m

D
D

m
a

M A N U A L S

FOR

STUDENTS OF MEDICINE.

COMPARATIVE ANATOMY

AND

PHYSIOLOGY.

F. JEFFBEY BELL, M.A.,

PROFESSOR OF COMPARATIVE ANATOMY AT KINO'S COLLEGE.

ILLUSTRATED WITH 229 ENGRAVINGS.

PHILADELPHIA :

LEA BROTHERS & CO.

(LATE HENRY C. LEA'S SON & Co.)
1885.

1

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

AS A LITTLE TOKEN OF RESPECT

FOR 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 because
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, arid I have to
return my thanks to the Publication Committee of
the Zoological Society ; to Professor Flower, who
only add.ed 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. JEFFREY BELL.

King's College , Mai/, 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

XI. 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
AND 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 leara 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 MATTER.

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 tirst 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
tinie 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 anc l its " atomic composition " has
been determined to be C 160 H 308 N 5 PO 35 , 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 afiects 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 (0) 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 life, 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 or
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 shows 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 words, 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 Browiiiaii 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 w r as 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

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 p9astid.
This pBastid, 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 :

10 COMPARATITE 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. Purifying. Gills, lungs (carbonic acid) ; kid-

neys (nitrogenous products).

(2) Organs of external relations.

iv. L<ocomotor. 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.

Copulatory : penes, etc., which are accesso

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

etc., brain.
()8) 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 Buffon, " Ce n'est qu'en
comparand 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.] HOMO LOGY. II

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 homogeiiy (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 Palaeont-
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 j udgment can be afforded, and it must
be used unceasingly and unsparingly.

EVOLUTION.

The great maze and mass of facts which are found

O

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 Phylogeiiy.

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 ob*
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 metacarpal 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.] 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

t/

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 CoMPARATirE 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 tartrate
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 Volvox
offers (5) an exception to the statement that pi ants 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.] AXIMALS AXD PLAXTS. 1 7

Cilio-Hagellata 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 Droseraceae we find
plants (a) whose leaves have in some forms the power
of moA r ement w r hen excited ; (ft) 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

animals

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

i8

CHAPTER n.

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.

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

n

^&\V

y ?.*?>

Fig. 1. Amoeba.
w, Xuclcus ; 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
eiidosarc. 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 nucleolus. 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, FO
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 amoeba 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.

w

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 amoeba
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 amoeba,
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 gemmation.

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.

CHAPTER 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. \Vhat 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 bv further investigation. What is called a

j

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 Jby^_ 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.

Tn 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,

if

which is known as the oyujon. 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
of labour.

The first or lower group of organisms are asso-
^ ciated together as the B * > 'ft ti y 1>n the second, or
those that come after them, form the division of the
^ TTffetayioai Did we desire to use less objective terms,
we might adopt for these groups the corresponding
jr terms of Cvtozoa and H * s *-*7iftP (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. III.] GROUPS OF PROTOZOA. 25

I. PEOTOZOA.

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 Oregariiiida ; the others, one of which is dis-
tinctly higher than the other group, may be called the
Sarcodiiia 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
pseud opodia, and they reproduce themselves either by
division or by budding.

In the Infusoria the amcebiform 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 Gregariiie 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

o

higher animals in which nutritious matter is abundant,
they obtain such food as they require by the mere

\

Fig. 2 A. Gromia, showing the test 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.

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 racuolated ectosarc, the finely
granulated endosare, the nucleus, contractile vacuole, and pseu-
dopodial filaments. (Aiter Leidy.)

Leaving out of onr consideration those simple and
incompletely known forms in which no nucleus is
developed in the protoplasm (ITIonera),* 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. Xiphacsutha, showing the siliceous skeleton. (After W.

Tuomsou.)

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

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 pseuclopodia
(Fig. 2 B) are fine, alter but little in form, and rarely
anastomose with one another ; lastly, the Kadiolaria
(Fig. 2 c) have a chitinous " central capsule," around
which flows the protoplasm, and with which there is

c~r

B

Fig. 3i.Paramceciumaurelia; 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 (flagelEum)
(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 PHYSIO LOGY,

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

Fig. 3 ii. A, Noctiluca miliaris ; B, with buds ; c, section.
n, Nucleus ; /, flagellum ; 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. Flagellaia ; a number of forms are grouped
by some writers under this head ; of such as are
almost indubitably animal, Noctiluca (Fig. 3 IL), the
animalcule which causes much of the diffused phos-
phorescence of the sea, is one of the best known.

I

Chap. III.]

METAZOA.

3 1

} nr. Acineta
tuber osa.

III. Suctoria : in these parasites (e.g. Acineta,
Fig. 3 in.), the month 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

i testicular reservoirs of the

earthworm.

II. THE METAZOA.

STRUCTURE AND EARLY HIS-
TORY OF THE EGG-CELL.

The key to the structure
of the higher animals, or
Itletazoa, is to be found in
a knowledge of the early
history of the egg from
Avhich, 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 (6), 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

Fig. 4. Ripe Ovum of Cat.
(After Klein.)

a, Envelope ; 6, 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

Fig. 5. Segmentation of AmpUioxus.

A, Stage with two equal segments; B, with four; c, with eight; r, segments
enclosing a segmentation cavity ; E, somewhat older 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 inorula
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 in-
vag ination)
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 hypoblast (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
arcliciiterou, 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, bypoblast.

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 mesoblast; it arises in
I 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
eiiteroii, 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 cosloni, and all
animals that possess it may, whether they secondarily
lose it or not, be spoken of as the Ccelomata.

The acoelomate Metazoa are the sponges (Pori-
fera), and the great group to which belong hydro,
the jelly-fishes, and the sea-anemones (Cflelenterata).
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-
cellular 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. Myxospongise, in which there is 110 hard
skeleton ; e.g. Halisarca.

2. Calcispongiae, in which a support for the
body is furnished by calcareous spicules ; e.g. A scon,

Fig. 7. Calcareous
Sponge. Ascetta primor-
<7 in 7 is. (After Haeckel,
x SOdiarus.)

36 COMPARATIVE ANATOMY AND PHYSIOLOGY.

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

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

4. Ceratospoiigiae, 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, endoderiu ; m, mouth ; be, enteric cavity ; t, tentacles.

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

In the Coelenterata 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.]

CCELENTERA TA.

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 the
grade of animal life, ready to our hand.

Pig. 9 Perigonimus vestitus, showing Tropliosoiues and Gonosomea

(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 Ccelenterate has a terminal mouth (m)
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 Hiucks.)

Chap. III.]

MEDUS/E.

39

S5

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 trophosomes. 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 K. Hertwig.) (See
Fig. 54.)

40 COMPARATIVE ANATOMY AND PHYSIOLOGY.

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

The Coeleiiterata fall into two well-marked
divisions, Hydrozoa and Aiitliozoa ; in the

former the mouth is placed on 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 (mesenteric 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 (\UiUtm). In consequence of the presence
of this fringe it may be spoken of as fh r^pqplftt>
division ; in it the sense organs are never protected
by any lid or cover, and they are therefore known as
the yaked-eed Medussa

and as the generative sacs never form projecting
pouches, they are by some spoken of as f!yypf.orarpa

I. Crpsp^fioJfr T^ft 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-
medusse. 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-
coral liiiae. Such are Millepora and Stylaster.

Chap. III.]

JELL Y-

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 Siplioiiopliora,
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 pair 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 1

<*---,

TIT,

Fig. 12. PTiysophora hydrostatica.

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

42 COMPARATIVE ANATOMY AND PHYSIOLOGY.

often spoken of as the An rag pfylr > t'n l ; the term
Steganophthalmata refers to the fact that their sense-
organs are placed in protected recesses on the margin

A -,,^ 7 > fffi

1'ig. 13. ^urelio aurita.

of

of the disc (covered-eyed medusae), and that
Phanerocarpa must be altered from its original signifi-
cance to mean only that the generative glands are
large and obvious. They are ordinarily free, but
Lucernaria is fixed; the common Aurelia (Fig. 13) is
a typical example of the group, while Rhizostoma is

Chap. III.]

ANTHOZOA.

43

A. Tubipora tnusica.

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. Antliozoa.-
Among the Anthozoa
we find the sea-
anemones and the great
bulk ol those ccelen-
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
Octactiiiise, and the
Hexactiiiiae.

I. The Octactiiiise
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 M B _ perinaiula

each polyp (Fig. 14 A) ; (Pteroides) spinosa.

44 COMPARATIVE ANATOMY AND PHYSIOLOGY.

in the sea-pen (Pennatula) (Fig. 1 4 B), the tissue which
connects the polyps together is horny, in the noble red

Fig. 14 c. Gorgonia fldbellum.

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

II. The HexactmisB ; 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, otf, 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) the
septa are alone calcified.

There still remains a division of the Coelenterata
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
Cteiiophora. The glassy globe
called Cydippe (Fig. 15) is found
on our own shores, while Yen us'
girdle (Cestus veneris) is an ex-
ample of that acquired bilateral
symmetry to which we have already
referred (Fig. 16).

Fig. 15. Cydippe pileus.

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 ccelom, 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 (Cestus veneria).

embryologists. All the Metazoa that possess this
body cavity may be spoken of collectively as the
Cflplomata. In some cases the ccelom remains
throughout life in a vory 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 eiiteroccele, or a schizoccele.

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 (stomodceiuii) which is lined
by epiblast ; (2) a mid-intestine (meseuteron) lined
by hypoblast ; and (3) an anal passage (procto-
clceum) 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
would 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
be 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 praestomiiim.

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 prsestomium
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. Platylielmiiithes. Of the three divisions of
flat-worms, two are degraded by parasitism ; such are
the divisions to which the tape-worms (Tsenia), 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 cceloin is not always
apparent (Acoela), or it may become secondarily
obscured. The enteric tract is straight, or branched.
Planaria, Dendrocoelum, and Mesostomnm 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
Aspidogaster, 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 C<- 1MPARA Tll'E A NA TO MY A ND Pin 'SIOLOG V.

with their constantly ento-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
Caryophyllseus 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

T\'g. 17. Tcenia, showing tbe head and four suckers,
the unjoiutei neck, and the early joints (Strobila).

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.]

- 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 (IVeinato-
helnriiitlies). 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

%r

other worms are the Acan-

thocepliali, 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. EchinorhynchiiR no-
dulatus (nat. size and en-
larged). (After Busk.)

52 COMPARATITE 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 JSTematoids, 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

-, ,-, * , T

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 metaineres, 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.

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
firm 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 pnestomial nervous mass with some few
nerve-fibres eiven off from it. Centres of nervous

O

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 ganglicilic
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-fibres 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 AIIIMI-
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 (seta?) ; 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 diiferent
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 ccelom, and also to the
exterior ; these are the renal organs (nepliridia).

The division of ringed worms in which the seta?

O

are numerous on each parapodium is called the
Polyclia^ta; of these, some, like the sea-mouse
(Aphrodite), Polynoe, and Nereis are free -swimming,

Chap III ]

CHJETOPODA.

55

Fig. 20. Tercbella 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 Tubicolse (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 setae
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 setae 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 ; JSTais 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 Sipunculus is the best known example ; for
these the old term of Gephyrea 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 boclv, 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 Echiisodermata 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 cnatliopoda (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 Lancelot (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 Ecliiiio.

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. III.]

ECHINODER MS.

59

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

Fig. 2L. Astropecten irregularis.
m, Mndreporite.

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 cnelornate 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
tube-feet) which are
connected with the canal ;

Fig-. 22. Pentaorinus Wymlle-fkomsoni. (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.]

ECHINODERMS.

61

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

i*. 23. Diagram of a Cross-section of an Avin of a Common Starfish

(wisterias rubens).

On the left side the section is suppled to pass between two of the ambulacra!
ossicles, but on the right side through one of them (no); ag, ambulacra!
groove ;n, radial nerve; 6, radial blood-vessel ; M.-, radial water-vessel; a,
anjpullaa ; t, tentacles or suckers; ap, adambulacral plates ; sp, spines : par,
paxillrs, arising from limestone plates; or, ovary; gp, genital pore ; gr,
genital blood-vessel ; br, respiratory processes ; pc, ca?ca 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 COMPARATH'E ANATOMY AND PHYSIOLOGY.

Fig. 24. Pentacrinoid Larva of
Antedon.

A, Quite younfr larva, before the open-
ing of the cup, and the appearance
of the radial plates ; B, Nearly
mature : b, hasals ; o, orals ; r, first
radials. CAfter 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-
noclerm is a colony of
bilaterally s } 7 m metrical
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 Echiiiodermata 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-
tacrimis, and Anted on.

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 ra dials.

In the higher grade of the Echinodermata, the
Echinozoa, these plates are often obscured. In the
regular Sea-Urchins (Echiiioidea) 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 Spatangidoe, 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 ambulacra!
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.

Astropliytidae) the .arms become more or less
branched. Lastly we have the class of the IIolo-
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
Arthropoda are more nearly allied to the Annulata
than to any other group of the worms, but they are

Fig. 25. Peripatns capcnsis.

Showing the elongated bilaterally symmetrical body, with the ringed antennse.
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 giiathites. 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
prsestomium was provided with tactile antennae, and

Chap, in.] ARTHROPODA. 65

afc 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 Protraclieata.

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. Arachiiida, 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
P 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 Branohiopoda.

1, Nelialia liipes (shell removed on one skle) ; 2, Estheria sp. ; Sa, 'dorsal : 3Z>,
ventral aspect of Lepidurus angassi ; 4, larva of Apus canciformis ; 5n, , adult
female of Branchipus stagnalis ; 5b, 5c, larvae ; 6, larva of Artemia salina.

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. HI.]

CRUSTACEA.

6 7

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 Oiiatliites,
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 (IVauplitis
larvae) (Fig. 26 : 4, 56, 6).

1. The Braiicliiopoda 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.

Fig. 28. Various Entornostraca.

1, Daphnia pulex; 2, Candona hispida; 3a, adult female of Cyclops quadri-
cornis ; b, c, d, larvre ; 4, Cetochilus septentriormlis ; 5, Sap'phirina ovato
lanceolata ; 6, Nicothoe astaci (parasitic on the 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.

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
and Cythere are examples.

4. Although the Cirripedia are, when adult,
greatly altered by their fixed or parasitic habit, they
leave the egg as NaupKiform larvse ; 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, Rhizo-
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 Malaeostraca have almost constantly

Fig. 29. S^uilla 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-
though

narily,

not always, are
set free at a
later than the
nauplius stage.

l.The Pod-
op lull a I mat a
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-
drioplitlial-
niata have the
eyes sessile, and
no carapace is
developed ; the
Aiiipliipoda (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

Lsemodipoda 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 Araclmida 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-
br a ii chi at a.
These are to-day
represented by the
king-crab (Limulus ;
Fig. 30). In them
the respiratory la-
mellae contain blood,
and the hinder por-
tion of the body is
fused into a single

O

mass, while the ter-
minal spine is of
great length.

2. IE rob raii-
cliia t a Such are
the scorpion (Scor-
pio ; Fig. 31) and the
spiders (Mygale). In
these the respiratory
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.

A, Female, nat. size; B. male, nat. size; o,
head of male, enlarged.

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. L.ipobraiicliiata. 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 Acariiia (mites, ticks), with an
imsegmented abdomen, and often a sucking mouth;
the Pedipalpi (Harvestmen), with a segmented
abdomen; and the Pycnogonida (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
taenoides, 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
Traclieata ; 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. Hexapoda 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. Aptcra, 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 trachece,
however well de-
veloped, and they are Fig. Si.OrcTicsella cincta, enlarged,
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

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 ; B, 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. Man dib ill at a. 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. Orthoptera : cockroaches, grasshoppers, and
locusts. With the anterior pair of wings converted

Chap, in.] INSECT A. 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 imagiiial 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 tin's
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.

. Haustcllata. 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. Lepidoptera (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,

1 f

a

d

B

Fig. 36. Diagrams of the Typical Structure of a Mollusc. A, from

above ; B, from below.

a, Tentacles of head ; 6, 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, osphradium (Sprengel's olfactory organ); i, ctenidia (gills); k,
generative orifice (paired) ; I, aperture of one of the nephrldia (excretory
organs) ; m, anus ; , foot where it extends beyond the visceral mass ; o,
mouth ; p, plantar surface of foot. (After Ray Lankester.)

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

So far the creature presents no characters other
than such as we might expect to find in any co3lomate
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 (cteniclia) (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 Lamellibraiichiata or mussels
and oysters. In these the primitively single shell is
divided into two bilaterally symmetrical halves, and
the two diA'isions 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 (Simipalliata),
as in Pholas, Solen, and Mactra.

B. In the higher division of the Mollusea the

Fig. 37. Mi/a arenaria, a Siphonate Lamellibranch.

ex, Excurrent ; in, inourrent 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 Qlossopliora, 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. Oastropoda, 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

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..

a. Isopleura. In these the two sides of the
body are equally developed, and
many of the characters of the
primitive mollusc are retained
unchanged. Here we have the
Polyplacopliora, 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 TVeomeniidse, and the
Clitrtodermatidti 1 , in which
the shell is represented by spicules
only.

. In the Aiiisopleura 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 Etithyiieura, which either, like Aplysia and
Doris, continue to breathe by gills the oxygen dis-
solved in water, or like the pond-snail (Lymnoeus),
the garden-snail (Helix), and the slug (Lirnax), 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

V '

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

Fig. 38. Chiton mag-
nificus.

82 COMPARATIVE ANATOMY AND PHYSIOLOGY.

symmetrical ; in the Azygobraiicliiata, 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
Heteropoda, 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 (mesopodiwin)
Fig-. 39. shell i g converted into a pair of flapping fin-
of Dentalium like organs by means of which these
num P 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 Tetral>raiicliiata, to
which many fossil forms belong, is represented to-day
by a single genus, Nautilus ; they receive their

Chap. III.]

CEPHALOPODA.

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 Dibraiicliiata 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; .7, shell-muscle; -ii. Siphuncle; fr, funnel or siphonal
tube; n, hood;p, tentacles; s, eye; x, eepta 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 COMPARATLl'E ANATOMY AND PHYSIOLOGY.

Mollusca form three very distinct branches or phyla,
the common ancestor of which is to be sought for

o

only in a simple worm. Of equal valne with
these is another phylum, which may be most conve-
niently spoken of as that of the Chordata, 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, parti}'" 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 IVemertiiiea.
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 diagrarnatic 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

F g. 41. The Common
Cuttlefish.

Chap. III.]

ENTER OPNE 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) ; the mouth (m) ;
and the anus (a). (After A. A. W. Hubrecht.)

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

So, again, in another group of " worms," the
Euteropiieiisti, the sole representative of which is
the remarkable Balanoglossus (Fig. 43), the anterior
portion of the eiiteron 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>otli 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. ^3. Young Balanoglossus seen from the side, placed Oil the
br. Branchial slits; x!2. (After Pagenstecher.) ventral Surface

not far from the

front end of the body. The Cliordata 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 Urocliordata 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.] CHORDATA. 87

there would seem to be degradation ; these are the
Cephalocliordata ; and, lastly, we have the true
Vertebrata 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. Urocliordata. 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 iiotocliord, and the essen-
tial resemblance between their gill clefts and those of
the Cephalocliordata ; 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

O

only, Perennicliordata (e.g. Appendicularia) ; in
the rest, Caducicnordata, 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. 44r).

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.

C. 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 vertebra?,

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

Fig. 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.

a. Cyclostomata, or Round-Mouths ; these are
the lampreys (Petromyzon), and hags (Myxine). There
is here 110 niandibular arch, no appendages in the
form of limbs, and the olfactory organ is single and
median. The hags are parasitic in habit.

. Criiathostomata. In this division all the
remaining Vertebrata are included ; in them an ante-
rior gill-arch becomes inaiidibiilar, 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.]

ICH TH YOPSIDA .

8 9

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
Yertebrata.

I. I <>Ii tli y-
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. Elasmoforanctiii (sharks and rays). In
" cartilaginous fishes " the gill slits are, in the simplest^
naked, i.e. not covered over by any fold (operculum),
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 fins are broad and
paddle-like (Fig. 45), or elongated and filiform.

3. The Oanoidei 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 Elasmobraiichs, Dipnoi,
and Amphibians, is so also in Ganoids, but is only
incompletely so in some Teleostei (Butirmus) ; 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, arid 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.

the gill chamber is
covered in by a
bony plate, oper-
culuin ; 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 O3so-
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 m e m -
brane ; spiracles
are persistent, and
the body is either
naked, or has bony
plates developed in
the dermis (Fig. 46).

* Chondrostei.

ii!:,!, 1 ; .. :!',. '. 'i: ii "' '

92 COMPARATIVE ANATOMY AND PHYSIOLOGY.

&. Teleostoidei ; * represented by the North
American bow-fin (Amia), 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. Pliysostomi, where the air bladder, which is
an outgrowth of the ossophagus, 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.

. Pfiysok9isli. 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 uiifrequeiitly 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 Dioclon, 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 riot trees, like Anabas ; some can take
leaps out of the water, like the " flying gurnards "
(and the physostomous Exoccetus) ; 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. Urodela; 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. Aiiura (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. Vsecilise 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.] SAUROPSIDA ; MAMMALIA. 95

large sac-like structure similar in origin and primitive
position to the bladder of the frog ; this allaiitois
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 amiiion ; 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 Amphisbaenidae)
have the occipital condyle double, may necessitate a revision of
current ideas as to the origin of the Mammalia.

96 CoMPARATirE 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. Beptilia. - 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 arehes.

1, 2. Lacertilia, or lizards, and Ophidia, 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.
Cyclodus) ; 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 Opliidia, in which the hind limbs are rarely
present, and then are only short and inconspicuous.
They are divisible into the Etirystomata, 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. Cheloiiia, 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 derm is 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 w T hich 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 maxilla?, premaxillse, 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. Cariaiatrc, 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 Mammalia, 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 (Ornitho-
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
Fig. 48. Pelvic Arch of epipufoes (see page 348) are

lar S 6 ' aild the coracoids

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

Chap, in.] MAMMALIA. 99

the primitive type, that they are without true
teeth.

B. Uletatheria (Didelphia). These are the
Marsupials ; they have true teats, but no placenta ;
the epipnfoes are large, but the coracoM 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 epipubes, 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 Uiigulata,
form a well-marked division, in which the group of
the even-toed forms (Artiodactyla), 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 Camivora 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 Homiiiidse,
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. Brachiopoda.
-These were placed
by earlier naturalists
with the Mollusca, from

"Fig. 49. Crania anomala. b, Arms. i i i j_-u

(After Davidson.) which, however, they

are to be distinguished

in consequence of the segmentation of the larva, the
dorsal and ventral positions occupied by the two

Chap. III.]

BRA CHIOPODA : BR } r ozoA .

101

unequal valves which make up their shell, and by the
characters of their nervous system. The so-called
arms (Fig. 49 ; 6) are outgrowths of the prse-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 larvse, appear to
have been descended from an ancestor common to
them and the Chaetopoda. 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 Chsetognatlia (as represented by Sagitta)
are forms that have relations to the Chastopoda 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. Myzostomnin is a form with some points of
resemblance to the Chtetopoda ; 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 TV.

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.] 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 iiitracellular 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 Amoaba, where the protoplasmic body
seems to engulf its nutriment by flowing and closing
around it. And this iiigestioii of food does not
take place at any definite point in the body of the
Amceba, 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 Infusoriaiis, where a definite orifice
("cytostome ' ; ) acts as the sole entrance for food
into the body : in many cases this so-called u 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

104 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 Paramoecium (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.

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
G-regarinida, which live in cavities rich in nutrient
matter, such as the intestine of the lobster, or the

testicular re-
servoirs of the

earthworm : in

'''//m/iiii'iiim\\VAw\vu\wv,*w"' _

Fig. 51. Anoplophrya prolifera. (After Clapa- SUC " * orms as
rede and Laclimanu.) these nutl'i-

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.] INTRACELLVLAR DIGESTION.

I0 5

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

\*^-

4b?

F

a

^

the body walls of
a Hydra we

find that the en-
dodermal cells
vary consider-
ably in size, and
that, while some
are provided

with n c;i n o-l p 1 nn cr Fig. 52. Longitudinal Section of the Body of

-L v A .1 Ck oi. i. 1 . -A *-' J. \J 1.1 ta , . -. T - i i i* fiii* j_*

a Hydra, killed in full digestion.

Tiagellum, OtlierS eCi Ectoderm ; en, endoderm ; mp, muscular pro-
ova rli G-i-iTTr'tlTr cesses; d, a diatom ;/, food particles. (After
aiSLlllCtiy T.J.Parker.)

auiOBboid 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 endodermal 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 nutrient
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 , . ,

Bath Sponge, showing the collared-cells and A we

flagellum.

K, Excurrent canals ; i Incun-ent canals. (After
Scmilze.)

have to do not
with an amoeboid
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

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

O

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

loS 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 Crelenterata,
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 (tropliosomes), 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 (goiiosomes). The Stylasteridse, 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 Rhizostomiclse) 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

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,

Fig. 54. Section of Saga rtia
parasitica.

t, 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.)

Ill

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.

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

iT2 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 Ccelognrgia plumosa,
showing the long delicate cilia of the siphouoglyphe (si). (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 Mesenterial 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. 1-13

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 Ctenopliora 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 Cceleiiterata ,
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

* O

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

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 JVematolielmiiitlies 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.

[cex

portion of their intestine which is known as the
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 ; 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 calciferota s
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 tbe Alimen-
tary Canal of an
Earthworm.
(After Ray Lan-
kester. )

m, Mouth ; pTi, 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

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

Eelations of the Intestine.

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

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 esophagus 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 ANNULATA. 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 typlilo-
sole, or blind tube. Around the intestine are a
number of granular greenish cells (Fig. 57; w), 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 w T orms ; 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

n8 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 c?eca is not, however, confined to
the leech, for it is found also in the sea-mouse (Aphro-
dite), where the very numerous cseca 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 Crepliyrea 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
(Colin).

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-

O '

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 aproctous 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 Vermes presents an arrangement which
is exceedingly common among the Vertebrata ; that is,
the possession of an enlargement or cloacal 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 (epistoine) 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 011 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 " odoiitophore " of starfishes, or
the various kinds of mouth papillse 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

^5

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 hi
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. Astropecten) and in all
Ophiuroids, the anus is lost, so that here we have an
example of the fact that the absence of an anus 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
^urface ; 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 caeca " 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 resophageal 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 Artltropoda
is the conversion of one or more pairs of its appendages
to the service of the mouth ; they become, in fact,
mouth-organs (girathites), 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.j 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
mandible, for here the basal portion is very strong,

=3H5k/ *

B

en.

If. .

Fig. 58. Mouth Organs of the Crayfish.

A, Mandible ; B, flrst maxilla; c, second maxilla : bp, basipodite ; i,endopodite ;
cxp, coxopodite ; p, palp of mandible ; sg, scapliognatliite. (After Huxley .1

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 endopodite. So
far as the digestive process is concerned, the second
pair of maxilla* 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 maxillae 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 so 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 011 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).

this stomach there is developed a hexagonal frame-
work of calcareous and 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) t 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 stomodfleum and the
proctodceuin, 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 a 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 maxillse of the crayfish.

The true Insects or Arthropoda Hexapoda have,
likewise, three pairs of mouth appendages, and these,
again, are known as mandibles and maxillse ; 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
jmathites work from side to side and are fit to act as

O

cutting 1 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 1 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
undifferentiated or generalised condition ; though not

chap, iv.j 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 mandibulate 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 max ill se.

In front of the mouth there is an upper lip or
In l> rii in, 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 gnlea (ga), and an
internally placed laciiiia (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 mentum (m), and
snbmentiini ; these, with the anteriorly lying

* cct

Fig. 60. Mouth-parts of the Cockroach.

m, Menturu ; sm, syibmentum ; li, ligula ; pg, paraglossa ; pp, palp of second
maxilla ; md, mandibles ; ca, cardo ; st, stipes ; ga, galea ; la, 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 lafoium; 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 Triclioptera (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 Hymeiioptera (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 011 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,
\vhich, 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

u

produce a sucking-tube (Fig. 61).
In the Diptera (or flies
and fleas), what were bristles in

rt the bug now form sharp, cut-

Fig. 61. Mouth Orgau of , . , ... 1^1

ting, lancet-like organs, and the

second pair of maxillae

Nepa.

again

md, Mandible ; mx, flrst pair

of niaxillffi ; mx', second . 1 .

pair (labium); li, .ligula. form the Suctorial tUDC | 111 SOine

(After Savigny.) . .

cases (Pangoma) 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

chap, iv.] ENTERON OF INSECTS. 133

soldiers, but reduced in the workers ; the Ephe-
meridse or day-flies, which want to eat no food in the
aduit 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 chit in, and, like that form, it has also a
considerable portion of the hinder region formed by
the proctodeal invagmation. The chitinous layer
extends through the funnel-shaped pharynx, the
narrower oesophagus, 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 creca 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

' O O

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 MOLLUSC A. 135

The rest of the Mollusca, or Cephalophora, are
provided with a rasping organ, which lies on the floor
of the pharynx, the odoiitophore. 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
calcified.

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 radaila.
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 011 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 uiiciiii. 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 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 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 repeated.
For example, when, as in ^Eolis, there are no lateral
teeth, we write the formula . 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-

O '

times conical and upstanding ; when the rachidian
teeth are, as sometimes happens, absent, another part
of the digestive tract may, as in the Bullidse, be pro-
vided with calcareous plates which replace them
functionally. In a few (e.g. Bhodope) 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.] ENTERON OF MOLLUSCA. 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 resopha-
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. Csecal 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

133 COMPARATIVE ANATOMY AND PHYSIOLOGY.

similarly named part in the Vertebrata ; 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 LamelHbraiicliBata (or Acepliala) 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
BaSanoglossus, 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 Chordata we find very simple
arrangements of the digestive tract in the Cepfia-
Jochordata; the mouth of the Lancelet is placed

ch.i P . iv.] ENTERON 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 cf 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 amis, 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 Kemertinea, the
enteric epithelium is ciliated. In the Uroclior-
data. 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 vaiies 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
oesophagus. 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 Salpida?, 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 (&iiatliostomata). 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

of

young Dog-fish, showing the spines
of the skin under the jaw, and the
teeth above. (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 Yertebrata, 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 size, and without any special masticatory

Chap. IV.J

TEETH OF FISHES.

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

Fig. 63. Lower pharyngeal Bone of Scarus,
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 Lepiclosteus 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 Yertebrata 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.

'45

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
vomerine teeth, and other amphibians have them ou
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 maxillae and pre maxillae.
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 Hatter ia ; 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 (Odontoriiithes).

Curiously resembling the arrangement of the turtle,
and having, of course, much the same function, are
the tough horny plates oil 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, are 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-
ciently close to
form a service-
able canal, along
which the poison
from the poison
gland may make
its way into the
wound. In the
latter the single
large maxillary
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

Fig. 65. Transverse Section of the Poison Fang
and Eeserve Set of a Viper.

1, Tooth in use; 2, the tooth which will succeed it ; 3 to
10, tooth-sacs numbered in the order of their succes-
sion. (After C. S. Tomes.)

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 premaxillse, 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
monophyodoiit, 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.-j 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 tuberculated 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 lioniodoiit 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, pill, 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 Tipper
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

I j c - . pm , m = 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

* 272' l7l' Pm 272' * 3l = 32 '

And that of the cat :

3.3 1.1 3.3 l.l

c pm m = <jU.
? 3.3 1.1 2.2 1.1

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
(Suidte), 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) ; Fi * f - gJg 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 has already been said, these two tissues
differ greatly in hardness ; they will, therefore, wear
down unequally and so give rise to a roughened sur-
face well adapted for grinding. In addition to the

Fig. 67. Teeth of Wolf.

i 1 to i 3, Incisors ; c, canine ; p 1 to u 4, premolars ; m I to m 3, molars ; h, heel
of the first molar or lower " carnassial" tooth.

outer investment of cement, we find it also filling up
the interspaces (Fig. 68). In the Carnivora the
lower jaw is so articulated to the skull as to be
able to work from below upwards ; in the herbivorous
forms the jaw works from side to side.

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

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

Chap. IV.]

TEETH OF GET ACE A.

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-w hale 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

Fig. 68. Teeth, of Horse ; A, Upper Jaw ; B,
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 Chamseleons, 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

(Neetarinidse). 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 ; m, mesial ridge of palate ;
J, lower jaw-hones ; 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
upper 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 instead, 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
diluted, 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 subliiigna)
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

* S to ^ e gathered
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 we'll 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

Fig. 71. Side View and Lower Surface of the
Tongue of a Galago.

Chap. IV.]

SALIVARY GLANDS.

159

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 submaxillary

glands has certainly poisonous effects,

but 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 Fi - 72 ,-~ H l ad , of

(jr6co \V oodpcck^r

absent ; in the wood-peckers, where seen from below,

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 buccal glands ; these, from their posi-
tions, are distinguished as the parotid, submaxil-
lary, and sublinguals. From our knowledge

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 archeiiteron, the proctodeal portion being, as
a rule, exceedingly short. The general tract may be
conveniently divided into the oesophagus, stomach,
intestine, and rectal portion ; this last, in all except

Chap. IV.] IXTESTINE OF VfRTEBRATES. l6l

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-

o *

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

1 62 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
vertebrae 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 O3sophagus
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.j 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 \valls of the stomach are richly pro-
vided with muscle in the mullets.

The simplicity of the oesophagus 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
proveiitriculus) 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 teiidiiiea.

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

B

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.

'65

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,

' V *

therefore, be distin-
guished from one
a 11 o t h e r 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
Gymiiura, and very
much more so in the

rabbit. As the trails- Fig. 74. Inner Face of the Wall of the

Stomach of the Horse.

verse axis increases

, , , , . . . f. Cardiac ; o, pyloric sac : c, duodenal dilata-

in length; a diVlSlOll tion. (After Chauveau.)

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 oesophagus is
further remarkable for being continued on, in the
form of a groove, into the pyloric division ; in addition

Fig. 75. Stomach of a Ruminating Animal.

A, Exterior ; B, interior ; a, oesophagus ; b, rumen or paunch ; c, reticulum or
honey-comb bag ; d, psalterium or nianyplies ; e, aboruasum or rennet-
stomach.

. iv.] 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
paunch, 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 lamella 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-bag* 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 larg^e 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 Butiriiius (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 Wieclers-
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 Amphisbceiia
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 Abdomiual 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.

i yo 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
iteum ; 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 urine-genital orifice ;
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 faforicii of
Birds, and the foursse aiiales 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
c SBC u in ; 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 cseca 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 vitelliiie 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

o

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
1'actor 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 PHYSIO LOGY.

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 Garni vora,
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 1 8 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 very
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.

173

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

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

ir, Inferior vena cava; p, vena portse; it, umbilical vein of the foetus, represented
by the round ligament in the adult, lying in the umbilical fissure; II, left
lateral lobe ; Ic, left central lobe ; re, right central lobe ; rl. right lateral lobe ;
s. spigelian lobe : c, caudate lobe ; g, gall bladder ; di\ remnant of ductus
venosus : llf, left lateral fissure ; cf, 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 bilobed 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.] LITER 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
oxy elation is less extensive. The observations that
have been as yet made on the " glycogenic " functioii
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

Tj6 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

o

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 Infusorians,
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 any part of its body, is doubt- Fig. 79. Acineta tube-

-, i * ,1 ' f-^ -I rosa with the pseu-

JeSS true also OI Other Lestoda, dopoHa extended and

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 larvse
(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 root3
which it insert? into the bo Jy 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 dip-es-

9 o

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 JVematoitl 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 oesophagus 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-oesophageal
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 Arthropoda,
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-
hmd-lips ; the anterior pair, or mandibles, are alone
well developed, and form piercing processes. In
Corycpeus the suctorial tube is not developed. In both
of these there are swimming feet. In Lernsea, 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 Mollusca ; 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 liremoglobin, 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 Vertebrata, 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 amoeboid 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 amoeboid 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
fasting 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 cyclostomatous
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 T ^- millimetre by -~ ; they
are smallest in the Chevrotain (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 Soleii (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
larse number of worms the fluid in the vessels is, on

O

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 i jicn '>
Lankester). It is, however, only quite recently that
observations have been directed to the presence of
corpuscles in the blood of Amiulata, and since then
they have been observed in several members of the
group (Eunice, Nereis).

BLOOD-VESSELS AXD 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 orsfan, 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 Mollusca and Arthropod a 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 oif from the
ccelom, 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 ccelom contains char-
acteristic corpuscles, some of which are coloured.
Curiously enough, haemoglobin has been detected in
the water-vascular system of an Ophiuroid (Fcettinger).

In the Protozoa there is, of course, 110 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 Coeleti-
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 gastro vascular

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 xrften indicate all
that can be seen of a coelom. 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 1 1 4),
which perform the same function as the gastro vascular
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 affordsus 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 Nemertiiiea,
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, is
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 aortae (/>); 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 forwardsit 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

Fig. 81. Diagrams to show the arrangement of the great Blood-vessels
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, \vhich 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 (Coiycseus) 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

ji a,

m

era

Fig. 82. Diagrams to show the arrangement of the great Blood-vessels
in the fresh-water Mussel (c) ; aud 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.] HEART OF ARTHROPODA. 189

supply the front part of the head, and the hepatic,
which go to the chief viscera. The posterior artery
runs 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 bod} r . 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
a ^ain 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 Peripatns is described

V

hy 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 Hexapocla the fardiac 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 abso-
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 (Aiiodon), 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 lacuiife 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-splaiiclmic, 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 CHORDATA. 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 Appeiidicularia fureatn. 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 Cephalochordafa we find an arrange-

^^

ment which reminds us of what obtains in the
Annulata, 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 aortas, or first
N 16

194 CoMrARATirE ANATOMY AND PHYSIOLOGY.

passes through tlie already-mentioned anterior en-
largement.

A very definite system obtains throughout the
Vertel>rata ; 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.

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 venosiis, 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 (Me William),
in which the terminations of the jugular veins, are
contractile ; and on the other, by the Elasmobranch
fishes, the Dipnoi, and the Amphibia, in which the
basal portion of the aortic system (conns arteriosiis)
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

196 COMPARATIVE ANATOMY AND PHYSIOLOGY.

blood from the body generally, opens into the 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
au.ricle (atrium or sinus venosus of
human anatomy).

The ventricle remains an un-
divided chamber throughout the
Amphibia and all Reptiles except
the Grocodilia, 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 cenogeiiy,
and is, no doubt, dependent on the requirements of
the organism.

The ventricular is separated from the auricular

Fig. 83. Heart
Squatina.

of

A, Auricle; B, arterial
cone ; aaa, branchial
arteries ; o, orifice of
ventricle, v. (After
GegenbaurJ

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

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 011 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 (cliordse
teiicliiieae) 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. 8-i. Heart of the
edible Frog (Rana
esculcnta), to sho>v
the auricle arid ve
tricle opened from
the left side.

s, Septum atrioruiii ; la,
left auricle ; ra, right
auricle ; w, auriculo-
vcntricula,' valves ; 0,
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 chords
teiidinese 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
rnithorhy nchus, the
valve of the right side
is, as in the Sauropsida,

Fig. 85. Left aimculo-ventricular , ,

valve of the Swau, showir.g the muscular membranous

chordae tendinese arid the musculi -h'ocjnp Vipirio- rmlv nnrn
papillares (pp). (After Wieder- 1R S nl ^ CO111 '

sheim.) 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.

Chap, v.] HEART OF REPTILES. 199

An indication of a division of tlie 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 cavum pulmonale (Fig. 8G ; 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 pulmonale ; 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 aortte. 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

L Ao

Jt.Ao.

Fig 1 . 86. Diagram of the Ventricle and connected parts in the Turtle ;
showing the transversely elongated ventricle, with, the right and
left auricles (RA, LA) lying towards the left, the auriculo-
veiitricular valves (uu) formed by the inter-auricular and septum.

RA, Right auricle; LA, left auricle; v, the right; 1/, 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 rierlit aorta ; x, arrow
showing the course taken by the blood from the left ; and w, from the right
auricle into the ventricle ; w, showing the course of the blood from tbe cavum
venosum into the cavum piilmonale ; z, from the latter into the pulmonary
artery ; a, the incomplete septum marking off the cavum piilmonale (cp) ; PA,
pulmonary artery ; RAO, LAO, right and left aorta? ; v, 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

o J

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 haemo-
globin ; that, under appropriate conditions of warmth
and moisture, the heart of a frog or a tortoise will,
after removal from the body, 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 off
vessels which are known as the arteries ; in the least
modified Fishes, and in the Ganoids, the common trunk
(coiius or trillions arteriosws) is, like the venous
sinus, contractile, but in the bony fishes this con-
tractile power is altogether lost, and the bulbus
aortas 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 Lepidosteus eight or nine) longitudinal rows
of pocket-like valves ; in Lepidosteus 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, Ceratoclus
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-

Fig. 87. Diagram of
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
fine 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 plexiforin
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 Reptilia, 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.

.-As

t

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
farther, for
in them the
aortic trunk
single

is

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 (Fi<*.
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 circiilus
cephaJicios, by means of which the supply of blood
to the head is the better regulated ; they supply the
head and neck.

Iff

Fig. 88. Heart of Lacerta muralis.

Auricles ; v, ventricle ; tr, arterial cone (or truncus
anonymus) : l, 2, first and second arterial arches ;
RA, root of aorta; AO, aorta; As As', subclavian
arteries; Ap, pulmonary artery ; vp, pulmonary vein;
J, jugular ; vs. subclavian veins; ci, vena cava in-
ferior ; these last three pass into the sinus venosus
(s) which lies beneath the right auricle. (After Wie-
aersheim.j

Chap. V. |

ARTERIAL ARCHES.

205

2. Sufoclaviaii. 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

..b

Fig 89. Diagrams to show the Metamorphosis of the Arterial Arches.

A, Lizard ; B, Snake ; c, Bird ; D, 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 (d) ; 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 (<j) < 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 the 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 COMPARATIVE ANATOMY AND PHYSIOLOGY.

4. The aorta proper gives off large branches, such
as the cceliaco-meseiiteriic, hepatic, renals to

the viscera of the body cavity, and two large hinder
(iBiac) 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 1 * 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 subintestiiial 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.]

VJ.SSELS 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
pair remains
throughout the

AA, Abdominal aorta; r.A.
HA, right and left roots of
the aorta, connected by
the collecting vessels (ss')
witli the branchial ves-
sels Ab ; cc', carotids ; S&,
subclavian artery ; KI/,
gill clefts ; si, sinus ve-
nsus : A, atrium ; v, ven-
tricle ; B, bulbus arterio-
sus ; vm, omphalo-mesen-
ter'c veins ; Ara.oniphalo-
niesenteric arteries; ic
ic, common iliac arteries;
EK, external iliac arteries,
A/?,allantoic arteries ; Acd,
cnudnl artery ; vc, HC, an-
terior and posterior cardi-
nal veins; sb', subclavian
vein ; D, dnctus fuvieri.
(After Wiedersheim.)

Fie. 90. -Diagram to explain the Arrange-
ment of the Embryonic vascular System.

2O8 COMPARATirE ANATOMY AND PHYSIOLOGY.

vertebrate series as the jugular veins. In the pen-

taclactyle 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 sutoclaviaii) 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.] 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

O

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
o16

2io COMPARATIVE ANATOMY AND PHYSIOLOGY.

now see that Birds and Mammals have independently
acquired the hot-Ulooded 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 Amcefoa) 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.] 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 Coeleiiterata,* 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 Turbellaria, 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 TVematoid 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 Metazoa 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 SUBGEMTAL PITS of Aurelia have no connection at all with
the geni-tal 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 Vertebrata, 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 Oligochaetous
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.

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

,cu

[J lyol-Asc*

Fig. 91. From a Trarsverse Section of a Leech ; to show cu, cuticle ;
v, intra-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.

Schizonemertini (such as Cerebratulus) the transverse
furrow of the Palseonemertine gives place to a deep
longitudinal slit, which penetrates the muscular tissue

Fig. 92. Diagrams of the Eespiratory Ciliated Ducts of (A) Carinella
annulata; (B) C. inexpectata ; (c) Polia curta ; (D) Cerebratulvs.
(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.]

TRACHEA.

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 oesophagus, 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 Eiitero-
piieusti (Balano-
glossus) and in the
Cliordata. (See
page 231.)

In the traclie-
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. Trachea? of Insect, showing the
Spiral Fibre.

216 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 renfling 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 Iarva3 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. Culicidre), 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

2i8 COMPARATIVE ANATOMY AND PHYSIOLOGY.

oxygenated sea-water by a thin wall of membrane
through which oxygen passes inwards, and carbonic
acid outwards. In the Opliiuroidea, 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
biirsal 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 coalom by nothing more than a thin wall. In.
the Ecliinoidea, 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 Polychretous 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 TiiMcolse 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
Liamellibraiichiata, where the Cteiiidium (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-
seen ding
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 ; /, intes-
tine ; br, gills; v, ventral vessel; d, dorsal; v', visceral
vessel ; p, vessel around intestine ; a, &, vessels of gills.
(.After Gegenliaur.)

220 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 iiiteriamellar junc-

i! ''

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 iuterlamellar junctions and large vertical
vessels of Anodon ; c, a more highly magnified view of B. (After
E. HolmanPeck.)

tioiis (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.] GlLLS OF MOLLUSCA. 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 dlochidia, 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 Aiiisobranchiata
the left gill is, OMdng to the torsion of the body, much
smaller than the right ; in others, as the Hetero-
poda, the left gill disappears altogether, and in some
the right gill has only one lamella developed ; this is
the so-called semi-pinnate gill. In other Gastro-
pods, such as Lymnaeus, 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 Nudibranchs
leads us, physiologically, to the vague respiration of
the gymnosomatous 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 often then, water is

Chap. VI.]

GILLS OF CRUSTACEA.

223

taken in and driven out, the more often is the animal
mechanically helped on its course.

In no group are gills better or more characteristi-
cally 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 Braiichiopoda) a
number of the appen-
dages are nothing more
broadened thin
within which

than

plates

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
their 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 ; c/) ;
these gills are set in three sets, the lowest of which
are (in the nomenclature of Huxley) podobraiichs,
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
arthrobraiichs, 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 ; t, intestine ; n, ventral nerve-
cord ; (I, 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-
foranclis, 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.

Pleurobranchs.

1 4

1 4

1 4

1 4

1 4

1 = 4

1 4

8 + 8 + 8 8 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 the segments
one farther back, and to call vi. what Prof. Huxley calls vii.

Somites and

Arthobranchs.

their appen- p dobranchs.
uages.

Anterior. Posterior.

VII.*

1

1

1

VIII.

1

1

1

IX.

1

1

1

X.

1

1

1

XI.

1

1

1

XII.

1

1

1

XIII.

1

1

1

XiV.

1

1

+ 1

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 scaphognatliite, 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 Araclmida, 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 lamellse 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 " Imig'-
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 " pectiiies." 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 lamellse 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,
remarkable series of chanes so far followed does not

B

Fig. 97. A. View of the lower Margin of the right latnelliferous Appen-
dages of the eleventh segment of Limulus polyphemus.

A. Z, Proximal lamella ; lx. one hundred and fiftieth lamella ; ex, external lappet of
the bifid distal prolongation of the appendages.

B. A similar view of the corresponding appendage of Buthus fcoc7ti.
Ix, 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 larnellas exposed by the removal of the integument of
the axis, the remnants of which are seen at m ; oc, the projecting portion of
axis ; I, proximal lamella. (After E. Ray Lankester.)

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
" lung." 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 biood- 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,

O

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, it' 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 Vertebrata the
lungs have undoubtedly the power of acting as
hydrostatic supports for the bod} 7 , when immersed in
water.

The Cliordata present us with the most interest-
ing and instructive series of arrangements, for while

Chap, vi.] CHORDATA. 231

the lowest members of all three divisions are
branchiate, the higher Yertebrata 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 Cephalochordata
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 chordate tail throughout life (Appeiidi-
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 Ampliioxus, 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 Cephalopods, 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 Verteforata
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 Heptaiiclms ; 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 Amphipiious 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-

luin), 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 (6), 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 lamella?
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

98. Gill of
Perch, to show
the form of the
gills and the ar-
rangement of the
blood-vessels.

a, Branchial artery ;
&, branchial arch
(seen in cross sec-
tion); c, branches of
the branchial vein
v d, bi-anches of
branchial artery.

234 COMPARATIVE ANATOMY AND PHYSIOLOGY.

a

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. Suprabranchial organ of Anabas

scandens.
a, Suprabranchial organ.

Chap, 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 Suclis
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
oxvdation 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 Amia (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 considerablv 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 Amira 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 ISTotodelphys lead to
the branchial vesicles which have been found in the
Csecilise (Peters).

In addition to or in place of the gills, all Amphibia
have a paired outgrowth from the esophagus, 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.]

MENOBRA NCH us.

237

3

a
I

8

a

bn

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

Fig. 101. Diagrams to illustrate the Development of tbe Lungs.

PD, Primitive intestine; ss', lung sacs; t, trachea; Z>, bronchus (in A, B, and
c) ; L0 (in D), primitive; ~LQ (inE), secondary pulmonary ve&iclee. (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 Yertebrata, 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 Reptilia; 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 super] acent 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

V1230/'

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 1600-22 X ,, _. . _ ...

to 1-12 (mo + 200 or lSr} Blrfs tllat fly hl h

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 '84 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

O O

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.

chap, 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 Dinoriiis,
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.

Verteb rates 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 the
posterior nares to
the opening of the
trachea (see page
344) ; by
these means
water is pre-
vented, at
least in part,
from passing
into the
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, hypar-
terial broncbial systems ; v, ventral ; d,
dorsal branches.

Chap. VI.]

CETACEA ; SIRENIA.

243

B

-B

v.

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). Here, then, milk
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 sac's with elastic walls ; the possession of
these sacs must, in addition to their air-containing
function, diminish to a cei'tain 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. Brou chial Tree of a
Mammal (Horse).

A, Eparterial ; B, hyparterial ventral
(y1;(ff) hyparterial dorsal bronchi;
pa, pi\ pulmonary artery and
vein. (After Aeby.1

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-iii 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 Pereniiibranchiata 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.] REPTILES; BIRDS. 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 CoMPARATirE 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 we 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 a^ent for removing waste.

* o o

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 CElenterata or the Echiiioclermata. 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 cseca 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 Vernies 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-

o

mental or kidney-like organs (iicphridia) 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. VII.]

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.
wider portion, the walls of

are muscular, may be

upon as

The terminal and

analogous to a

which

looked

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 Platylielniintlies,
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-
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 Deii-
droccela, as represented by
Polycoelis, the fine canals appear to be absent.

If we take the liver fluke as a type of the
TTrematoda, we again find that the system of
excretory vessels is diffused throughout the whole

Fig. 103. A single Kephri-
dium of ;l?iac7iajtt(.

a, Internal orifice, funnel-
shaped and surrounded by
cilia. It opens into one
segment, passes through
the septum () into the
next segment, and opens to
the exterior by e, external
oriflce. (After Vejdovsky.)

250 COMPARATIVE ANATOMY A.VD 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 Monoccslis fusca, showing: tlie numerous
Infuudibula, and the branching 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 ; Lieberklihn 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.] CES TO DA ; Ro TA TOR I A. 2$l

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
" segments! 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 guanin (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 ccelom,
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 Gepliyrea ; 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 Sipimculus) ; 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 and 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, w r hich
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 flatter,
has its walls produced into a number of small saccular
outgrowths. On the inner surface of the cells of this
green part, and of the succeeding w T hite 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 tip 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 organ of

254 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Crustacea. The simplest is that which obtains in
many Copepoda, 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 Bojamis 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 quite independently

chap, vii.j MOLL use 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 Hefceropods 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 Mai-
pighian 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

Fig. 105. Respiratory and Renal
Organs of Sepia.

a. Aorta ; v, vena cava ; v*, posterior
vcnie cavsB, c, heart ; rf, enlargements

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 nephiidia 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
legs are very rudimentary. A typical nephridium
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 bodv.

/

No definite information has been acquired as to the
possession of a renal gland by Amphioxus. The
Urocliordata 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 Vertebrata 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 Proiiephros (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 glomerulus.

The mesonepliros (Wolffian 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 Cyclostomata the mesonephros is
found in its simplest condition, for it there consists of
a segmental duct with tubes (Fig. 106; , 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 Petromyzoii, where the
whole kidney is more compact.

In the Elasniobraiicliii 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 proiiephros 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 Vertebrata.

In the sturgeon, among the Ganoids, the kidneys
extend from just behind the skull as far as the cloaca,
and differ in width in different regions ; in them, and

e

Fig. 106. Mesonepbrus of Bdellostoma.

A. a, Segmental duct; b, segmental tube : c, grlomrrulus. B. A part more highly
magnified, showing one duct with its afferent vessel d, and its efferent e.
(After J. Mailer.)

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 niesonephros is of considerable
length, and gives off a number of ducts (Fig. 107) ; in
the frog, which may be taken as a type of the Aiiura,
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 ittetanephros, 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
complexity of internal struc-
ture, we observe, as we pass
from Reptiles to Birds or
Mammals, that the length of
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

advancing

Fig. 107. Diagram of the
Mesonephros of an Uro-
dele. HO, testis ; N,
kidney. ( Modified, from
Sprengel.)

Chap, vii.] SAUROPSIDA ; MAMMALIA.

261

Cv Ao

FK

kidneys of the Amniota 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

/ *

one lies a good deal
in front of the other ;
this difference of
level between the
two kidneys, which
is clearlv an arrange-

inent 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.
In Birds the lobes
fit into 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

;. 108. Urogenital Apparatus of a Male
Frog.

, Kidneys ; ur ur, ureters ; + tbeir point of
origin ; s s', their opening into the cloaca Cl,
H o, testes ; FK, fat body ; AO, aorta ; cr,
vena cava inferior ; vr, efferent veins.
(.After Wiedersheim.)

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.

/"^

j ^S |>

OW

iifi

tfpo

M

Fig. 109. Diagrams of the Urinary Tubules of

A. The frog; B, tortoise; c, pigeon (after Hufnerl; D, man (after Ludsvi ?) ; I.,
glomerulus ; n. to vn., 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 metanephros,
becomes developed from part of the "Wolffiaii duct,
which, when this ureter is present, only carries away
the secretion of the testes.

In the Cyelostomata, 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 Iclithyopsida
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-urinary 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 Amiiiota, 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 Metatlieria
the ureteric ducts open into the base of the bladder ;
in the Eutheria they open at various points along its

264 COMPARATIVE ANATOMY AND PHYSIOLOGY.

Fig. 110. Renal and Generative Organs of
Ericulus setosus.

course ; thus in
Gymnura they
open into, and in
Ericulusnear, the
neck (Fig. 110) ;
in man they enter
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.
The bladder itself
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. (See
page 170.)

r. Kidneys; /, testes ;
ut, ureter ; rd, vas
deferens ; b, bladder ;
pg, prostate glnnd;
eg, Cowper's gland ;
lp, levator penis; cc,
cystic urethra; pr.
prepuce, divided and
reflected ; p, penis.
(AJter Dobsou.)

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.

addition to the various secretions, such as saliva
and bile, or excretions such as the uric, there are
others which, though dependent, of course, 011 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 dog 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 Mygale avicnlaria 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
au outer layer of muscular tissue, are situated in
the terminal segment of the body ; their ducts open
at the tip of tlje 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 capillaiy 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.

Yarjous Fihes 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 (Gimther).
The integument of many Anipliil>ia is richly pro-
vided with glands, which secrete a viscid fluid possessed

Chap, viii.] SILK ORGANS. 267

of more or less well-marked irritating properties ; a
familiar example of tliis 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 FO-callecl " 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 entrapping
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 larvae of the Lepidoptera, where they
have the form of two long csecal 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 labiuni. 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 (Myrmeleon) 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 JLamelJibranchiata, where the
foot secretes a soft substance, which becomes hard and
chitinous on exposure to the air ; this 4 * 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 Gloehidia, or young of the fresh- water
mussel, is one of attachment,

Offensive organs of a somewhat similar character
are to be found in certain Holothurian. 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 organ* 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. \U\.]ELECTRICANDPffOSPORESCENTORGANS. 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 Elateridse, 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; bj many
Medusae, by the Pennatulidse, 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 Polyehseta ; 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 Pyrosonia 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 Pennatula 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
(Elateridre), 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.j PIGMENTS. 271

pigments, 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
"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 (steiitoriii) ; in some corals and hydroids
there is a red pigment (polyperythrin) ; chloro-

cruoriii has been obtained from various Poly ch seta ;
peiitacriniii from Pentacrinus ; and antedoiiin

from Antedon and a Holothurian ; the terms crus-
taceorubriii, nplysiopurpuriii, iaaitliiiiiii,
and bonellein explain themselves. Zooxaiithiii,
zooerythriii, zoofulviii, and turaciii 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
algse, 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 anima'
with which they live ; such are the so-called yellow
cells of Anthozoa and Racliolaria. Where no cell-
nucleus is seen to be associated with the green cor-
puscles, as is the case in Spongilla and Hydra, we
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. VIIL] METALLIC COLOURS. 273

well seen in the wings of various insects, the scales of
which are marked by extremely delicate stria?, 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 Gado\v, 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 Ameeba 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 Amoeba 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 deal
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; buttheSarcodina are not
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.] SKELETOA T S OF RHIZOPODA.

275

various Rhizopods, and may take on the most different
forms, and even become pigmented. In marine
Rhizopods the test becomes much firmer, owing to the
deposition in its substance of calcareous salts ; and as

\

Fig. 111. Gromia terricola, 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 Foraminifera, become

276 COMPARATIVE ANATOMY AND PHYSIOLOGY.

very elaborately coiled, and attain, as in the fossil
Nummulites, 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 Radiolaria 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-
jnetriche, 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 Sarcoclina 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.]

XlPHA CA NT HA .

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. - Xivhacanttia 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.
Rarely, 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 on cyst at ion 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 Linnseus, " Petrefacta
montium calcariorum non filii sed
parentes sunt, cum omnis calx oriatur
ab animalibus," is supported by our
recently acquired knowledge that
Diatoms and Globigerinse live on the
surface of the sea, and that their cases
and tests sink to the bottom when
their inhabitants and makers die. Some rocks,
such as chalk-cliffs, are full of the tests of Globigerinse,
and the " Numuiulitic Limestone " of Nummulites.
Casts of Foraminifers have been found in ^reensand ;
a silicate of iron and alumina has been found filling
casts of recent Foraminifera, so that as ct 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 (Myxospongise)
skeletal structures have been observed, and these, as in
the Protozoa, are of an organic nature simply (fibrous

Fig. 113. Tintin-
nus lagenula,
showing the
Lorica below,
and the Crown
of Cilia.

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 Lithistidae are quite hard and strong.
In most cases the
inorganic skeleton is
spicular, and not
continuous ; but in
some, as " Venus's
Flower Basket' 1
(Euple3tella), a deli-
cate framework of
siliceous particles is
left after all the or-
ganic material has
been removed (Fig.
114).

The spicules vary
considerably in form,
being uniaxial or
needle - shaped, tri-
axial (this is the cha-
racteristic form in the Calcispongise), 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 the

Fig. 114. Section through the Wall of
Euplectella ( x 75).

p, Pores; ?/, flagellated chambers. (After
Schultze.)

280 COMPARATIVE ANATOMY AND PHYSIOLOGY.

1 gemmulea " 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 kins:-

o

dom in which, from such a point of view,
skeletal structures are of more impor-
tance than in the Coelenterata, and both
Hydrozoa and Aiithozoa 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

j.- ,1 T ,1 . n

tissue in the mesoderm ; this may be
called the supporting lamella, and, as we
may suppose, 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.

lonemo. sie-
boldi. (After
Scliultze.)

Chap. IX.]

SPICULES OF ANTHOZOA.

281

non-spiculate Anthozoa take up foreign bodies into, and
thereby strengthen, their ectoderm.

The next stage iaseen in Alcyoniurn, 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.

B

Fig. 116. -A, Triaxial spiculeof Calcisponge (Ascetta Wanca) ; B, Simple
Acerate Spicnle of Reniera ; c, Six-rayed Spicule of the Hexactinel-
lidge.

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 Eed Coral, magnified.

Chap. IX.]

PARTS OF CORALS.

283

the common Fimgia, 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 epitlieca ; the space be-
tween the calcined septa are the
loculi; the septa may unite in the
centre to form a pseudo-coluan-
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 synapticnlae 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 costSB.

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 Hydrocorallina?, 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
Tiibipora musica,
viiih. 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 tabulae, and the upper chambers are alone
living. The Millepores are common on coral reefs, the

Fig. 118 B, Coral. Caryophyllia cyathus.

Stylasters 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 Verities,
where, when a protecting tube is developed, it is often

Chap. IX.]

D ENDROPH YLLIA.

285

largely composed of foreign material ; nor is there any
complete internal skeleton. The cuticle may be soft

.b'ig. 118 c. Coral. Dendrophyllia ramosn,

and ciliated, as in the Turbellaria, 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 jointed and have muscles inserted

286 COMPARATIVE ANATOMY AND PHYSIOLOGY.

into it. In the Annulata 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 operculum 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. Schultze.)

Chap. IX.] HOOKS OF CESTODA. 287

hooks, or of bristles. The former are well developed
in some of the Cestoda, as in Trenia soliuni (the

Fig. 12U. berpiila vermic'i/an's, showing the 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 Turbellaria, where the absence of
nematocysts is often atoned for by the presence of
small, rod-like structures, the so-called rtiabdites.
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 Oligocltseta (e.g, Lumbricus, the
earthworm) these setee 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 tube.

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 Ecliinoderms, with the exception of the
Holothuroidea, have a well-developed skeleton, and
such is found also in some Holotliurians. It is formed

Chap. IX.] TEST OF ECHINOIDEA. 289

of an organic basis, which becomes impregnated with
calcareous salts, and, in thin sections, has a very
characteristic reticular appearance.

It is particularly well developed intheEchiiioidea,
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 iiiterradial in position ; the
former are perforated at the outer edge to allow of the
passage of the ambulacral tubes or suckers ; in the
fossil Palsechinoidea 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 Astlienosoma, 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 Diadema,
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

2 QO COMPARATIVE ANATOMY AND PHYSIOLOGY.

the larger primary are surrounded by smaller and
more delicate secondary spines. Oil the lower

o

< 'g 8

11 1 J ^
'

Sr

-I

(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 sucli 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 Hiadrepo-
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
echinids 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-ceiitral, and, in the study of the morphology of
the apical area of Echinoderms, it is necessary to always
bear in mind (I) 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
Cidarida, 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 one
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 centre-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-
tedoii, the interradially-
placed basals become ob-
scured, but in other forms
among the Crinoids they are
evident throughout life.
Careful investigation. into the

O

structure of the skeleton of
Asteroids and Ophiuroids

Fig. 122. Pentacrinus Wyville-Tliomsoni. (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 ainbulacral plates of an echinid, but the

lr

ao

Fig. 123 A. Cross Section of an Arm of a Starfish (Asterias riibcn?).

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) ; act, ambulacra!
groove; n, radial nerve; 6, radial blood-vessel; w, radial water-vessel; a,
ampullae; t, tentacles or suckers ; ap, adamlnilacral plates; s.p, spines ; pa~r,
paxillae, arising from limestone plates; or, ovary; gp, genital pore; yi\
genital blood-vessel ; fcr, respiratory processes ; pc, caeca of the intestine.
(After P. H. Carpenter.)

tube-feet pass out between them ; attached to each
ambulacral ossicle is a smaller 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-
and infero-marginal plates. All, some, or none

Chap. IX.]

SKELETON OF ECHINODERMS.

295

a.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 (o) 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 (s) 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 tliis 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

Fig. 123 B. Cross Section of
an Arm of an Ophiuroid.
(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 vertebrse of the harmless lizard. (See page 322.)
Arms thus broken are in time renewed. While in
the Opliiurida the arms are nearly always un-
divided, however long they may be, the Astroptiytida
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 Holotliiiriaiis 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 more or less with
one another, so much as one that has 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 turriform in shape, and the
surface of the body is sometimes quite rough to the
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 unfrequently 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 podi-
cellariae, as they were called by those who be-
lieved them to be independent and parasitic animals ;
the sessile pedicellarise 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 pedicellarise are used for the purpose of cleansing
the neighbouring spines of foreign or f?ecal 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 Artliropoda 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 Eiitoinostraca, 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 stomodeeal 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.

7 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 Peripatiis, as in the larvae of insects,
it is soft, and in the latter it is sometimes very thin.
It is generally stouter in JJJyriopods and Araeli-
nida, and its degree of stoutness varies considerably
in Insects. Among the Crustacea it is compara-
tively soft in Eiatomostraca, 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 Arachiiida, 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
tegumeiitary skeletal parts ; these ingrowths are
known as the apo denies. 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 larvae 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
(t erg 11 m) of which is continued into two lateral
(pleiiral) 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 epiineron.
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 s\viinmerets, 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 (telsoii),
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 iii the size and strength of the endopodite,

302 COMPARATIVE ANATOMY AND PHYSIOLOGY.

which constantly consists of five joints, known as the
ischio-, me so-, carpo, pro-, and dactylo-po.
dites. 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 Oiiatliites, 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
sqiiame, and the endopodite is many-jointed ; in the
antennule 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
Malacostraea, and the Myriopoda, all the seg-
ments of the body bear appendages; in the Copepoda
and Arachiiida 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 papillae, 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 Ciriipeclia
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
Arachnida there are no appendages in front of the
head, antennae being absent. In Peripatus the an-
tennae do not belong to the series of ventral appendages
of the segments of the body. In Myriopods and In-
sects there is a single pair of antennas. In the
Arachnida the basal parts of the circum-onil appen-
dages alone take part in the service of the mouth ;
the most anterior pair are pincer-shaped at their free
end (cholicersc) ; in some Myriopods one of the
anterior pairs of appendages become poison- claws, as
are the chelicerse in spiders. In the parasitic Pen-
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
trochanter, 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 cephalothoracic 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 (Nudibranchs). 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 mountain 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 layers 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 Jow 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 chitinoiis
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 Cephalophora 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 conch ologists 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 creat

* o

Chap. IX.]

STRUCTURE OF SHELLS.

37

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 mam-
ruillated ; it is made up of ichorls (,-), separated
by sutures (sit) ; bw, body whorls There is an internal
axis or coluiuella (i), an outer Up Co), an aperture (.),
and an anterior (c) and posterior (pc) caiial. (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 the Shell of the Pearly Nautilus, showing the
successive Chambers occupied by the Animal.

a, Mantle ; b, dorsal fold; g, muscle; ii, siphuncle; k, funnel; n, hood;/), ten-
tacles; s, 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 sipliuiicle

(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 cb priori as to function from structure.
Physiology, like other branches of science, must pro-
ceed rather by observation and a 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, Gvroceras with discoidal, Trochoceras with

' ,

spiral, and Baculites with straight shells.

Among the Dibrancliiata, 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-
mocoiie) 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
giiarcl. 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,

. 127. A, Section of Spirula australis, to show the internal shell ; B,
Belemnite restored ; c, Pen of the Loligo ; u, Argonauta 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 Doliurn and other large-mouthed

Chap, ix.] FORMS OF SHELLS. 31 T

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 Nudibranchs, 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 oesophagus,
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 Bracliiopoda 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-
cardiiies 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, Megeiiia ; F, Argiope; 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 Vertebrata at some period of their lives, the

Chap. IX.] NOTOCHORD. 313

Cephalocliordata 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 iiotocliord 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 notochorcl 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 ; 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 Tuiiicata and the protozoic Cilio-flagellata ;
scattered calcareous spicules are not unfrequeiitly
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 coltiaam. In the lowest stages, such

O '

as are found in the Cyciostomata, Chimera

among the Elasmobranchs, and the Dipnoi, the noto-
chord remains imconstricted, 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 Biaemal 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 aniphiccelous ver-
tebrae ; in better developed forms a smaller amount
of notochord is persistent, and we then get either
proccelous or opisthoccelous 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.

are articulated with one another by means of pro-
cesses directed forwards and backwards, the zyga-
popliyses, 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
Opliidia 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 (zygosplieiies),
which fit into corresponding
depressions (zygaiitra) 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 lacertiliaii
Iguanae, who are known to swim by the movements
of their tails.

In Hatteria, the Geckos, and some fossil lizards,
the notochord is persistent between the vertebrae,
the centra of which are, therefore, amphicoelous.
Among the Crocodiles a progressive loss of the inter-
vertebral portion oi the notochord may be made out,
such forms as lived before the period of the Chalk
having amphicoelous, while cretaceous and post-cre-
taceous crocodiles have proccelous, vertebrae. Among
Birds, the fossil Archaeopteryx and Ichthyornis
appear to have had amphiccelous vertebrae, but in all

Pig. 129. Section through the
Vertebra of a Frog, magni-
fied.

ch, Notochord; cfis, its sheath;
o,c, different kinds of
(After Ecker.)

3i 6 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

zys

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. I X.] VER TEBR&. 317

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 vertebra 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
vertebra?, 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 (Cholcepus 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-

4J
SVJ

01

a
o

o

r)
o

-u
OJ
i

a>

Chap. I X. ] VE R TEBR AL 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 defined 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 ; Main's has
forty - six. Connected

with and intermediate to Fig. 132. Anterior Surface of First
,1 i j -i Sacral Vertebra of Man.

the several caudal ver-

.. c, Centrum ; ?fff, neural arch ; p, rleuva-

tebrae are V - shaped popbysfc.

(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 Logger Leaded 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
bo bent on one another, but that it is elastic in virtue

Chap. IX.] VER TEBR A L COL UMN. 3 2 I

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 vertebrae 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

t

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 oiily 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 Sauropsida. In Birds, where the whole support
of the body falls 011 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 Plesiosauria had more than forty cervical
vertebne.

v 16

322 COMPARATIVE ANATOMY AND PHYSIOLOGY.

entirely on 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.

3 2 3

Sk-

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.

I n many
fishes there is
very constantly
developed a
haemal as well 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 haemaphyphyses, 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

CO

(t.

Fig. 135. Skeleton of Eagle (reduced).

c' Coracold ; ca, carpus ; del', phalanges of the chief digit of the wing ; d", pha-
langes of smaller digit; d'", pollex; dr, dorsal ribs; /, femur; /?, fibula;
fu, f urcula ; h, huraerus; in, metatarsus; ma, mandible ; me, metacarpus;
co, ploughshare bone ;p, pelvis ; pa, phalanges of foot ; pt, patella ; r, radius;
sr, sternal ribs ; s, scapula ; st, sternum ; ti, tibia, tm, tarsometatarsus ; u,
ulna ; up, uncinate processes of ribs. (After Milne-Edwards.)

Chap, ix.] 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 notochord 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 basilar 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 coiidyle, 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 trabeculre, 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 splieiioidal 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.

tr

p ts

(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 Vertebrata (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-
gion, but it
never enters
into direct
union with

fl--\ -jMBWfcU- ; iv i1ilKIiW/W//MWJ^ ; ' ;| J] / ~" ' Cf
- > \ KEfJjfcxbr? ti'.'IsLar/ ?

she-

cT

e

the

though

cranum

in?-.:

the
this
the
i s

form of
part of
skull
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 buccal.
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 Trabeculse (tr),
with their Central space (pis).

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, quacl-
rata ; cl, cochlea : she, semicircular canal of ear ;
9, foramen of exit of the ninth nerve ; nc, 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 ; wn, 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 (p/, ft) may be called the pterygo quadrate
bar, the lat-
ter the Meo
kelian car-
tilage. The
chief means
of connection
between these
bars and the
cranium is
not the me-
tapterygo-
idal region,
or hinder and
upper part of
the first arch,
but the upper

part of the second arch (ny), which forms the hyo-
maiidibular.

In such a skull, then, as that