
fellow of the other side in the subpha-
ryngeal ganglion and part passes on
to a pedal ganglion (P). Each of these
bilateral ganglia gives off a thick nerve-
trunk, which passes backwards, and
which, like the ganglia themselves,
is connected with the trunk of the op-
posite side by transverse anastomoses set at regular distances ; nerves
are given off to the foot from corresponding points. The outer chord.

Fig. 180. Nervous system of
Chiton cinereus. C Cerebral
nerve-chord. P Pedal ganglia.
pi Pallial nerves, p Pedal nerves.
B Buccal ganglia (after H. v.
Jhering) .

NEEYOUS SYSTEM OF MOLLUSCA.

345

exteiidiug over the oesophagus^ passes along the sides of the body in-
ternally to the branchise, and forms thepallial nerve-trunk {yl). There
is also a group of four small ganglia below the oesophagus (sublingual
ganglia). The two trunks of the pedal nerves are thicker than the
commissures which pass to them from the nervous band ; they must
consequently be regarded as central parts of the system. They seem
to be the longitudinal trunks of a ‘^ventral medulla/^ in which the
ganglionic cells are not definitely distributed into distinct groups any
more than they are in the Gephyrea. The structure of this chord re-
quiresj however, to be further investigated. The principal point
in the arrangement of the nervous system in the Placophora is that
we may recognise in it just the same relation of parts as in the
Solenogastres, and specially in Chaetoderraa (§ 121).

§ 267.

The relatively feeble development of the cerebral ganglia in the
Lamellibranchiata is due to the absence of a head, and its related
sensory organs. These ganglia (Fig. 181, a ) are often placed so
much to the side that there
is a long commissure be-
tween them (Lu cilia, Pano-
pa3a, Anodonta, Unio,

Mytilus, Area, Cardium,

Pholas, etc.). A few smaller
branches only are given off
in addition to the large vis-
ceral commissures. There
are two pedal ganglia in-
stead of the pedal nerve -
trunks, the nerves from
which are distributed in the
ventral portion of the body,
and especially in the foot.

They are placed at the root
of the foot, and are some-
times deeply imbedded in
it. The commissural chords
vary in length in propor-
tion to the development of
the foot, and the distance
between it and the anterior
parts of the body. When
the foot is feebly developed,
or when it is produced very
tar forwards, the cerebral

Fig. 181. Nervous s.ystem of Cytherea
Chione. a Snpra-cesophageal (cerebral) gau-
glion. h Pedal ganglia, c Visceral gaugliou.
d Ganglia of the respiratory tubes (si phonal
ganglia), ma Anterior, mp Posterior adductor
of the shell, p Foot, t Edge of mantle, t' Nerve
to edge of mantle. hr Branchiae, i Enteric
canal, li Liver, r Hind-gut. tr Respiratory
siphon, ta Cloacal siphon (after Duvernoy).

and pedal ganglia may be markedly approximated (Solen, Mactra).
They may indeed come to be directly approximated, as in Pecten
(Fig. 1 82, r), where the smaller pedal ganglia are placed between

34G

COMPx\.RATIYE AXAT0]\IY.

tlie cerebral ganglia (a), wbicli are connected together by a deeply-
curved commissure. The extent to v/hich the pedal ganglia are
developed is dependent on the development of the foot. As a
rule they are intimately connected_, though always separate. The
peripheral nerves of the cerebral ganglia are principally dis-
tributed to those parts of the body which lie nearest the mouth_,
while they also give olf branches to the mantle. In some forms
these pallial nerves (Fig. 181; f) have the appearance of two large
trunks, in which case they are connected with other nerves given
off from the visceral ganglia at the edge of the mantle; and the
connected trunks either form a single larger marginal nerve, or

a regular plexus of nerves. The
visceral ganglion is often the largest
of the whole nervous system. It
lies behind the posterior adductor
(Fig. 181, c; Fig. 182, c), and is
connected with the cerebral ganglia
by long commissures. We may
regard this ganglion as consisting’
of two halves connected together
by a short transverse commissure,
which become more or less ap-
proximated, and finally form a
single quadrangular knot, accord-
ing as the bilateral gills of these
animals are free or fused together.
This fact points to the relation be-
tween this ganglion and the gills,
which indeed is made still clearer
by the large nerve-trunks which
Fig. 182. Nervous system of the are given off from it, and which

LamelUbranchiata. A Of Teredo, innervate the brancliiK. In addi-
B Of Anodonta. C Of Pecten. i i i i .

a Supra-oesophageal (cerebral) ganglia. tion to the branches wllich gO to
h Infra-oesophageal (pedal) ganglia. the neighbouring portions of the
c Branchial or visceral ganglia, mantle, there are two other large

nerves which pass to the edge of
the mantle in many Lamellibranchiata, and take part in forming the
above-mentioned plexus.

When siphons are developed, large nerves are given off from the
visceral ganglia, which do not only ramify all along the respiratory
tubes, but also go to form a special ganglion, which is placed at the
base of the siphons (Fig. 181, d) (So'leu, Mactra, Mya, Lutraria,
Cytherea, etc.). Very little is known about the nerves which pass
from the visceral ganglia to the various organs of the body. Such
nerves have been observed in Pinna and Anomia, as well as in Area
and Solen, where they arise either from the ganglion or the commis-
sures. It is a difficult matter to compare the chords which pass to the
visceral ganglion (cerebro- visceral commissure) with the pallial nerves
of the Placophora, because of their relation to a ganglion ; on the

C- li / A

NEEVOUS SYSTEM OF MOLLUSCA.

347

otlier liand^ the same nerves in ChaBtoderma (§ 121) are connected
with a terminal ganglion.

The nervous system of the Scaphopoda is almost exactly similar to
that of the Lamellibranchiata.

Duvernoy, Sur le systeme nerv. des Moll, acephales. Mem. Acad, des Sc. Paris.
T. XXIII.

§ 268.

Owing to the distinctness of the head, and the development in it
of several sensory organs, which are often very highly differentiated,
the nervous system of the Gfastropoda is distinguished broadly from
that of the preceding divisions by the larger size of its cerebral
ganglia. These are connected with
the ventral parts of the system by
a commissure ; in the lowest of the
Prosobranchiata — the Zeugobran-
chia — the nervous system has many
points of similarity with that of the
Placophora. For instance, a rudi-
ment of a ventral ganglionic chain
can be made out in Fissurella
(Fig. 183) and Haliotis, inasmuch
as the nerves from the pedal ganglia
— pedal nerves — are united by
transverse commissures. The pallial
nerves are not given off directly
from the cerebral ganglia, but
run below the pedal nerves, and
appear to arise from a nervous
mass common to themselves and
these latter. The epipodium is
supplied by the branches of the
pallial nerves [fl). Double commis-
sures pass from the cerebral ganglia
to the ganglionic mass, which gives
off these nerves; one of them is
connected with the pedal nerve-
trunks, and the other with the
pallial nerves, that is, with the vis-
ceral nerves given off from the same
point. In the rest of the Proso-
branchiata there are no indications

whatever of any ventral ganglionic chain. There are pedal ganglia
in which the ganglionic elements, distributed along the nerve-trunks
of Haliotis and Fissurella, are distinctly concentrated. At the same
time, the relations of these nerves must be more exactly investigated.
A ganglion seems to be given off from the secondary pedal ganglia,
and to pass laterally into the commissures, and so to be as much

Fig. 183. Nervous system of Pissu-
rella. C Cerebral ganglia, cs Their
commissure, 'p Pedal nerves, pi Pallial
nerves. A Visceral ganglion. B Buc-
cal ganglia (after H. v. Jheriug).

348

COMPAKATIVE AXATOMY.

connected witli the cerebral as with the secondary pedal ganglia.
It gives off the pallial nerves ; these undergo degeneration in pro-
portion to the extent to which other nerves are given off from the
same commissural ganglia.

In some of the Prosobranchiata some of these nerves are remark-
able on account of the course that they take. They are present
in Haliotis, where they pass off from the* common pedal ganglionic
mass (the pallio-pedal ganglia). In other cases they are given off
from the commissural ganglia (Fig. 184, Co). A nerve is given off on
the right side which passes above the visceral
mass to a ganglion, which supplies the body-
wall (supraintestinal ganglion) {s]j). From the
left commissural ganglion a nerve passes below
the viscera to a subintestinal ganglion {sh),
which, like the former, is connected by a com-
missure with an abdominal ganglion (A).

The two nerves, therefore, which are given
off from the commissural ganglia cross over
one another ; this peculiarity — namely, that
the right nerve passes to the left, and the left
to the right side — makes it difficult to explain
the genesis of these nerves. It is probably
due to changes in position, which have not
affected the internal parts only, for the ganglia
on these nerves give off branches to the body-
wall. Although a large number of the Proso-
branchiata are distinguished by this crossing
of the nerves (Chiastoneura), it does not ob-
tain in another division, in which the commis-
sure to the abdominal or visceral ganglion
takes a straight course backwards (Ortho-
neura), except when the ganglion appears to
be fused with the right commissural ganglion
(Nerita). The commissural are generally separated from the pedal
ganglia, and, in the Heteropoda, are widely so (Carinaria), and in
this case the commissures are proportionately elongated. The
same thing happens also to the cerebro-pedal commissures in the
Heteropoda.

There is a commissure between the pedal ganglia in the Tecti-
brauchiata, which are consequently pushed more to the sides of the
body. The primitive visceral ganglia are also placed at the sides, or
between the pedal and cerebral ganglia (Umbrella, Gasteropteron) ;
there maybe commissural ganglia present, just as there are typically
in the Prosobranchiata, which send off connecting chords to one or to
a pair of ganglia which supply the gills; the ganglia appear to
correspond to the above-mentioned abdominal ganglion (Aplysia,
Acera) .

The pedal ganglia are still more widely separated from one
another in the Nudibranchiata, and are thereby approximated to

Nervous system
of Paludina vivipara.
C Cerebral, P Pedal,
Co Commissural gan-
glia. B Buccal ganglia.
A Abdominal ganglion.
sp Supra, sh Subintes-
tinal ganglion, p Pedal
nerves, o Otocyst (after
II. V. Jhei'ing).

NERVOUS SYSTEM OE MOLLUSCA.

3T9

tlie cerebral (Fig. 185) ; the visceral ganglia also get to be connected
with these latter owing to the abbreviation and complete disap-
pearance of the commissures between them (^olidia). A dorsal
plexus of ganglia_, which lies on the pharynx, is formed by the more
intimate fusion of these ganglia ; in each half of this plexus the
various ganglia which compose it can be more or less distinctly
made out ; several commissures are given off, which surround the
pharynx (Doris, Tritonia). This connection of the ganglia of either
side may lead at last to the complete fusion of the ganglionic masses
of either side into a single one, and in correlation with this fusion
the previously separate commissures may be represented by one
only (Tethys). This appears to be a lower stage, but it must not
be regarded as an early arrangement,
but as the final one in a series of diffe-
rentiations, exactly similar to what we
found to obtain in the Arthropoda. In
the same way as the nerves, which
pass off from the single nervous mass,
indicate that it is made up of separate
portions, the circum-oesophageal com-
missures prove that the ventral ganglia
have become more dorsal in position.

The nervous system of the Branchi-
opneusta is similar in many points to
that of the Tectibranchiata, and similar
relations can also be made out in the
Both divisions are
the development of
glia into several por-
be distinguished as
such even from the exterior.

§ 269.

The genetic relations of the nervous
system of the Pteropoda to that of the
Gastropoda may be seen in the Gymnosomata ; the Thecosomata
differ in the length of their cerebral commissures. The cerebral
ganglia either retain their lateral position or^ approach the pedal
ganglia with which the visceral ganglia are also fused. The central
ganglionic mass lies therefore below the pharynx. The arrangement
in the Gymnosomata is more primitive; the cerebral ganglia are
placed superiorly, and visceral ganglia are evidently present. The
pedal ganglia always innervate the fins formed from the foot. In
the Gymnosomata the cerebral ganglia give off considerable nerves
to the tentacles, at the base of which they fgrm ganglia. The
ganglia of either side are connected with one another by commis-
sures (Clio).

The three ganglionic masses already pointed out in the Mollusca

N ephropneusta.
characterised by
their cerebral gan
tions, which can

Fig. 185. Central nervous system
of one of the -(Eolidiae (Fiona
atlantica). A Supra-CBsopliageal
mass, formed by the anterior cere-
bral, and posterior visceral or
branchial ganglia. B Pedal gan-
glia. C Buccal ganglia. D Gastro-
oesophageal ganglia. a Nerve to
the superior (hinder) tentacles.
b Nerve to the inferior tentacles,
c Nerve to the generative organs.
d Pedal nerves, e Commissure of
the pedal ganglia, e' Commissure
of the visceral ganglia (after R.

Bergh).

350

COiMPArvATIYE ANATOMY.

are also found in the Cephalopoda ; but they are more closely
approximated in consequence of the shortening of their com-
missures. They form a ring around the oesophagus. In the
Dibranchiata this ring is enclosed by the cephalic cartilage in
such a way that the peripheral nerves pass out through foramina
in it.

In Nautilus, the upper part of the oesophageal ring is formed
by a transversely disposed nervous mass (Fig. 186, aa)) the nerves

of the higher sensory organs are given off
at the sides. It corresponds to the cerebral
ganglia, but these also extend some distance
ventrally (bh), and give off, in that region,
nerves which pass to the tentacles {tt').
The ventral segment only of this ring is
homologous with the pedal ganglion, as it
gives off the nerves for the funnel. A
second ventral mass (cc) is attached to the
lower nervous mass, which, as we have
already said, is partly formed from
the cerebral ganglia ; this corresponds to
the visceral ganglia, and gives off nerves
for the mantle {m), in addition to two small
trunks which accompany the vena cava, and
pass to the gills and vascular system. Each
of these two nerves forms a ganglion (d),
which, again, supplies the generative appa-
ratus.

In the Dibranchiata the nervous system
is much more concentrated. The cerebral
ganglionic masses are placed much more
to the sides and ventral surface, so that
they unite in the middle line in front of
the part which corresponds to the pedal
ganglia. The visceral are completely fused
with the pedal ganglia, and the only rem-
nant of the primitive independence of these
ganglia is a narrow point through which
the pedal artery passes ; the separate character of these ganglia
is much more distinct in the Tetrabranchiata. That portion of
the cerebral ganglionic masses which still remains above the oeso-
phagus is differentiated into several portions. The part which
has reached the ventral surface gives off the nerves for the arms,
and these unite to form a ganglion at their base. The pallial
nerves are given off from the visceral masses, and each of these
nerves forms a large ganglion (ganglion stellatum).

Compared with the rest of the Mollusca, the Cephalopoda have
much more highly differentiated central organs from a histological
point of view ; this is due to their greater size. In all parts of the
nerve-centres it is possible to distinguish an outer gray layer, formed

Fig. 186. Nervous system
of Nautilus pompilius.
a Superior, h Inferior gan-
glionic masses of the oeso-
phageal ring. c Visceral
ganglia, d Genital ganglia.
m Pallial nerves, t t' Ten-
tacular nerves (after Owen).

NEEVOUS SYSTEM OF MOLLUSCA.

351

of ganglionic cells, from tlie inner wliite medullary substance, wbicb
is formed principally of fibres.

Cheron, Kecli. p. servir a Thistoire du systeme nerveux des Cephalopodes
dibranchiaux. Ann. sc. nat. Y. Ser. T. Y.

OwsjANNiKOW und Kowalevsky, Ueber das Centralnervensystem und das Gehor-
organ der Cephalopoden. Mem. Acad, de St. Petersbourg. YII. Ser. T. XI,

Visceral Nerves.

§ 270.

Owing to tlie relations between the visceral nervous system and
the central nerve- organs, we were obliged to consider parts of this
visceral system while dealing with the central one ; and we had to do
with an example of the difference in the significance of the central
organ, when the ganglia which belong to the peripheral parts
become a part of it. In addition, however, to this hinder part of
the visceral nervous system which is united to the nerve-centres, and
which is principally distributed to the circulatory and excretory
organs, as well as to the genitalia and gills, there is another portion
which innervates the digestive canal.

In the Lamellibranchiata fine filaments arise from the cerebral
ganglion and surround the mouth ; these are the earliest signs of a
portion of the nervous system, which is still more differentiated in
the Gastropoda. The development of complicated mouth-organs
appears to be correlated with the development of this system. Two
nerves arise from the cerebral ganglion and pass to ganglia, which
are placed on the buccal mass, and are connected together by a
commissure. These buccal ganglia (Fig. 183, B; Fig. 184, B)
supply the organs of the mouth, and give off nerves to the gut. The
commissures differ a good deal in character. As a rule the ganglia
do not fuse. The same arrangement is found in the Pteropoda ; and in
Nautilus, among the Cephalopoda, the two buccal ganglia are con-
nected at their side with pharyngeal ganglia, and are connected by
commissural chords with the cerebral ones. There is but one buccal
ganglion in the Dibranchiata, and behind it there is a large supra-
pharyngeal ganglion (Sepia).

The nerves given off from the buccal ganglia have various small
ganglia on their enteric branches.

Sensory Organs.

Tactile and Olfactory Organs.

§ 271.

The sensory organs of the Mollusca are very similar to those of
the Vermes. All parts of the body, with the exception of the hard

352

COMPARATIVE AAEVTO:\rY.

ones, are capable of feeling when toucbed ; the anatomical arrange-
ments for this sense are found in various parts of tlie body, through
which they are more or less widely distributed ; they have the form
of fine setiform prolongations from cells, which can be seen to be
connected with nerves. These structures are most well marked on
those parts of the body which function especially as tactile organs ;
they are generally supplied with nerves of some size, and form the
tentacles.

They are very common on the edge of the mantle in the
Lamellibranchiata., where they 'are either found all over it, in
which case they are arranged in several rows (as in Mactra, Lima,
Pecten, etc.), or sometimes they are confined to certain regions ;
they are not unfrequently found on the siphons, and in either case
they serve to watch over the particles that get into the mantle-cavity
with the water. They are highly contractile, and are supplied with
filaments from the marginal nerves of the mantle.

The processes found on the epipodium, and on the edge of the
mantle in many Gastropoda, as well as the dorsal cirri of the Nudi-
branchiata, may function as organs of this kind.

It is doubtful whether the pair of lobes at the sides of mouth of
the Lamellibranchiata are organs with this function, but on the other
hand we find a very large number of tactile organs of this kind on
the cephalic tentacles which are so commonly present in the Gastro-
poda. The tracts, which carry the nervous end-organs, are very
often specially differentiated in these animals.

Although there is not much difficulty in the view that the above-
mentioned organs function in the perception of tactile impressions,
it is almost impossible to say what is the physiological duty of a
number of other organs, w^hich are clearly sensory, and are connected
with the integument. These enlargements are generally formed by
ciliated regions to which a nerve passes, and at which it often forms
enlargements. It is doubtful what part of the surrounding medium
acts on these organs, and we have to make a somewhat far-fetched
analogy to be able to regard them as olfactory organs.

In the Gastropoda they are found near the respiratory organs,
and in the Heteropoda I found them very widely distributed in this
region. The same I found to be the case in the Pteropoda. In the
Gymnosomatous forms of this division a ciliated organ of this kind
is placed superficially, and close to the gills ; in Pneumodermon it is
wheel-shaped. In the shelled foians it is a transverse ridge, which
is placed in that region of the mantle-cavity by which the water
l^asses to the branchiae.

In the Opisthobranchiata the hinder pair of tentacles (rhinophor)
appear to have the function of an olfactory organ ; in correlation
with this function they vary greatly in character, and the surface of
these tentacles may be seen to be increased by the formation of
ridges and various other arrangements. They seem to be in all cases
ciliated., If we r effect on the fact that respiration is largely carried
on by organs which arise from the back of the animal, we shall see

VISUAL OEGANS OF MOLLUSCA.

353

that the tentacles, when functioning as olfactory organs, have just
the same relations as the respiratory ones ; and with this we may
correlate the great distance backwards at which the tentacles are
sometimes found.

In the Cephalopoda the olfactory organs are more definite in
character. Behind the eyes we find two small pits, or papillae, which
are level with the surface ; these are ciliated. The processes of the
more deeply-placed olfactory cells stand up between the ciliated
ones ; they are supplied by a nerve which arises close to the optic
nerve.

Visual Organs.

§ 272.

Visual organs are found in all Mollusca that are endowed with
the power of active movement. On the other hand they are always
atrophied in the fixed forms, although they are present during the
larval stages. This is also the case in the Placophora, where the
larvae indicate by the possession of a pair of pigment spots that they
are provided with eyes, which are atrophied later on.

Structures of this kind are found on the nerve-centre, and in the
head of the Lamellibranchiata during the larval stages; they are
even provided with a refractive body, but they undergo degeneration
later on. The same thing happens in the Scaphopoda.

It is different with the organs, which are ordinarily found on
the edge of the mantle, in the higher, divisions of many Lamellibran-
chiata. They are carried on special optic-stalks (Area, Pectunculus,
Tellina, Pinna, etc.) ; in many cases (Pecten, Spondylus) they were
even observed by the older investigators, owing to the emerald-
green colour of the tapetum, which is found at the base of the eye.
Although these organs have many peculiarities of structure, they
agree in all essential points with the optic organs of the rest of the
Mollusca. They are supplied with nerves by the small trunks which
pass along the edge of the mantle. These organs vary greatly in
the extent to which they are developed, and are sometimes replaced
by mere pigment spots. This arrangement must be regarded from
that point of view, on which we have already insisted, when we
pointed out that sensory organs might be differentiated from simple
nerve-endings at any point of the integument ; these eyes, therefore,
on the edge of the mantle, can only be compared with the optic
organs which are found on the head, from a physiological point of
view ; morphologically, they are special structures, and are, like the
similar organs found in Vermes, due to adaptation.

We never find more than one pair of cephalic eyes in the Gas-
tropoda. They are often replaced by mere spots, placed on the
supra-oesophageal ganglion, and disappear altogether when the
animal loses its power of free movement (Vermetus). When it is

354

COMPAEATIVE AA^ATOMY.

simplest tlie eye lies beneatli tlie integument (as in many Opistlio-
brancliiata) . In others it is embedded in the dermo-muscular
tube^ and retains its superficial positioUj while at the same time
an elongation of the optic nerve is developed. This sub-integu-
mentary position must be regarded as a secondary one, for in the
Mollusca, just as in the Vermes, the integument takes part in the
formation of the eye. The eye-bearing region of the body is
ordinarily found to be the base of the tentacle (Prosobranchiata),
which may be converted into a special eye-stalk (ommatophor).
Or the eye may rest on a process formed from the tentacle
(Strombus, Pterocera), or this process may become separated
from the tentacle, and become independent. Owing to the pos-
session of this optic-stalk the eye is capable of movement; in
the Heteropoda this is effected by the muscles that are attached to
the wide capsule which encloses the bulb of the eye (Fig. 187, o).
Thanks to these muscles the bulb is enabled to vary its position ; in

form, the bulb is generally
rounded or oval ; in the
Heteropoda it is very pecu-
liar (Fig. 187).

The bulb has a thin outer
covering, which passes an-
teriorly into the cornea
(pellucida), which is formed
from the integument. In
the posterior portion the
optic nerve widens out, and
is generally provided with
a ganglionic enlargement
(r). Internally to it we find
the retina, with the end
organs of the optic nerve ;
these constitute a layer of rods which are turned towards the cavity
of the eye, and are separated by a layer of pigment from the
external layer of the retina. Behind the cornea there is a lens, which
either fills up the cavity of the eye, or has a gelatinous substance
posteriorly to it, which represents a vitreous body.

The sensory layer is formed from the ectoderm, and the lens also
is an integumentary structure, inasmuch as it is developed from a
cell, which gradually secretes the substance of the lens in stratified
layers.

§ 273.

The eye of the Cephalopoda resembles, in many points, the optic
organ of the Gastropoda. We know that it is gradually differen-
tiated from the ectoderm. In Nautilus, each bulb is carried on a kind
of optic stalk, and forms a lateral projection (vide supra. Fig. 175, o) ;
this is indicated in some of the Hibranchiata, but in them the bulb
may be supported by processes of the cephalic cartilage, and lie in

/

Fip;. 187. Upper part of the nervous system,
with the sensory organs of Pterotrachea.
fjs Cerebral ganglia (Cerebrum) . c Commissures.
0 Optic capsule. 1 Lens, cli Pigment layer,
r Ganglionic enlargement of the optic nerve.
a Auditory organ.

VISUAL OEGAUS OF MOLLUSCA.

355

a kind of orbital cavity. In Nautilus the optic capsule is continuous
with the stalk ; in the Dibranchiata it is placed in the cartilaginous
orbits^ where it encloses the ganglionic enlargement of the optic
nerve (Fig. 188^ go), which is represented in Nautilus by a layer
which extends over a larger portion of the bulb. Anteriorly, the
optic capsule forms a thin investment (0), which is known as the
cornea ; and the refractive media of the bulb are placed behind it.
This cornea is absent from the eye of Nautilus, as is the lens. In
front of this cornea the optic capsule is therefore directly continuous
with a mem-
brane which is
connected with
the integument
of the optic
stalk, and which
has a pupil-like
orifice which
leads into the
interior of the
bulb.

In the Di-
branchiata this
direct communi-
cation between
the internal
cavity of the
bulb and the
surrounding me-
dium is broken
by a lens {L) ;
as, however, in
many forms (Loli
gopsis, Histio-
theutis, etc.), the
transparent por-
tion of the optic
capsule is alto-
gether absent or

is perforated (Sepia, Loligo, Octopus), the anterior surface of the
bulb enclosed in the capsule is bathed by water. This space, which
communicates with the exterior, is not only continued through the
optic cleft as far as the lens, but also extends more or less around
the bulb.

In many the integument is thrown into folds around the cornea ;
these folds form eye-lids,^^ and are either limited in position or
extend around a larger portion of the bulb, when they form a defensive
organ for the eye by the possession of occlusor muscles.

The base of the bulb is formed by a cartilaginous capsule
(Fig. 188, 7f) ; around the pupil this becomes converted into the

2 A 2

Fig. 188. Horizontal section of the eye of Sepia (Diagram-
matic). KK Cephalic cartilages. C Cornea. L |Lens.
ci Ciliary body of the lens. Ri Internal layer of the retina.
Re External layer. P Layer of pigment, o Optic nerve.
go Ganglion. 7c Pupillary cartilage, ih Cartilage of the iris.
w White body, ae Argentea externa (after Hensen).

356

COMPAEATIYE ANATOMY.

cartilage of tlie iris (iJi). Outside and behind this optic capsule is the
ganglion of the optic nerve, in the periphery of which there is a
whitish organ {w) which projects more or less forwards. Behind
this there is a longitudinal layer of muscles, and lastly, a silvery
membrane (argentea externa) {ae), which reaches as far as the
edge of the pupil, and invests the bulb on its inner face, or
the one turned towards the above-mentioned cavity. Internally
to it there is the argentea interna. The bundles of nerves
wdiich arise from the ganglion {go) behind the cartilaginous cap-
sule, pass to the retina by a large number of pores in it ;
the retina lies within the cartilaginous capsule, and is con-
tinued forwards as far as the edge of an organ which carries the
lens. This retina is formed of essentially the same layers as the
same portion of the eye of the Grastropoda ; an internal (Ri) portion,
which contains the perceptive apparatus, is separated from the
external part {Re) by a layer of pigment (P). A layer of connec-
tive tissue extends inwards to the lens (P) from the layer of muscular
fibres ; this completely separates the eye into two parts, an anterior
smaller and a posterior larger one ; these unite to form an oval
body, the long axis of which corresponds to the optic axis. There
are epithelial thickenings on the anterior, as well as on the posterior,
surface of this layer of connective tissue ; they unite to form a
system of lamellae, which invests the edge of the lens, and is known
as the ciliary body {cl) (corpus epitheliale) . The cavity behind
the lens is filled up by a fluid.

Hensex, Zeitschr. f. wiss. Zool. Bd. XV.

[Laxkester, E. Ray, Develt. of Cephalopoda. Quart. Jouru. IMicrosc. Sci. 1875.]

Auditory Organs.

§ 274.

It is possible, apparently, to derive the organs, which seem to be
auditory in the Mollusca, from the vesicles or otocysts which we met
with in the Yermes. They contain otoliths, and
their inner wall is provided with nervous end-organs,
which are formed by modified epithelial cells. These
cells are derived from the ectoderm, for the otocyst
itself is formed from that layer ; this has been ob-
served in the Gastropoda. What we know of the
development of the ear in the Cephalopoda agrees
with this statement.

The auditory nerve is generally given off from
the cerebral ganglion. In its most primitive form
the auditory vesicle is attached to this ganglion, and
it is only when it is separated from it that we are
able to make out a true auditory nerve. The otocyst,
however, varies'greatly in position; thus, it may be attached to the
pedal ganglion; but even then the auditory nerve can always be

Fig. 189. Audi-
tory orgau of
Cyclas. c Au-
ditory capsule,
e Ciliated epithe-
lial cells. 0 Otolith
(after Leydig.)

AUDITOEY OEGAXS OF MOLLUSCA.

357

followed up to tlie cerebral ganglion ; it sometimes passes along the
cerebro- pedal commissure. This change in position is sometimes
associated with a change in the position of the cerebral ganglia
themselves.

In the Lamellibranchiata and Scaphopoda the auditory vesicles
are attached to the pedal ganglion ; they are either close to, or at
some distance from it (Naiades), or are even placed more deeply in
the substance of the foot (Cythera). In the Gastropoda the
otocysts vary greatly in position ; but the primitive position — that
is,, close to the cerebral ganglia — is the most common one; in the
Heteropoda, and many Opisthobranchiata, they are always placed
close to these ganglia.

The otoliths are either numerous, and consist of small crystalline
structures, which form an otoconia, or there is but a single spherical
otolith, which is derived from one cell of the rudimentary wall of
the auditory vesicle, and which forms a concentrically-striated con-
cretion. Dentalium, the lower forms of the Lamellibranchiata, and
Gastropoda, and also all the Pteropoda, possess otoconia. In the
larval stages, however, of these Mollusca, the otoconia first appear,
and then, later on, the spherical otolith ; this apparently disappears
afterwards. But when the adult form is provided with an otolith,
an otolith is found in the larva, and is never preceded by the
otoconia.

We have not as yet any connected statements about the struc-
ture of the end-organs in the otocyst. The most important fact
that we do know is that part of the ciliated epithelium is repre-
sented by cells with fine rod-like processes, which appear to be
true auditory fibres. They form the auditory organ, and are con-
nected with the nerves, while the ciliated cells, which are grouped
in tufts, form an accommodating arrangement which acts on the
otoliths.

We have a permanent proof that the otocyst is developed
from the ectoderm in the Cephalopoda in the presence of a fine canal,
which, in many of them, leads from the auditory vesicles to the
surface of the body. In Nautilus the otocysts lie on the cephalic
cartilage; in the Dibranchiata they are enclosed by it. It is
possible, therefore, to distinguish a membranous and a cartilaginous
labyrinth, analogous to the similar parts found in the case of the
Vertebrata.

In the Octopoda the auditory vesicles are simple in form ; in the
Decapoda they are complicated by the formation of diverticula and
processes. At the same time they are more closely connected with
the cartilage, whereas, in the Octopoda, they lie somewhat freely in
their cavity. The otolith, which is placed in a watery fiuid, varies
in form — it may be flattened or rounded, or may be broken up into
smaller acicular pieces. The terminations of the auditory nerves
either form the auditory plate, which is a thickened portion of
the epithelium, from which the cells send hair-like processes (audi-
tor}^- hairs) (Sepia); or an auditory crest,^^ which generally takes

358

COMPAEATIVE ANATOMY.

a curved direction, and wliicli is likewise covered by modified
epithelium.

Lacaze-Duthiers, Otocystes dcs Mollusques. Arch, de Zoologie. I. p. 97.—
Ranke, J., Das Gehoi’organ etc. bei Pterotrachea. Zeitschr. f. wiss. Zool.
XXV. Suppl. — V. Jeering, Die Gehorwerkzeuge der Mollusken. Erlangen,
1876. — SiJiROTH, Ueber die Siunesorgane unserer einheim. Weichthiere.
Zeitschr. f. w. Zool. Bd. XXVI.

Alimentary Canal.

§ 275.

Owing to the complete separation of the wall of the body from
that of the enteric canal in the Mollusca, the latter canal comes to
be embedded in a coelom. It forms coils or loops in this cavity, as
it is always longer than it ; at the same time the characters of its
anal opening are well worthy of note. We find, indeed, that it is
only in the Placophora and Lamellibranchiata that it traverses the
body in such a way that the aboral and anal ends of the body are the
same. In the Scaphopoda, Gastropoda, Pteropoda, and Cephalo-
poda it always ends at some distance from the aboral end of the
body; it is looped or coiled. If we suppose that the gut was
primitively arranged in a symmetrical manner, and that the anus
was placed in the aboral region, and that, therefore, this change in
the position of the anus was acquired afterwards, then we must also
suppose that this arrangement obtained at a very far-distant period,
inasmuch as there are no signs of it in the development of the indi-
vidual. The real cause of this change
in position must be sought for in the
universal possession of a shell. The
development of the dorsal mantle and its
shell, together with that asymmetrical mode
of development of both of these structures
which obtains in most Mollusca, makes it
easy to see how they affected the organism.
We must distinguish two modes of develop-
ment. In the first, the dorsal development
of a part of the body which was protected
by the shell, increased the space available
for the growing gut, which formed more
or less complicated loops or coils. At first,
of course, this only enabled it to depart
from its straight course. In the second, the
development of a mantle- cavity, the appear-
ance of which was correlated with that of the mantle and shell,
affected the position of the gut. When the mantle-cavity is
developed in the hinder region of the body, as it is in the Thecoso-
matous Pteropoda and the Cephalopoda (Fig. 190), the position of

Fig. 190. Diagram of the
relations of the enteric
canal. A In Pteropoda.
B In Cephalopoda,
p Foot, t Arms or Tentacles.
hr Branchiso.

ALIMENTAllY CANAL OF MOLLUSCA.

359

tlie anus is^ relatively speaking, least altered. It may still lie
more or less in the middle line. When the branchial cavity is more
anterior in position, and asymmetrical — as it is in most of the shelled
Gastropoda — the anus approaches it, inasmuch as its function is
here least interfered with.

The enteric tube is divided into separate portions in just the
same way as in Vermes.

Although embryological inquiries have not yet arrived at any
very definite results, this much seems certain : the mid-gut is
derived from the endoderm, and the fore-gut from the ectoderm.
The hind-gut is laid down with the mid-gut, and has therefore the
same history as it has.

§ 276.

As to the characters of the various divisions, we find that in the
Placophora the digestive tube is coiled several times; but the anus
still retains its aboral position, inasmuch as the above-mentioned
causes, which lead to a change in position, do not obtain in this group.

The Lamellibranchiata likewise retain the most primitive relation
of parts. The mouth, which is placed in the Dimyaria between
the foot and the anterior adductor, is continued into a short
portion which functions as the oesophagus; this passes into a
widened portion, or stomach. The efferent ducts of the liver
open into this gastric mid-gut. In many Lamellibranchiata this
stomach is remarkable for the possession of a csecal diverticulum,
which is often of some size, and can be shut off by a valve ;
this is placed in the pyloric region. In many forms we meet
with a peculiar structure in the caeca, or, when they are absent,
in the enteric canal itself; this, which is known as the ^^crystal-
line style,^^ is to be regarded as a secretion from the enteric
epithelium. The hind-gut, which forms by far the largest portion
of the whole tract, makes one or more coils, and then passes towards
the back of the animal ; as a rule it is of the same calibre through-
out, but it is sometimes differentiated into narrower and wider tracts.
It is closely surrounded by the other organs (liver, generative
glands) of the visceral sac ; its terminal portion passes nnderneath
the middle-line of the shell as far as the hinder margin of the body ;
in a large number of Lamellibranchiata it traverses, as it does so, the
pericardium and heart (Fig. 176, i), and then ends, behind the
posterior adductor, at the anus ; this is placed on a process at the
aboral end of the body, which projects freely into the man tie-cavity
(Fig. 181, r). Hei’e again the position of the anus is correlated with
the characters of the shell, which is in the form of two lateral valves.

§ 277.

In all Mollusca^ except the Lamellibranchiata^ part of the fore-
gut is differentiated into the so-called pharynx or buccal mass;
this is generally a large organ, the structure of which is well adapted

3G0

COMPAEATIYE ANATOjMY.

to its function ; this function is to seize and comminute the food
of the animal. The apparatus^ which forms the chief portion of this
arrangement, is a cuticular membrane which rises up from the lower
wall ; on it there are small teeth or hooks^ which are directed back-
wards and arranged in transverse rows. The teeth vary very greatly
in arrangement (Fig. 101 ^ a r r/)_, form, and number; they are not

only different in the larger di-
visions, but also in the orders
and families, and even in the
species, so that the genetic affi-
nities of the groups is indicated
by the form of these parts ; and
for this reason they have been
much used in classification. As
a rule they consist of a median
longitudinal row (a), with which
symmetrically - arranged lateral
denticles are connected. The
organ formed by the whole of these booklets is known as the radula.
In many (Turbo, Patella) it projects some way into the coelom, and
is enclosed by the sac-like sheath formed by a diverticulum of the
wall of the oesophagus ; it may even be longer than the body itself.
This organ may become very broad, and reach to the sides of the
pharynx. In the Heteropoda it is so far more highly developed
that the outermost uncini of the transverse rows may not only be
very long, but also be articulated in such a manner as to be movable.
When, therefore, the radula is protruded, these teeth are erected,
and when it is drawn back they come together like pincers, and so
act as seizing organs.

There are special muscles for moving the radula which, with the
cartilage on the wall of the pharynx (p. 361), help to increase the size
of this organ (Fig. 200, H). The size of the radula is therefore closely
correlated with that of the buccal mass. This organ is found in all
divisions of the Mollusca excepting the Lam ellibranchiata, although in
some (Thecosomatous Pteropoda) it is feebly developed. In rare cases
the radula and pharynx are altogether absent (Tethys). The radula is
relatively small in the Cephalopoda (Fig. 192, Cr), where the oral
opening is marked by the presence of strong jaws. These are two
strong pieces (Fig. 192, C) which resemble the beak of a parrot, and
are provided with sharp edges ; the lower one (ni) projects in front
of the upper one (m). The soft edges of the lips cover the bases
only of these jaws (Fig. 179, o]/. m').

In the Gastropoda also the wall of the mouth has a firm invest-
ment, which forms a kind of jaw. In the Nephropneusta there is
a single semi-lunar piece which is beset by toothed ridges at its
free edge. In many of the Braiichiopneusta lateral pieces are added
on to this unpaired one, which work horizontally on one another.
These paired jaws are also present in the Prosobranchiata, but are
best developed in the Opisthobranchiata.

/r

Fig. 191. A row of denticles from the
radula of Littoriua littorea. a Median.
1) c d Lateral denticles (after Gray).

ALIMENT AEY CANAL OE MOLLUSC A.

361

The mouth of the Gastropoda is surrounded by the lips^ whicli
meet in front of the entrance into the pharynx. These lips are folds
of the integumenC which can be retracted or protruded with the
pharynx. In some of the Prosobranchiata this arrangement is
carried so far that the
fold_,which^ as we have
seen, ordinarily forms
the lips, gives rise to
a more or less elon-
gated sheath, in which
a proboscis, within
which again is the
pharynx, lies freely.

When this proboscis
is protruded, the inner
wall of the sheath,
which encloses it, is
gradually everted (Do-
linin, Cassis, Conus,

Voluta, Buccinum,

Harpa, Murex, etc.).

It is clear then that
this, the most anterior
portion of the intesti-
nal tract, may be very
highly specialised.

Fig. 192. A Pharynx of a Gastropod (PI euro -
branchus) ; vertical longitudinal section. B Trans-
verse section of the same, taken along the vertical
line drawn in A. oe OEsophagus. I Lips, r Radula.
li Cartilage. C Pharynx of a Cephalopod (Loligo) ;
vertical longitudinal section. t Arms. m Upper,
m' Lower jaw-piece, fj Tongue, r Radula. oe CEsophagus.

§ 278.

Behind the pha-
rynx in the Gastro-
poda the fore - gut
forms the oesophagus,
and part of it widens out into a stomach ; this is succeeded by the
mid-gut, which often traverses the visceral sac in the form of a
single loop ; it passes on to the hind-gut, from which it is indistinctly
separated.

The canal is modified in character by the enlargement of various
parts of the oesophagus; in this way a special portion is formed
which functions as a crop. This is either spindle-shaped, as it is in
many of the Prosobranchiata (it is very long in the Heteropoda), or
it forms a unilateral diverticulum, Avhich may develop into a csecal
appendage (Lymnasus, Planorbis, Buccinum). Parts of the fore-gut
may also widen out into stomachal enlargements, which are separated
off from the neighbouring tracts by constrictions. These form
divisions, set one behind the other.

This separation distinctly corresponds to a division of function ;
this is also shown by the varying characters of the cuticular struc-
tures of the different portions. For example, in Aplysia, one portion

362

COMPAEATIYE ANATOMY.

is beset with pyramidal pieces of cartilagiiions consistency, and
another is provided with firm horny hooks. Similar kinds of hooks
are found in the simple stomach of Tritonia ; in Scyllaea there is a
broad girdle of sharply-pointed plates in the same region, while
in the stomach of those Pteropoda in which the mouth-parts are
rudimentary, there are strong radulae. The presence of these organs
shows that these portions of the gut merely function in preparing
the food for digestion.

The widened mid-gut is no less modified, both in form and in
the differentiation of its various j)arts. In many it is faintly dis-
tinguishable. In others it forms a stomachal caecum, so that the
cardiac and pyloric ends get to lie close together ; this is the more
common form. The stomach may be broken up into several
divisions. For example, the cardiac and pyloric portions are often
separated by a longitudinal fold, which projects into the stomach
(Littorina).

Of the peculiarities of the rest of the enteric tube, the frequent
enlargement of the hind-gut must be mentioned. In many Nudi-
branchs the whole gut undergoes more marked modifications
(^olidias) ; in proportion as the liver takes on its functions it
becomes more or less degenerated, the increased size of the liver
compensating for the great abbreviation of the gut (vide infra,
p. 365).

In many of the Gastropoda there are glands connected with the
anus j these are sometimes of some size (Murex, Purpura) ; but it is
not yet known what their significance is.

The anus is lateral or dorsal in position according to the size of
the shell and the development of the mantle-cavity. When the
shell is absent, and a mantle-cavity also, the anus may be on the
dorsal surface, and even in the middle line of that surface, as it is
in some of the Nudibranchiata (Doris) (Fig. 200, a). In others it
retains the lateral position which it acquired owing to the earlier
possession of a shell (^olidia).

§ 279.

In the Cephalopoda the pharynx (Fig. 199, is continued into
a narrow oesophagus, which, after passing through the cephalic
cartilage, is either continued, without any change of diameter, into
the stomach (Loliginidge), or it is provided somewhere along its tract
with a crop -like enlargement, which is often of considerable size
(Nautilus, Octopoda). A stomach (Fig. 193, v) is formed by an
oval or rounded enlargement, which is sometimes (Nautilus, Octopus)
provided with strong muscular walls. On each side of the stomach
there is a layer of muscles, which are arranged in a radial manner,
and in the centre of which there is a tendinous plate; this is
especially noticeable in the Nautilus.

The pylorus, which is placed close to the cardia, leads into the

ALIMENTAEY CANAL OF MOLLUSCA.

363

mid-gtit, wliicli is provided witli a caecum ; this
inner wall thrown into longitudinal folds for
the first part of its length. It generally takes
a straight course (it is slightly coiled in
Nautilus and the Octopoda only) forwards^
to a short hind-gut (Fig. 193, which opens
to the exterior at the base of the funnel. In
many Cephalopoda there are two or three
valves or valvular processes around the anus,
the muscles in which are remarkably well
developed.

The C90ca (Fig. 193, c) at the commence-
ment of the mid-gut vary not only in external
form, but also in the characters of their in-
ternal surface. The csecum is either rounded
(Nautilus, Eossia, Loligopsis) or elongated ;
in the latter case it is often spirally coiled
(Sepia, Octopus). When the caecum is very
long, it is often coiled several times (e e).
The inner surface is either thrown into pro-
cesses, which are arranged in a lamellar
fashion (Nautilus), or these processes may
form circular folds arranged in conformation
with the spiral form of the caecum. Two of
the largest folds receive the efferent ducts of
the liver, and project so far into the lumen of
the enteron as to form at times a valve-like
apparatus which shuts off the cgecum. It is
probable that this csDCum has a secretory
function, as in those forms (Loligo vulgaris)
in which the folds are absent a large number
found in its walls.

portion also has its

Fig. 193. Digestive appa-
ratus of Loligo sagit-
tat a. oe (Esophagus.
V The stomach, opened
longitudinally. x A
probe passed through
pylorus, c Commence-
ment of the CcCCum.
e e Its spiral portion.
i Hind-gut. a Ink-bag.
1) Its opening into the
rectum (after Home).

of glands are to be

Organs appended to the Enteric Canal.

1) Appendages of the fore-^gut.

§ 280.

Of those glandular organs which are connected with the enteric
canal, the salivary are found only in those forms in which the
pharynx is developed } it is possible therefore to make out a certain
connection between these structures. In the Gastropoda they are
always placed on each side of the fore-gut, and open into the
pharynx. Sometimes they form short caecal tubes (Pteropoda),
which are sometimes hidden in the very substance of the pharynx

364

COMPARATIVE ANATOMY.

(many Opistliobrancliiata) . WRen they are more liigRly developed
tlieir duct is elongated, so tliat the secreting portion comes to lie
some way further back ; sometimes it is placed on the oesophagus,
and at others on the stomach itself. In this case the glands are
rounded, elongated, and generally flattened tubes (Prosobranchiata,
many Pulmonata), which may be broken up into several smaller
parts, or have the form of ramified organs ; the glands found on
the stomach of Pleurobranchns are examples of this latter kind. We
not unfrequently meet with two pairs, the efferent ducts of which
are either separate all along their course, or the ducts of the hinder
pair unite with one another. Even when there is but a single pair
of glands they may be often observed to fuse into one mass, the
double nature of which is shown by the presence of two efferent
ducts. The salivary glands of many Prosobranchiata are differen-
tiated functionally (Dolium, Cassis, Cassidaria, Tritonium), for part
of the gland secretes free sulphuric acid. The glands of some Opis-
thobranchiata (Pleurobranchus, Doris) are differentiated in the same
way.

Among the Cephalopoda, Nautilus is provided with a paired
glandular mass, which is placed inside the pharynx. These glands
are also present in many Dibran chiata (Octopus, Eledone), as
are others, which are short and lie just behind the pharynx ; these
have an efferent duct which penetrates the wall of the pharynx, and
unites with its fellow of the opposite side immediately in the
orifice of the duct (Fig. 199, gls s). In addition, there are glands
behind these which lie at the sides of the oesophagus, and behind the
point at which it penetrates the cephalic cartilage. They are either
simple or lobate ; as a rule their efferent ducts unite into a single
one, internally to the cephalic cartilage, and this duct opens into the
pharyngeal cavity in front of the lingual ridge (Fig. 199, gls /).

Panceei, P., Gli organ! e la secretioue dell’ acido solforico nei Gasteropodi. Atti
della E. Accad. delle Sc. fisiche. Napoli. Yol. III.

2) Appendages of the Mid-gut.

§ 281.

Appended organs are very generally found on the mid-gut of the
Mollusca; they represent the liver,^'’ and are differentiations of the
wall of the enteron, from which they are developed, under the form
of diverticula, the primary origin of which is the sacculation of the
endoderm.

In the Lamm ellibran chiata the liver is a gland which sur-
rounds the stomach and a large portion of the rest of the enteron.
It forms numerous acini, which are connected together into large
lobes, and open at various points either into the stomach, or into the
succeeding division of the enteron.

LIVEK OF MOLLUSCA.

365

111 the Placophora it forms a pair of symmetrically disposed
branched tubes.

In the Gastropoda this gland is no less well developed. In the
shelled forms it occupies the largest portion of the visceral sac
within the shell ; it is always made up of a number of large lobes,
and embraces more or less of the enteron. The bile-ducts from the
lobes either open separately or together into the first portion of the
mid-gut, and sometimes also into the stomach-like enlargement.
The number, as well as the relative size of the separate portions of
the liver, varies greatly. As a rule, however, when the liver increases
in size it becomes less complex, while the smaller the lobes the more
numerous they are.

This mode of arrangement along a large portion of the enteric
canal leads to certain changes in this
portion of the enteron in one division of
the Opisthobranchiata. The ducts of the
several lobes of the liver widen out, and
so form diverticula of the stomach ; when,
therefore, there are a large number of
Impatic tubes opening into the stomach,
its inner surface has a reticular appear-
ance (Doris, Doridopsis). Owing to this
change, which is easily explicable by a
reference to the origin of the liver, the
glandular portion of this organ becomes
apparently a mere covering for these
irregular diverticula.

This is indeed the origin of that
arrangement of the digestive system in
the .dGolidias and others, to which we
have already called attention (§ 278).

The liver has the form of wide cascal
appendages, which arise from the mid-
gut (Fig. 194, m)j or so-called stomach.

They are either directly connected, when
the appendages open at once into the
mid-gut, or indirectly, when they still
form wide diverticula of it (Fig. 194);
and these, too, may be due to certain
changes in a part of the liver. These appendages traverse the
coelom and send blind processes into the dorsal cirri, when such
are present. These processes are more or less branched, according
to the number present, and they may, further, anastomose with one
another. The size, just as much as the number and general form
of the enteric appendages, may vary. Sometimes they are mere
diverticula of the enteron, and may communicate with it by wide
openings, and even be large enough to take in masses of food ; or,
again, they may form narrow canals which do not take any direct
share in the reception of the food. There are intermediate stages

Fig. 194. Enteric canal of
^olidia papillosa. p/i Pha-
rynx. m Mid-gut, with its he-
patic appendages, {K) all of
which are not figured, e Hind-
gut. an Anus (after Alder and
Hancock).

366

COMPAEATIVE ANATOMY.

between these extreme forms. The fact that these enteric caeca
always have a glandular investment is of importance as bearing
on their real character. These branches, therefore, are not merely
physiological equivalents of a liver, but are to be regarded as modi-
fications of a true liver, which have come to take a share in increasing
the enteric canal, owing to the enlargement of the lumen of their
canals. The same organ, which in other Gastropoda has the appear-
ance of a liver, becomes a part of the digestive tract in the ^olidias,
and retains its primitive significance in its walls only, or, it may be,
in parts only of these walls. The share taken by the cavities derived
from the enteron in the functions of this tract explains how it is
that the mesial enteric tube is so short. In other divisions also of
the Opisthobranchiata the liver has the form of wide tubes — as in
Phyllirhoe, Limapontia, etc. In all these structures the liver appears
to be degenerated, and there is no early stage of differentiation ; this
is because the -^olidioe are derived from shelled Gastropoda.

In the Pteropoda the liver is broken up into a large number of
small C80cal tubes. In Pneumodermon these tubes are collected
together into branched groups, and the wide orifices of their ducts
are scattered on the wall of the stomach, and give it an almost
sieve-like appearance. In the rest of the Pteropoda acini of simpler
character beset a portion of the enteron ; they form a well-defined
mass, which is traversed by this tube (Fig. 201, h).

The liver of the Cephalopoda is always a large, and, frequently, a
compact gland ; in Nautilus it consists of four loosely connected
lobes, each of which gives off an efferent duct. In the Dibranchiata,
there are only two lobes, and these are either distinctly separated
(Sepia), or only partly connected together (Eossia). In Sepiola
and Argonauta the two lobes are more intimately connected ; in the
Loliginidas and Octopoda they form a single mass, which is traversed
by the oesophagus. In no case does the liver give off more than
two efferent ducts, which point to the primitive existence of two
lobes, and which, just as in Nautilus, always open into the end of the
cmcum.

There are peculiar glandular lobules on the hepatic ducts at the
openings into the caecum, and also in the liver itself ; these differ in
structure from the acini of the liver. The glands at either of these
spots have been regarded as pancreatic glands, but it must be
noted that they have no close affinity to the organs of that name
found in the Vertebrata. In the Gastropoda also (Aplysia, Doris)
special glands have been observed in the neighbourhood of the liver.

3) Appendages of the Hind-gut.

§ 282.

Various glandular organs belonging to this division are found in
the Gastropoda only, where their significance has not been made

CCELOM OF MOLLUSCA,

367

out. In the Cephalopoda, the Dibranchiata are ordinarily provided
with an ink-bag, which is an organ of this kind; in many (Loliginidae)
this opens together with the hind-gut, and may, therefore, repre-
sent a structure which has been developed from it, although, indeed,
in other Cephalopoda, it opens below or behind the anus. It forms
an elongated sac, the contractile walls of which form internally-
projecting lamellse (Fig. 193, a), and which secretes the well-known
black fluid.

Ccelom.

§ 283.

A coelom appears very early in the differentiation of the
Molluscan body. The complications which affect the coelom, owing
to the coils of the enteric canal which are embedded in it, and the
appended organs which are developed from its walls, are further
increased by the formation of other organs, and, especially, of the
generative system ; the cavity is thereby broken up into a number
of segments varying in size. As a rule, the coelom is continued into
the processes of the body, as, for example, into the mantle-flaps of
the Lamellibranch, and the mantle of the Gastropoda. Less im-
portant appendages of the body are also frequently connected with
the coelom.

As a rule, the vascular system is freely connected with the coelom,
which therefore forms a portion of the haemal system. This
arrangement is more or less well marked ; and more or less wide
spaces are shut off from the coelom, according to the degree to which
the vascular system is developed. When the wider spaces of the
coelom are connected with the vascular system, these portions of the
haemal system form lacunm ; when these spaces are broken up, either
by the organs which are embedded in them, or by the bands of tissue
which connect together the walls of these organs, they become con-
verted into narrow, and often into vascular canals, which, moreover,
are frequently arranged in a regular manner. In the Lamellibran-
chiata and Gastropoda there are all kinds of stages of this kind ;
in the Cephalopoda the blood-vascular system is very complete, and
true lacunar spaces are, for the most part, confined to the visceral
sac. By means of the excretory organs (§ 289), the coelom, as in
many Vermes, communicates with the surrounding medium.
The water is thereby enabled to mix with the blood. In
addition to the communications which the coelom has with the
exterior by means of the excretory organs, it is able to communicate
with it directly, owing to the presence of pores in the foot of
the Lamellibranchiata and Gastropoda; the fluid is able to
escape from the coelom through these pores. This has been definitely
made out both in the Lamellibranchiata (Mactra, Cardium, Solen)
and in the Gastropoda (Pyrula). This fluid is of especial importance
in locomotion, inasmuch as the animal is enabled to swell out its

368

COMPAEATIVE AXATOIMY.

body by taking in tlie water. By its aid, retracted parts can be
protruded, limp parts erected, and tlie whole musculature of the
body-wall, and especially that of tlie foot, is thus made very
important. The power of protruding certain parts, which have
been withdrawn into the shell, and especially the foot, depends on
these relations, which are definitely known to exist in the case of the
Lamellibranchiata and Gastropoda, as also in that of the Pteropoda.
As yet it is not definitely known whether water is taken into the
Inemal system in the Cephalopoda.

Vascular System.

§ 284.

In all the Mollusca, excepting the Scaphopoda, the vascular
system is arranged in almost exactly the same way, so far as all
essential points are concerned. These are, first, the presence of a
dorsal longitudinal trunk, a portion of which is developed
into a central organ (ventricle) ; secondly, transverse
vessels are connected with the longitudinal one, which,
when lateral gills are present, carry the blood from them
to the heart, and are further differentiated into organs
of circulation by becoming auricles for the ventricle.
This dorsal development of the chief portions of the circulatory
system is a point in which they resemble the Vermes (cf. p. 168).

The symmetrical arrangement of the auricles in Mollusca which
are in other points markedly divergent, shows us that this is a
peculiarity which lies at the very bottom of their organisation ;

Fig. 195. Diagram to show the relations of the Circulatory Centres in the
Mollusca. A Part of the dorsal vascular trunk and ti’ansverse trunks of a Worm.
Jj Heart and auricles of Nautilus, C of a Lamellibranch or of Loligo, D of
Octopus. F Heart and auricle of a Gastropod, v Ventricle, a Auricle, or Arteria
cephalica. ai Arteria ahdominalis. The arrows show the direction of the current

of blood.

while the presence of two pairs of auricles, which open behind one
another into the ventricle, in the tetrabranchiate Cephalopoda,
points to a metameric arrangement of the vascular system,
just as much as does the presence of several transverse trunks in the

YASCULAE SYSTEM OF MOLLUSCA.

369

Annulate Yermes. These vessels still retain so much of their
primitive character that they are not called auricles^ hut branchial
veins.

This homology of the two pairs of auricles with two transverse
trunks of a dorsal vessel (Fig. 195_, A and B) points to a primitive
condition, which, while it is characteristic of the Nautilidas, agrees
also with the iDalseontological relations which these Cephalopoda
have to the rest of the extant members of their group. The presence
of but one pair of auricles appears, on the other hand, to be a degene-
ration (Placophora, Dibranchiate Cephalopoda, Lamellibranchiata,
and many Gastropoda), which corresponds to the reduction of the
gills. The key, therefore, to the comprehension of the mode of
formation of the ventricle and auricles in the Mollusca is to com-
pare them with more indifferent organs. When part of the dorsal
vessel is converted into the ventricle, the parts that are continued
from it form arterial trunks, and these, when they retain their
primitive course, are distinguished as anterior and posterior aortae
(aorta cephalica, et aorta intestinalis sive abdominalis). In some
of the Cephalopoda, the Octopoda (JD), we meet with an important
change in position, for the trunk of the dorsal vessel has made a
loop-like turn, so that the two arterial portions {ac and ai) run in
the same direction for a certain distance. The points at which they
arise from the ventricle are thereby approximated. The circu-
latory system of those Gastropoda, which are characterised by the
fact that only one arterial trunk is given off from the ventricle,
may be derived from a similar relation of parts (F7). This single
arterial trunk divides into two branches, which are distributed in
just the same regions as are the two arterial trunks {ac and ai)j
which are given off from either end of the ventricle in the Cepha-
lopoda and Lamellibranchiata. It may, therefore, be regarded as
developed from the two arterial trunks, which were primitively
disposed along a single axis. The final reduction of the auricles
to one is due to a reduction in the number of the gills, and
is correlated with the union of the anterior and posterior arterial
trunks.

The auricles and ventricle appear, therefore, to have been derived
from different portions of a primitive vascular apparatus, and point
to a metameric arrangement; taken in conjunction with the signs
of metamerism exhibited in the nervous system (p. 344), they lead
us to suppose that the ancestors of the Molluscan phylum were
segmented organisms.

§ 285.

The heart, in the Placophora and ‘Lamellibranchiata (Fig. 196, -y),
lies in the middle line of the body, just below the back, and is sur-
rounded by a pericardium ; it receives blood from two lateral
auricles {a) while the above-mentioned vascular trunks arise from it
anteriorly and posteriorly. In the Placophora the heart is placed a
good way back, so that the anterior arterial trunk is of a considerable

2 B

370

COMPAEATIVE ANATOMY.

length. In most of the Lamellibranchiata the heart is divided into
two limbs^ which embrace the hind-gnt (i), and which give off the

aorta after they unite. In Area this
passage of the hind-gut through the
heart leads to the formation of two
ventricles, which are represented
by two completely separate cham-
bers, each of which is provided with
an auricle. Each ventricle gives off
an aorta, which unites with its fellow
of the opposite side before it gives
off any branches, so that there is a
single aorta all the same. The same
holds for the posterior arterial trunk.

The anterior arterial trunk passes
as far as the region of the mouth,
where it gives off branches and opens
into wide haemal spaces. The pos-
terior arterial trunk, the length of
which is dependent on the develop-
ment of the hinder portions of the
mantle, which represent the siphons,
also passes into haemal spaces or
lacunae.

Spaces of this kind, which are essentially marked off by connec-
tive tissue, ramify not only in the mantle, but also between the
viscera. They may be distinguished into larger or smaller recep-
tacles for blood, according to their width; while, further, they take
the place of a capillary and of a venous system. Larger sinuses
are regularly present at the base of the gills, and a median
azygos one — which collects together the venous spaces in the foot
— extends betw^een the two adductor muscles. All of these haemal
spaces are connected together, and form in different parts a more or
less wide mesh-work. The two lateral spaces also communicate with
the organs of Bojanus (§ 290).

If we follow out the course which is taken by the blood sent from
•the arteries to the periphery, although, indeed, many points about it
have not been definitely made out, we find that some of it goes to
the mantle, and some to the visceral sac. Thence some of it passes
into the branchial sinuses, and from these either directly into the
gills, or to the glands of Bojanus, before it passes to the respiratory
organs. The latter course is that which is taken by most of the
blood. Since, however, the haemal spaces at the base of the gills
are in direct communication with the auricles of the heart, part —
although a small part — of the blood is able to return to the heart
without going to the gills. The blood from the mantle also comes
to the heart, and passes at once into the auricles ; but this cannot be
regarded as absolutely venous blood, on account of the respiratory
function of the flaps of the mantle. As all the blood which comes

Fig. 196. Vertical section through an
Auodonta. v Ventricle, a Auricles.
pp' Pericardial cavity, i Hind-gut.
m Mantle, hr hr' Branchia?. / Foot.

VASCULAR SYSTEM OF MOLLUSCA.

371

from tlie gills is received into tlie auricles^ all of the blood finally
returns, by various ways, to tlie ventricle.

The character of the circulation in the glands of Bo j anus is
worthy of remark. These excretory organs are placed in the
way of the blood which is going to the gills, and which, there-
fore, is venous; and herein we get a kind of primitive portal
circulation.

§ 286.

In the Gastropoda, the heart, which is here also enclosed in a
pericardium, is often provided with two lateral auricles (Haliotis,
Fissurella, Nerita). Trochus also has two auricles, but the left one

Fig. 197. Organisation of Paludina vivipara. c Head, t Tentacles, p Foot,
op Operculum, o Eye. a Auditory organ, n Cerebral ganglion, n' Pedal ganglion.
n” Branchial ganglion, n'” Buccal ganglion. (The commissure from the cerebral
ganglion is not figured.) 'ph Pharynx, ce (Esophagus, hr Branchiae, r Renal organs.
s Venous sinus. Sv Venous sinus at the base of the branchiae. / Branchial artery.
at Auricle of the heart, v Ventricle, ap Posterior artery (visceral artery), aa Anterior

artery (after Ley dig).

is somewhat reduced ; in the rest of the Gastropoda, as also in the
Pteropoda, there is but one auricle (Fig. 197, at). The dorsal position
of the heart is modified by the asymmetrical development of the
visceral sac ; it is always placed close to the respiratory organs, and
the thin- walled auricle is always directed towards them. It is
placed in front of them in the Prosobranchiata, and behind them in
the Opisthobranchiata. It has in many Gastropoda the same rela-
tion to the rectum as it has in the Lamellibranchiata (Turbo,
Nerita, Neritina); in some, indeed, the ventricle is divided (Haliotis,
Fissurella, Emarginula).

An aorta arises from the ventricle and gives off a visceral artery
which passes backwards {ap)j while the trunk is continued on as an
aorta cephalica (aa). This latter passes directly to the anterior

2 B 2

372

CO:\IPAEATIYE AXATO:\IY.

parts of the body, and generally gives off a large branch to the foot ;
this sometimes looks as if it were a continuation of the chief trunk.
In addition to this, it gives off on its course branches which pass to
the stomach, salivary glands, and so on ; it either ends simply, or
after several ramifications, in the region of the pharynx. In the
Pteropoda it is more widely distributed, for in them the pedal branch
has the characters of a continuation of the chief trunk, and in the
head it divides into two large terminal branches which ramify largely
in the fins. The visceral artery, which corresponds to the posterior
artery of the Lamellibranchiata, is but slightly broken up iu the
Pteropoda and lower Gastropoda. Like the cephalic artery, it loses
itself in larger hgemal spaces. In most of the Prosobranchiata it is
greatly developed, and much broken up [ap) ; this is the case also in
many of the Tectibranchiata (Pleurobranchus), but in others it is
replaced by several smaller arteries (Aplysia). The visceral arteries
of the Nudibranchiata are branches of the principal trunk (Doris).

The afferent vessels differ according to the number, form, and
disposition of the respiratory organs. In many of the Nudibran-
chiata the blood is collected from the body-cavity near the auricle.
In others, which have distinct respiratory organs, there are definite
canals, or even vessels with special walls, which convey the blood
from the venous passages to the respiratory
organs. ^Yhen these are most simple in character,
the blood passes to the auricle without going
through branchial veins. This is the case, too,
in many of the Pteropoda. When the gills are
more highly developed the returning blood is
collected into special venous trunks, which open,
either separately or together, into the auricle.
These branchial veins are always so arranged as
to be adapted to the size as well as to the posi-
tion of the branchiae. In many of the Nudi-
branchiata (^olidia, Scyllaea, Tritonia), large
vessels pass off from the gills and unite together
to form larger trunks, which give rise to a median
or two lateral branchial venous trunks, connected
with the auricle of the heart. When the bran-
chiae are scattered over a larger portion of the
surface of the body, this afferent system of bran-
chial vessels is well developed, but when they are
limited in extent it is reduced in size (Doris,
Polycera). Tritonia is an example of the former
arrangement (Fig. 198), for in it two lateral bran-
chial venous trunks {ss) pass by means of a trans-
verse trunk to the hearL The transverse canal
forms a kind of double auricle [a), although indeed it has only one
opening into the ventricle [v).

The paths by which the blood goes to the branchise are always
more or less lacunar. In many of the Opisthobranchiata the blood

Fig. 198. Part of
the circulatory or-
gans of Tritonia.
s Y enous sinuses
laid open. The wall
is perforated by
the openings of
the branchial veins.

V Yentricle.

VASCULAE SYSTEM OF MOLLUSCA.

373

from tlie coelom is collected into canals, wliicE run in tlie integu-
ment; and thence it is distributed to the gills. All the blood, how-
ever, does not pass to them, for some is returned to the heart after
having passed through the integument.

In Helix and Limax the hsemal spaces, which pass into the wall
of their branchial cavity, and which form a system of canals which
carries blood to the res-

piratory organs, are de-
veloped in a vascular
manner. They break
up into a rich network
in the integument, and
this network gives off a
number of large trunks
with distinct walls,
which unite to form a
pulmonary vein ;
and this vein passes
into the auricle. The
network of pulmonary
vessels may be regarded
as a large blood sinus,
which extends into the
walls of the lungs, and
which is broken through
at various points by
islets of firm substance.

§ 287.

The heart of the
Cephalopoda is placed
at the base of the vis-
ceral sac, and is formed
of a rounded or trans-
versely - oval ventricle
(Fig. 195, HC; Fig. 199,
r), which receives blood
from as many bran-
chial veins as there are
branchiae. That is to

Fi{^. 199. Anatomy of Octopus. Mautle-cavity and
visceral sac, opened from the ventral surface, ph Pha-
rynx. gls s Superior salivary glands, gls i Inferior-
salivary glands. 0 Eye. i Funnel, hr Branchiae,
oy Ovary, od Oviduct, c Heart, r br Branchial veins.
a Arteria cephalica. vc Venae cavae. av Venous
appendages (after Milne-Edveards.)

say, in Nautilus there are four, and in the rest of the Cephalopoda
two branchial veins opening into the ventricle. These veins are
generally considerably widened out in front of their opening into the
ventricle (Fig. 199, v hr), and this enlargement is homologous with
the auricle in the Gastropoda and Lamellibranchiata. Two arterial
trunks always arise from the heart ; one, the larger, passes straight
forwards ; this is the arteria cephalica (Fig. 199, a) ; at some dis-
tance from it a smaller trunk, the arteria abdominalis (n') is given

374

COMPAEATIVE ANATOLIY.

off, and this generally passes backwards. This, wbicli is tbe general
mode of arrangement, shows distinctly how the Cephalopoda agree
with the two other classes, while it points to their having a close
affinity to those Mollusca which are distinguished by the possession
of two auricles.

The cephalic artery first of all gives off large branches to the
mantle, and some to the intestinal tract and to the funnel ; when it
reaches the head it gives off the optic arteries and supplies the oral
regions, and divides into larger branches, the number of which
depends on that of the arms. In some of the Cephalopoda the
brachial arteries spring from a circular vessel, which is developed
around the commencement of the oesophagus. The abdominal artery
is more varied in character; in some of the Sepiadee (Fig 202, a')
and Loliginidge, it arises opposite to the cephalic artery, and has,
therefore, the same relations as the visceral artery in the Lamelli-
branchiata ; but, in the Octopoda, it arises from the anterior region
of the heart, close to the aorta (Fig. 199); it very soon breaks u])
into several branches for the enteric tube and generative organs. In
the Loliginidge, however, it gives off two additional branches for the
fins ; in Ommastrephes a special enlargement (which is perhaps an
accessory organ of circulation) may be observed on these vessels.

All over the body the terminal branches of the arteries communi-
cate with the veins by means of a well- developed system of capil-
laries. In the greater part of the body, at any rate, this system
takes the place of the lacunar blood-passages which were found in
the other Mollusca, and of which it seems to be a differentiation.

The venous roots from the capillaries are collected into larger
trunks, which have sometimes the characters of true veins, and are
sometimes widened out into large spaces, so that they are inter-
mediate between true vessels and mere lacunge. Of the more special
characters of the venous system we have to note that the brachial
veins are united into a circular sinus, placed in the head ; this is
supplied also by neighbouring smaller venous trunks, while it
gives off a large hgemal canal (vena cephalica, or great vena cava)
(Fig. 202, vc)j which passes backwards to the branchial region. At
the gills it breaks up into two (Dibranchiata) or four (Tetrabran-
chiata) venous trunks ; these take up the other veins which come
from the mantle and viscera [vc'‘) and pass to the base of the gills.
In the Dibranchiata the branchial arteries acquire a muscular in-
vestment and form a contractile portion, or branchial heart
(Fig. 202, VC '), which pulsates rapidly, and acts as an accessory organ
of the circulation. Special appendages are attached to the branchial
artery in front of these branchial hearts ; these, which are diverti-
cula of the walls of the vessels, are bathed by the venous blood,
which passes into the branchige, in just the same way as are the organs
of Bojanns in the Lamellibranchiata (vide Excretory Organs, § 289).

Although the venous blood-receptacles of the Cephalopoda which
we have described may be regarded as a venous system, provided
with closed walls, true blood lacunge are not absent. They are dis-
tributed in just the same way as in the other classes of the Mollusca.

EXCRETORY ORGAKS OE MOLLUSCA.

375

The ccelom is such a blood-space; all the organs in it are bathed by
venous blood. Various veins open into it, and it is also connected
by two canals with the large vena cava (vena cephalica).

Milne-Edwakds et Valenciennes, Nouv. obs. sur la constit. de I’appareil de la
circulation chez les Moll. Mem. Acad, des Sc. T. XX. Milne-Edivards,
Voyage en Sicile. T. I.

§ 288.

The blood fluid of the Mollusca is, as a rule, colourless, and
often has a bluish or opalescent appearance. In many Cephalopoda,
however, it is violet or green, and in some Gastropoda (Planorbis)
the blood is red — the plasma being coloured.

The form-elements of the blood are always colourless, and have
the character of indifferent cells, which give off all kinds of pseudo-
podia-like processes during their amoeboid movements; this has
been observed in the Lamellibranchiata and Gastropoda.

A rounded organ extends alongside the branchial arteries of the
Cephalopoda; its function is not known, but it may perhaps be an
organ wdiich is of importance in the development of the form-
elements of the blood.

[Lankester, E. Ray, Distribution of Haemoglobin in the Animal Kingdom
(red corpuscles of Area and Solen). Proc. Royal Society, 1873.]

Excretory Organs.

§ 289.

In addition to the various organs which have been already noted
as present in the integument, and which serve as excretory organs,
there are others which open on to the surface of the body, and
which are much more important from this point of view.

In the Placophora there is a glandular organ which is placed
close to the anus; but it is not certain that it is comparable to the
excretory organ of the Conchifera. Its internal orifices have not
been observed. For the present, therefore, we must regard this
organ as not belonging to the same series as the excretory organs of
the rest of the Mollusca.

These typical excretory organs of the Mollusca are
homologous with the organs found throughout the Vermes^
and there called renal; which organs form the looped
canals (nephridia) oftheAnnulata. In the Mollusca also they com^
mence by an external orifice, and open into the coelom after a longer
or shorter course. The internal opening is distinguished by special
arrangements; most commonly, perhaps, indeed, generally, by an in-
vestment of cilia, whereby they call to mind the ciliated funnels of the
looped canals of Vermes. Owing to the presence of these organs, the
internal cavity of the body ‘ communicates with the surrounding
medium. They are able, therefore, to bring water into the body,
while, like their homologues among the Vermes, they may preside

376

COMPAEATIVE AXATOMY.

over otlier fimctions also. Tims tliey may have relations to the
generative organs, as is clearly the case in some of the Lamellibran-
chiata. There is good reason also for thinking that the efferent
ducts for the generative products in the Cephalopoda are derived
from these excretory organs. They have not, therefore, any exclusive
relation to excretion. When they are excretory in function, the walls
of the canals, which are otherwise simple, undergo a certain amount
of metamorphosis, and may then be seen to have a glandular
structure. In these cases they may be regarded as kidneys,^^ on
account of the chemical constitution of their products. In this case,
if examined under the microscope, they are seen to be provided with
secreting cells, the contents of which are formed by granular or con-
centrically striated concretions, similar to those which are of such
importance in the renal secretions of other groups of animals.

Where the internal orifice has been observed, it has been seen to
lead into the pericardial sinus, through the wall of which the duct
passes. If it be true that the excretory organ is derived from a
looped canal, it is highly probable that the wall of this pericardial
sinus is derived from a dissepiment, such as those which carry the
openings of the looped canals in the Annelides. Many facts, however,
are wanting to confirm this supposition, and particularly those which
would explain how the change in the position of this dissepiment
has been brought about.

§ 290.

In the Lamellibranchiata the excretory organ is known as the
Organ of Bojanus; it is always paired, although sometimes it is
fused into one mass along the middle line ; it lies on the dorsal side
of the body, close to the base of the gills. Its substance is made up
of a yellowish or brownish coloured spongy tissue, the interspaces
in which often run together, and generally form a large central
cavity. From this cavity a pore, on either side, leads into the peri-
cardium, while another leads into the efferent duct. The latter either
lies close to the genital pore, or is confiuent with it, or, lastly, the
generative organs open into the organ of Bojanus, so that the
generative products are passed out to the exterior through it (Pecten,
Lima, Spondylus). Area and Pinna have the efferent ducts united;
Cardium, Chama, Mactra, Pectunculus, Anodonta, Unio, etc., have
the orifices of the excretory and generative organs separate. The
walls, which rise up in folds, or the meshwork-like tissue of the
organ, is thickly invested with secreting cells, which secrete the
already-mentioned concretions, in many of which the characteristic
excretion — uric acid — has, of course, not yet been observed. As
to its relations to the vascular system, see p. 370.

The Scaphopoda resemble the Lamellibranchiata in the possession
of a paired excretory organ.

§ 291.

In the Gastropoda the excretory organ varies still more in
character. A paired excretory organ — the predecessor of the per-

EXCEETOEY OEGAXS OE MOLLUSCA.

377

manenfc kidney — Eas been made out in the Pnlraonata. In adult
animals the organ is^ as a rule, unilateral. This double rudiment
points to its being the same as the paired organ of the Lamelli-
branchiata. The recent discovery of a paired excretory organ in
Haliotis, Fissurella, and Patella, in which forms the left organ is
more or less rudimentary, seems to be decisive on the matter. The
degeneration of one of the two organs appears to be correlated with
the degeneration of other paired organs, as, for example, the gills.
So far as exact observations show, it opens by one pore into the
pericardial sinus, and by the other to the exterior. In the majority
of the Gastropoda, uric acid has been detected in the organ. This
is especially true of the Pulmonata, in which the kidney, which is
placed between the heart and pulmonary veins, can be easily recog-
nised on account of its whitish or yellowish colour. It is lamellar or
spongy in structure, and the lamellse, or bands which make it up, are
covered by large secreting cells, in which firm concretions of various
forms can be made out. The long efferent duct, which commences
some way back, opens into the pulmonary cavity, which in Helix
appears to be a widened terminal portion of this duct.

In the Prosobranchiata the kidney lies between the gills and the
heart ; it has the same position in some of the Opisthobranchiata.
As a rule an efferent duct passes forwards and accompanies the hind-
gut, below which, and often not far behind the anus, we find its
orifice.

In many of the Opisthobranchiata it appears to have lost its
excretory function (as in Polycera), or the excretion is fluid. In this
case the kidney (as in Phyllirhoe, Actgeon, etc.) has the form of a
long transparent tube, which extends some way back behind the
heart along the middle line of the body, and is placed close to the

Fig. 200. Diagram of Doris, to show the excretory organ E. o Mouth. I Lips.
B Buccal mass, ce CEsophagus. v Stomach, i Intestine, a Anus. at Auricle.
V Ventricle (after A. Hancock).

back. It opens into the pericardial sinus by a ciliated orifice, and
by a contractile one on to the surface of the body.

In many of the Opisthobranchiata a greatly-developed csecal-sac
is given off by this organ, which gives off secondary diverticula
(Fig. 200, B), and so gradually forms a ramified tube. Structures of
this kind, which resemble a ramified gland, have been observed in
Doris, Dendronotus, ScyllEca, etc. A canal (B) is continued from the

378

COMPAEATIVE AXATOMY.

pericardial orifice (?’") into the interior of the tube, where it opens
(r), so that it only communicates with the exterior in a round-
about manner.

In the Thecosomatons Pteropoda^ and also in the Heteropoda, the

organ which is regarded as the
kidney, because of the similarity
between its two orifices and
those of the Prosobranchiata, is
also remarkable for its spongy
character. In Carinaria, among
the Heteropoda, it is provided
with a distinct investment of
secreting cells ; in all the rest
there is a clear cellular layer
instead. The framework of
the kidney is stiff; but in
Atlanta and the Firolidm it is
contractile, and performs en-
ergetic and spasmodic contrac-
tions. In the Thecosomatons
Pteropoda also, the kidney can
act in this way, e.g. in Chreseis
(Fig. 201, re).

As the glandular nature of
this organ is doubtful when the
secreting cells do not contain
concretions, greater weight may
be laid on its relations to the in-
gestion of water, which has been
best observed in these cases.
The movements of the organ
are not limited to merely open-
ing and closing its external
orifice, but they also drive the
water inwards and mix it with
the blood which is returning to
the respiratory organs from the
general circulation ; this organ is
always placed in the course taken
by this current. Although the
ingestion of water by the excre-
tory organ has not been directly
observed except in these divi-
sions, there is no reason for
concluding that it does not also
obtain in the rest of the acpiatic Gastropoda. It is in the Nephro-
pneusta only that the parts concerned have other relations, but even in
them the kidney has the same relations to the system of blood-canals,
for theflnidof the blood maybe observed to pass out by the renal orifice.

Fig. 201. Organisatiou of Chi’escis.
j)p The fins, ce CEsophagns. v Stomach,
r Uind-gut. /i Liver, a Auricle. cVen-
tricle. re Kidney, x Its internal orifice.
x' External orifice. h Ciliated organ.
rj Hermaphrodite gland, g' Hermaphro-
dite duct, g" Penial pouch, m Points to
the position of the retractor muscle.

EXCEETOEY OEGAXS OF MOLLUSCA.

379

§ 292.

The great variations in the more special characters of the excre-
tory organ, which are seen in the Gastropoda, prevent our being
astonished at the fact that this same organ is otherwise modified in
the Cephalopoda. All the Cephalopoda possess in their visceral mass
closed sacs, which open into the mantle-cavity. As the efferent ducts
for the generative products resemble the excretory canals in having
that part of them which surrounds the germ - glands connected
with the coelom, it is pro-
bable that these efferent
ducts have been derived
from primitive excretory
organs ; in this case the
Cephalopoda must have
had a larger number of
these organs, of which
some alone have retained
their primitive significance.

Four of these organs are
found in Nautilus, and two
in the Dibranchiata. Their
orifice is sometimes placed
on a papilliform process
(Fig. 178, r). The large
branchial vascular trunks

project into these sacs, and
their walls, therefore, are
irregular in form. So far,
however, as these vessels
project into the sacs, their
walls must be regarded as
belonging to the sac. In
connection with the bran-
chial arteries the wall of

Fig. 202. Circulatory and excretory organs of
Sepia, hr Branchiae. c Heart, a Anterior
artery of the body (aorta), a' Posterior artery.
V Enlargements of the branchial veins, repre-
senting the auricles of the heart, v' Branchial
vein, running alongside the branchiae, vc An-
terior large vena cava, vc' Branchial arteries
(branches of the vena cava). vc" Posterior
venae cavae. re Spongy appendages of the
branches of the venae cavae. x Their diverticula.
The arrows indicate the direction of the blood-
current (after J. Hunter.)

each sac presents numerous

ramified appendages, which project into the lumen of the sacs (cf.
Fig. 178, B; Fig. 202, re) ; they are formed by csecal diverticula of
tlie* vessel, and a superjacent glandular investment. In Nautilus these
appendages of the four venous trunks are clothed by tubular glands
which open into the connected sac. It is still doubtful what we
should regard the appendages on the other blood vessels, which
project into the pericardial sinus, as being. As this sinus communi-
cates with the mantle-cavity, perhaps they also represent excretory
organs. In the Dibranchiata the venous appendages appear to
have a somewhat different structure. Concretions of phosphate
of calcium appear to be the principal products of this appa-
ratus ; in the Sepiadse this organ (Fig. 201) is very large, and

380

COMPAEATIVE AXATOMY.

extends even on to the smaller roots of these veins. Owing to this
arrangement^ this secreting apparatus has relations to the venous
current of blood which passes to the gills ; it has therefore the
same relation as the excretory organ of the Lamellibranchiata and
Cephalophora.

It is less certainly proved that the sacs which contain the excre-
tory venous appendages have any internal communications. While
some authors assert that there is such a connection with the blood-
vascular system, and especially with the pericardial sinus, others
deny it. These organs still, therefore, require much elucidation.

Hancock, A,, On the Structure aud Homologies of the Renal Organ in the
Nuclibranchiate Mollusca. Transact. Linnean Soc. Yol. XXIY.

Generative Organs.

§ 293.

Eeproduction is never effected in the Mollusca by any of those
asexual modes which we saw in the Arthropoda to be ultimately due
to sexual differentiation. It is exclusively dependent on the complete
activity of both sets of generative organs. In several divisions the
two organs have been seen to arise from different layers of the
embryo; the male having been found to have relations to the
ectoderm, and the female to the endoderm. These organs are fairly
distinct in the different classes of the Mollusca, so that it is only
possible to derive them from a ground-form common to the whole
group, by looking for this form at a very low stage of differentiation.

In the Placophora there is an unpaired germ-gland, from
which paired efferent ducts lead to the laterally and posteriorly
placed genital pores. Owing' to the presence of separate efferent
ducts, the arrangement is of a higher grade than it is in the Lamelli-
branchiata. In most, the sexes appear to be distinct.

In the Lamellibranchiata, individuals in which the two sexes
are united are only found in a few diverse genera, or even in
isolated species ; they probably represent the remnant of an
arrangement which was formerly found in the whole class.
In the Oysters we find an intermediate step towards a separa-
tion of the sexes, inasmuch as these organs are not active at
the same time in the same individual; but the male and female
organs are alternately so. The germ-glands are paired and
placed at the sides ; they also open separately. They generally
occupy a large portion of the coelom, and are often intimately con-
nected with other organs.

Gradational differences can be made out in the relations of the
two kinds of germ-glands in hermaphrodite forms, which indi-
cate the lines along which the separation of the sexes has been
effected. In some (e.g. Ostrea) the germ-gland is a hermaphrodite
organ in the fullest sense of the word. The follicles which produce

GENEEATIYE OEGANS OF MOLLUSCA.

381

tlie ova_, and those which produce the semen, are united together, and
the efferent ducts are common to both sets of products. In Pecten
also (P. varius) the latter arrangement obtains, but the germ-
gland itself is differentiated into a male and a female portion. The
former is placed in front and above, the latter behind and below.
When, finally, as in other forms, the separate germ-glands have
ducts which open separately, they are more highly differentiated
(Pandora). In many genera hermaphroditism predominates in some
species, while others have the sexes separate (Cardium).

The efferent ducts of the germ-glands are feebly developed,
and the glandular lobules are often placed close to their common
orifice. There are, therefore, no accessory organs of any kind.
They open in various ways on either side. The genital canal is
sometimes united with the excretory organ, so that it looks like a
differentiation from it, and the generative products are passed out
through it (e.g. Pecten, Lima, Spondylus) ; the genital canal is
sometimes united with the orifice only of this organ (e.g. Area,
Mytilus, Pinna) ; lastly, it sometimes opens by a proper papilla
(e.g. in Ostrea, Unio, Anodonta, Mactra, Chama).

From these facts it is clear that the excretory apparatus is of
great importance in aiding in the formation of the efferent ducts of
the generative products. The genital canal, inasmuch as it opens into
the excretory organ, appears to be a differentiation of the latter,
which is extended to the germ-glands in which the generative pro-
ducts are developed; and the gradual separation of the genital canal
from the excretory organ implies a further stage of differentiation,
which leads to a complete separation of the genital canal, and con-
sequently of the generative organs, from the excretory organ. This
arrangement, which is seen in all the higher Mollusca,
must therefore be derived from a primitive functional con-
nection between the generative and excretory organs; this
relation is, later on, simply indicated in a faint manner by the
approximation of the external openings of these organs.

The Lamellibranchiata, by showing the lines along which the
differentiation of the efferent ducts of the generative organs has
been effected, are not, so far as these relations are concerned, so
very distant from the Vermes or the Brachiopoda. Some of the
Vermes present us with relations identical with those of the Lamelli-
branchs; while others, having the efferent apparatus greatly, and
apparently specially, complicated (Platyhelminthes), make it at first
sight more difficult to give any explanation of this matter.

§ 294.

The generative organs of the Gastropoda and Pteropoda are in
many ways more highly differentiated. Although there is a her-
maphrodite gland,^^ as in the Lamellibranchiata, which is found in
a larger number of forms, the apparatus is considerably compli-
cated, and, as a rule, has copulatory organs connected with it. The

382

COMPAEATIVE ANATOMY.

generative gland is, moreover, always unpaired, and is placed, and
opens, asymmetrically, so that, as compared with the Lamellibran-
chiata, it must have undergone unilateral degeneration.

The hermaphrodite gland varies greatly in character. It is

always made up of a
number of lobes (Fig.
203, A), which form the
ovarian germs at their
furthermost blind ends
(a), while the seminal
masses are developed at
some distance from the
end (b). These spots are
not, however, separated
from one another, but
the common cavity of a
lobule is the spot at
which the various pro-
ducts are developed.
They are, therefore, cells
derived from epithelial
structures, which become
ova at one point, and at
another give rise to seminal filaments. This double production does
not seem to take place simultaneously, as a rule, so that the same
lobule or the same gland produces ova at one time, and sperm at

Fig. 203. Follicles of the hermaphrodite
glands of Gastropoda. A Of Helix horten-
sis. The ova (aa) are developed on the wall of the
follicle ; and the seminal masses (b) internally. B Of
Aeolidia. The seminal portion (b) of a follicle is
beset peripherally by ovarian saccules (a), c Com-
mon efferent duct.

another.

A differentiation may be made out in the lobules when the
ovarian portions form diverticula (B, a) ; in this case they are
grouped in a rosette around the seminal portion (h), and have the
appearance of secondary acini. The variations in the forms of the
hermaphrodite gland are due to the way in which the separate
lobules are united together ; thus each lobule may have its proper
duct, and the whole gland look like a greatly-ramified organ (Opis-
thobranchiata) ; or the acini may open in a row on one side of a duct,
as in some Pteropoda (Cymbulia, Tiedemannia) ; or they may be
grouped into racemose or lobate masses of glands, of which there
may be a number (Phyllirhoe), or they may form a single, more or
less compact, gland (some Pteropoda, as Pneumodermon, Hyalea ;
most of the Opisthobranchiata and Pulmonata).

§ 295.

The efferent ducts may be arranged in one or other of these
modes in the hermaphrodite Gastropoda.

1) There is a common efferent duct for the semen and ova, which
represents, therefore, a vas deferens and oviduct, and carries both
sets of products from the hermaphrodite gland to the generative
orifice. A caecal diverticulum forms the uterus, and also serves for

GEXERATIVE OEGANS OF MOLLUSCA.

383

tlie reception of tlie copulatory organ. At the generative orlfice_, the
semen either passes directly into the subjacent and eversible copnla-
tory orgaiij or, when this is some distance from the orifice, it is
carried to it by a ciliated groove. This arrangement is found in some
Opisthobranchiata, and in all the Thecosomatous Pteropoda.

2) The efferent duct of the hermaphrodite gland is common for a
short distance only; it then divides, and each canal goes on its own way
to the generative orifice. At the point of division it may be connected
with secondary organs, or undergo a simpler
kind of differentiation as expressed by modi-
fications of the calibre of the canals. This
may also happen to the common tube an-
teriorly to its bifurcation. In the Opis-
thobranchiata it is often widened out for
a considerable distance, and so serves as a
receptacle for the generative products which
are about to be passed out. In the Neph-
the common efferent
two portions. The
upper one (ve) from the hermaphrodite
gland {z) is simple, while the lower is
divided into two cavities for some distance ;
the narrower of these, which accompanies
the wider one in the form of a half-groove,
serves as a duct for the sperm, while the
wider one {u) belongs to the female appa-
ratus. At its upper end this latter has an
albuminiparous gland (Ed) attached to it,
and in the Helicin^ is beset with diverti-
cula, in which the ova obtain their envelope.

As the other canal is not completely shut
off from this uterus (u), the separation of
the ducts is not complete. It is only at
the termination of the uterus that the vas
deferens is continued on as an independent
canal (vd), which passes to the eversible
penis (p), which here forms a portion of the
efferent ducts. The more distal portion of
the canal secretes a substance which unites
the seminal masses into a seminal rope
(Spermatophor). The uterus, finally, is continued into a terminal
portion, which is known as the vagina ; this extends as far
as the common generative orifice, and may have a number of
appendages {ps, d) on its sides. In addition to a receptaculum
seminis [Rs), the Helicinae have a group of larger glandular tubes
(d), which are connected to a thick-walled tube [ps). This latter
appendage is eversible, and contains a pointed calcareous concretion
(spiculum amoris), which appears to be moulded in its internal
cavity.

ropneusta (Fig. 204)
duct is divided into

Fig. 201. Generative ap-
paratus of Helix hor-
tensis. 2; Hermaphrodite
gland, ve Hermaphrodite
duct, u Uterus. Ed Al-
bumiuiparous gland, dd Ac-
cessory glands, pc Dart-
sac. Rs Receptaculum
seminis. vd Seminal duct.
p Penis, fi Flagelliform
appendage.

384

CO^IPAEATIYE AXATOMY.

lu the other hermaphrodite forms the two ducts are, as a rule,
separated earlier, and the common canal undergoes merely unim-
portant modifications. The separate canals are modified in various
ways, the vas deferens is very long in most of the Opisthohranchiata,
and is consequently arranged in a number of coils. It is frequently
united with a superiorly attached, and occasionally wider, gland,
before it reaches the copulatory organ. The oviduct is not so long,
and is but seldom much widened out. At the end, however, of the
female efferent apparatus there are a number of accessory organs.
The two sets of efferent ducts either open into a common space
(generative cloaca), which is generally placed on the right side of,
and towards the anterior region of the body, or the two canals open
into a shallow depression, or they may open separately from one
another, and on the surface of the body.

§ 296.

The appendages of the generative apparatus may be
divided into those which belong to the male, and those which belong
to the female systems. The receptaculum seminis is one of the most
important of the female organs. It is a rounded or pear-shaped
vesicle, inserted by a hollow stalk into the vagina, and receives the
semen at the time of copulation (Fig. 204, Bs.) Sometimes there
are two of these appendages (Pleurobranchus), which maybe placed
at some distance from the vagina, and on the narrower oviduct
(Doris). In the Pteropoda and Opisthobranchiata the vagina is pro-
vided with a wide diverticulum, the glandular walls of which are set
in folds, and which functions as a uterus. A special glandular organ,
which resembles in arrangement the albumin ip ar on s gland, opens
into it. When this organ is absent, the wall of the uterus appears
to take on its function. Lastly, we must mention the copulatory
pouch of the Pteropoda, which appears to be a diverticulum of the
vagina, and receives the penis during copulation (Hyalea).

The male apparatus, also, is provided with organs of this kind,
which, in their simplest form, are widened parts or caecal structures
for the collection of the sperm. That elongation of the vas deferens,
which we have already mentioned, is physiologically an organ of this
kind. Similar arrangements are found in both the Gastropoda and
the Pteropoda. The glandular organs attached to the vas deferens
are also organs of this series ; they are ordinarily known as prostatic
glands.

Lastly, the male apparatus is connected with a copulatory
organ, which is either the modified and eversible end of the seminal
duct, which when not in use projects into the coelom, or it is a special
organ, which has no direct connection with the vas deferens, and
which, when not in use, forms a retracted tube. The organ is either
connected with the generative orifice, as in many Nudibranchs,
or is separated from it. In the Tectibranchiata (Aplysia, Bulla,
Bullaea, etc.), the penis opens at a great distance from the common

GENEKATIVE 0EGAN8 OF MOLLUSCA.

385

generative orifice; a ciliated groove conducts tlie semen, which
passes out from the generative orifice, to the copulatory organ.

Among the Pteropoda,Pneumodermon has the penis merely repre-
sented by a papilla placed within the generative orifice ; while in the
Thecosomata there is an eversible organ below the opening of the
vagina.

§ 297.

In many hermaphrodite Gastropoda the germ-glands may be
seen to alternate in function, so that at one time they represent a
male, and at another a female organ. This fact is an indication of
that separation of the sexes which is completed in most of the
Prosobranchiata.

Notwithstanding the sexes being separate, we find the lowest
conditions in the Prosobranchiata, for in many the apparatus is merely
represented by the germ-glands. They repeat, therefore, the same
characters as those which we met with in the Lamellibranchiata.
There are no efferent ducts in Haliotis and Patella. As in many of
the Lamellibranchiata, the germ-gland appears to pass out its secre-
tion through the excretory organ. In Eissurella this arrangement
is still more definite in character, owing to the connection between
the efferent duct and the excretory organ.

The generative organs of the two sexes have very much the same
general characters, so that it is often only possible to distinguish
them microscopically by the presence of the copulatory organ in
the male. The male and female germ-glands are, like the herma-
phrodite glands of many hermaphrodite Gastropoda, hidden in, or
placed close to, the liver.

In the female organs an oviduct, which is ordinarily coiled, arises
from the ovary, and turns towards the hind-gut, where it widens
out and forms a uterus. A short vagina is given off from this, which
passes to the generative orifice, which is placed close to the anus.
Accessory organs are rare in the dioecious Gastropoda. Where they
are best known they consist of an elongated spermatheca (recepta-
culum), which opens into the end of the saccular uterus, and which
has the duct of an albuminiparous gland connected with it(Paludina).
In the Heteropoda the seminal receptacle only is present, and it is
either attached to the end of the uterus (Atlanta), or united to the
vagina in front of this organ (Ptero trachea).

In the male organs, the duct (vas deferens) is either quite simple
or is provided, on its way to the penis, with a swelling, which
functions as a seminal vesicle. The end of the vas deferens opens
on the surface, and on the right side, of the body. There is no
copulatory organ in Haliotis, Patella, or Trochus. Where it is present it
is formed by a process of the dermo-muscular tube, and is a massive
broad body, which is frequently curved at its tip ; it is placed on the
right side of the body, or on the head, at the base of the right
tentacle, or at times (Heteropoda) close to the anus. A cihated
semi-canal, which often extends for some distance on the surface of

2 c

386

COMPAEATIYE ANATOMY.

tlie body, is attached to this organ ; it may be directly continuous
with the copnlatory organ, and forms a groove which reaches to it
(Dolium, Harpa, Strombus), or it may traverse the penis in the form
of a canal (Bncciniim, Littorina, Paludina).

§ 298.

The sexes are quite separate in all the Cephalopoda. The male
and female organs greatly resemble one another in their general
mode of arrangement; the most essential point in which is that
the germ-glands are not directly continuous
with their efferent ducts. This fact is of
importance, as it seems to point to the
use of an organ which primitively did
not belong to the generative appara-
tus. In any case, this arrangement is alto-
gether different to that seen in the germ-
glands of the Gastropoda and Pteropoda,
where the secreting portions of the glands
gradually pass into the efferent ducts (cf.
supra, § 292). In the Tetrabranchiata the
efferent ducts are not perfectly continuous.
The oviduct and sperm-duct both lead into a
wider cavity, from which these ducts start
again.

Of the female organs, the ovary is formed
by a Ibbate gland, which is invested by a
s]3ecial sac, with which it is connected at one
point only. The duct (oviduct) is, as a rule,
single. In the
tata it is double

points to its having been primitively double ;
in the rest — as also in Nautilus — this double
character has disappeared owing to the
atrophy of one of the oviducts. The oviduct
is attached to the covering of the ovary, so
that the ova only reach the duct by passing
through the space enclosed by this covering. The oviduct ordinarily
opens at the commencement of the funnel ; it only opens some way
back in the branchial cavity in those forms in which the male has a
copnlatory arm ; hectocotylism is the cause therefore of a functional
adaptation. At one point the oviduct has a pad-like circular invest-
ment of glands, formed by tubes set radially to the axis of the
oviduct (Octopoda). In Nautilus, there is a larger number of these
glands, and they extend almost as far as the opening of the duct.
Where these glandular organs are absent their place is taken by a
secreting apparatus placed close to the opening.

A pair of so-called nidamental glands'’"’ are the accessory
organs of the female apparatus ; they consist of elongated lamellar

Octopoda and in Loligo sagit-
(Fio’. 199, od), a fact which

Fig. 205. Male genera-
tive organs of Octopus.
t' Testis, c Its capsule.
ve Efferent duct, ve' En-
largement, which serves
as a seminal vesicle, g Ac-
cessory glands. h~S Need-
ham’s Pouch (after Cants
and D’ Alton).

GENERATIVE ORGANS OF MOLLUSCA.

387

tubes, wliicli are placed in tlie anterior region of the animal ; tbeir
short efferent ducts open beside the generative orifice. Their
secretion appears to cement the ova together; and these, in most
Cephalopoda, are united into racemose groups. In front of the
nidamental glands there is a pair of smaller glandular organs, which
consist of closely-coiled tubes, and have the same function as the
other set of glands.

A similar capsule (Fig. 205, c) to that found around the ovary
encloses the testis {t') ; this organ is made np of a number of
branched caecal tubes united together. These tubes are likewise
attached to the wall of the capsule, so that in this case also the
germinal matters first ]oass into the capsule, and then into the vas
deferens, which is continuous with it. This duct is a much-coiled
and gradually- widening canal (re), so that it forms a seminal
vesicle. Glands are embedded in the walls of its widened portion,
and in many cases part of the wall is converted into a larger
glandular organ, so that this part has yet another function. One or
two other distinct glandular appendages (g) are found in various
Octopoda. All these glandular differentiations of the wall of the
vas deferens secrete matter which is mixed with the sperm, and
which is used to form the special seminal ropes. At the end of the
glandular portion, or after uniting with these ducts, the seminal
duct is considerably enlarged, or pushed out on one side (Sepia,
Loligo) ; this modification may also be converted into a considerable
appendage {hN) (Octopus). These pouches of Needham serve as
receptacles for the seminal ropes or spermatophors, which are
formed in the glandular portion of the seminal duct. The rest of
the duct is continued, with but little change, into a papilliform pro-
cess, which is placed on the left side in the mantle-cavity (Fig. 178,
g), or it opens to the exterior at the base of such a papilla. We
have already described the way in which single arms become func-
tionally connected with the generative apparatus in many Cephalopoda
(5 254).

The formation of spermatophors, which is a comparatively
rare thing in the Gastropoda and in the other divisions, is the rule
in all the Cephalopoda, and is in them most complete. As a rule,
one of these seminal ropes forms a long cylindrical structure, on
which several envelopes may be distinguished. The contents of the
tube are only partly seminal, for in each spermatophor there is a
special substance which occupies the hinder portion, and which we
may call the exploding mass. The sperm is invested in a tubular
manner by a special coat, and is placed in the anterior division of
the spermatophor. Behind it is the anterior flattened end of a long
and spirally-coiled band, which extends through a large portion of
the spermatophor, and is continuous at its hinder end with the outer
coats. The substance of this spiral band is the exploding mass.
When it comes in contact with the water, the spiral band immediately
begins to elongate, and drives before it, and to the front end of the
spermatophor, that part which encloses the semen.

2 c 2

Eiglitli Section.

Tunicata.

General Review.

§ 299.

In conceding to tlie division o£ the Tunicata, formerly by general
consent reckoned among the Mollusca, the rank of a special phylum
of the Animal Kingdom, we do no more than assign its legitimate value
to the very peculiar organisation of these animals. Their pecu-
liarities separate them not only from all the classes of the Mollusca,
but also from all the other animal phyla, although it must be ad-
mitted that they present certain distant affinities to some Vermes
(the Enteropneusti), and that equally close relationships are readily
to be observed between them and the lowest Vertebrata. We shall
return to the subject of these relationships in treating of the Verte-
brata; here it is only necessary to remark that the absence of a
clearly-marked metamerism of the body forbids the intimate asso-
ciation of the two groups, although indications of the formation of
metameres in certain regions of the body may be j)ointed out in
some Tunicata.

In the position of the most important organs and their primitive
relations we find the most obvious indications of Vertebrate affinity.
Thus we have the enteron, with its foremost division adapted to the
functions of a respiratory organ. A movable appendage of the body,
present in the adult state in only one division, but in others in
the larval stage, contains an organ of support, which exhibits the
closest resemblance to the primitive axial skeleton of the Vertebrata.
A further general characteristic of the group is seen in the hyaline
body-covering, which often attains a considerable development, and
is known as the mantle.

TUNICATA.

389

The following divisions are recognised :
Copelata.*

( Appendiculariae) .
Oikopleura, Fritillaria.

Acopa.

1) Ascidiae.

Simplices.

Cynthia, Phallusla, Molgula.
Sociales.

Clavellina.

Compositae.

Amarroecium, Botryllus.

2) Lucise.

Pyrosoma.

3) Cy clomyaria.

Doliolum

4) Thaliadae.

Salpa.

Bibliography.

Huxley, Observations on the Structure of Salpa and Pyrosoma. Transact. Royal Soc. Lond. 1851. —
Vogt, C., Recherches sur les animaux infdrieures de la Mdditerranee. II. Memoires de I’Institut
de Geneve. 1852.— Ganin, Zeitschr. f. w. Zool. Bd. XXV.

Copelata : Gegexbaub, Zeitschr. f. w. Zool. VI. — Huxley, Further observations, etc. Quarterly
Journal of Microscop. Sc. 1856. — Fol, H., Etudes sur les Appendiculaires. Mem. Soc. de
physique et d’hist. nat. de Genbve. XXI.

Ascidise ; Savigny, Memoire sur les animaux sans vertebres. II. Paris, 1816. — Schalck, De asci-
diaram stnictura. Halae, 1814. — Milxe-Edwaeds, Observations sur les ascidies compos^es.
Paris, 1841.— Eschhicht, Videnskab. Selsk. Abhandl. IX. 1842. — Van Beneden, Memoire sur
I’embryog^nie, I’anatomie, et la physiologic des Ascidies simples. M^m. de I’Acad^mie Royale de
Belgique. T. XX. 1846. — Metschnieoff, E., Ueber die Larven und Knospen von Botryllus. Bull.
Acad, des Sc. de St. P^tersbourg. T. VI. — Krohn, A., Fortpflanzung von Botryllus. Archiv f.
Naturgesch. 1869. — Kowalevsky, Entw. d. einf. Ascid. Mem. Acad. imp. de St. P^tersb. VII.
Ser. X. — The same, Ueber die Knospung der Ascidien. Arch. f. mikr. Anat. X. — Kupffer, C.,
Z. Entw. d. einf. Ascidien. Arch. f. mikr. Anat. VIII. — Hebtwig, R., Zur Kenntnis des Baues
der Ascidien. Jenaische Zeitschr. VII. — Gi.iBD, A., Rech. sur les Asc. comp. Arch, de Zool. I.
— Lacaze-Duthieks, H. de, Les Asc. simples des cotes de France. Arch. d. Zool. III.

Lucise : Kowalevsky, A., Entw. von Pyrosoma. Arch. f. mikr. Anat. XI — Panceei, P., Gli
organi luminosi dei Pirosomi. Atti del R. Accad. delle Sc. fis. e math. Napoli. V.

Cyclomyaria : Kbohn, Ueber die Gattung Doliolum. Arch. Nat. 1852. — Gegenb a ub, Doliolum.
Zeitschr. f. wiss. Zool. 1854.— Kefeestein u. Ehlebs, Zoolog. Beitriige. Leipz. 1861.

Thaliadse : Eschricht, Undersogelser over Salpeme. Kjdbenhavn, 1841. — Sabs, Fauna litoralis
Norvegise. I. — Kbohn, A., Ann. sc. nat. III. vi. — Mulleb, H., Zeitschr. f. w. Zool. IV.—
Salensky, Embryonal. Entw. d. Salpen. Z. f. wiss. Zool. XXVII. — The same, Ueber die Knos-
pung der Salpen. Morphol. Jahrb. III.

* These t-wo classes were formerly separated according to whether they had or had
not a propelling tail, as the names of the classes showed ; I have retained the nomen -
clature without giving an importance to this character, which does not belong to it ;
the larvae of many Acopa have this directive organ. A much greater difference between
the two divisions is to be found in the characters of their spiracles. In the Copelata
these open to the exterior. In the Acopa they open into a cavity, which is formed
from a part of the rudimentary spiracle of the Copelata. This is the most essential
characteristic.

390

COMPARATIVE ANATOMY.

Form of the Body.

§ 300.

The form of the body of the Tunicata undergoes in the various
divisions such remarkable modifications, that the resulting forms
compared in their extreme conditions of development appear to
have no relationship whatever to one another. In the Copelata, the
lowest Tunicata at present known, the body exhibits two divisions,
of which one encloses the most important organs, whilst the other
has the form of a very wide and long movable appendix — the pro-
pelling tail. The anterior division of the body contains the intestinal
tract together with its respiratory portion. The gut opens to the
exterior by the ventrally-placed anus. Two slits break through the
body-wall from the respiratory cavity. On the whole a bilateral
symmetry predominates, and accordingly two antimeres are recog-
nisable. The tail portion of the body in the Copelata, attached
ventrally to the fore-portion, projects at an angle from the latter,
and thus has the appearance of a simple appendage. How the earlier
stages explain this condition must be described below.

A form coming near to this condition is possessed by the laiwm
of the Ascidians, in which the tail is simply a prolongation of the
aboral end of the body, and so far appears to present us with a more
primitive condition. A similar process is found also in the young of
Doliolum, whence it may be supposed that all Tunicata have descended
from a common ancestor provided with such a tail-like region of
the body. In Doliolum, as the power of swimming becomes associ-
ated with modifications of the respiratory cavity, the propelling tail
dwindles. In the Ascidians free locomotion is lost when the pro-
pelling organ atrophies. The complete animal takes on a fixed
mode of life. Whilst a greater complication of the structure of the
organism is acquired the external appearance becomes simplified.
The tube-like body exhibits two openings, approximated to one
another. The orifice of entrance corresponds to that of the Copelata.
The second opening leads into a chamber having the function of a
cloaca, which has arisen from a development in connection with the
primitive respiratory slits. This condition of parts holds good also
for the higher divisions of the Tunicata, amongst which the Cyclo-
myaria and Thaliada appear as swimming forms, moving themselves
by the action of the walls of the body. Their body, which is on the
whole of a cylindrical form, has its incurrent orifice at one pole of the
long axis, and the orifice leading from the cloaca at the other or
aboral pole.

§ 301.

Complications of the external form of the Tunicata appear in
connection with the very common process among them of a sexual

FOEM OF THE BODY OF TUYICATA.

391

reproduction whicb. leads to tbe production of Colonies or Stocks.
Gemmation is the fundamental process in this phaenomenon. It pre-
dominates in the Acopa. In many Ascidia a new individual buds
from the body of the adult animal, the parent sending* forth a
runner (stole) which is composed of form-elements belonging both
to ectoderm and endoderm. From this there is gradually differen-
tiated an organism identical with the parent animal. In this way
colonies of Ascidians (Ascidiae sociales) are produced. In others this
proceeding appears with its successive steps compressed at a very
early period of life, and accordingly there buds forth from the
embryonic body of one Ascidian a second (Didemnum). Thus we
get two persons closely united to one another. From this case, it is
easy to derive those conditions in which the sessile young individual
gives rise by budding to a number of persons (Botryllus). In these
several generations, all produced by budding, follow one another.
From the first a bud grows out, which, like Didemnum, gives rise by
budding to two persons, and from these in like manner four arise.
When the mother-animal dies down, the eight budlings are left in
commnnication with one another by means of the cloaca, and form a
rosette-shaped group. Such and similar processes give rise to the
colonies which we find among the Ascidiaa compositee. The com-
bination of the scattered groups takes place through the agency of
a tissue belonging to the integument which in those persons which
live isolated lives is known as the mantle (outer mantle).

By means of a peculiar method of grouping, the persons of the
Luciae form cylindrical colonies. The wall of the hollow cylinder is
formed by the ascidian-like persons and their common investment.
On the outer surface of the cylinder are found the entrance-
orifices ; opposite to them, and opening into the internal cavity, are
the excurrent orifices. The multiplication of the persons of the
colony is carried on by budding. The formation of new colonies is
provided for by the sexual reproduction. From the egg arises an
embryo, from which at once four persons bud forth. They remain
invested by the mantle of the first, and constitute as soon as they are
born a new colony.

Since the persons which develop from an egg never in the com-
pound Ascidians acquire sexual organs, and since these organs on
the contrary appear in those persons which arise by gemmation, w^e
have here an example of the phtenomenon known as Alternation of
Generations.

What is performed in the Ascidiae by means of offshoots starting
from the surface of the body, is carried out in the Cyclomyaria and
Thaliadae by a special organ — the germ-stock or stolo prolifer. It
exists also in the Luciae, but has there a more limited functional
activity. In the Cyclomyaria it appears as a process springing
usually from the dorsal surface of the body in the neighbourhood of
the excurrent orifice ; in the Salpae and in the Pyrosdmae it has a
ventral origin, and agrees with the condition found in Cyclomyaria
only at the first, for, instead of budding out externally, it takes up a

392

COMPARATIVE AJfATOMY.

position, varying in different cases, within a cavity generally in the
neighbourhood of the intestine. In its relation to the formation of
buds the stolon of the Salpas also behaves differently from that of
Doliolum. In the latter, bud-generations sprout from the stolon ;
they are arranged in rows, are often dimorphic, and remain attached
to the stolon by short processes. In the Salpm similar buds arise by
outgrowth from the stolon, but each bud encloses within its base half
of the whole area of the stolon ; so that when two rows of such buds
are formed the material of the stolon itself is transferred to the body
of the buds. The maturity of the chain-like connected series of
young secondary buds (into which the primary bud divides) results,
in consequence of this disposition, in a separation (Fig. 206, n) from
the parent of the portion of the stolon concerned in its production.

Fig. 206. Asexual form of Sal pa
pinnata (solitary form), n Em-
bryo-chain escaping to the ex.
terior.

Of

Fig. 207. Sexual form of Salpa pinnata
(chain form), t Connecting bar. a Incurrent
orifice. h Excurrent orifice. c Ganglion.
d Gills. / Heart, h Ventral groove, r Hepatic
pouch. vhn Embryo wdth embryonic organs
(both figures after C. Vogt).

This arrangement brings about an alternation of generations,^^
seeing that the forms provided with a stolon are always permanently
devoid of sexual organs. On comparing this- series of pheenomena
with that occurring in the Ascidim, we find, just as in the Ascidias,
proliferous outgrowths, namely, the stolons. The outgrowth in
question is here confined to a definite part of the body. In Pyro-
soma a proliferous stolon is present, sunk in the mantle, and from
every such stolon only a single bud is developed; side by side with
this are developed sexual organs. Accordingly it is not possible to
suggest that the stolon belongs to the sexual apparatus. In the
Salpm and in Doliolum, contrary to what we find in Pyrosoma, the
proliferous stolons give rise to copious generations of buds. In this
case, however, the absence of a sexual apparatus is the accompani-

INTEGUMENT OF TUNICATA.

393

meut of the richer gemmation, and we must regard the sexual appa-
ratus as having been atrophied. This atrophy of the sexual apparatus
is referable to the increased reproductive activity of the stolon.
Ill the Salprn the progeny of the sexless generation are inva-
riably sexually complete, and so we have an example of pure
alternatio generationis,'’^ whilst in Doliolum the asexual reproduc-
tion is only played out after a numerous series of stolon-bearing
generations have succeeded one another. In this manner the condi-
tion of the Cyclomyaria approaches more closely to that of the
original Ascidian-gemmation, and this both in the external position
of the stolon and in the mode of connection of the buds with the
stolon. The internal stolon of the Salpac, on the contrary, is widely
separated from the primitive arrangement, not only by its position,
but also by the assumption of the material of the stolon by the buds.

Integument.

§ 302.

The investment of the body of the Tunicata consists in its
primitive condition of a cell-layer formed from the ectoderm. It
persists in this condition in the Copelata, in which it appears to
represent, at any rate to a large extent, the body- wall. The flattened
cells here form but a single layer. This simple arrangement gives
place, in the higher division, to a complication ; the primitive
arrangement only occurring in a transient form in the earlier stages
of development.

A layer of material secreted from the cells of the ectoderm forms
a covering enclosing the body, and known as the ^^mantle.^^ This
condition is not altogether a new one, for in many Copelata a fore-
runner of the mantle is to be found. In them the cells in the
neighbourhood of the incurrent orifice attain considerable dimensions,
and secrete a slimy but consistent substance, which forms in large
quantities, and gradually enclosing the body more or less completely,
assumes the appearance of a basin-like structure of relatively large
dimensions. It was described by earlier naturalists as the ‘‘ house,^^
and functions as a protective organ for the body (Oikopleura).

The secretory activity, which is here confined to a particular
region of the body-surface, is spread in the other Tunicata over the
whole superficies. Its products constitute the outer mantle,
which in its essential character belongs to the category of cuti-
cular structures. In consequence of the migration into it of form-
elements from the ectoderm, the resulting tissue assumes the
characters of the connective substances. The primarily homo-
geneous layer becomes converted into an intercellular substance.
The cells scattered in it present a great variety of characters. Often
the mantle thus formed attains a preponderancy over all other
organs, and serves, in consequence of its rigidity, as a supporting

394

COMPAEATIVE ANATOMY.

organ for the enclosed parts. The consistency of this covering varies
from the softness of jelly to the hardness of cartilage. It is
generally of glass-like transparency^ and in the Ascidians often
variously coloured. Complications of the structure of the mantle
are brought about in many Ascidians (Phallusia) by the penetration
into it of great numbers of blood-vessels. Very remarkable is the
development of the mantle into two valves, movable on one another
like the shells of the Lamellibranchiata, and which are able to open
and shut as do the latter (Chevreulius). In the colony-forming
kinds, this mantle-layer is common to all the persons, enclosing
them together.

The formation of this mantle leads to a suppression of the
differentiation of all other tegumentary organs ; we have never-
theless to notice in the Copelata the formation of numerous other
organs of this category — unicellular glands, hair-like processes, etc.

A mass of cells is also developed from the ectoderm in the
Pyrosomse, and occupies a position in the neighbourhood of the in-
current orifice; it is the paired luminiferous organ of these animals.

Hektwig, 0., Ueber den Ban und die Entwickelung des Tuuicatenmantels.

Jenaische Zeitschr. Bd. YII.

Skeleton.

§ 303.

In most Tunicata, the mantle, in virtue of its rigidity, functions
as the supporting organ of the otherwise soft body. Besides this
\ve meet with a peculiar organ of the greatest morphological import-
ance. In the tail-like propeller of the Appendicular iae there exists
an axial organ which extends forward as far as the fore-body. It is
made uj3 of cells, which secrete a fairly resistent chord consisting of
homogeneous substance enclosed in a continuous sheath, the remains
of the cells being found at a later period still resting upon the chord.
This chord acts by means of its elasticity, bringing the propelling-
tail back to its previous position after it has been bent by the con-
traction of its muscles. Such an axial organ (Fig. 208, cli) is retained
by all those Tunicata-larvee which possess a mobile propelling tail,
both Ascidians and Cyclomyaria. It disappears with the tail. Its
relations of position show it to be homologous with the chorda
dorsalis of the Vertebrata, and we may therefore designate this
structure also as chorda.

Muscular System.

§ 304.

The disposition of the musculature presents very different
features in the various Tunicata. The Copelata, for instance.

NEEYOUS 8YSTE]\I OF TUFTICATA.

395

possess a complete muscular layer with longitudinal fibres^ on the
propulsory tail only. It can be divided into a dorsal and a ventral
band of varying breadth^ covering in the chorda from above and
below. Muscles are entirely absent from the anterior division of the
body which contains the viscera.

'In the Ascidians the musculature forms a sac lying beneath the
ectoderm_, and is separable in Cynthia into several layers according
to the course of the fibres. In others the muscular layer is feebler,
composed of intercrossing bands (A. composite). The Pyrosomm
have no muscles excepting around the excurrent and incurrent
orifices of the body. The musculature of the Cyclomyaria is broken
up into isolated annular bands ; and in the Salpee also it forms hoops,
which are, however, here and there joined to one another. This
hoop -like formation arises from the differentiation of a primitively
continuous muscular layer. Gaps arising in this become gradually
larger until the breaking up of the layer into separate hoops is
brought about. At the incui*rent and excurrent opening of the
Ascidians the musculature has also an annular disposition and has
the characters of a sphincter.

The form-elements of the musculature are transversely striated.

Nervous System.

§ 305.

The central apparatus of this system occupies a dorsal position
in all Tunicata^ and proceeds from a differentiation of the ectoderm^
as has been ascertained from the study of the development of
Ascidm and of Salpm. In the general relations of the parts there are

A' 0

Fig. 208. Ascidiau embryo with only a part of the tail 0. N Nerve-centre, forming
a cavity in front N', produced behind into a nerve-cord n. 0 Eye. a Auditory organ.
K Embryonic foundation of the respiratory chamber, d Ditto of the digestive canal.
0 Ditto of the mouth, ch Chorda (after Kupffer).

agreements with what we find in the lower Worms. The in-sinking
of the ectoderm forms a tube which remains open for a time, and
then becomes pinched off from the surface-layer and extends itself
in Ascidian larvae (Molgula) as a chord reaching throughout the length
of the tail (Fig. 208, n). A central canal perforates the chord, and

30G

COMPARATIVE AXATOMY.

can be traced into tlie anterior larger mass (N). A division of this
anterior mass into three consecutive lobes, produced by an unequal
thickening of the wall of the tube, the foremost of which is in
Ascidiae and Salpee in close connection with the origin of the visual
organ, is also to be made out in the Copelata, in which the funda-
mental structure sketched out in the Ascidian larvae undergoes a
furtlier and permanent complication. In them we find the nervous
system made up of an anterior elongated ganglion (Fig. 209, n)
which exhibits three dilatations (App. flagellum), and passing back-
wards in the form of a chord (n') to the base of the tail, is continued
along that organ to its extreme point. At the root of the tail a
ganglionic swelling is formed on the chord, and two others follow
after this (A. furcata). The first appears to be the more con-
stant. This apparatus, in its rudimentary condition a
continuous one, as appears from the Ascidians’ tadpole, must
be regarded as the central-organ of the nervous system.
It is noteworthy that in the Copelata, too, it encloses a canal
which extends from the foremost ganglion to the ganglion at
the root of the tail. The central elementary parts (the nerve-
cells) are not, however, equally distributed, but form the
ganglia only, to which the rest of the chord serves as commissure.
The continuation of the nerve-chord in the tail lies to the left of the
chorda, if we consider the two surfaces of the tail (as its relation
to the rest of the body requires), as dorsal and ventral respectively.
This asymmetry is in the Ascidian larvae either developed late or not
at all, so that as the primitive condition we are justified in assuming
a median dorsal nerve-chord. The form of the nerve-centre
thus determined is one which is in the highest degree noteworthy,
since it is not presented in any other division of invertebrate animals,
in all of which the prolongation of the central organ takes place in
a ventral position.

Peripheral nerves pass out from the anterior ganglion, branching
laterally around the incurrent orifice of the branchial chamber.
Others pass backwards to the spiracula. In the tail, nerves pass
out from the ganglia, as too in the Ascidian larva? there are nerve-
branches on the caudal portion of the nerve-centre; terminally the
chord gradually breaks up into a number of branches.

§ 306.

The atrophy of the tail, or its complete absence, brings about in
connection with the further elaboration of the fore-end of the body
by the development of the branchial slits, a considerable change in
the nervous centre. In the Ascidians the caudal division of the
nerve-centre disappears, and in Pyrosoma and Salpa the embryonic
foundation of the organ is limited to the anterior portion which is
accordingly more voluminous in proportion. The three vesicular
divisions which still appear as embryonic rudiments in the Salpju
give place to a single ganglion-mass. The Ascidians have this

SENSE-OllGANS OF TUNICATA.

397

body located between tbe incurrent and the excurrent orifices
(Fig. 210^7^), and it occupies the same position indicating the dorsal
region in all Acopa. Whilst in Ascidians the peripheral nerves are
given off more abundantly from the fore and hinder divisions of the
often elongated ganglion, we find in the Cyclomyaria and Pyrosoma
nerve-branches given off laterally also, and in the Salpac the ganglion
gives off numerous nerves all around in a radial manner.

The arrangement of the whole apparatus is very widely different
from that of the higher Worms, Arthropods and Mollnsca, and it is
only among the Yertebrata that a comparable condition is to be
found. To this subject we shall have to return hereafter.

Organs of Sense,

§ 307.

As sensory apparatuses of an indifferent nature, possibly serving
a tactile function, certain cells are described in the integument of
many Tunicata (Salpa), and appear to be nerve-endings. From such
cells filamentous processes stretch towards the surface, as for instance
in the denticnlations of the incurrent orifice in Doliolum, and in the
margin of the same orifice in Salpae. The processes found in many
Ascidians around both orifices of the body are no doubt provided
with similar organs.

A more differentiated organ of sense is the so-called ciliated
groove, which is formed on that surface of the ganglion which is
turned towards the branchialchamber(Salp8eand Pyrosoma), and in all
Tunicata, even when it takes up a position in front of the ganglion,
retains a connection with the nerve-centre, which it gains at a very
early period of development. By the upgrowth of the margin of
this groove turned towards the branchial chamber, a variety of
forms, some stalked, are brought into existence, whilst dilatations of
the groove give rise to modifications of another kind. The signifi-
cation of this organ, clothed as it is with flagellate cells, must be
that of an Olfactory organ, or at any rate of an organ, the
function of which is to test the quality of the water coming into the
branchial chamber.

With more certainty can Visual organs be distinguished. They
have been observed in the larvae of Ascidians as well as in
Pyrosoma and Salpae. They take their origin in the anterior
vesicular enlargement of the central nervous system (Fig. 208, A'),
and in fact, on the dorsal portion of the wall of this vesicle. A dark
mass of pigment sunk in the wall carries on its surface a hemis-
pherical refractive body, over which a second is stretched. In the
neighbourhood of the pigment-mass, the cells are radially directed
towards it, and accordingly represent a division of the central
nervous system belonging to the eye. Probably processes of these

308

COMPAEATIYE ANATOMY.

radial cells are simk in tlie mass of pigment directed against the
light-breaking body perched upon it. In Pyrosoma the visual
organ appears as a division of the ganglion^ a pigment-clothed
outgrowth. On one spot of this, free from pigment, is placed a
mnlti-lamellar refractive apparatus.

Although also sessile on the ganglion, the eye of the Salpte
seems to be more conspicuously elevated, and at the same time
divided into several portions. Whether we have here the same type
as in the Ascidian larvce is not yet ascertained.

Auditory organs are known in the Copelata, Cyclomyaria,
and many Ascidian larvas. In the first named, a vesicle furnished
with an otolith is affixed to the left side of the anterior ganglion,
and has on its inner wall fine hairs wdiich appear to hold the otolith.
Also on the left side, but at a greater distance from the ganglion, is
a similar vesicle to be found in one generation of the Cyclomyaria.
A nerve passes from the ganglion to the vesicle. In the Ascidian
larvae there is also in the same chamber of the ganglion which
surrounds the rudimentary eye an otolith, which is supported by
fine hairs (Fig. 208, a),

j\[. UssoFF, Beitr. z. Kenntniss der Organisation der Mantelthiere. Berirht der
K. Ges. der Frennde der Natnrforscliung. Moskan, 1876 (Russ.).

Alimentary Canal.

§ 308.

This system of organs forms the part of the body which is the
most peculiar in the Tunicate phylum, and enables one to draw a
sharp line between that phylum and most of the other divisions of
the Animal Kingdom. Its peculiarity consists in the elaboration of
the anterior portion of the gut into a respiratory organ, similar to
that which we saw among the Worms in the Enteropneusti. The
water taken in not only brings nutrient matters, but serves also
for respiration, finding its exit through special openings (spiracula)
which are cut in the wall of this division of the gut. As a con-
sequence, peculiar arrangements are brought into existence which
have the function of directing the food particles entering the
respiratory chamber to the commencement of the proper alimentary
canal. The embryonic foundation of the entire gut proceeds from
the entoderm, which, however, only corresponds to the rudiment of
the respiratory portion to begin with, and from this, as a secondary
process, the proper digestive tract buds forth. We shall therefore
treat these two divisions of the primitively uniform alimentary canal
separately from one another ; the more so since considerable modi-
fications which the respiratory chamber undergoes, bring about in
their turn modifications of the form of the body.

ALIMENTAKY CANAL OF TUNICATA.

399

Respiratory Chamber (Branchial Sac).

§ 309.

The simplest condition — that found in the Copelata — must serve
as the starting-point. The incurrent orifice (Fig. 209, o) homolo-
gous with a mouth, occupies the foremost region of the body, and
leads into a rapidly widening space, triangular in section {k) . The
broad ventral surface is posteriorly somewhat expanded so that it
forms two lateral cornua. These gradually narrow each into a
tube-like continuation which breaks through the body-wall on the
ventral surface (k') and forms a branchial aperture (spiraculum) .
The dorsal continuation of the respiratory chamber is continued

Fig. 209. An Appendicularia seen laterally, n Nerve-centre, n' Nerve-chord.
nt. Otocysfc. o Incurrent orifice, fc Respiratory chamber, e Ventral groove. / Ciliate
• ti’act. i Digestive canal, a Anus, h' Spiracle, t Testis, ov Ovary, c Root of the

tail (after H. Fol) .

without any sharp line of demarcation into the beginning of the
proper digestive canal {{),

The two branchial apertures serving for the ejection of the water
are cylindrical tubes, each of which takes its origin as an outgrowth
of the wall of the respiratory chamber, into which an insinking of
the outer body- wall pushes its way. The tube is beset with a ring
of ciliated cells, which excite a streaming of the water directed at
will now from the mouth through the respiratory chamber and the
spiracula to the exterior, now in the reverse direction from without
through the spiracula into the branchial pharynx towards the mouth .
Mouth and spiraculum act here both as incurrent and excurrent
orifices for the water.

On the ventral surface of the respiratory chamber is found a
deep groove, the ventral groove (e), open by small slits to the
respiratory chamber. In front two bands of cilia (/) stretch from
this groove towards the dorsal surface, embracing the entrance to
the respiratory chamber. These structures are all connected with
the taking in of food.

400

COMPArxATIVE ANATOMY.

§310.

The rudiment of the branchial chamber or branchial-gut gives
rise in the Acopa to very highly elaborated differentiations which
are in harmony with the condition presented by the Copelata. In
the latter, two outgrowing sacs are formed which are not placed
in communication with the exterior, until each is met by an ingrowth
of the ectoderm ; and so, too, in the Ascidians, two lateral sacs arise
by a pinching off of the branchial gut. They communicate for a
time with this their point of origin, but subsequently separate from
it, and grow up around the walls of this part of the gut dorsally,
until they meet and unite with one another. As a result we have a
cavity formed around the branchial chamber by the lumen of these
united sacs, the peribranchial space (perithoracic chamber of authors) .
An insinking of the surface of the body approaches the point of
union of the two halves of the peribranchial space, and forms, when
it has finally broken through, a communication to the exterior, the
excurrent orifice. On the ventral face the separation of the two
spaces persists. During the concrescence of the two sacs which
grow up round the branchial chamber, and of the superficial in-
sinking, the anal aperture is gradually brought into the area of this
space. This region then constitutes the cloaca (Fig. 210, d). In the

walls of the branchial chamber there now
arise clefts leading into the peribranchial
space ; in fact, branchial slits, which con-
sequently have quite a different importance
from that of the two primary spiracula. '

Gradually, the entire wall of the respi-
ratory chamber breaks up into a lattice-
work, the fine slits of which, arranged in
rows, are beset with cilia. In the rods of
the lattice-work blood-channels are exca-
vated. Water passes through the slits into
the peribranchial spaces formed by the out-
growth of the above-mentioned sacs, whence
it is conducted to the cloaca, and thence to
the common excurrent aperture.

In the compound Ascidians the excur-
rent apertures of a number of individuals
are united to form a common cavity, so that
each group of individuals possesses a single
excurrent aperture placed in the centre and
surrounded by a number of incurrent aper-
tures. The entrance into the respiratory
chamber is, particularly in the Ascidians, surrounded by tentacular
organs, which are partly in the form of external processes, partly
placed at a distance from the orifice, and are pointed towards it.
The lattice-work of the gills affords an endless variety in the
arrangement of its component rods, and in the form and number of

Fig. 210. Diagram of an
Ascidiau. o Incurrent ori-
fice. i Respiratory chamber,
c Ventral groove, n Gan-
glion. d Digestive canal.
cl Cloaca. g Generative
gland.

ALIMENTAEY CANAL OF TUNICATA.

401

its rows of slits ; as well as outgrowths of diverse kinds, which
are sometimes ridge-like, sometimes papilliform, and give rise to
numerous complications by the formation of anastomosing processes.
The most remarkable are the tongue-like appendages languets ”)
found in Ascidians and in Pyrosoma, which form a long row along
the dorsal surface. Opposite to them lies the ventral groove,
already mentioned above.

The branchial chamber of the Ascidians appears, from what has
just been stated, to be, in respect of the structure of its walls, a
very difPerent organ from that of the Appendicularige, and must
have been formed only after a long series of modifications.

The same is essentially true for the other Acopa. The Pyrosomse,
in other respects very closely allied to the Ascidians, present an
aboral position of the cloacal opening in conjunction with the arrange-
ment of the several persons forming a colony around the walls of
a hollow cylinder. The persons arranged in the wall of such a
cylinder are placed with their incurrent orifices on the outer surface,
whilst the cloacEO open into the cavity of the cylinder, the orifice of
which serves as the common outlet of all the cloacae.

In the Cyclomyaria the body, which in its mature form is tub-
shaped, has a wide internal cavity. The gill, which traverses this cavity
obliquely, and is formed by a membrane perforated by a pair of clefts,
divides the internal cavity into an anterior and a posterior division.
The anterior is the branchial chamber, into which the incurrent
orifice leads ; the posterior space, into which the mass of the viscera,
covered by the body-wall, protrudes, is the cloaca, and corresponds
to the chamber formed by concrescence around the primitive branchial
chamber in the Ascidians. The Salpse have a similar disposition of
parts. The gill is, however, in them more completely detached from
the wall of the respiratory chamber, and forms a rafter stretching
obliquely from the dorsal wall of the respiratory chamber in front,
to the ventral wall behind, on each side of which the respiratory
chamber is widely open to the hinder space representing the cloaca.
The excurrent orifice proceeding from this has a more dorsal position,
and is not unfrequently drawn out into a tube-like form (Fig. 212, h).
In consequence of the reduction, in this case, of the gills to the
rafter-like septum, there is no formation of actual gill-slits, and the
water taken into the branchial chamber streams laterally on each
side of the median branchial septum into the cloacal chamber.

The taking of water into, and its ejection from, the body, has in
both Cyclomyaria and Thaliadm a close connection with locomotion.
This function is here in fact connected with respiration, and the
position of the incurrent and excurrent apertures are consequently
of importance.

The water taken in in front is, after it has passed the respiratory
chamber, driven on towards the aborally-placed excurrent aperture by
the action of the muscular rings of the body- wall, and the stream thus
expelled works as a vis a ter go, and moves the body forwards by
jerks.

2 D

402

COMPAEATIVE AXATOMY.

A plisenomenoii wortliy of the greatest attention appears in tlie
Acopa in tlie disposition of tEe branchial slits, and is in the Salpae
alone obliterated by the peculiarities of their organisation. This
is the disposition of the slits in the manner of metameric
structures. In Doliolum they form two rows of transverse clefts,
and in Pyrosoma and Ascidians their transverse arrangement is also
perceptible, although in the latter several or many slits occur in a
cross series. Although this disposition is exhibited merely in a part
of the gut, yet we are able to recognise in it a condition which
supports the interpretation of it as an instance of metamerism.
Metamerism exhibits itself here in fact without the participation of
the entire organism, and it is not difficult to understand how, under
ceiTain conditions, other parts of the body might take part in it.

§ 311.

The close relationship among themselves of all divisions of the
Tunicata is further exhibited in the existence of peculiar organs
belonging to the respiratory chamber which are connected with the
nutritive functions of the animal. These organs are the ventral
groove and the ciliated tracts. The ventral groove (hypo-
branchial groove) (Fig. 211, Bn), also called Endo style, is a groove
lying in the ventral median line of the branchial chambers wall,
possessing up-standing margins (*) (ventral folds), and terminating

B

Fig. 211. Diagram to exhibit the relations of the respiratory chamber to the ventral
groove. J. in Balanoglossns. B in Tunicata. r Eespiratory chamber, "/i. Ventral

groove. * Ventral folds.

at its anterior as well as at its posterior extremity in a blind
dilatation.

The sides of the groove, which is at first in the Salpge very broad,
but later becomes narrow as in the other Tunicata, do not sink
equally below the surface in all parts, but form in various regions a
variety of prominences which may be described as longitudinal
ridges parallel with the groove. Between these are more or less
deeply-cut fissures, so that the contour of the groove in cross-section
is described by a much contorted line. The epithelium of the
respiratory chamber exhibits even at the free edge of the groove

ALIMENTARY CANAL OF TUNICATA.

403

remarkable modifications. The cells form up-standing longitudinal
ridges. At the bottom of the groove, between the two most deeply
embedded ridges, there are cells provided with long vibratile filaments
which may even project into the respiratory cavity. The Copelata
possesses the simplest arrangement of this organ. In many there is
only a single cellular ridge. Two are recorded in Doliolum. Three
occur together, with other complications, in the Ascidim and Salpm.
The margins of the groove as a rule lie in close apposition, so that
the groove is closed except at one part near its foremost extremity.
At this spot commence the ciliate tracts which embrace the
entrance to the respiratory chamber. They are clefts beset with cilia-
bearing cells which take a dorsal course and lead either to the
oesophagus (Copelata) or to the neighbourhood of the great ganglion,
taking a spiral turn on the way (Doliolum), or end in a ciliated
groove (Salpae). A similar division of the foremost section of the
intestinal tract was noted in the Enteropneusti (Fig. 211, A). Two
longitudinal folds (*) separate this region into a respiratory (r) and
an alimentary (n) portion. The latter seems to be strictly comparable
with the ventral groove of the Tunicata, which in its early stage of
development is also of very considerable dimensions relatively.

The function of the ventral groove is that of a glan-
dular organ. The cellular ridges secrete a slimy substance which
is carried forwards to the open part of the ventral groove by the
cilia placed along its floor, and from thence is further moved along
the ciliate tracts. At the same time the mucous masses project into
the lumen of the respiratory chamber, arrest the nutrient particles
taken in with the water, and become formed with these j)articles
into a string which passes into the oesophagus. Since too the free
margins of the ventral groove are beset with cilia, and a ciliate tract
can be traced as far as the oesophagus, the mucus also which passes
out of the slit of the groove is caught up by the cilia, and directed,
together with the nutrient particles sticking to it, towards the
oesophagus. Briefly, the ventral groove secretes mucus, which is
destined to catch the nutrient matters suspended in the water, and
to be carried with them by the ciliate tracts to the oesophagus. The
entire arrangement has therefore a nutrient signification.

Fol, H., Ueber die Scbleiradruse, etc. der Tunicaten. Morph. Jahrb. I. p. 223,

Digestive portion of the Enteron.

§ 312.

At the bottom of the foremost section of the entire enteron or
tractus intestinalis, namely, of that section which is modified as a
respiratory chamber, commences the tract which serves exclusively the
functions of nutrition. As a rule several divisions of it may be dis-
tinguished by differences in its breadth. An anterior, narrow section

2 D 2

404

COMPAEATIVE ANATOMY.

forms an oesopliagus^ whicli lias a funnel-like commencement in tlie
Copelata. A second, generally wider, section may be regarded as
stomach, and corresponds to a mid-gut. In Ascidians it is broken up
into numerous smaller chambers by a number of folds and sagittate
up-growths of the wall; in Copelata it has a cascal appendage. Caecal
structures are also found on the stomachs of many Salpee. The
section thus constituted is usually of considerable length in the
Ascidians, and returns on itself in the form of a loop, from one arm
of which the hind-gut proceeds. These two sections of the canal
are quite short in the Copelata and also in the Cyclomyaria, where,
as in the Ascidians, they are but little differentiated from one another.
In many Ascidians, the single or double looji of the alimentary
canal has a position at the side of the respiratory chamber in the
outgrowth of the body-cavity which there surrounds the respiratory
sac ; others have the digestive tract confined entirely to the region
behind the respiratory chamber, the various forms of which appear
to determine this relation.

The Salpm have their digestive tract, together with its appen-
dages, compacted into a mass (the nucleus).

As accessory organs of the digestive tube we have, besides
the outgrowths already mentioned, other glandular tubes in all the
higher divisions. These tubes open into the section of the canal
which serves as stomach. There can be no doubt that they furnish
a secretion adapted to the purposes of digestion. In form and
arrangement they exhibit many variations. Sometimes they form
net-like anastomoses.

CiiAXDELON, Th., Recli. sur uue annexe clu tube dig. des Tuniciers. Ball. Acad.

Belg. XXXIX.

Vascular System.

§ 313.

In the arrangements of their circulatory organs, the two great
divisions of the Tunicata differ from one another. In the Copelata
a heart only is known, and that is absent from one genus. It con-
sists of a short sac with its ends attached to two cells, whilst its
delicate walls present two longitudinal slits placed on opposite sides
to one another. The circulation of the blood is provided for by
the pulsations of this pouch; in fact, without the existence of
vessels, a distinct direction is given to the movement of the blood
in the body-cavity which can be recognised. In the Acopa a vas-
cular system in connection with the heart is present, which in certain
parts has a lacunar character. It appears then that a portion of
the primitive body-cavity is ada23ted to the purposes of a blood-
carrying system.

In the Ascidians the elongated heart lies in the neighbourhood
of the digestive organs, and bending inwards at each end gives rise

VASCULAE SYSTEM OF TUNICATA.

405

to a vessel. One of these two vessels taking a ventral course breaks
up into a net- work which is distributed over the branchial lattice -
work, whilst the other passes to the intestine and to the generative
organs and branches out upon them. The same main -vessel sends
also a branch to the mantle and twigs to the wall of the coelom
(body-cavity). The blood distributed in this set of vessels is col-
lected again into a longitudinal trunk lying on the dorsal side of
the branchial sac, which also receives vessels from the intestine and
from the generative organs. Whether these dispositions which have
been observed in the solitary Ascidians have a more general distri-
bution is not yet ascertained.

In the Salpae the short, thin- walled heart (Fig. 212, c), generally
having the form of a tube, constricted at intervals, is in connection
with a large vascular canal (v), which runs along the ventral surface,
and also at the opposite end the heart is directly continuous with a
large vessel; the latter, in those forms which possess a so-called
nucleus (t’i), breaks ujd at once
into a reticular system, which
is distributed in this body, and
represents the intestinal ves-
sels of the Ascidians. In other
Salpse (those without nucleus)
it appears to divide into many
branches, which run towards

the dorsal surface and end in
a longitudinal canal. This
dorsal vessel {v') is placed in
communication with the ven-
tral stem by a number of trans-
verse canals (r), which freely
anastomose with one another.
There exists a further direct

Fig. 212. Circulatory system of Salpa
maxima, a Incurrent orifice, h Excurrent
orifice, hr Branchial septum, hr' Attach-
ment of the gills, vi Visceral mass (nu-
cleus). c Heart, v Ventral vascular stem.
v' Dorsal vascular stem, v" Communicating-
transverse stems. (The finer ramifications of
the vessels are not represented.) (After
Milne-Edwards.)

communication between the anterior portion of the dorsal vessel and
the hinder vessel proceeding from the heart, through a number of
vessels which run through the gills and break up there.

The most important peculiarity of the vascular system of the
Tunicata is assuredly the existence of the two longitudinal stems
which pass along the branchial sac, and which farther on meet on
the intestine.

Let us supj)Ose, starting from the Ascidians, the intestine to be
continued in the direction of the long axis of its anterior division,
the branchial sac, so that the anus came to be placed at the aboral
pole of the body, then we should find the arrangement of the
vascular apparatus to be similar to that of many Worms, oven in
respect of the detail that the branches of both longitudinal trunks
are divisible into visceral (to the branchial sac and intestine) and
parietal series (to the body- wall).

The heart belongs to the ventral longitudinal stem.
It is in fact a differentiated portion of that stem. In this

406

CO:\IPAEATIVE ANATOMY.

fact we liave a special divergence from the arrangement obtaining
in all other invertebrate animals, for in them the central organ of
the circulation is invariably a specialisation of a part of the dorsal
vascular stem. Nevertheless there can be no mistake about the
connection with that of the Worms in the disposition of the whole
ap]3aratus.

Peculiar to all Tunicates is the alternating direction of the
blood-stream set in motion by the heart. In consequence
of this change of direction, it is not possible in them to speak of an
arterial and a venous division of the vascular system. When the
heart has completed a certain number of pulsations in one direction,
suddenly a moment of quiescence occurs, and
then the peristaltic movements of the cardiac
tube begin again, but in the opposite direction.
This condition of indifference is a further obstacle
to the close assimilation of the vascular system
of the Tunicata to that of any one of the other
great divisions ; at the same time, it calls to mind
a similar phaenomenon of the reversal of the blood-
stream in the Gephyraea (Phoronis).

Fig. 213. Organisa-
tion of an Ascidian
( Am ar oecium pro •

liferum) . sh Bran-
chial sac. V Stomach.
i Intestine, c Heart.
t Testis, vd Efferent
duct of the testis.
0 Ovary, o' Eggs in
the body - cavity.
The arrows indicate
the direction of the
stream of water at
the orifices of the
body (after Milne-
Edwards) .

Organs of excretion have as yet been
recognised in the Tunicata to a limited extent.
In many Ascidians (Molgula, A. conchilega, com-
planata) a tubular organ is found in the neigh-
bourhood of the branchial chamber, or placed
farther back in the body, which exhibits, besides
other cells, some containing concretions. In one
species the murexide reaction has been obtained.
No openings have been discovered in the organ,
so that the arrangement appears to represent
that condition in which excretionary matters are
separated in the organism, and form concretions
which are not removed to the exterior.

Sexual Organs.

§ 314.

Only one division of the Tunicata is provided
with sexual organs universally — the Copelata.
In the others, in consequence of the elaborate
asexual reproduction, a large proportion of indi-
viduals are devoid of sexual organs j the absence
of which is explained by the production of germs,
a modification of the process of multiplication by
budding (cf. p. 391).

The hermaphrodite arrangement, common among Tunicata, is
often found at a very low stage of development. The Ap]ien-

SEXUAL OEGAXS OF TUXICATA.

407

diculariee have no efferent ducts for their sexual glands^ which are
sometimes paired and sometimes single. In the Acopa the sexual
elements are discharged into the cloaca. The male organ has the
form of a sperm-producing cmcum, which in Doliolum, and also in
many Ascidians, retains this simple form ; whilst in Pyrosoma it
acquires a rosette-like shape^ and in most Ascidians^ and also in the
Salpm, is produced into branches^ and forms a kind of multilobular
gland. In many Ascidians (Molgula) the testes surround each of
the two ovaries as a number of separate glands^ and open to the
exterior with separate efferent ducts. The ovaries too have often a
multifid character, at least in many Ascidians ; in others they are
only represented by a group of eggs of different stages of develop-
ment, each of which is enclosed in a kind of capsule. In many only
a few such eggs, joined together by a common stalk, are present ;
and in the Salpge and Pyrosoma we have only a single egg, the stalk
of which exists in the early stages of growth, but gradually shortens.
The development of the sexual products occurs here at different
periods for the two sexes ; the male organ only reaches its maturity
after the egg has commenced to develop as an embryo.

The presence of an efferent duct for the sexual products appears
to depend upon the greater or less distance of the sexual glands
from the clcaca. The whole apparatus, however, requires a very
much more careful investigation than it has yet received.

Nintli Section.

Vertebrata.

General Review of the Group.

§ 315.

The most essential characters of the Vertebrata are the possession
of a skeleton traversing the longitudinal axis of the body, and the
division of the body into a number of metameres (primitive ver-
tebra). They differ from the Tunicata, with which alone of all
the divisions of the Invertebrata they have any well-marked rela-
tions, by this metamerism. They have more distant relations to the
Vermes, but then this group obviously has relations to most of the
other phyla.

The axial skeleton separates a dorsal from a ventral division of the
body. The former contains the central nervous system, the latter the
digestive canal, which is continued on from a respiratory chamber,
and which, with the organs differentiated from it, is embedded in a
coelom. Two regions therefore can be made out, which extend
along the body ; the upper one is neural, and the lower enteric;
the chief trunks of the system of canals for the nutrieut fluid
belong to the latter region.

The various divisions are set in order in the following review :

A. Acrania.

Leptocai’dii.

Amphioxus.

B. Craniota.

I. Cy clostomata.^

Myxinoiclea.

Bdellostoma, ilyxine.

Petromy zontes.

Petromyzon.

* The Cyclostomata should be placed apart from all the rest of the Craniota, for
their whole organisation shows that they branched off very early from the Craniota.

VEETEBRATA.

409

II. Gnathostomata.
a) Anamnia.

1) Pisces.

Selachii.

Squali.

Hexanchus, Heptanchus, Acanthias, Scymnus, Galeus, Scyllium,
Squatina.

Raj re.

Raja, Torpedo, Trygon.

Holocephali.

Cbimrera.

Dipnoi.

Monopneumones.

Ceratodus.

Dipneumones.

Protop terns, Lepidosiren.

Ganoidei.*

Sturiones.

Acipenser, Spatularia.

Polypterini.

Polypterus.

Lepidosteini.

Lepidosteus.

Amiadini.

Amia.

Teleostei.

Physostomi.

Abdominales.

Clupea, Salmo, Esox, Cyprinus, Silurus, Mormyrus.
Apodes.

Murrena, Congei% Gymnotus.

Physocly sti.

Anacanthini.

Gadus, Plem’onectes.

Pliaryngognatlxi.

Belone, Hemirhamphus, Chroinis, Labrus.

Acanthopteri.

Perea, Labrax, Trigla, Scroprena, Anabas, Mugil, Scomber,
Zeus, Traebypterus, Gobius, Cyclopterus, Blennius,
Lophius.

Plectognathi.

Ostracion, Diodon, Orthagoriscus.

Lopbobranebii.

Syngnatbus, Hippocampus.

2) Ampbibia.t

Urodela.

Perennibranebiata.

Sii’edon, Menobranebus, Proteus.

Caducibranebiata.

* I regard each of these divisions of the Ganoidei as highly independent. They
represent the last shoots of very divergent series of forms, of -which that of the Polyp,
terini has many points of relationship to the Dipnoi ; the Amiadae, on tho other hand,
are the nearest allies of the Teleostei (Clupeidae). It would, perhaps, be best to
separate them completely from the Ganoidei. The Sturiones show the greatest
resemblance to the Selachii.

I must regard the Selachii as being nearest to the ancestral form of the Gnathosto-
matous Yertebrata. The Holocephali, as well as the Dipnoi and Ganoidei, appear to
have branched off from them, while the Teleostei again are a branching off from the
Ganoidei.

f The living Amphibia foi’m only a very small group, which in many parts indi-
cate considerable retrogression ; but few fossil forms can be safely placed in it. The
palaeontological records of the Amphibian phylum are fragmentary in the highest degree.
There are many reasons for placing the Archegosauria with them, but yet there are
many points in which these forms resemble the Reptilia.

410

COMPAEATIVE ANATOMY.

a) Au amnia {continued).

Derotremata.

Cryptobranchus, Menopoma.

Salamandrina.

Triton, Salamandra.

Anura.

Pelobates, Bombinator, Hyla, Ceratophrys, Rana, Bofo.
Gymnopbiona.

Coecilia.

b) Amniota.

1) Sauropsida.

1. Reptilia.’^

Cbelonii.

Sphargis, Trionyx, Clielonia, Cbelys, Cbelydia, Emys, Testndo.
Saurii.

Ascalabota.

Platy dactyl us, Hemidactylus.

Rhyncbocephala.

Spbenodon.

Lacertina.

Iguana, Calotes, Draco, Phrynosoma, Urorcastix, Lacerta,
Ameiva.

Monitores.

Monitor, Psammosaurus.

Scincoidea.

Scincus, Seps, Anguis.

Cbalcidea (Ptychopleura).

Chalcis, Zonurus,

Cbamseleonida.

Cbamreleo.

Ampbisbsenida (Annulata).

AmpMsbsena, Lepidostemum.

Ophidii.t

Eury stomata.

Python, Boa, Coluber, Tropidonotus, Dryophis, Dipsas, Hydro-
phis, Crotalus, Trigonocephalus, Vipera.

Stenostomata.

Typhlops, Uropeltis.

Crocodilini.

Alligator, Crocodilus, Ramphostoma.

2. Aves.t
Ratitse.

Struthio, Dromceus, Apteryx.

Carinatas.

Gallinaceae.

Megopodius, Penelope, Crax, Cryptui’us, Lagopus, Tetrao, Pavo,
Numida, Gallus, Phasianus.

^ The various divisions of this class appear to be the very divergent terminal twigs of
a branch of the Vertebrata, which was in former times largely represented. Many of the
fossil divisions which are ascribed to the Eeptilia, such as the Enaliosauria, apparently,
however, branched off from the Vertebrate phylum before the Amphibia. In another
group of fossil Saurii we find forms intermediate* between the Eeptilia and Birds ; and
that most markedly in the characters of the skeleton of the foot. These are the
Ornithoscelida. By uniting Eeptiles and Birds into one division of the Sauropsida,
as Huxley has done, we put these relations in their proper light.

f The Ophidii form a division nearest to that of the Saurii, and derived from it, and,
with it, equivalent to the Chelonii or Crocodilini ; and they were thus put together by
Stanniusas Streptostylica.

J The class of Birds which arose from reptilian forms is one which is divided by
the most important points in its organisation into groups which diverge very slightly
from one another, for the characters of these subdivisions present distinguishing points
of very slight importance in comparison with those of the other divisions of the groups
of the Vertebrata. Through the Saururi (Archa?opten’x) they are directly connected
with the Ornithoscelida.

VEETEBEATA.

411

b) Am 11 iota [continued],

Columbse.

Columba.

Grallatores.

Otis, Dicholophus, Grus, Ardea, Ciconia, Vanellus, Charadrius,
Scolopax, Fulica, Galliuula, Rallus.

Natatores (Palmipedes).

Procellaria, Sterna, Larus, Phaeton, Plotus, Pelecanus, Carbo,
Anser, Anas, Cygnus, Phoenicopterus, Mormon, Uria, Alca,
Aptenodytes.

Passeres (Insessores) .

Fringilla, Alauda, Tardus, Sylvia, Motacilla, Parus, Muscicapa,
Lanins, Stumus, Corvus, Hirundo, Oerthia, Trochilus,
Upupa, Merops, Coracias, Alcedo, Buceros.

Picides.

Picas, Yons,

Psittacides.

Psittacas, Strygops, Nestor.

Rapaces.

Gypogeranus, Falco, Buteo, Aquila, Gypaetus, Vultui’, Cathartes,
Harpyia, Surnia, Strix.

2) Mammalia.

Ornithodelphia (Monotreniata),

Ornithorhynchas, Echidna.

Didelphia* * * § (Marsupialia),

Botanophaga.

Halmaturas, Dendrolagus, Phascolomys, Phascolarctus, Phalan-
gista.

Zoophaga.

Perameles, Dasyurus, Thylacinus, Didelphys, Chironectes.
Monodelphia (Placentalia),

Edentata.t

Myrmecophaga, Manis, Chlamydophorus, Dasypus, Bradypus.
Ungulata.

Artiodactyla.

Sus, Dicotyles, Moschusj Camelopardalis, Cervus, Antilope,
Capra, Ovis, Bos.

Tylopoda.

Camelus, Aachenia.

Perissodactyla.

Tapirus, RhinoceroSj Equus,

Sirenia.

Manatus, Halicore.

Prosimii.t

Stenops, Lemur, Otolicnus, Tarsias, Galeopithecus, Chli‘omys.
Rodentia.

Scinrus, Arctomys j Mas, Hypudaeus, Cricetus, Georhychus,
Spalax, Pedetes, Dipus, Lagostomus, Myopotamus, Castor,
Hystrix, Coelogenys, Cavia, Lagomys, Lepus.
Proboscidea. §

Elephas.

Lamnungia. ||

* I regard the division of the Marsupialia as equivalent to the monodelphous
Mammalia, for not only are there found in it representatives of most of the orders of
Monodelphia, hut, further, there are in the Monodelphia many indications which point
to their having arisen from a didelphous form. The Marsupialia, or uniting with them
the Monotremata, the Implacentalia, consequently represent the ancestors of the
Placentalia.

f The great variations which the relations of the placenta present in various
Edentata weaken somewhat the value of the placental classification, although the
various orders are generally distinguished by the similar characters of their placentai

J The Prosimii form a stem-group, in some divisions of which peculiarities are
retained which are found in various other of the following orders. Thus there are
characters which we meet with in Insectivora, Eodentia, Carnivora, and Primates.

§ and II The Proboscidea and Lamnungia are representatives of orders which it is
very difficult to associate with the rest. They have genetic affinities to the Rodentia ;
Ifyrax has also relations to the Ungulata.

412

COMPAEATIVE ANATO^^IY.

b) Amniota [continued).

Hyrax.

Fera.

Carnivora.

Felia, Hygena, Proteles, Canis, Herpestes, ViveiTa, Lutra,
Mustela, Meles, Nasua, Procyon, Ursus.

Pinnipedia.

Plioca, Otaria, Trichechus.

Cetacea.*

Delphinus, Physeter, Balsenoptera, Balsena.

Insectivora.

Chrysochloris, Talpa, Sorex, Myogale, Erinaceus.

Chiroptera.

Pteropus, Rhinolophus, Glossophaga, Vespertilio, Vesperugo.
Primates.

Hapale, Callitlirix ; Ateles, Mycetes, Cebus ; Cynocephalus,
Inuus, Cercopitbecus; Troglodytes, Hylobates, Pithecus;
Homo.

Bibliography.

Owen, E., On the Anatomy of Vertebrates. Vols. I.— III. London, 18GG-68.— Huxlet, T. H., A
Manual of the Anatomy of Vertebrated Animals. London, 1871.

Leptocardii : Muller, J., TJeber den Ban und die Lebenserscheinungen des Branchiostoma lubri-
cura. Abhandl. d. Berl. Acad. 1814. — Goodsib, Transact. Royal Soc. of Edinbui'gb. T. XV. i.

• — Quatreeages, Ann. sc. nat. III. iv. — Kowaletskt, A., Entwickl. des Amphioxus. Mem.
Acad, des Sc. de St. Petersbom-g. Ser. VII. T. XI. — Rolph, Unters. iiber d. Ban d. Amphioxus.
Morphol. Jahrb. II.

Cyclostomata: Muller, J., Vergl. Anatomic der Myxinoiden. Abhandl. d. Berl. Acad. 1835-45.—
The same, TJeber den Ban und die Grenzen der Ganoiden. Ibid. 1846. — Rathke, Bemerkungen
iiber den inneren Bait der Pricke. Danzig, 1825. — The same, TJeber den Bau des Querders.
Beitr. z. Gesch. der Thierwelt. IV. Halle, 1827.— Laxgeehaxs, Unters. iiber Petromyzon Planeri.
Freib. 1873.

Pisces: Monro, A., The structure and physiology of Fishes. Edinburgh, 1785. — Cuvier et
Valenciennes, Hist. nat. des poissons. 'XXTI. vols. 1828-48. — Agassiz, Recherches sur les
poissons fossiles. 5 vols. av. Atlas. 1833-43. — Agassiz et Vogt, Anatomie des Salmones.
Neufchatel, 1845. — Lea'dig, Beitriisre zur mikroskop. Anatomie und Entwickelungsgeschichte
der Rochen und Haie. Leipzig, 1852. — Owen, Description of Lepidosiren annectens. Transact.
Linn. Soc. XVIII, — Bischoff, Lepidosiren paradoxa. Leipzig, 1840. — Hyrtl, Lepidos. parad.
Abhandl. der bohm. Ges. d. Wiss. 1845. — Peters, Lepidosiren. Arch. f. Anat. u. Phys. 1845. —
Gunther, Ceratodus. Phil. Trans. 1871.— Balfour, F. M., Development of Elasmobranch Fishes.
Journal of Anat. Vols. X. XL

Amphibia : Cuvier, in Recueil d’observations de Zoologie et d’Anat. comp. I. 1805. — Rusconi et
Configli-achi, Del Proteo anguineo di Laurenti monografia. Pavia, 1818. — The same. Hist,
naturelle, developpement et metamorphose de la Salamandre terrestre. Pavie, 1854. — Muller,
J., Beiti’iige zur Anatomie der Amphibien. Zeitschr. f. Physiol. IV. 1832.— Duges, Recherches
sur I’osteologie et la myologie des Batraciens. Paris, 1834. — Mayer, Zur Anatomie der Amphi-
bien. Analecten f. vergleichende Anatomie. Bonn, 1835. — Calori, Sulla Anatomia del Axolotl.
Mem. della Accademia delle sc. dell’ istituto di Bologna. III. 1851.— Rathke (Coecilia annulata).
Aich. f. Anatom, u. Phys. 1852. p, 334.— Leydig, TJntersuch ungen iiber Fische und Reptilien.
Berlin, 1853. — Vaillant, L. (Siren lacertina). Ann. sc. nat. IV. xviii. — Fischer, J. G., Perenni-
branchiaten u. Derotremen. Hamburg, 1864.— Hyrtl, J.. Cryptobranchus japonicus. Vindo-
bonae, 18G5. — Van her Hoeven, Ontleed-en deerkundige Bijdragen tot de Kenniss von Menobran-
chus. Leyden, 1867.— WiEDERSHEiir, R., Salamandra perspicillata, etc. Genua, 1875.— Gotte,
Entwickelungsgeschichte der Unke. Leipzig, 1875.

Reptilia : Boj.vnus, Anatomia testudinis europapje. Vilnae, 1819.— Duheril et Bibron, Erpetologie
gen^rale. Paris, 1834-54.— Ditv'ernoy (Serpens). Ann. sc. nat. I. xxx.— Rathke, Entwicke-
Inngsgesch. der Katter. Konigsberg, 1837.— The same, Entwick. der Schildkroten. Braunschw.
1818. — The same, Ueber die Entwick. und den Korperbau der Ki’okodile. Braunschw. 1866. —
Calori (Uromastix). Mem. della Accad. delle sc. dell’ist. di Bologna. III. ii. 1863.— Gunther,
(Ilatteria), Phil. Tr. 1867. II. — Leydig, Die in Deutschland lebenden Alien der Sauricr,
Tubingen, 1872. — Wiedehsheim, R., Z. Anat. v. Phyllodactylus europ. Morph. Jahrb. I.

Aves : Tiedemann, Anatomie und Naturgesch, der Vogel. Heidelberg, 1810-14.— Owen, On the
anatomy of the southern apteryx. Transact. Zool. Soc. Vols. II. HI.— The same. Art. Aves in

* The Cetacea are apparently of the same phylum as the Pinnipedia, •which phylum
is derived from the Carnivora ; this is sho-wn by certain fossil forms (Zeuglodon). But
the peculiarities of the Cetacean organisation are too great for them to be put under
the Fera ; they form the end of a series.

FORM OF THE BODY OF VERTEBEATA.

413

TotM’s Cyclopoedia. I. — Milne-Edwards, Aeph., Recli. sur les ossemens fossiles ties oiseaux
Paris, 1866. — Alix, E., Appareil locomoteur ties oiseaux. Paris, 1874.

Mammalia : Meckee, J. Fe., Ornithorhynclii paradox! clescriptio anatomica. Lips. 1826.— Veolik
(Dendrolao-us), Verhandel. d. Kon. Acad. Amsterd. V. — Guelt, Handb. d. verg’l. Anat. tier
Haussaugethiere. 4 Aufl. Berlin, 1860. — Franck, L., Anatomie der Hausthiere. Stuttgart, 1871.
— Brandt (Lama), M^m. Acad. St. P^tersbourg, 1841. — Owen (Giraffe), Transact. Zool. Soc. II. —
The same (Rhinoceros), Transact. Zool. Soc. IV. ii.— Milne-Edwabds, Alph. (Moschimd), Ann.
sc. nat. V. II. — Murie, J. (Manatus). Tr. Zool. Soc. VIII. — Camper, Observations sur la structure
intime et le squelette de Cetacees. Paris, 1820. — Rapp, Die Cetaceen. Stuttgart u. Tubingen,
1837. — Vrolik, W., Natuur- en ontleedkund. Beschouwing van d. Hyperoodon. Haarlem, 1848.
Escheicht, Untersuch. iiber die norclischen Walthiere. Leipzig, 1849. — Murie, J. (Globio-
cephalus, Otaria, Trichechus), Tr. Zool. Soc. VII. VIII. — Rapp, Anatom. XJntersuchungen iiber
die Edentaten. 2 Aufl. Tubingen, 1852. — Owen (Myrmecophaga jubata), Tr. Zool. Soc. IV. —
Hyrtl (Chlamydophorus truncatus), Denkschr. d. Wien. Acad. IX. 1855. — Pouchet, G., Mdm.
sur le grand Fourmilier. Paris, 1874. — Pallas, Nov. spec, quadrup. e gliriumordine. Erlangen,
1778. — Camper, Descript, anat. d’un Elephant male. Paris, 1802. — Burmeister, Beitrage z.
nahem Kentniss der Gattung Tarsius. Berlin, 1846. — Van dee Hoeven (Stenops), Verhand. d.
Acad. Amst. VIII. — Owen, Monograph on the Aye-Aye. London, 1863. — Peters (Chiromys),
Abhandl. d. Berliner Acad . 1865.— Murie, J. (Lemuridae), Tr. Zool. Soc. VII. — Tyson, Anatomy
of a Pygmy. London, Sec. edit. 1751. — Veolik, Rech. d’anat. comp, sur le Chimpanse. Amster-
dam, 1841. — Duveenoy, G, L., Caract. anat. des grands singes. Archives du Museum. VIII. —
Flower, Osteology of the Mammalia. London, 1870. — Turner, W., Lectures on comp. anat. of
the Placenta. Edinburgh, 1876.— For human anatomy, reference must be made to the manuals on
the subject,

Form of the Body.

§ 316.

The external metamerism disappears^ but dorsal and ventral
surfaces can generally be distinguished; the entrance into the
nutrient canal is placed near the anterior pole of the long axis of the
body^ and on the ventral surface ; the anus is also ventral^, but is at
some distance from the aboral pole. Three great regions can be
made out even in the body of the lowest divisions. The anterior one
contains a respiratory chamber formed from the nutrient canal^ and
is consequently distinguished by clefts in the sides of the body-wall.
It carries the higher sensory organs, and in the Craniota gives rise,
by concrescence and differentiation, to a head.

The second portion, which in Amphioxus is not sharply marked
off on the dorsal surface from the preceding one, forms the trunk,
which encloses the coelom and its contents ; this is only marked off
from the last or caudal portion of the body by the anus ; so that
from the outside there is not much difference between them.

We have already met with these divisions in the Tunicata. In the
Ascidian larvae (cf. Fig. 208), the most anterior one, which later on
forms the principal portion of the body, contains the rudiments of
the respiratory cavity, and the portion of the nervous system which
contains the sensory organs. Connected with this is a faintly
separated tract with the enteric tube, and this passes almost directly
into the caudal portion. The earliest characters of the embryonic
head, or of its equivalent in all Vertebrata, point to its being,
phylogenetically, the most ancient portion of the body, and serve as
a finger-post to the path of development of the Vertebrate body.

With the development of the head and of the organs differen-
tiated in and on it, the body of the Vertebrata acquires characters,
which, externally, separate it well off from the Invertebrata ; the
value of these is clear when we take note of the large number

414

' COMPARATIVE ANATOMY.

of metameres that liave been suppressed in the course of this
differentiation. Further differentiations are brought about by the
development of paired appendages. The hinder ones in the Gna-
thostomata separate the trunk and tail more distinctly from one
another, as do the anterior ones in the case of the head and trunk.

When the anterior appendages are separated from the head, as
they are even in the Selachii among the Fishes, a cervical region
is differentiated from the trunk, and connects it with the head.
We meet with this arrangement in the Amphibia. The trunk is
affected by further differentiations, and in the Amniota is separated
into cervical, thoracic, and lumbar regions.

The caudal portion of the body undergoes a gradual metamor-
phosis. In the Fishes it is scarcely marked off; in the Amphibia
(Urodela) and Reptilia (Saurii, Crocodilini) it is separated from the
trunk by the hinder pair of appendages, and by its diminished bulk.
Although it is atrophied in Birds, it is in the Mammals only that it
acquires the character of an appendage of the body, in consequence
of its great decrease in bulk, even though it may still be of some
length.

Appendages.

§ 317.

We must divide the appendages which are given off from the
body of the Yertebrata, and which function principally as locomotor
organs, into paired and unpaired parts. The unpaired ones are
developed from a vertical membrane — a continuation of the integu-
ment— which extends from the head to the anus. When firm
structures and special muscles are developed in this membrane, this
purely dermal process is converted into a fin. This organ either retains

its primitive continuity
of arrangement (Fig. 214,
A), or by the atrophy of
some, and by the in-
creased development of
the remaining portions,
is broken up into several
parts. These are dis-
tinguished according to
their position as dorsal,
caudal, and anal fins
(Fig. 214, B cl c a)
function chiefly as
ing organs ; the caudal
fin alone has any higher locomotor significance in so far as the
caudal portion of the body plays an important part in locomotion.
These organs, which are commonly found in Fishes, are also seen in
the early stages of development in the Amphibia, in some of which

• They
direct-

Fig. 214. Diagram of the unpaired fins. A Primi-
tive stage. B Differentiated stage, d Dorsal fin
(d' Fatty fin) . c Caudal ; a Anal ; p Thoracic j
V Yentral fins.

APPETOAGES OF THE VEETEBIUTA.

415

(many Urodela) they are indeed permanently present^ but supporting
organs are not developed in them.

In the Heptilia indications of the vertical dermal fringe can
sometimes be just made out, but in most it is altogether absent, as
it is also in the higher classes ; the vertical fin-like structures seen
in many of the Cetacea must be regarded as organs which have been
acquired by that order independently. The same remark applies to
the horizontal caudal fin of these Mammals.

§ 318.

Unlike the arrangement seen in many divisions of the Inverte-
brata, where paired appendages are found on all, or at least on a
large number of metameres, they are — and, so far as we know, with-
t out one exception — confined to an anterior and a posterior pair in
the Vertebrata.

They appear to be homodynamous organs, which gradually get
to vary greatly in form in correlation with their great variety of
function. They are probably derived from metamorphosed respira-
tory appendages of the head (branchial arches and rays), so that
they are not absolutely new arrangements.

They are absent in the Acrania and Cyclostomata, but are
generally present in the Gnathostomata. Although in some divisions
of these latter the appendages are atrophied, this atrophy is in every
case a secondary arrangement, which presupposes the fully-developed
condition. This is proved by the various stages of atrophy, which may
be seen in the appendages and in the parts of which they are composed.

In the lower condition seen in Fishes the appendages appear each
to form a single whole, undivided by external jointing into a number
of parts, while their increased surface is of importance as bearing on
the directing function of the organ. The anterior and posterior
appendages, which are here known as thoracic and ventral fins,
have essentially the same structure, although the thoracic fins are,
as a rule, much larger, in consequence of their position in the larger
part of the body. Their more powerful structure may be also ex-
plained by the fact that they take the initiative, and are consequently
of greater functional importance than the hinder appendages.

Owing to their similar mode of aquatic progression the ap-
pendages of the fossil Enaliosaurii, as shown by the remains of their
skeletons, resembled the fins of Fishes — at any rate in the absence
of any transverse segmentation.

Among the Amphibia we find the appendages transversely seg-
mented, for now several parts are sharply marked off from one
another. In the fore-limb we divide these into upper arm, forearm,
and hand ; to which the thigh, leg, and foot correspond in the hind-
limb. This division is correlated with the greater elongation of the
two first segments, which stand, in relation to one another, as the
arms of a lever, and are therefore set at an angle to one another.

In addition to the differentiation herein implied, the terminal

416

COMPAEATIYE AXATOMY.

division undergoes differentiation ; a number,, generally not more
than five^ terminal joints can be distinguisbed as fingers and toes.
As a part of tlie body wliicb generally projects towards the exterior
is more under modifying influences than any other part, we find a
large number of adaptations in them ; few parts of the body present
so many metamorphoses as these terminal parts of the appendages —
the hand and foot.

The primitive union of the fingers or of the toes into a swim-
ming-plate, represented by the hand and foot, is retained in the
natatory membrane of many Eeptiles, in the hind-limbs of many
Birds, and also in a number of Mammals, where it is always con-
nected with the adaptation of these appendages to the function of a

swimming organ.

The angulation of the limb at-
tainedto,in connection with terres-
trial locomotion, and which is also
advantageous in aquatic locomo-
tion, becomes gradually different
in the case of the two extremities;
the difference corresponds to the
functions performed by the an-
terior and posterior extremities
when moving about on land.

These relations are distinctly
seen even in the Amphibia (B) ;
but the difference in position
between the upper and forearm,
and thigh and leg, is not well
marked. The upper arm and
thigh are turned outwards to
almost the same extent. There
is a greater difference between
them in the Eeptilia ((7), and this
is still more marked in the Mam-
malia, where the planes in which
the angles of the limbs of either
side are set are parallel to the
vertical median plane of the
body. This gives greater inde-
pendence to the limbs, which
have now become supports for
the body, as they raise it up
from the ground. Together with
this change in the aspect of the
planes, in which the angle formed by the extremity lies, the angles
between the equivalent portions in each limb in the Mammalia
cease altogether to agree with one another (D) ; in fact they point
in an opposite direction in the case of the fore and hind limbs
respectively. The angle between the upper and forearm is open

Fig. 215. Diagram to show the clifferen-
tiation and alteration in the direction of
the axes of the limbs in the Yertebrata.
.4 Fish. D Amphibian (the side-view,
which one has been obliged to give so as
to compare it with the rest, gives to this
and to the next figure the appearance of
th.e body being raised up). 0 Reptile.
X) Mammal, a Shoulder girdle, p Pelvic
girdle.

IXTEGUMENT OF VEETEBEATA.

417

towards the anterior^ and that between the thigh and leg towards
the posterior,

In addition to these general modifications of the appendages
there are other changes, which are confined to smaller divisions, and
are explicable by special variations in physiological activity. When
the hind-limbs are greatly developed, they perform the more com-
plicated function of a springing organ, as in frogs ; or they may be
converted into the chief organs of support for the body, in such a
way that the fore-limbs, so far at least as terrestrial locomotion is
concerned, may get to play a subordinate part, or even lose this
function altogether. This arrangement obtains in Birds, where it
has been attained to through a large number of intermediate steps,
which have been made out in fossil Eeptiles ; the fore-limbs in the
Carinat80 have taken on the function of a flying organ.

Integument,

§ 319.

In the primitive stage in the Vertebrata the investment of
the body has the character of a cellular layer, the external
germinal layer — the ectoderm. At a further stage in develop-
ment. this cellular layer is connected with a layer of connective
tissue derived from the mesoderm, and the two together form the in-
tegument of the Vertebrata, and take an equal share in the formation
and further development of various organs.

There are two layers in this integument (cutis), as might be
inferred from its mode of origin : a superficial epidermis which is
homologous with the epithelial structures in the Invertebrata, and
which is directly derived from the ectoderm, and a deeper-lying
layer of connective tissue, the corium, the deepest, and looser,
layer of which forms the sub-integumentary tissue. The corium is
strengthened by the formation of plexuses between its fibrous bands.
The blood-vessels, and nerves of the skin, together with various
sensory organs, are scattered in it, as are also glandular organs.

The corium is frequently pigmented. It varies greatly in
thickness, and in microscopic structure. One of the more notable
variations is a lamellar striation seen in Fishes, Amphibia, and
Eeptiles, where perpendicular fibrous bands divide the layers into
partitions. Among the special structures are the wart-like elevations
seen on its surface, which vary from low hillocks to long conical
processes. These dermal papillse give rise, in different divisions
of the Vertebrata, to a number of variously complicated organs.

Contractile form-elements (smooth muscular fibres) are also
found in the corium of Birds and Mammals. Another modification,
which obtains in the cutis, is due to a change in texture ; parts of
it are ossified into hard structures, and bony plates are developed in

2 E

418

COIMPARATIVE Ai^ATOMY.

tlie dermiSj wliicli vary greatly in form^ and unite to form a dermal
skeleton. Lastly, tliere are glandular organs connected with the
cutis, which are developed from the epidermis, and are therefore to
he regarded as epidermal organs.

§ 320.

In Amphioxus the epidermis is a single layer ; in other forms
it is made up of a number of layers of cells, which invest the
corium, with its elevations and depressions. Even in the Yertebrata
we find a ciliated epithelium as a heritage from a lower condition ;
but it is limited to the embryonic stages in Fishes, and to
certain parts of the body in the larval stages of the Amphibia.
The lower layers of the epidermis, which lie closer to the
corium, are seen to be younger, and these replace the effete
portions of the superficial layers. The cells of the epidermis vary
greatly in consistency, form, and mode of connection. Pigmented
cells are not nnfrequently found between the rest. They are some-
times able to produce a change of colour by the movement of their
l^rotoplasm (chromatophores) ; this has been observed in Fishes and
Amphibia. In the aquatic Anamnia the whole of the epidermis is
less consistent, and, owing to the softness of its elements, the whole
layer is often gelatinous ; so much so, indeed, that it was for a long
time considered to be a mucous layer secreted by glands.

There is another arrangement which forms a contrast to the con-
dition of the epidermis in the Anamnia ; this is brought about by the
cornification of the cells which is first seen in the Amphibia, and
is commonly found in the Amniota. The cells form resisting plates
or fibres, which give rise to firm structures becoming overlapped
by, and being more or less marked off from, one another. The
process of cornification never affects any but the superficial layers,
the deeper-lying parts being always indifferent. When the cornified
layers increase in thickness, various kinds of plates, knobs, and
scale-like structures (Reptilia) are developed. The corium takes
part in the formation of these structures, for it is almost always
provided with elevations which correspond to those epidermal
formations, which are developed from enlarged papillae. The scales
of the Saurii and Ophidii are therefore processes of the whole cutis.
In Birds this cornified covering is retained in some parts only of
the body ; as beaks on the jaws, and as scales, plates, knobs, and so
on, on the feet. Larger horny plates are connected with a bony
dermal skeleton in the Chelonii, and in some families of the
Edentata among Mammals. The cornification of the epidermis,
which obtains in some divisions or in still smaller groups, cannot be
directly referred to the organisation of the Beptilia ; it is rather due
to adaptations to definite external conditions. However, we do meet
with horny epidermal structures in many parts of the body, which
must be regarded as acquired arrangements in consequence of their
wide distribution and constancy of character. These are the nails

lOTEGUMENT OF VEETEBEATA.

419

and claws found at the ends of the limbs. There are indications of
these in the Amphibia themselves (Salamander) ; they are generally
present in Eeptiles and Birds ; nails of this kind are not unfrequently
retained even on some of the fingers of the bird^s hand_, which is
used as an organ of flight. They are much larger in many Mammals^
where they form hoofs.

Epidermal Structures,

§ 321.

Other differentiations beside the cornified structures already
mentioned affect the epidermis. The most important of these are
feathers and hairs; and that on account of their distribution in
the two higher classes of the Vertebrata^ and of their peculiar
appearance. It is usual to regard
them as organs very closely allied,
as they have many points in com-
mon. They seem, nevertheless, to be
divergent structures. The earliest
rudiment either of a feather
or of a hair is a thickening of
the epidermis; that is to say,
it forms a knob-like thicken-
ing (Fig. 216, A), into which
there grows a papilla from the
cutis. This process is small in the
case of the hair, but larger in that
of the feather. They resemble
those elevations which ara found
in the Eeptilia. The first sign of
the feather is the growth of the
knobs into papilliform processes
{B Gy feather processes), which are
made up of an outer epidermal
layer {G e), and a subjacent papilla.

The arrangement also of these first
rudiments of the feathers in de-
finite areas (feather-tracts, pterylia)
is much the same as that of the
scales in Eeptiles. The feather,

therefore, is, in this simple condition, a mere process of the epidermis
and subjacent cutis. The depression of the embryonic feather which
carries the cuticular papilla, and the consequent formation of a
'^feather follicle,'’^ is a later phsenomenon, as is also the differentia-
tion of the feather into rachis and vexillum. This differentiation
does not obtain until the feather sheath is protruded, which sheath
is itself an epidermal layer derived from the earliest rudiment.

Fig’. 216. Diagrams of the earliest
rudiments of feather and hair. In
section. A Thickening of the epider-
mis. B Uprising of a papilla. C Pro-
cess of the feather. D Depression of
the epidermis. E Differentiation of
the rudiment of the hair. F Hair-
follicle and hair, e Epidermis, p Hair.
p' Eoot of the hair, ws Eoot-sheath.
g Sebaceous glands. The corium is
represented by the dark lines.

420

. COMPARATIVE ANATOMY.

Various changes affect the morphological characters of the feather,
after the development of its shaft and vexillum ; but these are
beyond the scope of this book.

A follicle is not formed till somewhat late in the course of the
development of the feather ; it then contains that portion of the
rachis which is known as the ‘‘ quill/^ and the vascular papilla which is
continued into it ; the hair, however, is characterised by the early
appearance of the follicle. In this case then the papilliform thicken-
ing of the epidermis is a very early stage, and one of short duration ;
for the hair is not formed in this first elevation, but in a follicle
which dips down from the epidermis into the cutis (Fig. 216, D E),
and at the base of this follicle the cutis-papilla {F) grows up. The
shaft of the hair {F lus) is differentiated from the invaginated epi-
dermis by the cornification of its cells, while other cellular parts of
the follicle form the root-sheaths.

The various forms of hairs, whether woolly or contour hairs, setse, .
or spines, are merely modifications of one and the same early
condition.

§ 322.

The glands differentiated from the epidermis are, when simplest,
modifications of single cells, the protoplasm of which is differentiated
into fine granules, which are passed out to the exterior. These
mucous cells (goblet-cells), which are placed among the other
epidermal cells, form unicellular glands (Fishes). They are found
also in the Amphibia, but in them there are, in addition, more com-
plicated glandular organs. These latter have the form of flask-
shaped tubes, which are scattered over the integument; several
forms of these maybe distinguished. In many cases they get to be
very large, and form knob-like projections, which give the integu-
ment a roughened or wart-like appearance (Toads, Salamander).
Sometimes a large number of integumentary glands are collected
together, and characterise certain regions of the body (parotid).

The integumentary glands are less widely distributed in the
Reptilia. In the Saurii the so-called ‘‘ crural pores ” lead into
glands, which look like compound tubes, and which secrete cells
which harden and fill up the lumen of the glands. In Birds the
number of integumentary glands is still less. A number of glands
unite to form the anal glands (glandula uropygii), which are especially
large in the Natatores ; their secretion serves to oil the feathers. In
the Mammalia they are divided into two distinct groups ; sweat glands
and sebaceous glands, which are frequently connected with the hair-
follicles. These two sets of glands are more easily distinguished by
their anatomical characters than by the quality of their secretion,
which is exactly known in a few cases only, although indeed the
same form of gland may have a different function in different
regions. The simpler tubes, which are coiled at their ends, are called
sweat-glands, while the sebaceous glands are generally lobate. A
number of them are often connected with one hair-follicle, and may
become so greatly developed in relation to it, that the hair-follicle

INTEGUMENT OE VEETEBEATA.

421

looks like an appendage of the gland. The sebaceous glands
undergo the most various modifications in form, size, and number,
as well as in the quality of their secretion. Both sets of glands
frequently secrete odorous matters of various kinds, which play an
important part in the economy of the animal. Glands of this
kind are developed in the most diverse regions of the surface of the
body in many orders of the Mammalia.

§ 323.

The most important modification of the integu-
mentary glands in all Mammals is the development . of
milk-secreting glands, which enter into relation to the reproduc-
tive function. They are regularly, and, as a rule, symmetrically
arranged on the ventral surface of the body. Each mammary
gland consists of a complex of separate glandular tubes, the ducts
of which are either quite separate or united together.

In the Monotremata these organs differ but little from the other
kinds of integumentary glands. Each of the two organs here
present is made up of a
group of tubes, which
pass separately through
the skin. The area on
which they open is merely
distinguished by the ab-

sence of hairs, and

in

Ornithorhynchus is on
the same level as the
SUIT oundin g in t eg u m en t .

In Echidna it is placed
in a pouch-like depression
(mammary pouch), which
appears to serve as a re-
ceptacle for the young.

In the rest of the
Mammalia nipples are
present; these are special
arrangements which were
gradually developed by
the process of sucking,
and which afford the
young a more suitable
connection with the mam-
mary apparatus, while at
the same time they make
each complex of milk-glands distinguishable externally.

There are two very different conditions of the perfected
nipple. They are both preceded by a similarly indifferent stage
(Fig. 217, A)j in which a nearly flat glandular area {b) has a number

Fig. 217. Diagram of the development of the
nipple ; vertical section. A Indifferent stage ;
glandular area flat. B Elevation of the glandular
area with the nipple. C Elevation of the peri-
pheiy of the glandular area into the pseudo-
nipple, a Periphery of the glandular area, h Glan-
dular area, gl Glands.

422

COMPAEATIVE ANATOMY.

of glands at its base wliicb grow into tbe corium; this area is
marked off from the surrounding integument by a circular elevation
(a). This arrangement corresponds to the mammary pouch in
Echidna. In most of the Mammalia this arrangement is not a per-
manent one ; it soon becomes level again, and the glandular area has
then its central part (E), in which the glands open, raised up into a
papilla or nipple; at the tip of which a number of gland-ducts
always open.

In the other arrangement the mammary pouch is persistent.
Owing to the continued elevation of the periphery of the gland (a) the
glandular area is more and more depressed, the edge of the mammary
pouch is developed into a pseudo-nipple, from the tip of which a
single canal passes to the glandular area {G).

This arrangement has been observed in some of the Ungulata.
Intermediate stages between the two arrangements can be made out
in the Marsupialia (Halmaturus) andEodentia (Murina). The number
of mammary glands which are distinguished by their nipples
varies in different divisions. They generally correspond to the
average number, or to the maximum number of young produced at
one birth. They vary even within the limits of the same order ; and
they also vary in position. As a rule they form two rows, which,
when there is a large number present, extend from the inguinal to
the pectoral region (Carnivora, Suina). In many of the Didelphia
they are arranged in a circle on the abdomen. When the number
is not so large, they either occupy an abdominal position, as in many
Didelphia, or they are only found in the lumbar region (Perisso-
dactyla, Euminantia, Cetacea), or, finally, they are limited to the
pectoral region (Elephant, Sirenia, many Prosimii, Chiroptera, and
Primates). When more than one pair is present some glands are
sometimes aborted, so that there are rudimentary organs present,
together with well-developed and functionally active ones; and
these may be recognised by their rudimentary nipples. In a
similar way the whole apparatus is atrophied in the male.

The most important adaptation of the integument to the function
performed by the mammary glands, is the formation of the folds
of integument found in the Marsupialia; these form a sac, the
marsupium, which encloses the mammiferous region of the ab-
domen. The extent of its development appears to vary inversely
with the extent to which the young are developed at birth.

Dermal Skeleton.

§ 324.

The function of the integument as a defensive organ for the
body is increased in value by the formation of hard structures.
When these parts are of some size they give rise to a dermal

INTEGUMENT OE VERTEBEATA.

423

skeleton. In many cases we know but little as to tke development
of these structures, but they may all be reckoned among osseous

Fig. 218. Vertical section through the skin of an Embryonic Shark. C Coriimi.
c, c, c Layers of the corium. d Uppermost layer, p Papilla. E Epidermis, e Its
layer of columnar cells, o Enamel layer.

formations, to which indeed they completely correspond in the
higher divisions.

The dermal denticles (placoid scales), which are distri-
buted over the whole of the in-
tegument in the Selachii, may be
regarded as the structures from
which the various forms have
been derived. In them we distin-
guish a basis, which is inserted into
the corium, and is ordinarily rhom-
boidal in shape, and a portion, which
stands out from it, and which ordinarily
has its apex directed obliquely. This
is covered over by the epidermis. In
some parts, as, for example, on the head,
they often have a bombous surface,
and are set irregularly ; while on the
trunk they are generally set in per-
fectly regular and obliquely running
rows (Fig. 219). They are developed
on papillae of the corium (Fig.218,
j?), which are covered over by a
layer developed from the epider-
mis: this secretes an enamel-like
substance on the projecting por-
tion of the papilla, while the body
of the papilla is ossified from the
tip downwards. The epidermis and corium, therefore, both share
in the formation of these structures. There is a central cavity in the

Fig. 219. Dermal denticles of
Centropborus calceus (a little
magnified).

424

COMPARATIVE ANATOMY.

papilla, wlience fine branched canals radiate out to the surface.
The placoid scale has therefore the structure of dentine, is covered
by enamel, and is continued at its base into a plate formed of
osseous tissue ; as they agree with teeth in structure they may be
spoken of as dermal denticles. In the Rays these structures have
altogether disappeared (Electric Ray), or are replaced by larger
structures, which are grouped together in the form of spines or
larger bony teeth, or are separate from one another (Spiny Rays).

The dermal denticles of the Shark are very generally converted
into larger bony plates in the Ganofdei ; in the Rhombifera they
have not only the same arrangement on the body, but have essen-
tially the same minute structure. In the Sturiones larger bony plates
alternate with smaller ones. They generally retain exactly the
rhomb-form, which is lost in the rest of the Ganoi'dei (the Cyclifera).
The common flat and thin scales of the Teleostei follow on here.
They appear to differ in many points from the ganoid scales, and re-
present an offshoot from the type, which obtains in the Ganoi'dei, and
which may be derived from the Selachii ; this offshoot is characterised
by its variety of form.

In many Teleostei the scales undergo complete degeneration.
On the other hand they give rise to parts which differ somewhat
from scales, and which are formed by the fusion of the dermal
denticles ; such are the plates and spines of the Plectognathi, where
the plates may become more firmly united together and form a
connected carapace (Ostracion, Lophobranchii).

Parts which are likewise formed from the concrescence of dermal
denticles are found in the integument which covers the appendages
of the Gano'idei and Teleostei. To compensate for the atrophy of the
internal or primary skeleton of the limbs, these bony plates form a
number of rays, which often branch dichotomously at their ends, and
unite to form an organ of support for the fins (secondary skeleton of
the fin). The ray which occupies the anterior edge of the fin is
frequently massive, or gives rise to a strong spiny ray, which may be
connected with the internal skeleton. This ray may not only be
larger than the rest of the rays, but it may even, as in certain
Siluroids, represent the whole of the pectoral fin.

Hertwig, 0., Ueber d. Bau u. die Entw. der Placo'idscbuppeu u. der Zahne der
Selachier. Jen. Zeitschr. Bd. VIII. — The same, Ueber das Hantskelet der
Fische. Morph. Jahrb. II.

§ 325.

The ossifications of the integument are of special importance in
those regions of the body where parts of the internal skeleton come
to the surface. These ossifications are developed in just the same
way as the bony plates on other regions of the surface of the body,
and may likewise be derived from the indifferent stage represented
by the dermal denticles. Although the various kinds of dermal bones
v/hich are found on the trunk have an importance which is limited to

IXTEGUMENT OE VERTEBEATA.

425

the fishes, there are others which are of more importance; these
are the bony plates which are definitely arranged, and constantly
present, on the head, where they form the earliest rudiments of the
bony skull, or, at first, of the roof of the skull (cf. Fig. 220). These
dermal bones are inherited by all Vertebrata that are pro-
vided with a bony skull, and are connected with other
ossifications, which do not appear till later, in the carti-
laginous skull. This arrangement is first seen in the Sturiones.
There are a number of smaller bony
plates in addition to the large ones,
but most of these have no general sig-
nificance. On account of these rela-
tions to the internal skeleton, their
more special characters will be ex-
pounded when we come to treat of it.

Other skeletal parts besides the bones
of the skull are derived from ossifica-
tions of the integument ; the clavicle,
for example, has a similar origin.

Lastly, there is another category of
bones which are likewise derived from
placoid scales; the bones around
the mouth have been recognised
as having their origin in tooth -bear-
ing plates derived from fused
placoid scales.

§ 326.

We meet with dermal bones in the
higher classes ; in the Amphibia, and
also in the fossil Archegosaurii, in
which there were dermal bones in the
form of scutiform plates. We find
only scattered dermal bones in a ru-
dimentary form in extant Amphibia.

In Ceratophrys there is an osseous
shield in the skin of the back; in
Brachycephalus there are three which
are united to several vertebrae. The bony scales which are pretty
generally found in the Cceciliae, and which are set in pouch-shaped
depressions, do not apparently belong to this set of structures.

They are more common in the Reptilia, which so far approach
the old Amphibian phylum. In the fossil Teleosaurii, as in the living
Crocodilini, there are dermal bones distributed over the whole
integument, which form a kind of carapace ; in the Scinco'idea we
generally meet with interlocking bony plates in the integument.
Similar kinds of dermal ossifications in the Chelonii form a special,
though well-developed form of, dermal skeleton, in consequence

Fig. 220. Head of Acipeuser
sturio; seen from above, to show
the osseous plates covering the
cartilaginous cranium, which is
shaded dark.

426

COMPARATIVE ANATOMY.

of tlieir connection witli tRe internal skeletal parts. They not only
form a dorsal shield on the dorsal surface, but a ventral one on
the ventral surface (plastron). In the dorsal shield we can make
out a median row of bones, which are fused with the spines of the
vertebrm, and project from them. At the sides there are larger
plates, which are fused with rib -like processes, and in addition to
these there are special marginal plates around the edge of the
shield. These are wanting in Trionyx. Four paired pieces and one
unpaired piece can be made out, as a rule, in the plastron. All of
these parts are variously developed in different families of the
Chelonii.

Although the dermal bones of the Reptilia may probably be
rightly regarded as derived from the bony carapace of Fishes, we
must regard the ossifications which are found in the Edentata as
independent arrangements, which have had their origin in fresh
adaptive modifications.

Internal Skeleton.

§ 327.

The internal skeleton is of greater morphological importance than
the skeletal structures formed from the integument ; it is connected,
on the one hand, with arrangements which are found in the Inver-

tebrata, and on the other, and by a long
series of very varied arrangements, it can
be followed out through all divisions of
the Vertebrata.

At first the internal skeleton has the
form of a rod-like structure which tra-
verses the whole length of the body, and
is, when simplest, made up of indifferent
cells, and surrounded by a cuticular
structure which is formed from a secre-
tion of these cells. This primitive organ
of support is the chorda dorsalis or
notochord ; we have already met with it
in the Tunicata (cf. §303). The invest-
ment formed by it is the chordal
sheath (cs).

The earliest rudiment of the noto-
chord is placed just below the central
nervous system ; it has not always the
same relations to the germ-layers, al-
though it must be derived either directly
or indirectly from the mesoderm. The compact, and, in all cases,
primitively unjointed condition of the notochord speaks to its having
been inherited from an unjointed condition of the organism, and this
is what might be supposed from its early appearance in the embryo.

Fig. 221 a. Section through,
the vertebral column of Am-
moccetes. Ch Chorda. cs
Chordal sheath. m Spinal
chord, a Aorta, v Veins.

INTEENAL SKELETOK OF VEETEBEATA.

427

Tlie notocEord Eas always tlie same topograpEical relations to
tlie most important of tlie otEer organs. Above it, is tEe central
nervous system, and below it, tEe respiratory and nutrient apparatus.
Processes are given off from tEe connective tissue surrounding tEe
cEord wEicE enclose tEe so-called dorsal and ventral cavities ; tliese
processes pass into tlie musculature of tEe body, wliicE is thereby
broken up into a number of segments, set one behind the other.
In Amphioxus these segments are so far asymmetrical that they
are found alternately on either side.

TEe low condition of the axial skeleton, which is represented
by the chord, is permanent in the Leptocardii, where it merely
presents special histological modifications. In all the rest of the
Vertebrata the chord is the sole axial skeleton
in the earliest stages of development only ;
new differentiations appear, and it becomes
of less physiological importance. These differ-
entiations affect the notochord as well as the
tissue Avliicli surrounds it, and which has been
called the ‘^^skeletogenous layer‘d or '^skele-
togenous tissue,^'’ on account of its relations
to the future skeleton. The cells of the
chord form a tissue resembling cartilage, and
the sheath becomes a more independent por-
tion— it forms a supporting organ — owing to
the thickening of its layers. Cartilaginous
tissue forms around the chord (Fig. 221, h lx),
and that segmentation of the axial skeleton into
separate segments, the so-called vertebr£e,
which was before merely indicated, now be-
comes obvious. This segmentation of the
axial skeleton is an expression of the
metamerism of the whole body; a series
of these vertebree make up the vertebral
column. In each vertebra we call that por-
tion which surrounds the notochord the cent-
rum, and the outgrowing portions which enclose the dorsal and ventral
cavities of the body, and which are given off directly or indirectly
from the centrum, the arches. These again are distinguished as
upper or lower arches, according to their relations to these two
cavities.

As the axial skeleton becomes segmented, a welEdefined portion
in the most anterior segment forms the primitive Cranium in the
Cranio ta.

An inferior system of arches, which encloses the most anterior
portion of the intestinal tract, which functions as a respiratory organ,
is distinguished as the branchial or visceral skeleton. The
cranium and visceral skeleton make up the most anterior portion of
the whole skeleton — the skeleton of the head. The other skeletal
structures which are connected with it are represented by the more

Section

spinal

young

Fig. 221 b.
through the
column of a
Salmon. Ch Chorda.
cs Chordal sheath,
m Spinal chord. Tc Supe-
rior, k ' Inferior arch
(in rudiment), a Aorta.

V Veins.

428

COMPAEATIVE AXATOMY.

or less homogeneous vertebral column, which extends to the caudal
end of the body. The upper arches remain in close connection with
the centra. Movable girder-like pieces are, however, separated off
from the lower arches in the region which encloses the coelom ; these
are the ribs.

Lastly, there are the skeletal portions of the appendages
which are connected by special organs — the pectoral and pelvic
girdles — with the skeleton of the trunk.

The cartilaginous stage of the primitive skeleton is found
in all of the higher divisions, but in them it has no function
after a short time, for it is gradually replaced by osseous tissue,
whereby the skeletal parts come to have a greater physiological
importance. In correlation with this we note a greater differen-
tiation in morphological points. Even in the osseous skeleton,
however, the cartilage is of great importance. A modified form of
cartilage, which is characterised by the deposit of calcareous matter
in it, is also of importance. This form is not only antecedent to the
ossification of the parts of the skeleton, which are laid down in
cartilage, but — as is seen in that superficial calcification of the car-
tilaginous skeleton of ihe lower Gfnathostomata — is also sometimes
a permanent arrangement.

Vertebral Column.

§ 328.

The separation of the rachis into skull and vertebral column is
not completely effected in Amphioxus ; the whole axial skeleton is
represented by the notochord. In the Craniota they begin to be
separated. The lowest characters of the spinal column obtain in
the Cyclostomata, where the more highly-developed notochord, wnth
its sheath, forms the chief portion of it. Around the sheath there is
cartilaginous tissue, which is continued into lateral ridges as well as
into the wall of the dorsal canal. This tissue has its origin in the
continuous differentiation of the skeletogenous layer, and must not
be confounded with the cartilages, which ordinarily form the
vertebral segments. Speaking exactly, therefore, the spinal column
is not here separated into distinct vertebrse ; of which, indeed, there
are indications only in Petromyzon, where cartilaginous pieces, which
correspond to the superior arches, are enclosed in the wall of the
more anterior division of the dorsal canal. We meet also with
indications of inferior arches.

The notochord also retains its primitive characters in Chimmra
and the Dipnoi'. In the Chimterm circular calcifications of the large
chordal sheath point to a segmentation of the notochordal tube, but
they are more numerous than the primitive vertebrm, which are
merely represented by the arches on the chordal sheath. In the
most anterior region they grow round the chord and give rise by
fusion to a larger undivided piece. In the Dipno'i a strong tube.

VEiriEBE/E OF VEETEBEATA.

429

derived from tlie skelefcogenous layer, is developed around tlie
primitive sheatE, and on this the cartilaginous and superficially
ossified arches are set.

The axial skeleton of the Selachii is much more highly developed.
The rudiments of the superior and inferior cartilaginous arches
appear around the notochord; these grow around it and so form
cartilaginous circular centra. That part of the cartilage which
encloses the chord is marked off from the peripheral part, which
is continued into the arches, and the former represents, just as in the
Dipnoi, a kind of cartilaginous sheath (skeletogenous chordal sheath),
which is deposited on the cuticular sheath.

The vertebral column of the Selachii varies greatly in structure
according to the mode of growth of the notochord and its skeleto-
genous sheath.

A B c B

Fig. 222. Diagram of tlie changes produced in the notochord by the skeletogenous
layer (longitudinal sections), c Chorda, cs Chordal sheath, s Skeletogenous layer.
V Bodies of the vertebrae. iv Intervertebral portion. g Intervertebral joint.
A The chordal tube, when all its parts are equally well developed (Fishes). B Interver-
tebral growth of the chorda. Formation of amphicoelons vertebrae (Fishes). C In-
tervertebral constriction of the chorda by cartilage ; while the rest of the chorda is
retained in the vertebrae (Amphibia). D Intervertebral constriction of the chorda
(Reptilia, Aves). E Vertebral constriction of the chorda, where part of the interver-
tebral portion is retained (Mammalia).

The cartilage sometimes forms a cylindrical tube, in which the
vertebrae are merely represented by the arches and circular parts of
the skeletogenous sheath. The notochord is sometimes developed
between the vertebrae (Fig. 222, B), and retains its earlier size at the
points where the vertebra (t;) and arches were first laid down around
it. This arrangement gives rise to biconcave (amphicoelons) vertebrae
(B), the depressions in which are filled up by the intervertebral
chord. This is the way in which the vertebrae of nearly all other
fishes are formed.

§ 329.

In the Ganoidei the vertebral column, when simplest in organisa-
tion, resembles that of the Selachii. Just as in the Selachii and

430

COMPAEATIVE ANATOMY.

Cliimseras special cartilages are intercalated, wEicli aid the superior
arcEes, wliicE are connected with the bodies of the vertebrge, in
closing the vertebral canal.

Ill the Sturiones the skeletogenoiis sheath forms a considerable
tube, and the separation of the column into vertebras is only indicated
by the superjacent arches. The vertebral column of the other Ganoidei
is sharply marked off from this, its lowest form. Amia resembles
the Teleostei. A small portion of cartilage is retained at the point
where the arches are connected with the centra of the vertebrae ;
but this is absent in Polypterus, so that in it the arches and the
centra are united together by bone.

Lepidosteus is the most divergent form, for in it the cartilage
becomes constricted between the vertebrm. In
the cartilage which forms the constrictions, an
intervertebral articular cavity is formed, so that
the opisthocoelous vertebrm articulate with one
another. So far they resemble the Amphibia,
but, later on, the remnant of the vertebral por-
tion of the notochord disappears, and a bony
centrum is developed, which is connected, and
continuous, with the upper arches.

The vertebral column of the Teleostei is cha-
racterised by the reduction of the cartilaginous
rudiment. This reduction may be seen to bo
gradual, and may indeed be seen in one and the
same vertebral column in certain stages of de-
velopment ; where, that is, the cartilage may be
seen to diminish in quantity as we go from before
backwards. As a rule, four cartilaginous pieces,
belonging to the superior and inferior arches
(Fig. 221 b, may be seen around the chord,
and these take a certain share in the formation
of the arches. They very rarely form complete
superior arches. When osseous substance is
developed, these cartilages are generally retained
in the middle of the centrum, so that on making
a vertical section through it we get an obliquely set cross (cf.
Fig. 223, lili), the arms of which are directed towards the bony
arches. The notochord is always well developed between the
vertebrm, so that the centra are amphicoelous.

§ 330.

The vertebral column of Fishes can only be divided into two
regions, the body and the tail. They are distinguished from each
other by the characters of the inferior processes of the vertebrae,
while the upper arches are connected with the vertebrae in the
same manner throughout ; and are generally distinguished by the
possession of median (spinous) processes. In the region of the

Fig. 223. Vertical
section tlirongh the
middle of a vertebra
of Esox lucius.
ch Notochord. cs
Chordal sheath. Tc Ic '
Cartilaginous cross.
h Corresponds to the
upper, and P to the
lower arches (in rudi-
ment). h Osseous
transverse process.
n Spinal canal.

VERTEBEzE OF VEETEBEATA.

431

trunk, the lower arches are divided into ribs, and supporting
transverse processes (parapophyses). In the tail o£ the Selachii and
Ganoidei they are continuously connected with the centrum, and run
out into spinous processes, just like the upper arches.

In the Teleostei the costiferous transverse processes gradually
converge, in the caudal region, and form inferior arches, which are
not homologous with those of the Selachii and Ganoidei, although
they also form spinous processes.

In the Chimserae, Dipnoi, and many Teleostei, the caudal portion
of the vertebral column ends by gradually diminishing in size, but in
most fishes it presents great modifications, which are correlated with
the development of the caudal fin. These modifications first affect
the lower arches, which, in the Sharks, form spinous processes, which
are greatly widened out at their ends, and with which the caudal fin,
which is most developed in its ventral region, is connected. ‘ In many
Sharks, and still more in the
Sturiones, this caudal skeleton
is differentiated in an unequal
fashion. The inferior spinous
processes are more largely de-
veloped; this is correlated with
the degeneration of the superior
spinous processes, and of the
superior arches of the terminal
caudal vertebrae; this of course
produces an up -turning of the
caudal end of the vertebral
column; and in this way the
inferior lobes of the caudal fin
of the shark get to be terminal
in position.

In the Teleostei this up -turn-
ing affects also the axial portion of the vertebral column. As a
number of the terminal centra of this column are generally fused
together, and, like their upper arches, feebly or not at all developed,
while their inferior arches still persist, the up -turning must be the
more marked in proportion as the inferior arches become more
numerous and larger than the superior ones. This condition
(Fig. 224) is carried still farther by the atrophy of a large number
of vertebrse, so that nothing remains of them but^ their inferior
arches (Phy sostomi) .

Finally, the vertebrae completely disappear, and the remains of
the inferior arches of the caudal region are connected, in the form of
vertical plates, with a single vertebra which represents the end of
the vertebral column ; a style-shaped process (urostyle) of the
column is directed upwards, and contains the end of the notochord
(Acanthopteri).

Supporting organs, formed from the integument, are connected
with the parts thus formed by the vertebral column, and they

Fig. 221'. End of tlie caudal portion of
the vertebral column of a young Cypri-
noid. V Centrum, n Superior; h In-
ferior arches (the cartilaginous parts
are dotted), c End of the notochord.
d Covering bony lamella, r Bony rays
of the caudal fin.

m

COMPARATIVE ANATOMY.

are continued into tlie caudal fin. In tlie Selacliii tlie fin-rays are
formed by tbe so-called horny filaments^ and in the Ganoidei and
Teleostei by ossifications.

Like the caudal fin, the other unpaired ones have their supporting-
organs formed partly by the axial skeleton and partly by the integu-
ment. In the Selacliii jointed pieces of cartilage pass from the
spinous processes into these fins, and gradually acquire an inde-
pendent significance. In the Ganoidei and Teleostei they become
distinct bony pieces, which are known as supports for the fin-
rays;"” these are quite separate from the spinous processes. They
are connected with the fin-rays; these are jointed structures, which
are sometimes made up of separate bony plates, and are sometimes
represented by solid bony rods (spinous rays).

§ 331.

In the Amphibian vertebrae the cartilaginous rudiment of the
arches grows around the notochord, and forms constrictions in it by
means of intervertebral enlargements (Fig. 222, C). In many of
them the notochord is destroyed at these points. In the Anura the
notochord remains persistent in the middle of the centrum ; the
only exception to this rule is to be found in those forms in which
the centrum is developed on the surface of the notochord (Hyla,
Bombinator, Pelobates, etc.) ; when the articulating cavities are
developed, the articulating ends of the centra are developed from the
intervertebral cartilage. These intervertebral articulations are incom-
plete in most of the Urodela, where we find the articulating processes,
derived from the centra at every stage of development.

In the rest of the Urodela the intervertebral cartilage is only
feebly developed, so that the notochord is but slightly or not at all
constricted by it. It persists all along the vertebral column, and
is alternately constricted and widened out in Menobranchus, Siredon,
and Menopoma. In the latter the cartilage takes a markedly small
share in the formation of the vertebra; indeed, a series, in which the
intervertebral cartilage may be seen to undergo gradual degenera-
tion, can be followed out from the Salamandrina up to Proteus. In
proportion to this degeneration the vertebra is formed by deposits
of bony layers, which may even be laid down directly on the chordal
sheath itself.

No separate rudiments of superior and inferior arches can be seen
in the trunk ; they seem to have been fused into a common mass of
cartilage. Henceforward, therefore, that arrangement w-hich we
saw in the Fishes disappears, and the rudiment of the cartilaginous
vertebra is formed of a single piece early in life.

The shortening of the hinder end of the vertebral column in the
Anura is the cause of the development of a small number of ver-
tebrae. When the tail disappears, a long dagger-shaped bony piece,

VEETEBE^ OF VEETEBEATA.

433

which is ordinarily known as the urostyle (Fig. 225, c)_, is formed
from the rudiments of a few vertebrae ; counting this then, no more
than ten vertebral segments can be made out. There are many
more in the Urodela ; Amphiuma has as many as
100; Menopoma, 48; Salamandra, 42; and the
Coeciliae, about 230.

The transverse processes {tr) are small in the
Salamandrina ; the anterior ones are generally
divided into two segments; in the Anura they
are larger, but not divided. The superior spinous
processes are always rudimentary. Articulations
between the arches of the vertebrae are very
common, and are effected by the formation of
paired articular processes.

The connection between the pelvic girdle and
the vertebral column does not only more dis-
tinctly mark off the caudal portion from the
region of the trunk, but a sacral portion is thus
represented by a vertebra, v/hich is generally
distinguished (and especially in Pipa) by the size
of its transverse processes.

Gegenbaur, Unters. iiber die Wirbelsiiiile der Ampbibien.

Leipzig, 1861.

§ 332.

The rudiments of the vertebral column are
developed around the chorda dorsalis of the Sau-
ropsida, as of the Amphibia. Arches, which enclose the spinal
canal, are given off by the cartilaginous centra. The notochord
is also constricted between the vertebrse (cf. Fig. 222, D), but
the whole of it eventually disappears (except in the Ascalobota).
The continuous rudiment is separated into centra in just the same
way as in the anourous Amphibia ; in the Saurii and Ophidii, the
centra are procoelous. In the Crocodilini and Aves the cartilaginous
portions of the rudiment, which lie between the centra in the cervical
region, are converted into a special intervertebral apparatus.

Articular processes extend from the superior arches to the
vertebra next in front and behind. They are greatly developed
in the cervical region of the Chelonii. The superior spinous
processes vary in size, especially in the dorsal region ; in the
Crocodilini and many Saurii they are present on the caudal ver-
tebrae. Transverse processes are either given off from the centrum
itself, or quite close to it. They are greatly developed in the dorsal
and caudal region of the Crocodile, but much more so in the Chelonii,
where they are surrounded by the bony plates of the dorsal shield,
which have been developed in the integument. They are seen to be
divided into an upper and a lower portion in the Ophidii. In the

bral column and pel-
vis of the Frog.
tr Transverse pro-
cesses. s Sacral ver-
tebra. c Urostyle.
il Ilium, is Ischium.
f Femur.

431

COMPAEATIVE AIS^ATOMY.

vertebra of V u 1 1 u r
cine reus, c Centrum.
^jArch. s Spinous pro-
cesses. CO Euclimen-
tarj rib.

Eeptilia and Aves ribs are found along tlie whole of tlie dorsal
portion of tlie vertebral column, and are absent only in tlie Chelonii.

The movable cervical ribs of the Eeptilia
unite with the vertebrae in the Aves (Fig. 226,
co), and the two together bound a foramen
trans versarium .

In the caudal region of the column of the
Saurii, Chelonii, and Crocodilini we meet with
inferior arches, which are always attached be-
tween two centra, and take part in the forma-
tion of a caudal canal. They are rudimentary
in Birds. The inferior processes of the Ophidii,
Saurii, and Aves, which are given off directly
from the centra, are quite different from these.
On comparing the vertebral column of the
Eeptilia and Aves with that of the Amphibia we can see that it is
divided into a larger number of regions. A cervical and a lumbar

region are more
distinctly marked
off, owing to the
connection between
a number of ribs
and a sternum. The
lumbar region con-
tains the pre-sacral
group of vertebrm,
which have only
short ribs ; it is dis-
tinct in the Saurii
and Crocodilini.
The absence of any
sternal connections
in the Ophidii is
the cause of there
being no difference
between the tho-
racic and cervical
regions in these
forms, as well as
of the impossibility
of distinguishing a
lumbar one. In the
Chelonii also the
vertebrse of the
trunk are similar in
character through-
out. These regions
are not, however,

very sharply marked off from one another, inasmuch as in the Saurii

Fig. 227. Sacral i3or- Fig. 228. Sacral portion of
tion of the vertebral the vertebral column of a bird,
column of a Eeptile,
with the adjacent pre.
and post-sacral vertebrae.

Both of these diagrammatic figures are drawn as if from
the ventral surface, and show the nervous plexus on the
left. In both the figures a is the first sacral vertebra, h the
second sacral vertebra. . . . 1, 2, 3,4Presacral. 1', 2',3',4'
.... Post-sacral (or caudal) vertebrae.

VEETEBE^ OF VEETEBEATA,

435

and Crocodilini, as well as in Aves^ the last ribs of tbe cervical
region differ but little in length from the succeeding ones^ which
are connected with the sternum. The same applies to the lumbar
region in the Saurii ; in Birds this region is united to the true
sacral portion. The sacral portion of the column is increased in
size, inasmuch as in Eeptiles there is, at the least, a second vertebra
(Fig. 227, a h) in addition to the one found in the Amphibia. These
vertebrae attain firmer connections, and are completely fused in
Birds, where a large number of pre-sacral and post-sacral ver-
tebrae are attached to the primitive sacral vertebra (Fig. 228,
a b), and the whole set is united to the ilium. In the so-called
sacrum of Birds, thoracic as well as lumbar and caudal
vertebrae may be recognised; in the Struthiones the whole
number is as great as twenty- three. The two true sacral vertebrae
are very distinct in the Gallinae, many Natatores, and in birds of
pi-ey-

The caudal region is the one which varies most in character ; in
the Chelonii and Aves it is considerably reduced.

In Carinate Birds, the caudal vertebrae are not only reduced in
number, but four to six of them, which were separate in the embryo,
are fused together; and the bone thus formed, which is of some
size, and is ordinarily known as the ploughshare-shaped bone,^^
forms the terminal portion of the vertebral column ; it is generally
produced into an upright plate, in adaptation to its relations to the
rectrices feathers.

§ 333.

In the Mammalia the cartilaginous rudiment of the vertebral
column grows around the chorda dorsalis, and is constricted at
points, each of which corresponds to a centrum ; the notochord is
therefore retained for some time between the vertebrae (Fig. 222, E),
The intervertebral circular disc is developed out of the surrounding
tissue, and the remnant of the notochord is preserved in it, under
the form of the gelatinous nucleus.^'’ The cartilage is continued
from the centra into the superior arches. Independent ossifications
ap23ear in the centrum as well as in the arches, and the bony pieces
thus formed do not fuse until growth is finished.

On most vertebrae the arches develop spinous processes. In the
long-necked Ungulatas (Giraffe, Camel, Horse), they are not found
on the cervical vertebrae, but are greatly developed on the dorsal
ones. This holds also for the Cetacea, where the caudal region is
also of great size. As a rule articular processes are developed; they
have undergone degeneration in the Cetacea only. We are in the habit
of calling all kinds of structures transverse processes, whether they
spring from the arches or from the centra. These processes are
simpler in the cervical and thoracic regions. In the former they
are somewhat complicated by being fused with rudimentary ribs,
which unite with them and aid in the formation of a foramen trans-
versarium. They likewise carry ribs in the thorax, and here they

2 F 2

436

COMPARATIVE ANATOMY.

are attaclied on tlieir ventral surface. They may, however, also carry
terminal ribs, as in the posterior thoracic vertebra of the Cetacea.
As the thoracic pass into the lumbar vertebra© the transverse process
is very commonly differentiated into three special processes. The
mamillary processes form knobs, which are sometimes of great
size, and which are directed forwards ; they may get to be placed
at the base of the anterior articular processes. The accessory
processes are directed backwards and upwards; and a third process
is lateral, though often directed downwards ; these form the
transverse processes (lateral processes) of the lumbar vertebra.

The various divisions of the vertebral column are more sharply
differentiated from one another in Mammals than in Birds and
Reptiles. The cervical region is especially so ; it is distinguished by
the constant possession of seven vertebrae, which are more definitely
marked off from the thoracic region, owing to the fact that their
rudimentary ribs rarely increase in size as they approach the thoracic
ribs. The increase in number of the cervical vertebra in Bradypus
to eight, or nine, is explained by the fact that the thoracic vertebra©
2©ass into the cervical region, while the diminution to six in Choice pus
and the American Manatee is similarly explained by the complete
development of the rib of the seventh cervical vertebra.

The lumbar region is distinguished by the absence of movable
ribs. In the sacral region one vertebra only carries the ilium, as a
rule; but a second one has frequently the same relations. It is
more rarely that a third vertebra is connected with the ilium. The
fusion of these vertebrae with one another, and with one or
more caudal vertebrae, gives rise to the formation of a compact
region, which is known as the Os sacrum;^’ in this we must
distinguish the true sacral vertebrae from the false ones which are
developed from the caudal vertebrge. In the Edentata the number
of the sacral vertebrae is increased by the union of the ischium with
the caudal vertebrae. In this way the sacral region gets to extend
over eight or nine vertebrae.

In the Mammalia also the caudal region of the vertebral column
is the most variable of all regions; in most divisions it may be
either greatly developed, or may undergo considerable reduction.
Thus in the Simiae the number of vertebrm may be as high as thirty,
while in some again it may sink below the number, which has been
retained by man.

In this point the last portion differs from the most anterior or
cervical region ; while the intermediate portion is, as regards its
numerical relations, less constant than the cervical, but at the same
time less variable than the caudal portion of the vertebral column.
The number of dorso-lumbar vertebrge is very high in some of the
half-apes (23-24 in Lemur) ; in Choloepus it is 27; in the Perisso-
dactyla, 24. It is highest in Hyrax (29). It is lower in all the other
divisions.

Within the larger divisions the common ancestry of the different
genera is implied by the great constancy of the whole number of the

VEMEBE^ OF VEETEBEATA.

43?

dorso-liimbar vertebrae. In tbe Marsupialia and Artiodactjla tbis is
always 19 ; the same number, or 20, predominates in most Rodents,
Carnivora (21 in Paradoxurns and Procyon), and the majority of
Primates. In some of the latter, however, it falls to 17 or 18, in
which number they agree with most of the Chiroptera.

The variations within the dorso-lumbar region depend on the
ribs, for when they undergo degeneration the lumbar vertebrm are
increased in number.

§ 334.

Varied as are the differentiations undergone by the vertebrse, the
extreme conditions are usually connected by intermediate forms. It
is in the first two vertebrae only that an arrangement obtains
which is limited to them, and them only; and this arrangement is
due to the way in which the skull is connected with, and moves on,
the spinal column.

In Pishes we sometimes meet with peculiar arrangements in
the way in which the skull and first vertebra are connected
together ; in the Rays there are articular facets, as there are also in
the Telecstei, where they are placed on lateral processes. Another
set of modifications commences in the Amphibia. The first cervical
vertebra is circular, as it generally has no transverse processes,
which are only present when it is fused with the succeeding vertebra
(Pipa). This first vertebra is known as the atlas. In the Reptilia
the centrum of the atlas is separated from its arch, and is placed in
front of the centrum of the second, which is distinguished as the
axis, and it is more closely connected with the centrum of
the axis than with its own arch. Owing to this arrangement
a special piece is developed, which connects the tivo sides of the arch
on the ventral surface. In the Crocodilini the arch is also closed by
bone on its dorsal surface. In the Ophidii, the part which cor-
responds to the centrum of the atlas fuses wdth the second cervical
vertebra, and forms its odontoid process. The same arrangement
obtains in Birds, where the ventral connection of the arch becomes
very large indeed as compared to this odontoid process."’^

The arrangement seen in the Reptilia is represented in an em-
bryonic stage in Mammalia. In the Monotremata it lasts some time
longer than in the rest. In the Marsupialia, however, it is often a
permanent condition, owing to the separation of the centrum of the
atlas from the axis. In other cases the centrum of the atlas passes
completely into the odontoid process of the axis. The inferior union
of the arches is, in the Marsupialia, merely represented by a liga-
ment, or there may be a distinct bone in its place. In the Mono-
delphia there is a bony ventral clasp between the two halves of the
arch.

438

COMPAEATIVE ANATOMY.

Kibs.

§ 335.

The name of ribs is given to those parts of the skeleton which
have been developed from the inferior arches of the vertebrm^ and
which are either permanently, or transitorily, articulated to the
vertebral column. As a rule they clasp round and enclose a sub-
vertebral cavity. This cavity is divisible into two portions, which
differ in size, and in the organs which they contain. The anterior
one is the coelom. The posterior one is continued into the tail, and
forms the narrow caudal canal, which is sometimes divided hori-
zontally into two parts. These relations may be seen in Fishes, where
also the division of the body into regions is in its most indifferent
condition.

A comparison of the contents of the two tracts of the sub-
vertebral cavity will explain their difference in size. While in the
caudal canal there is nothing but blood-vessels, or, at most, parts of
the kidneys, which are always organs that vary but little in size,
great variations in volume can be made out in the organs of the
coelom, and these variations may be often seen to be due to a regular
succession of states, in which the organs are being filled or emptied.
The arrangements which can be made out in the lower arches are
correlated with this characteristic. They are seen to be direct
processes of the vertebra in the caudal region, and are immovable.
In the abdominal region, however, in adaptation to the varying
size of the cavity they enclose, they are segmented off from the
vertebrge, and are more or less movably articulated to the centra or
its processes (transverse processes).

The ribs are therefore nmgarded as differentiations of
the inferior system of arches; a number of arches, varying
according to the extent of the coelom, have been converted into the
more free form of rib. This view, which explains the origin of the
ribs, proves to us that the inferior arches, which resemble the
ribs, but which do not now enclose the coelom, are not primitive
structures, but that they once were ribs; this necessarily pre-
supposes that the coelom once extended much farther back.

§ 336.

As we have already treated of the indifferent lower arches
when speaking of the vertebral column, we have now only to deal
with the ribs and the organs derived from them. They are not
completely absent in any of the Vertebrata, save the Leptocardii,
Cyclostomata, and the Chimmrm. In all the rest they are either in a
rudimentary or in a well- developed condition; in the latter case they
may become connected together on the ventral surface, and a special
skeletal piece, the sternum, is then developed.

BIBS OF VERTEBBATA.

439

All tlie trunk vertebrae may carry ribs. In Fishes they generally
extend as far as the caudal region^ without any change in form.
They never become united below^ for when they are connected with
other parts of the skeleton_, these parts belong to the dermal skeleton
(Clupeidae). They are rudimentary in the Selachii^ and are
ordinarily represented by short pieces of cartilage only ; they are
larger in the Sturiones (Acipenser). They are not placed on the
exact boundary of the coelom_, but at a certain depth in the muscles ;
but this does not affect the explanation that we have given of the
origin of the ribs.

The ribs are better developed in those Ganoids that have a bony
skeleton. In the caudal portion of the vertebral column^ as in the
Selachii and Sturiones, they gradually pass into the lower arches,
which are at first connected with the centra in just the same way as
are the true ribs in front of them.

In the Osseous Fishes the ribs vary very greatly in character.
They are frequently rudimen-
tary or completely absent
(Lophobranchii, Gymnodonta,
etc.). It is easy to under-
stand, from what has been
said above as to the origin
of the lower arches in the
Teleostei, that these lower
arches may carry ribs (Fig.

229, G). In a few divisions
of the Physostomi the most
anterior ribs undergo changes,
for bones connected with the
air-bladder are developed
from them to form a chain
which extends to the auditory
organ (Cyprinoids). In Polypterus, rib-like structures are placed
between the dorsal and ventral muscles, on the side of the trunk,
and these extend to the integument. They are found also in Amia
and the Physostomi, where they are sometimes so large that they
have been regarded as the true ribs. As a rule they are bifurcated
at their origin.

Among the Amphibia the most completely developed ribs are
found in the Gymnophiona, where they are found on all but the
first and last vertebra. In the Urodela they are rudimentary, and
form short pieces, which are movably attached by two processes ;
posteriorly they are more simple in form. Just as in the Selachii,
they extend into the muscles. The transverse process of the sacral
vertebra also carries a rudimentary rib, by which it is connected
with the pelvis. In the Anura, likewise, they are rudimentary or
completely absent.

Fig. 229. Differences in the arrange*
ment of the ribs and transverse processes in
the Teleostei. c Centrum, o Upper arches.
u Transverse processes, r Ribs.

440

COMPARATIVE AXATOMV.

§ 337.

Among tlie Reptilia, the Chelonii resemble tlie anourous Am-
phibia ; there are no ribs in the cervical region of the vertebral
column_, and it is doubtful whether the rib-like processes, which in
the trunk are connected with dermal bones, are not really transverse
processes. In the rest of the Reptilia, ribs are found on nearly all the
trunk vertebrge. In the Saurii and Ophidii there are no ribs to the
atlas, and in the former there are none on the second cervical
vertebra either. In the Saurii, some of the ribs of the trunk are
connected with a sternum, and so make a great difference between
the various costiferous portions of the vertebral column; in the
Ophidii, however, the ribs, from the second cervical vertebra as far
as the end of the trunk, have pretty much the same characters. They
are all characterised by their free articulation with the vertebral
column.

In the Saurii and Crocodilini, each rib connected with the sternum

to

Fig. 230. Cervical and thoracic vertebi’80 of Croc 0 cl ill! s. c Kibs. c' Rib of the atlas.

si Sternum.

is always divided into several portions ; of these the upper, or ver-
tebral, is, as a rule, the only portion that is completely ossified. The
sternal end ordinarily remains cartilaginous, and but few ribs are
directly attached to the sternum. Not unfrequently a large number
are connected with a cartilaginous arch which is attached to its
hinder end. The more posterior cervical ribs, even, may be divided
into two segments, and so form an intermediate stage towards the
arrangement seen in the thoracic ribs.

In Birds, the union between the rudimentary cervical ribs and
the vertebral column leads to most of them being completely fused
with it ; on the other hand, the hinder ones are less firmly connected,
and their mode of union is intermediate in character between that of
those in front of them, and of those thoracic ribs which reach to the
sternum. As in the Saurii, these latter are not numerous, and are
similarly divided into a vertebral and a sternal piece (os sterno-
costale). The vertebral pieces are distinguished by backwardly
directed processes (processus uncinati), which are applied to the body
of the succeeding rib, and so increase the strength of the thorax.
This arrangement obtains also in some Saurii (Sphenodon) and in the

PvIBS OF VEETEBEATA.

441

Crocodiliiii. These processes are not laid down in cartilage, but are
secondary ossifications. The Inmbar portion of the vertebral colainn,
which is fnsed with the sacmm, has no ribs in Birds, but distinct
rudiments may be seen on the true sacral vertebrae, so that even
here the ilium is not united directly to the vertebrae, but to the
rudimentary ribs that are attached to them. Similar rudiments
may be made out in the Crocodilini. When the caudal region is
well developed, the same structures as those which have been already
alluded to as rudimentary ribs may be seen to enclose the caudal canal.

With regard to the connection between the ribs and the vertebras ;
in the Saurii, Crocodilini, and Aves, there are generally two points
of attachment, for the rib is articulated to the centrum by a
capitulum (/3), and to the transverse process by a tubercle (a). The
posterior ribs gradually get to have only one point of attachment.

In the Mammalia the cervical ribs have passed completely into
the vertebrse, and it is only now and again that we meet with a free
rib. The thoracic ribs, which vary in
number, are divided into the two above-
mentioned pieces, and so show that ossi-
fication has not equally affected the whole
rib, but that a sternal portion remains in
its cartilaginous condition. When this
portion is ossified (Edentata, Cetacea) it
forms a separate piece; in Ornithorhyn-
chus, it is again divided in the case of
the last five ribs; this happens also in
Manis.

The anterior ribs alone reach to the
sternum. The hinder ones are either
connected with the sternal end of the one next in front, or project
freely, and so get to resemble the rudimentary forms, to which
class of ribs the hindermost ones in the Cetacea do belong; these
are not even connected with the vertebral column. In the lumbar
region the ribs are fused with the transverse processes. It is not
certain whether the transverse process itself does not represent the
rib. The rudiments of the ribs are much more distinct in the first
two or three sacral vertebrge, where, as in the lower classes, they
are the means of connection between the sacral vertebrae and the
ilium. In this region they have the form of ventral pieces attached
to the transverse processes. Finally, in long-tailed Mammals, the
rudiments of ribs having the form of inferior arches occur. The
double mode of union, which is generally seen in the case of the
cervical ribs, obtains also in the thoracic ribs, but it is single in
the case of the hinder ones.

Fig. 231. Thoracic vertebra of
Buteo vulgaris, c Ceutrum
of the vertebra. s Superior
spinous process, tr Transverse
process, io Rib. a Tuberculum.

^ Capitulum.

U2

COMPAEATIYE AXATOMY.

Sternum.

Fig. 232. Sternum and Shoulder-
girdle of Rana temporaria. p Body
of the Sternum, sc Scapula, sc' Supra-
scapular. CO Coracoid fused in the
middle line with its fellow of the
opposite side (s). cl. Clavicle, e Epi-
sternum. The cartilaginous parts are
shaded.

§ 338.

Tlie Sternum forms the ventral portion of the framework of
arches^ which is formed by the ribs. It is developed from a rudi-
ment similar to that of the ribs,
as a band of cartilage, which con-
nects together the proper ribs on
each side. It appears, therefore,
as a paired portion of the skeleton,
and its later characters are due
to its fusion along the middle
line. We first meet with it in
the Amphibia. We must there-
fore suppose that in these forms
there was once a stage, in which
the ribs were united by a sternum.
Of this stage nothing has been re-
tained, in addition to the rudiments
of the ribs, except the part which
represents the rudiment of the sternum; and the preservation of
this rudiment is explained by the fact that it is connected with the
shoulder -girdle. Thus in the Salamandrina it has the form of a

broad thin plate of cartilage, in which
there are deep grooves for the attach-
ment of the coracoids. In the Anura
(Fig. 232, p) it is placed at the hinder
edge of the coracoids (co) which are
united together in the middle line, and
forms a partly ossified appendage to the
shoulder-girdle, the hinder end of which
persists as a broad plate of cartilage.

The sternal plate of the Saurii and
Crocodilini resembles the broader form
of Amphibian sternum. It is generally
rhomboidal in form, and has the same
relations to the shoulder-girdle (Fig.
233, s). As a rule but few pairs of ribs
are connected with the sternum, which
frequently retains its cartilaginous con-
dition (Fig. 233, s) ; at its hinder edge
it gives off one or two processes, which
also receive ribs. The paired condition
of this second portion of the sternum
must be regarded as a continuation of the embryonic arrangement.

The sternum of Birds is always ossified ; it represents the more
highly developed sternal plate of Eeptiles, but the hinder portion is
no longer developed. It has, moreover, but few pairs (no more than

Fig. 233. Sternum and Shoul-
der-girdle of Uromastix
spinipes. s Sternal plate,
which supports at its sides
several pairs of ribs, and is
provided posteriorly with two
processes, sc Scapula, co Co-
racoid. cl Clavicle, t Epi-
sternum. The cartilaginous
portions of the sternum and
coracoids are dotted.

STEENUM OF VEETEBEATA.

443

six) of ribs connected with it. In the Eatitse it forms a broad piece
of bone, which is very convex anteriorly. In the Carinatao, however,
it is distingnished by a projecting keel (Fig. 234, crs) on the
anterior and convex face of
the sternum, which serves to
increase the surface from
which the muscles take their
origin. The form of the
sternum is therefore correlated
with the development of the
muscles, while the size of the
sternum and of its keel corre-
sponds to the power of flight
of which its possessor is
capable. The hinder end
often presents paired fora-
mina, which are closed by
membrane (Birds of Prey and
Water Birds) ; when the peri-
phery of these foramina is
broken through towards the
hinder edge of the sternum,
clefts are formed, between

which the so-called abdominal processes project (Fig. 235). The
sternum of Birds has very much the same relations as in Eeptiles,
owing to its connection with the shoulder-girdle.

The sternum of Mammals is distinguished from that of the
preceding classes by the large
number of segments that are
separately ossified in it. It is
made up of bones, which are
placed one behind the other,
although they were all laid
down in a continuous cartilage.

They are not unfrequently
developed from paired centres
of ossification, and frequently
fuse into one piece.

The form of the sternum

Fig. 234. Sternum
of Buteo vulgaris
(seen a little from
one side), crs Crista
sterni. / Furcula.
c Coracoid.

Fig. 235. Steruum
of Numida melea-
gris (seen from iu
front) . crs Crista
sterni. c Coracoid.

is affected by its relations to the
shoulder-girdle. The anterior
piece is connected with the cla-
vicles, and is distinguished by
its greater breadth ; it forms
the manubrium. A ridge-like process (Fig. 236, c) is developed on the
anterior face of this segment in aerial Mammalia ; this is physiologi-
cally similar to the keel of Birds. When the clavicles are absent,
the anterior end of the sternum is generally much diminished in
size. The hinder end is always drawn out into a median piece, which
frequently remains cartilaginous (Fig. 237, x) (xiphioid process).

Fig. 236. Steruum
of Vespertilio mu-
riua. s Sternum.
c' Crest, cl Clavicle,
c Ribs.

Fig. 237. Sternum of
Cervus capreolus.
se Costal cartilages.
X Xipbioid process.

444

COMPAEATIYE ANATOMY.

A special piece, tlie episternum, is very commonly present in
connection with the sternum ; this has one of t’wo forms according
to the way in which it is developed and connected.

In one the epistermim is merely represented by bony structures,
which are placed on the ventral face of the sternum. Thus, in the
Eeptilia, it has the appearance of a cross, or of a T-shaped bone
(Fig. 23o, t), the two arms of which support the clavicles, while the
median piece is attached to, or even fused with, the sternum (Asca-
labota). In the Crocodilini the transverse piece of the episternam
and the clavicles are both wanting, as is the whole episternum in
the ChamseleonidEe ; nor is it present in Birds.

The second type of these structures is made up of skeletal parts,
which are preformed in cartilage, and wEich lie in front of the
sternum. There is a structure of this kind in the anourous
Amphibia (Fig. 232, e), where it is represented by a bone, which is

separated from the sternum by the
coracoid, and is therefore set in
front of the shoulder-girdle.

In the Alammalia the episternum
always forms a uniting piece be-
tw^een the sternum and clavicle.
In the Monotremata it has the
form of a bone attached to the
sternum, and drawn out into tw^o
processes. In the Marsupialia (Di-
delphys) the lateral arms (Fig. 238)
remain movable, but the median
portion is fused with the sternum.
This leads to that reduction of the
episternum which is seen in other
JMammals ; in which the lateral
pieces alone are present in cartilage,
or form bony parts, wBich are attached to the sternal end of the
clavicle (Bodentia, Insectivora, Edentata). In the Primates these
episternal structures form the intermediate cartilages of the articu-
lation of the sternum with the clavicle.

Gegenbaur, C., TJeber die episternalen Skelettheile imd ilir Yorkommen bei den
Siiugetbieren und beim Menscben. Jen. Zeitscbr. I. — Parker, W. K., Structure
and development of the Shoulder-girdle and Sternum. Ear Soc. 1868.

Fig. 238. Episternum, with the parts
connected to it, of a young Opossum.
st Anterior end of the sternum (ossi-
lied). ep Episternum (cartilaginous).
cl Clavicle, c The first two ribs.

Cephalic Skeleton.

§ 339.

The indifferent stage in which the head is found in the Acrania
makes it impossible to distinguish any distinct cephalic skeleton.
It is, however, true that the head of the Craniota is not an absolutely

CEPHALIC. SKELETON OF VEETEBEATA.

445

new structure, and to tLe same extent tLe cepEalic skeleton is not
new. If the anterior respiratory portion of the body in Amphioxus
corresponds potentially to the head of the Craniota, then the
skeletal parts in it must be potentially homologous with a cephalic
skeleton. This applies to that region of the notochord, and to
the tissue derived from it, which encloses the anterior portion of
the central nervous system, as well as to the framework of the
respiratory cavity.

In the Craniota this anterior portion of the body is different
on the dorsal, as well as on the ventral, surface from what is behind
it ; owing to the changes which take place in its functional import-
ance, in consequence of its relations to a large number of other
organs, it gets to differ in many special points, owing to the exist-
ence of which it can be distinguished as the head ; and it conse-
quently becomes of greater importance than the rest of the body.
It stands in relation to the entrance of the alimentary canal ; it con-
tains the most important of the sensory organs ; and affords a shelter
to that portion of the central nervous system which is developed
into the brain. These relations are, moreover, the cause of the
metamorphosis which affects it.

In the skeleton of the head we may distinguish — 1) the Skull,
and 2) the Branchial skeleton.

1) The Skull (cranium) is the name given to that part which
is a continuation of the dorsal column, and is connected with the
axial skeleton. It has many things in common with the former, for
it corresponds to a number of vertebral centra and upper arches.
This similarity is not only implied in its texture, but also in its
structural relations, and in its relations to the central and peripheral
nervous system. The chorda dorsalis is continued into the basal
portion of the cranium, where it may either remain permanently, or
disappear after a short time. With increased development of the
higher sensory organs the cranium becomes of more importance.
A hinder segment encloses on either side the auditory organ, and
may be distinguished as the auditory capsule. Going forward, there
is, on either side, a depression which affords shelter for the eyes
(orbit), while in the most anterior part there are cavities for the
reception of the olfactory organ. The primitive condition of this
cranium is cartilaginous ; it forms the primordial cranium.^"’

2) With this cartilaginous skull there is connected, at the com-
mencement of the alimentary canal, a system of arches, which is
likewise primitively cartilaginous ; these branchial arches are, on
the whole, similar to the ribs of the vertebral column, but at the
same time they are not altogether homodynamous with ribs. The
various arches differ in form, but they all give indications of having
been primitively similar. Their difference in form is due to a
differentiation which has been brought about by adaptation to
different conditions.

446

COMPAEATIYE AXATOMY.

§ 340.

The relations between the cephalic skeleton and the vertebral
column were_, in past times, the cause of attempts to show that the
former was made up of various segments resembling vertebrte ; from
this point of view the cephalic skeleton seemed to be merely a modi-
fication of the vertebral column. It was indeed believed that the
characters of the various segments of the osseous cranium would
be of aid in this comparison ; as a fact, however, these are of very
uncertain value, as in dealing with them we have to do with a
condition which has already been much modified. In other words,
the cephalic bones which have been said to belong to three, four, or
five so-called cranial vertebraB,'’^ have had very different origins, and
are for the most part structures which primitively did not belong to
the cranium.

The examination of the primordial cranium of the lower Yerte-
brata, especially with reference to the nerves which pass out from it,
shows that, although indications of its primitive composition out of
metameres, homodynamous with vertebrm, can be made out, as
might be supposed indeed, yet this metamerism of the cranium is not
the same as that segmentation which is faintly indicated by the bony
cranium.

The view here adopted is chiefly based on the following points :

1) It can be shown that the arches of the branchial skeleton
form inferior arches which belong to the cranium.

2) It follows that a general agreement may be made out between
the visceral arches and the inferior arches of the vertebral column.

3) The cranium is comparable to a portion of the vertebral
column, which contains, at least, as many vertebral segments as there
are branchial arches.

4) In the cranium itself there are a number of important points
in which it resembles the vertebral column.

a) The chorda dorsalis which forms the base-work of the
vertebral column passes through the cranium in just the
same way as it passes through the vertebral column.
h) All the nerves which pass out in this segment are homo-
dynamous with the spinal nerves.
c) The differences seen in the cranium, as compared with the
vertebral column, may be explained as adaptations to certain
conditions, which are external to the cranium; that is,
as acquired arrangements. They lead us, therefore, to
presuppose a condition, in which the cranium had not yet
acquired these peculiarities, and when therefore it did not
differ very greatly from the vertebral column.

5) The differentiation of the cranium appears therefore to
be due to the concrescence of a number of vertebrm : such

SKULL OF YERTEBEATA.

447

concrescence may also obtain in the vertebral column. Modifi-
cations in the region^ which thus became continuous, were brought
about partly by influences which affected it directly from with-
out, and partly by influences which affected it from within (the
development of the brain).

6) Inasmuch as those nerves only can be seen to have any
likeness to spinal nerves which are found in that division of the
cranium which is traversed by the notochord, it follows that this
portion only can be derived from vertebrae ; the branchial skeleton
also belongs to this portion. We must therefore distinguish between
this |3ortion of the cranium which is vertebral, and the anterior,
or evertebral, portion, which does not exhibit any relations to the
vertebrae. This latter is rather a secondary formation, although it
has had its origin in the vertebral portion.

The number of vertebrae, which enter into the composition of the
cranium, are, so far as we yet know, nine at least. But this is by
no means saying that the number may not have been much larger.
Such a supposition is borne out by a large number of facts which are
known to us with regard to the distribution and mode of origin of
the nerves in the Selachii ; and these point also to the atrophy of
visceral arches. The arrangement in Amphioxus is just as much to
the point, for in it a large number of branchial arches are persistent.
All that portion of the primitive spinal column (notochord and
perichordal tissue), which extends along the branchial framework, of
Amphioxus would then be homologous with that portion of the axial
skeleton which, in the Craniota, has passed into the cranium.

Gegenbaur, C., Untersucli ungen zur vergl. Anafc. der Wirbelthiere. III. Das
Kopfskelet der Selachier als Grnndlage zur Beurtheilung der Genese des
Kopfskeletes der Wirbelthiere. Leipzig, 1872.

Skull.

§ 341.

The skulls of the Craniota are divided into two very different
groups. In the one the above-mentioned internal branchial skeleton
is well developed, and its most anterior segments take on the form
of maxillary organs, and affect the form of the cranium by being
either directly, or indirectly, connected with it. This arrangement
obtains in the Cnathostomata. The other form is represented in the
Cyclostomata, in which it is possible to make out indications of the
same arrangements in the cephalic skeleton as those which are seen
in the Cnathostomata ; but the differences are so great, that it is
impossible to compare all the parts with anything like exactness.

The notochord is continued into the base of a capsule which
encloses the brain, and which is, in comparison with other parts of the
skeleton which are to be regarded as belonging to the skull, of a
remarkably small size. In Petromyzon two enlargements which

448

COIMPAEATIYE AJSY4TO]\IY.

contain the auditory organ (auditory capsules) (/) are attached to the

sides of this capsule (Fig. 239^

Fig. 239. Skull and commencement
of the vertebral column of Petro-
myzon marinus. A Median sec-
tion. B Seen from above, a Noto-
chord. h Spinal canal, c Eudiments of
the vertebral arches, d Cartilaginous
cranial cavity, d' Membranous por-
tion of the cranial cavity, e Basis
cranii. / Auditory capsule, g Nasal
capsule. g' Palato-nasal passage.
gr Its blind end. h Continuation of
the bony palate, i Posterior cover-
ing-plate of the mouth. Jc Anterior
covering-plate. Z Labial ring, m Its
appendage (after J. Miiller).

d), and below them two divergent
bars are given otf^ which take a
carved direction forwards. An-
teriorly, these are connected with
a process which is given off from
the cerebral capsule. An unpaired
nasal capsule (g) is attached to the
upper and anterior portion of this
latter ; this capsule differs greatly
in form in the Myxinoidea and
Petromyzontes ; below this there is
a broad plate of cartilage, which
has a complicated apparatus placed
below it, which covers in the oral
opening from above {i h I m), and
forms a firm framework for the
palato-pharyngeal cavity. Poste-
riorly the capsule of the skull is
continuous with the spinal column.

The second form of skull is dis-
tinguished by being connected with
an apparatus which encloses the
opening of the mouth, and which
is developed from a branchial arch ;
this portion is more or less con-
nected with the cranium. Part of it

forms the lower jaw, and is always freely movable (Gnathostoinata).

This arch is differentiated into two pieces, which function as
jaws; an upper one, the palato-quadrate, and a lower one, the

cartilaginous lower jaw.
The palato-quadrate is
articulated with the base
of the skull; but it
is enlarged horizontally
backwards, and is con-
nected with the second
arch also, the upper
portion of which is
movably articulated with
the skull. The hyoid
forms the lower portion
of this second arch.
As this upper portion
of the second arch is

Fig. 240. Skull and visceral skeleton ofaSelacbian
(Diagram), occ Occipital region, la Wall of the
labyrinth, eth Ethmoidal region. n Nasal pit.
a First, h c Second labial cartilage, o Superior,
n Inferior portion of the mandibular arch I. II Hy-
oid arch. Ill — VIII (1 — 6) Branchial arches.

often greatly developed,
it gets the appearance of a supporting apparatus for the
two primitive jaw parts, which are developed from the first arch,
and is known as the hyomandibular. The rudiments of other

SKULL OF VERTEBEATA.

449

arches are embedded in the upper and lower lip, in front of the
mandibular arch ; they are the labial cartilages.

The parts, therefore, which are derived from the branchial
skeleton and enter into closer relation with the skull are these :

1) The two labial cartilages (Fig. 240, a and h c) ; the anterior
one consists of one, the posterior of two pieces.

2) The mandibular arch (I), which consists of a superior piece — ■
palato- quadrate (o), and of an inferior piece — lower jaw {u),

3) The hyoid arch {II) ; of this the upper portion only (hyo-
mandibular) has any intimate relations to the skull.

We find various cartilaginous rods attached to all the arches,
except the labial cartilages ; these are directed backwards, and afford
support for the branchial pouches ; they are known as branchial
rays. They undergo various modifications; they are diminished
in number on the palato- quadrate, but may be seen in the wall
of the spiracular cleft, which represents a rudimentary branchial
pouch (spiracular cartilage).

We shall treat of these portions of the branchial skeleton with
the skull ; the other arches (III — FIZT) will be considered later on
(§ 353).

The arrangement we have described may be seen in the cephalic
skeleton of the Selachii. All its parts are formed of cartilage, which
is, as a rule, covered over by a thin calcified layer, which however is
never ossified. In the cartilaginous cranial capsule the most anterior
portion is formed by the ethmoidal region. On its inferior surface
there is a nasal pit (n) on either side. The cranial cartilage is
frequently continued forwards, and between these pits, into a process
(rostrum). The succeeding portion, with its cavities, forms the
orbits, which is overhung by cartilage above and behind. The next
portion (la) is the broadest ; at the sides this encloses the auditory
labyrinth, and is continuous at its hinder surface with the last of the
primary regions (occ) ; in some Sharks (Notidani) this hindermcst
region is continuous with the vertebral column.

Teeth are developed in the mucous membrane which covers the
palato- quadrate, as well as in that which covers the lower jaw;
and this explains why these cartilages are so well developed.
The hyomandibular is attached posteriorly to the palato- quadrate,
and is either directly continuous with the lower part of the hyoid
arch (Notidani), or is more freely attached to it. The hyomandibular
is consequently greatly developed in the Sharks, and is converted
into a kind of suspensorium, by gradually gaining a closer connec-
tion with the lower jaw. Finally the hyoid portion ceases to appear
to be a continuation of the hyomandibular ; in the Rays, indeed, it
loses all connection with it, and the hyomandibular is then merely
the support of the jaws.

The skull of the Chimaerm is very different to this, for in it
there is a continuous connection between the palato- quadrate portion
of the first arch and the cartilaginous cranium. The mandibular
portion, which is articulated with a process of the cranium, is the

2 G

450

COMPAEATIYE ANATOLIY.

only part of tlie first arcli whicli is movable. The partly ossified
skull of the Dipnoi has just the same characters.

§ 342.

The cartilaginous cranium is most completely retained in the
Sturiones (Fig. 241) ; in the rest of the Gano’idei it is partly so ; in
the Teleostei^ and especially in Salmo and Esox^ it is more or less
so. In most forms the ethmoidal portion remains cartilaginous.
Henceforward^ and all through the Vertebrate phylum, a cartilaginous
cranium can be made out in the earliest rudiments of the skull j
howsoever modified, it may be derived from the primitive condition,
and may therefore be regarded as a remnant of that condition.

The degenerations which affect this primordial cranium are chiefly
due to the ossifications which take place in it. Bony parts, which
perform the function of organs of support and protection, better than
cartilage does, take its place ; and the development of these bones.

Fig. 241. Cephalic skeleton of Acipenser sturio; after the removal of the cover-
ing bones, r Rostrum, n Nasal cavity, o Optic foramen, tr Trigeminal foramen.
sp Spinous processes of the anterior portion of the vertebral column, which is fused
with the cranium, p Palato-quadrate. on Mandible. Hm. Hyomandibular. s Sym.
plectic. hr Branchial arches, r Ribs.

which enter into connection with the cartilaginous cranium, is clearly
the cause of the degeneration of the cartilaginous tissue. A higher
and more complete arrangement has ousted the lower one.

Bony pieces become connected with the cartilaginous pieces of
the branchial skeleton, as well as with the cartilaginous cranium ;
in this way the whole cephalic skeleton gradually passes from a
cartilaginous into an osseous condition. The osseous elements which
go to make it up are almost all derived from the dermal structures,
which we met with as dermal denticles in the Selachii. S ome of
these bones appear on the outer surface of the carti-
laginous cranium, and form the covering bones of the skull
(cf. supra, § 325). They are plates which are formed in the
integument, and owe their origin to the growth of placoid
scales; in the Sturiones, already a number of them have
taken up the same position as that which they retain in
the higher divisions.

SKULL OF VERTEBEATA.

451

Others are formed in the mucous membrane of the buccal cavity,
that is, in those parts of the cartilaginous arches of the mandibular
and branchial skeleton which aid in forming the boundaries of this
cavity. The origin of these bones may be seen, in the
Amphibia, to be due to the concrescence of tooth-like
structures, which are, indeed, similar to the dermal
denticles, and which
are also found on
the covering of the
buccal and branchial
cavities in the Sela-
chii.

Dentigerous plates
are formed by the fusion
of some of these den-
ticles, and these afford
points of support for
the necessary primordial
skeletal deposits, and
gradually become con-
nected with them. In
some of these plates the
denticles are retained ;
in others they disap-
pear, or are no longer
developed, so that their
product — that is, the
bone — is alone de-
veloped ; and this, by
investing or growing
around the cartilage,
forms a perichondrial
ossification. The origin,
therefore, of a large
part of the osseous ce-
phalic skeleton can be
explained by the relations, which the ossifications of the external
integument, and of the investment of the buccal cavity, have to it.
The phylogeny, however, of a few of the cephalic bones is as yet
unknown.

Hertwig, O., Ueber das Zahnsystem der Ampbibien. Arch. f. mikr. Anat.

Bd. XI. Supplement.

§ 343.

With regard to the various bones, we divide the primordial
cranium into the regions which we have already distinguished. The
occipital region is made up of four bones. The basi-occipital is a
direct continuation of the centra of the vertebrae (Fig. 242, Oh).
In its hinder portion there is a cavity which is filled by the noto-

Fig. 242. Skull of Salmo salar. A Side view.
B Vertical median section. The cartilaginous parts
are hatched, while the bones derived from the
primordial cranium are dotted. Oh Basi-occipital.
01 Exoccipital. Os Supra-occipital. 8q Squa-
mosal. Ep 0 Epiotic. Pr 0 Petrosal. Sh Basi-
sphenoid. Als Alisphenoid. Or 8 Orbito-sphenoid.
Fa Prefrontal. Fp Postfrontal. Fr Frontal.
An Nasal. Ps Parasphenoid. Fo Vomer. P® Pre-
maxilla, gl Articular surface for the Hyoman-
dibular. Eth Ethmoidal cartilage, vag Foramen
for the exit of the vagus.

452

COMPARATIVE ANATOMY.

chords and wliicli corresponds to tRe anterior cavity o£ tRe first
vertebral centrum. At its sides tRe exoccipitals [01) are attacRed to
it ; tRese always enclose tRe greater part of, and sometimes nearly
all, tRe posterior foramen magnum. TRe supra- occipital [Os) enters
into it above ; this is often distinguisRed by a large vertical process,
wRicR resembles tRe spinous processes of tRe vertebral column.

TRe next segment partly, at any rate, encloses tRe labyrintR, and
tRe bones wRicR compose it are named in accordance witR tRis rela-
tion. TRe most constant, and tRerefore tRe most important, is tRe
petrosal (pro-otic) ; tRis contains, or, at least, limits posteriorly, tRe
foramen of exit of tRe trigeminal nerve. It extends to tRe base of
tRe skull, wRere it may become united witli its fellow of tRe opposite
side, witliin tRe cavity of tRe skull. TRe epiotic forms a second
piece; tRis is attacRed above to tRe exoccipitals, and generally forms
a projecting process (Fig. 243). A tRird one, tRe opistRotic (inter-

calare), generally lies to tRe sides
and front of tRe exoccipital, and
varies greatly in cRaracter (Figs.
243-6). As a rule tliis piece Ras
no relations to tRe labyrintR,
witR wRicR, Rowever, otRer bones,
sucR as tRe exoccipital, very
frequently become connected.
Lastly, an external covering
piece of tRe primordial cranium
belongs to tRis region, and
gradually becomes closely united
witR it. It takes part in tRe for-
mation of tRe articulation for tRe
Ryomandibular, and often develops a process wRicR projects back-
wards and' outwards. TRis is tRe squamosal (Figs. 242, A Sq ; 243, 6).

Great differences may be seen to obtain in tRe development of tRe
bones of tRe next portion, wRicR depend on tRe size of tRe cavity of
tRe skull. WRen tRis extends far forwards, tRe walls of tRe primor-
dial cranium are correspondingly more complete, wRile, wRen it is
reduced in size, its walls are diminisRed, and are partly replaced by
membranous structures. TRus, in many cases, tRere is a mem-
branous interorbital septum ; or bones, wRicR are well developed in
otRer forms, are present in a rudimentary condition.

TRe ossifications of tRis segment are tRe alispRenoid (spRenoi-
dale laterale posterius) at tRe sides and beRind, and tlie orbito-
spRenoid (spRen. later, anter.) in front. In Amia, as in many Tele-
ostei, tRe latter are separated from one anotRer, wRile in otRer forms
tRe pieces of eitRer side unite at tRe base of tlie cranial cavity, and
at last may fuse into one piece, or may be rudimentary. At tRe base
of tRis segment tRere is a basi-spRenoid, wRicR is derived from tRe
cartilage of tRe primordial cranium ; tRis is, as a rule, a bone of no
great size; it is connected superiorly witR tRe alispRenoid. WRen
tlie base of tRe skull is traversed by a canal for tlie optic muscles.

Fig. 243. Posterior portion of a cranium
of Gadus (side view). 1 Basi-occipital.
2 Exoccipital. 3 Supra-occipital. 5 Para-
sphenoid. 6 Opisthotic. 6' Squamosal.
7 Epiotic. 15 Pi'ootic. 12 Postfrontal.
11 Frontal, c Articular surface for the
hyomandibular.

SKULL OF VERTEBRATA.

453

wliicb. passes obliquely backwards from tbe orbit, this bone forms a
pillar between tbe canals of either side. Not unfrequently it seems
to disappear altogether. On the ventral side of, and along the pri-
mordial cranium there extends the large parasphenoid (Figs. 242, Ps ;
243, 5), which has been already seen in the Sturiones.

It is seldom that the whole of the primordial cranium is retained
on the upper surface ; as a rule, there are spaces in it, which are
overlaid by covering bones. Nearest to the posterior primary region
there are the two parietals (Fig. 244, 7) ; these are sometimes sepa-
rated from one another by an anterior process of the supra-occipital
(3). In front of them there are the frontals, which are often re-
placed by a united frontal (11). At the sides of this are the two post-
frontals (12), which extend as far as the squa-
mosal, and take part in the articulation of the
hyomandibular.

In the ethmoidal region there is a median
piece, ethmoidale medium (16), to the sides of
which the lateral ethmoid s are attached (14)

(frontalia anteriora. Cuvier). The latter
form the base of the nasal capsules. The
median portion of the ethmoids is often per-
manently cartilaginous. The vomer forms the
covering piece of the base of the ethmoidal
region ; posteriorly this is united to the para-
sphenoid ; it is paired in Lepidosteus.

§ 344.

The mandibular apparatus of the Selachii
is not completely retained in the Gano'idei and
Teleostei, owing to the presence of bony struc-
tures in its place. It is complicated by the
union of, the hyomandibular with the bones
formed from the palato-quadrate cartilage.

The primitive relations, as they are seen in the
embryonic condition, show that it has been derived from the arrange-
ments which obtain in the Selachii. While the anterior ends of the
right and left palato-quadrate bones are connected by ligament in the
Selachii and Sturiones, in the rest of the Gano’idei and in the Tele-
ostei they are attached to the sides of the primordial cranium, and
are separated from one another by the ethmoidal region, the basal
surface of which enters into the formation of the boundaries of the
buccal cavity.

The hyomandibular (Fig. 245, Hm) is almost always a large bone,
which articulates with the squamosal and postfrontal. The sym-
plectic (Fig. 241,6-) is segmented off from it; in the Selachii this is
represented by a process, but in the Sturiones it is an inde-
pendent piece ; the lower portion of the hyoid arch is inserted at
the point where the symplectic is connected with the hyomandibular.

Fig. 244. Skull of a
Gadus from above.
3 Supra-occipital. 4 Epi-
otic. 6 Squamosal.
7 Parietal. 11 Frontal.
12 Postfrontal. 14 La-
teral ethmoid. 16 Me-
dian ethmoid.

454

COMPAEATIVE AKATOMY.

Tlie quadrate (Q) is segmented off from the palato- quadrate carti-
lage ; it carries the glenoid cavity for the mandible. The angulated
ectopterygoid (Ejjt) is attached to the front of the quadrate ; and
between it and the hyomandibular there is the flattened and
ordinarily quadrangular metapterygoid (Mt). The entopterygoid is
nearer the middle line than the ectopterygoid ; Anally the palatine
forms the most anterior end of the palato- quadrate cartilage ; it is
generally movably connected with the skull.

In front of the palatine there are two other bones^ which are
not preformed in cartilage; the hinder one^ which is generally
attached to the palatine, is called the maxilla (Fig. 245, Mx), and the
anterior one the premaxilla (Px). They appear as new pieces, which
henceforward play an important part. But it is exceedingly probable
that they have their origin in the two upper labial cartilages of the

Fig. 245. Side view of the cephalic skeleton of Salmo salar (cf. Fig. 242,

Fr Frontal. N Nasal, n Nasal pit. Pa Parietal. Sq. Squamosal, i i i i Infra-
orbital bony ring. Hm Hyomandibular. Sy Symplectic (shown as if it were visible
from the exterior). Jit Metapterygoid. Fpt Ectopterygoid. Q Quadrate. 3/<r Max-
illa. Pre Premaxilla. Art Articular. Arig Angular. D Dentary. Op Operculum.

PrOp Preoperculum. 8op Suboperculum. Jop Interoperculum, lig Ligament.

Selachii. Sometimes they are freely movable, and even protrusible ;
in other cases they are more firmly connected with the skull. This
is especially the case with the premaxilla, which is frequently united
firmly to the most anterior portion of the ethmoidal region. They
both bound the orifice of the gape, but when the premaxillm are
increased in length, the maxillfe may be shut out from any share in
it; while conversely, when the premaxill^ are shortened, the maxillm
acquire a larger share in this relation to the mouth.

In the lower jaw the cartilaginous rudiment, or MeckeFs
cartilage, is very completely retained. It gives rise to the dentary
(D), which, as it were, ensheaths the cartilage. The articular (Art)
is formed from the condylar portion of the cartilage, and below it
there is the angular (Ang). On the inner face of the bony lower
jaw a special bone is sometimes developed as a covering-piece for
the cartilage ; this is the opercular.

SKULL OF VERTEBEATA.

455

§ 345.

The skeleton of the operculum is one of the most important of
the skeletal parts which are connected with, although they did not
primitively belong to, the mandibular apparatus. In the Selachii
there are cartilaginous pieces, which are sometimes bifurcated, in the
place of this bony skeleton; these are set on both segments of the hyoid
arch, where they form branchial rays. This cartilage is covered in
by a common membrane, as is also the bony apparatus; the mem-
brane is adapted to this latter, and converts it into a defensive
arrangement, which extends over the postjacent branchial clefts.

In the Sturiones the first to appear is the largest of these bones
— the operculum ; and to it, in the rest of the Granoids, and in the
Teleostei, the other bones are attached (Fig. 245). The preoper-
culum (Pr Op) is developed in the cartilage which connects
together the hyomandibular and symplectic. It is often more closely
united to the constituent parts of the suspensorium. Behind
the preoperculum is the suboperculum {Sop), which is present,
in addition to the operculum, in Ceratodus ; and then below it there
is the interop erculum {Jop), which is connected with the lower
jaw by a ligament (%).

Accessory bones are formed from other pieces, which are
developed from parts of the dermal skeleton ; the most important
of these are the infraorbital bones (cf. Fig. 245, i Hi). They form
a curved series around the lower edge of the orbit ; the hindermost
piece is attached to the postfrontal, and the foremost to the lateral
ethmoid. Some of these bones acquire a considerable size in the
Cataphracta (Trigla).

The nasals also belong to this series of bones, in consequence of
their inconstant presence ; there are also many other pieces that are
connected with the so-called mucous canal system, and which are
modifications of scales.

Vrolik, a. J., Ueber die Verknocberung u. die Kuocbeu des Schiidels del’
Teleostei. Niederliind. Archiv f. Zoologie. I. — Parker, W* K. Develop*
ment of the Skull in the Salmon. Philos. Transact. 1873.

§ 346.

In the skull of the Amphibia the primordial cranium is some^
times greatly developed. It very frequently, however, loses its roof,
and also its floor, owing to the formation of spaces in the cartilage.

The palato- quadrate is directly connected with the primordial
cranium ; it is attached posteriorly to the auditory capsule of the
skull, while anteriorly it forms an arch around the orbits, and either
projects freely forwards (as in the Urodela), or is connected with
the cranium in the ethmoidal region. Behind, and at the sides, it
carries the glenoid cavity. It presents therefore those relations
which were seen in the Chimaerae, and in the Dipnoi ; and there are^

456

COMPAEATIVE ANATOMY.

indeed, many ossifications in tlie cranium of the Amphibia which
resemble those in the Dipnoi.

The primordial cranium gives rise to a few bones only. In the
posterior primary region the exoccipitals alone are present (Fig. 246);
each of these gives rise to a condyle (co). The next region of the
auditory capsule presents large lateral processes, w^hich are attached
more exteriorly to the hinder portion of the palato- quadrate. The
anterior portion of this segment is provided with an ossification, the
prootic. This contains the anterior portion only of the labyrinth,
the hinder portion of which is contained in the exoccipital ; it also
forms a foramen for the trigeminus. There are sometimes indica-

Fig. 246. Skull of the Frog. A from above. B from below. C from behind. D from
the side. In A and B the covering bones are removed from the right half of the
cranium, so that the whole of the primordial cranium and its ossifications can be seen,
and, in A, the spaces in the roof of the cranial cavity. Pa Fr. Fronto-parietals.
Ka Nasal. Ps Parasphenoid. Tij Tympanic. Pt Pterygoid. PI Palatine. Vo Yomer.
J Jugal. Mx Maxilla. Px Premaxilla, o Exoccipital. Pe Prootic. co Occipital
condyle. Co Columella, fo Fenestra ovalis. Exits of the nerves : 0 Optic. Tr Tii.
geminus. Vg Vagus. In the lower jaw : da Dentary. a Angular. Art Articular.

tions of an epiotic. The fenestra ovalis is a cleft in the region of
the labyrinth, which is covered over by a small piece of bone.

In the anterior portion of the orbital region there are sometimes
more or less extensive ossifications. They either embrace the lateral
wall only of the cranium (Siredon), or form a circular piece of bone, to
which Cuvier gave the name of girdle-bone.'’^ This bone may
extend into the ethmoidal region, and even reach to the base of the
nasal capsules.

The paired parietals and f rentals are covering bones. In the
Anura these fuse together on either side to form a fronto-parietal
{Pa Fr). In front of these, and separated from one another by
the f rentals, are the nasals, which we now meet with as constant

SKULL OF VERTEBKATA.

457

bones for tlie first time. At tbe base of tlie skull is the parasphenoid
{Ps), which still retains very much the same characters as it had in
Fislies_, and in front of this, and in the ethmoidal region, there is a
paired bone [vo), which is regarded as the vomer.

The palato- quadrate is more simple in character than in Fishes.
The whole piece sometimes remains for the most part in a cartilaginous
condition. An ossification at the point where it articulates with the
lower jaw corresponds to the quadrate of Fishes. In many, the
palato- quadrate is divided into an anterior and a posterior portion
(Triton). It is not completely united with the cranium, for there is
a distinct articular surface on its lower portion, between it and the
cranial capsule (Rana).

There are two covering bones on the palato- quadrate cartilage ;
the upper one (Ty) is distinguished, in the Frog, by the possession
of a strong process, which is directed forwards, and which probably,
though by no means certainly, corresponds to the squamosal of
Fishes. As it aids in supporting the tympanum, it may be called
the tympanic. The lower bone is the pterygoid (Ft), and it extends
forwards along the cartilaginous arch. Its anterior end reaches as
far as the palatine, which lies transversely behind the vomer (PI).
In some of the Amphibia another bone is continued forwards in
front of the glenoid cavity ; this is the so-called jugal (quadratojugal).

The premaxillae (Px) and maxilla (Mx) appear as covering bones
of the primordial cranium ; relations which obtain in many Fishes
lead to this condition. The maxilla varies greatly in the extent
to which it is developed laterally ; in the Anura it ordinarily extends
as far back as the jugal. The premaxilla is connected with the
primordial cranium by a process which projects forwards in the
middle of the nasal region.

These maxillary bones did not primitively bound the opening of
the mouth, as the presence of special cartilages (rostral and adrostral)
in front of the primordial cranium of the larvae of Anura distinctly
shows.

In the lower jaw the primordial cartilage is present, as in Fishes,
and bony parts are developed in connection with it, which essen-
tially correspond to those of Fishes.

Parker, W. K., Development of the Skull in the Frog. Philos. Trans. 1871.—
WiEDERSHEiM, R., Das Kopfskelet der Urodelen. Morphol. Jahrb. III.

§ 347.

The skulls of the Sauropsida have much in common with one
another, whilst they are far removed from the skulls of the Am-
phibia, and from those of the Mammalia.

The primordial cranium, as a rule, has the roof incomplete, but
it is much more completely ossified than in the Amphibia, while the
great size of the bones which are developed in, and from, the palato-
quadrate cartilage leaves but a small portion of the true cranium

458

COMPAEATIYE ANATOMY.

visible. The greater development of tbe cranial capsule in Birds is
the cause of its parts being more distinctly visible than they are in
Eeptiles.

In the occipital region we can make out tbe four bones wbicb
were present already in Fisbes. Of tbese, tbe basi-occipital and tbe
exoccipitals take part in tbe formation of a single occipital condyle.
Tbe relation that tbe bones bave to tbe foramen magnum varies a
good deal. In tbe Cbelonii tbe supra-occipital is continued into a
large crest. As in tbe Amphibia there is a
fenestra ovalis in the osseous auditory cap-
sule; and in addition to it there is a fenestra
rotunda, wbicb is closed by membrane. In all
Birds and Eeptiles tbe petrosal (prootic) lies
in front of tbe exoccipital ; its anterior edge
is distinguished by tbe foramen for tbe third
branch of the trigeminal. Another bone (opis-
tbotic) unites with tbe prootic to form the
boundary of tbe binder portion of tbe fenestra
ovalis ; but it does not remain separate in any
but the Cbelonii, as in other forms it fuses
with tbe exoccipital. In addition to these,
there are several, and in Birds numerous, ossifications wbicb are
independent for a short time ; these cannot be definitely regarded
as similar to any distinct cranial bones in the rest of tbe Vertebrata.
In Birds all tbe constituents of tbe auditory capsule fuse with tbe
neighbouring bones, as well as with one another.

In tbe Opbidii tbe squamosal (Fig. 249, C Sq) is a projecting
bone, wbicb carries tbe quadrate. In other Eeptiles, and in Birds it
has tbe same position, but is more embedded between tbe auditory
capsule, parietals, and post-frontal, and is partly set in the roof of
the tympanic cavity.

Tbe sphenoidal segment varies in size according to tbe extent of
the cranial cavity. A basi -spbenoid is generally present, as is also tbe
presphenoid, wbicb is ordinarily small ; the paraspbenoid is no longer
developed. Two bones, however, wbicb are seen at the base of tbe
temporal region in Birds, and wbicb fuse with one another (basi-
temporals), may be regarded as parts of a paraspbenoid. In Birds
there is an alispbenoid, and also an orbit o- sphenoid — in tbe Eatitae, at
least — wbicb represent the side-walls of the skull. Tbe Crocodilini,
also, are provided with an alispbenoid. In most of tbe Saurii,
however, there is an interorbital membranous septum, in wbicb
indications merely of this bone can be made out.

In the Saurii (Lacerta, Varanus, Podinema) there is a bone wbicb
extends from the parietal to the pterygoid (columella) (Fig. 248,
A co) ; in tbe Cbelonii this is represented by a broad plate of bone,
which passes straight down from the parietal, and wbicb takes a
share in tbe limitation of tbe cranial cavity; in tbe Opbidii there is
a similar process, wbicb embraces tbe cranial cavity, and extends on
to the frontal.

Fig. 247. Skull of a
Chelonian seen from
behind. 1 Basi-occipital.
2 Exoccipital. 3 Supra-
occipital. 5 Basi-sphe-
noid. 15 Prootic.

17 Quadrate.

SKULL OF VEKTEBRATA.

459

The covering bones are : the
and Aves), or unpaired (Ophi-
dii, Sanrii, Crocodilini) (Fig.
248, Fa). The frontal also
is unpaired in most of the
Saurii, and in the Crocodilini
(Fig. 248, B Fr). It is paired
in Lacerta and Monitor (Fig.
248, A Ft), and in the Ophi-
dii, Chelonii, and Aves. In
the Eeptilia the postfrontals
limit the posterior edge of
the orbit (Figs. 248, Pf’, 249,

B G Ff).

In the middle of the
ethmoidal region there is a
considerable remnant of the
primordial cranium (Chelonii).
The lateral ethmoids (pre-
frontals) bound the anterior
edge of the orbit in Reptiles ;
in Birds they appear to be
connected with the median
portion of the ethmoid. The
vomer is paired in the Ophidii
and Saurii (Fig. 250, Vo). The
nasals are almost always absent
from its upper surface in the
Chelonii, and in some of the
Saurii. A new covering bone
which is seen on the outer
face of the ethmoidal capsule i
Saurii, Crocodilini, and Aves (F

parietals, which are paired (Chelonii

Fig. 248. Skull of Eeptilia ; seen from
above, yi Monitor. B Crocodile. Os Supra-
occipital. C Occipital Condyle. Pa Parietal.
Pf Postfrontal. Fr Frontal. Prf Pre-frontal.
L Lachrymal. N Nasal. 8q Squamosal.
Qj Quadratojugal. Ju Jugal. Q Quadrate.
Mx Maxilla. Px Premaxilla, co Columella.

3 the lachrymal ; it is found in most
igs. 248, 249, L).

§ 348.

The anterior portion of the primitive palato- quadrate cartilage
undergoes atrophy very early, so that the bones which belong to it
are partly developed on the skull itself. The hinder portion of
the palato- quadrate persists as the quadrate (Fig. 249, Q). The
quadrate is movable in the Saurii, Ophidii, and Aves, while in
the Chelonii and Crocodilini it is firmly united to the skull. The
whole complex of bones, which is differentiated in the palato -
quadrate cartilage, is intimately and immovably connected with the
cranium, while, when the quadrate bone is movable, some, at least,
of these bones are also so.

Another character is due to the development of the nasal
cavity (see also § 413). The bones, which in Fishes are placed
at the sides of the base of the skull, extend to the middle

400

COMPAEATIVE AXATOMY.

line, so that the base of the skull is more or less shut off from taking
any part in bounding the buccal cavity. The nasal cavities, which
in the Amphibia lead into the buccal cavity at the very anterior
edge of the skull, have their internal orifices always placed farther
back in the Keptilia, owing to the union of the horizontal processes
of the maxillae, palatines, and pterygoids in the middle line, and in
front of them. In this way the nasal is more completely shut off
from the buccal cavity, and forms an upper cavity, the base of which
is the roof of the mouth — that is, is the ‘^hard palate.^^ These

Tr

Fig. 249. Side views of Skulls. J. Struthio. L’ Crocodilus. C Python. OZ Ex-
occipital. 0*' Supra- occipital. Pt Pterygoid. Pa? Palatine. Tr Transverse. Co? Colu-
mella. /ou Fenestra ovalis. B Foramen for the trigeminal nerve. The other letters
as in the preceding figures.

changes are least marked in the Saurii, Ophidii, and Aves, more
so in the Chelonii, and most completely so in the Crocodilini.

The parts that form the suspensorium in Fishes (hyomandibular
and symplectic) have undergone just the same fate as in the
Amphibia; or, in other words, they have no longer any relation to
the cephalic skeleton. The columella auris and the parts con-
nected with it are developed from its rudiments, partly a bony, and
partly a cartilaginous skeletal bit which has entered into the service
of the auditory organ.

When the quadrate is movably united with the skull (Ophidii,

SKULL OF YERTEBEATA. 461

Saurii, Aves), articulations, more or less well developed, appear in
those parts of the maxillo-palatine apparatus which are attached to it.
These are not present in the Orocodilini and Chelonii, where the
quadrate is jammed between the squamosal and the bones of the
auditory capsule. Sphenodon presents an intermediate arrangement;
its skull is on the Saurian type, but the quadrate is firmly united
with the pterygoid and squamosal.

§ 349.

Two sets of bones, which extend forwards, are attached to the quad-
rate, just as in the Amphibia. Nearer the middle line we find — in
Birds, Serpents, and Lizards

— the pterygoid (Fig. 250, A B

Ft) articulated with the base
of the cranium. The two
bones are united suturally
in the middle line in the
Chelonii and Orocodilini, and
are also firmly attached to
the base of the skull (Fig.

251, Ft) ; in the latter they
bound the posterior nares.

In the Ophidii, Saurii, and
Orocodilini, there is an ex-
ternal pterygoid (Os trans-
versum, Figs. 250, A Tr ;

251, B Tr) which connects
the pterygoid with the
maxilla ; it is uncertain
whether this corresponds to
the ectopterygoid of fishes.

In front of the ptery-
goid lie the palatines (Fat) ;
in the Chelonii and Oroco-
dilini they are united su-
turally in the middle line,
but in the Ophidii, Saurii,
and Aves, they are separated
from one another and form
the lateral boundaries of
the posterior nares (Fig.

250, Fal). In the Chelonian
skull the vomer (Fig. 251, A Vo) reaches to the roof of the cavity
of the mouth and lies between the palatines, while above the nasal
cavity the palatines unite together at the base of the cranium. In
Birds the palatines are generally long and flat bones (Fig. 250, B
Fal), which are connected at their anterior end with a process of
the maxilla (Mx), or fused with a process of the premaxilla.

Fig. 250. View of the base of the skull. A Of
Monitor. B Of Struthio. Oh Basi-occipital.
C Occipital condyle. 01 Exoccipital. Sph Basi-
sphenoid. Q Quadrate. Pt Pterygoid. Tr Trans-
verse. PaZ Palatine. F Vomer. Qj Quadrato-
jugal. Jw Jugal. Maj Maxilla. ' Its median
process. Px Premaxilla.

162

COMPAKATIYE ANATOiSIY.

Ill most Saurii (and in Clielys among tlie Chelonii), as in tlie
Birds, the premaxillae are fused together; in the latter they are
distinguished by the possession of a long frontal process. Their
size is correlated with that of the beak, the form of which is largely

due to the share

^ B that they take in it.

They are rudimen-
tary in the Ophidii
(Fig. 249, C Fx)
and small in the
Chelonii. The max-
illa therefore forms
the greater portion
of the boundary
of the edge of the
lower jaw (Mx) ;
this bone is of great
size in the Croco-
dilini and Saurii,
and very large in
the Ophidii, in
which also it is
capable of a large
amount of move-
ment.

There is a lateral
series of bones at-
tached to the quad-
rate. The first of

Fig. 251. View of the base of the skull. A Of Chelonia.
B Of Crocodilus. 01 Basi-occipital. 01 Exoccipital.
C Occipital condyle. Sph Basi-sphenoid. Opo Opisthotic.
Pt Pterygoid. Pal Palatine. Yo Vomer. Q Quadrate.
Qj Quadratojugal. Tr Transverse. Mx Maxilla. Px Pre-
maxilla. Pa Parietal. Pfr Post-frontal. Fr Frontal.
Oh Posterior nares. E Eustachian tube.

bone is united to the skull.

these is the quad-
rato-jugal ; this is
absent in the Ophi-
dii. In the Saurii
it is connected with
the quadrate at the
point where this
It is continued forwards into a second

piece, which is partly connected with a post-frontal, and partly
with a jugal which passes round the lower edge of the orbit. In
Birds the qnadrato-jugal (Fig. 250, B Qj) is a slender piece of
bone, which arises from the side of the mandibular joint of the
quadrate. In the Chelonii and Crocodilini it is connected with a
larger portion of the quadrate, and supports the jugal, which aids in
bounding the orbit.

In all cases the lower jaw is articulated to the quadrate, and is
made up of the same parts as in Fishes, to w^hich, however, a supra -
angular and a complementary bone are added on.

In the Chelonii and Aves the two dentaries fuse very early ; in
Birds the other bones retain indications merely of their primitive

SKULL OF VERTEBKATA.

463

separation. Tke two halves of the jaw are movably connected
together in the enrystomatons Ophidii.

Parker, W. K., Sfcrucfcare and development of the skull in the Ostrich tribe.
Philos. Trans. 1866. — The same, On the sfcrnctnre and development of the
sknll of the common Fowl. Phil. Trans. 1869.

§ 350.

In the Mammalian sknll the cartilaginous primordial cranium is
ordinarily developed in the basal region only, and limited to the early
stages of development. That part of the skull which is derived
from the cartilaginous cranium is in this group also to be distinguished
from the parts which are developed from other elements, but it
becomes intimately con-
nected with them. As
a capsule for the brain
it is itself larger, and
is enclosed by a larger
number of bones. It is
more distinctly divided
into different segments
than it is in the lower
divisions, but this must
be regarded as a secon-
dary adaptation (§ 340).

In the occipital seg-
ment the lateral pieces
(Fig. 252, 01) always
unite with a part of the
basi-occipital (Fig. 253,

Oh) to form the posterior
occipital condyles, by
which they bound the
foramen magnum, while
superiorly they enclose between them the supra-occipital {Os), This
latter may be shut out from the edge of the foramen magnum. The
four pieces almost always fuse together, but they may remain long
separate (Marsupialia). In many Mammals (several Marsupials,
Ungulates, etc.) the exoccipitals send down long processes {^pm)
(Paramastoid processes).

In the region of the auditory capsule separate ossifications in the
cartilaginous portion can be seen in the earliest stages only. They
form centres of ossification which partly correspond to the bones in
Fishes and Reptiles; these soon fuse into a single piece, the
petrosal (Pe), the greater part of which gets to be placed at the base
of the cranium as the skull grows out laterally. The lateral portion
of the petrosal is overlaid by other bones, which are developed from
the metamorphosis of the branchial skeleton, and becomes converted
into the middle wall of the tympanic cavity, in which there is a

Fig. 252. Lateral view of the cerebral portion of the
Skull of a Goat. 01 Exoccipital. Os Supra-occipital.
Jp Interparietal. Fa Parietal. Pe Petrosal.
8q_ Squamosal. Ty Tympanic. 8ph Basi-sphenoid,
As Alisphenoid. Ors Orbito-sphenoid. Fr Frontal.
Na Nasal. L Lachrymal. Ju Jugal. Mx Maxillare
superius. Pal Palatine. Pt Pterygoid, pm Para-
mastoid process, si Styloid process.

464

COMPAEATIYE ANATOMY.

fenestra rotunda as well as a fenestra ovalis. The posterior portion
of the petrosal_, which is likewise ossified from an independent centre^
is attached to the sides of the exoccipitals^ and is distinguished as
the mastoid portion, in consequence of its carrying the mastoid
process in Man. Superiorly, the squamosal (Sq) is attached to the
petrosal, and is sometimes fused with it to form the temporal bone,
of which it then forms the squamous portion.^^ In some forms it
is completely shut out from the cranial cavity ; in others a small
portion only can be seen from the inner surface of the skull (Cetacea,
Euminants). It is in the Primates only that this portion is of any
great size, and this leads to the well-known arrangement which is
seen in Man. A process (zygomatic process) of the squamosal which
is directed forwards, takes part in the formation of the jugal arch.

In front of the temporal region is the sphenoidal region, which
is made up of two j^erfectly developed segments. The basal

piece of the hinder
one (ba si-sphenoid)
(Fig. 253, Sqjh) abuts
directly on the basi-
occipital, and carries
at its sides the al^e
temporales (ali- sphe-
noids). The presphe-
noid with its alse orbi-
tales (orbito-sphenoid)
(P.S-) lies in front of
the basi - sphenoid.
The two median pieces
are separate through-
out life, or for a long
time, in Mammals.
In Man they fuse very
early to form the so-
called body of the
sphenoid.

In the roof of the skull we again find the well-knoAvn covering
pieces, which increase in extent as the cranial cavity grows larger.
The parietals (Figs. 252, 253, Pa) are often fused together (in the
Monotremata, many Marsupialia, the Ruminantia, and Solidungula).
Between them, a special bone, which marks off the supra- occipital,
projects from behind forwards; this interparietal is generally, as
in the Primates, fused with the supra-occipital (Figs. 252, 253, Jj>),
but, at times, with the parietals (Rodentia and Ruminantia).

The frontals (Fr) are attached to the alae orbitales; they are
always paired; they are fused in some, as in Elephas, Rhinoceros,
and also in the Prosimiac, Insectivora, Chiroptera, and Primates.

The most anterior segment of the primordial cranium presents
the most important modifications. It is developed into the wall of
the nasal cavity owing to the formation of various laterally and

Fig. 253. Vertical median section through the same
skull. Oh Basi-occipital. Ps Presphenoid. Eth Eth-
moid (vertical plate of the cribriform bone, the anterior
edge of vrhich is continued into the cartilaginous par.
titions between the nares, which is not seen in this
figure). Eth Turbinate bones. Fo Vomer, s/ Frontal
'sinus. The other letters as in the preceding figure.

* SKULL OF VERTEBEATA.

465

inwardly projecting processes. Below it^ lie tlie skeletal parts of
tlie niaxillo-palatine apparatus^ and to these a median lamella of
cartilage, which forms the wall of partition of the nasal cavity, is
sent down. The vomer is developed as an investing bone on this
plate (Fig. 253, Vo). Two ethmoidal pieces are formed by the
ossification of the two lateral halves of the ethmoidal cartilage, and
of the lamellar processes (turbinate bones), which are given olf from
it. They bound a portion of the cranial cavity in front of the pre-
sphenoid, and are fenestrated to allow for the passage of the olfactory
nerve. In Ornithorhynchus there are only two orifices in this region,
but they are many more in all other forms, so that this portion is
converted into the cribriform plate. An unpaired bone is formed by
the fusion of the two lateral halves with the median piece (lamina
perpendicularis) (Fig. 253, Vth).

The turbinals vary greatly in character, and by the development
of multi-ramified lamellEe aid in increasing the size of the nasal
cavity. The ethmoidal segment is, as a rule, covered by other bones,
and especially by those which form the niaxillo-palatine apparatus ;
and that to such an extent that no part of it at all can be seen
from the outside. Except in some Edentata, it is in the Primates
only that a portion of the lateral surface reaches to the median
boundary of the orbit, where it forms the lamina papyracea.^^

The lachrymals and the nasals form the investing bones of the
outer surface of the ethmoidal region. The former (L) are less
constant, and often seem to pass into the neighbouring bones, so
that they, as for example in the Pinnipedia, are absent as separate
pieces. They are wanting, also, in the Delphinoidea. As in Reptiles
and Birds they form part of the anterior wall of the orbit, and also
appear on the facial surface of the skull, from which they extend
backwards to the median wall of the orbit, in the Primates.

As to the nasals (No), they exhibit merely subordinate variations,
which are expressed partly by degeneration (Cetacea), partly by a
considerable increase in size. Their size is proportional to that of
the nasal cavity, and is correlated with an elongation of the facial
portion of the skull. They are small in the Primates.

§351.

The most important peculiarities in the Mammalian skull are
seen in the part which is developed out of the primitive branchial
skeleton. A bone which corresponds to the quadrate lies on the
outer surface of the auditory capsule. It forms an auditory ossicle,
the incus.

The skeletal parts, which are developed in front of the quadrate,
and along the base of the skull, are intimately connected with the
cranium.

The pterygoids (Fig. 253, Pt) are generally flat pieces of bone,
which are placed on the inner surface of the large processes which

2 H

466

COMPAEATIVE AITATOMY.

are developed from the basi-sphenoid. They form the side-walls of
the posterior nares, and may also limit these orifices below by
uniting together in the roof of the palate (in Echidna^ Dasypns, and
some Cetacea). In most Mammals they are permanently distinct,
as they are also, for a very long time, in the Primates, before they
unite with the above-mentioned processes of the sphenoid to form
the medial lamellae of the descending pterygoid processes. The
jDalatines generally form the inferior boundary of the posterior
nares, and the hinder portion of the hard palate. The maxillae
vary in length according to the extent of the facial region, and
always form the largest portions of the upper jaws. The premaxillae
vary more considerably ; as a rule, they also take part in the forma-
tion of the side walls of the nasal cavity. They are rudimentary,
or, as compared with the maxillae, feebly developed in many
Chiroptera and Edentata. They bound the foramen incisivum. In
the Apes they fuse with the maxillae ; this union takes place so early
in Man that their existence was justifiably doubted for a long time.

The outer series of bones, which is present in the Sauropsida, and
which extends from the quadrate to the maxilla, is reduced in Mammalia
to the jugal; this bone unites the jugal process of the squamosal with
the maxilla, and so forms the jugal arch. In a few forms the jugal
is absent (Sorex), or, though united with the maxilla, does not reach
the jugal process (Myrmecophaga, Bradypus). -When it is united
with a process of the frontal it gives rise to a posterior wall for the
orbit, and so separates this region from the temporal fossa ; there
are various stages of this arrangement. This process is most com-
plete in the Primates, where the orbital fissure represents the re-
mains of the wide communication which exists between the orbit
and the temporal fossa in other Mammals.

In the Mammalia a tympanic bone is developed on the outer
face of the petrosal ; this serves as a support for the tympanic
membrane. It is not certain that this bone is homologous with the
one of the same name which we found in the Amphibia. At first it
always forms a bony incompletely-closed ring (annulus tympanicus)
(Fig. 254, at)j which grows out into very various forms. In the
Monotremata and Marsupialia, as well as many Insectivora, etc.^ it
is never more than a simple ring. In many forms it is never
united to the petrosal; in the Cetacea it is very loosely so. In
many it forms a bony capsule which is continued into the external
auditory meatus. A bulla of this kind is most common in the
Marsupialia, Eodentia, Ferae, and in the Artiodactyla. In some
Marsupials, where the tympanic does not pass beyond the annular
condition, there is an apparently similar bulla, but this is formed
by an extension of the bases of the alae temporales (Dasyurus,
Petanrista, Perameles). When the tympanic is fused with the
petrosal and squamosal it takes part in the formation of the temporal
bone (Primates).

SKULL OF VEKTEBEATA.

467

§ 352.

Very early in development, tLe primitive cartilage o£ the lower
jaw turns off from that line of differentiation which obtains in the
rest of the Vertebrata. The part, which, in others, forms the articular
bone is converted into one of the auditory ossicles, the malleus (Fig.
254, m); the Meckelian cartilage {f)j which is never developed beyond
the cartilaginous stage,
is continuous with it.

The dentary forms an
investing bone on the
outer surface of this car-
It abuts in the
line on its fellow
of the opposite side, and
unites with it to form
the lower jaw ; this is
articulated to the skull
on the lower surface of
the jugal process of the
squamosal. It repre-
sents therefore a new
formation, though the
primitive one has not
disappeared, but per-
sists in other relations.

MeckeFs cartilage {]p) is
retained for some time
longer on the inner sur-
face of the lower jaw,
but then disappears; the only portion of it which persists is the
part which is placed within the tympauic cavity, and which extends
to the Glaserian fissure, where it is ossified to form the processus
folianus mallei. The early differentiation, and the, at first, relatively
large size of the auditory ossicles, shows that they must be regarded
as skeletal parts, which in a lower stage were much more developed
in size. *

The two halves of the lower jaw are permanently separate in a
large number of Mammals ; in others they unite early (Perissodactyla,
Chiroptera, Primates). Low morphological conditions are implied
by the straight mandibles of the Monotremata, in which there is
no distinct coronoid process ; in some others, also, this process is
merely indicated (Cetacea).

The piece which is developed from the upper portion of the
primitive hyoid arch (hyomandibular of Fishes) appears to form the
rudiment of a third auditory ossicle — the stapes.

tilage.

middle

Fig. 254. Lateral view of the skull of a human
foetus, with its auditory ossicles. Part of the upper
wall of the tympanic cavity and the tympanic mem-
brane have been removed, at Tympanic ring from
which a piece has been removed superiorly, m Mal-
leus. ma Manubrium of the malleus, p Meckel’s
process, extending along the inner side of the
lower jaw. i Incus, s Stapes, st Styloid process.
1st Stylohyoid ligament extending to the anterior
cornu of the hyoid, t Mastoid foramen.

2 n 2

468

COMPAEATIVE ANATOMY.

Brancliial Skeleton.

§ 353.

A ventral system of arclies is connected witli tlie most anterior
portion of the axial skeleton, and forms the organs of support for
that portion of the alimentary canal which functions as a respiratory
cavity. The number of arches, and the backward extension of this
apparatus, depends on the size of this respiratory cavity. We meet
with two very different types of these structures.

The first type is found in the Acrania (Amphioxus). In this
framework there is a cartilaginous arch around the mouth — that is,
in its most anterior portion ; this arch is beset with cartilaginous
rods which are directed forwards. The rest of the apparatus is
formed of a homogeneous substance, which forms, as in Balano-
glossus (cf. § 112), a complicated lattice-work. The branchial bars
of either side are independent of those on the other ; that is to say,
they are not united along the ventral line.

We cannot derive the second type, which obtains in the
Craniota, directly from this. In its earliest stage it is made up of
cartilaginous pieces only ; these do not form so large a number of
arches as exist in Amphioxus, and are, while completely symmetrical
as regards their arrangement, united ventrally by a copula.

In the Cyclostomata the branchial skeleton is made up of com-
plicated cartilaginous bars, which are connected interiorly with one
another, as well as with either side of the spinal column superiorly ;
owing to their superficial position they may be spoken of as forming
an external branchial framework. Very evident signs of this
are retained by the Selachii, but in them there is another, or

internal, organ of sup-
port; and this is found in
all the rest of the Verte-
brata.

The various arches pre-
sent indications of their
primitive similarity ; this
disappears in consequence
of the gradual change in
their functional relation,
which is due to a divi-
sion of labour. W e were
obliged to speak of some
of these arches in deal-
ing with the cranium; so
that now they need be but briefly considered. The first of them
surrounds the entrance to the alimentary canal, and is divided into
two pieces; one, superior, the palato- quadrate (Fig. 255, o), and the
other inferior, the primitive lower jaw (u). The succeeding pairs

Fig. 255. Skiill and branchial skeleton
of a Selachian (Diagrammatic), ah c Labial
cartilages. I Mandibular arch, o Upper, u Lower
portion. II Hyoid arch. Ill — VIII Branchial
arches.

BEANCHIAL SKELETON OE VERTEBEATA.

469

of arcEes either retain their primitive function of being supports
for the branchial arches, or nndergo a number of modifications.

All these arches evidently had the same original function. Their
relation to the respiratory apparatus has not disappeared in the first
pair only, which has been converted into jaws ; the hinder arches
also have gradually lost their functional and anatomical characters ;
it is, therefore, reasonable to suppose that this is but the last of a
series of reductions, which first commenced on a much larger
number of arches. If this is so, the branchial skeleton of the
Craniota is the remnant of an apparatus, in which there were
primitively a far larger number of arches. This view is supported
by a comparison with Amphioxus, as well as by the considerations
to which we are led by a study of the branchial apparatus and of
the peripheral nervous system.

As we pass through the Fishes to the Amphibia we may note
how this apparatus gradually loses its primitive relations ; while in
the Reptilia and all higher forms it has no relation at all to the
respiratory organs.

§ 354.

Ail the branchial arches are united ventrally by azygos pieces — »
the copulse. The various arches are always segmented into a
number of pieces, which are generally movably united with one
another. The upper portion of the hyoid, as above described, as well
as of the mandibular arch, enters into relations with the cranium;
these arches thus lose all connection with the other arches, with which
the lower portion only of the hyoid arch is still connected.

The succeeding arches are either slightly connected with, or
only indirectly united to, the cranium ; this is effected by their
being attached to the base of the skull, or, when their point of
attachment is more extended, to the commencement of the vertebral
column. In many Selachii the hyoid arch has the same conformation
as the branchial ones (Fig. 255, II). As a rule its copula is in-
creased in size, and affords a support for the tongue. In the
Selachii and Chimmrm this arch retains its primitive function of a
branchiferous portion of the skeleton. This relation disappears in
the Gaiioidei and Teleostei, where this gill is rudimentary, and the
rays of the upper portion, which is converted into the hyomandi-
bular and symplectic, are represented by the opercular apparatus
(§ 345).

The lower portion of the hyoid arch, or true hyoid, has then
bony instead of cartilaginous rays (Fig. 256, I r, branchiostegal
rays), and a membrane extends between them which covers over tlie
whole of the branchial apparatus. The hyoid arch thus develops
an organ of defence for the respiratory apparatus.

There are five pairs of arches connected with the respiratory
apparatus; occasionally there are six or seven (Notidani). No more
than five are ever found in the Osseous Fishes. While the
anterior arches {I II III) are always provided with copul[e (f g))

470

COMPARATiYE ANATOMY.

the hinder ones {IV V) are united to a single piece (a), and are always
degenerated, both in size and number. The last pair of all {VI), which
merely consists of a single piece on either side, carries no gills ; in
the fifth arch also there are often gill-lamellae on one side only ; in

1/

Fig. 256. Hyoid and branchial arches of Perea fluviatilis. I — FI Arches; the
first (I) is converted into an organ for the support of the hyoid; the next four (II — V)
are branchial arches, and the last (FI) forms the infra-pharyngeal bone, a h c d
Segments of the arches. The uppermost piece (d) forms the supra-pharyngeal bones,
r Branchiostegal rays, f g h Copulse (after Cuvier).

the last, however, dental structures are more completely developed, so
that this piece is often capable of functioning as a masticatory
organ. In the Pharyngognathi the rudiments of the last arch, on
either side, are fused into one piece.

We meet with other modifications of the posterior branchial
arches in the Labyrinthobranchiata, and in various Clupeidae ; these
are due to the conversion of various segments of the arches into the
walls of spaces into which water is received.

Just as the hyoid arch of the Selachii is provided with cartila-
ginous appendages, so also the succeeding arches are beset with
cartilaginous rays which support the walls of the branchial pouch.
Even these structures are rudimentary in the Ganoi'dei and Teleostei,
where they form fine cartilaginous lamellae, placed between the rows
of the branchial folds.

§ 355.

The branchial skeleton of the Amphibia is considerably reduced j
such forms as undergo a metamorphosis have the gills reduced, and

ISEANCiilAL SKELETON OF VEETEBEATA.

471

present at the same a gradual change in this apparatus. It is
retained in the Perennibranchiataj and undergoes slight changes
only in the Derotremata. It is made up of four or five pairs of
arches ; the first of which, as in Fishes, forms a hyoid arch (Fig.
257, M. The sue

ceeding arches are
united to a com-
mon copula. The
posterior ones do
not severally ex-
tend as far as it^
but are connected
together on either
side. In correlation
with the reduction

Fig. 257. Hyoid, and bran,
cbial arches of a larva of
Salamandra maculosa.
1) Hyoid arch, c c Supports
of the branchial arches.
d Appendage of the Copula.

Fig. 258. Hyoid of Bufo
cinereus. a Copula, h Cor-
nua of the hyoid, c Eem-
nants of the branchial
arches (after Duges).

of the arches_, thee o-
pulm are increased
in size. The only
portion which re-
mains complete
after metamorpho-
sis is the hyoid (Fig. 258, h). It is united with the copula {a),
which is generally of some size, and which is converted into the
body of the hyoid. A larger piece of the second arch is retained
in the Salamandrina, and a small portion of the third arch ; in the
Anura, however, there is a cartilaginous plate, which is made up
of all the branchial arches on either side, and which fuses with
the copula into one piece. Eod-shaped pieces (columellee), which
are developed from the ends of the j^rimitively paired plates, are
attached to this (Fig. 258, c).

The changes in the branchial skeleton, which are perceptible
when it changes its function, afford a striking example of the great
infiuence which adaptation to external conditions of life exercises on
the internal organisation.

§ 356.

The degeneration which is seen in individuals among some of the
Amphibia is an inherited arrangement in the higher classes. Except
those parts which enter into the composition of the auditory organ^
all the parts which were developed at one time from the large
branchial skeleton of Fishes, are converted into that support for the
tongue, which is known as the hyoid bone. The copula forms its
body,^^ and to this the rest of the arches are attached, under the
form of cornua/^ As a rule the remains of two arches — the hyoid
portion of the primitive hyoid, and parts of the first branchial arch
^are used for this purpose.

The simple body, which rarely consists of several pieces, is
beset, in Reptiles, with portions of two or three arches; these
are often very rudimentary. They are either single, or divided

472

COMPAEATIVE AEATOMY.

into two pieces. The arches are most numerous in the Chelonii, where
there are as many as three^ and next to these come the Saurii;
in the Crocodiliui the broad curved body of the hyoid has but a
single pair of arches. In the Ophidii the apparatus is reduced to a
cartilaginous remnant and even these remnants of an arch are lost
in various forms (Tortrix, Typhlops, etc.).
Two pairs of arches can be made out in Birds.
The rudimentary first arch fuses to form the
so-called entoglossal bone (Fig. 259, 2), pos-
teriorly to which lies the true body of the
hyoid. The second arch, however, is well
developed, and gives rise to the cornua (4 5),
which are formed of two large pieces, which
generally curve backwards behind the skull,
without being directly connected with it. Be-
hind the copula there is the remnant of a
second one, which forms the hyoid process (3).

In the Mammalia two arches are per-
manently connected with the single body of
the hyoid. The anterior cornua are the
largest, and are connected with the petrosal ;
they are made of several (three) pieces. When
the median piece is merely connected by a
ligament, this portion is divided in such a
way that the uppermost piece, if connected
with the petrosal, as it is in the Orang and in
Man, forms the styloid process of this bone ;
when this is the case, the remaining portion
is formed by the stylo-hyoid ligament, and the rest of the arch is
attached to the body of the hyoid as a small, and sometimes even
nnossified piece. In mpst Mammals, the posterior cornua, which are
always formed of a single piece, are the smaller ; occasionally, as in
various Eodents and Edentates, they are altogether wanting. In
the Primates they are larger than the remnants of the anterior
cornua. They are connected with the larynx, the thyroid cartilage
of which is attached to them by ligaments.

Fig. 259. Hyoid appara-
tus of the domestic fowl.
1 Body of the hyoid
(copula). 2 Eutoglossal.
3 Hyoid process. 4 An-
tei’ior, 5 Posterior por-
tion of the cornu of the
hyoid.

Skeleton of the Appendages.

§ 357.

The two pairs of appendages in the Vertebra ta, however much
they vary in the extent to which they are developed^ have their
skeleton arranged in very much the same way j this points to their
being homodynamous structures. In this skeleton we may dis-
tinguish an arched piece, which lies in the trunk, and which in its
lowest condition forms a band of cartilage ; according to the position

APPENDICULAE SKELETON OF VEPTEBPATA.

473

whicli it occupies, it is known as the thoracic (shoulder), or
pelvic (hip) girdle.

The skeleton of the free appendage is attached to the extremity
of the girdle. When simplest, this is made up of cartilaginous
rods (rays), which differ in their size, segmentation, and relation to
one another. One of these rays is larger than the rest, and has a
number of other rays attached to its sides. I have given the name
of Archipterygium to the ground-form of the skeleton, which
extends from the limb -bearing girdle into the free appendage.
The primary ray is the stem of this archipterygium, the characters
of which enable us to follow out the lines of development of the
skeleton of the appendage. Cartilaginous arches beset with rays
form the branchial skeleton. The form of skeleton of the appen-
dages may be compared with them ; and we are led to the conclusion
that it is possible that they may have been derived from such forms.
In the branchial skeleton of the Selachii the cartilaginous bars are
beset with simple rays (Fig. 260, a h). In many, a median one is

a i ^ ^

Fig. 260. Diagrams to illustrate the homodynamy of the appendicular skeleton with
that of the branchiae, ah c d Branchial arches of Selachii. e Archipterygium.

developed to a greater size. As the surrounding rays become
smaller, and approach the larger one (c), we get an intermediate
step towards that arrangement in which the larger median ray carries
a few smaller ones (d). This differentiation of one ray, which is
thereby raised to a higher grade, may be connected with the primi-
tive form of the appendicular skeleton; and, as we compare the
girdle with a branchial arch, so we may compare the median ray
and its secondary investment of rays with the skeleton of the free
appendage.

We meet with greater difficulties when we come to examine the
topographical relations of the appendages. If the comparison of
the skeleton of the appendages shows that it is similar to the
branchial skeleton, and that therefore it is possible to derive the
appendages from branchial arches, we must further suppose that the
two appendages were primitively branchial arches, which carried
rays, and that they have been differentiated in a different way
to the other branchial arches, and have been separated off
from the branchial apparatus. The hinder one altered its position
more than the anterior one, and this, of course, happened during
changes which affected the rest of the organism. The anterior

474

COMPAEATIYE ANATOMY.

appendage lias still some relations to tlie liead^ as is shown by
the muscles which are supplied by cerebral nerves^ while, in
Fishes, it, and its arch, lie just behind the branchial arches. In
this respect the hinder appendages are quite independent. They
must be supposed to have travelled to a greater distance, if we are
right as to the homodynamy, which a comparison of the skeletons
leads us to infer. The anterior appendage has, however, clearly
also undergone great changes in position; this is evident when
we note how, owing to the continual increase in the number of the
cervical vertebrte, it moves farther and farther back as we pass from
Fishes to Birds. As there are no facts known to us which
point to the formation of new vertebrae, which could only be brought
about by the intercalation of new metameres of the body, this
distinct change in position must be explained as due to the continual
retrogression of the appendages ; in other words, we are led to
postulate just the same process in its case, as in that of the hind
limbs. These considerations merely point to the manner in which it
is possible that the appendages were developed, and there are still
many questions which cannot be safely answered until a compara-
tive examination of tbe muscles and nerves which belong to the
appendages has been made.

Gegenbaur, C., Zur Morphol. der Gliedmasseu der Wirbelthiere. Morphol.

Jahrb. II.

Anterior Appendages*

Shoulder- Girdle*

§ 858*

in its simplest form the shoulder- girdle is a piece of cartilage^
which, in the Selachii, forms an arch on each side united to its
fellow along the ventral line, and placed just behind the branchial
apparatus. Owing to its attachments to the muscles of the appen-
dages, definite sculpturing may be made oiit on the arch; this is
most distinct in the Rays.

In the Gano'idei the two halves of the cartilaginous arch are
completely divided; a new apparatus is connected with the primary
shoulder-girdle, represented by the cartilage ; this non- carti-
laginous part is formed of bones which primitively belonged to the
integument, and in the course of its differentiation up to the
Mammalia it plays an important part.

We must therefore distinguish the secondary from the primary
shoulder-girdle. The latter is always cartilaginous in the Stnriones,
though various bony plates of the integument are developed on it ;
the two lower ones I have shown to be the clavicle and infra-clavicle,
and the two upper ones the supra-clavicles. In the primary thoracic
cartilage wider spaces are developed from the canals which are found

SHOULDER-GIKDLE OF VERTEBRATA.

475

girdle, and thoracic fin of Gadns. c Clavicle.
a h Supra-clavicles. d Accessory piece.
e Coracoid. / Scapula, g Basalia of the fin.
h Bays of the secondary skeleton of the fin.

in SelacRians. In tlie rest of tlie Ganoidei and Teleostei a part alone
ordinarily remains cartila-
ginous,, and the rest is ossi-
fied, but the whole piece
appears to decrease in size.

As a rule two bones (/ e) are
developed from it in the
Teleostei, and with these, parts
of the skeleton of the fin
may be closely connected.

The clavicle, however, which
is small in the Sturiones, has
increased in size (Fig. 261, c).

It is connected along the
ventral median line with that
of the opposite side, and by
the supra-clavicles (a h) with
the skull. The primary shoulder-girdle, in fact, undergoes degenera-
tion, and forms a mere appendage to the clavicle, which becomes the
chief support of the anterior extremity.

§ 359.

The clavicle, developed on the carti-
laginous shoulder-girdle of Fishes, is
reduced in the higher Vertebrata. The
primary apparatus, however, becomes of
greater importance owing to its con-
nection with the sternum, and the greater
power of movement possessed by its
uppermost (dorsal) portion, which is no
longer firmly connected with the axial
skeleton. That region of the girdle at
which the free limb is connected with it, is
distinguished by the formation of a cavity
which receives the articular head of the
humerus, and divides the primary shoulder-
girdle into two parts.

The dorsal portion forms the scapula ;
the ventral is divided into a hinder piece,
the coracoid, and an anterior piece, which
is ossified from the scapula, when it is
ossified — the precoracoid.

Among the Amphibia, the shoulder-
girdle of the Urodela forms a skeletal
piece on either side ; it is largely carti-
laginous, and is only ossified in the

region of the glenoid cavity. The widened dorsal end of the
scapula, the supra-scapula, is almost always cartilaginous, or has an

coid. co' Coracoid, d Clavicle,
c Episteruum. st Sternum. The
cartilaginous portions are
dotted.

47G

COMPAEATIVE AXATOMY.

independent periosteal ossification. Tlie ossification sometimes ex-
tends from the scapula on to the precoracoid. In the Anura the
two ventral processes (Fig*. 262, A co co) on either side of the
shoulder-girdle are united by their cartilaginous ends, and may also
become united in the middle line (Eana). In this case there is a
foramen on either side, in the ventral portion of the shoulder-girdle.
The coracoid (co') is ossified independently, while the jDrecoracoid
becomes closely related to the clavicle (d).

In the Reptilia, likewise, each half of the shoulder-girdle forms a
single piece, which closely resembles in form the same part in the
Amphibia. The coracoid, which is generally broad, is not unfre-
quently fenestrated (Saurii). A process of the scapula, which is
merely indicated in the Amphibia, is converted into the acromion,
and unites the scapula with the clavicle (Fig. 262, C cl). In the
Chelonii the scapula is generally a cylindrical bone (B s), which
forms an angle with, and is directly continuous with, the precoracoid
(B co) at the glenoid cavity. The end of this precoracoid is
connected by a ligament with the cartilaginous end of the coracoid.

In the Crocodilini the precoracoid has completely disappeared,
so that the scapula and coracoid alone make up the shoulder-girdle.
In Birds there is a somewhat similar arrangement ; the small and
slightly curved scapula is united to the strong coracoid at the
glenoid cavity ; the coracoid itself is, as in Eeptiles, attached to the
plate of the sternum. The Eatitm indicate their closer affinity to
the Saurii by the presence of a rudiment of the precoracoid.

Among Mammals the coracoid is complete in the Monotremata
only. In the rest, the only sign of it is the process (coracoid process)
which is given off from the scapula, and lies in front of the glenoid
cavity ; it is in rare cases only that the sternal end of the coracoid
persists. I have discovered it, however, in Sorex and Mus, where it
forms a piece of cartilage attached to either side of the manubrium
sterni. The scapular remnant of the coracoid still continues to take
part in the formation of the glenoid cavity, but this share decreases
as that of the scapula increases, so that at last this latter bone alone
forms the support for the anterior extremity, which thereby acquires
a greater power of free movement. The |)rimitive independence of
the remnant of the coracoid is implied by the presence in it of a special
centre of ossification, which persists so long as it is not completely
fused with the scapula.

In form, the scapula of Mammals resembles that of the Eeptiles,
but owing to the presence in it of new constituents it differs from the
latter in some essential points. In the Monotremata there are indi-
cations of a spine, the end of which forms the acromion. In the rest
of the Mammalia the lateral edge of this broad piece is developed
into a larger ridge, which now, owing to the development of
the median ridge also, into a projecting plate of bone, or spina
scapula, marks off a superior and an inferior fossa. The
anterior end of the spine is always developed into an acromial
process. The most important of the other changes which occur in

ANTEKIOK EXTREMITY OF VERTEBKATA.

477

ifc is tlie enlargement of the base of the scapula, which obtains in
the Chiroptera and Primates.

§ 360.

Owing to the development of the primary shoulder-girdle the
secondary apparatus, which forms the clavicle (§ 358), is either placed
completely in the background, or used for purposes other than those
which it had in Fishes. The Anura only among the Amphibia are
provided with a clavicle (Fig. 262, A d), which forms an investing
bone for the precoracoid. It is seldom separated from the shoulder-
girdle, and this separation is never complete in any forms below the
Reptilia {B d). In them it forms a bone which connects the acromial
process of the scapula with the episternum {B c). In Birds the
clavicle has the same relations ; it is small in Dromaeus, and absent
in all other Ratitm ; in the Carinatm, however, the clavicles soon
unite into an unpaired bone, the furcula, and are connected with
the keel of the sternum by ligaments (Fig. 234,/).

The independent appearance of this portion of the skeleton, the
primitive origin of which was that of an investing bone' for a piece
of cartilage, leads to a histological change in the Mammalia ; the
clavicle is in them largely formed from a cartilaginous rudiment, which
is similar in many points to all other bones, which are preformed in
cartilage. This bone, however, is retained in some Mammals only ; —
in those, namely, in which the anterior limbs are capable of a large
amount of movement. It disappears so far as to leave no signs of
its presence in the Ungulata ; in other forms there are only rudiments
of it which are sometimes merely formed by ligamentous bands
(Carnivora).

Anterior Extremity.

§ 361.

All the varied forms, which the skeleton of the free appendages
exhibits, may be derived from a ground-form which persists in a few
cases only, and which represents the first, and consequently the
lowest, stage of the skeleton of the fin — the Archipt erygium.
This is made up of a stem, which consists of jointed pieces of
cartilage, which is articulated to the shoulder-girdle, and is beset on
either side with rays, which are likewise jointed. In addition to
the rays on the stem there are others which are directly attached to
the limb-girdle (cf. Fig. 260, d).

Ceratodus has a fin-skeleton of this form ; in it there is a stem
beset with two rows of rays. But there are no rays on the shoulder-
girdle. This biserial investment of rays on the stem of the fin may
also undergo various kinds of modifications. Among the Dipnoi, Pro-
topterus retains the medial row of rays only, which have the form
of fine rods of cartilage; in the Selachii, on the other hand, the

478

COMPAEATIYE ANATOMY.

lateral rays are considerably developed. The remains of tbe medial
row are ordinarily quite small (Fig. 263, R % but they are always

sufficiently distinct to justify us in sup-
posing tliat in biglier forms tbe two sets
of rays might be better developed. Rays
are still attached to the stem, and are
connected with the shoulder-girdle by
means of larger plates (p ms). The
joints of the rays are sometimes broken
up into polygonal plates, which may, ,
further, fuse with one another ; concres-
cence of this kind may also affect the
pieces which form the base of the fin
ms). By regarding the free rays,
which are attached to these basal pieces,
as belonging to these basal portions, we
are able to divide the entire skeleton of
the fin into three segments — pro-, meso-,
and metapterygium.

The metapterygium (mt) represents
the stem of the archipterygium and the
rays on it. The propterygium (p) and
the mesopterygium (ms) are evidently
derived from rays which still remain
attached to the shoulder- girdle.

The peculiar form of the fin in the
Ray is due to the great development of
the propterygium ; the arrangement in
Squatina leads towards this. One ray
is here converted into a support for
rays, and forms, by gradually reaching
forwards, a stem for the propterygium,
just as the metapterygium in the stem
of the archipterygium possesses one. The Chimserae agree in all
essential points with the Sharks.

Fig. 2G3. Skeleton of the
thoracic fin of Acanthias
vulgaris, p Basale of the
protopterygium, mt Of the
metapterygium. B Median
edge of the fin. The line
di’awn through mt indicates the
series which formed the stem
of the archipterygium. The
dotted lines correspond to the
rays, which are mostly arranged
at the sides {R E), and are
rudimentary only on the medial
side {R').

§ 362.

The skeleton of the thoracic fin in the G-anoidei may be derived
from a condition which is similar to that which obtains in the Shark ;
it is the same fin, with the peripheral parts reduced (cf. Fig. 264).
In correspondence with this, a few rays only are attached to the
stem of the fin (J5), and those which are set on the shoulder-girdle
are also rudimentary. In the Teleostei the peripheral portion of the
skeleton of the fin is still further reduced, and as a rule nothing
remains of the primary fin-skeleton except four or five elements
which are very similar to one another (Fig. 261, g) ; a very variable
number of small and always cartilaginous pieces are attached
peripherally to them. These, then, serve as supports for the

APPENDICULAE SKELETON OF VEETEBEATA.

479

secondary skeleton of tlie fin-rays (li). Basal pieces can be seen
in a few only, and it is difficult to refer these even to their
primitive significance. The arrangements which
obtain in the Gano'idei would lead ns to regard
the basale of the metapteryginm, and the basalia
of some of the rays, as being the most constant
constituents of these pieces. In consequence of
their having the same function they have the
same form, so that it is impossible to show that
they have any connection with the primary stage,
except by referring them back to the skeleton of
the Ganoid fin.

In many divisions of the Teleostei these
pieces undergo great changes, in addition to
being diminished in number. They are, for
instance, intimately attached to the shoulder-
girdle, and immovably connected with the parts
of which it is made up (Cataphracti).

In this way we are able to make out a con-
tinuous series from the well-developed skeleton
of the fin in the Selachii to that which is found
in the Teleostei ; the most important changes
consist in the gradual reduction of smaller or
larger parts. Eeduction first affects the periphery, and then the
base, so that the latter is the most constant portion. The decrease
in size which the primary skeleton suffers is made up for by the
appearance of ossifications of the integument, which consist, as
in the unpaired fins, of jointed or firm bony rays, and are developed
on both surfaces of the fin.

Gegenbaur, C., Untersuclningen zur vergleich. Anatomie der Wirbelthiere. II,
Leipzig, 1865.

Fig. 264. Primary
skeleton of the
thoracic fin of Aci-
penser ruthenus,
after the removal
of a portion of the
secondary skeleton.
B Basale of the meta-
pterygium. R Bony
marginal ray of the
secondary skeleton
of the fin, only
figured in part.

§ 363.

In the skeleton of the fore-limb of the higher Vertebrata we are
able to recognise the stem of the archipterygium, with rays attached
to one side of it ; no rays are now attached to the shoulder-girdle,
the stem only is so attached. The arrangement of the joints of the
rays in rows set obliquely to the stem — which is just the
same arrangement as that of the primitive rays — is obscured by
subsequent transverse jointing, but it can be recognised without
difficulty in the lower forms. This jointing gives rise to new pieces ;
transverse rows of the joints of the rays, as well as the corresponding
joints of the stem, being developed into longer pieces. This change
is due to a change in function, in consequence of which the ap-
pendage is converted from a swimming organ into a compound system
of levers.

In Ichthyosaurus among the Enaliosaurii the basale of the
archipterygium is first of all differentiated from the rest of the

480

CO:\[PAIiATlVE AXATOMY.

appendage as a large bone, wbicli forms a piece of about tlie same size
as the rest of the appendage ; this may be called the humerus. In
Plesiosaurus two succeeding pieces, which in Ichthyosaurus are still
indifferent, are also increased in size ; these correspond to the fore-
arm : radius and ulna; these are succeeded by two transverse rows of
smaller pieces, which form a carpus, and these again by longer rows
of bones, which represent the metacarpus and the phalanges of the
fingers. The segmentation which affects the appendage after the stem

and rays are broken up into several pieces,
may here be seen in its different stages.

The arrangements which are presented
by the Amphibia are similar in character;
for although one finger is atrophied, we can
fill up the void by the aid of the arrange-
ments seen in the hinder limb, where they
are complete. The stem of the archiptery-
gium must, therefore, be sought for in a
lateral series of skeletal pieces, which ex-
tends from the humerus, through the ulna
to the fifth finger, and in the carpus con-
sists of two pieces. The other skeletal
pieces are arranged on these rays. One
ray begins with the radius, and extends
into the first finger. A second, third, and
fourth begin in the carpus, and end in the
second, third, and fourth fingers. The
primitive carpus is therefore composed of
ten pieces; five carpals carry the fingers,
three are attached to the bones of the fore-
arm; these are the radial, intermedium, and
ulnare ; two centralia (cc) are enclosed by
these two sets.

The change in the function of this appendage is connected with
a rotation of the humerus on its own axis, and this rotation may be
observed in the individual development of higher forms. It brings
about a difference in the position of the limb as compared with that
of lower forms.

Fig. 265. Diagram of ibo
fore-limb of an Amphi-
bian. The dotted lines
indicate the rays, which
remain attached to the stem
of the Archipterygium.

§ 364.

A more or less complete copy of the typical form of limb derived
from the archipterygium is retained in all divisions of the Verte-
brata. In all there are often unmistakable traces of the characteristic
relations, in opposition to which numerous deviations, due chiefly to
reduction and concrescence, make their appearance. These modifi-
cations are clearly due to the varied uses to which the limbs are put,
just as the complete atrophy of some parts, or even of the whole limb,
are due to their being no longer required.

ANTEEIOE EXTEEMITY OF VEETEBEATA.

481

In the Amphibia the two upper portions are greatly developed^
but, except that the radius and ulna are fused in the Annra, they
present no such striking differences as those which are seen in the
carpus.

Some of the primitive carpalia disappear in the distal row ; with
this is generally correlated a shortening of the fingers^ which are
commonly limited to four ; or, again, two or three carpalia may be
fused together (Frogs, etc.). Concrescence may likewise be seen
to affect the proximal series of carpalia.

In the Eeptilia, the various portions of the skeleton of the arm
are least altered in the Chelonii, which have not only nine carpal
bones, but all five fingers. In the Saurii two of the three carpalia
of the first row are fused together ; those of the second row are also
greatly modified, and are reduced in number when any of the fingers
disappear. The carpus is still more altered in the Crocodilini. The
radiale has become much larger than the ulnare, and the second row
of carpalia is merely represented by a few elements, which are
always partly cartilaginous. The two ulnar fingers are conse-

Fig. 266. Skeleton of the arm of Ciconia alba. Humerus, it Ulna, r Radius.
c c' Carpus, m Metacarpus, p p' p" Phalanges of the first three fingers.

quently shortened in comparison with the three radial ones. In the
limbs of the snake-like Saurii there are all stages of reduction.
The Opf^ii are distinguished by the complete absence of these
parts.

In birds, where the whole of the fore-limb is converted into an
organ of flight, the reduction of the manus is still more marked.
Two bones only (Fig. 266, cc') are well developed in the carpus,
while a 23iece of cartilage, which corresponds to the second row in
the carpus, soon fuses with the base of the metacarpus. Three
fingers are always more or less developed in the manus ; in the
Saururas these were permanently separate, but in the Eatitse and
Carinata3 the metacarpals (m) of the second and third, and generally
also that of the first, are fused into one piece of bone. On the
third finger there is a rudiment of a fourth one.

As compared with the Saurii, the number of phalanges is
reduced in Birds. In the Saurii, starting from the first finger of
the radial side, which has two phalanges, we find one more in each
finger as far as the fourth, which has five ; but the fifth finger has
not so many. In the Crocodilini this increase stops at the third ;
in most Birds the second finger has only two phalanges (p'), the

482

COMPAEATIVE ANATOMY.

first and third one only (p p ; the first and second fingers rarely
have an extra phalanx.

Furbringer, M., Die Knoclien imd Muskeln der Extremitaten bei den schlan-
genartigen Sauriern. Leipzig, 1870.

§ 365.

The greater variation in adaptive relations to various conditions
is implied by the greater variations in the structure of the skeleton of
the Mammalian forelimb. Its elements somewhat resemble the
lower condition^ such as is seen in the Chelonii, with regard to the
number of the carpal bones. Although the manus is often modified
by the atrophy of certain fingers, the extremity, even in the lower
divisions of the Mammalia, has very various uses. Owing to the
greater power of movement possessed by the two bones of the fore-
arm, and the connection between one of them (the radius) and the
manus, the anterior extremity loses its lower function of an organ
of support, and is converted into a prehensile organ. This phseno-
menon is seen in the Didelphia, as well as in the Monodelphia ; it is
most complete in the Primates. The carpus has the three primitive
pieces of the proximal row. A centrale, also, is not unfrequently
present (Eodentia, Insectivora, Lemurs, Orang, and, for a short
time, Man). The distal row of the carpus have the two ulnar bones
fused into an uncinate (cf. Fig. 268, I II). The pisiform is a
special bone, which is attached to the ulnar edge of the carpus ; it
is very large in many forms. It is also found in
the Eeptilia, and may be shown to be the soli-
tary remnant of a numerous series, which was
possessed by the Enaliosaurii.

The modifications derived from this series of
forms are very closely correlated with the function
of these parts. When the arm is used as an organ
of fiight (Chiroptera), we find that its different
portions are considerably elongated ; and so,
again, they are shortened, and various parts be-
come very large in those numerous cases in
which the arm acquires a special function, as in
digging and so on ; the Monotremata, many
Edentata, Talpa, etc., are examples of this. In-
stead of this great increase in size, which is seen
in various parts of the skeleton of the arm, there
may be atrophy, as is the case with the fore-limb
of the Cetacea. It forms a paddle, the separate
parts of which have but little power of move-
ment, and the various bones of these parts may
lose all their articulations, and become united into an unjointed
fin-like mass (Fig. 267).

In another series, several of the fingers are atrophied, and the

Fig. 267. Anterior
extremity of a young
Dolphin, s Scapula.
h Humerus, r Ea-
dius. u Ulna, c Car-
pus. m Metacarpus,
p Phalanges.

ANTEEIOE EXTEEMITY OF VEETEBEATA.

483

fore -limb becomes a mere organ of support and locomotion. It is
clear that this is not a primary condition from the relative position
of the bones of the fore-arm_, which requires us to presuppose a
condition in which they were capable of pronation and supination.
As the limb ceases to have more than one function, this power is
lost ; the radius and ulna are connected immovably, and this may
lead to the atrophy of various parts of these bones, or to their more
complete fusion with one another. This* is the case in the Artio-
dactyla, where the distal end of the ulna is rudimentary in the
Kuminant forms. In the Tylopoda and Solidungula this end of the
ulna has quite disappeared, while the upper end is united with the
radius into one bone.

The fingers may take on one of two sets of characters. In

Fig. 268. Skeleton of the manns of various Mammals. I Man. II Dog. Ill Pig.
IF Ox. F Tapir. FI Horse. r Kadiiis. u Ulna, a Scaphoid, h Lunar,
c Cuneiform, d Trapezium, e Trapezoid. / Magnum, g Uncinate, p Pisiform.

either case the pollex is absent, and it is not functional even in the
digitigrade carnivora (Fig.% 268, II). Of the remaining digits,
however, the third and fourth are so greatly developed in the
Artiodactyla (III IV), that the other two (2 and 5) often do not
touch the ground (Suina, Moschidae). The fifth finger is next lost,
so that the third and fourth only are well- developed, and the second
forms a mere appendage (Anoplotherium). The third and fourth
fingers become still larger when their two metacarpals are fused
together (IV), while the second and fifth fingers become rudi-
mentary (Oxen, Sheep, Deer, etc.). The Perissodactyle series also
begins with the four-fingered form, but in them one finger ouly
(the third) is markedly larger (Tapir) (F). When the fifth, which
is already the smallest, disappears (Palaeotherium), the second
and fourth are attached to the third in the form of appendages

2 I 2

484

CO^IPAEATIYE ANATOLIY.

(Hipparion), and wlien the two lateral fingers are reduced to their
metacarpals alone, these are attached to the large metacarpus of the
third finger as mere splint-bones ^.nd the third finger

becomes the sole support of the limb (Equus).

The number of phalanges in the different fingers is increased in the
Cetacea only ; all other Mammals have two in the pollex, and three
in all the other fingers.

Posterior Appendages.

Pelvic Girdle.

§ 366.

The relations of the pelvic girdle are also correlated with
differences in the functions of the extremity. The homology
between the two skeletal portions is consequently more fully recog-
nisable as the functions of the two extremities are more nearly
the same, and the extent to which they are differentiated from
one another less.

A single piece of cartilage forms the groundwork of the pelvic
girdle. In the Selachii this is rarely enlarged in a dorsal direction.
In the Gano’idei and Teleostei the two halves of the ossified portion
are connected in the middle line. They undergo considerable varia-
tions in position, for they may be placed more or less anteriorly and
close to the shoulder-girdle (Pisces thoracici), or may even be united
with it (Pisces jugulares).

In the Amphibia the two bones of the pelvis are connected with
the vertebral column; at the same time they may be seen to be
divided into two pieces at the point where they are connected with
the femur ; the dorsal one, which is attached to a transverse pro-
cess (that is, to a rudimentary rib), forms the ilium ; the ventral one,
which is connected along the middle line with its fellow of the
opposite side, is known as the ischio-pubic bone (Urodela). There is
reason, however, for supposing that it merely corresponds to an
ischium. This arrangement is modified in the Anura (cf. Pig. 225),
for the long and slender ilia (il) are united with the ischio-pubic
bones (is), which are converted into a vertical disc, and fused with
one another.

The ilium of the Reptilia is greatly developed; in Chamaeleo it
resembles a scapula, and is continued into a process, which is com-
parable to a supra- scapula. In the Saurii it is elongated (Fig. 269,
Jl) ; in the Crocodilini it is shorter and broader (Fig. 210, Jl). The
bone is directed forwards, so that it is connected with
the sacrum behind the acetabulum. In the Saurii and Che-
lonii the ventral portion of the pelvis is continued from the aceta-

PELVIC GIRDLE OF VERTEBRATA.

485

bulum into two divergent pieces (Fig. 269)^ wRich enclose a large
opening (foramen obturatum). The anterior process is called the
pubis (P), the posterior one the ischium (Js). The two bones of either
side are more or less connected together along the middle line,
but this connection may disappear. The pelvis of the Crocodilini

Fig. 269. View of the left side of the Fig. 270. View of the left side of the

pelvis of Monitor. J? Ilium. Js Ischium. pelvis of Alligator lucius. x y Two

P Pubis, a Hinder end of the ilium, h Its limbs of the ischium, which unite with r s,
pnterior process. two processes of the ilium, to enclose a

foramen (o) at the base of the acetabulum.
The other letters as in Fig. 269.

(Fig. 270) differs from this in many points^ for a single bone (Js) is
given off ventrally from the acetabulum, and is connected by means of
two processes with the ilium (x y). It appears to represent an
ischium only, while a bone, which takes no part in the acetabulum,
but articulates with the ischium (p), and converges, like its fellow,
towards the anterior wall of the abdomen, represents the pubis.

. The pelvis in the fossil Dinosaurii was of the same character ;
the ilium was distinguished by a process which was directed forwards,
and of which there is an indication only in the extant Saurii and
Crocodilini (?>). The acetabulum was similarly incomplete, and was
connected with a long ischium, which was directed obliquely back-
wards and downwards, and was not united with its fellow of the
opposite side. A long pubis, which also ended freely, was given off
from the anterior margin of the acetabulum, and ran parallel to the
ischium.

This relation of parts is the same as that which characterises the
Avian pelvis (Fig. 271). In them the ilium (Jl) does not only
extend a long way back (a a), but its anterior process is converted
into a broad plate (h h). This extends along the lumbar region of
vertebral column, and even into the thoracic region, and so presses
a very large number of vertebrse into the pelvic region. The ischium
(Js) runs backwards from the incomplete acetabulum, and in a
direction which is nearly parallel to that of the hinder portion of the
ilium ; the small pubis, which has a slight share in the formation

486

COMPARATIVE ANATOMY.

Fig. 271. View of the left side of a Bird’s pelvis.
The dotted portion represents that part of the three
pieces of the pelvis, which extends backwards by the
development of cartilage. The dotted line marks off
that portion of the ilinm (hh) which grows forwards
without the addition of any cartilage. The letters as
in the preceding figures.

of the acetabulum^ takes the same course ; its ends project farther back
than those of the ischium, and generally conyerge ; in Struthio they

even form a symphy-
sis. There are various
kinds of connections
between the ilium and
ischium, and between
these and the pubis.

The pelvis in the
Mammalia is very dif-
ferent. The primi-
tive connection
with the sacrum
is always in front
of the acetabulum.
The ilium, however,
is directed from be-
fore backwards, and
the hinder edge of the
Bird^s ilium corresponds to the anterior edge of the MammaTs
ilium. Two different positions therefore for the ilium are derived
from the Amphibia. In the Amphibia it is directed laterally and
interiorly away from its connection with the sacrum, in Reptilia
and Aves obliquely forwards, and in Mammalia obliquely back-
wards. The ventral portion of the pelvis encloses an obturator

foramen, and is united ven-
trally with that of the other
side.

The primitive pelvic carti-
lage gives rise to the ilium and
ischium ; the pubis is derived
from a separate rudiment,
which is united with the
ischio-iliac rudiment in the
acetabulum (Man). This leads
us to think that the pubis is
an independent piece of the
skeleton, which has retained
its independence in the Cro-
codilini. The ilium of the
Mammalia is connected with
a few vertebrae. The ischium
also may be united with the
false sacral vertebrce (Dasy-
pus, Bradypus). When the
two ventral pieces are united at the ischio-pubic symphysis, as they
are in the Marsupialia, many Rodents, Artiodactyla, and Peris-
sodactyla, the pelvis is elongated in form. In the Insectivora
and Carnivora the greater part of the symphysis is formed by

Fig. 272. View of the left side of the pelvis
of a Dog. il Ilium, is Ischium, p Os
pubis. vl Penultimate lumbar vertebra.
VC Caudal vertebra.

POSTEEIOE EXTEEMITY OF VEETEBEATA.

487

tlie two pubic bones, and tins is still more marked in tbe higher
orders.

An independent adaptation, which is seen in various Mammals
(Insectivora and Chiroptera), is the presence of a ligamentous con-
nection instead of the pubic symphysis ; this may be very wide in
the female (Erinaceus).

When the posterior extremities are absent, the pelvic girdle also
undergoes atrophy. There are rudiments of it in the Cetacea.

In the Monotremata and Marsupialia there are two bones in
front of the pubes ; these marsupial bones are directed forwards ; in
Thylacinus they are reduced to small rudiments in cartilage.

Gegenbaur, C., Beitrage zur Kenntniss des Beckens der Vogel. Jen. Zeitsckr.
VI.— Hoffmann, 0. K., Beitrage zur Kenntniss des Beckens der Amphibien
u. Beptilien. Niederland. Arch. III.

Posterior Extremity.

§ 367.

We find just the same arrangements in the hind-limb as we have
described as existing in the fore-limb. In Fishes the hind limb forms
the ventral fin. In the Selachii its skeleton has the same characters
as that of the thoracic fin ; the most striking difference is that the
rays are arranged in a simpler manner. The basale of the stem is
generally greatly elongated. The joints which succeed the basal
piece undergo a special metamorphosis in the male, where they are
converted into copulatory organs.

The skeleton of the ventral fin in the Cano’idei may be derived
from this by supposing that there has been a peripheral reduction,
very similar to that which we saw in the skeleton of the thoracic
fin j and the Teleostean fin can be derived from the Ganoid. This
is generally much simplified, both as regards the size and the
number of its separate pieces, in consequence of the feebler develop-
ment of the whole ventral fin. In both divisions the dermal skeleton
takes part in increasing the surface of the ventral fin, just as it has
been shown to do in the thoracic one.

When we come to compare the hinder extremity of the higher
Vertebrata with the ventral fin of Fishes, we must again begin with
the archipterygium, which seems to be the lowest stage of this
extremity also. The segmentation of the extremity into successive
pieces is a repetition of the arrangement which we met with in the
skeleton of the arm. We distinguish the femur, tibia, and fibula;
and lastly, in the foot, a tarsus, metatarsus, and phalanges. The
four inner toes, and the parts that carry them, may be again
regarded as joints of the rays which are given off from a row of
bones extending from the femur, through the fibula, to the
outermost toe. The tarsus is made up of ten pieces, three oE which

488

COMPAKATIYE Ai^ATOMY.

are attached to the leg; these are the fibulare^ intermedium^ and
tibiale. There are two centralia ; and five distal tarsalia carry the
bones of the metatarsus (cf. Fig. 265).

In the Enaliosaurii the skeletal portions of the hinder extremity
are an exact repetition of those of the anterior one ; and even in

some of the Amphibia (Urodela) we meet
with an arrangement which is the same
in all essential points^ so that we need
not describe them specially. In most
IJrodela_, all the five terminal pieces^ or
toes, are retained in the hind-limb ; this
is more distinctly like the primitive form
than is the skeleton of the fore-limb.
In Cryptobran chus, Menopoma, and
others, the two centralia even are per-
sistent. But in the Anura there is a
very great change; the tibia and fibula
are fused. In the place of the three
proximal tarsal bones there are two
long bones, which are, however, often
fused at their ends ; they are ordinarily
known as the astragalus and calcaneum.
The distal row of tarsal bones is also
greatly reduced. Finally, we must note
the presence of a rudiment of a sixth toe.

§ 368.

In the Chelonii there are unimportant
modifications in the larger pieces of the
extremities ; in addition to this we must
note the gradual concrescence of some
of the bones of the tarsus, which is of
great importance as explaining the ske-
leton of the foot in Birds, as well as in
other Eep tiles. An intermedium is united
with a tibiale to form an astragalus ; and
the centrale is attached to, or even com-
pletely fused with, this bone. The fourth
and fifth tarsalia similarly form a single
bone, the cuboid. Owing to the forma-
tion of a single piece out of the bones
of the first tarsal row, and the firm union that is effected between
this piece, and the tibia and fibula, the foot gets to be articulated
in a peculiar manner. It moves on an intertarsal joint. The
skeleton of the Crocodile^s foot is somewhat different. The tibia
and fibula articulate with two bones, of which the fibulare has the
greater power of movement. The larger bone, connected with the
tibia, corresponds to the similar bone in the Chelonii. A piece of

losa. The dotted lines are
drawn through the rays, to
which the different pieces be-
loner.

POSTEEIOE EXTEEMITY OE VEETEBEATA.

489

cartilage, which is more closely connected with the metatarsus, is
articulated to it ; while a cuboid is articulated to the fibula. Owing
to the independence of the fibula, we have here a peculiarity,
which is only seen again in the Mammalia. In the Saurii, the
tarsal bone developed out of four primary elements (Fig. 274, A ts)
has no signs of its constituent parts even in the embryo. It is
immovably connected with the tibia and fibula, while the distal
bones of the tarsus (ti) are more or less
connected with the metatarsus. This
appears to have been most complete
in the fossil Saurii (Ornithoscelida).

In these arrangements we may per-
ceive an outline of what obtains in the
foot of the Bird, which, in its em-
bryonic condition (Fig. 274, B), pre-
sents us with those characters which
are permanent in many Eep tiles. The
fibula ( jj) extends to the tarsus. This
is formed of two pieces of cartilage ;
the upper one (ts) is undoubtedly homo-
logous with the bone, which is made up
of four elements in the Eeptilia ; the
lower one (ti) corresponds to the distal
series of tarsal bones. The metatarsus
is made up of five bones, which were
primitively separate, but only four of
these (Bj I — IV) carry toes, while the
fifth is very small and completely fused
with the distal portion of the tarsus.

The difference between the adult and .
embryonic arrangements consists in
the degeneration of the fibula (Fig.

275, b'), which later on is attached to
the tibia, as a small appendage (h')j
and which never reaches the tarsus (b).

The proximal tarsal cartilage fuses
with the tibia, and forms its articular
condyle; the distal one unites with
the single piece (c), which is formed
from the fusion of the three longer
metatarsal bones, and in which no
permanent signs of separation can be

made out except what is implied by the separate condyles at its
distal end (Fig. 275, c'). The metatarsal of the hallux remains dis-
tinct, and generally forms a small appendage of the large tarso-
metatarsus. The arrangements, therefore, which are seen in the foot
of the Eeptile are still further developed in that of the Bird, for the
parts which in the former are merely united firmly together, are fused
in the latter; the foot still moves on the same intertarsal joint.

Fig. 274. Skeleton of the foot of
a Eeptile (Lizard) (X) and a Bird
(B) ; the latter is in its embryonic
condition. / Femur. t Tibia,
p Fibula, ts Upper, ti Lower
piece of the tarsus, m Metatarsus.

I — V Metatarsalia of the toes.

490

COMPAEATIYE AE’ATOMY.

With regard to the toes, we find five to he the dominant nmnher
in the Keptilia ; it is in Birds only that they fall to four, or three, or
even to two (Struthio). The phalanges of the
toes generally increase in number from within
outwards; there are two on the hallux and five
on the fourth toe. This holds for the Saurii,
Crocodilini, and Aves. There are not so many
in the Amphibia or Chelonii. Amongst the
Eeptilia the limbs are reduced in the snake-like
Lizards, and in all Ophidii, among which the
Peropoda only are provided with any rudiments
of them at all.

Gegenbaue, C., Untersuchurigen zur vergleicli.
Leipzig, 1864.

§ 369.

Ariat. I.

Fig. 275. Hinder ex-
tremity of Buteo
vulgaris, a Femur.
b Tibia, h' Fibula,
c Tarso-metatarsus.
c' The same piece
isolated, and seen
from in front, d d'
d” d"' Four toes.

The special differentiations in the skeleton of
the hind-limb of Birds and Reptiles do not re-
semble those which are seen in the Mammalia.
As a rule it is less altered than the fore-limb.
In the Perissodactyla, many Rodents, etc., the
femur is distinguished by the possession of a
third trochanter. The tibia is the most im-
portant bone of the leg ; the fibula is often rudi-
mentary, especially in the Ungulata. In the
Artiodactyla the distal end remains ; it is articu-
lated to the tibia and to the tarsus (astragalus),
and appears to enter into the composition of the
latter. In some (as in Rodents and Insectivora)
the tibia and fibula are complete, and are fused
together.

The tarsus is the most characteristic part ; it is attached by two
pieces to the leg, but, as a rule, only one of these forms the ankle-
joint. The process on the second bone (calcaneum), of which there
were indications in the Crocodilini, is still more developed. The
centrale remains separate, but passes to the inner edge of the foot,
where it forms the navicular. In some of the Prosimise it unites
with the calcaneum to form a long process (Macrotarsi). Of the
five distal bones the two outer ones are always replaced by the
cuboid, while the three inner ones generally remain distinct (cunei-
form). When the number of toes is diminished, these latter bones
are often reduced; they may even fuse with the metatarsus, as in
Bradypus. The cuboid also may be united to the navicular
(Ruminantia) .

In addition to its primitive function as an organ of support
and of movement, the foot may be developed into a grasping organ ;
when this happens, the foot comes to resemble in many points the
end of the fore-limb, or hand. But in all essential points of

MUSCULAR SYSTEM OF VERTEBEATA.

491

structure it is still a £oot^ so long as we hold to the anatomical
conception of what hand and foot are^ and do not put functional
relations into the fore-ground; and if we do, then the proboscis
of the elephant is a hand also.

This change in the character of the foot is seen in many Mar-
supials, Prosimise, and Primates. The chief change lies in the
development of the first toe in a manner similar to that in which
the thumb of the hand is developed. In Man also there are indica-
tions of the foot having once been a prehensile organ. When this
function is lost, the hallux is shortened in proportion to the extent to
which the whole of the sole of the foot ceases to take part in loco-
motion. The shorter hallux is then functionally inoperative (Digiti-
grade Carnivora). It disappears altogether in the Ungulata, where
the limbs do nothing but aid in locomotion and support the body.
The characters of the metatarsus and digits are parallel in character
to those of the fore-limb in the Artio- and Perissodactyla. In the
latter, the gradual conversion of the foot from a four-toed to a one-
toed condition has been recognised as obtaining in the same palaeon-
tological series as that which has already been pointed out in
reference to the fore-limb.

Muscular System.

§ 370.

The muscular system of the Vertebra ta is differentiated from the
mesoderm in the embryo, and is divided in a way corresponding to
the metamerism of the whole body. Before the skeleton is differen-
tiated the musculature below the integument unites with it to form
a dermo-muscular tube, similar to that of segmented Invertebrata in
many points, although not immediately derivable from one of those
forms.

Its relations to the skeleton, and the formation of a muscular
system connected with the skeleton, are therefore acquired in pro-
portion to the extent to which they are concerned in the develop-
ment of the skeleton. In Amphioxus, where the skeleton is
essentially formed by the chorda dorsalis, the musculature, in the
region of the trunk at any rate, has none of these relations ; it is
only in that portion of the body which encloses the respiratory
cavity that it seems to have any relations to the visceral skeleton.
The whole of the musculature is arranged in two lateral longitudinal
masses, which are separated by connective tissue into dorsal and
ventral masses. These longitudinal bands of muscle are separated
by septa of connective tissue into a series of metameres (myocom-
mata) ; and each septum serves for the origin as well as for the
insertion of the fibres, which take a straight course between them.
While this mass of muscle extends along the whole of the dorsal
region of the body, it is modified in the ventral surface of the

492

COMPAEATIVE ANATOMY.

anterior portions of the body, owing to its relations to the branchial
skeleton.

In the Cyclostomata also the greater part of the muscular
system has no direct connection with the skeleton, for the superficial
layers are here also merely connected with connective tissue, and the
septa that form the metameres are to be found over the whole of the
dorsal and caudal regions- of the body. On the head, however, and
on the visceral skeleton, we may see that some of the muscles
are connected with the skeletal parts and differentiated in a special
manner.

When the skeleton is formed it is necessarily connected with
the musculature, since the skeletal parts grow down between the
masses of muscles, following the septa of connective tissue. The
primitive similarity, therefore, between the parts of the mus-
cular system disappears, and a differentiation commences ; this is
implied, on the one hand, by the formation of a muscular system
connected with the skeleton, and on the other by the special
development of the remainder of the muscular system, which is *
not so connected, into a dermal musculature.

The whole of the muscular system requires, however, to be
systematically investigated before we can know as much about it as
v/e do about the skeleton. We must, therefore, confine ourselves in
this description to a mere sketch, many details in which must be put
in with great diffidence.

Dermal Muscles.

§ 371.

As we regard the dermal muscles as primitively forming a
common complex with those which belong to the skeleton, we must
distinguish from it those which belong to the integument as such.

Among the Cyclostomata some of the muscles of the trunk have
no connection with the parts of the skeleton, and appear, therefore,
to be essentially dermal muscles ; even in the lower Glnathostomata,
the greater portion of the large lateral masses of muscles on the trunk
are only connected to the skeleton by the tendinous intermediate
bands, which pass off from it; it has not, therefore, yet become
a part of the skeletal musculature, in the sense that it forms bundles
of muscles which are attached to the skeleton by their two ends of
origin and insertion. This more indifferent condition enables us
to understand how it is that there are no distinct dermal muscles.
At the same time there are distinct layers of dermal muscles in the
outer wall, at least, of the respiratory cavity in the Selachii, where
they form part of a common constrictor.

In many other parts also of the body there are subcutaneous
muscles, which are not connected with the large lateral muscles ; the
layer which runs along the lateral line in the Teleostei, and wEich is

MUSCULAE SYSTEM OF VEETEBRATA.

493

distinguislied by its deeper colour^ may be regarded as belonging to
tliis system. In the Amphibia there are dermal muscles in the
head, which act on the nasal orifices, and, in the Anura, in the
anal region also. The muscles lying on the external nares are better
developed in the Reptilia. The dermal muscles are of great func-
tional importance in the Ophidii, as they produce a movement of the
scales, which is of use in locomotion.

In Birds there are large flattened dermal muscles in various
parts of the body ; as in the Eeptilia (Chelonii) there is a continuous
layer of muscles in the neck; other dermal muscles take their origin
from the skeleton, such, for example, as the musculi patagii, which
pass into, and extend the membrane of the wing. The muscles
which serve to move the remiges and rectrices also belong to this
category.

The dermal musculature is more highly developed in the Mam-
malia. There is generally a large muscle below the integument of
the trunk, which covers the dorsal region of the body, and is con-
tinued on to the head and neck ; this is inserted by tendinous
pieces into different portions of the integument, while anteriorly it
is also inserted into the humerus. It is best developed in Echidna,
Dasypus, and in Erinaceus, where it forms the muscle by which the
body rolls itself up. In most of the Apes the large dermal muscle
is distributed over the same parts as in other Mammals, but it is
more distinct in its anterior portion. In the Orang and Chimpanzee
this latter is represented by a muscular plate, which occupies the
sides of the neck, and is continued on to the face ; in Man it is
reduced to the platysma myoides.

Musculature of the Skeleton.

§ 3V2.

The differentiation of the muscles, which is due to the connec-
tion between the muscular system and the skeleton, is very inti-
mately connected with that of the skeleton, inasmuch as both parts
have always reciprocal relations, owing to their being both formed
for the same function. When, therefore, any portion of the skeleton
is increased in size, the muscles that go to it are also increased, and,
when another part undergoes atrophy, its muscles are correspond-
ingly diminished. So, also, the greater functional independence of
the muscles is bound up with a greater differentiation.

This differentiation gives rise to a number of systems of muscles,
each of which can be again divided into subordinate complexes of
more or less distinct muscles. The muscles of the trunk, of the
cephalic skeleton, and of the appendages, may be distin-
guished as systems of this kind.

The above-mentioned primitive musculature gives rise to the

494

COMPAEATIVE ANATOIMY.

muscles of tEe trunk, or lateral trunk-muscles. Tliey consist
of two masses of muscle wkicli occupy the sides of the body, and
extend from the head to the caudal end (m. laterales) ; they are
separated from one another along the middle line of the dorsal and
of the ventral surface. In the Myxinoidea, among the Cycle -
stomata, the ventral portion of these masses of muscle is distin-
guished by the oblique course of its fibres. It is doubtful whether
this represents a new system or no. Each half is divided into a
dorsal and a ventral portion, which are separated from one another
along a horizontal plane drawn through the vertebral column ; so
that, altogether, there are four lateral muscles.

Each of the four lateral trunk-muscles is divided, in Fishes, into
a number of segments by tendinous bands which correspond in
number to the vertebrae (ligamenta intermuscularia) ; these may be

easily made out on the surface of
the body, owing to their free edges
forming distinct tendinous bands.
The muscular fibres between any
two intermuscular ligaments take
a parallel course, and the ligaments,
therefore, afford insertion as well as
origin to one muscular segment.
The muscles, therefore, are only in-
directly connected with the skeleton.
The tendinous septa first keep to
one plane, but they then curve,
and curve in such a way that in
each dorsal muscle we can recog-
nise a lower layer, which is made
up of cones, one within the other,
and with their apices directed for-
wards (Fig. 276, A a)i and an upper
one (&) which is made up of parts
of cones. The apices of these in-
complete cones look backwards. In the ventral muscles the rela-
tions are so far reversed that the cones {a‘) are placed above the
incomplete cones {h‘). In a vertical section through the tail of a
Fish we see, therefore, two systems of concentric rings on either
side, which project into one another (these are the sections of hollow
cones) ; while above the upper ones, and below the lower ones, there
are curved lines of varying length (the sections of the incomplete
cones).

In the Perennibranchiata, and in the larval stages of the other
Amphibia, we meet with essentially similar conditions; we meet,
that is, with the same zigzag lines of the ligamenta intermuscularia,
only they are not so markedly sinuous. When these ligaments take
a more direct course the cones are not formed. In the adult Sala-
mandrina the ventral portion of the lateral muscles in the trunk
undergoes certain changes, and it is in the tail only that we can

Fig. 276. A Transverse section through
the caudal muscles of Scomber
scomber, a Superior, a' Inferior
lateral muscles, h and V Section of
imperfect upper and lower investing
cones. d Centrum of the vertebra.
B Zigzag lines of the superficial ends
of the Lig. intermuscularia in the tail
of Scomber (after J. Muller).

MUSCULAE SYSTEM OF YEETEBEATA.

495

observe a symmetrical arrangement of tbe upper and lower halves ;
the persistent dorsal portion is^ however, still separated by inter-
muscular ligaments into separate pieces, and is quite fish-like in
character.

§ 373.

In the Amniota, other muscles are derived from the ventral
portion of the lateral musculature of the trunk, but it is retained, with
modifications, in the tail of the Eeptilia and Mammalia, where it
undergoes the same changes as the dorsal portion, which is still
continued with great regularity on to the tail.

In the Saurii, it is still possible to make out a separation of the
dorsal lateral muscles by the intermuscular ligaments, but in all the
rest of the Amniota they are still further differentiated, and give
rise to a series of separate dorsal muscles. In the Mammalia we
find them divided into a superficial and a deep portion. The former
consists of the splenius, which is limited to the cervical region, and
is partly inserted on to the skull, and partly on to the transverse
processes of the anterior cervical vertebrae. The sacrospinalis is also
one of the superficial muscles ; it is broken up into a median and a
lateral portion (iliocostalis and longissimus) . They both contain
masses of muscle which have their origin in the sacrum and ilium.
As the muscle passes up to the skull, accessory masses are added on
to it, which take their origin partly from the ribs, and partly from the
transverse processes. The insertions of the iliocostalis and longis-
simus extend to the ribs, and those of the latter muscle to the
transverse processes also. The deep layer is formed by the
transverse -spinalis, which is formed of a system of muscles which
arises from the transverse processes, and is inserted into the spinous
processes ; it is more or less broken up into various layers
(semispinalis, multifidus) .

Those parts of these muscles which reach the neck are often
developed in size in proportion to the freedom of movement
possessed by this region, and they may therefore be described as
special muscles. The same remark applies to those ends of these
muscles, which become developed into independent muscles, and
extend to the skull. The trachelomastoid is the cranial portion of
the longissimus, and the biventer and complexus of the semispinalis.
The musculi spinales and interspinales belong to this group. The
rectus capitis posticus major forms the most anterior spinalis muscle ;
and the rectus capitis posticus minor is the first of the interspinales.

The small muscles which move the vertical fins of Fishes must
be regarded as being derived from the primitive lateral muscles of
the trunk.

§ 374.

The intercostal muscles must be regarded as a group derived
from the lateral trunk muscles. In Fishes these muscles are not

496

CO]\rPAEATIYE ATTATOIMY.

differentiated, inasmucli as the muscles between tlie ribs, or their
equivalents, are still portions of the lateral muscles. The ribs them-
selves lie in the intermuscular ligaments. In the higher divisions
of the Vertebrata a more distinct differentiation obtains. These
muscles are best developed in the Ophidii. The muscles which are
found between the rudimentary ribs, which are fused with the
vertebra, or between the transverse processes (intertransversarii),
also belong to this group of intercostal muscles. So, too, do the
levatores costarum, and the muscles which lie on the inner surface
of the wall of the thorax (thoracici interni), and the scaleni. The
size of all these muscles varies very greatly according to the extent
and 23ower of movement of the ribs ; special retractors may be added
to the elevators of the ribs, as in the Ophidii.

The broad ventral muscles must also, in all probability, be
regarded as belonging to the system of intercostal muscles ; these
are found in those regions of the ventral wall where there are no
ribs. They consist of the obliquus ext emus, obliquus internus,
and transversus abdominis. The obliquus externus corresponds to
the intercost, externus, and the internus to the intercost, internus.
The tendinous bands found in many Amphibia, and in the Saurii,
must be regarded as remnants of the primitive intermuscular liga-
ments. The obliquus externus generally takes its origin from a
large portion of the thorax ; in the Eeptilia it is divided into several
layers.

In the Amphibia the transversus abdominis is a large muscle,
as it is also in all Eeptilia except the Ophidii, where it is absent. It
extends as far forward as the thoracic region. In Birds it extends
as far as the hinder edge of the sternum, but in Mammals it has a
wider area.

The rectus abdominis appears to be the proportionately least
altered portion of the primitive musculature ; its fibres retain their
primitive course, and its inscriptiones tendineee are remnants of its
primitive septa. It generally extends from the sternum to the
pelvis in the AmjDhibia, but when the sternum is shortened it is
continuous with the sterno-hyoid (Amphibia).

In the Crocodilini the transverse tendinous bands are ossified,
and form the so-called abdominal ribs."’^ The M. pyramidalis
must also be regarded as part of the recti abdominis ; it is found in
the Salamandrina, Crocodolini, Eatitee, and also in many Mammals.
In the Monotremata and Marsupialia it is largely developed. So
much so, indeed, that it nearly reaches to the sternum, and so covers
the rectus; it has its origin in one edge of the marsupial bone.

§ 375.

The branchial skeleton of Fishes is provided with a special
system of muscles, which is repeated between each of its segments.
As the primary pieces of the jaw also belong to this skeleton, their
muscles must be regarded as differentiations of the muscular system

MUSCULAR SYSTEM OF VERTEBEATA.

497

of the branchial skeleton. Part of these muscles have their origin
on the skull, others belong to the different arches, while others again
are arranged transversely, and act so as to approximate the arches of
either side to one another. Muscles pass off from the branchial
arches to the branchiostegal rays. They are well developed in the
Selachii, but rudimentary in Osseous Pishes, where they appear to
be converted, on the hyoid arch, into the muscles of the operculum,
and of the dermal rays of the gills. The Amphibia are provided
with a similar musculature during their larval stages ; this is partly
derived from the muscles of Fishes, and is retained by the Perenni-
branchiata throughout their life. When the branchial framework
disappears, and the hyoid becomes more independent, part of the
branchial musculature is taken up by it.

As to the muscles of the jaw, it can be shown that an adductor
of the two parts of the mandibular arch in the Selachii undergoes a
certain amount of differentiation into several parts, and forms the
rudiment of the muscles of mastication. When the palato- quadrate,
or the bones differentiated in it, are fixed to the cranium, these
muscles are inserted into the lower jaw. In the Amphibia and
Reptilia an inner portion of this mass of masticatory muscles is
differentiated as the pterygoid, and this again may be divided into
two (Pt. externus and internus) (Saurii) ; the differentiation of the
temporal and masseter muscles is indicated by their arrangement in
layers. In both classes the depression of the jaw is effected by a
muscle, which forms a short but powerful belly on the hinder edge
of the lower jaw. It corresponds to the posterior belly of the
digastric of Mammals. The Ophidii are distinguished by an increase
in the number of the muscles ; in addition to adductors of the
lower jaw, special muscles, which move the quadrate and the various
bones of the palatine arcade, may be seen to be largely developed
in the Eurystomata. In Birds there are similar muscles, which
elevate the pterygoids and the quadrate, and produce the movement
of the maxillary apparatus. The temporalis is the largest of the
proper mandibular muscles, and the adductor, which is present in
the lower divisions, where the two halves of the jaw are movable,
is replaced by a muscle which extends transversely between the two
halves of the jaw, and has a different function.

The masticatory muscles of the Mammalia are similar in number,
origin, and insertion to the same muscles in Man ; as a rule they
are larger, but otherwise they do not differ in any points except
those which are due to the form of the surfaces of origin and
insertion provided by the proper bones.

§ 376.

Of the paired appendages, the fins of Fishes possess a number
of muscles on the girdle, as well as on the free portion, but it has
not been possible to compare these muscles with those of other

2 K

498

COMPAEATIYE ANATOMY.

Vertebrates. They are divided into those which pass to the girdle,
and those which belong to the appendage itself.

When the appendages are metamorphosed, the musculature
undergoes changes also; the muscles are, indeed, simplified in
number, but are entrusted with more functions, in consequence of
the greater freedom and independence of the skeletal parts.

As compared with the Fishes the most important change is the
extension of the musculature of the shoulder-girdle and
of the anterior extremity over the dorsal and ventral
surface of the body; this obtains in all the higher Vertebrata.
The muscles developed from the superior lateral trunk-muscles are
covered over by muscles which go to the limbs, and which are sub-
stituted in Fishes by a mass of muscle which arises from the head.
These are slightly differentiated in the Perennibranchiata, and more
so in the Caducibranchiata ; they form those muscles, which repre-
sent the cucullaris and the sterno-cleidomastoid in the higher
divisions. They are supplied with nerves from the head. Other
muscles, which are probably derived from the muscles of the trunk,
and which pass to the appendages, partly from the back, and partly
from the thorax, are added on to them.

The other muscles, which belong to the limbs themselves, are
derived from the layers, which in Fishes are more similar to one
another, and which cover the dorsal and ventral faces of the skeleton
of the thoracic fin. The musculature undergoes great changes
owing to the reduction of this fin, and to the modifications under-
gone by the parts that are persistent; the changes, therefore, in
the anatomical characters of the musculature in the various divi-
sions run parallel to the functional changes in the value of the
appendages.

In the hinder limbs the relations of the pelvic girdle to the axial
skeleton are, at first, the factors which affect the characters of the
musculature. The absence of any connection between these skeletal
parts is the cause of the greater independence of the pelvic girdle
in Fishes ; so far as this affects the musculature it is made up for by
its more indifferent character. The more intimate connection be-
tween the pelvic girdle and the axial skeleton in the Amniota
diminishes its powers of movement, and, consequently, the develop-
ment of the muscles by which this is effected. The muscles which
belong to the limb itself have their origin in the pelvic girdle, or in
the skeleton of the limb ; they seem, on the whole, to be divided
into the same groups as those of the fore-limb, with the exception
of such modifications as are due to the difference in the function of
the two limbs.

§ 377.

The subvertebral muscles form a special group. They lie
below the vertebrae and their lateral processes, so that in the thoracic
region they lie within the thorax.

The musculus longus forms an anterior portion of the muscles

ELECTKIC OEGANS OF YEETEBEATA.

499

below the vertebral column ; this is first seen in Eeptiles ; it
generally commences within the thoracic cavity, and extends along
the neck up to the skull. It breaks up into several portions, which
are distinguished as longus colli, or longus capitis, according to the
point of insertion.*

Another subvertebral system of muscles appears to lead to the
formation of the Diaphragm. This arrangement does not obtain
in Fishes, and it is doubtful whether the separate bands of muscle,
which embrace the oesophagus in the Amphibia, can be looked upon
as forming a rudimentary diaphragm. Among the Eeptilia, the
Chelonii have a muscular layer over the lamella of the peritoneum,
w’hich encloses the lungs ; this takes its origin partly from the
centra of the vertebrae, and partly from the rib-like transverse
processes. In the Crocodilini there is no diaphragmatic muscle, for
it is not possible to regard the highly- developed peritoneal muscu-
lature as being a formation of this kind, as it has its origin in the
anterior wall of the pelvis. There are indications of a muscular
investment of the lungs in Birds ; it is best developed in Apteryx.

It is in the Mammalia only that there is a well- developed
diaphragm forming a partition between the thoracic and abdominal
cavities. The oblique direction, taken by the muscle in the Eeptilia
and Aves, is necessarily converted into a transverse one. The
muscular portions arise in part from the vertebral column, and in
part from the ribs ; they pass into a centrum tendineum, which is
occasionally absent (Delphinus).

Humphry, G. M., Observations in Myology. Cambridge and London, 1872, —
Furbringer, M., Vergl. Anat. der Scbultermuskeln. Jen. Zeitschr. VII. YIII.
Morphol. Jahrb. II. — De Man, Vergl. myolog. u. nenrolog. Stud. Leiden,
1873. — Vetter, B., Vergl. Anat. der Kiemen. u. Kiefermuskulatur der
Fische. Jen. Zeitschr. VIII.

Electric Organs.

§ 378.

The so-called electric organs are special apparatuses which are
found in a very few Fishes ; they are of importance, from an
anatomical point of view, in consequence of the large masses of
nerves which end in them, and from a physiological point of view, in
consequence of the development of electricity in them. The endings
of the nerves have very much the same relations as have the ends
of motor nerves in muscular fibres, while there are many points in
the development of these organs which point to their having had
their origin in metamorphosed muscles. There is sufficient reason,
therefore, for regarding these organs as belonging to the muscular
system, although we do not yet know anything of their earlier
stage, in which they probably appeared as muscles.

500

COMPARATIVE ANATOMY.

The Fishes which are provided with these organs belong to
the genera Torpedo and Narcine among Rays, Gymnotus among
Eels, and Malapterurns among the Siluroids; Mormyrns also has
similar organs. There is a pseiido-electric apparatus in Raja.

Although these
organs differ greatly
from one another in
position, and in their
broader anatomical
details, in the dif-
ferent genera, they all
agree in being com-
posed of alveoli of
various forms, which
are bounded by con-
nective tissue, and
filled with a jelly-like
substance. The nerves
pass to one surface of
these alveoli, where
they form fine net-
works, and give rise,
finally, to an electric
plate,^^ which rejDre-
sents the ends of
these nerves.

The relation
this plate to
whole apparatus,
its relations to
nerves, are described
in the following ac-
count of what is seen
in the Torpedo.
The electric organ
(oe) is placed between
the head, the bran-
chial sacs (Fig. 277,
hr), and the proto-
pterygium of the tho-
racic fin ; it is as deep
as the whole body,
and is invested by a

of

the

and

the

Fig. 2'77. A Torpedo, with the electric organs dis-
sected out. On the right the surface only of the
organ (oe) is shown. On the left side the nerve-trunks
passing to the organ are dissected out, and part is followed
some way into the organ. The cavity of the skull is
laid open, and the brain displayed. I Fore-brain.
II ’Tween-brain. Ill Mid-brain. IV Electric lobe.
V Vagus, tr Trigeminal. tr' Its electric branch.
0 Eyes, f Spiracular cleft, t Mucous tubes of the
skin, hr Branchice; on the right they are covered
over by a common layer of muscles, on the left the
separate branchial sacs are shown.

tendinous membrane,

which is covered by the integument above and below. Each organ
is made up^of a number of parallel prisms, which again consist of a

series of elements set

in

rows one on the other : these are

the above-mentioned alveoli. They are closely united with one
another by connective tissue ; they all receive interiorly the nerves

NEEVOUS SYSTEM OF VEETEBEATA.

501

wLicE pass into the prisms, and the free surface of the electric
plates are directed dorsally. Five large nerve-trunks pass to the
organ ; these rami electrici belong to different cranial nerves, but
principally to the vagus ; they are distributed between the prisms.

In the other Electric Fishes these organs agree with what has
been described, so far as their more minute characters are concerned,
but they differ in position, and in the characters of the alveoli. In the
Electric Eel, for example, the organs lie in the caudal region of the
body, just below the external integument. In Malapterurus the
organ extends over the whole surface of the body, just below the
integument; and in the Mormyri again it is found in the tail. There
are corresponding differences with regard to the nerves, so that
we may conclude that these organs are morphologically different,
notwithstanding their histological and physiological similarity.

ScHULTZE, M., Zur Kenntniss d.elektr.Org. d.Fisclie. Abh. d.Naturforsch.Gesellsch.

Halle, 1858.

Nervous System.

§ 379.

The central organs of the nervous system are placed above
the axis of the spinal chord, in the canal formed by the superior
system of arches of the axial skeleton. They consist of symmetrically-
arranged nervous masses, which are similar in character throughout,
in the Acrania only ; in the Craniota they are differentiated into
two large portions, the brain, and the spinal chord. Although
the latter has without doubt a great similarity to the ganglionic
chain of segmented Invertebrata, it is quite impossible to derive the
spinal chord from it ; the central nervous system of the Yerte-
brata is rather to be regarded as representing the superior,
or cerebral, ganglia of the Invertebrata in an extremely
high state of development. The earliest rudiment is derived from
a differentiation of the ectoderm. The medullary plate,^^ which is
formed in this way in the Invertebrata, does not extend along the whole

of the rudimentary body, or if it
does so at first, it does not keep
pace with thegro wth of thebody
(Ascidiae) ; in the Vertebrate
embryo, however, it is of very
nearly the same length as the
body, so that the central nerv-
ous system extends throughout
the whole length of the body.

The medullary plate forms
a groove by the uprising of
its edges (it;), which are continued into the neighbouring ectoderm
(epiblast) (Fig. 278, li) ; this is gradually converted into a closed

Fig. 2V8. Diagrammatic section through
the embryo of the Fowl (end of the first
day), ch Notochord, u Primitive vertebrm.
sp Lateral plates. w Medullary groove.
w and h Epiblast. d Hypoblast (after
Remak).

502

COMPAEATIVE ANATOMY.

tube. This gradually sinks away from the surface of the body,
owing to the growth over it of the epiblast, and of parts differen-
tiated from the mesoderm. The medullary tube, which is formed
in this way, remains as a simple chord in Amphioxus, the most an-
terior region of which contains an en-

alargement of its central canal. But in
the Craniota, diverticula appear in the
\ most anterior portion before it is com-

;A pletely closed (Fig. 279, a); these form

!\ the rudiments of the brain, while the re-

i mainder of the medullary tube is equally

L j differentiated throughout, and forms the

rudiment of the spinal chord.

W In addition to their position, which

M is always an important point in com-
' \ parison, the rudiments of the nervous
centre of the Vertebrata have, in common
O with those of various In vertebrata, certain
|;1 relations to the higher sensory organs
W (and especially to the optic organ) ; and
in this respect the Tunicata are those
which exhibit the closest affinities. In

Fig. 279. Embryo of the Dog ,, oq hi fhp Vpvtpbrata flip wholp of
seen from behiucl, with the nidi- 111 lUe VeiteOiata, tlie WllOie 01

ments of the central nervous the medullary tube IS iiot closed in the

system, of which the medullary same way; ill the cerebral region, it is

connection with the exterior for a long

of the three primitive cerebral time. Ill comparing the nervous system

vesicles, a' Sinus rhomboidaiis of V ertebrates with that of the Tunicata

in the lumbar legion, c Lateral chord, which is Continued along the

plates, which bound the rudi- , -i •

mentary body, d Epiblast and clorsal region, and Oil to the tail in the

mesoblast. / Hypoblast (after larv83 of the Ascidiae, and in the Appen-
Bischoff). dicularia (§ 305), is of importance; this,

which is distinguished by its ganglia,
appears to indicate the path by which the hinder portion of the
central nervous system of the Vertebrata was phylogenetically
developed, and gradually converted into the spinal chord. As there
is such a great difference between the true central organs and this
chord, that it is impossible to regard the chord as a true continuation
of the central organ, or even as a portion of it, which merely
differs in consequence of its position, it must be supposed that the
brain, or the most anterior portion of the medullary chord in
Amphioxus, represents the more primitive portion of the nerve-
centre; while it is also the first to appear in the embryo. The
similarity between the rudimentary spinal chord and brain would
then be merely a condition, which had been acquired by the Verte-
brata, and which had its starting-point, phylogenetically, from
a chord which was continued on from the primary nerve-centre,
such as we now meet with in the Tunicata. According to this
view the whole medullary tube is not derived, phylogenetically.

EEAIN OF VEETEBEATA.

503

from a mere elongation of a shorter nerve-centre^ but from the
gradual development of a nerve- chords which primitively formed a
peripheral apparatus only. The differences in the characters of the
brain (exclusive of the medulla oblongata) and of the spinal chord,
so far as regards the arrangement of the white and gray substance,
serve to confirm this view, which is also supported by other facts.

A. Central Organs of the Nervous System,

a) Brain.

§ 380.

Three successive portions are developed from the rudiments of
the brain (Fig. 280 a), the cavities in which communicate with one
another. The last of these passes freely into
the medullary tube behind it. These primi-
tive cerebral vesicles give rise to new seg-
ments, so that we can soon distinguish five.

The first is known as the Fore- brain or
Prosencephalon (Fig. 280, a)', the next as
the Twixt-brain or Thalam encephalon (5);
the Mid-brain or Mesencephalon (B C c)
forms a third swelling ; and this is succeeded
by the Hind-brain or Met encephalon (d),
and the After-brain or Myelencephalon (e),
which is directly continuous with the spinal
chord, and with the metencephalon. The
metencephalon forms the most anterior por-
tion of the roof of the myelencephalon, and is
not therefore as distinct as the rest of the
cerebral vesicles. At first, the vesicles are
placed one behind the other, and lie in the line
of the longitudinal axis of the spinal chord,
but they soon come to be set at an angle to
one another. This is due to the unequal
growth of the upper and lower portions, for
the upper ones increase greatly in size. Those
parts which are least developed become covered
over by the growth of some of the upper parts.

Between the prosencephalon and thalamen-
cephalon the wall is thinned out, and a fissure-
like portion developed (primitive cerebral
cleft. Fig. 280, s), into the interior of which a
process from the envelopes of the brain is continued. This is not a
true lacuna, but is merely due to the gradual thinning-out of the
wall of this portion. The epiphysis (pineal gland) is developed
from a part of this roof.

The lower portion of the thalamencephalon forms the floor of the

Fig. 280. Vertical and
median sections through
a Vertebrate brain. A Of
a young Selachian
(Heptanchns). B Of the
embryo of an Adder.
C Of the embryo of a
Goat, a Prosencephalon.
h Thalamencephalon. c
Mesencephalon (inj.it is
marked by d). d Meten-
cephalon. e Myelence-
phalon. s Primitive cere-
bral cleft. Hypophysis.

504

COMPAEATIVE AXATO:\[Y.

second cerebral vesicle, and gives rise to a diverticulum, wbicb is
found in all Craniota, and is known as the infundibulum. From the
lower side of the head a depression of the ectoderm grows towards
this diverticulum; later on, the ingrowth becoming pinched off, forms
a portion of the cerebral appendix attached to the infundibulum
(hypophysis). The range of the position of the depression for the
hypophysis as far forward as the entrance into the cavity of the
mouth enables us to recognise in this structure an organ, which
primitively did not belong to the nervous system at all, and the
function of which is still a matter for speculation.

Just as the upper wall between the fore- and twixt-brains gets
thinned out, so too the roof of the myelencephalon is thinned out, in
such a way that no roof remains but such as is formed by the outer-
most vascular layer of the nerve-centre, the pia mater. The large
cavity which is thus roofed over forms the fourth ventricle.

The ventricles, or cavities in the portions
derived from the primary cerebral vesicles, are
connected with one another in just the same
way as the cavities of the cerebral vesicles.

The brain of the Cyclostomata is the
simplest in form; among them the lowest
grade is occupied by the Myxinoidea, where
the various segments have very nearly the
same characters.

A portion, which is developed from the
fore-brain, and which gives off the olfactory
nerves (bulbus or lobus olfactorius), generally
forms large lobes, which, in the Selachii, are
connected with the brain by a more or less
long tractus olfactorius (Fig. 281, h). The
ventricle of the prosencephalon is continued
into them. They may also be fused with the
prosencephalon, which is larger than the other
divisions, in the Selachii {g), and gives indica-
tions of a separation into two, four, or more
paired pieces. It is large also in the Ganoidei (Fig. 282, g), while in
many Teleostei it is greatly reduced in size, as compared with the
other regions of the brain.

In the Selachii the thalamencephalon is distinctly separated from
the mesencephalon (Fig. 281, d), but in many Teleostei it is intimately
connected with it. The anterior portion of its roof contains the
above-mentioned cleft, and this portion is not unfrequently de-
veloped into an elongated tract, which forms a longitudinal com-
missure between it, and the prosencephalon (many Sharks and
Ganoids). The remainder of the primitive roof, which contains the
hinder portion of the cleft, is sometimes very large, and divided
into two hemispheres; this is the case in the Selachii and many
Teleostei. The floor of this segment, which surrounds the infun-
dibulum, and forms the lobi inferiores at the base of the brain, is

Fig. 281. Brain of a
Shark (Scyllinm catu-
lus). h Olfactory lobes.
g Prosencephalon. d
Thalam- and mes-ence-
phalon. h Metencepha-
lon. a Myelencephalon.
0 Nasal capsnles (after
Busch).

BEAIN OF VERTEBEATA.

505

simple in the Cyclostomata, and in tEe SelacEii presents indications
merely of its divisions. It is not developed to any great size
except in tlie Teleostei. The succeeding mesencephalon is small in
the Myxinoids, but larger in Petromyzon. In the Selachii it ap-
pears to be united to the thalamencephalon, while the portion which
corresponds to it, in position at any rate, is regarded as the cerebellum.
This portion is very greatly developed, so much so, indeed, that it
covers over those parts of the brain that lie in front of and behind
it (Fig. 281, 5). In the Teleostei this part of the brain is pro-
portionately more developed, and sometimes has the form of a
protuberance directed forwards or upwards. The rest of the brain,
which lies behind the mesencephalon, must be regarded as one piece.
It is of great importance as being the region from which
most of the cerebral
nerves take their origin.

Its roof is unequally de-
veloped. That is to say, in
the hinder, and larger, por-
tion, it is soon atrophied, so
that the internal cavity
(sinus rhomboidalis), which is
widened out anteriorly, is
only covered over by mem-
brane. In the Selachii and
Chimaerae the edge of this
sinus is thickened anteriorly
(lobi nervi trigemini). It is
simpler in the Ganoidei and
Teleostei. But in all Fishes it
is continued mesially into a
transverse lamella (Fig. 282,
h c), which covers in the sinus
anteriorly, and has the mesen-
cephalon projecting over it,
when this portion is very
large. This transverse la-
mella appears to correspond to the cerebellum of the higher Verte-
brata, while the base and sides of the sinus are formed by the myel-
encephalon (medulla oblongata). As we pass from the Selachii
to the Teleostei we note that the medulla oblongata diminishes in

Fig. 282. Brain of Polypterus bichir.
A From above. B From the side. C From
below, h Lobi olfactorii. g Prosencephalon,
/ Thalamencephalon. d Mesencephalon, he
Metencephalon. a Myelencephalon (medulla
oblongata), ol N. olfactorius. o N. opticus
(after J. Muller) .

Size j in many

Sharks it forms the largest portion of the brain.
And this corresponds to a primitive stage, in which it and the
mesencephalon form the largest part of the whole brain.

When it is still more developed, swellings may be observed
on the sides of the fourth ventricle ; these are set in series, and
correspond to the points of origin of the roots of the vagus
(lobi nervi vagi). The lobi electrici of the Torpedines are dif-
ferentiations of this kind, which, however, unite above the narrowed
ventricle (cf. Fig. 277, iv).

506

COMPAEATIVE ANATOI^IY.

§ 381.

The brain of the Amphibia resembles that of the Fishes in many
points. The prosencephalon (Fig. 283^ h) is divided into two hemi-
spheres, and shows signs of being enlarged backwards. The cavity
within it is divided into two lateral ventricles, one for each half, and
these are continued forwards into the olfactory lobes {a). At first,
these lobes are placed at the side of the prosencephalon (5), and

are directly attached to it, but they may
become closely fused with the prosence-
phalon, and with one another. The olfactory
nerve arises on their lower surface, some way
back, and near the prosencephalon. The
thalamencephalon is differentiated during
the larval stage from a portion which is
common to it and the mesencephalon. In
front of it is the primitive cerebral cleft,
which is more or less continued into the
thalamencephalon, and carries the epiphysis
cerebri. The cleft extends anteriorly into
the lateral ventricles, which are enclosed
by the two hemispheres of the prosence-
phalon. On the lower surface of this por-
tion there is an eminence, which corresponds
to the lobi inferiores.

In the Urodela the mesencephalon re-
mains at a certain stage, which is temporarily
presented by the Anura; it is in the latter only
that it becomes of any size, and is divided
into two halves (c) . The metencephalon, how-
ever, retains its primitive form of a lamella,
which bridges over the fourth ventricle {d).

In the brain of the Eeptilia the angula-
tion which we already observed in Fishes has
been much increased in the region of the
thalam- and mes-encephalon, owing to the
increased development of the upper parts ;
this produces a change in the relative position of the parts, which
is still more marked in the higher divisions (compare the sections
in Fig. 280). The prosencephalon is more largely developed, and has
the form of two hemispheres, covering the thalamencephalon, and
broadest behind. The olfactory lobes are attached directly to them.
The lateral ventricles are very large, and communicate at the cerebral
cleft with the third ventricle, which is placed between the two
halves of the thalamencephalon, and is provided with a large in-
fundibulum. The mesencephalon is divided by a groove into two
hemispheres, which sometimes project very far forwards. The
metencephalon varies a great deal ; in the Ophidii and Saurii it

Fig. 283. Brain and spinal
chord of the Frog. A from
above, B from below.
a Lobi olfactorii. h Prosen-
cephalon. c Mesencephalon.
cZ Metencephalon. eMyelen-
cephalon. s Fourth ven-
tricle. m Spinal chord.
t Filum terminale of the
chord.

BEAIN OF YEETEBEATA.

507

remains at a low stage of development, and forms a small lamella,
wBicli is raised up, however, in a vertical direction ; in the
Chelonii (Fig. 284, A IV) and Crocodilini it is broader, and in the
latter it is distinguished by the large size of its median portion.

Fig. 284. Brain of a Chelonian (after Bojanus). B Of a Bird. Vertical median
sections. I Prosencephalon. Ill Mesencephalon. IV Metencephalon. V Myelen-
cephalon. ol Olfactory, o Optic nerve, h Hypophysis, a (in A) connection between
the two hemispheres of the mesencephalon, c Anterior commissure.

This condition connects the Eeptilia with the Aves, which are distin-
guished by the great proportionate size of their prosencephalon, the
hemispheres of which are often greatly broadened out. They are con-
nected by a fine anterior commissure (Fig. 284, B c), and enclose a gan-
glionic mass, which projects inwards from the side wall, and convert-
ing the primitive cavity into a narrow space which is covered over by
the thin-walled roof of the hemispheres, itself forms the largest part

of the prosencephalon. These
masses may be observed in as
low forms as the Amphibia, and
in the Eeptilia they are very
large (Fig. 286, A st). The small
thalamencephalon, which is
completely covered over by the
hemispheres of the prosence-
phalon, has its roof divided. The
mesencephalon, which is very
large in the embryo, is divided
into two pieces, which are pushed
down to the sides of the brain
(Fig. 285, c), and have the general
internal cavity continued into
them. The large median portion
laminated, and, owing to its siz(
myelencephalon.

Fig. 285. Brain of the Domestic Fowl.
A From above, B From below, a Bnlbi
olfactorii. b Hemispheres of the prosen-
cephalon. c Mesencephalon, d Meten-
cephalon. d' Its lateral parts, e Myel-
encephalon (after 0. G. Cams).

of the cerebellum is transversely
3, covers over the whole of the

§ 382.

In the Mammalia the brain closely resembles that of the lower
forms in its earliest stages only (cf. Fig. 280), for, owing to its special
line of differentiation, it is very different to that of Birds and

508

COMPAEATIVE ANATOMY.

Eeptiles. The most important changes are seen in the prosen-
cephalon, which has the olfactory lobes, which are as a rule still
hollow, connected with its lower surface ; these lobes are more or
less covered over by the hemispheres, according to the extent to
which they are developed. The prosencephalic hemispheres are
always separated by a fissure, which is very deep anteriorly. They
are primitively connected together by a commissure which is placed
in front of the primitive cerebral cleft ; through the cleft there is a
passage into the cavities of the prosencephalon, or lateral ventricles.
When they are more highly developed, the hinder portions of the
hemispheres increase in size; the cleft, which was at first small, is
laterally extended and disappears from the surface, being completely

Fig. 286. Differentiation of the prosencephalon. A Brain of a Chelonian, B Of
a foetal Calf, C Of a Cat. In A and B the roof of the prosencephalic ca\dty is
removed from the left side, and the fornix from the right. In C the whole of the
lateral and posterior portions of the right prosencephalic lobe are removed, and so
much on the left as is necessary to display the upward bend of the cornu ammonis.
In all the figures I marks the prosencephalon. II Thalamencephalon. Ill Mesence-
phalon. IV Metencephalon. V Myelencephalon. ol Olfactory bulb (shown in A as
communicating with the cavity of the prosencephalon), st Corpus striatum. / Fornix.
h Pes hippocampi major, s r Ehomboidal sinus, g Geniculate process.

covered over by the binder wall of tlie lateral ventricles, wbicb have
grown out at the sides and behind. With this is correlated the differ-
entiation of the primitive commissure into a system of commissures,
the lowest stage of which is represented in the Monotremata and
Marsupialia. The primitive commissure is first differentiated into
an inferior and a superior portion ; the former is the anterior com-
missure ; the latter forms a slender bridge, which is placed above the
anterior edge of the thalamencephalon, and below which there is on
either side the entrance into the lateral ventricles ; these last extend
downwards and backwards. In their anterior region the corpus
striatum forms a projection (Fig. 286, B C st), and posteriorly there
is a rounded process, which is connected with the upper portion of

• BEAIN- OF VEETEBKATA.

509

tlie system of commissures, and which forms the posterior boundary
of the margin of the cleft, which always lies above the thalamen-
cephalon (cornu ammonis sive pes hippocampi major) (G U).

The superior commissure is converted into two different, but
connected, structures. The sides of one bound the entrance into
the lateral ventricles superiorly, and it passes, at its sides and below,
into a band which is placed on the hippocampus major. This portion
(fornix) {B Of) commences anteriorly by ascending columns, is
somewhat broadened out over the thalamencephalon, and is con-
tinued into the posterior descending columns. Superiorly it is
connected with a portion of the commissural system, the corpus
callosum, which, at first continuous with it, becomes anteriorly
separated from it. The backward extension of these commis-
sures is dependent on the development of the hemispheres of the
prosencephalon, which are feebly developed in Eodents, Edentates,
and Insectivora. According as they increase in size, the anterior
commissure is diminished in area. In the Monotremata and Didel-
phia it is very much so, and is converted into a thin chord placed
in front of the columns of the fornix. The more the hemispheres
of the prosencephalon are enlarged backwards, the more do they
overlie the parts behind them.

In many Mammals the hemispheres are smooth, and the surface
of the prosencephalon is then in a simple stage, corresponding to
its embryonic characters; these may be complicated by gyri and
sulci. The gyri are at first arranged regularly and symmetrically,
and only become asymmetrical when they are more largely developed,
as they are, for instance, in Man. But even in this case they may
be divided into groups, the boundaries of which are formed by the
earliest, which are in some Mammals the only, sulci present. The
gyri are similar in character in their earliest stages only. When
they are complicated, the arrangements differ in different divisions
of the Mammalia, and serve as indications of the degree of affinity
that there is between such divisions.

The thalamencephalon is divided into two masses, which lie
immediately behind the corpora striata of the lateral ventricles of the
prosencephalon, the thalami optici. The epiphysis is placed at the
hinder end of the cleft which separates them. The cavity of this
portion is reduced to a small space between the two thalami, and is
continued downwards into the infundibulum, which is carried by the
tuber cinereum.

The mesencephalon, which for a long time forms the largest
division of the brain (Fig. 280, G c), has its primitive lumen gradually
converted into a narrow canal (aqueductus Sylvii), which unites the
third with the fourth ventricle. Its surface is divided by shallow
longitudinal and transverse grooves into four bodies (Fig. 286, B G
III), in consequence of which the corpus bigeminum becomes the
corpora quadrigemina. This division into four lobes is very slight in
the Monotremata.

The metencephalon (cerebellum) only resembles that of the

510

COMPARATIVE ANATOMY.

Fishes and Amphibia during the earlier stages of development.
The simple lamella is developed into a large body^ in which, as in
the Crocodilini and Aves, the first part to be differentiated is the
middle. In the Marsupialia, however, this forms a delicate trans-
verse commissure for some time, while the lateral parts are developed
to a greater size. Transverse lamellee are develo]3ed in both regions,
and are arranged in various groups. The median portion is always
the larger in the Monotremata, and it is large in the Marsupialia,
Edentata, and Chiroptera. It is not till we come to the Carnivora
and Ungulata that we find the lateral parts, or hemispheres of

the cerebellum,^'’ de-
veloped to a greater
size; in most Primates
they are so much the
larger, that the median
portion diminishes,
and is known as the
vermis.'’^

As the prosence-
phalon increases in
size, it gradually
covers the other di-
visions of the brain.
In many Marsupials,
and in Rodents (cf.
Fig. 287, A) and In-
sectivora., it does not
reach to the corpora
quadrigemina ; and,
in most of the other
Mammalia the meten-
cephalon is altogether
very nearly free ;

Fig. 287. Brain of the Babbit. A From above,
B From below. Zo Olfactory lobes. I Prosencephalon.
Ill Mesencephalon. IV Metencephalon. V Myelen.
cephalon. h Hypophysis. 2 Optic. 3 Oculo-motor.
5 Trigeminus. 6 Abducens. 7 8 Facial and auditory
nerves. In A the roof of the right hemisphere is
removed so that we can see into the lateral ventricle,
and make out the corpus striatum in front, and the
fornix with the commencement of the pes hippocampi
major behind.

or

in the Primates, however, this portion is altogether below the
posterior lobes of the prosencephalic hemispheres ; in this point the
anthropoid Apes most closely resemble Man. When the hemi-
spheres of the metencephalon increase in size, a transverse commis-
sure is developed on the lower surface of the primitive myelencephalon
— the pons Varolii; this appears to unite the anterior portion of the
myelencephalon more closely to the cerebellum. This commis-
sure is feebly developed in the Monotremata and Marsupialia, and
most largely in the higher Primates.

Mihalkovics, V. V., Entwickelungsgesch. des Gehirns. Leipzig, 1877.

SPINAL CHORD OF VERTEBRATA.

511

b) Spinal Chord.

§ 383.

The spinal chord, which is continuous with the medulla oblongata
is formed from the de-
velopment of the lateral
halves of the wall of the
primitive medullary tube.

As the lateral parts in-
crease in size an anterior
longitudinal fissure is de-
veloped. The primitive
lumen of the tube is con-
verted into the central
canal .

The central appara-
tus of the spinal chord
occupies the inner parts,
and forms a gray medul-
lary mass, which is seen,
in cross section, to have
the form of cornua, which

Fig. 288. Transverse section through the spinal
chord of a Calf, a Anterior, h Posterior longi-
tudinal fissure. c Central canal. d Anterior,
e Posterior cornua. / Substantia gelatinosa.
g Anterior column of the white substance, h Lateral .
i Posterior column, h Transverse commissures.

pass forwards and back-
wards (Fig. 288, de).

Owing to the distri-
bution of the central
apparatus in the central
portions of the spinal
chord, that is, in the gray columns which pass off from the region
of the central canal (c), the white medullary mass, which consists
of nerve-fibres, is chiefiy placed towards the
exterior, forming longitudinal columns, which
are partly separated from one another by the
anterior and posterior longitudinal fissures (ah),
and partly by the points at which the roots of the
nerves pass out {ghi). This arrangement of
the white matter is a peculiarity of the spinal
chord, and is also a point of difference between
it and the ventral chord of the Annulata and
Arthropoda, which is of great significance.

In the Cyclostomata the spinal chord forms a
flat band, as it does also in Chimsera ; in most it
is more cylindrical in form, and gradually
diminishes in size as it passes towards the end
of the spinal canal. There are often special
enlargements at the points where the larger
nerves are given off ; they are very strikingly

Fig. 289. Braiu and
spinal chord of Ortha-
gosiscus m.ola (after
Arsaky). B Brain and
anterior region of the
spinal chord of Trig la
adriatica (after Tiede-
mann) .

512

COMPAEATIYE ANATOMY.

developed iu some species of Trigla (cf. Fig. 289^ B), as is a smaller
number of them forming the exceptionally short spinal chord of
Orthagoriscus, etc. (A).

As the size of the spinal chord is influenced by the masses of
nerves which are given off from it, we find that in the four higher
classes of the Vertebrata the great development of their extremities
and the large size of the nerve- chords that go to them are correlated
with the increased development of the size of the spinal chord in various
regions. In this way the cervical, or thoracic, and lumbar enlarge-
ments are formed ; they are very large in some cases (e.g. Chelonii
and Aves). The primitive medullary cavity, which persists as the
central canal, remains open in the lumbar swelling of Birds, and thus
a sinus rhomboidalis is developed ; this is similar to the sinus which
is always found in the medulla oblongata. It is found for a short
time in the embryos of Mammals also (Fig. 279, a).

As a rule the spinal chord extends through the whole of the
spinal canal; but in the Amphibia (Anura) and Aves, and most
markedly in many Mammalia, it is pressed more forwards, owing to
the unequal development of the enclosing and enclosed parts, so that
the nerves which are given off from it for the hinder parts of the
body, have to run for some way in the spinal canal before they get
to their orifices of egress.

c) Investments of the Central Nervous System.

§ 384.

As the cavity of the skull is adapted to the brain, which it
encloses, this latter at first fills up the cranial cavity. The same is
true of the spinal chord and spinal canal. The surface of the whole
of the central nervous system is separated from the walls of its case,
which are formed by the skeleton, by parts which either belong to
the skeleton, or to the nervous system, or which are interstitial in
character. These form the coverings of the brain and spinal chord.

The periosteal investment of this skeletal cavity develops the
dura mater. In the lower divisions this membrane appears to bo
a mere periosteal (or perichondrial) layer ; it is not much larger till
we reach the Reptilia, where it first acquires the character of an
independent formation. In the cranial cavity of Birds it forms a
process between the hemispheres of the prosencephalon (falx
cerebri), and this is found in most Mammals also, where it is accom-
panied by a process — the tentorium cerebelli — best developed in
the higher orders, which projects between the cerebellum and the
posterior lobes of the prosencephalon. In many Mammalia (Car-
nivora, Perissodactyla, etc.) the tentorium is ossified. The spinal
portion of the dura mater is less peculiar in character. In the Mam-
malia it is separated from the periosteum as high up as the occipital
foramen, and forms a sac, which loosely envelopes the spinal chord.

PERIPHEEAL NERVOUS SYSTEM OF VERTEBRATA. 513

The pia mater^ part of the nervous system, is a layer of con-
nective tissue covering the latter, in which the blood-vessels of
the nervous centres run. It extends into the depressions between
the different portions. It sends out convoluted vessels (retia) from
the large cerebral cleft, which are connected with the roof of
the cleft ; these pass into the interior of the lateral ventricles of the
prosencephalon. It is continued over and forms a roof for the sinus
rhomboidalis of the myelencephalon, where it often forms a vascular
plexus.

The arachnoid membrane is the most variable. In those
Fishes, in which the brain fills the cranial cavity, it is a thin layer of
connective tissue which scarcely deserves the name of a membrane,
for it is as intimately connected with the pia, as with the dura mater.
When a larger space is formed between the brain and the wall of
the skull, this tissue is either converted into a network filled with
lymph (Squatina), or into gelatinous tissue (Scymnus), or it gives
rise to fat cells (many Teleostei). In the higher Vertebrata the
arachnoid is generally a delicate layer of connective tissue; in the
Mammalia it is differentiated as it is in Man.

B. Periplieral Nervous System.

§ 385.

The nerves which are given off from the central organs are
divided into spinal and cerebral nerves ; in the Acrania there is no
difference between these two kinds. In Amphioxus only one
anterior and larger trunk is remarkable for its course, and its
numerous ramifications in the anterior end of the body. It is
clearly comparable to one of the cerebral nerves of the higher
Vertebrata, but it must be here noted that, compared with the
Craniota, the whole organisation of Amphioxus is in an indifferent
condition. The other nerves of the medullary tube (with the excep-
tion of those for the nasal pit and eyes) are similar in character to
spinal nerves, and are remarkable for the fact that they are given
off alternately from the medullary chord. The similarity in the
characters of these nerves leads to the supposition that the difference
between the cerebral and spinal nerves, which is seen in the
Craniota, is a condition which was acquired by them, when the head
was developed. The nerves of Amphioxus have no ganglia, and are
formed by single roots ; this, again, is a point in which they differ
markedly from the Craniota. As Amphioxus has no head,^^ in the
sense in which we speak of it in the Craniota, we cannot suppose
that the nerves are separated into cephalic and spinal nerves. The
most we can do is to regard the nerves which belong to the region
in front of the posterior boundary of the branchial cavity, as the
indifferent equivalents of the cephalic nerves of the Craniota, and
the rest of the nerves behind these as spinal nerves.

614

COMl^AEATlVE ANATOMY.

a) Spinal Nerves.

§ 386.

The metamerism of the vertebrate bocly^ which is first seen in
the formation of primitive vertebrae, is quite as much expressed by
the characters of the spinal nerves, and by their mode of distribu-
tion. A pair of nerves corresponds to each vertebral segment.
Each nerve is formed by the union of two roots which have their
origin in the lateral halves of the spinal chord. The superior or
sensitive root forms a ganglion, before it unites with the inferior or
motor root ; and the fibres thus formed mix with those of the lower
root, and form the trunk of a spinal nerve. In the Selachii the
inferior and superior roots pass through separate foramina in the
spinal canal. As a rule, the nerves leave the spinal canal between
two arches.

Each spinal nerve divides into two chief branches — a ramus
dorsalis supplies the muscles and skin of the back, a ramus ventralis
goes to the sides, and ventral wall of the body, and gives off a ramus
visceralis to the viscera. This latter forms the connection between
the so-called sympathetic, and the cerebro- spinal nervous system.

In Fishes we always find the spinal nerves at the intermuscular
ligaments. They follow exactly the metamerism of the body, so
long as this continues to be expressed.

The size of the nerves is correlated with the development of the
parts which they supply. When the extremities are formed, their
rami ventrales become very large. A number of rami ventrales of
the anterior spinal nerves (cervical nerves) are then formed into a
plexus (plexus brachialis), from which the nerves for the anterior
extremities are given off ; in the same way, the nerves for the hinder
extremities are given off from a plexus (plexus lumbalis and p. sac-
ralis), which is formed more posteriorly, either in front of, or in the
pelvis. These plexuses change their position when the limbs do so
(cf. pp. 473, 474).

The brachial plexus of the Amphibia is formed of three or four
nerves (in the Frog, the 2nd, 3rd, and 4th spinal nerves). In the
Ileptilia the brachial plexus is generally made up by the cervical
nerves from the sixth to the ninth ; in Yaranus, from the seventh to
the tenth ; and in the Alligator the first thoracic nerve also enters
the plexus. In Birds it is formed from the last cervical and
first thoracic nerve, or from the 1 1th and 12th cervicals, or 1st and
2nd thoracic nerves. In the Mammalia, the last 3, 4, or 5 cervical
nerves, and the 1st, and sometimes also the 2nd, thoracic nerves,
contribute to form the plexus.

The nerves for the hinder extremities are given off, in the
Amphibia, from a plexus which is generally formed by three nerves.
The anterior of the nerves thus formed is the crural ; the sciatic is

CEEEBEAL NEEVES OF VEETEBEATA.

515

larger, and is made up from nearly all the brandies that pass into the
plexus ; this nerve is the chief nerve of the extremity in the higher
Vertebrata also.

The crural and sacral plexuses are more distinct from one another
in Eeptilia and Aves. In the former, four nerves generally pass into
these plexuses. In Birds there are generally seven or eight nerves
set apart, and most of these are for the sciatic nerve ; in the Mammalia,
again, it is composed of a much smaller number.

b) Cerebral Nerves.

§ 387.

The cerebral nerves, which by a descriptive anatomist are taken
in the order of their position, are seen to break up into two very
distinctly marked divisions, when examined after the comparative
method. One division, the larger, contains nerves which more or
less agree with, or might even be derived from, spinal nerves, while
the other contains those which have not the faintest resemblance to
spinal nerves.

This latter division contains two specific sensory nerves, the
olfactory and the optic.

The olfactory is formed of a complex of nerve-filaments, which
arise from the olfactory lobe, and are distributed in the olfactory
mucous membrane. According as the olfactory lobe is near, or far
from this membrane, these nerves form a trunk on either side (as in
many Fishes, and in the Amphibia, Eeptilia, Aves, and Monotreme
Mammals), or leave the cranial cavity separately, by passing through
a lamina cribrosa^^ (Selachii and Mammalia).

The optic nerve, which arises from the thalam- and mes-ence-
phalon, is formed, as is part of the eye, from a vesicle (optic vesicle),
which is developed from the primitive prosencephalon ; it forms the
stalk of this vesicle. After the differentiation of the vesicle of the fore-
brain, it is connected with the thalam- and mes-encephalon. In the
Cyclostomata the optic nerve of each side passes to the eye on the
same side ; the nerves of either side do not exhibit any union with one
another excepting close to their point of origin. But in the Gnatho-
stomata a larger portion of the optic nerve is prominent on the base of
the brain, and here the fibres exhibit a crossing over from one side to
the other (chiasma). The fibres which pass to this point constitute
the optic tract, and form a portion of the brain, which in the Cyclo-
stomata has not attained to the surface. The chiasma, therefore, is
not a new formation, but a differentiation. In the Osseous Fishes,
the fibres cross completely ; the optic nerve of the right eye passes
to the left, and that of the left to the right, each passing above or
below the other. More rarely, one optic nerve perforates the other
(Clupea), or the different bundles of nerves pass through separately.

2 L 2

51G

COMPAEATIVE ANATOMY.

Ill the Selachii and Gano’idei, some only cross over, and this is the
arrangement which is seen in all the higher Vertehrata.

Neither of these sensory nerves have any character in which
they resemble spinal nerves, nor can they be referred to metameres.
They belong to that portion of the cranium which is not formed by
the concrescence of vertebral segments (cf. § 340) ; and they may
correspond to those nerves which we found passing to the same
organs in the Invertebrata.

§ 388.

The second division consists of the nerves which are arranged
on the type of the spinal nerves. It is sometimes possible to dis-
tinguish two roots : their dorsal branch is often very feebly developed,
in correlation with the small size of the area to which it is dis-
tributed. The ventral branch is consequently the important one ; it
sends nerves to the branchial arches, and the parts derived from them.
The visceral branch goes to the wall of the pharynx. The nerves
of this division arise at the base of the fourth ventricle, and partly,
also, from its continuation in the aqueductus Sylvii. They are
given off from the myelencephalon, and leave the cranial cavity by
perforating the vertebral portion of the cranium (§ 340). While these
relations are best seen in the cephalic nerves of the Selachii, which
most resemble the primitive condition, they undergo greater changes
the farther the organism has risen above this low stage, or has been
differentiated in another direction.

We may note a number of special characters in the various
nerves, proceeding on the supposition that the series are homody-
namous with spinal nerves. Some branches of a nerve become much
larger than others, which undergo degeneration ; or the roots of a
nerve take an independent course, and give the appearance of
independent nerves. In this way a nerve is broken up ; but other
nerves may undergo concrescence, so that what were primitively
complexes of nerves acquire the form of a single nerve.

This latter arrangement is seen in two groups of the cerebral
nerves, which I have distinguished as the trigeminal and as
the vagus group; the names of which are taken from that of the
most important nerve in each group.

§ 389.

The trigeminal group supplies the anterior, and larger
portion, of the head. The following nerves belong to it :

First, the trigeminus, the largest nerve of the group, which, in
correspondence with the great diforentiation of the regions to which
it is sent, has the characters of a greatly developed spinal nerve
(Fig. 290, Tr), Its dorsal branch is formed by the ophthalmic, which
supplies the orbits and the ethmoidal region. A branch for the cranial
cavity, which is found in the Teleostei, also belongs to the dorsal

CEEEBEAL NEEVES OF YEETEBEATA.

517

brancli. The ramus maxillaris superior always runs on the floor of
the orbit^ and gives off sensory branches to the maxillary region. Its
infraorbital branch is the largest^ especially in the Mammalia. The
E. max. sup., with the ramus maxillaris inferior, represents a ventral
trunk j in the Selachii this is very clearly the nerve of the mandi-
bular arch, and appears as the largest portion of the trigeminus. It
is distributed to the muscles of the jaw, to the integument, and to a
large part of the mucous membrane of the mouth (ramus lingualis).
The intestinal trunk is represented by a palatine trunk of the second
branch, which, in Fishes, passes directly to the palate, but in the
higher Yertebrata only reaches the palate by means of its connection
with a sympathetic ganglion (sphenopalatine ganglion).

The nerves of the optic muscles — the oculo-motorius and tro-
chlearis — in origin and distribution, belong to the trigeminus, and
seem to be parts which have been separated off from it. Although
the statements that the nerves for the optic muscles are given off from
the ophthalmic branch of the trigeminus in Lepidosteus and Lepi-
dosiren, and that in the Salamandrina the trochlear is replaced by a
branch of the same nerve, require confirmation, and whilst what really
has occurred in these cases is, perhaps, that the nerves of the optic
muscles have become united with the trigeminus, and not that they
are completely wanting, yet the supposition that processes of segre-
gation have been the cause of these nerves arising separately from
the myelencephalon in the other Yertebrata, cannot be rejected.
The fact that the ramus inferior of the trigeminus has no motor
elements, and that the muscles in its area are supplied by apparently
independent nerves, will always be of great importance.

The second nerve of the trigeminal group is the facialis and
acusticus. The latter seems to be homologous with a purely sensory
dorsal branch of a spinal nerve ; its terminal area has been carried
below that plane in which we must suppose it to have primitively
ramified — that is, below the surface of the body ; and this change
has accompanied the removal of the vesicles of the labyrinth from
the integument, and their passage into the interior of the wall of the
skull (cf. infra, auditory organ). This points to a dorsal branch
having primitively passed upwards through the wall of the skull, and
agrees with the course of the dorsal branches of the other cephalic
nerves, and with the ophthalmic branch of the trigeminal.

The facialis (Fig. 290, Fa) has the arrangement of a ramus
ventralis belonging to the hyoid arch. It supplies the integumen-
tary as well as the muscular portions of this segment, and is, there-
fore, primitively a mixed nerve. In the Teleostei it enters into con-
nection with the trigeminal; in many Sharks, also, it is fused
with it. So, also, in the anourous Amphibia it is united to the tri-
geminal. This union, however, is effected during their ontogenetic
development. In the Urodela, as in the higher Yertebrata, it is
always distinct, and in the Mammalia its sensory elements are
apparently absent. In these latter its area of distribution is very
great, owing to the increase in size of the facial musculature ; while

518

COMPAEATIYE ANATOMY.

its stapedial, digastric_, and styloliyoid, as also its auricular brandies,
belong to tbe primitive byoidean area. Its visceral branch appears
to be the palatine of Fishes ; in the Mammalia this is represented
by the petrosus superficialis major, and passes to the muscles of the
velum palati through the spheno-palatine ganglion. The chorda
tympani forms a connecting twig between the facial and the third
branch of the trigeminal; this is found even in Fishes.

One of the nerves for the optic muscles, the abducens, must also
be regarded as part of the facial nerve, as is clear from its area of
origin. It supplies, as a rule, the rectus externus, and in Petromyzon
the rectus inferior also. The position of the rectus externus explains
how it is that it belongs to a different nervous area from that of the
other optic muscles.

§ 390.

The first nerve of the vagus group, the glossopharyngeal, is
the simplest in character. In the Selachii it is distinct, as it is also
in the greater number of the Teleostei; in Chimoera, however, it
leaves the cranial cavity in company with the vagus, with which
nerve it is united in the Cyclostomata and in Lepidosiren. It has
the same relations in the Amphibia, but in the Amniota it is
ordinarily distinct.

In Fishes (many Sharks) it has a dorsal branch, which takes an
upward course within the cranium, and there ramifies on the surface.
The chief trunk (Fig. 290, Gj)) is the ventral one; this is distri-
buted all along the first branchial arch ; it gives off a pharyngeal
branch to the wall of the pharynx, which represents its visceral
branch. AYhen the first branchial arch is metamorphosed, this
arrangement is so far modified that the pharyngeal branch and
the lingual, which ends in the mucous membrane of the tongue, form
the chief portion of the nerve.

The vagus is closely attached to the glossopharyngeal at the
point where it leaves the myelencephalon ; to fully understand this
nerve it is necessary to have a knowledge of its simplest characters,
as they are best seen in Sharks (cf . Fig. 290) . The vagus is here made
up of a large number of separate roots, which have their origin in
the myelencephalon, nearly as far back as the fourth ventricle ; the
anterior ones, which have their origin just behind the glossopharyn-
geal, are the larger. The posterior ones become gradually smaller
and smaller than those in front of them. The last of the series are
collected into a small trunk, which passes forwards, and is attached
to one of the more anterior ones. The common trunk thus formed
passes through the wall of the cranium obliquely backwards and
outwards; on its course it gives off a small dorsal branch for the
occipital region.

When it has passed out of the cranium the vagus trunk gives off
a number of branchial branches, in correspondence with the number

CEEEBEAL NEEVES OE VEETEBEATA. 519

of brancliial arches (Fig. 290). The first branchial branch (5/)
passes to the second branchial arch^ and also gives off a fine twig to
the first arch. In this point the branchial branches of the vagus

Fig. 290. Cephalic nerves of Hexanchus griseus. On the right is figured as
much of the course of all the cephalic nerves as can be seen from above. The cranial
cavity and spinal canal are both laid open, so that the brain and spinal chord are
exposed. The right eye and its muscles is removed. On the left, the roof, only, of
the orbit is removed, so that the bulb and its muscles are exposed to view. The
region of the labyrinth, and of the occipital portion of the cranium, has been removed
as far as the levels of the nerve-trunks, which pass out through them. A Anterior
fontanelle (cranial lacuna). iV Nasal capsule, I?o Bulbus olfactorius. T/ First branch
of the trigeminal. a Its terminal twig in the ethmoidal region. Tr" Second
branch. Tr'" Third branch, tr Trochlearis, Fa Facialis. Op Glossopharyngeus.
Vg Vagus. L Eamus lateralis. J Eamus intestinalis. os Muse, obliq. oc. sup.
ri M. rectus internus. re M. rectus externus. rs M. rectus superior. S Spiracular
cleft. Pq Falato-quadrate. Hm Hyomandibular. r Branchial rays. 1 — 6 Branchial
arches, hr^ — Branchiae.

resemble tbe glossopharyngeal and the facial^ which likewise send
off fine twigs to the arches next in front of them. A pharyngeal
branch is given off at the point where this branchial branch divides.
The vagus trunk is continued on, as an intestinal branch {g), to

520

COMPAEATIVE ANATOMY.

tlie alimentary canal ; it ramifies on tlie pliarynx and stomaclij and
also gives off brandies to tbe heart. Before tlie vagus trunk gives
off the branchial branches, it sends off a large dorsal branch, which
passes backwards in the dorsal region ; this is the lateral branch (A),
which ramifies in the skin alongside the lateral line of the body,
and extends as far as the tail.

While the nerve-roots, that make up the vagus trunk, leave the
myelencephalon together, there are other roots, which belong to
the vagus, and which arise from the myelencephalon, underneath those
already mentioned; of these there are, at most, five, and, generally,
only two or three filaments, each of which reaches the exterior by a
special canal in the wall of the cranium. Some pass to the muscles,
and some are connected with the first of the spinal nerves ; they
may be called the inferior roots of the vagus, while those before
mentioned are the superior roots. The foramina of the lower roots
lie in the same row as the foramina of the inferior roots
of the spinal nerves; the foramen for the complex of superior
roots is placed higher up, and is in a line with the foramina of
the superior roots of the spinal nerves.

§391.

From the description that we have just given, it is clear that the
whole vagus must be regarded as a complex of a large
number of nerves, which are homodynamous with spinal
nerves. This is indicated by the various inferior roots, which
pass out separately, but much more emphatically by the distri-
bution of the trunk, formed by the superior roots. While each
branchial branch of the vagus has exactly the same characters
as a ventral branch of a spinal nerve; and while, moreover,
the branchial arches supplied by it must be regarded as arches,
which were primitively part of the cranium (§ 340) ; and while,
lastly, each of the other arches (mandibular, hyoid, and first bran-
chial) is innervated by a nerve, in just the same way as is a meta-
raere of the trunk by a spinal nerve ; the series of the superior roots
of the vagus are, on their side, the distinct equivalent of a number
of separate nerves, the sum total of which must correspond to the
maximum number, at any rate, of the arches supplied by them.
This view of the real character of the vagus, advanced by me, is shown
to be the correct one by the developmental history of these nerves,
which has lately been made out in the Sharks. As there is reason
for supposing that, even in the Selachii themselves, there has been a
great reduction in the primitive number of the gills, inasmuch as a
process of this kind, although, indeed, affecting a small number of
gills only, can be observed within the limits of the group, it follows
that the extension of the vagus on to a portion of the alimentary tube
is explicable as due, not so much to an incursion of the nerve into
a territory which was not originally within the area of its distribution,
as to the modification of a region, which formerly did carry branchial

CEEEBKAL NEEVES OF YEETEBEATA.

521

clefts in its \valls_, and belonged to the pharynx^ into a portion of the
intestinal tract, serving exclusively for the reception of food. Nor
is there anything remarkable in the existence of cardiac branches,
when the fact is recognised that the heart was derived from a region
which was partly within the area supplied by the vagus.

The ramus lateralis appears to be a sensory branch of the vagus,
which was some time in being developed proportionately to the
growth of the sensory apparatus of the lateral line, which is inner-
vated by it.

Taking therefore the vagus as a whole, we see that in it, just
as — though to a less extent — in other nerves (e.g. the facial
and trigeminal of the Amphibia), a number of nerves are united
together; and these nerves show both by their origin and by
their peripheral relations that they are the remnants of nerves
which were primitively distinct; our view, therefore, of the
characters of the vagus agrees very closely with the in-
dications given us by the posterior portion of the cranium.

The phaenomenon of the concrescence of separate nerves is
carried still farther in the vagus of the Selachii, and destroys all
signs of individuality, for in most of them (all the Eays) the
separate roots come nearer to one another ; this arrangement is the
dominant one in all other Fishes also.

In the Teleostei some of the relations of the vagus are much
changed. A few filaments of the hinder roots fuse with an inferior
root, and form a large nerve, which passes out from the cranium, and
seems to go to the muscles of the shoulder- girdle. The relations of
this nerve require more accurate investigation.

In other points the peripheral relations of the vagus are the same
as those which we have described. A dorsal branch, which is present
in some of the Teleostei, must be more particularly mentioned. It is
united to a dorsal branch (R. recurrens) of the trigeminal, and goes to
the base of the dorsal fin, receiving connecting branches from some
spinal nerves as it does so.

§ 392.

In the Amphibia the vagus has the same relations as in Fishes,
so long as the gills are present ; it even gives off a lateral branch,
which in the Caducibranchiata has the same fate as the branchial
branches, when the gills are atrophied.

In the Amniota the only portion of the vagus which is present is
the part which is developed from the anterior portion* of the series
of superior roots of the Selachian vagus ; the trunk, which they form,
is only distributed as far as the stomach in the intestinal tract, while,
owing to the absence of gills, the branchial branches have dis-
appeared, or — and this is probably more correct — have been partly
converted into pharyngeal branches. In Fishes the air-bladder,
which is differentiated from the enteric tube, receives branches from
the vagus, and so also the respiratory apparatus of the Amphibia
and of the Amniota, which has a similar origin, also receives branches

522

CO:\IPAEATIVE ANATOMY.

from the vagus ; when a larynx and laryngeal muscles are developed,
some of these nerves give rise to constant branches. The cardiac
relations of the vagus are retained, and, as the intestinal terminal
area of the vagus becomes gradually removed from the region of the
head, this branch is converted into a long nerve-trunk.

The hinder portion of the roots which belong to the vagus in the
Selachii are, in the Amniota, united into a small neiwe-trunk — the
accessorius Willisii — which is partly connected with the vagus, and is
partly distributed to the muscles of the shoulder-girdle. The root-
fibres which form the nerves arise from parts which are placed much
farther back than the medulla, especially in the Mammalia, where
they are found between the origins of the upper and lower roots of
the spinal nerves ; in Man, indeed, they extend as far back as the
sixth or seventh pair.

Lastly, the inferior roots of the area of the vagus form a special
nerve-trunk in the Amniota ; this is the hypoglossus, which supplies
the muscles of the tongue. It retains its primitive characters in
so far as that it is made up of several root-fibres, which moreover
pass out separately from the skull, and are arranged in pairs in
the Mammalia.

The posterior complex of nerves given off from the myelence-
phalon is therefore the most variable in character. Probably derived
from as many separate nerves as there were primitive branchial arches,
it is found in its most indifferent condition in the Selachii ; it sepa-
rates off a hinder portion in the Teleostei, which forms a special nerve;
and in the Amniota forms three different nerves — vagus, accessorius,
and hypoglossus.

Gegenbaur, C., Ueber die Kopfnerven von Hexanchus und ihr Yerbaltniss zur
Wirbeltbeorie des Schadels. Jen. Zeitschr. Bd. YI.

c) Visceral Nervous System.

§ 393.

After the visceral branches are given off from the cerebro-spinal
nerves, they become connected together, each uniting itself to the one
next behind it ; they thus form a commissure, which runs along the
vertebral column, and is continued to the basis cranii; this is the
subvertebral chord of the visceral nervous system, or sympathetic.
There are ganglia at the points where the rami viscerales of the
cerebro-spinal nerves are connected with the chord; the ganglia of
the sympathetic chord. From these ganglia nerves, which are
made up of fibres of the sympathetic and of cerebro-spinal fibres,
pass to their proper arese of distribution. The various nerves, whether
passing directly to the viscera, or first traversing the sympathetic
chord, are generally collected into trunks set apart for the chief
divisions of the viscera, and are known as cardiac, splanchnic, or

SENSOEY OEGANS .OF YEETEBEATA.

523

other nerves. They form plexuses^ which contain a large number of
ganglia, and there are also separate ganglion cells, in large numbers,
on the course of the sympathetic nerves.

These plexuses are distributed on the enteric tube, and on all the
organs derived from it, as also on the vascular system, and urino-
genitary organs.

This portion of the nervous system appears to be absent in the
Acrania, and among the Cyclostomata it is not present in the Myxi-
noidea, where the vagus seems to supply the enteric area of the sym-
pathetic. From the Fishes onwards this system is always present,
although much modified. The fibres of the sympathetic appear to
be elements, which permanently retain a lower grade of development,
just as do the fibres of the cerebro-spinal nerves of the Cyclostomata.

Sensory Organs.

§ 394.

All the sensory organs of the Vertebrata are formed from differen-
tiations of the integument. The kind of share which is taken by the
integument differs with the quality of the organ. As in the Inverte-
brata, so here we can separate the sensory organs
into those which preside over a specific sen-
sation, or higher sensory organs, and those
which appear to serve for the various percep-
tions, which are indifferent in character and
may be regarded as belonging to the sense of
touch.

There are many organs which cannot be
reckoned among the known specific sensory
organs, but which are remarkable for the high
grade of their differentiation, so that their ar-
rangements do not allow us to regard them as
mere ^Gactile organs;^^ these seem to justify us
in assuming the existence of specific sensory
organs, other than those known to us familiarly.

Organs of this kind are most varied in
character among Fishes, and as many of these
arrangements are repeated in the Amphibia it
is probable that they are correlated with an rig. 291. Goblet-shaped
aquatic life. The most important organs of organs from the mucous
this kind are the following : membrane of the palate

1. Goblet-shaped organs. Large struc- trunk, b Goblet (after
tures embedded in the epidermal layer, and F. E. Schulze),
surrounded by long spindle-shaped cells ; they

enclose rod-shaped end-organs of nerves ; they have been observed
in the skin of the Teleostei and of the Sturgeon, and appear to

524

COT^rPAEATIVE ANATOMY.

be found in tlie Amphibia also. They are also found on the head
of Eeptiles.

2 . M n c o n s c a n al s . A system of tubes ■which branches regularly
in the head of Fishes, runs in the corium, and opens to the exterior
by branched canals, and at definite points. Near its opening the
tube contains the end-organ of a nerve-twig. A similar canal
extends from the head, along the side of the body, as far as the tail.
In the Ganoidei and Teleostei the nerve-endings of the system of
tubes are protected, both in the head and along this lateral line,
by an apparatus formed by the dermal skeleton, being embedded
either in modified scales, or being embedded in passages in the larger
covering bones of the head. The presence of goblet-shaped organs,
or similar structures along the lateral lines of the Amphibia (larval
forms and Perennibranchiata), points to a connection between these
organs and the mucous canals of Fishes.

3. Gelatinous tubes. Thin- walled tubes of varying length,

filled with a gelatinous substance, open by fine pores, and carry at
the opposite end nerve-endings, which are placed in an ampulla-
like enlargement of varied form. These organs are found in
great quantity on the head of the Selachii, where they are generally
placed near the rostrum, but they are also found on more distant
parts ; thus, for example, in the Rays they extend as far as the
pectoral fins (Fig. 277, ^).

In the higher Vertebrata the nerve-endings in the integument are
less complicated ; as, for example, the corpuscula tactus in the pap ill te
of the cutis, which are observed from the Amphibia onwards.

Owing to the modifications undergone by different parts of the
body in consequence of the development in their integument of end-
organs of sensory nerves, special apparatuses are developed, which
function as tactile organs. The various arrangements of this kind
are wonderfully different, but as they are structures which owe their
origin to special adaptations, we can only mention them briefly. In
Fishes these organs are frequently represented by the beards
around the mouth, in which there are a number of goblet-shaped
organs. These are found in the Sturgeons, and many of the Cypri-
noids. In the Triglidge certain rays, which are separated off
from the thoracic fin, and are well supplied with nerves, function
principally as tactile organs. In Birds the sense of touch is not
unfrequently found in the soft tip of the beak ; this is the case in the
Rails, Ducks, etc. In the Mammalia, again, we find as tactile organs
stiff setiform hairs placed on the upper lip, or even above the eye ;
these hairs are not only greatly elongated, but are distinguished
from other hairs by the lai"ge supply of nerves to their follicles.
Finally, in many Mammals the limbs themselves, owing to the
rich supply of nerves on their volar and plantar surfaces, and to the
power of movement possessed by their terminal joints, have similar
functions.

Leydig, Ueber Organe eines secbsten Sinnes. N. A. Acad. Leop. Carol. Yol. XXXIV.

— JoBERT, Les organes du toucher. Ann. sc. nat. Ser. V. Tom. XVI.

OLFACTOEY OEGANS OF VEETEBEATA.

525

§ 395.

As it is more and more impossible to form any opinion about tlie
sense of taste, the farther the organism examined is removed
from Man, it is impossible to speak with much certainty about the
gustatory organs of most of the Vertebrata. We can only there-
fore regard the end-organs of nerves, which are placed in the
mucous membrane of the mouth, as belonging in a general way
to this apparatus. In Fishes, these end- organs have no specific
character; they resemble rather the goblet-shaped organs which
are scattered in the external integument ; this is easily understood
when we reflect on what is the origin of the buccal cavity. Those
which are found in the palatine region are the most exactly known
(cf. Fig. 291) ; in the Cyprinoids the mucous membrane of this
region is interwoven with a large number of muscular fibres. In
the Amphibia the tongue appears to be the principal seat of these
structures, which are also known as gustatory goblet-cells;^^
although these cells are not ordinarily found on the tongue of
Eeptiles and Birds, the tongue of the Mammalia is provided with
them, and they are placed on the sides of the papillae circumvallatae.

Olfactory Organs.

§ 396.

In all Vertebrata the olfactory organs appear as shallow pits,
placed on the head, and enabled to receive excitations from the
surrounding medium by means of the rod-shaped end-organs of the
olfactory nerve. The sensory organ is therefore represented by a
differentiated portion of the integument. Although we cannot
certainly say that these structures have in aquatic forms — Pisces and
Amphibia — the same function as they certainly have in the air-
breathing forms, yet we are perfectly justified in giving them the
same name at any rate, for we see that they pass, and that in a
continuous series, into the more complicated organs of the higher
Vertebrata, which undoubtedly do serve as olfactory organs.

In the Leptocardii the olfactory pit is unpaired, as it is also in
the Cyclostomata, where, however, it is converted into a deeper tube
(Fig. 239, g‘)j which ends blindly in Petromyzon (yr), but in the
Myxionoidea has the form of a canal which passes through the palate ;
the walls of this canal are supported by a tube of cartilaginous rings.
In the Grnathostomata there are paired olfactory pits. In Fishes
they remain mere pits, or are but slightly deepened. In the Selachii
two processes project from the margin of a nasal orifice towards
one another, and divide the primitively simple orifice into an afferent
and an efferent orifice. In the Osseous Fishes this arrangement is

52G

COMPAEATIYE AJ^ATOMY.

carried still fartlier, for a bridge of integument is drawn over tlie
pit, and the two separate openings are sometimes widely separated.
Both orifices — but most commonly the anterior ones — may project
forwards in the form of tubes. The investing mucous membrane
sometimes forms radial, sometimes parallel folds, by which its
surface may be considerably increased. The whole surface is supplied
with terminal branches of the olfactory nerve. , There is another
kind of modification, in which the surface is so much increased
towards the exterior, by the extension of the olfactory mucous
membrane over a papilliform j:)rocess, that the pit-like appearance
is altogether destroyed.

In many Selachii, and in the Chimserge the olfactory pit is
connected with the mouth, owing to the formation of a groove
(nasal groove), which passes from the pit to the angle of the
mouth (Fig. 292). The groove is frequently covered over by a

median dermal fold, and is not un-
frequently converted into a deeper
canal (Bays). In this arrangement
we may observe a step towards
that which obtains in other Ver-
tebrata, where the olfactory pits
are only superficial in- position
during an early period of em-
bryonic life. In them the arrange-
ment which is permanent in Fishes
disappears, and the processes which
go on during further development
cause the nasal pits to sink below
the surface. This is effected by the
great growth of the parts which limit
the pit in front, at the sides, and
in the middle line ; by the growth
of the edges of the groove towards one another a canal is developed
which leads from the pit to the primitive cavity of the mouth—
that is, from without, inwards ; this canal opens behind the margin
of the jaws.

We find this arrangement in the Dipnoi and Amphibia. The
internal opening of the nasal canal is placed, in the former, as also
in the Perennibranchiata, just within the soft margin of the mouth'.
In the Salamandrina and Anura it is bounded by firm parts of the
jaw-skeleton.

When a nasal canal is formed, the primitive nasal pit itself is
removed from the surface. The surface of the pit then becomes com-
plicated by processes from the ethmoidal cartilage (turbinate bones).
In the Amniota there are yet further modifications ; owing to these,
the upper part of the primitive buccal cavity is converted into a space
which takes in the nasal pit, and in its upper portion the olfactory
membrane is spread out. The primitive olfactory pit cannot,
therefore, be any longer distinguished as a separate organ ; and it

Fig. 292. Inferior surface of tlie Lead
of Scyllium. Mouth, o Entrance
to the nasal pit. n Nasal valve in its
natural position. n' Nasal valve
turned back, r Nasal groove. The
dots represent the orifices of the
mucous canals.

VISUAL ORGANS OF VERTEBRATA.

527

will be best, consequently, to consider this
nasal cavity, wben we come to treat of
the mouth. Glands are differentiated
in the mucous membrane of the nasal
cavity, which are of a relatively large
size in the Amphibia, although not absent
in the Mammalia. Connected with the
process by which the primitive nasal pit
is carried far inwards, is the development
of an organ, which appears to be a part
separated off from the nasal pit. This is
the organ of Jacobson. It forms a
tube which lies at the base of the nasal
cavity, and ends blindly behind (Fig. 293,
J) ] the olfactory fibres in its walls are
provided with end-organs. These organs
are found in Reptiles and Mammals, and
open into the buccal cavity by the ducts
of Stenson.

new arrangement of the

Fig. 293. Section through the
nasal cavity and Jacobson’s
organ, sn Septum (after J. v
Lacerta).

Visual Organs.

§ 397.

The eye in the Vertebrata appears to have essentially the same
structure as in the more highly developed groups of lower
animals ; but the ontogeny of the organ shows that it belongs to
another type, and this is also obvious from its minute structure.
We cannot, therefore, connect it directly with the relatively well-
developed stages of the eye in other animal phyla ; the only indi-
cations of any connection are to be seen in the Tunicata. In the
larvrn of the Ascidias, as in Vertebrata, the eye is not directly de-
veloped from the ectoderm, but from the anterior portion of the central
nervous system. What is known as the eye in Amphioxus is of a
much lower grade ,* it is a spot of pigment which varies in character,
and is attached to the anterior end of the central nervous system.

The central nervous system chiefly, and secondarily the integu-
ment, are concerned in the constitution of the Vertebrate eye. The
former gives rise to the apparatus which perceives, the latter to the
apparatus which refracts, the light. The earliest rudiment of the eye
is a diverticulum, which is developed from the sides of the prosen-
cephalon (Fig. 294, A a)j and which has the form of a vesicle, connected
by a stalk {h) with the rudiments of the brain (c). The ^‘'primitive
optic vesicle ” lies below the ectoderm ; the ectoderm next gives rise
to a thickening (J5), which pushes in the anterior wall of the vesicle
towards the posterior one. Below this thickening a process of the
mesoderm grows towards the optic vesicle, and puts the side-walls
also of that vesicle in continuity with the epidermic thickening.
The effect of these processes is to bring the anterior and posterior

528

COMPAEATIVE ANATOMY.

Fig. 294. A Vertical section through the rudimeutary
head of a Fish. c Brain, a Primitive optic vesicle.
h Its stalk, d Integumentary layer. B Formation of the
secondary optic vesicle, p Outer, r Inner layer of the
primitive optic vesicle, e Epidermis pushing the lens (1)
into the secondary optic vesicle. The vitreous body is seen
behind (after S. Schenk).

walls of tlie primitive optic vesicle close to one another ; the whole

then forms the se-
condary optic ves-
icle, and is cup-
shaped; the mouth
of the cup is
filled by the ec-
todermal thicken-
ino*. This latter
forms the rudiment
of the lens {!).
While the stalk of
the primary vesicle
is being converted
into the optic nerve,
the tissue behind,
which is enclosed
by these parts, is
converted into a
substance, which gradually fills up the greater part of the secondary
optic vesicle, and forms the vitreous body. The innermost layer of
tissue around the secondary optic vesicle is converted into a vas-
cular membrane, the choroid, while a firm fibrous layer outside it
forms the sclerotic, and invests the secondary optic vesicle ; this
grows out anteriorly as far as the connection between the lens and
the ectoderm. As a result of the extension of
this process, the lens becomes cut off,andatrans-
parent portion of the sclerotic now intercalated
in front of it forms the cornea, which at the
same time becomes connected with the rudi-
mentary piece of integument (conjunctiva)
which lays in front of the lens.

The eye then is a rounded capsule (bulbus
oculi), the investment of which (sclerotic) ex-
tends over the optic nerve, and is thence con-
tinued into the dura mater, while anteriorly
it is continued into the cornea. Within this
capsule is the secondary optic vesicle, which
is developed from the invaginated primary
one, and which is separated from the sclerotic
by the choroid. The secondary vesicle, in
which there is a lateral cleft owing to the
ingrowth of the ^Witreous body,^^ embraces the lens anteriorly.
These two, layers {a h), which pass into one another at this
anterior margin, and at the lateral fissure (Fig. 295, 6*), are not
similarly differentiated ; the inner one (5), which is greatly thick-
ened at a very early stage, has its hinder portion converted into
the retina, while the outer, and thin one (a), forms the tapetum
nigrum. When the tapetum nigrum becomes pigmented, a pale

Section
through the secondary
optic vesicle of a Fish’s
embryo, taken vertically
to the “choroidal fis-
sure s.” a Outer, h Inner
lamella of the optic
vesicle, c Vitreous body.
d Lens (after S. Schenk).

VISUAL OEGAU OF VERTEBRATA.

529

line can be made out on tbe lower and inner side of tbe rudimentary
bulb ; tbis extends from tbe optic nerve to tbe free anterior edge
of tbe cboroid. It corresponds to tbe fissure (choroidal fissure)
winch was formed when the rudiment of tbe vitreous body grew
into tbe secondary optic vesicle (s), and which must therefore affect
tbe retina and tbe pigmented layer of the choroid (tapetum nigrum).

A large number of changes subsequently affect tbis rudiment of
tbe eye. Tbe anterior edge of tbe secondary optic vesicle grows
out, together with tbe tissue that forms the rudimentary cboroid,
and gives rise to tbe iris, which bounds tbe pupil. When tbe pro-
cess of the cutis pushes its way into tbe secondary optic vesicle,
blood-vessels pass (in tbe Mammalia) into tbe cavity ; these are dis-
tributed in tbe periphery of tbe rudiment of the vitreous body, so
that they must have a large share in tbe nutrition and growth of
this structure. Tbe lens, also, of tbe Mammalia, is invested by a
vascular capsule of connective tissue, which disappears again before
birth ; in some, however, it does not disappear so early.

Muller, W., Die Stammesentwickelung des Aiiges der Wirbelthiere. Leipzig,
18V5. — Kessler, L., Zur Entwickelung des Auges der Wirbelthiere.
Leipzig, 1877.

§ 398.

As to tbe form of tbe bulb, its anterior segment is much flat-
tened in Fishes (Fig. 296). The aquatic Amphibia have the bulb
flattened anteriorly ; the Ophidii and Crocodilini, among the Reptilia,
are characterised by a more considerable curvature of the cornea.

In most Birds (Fig. 298) the bulb is divided into an anterior and
a posterior segment; the former carries the very convex cornea.
This form of eye is most marked in the Raptores, but the cornea is
flattened in the Natatores and Grallatores. Among Mammals, also,
the spherical form of bulb may undergo great variation in form.

The Sclerotic may be formed of various kinds of connective
substances ; in fact, it may be made up of connective tissue, of bony
parts, or of cartilage. This latter is found in the Selachii, Chimaerae,

Fig. 296. Eye of Esox
lucius. Ilorizontal sec-
tion, c Cornea, p Pro-
cessus falciformis. s' s' Os-
sifications in the sclerotic.

Fig. 297. Eye of Moni-
tor. Horizontal section,
c Cornea. p Processus
falciformis.

Fig. 298. Eye of Falco
chrysaetos. Horizontal
section, p Pecten (after
W. Sommering).

and Gano'idei, and alsoGn the Amphibia,
the most varied in the Osseous Fishes.

These arrangements are
2 M

530

COMPARATIVE ANATOMY.

lu the Saurii, Clielouii, and Aves, the anterior ^Dortion of the
sclerotic, which abuts on the cornea, is sup23orted (Fig. 296, 6*') by a

circlet of flat pieces of bone (sclerotic ring).
In all Mammals except the Monotremata
the sclerotic is formed of connective tissue;
it is very thick in the Cetacea (Fig. 299, s).

The choroid is made up .of several
layers, which, as a rule, have the same
characters as in Man. Anteriorly it gives
rise to the folded ciliary processes ; these
are feebly develop) ed in the Selachii and
Ganoidei (Sturio), and absent in most of
the Teleostei ; the choroid is then continued
on as the iris, which bounds by its inner
margin the 2^upil, which varies in form.

The ta}3etum lucidum is a special modi-
flcation of the choroid; this forms a spot of varying size, which
is generally greenish or bluish in colour, and has a metalhc lustre ;
it is sometimes produced by groups of spicular crystals placed in the
cells of the tapetum (Selachii), or by a flbrous tissue (Carnivorous
Mammals and Ruminants). It is owing to its presence that the eye
can be seen in the dark.

Avascular plexus, which lies outside the choroid of Fishes, forms
the so-called choroid gland. In the anterior portion of the choroid
there is a muscular layer, which forms the ring known as the ciliary
ligament. The musculature is continued hence into the iris, in
which there are radial and circular flbres. In Fishes, Amphibia,
and Mammals, this musculature is composed of smooth fibres ; in
Reptiles and Birds, of transversely striated ones.

The retina, which is placed on the choroid, extends forwards as
far as the commencement of the ciliary body, where it ceases to be
developed. The optic nerve is distributed, and ends, in it. The optic
fibres occupy the innermost layer of the retina, which is merely sepa-
rated from the vitreous body by a thin membrane. It is followed by
a number of layers, of varying structure, the last and outermost of
which is made up of rod-like and cone-like structures, the bacillar
layer. These end-organs, which are similar to the rods of the
invertebrate eye, are, therefore, turned away from the opening of
the eye in the Vertebrata; the Vertebrate eye is therefore
distinguished from the optic organs of the Invertebrata
by a very essential point, which must not be left out of con-
sideration when we are discussing their genetic relations.

Connected with the development of the secondary optic vesicle is
the formation of a special organ, which makes its way into the
vitreous body at the point at which the optic nerve passes into the
retina; it has no connection with the choroid, but forms a vascular,
darkly-pigmented, process. A structure of this kind is found in
the eyes of many Teleostei, and is known as the processus falciformis
(Fig. 296, p). Its end, which in many Fishes is distinguished by a layer

Fig. 299. Eye of Balaena
mysticetus. Horizontal
section (after W. Sommer-

ing)-

VISUAL OKGAN OF VEETEBEATA.

531

of smooth muscular fibres^ is provided with a swelling (campanula
Ilalleri), which is attached to the hinder part of the capsule of the
lens. These processes are also found, in a somewhat modified condi-
tion, in the eyes of Eep tiles and Birds. In the Saurii there is a
curved and thickened fold which extends to the margin of the
capsule of the lens, at the side of which there may be several other
folds (Fig. 297, p). This structure is feebly developed in the eye of
the Crocodilini. In Birds it is remarkable for the increase in the
number of its folds, and is distinguished as the pecten^^ 298,

In many Natatores and Girallatores it reaches as far as the capsule
of the lens. In the Struthiones the end of the pecten is widened out
into a pouch (marsupium). In Apteryx, as in the Mammalia, it is
absent. The point, at which the ojDtic nerve enters, varies with the
characters of this process, for when it is widened out at its base the
nerve is placed more to the side of the eye.

With regard to the lens, the difference in its form, in accordance
with the surrounding media, is a noteworthy point. In Fishes it is
very large, and quite spherical, as it is also in the Amphibia, and in
aquatic Mammalia; while in others, and in Birds and Eeptiles, we
meet with more flattened forms of lenses ; the amount of flattening
is, of course, very various. The internal cavity of the eye is divided
into an anterior and a posterior space by the attachment of the lens
to the ciliary portion of the choroid. The vitreous body fills the
hinder one; the anterior one, which lies between the anterior
surface of the lens and the cornea, is often but a very small portion
of the whole eye. It is filled by the aqueous humour.

§ 399.

Accessory organs, which partly serve to move, and partly to
protect the bulb, are connected with the eye. The movements of
the eye are generally effected by six muscles, of which four are
straight and two oblique. They are atrophied in the Myxinoidea.
In many Teleostei the straight ones are embedded in a canal at the
base of the skull ; this is in adaptation to their length, which again
is due to the large size of the bulb. They take their origin from a
point which is placed some way behind that at which the optic nerve
passes out; it is in the higher divisions only that they acquire
relations to this point. In the Amphibia and Eeptilia there is
a retractor of the bulb, in addition to the four straight muscles.
This is retained also by most of the Mammalia, and breaks up into
several portions (in Carnivora into four), which pass to the bulb from
the point at which the optic nerve enters the orbit. In the Mam-
malia the superior oblique, which, like the inferior oblique, ordi-
narily arises from the median wall of the orbit, is altered in character.
It has the same origin as the straight muscles of the eye^ and the
tendon of insertion passes to the bulb through a pulley, and at an
angle.

Of the protective organs of the eye, the eyelids are folds of the

2 M 2

532

COMPAEATIVE AE-ATOMY.

integument. The inner lamella of these folds is a continuation of
the conjunctiva which extends on to the bulb, and which is
continuous with the integument at the margin of the lid. Eyelids
of this kind are found even in Fishes. In the Selachii there are
two slightly projecting and movable folds, which appear to be
indications of an upper and lower eyelid ; in many Sharks there is
also a third fold at the anterior angle of the eye, which can be
drawn over the outer surface of the bulb (nictitating membrane).
In the Ganoidei and Teleostei the immovable folds are alone present,
or there may be merely indications of them ; they are ordinarily
distinguished as the anterior and posterior eyelids. Most commonly
the integument passes at once into the cornea. In the Perenni-
branchiata and Derotremata there is a connection of this kind.
Many Salamanders, and the majority of the anourous Amphibia,
are provided with horizontal eyelids, of which the lower, and more
movable one, functions as a nictitating membrane.

In the Keptilia and Aves there is an upper and a low^er movable
eyelid in addition to the nictitating membrane. In some Saurii
(Ascalabotse) and in the Ophidii, the eyelids are developed as an
annular fold, which continues to grow until at last it forms a pellucid
membrane which lies in front of the eye, and which completely sepa-
rates the cornea from the external medium. The circular rudiment of
this structure corresponds to the circular eyelid of the Chamaeleons.
There is a muscular apparatus for the horizontal eyelids, as well as
for the nictitating membrane. Whilst the two horizontal eyelids
persist in the Mammalia, the nictitating membrane undergoes
degeneration. It is supported, as are the two other eyelids, by a
cartilaginous lamella. It is generally reduced to a fold, which is
placed at the anterior (inner) angle of the eye ; in the Primates it
has lost its primitive significance, and forms the plica semilunaris.

A glandular apparatus for the eyelids is first differentiated
in the Amphibia and Reptilia. In Reptiles and Birds, and also in
Mammals, there is a gland which opens below the nictitating
membrane (Harderian gland, or gland of the nictitating membrane),
which is placed at the inner angle of the orbit ; it is not present in
the Primates. Its secretion is different from that of the lachrymal
gland.

The Lachrymal glands, which are placed at the outer angle
of the eye, are first seen in Reptiles, where they are smaller than the
Harderian gland ; they have the same characters in Birds. They
are larger in the Chelonii and Mammalia (except the Cetacea),
where the lachrymal gland consists of a complex of separate glands,
which are generally united into larger masses.

A special efferent duct into the nasal cavity is formed for the
secretion of these glands, which is passed out below the upper eye-
lid. A canal of this kind, formed by an epithelial thickening on
the surface of the head, is present even in the Amphibia. In the
Amniota the development of the lachrymal duct is connected
with that of the face. The groove, which is formed between the

AUDITOEY OEGANS OF VEETEBEATA.

533

processes of tEe upper jaw^ and the external nasal processes by the
dilferentiation of these parts, and which leads from the region
of the inner angle of the eye towards the edge of the nasal pit,
sinks deeper down as these processes are developed (lachrymal
groove) ; it now becomes grown over by their edges so as to form a
canal, which, when the nasal cavity is developed, opens into it just
below the inferior turbinated bones. In the Eeptilia (Lacerta) it
opens near the posterior nares. At the inner angle of the eye
this lachrymal canal is divided into several smaller canals; there
are a larger number (3-8) of such canals on the lower eyelid of the
Crocodilini, but a smaller number (2) in Birds and Mammals.

Auditory Organs.

§ 400.

The Auditory Organ of the Vertebrata, which has been ob-
served in all, except the Acrania, is derived from the ectoderm.
During the earliest embryonic period it is laid down in the form of
a thickening above the myelencephalon, which extends inw^ards. A
superficial organ of this kind, which must have carried the endings
of an integumentary nerve, must be regarded as the starting-point
of the great differentiation, which commences so early. The
earliest rudiment gives rise to a vesicle, which communicates with

Fig. 300. Development of the labyrinth of a Fowl. Vertical sections of the
rncliinentary skull, fi Fit of the labyrinth. Iv Vesicle of the labyrinth, c Euclimeut
of the cochlea. Ir Eecessus labyrinthi. csp Posterior semicircular canal, cse Ex-
ternal semicircular canal, jv Jugular vein (after Eeissner).

the exterior, and which is gradually cut off (Fig. 300), and enclosed
by the hinder lateral portion of the cartilaginous cranial capsule,
when that is differentiated. The primitive otocyst is the foundation
of a complicated cavitary system, in the walls of which the auditory
nerve is connected with its end-organs. From this is developed the
membranous labyrinth. The surrounding portions of the skull form
the cartilaginous, or osseous labyrinth.

The simplest condition of the labyrinth is found in the Cyclo-
stomata. In the Myxinoidea a tract, which remains connected at two

534

COMPAEATIYE A^-ATOMY.

points witli tlie primitive vesicle, is differentiated from it, and
forms a semicircular canal, so that the whole labyrinth has a
circular form. In the Petromyzontes there are two of these canals,
each of which commences with an ampulla-like enlargement, while
the rest of the vesicle of the labyrinth forms the membranous
vestibule ; in this there is a special diverticulum, which is the
rudiment of a new differentiation. In the Gnathostomata a third
canal is developed, so that henceforward three semicircular canals
open into the vestibule.

When the vesicle of the labyrinth sinks beneath the surface, its
stalk-like basal piece remains open on the roof of the skull, in the
Selachii, and swells out below the integument into a saccus endo-
lymphaticus. This corresponds to the recessus labyrinthi (ductus
endolymphaticus), which passes up as far as the roof of the skull in
the Teleostei, and may undergo various metamorphoses. One of these
metamorphoses has been regarded as leading to the growth of this
portion into a tube which covers the brain (Urodela), or extends to
the base of it (Anura). In the Ophidii and Saurii it reaches to the
roof of the skull, being filled in the embryo with crystals of lime, and
widened out. In Phyllodactylus it extends beyond the skull, and
may even pass into the cervical region, being swollen out in parts.
The connection between these structures and the primitive stalk of
the otocyst is denied, so that the recessus labyrinthi is regarded as
an independent structure. Most of its relations, however, require to
be more carefully investigated. In Birds it is an open cavity (r /),
for a short time only ; so, too, in Mammals, where later on it forms
the aqueductus vestibuli. The vestibule and semicircular canals are
very large in aU Fishes ; in the Selachii and Dipnoi they are com-

rig. 301. Auditory organ of Cyprinns carpio. a Membranous vestibule, b Ampulla
of the posterior and external semicircular canal, c United anterior and posterior canal.
d Posterior. e Anterior. / Canalis sinus imparis. g Sinus auditorius impar.
fc Claustrum. i h I Chain of connecting ossicles, m n Air-bladder o Air-duct.
p q r s Spinous processes of the anterior vertebrae. The numbers indicate the different
bones of the skull. 1 Basi-occipital. 2 Exoccipital. 3 4 Supra-occipital. 6 Petrosal.

7 Parietal. 10 Alisphenoid. 11 Frontal (after E. H. Weber).

pletely surrounded by the walls of the skull, while in the Teleostei
the median portion projects freely into the cranial cavity (Fig. 301).
Of the three semicircular canals, two — an anterior (e) and a pos-
terior one {d) — are placed in the direction of two planes, which cross

AUDITOEY OEGANS OF VEETEBEATA.

535

one another more or less perpendicularly; the third, and outer one, lies
in a more horizontal plane, and is provided with an ampulla on its
posterior limb. The two vertical canals have a common piece (c)
which opens into the vestibule, and ampullae at the two other ends.

Even in Fishes the vestibule of the labyrinth is divided into
several portions. An upper one is directly connected with the
semicircular canals (utriculus, alveus communis), and with the
subjacent sacculus. The sacculus and utriculus contain otoliths,
which are constant in the same, but different for different, divisions ;
they are often very large. The branches of the auditory nerve pass
into the end-organs which are to be found in the walls of both
cavities, as well as in the ampullae of the semicircular canals; in
the ampullae they are placed on a transverse ridge (crista acustica) ;
in the saccules they form the maculae acusticae.

Of the numerous modifications which may be observed, the
connections between the membranous vestibule and the air-bladder
are worthy of remark ; the arrangement is effected in various ways ;
it is simplest in some of the Percoidea, and Sparoidea, where the
vestibule is continued into spaces in the skull, which are merely
covered by membrane ; to these spaces processes of the air-bladder
are attached. The relations are more complicated in many families of
the Physostomi. In the Cyprinoids the sacculus {a) extends back-
wards, and is connected with that of the other side by a sinus impar.
This gives off a membranous saccule (atrium sinus imparis) on either
side, which passes to an opening on the posterior portion of the
skull, which is partly closed by a small bone. This is connected by
masses of ligament with a series of bony pieces {ihl) of various
forms, the last and largest of which is attached to the anterior end
of the air-bladder (m). These ossicles are modifications of ribs, and
form a continuous chain between the vestibule and the air-bladder.
In the Siluroidea and Clupeidea connections with the air-bladder
are effected in a different manner.

§ 401.

In and above the Amphibia, the labyrinth is greatly diminished
in size from what it is in Fishes. It is still of a relatively large
size in the Amphibia, and is smallest in the Mammalia. The dif-
ferences which are seen in it are partly due to the way in which the
two cavities of the vestibule, the utriculus and sacculus, are connected
together, and to the course taken by the semicircular canals which
spring from the former. The posterior canal may sometimes be set
at an angle to the external one (Birds).

There is a great difference between the portion of the labyrinth,
just described, and which is very similar in all forms, and that part
which is only developed as an independent structure in the higher
divisions; this, which is known in Mammals as the cochlea, on
account of its form, presents a continuous series of differentiations

536

COMPAEATIVE ANATOMY.

from tlie lower divisions upwards. In Fishes there is an indication
of it in the form of a diverticulum, generally a small one, of the
sacculus. In the Selachii it contains a number of small otoliths;
in the Teleostei one larger one (asteriscus). In the Amphibia this
diverticulum of the sacculus is more independent, but it is still
connected as before, and is still directed backwards.

This portion, which carries the end of a branch of the auditory
nerve, is still further differentiated in the Eeptilia and Aves, where
the diverticulum, which forms it (Fig. 300, C I) E c), is a short
conical piece, which is directed downwards from the median wall of
the labyrinth, and converges towards its fellow of the opposite side.
Its end is somewhat bent, and it forms the lagena.^^ Among
Mammals this stage of the organ is seen in the Monotremata only ;
in the rest this stage is not the permanent one, for the organ is
converted into a spirally- coiled canal. At first it is only formed by
a prolongation of the sacculus, but special differentiations appear in
it, and this cochlear canal, which is formed from the sacculus, is
permanently connected with it by a narrower portion only (canalis
reunions. Fig. 302). The organ, which thus becomes more indepen-
dent, is sur-
rounded on
two sides of
its course by
lymphatic ca-
vities, which
accompany it
in its coils,
and pass into
one another
at the apex
of the cochlea.
One cavity
is connected
with the os-
seous vesti-
bule, the other
is shut off
from it at
its commence-
ment, and is
only con-
nected with

the cavity of the vestibule indirectly ; that is by the communication
at the apex of the cochlea. Three cavities, therefore, can be dis-
tinguished in the Mammalian cochlea ; but one only, the ductus
cochlearis, is connected with the vestibular labyrinth. The other
two form the scalae — the sc. vestibuli and sc. tympani. The two
scalse occupy the periphery of the coils of the ductus cochlearis,
at the base of which the end- organs of the cochlear nerves (organ

Fig, 302. Diagrams in explanation of the labyrinth. / Fish.
// Bird. Ill Mammal. JJ Utriculus. ^ Sacculus. US Utriculus
and Sacculus. Cr Canalis reuniens. J? Recessus labyrinthi.
UC Commencement of the cochlea. C Cochlea. L Lagena.
K Csecal sac at apex. C Csecal sac of the vestibulnm of the
cochlear canal (after Waldeyer).

AUDITORY ORGANS OF VERTEBRATA.

537

of Corti) are spread out. As the scalae arise as spaces in the tissue
which accompanies the ductus cochlearis, they are similar to the
cavities between the membranous semicircular canals and their
bony wall, or between the membranous and osseous vestibules, and
are filled with perilymph.

In the Amphibia, and all higher forms, spaces appear in that part
of the walls of the bony labyrinth which lies on the outer surface of
the cranium; these in a different fashion effect a communication
between it and other arrangements that are connected with the
auditory organ. The fenestra ovalis, which is always closed by a
plate-like piece of bone, is a hole of this kind in the osseous vesti-
bule. A second opening, which appears first in Reptiles, and which is
correlated with the further development of the cochlea (fenestra
rotunda), lies in the wall of the scala tympani, and is closed by a
membrane.

Both these arrangements are related to the development of
external conducting organs.

Eetzius, G., Anatom. Untersnch. I. Stockholm, 1872. — Hasse, C., Anatomische
Studien. Leipzig, 1870-73.

§ 402.

Other parts are gradually added on, as accessory organs, to
the auditory organ, although primitively having no relation to
it. The first branchial cleft, which in the Selachii and Ganoidei
persists as the spiracle,^^ enters into close relation with the
wall of the labyrinth in the Amphibia. As it grows over this
wall it is converted into a cavity, the wider portion of which forms
the tympanic cavity; this is bounded in the middle line by
the wall of the labyrinth ; the portion which leads into the primi-
tive buccal cavity forms the Eustachian tube. It reminds us
of its primitive (spiracular) condition by at first communicating
freely with the exterior. The cleft is, however, soon closed,
which leads to different arrangements. In the Coeciliac and Urodela
the cleft is closed by the superjacent muscles, so that there is no
tympanic cavity. One division of the Anura (Pelobatidac) presents
the same arrangement, for in it there are only slight indications of
the outgrowth of the mucous membrane of the pharynx into this cleft.
In most Anura, however, that membrane does form an outgrowth,
and leads into a tympanic cavity, which is closed externally by a
tympanic membrane. Among the Reptilia, the Ophidii, and
Amphisbacnoidea have no tympanic cavity ; in Chamaeleo there is
no tympanic membrane ; but both these parts are present in all
other Reptiles, and in Birds.

In the Crocodilini and Aves, the inner openings of the Eustachian
tubes are united into a common canal, as is the case also in Pipa
among the Amphibia.

Those parts of the visceral skeleton which are connected with the

538

COMPAEATIYE ANATOMY.

bony labyrintli^ unite to form tbe apparatus of the auditoryossicles;
tlie lioniologies of wbich have not yet been definitely made out for
the different classes. The first portion is formed by an ossicle
(operculum) which closes the fenestra ovalis ; in the Urodela this is
either flat, or provided with a stalk-like process. Sometimes it is
cartilaginous and its stalk ossified (Siredon) ; sometimes the reverse
is the case (Menopoma). In the Coeciliae they are both ossified.
The same arrangement obtains in the Ophidii (Eurystomata), where
a small piece of bone (columella) reaches to the quadrate.

When there is a tympanic membrane present the columella is
connected with it; for its cartilaginous end, which has often a
peculiar form given to it by processes, sinks into that membrane.
The lining tissue of the tympanic cavity then surrounds part of the
columella, and causes this bone to appear to be more or less placed
in the tympanic cavity. These arrangements are first seen in the
Anura, and are still further developed in the Saurii, Chelonii, Cro-
codilini, and Aves. The process of the columella is in some Birds
(Drommus) connected with its plate by two limbs ; in other cases it
is simple, or is connected with the plate by one enlargement only.

Just the same relations are seen to obtain in the columella of the
Mammalia ; with this modification, however, that it is never directly
attached to the tympanic membrane. It is converted into the stapes,
the form of which, in the Monotremata and many Marsupials, calls
to mind the columella. In the monodelphous Mammalia it is
ordinarily divided into two limbs, which carry the plate. The
other auditory ossicles are the incus, which is connected with the
stapes, and the malleus, which is attached to the tympanic mem-
brane by a styliform process. A connection between the tympanum
and the fenestra ovalis, which was previously effected by a single
bone — by the columella alone — is now effected by it, and two other
bones. This chain of auditory ossicles is, for the most part, at
any rate, placed in the tympanic cavity, for it is covered by the
mucous membrane which is continued into that cavity from the
pharynx, through the Eustachian tube. The tympanic cavity itself
has, however, another relation, for it is principally formed by the
tympanic bone, in addition to the boundaries provided by the wall of
the labyrinth ; this tympanic bone commences as the framework of
the tympanum.

§ 403.

The external ear is derived from a prolongation of the edges
of the first branchial cleft. In the Amphibia, Keptilia, and Aves,
these parts are either altogether absent, or are only present in
individual cases, where they have been developed in consequence of
various kinds of adaptive changes. Thus, in the Crocodilini a fold
of the integument forms an operculum above the tympanic mem-
brane, and in the Owls there is a movable membranous valve.
As early as the Saurii the tympanic membrane is removed
some way from the surface, so that there is a short external

ALIMENTARY CANAL OF VERTEBRATA.

539

auditory meatus/^ The external auditory meatus in the Mammalia
is different, for its deeper portion is formed
by the tympanic bone. The external
ear, the cartilaginous support of which is
continuous with a narrow auditory meatus,
is attached to this. There is no external
ear in the Monotremata. The ‘'^external ear’^
may be much modified, either in form, or
in its relations to the muscular appai’atus,
which moves it. In addition to the muscles,
which move the whole of the external ear,
and which are sometimes of much power even
in Man, there are others which are placed
in the cartilage of the ear itself ; these are
partly represented, though of course as rudi- *

mentary organs, in the human ear. This
external ear is still more atrophied in aquatic
Mammalia. Reduced in Otaria, it is alto-
gether absent in the rest of the Pinnipedia,
as it is also in the Sirenia and Cetacea.

Alimentary Canal,

§ 404.

The alimentary or enteric canal of the
Vertebrata forms a tube, which runs be-
low the axial skeleton, and in which two
chief portions can be distinguished mor-
phologically, as well as physiologically,
at a very early epoch. The most anterior
portion is directly connected with the
body-wall, and, as it is perforated by
branchial slits, it functions as a respira-
tory organ, for respiratory apparatuses are
developed in the vascular arches between
the clefts. This portion does not, therefore,
belong exclusively to the digestive organs,
although it is used in the ingestion of food.
It forms the respiratory cavity, at the
end of which the nutrient canal, in the
strict sense, commences; this is separated
from the body-wall by the pleuro-peritoneal
cavity. The Vertebrata have these
two portions of the enteric tube in
common with the Tunicata. In the
Acrania the respiratory chamber of the
enteric tube occupies a very large portion,
which, as in the Ascidise, represents a large

Fig. 303. Amphioxus
lanceolatus (x2^).
a MouthjSurrounded by cirri.
h Anus, c Abdominal pore.
d Branchial sac. e Gastric
portion of the enteron.
/ Cmcum. g Hind-gut.
h Coelom, i Notochord, be-
low which is the aorta, which
accompanies it for nearly
its whole length, h Aortic
arches. I Aortic heart.
m, Enlargements of the
branchial arteries, n Heart
of the vena cava, o Heart
of the portal vein (after
Qnatrefages) .

540

COMPAEATIVE ANATOMY.

part of the body. This space is gradually reduced in size in the
Craniota ; it still, however, retains its respiratory function, but
many other organs are also differentiated in it ; these are, largely,
accessory organs for the ingestion of food.

Respiratory Ante-chamber (Cephalic enteron).

§ 405.

In Amphioxus this portion is bounded, in its most anterior
region, which is close to the cavity which carries the mouth, by a
ciliated apparatus ; there are a number of movable processes also
at that point, which are directed towards the lumen of the tube,
and so prevent the entrance of foreign bodies. The ante-chamber
(Fig. 303, d)y which occupies about two-fifths of the whole length
of the body, has its walls broken through by a large number of
obliquely-set clefts ; these form a complicated framework, the sup-
ports of which have been already (§ 353) mentioned. The water,
which is taken in by the mouth (n), passes through the clefts, and so
to the exterior. But as two lateral dermal folds are gradually con-
tinued ventrally over the surfaces on which the clefts are placed, and
become united below, a peribranchial cavity is formed, which opens
by a special pore (c). It should here be remembered that there was
something similar to this in the Ascidise (§ 310). But it would not
be correct to suppose that the two structures are morphologically
identical. A vascular plexus is distributed in the walls of the clefts,
the water that streams past effects respiration, the clefts function as
branchial clefts, and the whole cavity represents functionally a
branchial cavity.

There are many special points in this arrangement in Amphioxus,
such as the want of symmetry in the branchial frame, and its inde-

Fig. 304. Vertical median section of a larva of Petromyzon. o Mouth, v Velum.
li Hypobranchial groove. n Spinal chord, ch Notochord. a Otocyst. c Heart
(after a drawing by Calberla) .

pendence of the metamerism of the body; so that there is altogether
a great difference between this apparatus and that of the Craniota.

BRANCHI^ OF VEETEBEATA.

541

The region of the body which is occupied by the branchial cavity
corresponds to a head, for the nerves which go to it arise from the
myelencephalon in the Craniota. Viewed thus the branchial cavity
represents a cephalic enteron. Its nutrient and respiratory signifi-
cance is the cause of various differentiations in it, which are partly
arrangements which are peculiar to the Vertebrata, and partly
arrangements which have been inherited from a lower condition. In
addition to the branchial clefts, the ventral groove (hypobranchial
groove), which is developed on the ventral surface of the branchial
cavity, belongs to the latter series; this has just the same relations
as in the Tunicata (cf. p. 402) ; it is found in the larv^ of the
Petromyzontes, where it forms a grooved depression, enclosed by
ridge-like edges (Fig. 304, h). In Amphioxus this structure is also
present. Its presence in various stages of metamorphosis in all
Craniota, not only brings these forms into closer connection, but is an
indication of their genetic relations to the Tunicata, which must not
be forgotten (cf. § 416).

Branchiae.

§ 406.

In the Craniota the branchial clefts are universally much reduced
in number, as are also, in correspondence with this, the arches of the
branchial skeleton. This phaenomenon must be regarded as the
degeneration of a primitively larger number of these struc-
tures, such as is found in Amphioxus; it is compensated for
by the increased size of the surfaces which carry the
respiratory vascular plexus. This increase in size is implied by
the development of the gills, whereby the blood-vessels, which, in
the Acrania, are distributed over a large number of arches, are
limited to a less extensive region, and are therefore arranged on a
smaller number of arches. The essential point in the formation
of branchiae in these animals is the increase of the surface which
is directed towards the respired medium, and this increase may be
effected by means either of lamellae or of cylindrical processes.
The branchial arches are provided with various forms of these
organs, which enclose the well-developed respiratory vascular network.
We find that in the Cyclostomata these organs have special characters,
which have but little resemblance to what is found in Amphioxus ;
their earliest condition is most like what is seen in the Gnatho-
stomata, for the branchial clefts are simply spaces in the body-wall
(Fig. 304). They are differentiated into tubes, the median portion
of which has its lumen widened, and forms a branchial pouch
(Fig. 305, hr). Branchial lamellae are raised up from the wall of
the branchial pouches in the form of leaf-like folds, in which the
respiratory vascular plexus is spread out. Each branchial pouch
is connected, by an ^Gnternal branchial duct,^^ with the anterior

542

COMPAEATIVE ANATOMY.

section of the enteric tube. An external branchial duct {br') leads to
the exterior. There are several variations in the characters of the two
canals which spring from each branchial pouch. The inner ones either
open each separately into the digestive tube (Bdellostoma^ Myxine)

(Fig. 305)_, or they all unite into a median
respiratory tube which runs below the diges-
tive tube, and being connected in front with
the digestive tube carries water to each of
the branchial pouches (Petromyzon). The
external branchial ducts either open sepa-
rately on the sides of the body (Bdello-
stoma^ Petromyzon), or all the ducts of one
side are united into a branchial pore (s)
which lies behind the branchial apparatus ;
on the left side a special canal (c), which
comes from the oesophagus (ductus oeso-
phago-cutaneus), also opens into the same
pore (Myxine). These different forms may
be derived from one another; in the case
both of the inner and the outer branchial
ducts that condition should be regarded as
the primitive one, in which there is a direct
connection between the respiratory chamber
and the surface ; while, on the other hand,
the formation of the respiratory tube,
and the union of the external branchial
ducts, is the result of a subsequent differ-
entiation.

§ 407.

In Fishes, the branchial pouches are
more closely related to the skeleton. The
phaenomena seen in them lead to the con-
clusion that each arch of the primitive
branchial skeleton carried gills. The upper
part of the first (mandibular) arch is not
excluded from this ; as is clear from the
frequent presence of a gill in the opening,
which is found in many Selachii — the so-
called spiracular cleft — between the first
and second arches (mandibular and hyoid
arches). The spiracular canal, which repre-
sents a degenerate branchial pouch, is suc-
ceeded by the true branchial pouches, of
which there are, as a rule, five ; and rarely six or seven (Notidani).
The wall of the first pouch is supported in front by the hyoid arch,
and behind by the first (i.e. by the third primitive) branchial arch ;
the other pouches have just the same characters. In each of them
a septum (s), supported by cartilaginous rays, extends outwards from
the internal branchial skeleton, and serves as the posterior wall of the

seen from the ventral sur-
face. 0 CEsophagus. i In-
ner branchial ducts. brBran-
chial pouches, hr' External
branchial ducts, which unite
on either side into a common
branchial duct, which opens
at s. c Ductus oesophago-
cutaneus. a Auricle, v Ven-
tricle. ah Branchial artery,
giving off a branch to each
gill, d Lateral wall of the
body turned outwards and
backwards (after Joh.

Muller).

BlUNCHIiE OF VEETEBEATA.

543

poiicli ill fronts and the anterior wall of the one behind. While the
pouches communicate with the pharyngeal cavity by narrow orifices^
which are bounded by the cartilaginous branchial arches, they also
open on the side (or in Eays, on the ventral surface) of the body by
just as many clefts. Eows of branchial lamellae lie in the walls of
the branchial pouches; in the embryonic condition these lamellae
develop filamentous prolongations, which form the external gills.
These are also found on the spiracular cleft. The anterior wall
only of the last branchial pouch is provided with a gill (Fig. 306, A).

From this arrangement we may derive what we find in the
Ganoidei, and from that what we find in the Teleostei. The spiracular
gill, which has no respiratory function in the adult state of the
Selachii, is the first
to undergo the
greatest degenera-
tions. In some
Ganoidei, which
possess a spiracle
(e.g. Acipenser),
the gill is con-
verted into a
Ps eudobranchia
(a gill which has
lost its proper
branchial artery
and vein) ; this is
not found in Poly-
pterus nor Amia.

In the Osseous
Fishes it also ap-
pears to be wanting,
or has lost all re-
semblance to a gill.

The anterior

series of branchial lamella3 of the Selachii, that, namely, which is
attached to the posterior face of the hyoid arch, is also found among
the Ganoidei, where it forms a respiratory opercular gill (Acipenser,
Lepidosteus). It is found also during the embryonic stages of the
Teleostei, but it does not persist. It sometimes consists of a short
row of lamellse, which is attached to the operculum ; sometimes it
is carried to the base of the skull, and sometimes it is hidden
below the mucous membrane. Even when in this condition, rudi-
mentary cartilaginous rods may be found in it. When still more
degenerated, it forms a glandular structure, which is made up of
several lobules (Esox).

When all trace of the external branchial skeleton disappears,
the septum which arises from each of the inner branchial arches
disappears also, or is reduced to a slender fringe. Owing to this,
the rows of branchial lamellse in the Ganoidei and Teleostei come
into close relation with their respective branchial arches, and are

Fig. 306. Horizontal section through the branchial cavity.
A Of Scyllium. J5 Of Barbus. The floor of this cavity
is shown. I Tongue. oe Oesophagus. s Septa of the
branchial pouches, h Gills, op Operculum.

544

COMPARATIVE AR^ATOMY.

consequently placed in two rows on each arcli as it passes between
two branchial pouches (Fig. 306, B h). The anterior row of branchial
lamellae in a Teleostean or a Ganoid corresponds, therefore, to the
gill on the hinder wall of the branchial pouch of a Selachian, and the
hinder row of lamellae in a Teleostean gill corresponds to the anterior
gill in the branchial pouch of a Selachian.

These relations are shown in the following diagram, in which b
is the indifferent stage of the rows of branchial lamellae, B is the
differentiated arrangement in the various divisions. ^ represents any
row of branchial lamellte, which is specially modified or reduced :

Selachii: /3 B^- B^ B* B^

Ganoi'dei ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

(Sturio, Lepidosteus) — v—

and Teleostei : — B^ B^ B^ B*

By the degeneration of the septa between the branchial pouches,
the whole gill apparatus is made more compact, and no longer
therefore extends back into the region of the trunk, as it does in the
Selachii ; it is confined to the base of the skull. Whilst in the
Selachii the projecting septum (A s) forms an organ of protection
for the succeeding branchial pouch, a similar organ is formed in
the Chim80r80, Ganoi'dei, and Teleostei, from a single arch — namely,
from the hyoid ; the integument on this arch grows backwards and
covers all the gills, and is developed, in the Ganoidei and Teleostei,
into the opercular apparatus and the bran chiostegal membrane, with
their various skeletal pieces (§ 354) {B op),

§ 408.

In the Teleostei four arches are ordinarily beset with branchial
lamellm, the fourth arch having a single row only, or there are but
three arches which carry lamellse. When the lamellge of the fourth
arch, and the posterior row on the third arch disappear, the fourth
branchial cleft is ordinarily closed. Perhaps one of the most im-
portant of the modifications wPich affect the lamellm themselves is
seen in the villous gills of the Lophobranchiata. In some divisions of
the Teleostei, the branchial arches seem to be so metamorphosed as
to be able to retain the water in the branchial apparatus. The
organs of the Labyrinthobranchiata are of this kind; separate
branchial arches or parts of such, are modified to form coiled lamella-
like processes, which give rise to a portion which is placed above the
gills (Anabas, Polyacanthus). Another apparatus, which is found in
various Clupeidm, consists of a spirally- coiled tube (branchial coil),
which is formed by a diverticulum of the superior pharyngeal mucous
membrane. This tube is generally connected with the superior
segment of the fourth branchial arch, and has processes of its
skeletal parts in its walls (Heterotis, Lutodeira, Meletta, etc.). The

BRAXCHLE OF VERTEBRATA.

545

arborescent processes of the branchial arches, which are placed
in special prolongations of the branchial cavity, where they support
a respiratory vascular plexus, also belong to this series (Hetero-
branchus, Clarias).

Diverticula of its investing mucous membrane have the same
respiratory function as the cavity itself. Thus, in Saccobranchus, a
long tube extends from the branchial cavity, on either side, as far
as the lateral trunk muscles ; in Amphipnous there is a similar sac
behind the head, which opens just above the first branchial cleft.
Both these organs contain respiratory vascular plexuses.

§ 409.

External gills in the form of integumentary structures were not
primitively possessed by the Vertebrata, for the so-called external
gills of the Selachian embryo are nothing more than filaments of the
internal gills which protrude through the branchial cleft. Gills, how-
ever, may come to the surface, and take the form of tegumentary pro-
cesses; such gills may be seen in the young stages of Polypterus; cer-
tain gills of Protop terus, and the gills of the Amphibia generally are
of this character. In the Amphibia the gills have the appearance
of two or three pairs of branched processes, which spring from as
many branchial arches. In the Perennibranchiata this apparatus is
permanently functional. In the rest of the Amphibia (Caducibran-
chiata) these external gills disappear ; in the Anourous forms, where
they are found for a short time only, they are replaced by shorter
internal gills. A membrane which grows from before backwards
covers the gills, so that there is only one efferent orifice. The
orifices on either side may continue to grow out, and get nearer to
one another, so as to unite into a single ventral orifice.

When the larval stage ceases, the inner and outer gills of the
Derotremata and Salamander are atrophied ; in the latter, as in
the Anura, the branchial clefts are completely closed, but in the
Derotremata a cleft is left on either side.

When the gills disappear, the branchial cavity, which constitutes
the respiratory antechamber, is converted into the primitive buccal
cavity, which is limited by essentially the same parts as it was
before.

Branchial Clefts, and Palate of the Amniota.

§ 410.

In the Amniota, also, the arrangement which has been trans-
mitted from their branchiferous ancestors is retained during certain
stages of embryonic life, in the form of clefts in the wall of the
pharynx. These branchial or visceral clefts are never more
than four in number, and they appear in such a way from before

2 N

546

COMPARATIVE ANATOMY.

backwards^ that when the last has appeared the anterior ones have
already undergone certain changes. They are all gradually
atrophied, and completely disappear, except the first, part of which
is converted into the middle and outer ear (cf. supra, § 402).

The degeneration of the embryonic branchial clefts is an impor-
tant point of difference between the Amniota and the Anamnia, but,
in addition to this, there is a new peculiarity which is due to a
differentiation of the primitive buccal cavity. This leads to the
formation of the secondary nasal cavity, and of the secondary
buccal cavity. The remnant of the primitive buccal cavity, which
lies behind, and is not affected by this process, forms the Pharynx.
The cartilaginous portion of the ethmoid, which separates the two
nasal cavities, and is broad in the Amphibia, is developed in the
Amniota into a thin vertical lamella (Fig. 307, e) — the internasal
septum. It remains partly cartilaginous, and is partly converted into,
and develops bony structures, which were
treated of under the cephalic skeleton.

A second change is brought about by hori-
zontal ridges or processes, which are given off
from the maxillary process of the first arch,
and which gradually form a plate (Fig. 307, p),
the palate, which divides the primitive buccal
cavity into two compartments. This plate
forms the floor of the upper, or nasal cavity [n)j
and the roof of the lower one (m). When the
internasal septum reaches this palatine plate
it separates the nasal cavity into two por-
tions, into each of which the nasal canal
now opens, while its external orifice is coin-
cident with that of the bifid nasal cavity. The
posterior orifices of the nasal cavity, the choanm, which are
separated by the palatine plate from the buccal cavity, and by
the vertical internasal wall from one another, open into the
pharynx.

Very various stages in the arrangement of these palatine plates
may be observed. In the Ophidii, Saurii, and Aves, the process of
separation is less complete, the posterior nares form a longitudinal
cleft, owing to the palatine processes uniting anteriorly, but being
separated from one another posteriorly. They are sometimes
separate in Birds, in which case they are exceedingly small. In the
Crocodilini they are placed farther back than in any other forms,
while in the Mammalia they do not open into the secondary buccal
cavity, but into the pharynx. This latter region is thereby — as also
by the opening into it of the Eustachian tube, which is developed
from the first visceral cleft — shown to be a portion of the primitive
respiratory antechamber.

In Reptiles and Birds the palate is supported by pieces of the
Skeleton (vide supra) ; in Mammals the hinder portion is formed
of soft parts, which form the velum palatinum.^^

Fig. 307. Diagram of
the differentiation of the
primitive buccal cavity
into nasal cavities {n n),
and a secondary buccal
cavity {m). jo Palatine
plate, c Internasal wall.

NASAL CAVITY OF VEETEBEATA.

547

Nasal Cavity.

§411.

While the nasal cavities are increased in lengthy owing to their
being shut off from the buccal cavity by the palate, the increase in
the size of the facial portion of the head also affects them ; they
increase both in length and height, and thus become large spaces.
The olfactory nerve ends in their superior and posterior portion
only (regio olfactoria), while the inferior and anterior portion
principally serves as an air-passage,^^ and consequently comes into
relation with the respiratory organs (regio respiratoria). The whole
differentiation therefore of the nasal cavity is seen to be connected
with the development of the lungs, and their increased physiological
importance. The increase in the extent of the internal cavity is
effected in various ways. The lateral wall of the nasal cavity,
which is developed from the primordial cranium, always takes part
in this process; the turbinate bones are lamellar, folded, and
coiled processes of this wall.

In the Eeptilia there is only one turbinate bone; this extends
backwards from a cavity, which commences at the external nasal
orifice, and is generally horizontal in position ; it is feebly developed
in the Chelonii, and best developed in the Crocodilini. It is very
varied in character in Birds. Sometimes it is simple (Columbae),
sometimes complicated by coils (Eaptores), or it may be cleft into
several lamellas (Strathio). A turbinated structure is connected
with the internasal septum in front of, and below this bone, and is
by this connection distinguished from the turbinate bones, which
are always lateral in position. This pseudo-concha separates the
vestibule of the nose from the internal nasal cavity.

Above the turbinate bone, and, as a rule, at the upper blind end
of the nasal cavity, there is another process which corresponds to a
depression formed in the wall of the nasal cavity by an air sinus.
Part of the olfactory nerve ends on this process, which is not found
in the Columbidm. In the Mammalia three turbinate bones may be
distinguished. The lower one corresponds to the single bone in the
Eeptilia and Aves ; it varies very greatly owing to the way in which
its lamellae are ramified and variously coiled, e.g. in the Carnivora (it
is most complicated in Lutra and Phoca). These bones are least
developed in various Marsupials (Macropus, Phascolomys), in the
Apes (they are simplest in the Platyrrhini), and in Man. In the
Cetacea the cavity has undergone degeneration in consequence of
the loss of its olfactory function. The orifice on the upper surface
of the skull leads into a vertical canal, which is divided by the inter-
nasal septum, and which can be shut off from the pharyngeal cavity by
an occlusor muscle; there are no signs of any turbinate bones
in it.

2 N 2

548

CO]\[PAEATIVE A^s^ATOMY.

§412.

There are accessory organs belonging to the nasal cavity. These are :

1) Accessory cavities of the nose. These are formed by
the sinking of the mucous membrane of the nose into parts of its
firm wall. They are first seen in the Crocodilini, where there is a
cavity in the side walls of the nasal cavity, which communicates
with it. In Birds we frequently meet with connections between the
nasal cavity and the spaces in the neighbouringbones. In the Mam-
malia the nasal cavity communicates with a number of cavities in
different bones of the skull, the most important of which are the sinus
frontales. These are cavities which are placed in the frontal bone,
and which are either single, or divided into smaller portions ; they
are very greatly developed in the Ruminantia. There are other
communications with the sphenoid ; these are greatly developed in
the Elephant, for example, where the cavities extend through the
parietal and temporal bones as far as the occipital condyles. Lastly,
there are connections between the nasal cavity and the maxilla;
these form the sinus maxillaris, which is developed in Marsupials
and Ruminants, and very largely in the Solidungula. In Primates
they are less extensive, and they are not present in most Carnivora,
Edentata, or Rodentia.

2) Glands. There are larger glands connected with the nasal
cavity in addition to the glandular structures which are ordinarily
found on the mucous membrane of the nose. When they are more
developed they may also extend outside the nasal cavity. Such
nasal glands are found in the Amphibia and in the Ophidii, as also
in the Saurii and Crocodilini; in the former they lie outside the
upper jaw, and in the latter they are enclosed in a maxillary sinus.
In Birds also there is an external nasal gland, which is sometimes
placed on the frontals, and sometimes on the nasal bones. Among
the Mammalia also we find a gland on the sides of the face, but it is
absent in several orders.

3) Organ of Jacobson. This is a canal placed at the base of
the nasal cavity; it is generally attached to the nasal septum, and
communicates at the palate with the buccal, though it is shut off
from the nasal, cavity ; its walls, which form various kinds of pro-
cesses, carry the ends of a branch of the olfactory nerve, which
passes down the sides of the septum. In the Ophidii and Saurii the
canal is partly enclosed by the vomer; in the Mammalia these
organs are elongated, and are continued, as the ducts of Stenson,
through the incisor canals, to the surface of the palate ; they are
best developed in the Ruminantia and Rodentia (§ 396).

Buccal Cavity,

§ 413.

\Yhen the primitive antechamber of the enteric tube is divided
into the nasal and buccal cavities, by the formation of a palate, a

BUCCAL CAVITY OF VEETEBEATA.

549

number of organs, wbicb were seen in tbe primitive arrangement,
are assigned to the buccal cavity, while other organs appear only
as later developments. The teeth, the tongue, and various glandu-
lar organs belong to the former series. The soft palate, or velum
palati, which is only found in Mammals, is a new organ. This
muscular apparatus forms the posterior boundary of the buccal
cavity, and separates it from the pharynx. The uvula is a median
prolongation of the velum palati ; this structure is apparently found
in the Primates only.

In Keptiles and Birds the anterior and lateral boundaries of the
buccal cavity are formed by the margins of the jaws, which are
invested by the integument, and by the hard structures which are
found on the jaws. In the Saurii and Ophidii the integument de-
velops pad-like lips along the edge of the jaw. In all Mammals,
except the Monotremata, the integument is separated from the edge
of the jaws, and invests a complicated muscular layer which has its
origin in them ; this layer forms the ground-work of the lips, and
gives them their mobility. In this way a space is developed which
lies in front of the buccal cavity — the vestibulum oris. The lateral
portions of this space form the cheek-pouches, and, when capable of
great extension, develop into the pouch-like diverticula of many
Mammals (buccal pouches of the Eodents and Apes).

Organs of the Buccal Cavity.

§ 414.

Of these organs those hard structures which serve for the pre-
hension and comminution of food are of
various kinds. Some of them are formed
by the cornification of epithelial cells. The
sucker-shaped mouth of the Cyclostomata
(Fig. 308) is beset with horny teeth of this
kind, which are also present on a tongue-
like organ in these animals. In the Am-
phibia the edges of the jaws are provided
with a similar covering ; these structures,
which are partly limited to the larval stages,
are formed of a number of closely-set den-
ticles (Anura) ; in Siren some are retained
throughout life.

The larger horny coverings on the mar-
gins of the jaws of the Chelonii, Aves, and
Monotremata differ somewhat from these

horny denticles ; they are compensatory its “ horny teeth ” (after
arrangements, the presence of which is due Heckel and Kner).

to the absence of true teeth. Although

these structures are used for the comminution of food, they have
nothing to do with true teeth ; they are purely epidermal structures,
as is also the whale-bone of Whales.

550

COMPAEATIVE ANATOMY.

Tlie true teeth are the product of the mucous membrane of the
mouth, which is formed of connective tissue as well as of epithelium.
In the Selachii their structure and mode of development is exactly
the same as that of the dermal denticles, with which also they
have many external points of resemblance ; as, therefore, the matrix
of the two is continuous, and as in many Selachii these integu-
mentary scales are distributed over other portions of the wall of the
buccal cavity, we may conclude that the teeth and scales were
primitively identical. The teeth, which are developed on the
edges of the jaws are, therefore, however much they
are differentiated, nothing more than large structures
of the same kind as those which are found in the
integument. The change in them, as compared with
these integumentary scales, is clearly due to adapta-
tion to new functions, while their first appearance was
contemporaneous with the differentiation of the primi-
tive mandibular arch. The presence of these structures
in the primitive bn c cal cavity is explicable from the
fact that it was formed by an invagination from the
exterior.

All teeth are developed in essentially the same way ; this has
been already described above (p. 423), when we were speaking of the
dermal denticles of the Selachii. The dental papilla, formed of
connective tissue, develops the dentine from an epithelial-like
superficial layer (odontoblasts) ; and on this an epithelial layer
deposits the enamel. When the teeth are developed on the surface,
these layers are continuous with those of the surrounding mucous
membrane. When the rudiment of the tooth is sunk into the
mucous membrane, an epithelial thickening (enamel ridge) is
developed, which grows into it ; the portion of it which covers the
dental papilla is separated off, and forms the enamel organ. The
cement, or bony layer, is added to these two substances, and forms a
third layer.

We have described above how the presence of teeth in the buccal
cavity, and their deposition on the cartilaginous skeleton of its wall
leads to the formation of bone (§ 342). These bones are derived
from dentigerous plates, and therefore each of them may carry
teeth. In the Glanoidei and Teleostei, for instance, there are teeth
on the palatines, vomer, and parasphenoid, in addition to the maxillary
bones ; and, also, on the hyoid and branchial arches. It is generally
the last of the branchial arches reduced to a simple plate, which
is distinguished by the possession of teeth (pharyngeal teeth ;
Fig. 256, VI). Teeth are much more common on the upper pieces
of the branchial arches.

In the Amphibia there are still teeth on the palatine and vomer;
more rarely on the parasphenoid; among the Eeptilia, the Ophidii and
Saurii alone have teeth on the palatine and pterygoid ; while in the
Crocodilini, as in the Mammalia, the teeth are confined to the
maxillary bones.

TEETH OF VEETEBEATA.

551

111 tlie Selacliii they are partly movable, and are arranged in
rows of different ages. In most Fisbes they retain their superficial
position, and, where they are more firmly united,
this is effected by their fusing with the bones
which carry them. This is the case also in the
Amphibia, where the earliest dental structures ^ j

form their proper bones by fusing together at
their bases. In the Eeptilia the teeth are formed
independently, like the later teeth of the Am-
phibia; sometimes they are mere excrescences
(pleurodont Lizards) ; sometimes the developing
teeth are sunk into their proper bones. In some of
the Saurii, the teeth are attached to the edge of
the jaw (acrodont Lizards). In the Geckos and
Ophidii, and in all Crocodilini, the developing
teeth are partly surrounded by the edges of the
jaws, and are, therefore, embedded in alveoli. A
similar arrangement obtains in the Mammalia. A
mass of epithelium grows into the mucous mem-
brane of the edge of the jaw, and forms a cap
around a papilla, on which the rudiment of the Diagram

tooth is developed; as this follicular structure is of teeth.^^ A^prwess
surrounded by the jaw the tooth is completely of the epithelial
differentiated within the jaw, and only breaks layer is sunk into the
through the mucous membrane as it is gradually
developed ; the saccule which forms it is nipped organ (e) over each
off from this mucous membrane. papilla (p).

The teeth vary very greatly in form, so that
there is every intermediate stage between broad plate-like struc-
tures, and long, fine, spicular forms ; this variety of character is
most common in Fishes. The teeth of the Amphibia are more
similar in form ; in the extant members of the group, at any rate,
they are generally simply conical, or faintly notched. Among the
Eeptilia greater differences are seen in the Saurii, and partly too
in the Ophidii, in some of which a certain number of teeth are
connected with a special poison apparatus. In the Crocodilini, also,
the conical form is the most common; in them the new teeth are
always placed below those which are already developed, and are
covered by them.

Birds have no teeth. But as fossil forms — the Odontornithes
(Ichthyornis, Hesperornis) — are known in which the jaw did carry
teeth, their absence in extant forms must be regarded as having
been acquired within the limits of the class.

Among Mammals, the individual tooth varies very greatly, so
that a single dental apparatus contains various forms of teeth.
These, again, have different functions in relation to the food in-
gested, and vary greatly in character according to the kind of
food; it is in the Delphinoidea only that the lower condition, in
which all the teeth are similar, is retained ; in the Balaenoidea the

552

COMPAEATIYE ANATOMY.

teetli are merely formed in rudiment^ and are atrophied while stil
within the alveolar cavities.

The replacement of the worn-out, and subsequently-shed teeth,
is effected in Fishes by the continual development of new teeth close
to the old ones. Teeth are, therefore, developed by a process which
is continued throughout the whole life of the animal, and is always
being renewed. Even in the Amphibia and Keptilia we also meet
with successional teeth, so that by the continual development of
fresh teeth the dental apparatus is kept complete. In most
Mammals this process is limited to a single chauge, the first (milk)
dentition being replaced by a second one, in which there is a larger
number of teeth (Diphyodonts). This change does not happen in
the Cetacea (Monophyodonts). In the Marsupialia the diphyodont
condition is in a rudimentary stage, for it is confined to one tooth
only on either side of the jaw. This is the case also in several other
Mammals (Elephas, Halicore), while the Eodentia would seem to
belong to this series. The two series are therefore connected, and
the change of teeth in the Mammalia may be regarded as a process
which has been developed from a polyphyodont condition.

Tomes, Ch. S., Manual of Dental Anatomy, Human and Comparative. London
1876.

§ 415.

The tongue is a second organ of the buccal cavity. In Fishes it
is generally a process formed by the investment of mucous mem-
brane of the body of the hyoid. It is flat, and movably connected
with the general branchial skeleton only. Like the other skeletal
portions of the wall of this cavity it frequently carries a number of
teeth. This organ is not provided with a special musculature below
the Amphibia, where it forms a thick, and in many, a protrusible
structure. It is not developed in Pipa and Dactylethra. As a rule,
the anterior end alone is connected with the floor of the buccal
cavity, and the postenor, and more movable portion, is drawn out
into two processes. In the Eeptilia there is likewise a muscular
tongue, which, in the Ophidii and Saurii, can be drawn out of a
special sheath. The epithelium of the tongue, which is ordinarily a
delicate organ, frequently develops scales and knobs on its upper
surface, while the anterior end is- drawn out into two fine points
(Fissilingues) (Fig. 310,2;). In the Chelonii, and especially in the
Crocodilini, the tongue is broad and flat. In Birds, the, anterior
end of the tongue is ordinarily covered by a cornified layer of
epithelium, and is sometimes beset by lateral barbs (Woodpeckers),
or fine setae (Toucan) ; it is in the Psittacidae only that the tongue
forms a larger fleshy organ. In the Mammalia we find that the
tongue is very large, owing to the greater development of its
musculature, while at the same time its investing mucous mem-
brane is provided with a number of differentiated papillae. The
function of the organ is chiefly that of aiding in the ingestion of

TONGUE OF VEETEBEATA.

553

food. In many Prosimii and Chiroptera, as also in the platyrrhine
Apes, there is a process below the tongue which is sometimes
double ; this is the so-called snblingua.

The glandular organs connected
with the buccal cavity are developed
from its mucous membrane. When
they are largely developed, and are
placed outside the mucous membrane,
their efferent ducts pass into it.

They may therefore be regarded as
greatly developed glands of the mucous
membrane. Larger glands of this kind
are placed between the nasal capsules,
and on the palate, in the Amphibia.

When they are much larger they may
extend on to the skull (intermaxillary
glands). The labial glands, which are
set along the edges of the jaws in the
Keptilia, must be mentioned (Ophidii
and Saurii). The poison-gland of the
Ophidii is a larger organ, but is merely
formed by a modification of simple
glands. In the Chelonii there is a
pair of glands below the tongue, which
are regarded as salivary glands. The
Saurii also are provided with similar
groups of separate glands. Larger
glands of this kind, which aid in the
production of a buccal fiuid, are also found in different regions.
They are constant in Birds and Mammals, and are distinguished
as sub-maxillary, sublingual, and parotid glands. In Birds the last
of these open at the angle of the mouth, but in Mammals, in the
vestibulum oris. These glands
are not developed in the Cetacea, A

and but feebly in the Pinnipedia.

The three pairs are largest in
Herbivora; sometimes one and
sometimes another pair being the
best developed.

Fig. 310. Hyoid apparatus, with the
tongue and trachea of Varanus.
e Median piece of the hyoid.
hf Anterior, li" Posterior cornu
of the hyoid. m m' Muscles.
tr Trachea, z Tongue.

§ 416.

Fig. 311. Sections through the body of
young larvaa of Petromyzon to show the
ventral groove, d Branchial cavity (after
Calberla).

Mention has still to be made
of the hypobranchial groove
and its derivates. It is an organ
differentiated from the primitive cephalic enteron (cf. p. 540). In
Amphioxus it extends all along the branchial cavity. Among the
Cyclostomata it has only been observed during the early larval con-
dition of Petromyzon (Fig. 304, h). As it does not extend along

554

COMPAEATIVE A^-ATOMY.

tlie whole of the branchial cavity, it appears to have been reduced,
in comparison with the same organ in the Tunicata. When the
organ which functions as a tongue is differentiated, the groove under-
goes still further reduction, and is converted into a canal, which is
gradually cut off from the superior cavity (Fig. 311), and is at last
completely separated from it. In the adult animal it is converted into
a complex of follicles, covered with epithelium, which extend from
the second to the fourth pair of branchial sacs. They form an
organ with unknown physiological relations — the thyroid gland.'

In the Gnathostomata a groove, remaining for some time, is
no longer developed, but at the homologous region a process of the

lumen of the cephalic enteron is
nipped off, and forms an azygos
follicle invested by epithelium. By
a process of gradual gemmation this
is broken up into a number of sepa-
rate follicles, which are united to-
gether by connective tissue. In
Fishes the organ is placed not far
from the point at which it was
formed ; that is, at the anterior end
of the trunk of the branchial arteries,
and between it and the copula of the
hyoid arch. In the Amphibia the
thyroid is placed near the larynx,
where it forms a paired coil (unpaired
in Proteus), and is set on the inner
surface of the posterior cornua of the
hyoid. It is sometimes broken up
into several groups. In the Keptilia
it is unpaired, and lies in front of the
aortic arches ; in Birds, however, it
is paired (Fig. 312, t), and lies
near the commencement of the
carotids. In both these divisions, therefore, it is removed some
way from the point at which it was developed. This appears to be
due to the shifting backwards of the great arterial trunks. Among
Mammals it is separated into two parts in the Monotremata, many
Marsupials, and various other forms ; while in the rest its two
lateral masses are united by a median bridge (isthmus). It always
lies just below the larynx, and on the trachea.

The preservation of this organ, which lost its primitive signifi-
cance even in the lower Yertebrata, throughout the long series of
higher forms, is explicable from the fact that it has been inherited
from what is phylogenetically a very early period ; it is an arrange-
ment, indeed, which was physiologically of great importance to the
ingestion of food in the Tunicata.

Muller, W., Die Hypobranchialriune der Tunicaten, etc. Jen. Ztschr. Bd. VII.

The same, Entw. d. Schilddriise. Jen. Ztschr. Bd. VI.

Fig. 312. Thymus (t/i) and thyroid
(f) of a mature embryo of Buteo
vulgaris, tr Trachea.

ALIMENTAEY CAISTAL OF YEETEBEATA.

555

Alimentary Canal proper (Enteron of tlie Trunk).

§ 417.

That portion of the tractus intestinalis, which serves exclusively
for the ingestion and alteration of food, commences at the hinder
end of the cephalic enteron ; this, which is the digestive tube in the
strict sense, has an apparatus differentiated from it at its anterior
boundary, which forms an air-bladder in Fishes, where it is in an
indifferent condition, and a respiratory apparatus consisting of lungs
and trachea in the Amphibia and all higher forms.

The most anterior portion of the digestive tube is not sharply
marked off from the cephalic enteron. As they are both innervated
by the vagus, there is reason for supposing that this portion was
primitively derived from the respiratory portion of the primitive
enteric tube after the atrophy of a large number of the hinder
branchial clefts, and that therefore it corresponds to the posterior
portion of the respiratory antechamber, which is so much larger in
Amphioxus.

In the Craniota, not only some of the peculiar relations of the
rudimentary enteron, but also later stages in the development of this
tube, are due to the relations of the egg to the general rudiment of
the embryo, and to an increase in the quantity of the yolk.

In the Selachii, the rudimentary enteron grows round the yolk,
but it is the groove-like portion only of the general rudiment, that
lying below the axial skeleton of the embryo, which is converted
into the enteron ; this is gradually shut off from the rest, or yolk-
beai’ing portion, which then appears as an appendage of the
enteron, the yolk-sac. This, which is at first placed apparently
outside the body, but which is surrounded by a continuation of the
integumentary layer, is merely connected by a stalk with the enteron
(external yolk-sac), and is gradually taken into the body (internal
yolk-sac). As the yolk is gradually used up, the yolk-sac is atrophied.
The Teleostei (and Ganoidei) are provided with a smaller quantity
of the nutrient material for the embryo, which constitutes the yolk.
Owing to the larger size of the yolk of the egg in Eeptiles and Birds,
there is a similar contrast between the enteric canal and the yolk-
sac, but the latter is not covered by the integument, for the parts
which in the Anamnia enclose it go to form the amnion, and another
of the foetal coverings of the egg. In the Mammalia also, where the
material of the egg is very greatly reduced in quantity, the rudi-
mentary enteron becomes nipped off from the embryonic bladder,
which represents the yolk-sac (Fig. 319). This arrangement may be
deduced from a condition which was distinguished by the possession
of a large quantity of yolk -material. The want of a large quantity of
yolk in the Mammalia is compensated for by the development of
the foetus within the maternal organism, and the more or less dose

556

COMPAEATIYE ANATOMY.

connection between tbe foetus and tlie uterus. A rudiment of tbe
yolk-sac is retained as tbe umbilical vesicle/^ wbicb is not taken
into tbe body-cavity, as it is not of any use in tbe nutrition of tbe
embryo, but is after birtb separated from tbe young animal with
tbe coverings of tbe egg.

Tbe divisions of tbe alimentary canal are tbe same as those in
tbe Invertebrata ; fore-, mid-, and bind -gut.

Fore-gut.

§ 418.

Tbe first portion of tbe alimentary canal proper is an exceedingly
short piece in Ampbioxus, and is placed directly in front of a
diverticulum wbicb is directed forwards, and is regarded as tbe liver.
If we consider that tbe liver is always derived from tbe portion
wbicb is to be regarded as tbe mid -gut, and that it forms its
anterior boundary, it follows that tbe fore-gut is exceedingly small
in many of tbe Craniota also. This character is seen in tbe Cyclosto-
mata, Cbimserae, and various Teleostei. Tbe rest of tbe Craniota difier
in this point, for their fore-gut forms a large piece, wbicb can be
divided into oesophagus, and stomach. In any case these parts
appear to have been acquired by tbe Gnatbostomata only, among the

Vertebrata. Tbe distribution of tbe
B vagus on their walls is of importance

as bearing on their origin ; and there
is in consequence of this distribu-
tion reason for supposing that tbe
tract in question has been developed
from a portion wbicb primitively
belonged to tbe cephalic enteron.
The reduction of a larger number
of branchial clefts, and tbe conver-
sion of a portion of the respiratory
antechamber to a purely nutrient
function, is in agreement with
this distribution. On tbe other
band, tbe extension of this tract,
and especially of tbe portion wbicb
represents the stomach, as well as
its position in tbe coelom, are due
to tbe great quantity of food in-
gested. Tbe stomach is almost
always separated from tbe mid-gut
by a fold of tbe enteric wall (pyloric
valve) .

In Fishes, the oesopbagus, wbicb is very wide and provided with
longitudinal folds of tbe mucous membrane, generally passes at

Fig. 313. Digestive canal of Fishes.
A Of Gobius melanostomus. B Of
Salmo. 0 CEsophagus. v Stomach.
i Mid-gut. ap Appendices pyloricae.
?• Hind -gut.

FORE-GUT OF VERTEBRATA.

557

B

ouce into tlie stomach, which can only be distinguished from it by
the differences in the characters of its mucous membrane. As a
rule, the stomach (Fig. 313) forms a csecal sac, which is directed back-
wards, and from which a narrow portion (pyloric tube) which bends
forwards, can be distinguished ; this leads ta the mid-gut {i) . This
is the case in all Selachii and Ganoi'dei, and in many Teleostei,
while the rest vary greatly in the absence, or the great development
backwards, of the ca0cal sac.

Among the Amphibia we find a lower stage in Proteus, for the
enteric tube, which has a perfectly straight course, has no stomachal
enlargement at all. In the other Urodela, however, the stomach
forms a wider portion of the enteron ; and this is the case also in the
Anura, where the stomach is sometimes,
indeed, placed transversely (Bufo).

Among the Reptilia, the fore-gut is of
a lower stage in the Ophidii and Saurii,
owing to the greater width of the oeso-
phagus and the straight course of the
stomach. However, there is an arrange-
ment in the Saurii which calls to mind
the pyloric tube of the Selachii, and from
this the stomach might gradually acquire
a transverse position. In the Chelonii and
Crocodilini the oesophagus is more sharply
separated from the stomach, which in the
former has a large and a small curvature,
owing to the great elevation of the pyloric
portion. Owing to the approximation of
the cardiac end of the stomach to the
pylorus, this portion is rounded in the
Crocodile, and is also distinguished by
a tendinous disc on each face of its
muscular wall ; in this point it resembles
the stomach of Birds.

In the fore-gut of Birds there is a
greater division of labour. The influence num.

of adaptation to the mode of life, and here

especially to the mode of nutrition, is most clearly shown by the varia-
tions in the different arrangements. The oesophagus, which is of the
same length as the neck, is either of equal calibre along its whole
course, or is provided with a widened portion (Fig. 314, A), or with
a caccal diverticulum (J5), which looks like an appendage. Portions
(f) of this kind, which are characterised by modifications of the
glandular organs of the mucous membrane, form a crop (ingluvies).
This is best developed in carnivorous and graminivorous Birds ; in
the former, indeed, it generally forms a spindle-shaped enlargement,
while in the latter it forms a unilateral diverticulum, which is dif-
ferentiated into a C80cal appendage, in many provided with a narrow
connecting piece.

Fig. 314. A Fore-gut of u
Eaptorial Bird (Buteo).
B Of a Fowl, oe CEsophagus.
i Crop, pv Glandular stomach.
V Muscular stomach, d Duode-

558

COMPAKATIVE ANATOMY.

The next portion of the oesophagus, which is generally narrower,
passes into the stomach, in which two divisions can be made out ; the
first is known as the proventriculus {A B pi') ; its walls are greatly
thickened by a glandular layer. The second portion is characterised
by the great development of its muscular layer, the strength of
which varies with the mode of life of the animal. Where it is
greatly developed we may observe a tendinous disc on either
side {A B). In the Paptores, as also in many Natatores that live on
animal food, the muscular layer is feebly developed. It is very
strong in the graminivorous forms (Gallinse, Anatinac, Columbas,
Passeres). This portion, which serves for the comminution of food,
aud compensates for the absence of masticatory organs, may be
provided with other arrangements also which serve the same purpose;
its inner surface may be covered by a firm horny layer, which is
often of considerable thickness, and functions as a radula. It is
produced by a glandular layer, the secretion of which passes into
this firm stiff condition.

In the Mammalia the fore-gut is more completely divided, owing
to the sharper delimitation of the oesophagus from the stomach, than
it is in almost any other division. In many cases the shape of the
stomach is of a low type. In the Phocidse it retains its position
parallel to the long axis of the body, while in other Mammals a
position transverse to this axis is the common one.

We must regard a number of peculiarities, which sometimes
consist in an enlargement of the internal space, at others of a
differentiation of the primitively single, and, as we must suppose,
uniformly functional stomach, into several portions of different
function, as the results of adaptation to the material of nutrition.

The first relation is implied by the transverse position of the
stomach, in consequence of which the great curvature gets to be
much the larger, and, forming a swelling behind the cardiac portion,
gives rise to the fundus of the stomach. This is absent in most
Carnivora, but is developed in the Monotremata, Marsupialia, Ko-
dentia, and Edentata, and is found also in most of the Primates.

When the fundus is more largely developed the stomach may
be divided into several portions, but this division is not unfrequently
implied by the characters of the mucous membrane only (Equus).
This arrangement is carried farther by the development of a trans-
verse constriction ; thus, in many Rodents, the stomach is divided
into a cardiac and a pyloric portion, to which smaller diverticula
may be added on. Similar stomachs of a more complicated character
may be seen in many Marsupials (Halmaturus), and in the Cetacea.
The fundus is always a considerable enlargement, which, in the
Cetacea, is succeeded by a number of diverticula, which are attached
to the pyloric portion ; these give the stomach the appearance of
being made up of from four to seven spaces which communicate
with one another by connecting pieces of varying width.

In the Ruminantia the complication is due to the share taken by
the oesophagus, the cardiac end of which bulges out on one side and

rOEE-GUT OF VEETEBEATA.

559

fuses with the stomachy of which it forms two divisions. The first
has the character of an enlarged fundus^ and is known as the rumen
or paunch (Fig. 315, Z) ; it functions essentially as an organ for the
reception of the large quantity of food that is ingested. Just below
the cardia it is connected with the second division, the reticulum
(ZZ), which is succeeded by the psalterium (omasus) ; this third
portion is wanting in the Tragulidse and Tylopoda. The last
portion, which is formed from the pyloric part, is attached to this ;
it forms the ab omasus, in ^
the mucous membrane of
which the rennet glands
are placed. A groove
(oesophageal groove) which
leads from the oesophagus
into the reticulum, and is
shut off by a valvular pro-
cess (Fig. 315, 5 s) from the
first two divisions of the
stomach, represents that
portion of the oesophagus
which has entered into the
formation of the stomach,
and formed the first two por-
tions of that organ by bul-
gingoutononeside. Thanks
to its presence the food that
has passed from the reti-
culum into the oesophagus,

and from thence into the month, can be directly returned, after it has
been sufficiently masticated, into the psalterium and abomasus,
while, when the groove is open, the fodder passes easily into the
paunch and reticulum. The influence of the food in determining
the size of the various portions may be seen from the differences
between the paunch and the abomasus at different periods of life.
The abomasus is relatively large in the calf, while later on the
paunch may be as much as ten times larger than the abomasus,
and even more than that.

7^

Fig. 315. Stomach of an Antelope. A From
in front. B Opened from behind, oe (Esopha-
gus. /Kumen. //Reticulum. ///Psalterium.
/V Abomasus. p Pylorus, s (Esophageal
groove.

Mid-gut.

§ 419.

The mid-gut (small intestine) which is generally separated from
the stomach by a circular fold, the pyloric valve, is characterised at
its commencement by having glandular organs (liver and pancreas)
connected with it. With regard to length it is the most variable
portion of the enteric tube. It is straight in the Cyclostomata,
some Teleostei, and in Chimaera. In the last it is distinguished by

560

COMPAEATIVE ANATOMY.

a spiral fold, wMcli is greatly developed in the Selachii, and divides
the greater part of the mid-gut into a number of more or less
closely applied coils (Fig. 316, C vs). In Carcharias this fold has
the form of a rolled-up sheet of paper. This spiral valve is
retained in the G-anoidei : it is reduced to almost nothing in
Lepidosteus. It is not present in the Teleostei.

An enlargement may be observed at the commencement of the
mid-gut of the Selachii ; in the Sturiones there is a large, and
externally much diverticulated glandular organ at the same point ;
it is divided internally into a number of spaces corresponding to the
diverticula. In Lepidosteus the various portions are more sharply
separated from one another, and have the appearance of groups of
short csecal tubes which beset the pyloric portion of the mid-gut,
and, as in many Teleostei, form the appendices pyloricae (Fig. 316,
A B a/p). They beset a certain portion of the mid-gut and vary in

Fig. 316. Enteric canal of Fislies. A Of Salmo salvelinus. B Of Trachinus
radiatns. C Of Squatina vulgaris, oe (Esophagus, v Stomach, dp End of
the air-duct, p Pylorus, ap Appendices pyloricae. d Ductus choledochus. vs Spiral
valve, i Mid-gut. c Hind-gut, x Its appendage.

number and size. They sometimes open separately into the gut,
sometimes are united into larger trunks, and give rise to branched
structures. They are most numerous in the Gadidae and Scom-
beroidae. In many Fishes the different caeca are held together by
connective tissue and united at a common efferent duct, in which
case they have the appearance of a compact gland (Scomberoidae),
while their affinity to the gland in the Sturgeon is implied by the
frequent union of their orifices.

In many Teleostei the mid-gut is much longer than the tract
of the coelom which is given up to it, and it is then arranged in
coils (Fig. 316, B i)j or in several ascending and descending loops.
This implies an adaptation to the cavity of the coelom, whilst the
elongation of the tract, which is always derived from a straight

MID-GUT OF VEETEBEATA.

561

rudiment^ is an adaptation to the functions required of it by the
ingesta.

In the Amphibia the simple condition of the mid-gut is very
rarely permanent (Proteus). It generally forms, as it does also in
Keptiles, a longer tube, and, consequently, a
number of coils. (Fig. 317, i.) In the Ophidii
these are least, in the Chelonii they are con-
siderably, and in the Crocodilini they are still
more developed. The mid-gut is very greatly
elongated in the larvae of the anourous Amphibia,
where this portion forms a long loop arranged
in spiral coils. It is reduced when the mode of
feeding is changed during the final stages of
larval life, and this leads to an abbreviation of
the length of the enteron.

The length of the mid-gut in Birds also varies
very greatly according to the characters of their
food. It is arranged in loops, the first of which
(duodenal loop) is the best developed, and always
contains the pancreas.

The mid-gut of Mammals is seen no less dis-
tinctly to vary in length according to the kind of
food that is eaten ; so that there are different
conditions in Carnivorous and Herbivorous forms.

The surface of the mid-gut is increased by
various arrangements of its mucous membrane,
as well as by its increase in length. In the lower
groups there are coarser folds (spiral valve of
the Selachii), but in the Amphibia and Eeptilia
by far the most common arrangements are fine
longitudinal folds of the mucous membrane. These obtain also in
the Birds, but in them they generally form unequal elevations, and
may be united by transverse lines. Fine folds arranged in zigzag
lines are seen in the Amphibia and Eeptilia, and are found also on
the mid-gut of Birds. In Mammals, these longitudinal folds of
the mucous membrane are commonly found in the Cetacea ; but in
most of the other Mammalia the mucous membrane is smooth, or
raised up into transverse folds, which are very generally beset with
villi. When the folds are feeble we find that these villi are greatly
developed in Birds also, while when the folds are present the villi
are merely smaller elevations.

Fig. 317. Enteric
canal of Menobran-
chus lateralis.
p Commencement of
the fore-gut with the
Pharynx. oe QEso-
phagus. V Stomach.
i Mid-gut. r Hind-
gut.

Hind-gut.

§ 420.

The end- or hind-gut is the smallest of all in the lower
divisions, and is merely represented by a short and somewhat wider

2 o

562

COMPAEATIVE ANATOIMY.

piece (Fig. 313, r; 316, (7 c). In the SelacFii it is provided with a
special glandular appendage (Fig. 316, Ox). It is only in tlie Amphibia
that, owing to its greater length and width, it becomes of some im-
portance, but in them, as in the Eeptilia, it retains its straight
course in correspondence with its shortness. In consequence of this
straight course it has got the name of rectum. It is generally
separated from the mid-gut by a transverse fold or valve. Many
Eeptiles are provided with a caecal appendage, which, in the Ophidii
is feebly, and in the Saurii is better developed. The caeca in Birds
are much more independent. In this group, also, the hind-gut is
short and straight (Fig. 320). The caecum is generally paired, and
is absent in a few families only (e.g. Woodpecker, Psittacus, etc.).
They vary greatly in the extent to which they are developed, so that
they may form short papilliform appendages, or very long tubes
(Apteryx, Gallinae, Anseres).

The hind-gut is longest in the Mammalia, where it forms the
large intestine, and is distinguished, as such, from the mid-gut, or
small intestine. Owing to its greater length it is arranged in coils,
so that the terminal portion, only, has the straight course taken by
the hind-gut of other Vertebrata. The anterior portion ordinarily
forms a loop which bends from the right side of the abdominal cavity
forwards, and then to the left, and then again backwards to be
continued into the rectum. This loop is sometimes broken up into
secondary loops.

At the boundary between it and the small intestine caecal struc-
tures are likewise developed, but these are rarely arranged in
pairs (Fig. 318, c d), and are commonly single. The size of this

caecum may be shown to depend
on the food. In the Carnivora
it is short, and sometimes com-
pletely absent (Ursina, Muste-
lina) : it is very large in the
Ilerbivora, where its length is
compensated for by that of the
colon.

The caecum itself may be
affected by differentiations. Its
terminal portion is frequently
diminished in size (e.g. in various
Prosimiae and many Eodents)
(Fig. 3 1 8, c) . In various Primates,
and in Man, the terminal portion, which, at first, is as wide as the
rest, is not developed in proportion to it ; it thus becomes more and
more distinct from the other portion, which continues to grow
wider, until at last it forms a mere appendage to it — the appendix
vermiformis.

The hind-gut primitively opens into the same space as the urinary
and generative ducts, the cloaca. This arrangement, which ob-
tains in the Selachii, Amphibia, Eeptilia, and Aves, is permanent in

Fig. 318. Csecum and colon of Lago-
mys pusillus. a Small intestine.
h Opening of the larger (c), and of the
smaller (d) csecum. e f g Diverticula of
the colon (after Pallas).

APPENDAGES OF THE MID-GUT OF YEETEBEATA. 563

the Monotremata only, among Mammals, in the rest of which it is
confined to the embryonic stage, and subsequently the hind^gut
opens to the exterior by means of an anus,

Organs appended to the mid-gut.

§ 421.

Two large glandular organs, the liver and pancreas, are
connected with the beginning of the mid-gut ; they are both dif-
ferentiated from the walls of the rudimentary enteron.

In Amphioxus an organ, which must be regarded as the liver,
has the form of a caccal tube (Fig. 303,/), which arises close to the
commencement of the alimentary canal, and is directed forwards
(Fig. 303,/). It is provided with an epithelial investment of a
greenish colour. A similar condition is
seen in the Craniota during the earliest
stages of development, in which the rudi-
ment of the liver has the appearance of a
paired diverticulum (//) of the enteric
tube, lying behind the rudimentary stomach
(Fig. 319, d). It is partly formed by the
epithelial layer of the rudimentary enteron
(endoderm), and partly by the external layer
developed from mesoderm. As Eeptiles,

■ Birds, and Mammals agree in this point,
this condition must be regarded as a funda-
mental one, while at the same time it calls
to mind the morphological characters of
the hepatic organ in Amphioxus and many
Invertebrata (Vermes, Mollusca).

Owing to the thickening of the splanch-
nopleure and its large connection with the
venous portion of the vascular system,
together with the simultaneous thickening
of the endoderm, relations are produced,
which distinguish the liver of the Craniota
from that of the Acrania, as well as from
that of the Invertebrata. While the first
rudiment of the liver appears as a diver-
ticulum, the later differentiations are
brought about by the thickening of the
endoderm, and give rise to solid chords of
cells which grow into the layer of mesoderm, and the vascular
apparatus embedded in it ; these give off new buds, and are finally
connected together in a retiform manner. The parenchyma of the
liver is formed by these primitively solid chords, and their secondary

2 o 2

Fig. 319. Rudiment of the
enteric canal and its ap-
pendages in an embryo
of the Dog, seen from the
ventral surface, a Diver-
ticula of the enteric tube
towards the visceral clefts.
h Rudiment of the pharynx
and larynx, c Of the lungs.
d Of the stomach. / Of the
liver, g Walls of the yolk-
sac in connection with the
mid-gut. h Hind-gut (after
Bischoff).

564

COMPAEATIVE ANATOMY.

and otlier processes, wEile tEey give rise to tlie bile- ducts by tbe
formation of intercellular passages, wbicb run in tbe axes of tbe
epithelial chords. Tbe hepatic lobes, wbicb are formed on either
side, fuse with one another into a single organ. Tbe two primitive
diverticula, after they have formed tbe bile-ducts in tbe parenchyma
of tbe liver, and have been continued into tbe network of cellular
chords, form tbe efferent ducts of tbe liver.

Tbe liver, which is thus differentiated from tbe enteron, forms
a compact, and ordinarily, very large organ ; it is embedded in a
fold of tbe peritoneum, wbicb extends from tbe anterior portion of
tbe enteric tube to tbe anterior wall of tbe abdomen.

In Fishes, tbe liver generally forms a single, undivided mass, but
sometimes it consists of two, or more lobes. There are two large

portions in tbe Amphibia ; it is gene-
rally simple in tbe Opbidii, and is
merely notched at tbe margin in tbe
Saurii ; in tbe Crocodilini and Cbelonii
it is again divided into two lobes,
wbicb in tbe latter are widely sepa-
rated from one another, and united
by a slender transverse bridge. Ordi-
narily two lobes are, sometimes more,
sometimes less, indicated in tbe Mam-
malia. In the Carnivora, Rodentia,
some Marsupialia, Simiae, and others
we find, indeed, multilobate forms, but
these may be referred to two larger
primary lobes.

There are various modifications in
the character of tbe efferent ducts
(ductus bepato-enterici) in relation to
their primitively double character ; for
either tbe first condition persists, or
tbe two ducts are gradually fused to-
gether, that is to say, tbe diverticulum
of tbe enteron is converted into a
single duct, or, lastly, tbe primitive
ducts are atrophied, and secondary
canals are converted into efferent
ducts; in this case there is a large
number of ducts (in tbe Saurii and
Opbidii). A unilateral caecal diverti-
culum, the gall-bladder (Fig. 320,/)
is placed on these ducts ; it has very
various relations, and is by no means
a constant structure.

Tbe pancreas is developed in the
same way as tbe liver — from a diverticulum of tbe wall of tbe
enteron, wbicb is developed behind the rudiment of tbe liver. Tbe

Fig. 320. Enteric canal of Ardea
cinerea. i (Esophagus and crop,
pr Proventriculus. v Gizzard.
v' Antrum pylori, d Duodenal
loop, it Mid-gut. h Hind-gut.
c Part of the single caecum.
cl Cloaca and Bursa Fabricii.
h Liver. dh Ductus hepato-en-
tericus. / Gall-bladder, p Pancreas.
dp Pancreatic duct.

MESENTEEY OF VEETEBEATA.

565

epitlielial layer of tEe mdimentary enteron forms tEickenings from
wliicE tEe glandular lobules and tEeir efferent ducts are developed
by a process of gemmation, wEile tEe pancreatic duct is derived
from tEe first rudiment of tEe gland. TEis organ, wEicE is never
absent except in some divisions of FisEes, is always placed close to
tEe commencement of tEe mid-gut, or close to tEe stomacE;
its duct is frequently united witE tEat of tEe liver, or passes into
tEe enteric canal in company witE it. Not unfrequently tEere
are two ducts (CEelonii, Crocodilini, Aves [Fig. 320], and some
Mammalia), one of wEicE is, as a rule, connected witE tEe ductus
Eepato-entericus.

Mesentery.

§ 422.

As tEe enteric canal is developed tEe peritoneal fold, wEicE en-
closes it, is developed also ; it fastens tlie canal to tEe Einder wall
of tlie abdomen. TEis double lamella, wEicE contains tEe enteron,
forms tEe mesentery; tEat portion of it wEicE goes to tEe stomacE
is known as tEe mesogastrium. TEis does not, Eowever, merely
enclose tEe stomacE, like tlie mesentery of tEe greater part of tEe
mid-gut, but Eas its two lamellse continued from tlie stomacE into a
double fold, wEicE extends to tEe anterior wall of tlie abdomen,
wEere it is again connected witE tlie peritoneum of tEe abdominal
wall. The liver is contained in this continuation of the meso-
gastrium to the anterior wall of the abdomen, so that this organ Eas
not only a peritoneal investment, but is also connected with the
enteric tube (and especially the stomacE and the first part of the
mid-gut), and with the ventral wall of the coelom. As long as the
enteric tube retains its primitively straight course the relations of the
mesentery are simple, and peculiarities in it are only due to the
partial absorption of large tracts, as is the case, for example, in
Fishes. The increased size also of the liver brings about changes in
the characters of the fold which passes from the stomacE to the
anterior wall of the abdomen ; this fold, where it forms the connection
between the latter and the stomacE, is known as the small omentum.
TEat portion of it which extends to the wall of the body forms the
suspensory ligament of the liver. Other changes are effected in it
by its relation to the diaphragm, by the curvature of the stomacE,
and by the elongation of the mid-gut, which cause the mesentery to
be arranged in frill-like folds. These relations are seen as early as
FisEes ; they are still simple in the Amphibia, and in the OpEidii and
Saurii ; in the CEelonii and Crocodilini they are especially modified
by changes in the form and position of the stomacE.

The changes of the mesogastrium are most considerable in the
Mammalia. As the stomacE alters its position this membrane grows
out into a wide sac (bursa omentalis), which either hangs down over

566

COMPAEATIYE ANATOMY.

tlie loops of tlie mid-gut, as in most Mammals, or partly conceals tlie
stomacli (Ruminantia) .

The mesentery of the hind-gut retains its primitive characters in
Vertebrata with a short hind-gut. When that portion of the hind-
gut which is known as the colon increases in length, as it does in
the Mammalia, the mesentery or mesocolon accompanies it, and is
at the same time raised by one portion as far as the root of the
mesogastrium so that the two arise close together. This gives rise
to that gradual connection between the mesocolon and the posterior
fold of the mesogastrium, which is seen in the Primates, and which
ends with the condition seen in Man, where a portion of the colon
(C. transversum) is enclosed in the hinder wall of the omental sac.
At the same time the anterior and posterior walls of the omental
sac grow out, so that the great omentum is formed, which consists
of four lamella) of the peritoneum.

Pneumatic Organs of the Enteric Tube.

§ 423.

As the enteric tube is the road for everything that is taken in
from the exterior, the water which serves in respiration as well as
the substances that will be converted into food in the organism, so,
too, the enteric tract may take in air, which is collected in special
spaces which are differentiated from it, and which, therefore, repre-
sent portions of the primitive enteric tube. This ingestion of air
leads us to suppose that sometimes, at any rate, the animal came to
the surface of the water ; indicating a not unimportant stage
between those in which life was passed in water exclusively, and
those in which life was also possible out of this medium.

The organs which are formed when air is taken in are known as
air-bladders. It is still uncertain what, is the practical use of
these organs to the whole organism, but, as they are found in so
many forms, they must be regarded as important parts. The arrange-
ment of air-spaces in the body of aquatic animals must have some
influence on the specific gravity of their bodies, so that there is
good reason for supposing that these organs have a hydrostatic
function.

A great change occurs in this character when the relations of
the circulatory system are changed. The organs have a respiratory
function, for the air in them exchanges its gases with those in the
blood which is brought to the wall of the organ, so that the blood
which passes away from it is richer in oxygen. The organ, there-
fore, becomes one of the respiratory organs, and is called the lung.
The first point in this metamorphosis is not the change which has
occurred in the blood-vascular system, but rather the commence-
ment of an exchange of the gases in the blood of the walls of the

AIE-BLADDEE OF VEETEBEATA.

567

organ,, and the air contained in the organ itself. This must happen
wlienever the blood brought to the organ is less rich in oxygen than
the air in the organ. The changes in the vascular system do not
occur till this has happened.

The pneumatic organs of the enteric tube are divided^ therefore,
into two series which are functionally very different, though morpho-
logically homologous, and each series undergoes a large number of
differentiations.

a) Air-Bladder,

§ 424.

This organ is not found in Amphioxus, nor in the Cyclostomata.
In some Sharks (Galeus, Mustelus, Acanthias) there is a diverticulum
of the dorsal wall which opens into the pharynx, and which may be
regarded as the rudiment of an air-bladder. All the Ganoidei, and
very many of the Teleostei, have air-bladders. If we examine the
arrangements which obtain in the Granoi'dei more closely, we find
that there is a single or paired sac, which is connected with the
pharynx by an air- duct of varying length. This opens on to the
upper wall of the fore-gut, and, generally, at the same point as that
at which we find the short ceecal sac in the Selachii. In Acipenser
the opening is placed very far back, and the air-bladder is connected
with the stomach. In Polypterus we find a paired air-bladder
(Fig. 321, A), which opens on to the ventral wall of the oesophagus ;
in Lepidosteus it is placed on the dorsal surface, and, though single
externally, is divided into two longitudinal halves by the trabeculao
which traverse it, and each of these halves is again divided into
smaller cellular cavities by a number of processes and bars; the
inner surface is thereby greatly increased in size. In Amia, also,
the cellular air-bladder is divided by a fold, and is continued
anteriorly into two short cornua. In the three last-mentioned
Ganoids the bladder opens into the enteron by a short and some-
what narrow air-duct which leads to a longitudinal cleft. Even in
the Ganoidei, therefore, there are great variations in the characters
of the air-bladder, and their significance must be estimated in rela-
tion to the fact that this division now consists of but few living
forms. It is very significant that in the various stages of the
air-bladder in the Ganoidei all the essential arrangements are
recognisable which are presented by this organ, either under the
form of the air-bladder of the Teleostei, or the lung of the higher
Vertebrata.

In one division of the Teleostei the air- duct is persistent (Phy-
sostomi); in the rest it is developed for a time only, for it disappears
again after the development of the air-bladder (Physoklisti) ; in
many, finally, the air-bladder ceases to be developed. This last
character is, moreover, very variable, even within the limits of

568

COMPAEATIYE ANATOMY.

single genera. Tlius, some species of tlie genus Scomber do, and
others do not, possess an air-bladder.

There are great variations in the mode of connection between
the air-duct and the gut. It may open at the sides, or above,
and in all regions of the fore-gut from the pharynx as far as the
end of the stomach. It varies very greatly in form also. In the
Cyprinoids it is divided transversely into two portions which lie
one behind the other, the air-duct being given off from the hinder
one (cf. Fig. 301, m n). In others there are lateral diverticula,
which may become simple or branched processes (Fig. 321, B 0 a).

The air- duct, which is
often very narrow and
long in the Physostomi,
is ill adapted for the
passage of air ; in the
Physoklisti air cannot of
course be taken in in
this way. In the latter,
therefore, the air in the
air-bladder must be re-
garded as secreted from
the walls of the bladder,
while in many Physos-
tomi the air- duct can
only serve as an occa-
sional outlet for this air.
In texture the walls of
the organs resemble those
of the gut, but there
are at the same time a
number of special dif-
ferentiations which it is
beyond our purpose to
speak of here. The various adaptations of the air-bladder to other
organs, as, for example, its connection with the auditory organs of
many Physostomi, are differentiations of this kind (cf. supra, § 100).

In the Dipnoi the air-bladder is more lung-like in character.
Although in its external characters the organ is just like an air-
bladder, yet there is an essential difference owing to the presence on
it of afferent veins and efferent arteries ; and, owing to this change,
the air-bladder is henceforward to be regarded as a respiratory
organ. In Ceratodus, where, indeed, it only occasionally functions
as a lung, it is formed of a single sac, which extends along the
whole of the dorsal region of the coelom, and presents indications
merely of longitudinal division ; in Lepidosiren and Protopterus
it is divided into two halves.

Polypterus bichii' (after J. Muller). B Of
Johnius lobatns. C Of Corvina trispinosa
(after Cuvier and Valenciennes), a Appendages of
tbe air-bladder, h Its orifice.

LUNGS OF VERTEBEATA.

569

b) Lungs.

§ 425.

As the pneumatic organ appended to, and differentiated from,
the primitive wall of the enteron takes on its respiratory form, we
meet with a gradual differentiation of that portion of it which cor-
responds to the ductus pneumaticus, and which gives rise to air-
passages. These are the means of communication between the
lung and the pharynx, and are divided into several portions which
take on new functions ; the most important
of these is an organ for the production of
voice. The lungs themselves are now paired
organs. This paired condition, however, is
less due to a further development of that
division, which is indicated in many air-
bladders, or even to the completion of such
division, than to the development of the
organ in a manner which is adapted to its
position. As it is always connected with
the ventral surface of the pharynx, it is
easy to see how it is that an air-carrying
organ developed from this point would
grow out on either side. When filled
with air the two halves must be blown
out into a dorsal position, the result of
which must necessarily be the formation
of two completely separate lungs, which
are only connected together by the ventral
air-passage.

We have as the first point in the diffe-
rentiation of the trachea a short wide
canal which connects the two lungs with
the pharynx. As this canal increases in
length it develops, in its wall, cartila-
ginous organs of support, and is divided
into two branches, which pass to the
lungs. A paired and an unpaired portion
can therefore be distinguished in the air-
passages. The supporting organs of these
canals, which are generally very short in
the Amphibia, are formed by two lateral
bands of cartilage (Fig. 322, A u), which
are continued as far as the commence-
ment of the lungs (b) (Proteus); in others

(B) the upper ends (a) of these two pieces are separated off and
form the groundwork of a special portion, which is henceforward

Fig. 322. Cartilage of the
larynx in the Amphibia and
Eeptilia. A Of Proteus.
B Of Salamandra. C Of
Eana. D Of Python.
a Aryteenoid cartilage.

h Supporting cartilage,

which in A P and C forms
the skeleton of the paired
and unpaired portions of the
trachea ; in P it is merely
represented by the com-
mencement of the unpaired
portion of the trachea (after
Henle).

570

COMPAEATIVE ANATOMY.

seen to be entrusted with tlie function of a vocal organ, and is
known as the larynx. Part, therefore, is thus differentiated from
the rest of the air-passage. The rest has very much the same
characters in its unpaired portion, or trachea, and its paired or
bronchial portion : the larynx varies much more considerably. In
the Amphibia the two cartilages above mentioned, and known as
the aryt^enoid cartilages (u), form a support for the two folds which
bound the entrance into the larynx. The change in the position of
the cartilages, which is effected by muscles, opens and closes the
entrance into the larynx. Functionally, therefore, they are of more
.importance than the more indifferent parts which have the character
of supporting organs. These arytaenoid cartilages rest on the
anterior ends of the two longitudinal ridges of cartilage, which
in other forms are connected on the ventral surface by transverse
processes directed towards one another, and which thereby gave
rise in many Amphibia to an unpaired portion of the supporting-
framework of the larynx {C c).

In the Reptilia the two longitudinal ridges are more perfectly
connected transversely, but a low condition is implied by their
continuous connection with the arytaenoid cartilages ; and this is
especially the case in some of the Ophidii. In others, these cartilages
are more completely separated (D a). This character is even found in
the Saurii, but in them the portion which carries the arytaenoid carti-
lages is converted into a circular, and generally a closed piece. A
second portion of the larynx may then be distinguished, which forms
the circular cartilage, and is to be found in course of formation as
early as the Amphibia {G c). In the Chelonii and Crocodilini this
is more completely separated from the tracheal skeleton, and its
anterior portion is considerably widened out. Not unfrequently it is
possible to make out in it indications of its connection with several
rings of cartilage. In Birds, this circular piece is made up of one
anterior and broader, and two ^Dosterior and more delicate portions ;
a small piece is, further, placed on the latter and carries the
arytsenoid cartilages.

Lastly, in the Mammalia the large circular piece of the Reptilia
is divided into two portions, for the anterior upright plate forms the
thyi-oid cartilage, while there is a second piece, which is circular
and generally very massive posteriorly (cricoid cartilage), and this
carries the aryteenoid cartilages on its posterior and more elevated
portion.

§ 426.

Other parts, which serve more or less for the production of the
voice, are connected with this laryngeal skeleton. Of these, the folds
of mucous membrane which are placed at the sides of the entrance
into the larynx are worthy of note, for they are converted by tension,
and by the development of elastic tissue into vocal chords. They
enclose a fissure— the glottis, which can be altered in width owing
to the attachment of the vocal chords to the movable arytaenoid

LU^^GS OF VERTEBEATA.

571

cartilages. These chords are found in most of the Anura^ in the
Geckos and Chamaeleons among the Saurii^ and in the Crocodilini.
They are not found in the Ophidii.

In Birds, the vocal organ is placed in the lower portion of the
trachea, and forms the so-called lov/er larynx. This arrangement
compensates for the absence of vocal chords in the true larynx.
Among Mammals the chords are atrophied in the Cetacea only ; in all
essential points they present just the same arrangements as in Man.

As the various cartilaginous pieces are differentiated from the
primitive laryngo-tracheal muscle, muscles are separated off to move
the portions which have become free. In the Reptilia these are
replaced by a constrictor and a dilatator muscle, which are also
present in a modified form in Birds. In the Mammalia, the
musculature is complicated in comparison with the simpler arrange-
ment found in Reptiles ; this is seen partly in the number, and partly
in the arrangement of the muscles. They correspond in all essential
points to what obtains in Man.

A process which hangs over the entrance to the larynx from in
front — the epiglottis — is merely indicated in the Reptilia by a process
of the supporting cartilage ; this is sometimes of no small size. It
is well developed in Birds. Many Birds, however, are provided with
a special epiglottis, the cartilage of which is only suturally connected
with the supporting cartilage. But these arrangements never
succeed in completely covering the entrance into the larynx. The
cartilage of the epiglottis is more completely separated in the
Mammalia, where it forms a protective apparatus during the passage
of the food over the entrance to the larynx. In the Sirenia it is
atrophied, while in the Cetacea it is converted into a long spout-like
piece, which unites with the similarly elongated arytsenoid cartilages
to form a cone, which projects into the internal nares, through
which the air passes in and out.

In some of the Amphibia the portion of the air-passage which
commences at the larynx is more distinctly divided into the trachea
and its two branches, the bronchi; these pass directly into the
walls of the pulmonary sacs. The ends of Hhe laryngo-tracheal
cartilages extend into these, either in the form of finer processes
(Menobranchus, Menopoma), or as broader pieces which give off
lateral processes (Bufo). As the transverse processes grow towards
one another at the anterior end of these ridges (cf. Fig. 322, G h),
they represent the earliest cartilaginous rings. These rings are
developed on the trachea of the Reptilia, which is generally long,
and either are, or are not complete. In the Ophidii and Saurii a
remnant of the primitive arrangement is indicated by the connection
of the rings with one another by means of longitudinal ridges.

The trachea of Birds is always distinguished by its great length,
and the rings, which are generally completely closed, are always more
largely differentiated. The two bronchi have the same structure.
We frequently meet with enlargements at various points in the
trachea (Natatores), while in many Birds this tube does not keep to

572

COMPARATIVE ANATOMY.

a straiglit course. Tliis is the case in the Penelopidae^ various Swans,
and in the Crane. In the last named a loop of the trachea is even
enclosed in the sternum.

The most peculiar character is the formation of a lower larynx
(Syrinx) in the Carinatse ; as a rule this is formed by the end of the
trachea, and by the commencement of the bronchi. The variations

in the form of this portion are due to
lateral compression, or to the fusion of
several rings at the end of the trachea;
this latter is divided by a bony ridge
(pessulus), which rises up at the angle
of furcation of the bronchi and forms
the tympanum. A membrane (mem-
brana tympaniformis interna), is
stretched, as if on a frame, from the
median surface of either bronchus to
the pessulus. The membrana tympa-
niformis externa is stretched between
the last tracheal and the first bronchial
ring, or between a pair of modified
bronchial rings. In the male Anatidm
we find various kinds of vesicular,
and asymmetrical enlargements of the
tympanum. In Singing Birds, a fold,
the membrana semilunaris, projects
inwards. As there are vocal membranes, which are elastic folds of
the mucous membrane, on either bronchus, a double glottis is
marked ofi. The action of a special supply of muscles varies the
tension of the vocal chords, and narrows or widens the glottides.
Several pairs of muscles pass into the trachea, which depress it, and
relax the vocal chords. In addition to these, the Singing Birds are
distinguished by the possession of a muscular apparatus, formed of
five or six pairs of muscles (Fig. 323, a — -/).

§ 427.

In, and above the Amphibia the lungs, which commence at the
ends of the air-passages, form the respiratory organs of the higher
Vertebrata. They do not form the sole organ quite at once, for in
all Amphibia there are branchiae either during the larval stage, or
throughout life (Perennibranchiata) . There is a series of similar
differentiations in the anatomical characters of the lungs, as in the
air-passages which lead to them. Simple sacs are gradually re-
placed by complicated organs, the respiratory surface of which is
continually increased by the formation of smaller internal cavities.

Among the Amphibia the lungs are exactly like those of the
Dipnoi ; in the Perennibranchiata their internal cavity has its
surface only slightly increased. In Proteus and Menobranchus
they are formed of simple, but very long tubes, which are slightly

Fig. 323. Lower larynx. Muscular
organ of voice of the Crow.
A From the side, B From in
front, a — f Muscles which move
the lower larynx. g Membrana
tympaniformis interna.

LUNGS OF VERTEBEATA.

573

widened out anteriorly, but always end in an enlargement. The
plexiform processes in the walls of tbe lung of Cryptobranclius are
more considerable, but they are very small in Triton. This is
frequently the case also in the other Salamandrina, but in the
Anura they are divided into smaller spaces by a close-set network.
The lung is thereby enabled to expose a larger quantity of blood for
the exchange of gas. This relation is still more complete in the
Reptilia. Although many (most Saurii) have very simple lungs, yet
in the Ophidii, as well as in the Crocodilini and Chelonii, each lung
is divided into a number of large cavities, which again are divided
into smaller ones of various kinds. In the Ophidii the lungs may
be seen to be adapted to the elongated form of body and to be
themselves elongated; the more or less atrophied condition of one
lung is a similar adaptation. The elongation of the lung is accom-
panied by a peculiarity of the last portion of the lung, which is
generally a good deal enlarged, gets simpler in structure and loses
its respiratory function. Similar portions, which have lost their
respiratory function, may be seen in the Saurii. As in the Ophidii,
the most anterior portion, or that which is above the point at
which they are connected with the air-passages, carries a more
closely-set meshwork, while the hinder end has its internal surface
less largely increased. In the Chamseleons special cmcal tubes
are given off from this portion, and project far into the coelom.
They are rudiments of an arrangement which has other functional
relations in the Birds.

In Birds conical prolongations are developed on the surface of
the lung during the embryonic period, and these become con-
nected with other organs and form cavities for the passage of
air. In the adult this pneumatic apparatus is formed of mem-
branous sacs embedded between the viscera, or of tubes which
extend into parts of the skeleton. In the latter case air-cavities
take the place of the medulla, which disappears, and so diminish
permanently the specific gravity of the animal. In the same way,
the specific gravity of the animal may be diminished at will by
the blowing out of the sacs between the viscera, and this, like the
other arrangement, is of assistance in flight.

As to their minute structure, the lungs of Birds have their finest
cavities connected with one another. The parenchyma of the lung
is spongy. In the Mammalia the lobate arrangement is continued
into the smallest portions of the lungs, while larger lobes may be
made out externally.

There are greater peculiarities in the position of the lungs. In
the Amphibia, and in the Saurii and Ophidii, they project into the
coelom. In the Chelonii and Aves they are placed on the dorsal
wall of the thorax, and are invested anteriorly by the peritoneum.
In the Crocodilini each lung is placed in a serous sac, by which it is
invested. The same is the case in the Mammalia, where the lungs,
which are covered by a pleural investment, occupy the lateral halves
of the thoracic cavity.

574

COMPAEATIYE AE'ATOMY.

Coelom.

§ 428.

In all Vertebrata^ just as in many of tlie Invertebrata^ a bollow
space is differentiated around the enteron of the trunk; this is
effected by the cleavage of tlie mesoderm. It is^ therefore^ a
space which appears in the middle germinal layer, and separates the
epithelial lining of the digestive tract and the parts developed from
it from the organs differentiated from the outer germ-layer. The
limitation of this process of separation to the region of the trunk
appears to be correlated with the formation of branchial clefts in
the cephalic enteron, for this prevents any continuation forwards of
the cleft, at any rate laterally. As in the Invertebrata, the coelom
forms a cavity, which is a part of the vascular system, in so far, that
is, as the lymphatic portion is connected with it. The direct com-
rannication with the exterior which obtains in many Invertebrata
is not completely wanting, although it is not greatly developed. It
is represented by an abdominal pore which is placed near the
anus, and which is generally paired ; this is seen in the Cyclostoma
and among Gnathostoma also, in the Selachii, Chim^erae, Ceratodus,
and many Teleostei ; it is last seen in the peritoneal canals of the
Crocodile. In the Chelonii there are merely indications of it. We
should also note that there is a free communication between the
coelom and the excretory apparatus, for a connection with the con-
dition of lower organisms is thereby demonstrated (vide Excretory
organs).

The whole of the inner surface of the coelom is invested by a
layer of epithelium, which is greatly developed in a certain region,
where it forms the germinal epithelium. The female germinal
glands are differentiated from this tract. In the anterior portion of
the coelom of the lower divisions ciliated epithelium is distributed
over certain regions. The epithelium of the coelom unites with a
subjacent layer of connective tissue to form a special membrane, the
peritoneum, which is continued on from the walls (where it forms
the parietal layer) to the parts (viscera) which are placed in, or
project into the coelom ; and these parts it also covers (visceral
layer) .

In the Anamnia, the coelom is a single cavity, as it is also in
most of the Eeptilia, although in the Crocodilini we may observe
that an anterior portion is being separated from a hinder one. In
the Mammalia this separation is complete. The diaphragm separates
the hinder portion of the coelom, or abdominal cavity, from an
anterior one, which contains the two lungs, and which is divided into
two lateral halves (pleural cavities), which contain the lungs, by a
median wall or partition, which contains the pericardium.

VASCULAR SYSTEM OF VERTEBRATA.

575

Vascular System.

§ 429.

The nutrient fluid of the Vertebrata moves along closed canals,
which have proper walls ; it is very rarely that this tract becomes
lacunar in character. In this point it differs from the arrangements
which obtain in the Mollusca, though it is more nearly similar to those
of the Vermes. Its cavities form a system of canals — a vascular
system. This portion is derived from the mesoderm, which also
develops the parts that carry the vessels. The chief trunks are
placed in the middle line, and give off branches in accordance with
the segmentation of the body ; looked at in the broadest way, the
arrangement of these vessels calls to mind many arrangements which
obtain in the Invertebrata. This may be still more distinctly seen
in the relations of the longitudinal trunks to the respiratory portion
of the enteric canal. But there is a great difference when a central
organ of circulation is developed. In the Arthropoda, Mollusca,
and most Vermes, this is developed from a dorsal vessel, or from
part of one, but in the Vertebrata it is formed from a ventral
portion. The double character of the early rudiment of the heart,
such as has been made out in the higher Vertebrata (Rabbit),
cannot be referred to any definite vascular organ, for none of the
kind is known to us.

There are considerable differences between the circulatory centres
of the nutrient fluid in the two great groups of the Vertebrata,
so that we must separate the apparatus which obtains in Amphioxus
from that of the Craniota. In the former, all the larger vascular
trunks are contractile, and so call to mind the arrangements which
obtain in the Vermes. The contents of the vascular system are
driven on at various points, without one centre being at all aided by
the others. The following is a description of the arrangements of
these vessels. Below the respiratory portion of the enteric canal there
is a longitudinal canal, which gives off branches, at regular distances,
to the branchial framework; these branches are branchial arteries.
They are collected into a trunk, which is placed above the gills — the
aorta ; and from this they are distributed through the body. At the
root of each branchial artery there is a heart-like formation, or con-
tractile enlargement. The most anterior pair of branchial arteries
passes into two arches, which are likewise contractile, and which snr-
round the mouth; this pair is connected with the commencement
of the aorta. From this vascular trunk arterial blood-vessels are
distributed in the body.

Our knowledge of the vessels, which distribute the blood, is at
present very slight, while the afferent vessels also require to be
more accurately investigated. It seems to be certain, however, that
there are contractile tracts in this portion also of the vascular

576

COMPAEATIYE ANATOMY.

system. In any case the whole apparatus agrees only generally
with the vascular system of the Craniota^ and it may be regarded
as an expression of the great difference which there is between
these two divisions, as shown by many other characters.

Tlie blood-fluid of Amphioxus is colourless, and its form-elements
are very small, indifferent, cells.

§ 430.

The Craniota possess a single organ, the heart, which effects the
circulation of the nutrient fluid ; they are also distinguished by a
differentiation of the circulatory vessels, which is of such a kind that
the nutrient fluid itself may be divided into two categories. Part
of the blood which passes through the body into the tissues, and
which, during the circulation, filters through them, is collected into
special tracts, and is again restored to the principal stream. This
fluid is the lymph, and its vessels form the lymphatic system.
The vessels which pass directly from the heart, and which lead back
again to it, form the blood-vascular system. The lymph-vessels
which are distributed in the wall of the enteron take up the
formative material, or chyle, which has been formed by the process
of digestion, and carry it into the blood-stream. They make up,
therefore, for the loss which the blood has suffered in conse-
quence of the metastasis which is continually taking place in its
circulation through the body. The lymphatic system is an im-
portant part of the whole vascular system. That system gains an
important step by this elaboration, and, owing to its division into these
two parts, is seen to be more highly developed than are the arrange-
ments which obtain in the Acrania, and in all the Invertebrata.

The separation of the nutrient fluid into blood and lymph is
accompanied by a differentiation of the form-elements of these
fluids. Those of the lymph are indifferent cells, which are similar
to the blood-cells of most Invertebrata. The form-elements of the
blood are, in their earliest stages of development, similar in cha-
racter, but are afterwards converted into coloured corpuscles of a
definite form, although this form is different in different divisions.
Owing to their large quantity they give the blood the appearance of
being coloured, as compared with the colourless lymph.

In all points but their size the lymph-cells of the Vertebrata
are similar to one another. The blood-cells, however, which are
much more differentiated, differ a good deal. They are all cellular
in character, so far as this depends on the nucleus, although,
indeed, it is only present in Mammals during their foetal stages.
As a rule, the blood-cells are flattened and discoid. In Fishes,
Amphibia, Heptiles, and Birds, they are oval and biconvex,
for the centre of each surface protrudes slightly. They form
biconcave rounded discs in the Mammalia, but in some they are
oval (e.g. Tylopoda). In size, those of the Dipnoi and Amphibia
(especially Proteus, Siren, etc.) are the largest. Owing to the

IIEAET AND AETEEIES OF VEETEBEATA.

577

important functions of the blood-corpuscles in the economy of the
Vertebrata, as carriers of gases^ their number, size, and the surface
which they thereby represent, is of the greatest importance. In
the higher divisions the relative quantity of blood varies but little,
and there is not any great difference between the relative volumes
of the plasma and blood-corpuscles. With regard, however, to the
distribution of the whole substance of blood-corpuscles in larger or
smaller form-elements, there is a great difference between the cold
and warm-blooded divisions, and, again, between the Eeptilia and
Amphibia ; from this point of view, the latter occupy a much lower
position.

Welckek, H., Zeitschr. f . rationelle Med. XX. p. 290 j imd Arcli. f. Mikrosk. VIII.

Heart and Arterial System.

§ 431.

At a certain stage the heart of all Craniotais formed of a single
tube. As it gradually gets longer than the space set apart for it, it
is arranged in an S-shaped loop, and so takes on the form which the
heart has later on. As it changes its form, it is divided into two
parts. The hinder one receives the blood and sends it to the
anterior one, which conducts it into the vascular arches, and so into
an arterial trunk which runs along the axial skeleton, from which it
is distributed in the body. The first portion of the heart is called
the auricle, the second the ventricle. A special cavity, which is
similar to the heart when it is first formed, encloses the auricle and
ventricle; this is the pericardial cavity, and its walls form the
pericardium.

We find this simple condition of the heart in Fishes. A
ventricle and an auricle form the two chief portions. The
latter receives blood from a sinus venosus, which is placed just
behind it, and is only partly outside the pericardium. As a rule it
has diverticula on either side, which grow out towards the chamber
in front of them (auriculgc) . The walls of the heart vary in character
according to their function. The wall of the auricle is provided
with a very thin muscular layer which projects inwards in a retiform
manner. Its only function is to drive the blood into the ventricle.
The ventricular wall has a more important function, and is conse-
quently provided with a powerful network of muscular bands. This
projects inwards, and so diminishes the size of the proper cavity of
the ventricle. But this is made up for by the connection between
the spaces in the network of the ventricular wall and the cavity of
the ventricle itself. There are two membranous valves at the
ostium atrio-ventriculare (Fig. 324, o), which prevent the blood
from returning to the auricle. There are generally as many as
three such pouched valves at the ostium arteriosum of the ventricle.

578

COMPAEATIYE ANATOMY.

The mechanical arrangements by which the stream of blood is
directed are therefore, at first, essentially the same at both ostia of
the ventricle. The cavity of the ventricle is
continued into the arterial trunk which is
given off from it, and commences by an en-
largement (bulbus arteriosus). That part
of the ventricle which is attached to this bulb
has an elongated form in the Selachii and
Chimser^e, and in its wall the musculature
has gained a circular arrangement. This
portion of the heart is the conus arte-
riosus (B). It is marked off from the bulb
by three pouch-like valves ; behind these
there are a number of valves in the conus
arteriosus, which are set in longitudinal rows.
There is a similar arrangement in all the
Granoidei. In the Teleostei it is rarely pos-
sible to distinguish the conus arteriosus,
and, as a very general rule, there are only
two valves between it and the bulbus. In
Ceratodus there are rudiments of two rows of
valves behind the four pouched valves that are
placed on the boundary line of the conus. In
Protopterus there are longitudinal folds which
foreshadow a division of the cone.

The atrophy of the conus arteriosus in the
Teleostei is accompanied by a development of
the bulb, and the smooth muscular tissue in its walls is increased in
quantity. It thus gets the appearance of an independent portion,
which must not be confounded with the conus arteriosus. In all
Fishes, except the Dipnoi, the heart contains venous blood only.

Fig. 324. Heart of
Squatina vulgaris.
The anterior wall of the
ventricle and of the
conus arteriosus is re-
moved. A Auricle.
V Ventricle. B Conus
arteriosus, o Ost. atrio-
ventriculare. a Bran-
chial arteries.

§ 432.

The trunk of the branchial artery is placed in all Fishes below
the branchial framework, along the arches of which it sends out its
branches. In the earlier stages of development these pass on each
side directly into a longitudinal vessel which runs along the base of
the skull, and from which an artery is continued forwards to the
head, and especially to the brain and eye (carotis interna). Pos-
teriorly the two longitudinal trunks (roots of the aorta) are united
into an unpaired larger trunk — the aorta (cf. Fig. 325). When the
branchial lamellse are developed on the branchial arches, a vascular
network is gradually developed in them by the arterial arches.
When this network is well developed the arterial arches disappear.
Each of them is replaced by a network of capillaries, which is
supplied by a branch of the branchial artery, and which gives off
an artery, or branchial vein, which takes part in the formation of

HEAET AND AETERIES OF VEETEBEATA.

579

tlie aorta. The blood which is brought to the gills from the
stomach by the branchial arteries is venous bloody for it is returned
to the heart from the systemic circulation ; as it passes through
the capillary network of the gills it again
becomes arterial blood, and passes as such
through the branchial veins into the aorta,
and so to the systemic circulation.

The number of branchial arteries given
oif from the arterial bulb is the same as
that of the functionally active gills. It
is largest in the Cyclostomata and the
Selachii. There are five pairs in the
Ganoi'dei also, while in the Osseous Fishes
it is only during the embryonic stage that
there is a larger number (6 or 7) of arterial
arches. The two anterior ones belong to
the mandibular and hyoid arches, and have
either no relations to the gills, or the gill
of the hyoid arches is functional for a time
only (opercular gill). When the hinder-
most gill, which belongs to the more or less
rudimentary last branchial arch is atrophied,

the arterial arches are reduced to four, and in some cases to three

Fig. 325. Diagram of the
arterial arches. 1 — 5.
a Branchial artery, a" Aorta,
c Carotid.

pairs.

These branchial arteries are
either arise in pairs from the
simple primary trunk, which
ends when it has given off
the last pair, or they are
given off on either side of a
common short trunk; this is
especially well seen in the
posterior branchial arteries of
the Selachii, and in various
Ganoids and Teleostei ; or, the
primary trunk of the bran-
chial artery divides at its
origin into two equal and
lateral branches, from which
the various branchial arteries a:
the Myxinoidea).

given off in very various ways. They

Fig. 326. Head of an embryonic Teleo-
stean, with the rudiments of tbe vascular
system (Diagrammatic), a Auricle, v Ven-
tricle. dbr Branchial artery. c Carotid.
ad Aorta, s Branchial clefts, n Nasal pit.
sv Sinus venosus. dc Ductus Cuvieri.

re given off (e.g. Bdellostoma among

§ 433.

The appearance of lungs is of the greatest importance in effect-
ing a change in these relations, for they produce great changes in
the arrangement of the large vascular trunks by taking on the
function which was previously performed by the gills. This change
affects also the structure of the heart ; the Dipnoi afford an interesting

2 p 2

580

CO]\IPAEATIVE ANATOMY.

example of this, for it is in them that the cavities of the heart
are first divided. In Lepidosiren a network of muscular bands is
continued from the wall of the auricle right through it, and forms a

kind of partition. The auricle is thereby
divided into a right and left portion, but
there are many points by which these
two parts communicate with one another,
and they also open by a common opening
into the ventricle. The venous sinus now
opens into the right side of the auricle, and
a pulmonary vein passes into the left side.
The ventricle, also, is partly divided by
muscular processes ; and, in correspon-
dence with this, the lumen of the arterial
bulb is divided into two cavities, each
of which gives rise to special arteries.
These form three vessels which extend
along the anterior branchial arches ; the
most anterior on either side is connected
with the second one, and unites with its
fellow of the opposite side to form an
aorta (ao). These two vessels have no
relation to the gills. The third arch
gives rise to branchial arches, but it is
also connected by a narrow duct {b) with
the root of the aorta, and it is continued on as the pulmonary
artery (j:?). This arch forms, therefore, a branchio -pulmonary
artery, and the two anterior arches, as they do not give off any
branchial vessels, represent aortic arches.

The circulatory apparatus of the Amphibia presents the same
conditions. In most, the auricle is completely divided, but in some,
as in Proteus, it is not so ; in Salamandra, also, there is a hole in the
septum. The systemic veins open into the right, the pulmonary
into the left auricle. The ventricle is single, and there are indica-
tions merely of a partition. There are membranous pouched valves
at the ostium atrio-ventriculare, which have the same character
as in Fishes. A muscular arterial bulb (Fig. 328, ba) is given off
from the ventricle, and its separation into two parts, which we saw
commencing in Lepidosiren, is completely effected. At first it gives
off as many as five pairs of arterial arches, but these are reduced to
three or four. As in Fishes, a network of branchial vessels is de-
veloped leading from the heart. Owing to this the arterial arches
are divided into efferent and afferent vessels; a branchial artery,
and a branchial vein, between which is the capillary network. The
branchial veins form the roots of the aorta.

Each branchial artery is, however, connected with its own
branchial vein by a ductus arteriosus, which is a portion of the
primitive aortic arch. When the lungs are developed, the last
branchial artery either gives off a twig to the pulmonary artery.

a Trunk of the branchial
arteries. 12 3 Arterial arches;
the first two are united to form
the aorta, p Pulmonary artery.
h Ductus Botalli. hr Branchial
clefts. b/ Accessory gill.
ao Aorta. c Coeliac artery.
oe CEsophagus (after Hyrtl).

HEART AND ARTERIES OF VERTEBRATA.

581

just as ill Lepidosiren, or tlie pulmonary artery (p) is continued on
directly from tRe branchial artery.

The disappearance of the gills
causes certain changes in some of the
Amphibia, in this vascular apparatus,
which is persistent only in the
Perennibranchiata. The direct con-
nection (cf. Fig. 328), which already
exists between the branchial arteries
and veins, is developed in such a way
that some of the arterial arches are
continued directly from the heart into
the roots of the aorta. The last arch,
which already gives off the pulmonary
artery, is developed into the trunk of
this artery, and is either connected by
small vessels only (ductus arteriosus)
with the root of the aorta, or is cut
off from it, and forms a separate
vessel. In this way several arterial
arches are connected with the root
of the aorta just as in Lepidosiren,
while one of the primitive vascular
arches is converted into the pulmonary

artery. The two kinds of blood are enabled to commingle, owing
to the arrangement of the heart, and the characters of the large
vascular trunks.

Fig. 328. Heart and arterial trunks
of a larva of Triton, ct Auricle.
V Ventricle. ha Arterial bulb.
1 — 4 Arterial arcbes ; the first
three are the branchial arteries.
c Carotid, p Pulmonary artery.
ao Aorta. vh Branchial veins
(after M. Eusconi).

Brucke, E., Deukschr. d. Wien. Acad. I.

§ 434.

A great step in the differentiation of the circulatory organs is
seen in the Reptilia, where the heart is placed at a greater distance
from the head. It is gradually moved backwards from the point at
which it is formed, and is embedded in the thoracic cavity; this is
its position in all the Amniota. The ventricular portion is generally
elongated, but it is broad in the Chelonii (Fig. 330), and several
Saurii. The two auricles are always separated from one another by
a septum, as in the Amphibia ; the right one receives the systemic,
and the left the pulmonary veins. The former is always the larger.
The strong muscular wall of the ventricle is continued into a net-
work which diminishes its cavity, just as in Fishes and Amphibia ;
this is especially the case in the Ophidii, Chelonii, and Saurii. The
ventricular partition is chiefly formed by this network, but some of
the muscular bands are more largely developed. The right half
of the ventricle contains venous, and the left arterial, blood, so that
the two portions can be distinguished. The incompleteness of the
separation of the two cavities is partly, at least, made up for by

582

COMPARATIVE ARTATO^^IY.

various arrangements. For example^ there is a muscular process,
vhicli is able to partly shut off the cavity which gives off the

branchial arteries from the rest of the
ventricular cavity. The ventricle is com-
pletely divided in the Crocodilini.

The membranous valves of the ostium
atrio-ventriculare are greatly developed
in the right half of the heart. In the
Crocodilini there is only one of these
valves on the right side, and it extends
along the septum of the ventricle; the
other valve is replaced by a process of
the lateral muscular wall of this chamber.
The arterial bulb, which is single ex-
ternally, is apparently given off from
the right ventricle. It is, however,
divided into a series of canals, which are
connected with both ventricles. There
are pouched valves at the root of the
arteries.

Of the five primitive arterial arches,
the two first have disappeared, and the
rest undergo various metamorphoses in
the different divisions. In the Saurii
the third persists on both sides, and is

Fig. 329. Heart and arteries
of an Ophidian (Boa).
d Right, s Left auricle.
c Carotid, ad Eight, as Left
aortic arch. p Puhnonarj^
artery.

connected on the right with the fourth

arch, which, like the two branches of
the third arch, is given off from the vessel that arises from the
left ventricle. The left half of the fourth arch is connected with
the third arch on its own side, and so corresponds to the right
ventricle. The fifth arch on either side is partly converted into the
pulmonary arteries, which primitively spring from it only, and which
are given off from the trunk of the pulmonary artery in consequence
of the differentation of the primitive aortic bulb. There are, there-
fore, two aortic arches on either side, one of which, the second on
the left, conveys venous blood. It is, however, connected peri-
pherally with the other arches, so that the two kinds of blood must
be mixed. In the Ophidii the first pair of arches that persists in
the Saurii is not generally connected with the second pair, but the
continuation of this portion is converted into the internal carotid.
In the Cheloniithe right arterial (Fig. 330, ad), and the left venous
aortic arch {as), are connected by a ductus Botalli with the pulmonary
arteries, which are developed from the last pair of primitive arteries
{pd ps). This disappears in the Crocodilini, so that in them a vessel,
which gives off the right aortic arch and the carotids, arises from the
left ventricle, while a left aortic arch and the pulmonary arteries
arise from the right ventricle. The primitive connection between
these vascular trunks is retained in the arterial bulb of the Crocodilini,
where the foramen Panizzse is the means of communication between

HEAET AND AETERIES OF VEETEBEATA.

583

the arterial and venous trunks. The two kinds of blood are not^
however, able to commingle to any great extent.

The vascular apparatus of Birds is very closely similar to that of
Eeptiles, and especially of the Cro-
codilini. But there is this considerable
advance, viz. that the arterial and venous
blood are completely separated in the
heart, as well as in the great arterial
trunks. The auricles of the heart appear
to be smaller owing to the smaller size
of their anterior (ventral) portion. The
musculature of the wall of the ventricle
is greatly thickened, especially on the
left side. The right ventricle is folded
around the greater part of the left one.

The atrio- ventricular valve of the right
ventricle is formed by a muscular ridge
(muscular valve) which projects from the
wall of the ventricle, and surrounds the
ostium peripherally; the second or mem-
branous valve, which is found in the
Crocodilini, is but seldom represented,
even in rudiment. At the left ostium
there is a valve which is connected with
the wall of the ventricle by tendinous

fibres. The arterial arches are reduced in much the same way as
in the Reptilia, especially the Ophidii and Crocodilini. But the left
aortic arch is not permanently developed. There is, therefore,
merely a single right aortic arch, which arises from the left ventricle.
Two arterial trunks, art. brachiocephalicae, are given off with it ;
these are divided into a common carotid, and a subclavian. In
some Raptores there is a permanent remnant of a left aortic arch,
in the form of a ligamentous chord; this indicates the course taken
by the vessel. The pulmonary artery, which is derived from the
last of the primitive aortic arches, is consequently the only arterial
trunk which is given off from the right ventricle.

Fig. 330. Heart and arterial
trunks of a Chelonian
(Chelydra). The letters as in
the previous figure.

Fritsch, G., Zur vergl. Anatomie der Amphihienherzen.
Physiolog, 1869.

Arch. f. Anat. u.

§ 435.

The heart of the Mammalia resembles that of the Birds in
having its two halves completely separated ; but there is a great
difference in its structure, and in the arrangement of the great
vessels. It is in rudiment only that there is any close similarity
between either the heart or the general arterial system, which, as in
Birds, is derived from a number of paired arches. During the
embryonic stage the two auricles communicate ; in the Marsupialia
this is effected by a narrow slit, and in the placental Mammals by a

584

COMPARATIVE ANATOMY.

larger space (foramen ovale). Owing to tliis connection, tlie blood
which passes into the right ventricle from the umbilical vein through
tlie vena cava inferior, is able to pass into the left ventricle, whence
it is distributed through the body by the aorta. In the Mono-
delphia this opening is gradually closed by the forward growth of a
wall, or partition, towards the left auricle (valvula foraminis ovalis) ;
after birth, therefore, the two auricles are completely separated.
The boundary of the primitive foramen ovale may, even later, be seen
as a circular process. The most anterior (ventral) portion of each
of the two auricles in the Mammalia forms a small prolongation,
which has a different form on either side, the appendages of
the auricles.^^ They correspond to the greater part of the auricles
of the lower classes, for the hinder portion of the cavity of the
auricles, on the right side, at any rate, is formed by a venous sinus,
which in the lower forms is separated from the auricle (cf. Venous
System). These appendages, therefore, in the Mammalia are the
remnants of the anterior portion of the auricle.

The at rio -ventricular valves are much altered in character. In
the earliest stages their place is taken by membranous folds, as in
Fishes, Amphibia, and Reptilia. The ventricles have a compara-
tively small cavity, and their wall is formed by the same spongy
muscular tissue as that which we have found in a well-developed con-
dition in the above-mentioned classes. The bands gradually thicken,
and part goes to form the more compact cardiac wall. That portion
of this network which passes inwards and bounds the lumen of the
ventricular cavity, and which is inserted into the edges of the
venous ostium, is connected with these membranous valves. As
the free edge of the valve ceases to grow, and as the only part
which is left is that which is connected with the muscular bands, the
valve becomes intimately connected with the wall of the ventricle,
so that the muscular bands from the latter pass into a membrane
which is given off from the ostium. This condition, which is
transitory in most Mammals, is permanent in the right ventricle of
the Monotremata (Ornithorhynchus). The muscular bands which pass
off from the wall of the ventricle are continued into a membranous
valve. In all other Mammalia this condition is replaced by another
one. The muscular bands are withdrawn towards the wall of the
ventricle, and so form the musculi papillares, while their more
anterior portion, which passes into the valve, is represented by the
chordm tendineae. The rest of the spongy muscular network of the
wall of the ventricle forms the trabeculae carneas. The atrio-ven-
tricular valves and the chordae tendineae are, therefore, differentia-
tions of the primitive muscular network, which was connected
with the primitive valves. The similar valves in the left ven-
tricle of the Bird^s heart are formed in just the same way. The
great arterial trunks in the Mammalia are differentiated from
the embryonic arrangements in a somewhat different manner. The
two first arches disappear completely ; the third, as before, forms
part of the carotid. The fourth is continuous on the right side with

HEART AND ARTERIES OF VERTEBRATA.

585

tlio subclavian, while on the left it gives rise to the aortic arch,
from which the left subclavian springs independently. The carotids
and the right subclavian are united to the commencement of the
aorta. In the Mammalia, therefore, the chief trunk of the arterial
system is formed byaleft aortic arch. The pulmonary artery is
formed from the fifth arch, which is during foetal life connected
with the aortic arch of the left side by a ductus arteriosus
(ductus Botalli). The blood, therefore, of the superior vena cava,
which passes into the right ventricle, is in this way kept away from
the lungs, and is carried into the descending aorta, which till birth,
therefore, conveys mixed blood. After birth the communication
between the pulmonary artery and the aorta descendens disappears,
and the connecting portion (h) of this vessel is converted into a
chord (ligamentum Botalli).

Sabatier, A., Etudes sur le cceuu et la circulation centrale dans la scrie des
vertebres. Montpellier, 1873.

§ 436.

In all Vertebrata the systemic arteries arise, in their earliest
condition, from the bulbus arteriosus. In those forms that breathe
by gills the system of arterial arches, which arises from the bulb
(primitive aortic arches), is, as has already been remarked (§ 432),
broken up into the vessels of the branchial circulation; and the
system of systemic arteries is only derived from the efferent
branchial vessels (branchial veins). The current of blood, which at
first is sent to the aorta through the arterial arches, is driven into
new passages when the gills are developed, so that it is only dis-
tributed to the body after taking a roundabout course, which is due
to the conditions imposed by the respiratory process.

In the Myxinoidea almost all the branchial veins unite to form a
sub vertebral aorta, which is continued backwards as the chief artery
of the body, but which is also continued forwards as the arteria
vertebralis impar."’^ Two lateral longitudinal trunks are collected
from the branchial veins in just the same way, and each trunk sends
a branch anteriorly into the arteria vertebralis impar, while its other
branch goes to form a carotid. The two carotids divide into an
external and an internal branch, which supply the head. In Petro-
myzon the aorta is not continued forwards, so that the carotids, which
are formed in just the same way as in the Myxinoidea, form the sole
anterior arteries. In the Selachii and Chimaer^e, the aorta is formed
from a trunk which is developed on either side by the union of the
branchial arteries. There is the same arrangement in the Ganoi’dei
and Teleostei. The carotids arise from the first branchial vein, or
from the anterior end of the paired arterial trunk, which collects the
branchial veins on either side to form the roots of the aorta, and
then unites with its fellow of the opposite side to form the aorta, or
enters anteriorly into a transverse anastomosis which marks off an

586

COMPAEATIVE ANATOMY.

arterial cii^culus cephalicus at the base of the skull. A special optic
artery is given off from the vessels of the rudimentary gill, with
which a direct branch of the first branchial vein (Selachii), or a
branch of the same vessel which surrounds the body of the hyoid
(Teleostei), is connected. There are numerous modifications in the
mode of origin and arrangement of the different vessels, the most
important of which are found in the carotid and optic arteries.

This portion of the vascular system is arranged in a similar
manner in the Amphibia. In the Perennibranchiata the cephalic

arteries arise from the anterior
portion of the roots of the aorta ;
in the Caducibranchiata, from
the first permanent aortic arch,
or, they are continuations of the
anterior arch itself (Fig. 331, c).
In this case, an artery which
goes to the tongue (Z) represents
an external carotid. After this
vessel is given off, there is a
swelling (c) on the carotid trunk
in the Frogs and in the Sala-
manders j this is the so-called
carotid gland. The lumen of
the vessel is here traversed by a
network of bands, which break
it up into a number of narrower
passages, just as if a capillary
network had been intercalated
in the course of an artery. The
carotid gland appears to be de-
rived from an arrangement of
this kind, the branchial vascular
network not having been com-
pletely atrophied. The next pair
form the aortic arches (ad as),
which converge backwards, and
finally unite into an unpaired aortic trunk (a). Each aortic arch
gives off a subclavian (sd ss). Just before they unite, a large
visceral artery (m) is given off from the left aorta. The pulmonary
artery represents a last aortic arch. Before it goes to the lungs (p)
it gives off a large cutaneous branch (czii)^ which ramifies on the
back and neck as far as the posterior region of the head, and affords
a distinct proof of the respiratory function of the integument.

In the earliest stages of the Amniota we meet with many similar
arrangements of the arterial system. The internal carotid, which
supplies the brain and the eye (Fig. 332, A Bj c'), is seen to be a
forward prolongation of the roots of the aorta on either side. The
external carotid is a branch of the third primitive aortic arch. If
this arch loses its connection with the fourth, the two carotids are

Fig. 331. Arterial system of the Frog,
ha Bulbus arteriosus. c Carotid.
C Carotid gland. I Lingual artery,
ad Eight, as Left aorta, a Aortic trunk.
m Visceral artery. sd Eight, ss Left
subclavian, oes (Esophageal branches,
p Pulmonary artery, cut Its cutaneous
branches, occ Posterior cephalic branch.

HEAET AKD AETEEIES OF VEETEBEATA.

587

given off on either side of a common trunk (G) . They generally appear
as two trunks, which pass along the sides of the neck in company
with the vagus. In the Saurii they do not lose their connection
with the next arterial arch, and retain, therefore, their primitive
relations. In many Ophidii the right common carotid is atrophied,
and may even completely disappear.

Fig. 332. Development of the great arterial vascular trunks, as seen in the emhryoes,
Of a Eeptile (Lizard), D Of a Bird (Fowl), and C Of a Mammal (Pig). The
two first pairs of arterial arches have disappeared in all three j in A and B the third,
fourth, and fifth are still persistent ; in C the two last are alone complete, and the
third is no longer connected with the fourth pair. A branch (p) of the fifth forms the
pulmonary artery. Its trunk from this point to the aorta forms the ductus Botalli.
c Carotis externa, c' Carotis interna. In A and B this forms the anterior prolongation
of the root of the aorta, and in C a common trunk with the external carotid.
a Auricle, v Yentricle. ad Aorta descendens. s Branchial clefts, m Rudiment of
the fore-limbs, n Nasal pit (after H. Rathke).

In Birds also tkis artery arises in company with a subclavian
from a common trunk (art. bracliiocephalica), but it leaves its
primitive course, and lies in the middle line of the inferior surface
of the cervical vertebrae ; on the left, however, it retains its original
course. In others, again, the two carotids both leave their old
course, and this leads on towards that third form, in which the
two closely-approximated vessels are fused together. In this case
no part of the right carotid runs by itself, but a vascular trunk
arises on the left side, which runs in the middle line, and which
passes to the head, under the name of the primary carotid. Many
Birds have this character in common with the Crocodilini. A single
carotid trunk (Fig. 329, c), which obtains in the Ophidii and various
Saurii, must be regarded as differing from this, although it also
passes into two cephalic arteries anteriorly. This arrangement is
due to the approximation of the points of origin of the two carotids
on the right aortic arch. A common arterial trunk is given off from
the united point of origin. Another peculiarity is the presence of
an unpaired subvertebral artery, which runs forward from the right
aortic arch along the vertebral column (Fig. 329, sv).

588

COMPAEATIVE A^s^ATOMY.

There are various kinds of modifications in the Mammalia, which

Fi^'. 333. Heart and great vessels of Buteo
vulgaris, tr Trachea, i Crop, ae Com-
munication between the air-sacs and the
lungs. h Bursa Fabricii. ao Aortic arch.
aad Art. anonyma dextra. aas Art. anonyma
sinistra. ps Art. pnlmonalis sinistra,
c Carotid. am Visceral artery. vci Com-
mencement of the inferior vena cava.
van Vena coccygoo-mcseutcrica.

are due to similar changes in
the vascular trunk during em-
bryonic life. Among others
these specially affect the ter-
minal branches of the carotids;
the internal carotid, as in
various Lizards and Birds, is
not sent to the cranial cavity
and sensory organs only.

The arteries of the fore-
limbs arise from various and
very different points, so that
transmission appears to be of
less importance than adapta-
tion in the development of
these vessels.

The trunk of the aorta is
continued along the vertebral
column, without any change
in character, as far as the por-
tion which is set apart for the
caudal region — caudal artery;
when the tail is shortened, it
forms the median sacral artery.
When the so-called chevron
bones are present, the terminal
portion of the aorta always
lies in the canal formed by
them. But in various Fishes
it may be enclosed in a canal,
formed by processes from the
centra of the vertebrae, in the
region of the trunk also ; this
is the case in the Sturgeon,
and in some Teleostei.

The aorta gives off arteries
(arteriae intercostales) for the
metameres in regular succes-
sion, as well as the vessels
which go to the viscera, and
also those which go to the hind-
limbs, when these are well
developed. In Fishes there
is ordinarily only one large
arterial trunk for the viscera

(A. coeliaco-mesenterica) ; but
in some there is a mesenteric artery also. The aorta gives off a
larger number of arteries for the renal and generative organs. As

VEINS OF YERTEBEATA.

580

in the Amphibia, the Art. coeliaco-mesenterica arises from the end
of the left aortic arch in the Reptilia (Saurii, Chelonii) ; this arch
is only connected by a narrow tract with the right one ; or there
are several visceral arteries (some Sanrii) ; these are especially
nnmerous in the Ophidii, in consequence of the elongated form of
their body. In the Crocodilinii, also, independent mesenteric arteries
are given off from the unpaired aorta in company with the arteries
from the left aortic arch.

In Birds, where the left aorta disappears, the aortic trunk is the
sole vessel from which the visceral arteries are given off.

In Mammals the coeliac and superior mesenteric artery are the
chief arteries of the enteric canal. In the placental forms the inferior
mesenteric is also a large vessel.

The large number of renal arteries found in Fishes are found also
in the Amphibia and in most Reptiles ; in Birds also there are several
renal arteries, the middle one being given off from the ischiac artery.
It is very rarely that there are a number of these arteries in the
Mammalia.

The arteries of the hind-limbs are not direct branches of the
posterior aorta until these parts are very largely developed. The
two chief trunks (iliac arteries) of this region are not always the
same. As is clear from their topographical relations to the pelvis,
different branches may supply the area of these arteries. In the
Sauropsida the ischiac are the chief trunks of the hinder extremities.
In the Mammalia the chief trunks are formed by the crural artery,
and there are numerous modifications in its more special characters
which are of less importance.

Venous System.

§ 437.

The venous system of the Vertebrata exhibits no less important
phenomena in the various modifications that obtain in it, as we pass
from the Fishes to the Mammalia, than does the arterial portion of
the circulatory system. Our knowledge is in many points as yet
incomplete. The blood which returns to the heart is, in Fishes,
collected into four longitudinal trunks, two anterior and two
posterior. Those of either side pass into a transverse trunk (ductus
Cuvieri, Fig 334, dc), which opens with that of the opposite side into
a sinus (si;) which is placed behind the auricle of the heart. The
anterior pair, which chiefly collects the venous blood of the head,
forms the jugular veins [j), which are placed above the branchial
arches; the hinder pair, which receives the blood from the walls
of the trunk and from the renal and generative organs, forms the
cardinal veins (c); an unpaired caudal vein runs below the artery in
the caudal canal ; in the Cyclostomata, Selachii, and some Teleostei,
this divides into two branches which are continued into the cardinal

590

COMPAEATITE AAUTOMY.

veins of their own side. In many Teleostei, this caudal vein is
continued into the right cardinal by a large, and into the left
cardinal by a smaller branch ; in this case the
left cardinal vein is also smaller than the right
one. This leads to the condition in which the
whole of the caudal vein passes into the right
cardinal ; this has been observed in a number of
Teleosteans.

As the caudal vein sends off branches into
the kidneys, which either break up completely or
partially in this organ, these branches form the
ven83 renales advehentes, and send their blood
into the cardinal veins through the venae reve-
hentes. In this way the renal portal system
is developed. A second vascular apparatus of
similar character has its roots on the digestive
canal ; its venous blood is carried to the liver by
a trunk which is known as the portal vein. It
is distributed in this organ, and is carried to the
common venous sinus by hepatic veins, which
are generally united into several trunks.

In this arrangement of the venous system in Fishes we may
distinguish the paired, and ordinarily symmetrical, portion from the
unpaired portion, which is solely represented by the hepatic veins.

We will first follow out the former in its
changes throughout the Vertebrate series, for
in all of them its essential characters, at any
rate, may be observed in the early stages of
development, as a transmitted arrangement,
and, since it is the groundwork of the embry-
onic venous system, it furnishes the starting-
point for all later metamorphoses.

Fig. 334. Diagram
of the primitive
venous system,
j Jugular, c Cardinal
vein. dc Ductus
Cuvieri. h Venae
hepaticae. sv Sinus
venosus.

438.

Fig. 335. Anterior por-
tion of the venous sys-
tem of an embryonic
Ophidian, u Ventricle.
ha Bulbus arteriosus.
c Auricle. DC Ductus
Cuvieri. vc Cardinal
vein, -yj Jugular. Um-

bilical vein. U Primitive
kidney. I Rudiment of
the labyrinth (after H.

Rathke).

In the Amphibia and Eeptilia the venous
sinus receives the two jugular veins, which
have the same area of origin as in Fishes.
They persist in all the higher Vertebrata,
while the hinder pair — the cardinal veins
(Fig. 335, vc) — have only the same characters
as in Fishes, during the earliest stages of em-
bryonic life. They are the veins of the primi-
tive kidneys (Z7). Their anterior portion is
obliterated, while their posterior portion re-
ceives veins from other regions, and forms the venas renales adve-
hentes. Even before the disappearance of that part of the cardinal
veins which opens into the ductus Cuvieri, four other trunks are
developed in the Eeptilia, which chiefly receive the intercostal veins.

VEINS OF VEETEBRATA.

591

and are known as the venae vertebrales. The anterior and posterior
ones on either side are united, and open into the jugular vein of
their own side. Their connection with the left jugular disappears
later on, when the left vertebral veins develop transverse ana-
stomoses, and become connected with the right ones, and, like them,
open into the right jugular.

When the cardinal veins cease to be connected with the ductus
Cuvieri they form prolongations of the jugular veins, which receive
the subclavians which come from the fore-limbs, and are known as
the superior venae cavae. The vertebral veins, which collect the
blood from the walls of the body, are not large after the embryonic
period, and they are generally considerably atrophied. They cease
to have a paired arrangement (Ophidii), and the greater part of their
area is occupied by the vena cava inferior.

We meet with similar arrangements in the Birds. A pair of jugular
veins, which are often unequally developed, form the chief trunks
for the blood returned from the anterior parts of the body. At the
base of the skull they are generally connected with one another by
a transverse trunk, into which the veins from the cervical vertebral
column, as well as from the head, may enter. When the left
jugular is atrophied, this transverse trunk forms the vessel by which
the blood is conveyed into the right jugular. The vertebral veins
are now inconsiderable vessels. The jugulars unite with the veins
of the anterior extremities, which form the subclavians, and the
trunks thus formed are again known as the superior veneo cavae.
As these still receive the posterior vertebral veins, a portion is
separated off from them, which may be seen to be derived from the
transverse trunks (ductus Cuvieri), which are persistent in Fishes.
These venae cavae, however, open separately into the right auricle,
for the sinus, which is persistent in the Reptilia, here forms a portion

T n. la: Tw

Fig. 336. Relations of the great venous trunks on the heart. I Reptile
(Python). II Bird (Sarcorhamphus). Ill Marsupial (Halmaturus) . IV Pig.
They are all seen from behind, i Vena cava inferior, s Vena cava superior sinistra,
d Vena cava superior dextra. up Pulmonary artery, a Aorta, sv Sinus venosus.

of the auricle (Fig. 336, I, sv). The vertebral veins in Birds pass
along a canal which is enclosed by the ribs, by which point they are
seen not to be the same vessels as the cardinal veins.

592

COMPAEATIVE ANATOMY.

§ 439.

The embryonic venous system of the Mammalia is completely
similar to that of the lower Vertebrata. Two jugular veins (Fig. 334)
receive the cardinal veins, and the common trunks on either side
pass into a venous sinus, which is connected with the auricle, and,
later on, forms a part of the right one. Two distinct venous trunks
then open into this auricle, each of which is continued into an
anterior and larger, and a posterior and smaller, trunk. When the
anterior extremities are developed, the subclavian veins (s) fall into
the anterior ones (Fig. 337, A), and the two venous trunks thus
formed are distinguished as the superior venm cavse.

When the system of the inferior venae cavae is developed, the area
of the cardinal veins is diminished, for part of the blood which was col-
lected by the cardinal
veins is now carried
to the inferior venae
cavae. The cardinal
veins also undergo de-
generation, owing to
some of their roots
passing into the new
longitudinal trunks,
which, as in the
Eeptilia, represent the
vertebral veins, and
are continued into the
end of the cardinal
veins, which open into
the ductus Cuvieri.
Owing to the decrease
in the size of their
area, these vertebral
veins (Fig. 337, A B, v)
appear to be branches
of the trunks derived
from the ductus Cuvieri and the jugular veins, or superior venaa cavae.
These are found in the Monotremata, Marsupialia, many Eodentia,
and Insectivora. In others, part of the area of the left superior cava
{B) is handed over to the right one (cs), owing to the development
of a transverse anastomosis; the left superior vena cava is now
atrophied (Eodentia, Euminantia, Solidungula) . When this arrange-
ment is complete, the greater part of the trunk of this vein disap-
pears, and the only part that remains is the terminal portion, which
primitively formed the left ductus Cuvieri, and which is placed
between the left ventricle and auricle [Gy cor)] the cardiac veins open
into it, and it forms the sinus of the coronary vein of the heart. In
Man, even, a semilunar fold separates this sinus from the true

J D C

Fig. 337. Diagram of the primitive paired veins in
Mammals. A The vertebral have taken the place of
part of the cardinal veins, -which are indicated by-
dotted lines. B The left jugular has its lower portion
atrophied, and its area is united -with that of the right
jugular by a transverse ti’unk. C The left jugular vein
has completely disappeared, with the exception of a
rudiment on the heart. j Jugular. s Subclavian.
cs Vena cava superior, c Cardinal vein, v Vertebral
vein, cor Coronary vein, az Vena azygos.

VEINS OF VERTEBEATA.

593

coronary vein, while tlie valvula Thebesii, which is placed at its
opening into the right auricle, forms, for a long time, the valve of
the left superior vena cava. The right superior cava is now the
sole anterior trunk (Cetacea, Carnivora, Primates).

When the trunk of the left superior vena cava is reduced, the
cardinal veins, or the vertebral veins developed in their area, undergo
great changes. While in the first case they open into the vena cava of
their own side (A), and while in the second they pass separately into
the left side of the right auricle, owing to the development of a right
vena cava (R); when the vascular passage which leads directly to the
heart is reduced, they become connected with the right vertebral
vein. The left vertebral vein is connected with the right one by
transverse anastomoses, and when the connection between its upper
end and the left superior cava is broken, it is converted into a
vena hemiazygos, while the right one, which still retains its
primitive position, becomes the vena azygos (Fig. 339). When
the two superior cavae persist, the two vertebral veins are not, in all
cases, unchanged; one trunk often becomes much larger than the
other, which may even be so far reduced as to disappear altogether.
In this case a vena azygos receives the intercostal veins of both
sides ; and this sometimes opens into the left, and sometimes into
the right superior vena cava, or even into the only one that is
present, as, for example, in the Carnivora (Fig. 337, (7, az).

In most Mammals the roots of the jugulars are formed of a
number of veins from the external and internal cephalic regions,
one of which conducts a part of the blood from the cranial cavity
through the jugular foramen. It forms a small vessel only, for the
greater part of this blood passes out in a canal (canalis temporalis),
which is either placed between the petrosal and squamosal, or in the
latter only. When the foramen jugulare is enlarged, the vein, which
in other cases is a small one, increases in size, and gradually becomes
the most important of all the vessels which come from the skull ; in
this case it forms, as it does in the Primates, the internal jugular
vein. The other veins gradually unite to form the external jugular,
which is the most important one in most Mammals.

§ 440.

The second large venous tract is very small in Fishes, for in them
it is merely represented by the hepatic veins, which are united into
one or more trunks, and open into the common venous sinus. When
the tract of the cardinal veins is diminished in extent, a new tract is
formed in connection with' the hepatic veins — that of the inferior
vena cava (Amphibia). This venous trunk collects blood from the
kidneys, and is, therefore, the vena renalis revehens (Fig. 338, A, ci).
The blood from the hinder extremities passes into an iliac vein
(A, -i), which, in the Urodela, receives on either side a branch of the
divided caudal vein. It breaks up in the liver, and forms a vena
renalis advehens. A branch of the iliac vein passes to the middle

594

COMPAEATIVE ANATOMY.

line of the abdomen^ and receives veins from the so-called urinary
bladder (A, o), after which it is united with its fellow of the opposite
side to form a single trunk which passes to the liver, and which is
therefore connected with the portal system (a) ; this is the epigastric
(abdominal) vein. The veins of the digestive canal and of the spleen
are united into a portal trunk, which breaks up in the liver.

In the Eeptilia also the hepatic and the efferent renal veins form
au inferior vena cava (B, ci), which opens into the common venous
sinus below the right superior cava. But there are various modifi-
cations in the different divisions of the Eeptilia, and it is in the
Saurii and Ophidii only that there is any close similarity to the
arrangement of the venous system which' obtains in the Amphibia.
The caudal vein divides into two trunks, which receive the veins from
the hinder extremities in the Saurii, and form the venae renales

Fig. 338. Posterior portion of the venous system. A Of the Frog. B Alligator.
C Bird. 22 Kidneys, c (azygos trunk) Caudal vein, c Crural, i Ischiac vein.
V Yen® vesicales. a Epigastric (abdominal) vein, m Vena coccygeo-mesenterica.
ra Vena renalis advehens. rr Vena renalis revehens. ci Vena cava inferior, h (in
A and C) Vena hypogastrica, (in B) End of the epigastric vein in the liver.

advehentes. The veins of the vertebral column are connected with
these. Similar arrangements obtain in the Crocodilini, where the
caudal vein (B, c) is also divided, but this vessel then forms a transverse
trunk which gives off the venae renales advehentes {ra). In all these
forms the venae renales revehentes form a trunk which runs in front
of the vertebral column, and there is a renal portal system in the
kidneys ; this appears to be absent in the Chelonii only.

Another venous tract in the Eeptilia is represented by the venae
epigastricae sive abdominales. When the allantois is developed,
a pair of veins is developed from the vascular network that ac-
companies it, and this primitively opens at the same point as the

VEINS OF VEETEBEATA.

595

ends of the ductus Cuvieri (Eathke : King Snake). These umbilical
veins receive veins from the abdominal wall, and. are also connected
with the formation of the hepatic portal circulation. In the Ophidii
this umbilical vein disappears after the veins of the abdominal wall
which open into it have broken up into a plexus, but in the Saurii
the terminal portion of one umbilical vein persists, and unites with
the abdominal veins that open into it to form an epigastric vein ; this
receives veins from the urinary bladder, and passes forwards to the
liver.

In the Crocodilini and Chelonii the ends of the umbilical venous
trunks persist, and form, as they are continued into the veins of the
abdominal wall, a part of the epigastric veins. Like the single veins
in the Amphibia and Saurii they also go to the liver, and, in the
Crocodilini, they are connected with branches of the portal vein. In
the Chelonii, those of either side unite into a transverse trunk, which
receives the various venae intestinales, which are not, in them, united
into a portal venous trunk. In both cases they are distributed in the
liver, and belong therefore to the portal venous system. In the
Crocodilini, as in the Chelonii, the epigastric veins (E, a) are given
off from the two branches of the caudal vein (c), and receive the
crurals (c) ; they also receive the ischiac veins more anteriorly. But,
since in the Crocodilini, the venae renales advehentes also arise from
the caudal vein and its connection with the ischiac vein, part of the
venous blood from the hinder portion of the body is carried into the
renal portal circulation, and the rest into that of the liver. But in
the Chelonii, where there are no advehent renal veins, all the blood
from the hinder end of the body is carried into the liver, for the
vertebral vmns, in these forms, also open into the epigastric veins.

§ 441.

Several of the veins which are found in the Eeptilia are not
permanent structures in Birds. The inferior vena cava (Fig. 338,
G, ci) is indeed still made up of two trunks from the kidneys, but
these receive the veins of the hind-limbs (c), and might from their
size be taken to be the continuation of these veins. Two hypo-
gastric veins {h) are connected with these trunks in addition to the
vessels which arise from the kidneys. They are united by a trans-
verse anastomosis at the root of the sacrum; this anastomosis
receives the caudal veins (c) from behind, and gives off in front a
vena coccygeo-mesenterica (m), which goes to the mesenteric vein.
This vena coccygeo-mesenterica is a wide trunk in the Crocodilini
also, where it anastomoses with the transverse trunk, which unites
the two branches of the caudal vein ; part of the venous blood
from the tail or hinder extremities is conducted away from the renal
portal circulation by it.

In the Mammalia there are no indications whatever of a renal
portal system. The umbilical and omphalo-mesenteric veins have
the same relations as in Eep tiles, though there are several variations

2 Q 2

596

COMPAEATIYE AXATOMY.

ill some, even of tlie larger, trunks. Tke inferior vena cava (Fig.
339, ci)y which collects the blood from the kidneys and generative
glands, is developed very early ; it accompanies the united umbilical
veins, and, when the right one disappears, it receives the left.
After the cardinal veins (c) disappear, the veins of the pelvis {luj) .
are connected with the end of the trunk of the vena cava, as are
also the veins of the hinder extremities (^7), and the caudals. At the
time when the umbilical is the largest of the veins, the inferior cava
appears to be merely a branch of it. Where the umbilical vein
enters the liver, the hepatic vessels are formed,
while at the same time similar branches from
the liver pass to the point where the umbilical
unites with the vena cava inferior ; these form
the hepatic veins. As the blood which is
returned to the heart from the umbilical veins
passes through the liver, that portion of them
which lies between the afferent and efferent
veins is atrophied, and forms the ductus
venosus Arantii. That portion of the omphalo-
mesenteric vein which receives the mesenteric
veins is then converted into the trunk of the
portal vein, while the hepatic branches of the
umbilical form the branches of the portal vein
after the obliteration of the ductus Arantii.
The inferior vena cava is thus converted into
the chief hinder trunk, into which open the
veins of the pelvis, of the hinder extremities,
of the renal and generative organs, while the
veins of the digestive canal and spleen form
the portal vein.

§ 442.

The blood-vessels are ordinarily distri-
buted in the body by the gradual branching
of the different trunks, until at last the finest
branches of the arteries and veins give rise
to the capillary system, which connects the
two kinds of blood-vessels with one another.
To say nothing of the various special arrangements in certain organs,
a somewhat different method of distribution obtains in the blood-
vascular apparatus of several regions of the body. A vein or artery
suddenly breaks up into a tuft of fine branches, which either do, or
do not anastomose, and which lose themselves in the capillary
system, or are again soon collected into one trunk. This distribution
of the vessels has been long known as a ret e mirabile. Its function
is clearly to slacken the blood-current, and to increase the surface of
the walls of the vessels, so that there must be a change in the amount
of nutrient fiuid diffused by osmosis. If from this rete a vascular

Fig. 339. Diagram of
the chief trunks of
the venous system
of Man. cs Yena cava
superior, s Yena sub-
clavia. je Jugularis ex-
terna. ji Jugularis in-
terna. az Yena azygos.
ha Yena hemiazygos,
c Indication of the car-
dinal veins. ci Yena
cava inf. h Yenje hepa-
ticae. r Yenm renales.
il Yena iliaca. hy Yena
hypogastrica.

LYMPHATICS OF VEETEBEATA.

597

trunk is given off similar to tke one tkat was broken up, it is called
bipolar or amphicentric ; if tlie rete remains broken up, then it is
known as a diffuse, unipolar, or monocentric rete mirabile. Some-
times arteries only, sometimes veins only (rete mirabile simplex),
sometimes both kinds of vessels are united witb one another (rete
mirabile geminum seu conjugatum) to form the rete.

Arterial retia are found in the pseudobranchia, in the choroid of
the eye of Fishes, and, in very various forms, on the air-bladder. In
Birds and Mammals, retia are not unfrequently found in the area of
the carotids and their branches. They are very common on the limbs
of the Mammalia (Monotremata, Edentata). In the area of the
visceral arteries there are both arterial and venous retia ; thus, in
the Pig, the mesenteric artery forms an arterial rete. Arterial retia
are very common on the terminal branches of the renal arteries,
where they form the Malpighian glomeruli, from which, as we all
know, another artery is given off to be distributed in the capillaries
on the urinary tubules (cf. Fig. 343, B).

Lymphatic System.

§ 443.

The presence of a system of canals connected with the blood-
vascular system — in which the nutrient fluid, which has passed out
from the capillary portion of the hsemal system, is conveyed again
to the blood stream as lymph, after having filtered through the
tissues — is an arrangement which is peculiar to the organisation
of the Craniota. It appears to be correlated with a high develop-
ment of the body, for it is wanting in Amphioxus, and in embryo-
logical development it only begins to appear at a relatively late
period, and not until the blood-vascular system has been differen-
tiated into its arterial and venous portion, and is in full function.
That portion of the lymphatic system which has its root on the
digestive canal is of especial importance, for it receives the chyle, or
nutrient material, which has been prepared from the chyme by the
process of digestion, and conveys it to the blood-vessels.

In addition to the function of conveying the lymph, this system
of canals has yet another duty, which complicates its anatomical
relations. The points at which the form-elements of the lymphatic
fluid, or lymph-cells, are developed, are embedded in its vessels;
these lymph-cells are carried to the blood, and are gradually con-
verted into its form-elements.

In the lower divisions of the Vertebrata this lymphatic system
has not much independence, for its vessels are chiefly formed of
wide spaces, which enclose other organs, and especially arteries. The
sheath of connective tissue of the artery also encloses the lymphatic
vessel. The veins also may be surrounded by wide lymph -spaces ;

598

COMPAEATIVE A:N'AT0MY.

thus, for example^ tlie abdominal vein of tbe Salamander is enclosed
in a lympbatic vessel.

There are, however, other vessels in the lower divisions besides
those which accompany the blood-vessels — in the skin, or on
portions even of the digestive canal, or other
viscera. Peripherally, the lymphatic vessels
anastomose largely, and form capillary net-
works or similar spaces. These gradually give
off wider spaces, either in the form of canals,
or of sinuses with irregular boundaries, the
place of which is taken, in the higher Verte-
brata, by vessels allied in structure to veins.

Although we may note that, as we pass
from the lower to the higher Vertebra ta, there
is a gradual differentiation from spaces, similar
to those of the lacunar system of the Inverte-
brata, to a definitely developed canalicular
system, so that the interstitial nature of the
lymphatic ducts is well marked at the periphery
only; nevertheless, in the coelom we have an
arrangement which indicates the origin of the
lymphatic vessels from a lower condition — for
the coelom is a lymphatic cavity. In this point
the coelom of the Vertebrata resembles closely
that of the Invertebrata. The communications
between the coelom and the pericardial cavity,
which obtain in various Fishes (Sturio, Selachii), are indications of
the same thing ; as are also the pleural cavities of the Mammalia,
which are merely differentiations of the general coelom.

§ 444.

In Fishes the chief trunks have the form of lymph-sinuses.
There are generally two pairs of them, or one unpaired one is placed
below the vertebral column. The unpaired trunk divides into two
branches. Smaller sinuses, and narrower canals, form the lymphatic
Vessels which are collected into these trunks. They are generally
connected with the venous system at two points. A cranial lym-
phatic sinus opens into the jugular vein of its own side, and two
sinuses, which receive lateral trunks, are connected by a trans-
verse anastomosis with the caudal vein near the last caudal
vertebra.

The subvertebral lymphatic cavity of the Amphibia forms a portion
of the system which is about the same size as a very large subcutaneous
series of lymphatic cavities, which are present in these animals ; in
the anourous Amphibia especially, this latter series extends over a
large portion of the surface of the body. The lymphatic vessels of
the digestive canal (chyle-vessels) open into it, as do those of the
other viscera, while it is connected also with the lymphatics of the

Fig. 340. Portion of the
Aortaof a Chelonian
(Chelydra) surrounded
by a lymphatic space,
a Aorta, h Outer wall
of the lymph space,
which is removed at h'
BO as to expose the blood-
vessel, c Trabeculse.

LYMPHATICS OF YEETEBEATA.

599

extremities. In the Eeptilia the subcutaneous lymphatic cavities
are more varied and numerous, and the system is more intimately
related to the arteries ; the lymphatic vessels are sometimes wide
spaces (Fig. 340), which surround the arteries and are traversed by
trabeculm ; sometimes they form plexuses which accompany these
vessels. When their trabeculae are more largely developed, the
lymphatic cavity is broken up into several anastomosing canals.
The space which surrounds the aorta is broken up, in the Crocodilini
and Chelonii, into two trunks which surround the veins of the
anterior extremity ; and lymphatic vessels from the head, neck, and
extremities open into them. The lymphatic trunks of Birds have
the same characters, but, in them, both the large trunk in front of
the aorta (thoracic duct), and the small vessels are more inde-
pendent. As in the Eeptilia, the thoracic duct opens into the
superior vensc cavm (venge brachiocephalicas). At the commence-
ment of the tail the lymphatic system is also connected with the
ischiac veins, or with the afferent renals, in which point they
resemble the Amphibia and Eeptilia.

In the Mammalia the walls of the lymphatic system are still
more differentiated, although it often happens that in them also the
sheath of the arteries bounds the course of part of the lymphatic
current. Where they do not accompany the blood-vessels they
form frequent anastomoses, or wide-meshed plexuses, and are dis-
tinguished by valves, as are the same parts in Birds. The lymphatic
vessels of the hinder extremities, as well as the chyle-ducts, unite
into a chief trunk in the abdomen, which is rarely paired, and the
origin of which is frequently distinguished by a considerable enlarge-
ment (cisterna chyli). Thence they are continued into a thoracic duct,
which opens into the commencement of the left
brachio-cephalic vein ; the trunks of the lym-
phatics of the anterior parts of the body (of the
head and anterior extremities), and of the wall
of the thorax, open into, and on either side of
the same vein.

The lymphatic trunks are generally widened
out near their opening into the veins, and the
wall of these enlargements is distinguished by
its muscular investment; it executes rhythmic
contractions. These arrangements are known as
lymphatic hearts. They have been occa-
sionally observed on the caudal sinus of Fishes,
but they are more accurately known in the Am-
phibia (Eanidm) and Eeptilia (Chelonii) ; in the
former they are found on both the anterior and
posterior openings into the veins, but in the
urodelous Amphibia and in the Eeptilia only the
posterior lymphatic hearts have been made out.
found also in the Eatitm (Struthio, Casuarius), and some Natatores,
but in other Birds they have no muscalar investment, and form mere

Fig. 341. Caudal
sinus. a a Anas-

tomosing transverse
trunk. h Lateral

vessels c, and origin
of the caudal vein
d. Of S i 1 u r u s
glanis (after Hyrtl).

These latter are

600

COMPAEATIYE A]^ATOMY.

vesicular enlargements. Finally, in tlie Mammalia sucli structures
do not seem to be developed.

§ 445.

The apparatuses that produce the lymph- cells are simple in
Fishes, where they are placed in the course of the various lymphatics;
the cells are produced in the meshes of reticular connective tissue.
Where more largely developed, this arrangement gives rise to local
enlargements, which accompany the arteries, in consequence of the
relation between these vessels and the lymphatics. This arrangement
obtains even in the higher Vertebrata, although the cells are not
always developed in the sheaths of the arteries. Follicular enlarge-
ments are formed beneath the mucous membrane of the enteric canal,
the lymphatics of which are connected with these cell-forming
regions. They are either scattered, or variously grouped together
(closed glandular follicles). At the commencement of the wall of
the enteron these structures form the tonsils already mentioned ; in
different parts of the mucous membrane of the mid-gut they are
placed closer to one another, and form the so-called Peyerian
Glands, which are present in the Eeptilia, but are only found to
any great extent in the Mammalia.

When a number of these lymphatic follicles are united together
they form larger structures, lymphatic glands, which are also
placed on the course of the lymphatic vessels. In Fishes, Amphibia,
and Eeptilia there are not, so far as we know, any true lymphatic
glands. In Birds, also, they seem to be confined to the neck; it is
in the Mammalia only that they are generally present, and in them
they are found in other parts of the body, as well as in the chyliferous
portion of the lymphatic system of the mesentery. In some
Mammals (e.g. Phoca, Canis, Delphinus) the mesenteric glands are
united into a single mass, the so-called pancreas Aselli.

The Spleen is also one of the organs that form lymph-cells ;
in its histological structure it only differs from the lymphatic glands
by the fact that the cells formed in it pass directly into the blood-
vessels. Essentially it is formed of a fine Jacunar system inter-
posed between the efferent and afferent blood-vessels ; this forms
the greater part of the so-called parenchyma of the spleen.

The spleen is found in all Vertebrata save Amphioxus, and is
always placed in the region of the stomach, and generally close to
the fundus. It forms an elongated or rounded organ of a dark-
red colour, which is sometimes, as in various Selachii, broken up
into a number of smaller lobules, some of which are, in other cases,
converted into secondary spleens.

§ 446.

An organ which is very generally present, and which resembles
the lymphatic glands in various points of structure, cannot bo

LYMPHATICS OF VEETEBEATA.

601

passed ovei% altliougli its relations to tHe lymphatic system are still
very uncertain ; this is the thymus. This is an organ which is also
made up of glandular follicles, and which is divided into larger and
smaller lobes ; its smallest vesicles are filled with cells. In the
Selachii this organ is placed on the branchial sacs, and between them
and the dorsal muscles. In the Sturgeon, and some Teleostei, the
similar follicles that are found on the superior hinder boundary of
the branchial cavity are regarded as the same organ. In the
Amphibia the thymus is a small swelling, placed behind the angle of
the lower jaw. It has the same characters in the Eeptilia. In the
Ophidii and Chelonii it is placed on the carotid, and above the heart.
In the Crocodilini, as in Birds (Fig. 312, tli), it extends from the
pericardium to the lower jaw. The lower portion is the larger in
the Mammalia, so that it rarely passes beyond the thoracic cavity.
In all cases it is best developed in early life, after which it undergoes
atrophy, and it is very rarely that it retains its earlier size in the
adult stage (Pinnipedia).

In the higher divisions of the Vertebra ta there is an organ which
lies in front of the kidneys and on either side of the body ; it is con-
sequently called the supra-renal gland, but we have no information
at all as to its function. In the Anamnia these structures are re-
placed by the investment of the sympathetic ganglia by means of a
cortical layer made up of cell-containing tubes ; these form yellowish
or whitish bodies, and are scattered over a larger portion of the
body, whereas in the Amniota they form a mass on either side,
while nerve-elements can be made out in their medullary substance.
The relatively large size of this organ during foetal life is a note-
worthy point. The function of these organs cannot be regarded as
at all definitely known ; nor are we aided in our inquiry by classing
it as one of the blood -vascular glands — a term which is altogether
obscure, and consequently objectionable.

Excretory Organs.

§ 447.

The arrangements which are found subserving the purpose of
excretory organs among the Invertebrata obtain also, in their most
essential relations, in the Vertebrata, and are therefore indications
of the affinity between the vertebrate phylum and lower forms, which
in other morphological details are very remote. Organs of this
kind have as yet been vainly sought for in Amphioxus ; but in all
the Craniota they are found to exist, and to be formed on the same
type. The type has been obliterated by gradual differentiation,
but it is revealed by the study of individual development. The
simplest stage is represented by a canal which runs in the dorsal
wall of the coelom, and opens to the exterior posteriorly, and in the

602

COMPAEATIYE ANATOMY.

region of the anus, while anteriorly it opens into the coelom by an
internal orifice. Although it is clear that this arrangement has much
in common with the excretory organs of the Yermes, yet the pecu-
liarity must not be overlooked, that although the vertebrate body is a
metameric one, this archinephric duct is not a metamerised organ;
it is not therefore completely homologous with the metameric looped
canals of the Annulate Yermes. It must consequently be derived
from a still lower condition, that is, from one in which the organism
was not divided into metameres ; so that it represents, as does also
the unsegmented chorda dorsalis, one of the, phylogenetically, oldest
organs of the Yertebrate body.

This archinephric duct has been observed to be derived from the

mesoderm ; in its rudimentary condition
it has the form of a solid chord of cells,
or is differentiated as a groove from
the epithelium of the peritoneal cavity
(Teleostei). The rudiments of the canals
(Fig. 342, t) are derived from the same
parts ; these, either permanently, or for
a time only, open by an infundibular
orifice into the abdominal cavity, while
they are also connected with the archi-
nephric duct (Selachii, Amphibia). They
develop coiled glands on their course,
and so form the secreting portion of the
primitive kidneys. At definite points
a coil of arterial vessels (glomerulus)
pushes its way into a dilatation of these
metamerically arranged canals, and gives
rise to a Malpighian body, lying in a cap-
sular enlargement. This last arrangement
obtains in all forms of the renal organ,
however much it may be modified in
various members of the vertebrate group.

The fundamental form of this primi-
tive kidney must be regarded as being a
longitudinal canal, which receives trans-
verse canaliculi, which open by ciliated
infundibula into the abdominal cavity ; this is the form which the
rudimentary apparatus really has in the Selachii. The connection
with the coelom, the epithelial investment of which always gives
rise to a large portion of this system, allows us to compare it with
the excretory organs of many Yermes, and points to those distant
forms in which these organs are the sole cavitary organs that are
developed from the mesoderm (Platyhelminthes) . The metameric
arrangement of the open transverse canals is due to the general
metamerism of the vertebrate organism. It must not, therefore, be
regarded as the same as that of the looped canals of the Annelids, or
even as derived from it, for those canals open to the exterior on the

Fig. 342. Section of an Embryo
of Pristiurus. ug Archi-
nephric duct, t Rudiment of a
funnel-sbapedorgan. dEnteron.
in Medullary tube. ch Noto-
chord. a Aorta, v Veins.

EXCEETOEY OEGAXS OF VEETEBEATA.

603

surface of tlie metameres tEemselves (§ 145), and not into a longitu-
dinal canal. This canal is the part which, by being the first part to
appear in the Vertebrata, defines the type of the whole apparatus.

But, just as in a large number of Invertebrata the excretory
organs partly lose their function, and serve as efferent ducts for the
generative products, so too among the Vertebrata do we meet with
a process of this kind, by which great changes are effected in the
primitive excretory apparatus. It loses, and that often very early
in life, its primitive arrangement. And when this is not seen in the
embryo, it must be regarded as due to
new, acquired, relations.

§ 448.

In the Cyclostomata, Teleostei, and
Amphibia, a special portion of the primi-
tive kidney appears at the most anterior
end of the archinephric duct, which de-
serves especial mention, as it does not
only appear earlier than the rest of the
primitive kidney, but is generally sepa-
rated by some distance from it. This
portion is made up of a small number
of canaliculi, which commence by ciliated
infundibula, and are generally set in a
coil. There may be only one canaliculus.
A Malpighian body may sometimes be
observed on the canaliculi. In the
Amphibia this pro nephron undergoes
atrophy, while in the Amniota it does
not seem to be even rudimentarily de-
veloped. It persists, however, in the
Cyclostomata, where it is provided with
a tuft of ciliated infundibula, which
project into the abdominal cavity.

Among the Cyclostomata the primi-
tive kidney is seen at its simplest in
Bdellostoma. An elongated canal (Fig.
343, A Bj a) gives off short transverse
canaliculi at various points (5) ; the
blind end of which (c) is constricted off,
and encloses a glomerulus (B). The

Fig. 343. A Portion of the
kidney of Bdellostoma.
a Urinary duct, h Urinary
canaliculi. c Terminal capsule.
B Portion of the same more
highly magnified, a c As before.
In c there is a glomerulus.
d Afferent, e Efferent artery
(after J. Muller).

transverse canaliculi form the secretory
apparatus (urinary canaliculi) ; the archi-
nephric duct is the collecting tube, and functions as the ureter.
In the Myxinoidea and Petromyzontes, the kidneys, which are set
along the posterior third of the coelom, are larger, but the urinary
canaliculi have exactly the same relations. In both forms the
ureter takes a lateral course to the abdominal pore ; but in the

604

COMPAEATIYE AEATOMY.

Petromyzontes it is first connected with the one on the opposite
side, to form an unpaired and wider portion. We do not yet know
its relation to tlie metameric ciliated infundibula.

In the Selachii the primitiye arrangement is limited to the early
stages of development. The primitive kidney extends along the
dorsal wall of the coelom, and is made up of separate canaliculi,
which commence by ciliated infundibula (Fig. 344, i), which open

into the abdominal cavity. Each canal,
after having broken up so as to en-
close a glomerulus {m), is continued
on into the archinephric duct. These
canals increase in length, so that each
of them forms a coiled lobule (r) ; each
kidney, therefore, is composed of a
series of these coils, which are col-
lected together into the archinephric
duct (u). This duct opens into the
cloaca. Changes may occur in the
glandular portion of these kidneys, as
well as in their efferent ducts. The
anterior portion, which is made up of
a number of lobules, does not undergo
any very great development, as does
part of the hinder portion. This,
which is made up of a varying, but
large number of primitive lobules
(13-14 in Acanthias), is converted
into a larger organ, the canaliculi in
which may be seen to increase in
number by budding off new ones.
This portion retains its renal function,
while the anterior part is atrophied,
and, in the male, enters into con-
nection with the generative gland.

The ciliated funnels (nephrostomata)
are retained in some Sharks only ; they disappear in all the Eays,
and many of the Sharks. Where they are retained they are reduced
in number.

Of the changes which obtain in the primary archinephric duct
the most important is its division into two parts. This commences
at its anterior end, and extends backwards, so that there come to be
two canals. One commences at the anterior abdominal orifice of
the primary duct, and has no further relations to the kidney. This
is the Mullerian duct. The other canal retains its connection
with the primitive kidney, and forms the secondary archinephric
duct. But even this portion may undergo certain changes, inasmuch
as in the male it is converted into the seminal duct. The efferent
ducts from the posterior portion of the kidney are then collected
into a common ureter, which opens into the sinus urogenitalis, into

Fig. 344. Portion of the kidney
of an Embryo of Acanthias (dia-
gram). i Ciliated funnel, m Mai-
pighian body, r Eenal lobules,
u Archinephric duct.

EXCEETOEY OEGANS OF VEETEBEATA.

605

wEicE, Eowever, several ureters may open separately. In tlie
females, also, tEe efferent ducts from tEe anterior and aborted
portion of tEe primitive kidney are connected witE tEe ureter.

In tEe Ganoidei and Teleostei tEe kidneys Eave tlie same posi-
tion. TEe primitive kidney appears to be considerably increased in
size, wEile the efferent ducts are not so completely differentiated as
in tEe SelacEii, wEere tEey were tEe cause of mucE complication ; in
tEe Ganoidei, Eowever, tEe presence of a nepErostome, witE a wide
abdominal orifice on the efferent duct, speaks to the commencement
of the process by which the Mullerian duct
is differentiated; the ureter, therefore, no
longer corresponds to the arcEinepEric duct.

In the Teleostei the secondary portion
of the gland first appears on the anterior
division of the arcEinepEric duct, and
forms that portion which, in many, extends
as far as the head (head-kidney). The
hinder portion, which is developed later,
becomes connected with this. The whole
forms a compact glandular organ, which is
covered by the peritoneum, and extends
along the vertebral column; it varies in
size in different regions. Its differentiation
into lobes is generally implied by the
greater development of certain regions.

The efferent ducts (Fig. 345, u) either pass
along the anterior surface, or more to the
sides ; they generally unite into an unpaired
portion, which opens behind, or below the
generative orifice. TEe ducts are widened
at different points, either in the common,
or in the separate portions; these struc-
tures function as urinary bladders,^^ but
they Eave no morphological connection
with the urinary bladder of the higher
Yertebrata.

TEe renal organs of the Amphibia Eave
many points in common with those of the

SelacEii. TEe rudimentary ducts are always provided with func-
tionally active nepErostomata. The primary ureters form lobules
by becoming arranged in coils. In the Coeciliae they are all of
much the same size, but in the Urodela and Anura the hinder ones
are increased in size and number, so that this portion becomes
much larger than the anterior part. The nepErostomata, also, are
greatly increased in number in this region, and are persistent.
In the Urodela the anterior portion of the kidneys receives the
efferent ducts of the testes, while in the Coecili£e and Anura
different parts of the kidneys are connected with these organs.
The primary arcEinepEric duct is differentiated so as to give rise

Fig. 34'5. Kidneys of Sal mo
fario. E Kidneys, -u Ureter.
V Yesicular enlargement.
ur Its efferent duct, rr Yenm
renalesrevelientes. d Ductus
Cuvieri. s Yena subclavia
(after Hyrtl).

606

COMPAEATIYE ANATOMY.

to a Mnl]erian_, and a secondary arcliinepliric duct (Fig. 348). The
latter serves as the efferent duct of the kidney, or ureter, in the
Coecilias, Urodela, and female Anura, while in the males of many
of these latter the primary archinephric duct appears to retain its
original function. They open independently into the cloaca.

Muller, W., Das Urogenitalsystem der Cjclostomen. Jen. Zeitschr. IX. —
Semper, C., Das Urogenitalsystem der Plagiostomen. Arbeiten ans dem
zool. Institut zn Wurzburg, II. — Spexgel, J. W,, Das Urogenitalsystem der
Ampbibien. Ibid. III.

[Balfour, F. M., A Monograph of the Development of the Elasmobranch Fishes.
London, 1878.]

§ 449.

The primitive kidney is likewise developed in the Amniota.
For some time in development it extends through the coelom, and

projects into it from the dorsal
wall of this cavity. The archi-
nephric duct is again (Fig. 346,
the first part to be developed.
The urinary canaliculi {u)y which
form the glandular portion of the
organ, open into it.

The hinder portion of the
primitive kidney, which has
always the same function, is well
developed even in the Selachii,
but still more so in some of the
Amphibia ; this is effected by
the increase in the number of
the urinary canals, and by the
formation of special efferent
ducts. These processes indicate
prophetically the relations of
these parts in the Amniota. In
the Reptilia the additional por-
tion of the urinary canals is
directly connected with the
hinder portion of the primitive
kidney (Lacerta), but it is not
connected with it to form the same, but a new organ — the per-
manent kidney. For a long time it is present in company with
the primitive kidney, but it has its own ducts (ureters), and it takes
on the function of the primitive kidney, in proportion as the latter
is atrophied, or converted to the purposes of the generative system.
In Birds the rudiment of the permanent kidney appears to be
formed independently ; and this is still more the case in the Mam-
malia. We see, therefore, that the so-called permanent kidney of
the Amniota is at first an organ which is connected with and forms
part of the primitive kidney, and that it is gradually separated from

V Cardinal veins. u Primitive kidney.
ug Archinephric duct. e Germinal
epithelium. P Pleuroperitoneal cavity
D Enteric groove.

EXCEETORY ORGANS OF VERTEBRATA.

607

it both in space and time. No rudiment of the nephrostomata has
been observed, nor is the archinephric duct divided as in the
Anamnia; the Mullerian duct has a separate rudiment.

The kidneys of Reptiles and Birds somewhat resemble those of
Fishes in their size and position. They are placed far back and close to
the cloaca; in the Snakes only are they placed farther forwards, while
at the same time they are longer. They vary very greatly in form, in
consequence of the development of lobes. In Birds they are placed in
depressions between the transverse processes of the sacral vertebrm,
and are generally divided into three portions, which are sometimes
connected with one another, and which may vary greatly in size.
The ureters (Fig. 349, u) are generally placed on the inner edge of
the kidney, and receive at various points larger urinary canals
(Ophidii, Chelonii) ; or these canals are enclosed in the parenchyma of
the kidney, and do not leave the organ except at its termination
(Saurii, Crocodilini). In Birds a large part of the canal is outside
the kidney. In all cases they open separately into the cloaca, or
into a sinus urogenitalis, into which the genital ducts also open.

The kidneys of the Mammalia vary in several points, and espe-
cially as to the characters of the orifice of the ureters, after the
differentiation of the rudiment which is known as the renal
canal.

The kidneys, which are developed at the blind end of the
urinary canal,^^ are, after they are differentiated, placed behind
the primitive kidneys. At first they appear to have a smooth
surface, which becomes uneven when the glandular parenchyma is
developed into separate lobes. In either lobe the urinary canaliculi
are united together at a papilliform process, with which the common
efferent duct of the lobe is connected. It forms the pyramid, and a
number of these unite to form the pelvis of the kidney, from which
the ureter is given off. The permanently distinct lobes are very
numerous (about 200) in the Cetacea. There is a smaller number
in the Pinnipedia. In many Carnivora, also, the lobes are separate
(Ursus, Lutra), while in others they are fused. This gives a
knobbed appearance to the surface of the kidney (e.g. in Hyaena,
Bos, Elephas). In others there is a condition of this kind for some
time, but when the cortical substance of the lobes is completely
fused, the surface of the kidney becomes smooth, although the
grooves that remain indicate its primitive division into lobes.
Within the organs, however, the division is more or less completely
retained, and the number of primitive lobes is implied by the
greater or less extent to which the papillge are fused together.
This fusion, further, may affect some, or all the lobes, so that the
number of renal papillae may be much reduced; at last, indeed,
they may all unite into one (Marsupialia, Edentata, Rodentia, several
Carnivora and Primates).

The ureters formed from the renal canals, after they are
separated from the archinephric duct, primitively pass into that
portion of the allantois which runs in the abdominal cavity of

608

COMPAEATIVE A^TATOMY.

tlie embryo, and is connected witli tlie primitive cavity of tlie
pelvic portion of tbe enteron (urachus). This is gradually con-
verted into a fusiform widened organ — the urinary bladder,
while the continuation of the urachus into the umbilicus, and from
thence into the umbilical chord, is obliterated. The former portion
forms the ligamentum vesico-umbilicale medium. The primitive
(fusiform) form of the urinary bladder is retained in some Mammals
(Seals), while in others it is gradually modified, and with these
modifications there are correlated differences in the way in which
the ureters open. Thus in many Eodents they open high up on
the posterior wall of the bladder (Fig. 354, O', u).

The other characters of the efferent ducts are common to them
and the generative apparatus, with which, therefore, they will be
treated.

Generative Organs.

§ 450.

In the Vertebrata, the reproductive organs are shared by
different individuals ; the separation of the sexes is the rule,
although there are various exceptions to it in the class Pisces.
In the higher divisions also there are various arrangements which
are indications of hermaphroditism. But it seems to me that the
point which is of real importance in this matter is the repro-
ductive material, and that the characters of the efferent ducts are
of no importance, for they were not primitively part of the gene-
rative apparatus.

Our knowledge of the earliest development of the male generative
matter is not quite definite, but we know certainly that the female
elements are derived from the epithelial layer which invests the
abdominal cavity. In this there are points of likeness between
the Vertebrata, and the Vermes among the Invertebrata. In
Amphioxus follicular structures, covered by a layer of epithelium,
and forming diverticula of it, are developed at various points in
the coelom, or in the cavities connected with it ; these structures
are the germ-glands. The ova are formed in them, between
indifferent and flattened cells, which form the stroma of the organ.
In this character Amphioxus is very different to the Craniota, where
the germ-glands are always developed at a sharply defined and
less extensive region. The epithelial investment of the abdominal
cavity retains its primitive character along a tract which corre-
sponds to the rudiment of the primitive kidney longer than it does
in other regions ; and this epithelial layer may be distinguished as
the germinal epithelium (Fig. 346, e). At the side of the
mesentery in this region there is an elevation of varying leugth,
which is formed by a thickening of the connective tissue — the
genital ridge. The epithelium dips into this, and forms the rudi-
ments of the ova. Of a group of cells which grows inwards, one

GENEEATIYE ORGANS OF VERTEBRATA.

609

cell becomes an egg, while the rest form a cellular layer around it —
the follicular epithelium_, which unites with the surrounding con-
nective tissue to form the ovarian follicle. Each invagination of the
germinal epithelium either forms a single follicle, as in the Anamnia
(Selachii), or these groups of cells grow out and form the rudiments
of a number of follicles^ as in the Craniota.

The cells of the ovarian follicle that are set around the egg
generally remain indifferent^ and aid in the nutrition of the egg as
well as in the formation of the yolk-sac^ which surrounds it. The
egg itself, and the cells of the follicle which surround it, undergo
more or less considerable modifications. When the egg and the
follicle increase equally in size, the follicle-cells form a simple epi-
thelial layer, as in Fishes, Amphibia, Reptiles, and Birds. But in
the Mammalia they multiply while the egg-cell remains relatively
small, and for a long time they fill up by far the greater part of the
follicle. As this follicle grows a cavity is gradually formed in its
interior which is filled with fluid ; this causes the cellular layer of
the follicle to be extended around its wall (membrana granulosa),
while at one point, which is somewhat thickened, it encloses the egg.

The changes which obtain in the egg-cell relate to the yolk,
and they are accompanied by an increase in the size of the egg.
This may be seen in the Teleostei, where the granules of the yolk
often undergo great metamorphoses. The same happens to the eggs
of the Amphibia. In the Selachii, Reptilia, and Aves the yolk-
granules are greatly increased in number, and are specially differ-
entiated. Owing to the number present the ripe egg is of a
considerable size.

The region invested by the germinal epithelium is the point
at which the male germ -glands are also developed, but it
seems that this epithelium does not take any direct share in the
formation of the testes. The earliest differentiation of the glandular
tubes (seminal canals), which make up the testes, has not yet been
observed ; the view that they are formed from a portion of the
primitive kidneys is beset by the difficulty of their having no
relationship of any kind with these organs.

The form-elements of the sperm are developed by the dif-
ferentiation of the epithelium of the seminal canals. In all Verte-
bra ta these are movable filaments which are given off from a thicker
portion of varying form — the so-called head. This head is dis-
coidal or elliptical, as in many Mammals and Fishes, or elongated,
as in the Selachii, Amphibia, and Aves. In the latter it is fre-
quently coiled in a corkscrew fashion. The seminal filament of
some Amphibia (S alamandrina and Toads) is distinguished by an
undulating membrane.

§ 451.

The germ-glands are developed from the structures known
as genital ridges. Sometimes more and sometimes less of this ridge
is converted into the ovary or testis. The simplest condition is seen

610

COMPAEATIYE AXATOMY.

in the Cyclostomata. The ovaries o£ the Petromyzontes have the
form of paired lamellse^ which extend along the coelom, and are
thrown into a large number of folds, in which the ova are developed.
The. testes are similar in character. In the Myxinoidea the germ-
glands are unpaired, and arise from the right side of the mesentery.
Both sets of generative products are passed into the coelom, whence
they reach the exterior through the abdominal pore.

The ovaries of some Teleostei have almost the same characters ;
thus, in the Salmonidse the eggs are passed into the abdominal
cavity, and are evacuated through the abdominal pore. The
same is the case in Lsemargus borealis among the Selachii,
where the ovaries contain much smaller eggs, and are themselves
much larger. In the rest of the Fishes there are efferent ducts
in both sexes, which are largely — perhaps always — due to the
differentiations which affect the primitive kidneys (cf. § 448).

In this relation the Ganoidei are of a low grade, for their
germ-glands have no direct ducts, and their products are passed
into the coelom. In both sexes the products escape by an apparatus
which is homologous with the Mullerian duct, consisting of a canal of
varying length, and provided with an infundibular orifice, which is
attached to the ureter (secondary archinephric duct) ; this takes up
the generative products. This fact must be regarded as one of
special importance, for we learn from it that the Mullerian duct
may be turned to use in the male. The presence of this duct in
both sexes leads to a correct apprehension of the real facts of the
case, and renders it unnecessary to regard the presence of the
rudiments of these organs in the male as due to a primitive
hermaphroditism, which, cannot be shown to have obtained at the
required stage of development.

Two different arrangements can be derived from that which is
dominant in the Ganoidei. One is seen in the majority of the

Fig. 347. Generative organs and enteric canal of Clnpea harengns. oe CEsophagns.
Stomach. ap Appendices pyloricae. i Enteron. a Anns, vn Air bladder.
pn Air duct, s Spleen, tt Testes, vd Their efferent duct, g Genital pore.
hr Branchise (after Brandt) .

Teleostei, and the other in the Selachii, and from thence in the
Amphibia and all Amniota.

The male organs in all Teleostei and the female organs in all

GEJ^EEATIVE OEGANS OF VEETEBEATA.

611

except tEose above mentioned_, bave the tubular form. The germinal
region is often limited to one portion of the tube, whence it extends
more or less considerably, according to the degree to which its
products are developed. The lateral efferent ducts of these genital
tubes (Fig. 347, tt), are united into a common duct, which opens by
the genital pore. In this arrangement the germ-gland is not
ordinarily represented by the whole apparatus, but by the germi-
nal region only, which projects on the inner wall of the tube,
and is often lobed or branched. The germinal region is probably
invested by the Mullerian duct, which is converted into a tubular
form, but this has still to be established by embryological observation.

In a number of Teleostei hermaphrodite arrangements have been
observed, a testicular as well as an ovarian tube being developed ;
this is best known in species of the genus Serranus,

§ 452.

In the Selachii the arrangement which obtains in the female
Ganoi’dei is retained and further developed. The germ-glands
are generally developed on a certain portion only of the genital
ridge, while the rest of the organ has its stroma increased in
thickness, and converted into a special tissue (epigonal organ). As
a rule, the ovaries are paired, and lie some way forward. In many
the left one is rudimentary (Mustelus, Galeus, Scyllium, Pristiurus,
Carcharias). The long oviducts, which are developed from the
Mullerian ducts, unite with their fused abdominal mouths to form a
wide infundibular opening; this is correlated with the great size
of the eggs which it has to take up. The hinder end of each ovi-
duct is differentiated into a portion which is distinguished by its
greater width, and often by its thicker walls ; this generally func-
tions as a uterus, and opens into the cloaca. In the Selachii, as well
as in the Chimacrac, a glandular portion is differentiated close
to the abdominal end of the oviduct. The generative organs
of these two groups, and of the Dipnoi, agree in the most essential
points.

In these divisions the male organs are generally represented by
small testes, the ducts of which are connected with the anterior
portion of the excretory organs, so that this portion of the primitive
kidney, with its efferent ducts, is adapted to the service of the
generative apparatus. After several coils the vas deferens passes
to the cloaca; in Chimsera it first unites with its fellow of the
opposite side ; it generally opens with the ureter into a sinus
urogenitalis, which opens by a papilliform process into the cloaca.
Part of the Mullerian duct remains connected with the infundibular
ostium, at the same point as that at which it is found in the female.
At the hinder end also a portion remains connected with the cloaca,
in the males. The Mullerian duct in Chimeera is retained in the same
way. In the males of the Selachii and ChimaeraB certain parts of the
posterior appendages are converted into copulatory organs (p. 487).

612

COIMPARATIVE ANATOMY.

The generative apparatus of the Amphibia is very similar to that
of the Selachii. The ovaries (Fig. 348, A, ov) form lamellae, which
project into the abdominal cavity, and vary in size according to the
number of the eggs which are being developed. In the Urodela
they enclose a cavity, which is broken up into several spaces in the
Anura. The Mullerian duct forms the oviduct (od), which com-

Fig. 348. Urogenital system of the
Amphibia (Triton). Diagrammatic.
A Female. B Male, r Kidneys ; on the
surface of which the nephrostomata are
indicated. sug Ureter. od Oviduct,
w Mullerian duct. ve Efferent duct of
the testes, t Testes, ov Ovary, u Uro-
genital orifice (partly after Spengel).

mences a long way anteriorly,
by an infundibular orifice, and
which always opens separately
into the cloaca. It is generally
greatly increased in size at the
breeding season; this results in
its being thrown into a number
of coils. In the oviparous species
(Salamandra) the terminal por-
tion of the oviduct functions as
a uterus.

The testes are placed in the
same region as the ovaries. They
either form a compact organ, or
consist of a series of larger or
smaller (and consequently more
numerous) bodies. The latter is
the case in many Coecilige, while
in others there are intermediate
stages towards the more compact
form. A longitudinal collecting
duct receives the efferent ducts
of the different portions of the
testis, and gives off again trans-
verse canals, which correspond
in number to the primary divi-
sions of the kidney and are con-
nected with them. The kidney,
therefore, is the efferent duct
for the sperm, which is passed out
through the ureter (secondary
archinephric duct) . In the Anura,
the sperm from the testis is
passed to the kidney by a net-
work of fine canals which are

placed between these two organs. The canals, however, which pass
into the kidneys from the longitudinal collecting duct, traverse the
kidneys, without being connected with the Malpighian corpuscles,
and open directly into the ureter. Bufo is the sole exception ; in it
there is a connection between the vasa efferentia and the Malpighian
corpuscles. In the Urodela the anterior portion of the kidney
(genital portion) is connected with the generative organs. Trans-
verse canals (o e), are given off from a collecting duct, which is

GENERATIVE ORGANS OF YERTEBRATA.

6l3

placed in, or on tlie testes {B, t) ; these pass tlirough the mesor-
chinm to a longitudinal canal, from which canals are again given off,
and these pass into the so-called renal portion. The sperm, there-
fore, passes through a certain portion only of the kidney, and only
passes to the common ureter by the ducts which are given off from
this portion ; this ureter is developed from the secondary archi-
nephric duct. In proportion as this portion of the primitive kidney is
freed of renal secretion it is converted to the uses of the generative
apparatus, so that the two secretions are not commingled except in
the ureter.

In the males, the Mullerian duct remains free anteriorly,
but it is generally closely connected with the secondary archi-
nephric duct. It is either complete {m)y and even has a coelomic
ostium, or parts only are canalicular, and the rest is converted
into a solid chord at various points. This is most commonly the
case in the Anura, but in Bufo it is very well developed. In
the Coecilise the hinder portion has its walls provided with well-
developed glands, in consequence of which this portion is still
functional.

In many Anura (Bufo) there is a peculiar large organ contain-
ing ova-like cells on the testis; this was formerly regarded as a
rudimentary ovary. We do not know what function it has, any
more than we know that of the so-called fatty bodies which are
found attached to the anterior end of the germinal gland of the
Anura.

Since the generative organs open into the cloaca this organ func-
tions as part of the generative system. In the female Urodela
(Salamandra) the cloacal glands take up the sperm, and function as
receptacula seminis. In the Coecilise the cloaca of the male can be
everted, and serves as a copulatory organ.

Semper, C., Urogenitalsysteni der Selachier. — Spengel, Urogenitalsysteni der
Amphibien, 1. c.

§ 453.

The generative apparatus of the Sauropsida resembles that of
the Amphibia in the more important points, and has, therefore,
some of their arrangements more highly developed. The ovaries are
racemose organs, which are placed in front of, or at the sides of, the
vertebral column, and form large organs, which vary in size accord-
ing to the extent to which the eggs, which are very large in this
division, are developed. In the Ophidii the ovaries are placed at
different levels. The right one is the larger, and is generally placed
in front of the left one. In Birds the right ovary is atrophied. In
the embryo it is as large as the left one, but while the left is
developed it remains at a lower stage, and may at last disappear
completely. Rudiments of it are found in the diurnal Raptores.

The oviducts are again developed from the Mullerian ducts, and
when fully developed are large, and ordinarily coiled canals which

614

COMPAEATIYE AAUTOMY.

commeuce by a wide abdominal moutb. The mucous membrane
wliicli invests them is set in a number of longitudinal folds; tbe
lower portion, in addition to the greater thickness of its muscular
wall, is distinguished from the other and longer portion by the larger
size of its folds and villi ; this is especially the case in Birds. This
differentiation of the oviduct corresponds to the different function of
the different parts ; the longer and more anterior portion secretes the
albumen, and the thick- walled terminal part forms the shell. This
portion is connected by a short and narrower piece with the cloaca.
In correlation with the atrophy of the right ovary the oviduct of the
same side is also atrophied in Birds ; not unfrequently, however,
remnants of it are found near the cloaca. While the oviducts open
by one orifice in the Ophidii and Saurii, as well as in Birds, in the
Chelonii they open into the neck of the so-called urinary bladder ;
this foreshadows the relation which is the typical one among the
Mammalia. In many Ophidii a diverticulum of the posterior wall of

the cloaca receives the openings
of the oviducts. A remnant of
the primitive kidney is retained
behind the ovaries (this has been
observed in the Saurii and Aves).

The testes, which are gene-
rally oval, are attached to the
vertebral column by a fold of
the mesenteries ; this is either
effected in front of, or between
the kidneys. Their size is closely
correlated with their physio-
logical activity ; this is especially
the case in Birds. In the Ophidii
they are arranged in the same
way as the ovaries. The vasa
efferentia pass to a parorchis,
which generally consists of a
few canals only, and from which
a vas deferens extends to the
cloaca. In the Crocodilini it
is straight, in the Ophidii, Saurii,
and Aves it is arranged in a
number of smaller coils, while
in the Chelonii (Fig. 349, e) it
forms a complex of coils. In
many Saurii and Aves, as well
as in the Crocodilini, its hinder
portion is widened out.

In the Saurii the vasa defe-
rentia still unite with the ureters
to open into the cloaca ; in the Chelonii they open into a sinus uro*
genitalis, which is formed by the neck of the urinary bladder.

Fig. 349. Urinary and generative organs
of a Chelonian (Chelydra serpentina),
r Kidney, u Ureter, v Bladder, t Testes.
e Secondary testes and vas deferens.
ug Opening of the nrogenital sinns into
the cloaca, cl Cloaca, opened from be-
hind. p Penis, s Groove of the penis.
re Hind gut. c d Cloacal cgecal sacs.

GENERATIVE ORGANS OF VERTEBRATA.

615

Sometimes eacR spermatic duct opens on a papilliform process (Saurii^
Aves).

A rudiment of tlie Mullerian duct may sometimes be seen in the
form of a filament passing* forwards from the anterior end of the
secondary kidney (Saurii), while further remnants of the anterior
portion of the primitive kidney which are not converted into the
secondary testis may be recognised.

§ 454.

In the Mammalia the generative apparatus undergoes great
metamorphoses j owing to the further development of various portions
of the efferent ducts and the formation of a number of accessory
organs. In the female apparatus these are largely correlated
with the relations that obtain between the embryo and the maternal
organism. As this is least marked in the Monotremata_, they under-
go the least amount of modification, and have therefore direct
relations to the lower divisions of the Yertebrata, and especially to
the Sauropsida. The oviducts (Fig. 350, t) open separately into a

Fig. 350. Female generative organs of
Ornithorhynclius. o End of the ovi-
duct and ovary, t Oviduct, u Uterus,
u' Point at which the orifice of the uterus
projects upwards, close below the opening
of the ureter. vu Urinary bladder.
sug Sinus urogenitalis. cl Cloaca.

Fig. 351. Female generative organs of
Halmaturus. ov Ovary, od Oviduct.

Uterus, cv Vaginal canals, cug Sinus
urogenitalis. vu Urinary bladder.
ur Ureter, * Opening of the bladder.

sinus urogenitalis, which communicates with the cloaca {cl). The
lower end of the oviduct, which is distinguished by the greater
thickness of its muscular wall, forms a uterus {u) ; but this merely
corresponds to the structures which likewise function as a uterus in
many Anamnia and Sauropsida.

In the Marsupialia the efferent ducts of the female are connected
together on the outer surface, and each of them gives rise to an
oviduct and uterus, as well as to a new portion, or vagina, which

G16

COMPAEATIYE ANATOMY.

opens into tlie sinus urogenitalis. The upper portion^ which
commences by a very wide coelomic orifice, forms an oviduct
(Fig 351, od), while the next and thicker- walled portion forms a
uterus (w). Each of the two uteri open by a papilliform process into
a portion, which from the exterior appears to be common to them
both, and which is formed by the union of the two Mullerian
ducts. A curved vagina is given off from this on either side
(Didelphys), or the commencement of the tube is replaced by a
csecal vaginal sac which is pushed out backwards, and is usually,
though not always, divided internally by a median partition ; from
this sac the distinct ^Waginal canals^"’ (cv) pass in a curved direction
to the urogenital sinus (cug) (Halmaturus).

In the monodelphous Mammalia the archinephric ducts are united
with the Mullerian ducts to form a common chord (genital chord) . The
connection between the two Mullerian ducts, which is well marked in
Halmaturus, is effected in them at about the middle point of the duct,
and thus they become connected during embryonic life. A portion
of these ducts have their cavities fused, while they are separate in
front of, and behind this point ; this is an indication of the common
sac, which gives off the vaginal canals in the Marsupialia. But in
the Monodelphia the lumina are fused as far as the end of the genital
chord, and so form a single canal (genital canal) which opens into
the sinus urogenitalis. There are, therefore, two canals, which are
separated from one another at their commencement, but which unite
into an unpaired portion of varying length ; these canals are derived
from the Mullerian ducts, which are separate in the early stage of the
embryo. The parts, which are distinguishable even in the Marsu-
])ialia, are due to the differences in the extent to which the walls of the
different parts are differentiated, and the modifications in them are
essentially due to the greater or less extent of the two tubes. The
uterus undergoes a number of changes, most of which are due to
adaptations to its relations to the foetus. Two completely separated

uteri open into a
^ 77 r vagina in many Bo-

dentia (Lepus, Sciurus,
Hydrochoerus, etc.),
and in Orycteropus
(Fig. 352, A). In

other Bodentia the two
uteri are only united
for a short distance
into a common open-
ing into the vagina (e.g. Cavia, Coelogenys, Mus). This leads to the
arrangements seen in the uterus of the Insectivora, Carnivora, Cetacea,
and Ungulata, where a single uterus is continued into two separate
cornua (B), which are continued into the oviducts. When the common
portion of the uterus is elongated, the cornua are shortened ; this is
the case in the Chiroptera and Prosimiee ; in the Simiae, as in Man,
there is a single uterus (0), which receives an oviduct on either side.

Fig. 352. Various forms of the uterus A B C. u Uterus
od Oviduct. V Vagina.

GENERATIVE ORGANS OF VERTEBRATA.

617

The cornua of the uterus, and the common uterus itself, vary very
greatly in length; so, too, does the vagina, the mucous membrane
of which may be variously modified. In many Rodents (Lagostomus)
a certain portion retains its original double nature. Its opening
into the urogenital sinus is sometimes distinguished by a temporary
fold of mucous membrane, which is known as the hymen. This has
been observed in the Ruminantia, Carnivora, etc. ; but it is in the
Simise only that it has the same relations as in Man. The primitive
Mullerian duct, which only served for the passage of the generative
products, is therefore differentiated into three parts, owing to the
great physiological changes that happen to it ; and of these parts the
first, or Fallopian tube, alone retains its primitive relations.

The ovaries, which are generally small, vary greatly according
to the relation that obtains between the follicles and the stroma of
the ovary. In a large number of Mammals they are racemose in
form. They seldom retain their primitive position, and generally
travel towards the pelvic basin, or, with their oviducts, are com-
pletely enclosed in it. They are always in close relation to the oviduct,
or rather to its infundibular coelomic mouth, for a process of the
margin of the ostium extends to the ovary. The mesenteric folds
(ligamenta uteri lata), which support the ovaries and oviducts, not
uufrequently unite with the pouch that encloses the ovary to form
the mouth of the oviduct (as in the Carnivora).

Remnants of the primitive kidneys and their ducts, which are
enclosed with them in the genital chord, are retained at the sides
of the uterus, or in the folds of the peritoneum, which connect the
ovaries with the uterus. The so-called canals of Gartner are
formed by remnants of the archinephric ducts, which accompany
the uteri in Echidna, and open into the urogenital sinus; in other
forms, portions only of these canals persist. A rudiment of the
primitive kidney, which is placed near the ovaries, is known as the
parovarium.

§ 455.

In the male generative apparatus of the Mammalia the
testes have, at first, the same position as the ovaries — that is, they
are placed at the inner edge of the primitive kidneys. A chord
extends from the archinephric duct to the inguinal region of the
abdominal wall (n). The primitive kidneys are partly united with
the testes, and there form the parorchids (epididymes) . As in
the female, the archinephric duct unites with the Mullerian duct
to form a genital chord, which passes to the urogenital sinus,
developed from the lowest portion of the allantois. It forms
the vas deferens ; the Mullerian duct is atrophied, its terminal
portion only being, as a rule, converted into a permanent organ,
corresponding to a sinus genitalis, the so-called uterus masculinus ;
this generally opens into the urogenital canal between the orifices
of the seminal ducts.

618

COMPAEATIYE ANATOMY.

The apparatus thus formed is variously modified in different
parts. The testes do not retain their primitive position anteriorly
to the kidneys in any Mammals except the Monotremata. In the
Cetacea, Hyrax, Elephas, and various Edentata, they are placed a
little to the side of, or below the kidneys. In others they are found in

the inguinal region of the ab-
dominal wall, which they pass
through (many Eodents, the
Camelidae, and various Car-
nivora [Lutra, Yiverra]). In
others, finally, they travel still
further by means of the ingui-
nal canal, descending through
the wall of the abdomen into
a diverticulum, the scrotum,
which is formed from the in-
tegument. The space which is
formed (canalis vaginalis) when
the testis passes into the scro-
tum, by the peritoneum which
grows out with the descendiug
testis, is permanently open in
most Mammals, so that the
cavity around the testis is in
communication with the ab-
dominal cavity. As the testes
pass down the inguinal canal
the abdominal wall is driven
in front of them. When the
vaginal canal remains open
the testes may return again
to the abdominal cavity ; this
ordinarily happens in many
Mammals during the breeding
season (e.g. Marsupialia, Eo-
dentia, Chiroptera, Insectivora,
etc.). The scrotum of the
Marsupialia is remarkable for
its position in front of the
genital orifice. It is a special
structure, while in the Mono-
delphia the scrotum is de-
veloped from the boundary of
the primitive urogenital orifice.
The lower end of the vas deferens is always simple in the
Monotremata and Marsupialia, Carnivora and Cetacea. In the rest
it gives rise to glandular structures, which are known as 'Wesiculse
seminales,^^ as the sperm may be collected in them {gl). These
organs are greatly developed in the Insectivora and many Eodents ;

Tig. 353. I Urinary and generative organs
of Cricetus vulgaris. R Kidneys.
u Ureter, t; Urinary bladder. T Testes.
8p Vasa spermatica. d Y as deferens, gl
Vesiculae seminales. gl' gl" Prostatic glands,
w Muscular portion of the urogenital sinus.
ic Corpus cavernosum penis, he Corp. cav.
urethrae. c Cowper’s glands. t Tyson’s
glands, p Prepuce. ^ Gians penis. JINeck
of the bladder, and commencement of the
urogenital sinus, opened in front. * Opening
of the ductus ejaculatorii. Ill Gians penis
seen from in front.

GENEEATIVE OEGANS OF VERTEBEATA.

619

in the former they are often broken up into several large lobes,
while in the latter they are distinguished by their length and by
the diverticula which are found on them. The terminal portion
also of the vas deferens often has a glandular structure.

Besides the seminal ducts, the short terminal portion of which
receives the vesiculae seminales, and is known as the ductus ejacu-
latorius, rudiments of the Mullerian ducts open
into the urogenital sinus in many Mammals.

They either consist of a single or of a paired
diverticulum, which corresponds to a rudimentary
sinus genitalis of the female, or, rather, to its
vaginal portion, so that it is not very exact to
call it a uterus masculinus. Part of it some-
times forms a portion of the male genital sinus, for
the seminal ducts open into it. These organs are
largest in the Rodentia (Fig. 354, ^), although,
indeed, they are not altogether wanting in other
forms ; in Man they are represented by the
prostatic vesicle.

Lastly, the urogenital canal is provided with
yet another set of glandular organs, the prostatic
glands. These may be of a considerable size,
and form paired lobate structures (Rodentia,

Elephas, Insectivora) (Fig. 353, gV gV), or they
are formed of a number of smaller tubes, which
are connected by layers of smooth muscular
fibres to a mass which is attached to the wall
of the urogenital canal. By the further develop-
ment of the musculature, which is found on
these glands in other forms also, the prostate is
converted into a circular body.

Fig. 354. Urogenital
. ^ canal and urinary

V 45o. bladder of Lepus

Cuniculus. J.From

In the lower divisions the efferent ducts of behind. B The

the urinarv and generative apparatus unite with posterior wall of the

the terminal portion ot the enteric canal to open q

into the cavity which has already (p. 562) been view. v Bladder,
called the cloaca ; but it is doubtful whether ^ Ureter, d Seminal
this should be regarded as the primitiye condition, f 4 UrogeS
for we might take the arrangement which obtains canal,

in the Cyclostomata, Ganoidei, and Teleostei, to
be such, where the urogenital organs and the tractus intestinalis
open separately. In them, the anus is in front of the urogenital
orifices, although, and especially in the Ganoidei, there is a depres-
sion into which both these orifices open ; this depression is an early
indication of a cloaca. The cloaca is well developed in the Selachii,
and the orifices of the urogenital apparatus, which lie, in other forms,
behind the anus, are there placed on the dorsal wall of the cloaca.

620

COMPAEATIVE A^s^ATOMY.

This relation is henceforward the common one ; in the Amphibia,
Eeptilia, and Aves, there is a cloaca of pretty much the same kind ;
in Birds it is provided with a diverticulum, the bursa Fabricii
(Fig. 333, h)j which is attached to its hinder wall. The cloaca must
be regarded as being inherited by all the Mammalia, although it is
in the Monotremata only that it persists without any great modifi-
cation ; in the rest it undergoes considerable changes. The most
important of these is the share which it takes in the differentiation
of a copulatory organ, and which was faintly indicated in the Am-
phibia j these changes end by giving rise to a urogenital orifice
distinct from the anus. The allantois is one of the most im-
portant of the organs which are differentiated from the cloaca ; it is
developed from the anterior wall of the cloaca, that is, from the
part of the primitive cavity of the hind- gut that represents it. In
Lepidosiren and in the Amphibia this organ forms a body which
springs from the anterior wall of the cloaca by a short stalk ; in the
latter it is continued into two anteriorly placed diverticula ; it lies
freely in the coelom. It is known as the urinary bladder, and
seems indeed to function as such, although the ureters open some
way from it. Blood-vessels are distributed on its thin wall ; the
arteries come from the pelvic vessels, and the veins pass to the portal
vein.

In the Amniota this organ is very greatly developed during the
embryonic stages, and becomes a large sac which grows out far
beyond the embryo, and is provided with a large number of vessels ;
it envelops the embryo, already covered by the amnion. In
Eeptiles and Birds it gradually atrophies as the abdominal wall is
closed in, and disappears altogether, In the Saurii and Chelonii
only the portion of the allantois within the abdominal cavity is
retained ; in them it is widened out into a sac, which is provided
with diverticula on either side (Fig. 349, v).

In the Mammalia this organ has different relations to the develojD-
ing organism. As in the Keptilia and Aves it grows out into a
vesicle, which communicates with the cavity of the hind-gut by a
stalk which runs inside the umbilical chord. That portion of the
chord, which passes into the coelom (urachus) is partly converted
into a ligament (Big. vesico-umbilicale medium), partly into the urinary
bladder, and partly into a sinus urogenitalis, where the orifices of
the generative ducts pass into it. In the Monotremata and Marsupialia
the peripheral portion appears to have the same I’elations as in the
Sauropsida, while in other Mammals it aids in the formation of the
chorion,^^ which is connected by villous elevations with the mucous
membrane of the uterus. When these vascular villi of the chorion
are further developed, the foetal blood passing along the vessels of the
allantois, acquires a distribution in the peripheral regions of the sac.
This effects exchanges with the blood which is distributed in the
mucous membrane of the uterus. As it becomes more intimately
connected with the uterine mucous membrane a placenta is
developed; this varies greatly in character according to the way

GENERATIVE ORGANS OF VERTEBRATA.

621

in wRich^ and tRe extent to wRicR^ tRe cRorion is connected witli tRe
mucous membrane of tRe uterus^ and according to tRe modifications
undergone by tRe latter organ.

§ 457.

TRe copulatory organs form anotRer series of parts formed
by tRe differentiation of tRe wall of tRe cloaca. In tRe SelacRii,
indeed, organs wRicR did not belong to tRe generative apparatus — •
parts of tRe Rinder appendages — are used as organs of copulation
and modified accordingly, but new organs begin to be differentiated,
wRicR in tRe AmpRibia are faintly indicated by the presence of a
papilla which projects into the cloaca. These belong to one of two
typical forms ; in one the organs are connected with the posterior,
and in the other with the anterior wall of the cloaca.

One of them is dominant in the Saurii and OpRidii. The copu-
latory organs first appear as external appendages,
placed just behind the cloaca ; later on these are
invaginated in a tubular fashion (Fig. 355, |9),
and are only protruded during copulation. When
protruded, each of these organs is continued into
two more or less blunt ends, which vary in form.

At the sides there is a groove, which is continued
on from the cloaca, and which has a spiral course
posteriorly, and is then directed towards the
middle line; this serves to convey the sperm.

The largest of the muscles supplied to it are the
retractors, which are inserted into the blind end
of the tubes. Glands open near the root of the
tubes {gl).

The second form contains a number of dif-
ferent structures, all of which, however, pro-
ceed from the anterior wall of the cloaca, and
must be regarded as modifications of one and
the same arrangement. One form of these
organs is found in most of the Ratitse, and
in the PenelopidiB and Natatores (Anser). It consists of a tube
which is supported by two fibrous bodies, and which, when pro-
truded, forms a groove which leads from the cloaca. The terminal
portion of the organ is retracted by an elastic ligament.

Another form is seen in the Chelonii and Crocodilini, and in
Struthio ; this is distinguished from the previous one by the fact
that it is not protrusible. This organ is likewise supported by two
fibrous bodies, which are intimately connected together, and covered
by mucous membrane (Fig. 349, _p). On the dorsal surface there is
a groove between the two bodies (5), which appear to be invested by
cavernous tissue, at the root in the Crocodilini and Chelonii, and
along the whole length in Struthio. As this tissue is more abundant
at the anterior end of the fibrous bodies (in Struthio it is formed
from the continuation forward of a third elastic body, which is

Fig. 355. Cloaca of
Py t h 0 n, opened from
in front. R Hind-
gut. u Orifices of the
ureters, gl Glands,
which open at *.
p Penial tubes, one
of which is laid open
longitudinally.

622

COMPAEATIYE ANATOMY.

placed below tlie two fibrous ones) it forms an erectile welt^ wbicb
represents a penis. Special muscles, wbicb are inserted into tbe
fibrous bodies, act as retractors of tbe penis; in Strutbio tins
organ is provided witli special elevator muscles, and is bidden in a
diverticulum of tbe cloaca.

Tbe copulatory organs of tbe Mammalia also belong to tbe second
type ; those of tbe Monotremata differ markedly from tbe organs in
other Mammals. Their copulatory organs consist of a short penis,
wbicb is formed of two erectile bodies, and wbicb lies in a pouch
wbicb opens into the cloaca. By means of a muscle this can be
approximated to tbe urogenital canals, and so takes up the sperm
through an orifice wbicb is placed at its root, near tbe opening of the
urogenital sinus. Owing to tbe special mode by which a portion of
tbe wall of tbe cloaca is differentiated, this organ comes to be
exclusively related to tbe generative apparatus, while tbe urine
passes out through the cloaca.

When tbe cloacal aperture is differentiated into two orifices, the
copulatory organ becomes more closely related to tbe urogenital
sinus. During tbe embryonic stage a fold begins to be raised up
around tbe cloacal orifice, and a process is developed on tbe anterior
wall of tbe cloaca, wbicb carries on its posterior surface a groove
wbicb leads to tbe opening of tbe urogenital canal. As tbe embryo
continues to grow, the cloaca becomes shallower, and tbe wall of
partition between the orifice of tbe bind-gut and tbe urogenital
canal, wbicb is formed from tbe lower end of tbe urachus, becomes
more distinct. At last tbe orifices wbicb were formerly placed on
tbe floor of tbe cloaca come to tbe surface. Tbe anterior fissure at
tbe base of tbe genital protuberance forms tbe opening of tbe uro-
genital sinus, while tbe binder orifice forms the anus. In many
Mammalia tbe two orifices are always close to one another, and may
even be enclosed by the same fold of integument ; in tbe female sex
the two orifices are ordinarily close together. This is most markedly
the case in tbe Marsupialia (where there is even a common sphincter
for the anus and urogenital orifice) and Kodentia ; in which forms,
indeed, it obtains in the males also.

§ 458.

Tbe urogenital sinus is developed to a different extent in tbe
two sexes, and this is due to tbe difference in their functions. In
tbe male tbe urogenital sinus and genital protuberance grow out
into a narrower, but ordinarily long canal (the so-called urethra),
with tbe walls of wbicb erectile organs are connected. They form
tbe penis. In tbe female there are parts wbicb are similar to,
though less largely developed than, this organ and its erectile
bodies ; they form tbe. clitoris, an organ wbicb corresponds to the
penis.

Tbe erectile organs of tbe Marsupialia are formed of two bodies

GENERATIVE ORGANS OF YERTEBRATA.

623

wliicli are derived from the genital protuberance and surround the
urogenital canal ; in some they are divided at their free end
(Fig. 356, a h) and form the glans penis. The urogenital canal is
continued on to each half in the form of a
groove [s)j and these grooves may unite together
to form a canal. In others (Halmaturus) these
erectile bodies are connected with two others,
with which they unite to form a cylindrical penis,
and bound the urogenital canal. The first-
mentioned erectile bodies generally fuse very
early in other Mammalia to form a corpus caver-
nosum urethrae which surrounds the urogenital
canal (urethra), and of which the most anterior
end, which varies greatly in form, forms the glans
penis. The two other erectile bodies (corpora
cavernosa penis), which in the Marsupialia are
not firmly connected with the pelvis, are con-
nected with the ischium; they pass above the
corpus cavernosum urethrae, but do not extend
into the wall of the urogenital canal. In most
Mammals the penis thus formed extends for-
wards from the symphysis pubis along the median line of the
abdomen, and ends at a varying distance from the umbilicus ;
in others (Chiroptera, Primates), it is free and hangs down
from the symphysis pubis. In either case, the integument covers
it and forms a fold in front of, and around the glans — the
prepuce.

In the female, the genital protuberance is never developed to
the same extent as in the male; it forms the clitoris, which carries
on its lower surface the opening of the urogenital sinus, which is
bounded by lateral folds. The clitoris is generally more largely
developed in the embryo than in the adult, as it projects from the
pubic fissure and is afterwards withdrawn into it. In some Apes,
however (Ateles), the clitoris continues to be developed and become
an organ of some size. Two erectile bodies (corpora cavernosa
urethrae) lie in the walls of the urogenital sinus and surround it
as far as the clitoris, at the base of which there is another pair of
erectile bodies. The end of the clitoris is generally provided with
a gland, and is also covered by a prepuce. Sometimes this organ
is provided with special muscles, which are mostly differentiated,
as are also those of the erectile bodies, from a common occludor
of the cloaca, such as is seen in the Marsupialia. In addition
to these, many Mammals have muscles which raise, or retract the
penis.

Glandular organs open into the urogenital sinus of both
sexes. There are others besides the prostatic glands already men-
tioned (p. 619) ; there may be one or more, or as many as four
pairs (Marsupialia) ; they lie at the root of the penis (Fig. 353, c).

penis of Didelphys
philander. a h
Halves of the glans.
s Groove on its inner
surface. x Region
of the anus which is
placed behind the
orifice of the prepuce
(after Otto).

624

CO:\rPARATIYE ANATOMY.

Cowper^s glands are connected with the portion which is enclosed
by the erectile bodies. These are not always present (Cetacea,
Carnivora). In the female they open into the vestibulum vaginas
(glands of Duverney or Bartholin). The glands of the prepuce
(Tyson^s glands) are developed into large organs in many Mam-
mals, and especially in the Kodentia, among which they are best
developed in Castor (Fig. 353, t).

INDEX.

AcalephaD — Tentacles, 101 ; Ectoderm,
103; Urticating capsules, 163; Skele-
ton, 106 ; Nervous system, 108 ;
Gastric filaments, 118 ; Sexual or-
gans, 119.

Acanthias — Fin, 477 ; Kidneys, 603.

Acanthocepliali — Integument, 136 ; Mias-
cular system, 143 ; Muscular fibres,
144 ; Nervous system, 147 ; Enteron,
159 ; Lemnisci, 174 ; Generative
organs, 176.

Acantbometridae — Skeleton, 82.

Acanthopteri — Urostyle, 431.

Acarina — Metameres, 237 ; Cerebral
ganglia, 256 ; Caeca, 269 ; Hind-gut,
270; Malpighian vessels, 276 ; Germ-
glands, 298.

Acera — Ganglia, 348.

Acervulinae — Supporting organs, 81.

Acheta — Testis, 304.

Achetida — Auditory organ, 262.

Acineta — Figure, 88.

Acipenser — Dermal bones, 425; Eibs, 439;
Cartilaginous cranium, 450 ; Dermal
denticles, 450 ; Thoracic fin, 479 ;
Spiracular cleft, 543 ; Pseudobran-
chia, 543 ; Air-bladder, 547.

Acopa— Form of body, 390; Gemmation
in, 391 ; Ganglia, 397 ; Branchial sac,
399; Branchial slits, 402; Sexual
organs, 407.

Acrania — Auditory organs, 533; Eespi-
ratory cavity, 541.

Acridida — Auditory organ, 262.

Acrodont Lizards — Teeth, 557.

Acrocladia — Spines, 206.

Actseon — Excretory organ, 377.

Actinosphaerium — Figure, 84.

Adder — Epigastric veins, 595.

.^Eginidm — Tentacles, 102, 107 ; Mar-
ginal vesicle, 110 ; Gastrovascular
system, 115 ; Generative organs, 122.

iEginopsis — Tentacles, 102.

.^olidia — Ganglia, 349 ; Liver, 362 ; Ves-
sels, 372 ; Hermaphrodite glands,
382.

.^Equoridae — Generative organs, 122.

-^schna — Tracheal gills, 290.

Alciopidae — Visual organs, 155 ; Auditory
organs, 156; Excretory organs, 176.

Alcyonaria — Generative organs, 122.

Alcyonella — Nervous system, 146.

Alcyonidae — Digestive persons, 117.

Alcyonium — Skeleton, 106 ; Figure of,
116.

Alepas — Mantle, 236.

Alligator — Pelvis, 485 ; Spinal nerves,
514.

Alveolina — Shells, 81.

Amia — Sphenoids, 452.

Ammocoetes — Vertebral column, 426;
Notochord, 426.

Ammonites — Shell, 333.

Ammothoe — Digestive organs, 269.

Amniota — Muscles of paired appendages,
498 ; Glossopharyngeal, 518 ; Vagus,
521 ; Nasal canal, 526.

Amoeba — Figure of, 78; Supporting or-
gans, 81 ; Contractile vesicle, 86.

Amoebidae — Protoplasm of, 76.

Amphibia — Anterior appendages, 414 ;
Limbs, 415 ; Corium, 417 ; Epidermis,
418 ; Tegumentary glands, 420 ; Der-
mal bones, 425 ; Vertebral column,
429 ; Vertebrae, 432 ; Atlas, 437 ;
Sternum, 442 ; Dermal denticles,
450; Skull, 450; Jugal, 457; Bran-
chial skeleton, 469; Branchial arches,
470 ; Hyoid, 471 ; Shoulder-girdle,
475 ; Fore-limb, 480 ; Eadius, 480 ;
Ulna, 480 ; Carpalia, 480 ; Pelvic-
girdle, 484; Hind-limb, 488; Toes,
490 ; Dermal muscles, 493 ; Trunk
muscles, 494; Intercostal muscles,
496; Muscles of the branchial skele-
ton, 497 ; Masticatory muscles, 497 ;

2 S

626

INDEX.

Brain, 506 ; Spinal nerves, 514 ;
Brachial plexus, 514 ; Olfactory-
nerve, 515 ; Glossopharyngeal, 518 ;
Vagus, 521 ; Goblet- shaped organs,
524 ; Lateral line, 524 ; Gustatory
organs, 525 ; Olfactory organs, 525 ;
Nasal canal, 526; Optic bulb, 529;
Sclerotic, 529 ; Optic muscles, 531 ;
Eyelids, 532 ; Lachrymal ducts, 532 ;
Labyrinth, 535 ; Eustachian tube,
537 ; External ear, 538 ; External
gills, 545 ; Pharynx, 546 ; Nasal
glands, 548 ; Teeth, 550 ; Tongue,
552 ; Thyroid gland, 554 ; Stomach,
557 ; Mid-gut, 561 ; Hind-gut, 562 ;
Cloaca, 562 ; Liver, 564 ; Mesentery,
565 ; Lungs, 569, 572 ; Blood-cor-
puscles, 576 ; Heart, 580 ; Branchial
arteries, 580 ; Pulmonary arteries,
581 ; Kenal arteries, 589 ; Venous
sinuses, 591; Inferior venae cavae,
592; Lymph sinuses, 598; Lympha-
tic hearts, 599 ; Glands, 600 ; Archi-
nephric duct, 602 ; Kidney, 603 ;
Pronephridion, 603 ; Mullerian duct,
604, 612 ; Ureter, 604 ; Ova, 609 ;
Sperm, 609 ; Generative organs, 612 ;
Oviduct, 612 ; Allantois, 621 ; Urinary
bladder, 621 ; Copulatory organs,
621.

Amphiglena — Auditory organs, 156.

Amphinoma — Appendages, 136 ; Nerve
trunks, 149.

Amphioxus — Nerve fibres, 32 ; Branchial
skeleton, 142 ; Trunk, 413 ; Epi-
dermis, 418 ; Notochord, 427 ; Ver-
tebral column, 428 ; Cephalic skeleton,
442 ; Cranium, 447 ; Branchial ske-
leton, 468 ; Muscular system, 491 ;
Medullary tube, 502 ; Peripheral
nervous system, 513 ; Eye, 527 ;
Figure of, 539; Eespiratory cavity,
540; Branchial cavity, 540; Branchiae,
541 ; Hypobranchial groove, 553 ;
Enteron, 555; Fore-gut, 556; Liver,
556; Hearts, 575.

Amphipnous — Branchial lamella, 545.

Amphipoda — Branchiae, 241; Ventral
chord, 255 ; Heart, 280.

Amphistoma — Visual organ, 154 ; Shell
gland, 183.

Amphiuma — Vertebrae, 433.

Anamnia — Epidermis, 418 ; Supra-renal
gland, 601.

Anatidae — Stomach, 558 ; Syrinx, 572.

Animals — Cells in, 15 ; Classification of,
66 ; Phyla of, 69.

and Plants — Boundary between,

68, 75.

Anisobranchiata — Branchiae, 337.

Annehdes — Excretory organ, 46; Vascular
system, 50; Parapodia, 135; Cilia,
137 ; Eod-like bodies, 140 ; Tubular
glands 141 ; Muscular system, 143

Nervous system, 149 ; Visceral nerves ,
151 : Tactile setae, 151 ; Visual organs,
155 ; Auditory organs, 156 ; Hind-
gut, 164; Coelom, 166; Blood-vessels,
171.

Annulata — Musculature, 42; Nervous
system, 42 ; Excretory organs, 42 ;
Homology in, 64; Metamerism in,
130 ; Integument, 136 ; Setae, 140 ;
(Esophageal ring, 148 ; Eyes, 154 ;
Alimentary canal, 162 ; Coelom, 165 ;
Vascular system, 167 ; Excretory
organs, 176 ; Spermatophores of, 191.

Anodonta — Branchiae, 336 ; Muscles, 342 ;
Nervous system, 346; Heart, 370;
Organs of Bojanus, 370 ; Renal ducts,
376 ; Genital canal, 380.

Anomia — Foot, 321 ; Branchiae, 336
Visceral ganglia, 346.

Anoplotherium — Manus, 483.

Anseres — Caeca, 562.

Ant — Salivary glands, 23.

Antedon — Pinnulae of, 195.

Antelope — Stomach, 558.

Anthozoa — Form of, 99 ; Tentacles, 102 ;
Skeleton, 106 ; Muscular system,
108 ; Gastric system, 110 ; Generative
organs, 122.

Antipatharia — Generative organs, 123.

Antipathidae — Skeleton, 106.

Anura — Ribs, 439 ; Maxilla, 457 ; Bran-
chial arches, 471 ; Shoulder-girdle,
476; Pelvic -girdle, 484; Hind-limb,
488; Sixth toe, 488; Dermal muscles,
493 ; Mesencephalon, 505 ; Facial
nerve, 517 ; Ductus endolymphaticus,
534 ; Columella, 538 ; External gills,
545 ; Horny teeth, 549 ; Kidneys,
604 ; Testes. 612 ; Fatty bodies, 613.

Apes — Dermal muscles, 493 ; Turbinate
bones, 547 ; Buccal pouches, 549 ;
Sublingua, 553 ; Clitoris, 623.

Aphides — Wax-secreting organs, 250;
Salivary glands, 274 ; Pseudova, 302.

Aphroditaceae — Elytra, 135 ; Maxillary
apparatus, 162 ; Mid-gut, 162.

Aphrodite — Origin of nerves, 151 ; Ali-
mentary canal, 162 ; Liver, 165.

Aphroditidae — Setae, 140.

Aplysia — Ganglia, 348 ; Vessels, 372
Copulatory organ, 384.

Appendicularia — Tail, 394 ; Axial organ,
394 ; Branchial sac, 399 ; Sexual
organs, 407.

Aptera — Gnathites, 245 ; Ventral chord,
258; Malpighian vessels, 276;
Tracheae, 288.

Apteryx — Diaphragm, 499; Accessorius
Willisii, 522.

Apus — Shell, 235 ; Ventral chord, 254 ;
Caeca, 268 ; Salivary glands, 274 ;
Generative organs, 290.

Arachnida — Form of Body, 237 ; Appen-
dages, 244; Spines, 249; Ventral

INDEX.

627

ganglia, 256; Visceral nervous sys-
tem, 259 ; Eye, 265 ; Enteric canal,
269; Salivary glands, 273; Liver,
275; Malpighian vessels, 276; Cir-
culatory system, 281 ; Tracheee, 290 ;
Germ glands, 292; Generative organs,
296 ; Seminal elements, 304.

Aranea — Gnathites, 244 ; Casca, 269 ;
Hind-gut, 270; Salivary glands, 273;
Hepatic tubes, 278 ; Circulatory sys-
tem, 281 ; Tracheae, 290 ; Copulatory
organs, 299.

Area — Byssus gland, 329 ; Visceral gang-
lia, 346 ; Visual organs, 353.

Arcella — Supporting organs, 81 ; Tests,
83 ; Contractile vesicles, 86.

Archegosauria — Dermal bones, 425.

Ardea — Enteric canal, 565.

Arenicola — Appendages, 136 ; Auditory
organs, 156; Fore-gut, 163; Hind-
gut, 163 ; Vascular system, 169 ;
Central organs of blood-vascular
system, 169.

Argiope — Figure of larva of, 307.

Argonauta — Hectocotylised arm, 327 ;
Shell, 335 ; Liver, 366.

Argulus — Caeca, 268.

Argyroneta — Tracheae, 290.

Aricia — Dermal glands, 141.

Artemia — Eye, 265 ; Heart, 280.

Arthropoda — Connective tissue, 24 ; Tests,
38; Muscles, 40; Nervous system,
42; Vascular system, 50; Homo-
dynamy in, 64 ; General review,
228 ; Classification, 229 ; Biblio-
graphy, 232 ; Appendages, 237 ;
Integument, 248 ; Muscular system,
252 ; Cephalic ganglion, 252 ; Ventral
ganglionic chain, 252 ; CEsophageal
ring, 252 ; Peripheral nervous system,
252 ; Visceral nervous system, 259 ;
Visual organs, 263 ; Alimentary
canal, 267 ; Salivary glands, 273 ;
Coelom of, 278 ; Fat bodies, 278 ;
Vascular system, 279; Dorsal trunk,
279 ; Venous ostia, 279 ; Pericardial
sinus, 279 ; Blood-fluid, 280 ; Gener-
ative organs, 291 ; Germ glands, 292.

Arthrostraca — Auditory vesicles, 261 ;
Salivary glands, 274.

Artiodactyla — Dorso-lumbar vertebrae,
437 ; Fore-limb, 483 ; Pelvis, 486.

Ascalabotae — Eyelid, 532.

Ascaris — Muscular fibres, 144; Female
organs, 184.

Ascidiae — Form of body, 390; Muscular
system, 395; Median dorsal nerve-
chord, 396 ; Peripheral nerves, 397 ;
Branchial sac, 400 ; Heart, 401 ;
Languets, 401 ; Branchial slits of,
402; Ventral groove, 403; Enteron,
404 ; Sexual organs, 407.

Ascones — Gastrula of, 93.

Asellus — Benal concretions, 278.

Asiphonia — Respiratory cavity, 319.
Aspidisca — Carapace, 83.

Aspidogaster — Pharynx, 158.

Astacus — Respiration in, 269.
Asteracanthion — Arms, 195, 199 ; Am-
bulacral feet, 195.

Asterida — Nervous system, 208 ; Visual
organs, 210 ; Alimentary canal, 213 ;
Coelom, 217 ; Blood - vessels, 218 ;
Generative organs, 226.

Asteriscus — Arms, 195, 199 ; Alimentary
canal, 213.

Asteroidae — Rays of, 195; Ambulacral
plates, 202 ; Paxillae, 203 ; Dermal
skeleton, 205; Internal skeleton, 205 ;
Spines, 206 ; Pedicellariae, 206 ;
Muscular system, 207 ; Nervous sys-
tem, 208 ; Interradial emea, 215 ;
Stone- canal, 221 ; Generative organs,
224.

Astraeidae — Skeleton, 106.

Astropecten — Alimentary canal, 213 ;
Interradial caeca, 215; Stone-canal,
221.

Ateles — Clitoris of, 623.

Atlanta — Shell, 323, 332 ; Foot, 324 ;
Excretory organ, 378 ; Receptaculum
seminis, 385.

Aulacostomum — Mid-gut, 162.

Aurelia — Disc of, 24; Figure of, 98;
Discophora form, 99 ; Muscular
system, 108; Gastrovascular system,
115.

Aves — Corium, 417 ; Scales, 418 ; Claws,
419 ; Feathers, 419 ; Vertebral
column, 429 ; Centra of vertebrae,
433 ; Sacrum, 434 ; Caudal vertebrae,
434; Cervical vertebrae, 434; Axis,
437 ; Odontoid process, 437 ; Ribs,
440 ;' Covering bones of skull, 459 ;
Lachrymal, 459 ; Palato-quadrate,
459; Pterygoids, 461; Palatines, 461;
Premaxillae, 462 ; Dentaries, 462 ;
Shoulder. girdle, 476 ; Clavicles, 477 ;
Phalanges, 471 ; Pelvis, 486 ; Tarsus,
488 ; Brain, 505 ; Spinal chord, 512 ;
Olfactory nerve, 515 ; Gustatory
organs, 525 ; Olfactory organs, 525 ;
Sclerotic, 530; Eyelids, 532; Lagena,
536 ; Tympanic cavity, 537 ; Colu-
mella, 538; External ear, 538 ; Cloaca,
562, 620 ; Pancreatic ducts, 565 ;
Larynx, 570 ; Lower larynx, 571 ;
Epiglottis, 571 ; Ova, 609 ; Sperm,
609 ; Bursa Fabricii, 620.

Avicula — Byssus gland, 329 ; Branchiae,
336.

Balanida — Mantle, 236 ; Branchiae, 243 ;
Dermal skeleton, 249; Cement glands,
286 ; Hermaphroditism in, 293.
Balanoglossus — Enteric branchiae, 163 ;
Vascular system, 170 ; Ventral
groove, 402.

2 s 2

628

IlS-DEX.

Bdellostoma — Respiratory organs, 542 ;

Branchial arteries, 579; Kidney, 603.
Bees — Stomach, 262 ; Parthenogenesis
in, 302,

Belemnites — Shell, 334 ; Phragmocone,
334; Pro-ostracum, 334.

Beroe — Generative organs, 123.

Beroidse — Gastrovascular system, 117.
Birds — Fore-limbs, 417 ; Tegumentary
glands, 420 ; Sternum, 442 ; Squa-
mosal, 458 ; Foot, 489 ; Toes, 490 ;
Dermal muscles, 493 ; Intercostal
muscles, 496; Masticatory muscles,
497 ; Dura mater, 512 ; Falx cerebri,
512 ; Brachial plexus, 514 ; Sacral
plexus, 514 ; Crural plexus, 515 ;
Optic bulb, 529 ; Pecten, 531 ; Lens,
531 ; Lachrymal glands, 532 ; Lachry-
mal ducts, 533 ; Ductus endolympha-
ticus, 534 ; Labyrinth, 535 ; Palate,
546; Turbinate bones, 547 ; Nasal
cavities, 548; Nasal glands, 548;
Buccal cavity, 549 ; Teeth, 551 ;
Tongue, 552 ; Buccal glands, 553 ;
Thyroid gland, 554; Stomach, 557;
Crop, 557 ; Proventriculus, 558 ;
Mid-gut, 561 ; Hind-gut, 562 ; Caeca,
562 ; Liver, 563 ; Lungs, 573 ; Heart,
583 ; Arterial arches, 583 ; Carotids,
587; Visceral arteries, 589; Verte-
bral veins, 591 ; Renal veins, 594 ;
Inferior vena cava, 595 ; Arterial
retia, 597 ; Lymph sinuses, 599 ;
Lymphatic glands, 600 ; Thymus,
600 ; Renal organs, 604 ; Ova, 609 ;
Ovaries, 613 ; 0\dducts, 614 ; Testes,
614.

Boa — Heart, 582.

Bohadschia — Cuvierian organs, 216.
Bombinator — V ertebrae, 432.

Bombyx — Nervous system, 259.

Bonellia — Cerebral mass, 148 ; Alimentary
canal, 161 ; Vascular system, 170 ;
Excretory organs, 174.

Borlasia — Blood-corpuscles, 172.
Bothryocephalus — Male organs, 181 ;

Female organs, 182.

Botryllus — Colonies, 391.

Bovrerbankia — Alimentary canal, 160.
Brachiata — Arms, 196.

Brachionus — Excretory organ, 174.
Brachiopoda — General review, 306 ; Clas-
sification, 306 ; Bibliography, 307 ;
Form of body, 307 ; Mantle-folds,
307 ; Shells, 307 ; Tentacles, 308 ;
Integument, 308 ; Arms, 309 ; Gills,
309 ; Muscular system, 309 ; Nervous
system, 310; Sensory organs, 310;
Alimentary canal, 310 ; Ileoparietal
band, 311 ; Excretory organs, 312 ;
Generative organs, 313.
Brachycephalus — Dermal bones, 425.
Brachyura — Branchi0e,243 ; V entral chord,
254 ; Heart, 281 ; Male organs, 295.

Bradypus — Cervical vertebrae, 436; Jugal,
466 ; Pelvis, 486 ; Cuneiform, 490.

Branchiata — Appendages, 238 ; Nervous
system, 255 ; Heart, 280 ; Germ
gland, 292.

Branchiobdella — Vascular system, 168 ;
Excretory organs, 176; Generative
organs, 189.

Branchiomma — Visual organs, 156.

Branchiopoda — Shell, 235; Gnathites,
239 ; Branchiae, 242 ; Dermal skele-
ton, 249; Caeca, 268; Liver, 274;
Germ glands, 294.

Branchipus — Larva, 234 ; Eyes, 265 ;
Germ glands, 294.

Brisinga — Rays, 196 ; Anus, 203 ; Coelom,
216 ; Generative organs, 225.

Bryozoa — Form of body, 129; Gemmation,
132 ; Tentacles, 134, 136 ; Cilia, 138 ;
Cells, 139 ; Muscular system, 143 ;
Nervous system, 148; Tactile setae,
152 ; Alimentary canal, 161 ; Liver,
165 ; Coelom, 166 ; Generative organs,
179.

Buccinum — Siphon, 322 ; Crop, 361 ;
Penis, 386.

Bufo — Branchial arches, 471 ; Stomach,
557 ; Testes, 612.

Bugs — Stomach, 272; Cement glands,
303.

Bulla — Copulatory organs, 384.

Bullaea — Copulatory organs, 384.

Bursaria — Contractile vesicles, 86.

Buteo — Thoracic vertebrae, 441 ; Sternum,
443; Hind-limb, 490; Thyroid gland,
554 ; Thymus, 555 ; Stomach, 557 ;
Heart and vessels, 588.

Caducibranchiata — Muscles of append-
ages, 498 ; Vagus, 522; External gills,
545 ; Cephalic arteries, 586.

Calcispongiae — Skeleton, 105 ; Amphi-
discs, 105 : Gastric system, 112.

Calf — Prosencephalon, 508 ; Spinal chord
511.

Callianiridae — Tentacular organ, 103.

Calveria — Pedicellariae, 206.

Calycozoa — Organisation of, 99.

Calymnidae — Tentacular organs, 103.

Calyptraea — Branchiae, 338.

Camelidae — Testes, 618.

Campanularia — Tentacles, 93, 101 ; Cormi
of, 93 ; Tests, 104.

Campanulariae — Buds of, 95.

Campodea — Feet, 246 ; Tracheae, 288.

Canis — Pancreas Aselli, 600.

Capitella — Nervous system, 149 ; Vas-
cular system, 170.

Caprella — Heart, 280.

Carabidae — Ventral chord, 254; stomach,
262 ; Ovary, 301.

Carcharias — Spiral valve, 560; Ovaries
611.

Carchesinae — Stalk, 83.

IJ^DEX,

629

Carcinus — Nervous system, 254.

Cardium — Nervous system, 345 ; Coelom,
367.

Caridina — Male organs, 295.

Carinaria— Shell, 325, 332; Cutis, 328;
Branchiae, 338 ; Commissures, 348 ;
Excretory organs, 377.

Carinatae — Fore-limbs, 417 ; Ploughshare
bone, 435 ; Sternum, 443 ; Clavicle,
477 ; Furcula, 477 ; Syrinx, 572.
Carmarina — Generative organs, 122.
Carnivora — Nipples, 422 ; Dorso-lumbar
vertebrae, 437 ; Tympanic bone, 466 ;
Clavicle, 477 ; Symphysis pubis, 486 ;
Cerebellum, 510 ; Tentorium cere-
belli, 512 ; Tapetum, 530 ; Optic
muscles, 531 ; Turbinate bones, 547 ;
Caecum, 562; Liver, 564; Venae
cavae, 593 ; Renal organs, 605 ;
Uterus, 615 ; Hymen, 616 ; Ovaries,
617 ; Vas deferens, 618 ; Cowper’s
glands, 624.

Caryophyllaei — Segmentation of, 129.
Cassiopeia — Discophora form of, 99.

Cassis — Proboscis, 361 ; Salivary glands,
364.

Castor — Tyson’s glands, 624.

Casuarius — Lymphatic hearts, 599.

Gat — Prosencephalon, 508.

Catallacta — Cells of, 19.

Cataphracta — Infraorbital bones, 455 ;
Thoracic fin, 479.

Caterpillar— Spines, 250; Enteron, 271;

Tracheae, 287.

Cavia — Uterus, 615.

Cecidomyiae — Parthenogenesis in, 302.
Centrophorus — Dermal denticles, 423.
Cephaea — Discophora form of, 99.
Cephalopoda — Cartilage cells, 26, 27 ;
Head, 324 ; Tentacles, 326 ; Suckers,
327 ; Chromatophores, 328 ; Integu-
mentary glands, 329 ; Shell, 332 ;
Respiratory organs, 340 ; Internal
skeleton, 341 ; Muscles, 343 ; Ner-
vous system, 350; Buccal ganglia,
351 ; Olfactory organs, 353 ; Eye,
354 ; Auditory organs, 356 ; Alimen-
tary canal, 358 ; Jaws, 360 ; Radula,
360; Pharynx, 362 ; Salivary glands,
364 ; Liver, 366 ; Ink-bag, 367 ;
Coelom, 367 ; Heart, 373 ; Excretory
organs, 379; Nidamental glands, 387 ;
Generative arms, 387 ; Spermato-
phores, 387.

Cerambicidas — Ventral chord, 258.
Ceratodus — Operculum, 455 ; Fin, 477 ;
Abdominal pore, 574 ; Conus arte-
riosus, 578.

Ceratophrys — Dermal bones, 425.
Cercarise — Stylets, 141.

Cereanthus — Colonies, 100; Pore, 117;

Generative organs, 123.

Cervus — Sternum, 443.

Cestoda — Form of body, 129 ; Segmenta-

tion, 130; Cystic form, 130; Cysti-
cercus form, 131 ; Coenurus, 131 ;
Echinococcus, 131 ; Cilia, 137 ; Aci-
culi, 140 ; Suckers, 143 ; Pigment
spots, 154 ; Enteron, 159 ; Excretory
organ, 172 ; Generative organs, 180 ;
Genital pore, 184.

Cetacea — Nipples, 422; Vertebral column,
435 ; Ribs, 441 ; Nasals, 465 ; Ptery-
goids, 466 ; Fore-limbs, 472 ; Sym-
physis pubis, 484 ; Sclerotic, 530 ;
Lachrymal glands, 537 ; Auditory
meatus, 539 ; Milk teeth, 549 ; Buc-
cal glands, 553 ; Stomach, 558 ; Mid-
gut, 561 ; Venm cavae, 593 ; Renal
organ, 605 ; Uterus, 615 ; Testes,
618; Vas deferens, 618; Cowper’s
gland, 624.

Chgetoderma — Ventral surface, 130 ; Ner-
vous system, 152 ; Enteric branchiae,
164.

Chaetognathi — Muscular system, 142 ;
Muscular fibres, 145; Lateral line,
143 ; Alimentary canal, 160 ; Gene-
rative organs, 185.

Chaetopoda — Tentacles, 133, 134; Para-
podia, 134; External Branchiae, 135;
Cilia, 138 ; Dermal glands, 141 ;
Longitudinal nerve-trunks, 149 ; Vis-
ceral nerves, 151 ; Tactile setae, 152 ;
Visual organs, 155 ; Muscular
stomach, 162 ; Fore-gut, 163 ; Vas-
cular system, 169 ; Excretory organ,
178 ; Generative organs, 189.

Chama — Respiratory cavity, 320; Renal
ducts, 376 ; Genital canal, 381.

Chamaeleon — Episternum, 444 ; Ilium, 484;
Eyelids, 532 ; Tympanic membrane
538.

Charybdea — Marginal vesicles, 110.

Chelonii — Scales, 418 ; Dermal bones,
425 ; Plastron, 426 ; Processes of
vertebrae, 433 ; Caudal vertebrae, 434 ;
Ribs, 439 ; Columella, 458 ; Covering-
bones of skull, 459; Ethmoid, 459;
Palato-quadrate, 459 ; Dentaries, 462 ;
Hyoid, 472; Shoulder-girdle, 475;
Fore-limb, 481; Toes, 490; Meten-
cephalon, 507 ; Spinal chord, 511 ;
Sclerotic, 530; Lachrymal glands,
532 ; Turbinate bones, 547 ; Horny
teeth, 550 ; Tongue, 552 ; Stomach,
557 ; Hind-gut, 561 ; Liver, 564 ;
Mesentery, 565; Larynx, 570; Lungs,
573 ; Abdominal pore, 574 ; Heart,
581 ; Arterial arches, 582 ; Arteriae
intercostales, 589 ; Renal veins, 594 ;
Lymph sinuses, 599 ; Lymph hearts,
599; Thymus, 600; Testes, 614; Allan-
tois, 670 ; Copulatory organs, 621.

Chelydra — Aortic arches, 583 ; Lymphatic
system, 598 ; Generative organs, 616.

Chelys — Premaxillae, 462.

Chevreulius — Mantle, 394.

630

mDEX.

Chiastoneura — Nervous system, 348 j
Commissures, 348.

Chilodon — Pharynx, 85.

Chilopoda — Tracheae, 288.

Chimaera — Vertebral column, 428; Caudal
region, 431; Vertebrae, 439; Hyo-
mandibular, 450; Fin, 478; Spinal
chord, 510 ; Branchial lamella, 544 ;
Fore-gut, 556 ; Mid-gut, 559 ; Ab-
dominal pore, 574; Conus arte-
riosus, 578 ; Aorta, 585 ; Oviducts,
611 ; Copulatory organ, 611.

Chimpanzee — Dermal muscles, 483.

Chirodotae — Alimentary canal, 214; Lungs,
215 ; Excretory organs, 224.

Chiroptera — Nipples, 422 ; Dorso-lumbar
vertebrae, 437 ; Premaxillae, 460 ;
Central e, 482 ; Symphysis pubis,
484 ; Cerebellum, 510 ; Sublingua,
553 ; Uterus, 615 ; Testes, 618 ;
Penis, 622.

Chiton — Spicules, 330 ; Nervous system,
344.

Chreseis — Tentacles, 326 ; Shell, 332 ;
Excretory organs, 377.

Chrysomitra — Generative organs, 97.

Cicadae — Organs of voice, 250; Hind-
gut, 273; Malpighian vessels, 277.

Ciconia — Carpus, 481.

Cicindelae — Stomach, 272.

Cidaris — Skeleton, 201, 205 ; Spines,
206 ; Stone-canal, 222.

Ciliata — Cilia, 77 ; Mouth, 84 ; Peristoma,
85 ; Pharynx, 85 ; Nucleolus, 89.

Cirratulus — Ganglia, 150.

Cirripedia — Mantle, 326; Cirri, 239;
Branchiae, 243 ; Dermal skeleton, 249 ;
Optic nerve, 253 ; Ventral chord, 253 ;
Eyes, 264; Enteron, 269; Liver,
274; Cement-glands, 285; Herma-
phroditism in, 293 ; Seminal ele-
ments, 304.

Cladocera — Shell, 235 ; Eye, 265 ; Heart,
280 ; Germ-glands, 294.

Cladolabes — Polian vesicle, 223.

Cladonema — Tentacles, 101.

Clarias — Branchial coil, 545.

Clepsine — Ganglia, 150 ; Mid-gut, 162 ;
Vessels, 167; Excretory organ, 177;
Generative organs, 187.

Clio — Kespiratory organs, 339; Nervous
system, 349.

Clupea — Optic nerve, 515 ; Branchial coil,
545 ; Generative system, 610.

Clupeidae — Vertebra9,439; Labyrinth, 533.

Clubiona — Tracheae, 291.

Clypeastrida — Skeleton, 204 ; Mastica-
tory apparatus, 206, 214; Stone-
canal, 222.

Coccidae — Pseudova, 302.

Coleoptera — Thorax, 237 ; Gnathites,
245 ; Elytra, 247 ; Wings, 248 ;
Dermal Skeleton, 249 ; Spinning ves-
sels, 250; Ventral chord, 254; Audi-

tory organ, 262 ; Stomach, 272 ;
Mid-gut, 272 ; Salivary glands, 273 ;
Nervus recurrens, 259; Malpighian
vessels, 277 ; Tracheae, 288 ; Ovary,
302 ; Bursa copulatrix, 303 ; Testis,
304; Copulatory organs, 305.

Coleps — Carapace, 83.

Collembola — Gnathites, 245 ; Ventral
chord, 258; Excretory organ, 276;
Tracheae, 288.

Collidae — Skeleton, 82.

Columbae — Stomach, 558.

Coeciliae — Dermal bones, 424 ; Vertebrae,
433 ; Eustachian tube, 537 ; Auditory
ossicles, 538 ; Kidneys, 604 ; Ureter,
604 ; Testes, 612 ; Cloaca, 613.

Coelenterata — Classification, 90, 91 ; Meso-
derm, 90 ; Coenenchyma, 93 ; Biblio-
graphy, 91 ; Form of, 91 ; Gastrula,
92 ; Hydranth, 93 ; Sensory organs,
109; Alimentary canal, 111; Ova,
124 ; Seminal filaments, 124.

Coelogenys — Uterus, 615.

Comatula — Centro-dorsal, 204 ; Pinnulae,
204 ; Nervous system, 209 ; Alimen-
tary canal, 213 ; Blood-vessels, 218 ;
Generative organs, 226.

Conchifera — Shell-gland, 331.

Conus — Proboscis, 361.

Convoluta — Auditory organ, 156.

Copelata — Form of body, 390; Mantle,
393 ; House, 393 ; Nervous system,
396 ; Auditory organ, 398 ; Branchial
sac, 300 ; Enteron, 403 ; lieart, 404 ;
Sexual organs, 406.

Copepoda — Appendages, 238 ; Gnathites,
239; Optic nerves, 253; Ventral
chord, 254 ; Eye, 226 ; Caeca, 268 ;
Enteron, 269 ; Excretory organs, 276 ;
Heart, 280 ; Germ-glands, 293.

Corallium — Skeleton, 106 ; Generative
organs, 123.

Cordylophora — Tentacles, 93.

Corethra — Stigmata, 289.

Coronula — Nervous system, 254 ; Liver,
274.

Corvina — Air-bladder, 468.

Corycaida — Ventral chord, 254; Eyes,
265 ; Germ-glands, 293.

Corynactis — Urticating capsule, 104.

Coryne — Tentacles, 93 ; Buds, 95.

Crabro — Stomach, 272.

Crane — Trachea, 572.

Craniota — Cranium, 427; Branchial skele-
ton, 427, 445, 468 ; Appendicular
skeleton, 428 ; Vertebral column,
428; Cephalic skeleton, 445; Skull,
445, 447; Nervous system, 501;
Medullary tube, 502 ; Peripheral
nerves, 513 ; Glossopharyngeal, 518 ;
Visceral nerves, 523 ; Respiratory
cavity, 541 ; Branchiae, 555 ; CEso-
phagus, 556; Stomach, 556; Liver,
563; Heart, 576, 577 Lymphatic

INDEX.

631

hearts, 576, 598 ; Blood-vascular
system, 576 ; Excretory organs, 601 ;
Archinephric duct, 602.

Crania — Caeca, 311.

Craspedosoma — Generative organs, 299.

Crayfish — Green-gland, 286 j Male organs,
295.

Crepidula — Branchiae, 338.

Cricetus — Generative organs, 618.

Crinoida — Pinnulae, 196 ; Arms, 199 ;
Tentacles, 200 ; Dermal skeleton, 203;
Muscular system, 207 ; Alimentary
canal, 213 ; Coelom, 217 ; Blood-
vessels, 218; Stone-canal, 222; Ex-
cretory organs, 224.

Crocodilini — Centra of vertebrae, 433 ;
Processes of vertebrae, 433 ; Caudal
vertebrae, 434 ; Axis, 437 ; Bibs,
439 ; Sternum, 442 ; Episternum,
444 ; Covering-bones of skull, 459 ;
Lachrymal, 459 ; Palato-quadrate,
459 ; Pterygoids, 461 ; Palatines,
461; Maxilla, 462; Hyoid, 472;
Shoulder-girdle, 47 6 ; Carpus, 481 ;
Phalanges, 481 ; Ilium, 484 ; Pelvis,
485 ; Tarsus, 488 ; Toes, 490 ; Cal-
caneum, 490; Abdominal ribs, 496;
Diaphragm, 499, 574; Metencephalon,
507 ; Optic bulb, 529 ; External ear,
538; Turbinate bone, 547 ; Nasal
glands, 548; Teeth, 550; Stomach,
557 ; Mid-gut, 561 ; Liver, 564 ;
Mesentery, 565; Larynx, 570; Lungs,
573; Abdominal pore, 574; Coelom,
574; Heart, 582; Arterial arches,
582 ; Carotids, 587 ; Arteriee inter-
costales, 589 ; Kenal veins, 594 ;
Lymph sinuses, 599 ; Thymus, 600 ;
Testes, 614 ; Copulatory organs, 621.

Crow — Syrinx, 572.

Crustacea — Blood-corpuscles, 29 ; Der-
mal branchiae, 46; Form of body,
234 ; Nauplius stage, 235 ; Head,
235 ; Cephalothorax, 235 ; Appen-
dages, 238 ; Branchiae, 240 ; Integu-
ment, 248 ; Spines or setae, 249 ;
Muscular system, 251 ; Optic nerves,
253 ; Visceral nervous system, 254 ;
Tactile rods, 260 ; Olfactory organs,
261 ; Simple eyes, 265 ; Compound
eyes, 265; Faceted eye, 267; Ali-
mentary canal, 267 ; Masticatory
stomach, 268 ; Salivary glands, 273 ;
Liver, 275; Excretory organs, 276,
285 ; Fat bodies, 278 ; Copulatory
organs, 299 ; Seminal elements,
305.

Cryptobranchus — Hind-limb, 488 ; Lungs,
573.

Cryptochiton — Spicules, 330.

Cunina. — Cartilage cells, 26 ; Gelatinous
disc, 107 ; Marginal vesicles, 110,

Curculionidse — Ventral chord, 254.

Cuvieria — Integument, 207.

Cyanea — Marginal filaments, 102.

Cyclas — Byssus gland, 329 ; Auditory
organ, 356.

Cyclifera — Bony plates, 424.

Cyclomyaria — Chorda, 394; Axial organ,
394 ; Auditory organ, 398 ; Enteron,
404; Gill, 406.

Cyclops — Nauplius stage, 234 ; Appen-
dages, 238.

Cyclostoma — Lung, 339.

Cyclostomata — Nerve fibres, 33 ; Verte-
bral column, 428; Vertebrae, 438;
Skull, 447 ; Branchial skeleton, 468 ;
-Muscles, 492 ; Brain, 504 ; Spinal
chord, 510 ; Optic nerves, 515 ; Glos-
sopharyngeal, 518 ; Horny teeth,
542 ; Hypobranchial groove, 553 ;
Fore-gut, 556 ; Mid-gut, 559 ;
Branchial artery, 579 ; Caudal vein,
579 ; Kidney, 603 ; Cloaca, 619.

Cydippidae — Axes, 101 ; Grappling lines,
103 ; Gastric system, 117.

Cymbulia — Hermaphrodite glands, 382.

Cymbulidae — Foot, 324.

Cynips — Stomach, 272.

Cynthia — Muscular system, 395.

Cyprinoids — Caudal region, 431 ;V ertebrae,
439 ; Beards, 525 ; Gustatory organ,
525 ; Auditory organ, 534 ; Sacculus,
534.

Cythera — Nervous system, 345; Audi-
tory vesicle, 357.

Dactylethra — Tongue, 552.

Dactylogyrus — Visual organs, 154.

Daphnia — Figure of, 268 ; Caeca, 268 ;
Respiration in, 269.

Daphnida — Branchiae, 243.

Dasypus — Pterygoids, 466; Pelvis, 486;
Dermal muscles, 493.

Dasyurus — Tympanic bone, 466.

Decapoda — Cephalothorax, 235 ; Appen-
dages, 239 ; Mandibles, 239 ; Maxillae,
239 ; Maxillipeds, 239 ; Locomotor
appendages, 239 ; Feet, 240 ; Bran-
chiae, 242; Ventral chord, 254; Au-
ditory vesicles, 261; Masticatory
stomach, 268; Liver, 274; Heart,
281 ; Male organ, 295 ; Seminal
elements, 305.

Delphinoidea — Teeth 551 ; Pancreas
Aselli, 600.

Dentalium — Auditory vesicle, 357-

Derostomum — Auditory organ, 156.

Derotremata — Branchial arches, 471 ;
Eyelids, 529 ; External gills, 545.

Desmosticha — Ambulacra, 197 ; Integu-
ment, 204.

Dibranchiata — Funnel, 325 ; Arms, 326 ;
Internal skeleton, 342 ; Nervous
system, 350; Eye, 355; Salivary
glands, 364 ; Liver, 366 ; Ink-bag,
369; Auricles, 369; Veins, 374;

G32

INDEX.

Branchial heart, 374 ; Excretory
organs, 379.

Dictocyrta— ^Shell, 83.

Dicyema — Organism of, 68, 69.

Didelphia — Nipples, 422; Manus, 482.

Didelphys — Vagina, 621 ; Erectile bodies,
623.

Didemnum — Colonies, 391.

Difflugia — Supporting organs, 81.

Digitigrada — Hallux, 491.

Dimyaria — Internal skeleton, 342 ; Mouth,
359.

Dinosaurii — Pelvis, 485.

Diphyidae — Nectocalyces, 96 ; Generative
organs, 121.

Diplopoda — Nervous system, 255.

Dipnoi — Vertebral column, 428 ; Caudal
region, 431; Nasal canal, 521 ; Vesti-
bule, 534; Air-bladder, 568; Blood-
corpuscles, 576; Conus arteriosus,
578; Oviduct, 611.

Diptera — Thorax, 237; Seise, 246; Labrum,
246 ; Wings, 248 ; Halteres, 248 ;
Ventral chord, 259 ; Auditory organ,
262; Stomach, 272; Mid-gut, 272;
Salivary glands, 274; Malpighian
vessels, 277 ; Tracheal vessels, 289 ;
Ovary, 302 ; Cement glands, 303.

Discophora — Marginal filaments, 102 ;
Gelatinous disc, 107 ; Muscular
system, 108 ; Marginal vesicles, 110 ;
Gastro-vascular system, 114 ; Gastric
filaments, 116; Generative organs,
122.

Distoma — Enteron, 158 ; Excretory or-
gans, 174; Shell-gland, 183.

Dog — Manus, 483 ; Pelvis, 486.

Doliolum — Larvae, 390; Sensory organs,
397 j Branchial slits, 402; Ventral
groove, 403 ; Sexual organ, 404.

Dolium — Siphon, 322; Proboscis, 361;
Salivary glands, 364 ; Penis, 385.

Dolphin — Fore-limb, 482.

Dorididae — Branchiae, 338.

Doridopsis — Liver, 366.

Doris — Branchiae, 338 ; Arms, 362 ; Sali-
vary gland, 364; Liver, 365; Ves-
sels, 372; Eeceptaculum seminis,
384.

Dromaeus — Auricle, 477; Columella, 538.

Dysdera — Tracheae, 291.

Dyssycus — Gastric system, 112.

Dytiscus — Visual organ, 263; Stomach,
272 ; Hind-gut, 273.

Ecardines — Shell, 308 ; Vascular system,
312 ; Generative organs, 313.

Echidna — Pterygoids, 466 ; Dermal
muscles, 493 ; Canals of Gartner, 617.

Echinida — Masticatory organs, 214 ;
Stone-canal, 222.

Echinocucumis — Integument, 207 ;
“ Lungs,” 216.

Echinoderes — Nervous system, 146 ; Ex-
cretory organ, 174.

Echinoderma — General review, 192 ;
Classification, 193 ; Bibliography,
194 ; General form, 194 ; Larvae, 194 ;
Muscular system, 207 ; Nervous sys-
tem, 208 ; Sensory organs, 210 ;
Alimentary canal, 211 ; Vessels, 217 ;
Water vessels, 219 ; Stone-canal, 220 ;
Excretory oi'gans, 224 ; Generative
organs, 224.

Echinoida — Dermal branchiae, 200 ; Cal-
careous skeleton, 202 ; Dermal
skeleton, 205 ; Internal skeleton,
205 ; Pedicellariae, 206 ; Masticatory
apparatus, 206, 214 ; Spines, 206 ;
Muscular system, 207 ; Nervous sys-
tem, 209 ; Blood-vessels, 218 ; Stone-
canal, 222 ; Generative organs, 225.
Echinorhynchus — Aciculi, 140 ; Genera-
tive organs, 186.

Echinothurida — Dermal skeleton, 205 ;

Masticatory organs, 214.

Echinus — Figure, 204; Pedicellariae, 206;
Nervous system, 209 ; Blood-vessels,
218 ; Stone-canal, 222 ; Generative
organs, 226.

Echiurus — Ganglion cells, 148; Alimen-
tary canal, 161 ; Vascular system,
171 ; Excretory organs, 175.

Edentata — Scales, 418; Carapace, 426;
Sacral vertebrae, 436 ; Episternum,
444 ; Turbinate bones, 465 ; Prc-
maxillae, 466; Centrale, 482 ; Cerebral
hemispheres, 509 ; Cerebellum, 510 ;
Stomach, 558; Arterial retia, 597 ;
Eenal organs, 605 ; Testes, 618.
Eledone — Suckers, 327 ; Copulatory or-
gans, 328 ; Salivary glands, 364.
Elephas — Nipples, 422; Milk teeth, 552;
Eenal organs, 605 ; Prostatic glands,
619.

Elysia — Branchiae, 339.

Enaliosaurii — Hind-limb, 488.
Enchytroeus — Central ganglia, 149.
Enteropneusti — Branchial skeleton, 142 ;

Alimentary canal, 398.

Entomostraca — Nauplius stage, 235 ;
Branchiae, 243; Eye, 264; Masticatory
stomach, 268 ; Liver, 275 ; Fat
bodies, 278 ; Excretory organ, 285 ;
Shell-gland, 286.

Epeira — Eye, 266; Tracheae, 291.
Ephemera — Tracheal gills, 290.
Ephemerida — Salivary glands, 274; Ec-
dysis, 289; Tracheal gills, 290 ; Ovi-
duct, 302.

Epistylis — Stalk, 83.

Equus — Digits, 484 ; Stomach, 556.
Erinaceus — Dermal muscles, 493.

Ervilia — Pharynx, 85.

Esox — Vertebrae, 430; Cartilaginous cra-
nium, 480 ; Eye, 529 ; Opercular gill
543.

INDEX.

633

EucopidaD— Marginal vesicles, 110.
Eiidendrium — Tentacles, 93, 101 ; Cormi,
91 ; Figure of, 94 ; Tests, 104.
Eunice — Appendages, 136; Vascular
system, 169 ; Generative organs, 190.
Euniceidge — Longitudinal nerve-trunks,
149.

Euphausia — Eye, 264.

Euplectella — Skeleton, 106.

Euplotes — Carapace, 83.

Euryalida — Arms, 196; Integument,
203.

Eurylepta — Alimentary canal, 158.

Eury stomata — Auditory organ, 538;
Columella, 538.

Fabricia — Auditory organs, 156.

Falco — Eye, 529.

Fibrospongiae — Skeleton, 106.

Firolidm — Kidney, 377.

Fishes ~ Corium, 417 ; Tegumentary
glands, 420; Vertebral column, 429 ;
Articular processes, 437 ; Kibs, 438 ;
Hind-limb, 487 ; Intercostal muscles,
496 ; Muscles of branchial skeleton,
497 ; Do. of paired appendages, 498 ;
Electric organs, 500 ; Cerebellum,
505 ; Medulla oblongata, 505 ; Ar-
achnoid, 513 ; Spinal nerves, 514 ;
Olfactory nerve, 515 ; Vagus, 517 ;
Visceral nervous system, 523 ; Beards,
524 ; Olfactory organs, 525 ; Eye,
528; Optic bulb, 529; Lens, 531;
Eyelids, 532 ; Labyrinth, 535 ;
Branchial pouches, 542 ; Enteron,
555 ; CEsophagus, 556 ; Liver, 564 ;
Mesentery, 565 ; Air-bladder, 567 ;
Heart, 577 ; Bulbus arteriosus, 578 ;
Branchial artery, 578 ; Carotids,
578 ; Arterial arches, 579 ; Aorta,
579 ; Arteriae intercostales, 589 ;
Venous system, 590; Ductus Cuvieri,
590 ; J ugular veins, 590 ; Cardinal
do., 590 ; Caudal do., 590 ; Portal
system, 590 ; Renal portal system,
590 ; Inferior venm cavae, 593 ;
Lymph sinuses, 598 ; Lymphatic
hearts, 599 ; Lymphatic glands, 600 ;
Ova, 609 ; Sperm, 609 ; Efferent
ducts, 610.

Fissurella — Branchiae, 337 ; Heart, 371 ;
Excretory organ, 377 ; Generative
glands, 385.

Flagellata — Protoplasm, 76.

Flea — Ovary, 301.

Flies — Nervous system, 257.

Flustra — Hind- gut, 161.

Foraminifera — Protoplasm, 76; Shells,
81.

Forficulidae — Elytra, 247.

Formica — Masticatory stomach, 272.

Fowl — Hyoid, 472 ; Brain, 507 ; Auditory
organ, 533 ; Stomach, 557 ; Develop,
ment of arteries, 587.

Frog — Vertebral column, 433 ; Brain,
505 ; Branchial plexus, 514 ; Cephalic
arteries, 586; Arterial 'system, 586;
Renal veins, 594.

Fuugiae — Colonies, 100; Skeleton, 106.

Gadus — Skull, 452; Shoulder-girdle, 475.
Galeodea — Nervous system, 256; Ganglia,
256; Digestive organs, 270; Lungs,
291 ; Generative organs, 297.

Galeus — Ovaries, 610.

Gallinae — Caeca, 562.

Gammarus — Heart, 280.

Ganoi'dei — Vertebral column, 429; Trans-
verse processes, 431 ; Fin rays, 432 ;
Ribs, 439 ; Cartilaginous cranium,
450; Mandibular apparatus, 453;
Operculum, 455 ; Shoulder-girdle,
475 ; Thoracic fin, 478 ; Pelvic-girdle,
484 ; Ventral fin, 487 ; Olfactory
lobes, 504 ; Thalamencephalon, 504 ;
Sinus rhomboidalis, 505 ; Medulla
oblongata, 505 ; Optic nerves, 517 ;
Ciliary processes, 530 ; Spiracle, 537 ;
Spiracular cleft, 513; Teeth, 550;
Enteron, 555 ; Spiral valve, 560 ;
Air-bladder, 567 ; Conus arteriosus,
578; Aorta, 585; Kidneys, 601;
Mullerian ducts, 610 ; Cloaca, 619.
Gasteropoda — Velum, 319 ; Foot, 321, 323 ;
Mantle, 322 ; Siphon, 322 ; Tentacles,
326, 352 ; Integumentary glands, 329;
Shell, 332; Operculum, 335 ; Branchiae,
337 ; Internal skeleton, 341 ; Cerebral
ganglia, 347 ; Buccal ganglia, 351 ;
Olfactory organs, 352 ; Visual or-
gans, 353; Auditory organs, 356;
Alimentary canal, 3’58 ; Jaws, 360 ;
Circulatory centres, 368, Auricles,
369, 373 ; Heart, 371 ; Vessels, 371 ;
Blood-fluid, 375; Excretory organ,
377 ; Generative organs, 381 ; Effe-
rent ducts, 382.

Gasterostomum — Pharynx, 158.
Geckotidae — Teeth, 551.

Geophilus — Nervous system, 225.
Gephyrea — Incomplete metamerism, 310 ;
Respiration in, 136 ; Integument,
136 ; Cilia, 137 ; Dermal glands, 141 ;
Cerebral mass, 148 ; Alimentary
canal, 161 ; Vascular system, 170 ;
Excretory organs, 174; Generative
organs, 189.

Gerardia — Generative organs, 122.
Geryonidae — Nervous system, 109 ; Itlar-
ginal vesicles, 110.

Glomeris — Enteric canal, 271 ; Tracheaa,
288.

Glycera — Visceral nerves, 151 ; Vascular
system, 170.

Glyceridae — Blood-corpuscles, 172.
Gnathostoma — Nerve-fibres, 33 ; Cranium,
447 ; Lower jaws, 448 ; Palato-quad-
rate, 448 ; Dermal muscles, 492 ;

634

INDEX.

Optic nerves, 515 ; Semicircular
canals, 534; Hypobranchial groove,
554.

Goat— Skull, 463.

Gordiacea — Alimentary canal, 160 ; Ex-
cretory organs, 173.

Gordius — Alimentary canal, 160; Ex-
cretory organs, 173 ; Generative
organs, 185.

Grallatores — Optic bulb, 529 ; Pecten,
531.

Gregarinae — Sensory organs, 42; Form
of, 76 ; Cuticular layer, 78 ; “ Head,”
80; Nutrition in, 83 ; Figure of, 87;
Eeproduction in, 87 ; Pseudonavi-
cellae, 87.

Gymnolaemata — Cells, 139.

Gymnopbiona — Ribs, 439.

Gynmosomata — Cepbaloconi of, 326 ; Re-
spiratory organs, 339.

Gymnotus — Electric organs, 500.

Haliotis — Epipodium, 324 ; Branchiae,
337 ; Ganglionic chain, 347 ; Heart,
371 ; Excretory organs, 377 ; Germ-
gland, 385.

Halisarca — Ectoderm, 103; Sexual organs,
119.

Halmaturus — Nipples, 422 ; Veins, 591 ;
Erectile bodies, 623.

Harpei — Siphon, 323 ; Proboscis, 361 ;
Penis, 386.

Hedessa — Caeca, 268.

Helicinae — Shell, 332 ; Lung, 339 ; Sper-
matophores, 383 ; Receptaculum
seminis, 383.

Heliozoa — Protoplasm, 76.

Helix — Tentacles, 326 ; Haemal spaces,
373 ; Hermaphrodite glands, 382 ;
Generative apparatus, 383.

Hemiptera — Setae, 246 ; Wings, 248 ;
Ventral chord, 259; Stomach, 272;
Mid-gut, 272 ; Malpighian vessels,
277 ; Generative organs, 302 ; Testis,
304.

Hermella — Tentacles, 134 ; Appendages,
136 ; Ganglia, 150 ; Vascular system,
170.

Heterakis — Alimentary canal, 159.

Heteropoda — Gelatinous tissue, 25; Shell,
322, 332; Foot, 324; Cutis, 328;
Branchiae, 338 ; Muscular system,
342 ; Commissures, 348 ; Olfactory
organ, 352 ; Eye, 354 ; Auditory
vesicles, 357; Radula, 360; Crop,
361; Excretory organs, 378; Recep-
taculum seminis, 385 ; Penis, 385.

Heterotricha — Cilia, 79.

Hippolyta — Auditory vesicles, 261.

Hirudinea — Respiration in, 136; Integu-
ment, 136 ; Dermal glands, 141 ;
Muscular system, 142; Muscular
fibres, 144; Nervous system, 149;
Ventral ganglia, 150; Visceral nerves

151 ; Cup-shaped organs, 153 ; Vis-
ual organs, 154 ; Fore-gut, 162 ;
Enteric glands, 164 ; Vascular sys-
tem, 167 ; Excretory organs, 177 ;
Generative organs, 186 ; Generative
products, 190, Spermatophores, 191.

Hirudo — Vascular system, 167 ; Excretory
organs, 177 ; Generative organs, 187.

Histiotheutis — Eye, 355.

Hoemopis — Mid-gut, 162 ; Generative or-
gans, 187 ; Generative products, 190.

Holothuria — Alimentary canal, 214; Polian
vesicles, 223.

Holothuriae — Integument, 207.

Holothuroida — Ambulacra, 197; Tentacles,
200 ; Calcareous skeleton, 202 ; In-
tegument, 206 ; Internal skeleton,
207 ; Muscular system, 208 ; Nervous
system, 210 ; Sensory organs, 210 ;
Alimentary canal, 211, 214; “ Lungs,”
215; Coelom, 217; Blood-vessels, 218;
Stone-canal, 221 ; Polian vesicles,
222 ; Water- vessels, 223 ; Excretory
organs, 224 ; Generative organs, 226.

Holotricha — Cilia, 79.

Homarus — Male organs, 295.

Hysena — Kidney, 605.

Hyaleidae — Foot, 324; Respiratory or-
gans, 339 ; Hermaphrodite glands,
382.

Hyalonema — Skeleton, 106.

Hydra — Neuromuscular cells, 30 ; Cormi,
94.

Hydree — Generative buds, 120.

Hydractiuia — Tentacles, 93, 101 ; Buds,
95; Polymorphism in, 95; Tests, 104.

Hydriformes — Muscular system, 108.

Hydrobius — Generative organs, 303.

Hydrochserus — Uterus, 615.

Hydroida — Musculature, 39 ; Tentacles,
93; Medusae, 93; Buds, 95; Tests,
104 ; Supporting lamella, 107 ;
Nervous system, 109 ; Gastrovascular
system, 114 ; Ova, 124.

Hydroid Polyps — Flagellate cells, 21.

Hydromedusae — Tentacles, 101 ; Muscu-
lar system, 108; Marginal vesicles,
110 ; Gastric system, 115; Pigmented
investment of stomach, 118 ; Separate
sexes, 120.

Hydrophilus — Development, 245.

Hymenoptera — GnatMtes, 246 ; Liuguae,
246 ; Paraglossae, 246 ; Wings, 248 ;
Dermal skeleton, 259; Wax organs,
250; Spinning vessels, 250 ; Ventral
chord, 259 ; Stomach, 272 ; Malpi-
ghian vessels, 277; Tracheal vesicles,
289 ; Generative organs, 302 ; Testis,
304.

Hyperida — Optic nerves, 253 ; Auditory
vesicles, 261.

Hypotricha — Cilia, 79.

Hyrax — Dorso-lumbar vertebrae, 436 ;
Testes, 618.

INDEX.

635

Ichneumonida — Mid-gut, 272 ; Cement-
glands, 303.

Ichthyornis — Teeth, 551.

Infusoria — Sensory organs, 42 ; Alimen-
tary canal, 47 ; Protoplasm, 77 ;
Nucleus, 77; Cuticular layer, 78;
Figure of, 79; Skeleton, 83; Con-
tractile vacuoles, 85; Conjugation,
89.

Insecta — Alimentary canal, 48 ; Head,
235 ; Metameres, 237 ; Thorax, 237 ;
Gnathites, 244 ; Mouth organs, 245 ;
Feet, 245 ; Mandibles, 245 ; Maxillae,
245 ; Feet, 246 ; Dorsal appendages,
247 ; Wings, 247 ; Integument, 248 ;
Spines, 250; Spinning vessels, 250;
Nervous system, 257 ; Ganglia, 258 ;
Visceral nerves, 259 ; Nervus recur-
rens, 259; Nervi transversi accessorii,
260; Tactile rods, 260; Olfactory
organs, 261 ; Auditory organs, 261;
Simple eye, 265; Compound eyes,
266 ; Faceted eye, 267 ; Enteric
canal, 270 ; Malpighian vessels, 271,
273, 276; Hind-gut, 273; Salivary
glands, 274; Subneural cavity, 279;
Blood-cells, 280 ; Alae cordis, 283 ;
Circulatory organs, 283 ; Aorta, 283 ;
Ecdysis, 289 ; Germ-glands, 292 ;
Generative organs, 300, 304; Par-
thenogesis, 302 ; Pseudova, 302 j
Seminal elements, 305 ; Copulatory
organ, 305.

Insectivora — Centrale, 482 ; Symphysis
pubis, 486 ; Fibula, 490 ; Cerebral
hemispheres, 509 ; Venae cavae, 592 ;
Testes, 618; Vesiculae seminales, 618 ;
Prostatic vesicles, 619.

Isopoda — Branchiae, 241 ; Ventral chord,
255 ; Compound eyes, 267 ; Chyle
intestine, 268; Liver, 275; Heart,
280; Female organs, 294; Seminal
elements, 305.

Ixodes — Generative organs, 298.

Janthina — Branchiae, 338.

J ulus — Salivary glands, 274 ; Malpighian
vessels, 276 ; Benal concretions, 278 ;
Circulating system, 284 ; Generative
organs, 299.

Labyrinthobranchiata — Branchial arches,
470.

Lmmargus — Ovaries, 610.

Lacerta — Columella, 458 ; Frontals, 458.
Lagomys — Caecum, 562.

Lagostomus — Uterus, 616.
Lamellibranchiata — Velum, 318; Diagram
of, 319 ; Foot, 321 ; Tentacles, 326 ;
Integumentary glands, 329; Byssus
gland, 329 ; Shell, 331 ; Hinge of
shell, 332 ; Branchiae, 336 ; Muscles,
342 ; Nervous system, 345 ; Visceral
nerves, 351 ; Tentacles, 352 ; Visual

organs, 353 ; Auditory vesicles, 357;
Alimentary canal, 358; Stomach, 359;
Hind-gut, 359; Liver, 364; Coelom,
367 ; Circulatory centres, 368 ; Auri-
cles, 369, 373 ; Vessels, 372 ; Form-
elements of blood, 375; Organs of
Bojanus, 376 ; Generative organs,
380.

Lamellicorniae — Ventral chord, 258; Tra-
cheal vesicles, 289.

Lemur — Dorso-lumbar vertebrae, 436.

Lepadidae — Dermal skeleton, 249; Ven-
tral chord, 255 ; “ Cement glands,”
285 ; Hermaphroditism in, 291.

Lepidoptera — Thorax, 237; Mouth organs,
246; Feet, 246; Wings, 248; Spinning
vessels, 250; Nervus recurrens, 259;
Malpighian vessels, 277 ; Tracheal
vesicles, 289 ; Ovary, 302 ; Cement-
glands, 303 ; Bursa copulatrix, 303.

Lepidosiren — Vagus, 517 ; Air-bladder,
568; Heart, 580; Aortic arches, 580;
Allantois, 620.

Lepidosteus — Opercular gill, 543 ; Spiral
valve, 560.

Lepisma — Ventral chord, 258.

Lepus — Uterus, 617; Urogenital canal,
619.

Leptocardii — Notochord, 426; Olfactory
organs, 525 ; Vertebrm, 438.

Leucifer — Auditory vesicles, 261.

Libellulidse — Tracheal gills, 273; Salivary
glands, 274; Kespiration in, 290.

Limax — Tentacles, 326 ; Byssus gland,
329 ; Shell, 332 ; Lung, 339.

Limicolse — Excretory organs, 177 ; Gene-
rative organs, 185.

Limnadia — Shell, 235 ; Respiration in,
269 ; Liver, 274 ; Ovary, 294.

Limnadiaceae — Branchiae, 241.

Limnetis — Branchiae, 241.

Linens — Proboscis, 141.

Linguatulidae — Tracheae, 291.

Lingula — Gill, 308; Stalk, 308; Alimen-
tary canal, 311 ; Caeca, 311 ; Vascular
system, 312.

Lisarmatidae — “Lungs,” 215; Excretory
organ, 312.

Littorina — Denticles, 360 ; Stomach, 362 ;
Penis, 386.

Lizard — Foot, 489 ; Arteries, 587.

Lizzia — Tentacles, 101.

Locustidae — Auditory organs, 262.

Loemodipoda — Compound eyes, 267.

Loliginidae — Fins, 325 ; Arms, 326 ;
Calamus, 335 j Respiratory organs,
340 ; Eye, 355 ; Pharynx, 361, 362 ;
Digestive organs, 363 ; Liver, 366 ;
Ink-bag, 367 ; Circulatory centres,
368; Arteries, 374; Oviduct, 386;
Pouches of Needham, 387.

Loligopsis — Eye, 355 ; Caeca, 363.

Lophobranchii — Carapace, 424 ; Ribs
438 ; Vertebrae, 439.

G36

I^^DEX.

Lopliopus — Cells, 139.

Lottia — Branchiae, 337.

Loxosoma — Tentacles, 134.

Lucernaria — Marginal filaments, 102 j
Gastro vascular system, 112.

Luciae — Colonies, 391.

Lucina — Nervous system, 345.

Luidia — Alimentary canal, 213.
Lumbricidae — Ganglia, 150 ; Visceral
nerves, 151 ; Excretory organs, 177 ;
Generative organs, 185.

Lumbriculus — Vascular system, 169.
Lumbricus — Muscular stomach, 162 ;
Vascular system, 167 ; Excretory
organs, 178.

Lutra — Testes, 618.

Lutraria — Ganglia, 346.

Lycosa — Circulatory system, 284 ; Tra-
cheae, 291.

Lymnaeidae — Lung, 339.

Lysidice — Maxillary apparatus, 162.
Lymnacus — Crop, 361.

Macgillivraya — Velum, 321.

Macropus — Turbinate bones, 547.
Macrotarsi — Calcaneum, 490.

Macrostoma — Generative organs, ISO.
Macrura — Ventral chord, 254; Heart,
281 ; Male organs, 295.

Mactra — Siphons, 320; Ganglia, 345, 346 ;
Tentacles, 352 ; Coelom, 367 ; Genital
canal, 381.

Madrepores — Skeleton, 106.

Madreporinae — Generative organs, 122.
Malacostraca — Development, 234 ; Nau-
plius, 235; Appendages, 239; Ventral
chord, 253 ; Auditory vesicles, 261 ;
Caeca, 268.

Malapterurus — Electric organs, 500
Malleus — Byssus gland, 329.

Mallophagus — Generative organs, 303.
Mammalia — Stomach, 9 ; Blood-corpus-
cles, 29 ; Corium, 417 ; Hoofs, 419 ;
Hairs, 420; Anal glands, 421;
Mammary glands, 421; Vertebral
column, 435 ; Processes of vertebrae,

436 ; Cervical vertebrae, 436 ; Caudal
vertebrm, 436 ; Odontoid process,

437 ; Bibs, 441 ; Cervical ribs, 441 ;
Sternum, 442 ; Clavicles, 442 ; Xiphoid
process, 442 ; Episternum, 444 ; Pri-
mordial cranium, 463 ; Tympanic
bone, 466 ; MeckeTs cartilage, 467 ;
Glaserian fissure, 467 ; Stapes, 467 ;
Hyoid, 472 ; Coracoid, 476 ; Scapula,
476; Manias, 482; Pelvis, 485;
Hind-limb, 490 ; Dermal muscles,
493 ; Dorsal muscles, 495 ; Inter-
costal muscles, 496 ; M. pyramidalis,
496 ; Masticatory muscles, 497 ;
Diaphragm, 499, 574; Corpus stria-
tum, 508 ; Cornu ammonis, 508 ;
Fornix, 509; Cerebral hemispheres,
509 ; Gyri, 509 ; Sulci, 509 ; Tha-

lami optici, 509 ; Epiphysis, 509 ;
Mesencephalon, 509 ; Aqumductus
Sylvii, 509 ; Corpora quadrigemina,
509; Cerebellum, 510; Pons Varolii,
510 ; Spinal chord, 512 ; Falx cerebri,
512 ; Tentorium cerebelli, 512 ; Ar-
achnoid, 513 ; Brachial plexus, 514 ;
Crural plexuses, 515; Sacral plexuses,
515 ; Olfactory nerve, 515 : Facial
nerve, 517 ; Accessorius Willisii,
522; Tactile organs, 524; Gustatory
organs, 525 ; Nasal canal, 527 ; Organ
of Jacobson, 547, 528 ; Stenonian
ducts, 527, 548; Eye, 528; Lens,
529 ; Optic bulb, 529 ; Sclerotic,
530 ; Optic muscles, 531 ; Eyelids,
532; Harderian gland, 532; Lachry-
mal glands, 532 ; Lachrymal ducts,
533 ; Ductus endolymphaticus, 534 ;
Labyrinth, 535 ; Cochlea, 536 ;
Columella, 538; Stapes, 538; Audi-
tory meatus, 539; Palate, 546;
Eustachian tube, 546 ; Pharynx, 546 ;
Velum palatinum, 546, 549 ; Nasal
cavity, 547 ; Turbinate bones, 547 ;
Teeth, 550 ; Milk teeth, 552 ; Tongue,
552 ; Buccal glands, 553 ; Thyroid
gland, 554 ; Stomach, 558 ; Mid-gut,
561 ; Hind-gut, 562 ; Caecum, 562 ;
Cloaca, 563, 620; Liver, 563, 564;
Mesentery, 565 ; Larynx, 570 ; Epi-
glottis, 571 ; Lungs, 573 ; Coelom,
574; Blood-corpuscles, 576; Heart
583 ; Arterial system, 583 ; Auricles,
583 ; Atrio-ventricular valve, 584 ;
Aortic arches, 584 ; Ligamentum
Botalli, 585 ; Aorta, 588 ; Ischiac
arteries, 589; Vertebral veins, 592;
Vena azygos, 593 ; Jugulars, 593 ;
Inferior vena cava, 595 ; arterial
retia, 597 ; Pleural cavity, 598 ;
Lymph sinuses, 599 ; Peyerian
glands, 600; Lymphatic glands,
COO; Thymus, 600; Benal organs,
605 ; Urachus, 606; Urinary bladder,
606; Ova, 609; Sperm, 609; Genera-
tive organs, 615 ; Ovaries, 617 ;
Male organs, 617 ; Mullerian duct,
617; Placenta, 620; Chorion, 620;
Prepuce, 624 ; Clitoris, 624 ; Cowper’s
glands, 624.

Man — Mastoid, 464 ; Hyoid, 472 ; Cen-
trale, 482 ; Manus, 483 ; Pelvis, 486 ;
Dermal muscles, 494; Masticatory
muscles, 497 ; Choroid, 530 ; Auditory
meatus, 539 ; Turbinate bones, 547 ;
Caecum, 562 ; Appendix vermiformis,
562; Venous system, 596; Uterus,
615 ; Hymen, 616 ; Prostatic vesicle,
619.

Manatee — Cervical vertebrae, 436.

Marsupialia — Nipples, 422 ; Odontoid pro-
cesses, 437 ; Episternum, 444 ; Para-
mastoid, 463 ; Pelvis, 486 ; Marsupial

INDEX.

637

bones, 487 ; Hallux, 491 ; M. pyra-
midalis, 496 ; Cerebral commissures,
508; Cerebellum, 510; Stapes, 538;
Turbinate bones, 547 ; Sinus maxil-
laris, 548 ; Milk teeth, 552 ; Thyroid
gland, 554; Stomach, 558; Liver,
564; Auricles, 583; Venae cavm, 592;
Renal organs, 605 ; Oviducts, 615 ;
Copulatory organs, 622 ; Erectile
bodies, 623 ; Scrotum, 618 ; Yas
deferens, 618 ; Testes, 618.

Medusae — Gelatinous tissue, 25 ; Cartilage
cells, 26 ; Cartilaginous skeleton, 38 ;
Nervous system, 109.

Melolontha — Mid-gut, 272 ; Heart, 283 ;
Generative organs, 304.

Menobranchus — Vertebrae, 432; Enteric
canal, 561 ; Trachea, 571.

Menopoma — Vertebrae, 432 ; Hind-limb,
488 ; Trachea, 571.

Mermis — Excretory organ, 174; Genera-
tive organs, 185.

Mesostomum — Nervous system, 145; Vis-
ual organs, 154.

Microstomeae — Generative organs, 183.

Mites — Renal concretions, 278.

Mnemidae — Axes, 101.

Moeandrina — Colonies, 100.

Molgula — Nervous system, 395; Sexual
organs, 407.

Molpadia — “Lungs” 216; Cuvierian or-
gans, 216; Polian vesicles, 223.

Mollusca — Connective tissue, 24 ; Carti-
laginous skeleton, 38 ; Dermal bran-
chiae, 46 ; General review, 315 ; Clas-
sification, 316 ; Bibliography, 317 ;
Form of body, 318 ; Velum, 318 ;
Appendages, 325 ; Integument, 328 ;
Cilia, 328; Shell, 330; Branchiae, 335 ;
Nervous system, 343 ; Sensory organs,
351; Tentacles, 352; Visual organs,
353 ; Auditory organs, 358 ; Alimentary
canal, 358; Pharynx, 359; Radula,
360 ; Liver, 364 ; Pancreatic gland,
356 ; Coelom, 367 ; Vascular system,
368 ; Blood-fluid, 375 ; Excretory
organs, 46, 375 ; Generative organs,
380.

Monitor — Frontals, 459; Pelvis, 485; Eye,
529.

Monodelphia — Stapes, 538 ; Genital canal,
615 ; Scrotum, 618.

Monotremata — Mammary glands, 421 ;
Odontoid process, 437 ; Episternum,
444; Coronoid process, 467; Coracoid,
476 ; Scapula, 476 ; Marsupial bones,
487 ; M. pyramidalis, 496 ; Cerebral
commissures, 508 ; Cerebellum, 510 ;
Olfactory nerve, 515 ; Eye, 530 ;
Lagena, 536 ; Stapes, 538 ; Auditory
meatus, 539 ; Horny teeth, 549 ;
Thyroid gland, 554; Stomach, 558;
Cloaca, 563, 620 ; Foramen ovale, 584 ;
Venae cavae, 592 ; Oviducts, 615 ;

Testes, 618 ; Vas deferens, 618 ; Copu-
latory organs, 622.

Mormyrus — Electric organs, 500.
Moschidae — Manus, 483.

Muelleria — Cuvierian organs, 216.

Murex — Siphon, 322 ; Dermal glands, 329 ;

Proboscis, 361 ; Anal glands, 362.
Mus — Uterus, 615.

Mustelina — Caecum, 562.

Mustelus — Ovaries, 611.

Musca — Proboscis, 260; Malpighian ves-
sels, 277.

Muscidae — Heart, 283.

Mya — Ganglia, 346.

Mygalida — Lungs, 291.

Myrianida — Appendages, 136; Visua
organs, 135.

Myriapoda — Head, 235 ; Form of, 237 ;
Gnathites, 244 ; Integument, 248 ;
Nervous system, 255; Tactile rods,
260 ; Eyes, 265 ; Enteric canal, 260 ;
Salivary glands, 274; Malpighian
vessels, 276 ; Renal concretions, 278 ;
Circulatory system, 283 ; Tracheae,
280 ; Germ-glands, 292 ; Generative
organs, 299 ; Seminal elements,
305.

Mysis — Branchiae, 241 ; Auditory vesicles,
261 ; Heart, 281 ; Female organs,
294 ; Seminal elements, 305.
Mytilidae — Respiratory cavity, 320.
Mytilus — Byssus gland, 329 ; Branchiae,
336 ; Genital canal, 381.
Myrmecophaga — Jugal, 466.

Myxinoidea — Auditory capsules, 448 ;
Trunk muscles, 494 ; Brain, 504 ;
Visceral nervous system, 523 ; Laby-
rinth, 533 ; Systemic arteries, 585 ;
CaroGds, 585 ; Kidney, 603 ; Ovaries,
610.

Myzostoma — Intestinal tube, 163.

Naiades — Byssus gland, 329 ; Auditory
vesicles, 359.

Narcine — Electric organs, 500.

Nassula — Pharynx, 85.

Natatores — Anal glands, 420 ; Optic bulb,-
529 ; Pecten, 531 ; Stomach, 558 ;
Trachea, 571 ; Lymphatic hearts,
599 ; Copulatory organs, 621.
Nauplius — Appendages, 238 ; Eyes, 264.
Nautilus — Funnel, 325 ; Copulatory
organs, 327 ; Shell, 333 ; Respiratory
organs, 340 ; Cephalic cartilage, 342 ;
Nervous system, 350, 351 ; Eye,
354 ; Otocyst, 357 ; Pharynx, 362 ;
Caeca, 363 ; Circulatory centres, 368 ;
Branchial veins, 373 ; Excretory or-
gans, 379 ; Oviduct, 386.
Nemathelminthes — Muscular system, 142 ;
Muscular fibres, 144 ; Nervous sys-
tem, 147 ; Alimentary canal, 157 ;
Genital products, 190.

Nematodes — Muscular system, 142 ;

638

INDEX.

Lateral line, 143 ; Tactile papillae,
153 ; Generative organs, 184 ; Semi-
nal elements, 191.

Nemertes — Proboscis, 141 ; Visual organs,
154.

N emertina — Incomplete metamerism, 130 ;
Proboscis, 140, 141 ; Nervous system,
147; Tactile setae, 152 ; Cephalic pits,
153 ; Visual organs, 154; Alimentary
canal, 157.

Neomenia — Nervous system, 152; Enteric
branchiae, 164.

Neophanta — Visual organs, 155.

Nepa — Respiratory tube, 289.

Nephelis — ^Vascular system, 167 ; Excre-
tory organs, 177 ; Generative pro-
ducts, 190.

Nephropneusta — Shell, 323 ; Nervous
system, 349 j Jaws, 360 ; Excretory
organs, 378.

Nerita — Heart, 371.

Neritacea — Branchiae, 338.

Neritina — Heart, 371.

Neuroptera — Thorax, 237 ; Gnathites, 242 ;
Enteric canal, 272 ; Ovary, 302 ; Male
organs, 304.

Nodosaridae — Shells, 81.

Notidani — Branchial arches, 469 ; Clefts,
492.

Notommata — Excretory organs, 175.

Nudibranchiata — Branchiae, 338 ; Tenta-
cles, 352 ; Intestine, 362 ; Vessels,
372.

Numida — Sternum, 443.

Oceanidae — Marginal vesicles, 110.

Octactiniae — Tentacles, 100 ; Transverse
axes, 100 ; Tentacles, 102 ; Gastric
system, 117.

Octopoda — Tentacles, 327 ; Internal
skeleton, 340 ; Muscles, 343 ; Oto-
cysts, 357 ; Pharynx, 362 ; Liver,
366 ; Oviduct, 386.

Octopus — Mantle, 325 ; Copulatory or-
gans, 327 ; Eye, 355 ; Salivary
glands, 363 ; Circulatory centres,
368 ; Figure, 373.

Odontornithes — Teeth, 551.

(Erstedia — Nervous system, 147; Auditory
organ, 156.

Oikopleura — “ House,” 393.

Oletera — Ovaries, 296.

Olynthus — Gastrula, 93 ; Pore canals,
112.

Ommatoplea — Nervous system, 147.

Onchidium — Lung, 339.

Oniscus — Liver, 275 ; Generative organs,
294, 295.

Onychoteuthis — Suckers, 327.

Ophidiaster — Generative organs, 225.

Ophioderma — Generative organs, 224.

Ophidii — Scales, 418 ; Centra, 433 ; Pro-
cesses of vertebrae, 433 ; Caudal ver-
tebrae, 434 ; Axis, 437 ; Ribs, 440 ;

Squamosal, 458 ; Columella, 458 ;
Covering-bones of skull, 459; Vo-
mer, 459 ; Palato- quadrate, 459 ;
Quadrate, 460; Pterygoids, 461; Pala-
tines, 461 ; Maxilla, 462 ; Hyoid, 472 ;
Fore-limb, 481 ; Toes, 489 ; Dermal
muscles, 493; Intercostal do., 496;
Masticatory do., 497 ; Brain, 507;
Optic bulb, 529 ; Ductus endolymph-
aticns, 534; Tympanic cavity, 537;
Palate, 546 ; Nasal glands, 548 ;
Organ of Jacobson, 548 ; Teeth, 550,
552 ; Labial glands, 553 ; Stomach,
557; Mid-gut, 561; Hind-gut, 562;
Liver, 564 ; Mesentery, 565 ; Larynx,
570 ; Lungs, 573 ; Heart, 581 ; Caro-
tids, 587 ; Epigastric veins, 595 ;
Ovaries, 613 ; Testes, 614 ; Copula-
tory organs, 621.

Ophiothrix — Figure, 202.

Ophiura — Anus, 199 ; Generative organs,
225.

Ophiurida — Larva, 195; Rays, 196; Anus,
199; Calcareous skeleton of larva,
202 ; Integument, 203 ; Alimentary
canal, 212 ; Stone-canal, 222 ; Gener-
ative organs, 224.

Ophrydiae — Anal opening, 85.

Opilionea — Visceral nerves, 259.

Opilionida — Eyes, 265 ; Caeca, 270 ; He-
patic tbues, 275 ; Malpighian vessels,
276; Circulatory system, 284; Tra-
cheae, 291 ; Germinal glands, 297.

Opisthobranchiata — Shell, 323; Tentacles,
326 ; Urticating cells, 329 ; Rhino-
phor, 352 ; Visual organs, 354 ; Jaws,
360 ; Salivary glands, 364 ; Heart,
371 ; Vessels, 373 ; Circulation in,
373 ; Excretory organs, 377 ; Uterus,
383.

Orang — Central e, 482.

Orchesella — Ventral chord, 258.

Oreaster — Arms, 195.

Ornithorhynchus — Ethmoid, 465 ; Atrio-
ventricular valve, 584.

Ornithoscelida — Tarsus, 489.

Orthagoriscus — Spinal chord, 511.

Orthoceratites — Shell, 332.

Orthoptera — Thorax, 237 ; Gnathites,
245; Dermal skeleton, 249; Ventral
chord, 258 ; Nervus recurrens, 259 ;
Auditory vesicle, 262 ; Tympanum,
262 ; Masticatory stomach, 272 ; In-
gluvies, 272; Mid-gut, 272; Hind-
gut, 273 ; Salivary glands, 27 4,
Ovary, 302; Testis, 304; Seminal
elements, 305.

Orycteropus — Uterus, 616.

Ostraclon — Carapace, 424.

Ostracoda — Appendages, 238 ; Gnathites,
239 ; Branchiae, 243 ; Dermal skele-
ton, 249 ; Seminal elements, 305.

Ostrea — Muscles, 343 ; Genital canal, 381.

Owls — External ear, 538.

I^^DEX.

639

Ox — Manns, 483.

Oxyuris — Alimentary canal, 159.

Pagurus — Ventral chord, 254; Male organs,
295.

Palinurus — Ventral chord, 254; Liver, 275.

Palaeotherium — Manus, 483.

Paludicella — Alimentary canal, 169.

Paludina — Nervous system, 348 ; Figure,
371 ; Albuminiparous gland, 385 ;
Penis, 386.

Pandora — Generative organs, 381.

Panorpa — Masticatory stomach, 272.

Paradoxurus — Dorso-lumhar vertebrae,
437,

Paramoecium — Trichocyst, 79 ; Pharynx,
85 ; Figure, 85 ; Contractile vesicles
86.

Passeres — Stomach, 558.

Patella — Branchise, 337 ; Eadula, 360 ;
Excretory organs, 377 ; Germ-glands,
385.

Pecten — Byssus gland, 329 ; Branchiae,
337 ; Muscles, 343 ; Ganglia, 346 ;
Tentacles, 352 ; Visual organs, 353 ;
Organ of Bojanus, 376; Generative
organs, 381.

Pectinaria — Branchial tentacles, 135.

Pedicellina — Tentacles, 134 ; Nervous
system, 146.

Pelagia — Larvae, 99 ; Marginal filaments,
102 ; Marginal vesicles, 110.

Pelobates — Vertebrae, 432 ; Tympanic
cavity, 537.

Penelopidae — Copulatory organs, 621.

Pennaria — Buds, 95.

Pennatulida — Skeleton, 106; Generative
organs, 123,

Pentacrinus — Ambulacral groove, 204.

Pentacta — Cuvierian organs, 216.

Pentastomum — Generative organs, 298.

Perameles — Tympanic bone, 466.

Perennibranchiata — Branchial arches,
471 ; Muscles of branchial skeleton,
497 ; Eyelids, 532 ; External gills,
545; Lungs, 572; Cephalic arteries,
586.

Peripatus — Form of, 237 ; Appendages,
238, 244 ; Spinning glands, 250 ; Mus-
cular system, 251 ; Nervous do., 255 ;
Enteric canal, 270; Salivary glands,
273 ; Circulatory system, 283 ; Tra-
cheae, 286.

Perisossodactyla — Nipples, 422 ; Dorso-
lumbar vertebrae, 436 ; Manus, 483 ;
Pelvis, 486 ; Third trochanter, 490.

Peritricha — Cilia, 79.

Perlida — Tracheal gills, 247, 289; Ec-
dysis, 289.

Petalosticha — Ambulacra, 199.

Petaurista — Tympanic bone, 466.

Petromyzon — Auditory capsule, 448; Ol-
factory organ, 525 ; Ear, 533 ; Kespi-
ratory organs, 542 ; Aorta, 585.

Phalangium — Generative organs, 298.

Phallusia — Mantle, 394.

Phary ngobranchiata — Branchial arches ,
470.

Phascolomys — Turbinate bones, 547.

Phascolosoma — Alimentary canal, 161.

Pheruseidae — Branchial tentacles, 135.

Phoca — Turbinate bones, 547; Stomach,
558 ; Pancreas Aselli, 600.

Pholas — Nervous system, 345.

Phronima — Optic nerves, 253; Heart, 280.

Phryganida — Tracheal gills, 247, 273.

Phrynida — Gnathites, 244 ; Nervous
system, 256 ; Eyes, 265 ; Enteric
canal, 269 ; Lungs, 291.

Phylactolaema — Muscular system, 144.

Phyllodoce — Visceral nerves, 187; Pro-
boscis, 163.

Phyllopoda — Shell, 235; Appendages,
238 ; Ventral chord, 254; Eyes, 265;
Liver, 275 ; Heart, 280.

Phyllbsoma — Liver, 275; Germ-glands,
294.

Physemaria — Ectoderm, 103.

Physoklisti — Air-bladder, 567.

Physophoridae — Nectocalyces, 96 ; Air-
sac, 97 ; Generative organs, 121.

Physostomi — Caudal region, 431 ; Ribs,
438; Air-bladder, 567.

Pinna — Visceral ganglia, 346; Visual
organ, 353 j Renal ducts, 376; Genital
canal, 381.

Pinnipedia — Auditory meatus, 539 ;
Buccal glands, 553 ; Thymus, 600 ;
Renal organs, 605.

Pipa — Sacrum, 433; Atlas, 437; Eusta-
chian tube, 537 ; Tongue, 551.

Piscicola — Vascular system, 167 ; Gene-
rative organs, 184.

Placentalia — Foramen ovale, 584.

Placophora — Groove, 318; foot, 321;
Tentacles, 326 ; Spicules, 330 ; Bran-
chiae, 336, 337 ; Nervous system,
344 ; Liver, 365 ; Auricles, 369 ;
Excretory organs, 375 ; Generative
organs, 380.

Planaria — Tentacles, 132 ; Liver, 165 ;
Coelom, 165 ; Excretory organs, 177 j
Genital pore, 184.

Planorbidse — Lung, 339 ; Crop, 361.

Planorbis — Blood-fluid, 375.

Platyhelminthes — Form, 129 ; Mouth, 129 ;
Integument, 136 ; Dermal glands,
141 ; Muscular system, 142 ; Mus-
cular flbres, 144; Nervous system,
145 ; Sexual organs, 184 ; Alimentary
canal, 157; Liver, 165; Excretory
organs, 175 ; Generative organs, 179.

Platyrhini — Turbinate bones, 547.

Plectognathi — Bony plates, 424.

Pleione — Origin of nerves, 151.

Pleurobranchus — Jaws, 360 ; Salivary
glands, 364; Vessels, 372; Recepta-
culum seminis, 384.

GIO

IXDEX.

Pleuroclonts — Teeth, 551.

Pleuroma — Generative organs, 293.
Pleurophyllidia — Branchia?, 338.
Plumatella — Alimentary canal, 160.
Plumularia — Colonies, 89.

Pueumodermon — Respiratory organs, 339 ;
Olfactory organs, 352; Liver, 366;
Hermaphrodite glands, 382 ; Copu-
latory organ, 385.

Poecilopoda — Development, 234 ; Abdo-
men, 235 ; Branchim, 242 ; Liver,
275; Circulatory system, 282 ; Gene-
rative organs, 296.

Podinema — Columella, 458.

Podophthalma — Eye-stalk, 267.
Podophrya — Nucleus, 89.

Polyacanthus — Branchial lamellae, 544.
Polycera — Vessels, 372.

Polydesmus — Renal concretions, 278.
Polygordius — Cilia, 138; Muscular sys-
tem, 142 ; Cephalic pits, 153.
Polymorphina — Shells, 81.

Polypterus — Vertebral column, 429 ;
Ribs, 438 ; Brain, 505 ; External
gills, 545.

Polyzoa — Skeleton, 82.

Pontolimax — Branchiae, 339.

Porifera — Ectoderm, 103; Skeleton, 105;

Sexual organs, 119.

Porpita — Air-sac, 97.

Priapulus — Alimentary canal, 161.
Primates — Nipples, 422 ; Episternum, 444;
Interparietal, 464; Turbinate bones,
465 ; Tympanic, 466 ; Hyoid, 472 ;
Manus, 482; Hallux, 491; Cerebellum,
510; Pons Varolii, 510; Uvula, 549;
Stomach, 558 ; Venae cavae, 592 ;
Renal organs, 605 ; Penis, 623.
Pristiurus — Ovaries, 64.

Procyon — Dorso-lumbar vertebrm, 437.
Propoms — Auditory organ, 156.
Prorhynchus — Alimentary canal, 157.
Prorodon — Contractile bands, 80 ;
Pharynx, 85.

Prosimii — Nipples, 422 ; Caeca, 562,
Uterus, 616.

Prosobranchiata — Epipodium, 324 ; Cere-
bral ganglia, 347; Ommatophor, 359;
Jaws, 360 ; Crop, 361 ; Salivary
glands, 364 ; Germ-glands, 385.
Proteus — Vertebrae, 432 ; Thyroid gland,
554; Stomach, 557; Mid-gut, 561;
Larynx, 570 ; Lungs, 572 ; Blood-
corpuscles, 577 ; Auricle, 580.
Protista — Cilia, 39 ; Conjugation, 52 ;
Spores, 87.

Protopterus — Fin, 477 ; External gills,
545 ; Air-bladders, 568 ; Conus arte-
riosus, 578.

Protozoa — Review of, 75; Bibliography,
77 ; Supporting organs, 81 ; Respira-
tion in, 85 ; Sensory organs, 86.
Protracheata — Enteric canal, 269; Tra-
cheae, 286 ; Generative organs, 296.

Protula — Tentacles, 133,

P seudoneuroptera— Thorax, 239; Gnathites,
■ 245; Feet, 246; Ventral chord, 258;
Enteron, 272 ; ^Malpighian vessels,
277 ; Tracheal system, 289.

Psittacidae — Tongue, 551 ; Caeca, 562.

Psolus — Ambulacra, 199.

Pteraster — Arms, 196.

Pterocera — Siphon, 323.

Pteropoda — Velum, 322; Mantle, 322;
Foot, 323, 324; Shell, 332, Muscles,
343 ; Nervous system, 349 ; Buccal
ganglia, 351 ; Tentacles, 352 ; Olfac-
tory organs, 352 ; Auditory vesicles,
357 ; Alimentary canal, 358 ; Radula,
360 ; Stomach, 362 ; Liver, 366 ;
Heart, 371 ; Vessels, 372 ; Generative
organs, 381 ; Hermaphrodite glands,
383 ; Receptaculum seminis, 384 ;
Copulatory organ, 385.

Pterotrachea — Shell, 323; Sensory organs,
354 ; Ommatophor, 359 ; Recepta-
culum seminis, 385.

Pterygota — Gnathites, 245; Ventral chord,
258 ; Trachem, 288.

Pulex — Ventral chord, 259.

Pulmonata — Shell, 332 ; Salivary glands,
364; Excretory organs, 377 ; Herma-
phrodite glands, 383.

Pupipara — Ventral chord, 259.

Purpura — Dermal glands, 329 ; Anal
glands, 362.

Pycnogonida — Nervous system, 256; Eyes,
266; Caeca, 269; Circulatory system,
284 ; Tracheae, 291 ; Generative
organs, 299.

Python — Skull, 460; Larynx, 569; Veins,
591 ; Copulatory organs, 621.

Pyrosoma — Stolo prolifer, 396 ; Lumi-
nous organs, 294 ; Muscular system,
395; Nervous system, 396; Languets,
401 ; Branchial slits, 402 ; Sexual
organs, 407.

Pyrula — Coelom, 367.

Radiolaria — Central capsule, 76, 82;

Shells, 81 ; Skeleton, 82 ; Figure,
82 ; Spores, 87.

Rana — Sternum, 442 ; Palato-quadrate,
457; Shoulder-girdle, 476; Larvnx,
569.

Raptores — Optic bulb, 529 ; Turbinate
bone, 547 ; Stomach, 558.

Ratitae — Sternum, 443 ; Shoulder-girdle,
476; Precoracoid, 476; Clavicle, 477;
M. pyramidalis, 496 ; Lymphatic
hearts, 599; Copulatory organs, 621.

Rays — Dermal denticles, 424; Articular
processes, 437 ; Hyomandibular, 449;
Shoulder-girdle, 474 ; Protoptery-
gium, 478 ; Electric organs, 500 ;
Vagus, 521; Gelatinous tubes, 524;
Nasal groove, 526; Branchial clefts,
543.

INDEX.

G41

Reuilla — Pore, 117.

lleptilia — Vertical fringe, 415; Corium,
417 ; Scales, 417; Dermal bones, 425 ;
Vertebral column, 429; Atlas, 437;
Axis, 437 ; Kibs, 440; Sternum, 443 ;
Episternum, 444 ; Squamosal, 458 ;
Postfrontals, 459 ; Nasal cavities,
460; Hard palate, 460; Branchial
skeleton, 469 ; Hyoid, 471 ; Shoulder-
girdle, 476; Ilium, 484; Fore-limb,
481 ; Toes, 490 ; Dermal muscles,
493 ; Intercostal muscles, 495 ; Masti-
catory muscles, 497 ; Brachial plexus,
514; Crural plexus, 515; Sacral
plexus, 515; Goblet-shaped organs,
524 ; Gustatory organ, 525 ; Lens,
531 ; Optic muscles, 531 ; Eyelids,
532 ; Lachrymal glands, 532 ; La-
chrymal ducts, 533 ; Lagena, 536 ;
Tympanic cavity, 537 ; External ear,
538; Buccal cavity, 549; Teeth,
550 ; Tongue, 552 ; Labial glands,
553; Thyroid gland, 554; Enteron,
555 ; Stomach, 597 ; Hind-gut, 562 ;
Cloaca, 563; Epiglottis, 571; Trachea,
571 ; Lungs, 573 ; Blood-corpuscles,
576; Heart, 587; Jugular veins, 590;
Venae vertebrales, 591 ; Cardinal
veins, 591 ; Superior venae cavao, 591 ;
Ductus Cuvieri, 591 ; Penal veins,
594; Lymphatic hearts, 599; Peyerian
glands, 600 ; Thymus, 600 ; Eenal
organs, 609 ; Egg, 609 ; Yolk, 609 ;
Cloaca, 620.

Rhizocephala — Mantle, 236; Eyes, 264;
Enteric canal, 269.

Bhizocrinus — Figure, 197.

Bhizopoda — Sensory organs, 42 ; Aliment-
ary canal, 47 ; Protoplasm, 76; Pseu-
dopodia, 78 ; Spores, 87.

Bhombifera — Bony plates, 424.

Bhynconella — Excretory organs, 313.

Bodentia — Nipples, 422 ; Episternum,
444 ; Interparietals, 464 ; Hyoid,
472 ; Centrale, 482 ; Pelvis, 486 ;
Fibula, 490; Cerebral hemispheres,
509 ; Buccal pouches, 549 ; Teeth,
552 ; Stomach, 558 ; Caecum, 562 ;
Liver, 564; Venae cavae, 592; Renal
organs, 605 ; Uterus, 616 ; Testes,
618; Vesiculae seminales, 618; Pros-
tatic glands, 619 ; Copulatory organs,
622 ; Tyson’s glands, 624.

Rotalia — Figure, 78.

Rotatoria — Segmentation, 129; Cilia, 138;
Wheel organ, 138 ; Dermal carapace,
139; Tubes, 139; Nervous system,
146 ; Tactile setae, 152 ; Visual
organs, 154; Alimentary canal, 160;
Glandular organs of enteyDn, 174;
Coelom, 166; Excretory organs, 174 j
Generative organs, 190.

Ruminantia — Interparietal, 464; Cuboid,
<190 ; Tapetum, 530 ; Accessory

cavities of nose, 548 ; Sinus maxil-
lary, 548; Stenonian ducts, 548; Sto-
mach, 559.

Saccobranchus — Branchial tubes, 545.

Saenuris — Vascular system, 168.

Sagitta — Nervous system, 149 ; Eyes, 154.

Salamandrina — Vertebrae, 432, 433; Hind-
limb, 488 ; Trunk muscles, 494 ; M.
py ramidales, 496 ; Lungs, 573 ; Heart,
581; Lymph spaces, 598 ; Vagus, 517;
Eyelids, 532 ; External gills, 54^

Salmo — Notochord, 427 ; Cartilaginous
ci’anium, 450 ; Mandibular apparatus,
454 ; Enteron, 556 ; Kidneys, 604 ;
Ovaries, 610.

Salpa — Stolo prolifer, 391; Asexual form,
392; Sexual form, 392 ; Muscular sys-
tem, 395; Nervous system, 395; Sen-
sory organs, 397 ; Ciliated groove,
397; Visual organs, 397; Ventral
groove, 402 ; Enteron, 404 ; Nucleus,
404 ; Heart, 405 ; Sexual organs, 407.

Saltatoria — Thorax, 237.

Saphenia — Tentacles, 102.

Sapphirina — Female organs, 294,

Sarcoptes — Trachese, 291.

Sarcorhamphus — Veins, 591.

Sarsiadae — Gastrovascular system, 115.

Saurii — Centra of vertebrae, 433 ; Pro-
cesses of do., 433; Caudal do., 434,
Ribs, 438 ; Sternum, 442 ; Columella,
458, 538 ; Vomer, 459 ; Lachrymal;
459 ; Pterygoids, 461 ; Palatines, 461 i
Maxilla, 462; Shoulder-girdle, 476 j
Carpus, 481 ; Ilium, 484 ; Pubis, 485 ;
Tarsus, 489; Intercostal muscles, 496;
Masticatory do., 497 ; Brain, 507 ;
Sclerotic, 530 ; Eyelids, 532 ; Ductus
endolymphaticus, 534 ; External ear,
538 ; Nasal glands, 548 ; Organ of
Jacobson, 548; Teeth, 550; Tongue,
552 : Labial glands, 553 ; Hind-gut,
562; Liver, 564; Mesentery, 565;
Lungs, 573 ; Heart, 581 ; Arterial
arches, 582 ; Carotids, 507 ; Arterise
intercostales, 589; Epigastric veins,
595; Oviducts, 618; Testes, 614;
Vasa deferentia, 614; Mullerian
duct, 615 ; Allantois, 620 ; Copula-
tory organs, 621.

Sauropsida — Vertebral column, 433 ;
Skull, 457 ; Ischiac arteries, 589.

Scaphopoda — Appendages, 326 ; Nervous
system, 347 ; Visual organs, 353 ;
Auditory vesicles, 357 ; Alimentary
canal, 358.

Schizopoda — Branchiae, 241 ; Heart, 281;
Female organs, 294 ; Male organs,
295 ; Seminal elements, 305.

Schizostomeae — Alimentary canal, 157.

Scincoidea — Dermal bones, 425.

Sciurus — Uterus, 616.

Scoleina — Dermal glands, 141 ; Nerve

2 T

(542

INDEX.

trunks, 149; Ganglia, 150 ; Yascular
system, 168 ; Excretory organs,
176.

Scolopendra — Salivaiy glands, 274; Liver,
274; Malpiglnan vessels, 276; Cir-
culatory system, 284; Generative or-
gans, 299.

Scomber — Trunk muscles, 494; Appen-
dices pyloricse, 560.

Scorpionea — Metameres, 237 ; Chelicera},
244; Integument, 248; Poison glands,
250; Nervous system, 256; Eye, 265;
Enteric canal, 269; Hind-gut, 270;
Salivary glands, 274; Hepatic tubes,
275 ; Malpighian vessels, 27 6 ; Cir-
culatory system, 281; Lungs, 291;
Generative organs, 297.

Scutigera — Tracheae, 288.

Scylloea — Branchiae, 338 ; Stomach, 362 ;
Vessels, 372 ; Excretory organs, 377.

Scyllium — Brain, 501; Olfactory organs,
525 ; Branchial cavity, 543 j Ovaries,
611.

Scymnus — Arachnoid, 513.

Scyphostoma — Gastrovascular system,

116.

Segestria — Tracheae, 291 ; Ovaries, 297.

Selachii — Cartilage cells, 26 ; Anterior
appendages, 414 ; Dermal denticles,
423 ; Vertebral column, 429 ; Trans-
verse processes, 431 ; Fin-rays, 432 ;
Bibs, 439 ; Cranium, 447 ; Palato-
quadrate, 448 ; Hyoid, 448 ; Mandi-
bular apparatus, 453 ; Branchial
skeleton, 468 ; Hyoid arch, 469 ;
Copula) of hyoid, 469 ; Archip-
terygium, 473; Anterior appendages,
474 ; Shoulder-gii-dle, 474 ; Fin,
477 ; Pelvic -girdle, 484 ; Ventral fin,
487 ; Dermal muscles, 494 ; Muscles
of branchial skeleton, 497 ; Olfac-
tory lobes, 504; Thalamencephalou,
504 ; Mesencephalon, 505 ; Sinus
rhomboidalis, 505 ; Medulla oblon-
gata, 505 ; Optic nerve, 515 ; Ce-
phalic nerves, 516 ; Vagus, 517 ;
Facial nerve, 517 ; Glossophar-
yngeal, 518 ; Gelatinous tubes, 524 ;
Olfactory organs, 525 ; Sclerotic, 529;
Ciliary processes, 530 ; Tapetum,
530; Eyelids, 532; Vestibule, 534;
Otoliths, 536 ; Spiracle, 537 ; Bran-
chial pouches, 542 ; Spiracular cleft,
542 ; Branchial lamellae, 544; Teeth,
550 ; Enteron, 555 ; Stomach, 557 ;
Spiral valve, 560 ; Hind-gut, 562 ;
Air-bladder, 567 ; Abdominal pore,
574 ; Conus arteriosus, 578 ; Bran-
chial arteries, 579 ; Aorta, 585 ;
Spleen, 600; Thymus, 6C0; Archine-
phric duct, 602 ; Kidney, 603 ;
Nephro stomata, 603 ; Mullerian duct,
603; Eggs, 609; Yolk, 6C9; Sperm,
CC9 ; Ovaries, 610, 611 ; Germinal

glands, 611 ; Epigonal Organ, 611
Copulatory organs, 611 ; Cloaca,
619.

Sepia — Mantle, 325 ; Copulatory organs,
327 ; Eespiratory organs, 340 ; In-
ternal skeleton, 341 ; Buccal ganglia,
351 ; Eye, 355 ; Auditory plate, 357 ;
Caeca, 363; Liver, 366; Pouches of
Needham, 357.

Sepiadae — Fins, 325 ; Arms, 326 ; Os
sepiae, 334 ; Arteries, 374.

Sepiola — Liver, 366.

Sergestes — Auditory vesicles, 261.

Serpula — Tentacles, 133; Stalk of oper-
culum, 134 ; Ganglionic chain, 149.
Serranus — Hermaphrodite arrangements,
611.

Sertularia — Colonies, 93 ; Tests, 164.
Shad — Beard, 524.

Shark — Skin, 423 ; Dermal denticles, 423 ;

Caudal region, 431.

Sialida — Salivary glands, 274.

Siredon — Vertebrae, 432 ; Auditory ossi-
cles, 538.

Siphouiata — Siphons, 320.

Siphonophora — Division of labour in, 95 ;
Colonies, 95 ; Nectocalyces, 96 ;
Figure, 96 ; Nutritive persons, 93 ;
Protective persons, 97 ; Tentacular
persons, 97 ; Generative persons, 97 ;
Cilia, 104 : Muscular system, 108 ;
Gastric system, 114 ; Pigmented in-
vestment of stomach, 118 ; Separa-
tion of sexes, 120 ; Generative
products, 121.

Silurus — Lymph sinuses, 599.

Simiae — Liver, 564; Uterus, 616; Hymen
616,

Singing Birds — Syrinx, 572.
Siphonostoma — Tentacles, 133, 135 ;

Degeneration in, 236 ; Ovaries,
293.

Sipunculus — Cerebral mass, 148; Gan-
glion cells, 148; Alimentary canal,
161, 162; Vascular system, 171;
Excretory organs, 176.

Siren — Blood-corpuscles, 576.

Sirenia — Nipples, 422 ; Caudal vertebrae,
436.

Smynthurus — Tracheae, 288.

Solaster — Arms, 196; Alimentary canal,
213.

Solen — Ganglia, 345.

Solenogastres — Ventral surface, 130;
Aciculi, 139; Nervous system, 151;
Groove, 318.

Solidungula — Fore-limb, 483 ; Venae
cavae, 592.

Sorex — Jugal, 466; Coracoid, 476.
Spatangidae — Semitae, 201; Dermal skele-
ton, 205 ; Spines, 206 ; Muscular
system, 207 ; Blood-vessels, 218 ;
Water-vascular ring, 220 ; Stone-
canal, 222 ; Generative organs, 226.

IXDEX.

043

Sphnoroclonmi — Integument, 138.

Sphinx — Digestive organs, 270.

Sphenodon — Eibs, 440 ; Quadrate, 461.

Spiders — Chelicerse, 244; Spines or seta?,
250 ; Spinning glands, 250; Nervous
system, 256; Cerebral ganglia, 256;
Eye, 265 ; Digestive organs, 269.

Spio — Dermal glands, 141.

Spirochona — Carapaee, 83.

Spirorbis — Stalk of operculum, 134.

Spirostomum — Contractile bands, 80;
Nucleolus, 87.

Spirula — Shell, 334.

Spondylus — Shell, 336.

Spongia? — Gastrnla, 92 ; Tentacles, 101 ;
Integument, 103 ; Alimentary canal,
111 ; Gastric system, 112 ; Lipo-
gastria, 113; Sexual organs, 119;
Ova, 124.

Spongilla — Amphidiscs, 106.

Squatina — Protopterygium, 478; Arach-
noid, 513 ; Enteric canal, 560 ; Heart,
578.

Squilla — Branchiae, 241 ; Nervous system,
253.

Stauridium — Tentacles, 93.

Stentor — Contractile bands, 30; Shells,
83 ; Anal opening, 85.

Stephanoceros — Wheel organ, 138.

Sternaspis — Respiration in, 136 ; Cere-
bral mass, 148 ; Ganglion cells, 148 ;
Vascular system, 171 ; Excretory
organs, 176.

Stomapoda — Liver, 275; Heart, 281.

Strepsiptera — Wings, 248; Male organs,
304.

Strombus — Siphon, 323.

Struthiones — Sacrum, 435 ; Skull, 460 ;
Toes, 490 ; Pecten, 530; Marsupium,
531 ; Turbinate bones, 547 ; Lym-
phatic hearts, 599; Copulatory or-
gans, 621.

Sturiones — Dermal bones, 425; Vertebral
column, 430; Ribs, 439; Cartilagi-
nous cranium, 450 ; Mandibular
apparatus, 453; Operculum, 455;
Goblet-shaped organs, 523 ; Ciliary
processes, 530; Branchial lamellae,
544 ; Thymus, 600.

Suina — Nipples, 422; Manus, 483.

Suctoria— Pseudopodia, 83.

Swans — Trachea, 572.

Sycaltis — Skeleton, 105.

Sycones — Ectoderm, 103 ; Gastric sys-
tem, 113.

Syllidae — Parapodia, 134; Visual organs,
155.

Synaptao — Ambulacra, 198 ; Tentacles,
200 ; Calcareous anchor, 206 ; Inte-
gument, 206 ; Inteimal skeleton, 207 ;
Muscular system, 208 ; Alimentary
canal, 214 ; Figure of, 223 ; Polian
vesicles, 223 ; Water- vessels, 223 ;
Hermaphrodite organs, 226.

Syueoryne — Tentaeles, 93 ; Figure of, 93.
Syncorynida? — Cormi, 94.

Taenia — Segmentation, 129 ; Cystic form,
131; Aciculi, 140; Female organs,
182.

Tamoya — Marginal filaments, 102.

Tanais — Auditory vesicles, 261.

Tapir— Manus, 482.

Tardigrada — Eye, 266; Generative organs,
298.

Talpa — Manus, 482.

Teleosaurii — Dermal bones, 425.

Tegeneria — Trachea?, 291.

Teleostei— Bony plates, 424; Vertebral
column, 430 ; Transverse processes,
431 ; Caudal region, 431 ; Fin rays,
432 ; Articular processes, 437 ; Ribs,
439 ; Cranium (cartilaginous) 450 ;
Sphenoids, 452 ; Mandibular appa-
ratus, 453 ; Operculum, 455 ; Bran-
chial arches, 469 ; Shoulder-girdle,
474 ; Thoracic fin, 478 ; Pelvic-
girdle, 484 ; Ventral fin, 487 ; Dermal
muscles, 492 ; Muscles of branchial
skeleton, 497 ; Olfactory lobes, 504 ;
Thalamencephalon, 504 ; Mesen-
cephalon, 505 ; Sinus rhomboidalis,
505 ; Optic nerves, 515 ; Facial
nerves, 517 ; Glossopharyngeal, 518 ;
Vagus, 521; Goblet-shaped organs,
523 ; Mucous canals, 524 ; Lateral
line, 524 ; Olfactory organs, 525 ;
Sclerotic, 529 ; Optic muscles, 531 ;
Eyelids, [532 ; Ductus endolympha-
ticus, 534 ; Asteriscus, 536 ; Bran-
chial clefts, 543 ; Spiracular cleft,
543 ; Branchial lamella?, 544 ;
Teeth, 550 ; Enteron, 555 ; Fore-
gut, 556 ; Stomach, 557 ; Mid-
gut, 559 ; Air-bladdei’, 567 ; Bulbus
arteriosus, 578 ; Branchial arteries,
579 ; Aorta, 585 ; Thymus, 600 ;
Kidney, 603,604; Yolk, 609; Ovaries,
610 ; Cloaca, 619.

Tenthredinem — Feet, 246; Cement glands,
303.

Terebella — Tentacles, 133; Branchial do.,
135 ; Vascular system, 169.

Terebratula — Circlet of cilia, 307 ; Mus-
cular system, 307 ; Nervous system,
310 ; Excretory organs, 313.

Termes — Nervous system, 257.

Termites — Malpighian vessels, 277.

Testicardines — Stalk, 308 ; Shell, 308 ;
Skeleton of arms, 308 ; Alimentary
canal, 311 ; Generative organs, 313.

Tetractinia — Tentacles, 99.

Tetrarhynchus — Aciculi, 140.

Thalassema — Excretory organs, 176.

Thaliadae — Stolo prolifer, 391.

Thanmantias — Gastrovascular system,
115.

644

I^s^DEX.

Thecidiam — Mefcameres, 307 ; Generative
organs, 313.

Thecomednsce — Organisation, 98 ; Scy-
phostoma form, 98; Strobila, 99;
Ephyra form, 99; Discophora of, 99.

Thecdsomata — Velum, 318, 321 ; Head,
324.

Thelyphonus — Nervous system, 256.

Thomisus — Trachea, 291.

Thoracosti’aca — Optic nerves, 253 ; Ven-
tral chord, 254; Heart, 281 ; Female
organs, 291.

Thylacinus — Marsupial bones, 407.

Thysanopoda — Branchiae, 212.

Thysanozoon — Tentacles, 133; Alimentary
canal, 158.

Thysanura — Gnathites, 245; Feet, 246;
Ventral chord, 258 ; Eye, 265; Enteric
canal, 272 ; Tracheae, 288.

Tinea — Goblet-shaped organs, 523.

Tintinnus — Shells, 83.

Tipulidao — Eye, 267.

Toads — Sperm, 669.

Tomopteris — Generative organs, 189.

Torpedo — Electric organs, 500, 505.

Tortrix — Hyoid, 472.

Toucan — Tongue, 552.

Tracheata — Development, 234 ; Appen-
dages, 238, 243 ; Antennae, 244; Ner-
vous system, 255 ; Salivary glands,
273; Liver, 275; Malpighian vessels,
276, 286; Fat body, 278; Renal con-
cretions, 278 ; Circulatory system,
282 ; Tracheae, 286.

Trachelius — Contractile vesicles, 86.

Trachynema — Tentacles, 102, 107 ; Mar-
ginal vesicles, 110.

Tragulida — Stomach, 559.

Trematoda — Sporocyst, 131 ; Cercariae,
131 ; Cilia, 137 ; Aciculi, 139 ; Stylets,
141 ; Dermal glands, 141 ; Suckers,
143 ; Nervous system, 146 ; Visceral
nerves, 151 ; Visual organs, 153 ;
Alimentary canal, 157 ; Glandular
organ of enteron, 164 ; Liver, 165 ;
Excretory organs, 173 ; Generative
organs, 179 ; Pore, 183.

Tremoctopus — Hectocotylised arms, 327.

Tricocephalus — Muscular system, 143.

Trionyx — Plastron, 426.

Tristoma — Excretory organs, 173.

Triton — Lungs, 573 ; Heart, 581.

Tubicolae — Tube, 134 ; Setae, 110 ; Visual
organs, 155 ; Hind-gut, 163 ; Vascular
system, 169.

Tubifex — Generative organs, 189 ; Sper-
matophores, 191.

Tubipora — Skeleton, 106.

Tubularia — Buds, 95 ; Tentacles, 101 ;
Tests, 104; Supporting lamella, 107;
Pigmented investment of stomach,
108.

Tunicata — General review, 388 ; Biblio-
graphy, 389 ; Classification, 389 ;

Form of body, 390 ; Integument,
393 ; Mantle, 393 ; Skeleton, 391 ;
Muscular system, 391 ; Nervous sys-
tern, 395 ; Sensory organs, 397 ; Ali-
mentary canal, 393 ; Ventral groove,
402 ; Sexual organs, 406.

Turbellaria — Mouth, 129 ; Tentacles, 132 ;
Epidermis, 137 ; Rod-like bodies,
140 ; jMuscular system, 142 ; Nervous
system, 146 ; Visceral nerves, 151 ;
Tactile setse, 152 ; Visual organs, 153 ;
Auditory organs, 156 ; Alimentary
canal, 157 ; Glandular organs of en.
teron, 161; Excretory organs, 173;
Generative organs, 179 ; Pore, 184.

Tylopoda — Fore-limb, 483 ; Stomach, 559 ;
Blood-corpuscles, 576.

Ungulata — Nipples, 422; Vertebral
column, 435 ; Paramastoids, 463 ;
Clavicle, 477 ; Fibula, 494 ; Hallux,
491 ; Cerebellum, 510 ; Uterus, 616.

Unio — Muscles, 312; Nervous system,
345 ; ERerent renal ducts, 376, Geni-
tal canal, 381.

Urodela — Vertebras, 432, 433 ; Shoulder-
gh’dle, 475; Pelvic-girdle, 484 ; Hind-
limb, 488 ; Brain, 505 ; Ductus en-
dolymphaticus, 534; Auditory os-
sicles, 538 ; Stomach, 537 ; Iliac
vein, 593 ; Lymphatic hearts, 599 ;
Kidneys, 604 ; Ureter, 601 ; Cloaca,
613.

Uromastix— Sternum, 412.

Ursina — Caecum, 562.

Ursus — Renal organs, 605.

Vaginicola — Shells, 83.

Varanus — Columella,458 ; Brachial plexus,
514; Tongue, 552.

Vegetable Kingdom — Cells, 15 ; Tissues,

21.

Velella — Generative persons, 97 ; Air-sac,
98 ; Hepatic canals, 118 ; Generative
products, 121.

Vermes — Gastrula, 35 ; Dermal branchiae,
36 ; Excretory organs, 46 ; Alimen-
tary canal, 47, 48 ; Respiratory or-
gans of the enteron, 49 ; Vascular
system, 50 ; Gemmation in, 61 ; Ho-
mology in, 64 ; Classification, 125 ;
Bibliogi’aphy, 127 ; Form of body
128 ; Segmentation, 129 ; Muscular
system, 142 ; Visceral organs, 153 ;
Auditory organs, 156 ; Alimentary
canal, 156 ; Coelom, 165 ; Generative
products, 190.

Vertebrata — Branchial clefts, 7; Lungs,
10; Connective tissue, 24; Osseous
tissue, 28 ; Blood-corpuscles, 29 ;
Cartilaginous skeleton, 38 ; Muscu-
lature, 40; Excretory organs, 47 ;
Respiratory organs of the enteron,
48 ; Homology in, 64’; General

INDEX.

645

review, 408; Classification, 409; Bib-
liography, 412 ; Form of body, 413 ;
Head, 413 ; Appendages, 414; Limbs,
415 ; Integument, 417 ; Ectoderm,
417 ; Dermal papillae, 417 ; Dermal
bones, 425 ; Internal skeleton, 426 ;
Eibs, 438; Thoracic-girdle, 473 ;
Pelvic-girdle, 473 ; Archipteryginm,
473 ; Pore-limb, 479 ; Muscular sys-
tem, 491 ; Nervous system, 501 ;
Brain, 503 ; Pineal gland, 503 ; In-
fundibulum, 504; Pia mater, 513;
Arachnoid, 513 ; Spinal nerves, 514 ;
Vagus, 516; Olfactory organs, 525;
Eye, 527 ; Iris, 530 ; Fenestra ovalis,
537 ; F. rotunda, 537 ; Tympanic
cavity, 537 ; Auditory ossicles, 538 ;
Enteric canal, 539; Gall bladder,
564; Pancreas, 565; Coelom, 574;
Vascular system, 575 ; Systemic
arteries, 585 ; Spleen, 600 ; Excre-
tory organs, 601; Generative organs,

606 ; Germinal epithelium, 606 ;
Male germinal glands, 609 ; Sperm,
609 ; Genital ridges, 610.

Virgnlaria — Generative organs, 123.

Viverra — Testes, 618.

Volnta — Proboscis, 361.

Volvocineae — Cells, 19.

Vortex — Generative organs, 180.

Vorticellinm — Stalk, 83 ; Anal opening,
83 ; Nucleus, 89.

V ultur — Cervical vertebra), 434.

Waldheimia — Nervous system, 310 ; Vas-
cnlar system, 382.

Wasps — Mid-gut, 272 ; Parthenogenesis,
302.

Whales — Whalebone, 549.

Woodpeckers — Tongue, 552 ; Caecum,
562,

Zoothamnlum — Contractile bands, 80.

CSAELES DICKENS AND EVANS, CEYSTAL PALACE PEESS.