MA.

SAUNDERS.

Medical Books.

(3 S. i..th St.

Phila.

THE LIBRARY

OF
THE UNIVERSITY

OF CALIFORNIA

PRESENTED BY

PROF. CHARLES A. KOFOID AND
MRS. PRUDENCE W. KOFOID

MANUALS

STUDENTS OF MEDICINE.

COMPARATIVE ANATOMY

AND

PHYSIOLOGY.

P. JEPFEEY BELL, M.A.,

PROFESSOR OF COMPARATIVE ANATOMY AT KING'S COLLEGE.

ILLUSTRATED WITH 229 ENGRAVINGS.

PHILADELPHIA :

LEA BROTHERS & CO.

(LATE HENRY C. LEA'S SON & Co.)

1885.

SEPTIMUS W. SIBLEY, F.R.CS.,

AS A LITTLE TOKEN OF RESPECT

FOll THE

SKILL AND SYMPATHY WITH WHICH HE EXERCISES
HIS BENEFICENT ART.

PEE FACE.

THE reader who is sufficiently acquainted with the
progress in vertebrate physiology during the last
phase of physiological methods, and who knows how
scattered and incomplete are the investigations which
have been made by the same kind of physical and
chemical inquiries on invertebrate animals, will not
expect to find in the present volume any complete
statement of the physiology of animals, in the sense
in which that term is now used. Such observations
as have been made without especial reference to the
vital processes of man are, for the most part, very
valuable and suggestive } but the time to write a text-
book of Comparative Physiology, as we now understand
it, has not yet arrived.

All that I have attempted to do in this little book
has been to illustrate the details of structure by a
notice of such experimental inquiries as I have con-
vinced myself, or have adequate reason to believe, are,
in their broad outlines, correctly stated. I have much
more attempted to make use of what were long since
called the experiments that Nature makes for us, by

mi COMPARATIVE ANATOMY AND PHYSIOLOGY.

referring to, sometimes perhaps insisting on, the dif-
ferent methods by which similar results are attained by
different animals. That which I have most constantly
kept before myself, and which I hope the student
will faithfully bear in mind, is, that there has been an
evolution of organs as well as of animals, and that he
who desires to understand the most complicated organs
must first know the structure of such as are more
simply constituted.

In pursuit of this object, I have written about
organs rather than about groups of animals ; but I
have added an index in which the various parts of
an animal are collected under the head of its name ;
so that the student who desires to use this manual as
a zoological text-book will have no difficulty in
selecting the portions of the chapters which bear on a
particular form or set of forms.

I have departed a little from the ordinary method
of writing a handbook, in somewhat plentifully inter-
spersing the names of my authorities for various
statements. I have done this, not only because it
recommends itself to my sense of justice, but becau.se
zoological science is just now advancing so rapidly
that many observations and suggestions have to be
incorporated, even in a text-book, before they become
the general property of zoological workers. My
indebtedness to the personal teaching and the pub-
lished writings of Professor Ray Lankester must be

PREFACE. ix

by no means thought to be limited to the statements
with which his name will be found to be connected ;
indeed, I owe him more than I can well express.

I have been careful to acknowledge the source
whence the illustrations are taken, and I have to
return my thanks to the Publication Committee of
the Zoological Society ; to Professor Flower, who
only added one more to a number of acts of personal
kindness when he generously put at my disposal all
the wood-blocks which were in his own possession ;
and to those other friends who have allowed me to
copy figures from their works.

As this manual is written on lines that are rarely
followed, I shall be greatly obliged for any suggestions
as to its improvement, or for corrections of any errors
which may have found their way into it.

F. JEFFKEY BELL.

King's College, May, 1885.

CONTENTS.

CHAPTER PAGE

I.— INTRODUCTORY 1

II.— AMCEBA 18

III.- THE GENERAL STRUCTURE OF ANIMALS ... 23

IV.— ORGANS OF DIGESTION 102

V.— THE BLOOD AND THE BLOOD-VASCULAR SYSTEM . 181

VI.— ORGANS OF RESPIRATION 210

VII.— ORGANS OF NITROGENOUS EXCRETION . . .247

VIII.— ORGANS OF SPECIAL SECRETIONS .... 265

IX.— PROTECTING AND SUPPORTING STRUCTURES . . 274

X.— ORGANS OF MOVEMENT .370

XL— VOCAL ORGANS 387

XII.— THE NERVOUS SYSTEM AND ORGANS OF SENSE . 393

XIII.— ORGANS OF REPRODUCTION 472

XIV.— THE DEVELOPMENT OF THE METAZOA . . .525

COMPARATIVE ANATOMY
PHYSIOLOGY.

CHAPTER I.

INTRODUCTORY.

Comparative anatomy is the science of the
structure of animals, considered in their relation to
one another; comparative physiology deals with
the functions of the parts of which these animals are
made up, and, by examining different forms that
present various kinds of activities, it throws light on
the essential properties of living matter.

The study of animals is but a part of the wider
science of the study of organised matter generally,
the science of biology, which takes plants as well as
animals for the objects of its investigations. Under
the head of biological studies we have, therefore, to
group (a) those which regard organisms as working
machines, capable of performing various functions ;
these studies are physiological, whether animals or
plants be separately or simultaneously examined ;
(b) in the second place, the parts of which the orga-
nism is made up may be investigated, and our studies
are then said to be anatomical, if we concern our-
selves with isolated types, as does the student of
human anatomy ; or they are morphological, when we
compare organisms and their parts one with another,
and try to draw out the significance of isolated facts,
and to learn their bearing on the general scheme of
the organisation of living matter.
B— 16

2 COMPARATIVE ANATOMY AND PHYSIOLOGY.

The present work is concerned only with Animals ;
but, as there is a fundamental resemblance between
Plants and Animals, it is in the first place necessary
to enquire into the characters and modes of activity
of living matter, pure and simple, without any ques-
tion as to whether it be animal or vegetable.

LIVING MATTEE.

Animals and plants have at least this in common,
that they are both fashioned out of a material which,
in all its essential characters, is common to them
both ; and, whether one would be a zoologist, or student
of animals, or a botanist, or student of plants, it is,
in the very first place, necessary that he should have
some clear and exact comprehension of what are the
characters and what are the modes of action of that
primary fashioning substance which forms the material
basis of living creatures, and which is known as
protoplasm. The fact that the sciences of zoology
and botany have to do with this " physical basis" of
living matter separates and distinguishes them at once
from such studies as chemistry or physics, with which
the phenomena of life have no necessary connection.

Living is distinguished from not- living: matter
by several important and easily recognisable charac-
ters. It would seem to have a fundamental and
characteristic composition ; it has the power of con-
tinuing to exist by taking into (nutrition), and
making part of itself (assimilation) other living or
even not-living matter. Nutrition and assimilation
lead to growth, and this growth is succeeded by a
stage in which the additional material obtained is
used for the purposes of reproduction. After a
time a living organism may be seen to be unable to
withstand the action of the surrounding forces in the-
midst of which it has lived, grown, and reproduced
itself; in other words, its activity diminishes and

Chap, i ] CHARACTERS OF LIVING MATTER. 3

diminishes, until at last it dies. From this dead
matter, living material can never, by any process now-
known to us, be produced ; for, so far as we know,
living matter can only proceed from other living
matter.

As the chemist is only able to acquire definite in-
formation with regard to the chemical composition of
living matter by the use of certain treatments which
deprive it of life, we cannot speak with certainty of
more than the broad outlines of its composition ; but
this, at least, may be said : in living matter (proto-
plasm), the four chemical elements, oxygen, hydro-
gen, nitrogen, and carbon, are always found, and with
them there would seem also to be associated small
quantities of sulphur and phosphorus. It is possible,
if not certain, that protoplasm is a compound of a
number of the so-called proteid bodies, and it is quite
certain that what chemists call its " atomic composi-
tion " is very high. One of the most complex bodies
known to us is that constituent of the brain which is
called protagon ; and its " atomic composition " has
been determined to be C^B^NgPO^, or no less than
509 atoms. When such a body is active, fresh chemi-
cal changes are always taking place within it ; it is in
a condition of unstable equilibrium; the result of
such change, so far as it aftects the living matter, is
loss or waste ; in addition to this, living matter is
always taking up fresh oxygen, and forming carbonic
acid, of which it has to free itself. These activities
combined require, as may be supposed, the addition
of fresh material from without; that is to say, living
matter demands food. The food so taken in may or
may not be similar in composition to the organism
itself ; but, as the living creature has wasted through
all its parts, the fresh material has not merely to be
taken in, it has also to be assimilated. When a crystal,
placed in a solution of its own material, grows, it does

4 COMPARATIVE ANATOMY AND PHYSIOLOGY.

so by merely laying on the fresh molecules outside
those already formed ; protoplasm, on the other hand,
makes the fresh food, which may or may not, indeed
need not, have the same composition as itself, an
essential part and parcel of itself.

In the next place we observe, that while a crystal
under the conditions just now mentioned will grow
so long as it is supplied with matter of similar
chemical constitution, living matter only grows
when assimilation goes on at a quicker rate than
destruction or waste. Save for the difficulties of ex-
perimenting, there is no reason why all the sulphate
of copper in the world should not (a) be brought into
one huge crystal, and (£) so remain. It is not so with
living matter ; for every organism there appears to be
a limit of growth, and when that is reached, all the
succeeding matter assimilated goes for a different pur-
pose. The organism, ceasing to grow, begins to repro-
duce its kind, and, in the very simplest cases, produces
an individual exactly similar to itself. This act of
reproduction appears to be, next to sustentation, the
primary work of every organism, and when that is
completed, we often observe that the parent organism
begins to lose its activity ; it becomes the prey of other
living organisms ; or, undergoing gradual decay, the
complex mass of albuminous matter, which we call
protoplasm, and associate with Hfe, falls away into
constituent molecules of a less high degree of chemical
complexity.

Assimilation, growth, reproduction, death, are,
as here explained, four phases in the history of living
matter which at once and sharply distinguish it from
crystalline or other dead material.

Nor is this all ; if we set one crystal against
another of similar composition, or if we try to rouse
or stimulate a crystal, we get no response. With living
matter the case is very different ; roused either by

chap, i.] CHARACTERS OF LIVING MATTER. 5

some apparent friend or enemy in the water, or by a
touch from our needle, as we observe it under the
microscope, a mass of living matter will be found to
be irritable. In consequence of this irritability

it undergoes some change converting latent into
actual energy, and this is most frequently and most
easily seen to be some change in space, or in the
relations of its parts ; these are due to what is known
as the contractility of living matter. In other
cases, the production of heat, light, or electricity, is
the expression of irritability.

We have next to observe, that within the area of
any given mass of protoplasm, there may be move-
ments of its parts ; some of the granules seem to
stream in a- more or less regular course between
those on either side of them, in a way which can best
be understood by supposing the observer to be raised
above and to be able to note the movements of a great
crowd of passengers in a busy street ; some move
faster than and overtake others, some collect into
more or less small crowds ; others, having moved on-
ward for a certain distance, turn aside or turn back.
This streaming movement of protoplasm is highly
characteristic, and affords a proof that the problem of
the motile activity of protoplasm can only be explained
by the study of the parts of which it is made up.

Lastly, thin layers of non-granular protoplasm
are sometimes to be observed gliding' over firm
bodies ; by these means the whole mass is enabled to
progress in a forward direction.

The study of streaming movements shov/s us that
the constituent particles do not move around any fixed
point, but freely as the particles of a fluid substance.
So far as we can see, these movements are not the
result of any external cause ; did we choose to allow
that a simple mass of protoplasm had a " will," we
might well call them " spontaneous" or " voluntary;"

6 COMPARATIVE ANATOMY AND PHYSIOLOGY.

without going so far, we must allow that they appear
to be due to the protoplasm itself; they are self-
moved or automatic.

Living matter, then, is irritable and automatic ;
irritability finds expression in contractility, or in the
production of such forces as heat, light, or electricity.

With regard to its general physical and chemi-
cal characters, we have to note that it is possessed of
great cohesive powers, and yet is very extensile ; it
does not mix with water, but it swells by imbibition ;
it may expel the contained fluid in the form of
rounded vacuoles, and bubbles of gas are sometimes
apparent in it. It is ordinarily colourless, and re-
fracts light more strongly than water ; it is in most,
and probably in all cases, slightly alkaline in reac-
tion.

Before we leave the general consideration of pro-
toplasm, we must point out two foreign elements
which have to be considered. The first of these is the
presence in protoplasm, as we ordinarily observe it, of
various more simple chemical compounds, which have
the form of granules ; these, which may be fatty or
starchy bodies, are conveniently grouped together
under the head of metaplasm; they may be re-
garded as owing their origin to the changes that are
constantly taking place in the molecular constitution of
the protoplasm, or, in other word?, as waste products
not yet eliminated.

The second is a general motion of a protoplasmic
mass, especially when of particularly small size (e.g.
bacteria) ; this movement of the body as a whole is
not a vital, but a purely physical phenomenon, as may
be demonstrated by the simple experiment of rubbing
up a little gamboge in a drop of water, when exactly
the same movement is to be observed. This approxima-
tion and separation of small particles is a phenomenon
which has attracted the attention of the physicist, by

Chap, i.] THE CELL. 7

whom it must be explained ; it was, however, first
observed by an eminent botanist, and is consequently
known as the Browiiian movement.

The term cell is not unfrequently applied to
every separate mass of living matter, but, in conse-
quence of the associations connected with this term,
it is better to make use of the more elaborate though
perhaps more intelligible nomenclature which enables
us to distinguish between the different characters of
"elementary organisms." When attention was first
directed to these objects, the botanist observed that in
each mass of protoplasm there was a portion which,
by various characters, could be easily distinguished
from the rest, and which might be very appropriately
spoken of as the nucleus ; in addition to this, he
saw that the outer portion of the protoplasm was en-
closed as in a wall ; he spoke, therefore, of the whole
as a cell, with a cell wall, and a contained nucleus.

Later on it was found that the protoplasm (or
" sarcode," as it was originally called) of animals was
not to be distinguished from that of plants, and it
was then also seen that it was only in very rare cases
that this animal protoplasm was enclosed in a cell
wall. Thereby the very first conception of a cell was
destroyed, but the name was still retained as a con-
venient term.

Still later researches revealed the at first
astonishing fact that organisms could and did exist
in which that specially modified portion of the proto-
plasm which had been called the " nucleus " was, to
all appearance, altogether absent ; some naturalists,
and especially some physiologists, now regard the
nucleus as no essential part of the cell. On the other
hand, it seems better to recognise in our nomenclature
the present conditions of our knowledge, and to use
for the " elementary organism " some other definite
term than that around which so many battles have

8 COMPARATIVE ANATOMY AND PHYSIOLOGY.

been fought, and with which, perhaps, no few super-
stitions are or have been connected.

We will, therefore, follow those who have agreed
to the suggestion of Prof. Haeckel, and will use for
the elementary organism, whether or no provided with
a nucleus, the useful and suggestive term of plastid.
This plastid, or unit of organic structure, is com-
posed of protoplasm ; it may be without a nucleus,
when it is a cytod (or cell-like body), or it may have
within it a denser mass, which is very feebly, if at all,
contractile, the nucleus; in which case it is a cell.
This nucleus is ordinarily provided with one or more
smaller nucleoli, and, possibly, always has a distinct
investing membrane. It would appear to have a
special chemical composition, inasmuch as while a cell
when treated with a ten per cent, salt solution leaves
a precipitate, no such precipitate is stated to be found
when a cytod is subjected to the same reagent. The
body so precipitated has been called nuclein.

Protoplasm, then, is presented to us in the form
of plastids, and these plastids may either be without
(cytods) or have (cells) distinct nuclei. All organisms
are composed of one or more cells, or, in other words,
are either unicellular or multicellular. The former,
as much as the latter, are capable of exhibiting all
the .essential phenomena of life.

TISSUES AND ORGANS.

"When we examine the different stages in the
history of a developing animal, or compare a series
which commences with low and passes through more
highly developed forms, we find a gradual increase in
the complexity of the parts ; of this we have already
had an example in comparing the cytod with the cell,
and we shall observe it in every chapter of this work.
This increase in complexity is termed the process of
differentiation.

Chap. I.] TISSUES AND ORGANS. 9

In making a general survey of animals we find
that the lowest consist only of simple cells ; later on,
the cells are found not to live an independent
existence, but to be associated one with another, and
different groups of cells are seen to be differentiated
in various ways. The result of this is that sets of
cells come to have different characters (some are
contractile, others irritable, and so on), and these
different sets are what are known as tissues ;
secondly, we observe that these tissues become
connected with one another in different proportions
and relations, so as to give rise to those parts of the
adult which take on particular duties, and are known
as organs.

Looked at in a general way, and without taking
any notice of exceptional cases, we observe that
there are tissues in an animal which are not found in
a plant ; these, which are distinguished as the animal
tissues, are such as have a relation to movement or
sensation ; in other words, the muscles and nerves are
animal tissues. On the other hand, plants preserve,
protect, and sustain themselves, and the corresponding
tissues in animals are always spoken of as the
vegetative ; .of these we may find convenient
examples in that outer layer of the body which is
spoken of as epithelium, or that supporting tissue
which is known as bone.

The classification of organs is a little more complex,
but it will be convenient to give it now, so that time
and space may be saved in the future.

In the first place it is clear that the vegetative
functions fall under three great heads ; an animal has
to care for itself, to adapt itself to or move through
its surroundings, and to reproduce its kind. And, in
the second place, it is just as obvious that it has to
perceive what is going on around it, and to act
accordingly. We have, then :

io COMPARATIVE ANATOMY AND PHYSIOLOGY.

(1) Organs of internal relations.

i. Protective.— Examples : Skin, shell.

ii. Nutritive. — Examples : Digestive tract (nu-
trient) ; heart and blood-vessels (circulatory).

iii. Purifying1.— Gills, lungs (carbonic acid) ; kid-
neys (nitrogenous products).

(2) Organs of external relations.

iv. LiOCOmotor. — Limbs, etc. (compounded of

skeletal and muscular tissues).
v. Prehensile. — Limbs, etc. (compounded of

skeletal and muscular tissues).
vi. Offensive.— Teeth, claws, electrical, odorous

organs.

(3) Reproductive.

(a) Germ -producing glands : testes, ovaries, which are

essential.
(/3) Copulatory : penes, etc., which are accessory.

(4) Sensory.

(a) Organs which receive impressions ; eye, ear,

etc., brain.
(j8) Organs which Stimulate other organs ; brain.

METHODS OF COMPARISON.

When an anatomist has acquired a positive
knowledge of a certain number of selected forms of
life, he proceeds to convert his empirical acquaintance
with facts into science by reasoning upon the infor-
mation which he has acquired. In this operation he
makes great use of the fertile method of comparison,
remembering the words of Buflbn, " Ce riest qu'en
comparant que nous pouvons juger." Like things,
however, must be compared with like, or confusion
will inevitably result. We must, therefore, lay down
certain rules to guide us in these kinds of enquiries,
for, though no one would attempt to compare a heart
with a lung, many would, at first, more willingly
compare the leg of a man with that of a cockroach,
than the fin of a perch with the wing of the sparrow ;
yet the latter is the more justifiable proceeding.

Chap. I.] HOMOLOGY. 1 1

The reason for this is plain, the moment we clearly
understand what object the comparative anatomist
has before him ; it is that of coming to some general
conclusions as to identity or community of structure ;
for this purpose, then, he is not to compare parts
that have the same function, but those that are formed
in the same kind of way. The physiologist, on the
other hand, looking at organs as parts of a machine,
examines together those that do the same thing.
When we compare parts morphologically, we must not
be content merely with an analogy between them,
we must be careful that there is a homology or
real resemblance.

The first criterion of homological parts is their
development from similar embryonic structures ; such
are the wing of a bird and the leg of a horse. But
a further question now arises ; why have these wings
and legs, which in their completed condition are so
different from one another, a similar structure in the
embryo 1 The answer to this is given by the doctrine
of descent, which supposes that the bird and the
horse had in the past a common ancestor, provided
with limbs simpler in structure than those of either
bird or horse, but having essentially that which they
have now, or which they have had, and from which
they are both derived ; a true and complete homology
of parts is, then, only to be found between animals
which have had a common ancestor provided with the
part to be compared. This complete homology may
be conveniently spoken of as homogeny (Ray
Lankester).

It is very necessary to have before the mind this
idea of community of descent, because we shall con-
stantly meet with cases in which, with a very close re-
semblance in structure and mode of development, there
is not a complete identity in descent. For example,
all mammals and all birds are provided with four

12 COMPARATIVE ANATOMY AND PHYSIOLOGY.

cavities in their hearts : two auricles and two ven-
tricles ; but it is certain that, whatever was the animal
that was the nearest common ancestor to the two, it
had only one ventricle. The right and left ventricles
of the hearts of birds and mammals are, then, not
homologous in the sense of being homogenetic ; they
have been acquired independently by the two groups,
in consequence of certain physiological needs ; they
are the result of similar modifying forces, and are
homoplastic, but not homogenetic parts.

DEVELOPMENT.

The last point to which the student must be intro-
duced is one of the very greatest importance. If we
study the animal kingdom throughout, we find that,
starting from the simplest mass of protoplasm,
we are gradually led to the complex and elabo-
rate structural and functional arrangements which
are found in so highly organised an animal as
man himself. If, on the other hand, we study the
developmental history of a highly organised form,
we find that it starts from a simple mass of pro-
toplasm, the egg, or ovum, as this plastid is called ;
this cell gradually becomes more and more elaborated,
and takes on the more complex arrangement which
may be seen in its parent ; we observe, that is, that not
only are there a number of stages in the different re-
presentatives of the animal world, but that there are
also a number of stages in the structural history of
every individual ; and we may go yet a step farther,
and say that in a broad and general way there is a
complete parallelism between the two.

The results of the investigations and considerations
which flow from a study of the facts here indicated
are best expressed in an aphorism, which may at once
be laid to heart, and which will be abundantly proved
by a study of development and comparative anatomy :

Chap, i.] EVOLUTION. 13

" The history of the individual is a compressed epitome
of the history of the race."

Those, therefore, who desire to obtain a complete
knowledge of animals or, indeed, of any one animal,
must not be contented with an account of the anatomy
of the adult ; they must direct their attention also
to its development, and become the students of
Embryology, while they must no less take care to
study the history of the animal, or of its allies, in the
past ages of the world, or to know something of its
Palaeontology.

These two branches, Embryology and Palseont-
ology, are of the greatest assistance in an endeavour
to obtain some clear idea of the morphology of animals ;
but a weapon no less sure and no less important is that
of comparison, by means of which similar parts in
different organisms are studied and explained ; no
better aid to safe judgment can be afforded, and it must
be used unceasingly and unsparingly.

EVOLUTION.

The great maze and mass of facts which are found
in works on zoology or comparative anatomy are
hardly to be held together without the bond of phi-
losophy ; the grouping of facts, and, still more, the
grouping of animals, must be always more or less un-
intelligent, mechanical, and artificial, unless we make
some use of some kind of explanation. That which
we shall use here will be founded on the belief that
there is a blood relationship, or relationship by descent
and inheritance, between every member of the animal
kingdom; and that, were it possible to know all the
facts, we could make a genealogical tree for animals,
which should be as exact and definite as the family
tree which is drawn up by the genealogist or the
herald. That division of biology which busies itself
with genealogical problems is known as Phylogeny.

14 COMPARATIVE ANATOMY AND PHYSIOLOGY.

All the steps in the differentiation and elaboration
of organised beings are examples of that process of
evolution, which, when based on a belief in the
existence of a blood relationship between animals, is
known as the doctrine of descent. In an attempt to
understand how this has worked, we make reference to
two different series of facts. We have, in the first
place, to make certain generalisations as to the way in
which the differences have been brought about, and we
have, in the second, to consider what are the essential
properties of living matter which may be regarded as
the determining factors in the evolution of organised
material

The generalisations made from a number of obj
served facts may, if this definition be borne in mind,
be called the laws of evolution; they have been thus
enunciated by Professor Huxley :

(1) There has been an excess of development of
some parts in relation to others.

(2) Certain parts have undergone complete or
partial suppression.

(3) Certain parts, which were originally distinct,
have coalesced.

Let us apply these laws to a concrete example, and
select for study the fore-foot of a camel In the more
primitive mammalia there were five fingers or digits,
each connected by a metacaqDal or palm bone with
the wrist, and these five sets of digits and metacarpals
were of subequal size. In the hoofed group of animals
the first of these, or thumb, disappeared, as in the case of
the modern pig ; the two that were now outermost, the
second and fifth, became smaller and smaller, as in the
sheep or deer, and finally, as in the camel, disappeared
altogether. Here we have various stages of law 2.
This loss of the outer was accompanied by an increase
in the size of the median digits and metacarpals
(law 1), and in the more or less complete fusion of

chap, i.i HEREDITY AND VARIABILITY. 15

the third and fourth metacarpals one with the other
(law 3) ; the result of this last process being the
formation of a bone which at its lower end only
gives any obvious indication of its primitively double
nature.

The characteristics of protoplasm which appear to
be the determining factors of evolution, are (1) its
power of producing an organism like to itself; and (2)
the fact that no child or parent, or any two children,
are exactly similar one to another. The first of these
principles is known as that of heredity, the second
as that of variability. It is obvious that the
second principle only comes into action because of the
differences in the surroundings of every individual
plastid ; the greater the homogeneity of the surround-
ings, the greater the likenesses between the plastids.
The law of heredity may consequently be compared to
the first law of motion (Gasquet).

Organisms, therefore, tend to resemble their
parents, but, being more or less differently affected by
surrounding media and objects, diverge more or less
from the parent stock ; the greater the differences
in environment, the greater the differences between
parent and child. This is a fact so well known to
us all that we need not enlarge upon it here.

ANIMALS AND PLANTS.

While the conviction that there is an essential
unity between animals and plants may be taken as
one of the most important results of modern biology,
we have to note that along the two lines of organisa-
tion the constituent protoplasm has, on the whole, de-
veloped special characteristics. In other words, we are
not able always to say definitely whether a given uni-
cellular- organism is an animal or a plant ; but we can
always with certainty point to the differences which
distinguish a rose from a bee.

1 6 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Thus the form (1) of a plant is diffuse and arbores-
cent, that of an animal oblong and rounded. A plant
lives on (2) carbonic acid and mineral salts, but an
animal requires albuminoid foods. These foods are in
the plant taken in (3) by the porous tissues, and there
is no distinct mouth as there is in all but the lowest,
and in the majority of parasitic animals. The secre-
tions (4) of a plant are non-nitrogenous, while some of
the waste products of an animal always contain
nitrogen. In their habits (5) we find that plants are
fixed, and animals locomotive. And, lastly, (6) as to the
characters of their cells, we find that plants have a cell
wall formed of that ternary compound which is known
as cellulose, while the wall of an animal cell, when
present, is derived directly from the cell protoplasm.

To nearly all the statements now made an ex-
ception may be found : thus (1) cacti and fungi are
certainly not arborescent or diffuse, while polyps as cer-
tainly are. (2) Fungi appear to require some more
complex compound than merely carbonic acid and
mineral salts, but such a body as ammonium tart rate
will give the nourishment required ; every animal
known to us requires albuminoid food, and dies when
deprived of it. (4) It is quite true that plants do
not give off nitrogenous excreta ; but their protoplasm,
it must always be remembered, is capable of forming
them ; on the other hand, all the excreta of an animal
are not nitrogenous; Ascidians (and, if they are
truly animals, some of the Cilio-flagellata) form cellu-
lose. The latter and some low worms have been ob-
served to form starch, and sugar is a ternary compound
formed by various animals. The well-known Yolvox
offers (5) an exception to the statement that plants are
fixed, and polyps and, to a large extent, stalked Echi-
noderms, to the statement that animals are locomotive.
Lastly, some of the lowest plants, such as Myxomycetes,
have their protoplasm naked, while the just-mentioned

chap, i.] ANIMALS AND PLANTS. 17

Cilio-flagellata have cellulose in their cell-walls, and
the so-called matrix of cartilage cells does not appear
to be directly formed from the cells themselves.

This enumeration of differences or resemblances is,
after all, unsatisfactory, and will, with the progress of
knowledge, come, no doubt, to be regarded as mis-
leading ; for the present, it will not fail in its object
of impressing on the student the broad and general
characteristics of animals and plants as we now know
them ; but there must be added to it a reminder that
among the higher members of the Droseracese we find
plants (a) whose leaves have in some forms the power
of movement when excited ; (#) the glands of their
leaves are able both to digest and to absorb animal
matters ; and (7) the normal electrical current is,
when these leaves are irritated, disturbed in the same
manner as is that of a contracting animal muscle.

The general relation of animals to plants is well
shown in the following table (Brass) :

^Plants-

use up form

carbonic acid, water, nitrates, oxygen, carbohydrates, fat, albumen,

form use up

The fact that, in sunlight, green plants (that is,
plants containing chlorophyll) give off oxygen has led
some to think that plants take in carbonic acid and
exhale oxygen ; but plants as much as animals give off
carbonic acid as a waste product. If or when an ani-
mal contains chlorophyll grains, it as much as a plant
will give off oxvgen under the influence of sunlight.
c— 16 '

18

CHAPTER IT.

AMCEBA.

IT has been wisely said that "the highest laws of our
science are expressed in the simplest terms in the
lives of the lowest orders of creation " (Paget) ; and
it will be well, therefore, to commence our studies
with a close investigation into the characters of one of
the simplest of living animals.

The word Amoeba is a generic term,* which is
applied to a number of forms, which have in common
the following characters; they are more or less
minute specks of nucleated protoplasm, without any
wall or membrane limiting their surface, and they are
capable of pushing out processes of their body sub-
stance from any part or point of it. They are some-
times as much as one-hundredth of an inch in
diameter, but they in all cases require the assistance
of a microscope of high powers for their satisfactory
study.

If we place one on a glass slide, and, after allowing
it to become used to its new position, examine it
under the microscope, we shall at once see how ap-
propriate is the name that has been given it. Its
form is never constant for more than a few moments
together, as we can best demonstrate by making a
sketch of its shape once every minute for some five or
six times.

These changes in form are, we know, expressions
of the irritability and contractility of the protoplasm.

* The possibility that a number of so-called Amoebae are
stages in the life-history of animals or plants does not affect the
question here dealt with.

Chap. II.]

AMCEBA.

T9

Looked at more closely, we see in it evidences of dif-
ferentiation of structure; the mass, small as it is,
is not homogeneous ; the outer portion is denser and

Fig. 1.— Amoeba.
n, Nucleus ; cv, contractile vacuoles.

clearer (Fig. 1) than the inner, which is more fluid
and granular. Although these two portions are not
sharply marked off from one another, it is convenient
to have definite names by which to distinguish them,
and we will speak therefore of an ectosarc, and an
endosarc. Within the endosarc we see a disk-
shaped or rounded body which retains its form, while

20 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the protoplasm around it is changing ; this is the
nucleus (n), and within it is a smaller body, the
little nucleus, or niicleolus. In the ectosarc we
have to observe a space which opens slowly, and con-
tracts rapidly ; its power of contraction may be seen
to be independent of that of the general mass of proto-
plasm. This space (the contractile vacuole, cv)
appears, though we cannot speak with certainty, to
be a kind of pump, whereby water is taken into and
forced out of the body ; the water that enters must
bring with it a certain quantity of oxygen, which is
a prime necessity of every living organism, whether
it be plant or animal ; while the water that is forced
out of the body must carry with it a certain quantity
of those waste products which always appear when a
living body is in active function.

The contractile vacuole, then, would appear to
effect for the amoeba the two processes of respiration
and of purification, which, in higher animals, are per-
formed by definite organs.

It will at once be noticed that there is no special
point by which food enters, or what is useless in that
food escapes from the amoeba ; in other words, there
is neither mouth nor anus. But it will almost as
soon be seen that this naked cell has no need of
either the one or the other ; it flows around the food
it needs, and it flows away from the waste or useless
matter which is of no further use to it.

Just as there is no special inlet for the food, so
there is no part of the cell which can be said to be es-
pecially digestive in function. We can best see what
happens to the food when it is a green-coloured plant ;
when such is under observation we find that it
gradually breaks up within the amoeba, that it
gradually loses its green colour, and finally disappears ;
if it be a diatom that has been flowed around, we
may observe in time that the undigested case will be

chap. ii. j AMCEBA. 21

left behind. The cell, then, of which the amoeba
consists, is capable of taking in food, and of making
it part of itself ; it can, in fine, effect all the opera-
tions of nutrition.

The flowing around food is only an expression of
that general locomotor activity of the amaba which
finds a more general expression in those remarkable
changes in form to which we have already directed
attention. These, when studied in detail, are found
to be effected in the following fashion. At some
point of the body where the contour is smooth and
rounded a little knob of ectosarc may be seen to be
protruded, and to widen out as it increases in size ;
the cavity in its interior which is thus formed becomes
filled with endosarc which flows into it. The pro-
trusion is at first broad or lobate, and it may so
remain ; or it may increase in length and diminish
in proportionate breadth, or it may even become
branched at its free extremity. Such an out-pushing
of the substance of the naked cell is spoken of as a
pseudopodium (false foot). When, as often
happens, several small pseudopodia, or one or a few
of large size are given off close to one another, and if
the pseudopodia are not at the same time protruded
from the opposite surface of the cell, then the whole
mass follows the pseudopodia, and there is a general
movement of the amoeba ; at such a time we can
distinguish an anterior from a posterior end.

The amoeba, then, feeds, grows, and moves about,
takes in oxygenated water, and gets rid of waste
material; exhibits, in fine, all the essential pheno-
mena of internal and external relation ; it does not
exhibit anything more than a general irritability, but
as it does answer to stimuli from without, it presents
us with a copy, as it were, of the changes that occur
in ourselves when we are acted on by external stimuli.
It performs all the actions that are essential to our

22 COMPARATIVE ANATOMY AND PHYSIOLOGY.

idea of an individual living for itself. But it does
more than this ; it performs also the function that is
necessary for the continuance of the species of which
it is a representative. It reproduces itself.

In the simplest case the act of reproduction is
effected thus; the nucleus elongates, becomes con-
stricted in its middle, and divides into two. As this
division is being effected the surrounding protoplasm
becomes divided into two masses, each of which
accompanies one half of the nucleus. As a result of
this process we have two individuals where before
we had one, and they differ only from the amoaba
which we have been previously studying by their
smaller size ; as our first amoeba has altogether dis-
appeared, it is, to all practical purposes, dead ; and
we have, then, in this, the simplest condition of
reproduction, the death of the parent absolutely co-
temporaneous with the appearance of a new generation.
This process of reproduction is that which is known
as fission.

Another method is also observed in the amceba,
which may be regarded as a modification of that of
fission. A small portion (bud) of non-nucleated
protoplasm is gradually separated off from the rest of
the mass ; this increases in size, and develops within
itself a new nucleus, so that it becomes exactly
similar to its parent, which, in this case, continues to
exist. Here we have reproduction effected by bud-
ding, or gem million.

Notwithstanding all the functions performed by
this minute mass of protoplasm, it will be observed
that there is nothing in the cell to which we could
correctly give the name of an organ. We are in the
presence of life, but hardly of organisation.

CHAPTEK IIL

THE GENERAL STRUCTURE OF ANIMALS.

BEFORE proceeding to a comparative account of the
structure and functions of the organs of different
animals, it will be necessary to introduce the student
to the broader characteristics of the groups into which
the animal kingdom has been divided. What fol-
lows in this chapter is to be regarded as having that
aim alone ; it is in no way to be looked upon either
as a classification of animals, or even as an intro-
duction to it, and it is to be used rather as a kind of
guide to the relative position of any animal that may
be mentioned in the succeeding chapters. So far as is
possible in the necessities of the case, it has been so
prepared as to hinder rather than to aid the student
in any attempt to commit to memory a system of
classification ; for it is certain that there is nothing less
fruitful in good result than a parrot-like acquaintance
with what is only a compressed epitome of the more
certain results of zoological enquiries, but which, it is
to be remembered, may at any time be profoundly
modified by further investigation. What is called a
classification of the animal kingdom is nothing more
or less than a precis of our knowledge at a given mo-
ment, and, at its best, can never be more than rela-
tively correct.

On the other hand, the sketch that follows may be
of use as indicating the general course of development,
taken along different lines by different kinds of
animals.

The simplest animals essentially resemble an

24 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Amoeba in this particular, that, for the whole period
of their lives, all the functions of the organism are
performed by a single cell ; and, even where cells
remain collected into a colony, each individual member
of that colony performs all its own duties, and affords
no assistance to the rest ; there is no division of
labour.

In the higher animals a very different phenomenon
is seen; here again the whole organism is, indeed,
composed of cells or cell-derivates ; but, howsoever com-
plex it may become, it starts always on the cycle of
its existence under the form of a single cell. This cell,
which is known as the ovum or egg-cell, undergoes a
series of divisions by means of which, two, four, eight
.... cells are produced, and these become arranged in
definite fashion, and take on more or less well-defined
functions. Here, then, different parts of the organism
have different duties, or, in other words, there is
division of labour.

The first or lower group of organisms are asso-
ciated together as the Protozoa; the second, or
those that come after them, form the division of the
JVIetazoa. Did we desire to use less objective terms,
we might adopt for these groups the corresponding
terms of Cytozoa and Histozoa (Maupas), which
conveniently direct attention to the essential differ-
ence in the cells of the protozoan, and the tissues
of the metazoan organism.

In attempting to arrange either of these divisions,
we are met at once by the fact that the changes which
have taken place in organisms have been in two lines
or directions ; there has been progress, and there
has been degeneration. The former we shall find
to be more intimately associated with a free and active
life, and a ready power of adaptation to changed cir-
cumstances ; the latter to a fixed and often to a para-
sitic mode of existence.

Chap, in.] GROUPS OF PROTOZOA. 25

I. PROTOZOA.

For our purposes we shall find it convenient to
divide the Protozoa into three great groups, one of
which has become degraded by parasitism ; these are
the Sporozoa, of which the best known division are
the Gregarinida ; the others, one of which is dis-
tinctly higher than the other group, may be called the
Sarcodina and the Infusoria.

Of the Sarcodina, the best type is the common
Amceba, which we have already studied ; like it, all
the members of the group move about and take in
their food by means of those movements of the proto-
plasm of the cell which result in the formation of
pseudopodia, and they reproduce themselves either by
division or by budding.

In the Infusoria the amcBbiform character is
lost, and the cell has and retains a definite form ; the
ectosarc ordinarily sheds out a structureless mem-
brane. This encloses the softer protoplasm which
makes up the rest of the organism, giving oft'
delicate processes which make their way through
the limiting membrane : these processes, or cilia,
are typically developed, are portions of proto-
plasm which retain their contractile power, and form
the chief means of progression. Owing to the pre-
sence of the covering membrane or cuticle^ it is neces-
sary that there should be at some point an opening in
the cell (cytostome), by means of which food may,
at any rate, enter ; this opening is ordinarily spoken
of as the mouth ; in addition to it there is sometimes a
second orifice developed, which has the function of an
anus (cytoproct).

The third division of the Protozoa are the de-
graded parasitic forms, of which the Gregarine is
an excellent example. Though these cells are covered
in by a distinct membrane, there is no orifice or

26 COMPARATIVE ANATOMY AND PHYSIOLOGY.

mouth by which the food can enter ; living as they do
in the digestive tract or other cavities of the bodies of
higher animals in which nutritious matter is abundant,
they obtain such food as they require by the mere

JFig. 2 A.— Gromia, showing the teat and the protruding
protoplasm.

physical process of osmosis. Similarly, having ceased
to lead a free life, and abiding now in closed spaces,
they have lost the cilia which were possessed by the
infusorian and exhibit instead a slow serpentine
movement which is effected by the ectosarc.

The Sarcodina are conveniently divided into three
great divisions :

Chap. III.]

GROUPS OF PROTOZOA.

27

I. Rhizopoda; example: Amoeba, Gromia, Nummu-

lites.

II. Heliozoa ; example : Actinophrys (Sun animal-
cule).
III. Radiolaria ; example : Acanthometra, Chilomma.

Fig. 2 B.— Actinophrys sol, showing the yacuolated ectosarc, the finely
granulated endosarc, the nucleus, contractile vacuole. and pseu-
dopodial filaments. (After Leidy.)

Leaving out of our consideration those simple and
incompletely known forms in which no nucleus is
developed in the protoplasm (Wlonera),* we may dis-
tinguish the naked Amoeba-like Rhizopoda from those

* It is possible that in such forms the nuclear substance is
diffused through the protoplasm (Gruber).

28 COMPARATIVE ANATOMY AND PHYSIOLOGY.

in which a covering or test is developed ; this test
may be chitinous (Gromia), or chitinous and calca-
reous, or, in rare cases, siliceous; and it may have either

Fig. 2 c. — Xiphacs/ntha, showing the siliceous skeleton. (After W.
TJUOIUSOH.)

a single large orifice (Fig. 2 A), or the test may be per-
forated with a number of holes (Foraminifera),

and may attain to a large size (Nummulites), and great
complexity of form.

Chap. III.]

GROUPS OF PROTOZOA.

29

The Heliozoa either have the body naked or a
siliceous skeleton is developed ; the body is very com-
monly spherical in shape, while the pseudopodia
(Fig. 2 B) are fine, alter but little in form, and rarely
anastomose with one another ; lastly, the Radiolaria
(Fig. 2 c) have a chitinous " central capsule," around
which flows the protoplasm, and with which there is

Fig. 3 I. — Paramcecium aurelia ; A, from the side ; B, from below ; c, two in

conjugation,
n, Nucleus ; b, mouth ; cv, contractile vacuole.

ordinarily connected a delicate and often elaborate
siliceous skeleton. The pseudopodia are less constant
in form than in the Heliozoa, and enter into anasto-
moses with their neighbours.

The Infusoria are ordinarily ciliated, but in
some (Flagellata) the cilia are replaced by a single
long whip-like process of protoplasm (flagellum)
(Fig. 3 ii.), and in others which are parasitic on
(ectoparasitic) the bodies of other infusorians, the
cilia are lost and replaced by tentacle-like sucking
tubes (Fig. 3 in.).

30 COMPARATIVE ANATOMY AND PHYSIOLOGY.

I. Ciiiata, as Paramoecium, Vorticella, and
others ; the cilia are either regularly distributed over
the cell, and are, for the most part, subequal in size
(Paramoecium) (Fig. 3 i.) ; or some are much larger
than the rest (Stentor) ; or the cilia are ordinarily con-
fined to a spiral circlet around the mouth (Vorticella),
and are only occasionally found on other parts of the

an

Fig. 3 ii.— A, Noetiluca miliaris ; B, with buds ; c, section.
n, Nucleus ; /, flagellura ; t, tentacle ; d, denticle ; an, anus.

body ; or, finally, they may be limited to the so-called
ventral surface (Euplotes) ; in the Peritricha, as the
group to which Vorticella and its allies belong is called,
there is often an elongated aboral stalk, which some-
times exhibits a remarkable power of rapid contraction.
II. Flagellata ; a number of forms are grouped
by some writers under this head ; of such as are
almost indubitably animal, Noetiluca (Fig. 3 n.), the
animalcule which causes much of the diffused phos-
phorescence of the sea, is one of the best known.

Chap. III.]

METAZOA.

Fig. 3 in. — Acineta
tuberosa.

III. Suctoria: in these parasites (e.g. Acineta,
Fig. 3 in.), the mouth is lost and the sucking tubes
protruded from the protoplasmic
mass serve to convey food into
the body. A study of their
development reveals the interest-
ing fact that they commence life
as ciliated embryos, and suggests
the idea that they are descended
from ciliate infusoria.

The Sporozoa will, for the
purposes of this book, be repre-
sented by the Gregarinida. The
forms best adapted for study are
the gigantic Gregarine found in
the intestine of the lobster, and
remarkable for being, though but
a single cell, as much as two -thirds of an inch in
length • and the much smaller species found in the
testicular reservoirs of the
earthworm.

II.— THE METAZOA.
-C STRUCTURE AND EARLY HIS-
TORY OF THE EGG-CELL.

The key to the structure
of the higher animals, or
Metazoa, is to be found in
a knowledge of the early
history of the egg from
which, as has been already said, they all arise.
This cell, when mature, consists of a mass of proto-
plasm (Fig. 4, c), with a central nucleus (b), and con-
tained nucleolus, and in most, though not in all cases
(Hydra), it has a definite investing membrane (a).
Under normal circumstances this egg-cell is fertilised

Ripe
(Afte

er Klein.)
a, Envelope ; b, nucleus ; c,
protoplasm.

32 COMPARATIVE ANATOMY AND PHYSIOLOGY.

by the male element (chap, xiii.), and then commences
to undergo a process of cleavage, or division. It first
divides into two cells, which are, in the simplest cases,
equal in size ; each of these again divides, so that there

Tig. 5. — Segmentation of Amphioxus.

A, Stage with two equal segments; u, with four; c, with eight; D, segments
enclosing a segmentation cavity ; E, somewhat c Ider stage in optical section.
(After Kowalevsky.)

are four, then eight, and so on. After a time the pro-
cess of segmentation (Fig. 5) comes to an end, and
then we have a mass of segments, which are either
closely applied to one another, and so have a kind of
mulberry-like appearance (hence the name of momla
applied to this stage) ; or, as is more common, the
segments separate from one another during the
process of division, and give rise within to a space,

Chap. TIL]

THE GASTRULA.

33

the segmentation cavity; the cells bounding
this cavity then undergo a further change, by means
of which the single becomes replaced by a double
layer, one of which is interior to the other.

This two-layered condition is brought about in
one of two ways ; either the cells of one half of the
sphere are pushed into the contained space, and, by
approaching the other half, more or less completely
obliterate the
segmentation
cavity, or the
cells undergo a
transverse and
concentric clea-
vage, by means
of which each
cell becomes
two, and the
single is con-
verted into a
double layer.
Whether the
former process
(that of iii-
vagi nation)
or the latter (delamiiiation) takes place, the cell-
layers are regarded as comparable, and receive the same
names ; the outer is known as the epiblast (Fig. 6, ep),
the inner as the hypotolast (hyp). Similarly the con-
tained cavity, which is clearly the segmentation
cavity in the latter mode, and an altogether new
formation in the former, is spoken of as the
arclieiiteroii, while the narrow opening to the ex-
terior is the blastopore (o). The whole organism is
now said to be in the Gastmla stage (Fig. 6).

No known animal remains at quite the low and
undifferentiated condition of a Gastrula ; and, indeed,
D— 16

Fig. 6. — Diagram of a Gastrula.

o, Blastopore; ep, epiblast ; hyp, Lypoblast,

34 COMPARATIVE ANATOMY AND PHYSIOLOGY.

in most cases yet another germinal layer, as the

epiblast or hypoblast is respectively called, is developed
between the two we know already. It is appro-
priately spoken of as the mesol*last; it arises in
various modes, into the distinctions of which we need
not enter here ; it will suffice for us to know, that
in all the higher Metazoa the greater part of the
organism is fashioned out of it.

In all cases the outer and inner layers undertake
the functions which their position entails on them ;
the cells of the epiblast become converted into the
parts which cover in and protect the rest of the body,
and give rise also to those organs by means of which
the organism becomes acquainted with what is going on
around it, sensory organs and nervous system.
The hypoblast remains always in connection with the
enteron, or digestive tract, forming the lining of its
walls, of the glands that are therein developed, and of
such outgrowths as may arise from it. In the lower
divisions of the Metazoa the mesoblast does not take
any large share in the formation of the organs ; it
remains in a more or less indifferent condition. In
the higher forms it becomes quite the most important
layer in the body, taking on as it does the duty of
developing the skeleton, the muscles, the blood, and
vascular system, the excretory organs, and the con-
necting tissues ; it always, also, becomes primarily
cleft or divided, so that a cavity is developed within
it ; this is the true body cavity, or ccelom, and all
animals that possess it may, whether they secondarily
lose it or not, be spoken of as the C«plomata.

The acoalomate Metazoa are the sponges (Pori-
fera), and the great group to which belong hydra,
the jelly-fishes, and the sea-anemones (Ccelenterata).
The simplest sponges show hardly any advance on
the typical Gastrula, the amount of mesoblastic tissue
developed being small; but they are remarkable at

Chap. III.]

SPONGES.

35

once for a character which sharply distinguishes from

all other animals. It happens to many Gastrulse

that, their blastopore closing up, they develop an

investment of cilia on their epi-

blast, and swim about for a

time freely in the water; but

these cilia are confined to the

outer surface. In the sponges

it is otherwise, the ciliated cells

early become internal to the non-
ciliated, and some are retained

throughout life in the so-called

" ciliated chambers." When we

come to examine into the activity
of a living sponge we find no
advance on that of a Protozoon,
save so far as the division of

labour is here first clearly seen ;

we find, that is, that the multi-

celluiai\ organism feeds, grows,

respires, reproduces itself, and

dies ; and we find, too, that, like

many Protozoa, it forms for

itself firm supports in the way

of a skeleton, but we find no

cells that are specially sensory,

and none that are obviously

muscular ; there is the general

irritability and contractility

which living protoplasm always exhibits, but there

are no special organs for either function.

The Porifera, or sponges, fall into the following
divisions :

1. Ittyxospoiigiae, in which there is no hard
skeleton ; e.g. Halisarca.

2. Calcispoiigiae, in which a support for the
body is furnished by calcareous spicules ; e.g. Ascon,

Fig. 7. — Calcareous
Sponge. AKcetta pn'mor-
ctoa7f*. (After Haeckel,
x SOdiams.)

36 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Leucon, Sycon. The commonest British form is
ordinarily known as Grantia. (See Fig. 7.)

3. Silicispongiae, in which part of the skeleton
is made up of spicules of silica ; e.g. the common
fresh-water sponge (Spongilla), Chalina, Euplectella.

4. Ceratospongise, in which the skeleton is

Fig. 8. — A, Hydra v 'ridis, attached to Duckweed ; B, a Single Specimen

magnified ; c, Hydra in Diagramatic Section,
cc, Ectoderm ; en, endodenu ; TO, mouth ; be, enteric cavity ; t, tentacle?.

completely horny or fibrous, and devoid of siliceous or
calcareous spicules ; e.g. the bath-sponges (Euspongia).

In the Cvceleiiterata it is otherwise; in many
forms both nervous and muscular tissues are to be
recognised not only by the aid of the microscope,
but by the activity of these animals, and by their
reactions when subjected to physiological experiment.

Henceforward, then, we have to do with forms
which possess, in some shape or other, all the
essential tissues of even the most complicated

Chap. III.] CCELENTERATA. 37

organisms ; differentiation will lead to greater sub-
division of labour, and greater complexity of struc-
ture, but all the materials are, even so low in tlie
grade of animal life, ready to our hand.

"Fig. 9. — Perigonimus vestitus, showing Trophosomes and Gonosoines
(After Allman.)

A ccelenterate animal, then, is one in which the
archenteron of the gastrula, even when secondary
outgrowths are developed from it, remains always as
the only cavity in the body, in which the mesoblast
is but imperfectly differentiated, but in which organs
of offence, locomotion, and sensation are added on to
the structures of the original gastrula form.

38 COMPARATIVE ANATOMY AND PHYSIOLOGY.

In its simplest known condition, e.g. Hydra
(Fig. 8), a Coelenterate has a terminal mouth (ra)
which leads into a digestive cavity (be), and around

which ten-
tacles (t) are
developed;
these tentacles,
which serve as
organs of pre-
hension, sen-
sation, and
offence, are
hollow, con-
tinuations of
the enteric
cavity passing
into them.
There is no
second orifice
to the enteron,
and reproduc-
tion is effected
either by gem-
mation, or by
the formation
of ova and
spermatozoa.

In the more
complicate d
members of

the group the hydriform body gives off buds, and be-
comes one of a colony (Fig. 9) ; and the separate
" persons " of this colony are connected together by a
common trunk, which is hollow within, and continuous
with the enteric cavity of each person ; in the simplest
stage of these colonial formations each person performs
the same duties, but in the more complex different

Fig. 10.— Figure of the Medusa of a Hydroid.
(After Hincks.)

Chap. III.]

MEDUSA.

39

persons take on different duties; when these, again,
are at their simplest stage, we find that while some
nourish the colony (tropliosomes), they take no
share in reproducing it; this office is performed by
other persons (gonosomes), which depend for their
nourishment on the neighbouring tropliosomes. Di-
vision of labour among the persons of the colony may
go still farther, and groups become formed of which
some have nutrient, others locomotor, others protective,
and others prehensile or offensive functions (Siphono-
phora; e.g. Portuguese man-of-war) (Fig. 12). Where
the Ccelenterate is fixed, we observe, in one division,
that the generative persons become free-swimming,
and, while retaining the
essential characters of the
division, become greatly
altered in form, in adap-
tation to their new mode
of life ; such persons are
spoken of as Medusae
(Fig. 10). Finally, we
find that, in some cases,
the fertilised ovum of a
medusa gives rise not to
a fixed hydra-like body,
but directly to a medusa
form. The tentacles are
set round the mouth in a
circle, and the parts of
the body are similarly
arranged in a fashion of
symmetry, which is called
radial ; where, however,
the free mode of life has obtained for a long period of
time, we sometimes find that there is only one axis of
the body on either side of which exactly corresponding
parts are to be found ; in other words, a bilateral

Fig. 11. — Longitudinal section
through Sagartia parasitica,
showing a meseuteric septum
with the body wall to the right,
and the enteric wall to the left.
(After O. and E. Hertwig.) (See
Fig. 54.)

4o COMPARATIVE ANATOMY AND PHYSIOLOGY.

takes the place of a radial symmetry ; e.g. Venus' girdle
among the Otenophora. (See Fig. 16, page 46.)

The Cceleiiterata fall into two well-marked
divisions, Hydrozoa and Anthozoa ; in the
former the mouth is placed 011 a projecting oral cone,
while in the latter it is sunk below the level of the oral
circlet of tentacles, and the cavity developed from the
enteron, and separating its wall from the body wall,
is traversed by partitions (meseiiteric septa)
(Fig. 11), of which a certain number extend across
the whole of the cavity, while others only project for
a shorter or longer distance into it.

CCELENTERATA.

A. Hydrozoa.— The hydrozoa fall into two well-
marked divisions, in the first of which the medusa
form, when developed, always has an infolded rim of
the body running round the inner edge of the mouth
of the bell (velum). In consequence of the presence
of this fringe it may be spoken of as the Craspedote
division ; in it the sense organs are never protected
by any lid or cover, and they are therefore known as
the Naked-eyed Medusse (Oymnophthalinata),
and as the generative sacs never form projecting
pouches, they are by some spoken of as Cryptocarpa.

I. Craspedota.— The Craspedota fall into three
groups ; in the first the organism is always hydri-
form. or the nutrient persons are hydriform, and
the generative medusiform, or the organism is always
medusiform. They may, therefore, be called Hydro-
medusae. Examples of these are : Hydra, Cordylo-
phora, Hydractinia, Sarsia, Oceania.

In the second group we have those colonies of
hydriform persons in which the common stem becomes
richly impregnated with calcareous salts, and they
therefore may be known as Hydroid Corals or Hydro-
< 01 alliii;r. Such are Millepora and Stylaster.

Chap. III.]

JELLY-FISHES.

In the third group
we have those free-
swimming colonies to
which reference has
already been made as
examples of the highest
form of division of
labour : they are called
the Siphonophora,
and Velella, Diphyes,
Physalia, and Physo-
phora (Fig. 12) belong
to this group.

Scypliomedusse.—
In the second great
division of the hydrozoa
we have the forms
which are best known
as the Medusae, or jelly-
fishes par excellence.
With one exception,
they all pass through
a stage which, at first
somewhat hydriform in
appearance (Scypliis-
toma-stage), is re-
markable for under-
going transverse di-
vision ; each of the
segments so formed
separates and forms an
independent medusa.
When adult they are
always medusiform in
appearance, and, as they
rarely have a velum to
their disc, they are

Fig. 12. — PhysopTiora hydrostatica.

a, Air-bladder; m, nectocalyx ; jjr, gener-
ative persons; ?»., nutrient, persons (in
the form of sucking tubes) ; t, tentacu-
lar persons. (After Cuvier.)

42 COKJ-ARAWE ANATOMY AND PHYSIOLOGY.

£. 13.-- Aurelia aurita.

large and obvious Th «eneptive glands are

Chap. III.]

ANTHOZOA.

43

an example of the
forms in which the
original mouth is lost,
and replaced by a
number of small aper-
tures developed on the
long arm - like out-
growths of its lips.

B. Anthozoa.—
Among the Anthozoa
we find the sea-
anemoiies and the great
bulk ol those cceleri-
terates which form
coral.

According as they
possess eight, and eight
only, or six, or some
multiple (often a large
one) of six, we divide
the Anthozoa into the
Octactiniae, and the
HexactiiiisB.

I. The Octactinise
have never more than
eight tentacles, and
these are flattened and
serrated at their edges.
In Alcyonium ("dead
men's fingers ") cal-
careous spicules are
scattered in the body ;
in Tubipora ("organ-
pipe coral") the spi-
cules collect and form
a continuous tube for
each polyp (Fig. 14 A) ;

Fig. 14 B.— Pennatula
(Pteroides) spinosa.

44 COMPARATIVE ANATOMY AND PHYSIOLOGY.

in the sea-pen (Pennatula) (Fig. 14 B), the tissue which
connects the polyps together is horny, in the noble red

Fig. 14 c. — Gorgonia fldbellum.

coral it is calcified, while in the sea-fans (Gorgonia) an
elegant hard network is developed (Fig. 14 c).

II. The Hexactinise ; the six tentacles or mul-
tiples of that number are filiform, and their edges
smooth. Some, like the common sea-anemone,
remain single throughout life, but, in most, buds are
given off, and a colony is formed. The deposition of

Chap. III.]

CTENOPHORA.

45

calcareous salts often gives rise to large masses of
" stony " coral, of which the brain-coral (Mseandrina)
is a good example ; in other cases (e.g. Fungia) ths
septa are alone calcined.

There still remains a division of the Ccelenterata
which, though it has been definitely placed by some
naturalists with the Hydrozoa,
and by others with the Antho-
zoa, is possibly an independent
group ; in these, the eight canals
derived from the enteron run
at equal distances close to the
surface of the body, and along
these there are formed bands of
cilia, which have, in consequence
of their comb-like appearance,
gained for these forms the name
Ctenopliora. The glassy globe
called Cydippe (Fig. 15) is found
on our own shores, while Venus'
girdle (Cestus veneris) is an ex-
ample of that acquired bilateral
symmetry to which we have already
referred (Fig. 16).

Fig. 15.— Cydippepileus.

THE HIGHER METAZOA.

In the remaining Metazoa a cavity distinct from
the archenteric cavity becomes developed, and the
mesoblast becomes the seat of those important changes,
by means of which nearly all the tissues of the body
are derived from it. In the midst of this mesoblast
a cavity arises by cleavage or fissure, or from the
archenteron there are given off out-growths which,
in time, become shut off from the parent space, and
occupy the middle of the mesoblast. The cavity
formed in either of these ways is spoken of as the body
cavity or ceelom, and the result of its appearance is

46 COMPARATIVE ANATOMY AND PHYSIOLOGY.

that the mesoblast becomes separated into two layers,
one of which applies itself to the epiblast, and
the other to the hypoblast ; in this way we get
the somatopleure and splanclmopleure of

Fig. 16.— Venus' Girdle (CeJus veneri*),

embryologists. All the Metazoa that possess this
body cavity may be spoken of collectively as the
Cflelomata. In some cases the coelom remains
throughout life in a very rudimentary condition, and in
a few it cannot be said to be developed at all, while in
others it would seem to have been lost by degeneration.
According to its mode of origin, as an out-growth
from the enteron, or by cleavage of the mesoblast, it
is spoken of as an enteroccele, or a scliizocoele.

Chap, in.] METAZOA. 47

The archenteron ordinarily closes up, so that the
blastopore disappears ; a fresh mouth, and in most
cases also, an anus, are developed at either end of
the tube ; these are lined by inpushings of the epi-
blast; the epiblastic pits, deepening and elongating,
finally become continuous with the original or arch-
enteric cavity, which is, it wil] be remembered,
lined by hypoblastic cells. In a fully developed di-
gestive tract we have now to distinguish three regions :
(1) a mouth passage (stomodceum) which is lined
by epiblast ; (2) a mid-intestine (mesenteroii) lined
by hypoblast ; and (3) an anal passage (procto-
<8<rum) lined again by epiblast.

The greater number of the Metazoa are free
animals, and no doubt the ancestors of all the terres-
trial were aquatic forms; organisms moving freely
in such a medium as water would clearly have
one end which was anterior and one which was pos-
terior, and as these would be differently affected by
the water through which they moved, the one end
wrould become differently constituted to the other ; the
anterior end would be that at which food would be
taken in, and at which the prey or an enemy would
l>e first met. This end would then be primarily the
sensitive end, and we find that it is here that sense
organs of various kinds are best developed. In other
words, we have henceforward to look for a definite re-
gion, specially sensitive in function, developed in front
of the mouth ; this may be called the praBStomiiim.

On either side of the moving body the water would
exert equal pressure, and the two sides would come to
exhibit similar characters, or bilateral symmetry
would become apparent. In shallow waters one as-
pect of the body would be more exposed to the in-
fluence of light than the other, and we should there-
fore distinguish between an upper or dorsal and a
lower or ventral surface.

48 COMPARATIVE ANATOMY AND PHYSIOLOGY.

The origin, then, of the higher Metazoa is to be
looked for in an animal in which an anterior end with
a prsestomium is to be distinguished from a posterior
end ; in which the two sides are similar to one another,
and the dorsal slightly different from the ventral
surface. Forms of this kind are still to be found
among the lowest Worms.

Various organs must, of course, be developed with-
in such an organism ; in the simplest cases some of
the cells of the hypoblast retain the power possessed
by the Amoeba of taking solid food into the substance
of their own bodies ; the organism being small, no
special means of circulating the nutriment thus
obtained are required ; and, just as in the Amoeba,
respiration is carried on by the general surface of the
body, and by the water brought in with the food. On
the other hand, even in Amoeba, we found a con-
tractile vacuole, and we may, therefore, well suppose
that in this complex of cells there must be some
special means for the removal from the body of its
waste nitrogenous products. At any rate, the meso-
blast is on either side channelled by a delicately
walled canal which has openings into the spaces in
the mesoblast, and communicates by a pore with the
exterior. As the organism is to give rise to cells from
which other organisms are to arise, some part of
its body must be set apart as generative cells ; in
the simplest cases these are mere masses of cells in
simple pouches, which pass directly into the water.

Of the cells in the region of the praestomium
some will be more particularly modified for the
reception of impressions from the outer world, and will
form a rudimentary nervous mass, with which a few
nerve-fibres will be connected ; as the creature is
capable of moving from place to place, we have,
further, to look for the presence of muscular
tissue.

Chap, in.] FLAT- WORMS. 49

The lowest Metazoa are grouped into a somewhat
heterogeneous mob, which is known as the division of
the Verities or Worms. Of these the lowest are the
Flat-Worms.

A. Platyhelmintlies. — Of the three divisions of
flat-worms, two are degraded by parasitism ; such are
the divisions to which the tape-worms (TaBnia), and
the flukes (Distomum) belong.

I. The Turfoellaria are the simplest forms, and
are free living; the body is soft and small, covered
with cilia, and without an anus ; the entrance to the
digestive tract is often provided with a proboscis, and
the generative apparatus may be simple, or may be
greatly complicated. A distinct crelom is not always
apparent (Accela), or it may become secondarily
obscured. The enteric tract is straight, or branched.
Planaria, Dendrocoelum, and Mesostomum are ex-
amples of this division.

II. The Trematoda are flat-worms that have
taken to a parasitic mode of life, but are by no
means so profoundly modified as the members of the
group next to be considered. They either live on
the bodies of other animals (ecto-parasitic), like
A spidogaster, which is found in the gill chamber of the
fresh-water mussel ; in this case they exhibit no
" alternation of generation." Or they live within the
bodies of other animals (pento-arasitic), like Disto-
mum hepaticum (the liver-fluke) ; in this case they
pass different stages of their existence in two different
animals. The ciliated covering is lost, and suckers are
developed, which serve as organs of attachment, and,
to a certain degree also, as organs of locomotion ; the
sexes are ordinarily united in the same individual, and
the accessory parts of the generative apparatus are
greatly complicated.

III. The Cestoda, or tape-worms, are flat-
worms which are still further modified in accordance

E— 16

50 COMPARATIVE ANATOMY AND PHYSIOLOGY.

with their constantly eiito-parasitic habit of life, and
they, like the endo-parasitic Trematoda, ordinarily pass
through different stages of their development in
different hosts. While the simplest forms, like the
Caryophyllaeus of the carp, exhibit no kind of jointing
or division of the body, and Ligula has the jointing
affecting only the internally placed generative organs,
most consist of a more or less large number of joints ;
Tsenia echinococcus having three or four,T. solium about

Fig. 17.— Tomia, showing the head and four suckers,
the unjointed neck, and the early joints (Strohila).

a thousand, and Bothriocephalus latus having, it is said,
as many as 10,000 joints, and attaining to a length of
twenty-five feet (Fig. 17). As these joints increase in
size and approach maturity, the ova become fertilised,
and commence to develop ; on the joints breaking off
and escaping to the exterior, the ova within are set
free, and if eaten by the other host proper to the
tape-worm, they go through the earlier stages of
their development within its body. In these parasites
the digestive tract is altogether aborted.

We have been carried away by these degraded
forms from the general line of development ; we return
to it, however, only again to find ourselves confronted

Chap. III.]

ROUND- WORMS.

with a group, the great majority of the members of
which are, in their sexual state at least, endo-parasitic.
These are the round-worms or thread-worms (Nemato-
lielmi lit lies). They are remarkable, as compared
with the soft-bodied Turbellarians, for the great
development of that horny material which is, as chi-
tin, so richly present in the integuments of many
Metazoa. The intestine forms a straight tube, and
is surrounded by a comparatively spacious body cavity.
The whole body is, as
their popular name implies,
greatly elongated. Examples
of this group are Gordius,
Ascaris, Filaria, and Tri-
china.

More closely allied to the
round- worms than to any
other worms are the Acan-
thocephali, of which Echi-
norhynchus (Fig. 18) is an
example. They are internal
parasites, which, like most
tape-worms and flukes, live,
at different stages of their
life-history, in different hosts. They are provided with
a protrusible proboscis, which is armed with recurved
hooks of considerable strength.

The Rotatoria or Wheel- Animalcules exhibit
certain characters which we shall again meet with in the
larval stages of some of the higher forms. The anterior
end carries a disc, the edge of which is ciliated (this is
the so-called "wheel-organ"), and in the centre of
which the mouth is placed (Fig. 19). A special
apparatus for comminuting the food is found in the
stomach. The " water- vessels," or organs by means
of which, in all probability, waste nitrogenous matters
are excreted, are very distinct, and are provided with

Fig. 18.— Echinorliynclius no-
dulatus (nat. size and en-
larged). (After Busk.)

52 COMPARATIVE ANATOMY AND PHYSIOLOGY.

delicate branches with terminal orifices ; the two
vessels open into a special enlargement or bladder, the
walls of which are contractile, so that the fluid stored
up in it can be forced to the exterior. The sexes, as
in Nematoids, are ordinarily separate, and the males
can be distinguished from the females by their smaller
size. Rotifer, Brachionus, and Melicerta are examples
of the Rotatoria.

Most of the forms with which we have already
had to do are small in size, and
it will have been noted that, where
the body attained, as in the case
of certain tape-worms, to a con-
siderable length, that body was not
an individual whole, but was broken
up into joints or segments.

In the great group of worms
which we are now going to consider,
this segmentation of the body is
very distinctly exhibited, and affects
not only the external form, but the
great majority of the internal
organs ; this phenomenon becomes
the more comprehensible when we
learn that at one of its very earliest
stages in development the mesoblast
itself becomes regularly segmented. In. the simpler
conditions the segments, which we will henceforward
call metameres, are, for the greater part, exactly
similar in character, and only those at either end of
the body differ much from the rest. Later on we shall
see that, just as in the simpler animals, different parts
take on different duties, and division of labour becomes
as apparent among the metameres as it was in the
various persons of the colonial Coelenterata.

Now, also, we find that organs for which, in the
smaller and simpler forms, there was no necessity,

Fig. 19.— Brachionus;
to show the Ciliated
head-disc.

Chap, in.] HIGHER METAZOA. 53

gradually become elaborated. The body is now too
large be to able to do without an apparatus by means of
which the nutrient material obtained by digestion, or
the store of oxygen necessary for the activity of the
protoplasm of its constituent cells, may be carried
about from part to part, and we have therefore a
system of circulating vessels. In many, also, the
iirm covering of the body necessitates the develop-
ment of special outgrowths into which the vessels
pass, charged with the carbonic acid which is con-
stantly associated with the activity of living proto-
plasm ; in these outgrowths the blood gives up
carbonic acid, and receives oxygen in its place ; in
other words, a respiratory is added on to a
circulatory apparatus. In the majority, again, the
body is too large to be able to move about without
the assistance of special muscular processes or limbs,
and these are not unfrequently strengthened .and sup-
ported by those chitinous secretions which we call
setae (bristles).

Elaborate and complex activities of such a kind
as these require to be brought into relation with one
another, or, in other words, to be co-ordinated,
and performed in regular and systematic fashion ; it
is not now sufficient for the organism that there
should be a prsestomial nervous mass with some few
nerve-fibres given off from it. Centres of nervous
activity must be developed in various parts of the body,
and we find, therefore, that collections of nerve-cells
are found in different metameres ; these ganglionic
masses are connected together by fibres, and so
it results that there runs down the ventral surface
of the body a chain of ganglia. From each
of these ganglia nerve-fib'res pass to the muscles
and other organs of the body, and to them there
come other fibres which have one end in the
skin, and which convey to the central apparatus

54 COMPARATIVE ANATOMY AND PHYSIOLOGY.

some information of what is going on in the world
around it.

General sensibility of this kind is, however, soon
found to be insufficient for the needs of the organism ;
sight and hearing are possessed, no doubt, by lower
forms, but we shall soon find creatures with elaborate
eyes, and well-defined auditory organs, while obscure
indications of an olfactory sense are, a little later,
to be detected.

The organisation of the ringed worms or Amui-
lata attains its highest degree of complexity in the
free-swimming marine forms. Here the ringed body
has on most of its metameres a single or double pro-
jection on either side (parapodium), from which
there project a number of bristles (set«) ; at the
anterior end, the tentacles are aided by a number of
elongated feelers, and a pair of well-developed eyes,
and sometimes, too, auditory vesicles are to be found
there. The mouth is provided with strong horny
denticulated jaws, which are moved by special
muscles, and which serve to break up the food; dif-
ferent parts of the digestive tract take on different
functions, and pouches, which may again be branched,
sometimes appear at the sides. A fluid circulates
through the body in a system of closed vessels, and
some of these vessels have their walls provided with
muscles by means of which the current, which is
always regular in direction, is propelled onwards.
At the sides of the body thin outgrowths of its wall
serve as gills (branchiae), and most of the meta-
meres are provided with a pair of coiled tubes which
open into the spacious crelom, and also to the
exterior ; these are the renal organs (nephridia).

The division of ringed* worms in which the setze
are numerous on each parapodium is called the
Polychseta; of these, some, like the sea-mouse
(Aphrodite), Polynoe, and Nereis are free-swimming,

Chap III.]

CH&TOPODA.

55

Fig. 20.— Tenbella emmalina.

and form the group of the Vagantia ; others give up
their free mode of life and settle down like Sabella
and Serpula into tubes ; in these TuMcolse (Fig. 20),

56 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the hinder part of the body is less elaborately de-
veloped than the anterior, which can be protruded
from the mouth of the leathery, sandy, or calcareous
tube. The lowest forms of the division have no setse
at all, and Polygordius, which may be taken as the re-
presentative of the Achseta, retains throughout life a
circlet of cilia at its anterior end.

In another and lower division of the Annulata we
find that the setse are never more than eight at the
most in each bundle ; and such forms may be distin-
guished from the Polychseta, and known as the
Oligochaeta. Of these the best known form is
the common earthworm (Lumbricus), but all are
not, like it, terrestrial in habit ; Nais and the
blood-worm (Tubifex) are inhabitants of fresh
water.

Most appropriately, perhaps, associated with the
Annulata, but exhibiting a number of characters that
bring them into relation with the flat- worms, are the
leeches or H imdinea ; living on the blood of other
animals, as many of them do, they have the integu-
ment often developed at one or two points into
suckers, by means of which they attach themselves
to other animals, or to firm bodies, from which they
can extend themselves to seize or attach themselves
to their prey.

Most closely allied to the Annulata, but best kept
in a separate division, are those marine worms of
which Sipunaulus is the best known example ; for
these the old term of Oepliyrea may be retained,
without prejudice to our views of the value of the
ideas which gave rise to the name. The body ex-
hibits no external segmentation ; they are remark-
able for possessing excretory organs of the kind
found in the Annulata, as well as those seen
in Rotifers ; in some cases the anus is not at
the hinder end of the body, but the intestine is so

Chap. III.] GROUPS OF HlGHER METAZOA. 57

coiled on itself that its orifice comes to lie at the
side, and in the anterior half of the body.

The difficulties arising from our imperfect know-
ledge, and the generalised characters of the lower
forms which are associated together under the head of
the Vermes, disappear, for the most part, when we
rise above them in the scale of animal organisation.

No one, for example, can fail to see that a starfish
is no close ally of a crayfish, or a snail of a frog ; on
the other hand, a sea-urchin and a starfish are as
clearly allied to one another as is the crayfish to the
crab, the mussel and snail to the octopus, and the
shark to the frog, the pigeon, or the rabbit.

While the bases or origins of these several forms
are obscure enough, the apex stands sharply out, and
we may compare the four series of forms of which
mention has just been made to four great branches
arising from a common trunk. Each of these branches
may be called a phylum. In one the body wall
becomes richly impregnated with calcareous salts,
which sometimes form projecting spines, the original
bilateral symmetry yields to an acquired radial one,
and locomotion is typically effected by a special series
of suckers connected with a system of water-tubes ;
this is the phylum of the Echinodermata or star-
fishes. In another the soft body becomes invested in
and protected by a hard shell which is secreted by a
special outgrowth of the body called the mantle ;
the ventral surface is drawn out into a muscular foot,
and a series of delicate filamentous processes grow out
on either side of the body ; this is the phylum of the
Mollusca, or shell-fish.

In yet another series we find a closer resemblance
to the Annulata than is exhibited in any other of the
higher phyla. Some or all of the metameres become
provided with appendages, which are most often
jointed, and one or more of these pairs of appendages

58 COMPARATIVE ANATOMY AND PHYSIOLOGY.

become specially modified to the purpose of the
mouth. This phylum, which we will call that of the
Arthropoda, might, if constancy of nomenclature
were not a matter of convenience, be more appro-
priately designated as the Cnathopoda (Lankes-
ter). Lastly, there is an important phylum for
which, in the light of recent researches, it seems well
to adopt some other name than the ordinary desig-
nation of Vertebrata. This phylum is remarkable
for the development along the dorsal area of a rod,
which, at first hollow, subsequently becomes solid,
and forms a primitive and, in some cases, permanent
support for the overlying nervous system. In recog-
nition of the presence of this cord we will speak of
the phylum as that of the Chordata ; here are in-
cluded the degenerated Tunicates, the primitive and
somewhat modified Lancelet (Amphioxus), and the
great group of fishes, reptiles, birds, and mammals in
which a vertebral column, more or less well de-
veloped, encloses and protects the spinal cord ; these
are the true Vertebrata (Balfour).

It is a matter of little importance which of these
phyla is first considered in greater detail, but, as the
most aberrant are the Echinodermata, it is, perhaps,
convenient to dispose of them first of all.

One of the best known types of the Echino.
dermata is presented to us by the starfish (Asterias),
in which no bilateral symmetry is at first apparent in
the adult, though it is quite well marked in the larva.
There is a central rounded disc from which are
given off five rays or " arms ; " in other words, we
have the bilateral symmetry overshadowed by an
acquired radial symmetry (Fig. 21). On the prin-
ciples on which we have already worked, this mode
of symmetry in a freely moving animal is not, at once,

Chap. TIL] ECHINODERMS. 59

explicable. To understand it we must make use of
the method of comparison, and appeal to palaeonto-
logical evidence. When we do this we find that the
oldest forms were, like the still extant Pentacrinus,

Fig. 21. — Astropecten irregularis.
7w,Madreporite.

fixed on a stalk (Fig. 22) ; in other words, the ances-
tors of the Crinoids being fixed forms had to develop
their organs in different directions around a common
centre, so that, from whatever point prey or enemy
approached them, they would be prepared for and
ready to meet them.

In the great majority of this group we observe
for the first time among the coelomate Metazoa a
hard supporting structure to which we can apply the

60 COMPARATIVE ANATOMY AND PHYSIOLOGY.

term skeleton"; this skeleton consists of a large
number of firm calcareous plates closely soldered

together. Within, or just
outside, these plates there
runs down every arm, or
branch of an arm, a canal
which contains water, and
from this canal there are
given off more or less deli-
cate tubes (the so-called
til toe-feet) which are
connected with the canal ;

Fig. 22.— Pentaorinus Wyville-fhomsoni. (After Wyville Thomson.)

all the canals communicate with one another by
means of a ring which surrounds the mouth. Owing
to the appearance presented by a dried starfish the
earlier naturalists spoke of the areas in which these

Chap. III.]

ECHINODER MS.

61

tube-feet were placed as the "walks" or ambulacra,
and we may, therefore, speak of the ossicles or plates
which specially support and protect the tube-feet as
the ambulacra! plates or ossicles. Accom-
panying the radial water-vessel is a nerve-trunk and

an

Fig. 23. — Diagram of a Cross-section of an Arm of a Common Starfish
(Asterias rvbens).

On the left side the section is supposed to pass between two of the ambulacra!
ossicles, but on the right side through one of them (ao) ; ag, ambulacra!
groove ; n, radial nerve ; b, radial blood-vessel ; w, radial water-vessel ; a,
ampullae ; t, tentacles or suckers; ap, adambulacral plates ; sp, spines ; pax,
paxillie, arising from limestone plates ; or, ovary ; gp, genital jpore ; gv,
genital blood-vessel ; br, respiratory processes ; pc, caeca of the intestine.
(After P. H. Carpenter.)

a blood-vessel ; while in the arm of the starfish we
find also generative sacs, and processes of the digestive
tract ; all of which enter, like the water-system, into
the cavity of the disc.

If, therefore, we make a transverse section (Fig.
23) throughout the arm of a starfish at a short
distance from the disc we should cut through digestive,
circulatory, ambulatory, generative and nervous

62 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Fig. 24.— Pentacrmoid Larva of
Antedon.

A, Quite young larva, before the open-
ing of the cup, and the appearance
of the radial plates; B, Nearly
mature : 6, basals; o, orals; r, first
radial s. (After Carpenter.)

organs, or should, in other
words, have before our
eyes representatives of all
the more important sys-
tems of organs in the body.
It is this phenomenon
which has led to the
theory once held by Cuvier,
re-presented by Duvernoy,
and, in our times, sup-
ported with much vigour
by Haeckel, that the Echi-
noderm is a colony of
bilaterally symmetrical
metazoic animals which
have become connected to-
gether by their anterior
ends. It is more in
accordance with the facts,
as at present known to
us, to suppose rather that
the radiate form has been
brought about by a return
to a fixed habit, and that
this mode of symmetry
has been retained by in-
heritance. In those forms
which stand farthest from
the Crinoids the radial is
again obscured by a
secondarily acquired bila-
teral symmetry (Spatan-
gus, Synapta) ; a close in-
vestigation into the char-
acters of most members of
the phylum enables us
to distinguish a plane

Chap. III.] ECHINODERMS. 63

which exactly divides the body into two similar
halves.

The Echinoderniata are sharply divisible into
two grades ; in the lower of these the animal is either
fixed by a stalk throughout life, or, as in the case of
the Rosy Feather star (Antedon rosacea) of our own
shores, the larva is fixed by a stalk (Fig. 24). This
grade may be called that of the Pelmatozoa ; to
it belongs the order of the Crinoidea, with others now
extinct ; representatives of it are Rhizocrinus, Pen-
tacrinus, and Antedon.

In the organisation of these forms attention should
be directed to the presence of the cuplike central
portion ; this calyx consists essentially of a central
plate and two sets of alternating plates five in num-
ber ; these are the foasals and the radials.

In the higher grade of the Echinodermata, the
Echinozoa, these plates are often obscured. In the
regular Sea-Urchins (Echinoidea) the two sets of
five plates can always be made out, but the central
plate is excavated to make room for the anus ; five
of the plates become perforated by the genital ducts
(basals), while the other five (radials) are similarly
perforated by the ocular tentacles. Cidaris, Echinus,
Echinometra are examples of the regular Echinoidea ;
by Clypeaster and the flattened Laganum we pass to
the edentulous Spatangidse, where a secondary bi-
lateral symmetry becomes very apparent.

In the true starfishes (Asteroidea), of which
Asterias, Linckia, Oreaster, and Astropecten are
examples, and in the Ophiuroidea, of which Ophiura,
Ophiocoma, and Ophiothrix are representatives, the
calycinal plates are often obscured, and the ambulacral
suckers are limited to the lower surface of the body
and do not extend, as in Echinus, from mouth to
apex ; in the latter the ambulacra are covered in by
a ventral plate, and in one division (that of the

64 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Astrophytidse) the arms become more or less
branched. Lastly we have the class of the Holo-
thuroidea, which are more nearly allied to the
Echinoids than to the Asteroids ; in these all signs of
the calycinal system have disappeared, the calcareous
skeleton is greatly reduced, and often consists merely
of scattered and minute calcareous plates, which are
sometimes altogether absent. In many cases the
tube-feet cease to be arranged in five regular rows,
and may, as for example in Synapta, disappear alto-
gether ; when this happens there remains no external
Character which speaks to the five-rayed ancestry of
these extreme forms ; in other words, here again
external bilateral symmetry is re-acquired. Holo-
thuria, Cucumaria, Synapta, are the best known
examples of this group.

It is impossible to escape from the belief that the
Artliropodsi are more nearly allied to the Annulata
than to any other group of the worms, but they are

Fig. 25.— Peripatus capensis.

Showing the elongated bilaterally symmetrical body, with the ringed antenna?,
and the incompletely jointed paired appendages with a pair of terminal
claws.

sharply distinguished from them by the fact that, in
all cases, one or more of the appendages of the body
are converted into organs which may be called mouth-
organs, jaws, or gnathites. Some idea of the primi-
tive form may be gathered from Peripatus, which
is the simplest Arthropod known to us. The body
was elongated, distinctly bilaterally symmetrical, the
praestomium was provided with tactile antennae, and

chap, in.] ARTHROPODA. 65

at the sides of the body there were a number of
appendages which were only incompletely ringed, but
the presence of which afforded evidence of metameric
segmentation. The mouth was near, though not
quite at, the anterior end of the body, and at its side
were a pair of slightly modified appendages ; the
anus was posterior and terminal. The excretory
organs were on the type of the Annulata, and were
arranged metamerically. Peripatus may form the
type of the Protracneata.

In all the remaining Arthropoda, some of which
in all probability did not have a Peripatus-like an-
cestor, but have acquired a form similar to that of the
descendants of such an ancestor, owing primarily to
similar external conditions and similar necessities of
life (homoplasy, see page 12), the appendages are dis-
tinctly jointed, so that the separate parts can be
moved on one another ; the mouth is often some way
from the anterior end, and excretory organs of the
annulate type are never found.

In the simpler forms the greater number of meta-
meres remain distinct, but in all divisions there is a
marked tendency for the metameres at the anterior end
to fuse into a head, and in some cases also into
a thoracic region.

They are divisible into three great groups : A.
Crustacea, B. Arachnida, C. Tracheata. In
all three chitin is largely developed in the integument ;
and they are all, in addition, remarkable for the total
absence of those delicate protoplasmic processes which
we have learnt to know as cilia.

A. The great majority of the Crustacea are
aquatic forms, and they either breathe the oxygen dis-
solved in the water in a vague manner (that is to say,
no special respiratory organs are developed, and the
exchange of gases is effected through the walls of the
body), or they are provided with outgrowths of the

F— 16

66 COMPARATIVE ANATOMY AND PHYSIOLOGY.

body wall, which are known as gills or branchiae ;

the presence of these has caused the name of
Branchiata to be given to this division of the Arthro-
poda. The greater number of the segments carry a
pair of appendages, and the great majority of these
are, in the lower forms, exactly similar in character
(Fig. 26, 5a). The metameres remain separate, and are

Fig. 26. — Various Branchiopoda.

1, Nelmlia bipes (shell removed on one side); 2, Estlieria sp. ; 8nt 'dorsal; S6,
ventral aspect of Lepidurus angassi ; 4, larva of Apus cancifornus ; 5«, adult
female of Branchipus stagnalis ; 5b, 5c, larvif ; 6, larva of Artemia saliua.

often very numerous ; in the higher forms they tend,
in a most remarkable manner, to be limited to about
twenty, and the dorsal parts of the hard exoskeleton
become fused in the anterior region (Fig. 27).

In all, the mouth is moved so far back from the
anterior end of the body that two pairs of appendages
(antennae) lie in front of it.

They are divisible into the Eiitomostraca, so
called from the slight amount of fusion of the

chap, in.] CRUSTACEA. 67

exoskeleton of the separate metameres, and the
Malacostraca, which were so called because their
covering is soft as compared with the hard shell of
the oyster or the snail. In both divisions we find
members which .have become parasitic in habit, and

Fig. 27. — The Common Prawn (Palcemon serratus).

in which, consequently the characteristics of ar-
thropod organisation are more or less modified and
obscured.

In the Entomostraca we never have more than
three pairs of appendages converted into Onatliites,
or jaws; the appendages behind the genital orifices
never carry appendages (Fig. 26; 5«), and the young
nearly always make their appearance as unsegmented
larvae with two or three pairs of appendages, of
which two are constantly biramose (Naiiplius
larvae) (Fig. 26 : 4, 56, 6).

1. The Brancliiopoda have, as their name

68 COMPARATIVE ANATOMY AND PHYSIOLOGY.

implies, the function of respiration undertaken by some
of the appendages ; the body is often provided with a
fold (Fig. 28 ; 1, 2), which is derived from the dorsal
portions of the anterior metameres, and forms a back-
wardly-directed free carapace. Such are Apus and
Daphnia ; Nebalia forms a link of connection with the
Malacostraca.

1, Daphnia pulex ;

Fig. 28. — Various Entomostraca.

; 2, Candona hispida; 3a, adult female of Cyclops quartri-
pornit; 6, c, d, larv;«; 4, Cetochilus septentrionalis ; 5, Sappbirina ovato
lanceolata ; 6. Nicotboe astaci (parasitic on tbe gills of the lobster) ; 7, Nau-
plius stage of copepod. (From Woodward.)

2. The Copepoda have a small stout body,
covered by a carapace ; one pair of the antennae are
large and oar-like (Fig. 28 ; 3a), and retain the primi-
tive locomotor function that they had in the
nauplius stage. Cyclops and Cetochilus are free-
swimming forms ; some, like Sapphirina (Fig. 28 ; 5)
are temporary parasites ; others, like Nicothoe(Fig. 28;
6), which lives on lobsters and crayfishes; Dichelestium,
which is found on the sturgeon, and Lernsea, which lives
on the cod and other fishes, are still more modified ;

Chap. III.]

CRUSTACEA.

69

while the extreme modification is seen in Argulus, a
common parasite on the stickleback.

3. In the Ostracoda the carapace forms a com-
pletely bivalve shelly covering for the body, the
abdominal region of which is greatly reduced. Cypris
arid Cythere are examples.

4. Although the Cirripedia are, when adult,
greatly altered by their fixed or parasitic habit, they
leave the egg as Naupliiform larvae ; these become
attached by their anterior

ends, and enclosed in a
sac-like mantle formed by
the integument ; this
either remains soft, as in
Alcippe, which lives in
cavities, and is thereby
protected, or undergoes
calcification, when a
greater or less number of
plates become developed.
The anterior region is
either broad, as in the
acorn shell (Balanus), or
drawn out into a stalk, as
in the barnacle (Lepas).

5. The Ceiitrogo-
iiida, or, as they are
often called, Rliizo-
cepliala, are usually
found on the bodies of
higher Crustacea after the
nauplius stage is passed.
They are endoparasitic,

and, later on, form a sac without limbs on the outer
surface of their host's body. To this group belong
Peltogaster and Sacculina.

B. The Malacostraca have almost constantly

Fig. 29.— Squilla mantis.

yo COMPARATIVE ANATOMY AND PHYSIOLOGY.

twenty segments to their body, and all but one of
these bear appendages ; as many as six may be con-
verted into
gnathites, and
the larvae ordi-
narily, though
not always, are
set free at a
later than the
nauplius stage.

l.The Pod-
optitlialmata
are so called
from the fact
that their eyes
are placed on
stalks (Fig. 29);
in them some of
the dorsal por-
tions of the
thoracic m eta-
meres take part
in the formation
of a carapace.
Such are cray
fishes, lobsters,
shrimps, and
crabs.

2. The He-
d ra op lit li al-
ma ta have the
eyes sessile, and
no carapace is
developed ; the
A in phi pod a (e.g. sandhopper) are the least modi-
fied ; some of the Isopoda (such as the wood-louse)
are fitted to and do dwell on land, while the

Fig. 30. -Limulus moluccanus.

Chap. III.]

Fig. 31.— Scorpio occitanus.

72 COMPARATIVE ANATOMY AND PHYSIOLOGY

Laemodipoda are modified by parasitism, and have
the abdominal region rudimentary (e.g. Cyamus, which
is found on the skin of whales).

Fig. 32.— Ammothoa pycnogonoides.

B. The Arachnida are arthropods, in which the
mouth is never placed so far back that any of the
appendages become antennary organs ; the second and
succeeding four (at most) pairs of appendages have
their basal portions ranged round the mouth, the
functions of which these parts subserve. The free
portions of the six anterior appendages take on various

Chap. III.]

ARACHNIDA

73

duties. Respiration is effected by flattened processes
attached to the appendages behind the generative
pores (which are
always placed com-
paratively far for-
wards), and they
either carry blood
or contain air, or
disappear and are
replaced by tracheae.
The hinder part of
the body never
carries jointed ap-
pendages.

1. Haemato-
branctiiata. —
These are to-day
represented by the
king-crab (Limulus ;
Fig. 30). In them
the respiratory la-
mellse contain blood,
and the hinder por-
tion of the body is
fused into a single
mass, while the ter-
minal spine is of
great length.

2. jErobran-
ctiiata. — Such are
the scorpion (Scor-
pio; Fig. 31) and the
spiders (Mygale). In

flipop fhp rpcnriratrvrv A» Female, nat. size; B. male, nat. size ; o,

inese T-ne respiratory head ()f lualei eniarged.

lamellae are sunk into

depressions of the body, and contain air (the so-called

lungs or lung-books). The hinder portion of the

Fig. 33. — Pentastomum tcenioides.

74 COMPARATIVE ANATOMY AND PHYSIOLOGY.

body is either elongated and distinctly jointed, with a
short terminal spine, or is greatly contracted, and
globose in form.

3. Upoforanchiata. — In these the respiratory
lamellae are lost, and if any special respiratory organs
are developed, they are in the form of tracheal tubes.
Here belong the Acarina (mites, ticks), with an
nnsegmented abdomen, and often a sucking mouth;
the Pedipalpi (Harvestmen), with a segmented
abdomen; and the Pyciiogonida (no-body crabs),
in which prolongations from the gastric cavity extend
into the enormously long legs (Fig. 32).

Appended to this group, but considerably altered
by parasitism, so that when adult they have elongated
worm-like bodies, with but two pairs of mouth hooks
to represent the appendages, are the Peiitastomida,
the best known example of which is the Pentastomum
tsenoides, which is found in the frontal sinuses of
dogs' skulls (Fig. 33, A, B, c).

C. The third division of Arthropoda is that of the
Tracheata ; in them there is always one pair of
antennae in front of the mouth, the gnathites may be
very profoundly modified ; respiration is effected by
means of air tubes (tracheae), which are regularly
arranged and richly developed within the body. They
are divisible into a lower and a higher group, of which
the former has comparatively few representatives ;
the other more than all the rest of the animal
kingdom.

I. Myriopoda or Centipedes and Millipedes. —
In these most of the metameres are separate and
distinct, or are united by pairs, and all are provided
with a pair of jointed appendages. The mouth organs
are not greatly modified ; they are all terrestrial.

II. Hcxapoda or Insects. — In the vast assem-
blage of forms associated under this head, the
appendages of the adult are never functionally

Chap. III.]

INSECT A.

75

developed behind the region of the thorax ; one pair
of appendages form the prse-oral antennae, and the
metameres do not exceed twenty in number. They
are sharply divisible
into two great sub-
divisions, according as
they are or are not
provided with wings ;
with the latter, of
course, we must as-
sociate those in which
wings are found in
one sex only, or are
rudimentary, or of
whose ancestral ex-
istence (as in the case
of parasites), we have
sufficient evidence.

A. Aptera, or
true wingless forms
such as the spring-
tails (Podura), and
bristle-tails (Lepisma).
In the simplest of
these the mouth or-
gans can work either
from side to side, or
from before back-
wards ; the tracheae,
however well de-
veloped, and they are
often only poorly so,

never anastomose with one another (Fig. 34).

B. Pterygota. — Here belong all the remaining
insects, which are either winged, that is, provided
with two pairs of membranous dorsal outgrowths in
the region of the thorax, which can be moved by

Fig. 34. — Orchcsella cincta, enlarged.

7 6 COMPARATIVE ANATOMY AND PHYSIOLOGY.

muscles and serve for flight; or one pair only is
developed, or is developed in one sex only, or both
pairs are more or less rudimentary. The organs of
the mouth are adapted for biting and cutting, or for
sucking, and the abdominal metameres are often more

Fig. 35.— Cockroaches : A, Male ; u, Female ; c, Young.

or less reduced ; the generative pores are placed far
back, and respiration is always effected by tracheal
tubes or modifications thereof.

a. Mandibulata.— In this series the mouth
organs are adapted for cutting and biting, and move
from side to side; or are converted into licking
organs.

1. Ortlioptera : cockroaches, grasshoppers, and
locusts. — With the anterior pair of wings converted

chap, in.] INSECTA. 77

into wing covers, and the posterior often functional
in the males only. No true metamorphosis (Fig. 35).

2. Neuroptera: dragon-flies, termites, — With
two pairs of membranous wings. A true metamor-
phosis, or the life history consisting of three periods,
an active larval, a quiescent pupal, and an active
perfect or imaginal condition.

With this group may be placed the Triclioptera
(caddis-flies).

3. Coleoptera; beetles, cockchafers, lady-birds.
— Anterior pairs of wings converted into wing-covers ;
these are distinctly horny. True metamorphosis.

The parasitic Strepsiptera come nearest to this
order.

4. The Hymenoptera (bees, ants) have the
mouth organs adapted for licking, as well as for
biting and cutting. Both pairs of wings functional.
Metamorphosis complete.

£. Haustellata. — In this series the mouth
organs move from before backwards, or serve as
stabbing or sucking organs.

5. Hemiptera (bugs, aphides, lice). — Mouth-
organs stabbing and sucking. Anterior pair of wings
functionless ; in parasites both may be rudimentary.
Metamorphosis generally incomplete.

6. Diptera (flies, fleas). — Mouth organs stabbing
and sucking ; anterior wings functional, the posterior
possibly represented by the small knobbed " balancers "
(halteres). Metamorphosis complete.

7. L-epidoptera (butterflies, moths). — Mouth
organs form a sucking apparatus, with no power of
stabbing ; both pairs of wings functional. Metamor-
phosis complete.

The Mollusca form a well-marked phylum, the
essential characters of which would be represented in

78 COMPARATIVE ANATOMY AND PHYSIOLOGY.

some such schematic Mollusc (Lankester) as that here
figured. The oblong body is bilaterally symmetrical,
and the prsestomium, as in Peripatus, is provided
with a pair of tentacles (Fig. 36, A, a) • the mouth
(B, o) is on the lower surface, and near, though not
at the front end, while the anus (m) is median, dorsal,

* I

d

Fig. 36.— Diagrams of the Typical Structure of a Mollusc. A, from
above ; B, from below.

a, Tentacles of head ; 5, Head ; c, edge of mantle ; e, outline of foot seen through
the mantle, which is supposed to he transparent; /, edge of shell-follicle;
g, shell ; h, osphradiiiiu (Sprcngel's olfactory organ); i, ctenidia (gills); fc.
generative orifice (paired); I, aperture of one of the nephrldia (excretory
organs) ; m, anus ; n, foot where it extends beyond the visceral mass ; o,
mouth ; p, plantar surface of foot. (After Ray Lankoster.)

and posterior ; right and left of this anal opening we
find the orifices of the excretory organs (I), and near
them those of the genital ducts (k).

So far the creature presents no characters other
than such as we might expect to find in any ccelomate
Metazoon ; in addition, there are four characters of
greater significance. The ventral surface is produced
into a more or less triangular muscular outgrowth,
which is known as the foot ; the dome-like dorsal
surface, which contains the chief mass of the viscera,

Chap. III.] MOLLUSCA. 79

is protected by a hard body, the shell (#), and this
shell is derived from a primary shell-sac (/) ; the
walls on either side of the middle line of the body are
produced into free folds, the pair of which make up
the mantle, and on either side of the body there are
given off comb-like processes (ctenidia) (i), which
are ordinarily known as the gills. Indications of
metameric segmentation are rare, and are only
obscurely indicated in the majority of the cases where
they are to be detected.

The Mollusca may be primarily divided into those
in which the region of the head is reduced or lost,
and those in which it takes on more special characters.
The former are conveniently known as :

A. JLipocephala. — This division contains only
the group *of the JLamellibranchiata or mussels
and oysters. In these the primitively single shell is
divided into two bilaterally symmetrical halves, and
the two divisions of the shell are only different
(Oyster : Myodora) in size and character, when one
side comes to be that on which the animal ordinarily
reposes, or when it ceases to live in an upright
position ; the foot may, as in boring forms, be of con-
siderable size, or it may be greatly reduced, as in the
oyster, which remains for long periods at the same
place.

This shell is brought together by special adductor
muscles, of which two pairs are found in many adults,
and have been observed in the young of some which
(oysters) have only one pair in adult life. The
ctenidia, which commence as separate ciliated filaments
in two rows on either side, ordinarily undergo a large
amount of fusion or concrescence, whereby they are
converted into perforated plate-like structures which
have, among others, a respiratory function. In some
the mantle never extends beyond the limits of the
shell, and these are the :

8o COMPARATIVE ANATOMY AND PHYSIOLOGY.

I. Asiphoniata, such as the mussels and the
oysters.

II. In others the mantle is produced into two more
or less elongated siphons (Fig. 37, Siphoniata) and
these siphons are either not retractile as in the cockle
(Cardium), and the immense Tridacna ; or the siphons
can be retracted by special muscles (Sinupalliata),
as in Pholas, Solen, and Mactra.

B. In the higher division of the Mollusca the

Fig. 37. — Mya arenaria, a Siphonate Latnellibranch.
ex, Excurrent ; in, incurrent siphon ; a, anterior ; a', posterior adductor muscle;
gg, branchiae ; /, foot ; t, labial tentacles ; o, mouth ; s, stomach ; d, Intestine;
p, muscle of the foot.

cephalic tentacles and eyes are retained, and within
the cavity of the pharynx there is developed a special
rasping organ or tongue, the presence of which
justifies the name Glossopliora, which is applied to
this series. In a number of these the foot becomes
divided into three well-marked regions, but in the
lowest group,

1. Gastropoda, the foot is ordinarily simple, and
only constricted into three regions ; it -is broad and
flattened. In a large number the body undergoes a
twisting round its central axis, in consequence of
which the two sides of the body come to be unequally

Chap. III.]

MOLLUSC A.

Si

the

or asymmetrically developed. The appearance of this
torsion allows us to divide the Gastropoda into a
lower or more primitive, and a higher or more
differentiated series.

o. Isopleura. — In these the two sides of
body are equally developed, and
many of the characters of the
primitive mollusc are retained
unchanged. Here we have the
Polypfacophora, represented
by the Chitons, in which the shell
is broken up into eight pieces ar-
ranged in a fashion to which it is
difficult to refuse the name of
metameric arrangement (Fig. 38) ;
and the Neomeniidse, and the
Chaetodermatidae, in which
the shell is represented by spicules
only.

£. In the Anisopleura we

have an exceedingly interesting phenomenon ; while
the body undergoes torsion, the nerve- cords that run
down the sides of the body may or may not be impli-
cated in the change. Where they are not we have
the Euthyneiira, which either, like Aplysia and
Doris, continue to breathe by gills the oxygen dis-
solved in water, or like the pond-snail (Lymnceus),
the garden-snail (Helix), and the slug (Limax), have
their gills aborted, and a breathing chamber or lung
formed by the apposition of part of the edge of the
mantle to the side of the body.

In the Streptoneura the nerve-cords are impli-
cated in the general torsion of the body, and form a
figure of eight loop ; in the Zygobranchiata, of
which the ear-shell (Haliotis) and the limpet (Patella)
are examples, the right and left gills become re-
spectively the left and right, and are equal and
c— 16

— Chiton mag-
nificus.

82 COMPARATIVE ANATOMY AND PHYSIOLOGY.

symmetrical ; in theAzygol>raiicliiata,such as Palu-
dina, Dolium, the cowry (Cyproea), and the whelk
(Buccinum), the left gill and excretory organ become
aborted ; some members of this division, the so-called
Heteropocla, become modified to a free-swimming
life, as Atlanta or Firuloides.

II. The ancient group of the Scaphopoda
exhibits some primitive characters, but is specially
remarkable for its elongated elephant-
tooth-like shell (Dentalium) which is
open at either end (Fig. 39).

III. The Pteropoda closely ap-
proach in many important characters
the next succeeding group, but they
are most conveniently kept separated
from them. The anterior portion of the
foot (propodium) surrounds the head,
and the median part (mesopodiuiii)
Fig. 39. —Shell ig converted into a pair of napping fin-
ofDentaiinm like organs by means of which these

elephant!- -,. v, . J , -,-,-,

num. ordinarily minute creatures are enabled

to swim about on the surface of the

ocean. According as they have or have not a shell,

they are called Thecosomata (Hyalea, Cymbulia),

or Oymnosomata (Clione, Pneumodermon).

IV. The last and highest division of the Mollusca
is formed by the Cephalopoda ; the propodium is
here produced into a number of long tentacular pro-
cesses or arms, on which suckers are not unfrequently
developed ; the mesopodium of either side unites with
its fellow to form an incompletely or completely
closed siphonal tube, which serves as the chief organ
of locomotion. The shell is external or internal,
coiled or simple, or completely absent.

a. The ancient group of the Tetrabrancliiata, to
which many fossil forms belong, is represented to-day
by a single genus, Nautilus ; they receive their

Chap. HI.] CEPHALOPODA. 83

name from the possession of two pairs of gills, with
which, an exceptional circumstance among Molluscs,
are associated two pairs of ante-chambers to the ven-
tricle. The siphon is incomplete, the propodial ten-
tacles are numerous and devoid of suckers ; the shell
in external, chambered, and coiled (Fig. 40).

B. The Diforancliiata have either eight arms as
in the Octopus, or ten as in the squid (Loligo), or Sepia ;

Fig. 40.— Section of the Shell of the Pearly Nautilus, showing the coil

of chambers, and the animal in the largest, or that last formed (z).
a, Mantle; b, dorsal fold; o, shell-muscle; ii, Siphuncle; k, funnel or Bi phonal
tube; n, hood;p, tentacles; s, eye; x, septa between the chambers.

there is only a single pair of gills and auricles, and
the arms are provided with suckers (Fig. 41).

It has long been the custom to divide the members
of the Animal Kingdom sharply into the two great
groups of " Vertebrata " and " Invertebrata"; we have
seen, however, that the most scientific separation is
that into uni-cellular and multi-cellular organisms,
Protozoa and Metazoa ; and, next, that the lower
Metazoa have no signs of that body-cavity or ccelom
which becomes so well marked a part of the organisa-
tion of the higher forms ; and, lastly, we have seen
that the Echinodermata, the Arthropoda, and the

84 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Mollusca form three very distinct branches or phyla,
the common ancestor of which is to be sought for
only in a simple worm. Of equal value with
these is another phylum, which may be most conve-
niently spoken of as that of the Chorda ta, distin-
guished from the rest by the association of two
characters, the temporary or permanent possession
of a rod underlying the central
dorsally-placed nervous system,
and the similarly temporary or
permanent possession of clefts
or passages at the sides of the
head and neck, which open to
the exterior (visceral clefts).
Either one of these cha-
racters may be seen in certain
members of that heterogeneous
mob, which, partly from the
nature of things, and partly
from the imperfect condition of
our knowledge respecting them,
must be retained in the group
of Vermes.

Among, or standing near to,
the Platyhelminthes, are some
elongated, free-swimming, ma-
rine forms which are known as the Nemertinea.
These worms are provided with a dorsal proboscis, which
is enclosed in a sheath. The relations of this proboscis
to its sheath are shown in Fig. 42 A, while Fig. 42 B ex-
hibits in diagramatic form the relation of certain parts
in one of the lowest of fishes (the lamprey) ; a comparison
of the relations of these structures (proboscis and its
sheath on the one hand, and chorda dorsalis on the
other) with (a) the dorsal surface of the body and (£)
the digestive tract, reveals very striking resemblances,
which come to be of still greater significance when we

41. — The Common
Cuttlefish.

Chap. III.]

ENTEROPNE us TI.

combine with them the knowledge of the fact that, in
certain Nemertines, the nerve cords, instead of lying
at the sides of the body, tend to take up a dorsal posi-
tion. Whether or no Hubrecht is right in regarding
the Nemertinea as giving us indications of where to
look for the ancestral form of the Chordata, it is
clear that we must sharply distinguish them from the
group of the Platyhelminthes, with which they have

Fig. 42. — A, Diagram to show the relation of the proboscis (pbs) to the
surface of the body and to the sheath of the proboscis (pbs) , in the
Nemertinea; (B) diagram of Petromyzon (the lamprey) showing the
hypophysis cerebri (hyp) ; the chorda dorsalis (ch) j the mouth (m) ;
and the anus (a). (After A. A. W. Hubrecht.)

been hitherto very closely associated. Lineus, Cari-
nella, Polia, are examples of this group.

So, again, in another group of " worms," the
Enteropncusti, the sole representative of which is
the remarkable Balanoglossus (Fig. 43), the anterior
portion of the enteron divides into a ventral and a
dorsal portion ; the former retains its nutrient office,
but the latter has chitinous lamellae developed in its
walls ; between these clefts (br) appear, which finally
open on the surface of the body ; blood-vessels are richly
distributed to the walls of the arches, and the water
taken in by the mouth passes through the clefts to
the exterior. In Balanoglossus, therefore, just as

86 COMPARATIVE ANATOMY AND PHYSIOLOGY.

much as in a fish, we have gills developed at the

sides of the anterior region of the digestive tract.

With regard to the Chordata, however, it is to be

distinctly borne in mind that l>oth these organ-;

(notochord and gill-slits) are to be found, and we may,

therefore, look
for the ancestral
or ideal Chordate
in an elongated,
bilaterally sym-
metrical, meta-
merically seg-
mented animal,
in which the cen-
tral nervous sys-
tem, dorsal in
position, was
supported by a
rod of firm tis-
sue, in which the
sides of the body
and pharynx
were perforated
by gill slits ; and
in which the
mouth was

Fig. 43.— Young Balanoglostus seen from the ride. P^ced on the
br. Branchial slita; x!2. (After Pagenstecher.) ventral surface,

not far from the

front end of the body. The Chordata fall into
three well-marked groups ; in one degeneration has
proceeded to an extent so considerable, that in many
all indications of a chordate ancestor are completely
lost; these are the Urochordata or so-called
Tunicata. In another, many primitive characters,
such as the original segmentation and the notochord,
are retained unchanged, but in some few points

chap, in.] CHORD ATA. 87

there would seem to be degradation; these are the
Ccphalochordata ; and, lastly, we have the true
Verteorata or Craniata.

A. Cephalochordata. — Of these the only exam-
ple is the Lancelet or Amphioxus, in which the noto-
chord, pointed at either extremity, extends from one
end of the body to the other ; the number of gill slits
is very great, and they are covered over by an out-
growth of the body wall which grows down on either
side, and unites along the ventral line, leaving a pore
for the exit of the water (atrial pore). The original
segmentation of the muscles of the body is not ob-
scured ; the mouth is over-hung by a projecting hood,
and furnished with a number of tentacles (cirri) ; the
liver is represented by a very slight, blindly ending
outgrowth of the enteric tube, and renal organs are very
obscurely indicated ; there is no centralised heart, and
appendages are completely wanting. The eye is only a
pigment spot, and no signs of an ear have been detected.

B. Urochordata. — In no division of the animal
kingdom has the value of the study of development
been of more -importance than in this, for it has
revealed the presence of a notochord, and the essen-
tial resemblance between their gill clefts and those of
the Cephalochordata ; while in none has the applica-
tion of the principle of degeneration (Dohrn ; Lan-
kester) been more instructive.

In but few forms is the notochord retained
throughout life, and in these it is found in the tail
only, Perennichordata (e.g. Appendicularia) ; in
the rest, Caducichordata, the caudal notochord is
present in the larva only, or is never developed at all ;
in these, just as in Amphioxus, outgrowths of the body
wall enclose the true sides of the body, and give rise
to an atrial chamber, by whose pore the water of re-
spiration, and often also the waste matters of digestion
finally make their way to the exterior (Fig. 44).

88 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Some of the Caducichordata remain solitary
throughout life, e.g. Ascidia, or Boltenia (which is
remarkable for its long stalk) ; others become fused
into a common colony, as Botryllus, Pyrosoma, colo-
nies of which may be more than a foot long, and Salpa,
the chains of which are sometimes several feet long.

0. In the true Vertebrata the anterior end of
the central nervous system is enlarged into a brain,
which becomes surrounded and protected by a carti-
laginous capsule or skull ; supporting and protect-
ing arches, which finally become distinct vertebrae,

are developed
around and
above the noto-
chord, which, in
the adults of the

Pig. 44.— Pyrosoma ; A, The atrial or excurrent higher forms,

opening- is completely

aborted. Optic,

auditory, and olfactory organs are developed; there
is a centralised heart and a distinct liver appended to
the enteric tract. They are divisible into two groups,
distinguished by the fact that, in the higher, an an-
terior gill-arch becomes modified to form jaws at the
sides of the mouth.

o. Cyclostomata, or Round-Mouths ; these are
the lampreys (Petromyzon), and hags (Myxine). There
is here no maiidibular arch, no appendages in the
form of limbs, and the olfactory organ is single and
median. The hags are parasitic in habit.

ft. Gnathostomata.— In this division all the
remaining Yertebrata are included ; in them an ante-
rior gill-arch becomes niandibular, two pairs of
lateral appendages are typically developed, and the
nasal sac is double.

In all divisions of the animal kingdom we may
observe groups which seem to stand near the ancestral

Chap. III.]

ICHTHYOPSIDA .

forms, and
others in
which, a given
complexity of
structure hav-
ing been at-
tained, there is
a profusion in
the elaboration
of the details.
This truth is
well exempli-
fied in the
groups of the
Vertebrata.

I. Ichthy-
opsida; these
are the true
Fishes, and the
Amphibia (or
frogs and
newts). In
them respira-
tion is always
effected by gills
during some or
the whole of
their life, the
heart never
has more than
three cham-
bers, and there
are always two
aortic arches at
least given off
from it.

a. Pisces.

90 COMPARATIVE ANATOMY AND PHYSIOLOGY.

1. Elasmobranchii (sharks and rays). — In
" cartilaginous fishes " the gill slits are, in the simples^
naked, i.e. not covered over by any fold (opercnluni),
neither the skull nor the jaws are ever protected by
ossifications of the investing membrane (membrane
bones) ; the notochord has the outer sheath pro-
vided with rings of ossification, or distinct vertebrae
become developed. The skin is either naked, or
covered with calcified tooth-like papillae.

2. Dipnoi; e.g. Lepidosiren, Ceratodus. — In
these the cartilaginous brain capsule becomes invested
by bones developed in the covering membrane, and
the digestive tract gives off a single or incompletely
divided air sac, which is more or less richly supplied
with blood-vessels, and may undertake the office of a
lung, the possession of which enables the fish to live
in mud. The pectoral and pelvic tins are broad and
paddle-like (Fig. 45), or elongated and filiform.

3. The Ganoidei and (4) Teleostei are the
two groups of the Pisces in which we observe that
elaboration of the details to which reference has
already been made; a cod, a sole, or an eel stand
almost as far from the primitive vertebrate as the
snake, the hawk, or the bat. The former group
retains certain more primitive characters which are
only rarely or rudimentarily possessed by the latter ;
thus the arterial trunk (see page 195), which is
muscular and contractile in Ela^mobranchs, Dipnoi,
and Amphibians, is so also in Ganoids, but is only
incompletely so in some Teleostei (Butirinus) ; the
spiral valve which is found in the intestine of
Elasmobranchs is retained in the Ganoids, though not
well developed in the Sturgeon and its nearest allies ;
it is lost in most Teleostei, though found in Butirinus
(Stannius), in Chirocentrus, and perhaps represented
in rudiment in the smelt (Huxley).

In both groups the ends of the gills are free, and

Chap. III.]

G A NO I DEI.

91

the gill chamber is
covered in by a
bony plate, oper-
< n lima ; the renal
ducts do not open
into a depression
(cloaca,) common
to them and the
anus. In all Ga-
noids, and in one
great division of
the Teleostei, the
air sac on the dor-
sal surface of the
body opens by a
duct into the oeso-
phagus.

The recent Ga-
noidei fall into two
divisions :

a. Selachoi-
dei ; * such are
the Sturgeons (Aci-
penser) and Poly-
odon ; in these the
skull consists of
persistent cartilage,
overlaid by bones
developed in the
investing mem-
brane ; spiracles
are persistent, and
the body is either
naked, or has bony
plates developed in
the dermis (Fig. 46).

* Chondrostei.

92 COMPARATIVE ANATOMY AND PHYSIOLOGY.

0. Teleostoidei ; * represented by the North
American bow-fin (Anna), and gar-pike (Lepidosteus),
and the North African Polypterus ; in these the
hinder part of the cartilaginous cranium always under-
goes ossification, the spiracles close up, or are covered
by a bony plate, and the scales, which are never
formed of true bone, are large, and may be covered by
a layer of enamel. Vertebrae are developed around
the notochord.

The Teleostei, or bony fishes, always have ossi-
fied vertebral centra, and more or less of the primitive
cranial cartilage is finally replaced by bone ; scattered
bony plates are developed in the dermis, or the in-
tegument is protected only by thinner scales, or the
body is naked ; they are divisible into :

a. Physostomi, where the air bladder, which is
an outgrowth of the oesophagus, almost always remains
connected with it by an open duct, and the hinder
pair of fins, if retained, as in the salmon, are always
abdominal in position ; here we find catfishes (Silurus),
carps, pikes (Esox), and salmons, as well as the finless
eels.

£. Pliysoklisli. — In these the air bladder be-
comes shut off from the oesophagus, or is aborted, as
in the sole ; the ventral fins, which are rarely ab-
dominal (Notacanthus), are ordinarily thoracic or
jugular in position ; not unfrequently they are rudi-
mentary or lost. The fin-rays are either all jointed
as in the cod, or some are entire, as in the perch.
Some forms are asymmetrical and flattened like the
sole ; some swollen and globular like the sun-fish ; some
greatly elongated like the pipe-fish; some with a
prehensile tail like the sea-horse ; some have the body
scaleless ; others, like Diodon, have erectile spines ;
some can live in semi-fluid mud (Ophiocephalus) ;

* Holostei.

Chap, in.] AMPHIBIA. 93

some can make overland journeys, and go up inclined
surfaces, if not trees, like Anabas; some can take
leaps out of the water, like the " flying gurnards "
(and the physostomous Exocoetus) ; some, like Chseto-
don, have a minute mouth, while the sword-fish has
its upper-jaw converted into a powerful piercing
organ, and another (Toxotes) has acquired the habit
of throwing a drop of water at the insect it desires to
obtain. Other examples might be given of the pro-
fusion of variation within the limits of Teleostean
organisation.

Even the lowest of the Amphibia are dis-
tinguished from the highest of fishes, such as Cera-
todus or Lepidosiren, by the fact that their fore and
hind limbs are arranged on the same plan as in the
higher vertebrata (see page 350), and these limbs
terminate typically in five digits, so that, like the
higher forms, they are pentadactyle ; if, further,
fins are developed, they never have fin-rays.

1. Uroclela; in the lowest of these (Proteus,
Menobranchus) (Fig. 47) external gills persist through-
out life; in the next grade (Amphiuma, Menopoma)
the gills are lost, but the gill-clefts remain ; while in
the highest (Salamandra, Triton) the gills disappear
in the adults, and the clefts close up. All retain the
tail, which in the

2. Anura (or frogs and toads) is only found
during the tadpole stage, when also respiration is
effected by external or internal gills, which disappear
in the adult, to be functionally replaced by lungs.

3. Caeeilise are still more modified forms, in
which the limbs are lost, and the body is elongated
and serpentiform.

The two higher divisions of the Vertebrata are
the Sauropsida and the Mammalia, which may
be grouped together as the Amniota. They are
characterised by the very early development of a

94

Chap, in.] SAUROPSWA ; MAMMALIA. 95

large sac-like structure similar in origin and primitive
position to the bladder of the frog; this allantois
takes on respiratory functions in the developing reptile
or bird, and a nutrient one in the higher Mammalia.
From either end of the body there grows out a fold,
which passes over the body of the embryo and unites
above it with its fellow ; this fold, which is double,
forms the amnion; the two layers of the amnion
separating from one another give rise to a cavity
between them which is more or less occupied by the
allantois ; in the Bird the allantois is comparatively
larger than it is in the Mammal.

The differences between the Sauropsida, or reptiles
and birds, and the Mammalia are well and sharply
marked, and it is almost impossible to suppose that
their common ancestor was not more amphibian than
amniote in character. Thus, the Sauroids have scales
or feathers, the Mammals hairs ; the skull is always
articulated to the atlas by a single condyle in the
Sauroid, and by two in the Mammal ; the quadrate
bone, which is external to the ear in the Sauroid, is
enclosed by the otic capsule in the Mammal \ the red
blood corpuscles of a Sauroid are, and of a Mammal
are not, nucleated ; the connection between the
cerebral hemispheres of a Mammal is more intimate
than in a Sauroid, and while the eggs of the latter are
large, and provided with a quantity of yolk, those of
the Mammal are much smaller,* and nutrition is
afforded to the young by milk, the secretion of certain
modified tegumentary glands.

The recent investigations of palaeontologists have

* It has been recently stated that the ova of the lowest
Mammals are large, and that they are hatched outside of the body.
This observation, coupled with the facts that certain fossil Reptiles
(TheriomOrpha) give well-marked indications of mammalian affini-
ties, and that some Reptiles (e.g. some of the Amphisbsenidae)
have the occipital condyle double, may necessitate a revision of
current ideas as to the origin of the Mammalia.

96 COMPARATIVE ANATOMY AND PHYSIOLOGY.

afforded us a complete series of intermediate stages
between the reptiles and birds, and they are justly
united in the common group of the Sauropsida.

A. Reptilia. — Sauroids with horny or bony
plates, but no feathers, with more than three digits in
the manus, of which three at least bear claws, with at
least three digits in the pes, and with unankylosed
metatarsals. The blood is ordinarily cold, and there
is at least one pair of aortic arches.

1, 2. Lacertilia, or lizards, and Op hi ilia, or
snakes, have the quadrate movable, the penis double,
and the anus a transverse slit. Some of the Lacer-
tilia, such as Lacerta (the common lizard), are
the least modified of all Sauroids, and the Geckos
retain a primitive character in the persistence of
remnants of the notochord. Others are specially
modified, like the flying lizard (Draco), others have
ossified scutes approaching those of crocodiles (e.g.
Cycled us) ; Hatteria is remarkable for the possession
of " uncinate processes " on the ribs (see page 346),
such as are seen in crocodiles and birds. Some, like
the blind-worm, lose their limbs, but all have a pectoral
arch and a urinary bladder, both of which are absent
from the Opliiclia, in which the hind limbs are rarely
present, and then are only short and inconspicuous.
They are divisible into the Eurystomata, in which
the mouth-cavity is capable of dilatation, and the
Stenostomata, in which the facial bones are im-
movably connected with one another. Among the
former we find vipers, rattlesnakes, and water snakes,
which are venomous ; and adders, boas, and pythons
which are not so. Typhlops and Uropeltis are
examples of the Stenostomata.

3. Chelonia, or turtles and tortoises. — In these
the quadrate is immovably connected with the side
of the skull, the penis is simple and solid, and the
anal orifice rounded. The bony plates developed in

Chap, in.] REPTILES : BIRDS. 97

the dermis are definitely arranged, and form a
" carapace," which is generally, though not always
(Trionyx), covered by horny epidermic plates, which
form the " tortoise-shell." They exhibit a primitive
character in the retention of the five digits in either
limb, but diverge from the typical organisation in the
loss of teeth ; an interesting series of modifications, in
relation to their mode of life, are exhibited by the
limbs. .In the tortoises, which are terrestrial, the
digits are free ; in the amphibian terrapenes there is a
partial web, which is more complete in the Triony-
chidse ; while the marine Cheloniidse have the digits
completely covered by skin, so that they form
flattened swimming fins.

4. Crocodilia, or crocodiles and alligators, are
the only reptiles in which the heart is four-chambered ;
like the Chelonia, they have the quadrate immovably
connected with the side of the skull, the penis is
simple and solid, and the anal orifice is rounded. The
teeth are set in distinct sockets, and are never found
on any bones but the maxillae, premaxillae, and
dentaries. They have returned to an amphibious or
aquatic mode of life, in correlation with which their
feet are webbed, the nostrils can be closed, and the
tympanic membrane of the ear covered over.

B. Aves, or birds, are Sauroids with feathers,
with never more than three digits in the manus, or
four in the pes ; three of the metatarsals are ankylosed
with one another, and with the distal tarsal bone.
The blood is hot, and there is only a single systemic
aorta. All recent forms are toothless. Physiologically,
if not also morphologically, the recent forms are
divisible into :

I. Ratitae, in which the ventral surface of the
sternum is broad and flattened, and the fore-limb does
not form a functional wing ; such are the ostrich and
the cassowary.
H— 16

98 COMPARATIVE ANATOMY AND PHYSIOLOGY.

II. Cariiiatse, in which the ventral surface of
the sternum is typically provided with a median keel,
and the fore-limbs may serve as functional wings.
Among them we find the singing birds, parrots, owls,
eagles, geese, pigeons, and gulls ; and here, as among
the Teleostei, we find the most varied elaborations in
the details of a structural organisation, which is, in its
essential points, extraordinarily similar throughout
the group. The extinct Odontornithes (e.g. Hes-
perornis) were true birds with teeth in their jaws.

The Ulammalia, or last division of the Verte-
brata, are all distinguished from the Sauropsida by
the possession of two occipital condyles, and by the
fact that the single aortic arch is the left and not the
right member of a primitive pair. They are all more
or less hairy, and have mammary glands ; the quad-
rate becomes the malleus among the auditory
ossicles, the blood is hot> and the red blood corpuscles
are without a nucleus, while
the cerebral hemispheres have
a corpus callosum. (See
page 426.) They exhibit three
Well-marked grades of develop-
ment :

A. Prototheria (Ornitlw-
delphia), in which the mammary
glands are without teats, the
young are not nourished within
the uterus of the mother by
means of a placenta* the
epipufoes (see page 348) are

larSe' and the coracoids
complete. Here are placed the
duck-bill (Ornithorhynchus) and the Echidna (Fig. 48),
which have so far diverged, like the Chelonia, from

Chap, in.] MAMMALIA. ' 99

the primitive type, that they are without true
teeth.

B. Metatheria (Didelphia). — These are the
Marsupials; they have true teats, but no placenta ;
the epipufoes are large, but the coracoicl rudimen-
tary. The Marsupials exhibit a great range of varia-
tion and structure among themselves ; some are car-
nivorous, like the Opossum, the Dasyurus, and the
Thylacine ; others herbivorous, like the kangaroo
(Macropus) and the wombat (Phascolomys).

C. Eutheria (Monodelphia). — Here stand the
rest of the Mammalia, which, without any known
exception, have teats, a placenta, rudimentary or
no epiptifoes, and a rudimentary coracoid. The
least differentiated are the Insectivora (e.g. hedgehog,
mole), to which are most closely allied the Chiroptera
(bats), and the Rodents (rat, rabbit) ; in these the
yolk sac takes a larger share in the formation of the
placenta than it does in other mammals.

The Edentata form, at the present day, an isolated
group, represented by the sloths, anteaters, and arma-
dillos, by the pangolins (Manis), and by the ant-bear
(Orycteropus). The hoofed animals, or Ungfulata,
form a well-marked division, in which the group of
the even-toed forms (Aitiodactyla), such as the
pig, deer, and cow, is very distinct from that of the
odd-toed (Perissodactyla), such as the tapir, rhino-
ceros, and horse. With the Ungulata, the coney
(Hyrax) and the elephant may be associated (Flower).
Of aquatic forms, the Cetacea, or porpoises, toothed
whales, and whalebone whales seem to stand nearest
to the Ungulates. Of the affinities of the other
aquatic mammals, the Sirenia, or manatee and
dugong, we can only with confidence say that they
are not with the Cetacea. The true Carnivora are
the dogs, cats, and bears, and with these are closely
allied the walruses and seals.

ioo COMPARATIVE ANATOMY AND PHYSIOLOGY.

By the almost universal consent of zoologists, the
highest " order " of the Mammalia is that of the
Primates; of these, the lowest suborder is that of
the L.emuroidea (of which some naturalists would
make a separate order), the highest that of the
Anthropoidea, which is divisible into five " fami-
lies," the highest of which is the Ilomiiiidsr,
represented by the single genus Homo.

While Man is said to be the highest of animals,
it is not to be forgotten that in the other divisions
of zoologists there are forms in which structural
characters are at least as perfectly elaborated, when
we bear in mind their ancestral history and the
relation of structure to function. The horse, the
whalebone whale, the woodpecker, or the boa con-
strictor, are, to cite only a few examples, forms in
which structural organisation is as, if not more, com-
plete, and as differentiated as it is in man.

There remain to be considered very briefly several
groups of animals which, in the present state of our

knowledge, cannot be
satisfactorily placed
with any of the great
phyla which we have
just been describing.
Of these the more im-
portant are :

1. Bracliiopoda,
— These were placed
by earlier naturalists
with the Mollusca, from
which, however, they
are to be distinguished
in consequence of the segmentation of the larva, the
dorsal and ventral positions occupied by the two

Fig. 49. — Crania anomala. b, Arms.
(After Davidson.)

Chap, in.] BRACHIOPODA: BRYOZOA. 101

unequal valves which make up their shell, and by the
characters of their nervous system. The so-called
arms (Fig. 49 ; b) are outgrowths of the pree-oral disc
of the larva, at the edges of which the tentacles or
cirri are set. - This great development of their arms is
to be correlated with the fixed habit of the adult.

Fig. 50. — Bugula purpurotincta. Nat. size. (After Hiucks.)

Terebratula and Lingula (which is stalked) are ex-
amples of this isolated and geologically ancient group.

2. The Bryozoa have likewise been placed with
the Mollusca; they are clearly degenerate forms
which, by the characters of their larvae, appear to
have been descended from an ancestor common to
them and the Chsetopoda. Balfour has suggested
that they become fixed by their prse-oral lobe. They
live in colonies, and are the forms that are popularly
known as sea-mats or sea-mosses (Fig. 50).

3. The Chsetogrnatha (as represented by Sagitta)
are forms that have relations to the Chsetopoda and

102 COMPARATIVE ANATOMY AND PHYSIOLOGY.

to the round worms, but differ from them remarkably
in the mode of development of their body cavity,
which is an enterocoele.

4. Ulyzostomiim is a form with some points of
resemblance to the Chsetopoda; its characters, how-
ever, are still obscure, partly, no doubt, on account of
its having taken to the habit of living parasitically
on Crinoids, on which alone it has as yet been
detected.

CHAPTER IV.

ORGANS OF DIGESTION.

THE activity of a living organism has for one of its
chief results destruction and loss of tissue ; this loss
can only be made up for by the act of taking in fresh
material from the outer world. In the necessary
nutrition of an organism, we find that the first
process is that of digestion, by means of which
substances foreign to the organism become assimi-
lated to it, and are rendered capable of being
absorbed, and of passing into that stream whence
the different parts of a body take, as they require,
the food which is needed to make up the losses caused
by their several activities. Organisms are, in other
words, metabolic.

It is to be carefully borne in mind that the essen-
tial step in the nutrition of an animal is that of
assimilation, and it, indeed, is the only process
which obtains in the case of the lowest and simplest
organisms. In other words, a simple mass of proto-
plasm, such as an Amoeba, takes up from without
food material into its own substance, and this, as
we have already learnt, is effected directly ; the
material thus taken in is acted upon by the living

chap, iv.i INTRACELLULAR DIGESTION. 103

protoplasm of the cell, which is capable of separating
out from the food such parts as are nutritious, and
of converting them into protoplasmic matter ; what
is useless is discharged, or got rid of.

This direct mode of assimilation by a living cell is
spoken of as intracellular digestion; it is the
only mode of nutrition which is known to obtain
in the Protozoa, but it is very important. to observe
that the phenomenon is by no means limited to that
division of the animal kingdom ; it obtains also in
various lower groups of the Metazoa, and even after a
distinctly defined mouth has become developed. It
is, therefore, associated with a number of characters
which indicate an advance in the complexity of
organisation; and, on the other hand, it is found
also in forms which have, under the influence of a
parasitic habit, become degraded as compared with
their ancestors. The simplest mode of seizing food is
observed in the Amoeba, where the protoplasmic body
seems to engulf its nutriment by flowing and closing
around it. And this ingestion of food does not
take place at any definite point in the body of the
Amoeba, but now at one spot, and now at another.
When the form of the body becomes more definite,
the protoplasmic processes act as organs by which
the food is drawn towards the central body-mass.

A much more elaborated mode is to be seen in
the ciliated Infusorians, where a definite orifice
(" cytostome ") acts as the sole entrance for food
into the body ; in many cases this so-called <l mouth "
has also an anal function, but in a few forms it has
been distinctly observed that a second orifice is pre-
sent ; by means of this " cytoproct," the undigested
portion of the food passes from the body. The
presence of a definite oral orifice is no doubt to be
associated with the greater elaboration of the organi-
sation of an Infusorian, and we find also that some

io4 COMPARATIVE ANATOMY AND PHYSIOLOGY.

of its most characteristic organs (the cilia) are
specially modified in the neighbourhood of the
" mouth." In other words, we have here the first
sign of a correlation between the digestive orifice and
the organs which are locally connected with it, and
which are also in relation with the outer world. The
cilia around the mouth of Paramcecium (Fig. 3 I.) are
much longer than those which fringe the greater part
of their body, and give rise to more powerful currents,
by means of which food particles are floated towards
the orifice.

0 Some of the ciliated Infusorians, such as Opalina
ranarum and Anoplophrya are endoparasitic, and in
these the mouth is lost (Fig. 51), as it is also in the
Gregarinida, which live in cavities rich in nutrient
matter, such as the intestine of the lobster, or the

testicular re-
servoirs of the
earthworm ; in

Pig. 51.— Anoploplirya prolifera. (After Clapa- SUC^ ^Orms ^
rede and Lachmann.) these nutri-

ment enters

into the substance of the cell by the mere physical
process of diffusion or osmosis.

In the ectoparasitic Suctoria, where the mouth is
likewise lost (Fig. 3 in.), processes of the body are
drawn out into sucking tubes with knobbed'ends ; these
tubes retain the extensile and contractile power of
simple protoplasm, so that they are able to elongate
themselves in such a way as to touch their prey,
which is ordinarily a ciliated infusorian, and to con-
tract themselves so as to draw the prey nearer. The
knob is enabled to bore its way beneath the cuticle,
and then, in the words of Stein, " a very rapid stream,
indicated by the fatty particles which it carries, sets
along the axis of the tentacle, and, at its base, pours
into the neighbouring part of the body of the Acineta."

Chap, iv.] INTRACELLULAR DIGESTION.

I05

No movement of the wall of the tentacle has been
observed, and the cause of the production of this
stream is still unknown.

As has been already observed, the simplest mode of
digestion (the intracellular) is not confined to the
Protozoa ; it has
been observed in
Sponges, Coelen-
terata, and the
lowlier worms.
A clear idea of
what is under-
stood by this
method will be
obtained from
the consideration
of a single case.
When a section
is made through
the body walls of
a Hydra we
find that the en-
dodermal cells
vary cons i d e r -
ably in size, and
that, while some
are provided

with a Single long Fig. 52.-Longitudmal Section of the Body of
,, ,, a Hydra, killed m full digestion,
tiageilum, Others ec< Ectoderm; en, endoderm; mp, muscular pro-
are distinctlv cesses; d, a diatom ;/, food particles. (After

amceboid in

form, and give off large pseudopodia (Fig. 52) ; within
these cells dark-coloured granules of various sizes are to
be detected, and these food-particles are sometimes
found to be " half in and half out of the protoplasm "
(T. J. Parker). In such a form, therefore, as the Hydra,
there would not seem to be, as in Man and most of

io6 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the Metazoa, any secretion poured out into the gastric
cavity from the cells which line this space, but a
number of the cells would appear to retain the power
of separately assimilating the food material. Obser-
vation of these endoderinal gastric cells shows that they
vary considerably in size according to the fasting or
well-fed condition of the animal ; and we are entitled
to suppose that these cells become smaller after the
process of digestion, in consequence of their having
given up part of their acquired material to the other
cells of the body ; cells which, be it understood, have

lost the power
of independently
assimilating nu-
triment. We
have here to do
with nutrient
cells, just as in
the Ccelenterata
(page 39) we ob-
served nut rien t
persons (trophos-
omes)ina colony.
A history,
not unlike that
of Hydra, may
be told of a
Sponge; but
here, it is interes-

Fig. 53.— Flagellated Chambers (c) of Turkey ,« .„ ,

tmg to note, we

have to do not

with an amceboid

ingestive cell,
but with another form which, no less, has its representa-
tive among the Protozoa ; we find, that is, a " collared-
cell" taking the place of the amoeboid cell (Fig. 53). In
the "ciliated "or "flagellated "chambers which are found

Bath Sponge, showin? the collared-cells and

flageiium.

K, Excurrent canal s ^ Current canals. (After

Chap, iv.] INTRACELLULAR DIGESTION. 107

along the course of the canals which traverse the body
of a sponge we find a single layer of 'cells, each of which
is provided with a long whip-like process (flagellum),
and has the free edge of its protoplasm converted into
a collar-like fringe. By the action of the flagellum
currents are set up around the cell, and directed to the
space surrounded by the collar ; these currents of
water bear with them minute food-particles, which
thus make their way into the substance of the cell.
Such flagellate cells recall the Flagellate Infusoria,
among the Protozoa. Finally, in the case of the lower
worms, we have the evidence which is afforded by
Mesostomum ehrenbergii, a Turbellarian which lives
on the small Annelid Nais. In observations on intra-
cellular digestion no method is more fruitful in its
results than that which consists in feeding an animal
with some finely-divided colouring matter such as
carmine ; Mesostomum, however, has been found to
reject this substance, and the ingenious expedient had
to be resorted to of first feeding the Nais with carmine,
and then inducing the turbellarian to eat the annelid.
This experiment, which was completely successful,
afforded certain evidence as to the persistence of the
intracellular mode of digestion in this animal, for a
large quantity of the coloured material was found in its
digestive cells.

The phenomenon of intracellular digestion has
been now seen to be very widely distributed among the
lower Metazoa, and observations are continually being
made in confirmation of the facts here described.
With a single exception, no observer has as yet seen
any combination of this primitive method of taking in
food with the more complex one of the presence of a
set of cells which secrete a special gastric juice ; we
may expect, however, to find that the sharp distinction
between the lower and the higher methods will be
bridged over by other forms than the fresh-water

io8 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Medusa (Limnocodium) ; in this remarkable creature
the cells which line the mouth of the gastric tube have
the function of secreting cells, while it is only in those
that lie at the opposite end of the tube that the intra-
cellular method has been observed (Lankester).

This power of intracellular digestion is not con-
fined to the cells that line the gastric or endodermal
cells ; Metschnikoff has observed that some of the
organs (nematocalyces) of the hydroid Plumularia
may be fed with powdered carmine, when the dust will
enter into the substance of the cells of the ectoderm,
which, like the endodermal cells of Hydra, have re-
tained the power of protruding pseudopodia. The
mesoderm, likewise, in the form of the wandering cells
of sponges, and in the larvae of Echinoderms, where
some of the organs disappear, or are not continued on
into the structure of the adult, exhibits this same
property. Even in the higher Metazoa the white
blood corpuscles have been observed by Koch to have
in their midst bacilli which they have taken into
their own substance ; and in inflammatory processes
large connective tissue-cells may be observed eating up
blood corpuscles, carmine granules, and pigment
particles.

The lowest and simplest condition of the wall of
the gastric cavity is to be seen in the lowest Ccelenterata,
which present a far more primitive arrangement than
do most of the sponges ; there is, indeed, hardly a
perceptible advance on what is found in the typical
gastrula, and such as there is, is due to the presence of
the tentacles around the mouth ; the central, or axial
sac, lined with endodermal cells, is continued into the
tentacles.

If we bear this arrangement carefully in mind, we
shall be able to refer to it the greater number of
arrangements which are to be found in the higher
Ccelenterates ; we have, in other words, to look for an

Chap. IV.] CCELENTERATA. 1 09

axial gastric cavity, with which there communicate
passages or canals. The stomach may be enlarged in
some, and diminished in other directions, and the
canals may be greatly developed in number, and pro-
vided with outgrowths or pouches ; but the essence of
the arrangement is still apparent.

When, as so frequently happens, a number of
hydroid polyps become connected with one another by
a common trunk and form a colony, the gastric cavity
of each polyp is brought more or less into relation
with those of the rest ; for each cavity is continuous
with the canal which runs in the centre of the stem
or trunk of the colony and the cells which line this
passage are provided with cilia. The facts that some
polyps occupy positions moie easily accessible' to food
currents than others, and that the less fortunately
situated can draw on theii fellows, lead, in a number of
cases, to a division of labour , those best adapted for the
business of nutrition come to limit their activities to
this important duty (trophosomes), while others,
fed at their expense, devote themselves to the equally
important duty of developing the generative products,
and so take on the especial function of reproducing the
species (gonosomes). The Stylasteridae, on the other
hand, afford us examples of zooids which, having ceased
to be nutrient, have become reduced to mere tentacles,
the duties of which they alone perform (dactylo-
zooids).

It is now necessary to direct attention to a portion
of the gastric apparatus of Hydra, which was, for the
moment, neglected ; the mouth of Hydra, or indeed of
any hydroid, is not a mere space in the wall of the
body, but forms a conical process, at the tip of which is
set the orifice, so placed that when a hydra is looked
at from the side, the mouth cone only can be seen, and
the wide mouth itself is hidden.

If we pass now to the other extreme of the series

no COMPARATIVE ANATOMY AND PHYSIOLOGY.

and examine a free-swimming Medusa (Fig. 10) we
find that the mouth and the stomach form a free
projection hanging downwards, sometimes in the shape
of a tube of some length \ around this mouth we again
find tentacles, and if we examine the first portion of
the gastric apparatus we find it is widened out to form
what may be called a stomach ; connected with this
last there are a large number of canals which channel
the substance of the disc of the umbrella, and carry
into it the nutriment prepared by the gastric cells ;
these canals have, therefore, a circulatory function, and
are, consequently, appropriately spoken of as the
gastrovascular canals. They either run simply
or are ramified, and are again brought into connection
with one another by opening into a canal which runs
round the edge of the umbrella ; from this canal
cavities sometimes pass into the tentacles which fringe
the margin of the disc.

The tentacular processes set around the mouth are
often of considerable size, and are in certain forms
broken up into a number of processes ; in one group
(that of the Rhizostomidse) this is carried to an ex-
treme, for the oral tentacles take the place of the
mouth, which, in the adult, is closed up, and they
become provided with digestive cells and openings to
the exterior ; so that in these forms a number of small
secondary orifices take the place of the single large
primitive mouth.

The other great division of the Ccelenterata, that
of the Anthozoa, presents us at once with an impor-
tant distinctive character ; for the mouth is not placed
on a projecting cone, but is depressed below the level of
the surrounding platform developed from the body wall
(Fig. 54). The second distinction is perhaps the more
important ; the tube into which the mouth leads is
widely open at the lower end ; in other words, we
have here the appearance not of a system of canals

Chap. IV.]

ANTHOZOA.

in

JLbt

channelling the surrounding tissue, but rather of a
series of chambers separated from one another by nar-
row septa, while even these are perforated by two
holes (Fig. 54 ; I}.

The mouth is an elongated slit, which sometimes
becomes constricted in its middle, so that we have
essentially two orifices.
On the ventral side of this
slit a groove is often deve-
loped, which leads into the
gastric cavity ; the cells
which line the sides of
this groove (the " siphono-
glyphe" of Hickson) (Fig.
55 ; st), are ciliated, and by
the action of these cilia
the food is carried to the
digestive region of the
body ; the presence of this
groove or the size to which
it is developed have been
observed to vary with the
size of the animal, or of
the colony of which the
polyp is a part; or, in
other words, to depend upon the demand for food
which is made by the Alcyonarian (Fig. 55).

The great size of this mouth slit, and the fact that
it is often constricted in its middle, are of considerable
Importance as bearing on the early history and func-
tion of the blastopore, or opening into the gastrula ;
in simple or archaic forms, such as Peripatus, the
blastopore is a greatly elongated slit which closes up
in the middle, and forms the mouth at one end and
the anus at the other.

ft In the Anthozoon Peachia the mouth slit is
similarly converted into two openings, one of which

artia

Tentacle ; I, internal septal stoma ;
Im, longitudinal muscle ; tm, trans-
verse muscle ; pm, parietal mus-
cle ; v, mesenterial filaments ; w,
Acontia. (After O. and R. Hertwig.)

1 12 COMPARATIVE ANATOMY AND PHYSIOLOGY.

has the function of an ingestive, and the other of an
egestive passage (Sedgwick).

The walls of the digestive tract are not, as in the
Hydrozoa,in direct contact with those of the body ; the

Fig. 55. — Transverse Section of a Polyp of Coelogorgia plumosa,
showing the long delicate cilia of the siphonoglyphe ($t). (After
Hickson.)

intermediate space is traversed by delicate plate-like
septa, some of which extend across the whole, and others
only partly project into the perigastric cavity (Fig.
54). The axial gastric space communicates at its
lower end with the compartments of the perigastric ;
and the septa project more or less inwards at this
point. Along the free edges of these septa there are
placed special filamentous structures, which are known
as the HEeseiiterial filaments, the name of mesen-
tery being applied to such septa as reach the walls of
the gastric tube. The only physiological experiments
yet made on those filaments are those of Krukenberg,
which demonstrate that these constituent cells act on

Chap, iv.] DIGESTION IN METAZOA. 113

proteids by the method of in trace! lular digestion,
and they appear to be the only part of the organism
which is entrusted with this duty.

The Ctenophora have the spaces in connection
with the axial gastric cavity narrowed to four canals,
and there are two pores at the aboral pole of the body.
There are never more than two long tentacles, and
when these are lost, as in Beroe, the mouth is much
wider than in the tentaculate forms.

For the rest of the Metazoa, with the exception of
the already mentioned Turbellaria and Trematoda
(e.g. liver fluke), the intracellular mode of digestion
has not been observed. As in some of the Coelenterata,
we have a higher mode ; the cells of the endodermal
lining of the gastric tube have now ceased to act in-
dependently of one another; certain of them are
set apart for the function of secreting a ferment,
which, passing from them into the digestive cavity,
there acts upon the food ; the albuminoids contained
in it are converted into substances capable of passing
through the wall of the intestine. Special salivary
glands are, in many, developed for the purpose of con-
verting starch into sugar. There is some evidence,
however, that certain cells continue to take up nutri-
ment into their own substance ; even in the frog some
of the cells of the small intestine have been observed
to send out short processes into the enteric cavity
(Thanhoffer), recalling thereby the amoeboid cells and
the intracellular mode of digestion which is seen in
Hydra.

Among the flat-worms we need here only consider
the Turbellaria and flukes, as the tapeworms obtain
their nutriment in a very special way. (See page 177 ;
Digestion of parasitic animals.) A mouth is always
present, but is by no means constant in position, as
it may be far forwards, at the middle of the body,
or far back (Opisthomum). In a number there is
1—16

ii4 COMPARATIVE ANATOMY AND PHYSIOLOGY.

a protrusible proboscis, formed from the anterior
portion of the digestive tract; in some the mouth
does not lead into any distinct gastric space
(Convoluta), or there may or may not be a central
space (Mesostomum) : such forms, of course, obtain
their nutriment by intracellular digestion. The
tube, when distinctly formed, may be simple through-
out, and blind at the end opposite the mouth ; or
there may be a muscular pharynx, and the tube
may have a vent or anus. The tube may be
bifurcated in its hinder part (some Trematoda), or
may give off a large number of branches, which, as
in the fluke, ramify through the body, and either
end blindly, or communicate with one another ; in
the latter cases the gastric canals have also a circu-
latory function, just like the gastro vascular canals of
the Medusae. (See page 110.)

The Nematohelmintlies have the mouth at one
end of their elongated body, and the anus not far
from the opposite end ; the digestive tube is perfectly
straight, and is strengthened anteriorly by a deposit
of chitin. The mouth, which sometimes (Gordius)
disappears during the course of development, but not,
curiously enough, until the worm has ceased to live an
endoparasitic life, is only provided with circumoral
bristles in such (Anguillulidae) as never pass any
part of their lives within other animals. Anteriorly
the tube is often widened out, well supplied with
muscles, and converted into a sucking apparatus.

The Earthworms afford an example of how an
animal may atone for the absence of certain organs
by what may be really regarded as artificial means ;
though they live on all kinds of food, and especially
on leaves, they are without any organ by means of
which their food may be broken up ; to effect this
they swallow small stones, which, acted on by the
contraction of the muscles in the walls of that

chap, iv.] DIGESTION IN EARTHWORMS.

portion of their intestine which is
gizzard, are able to pound the food
which has been taken into it; the
same phenomenon is known to be
observed in grain-eating birds. But
this is not the only method by means
of which the earthworm, with its un-
armed mouth, is able to act on the so
often dry food on which it lives j as
Mr. Darwin pointed out, we observe
in them a case of extra-stomachal di-
gestion, which, so far as is known, is
unique in the animal kingdom. Before
proceeding to swallow its food, the
worm bathes it in a fluid secreted by
the glands of the mouth ; this has not
merely a lubricating, but a distinct
chemical action, the contents of the
cells and the starch granules being, in
some observed cases, dissolved out be-
fore the leaves were taken into the
mouth. The parts of the leaves thus
acted on seemed to be sucked into the
mouth by the action of the muscular
pharynx (Fig. 56) • as the food passes
down the completely straight intestine,
it meets in the oesophagus with the
secretion of three pairs of calciferous
glands, in which we find crystals or
concretions of carbonate of lime. It
would appear that these glands are
first of all excretory organs, but the
excretion seems to have a definite
action on the food, and to prepare it
for the action of the gastric juices.

The secretion of the cells of the
intestine, by the action of which the

Fig. 56.— Diagram
of the Alimen-
tary Canal of an
Earthworm.
(After Ray Lan-
kester.)

m, Mouth ; ph, pha-
rynx; ces, oesopha-
gus ; eg, calcareous
glands ; cp, crop ; g,
gizzard ; i, intestine.

n6 COMPARATIVE ANATOMY AND PHYSIOLOGY.

food is brought into condition suitable for absorption,
is unable to exercise its activity unless the food on
which it acts is alkaline in reaction ; in other words,
its activity is arrested in an acid solution. Now, the
process of the decay of leaves is accompanied by the
formation of several acids, which must necessarily be
neutralised before the digestive fluids can act on the

a

Fig. 57.— Transverse Section of Earthworm to show the Position and
Relations of the Intestine.

«, Cuticle ; 6, hypodernris ; c, layer of circular muscles : d, layer of longitu-
dinal muscles; i, enteric cavity; m, "green layer"; n, dorsal vessel ;
o, " liver." (After Clapartde.)

ingested leaves; this neutralisation appears to be
effected by the calcareous concretions on which the
so -called humus acids readily act ; the result of their
union is an alkaline liquid.

Below the calciferous glands the oesophagus widens
out into a crop, and this is succeeded by a gizzard,
which is provided with powerful transverse muscles,
and ordinarily contains, as has been already stated,
small stones and grains of sand ; by the powerful con-
traction of its muscular walls and by the aid of these

chap, iv.] DIGESTION IN ANNUL AT A. 117

stones the gizzard becomes the organ of the earth-
worm in which the food is triturated, or ground up.
Beyond the gizzard the intestine runs straight back-
wards to the anus, which is placed quite at the end of
the body. In this intestine we first meet with a
structure which will reappear in other groups, and
affords us the first example of a method by which the
absorbing capacity of the intestine may be increased
with the greatest economy of space. A transverse
section of an intestine reveals the presence of a fold
which runs along the median dorsal line and projects
• into the enteric cavity. This is the so-called typhlo-
sole, or blind tube. Around the intestine are a
number of granular greenish cells (Fig. 57; m), which
become specially aggregated together on the dorsal
surface to form the so-called " liver " (o) ; the function
of this aggregation of cells is unknown, but it is un-
doubtedly misleading to apply to it a term of such
definite significance as that by which it is known.
This remark will apply also to the so-called livers of
other invertebrate animals.

We may easily pass from the intestinal tract of
the earthworm to those of the other ringed worms.
The absolutely unarmed condition of the mouth is not,
of course, to be expected in a blood- sucking or vora-
cious form, and thus it is that we find the leech
provided with three chitinous "jaws," hardened by a
little carbonate of lime, the edges of which are
minutely serrated, and which are provided with a
special system of muscles by means of which they are
able to work on one another ; so, again, one or more
pairs of hard chitinous or even calcareous teeth are
developed in the free-living marine worms ; these,
which are generally hooked and serrated on their con-
cave edge, work from side to side.. The earthworm is
enabled to push its pharynx forwards when seizing
food, but the voracious sea-worms can protrude their

u8 COMPARATIVE ANATOMY AND PHYSIOLOGY.

pharynx to some considerable distance and so give to
it the function of a proboscis.

The region of the crop of the earthworm is, in the
leech, specially modified in relation to the blood-suck-
ing habits of that form ; from either side it gives off
as many as eleven tubes or blind diverticula, which
occupy a very large proportion of the body cavity, and
appear to serve as strainers of the watery portion of
the blood which is pressed out through their walls.
The development of caeca is not, however, confined to
the leech, for it is found also in the sea-mouse (Aphro-
dite), where the very numerous caeca are branched
towards their free ends ; in many other marine
worms the intestine has a more or less sacculated
appearance, owing to the tube being constricted at the
points where the septa between the body-segments are
developed.

The Cepliyrea contrast strongly with the Annu-
lata so far as the arrangement of their intestine is
concerned, for this, in place of being straight, is
ordinarily coiled, and the anal opening is often found
within the limits of the anterior third of the body.
The most anterior portion of the tract has here, again,
the function of a proboscis, and is sometimes sur-
rounded by retractile tentacles ; in Bonellia, a form
which in adult life lives in mud or shells, the proboscis
is of great length, and is divided into two lobes at its
free end ; along the ventral surface of this organ there
runs a ciliated groove which reaches to the mouth,
and the whole apparatus is capable of being retracted
with great rapidity.

The Rotatoria obtain their food from the cur-
rents of water which are set in motion by the cilia on
their " wheel-organ " or disc ; and comminute it by
means of a system of hard parts which is placed in an
anterior enlargement of the intestine, and consists
typically of two hammer-like pieces which are set

chap, iv.] ROTIFERS. 119

laterally, and are caused, by the contraction of the
muscles connected with them, to work upon two
centrally set pieces, which may be regarded as forming
an anvil. Notwithstanding the minuteness of these
forms, it has been possible to form some idea as to
the character of the secretions of their digestive cells ;
red monads swallowed by them exhibiting a bright red
colour in the stomach, thanks, apparently, to the acid
reaction of the gastric juice of these forms ; in the
other parts of the intestine they have been seen to be
of a dark or brown-red colour, owing to the neutral or
alkaline reactions of the contents of that region
(Cohn).

The characters of the digestive tract of the
Rotifers present us with several instructive phe-
nomena, for we find that in the males, which are
always smaller than the females, the intestine is
nothing more than a solid cord of cells, while some-
times there is in the females themselves an indication
of degradation in the arrested development of the
terminal portion of the gut, and the consequent return
to the lower aproctoiis condition or stage in which
an anus is wanting. Nor is this all ; while the males
of Nematoid worms are distinguished from the females
by having the generative ducts opening to the exterior
by a passage common to them and the intestine, the
Rotifer among Yermes presents an arrangement which
is exceedingly common among the Vertebraba ; that is,
the possession of an enlargement or cloaca! chamber
into which there open not only the digestive and
generative tubes, but also the canals of the excretory
system.

The fixed Bryozoa likewise obtain their food by
means of the currents of water which they set in.
motion with the cilia that cover the surfaces of their
protrusible tentacles; in a number the mouth is
guarded by an outgrowth (epistome) which has a

120 COMPARATIVE ANATOMY AND PHYSIOLOGY.

singular resemblance to the foot of Molluscs. (See
page 78.) Though the epistome is probably a guard
for the mouth in most of the Bryozoa that possess it,
it is undoubtedly an organ of locomotion in the re-
markable genus Rhabdopleura. The enteric tract is
folded on itself, so that the anus is always near the
anterior end ; it is, indeed, placed either within
(Endoprocta) or without the circlet of tentacles
(Ectoprocta).

Among the Echinodermata, where the mouth
is ordinarily placed in the centre of the disc, we
find that there are either no masticatory organs,
as in the Crinoids, or that the hard skeletal pieces
are specially modified in the region of the mouth
to form the so-called " odontophore " of starfishes, or
the various kinds of mouth papillae which are found
in the Ophiuroids. These are ordinarily said to have
a masticatory function, but their small size and feeble
development justifies us rather in looking upon them
as mere filters. In the regular Echinoids, or those in
which the spherical form of the body is retained
throughout life, a very elaborate system of penta-
merally arranged parts is developed, the appearance of
which, en masse, has given rise to the popular term of
"Aristotle's Lantern." Each fifth part of this lantern
consists of a hard tooth, bevelled at the free edge like
that of a rabbit or a rat, so as to keep constantly a
sharp free edge ; this is supported in a framework,
and connected by muscles with an arched piece
(auricle) developed on the interior of the test. In
Echinanthus and its allies this " dental pyramid " is
less complex, and in the Spatangoids it has disap-
peared altogether ; so that these last are reduced to
living on such organic material as is to be found in the
sand, which they scoop up by the aid of their spout-
shaped mouths. Holothurians, likewise, are without
any special dentary organs, though the walls of their

Chap. IV.] ECHINODERMS. 121

oesophagus are ordinarily strengthened by the deposit
of calcareous plates, which are sometimes very regu-
larly arranged.

The walls of the intestines of Echinoderms are in
all cases remarkably thin, and but feebly provided with
muscular tissue, a somewhat remarkable arrangement,
when we reflect that the movement of food in their
digestive tract can be by no means aided by the
pressure of their body walls on the enteric tube
within.

In the Crinoidea the anal is always near the oral
orifice, and is placed on a projecting cone ; in Holopus,
as in some starfishes (e.g. Astropecteii) and in all
Ophiuroids, the anus is lost, so that here we have an
example of the fact that the absence of an aims is not
always to be regarded as a primitive condition.
There can be no reasonable doubt that the Crinoids
are older than the rest of the Echinoderms, and it is
only in the most aberrant of these that we find an
anus absent. Where an anus is present it is, except
in Crinoids, placed typically at the opposite pole of the
body to the mouth ; but in the irregular Echinids we
find a most interesting series in the way of modifica-
tion ; thus, in Rhyncopygus it is on the " back," but
not at the apical pole ; in Echinolampas it is at the
edge of the test, where the upper passes into the under
surface ; while in Echinoneus it is quite close to the
mouth, and, therefore, completely on the ventral
surface.

The intestine is either saccular, as in the aproctous
Echinoderms, or spirally coiled as in Crinoids and
developing starfishes, or looped as in Holothurians ;
in the proctuchous Asteroids and in some Echinids
it is provided with caeca, which in the former are
paired, and extend some way down the cavity of each
of the arms. These so-called "hepatic cseca " have
been found to have on fibrin the action of peptic

122 COMPARATIVE ANATOMY AND PHYSIOLOGY.

ferments, part of the fibrin being converted into
peptones ; and on starch that of salivary fluids.
As in many other Invertebrata, the term hepatic has
been applied to regions of the digestive tract, rather
on account of the brown coloration of these regions,
than from the definite experimental knowledge that
their secretions have in any way the functions of
a human liver. Some starfishes are capable of pro-
truding the oasophageal portion of their intestine, and
of engulfing prey, which they then draw into their
bodies.

It cannot be too much insisted on that one of the
most prominent characteristics of the Artliropoda
is the conversion of one or more pairs of its appendages
to the service of the mouth ; they become, in fact,
mouth-organs (gnathites), and are, from a physio-
logical point of view, to be regarded as part of the
digestive apparatus.

There is, perhaps, no investigation which can be
more interesting than the study of the modifications
undergone by these parts, whether we examine a
single individual, such as the lobster, with its six
pairs of mouth organs, or extend our survey over
the whole series of arthropod ous forms ; in the one
case we observe the modifications undergone by
similarly constituted parts as they take on different
parts in the duty of performing a common function,
and, in the other, we see a multitude of changes, con-
ditioned by differences in affinity and in habit. The
remarkable phenomena associated with the parasitic
mode of life of some members of this phylum will be
considered later on. (See page 179.)

When we examine a lobster or a crayfish, we find
that six pairs of appendages enter into the service of
the mouth, and that in most of them we can make
out the leading points in organisation, which are cha-
racteristic of a " typical " appendage. (See page 301.)

Chap, iv.] MOUTH ORGANS OF CRAYFISH.

123

There is, in other words, a basal portion and two
branches more or less well developed.

Of all, the most modified is the first of the six, "or
inaudible, for here the basal portion is very strong,

Fig. 68.— Mouth Organs of the Crayfish.

•*, Mandible ; B, first maxilla ; c, second maxilla ; bp, basipodite ; en, endopodite ;
cxp, coxopodite ; p, palp of mandible ; eg, scaphognathite. (After Huxley.)

and gives rise to two toothed ridges ; of these the
lower projects farther than the upper, and has a more
sharply serrated edge ; of the two branches of a
typical appendage, the endopodite is alone developed,
and that feebly, for it consists only of three compara-
tively short joints (palp).

124 COMPARATIVE ANATOMY AND PHYSIOLOGY.

From the appendage next behind, or first pair
of maxillae, the outer branch, or exopodite, is

still absent, but the basal portion is well developed,
though not so stout or so strong as in the case of the
mandible ; both its joints are flattened out and pro-
vided with a number of bristles, which are also present,
though less numerous and not so strong on the un-
jointed piece which represents the eiidopodite. So
far as the digestive process is concerned, the second
pair of maxillee are still chiefly represented by
the two basal joints of the typical appendage, the
endopodite being still small and undivided, while the
exopodite, though developed, has duties to perform
in relation to the respiratory organs. (See page 225.)
Behind the maxillse we find three pairs of maxil-
lipedes, or foot-jaws, the most posterior of which is
the largest, and in a state of repose covers over the
five pairs of mouth organs that lie in front of it. In
the two more posterior pairs we do not observe that
increase in size or flattening out of the basal portion
which we saw in the maxillae ; but in the first
maxillipede the most important part is taken by
the two lamellar joints, of which that portion is
composed, while the endopodite consists only of two
in the place of the five distinct joints which are found
in the succeeding pairs.

All these appendages are <3O articulated to the
walls of the body that they work on one another from
side to side ; it is clear that they can only cut or tear
the food on which the great forceps have already
seized, and, for the purposes of digestion, the food has
to undergo a further comminution, comparable in a
sense to that which is performed by the grinding, as
distinguished from the cutting teeth of man.

The mouth, which is a narrow elongated slit, leads
by a short wide gullet into a capacious stomach,
divided into an anterior and a posterior chamber, and

chap, iv.] GASTRIC MILL OF CRAYFISH.

125

separated from the intestine, which lies behind it, by a
filtering apparatus of valves and bristles. Within

Fig. 59.— The Parts of the " Gastric Mill " of the Crayfish in situ (A),
and disarticulated (B).

c, Cardiac ossicle; pc, pterocardiac ossicle; zc, zygrocardiac ossicle ; It, lateral tooth ;
p, pylpric ossicle; uc, urocardiac ossicle with (&) accessory tooth; pp, pre-
pyloric ossicle with (mt) median tooth. (After T. J. Parker.)

this stomach there is developed a hexagonal frame-
work of calcareous arid chitinous pieces, some of which
are provided with powerful grinding teeth. The fore
(Fig. 59 ; c) and hind (p) sides of the hexagon give
off, the one forwards and the other backwards, an
elongated ossicle (uc, pp), each so placed in relation to

126 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the other that, when at rest, the two make an open
angle towards the dorsal aspect, while the more pos-
terior has at its lower edge a strong tooth (mt) which
takes a backward direction. To the fore and hind
bars of the hexagonal framework there are attached
strong muscles, which, by contracting, draw these two
bars away from one another. This separation of the
terminal naturally requires an approximation of the
lateral faces (pc, zc), two of which bear strong teeth.
While these teeth are thus brought closer to one
another, the angulated bar which connects the fore and
hind pieces of the hexagon becomes straightened out ;
the result of this straightening is seen in the downward
and forward movement of the tooth which is developed
on the hinder median bar (mt) and which is thereby
brought into closer relation with the approximating
teeth on the side-pieces. (See Fig. 59.) This elaborate
" gastric mill " must break up the food-masses taken
in by the crayfish ; but, as if this were not enough,
the hinder part of the so-called pyloric region of the
stomach is provided with cushions covered with hairs,
and longitudinal ridges with still finer hairs, which
form a most efficient filtering apparatus. This may,
from a physiological point of view, be compared with
the sieve of hairs which lies at the entrance to the in-
testine of the fish-eating bird, the darter (Plotus).

As far as this filter the whole of the enteric tract
will be found to have its inner face lined with chitin ;
the next succeeding portion, which forms the com-
mencement of the delicate " intestine," has no such
internal layer ; but this in the crayfish, though not in
the lobster, is quite short. On it there follows the
remainder of the " intestine," and this will be found
to be again lined with chitin.

When we come to ask ourselves why so much, yet
not all, of the enteric tract of the crayfish is thus lined
by the same dense body as that which forms the outer

Chap, iv.] MOUTH ORGANS OF TRACHEATA. 127

covering of a crayfish's body, we are compelled to
turn to the history of development for an explanation.
When we do this, we find that the epiblastic infoldings,
which form respectively the stomodoeum and the
proctodoemn, are carried very far inwards, and that
only a small portion of the archenteron, or region
primitively lined by hypoblast, remains in the adult
organisation. The developing lobster, as compared
with the developing crayfish, has a much shorter proc-
todeal invagination.

While it is absolutely true of all the animals here
spoken of as Arthropoda that some of their appendages
are converted into mouth organs or gnathites, the
number is by no means always so large or the arrange-
ments so complicated as those which we have just
found to obtain in the crayfish. In Peripatus, for
example, one pair only of appendages are modified to
serve as jaws, which have the special function of
cutting blades. In the Scorpion, where there are no
appendages in front of the mouth, there are only two
pairs specially adapted to the service of the mouth,
and these have their free ends pincer-shaped, and not
converted into cutting or biting organs ; this arrange-
ment will be the more clearly understood when one
remembers that these animals suck the juices rather
than eat the tissues of their prey.

The differences between the Chilopodous and
Chilognathous Myriopoda allow us to say little
that is true of them both ; in both, however, we see
here, as in other parts of their organisation, characteris-
tics of a less high degree of differentiation than those
that obtain in the crayfish, on the one hand, or the
cockroach on the other ; there are two or three pairs
of gnathites, and these are always jointed. One is
often converted into u poison gland in the Chilopoda,
and in them also, as in the scorpion, the basal por-
tion of some of the succeeding pairs of appendages

128 COMPARATIVE ANATOMY AND PHYSIOLOGY.

surround the orifice, and aid the work of the
mouth.

In the lower Crustacea (Entomostraca) we never
find more than three pairs of appendages converted
into gnathites, and these are, in a general way, com-
parable to the mandibles and maxillae of the crayfish.

The true Insects or Arthropoda Hexapoda have,
likewise, three pairs of mouth appendages, and these,
again, are known as mandibles and maxillae ; but the
mandible is never provided with the three-jointed
" palp," which is found in the crayfish.

No series of structural changes in relation to the
different modes of taking in food is more interesting
than the really remarkable variations which are found
in the size and shape of the gnathites of insects, and
nowhere, perhaps, do we see more distinctly the in-
fluences of those two prime factors in organic evolution,
heredity and adaptation.

As Meynert has pointed out, we find in winged
insects two chief types of mouth organs ; in some the
mandibles are hinged on to the sides of the head, and
the first pair of maxillse have a less perfect articu-
lation ; sometimes, indeed, the latter merely slide on
the sides of the hard parts which bound the mouth ;
in others the mandibles and maxillse are not arti-
culated, but can be withdrawn inwards, or protruded
outwards.

In the former the articulation is such that the
gnafchites work from side to side and are fit to act as
cutting: or biting organs ; in the latter they can be
pushed into an object or laid side to side, so that they
form stabbing or sucking parts.

It is of supreme interest to observe that among
the members of the lower grade of insects, or that in
which wings are never developed (Aptera), the
mouth organs sometimes (Campodea) remain in an
undifierentiated or generalised condition ; though not

chap, iv.] MOUTH ORGANS OF INSECTS. 129

articulated to the sides of the head, they can be
moved by muscles from side to side, while, thanks to
the absence of the articulation, they can be pushed out
or drawn in ; they are, in fine, capable of acting either
as cutting or as stabbing organs, and it is in them,
therefore, that we must look for that indifferent ar-
rangement from which both the mandifoiilate and
haustellate type of mouth organs have had their
origin.

The former is well seen in the ancient group of
the Orthoptera, and is easily demonstrated by the
familiar example of the cockroach. In this form it is
quite easy to recognise the three pairs of gnathites
which, in insects, form those organs of the mouth
which are derived from modified appendages, the one
pair of mandibles, and the two pairs of maxillse.

In front of the mouth there is an upper lip or
labriim, which has the form of a movable flap ;
behind it lie the mandibles (Fig. 60 ; md), modified
appendages, of which no part other than the basal
remains, all signs of a palp having completely dis-
appeared; these work from side to side, and have
their inner edge strongly toothed, so that they act as
efficient biting organs.

Behind these we find the first pair of maxillae,
organs of some size and complexity ; the basal piece
or cardo (ca) is articulated to the head, and has,
at right angles to its long axis, the second joint or
stipes (st) ; this can move in a lateral direction, and
is continued forwards into a soft galea (go), and an
internally placed lacinia (la), the inner edge of
which is toothed, though not so strongly as the
mandibles. Attached to the outer side of the stipes
is the so-called palp.

The second pair of maxillse are still further modi-
fied, presenting as they do confluence of the basal
portions, which in most air-breathing Arthropods is
J— 16

130 COMPARATIVE ANATOMY AND PHYSIOLOGY.

still more complete. The result of this fusion is the
formation of a median piece, which is incompletely
divided into two, forming the mentiim (m), and
sub in en tii an ; these, with the anteriorly lying

ccl

Fig. 60.— Mouth-parts of the Cockroach.

m, Mentum ; sm, submentum ; K, ligula ; py, paraglossa ; pp, palp of second
maxilla ; md, mandibles ; ca, cardo ; at, Btlpes ; ja, galea ; Ja, lacmia ; p, palp.

ligiila (li) and paraglossa (pg), make up the part
which by entomologists is most unfortunately spoken
of as the lower lip or labium; unfortunately, be-
cause it is formed from the modification of the
proximal parts of an appendage, and is not strictly
comparable to the labrum or upper lip, which is a
part of the exo-skeleton of the head. The palp (pp)
of the second pair of maxillae is smaller than that of
the first

Chap, iv.] MOUTH ORGANS OF INSECTS, 131

The mouth organs of the Neuroptera are

strictly comparable to those of the Orthoptera ; but
we see an advance in the fusion of the lateral halves
of the labium, while the biting mandibles are grooved
on their inner face, and the first pair of maxillae are
slender, and are so arranged as to close the groove,
and to give rise to a pair of organs which serve as
tubes for the passage of the juices of the prey which
they have first bitten.

In the allied Trichoptera (caddis flies) the
mandibles are reduced to membranous rudiments, and
the maxillae and labium are closely united, and at
their base come to be tubular in form.

In the Coleoptera (beetles) the biting powers of
the mandibles seem to reach the maximum of their
development, and the labium has the mentum and
submentum united into single piece.

In the Hymenoptera (bees and wasps) the
mandibles still retain their biting function, but the
maxillae are modified to serve as licking and sucking
organs ; the ligula and the first pair of maxillae are
greatly elongated, and the latter apply themselves to
the sides of the former, giving rise with it to a
tubular apparatus, which comes into play after the
elongated ligula (or its accessory piece) has licked up
the honey on which their possessor depends.

The conversion of the mouth parts into a sucking
organ is most completely seen in the butterfly
(Lepidoptera) ; the mandibles are reduced to
mere rudiments, and the first pair of maxillae
are greatly elongated ; the inner face of each of
these last is deeply grooved, and the edge of the
grooves minutely denticulated in such a manner that,
when one maxilla is applied to its fellow of the
opposite side, it combines with it to form a closed
tube ; the labium is reduced, and its palps are often
very small or evanescent. The sucking tube may

132 COMPARATIVE ANATOMY AND PHYSIOLOGY.

become of considerable length when the Lepidopteron
feeds on the honey of plants, such as orchids, in
which the nectaries are at a considerable distance
from the outer edges of the flowers ; in, for example,
Amphonyx, one of the Sphingidse, the proboscis is
nine and a quarter inches long, or about three times
the length of the animal's body. In some Lepidoptera
the proboscis is enlarged to pierce vegetable tissues,
and, as in the orange-sucking
Ophideres, it has externally the
form and function of a bayonet-
shaped saw (F. Darwin).

In the blood- or juice-sucking
Hemiptera (bugs, aphides) not
only the mandibles but also
the first pair of maxillae are re-
duced to fine setiform processes,
vvhich, being moved by muscles,
are enabled to serve as stabbing
organs ; they are ensheathed in
the elongated labium (rostrum)
the sides of which curve up-
wards in such a way as to
produce a sucking-tube (Fig. 61).
In the Diptera (or flies
and fleas), what were bristles in
the bug now form sharp, cut-
ting, lance c-like organs, and the
second pair of maxillse again
form the suctorial tube ; in some
cases (Pangonia) the proboscis
is more than twice as long as the body.

Allied to various orders of insects are forms which,
in correlation with their modes of life, have their
gnathites still more considerably altered from the Or-
thopterous type ; thus, among the white ants (Termi-
tidse) the mandibles are functional in the so-called

Fig. 61.— Mouth Orgaii of
Nepa.

md, Mandible ; mx, first pair
of maxilla; ma/, second
pair (labium); li, ligula.
(After Savigny.)

Chap, iv.] ENTER ON OF INSECTS. 133

soldiers, but reduced in the workers ; the Ephe-
meridse or day-flies, which want to eat no food in the
adult stage, have the gnathites almost completely
aborted. The Mallophoga, or so-called " Mandibulate
lice," which are found on the skin of birds and mam-
mals, and feed on their feathers and hairs, have the
mandibles hooked and the maxillae small.

Like the crayfish, the cockroach has a large
portion of the anterior region of its digestive tract
lined with chitin, and, like that form, it has also a
considerable portion of the hinder region formed by
the proctodeal invagination. The chitinous layer
extends through the funnel-shaped pharynx, the
narrower 03sophagus, the crop-like enlargement, and
the proventriculus ; the last has the form of a trun-
cated cone, and its walls are thick and well-provided
with muscles; its internal lining is raised up into
ridges which serve as teeth, and between these ridges
there are pouches. The next succeeding portion has
no chitinous lining, and its anterior end has connected
with it eight blind prolongations (the so-called pyloric
caeca), which are not all of the same length, and
which vary in size according to the periods of digestive
activity ; it is, apparently, in this cavity that the
food undergoes the changes which convert it into
. chyle, and the caeca are only to be regarded as out-
growths which increase the capacity of the ventriculus.
The intestine behind is lined throughout with chitin,
and the smaller is separated from the wider portion
by a valve ; the whole tract ends in a terminal anus.

As may well be supposed, the different parts of
the digestive tract present very different characters
in the various groups of insects • in the mandi-
bulate forms (Neuroptera, Coleoptera) the stomach
is provided with a series of more or less powerful
chitinous ridges, by means of which the food is
comminuted; in the sucking insects the gizzard

134 COMPARATIVE ANATOMY AND PHYSIOLOGY.

is aborted, but the crop is swollen out into a simple
sac (bees), or into two hemispherical sacs (blowfly),
or its attached portion forms a short narrow tube
and its free part a swollen bladder-like enlarge-
ment (butterfly). This organ may extend far back
into the abdomen, and, as it has thin walls and no
muscular attachment to the body wall, its size is
probably increased and diminished by the contractions
of the hinder parts of the body ; this so-called " suck-
ing stomach " appears to act as a reservoir for food in
the Diptera.

At the anterior end of the tract there open the
ducts of the salivary glands, which are ordinarily
developed in insects, but best seen in the haustellate
forms ; they vary greatly in form and size, and are
by no means always confined to the function of
digestive glands, as the mosquito, the bug, or the
tse-tse fly are sufficient to bear witness. Many larvae
have well-developed glands which open just behind the
mouth, and which secrete a body which in air hardens
into a fine silky thread. Glands are often developed in
the walls of the rectum or large intestine, and have a
secretion which is frequently of a pungent, if not of a
disagreeable, odour. The Malpighian vessels which
are connected with the hinder portion of the tract
and open into it are not digestive glands, but organs
of renal excretion. (See page 256.)

We find a very different arrangement of mouth
organs in the Mollusca to that which we have just
been studying in the Arthropoda ; the great majority
are without any seizing organs of any kind, and the
lowest, the Lamellibranchiata, have no means by
which they can comminute their food ; they live,
therefore, on the minute organisms which are brought
to them with the water of respiration, and which are
felt for and guided to the mouth by the blunt "labial
tentacles " that lie on either side of it.

Chap, iv.] ODONTOPHORE OF MOLLUSCA. 135

The rest of the Mollusca, or Cephalophora, are
provided with a rasping organ, which lies 011 the floor
of the pharynx, the odontophore. But, anteriorly
to this, and at the edge of the mouth, there are one
or more horny plates, with a sharp cutting edge; these
are best developed in the cuttle-fishes, where they
have the appearance of a parrot's beak turned upside
down ; in the nautilus these beak-like plates are
calcined.

The characteristic organ of the digestive system of
such Mollusca as have not suffered degeneration of
the head is the just-mentioned odontophore. This
consists essentially of an overlying chitinous sheet,
the surface of which is produced into a variable
number of more or less sharp processes, the so-called
teeth ; this, then, is a rasping organ, or radii I si.
Underlying are connective and muscular fibres, and
supports are afforded for it by the development
beneath of masses of cartilage ; as muscles are inserted
into the anterior and posterior faces of these cartila-
ginous supports, it is clear that, by their alternate
contraction and relaxation, they will draw the radula
backwards and forwards. The whole apparatus is
developed in a blind diverticulum, lying on the
ventral surface of the cavity of the mouth. The
teeth, which may be very variously arranged, are
greatly strengthened by the deposit of silica ; and as
they and the chitinous sheet are worn away they are
replaced by the hinder part of the radula, which
passes forwards on its bed ; the replacement of the
effete parts being effected, in other words, in a way
comparable to that of the human nail. The radula is
ordinarily divisible into a central piece, with a lateral
piece on either side ; the teeth on the former are
spoken of as the racliidiaii teeth, and those on the
lateral pieces as the iincini. The arrangement of
these teeth varies very greatly, and, for the purpose

136 COMPARATIVE ANATOMY AND PHYSIOLOGY.

of succinctly stating their numbers and positions, the
following method of formulation is used.

The central teeth of the rachidian series are
denominated by the sign 1, when present, and 0 when
absent; the admedian teeth by the signs 1,2,3...,
according to the number present ; while the lateral
teeth are noted by the sign 0 repeated as often as
there are lateral teeth on either side ; when the
number of admedian or of lateral teeth is very large,
the sign x is used in place of 1, 2, 3. .. , or 0 repeated.
For example, when, as in ^Eolis, there are no lateral
teeth, we write the formula 0.1.0; that of Amphis-
phyra, is 1 . 1 . 1 ; that of Aplysia, 13 . 1 . 13 ; and that
of Oncidium, 54. 1 . 54 (Woodward).

The whole mass of the odontophore may be of
considerable size, and, in the limpet, the radula is two
or three times the length of the body; the number
of separate teeth may be very great, as among
the snails, where 167 transverse rows of 135 teeth
each will give some twenty thousand teeth ; in some
species of Helicidae, the aggregate exceeds thirty-
nine thousand (39,596).

The teeth are sometimes large and hooked ; some-
times conical and upstanding; when the rachidian
teeth are, as sometimes happens, absent, another part
of the digestive tract may, as in the Bullidae, be pro-
vided with calcareous plates which replace them
functionally. In a few (e.g. Rhodope) the odonto-
phore is lost.

In a number of cases, the muscles that move the
radula are not confined to those that are inserted into
the supporting cartilages, but there are others that
pass to the walls of the head ; the contraction of
these is the cause of the licking movement which a
protruded radula may be often seen to perform.

In some, especially slugs and snails, a hard horny
plate is developed on the roof of the mouth cavity,

Chap, iv.j ENTERON OF MOLLUSC A. 137

and aids the radula in its work of trituration, just as
the hard pad which takes the place of the upper in-
cisors of the sheep serves as a resistent structure
against which the lower incisors may bite.

At the sides of the anterior portion of the digestive
tract glands of various forms are ordinarily found ;
these are known as salivary glands ; but the inappro-
priateness of the name is not only obvious from the
observed fact that in the slug the secretion of these
glands has no influence on starch, but is made the
more striking so soon as we know that, in several
genera, the secretion of these glands contains a com-
paratively large amount (nearly three per cent, in
Dolium) of free sulphuric, and a smaller quantity of
hydrochloric, acid. Further, we have to note that
these buccal glands are found in marine as well as in
terrestrial forms, whereas among the vertebrata the
salivary glands are only well developed in terrestrial
forms.

The intestine is considerably coiled ; the oasopha-
geal portion is sometimes produced into a " crop," as
in Lymnseus or Octopus \ the succeeding portion may
be simple, and have its walls thin or muscular, or it
may be broken up into several chambers ; in Scyllsea
it is armed internally with horny cutting blades, and
in Aplysia with blunt horny spines, behind which is
an armature of sharp hooks. It is only behind such
gizzard-like enlargements that the digestive ferments
are secreted. The anal orifice is, in those Cephalo-
phora that have lost their original bilateral symmetry,
brought far forwards, and situated near the mouth,
or is placed at the side of the body. Ca3cal pouches
or tubes are developed in various ways along the tract
of the intestine, and some of them become charged
with dark-coloured cells, and have been regarded as
forming a " liver ; " there is, however, no reason for
associating with these structures the functions of the

138 COMPARATIVE ANATOMY AND PHYSIOLOGY.

similarly named part in the Yertebrata ; and, indeed,
where best studied, they have been found to have
rather the function of the pancreas. In the dibran-
chiate Cephalopoda a rectal caecum secretes an inky
fluid, which was formerly used for writing and for
the manufacture of sepia ; this is the so-called ink-
bag. The secretions of the Octopus have been found
to be all acid.

In the Lamellibranchiata (or Acephala) the
odontophore is completely absent ; the intestinal tract
is comparatively simple, but varies in the extent of
its convolutions ; in its walls, or in an appended
caecum, is the so-called crystalline style or stalk, a
transparent rod-like structure of unknown function.
Its absorbent surface is sometimes increased by the
development of a typhlosole, as in the earthworm,
and the terminal portion very frequently passes
through the dorsally-placed heart.

In all Chordata we observe that, as also in
Balanoglossus, the anterior posterior of the diges-
tive tract is primarily divisible into an upper and a
lower portion, one of which serves as the means of
passage for the water of respiration, and the other as
the food passage. Postponing for the moment (see
page 231) the consideration of the former, and insist-
ing only on the significance of this arrangement as a
leading point in the morphology of the Chordata,
we observe that in the Tunicata the exclusively
nutrient region of the enteric tract commences at
the bottom of the respiratory part by a rounded
or funnel-shaped opening; the tube, which varies in
calibre in different parts, is often looped, and in
such cases the anus comes to lie not far from the
mouth.

Among the Cliordata we find very simple
arrangements of the digestive tract in the Cepha-
locliordata ; the mouth of the Lancelet is placed

Chap, iv.] ENTER ON OF CHORDATA. 139

on the ventral surface of the body, not far from the
anterior end, is over-hung by a hood, and supported
by cartilaginous bars, which bear ciliated cirri, the
gill-like appearance of which gained for the animal
the misleading name of Branchiostoma. As these
cirri are moved by muscles they are enabled to direct
food to the mouth, and to serve as a filter against the
entrance of sand and other useless or dangerous
bodies. As in the rest of the lower Chordata, this
mouth serves as the orifice of entrance for the water
of respiration, which makes its way to the exterior
through numerous spaces in the wall of the more an-
terior region of the digestive tract. The part of the
tract behind the gill chamber is of some width, and
gradually narrows as it approaches the anus, which
is situated on the ventral surface not far from the
hinder end of the body, At its anterior end it gives
off a forwardly directed short blind process, which is
known as the liver. As in the Nemertinea, the
enteric epithelium is ciliated. In the Uroclior-
clata a large part of the anterior region of the
enteron is again converted into a respiratory cham-
ber ; and it is the succeeding portion only that is
limited to the duties of a digestive apparatus. The
tube varies in width in different regions and is
ordinarily coiled on itself, so that the anal is not far
from the oral orifice ; the food passes into it along
a groove which lies on the ventral surface of the
respiratory chamber, the sides of which are ciliated,
and the cells of which secrete a mucous substance
which entangles the food and carries it into the
03sophagus. The anterior orifice of the oesophagus
is generally funnel-shaped, and provided with cilia ;
the succeeding portion has a diverticulum, which is
spoken of as the liver, and it may be further provided
with other glandular organs. In some cases the
digestive tube is coiled into a closely compacted

140 COMPARATIVE ANATOMY AND PHYSIOLOGY.

\ mass, as in the Salpidse, where it forms the so-called
" nucleus."

In those Vertebrates that breathe by gills the
water of respiration enters by the mouth ; in the air-
breathing forms the air enters the mouth by the nostrils,
so that in their case also the most anterior portion of
the digestive tract serves as an ante-chamber to the
respiratory organs. Leaving these functions aside
for the moment (see page 231) and confining ourselves
to the mouth as a part of the digestive apparatus, we
observe that it is rounded in shape in the lowest,
the Cyclostomata (lampreys and hags), and merely
supported by cartilages ; in all the rest it is more or
less slit-like, and a pair of branchial arches give rise
to jaws (Gnathostomata). These jaws are either
covered by connective tissue, or horny plates (tor-
toises, birds, monotremata), or, as in all Mammals,
except the lowest and the whales, they are guarded and
aided by movable muscular structures, which are
known as lips. These aid in the taking in of food,
or in retaining it when it has entered the mouth
cavity ; in the production of sounds, and especially of
human speech ; and they are an important factor in
the production of the expression of the emotions.

In the vertebrate series we apply the term teeth
to those hard bodies which are set in the mouth, and
are developed from cells of epiblastic origin ; in their
simplest condition these organs are more or less
simple spiny bodies, exactly comparable to the spines
which are found on the skin of many sharks. Nor
is the community a community of structure
merely; from within the limits of the history of
an individual it is possible to draw sufficient evidence
to prove that there is a community of origin
between what have been well called dermal
denticles and what we call teeth. The accom-
panying figure, which represents a section of the lower

Chap. IV.]

TEETH OF VERTEBRATES.

141

jaw of a dogfish, at a stage previous to that at which any
lip is developed, shows the direct continuity of struc-
tures, which, in the adult, seem to be very different
from each other. When we consider the different re-
lations to the surrounding parts which would be entered
into by the spines that
came to lie within
the area of the mouth,
we see at once that,
by being brought into
contact with the food
the spines would be
led to increase in size
and strength \ but this
increase in activity
would be necessarily
accompanied by a
richer nervous and
vascular supply; and
this, reacting on the
spines, would lead to
greater differentia-
tion, which has taken the form of greater definiteness
in arrangement and structure.

In commencing, therefore, a review of the teeth
of vertebrates, we find that we start with a general-
ised or non-differentiated condition ; as we pass on
we shall find that the teeth become more and more
limited to certain bones, and diminish in number ; in
other words, there is a gradual reduction. Concur-
rently with this, we have to note that, when a group
of animals becomes especially adapted to a certain
mode of life, or presents marked aberrations from the
general plan of structure, they become edentulous,
or lose their teeth ; such, for example, are the pipe-
fishes among Fishes, toads among Amphibia, Chelonia
(turtles and tortoises) among Reptiles, all recent

62. — Section of Lower Jaw of
young Dog-fish, showing the spines
of the skin under the jaw, and the
teeth ahove. (After C. S. Tomes.)

142 COMPARATIVE ANATOMY AND PHYSIOLOGY.

birds, the duckbill and the echidna, and some of
the whales among Mammals; this is a phenomenon
not confined to Vertebrata, for it may be observed in
the Spatangoids among Echinoidea, where the
" Lantern of Aristotle " is altogether absent, and in
the tubicolous Chsetopods, which have lost the
strong jaws of their free-swimming allies.

In correspondence with the great diversity of
mode of life and of details of structure among fishes,
we note in that group the very greatest differences
in the disposition and size of the teeth; seeing, indeed,
here an excellent illustration of the law that com-
mencing structures are subject to great variability.
Here, too, we find an example of spines on the skin
taking on the function of teeth ; the true teeth, that
is to say, the hard structures within the area of the
mouth, are, in the saw-fish (Pristis), quite small and
blunt ; the sides of the enormous snout are, however,
provided with large dermal spines, set at regular
distances from one another, and each implanted in a
special socket.

When well developed, as in the dog-fish, the teeth
are set in several concentric rows ; those of the outer
are alone functional, and they, as all, are not
attached to the jaw, but are only fixed in
the covering membrane ; this membrane appears
to move over the surface of the jaw, and
thereby the teeth which have been in use for a time
are removed from the edge of the jaw, and the
next succeeding series come to occupy their position,
and to take on their function.

A large number of small teeth are likewise to be
found in many bony fishes (Teleostei), and here, where
a number of distinct bones are developed^ we often
find every bone within the mouth bearing teeth ; as
may readily be supposed, such teeth are generally
of small sizej and without any special masticatory

Chap. IV.]

TEETH OF FISHES.

143

a

function ; indeed, in very many fishes the food is
swallowed whole.

Owing to the fact that these Vertebrates are not
able to put their fins to the duty of seizing their food
in the way in which many higher forms use their
anterior pair of limbs, the teeth may often be observed
to have a special prehensile function. This power is
sometimes developed to an extraordinary degree ; all
the numerous teeth in the mouth of the pike are
directed backwards, and so prevent or oppose the
escape of any prey which has been taken into the
mouth ; an extension of this arrangement has been de-
scribed in the angler (Lophius), where some of the
larger teeth in the front of the mouth are so attached
to the edge of the jaw that they spring up again as
soon as the food
which has pressed
them downwards
into the mouth
has passed them
and entered the
oral cavi ty
(Tomes). By
this means the
prey is caught as
in a trap.

Where the
teeth are used for
the purposes of
breaking up the food or the shell in which it is con-
tained, they become of considerable size, as in the
Wrasses ; in the parrot- Wrasses (Scarus) the teeth un-
dergo fusion with their neighbours ; in the sheep's-head
(Sargus) the teeth in the front of the mouth are cutting
organs, and those at the sides larger, and have their
surface rounded, so that, as they move on one another,
they act as grinders. Where a number of teeth are

'ig. 63.— Lower pharyngeal Bone of Scants,
showing Teeth of different Ages. Two-thirds
natural size. (After C. S. Tomes.)

144 COMPARATIVE ANATOMY AND PHYSIOLOGY.

required they are not always confined to the bones
of the skull ; thus, in the just-mentioned Scarus, the
lower pharyngeal bones unite, and they, like the
upper pharyngeals, are armed with crushing teeth (Fig.
63) ; here, then, we have an instance of the bones of the
branchial arches (see page 328), being tooth-bearing.
Another example is afforded by the carp, in which fish
the bones of the skull are all devoid of teeth, which
are confined to the lower pharyngeals ; these, as in
the case of the incisors of the sheep or ox, do not
work on upper teeth, but on a hard process, which,
in the carp, is developed on the occipital bone of the
skull.

In other fishes the teeth are exceedingly deli-
cate, as in Chcetodon, which has gained its name
from the bristle-like character of these organs. In
a few cases the teeth are placed in distinct sockets,
as in the file-fishes, of which Balistes is an example ;
in Lepidosteus the socket is not complete, and the
tooth becomes anchylosed to its walls.

Lepidosiren presents an arrangement not unlike
that which is found in Rodents among Mammals, for
the front edge of the teeth is harder than the rest,
which therefore wears down sooner, and leaves a sharp
cutting edge. In no group of the Vertebrata are these
organs of greater value to the palaeontologist than
among Fishes, as the discovery in Australian rivers
of Ceratodus, which had been thought to have been
extinct since the time of the deposit of the older
secondary rocks, is sufficient to bear witness.

In a few cases there are differences between the
teeth of males and females, as in the skate (Fig.
64 ; A and B) where those of the male are more
pointed than those of the female.

In the male salmon, at the breeding season, the
lower jaw is produced into stout hooks, and in corre-
spondence with this the anterior end of the upper jaw

Chap. IV.J

TEETH OF AMPHIBIA.

is also enlarged, and the premaxillary teeth are four
times as large as those of the corresponding region in
the female.

Small and simple as are the teeth of most recent
Amphibians, they are, as compared with those of
most fishes, greatly reduced in number ; this, no doubt,
is largely to be explained by the development of the

Fig. 64.— Teeth of Skate. A, Male ; B, female.

fore-limbs into organs which are capable of seizing and
holding the prey, or of pushing it into the mouth ; we
find, too, that the great majority of the teeth are now
found on the membrane bones (see page 329), at the
sides of the mouth only; though, indeed, the frog has fine
vomeriiie teeth, and other amphibians have them on
the palatine, or the pterygoid bones.

Whether we pass now to the Reptiles or to the
Mammalia, we get still more marked indications of
this reduction ; in all the latter which have teeth, and
in all crocodiles and many lizards, teeth are found only
on the lower jaw, and on the maxillse and premaxillse.
K— 16

146 COMPARATIVE ANATOMY AND PHYSIOLOGY.

As may be supposed, the teeth of the crocodile are of
great size and strength.

An instructive example of a quasi- edentulous con-
dition is found in the lizard of New Zealand, which is
known as Hatteria ; the teeth at the sides of the jaw
are not replaced when they are worn down; but the
bone itself, which is exceedingly dense, takes on the
function of a cutting organ. In forms which are perma-
nently edentulous, like the tortoise or the pigeon, the
edges of the jaws become invested in horn, the shape
and form of which varies with the habits of the
animal ; in some birds (wild duck) the edges of the
horny case become serrated and give rise to the
appearance of tooth- like structures ; in some cases
(Odontopteryx) the edge of the underlying bone be-
comes denticulated ; these are adaptations to modes of
life, and must be carefully distinguished from the
actual possession of true teeth such as characterises a
large group of extinct birds (Odontomitlies).

Curiously resembling the arrangement of the turtle,
and having, of course, much the same function, are
the tough horny plates on the jaws of tadpoles ; the
history of these plates would, however, seem to be very
different from that of the similarly disposed parts in
the higher forms ; that is to say, the beak of the
tadpole, and, doubtless, the horny apparatus of the
lamprey, aie structures which preceded, and not suc-
ceeded, the possession of teeth.

A phenomenon similar to that seen in Lophius is
to be observed in Snakes ; here, again, the organs of
prehension being absent, owing to the disappearance
of the limbs (see page 96), the teeth are directed back-
wards ; when, therefore, living prey, such as a frog, has
entered into the cavity of the mouth, it is prevented
from escaping out of it by the erection of the teeth.
Some snakes kill their food by constriction, and swal-
low it at leisure ; others swallow it whole, and in them

Chap. IV.]

TEETH OF REPTILES.

the bones of the skull are loosely connected with one
another, and so allow of the enlargement of the cavity
of the mouth ; others, finally, kill their prey by biting
and simultaneously injecting poison into the wound.
In these last (the venomous snakes) there may be
several not very long teeth in the maxillary bone, or
there may be
but one maxil-
lary tooth, which
in such cases is
long and mov-
able. In the
former the fangs
are distinctly
grooved along
some part of
their length, but
the sides of this

groove are suffi- •"••-""Hr-WMte /,
ciently close to
form a service-
able canal, along
which the poison
from the poison
gland may make
its way into the

WOlllld In the ^^* ^* — Transverse Section of the Poison Farg

latter the single ], Tooth in use; 2, the tooth which will succeed it; 3 to
IQTTCO ma villar-tr 10, tooth-sacs numbered in the order of their succcs-

large maxillary sion- (After c. s. Tomes.)
tooth appears,

from the outside, to be solid and ungrooved ; the real
fact is, however, that the two edges of the groove have
completely united to form a closed canal, the existence
of which becomes apparent in a transverse section of
the tooth ; the opening of the canal is not placed quite
at the tip of the sharp fang, but, just as in a subcu-
taneous injection syringe, the orifice is a little behind

148 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the tip which, as it were, guides and saves the
poisonous fluid.

As a further aid to the more extreme venomous
forms, which can only subsist by this mode of attack-
ing their prey, and which are eminently liable to have
their organ of offence broken in the act of "striking,"
the reserve teeth are arranged in a manner which
seems to be unique in the animal kingdom. Instead
of a single series of reserve teeth set in one and the
same line with the tooth in active function, there are
two rows, in each of which the pair of teeth are
almost of the same age and grade of development.
When, therefore, the active tooth is lost, that in the
other line, which is lying beside it (Fig. 65), is ready
at once to move forwards into a little different
position, and to take on its function ; by this means
the fang is replaced with a minimum loss of time (C.
S. Tomes).

When we come to the Mammalia, where, as
has been already said, teeth are never found except in
the mandibles, maxillae, and premaxillae, we are met,
at the outset, with an arrangement of which, at
present, it seems impossible to afford any altogether
satisfactory explanation. There are never more than
two sets of teeth, one of which is temporary or
milk, and the other permanent* the teeth of
these two sets differ in form, size, and number.
In some cases the milk teeth are never of any use ;
such mammals may be conveniently spoken of as
monophyodont, while those in which there are two
sets may be similarly called diphyodont. At the
same time it must be carefully borne in mind that
that there is no sharp delimitation between these two
groups. In marsupials and guinea-pigs there is only
one milk molar ; in the rabbit the milk incisors dis-
appear before birth, and among edentates only one
species (Tatusia peba) is known to have milk-teeth.

Chap, iv.] TEETH OF MAMMALS. 149

The definite diphyodont arrangement is Lest seen in
the higher Mammals.

The possession of this double series is not, how-
ever, the only remarkable character of the teeth of
Mammals ; while there are only inconsiderable, if any,
differences in the form of the teeth of any given fish
or reptile, and such differences are characteristic only
of small groups, we find that for a large number of
Mammals, though by no means in all, the teeth in
different regions of the mouth have distinctly and
definitely different forms and function; (1) in the
anterior portion we find sharp cutting teeth ; (2) at
the sides we sometimes see strong seizing or holding
or offensive organs, and, farther back (3) we see that
the upper surface of the tooth becomes widened out
and tubercnlated so as to form a more or less suitable
grinding surface. Looked at in a general way, these
three kinds or forms of teeth may be grouped as (1)
incisors, (2) canines, or (3) molars. The molars
are spoken of in diphyodonts as premolars or
molars, according as they are or are not preceded by
milk or deciduous molars. Mammals with variously
formed teeth are conveniently known as hetero-
donts ; while a nomodont dentition is ascribed to
such forms as the edentates, or the toothed whales, in
which all the teeth have exactly the same character.

When a homodont dentition obtains, the number
of teeth in the jaws may be very great, some dolphins
having as many as two hundred (Pontoporia) ; in the
other forms the number of teeth is strictly limited, no
known living mammal having more than forty-eight
teeth (Megalotis).

In comparing the teeth of one heterodont with
those of another, it is very convenient to make use of
the set of symbols which make up the "dental for-
mula ; " here the letters i, c, pm, and m, represent
the different categories of teeth, while the fraction

150 COMPARATIVE ANATOMY AND PHYSIOLOGY.

sign is used to represent the disposition in the upper
and lower jaws. Making use of this method of
formulation, we may represent the typical dentition
of a heterodont mammal thus :

. 3.3 1.1 4.4 3.3

t — , c — , nm — , 7» — = 44.
3.3 1.1' " 4.4' 3.3

This is the dental formula of the low insectivorous
mammal Gymnura.

The dental formula of man is :

2.2 1.1 2.2 3.3

And that of the cat :

3.3 1.1 3.3 1.1

« s73 c TTi pm 272 m i7i = 30<

There are a number of certain modifications in
the form or structure of the tooth which at once
attract attention. While those Mammals, such as man,
which bite their food have sharp incisors, those that
gnaw it have the greater part of the surface of the
incisors devoid of that hardest part of the tooth
which is called the enamel, and so maintain an edge,
the softer dentine always wearing down faster than
the enamel which is at the front and sides. The
teeth which are set at the outer angles of the jaw
(the canines) are especially large in those forms which
seize on a living, and require to hold a struggling
prey \ and it is these organs which are most frequently
converted into weapons of attack, though in the most
prominent case of all, that of the elephant, the tusks
belong to the incisor series. In the male boars
(Suidse), the canines attain to a considerable length,
being in the Babirussa, an animal not so large as the

Chap, iv.] TEETH OF MAMMALS. 151

domestic pig, from eight to ten inches long ; they are
often exceedingly sharp,, and are capable of inflicting
on those whom they attack severe and deep wounds.
In the males of the anthropoid apes the canines are
always much larger than in the females, and as they
are not developed till later, we are justified in
believing that they are sexual weapons of attack, by
which the males are aided in fighting with one
another for the possession of the females.

The molars again present us with indications of
differences in the form of the teeth corresponding to
differences in the character of the food ; at the same
time, the very greatest care must be taken in esti-
mating the kind of food from the form of the teeth,
and at all times, where it is possible, the general
arrangements of the alimentary canal must be steadily
borne in mind. Four chief types may, however, be
easily distinguished (1) carnivorous, (2) insecti-
vorous, (3) frugivorous, (4) herbivorous. The
molar or premolar tooth of the dog, which has, since
the time of Cuvier, been distin-
guished as the carnassial, is
modified to form a sharp blade,
and is continued behind into a thick
tubercle (Fig. 67; A).

The typical insectivorous tooth
is distinguished by the development
of four or five sharp cusps (Fig. 66) ; F3£ gj Tootl1
in the frugivorous forms the cusps
are not so distinct, nor so sharp, and are more con-
nected with one another by more or less distinct
ridges.

The most essential point in the arrangement of
the herbivorous or grinding molar tooth is the dis-
position of the several constituent tissues of which it
is made up ; the large squarish solid structure, formed
of pillars and ridges, is composed of enamel and

152 COMPARATIVE ANATOMY AND PHYSIOLOGY.

dentine ;* as lias already been said, these two tissue
differ greatly in hardness ; they will, therefore, wes
down unequally and so give rise to a roughened su
face well adapted for grinding. In addition to tl;

Fig. 67.— Teeth of Wolf.

i 1 to i 3, Incisors ; c, canine ; p 1 to » 4, preraolars ; m 1 to TO 3, molars ; h, he
of the first molar or lower " carnassial" tooth.

outer investment of cement, we find it also filling u
the interspaces (Fig. 68). In the Garni vora th
lower jaw is so articulated to the skull as to lc
able to work from below upwards ; in the herbivoroi;
forms the jaw works from side to side.

Some of the Cetacea (dolphins, whales) are dii
tinguished by the total absence of teeth, and th

* For an account of the minute structure of the teeth, s<
Klein's "Elements of Histology," chap. xxi.

Chap. IV.]

TEETH OF CETACEA.

153

special armature of the mouth by " whalebone ; "

but indications are presented both by palaeontology

and embryology which are sufficient to justify us in

believing that this

order was primi-

tively provided

with a heterodont

dentition. While

the dolphin has a

number of teeth

in both jaws ; the

sperm-whale has

well-developed

teeth in the lower

jaw only; the

bottle-nosed whale

has never more

than four func-

tional teeth, and in

the narwal (Mono-

don) it is the male

only that has the

ordinarily single

tusk well de-

veloped ; this may

be as much as nine

feet long. Among

the whale-bone

whales teeth are

never more than

rudimentary, but

in the adult their

place is taken by plates of " baleen," which are

set nearly at right angles to the axis of the mouth,

and have their free ends frayed out into a number

of stiff hairs, which make a most efficient strainer;

the whale, in taking into its enormous mouth a

Lower Jaw.

154 COMPARATIVE ANATOMY AND PHYSIOLOGY.

quantity of sea-water, simultaneously takes in a large
number of the smaller marine animals; raising its
tongue it drives out the water, but retains behind the
filter the food that came in with it (Fig. 70).

In all vertebrates, with the exception of some
fishes and a few amphibians (e.g. Pipa), there is
developed in the floor of the mouth a tongue, which
owes its primitive origin to the mucous membrane
which covers the branchial arches. It is never of

Fig. 69.— Last Molars of (A) African, (B) Indian Elephant.

large size in fishes, and in them is never supplied with
muscular tissue as it is in the higher forms ; in some
cases it is provided with a horny sheath.

In many forms, as in some Amphibians (e.g. the
frog), where it is attached by its anterior end to the
symphysis of the lower jaw ; in Chameleons, where it
is knobbed at its extremity ; and in various other
lizards, where it is cleft anteriorly, as it is also in
Ophidia, it is capable of considerable protrusion, and
may be used as a prehensile organ.

Among Birds the tongue is protruded with
great rapidity by the wood -pecker (Picidse), and
by the humming-birds (Trochilidse), and sun-birds

Chap, iv.] BALEEN. 155

(Nectarinidse). In the wood-pecker some of the hyoid

Fig. 70.— Figures to illustrate Position and Structure of the Baleen.
(From Murie, after various authors.)

A, Hind view of skull of right whale ; w, whale-bone ; ra, mesial ridge of palate ;
J, lower jaw-none* ; B, arch of baleen plates, as seen in cross section of the
mouth ; c, vertical section, showing the intermediate substance; is, Baleen
plates ; b, frayed out at their free edges ; D, transverse section of whale-bone,
magnified.

156 COMPARATIVE ANATOMY AND PHYSIOLOGY.

bones, or bones that support the tongue, are of con-
siderable length, form a curve with its concavity
forwards, and then, trending forwards, lie on the
iipper surface of the skull, and reach as far as the
nasal region. Attached to these bones, and attached
also to the anterior end of either mandible, is an
extensor muscle, which lies on the inner or concave
side of the loop ; on its contraction the loop is drawn
up, and as the only mobile point of the hyoid bones is
that at which their anterior extremities are inserted
into the tongue, it is clear that the upward movement
of the bones must result in the forward movement of
the tongue ; in some other Picidse the hyoid bones
are movable in their sheath, but the result is the same.
Essentially similar arrangements are to be seen in the
humming-birds and sun-birds. The mechanical prin-
ciple, that the longer the hyoid bones the greater the
force and extent of the protrusion of the tongue, is
supported by what has been observed in Picus as
compared with Zosterops (Gadow).

The tongue, thus protruded, is in the woodpecker
invested in a horny sheath, which ends in a slender
point, and is provided on either side with backwardly
directed prickles, which serve to draw from the holes
in the wood the insects on which this bird feeds ;
their capture is aided by the slimy secretion of the
salivary glands, which are compressed on the con-
traction of the extensor muscle. (Compare the account
of the Great Ant-eater, page 160.)

In the sun-birds the horny part of the tongue
forms two tubes ; in the honey-eaters there may be as
many as eight, and the inner or outer margins
become frayed out into fine processes; in the
humming-birds there are tubes, the edges of which
are ordinarily entire. It has been pointed out by
Gadow, that while the sucking in of honey is an easy
process when there is sufficient fluid to fill the anterior

Chap, iv.] TONGUE OF VERTEBRATES. 157

opening of the entire tubes, air would, when there
is not, rush in inste'ad, and that the advantage of the
frayed margins lies in the fact that the honey will
ascend to the tubular portion by capillary attraction.
It is important to note that these arrangements are of
a homoplastic character, inasmuch as the humming-
birds are not close zoological allies of the sun-birds ;
in other words, we have here again an example of
how similar structures are gained by forms which live
under similar external conditions.

In the Parrots the end of the fleshy tongue is
dilated, and, from its prehensile function, has been
compared to a human finger; on its lower surface
there is a broad nail-like horny plate, which is free at
its anterior border ; in the brush-tongued parrots
there are spinous papillae on the upper surface, which
stand upright, or project forwards when the tongue is
protruded.

The tongue of Mammals is well provided with
muscular tissue, so that it is not only protrusible (it
is of great length in insectivorous forms like the
ant-eater), but is capable of a licking movement;
and in some, such as the lion, it is armed with strong
horny papillae that are of considerable assistance in
removing flesh from bones. In man, in addition to
the kind of functions just enumerated, it is of impor-
tance as an accessory organ of articulate language. In
some mammals a tongue like structure (the sufolingna)
is developed beneath the tongue; in the lemurine
Galago (Fig. 71), the front end of this organ is provided
with five stiff denticles, arranged in comb-like fashion,
and having, apparently, the function of keeping clean
the incisor teeth (Flower). In the dog, and some other
Carnivora, the protrusible tongue is supported by
a fibrous and muscular body, the so-called "lytta,"
or worm of the dog's tongue.

The walls of the cavity of the mouth are provided

158 COMPARATIVE ANATOMY AND PHYSIOLOGY.

with glands, which are outgrowths of the walls them-
selves ; of these the most important and best known
are the salivary glands par excellence ; but these
are only specially modified forms of the simple
tubular, which, with the compound tubular glands,
are alone found in the lower Vertebrata, and which
have at first no other function than that of lubri-
cating the tongue, the mouth cavity, and the food ;

and they are,
therefore, only
feebly, if at all,
developed in
fishes.

In the Am-
phibia the most
important is the
intermaxillary or
internasal gland
(Wiedersheim),
an idea as to the
function of which

Fig. 71.-Side View and Lo*er Surface of the is to be gathered
Tongue of a Gaiago. from the fact that

in the water-
living Axolotl it is but feebly developed, while in
the metamorphosed and pulmonate form known
as Amblystoma it is of much larger size. In this
group, further, where the tongue is so often used
as an organ of prehension, the lingual glands are
well developed, and their secretion is driven on to the
surface of the tongue with every contraction of that
organ » In the chamseleons, which likewise catch
their prey by the aid of their tongue, the labial and
palatine glands are well developed ; the median of
these latter may be regarded as the homologue of the
intermaxillary of the Amphibia. The glands that
lie in the floor of the mouth are ordinarily well

Chap. IV.]

SALIVARY GLANDS.

developed in the Lacertilia, and very richly so in the

poisonous Heloderma of Mexico ; in correspondence

with the position of these glands the

reptile is said to turn on its back

when striking its prey (Fischer) ; the

secretion of these sub maxillary

glands has certainly poisonous effects,

}}ut the blood of a guinea-pig killed

by it does, unlike the blood of the

victim of the bite of a colubrine

snake, but like that of the victim of

a viperine snake, coagulate after

death (Fayrer). In the venomous

Ophidia the poison is supplied by

the labial gland, which lies along the

edge of the upper jaw, and has its

duct opening into the maxillary

tooth.

The salivary glands are small in
river tortoises, and absent in the
marine Chelonia, and in the alligator,
where the glands are confined to the
tongue.

In Birds, again, lingual glands
are well developed, but, as may be
supposed, the labial are altogether
absent ; in the wood-peckers, where
the tongue is protruded with great
rapidity (see page 156), the sub-
linguals are enormously developed,
and provide a sticky secretion, which
acts like bird-lime.

In the Mammalia, three pairs of large, ordinarily
acinous, glands predominate over the smaller and
more scattered bticcal glands ; these, from their posi-
tions, are distinguished as the parotid, submaxil-
lary, and subliiiguals. From our knowledge

seen from below,
showing the large
Sublingual Glands
(i, i), the Hyoid
Bones (e), and
the Base of the
Tongue (/). (After
Macgillivray.)

160 COMPARATIVE ANATOMY AND PHYSIOLOGY.

of the influence which the secretion of these glands
has on starchy foods in ourselves, it is often thought
that therein lies their prime function. Certain con-
siderations seem to show that this is not a correct
view ; in the Cetacea and other aquatic forms, and in
the blood-sucking Desmodus, the glands are con-
siderably reduced in size ; the dog " bolts " his food,
or, in other words, does not subject it to the influence,
which is not exerted in a moment, of the salivary
secretions ; while the kangaroo, which dwells in the
arid plains of Australia, has some of the glands of
great size. It would seem, therefore, that the prime
function of these structures is to afford a supply of
water for the solid food, and a further physiological
advantage is gained by the fact of this water being
supplied at or near to the temperature of the mam-
malian body.

The well-known fact in human physiology that
the secretion of the parotids is the most watery, and
that of the submaxillary and sublinguals more viscid,
is paralleled in comparative anatomy by the large size
of the parotids in animals, such as ruminants, and
other herbivorous forms, which masticate dry foods ;
and the great size of the submaxillary in such
animals as the Echidna, or the ant-eater, which
require a viscid fluid for the purpose of catching their
insect prey. In the last-mentioned mammal some of
the fibres of the stylo-hyoid muscle encircle the sub-
maxillary ducts, which are thereby constricted when
the muscle contracts, and in this way the ejection of
the fluid is assisted (W. A. Forbes).

With the exception of the buccal cavity, the greater
part of the enteric tract of a Vertebrate is developed
from the archenteron, the proctodeal portion being, as
a rule, exceedingly short. The general tract may be
conveniently divided into the oasophagus, stomach,
intestine, and rectal portion ; this last, in all except

Chap, iv.] INTESTINE OF VERTEBRATES. 161

some Fishes and the higher Mammalia, opens into an
epiblastic pit, the cloaca, into which there also open
the ducts of the renal and generative glands. (See page
263.) Primitively, as in Amphioxus, this tract is quite
straight, and has no definite outgrowths along any
parts of its course ; later on it undergoes considerable
modifications ; its anterior region gives rise to an
outgrowth, which serves at first as an air bladder,
and later on becomes converted into a pair of lungs,
which serve as the definite respiratory organs of all
higher vertebrates (see page 236) ; the stomach ceases to
have its long axis parallel to the long axis of the body,
and at the same time becomes enlarged and more or
less complicated ; the intestine buds off two glands of
high physiological importance, the liver (only feebly,
if really, represented by a caecum in Amphioxus) and
the pancreas ; the intestine becomes narrower an-
teriorly than it is posteriorly, so that one may distin-
guish a small and a large intestine ; between
these a blind outgrowth, or caecum, which in Birds
is often double, is nearly always developed ; while
the inner face of the walls of part of the intestine is
thrown into folds, whereby the extent of the absorbing
surface of this region is very greatly increased.

So far as the intricacy of the tract is concerned,
we find it to be a very general rule, not only in Ver-
tebrates, but in all animals, that carnivorous, as
compared with herbivorous, forms have a simpler
enteric tract ; thus, the gar-pike (Belone) has the
intestine short and straight, and the herbivorous tad-
pole has a more complexly coiled intestine than the
insectivorous frog ; the proportion of the length of
the intestine in the cat is to the body as from 3 to
5 times to 1, while in the pig the proportion is as
12 to 1 ; so, too, the milk-fed calf has a much less
complex stomach than the grass-eating and ruminating
cow.

L— 16

162 COMPARATIVE ANATOMY AND PHYSIOLOGY.

In Amphibians and Reptiles the oesophagus is ordi-
narily wide, and but few adaptive modifications are
to be noticed in it. Oligodon, an egg-eating snake,
presents a remarkable arrangement. It will be easily
seen that, were this animal to break in its mouth the
eggs on which it feeds, the greater part of the contents
would almost certainly escape. We find, however,
that the teeth in the mouth are quite rudimentary,
and that through the upper wall of the gullet there
project the elongated inferior spines of several of the
vertebrse of this region ; the tips of these spines are
coated with a substance of great hardness, and the
eggs, after they have reached a position in which
their contents can be safely disposed of by the animal,
are broken by them.

The most important modification of the ossophagus
is to be found in birds, and especially those that are
grain-eating ; there is here developed a considerable
enlargement, the so-called "crop," which may not
(cassowary), or may (pigeon), be provided with special
glands in its walls. The object of the former arrange-
ment would appear to be merely that of a kind of
reserve-pouch, for it is found in fish-eating forms and
in birds of prey. On the other hand, the glandular
crop is a necessity for such birds as live on food which
is so difficult of digestion as is grain*

Throughout the whole of the series we meet again
and again with examples of a stomach only slightly,
if at all, distinguished by its size from the rest of the
tract ; and this is not always due to the carnivorous
habits of the animal, but rather, as we must suppose,
to heredity, and to the fact that some other part of
the intestine performs the more important part in the
functions of digestion. Ceratodus is a striking
example of this, for here the stomach is almost in
a straight line with the general axis of the body,
while the interior of the small intestine is elaborately

Chap, iv.] GIZZARD OF BIRDS. 163

developed. Two leading types of stomach have been
distinguished in bony fishes ; the so-called siphonal, in
which the two halves of the stomach are bent upon
themselves, as in the salmon, and the csecal form, in
which the upper or cardiac portion gives off a long
blind sac. The walls of the stomach are richly pro-
vided with muscle in the mullets.

The simplicity of the resophagus is found to extend
to the stomach in most Amphibians and Reptiles, and
the chief modification which we observe is an increas-
ing tendency for the stomach to be set at right angles
to the long axis of the body. In the crocodile the
walls are very muscular, and call to mind those of
some birds. In the latter group, the members of
which are, it will be remembered, toothless, the so-
called stomach is divided into two more or less
distinct portions. The anterior of these (the
proventriculus) has, as a rule, walls which are
richly provided with glands that exert a chemical
action on the food, while the posterior portion, or
gizzard, has walls of considerable thickness, owing
to the great development of its muscles ; it seems to
have only a mechanical action on the food. Two
chief types of gizzard may be distinguished, the
simpler, which is found in such birds as are carni-
vorous or insectivorous or live on soft fruits, has the
walls much less thick than in the other type ; the
muscles radiate out from two tendinous centres,
which are placed one on either side of the stomach,
and the muscular fibres pass from one to the other.
The crocodile also has such centra tendiiiea.

The complicated form of gizzard, or gizzard par
excellence, is distinguished from, the more simple by
the great development of its lateral muscles, and by
the conversion of its lining wall into a strong horny
layer, which is particularly thick, and may even form a
hard pad on either side (Fig. 73). The tendinous patches,

164 COMPARATIVE ANATOMY AND PHYSIOLOGY.

and the fibres that they give off, are very strong. We
have already noted the presence of small pebbles in
the gizzard ; the duty of these is to act as grinding
stones. On the contraction of the muscles the cavity

Fig. 73.— Horizontal Sections of the Gizzard of a Goose, in contraction
(A) and relaxation (B). (After Garrod ; Proceedings of the Zoolo-
gical Society, 1872, page 527.)

of the gizzard is, of course, diminished in extent, and
the food contained in it crushed against the stones.
It sometimes happens that the lower end of one pad
and the upper end of the other are respectively better
developed than the rest ; when this happens a slight
sliding is added to the crushing movement, which
must have considerable influence in. breaking hard
grains.

Chap. IV.]

STOMACH OF MAMMALS.

165

In the heron, which lives especially on fish, which
it swallows whole, the stomach is of great size and
extent, reaching nearly to the anus, and occupying
the greater part of the abdominal cavity.

In the Mammalia the stomach is often set more
or less at right angles to the longitudinal axis of the
body, and its upper is shorter than its lower curva-
ture, which may,
therefore, be distin-
guished from one
another as the
greater and the
less ; the enlarge-
ment of the cardiac
side of the stomach
is a little more
marked in the dog
and in man than in
the insectivorous
Gymnura, and very
much more so in the
rabbit. As the trans-
verse axis increases
in length, a division
into two parts be-
comes more or less well pronounced ; this is best seen
in the interior, and is very well marked in the case of
the horse (Fig. 74), where the white-coloured cardiac
portion, with its thick epithelium, is separated by a ridge
from the redder pyloric sac, with its softer epithelium
and its contained gastric glands. This separation of
the stomach into a reservoir and a digestive portion is
carried to an extreme in the ruminating Ungulates.

In the peccary the stomach may be divided into
cardiac and pyloric regions, but the esophagus is
further remarkable for being continued on, in the
form of a groove, into the pyloric division ; in addition

Fig. 74.— Inner Face of the "Wall of the
Stomach of the Horse.

a. Cardiac ; &, pyloric sac ; c, duodenal dilata-
tion. (After Chauveau.)

Pig. 75.— Stomach of a Eliminating Animal.

A. Exterior; B, interior; a, oesophagus ; b, rumen or paunch; c, reticulum or
lioney-comb bag ; d, psalterium or manyplies ; e, abomasum or rennet-
stomach.

chap, iv.i STOMACH OF RUMINANTS. 167

to this, a well-marked ridge divides the cardiac part
into two regions. In the chevrotain (Tragulus) the
outer portion of the cardiac division forms a large
paimcti, and is incompletely separated from a portion
lying just below the oesophagus, the inner surface of
which is raised up so as to form a honeycomb-like
arrangement of ridges ; from this the pyloric diges-
tive portion opens by a narrow tube-like piece. In the
horned ruminants (Fig. 75) this tube-like piece becomes
more distinctly separated from the rest of the pyloric
portion, and its inner surface becomes raised up into
a number of folds of mucous membrane, which are
closely appressed to one another, and permit only of
the passage of the most finely comminuted food. All
gradations in complexity are to be observed in the
size and number of these lamellae of the psalteriiim.

In the desert-dwelling camels, and in their allies,
the llamas of South America, part of the cardiac
region is converted into a number of pouches, which
are provided with sphincter muscles, by which they
can be shut off from the rest of the cavity ; these
pouches make up the so-called water-bag1 of these
animals.

In the blood-sucking bat (Desmodus, Fig. 76),
where little digestive secretion is required, on account
of the nature of the food, the pyloric portion of the
stomach is very short, but the cardiac is converted
into a wide caecum, the length of which may be double
that of the body of the animal, and which, no doubt,
serves as a reservoir for the blood that is sucked in as
food.

The region beyond the stomach is known as the
intestine; it is characterised by having at its com*
mencement the orifices of the bile ducts from the liver,
and it is often sharply divisible into a narrower small,
and a wider large intestine. The capacity of the
internal surface is increased, as in the stomach, by the

1 68 COMPARATIVE ANATOMY AND PHYSIOLOGY,

development of longitudinal and transverse folds : the
most remarkable of these ingrowths is one that we
cannot refrain from associating with the typhlosole of
the earthworm and of the fresh-water mussel ; when
best developed, as in the elasmobranchs, this fold

Fig. 76. — Stomach of Desmodus, in the form of a long caeca! process.

forms a spiral valve. In a more rudimentary con-
dition it is presented as an incomplete spire in the
lampreys, as consisting of thin whorls in the chi-
msera, or of varying degrees of complication in various
ganoids, and as sometimes forming a closely appressed
series of folds in the skate. Among the Teleostei it
is very rarely developed, though a projection of this
kind is certainly to be seen in some specimens of
Chirocentrus, and has been expressly said to be found
in Butirinus (Stannius). In the Teleostei there appear
tubular outgrowths of the upper end of the small
intestine, which may be supposed to take the place of
the absent spiral valve ; these pyloric appendages*

Chap, iv.] ENTERON OF VERTEBRATES.

169

the number of which varies considerably in different
forms, have been sometimes found to be full of food
(Spatularia Wieders-
heim), and are cer-
tainly developed in
inverse proportion to
the spiral valve itself.
They are only one of
the many means by
which vertebrate ani-
mals increase the ab-
sorbing surface of their
intestine without en-
croaching greatly on
the space occupied by
the other organs of the
abdominal cavity.

In some Amphibia,
as in Siren, the differ-
ence in calibre be-
tween the small and
large intestine is not
very well marked ; in
others, as the frog,
there is a considerable
difference. The same
kind of difference is
seen among the Rep-
tilia ; for the Ophidia
and the Amphisbcena
have the greater part
of the tract of the same
width, and the whole intestine is less coiled than in forms
with a shorter body, such as the Chelonia or the Croco-
dilia. In Birds the first duodenal loop of the intestine
always encloses the pancreas ; the intestine is propor-
tionately long, but while nesting-birds, such as the

Fig. 77. — Diagram of the General Ar-
rangement of the Abdominal Portion
of the Intestine of Mammalia, seen
from in front ; the small intestine is
greatly abbreviated. (After Flower.)

p, Pylorus ; d, duodenum ; i, ileum ;
r, rectum.

170 COMPARATIVE ANATOMY AND PHYSIOLOGY.

sparrow, in which the yolk is almost or altogether
used up before the bird leaves the egg, rapidly acquire
the enteric proportions of the adult, those which leave
the egg, like the chick, before the yolk material has
been all used, are longer in acquiring the proportions
of intestine which obtain in adult forms (Gadow).

In the Mammalia the small intestine is artificially
divided into a duodenum, a jejunum, and an
ileum ; the large intestine is of greater length than
in other Vertebrates, and the terminal portion only is
straight or rectal (Fig. 77).

The rectum in. many Fishes does not open into a
cloacal pit, but lies in front of the u rino-genital oritice ;
in all other Vertebrata, except the Eutheria, the rectal
and renal orifices open into a cloaca, while in the
higher Mammals the anus lies behind the urinogenital
opening. In the Chelonia, where the walls of the
abdomen exert little or no influence on the move-
ments of the intestine, part of the terminal portion of
the gut may be enlarged, and its walls provided with
abundant muscular tissue (elephant tortoise). The
walls of the rectum are sometimes provided with
glandular appendages, such as the foursa fabricii of
Birds, and the bursar anales of Chelonia ; the func-
tion of these organs is unknown.

The large is sometimes separated from the small
intestine by a blind ingrowth or diverticulum, the
ceecum ; this is first seen in the Reptilia, where it
varies somewhat in size in different forms, but it is
never of any considerable extent. In most Birds the
caecum is double, and the length of these outgrowths
are in direct relation with that of the intestine, and
therefore with the habits of the bird ; they are, in
other words, longer in herbivorous than in other forms ;
it has, however, been pointed out by Gadow, that
when the intestine is narrow the tract is long and
the caeca short, while other short caeca may also be

Chap, iv.] C&CA OF MAMMALS. 171

associated with a short and wide intestine. With
these caeca there must not be confounded the frequently
present outgrowth on the small intestine, which is the
curiously permanent remnant of the vitelline duct,
by which food is obtained from the yolk.

The caecum of Mammals presents some very re-
markable variations ; and we find many exceptions to
the generalisation, that it is short in carnivorous and
long in herbivorous forms. While its shortness or
absence in the true Carnivora would seem to afford a
support to the generalisation just indicated, it is more
probable that its small size or absence in insectivorous
forms, is rather to be associated with the dangers to
which the hard chitinous coverings of their food
might expose animals that fed on insects ; the same
explanation will apply to the insectivorous bats, and
one of the same sort to the frugivorous species of
Chiroptera, or to the fish-eating otter. At the same
time, it is not to be thought that all frugivorous
mammals are without caeca ; not only has man a
caecum, but the fruit-eating monkeys have one also.
Herein we find one of the great arguments against
prophesying as to the habits of an animal from what
we can see as to its structure, and find a sufficient
answer to those who assume that the parts of an
animal are in accordance with its habits. The tele-
ologist takes no account of that influence which, as we
have shown again and again, is no less an important
factor than adaptation, the factor of heredity.

It is clear enough that a cherry-stone impacted in
a narrow caecum may produce inflammation, and yet
an animal that has been so successful in the struggle
for existence as has man, has not only a caecum
(which is about two or two and a half inches in. width),
but this is continued into an appendix vermiformis,
which is not so much as half an inch in width. So
far as man is concerned, we may well suppose that he

172 COMPARATIVE ANATOMY AND PHYSIOLOGY.

has inherited this structure, for it is found also in all
the anthropoid apes, while its wider distribution
among the ancestors of the present Mammalia is
spoken to by the presence of a vermiform prolongation
to the caecum in the rabbit. An appendix which is
physiologically, even if it be not morphologically com-
parable to that of a man, is to be seen in the wombat
among the Marsupialia.

Though it is not correct to say that the caecum is
largest in herbivorous mammals, for it is absent in the
bears, who are the most herbivorous of the Carnivora,
yet in many cases there is a certain relation between
the size of the caecum and that of the stomach ; for
example, in the rabbit the stomach is simple, while
the caecum is enormous (more than 18 inches in length),
and its absorbent surface is increased by the develop-
ment in its interior of a large spiral valve ; similarly,
the horse has a very large caecum ; on the other hand,
the Ruminantia, in which the stomach is so complexly
arranged, have a simple caecum. It would, therefore,
appear that when the absorbing surface of the stomach
is not greatly increased by adaptive modifications, a
special region of the intestine is entrusted with the
functions that the stomach cannot perform.

In Hyrax there are two caeca.

In all Vertebrates the liver appears to arise as a
bud or outgrowth from the wall of the enteron, and
this embryonic condition is that which is seen in the
lancelet, where, probably, the organ has no definite
function ; in most of the higher forms the secretion
from the cells is stored up in a receptacle connected
with the bile ducts ; this is known as the gall
bladder, and it is generally, though not invariably,
attached to the right lobe. The primitive bud veiy
early becomes double, though this is effected in differ-
ent ways ; in the chick the two diverticula are at first
unequal, in sharks and Amphibians the primary

Chap. IV.)

LIVER OF MAMMALS.

'73

outgrowth becomes bilobed, while in the rabbit two
diverticula are developed, but not simultaneously.

The fat formed in the liver is, in many Fishes, fluid ;
or, in other words, oil ; in these animals the organ is
often of large size in proportion to the rest of the

iff

Tig. 78.— Diagramatic View of the Inferior or Visceral Surface of a
Multilobed Liver of a Mammal extended transversely.

tr, Inferior vena cava; p, vena portae; it, umbilical vein of the foetus, represented
by the round ligament in the adult, lying in the umbilical fissure ; 77. left
lateral lobe ; Ic, left central lobe : re, right central lobe ; rl. right lateral lobe ;
s, spigelian lobe ; <-, caudate lobe ; g, gall bladder ; dr, remnant of ductus
venosus ; llf, left lateral fissure ; c/, central fissure ; rlf, right lateral fissure.
(After Flower.)

intestines, and much less firm in its consistency than
in the higher divisions of the Vertebrata.

With regard to the liver of Mammals (Fig. 78),
various attempts have been made to form a satisfactory
system of nomenclature for its lobes, but it remained
for Flower to suggest one which should gain universal
acceptance,

We are taught by embryology that the liver
ordinarily arises by two lateral outgrowths from the
intestine, so that it is primitively bilobed ; in the
young (foetus) the umbilical vein divides the liver

174 COMPARATIVE ANATOMY AND PHYSIOLOGY.

into two parts, and this vein is retained in a rudi-
mentary condition in the adult as the round, liga-
ment. It may, therefore, be taken as lying in the
middle line of the liver ; the parts or lobes that lie on
one side are the right lobes, those that lie on the
other are the left lobes. The fissure in which the
remains of the umbilical vein lie is called the
umbilical fissure, but it is not always the deepest ;
where a lateral fissure is as deep, or deeper, the
liver appears to lose its bilobate character, and seems
often to consist of three chief parts. The characters
and relations of the umbilical fissure must, therefore,
be carefully borne in mind if we desire to retain the
proper morphological conception of the liver as an
originally bilobecl organ. A well-marked lateral
fissure being frequently found on either side of the
umbilical, it results that the organ is often found to
consist of four chief lobes ; these, from their topo-
graphical relations, may be spoken of as right
central, left central, right lateral, and left
lateral ; while the whole mass of lobes on either
side of the primitive middle line may be called re-
spectively the right and left segments.

So far, then, we find that the liver is an organ
consisting of two segments, each of which may be
divided into two or more lobes. All the more impor-
tant modifications of the liver affect its right segment;
with the right central there is very frequently con-
nected a reservoir or gall bladder ; the right lateral
often develops a prolongation on its lower surface,
which is known as the spigelian lobe ; another
accessory to be developed from the right lateral is the
so-called caudate lobe.

While all these parts are to be found in the human
liver, we find some considerable variations exhibited
in different mammalia ; in some cases (Cetacea, Peris-
sodactyla, and some other Ungulata, etc.) the gall

chap, iv.] LIVER OF VERTEBRATES. 175

bladder is absent, though the ducts may be enlarged
at their extremity; sometimes (hippopotamus) it is
present or absent, and, in the lemurs, it may be seen
on the convex aspect of the liver. Sometimes the
segments of the liver are greatly subdivided, and there
are considerable differences in the depth of the fissures;
thus, in the porpoises, the two segments are subequal
and no further divided, while in the seal there are a
number of minute notches.

The liver is often adapted to the form of the body
of its possessor, being elongated in elongated, truncated
in shorter forms ; in the lower Vertebrates it largely
retains its primitive bilobate character. The ducts by
which its secretion passes into the intestine vary con-
siderably in number and arrangement, and even closely
allied forms may or may not be provided with the
reservoir which is known as the gall bladder.

In Vertebrates with " hot blood " the bulk of the
liver is, in proportion to the size of the animal, less
than in the so-called cold-blooded orders ; this, no
doubt, is to be associated with the greater demand
which is made by the former on the store of fat which
accumulates in the liver, and which is more rapidly
used up by them than by those animals in which
oxydation is less extensive. The observations that
have been as yet made on the " glycogenic " function
of the liver have been directed rather to a study of
the mode of production of this starchy compound
than to the differences which obtain in different groups
of Vertebrates.

A special outgrowth of the wall of the intestine
gives rise in most Vertebrates to an important diges-
tive or ferment-producing organ, the pancreas. It
is ordinarily connected with the duodenal region of
the intestine, into which its duct often directly opens ;
in other cases, as in the frog, the duct opens into the
bile duct. The details of the comparative physiology

176 COMPARATIVE ANATOMY AND PHYSIOLOGY.

of this organ still await investigation, but sufficient is
now known as to its importance to lead us to hope for
much instructive information as to the action of its
secretion in different animals.

Although it is absolutely certain that structural
characters are profoundly modified by changes in
function, or, in the words of John Hunter, " I dare
say the different manner of living gives rise to the
different formation of the viscera," it is, on the other
hand, a fact beyond contradiction that in two purely
herbivorous animals, such as, for example, the horse
and the cow, or piscivorous forms as the seal and the
porpoise, we find anatomical structures which are
strikingly different ; to understand this we must again
invoke the principle which, as we have already said,
stands equal in value to that of the power of varying
with varying circumstances ; certain modifications of
structure are impossible to certain animals, on account
of the influence of heredity ; in other words, descent
as much as environment has to be taken into account
in the study of the morphological characters of the
parts of any organism.

The first definite evidence as to the influence of
food on the structural characters of the digestive canal
was given by John Hunter, when he fed a sea-gull for
a year on barley, and found that the muscular tissue
of the gizzard became enormously developed.

It is stated that "this experiment is annually
repeated by Nature; that the herring-gull, Larus
tridactylus, of the Shetland Islands, twice every year
changes the structure of its stomach according to its
food, which consists during the summer of grain and
in winter of fish." Somewhat similar observations
have been made on the raven and the owl, and the
converse experiment, or that of converting the stomach
of the grain-eating pigeon to the carnivorous type, has
been effected by Holmgren.

Chap IV.]

PARASITIC HABITS.

177

The influence of the parasitic mode of life on
the organs of digestion is exceedingly well marked.
One of the most obvious and common results is the
loss by endoparasites of a mouth ; this, among the
Protozoa, obtains in the Gregarines, which, living in
organs or cavities of other animals (such as the intes-
tine of the lobster, or the testicular reservoirs of the
earthworm) that are rich in nutrient fluids, obtain
their necessary nourishment
through their cuticle by the merely
physical process of osmosis.
Among the ciliated .Infusori an s,
Opalina is mouthless. The same
phenomenon is seen among the
Metazoa in Echinorhynchus and
the Cestoda, which in their adult
condition live always in the di-
gestive cavity of Vertebrates.
What is certainly true of Tsenia
serrata (Fredericq), namely, that
no digestive ferment is to be found
in anv part of its body, is doubt- Fig. 79.— Atineta tube-
less true also of other Cestoda, ^VaextendelZd
and is to be explained by the retracted,
fact that these animals live in the
midst of food which is being made ready to pass
through animal membranes.

In another large set of cases food is obtained by
suction; among the Protozoa this is seen in the ecto-
parasitic Acinetse (Fig. 79), where elongated tubular
processes of protoplasm arise from the surface of
the body ; these tentacles, as they are often called,
are capable of very rapid protrusion ; their knobbed
ends widen into sucking discs, and are able to pene-
trate the cuticle of their prey, which are ordinarily
ciliated Infusoria ; the semifluid endosarc is then drawn
up through the granular axis of the sucking tube.

M-16

178 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Among the Metazoa sucking tubes are best developed
in the Rhizocephala; these parasitic Crustaceans begin
life in the shape of free-swimming naupliiform larvae
(see page 534), attach themselves to the bodies of
higher Crustacea, and, losing all appearance of having
true appendages, develop at their anterior end a

Fig. 80. — Sacculina, carcini. A, Adult form showing the tuft of roots
which it insert* into the boJy of its host. B, The naupliiform larva.
(After Haeckel.)

number of filamentar processes. These make their
way into the body of the host, and by endosmosis
take up nutriment, which they pass on to the shapeless
body.

The mouth and the commencement of the diges-
tive tract are often adapted for sucking, as in the
liver-fluke, where the contractions of the protractor
and retractor muscles of the pharynx effect this pur-
pose ; in the JVematoid worms, where, as in the
fluke, the intestine is complete, and is, moreover,
provided with an anus, muscles set radially around
the oesophagus extend to the walls of the body ;

Chap, iv.] SUCTORIAL HABITS. 179

on their contraction the cavity of the resophagus is
greatly increased, and a vacuum is thereby formed;
here, then, the tube acts as a sucking-pump. The
great care needed in making any generalisations in
comparative physiology is well spoken to by the
curious fact that among the parasitic Nematoids a
mouthless condition obtains only in the free-living
stages ; thus Gordius has a mouth when parasitic,
but when it leads a free life the mouth is lost, and the
worm is dependent on the store accumulated in its
earlier stage ; in its ally, Mermis, the peri-o3sophageal
muscles are lost in the free stage of existence, though
the mouth remains. In the leech the sucking action
is effected in essentially the same way as in the
Nematoid. In the scorpion, where the mouth is ex-
cessively minute, the pharynx is pear-shaped, and has
attached to its wall transversely set muscles, which,
on contraction, increase the extent of the cavity, and
so cause a vacuum which results in an up-flow of the
fluids of the prey which it has stung to death. A
large number of Araclmida have a distinct sucking
apparatus.

The changes induced by parasitic habits on the
conformation of external parts of the body are, as
may be supposed, most striking in the Artliropoda,
Among the Entomostraca a very instructive series of
gradations may be made out. The gnathites of
Cyclops are in Caligus, which is a temporary parasite
on fishes, enclosed in a tube formed by the fore-and-
hind-lips ; the anterior pair, or mandibles, are alone
well developed, and form piercing processes. In
Corycseus the suctorial tube is not developed. In both
of these there are swimming feet. In Lerneea, which
may be found on the gills of the cod, to which the
adult females are permanently attached, the swimming
feet are small. In Achtheres, which is found on the
perch, these feet are wanting, and a pair of gnathites

180 COMPARATIVE ANATOMY AND PHYSIOLOGY.

unite to form a single sucker, or disc of attachment.
As we pass from Cyclops to Lernsea or Achtheres, the
form of the body becomes less and less obviously seg-
mented, and more and more bizarre in appearance. In
Argulus, the common parasite of the stickleback and
other fishes, the changes are still more marked ; the
suctorial tube is large, and has within a pair of finely
toothed mandibles and style -shaped maxillae ; in
front of the mouth is a pointed spinous tube, contain-
ing the ducts of what are supposed to be poison
glands ; the anterior pair of maxillipeds (Glaus) form
large sucking discs on either side of the mouth.

Among the higher Crustacea the parasitic forms
belong to the group of the Isopoda ; the Bopyridse
have a sucking proboscis, and their mandible is with-
out a palp ; some, like Entoniscus and Cryptoniscus,
are ordinarily lernseoid in form, when adult ; one
species of the latter, which is parasitic on a Sacculina,
which is parasitic on a Pagurus, has so peculiar a
form as to have received the specific name of plana-
roides.

Among the Arachnida, many of the mites
(Acarina) are parasitic, and the bases of the two an-
terior pairs of appendages form a sucking proboscis,
as in Demodex, which dwells in the hair follicles of
various mammals ; the females of itch-mites are able
to bore under the skin ; in the blood-sucking ticks the
proboscis is provided with a number of hooks. In
Pentastomum, which has two hosts, and is endo-para-
sitic in both of them, the only signs of appendages to
the body are the two hooks on either side of the
mouth.

Parasitism is very rare among Molliisca ; Mont-
acuta lives among the spines of Spatangus, Stylifer on
sea-urchins, among corals, or in starfishes, but neither
of these are specially modified. Entoconcha, parasitic
in Holothurians, is merely known as an ovigerous sac.

Chap, v.] THE BLOOD. 181

No Vertebrate is truly parasitic, for Myxine
(the " Borer ") penetrates the body of other fish for
the direct purpose of feeding on their flesh ; and the
relation of Fierasfers to the Medusae, Echinoderms, and
Molluscs, with which they have been found, is only
that of a guest which makes use of the currents of
water which the host intended for its own purposes
(SYMBIOSIS).

CHAPTER V.

THE BLOOD AND THE BLOOD-VASCULAR SYSTEM.

THE result of the process of digestion is the formation
of a quantity of material which can be usefully taken
up by the different cells of which the body is com-
posed, and used by them for the purposes of repair,
growth, and reproduction. In a majority of cases the
material has yet another function, inasmuch as it
becomes the vehicle for the oxygen which is constantly
necessary to cell-activity ; it is respiratory as well
as nutrient.

This material takes the form of a liquid in which
cells or corpuscles float ; and these cells are either
colourless, or tinged red by haemoglobin, and
are either amoeboid or constant in form. The
liquid is known as the plasma, and it is either
colourless, or tinged red, green, or blue.

The different characteristics of the various parts of
this nutrient and respiratory medium are most clearly
seen in the Yertebrata, with which, therefore, we will
commence.

The blood is a fluid containing white and red cor-
puscles, a certain amount of dissolved albuminous
and mineral bodies, and about half its own volume of

1 82 COMPARATIVE ANATOMY AND PHYSIOLOGY.

gases \ the red would appear to be derived from the
white corpuscles, and some, at any rate, of these last,
owe their origin to the colourless amreboid corpuscles
of the lymph; this lymph, again, is finally depen-
dent for the production of fresh corpuscles on the
thicker milky fluid which is found in the lymphatics
of the intestines (chyle) ; and this chyle is, of course,
due to the metamorphosis of the food taken into the
digestive tract, and there converted into peptones and
other diffusible bodies.

The white or colourless corpuscles are amreboid in
form, vary a good deal in size, but are constantly
smaller in Mammals than in other Vertebrates, and
while, on the whole, less numerous than the red cor-
puscles, they vary in number according to the state of
lasting or repletion.

In all Vertebrates save Mammals the red blood
corpuscles are provided with a nucleus ; in all Mam-
mals, except the camel and the llama, the red discs are
circular in form, as they are also in the cyclos tomato us
fishes ; in the remaining Vertebrates the discs are
elliptical. These red corpuscles differ very greatly in
size ; largest in the urodelous amphibian Amphiuma,
where they measure about -^ millimetre by -^ ; they
are smallest in the Qhevrotain (Tragulus), where they
are only —^ millimetre in diameter. The interesting
researches of Gulliver have shown that, within the
limits of any given natural group, the corpuscles are
largest in the largest, and smallest in the smallest
species of the group. The average number of red
corpuscles in a cubic centimetre of human blood has
been estimated at five millions ; in the goat there have
been found in the same quantity of fluid eighteen
millions ; in the rabbit, three and a half millions ;
in a cock, two to three millions or more ; in
bony fishes seven hundred thousand to two millions,
and in various Elasmobranchs from one hundred

Chap, v.] BLOOD CORPUSCLES. 183

and forty thousand to two hundred and thirty thou-
sand (Malassez).

In Amphioxus and the Urochordata, there are
no red blood corpuscles, and, as a rule, these are not
found in what have been called invertebrates ; they
have, however, been observed in Solen (a Mollusc) ;
Glycera (a Chsetopod) ; Amphiporus (a Nemertean) :
and Phoronis (a Gephyrean).

In most Invertebrates the blood corpuscles
are either limited to the fluid in the body cavity, or
are found more or less numerously represented in
the fluid contained in the vessels. In all these cases
they are single cells, generally amoeboid in character,
and they vary considerably in size and number.

In Echiiioderms, Arthropods, and Molluscs,
the corpusculated fluid is contained in a system of
more or less completely closed walls (vide infra) ; in a
large number of worms the fluid in the vessels is, on
the other hand, said to be non- corpusculated, and, at any
rate where corpuscles are found, they are often, as in
the earthworm, rare, and of small size (-g-oVo ijicn j
Lankester). It is, however, only quite recently that
observations have been directed to the presence of
corpuscles in the blood of Annulata, and since then
they have been observed in several members of the
group (Eunice, Nereis).

BLOOD-VESSELS AND HEARTS.

In all the forms already mentioned, part of the
blood at least is contained in a system of more or less
completely closed vessels, by means of which it is
conveyed from part to part of the body ; these vessels
make up the blood- vascular system. When best
developed this system has, on some parts of its course,
a contractile organ, by means of which the fluid is
pumped along ; this is the heart. In the Vertebrata,
the vessels given off from the heart (arteries) do not,

184 COMPARATIVE ANATOMY AND PHYSIOLOGY.

as in the crayfish, open into spaces in the ccelom,
thence to be taken up again by the vessels which pass
to the heart (veins), but they are conveyed through
networks of fine hair-like vessels (capillaries), which
are completely closed. So, again, the chyle, or the
direct result of the products of digestion, is contained
in vessels which make their way into the veins (the
more or less completely closed lymphatic system).

In the MoUusca and Arthropoda the blood-
vascular system is not completely closed ; in other
words, the blood makes its way from the arteries into
spaces or cavities in the body cavity, and from these
incompletely closed spaces it is again taken up by the
veins ; it is clear, therefore, that no closed or proper
lymphatic system is here needed ; the results of the
process of digestion make their way through the walls
of the intestine directly into the ccelom, and thence
into the sinuses, and in this way they replenish the
store of corpuscles in the blood of the crayfish or the
mussel.

In the Echinodermata the blood-vascular sys-
tem would appear to be completely shut off from the
ccelorn, and, as it would seem to be connected with the
system of water vessels, it is, no doubt, diluted by
sea-water ; but the fluid in the coelom contains char-
acteristic corpuscles, some of which are coloured.
Curiously enough, liEemoglobin has been detected in
the water-vascular system of an Ophiuroid (Foettinger).

In the Protozoa there is, of course, no blood,
but even in the Amoeba we observe currents within
the protoplasm, and some of these are, no doubt,
richer in nutriment than others, so that by their
movement the distribution of nutritious material is
equalised over the whole organism ; in Paramcecium
there is an advance on this, inasmuch as in it currents
of definite directions are to be detected. In the
Sponge the currents of water that traverse its walls

chap. v.] ' BLOOD-VASCULAR SYSTEM. 185

bring in oxygen and food material. In the Coelen-
terata there are, as we know, out-growths of the
enteric cavity, which, in reference to their functions,
are spoken of as parts of the gastrovascular

system ; along these the digested material and the
water taken in by the mouth pass to the different
cells which line them. But the only agent in the
propulsion of the material is the pressure due to the
movements of parts of the body.

In the Turfoellaria we observe no system of
vessels ; the nutrient fluid either makes its way from
cell to cell, or passes through the clefts and passages
in the tissues of the body which so often indicate all
that can be seen of a coalom. In the dendrocoelous
Turbellaria and in the Trematoda the absence of a
system of blood-vessels is, no doubt, largely made up
for by the branches of the gastric cavity (see page 114),
which perform the same function as the gastrovascular
system of the Ccelenterata. In the Tapeworms the
only indication of a system of nutrient vessels are the
delicate canals that lie internally to the longitudinal
excretory vessels, and contain a homogeneous plasmatic
fluid ; these are the plasmatic canals of Sommer.
From the Rotatoria, on account of the small size of
their bodies, a system of blood-vessels is wanting.

The origin of the closed system of vessels is
involved in great obscurity, but it is, at any rate, to
be partly ascribed to the increase in size of the
organism ; this increase demands the possession of a
means of providing for the course of the circulating
medium, and affords us another example of that division
of labour which we constantly note as we ascend the
scale of organisation. This explanation does not, at
first sight, appear to apply to all of the Jtf emertiiiea,
for in the smallest members of that group we find a
comparatively elaborate system of not only closed but
also contractile vessels ; further investigation reveals,

1 86 COMPARATIVE ANATOMY AND PHYSIOLOGY.

however, the important difference in the function of
the contained fluid in the lower as compared with the
higher Nemertinea. In the former haemoglobin in
distributed in the nerve tissue, but is absent from the
blood, so that that fluid has only nutrient functions
in the Schizonemertini, and not both nutrient and
respiratory duties as in the higher Hoplonemertini.
There are three chief longitudinal vessels, two lateral,
which are connected with one another at the anterior
end of the body, and one median, which is connected
with the two lateral a little behind the region of
the mouth. All these are contractile, but they are
of the same calibre throughout, or, in other words,
there is no special portion which is enlarged to act
as a pumping organ or heart.

When we pass to those higher forms of Worms
in which metarneres are developed, transverse branches
or lateral vessels unite the median with a now
ventrally placed trunk, and some of these lateral
vessels become contractile (so-called hearts of
Ssenuris and others).

The dorsal vessel (d) of such forms as, for example,
the earthworm, is retained in the crayfish as the an-
terior (Fig. 81; aa')and posterior aortse (pa); the trans-
verse vessels are indicated by the short arteries at and
hp, which supply the anterior regions of the body and
the viscera; one transverse vessel is still complete, and
forms the descending sternal artery (st.a) which
opens into the backwardly and forwardly directed
abdominal artery (si.a ; iaa) the representative of the
ventral vessel of the earthworm. In the Anodon (Fig.
82 c) the spaces or sinuses are much more developed,
and no indications of a ventral vessel are now to be
seen ; the dorsal is, however, shown by the heart (H)
with its anterior and posterior aortse (aa',pa) ; while
the terminal parts of the transverse vessels become
enlarged to form the auricles of the heart or ventricle (a).

Chap. V.]

HEART OF ARTHROPODA.

In the fish (Fig. 82 D) the enlargement for the heart (H) is
found on the ventral vessel; passing for wards it branches
on either side into* the branchial vessels, and these unite
and pour their blood into the dorsal (br) aorta (da).

The blood-vascular system of the Arthropoda
is distinguished by the fact that the blood which
comes to it to be pumped through the body does not

d

a a/

Fig. 81. — Diagrams to show the arrangement of the great Blood-
in the Earthworm (A) ; the Crayfish (B).

reach it directly by distinct vessels ; the heart is sur-
rounded by an imperfectly closed space, the
pericardial sinus. When the contractile cham-
ber which is called the heart dilates, the blood in the
surrounding sinus flows into it through two or more
spaces or holes in its walls. The heart may be short,
as in Daphnia, and have only a pair of orifices ; or it
may be greatly elongated, as in Artemia, where there
are twenty pairs ; or it may be much more compact,
as in the crayfish, where there are three pairs
of large ostia, one superior, one lateral, and one
inferior, which are guarded by valves that prevent

1 88 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the return of the blood to the sinus, as well as an
irregular number of smaller holes. In the occasionally
parasitic copepods (Corycseus) and the degenerated
Cirripedes there is no heart.

The vessels arising from the heart likewise differ
considerably in their arrangement \ in the Entomos-
traca there is an anterior artery only, which may branch

Fig. 82. — Diagrams to show the arrangement of the jrreat Blood-vessels
in the fresh-water Mussel (c) ; and the Fish (D).

more or less at its free end ; but the greater part of
the blood makes its way through definite spaces without
distinct enclosing walls, the so-called lacunae.* In
the Malacostraca a posterior aortic artery is given off,
in addition to the anterior; and in the crayfish, for
example, we may further distinguish two pairs of
anteriorly directed trunks ; the antennary, which

* Our knowledge of the vascular system of Arthropoda, or
Molluscs, is in an unsatisfactory condition; the "lacunar con-
nection between the arteries and veins, which is confidently de-
scribed and discussed by all zoologists, has never yet been demon-
strated to exist in a manner satisfying the requirements of modern
histology " (Lankester).

chap. v.i HEART OF ARTHROPODA. 189

supply the front part of the head, and the hepatic,
which go to the chief viscera. The posterior artery
rims backward along the dorsal surface of the tail, and
gives off on its course a downwardly-directed vessel
(sternal artery), which on reaching the ventral region
divides into an anterior and a posterior abdominal
artery. As these several vessels ramify, they break
up into smaller vessels, and then finally open into
spaces among the various organs of the body. The
largest and most important of these is the great
sternal sinus, which lies in the region of the
entrance to the gills, into the spaces in which the
blood passes to receive a fresh supply of oxygen (see
page 224) ; thence the blood returns by the branchio-
cardiac veins or canals to the pericardial sinus, to
again pass into the heart, and resume its journey
through the body.

It will be observed that the blood goes to the
organs of the body from the heart before it goes to
the gills. Such a heart is known as a systemic
heart, in contradistinction to the branchial heart of
fishes, for example, in which the blood pumped from
the heart goes firstly to the gills, and secondly to the
other organs of the body.

The vascular system of Peripatus is described
by Balfour as consisting of a dorsal vessel shut off
from the body cavity by a continuation of the endo-
thelial lining of the latter. It has definite walls,
but it is not clear whether they are muscular. It ex-
tends from near the hinder end of the body to the
head, and is largest behind. Between the skin and
the outer layer of muscles there is a very delicate
ventral vessel.

In the Myriopoda the heart extends through
the whole of the body, and is made up of a number of
chambers separated from one another by valves pro-
vided with orifices for the entrance of the venous

190 COMPARATIVE ANATOMY AND PHYSIOLOGY.

blood, and giving off in regular metameric fashion a
pair of arterial vessels. Anteriorly the cardiac
trunk is continuous with a vessel which lies on the
upper surface of the ventrally placed nerve cord.

In the Hexapoda the Cardiac tube is confined
to the abdominal region of the body, and there is a
smaller number of separate chambers. The anterior
tube is known as the aorta ; its further ramifications
are not known. In them and in the Myriopoda
the pericardiac sinus has connected with it several
pairs of ordinarily fan-shaped muscles (alse cordis) ;
as these are not directly attached to the cardiac tube,
they cannot, as has been sometimes supposed, have
any function in the way of dilating the heart ; it is
probable, however, that they enlarge the extent of the
pericardiac sinus, and thereby assist or accelerate the
flow of blood into it. The inner lining of the heart is
elastic, and the outer muscular coat does not contract
simultaneously, but from behind forwards (Lowne).

The blood-vascular system of the Araclmida, as
represented by Limulus and Scorpio, is more com-
plete than that of any other Arthropod ; fine vessels
given off from the arteries form a true capillary
system, and the veins are definite and distinct. The
heart is elongated, and consists of eight chambers,
each provided with a pair of apertures guarded by
valves ; it is continued forwards into an anterior, and,
in the scorpion, backwards into a posterior aorta. In
the scorpion each cardiac chamber gives off an artery
on either side, and several pairs are given off from the
posterior aorta. Anteriorly, the aorta forms a collar
round the cerebral nerve-mass, and is continued into
a ventral artery which lies above the ventral nerve-
cord ; this artery is intimately connected with the
nerve-chain in the scorpion, and in Limulus it absor
lutely surrounds it.

In the spiders and other Arachnids the number of

Chap, v.] HEART OF MOLLUSCA. 191

cardiac chambers is 'reduced, and in the mites appear
to be altogether absent.

The circulatory system of the Mollusca presents
a remarkable difference from that of the Arthropoda,
in so far as the blood never passes into the peri-
cardiac sinus. The heart is again formed from part
of the dorsal vessel, and in the least modified forms,
or such as still present a bilateral symmetry, a pair,
or two pairs (Nautilus), of transverse vessels open
directly into the central or axial portion of the heart ;
the ends of these vessels nearest to the axial portion
are enlarged in size and modified to form auricles,
while the altered part of the dorsal trunk serves as a
ventricle, from which the blood passes forwards by
an anterior, and backwards by a posterior, aorta.

A simple arrangement of this kind is well seen in
the mussel (Anodon), or in the squid (Loligo). What
is probably a still more primitive arrangement is
presented by the Nautilus, in which there are two
pairs of transverse vessels, and therefore two pairs of
auricles. In the Octopus the aortic vessel, which in
the mussel was directed backwards, now takes a
forward course, or at first runs parallel to the true
anterior aorta ; in those Gastropods that have suffered
a more or less well-marked torsion of the chief viscera
(see page 81), there is but a single auricle, and the great
vessel arising from the front end of the ventricle early
divides into two branches ; of these one, like the
anterior aorta of the mussel, supplies the front end of
the body, while the other is, in like manner, distributed
to the chief viscera.

The circulation is, to a large extent, effected
in a manner which has been called lacunar ; but,
as has been already pointed out, our knowledge of
these lacunae is in a very elementary condition. The
statement that water is taken up into the blood-
vascular system by pores in the foot does not appear

192 COMPARATIVE ANATOMY AND PHYSIOLOGY.

to rest on good foundation and is disproved by
a number of observations. The system of vessels
is better developed in the Cephalopoda than in other
Molluscs ; the definite arteries are more numerous,
and their finer ramifications are more distinctly capil-
lary in nature. The contractile power of the gills is,
no doubt, of some aid in the propulsion of the
blood ; the walls of the vessels connected with them
are, in the two-gilled Cephalopods, provided with
muscles, and the name of branchial heart has
been given to the enlarged portion of these arteries.

The three great divisions of the Chord ates must
be dealt with separately. The Urochordata are
remarkable for an arrangement which, though not
unique in the animal kingdom (for it has been ob-
served also in the embryos of certain gastropods), is a
very striking characteristic, and most instructive pheno-
menon. It has been observed that in them the pul-
sations of the heart, having resulted in the movement
of the blood current in a forward direction, are, after
a pause, reversed, so that the blood flows backwards
instead of forwards. After this backward movement
has obtained for a time there is another pause, and
this is succeeded by a forward movement of the
blood.

The heart of Tunicates has the shape of a tubular
or fusiform sac, and gives off a large vessel at either
end. In distinction to the forms already considered,
and in agreement with what obtains in -the Verte-
brata, the heart appears to be an enlargement of a
ventral, and not of a dorsal, vessel. The trunks
which arise from it break up into vessels which,
according to the area of their distribution, may be
grouped as branchio-cardiac, cardio-splanchnic, or
splancho-branchial ; and, in addition to these, there
are a number of anastomosing vessels in the test.
When the heart contracts from behind forwards

Chap, v.] HEART OF CHORD ATA. 193

(that is, from its ventral towards its dorsal end) it
contains almost pure arterial blood, and may, there-
fore, be regarded as a systemic heart ; on the other
hand, when the contractions are reversed in direc-
tion, the blood is nearly all impure, and, as it
largely passes to the gills, the heart may now be said
to be branchial or respiratory.

The cardiac tube is sometimes constricted at
various points, but is never divided into distinct
chambers. The most remarkable condition is pre-
sented, not only among the Tunicata, but among all
known animals, by Appendicularia furcata. In
it the heart consists of but two cells, which are con-
nected with one another by from twelve to twenty-
five processes, between which there are open spaces.

In the Ceplialocliordaifa we find an arrange-
ment which reminds us of what obtains in the
Anmilata, inasmuch as there is no centralised con-
tractile heart, but the blood is only moved forwards
by the contractility of some of the great vessels. The
vessel of largest calibre is found in the neighbourhood
of the anterior gill- clefts ; into this the blood passes
from the gills, and from it it goes into a vessel
which is connected with the right of the two so-called
aortic trunks. These two trunks unite with one
another behind the branchial area, and form a single
dorsal vessel, or " aorta," which extends backwards
along the body ; at the hinder end it is continuous
with a ventral vessel, which, on. its way forwards to
the gills, gives off some branches to the rudimentary
liver (see page 161), and so forms a kind of rudimentary
portal system. (See page 206.) The blood from the liver
returns to the great ventral trunk, and, with the rest,
makes its way to the gills to receive a fresh supply of
oxygen. The branchial vessels form dilatations on
their course (bulbilli), and the contained blood either
makes its way directly into one of the aortae, or first
N— 16

194 COMPARATIVE ANATOMY AND PHYSIOLOGY.

passes through the already-mentioned anterior en-
largement.

A very definite system obtains throughout the
Verteforata ; there is always a centralised ventrally
placed heart, which consists of at least two chambers,
an auricle and a ventricle; from the latter, one, or a
pair, or several pairs of arterial vessels (aortic
arches) are given off; these divide into smaller and
smaller arteries, which end in the capillaries found
in all organs and parts of the body ; the capillaries
pour their blood into the small veins, and the small
veins into larger ones, three, or less than three, of
which open into the auricular region of the heart.

Where respiration is effected by gills the blood
goes through the aortic arches directly to these organs,
distributes itself in the fine gill-capillaries, and then,
re-collecting, distributes itself through the body ; the
heart, therefore, of a lowly Vertebrate is branchial,
and not systemic like that of the gill-bearing cray-
fish ; when lung-like structures are superadded to the
gills, the heart becomes incompletely divided into two
halves, and where lungs altogether take the place of
gills there is a tendency, which in the higher forms
becomes an accomplished fact, for the heart to become
divided into two separate parts ; one of these, that
on the right side, collects the blood from the body,
and sends it to the respiratory organ, or acts the
part of a branchial heart, while the other (left
side) receives the blood from the lungs and pumps
it into the body, or, in other words, acts as a systemic
heart.

The heart is placed in a membranous pouch or
bag, the pericardium, and ordinarily hangs freely
in it, though sometimes, as in the eel, the heart is
attached to it by fibrous bands ; the successive cham-
bers are separated from one another by valves, and the
ventricle is likewise separated from the aortic system

chap, v.] HEART OF VERTEBRATES. 195

by similar structures ; these are numerous in the
lower, but reduced to a single set in the higher forms.

The blood, on returning from the body to the heart,
is in the lower Vertebrata collected into an enlarge-
ment of the venous system which is known as the
sinus venosus, and the walls of this chamber are,
like those of the auricle and ventricle, contractile.
Contractility, then, is not, as in the higher members of
the group, confined to the centralised heart ; this is
well illustrated, on the one hand, by the Myxinoids, in
which the portal vein, and by the eel (McWilliam),
in which the terminations of the jugular veins, are
contractile ; and on the other, by the Elasmobrarich
fishes, the Dipnoi, and the Amphibia, in which the
basal portion of the aortic system (conus arteriosus)
is also contractile (Fig. 83). Here, as elsewhere, we
have evidence of gradation in the division of labour.

The auricle, which is a single thin-walled sac in
most fishes, becomes more or less divided into two in
the Dipnoi; along the left-hand division of the heart
there flows, in addition to the blood from the veins,
that which has been returned from the rudimentary
lung (see page 232) ; along the right side the rest, or
purely venous, blood passes. Now, the arterial cone
or trunk arises rather from the left than from the right
side of the ventricle, which is incompletely divided
into two halves, so that the blood which first leaves it
is the blood from the left auricle ; this will, of course,
go to the farthest gill vessels, or those of the first and
second arch ; the last arch of all, the fourth, will, of
course, receive the most impure or venous blood, and
it is the one which, in Ceratodus, sends off a trunk
to the lung.

This division of the auricle, which is hinted at
even in Chimsera (Lankester), becomes complete in
the forms which constantly breathe air by means of
distinct lungs, and the sinus venosus, which brings the

1*96 COMPARATIVE ANATOMY AND PHYSIOLOGY.

blood from the body generally, opens into tlie right,
and the lung vein into the left compartment ; in all
the higher forms, however, the two halves are more
or less connected during embryonic life, and just as
the tadpole has only a single auricle, so even in man
there is a communication between
the right and left sides (foramen
ovale of the interauricular sep-
tum), which, in exceptional cases,
remains open in the adult con-
dition, and thereby produces the
affection known as cyanosis. In
the Mammalia the sinus venosus,
which in Ceratodus (Lankester),
though not in most lower Verte
brata, is not sharply separated off
from the auricle, becomes, when
foetal development is over, com-
pletely merged into the right
auricle (atrium or sinus venosus of
human anatomy).

The ventricle remains an un-
divided chamber throughout the
Amphibia and all Reptiles except
the Orocodilia, so that it is clear
that the presence of two ventricles
in Crocodiles, Birds, and Mammals,
is not a homogenetic, but a homo-
plastic arrangement. (Compare page 12.) An in-
teresting example of the " falsification of the embryo-
logical record," is afforded by the development of the
ventricles, inasmuch as in those forms where they are
distinct, they become so before and not after the
auricles ; it is a case of what Haeckel calls cenogeny,
and is, no doubt, dependent on the requirements of
the organism.

The ventricular is separated from the auricular

Fig. 83. - Heart
Squatina.

of

L, Auricle; B, arterial
cone; aaa, branchial
arterios ; o, orifice of
ventricle, v. (After
Gegenhaur.)

Chap. V.]

HEART OF VERTEBRATES.

197

portion of the heart by membranous valves, just as

the auricle is shut off from the venous sinus by

similar structures ; these are in fishes ordinarily,

though by no means always, two in number; and their

function is clearly to close the way back into the

venous system, and thereby to aid in forcing the

blood forwards, on the contraction

of the walls of the cavity. In the Tft

Amphibia (Fig. 84) the two valves

are fibrous, and are connected by

fibres with the wall of the ventricle,

so that when this part of the organ

contracts it draws down the valves ;

when the auricular chamber (atrium)

becomes divided, each opening into

the ventricle is provided with

valves.

In the turtle the auriculo- ven-
tricular valves are formed by the
development of the ventricular edge
of the auricular septum into two
folds, which, on the contraction of
the ventricle, meet a ridge on the
correspondingly opposite side of the
ventricle ; in the crocodile these
folds are distinct valves. In the Bird
we find a difference between the valves of the right
and left side ; on the right two folds of muscular
tissue, close together at their auricular end, diverge
from one another, and extend far down into the right
ventricle ; on the left the muscular is largely replaced
by fibrous tissue which gives off fine tendons (chordae
teiidinese) to projecting muscular processes of the
wall of the ventricle (musculi papillares) (Fig.
85) ; these tendinous chords are grouped into
three masses, and there are three muscular elevations.

Though it is possible to derive the arrangement of

Fig. 84.— Heart of the
edible Frog (Rana
esculenta), to sho*
the auricle and ve
tricle opened from
the left side.

s, Septum atriorum ; la,
left auricle ; ra, right
auricle ; w, auriculo-
ventricula.- valves ; o,
orifice of arterial cone
or trunk : v. ventricle.
(After Ecker.)

198 COMPARATIVE ANATOMY AND PHYSIOLOGY.

valves which is found in the bird from that which
obtains in reptiles, no such comparison is possible in
the case of the mammalian valves ; in most of the
Mammalia we find that the valves are membranous
and not fleshy, and that there are three (tricuspid)
valves in the right auriculo-ventricular orifice, and

two in the left (mitral
valves) ; as in the left
side of the heart of the
bird, there are chorda?
tendiiiese and musculi
papillares, but the num-
ber of these is not con-
fined to three. In the
rabbit the valve of the
right side is continuous,
and not produced into
three cusps. In. the
Ornithorhynchus, the
valve of the right side
is, as in the Sauropsida,
muscular membranous
tissue being only com-
paratively feebly de-
veloped in it.

As in the other parts of the heart, so in the
ventricle, we find an instructive series of gradations.
Single and undivided in all Fishes except the Dipnoi,
it is always much stronger, and has much thicker
walls, which are largely composed of muscular tissue,
than has the auricular region ; for while the office of
the contraction of the auricle is merely to drive the
blood into the adjoining chamber, the ventricle has
the chief part to play in forcing it through the body.
In the Teleostei and some Ganoids the wall of the
ventricle is arranged in two layers, between which is
a lymphatic space.

Fig. 85.— Left auriculo-veutricular
valve of the Swan, showing the
chordae tendinese and the musculi
papillares (pp). (After Wieder-
sheim.)

Chap, v.] HEART OF REPTILES. 199

An indication of a division of the ventricle into
two parts is seen in the Dipnoi, but in the Amphibia
there is no septum; in the uni- ventricular Reptiles
(that is, in all but Crocodiles) there is no complete
division of the cavity, but the muscular walls form
internal projections which are functionally of some
importance ; the most valuable of these is the
prominent fold which lies just beneath the entrance to
the pulmonary artery, and almost separates off the
part of the ventricular cavity which lies beneath it,
from the rest of the cardiac chamber ; in consequence
of this being the region whence blood passes directly
to the lungs by the pulmonary arteries (PA), it is
known as the cavtim piiliiioiialc (Fig. 86 ; Cp\
and it occupies the right extremity of the heart. As
the ventricle contracts, the blood in this cavum is
forced into the pulmonary artery, and as it is the
blood which has entered the ventricle from the right
auricle (RA), it is, of course, venous blood, or blood
that requires oxygenation. As the contraction con-
tinues, the wall of the ventricle and the edge of the
septum are brought closer together, so that the blood
which is the last to leave the cavity is prevented from
making its way into the cavum pulnionale ; this blood
is that from the left side of the heart, that is, from
the left auricle (LA) ; in other words, it is blood
which has just returned from the lungs, and requires
no further oxydation, and it passes altogether into
the systemic aortse. An inspection of the figure of
the heart will, however, show that some venous blood
must pass into the same vessels, so that the blood
in the systemic vessels of the tortoise is not pure
arterial blood, but is a mixture of partly oxygenated
blood and of blood that has already made a passage
through the body.

In the Crocodilia, Birds, and Mammals there is a
complete interventricular septum, so that within the

200 COMPARATIVE ANATOMY AND PHYSIOLOGY.

heart itself the freshly oxygenated or arterial, and the
impure or venous blood never commingle ; in the
Crocodile, where an aortic arch is in communication
with each half of the ventricle, the arterial and
venous blood commingle outside the heart at the
point of union of their two vessels (the so-called
foramen Panizzse) ; in birds and mammals there is but

Ji.AO.

Fig. 86.— Diagram of the Ventricle and connected parts in the Turtle ;
showirg the transversely elongated ventricle, with the right and
left auricles (RA, LA) lying towards the left, the auriculo-
veutricular valves (uu) formed by the inter-auricular and septum.

RA, Right auricle; LA, left auricle; », the right; t/, the left median auriculo-
ventricular valves; SL., arrow showing the course of the blood in the left
aorta; t, arrow showing the course of the blood in the right aorta ; x, arrow
showing the course taken by the blood from the left ; and to, from the right
auricle into the ventricle ; y. showing the course of the blood from the cavuiu
venosum into the cavum pulmonale ; z, from the latter into the pulmonary
artery ; a, the incomplete septum marking off the cavum pulmonale (cp) ; PA,
pulmonary artery ; RAO, LAO, right and left aortoe ; t>, cavum venosum. (After
Huxley.)

a single aortic arch, which arises from the left ven-
tricle, and the blood from the right never, therefore,
passes into the aorta.

The differences between the arrangement of the
auriculo-ventricularvalves have been already described,
and we now need only point out that there must be a
difference in the way in which these valves perform
their office; in the Sauroid they are muscular, and
therefore actively close the entrance to the auricles by
contracting when the ventricles contract, while in the
majority of Mammals the membranous flaps are

Chap. v.] HEART OF VERTEBRATES. 201

floated upwards by the pressure of the blood contained
in the ventricles, when acted on by the contraction of
the walls of these cavities.

The peculiarities of the muscular tissue of the
heart of vertebrates are dealt with in works on human
or general physiology ; but it must be pointed out that
this tissue is remarkable for the possession of hsemo-
globin ; that, under appropriate conditions of warmth
and moisture, the heart of a frog or a tortoise will,
after removal from the bodv, continue to beat auto-
matically for a number of hours ; and that minute
threads of the tissue from certain regions possess the
same peculiarity.

In the Mammalia the muscular tissue of the heart
is supplied with proper blood-vessels (the coronary
arteries), which arise directly from the aorta, and
after branching elaborately, unite into the coronary
veins which open into the right auricle. In some,
especially Ungulates, a bone, which in the ox may be
as much as an inch in length, is developed in the walls
of the heart. An analogous development obtains in
the penis, where a bone is sometimes present.

The ventricular portion of the heart gives oft1
vessels which are known as the arteries ; in the least
modified Fishes, and in the Ganoids, the common trunk
(conns or trimcus arteriosus) is, like the venous
sinus, contractile, but in the bony fishes this con-
tractile power is altogether lost, and the bulbus
aorta', as it is there called, becomes simpler in con-
struction, while the valves which prevent the blood
from flowing backwards are ordinarily reduced to
two ; the loss of the valves is clearly correlated with
the loss of contractility, for there is not in the walls
of this bulb any means by which the column of
blood can be compressed, and thereby tend to be driven
back into the ventricle. Where contractility is, on
the other hand, retained, we find three (dog-fish) or

202 COMPARATIVE ANATOMY AND PHYSIOLOGY.

more (in Lepidostens eight or nine) longitudinal rows
of pocket-like valves ; in Lepidostens there are four
well-developed and four smaller
valves in each of the nine planes,
so that were they all complete there
would be as many as seventy-two.

Among the Dipnoi, Ceratodus
has one or more rows of well-
developed pocket valves, but the
fact that the number is inconstant
shows that a change is impending ;
such a change is found in Protop-
terus, where the valves are few in
number and minute in size, while
their place is taken by a longitu-
dinal fold, which extends down the
greater part of the cone, and very
possibly owes its origin to a fusion
of a row of valves. By means of
the valve the cone is divided into a
right and a left half, and the blood
that has just returned from the
body is now carried to the third
and fourth arches, the latter of
which gives off a large pulmonary
artery, or vessel which goes direct
to the lungs.

The essential parts of this ar-

g.

the Arterial Circu-
lation in Fishes.
(AfterWiedersheim.)

rangement are seen among some of

the Amphibia ; but, as may be sup-
posed from what has already been
said of the arrangement of the ventricle in the lower
Reptilia, no functionally independent arterial cone is
to be observed in them ; nor is it seen in the adults of
the higher Vertebrates, though even there it is at first
a distinct part of the heart, and is undivided both
within and without.

Chap, v.] ARTERIES OF VERTEBRATES. 203

From this cone or bulb cf the heart there pro-
ceeds a vessel which soon breaks up into a number
of arches (Fig. 87); in Fishes the number of these
is in correspondence with that of the gill clefts.
Within the substance of the gill plate the artery
(branchial artery) breaks up into a plexus of
tine capillaries, and these become collected into a
common trunk on either side which passes forwards
to the brain and backwards to the rest of the
body ; behind the heart, the two trunks unite into
a single median and dorsal aorta, whence vessels
(arteries) are given off to the different organs and
regions of the body.

When, as in the Dipnoi, a pair of lung sacs
become developed, one of the branchial vessels (the
fourth) gives off on its way from the gills a large
trunk which passes directly to the lungs, whence
the blood is returned directly to the left side of
the heart. When the gills are lost altogether the
branchial capillaries lose their function, and, for the
greatest part, become aborted, though the frog re-
tains in its so-called carotid gland the plexiform
arrangement of the capillaries which was of use to
it in its gill-bearing tadpole stage. As the arterial
cone is retained by the Amphibia, the general re-
lation of the great vessels to the ventricle is the same
as in Fishes, and the only differences that obtain are
such as are due to the differences in function of differ-
ent vessels, which influence their size and distribution.

In the Beptilia, as has been already explained,
the orifices of the great vessels, which are ordinarily
guarded by merely two semilunar valves, are brought
into closer relation with certain parts of the ventricle;
the arterial cone (Fig. 88 ; tr) becomes shorter, and is
divided internally by septa.

In the lizard (Fig. 88) three arches arise from
the heart ; the two anterior are aortic, the third

204 COMPARATIVE ANATOMY AND PHYSIOLOGY.

pulmonary. While three arches arise from the
heart in many reptiles, only two are directly given
off in Ophidia, one of which is aortic and one pul-
monary. In the bird and mammal this reduction is

carried still

</ n farther, for

in them the
aortic trunk
is single
throughout
its whole ex-
tent, the left
half being re-
duced in the
former, and
the right in
the latter.
These reduc-
tions are best
explained by
a study of the
figures of
Rathke (Fijj.
89).

The fol-
lowing are
the names
and areas of
distribution of the more important arteries :

1. Carotids. — These may be double, when they
are known as external and internal, or one or other
may be reduced or disappear ; in some Fishes the
carotids are not direct continuations of branchial
vessels, but the latter first unite to form a cireulus
cephaliciBS, by means of which the supply of blood
to the head is the better regulated ; they supply the
head and neck.

-As

A(7

Fig. 88. Heart of Laoerta inuralis.

A, Auricles ; v, ventricle; tr, arterial cone (or triincus
anonym us): 1, 2, first and second arterial arches;
RA, root of aorta; AC, aorta; AS AS', subclavian
arteries; Ap, pulmonary artery ; rp, pulmonary vein;
j, jugular ; vs. subrlavian veins; ci, vena cava in-
ferior : these last three pass into the sinus venosus
(8) which lies beneath the right auricle. (At ter Wie-
dersbciiuj

Chap. V.

ARTERIAL ARCHES.

205

2. Subclavian.— This is given off from the aortic
arch, and supplies the fore-limb of either side.

3. Pulmonary artery (see Fig. 88), which
supplies the lungs ; this in the Amphibia, where

Fig 89. — Diagrams to show the Metamorphosis of the Arterial Arches.

A, Lizard ; B, Snake ; c, Bird ; u. Mammal. The five primitive arches are shown
from below. The first and second arches are always obliterated and their
trunks carried on as the internal («) and external (6) carotids. These are both
continuous with the third arch (c) which forms the common carotid, and in
A arises directly from the arterial trunk ; in A also the outer trunk between
the third and fourth arch persists as the ductus Botalli (rf) ; in A and B the
two arches of the fourth series are seen to be subequal, to cross one another
just outside the heart, and to unite behind it into a dorsal aorta (<7) ; in c the
left fourth arch becomes the subclavian artery, and takes.no part in forming
the dorsal aorta; in D the same happens to the fourth arch on the ripht side.
In all the primitive fifth arch supplies the lungs; but in mammals tne right
kalf disappears. (After Rathke.)

respiration is largely cutaneous (see page 326), gives off
a large cutaneous branch, which is distributed in
the skin.

206 COM PAR ATI]/ E ANATOMY AND PHYSIOLOGY.

4. The aorta proper gives off large branches, such
as the coeliaco-meseiiteric, hepatic, rcnals to

the viscera of the body cavity, and two large hinder
(iliac) arteries which pass to the hinder limbs. The
final termination of the aorta is the caudal artery,
and the size and extent of this and its branches are,
of course, in direct relation to the size of the tail.

As the arterial vessels get smaller and smaller the
blood from them flows into the capillaries, and thence
begins to make its way back to the heart by the
veins.

1. The blood brought back by the hind-limb enters
the pelvic vein, and then passes either by the iliac
or the anterior abdominal into the vena cava
inferior ; in all Vertebrates, except Mammals and
Birds, the blood that passes along the iliac veins breaks
up again into a system of smaller veins within the
substance of the kidneys, forming in them a renal
portal* system. Again collecting, the renals, just as
in mammals, pass into the inferior vena cava.

3. The blood from the intestinal viscera is collected
into a portal* vein, which breaks up in the substance of
the liver into a portal system ; this hepatic portal
system obtains in all Vertebrates. The blood brought
from the liver by the hepatic veins likewise passes
into the inferior vena cava.

It is impossible to understand the arrangements of
the collecting portion of the heart and of the superior
veins without a knowledge of what obtains in all
Vertebrates at an early period of development, and
in Fishes throughout their lives. The unpaired vena
cava is preceded by a single subintestinal vein,
which collects the blood from the yolk sac, and this by

* The term PORTAL SYSTEM owes its name to the fact that in
man the so-called portal vein enters the substance of the liver
by its fissure (or PORTA, as the older Latin-speaking anatomists
called it).

Chap. V.]

VESSELS OF EMBRYO.

207

a paired system of
cardinal veins,
of which the ante-
rior pair collect
the blood from the
head, and the pos-
terior from the
body generally
(Fig. 90; vc and
HC). As these lie
at the sides of the*
body the blood can
only pass to the
heart by a trans-
verse vessel, the
diictus Cuvieri
(D). These two
pairs of anterior
and posterior car-
dinal veins are
possessed by fishes,
and the anterior

par
throughout

remans
the

AA, Abdominal aorta ; RA.
RA, right and left roots of
the aorta, connected by
the collecting vessels (ss')
with the branchial ves-
sels Ab; cc', carotids ; s6,
subclavian artery; KL,
gill clefts ; si, sinus ve-
nosns ; A, atrium ; v, ven-
tricle ; B, bulbus arterio-
su* ; VJH, ompbalo-me?en-
tcric veins: Am.omphalo-
mesenteric arteries ; ic
ic, common iliac arteries;
EE.cxternal iliac arteries,
A/?,alIantoic arteries ; Acd,
caudal artery ; vc, HC, an-
terior and posterior cardi-
nal veins; s//, subclavian
vein ; D, diictus Cuvieri.
(After Wiedersheiiu.)

Pig. 90. -Diagram to explain the Arrange-
ment of the Embryonic vascular System.

208 CoMPARATfPM ANATOMY AND PHYSIOLOGY.

vertebrate series as the jugular veins. In the pen-
tadactyle vertebrates the chief afferent function for
the hinder part of the body is undertaken by the just
mentioned vena cava inferior, the posterior cardinals
becoming the pelvic or hypogastric veins, while
the Cuvieriaii ducts come to be the terminations of the
jugular veins, and so lead into the sinus venosus
whether that remains distinct from the atrium, or, as in
higher forms, becomes a portion of the right auricle.

In addition to the important pulmonary veins
which open into the left auricle, the vessel that brings
the blood from the fore-limb (the subclavian) comes
to be almost as important as the jugulars.

In the Mammalia the relations of the superior veins
become considerably altered ; thus the left jugular, after
having entered into an anastomosis with the right,
becomes atrophied in various Ungulates and Rodents ;
in Man and the other Primates, in the Carnivora, and
in Cetacea, the left jugular ceases to have, as a rule,
any rudiment left in the adult condition. Rudimentary
left ducts of Cuvier have, however, been occasionally
observed in the human subject (Quain, Marshall).

Although there is a general body of truth in the
statement that arterial vessels gradually become
smaller, and veins larger, as the one leave and the
other approach the central heart, yet, as we have
already seen in the case of the portal circulation in the
liver and kidney, veins do break up into smaller
vessels to again unite into a larger one ; when this
phenomenon obtains either with an artery or with a
vein in any other part of the body than the organs
just mentioned, such a plexus of vessels is ordinarily
spoken of as a rete mirabile.

The function of such retia is not far to seek, the
moment we remember the physical law that the
passage of a fluid is slower through a narrow than a
wide current ; in other words, the exchange of gas or

Chap, v.j RETIA MIRABILIA. 209

the escape of the blood corpuscles (diapcedesis), or, in
general terms, the relation of the moving nutrient and
respiratory currents to the surrounding cells is im-
proved by such a slowing. This will explain the
presence of a rete mirabile in the reduced anterior gill
(pseudobranch) of many fishes, or in the walls of their
air bladders, for by this means the exchange of oxygen
is the more successfully effected ; a similar reason may
be given for the great development of retia in the
thoracic and costal regions of the Cetacea, or the very
general distribution of small plexuses in the glomeruli
of the kidney. (See page 258.)

On the other hand, the mere mechanical advantage
of the slowing of the blood current must be of impor-
tance in forms such as the Ruminantia, in which the
head is often for a long period at a lower level than
that of the carotids, and by their presence the dangers
of a flow of blood to the head may be averted ; the same
kind of explanation may, no doubt, be applied to the
retia in the course of the abdominal vessels, the pres-
sure on which must vary greatly with the extent of
distension of the walls of the intestinal tract.

It is more difficult to explain the function of the
retia mirabilia in the eyes of Fishes and Birds ; or in the
course of various vessels in monotremes and edentates.
In the latter case the lowly position of these Mammals
may, perhaps, be of significance.

The earlier division of Vertebrates into hot-blooded
and cold-blooded forms disappeared before the pro-
gress of morphological discovery ; in addition to the
evidence that we now have as to the relationship of
birds to reptiles, we have further to support us the
now generally recognised difference between homoplasy
and homogeny ; in other words, it is now clear to us
that, with given structural arrangements, two forms,
not closely allied, may, under similar external con-
ditions, acquire a physiological resemblance ; and we
0—16

2io COMPARATIVE ANATOMY AND PHYSIOLOGY.

now see that Birds and Mammals have independently
acquired the hob-blooded condition. In connection
with this it is important to observe that they are the
only Vertebrates in which the arterial and venous
blood do not commingle at some point in the circulatory
system. The term cold-blooded is so far inaccurate, that
the blood is no colder than the surrounding medium,
whether that be air or water ; and, as its temperature
varies, it is better to use the term " poikilothermic."

CHAPTER VI

ORGANS OF RESPIRATION.

THE essential object of the process of respiration is
the taking in of oxygen either from the atmospheric
air, which may be said to be oxygen dissolved in
nitrogen, or from water, which holds in solution a
larger quantity of oxygen than of nitrogen; for,
whereas atmospheric air consists of twenty volumes of
oxygen -to eighty of nitrogen, water contains about 33
per cent, of oxygen. In the simplest cases (that of naked
cells, such as Amoeba) the oxygen passes directly
into the protoplasmic substance of the cell, and the
chief product of the oxydation of the tissue, carbonic
acid, passes freely out. In the Infusoria the oxygen
has to make its way through the cuticle, and some, no
doubt, enters with the drops of water that are taken
in by the mouth ; we are as yet altogether ignorant of
the conditions under which animal membranes allow of
the entrance of oxygen and the exit of carbonic acid.
We have already seen that the cilia are of use in driving
food particles towards the mouth, and it is clear that
they are also of assistance in the respiratory process by
producing currents around the cell, and thereby bring-
ing fresh supplies of oxygenated water to its surface.

Chap, vi.i LOWER METAZOA. 211

The cilia of the flagellated chambers of the
Sponge (Fig. 53) have clearly the same function
among the Porifera, and the currents, which we have
already learnt to be food-bearing currents, are the
means also by which oxygen is brought to the cells
that line the surface of the canals, and, while they
bring oxygen, they carry away carbonic acid. Similarly,
among the Ccelenterata,* the necessary oxygen
enters in the same way, and passes by the same
passages as the food-currents. Among the lower
worms, such as the Turfoellaria, oxygen is, no
doubt, obtained in the same way, but here in all
probability the soft ciliated integument also affords a
means by which oxygen may be carried to the cells.

It would be difficult to point to any part of the
body of a Nematoid worm which could be reason-
ably supposed to have a respiratory function ; on the
other hand, such experiments as those of Oerley, who
placed a number of specimens of Anguillula aceti (the
vinegar paste-worm) in a small vessel and covered
them with a layer of oil an inch thick, and found that
after two months the greater number were still alive,
prove that, in these worms at any rate, there is but a
very feeble demand for fresh supplies of oxygen.

Where a definite blood-vascular system is de-
veloped, the consideration of respiratory problems is
rendered easier ; for, just as the currents of water in a
sponge carry in oxygen as well as food, so does the
blood of the higher Ifletazoa serve as an oxygen
carrier as well as a store of food material for the
different cells of the body. The oxygenating office is
again rendered still more effectual when the blood

* It has been recently pointed out by Lankester that the so-
called SUBGENITAL PITS of Aurelia have no connection at all with
the geni-fcal glands. They are spacious cavities, opening to the
exterior by comparatively small pores, and they serve, he thinks,
as receptacles for respiratory water. They are four in number,
and are set close to the mouth, one in each quarter of the disc.

212 COMPARATIVE ANATOMY AND PHYSIOLOGY.

is impregnated with haemoglobin (see Power's
" Human Physiology,"), which has a remarkable
affinity for oxygen. This haemoglobin is either found in
special corpuscles, as in the Yertebrata, some Annelids
(Glycera, Capitella), some Gephyreans (Phoronis,
Thalassema, Hamingia), some Lamellibranchs (Solen,
Area), or it is diffused in the liquid of the blood, as in
most, though not all, Polychsetous and Oligochsetous
Annelids and Hirudinea, some Nemertines, some
Crustacea, the mollusc Planorbis, and the larva of the
insect Chironomus.* It has been found in a special
system of vessels in the Crustacean Lernanthropus and
Clavella, and in the corpuscles of the water vascular
system of Ophiactis virens. In various forms it is
found diffused in muscular tissue. Its most remark-
able position, however, is in the nerve tissue ; this has
been observed in the sea-mouse (Aphrodite aculeata)
and in a number of Nemertinea ; in the latter it is
absent from the nerve-tissue of those forms in which
the blood is impregnated with it.

f) Among the higher worms in which no special
respiratory apparatus is developed, it will probably be
frequently observed, as has already been the case with
the leech, that the epithelial covering of the body is
interpenetrated by capillaries (Fig. 91); "the true
respiratory organ of the leech is clearly this vascular
epiderm, and amongst respiratory organs it stands
alone in the nearness with which the absorbent blood-
vessels succeed in bringing themselves through all
structural obstacles into direct contact with the
oxygenating medium " (Lankester).

When specialised respiratory organs are developed
they may arise in various ways, either as in-pushings
of the surface, which may become slits or tubes, or as

* The larger number of these cases have been observed by Ray
Lankester. See the Proceedings of the Royal Society, 1873, No.
140.

Chap, vi.] GILLS. 213

out-pushings of the body (external gills) ; or water may
pass through or into the intestine and be pumped out
of it again, or the walls of the intestine may give rise
to cavities into which air is received ; or, lastly, the
body wall may give rise to an air chamber by folding
over and becoming attached, for a more or less con-
siderable extent, to the surface of the body, as is the
case in the snail.

In a general way we apply the term gill to an

eji ,cu

Fig. 91. — From a Transverse Section of a Leech.; to show cu, cuticle ;
v, infra-epithelial blood-vessel ; ep, epithelial cells. (After Ray
Lankester. )

organ which is fitted to take up the oxygen dissolved
in water, and lung: to that which breathes the oxygen
of atmospheric air ; but these terms can only be used
in a very general sense.

The simplest form of in-pushing is seen in the
Nemertinea, where the so-called side-organs or
ciliated furrows are in Carinella annulata (Fig. 92 ; A)
mere pits ; this pit in C. inexpectata forms a more
complicated groove, which leads into a ciliated duct
(Fig. 92 ; B), which ends blindly among the cells of the
brain ; in both these species the nervous centres are
quite close to the surface (page 398), and the tissue is,
as we have just learnt, impregnated with haemoglobin.
In Polia, where the brain is more deeply placed,
the ciliated duct is longer (Fig. 92; c). In the

214 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Sehizonemertini (such as Cerebratulus) the transverse
furrow of the Palseonemertine gives place to a deep
longitudinal slit, which penetrates the muscular tissue

Fi&. 92.— Diagrams of the E espiratory Ciliated Ducts of (A) Carinella
annulata; (B) C. inexpectata ; (c) Polio curia; (D) Certbratulus.
(After Hubrecht.)

of the head, and is continued below into the ciliated
canal which penetrates the lobe of the brain (Fig.
92; D). The respiratory function of these slits has
been pointed out by Hubrecht, who has made experi-
ments in demonstration of his hypothesis, and who

Chap. VI.]

TRACHEJE.

215

has directed attention to the deeper slits and larger
amount of haemoglobin in the Schizonemertini, as
being correlated with their habit of dwelling in mud
and other places in which the supply of oxygen is
small; the ciliated cells of the groove clearly serve to
drive in currents of sea-water.

In c and D (Fig. 92) it will be observed that the
lower part of the ganglionic mass is shaded more lightly
than the rest ; the cells that form this portion are
derived from the ossophagus, from the walls of
which, however, they subsequently become separated ;
and we observe, therefore, that an out-pushing from
the gullet goes to
meet the epiblastic
in- pushing from the
surface.

An essentially
similar phenomenon
is to be observed
in the Entero-
pneusti (Balano-
glossus) and in the
Choi-data. (See
page 231.)

In the trache-
ate Artliropoda
we have examples of
in-pushings of the
surface adapted for

the entrance not of water, but of air ; they are seen at
their simplest in Peripatus, where they are
distributed over the whole of the body. The tracheal
orifice leads into a pit, which traverses the dermis,
and widens out at its inner end ; from this the
tracheae arise as minute tubes, which rarely branch,
and only have a faint indication of the presence of the
spiral fibre, which, in higher Tracheates, gives so

Fig. 93.— Tracheae of Insect, showing the
Spiral Fibre.

2i6 COMPARATIVE ANATOMY AND PHYSIOLOGY.

characteristic an appearance to these tubes (Fig. 93).
These tracheae are distributed to all parts of the
body, running alongside the nerves and entering with
them into the central nervous system ; they are
particularly well developed in the head.

In the higher forms the tracheal orifices, which are
definitely known as stigmata, are not distributed over
the whole body ; they would appear to have been primi-
tively arranged by pairs in each metamere, and in some
Myriopods that condition is essentially retained. As
the stigmata diminish in number, the tracheae tend to
branch, and in some Myriopods (such as lulus)
they unite with one another and form tracheal
anastomoses of much the same kind as those found
among Hexapod insects ; in the higher members of
the last-mentioned group the stigmata are reduced to
two or three pairs, but, thanks to the anastomosing
branches, all the parts of the body are still well supplied
with air vessels, the thinnest and finest of which
are without the spiral fibre. Just as in Birds
(see page 239), we find that in flying insects the air
passages or tracheal tubes often pass into swellings or
enlargements, the so-called tracheal vesicles ; these
may be small and numerous as in some Coleoptera, or
larger and less numerous as in butterflies and bees,
or reduced to two, which are, as in the fly, of enor-
mous size, and abdominal in position. The stigma
or spiracle, by which the air enters, varies consider-
ably in form and structure, and the same is true of
the apparatus by which it can be closed. Little is known
with certainty as to the mechanism of the respira-
tory movements of insects, but the high temperature
of the bee-hive, and the activity of such restless
creatures as the house-fly, speak clearly enough of a
large amount of oxydation, and of waste of tissue ;
in direct relation to the activity of the cellular organ-
ism is the amount of oxygen which it requires. It

chap, vi.] RESPIRATION OF INSECTS. 217

has been observed by Lowne that the blowfly makes
vigorous movements with its legs from sixteen to
thirty times a minute, and he judges that these are
respiratory movements from the consideration that,
as the valves of the anterior spiracles are closed for a
short time, the air must be necessarily driven through
the small tubes \ these movements are accompanied
by a contraction and a dilatation of the abdomen.
If placed under an exhausted receiver of an air-pump
and removed before death ensues, the refilling of the
tracheal system is observed to be accompanied by
violent movements of the legs and wings, so that
here, as in the crayfish (see page 225) and elsewhere,
the activity of the locomotor organs is seen to be in
direct relation with an increased supply of oxygen.
In the larvae of some Insects the tracheal system is
completely closed, or, in other words, there are
tracheal tubes but no stigmata; in these cases the
tubes are either placed not far from the surface of
the body, when the respiration may be said to be
vague, or the tracheae form out-growths (the so-
called tracheal gills) which project into the water
(e.g. Culicidae), and offer the exact converse of what is
the essential point in tracheal respiration ; in it the
air comes to the organs by internal tubes, in such
larvae tubes go to the water in order to obtain the
oxygen.

* We may appropriately pass from these external
^tracheal tubes to the form of respiratory organ in
which there is an out-pushing of the body wall to
form a so-called external gill. Physiologically this
condition is preceded by what obtains in the Starfish,
where the respiration is said to be vague ; from
among the spaces left between the ossicles of the
dorsal surface (Fig. 23) there project thin membranous
out-pushings of the lining of the body cavity, and
the fluid within is thus only separated from the

218 COMPARATIVE ANATOMY AND PHYSIOLOGY.

oxygenated sea-water by a thin wall of membrane
through which oxygen passes inwards, and carbonic
acid outwards. In the Ophinroidea, where the
calcareous skeleton consists of continuous plates, a
slit is to be seen on either side of the insertion of each
arm into the disc ; these so-called genital slits or
bursal clefts, as they are more appropriately
named by Ludwig, lead into membranous sacs (bursse)
into which sea-water is sucked, and from which it
is again driven out ; the thin walls of these sacs
project into the body cavity, and here again the
oxygenated sea-water is separated from the contents
of the crelom by nothing more than a thin wall. In
the Ecltinoidea, where the test is, again, con-
tinuous, the membranous sacs are either internal as
in Cidaris, where they lie at the apex of the lantern
of Aristotle, or they form five pairs of sacculated
projections which protrude from the five pairs of
slits in the margin of the mouth. In some Echinoids
the tube feet, or ambulacral suckers, take on a respi-
ratory function. In the Holotlmroidea, the respi-
ratory organs, when specially developed, are connected
with the intestinal tract. (See page 229.)

A simple condition of external gill obtains in
some of the Polyclisetous Annelids, where,
however, they are by no means always developed. In
Nereis, for example, they are simple, short filaments ;
in Cirratulus the filaments are simple, but are produced
to an extraordinary length ; in others, as the lug-worm
(Arenicola), they are comparatively short but are
elaborately branched (Fig. 94) ; in the tube-dwellers
they are confined to the anterior end, where they form
a pair of " respiratory plumes." In all cases these
gills consist essentially of a thin wall, the epithelium
of which is ciliated, and within which are blood-
vessels more or less well developed. In the plumes of
the Tufoicolse supporting cartilages are not rarely

Chap. VI.]

GILLS OF MOLLUSC A.

219

developed. The mechanism of these gills is simple
enough ; projecting into the water they are moved
about in it, and minor currents are produced around
them by the action of the cilia which cover their
surface.

A very simple type of gill is found in some of the
Lamellibranchiata, where the Ctenidiiim (see
page 79) consists essentially of two rows of hollow
filamentar processes of the body wall extended along
either side of the foot, and ciliated externally ; these
are the gill filaments; each filament, at its free
end, bends upwards in such a way as to leave a space
between the ascending and descending portions ; the
free end of the ascending portion of the outer filament
may either be free, as in the sea-mussel (Mytilus), or
become at-
tached to
the mantle
as in the
fresh - water
mussel (An-
odon) ; it is
plain, then,
that its as-
cending lies
externally
to its de-
scending
portion ; the

inner gill filament has its ascending portion internal to
the descending, and so it results that in transverse sec-
tion the two gill filaments have the form of a W, the
median upper point of which represents the point
where the two filaments are inserted (or arise from)
the body wall. The ciliated epithelial cells are often
particularly well developed at certain spots ; the cilia
of these " ciliated junctions " interlock with those

•n/

Fig. 94.— Transverse Section of Arenicola.

D. Dorsal ; v, ventral side ; n, ganglionic chain ; i, intes-
tine ; br, gills; v, ventral vessel; d, dorsal; t/, visceral
vessel ; p, vessel around intestine ; a, b, vessels of gills.
(After Uegenbaur.)

22o COMPARATIVE ANATOMY AND PHYSIOLOGY.

of a corresponding junction on a neighbouring
filament (Fig. 95 ; A) ; and in this way the several

filaments are connected
together. But the concre-
scence of the constituent
parts of a gill does not end
with the union of the
neighbouring filaments, the
ascending and descending
portions likewise become
connected with one another
by iiiterlame liar June-

IF!! „.£

.Jl- .J

"V k i

1

Fig. 95.— A, Part of three gill filaments of Area, showing the large
ciliated junctions; B, transverse section of a portion of an outer
gill plate, with solid interlamellar junctions and large vertical
vessels of Anodon ; c, a more highly magnified view of B. (After
K. HolmanPeck.)

tions (Fig. 95 ; B and c), and the bridges of union that
were primitively ciliated become fibrous and solid, till
at last we reach the continuous plate-like arrangements,
which are channelled by spaces, or by definite vessels
which lie outside the primitive blood-carrying hollow

Chap, vi.] GILLS OF MOLLUSC A, 221

gill filaments, and which are now the means of passage
for the circulatory medium. With this considerable
alteration in structure there would seem to have come
some change of function, and Peck is probably right
in believing that the ciliated gill plates of the Lamelli-
branchs, by causing a constant current of water to pass
over the animal, are as important aids to their nutrient
as to their respiratory processes. The spaces between
the gill filaments are sometimes used as incubatory
pouches, in which the Olochidia, or young of the
fresh -water mussel, lie during the early days of their
development.

The branchial outgrowths (ctenidia) of the
Cephalophora are, on the whole, simpler in structure
than in the more highly developed members of the
Lamellibranchiate group ; in most of the lowest forms
they are arranged in a circle, as in Neomenia, where
there is an oblong circlet of thirty filiform tubules, the
Chiton, or the limpet ; in Haliotis and others they
are distinctly bilateral, and in the Anisobranchiata
the left gill is, owing to the torsion of the body, much
smaller than the right; in others, as the Hetero-
]>ocla, the left gill disappears altogether, and in some
the right gill has only one lamella developed ; this is
the so-called semi-pi iinate gill* In other Oastro-
pods, such as Lymnseus, or Helix, the gills disappear
altogether, and a so-called lung is developed. (See
page 228.) In a number of naked Pteropoda re-
spiration becomes vague by a process of adaptative
degeneration ; or, in other words, the whole surface of
the body takes on the function of the lost gills. Among
the Cephalopoda the Tetrabranchiata (Nautilus)
have two pairs, and the Dibranchiata (Octopus, Loligo)
one pair of well-developed gills.

In the great majority of cases the gills of the
Mollusca are covered in by the mantle, and come,
therefore, to lie in a branchial chamber; into

222 COMPARATIVE ANATOMY AND PHYSIOLOGY.

this chamber the air is, under the simplest conditions,
drawn in by the action of the cilia which cover the
surface of the gills or gill plates ; in the Anodon, for
example, the currents of water enter into the lower
part of the chamber, which in the hinder region of the
body is separated from the upper by the union of the
gill plates of either side along the middle line ; the
water that enters by this lower inhalent passage passes
out by the upper or exhalent one. In a number of
Lamellibranchs the mantle which bounds these orifices
is produced into a more or less long siphon ; these
siphons are best developed in forms that burrow in
the sand, and which have the siphons directed upwards.
A similar kind of gill chamber is formed in many
Gastropods by the folding over of the mantle,
and in a number of flesh-eating forms a pair of
siphons are also developed. The absence of the
mantle-fold in such forms as the JVudibranehs
leads us, physiologically, to the vague respiration of
the gynmosomatous Pteropoda, where the gills have
become atrophied.

The most characteristic organ of the true
Cephalopoda is the so-called funnel, which is a
modification of part of the foot ; in these highly
developed molluscs we have again an example of the
relation of the respiratory to the locomotor activity
of the animal. When the muscles in the walls of the
investing mantle relax they allow water to enter into
the gill chamber on either side of this funnel ; when
they contract they not only press on the water in the
cavity, but also close the orifices by which it entered ;
the only passage, then, by which the compressed fluid
can escape to the exterior is by way of the funnel, the
walls of which, by contracting, aid in the expulsion of
the water ; and the final result of this expulsion is,
that, unless the Cephalopod is resting on the ground
it is driven backwards ; the more of ben then, water is

VI.]

GILLS OF CRUSTACEA.

223

baken in and driven out, the more often is the animal
mechanically helped on its course.

In no group are gills better or more characteristi-
3ally developed than in Crustacea, and in none do
we find better evidence of the association of locomotor
with respiratory activity. Among the lowest repre-
sentatives of the group (the BrancSiiopoda) a
number of the appen-
dages are nothing more
than broadened thin
plates within which
the blood circulates,
and outside of which is
the oxygenated water in
which they are bathed
(Fig. 96).

In Squilla and its
allies branched tufts of
gill filaments are at-
tached to the abdomi-
nal feet.

In the Decapoda,
such as the crayfish
or the lobster, the

gills are outgrowths of the sides of the body wall, but
fcheir relation to the locomotor function is still well
marked ; in this group the gills are placed in a gill
chamber, which, as it is formed by lateral folds of the
dorsal integument, reminds us, so far, of the simpler
arrangements of the Branchiopod (Fig. 96 ; d) ;
these gills are set in three sets, the lowest of which
are (in the nomenclature of Huxley) podobranehs,
for they are attached to the basal joints of the appen-
dages (in the crayfish from the second maxilliped to
the penultimate thoracic appendage) ; the next set,
which are arranged in two rows, are called the
arthrobranchs, from the fact that they are attached

Fig. 96. — Transverse Section of
a Branchiopod, showing the
leaf-like (phyllopod) gills
(br'), which are appendages
of the body.

c, Heart ; i, intestine ; n, ventral nerve-
cord ; d, fold of the iutegument.
(After Grube.)

224 COMPARATIVE ANATOMY AND PHYSIOLOGY.

to the membranous piece which connects the basal
joints of the appendages with the walls of the thorax:
the third and uppermost set consists of pleuro-
branctis, so-called from their attachment to the sides
of the thorax. Among different members of the
group we find a difference in the number of these
gills, and here, as elsewhere, in scientific investiga-
tions, much time is saved, and intellectual operations
considerably aided, by the use of formulae. The fol-
lowing table, taken from Huxley, may be regarded as
a type which is to be followed out when making the
dissection of any one of the higher Crustacea.

HYPOTHETICALLY COMPLETE BRANCHIAL FORMULA.

Somites and

lagVIL*

VIII.

IX.

X.

XI.

XII.

XIII.

XIV.

Arthol>ranchs.
Anterior. Posterior.

1
1

Pleurohranchg.

= 4

= 4

= 4

= 4

= 4

= 4
4

= 32

All these gills have essentially the same structure ;
they consist of an elongated stem, within which
run two distinct canals, into one of which the blood
passes from the body, and by the other of which it
returns on its way to the heart ; connected with these
canals are a number of comparatively short hollow fila-
ments with thin walls ; the blood, therefore, on pass-
ing into them, is separated only by a thin membrane
from the oxygenated water that is passing through the
gill chamber, and an exchange of carbonic acid and

* There are many reasons for beginning to count tl
one farther back, and to call vi. what Prof. Huxley call

to count the segments
"a vii.

Chap, vi.] GILLS OF CRUSTACEA. 225

oxygen is, consequently, easily effected. The podo-
branchs and pleurobranchs are more elaborately con-
stituted than the simpler arthrobranchs.

When we remember the well-known fact that the
Crustacea are altogether devoid of cilia, we find it at
first difficult to understand how water is driven
through the gill chamber ; we have only, however, to
make the experiment to see. that a current of water does
constantly enter at its hinder and pass out at the
anterior end. The apparatus by which this current is
produced is, again, a modification of one of the appen-
dages for the exopodite and epipodite of the second
maxilla (page 1 23) of either side is converted into a scoop-
shaped plate, the cavity of which is directed forwards,
and which itself fits into the anterior orifice of the
gill chamber ; this so-called scaphognatliite moves
backwards and forwards about 200 times a minute,
and with each backward and forward movement it
scoops out water at the anterior, and causes a fresh
supply of water to enter at the hinder end of the
animal ; moreover, the quicker the animal moves,
the quicker the action of the scaphognathite, and, in
consequence, the larger the inflow of oxygenated water.
Within the gill chamber the waving plumes of the
gills aid in the movement of the water, and the at-
tachment of the podobranchs to the ambulatory
(thoracic) and hinder mandibular appendages affords a
certainty that the more these appendages work the
greater will be the supply of oxygen that they will
receive.

A few Crustacea are modified to breathe air. (See
below. )

Among the Arachnida, processes of the body
projecting into the water are found in the only mem-
bers of the group that are inhabitants of the sea;
these are the King-crabs, represented to-day by Li-
mulus. In these forms there is no protecting gill
p— 16

226 COMPARATIVE ANATOMY AND PHYSIOLOGY.

chamber, and the gills project freely into the water, as
in the lower Crustacea ; they are attached to the five
pairs of abdominal feet, and are broad, flattened, and
provided with a number of secondary plates. These
gills are essentially formed by a hard supporting axis,
on which are placed some hundred and fifty flattened
lamellae (Fig. 97) ; these lamellae are exposed to the
surrounding water, and as they are hollow, and their
cavities contain blood, we have only further to know
that their walls are delicate to understand how it is
that these "gill-books" are respiratory organs.
Connected with these gills are the so-called " lung-
books " of the scorpion, which are adapted for aerial
respiration, and the exact characters of which have
been very carefully and ingeniously elucidated by
Lankester. In that Arachnid the ninth to the twelfth
segments bear appendages which are respiratory in
function, the appendages of the eighth segment, or the
first which is branchial in Limulus, being more espe-
cially modified as the so-called " pectines." These
respiratory appendages, or " lung-books," are, like the
" gill-books " of the king-crab, formed essentially by
a hard supporting axis, and a number of lamella set
on that axis (Fig. 97, B ; and in greater detail c) ; but
they differ, at first sight, altogether from those of the
king-crab in being no longer exposed freely, but
placed in recesses, which open to the exterior by a
narrow slit. This slit gives entrance not to blood but
to air, and, as it communicates with the cavities in the
lamellse of the lung-book, we expect to find that these
do not, as in Limulus, any longer serve as blood
passages ; the blood, indeed, is now found in the sac
which invests the lung-book. Great as are the struc-
tural changes, the ultimate physiological arrangement
is the same as before ; in other words, the lung-books
no less than the gill-books are respiratory organs, but,
instead of carrying the blood to the oxygen, they carry

Chap. VI.]

ARACHNIDA.

227

the oxygen to the blood. Among the Arachnida, thb
remarkable series of changes so far followed does not

Fig. 97. — A. View of the lower Margin of the right lamelliferous Appen-
dages of the eleventh segment of Limulus polyphemus.

A. I, Proximal lamella ; lx, one hundred and fiftieth lamella ; ex, external lappet of

the bifld distal prolongation of the appendages.
B. A similar view of the corresponding appendage of Buthus fcocTii.

lx, One hundred and thirtieth lamella ;
c. A. semi-diagramatic view of one of the respiratory Appendages of a

Scorpion, to show

I, The bases of the lamellce exposed by the removal of the integument of
the axis, the remnants of which are seen at TO : oc, the projecting portion of
axis ; I, proximal lamella. (After E. Ray Laukester.)

end with the scorpion ; some, like Mygale, have but
two pairs of lung-books, and other spiders have but

228 COMPARATIVE ANATOMY AND PHYSIOLOGY.

one. In others, like the mites, there are no specia-
lised respiratory organs, but a " vague respiration ; "
and, lastly, others, such as the pseudo-scorpions
(Chelifer), have replaced the lung - books by true
tracheal tubes.

A number of Mollusca have, like the common
snail, replaced an aquatic method of respiration by
one that is aerial ; this is effected by the large distri-
bution of blood-vessels to a part of the mantle, which
becomes so attached to the sides of the body as to leave
only a comparatively small orifice by which air can
enter ; the modified mantle chamber is called the
44 lung1.99 This arrangement is not separated by any
wide gulf from that which is found in the branchiate
Gastropoda, for some of these have the walls of the
mantle cavity more or less well provided with a lung ;
and others, like the marsh-snail (Paludina), have both
gill and lung. On the other hand, the water-snail
(Lymnseus) has no gill at all, yet constantly lives in
water, and uses its air receptacle, as do some fishes
(page 232), as a hydrostatic organ.

In some cases, as Semper has pointed out, certain
Mollusca may be truly spoken of as amphibious ;
Ampullaria, an ally of Paludina, has been observed
by him to use its gills and lungs in rapid alternation ;
" for a certain time they inhale the air at the surface
of the water, forming a hollow tube by incurving the
margin of the mantle, so that the hollow surface is
enclosed against the water, and open only at the top.
When they have thus sucked in a sufficient quantity
of air, they reverse the margin of the mantle, opening
the tube, into which the water streams. The changes
are tolerably frequent, once or twice in a few minutes,
depending, probably, on the temperature. No phy-
siological explanation of these rhythmic alterations
can, however, be at present assigned."

It is not only among the Mollusca that we have

Chap, vi.] LUNGS OF CRUSTACEA. 229

air-breathing forms closely allied to those that breathe
oxygen dissolved in water ; not only are there true
amphipnous Vertebrates (see page 236), but there are
among the Crustacea some terrestrial Isopods, in
which some of the appendages are placed in a cavity
of the abdomen, which is partly closed ; the cavity of
such of these appendages as are not rudimentary
opens to the atmosphere by a longitudinal slit. Among
the true crabs there are also some forms that con-
stantly live on land, such as the robber crab (Birgus
latro), the land crab (Gecarcinus), and others ; the
gills in the branchial chamber of these Crustacea are
always small, but a quantity of air is to be found in
the chamber ; in Birgus this chamber is divided into
a lower and smaller one, which contains small
gills, and an upper larger one, which never con-
tains any water, but always air, and which has
its walls not only richly supplied with blood-vessels,
but also produced into branched outgrowths, or villi,
in which the blood-vessels are particularly well de-
veloped.

It is not in the lower Metazoa alone that the
lining of the alimentary canal serves as a means of
entrance for oxygen ; even among the Vertebrata,
where, in the higher forms, the respiratory organs
(lungs) are really outgrowths of the enteric tract, we
know of a loach (Cobitis) which swallows air bubbles ;
among the Echinoderms, the surface of the mucous
membrane of part of the intestine is, in Stichopus
variegatus, so grooved as to display a large amount of
vascular surface to the action, of the inflowing water,
or, as in many (Holothuria, Cucumaria), special
branched organs, which extend throughout the greater
part of the length of the body, are developed from the
walls of the cloaca ; in such pneumonophorous holo-
thurians water is pumped in and out by the muscles
at the hinder end of the body. Such a form of anal

230 COMPARATIVE ANATOMY AND PHYSIOLOGY.

respiration is not confined to the Echinodermata, for
it is very common among the Arthropoda ; among the
Entomostraca it has been frequently observed; in
Leptodora, when the intestine is free of food, the
water appears to pass in a continuous stream into the
more anterior parts of the tract, and, when the
stomach is full, water is taken in and expelled by the
mouth ; anal respiration has been seen by Lereboullet
in the crayfish, and the rhythmical closing and
dilatation of the rectum may always be seen in that
animal, after the extirpation of the thoracic ganglia.

Among Worms, enteric respiration obtains in the
Rotatoria, in some, if not all, Gephyrea, and in some
of the aquatic Oligochseta ; Dentalium is the only
mollusc in which it has been definitely observed.

In certain polychsetous Annelids, a somewhat
complex arrangement has been detected by Eisig,
which is of especial interest, both morphologically and
physiologically, when it is compared with the function
and structure of the respiratory organs of Vertebrates.
The observation that a number of air bubbles constantly
escape from the mouth or anus of Hesione sicula led
him to detect the presence of distinct outgrowths of
the intestine, which clearly serve as air reservoirs, and
at the same time may be used as floats or hydrostatic
supports ; they are especially of use when the intestine
is filled with food, and structural evidence in favour
of their respiratory significance is afforded by their
rich supply of blood-vessels. In connection with this
it is very interesting to note that a fish will use up all
the air in its air bladder before it dies of suffocation,
and that, conversely, in the pulmonate Yertebrata the
lungs have undoubtedly the power of acting as
hydrostatic supports for the body, when immersed in
water.

The Cliordata present us with the most interest-
ing and instructive series of arrangements, for while

Chap, vi.j CHORDATA. 231

the lowest members of all three divisions are
branchiate, the higher Vertebrata pass from an
amphibious or amphipnous stage to one in which
outgrowths of the enteric tract, or lungs, are alone
the respiratory organs. In the Ceplialochordata
and Urocliordata respiration is always effected by
gills, and there are some very striking points of
agreement between the two sets of forms.

In both, the gill slits are formed by an ingrowth
from without, and an outgrowth from within ; in both
it is the anterior portion of the enteric tract which is
so affected, and in both the water of respiration enters
by the same orifice as the food. In those Tunicata
that retain the cliordate tail throughout life (Appendi-
cularia), the water that passes in at the mouth passes
out by a cylindrical tube on either side ; but, on the
other hand, water may enter by these " spiracula "
and pass out by the mouth.

In the rest, as also in Amphioxus, an outgrowth
of the body wall on either side gives rise to the
formation of a peribranchial chamber, into which the
water streams from the gills; the folds which form
the walls of this chamber unite along the greater part
of their length, but leave an orifice (atriopore)
by which the water can escape to the exterior. This
atriopore may either open, as in Ascidia, close to the
incurrent orifice or mouth, or it may be at the aboral
end of the body, as in Pyrosoma ; in compound
ascidians there is a single common excurrent orifice.
In some the water is forcibly driven out, and then,
just as in Cephalopoda, the excurrent stream aids in
locomotion. In Amphioxus the atriopore is on the
ventral surface, and not far in front of the anus.

The ancestors of the present race of Vertebrata
were aquatic forms that breathed the oxygen which
was dissolved in the water in which they dwelt ;
or in other words, they had gills. This mode of

232 COMPARATIVE ANATOMY AND PHYSIOLOGY.

respiration, the branchial, is retained to-day by the
lowest of the Vertebrata, and gills are to be found in
all Fishes, in all Amphibia at some period of their lives,
and in some Amphibia throughout the whole course of
their existence. None of the Sauropsida or of the
Mammalia ever breathe oxygen dissolved in water,
but' are air-breathing forms with lungs ; though the
change of function has been completed, the remnants
of gill clefts are observable in the earlier stages of
development

A most instructive series of gradations is to be
detected in Fishes; all adult 'forms have the gills in
pouches or recesses, but the young of some (Elasmo-
branchs, some Ganoids), like the tadpole at an early
stage, have protruding filaments or external gills ;
the lampreys have seven pairs of gill-clefts, as has the
shark Heptanchus ; Hexanchus, and most examples of
Myxine, have six ; most Elasmobranchs, five pairs ;
Chimsera has the first and fifth gill incomplete ; most
Teleosteans have four pairs of gills, but some have the
fourth incomplete ; the angler (Lophius) has three
pairs of gills, its ally Malthe has the third incomplete,
while in Amphipnous all the three pairs of gills are
more or less rudimentary.

Where gill respiration ceases to be effective, the
blind outgrowth from the anterior portion of the
intestine (the air bladder) may take on respiratory
functions ; Amia has a single sac lying on the dorsal
surface of the intestine ; in Lepidosteus (the gar-pike),
the sac is divided internally, though it is single
externally ; in Ceratodus the opening into the single
sac lies to the left of the ventral surface of the
intestine, while in Polypterus the sac is double, and
opens on the middle line of the ventral surface. As
the air bladder becomes better developed, it becomes
better supplied with blood. (See page 202.)

In the cyclostomatous Bdellostoma the ducts from

Chap. VI.]

GILLS OF FISHES.

233

the gill sacs open separately to the exterior, but in

the hag the ducts unite and open by a common orifice

on either side. A similar modification obtains in the

gnathostomatous fishes, for in the

shark -like elasmobranchs (sharks and

rays) each gill cleft is open to the

exterior, while in Chimsera and all

other Fishes the clefts are covered

over either by a fold of the skin

merely, or by bony pieces (opercu-

liini), so that there is on either side

but a single opening to the exterior.

The gills, when complete, consist
of two folds of mucous membrane,
which are ordinarily triangular in
shape (Fig. 98), are supported by a
branchial arch (b), and by cartila-
ginous rods, have a blood-vessel
passing into them and bringing blood
from the heart, which breaks up
into capillaries ; these capillaries
unite into another vessel which car-
ries the oxygenated blood back to
the body. A gill is said to be
incomplete when one of the two
folds is alone developed.

IntheCyclostomata and the shark •
like Elasmobranchs, the gills take the
form of pouches, and the lamellae
form the transverse folds on the
walls of the pouch ; the septa which
project from the branchial arches are
as long as the gills, and each gill
chamber is, therefore, in a shark, completely separated
from the one in front or behind it ; as we pass through
Chimaera and the Ganoids to the Teleostei, we find
that these septa become more and more reduced, so that

the form of the
gills and the ar-
rangement of the
blood-vessels.

a, Branchial artery ;
6, branchial arch
(seen in cross sec-
tion); c, branches of
the branchial vein
v ; d, branches of
branchial artery.

234 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the gill is attached at its base only (Fig. 98), and the
gill lamellae are free. With the disappearance of the
septa we have, of course, the loss of the separate gill
slits, and the whole of the gills of one side come to
lie in a common chamber, which is covered over by
the operculum, and has only one opening to the
exterior.

The water which brings the necessary oxygen to
the gills enters by the mouth ; as the mouth opens

the operculum
rises, and the
gills separate
from one an-
other, but the
membrane which
fringes the oper-
culum acts as a
valve to prevent
the entrance of
water through
the opercular
cleft (Bert).
When the mouth
closes, and the
pharynx con-
tracts, the water is forced through the pharyngeal
clefts into the gill clefts owing to the presence of a
valvular arrangement which shuts off the passage into
the mouth.

In the already mentioned Amphipnous and in
Saccobranchus, the true gills are rudimentary, and a
sac with contractile walls is developed, which takes
in water and expels it at intervals ; the walls of these
sacs are richly supplied with blood-vessels, which are
arranged as in a gill ; that is to say, the blood that
passes from them goes direct to the aorta ; in the
climbing perch (Anabas), the internal surface of the

Fig. 99. — Suprabrancbial organ of Andbas

scandens.
a, Supraforanchial organ.

ehap. vi.] AIR SACS OF FISHES. 235

accessory sac is produced into a number of convoluted
folds (Fig. 99), which retain their moisture, and are
able to take up oxygen direct from the air, during the
comparatively long periods that the fish lives out of
the water. Its ally Ophiocephalus, Cobitis, and
various fishes with spongy air-bladders, such as Sudis
and Erythrinus, swallow air directly ; so that it is not
among the Dipnoi only that oxygen dissolved in
nitrogen (atmospheric air), is used for the necessary
oxydation of the tissues of fishes.

It is, however, in certain Ganoids and in the
Dipnoi that we get the most certain proofs of aerial
respiration ; Lepidosteus has been observed to pro-
trude its head from the water, to emit a bubble of
air, and to make a swallowing movement, and a
similar phenomenon has been seen in Araia (B. G.
Wilder) ; the noise made by Ceratodus is explained as
being due to the swallowing of air, and the streams of
Australia in which it lives are known to become
liquid mud in the dry seasons of the year. Protopterus
has been brought from West Africa to this country
embedded in the mud balls in which it lives during
the droughts, and has been revived by being placed in
warm water.

Fishes differ considerably in the extent to which
they are able to live on land ; thus, an eel will live
much longer than a gudgeon when taken out of the
water. The careful experiments of Bert show that
this difference is due not to a difference in gill arrange-
ment, but to a difference in the demand made by the
tissues of the body for their supply of oxygen. Here,
again, we have an example of the danger of arguing
from anatomical peculiarities where our hypotheses
are not controlled by experiments. In any discussion
of respiratory phenomena in animals it is neces-
sary to bear in mind the fact that all living
tissues are capable of absorbing oxygen, and that

236 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the tissues of different animals differ in the amount
they require.

We may be especially convinced of the truth of this
dictum of Bert's by the study of the respiratory arrange-
ments of the Amphibia. If an adult frog is placed in a
dry atmosphere it speedily dies ; in other words, respira-
tion in a frog is cutaneous as well as pulmonary, and a
frog may be deprived of its lungs and continue to live ;
so, again, the axolotl may have both gills and lungs
removed and yet continue to live. But if these
experiments are made in summer death soon super-
venes ; in other words, the skin becomes more dry
owing to the larger amount of moisture which can be
taken up by an equal volume of warmer air, and is
unable to take up enough oxygen to suffice for the
needs of the organism.

In all Amphibia the gills are at first external, or
project, under various forms, from the sides of the
body ; there are ordinarily three pairs present, which
are placed one above the other ; among the Urodela,
Menobranchus (Fig. 100) and Proteus appear to
retain the gills throughout life; in Menopoma and
Amphiuma the gills disappear, but one or two gill
clefts persist ; in the rest of the Urodela the gills dis-
appear completely. In the Anura the external are
soon replaced by internal gills, which, on the cessation
of the tadpole stage, disappear, and the clefts, which
had been covered by an opercular membrane, close up
entirely. The bell-shaped gills of Notodelphys lead to
the branchial vesicles which have been found in the
Cacciliac (Peters).

In addition to or in place of the gills, all Amphibia
have a paired outgrowth from the oesophagus, which
lies on the ventral surface, and is provided with
special blood-vessels coming directly from and return-
ing at once to the heart.

These outgrowths are known as the lungs, and

Chap. VI.]

ME NO BRA NCH US.

237

238 COMPARATIVE ANATOMY AND PHYSIOLOGY.

they seem to be the direct descendants of the swim-
bladder of fishes, which we have already traced from
its position as a dorsal sac in Amia and Lepidosteus
to a completely divided ventral sac in Protopterus.

PI)

PD

PD

Fig. 101. — Diagrams to illustrate the Development of tbe Lungs.

PD, Primitive intestine; ss', lung sacs; t, trachea; b, bronchus (.in A, B, and
c); L0 (in D), primitive ; i,g (in a), secondary pulmonary vesicles. (After
Wiedersheim.)

Comparable with these changes in the coarser details
of its anatomy are the modifications suffered by its
internal surface, which becomes more and "more spongy
and broken up into internal spaces ; and the changes
which bring its blood-vessels into direct relation with
the heart. (See page 203.)

A similar set of changes affects the lungs, either
as we trace them through the ascending scale of the

chap. vi. j LUNGS OF VERTEBRATES. 239

pentadactyle Vertebrata, or through the developmental
stages of a given individual. The earliest rudiment of
the lung is a single outgrowth (Fig. 101 ; A), which soon
divides at its blind end (B), while the unpaired
portion remains to form the tube (trachea) by
which the two sacs communicate with the oesophagus;
each swelling gives rise to primary (D), and these
to secondary (E) vesicles.

This series of changes ceases at various points in
various forms, so that the lungs are smooth within in
Menobranchus, provided with a few simple ridges in
Siren, and with secondary as well as primary ridges in
Amphiuma. The internal network in which the
blood-vessels course is still more elaborately developed
in the frog, but the lungs are, when looked at from
without, apparently nothing more than simple sacs.

The same is true of the lower Kept ilia; but there
is this important advance, that the bronchus, or tube
which brings the air into the lungs, does not, as in the
frog, cease at the opening into the lung, but is con-
tinued into it, and gives off branches within it ; in
some chamseleons narrow blind outgrowths proceed
from the hinder end of the lung, and in Chelonians
and Crocodiles the common lung-chamber opens into a
number of pouch-like sacs. The lungs of the former,
like those of birds, are firmly attached on either side
of the vertebral column, and the dorsal surface is
marked by grooves which correspond in position to
those of the superjacent ribs.

The lungs of Birds, in addition to their greater
internal complexity, are more particularly remarkable
for being continued into a number of air sacs,
whence prolongations are given off in the form of air
tubes and passages, which extend through all the
organs, including even the bones, of the body. These
air sacs play a very important part in the economy of
the bird, for they not only diminish its specific gravity,

240 COMPARATIVE ANATOMY AND PHYSIOLOGY.

but also warm the air. It has been calculated by-
Bert that, in a bird weighing 1,600 grammes, and
having a volume equal to 1,230 cubic centimetres, or,

in other words, a specific gravity of 1 *3 (— — ), 200

\1230 '

cubic centimetres of air can be introduced ; as these
200 cubic centimetres weigh -22 of a gramme, it is clear
that the specific gravity of the animal will be reduced

- 10 /1600 + 0-22 1600-22V,,
10 1-1S (l230 + 200 OT 1«T>*

must often take into their lungs air at a very low
temperature, but with this cold air there is com-
mingled that which returns to the lungs from the
warm viscera, and by this means the temperature of
the respired air is raised ; yet again, such cold air, or,
still more, the air of a desert, is often of great dryness,
while that which returns from the air sacs has been
moistened by the walls of these outgrowths.

The maximum of complexity is attained by the
lungs of the Mammalia, which, occupying a com-
paratively smaller space in the body, have nevertheless
a much larger area of respiratory surface ; externally
the lungs are frequently subdivided into two or more
lobes. It has been calculated by Aeby that the
human lung contains from three to four millions of
pulmonary vesicles, and that in man the respiratory
mucous membrane has, in a period of repose, a
superficial area of 79 '28 square metres, which
can be extended to more than half as much again,
or 129 '8 4 square metres; the extent of respiratory
surface in the female is rather less than that of
the male.

The air is brought into the lungs from the nasal
passages by the trachea, and that tube, as we know,
divides into two bronchi, which, in the Amniota,

* I have corrected what appears to be an error in Bert's calcu-
lation.

chip, vi.] BRONCHI ; TRACHEA. 241

extend into the cavity of the lung. The bronchi are
short indeed in lizards and snakes, but in crocodiles
and chelonians they extend for some considerable dis-
tance, and retain the cartilaginous rings by means of
which the tube is kept open ; these tubes give off
smaller lateral tubes, and so give rise to the so-called
bronchial tree, some of the branches of which lie
above and some below the pulmonary artery (p ; Fig.
102 A) ; these may be conveniently distinguished as the
eparterial and hyparterial bronchi.

While this bronchial tree is comparatively simple
in Reptiles, it becomes much more complicated in Birds,
where both eparterial and hyparterial systems are
well developed and give off lateral branches, some of
which extend to the end of the lung.

The difference between the bronchial tree of a Bird
and a Mammal does not lie in, as is ordinarily said,
the dichotomous mode of division of the bronchial
tubes of the latter, which never does obtain (Aeby),
but in the great reduction in the eparterial bronchial
system (Fig. 102 B), of which the right and left halves
are but rarely both present ; as a rule, the right is
lost, while in Hystrix both right and left eparterial
bronchi disappear.

The trachea varies greatly in the extent and
characters of its development ; short in Amphibians, it
is of a considerable length in Reptiles, but in them the
cartilaginous rings are incomplete ; in Mammals the
rings are also always incomplete, but in Birds the sepa-
rate rings are not only complete, but tend to undergo
calcification, and in some cases, as in the Dinornis,
even ossification. The trachea is of great length in
birds, and while this may be often seen to be of
significance as an aid to the vocal organ (see page 391),
it has clearly the not unimportant function of forming
a long tube in which the air is slightly warmed before
it enters the lungs. In birds the lower, and in other
Q-16

242 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Vertebrates the upper, part of the trachea may be con-
verted into a vocal organ.

In the skulls of certain Vertebrates, such as the
crocodile and the whale, certain modifications of the

bones of the palate
bring about an elon-
gation of the nasal
passages and an ap-
proximation of
posterior nares
the opening of
v /jr \m trachea (see

the

to

the

page

344) ; by
these means
water is pre-
vented, at
least in part,

lungs. other adaptive
modifications to the
same purpose may be
conveniently consi-
dered here.

In the Whales the
glottis, or opening
into the trachea, is
produced into a fun-
nel - like projection,
which extends into
the soft palate, and is

embraced by its sides. By this means the trachea is
brought into direct connection with the nasal passages,
the air does not enter at all into the cavity of the
mouth, and the water flows on either side into the
gullet. A similar disposition of the glottis obtains
in the young of the Marsupials, which, born at an

B

Fig. 102 A.— Bronchial Tree of a Bird.

p, Pulmonary artery ; A, eparterial ; B, hyp?
term bronchial systems ; v, ventral ;
dorsal branches.

Chap. VI.]

CETACEA ; SIREWA.

243

Here, then, milk

B

age too early to allow them to actively suck the
mother, hang on to a long nipple, and have the
milk injected by the mother (by the contraction of
the cremaster muscle, and the consequent compres-
sion of the mammary glands),
flows on either side of the
air tube, and the latter is,
as in the whale, a direct
continuation of the air
passages in the head. It
is important to observe that
•there is no prolongation of
the air tube in the Sirenia,
but that their epiglottis is
large, and capable of com-
pletely closing the entrance
into the trachea; at the
same time it will be remem-
bered that the dugong and
manatee are herbivorous.

So, again, the Sirenia
differ from the Cetacea in
the manner in which they
obtain a large supply of air.
In the former the dia-
phragm, in place of forming
a more or less vertical par-
tition between the thoracic
and abdominal cavities, slopes backwards and upwards,
so as to largely increase the area of the thoracic cavity,
the extension of which is occupied by the large lungs.
In the dolphins and porpoises the nasal passages open
into lateral sacs with elastic walls ; the possession of.'
these sacs must, in addition to their air-containing
function, diminish to a certain extent the specific
gravity of the skull. The commonly received story
that a whale " blows " water is due to the fact that a

Fig. 102 B.— Bronchial Tree of a
Mammal (Horse).

A, Eparterial : B, hyj>arterial ventral
(»1; (<f) hyparterial dorsal bronchi;
pa, pv, pulmonary artery and
vein. (After Aeby.)

244 COMPARATIVE ANATOMY AND PHYSIOLOGY.

large quantity of warm air is rapidly expelled through
the single spiracle (the homologue of the two external
nares of other animals), or so-called "blow-hole," and
that the moisture in this air condenses into water
as it suddenly comes into a colder medium. A whale
no more breathes water than does a man on a frosty
day.

Pulmonary respiration, or the taking-in and the
expulsion of air from the lungs, is effected in very
various ways in different Vertebrates ; the air tube
being, as we know, ordinarily kept open by the car-
tilaginous or bony rings or supports which are found
in its walls, it is clear that air may either be driven
out or sucked in.

In the Dipnoi there is no cartilaginous trachea,
and the air enters in and passes out by a longitudinal
slit, the sides of which are separated from one another
by the contraction of the muscle that surrounds it.
In the Perennibranchiata there is no true trachea, but
on either side of the slit there is a small cartilage
with which a constrictor or a dilatator muscle is
connected (Wiedersheim). In the rest of the Am-
phibia there is a true trachea of no great length, the
opening into which is sometimes provided with muscles,
by means of which it can be enlarged or diminished
in size. In the frog, whose physiology has been more
fully studied, air is known to be forced into the lungs
by the action of the muscles of the floor of the mouth ;
this apparatus, appropriately known as the buccal
pump, acts in the following manner : the mouth is
shut, and the floor of the mouth depressed by the
contraction of the muscles connected therewith ; the
vacuum so formed is filled by the entrance of air
through the nostrils and nasal passages ; the nostrils
are then closed, and the entrance to the gullet barred,
while the floor of the mouth rises on the contraction of
the muscles connected with the hyoid ; the entrance

Chap. VI.] Rp.PTILES ; BlRDS. 245

to the air tube is widened by the contraction of the
dilatator muscles, and the compressed air, finding
there its only means of exit, enters the passage to the
lungs. By the elasticity of the walls of the lungs
themselves, and by the contraction of their muscles
and those of the body wall, the air that has thus
entered is soon afterwards driven out.

In the Chelonia, which, in accordance with their
sluggish habits, execute respiratory movements only
three times a minute (Bert), the thorax is dilated by
a special inspiratory muscle, and the limbs only take
part in the action when inspiration and expiration
succeed one another with more than an ordinary
rapidity. In the Ophidia the cavity of the thorax is
increased by the movements of the ribs, and as these
are also the locomotor organs of snakes, we have here
again an example of the relation between respiratory
and locomotor activity. In Lizards and Crocodiles,
where the belly ordinarily touches the ground, the
thorax is extended transversely much more than from
above downwards, for in all Amniota the enlargement
of the cavity is effected by the movements of the ribs.
The expulsion of the air is brought about by the con-
tractility of the walls of the lungs.

In Birds the lungs are fixed to the back and sides
of the thorax, the extension of which, in the movements
of expiration, is much greater in the vertical than in
the transverse direction. An inspection of the skele-
ton of a bird (Fig. 135) will show that the ribs con-
nected with the spinal column are set at an angle to
those which are connected with the sternum ; on the
contraction of the inspiratory muscles this angle
becomes more open, the sternum is more widely
separated from the back, and the thoracic region is
increased in extent ; there is, at the same time, a
certain amount of transverse extension. When the
thorax enlarges air is drawn in from the air sacs as

246 COMPARATIVE ANATOMY AND PHYSIOLOGY.

well as from the outer world ; and when the thorax
contracts, in the act of expiration, air is driven into
the air sacs as well as through the trachea outwards.

In Mammals the movements of the ribs are
greatly aided by the flattening out, or curving up-
wards, of the diaphragm, or muscular partition
which separates the thoracic from the abdominal
portion of the body cavity. The respiratory move-
ments of mammals have been fully studied in Man.
(See " Human Physiology," chap, v.)

Bert has collected a large number of statistics
with regard to the number of respiratory move-
ments executed per minute by various animals.
From this we learn that, on the whole, they are
more numerous in Mammals than in Birds. A rat,
for example, has been seen to make 320 movements
a minute, while the canary gives the highest (100)
number for birds. Rodents generally respire fre-
quently ; the dog and ox 15 times, the lion and horse
10, and a hippopotamus was on one occasion observed
to breathe only once in a minute ; some large birds,
such as ' the marabou, pelican, or condor, only 4 to 6
times a minute; a Crotalus 5 times, a lizard 12. An
active sea- lamprey gave a number of 120 ; rays and
dog-fishes from 40 to 50, Limulus 12, while Cepha-
lopods varied between 14 and 65 times a minute. On
the whole, carnivorous breathe less frequently than
herbivorous forms, and both than rodents ; smaller
forms more frequently than larger members of the
same group, and active more often than sluggish
species. It is, however, to be carefully observed
that these numbers give us no information as to the
quantity of air taken in, nor as to the number of
times in which the heart was beating per minute;

247

CHAPTER VII.

ORGANS OF NITROGENOUS EXCRETION.

VERY much doubt hangs over the function of the
organs which are said to have a renal function in the
lower animals, owing to the great discrepancies be-
tween the results attained to by those who have inves-
tigated the excreta of, or concretions in, these organs,
and the very great difficulties which lie in the way of
such chemical inquiries.

Nothing can be certainly said as to the renal
organs of the Protozoa, if we may use the term
renal organs in a general way, as applying to such
parts of the organism as purify the body of its nitro-
genous waste ; this, however, is certain, that in the
course of the molecular activity of a mass of proto-
plasm, nitrogenous products are formed which are in
the nature of waste products, and which are injurious
to the organism if not speedily removed from it. We
know that in most Protozoa there are one or more
spaces which, expanding, take up, and, contracting,
drive out, water. Knowing, as wre do, from our own
experience, the value of water as a diuretic agent, it
seems almost justifiable to suppose that, while this
water has no doubt a respiratory function in the
Protozoa, it acts also as an agent for removing waste.
The supposition that the office of the contractile vesicle
is to drive fluid out of the body is supported by the
discovery of Vorticellids, in which the contractile
vesicle is connected by a canal with the " vestibule "
which lies beneath the mouth opening ; on the con-
traction of the vesicle the contained water passes into
the mouth-opening, and so, of course, makes its way

248 COMPARATIVE ANATOMY AND PHYSIOLOGY.

to the exterior. In the same manner we may sup-
pose that in the Sponges, which certainly do get rid
of nitrogenous waste, the currents of water that pass
through the canals of the body wall carry away with
them waste nitrogenous formations ; but here, as in
the Protozoa, experimental evidence is still wanting.
We are hardly better off for information as concerns
the Ccelenterata or the Echiiiodermata. Of the
former class, indeed, the mesenterial filaments of
Actiniae, and a whitish layer on the lower side of the
umbrella of Porpita, have been stated to contain
guanin, which is a waste nitrogenous product ; and
the same compound has been said to be found in the
rectal caeca of the starfish, and in the Cuvierian
organs of certain holothurians ; as to the last, how-
ever, it is doubtful whether the true Cuvierian organs
were really examined, and, as to their function, the
great balance of evidence is in favour of their being
rather offensive than excretory organs.

In the Verifies we have the advantage of being
able to detect organs which, by their position, rela-
tions, and homologies, afford considerable support to
the view that they have a renal function. It will be
most convenient to first examine the so-called seg-
mental or kidney -like organs (iiephridia) of so well
developed a form as the earthworm.

In all but the first segment of the body we find
on the ventral surface and on either side of the middle
line, a convoluted tube, which opens by a funnel-
shaped orifice into the body cavity, penetrates
the membranous wall which separates one segment
from the next succeeding, and in the latter opens to
the exterior by a small pore. The ciliated funnel-
shaped opening, and the thinner-walled portion of
the coiled duct, may be looked upon as the receiving
portion of the organ ; the true excretory activity is,
no doubt, limited to the part where the walls are

Chap. VI T.]

PL A TYHELMINTHES.

249

glandular, and these glands, we may suppose, act on

the contents of the blood-vessels which are richly

distributed to the nephridium. The terminal and

wider portion, the walls of

which are muscular, may be

looked upon as analogous to a

ureter.

Bearing in mind that we
have in the earthworm to do
with a form in which meta-
meric segmentation is most
markedly expressed, and that
this metamerism has clearly
affected the nephridia, we are
prepared to find a very much
simpler condition of things
among the Platyhelminthes,
and, at the same time, to find an
arrangement which is more dif-
fused. InMonoccelis (Fig. 104),
for example, there is a plexus of
fine canals, which communicate,
on the one side, with large
principal canals, of which there
are two pairs, one external
and one internal, and on the
other with funnel-shaped pro- Fig.^ios.— A single *:«
cesses, the entrance to which is
guarded by a long cilium ; the "'
principal canals are connected
with one another by anastomo-
sing branches. In the Den-
drocoela, as represented by
Polyccelis, the fine canals appear to be absent.

If we take the liver fluke as a type of the
Trematoda, we again find that the system of
excretory vessels is diffused throughout the whole

dium of Anachceto-

gSTes

K

250 COMPARATIVE ANATOMY AND PHYSIOLOGY.

body ; the finest ducts are distributed through all
parts of the organism, and they pass into collecting
vessels, which, by the formation of anastomoses, give
rise to a most complicated plexus ; from these arise

Fig. 104. — Excretory System of Monoccelis fusca, showing the numerous
Infuudibula, and the brauchiug Tubes. (After Fraipont.)

the efferent ducts, which gradually unite into collect-
ing vessels; these, again, form a plexus, and from
these there again arise vessels which pass into a
median longitudinal trunk, which opens at the hinder
end of the body by an excretory pore. There are
no valves or muscular walls by means of which the
products are aided on their way to the outer world.
The contents of the vessels are stated to be colourless,
and to contain a number of small particles of high
refractive power ; Lieberkiihn says that he has been
able to detect the presence of guanin.

Among the Cestoda we find that, while the
young of some forms have a complicated system of
fine canals, the ordinary arrangement is that of two

Chap, vii.] CESTODA ; ROTATORIA. 251

longitudinal vessels which extend through all the
joints, which may or may not branch and form a
plexus in the head, and which open to the exterior by
a single excretory pore, which is placed in the terminal
joint ; sometimes the tubes open in the several joints
by secondary foramina (Fraipont), and in such cases
the terminal pore and vesicle become more or less
atrophied. These secondary orifices in a tapeworm
are not to be compared with the openings of the
" segmental organs " in the earthworm. The calcareous
concretions which are so frequently observed in tape-
worms have not as yet been certainly shown to have
the character of renal excretory products. The course
of the fluid in these vessels is directed by the valves
which are placed in the region of the head, and which
are so arranged as to prevent the fluid from passing
forwards ; the canals themselves are devoid of cilia,
and, as in the Trematoda, the propelling power is to
be sought for in the muscles of the body wall. The
fluid is said to contain substances which are chemically
allied to xanthin or guaniii (Sommer).

The simple unsegmented body of the Rotatoria
presents us with a correspondingly simple condition of
the excretory organs, .but their relations are here
more easily made out, owing to the development of a
definite body cavity. There are several distinct
ciliated and funnel-shaped openings into the coelom,
and these lead, by short and simple canals, into a
longitudinal vessel on either side ; this is more or
less coiled on its course, and opens into the cloaca.
(See page 119.)

Similar canals arising from the cloaca, and opening
by ciliated infundibula into the body cavity, are
found also in the Oepliyrea ; but these forms are
most remarkable and interesting for having, in addi-
tion to these cloacal outgrowths, others which, by
opening on one side into the body cavity and on the

252 COMPARATIVE ANATOMY AND PHYSIOLOGY.

other directly to the exterior, recall the characters of
the segmental organs of the earthworm ; there may
be one or a few pairs of these tubes, and their excretory
nature .is assumed from the presence in them of a
brown concretion (as in the so-called " brown tubes "
of Sipunculus) ; in certain forms they do, without
doubt, lose their excretory, and take on the function
of efferent ducts for the generative products, an
arrangement which is by no means confined to the
Gephyrea among animals ; in Bonellia, the tube which
functions as the uterus is developed on one side only
of the body.

It is of especial interest to observe that in the
developing leech three pairs of canals are developed
in the hinder end of the body, and are, at least, pro-
visional excretory organs, even if they are in no way
related to the cloacal outgrowths of lower worms.
The permanent nephridia of the leech attain to a
very high degree of complexity ; it is possible to
distinguish a vesicle and a gland, connected with
one another by a vesicle duct (Bourne). The cells
of the gland are all penetrated by ductules, and the
central portion of each of its four constituent lobes is
occupied by a duct which opens into the vesicle
duct ; a plexus of blood-vessels is found in the
gland, each cell of which is surrounded by a loop of
that plexus ; the wall of the vesicle is muscular, and
by its contractions the contents are expelled to the
exterior. The marine ecto-parasitic leech Pontob-
della is remarkable for the possession of a very
primitive disposition of the nephridia ; the organ is
single and continuous, and consists of a highly com-
plicated network of tubules ; those on one side of the
body are continuous with those of the other, and with-
out developing any vesicle, they open to the exterior
at regular intervals (Bourne).

In the lower Crustacea an excretory function is

Chap, vii.] CRUSTACEA. 253

ascribed to the so-called "shell gland" which forms
a looped organ in the dorsal middle line } but there
are as yet no physiological facts which confirm this
supposition. In the higher Crustacea, an organ
which, in its essential relations, calls forcibly to mind
the arrangement of the nephridia of the earthworm,
is found at the base of the second pair of antennae.
This is the so-called " green gland " of the cray-
fish, where it presents the following characters.

An orifice, large enough to admit a fairly stout
bristle, leads by a short canal into a wider sac, with
very delicate walls, which lies in front of arid below
the anterior portion of the stomach. Below this wide
thin-walled sac lies a smaller body, which is in com-
munication with it by a narrow coiled passage ; this
body consists of a yellowish-brown anterior portion,
which ends blindly, and of a green portion, which
lies between it and the duct. The former is spongy
in character at its anterior end, and the rest has a
number of lamelliform processes rising up from its
floor ; the green part, which is broader and natter,
has its walls produced into a number of small saccular
outgrowths. Ou the inner surface of the cells of this
green part, and of the succeeding white coiled tube,
small highly refractive bodies are to be observed,
which are no doubt of an excretory nature. The
blood-vessels which bring to the gland the materials
that are to be excreted by it arise from the antennary
and sternal arteries, and break up into fine capillaries
in the walls of the gland. The products excreted
are stated to resemble guanin, but it will be understood
that the small quantities which can be collected make
any chemical investigation a matter of considerable
difficulty;

Wassiliew, to whom we owe the latest description
of the green gland, believes that three stages may be
recognised in the differentiation of the renal orgari of

254 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Crustacea. The simplest is that which obtains in
many Copepocla, where there is merely a long
smooth tube, of the same calibre throughout ; in some
Phyllopods the tube is enlarged at certain points,
and more especially at its blind end ; while the third
and most complex stage is that which obtains in the
crayfish, where the tube is widened at various points,
has the constituent cells differing in structure and
function, and is folded on itself. We may suppose
that the lower terminal portion is glandular and ex-
cretory, and that the wide thin-walled sac acts as a
reservoir.

The organ of Bojanus in the lamellibranch
Mollusca offers many very striking points of re-
semblance to the green gland, but it differs most essen-
tially in retaining the primitive character of having an
opening into the body cavity. On the floor of the
space which surrounds the heart (pericardium) we
find, on either side of the ventricle, a small orifice
which leads into an elongated chamber, with thick
dark-coloured walls, and narrower at its hinder than
at its front end ; the walls give rise to spongy out-
growths, which project into the cavity, and which
contain blood spaces, and are invested by the secreting
epithelial cells ; at its hinder and narrower end this
thick- walled portion opens into a cavity which lies
above it, and which has thin walls ; this, which
opens on either side into a cloaca, or directly to the
exterior, is no doubt the portion of the organ which
has the function of a reservoir. Here, then, we have
again an arrangement which may be explained as
that of a tube, folded on itself, and having part
differentiated into a glandular secreting region and
part into a collecting region or reservoir. The gland
is said to secrete uric acid.

In the fresh-water mussel the products of the gene-
rative glands pass to the exterior qiiite independently

chap, vii.i MOLLUSC A. • 255

of the ducts of the renal organ, but in others the latter
are used as a means of passage for the genital
products, just as are the brown tubes in the Ge-
phyrea. In Spondylus the products are' discharged
into the renal cavity ; in Mytilus (the sea mussel)
there is a distinct genital duct, which opens, however,
on the same papilla as the renal ; while in Anodon
and others the two ducts are completely separated.
This use of. the renal ducts by the generative glands
is regarded by Hubrecht as a more primitive arrange-
ment, but it was, he thinks, preceded by one in
which the genital products first escaped into the peri-
cardium, whence they were taken up by the renal
organ.

In the lower Cephalophora the renal glands are
paired, and either open separately, as in Dentalium, or,
as in Proneomenia, the ducts unite posteriorly ; in the
more differentiated Gastropoda we find that the
organ of one or other side is affected by that torsion
of the body, which has so pronounced an influence on the
development of all the other organs of these molluscs.
In the Pulmonata the external orifice is obscured
by opening into the cavity of the air chamber, but as
this is merely formed by the folding over and attach-
ment of one edge of the mantle, there is no reason to
suppose that there is any real change in its essential
morphological characters. Sometimes the terminal
portion of the gland has muscular tissue developed in
its walls, and in some Heteropods and Pteropods the
whole organ is capable of contracting.

In the Cephalopoda there are either one
(dibranchiata) or two (tetrabranchiata) pairs of renal
organs (Fig. 105 ; rr). They are richly supplied with
blood-vessels, which enter into the lamelliform pro-
cesses that project into their interior ; they open by
a somewhat circuitous course into that portion of
the body cavity which surrounds the heart, and

256 COMPARATIVE ANATOMY AND PHYSIOLOGY.

communicate by ducts, or ureters, with the exterior.
The chief product of these excretory glands is stated
to be phosphate of calcium.

Among the air-breathing Arthropods we find
that the excretory organs are outgrowths of the
terminal portion of the intestine, which, varying
more or less in size and number, extend some way

into the body cavity ;
they are the organs that
are known as the Mal-
ptgliiaii tubes, and
uric acid has been repeat-
edly proved to be found
in them. It is possible,
but it is by no means
certain, that they are ho-
mologous with the rectal
excretory organs which
we have already found
in the Gephyrea and the
Rotatoria. They may be
completely wanting, as in
many of the Aptera and
Pycnogonida ; there may
be two as in the harvest-
men (Opilionida) where
they are considerably
branched ; or four, as in the blowfly, where they are
very short; or six, as in the greater number of insects.
Sometimes there is a much larger number, the cock-
roach having from twenty to thirty, and some Hymen-
optera as many as one hundred and fifty. They are
sometimes arranged in bundles, and where there is a
common duct leading to the exterior, its walls are
sometimes provided with muscular tissue, which aids
in the expulsion of the contents.

The Malpighian tubules are often of great size in

Tig. 105.— Eespiratory and Eejal
Organs of Septa.

a. Aorta ; v, vena cava ; v*, posterior
vcnas cavas, r, heart ; d, enlargements
of the veins (auricles) ; e, branchial
hearts ; 6 6, gills ; r r, kidneys.

Chap, vii.] RENAL ORGANS OF CHORD ATA. 257

the larvae of insects, and a large quantity of renal ex-
cretion is collected in the rectum during the pupal
stage. This phenomenon may, as Gegenbaur has
pointed out, be well brought into relation with the fact
that it is at this stage that the " most intense plastic
activity is going on in the organism in con-
nection with the development of the perfect body."
The blowfly, when -it first emerges from the pupa
case, excretes a. semi-solid mass of nearly pure
uric acid (Lowne).

Peripatus is remarkable for the possession of
organs which have a general resemblance to the seg-
mental organs or nephridia of the earthworm and other
Annulata, and are like them found in all the segments
of the body, but those in the three foremost pairs of
tegs are very rudimentary. A typical nephridinm
opens at the base of each leg ; the tube leading to the
opening is narrow, but is continued internally into a
large sac, which appears to act as a bladder or collect-
ing organ ; this sac is continuous with a coiled tube,
which opens by a funnel-shaped orifice into the cavity
of the body.

No definite information has been acquired as to the
possession of a renal gland by Amphioxus. The
Urochordata are remarkable for the fact that the
uric acid secreted from their blood is not carried away
to the exterior, but collected into spherical vesicles
of large size, which lie in a mass round the intestine ;
in Lithonephrya the cavity of the renal organ is
almost filled by a single large concretion ; in other
Molgulidse, where the presence of uric acid has been
definitely proved by the colour reactions given with
nitric acid and with ammonia (" murexide test "), the
renal organ has the form of a sac, which lies close to
the pericardium.

In the Verteforata we find that, with a general
resemblance to the nephridia of the ringed worms, the
R— 16

258 COMPARATIVE ANATOMY AND PHYSIOLOGY.

excretory organs present some very complex characters
as we ascend the series. In the examination of these
organs it will be found convenient to make use of cer-
tain technical terms.

The Pronephros (or head-kidney) is a small
glandular body, with one or more funnel-shaped
ciliated openings into the body cavity ; it is ordinarily
placed far forwards, and is provided with a duct, the
so-called segmental duct. Like many other renal
organs, it is provided with a special blood-supply in the
shape of a coil of vessels, or glonierulus.

The mesonephros (Wolfftan body) consists of
a series of glandular tubes which open by funnel-
shaped openings into the body cavity, and pour their
secretion into the common segmental duct.

The metanephros (kidney of Amniota) con-
sists of a complex of coiled tubes which open into a
special duct, which is derived from that of the
mesonephros.

All these three organs may be developed in one
and the same individual, but they are not, in higher
forms, in active function at the same time ; the
metanephros is developed in the Amniota only, though
an indication of its existence is to be seen in Elasmo-
branchs.

In the adult Cyclostoniata the mesonephros is
found in its simplest conditioL, for it there consists of
a segmental duct with tubes (Fig. 106; a, b) given off on
one side, and ending in a blind enlargement ; in this
enlargement an artery (d) breaks up into a plexus of
fine vessels which form the glomerulus (c), and thence
the blood passes into the efferent artery (e). An-
teriorly to this there is, in Myxine, a pronephros,
which disappears in the adult Petromyzon, where the
whole kidney is more compact.

In the Elasmotoranchii the segmental tubes in
the hinder part of the mesonephros unite with one

Chap. VII.]

FISHES; AMPHIBIA.

259

another before they open into the common efferent
duct, and the pronephros would appear to be absent
even at the very earliest stages. In them, as in Fishes
generally, the renal organs are of great length, as com-
pared with those of the higher Yertebrata.

In the sturgeon, among the Oanoids, the kidneys
extend from just behind the skull as far as the cloaca,
and differ in width in different regions ; in them, and

A I B

Fig. 106.— Mesonephros of Bdellostoma.

A. ff.,Segmental duct ; 6, segniental tube ; c, glomeruHie. B. A part more highly
magnified, showing one duct with its afferent vessel d, and its efferent e.
(After J. Mill lef.)

in the Teleostei, there is a great reduction in the
number of separate ductules which pass from the sub-
stance of the kidney into the efferent duct.

In the Urodela the mesonephros is of considerable
length, and gives off a number of ducts (Fig. 107) ; in
the ffogy which may be taken as a type of the Aimra,
the kidney is much shorter, and the efferent duct
(ureter) is closely applied to the lower third of the
kidney ; if, however, we make an examination of a
longitudinal section of the kidney under a low magni-
fying power, we shall see that the substance of that

260 COMPARATIVE ANATOMY AND PHYSIOLOGY.

organ consists of a number of delicate and convoluted
tubes, which, when mapped out in diagram, have
much the appearance shown
in the figure (Fig. 109; A).

Just as in nearly all Ich-
thyopsida the pronephros is
seen to be a purely larval
organ (Balfour), so in the Am-
niota the mesonephros makes
way for the metanephros,which
is here preceded for a very
short time by the pronephros.
The metanephros appears to
be a further development of
the hinder part of the mesone-
phros, and, like it, it retains
throughout life evidence of
being composed of a system of
tubules, which advance in com-
plexity as we rise in the series.
The truth of this will be
shown by a study of the mi-
nute anatomy of the kidney
of a tortoise (Fig. 109; B), and
a comparison of it with that
of a pigeon (Fig. 109; c), and
of a man (D).

So, again, with advancing
complexity of internal struc-
Fig. 107. — Diagram of the ture, we observe, as we pass

JMe.°neH0rOS t°efsti?. ^ fr°m RePtiles to Birds or

kidney. ('Modified 'from Mammals, that the length of
Sprengei.) the kidney diminishes, and

that it becomes limited to the lumbar region of the
body, while the ducts that open into the pelvis of the
kidney are reduced in number and increased in size.
The more important macroscopic differences in the

Chap, vii.] SAUROPSIDA ; MAMMALIA.

261

kidneys of the Ainniota obtain in the relative position
of the two organs ; in Snakes, for example, not only
are the kidneys elongated in relation to the general

form of the body, but ^ .

(rAo

TK

one lies a good deal
in front of the other ;
this difference of
level between the
two kidneys, which
is clearly an arrange-
ment for more con-
venient packing, may
be seen also in some
Mammals (rabbit) ;
secondly, the kidneys
vary considerably in
their external form ;
thus, those of the
lizard are only
slightly, those of the
Ophidia much more,
broken up into lobes;
this is a difference,
also, which obtains
between young and
old forms, for in the
latter the number of
lobes is much greater
than in the former.

FK

lOe.-Urogenit^ljLpparatus of a Male

N Kf Kidneys : urur, ureters : + their point of
origin ; 8 s', their opening into the cloaca cl,
H o, testes; FK, fat body;

vena cava inferior ;
tAfter Wiedersheim.)

AO, aorta; cv.
efferent veins.

fif i™4-^ 4-1^0. c.r nnaa
tit mtO the Spaces

between the trans-

verse processes of the vertebrae of the pelvic region,
The lobes of the kidney, after appearing, do, in many
Mammals, again fuse with one another, so that
while the Cetacea, in which this process does not
obtain, may have as many as two hundred lobes, the

262 COMPARATIVE ANATOMY AND PHYSIOLOGY.

outer surface of the kidney of a rabbit or of a man is
quite smooth ; this fusion affects also the inner sub-
stances of the lobes, and in man three, or only two,
tubes open directly into the pelvis of the organ.

Fig. 109.— Diagrams of the Urinary Tubules of

A, The frog; B, tortoise; c, pigeon (after Hufner); D, man (after Ludwig) ; I.,
glomerulus ; u. to vii., various regions of the tubule.

The duct that carries to the exterior the secretion
of the kidney is ordinarily known as the ureter ;
so long as the pronephros persists, the segmental
duct and the ureter are one and the same ; when
the mesonephros appears, the duct becomes divided
into two, the Miilleriaii duct (which has no con-
nection with the kidney, but forms the oviduct of the

chap, vii.] URETERS AND BLADDERS. 263

female, and undergoes more or less degeneration in
the male), and the Wolffiaii duct, which remains
connected with the mesonephros, and carries away its
products and those of the testes in the male. The
ureter, like the collecting ducts of the inetanephros,
becomes developed from part of the Wolffian duct,
which, when this ureter is 'present, only carries away
the secretion of the testes.

In the Cyclostomata, the ducts that have the
functions of ureters open to the exterior by a papilla
(the urinogenital papilla), which is placed .behind the
anus ; they first, however, open into a cloaca, into
which also the generative products make their way by
the abdominal pores. Among the Ichthyopsida
the renal and generative products ordinarily pass into
a cloaca, which is common to them and to the rectal
orifice of the intestine ; but in the Teleostei the
genito-urmary is distinct from and posterior to the
rectal, and the urinary pore is, as a rule, separate
from and behind the genital. The terminal portion
of the ureteric ducts of fishes is often enlarged, to
form a so-called bladder ; this, however, must not be
regarded as the homologue of that of Amphibians,
or of the Amniota, where the bladder is an out-
growth of the ventral wall of the cloaca. In the
Amphibia and in some Reptilia this bladder retains
its primitive position, or, in other words, does not
become part of the direct line of passage between the
kidneys and the exterior ; in the Ophidia, Crocodilia,
and Aves, the bladder is atrophied.

It is in the Mammalia only that the bladder is
found on the direct line of passage between the ure-
ters and the exterior, and is not so found in the lowest
division or Prototheria, where rather it occupies
the same position as in the frog ; in the Metatheria
the ureteric ducts open into the base of the bladder ;
in the Eiitheria they open at various points along its

264 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Fig. 110. — Renal and Generative Organs of

Ericulus sct(j8us. ,

course ; thus in
Gymnura they
open into, and in
Ericulus near, the
neck (Fig. 110);
in in an th ey e n te r
the walls of the
bladder at its
base, but run in
its walls for
about three-
quarters of an
inch before open
ing into the ca-
vity of the blad-
der ; in the rab-
bit they open far
up on the hind
wall, and in the
coney at the top.
Thebladderitself
varies consider-
ably in size and
form ; when, as
in the higher
forms, the cloaca
disappears, the
urino-genital ori-
fice is in front of
the anus,
page 170.)

Kidneys ; /, testes ;
ut, ureter ; ?-'d, vas
deferens ; 6, bladder;
pg, prostate gland ;
eg, Cowper's gland ;
lpt levator penis; cc,
cystic urethra; pr.
prepuce, divided and
reflected ; p, penis.
lAf ter Dobson.)

Chap, viii.] SPECIAL SECRETIONS. 265

The two important constituents of the urine of the
Vertebrata are urea and uric acid ; the former is
the preponderating constituent in the Mammalia,
the latter in the Sauropsida, and, as urea is readily
soluble in water, while uric acid is very insoluble, we
find that the renal products of the Sauropsida are or-
dinarily semi-fluid, and dry rapidly on exposure to
the air. The urine of carnivorous mammals is more
concentrated and more acid than that of man ; that
of herbivorous forms is ordinarily alkaline, but when
it is acid in reaction, uric acid is as abundant as
in the lion or the tiger (Garrod) ; the herbivora have
a large quantity of hippuric acid, which is only found
in small quantities in man.

CHAPTER VIII.

ORGANS OF SPECIAL SECRETIONS.

IN addition to the various secretions, such as saliva
and bile, or excretions such as the uric, there are
others which, though dependent, of course, on the
activity of protoplasmic cells, are special and peculiar
to different animals, and are not a necessary result of
protoplasmic activity ; such, for example, is the poison
of the scorpion, or the ink of the cuttlefish.

Poison or venom glands. — While in the
Ophidia or the mad clog the poison is due to a modifi-
cation of the proper salivary glands, we find special
glands developed in various Arthropods. Among the
Arachnida, the spider is provided with tubular
glands placed at the base of the. chelicerse, or first pair
of appendages, which open by a narrow duct at the
orifice at the end of these organs ; the two last joints
are movable on one another, and are thus enabled to

266 COMPARATIVE ANATOMY AND PHYSIOLOGY.-

bite their victim before injecting the poison ; though,
as in the case of the Tarantula, the ill effects of their
venom have been somewhat exaggerated, there is no
doubt that the poison of many spiders is capable of
inflicting mortal injuries; the statement that the
West Indian My gale avicularia is able to catch and
kill small birds appears to be true. In the Scorpions
the poison glands, which are oval in form, and have
an outer layer of muscular tissue, are situated in
the terminal segment of the body ; their ducts open
at the tip of the spinous process at the end of the tail,
which is recurved when the animal strikes a blow.
Among insects, many of the Hymenoptera are pro-
vided with racemose organs placed in the hinder part
of the body, which secrete a fluid, the irritating
effects of which appear to be due to the contained
formic acid ; the venom is injected by a sting, which
consists of a median piece grooved longitudinally, and
of two side pieces which, on becoming closely applied
to it, convert the groove into a capillary canal along
which the fluid flows. The venom is not always used
merely as a means of offence, many Hymenoptera
stinging other insects for the purpose of paralysing
them while they carry them to their young, to which
they will serve as food.

Various Fishes are provided with defensive organs
possessing venomous properties ; such are the dorsal
spines of the weavers, which are deeply grooved and
charged with fluid mucus. In Synanceia the free half
of each dorsal spine bears a pear-shaped bag in which
the milky poison is contained. In Thalassophryne,
from Panama, the sac is placed at the base of the
spine, and as it is without any muscular sheath, the
poison can only be ejected by the pressure exerted on
the sac when the spine enters the body (Giinther).
The integument of many Amphibia is richly pro-
vided with glands, which secrete a viscid fluid possessed

Chap, vni.] SILK ORGANS. 267

of more or less well-marked irritating properties ; a
familiar example of this is the common toad, the
handling of which is often succeeded by inflamma-
tion of the eyelid. Experiments with subcutaneous
injections of the dermal secretion of the Triton show
that it appears to have an effect on the heart, and
that of the salamander on the central nervous system.

Silk organs.- The result of the secretion of the
silk organs of Spiders is the well-known web ; but
the secreted product, when it first appears, is a vis-
cous transparent liquid, which rapidly hardens on ex-
posure to the air, and then forms threads. The silk is
produced in various glands, which, however different
in form, are always found distributed among the con-
tents of the abdomen ; the secretion makes its way
to the exterior through the so-called "spinnerets," of
which there are ordinarily three pairs ; these have the
form of obtusely conical papillae, the tips of which are
provided with a number of pores through which the
silk escapes to the exterior. This silk is used in very
various ways ; some spiders make cells or tubes for
themselves, some scatter the threads about, with the
obvious object of entangling an approaching prey,
while many make nets for the purpose of entrap] ring
victims. The so-called mason, or trap-door spiders,
spin a number of successive webs, which unite to form
a door for the pit in which they dwell. Clotho makes
a net-like tent, in which the young are concealed. In
many cases the webs are spun with considerable
rapidity, the common English spiders being able to
make one in about an hour.

Among the Iiisecta, silk-producing glands are
best seen in the larvaB of the Lepidoptera, where they
have the form of two long caecal tubes, placed one on
either side of the intestine, and opening by narrow
ducts at the base of a spinneret, which is developed
on the labium. As in the spider, the silk is at first

268 COMPARATIVE ANATOMY AND PHYSIOLOGY.

fluid, but soon hardens on exposure to the air. The
silk thus secreted may be used as a kind of attaching
rope, as in some moths (Tortrix), or it may form an
investment for the larva, as is the case with the silk-
worm. In the larvae of the ant-lion (Myrmeleoii) the
silk is secreted by the rectum, and escapes by a spin-
neret which is placed near the anus.

A secretion of somewhat similar character is made
by some of the JLamellibranchiata, where the
foot secretes a soft substance, which becomes hard and
chitinous on exposure to the air ; this " foyssus "
may consist of threads fine enough to be woven into
gloves (Pinna), or of coarser filaments, as in the sea
mussel, or they may form firmer chitinous plates.
The function of these byssal threads, as may be well
seen in the Glochidia, or young of the fresh- water
mussel, is one of attachment.

Offensive organs of a somewhat similar character
are to be found in certain Holothurians. Con-
nected with and opening into the cloaca are a number
of tubes, compacted together into a more or less large
mass, and occupying in some cases a considerable
portion of the body cavity. The secretion of these
Cuvierian organs is expelled, on irritation, in the
form of fine tubes, which are capable of considerable
extension, and which also swell up in the water.
These expelled threads have a remarkable power of
adhering to any object which they may touch, and of
more or less completely entangling it and preventing
its escape. An English example of a Holothurian
thus provided is afforded by the so-called " Cotton-
spinner " (Holothuria nigra) ; the tubes are known to
have an irritating effect on the human skin.

True electric organs are developed in Torpedo
and other rays, in the eel (Gymnotus), and in the
teleostean Malapterurus. They are either placed in
the head (Torpedo), or in the tail (Gymnotus), or over

Chap. VllLlELECTRICAA'DPlfOSPORESCENTORGANS. 269

the whole surface of the body (Malapterurus). They
are very richly supplied with nerves, and appear to be
modifications of muscular tissue, which they so far
resemble in physiological activity that they are under
the control of the fish ; are exhausted after a certain
period of activity ; and are brought into a tetanic
condition in which a number of discharges succeed
one another involuntarily, when their possessor is
treated with strychnine. In the Torpedo the organs
are made up of a number of hexagonal bodies, each of
which is divided into a number of cells by intervening
septa, between which is a clear gelatinous fluid, or
mucous tissue ; the Torpedo has about a thousand
electric prisms, and Gymnotus is said to have two
hundred and forty electric cells in one inch of its
electric organ. Though the effect of these bodies has
no doubt been exaggerated by travellers, it is clear
that they are capable of producing sufficiently remark-
able results.

Curiously allied in the details of their structure to
the organs just mentioned are the so-called eye-like
spots found in various fishes (Argyropelecus, etc.),
and best developed, apparently, in deep-dwelling
forms. The special activity of these organs does
not, however, exhibit itself in the production of
electricity, but of light ; they are phosphorescent
organs. Kolliker, more than a quarter of a century
ago, suggested that the luminous organs of insects,
such as the Lampyridse and Elateridae, were allied to
the electric organs of fishes. So far, however, as we
know anything as to the mode of activity of these
bodies, which are richly supplied with tracheae, and
appear to vary in brightness with the movements of
expiration and inspiration, we are led to suppose that
the oxygen taken in from the air is a factor of con-
siderable importance.

Phosphorescence is exhibited by such simple

270 COMPARATIVE ANATOMY AND PHYSIOLOGY.

forms as Noctiluca among the Protozoa, by many
Medusae, by the Pennatulidae, by Beroe and Cestus ;
among the Annulata, it has been observed in the
earthworm, where it appears to have its seat in the
clitellum, and in various marine Polychseta ; in
Polynoe the light is of a greenish colour ; in Poly-
cirrus pale-blue ; among the Tunicata, Doliolum has
been observed to be phosphorescent ; and the com-
pound ascidian. Pyrosoma is, as its name implies,
remarkably so. As these animals float in great
companies, they have been spoken of as a " shoal of
miniature pillars of fire gleaming out of the dark sea,
with an ever-waning, ever-brightening, soft bluish
light " (Huxley),

The physiology of phosphorescence is incompletely
known. Panceri observed in Pemiatula that the
activity was exhibited only by the eight longitudinal
bands of fatty substance placed on the outer wall of
the stomach ; and these bands are luminous after
removal from the body. They can be set in activity
by various stimuli, mechanical, chemical, and so on.
When exactly studied by electrical stimuli, there is
found to be a latent period of |ths of a second. The
fact that many deep-sea forms are coloured points to
the existence of light in great abysses of the ocean ;
this can only be due to phosphorescent animals, as we
cannot accept the supposition that sunlight can pene-
trate to any considerable depth.

The observations of Aubert and Dubois on Pyro-
phorus, one of the well-known phosphorescent beetles
(Elateridse), have revealed the remarkable fact that
the most persistent of the rays of light are the green,
and that, with increasing brightness, the last rays to
appear are those that are least refractive, whereas, as
a rule, they are the first to be seen. This light has
been observed to have an action on sensitised paper.

The colours of animals are due either to

Chap, viii.] PIGMENTS. 271

pignments, which are formed by protoplasmic cells,
or to the minute structure of the surfaces of parts
of their body, which .variously affect the rays of
which white light is composed ; or to these two
causes combined. Although, in most cases, the
pigment is superficial, it is not always so ; thus, the
11 colour " of a man's cheek is due to the haemo-
globin in the blood, as is shown by the yellow
colour of those affected with jaundice, in which
disease haemoglobin is converted into bile pigment ;
or the staining of the skin in syphilis, the poison of
which seems to be particularly destructive of the red
blood corpuscles. Similarly, the red eye of an albino
is due to the absence of pigment in the iris and the
retina, in consequence of which the red blood is
seen through the transparent tissues of the eye ; when
the retina is pigmented, but the iris free of pigment,
the red colour of the blood is, by interference, given
a blue shade, and the eye is said to be blue. When
pigment is laid down also in the iris, the red colour is
more or less completely obscured, and we get light-
brown or dark-brown eyes.

No distinctive white pigment has yet been de-
tected, and the whiteness of certain animals must be
explained as caused by the presence of air-cells or
spaces in which none of the impinging light is ab-
sorbed. Many of the colours seen in animals are due
to the admixture of different pigments ; red overlaid
by yellow giving, for example, orange, or, when thinly
spread out, pink of various shades, proportionate to
the amount of colouring matter present.

Many colouring matters are soluble in alcohol, and
not a few are fluorescent ; in some cases they have
been observed to present absorption bands when
examined with the spectroscope, and these bands are
definite and characteristic of the pigment. Among
the Protozoa a blue colouring matter has been observed

272 COMPARATIVE ANATOMY AND PHYSIOLOGY.

in Stentor (stentoriii) ; in some corals and hydroids
there is a red pigment (polyperythrin) ; ctiloro-

cniorin has been obtained from various Poly cb seta ;
pent acriai hi from Pentacrimis ; and antedoniii

from Antedon and a Holothurian ; the terms cms-
taceorubrin, aplysiopurpurin, ianthiiiin,
and bonellein explain themselves. Zooxanthin,
zooerythrin, zoofulvin, and turacin have been
extracted from the feathers of various birds.

The question whether the characteristic colouring
matter of plants (chlorophyll) is formed by animals
is complicated by the undoubted fact that a number
of lower organisms have associated with them green
algae, which are not so much parasitic as symbiotic,
inasmuch as the oxygen which they evolve in the
presence of sunlight is of advantage to the anima1
with which they live ; such are the so-called yellow
cells of Anthozoa and Radiolaria. Where no cell-
nucleus is seen to be associated with the green cor-
puscles, as is the case in Spongilla and Hydra, w«
have no reason for refusing to suppose that the
chlorophyll has been formed by the animal itself.

Some animals possess the power of changing more
or less rapidly in colour ; as, for example, the cuttle-
fish or the chameleon. This property is due to the
presence of chromatophores, or aggregations of
pigment surrounded by an envelope ; the latter is
provided with radiating muscles, by the contraction
or expansion of which the chromatophore becomes
flattened out, and the contained pigment displayed or
drawn into a denser mass, so as to appear merely as a
dark spot. In the chameleon, where the play of
colour is not so rapidly effected as in the cephalopod,
there are no radial fibres. Similar structures are found
less well developed in other lizards, and in some fishes*.

The effects of structure are best shown by what are
ordinarily known as metallic colours. These are

chap, viii.] METALLIC COLOURS. 273

well seen in the wings of various insects, the scales of
which are marked by extremely delicate strise, or covered
by a thin membrane. The rays of white light suffering
interference are broken up into their constituent parts,
and different colours are produced in different positions.
Similar phenomena are to be observed in the shells of
Lamellibranchs. The causes of the metallic colours of
birds has been carefully investigated by Gadow, who
points out that if we look at a feather in a direction
nearly parallel to its plane, having one eye between it
and the light, it appears black, as it does also when
placed between the eye and the light. If we keep the
feather steady, and move the eye from one to the
other of the just mentioned positions, we notice the
gradual appearance of all the metallic colours that the
feather is able to display. It is important to observe
that these colours do not appear at random, but that
the first to be seen are those that are nearest the red
end of the spectrum, and the last those that are nearest
the violet. No metallic feather ever exhibits a brown
or grey appearance, or, in other words, any colour
that is not spectral. These facts lead to the belief
that the changeable metallic colours are due to a
structure comparable to that of a prism ; this structure
is formed by a transparent sheath of remarkable
thinness (0-00085 mm. in Sturnus, 0-0022 mm. 'in
Galbula), which is either perfectly smooth and
polished, or has fine longitudinal ridges or numerous
minute dots on its surface. Below the sheath there
is a brown or dark pigment. As a very small part
of the orbit of a curve may be treated as a straight
line, the sheath may be regarded as consisting of a
number of small prisms ; the reason why colour is not
seen when the eye is between the object and the light
is that such a prism only produces colour on the side
farthest from the light, and therefore refracts no light
in the direction of the observer,
s— 16

274

CHAPTER IX.

PROTECTING AND SUPPORTING STRUCTURES.

WITH the exception of such simple forms as Prot-
amceba, even the lowest Protozoa exhibit some kind
of difference between the outer parts of the cell that
have to bear with the jars and dangers of external
agencies, and the inner parts that are protected from
them. In Amoeba itself we can recognise (Fig. 1)
a difference between the outer ectosarc and inner en-
dosarc. The former, from our present point of
view, may be merely said to be firmer ; but we have
to note that the ectosarc of such Amoebae as live in
moist earth is much firmer than in those which live in
water. But the group of which the Amo3ba is the
simplest representative is not without parts which
clearly form supports for the protoplasm of the cell :
these are firmer structures, which may be called
skeletons.

In the present chapter, then, we shall chiefly denl
with skeletal structures, whether internal or external,
but we shall have also to speak of other offensive
and defensive organs.

The ectosarc of the Amoeba leads to the definite
cuticle of the Infusorian; but the Sarcodina are nob
without external defensive structures. The simplest
condition may be found in such a form as Gromia
(Fig. Ill), where the ectosarc forms an organic layer of
a substance like chitin, which, while it envelops the
general body mass, may be itself flowed over by the
contained protoplasm. In such a test as this there is
a single large orifice at one end. This chitinous test
varies considerably in thickness and consistency in

Chap, ix.] SKELETONS OF RHIZOPODA.

275

various Rhizopods, and may take on the most different
forms, and even become pigmented. In marine
Rliizopods the test becomes much firmer, owing to the
deposition in its substance of calcareous salts ; and as.

Fig. 111.— Gromia tenicola, showing the Protoplasm extending round
the chitinous test. (After Leidy.)

the test becomes traversed by pore canals, through
which there pass processes of the protoplasmic body
that has formed it, we get a structure which more
easily falls in with our idea of a skeleton. This
skeleton may be rounded and simple, or else it may
give off fine projecting processes, or it may, as in
Orbitolites and other Foramiiiifera, become

276 COMPARATIVE ANATOMY AND PHYSIOLOGY.

very elaborately coiled, and attain, as in the fossil
JVummulites, to a considerable size (more than four
inches in diameter ; or, as in some recent species, to
a diameter of two inches). Other Rhizopods build up
their skeleton, as do many sponges, from the silica
dissolved in sea-water, and others, like a number of
sedentary worms, take up sand and other foreign
products, and weld them into a consistent skeleton.

In the Heliozoa there may be a more or less
gelatinous investment, which, as in the Rhizopoda,
may appear at times only, or be permanent ; or there
may be a definite skeleton, which is in no case calca-
reous. It is most often formed of silex, and its parts
are often disconnected. In rare cases a shell is formed
of sand only, or of sand and the tests of diatoms.

The Racliolaria are remarkable for the pos-
session of a so-called " central capsule," which is
membranous in structure, and is, like the test of
Gromia, perforated at one point only, where there is
a comparatively large space, or the membrane is per-
forated by several spaces, or a number of pore canals
(as in the test of the perforate Foraminifera). In
addition to this membranous central capsule, most,
though not all, Radiolaria have also a skeleton which
may or may not penetrate the central capsule. This
skeleton is made up of spicules, which either consist
of an organic substance, acanthin, as in the Acantho-
metridae, or of a siliceous compound. These spicules
are primarily arranged in a radiating fashion, and are
often connected by secondary spicules with one
another, the result being forms of the utmost delicacy,
and of great beauty (Fig. 112).

The great variety of skeletal structure which is
seen in the Sarcodina does not obtain in the Infusoria,
many of which are extremely active in movement. In
various divisions, however, we find that the cuticle be-
comes particularly hard, and the so-called lorica (Fig,

Chap. IX.]

XlPHACANTHA.

277

113) thus formed may be variously ornamented; the
stalk, well known in Vorticella, is not contractile in
Epistylis and others, The lorica may be produced

Fig. 112, -XivhacantJia murrayana. (After Wyville Thomson.)

into tooth-like or tail-like processes ; a shield-shaped
test, or a bivalved carapace may be developed, or the
body may become surrounded by a gelatinous capsule.
Karely, as in the Dictyocyrtidse, the investing test
becomes impregnated with siliceous bodies.

278 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Even naked Protozoa may become covered with a
firm cyst formed by the ectosarc, at such times, as from
choice or necessity they pass into a quiescent con-
dition. This power of eneystatioii is found also in
the lowest members of the vegetable kingdom, and is
a means of protection for the protoplasm at the time
that it is undergoing the important changes that pre-
cede the rejuvenescence of the indi-
vidual, or the production of progeny.

It is impossible to pass from the
Protozoa without reminding the stu-
dent of how large and important a part
they have played and are playing in
the formation of the earth's crust. The
aphorism of Linnaeus, " Petrefacta
montium calcariorum non filii sed
parentes sunt, cum omnis calx oriatur
Fig. us.— Tintin- ab animalibus," is supported by our

nus lagenula, . , -,-,-, -,

showing the recently acquired knowledge that
and the Or ovm Diatoms and Globigerinae live on th,
of Cilia. surface of the sea, and that their case.s

and tests sink to the bottom when
their inhabitants and makers die. Some rocks,
such as chalk-cliffs, are full of the tests of Grlobigerinse,
and the " Nummulitic Limestone " of Nummulites.
Casts of Foraminifers have been found in greensand ;
a silicate of iron and alumina has been found filling
casts of recent Foramlnifera, so that as a matter of fact
we at this present period find " greensand replacing
and representing the primitively calcareo-siliceous
ooze ; " and, lastly, the researches of the Challenger
show that at a depth greater than 2,500 fathoms a
substance known as " red-clay " takes the place of the
Globigerina ooze.

In all but the lowest Sponges (Myxospongiae)
skeletal structures have been observed, and these, as in
the Protozoa, are of an organic nature simply (fibrous

Chap. IX.j

SPICULES OF SPONGES.

279

sponges), or the organic substance becomes impregnated
with calcareous salts (calcareous sponges), or with
siliceous (siliceous sponges). Considerable variations
are, moreover, to be seen in the extent to which this
impregnation takes place, so that while the fresh- water
sponge (Spongilla) has but few and simple siliceous
spicules, the Lithistidse are quite hard and strong.
In most cases the
inorganic skeleton is
spicular, and not
continuous ; but in
some, as " Venus's
Flower Basket :'
(Euplejtella), a deli-
cate framework of
siliceous particles is
left after all the or-
ganic material has
been removed (Fig.
114).

The spicules vary

• i i_i • £ Fig. 114.— Section through the Wall of

considerably in form, Euplecteila ( x 75).

being uniaxial Or p, Pores; -f, flagellated chambers. (After

needle-shaped, tri-
axial (this is the cha-
racteristic form in the Calcispongiae), or quadriaxial ;
connected with these are bi, tri, quadri, and sex-
radiate spicules, which may by the loss of some, and
the greater development of other rays, take on the
most different shapes. Some spicules are multi-radiate,
and others curved. Some project beyond the body of
the sponge, as in the glass-rope sponge (Hyalonema ;
Fig. 115), where anchoring spicules as much as
eighteen inches long have been observed. In addi-
tion to these proper skeletal spicules, others which
are smaller take an important part in giving firmness
to the sponge body, and even, as in the case of tho

280 COMPARATIVE ANATOMY AND PHYSIOLOGY.

" gemmules " of the fresh- water -sponge forming the
" amphidiscs," which strengthen its protective coat
during the period of quiescence. Sponges free from
calcareous or siliceous spicules, and with
only a fibrous skeleton, have, in the
present period, some commercial value,
in consequence of their well-known use
to man.

From the share that they have had
in forming parts of the earth's crust,
there is no division of the animal king-
dom in which, from such a point of view,
skeletal structures are of more impor-
tance than in the Ccelenterata, and both
Hydrozoa and Anthozoa contain groups,
members of which form the hard struc-
tures which we call coral ; this con-
sists essentially of deposits of carbonate
of lime in the organic substance of the
body.

The division of the Anthozoa con-
tain the larger number of coral-forming
animals, and may therefore be first dealt
with. In the simplest forms, such as the
common sea - anemone, there are no
spicules at all. but the body wall is ren-
dered more or less consistent by the
development of fibrils of connective
tissue in the mesoderm ; this may be
called the supporting lamella, and, as we
it is thinner in the tentacles than in
the rest of the body, where it may become thrown
into folds ; from the body wall bands or septa, in
the midst of which is a more or less thin support-
ing lamella, project inwards, and some of them reach
the wall of the gastric cavity which lies in the central
axis of the polyp. (See Fig. 54.) In rare cases the

Fig. 115.— Hya*
lonema sie-
boldi. (After
Schultze.)

may suppose,

Chap. IX.]

SPICULES OF ANTHOZOA.

28!

non-spiculate Anthozoa take up foreign bodies into, and
thereby strengthen, their ectoderm.

Thenext stage isseen in Alcyomum, where definitely
formed but scattered spicules are found in the layers
•of connective tissue. Where the skeleton becomes
continuous it may be horny, and where, as in Gorgonia ,
a number of polyps are connected together, the skele-
ton of the common trunk forms a horny axis ; in the
mesodermal tissues of the polyp spicules with an

Fig. 116. —A, Triaxial spicule of Calcisponge (Ascetta blanca) ; B, Simple
Acerate Spicule of Reniera ; c, Six-rayed Spicule of the Hexactinel-
lidse.

organic basis are developed, which, on the death of the
animal, merely form a crust on the axis. In Isis the
axis is calcined at certain points only ; so that it is
alternately horny and calcareous. In the red coral the
whole of the axis is calcined (Fig. 117). In other
cases, as in the organ-pipe coral, the hard deposits are
laid down in the wall only of the polyp (Fig. 118 A),
and these tubes become connected with their neigh-
bour by lateral outgrowths, and so form a continuous
hard mass. In others, as in the only " coral " found
on our own shores (Caryophyllia) the deposits in
spicules is not confined to the wall, but extends
also into the septa (Fig. 118 B) ; in others, as in

282 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Fig. 117.— Section of Axis of the Ked Coral, magnified.

Chap. IX.]

PARTS OF CORALS.

283

the common Fungia, the spicules are found in the
septa only, while the body wall remains soft.
Finally, the axis common to a colony of polyps may
become calcined as well as the body wall and the
septa (Fig. 118 c), and we then get large masses
of hard stony-like structures which persist long after
the polyps that formed them are dead and decayed
(brain-coral).

In describing the skeletons of corals use is made
of the following terms : the wall of
the cup-like calcification is called the
theca, and consists sometimes of
an exo- and endo- theca; where the
theca is thin it is aided by an in-
vesting epitliecsi ; the space be-
tween the calcined septa are the
loculi ; the septa may unite in the
centre to form a pseudo-colum-
ella, or may be inserted into an
axial hard part of distinct origin,
which is the true columella; the
ridges or outgrowths on this are the
pali, and the synapticulse are the
plates that project transversely and
connect one septum with another,
sometimes divided into chambers which rise one above
the other, like storeys, and the floor of each of these
is a tabula. The ridges on the exotheca are known
as costse.

The hard connecting sclerenchyma may be compact,
as in the stony corals, or traversed by canals, as in the
red coral (Fig. 117).

Among the Hydrozoa continuous coralla are
found only in the Hydrocorallinre, where they are
formed by the ectoderm which covers the canals that
traverse and branch in the " ccenosarc," that is
common to the compound stock of polyps ; the

Fig. 118 A. — Coral.
Two tubes of
Tubipora musica,
•with their con-
tained polyps.

The loculi are

284 COMPARATIVE ANATOMY AND PHYSIOLOGY.

gastric cavities of the separate polyps communicate
with these canals. The coralla, though porous, are
very hard and stony, and the canals are separated into
storeys by tabula, and the upper chambers are alone
living. The Millepores are common on coral reefs, the

Fig. 118 B.— Coral. Cari,ophyUia cyathus.

Styl asters live in water from ten to seven hundred
fathoms in depth.

In all the other Hydrozoa the supporting tissue is
simply a supporting lamella mesodermal in position,
as in the common Hydra, or there is an outer chitinous
perisarc, as in many Hydroid polyps, which persists
after the death of the animal as the so-called coralline ;
or, as in the Medusae, the tissue lying between the
ectoderm and endoderm becomes gelatinous (Fig. 119),
or cartilaginous.

Spicular skeletons are not found among Vermes,
where, when a protecting tube is developed, it is often

Chap, ix.] DENDROPHYLLIA. 285

largely composed of foreign material ; nor is there any-
complete internal skeleton. The cuticle may be soft

.Fig. 118 c. — Coral. Dendrophyllia ramosi.

and ciliated, as in the Tiirbellaria, or become very
firm and appear to be formed of a chitiiious substance,
as in the Nematoidea, or still more in the Rota-
toria, where it maybe join ted and have muscles inserted

286 COMPARATIVE ANATOMY AND PHYSIOLOGY.

into it. In the Animlata it often becomes of con-
siderable thickness and is then traversed by pore-canals.
In the sedentary marine Annelids a tube is developed
as a means of protection ; the inner portion, which is

partly membran-
ous and partly
fibrillated, is
formed by special
glands in the
body wall ; out-
side this the tube
is often rendered
moreresistent by
the deposition of
calcareous
matter (as in
Serpula), or of
aggregations of
sand, mud, and
other foreign ma-
terial (as in Sa-
bella, or Amphi-
trite), which are
taken up by the
tentacles of the
worm, and laid
down on the tube
by the animal it-
self. Within this
tube the inhabitant may be retracted, and some (as
Sabella) form an operciilum by means of which the
entrance to it may be closed.

In the Sabellidse special cartilaginous supports are
developed within the gill tentacles ; this is not found
in the Serpulidse.

The cells of the integument often give rise to hard
projecting structures, which may have the form of

Fig. 119.— Gelatinous Tissue from the Disc of
Aurelia aurita; a, fibres: b, cells. (After
M. Sclxultze.)

Chap. IX.]

HOOKS OF CESTODA.

287

hooks, or of bristles. The former are well developed
in some of the Cestoda, as in Taenia solium (the

Pig. 120. — tierpula vermicular™, showing tlie coiled Tube, and the
Animal protruded.

tapeworm), where the head is surrounded by a
circlet of recurved chitinous hooks, by means of which
the animal fixes itself to the mucous membrane of the
intestine of its host ; the presence of these is the cause
of the difficulty found by the practitioner in attempting
to expel the parasites from the human intestine.

288 COMPARATIVE ANATOMY AND PHYSIOLOGY,

Tapeworms with hooked heads are found in carnivorous
mammals and in birds, where the cavity of the in-
testine is comparatively limited, but they have not
yet been seen in such Cestoda as live in herbivorous
mammals, where the intestinal tract is much more
spacious. The group term Acanthocephali, and the
generic name Echinorhynchus, refer to the presence of a
number of hooks on the " proboscis " of other parasites.

In the higher Nemertinea stylets are developed
at the base of the proboscis, and it is particularly
interesting to observe that, where not present, their
place is taken by stinging cells ; a similar correlation
is found among the Turfoellaria, where the absence of
nematocysts is often atoned for by the presence of
small, rod-like structures, the so-called rtiafodites.
In the Chsetopoda some of the gland-cells of the
integument secrete hard chitinous bristles or setae of
various lengths, which are protective and locomotor
organs ; in the Oligochaeta (e.g. Lumbricus, the
earthworm) these setse are few in number, and never
exceed, so far as is known, eight in all ; in the marine
Polychseta they may be more numerous and much
larger than in the earthworm ; they may be variously
denticulated or hooked at their free ends, and may,
in the tube dwellers, aid the animal in raising itself
up its tule.

The Polyzoa are provided with an organ of pro-
tection, which is in all cases external or of tegumentary
origin ; it may be soft and gelatinous, or harder and
chitinous, or calcareous. It has been, somewhat
unfortunately, called a cell ; it invests only the hinder
part of the body, but it may serve, in times of danger,
as a refuge for the more anterior portion, which can
be withdrawn into it.

All Echinoderms, with the exception of the
Holothuroidea, have a well-developed skeleton, and
such is found also in some Holothurians. It is formed

Chap. !£.] TEST OF ECHINOIDEA. 289

of an organic basis, which becomes impregnated with
calcareous salts, and, in thin sections, has a ver}
characteristic reticular appearance.

It is particularly well developed intheEchinoidea,
with the consideration of which it will be convenient
to commence. In recent forms the test ^corona)
is made up of ten pairs of rows of plates, five of which
are radial and five interradial in position; the
former are perforated at the outer edge to allow of the
passage of the ambulacral tubes or suckers ; in the
fossil Palcechinoidea the interambulacral plates were
not paired, but as many as five or six took the place
of the two which are now constantly developed in all
known living species. These plates of the corona,
which are covered by an epithelial lining and by the
extracoronal portion of the peripheral nervous
system, are ordinarily firmly attached to one another,
so that no part of the corona is movable ; in some,
however, such as Asthenosoma, the plates are mov-
able on one another, and the whole test is flexible.

The rows of pores may remain straight, as in Cidaris,
or three or more primary may unite to form larger
secondary plates, and the pores then become arranged
in arcs ; three pairs of pores go to form an arc in an
Echinus, and as many as twelve or thirteen in a
Heterocentrotus. The plates carry tubercles of vary-
ing sizes, and on these tubercles (Fig. 121; B) are
placed movable spines, which may be quite short, as
in Echinus, longer than the long axis of the body, as
in the piper (Dorocidaris), or very strong and massive,
as in Heterocentrotus. Sometimes, as in Diadem a,
these spines are not only protective organs in virtue of
their own strength and number, but are also capable
of inflicting painful burning wounds in a manner which
has not yet been satisfactorily explained. Sometimes,
as in Spatangus or Echinocardium, the spines become
very fine and silky. In most, though not in all cases
T— 16

290 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the larger primary are surrounded by smaller and
more delicate secondary spines. On the lower

(actinal) surface of the corona there is an orifice which
is the mouth, on the upper there is a special arrange-
ment of plates which form the so-called apical area.

Chap, ix.] APICAL AREA OF ECHINUS. 291

This is best studied in a regular form such as the
ordinary sea-urchin (Echinus), where it is found to
consist of two sets of five plates (Fig. 121 ; A), one of
which is radial and one interradial in position. The
former are spoken of as the ocular plates, and are
perforated by an ordinarily single orifice, through
which a tentacle protrudes. The interradial plates,
which are similarly perforated, and which are generally
larger than the radial, are called the genital plates,
from the fact that they have become secondarily
modified to serve as the means of exit of the generative
products ; for the comprehension of the morphology of
this apical part of the test it is, however, best to
strictly limit our nomenclature to the terms radial and
interradial. One of these interradial plates, that
which lies to the right of the anterior ambulacrum or
radial plate, as seen in Fig. 121, is specially modified
to serve as the entrance to the madreporic or stone-
canal. It is distinguished by the name of madrepo-
rite, and is characterised by its larger size, and its
perforation by a number of minute orifices.

Within the circlet of these radial and interradial
plates of the apical area, there is a space which is
ordinarily covered by a number of small irregularly-
shaped calcareous plates, in or near the middle of
which there is an orifice, the anal opening of the
digestive tract ; sometimes, as in Echinocidaris, the
number of anal plates is much smaller, and in the
genus just mentioned there are very often not more
than four ; in Diadema the anal area is very nearly
completely membranous, and the anal orifice is placed
at the end of a projecting tube. Among the irregular
echmids the anus leaves its apical position, and opens
either some way posteriorly on the upper surface, as
in Rhyncopygus, or at the margin of the test, as in
Echinolampas, or on the lower surface and quite close
to the mouth as in Echiiioneus.

292 COMPARATIVE ANATOMY AND PHYSIOLOGY.

The region of the anus in the regular echinoids is
primitively occupied by a single large plate, the
dorso-central, and, in the study of the morphology of
the apical area of Echinoderms, it is necessary to always
bear in mind (L) the dorso-central, (2) the radial, and
(3) the basal plates, which are interradial in position.

Within the test of the Echinus are five calcareous
arches (auriculae) which afford attachment to the
"Lantern of Aristotle"; these auricles are, when
present, radial in position in all Echinoids, except the
Cidaricla, where they are interradial.

In the adult Crinoid we distinguish a cup, or
calyx, which may, as in Pentacrinus, be permanently
fixed by a stalk, or, as in Antedon, be stalked in the
larval stages only. In both cases the calyx gives off
a number of arms, which consist of numerous small
calcareous joints, and have jointed appendages, the
pinnules, attached to them. However numerous
these arms may be, and there may be almost ono
hundred, we find that, as we trace them back to the
calyx, they form branches of one or other of its five
rays. These rays ordinarily consist (the common
Antedon of our own seas is an excellent example) of
three radial joints ; all these joints unite to a common
central piece, which is known as the centro-dorsal,
In the stalked forms this centro-dorsal is placed at the
top of the stalk, which consists of a large number of
small ossicles, and is at various points along its length
provided with jointed five-rayed outgrowths (cirri),
which have a claw at their free end. In Antedon and
others, where the stalk is lost in adult life, the cirri,
which vary a good deal in number, and are sometimes,
after a certain age, completely lost, are directly attached
to the centro-dorsal itself.

We apply the term centro-dorsal to the central
plate of the crinoidal calyx to distinguish it from the
dorso-central of the typical unaltered apical area of

Chap. IX. ]

CALYX OF CRINOIDS.

293

Echinoderms ; the plate is, in the Crinoids, modified
by the large share in its formation that is taken by the
topmost plate of the stalk.
In the development of An-
tedon, the interradially-
placed tmsals become ob-
scured, but in other forms
among the Crinoids they are
evident throughout life.
Careful investigation into the
structure of the skeleton of
Asteroids and Ophiuroids

Fig. 122.— Pentacrinus Wyville-Thomsoni. (Natural size.)

reveals the presence of the plates to which our atten-
tion has just been directed.

The following are the more important points in
the structure of the skeleton of the Starfish; the
arms, or rays, are made up of a number of ossicles

294 COMPARATIVE ANATOMY AND PHYSIOLOGY.

set in regular paired rows, at a more or less acute
angle to one another ; these ossicles are not perforated
like the ambulacral plates of an echinid, but the

Ir

T V-v \7 I

$M*&

Vr

^^x^=^v ^\ \

Fig. 123 A. — Cross Section of an Arm of a Starfish (Asterias rubcns).
On the left side the section is supposed to pass between two of the ambulacral
ossicles, but on the right side through one of them (ao) ; ap, ambulacral
groove; n, radial nerve; 6, radial blood-vessel; u\ radial water-vessel; a,
ampullae; t, tentacles or suckers ; ap, adambnlacral plates; s.p, spines ; pax,
paxillse, arising from limestone plates ; ov, ovary ; ftp, genital pore ; gv,
genital blood-vessel ; 6r, respiratory processes ; pc, cseca of the intestine.
(After P. H. Carpenter.)

tube-feet pass out between them ; attached to each
ambulacral ossicle is a smallei ad -ambulacral ossicle
(Fig. 123 A; ap\ which completes the side of the
groove and carries spines ; the rest of the wall
of the arm is strengthened by irregular plates,
which may be so formed as to leave considerable
interspaces, as in the starfish, or they may be larger
and more closely packed, and have only minute pores
between them, as in Linckia ; sometimes, as in
Oreaster, the plates that form the margin of the
arm form two regular rows of much stronger supero-
aiid infero-margiiial plates. All, some, or none

chap, ix.] SKELETON OF ECHINODERMS.

295

d.o

of these ossicles may bear spines of varying size and
strength ; where they are best developed we rarely
find that any of the arms have suffered injury, and
they are, no doubt, of very considerable importance
as protective organs. The disc is formed chiefly of
irregularly arranged intermediate plates, but the
radials and basals are, in some, cases, to be clearly
detected in young specimens ; near or at the centre of
the disc there is an anal perforation which is rarely
wanting ( Astropecten). On the
lower surface the large central
mouth is to a slight extent
aided in its work by the modi-
fication of the most central
ambulacral ossicles, which pro-
ject inwards at the angles of
the mouth.

In the Ophiuroidea a
cross section of the arms (Fig.
123 B) shows that the ambu-
lacral ossicles (ao) are covered
in on all sides, so that no
groove is apparent; pores in
the lower plate allow of the
passage of the tube feet ; the side plates («) ordinarily
bear spines (t), which are never of great length or much
size, and can be of little use as organs of defence ;
above, a single plate (u) roofs in the ambulacral ossicles,
but this is rudimentary in Neoplax, and absent in
Ophioscolex. The plates in the disc are propor-
tionately larger than in starfishes, are ordinarily set
in a close mosaic, and not unfrequently exhibit the
essential parts of the typical calyx, the dorsocentral
even being often apparent, owing to the fact that
there has been no resorption. of calcareous tissue to
make room for an anal orifice. The plates around the
mouth are so arranged as to give rise to five radially

( After p. H. Carpenter.)

296 COMPARATIVE ANATOMY AND PHYSIOLOGY.

arranged slits ; the edges of these slits often bear
small spines, while the oral faces of the ossicles carry
similar spines, the so-called oral papillae ; this
armature of fine spines serves no doubt as a filtering
apparatus to the digestive cavity of these aproctous
Echinoderms. We may correlate the injuries which
the arms are often seen to undergo with the absence
of defensive spines ; the Ophiuroid leaves an arm with
the foe from which it is unable to defend itself ; and
we may compare with this the arrangements of the
tail vertebrae of the harmless lizard. (See page 322.)
Arms thus broken are in time renewed. While in
the Ophiurida the arms are nearly always un-
divided, however long they may be, the Astrophytida
exhibit various stages of division ending in the great
complexity of the free termination of the arms which
obtain in the basket-fish, or gorgon's head (Astro-
phyton, Gorgonocephalus) ; in the Astrophytida the
spines are reduced to a minimum, and the integument
is thick and leathery.

Though many Holothiirians have a very thick
skin, and a deposit only of spicules in their integu-
ment, we cannot suppose that this is a retention of
the primitive condition, spoken to by the fact that in
all Echinoderms the skeleton commences in the form
of spicules, which gradually unite mere or less with
one another, so much as one that lias been secondarily
acquired. In some cases (Psolus) the calcareous
plates are quite large, firm, and connected, and, on
the other hand, the spicules sometimes disappear
completely from old and large, even where they are
present in younger and smaller, examples of some
species of Cucumaria. In Synapta the spicules take
on the form of anchors ; in Chirodota, of toothed
wheels. They are often turn-form in shape, and the
surface of the body is sometimes quite rough to tfie
touch, owing to the large numbers which are present

Chap, ix] PEDICELLARI&. 297

in and project from the integument. In these
Echinoderms, where the defensive powers of the
skeleton are slight or lost altogether, we again observe
that the creature is prone to acts of self -mutilation,
not unfrequeiitly ejecting, when attacked, the whole
of its viscera ; these are in time repaired, if the
animal is left to recover.

In the Echinoidea and Asteroidea a number
of the spines are not unfrequeiitly converted into
stalked or sessile snapping-like organs, the pedi-
rcllai'isr, as they were called by those who be-
lieved them to be independent and parasitic animals ;
the sessile pediceilarise are bivalve ; the stalked
have three or four valves ; they are supported
by the calcareous reticular tissue which is so
characteristic of the hard parts of Echinoderms,
and are moved by muscles. Their chief function
appears to be that of holding on to objects that come
into contact with them, or to such supports for the
progression of the animal as waving fronds of sea-
weed, until the suckers are able to be brought into
relation with the object; it has been observed that
their prehensile power only lasts for about two
minutes. In some cases it is probable that some of
the pedicellarisc are used for the purpose of cleansing
the neighbouring spines of foreign or faecal material ;
but, if we are to judge from the great differences
which obtain in their number, and their complete,
or almost complete absence from some species, the
close allies of which have a large number, we are
led to believe that their function is not important,
and that they have an inverse ratio of development
to the size and number of the spines proper (Fig.
121 ; c, D).

The Arthropoda are as definitely characterised
by the development of a chitinous, as are the Echino-
dermata by that of a calcareous skeleton; this is

298 COMPARATIVE ANATOMY AND PHYSIOLOGY.

likewise, in large part, external, so that while the
endoskeleton of Vertebrates is characterised by having
the muscles external to it, the exoskeletal parts of
an Arthropod are moved by muscles that lie inter-
nally to them. The chitinous skeleton may be thin
and soft, as in the simpler E lit 01110 straca, or
the parts of the different somites may fuse, to
form, as in the Crayfish, a firmer cephalothoracic
carapace ; or, as in the Ostracoda and other Ento-
mostraca, give rise to two more or less dense lateral
valves ; in other cases certain parts may become
very strong and thick, as is the case with the elytra,
or wing-covers, of the Coleoptera (Beetles). The
chitinisation of the epithelial layer is not confined to
the surface, and, just as in Echinoderms, spicules or
plates may be found in the walls of the digestive
tract, or in the generative glands ; so, too, chitin may
invade the stomodseal and proctodseal portion of the
alimentary tract of Arthropods, or give rise to a
definite series of internal supporting pieces, the
endosternites.

These chitinous layers are not formed of cells, but,
like the cuticle of a protozoon, are shed out by cells,
which they invest with a continuous layer ; the layer
is often seen to be laminated, or made up of a number
of superimposed secondary layers, laid down in suc-
cession. They are traversed by vertically-running pore-
canals, and are often strengthened, especially in the
Crustacea, by the deposition of calcareous salts.* A
firm outer coating of this kind, moulded to the form
of the body, would speedily limit the growth of an
Arthropod, were it not for the process of shedding, or
exuviation, which obtains during growth, and much
more frequently in young and rapidly-growing forms
than in those which have attained to their full size.

* In the Crayfish more than half of the whole weight of the
exoskeleton is due to the presence of calcareous salts.

Chap, ix.] SKELETON OF ARTHROPODA. 299

In this exuviation, or ecdysis, the internal chitinous
and calcareous parts are as much affected as the
external ; when it is completed the integument of the
animal is for a few days soft and moist, but a new
exoskeleton is developed with comparative rapidity,
while its function as a protective organ is spoken to
by the temporary timidity of animals, which, when
armed, are bold, and ready to resent attack.

The integument is smooth in the lower forms
only ; in Peripatus, as in the larvse of insects,
it is soft, and in the latter it is sometimes very thin.
It is generally stouter in Myriopods and Arach-
iiida, and its degree of stoutness varies considerably
in Insects. Among the Crustacea it is compara-
tively soft in Eiitomostraca, except where valves
are developed ; these attain to their greatest hardness
in the Cirripedia, which are fixed in the adult
stages of their lives, and are therefore unable to
escape from enemies ; they are stronger and more
compact in the sessile Balanus than in the stalked
Lepas ; some Cirripeds have, however, lost their hard
valves. In the Malacostracous Crustacea, where the
carapace has more definite relations than in such
Entomostraca as Apus, knobs, ridges, or immovable
spinous processes, all of which are defensive in
function, are very commonly developed. Among
the Arachnida, Limulus is remarkable for the long
caudal spine-like termination (Fig. 124) of its shield.

Internal hard pieces, which appear to have for their
chief function that of protecting the central nervous
system, whereby they may be compared to the verte-
bral column of Vertebrates, are best developed in the
higher Crustacea. Though topographically and func-
tionally internal, these parts are morphologically ex-
ternal, and they share in the general moulting of the
tegumentary skeletal parts ; these ingrowths are
known as the apodemes. In the crayfish, for

300 COMPARATIVE ANATOMY AND PHYSIOLOGY.

example, four apodemes are well developed between
every two thoracic somites \ the inner pair unite above

and below so as
to form a closed
canal, the ster-
nal canal ; with
these, on either
side, one of the
outer apodemes
becomes con-
nected, and, as
it also becomes
connected with
the apodemes
behind it, the
several parts are
united into a
continuous and
substantial in-
ternal support-
ing and protec-
tive mass, which,
in addition to its
other functions,
affords attach-
ment to mus-
cles.

The skeleton
may gain in pro-
tective or defen-
sive power by
the development
of spines, pro-
Fig. 124.— The King-Crab (Limulus moluccanus). cesses, or knobs,

which resist the

attacks of enemies, or are able to passively inflict in-
jury upon them ; sometimes, indeed, they almost come

Chap. ix. j SKELETON OF CRAYFISH. 301

to be reckoned among active agents of offence, as when
they are developed on the two terminal joints of the
great chelae or forceps, the last of which is movable on
the last but one. The protective power of the hard
exoskeleton is, inversely, spoken to by the softness of
the hinder end of the body of the hermit-crab, which
lives in an empty snail-shell, and protrudes only the
anterior portion of its body.

The larvse of various insects have often protective
spines or warts ; those of Crustacea, in the Zoea-stage,
have more or less long, anterior, dorsal, and lateral
spines.

The several parts of which the skeleton is made
up may be conveniently studied in one of the ab-
dominal segments of a crayfish. We here see that
there is a continuous ring, the convex dorsal region
(tergum) of which is continued into two lateral
(pleura.!) regions, which hang down on either side ;
beneath is a flattened ventral region (sternum), to
which are articulated two jointed appendages; the
piece between the articulation of the appendage and
the pleuron of either side, is known as the epimerou.
The appendage, when completely developed, consists
of a two-jointed basal protopodite, with which are
articulated an inner and an outer branch,, which are
known respectively as endopodite and exopodite ;
with these a third piece, epipodite, is sometimes con-
nected. In the crayfish the appendages of the abdo-
men are either flattened to serve as swimmerets, or
modified to act as accessory reproductive organs (see
page 496) ; the last pair of appendages are greatly
flattened out, and, with the last segment (telson),
which is, in all known cases but that of Scyllarus,
without appendages, forms the tail-fin. The four
hindermost pairs of thoracic appendages are con-
verted into walking limbs by a considerable in-
crease in the size and strength of the endopodite,

302 COMPARATIVE ANATOMY AND PHYSIOLOGY.

which constantly consists of five joints, known as the
ischio-, mcso-, carpo, pro-, and dactylo-po-
dit.cs. The pair next in front form the great " for-
ceps " or chelae, the propodite of which is produced
and articulated upon the dactylopodite, so as to form
a most efficient seizing organ. The six pairs next in
front form the Onathites, the modifications of which
have been already described in connection with the
organs of digestion (page 123). The two most ante-
rior appendages form the antennae and the anten-
miles ; in the former the exopodite forms a flattened
squame, and the endopodite is many -jointed ; in the
antenmile both endopodite and exopodite consist of a
number of joints. By some authors the eye-stalks
are regarded as representing the protopodites of an
appendage.

The paired appendages of the Arthropoda take on
very various functions in different groups, and vary
considerably in number ; in the Phyllopoda, the
IVIalacostraca, and the Myriopoda, all the seg-
ments of the body bear appendages; in the Copepoda
and Arachnid a they are absent from the hinder
part of the body ; and in the insects (Hexapoda),
there are but three pairs of definitely constituted ap-
pendages behind the gnathites, and these are always
attached to the thorax.

The appendages may be very simple, and may be
nearly all similar in function, as in Peripatus,
where the incompletely jointed appendages, provided at
their free end with their two-hooked claw, are nearly
all ambulatory in function ; one pair alone forming
gnathites, one oral papilla?, and the last of all the anal
papillae. In the Myriopoda, all behind the gnathites
are ambulatory in function ; in the Branchiopoda they
form branchial swimmerets, and, as in other Entomos-
traca, there are never more than three pairs of
gnathites ; in the Copepoda and Ostracoda the second

Chap, ix.] APPENDAGES OF INSECTS. 303

pair of antennae retain the natatory function which
they have in the Nauplius stage. In the Cirripedia
the six pairs of appendages behind the gnathites have
the exopodite and endopodite consisting of a large
number of joints, and they form the filamentous cirri
which are so characteristic of these animals. In the
Arachnid a there are no appendages in front of the
head, antennas being absent. In Peripatus the an-
tennae do riot belong to the series of ventral appendages
of the segments of the body. In Myriopods and In-
sects there is a single pair of antennae. In the
Arachnida the basal parts of the circum-oral appen-
dages alone take part in the service of the mouth ;
the most anterior pair are pincer-shaped at their free
end (clidicerse) ; in some Myriopods one of the
anterior pairs of appendages become poison- claws, as
are the chelicerae in spiders. In the parasitic Peii-
tastomida all signs of appendages are reduced to two
pairs of curved hooks in the region of the mouth.

The Hexapoda have only three pairs of gnathites
(see page 128) ; and these, as has been already pointed
out, present the most diverse modifications in different
orders. The legs are almost always well developed on
the three segments of the thorax, and are typically
five-jointed ; the most proximal is known as the coxa-
and this is succeeded by the ordinarily smaller
troclia liter, by the longer femur, and the still longer
tibia ; the last .joint or tarsus consists of several
pieces, the most distal of which, or that farthest from
the axis of the body, carries a pair of claws.

The number of legs (six) must be a great me-
chanical advantage to an insect, for three supports are
necessary to maintain a stable equilibrium. They may
become adapted to very various modes of progression
through earth or air. In the digging forms, such as
Gryllotalpa (the mole-cricket), the tibiae of the first
pair of legs are flattened, triangular, and toothed,

304 COMPARATIVE ANATOMY AND PHYSIOLOGY.

and are supplied with well-developed muscles. In
the aquatic forms, such as the water-beetles (Dyticus),
the coxse of the third pair of legs are flattened and
oar-like. Such as float on the surface of the water
have the contained air tubes enlarged to serve as float
bladders, or the legs are greatly elongated so as to
extend over a large surface of water. In climbing
insects the claws may be cleft or pectinated so as to
enable them to hold on to small objects ; or an at-
taching lobule may be developed between the claws.

The tergal portions of several successive seg-
ments may unite with one another, and thus give
greater firmness to the dorsal surface ; this process
may result in the formation of a free-projecting shell,
as in Apus, or this shell may become divisible into
two valves, as in the Ostracoda, or the fusion may
extend far back, as in the crayfish, or the scorpion,
where we have the so-called cephalo thoracic cara-
pace ; in Limulus the sides of the carapace are pro-
duced, and we get the well-known large shield of
these animals ; the same phenomenon in the crayfish
or the lobster results in the formation of a special
wall for the branchial chamber.

The most remarkable modifications are exhibited
by the Cirripedia, where the exoskeleton is ordi-
narily in the form of calcified valves, two on either
side of the body, and in Lepas with a dorsal median
piece ; in Balanus these valves are withdrawn into
a special shell.

In the Mollusca the characteristic organ of sup-
port and defence is an external calcareous shell,
which is formed by the mantle, and which, when
aided by the operculum, which is developed on the
base of the foot, becomes so completely an organ of
protection that many snails hibernate in their closed
shell ; the tenant of an exotic shell has been
bought, sold, and exhibited in a museum for the space

Chap, ix.] STRUCTURE OF SHELLS. 305

of four years before giving any signs of vitality (Helix
desertorum). The shell is sometimes, however,
merely chitinous and internal, as in the slug or the
squid, or it is, in earlier stages, rudimentary, and in
adult life completely lost (Nudibraiichs). In this
phylum there is no arrangement comparable to the
exuviation which obtains among the Arthropoda. The
dependence of these calcareous shells on the nature of
their surroundings is admirably spoken to by such
facts as the absence of shelled forms from such dis-
tricts as the Lizard, or parts of Asia Minor, where
there is no lime (Forbes) ; while, on the other hand,
the influence of the animal itself, and of other condi-
tions, is expressed by the greater density of the shells
of the mussel of the m6untam streams of Westmore-
land, as compared with the thinner shells of the Isis,
which, at Oxford, is richer in salts of lime than the
just mentioned more northern waters (Rolleston).

Shells vary considerably in texture and struc-
ture ; many are, as is well known, pearly within ;
these nacreous shells owe their characteristic
appearance to the alternate deposition of layers of
thin membrane and of carbonate of lime, and to the
irregular deposition of the thin layers, which, by
slightly overlapping one another, diffract the light and
so give rise to the iridescence of the internal surface.
This explanation of the physics of the appearance of
the shell may be applied also to the " pearls,"
whether formed naturally by the deposition of thin
layers around an organic or inorganic nucleus, or
artificially by the introduction of foreign material.
Where the lustre is duller than in nacreous shells, the
hard structures are said to be porcellaiious; in
Pinna and others the shell is said to be fibrous, owing
to the fact that the separate parts of each layer
correspond to one another, and fracture, therefore,
results in a number of vertical pieces. The laminated
u— 16

306 COMPARATIVE ANATOMY AND PHYSIOLOGY.

arrangement, or disposition of the la}^ers of the shell
in horizontal planes, is well seen in the oyster. Where
the possessor of the shell is in the habit of floating
(lanthina, Argonauta), the " shell " is comparatively
thin, and of low specific gravity.

In the most primitive Mollusca the shell is merely
spicular ; in Neomenia the spicula are arranged in a
single layer in the outer part of the integument, and
protrude their pointed ends from its surface ; in
Proneomenia the spicules are set in several layers,
placed in a chitinous interspicular substance. In both
cases the spicules consist of carbonate of lime. A
higher stage is found in the Chitons, where the shell
consists of eight plates which lie on the back of the
animal, and have an arrangement which we can
hardly resist from regarding as metameric. In the
Lamellibrancliiata the shell consists of two
valves, which lie to the right and left of the animal,
and are connected with one another by a chitinous
ligament, which runs along the middle line of the back ;
this ligament is elastic, and in consequence of this pro-
perty, the animal in a state of repose is able to keep
its two valves slightly separated without incurring any
expense of muscular activity. At the approach of
danger the valves can be approximated so as to pro-
tect their possessor, by the contraction of two (fresh-
water mussel) or one (oyster) pair of adductor muscles.
In the higher Cepiialophora the shell is always
single, whence they are often distinguished by the name
of univalves from the Lamellibranchs, which are the
bivalves. This shell may form a single conical cup,
as in the limpet (Patella), or it may be slightly coiled,
as in the cowry, or it may be greatly coiled and
consist of a number of chambers, as in the nautilus.
It is certain that many conchologists go too far in the
trust that they put in the characters of the shell, but
it remains as a matter of fact that in the great

Chap. IX.]

STRUCTURE OF SHELLS.

307

majority of cases the exact systematic position of a
mollusc may be determined by the shell alone, so
marked are the
differences, and
so deep - seated
the essential cha-
racteristics. Geo-
logists believe
that there is no
evidence more
worthy of con-
fidence than that
which is afforded
them by the
shells of any
given deposit.

The shell,
which owes its
growth to the
activity of the
outer cell-layers
of the mantle,
commences as a
pit or invagina-
tion of the outer
layer on the abo-
ral surface of the
larva ; this pit is
the shell gland
of Lankester, and
it secretes a vis-
cid body which
hardens on con-
tact with water ; this hardened substance is the earliest
rudiment of the shell, and even in the bivalved forms
it is at first a single saddle-shaped plate, which only
later becomes divided into two bilateral halves.

Fig. 125.— Shell of Triton, to explain the terms
used in the descriptions of Shells.

The -shell is fusiform, in shape : its apex (A) is niam-
niillated ; it is made up of whorls (w), separated
by sutures (su) ; bw, body whorls There is an internal

millated ; it is made up of whorls (w), separated
by sutures (su) ; bw, body whorls There is an internal
axis or'columella (i), an outer Up (o), an aperture (a),
and an anterior (etc) and posterior (pc) canal. (After
Woodward.)

308 COMPARATIVE ANATOMY AND PHYSIOLOGY,

In many cases lines of growth can be made out,
and the shell may continue to increase in size for so
long a time that a single valve of a Tridacna will be
found to weigh a hundred and fifty-six pounds.

In Nautilus the shell consists of a number of
chambers (Fig. 126), each of which is larger than that
which precedes it, and is formed by the animal as it
increases in size ; each of these chambers is separated

Fig. 126.— A Section of tbe Shell of the Pearly Nautilus, showing the
successive Chambers occupied by the Animal.

a, Mantle; b, dorsal fold; g, muscle; ii, siphuncle; fc, funnel; n, hood;p, ten-
tacies ; «, eye; x, septa ; z, last chamber.

from one another by a septum (x), and the whole
mass is spirally coiled on itself. The chambers are
connected with one another by a tube or siphuncle

(ii), the presence of which has given rise to the belief
that the whole series forms a kind of float by means
of which the animal is enabled to remain at will on the
surface of the water ; such definite observations,
however, as have been made on living specimens,
and the fact that though (like the shells of Spirula)
the shells are common enough, but the animals very
rare, lead us rather to believe that Nautilus is essen-
tially a dweller at the bottom of the ocean. Here,

chap, ix.] FORMS OF SHELLS. 309

then, we have another example of the danger of
arguing d, priori as to function from structure.
Physiology, like other branches of science, must pro-
ceed rather by observation and ct, posteriori argu-
ments.

Nautilus is the only existing tetrabranchiate
Cephalopod, but to that division belonged a number
of extinct forms, whose shells are found fossil ; such
are the Ammonites, with shells like those of the
Nautilus, Gyroceras with discoidal, Trochoceras with
spiral, and Baculites with straight shells.

Among the Diforanchiata, Spirula (Fig. 127; A),
whose body is so rare and whose shell so common,
alone has the shell coiled and divided into chambers ;
it is not, however, an external shell, like that of the
nautilus, but is internal. In the fossil Belemmite
(Fig. 127; B) the proximal end of the shell (phrag-
mocone) was divided into separate chambers, which
were connected by a siphuncle. The distal end of the
shell is dart-shaped and solid, and forms the so-called
giiard. In Sepia the shell is calcareous, straight,
flattened out for the greater part of its length, with
the apex only incompletely chambered.

In the squids, such as Loligo (Fig. 127 ; c), the shell,
now ordinarily known as the pen, is merely horny,
and consists of a shaft with two wings; in the Octopus
the shell is lost ; its ally Argonauta (Fig. 1 27 ; D)
fashions for itself a shell which both morphologically
and physiologically is a different structure from those
we have hitherto been considering, for it is formed
by a pair of the arms and not by the mantle cells, and
is confined to the female, where it serves to carry the
fertilised ova. A structure, the origin of which is
unknown, but the function of which is likewise
incubatory, is that which is known as the float of the
Gastropod lanthina.

The operculum is formed by the foot, is horny,

3io COMPARATIVE ANATOMY AND PHYSIOLOGY.

and sometimes impregnated with calcareous salts ; it
is very generally distributed among the Gastropoda,

Fier. 127.— A, Section of Spirula australis, to show the internal shell ; B,
Belemnite restored ; c, Pen of the Loligo ; D, Argonaut a in Natural
Position. (From Woodward.)

and may completely fit the mouth of the shell, as in
the common snail, or close part only of it, as in
Strombus; or, as in Dolium and other large-mouthed

Chap, ix.] FORMS OF SHELLS. 311

shells, it may be rudimentary, or absent. The operculum
is not represented in the Lamellibranchs. In a few cases
(e.g. the Lamellibranchiate Aspergillum) the protective
function is not assumed by the shell, which may be
quite small, but by the deposit of calcareous matter
on the siphon- shaped prolongations of the mantle.
(See page 80.)

Among the Gastropoda the shell is often greatly
reduced, as in the slug, or completely lost, as in Doris
and other Nudibranehs, where the integument is,
however, richly supplied with calcareous spicules, and
in Oncidium, where the integument is thick and
leathery. One large division of Pteropods are
without any shell, and in the thecosomatous forms it
is always thin and glassy.

The internal skeleton of Gastropods consists
merely of one or two pairs of cartilaginous plates,
which are found in the region of the pharynx. It is
better developed in the Cephalopoda, where it is
represented by a median cephalic cartilage, which
is pierced in the middle line by the resophagus,
and is produced on either side into plate-like supports
for the eye and ear; in some cases the orbit is
completely surrounded by cartilaginous pieces. In
the Dibranchiata the muscles which move the fins,
when those organs are developed, are inserted into
special cartilages, and other cartilaginous pieces are
more irregularly developed at the base of the funnel,
on the dorsal surface, or in the neck.

The Brachiopoda have a hard external shell,
which, though it consists of two valves, is not to be
compared with that of the lamellibranchiate mollusc,
for their valves are not right and left, but dorsal and
ventral in position ; they may be subequal, as in
Lingula, or the ventral may be prolonged into the beak-
shaped free end, which has gained for these animals
their familiar name of " lamp-shell ; " in the latter

312 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the two valves are hinged on one another (Testi-
cardines or Articulata). The dorsal valve may
give off hard processes, which project into the cavity
between the two valves, and which has a spiral or
looped form (Fig. 128) ; they serve as a means of
support for the highly-developed " arms " of the

Fig. 128.— Dorsal Valves of various Brachiopods, seen from within, to
show the loop (1). A, Terebratula ; B, Terebratulina ; c, Terebratella ;
D, Bouchardia; E, Megerlia ; F, Argiopej h, Hinge; I, loop; s,
septum. (After Davidson.)

Brachiopoda. In some (e.g. Terebratula) the sub-
stance of the shell is traversed by tubular prolonga-
tions of the contained mantle. Scattered calcareous
spicules are to be found in the integument.

Among the Chordata, a well-developed skeleton
appears only in the Verteforata, where it attains to
considerable complexity ; the more characteristic is
internal, but an external skeleton is sometimes also
present.

All Vertebrate at some period of their lives, the

Chap. IX.] NOTOCHORD. 313

Ceplialocliordata throughout their life, and the
Urochordata either permanently, temporarily, or
never, have an internal organ of support in the form
of a rod, the so-called notoctiord or dorsal rod,
which lies just beneath the central nervous system ;
the substance of which this rod is formed appears to be
allied to cartilage ; in Amphioxus it becomes much
more complex in structure than in other Chordata, so
that we have here an example of how an organ of an
animal which remains at a certain grade in the scale
of development, becomes more complex and elaborate
under its conditions than does the same organ in a
" higher animal," where it serves only a temporary
purpose. In the Urochordata the notochord is only
found in the tail ; it either is persistent, or is aborted
when the free-swimming larva settles down to a fixed
mode of life, or it is never developed at all ; as in the
rest of the division, it has a stout continuous sheath,
and, as it is elastic, it brings the tail back into posi-
tion when the organ has been bent by the muscles
attached to it. In the Urochordata, therefore, the
notochord, when present, may be regarded as having
also a locomotor function.

Amphioxus has no external skeleton, nor have
those Urochords that are tailed throughout life j in
the rest, the " outer mantle," or test, may become
very strong and rigid, so as to form a complete
organ of protection; it is remarkable for containing
cellulose, a starchy compound which, so common in
vegetable organisms, is only known among animals
in the Tunicata and the protozoic Cilio-nagellata ;
scattered calcareous spicules are not unfrequently
deposited in the cells of the mantle, but never form
a continuous layer. The tailed forms, such as Ap-
pendicularia, are able to rapidly secrete an invest-
ment, the so-called " house ; " in this, however, they
do not dwell permanently, but are described as

314 COMPARATIVE ANATOMY AND PHYSIOLOGY.

leaving one after a few hours, and as forming a
fresh one, when they again settle down.

In the greater number of the Vertebrata the
notochord is very profoundly modified in character,
and in the higher forms it disappears altogether from
the adult, its place being taken by the jointed
vertebral column. In the lowest stages, such
as are found in the Cyelostomata, Chimrera
among the Elasrnobranchs, and the Dipnoi, the noto-
chord remains unconstricted, but cartilaginous or
calcareous deposits become aggregated around it,
while above, and sometimes also below it, there
appear arches of cartilage, which protect the over-
lying nerve-cord and the subjacent blood-vessel.

In more complete Vertebrae it is possible to
distinguish a basal portion, or centrum, which
grows round the notochord, from the overlying
neural and the underlying haemal arches, which
enter into more or less close union with it. In many
Fishes the notochord remains well developed between
the separate vertebral centra, and these are, in the
simplest cases, excavated both in front and behind,
whence they are known as amphicoelous ver-
tebrae ; in better developed forms a smaller amount
of notochord is persistent, and we then get either
procoelous or opisthocoelous vertebrae, accord-
ing as the excavation is on the anterior or the pos-
terior face of the centrum ; sometimes, as in the frog,
the invasion of the notochord by cartilage or bone is
never complete, and in such cases a cross-section
of the centrum of the vertebra reveals the presence
of a central notochord (Fig. 129; ch). In the frog,
as in the Amphibia generally, the neural arch
and the centrum become firmly connected with one
another, and from the centrum there are given off
horizontal pieces of bone, which form the so-called
transverse processes. The separate vertebrae

Chap. IX. J

VERTEBRA.

3T5

are articulated with one another by means of pro-
cesses directed forwards and backwards, the zyga-
pophyses, as these articulating outgrowths are called.

The shape of the faces of the centra of the ver-
tebrae varies greatly, not only in different forms of
the Saiiropsida, but even in different parts of the
vertebral column of the same individual. In the
Ophidia these differences are seen to be associated
with their mode of life ; the anterior face is deeply
hollowed, and the posterior
rounded and convex ; the
convexity fitting into the
concavity of the next suc-
ceeding vertebra, and being
capable of rotation within it ;
in addition to this, the faces
of the neural arch are modi-
fied, the anterior being pro-
duced into two wedge-shaped
processes (zygosphenes),
which fit into corresponding
depressions (zygantra) on
the hinder face of the arch, and
thereby form a kind of peg-and«-socket joint (Fig. 130).
Vertebrae of this kind are found also in the lacertilian
Iguanse, who are known to swim by the movements
of their tails.

In Hatteriaj the Geckos, and some fossil lizards,
the notochord is persistent between the vertebrae,
the centra of which a,re, therefore, amphiccelous.
Among the Crocodiles a progressive loss of the inter-
vertebral portion of the notochord may be made out,
such forms as lived before the period of the Chalk
having amphiccelous, while cretaceous and post-cre-
taceous crocodiles have proccelous, vertebrae. Among
Birds, the fossil Arcbaeopteryx and Ichthyornis
appear to have had amphiccelous vertebrae, but in all

Pig. 129.— Section through the
Vertebra of a Frog, magni-
fied.

ch, Notochord; chs, its sheath;
o,c different kinds of bone.
(After Ecker.)

316 COMPARATIVE ANATOMY AND PHYSIOLOGY.

recent forms the centra of the vertebrae are exceed-
ingly well ossified ; they have, especially in the region
of the neck, an exceedingly characteristic form of
surface, for they are saddle-shaped, being convex from
side to side, and concave from above downwards on
their anterior face ; as exactly the opposite arrange-
ment obtains on the posterior surface, it is obvious
that the vertebrae are able to move on one another,
and the neck capable of that mobility which is so
notable and useful a possession of the bird, whose

ayi

Fig. 130.— Anterior and Posterior Surfaces of the Vertebrae of a Snake,
showing the form of the Centra, and the Zygosphenes (zys), and
Zygantra (zyt).

anterior appendages are of no use for seizing food
or other objects. An exactly analogous arrange-
ment to this may be observed among the Ophiu-
roidea ; in the larger number of these brittle stars the
several ossicles have a certain power of movement on
one another, but this is limited by the development of
processes and pits analogous to the zygosphene and
zygantra of the Ophidian vertebrae. In such Ophiu-
roids, however, as are, like Astroschema, capable of
twisting or twining their arms round a straight
Gorgonian, the saddle -shaped faces are well developed,
but the limiting pits and processes are absent.

In the Mammalia the faces of the centra are
often nearly plane, and from the intervening cartilage
there is developed (except in the Prototheria, where

Chap. IX.] VER TEBRM. 3 1 7

there are only occasional rudiments in the tail, and,
much more remarkably, in the Sirenia) disc-like plates
of bone, the so-called epiphyses, which, on the
arrival of maturity, fuse with the centra, and obscure
the line of union (neuro-central suture) between the
centra and the neural arches.

On the dorsal surface of these arches a spinous
process (neural spine) is often developed, and from
these muscles may have their origin ; forming only a
feeble ridge in the Amphibia, though prominent in
many fishes, they are often large in the Sauropsida,
and are of considerable importance in many Mammals.

It is in the highest forms that we can best distin-
guish the several so-called regions of the vertebral
column. In such a Mammal as the rabbit, it is, for
example, possible to make out (1) a cervical region,
in which the laterally placed ribs are never more
than rudimentary; the vertebrae of this region,
whether the neck is as long as in the giraffe, or as
short as in the porpoise, is always composed of seven
vertebrae, with the exception of the three-toed sloths
(Bradypus) which have nine; of another Edentate
(Manis), which has sometimes eight (W. K. Parker) ;
and of one two-toed sloth (Chokepus hoffmani), and
the Manatee, which have six. (2) A thoracic
region, with which are connected ribs that are movably
articulated with them, and some of which join the •
veiitrally placed sternum, and so form a kind of pro-
tecting cage for the thoracic viscera, and points of
attachment for the important costal muscles. (3) A
lumbar region, where the ribs are not movably
articulated, but, being shorter, leave space for the
coils of the intestine and the distension of the abdo-
men which occurs in gravid females of this group.
(4) A sacral region, the definition of which is sur-
rounded with considerable difficulties, but which is,
perhaps, best defined, with Gegenbaur and A. Milne-

Chap. IX.] VEK TEBRAL COL UMN. 319

Edwards, as the region in which the vertebrae have
additional (pleurapophysial) centres of ossification
for the attachment of the ilium (Fig. 132), and
which is denned posteriorly by the point of inser-
tion of the ischio-sacral ligament. There are ordi-
narily two true sacral vertebrae, but with them there
often become connected some of the (5) caudal or
tail vertebrae ; the whole fusing to form a single bone
of great strength. (See page 321.) The caudal
vertebrae vary greatly in number, according to the
length of the tail. The
greatest known number
among mammals is found
in the insectivorous
Microgale longicauda,
which may have as many
as forty-eight ; Manis has
forty - six. Connected

with and intermediate to Fig. 132.~ Anterior Surface of First

the several caudal ver- Sacral Vertebra of Mat3'

. . , , c, Centrum ; na, neural arch; p, pleura-

tebrae are V - shaped pophytta.

(chevron) bones, which

protect the vessels of the tail, and afford a larger
surface of attachment for the muscles.

The neural spines and the transverse processes
vary very considerably in length and size, according
to the functions and size of the muscles attached
to them ; the transverse processes, for example, being
long in the lumbar region of active jumping forms,
such as the hare' or the bandicoot.

The first vertebra, which in man supports the
head, has been on that account called the atlas, while
the second, on which the atlas moves, is distinguished
as the axis ; the centrum of the atlas is remarkable
for either fusing with that of the axis to form the
odontoid process, or, as in the Monotremata and
many Reptiles, it persists as an independent bony piece.

320 COMPARATIVE ANATOMY AND PHYSIOLOGY.

The importance of the vertebral column is well
illustrated by the arrangements which obtain in the

Fig. 133.— Skeleton and Carapace of the Loggerheaded Turtle
(from below).

Mammalia. We find that not only does the bony tube
afford a complete defence for the enclosed spinal cord,
and that the several parts of which it is composed can
be bent on one another, but that it is elastic in virtue

chap, ix.] VERTEBRAL COLUMN. 321

of the cartilaginous discs that lie between the several
vertebrae, and, in the more erect forms, by its
sigmoid curvature ; the bony outgrowths of the
several parts may be elongated or broadened to serve
as larger areas of attachment for the muscles.

The number of cervical vertebrae among the
Sauropsida varies with the length of the neck ;
the swan, for example, having as many as twenty-
five.* In all these hand-less Vertebrates the neck
is of great mobility ; freedom of movement to
the vertebrae on one another being allowed by the
already described saddle-shaped form of their centra.
In the Chelonia also, where much of the body is in-
vested in the firm carapace, the neck is very flexible,
and the shape of the centra varies greatly not only in
different species, but in the different cervical vertebrae
of the same animal ; the neural spines are never well
developed, and the head can be retracted or protruded.
The succeeding vertebras in the Chelonia have flattened
centra, but most are more remarkable for the possession
of a broad plate of bone, which is connected with the
apex of the neural arch, and forms one of the median
" neural plates " of the exoskeletal carapace • the ver-
tebrae of the tail can be moved on one another, and
are, like most of the vertebrae of most Reptiles, procce-
lous. The most important exceptions to this law
have been already noted.

While in the Amphibia only one vertebra enters
into relation with the ilium, or can be spoken of as
sacral, there appear to be two true sacral vertebrae in
the Saiiropsida. In Birds, where the whole support
of the body falls on the hind limbs, a number of pre-
sacral and post-sacral vertebrae fuse with the true
sacrals, to form a firm mass of attachment and sup-
port. Where the bird has, like the ostrich, to depend

* Some of the extinct tlesiosauria had more than forty cervical
vertebrae.

v— 16

322 COMPARATIVE ANATOMY AND PHYSIOLOGY.

entirely oh its legs, not only for support, but also for
means of locomotion, the number of vertebrae that so
fuse together may be greater than twenty. Chevron
bones are sometimes, as in lizards and crocodiles, de-
veloped in the caudal region, the length of which
varies considerably among reptiles ; but in flying
birds, where a tail would seriously affect the centre of
gravity, the caudal vertebrae are reduced in number,
and unite to form the ploughshare bone, with which
the so-called rectrices, or steering feathers, are con-
nected (Fig. 135; co). An exception to the rule that
the central portion of the vertebrae is that which is
most completely ossified is afforded by the caudal ver-
tebrae of many lizards. In these there is a central
unossified region, where, of course, the tail is much
weaker than at other points. If a lizard be seized by
the tail it will ordinarily escape, thanks to the fracture
of one of these centra, while the part of the tail left
with the lizard will grow again. Here we have, no
doubt, an example of a variation which has been
seized upon, on the principle of natural selection, and
has afforded these long-tailed but inoffensive forms
with a satisfactory, though undignified, method of
protection and escape.

In fishes the vertebral column can only be
divided into that which belongs to the trunk and that
which belongs to the tail. In some of the more gene-
ralised the notochord extends in a straight line to the
hinder end of the body, dividing the tail-fin into two
equal halves. In most Elasmobranchs this notochord
is bent upwards, and the lower half of the fin is much
larger than the upper. In the Teleostei the
notochord likewise becomes so bent up, but the rays
which support the fin become so arranged as to give
to the tail fin, when seen from the surface, the appear-
ance of being composed of equal upper and lower
halves, though, as a matter of fact, all the fin rays are

Chap. IX.]

SKELETON OF FISHES.

323

inferior to the notochord, or the modified urostyle
which has taken its place.

The unpaired dorsal fins of Fishes are connected
with the vertebral column by means of spines, which
are placed be-
tween the neu-
ral spines of the
vertebrae, and
are connected
with the dermal
spines or fin
rays which sup-
port and make
up the greater
part of the fins
themselves.

In many
fishes there is
very constantly
developed a
h?emal aswell as
a neural arch of
bone in connec-
tion with the cen-
trum, and these,
like the superior,
are produced
into a spine ;
in the hinder re-
gion of the body
interspinous
bones are de-
veloped between
these so-called hsemaphyphyses, and they serve to
carry the fin rays of the anal fin.

Articulated to the atlas is the skull, which, in the
first place, is the box or covering for the brain ; this

Fig. 135.— Skeleton of Eagle (reduced).

c' Coracoid ; ca, carpus ; dd', phalanges of the chief digit of the wing ; d", pha-
langes of smaller digit; d'", pollex; dr, dorsal ribs; /, femur ; JL, fibula;
fu, f urcula ; h, humerus; m, metatarsus; ma, mandible; me, metacarpus;
co, ploughshare bone ;p, pelvis ; pa, phalanges of foot ; pi, patella ; r, radius;
sr, sternal ribs; s, scapula; st, sternum; ti, tibia, tm, Tarsomctatarsus; u,
ulna ; up, uncinate processes of ribs. (After Milne-Edwards.)

Chap, i x. ] SKULL. 325

is the true cranium. With this there enters into
more or less complete union the cartilages or bones
that form the framework for the mouth, and give rise,
in higher forms, to the face.

The noto chord extends forwards below the two
hinder of the three primitive brain vesicles, and, on
either side, there appear masses of cartilage, homolo-
gous with those that form the arches of the vertebral
column. These parachordals unite with the noto-
chord to form a continuous toasilar plate, which
serves as a floor for the two hind brain vesicles ; this
grows up on either side, and unites above to form a
ring of cartilage which embraces the hindermost part
of the brain. Posteriorly each half gives rise to a
cartilaginous condyle, which articulates with the
atlas. This hindermost portion of the cranium may
be distinguished as the occipital region. In front
of the parachordals there appear two bars, which unite
behind, where they embrace the anterior end of the
notochord, and in front also, leaving a space in the
middle. These are the trabeciilae, and they form
the floor for the first brain vesicle, or fore-brain. As
the plates unite to form a solid floor they grow up at
the sides, but never form more than an imperfect roof
in this region, which, therefore, is not cartilaginous,
but membranous. This portion of the cranium may
be spoken of as the sphenoidal region. As the
cranium invests the brain, holes, or notches that will
be converted into holes, have to be left for the passage
of the cerebral nerves (Fig. 136 ; 5, 9).

In addition to these, the whole architecture of the
skull is profoundly affected by another set of elements,
which enter into more or less close contact with it.
These are the capsules of the three higher senses, smell,
sight, and hearing. At the anterior end of the sphe-
noidal region the olfactory cartilaginous capsule
becomes connected with the cranium, the anterior wall

326 COMPARATIVE ANATOMY AND PHYSIOLOGY.

(ethmoid) of which is perforated for the passage of
the olfactory nerves from the brain. In the occipital
region, on either side, the capsule for the ear (periotic
capsule) early becomes connected with the cranial
walls, and in the higher Yertebrata (see Fig. 136) the
periotic cartilages are, at a very early stage in deve-
lopment, con-
tinuous with
the basilar
plate. The
optic cartilage
(sclerotic)
lies in the
sphenoidal re-

CV

gion,

but it

never enters
into direct
union with
the cranium,
though the
form of this
part of the
skull is
greatly affec-
ted by the
size of the eye-
ball.

In addi-
tion to the (1)
cranial and
(2) sensory
cartilages

which take so large a part in the formation of
the skull, there is in all Gnathostomata yet a third
element, which may be distinguished as the touccal.
In the branchial and visceral clefts, which appear
just behind the brain, cartilaginous bars are developed

Fig. 136.— Cartfagiuous Cranium of a Chick of
the fourth Day of Incubation, showing the
investing Mass (iv), and the Trabeculee (tr),
with their Central space (pts).

cv, Cerebral vesicle (sliced off) ; e, eye ; Ig, anterior end
of investing mass formed from the parachordals ; 5,
notch for the passage of the fifth nerve ; q, quad-
rata ; cl, cochlea : she, semicircular canal of ear ;
9, foramen of exit of the ninth nerve ; we, noto-
chord. (After Parker.)

Chap. IX.] BUCCAL ELEMENTS OF SKULL. 327

for their support. Throughout the series, whether
gills are present or not, the first two take on, in addi-
tion or solely, quite another than a branchial function.
These (Fig. 137 ; Mrc, ny) send off from their
upper end a process, which is directed forwards ; the
anterior arch becomes segmented into an upper and a
lower piece, both of which, growing forwards, form
the rudiments of the upper and lower jaws. The
former (pi, Pt) may be called the pterygo-quadrate
bar, the lat-
ter the Mec-
kelian car-
tilage. The
chief means
of connection
bet ween these
bars and the
cranium is ,

not the me- £/ —TOT— v , ; R j
tapterygo- Tf Mn ***/ J

idal region, Fig i37._Head of Embryo Dogfish (11 lines long) .

Or hinder and T?% Trabecula ; Pi, Pt, pterygo-quadrate ; m, Pt, rneta-

e> pterygoid ; M?t, inandibular cartilaee ; ny, hyoid arch ;

Upper part OI sr l, first branchial arch, with four succeeding

,1 /> -i arches; sp, mandibulobyoid cleft; cl, hyo-lmtnchial

the first arch, cleft; Ci,c2,c3, cere bral vesicles. (After Parker.)

but the upper

part of the second arch (H?/), which forms the Ityo-

mandibular.

In such a skull, then, as that of the dogfish (which
has formed the basis for this account), the attachment
of the jaws to the skull is hyostylic (Huxley) ; in a
large number of fishes this hyostylic arrangement
obtains ; in a few (Notidanus), however, the meta-
pterygoid does enter into contact with the cranium,
and the jaw is then supported by elements of both the
inandibular and hyoid arches, or is ampliistylic. On
the other hand, in Chimsera, the Dipnoi, and all the
pentadactyle Vertebrata, the hyoid takes no share in

328 COMPARATIVE ANATOMY AND PHYSIOLOGY.

attaching the jaw to the skull, that attachment being
effected solely by mandibular elements, and being,
therefore, autostylic.

In the branchiate Yertebrata the number of
branchial arches corresponds with that of the
branchiae, and the separate bars become segmented ;
all the visceral bars save the mandibular have a
distinct median basal piece, which is known as
the foasiforaiicliial ; this passes on either side
into the liypobranchial, which is succeeded
by the ceratotoranchial, epibrancliial, and
pharyiigobranchial. When gills cease to be de-
veloped these bars undergo, as may be supposed, a
certain amount of atrophy, but, in all, the first
branchial arch is retained, while in tortoises and
lizards two arches may be detected. These basal
portions always fuse with those of the hyoid arch, and
the coalesced pieces make up the so-called body of the
hyoid, which forms a support for the tongue ; the
parts of the true hyoid arch form the interior, and
those of the first branchial the posterior or lesser
cornua of the hyoid of man.

"We have hitherto regarded the skull as compounded
of neural, sensory, and visceral portions, all of which
are formed by cartilage ; we have now to look at the
same structure from another point of view. It has
already been pointed out, that while the cartilage in
the occipital region of the skull forms a complete ring
in its hinder portion, the sphenoidai region is roofed
in by membrane ; this membranous roof is retained
throughout life by Myxine (the hag). In other
Cyclostomes and in Elasmobranchs the roof becomes
more or less completely cartilaginous, and this carti-
lage, which never becomes ossified, though its outer
layers may be calcified, is covered in by membrane.

In the more shark-like Ganoidei, the membrane,
though not the cartilage, undergoes ossification, and a

Chap. IX.J

SKULL OF PISHES.

329

number of investing membrane bones appear on
the roof of the skull ; in the Holosteous Ganoids ossifi-
cation commences in
the occipital region of
the cartilaginous cra-
nium, while there are
also membrane bones.
From this point for-
ward we have to dis-
tinguish between bones
that are preformed in
cartilage (cartilage
bones), and those
that are preformed in
membrane (mem-
brane bones).

At first, that is, in
the lower Vertebrata,
the membrane bones
are numerous, and
their relations are not
so constant and exact
as they are in the
higher forms. When
they become so we are
able to recognise that
the roof is formed by
two pairs of more or
less large bones, the
parietals abutting
on the occipital re-
gion, and thefrontals
in front of the parie-
tals. The base of the

skull is in the Ichthyopsida ossified in the occipital
region only, and the sphenoidal portion is under-
laid by a membrane bone, the parasphenoid

Fig. 138.— Head of Sturgeon, showing the
Membrane Bones, aud the Cartilaginous
Cranium, which is shaded dark. (After
Gegenbaur.)

330 COMPARATIVE ANATOMY AND PHYSIOLOGY.

(Fig. 139 ; par). The upper half of the mandibular
arch becomes invested by membrane bones only, the
jugal or quadratojugal, or both, which trend a
little inwards as they pass forwards (Fig. 139 ; q).
In front of these are- two bones which, typically, carry
teeth, the maxillae and the premaxillse.

Internal to this row of membrane bones is another,
of which the most anterior, the vomer (v), is formed
from membrane that did not overlay cartilage, just
like the maxillae and the premaxillse, while the
others, the palatines and the pterygoids (pt), are

formed from mem-
brane which gene-
rally invested carti-
laginous bars. Mec-
kel's cartilage is
likewise invested in
bones of membra-
nous origin, the most
important and con-
stant of which is
the tooth - bearing
dentary* At the
anterior end of the
skull, above the
olfactory capsule,
there appear the
paired nasals, with
which a lachrymal
is related in the
higher forms.

In Fishes a series of membrane bones may become
developed in connection with the branchial skeleton,
and form the support for the opercular flap of the
gills; such are the opercnlnm, siiboperculum,
and interoperculum. The most anterior of the
opercular bones is possibly the homologue of the

Fig. 139.— Skull of Frog, from below;
the Lower Jaw lias been removed.

e,o, Exoccipital ; po, prootic ; par, parasphe-
noid ; et sphenethmoid ; v, vomer; pm,
premaxilla- mx, maxilla; q, quadrato
Jugal ; pt, pterygoid ; SMS, suspensorium ;
palatine ; 1, optic foramen ; 2, foramen
of fifth nerve ; 3, foramen for ninth and
tenth nerves. (After Parker.)

chap, ix.j SKULL OF AMNIOTA. 331

membranous bone at the side of the skull, which is
known as the squamosal in the abranchiate Verte-
brata.

While in the Amphibia the posterior (occipital)
and anterior (ethmoid) portions of the base and sides
of the cartilaginous cranium undergo ossification, it is
riot till we reach the Amniota that we find the
central and lateral cartilaginous parts of the sphenoidal
region becoming bony ; when they do so we recognise
a basisphenoid, with an alisphenoid on either
side, and a presphenoid with corresponding later-
ally placed orbitosphenoids. Now, too, we can
distinctly see an ossified basioccipital, two ex-
occipitals, and a median supraoccipital, all of
cartilaginous origin, and surrounding the foramen
magnum. In the Saurcpsida the exoccipitals unite
with the basioccipital to form a single median
occipital condyle ; in the Mammalia the exoccipitals, as
in the dog, alone form the condyles, or some share is
taken by the basioccipital, but in either case the skull
is articulated to the vertebral column by two con-
dyles ; it is for this reason that some writers speak of
the Sauropsida as Monocondyla, and of the Mammalia
as Amphicondyla.

At the anterior end, the cartilaginous plates which
subdivide the nasal cavity may undergo more or less
ossification, and give rise to the " spongy bones "
of the nose ; they enter into connection with tho
ethmoid in the middle line, and may become united
with the nasals (naso - turbinals) or maxillae
(maxillo-turbinals) at the sides. (See Fig. 189,
page 442.)

In the walls of the cartilaginous ear- capsule there
appear centres of ossification, which are ordinarily
three in number ; of these the most constant is the
prootic, which alone is found in the Amphibia, though
in some fishes there are also epiotic and opisthotic

332 COMPARATIVE ANATOMY AND PHYSIOLOGY.

ossifications. In Birds these fuse with one another
and with the supra- and ex-occipital bones to an

Fig. 140. — Diagram of the Cranial Bones of a Mammal, showing the
Foramina of exit of the several Cerebral Nerves.*

1, Olfactory nerve ; 2, optic ; of, optic foramen ; da, foramen lacerum anterium, for
the passage of the third, fourth, sixth, and first branch of fifth ner\re (3, 4,
6, 5<x; ; fr, foramen rotundum, for the second branch of the fifth (5)3) ; fo,
foramen ovale, for the third branch of the fifth (ay") ; am/, stylomastoid

foramen for the seventh nerve (7) ; flp, foramen lacerum posterius, for the
ninth, tenth, and eleventh nerves ; MES, mesethmoid ; cf, condylar foramen
for the twelfth nerve ; CP, cribriform plate : PS, presphenold ; BS, basi-

sphenoid; BO, basioccipital ; os, orbitosphenoid ; AS, aiisphenoid ; E'O, ex-
occipital ; PEE, periotic ; F«, frontals ; PA, parietals ; so, supraoccipital.

extent of completeness which is greater than ordinarily
obtains among Mammals.

In addition to the important relations which the
cranium bears to the sense capsules, it has others even

* This diagram was, many years ago, shown to the author by
Prof. Flower, F.R.S., and it is here, for the first time, published
by his kind permission.

Chap, ix.} FORAMINA OF SKULL. 333

more important to the brain, which it contains and
protects ; of these the most important is its relation to
the cerebral nerves and outgrowths that pass out
from it. The distribution and arrangement of these
nerves form, in disputed cases, one of the best criteria
of the homologies of the different parts of the cranium.
Seen in its most elaborated condition, as it is found in
Mammals, the cranial bones and cerebral nerves have
the following relations. The so-called olfactory nerve
(Fig. 140 ; 1) perforates the cribriform plate of the
ethmoid ; the optic nerve passes through the optic
foramen in the orbitosphenoid bone (os) ; the third,
fourth, and sixth nerves, that go to the muscles of the
eye,- pass with the most superior division of the fifth
through the irregular or jagged space (/la) that lies
between the orbitosphenoid and the alisphenoid. The
two other branches of the fifth (or trigeminal) pass
through the round (fr) and oval (fo) foramina in
the alisphenoid, while the seventh has a passage at the
outer side of the periotic (PER), while between the
periotic, basioccipital, and exoccipital there is a pos-
terior foramen lacerum (flp) for the glossopharyngeal,
vagus, and hypoglossal (9, 10, 11) nerves; lastly, the
twelfth nerve passes through the conclylar foramen (cf),
while, as we have already learnt, there is a great
foramen at the hinder end of the cranium which
serves as the means by which the medulla oblongata
is allowed to be continuous with the spinal cord.

An examination of the interior of the cranium
similarly reveals the close connection that obtains be-
tween the containing case and the contained brain ;
and, indeed, our knowledge of the characters of the
brains of extinct forms is absolutely dependent on casts
of the internal configuration of such skulls as have
been preserved to us in the form of fossils.

Where, as in the lower Mammals, the cerebral
hemispheres are of no great size, and do not overlap

334 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the hinder cerebellum, the upper surface of the cranium
is straight and flattened \ as we ascend the scale, how-
ever, we find that the cerebral hemispheres growing
backwards come to overlap more or less the cerebellar
region ; concurrently with this the upper surface of
the brain becomes more or less arched, and the cranial
walls take on a corresponding form ; the most familiar
example of this is, of course, the brain of man, but it
is to be carefully noted that the skulls of the old-world
baboons, and of some of the lower and smaller new-
world monkeys, have the supraoccipital region thrown
farther back and down than it is in man himself. As
a result of this alteration in the position of the parts
of the brain case we find that the foramen magnum
looks downwards instead of backwards ; as a secondary
result we find that the skull of man balances more or
less completely on the occipital condyles, and, this
being so, there is not the same need for the development
of muscles and ligaments to support and hold up the
back of the head as there is in the dog or the horse ;
from this mechanical arrangement we get, further, a
marked diminution in the extent of the bony ridges
on the occiput to which these muscles are attached.

When a longitudinal section is made through the
skull of a Mammal and the form of the internal cavity
is revealed, it is seen that the bony ear-case projects
into the hinder part of the cavity, and that the wall of
the anterior boundary is perforated by the small holes
which give passage to the fibres of the olfactory nerve
(cribriform plate of the ethmoid) ; in the whales, where
the olfactory sense is in a rudimentary condition, the
holes in this plate are few and small ; the region of the
skull in which the olfactory lobes of the brain are con-
tained is known as the olfactory fossa, and this is
smaller or larger according to the size of the olfactory
lobes themselves (see page 425) ; this cavity is bounded
by the cribriform plate in front and below, and at the

Chap, ix.] MOUTH OF CYCLOSTOMATA. 335

sides, and has behind it a ridge of bone on the orbito-
sphenoid, and frontal bones by which it is separated
from the cerebral fossa, which, in all Mammals,
occupies a larger part of the cranial cavity ; this fossa
is more or less feebly divided into two by a ridge of
bone which corresponds to the sylvian fissure of the
brain ; in the more anterior division there lies the
frontal lobe (see page 426) ; behind this comes the cere-
bellar fossa, marked off anteriorly by the tentoriuni.
The flocculus of the cerebellum lies in a special depres-
sion on the inner face of the periotic, and the hypo-
physis cerebri on a pit (sella turcica) on the
superior face of the basisphenoid, which, as we have
already learnt, forms a portion of the floor of the brain
cavity.

Such, then, being the general disposition of the
parts of the skull in a well-developed Vertebrate, we
have now to investigate the arrangements which obtain
in various forms in relation to their habits of life and
their zoological affinities.

In the Round-mouths, where no branchial rods
are modified to form jaws, the sides of the buccal orifice
are supported by cartilaginous pieces, the so-called
labial cartilages ; the mouth is surrounded by a circu-
lar lip, in the posterior region of which is placed the
annular cartilage ; into the cavity of the mouth
there project a number of horny denticles ; on the
floor is the lingual cartilage, and in front of this is the
median ventral cartilage, which, possibly, represents
the basal median portion of the mandibular arch, which
in other Vertebrates is only to be detected in early
stages. It is very instructive to observe that there is
a close resemblance between the mouth parts of a
lamprey and those of a tadpole during the period when
the latter has a suctorial mouth.

The labial cartilages of the Cyclostomata appear to
be better retained by Elasmobranchs than by other

336 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Vertebrates, in which only those persist that take part
in the formation of the nasal cavities.

In the voracious sharks the mouth is of large size,
but the character of their food is related rather to the
size of their teeth than to the extent of their mouth ;
when the mouth is not terminal but ventral in position,
a shark has to turn on its side to seize its prey.
In some Rays, such as Pristis (the saw-fish) the
snout is produced into a long flattened weapon of
attack, which is armed at the sides with spinous
processes, and serves as an organ by means of which
the body of its prey may be torn open. In the
Teleostean sword-fishes, where maxillae and premaxill<e
are present, these bones are produced into a long
stabbing-organ, not only strong enough to pierce the
bodies of whales, but even the planks of wooden
ships. In many Ganoidei, such as the sturgeon, the
snout is of considerable length ; the function of this
organ is not completely understood, and the most
plausible hypothesis is that of von Martens, who
ascribes to the long snout of Polyodon the function
of a tactile organ, the necessity for which is to be
explained by the turbidity of the rivers in which it lives.

In the Teleostei the mouth may be very large, as
in the Angler (Lophius), or exceedingly small, as in
Chsetodon and Diodon ; in Chelmo, a form allied to
Chsetodon, the mouth is prolonged into a snout which
possibly serves as an apparatus for drawing from holes
or crevices the small animals on which it feeds
(Giinther). Where the teeth are of great size, and
adapted, say, for crushing shells, as in the sea-cat, the
jaws which carry them are of corresponding strength; in
the wrasses, with somewhat similar habits, the upper
pharyngeal bones are articulated with the basi-occipi-
tals, and no doubt afford a firmer fulcrum for the jaws.

In bony fishes the eye-ball is sometimes provided
with separate bony pieces for its protection.

Chap. IX.}

SKULL OF OPHIDIA.

337

Pmx

PL

In the Amphibia the mouth is wide, and in
some Anura the symphysis of the mandibles is strong,
for it has to serve as the
point of attachment of the
tongue (see page 154) ; in
some TJrodela, as in some
Reptiles and Birds, the
sclerotic cartilage undergoes
ossification, but presents only
an advance on what is seen
in the Teleostei.

Owing to our better
knowledge of the habits of
the higher Vertebrates, we
are more easily able to
associate the arrangements
of parts of their skulls with
the habits of their possessors. *
Nowhere do we find a better
series of mechanical arrange-
ments than among the
Optiidia; in the python
and those that swallow,
without poisoning, their
prey, the skull is wide be-
hind, and the quadrates are
at a considerable distance
from one another (Fig. 141 ;
QM). In other words, the
lower jaw is wide at its
point of attachment, and,

as we have learnt already (page 96), the quadrate
is movable upon the squamosal (sq), so that the
width of the mouth behind can be very considerably
increased.

Anteriorly, there is a corresponding arrange-
ment, inasmuch as the lower jaws are not firmly
w— 16

Fig. 141.— Lower Surface of
Skull of Python.

praz, Premaxillae ; MX, maxillse ;
vo, vomer; IT, transverse
bone ; pi, palatine ; Ft, ptery-
goid; Q«, quadrate; sq, sciua-
niosal; BS, basisphenoid;
BO, basioccipital. (After
Huxley.)

33 8 COMPARATIVE ANATOMY AND PHYSIOLOGY.

united by a symphysial suture, but are only con-
nected by an elastic ligament. Thanks to this dis-
position of the separate parts in front and behind,
the cavity of the mouth is capable of very considerable
enlargement.

In the venomous snakes the arrangements are

M/t

Fig. 142.— Skull of Crotalus.

Pmp, Premaxilla; Ma;, maxilla; La, lachrymal; vl, palatine; Ft, pterygoid ;
QU, quadrate; MM, lower jaw. (After Huxley.)

even more elaborate ; the maxilla, instead of being
elongated backwards, is shortened antero-posteriorly,
so that its longer axis is directed downwards ; it
carries one long tooth, the "poison fang," and is
excavated on its upper outer surface, where there lies
the duct of the poison gland ; the maxilla moves freely
on the lachrymal, which again can move en the
frontal. The palato-pterygoid bar (Fig. 142 ; P£, pt) is
jointed, and the pterygoid is further connected with

chap, ix.] SKULL OF OPHIDIA. 339

the maxilla by the transverse bone (tr). The long
quadrate is movable in a plane parallel to the Icng
axis of the skull, and when the mouth is shut is
directed backwards, so that the palato-pterygoid bar
forms a straight line, while the palatine and transverse
bones pull the maxilla backwards and upwards, and
so cause the poison fang to lie along the floor of the
skull. When the mouth opens, the point of attach-
ment of the lower jaw is necessarily drawn upwards ;
this action forces the lower end of the quadrate
forwards ; the quadrate acts on the pterygoid, and the
pterygoid on the palatine and transverse bones, which,
driving forwards what is before them, force the
maxilla outwards and downwards ; the poison fang is
thus caused to become vertical, or in a position to
inject the poison, which is simultaneously forced out
of its gland through the channel in the tooth, and so
into the body of the victim.

In the poisonous Mexican lizard Heloderma, the
poison glands are modifications of sublingual glands,
and the lower jaw is traversed by the channels
through which their ducts pass into the floor of t'he
mouth. In Hatteria, the teeth are lost after a
certain time, and their place is taken by the bones
that bear them, the margins of which are sharp and
of exceedingly dense structure, so that here the bones
take the place of the teeth. (See page 146.)

While in the Ophidia the two halves of the lower
jaw are connected by ligament, and in most Lizards
and all Crocodiles by sutures, they become completely
fused with one another in the Chelonia; the same
phenomenon is to be observed in Birds, and in both
cases it is, no doubt, to be correlated with the loss of
teeth and the acquirement of a horny investment or
beak. In the turtles the fossa on either side of the
skull, bounded externally by the jugal and quadrato-
jugal, is roofed over by an outgrowth of the parietal ;

340 COMPARATIVE ANATOMY AND PHYSIOLOGY.

this remarkable arrangement is, curiously enough,
found also in one species of the amphibian Pelobates,
and in the African rodent Lophiomys.

In the voracious Crocodiles the teeth are con-
fined to the maxillse, premaxillsR, and lower jaw, as in
Mammals, and these bones are, further, socketed for
the reception of the strong teeth. In connection with
their mode of life, which requires that they should,
while breathing air, hide as much of their body as
possible under the water, the anterior iiares are
placed at the end of their long snout ; the eye, for the
same reason, lies high up on the head, and the orbital
cavity is seen, therefore, from an upper rather than
from a lateral view of the skull ; the hinder openings
of the nasal passages, which, in the forms hitherto
described, lie in the anterior region of the roof of the
mouth, are in the crocodile placed very far back ; this
is effected by the development of transverse plates
formed by the maxillse, palatines, and pterygoids
which unite into a long floor for the nasal passages.

In many Birds the upper jaw is capable of a
certain amount of vertical movement, such as is
carried to an extreme in the parrots ; this freedom is
due either to incomplete ossification between the
ethmoidal and nasal regions, or to the formation
of true articulations between the large premaxillse
on the one hand, and the f rentals, jugals, and pala-
tines on the other; these latter, as in a number
of other birds, are not immovably connected with
the basisphenoid, but can slide backwards and
forwards, while the quadrate, with which the jugal
is connected, is movable, as in lizards or snakes.
When the quadrate is pushed forwards, as it is when
the lower jaw is depressed, it pushes the jugal forwards,
and that bone pushes the beak upwards. While the
maxillse are comparatively small, the three-rayed pre-
maxillse are of considerable size, and the processes by

Chap. IX.]

SKULL OF BIRDS.

which they are connected with the frontal and with the
palatines are very important factors in the attachment *
of the beak to the cranium ; especially is this the case
in those skulls in which ossification of the ethmoidal
region is incomplete.

In the wood-peckers, where the head is " employed

Fig. 143.-Side view of a Dissected Head
of a common Wood-pecker. (Half iiat.
size.)

u i, Upper and lower jaws ; t, barbed tip of
tongue ; thh, thyrobyal of right side, with
its muscle and sheath ; o, right orbit ; n.
right nostril; sgr, salivary gland; m, m,
muscles of neck ; o», oesophagus ; tr, trachea ;
rm, retractor muscles of tongue. (.After
Macgillivray.)

as a powerful hammer or axe, whose strokes can be
heard at a considerable distance " (Garrod), the bones
of the skull are thicker and stronger than in most
birds, and the upper jaw has its power of vertical
movement considerably limited. The articulation of
the palatine with the basisphenoid disappears, in
consequence of the feeble development of the hinder

342 COMPARATIVE ANATOMY AND PHYSIOLOGY.

end of the palatines. In correlation with the mode of
action of the tongue (Fig. 143 ; 't\ which is protruded
into the recesses from which the bird extracts the
insects on which it feeds, the hyoid bones that
support it (thJi) are greatly developed, the anterior
cornua passing backwards and then curving forwards
over the roof of the skull, in grooves of which they
lie. From the ends of these bones or their sheath,
there arises a muscle which is inserted into the lower
part of the same bones ; the contraction of this muscle
straightens the curve of the thyrohyals, but this can
only be effected by the protrusion of their free or
lower ends ; as the tongue is placed at this point, and
is supported by the bones, it is clear that it is
simultaneously protruded. (See page 156.)

In the Mammalia the quadrate, of which we
have till now heard so much, is converted into one
of the auditory ossicles, the malleus ; the covering
bones of the mandible are reduced to the dentary.
Man, so far as is known, is the only animal, in addition
to the frog and the sturgeon, in which the meiito-
meckeliaii cartilage is found on either side of
the mandibular symphysis. The Marsupials are re-
markable for having the angle of their lower jaw
developed into a strong inflection, which is absent
only in Tarsipes, and, among the Eutheria, has only
been observed in the insectivorous Centetes (the
Tenrec).

When we compare a carnivorous mammal, in
which the lower jaw moves up and down, with a
ruminant, in which it moves . from side to side, we
observe a difference in the characters of the glenoid
cavity by which the mandible is articulated with the
cranium ; in the dog, for example, this fossa is concave
from before backwards, and the projections in front
and behind limit the movements of the articular end
of the mandible to such as can be effected in a vertical

Chap. IX.]

SKULL OF ELEPHANT.

343

direction ; and this articular end or condyle is convex.
In the sheep, on the other hand, the condyle is broad
and flattened, and works upon a flattened or slightly
convex glenoid facet.

When a large surface of attachment for the

Fig. 144.— Section of the Skull of an Indian Elephant, to show the

Air Cavities.
6, Brain-case ; s, air sinuses ; n, nostril ; m, molar ; t, tusk.

necessary muscles is required, as in the elephant, the
skull is not composed of thick bone throughout, .but
a finer bony tissue with a number of intervening air
cells is developed between the " tables " of the cranial
bones ; by these means a wider area is obtained
without any proportional increase in the weight of
skull to be supported (Fig. 144).

344 COMPARATIVE ANATOMY AND PHYSIOLOGY.

In the dolphins, where the olfactory sense is greatly
reduced or perhaps altogether lost, no olfactory fossa
is to be seen in a longitudinal section of the cranium ;
in correlation with their habits, and the large size of
the mouth cavity, and the length of the premaxillse
and maxillae which form its roof, the nasals are
reduced in size and placed far up on the surface of the
skull ; the anterior nares consequently look upwards
instead of forwards, and are therefore the first to
appear above the surface of the water, when these
pulmonate aquatic animals come up to breathe air.
Just as in crocodiles, whose habits are so far essentially
similar, the ptery golds develop transverse plates,
the formation of which throws the posterior nares
very far back on the roof of the mouth, and thereby
approximates them to the opening into the trachea.

It is important to observe that quite a different
set of arrangements obtains in the Sirenia, which,
instead of being active or marine forms, are sluggish,
and live much at the bottom of shallow rivers ; here
the bones are exceedingly dense, the anterior nares
look forwards, but are of very large size, and no special
plates of the pterygoid prolong the nasal passages
backwards. When we compare the structural adap-
tations of the Sirenia and the Cetacea, it is impossible
to avoid seeing, on purely theoretical grounds, which
of the two has the advantage ; and this consideration
may be supported by the abundance of the species
and specimens of the latter as compared with the
former.

The ant-eater is another form in which the
posterior nares are placed very far back owing to the
development of pterygoidal plates to form a floor for
the air passages ; in this case an explanation is possibly
to be found in the fact that such a disposition, by
means of which the nares and the glottis are brought
into much closer proximity, would be an advantage in

chap, ix.i HIES. 345

forms where a powerful inspiration might lead to an
insect being carried from the tongue into the trachea.

In various Mammals the roof of the skull is in one
way or another enlarged ; in the sperm-whale, the
nasals, maxillaries, and supraoccipital unite to form a
large basin ; as the contents of this basin are, how-
ever, of an oily nature, we may believe that the
specific gravity of the bone is neutralised (even if no
advantage is gained) by that of the spermaceti, the
specific gravity of which is '843 at 50° C. In the
Rhinoceros and in the Uiigulata the roof of the skull
carries horns. (See page 369.) In the porcupines
many of the upper cranial bones are expanded, and
contain large air sinuses, as in the frontal region of
the sloths.

The morphological characters and homologies of
the ribs present a problem that is yet unsolved ; the
evidence as to their origin in the embryo being not a
little conflicting. In a number of forms they un-
doubtedly arise in the tissue of the protovertebrae
(see page 529) quite independently of the vertebrae
themselves, their connection with which is only
secondary. In the Cyclostomata, Holocephala, and
some rays, there are 110 ribs, and the supporting
function of these absent rods is undertaken, as in
Amphioxus, by the fibrous tissue which lies between
the dorsal and ventral muscular layers.

Among the Amphibia and the Amniota, the ribs,
when present, and typically developed, are connected
with the vertebrae by an upper and lower process ; in
the Csecilise, where, among amphibians, ribs are best
developed, these two processes are widely separated 3
in the Salamanders the proximal ends of the anterior
ribs are forked, and of the hinder single ; in the
Anura the ribs are reduced or quite rudimentary.

In the Amniota the ribs have a greater functional
importance, as is shown by the length of the more

346 COMPARATIVE ANATOMY AND PHYSIOLOGY.

anterior, and their union in many cases with the
sternum in the ventral median line. A definite
thorax may henceforward be recognised, and the
appearance of this cage associated with the complete
absence of branchial respiration. In front of the
thorax more or less well developed ribs may be
developed, but these never become connected with the
sternum ; in the Chelonia there are no cervical ribs.
The sternal ribs are jointed, and in Reptiles the
sternal portion remains cartilaginous ; where the
sternum is short many of the ribs are connected with
the cartilaginous rod that lies behind it. In Birds
the connection between the ribs and the sternum is
more complete, and the whole apparatus is firmer and
stronger (see Fig. 135), while in them, as in crocodiles,
the side walls of the thorax are strengthened by the
development of backwardly directed processes from
the ribs (uncinate processes) (up). Cervical ribs
are very rare among Mammals, though occasionally
found, as in Bradypus. The thoracic ribs are so
articulated as to be capable of a considerable amount
of movement, thanks to which the thoracic cavity
may be increased or diminished in size ; they are
either attached directly to the sternum, or connected
with it by ligament, or are quite free at their distal
end. In the Cetacea some of the hinder ribs lose
even their attachment to the vertebrae.

In the Ophidia, where the primitive condition of a
large number of similar ribs is retained, there is a
single articular head for connection with the vertebral
column ; this is of a kind to allow of considerable
freedom of movement, and the snake uses its successive
pairs of ribs as stilts by which it may move along the
ground.

In the Amniota the ventral ends of the ribs
become constricted off, and fuse with one another in
the median line ; the cartilaginous plate thus formed

Chap. IX.]

STERNA.

347

is known as the sternum. With the anterior end
of this sternum there ordinarily comes into relation
the ventral part of the anterior limb-arch (see infra) ;
and, farther back, we may find the ventral attach-
ments of the ribs. In the Chelonia and Ophidia
the sternum is lost, and this is to be correlated with
the great development of the
exoskeleton of the one, and
the mode of life of the others
of these orders of Reptiles.
In the Carinate birds the me-
dian portion of the sternum is
produced into a long and
strong keel (Fig. 145 ; cs), to
the sides of which are attached
the powerful thoracic muscles
which depress and elevate the
fore limbs ; an analogous cari-
nation is to be seen in Bats.
In the fossorial Insectivora,
such as the mole (Fig. 146),
the presternum is produced
far forwards, is keeled, and
widened out at the sides so
as to afford a large surface
of attachment for the mus-
cles that move their digging
limbs.

In all the gnathostomatous Yertebrata the body
is typically provided with two pairs of lateral ap-
pendages, or limbs, one of which lies, as a rule, some
distance in front of the other; these are the fore
and hind limbs. They are brought into connection
with the axial skeleton by means of arches, the
pectoral and pelvic arches.

The Pectoral arch consists essentially of a bar of
cartilage, which undergoes division into a dorsal part,

Fig. 145. — Sternum of Fregi-
lupus varius.

cl. Clavicle ; sc, scapula ; co, cora-
coid ; cs, keel of sternum.
(After Murie.)

348 COMPARATIVE ANATOMY AND PHYSIOLOGY.

or scapula, and a ventral portion, the coracoid ;

at their point of junction the proximal end of the fore
limb is attached. The pelvic arch is, similarly, a
cartilaginous bar, which may be divided into a dorsal

Fig. 146.— Sternum of common Mole.

iliac, and a ventral pubic piece ; and the head of the
hind limb is attached to the arch at the point where
its two halves unite.

The coracoid divides into two processes, the
prsecoracoid and coracoid proper; the pubic
bar is, similarly, in the higher forms, separable into
an anterior pufois and a posterior ischiuiu. In

chap, ix.] PECTORAL ARCH. 349

Fishes, where there is no sternum, the coracoids early
unite in the ventral median line ; where a sternum is
present the coracoid of either side enters into con-
nection with it. In the hind arch, where there is no
bar corresponding to the sternum, the pubes and ischia
may unite with their fellows at a symphysis, as in all
Mammalia ; in all Vertebrates above fishes, the ilia
enter into a more or less firm union with the sacral
region of the axial skeleton, or vertebral column.
(See page 321.)

Just as in the case of the skull, when covering
membrane bones are developed some enter into union
with the cartilage or cartilage bones of the pectoral
arch, but no such bones go to form part of the pelvis.
The most important of these bones is the clavicle,
with which in some fishes a siipraclavicle, con-
nected with the skull, and an mfraclavicle, united
with its fellow below, are added on ; in the Amniota
an interclaviole is sometimes developed. As we
ascend the scale we observe a reduction in the cora-
coidal region ; thus the precoracoid is absent in the
crocodiles, and among birds is found in a rudimentary
condition in the Ratitae only ; in mammals it is never
present, and the Prototheria alone have a fully de-
veloped coracoid ; as a rule the scapular end of the
bone is alone retained ; it is, indeed, from the hook-
like form of its remnant in man that the bone has
received its name. In a few (shrew, mouse) the
sternal end of the coracoid is persistent (Gegenbaur).

In Chamseleons and Crocodiles the clavicle is lost,
as it is also in many Katitse, where, if present, it is
only rudimentary ; in the Carinatse the two clavicles
unite to form a single bone, the furcula (" merry-
thought"), which becomes connected by ligaments
with and strengthens the ear in a sterni (Fig. 145).
Among mammals the clavicle may be well developed,
as in man and the bat, bony in its median region

350 COMPARATIVE ANATOMY AND PHYSIOLOGY.

only, as in the rabbit, and still more in the dog, or
absent, as in bears and all Ungulates. The iiiter-
clavicle is developed only in Prototheria.

In the Teleostei the pubic arch may be placed far
forwards, and be thoracic, when the hind-fins lie
below the fore-fins ; or jugular, when they lie in
front of them (Fig. 134). In the higher forms they
always retain their position between the abdomen and
the tail. In recent reptiles the ilium may extend
far back, while in birds, and in certain extinct
reptiles (Dinosaurs) it is developed anteriorly to the
acetabulum, or cavity of articulation for the head of
the hind limb. In the Sauropsida this cavity is never
completely bony, and the same is the case in Echidna
(though not in Ornithorhynchus) ; with this exception
the acetabulum is always a completely bony cup in
mammals.

In correlation with the posture or mode of pro-
gression, the ilium enters into more or less close union
with the sacral region of the vertebral column, and
the demands made upon the axis for further support
are responded to by the fusion of presacral or post-
sacral (or both) vertebrae with those of the true
sacrum to form a solid piece. Thus, in a bird
(Fig. 147) the whole arch is of groat size, while in the
Cetacea it is at most represented by the ischia.

Just as there are very many striking and sugges-
tive points of resemblance between the fore and hind
arches, so are the fore and hind limbs arranged on
essentially similar principles.

It will be most convenient to begin with what
obtains in the Amphibia and Amniota, or the penta-
dactyle Vertebrata. Either limb may be divided into
three regions : (a) arm, fore-arm, hand ; (ft) thigh, leg,
and foot. In the arm, and in the thigh, there is a
single bone: (a) humerus, or (ft) femur; in the
fore-arm and leg two : (a) radius and ulna ; (ft)

Chap, ix.] FORE AND HIND LIMBS. 351

tibia and fibula; the (a) hand (maims) is divi-
sible into wrist (carpus), palm (metacarpus), and
digits ; the (0) foot (pes), into tarsus, metatar-
sus, and digits. The digits are typically five in
number, and consist of a number of separate pieces,
articulated on one another ; the number of these
phalanges is inconstant, but, as we ascend the

Fig. 147.— Side View of the Pelvis of an Adult Fowl.

iZ, Ilium; is, ischium ; pft, puhis; dl, dorsal vertebrae ; cd, caudal vertebras;
AW, acetabulum. (After W. K. Parker.)

series, we observe a reduction ; they are sometimes,
though not always, provided with horny claws.

The humerus, which fits, by its anterior rounded
head, into the glenoid cavity formed by the edges of
the scapula and coracoid, acquires a greater freedom
of movement when, as in the Mammalia, the coracoid
is reduced in extent. It is generally characterised
by the possession of a more or less strong bony ridge
to which the deltoid muscle is inserted, the size of
which is, of course, in proportion to the use to which
the fore limb is put ; and the ridge, therefore, is very-
pronounced in fossorial and flying forms. In the
Carinatse the muscle that elevates the wing lies, with
those that depress it, in the pit formed by the sides of
the keel of the sternum, and its tendon passes over

352 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the coracoid and scapula to be inserted into the
humerus ; in other words, it works over a pulley.

While a flying organ is developed in the Carinatse
(Fig. 148) by the elongation of the humerus and
of the radius and ulna, the latter of which is
marked by the impressions of the secondary wing-
feathers ; while both are so articulated with the lower
end of the humerus as to allow of little movement

in

Fig. 148.— Skeleton of Fore Limb of a Flying Bird.
h, Humerus ; r, radius ; u, ulna ; c c, carpal bones ; ra, metacarpus ; 1, 2, 3, digits.

on one another, or, in other words, aid in the forma-
tion of a rigid rod ; by the reduction of the carpus
to two bones ; and by the fusion of the second and
third metacarpals and the reduction of the digits . a
very different modification of the homologous parts
is found in the bat. Here (Fig. 149) all the meta-
carpals, save that of the first digit (thumb), are greatly
elongated, as are too the phalanges of the third,
fourth, and fifth digits ; between these and the body
there extends a fold of thin membrane (the "wing
membrane "), by the expansion of which these Mam-
mals are enabled to float and move through the air.

Chap. IX.]

FORE LIMBS.

353

In Galeopithecus (the so-called Flying Lemur), the
fore and hind limbs are both elongated, and there
stretches between them, attached to the sides of the
body, a fold of skin which, unlike
the wing membrane of a bat, is hairy
on either side. Among lizards a
flying form is represented by Draco,
where the support for the flying
apparatus is afforded by the elonga-
tion of the ribs.

Another set of modifications is
to be found in the aquatic penta-
dactyle forms, such as the turtles,
crocodiles, and aquatic mammalia.
Here the essential modification con-
sists in the elongation of the manus
or pes to form a fin-like organ ; the
simplest and first change is seen in
river tortoises, where, as in the feet
of wading birds, the digits are
merely connected by a web ; as this
web extends, it gradually encloses
the separate digits and converts the
organ into a paddle, as in the marine
turtles, where the fore are larger
than the hind limbs, or the whales
and Sirenia, in which the hind limbs
are altogether aborted ; the Sirenia
have rudimentary nails, and agree
with other mammals in never having
more than three phalanges to their
digits. The whales present a more
extreme case, as all rudiments of
nails are lost, and the phalanges of some of the digits
may come to be as many as twelve or thirteen in
number (Fig. 150).

In the Mammalia we observe tha.t the limbs
x— 16

Fig. 149.— Manus of
Bat.

p, Pollex ; sc, scaphoid ;
mi to m*, the elon-
gated metacarpals of
the second to fifth
digits.

354 COMPARATIVE ANATOMY AND PHYSIOLOGY.

ordinarily become organs of support for the body,
and that in them more than in
other Vertebrates these ap-
pendages cease to lie along-
side the body, or in a plane
more or less parallel to its
long axis, and come to be set
in a vertical plane. This
change having been effected,
we note here, as elsewhere in
the organs of the animal body,
a reduction of superfluous
parts, and a consolidation of
what remains. In a large
number of Mammals, the
thumb (pollex) or great toe
(hallux), which never have
more than two phalanges, ex-
cept in some, though not all,
Cetacea, is completely lost.
Among the Ungulata the
tendency to a further reduc-
tion is seen in the sheep and
ox on the one hand, where
two digits persist, and in the
horse, where the whole weight
is carried by the middle or
third digit of each limb. The
historical evidence as to the
gradual reduction of the
second and fourth digits in
the horse may be regarded as
complete. (Compare Fig. 151.)
While reduction affects
the digits, consolidation is
more often seen in the metacarpal and metatarsal,
and carpal and tarsal bones ; the muscles that are

igf. 150. — Fore - arm and
Manus of the Round-
headed Dolphin.

Radius ; u, ulna ; c, carpal
hones ; mi, first ; ?>i5, fifth
metacarpal ; i to v, digits.

Chap. IX.]

FEET OF UNGULATA.

355

connected with the lateral digits gradually dis-
appear.

In the series of Artiodactyla (even-toed forms)
we find, to take the foot, four toes, distinct metatarsal,
and distinct tarsal bones (in the pig) ; in the Chevro-
taiiis (Tragulus), the second and fifth digits are still
smaller, and while their metatarsals are distinct, the
third and fourth
metatarsals have
united together,
two of the tar-
sals have united
together, and
one of the rest
has disappeared;
in the musk-
deer, as in the
true deers, the
outer digits are
not directly arti-
culated with the
other bones of
the foot, and the
outer metatar-
sals have, as in

Fig. 151. — Foot of Anchitherium (A); Hippa-
riou (B), and Horse (c).

ii, in, iv, digits.

them, disappeared ; the musk-deer, however, retains
what the deer have lost, the extensor muscle of the
fifth digit.

While the large number of what are really or
practically two- toed Ungulates is evidence that this
reduction of the digits has not been associated with
any diminution in the value of the limbs as locomotor
or supporting organs, we have palaeontological evidence
of the disappearance of a group of even-toed Ungulates
who tended to lose their lateral digits. When we ex-
amine the carpus of a deer we see that the carpal bones
have fused with one another, and have not disappeared

356 COMPARATIVE ANATOMY AND PHYSIOLOGY.

as the lateral digits, with which some of them were
connected, have lost their function ; as the middle digits
have grown larger and thicker they have seized on the
carpal bones, and thereby gained " a better and more
complete support for the body." In some fossil forms
(Xiphodon, Anoplotherium), " the relation between the
carpal and tarsal bones, and the remaining two middle
metacarpals and metatarsals, remains just the same as
it was in the tetradactyle ancestor"; the digits that
remain do not, in other words, gain further support
from the carpal or tarsal bones. Forms in which
inheritance has been stronger than modification
have disappeared, while in those which have lived on
or left descendants, an adaptive modification
has been effected (W. Kowalevsky).

As we ascend the scale of the Primates we find
an increasing tendency to throw the support of the
body on the hind limbs only ; thus, all the manlike
(anthropomorphous) apes are semi-erect; the
Gibbon (Hylobates) uses the tips of his fingers much as
an active man uses a walking stick (Huxley), the orang,
the gorilla, and the chimpanzee, support themselves on
their knuckles. Man is erect, and, in correlation with
this position, the tuberosity of the os calcis of the foot
is greatly broadened, the thigh and leg are in a
straight line, the pelvis becomes an open basin sepa-
rated by a wide space from the thorax, the vertebral
column takes on a marked S -shaped or sigmoid curva-
ture, the head is balanced on the atlas, and the spines
of the cervical vertebrae, which have no longer to give
origin to powerful muscles, are reduced in size. Owing
to the monopoly of support enjoyed by the hind-limbs,
the fore limbs become free to serve as prehensile
organs, and in man, where there are no great canines
(as in male gorillas) to serve as organs of attack, it is
to the arms only that such an animal can look for
offensive or defensive organs.

Fig. 152.— Skeleton of the Left Fore Limb of a Pig (A) ; Hyomoschus or
African Deerlet (B) ; Tragulus or Javan Deerlet (c) ; Roebuck (D) ;
Sheep (E) ; Camel (F). (After Garrod.)

358 COMPARATIVE ANATOMY AND PHYSIOLOGY.

While the results of the erect position show that
man has been able to adapt his altered mode of pro-
gression to the mechanical conditions of an organisa-
tion best suited for quadrupedal movement, it is to be
noted that (1) the space between the
thorax and the pelvis leads to, and is the
cause of, prolapse and other affections of
the uterus, and of hernia in both sexes ;
(2) the carotids which supply the most
important of organs, the brain, have to
carry their contents against the action of
gravity, and, for this reason, they are of
large size. Or, to put it in another way,
the erect position entails certain positive
disadvantages.

In other members of various divisions
of Vertebrates, by far the most important
part in support or locomotion is often
undertaken by the hind limbs; this is
especially well seen in hopping or jump-
ing forms, as, for example, the frog,
where the tarsal bones are greatly elon-
gated and the digits of considerable
length; in the jerboa, where the meta-
tarsals are very long ; or the kangaroo,
where the calcaneum (c) is very long, the
cuboid (CB) very strong, and the meta-
tarsal of the fourth digit greatly elon-
gated ; in other words, we have here a continuous
series of well-developed bones lying along one axis,
and affording a firm support (Fig. 153).

When the extremities are used as seizing organs,
the pollex of the maims and the hallux of the pes are
qpposable on the other digits ; such an arrangement
obtains in the higher Primates, but in man, where the
foot has more of a supporting than of a prehensile
function, this power of opposition is lost in many races

Pes

Chap, ix.] FINS OF FISHES. 359

by the hind limb, though it can be regained under the
stress of necessity, or by education ; the saddle-shaped
form of the articular surface of man's trapezium gives
the mechanical reason for the power of apposition of
the thumb, which he possesses in so marked a degree.

In the tendons of the digits extra bones (sesa-
moids) are not unfrequently developed, and their
presence is no doubt to be explained by a refer-
ence to the primitively multiradiate condition (see
page 361) of the vertebrate limb; of such bones the
most constant is the patella (knee-cap), which is
found in all Mammals save a few Marsupials ;
another, which is very frequently found in the carpus,
is the so-called pisiform (or pea-shaped bone of the
human hand). The sesamoids are, as will be imme-
diately explained, most commonly developed in asso-
ciation with the digits ; thus, in the dog they are
found on each metacarpal ; in the fossorial armadillos
there is a large sesamoid on the palmar side of the
metacarpus ; two large palmar sesamoids are found in
Ornithorhynchus ; while in the just-mentioned Mono-
treme, as to a less extent in Echidna also, there is a
large sesamoid in the tarsus which supports the spur
of the foot, that has so remarkable a likeness to what
is found in the fowls and some other birds.

The paired fins of Fishes are, at first sight,
difficult to bring into alliance with the pentadactyle
limb of the higher Vertebrata. If we take the dog-
fish as a type, we find that the pectoral are larger
than the pelvic fins, and more complicated in cha-
racter. We will commence, therefore, with an account
of the latter. They lie horizontally, and approach
one another at the ventral median line. A long basal
bar (Fig. 154; A, bp) is articulated to a process of the
ilium, and bears on its outer side a series of rays, which
are each divided into a larger proximal or basal and a
smaller distal piece, almost parallel to one another;

Fig. 154.— A, Eight Pelvic Fin and part of Pelvic Arch of an Adult
Female of ScylUum canalicula (nat. size). B, Eight Pectoral Fin and
part of Arch of an Adult ScylUum canilicula.

co, Coracoid ; sc, scapula ; pp, protopterygiurn ; mep, mesopterygiura ; mp,
nietapteryprium ; il, iliac process ; pp, pubic process, cut across below ; bp,
basipteryprium ; /«, anterior fin-ray ; fn, part of fin, supported by horny fibre.
CAfter Balfour, P.Z.S., ]8:-l, p. 663.) '

chap, ix.] FINS OF FISHES. 361

the most proximal articulates directly with the ilium
(il), and the most distal is, in the male, converted
into the clasper. (See page 519.) The outer portion
of the integument of the fin is supported by horny
fibres (fn). The pectoral fin (Fig. 154; B) is at least
twice as large as the pelvic, and is placed horizontally,
but the two halves do not approach one another ven-
trally ; there are three basal cartilages, called respec-
tively (Gegenbaur) protopterygium (pp), meso- (mep),
and metapterygium (mp) ; the latter carries most of
the cartilaginous rays, and these are divided into a
larger number of pieces than the corresponding rays
of the pelvic fin ; as with it, the greater part of the fin
is supported only by horny fibres.

According to the observations of Balfour, the
paired fins arise as ridge-like thickenings of the epi-
blast (see page 33) ; the mesoblast that invades the
ridge gives rise to a cartilaginous bar, which, at first,
lies parallel to the long axis of the body. On one
side (the outer) of this bar a thin plate extends out-
wards, and this, by becoming divided, gives rise to
the primary fin rays ; this simple condition is essen-
tially retained in the pelvic arch; in the pectoral,
however, the basal bar becomes rotated outwards, so
that it is now only connected by its anterior end with
the pectoral arch, and the bar, in place of being the
basal portion, now forms the hinder border of the fin ;
the plate attached to the bar becomes imperfectly
divided into a smaller proximal and a much larger
distal piece ; from the edge of each of these, rays are
given off; the smaller piece undergoes a second divi-
sion, by which we have, at last, the protopterygium
(pt) with one ray, and the mesopterygium with
a few ; the rest of the rays are attached to the meta-
pterygium, or larger distal piece.

On the supposition that a many-rayed limb of
the characters just described is that from which the

362 COMPARATIVE ANATOMY AND PHYSIOLOGY.

pentaclactyle limb of the higher Vertebrata has been
developed, we must suppose that the greater number of
the cartilaginous pieces have undergone reduction, and
that, in the Ichthyosauria for example, or in the frog,
where there is a rudiment of a sixth digit to the foot, the
number five is exceeded in consequence of the re-
duction not having been definitively impressed on the
organism by inheritance ; on the other hand, the
possession by a Mammal (e.g. man) of more than five
digits (polydactylism) must be regarded rather as an
abnormality than as a return to an ancestral condi-
tion, and this because the gap between a man and a
polydactylous ancestor is too wide for us to be able
reasonably to believe in an " atavism " so far-reaching.

In the Ganoidei or Teleostei, the pterygial portions
of the fins are reduced, but the reduction is atoned
for by the replacement of the horny fibres by osseous
tissue. In Ceratodus the fin takes the form of a
central axis of cartilaginous pieces, with rays on both
sides ; and in Protopterus it becomes filamentar, owing
to the loss of the lateral rays. Gegenbaur regards the
fin of Ceratodus as the most primitive arrangement
(archipterygium) ; but, as Balfour has pointed out, this
view of the matter is opposed by the facts that in
Elasmobranchs there are indications of rays on one
side only of the basipterygium, and that the support-
ing bar is, at first, basal, and not central.

Like the limbs of higher Vertebrates, the fins of
fishes are, at first and in most cases, locomotor in
function, wherein they are aided by the tail ; just as
the former are supporting organs, so, too, are the fins.
This may be seen by removing the fins of one side,
when the fish falls on to that side ; or by cutting off
both pectorals, when the body inclines forwards and
downwards. In mud-dwelling fishes the pelvic fins
are rudimentary or absent, disuse producing degrada-
tion. One of the most remarkable modifications of

chap, ix.i EXTERNAL SKELETON. 363

the fins is seen in Periophthalmus, which, thanks
especially to its large pectorals, is able to hop over the
mud. In some Gobies the ventral fins unite to form
a kind of suctorial disc, by means of which the fish
can attach itself to rocks. The sucking disc of
Cyclopterus lumpus is supported by the rudimentary
spines and rays of the ventral fins. In the flying-
fish (Exocoetus) the pectoral fins may extend as far
back as the caudal, and can be spread out so as to
act like sails. In cartilaginous fishes, where the
edges of the fins are softer than in the bony fishes,
these edges may perform an undulatory or screw-
like movement. When the lateral fins disappear,
the locomotor function falls altogether on the vertebral
column and unpaired fins.

The External skeleton of Vertebrates is, in
the simpler conditions, formed by scales, which are
developed in the cells of the integument. The most
generalised condition obtains among Elasmobranchs,
where, as we have already learnt, the internal skele-
ton is throughout life cartilaginous ; in such a form
as the dog-fish the whole of the external surface is
roughened, owing to the presence of projecting
pointed processes, which have not inappropriately
been called dermal denticles, so close and strik-
ing is their resemblance to the processes which, when
placed within the area of the mouth, are called teeth ;
like them, they consist essentially of dentine invested
in a layer of harder enamel. In the huge basking
shark the whole of the body is covered by denticles,
which, taken separately, are small enough, but which
en masse must be a very effective means of defence.
In the spinous shark (Echinorhinchus) the diffused ar-
rangement yields to "one in which large spinous tuber-
cles are scattered over the body, and the value of that
diffused arrangement is very eloquently spoken to by
the naked body of the torpedo, which has found a still

364 COMPARATIVE ANATOMY AND PHYSIOLOGY.

better mode of protection in its well-developed electric
organ. In various Elasmobranchs the more prominent
fins are provided with strong ^spines.

The Ganoidei received their name from the posses-
sion by some of them of bright shining scales, which
owe their appearance to the investing layer of enamel.
Such "ganoid scales" are, however, found in
perfection only in Lepidosteus and Polypterus among
recent members of the group; in the sturgeon, for
example, there are bony plates, and Spatularia is
naked. In the two Ganoids first mentioned the
scales overlap, and the whole body is protected by
a closely and firmly set coat of mail. Among fossil
forms we find the typical ganoid arrangement some-
times carried to a remarkable extreme, as in Pterich-
thys, where large bucklers are found not only on the
dorsal but also on the ventral surface. The allied
Dinichthys is thought to have reached a length of
more than fifteen feet ; and we see in it, as in other
gigantic forms, such as the Irish elk, that individual
protection has been only attained at the cost of the
disappearance of the species.

The simpler smaller scales that are found in some
Ganoids, and very commonly among the Teleostei,
may be, when we look at extremes, classed under
the head of cycloid scales, in which the free pro-
jecting margin is rounded, or as ctenoid, in which
the margin, or part of the surface, is denticulated or
comb-like ; between these, however, there are a
number of intermediate stages ; the ctenoid scales may
be supposed to have given rise to those in which part
of the surface is continued into fine non-denticulated
spines (sparoid scales of sea-breams). While some
fishes, such as Stomias, have the scales deciduate,
and others, not to speak of electric forms, are,
like the eel, scaleless, the Teleostei, almost as much
as the Ganoids, present us with examples of forms in

chap, ix.] EXTERNAL SKELETON. 365

which the whole or the greater part of the body is
amply provided with a defensive armature. Such
forms are Osteoglossum and its allies, in which the
body is closely covered with hard scales, the " cofier-
fishes " (Ostracion), where the hexagonal scales fit ,
like the pieces of a mosaic, or the globe-fishes (Diodon),
where the whole of the globular body is covered by
projecting and movable spines, which, standing out
on erection, must most effectually protect their pos-
sessor.

The protective function of the exoskeleton of the
true fishes is replaced in the Cyclostomata by the
rich supply of mucous glands to the integument ; in
the hag this power is carried to so great an extreme
that a single example placed in more than three cubic
feet of water is able to shed out so much mucus that
the whole becomes converted into a continuous viscid
mass; with this power of emitting a sticking secretion we
may compare the " cotton-spinner," where, however,
no observations have yet been made as to the amount
of the secretion.

The Amphibia best known to us have a soft
unarmed integument, but the Csecilise, among recent
forms, have small cycloid-like scales in their integu-
ment, a few Urodeles have flat bony plates, and the
extinct Labyrinthodonta would seem to have had a
plentiful supply of well-developed ventral plates.

Among the Reptilia we have thickenings which
may merely form epidermic scales* as in snakes and
lizards, or larger bony plates (scutes), as in croco-
diles, or very extensive pieces, as in tortoises and
turtles. In the Ophidia the separate scales are held
together by the continuous epidermic covering to
which they owe their origin, and the whole is ordina-
rily shed in one piece ; the most remarkable modifica-
tion undergone by them is to be seen in the rattlesnake,
where the cuticular scales at the hinder end of the

366 COMPARATIVE ANATOMY AND PHYSIOLOGY.

body are converted into transversely oblong plates,
which, when moved rapidly on one another, give out a
rattling or vibrating sound. Lizards may be scaleless,
like the chamseleon, which possibly makes up for the
absence of the protective covering, such as it is, by
the power of so adapting itself to the coloration of
the neighbourhood in which it finds itself as to be
almost invisible ; or the Amphisbaena ; or there may
be thin scales, as in the true lizards ; or, as in Cyclodus,
bone may be deposited in the dermis, and the bony
plates may, as in the skink, unite into a mosaic-like
arrangement.

In the Crocodilia there are scutes as well as scales ;
that is to say, the dermis undergoes ossification ; and
the separate scutes are covered by an epidermic thick-
ening or scale. In a few (as the Caiman), the scales
on both the upper and lower surfaces become, respec-
tively, so united with their neighbours as to give rise
to a dorsal or a ventral shield ; on the long tail the
upper and lower ossifications unite to form continuous
rings. In most crocodiles, however, the ventral
shields are absent, and the dorsal scutes do not unite
with one another to form such continuous pieces as
can properly be called shields.

The differences between the horny epidermic
hardenings and the osseous dermal thickenings are
best exhibited by the Chelonia, where, as is well
known, large continuous pieces, both of shell and bone,
are ordinarily exceedingly well developed. The
thinner epidermic plates form the so-called tortoise-
shell, the thicker dermal bones the plates of the
shield, or carapace, which enter into close connec-
tion with parts of the endoskeleton.

In Birds, the outer covering is in the form of
feathers ; a feather consists of a central quill, shaft,
or scapus (Fig. 155; d), from which on either side
there are given off flattened branches, or barbs;

Chap. IX.]

FEA THERS.

the latter similarly give off much finer radii or
barbules, which, interlocking by "cilia" and booklets
with those that are found on. neighbouring barbs,

c/

Fig. 155.— Feather from the Back of Argus giganteus.

a, Shaft (rachis) ; 6, aftershaft : c, branches to forna the vexillum, removed
from one side of both shaft and undershaft ; d, shaft (scapus). (After

form the connected vane or vexillum of the feather
(c) ; the shaft, which in its upper portion is often
called the rliachis, frequently gives off near its base
a smaller feather or aftershaft (b}. It has been
calculated (by Gadow) that the feather of an eagle
contains about two thousand barbs, five millions and a

368 COMPARATIVE ANATOMY AND PHYSIOLOGY.

half of barbules, and fifty-four millions of cilia and
booklets. These feathers are not irregularly arranged,
but are set along definite tracts (feather tracts) the

arrangement of which
(pterylosis) varies in
various birds, and has,
since the time of Nitzsch,
been made use of in
classification (Fig. 156).
The function of
feathers is not limited
to the diminution of
the specific gravity of
the bird, which they
effect by entangling air ;
the same process is also
of aid in preserving the
high temperature of
these creatures, in con-
sequence of the feeble
conductive power of air.
So far as the former
effect is concerned, we
have to note that the
Ratite birds, which
never soar into the air,
are without the barbules
by means of which the
barbs form a connected
vane.

The hairs of Mam-
mals, like the feathers

of birds, are epidermic in origin, but their mode of
development is somewhat different. As a general
account of the structure of hair has already
been given in chap, xxxiv. of Klein's " Elements of
Histology," it is here only necessary to give some

Fig. 156.— Pterylosis, or arrangement
of Feather-tracts on the under
surface of the body of a Cock
(Gallus bmikiva). (After Nitzsch.)

Chap, ix.] HAIRS OF MAMMALS. 369

account of their arrangement in different forms.
Hair is almost entirely absent from the body of adult
Cetacea, and only scantily developed in the Sirenia ;
this common character must not, however, be regarded
as any evidence of community of origin or closeness
of relationship, but rather as the result of exposure
to similar conditions. Sometimes, when the hair is

Fig. 157.— The Armadillo.

scanty on the body, as in the rhinoceros, a number of
hair-like shafts unite to form a horn. In forms which
live in very cold climates, like the musk-sheep (Ovibos
moschatus), the hair is exceedingly long and thick, and
serves as an efficient protection against the external
cold ; the most striking example of this is afforded by
the thick coating of the extinct mammoth, which
lived in cold regions, whereas its allies, the elephants,
which, in recent times, are confined to warm coun-
tries, have but little hair. The soft hair may be
replaced by firm and strong spines, as in the porcu-
pine or hedgehog, where, thanks to their power of
Y— 16

37 o COMPARATIVE ANATOMY AND PHYSIOLOGY.

erection, they form very efficient organs of defence
and protection.

Sometimes the hairs become specially endowed
with a tactile function, as in the " whiskers " of feline
and especially nocturnal carnivora ; reminding us so
far of the elongated delicate filaments of, no doubt,
similar functions, which are found on the bodies of
deep-sea fishes.

The hairs may be greatly elongated, and used, as
in horses, for switches, by means of which their bodies
are freed from offending insects.

The claws found on the digits of various lower
Vertebrates are, as " nails," almost constantly present
in Mammals, where they may be flat, as in man, sharp
offensive claws, as in Carnivora, large protecting hoofs,
as in Ungulates, or organs of support to arboreal
forms, such as the bat or the sloth ; they are wanting
in the Cetacea.

The only Mammals in which long dermal scutes
are now developed are the armadillos (Fig. 157), where
three or more zones may be present, and form a more
or less complete protective covering for these animals ;
such scutes were present in enormous numbers in the
extinct Glyptodon and Hoplophorus.

CHAPTER X.

ORGANS OF MOVEMENT.

IN the Protozoa, where division of labour never
proceeds so far as to lead to the formation of definite
tissues, the function of locomotion, like all the rest,
is simply performed by the protoplasm of the cell,
which, as we have already learnt, is contractile.
Thanks to this power of contractility, even an

Chap, x.j MOVEMENTS OF PROTOZOA. 371

amorphous mass like an Amoeba is enabled, by the
withdrawal of one and the protrusion of another part
of its substance, to move about from place to place. In
the ciliated forms movement is due to the contractile
action or play of those delicate processes of proto-
plasm which form the cilia ; between the fine processes
that we ordinarily call by that name, and the coarser,
more lobate, processes that are distinguished as pseudo-
podia, the connection is very close, and under certain
conditions one form may be observed to pass into the
other. Among certain stalked Infusoria, such as
Vorticella, we observe a mode of movement which is
more rapidly executed than that of ordinary trans-
lation ; a Vorticella, or its branched ally, Carchesium,
may be seen to suddenly lower its bell, owing to the
rapid contraction in length of its stalk ; the agent by
which this is effected is a modified portion of the
protoplasm in the stalk (the so-called contractile band),
which presents a striation that calls to mind that of a
muscular fibre. Though agreeing with it functionally,
the stalk differs from it morphologically, in that it
is a modification of only part of a cell, and not of a
whole cell, or of a set of cells. These bands are not
confined to the stalked Infusoria, but are found in
other forms both of the Ciliata and of the Gregarinida;
without • them, indeed, there can be but feeble move-
ments in the latter endoparasitic organisms, which are
without either cilia or pseudopodia.

Among the lower Metazoa we find that the
movements of the young are at first effected not by
muscular tissue, but by cilia ; the free-swimming larva
being provided with cilia, which may be scattered
over the whole of the body, or confined to certain
definite and characteristic tracts, such as circlets, one
or more in number, or wavy bands (Fig. 158). In all
groups, save that of the Porifera, the cilia are found
on the outer surface of the body or epiblast, and in

372 COMPARATIVE ANATOMY AND PHYSIOLOGY.

all but it, the members of which are always fixed
when adult, a definite tissue, or collection of cells,
becomes specially endowed with a contractile function,

and forms muscular
tissue, and a more
or less regularly
disposed muscular
system. (For the
minute structure of
muscle see " Klein's
Histology," chaps,
viii. and ix.)

In Hydra, among
the Ccelenterata,
the only indications
of muscular tissue
are the branched
prolongations in-
wards of certain of
the cells of the
ectoderm (neuro-
muscular cells of
Kleinenberg, or,
more shortly, Klei-
nenberg's cells) ; in
it the several cells
of the body still re-
tain their indepen-
dent contractility.
In higher forms the
epithelial ingrowths
become more independent, and in the Medusae they
become transversely striated. In these last they form
a sheet on the lower face of the disc or umbrella, which
in living specimens is repeatedly opening and closing ;
they are continued into the tentacles, and when a
velum is present they are largely developed in it.

Fig. 158. — Larva of Holothuria tubulosa in
its natural position.

The arrow indicates the axis of rotation, and
the cilia are seen to be arranged in a sinuous
band. (From Carpenter, after Selenka.)

Chap, x.] MOVEMENTS OF CCELENTERATA. 373

In the Actiniae it is possible to distinguish a system
of longitudinal from one of transverse muscular fibres,
and the presence of these two explains how it is that a
sea-anemone is able, when irritated, to diminish both
in length and breadth ; the longitudinal muscles are
the best developed, and may be seen to be arranged in
definite bundles; the transverse are strongest in the
region of the base of the polyp (Fig. 54). The ten-
tacles owe their contractility to the possession of
muscular fibres.

In the Ctenophora, which retain an external
investment of cilia along the lines of their " cteno-
phoral plates," the greater part in the production of
movements of the body is not effected by the muscles,
which are poorly developed in the ectodermal layer,
but by the contractile fibres which are developed in
the mesoderm, which is so richly developed in the
Ctenophora ; as seen in Beroe, these muscles are long
cylindrical cords, which are not united into bundles,
and are disposed radially, circularly, and longitudinally.
The greater number are, like Cydippe (Fig. 15), pro-
vided with a pair of long tentacles, in addition to which
other smaller or secondary tentacles may also be
present ; the axis of these is occupied by a cord of
muscular fibre; their most important office is, no
doubt, not one of locomotion, which is effected chiefly
by the ciliated paddles, but of prehension, for where
they are absent, the mouth is much wider than it is
in those that possess them.

Many of the lower Worms move by the elonga-
tion of the anterior end of their body, which is suc-
ceeded by a contraction by means of which the hinder
part is brought to its original point of distance from
the anterior ; in the performance of this operation
they are sometimes aided by one or more cup-shaped
suckers, by means of which a fixed point is gained ;
others, like the leech, fix themselves by their hinder

^^COMPARATIVE ANATOMY AND PHYSIOLOGY.

sucker, and sway about or elongate their body so as
to reach their prey. In the flat- worms and in the
leeches there are longitudinal, circular, and transverse
muscles. In all the rest of the Annulata, and
in the Gephyrea, there are only circular and longitu-
dinal bands in the body wall, the former of which are
the more external ; but, in addition to these, there are
smaller muscles which are of considerable importance
in locomotion, as they are inserted into the base of
the setse, and are the means by which these processes
are moved forwards and backwards, or used as parts of
a locomotor apparatus, working either as mere stilts as
in the earthworm ; or, as in the Polychneta, where they
are numerous, like oars in the free-swimming forms,
and as climbing hooks in those that live in tubes.

Among the Echinodemiata the most impor-
tant organs of movement are the contractile tube feet,
which are most valuable when, as often happens in
the Starfishes, or the Urchins, they are provided at
their free ends with a sucker-shaped enlargement by
means of which they can gain certain fixed
points to which they can draw their bodies. When
climbing up vertical, or almost vertical, heights, the
Echinoderin converts its pedicellarise (see page 297),
which are provided with special muscles, into organs
of locomotion, in so far that these pedicellarise seize
hold of waving fronds of sea-weed, which act, there-
fore, like the rungs of a ladder, up which one is
climbing by the use of the hands only ; it is of parti-
cular interest to observe that " the wonderfully tena-
cious grasp of the forceps is timed as to its duration
with an apparent reference to the requirements of the
pedicels (tube feet), for after lasting about two
minutes, which is about the time required for the
suckers (tube feet) to bend over and fix themselves
to the object held by the pedicellarise, if such should
be a suitable one, this wonderfully tenacious grasp

chap, x.] MOVEMENTS OF ARTHROPODA. 375

is spontaneously released " (Romanes and Ewart).
Ordinary Ophiuroids, which, according to the authors
just qiioted, are able to move along at the rate of six
feet a minute, have a certain wriggling power of
their arms, which, in the Astrophytidse, is converted
into a power of coiling for the purposes of attach-
ment, thanks to the fact that the faces of every one
of their arm joints are convex in one direction, and
concave in that which is at right angles to it. When
the spines are long, as in the piper (Dorocidaris),
where they are also of considerable stoutness, or in
Spatangus, where they are much more delicate, they
can be used as stilts, owing to the attachment of
muscular tissue to their bases.

In the Artliropoda the function of locomotion,
like so many other functions in that group, falls very
largely upon the appendages, which may either act as
walking or as swimming organs. In the Crustacea,
where all but the first pair are typically biramose,
this locomotor function is seen in the early Nauplius
condition (see page 534), when even the antennae take
part in performing that duty ; these appendages, being
jointed and provided internally with muscles, are able
to move in various directions. At first, and in the
lower forms, they act more or less like oars, beating
the water as they move backwards and forwards. In
the higher forms, such, for example, as the crayfish,
the more anterior of the locomotor appendages act as
walking, and the more posterior as swimming organs.
In an appendage, which, has been but little
modified, and which may be regarded as typical, such
as the pair formed on the third abdominal segment,
we see a doubly-jointed basal piece or protopodite,
bearing two terminal pieces, the outer exopodite and
the inner endopodite. These pieces, which are fringed
with long bristles, or setae, are flattened, and can act
like oars.

376 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Those appendages of the thoracic region which are
confined to a locomotor function have the form of an
elongated jointed bar, consisting of seven joints, which
have received the following names : the first two,
which appear to correspond to the protopodite of the
typical appendage, are called coxopodite and basi-
podite ; the remaining five, which may be supposed to
represent the endopodite, are
ischio, mero, carpo, pro, and
dactylo-podites.

While these ambulatory
appendages move in a plane
which is parallel to the long
axis of the body, those of the
abdomen swing backwards a ad
forwards, owing to the fact
that the abdominal segments
are, unlike those of the thorax,
which is invested by the hinder
part of the great carapace,
capable of being moved on one
another. This movement of

segments is brought about by two great bundles of the
muscle, which lie respectively above and below
the intestine, and are attached to the tergal and
sternal plates of the separate segments. It follows,
from their mode of attachment, that the contraction
of the upper muscles straightens out or extends the
abdomen (tail), and that a contraction of the lower
muscle tends to bend in or flex the same parts. An
alternate contraction and relaxation of these muscles
tends therefore to an alternate bending in and
straightening out of the " tail," and therefore to
repeated beats of the water, by means of which the
crayfish or lobster is driven through it. In the
performance of this locomotor action the " tail " is
greatly aided by the modification of the appendages of

Fig. 159.— Third Abdominal
Segment of the Lobster.

f, Tergura (dorsal piece) ; «,
sternum (ventral piece); pi,
pleuron ; p, protopodite ; ex,

• exopodite ; en, endopodite.

Chap. x.]. -LEGS OF ARTHROPODA. 377

the penultimate segment of the body, which, in place
of being comparatively small parts, as in the typical
third abdominal appendage, are widened out into
more considerable plates, which have a backward
instead of a downward direction ; these unite with
the terminal segment, which sometimes, though very
rarely (Scyllarus), bears minute appendages, to form the
powerful flapper of the Crayfish.

Peripatus, the species of which vary considerably
in the number of appendages, have these organs only
imperfectly jointed, and they move but slowly ; in
them, as in all Arthropods other than the Crustacea,
the limbs are uni-, and not bi-ramose, but, as often
happens, they are provided with a terminal claw.

The Myriopoda (Centipedes), as their name
indeed implies, have a large number of walking limbs,
each of which has essentially the same characters as
that which precedes and that which follows it ; in
the Millipedes a number of segments carry two pairs
of legs each. The Araclmida have four pairs of
walking limbs, which are completely lost in such
endoparasitic forms as Pentastomum. The Insects, or
as they are very appropriately called, the Hexapoda,
have three pairs of walking limbs ; these are typically
composed of a coxa, a trochanter, a femur, a tibia,
and a six-jointed tarsus, which ends in a pair of claws ;
the larval or caterpillar forms have, however, a more
or less larger number of walking appendages, or pro-
legs ; these are best and most numerously developed
among the Lepidoptera, but they are in all cases
rudimentary as compared with the legs of the adult.

A large number of insects have yet another set of
locomotor organs, in the shape of the dorsally-placed
wings ; of these there are never more than two pairs,
and of these both may be rudimentary, as in the
female cockroach ; or the anterior pair only may be
developed as in the Diptera (flies), or the hinder alone

378 COMPARATIVE ANATOMY AND PHYSIOLOGY.

be functional as in the Coleoptera (beetles). In no
case are they developed except on the second and
third segments of the thorax (rneso- and meta-
thorax).

A wing consists essentially of two flattened
A

81

Fig. 160.— Skeleton of Butterfly's Wing.

A, Costalvein ; B, subcostal ; mi m*, median branches ; ri 7-2, radial ; si s5, subcostal
branches; c, median vein ; D, submedian vein ; dc. discocellular veinlet ;
in, internomedian veinlet ; B, internal vein. (After Butler.)

membranes, the presence of which is due to the organ
arising in the form of a sac, which gradually becomes
elongated and flattened out ; through its substance
there pass blood-vessels and air tubes, the walls of
which are strengthened by chitin ; the chitin may
invade the rest of the wing, and convert it into a more

chap, x.] WINGS OF INSECTS. 379

or less horny body ; this process, when carried to an
extreme, ends in the stout wing-covers (Elytra) of the
beetle. These tracheal tubes are the " veins " of
entomologists, and the finer branches the so-called
nervures.

These wings, when expanded, beat the air by
being moved forwards and backwards on their point
of articulation to the thorax ; this is effected by special
flexor and extensor muscles, the number of which is
considerable, and each of which consists only of a few
fibres; in considering, however, the causes which give
their particular direction to the wings as they move
in flight, due attention must be given to the effects of
the resistance of the air which is beaten by the wing,
which, as a matter of fact, follows a figure of 8 course.
In studying the mechanism of the wing we have to
bear in mind that the essential points are a rigid
anterior nervure, and a flexible membrane behind
(Marey).

Insects vary considerably in the number of move-
ments of the wing per second, as may be seen by the
following table, which we owe to Marey :

Common Fly 330

Drone-fly 240

Bee 110

Wasp 130

Humming-bird Moth 72

Dragon-fly 28

Butterfly (Pontia rapes) .... 9

Among the Apterous insects, or those Hexapods
in which wings have never been developed, and which
must be carefully distinguished from those that have,
owing to parasitic habits and so on, lost wings, which
were possessed by their ancestors, the Collembola or
Spring-tails are remarkable for the possession of a
fork-like appendage to the hinder end of their abdomen,
which can be bent backwards, and act like a spring.

380 COMPARATIVE ANATOMY AND PHYSIOLOGY.

In the Mollusca the characteristic organ of loco-
motion is the foot, which is made up of muscular
fibres, which are transversely striated, but are
spindle-shaped cells, and so have the general form of
unstriated muscular tissue. This foot, which can be
withdrawn into the shell of such forms as Anodon by
retractor muscles attached anteriorly and posteriorly,
seems to be protruded or put into a state of erection
by an increased flow of blood into its substance, and
not, as has sometimes been supposed, by the intaking
of water from without. While it has a somewhat
conical or hatchet- shaped form in the fresh-water
mussel, and in those Lamellibranchs which move about
with some activity, it is very strong in boring forms
such as Solen, and long and curved in Trigonia, where
it is used as a leaping organ ; on the other hand, it is
very small in the scallop (Pecten), and quite incon-
spicuous in the still more sedentary oyster.

Among the Gastropoda the foot has often, as in
the common snail, a broad disc-like lower surface, and
is adapted for creeping or crawling. When the snail
is in movement waves of contraction may be seen
passing over the lower surface of the foot from behind
forwards, and it has been found that smaller have
greater locomotor power than larger forms. Within
limits, snails are able to carry weights, and it follows,
therefore, that unloaded snails do not make use of all
the activity of which they are capable.

The foot may become modified in a most remark-
able manner, as, for example, in the Heteropoda,
which are forms found only on the surface of the
ocean ; the animal swims with its shell downwards,
and its foot (Fig. 161 ; f) is converted into a high
crest-like fin, which is no doubt aided functionally by
the fin-like prolongation of the hinder end of the
body.

In the Pteropoda the sides of the foot become

chap, x.] MOVEMENTS OF CHORD AT A. 381

greatly enlarged, and form distinct epipodia, and
these, either independently or in conjunction with the
median part of the foot, become converted into
powerful muscular fins. In the Cephalopoda the
epipodia form a funnel, through which the water of
respiration is expelled to the exterior ; this expulsion
of the water forwards results in a backward movement
of the animal. In the Tetrabranchiata (Nautilus) the
edges of the epipodia
are not, as in th
Dibranchiata, fused
with one another, but
merely folded over.

Among the Chor-
clata locomotion is

effected by swimming, rte- 161.-Cfcrroewta cymlium.

i n m m* r> a- r»T>PPr>in o- m> Proboscis ; t, tentacles : /, foot ; d, disc ;
jumping, eeping, s, shell; g, branchiae.

walking, or flying, and

all these activities are presented by marine as well as

by terrestrial forms.

Swimming organs have the form of more or less
broad plates, which may or may not be supported by
bone. The simplest cases are presented by the mere
flattening or expansion of an organ ; this, for example,
obtains in the tadpole, the newt, or the insectivorous
form Potamogale velox, where the tail is flattened
from side to side and forms a powerful locomotor
organ. In the Cetacea, on the other hand, which
require to come repeatedly and rapidly to the surface
of the water, the tail is flattened from above down-
wards.

In more complicated cases, as in many fishes, the
tail, which is here also the most important organ of
locomotion, and has a screw-like movement, is aided
by the caudal fin when that is well developed ; the
paired lateral fins are in most cases rather organs of
support and direction than of locomotion ; but in some

382 COMPARATIVE ANATOMY AND PHYSIOLOGY.

cases, as in the Rays, movement is almost altogether
effected by the undulation of the margins of the enor-
mous pectorals. In flat-fishes and eels locomotion is
due to the undulations or curvatures of the whole
body.

In other aquatic Vertebrates, such as the marine
turtles, the penguins, or the whales, where the limbs
take some share in their movements through the water,

Fig. 162.— Exoccetus volitans.

the tendency is for the flipper to become a more or
less rigid organ, movable only at its point of attach-
ment to the body. The series of modifications which
lead to this arrangement are very well seen among
the Clielonia. In the marsh tortoises the digits are
united by a web, but each digit has a claw ; in the
mud tortoises the limbs are flatter, and there are claws
on only three of the digits, while in marine turtles
the still more flattened digits are united by a common
covering of skin into a more rigid paddle, and only
one or two claws are found. In the penguin the
wings are converted into firm paddles, movable only

Chap, x.] FLYING ORGANS. 383

at their base. In the Cetacea all the bones of the
fore limb are united in a common integument, and
form the " flipper."

Some forms escape with rapidity by making
bounds or jumps ; of these we have examples in the
frog, the kangaroo, or the Cape jumping-hare, in all of
which the hind limbs are much stronger and longer
than the fore limbs.

Creeping or crawling is best seen in the snakes,
which move along the ground by the backward and
forward movement of their ribs, which they use as
stilts.

Flying organs are found among fishes in Exoccetus,
where the pectoral fins are so greatly elongated as in
some species to reach as far back as the caudal fin ;
the fins are not actively moved, and seem to have no
power of turning the fish to the right hand or the
left ; they cannot fly far at a time.

Similarly modified pectorals are found in Dactyl-
op terus.

Among recent Reptiles, Draco volans, the dragon,
or flying lizard, is capable of short movements through
the air, owing to the prolongation of some of its ribs,
which, when covered with the skin, form a semi-
circular wing on either side of the body. The extinct
Pterosaurians (Pterodactyle) had the outer digit of
the manus as long or longer than the rest of the fore
limb ; and there is evidence that, as in the bats, the
integument was produced on either side into a mem-
brane, the outer edge of which was attached to this
digit, and so formed an expansion, by means of which
the creature was enabled to support itself in the air.

Among Mammals the organs of flight are best
developed in the Chiroptera (bats) (Fig. 163), where
they are formed by the modification of the skeleton,
and especially of the fore-arm (see Fig. 149), and by
the extension of the integument into the so-called

384 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Chap, x.] FLYING ORGANS OF MAMMALS. 385

volar membranes. These, when best developed, con-
sist of (1) an " antebrachial membrane," which extends
from the shoulder to the base of the thumb ; (2) the
" wing membrane," which extends from the sides of
the body along the fore-arm and between the elongated
digits of the manus ; (3) an " interfemoral membrane,"
which is attached to the hinder end of the body, and
to the sides of the leg as far as the heel, and in some
as far as the phalanges of the foot. The relation of
the antebrachial membrane to the power of flight is
spoken to not only by the extent of its development
in flying forms, but also by its reduction in such as
are best fitted for terrestrial progression. The most
important function of the interfemoral membrane
would appear to be that of acting as a rudder ; this
power is greatest when, as in the Molossi, the bat is
able to vary the extent of the membrane, for this
"must confer upon them great dexterity in quickly
changing the direction of their flight, as when obliged
to double in pursuing their swiftly-flying insect prey "
(Dolson).

Less well-marked powers of flight are possessed by
the aberrant insectivore Galeopithecus (the so-called
" flying lemur "), which has been observed to move
through seventy yards of air, and in which the two
pairs of limbs and the tail are connected together by
an expansion of the skin, which forms a parachute-
like enlargement ; this is not, however, merely mem-
branous, as in bats, but is hairy on either surface.
Among the Rodents the flying squirrel (Pteromys),
and among Marsupials the flying phalanger (Petau-
rus), have the fore and hind limbs connected by a fold
of skin, which, when the limbs are extended, forms a
similar kind of parachute, but it does not reach to the
tail, nor is their patagium provided, like that of Galeo-
pithecus, with any special muscles.

Organs of flight are, among the Yertebrata, best
z— 16

386 COMPARATIVE ANATOMY AND PHYSIOLOGY.

developed, as we know, in certain Birds • the skeleton
of the fore-arm is specially modified (see page 352), and
forms for the wing a firm anterior bar, comparable to
the anterior nervure of the insect's wing ; this bar is

moved by special mus-
cles, which are at-
tached near its base ;
but all of which lie
on the ventral or
lower surface of the
body, and thereby
enable the centre of
gravity of the bird
to be lower than it
would be were the
extensor muscles of
the arm placed, as in
other Vertebrates, on
the dorsal surface. A
large surface of at-
tachment for the pec-
toral muscles is ob-
tained by the great
development in flying
birds of the keel of
the sternum (see page
347), and the extensor
muscle works on a
pulley. The greater
portion of the wing is

not formed by membrane or integument, but by the
development of those integumentary structures which
we call feathers. These feathers overlap one another
in such a way that the wing is convex above and
concave below, and that pressure from below forces
the feathers more closely together. From this arrange-
ment it is clear that in the up and down movement of
the wing in the air, much greater effect is gained by

Fig. 164.— The Common Swift.

Chap. XL] VOCAL ORGANS. 387

the down-stroke than by the up-stroke ; for, in the
first place, the pressure of air on a concave surface is
always more effectual than that on a convex ; an
umbrella, for example, may be blown inside out, but
never outside in ; in the next place, the pressure of
the air against the separate feathers welds them into
a connected whole, while in the up-stroke the air not
only meets with a convex surface from which it may
flow away, but it is also able to escape between the
separate feathers. The influence, therefore, of gravity
is overcome by the greater value of the down-stroke,
and by the diminution of the pressure of air in the
less valuable up-stroke, which can be made more
rapidly than the down-stroke. A further inquiry
into the complicated question of the mechanism of
flight would lead us beyond the scope of this work.
In some cases the tail feathers, by being raised, de-
pressed, or turned a little to one side, are able to give
an upward, downward, or oblique course to the bird.

CHAPTER XL

VOCAL ORGANS.

UNDER the head of vocal organs we may group all
those which produce distinct and definite sounds to
the human ear, or which may be supposed to similarly
affect the auditory nerves of other animals.

These organs are never developed among the lower
Metazoa ; indeed, so far as we know at present, they
are confined to the Arthropoda and the Vertebrata.
Among the Crustacea they have been detected in
Palinurus.

In several orders of Insects they are confined to
the male sex, and appear, therefore, to be means, as

388 COMPARATIVE ANATOMY AND PHYSIOLOGY.

they are also no doubt in birds, by which the male
may attract the female.

They are so commonly developed in the Ortliop-
tera (grasshoppers and crickets), that the arrange-
ment which obtains in one member of this order may
be conveniently taken as a type. In Macrolyristes

imperator (Fig.
165 ; A and B)
we observe that
the hinder bor-
der of the right
wing (s) is thick-
ened in such a
way as to act as
a cord, and that
another part of
the wing (m) is
converted into a
tense membrane.
The left wing (B)
has its lower
surface rough-
ened like a file
along one line ;
this file is
brought to rub
upon the thick
cord (s) of the
right wing, and

so sets the membrane (m) in vibration ; vibrations are,
of course, conveyed to the air, and, being regular and
definite, they set up vibrations in the air which, on strik-
ing the auditory nerve, give rise to the sensation of more
or less musical sounds. Somewhat similar structures
are to be found on the wings of the locust, and in the
field cricket ; in the latter the two wings are similar
in structure, and their movement on one another can,

Fig. 165.— The Sound-producing Organ of the
Orthopterous insect Macrolyristes imperalor.

A, Upper view of right wing ; s, cord ; m, membrane ;
B, lower view of left wing ; b, roughened edge.

Chap, xi.] VOCAL ORGANS OF INSECTS. 389

therefore, be reversed. In the grasshoppers the sound-
producing organs are developed not on the wings but
on the legs, the upper joints of which are provided
with rather less than one hundred minute denticles
which scrape on the wings ; in the males of an allied
form (Pneumora), the legs are rubbed against a notched
ridge which is developed on either side of the abdo-
men, and the resonance is greatly increased by the
whole body being distended with air. In most cases
among the Orthoptera the males are alone vocal, and
the object of the use of these organs is, no doubt, that
of attracting the female.

In the hemipterous Homoptera, of which the Cicadas
are members, and of which the males are alone vocal,
the sound seems to be produced by the vibration of
membranes, placed on either side of the stigmata of
the metathorax, and set in motion by the respiratory
air.

The Hymenoptera, among which are the bees that
hum, would appear to produce sounds by the move-
ment of the abdominal segments on one another ;
these, as Mr. Darwin has observed, are marked with
very fine concentric ridges, such as are found also on
the thoracic collar, with which the head articulates.
Among the Coleoptera (beetles), there are forms such
as the carrion beetles (Necrophorus), and others
which make very distinct sounds ; these are ordinarily
produced by rasped ridges, which are placed on various
parts of the body and worked against the edges of the
elytra or wing-covers ; or parts of the leg work against
ridges on the abdomen ; or the elytra are ridged, as in
some of the water beetles ; or, lastly, two of the seg-
ments of the thorax may work on one another ; in the
latter case the ridges may be developed either on the
upper or on the lower surface. The vocal or stridu-
lating organs of Coleoptera appear to be equally or
nearly equally developed in both sexes, and it is rare

390 COMPARATIVE ANATOMY AND PHYSIOLOGY.

for the male to be much better provided with them
than the female.

Sound-producing organs are much less common
among butterflies and moths, and where present,
they seem to be due to the vibration of a membrane,
and not to the movement of a rasping organ, as in
beetles.

Among the Yertebrata, voice, or definite and more
or less musical sounds, are ordinarily produced by the
vibration of the column of air which passes down the
trachea and sets in movement the membranes (vocal
membranes), which lie on either side of that por-
tion of the trachea which is distinguished as the
larynx ; they are supported by definite cartilaginous
pieces (arytenoid cartilages), and bound a narrow
cleft which is known as the glottis. While this
simple condition is, for example, found, among the
Amphibia, in some frogs, others have well-developed
sacs connected with the larynx, which become swollen
out and project on either side of the head ; these sacs,
which are often better developed in males than females,
take an important share in increasing the noise made
by their possessor, which may sometimes be heard at
a great distance.

Among Reptiles, where the laryngeal apparatus is,
on the whole, comparatively simple, the chameleons
are provided with air sacs, which do not appear 011
the surface of the animal as they do in the edible
and some other, though not all, frogs.

Birds are remarkable for the fact that their vocal
organ is not, as in other Vertebrates, formed at the
top, but at the bottom of their trachea, and at the
point where the trachea divides into the two bronchi ;
the syrinx, as the organ of voice in birds is called, is
best developed in the Passeres, where a share in its
formation is taken both by the trachea and by the
bronchi (broncliio-traclieal syrinx).

Chap. XI.]

VOCAL ORGANS OF BIRDS.

391

Some of the lower rings of the trachea unite to form
a tympanic chamber ; the tracheal orifices of the two
bronchi are separated by a membranous septum, and
on either side there is a tympaniform membrane
formed on the inner side of the uppermost bronchial
rings ; the air which passes through the bronchial clefts
sets in vibration the membranes which bound them, and
the character of the note produced is affected, on
purely physical principles,
by the position of the
bronchial half-rings, and
by the length of the
column of air in the
trachea. The position of
these half-rings is not
constant, owing to the
fact that they are moved
by proper muscles, which
act on their ends, and so
rotate them.

In Steatoriiis (one of
the night-jars), the syrinx
is completely bronchial,
the fifteenth and sixteenth
bronchial rings being only

half-rings, as are also some that follow them ; the
space thus formed is filled in with membrane, which
can be rendered tenser by the contraction of the
lateral muscle of the trachea, which is attached to the
middle of the fifteenth ring. In those American
crows in which the syrinx is completely tracheal, we
have an arrangement which is essentially similar.
Among the Katite birds the syrinx is best developed
in Rhea ; in the American vultures the voice organ is
found in its simplest condition.

It is obvious that the length of the trachea must
have a very considerable influence 011 the character of

Fig. 166. — Larynx of Peregrine
Falcon. A, Front view; B, in
section.

392 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the note or notes emitted by a bird ; but as yet we
have no definite information as to the meaning of
those convolutions of the trachea which are so com-
mon in swans and ducks, and sometimes give to
the tube the appearance of a French horn (Darwin).
In some grouse resonance is aided by the development
of air sacs which are capable of inflation ; the great
throat-pouch of the European bustard appears to
have a similar function.

Among the Mammalia the larynx becomes re-
markably complicated, a number of special cartilages
being developed, which are connected together by
ligaments, and moved on one another by special mus-
cles ; the whole function of this apparatus is to alter
the form of the slit of the glottis, and to increase or
diminish the tension of the vocal cords. As this sub-
ject has been already dealt with in chap. xv. of the
" Elements of Human Physiology," we have here only
to point out that Mammals differ greatly in the sounds
that they make, the dog barking, the cat mewing, the
lion roaring, but that most agree in using the voice
more at the breeding season than at any other ; a few
mammals, such as the American Hesperomys cornutus,
and the gibbon (Hylobates), which, it is interesting
to observe, is one of the anthropoid or man-like apes,
may be distinctly said to sing. Man is remarkable
for his capacity for producing not only sounds, but
articulate speech, the wealth and extent of which is
much greater in the higher than in the lower races of
his species.

In Fishes, sounds, when produced, are of course
but rarely associated with the passage into the air
bladder ; but Ceratodus has been observed to make
grunting noises, which are possibly involuntary.
Various Cyprinoid and Siluroid fishes are known to
make sounds, and in Callomystax, Haddon has dis-
covered that the agent by which they are produced

Chap, xii.] THE NERVOUS SYSTEM. 393

are the anterior neural spines ; these rub on the suc-
ceeding and more solid portion of the vertebral
column.

CHAPTER XII.

THE NERVOUS SYSTEM AND ORGANS OF SENSE.

THE nervous system of an animal is the apparatus
by means of which it becomes acquainted with what
is going on around it, is enabled to distribute that
information throughout itself, or to bring it to some
central region, and to set itself in proper relation to
the surrounding medium. In consequence of this
relation to the outer world, we -find that the system
is, at first, superficial in position and diffused in
arrangement, that is to say, it at first lies in the outer
layer of the body, with which, indeed, it at all times
remains closely connected ; and that, primitively, the
system is more or less equally distributed throughout
the whole of the organism.

As we know, the Protozoa have no definite
nervous system, but we have already learnt than an
Amoeba is so far sensitive that stimuli applied to its
surface are followed by changes in the disposition of
its protoplasm. Nor have we any knowledge of a
nervous system in Sponges. (See page 431.)

In all other groups of the Metazoa we have
evidence of the presence of cells set apart for the
genera] function of informing the organism of what
is going on around it.

When we inquire as to what are the essential con-
stituents of a nervous system, we find that they are
either central (ganglionic) cells or conducting
fibres ; and, as we advance through the scale of
organisation, we observe that both cells and fibres

394 COMPARATIVE ANATOMY AND PHYSIOLOGY.

undergo aggregation, so that a diffused or scattered
arrangement makes way for one in which we have
definite nerve centres and well-marked lines, along
which, and along which only, nervous impulses will
pass. The most important of the aggregations of
nerve cells form the brain, or cerebrum, the most
important of the fibres the nerve cords; and just
as nerve fibres going to or coming from the latter are
associated with them, so there are secondary masses
of ganglia which are connected with the former.

In the next place, information from without is
gained from specially-modified cells, sense cells ;
these belong to the epithelial region of the body, and
are derivates of the epiblast.

The most generalised and widely-distributed sense
cells are those which belong to the sense of touch, the
so-called tactile cells; next we have those which,
only a little more complex, are confined to the ante-
rior region of the digestive tract ; these are the
gustatory cells, or those that subserve the sense of
taste ; thirdly, we have the more complicated organs
of the three higher senses, smell, sight, and
hearing^. When a brain is developed, all, or such
of these organs as are present, send to it by the
nerve fibres messages from the outer world ; in it
the messages are converted into more or less distinct
sensations, and from it fresh messages are sent out to
the different parts of the body.

The relations of the sensory cells to the epithelial
layer are particularly well seen in some of the
Coelenterata ; for example, in the sea-anemones
(Tealia; Fig. 168), some of the cells of the epithelial
layer have their free end continued into a fine
stiff" process, which projects outwards ; the inner or
basal end of such cells breaks up into finer pro-
cesses, which branch towards their ends. The free
projecting process may be justly regarded as a

Chap, xii.] NERVOUS SYSTEM OF CCELENTERATA. 395

sense hair, which, acted on by movements in the
water, and communicating with the body of the cell,
is able to bring the animal into relation with the
outer world.

In the sea-anemones the basal processes of the cells
have been observed to be continued into a layer of

Fig. 167.— Part of the submuscular pleius of Amelia aurita, showing
ganglionic cells. (After Schiifer.)

fibres, which are, to all appearance, nervous in nature.
Well-developed ganglionic cells are to be found
scattered in the layer of nervous fibres which sur-
rounds the mouth.

While in Aurelia and other Acraspedote Me-
dusae the central part of the nervous system consists
of isolated ganglia, ordinarily eight in number, the
Craspedote Medusse, or those in which the edge
of the bell is provided with a velum, have a more
definite central system; the epithelial coverings of

396 COMPARATIVE ANATOMY AND PHYSIOLOGY.

both the upper and lower surfaces of the velum have
some of the constituent cells converted into sense
cells ; the basal ends of these are of some length, and
pass into a nervous ring which runs round the edge
of the bell. The several sensory cells are thereby

brought into connection
with one another, and the
consentaneous action of
all parts of the jelly-fish
is thus ensured.

Underlying the epi-
thelium of the lower sur-
face of the bell, and placed
between it and the mus-
cles, is a network of nerve
fibres, among which there
are scattered ganglionic
cells (Fig. 167); this net-
work is connected with
the marginal nerve-ring.
Here, then, we have a
simple example of an
aggregated central ner-
vous system, together
with a peripheral
system of fibres and cells,
which is diffused over the whole of the under surface
of the bell of the medusa. Some of the Craspedota
(e.g. Carmarina) present us with an important advance
in structural differentiation, for some of what, in all
other particulars, resemble the sense cells, are found to
have lost their free projecting process, and to be now
moved a little away from the surface of the body.
Here, then, we have nervous epithelial cells which
are beginning to lose their superficial position, and
sinking deeper into the substance of the or-
ganism..

Fig. 168. — Transverse Section
through aTentacle of Tealia cras-
sicornis ; to show (a) Sensory
Cells with their free Projecting
Processes, and their Bases con-
tinued into the Nervous Layer;
(b), supporting cells.

Chap. XII.]

MEDUSM.

397

Experiments on Medusae show that the seat of
spontaneous activity is confined to the edge of the
bell in the Craspedote Me-
dusse, and to the region of
the marginal sense organs
in the Acraspedote forms;
if the extreme margin of the
bell of the former be com-
pletely removed, there is
immediately a total and
permanent paralysis of the
entire organ ; in the latter,
removal of the marginal
bodies is sufficient to pro-
duce a similar effect. The
results of these experiments
are, then, in
complete ac-
cordance with
the anatomical
facts. The dif-
fused plexiform
arrangement of
the nerve fibres
is, further, spo-
ken to by the
following ex-
periment : if all
the marginal
sense organs but
one be removed,
and if deep
sections be made in the substance of the bell,
so as to, at any rate, separate the nerve fibres at
many points of their course, it is found that the
bell is still capable of contraction ; or, in other
words, the stimuli sent out from the sole remaining

398 COMPARATIVE ANATOMY AND PHYSIOLOGY.

centre are able to diffuse themselves over the whole
substance of the jelly-fish.

We have, it is clear, to consider the nervous system
as at first forming a diffused network over the
whole body, and this truth must be constantly borne
in mind, for it applies not only to the Ccelenterata,
but also to the lowest worms. At the same time,

Fig. 170.— Diagrams to show the relative positions of the longitudinal
Nerve Cords in different genera of Nemerteans. The epidermal
tissues are left white, the muscles are darker, and the nerve cords
are darker still. A, Carinella ; B, Cerebr.itulus ; c, Langia ; D, Amphi-
porus; E, Drepanophorus. (After Hubrecht.)

we have to note the tendency of the nerve cells and
fibres to seek a more sheltered position than that
which can be afforded them by the surface of the
body ; nowhere, perhaps, are the various stages of
modification better seen than among the TVemertean
worms, of which Carinella is one of the lowest and
simplest examples.

It will be seen that in the figure (Fig. 170) Cari-
nella (A) has the longitudinal nerve cords just under-
lying the epidermal, and placed above the muscular
tissues.

Chap, xii.] NERVOUS SYSTEM OF NEMERTINES. 399

In Cerebratulus and Langia (B, c) they lie in the
midst of the muscular tissue ; while in Amphiporus
and Drepanophorus they are internal to it, as they
are in the greater number of invertebrate Metazoa.

Fig. 171.— Outer surface of a decalcified Plate of the Test of Brissopsis
lyrifera, from the greater part of which tbe connective tissue (ct)
has been removed, to show the course of the Peripheral Nerve-
fibres, and their ganglionic cells. Highly magnified. (After Loven.)

Carinella is, moreover, remarkable for the fact
that the centralisation of ganglia and nerve cords has
proceeded to a small .degree only. As in all Nemer-
tines, the ganglia are distributed over the whole
course of the longitudinal nerve trunks, and what, in
other forms, is an anterior cerebral enlargement, is here
merely represented by the enlargement of the front

400 COMPARATIVE ANATOMY AND PHYSIOLOGY.

end of the lateral trunks. Connected, finally, with
the two chief nerve trunks is a network of nervous
cells and fibres, which lies just below the dermis, and
forms a continuous layer over the whole of the worm.

In the Turfoellaria we find also that the nervous
system is superficial in position, and that the nerve
fibres so branch as to be distributed widely over the
surface of the body.

A similarly primitive condition obtains in the
I :< h i iKHh'iiii:! I :i • the epidermis consists not only of
supporting: cells, but of others which are sensory?
and have their basal ends continued into nerve fibrils,
which ordinarily run parallel to the surface of the
body ; with these fibrils small ganglion cells are con-
nected (Hamann) ; as a result of this, we have a
continuous sheath of nerve tissue investing the body
of a starfish or of an Echinoid (Fig. 171). In the
Ophiuroid and the Holothurian, the superficial nerve
plexuses have as yet been detected only on the tube
feet. By far the greater part of the nervous system
is superficial in the starfish, for the nervous band
that runs down the groove of every arm is placed just
below the investing epithelium ; and, in addition to
this, the more primitive histological condition is still
retained, for the ganglia are scattered among the nerve
fibres, and not collected into separate masses.

Having now sufficient evidence of the truth of the
statement that the nervous system is primitively
superficial in position ; that is to say, that at first the
nerve cells lie side by side with the epithelial cells, and
that they gradually come to lie just below the epithe-
lial layer, we may return to that plexiform disposition
of fibres which precedes the arrangement in definite
strands or cords. Evidence as to this is afforded by
the most primitive members now existing, both of
the Arthropoda and of the Mollusca. Of the
former, Peripatus is a striking example (Fig. 172).

chap, xn.j NER vo us S\ '.v TEM OF PERI PA rus. 40 1

A .' aln

The ventral nerve cords are widely separated from one
another, but are
connected together
by a large number
of commissures (co1),
of which there are
from nine to ten for
each segment of the
body. From the
outer borders of the
cords nerve fibres are
given off to all parts
of the body, the
whole of which is
consequently sur-
rounded by the ner-
vous system ; and we
have here, therefore,
what is essentially a
plexiform arrange-
ment, but one which
has, so to speak, be-
come regulated. A
further advance is to
be found in the fact,
that while the cords
are everywhere co-
vered by ganglion .
cells on their ventral
surface, the ganglia
are more especially
numerous at one
point in every seg-
ment of the body,

\\ here they form such an enlargement as that marked
fy] in Fig. 172.

Proneomenia may be taken as the simplest type of

A A--16

Pig. 172.— Anterior portion of the central
Nervous System of Peripaius, show-
ing the Anterior Cerebral Ganglia,
with the Lateral Nerve Cords con-
nected with one another by numerous
commissures (co). (After Balfour;)

E, Eye ; atn, antennary nerve ; co l, first com-
missure : orn, nerves for the mnutb ; ore/,
oral ganglion ; pn, pedal nerves; fc/',
first ganglionic eulai gement for the
pedal nerves.

APGr

402 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the Mollusca, but it is impor-
tant to note that along this
phylum we have persisting a
larger number of conditions
than are at present, at any
rate, known among the Ar-
thropoda. A reference to Fig.
ITS will show that, in Pro-
neornenia, there are, on either
side, two1 cords which run
down the whole length of the
body, and both of which ter-
minate in a ganglionic swel-
ling ; the two inner cords are
seen to be Connected with one
another by commissural fibres,
and each of these with the edge
of the cord that lies outside
it j as these latter give off
peripheral nerves it follows
that here again we have a
plexus of nerve fibres distri-
buted through the body. In
the case of Proneomenia we
have ganglion cells not only
accompanying the nerve fibres
throughout the whole of their
length, -but they are also, as
they are in some of the com-
missures of Peripatus, found
on the commissures which con-
nect these cords with one an-
other. Here, then, we have
yet another instance of the
plexiform disposition of nerve
fibres, and the diffused condi-
tion of ganglionic cells in a

.TPG
JVC—

Fig. 173. — Diagram of the
Nervous System of Proneo-
menia.

CX3, Cerebral ganglion ; sty, sub-
lingual gansrlla ; APG, PPG, PVO,
anterior pedal, posterior p -dal,
posterior lateral (visceral)
ganglia ; el, sublingual • con-
nectives ; Cpc, cerebropedal
connectives ; pe, longitudinal
pedal nerve trunks ; la, longi-
tudinal lateral nerve trunks.
(After HubrechU

chap xii.] NERVOUS SYSTEM. 403

lowly and little differentiated representative of a large
group of animals, in the higher members of which con-
centration is exceedingly well marked. (See page 411.)

With the exception, then, that in Peripatus and;
Proneomenia, the anterior end of the nerve cords is
enlarged into a cerebral mass, we should appear-
to be able to see no essential difference between them
and a Craspedote Medusa, save in the fact that the
Medusa has a complete nerve ring. In so far, how-
ever, as there is in both the Arthropod and Mollusc
just named, a commissure at the hinder end of the
body which connects the right and left cords with one
another, it is clear that the nerve system, if not a
ring, is at any rate a closed system ; that, in other
words, it may be compared to a ring drawn out length-
wise (Balfour). If this comparison be a just one we
are soon able to explain the reason why the anterior
end of a Nemertine or Arthropod or a Mollusc is better
developed than the rest of the nerve cord, for these
animals are all bilaterally, in place of being circularly
or radially, symmetrical ; and it follows, therefore, that
they do not advance in any direction indiscriminately,
as does a jelly-fish, but that there is one end which
is always directed forwards, and which first comes into
contact with friends, foes, or food. It is at that end,
naturally, that sense organs are first and best deve-
loped, and it is at that end, therefore, that the central
portion of the nervous system comes to be largest and
most highly developed.

In connection with this, the discovery by Kleinen-
berg of a nervous ring in the larvae of certain Annelids
is of great significance ; for though the adult poly-
chsetous worm is bilaterally symmetrical, and has a
central nervous system of the same character, the
larva has a rounded head-disc.

After the disappearance of the diffused or plexi-
forui arrangement of the nefVe fibres the system may

404 COMPARATIVE ANATOMY AND. PHYSIOLOGY.

fetill retain a very close connection with the surface
bi the' body ; the Annulata, for example, present us
with various arrangements of this kind, for while
Chsetopterus and Spio have the nerve cords out-
side the muscular layer of the body wall, and others,
such as Hermella, have them between or even in
the substance of these muscles, others again, like
the earthworm, have them placed inside the muscular
layers.

In the simplest condition of those form's which do1
not present the most primitive arrangements^ we find
a central gaiiglionic, or cerebral mass* with
which there are connected a number of nerve fibres,
which pass to different parts of the body ; such «i
disposition is found in some of the Turbellaria, and in
the .Rotatoria.

The most important advance is seen in the appear-
ance of the main or longitudinal cords, such as we have
already noted in Peripatus ; but even when these do
appear, we find that the cerebral mass still gives off a
number of fibres, which pass to the different sensory
organs that are situated at the anterior end of the
body. The two main trunks that pass backwards are
more or less intimately connected with one another on
the ventral surface of the gullet, so that we have now
to distinguish the carebral, or supracBsophageal
ganglia, the cesopliageal nerve cords or com-
missures, and the sufooesopliageal ganglia;
these last are, in their most primitive conditions,
similar to those that follow them (Fig. 174) ; at
first they are not closely united with one another,
but connected together by a pair of transverse com-
missural cords, as are the ganglia that follow them.

In the more primitive conditions, such as are
presented by Apus among the Crustacea, the cerebral
ganglia are merely formed by the nervous swel-
lings in the anterior region (primitive cerelrum)

Chap, xi r. ] NER vo us SYS TEM OF An THRO POD A .

405

(" archicerebrum" of Lan-
kester). Such an arrange-
ment is found also among
annulate worms.

In the greater number
of the Arthropoda we
not only see that the nerve
trunks lie internally to the
muscular layers of the
body wall, but also that
the cerebrum is no longer
primitive, but has other
ganglionic cells used with
it ; or, to use the words of
Rathke, as applied to the
developing scprpion, the
brain is " composed of
several pairs of ganglia
lying one behind the
other." Nor is this kind
of fusion confined to the
brain ; a longitudinal sec-
tion of part of the nerve
cords of a crayfish shows
that the ganglionic cells in
a segment have become
closely united together,
while, at the same time,
the cords are still distinct.
Nor is this all ; while
Apus has a distinct gang-
lionic enlargement in
every segment of its body,
we find that in higher
forms various ganglia be-
come connected together,
until at last, in the common

0(7

Fig. 174— Diagram of the Ante-
rior Portion of the Nervous
System of Apus, showing the
" archicerebrum " (c), and the
Ganglia of the Lateral Cord?.
(From Lankester. after Zad-
dach.)

x, Frontal nerves ; oc, optic nerves ;
CE, oesophagus ; a 1. nerve for first
antenna;«2, nerve for serondditto :
Mri, nerves for mandible ; MX, for
maxilla; wp, for maxllliped; T 1,
for first thoracic appendage.

406 COMPARATIVE ANATOMY AND PHYSIOLOGY.

crab, all the ganglia behind the cerebral become fused
into one large mass, which still retains evidence of its
composite character by giving off a large number of
separate nerve fibres (Fig. 175). A similar series illus-
trating the phenomenon of the fusion of nerve centres
may be observed jn Araolniida and Insecta.

We note, then, that the
loss of that plexiform arrange-
ment, of which we have
spoken so often, i§ succeeded
by an aggregation of ganglionic
cells, which form distinct
masses in every segment of
the body ; at first each mass
is composed of two distinct
halves. The anterior regioD.
becomes more and more pre-
dominant, and the " archicere-
brum," or simple anterior
enlargement, becomes a " syn-
perebrum," or compound one..
As the segments of the body,
which in the earthworm, for
example, are all alike and ha.ve
nearly all just the same func-
tions, become arranged in
groups which, as in the cray-
fish, take on different duties, or exhibit division of
labour, the nervous centres likewise become affected,
so that while Apus has a separate ganglionic mass
for each of its sixty segments, the crayfish has the
first six of its ventral ganglia fused together, and
the short-tailed crab has all the ventral ganglia in a
single mass (Fig. 175); so, again, the Myriopod has
ganglia in every one of its segments, the scorpion
has the first nine ventral ganglia united, and in the
short-bodied spider there is only one ventral ganglion.

Fig. 175.— Nervcras System
of a Crab.

c, Cerebral ganslion; o, optic;
a. antennary nerye ; c, O?PO-
phageal commissure, T, fused
ventral ganglion.

chap, xii.] NER vous SYSTEM OF ECHINODERMS; 407

We meet with the same phenomenon in Insects,
*but these Arthropods are of greater interest from
the point of view that the changes undergone by
them during their development afford support to the
view that the more primitive forms have a larger, and
the more complex a smaller, number of separate
ganglia. While the worm-like larva has a ganglionic
mass in nearly every one of its segments, the adult
insect has a varying number fused together.

As has been already pointed out in speaking of the
Ectimodermata, the nervous system of a starfish
is so far extremely primitive in character, that the
nerve cord which runs down the ainbulacral groove of
each arm lies just below the integument ; in the
Ophiuroids this superficial position is lost, owing to the
development of a calcareous plate, which forms a floor
for the groove. The great development of the test in
Echinoids leads to tl*e same result ; but here, as we have
already learnt, a compensating arrangement is effected
by the development of a plexus of nerve cells and
fibres which is superficial to the test. In Holo-
thurians the nerve cords are placed more or less deeply
in various forms.

In all cases these radially disposed nerve cords are
united with one another by a set of circular fibres,
which form the circumoral nerve ring, and it is
thanks to this that the apparently independent rays
of a starfish or of a brittle star are enabled to act in
concert : but, although the nervous system of an Echi-
noderm is hereby made a connected whole, it is im-
portant to observe that a single arm of a starfish, or
even a segment of an Echinus (Fig. 176) is capable of
exerting independent movements ; for example, single
rays of a starfish have been found to crawl as fast,
and in as definite a direction as entire forms ; if turned
on their back they succeed in righting themselves, and
sometimes, though not always, they attempt to move

408 COMPARATIVE ANATOMY AND PHYSIOLOGY.

away from injuries inflicted on them. If the nerve
ring be divided, without the separation of a ray from
the rest of the organism, the ray whose nervous con-
nection is so cut ceases to act with the rest of the star-
fish, but is capable, to a certain extent, of responding
to stimuli on its own account.

Fig. 176.— Separate Segment of an Echinus attempting to right itself
. after having been inverted. (After Komanes and Ewart.)

The Crinoidea must be dealt with separately from
the rest of the Echinoclermata, in consequence of the
difficulties presented by the conditions and relations
of their nervous system. When a transverse section
is made of one of the pinnules which hang down from
the sides of an arm of a Crinoid, a nerve cord (n ; Fig.
177) is seen to lie just underneath the epithelium of
the groove of the pinnule ; this clearly corresponds to

Chap, xii.] NERVOUS SYSTEM OF CRINOIDS.

409

a.'

the nerve in the arm of a starfish, and it has similar
relatio'ns to a nerve ring which runs round the mouth.
If we now look at the opposite side of the section, we
find another and larger
cord which gives oft
branches to the muscles
of the arms (a a) ; this
cord, if it be followed
up, will be found to
end in an organ, the
so - called " chambered
organ," which lies in
the centred orsal piece
(see page 292) of the
Crinoi('. Now, if the
visceral mass, part of
which is the circumoral
nerve ring, be alone
removed, the arms will
continue to move as
regularly as they did
before, and the Crinoid
will still be able to
swim about in the
water. If, on the other
hand, the five - cham-
bered organ be stimu-
lated, then, as Dr.
Carpenter has shown,
there is a sudden and
simultaneous flexion of
all the arms. The ex-
istence of these two

apparently independent nerve systems in a Crinoid
is a difficulty which the morphologist has not yet
been able to solve, but the anatomical and physio-
logical evidence in favour of the nervous nature

CL

Fig. 177.— Cross Section of a Pin-
«ule of the Arctic Feather-
star (Antedon (schncJiti) ; x 75.

a, Axial cord ; a', its branches ; ag, ambu-
lacral groove; b, radial blood-
vessel ; gv, genital vessel ; ov, ovary ;
•n, radial nerve ; pj, pinnule joint ;
w, water-vessel ; T, tentacle. (From
Carpenter, altered from Ludwig.)

4i o CoMPARAi^iy-E ANATOMY AND PHYSIOLOGY.

of the chambered organ and the axial cords appears
to be complete.*

The greater number of the Mollusca present us
with an arrangement of the nervous system which is
very different from that which obtains in Arthropods ;
this is due to the want of metameric segmentation,
and to the marked tendency of the ganglionic masses
to fuse with one another. Indications of a more
primitive condition of things are not, however con-
fined to Proneomenia (page 402); commissures con-
necting the two chief longitudinal trunks, and so
giving rise to a step-ladder-like kind of arrangement,
are to be observed in Chiton and in Haliotis.

In the L,amelliforanchiata (e.g. Anodon), when
the primitive bilateral symmetry of the body is re-
tained, we find two supraoesophageal ganglia, whence
nerve cords pass off on either side to the hinder end of
the body ; no ganglia are developed on the course of
these trunks, but, as in Proneomenia, at their termin-
ations only (visceral ganglia) ; these two ganglia
are sometimes almost separate, in other cases more or
less completely fused with one another, just as, at the
other end of the body, is the case with the supra-
oesophageal ganglia.

These last also give off a pair of cords, which in the
mussel extend some way down into the substance of
the foot, where they end in the pedal ganglia ; but
these pedal ganglia are not always so far distant from
the supracesophageal as in the mussel, their size and
position depending on that of the foot itself.

While the supracesophageal or cerebral ganglia of

* Prof. Milnes Marshall, who has lately repeated and extended
the observations of Dr. Carpenter, has suggested that the ant-
ambulacral or dorsal portion of the nervous system of a Crinoid is
modified from the antambulacral portion of the primitive nerve
sheath, which in the starfish still invests the whole of the body.
The "chambered organ," or "central capsule," still requires in-
vestigation from the morphological and embryological side..

chap, xii.] MOTOR AND SENSORY NERVES. 411

the Lamellibranchiata (Acephala) are always com-
paratively small, in consequence of the reduction of
the head of these Molluscs, they are always much
larger in the Cephaloptiora, which are provide^
with eyes and powerful £acti}e tentacles. The two
most important phenomena observable jn the charac-
ters of the nervous system ,of this group are th,e fusion
of the primitively separate ganglionj.c masses, and the
twisting undergone by the nerve cords of soine of the
Gastropoda. The former attains its most marfceql de-
velopment in the Cephalopoda, where the pedal fuse
with the visceral ganglia, and are closely approxi-
mated to the cerebral mass ; the latter, which may be
seen in the limpet (Patella), or the river-snail (Palu-
dina), results in the nerves which connect the cerebral
with the visceral ganglia passing from the right to
the left, a#d from the left to the right-hand side.

From the ganglionic masses and from the cords that
connect; £hem together m the way that }ias now been
described, nerves are giyen off to various parts of the
body. We have already seen that in the lower forms
the whole of the body is invested in a superficial
plexus qf nerve fibres and cells ; as the cells became
gradually aggregated intp plefinjte mass^s, the n,erves
that were given off from then} became likewise arranged
in a defjnjte and regular fashion} and took on definite
duties, and functions. Those nerves jthat pass to
muscles may be spoken of as the motor pr efferent
nerves, those that end in sensory organs, whether
general tactile organs or organs of more espepia} sense,
as sensory or afferent nerves ; that is tp say, they
bring messages to the central system, while the efferent
nerves carry messages away. The size and number of
these nerves depend, therefore, primarily on the size
of the parts to which they are distributed. Their
general arrangement may be well seen in a segmented
animal ; putting aside for a moment the nerves given

412 COMPARATIVE ANATOMY AND PHYSIOLOGY.

off from the supra cesophageal ganglia, we find that in
the earthworm, for example, several nerves are given
off from the cesophageal commissures, and that each
successive ganglion gives off two nerves on either side,
while one nerve on either side is given off by the cords
which connect the ganglia witl) one another. When
we come to a more differentiated form, si^ch as the cray-
fish, we find that no nerves are given off from the
commissures, but that three pairs of nerves are sent ofjf
from each of tfye ganglia that belong to one segment
only, while, when two op more ganglia have fused
together, a large number qf nerves are given off ii)
order to supply more than one segment of the body.

In addition tq the sensory and motor nerves therp
are others which are particularly related to the digesr
tive and cirpulatqry organs ; the§e are the so-calLed
visceral nerves, and, from a physiological standpoint,
if no£ indeeq1 also froin a morphological, they aro
comparable tq the system which, in Man and other
vertebrates, is spoken of as the sympathetic system.
While in thje lower worms these yisperaj nerves are
merely cords given off frqm tl^e cerebral ganglia, they
become more independent in the higher forms, owing
to the development of ganglia along their course ; a
well-marked ganglion of this kind may be seen on the
dorsaj surfape of the crop of the cockroach. The
general arrangement of the •' stomatorgastric " system of
this animal wi}i serve conveniently as a type, and may
"be thus describe.4 ; from the anterior part of the cere-
bral mass a cord arises on either side, which, after
passing forwards for a short distance, bends on itself
and unites with its fellow in a median ganglion. The
single cord given off from this ganglion passes back-
wards beneath the brain to .another median ganglion ;
with this last two lateral ganglia are connected ; the
second median ganglion gives off a cord which passes
backwards above the digestive tract to a third ganglion

chap, xii.] NERVOUS SYSTEM OF CRAYFISH. 413

or that already mentioned ; from this there arise two
trunks which give oft' nerve fibres to the anterior por-
tions of the digestive tract. While the median
ganglia and nerves form the unpaired system, the two
lateral ganglia are the most anteriorly placed repre
sentatives of a paired system of stomato - gastric
nerves and ganglia.

Other nerve cords connected with the sympathetic
system supply especially the air tubes (trachese), and
the muscles of their Orifices (stigmata) ; from the fact
that the nerve which runs above the ventral gangli-
onic chain gives off lateral branches which pass out-
wards, the system is known as that of the nervi
transversi accessorii. In the crayfish the ter-
minal ganglion of the ventral chain gives off' nerve
fibres which innervate the hinder portion of the
digestive tract.

The function of the several parts of the nervous
feystenl have been investigated in so few of the Inver-
tebrata, that it will be well to state at some length
what is definitely known as to the physiology of the
nervous system of the crayfish or the lobster.

We note in the first place that the presence of a
comparatively large cerebral mass is associated with a
large amount of influence over the rest of the ganglia;
thus, the limbs, which in ordinary circumstances move
in due order in such a way as not to oppose, but rather
to assist one another, cease to exhibit this harmonious
activity when the cerebral ganglia are removed \ in
other words, they are no longer co-ordinated ; but this
is not all ; the cerebrum appears to be the centre of
what, in our ignorance of all the circumstances of the
case, we call spontaneous activity, and this is very
pointedly spoken to by the loss of power in the selec-
tion of food, which follows on a removal of the cerebral
centres. The separate condition of the resophageal
commissures which unite the brain with the chain of

414 COMPARATIVE ANATOMY AND PHYSIOLOGY.

ventral ganglia is not only an anatomical fact, it has
also a physiological significance, for when that of one
side is removed, it is only the organs on that side of the
body which cease to react to stimuli, the appendages
on the other side alone appearing to be affected.

The ganglia just below the O3sophagus (the sub-
O3sophageal) appear to have a considerable function
as the centres of motor energy, for so long as they are
present the appendages move with . considerable ac-
tivity, but when they are removed the chelae " sprawl
helplessly," and the legs are often found doubled up
under the body. As might be supposed from the re-
lations of their nerve fibres to the muscles of the
gnathites, the same ganglia appear to be the centre
for the feeding movements ; after their extirpation,
the chelae or great forceps do not always carry the
food to the mouth, as they do regularly in the uninjured
animal ; it is a curious fact that even when they do
carry it there they do not give it up to- be swal-
lowed.

With regard to the general physics of the nerve
fibres, we know from Fredericq that motor excitations
produced by electrical currents pa,ss much more slowly
along the motor nerve of a lobster than that of a
frog, the proportion per second being as twenty-seven
metres in the frog to six in the lobster.

The student of vertebrate physiology will best
understand the leading differences between the ac-
tivities of the nervous system of the frog and of the
crayfish, by a comparative statement : " There is much
less solidarity, a much less perfect consensus among
the nervous centres in the crayfish than in animals
higher in the scale. The brainless frog, for example,
is motionless except when stimulated, and even then
does nothing to suggest that its members have a life
on their own account ; whereas the limbs of a cray-
fish, deprived of its first two ganglia, are almost

chap xii.] NERVOUS SYSTEM OF CHORDATA. 415

incessantly preening, and, when feeding movements
are started, the chelate legs rob and play at cross pur-
poses with each other as well as four distinct indivi-
duals could do " ( J. Ward).

This quotation will bring very forcibly to the
mind the value and meaning of gangl ionic masses in
the separate segments.

So far as our present knowledge extends, we are
led to the belief that the spinal cord of the lower
Vertebrates (as represented by the frog) has much
greater independence than that of the higher, as re-
presented by the dog, or by man. For example, if
the brain of a frog be removed, the animal will still
execute movements, to which it is impossible to re-
fuse the name of purposeful ; in the Mammal, on
the other hand, the movements which, under similar
conditions, are similarly excited, are irregular and
without order. Extirpation of the cerebral hemi-
spheres of a Mammal results in death after a few
hours, while the frog may be kept alive for an indefi-
nite period, if suitable care be taken of it.

The general functions of the various parts of the
brain have been discussed in the volume on "Human
Physiology" (chap. xiv.).

The Chordata are to be distinguished practically,
even if not morphologically, from the majority of the
so-called Invertebrata by the fact that the nervous cord
lies on the dorsal aspect of the body, and not on that
on which the mouth is situated; at the same time
it is to be borne in mind that in the Nemertinea the
nerve cords often tend to lie dorsally, and that in
Peripatus the two cords are, at the hinder end of the
body connected together by a commissure which lies
above or dorsally to the terminal portion of the in-
testine. Similarly, there are certain points in the
anatomy of the vertebrate brain, too complicated to
be here described, which afford some evidence in

416 COMPARATIVE ANATOMY AND PHYSIOLOGY.

favour of the view that the anterior portion of the
brain was once separated from that which lies behind
it by the digestive tract.

In no known Chordate, however, does the oeso-
phagus separate any one part of the nervous system
from the rest, and the whole mass is superior to or
dorsal in relation to it. In all Chordata also the
nerve cord has a central canal, and occupies exactly
the median axis of the body. The presence of this
canal is not to be explained without a reference to the
history of the development of the central nervous
system ; in this mode of development we find yet
another important characteristic of the Chordata.

The median strip of epiblast which is to give rise
to the nerve cord, instead of merely sinking away from
the surface of the body, becomes grooved along its
middle line ; the sides of the groove grow up and
unite with one another, so as to leave a central cavity ;
in most cases the tube is first formed, and only later
on separates off from the layer of epiblastic cells
which forms the covering of the body ; in Amphioxus,
however, the external layer covers over the so called
" medullary plate " which forms the nerve cord before
the groove has become closed up. It will be seen
that, owing to the formation and closure of this
groove, the cells that were primitively external come
to lie within those that were primitively internal.

In the Cephalocliordata the central nervous
system retains throughout life the form of a hollow
tube, and there is no distinct enlargement at the anterior
end which can be called a brain. In the U roeliordata
the typical arrangement is best seen in those which
retain the tail during the whole of their lives (Appen-
dicularia) ; in them we find an anterior swelling,
which becomes divided into two vesicles, with the
foremost of which an optic and an auditory organ
become connected ; the hinder vesicle is separated by

Chap. XII.]

BRAIN OF VERTEBRATA.

CH-

CV-

N..

-H3

a constriction from the cord that follows it. and from
which three pairs of nerves have been observed to be
given off. In this cord, as in that of the Yertebrata,
we find that the nerve fibres lie externally to the gang-
lionic cells, an arrangement of the histological elements
which is exactly the reverse of
what obtains in "invertebrates."

With the loss of the tail, the
nerve cord, which is found in the
tailed larva in the same position
as in the adult Appendicularia,
undergoes atrophy, and the fixed
or colonial Tunicate has a single
ganglionic mass which lies be-
tween the mouth and the atrio-
pore. (See page 231.) From
this ganglion nerves are given
off to the different parts of the
body.

In the Vertebrata a brain
is always present ; the primi-
tively single swelling at the an-
terior end rapidly becomes divided
into three brain vesicles, which
may be distinguished as those of
the fore-, mid-, and hind- brain.
These vesicles are, of course, hol-
low within, and their cavities
have received distinct names, the
reasons for which will certainly
be far from clear, unless we recollect that the termin-
ology of the parts of the vertebrate brain is based on
the nomenclature of anthropotornists. The cavity in
the fore-brain (Fig. 178; m) is known as the third
ventricle, and that in the hind-brain (iv) as the
fourth ventricle ; the often narrower cavity in the
mid -brain (it) is known as the iter a tertio
BB— 16

Fig. 178.— Diagram of the
Ventricles of the
Vertebrate Brain.

in, third ventricle ; it, iter ;
iv, fourth ventricle ; CH,
cerebral hemispheres;
cv, their cavity ; FM, fora-
men of Munro ; FB, f ore-
brain ; JIB, mid-brain :
HB, hind-brain.

4i 8 COMPARATIVE ANATOMY AND PHYSIOLOGY.

ad quartum ventriculiim, or more shortly as
the iter.

The walls of these cavities undergo further
changes ; the hind-brain becomes divided into two
parts, one of which lies behind, and at a little
lower level than the other; this is the medulla
oblongata, and it is directly continuous with
the spinal cord* The anterior half, which in the
frog is a narrow band, but in man forms a very
conspicuous part of the whole mass, is the so-called
little brain or cerebellum. The mid-brain does
not undergo transverse division ; its upper and late-
ral portions form the optic lobes, and the inferior
portion the so-called crura cerebri. The most re-
markable changes are undergone by the fore-brain
vesicle, which buds out a vesicle on either side, the
cavities in which are known as the lateral ventricles
(cv) ; these lateral outgrowths always become of con-
siderable size, and in the higher vertebrates form the
chief mass of the brain. They are the cerebral hemi-
spheres, and are the seat of the most important of
the functions performed by the brain ; they not only
increase in size, but by the development of grooves,
the presence of which permits an addition to the
quantity of grey or ganglionic material altogether
out of proportion to the increase in the area occupied,
they come to have not only a more complicated sur-
face, but also a much higher functional value.

The cerebral hemispheres are continued anteriorly
into the olfactory lobes (Fig. 179 ; o£), and these
into the so-called olfactory nerves. More pos-
teriorly, the fore-brain gives off another vesicle on
either side, and this vesicle travels away from the
brain, with which it only remains connected by its
stalk ; the vesicle forms the hinder part of the eye,
and the stalk becomes the so-called optic nerve.
The remainder of the fore-brain forms the thalamen-

Chap. XII.]

BRAIN OF FROG.

419

cephalon, or optic tlialami, so called from the fact
that when the brain is laid on its upper surface the
optic nerves rest on them as on a couch (thalamus).

Connected with the upper surface of the thala-
mencephalon is the pineal gland, which is not

01

CH

Fig. 179.— A, Brain of Frog from above ; B, from below.

1, Olfactory nerves; ol, olfactory lobes; CH, cerebral hemispheres; i-T,
lamina terminalis ; T^, thalamencephalon with pineal gland (PG) ; OpL, optic
lobes; cl, cerebellum; MO, medulla oblongata ; 2, 9ptic nerves ; OT, optic
chiasma ; TC, tuber cinereum ; H, hypophysis cerebri ; 3—10, cerebral nerves.
(After Ecker.)

nervous in nature, while the lower surface of the
same region of the brain is continued into the funnel-
shaped tnfoer cinereum (Fig. 179 ; TO), with the
base of which is connected the so-called pituitary
body; this, like the pineal gland, is not nervous in

420 COMPARATIVE ANATOMY AND PHYSIOLOGY.

nature, and is a structure which is not of cerebral
origin at all, but is derived from the epiblast which
lines the cavity of the mouth ; in its primitive con-
dition it forms an inpushing towards the lower
surface of the brain ; its base becomes solid, and
then disappears, so that the ingrowth becomes
completely separated off from the layer of cells
from which it took its origin. In the lower ver-
tebrates it does not, but in mammals it does, become
structurally united with the brain.

In Fishes the brain is always small ; in the pike,
for example, it is not more than T^^th part of the
weight of the whole body, whereas in Man it is about
•g^th of the total weight ; nor does it grow propor-
tionately with the growth of the body, or occupy the
whole of the cranial cavity. In the Cyclostomata
the walls of the cerebral hemispheres become greatly
thickened, so much so, indeed, that in Myxine they
become quite solid ; the olfactory lobes are propor-
tionately large, as is also the pineal gland ; the
region of the hind-brain is also of great size, propor-
tionately to the rest of the organ. In the Elasmo-
branchs the olfactory lobes are often carried
forwards on stalks, which are of great length in some
sharks ; these lobes may be broken up into smaller
lobules. The cerebral hemispheres are proportionately
large, and differ greatly in the size of the contained
ventricles, or, in other words, in the thickness of their
walls ; the surface of these hemispheres is sometimes
marked by a few shallow grooves. The cerebellum is
of large size, and is often grooved transversely.

During the process of development the brain
vesicles cease to lie in a straight line one behind
the other ; as a consequence of this " cranial
flexure," the fore-brain lies at a lower plane than
the mid- brain, and the long axes of the two are
set at an angle to one another. A little later the

chap. xii. i BRAINS OF FISHES AND AMPHIBIA. 421

wal) between the two parts of the fore-brain becomes
thinner, and a " primitive cerebral fissure " is apparent.
This condition of things is retained by some Ganoids
throughout life (Polypterus ; Fig. 180); these fishes
also possess the more primitive character of a large
thalamencephalon.

In the Teleosfei the brain is compressed, the
cerebral hemispheres are almost completely solid, and
the cerebellum is usually, though not always, of com-
paratively large size ; it is often prolonged into the
cavity of the mid-brain (valvula cerebelli) ; on the

Fig. ISO.— Brain of Polypteriis seeu from the Side,

t Olfactory nerves ; h, k, cerebral hemispheres • o, optic nerve ; d, optic lobes ;
e, hypophysis ;/r central fissure ; 6, c, cerebellum; a, mednlla oblongata. (After
J. Miiller.)

whole, the brain of the Teleostei exhibits many resem-
blances to that of Ganoids, and especially of Lepi-
closteus.

In the adult Amphibia, as in the adults of most
fishes, the several parts of the brain lie in the same
plane ; 011 the whole, the brain of the Anura is more
highly organised than that of the Urodela ; it is pro-
portionately larger than that of fishes, but is still
small. The brain of the Anura is different from
that of all other Vertebrates, owing to the fact that
the olfactory lobes of the adult are not separated
from one another, and, like that of the Urodela, the
cerebellum of the Anura is of extraordinarily small
size.

422 COMPARATIVE ANATOMY AND PHYSIOLOGY.

In the Amniota we find considerable advances
in the characters of the brain, which are chiefly due
to the angulation of its several parts, and the thicken-
ing undergone by the walls of the primary vesicles at
various points.

In the Reptilia the cerebral hemispheres are
always smooth on their surface, but they are now, and

Fig. 181.— Side views of the Brain of a Tortoise (A) and a Bird (B).

I, Olfactory nerves; \.ol, olfactory lobes; VFT, cerebral hemispheres; n, optic
nerves ; Tro, optic tract ; inf, infundibuluin ; H, hypophysis cerebri ; T,
temporal lobe; MH, optic lobes; HH, cerebellum; NH, medulla oblougata ;
R, spinal cord. (After Wiedersheim.)

henceforward, always large in proportion to the re-
maining parts of the brain ; the hemisphere of either
side is united to its fellow by a transverse band of
fibres (commissure), which lies just in front of
the third ventricle ; the optic thalami are similarly
united by a transverse commissure j the cerebellum

chap, xii.] BRAIN OF BIRDS. 423

is not always a narrow plate, and in the Crocodilia
the central portion forms a distinct " vermis."

Of the thickenings of the cerebral walls, the two
most important are the corpora striata in the
hemispheres, and the restiform bodies in the
medulla oblongata ; the former are the ganglionic
masses which become developed on the floor of the

01

Fig. 182 A.— Lateral view of the Brain of Eabbit, to show the large
olfactory lobes, and the termination of the hemispheres in front of
the Cerebellum. (After Huxley.)

A, Frontal lobes ; B, occipital lobes ; sy, sylvian fissure.

brain, and, as they extend inwards, they encroach on
the cavity of the lateral ventricle ; as may be sup-
posed, they are largest in the Crocodilia. The cor-
pora restiformia in a similar manner encroach on
the fourth ventricle.

In Birds the cerebral hemispheres are propor-
tionately still larger in size, and, as the optic lobes,
or so-called corpora bigemina (Fig. 181 (B) ; MH) are
now £°it at the sides and base of the brain, the
cerebral hemispheres so overlap them as to hide
them when looked for from above. The cerebellum
(Fig. 181 (B) ; HH) is much larger, and its lateral lobes,
or flocculi, may be distinguished from its central
body, or vermis; while in section this division of
the brain presents just the same appearance as the

424 COMPARATIVE ANATOMY AND PHYSIOLOGY.

OL

S'Jl

I.0o.

01

Fig. 182.— Lateral views of the Brain of : B, A Pig. c, A Chimpanzee,
drawn of nearly the same absolute size.

Ol, Olfactory ; A, frontal; B, occipital : c, temporal lobes; s.?/, sylvian fissure;
in, island of Reil ; s.o?% supraorbital ; s F, Btiperfor : M F, middle ; I F, inferior
frontal gyri ; A P, antor>>-parietal ; p p, postero-parietal gyri ; u, ml CM of
Rolando; p Pi, postero-parietal lol>ule; opf, occipito-temporal sulcus; MI,
angular gyms : 2, 3, 4, annectent gyri ; A T, M T, p T, temporal ; s oc, 11 oc,
ioc. occipital gyri. (After Huxley.)

Chap. XII.]

BRAIN OF MAMMALS.

425

so-called "arbor vitse" of the human brain, it is
marked externally by fairly deep transverse fissures.
The external surface of the cerebral hemispheres is
smooth, but the corporate striata are very well de-
veloped.

The most important and instructive changes are
to be seen in the brain of
the Mammalia ; these
depend chieflyon the great
development of the com-
missures, which connect
the two halves of the
brain with one another,
and on the gradually in-
creasing sizeof the cerebral
hemispheres which ends
in their having an extra-
ordinary predominance
over the other parts of
the brain ; hand in hand
with their increase in
size and extent is the
improvement of the in-
tellectual faculties. But
the cerebral hemispheres
do not merely increase
in bulk, their surface be-
comes marked by grooves. Figvl83TBrai™f rupai> t° sh°w

, , J ^ n the large Olfactory Lobe, the

and the amount of sur-
face thereby developed is,
as we have already said,
greatly extended without

any corresponding or proportionate increase in the
size of the cranial cavity.

The olfactory lobes lie more or less below the
cerebral hemispheres, and diminish in proportionate
size as we ascend the series ; the cerebral hemispheres

ungrooved Cerebral Hemi-
spheres, and the large Cere-
bellum. (After Garrod. P.Z.8.,
1879, p. 304.)

426 COMPARATIVE ANATOMY AND PHYSIOLOGY.

more arid more extend backwards, and at last com-
pletely overlie the cerebellum. As they increase in
size they become broken up into distinct lobes,
frontal, occipital, and temporal. The cerebellum
diminishes in proportionate size, and the flocculi
cease to be conspicuous at its sides. (Compare ol, in
Fig. 182; A, B, and c.)

Transverse commissures are always richly deve-
loped, the corpus callosum connecting the two
cerebral hemispheres, and the pons varolii, which
bridges over the hind-brain, being parts which are
developed in mammals only. The optic lobes are
divided transversely, so that the " corpora bigemina "
of the lower vertebrates are now the " corpora quacl-
rigemma " ; this mid-brain is proportionately small.

A very complete series of gradations of all these
differential characters is to be observed as we pass up
the scale of the Mammalia. This is to be seen, first
of all, in the proportionate increase in the weight of
the brain, as compared with the rest of the body, for,
while that of the rabbit is about y^th part, that of
man is ^th.

In the Prototheria the corpus callosum is always
small, and the cerebral hemispheres, which are smooth,
do not cover the cerebellum. The Metatheria
differ a good deal among themselves. Among the
Euttieria, the Insectivora exhibit a brain of very
low character; the cerebral hemispheres are often
quite smooth, the olfactory lobes are large, and project
in front of the hemispheres, which only just, if at
all, overlap the cerebellum behind (as in Tupaia;
Fig. 183). This latter has a large vermis. The
corpus callosum is thin and nearly straight, while
the corpora quadrigemina are proportionately large.
The pons varolii is very small. In the hedgehog
there is a single simple groove (sulcus) on either
hemisphere.

Chap. XII.l

BRAIN OP MAMMALS.

427

There early ap-
pears a fissure at
the side of the
cerebral hemi-
spheres, the sylvian
fissure (Figs. 182;
sy; and 184 (u) ; s),
which separates the
frontal from the oc-
cipital lobe ; this,
which is very shal-
low in the rabbit or
the musk-deer (Fig.
184; s), is deeper
in the pig or the
dog, and in man
divides into an an-
terior and a pos-
terior groove, be-
tween which is
placed the island
of Beil. The sur-
face of the hemi-
spheres is next

8, Pi

Fig. 184.— Brain of Musk-Deer. A, from the side ; B, from above.

(•sure of Sylvius ; ss, superior external gyrus ; m, middle ; ii, inferior external
1S75US ' 1^lppocaml)al syrus : o, supraort-ital gyrus. (.After Flower, P.Z.S.,

428 COMPARATIVE ANATOMY AND PHYSIOLOGY.

broken up into simple folds or gyri, by the forma-
tion of intervening fissures ; the arrangement of
these is better studied in a small than in a large
animal, for with increase in size the primitive pattern
is obscured by the increase of the convolutions
(Flower). These gyri may be distinguished as the
superior, middle, and inferior external gyri (Fig. 1 84 ;
8, m, i) ; below is the temporal lobe, separated by the
hippocampal sulcus (A). To these other grooves may
be added on, such as the supraorbital (Fig. 184; o),
and the complexity of the surface of the brain be
increased by the development of annectent gyri
between the primary folds of a simpler brain.

It is not, however, to the surface that the complexi-
ties of the brains of the higher Mammals are limited,
the inner as well as the outer face of the cerebral
hemispheres becomes convoluted. The corpus callosum,
which is at first a thin straight band of connecting
fibres, becomes thicker, especially in front and behind,
and so curved on itself that anteriorly it forms the
" genu " of human anatomy. Behind and below this
corpus callosum is the " fornix," and these two
structures are peculiar to mammalian brains ; the
former is developed from what is, morphologically, the
inner portion of the surface of each cerebral hemisphere,
and there is, therefore, a space left which is bounded
on either side by a thin wall (septum lucidum) ; this
space is known as the fifth ventricle, but the name
is an unfortunate one, inasmuch as this fifth ventricle
is not developed, as are the others, from the original
cavity of the cerebrospinal axis, but is merely a space
between two overgrown walls. The fornix is similarly
derived from the hinder part of the walls of the cere-
bral hemispheres.

The thickening in the floor of the cerebral hemi-
sphere of either side (corpus striafuiii) is much
more prominent in the Mammalia than in other

chap xii.] BRAIN OF MAN. 429

Vertebrates ; behind this is a less conspicuous thick-
ening (the hippocampus major, to which is
added on in the higher Primates the hippocampus
minor.

The average weight of the human brain is,
for males, between 46 and 53 oz., and for females
between 41 and 47 oz., but the range of difference is
much greater than this. As is well known, the brain
of Cuvier weighed 64 oz., or 4 Ibs., while that of an
anonymous sane man was only 34 oz., or but little
more than half that of the great anatomist ; but the
weight only must not be taken into consideration ; the
depth and extent of the convolutions must also be
estimated, and Wagner has found a difference of as
much as 15 per cent, in the extent of the surface of
the cerebral hemispheres of two selected n ales. But
that this, again, is not all is not only clear from the
consideration that a small well-made watch often
keeps better time than a kitchen clock, but by the
following facts :

(1) The anterior portion of the cerebrum is fed by
the carotid and the hinder by the vertebral arteries ;
as the former are much larger than the latter, it
follows that the anterior portion of the brain is better
supplied than the posterior, and that pro tanto the
advantage lies not in the greater size of the cerebral
hemispheres as a whole, but in the size of the anterior
portion, or that which lies in front of the ear.

(2) Though absolutely the human brain is, on the
average, heavier than that of all mammals except of
the elephant, wrhich weighs between 8 and 10 Ibs., or
of some whales, which may weigh as much as 5, while
the horse, for example, has a brain weighing only
23 oz., and an average-sized dog less than 7 oz., yet,
in the apparently more important relation of brain
weight to body weight, in which man presents the
proportions of T^, he is surpassed by some American

43° COMPARATIVE ANATOMY AND PHYSIOLOGY.

a,pes, in which it varies from -i- to T\, by the sparrow
in which it is ^T, and the titmouse in which it is j1^
(Bischoff).

On the other hand, when we compare man with
his nearest zoological allies, we find that not only is
the capacity of his skull and the weight of his brain
greater, but that there is a notable increase in the
complexity of the secondary gyri of the surface of
his cerebral hemispheres, as compared with those
of the apes.

The spinal cord differs from its anterior enlarge-
ment, the brain, in having the grey ganglionic mate-
rial placed internally to the white fibrous cords, which
act as the conductors of nervous stimuli, but, like it,
it is hollow internally, and the epithelium whicli
lines it is temporarily or permanently ciliated.
It is marked above and below by a median groove,
and, in all vertebrates, has paired nerves issuing from
it, each of which is connected with it by a superior
and an inferior root. It is cylindrical in all Verte-
brates except the Cyclostomata and Chimsera ; not
unfrequently it extends throughout the whole length
of the neural canal formed by the spinal column, but
in the sun-fish it is greatly shortened, so as to look
indeed like a mere appendage to the brain, and in the
anurous Amphibia, in Birds, and various Mammals
(among which are the hedgehog and man), the
terminal portion is filamentous, and is accompanied on
either side by a number of nerves, thereby giving rise
to the so-called cauda equiiia (horse's tail).

SENSORY ORGANS.

It has been already stated that all the organs
of sense have their primitive seat in that outer
layer of the body which, in the embryo, is called the
epiblast or ectoderm ; and we have already learnt
that the nervous system itself does, in most cases,

Chap. XII.]

SEWSOX Y ' ORGANS.

remain throughout the life of the animal in close local
contact with the outer world. In tracing the history
of the organs of sense we shall find that, whatever
their final position, they too are essentially of epiblastic
origin.*

Among the Hydroid polyps, where no nervous
system has as yet been made out, we observe that the
tentacles which surround the mouth are provided with
fine hair-like projections, which look not unlike a
trigger ; these processes are seen to be in connection
with cells which differ in character from their neigh-
bours by the possession of a coiled up thread; when

* Since the above was put into type, Prof. Charles Stewart
has favoured me with an account of his observations on sense cells
in sponges, and with the accompanying illustrative figure (Fig.
184 A). It is found that
" the external orifices
of the interradial canals
of Grantia compressa
are fringed with deli-
cate hair-like processes
of the soft substance of
the sponge. At first
sight these remind one
of the palpocils of Hy-
dra, which they closely
resemble in general
form and size " ; from
these, however, they
differ in important par-
ticulars. The processes
or hairs vary in length
from nfotfth to about Fig. 184 A.

ysV&th of an inch ;

their base is from ^^Wtt1 to T-jrtar&'th of an inch, and they taper
to a fine point. All such as can be well seen are found to have
a special relation to a subjacent branched cell ; this latter sends
outwards a delicate filament which traverses the axis of the pro-
cess. " Such an apparatus appears both by position and structure
to be specially impressed by varying conditions in the inrushing
water, particles in solution or suspension in this water inducing
molecular changes in the cell at the base of the process, and per-
haps leading to the contraction of neighbouring cells- In other
words, these processes seem to act as part of an automatic
mechanism for regulating the water-currents of the organism."

432 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the tentacle is stimulated we observe that these
threads are expelled, and that they are barbed ; it will
be within the knowledge of most of us that these
thread cells, as found in jelly-fishes, are efficient organs
of offence. Their relation to the trigger-like process
suggests that these projections are the first to feel the
pressure of any foreign body, and that the pressure
communicated by them to the thread-cell or nemato-
cyst, results in the projection of the contained thread.
Here, then, we seem to have
the earliest and simplest
kind of automatic tactile
or ;;aii including, of
course, in the term touch
the general sensation of
pressure from without. It
is, at the same time, neces-
sary to observe that, al-
though these trigger-hairs

aPPear to be the simPlest

Schultze.) sense organs of a multicel-

lular or metazoic animal, yet

that some of the unicellular Protozoa are not without
organs of offence that are physiologically comparable to
the threads of thread cells, for, if we add a drop of iodine
to the water in which a Paramo3cium is swimming, we
find that it immediately thrusts out from its body fine
stiff processes. If, then, these are comparable to the
threads of a hydroid, it is clear that, functionally also,
the ectosarc of an infusorian is comparable to the
sensory parts of the epithelium of a hydroid, and is,
like it, capable of responding to definite external
stimuli in a definite way. It is important to observe
that the first indication of tactile organs is associated
with the protection of the individual, as much as with
the function of paralysing the prey which is seized
upon for food. As the sensory cells remain superficial

Chap, xii.] TACTILE ORGANS. 433

in position in the Cceleiiterata, the absence of special
tactile organs in most of the members of the group is
not to be wondered at, for the tentacles, as a whole,
may be looked upon as having a general tactile sense.

Among the Turtoellaria, trigger-hairs in con-
nection with nematocysts have been observed ; in
many cases tufts of delicate hairs have been found
scattered over the whole body, but more especially
well developed at its sides. In some there are
definite tactile organs in the shape of tentacles, which
are best developed in the anterior regions of the body,
and on which the sensory hairs are particularly nume-
rous. Thysanozoon, which is remarkable for having
the dorsal surface covered with villiform projections
of the body wall, has a bundle of such hairs at the tip
of each villus. In the earthworm, the whole body of
which is very sensitive to tactile impressions, the
anterior end is most remarkably so ; in the polychsetous
Annelids specially modified sense-cells are largely
developed on the protruding antennae and tentacles
which are developed on the praBstomium, and are
supplied by nerves which arise directly from the
cerebral ganglia ; these, as well as those on other parts
of the body, are, like the antennas of the Arthropoda
and of some Mollusca, very important aids to the
organism, for they are capable of movement laterally,
or of protrusion forwards, or of both ; they are, in
other words, able to feel about, and not, as is the case
with the earthworm, compelled to wait for the arrival
of food or foe.

In the Hirudinea the widely distributed organs
of general tactile sense are purely of epidermic origin,
and are known to be supplied with nerve fibres ; at
the anterior end of the body these cells are aggregated
to form the so-called goblet-shaped organs. According
to Whitman, special papilliform aggregations are to
be found on every segment of the body.
cc— 16

434 COMPARATIVE ANATOMY AND PHYSIOLOGY.

When the outer surface of the body becomes
hardened by the dejposit in its wall of chitin, as in the
Arthropoda, or of calcareous salts, as in the Echino-
dermata, or by the development of a shell, as in the
Mollusca, the general tactile sense becomes more dis-
tinctly limited ; this is, perhaps, least noticeable in the
Ediinodcrmata, where the superficial plexus of
nerve filaments extends over the test and along the
projecting suckers, while special nerve cells are deve-
loped in the highly sensitive pedicellariae.

In the Arthropoda the special tactile organs are
seen at their simplest in Peripatus ; in it the dorsal
surface is raised up into delicate imbricated papillae,
from the tip of which there projects a fine process. In
others they take on the forms of projecting rods. As
we all know, we have only to stroke lightly the hairs
on our own arms to discover how easily tactile sensa-
tions are conveyed by more or less stiff processes to
the sensory cells that lie at their base. Where the
greater part of the integument is hardened it is clear
that projecting rods or "hairs" will, if they be
provided with nerve fibres, and continuous with
sensory cells, convey to the underlying and protected
nervous system any movement of their free ends ; the
movement, then, of these hairs becomes in an Arthro-
pod a sense of touch ; these rods are not confined to
the antennae, for they are developed on very various
parts of the bodies of Arthropods.

Sagitta, in which there is likewise a chitinous
cuticle investing the body, has a large number of
bundles of stiff setae scattered over the surface of its
integument (Fig. 186).

Among the Chordata we find that little is
definitely known as to the tactile organs of the two
lower groups ; the only sensory cells that have as yet
been recognised in Amphioxus are of a much sim-
pler character than those which we have just been

Chap. XII.]

ORGANS OF TASTE.

435

considering ; these, which are most numerous on the
cirri and in the neighbourhood of the mouth, lie side
by side with the ordinary epithelial cells, from which
they are to be distinguished by a stiff free process, and
a basal connection with a nerve fibre, calling to mind
again the simple sense cells of the Medusae. A very
ordinary character of tactile cells among the
Verteforata is their bulb-like arrangement (see
" Elements of
Histology," chap,
xv.) ; they are,
as may be sup-
posed, widely dis-
tributed over the
whole body, al-
though, of course,
they are much
more richly de-
veloped in some

Fig. 186.— Tactile Organ of Sagitta bipunctata,
showing the long stiff setae. (After O.
Hertwig.)

in

parts than
others, and in
some forms more
than in others.

Organs of taste. — Although we may well
suppose that some sense of taste is possessed by the
lower Metazoa, we have as yet very little definite
information as to organs to which it is reasonable to
ascribe such a function. In the Echinoidea (ex-
cepting Cidaris) Loven has described, under the name
of sphseridia* organs to which he assigns a
gustatory function. These are always set around
and confined to the region of the mouth (actinostome),
where they have the general appearance of transparent
solid bodies invested by pigmented cells and a
ciliated epithelial layer. Just as the auditory organs
of some Ccelenterates appear to be modified tentacles,
so do the sphseridia remind us in the most striking

436 COMPARATIVE ANATOMY AND PHYSIOLOGY.

way of the structure of the spines of Echinids. Their
small size and protected position, under large spines
or in special cavities of the test, prevent us from
regarding them as tactile organs, while their constant
approximation to the entrance into the digestive
tract justifies us, at present, in ascribing to them the
function of testing the food which is found in the
water in which their possessor lives.

Very little is definitely known as to the organs of
taste in other Invertebrata, although, of course, most
do, on observation, exhibit some kind of preference
for certain foods ; this was seen by Mr. Darwin even
in the omnivorous earthworm. In Insects the
maxillary palpi are probably the seat of the organ,
and Lowne has described those of the blowfly as
having their cavity filled with cells, which are supplied
by a branch from the great nerve trunk of the
proboscis. Freely projecting epithelial papillae, not
unlike the gustatory organs of tadpoles, have been
observed on the tentacles of various Molluscs ; the
cells of which these papillae are composed are ciliated,
and appear to be well supplied with nerves ; their gus-
tatory function seems to have been demonstrated.

Nothing is certainly known as to gustatory organs
in the Urochordata or Cephalocfiordata. In
Fishes, the organs of this sense are only feebly
developed, and, as often happens with organs in a
generalised condition, they are not so definitely localised
as in the higher forms. The cup-shaped organs have
at their edge long cylindrical cells, with more delicate
cells in the central portion ; they are not confined to
the cavity of the mouth, but are found also on the
skin (compare the account of the teeth of Elasmo-
branchs, page 141); those that are placed on the
mucous membrane of the palate are supplied with
branches from the glosso-pharyngeal nerve. In the
carps they are described as being most largely developed

chap, xii.] ORGANS OF TASTE. 437

on the palate, on the rudimentary tongue, on the
mucous membrane which covers the inner side of the
branchial arches, and the barbels ; around the mouth,
on the skin of the head, and the rest of the body they
are less numerously developed.

In the Amphibia the cells of this sense are
grouped into discs, the so-called gustatory discs ; those
on the tongue are placed on elongated papillae, but
such as have been observed on the mucous membrane
of the palate are not known to project above the
surface, except in the region of the vomerine bones,
where, as on the tongue, the papillae that bear them
may be distinguished as fimgiform. The Amphibia
exhibit a higher form of differentiation than the
fishes, inasmuch as the gustatory cells appear to be
confined to the region of the mouth. For the majority
of the Saiiropsida it is impossible to affirm definitely
the possession of a sense of taste, and it is very
probable that in many, as in some (e.g. Birds) almost
certainly, the sensations experienced are those of a
foreign body only ; are, in fact, mechanical, and not
chemical. In Lizards and in Crocodiles there are,
however, projections of the mucous membrane (papillae)
which are provided with goblet-shaped cells, and these
may, by analogy, be reasonably supposed to have a
gustatory function.

Just as the ant-eater, and other Mammals, prove
to us that the tongue may be a seizing organ, and is
not merely the bearer of the gustatory bulbs, so, in
man at any rate, the gustatory function is not
confined to the body of the tongue, for in ourselves
the soft and part of the hard palate are also capable of
taste. The greater number of gustatory sensations
are, nevertheless, experienced through the tongue,
and we may justly say that, in this particular, the
fish stands at one, and the mammal at the other end of
the series. The majority of the gustatory cells are

438 COMPARATIVE ANATOMY AND PHYSIOLOGY.

set upon papillae, and are most numerous on the cir-
cumvallate papillae at the back of the tongue ; in
rabbits and hares a large supply of taste bulbs is to be
found on an organ developed on either side of the
root of the tongue, which is broken up into ten to
fourteen valleys, in the recesses of which the bulbs are

Fig. 187.— A, Taste Bulbs of Babbit ; B, Transverse Section through
Taste Folds of Eabbit. ( Ai ter Engelmann. )

placed. In this sense, then, as in others, we find that
the terminal sense organs are withdrawn from the
surface, protected from rough contact, and excited
only by certain definite stimuli. This must not lead
us to suppose that the gustatory sense organs offer
any exception to the rule that all organs of sense have
their origin in the epiblast of the embryo.

Olfactory organs. — Till we reach the Arttiro-
poda and Ulollusca we do not find any structures

Chap, xii.] OLFACTORY ORGANS. 439

which can be definitely asserted to have an olfactory
function. In the higher Crustacea we find organs in
the antennules which, in the crayfish, are thus dis-
posed ; the outer branch (exopodite) has attached to
the greater number of its more distal joints tufts of
short delicate bristles, flattened or papilliform at their
free ends; these bristles have granular contents, and
are supplied by fine nerve fibres. In the Insecta,
where there is only one pair of antennae, the olfactory
organ is, to judge from the
accounts of Braxton Hicks
and Lowne, placed in the
third joint of the antennae
of the blowfly ; the surface
of this joint is described as
being " covered with minute
hairs, between which are a
vast number of pellucid dots,
about 17,000 or 18,000 on
each antenna, with about *'*&• 187 A.— olfactory Appen-

• 1,1 • i dage of Exopodite of an tennule

eighty large irregular spots of Crayfish ; x soo. a. Front, b.
of a similar character." The Side View. (After Huxley.)
smaller dots appear to be the optical expression of
the orifices of minute sacculi, and the larger the
common openings of compound sacculi. This third
antennal joint is described as being filled with a cellular
pulp, through which are distributed the fibrils of the
antennary nerve.

In the Mollusca the olfactory organ ("os-
phradiiim," Lankester) is remarkable for its
constant relation to the neighbourhood of the respi-
ratory orifice, and its as constant nerve supply from
the visceral commissures ; it appears to be absent in
air-breathing forms (e.g. the snail), and we may
suppose, therefore, that it has a function in the way of
testing the water which carries the oxygen necessary
for respiration. It has ordinarily the form of a short

44° COMPARATIVE ANATOMY AND PHYSIOLOGY.

canal, which either ends blindly, or is bifurcated at its
free end ; at this end, or at the point of bifurcation,
there is a small ganglion. The cylindrical canal
-consists of a network of coiled fibrous bands, and is
invested by elongated epithelial cells, which are
directly continuous with the integument ; these cells
ara very richly supplied with nerve fibres.

Among the Ch or data no definite olfactory organ
has been recognised in the TJrodiordata ; in the
rest it always stands in close relation to the respiratory
orifice, but in nearly all fishes it is not directly
continuous with the respiratory passages. The single
pit at the anterior end of the body of AmpMoxus,
though lined with a ciliated epithelium, can by no
means be certainly said to be an olfactory organ.
The Cyclostomata have but a single pit, whence they
have been distinguished from all other Vertebrata as
the Monorrhina ; notwithstanding the single con-
dition of this pit the nerve supply is double, and we
must not, therefore, yield to the temptation to regard
this condition as being a primitive one ; in this, as in
many other points, the existing Cyclostomata show
that they stand at some distance from the primitive
vertebrate stock ; their single nasal pit is, almost
certainly, the result of the fusion of two originally
separate sacs ; this view is supported by the observation
that, in the larval lamprey, the sac is more nearly
divided into two internally than it is in the adult.
The interior of the cavity is occupied by folds, some
of which project farther inwards than others, and all
of which are covered by a mucous membrane ; to this
are distributed branches of the olfactory nerves. In
the lampreys the sac is closed posteriorly, but in
Myxinoids it opens into the cavity of the mouth.

In all the rest of the Vertebrata the olfactory
organs arise from a pair of patches of epiblast in front
of the mouth, which, as they thicken, give rise to a

Chap, xii.] OLFACTORY ORGANS. 441

pit-shaped cavity ; the epithelial cells that line this
pit are the end organs of the olfactory sense, and the
whole layer forms the so-called Sdmeideriaii
membrane, which gradually becomes more and
more elaborately folded. The sac does not remain
pit-like in fishes, but becomes connected by a groove
with the angle of the mouth ; this groove, which may
become of some depth (rays), is covered over by a fold
of the integument, the so-called nasal valve (Fig. 188) ;
so that we are able to distin-
guish an anterior and a pos-
terior orifice, the hinder of
which is in close relation to,
but is not within, the cavity of
the mouth.

Tn the Dipnoi the hinder
orifices come to lie within the

buccal area, and the same is Fig m_Nasal Groove
true or all the pentadactyle V er- of the Dog-fish,

tebrata, in which, as we ascend «* 1Sg^ffyUfS\ r\
the series, we find the posterior S!froove- (AfterGegen-
nares coming to lie farther and

farther back, as the various bones of the roof of the
mouth form outgrowths which serve as a floor for the
nasal passages. We cannot resist the supposition that
this movement in the position of the posterior nares
is in relation, firstly, to the altered mode of respiration,
the lungs taking the place of the gills ; and, secondly,
to the needs of the organism. If we may judge from
the crocodile or the whale (page 242), the elongated
passage has not essentially any relation to the olfactory
sense ; the true olfactory portion remains throughout
the Vertebrata a closed pit, and the only advantage to
it that results from the elongation of the passage is a
mechanical one. The longer air passages allow of a
more forcible inspiration, and, in consequence, of a
more forcible taking in of odoriferous particles.

442 COMPARATIVE ANATOMY AND PHYSIOLOGY.

The nasal sacs, then, of different Vertebrates, differ
only in the extent of the complication of their internal
walls, and of the membrane which covers them. This
complication is chiefly effected by the development of

Fig. 189. — Longitudinal Section through a Dog's Nose, showing the
Spongy Bones.

o, Region of the olfactory sense ; 6, air chamber (" sneezing region ") ; c, a bristle
passed thi'ough the nostril into the nasal chamber; d, a bristle from the
nasal chamber into the passage by which the latter communicates with the
mouth. (After T. J. Parker.)

cartilages, which may become more or less completely
ossified, in the upper or olfactory region ; these arise
from the side walls of the cavity, and project into it;
such bones are known as the turMnate bones.
While Reptiles and Birds have one only, Mammals
have three (excepting the Cetacea, which have none) ;
these vary greatly in form and in the extent to which
they are developed, and, as they are covered by the

chap, xii.] SMELLING AND SCENTING. 443

olfactory membrane, we may estimate the comparative
complexity of the turbinate bones by the acuteness of
smell of their possessor. Many mammals, both those
that hunt (Felidse), and those that are hunted (Cer-
vidre), have a much more acute sense, and more com-
plicated turbinate bones, than has man (Fig. 189).

Like other specialised sensory organs, the olfactory
apparatus of Vertebrates is provided with character-
istic cells, which are to be found in the lamprey almost
as well marked as in man. (See " Elements of His-
tology," Fig. 166.)

In the physiology of this sense it is necessary to
distinguish between smelling;, which is a more or
less passive act, and scenting1, which is an active
operation. Although we cannot suppose that the
latter power is well developed among Fishes, yet the
fact that the nasal valve is provided with muscles,
taken in connection with what we know as to the
habits of sharks, for example, justifies in believing that
some fishes, at any rate, are capable of scenting as well
as of smelling. In the Sauropsida a more forcible in-
spiration of air must be the chief aid, but in Mammals
the addition of external movable cartilages supplied
with muscles results in a power to enlarge or diminish
at will the size of the entrance to the nasal passages.

The external cartilaginous "nose" once formed may
become adapted to duties altogether foreign to the
olfactory sense ; it may be prolonged into a snout
which, as in the pig, may be of real use as a digging
organ, or it may become, as in the elephant, greatly
elongated, and have the functions of a prehensile trunk,
or proboscis.

The sense of sight is at first a generalised property,
many Protozoa showing themselves to be sensitive to
light. The most primitive condition of an eye or optic
organ is presented by patches of pigment which are
more sensitive to light than is protoplasm generally.

444 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Such eye- spots are possessed by a number of the
lower Invertebrata. They are, perhaps, found at their
simplest condition in a starfish, where they occupy the
ends of the arms ; and these ends are, by a muscle run-
ning along the upper surface of the arm, turned upwards
so as to be exposed to the light. There are here (Fig.
190) a number of eye-spots, each of which is made
up of several epithelial cells j these undergo a

Fig. 190. — Four separate Eye-spots of a Starfish, showing the invaginated
Epithelial Cells and the Central Cavity ; below is the plexus of
Nerve Fibres. (After Hamann.)

certain amount of invagination, and give rise to a
central cavity in their midst ; with these cells nerve
fibres become connected, and in their substance a red
pigment is deposited. Here, then, we have nothing
more than a number of epithelial sensory cells, distin-
guished by the possession of pigment ; the cuticle, it
will be observed, is not thickened into a convex cornea,
and there is no reason to suppose that the fluid in the
central cavity has any refractive action on the rays of
light.

The Medusae, or such of them as have eyes
present us with a condition which is a little in ad-
vance of what obtains in the starfish ; for, speaking

chap, xii.] OPTIC ORGANS. 445

generally, we observe a distinction between the pig-
mented and the sensory cells ; the latter are not mere
cylindrical bodies, but have their peripheral portion
converted into an elongated process, not unlike a small
rod, while they are continuous behind with ganglionic
cells. In the simplest cases there is no cornea or
lens, or organ to concentrate the rays of light \ in the
more complicated the investing cuticle becomes convex
in shape, and has, no doubt, some such function ; so
that we have now to observe an apparatus which is
composed of parts that are respectively refractive,
light-absorbing, and light-perceiving. These eyes lie
at the base of the tentacles, and have been proved by
direct experiment to be really sensitive to luminous
impressions ; specimens of Aurelia (the common jelly-
fish), which, when uninjured, were found to swim
towards a beam of light flashed upon the water in
which they were kept, were, when the eye spots were
removed, observed to exhibit no change of manner oil
the application of a similar stimulus.

The earthworm is without any organs that can bo
called eyes, and, as a general rule, we find that bur-
rowing forms are always less well provided with optic
organs than their allies which live on the surface of
the land ; at the same time the worm is sensitive to
light, and ordinarily withdraws from it ; the sensitive-
ness is confined to the anterior region of the body.
This cannot but be regarded as a very striking phe-
nomenon, when correlated with the concentrated con-
dition of their nervous system, and the fact that in
Vermes with a more diffused arrangement of the
nervous system, eyes are found in various regions of
the body.

In the lower worms, simple eye-spots are not un-
frequently present, and, as often happens with organs
in a simple or indifferent condition, they are present
in large numbers ; some Tiirtoellaria, for example,

446 COMPARATIVE ANATOMY AND PHYSIOLOGY.

have several hundreds ; they are, as a rule, best de-
veloped in the region of the cerebral ganglia, and, in
some cases, even in these low forms, they are found
on the tentacles ; pigment cells are here also separate
from sensory cells, and the latter are continued into
nervous filaments, which pass to the optic nerve.
They are turned towards all directions, but exhibit an
advance in differentiation by lying below the epithe-
lium which invests the body. Pigment spots are not
confined to the adult forms, the larva of the liver-
fluke, for example, having on its back two curved
patches, the convex sides of which are opposed to and
placed close to one another.

In higher groups, the number of eyes ordinarily be-
comes reduced, but even among the Polychaetous
Annelids we find a form (Polyophthalmus) in which
a pair of eyes is developed on every segment, in ad-
dition to those on the head. This fact, especially when
taken into consideration with the presence of eyes in
the last segment of the body in Fabricia and some
other worms, is very significant, as showing us that
sensory organs, which are essentially of epiblastic
origin, may be developed and retained on any part of
the body in which their presence is useful to their
possessor.

When the eyes become reduced in number, there
may be several pairs in the more anterior region of
the body, as in the leech, which has ten pairs ; or they
may be found on the tentacles, as in Branchiomma, or
on the gills, as in Sabella. The next step in the re-
duction is seen in the scorpion and other Arthropods,
where there are a pair of " compound " and several
pairs of " simple " eyes \ and the final step is reached
in the higher members of all groups, where the eyes
are two in number only ; in various Entomostraca
(e.g. Leptodora) the two eyes become fused in the
adult.

chap, xii.] OPTIC ORGANS. 447

The simplest condition of the final stage is to be
found in the Nautilus, where the eyes retain the primi-
tive condition of having their central cavity open to
the exterior ; the cells which line this cavity, and
which are the direct continuation of the epithelial
cells which invest the body, are converted into sensory
(retinal) cells, and are connected by nerve filaments
with the optic nerve which is given off from the cere-
bral ganglion. A. higher stage than this is to be seen
in the snail, for here the cup becomes closed up, and
there is developed in its cavity a spherical body which
has the function of a lens, while the outer wall of the
cavity plays also a part in refracting the rays of light,
owing to its having been converted into a cornea.
Peripatus has an eye which does not essentially differ
from that of the gastropodous Mollusca.

The typical eye of a well-developed Polychaetous
Annelid presents an advance upon those of the just'
mentioned Mollusca by the following characters ; the
lens does not occupy the whole of the cavity of the
eye, but is placed anteriorly, while the rest is filled by
a vitreous humour ; the lens, therefore, is more
distinctly convex, and has a greater influence on the
impinging rays of light ; the layer of rods which
lines the cavity is bounded by a distinct and well-
marked layer of pigment.

Though the physiology of the eye of a crayfish
offers some considerable difficulties which cannot as
yet be satisfactorily explained, the morphological series
is so complete that, from its point of view, much may
be made clear.

The prime difficulty lies apparently in the large
number of lenses that seem to be present in a com.
pound eye physiologically, this arrangement is
preceded by what obtains in the Chsetognath Sagitta.
In this worm, the eye, which is completely covered by
the epidermis, consists of three biconvex lenses, each

448 COMPARATIVE ANATOMY AND PHYSIOLOGY.

of which is embedded in a central pigment body, and
surrounded by fine cylindrical optic cells, which form

Figs. 191 A, 191 c.— Figures of Eyes of Arthropoda.

A, Eye of larva of Dytiscus, showing the simplest condition of a single layer of
cells (p, a, r), continuous with those of the hypodermis (to) ; each cell is dis-
tinct, and some (r) are continuous with optic nerve fibres (o) ; I, lens. (After
Grenacher.)

c, Simple eye (sternum) of fly, showing the layer (gk) of vitreous cells distinct
from the retinal layer (rt) ; c, cuticle; hyp, hypodermis ; I, lens ; fg, fat cells;
tt\ trachea ; on, optic nerve ; og, optic ganglion ; st, rods of retina. (After
Grenacher.)

a retina ; each of these cells is sharply divided into
two portions ; that which lies nearest the lens is rod-
like, the rest is granular in character. As the lenses

Chap. XII.]

OPTIC ORGANS.

449

lie in different planes it follows that light passes to
the rods from very various points.

Among the Arthropoda the simplest cases are
seen, in the larvse of various insects (Fig. 191, A);

,f

Figs. 191 B, 191 D. -Figures of Eyes of Arthropoda.

B, A single cuticular lens of Limulns, to show the aggregation of cells to form a

retinula (rt). (After Lankester and Bourne.) I, lens; rZ, retinula; op, optic

nerve.
D, Part of the compound eye of Pliri/r/anea ; the retinal cells are seen to bo united

into a retinula (r), which is differentiated into a rhabdom (m) posteriorly ;

cc, crystalline cone ; /.facet of compound eye ; pg, pigment, (After Grenacher.)

there is a single lens, the hypodermic cells that form
the sensitive elements, and some of which are con-
tinuous with filaments of the optio nerve, are simple
and separate ; these cells may be called the retinal
cells. This condition may, as in Limulus (Fig. 191 B),
be complicated by the cells, instead of remaining
separate from one another, becoming aggregated into
D D — 16

450 COMPARATIVE ANATOMY AND PHYSIOLOGY.

definite groups, each of which may be known as a
retimila. The next stage in complication is brought
about by the cells of the sensitive layer becoming
divisible into an outer (vitreous) portion, and an
inner retinal part ; this may be effected without the
elements passing through the second stage, or that
of segregation into retinulse ; as happens, for example,
with the lateral eyes of the scorpion, and the more
simple eyes of adult insects (Fig. 191 c). In the
more complex cases the retinal cells form retinulse
(Fig. 191 D).

While the sensory parts thus become more compli-
cated, the refractive element, or cornea, which is formed
by the epidermis, may likewise lose its primitive sim-
plicity as a continuous investment to the eye, and be-
come divided into a number of facets, each of which
is in connection with its proper set of sensitive cells ;
and of these lenses there may be several hundreds.
We may distinguish, therefore, an eye with one lens
from an eye with many by calling them respectively
monomeniscous and polymeniscous.

The eyes of Arthropods (Fig. 191) are, therefore,
in the classification of Lankester and Bourne :

A. MonosticJsOMS (formed by a single layer of

cells).

o. Non-retimilate, as in the larvae of insects.
)8. Refill II Sate, i. Lateral eyes of scorpions,
ii. Lateral eyes of Limulus.

B. IMjplosticllOUS (formed by a double layer of cells,

one vitreous, and one retinal).

a. Non-retinillate.— Dorsal eyes of spiders, and
simple eyes of adult insects.

£. Retinillate. — Central eyes of scorpions, com-
pound eyes of insects and Crustacea.
i. Monomeniscoiis (with a single lens).
ii. Polymeniscous (with a number of lenses).

1. Separate vitreoiis bodies.

2. Aggregated vitreous bodies.

Among the Invertebrata the highest type of eye

chap, xii.] EYES OF CHORD ATA. 451

is to be found in the dibranchiate Cephalopoda,

and it is remarkable for being protected by a cartila-
ginous orbit ; the sides of the eye are protected by a
hard layer which has been called the sclerotic;
this, which, in front, passes into the transparent cor-
nea, is either entire, or perforated in its centre by
a more or less large aperture. Beneath this is a
chamber which is not so small as in the vertebrate
eye, and which sends down a narrow process on either
side. The hinder part of this chamber is, in the long
axis of the eye, occupied by the lens, which is bounded
on either side by the iris; the hinder part of the
lens projects into the hinder or inner optic chamber,
the posterior wall of which is formed by the retina.
In this retina, as in those of nearly all Invertebrates,
the sensitive portion or layer of rods is turned
towards the impinging rays of light, and the con-
nective elements are posterior to it. We shall shortly
see that the reverse of this arrangement obtains among
Vertebrates.

The eyes of the Vertebrata are constantly paired,
and lie, as an ordinary rule, on either side of the more
anterior portion of the head ; they are always divisible
into two portions, an anterior and a posterior cham-
ber, and the hind wall of the latter, far away as it
lies from the surface of the body, is the percipient
portion of the optic organ, and has an epiblastic
origin. In comparison with any other fact as to the
vertebrate eye, this one fact stands out pre-eminently,
and first deserves our attention.

We have already learnt that in the Chordata the
central nervous system arises as an infolding of the
epiblasfc, which gradually becomes separated from the
surface of the body ; as we know, the result of this
infolding is to reverse the relations of the outer and
inner strata of the epiblast, or, so to speak, to turn
them inside out (Fig. 192; A, B). The nervous tube

452 COMPARATIVE ANATOMY AND PHYSIOLOGY.

thus formed grows out at its anterior end into three
brain vesicles ; from the foremost of these an out-
growth is given off on either side which forms the

D

Fig. 192.— Diagrams to show (A, B) the relative position of the Eods
and Cones (r) and the Nerve Fibres (n) in an Invertebrate (A)
and a chordate Eye (B). c shows the inpushing of the Cells fr^m
which r and n have their origin, cl the final result in a chonlate
Spinal Chord ; D, the outgrowing Optic Vesicle from the Brain ; E,
the formation of the Optic Cup.

optic vesicle (Fig. 191 D) ; the outer wall of this
vesicle next gets pushed in (E), but the layers retain
the relations to one another that they had in the optic

Chap. xii.] EYES OF CHORDATA. 453

cup. The difference, then, in the position of the rods and
nervous elements of a chordate as compared with that
of an invertebrate eye is due to the primary alteration
in position, caused by the mode of formation of the
central nervous tube, and by the fact that the retina
is an outgrowth from an anterior enlargement of this
tube. The stalk of the vesicle forms the optic nerve.

The other or anterior half of the eye has a his-
tory which is essentially similar to that of the eyes
of most invertebrates ; the epiblast of the surface
thickens and gives rise to the lens and cornea, while
the mesoblast forms supporting and protective tissues.

The simple eyes of the Tunicate agree in the
important point of the position of the sensory layers,
with the typical eye of the Vertebrata ; Ampluoxus
has no well-developed eyes, but eye spots have been
observed in the larva ; the eyes of the Cyclostomes
remain in a condition which is embryonic as com-
pared with that of higher forms ; a lens, for example,
is absent. Throughout the rest of the Yertebrata the
eye has essentially the same structure as in man (see
" Elements of Histology," chap, xxxvi., and " Human
Physiology," chap, xv.) ; such differences as obtain are
of importance and interest as bearing on the adapta-
tion of the different parts of the eye to the different
media in which it is placed, or to certain differences
in its duties.

In Fishes, where the aqueous and vitreous hu-
mours have but little effect in bringing to a focus
the rays of light that have already entered the water,
the antero-posterior axis of the eye is short, and the
lens is very large and convex, while the cornea, owing
to the small amount of vitreous humour that is present,
is much flattened, and, inasmuch as it is without
eyelids, it is thereby less liable to friction than if it
projected outwards. The pupil is large, so as to admit a
large quantity of light ; in Anableps, which swims with

454 COMPARATIVE ANATOMY AND PHYSIOLOGY.

its head half in and half out of the water, the cornea
is divided into two halves by a horizontal line of
conjunctival epithelium, and the pupil is similarly
divided into an upper and a lower half. The eye is
kept in shape and position by the cartilaginous or
fibrous condition of the sclerotic ; in bony fishes plates
of bone are not unfrequently deposited in this enve-
lope or coat of the eye.

The eyes of the Urodela and Csecilife are small,
and this is especially the case in the latter group,
the members of which are of burrowing habits; in
both, the skin is completely continued over the eye,
and in the Caecilise it is often quite thick. In Proteus,
the skin which covers the lens is not at all trans-
parent, and this cave-dwelling animal is so far blind
that it is apparently only able to distinguish between
light and darkness ; in much the same way, probably, as
when a man turns his face to the light and closes his
eyes, he is still able to perceive the passage of an
opaque object, such as a hand, between himself and
the light. Just as the cornea of fishes is flattened, so
that of the amphibious newt and of the frog is pro-
vided with a muscle by which the eye-ball can be re-
tracted when the animal is in tho water.

In all Reptiles the eyes are small ; but, partly owing
to the length of their bodies, we are especially struck
with the small ness of the eyes of Snakes ; in them the
pupil is generally rounded, but in some nocturnal
species it has the form of a vertical slit. There are
no eyelids, or, in other words, the skin is continued
over the eye, and this part of the integument is shed
with the rest of the skin. In most other Reptiles
there are two eyelids, in addition to the ''nictitating
membrane " which is found in some sharks and Am-
phibia, as well as in Birds, and which is drawn over
the eye by special muscles. The eye of the crocodile
is small.

Chap, xii.] EYES OF BIRDS. 455

In Birds, even more than in Reptiles, we see the
influence of the terrestrial mode of life, or rather of
the different refractive powers of air and water, in
the more convex form of the cornea. It is clear that
the rarer the atmosphere the greater is the necessity
for a convex apparatus to collect the rays of light,
and we find an arrangement in the eyes of flying
birds by which this convexity may be attained. By
the contraction of the muscles at the sides of and
behind the eye, the fluids in its two chambers, and
thus the cornea, are pressed forwards ; in some,
pressure on the optic nerve is prevented, thanks to
the possession of bony plates in the sclerotic. The
crystalline lens is flattened, except in Apteryx and the
owls that fly by twilight ; the ciliary muscle, which is
of such importance in the accommodation of the eye
(see Power's " Human Physiology,"), consists in birds,
as in reptiles, of striated muscular tissue, whereas in
mammals the muscle is of unstriated tissue; owing
to the difference in the property of these muscles, the
eye of a swif i ly-moving bird is more rapidly brought
into focus than is that of the more slowly-moving
mammal. It is not, however, unnecessary, perhaps,
to point out that this possession of striated tissue in
the ciliary muscle of the eye is not to be looked upon
as a direct adaptation to the habits of a bird, inas-
much as it is possessed also by the more lethargic
reptile; all we can say of it is that it is a very
useful heritage. Projecting into the hinder chamber
of the bird's eye is a folded membrane richly provided
with blood-vessels ; this pecten, which is found also
in the eye of reptiles, has possibly a nutrient function,
but nothing is certainly known as to the office which
it fills. Compared with the size of their body, the
eyes of birds are large, and the anterior chamber is
remarkable for having its longitudinal axis as long as
or longer than that of the hinder chamber. Bony

456 COMPARATIVE ANATOMY AND PHYSIOLOGY.

pieces are found in the sclerotic of birds, as of
reptiles.

The eyes of Mammals agree in essential points
with those of man, no mammal above the Prototheria
having a bony sclerotic ; as we ascend the scale we
observe that the eye becomes more and more com-
pletely protected, owing to the formation by the
cranial bones of a bony orbit, which is to be seen
in the dried skull. The eyes are reduced in moles
and in burrowing rodents ; in the mole-rat (Spalax)
they are covered by the skin. In those that seek
their prey by night or twilight, the cornea is larger
and more convex, and the pupil broader than in
the rest; in such, too, the lens is nearly spherical.
In aquatic forms, just as in fishes, the cornea is
much less convex than in their terrestrial allies;
compared with the bulk of their bodies, the eyes
of the Cetacea are exceedingly small.

While the body of an albino is perfectly white,
it is often a matter of astonishment that the eyes
are red ; but a little reflection will show that this
redness is due to the blood in the vessels of the
eye, and that the colour is seen in the eye, though
not in other parts of the body,, in consequence of
the transparency of its tissues. It is not so fre-
quently or so easily recognised, that the " colour
of the eye " is dependent also on this blood. In
light-grey or blue eyes no pigment is deposited in
the iris, but there is pigment in the retina, the light
reflected from which is, owing to interference, of a
blue colour. When pigment is more thickly laid
down in the retina, and becomes also deposited in
the substance of the iris, we have dark-blue or brown
eyes ; and it is because tins pigment is ordinarily
laid down after birth that we have the somewhat
strange phenomenon of the blue eyes of the babe
becoming brown eyes in a child.

Chap, xii.] OPTIC ORGANS. 457

Movements of the eyes.— It is clear that when
the number of eyes becomes limited, the power of
sight of their possessor must either be very small,
or the eye must acquire a power of movement. To
obviate this inconvenience various means have been
resorted to. The eye may, as in polymeniscous
Arthropods, be provided with a large number of
lenses, so that the whole corneal surface extends
over more than half a sphere ; in addition to this,
the body may be provided with less useful lateral
eyes, as in the scorpion ; or, as in the crayfish or
the crab, the eye may be placed at the end of a
movable stalk. Phenomena of a corresponding kind
obtain among the Mollusca; the Lamellibranchiata,
which are without prsestomial eyes, often have a
number of small eyes (or pigment spots only) deve-
loped on the edges of the mantle, and these even are
sometimes placed on stalks. In some Gastropods the
eyes are placed at the ends of the tentacles, and as
these tentacles are capable of protrusion and retrac-
tion, the optic nerve is of sufficient length to be quite
straight only when the tentacle is protruded, while,
when that organ is retracted, the nerve is looped.
In Onchidium and some of its allies, a number of
simple eyes, resembling in essential arrangement those
of the Vertebrata, are developed on the surface of the
back of these shell-less and slow-moving molluscs ;
Semper has counted as many as ninety-eight of these
eyes on the back of an Onchidium.

In the Chitonidee, Moseley has recently detected
in some species more than ten thousand minute eyes,
placed on the exposed surfaces of their shells ; but it
is remarkable that these eyes, unlike those of Onchi-
dium, are on the type of the invertebrate, and not of
the vertebrate, eye. Scattered among them are tactile
organs, from which, it is supposed, the eyes have
arisen by modification. In some of the heteropodous

458 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Molluscs the optic bulb is moved by muscles, and this
is the kind of mechanism which obtains in the Verte-
brata, where four rectal and two oblique muscles are,
as in man, almost always developed.

Influence of light and darkness on the
development of the eye.— While our knowledge
of such terrestrial forms as the earthworm, the
probeus, or the mole, would lead us to think that a
diminution of light is constantly associated with a
degenerate condition or loss of the eye, it is very
remarkable that among aquatic forms we find species
which support, and others that unmistakably con-
tradict this hypothesis. Among Crustacea we have,
for example, Ethusa granulata, which in shallow
water has eyes of the ordinary character, but, when
taken from depths of 110 to 370 fathoms, is found
to have the stalked eye replaced by a calcareous
knob ; it is a case to which the words of Darwin
are altogether applicable : " The stand for the tele-
scope is there, though the telescope with its glasses
has been lost." Specimens of the same species,
taken from a greater depth (500 to 700 fathoms),
showed that the eye-stalk had undergone a change
of function, and had become converted into a
pointed rostrum, which probably serves as an organ
of touch. On the other hand, species of Munida
that were dredged at the same time as the Ethusa
were found to have exceedingly large eyes. What is
true of Crustacea is true also of fishes ; at moderate
depths the eyes are generally large, at greater depths
they may be very large or very small. Where the
eyes are small the fish has its tactile organs very
largely developed. Fishes or Crustaceans, however,
taken from very great depths (e.g. 1,900 fathoms),
have been found to have both the eyes rudimentary
and the special organs of touch absent. There is no
reason to suppose that the so-called eye-like organs of

chap, xii ] EAR OF MEDUSA. 459

some fishes (e.g. Scopelus) have any optic function.
They appear, from the accounts given by those who
have seen such fishes alive, to be phosphorescent
organs.

The ear.— Definite auditory organs are wanting
in various lower forms, which are, so far as we can
tell, without any sense of hearing ; an intermediate
condition, between that of absolute incapacity to hear
and the possession of this sense, is, perhaps, presented
to us by the earless earthworm, on which the vibra-
tions of air which are heard by man are absolutely
without effect, though the worms are very sensitive to
vibrations conveyed along solid objects ; in the ex-
periments made by Darwin, "the vibrations, before
reaching their bodies, had to pass from the sounding
board of the piano, through the saucer, the bottom of
the pot and the damp, not very compact earth on
which they lay with their tails in their burrows ; "
though the connection was so slight, yet the result of
striking a note on the piano was the immediate with-
drawal of the worms into their burrows.

Definite auditory organs of a low degree of or-
ganisation are to be found in some Medusae, but it
is a remarkable fact that, so far as we know at pre-
sent, distinct auditory and optic organs are never
developed in one and the same species. A very simple
condition is found in such a jelly-fish as Euchilota
or Tiaropsis, where the lower surface of the velum
is, at various points, indented by open-mouthed pits ;
the sides and base of this pit are formed by the
epithelial cells of the velum, and of these some on
the inner face become provided with projecting audi-
tory hairs, and so become special sense cells, while
others develop within their contents the calcareous
concretions which form the otolith ; the sense cells are
continuous by their bases with the lower nerve ring.
The epithelial cells on the outer surface of the pit

460 COMPARATIVE ANATOMY AND PHYSIOLOGY.

(which belong, of course, to the series of cells that
lie on the upper surface of the velum) have thick
membranes, and become filled with fluid. Mitrotrocha
is provided with no less than eighty such open pits.

The next stage in advance is seen in Phialidium
(Fig. 193), where the pit becomes closed. In other
Medusae the auditory vesicle appears to be a modified

Fig. 193.— An Auditory Vesicle of Phialidium.

d', Epithelium of the upper surface of the velum ; eft, of the under surface ;
nr', upper nerve ring; ft, auditory cells; hh, auditory hairs ; np, nervous
cushion formed by a prolongation of the lower nerve ring ; r, circular canal
at the edge of the velum. The; spot in the cavity represents an otolith.
(After 0. and R. Hertwig.)

tentacle. In some cases the sense organs appear to be
both eye and ear.

Throughout the whole of the Metazoa we find
that the auditory organs have more or less the form
of vesicles, which in lowlier forms, and in all primi-
tively, are open to the exterior ; within the vesicles
one or more hard bodies are developed, which, on being
agitated by the vibrations of the waves of sound, act
on the sensory hairs of the sense cells, which are
developed within the auditory cavity, and which are
connected with the central nervous system by the
fibres of the auditory nerve.

Chap, xii.] AUDITORY ORGANS. 461

Among the Eclunodermata, auditory organs
have only been satisfactorily observed in the deep
sea-dwelling holothurian Elasipoda, where they
are often present in large numbers, Kolga nana hav-
ing no less than fifty-six auditory sacs; as in such
Annelids as possess them, the sacs are set close to
the nerve cords, and have a large number of contained
otoliths or concretions.

We have more definite information as to the
Arthropoda and Mollusca; in the former they
are not always developed on the head ; Mysis, just like
Fabricia (among worms) with its posterior eyes,
showing us that, inasmuch as sensory cells are distri-
buted over the whole of the body, special sense organs
may be developed at any part, and pointing the moral
by having auditory organs in its terminal segment.
As a rule they are, as in the crayfish, developed at
the base of the antennules. Here the auditory sac is
permanently open, though the seta3 that protect it
prevent the entrance of much foreign matter ; within
this sac part of the wall is raised up into a ridge, and
the cells that form it are provided with delicate setae
at their free end, and with nerve fibres at their base
and within. The sac is filled with a gelatinous fluid
in which are to be found minute otoliths ; these last,
being set in motion by vibrations in the water which
strike on the guarded open slit of the ear sac, affect
the setae ; the setae affect the cells on the acoustic
ridge, and so the contained nerve fibres which are
in direct connection with the brain.

While the sac of the Crustacea resembles in some
respects the embryonic ear of the Vertebrata, that of
Insects presents other points of similarity. Placed
on the median segments of the body or on the legs,
the ears of the Orthoptera are remarkable for the
possession of a drum or tympanum ; this is merely a
modification of part of the chitinous integument of the

462 COMPARATIVE ANATOMY AND PHYSIOLOGY.

insect, and the bow on which it is stretched is merely
a part of the same integument that has become con-
siderably thickened. Taking for an example the
grasshopper, we find a number of small muscles
which are inserted into the bow, and by their exten-
sion or contraction increase or diminish the tension
of the tympanic membrane. The central portion of
the membrane is occupied by a cavity which commu-
nicates with the exterior by an open tube, and within
the cavity there is a ganglionic mass developed at the
end of an auditory nerve. Here, then, we have the
case of the sonorous vibrations impinging on a mem-
brane, which is held tense ; being conveyed to an air
chamber, which is in relation with the outer air, and
is therefore capable of adapting itself to any force that
may be brought to bear upon it ; and being carried
thence to the termination of a sensory nerve.

In some insects, though not in the Orthoptera,
rows of corpuscles have been observed on some of the
nervures of the wing, and as these are supplied witli
nerve filaments, Braxton Hicks has suggested that
they are auditory organs ; somewhat similar organs
found on the halteres of the Diptera have had the same
function ascribed to them by Lowne.

In the ears, as in some other parts of the organi-
sation of the Mollusca, we see arrangements which
are simpler, and others that are more complex than
those that obtain in the Arthropoda. The auditory
organ is often a simple closed vesicle, surrounded by an
investing membrane, and having in its cavity sensory
cells provided with projecting hairs (Fig. 194); the
central cavity is occupied by a single large concretion,
otolitli, or a number of smaller otoconia, as the
smaller concretions may conveniently be called. In
the Nautilus, the ears, as in most Lamellibranchs and
Gastropods, are attached to the pedal ganglia, but in
the Dibranchiata they are enclosed in the cartilage

Chap. xii.] EAR OF CHORDATA. 463

which protects the cerebrum and, as such portion of
the cartilage forms a special investment for the ear,
we have in these alone, among invertebrates, that dis-
tinction between the outer or cartilaginous, and the
inner or membranous ear capsule to which we are
accustomed in the ears of vertebrates ; in Cephalopods,
as in Crustacea and Vertebrates, an acoustic ridge or
crest is formed on which are set the auditory cells, and
in some of them, as in some of the lower Vertebrates, it
appears that the ear sac is permanently in communica-
tion with the outer world by a nar-
row open duct, the remnant of the
primitive involution of the epiblast
from which the organ was fashioned.

Amphioxus has no known
auditory organ, and that which is
found in the Urochordata would
appear to .have been independently Fig. 194,-Diapram
developed within the limits of the cycSs! E(After
group (Balfour). It lies on the Simroth.)
under surface of the anterior brain
vesicle (Fig. 195 ; a) ; the cells of the brain form an
acoustic ridge, the delicate hairs on whose cells hold
an otolith which projects into the cavity of the brain,
and is remarkable for being pigmented.

Though the sensory portion of the ear of higher
Vertebrates is at some distance from the surface of
the body, it is not to be supposed that it has not, like
all other organs of sense, its primary seat of origin in
the outer layer of the embryo or epiblast. In the
lower vertebrates the auditory capsule is closed, and
lies just below the skin, or sinks some way into the
walls of the brain case, as in Elasmobranchs, where
the duct either opens to the exterior by a minute
pore (ray), or is closed over by the skin (sharks) ; in
the higher forms a special auditory passage is deve-
loped.

464 COMPARATIVE ANATOMY AND PHYSIOLOGY.

The first sign of the development of the ear is the
formation, on either side of the hinder part of the head,
of a pit or depression (Fig. 196 ; au v), which gradually
deepens, and with which an outgrowth from the audi-
tory nerve (au n) comes into contact. The pit becomes
converted into an elongated sac, narrower above and
below. The upper end grows out and gives rise to
the recess of the membranous labyrinth, the lower

Fig. 195. — Larva of Ascidia mentula ; the Anterior portion of the tail is
alone represented.

Nl, Anterior swelling of neural tube; N, anterior swelling of spinal portion of
neural tube; n, hinder part of neural tube ;' c/i, notoohorcl: K, {branchial
region of alimentary tract ; d, its cesophageal and gastric region; o, eye; a,
otolith. ; o', moutli ; s, papilla of attachment. (After Kuppffer.)

end forms the cochlear canal, and the narrow duct
(canalis reuniens) by means of which the cochlear
canal communicates with the sacculus ; the median
portion of the auditory vesicles gives rise to the semi-
circular canals, and to the utriculus.

Fig. 197 (page 466) will make clear the relations
of the just named and of other parts in the complex
auditory organ of man.

External to the proper sensory portion in which
the branches of the auditory nerves terminate in the
special sensory cells, there is a median division of the
ear, in which are developed one or more bones that
convey the sonorous vibrations, and these are acted

Chap. XII.]

EAR OF VERTEBRATA.

465

on by a membrane (the tympanic membrane)

which is analogous to, though, of course, not homo-
logous with, the membrane which we have already found
in the ears of orthopterous insects. Externally to this
middle ear is the outer portion, which forms the
external auditory meatus, and is in higher forms
aided and protected by a conch, or so-called ex-
ternal ear.

Outer division of the ear, — The functions of

awn

-ftlEV

Fig. 196.— Section through the Head of a Lepidosteus, Six Days after

Impregnation

au v, Auditory pit ; au n, auditory nerve ; cJi, notochord ; by, hypoblast. (After
Balfour.)

the outer and middle portions is that of conveying the
sonorous vibrations to the sensory region internal to
them ; they are but poorly developed in the lower
forms for two reasons : first of all, the insinking of the
sensory portion which is so marked in the higher forms
is, as we may expect, less marked in the lower divi-
sions of the Yertebrata, for it is a process which has
only gradually been effected ; the second reason is to be
found in the fact that the trumpet-shaped arrange-
ments of the outer region, which are so useful in
bringing to a focus the vibrations of air, are not so
necessary when the animal lives in water. Evidence
as to the influence of this denser medium is afforded us
E E— 16

466 COMPARATIVE ANATOMY AND PHYSIOLOGY.

by the Cetacea, which, at most, have nothing more
than an exceedingly rudimentary external ear (pinna),
and this, when found in these mammals, is proportion-
ately longer in the foetus than in the adult (Howse).

Fig. 197.— Representation of the Human Ear.

e. External ear; me, auditory passage (meat us auditorius extermifO; I, bony
labyrinth ; vm, auditory nerve ; o, ossicles of the ear ; fo, fenestra ovalis ;
eu, Eustachiah tube, which opens into the pharynx ; t, tympanum ; c, bony
cochlea ; tin, tympauic membrane.

Fishes are without any external ear or tympanic
membrane, and, as has already been said, the Elasmo-
branchs have a canal which reaches to, and may open
on, the outer surface of the skull ; in many Teleostei,
where there is no such external opening, there are
spaces in the skull which are only covered by skin or
very thin bone in the neighbourhood of the ear;
in some of the bony fishes there is an exceedingly

Chap, xi LI EAR OF VERTEBRATA. 467

remarkable arrangement, by means of which sounds are
conveyed to the organ of hearing. The anterior ends
of the air bladder are attached to membranes which
close in spaces in the occipital region of the skull,
and on the other side of these membranes is the ear of
that side. This simple condition which obtains in the
perch and its allies, is complicated in carps and others
by the addition of three ossicles, which connect the air
bladder with the auditory region, and convey such
vibrations as affect the air in the air bladder.

In the lower Amphibia the ear cleft is merely
closed by muscles, but in the Anura there is a distinct
tympanic membrane, as there is also in most, though
not in all, Reptiles, In the simplest condition this
lies on the surface of the head, so that there is no ex-
ternal auditory meatus ; but in the lizards we have a
small pit external to the membrane, and we have,
therefore, the commencement of an external ear
passage.

In Mammals the tympanic bone of the skull
takes part in forming the walls of this meatus. The
pinna is represented by a fold of skin with combined
muscular tissue in crocodiles, and by a movable mem-
branous valve in some birds (owls). It is not found in
the Prototheria, but in all other Mammals there is a
well-developed outer ear, which becomes rudimentary
or aborted only in marine forms. In many mammals
the pinna can be moved by muscles, and directed,
therefore, to different points from which sound is sup-
posed to be coming. In the higher Primates these
muscles are ordinarily rudimentary, but their posses-
sion by some men is spoken to by the power that such
have of moving their ears.

Middle car. — Associated with the development
of a tympanic membrane is that of a contained tym-
panic cavity. This cavity is not a new formation, but
is due to the modification of the, in the pulmonate

468 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Vertebrates, now useless visceral clefts. While on its
outer side this cleft gives rise to the tympanic cavity
(Fig. 197 ; c£) and part of the external meatus, on its
inner side it continues to communicate with the
pharynx, and so forms the Eustachian tube (e,u),
by means of which air can enter into the otherwise
closed ear chamber. The vibrations of the tympanic
membrane have to be conveyed to the inner ear, and
this is effected by one or more bones, the outer end of
which is fixed to the tympanic membrane, while the
other impinges on the membrane which closes the en-
trance to the internal ear (fenestra, ovalis, Fig. 197).
In the Amphibia and Sauropsida there is but a single
bone in the middle ear ; the origin of this columella
is not satisfactorily established, but it is, probably, the
upper end of the hyoid arch. (See page 327.) In the
Mammalia this single bone is replaced by three,
which are known respectively as the stapes, incus,
and malleus ; the discussion of the homologies of
these bones has been deep and protracted, but a con-
sideration of it would be beyond our scope here. We
must be content to say that, according to the latest
views of Parker, the stapes is the uppermost element
of the hyoid, and that the incus and malleus belong to
the mandibular arch. According to these views it is
the incus, and not, as is ordinarily taught, the malleus,
that is the homologue of the quadrate bone of the
Sauropsida. In the Prototheria the stapes is not
hollowed out at one end so as to have the form of a
stirrup, nor is the incus of the " normal mammalian
form " ; Parker further finds that the malleus still
forms the hinder part of the mandible.

We have in this history of the auditory ossicles
(which must not by any means be confounded with
otoliths) one of the most remarkable examples of the
way in which parts useless for one are gradually ap-
propriated to another function. The cleft between

Chap, xii.] INTERNAL EAR. 469

the hyoid and mandibular arches (the hyomandibular
cleft) becoming useless as an organ of respiration has
been seized upon by the ear ; in Mammals, parts of the
mandibular arch that lies in front, and of the hyoid
arch that lies behind the cleft, are adapted to the use of
one and the same organ.

Internal ear. — This, which is the essential
portion, as those just described are the accessory
parts, of the organ of hearing, consists primarily of
the so-called membranous labyrinth, formed by the
sacculus, utriculus, and semicircular canals ; as we
ascend the scale we find a bony labyrinth fashioned
around this membranous one ; the space between them
contains lymph, and is known as the perilymphatic
space or cavity, while the fluid within the membranous
labyrinth is the endolymph.

The simplest stage obtains in the Myxinoid round-
mouths, where there is only a single semicircular
canal, at the base of which there is, on either side, a
swelling or ampulla. In this, as in the underlying
vestibule, special auditory cells are developed, which
are supplied with filaments from the auditory nerve.
In the other division of the round-mouths, that is, in
the lampreys, there are two semicircular canals, and
the vestibule into which they open has two blind
diverticula arising from it, in each of which auditory
cells are developed. In all the remaining Vertebrata
there are three semicircular canals.

In the Elasmobranch fishes, and in the Dipnoi, no
bony labyrinth is formed around the membranous, but
a promise, as it were, is offered by the excavation of
the cartilage around the labyrinth, in a form not un-
like the membranous internal ear ; in the bony fishes
the labyrinth is protected by bone, but there is no
proper bony labyrinth. A further point of advance is
to be observed in the Ganoidei and Teleostei, for in
them, as in Chimsera, the gradual insinking of the ear

470 COMPARATIVE ANATOMY AND PHYSIOLOGY.

is expressed by the fact that the most internal portion
now projects into the cranial cavity.

In no fish is there anything more than a rudiment
of the cochlea (Fig. 198 ; A, c). The highest condition
of the amphibian ear is seen in the Anura ; though
we cannot yet speak definitely of a bony labyrinth,

Fig. 198.— Diagrams to show the Relations of the Auditory Labyrinth in
the Vertebrate Series.

A, Pish ; B, bird ; c, mammal ; u, utriculus, with the three semicircular canals !
s, sacculus: c, cochlea; r, aquaeductus vestihuli ; 6, lagena; cr, canalis
reuniens. In c, r is seen to divide into separate passages for the utriculus
and sacculus ; the vestibule is seen to have a caecal sac at v ; k, coil of the
cochlea. (Af te r Waldeyer.)

yet we can, in the frog, see that the membranous laby-
rinth within has an influence on the contour of the
surface of its bony and cartilaginous capsule. Two
foramina, the f. rotund um and the f. ovale, are now
to be distinguished. Two tubular outgrowths are
given off from the periosteum of the perilymphatic
space ; both of them end in blind sacs, and one of
them (the ductus perilymphaticus) extends into the
jugular canal, and part of the neck of the sac is

chap, xii.j EAR OF MAMMALS. 471

continued into a tube which enters into connection
with the subarachnoid cerebral cavity. Eight different
sensory regions are now to be distinguished ; the
cochlear region has a commencing outgrowth or lagena,
and within is a space which is covered over by a very
thin membrane, the membrana basilaris.

In the Reptilia we observe several stages in
the outgrowth of the cochlea, and these are most
marked in crocodiles, which, in this character, as in so
many others, stand nearest to the birds. In these last
the lagena is quite prominent (Fig. 198; B, b), and
begins to take on a spiral course.

With the exception of the Prototheria, all Mammals
have their cochlea coiled into a heliciform spiral, the
canal of which is wound round the bony axis or
modiolus ; the coil may be flat, as in the Cetacea, or
very steep, as in some Rodents (guinea- pig) (Fig. 198 ;
c). The internal structure of the cochlea has been
fully described in the " Elements of Histology " (chap,
xii.) ; here it need only be said that the seal a vestibuli,
the membrane of Reissner, and the rods of Corti are
peculiar to the mammalian ear; as to
the last, we have so far evidence that
it has been developed within the
limits of the mammalian series that
we find them to be much more simply
arranged in the Prototheria than in
the higher mammals. The absence of
this organ from the ears of birds,
many of which are, as we know,
capable of being attracted by musical
sounds, makes it impossible for us at present to accept
the doctrine that these rods are physiologically impor-
tant as the means of distinguishing different notes of
music.

The otoliths found in the lymph of the membranous
labyrinth are ordinarily larger in fishes than in higher

472 COMPARATIVE ANATOMY AND PHYSIOLOGY.

vertebrates; and their number is, of course, proportional
to their size. Most bony fishes have two only, but
these are rather to be looked upon as calcareous
masses than as separate otoliths ; in Elasmobranchs
such otoliths are often grouped into masses of various
sizes and forms. In the Teleostei they are crystal-
line, but in Chimsera and the sturgeons they are
more chalky in character. Their function is, as in
invertebrates, that of aiding the vibrating fluid in
its action on the sensory cells of the auditory crest.

CHAPTER XIII.

ORGANS OF REPRODUCTION.

IN the preceding chapters we have considered the
various organs of the body by means of which an
animal is enabled to sustain or defend its own exis-
tence, to obtain information of what is happening around
it, and to adapt itself more or less successfully to the
course of events. So far as an individual animal is
concerned, no other organs than those with which we
have already dealt are necessary for the maintenance
of its own existence ; indeed, there are, we know,
individuals which do pass through the whole of their
lives, are born, grow, and die, without once putting
into active function the set of organs that remain to
be considered.

Unimportant as they may be to the individual, they
are of prime importance for the species to which
that individual belongs ; for they are the means by
which individuals are enabled to reproduce their
kind ; and they are of the more importance inasmuch
as, so far as we know, living matter never arises or is
formed except from pre-existing living matter. In

Chap, xni.] REPRODUCTION OF PROTOZOA. 473

the performance of that part of his life-work which
affects his race, the individual reproduces his kind.

This process of reproduction may be one of two
modes, it may either be sexual or asexual ; that is
to say, two different cell elements may unite to form
a single cell from which others arise, or one kind of
cell element alone may form a new individual.

The latter is obviously a more simple method than
the former, and it is the only one which is certainly
known to obtain in the Protozoa. Here, too, as
our previous studies would lead us to expect, there is
no distinct differentiation of any special organ ;* we
have the phenomenon, but not the organ.

As has been already pointed out in the case of the
Amoeba (see page 22), the simplest method of repro-
duction is that in which the mass of protoplasm under
observation divides into halves of about the same size.
Each of these halves is, save in size, a copy of the
parent; which, ipso facto, has disappeared. This
method of reproduction is known as Fission, and it
is exceedingly common among the lower organisms.
In some cases a process of non-nucleated protoplasm
separates from the body of the Amoeba ; and this bud-
like outgrowth, increasing in size and acquiring a
nucleus, shortly comes to have the form and structure
of its parent. This is the process by Oemmation.

Yet a third mode of reproduction, which may be
called, that of endo-spore formation, has been ob-
served in some of the Protozoa ; but it, like the
methods of fission and gemmation, does not require
the influence of another individual ; like them, it is
absolutely asexual. The protozoon, becoming qui-
escent, forms around itself an envelope or cyst, which
is at first transparent, and which completely encloses

* The action and influence of the nucleus of a cell is so obscure
that the part which it possibly takes in initiating cell-division
cannot be discussed in an elementary work.

474 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the rounded cell. The nucleus at the same time becomes
invested in a proper capsule. After a period of repose,
the nucleus begins to break up into a number of
smaller pieces, the wall now breaks, and each nuclear
portion (spore) uniting itself with a certain quantity
of the surrounding protoplasm, separates from the rest
of the mass, escapes, and begins to grow up into the
form of the parent cell.

Lastly, we sometimes observe that two individual
Protozoa of the same species become connected with
one another, the protoplasm of the two cells becomes
commingled, the whole quiescent, and invested in a
cyst. The enclosed contents break up into a number
of spores, which, on the eventual bursting of the cyst,
escape and begin to grow up into the form of their
parent. Here, then, not only is spore-formation pre-
ceded by encystation, but also by conjugation. At
the same time it is to be most carefully borne in mind
that the two individuals are, to all appearance, essen-
tially alike, and that there is no reason whatever for
regarding this conjugation as being a sexual act.

The life-history of the Gregarina presents us with
a case of development by spore formation, which may
or may not be preceded by conjugation. A single
Gregarine, or two conjugated forms, become spherical,
and a firm structureless cyst is gradually developed
around the protoplasm ; the nucleus disappears, and
the whole of the contained mass breaks up into a
number of small separate bodies (spores) ; these are
often spindle-shaped, and from their occasional resem-
blance to the diatom Navicula, have obtained the dis-
tinctive name of psetidonavicellse. This appears
to be the most general mode of spore formation. The
spores become each invested in a distinct envelope,
within which the protoplasm is contained. The suc-
ceding stages of development have as yet been very
insufficiently studied ; in the large Gregarine of the

chap, xiii.] DEVELOPMENT OF GREGARINA.

475

lobster the following stages have, however, been
observed.

The protoplasm, which has not been directly ob-
served escaping from the spore, is first seen as a small
amcebiform and apparently non-nucleated mass.
Passing into a quiescent condition, it becomes differ-
entiated into ectosarc and endosarc, and then gives rise

Fig. 200.— Figures of Gregarina of Earthworm.

A, Separate form ; B, encystment completed ; c, formation of pseudonavicellae.
(After Stein and LieberkUun.)

to two processes, one of which is stiff, and the other
actively motile ; in the latter granules are richly de-
veloped, and it is the first to become elongated and to
separate from the parent mass. It has now the form
and something of the movement of a thread-worm,
whence it has been called the pseudo - filaria ;
within this elongated mass a nucleolus and a nucleus
become apparent, the tube shortens, becomes divided
into protomerit and deutomerit, and, later on, deve-
lops a cuticle ; so that we have here a minute
example of the giant Gregarine. The stiff process has
meanwhile absorbed the remainder of the parent

47 6 COMPARATIVE ANATOMY AND PHYSIOLOGY.

protoplasm, has become motile, and been converted
into a pseudofilaria.

The statement that a kind of sexual reproduction is
observable among the Infusoria has, on account of
the detailed characters of the reported observations,
obtained considerable vogue. Put shortly, the account
comes to this ; the substance of the nucleolus becomes
converted into a number of curved rods, which repre-
sent the male element, while the nucleus breaks up
into small spherical bodies, which have been compared
to ovules. The resulting young are said to be at
first provided with knobbed tentacles and suckers
(acinetiform embryos), and to become gradually con-
verted into ciliate infusoria. Observations undertaken
with the express view of examining into these results,
have done anything but confirm them, for they have
resulted in the conviction that the rod-shaped bodies
of Balbiani are nothing but bacterioid parasites
(Vibrios), and that the so-called embryos are also para-
sites ; these last, indeed, have, on direct observation,
been seen to enter the body of one after escaping from
another Infusorian. What really does happen appears
to be this ; two individuals (of Paramoecium) conjugate,
and remain united for a day or longer ; the only result
of this conjugation is that the nucleus becomes more
finely granular, while the nucleolus breaks up into
four oval capsules. Of these, two in each individual
disappear, while the other two grow till they reach
two-thirds the size of the original nucleus, which they
then resemble in appearance ; one of these remains
as a nucleolus, and the other appears to fuse with the
primitive nucleus.

The process, then, instead of being in any way sex-
ual, falls rather under the head of rejuvenescence ;
the protoplasm, in other words, seems to undergo a
kind of re-arrangement in much the same way as, in
the political world, cabinets sometimes do.

Chap. XIII.]

SPERMATOZOA.

477

Where the method of reproduction becomes
sexual, we find sets of cells or glands which have a
different history and function ; these are the male
and female elements, and they may be found sepa-
rately in different individuals of the same species, or
they may both be formed in one individual ; in the

Fig. 201.— Figure of Spermatozoon of, A, Guinea-pig (not quite mature) ;
B, the same seen sideways; c, Horse ; D, Newt. (After Klein.)

latter case we have to do with hermaphrodite forms,
and these may be only structurally hermaphrodite, as
are the earthworm and the snail ; or they may also be
physiologically hermaphrodite, as the tapeworm, the
fluke, or the Ascaris nigrovenosa ; that is to say, the
male elements of one individual sometimes impregnate
the female cells of another individual, and in other
cases the two kinds of sexual cells of one and the same
individual come into union. It ordinarily happens

478 COMPARATIVE ANATOMY AND PHYSIOLOGY.

-c

that the set of cells which give rise to one are quite
separate from those which give rise to the other sexual
cells, but this is not always the case, as the herma-
phrodite gland of the snail and the generative cells of
the just-mentioned Ascaris are sufficient to bear
witness.

The broad differences between a male and a
female element may be easily apprehended ; starting
from cells which are essentially similar in character,
those which are to become the
male bodies subdivide, and
each cell gives rise to a large
number of smaller bodies,
which typically, though by no
means always, consist of a
rounded head (which repre-
sents part of the nucleus of
the original cell), and a more
or less long, actively moving
tail (Fig. 201). The female
cell (Fig. 202), on the other
hand, increases rather than
diminishes in size, and often

acquires considerable bulk from the large number of
yolk cells that are aggregated around it ; it frequently
also becomes invested in a membrane, the outermost
portion of which may, as in the familiar example of
the egg of the bird, form the basis for a shell, which
may be calcareous or chitinous.

The generative cells are, in their simplest condition,
nothing more than modified elements of the epithelial
layer which lines the body cavity, and it is only with
increasing differentiation of structure that they
become aggregated into definite masses holding a
certain topographical relation to the other parts of
the organism. The influence of the male on the
female element will be described shortly (page 482).

Fig. 202.— Eipe Ovtun of Cat.

a, Zona pellucida ; b, germinal
vesicle ; c, protoplasm.
(After Klein.)

Chap, xiii.] SPERMATOGENESIS. 479

We must first develop in detail the characters of the
parts whose general morphology has just been
sketched.

It is only lately that much attention has been
given to spermatogenesis, or the history of the
development of the spermatozoa, and it will be most
convenient, therefore, to give an account of a common
form (the earthworm) in which the process seems to

Fig. 203. — Figures showing the Mode of Development of the Sperma-
tozoon of the Earthworm. A, Spermatospore ; B, 5Toung Sperma-
tosphere, with eight Spefmatoblasts ; c, Spermatoblasts with
Central Blastophore ; D, Spermatoblasts with protruding Filament
(After J. E. Blomfield.)

have been worked out (by Blomfield) with great
exactness.

The testis of the earthworm is a body of irregularly
quadrate form, which is about one-tenth of an inch in
diameter, and is directly attached to, and seems to
form a modified part of, the epithelium lining the
body cavity; it consists of a mass of cells, each of
which, breaking away from the common mass, makes
its way into a special reservoir, there to undergo its
further development. Each of these cells may be
known as a spermatospore, and is distinguished
by the comparatively large size of its nucleus, and its
thin coat of surrounding protoplasm ; the nuclei of
these spermatospores undergo division, and the whole
mass increases in size. When eight segments have
been thus formed we get the spermatospliere,

480 COMPARATIVE ANATOMY AND PHYSIOLOGY.

which consists of eight spermatoblasts, with a
small central mass of inactive protoplasm (folasto-
phore). Division of the cells still continues, until at
last we get a spermatosphere, which consists of a
number of elongated spermatoblasts supported by the
blastophore (Fig. 203 ; c). The protoplasm around
the nucleus of each spermatoblast next collects into a
small cap, and then gives off' a delicate filamentous
process (Fig. 203 ; D), which, gradually increasing in
size, comes to form the tail of the spermatozoon.
Further changes in form are effected, and the con-
stituent spermatoblasts of the sphere fall away from
one another, to become, each of them, an actively
motile spermatozoon capable of fertilising a female
cell.

The essential points in this history have been
detected by various observers in other animals, many
of whom have, however, somewhat obscured the
subject by the number of new technical terms which
they have introduced.

Oogenesis, or the development of ova, has
been more thoroughly studied than spermatogenesis,
but the subject is rendered more complicated by the
fact that the egg cell either absorbs in early periods,
or is for a time surrounded by nutrient or yolk cells.
The egg cells of the earthworm form a coherent mass,
which occupies a similar position in the thirteenth to
that occupied by the testes in the tenth and eleventh
segments, and is only distinguished by the investment
of firm membrane, which surrounds the mass of cells,
or ovary, and separates it from the rest of the
epithelium of the body cavity ; the constituent cells
of this ovary do not, however, undergo the segmenta-
tion which affects the male elements.

Consisting, in the simplest cases (e.g. Hydra), of a
naked mass of protoplasm, the ovum, with its nucleus
(here called germinal vesicle) and nucleolus (or

Chap, xiii.] HISTORY OF THE OVUM. 481

germinal spot), brings to our mind the Amoeba,
with which our studies commenced ; and, if we observe
its early behaviour, we are the more struck with the
resemblance, for we often find it seizing on and
making part of itself the cells which surround it. In
the great majority of cases the cell becomes so far
differentiated that it develops around itself an in-
vesting (vitelliiie) membrane ; here, again, recalling
to mind the next stage in protozoic differentiation in
so far as protoplasmic pseudopodial processes pass
through the pores in the membranous wall (Toxo-
pneustes). In more elaborated stages the surrounding
cells of the ovary give rise to more specialised
membranes, and in some cases it appears to be
necessary to leave an orifice (so-called " micropyle "),
by means of which nutrient material or fertilising
elements may be allowed to enter and come into
contact with the substance of the egg.

The final act in the maturation of the ovum
appears to be the extrusion of the two polar
globules. The nucleus of the egg cell (the ger-
minal vesicle) moves towards the periphery of
the cell ; as it does so its surrounding membrane
becomes absorbed, and the contents altered in
character. What remains of the germinal vesicle
becomes spindle-shaped, and one end of the spindle is
protruded from the edge of the cell. The projecting
portion is next constricted from the rest, and so gives
rise to the first polar globule. The process is again
repeated, a second spindle being formed, and the
projection being again constricted off" to give rise to
the second polar globule.

Whatever be the real explanation of this pheno-
menon, it is, in the first place, clear that it bears
a very striking analogy to what happens in the
male cell, where a portion of the original protoplasm
becomes the inactive blastophor ; and we can hardly
FF— 16

482 COMPARATIVE ANATOMY AND PHYSIOLOGY.

help giving in our adhesion to so much of the
doctrines of Sabatier as are contained in his thesis
that all cells are originally hermaphrodite, and that
some by losing one element, and some the other,
become distinctively male or female cells. Balfour
has enunciated the striking and bold hypothesis that
the function of forming polar cells was acquired by
the ovum with the object of preventing partheno-
genesis ; the strongest support for this doctrine was
found by Balfour in the reported absence of polar
globules in the only two divisions of the animal
kingdom (Rotifera and Arthropoda) in which we
normally find development of ova without male
influence. On the other hand, Billet, with a full
knowledge of the facts, and of Balfour's hypothesis,
has lately recorded the presence of polar globules in
the Rotifera, and Grobben has given a less pointed
account of the formation of the same bodies in some
of the lower Crustacea.* The remaining portion of
the original nucleus returns to the centre of the egg,
where it forms the female promicleiis. The
mature egg, or female element, requires the addition
of the male element or spermatozoon, before it can
set out on the course of its development. When
brought into the neighbourhood of the male cell we
find that an egg will receive one or more spermatozoa,
but that, if fresh and uninjured, not more than two or
three pass into it ; if they do the future of the egg is
endangered. As a rule, only one spermatozoon enters
into and becomes a constituent of the protoplasm of
the ovum ; the tail of the male cell disappears, but its
head persists for a time as a distinct structure ; this

* The student will not fail to observe that, at the present time,
a well-conducted and carefully described series of observations on
a selected form may affect very deeply the speculations of previous
students. Conversely, philosophical speculations have a guiding
influence on lines of study.

Chap, xiii.] REPRODUCTION OF SPONGES. 483

may be call the male proiiucleus. Approaching
the female pronucleus it gradually fuses with it, and
thereby gives rise to a fresh structure, the so-called
segmentation nucleus.

Pausing for a moment to consider how far the
history now detailed has led us, we find that there has
been a fusion of cells which, although different in
final form, have arisen from parts which at first were
exactly similar.

In the lowest forms the generative cells are not
aggregated into any special masses, and though we
can say that here there are male and there female
cells, we cannot with accuracy speak either of testes
or of ovaria ; here, as with various other organs, we
find a diffused preceding a localised or concentrated
arrangement.

The Sponges afford an example of this, the
reproductive cells being, as a rule, scattered through
the mesoderm (see Fig. 53, page 106) ; to this state-
ment Myxospongia and Euspongia form exceptions ;
in the latter the ova are arranged in small groups, are
embedded in connective tissue, and hold a definite
topographical relation to the afferent canals. Here,
too, the ova are naked and amoeboid, and not yet
enclosed in a distinct membrane, as they are in most
of the higher Metazoa.

Asexual reproduction does obtain so far among
the sponges that buds may be given off from an
individual, and an increase in a sponge colony can be
effected in a way which is of some commercial impor-
tance. The method here referred to has been tried in
the Mediterranean, and in the Florida sponge fishery
with a certain measure of success, the greater com-
pleteness of which does not appear to depend on the
sponge as much as on suitable fishery legislation. A
piece of sponge, some two or three inches high, is
carefully cut off from the rest of the mass ; owing, as

484 COMPARATIVE ANATOMY AND PHYSIOLOGY.

it seems, to the injury done to the sponge by the
operation, no growth occurs during the first four
months, but during the next two months it will be
found to have grown two or three inches.

Taking the groups in order, we find a higher grade
of localisation in the Ccelenterata than we should
have been led to expect from what we know of the
sponges. In Hydra, for example, the testes are
always placed just below the circle of tentacles, and
the ovary nearer the foot ; in the sea-anemone the
generative glands, or, as they may be more shortly
called, the gonads, are developed on the sides of the
primary septa ; in the jelly-fishes they are found on
the walls of the gastro-vascular canals. It is clear,
then, that there is localisation, but this is still of a
diffused nature \ the generative elements are not, as
in the crayfish or the fowl, limited to one aggrega-
tion, but there are several cell aggregates, each with
a reproductive function.

This phenomenon is most striking in the case of a
colony of hydroid polyps, such as that presented by
Syncoryne. Here we find that, of the numerous
buds developed on the colony, some never attain
to nutrient functions, and never have the oral cone
or tentacles of a nutrient person (trophosome) ;
instead thereof, they become gradually fashioned into
the shape of bell-like Medusae, separate from the
colony, become free-swimming, and develop gonads
on the walls of their gastro-vascular canals. In other
cases the medusoid buds or gonosomes become
more or less completely developed, but never separate
themselves from the rest of the colony ; within such
buds gonads become developed.

This method of division of labour, some persons
of the colony undertaking nutrient and others gene-
rative functions, is, as may be supposed, particularly
well seen in the Siphonophora, where special sets of

Chap. XIII. ] GONADS CF CtSTODA. 485

persons, more or less medusoid in form, devote them-
selves solely to the duty of producing genital glands,
and obtain the necessary food from the nutrient
persons of the colony ; in a few cases these gonosomes
become free.

The Platyhelmintnes present an elaborate
and somewhat difficult arrangement of their sexual
organs ; this is no doubt to be partly explained as due
to their exhibiting an early stage in the consolidation
of the diffused reproductive cells ; we must, how-
ever, not fail to note that they present a distinct
advance in the possession of accessory repro-
ductive organs. The male is provided with a
copulatory organ or penis, and the female, which
may now have special ovarian ducts, has the ter-
minal portion of the efferent tube modified into a
special canal (vagina), into which the male organ
may be received. Nor is this all ; another portion
of the duct is widened out into a receptacle in which
the ova may pass through the earlier stages of their
development (uterus) ; and yet another is often
converted into a pouch, in which the male elements
may be stored till such time as the ova are ready for
fertilisation (receptaculum seniinis). The egg-
producing and the yolk-producing cells are, however,
still distinct, and the latter have not yet, as in the
case of a bird, for example, taken their place on some
part of the duct that leads from the ovaries.

This kind of arrangement is well seen in the Cestoid
Bothriocephalus latus (Fig. 204; AB), where the testes
(t) are seen to be represented by aggregations of cells
which are scattered through each segment j their pro-
ducts pass by narrow ducts (ve) into a common coiled
efferent vessel (vd) which opens at the anterior end of
the segment into the copulatory organ (cirrus, c). The
ovarian region (ov) occupies either side of the middle
line in the hinder region of the segment, while the

486 COMPARATIVE ANATOMY AND PHYSIOLOGY.

yolk-producing glands (d) lie outside of these ; from
the latter there are given off a number of fine canals,
which meet in the middle line (d') towards the hinder
end of the segment ; these canals communicate with

?© t

e

t

®
tm

•3
•

V'

Fig. 204 A. — Male Apparatus of Bothriocephalus latus.

t, Some of the testes, with (ye) their ducts opening into (vd) the vas deferens ;
c, cirrus ; cbt cirrus sheath; ov, ovaries; u, uterus. (After Sommer and
Landois.)

the oviduct (od), which leads into a large coiled
uterus (u), which opens near the anterior end of the
joint (u'J.

The liver fluke again presents us with a very
complex arrangement of parts ; there are two testes,
one of which is placed in front of the other in the
middle of the body j each consists of a large number
of blind tubes of varying lengths, which open into

Chap. XIII.]

Go NADS OF FLUKE.

three or four primary ducts ; these unite at a common
point, one for each testis. Thence there pass forwards
two deferent ducts, which open into a common seminal
reservoir. The reservoir is continuous anteriorly with

Fig. 204 B. — Female Apparatus of Bothriocephalus latus.

c. Cirrus; cb, cirrus sheath; ov, ovaries; d, yitellaria ; d', their ducts; od,
oviduct ; u, uterus ; u', its orifice ; v, vagina ; v', its orifice ; gl, shell-
producing glands. (After Sommer and Landois.)

a ductus ejaculatorius, which is coiled and looped, and
which forms a papilliform projection at the base of
the genital sinus, which is common to the male and
female ducts, and is placed just behind the ventral
sucker. The terminal portion is protrusible, and
functions as a penis ; it is ordinarily known as the
cirrus. In the female organs the germ glands are
quite distinct from the yolk glands ; the former make

488 COMPARATIVE ANATOMY AND PHYSIOLOGY.

up a single mass, which lies well forwards, and is
composed of a number of blind tubes ; the whole
ovary is not very large. The yolk glands occupy the
sides of the body, extend farther forwards than the
ovary, and reach to quite the hinder end ; they are
racemose in character, each lobule being made up of
a number of small vesicles, the ducts from which open
into a large longitudinal duct, of which there is one
on either side of the body. Just below the level of
the ovary each of these gives off a transverse duct,
which opens into a common median reservoir ; the
duct from this, after a somewhat irregular course,
during which it gives off the fine duct of Laurer and
Stieda which opens by a very minute pore on the
dorsal surface of the body, opens into the genital
sinus ; the terminal part of the duct serves as a
vagina. At the commencement of its course, where
it unites with the short oviduct, it is surrounded by a
complex of glands, the so-called shell glands, which
are so called from their secretion serving to form a
shell for the ova.

In the round worms (Neniatohelniintlies) the
sexes are nearly always separate, and the generative
products are developed in special tubes, which in the
female are, however, of considerable length. Owing
to their length, the female tubes are in most cases
coiled, but this is the only character which presents
any complexity ; the blind end of the tube serves as
the seat of production of the egg cells, and the re-
mainder has the function of an uterus, or of a vagina.
Here, again, we find that while the blind ovarian
portions of the tubes are double, the efferent portion
(vagina) has undergone fusion, and the generative
orifice is therefore single and median. The male
tubes have essentially the same structure, and are
only less complicated in character ; the blind end
of each tube is the seat of development of the

Chap. XIII.] GONADS OF WORMS. 489

male cells, while the rest forms merely an efferent
duct, or has its distal portion widened out into a
reservoir or seminal vesicle. The male orifice is,
however, associated with that of the intestine, and
two chitinous spicules are developed to serve as copu-
latory organs, and to aid in the entrance of the
spermatozoa, which here are always amoeboid in form,
and have no specialised mobile tail.

The Acanthocephali are, again, examples of
forms in which the sexes are separate and the males
provided with a copulatory organ, but they exhibit so
much advance in structure as is implied by the testes
being two definite sacs; these, however, are not
paired, but one lies in front of the other ; the testes,
like the ova, are developed on a special cord
(ligamentum suspeiisoriuiii), the exact signifi-
cance of which is very incompletely understood ; a
somewhat similar structure has been observed in the
Bryozoa ; in the Chsetognatha we find that the testes
are developed in the anenterous or caudal segment
of the body, and the ova in the segment in front ; in
the Rotifer* the males are always without an
intestine.

In the Nemertinea, the Oephyrea, and the
polychaetous Chsetopods the generative pro-
ducts are developed directly from the epithelial cells
lining the body cavity, and there is no definite region
of which one can speak as testicular or ovarian ; in
these cases, moreover, the sexes are, as a rule, sepa-
rate ; and we have herein some support for the view
that the hermaphroditism of many worms has been
secondarily acquired.

In the Hirudinea and the oligoehsetous
Cheetopods we find, on the other hand, that the
generative products are developed in certain segments
only, and here, too, we find that the two sexes are
united in the same individual. Taking as types of these

49° COMPARATIVE ANATOMY AND PHYSIOLOGY.

two groups the leech (Hirudo) and the earthworm
(Lumbricus), we have a striking example of the way
in which more lowly and more specialised characters
are often associated in the same individual. Starting
with the proposition that the generative organs are at
first irregularly distributed through the body cavity,
we are led to suppose that the more these sexual cells
are consolidated, or, in other words, the fewer the
segments in which they are developed, the higher the
grade of organisation. From this point of view we
should assign the higher position to the earthworm,
inasmuch as the testes are to be found in two seg-
ments only, while the leech has testes developed in
nine segments ; on the other hand, the arrangements
for the safe disposal of the testicular products are no
less indicative of superior organisation; from this
point of view we should assign the higher position to
the leech, inasmuch as it has, and the earthworm has
not, a special intromittent organ, or penis, by means
of which the male products are safely carried to the
female.

Notwithstanding the absence of a penis in most
Oligochsetes, different individuals copulate with one
another, and the male products of the one are received
by the other into special pouches, whence they are
expelled when the ova are mature and expelled also,
while some of the setse in the region of the body
where the orifices are placed are specially modified to
aid in copulation. The spermatozoa are often collected
into masses or spermatophores, which may or may
not be provided with a special investment.

While the generative products of the Nemertinea es-
cape directly to the exterior, and those of the Gephyrea,
and probably also of some Annelids, by means of the
nephridial canals, the Hirudinea and some Oligochseta
are provided with special ducts ; in the earthworm
the open funnel-shaped orifice of the efferent duct

Chap. XIII.] GONADS OF WORMS. 49!

becomes greatly enlarged, and two pairs of large sacs
(the already mentioned reservoirs), are developed,
which completely obscure, and, indeed, have been
very frequently mistaken for the true testes. The
only explanation which has been given of the pre-
sence of nephridial canals and efferent ducts in
the same segment is that of Lankester, who has sug-
gested that each segment was typically provided with
two pairs of segmental organs, the superior of which
ordinarily become aborted. Some worms (Eudrilus),
present indications of the presence of both sets of
organs, and in others (Ocnerodrilus), the ordinary
nephridia are not developed in the segments which
carry the generative ducts.

While both the leech and the earthworm have but
a single pair of ovaries, the former is provided with a
single median vagina, in place of a duct opening
directly to the exterior ; in the leech, moreover, the
ova, when set free from the ovary, are not taken up by
the open mouths of the oviducts, and as the wall of
the oviduct is directly continuous with the investment
of the ovary, they do not, as in most vertebrates, for
example, pass first into the body cavity ; they make
their way directly to the exterior.

As may be supposed from their radial symmetry
in adult life, the Echinodermata have their genera-
tive organs radially disposed ; in the more primitive
forms, such as Brisinga, the glands are arranged by
pairs, and extend along the whole length of each arm,
while the generative pore is placed in the proximal
region of each ray ; as centralisation increases, the
glands become less elongated, and the pores are placed
within the area of the disc ; in the Ophiuroids the
glands are completely confined to the disc, where they
form five racemose groups ; in the Echinoids the in-
terradially-placed pairs become fused, and only five
sets of genital glands are to be made out. In the

492 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Holothurians reduction is carried still farther, the
genital tubes finally uniting into a single tube which
opens near the anterior end of the body.

In keeping with their
other primitive characters,
the Crinoidea have a more
diffused arrangement of
the genital glands ; these
are situated in the axis of
each arm, and make their
way into the pinnulse that
are attached to it (Fig.
205).

It is ordinarily sup-
posed that the generative
products of all Echino-
derms make their way in to
the water in a more or less
casual manner ; in Aste-
rina, however, Ludwig has
observed that the males
twist their arms around
those of the female, and so
dispose themselves as to
ensure the escaping sper-
matozoa meeting with the
ejected ova. A somewhat
similar mode of copulation

Fig. 205.— Cross Section of a Pinnule
of the Arctic Feather-star (An-
tedon eschrichti] ; x 75.

has been observed by Jic-
keli in Antedon rosacea.
In addition to their

a, Axial cord; a', its branches ; ag, ara-
bulacral or food groove ; b, radial
blood-vessel; w, radial nerve; ov,
ovary ; pj, joint of pinnule ; w, water-
vessel ; T, tentacles. (From P. H. Car- ., .. .
penter.slightly altered from Ludwig.) power Ol SCXUal multi-
plication, the Echinoder-

mata are distinguished by their remarkable capacity
for repairing injuries, and of giving rise to new indi-
viduals from separated arms ; in those cases in which
the rays are very numerous (Brisinga, Labidiaster),

chap, xiii.] REPRODUCTION OF ECHINODERMS. 493

it would appear that the arms often break off for the
purpose of more effectually evacuating their genital
products. In some cases, ordinary five-rayed forms,
such as the common starfish, have been seen to,
as it were spontaneously, break off an arm ; from
such a single arm several new rays are budded off, and
as these only gradually grow, such a starfish has the
appearance of a comet, or body with a long tail. In
the case of the Ophiuroids, it is, owing to the cen-
tralisation of the organs, necessary that this mode of
multiplication should be effected not by the separa-
tion of a single arm, but by the division of the disc,
and Ophiuroids are not ^infrequently to be seen in
which there are three shorter, and two or three longer
arms, the members of either set being subequal among
themselves. The reproduction of arms is also to
be frequently noticed among the Crinoids, but we
have not yet sufficient information as to whether these
arms have been directly broken off by enemies, or
set free by their possessor in consequence of an in-
herited peculiarity, as observed by Jickeli in Antedon
rosacea, or from fear of danger. This kind of repro-
duction has not been observed, as may well be sup-
posed, among Echinoids, but it is well known that
Holothurians will, if terrified, eject their viscera, and
gradually redevelop what one must suppose to be im-
portant organs ; the Crinoids will replace the whole
of the viscera contained in their calyx (M. Marshall).

The various stages of reproduction and fusion
which are to be observed among Echinoderms can be
nearly all paralleled by what is to be seen in dif-
ferent groups of the animal kingdom. An annelid of
the southern seas (Palolo) is said to evacuate its
genital products by completely breaking into pieces,
when the ova and spermatozoa meet and unite in the
water ; the loss of a single arm may be compared to
the gradual break-up of the organism which is seen in

494 COMPARATIVE ANATOMY AND PHYSIOLOGY,

the common jellyfish (Aurelia, of. page 541); the loss
by reflex action on irritation of an arm is paralleled by
the tail of the lizard ; while the formation of buds by
the injured arm or disc reminds us of the capacity for
reproduction which is exhibited by the common
Hydra.

Asexual reproduction in Echinoderms may, to
sum up, be one of two things ; there may be fission
with repair, or a single arm may be separated off
and give rise by external gemmation to a fresh zooid.

It is of interest to note that these phenomena are
seen especially in those forms (Linckia) in which the
ossicles of the arms are not protected either by strong-
spines as in Astropecten, or by well-developed margi-
nal plates as in Pentagonaster.

A high grade of differentiation is reached by some
of the Arthropod a, for the germinal glands are
compact, and often, indeed, united into a single mass ;
the efferent ducts are not unfrequently single, and
their orifice median in position, while the distinction of
the sexes is often to be demonstrated by the posses-
sion of secondary characters, such as differences in
external configuration, or the characters of certain
pairs of appendages. It is but seldom that the sexes
are united in the same individuals, and the mode of
life ordinarily allows us to find an easy explanation
for this arrangement, when it does obtain.

On the other hand, there are some remarkable ar-
rangements obtaining in various Arthropods which
require immediate notice ; in the Crustacean Apus,
and in such insect forms as the bee (Apis), and the
plant-louse, Aphis, we find that for a large number of
generations the females are enabled to produce ova,
which grow up to the adult stage without the inter-
position of any male influence; here, too, as in the
case of the Rotifera (see page 482), the presence of
polar globules in the maturing egg-cell have not yet

Chap. XIII.] GONADS OF CRAYFISH. 495

quite been satisfactorily demonstrated. The distinctions
between the various stages of what was originally called
parthenogenesis will be considered later on.

To fix our ideas of the peculiarities of the Arthropod
organisation, we may commence with an account of the
generative organs of the crayfish ; the testes and ovary
are respectively compact glands united along the
middle line, and giving only indications of a primi-
tively bilateral origin; they occupy a definite and
constant position in the body, lying beneath or just
in front of the heart, and having the enteric canal be-
neath ; they both give off a duct on either side, which
opens to the exterior at the base of one of the paired
appendages which are connected with the thorax ; as
the female orifice is placed two segments farther for-
ward than the male, the oviducts are, as we may sup-
pose, shorter than the vasa deferentia which carry
away the products of the testes (Fig. 206).

The cells which line the walls of the ducts of the
testes are, at the ends of the final canals, found to
occupy swellings, the large cells of which undergo that
division into smaller bodies which is so striking a charac-
teristic of spermatogenesis \ the products of these
divisions are not, however, provided with merely one
protoplasmic tail ; in the Crustacea, as in the ISTema-
tohelmmthes, we have a large development of chitin
in various parts of the organism, and here, as there,
we have spermatozoa developed which are without
that protoplasmic flagellar process, which is to be
likened to a cilium ; in place of this there are a number
of stiff processes, the disposition of which has gained
for the spermatozoa the name of "radiate cells." In
some Decapoda the rays retain so much of the charac-
ter of the pseudopodia of an Amoeba that tfyey can be
withdrawn into the nucleated portion of the cell
(Owsjannikoff).

Immobile cells like these require some accessory

496 COMPARATIVE ANATOMY AND' PHYSIOLOGY.

organ by means of which they can be carried with

safety to the body of
the female, and here,
as in so many of their
other organs, we find
the crayfish convert-
ing some of its ap-
pendages into a suit-
able apparatus ; the
appendages of the
first two segments of
the abdomen, that is,
of the two segments
which lie immedi-
ately behind that on
the base of the ap-
pendages of which
are placed the male
orifices, have their
terminal portions
converted into styli-
form processes, with
their edges so folded
on themselves as to
form each a half-
canal.

In the ovary we
find here, as in so
many other cases,
that one cell in a
special set (ovisac)
grows to a compara-
tively large size at
the expense of those
that surround and
form a coat for it ;

when this ovum escapes from the ruptured ovisac it

Fig. 206.— Figures of the Male (A) and Fe-
male (B) Organs of Astacusfluviatilis.

ov, Ovary ; od, oviduct ; od', its orifice ; t, testis ;
vd, vas deferens ; vd', its orifice. (After
Huxley.)

Chap. XIII.] GONADS OF COCKROACH. 497

passes into the oviduct, where it is perhaps fertilised,
and, further, provided with a coat (comparable to that
by means of which the spermatozoa are aggregated into
spermatophores), one end of which is drawn out into a
short stalk ; by means of this stalk the developing ova
become attached to the small appendages of the
abdominal region, with which they remain connected
till they are converted into the likeness of the adult ;
a crayfish, or lobster, at this stage is said to be "in
berry." There is, then, no free-swimming larval stage
in the fresh-water crayfish.

In the cockroach, as in the earthworm, the true
character of the testes proper has been misunderstood,
owing to just the same causes ; it is in young males
only that the true testes, which have a dorsal position,
can be detected ; in the adult forms their products are
found in the reservoir which forms the double head of
the single short efferent duct, and as this reservoir is
a complicated structure (the so-caRed mushroom-
shaped gland), formed of a number of short blind
tubes, within which the spermatozoa go through the
later stages of their development, it has, not un-
naturally, been regarded as the true testis. The
matured spermatozoa are thread-like bodies pointed at
either end, which exhibit a wavy movement.

As has been pointed out by Waldeyer, structures
are to be seen in the ovaries of the Arthropoda which
correspond to the Graafian follicles of the Vertebrata
(page 508).

Gegenbaur is strongly of opinion that the hiass
of generative cells in the Arthropoda is primitively
single, and adduces many facts in support of this
view ; not only, however, is this arrangement contrary
to that which obtains in all other bilaterally sym-
metrical animals, but it is further opposed by certain
embryological facts ; for example, the Lepidoptera
(butterflies and moths) have, in the later stages of
G G— 16

498 COMPARATIVE ANATOMY AND PHYSIOLOGY.

embryonic life, an organ on either side of the heart ;
and, lastly, it would be as easy to derive the single
from the double, as the double from the single
arrangement, when we bear in mind that so primitive
a form as Peripatus has the two testes completely
separated (Fig. 207).

Fig. 207.— Male Organs of Peripatus.

sntia; pr, prostat
(After Balfour.)

te, Testes ; vd, vasa deferentia ; pr, prostates ; p, common duct of v<L
~;alfou '

We find, then, that the generative glands are
either distinctly double, united by an obvious bridge,
or converted into a more compact single mass, which
retains more or less obscurely indications of a primi-
tively double arrangement.

The male glands are not always rounded off as in
the crayfish ; in Squilla they are tubular, and from the
sides of the walls short caeca, in which the generative

Chap. XIII.] GONADS OF ARTHROPODA. 499

epithelium is found, are given off; in My sis the
caeca are fewer and more distinct, and in Oniscus
there are a few very long cseca.

In lulus there are two testicular tubes united by a
number of median branches, and provided at their
sides with about as many rounded testicular follicles.
In insects the testes are ordinarily found to consist of
a large number of separate tubes, but the form of the
compact mass varies very considerably, and no
observations seem to have been made on these parts
since the discovery of the character of the true testi-
cular organs of the cockroach.

While the spermatozoa of all Crustacea, with the
exception of the parasitic Cirripedia, have no power of
independent movement, those of the Insecta are wavy,
and one end is often rigid, those of Myriopods may be
rigid or motile, and those of the Arachnida, with
which Limulus must be classified, are often actively
motile.

The hermaphroditic arrangements which obtain in
the Cirripedia are to be explained by their fixed
mode of life, while the imperative necessity of
avoiding the dangers of repeated self -fertilisation has
in some cases been yielded to in the production of
minute (^ inch) and degraded males (comple-
mental males) (Darwin), which, as in the case of
the Gephyrean Bonellia, attach themselves to the body
of the hermaphrodite, or simply female Cirriped.

In other epizoic Crustacean parasites (Achtheres
percarum and other Siphonostomata) the male is con-
stantly smaller than, and is generally found attached
to, the female ; here, too, as in the case of the Rotifera,
the number of males is much smaller than that of the
females, and adult forms are often developed which
arise from non-fertilised ova.

In some Isopoda (e.g. Cymothoa) a parasitic habit
likewise obtains, and there is a curious mixture of

500 COMPARATIVE ANATOMY AND PHYSIOLOGY.

structural and functional hermaphroditism ; in the
younger stages the testes are enormous as compared
with the ovary, and two penes are seen to be
developed ; the spermatozoa developed in the glands
of these Crustacea, with more highly differentiated
ancestry than the Cirripeds, are, it is said, motionless.
Later on, the testes diminish in size, and the ovarian
region comes into functional activity. In the Cryp-
toniscidse the male elements are matured during the
larval stage, and male free-swimming larvse copulate
with females of fixed habit and a less high degree of
organisation ; the male larvae subsequently become
degraded, and take on the characters and develop the
glands of the female (Kossmann). We have here to
do with what may be called phenomena of pro-
tandrous liermapliroditism.

The differences between males and females are
especially well marked in many groups of Insects ;
as an ordinary rule the males are smaller than the
females, being, as it seems, developed more rapidly so
as to be ready to fertilise their often short-lived mate ;
where, on the other hand, the males fight with one
another, or carry the female through the air, they are
the larger of the two sexes. In many cases (Cicadas,
grasshoppers, etc.) the males are alone provided with
sound-producing organs, or, as so often happens
among butterflies, the males are much handsomer
in appearance ; sometimes, also, the females of one
species are of two distinct forms (dimorphic
females), and among the beetles we find that
males of one species may vary very greatly in the size
and character of their horns (Lucanidse).

Differences in size obtain also among some
Arachnids ; the male spider, for example, being very
much smaller than the female, and often exceedingly
agile in escaping from her ferocity ; in the spider, as
in the crayfish, one of the appendages is modified to

Chap. XIII.] GO NADS OF MOLLUSCA. $01

serve as an organ for conveying the sperm to the
female.

The cephalous Mollusca, such as the nvussel or
the oyster, retain the simple conditions of generative
glands, being, as are so many marine forms for which
the water serves, by its currents, as the carrier of the
products of the male to the eggs or egg receptacles of
the female, without any secondary sexual organs. In
general character and appearance also the male glands
closely resemble the female, and it is, no doubt, in
consequence of this that so many discussions have
arisen as to the monoecious or dioecious arrange-
ments of certain Lamellibranchs.

As seen in ordinary cases, the glands are placed
on either side of the body, and each has a separate
orifice ; with a continuous outer wall, each gland is
broken up into a number of separate pouches, and
some of the epithelial cells on their inner face become
converted into ova or spermatozoa. Small at most
periods of the year, they become greatly enlarged at
the breeding seasons, when they occupy a large part of
the spaces in the body ; the ducts are ordinarily short,
and the ova, on escaping, make their way into pouches
in the gill chambers, where they are fertilised by the
spermatozoa which are brought in by the currents of
the water of respiration. (See page 221.)

The elaborate investigations of Ryder seem to
have settled the problem of the sexual characters of
the oyster ; one difficulty in the determination arises
from the fact, that while the Portuguese and the
ordinary American oyster have the sexes separate,
the common edible oyster of Europe (Ostrea edulis)
has the sexes united. By a magnificent effort of
histological chemistry, Ryder has shown that if two
colouring matters (safranin-red and methyl-green)
are brought to bear on suitably-prepared sections of
the body of an edible oyster, the red-staining fluid

502 COMPARATIVE ANATOMY AND PHYSIOLOGY.

affects the ova, and the green the spermatozoa. Before
long we may hope to see this method of investigation
applied to other problems of a like nature.

In the Gastropoda we not unfrequently meet
with a hermaphrodite arrangement, and this even
among the lowest forms ; in Proneomenia Hubrecht
has observed differences of colour in different parts of
the elongated and double generative gland ; in spirit
specimens the light-yellow portions were found to be

ovarian, and the brown-
ish - grey parts spermi-
genous. Here, again, we
note just the same kind
of development as we
have seen before; the
germinal epithelium gives
rise to ova in one region,
and spermatozoa in an-
other.

In the higher Gastro-
pods we again often find,
as in the snail, a " her-
maphrodite gland,"
and here, too, the male and female products are devel-
oped in different portions of the same genital area, from
cells which were primitively similar in character ; the
spermatoblasts sometimes become free from the wall
at an early stage (Fig. 208), and in some cases the wall
of the gland is produced into a number of pouches.
From the common generative gland, or ovotestis,
there leads off a common duct.

In those Gastropods that are not hermaphrodite,
and in the Cephalopoda, where, too, the sexes are
separate, there is very generally a close resemblance
between the male and female essential organs, remind-
ing one altogether of what we have already noticed in
the Lamellibraiichiata.

Fig. 208. —Follicles of the Ovotestis

of Helix hortensis.
oo, Ova ; ss, spermatoblasts.

Chap. XIII] GONADS OF CEPHALOPODA. 503

As in the Lamellibranchiata, we find that a simpler
arrangement of ducts obtains among the Gastropoda
with separate sexes, the secondary glands and copu-
latory organs, which are so well developed in the
monoecious forms, being frequently altogether absent ;
this observation does not, however, apply to the
Cephalopoda, where we find several important and
instructive complications.

As in some worms and nearly all Vertebrates, the
oviduct is not directly continuous with the proper
wall of the ovary, but the ova are set free into the
cavity of the capsule which encloses the ovary, to
be thence taken up by the open mouth of the
oviduct. This is either single or double, and has a
considerable extent, or only the terminal portion, of
its walls provided with secreting glands. Near the
orifice of the oviduct there open the ducts of two
large glands, which lie on the branchial cavity, and
which secrete a viscous substance, by means of which
the ova are massed into groups ; these are the so-called
nidamental glands.

The vas deferens or duct from the testis, which
may or may not be double, is, like the oviduct, not
directly continuous with the gonad ; it is considerably
coiled, and glands or pouches are developed along its
tract ; of these the most important is that which is
ordinarily known as IVeedham's pouch, in which
are collected the masses of spermatozoa that have been
grouped together on their way down the duct (Fig. 209).

These spermatophores are tubular structures, which
may be about half an inch in length ; the spermatozoa
are grouped together, and the rest of the cavity of the
tube is occupied by a coiled body ; when the sperma-
tophore escapes by the penis into the water the ex-
ternal sheath becomes ruptured, and the coiled elastic
band within being set free forces the sac of sperma-
tozoa out of the containing sheath (Fig. 210).

504 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Though the germ glands of the higher Mollusca
are simple and similar in
structure, there is in some
a very complex system of
accessory organs. In the
hermaphrodite forms the
duct remains common for
a short distance only, and
its tract becomes compli-
cated by the development
of glandular bodies, the se-
cretion of which nourishes
or protects the ova, and of
pouches, in which the sperm
received during copulation
can be stored up till the
ova are ready for fertilisa-
tion. The male portion has
connected with it glands,
by means of which the
spermatozoa are massed into
spermatophores, and
the integument is invagi-
natedto form a penis, which,
when turned inside out,
forms a duct for the sperm.
In some cases also, as in
the snail, each individual
is provided with a gland
which secretes a chitinous
Pig. 209.- Male jDuct of Loiigo dart-like body (dart sac),

«, Penis ;^pouITLdham;c,vas which is thrown off from

e:)te each

,er snail at its mate

ct)0 capsule in the Preliminary stages

of copulation.
In some Cephalopods there is a still more remark-
able provision for the safety of the seminal products.

Chap. XIII.]

HECTOCOTYLUS.

505

Within the mantle cavity of a female Argonaut, Cuvier

discovered an elongated worm-like body, provided with

a number of suckers. This he regarded as a parasite,

and in consequence of its appearance conferred on it

the distinctive name of Hectocotylus Argonautse.

This " ver bien extraordinaire," as Cuvier called it, is

nothing more than one of the arms

of the male Argonaut, which has

undergone a remarkable alteration,

become greatly elongated, and had

conveyed within it the spermato-

phore. When fully developed it

breaks away from the individual

that has produced it and makes

its way into the mantle cavity of

the female.

There are various stages of

" hectocotylisation " intermediate

between this extreme form and

the much more simple extrusion of
the spermatophore which obtains
in Loligo ; in the latter the fourth
arm on the left side has its suckers
rudimentary ; in Octopus it is
the third on the right side, and
in it the free end is provided with
a spoon-shaped plate, to which
the spermatophore is conveyed.

The general lessons as to the
primitive origin of germinal cells may be well impressed
on the mind by a consideration of what obtains in the
Craniate Chordata. Among the epithelial cells
which line the body cavity, some become, at an early
stage, sharply distinguished from the rest, but of these
germinal cells it is not at first possible to say
which are male and which female; later on they
undergo their characteristic changes, the male cells

Fig. 210. — Spermato-
phore of Loligo pealii.

a, Sheath ; b, spermatozoa ;
c, coiled band. (After
W. K. Brooks.)

506 COMPARATIVE ANATOMY AND PHYSIOLOGY.

gradually breaking up, and the female becoming sur-
rounded with follicular cells and growing at their
expense. The epithelial cells that are about to be-
come germinal are, in the early stages of both sexes,
formed on a genital ridge, or " sexual eminence " ;
but, while this ridge becomes more prominent in the
female, it gradually disappears in the male. In
other words, the distinction between the two sexes
is, at this point, well marked at a very early period.

With the exception of Amphioxus, no Chordate
presents any distinct indications of the metameric
segmentation of its reproductive organs, and the
consolidation which was observed in so many of
the non-vertebrated forms is just as well marked
in this phylum. The higher the vertebrate, the
more compact its genital glands.

In Petromyzon the testis extends almost through-
out the whole length of the body cavity ; in the
Elasinobranchs it is best developed in the anterior
region of the body; and in most Ganoids it is
rather smaller than in Elasmobranchs ; in the
Teleostei the glands are known to vary much in
size at different times of the year, but they are
generally of considerable length. In the Csecilise
the testes are broken up into a number (about ten)
of separate parts, connected one with another by
ducts, and having, on either side, the appearance of a
string of pearls. In the Urodeles and Anura the
testes are more or less elongated or rounded, but
never form, externally, more than a single mass. As
we pass through Keptiles to Birds we observe the same
phenomenon, though it is necessary to note that the
size of the organs varies very much with the conditions
of virile activity \ for example, the testes of the
sparrow, which in January are only two millimetres
long, are in April 15 mm. long, and wider in pro-
portion.

Chap. XIII.] GONADS OF MAMMALIA. 507

The next important step in the differentiation of
the testes is the great change in their topographical
relations which is to be observed in the higher
Mammalia, a change which is of the more signifi-
cance, as it is one that is never suffered by the ova-
ries. In the Monotremata the testes depart but little
from their primitive position ; in the porpoises they
lie beneath the kidney ; in the hedgehog they never
leave the abdomen, though they descend as far as the
inguinal ring ; * in the horse they pass through the
ring, which, however, remains permanently open, and
they make their way into a sack which is formed by
the integument, and is known as the scrotum. In
man, finally, as in some other forms, the testes are
permanently enclosed in a scrotum, although in indi-
vidual cases of arrested development we have the
atavistic arrangement of the testes being throughout
life abdominal in position.

The ovaries, like the testes, become more compact
as the scale of vertebrate organisation is ascended. In
both sexes we may find that the gonad of one side
projects farther forwards and that of the other farther
back.

In rare cases, and in individual or specific varia-
tions (as in the toad, cod, herring), rudimentary testes
are to be observed in females, and rudimentary ova-
ries in males. On the other hand, Serranus, among
fishes, would appear to be ordinarily hermaphrodite,
and in individual cases only to have the sexes
separate.

The spermatozoa of Elasmobranchs have, as
a rule, larger heads than those of Teleostean fishes ;
in the triton, salamander, and some other Amphibia a
remarkable undulating membrane is developed on the

* The inguinal ring is the inferior or cutaneous orifice of the
inguinal canal, or space in the abdominal wall by which the
spermatic cord passes to the testis.

508 COMPARATIVE ANATOMY AND PHYSIOLOGY.

tail of the spermatozoon ; in some birds the head is
not straight, but is twisted into a spiral (Canary).
In the Mammalia there is nearly always a rounded
head and a long tail.

The ova of Elasmobranchii and Teleostei, Am-
phibia and Sauropsida, are always of some size, the
first and the last of these groups having much the
largest ova, or eggs most richly provided with food
yolk. In Amphioxus and in mammals * the eggs
are very much smaller, and, in consequence of the
absence of the disturbing element (the food yolk) the
early stages of development are very different. In
addition to the investing membranes, which are some-
times perforated by a micropyle, some eggs are pro-
vided with a calcareous coating or shell.

Calling to mind somewhat the arrangements which
obtain in the Arthropoda, the eggs of vertebrates are
enclosed in an envelope of cells which owe their
origin to the primitive germinal epithelium ; in the
Mammalia this cellular investment becomes more than
one layer deep, the cavity within enlarges, and the
ovum projects into the fluid therein contained ; this is
the structure which is ordinarily known as the
Oraafian follicle.

The numbers of ova formed within any one female
vary enormously, and, as may be supposed, the
largest numbers are found in the lowest forms, or, in
fishes ; thus the cod is credited with nine millions,
the haddock and plaice with six millions ; the Elasmo-
branchs have a very much smaller number, often less
than ten ; in the Amphibia there are often a consider-
able number, in the Sauropsida the number is smaller,
and no mammal has more than about ten matured at
the same time, while in many cases the birth of twins
even is only an occasional phenomenon.

These ova become invested in membranes, of
* The Monotremata have been lately stated to have large eggs.

Chap, xiii.] AMNION; ALLANTOIS. 509

which the most common are the vitelline and the
zona radiata ; in some cases a third, more internally
placed membrane, is also developed. In addition to
these, there appear in the Sauropsida and Mammalia
two very important foetal membranes ; the first of
these forms an enveloping and protecting sac for the
embryo, and is called the amiiion; it owes its
origin to two folds, which appear one at either end of
the embryo, rise up, curve over, and finally unite in
the middle line above. As each fold is double it
follows that, when the ends of each unite, two
envelopes are formed, and a cavity or space, which
later on becomes filled with the liquor amnii, is
developed between them. The inner of these envelopes
forms the true aiiiuion, and the outer the false
amnion or subzonal membrane (Fig. 211) .

The other covering is the allantois, and it has a
very different history. Early in the development of
the intestine there is given off from in front of the
region of the future anus a saccular outgrowth, which
gradually extends beyond the body of the embryo,
and over the greater part of it ; during the period of
foetal development this allantois remains connected by
a stalk with the body of the embryo, and over its
surface there extends a rich supply of blood-vessels,
the allantoic arteries and veins ; so that it has a
nutrient or a respiratory function, or both.

In the Sauropsida all the material for the develop-
ment of the embryo is contained within the egg itself,
which, at a very early period, becomes sharply divisible
into a blastoderm, the seat of the future essential
stages in development, and a yolk sac, or store of
nutriment, the material from which is acquired by the
embryo through the intermediation of the rich plexus
of vessels which are rapidly developed within and
around it. As the yolk sac diminishes, the allantois,
for a time, increases in size, its vessels attain to

/jj

Fig. 211.— Diagrams to show the Development of the Atnnion and
Allantois. (After Foster and Balfour. )

In A the folds of the amnion (at) are to be seen rising up on either end of the
embryo (e) ; in B they have nearly met, and in c they have entirely coalesced.
In D the allantois («0 is seen making its way out of the body of the embryo,
and in E it is seen at a later stage. In v the yolk sac is shown in position,
and in G just before its final absorption.

Chap, xiii.] OVIDUCTS OF VERTEBRATES. 511

greater physiological importance, and give to this out-
growth the function of a respiratory organ.

The fate and functions of the membranes of the
mammalian ovum will be considered a little later
(page 515).

In the lower Vertebrates there are no special
efferent ducts for the genital products ; as in the
Chsetognatha, these fall into the coelom, and make
their way into the surrounding water through the two
abdominal pores ; in the Cyclostomata these pores
represent, no doubt, the primitive mode of exit of the
ova and spermatozoa, but in the higher Teleostei,
where, again, they (or parts called by the same name)
have the same function, it is to be distinctly borne in
mind that proper ducts have been there developed,
and have subsequently undergone atrophy.

The arrangement of oviducts which obtains in the
frog is that which is most common, indeed almost
universal, among Vertebrates ; as in the earthworm,
the ova escapes from the ovary into the body cavity,
whence they are taken up by the open mor '* of the
paired oviducts. These two mouths are sometimes
(Elasmobranchs, most Ganoids) fused into a common
ostiuin, but the greater part of the ducts are
distinctly separated from one another. As has been
already pointed out (page 262) these ducts (Mullerian
ducts) are derived from the primitive excretory duct.

In L<epidosiren Hyrtl has described the ab-
dominal orifice of each duct as being funnel-shaped,
and this form of opening is commonly found in all the
pentadactyle Vertebrata. As we ascend the scale we
find the oviducts becoming more compact, in so far
that the orifice, which in the Amphibia and in the
lizard (Fig. 212; ot) lies far forward, and in front of
the ovaries, becomes in snakes and mammals set
much farther back ; in the latter the oviducts are
often spoken of as the Fallopian tubes.

Ot

-..Ob

, Kidney; B, blad-
der ; B', neck of
bladder ; B, rec-
tum ; cl, cloaca ;
K', rectal open-
ing into cloaca ;
ur', opening of
ureter into
cloaca; ov, ov-
ary ; ot. opening
of oviduct; od,
oviduct ; od', its
opening into tlie
cloaca". (After
Wietlerslieim.)

mm

Fig. 212.— Urogeuital Organs of the Lizard (Lacerta mumlis).

Chap, xiii.] OVIDUCTS. 513

In the Teleostei, and the teleostean-like Ganoids,
such as Lepidosteus, the oviducts have no patent
abdominal mouths, but the walls of the tubes are,
continuous with those of the gland, or the ducts
become more or less aborted ; here it would appear
that the oviduct is not the altered Mullerian duct, but
sufficient information on this head remains to be
acquired. When, as in eels and salmons, the ducts
become completely aborted, the ova fall into the body
cavity and escape to the exterior by the so-called
abdominal pores.

The different regions of the oviducal canal take on
very various functions in various vertebrates; one
important factor in determining the character of the
ducts is the size of the ova which pass through it, and
another is, of course, to be found in the different
conditions of size and age in which eggs are laid or
young brought forth. In the frog the whole of the
canal, with the exception of the penultimate portion,
is of the same calibre throughout ; in the terminal
enlargement the ova become grouped into masses,
which are held together by the gelatinous substance
secreted by the oviducal glands ; the ova are fertilised
outside the body of the female. In some Elasmo-
branchs, and in the Sauropsida, the ova as they pass
down the ducts become not only invested by a layer
of albumen, but surrounded by a shell ; this may be
horny, as in Elasmobranchs, Lacertilia, and Ophidia ;
or firmer and calcareous, as in other Reptiles and in
Birds.

In the Ophidia the right ovary is always larger,
and often much larger, than the left, and the left
oviduct shorter than the right ; in Birds, on the other
hand, the right ovary and duct become aborted during
development, though indications of their presence are,
of course, retained by some forms.

Where the ova are not only fertilised within the
HH— 16

514 COMPARATIVE ANATOMY AND PHYSIOLOGY.

body of the female, but also pass through the early
stages of development within it, the ducts become
much more elaborate than in. the oviparous forms ;
Vertebrates in which this obtains are, somewhat im-
properly, spoken of as viviparous. The most pro-
minent phenomenon is the enlargement of the distal
portion of the duct into a cavity of varying width, the
so-called uterus. In most of the viviparous fishes,
in the few viviparous Lacertilia (Zootoca, Anguis
fragilis), and in the viviparous Ophidia, the ova appear
to continue to develop within the uterine cavity with-
out any assistance or nourishment from the mother.

In a few Elasmobranchs, however, and in all the
higher Mammalia, means are taken by which the
young are brought into real relation with the parent,
arid are directly nourished by its blood ; in the
former (e.g. Mustelus Isevis) the vascular walls
of the yolk sac become raised into ridges which
fit into corresponding depressions in the vascular
uterine membrane. Here we have the lowest and
simplest representation of a placenta.

Of the aplacental Mammals little is definitely
known, and what information we have was long ago
furnished by Owen, who observed in Ornithorhynchus
that the ova lay free in the uterus, the lining
membrane of which was highly vascular ; in a foetal
kangaroo there were folds on the investing membrane
of the foetus, and corresponding depressions on the
walls of the uterus, but no organic connection was
observed between the parent and the young.

In the placental Mammals outgrowths of the
investing membranes of the egg become more or less
closely united with vascular outgrowths of that
portion of the oviduct which has the function of a
uterus, and give rise to a characteristic organ.

To make such structures intelligible it is necessary,
first of all, to give some account of the characters of

Chap, xni.] PLACENTA OF MAMMALS. . 515

these membranes ; it will be remembered that, in
speaking of the extra-uterine development of the
embryo of the Sauropsida (the other division of
the Amniota, as the Mammalia and Sauropsida are
often collectively called) attention was directed to
the amnion, the allantois, and the yolk sac. As in
the other division, the embryo, notwithstanding the
smallness of the mammalian egg, is early distinguished
into an embryonic and a vitelline portion, but the
yolk sac * is always smaller than in Sauroids ; the
false amnion, or subzonal membrane, has, moreover,
an important part to play, which, from the nature of
things, was impossible in the oviparous groups, and
the yolk sac has, in some mammals, a certain re-
lation to the uterine walls.

Dealing first with such a case (e.g. the rabbit)
we find the developing embryo soon becoming attached
to and embraced by the epithelium which lines the
uterus ', the epiblastic covering of the yolk sac
separates from the subjacent layers and unites with
• the false amnion to form a layer beneath the zona
radiata (page 509) ; a little later the latter and the
subzonal membrane fuse together. Later on,
this subzonal membrane fuses with the vascular walls
of the yolk sac, to form a fresh investing membrane,
the false chorion. The true chorion is formed
by the fusion of part 'of the vascular wall of the
allantois with part of the subzonal membrane.
Into the substance of this chorion there extend blood-
vessels, and from its surface there grow out a number
of delicate processes which fit into corresponding
vascular " crypts " which are developed on the inner
wall of the uterus ; the whole of the apparatus thus
formed, partly by the mother and partly by the foetus,
is known as the placenta.

* In mammals the yolk sac is generally known as the "um-
bilical vesicle."

516 COMPARATIVE ANATOMY AND PHYSIOLOGY.

As was first pointed out by Balfour, the types of
mammalian placentae fall primarily into two great
groups ; in one the yolk sac (or so-called umbilical
vesicle) takes a share in the formation of the placenta ;
this arrangement, which is probably the more primi-
tive, is retained by the Insectivora, Rodentia, and
Chiroptera. The other type, which is seen in all the
other forms, is characterised by the fact that the yolk
sac ceases to take any important share in the nourish-
ment of the foetus.

Placentas, in which the foetal portion is derived
chiefly from the allantois, and not from the yolk sac,
fall again into two great divisions ; in the simpler
forms the outgrowths (villi) of the chorion merely
project into the pits which are developed in the uterus
of the mother, and may, at birth, be drawn out from
them without injuring the uterine vessels ; these are
the non-deciduate placentas. In the other the
connection between the walls of the villi and those of
the pits becomes so intimate that the vessels of the
uterus are torn when the foetus is born; such pla-
centaa are called deciduate. The former, again,
belong to one of two groups ; in the horse or the pig
the chorionic villi extend over nearly the whole of
the surface of the placenta, and we have a diffused
placenta ; in others, such as the sheep or the cow,
the villi are collected into definite tufts or branches,
which are distinctively known as cotyledons, and
the placenta is said to be cotyledonary. The spaces
between the cotyledons, of which the cow and the
goat have from sixty to one hundred, but the deer
only from eight to twelve, are left bare of villi.

In the deciduata the placenta is ordinarily said to
be discoidal or zonary, but care must be taken in
the application of the former of these two terms, for
the placenta is disc-shaped in insectivores, rodents, and
bats, as well as in man and the apes ; it is convenient,

Chap, xiii.] PLACENTA OF MAMMALS. 517

therefore, to follow Balfour in applying the term
metadiscoidal to the disc-shaped placentae of such
forms as have the placenta formed by the allantois,
rather than by the yolk sac. From the diffused or
discoidal forms, or from both, there has been evolved
another form of placenta, such as is seen in the Car-
nivora and some other mammals ; in these the villi
are confined to a broad girdle-shaped region of the
chorion, and we have therefore an arrangement which
may well be spoken of as zonary.

The oviducts of all vertebrates are typically
paired, and in many cases, as in the Amphibia, they
are completely separate from one another along the
whole of their course, as they are also in the Reptilia,
although in the Chelonia their terminal portion is in-
vested in a common sheath.

In Teleostei and Elasmobranchs the two ducts
unite at their termination to open by a common orifice;
in the Teleostei they open to the exterior behind the
anus ; in Elasmobranchs, Dipnoi, and all the Sauro-
psida, they open into a pit or cloaca, which is common
to them, the renal, and the rectal orifices.

In the Monotremata, in which there is a distinct,
and the Marsupialia, in which there is a shallow,
cloaca, the oviducts open separately ; in the latter,
but not in the former, the lower part of the oviducal
tube is modified to form a vagina, which, however
modified in various forms, is essentially double. In
the higher Mammals, in which the common urino-
genital orifice is placed in front of the anus, and sepa-
rated from it by a more or less distinct perinseiiui,
the terminal portions of the two oviducts are always,
at least externally, and in most cases also internally,
single and united. As we ascend the soale of the
Eufcheria we find this confluence becoming more and
more extensive. In the great ant-eater there is a
remnant of the two walls of the separate tubes in

518 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the form of a longitudinal septum, which is about
an inch in length and terminal in position, so that
the vaginal tube opens by two separate orifices into
the urino-genital canal. In some examples of, though
apparently not in all, Indian elephants, the vagina
and the uterus above are divided into two by a still

Fig. 213.— Diagrams to Show the Different Stages of the Mammalian
Uterus.

A, Uterus duplex ; B, uterus bipartitus; c, uterus Incornis ; n, uterus simplex ;
od, oviduct ; ut, uterus ; \g, vagina. (After Wiedersueiin.)

more complete longitudinal septum ; in the rodent
Lagostomus the septum is retained in the proximal
or uterine, but is absent from the distal, portion of
the vagina. As an occasional occurrence, a " vagina
duplex " is found in the human subject.

In those Mammals in which the vagina is com-
pletely single we find (1) two quite distinct uteri,
each opening by its own os into the vagina (Fig. 213 ;
A) ; this condition obtains in the rabbit ; (2) the
undivided portion of the uterus is short, as in the

Chap, xiii.] MALE DUCTS. 519

hedgehog, and the remaining portions of the ducts, or
so-called cormia iiteri, are long ; (3) in the mare
the cornua are proportionately shorter ; and (4) in
the highest forms, as in man, the short proximal
portions of the oviducts ("Fallopian tubes") open
directly into the large single uterus.

As has already been pointed out (see page 263),
the male have a different origin to the female
generative ducts of Vertebrates, arising, as they do,
from the primitive Wolffian duct. On the whole
they present less important points of difference in
various groups than do the oviducts ; and they con-
stantly remain separate along the whole of their
course ; where there is a terminal portion which ap-
pears to be single, it is not part of the Wolffian duct.

In the ichthyoid vertebrates the seminal products
always make their way through the kidney ; in the
lizard the testicular duct (vas defereiis) and the
ureter only unite just before they open into the cloaca.
In Birds the two ducts run close to, but open sepa-
rately from, one another. In the Monotremata the
renal and seminal ducts open separately into the
cloaca, and the common urine-genital passage is distal
to it.

In the remaining Mammalia, where the ureters
open separately into the bladder, there is, again, no
relation between them and the Wolffian ducts ; in
place thereof, however, we find -a common passage,
the vasa deferentia, opening at some point on the
course of the urethra, which is itself the narrowest
proximal end of the allantois.

Copulation is not always effected among fishes,
some only discharging their genital products into the
water ; in the Elasmobranchs the hinder pair of fins
serve as organs for dilating the female orifice, and
between them there is placed a papilla, on which
the seminal ducts open, and this is inserted into

520 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the female. Union of the sexes is common among
the Amphibia, but it is in the Csecilise only that
an external copulatory organ is developed from the
cloaca ; this may be as much as five centimetres
long (Wiedersheim). In Reptiles the penis is nearly
always distinctly double ; in the lizard the two halves
ordinarily lie beneath the skin just behind the
cloacal slit, but they are here, as they are also in the
snakes, capable of protrusion and erection ; a spiral
groove runs along the inner face of each half, and
serves as the duct by which the semen is conveyed
from the vas deferens to the female. The penis of
the Crocodilia and of the Chelonia is feebly, if at all,
protractile, but an advance is to be noted in the greater
union of the two halves, and the development of blood
spaces or corpora cavernosa in its substance. Where
a penis is developed in birds, it has (except in the
ostrich) the form of a coiled protrusible tube, the
groove on whose upper surface leads into a canal.

Among the Eutheria the penis always presents a
certain number of common characters, in so far as the
outgrowth of the cloaca from which it is formed
always becomes separated by the perinseum from
the rectal orifice, that the primitive groove always
becomes a canal, and the basal or proximal end is
continuous with the urogenital sinus, or portion
common to the urethra and the vasa deferentia.
This penis is erected by a median and two lateral
corpora cavernosa, and at the free end, which is
covered by a fold of the skin (prepuce), there is
developed a glans: In addition to the corpora, an
os penis may be found, and this is sometimes, as
in the walrus, of great size. The portion of the
urethra found in the penis is known as the penial
urethra, and it sometimes, though by no means
always, traverses the body of the glans before opening
to the exterior.

chap, xiii.] ACCESSORY MALE ORGANS. 521

On the course of the urethra there are developed
the prostate, and frequently also the Cowperian

glands, the secretion of which appears to serve as a
vehicle for the semen ; the size of these, and espe-
cially of the former, varies not only greatly in
various species, but also at different ages, being
often much larger in advanced years. The glans
itself is also provided with preputial glands, and is
not unfrequently armed with spines. In Ornitho-
rhynchus it has a kind of claw at its end, and its
surface is covered with spiny hooks, which are
directed backwards, and serve as organs of attach-
ment.

The penis arises as an outgrowth of the wall of
the cloaca, and is at first grooved along its upper
surface ; the sides of this groove grow over and unite
in the male. On either side there is a fold of skin,
which, in males, unites below with its fellow to form
the scrotum, in which the testes are carried in a
number of mammals. In the female the just-men-
tioned parts remain as the clitoris and labia majora,
and do not, as a rule, increase greatly in size ; in
some, however, like the hyaena, they are as large as
the corresponding or homologous parts in the male,
so that the sexes cannot, externally, be distinguished
from one another, and a belief in the hermaphroditism
of the hyaena is consequently not uncommon. The
clitoris is perforated by the urethra in some of the
lemurs ; and in the seal the female has an os
clitoridis, the homologue of the os penis of the
male.

Internally the male ordinarily retains a small por-
tion only of the Miillerian duct. This uterus
masculiiius varies considerably in size; in the
rabbit, for example, it is of considerable extent.

Of the organs connected with the care of the young,
the most important are the mammary glands and

522 COMPARATIVE ANATOMY AND PHYSIOLOGY.

the marsupial pouch. The former are found in
all the Mammalia, but in the Monotremata they re-
main at a low or indifferent stage in so far as the ducts
do not emerge to a common orifice, but open separately,

F

Fig. 214. — Diagrams of the Arrangements of th« Ducts of the Mam-
mary Glands in various Mammals.

A, Adult echidna ; B, human embryo; c, adult Homo ; D, adult mouse; E, embryo
of Bos ; F, adult Bos. (After Klaatsch.)

either on the surface of the integument (Ornitho-
rhynchus), or in a special mammary pouch (Echidna)
(Fig. 214; A). In all other mammals, "true" or
" false " teats are developed, the former being found
when, as in man, the ducts of the gland converge and
open on an uprising projection (Fig. 214 ; c), or a

Chap, xni.] MAMMARY GLANDS. 523

single duct (as in Mus ; D) traverses the projection.
The false teat is found, for example, in the cow (T and
F), where the sides of the mammary pouch are drawn
out to form a canal, at the pit of which the mammary
ducts open.

Though the mammary glands are merely integu-
mentary glands set apart for the secretion of a special
fluid, milk, they are remarkable for being unlike any
gland found in any other vertebrate. The characters
of the secretion vary within limits ; the milk of the
mare, for example, being poor in fats, but so rich in
sugar as to easily ferment, and form a spirituous
liquor (koumiss) ; in other cases, e.g. the goat, the
milk has the odour of the animal that secretes it.
As a rule the glands are active in females only, but
medical observers have put on record a few cases of
"male lactation."

In the lower Mammals the number of teats is
much greater than in the higher forms, while their
presence on the thorax in some (man, Sirenia), and 011
the groin (cow) only in other mammals, speaks to
their having primitively extended along a large part
of the ventral surface. The Centetidse have twelve
pairs of teats (Dobson), the rabbit and the hedgehog
five. The teats are often found to correspond in
number to the maximum number of young pro-
duced at a birth ; in no known case do they exceed
fourteen.

In the Marsupialia the teats are often arranged in
a circle round a central larger teat, and the whole
mammary area is enclosed by a fold of the skin, the
marsupial pouch, into which the young, which
are born in an altogether helpless condition, and at a
period so early that they are unable to actively suck
the mother, are conveyed. These helpless babes are
fed by the mother, who forces the milk out of her
mammary glands by the contraction of the cremaster

524 COMPARATIVE ANATOMY AND PHYSIOLOGY.

muscle ; the young thus fed and fixed would, no doubt,
soon be suffocated were their respiratory tube of ex-
actly the same form as that of the adult ; to prevent
such a catastrophe the larynx is prolonged at its upper
end into a conical projection, which is inserted into
the cleft of the soft palate, and is thereby brought into
direct connection with the nasal air passages. By
these means the nutrient milk can pass with as much
safety into the gullet as the water does in the case of
the Cetacea. (See page 242.) Here we have an example
of how in different groups the same result is attained
to by the use of the same kind of means.

Very striking differences are to be observed among
various Vertebrates as to their care of their young.
Most Fishes lay their eggs in the water, where they are
fertilised by the male and left to hatch ; in very rare
cases does the female exhibit the least care for them ;
indeed, but two are known ; Aspredo, after deposit-
ing her eggs, is known to attach them to her belly,
where they stay till they are hatched, and Solenostoma
develops a pouch, within which are projecting fila-
ments to which possibly the ova become attached. The
male more frequently exhibits care for its young, the
stickleback and others forming and guarding a nest in
which the ova are deposited ; others carry the ova in
their pharynx, while the pipefish (Syngnathus), and
the seahorse (Hippocampus) develop a subcaudal
pouch in which the ova are carried till they are
hatched. The female Surinam toad carries her young
on her back, and the male Alytes obstetricans cuts the
gelatinous cords by means of which the ova are at-
tached to the body of the female, and twines the eggs
round his legs. Pythons incubate their eggs, and
during the process their temperature rises about 3° F.
(Forbes). Most Birds but the cuckoo and the American
Cow-bird (Molothrus) incubate their own eggs, and in
those cases where the young are unable, when hatched,

chap, xiv.] DEVELOPMENT OF METAZOA. 525

to forage for themselves, provide them with food. All
Mammals suckle their young, and the ascent in the
series appropriately finds its termination in man, who
alone has the idea of the family.

CHAPTER XIY.

THE DEVELOPMENT OF THE METAZOA.

THE organs, described in the preceding chapter,
whether essential, as the testes or ovaries, or accessory,
as the ducts, glands, and intromittent organs, have in
common the function of providing for the union of the
male cell or spermatozoon and the female cell or ovum.
In all Metazoa, save those which multiply by budding
or by the parthenogenetic development of the egg,
this union of a male and female cell is the first step
in the history of a new individual ; it may be effected
either within the body of the female, as in Birds and
Mammals, or it may be effected externally to it as in
the frog, the perch, or the starfish. The ovum having
freed itself of the polar globules, and received within
itself the male or fertilising element, proceeds to
undergo division or segmentation • this may regularly
affect all the parts of the egg, and the several seg-
ments may be all of the same size, or, as in the egg
of the hen, where there is an abundant supply of
yolk, division may go on actively in a small portion
only of the whole egg. It is particularly to be
borne in mind that the absence or presence of yolk
in small or larger quantities profoundly affects the
character of the segmentation.

Regular segmentation, such as is seen in Am-
phioxus, obtains only in ova in which there is none
or but little yolk (aleoitlia I ova ; Lankester) ; it is
most common among the lowest Metazoa, such as the

526 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Sponges and Coelenterata ; it is found in some round
and fiat worms, and in Echinoderms, but is very rare
in Insects, Molluscs, and Chordates, Amphioxus, in-
deed, being the only representative in the last-
mentioned phylum.

Fig. 215.— The Segmentation of Amphioxus.

A, Stage with two, B, with four equal segment?. In c each of the four segments
has been divided horizontally ; D, a later stage m which a single layer of
equal cells encloses a central segmentation cavity ; B, the cavity (sg) is seen
in optical section. (After Kowalevsky.)

The influence of food yolk on the segmentation has
been stated by Balfour in the following terms : " The
rapidity with which any part of an ovum segments
varies ceteris paribus with the relative amount of
protoplasm it contains ; and the size of the segments
formed varies inversely to the relative amount of

Chap, xiv.i HISTORY OF THE OVUM. 527

protoplasm. When the proportion of protoplasm in
any part of an ovum becomes extremely small, seg-
mentation does not occur in that part."

The yolk may be either concentrated in the centre
of the egg (centro-lecithal ova) as in many Arthro-
pods, or at one pole of the egg (telo-Iecithal ova),
as in the frog or the chick. In the former case the
segments may be all equal, or some may be larger than
others (unequal segmentation), or the segmentation
may affect only the surface of the egg ; in the latter
the segmentation may be unequal, as in the frog,
where there is not a large amount of yolk ; or partial,
as in the fowl, where, owing to the enormous quantity
of yolk that is present, segmentation occurs only in
the small area to which the protoplasm is confined ;
affording, that is, a proof of the validity of the law
just quoted.

The mass of cells resulting from the segmentation
of the ovum may remain closely packed together, and
resemble a mulberry (morula stage), or they may
separate from one another, and give rise to a central
segmentation cavity (Fig. 215 ; E, sg) (folastula
stage). In the next stage, instead of a single we
have two layers of cells, and this condition may either
be brought about by the simple inpushing of Borne
of the cells (invaginatioii), or some of the cells
may grow over the others, or the cells of the single
layer may undergo transverse division (delamina-
f ion), and the arrangement of the double layer be
thus arrived at.

As we have already learnt, the two-layered stage
with a central cavity is known as the Gastrula
stage ; the layers are known as the germinal layers
(epiblast and hypofolast), and the cavity as the
archenteron (Fig. 216). The opening into the
archenteron is known as the folastopore, and
appears, from what is known of the development of

528 COMPARATIVE ANATOMY AND PHYSIOLOGY.

such primitive forms as Peripatus or Amphioxus to
have been of considerable extent, just as is the
mouth of a sea-anemone ; like it, it tended to close up
in its middle, and to have an orifice at either end
(A. Sedgwick) ; and these two ends would therefore
represent the oral and anal orifices of the primitive
intestine. The fate of the blastopore is very various ;
it may remain as the permanent mouth, or as the

permanent anus ;
or it may close
up, and the
mouth or the
anus be subse-
quently formed
at the point of
closure, or it may
close up and ap-
parently have no
topographical re-
lation to either
the future mouth
or the future
anus.

In most,
though not all
(e.g. Hydra) Me-

tazoa, a third layer (the mesoblast) appears between
the epiblast and hypoblast, and gives rise to the sup-
porting tissues, the muscles, the connective tissue,
and the blood and lymph ; the epiblast covers the
surface of the animal, and is the seat of origin of the
nervous system and sense organs, the hypoblast lines
the alimentary tract, and its appended glands. As
has been already pointed out, a cavity (the ccelom)
appears in the midst of the mesoblast (see page 45)
in all Metazoa above the Ccelenterata.

The epiblast becomes divisible in all forms above

Pig. 216.— Diagram of a Gastrula.
o, Blastopore; hyp, hypoblast ; ep, epiblast.

Chap, xiv.j DEVELOPMENT. 529

the Coelenterata into two layers, the epidermic and the
nervous ; the former gives rise to a structureless
cuticle, or undergoes hardening and becomes chitinous,
horny, or calcified, thereby giving rise to the various
forms of external skeletons • or its cells may
undergo invagination, and give rise to the various
forms of tegumentary glands, of which the mammary
are the most specialised. In the region of the mouth
the epiblast very ordinarily folds inwards to form the
st OHIO <l;r mil, and in that of the anus to form the
proctodseum.

The nervous layer undergoes one of two kinds of
modifications ; its cells either become thickened along
certain tracts, which in the more primitive forms
remain permanently connected with the epidermic
layer, or, as in all the Chordata, the nervous layer is
grooved along its middle line, and, giving rise to the
medullary canal, becomes separated from the epider-
mis. (See page 416.) Other parts of the layer become
modified to form the organs of sense.

The mesoblast, the mode of development of which
varies greatly in different forms, arises in such primi-
tive forms as Peripatus or Amphioxus in the form of
paired outgrowths of the enteron ; in the segmented
Metazoa, such as the earthworm, the insect, or Am-
phioxus, the mesoblast on either side becomes divided
into a series of cubical masses, the mesofolastic
somites. Where the mesoblast arises from the ente-
ron, the somites are from the first hollow within; where,
as in the earthworm, the somites arise from special cells,
and are at first solid, a cavity is gradually developed
within them ; and the cavities of either side becoming
in time continuous with one another, give rise to the
general body cavity (ccelom). The outermost portion
of the mesoblast comes into contact with the epiblast,
and gives rise to the dermis and the muscles of the
body wall ; the innermost layer comes into relation

53° COMPARATIVE ANATOMY AND PHYSIOLOGY.

\vith the hypoblast, and forms the greater part of the
wall of the permanent digestive tract. In the Verte-
brata the upper dorsal portion of the mesoblast
surrounds the iiotochord, and gives rise to the
rudiments of the bodies of the vertebrae ; the rest
of the dorsal portion is converted into the so-called
muscle plates. Other parts of the mesoblast give rise,
in various ways, to the heart and blood-vessels, when
such are present, to the blood itself, and to the renal
and generative organs.

The hypoblast, in addition to forming the lining of
a more or less large portion of the digestive tract, and
of the organs (liver, lungs) which are developed as
outgrowths from it, does, in the Cephalochordata and
Urochordata at any rate, give rise also to the noto-
chord, which arises as a diverticulurn or outgrowth
on the dorsal side of the primitive enteron.

We have, then,

(1) The epiblast giving rise to the epidermis,
nervous system, stomodaeum, and proctodaeum.

(2) The mesoblast gives rise to the internal skeleton,
the muscles, connective tissue, vascular, renal, and
generative systems.

(3) The hypoblast forming the lining of the diges-
tive tract and its appendages, and, in the lower Chor-
data, the notochord.

While in a large number of Metazoa the fertilised
ovum, by a regular and steady series of differentia-
tions, gives rise to forms which essentially resemble
their parents, there is a not inconsiderable number of
cases in which the young live for a time an inde-
pendent life under a form very different to that of the
adult. Such forms are known as larvae ; a few only
can be here dealt with.

Among the Cliordata the best known is the
tadpole stage of the frog ; instead of a tailless four-
limbed animal with a large mouth, no gills or .gill

Chap. XIV.]

TADPOLES.

clefts, we have a creature with a long tail, a sucking
mouth with horny jaws, and two suckers below, and
with external gills. In a later stage of metamorphosis
the gills become covered by an operculum (0) \ two
buds between the body and the tail are the rudiments
of the future hind limbs ; later on the signs of fore

Fig. 217.— Structure of the Tadpole.

A, From the side : B, from below ; c, a later stage ; D, head of tadpole ; a 01 g2,
gills ; TO. mouth ; j, jaws ; H, nasal sac ; e, eye ; «, car ; o, operculum ; hi, rudimeu t
of hind limbs ; s, sucker ; Ip, upper lip ; cP to cfi, visceral clefts.

limbs become apparent, the gills are lost, and their
clefts disappear, while the tail gradually undergoes
atrophy, and the larva is converted into a small
quadruped tail-less frog, which breathes air by means
of lungs. A similarly tailed larval stage obtains in
some of the Urochordata, which, in adult life, are fixed.
Among the Arthropoda, Insects, as we well
know, present, when their metamorphosis is "com-
plete," three distinct stages ; in the earliest stage, or
that of the " larva" (Fig. 218 ; A), the product of the

532 COMPARATIVE ANATOMY AND PHYSIOLOGY.

developed egg has a more or less worm-like shape ; it
may be headless and legless (maggot), or have a
head, but 110 legs (grub), or be provided with head,

Fig. 218.— A, Larva ; B, Chrysalis ; c, Imago of Papilio machaon.

legs, and fore-legs (caterpillar). The larva grows,
and moults its skin as it grows ; after a time it ceases
from this active mode of life, and passes into a more
quiescent condition, as in the case of the butterfly, or,

chap, xiv.] LARVAL STAGES OF INSECTS.

533

even in this pupal stage, it continues to move about
actively ; during the pupa stage a number of changes
occur within the body, and organs, such as the wings,
which were ab-
sent from the
larva, are de-
veloped from
masses of indif-
ferent cells, the
so-called im-
aginal discs.
The most com-
plete series of
changes during
the pupal period
obtain in the
Flies; all the
organs of the
larva,except the
generative, un-
dergo degenera-
tion, while the
abdomen of the
imago is derived
from that of the
larva; the imag-
iiial discs, which
are formed of
minute cells,
and enclosed in
a structureless

capsule, grow Fig. 219.— Larval form of Cirripedia. 1, Naxiplius of
T-orvIrlUT 4- "U™r> Salanus ; 2, Larva of Chtliamalus stellatv* ; 3, Older
rapidly , \ Larva of Lepas avftralis. (After Woodward.)

in the lower

portion of the thorax become united by pairs, and give
rise to the legs ; those in the upper portion become
converted into the wings and halteres ; tlie cephalic

534 COMPARATIVE ANATOMY AND PHYSIOLOGY.

discs similarly give rise to the head and its ap-
pendages.

In the developmental history of the Crustacea
there are two larval forms or stages which are very
widely distributed among the different orders ; the
appearance of these has been of very considerable
assistance in determining the real zoological position
of such forms as the barnacle and the parasitic Cope-
poda, which, when adult, have an appearance altogether
unlike that of Apus or Astacus (Fig. 219).

Like many other larvae, these free-swimming forms
were, when first observed, thought to be distinct
animals, and received in consequence distinctive
names. The first is the stage known as that of the
Nauplius. In this the larva has an unsegmented
body and three pairs of appendages of which the two pos-
terior are biramose, a single median eye, and a distinct
digestive tract. In the lowest forms, the Phyllopoda,
this nauplius passes gradually into the adult stage, the
body becoming segmented, and fresh appendages ap-
pearing as the crustacean grows in size, and undergoes
its periodical ecdyses, or sheddings of the outer skin.

Among the higher Crustacea (Malacostraca) the
larvae are hardly ever found freely swimming in the
Nauplius stage ; they more frequently make their
appearance at a more advanced period, or that which
is known as the Zoea. Here we have a cephalo-
thoracic shield, which is often, though not always,
provided with long spiniform processes, the longest of
which projects upwards from the middle of the back ;
the tail region is developed, but, like the hinder part
of the thorax, it is without the appendages that are
already developed in the anterior region of the body ;
lateral eyes are present in addition to a median one.
This Zoea stage is often succeeded by others, in which
certain characters are greatly exaggerated, or in which
there are presented arrangements which are permanent

Chap. XIV.]

LARV/E OF CRINOIDS.

535

in less highly developed forms, but only transitory in
the higher ; these, however, differ in different orders,
and are beyond our consideration here.

Finally, it is to be borne in mind that some Crus-
tacea leave the egg in a form essentially similar to that
of their parent; of such forms the crayfish is an example.

Some remarkable larval forms obtain among the
Echinodcrmata, and the
wide distribution of species
which, when adult, are capable
of but a slight amount of loco-
motion, must be ascribed to
their possession of free-swim-
ming ciliated larvae. The most
instructive examples are pre-
sented by the Comatulidse,
which are members of the
group Pelmatozoa, but are
stalked in their larval stages
only, during which, therefore,
they have a certain resem-
blance to the permanently-
stalked Pentacrinus. After
passing through a short period
of free existence, in which the
cilia are arranged in four
transverse bands (Fig. 220),
and during which two sets of
five plates and a short calcareous stem become de-
veloped, the larva loses its ciliated bands, and becomes
fixed by the stalk (Fig. 221 ; A) ; at the free end of this
stalk the arms become developed, and below the cup-
like portion (calyx) there appear the jointed pro-
cesses which are known as the cirri. The calyx and
the top joint of the stem break away from the rest,
and we get the Comatulid which is capable of a certain
amount of locomotion.

Fig. 220.— Tortal view of the
Lnrva of the Common
British Feather-star (An-
tedon rosacea) ; x 20.
(After Wyville - Thom-
son.)

536 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Fig. 221.— Pentacrinoid Larvse of
the Feather - star (Antedon
rosacea).

A, Quite young, before the opening of
the cup, and the appearance of
the five radial plates; B, nearly
imture ; 6, basal ; o, orals ; r, first
radials. (After Carpenter. )

Among the Echinozoa
we often find arrangements
which exhibit more com-
pletely the characters of a
true metamorphosis, and
which are of especial inter-
est because they present a
bilateral symmetry, such as
is ordinarily obscured in the
adult. The simplest con-
ditions obtain in the Holo-
thurians. After passing
through the early stages
of development, the body,
which was originally co-
vered with cilia, has these
processes arranged in a
sinuous band at its edges.

The anterior portion of
the enteric tract, before
uniting with the hinder in-
volution, the orifice of
which forms the permanent
anus, buds off a vesicle,
which becomes completely
separated from the enteric
tract, and the cavity of part
of which forms the I>od3*
cavity. The vesicle elong-
ates, and sends outwards
a process which comes into
contact with the dorsal sur-
face of the body, or that
which is opposite to the
surface on which the mouth
opens ; this process, or di-
vert! julum, has an opening

Chap. XIV.]

LARV& OF ECHINOZOA.

537

to the exterior (Fig. 222; wp). The vesicle then breaks

up into three parts, the most anterior of which gives

rise to the water-vascular ring and its appended canals,

while the two more posterior (Fig. 222 ; rp, Ip) give rise

to the general body

cavity, the lining of

which is formed by

their walls. In Holo-

thurians generally,

though not always, the

connection between the

vascular system and the

body wall becomes

broken, and the madre-

poric canal hangs freely

in the body cavity.

Among other Echi-
nozoa the amount of
difference between the
larval and the adult
stage is much greater
than it is in Holothu-
rians ; the larvae are
more elaborately de-
veloped, and present
distinct evidences of
secondary adaptations
to their free mode of
life. The sides of the
body are not unfre-

quently produced into free arm-like processes, the
interior of which may (Pluteiis larvae, Fig. 223),
or may not (Brachiolaria), be supported internally
by delicate calcareous rods. Part only of the body of
such larvae passes directly into the substance of the
adult ; the rest is either -absorbed by the growing
echinozoon, or shrivels up and disappears. The

Fig. 222.— Diaenrammatic View of the
J arva of a Holothuriaa (from the
side).

m, Mouth ; g, gullet ; s, stomach ; a, anus ; c,
longitudinal ciliated band ; w, rudiment
f water-vascular ring; wp, water-pore;

rp, Ip, right and left peritoneal cavities,
from which the body cavity is developed.
(From P. H. Carpenter, after Beleoka )

538 COMPARATIVE ANATOMY AND PHYSIOLOGY.

connection between the peritoneal vesicle and the outer
world is permanently retained, and forms the so-called
madreporic canal.

A few Echinoderms (Hemiaster, Ophiacantha
vivipara, Chirodota rotifera) do not pass through any

Fi.j 223.— Pluteus paradoxvs, the Larva of an Ophiuroid, at a late stage,
in which both the Larval Arms with their supports and the rudi-
ments of the Disc and Radial Skeleton of Adult are to be seen.
(After J. Miiller.)

larval stages ; the eggs are received into incubatory
pouches, or are developed in the ccalom without pass-
ing through any larval stages, or leading a free-
swimming independent existence.

A very common form of free-swimming larva is
that which is known as the Trocliospliere, and
which essentially resembles the adult condition of a
Rotifer; it is found among the marine Chsetopocla,
some of the Gephyrea and Mollusca, and in the

Chap, xiv.j THE TROCHOSPHERE LARVA. 539

Bryozoa. It is characterised by the possession of a
circlet of long cilia, which separates the anterior
portion of the body of the larva (prseoral lobe) from
that which lies behind it (Fig. 224) ; this ciliated
circlet is retained throughout life by the Rotatoria.
In addition to it, other circlets may become developed.
The most common of these is that which appears in
the region of the anus (telotrochal larvae) ; in
others several bands of cilia are formed (polytrochal),
and these sometimes encircle the whole body, and are
sometimes dorsal and some-
times ventral in position.
The banded condition is
preceded by one in which
the cilia are equally distri-
buted over the whole body. „ __

The Trochosphere is
provided with a definite di-
gestive tract, the lining of
which is ciliated : has a

» . , ,, , , , Fig. 224. — Larval Chsetopods.

fairly well developed ner- o> Mouth . anug . ^ prffioral . w> post.
vous system and a sensory schLkC)Iliated bandf (After Hat"
apparatus in the preeoral

lobe ; there is also a paired excretory organ, which
opens into the body cavity by several funnel-shaped
orifices. As the postoral portion increases in length
the bands of mesoblastic cells undergo segmentation,
and the prseoral portion becomes proportionately
smaller. Later on it develops the tentacles charac-
teristic of the Chsetopod, and loses the band, or bands
of cilia.

All the Mollusca have not a free-swimming larva
which can be referred to this type ; in the common
fresh-water mussel the ova are developed under the
shelter of the gills ; here they become provided with a
bivalved shell, the free edges of which are toothed ;
the larva does not fix itself to its parent by these hooks,

540 COMPARATIVE ANATOMY AND PHYSIOLOGY.

but by the byssus-threads, which are secreted by a
gland at the hinder end of the body. After a time
the Olochidium, as the larva at this stage is called,
breaks away from the parent, and makes it way to
some of the fish that live in the same water. To the
gills or other part of these hosts it fixes itself by its
toothed shell, while the byssus gland becomes aborted,
as do also the sense organs with which the larva is
provided. Attached to and covered by the epidermis
of its host, the young mussel undergoes a series of
further changes and takes on the characters of the
adult.

When the Molluscan larva is referable to the
trochosphere type, it has, as Lankester was the first
to point out, two distinctive characteristics ; on the
ventral surface, between the mouth and the anus,
there is a projection which is the rudiment of the
foot, and on the dorsal surface there is an epiblastic
ingrowth which forms the shell gland. The larva of
Chiton is remarkable for having the posterior dorsal
region segmented.

The simplest of all known larvae are found in the
Coelenterata, where they have the form of a two-
layered oval or elongated body, covered externally
with cilia, and provided with a central gastric cavity,
but without a mouth. In the simplest cases this
Plan 11 1 a becomes fixed by one end, loses its cilia, and
begins to develop tentacles at its free end. In the
common jelly-fish (Aurelia) and in the vast majority
of the Acraspedota a very remarkable metamorphosis
obtains. The free-swimming planula having settled
down and become fixed (Scyphistoma stage) in
the form of a polyp with a central mouth (Fig. 225 ; A),
begins to undergo division into a number of saucer-
like rings set one below the other ; each of these
Strobila contains a portion of the gastric cavity, and,
as development proceeds, the edges of the saucers

Chap, xiv.j DEVELOPMENT OF AURELIA.

54i

become produced into eight lobes into which prolonga-
tions of the central cavity extend. After undergoing
some further development, each saucer in turn breaks
away from the common stock, and, as an Ephyra,
with a disc of gelatinous tissue, a layer of muscle,
and eight bifid tentacular lobes, swims about freely,
increases in size, and becomes gradually converted
into an adult .sexually
mature jelly-fish. Here,
then, we have an ex-
ample of "alternation
of generations " ; the
fertilised ovum gives
rise, through the plan-
ula, to the Hydra-tuba,
the parts of which
undergo by constriction
a serial multiplication,
and each part gives rise
to a sexually mature
form.

/Alternation of
generations. — This
complex process has,
from various causes,
been considerably ob-
scured, and various

terms have been applied to the various ways in which
this phenomenon has been observed. As seen among
the hydrozoic Ccelenterates, Annelid worms, and Tuni-
cata, it may be thus described in the words of Balfour :
" The simplest cases are those in which an individual
which produces by sexual means gives origin to asexual
individuals differently organised to itself, which pro-
duce, by budding, the original sexual form, and so com-
plete a cycle .... In all these cases the origin of
the phenomenon is easily understood. It appears, as is

Fig. 225. — Development of Aurelia
aurita.

A, Polyp stage ; B, commencement of trans-
verse cleavage : c, completion of the
same so-called Hydra-tube stage. (After

542 COMPARATIVE ANATOMI AND PHYSIOLOGY.

most clearly shown in the case of the Annelida, that
the ancestors of the species which now exhibit alterna-
tions of generations originally reproduced themselves
at the same time both sexually and by budding, though
probably the two modes of reproduction did not take
place at the same season. Gradually a differentiation
became established, by which sexual reproduction be-
came confined to certain
individuals, which in
most instances did not
also reproduce asexu-
ally. After the two
modes of reproduction
became confined to se-
parate individuals, the
dissimilarity in habits
of life necessitated by
their diverse functions
caused a difference in
their organisation ; and
thus a complete alter-
nation of generations
became established.
The above is no merely
speculative history,
since all gradations be-
tween complete alternations of generations and simple
budding combined with sexual reproduction can be
traced in actually existing forms."

When alternation of generations is fully expressed
among the Hydrozoa we find that the sessile hydri-
form colony gives rise to buds which gradually break
away from their colony and become free-swimming
(Fig. 226). Differing in some details from the
structure of the Medusa already noted, these forms are
still more interesting in that between them and the
ordinary hydroid polyp we find a series of stages which

Fig. 226. — Figure of Syncoryne with a
numV>er of Budding Medusae on it
at Different Stages (a to e) of De-
velopment. (After Desor.)

Chap, xiv.] ALTERNATION OF GENERATIONS. 543

have been variously regarded as grades of development
or oi degradation.

We find, that is, that the medusiforni buds do not
always become separated from the stock that has
produced them ; and while in some cases (e.g. Syn-
coryne itself, towards the end of the breeding season,
or Tubularia) the buds are fully formed medusae, in
others, though still bearing the sexual organs, they
are nothing more than projections from the sides of
the body, in which the medusoid characters are hardly,
if at all, apparent (Hydractinia). These stages of
difference in the medusoid buds are allied, 011 the one
hand, to the condition which obtains in the common
Hydra, where ova and spermatozoa are developed in
one and the same individual, and in which the young
do not pass through any larval stage ; and on the
other, to what is seen in Geryonia, for example, where
the hydriform condition isaltogether suppressed, and the
larva, after a certain amount of metamorphosis, passes
into the medusoid condition of its parent. In both of
these cases there is no alternation of generations.

A series of very interesting conditions are exhi-
bited by different Annelids. In Lumbriculus there
may be simple transverse division of the body, one
half of which acquires a new tail, and the other a new
head ; in Ctenodrilus it has been observed that the
anterior half of the body may again divide ; in Syllis
the generative products are developed in the posterior
half only of the body ; in Myrianida the same pro-
ducts are confined to the forms that arise by budding,
so that from a simple case of transverse division we
have come to a complete example of alternation of
generations.

In some cases (e.g. the fresh-water Nais) there is
not simple transverse division, but the formation first
of all of a so-called " zone of gemmation ; " here the
zone becomes converted anteriorly into an anal zone,

544 COMPARATIVE ANATOMY AND PHYSIOLOGY.

and posteriorly into a fresli head ; several zones of
gemmation may appear before the zooids break away
from the parent, and begin to develop generative
organs. In Protula, the parent reproduces sexually,
as well as the buds, but in Autolytus the genital
glands are confined to the zooids that have been de-
veloped by budding.

The most complicated alternations are found among
the Urochordata,* a large number of which multi-
ply by budding ; a simple case is presented by those
forms in which the bud arises as an outgrowth of the
body wall, together with a prolongation of part of the
intestine. From this outgrowth the organs of the
bud are fashioned, and the bud, breaking away, gives
rise to fresh buds. Both bud and parent develop
generative organs and reproduce themselves sexually.

In Botryllus the product of a fertilised ovum gives
rise to a single bud ; this gives rise to two, each of
which again develops two buds ; the four buds ar-
range themselves round a common cloaca, then give
rise to two or three buds, and these again to others.
These last, which may go on budding, are the first that
are provided with sexual organs.

In Pyrosoma the product of a fertilised ovum gives
rise, while still an embryo, to four zooids ; these re-
produce sexually, and so give rise to fresh colonies, or
multiply by budding, and so increase the size of the
colony.

The height of complexity is reached by Doliolum,
the embryo of which is at first tailed, but becomes
cask-shaped in form, like its parent. From its dorsal
surface there grows out a process or Stolon, at the
sides and along the dorsal middle line of which buds
appear. The former become converted into the spoon-
like forms of Gegenbaur, and become free ; their

* The account given by Balfour (" Comparative Embryology,"
voL ii.) has been closely followed here.

Chap, xiv.] DEVELOPMENT OF T^ENIA. 545

further history is as yet unknown. The dorsal buds
take on the form of the parent with sexual organs,
but do not themselves become sexually mature ; they
develop a stolon from their ventral surface, on
which appear buds that grow up into the sexual
forms.

The relations of these different stages is shown by
the following table :

Sexual generation.

First asexual form with dorsal stolon.

Spoon-like forms developed Second asexual forms developed as
as lateral buda (future median buds with ventral stolon,
history unknown). |

Sexual generation.

A somewhat different condition of things is found
among the endo-parasitic forms, where, as a rule,
the animal passes through its different stages in two
different hosts ; we may take as typical the histories
of the common tapeworm, and of the liver-fluke which
causes the " rot " in sheep (Distomum hepaticum).

Taenia solium is sexually mature in the intes-
tine of man, and the final joints of the tapeworm
consist merely of fertilised ova, which have al-
ready passed through the earlier stages of develop-
ment ; when the joints become free and escape to the
exterior, they break up, and the contents escape in
the form of embryos contained in a thick chitiiious
shell. If these are now swallowed by a pig, the
shell is digested by the gastric juices of the new host,
and a rounded embryo, which is provided with three
pairs of hooks, is set free ; by means of these hooks
the guest makes its way through the wall of the
stomach or intestine, and finally settles down in the
muscles of its host. The embryo now loses its hooks
and gradually acquires a bladder-like form, the central
cavity of which is filled with fluid, while circular and
JJ— 16

546 COMPARATIVE ANATOMY AND PHYSIOLOGY.

longitudinal muscular fibres are developed in its walls.
This bladder- worm (cysticercus), now has its outer
wall pushed inwards at the anterior end, and on the
involution so formed hooks and suckers become de-
veloped, in such a way that when, as next happens, the
involution is turned inside out, these hooks and suckers
lie on the outer surface of the so-called "head."

We have now a narrow head and neck with an
attached bladder (Fig. 227), the head being at this
time hollow, and having in it a
circular vessel which communicates
with four longitudinal fibres.

If, during the long time that
these " bladder-worms " remain alive,
the pig is killed for food, and after-
wards insufficiently cooked, they are,
when the pork is eaten, conveyed into

Fig.227.— Cyshcercus ,, , J

cellulose (after Von the human stomach. Here the

fhfnSd ($f*eX ^adder-like termination becomes ab-

(c), and Vesicle (a), sorbed, and the neck, increasing in

length, becomes divided into joints

which are constantly produced at the anterior end ;

the oldest joints (proglottids) are, in other words,

farthest from the head. In them sexual organs are

developed, and the cycle recommences.

Distomum hepaticiim, of which several hun-
dreds may occupy the liver of one sheep, is of extra-
ordinary fecundity, producing at least as many as one
hundred thousand ova ; these only pass through their
earliest segmentation phases in the warmth of the mam-
malian body, but when they escape and reach a
moderately warm and moist place, the egg commences
to develop rapidly within its firm shell. When ready
to escape as an elongated ciliated larva, the embryo
bursts the cap of its shell, and begins to move about
freely. If the pasture on which it has fallen is moist,
the larva soon finds a stream of water along which

Chap, xiv.] DEVELOPMENT OF FLUKE.

547

it may pass to the neighbourhood of its next host ;
this has been shown by Thomas to be the small air-
breathing snail which is known as r.ymiiseiis
trimcatuliis. Provided at its anterior end with a
papilla which acts as a most effi-
cient boring-organ, the larva forces
its way between the cells of the
wall of the lung of the Lymnseus,
and makes its way into the lung
cavity. In this position it loses its
elongated and acquires a rounded
form, giving rise to the so-called
sporocyst stage. The cells with-
in the body which have not yet
been used up in the formation of
any tissue, arrange themselves in
definite groups, each of which gives
rise to an elongated larval form
not unlike a gastrula (Fig. 228),
save that it is provided with a
definite pharynx, has an " annular
ridge," and two short blunt pro-
cesses behind. We have now the
Redia stage. The R6dia, be-
coming free, may make its way
into other organs of the snail's
body ; within this Redia fresh
Redise may be again developed, or
the germinal cells within it may, in- Fig._228. — Redia of D.
stead, give rise to yet another form.
At any rate, the final product
of redise, or daughter-rediae, is a
body of rounded form with a long tail (Fig. 229),
to which the name of cercaria has been long since
applied. The parasite in this stage makes its way
to the exterior, and, becoming enclosed in a firm cyst,
loses its tail; these cercarian cysts take up their

Hepaticum.
Thomas.)

(After

n, Pharynx ; m, contained
germs ; r, posterior pro-
cesses.

548 COMPARATIVE ANATOMY AND PHYSIOLOGY.

position at the roots of the grass, and so on, and in
time either die down, or are eaten by a sheep. When
the latter misfortune happens, they pass into the
stomach, and so to the gall ducts and liver, to grow
up afresh into the likeness of
the liver-fluke from which they
started.

Histories not unlike those of
these two divisions of the Platy-
helminthes are presented by the
round • worms or Nematoliel-
mintlies, and by the Echino-
rliyiiclii. The thread - worm
of the human blood (Filaria
sanguinis hominis), which appears
to be the cause of chyluria and
of some other diseases in the
countries of the Eastern Old
World, has been found to have
an intermediate host in the
mosquito, from whom it passes
into water ; when this water is
drunk the young return to the
human intestine. Dracunculus
medinensis lives in its adult con-
dition in the subcutaneous tissue
of the human leg and foot : its
larval stages being passed, as it
seems, in a fresh- water crustacean.
Trichina is an example of a form
which appears to have had its history modified ; in
societies that may be called cannibal (e.g. rats) no in-
termediate host would appear to be necessary ; in
the case of civilised man, the adult worms are obtained
from the flesh of incompletely-cooked pigs.

Fig. 229.— Cefcaria of D.
hepaticum. (After
Thomas.)

INDEX.

Acanthocephala, 51 ; hooks, 28? ;

gonads, 489

Acanthometra, 27 ; skeleton, 276
Acarina, 74 ; respiration, 228
Achseta ; cilia, 56
Achtheres ; gnathites, 179
Acineta, 31 ; suckers, 177
Acipenser, 91 ; skull, 326
Acraspedota, 42 ; nervous system,

395

Actinophrys, 27
JErobranchiata, 73
Alcippe, 69

Amblystoma ; oral glands, 158
Amia, 92

Ammothoa ; figure, 72
A.mniota, 93
Amoeba ; structure, 18 ; zoological

position, 27 ; respiration, 210 ;

ectosarc, 274
Amphibia, 89, 93; teeth, 145;

tongue, 154 ; oral glands, 158 ;

heart, 195, 202 ; respiration, 232,

236 ; kidneys, 259 ; glands, 266 ;

vertebral column, 314, 321 ;

skull, 330; mouth, 336 ; scales,

365 ; brain, 421 ; sensor v organs,

436, 454, 467 ; gonads, 506
Amphioxus ; segmentation, 32, 87
Amphipoda, 70
Amphiuma, 93 ; blood corpuscles,

Amphonyx ; proboscis, 132
Ampullaria ; respiration, 228
Anabas, 93 ; suprabranchial organ,

234
Anguillulidffi ; oral bristles, 114 ;

respiration, 211
Anisopleura, 81
Annulata, 54; digestion, 117;

gills, 218; skeleton, 286; eyes,

446 ; gonads, 489
Anodon, 80 ; blood-vessels, 191.
Anoplophrya ; figure, 104
^.ntedon, 63; nervous system,

409 ; copulation, 492

jj*— 16

Anthozoa, 43 ; mouth, 110 ; sipho-
noglyphe, 111 ; figure, 112 ;
yellow cells, 272 ; skeleton, 280 ;
muscles, 373 ; gonads, 484

Anthropoidea, 100

Anura ; respiration, 236 ; kidneys,
259 ; testes, 506

Aphrodite, 54; cseca, 118

Aplysia, 81 ; intestine, 137

Appendicularia, 87 ; heart, 193 ;
house, 313

Aptera ; zoological position, 75 ;
gnathites, 128 ; springs, 379

Apus ; nervous system, 405 ; ova,
494 ; carapace, 299

Arachnida ; organisation, 72 ; para-
sitic, 180; heart, 190; respira-
tion, 225 ; limbs, 377 ; gonads, 499

Area ; gills, 220

Arenicola; gills, 219

Argon auta; shell, 309

Argulus ; guathites, 180

Arthropoda, 64; gnathites, 122;
blood-vessels, 187 ; tracheae, 215 ;
stigmata, 216 ; renal organ, 256;
skeleton, 291; locomotion, 375;
nervous system, 405 ; sensory
organs, 434, 438, 418, 461 ; go-
nads, 494

Artiodactyla, 99 ; limbs, 357

Ascaris, 51

Ascefcta ; figure. 35

Ascidia, 88 ; atriopore, 231

Ascon, 35

Asiphoniata, 80

Aspidogaster, 49

Asterias, 63 ; figure of arm, 61 ;
skeleton, 294

Astropecten, 63 ; figure, 59 ; loss
of anus, 121

Atlanta, 82

Aurelia, 42 ; figure, 42 ; respira-
tion, 211 ; gelatinous tissue,
286 ; nervous system, 395

Axolotl ; oral glands, 158

Azygobranchiata, 82

53° COMPARATIVE ANATOMY AND PHYSIOLOGY.

Balanoglossus ; figure, 86

Balanus, 69

Balistes ; teeth, 144

Bdellostoma ; kidneys, 259

Beroe ; mouth, 113

Birds, 97; tongue, 154; oral
glands, 159 ; crop, 162 ; gizz.ird,
163 ; intestine,- 169 ; bursa f a-
hricii, 170 ; vitelline duct, 171 ;
arterial arches, 205 ; lungs, 239 •
kidneys, 261; metallic colours
273; vertebrae, 316, 321; jaws
340 ; furcula, 349 ; limbs, 352
feathers, 366; wings, 336 ; voice
390; brain, 423; spinal cod
430 ; sensory organs, 437, 442,
455, 470 ; gonads, 506

Boltenia, 88

Bonellia ; proboscis, 118

Bothriocephalus ; joints, 50 ; go-
nads, 455

Botryllus, 88 ; development, 544

Brachionus, 52

Brachiopoda, 100 ; shell, 311

Branchippoda, 67 j figure, 66 ; re-
spiration, 223

Brissopsis ; nervous system, 399

Bryozoa, 101 ; intestine, 120 ; cell,
288 ; gonads, 489

Buccinum, 82

Bugula ; figure. 101

Butiriuus, IK) ; intestine, 168

Caducichordata, 87 ; sen»e-cells,

431

Casciliee, 93 ; penis, 520
Calcispongiee, 35
Caligus ; gnathites, 179
Camels ; water-bag, 167 ; foot, 357
Cardium, 80
Carinella, 85: nervous system,

398

Carmarina ; nervous system, 396
Carnivora, 99 ; jaws, 342
Caryophyllseus.'SO
Centetes; lower jaw, 342; teats,

523

Centrogouida ; organisation, 69
Cephalochordata, 86 ; digestive

tract, 138; vascular system,193;

respiration, 231 ; renal organ,

257 ; gonads, 506

Cephalopoda ; ink-bag, 138 ; bran-
chial hearts, 192; gills, 221;

funnel, 222 ; renal organ, 255 ;

ear, 463 ; gonads, 503
Ceratodus ; teeth, 144; heart, 195,

202 ; fin, 362
Ceratospongias, 36 ; skeleton, 278

Cestoda, 49; nitrogenous waste,
250 ; hooks, 287 ; gonads, 485

Cestus, 45 ; figure, 46

Cetacea, 99; teeth, whale-bone,153 ;
salivary glands, 160 ; retia mi-
rabilia, 209 ; respiration, 242;
kidneys, 281 ; skull, 344 ; tail,
381 '

Cetochilus ; figure, 68

Chalina, 36

Chamselpon ; tongue, 154 ; chro-
matophore-', 27 '£

Chsetoderinatidae, 81

Chsetodon, 93 ; teeth, 144

Chsetognatha, 101

Chelonia, 96 ; fins, 97 ; intestine,
169; bursse auales, 170; fin,
353, 384. S«e Tortoise.

Chirocentrus, 90 ; intestine, 168

Chiroptera, 99

Chitonidee, 81 ; shell, 306 ; eyes, 457

Chordata, 86 ; digestive tract,
138 ; vascular system, 192 ; re-
spiration, 230; skeleton, 312;
nervous system, 415; eye, 452;
gonads, 505 ; larvae, 530

Cicada : vocal organ, 389

Ciduris, 63

Ciliata, 30

Cirripedia, 69 ; shell, 299, 304

Clione, 82

Clypeaster, 63

Cockroach ; zoological position,
76 ; figure, 76 ; gnathites, 130 ;
intestine, 133 ; gonads, 497

Coelenterata, 36, 40 ; trophosomes,
109 • gastro-vascular canals, 185 ;
respiration, 211 ; nitrogenous
waste, 248 ; gonads, 484. See
also Medusse.

Coleoptera ; characters, 77 ;
gnathites, 131; stomach, 133;
elytra, 298 ; vocal organ, 389

Copepoda, 68

Corallinm ; coral, 282

Cordylophora, 40

Crania ; figure, 100

Craspedota, 40

Crayfish ; mouth organs, 123 ; gas-
tric mill, 125 ; heart, 187 ; re-
spiration, 223; scaphognathite,
123, 225; green gland, 253;
skeleton, 301 ; nervous system,
413; sensory organs, 439, 449,
451 ; gonads ; 495

Crinoidea; organisation, 58; di-
gestion in, 121 ; skeleton, 292 ;
nervous system, 408; gonads,

INDEX.

55'

Crocodile, 97 ; stomach, 163 ; ver-
tebra, 315; skull, 340; scales,
355

Crotalus ; skull, 338.

Crustacea"; organisation, 65 ;
gnathites, 128 ; respiration, 223 ;
branchial formula, 224 ; nitro-
genous waste, 253; skeleton,
298 ; locomotor organs, 376 ;
gonads, 499 ; larvse, 534

Cryptocarpa, 40

(Jryptoniscidse ; gonads, 500

Ctenophora, 45; cilia, tentacles,
373; gastric cavity, 113

Cucunmria, 64 ; lungs, 229

Cyamus, 72

Cyclodus, 96 ; scales, 366

Cyclops ; zoological position, 68 ;
figure, 68 ; gnathites, 179

Cyclostomata, 88 ; mouth, 140 ;
gills, 223 ; kidney, 258 ; skeleton,
314, 328 ; gonads, 506

Cyrnbulia, 82

Cymothoa ; hermaphroditism, 500

Cyprcea, 82

Cytozoa, 24

Demodex, 180

Dendroccelum, 49

Dentalium ; figure, 82

Desmodus; stomach, 167

Didelphia, 99

Diodon, 92 ; exoskeleton, 385

Diphyes, 41

Dipnoi, 90 ; heart, 195 ; pulmo-
nary vessels, 203 ; lungs, i35

Diptera, 77 ; guathites, 132 ; sto-
mach, 134 ; imaginal disks,
533

Distomum, 49; digestion, 114;
gonads, 486

Dog ; tongue, 157 ; salivary glands,
160

Dogfish ; teeth, 141 ; fins, 360

Dolium, 82 ; salivary glands, 137

Dolphin ; f ore-limb, 354

Doris, 81

Draco, 96 : flying organs, 383

Dytiscus ; eye, 450

Earthworm ; digestion in, 115 ;
typhlosole, 117; blood, 183;
spermatozoa, 479 ; gouads, 490

Echidna, 98 ; teats, 522

Echinanthus, 63

Echinodennata, 58 ; digestion in,
120 ; respiration, 218, 229 ; renal
waste, 248 ; skeleton, 288 ; pedi-
cellariae, 297, 374; movements,

374 ; nervous system, 400, 407 ;

sensory organs, 434, 435, 444, 461 ;

gonads, 491 ; development, 535
Echinoidea ; organisation, 63 ;

skeleton, 289 ; sphaeridia, 435
Echinometra, 63
Echinorhynchus ; figure 50
Echinozoa, 63
Echinus ; zoological position, 63 ;

test, 290 ; nervous system, 408 ;

gonads, 491
Edentata, 99
Elasmobranchs, 90 ; heart, 195 ;

labial cartilages, 335; brain,

420 ; taste-organs, 436 : gonads,

506 ; placenta, 514
Elephant ; molars, 154 ; skull, 343
Entoconcha, 180
tntomostraca ; parasitic, 179 ;

blood - vessels, 188 ; skeleton,

298

Ephemeridse ; gnathites, 133
Ericulus; kidneys, 264
Euplectella ; zoological position,

36; skeleton, 279
Eurystomata, 96
Euspongia ; zoological position,

36

Eutheria, 99. See Mammalia
Euthyneura, 81
Exocoetus, 93 ; figure, 382

Fierasfer, 181

Filaria, 51

Firuloides, 82

Fishes ; teeth, 142 ; spiral valve,
168 ; heart, 201 ; circulus cepha-
licus, 205; gills, hair-bladder,
232; operculum, 233; kidneys,
258; poison glands, 266; eye-
like spots, 269; vertebrae, 314;
skeleton, 323; fins, 359; bran-
chial bars, 328; mouth, 336;
sounds, 392 ; brain, 420 ; spinal
cord, 430 ; sensory organs, 436,
440, 453, 466; eye-like organs,
458 ; gonads, 506 ; oviducts, 513 ;
copulation, 519 ; care of young,
524

Fbgellata, 30

Fluke ; digestion, 178 ; nephridia,
248 ; gonads, 406

Fowl ; pelvis, 351

Frog, 93; heart, 197; carotid
gland, 203 ; vertebras, 315 ; skull,
330; brain, 419; gonads, 506

Galago ; tongue, 157
Galeopithecus ; flight, 385

552 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Ganoidei, 90 ; snout, 336

Gastropoda; divisions, 80; diges-
tive tract, 135; blood-vessels,
191 ; gills, 221 ; lungs, 228 ; re-
nal organ, 255 ; foot, 380 ; go-
nads, 502

Gephyrea, 56 ; digestion. 118 ; re-
spiration, 230 ; nephrida, 251 ;
gonads, 489

Glossophora, 80 ; odontophore, 135

Gnathopoda, -58

Gnathostomata, 88 ; mouth, 140

Goose ; gizzard, 164

Gordius, 51 ; mouth, 114, 179

Gorgonia ; zoological position,
44 ; figure, 44 ; skeleton, 281

Grantia, 36 ; sense-cells, 431

Gregarina, 25 ; reproduction, 474

Gromia, 27 ; figure, 26 ; test, 275

Gymnosomata, 82

Gymnotus ; electric organs, 269

G>mnnra; teetb, 150; stomach, 165

Heematobranchiata, 73

Haliotis, 81

HaHs.rca, 35

Hatteria, 96 ; teeth, 146 ; vertebrae,
315 ; iaws, 339

Haustellata, 77

Hedgehog ; teeth, 151

Hedriophthalmata, 70

Helicidse ; teeth, 134 : gonads, 502

Heliozoa, 27, 29; skeleton, 276

Helix, 81

Heloderuia ; oral glands, 159

Hemiptera, 77 ; gnathites, 132

Hesione ; air sacs, 230

Hesperornis, 98

Heteropoda, 82

Hirudinea, 56 ; sensory organs,
433 ; gonads, 489

Histozoa, 24 ; nephridia, 252

Holopus, 121

Holothuria, 64 ; lungs, 229 ; cuvie-
rian organs, 268 ; skeleton, 296 ;
larva, 372 ; gonads, 492

Homo, 100; erect position, 356;
brain, 429; scrotum, 507; ovi-
ducts, 519 ; teats, 522

Horse ; teeth, 153 ; stomach, 165 ;
foot, 355

Hyalea, 82

Hy alone ma ; spicules, 279

Hydra; figure, 36, 40; digest'ou,
105; chlorophyll, 272 > sense-
cells, 432 ; gonads, 484

Hydractinia, 40

Hydrocorallinse, 40

HydromedussD, 40 ; coral, 283

Hymempteri, 77; gnathites, 130;

vocal organ, 389
Hyomoschus ; foot, 357
Hyrax, 99 ; caeca, 172

Ichthyopsida, 89

Infusoria, 29 ; digestion, 103 ; re-
spiration, 210 ; lorica, 278 ; cilia,
371 ; reproduction, 476

Insecta; gnathites, 128; enterou,
133 ; salivary glands, Malpighian
vessels, 134 ; heart, 190 ; respi-
ration, 216 ; poison glands, 266 ;
silk glands, 567 ; appendages,
303 ; wings, 378 ; vocal organs,
388 ; nervous system, 406 ; sen-
sory organs, 436, 449, 439, 461 ;
gonads, 497, 500 ; larvae, 531

Insectivora, 99 ; brain, 426

Isopleura, 81

Isopoda, 70 ; parasitic, 180 ; respi-
ration, 229 ; gonads, 499

Lacerta, 96

Lacertilia, 96 ; oral glands, 159
Lremodipoda, 72
Laganum, 63

Lamellibranchiata, 79 ; gills, 219 ;
glochidia, 221 ; renal organ, 254 ;
byssus, 268 ; shell, 306 ; foot,
380 ; nervous system, 410 ; eyes,
457 ; ear, 462 ; gonads, 501
Lancelet. Sen Cephalochordata

Leech ; mouth, 179 ; respiration,
212 ; nephridia, 252 ; movements,
373

Lenviroidea, 100

Lepidoptera, 77; gnathites, 131;
stomach, 134

Lepidosiren, 90 ; teeth, 144

Lepidosteus, 92 ; teeth, 144 ; air
sac, 235 ; ear, 465

Lepisma, 75

Leucon, 36

Ligula, 50

Limax, 81 ; shell, 305

Limulus, 70; respiration, 225;
carapace, 299; eye, 451; sper-
matozoa, 499

Linckia, 63 ; comet form, 494

Lineus, 85

Lion ; skeleton, 318

Lipobranchiata, 74

Lipocephala, 79

Lizard; heart, 204; arterial ar-
ches, 205 ; gonads, 512

Loligo, 83; shell, 309; spermato-
phore, 505

Lophius ; teeth, 142

INDEX.

553

Lumbricus, 56. See Earthworm.
Lymnseus, 81 ; lung, 228

Mactra, 80

Malacostraca, 69

Mallophaga; gnathites, 133

Mammalia, 98 ; teeth, 148 ; tongue,
157 ; salivary glands, 159 ; sto-
mach, 165 ; intestine, 170 ; caeca,
171 ; liver, 173 ; blood-corpuscles,
182 ; heart, 198 ; arterial arches,
205 ; jugulars, 208 ; lungs, 240,
246 ; kidney, 261 ; bladder, 263 ;
vertebral column, 316 ; skull,
332 ; jaws, 342 ; hairs, 368 ; nails,
331 ; flying organs, 383 ; larynx,
392 ; brain, 425 ; spinal cord,
430; sensory organs, 437, 443,
456, 471 ; gonads, 507 ; placenta,
514 ; vagina, 517 ; penis, 520 ;
teats, 521

Mandibulata, 76

Manis, 99 ; vertebrae, 317

Macrolyristes ; vocal organ, 388

Marsupials ; respiration, 243 ;
bladder, 263 ; lower jaw, 342

Medusae ; gastro-vascular canals,
110 ; nervous system, 395 ; eyes,
444 ; ear, 459 ; gonads, 484

Meuobranchus, 93 ; gills, 236

Menopoma, 93 ; gill clefts, 236

Mesostomum, 49 ; digestion in, 107

Metatheria, 99

Metazou, 31

Millepora, 40 ; coral, 283

Mole ; sternum, 348

Mollusca, 77 ; odoutophore, 134 ;
parasitic, 180; intestine, sali-
vary glands, 136; heart, 191 ;
respiration, 219 ; lungs, 228 ;
renal organ, 254; byssus, 268;
skull, 304; foot, 380; nervous
system, 410 ; sensory organs,
439, 447. 449 ; gouads, 501

Monera, 27

Monocrelis ; nephridia, 248

Monodelphia, 99

Mouotremata ; retia mirabilia,
209 ; bladder, 263

Musk-deer ; feet, 355 ; brain, 427 j
cloaca, 517

Mussel ; gills, 219 ; organ of
Bojanus, 255 ; byssus, 268

Mya ; figure, 80

Myodora, 79

Myriopoda, 74 ; gnathites, 127 ;
heart, 189 ; respiration, 216 ;
appendages, 377 : gonads, 499

Myxiue, 88, 181

M.yxospongiae, 35 ; ova, 483
Myzostomum, 102

Nais, 56

Nautilus; figure, 83; gills, 221;

shell, 308; funnel. 381
Nematoids, 50 ; digestion, 114,

178; respiration, 211; gouads,

488
Nemertinea, 84 ; blood-vessels,

186 ; respiration, 213 ; gonads, 489
Neomeuiidae, 81
Nepa ; gnathites, 132
Nereis, 54
Neuroptera, 77 ; gnathites, 131 ;

stomach, 133
Notacanthus, 92
Nudibranchs ; respiration, 222
Nummulites, 27, 28 ; test, 276

Oceania. 40
Octactiuiae, 43
Odontopteryx ; jaw, 146
Odoutornithes, 98 ; teeth, 146
Oligocheeta, 56
Opalina ; digestion in, 104
Ophideres; proboscis, 132
Ophidia ; vertebrae, 314 ; skull,

337 ; scales, 355 ; oviducts, 513
Ophiocephalus, 92
Ophiocoma, 63
Ophiothrix, 63
Ophiuroidea, 63 ; respiration,

218 ; skeleton, 295 ; gonads, 491
Orchesella, 75
Ornithodelphia, 98
Oruithorhynchus, 98 ; heart, 198 ;

pelvic arch, 350
Orthoptera, 76; guathites, 129;

vocal organs, 388
Orycteropus, 99
Ostracoda, 69
Oyster, 79; foot, 380; gonads, 501

Paludina, 82 ; gill, 228

Pangonia ; proboscis, 132

Paramoacium ; figure, 29

Parrots ; tongue, 157 ; skull, 340

Patella, 81 ; gill, 221

Peachia ; mouth of, 112

Peccary ; stomach, 165

Pedipalpi, 74

Pelmatozoa, 63

Peltogaster, 69

Pennatula, 70 ; phosphorescence,
270

Pentacrinus ; figure, 60, 63 ; skele-
ton, 293

Peutastomum ; hooks, 180

554 COMPARATIVE ANATOMY AND PHYSIOLOGY*

Perennichordata, 87

Perigonimus ; figure, 37

Peripatus; figure, 64: gnathites,
127; blood-vessels, 189; tra-
chese, 215 ; nephridia, 257 ; skin,
299 ; appendages, 302, 377 ; ner-
vous system, 401; sensory or-
gans, 434, 417 ; gouads, 498

Perissodactyla, 99

Petronayzon, 88

Pkolas, 80

Physalia, 41

Pbysoklisti, 92

Physophora ; figure, 41

Physostomi, 92

Pipa ; tongue, 153

Plinaria, 49

Platyhehnintb.es, 49 ; gonads, 485

Pueumodermon, 82

Podophthalmata, 70

Podura, 75

Polia, 85

Polychseta, 54 ; larva, 539

Polynoe, 54

Polyodon, 91

Polyplacophora, 81

Polypterus, 92 ; brain, 421

Porifera, 35

Primates, 100 ; limbs, 356

Pristis ; teeth, 142

Proneomenia ; nervous system,
401 ; gouads, 502

Proteus, 93; gill, 236

Prototheria, 98 ; vertebrae, 317

Protozoa, 25 ; digestion, 103 ;
parasitic, 177 ; contractile va-
cuoles, 247 ; phosphorescence,
270 ; movements, 371 ; repro-
duction, 473 ; skeleton, 274

Protracheata, 65

Pteropoda, 81

Pycnogonoidea, 74

Pyrophorus ; phosphorescence, 270

Pyrosoma. 88; phosphorescence,
270

R .bbit ; caecum, 172 ; brain, 42 1 ;
taste-bulbs, 438 ; placenta, 515

Radiolaria; yellow cells, 272;
skeleton, 276

Ratitse, 97 ; feathers, 368

Reptiles, 96 ; teeth, 145 ; salivary
glands, 159; oesophagus, 162';
intestine, 169 ; heart, 196 ; lungs,
239 ; respiration, 245 ; kidneys,
261 ; brain, 422 ; sensory organs,
437, 454, 467, 471 ; gonads, 512

Rhizocephala, 69

Rhizocrmus, 63

Rhizopoda ; skeleton, 276

Rhyncopygus ; anus, 121

Rodents, 99 ; teeth, 150

Rotatoria, 50 ; digestion, 118 ; re-
spiration, 230 ; nephridia, 251

Ruminants ; stomach, 166 ; retia,
209

Sabella, 55

Sacculina, 69; nutrition, 178

Sagartia, 39 ; section of, 111

Salamaudra, 93 ; ribs, 345

Salmon ; teeth, 144

Salpa, 88

Sargus ; teeth, 142

Sarsia, 40

Sauropsida, 95 ; uric acid, 265 ;
vertebral column, 314, 321 ;
skull, 331

Scaphopoda, 82

Scarus ; teeth, 142

Scorpion; gnathites, 127 ; mouth,
179 ; lung-books, pectiues, 226 ;
poison, 266

Sea-gull; gizzard, 176

Selachoidei, 91

Sepia, 83

Serpula, 55

Siuupalliata, 80

Siphonophora, 41

Sirenia, 99; respiration, 2 13; ver-
tebrae, 317; limbs, 353

Snakes ; teeth, 146 ; arterial ar-
ches, 205

Solen, 80 ; haemoglobin, 212

Spitangidse, 63

Spide/ ; poison glands, 265 ; silk
organs, 267

Sponge, 31 ; digestion, 106 ; re-
spiration, 211; nitrogenous
waste, 248 ; skeleton, 278 ; sense -
cells, 431 ; gonads, 483

Squatina ; heart, 196

Squilla ; figure, 69 ; testis, 498

Steatornis ; syrinx, 391

Steganophthalmata, 42

Stenostomata, 96

Stentor, 30 ; stentorin, 272

Strepsiptera, 77

Streptopeura, 81

Stylasteridae ; dactylozooids, 109

Suctoria, 31 ; digestion in, 104

Sun-birds ; tongue, 156

Sycon, 36

Synapta, 64 ; spicules, 296

Taenia, 49 ; digestion in, 177 ;
plasmatic canals, 185 ; develop-
ment, 545

INDEX.

555

Teleostei, 90 ; teeth, 142

Teleostoidei, 92

Terebella ; figure, 55

Termitidaj, 77 ; gnathites, 133

Tetrabranchiata, 82

Thecpsomata, 82

Theriomorpha, 95

Torpedo; electric organs, 269

Tortoise ; shell, 3t>6 ; brain, 4-2

Toxotes, 93

Tracheata, 74

Tragulus ; blood corpuscles, 182

Trichina, 50, 548

Trichoptera, 77 ; gnathites, 131

Tridacna, 80^ shell, 308

Triton, 93 ; shell, 307

Tubicolse, 55; setae, 288

Tubifex, 56

Tubipora : figure, 43 ; skeleton,
283

Tunicata, 86 ; eye, 452 ; ear, 464

Tupaia ; brain, 425

Turbellaria ; digestion, 107, 113 ;
circulation, 185 ; respiration,
211 ; nephridium, 249 ; nervous
system, 400 ; tactile organs, 433 ;
eyes, 445

Turtle ; heart, 197 ; skeleton, 320 ;
skull, 339

Typhlops, 96

Ungulata, 99 ; feet, 355
Urochordata, 86; digestive tract,

139 ; vascular system, 192 ; re-
spiration, 231 ; renal organ, 257 ;
uotochord, 313 ; larva, 544. Sea
Tunicata.

Urodela, 93; respiratioa, 236;
kidneys, 259 ; gonads, 506

Uropeltis, 96

Vagantia, 55

Velella, 41

Vertebra ta, 88; digestive tract,
140; teeth, 141; tongue, 157;
intestine, 161 ; liver, 175 ; blood,
181 ; vessels, 194 ; retia mirabilia,
208 ; haemoglobin, 212 ; respira-
tion, 231 ; renal organ, 257 ;
vertebral column, 314; skull,
323; limbs, 347; vocal organ,
390; brain, 417; spinal cord,
430; sensory organs, 435, 437,
440, 453, 463 ; gonads, 505

Viper ; fang, 147

Vorticellids, 30; contractile va-
cuoles, 247 ; movements, 371

Whales ; see Cetacea, 99

"Wolf; teeth, 152

"Woodpecker; tongue, 156; skull,

Xiphacantha ; figure,28 ; skeleton,
Zygobranchiata, 81

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