AN ELEMENTARY COURSE

PRACTICAL ZOOLOGY

MACMILLAN AND CO., LIMITED

LONDON . BOMBAY . CALCUTTA
MELBOURNE

THE MACMILLAN COMPANY

NEW YORK . BOSTON . CHICAGO
ATLANTA . SAN FRANCISCO

THE MACMILLAN CO. OF CANADA, LTD.

TORONTO

AN ELEMENTARY COURSE

OF

PRACTICAL ZOOLOGY

BY THE LATE

T. JEFFERY PARKER, D.Sc., F.R.S.

PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTAGO
DUNEDIN, NEW ZEALAND

AND

W. N. PARKER, PH.D.

PROFESSOR OF ZOOLOGY AT THE UNIVERSITY COLLEGE OF SOUTH WALES
AND MONMOUTHSHIRE, IN THE UNIVERSITY OF WALES

SECOND EDITION

With One Hundred and Sixty-seven Illustrations

MACMILLAN AND CO., LIMITED
ST. MARTIN'S STREET, LONDON

RICHARD CLAY AND SONS, LIMITED,

BREAD STREET HILL, E.G., AND

BUNGAY, SUFFOLK.

First Edition 1899.
Second Edition 1908.

PREFACE

IN the early part of 1897, my brother and I had arranged
to collaborate in writing a practical text-book of Elementary
Zoology, adapted more particularly to the requirements of
Students pursuing courses in the subject as laid down by
various examining bodies. We had, however, only reached
the stage of deciding on a general plan at the time of my
brother's death in November of the same year.

The following are the chief points on which we had
agreed : —

i. To adopt the method pursued in Huxley and Martin's
Elementary Biology of giving a connected account of each
example. 2. To give brief practical directions which should
serve mainly as a guide, the student being able to refer, in
case of difficulty, to the descriptive accounts preceding
them. 3. In the larger animals, to arrange for as much work
as possible to be done on one specimen : there is much to
be said in favour of this plan apart from the fact that the
average student cannot give sufficient time to the subject
to dissect a fresh specimen for each system of organs.
4. To begin the course of instruction by an introductory
study of one of the higher animals ; to include in this in-
troduction the elements of Histology and Physiology ; and
to select the Frog for the purpose : after trying various

79;

vi PREFACE

methods, I have found this plan to be the most satisfactory
in practice. 5. To give drawings and diagrams of difficult
dissections, and of details which the beginner cannot as a
rule make out satisfactorily for himself ; but otherwise to
limit the number of illustrations so as not to tempt the
student to neglect observing the things themselves. 6. To
include a short account of methods and technique, limited
to the barest essential outlines, sufficient for a student
working by himself to make out the things described, but
not going into such details as would naturally be learnt in a
properly organised laboratory.

In the meantime, my brother had in preparation a Biology
for Beginners, in which he intended to carry out the
plan, suggested in the preface to his Elementary Biology,
of giving a simple account, with practical directions, of one
of the higher animals and one of the higher plants, as
an introduction to the study of Biology. The animal he
selected was the Frog, and the manuscript of the greater
part of this section of the book was already finished and
the rest in rough draft. He had previously suggested that
some of this work might be utilised for our proposed Prac-
tical Zoology ; and I found that, with certain additions and
with modifications in the arrangement, the whole of it was
exactly the kind of introduction I had in view for the first
part of our book. Some of the illustrations that my brother
had intended to insert in the Biology for Beginners I have
found it advisable to omit, and even now the figures in the
introductory part are purposely nearly as numerous as those
in the rest of the book. But apart from these various minor
modifications, Chapters I — XII in Part I are almost en-
tirely taken from my brother's manuscript.

We felt that there could be no object in entirely re-
writing the descriptions of several familiar animals already

PREFACE vii

given in my brother's published works ; and, in Part II, .1
have intentionally made use of these descriptions, borrowing
very freely from the Elementary Biology, as well as (with
Professor Has well's permission) from the Textbook of
Zoology ; and to a less extent, from the Zootoiny.

The practical directions are mainly based on a series of
Laboratory-instructions I drew up some years ago for the
use of my junior classes, which consist principally of
students preparing for the Intermediate Science examin-
ation of the University of Wales, the Preliminary Scientific
examination of the London University, and the first ex-
amination of the Conjoint Board of the Royal Colleges of
Surgeons and Physicians. The time such students can
devote to an elementary course in the subject is limited ;
and throughout the book I have borne in mind that the
main object of teaching Zoology "as a part of a liberal
education is to familiarise the student not so much with the
facts as with the ideas of the science," but at the same
time that he should be provided with a sound basis of facts
so arranged, selected, and compared as to carry out this
principle.

Our original intention was to include one or more examples
of each of the larger phyla, and also to add a practical
exercise after each type, giving general directions for the
examination of an allied form for comparison. But I
found that this would be impossible within the space of a
single volume, and it was therefore necessary to limit the
descriptions mainly to those animals to which the students
for whom the book is chiefly intended have to give special
attention. This has resulted in rather a heavy balance on
the side of Vertebrates ; but on the whole, I think that if
sufficient work is done on the lower animals to illustrate
certain main facts and generalisations, a comparative study

viii PREFACE

of several Vertebrates forms as good a training as any for
beginners — more especially in the case of medical students.
I am indebted to Mr. H. Spencer Harrison, B.Sc.,
Demonstrator of Biology in this College, for much assistance
in testing and improving the practical instructions, as well
as for various suggestions and for help while the work was
passing through the press. The new figures were redrawn
from the originals by Mr. M. P. Parker.

W. N. PARKER.
UNIVERSITY COLLEGE, CARDIFF,
November •, 1899.

PREFACE TO SECOND EDITION

AFTER a practical experience of this book in my classes
for the last eight years, I have tried to increase its useful-
ness by slightly extending some parts and by making
various minor modifications throughout. Short accounts
are given of Monocystis, as an example of a sporozoan
parasite, and of Nereis, for comparison with the earthworm ;
while Bougainvillea is replaced by Obelia, and several
additional figures are inserted.

W. N. PARKER.

November ; 1907.

CONTENTS

PAGE

PREFACE v

PART I
CHAPTER I

SCOPE OF THE SCIENCE OF BIOLOGY — THE FROG : PRELIMINARY
SKETCH OF ITS STRUCTURE, LIFE-HISTORY, AND VITAL
FUNCTIONS ...................... I

HINTS ON DISSECTION AND DRAWING .......... 12

CHAPTER II

THE FROG (continued] : GENERAL INTERNAL STRUCTURE ... 16

PRACTICAL DIRECTIONS ................. 31

CHAPTER III

THE FROG (continued}'. THE SKELETON ......... . . 35

PRACTICAL DIRECTIONS ................. 53

CHAPTER IV

THE FROG (continued}'. THE JOINTS AND MUSCLES ...... 55

PRACTICAL DIRECTIONS ................ 64

CHAPTER V

THE FROG (continued} : WASTE AND REPAIR OF SUBSTANCE

—THE DIGESTIVE ORGANS — NUTRITION ......... 66

PRACTICAL DIRECTIONS

CONTENTS
CHAPTER VI

PAGE

THE FROG (continued] : THE VASCULAR SYSTEM — THE CIRCU-
LATION OF THE BLOOD 78

PRACTICAL DIRECTIONS 98

CHAPTER VII

THE FROG (continued] : THE MICROSCOPICAL EXAMINATION

OF THE SIMPLE TISSUES 104

PRACTICAL DIRECTIONS 119

CHAPTER VIII

THE FROG (continued] : THE MICROSCOPICAL EXAMINATION OF

THE COMPOUND TISSUES— GLANDS — SECRETION AND AB-
SORPTION 126

PRACTICAL DIRECTIONS 135

CHAPTER IX

THE FROG (continued] : RESPIRATION AND EXCRETION .... 141

PRACTICAL DIRECTIONS 152

CHAPTER X

THE FROG (continued] : THE NERVOUS SYSTEM 1 54

PRACTICAL DIRECTIONS 175

CHAPTER XI

THE FROG (continued] : THE ORGANS OF SPECIAL SENSE . . . 179

PRACTICAL DIRECTIONS I91

CHAPTER XII
THE FROG (continued]-. REPRODUCTION AND DEVELOPMENT. . 193

PRACTICAL DIRECTIONS 2IO

CHAPTER XIII

THE FROG (continued] : MEANING OF THE TERM SPECIES — THE

PRINCIPLES OF CLASSIFICATION— EVOLUTION — ONTOGENY
AND PHYLOGENY — HEREDITY AND VARIATION. — STRUGGLE
FOR EXISTENCE — SELECTION — ORIGIN OF SPECIES .... 215

CONTENTS xi

PART II
CHAPTER I "

PAGE

AMCEBA — UNICELLULAR AND MULTICELLULAR ANIMALS . . . 22Q
PRACTICAL DIRECTIONS 238

CHAPTER II

SPH^RELLA AND EUGLENA — MONADS AND BACTERIA — DIF-
FERENCES BETWEEN ANIMALS AND PLANTS — SAPROPHYTES 240
PRACTICAL DIRECTIONS 259

CHAPTER III

PARAMCECIUM : VORTICELLA AND ITS ALLIES — COLONIAL ORGAN-
ISMS 26l

PRACTICAL DIRECTIONS 277

CHAPTER IV

OPALINA : MONOCYSTIS — PARASITES — BIOGENESIS AND ABIO-
GENESIS — CLASSIFICATION OF THE UNICELLULAR ORGAN-
ISMS EXAMINED 28o

PRACTICAL DIRECTONS 293

CHAPTER V

HYDRA : OBELIA— SYMBIOSIS— ALTERNATION OF GENERATIONS

— CHARACTERS OF THE PHYLUM CCELENTERATA ..... 294
PRACTICAL DIRECTIONS 322

CHAPTER VI

THE EARTHWORM : NEREIS — CHARACTERS OF THE PHYLUM

ANNULATA . . . 326

PRACTICAL DIRECTIONS 354

CHAPTER VII

THE CRAYFISH — CHARACTERS OF THE PHYLUM ARTHROPODA . 360
PRACTICAL DIRECTIONS 386

xii CONTENTS

CHAPTER VIII

PAGE

THE FRESH-WATER MUSSEL — CHARACTERS OF THE PHYLUM
MOLLUSCA — ENUMERATION OF THE CHIEF PHYLA OF THE

ANIMAL KINGDOM 396

PRACTICAL DIRECTIONS 412

CHAPTER IX

CHARACTERS OF THE PHYLUM VERTEBRATA — THE LANCELET . 418
PRACTICAL DIRECTIONS 426

CHAPTER X

CHARACTERS OF THE CLASS PISCES — THE DOGFISH 430

PRACTICAL DIRECTIONS 472

CHAPTER XI

CHARACTERS OF THE CLASS MAMMALIA — THE RABBIT .... 482
PRACTICAL DIRECTIONS . 542

CHAPTER XII

THE MINUTE STRUCTURE OF CELLS — CELL DIVISION — STRUC-
TURE OF THE OVUM — SPERMATOGENESIS AND OOGENESIS
— MATURATION AND FERTILIZATION OF THE OVUM —
SEGMENTATION OF THE OOSPERM — EFFECT OF FOOD-YOLK
ON DEVELOPMENT — FORMATION OF THE CHIEF ORGANS
OF VERTEBRATES, AND OF THE AMNION, ALLANTOIS,

AND PLACENTA 558

PRACTICAL DIRECTIONS 603

INDEX 607

AN ELEMENTARY COURSE OF
PRACTICAL ZOOLOGY

PART I

CHAPTER I

SCOPE OF THE SCIENCE OF BIOLOGY THE FROG : PRELIMIN-
ARY SKETCH OF ITS STRUCTURE, LIFE-HISTORY, AND
VITAL FUNCTIONS— HINTS ON DISSECTION

Biology, Zoology and Botany. — There is a good deal of
misconception as to the scope of the science of Biology.
One often meets with students who think that while the
study of animals as a whole is Zoology and the study of
plants as a whole Botany, Biology is the study of a
limited number of animals and plants, treated as if they
had no connection with anything else, — even with one
another.

This is quite wrong. Biology is the master-science which
deals with all living things, whether animals or plants, under
whatever aspect they may be studied. Physiology, treated
for practical purposes as a separate subject, is a branch of
biology ; so is anatomy, to which the medical student

PRACT. ZOOL. B

2 SCOPE OF THE SCIENCE OF BIOLOGY CHAP.

devotes so much time ; so are botany and zoology, in the
ordinary sense of the words, i.e. the study of the structure,
the mutual relations, and the arrangement or classification
of plants and animals. But biology may also be pursued,
and very profitably pursued too, quite independently of
teachers, class-rooms, and examinations. The country boy
who knows the song of every bird, its nesting place, the
number of its eggs, the nature of its food, the lurking place
of the trout in the stream or the frogs in the marsh ; who
has watched the ants with their burden of grain, or the bees
with their loads of honey or pollen ; has begun the study of
biology in one of its most important branches. The in-
telligent gardener who observes the habits of plants, their
individual tastes as to soil, moisture, sunshine and the like,
is also something of a biologist without knowing it. So
also is the collector of eggs, shells, or insects, provided he
honestly tries to learn all he can about the things he collects,
and does not consider them merely as a hoard or as objects
for barter. Indeed, all that is often spoken of as natural
history, so far as it deals with living things— plants and animals
— and not with lifeless natural objects, such as rocks and
minerals, is included under the head of biology.

What then is the connection between biology in this wide
sense and the kind of thing you are expected to learn in a
limited number of lessons ? Simply this : — In the class-room
nature cannot be studied under her broader aspects : indeed,
much out-door natural history cannot be taught at all, but
must be picked up by those who have a love of the subject,
a keen eye, and patience. But there is one thing we can
do within the narrow limits of the class-room : we can con-
fine ourselves to some department of biology small enough
to be manageable : we can take, for instance, one or more
familiar animals and plants, and, by studying them in some

I THE STUDY OF ZOOLOGY 3

detail, get some kind of conception of animals and plants
as a whole. This book deals with the zoological side of
biology only ; and what we have now to do is, in fact, what
you have often done in the study of English : you take a
single verse of a poem at a time, analyse it, parse it, criticise
its construction, try to get at its exact meaning. If you have
any real love of literature this detailed study of the part will
not blind you to the beauty of the whole. And so if you
have any real love of nature, the somewhat dry and detailed
study we have now to enter upon should serve to awaken
your interests in the broader aspects of biology by showing
you, in a few instances, what wonderful and complex things
animals are.

One word of warning before we begin work. You must
at the outset disabuse your mind of the fatal error that
zoology or any other branch of natural science can be learnt
from books alone. In the study of languages the subject
matter is furnished by the words, phrases, and sentences of
the language ; in mathematics, by the figures or other symbols.
All these are found in books, and, as languages and mathe-
matics are commonly the chief subjects studied at school,
they tend to produce the habit of looking upon books as
authorities to which a final appeal may be made in disputed
questions. But in natural science the subject-matter is
furnished by the facts and phenomena of nature ; and the
chief educational benefit of the study of science is that it
sends the student direct to nature, and teaches him that a
statement is to be tested, not by an appeal to the authority
of a teacher or of a book, but by careful and repeated
observation and experiment.

The object of this book, therefore, is not only to give
you some idea of what animals are, but also to induce
you to verify the statements contained in it for your-

4 THE FROG CHAP.

self. The description of each animal you should follow
with the animal before you ; and if you find the account in
the book does not agree with what you see, you must
conclude, not that there is something wrong with your
subject, but either that the description is imperfect or
erroneous, or that your observation is at fault and that
the matter must be looked into again. In a word, zoology
must be learnt by the personal examination of animals : a
text-book is merely a guide-post, and all doubtful points
must be decided by an appeal to the facts of nature.

It matters very little what animal we choose as a starting-
point — a rabbit, a sparrow, or an earthworm — one will serve
almost as well as another to bring out the essential nature
of an animal, how it grows, how it is nourished, how it
multiplies. On the whole, one of the best subjects to begin
with is a frog : partly because it is easily obtained, partly
because its examination presents no difficulties which an
intelligent student may not be expected to surmount by
due exercise of patience.

Let us therefore begin our studies by catching a frog and
placing it in a convenient position for examination, as, for
instance, under an inverted glass bell-jar or even a large
tumbler.

External Characters. — Notice, first of all, the short,
broad trunk, passing insensibly in front into the flattened
head — there being no trace of a neck — and ending behind
without the least vestige of a tail : these constitute the axial
parts of the animal. In the ordinary squatting position the
back has a bend near the middle, producing a peculiar
humped appearance. The head ends in front in a nearly
semicircular snout, round the whole edge of which extends
the huge slit-like mouth. On the top of the fore-end of the

I EXTERNAL CHARACTERS 5

snout are the two small nostrils, one on each side of the
middle line ; and, some distance behind them, the large,
bright, prominent eyes, in which we can distinguish, as in
our own eyes, a coloured ring or iris, surrounding a
roundish black space or pupil. The eyelids, however, are
rather different from our own : the upper is fairly well
developed, but the lower is a mere fold of skin, incapable
itself of covering the eye, but produced into a thin
transparent skin, the nictitating membrane, which can be
drawn upwards over the eye. The entire absence of eye-
brows and eyelashes is a point worthy of notice.

Extending backwards from the eye is a large dark patch,
in the middle part of which is a circular area of tightly
stretched skin, reminding one of the parchment of a
tambourine : this is the drum-membrane, or tympanic
membrane, a part of the ear. Here again we see a striking
difference from our own organs : in ourselves the drum-
membrane, instead of being flush with the surface of the
head, is placed at the inner end of a deep passage or tunnel,
the entrance to which is guarded by the large eternal ear.
Of the latter there is no trace in the frog.

Attached to the trunk are two pairs of offshoots or appen-
dages, the arms and legs, or fore- and hind-limbs, in which
the resemblance to our own limbs will be at once obvious.
The arms are very short : each consists of an upper-arm, a
fore-arm, and a hand, the latter provided with four fingers,
which are slender and tapering and have no nails. The
legs, on the other hand, are very long : each consists of a
stout thigh, a long shank, with a well-marked " calf," and
a very curious foot. The ankle-region is long — almost like
a second shank — and has no heel : it is followed by five
toes, the first or innermost short, the second of moderate
length, the third longer, the fourth longer still, and the fifth

6 THE FROG CHAP.

of about the same length as the third. All the toes are
joined together by thin transparent webs, and, like the
fingers, have no nails. The name digit is conveniently
applied both to fingers and toes. Between the bases of
the thighs, at the hinder end of the trunk, is a small
aperture, the vent or anus.

In the squatting posture the body is raised upon the
arms, which are kept slightly bent at the elbows, with the
fingers spread out and directed forwards. In this position
the innermost of the four fingers correspond with our own
index-finger, the frog having no thumb. The hind-limb,
under similar circumstances, is bent into a sort of Z, the
knee being directed forwards and the ankle-joint backwards.
The toes are turned forwards, and the inner one, which is
the smallest of all, corresponds with our own great toe.

Owing to the bent position of the limbs, we cannot very
well, as in our own arms and legs, speak of their upper and
lower ends. It is therefore customary to call the end of a
limb, or of any division of a limb, which is nearest to the
trunk, the proximal end, that which is furthest away the
distal end. Thus the proximal end of the fore-arm is the
elbow region, the distal end of a digit is its tip.

The whole body, including head, trunk, and limbs, is
covered with a soft, slimy skin, of a brown colour, irre-
gularly spotted with brown or black on the upper or dorsal
surface, and whitish on the under or ventral surface. The
colouring is, however, not constant ; in a frog kept in the
dark the black spots increase to such an extent that the
whole animal becomes almost black, while if kept in full
daylight a corresponding brightening of the tints takes
place. Moreover, the spots and patches of brighter colour
are very variable : if you examine a dozen specimens you
will see at once that no two are alike in this respect. The

i MOVEMENTS AND GROWTH 7

large dark patch situated behind the eye and containing the
tympanic membrane, is, however, always present, and is one
of the chief distinguishing marks of the common British frog
as compared with other kinds, such as the " edible frog " of
the Continent.

Sexual Characters. — As in so many of the more familiar
animals there are two sexes of frogs, easily distinguished
from one another. If you examine several of them you will
find that a certain number have on the palm of the hand,
towards the inner side, a large swelling, rather like the
ball of our own thumb, but much more prominent and of
a black colour. Frogs having this structure are males ;
it is not present in the females.

Actions performed by the Living Frog. — Kept under
suitable conditions a frog very soon shows evidences of life.
If touched or otherwise alarmed it attempts to escape by
making a series of vigorous leaps — suddenly extending the
hind-legs and jumping to a considerable height. Thrown
into water k swims by powerful strokes of the hind-limbs.
It has thus, like so many living things with which we are
familiar, the power of voluntary movement.

If kept under observation for a sufficient time — weeks or
months — it will be found that frogs grow until they reach a
certain limit of size. Growth, in the case of the frog, is an
increase in size and weight affecting all parts of the body, so
that the proportions remain practically unaltered, and no
new parts are added.

Careful observation shows that the throat is constantly
rising and falling, and the nostrils opening and shutting.
These movements, like the expansion and contraction of
the human chest, are respiratory or breathing movements,
and serve to pump air into and out of the lungs.

It requires frequent watching and sharp observation to see

8 THE FROG CHAP.

a frog feed. It lives upon insects, worms, slugs, and the
like. Opening its mouth it suddenly darts out a tolerably
long, nearly colourless, and very sticky tongue • if the prey
is a small insect, such as a fly, it adheres to the end, and
the tongue is quickly drawn back into the mouth, the
whole operation being performed with almost inconceivable
rapidity.

Like other animals the frog discharges waste matters
from its body. Its droppings m faces, discharged from the
vent, are black and semi-solid. From the same aperture, it
expels periodically a quantity of clear fluid, the urine } which
is perfectly clear and colourless, and contains little beyond
water.

Sometimes a frog will escape from confinement, leaving
its damp box or vivarium for the warm, dry atmosphere of
an ordinary room. When this happens the animal is
usually found next morning dead and shrunken, and with
its naturally moist skin dry and hard. From this it may
be inferred that there is a constant evaporation of water
from the skin, which, under ordinary circumstances, is
checked by a damp atmosphere or by occasional immersion
in water.

Hibernation. — In- winter frogs bury themselves in damp
places and become sluggish, manifestations of life becom-
ing hardly apparent until the following spring, when they
emerge from their holes. In this way they escape the
dangers of frost which would otherwise be fatal to them.
This suspension of activity during winter is known as
hibernation, or the winter-sleep.

Reproduction and Development. — If you examine a
number of frogs towards the end of winter — about February
in England — you will find that the full-grown females are
distinguished from the males, not only by the absence of

i EGGS AND TADPOLES 9

the pad on the hand, but by the swollen condition of the
trunk, due to the interior being distended with eggs. After
a time the eggs are laid, being passed out o'f the vent by
hundreds ; each is a little globular body about ^ th inch in
diameter, half black and half white, and surrounded by a
sphere of clear jelly, by means of which the eggs adhere
together in large irregular masses, the well-known "frog-
spawn." As the eggs are laid, the male passes out of his
body, also by the vent, a milky substance, the milt or spermatic
fluid, which gets access to the eggs and impregnates or
fertilises them. Without impregnation they are incapable
of developing.

Neither male nor female takes the slightest care of the
eggs when once they are deposited and fertilised. They
are simply left in the water unprotected in any way ; and,
naturally enough, the mortality among them during the
course of development is very great, the majority being
eaten or otherwise destroyed, and only a very small per-
centage coming to maturity.

The first noticeable change in the spawn is that the
sphere of jelly surrounding each egg swells up so as to
acquire several times the diameter of the enclosed egg.
The egg itself, or embryo, as it must now be called,
gradually becomes entirely black, then elongates, and takes
on the form of a little creature (Fig. i, 1) with a large
head, a short tail, and no limbs ; which after wriggling
about for a time, escapes from the jelly and fixes itself, by
means of a sucker on the under side of its head, to a
water-weed. Great numbers of these tadpoles, as the free-
living immature young or larva of the frog are called, may
be seen attached in this way. At first they are sluggish and
do not feed, but, before long, they begin to swim actively
by lashing movements of their tails, and to browse on the

io THE FROG CHAP.

weeds. They are thus in the main vegetable-feeders, not
carnivorous, like the adult frog. On each side of the head
appear three little branched tufts or gills, which serve as

FIG. i. — Stages in the life-history of the Common Frog, from the newly-hatched
tadpoles (i) to the young frog (8) ; -za is»a magnified view of 2. (After Mivart.)

respiratory organs (2, 2a), the tadpole, like a fish, breathing
air which is dissolved in the water. After a while the gills
begin to shrivel up (3, 4), and the tadpole then comes
periodically to the surface to breathe, lungs having in the

I SUMMARY OF CHAPTER n

meantime made their appearance. A pair of little hind-limbs
appears at the root of the tail, and a pair of fore-limbs behind
the head (5, 6). As these increase in size the tail slowly
dwindles, the head and trunk assume the characteristic frog-
form, and the animal now comes on land and hops about
as a small tailed frog (7). As growth goes on, the tail further
diminishes and finally disappears altogether, the transforma-
tion or metamorphosis being thus completed (8).

Death and Decomposition. — Frogs may live for many
years, but, sooner or later, either in the ordinary course of
nature or by accident, they die. The heart stops beating,
the flesh undergoes what is called " death-stiffening,"
becoming hard and rigid, and all vital manifestations cease.
Before long the process of decomposition ensues, the flesh,
viscera, etc., soften and emit a bad smell, and in course of
time rot away completely, leaving only the bones.

Summary of Chapter.— The very brief and cursory study
we have made so far shows us (i) that a frog has certain
definite parts arranged in a particular way ; (2) that it
performs characteristic movements, some of them, such as
leaping and swimming, voluntary ; others, such as breathing,
involuntary ; (3) that it takes in solid food, consisting mainly
of vegetable matter in the tadpole, of living animals in the
adult ; (4) that it gives off waste matters • (5) that it
reproduces its kind by laying eggs, which develop only if
impregnated ; (6) that it undergoes a transformation or «
metamorphosis, the egg giving rise to a larva, the tadpole,
which, after living for a time the life of a fish, gradually
changes into a frog.

12 PRACTICAL WORK CHAP.

HINTS ON DISSECTION AND DRAWING

Instruments and other Requisites for Dissection.1 in

order to carry out the dissection of the frog and other animals success-
fully it is necessary to be provided with proper tools. The most
important are —

1 . Three or four sharp dissecting knives, or scalpels , of different sizes.

2. A large and a small pair of straight dissecting-forceps ; the small
pair should have a peg on one leg fitting into a hole on the other, to
prevent the points crossing ; the points should be roughened.

3. A large and a small, fine-pointed pair of dissecting scissors ; the
small pair for the more delicate work, and the large pair for coarser
work and for cutting through bones. For the latter purpose a pair of
bone-forceps is useful, but is not necessary in the case of such a small
animal as the frog.

4. A seeker^ i.e., a blunt needle mounted in a handle.

5. Three or four probes : a seeker or knitting needle, or a thin slip
of whalebone will answer for some purposes, but the most generally
useful form of probe is made by sticking the end of a hog's bristle into
melted sealing wax, and immediately withdrawing it so as to affix a
little knob or guard.

6. An anatomical blowpipe, or, failing this, a piece of glass-tubing,
6 or 8 inches long, with one end drawn out in the flame until it is not
more than TVth to ^jth of an inch in diameter.

7. An ordinary * * medicine- dropper" or " feeder " of a self- feeding pen
(see Fig. 25), made of a piece of glass-tubing about three inches long,
drawn out in the flame at one end, and thickened at the other so as to
form a collar, over which an india-rubber cap — an ordinary non-
perforated teat — is fixed. This is useful for washing fine dissections, as
well as for injecting.

• 8. A dissecting dish. Get a common pie-dish, about 6 or 8 inches
long, with rather low sides. Cut out a piece of cork-carpet or thick
linoleum the size of the bottom of the dish, and a piece of sheet-lead
of the same size, and fasten the two together by three or four ties
of copper wire or strong thread. Place this in the dish with the lead

1 A suitable box of dissecting instruments can be bought from most
scientific instrument makers for about £i. (For further apparatus re-
quired in connection with injection and microscopical work, see pp. 99,
119, and 135.)

i HINTS ON DISSECTION 13

(which is simply to keep the cork-board from floating in water) down-
wards. Or, place a few strips of sheet lead in the bottom of the
dish, and then pour in some melted paraffin-wax into which a little
lamp-black has been stirred, so as to make a layer half an inch or more
in thickness.

For larger animals than the frog, in addition to a larger dish, a
dissecting board will also be required. Get a piece of soft deal or pine
about 1 8 inches x n inches and f-inch thick, and nail round its edge
a strip of wood about | inch x \ inch, so as to form a projecting rim.

9. A magnifying glass. Any good pocket-lens or a common watch-
maker's glass will answer the purpose. As it is often desirable to
have both hands free while using the lens, a stand of some kind is
useful. One of the simplest is made by fixing a piece of \ -inch brass-
tubing, about 6 inches long, into a heavy block of wood, about 3 inches
in diameter, and coiling round it, in a close spiral, one end of a piece of
thick wire, which can be raised and lowered on the tube. Leave 6 or 7
inches of the wire standing out at right angles, and bend the free end
into a loop to carry the lens. Or, get a piece of narrow clock-spring,
about 13 inches long, and rivet one end of it to the outside of the
rim of a watchmaker's glass, and the other to a small piece of zinc or
brass ; on passing the spring round the head, the lens is kept in place at
the eye without exertion.

10. Medium and small-sized pins. Large blanket-pins are useful for
fixing down larger animals.

11. A small sponge and a duster.

12. One or more wide-mouthed bottles or jars, containing a preserva-
tive in which to place your subjects after each day's work. The most
convenient preservative for the purpose in most cases is the fluid sold as
formaline,1 which can be diluted as it is wanted. For preserving your

dissections from day to day, a I per cent, solution of formaline is strong
enough in many cases — i.e., I cubic centimetre of formaline to 99 c.c.
of water, or three-quarters of a dram of formaline to half a pint of water.
For permanent preservation, a stronger solution — 2 to 5 per cent.,
according to circumstances — should be used, or methylated spirit. If
formaline is not available, use strong methylated spirit (i.e., about 90
per cent.) diluted with one-third of its bulk of water.

13. A plentiful supply of clean water.

14. An ounce or two of chloroform.

1 A 40 per cent, solution of the gasforwtc aldehyde.

14 PRACTICAL WORK CHAP.

Rules to be Observed in Dissection. — Many of the parts and
organs of animals are bound together by means of a substance known as
"connective-tissue," and the main object of dissection is to tear away
and remove this substance so as to separate the parts from one
another.

The subject should be firmly fixed down in the dissecting-dish or on
the dissecting-board by means of pins, inserted obliquely, so that they
do not interfere with the dissection. The dissecting-dish must always
be used for finer dissections, which should be done under water ; only
just enough water being put into the dish to cover the dissection, which
should be washed under the tap from time to time.

When dissecting a part keep it on the stretch, and avoid fingering it or
damaging it with the forceps.

Never remove anything until you know what you are removing.
Dissect along and not across, such structures as blood-vessels and nerves.

See that your instruments are kept clean and sharp, and never use the
smaller scissors and scalpels for coarse work.

Drawing. — You should make a point of drawing as many of your
preparations, as well as of the living animals, as possible : an accurate
sketch, taken from nature, no matter how rough, is of more value in
teaching observation and in impressing the facts on your memory than
the examination and copying of more perfect drawings made by others.
Anyone can soon learn to make sketches of this kind, even without
having any previous knowledge of drawing.

Each sketch should be made to scale, and small objects should be en-
larged several times ; it is much easier to insert details in a large drawing
than in a small one. Mark the scale against each drawing — e.g., x 2,
x*.

Using a rule and compasses, first sketch in an outline of the principal
parts with a hard pencil ; if your object is bilaterally symmetrical, draw
a faint line down the middle of the paper, and then sketch in one side
first. When you have sketched in all the outlines correctly, go over
them again with a softer pencil, so as to make them clear and distinct.
Do not attempt any shading unless you have some knowledge of
drawing.

Then tint the various parts in different colours, using very light tints
except for such structures as vessels and nerves. You should keep to
the same colours for the corresponding organs or tissues in all the
animals you examine : thus you might in all cases colour the alimentary

i HINTS ON DRAWING 15

canal yellow, the arteries red, the veins blue, glands brown, cartilage
green, and so on.

Make your drawings on one side of the page only ; the opposite side
can then be used for explanations of the figures.

Never insert on your original sketches anything you 'have not actually
seen ; you can copy as many other figures as you like from various
sources, but these should be kept apart from your own original
drawings.

Directions for the examination of the external characters of the adult
frog, as described in this chapter, are given at the end of Chapter II.,
p. 31 ; and of the eggs and tadpoles in Chapter XII., p. 212.

CHAPTER II

THE FROG (continued) : GENERAL INTERNAL STRUCTURE

You have now seen that a frog can perform a number of
very complicated actions ; and, if you have any curiosity
in these matters, you will probably want to know some-
thing of the mechanism by which these actions are
brought about. Now, the best way to understand the
construction of a machine, such as a clock or a steam-
engine, is to begin by taking it to pieces ; and, in the
same way, you can find out the parts of which the
living machine we call a frog is made, and the way they
are related to one another, only by taking it to pieces, or
dissecting it.

Firs£n.otice, in addition to the external characters described
in the last chapter, that the various parts of the body
are strengthened or stiffened, as in ourselves, by a number of
bones, which together form the greater part of the skeleton.
It is quite easy to ascertain by feeling that the head con-
tains a hard skull \ the lower jaw, a lower-jaw-bone or
mandible ; that running through the back is a jointed •
back-bone or vertebral column • that the region of the
chest is protected by a breast-bone, or sternum • and that
each division of the limbs has its own bone or bones.

The Mouth-Cavity. — There are also several points to

CHAP, ii THE FROG : MOUTH AND PHARYNX 17

be observed in the interior of the mouth. All round the
edge of the upper jaw is a row of small conical teeth (Fig.
7). There are no teeth in the lower jaw; but on the
roof of the mouth, a short distance behind the snout, are
two little patches of teeth, called the vomerine teeth (vo. /).
Just behind these are two apertures, called the internal
nostrils (/. no) : a guarded bristle passed into one of the
external nostrils and pushed gently backwards and down-
wards, will be found to enter the mouth by the correspond-
ing internal nostril.

Behind the internal nostrils are two large hemispherical
projections, due to the roof of the mouth being bulged out
by the huge eyes, as can be readily made out by pushing
the eyes from outside.

On the floor of -the mouth is the large, flat tongue
(tng), remarkable for the fact that it is attached at its front
end, its hinder end being free and double-pointed. When
the frog uses it to catch insects it is suddenly thrown for-
wards, almost like a released spring. Just behind the back-
wardly-turned tips of the tongue is an oval elevation,
having on its surface a longitudinal slit, called the glottis
(gi\ which leads, as we shall see afterwards, into the lungs.

The back of the mouth narrows considerably, and the
soft skin or mucous membrane lining it is here thrown into
folds. A probe gently pushed backwards passes, as we
shall see, into the stomach. The narrowed region of the
mouth is the throat, or pharynx. On Jts upper wall, near
the angles of the mouth, are two pits : a guarded bristle
passed into one of these will be found to come into
contact with the corresponding tympanic membrane,
which will be pierced if sufficient force is used. The
pits are known as the Eustachian recesses or tubes (eus. t).

Dissection of the Frog : Skin and Muscles.— If a slit is

PRACT. ZOOL. C

i8 THE FROG CHAP.

made in the skin of the belly, and a probe pushed in under it,
it will be seen that the skin, instead of being firmly attached
to the underlying flesh, as in a rabbit or a sheep, is for the
most part quite loose, a spacious cavity lying between it
and the flesh. Not, however, a single continuous cavity
for the whole body : the probe, gently pushed in various
directions, is stopped, in front, at about the level of the
arms ; behind, at the junction of the thighs with the trunk ;
and at each side, along an oblique line joining the
armpit with the thigh. Moreover, by opening the
skin of the back, throat, and limbs, and inserting the
probe as before, similar cavities will be found in these
regions, all separated from one another by partitions,
along which the skin is firmly united to the underlying
flesh. It will be noticed also that the probe, when with-
drawn from any of these cavities, is wet. The cavities
contain a watery fluid, called lymph, and are hence known
as subcutaneous lymph-sinuses (Fig. *], d. ly. s, v. ly. s).

When the skin is removed it will be seen that under
the skin and separated from it by the lymph-sinuses is a
nearly colourless, semi-transparent, fibrous substance, the
flesh. At first this appears to be continuous over the whole
body, but, by careful dissection with a sharp scalpel, a
very delicate, transparent skin, called the fascia, can be
separated from the flesh, which is then seen to consist of
a number of separate bands (Fig. 2, my. hy, pet, ret. abd\
see also Fig. 16), covered as aforesaid by the fascia, and
separated from one another by a kind of packing substance,
also very delicate and transparent, and known as
connective-tissue. These bands or sheets are the muscles,
and the whole of the flesh is made up of distinct muscles,
readily separated from one another when once the requisite
anatomical skill is attained. Here and there — for instance

ii BODY-WALL 19

on the top of the head and the front of the shanks — there
are no muscles, and the bones are covered only by skin and
connective tissue.

Passing along the middle line of the belly is a dark
longitudinal streak (Fig. 2, abd. v) : this is a blood-vessel, the

FIG. 2. — A Frog with the skin (sk) of the ventral surface cut through and turned
back right and left, so as to expose the muscles. Of these the mylohyoid (my. hy],
pectoralis (pet), external oblique (ext. o(>l), and rectus abdominis (ret. add)
are lettered. On the right side (left in the figure), the posterior portion of the
pectoral muscle is cut away, its two ends (pet pet") only being left. The
cartilaginous extremity of the breast-bone (xiphisternum, x. st) is shown, as
well as the abdominal (abd. v), musculo-cutaneous (m. c. v), and brachial
(scl. v) veins, and the cutaneous artery (c. a).

abdominal vein. On each side of the body another vein (m.
c. v) is seen forming a loop, one limb of which is on the
turned-back flap of skin, while the other passes between
the muscles not far from the armpit : this vessel is the
musculo-cutaneous vein. Both these veins, and many others

C 2

20 THE FROG CHAP.

which will be seen in the course of the dissection, are thin-
walled tubes full of blood, as will be proved if you should
happen to cut one of them, when the blood will escape in
considerable quantity.

Between the right and the left fore-limbs the ventral
region of the trunk is protected by certain bones which
form part of the shoulder-girdle : projecting backwards from
this in the middle line is a flat, heart-shaped plate of a
softer, gristle-like substance, known as cartilage (compare
Fig. 12). Immediately between the thighs a cartilage called
the pubis, part of the hip-girdle (Fig. 14), can be felt.
Between the shoulder and hip-girdles the ventral body-wall
is soft, being formed only of muscle and connective tissue.

The Abdomen and its Contents. — By cutting through
the muscles of the belly or abdomen, a large cavity, the body-
cavity or cozlome, is exposed, in which are contained
numerous structures presently to be described. In order,
however, to open the whole of the cavity the ventral
part of the shoulder-girdle must be removed.

In the middle line, between the fore-limbs, and there-
fore covered in the entire animal by the shoulder-girdle, is
a pink conical body (Figs. 3 and 4, v) connected in
front with a thin-walled bag, (r. au, I. au] of a purplish
colour. The whole thing is the heart : the pink
posterior portion is called the ventricle ; the purple
anterior part consists of two chambers, the auricles.
The heart is enclosed in a transparent, membranous bag,
the pericardium (pcd\ which contains a lymphatic fluid.

Just behind or posterior to the heart are two
large masses (lr] which have a dark reddish-brown
colour; these are the right and left lobes of the liver.
They extend forwards, one on either side of the heart :
between them is a globular bag of a greenish colour (Fig. 3,

DISSECTION OF A MALE FROG

7~.pr.cis

FIG. 3. — Dissection of a Male Frog The enteric canal and liver are displaced to
the animal's left, and part of the liver is cut away. Some of the muscles are
cut away and certain nerves and blood-vessels traced into the head and
limbs.

abd. v. abdominal vein ; br. brachial artery, vein, and nerve ; ccel. mes. splanchnic
or coeliaco - mesenteric artery ; cp. ad. right fat - body ; dm. duodenum ;
ext.ju. external jugular vein;,/;;?, femoral vein; gl.bl. gall-bladder; hp.pt.
hepatic portal vein ; hp. v. hepatic vein ; Ir. liver ; m. c. v. musculo-cutaneous
vein ; my. hy. mylohyoid muscle ; pcd. pericardium ; /. c. hy. anterior horn of
hyoid ; pt. cv. postcaval vein ; pul. a. pulmonary artery ; pul. v. pulmonary
vein ; pv. pelvic vein ; r. au. right auricle ; ret. rectum ; r. kd. right kidney ;
r. Ing: right lung ; r. pr. cv. right precaval vein ; rn. pt. right renal portal
vein ; r. spy. right spermary ; s. int. ileum ; spl. spleen ; st. stomach ; s. v.
sinus venosus ; tr. a. conus arteriosus ; u. bl. urinary bladder ; ur. ureter ;
v. ventricle ; vs. sm. seminal vesicle.

22 THE FROG CHAP.

gl. bt), the gall-bladder. In front of the liver and left
and right of the heart are two thin-walled, transparent
sacs (r. Ing, L Ing ) with a honeycombed surface, the lungs.
Their appearance varies very much according to their state
of distension. When full of air they are an inch or more in
length in a full-sized frog, and protrude freely as soon as
the abdomen is opened : when empty they hardly show
unless the liver is turned aside.

Emerging from beneath the left lobe of the liver (beneath
in the present position of the animal, actually above) is a
wide, whitish »tube (Fig. 3, .r/), which almost immediately
turns to the right (the frog's right, not yours), so as to
form a U-shaped bend (st, dm}. This is the stomach,
which is connected with the pharynx by a short tube
called the gullet or oesophagus (compare Fig. 7, gut, st),
and which varies considerably in size according to
whether it is empty or distended with food. The stomach
becomes continuous with a narrower tube, the first part of
which (dm} passes forwards parallel with the stomach,
thus forming the narrow limb of the U, while the rest of
it (s. int) is thrown into a rather complex coil. This
tube is the small intestine ; the part in immediate con-
nection with the stomach (dm) is distinguished as the
duodenum and the coiled part as the ileum. Between
the stomach and duodenum, in the bend of the U, is a
small yellowish-white body of irregular form, the pancreas
(Figs. 7,/«, and 18, P).

The stomach and intestine are kept in place and
suspended to the dorsal wall of the body-cavity by a delicate
membrane, the mesentery (Fig. 5, mes\ which is folded in
correspondence with the various coils. As we shall see, the
mesentery is really a portion of a thin, moist membrane,
the peritoneum, with which the body-cavity is lined.

ii ABDOMINAL VISCERA 23

The small intestine becomes continuous, posteriorly, with
a much wider tube (Figs. 3 and 7, rct\ lying against the
dorsal wall of the abdomen, and called the large intestine or
rectum. It is continued into a short tube, the cloaca (d\
which passes backwards, between the backbone above and
the pubis below, to open externally by the vent. Thus the
mouth-cavity, pharynx, gullet, stomach, small intestine, rec-
tum, and cloaca form a continuous tube, opening externally
at each end, by mouth and anus respectively, and, for the
greater part of its extent, contained within the body-cavity.
The whole tube is known as the enteric or alimentary canal.

Attached to the mesentery, close to the anterior end of the
rectum, is a rounded body of a deep-red colour, the spleen
(Figs. 3 and 7, spt). Quite at the posterior end of the
abdominal cavity a very thin-walled and transparent
sac (u.bl) will be seen, connected with the ventral surface
of the cloaca, and varying much in size according to its
state of distension. This is the urinary bladder, which
communicates by an aperture (Fig. 7, bl'} with the cloaca,
and when distended will be seen to be bilobed and of
considerable size.

If your specimen should be an adult female, and the
time of year approaching the "breeding season, you will
already have observed, as the most prominent organs in the
body, two large, lobed structures of a dark colour, protrud-
ing one on either side, and partly obscuring the view of the
other organs. Each (Fig. 4, /. ovy] contains a great number
of small globular bodies, half black and half white, and is
suspended to the roof of the body-cavity by a sheet of
peritoneum. These bodies are the ovaries, or organs for
the manufacture of the eggs ; the rounded bodies of which
they are largely composed are the eggs themselves. To
each ovary is attached a yellow structure, produced into a

FIG. 4. — Dissection of a young Female Frog. The gullet (gu£) and rectum ^rc/)
have been cut through, and the enteric canal removed between these two points.
The liver is removed, with the exception of a small portion (Ir) surrounding the
postcaval vein (pt. cv). The ventricle of the heart (v) is turned forwards, and

CHAP, ii ABDOMINAL VISCERA 25

the abdominal vein (abd. v) is severed and turned backwards. The right ovary
and fat-body are removed, and the right oviduct (r. ovd) is slightly displaced
outwards.

abd. v. abdominal vein ; ccel. mes. splanchnic or coeliaco - mesenteric artery ;
cp. ad. corpus adiposum, or fat-body ; d. ao. dorsal ^ aorta ; gul. gullet ;
hu. cut end of humerus or upper-arm bone ; /. au. left auricle ; /. Ing. left lung ;
/. ovd. left oviduct ; /. ovd ' . its opening into the body cavity ; /. ovd" . its pos-
terior dilatation ; /. ovy. left ovary ; Ir. portion of liver ; pt. cv. postcaval vein ;
pt. cv . its anterior portion passing between the liver and the heart ; r. au. right
auricle; ret. rectum; r. kd. right kidney; r. Ing. right lung; rn.pt. renal
portal vein; r.ovd. right oviduct; r.otd". its opening into the body-cavity;
r. ovd". its posterior dilatation ; syst. tr. systemic arterial trunks at their
point of union ; u. bl. urinary bladder ; ur. ureter ; v. ventricle.

number of streamer-like lobes (cp. ad} ; this is the fat-body,
which serves as a storehouse of reserve nutriment.

By lifting up either of the ovaries there is seen beneath it
— in the natural position of the parts above or dorsal to it —
a greatly convoluted colourless tube (/. ovd, r. ovd} of about
the same diameter as the intestine. This is the oviduct,
through which the eggs pass from the ovary to the cloaca.
If the specimen is allowed to remain long in water the
oviducts will be found to swell and finally to become dis-
integrated ; this is due to the fact that in them is formed the
jelly in which the laid eggs are enclosed, and which, as
already mentioned (p. 9), swells in water.

In the male there is seen, on turning the intestines aside,
a pair of yellow ovoidal bodies (Fig. 3, r. spy} about half an
inch long, attached by peritoneum to the dorsal wall of the
body-cavity. These are the spermaries or testes ; they manu-
facture the spermatic fluid or milt by which the eggs are
impregnated. To the anterior end of each is attached a
fat-body (cp. ad}, like that of the female. In young speci-
mens of both sexes the reproductive organs — spermaries,
ovaries, and oviducts — are very small.

When the intestine is turned aside there will also be seen,
in both sexes, a pair of flattened, irregularly-oval bodies (Figs.
3 and 4, r. kd} lying in the posterior part of the abdominal
cavity just above or dorsal to the ovaries or spermaries.

26 THE FROG

CHAP.

These are the kidneys. With the outer edge of each is
connected a tube, the ureter (ur\ by which the urine,
formed in the kidneys, is carried to the cloaca (Fig. 7).

It has been pointed out that the abdomen is lined
by peritoneum, and that the various organs are suspended
by folds of the same membrane, called, in the case of the
enteric canal, the mesentery. The relations of this mem-
brane are best seen in a diagrammatic transverse section of

FIG. 5. — Diagrammatic transverse section through the trunk of a Frog, to show the

relations of the peritoneum.

abd. v. abdominal vein ; d. ao. dorsal aorta ; il. ilium ; int. intestine ; kd.
kidney ; m. muscles of back ; m'. muscles of abdomen ; vies, mesentery ; p. per.
parietal layer of peritoneum; p. per', the same, turning down to cover the
kidney ; //. cv.. postcaval vein ; sk. skin ; ,9. cu. ly. s. sub-cutaneous lymph-
sinuses ; spy. spermary ; s. v. ly. s. sub-vertebral lymph sinus ; u. st. urostyle
(part of the vertebral column); v.per. visceral layer of peritoneum, investing in-
testine ; v. per' . the same, investing spermary.

the body (Fig. 5), though many points can be perfectly
well made out from the actual specimen. The body-cavity
is lined by what is called the parietal layer of the
peritoneum (p. per), which adheres closely to the body-wall
except in the middle dorsal region, where it becomes closely
applied to the ventral surface of the kidneys and reproduc-

n PERITONEUM- NEURAL CAVITY 27

tive organs. Leaving these, the peritoneum of the right
side approaches that of the left, and the two, coming into
contact, form a double vertical sheet, the mesentery (mes\
which extends ventrally towards the enteric canal. On
reaching the latter, the two layers diverge again and
surround the canal, forming the visceral layer of the perito-
neum (v.per). The liver, oviducts, etc., are suspended and
covered in the same way. Thus the lining of the body-
cavity, the investment of the various organs contained in it,
and the folds by which they are suspended, are all parts of
one continuous membrane. The space left between the two
diverging layers of peritoneum, in the mid-dorsal region,
contains lymph, and is known as the sub-vertebral lymph
sinus (s. v. ly. s).

We have already noticed the abdominal and musculo-
cutaneous veins. Other veins of greater or less size will be
seen everywhere, passing, for instance, to the head and
limbs (Fig. 3), and in the mesentery. Running parallel
with many of the veins are smaller vessels, many of which
have pigment in their walls, and which are of distinctly
stouter texture. These are the arteries. They contain little
blood in the dead animal, and, owing to the stoutness
and elasticity of their walls, do not collapse when empty.
Hence they a-re quite easy to see in a frog from which all
the blood has been drained, while the thin-walled veins are
almost invisible under like circumstances. Finally, there
will be seen in many parts of the body, often lying parallel
to an artery and a vein, white cords, the nerves.

The Neural Cavity and its Contents. — By turning the
frog with its back upwards and cutting through the muscles
of the back and the arches of the vertebrae (see Fig. 6), as
well as, in front, the roof of the skull, you will see that the
backbone contains a distinct cavity, the neural canal, in

28

THE FROG

which lies a white rod, made of the same soft, pulpy
substance as the nerves, and called the spinal cord
(sp. cd\ which ends behind in a thread-like prolongation
(/ /), some distance in front of the thighs. It will also be
found that the neural canal is continued, with a slightly

ust.

FIG. 6. — Dissection of a Frog in which the entire neural canal («.<:.) has been opened
from above, and the brain (br) and spinal cord (sp. cd) laid bare. The brain con-
sists of olfactory lobes (olf. 1), cerebral hemispheres (crb. /?), diencephalon (dien),
optic lobes (opt. /), cerebellum (cl>lm), and medulla oblongata (med. obl\ which
will be referred to in Chapter X. The spinal cord ends in a delicate prolonga-
tion, the filum terminate (f. f). The nasal bones (na\ eyes (e), auditory
region of the skull (au), transverse processes of the nine vertebrae (v. I — v. 9),
urostyle (u. st) and ilia (il) are indicated in outline, and serve as landmarks.

^ fAfter Howes, slightly altered.)

increased diameter, into the skull, and that the spinal cord
becomes continuous with the brain (br), a complex organ
formed of several parts, which will be referred to hereafter.
General Structure of the Limbs. — A transverse section

SUMMARY OF CHAPTER

29

cut across one of the legs, at about the middle of the thigh,
will show in the middle of the cut surface the thigh-bone,
around it the flesh or muscle, and around this again the
skin. Similar cuts through various parts of both fore- and
hind-limbs show that these appendages of the body are solid,

s.inl

FIG. 7. — Dissection of a Male Frog from the left side. The left fore- and hind-limbs

and the left sides of the head and trunk have been cut away, the enteric canal

and liver are displaced downwards, and the mouth, pharynx, and cloaca laid

open.
an. anus ; 1. d. bile-duct ; b. hy. body of hyoid ; bl. urinary bladder ; bl'. its opening

into the cloaca ; c. art. conus arteriosus ; cbhn. cerebellum ; c 1. cloaca ; en. j,

centrum of third vertebra ; cp. ad. fat body ; c rb. h. cerebral hemisphere ;

d. ly. s._ dorsal lymph sinus ; du. duodenum ; ef>. cor. epi-coracoid ; eus. t.

Eustachian recess ; / < R. PA. fronto-parietal ; gl. glottis ; gul. gullet ; IL. ilium ;

IS. ischium ; kd. kidney ; / au. left auricle ; /. Ing. left lung ; Ir. liver ; M.MCK.

mento-meckelian bone ; n. a. /, arch of first vertebra ; olf. I. olfactory

lobe ; opt. 1. optic lobe ; O. ST. omo-sternum ; pcd. pericardium ; PMX. pre-

maxilla ; pn. pancreas ; p. na. internal nostril ; :pu. pubis ; ret. rectum ; r. Ing.

right lung ; s. int. ileum ; sp. cd. spinal cord ; SPH.ETfl. sphenethmoid ;

spl. spleen ; st. stomach ; s. ?/. sinus venosus ; tng. tongue ; ts. spermary ; ^^r.

ureter ; ur1 . its aperture into the cloaca ; UST. urostyle : v. ventricle ; v. ly. s.

ventral lymph sinus ; i>o. t. vomerine teeth ; vs. sem. seminal vesicle. (From

Parker and Haswell's Zoology.}

containing no cavities, except the sub-cutaneous lymph-
sinuses previously observed.

Summary. — We thus get a notion of the general plan of
construction of a frog as follows. It consists of a central or

30 THE FROG CHAP.

axial portion, the head and trunk, and of two pairs of lateral
offshoots or appendages, the fore- and hind-limbs. The trunk
is hollowed out into two cavities : the abdominal or body-
cavity (ccelome) below, and the neural canal above ; of these
the neural cavity alone is continued into the head. The
abdominal cavity contains the greater part of the enteric canal,
the liver, gall-bladder, pancreas, spleen, lungs, heart, kidneys,
urinary bladder, and reproductive organs. The neural
canal contains the brain and spinal cord. The anterior
end of the enteric canal is continued forwards into the head,
forming the mouth-cavity, and opens externally by the
aperture of the mouth ; its posterior end opens externally
by the anus. The enteric canal passes through the contain-
ing body-cavity, having no communication with it. The
lungs open into the pharynx, and thus communicate with
the exterior not only by the mouth but also by the nostrils.
The kidneys, bladder, and oviducts communicate with the
cloaca, and thus with the exterior through the anus. Neither
the neural nor the abdominal cavity has any communi-
cation with the exterior. The walls of the head and trunk
consist largely of muscles and bones covered with skin.
The limbs are solid outgrowths of the trunk, formed
mainly of muscle, with bony supports and a covering of
skin.

Organs. — Notice that the body consists of various definite
structures, or organs as they are technically termed, which
have various purposes <yc functions to perform. The enteric
canal, together with the liver and pancreas, are organs of
digestion : the lungs and skin, organs of respiration or breath-
ing ; the heart and blood-vessels organs of circulation, serving
as they do to propel and conduct the blood through the body ;
the kidneys, aided by the skin, organs of excretion, for getting
rid of waste matters ; the ovaries and spermaries, organs of

ii PRACTICAL DIRECTIONS 31

reproduction ; the muscles, organs of movement ; the brain
and spinal cord, together with the nerves, organs of control,
serving to direct or control the actions of the body ; the
skin, nose, eye, and ear, sensory organs, by which communi-
cations are kept up with the external world.

Tissues. — Notice also that the various organs of the body
are built up of different materials, or tissues as they are
called. We have already distinguished muscle, bone,
cartilage, connective-tissue and nervous tissue. Other
tissues we shall meet with in the course of a more careful
examination.

PRACTICAL DIRECTIONS.

To kill a Frog for Dissection. — Place a frog on a plate, and cover
it with a tumbler, or put it into a stoppered bottle. Soak a little bit of
cotton-wool or sponge in chloroform, and push it under the edge of the
tumbler, or drop it into the bottle. In a few minutes the vapour will
make the animal quite insensible, and a somewhat longer exposure will
kill it painlessly.

External Characters. — Observe the voluntary and the involuntary
respiratory movements of the living animal, and compare with a dead
frog when making out the external characters (pp. 4-8) and the position
of the various parts of the skeleton (p. 16).

Sketch the entire animal from the side or from above.

The Cavity Of the Mouth-— Gently open the mouth of a dead
frog as wide as possible, and make out the points described on p. 17.
Sketch.

The Body-wall. — Lay the frog on its back in the dissecting-dish, and
fix it firmly by sticking pins through the skin of the arms and legs.
With the forceps, held in the left hand, pinch up the skin of the
abdomen near the middle line between the thighs, and make a nick in it
with the points of the scissors. Then, holding the edge of the hole thus
made with the forceps, pass in a probe and push it forwards as far as it
will go without opposition. Note : —

The sub-cutaneous lymph-sinuses, and the underlying muscle.

With the scissors extend the incision made in the skin of the belly
forwards, in a straight line, to the chin. Holding up the edge of the

32 THE FROG CHAP.

skin with the forceps, cut through, with a scalpel, the partitions between
adjacent lymph-sinuses, so as to separate the whole of the skin of the
ventral surface from the muscle ; and, having done so, pin back the flaps,
right and left (see Fig. 2). Similar cuts should be made in the skin of
the limbs and back. Observe —

The fascia, the muscles of the body-wall, the abdominal and
musculo-cutaneous veins, and the shoulder-girdle and pubic region of the
hip-girdle.

The Abdomen and its Contents.— Pinch up the muscles on one

side of the abdominal vein with the forceps, and make an incision in
them by a single snip of the scissors. Then, holding the edge of the
wound with the forceps, extend the cut forwards to the shoulder-girdle
and backwards to the pubis. Keep the cut parallel to the abdominal vein,
and be careful not to wound the latter. You will find that the incision
thus made opens a large body-cavity or ccelome, in which a number of
structures, the abdominal viscera, are contained. Note that the body-
wall consists of three layers : (i) skin, (2) muscles, with their fascia, and
(^peritoneum.

So far, however, the cavity is not thoroughly opened. Lift up the side
of the abdominal wall to which the abdominal vein is attached, and
very carefully separate the vein by tearing through, with a needle or
the point of a scalpel, the connective tissue by which it is attached to
the inner face of the muscles : or, in order to prevent the possibility of
injuring the vein, cut through the muscles of the body-wall longitu-
dinally on the other side of the abdominal vein, so as to leave a narrow
strip of muscle attached to it. Then make two cross-cuts, starting from
the anterior end of the longitudinal incision, and extending outwards
towards the fore-limbs : take care not to injure the musculo-cutaneous
veins, and pin back the two flaps into which the soft abdominal wall is
now divided (Figs. 3 and 4). Next dissect away the muscles covering
the shoulder-girdle, so as to expose the bones : identify the bones
called coracoid and clavicle (compare a skeleton and Fig. 12). With
the s.trong scissors cut through these bones on either side as near as
possible to the shoulder-joint : then lift up the middle portion of the
shoulder -girdle, and carefully dissect it away from the underlying parts.

Having thus exposed the whole of the abdominal cavity, pour just
enough water into the dissecting-dish to cover the animal, first washing
away any blood which may have escaped from cut vessels. If your
specimen is a female, dissolve a little common salt in the proportion of

II PRACTICAL DIRECTIONS 33

I per cent, in the water (i gramme to 100 c.c.), or mix it with about
one-third of its bulk of methylated spirit, in order to prevent the
excessive swelling of the oviducts. (If, however, you wish to make out
the blood-vessels in this specimen without injecting them, it is as well to
defer putting water into the dish until a later stage of the dissection. )
Note—

1 . The peritoneum, parietal and visceral layers.

2. The pericardiitm, containing the heart. If not already opened,
the pericardium should be slit through, so that the auricles and ventricles
can be plainly seen. If the frog has been killed quite recently, you will
be able to observe the pulsation of the heart.

3. The right and left lobes of the liver, and the gall-bladder.

4. The two lungs : if contracted, inflate with a blowpipe through the
glottis.

5. The enteric or alimentary canal, consisting of gullet or oesophagus,
stomach, small intestine (duodenum and ileum], and large intestine or
rectum, communicating with the cloaca, which will be seen at a later
stage, and which opens to the exterior by the vent.

6. The mesentery.

7. T\\Q pancreas.

8. The spleen.

9. The urinary bladder. If collapsed, insert a blowpipe into the
vent and inflate. (You will very likely find some small parasitic flat-
worms, called Polystomum integerrimtm, in the bladder ; each worm
has a ring of suckers round the hinder end. )

10. In the male the spermaries and fat-bodies, and in the female the
ovaries, fat-bodies, and oviducts.

1 1 . The kidneys and iireters.

12. The mode of suspension of all these organs (p. 26), and the
position of the sub-vertebral lymph-sinus. In order to understand clearly
the relations of these parts, a thick transverse section Should be made
through another frog in the region of the kidneys and examined under
water (compare Fig. 5). .

Sketch the contents of the abdomen in situ.

The Neural Cavity and its Contents-— Now turn the frog with

its back upwards, and pin it firmly to the bottom of the dissecting-dish
or to the dissecting- board as before. Pinch up the skin, make a
longitudinal cut through it from the snout tc within a short distance of
the vent, and turn the flaps right and left. The muscles of the back
PRACT. ZOOL. D

34 THE FROG CH. n

will be exposed, and, in front, the roof of the skull, which lies just
beneath the skin. Carefully dissect away the muscles along the middle
line of the back until the vertebral column is seen. Compare a prepared
skeleton and Fig 8, and make out the arches of the vertebrae. Between
the first of these and the back of the skull is a slight space : insert one
blade of the strong scissors into this, directing the points backwards,
and cut through the arch of the first vertebra, first on one, then on
the other side, and finally detach and remove the little piece of bone.
The neural canal will then be exposed, in which lies the spinal cord,
ending behind in a thread-like prolongation (compare Fig. 6) : work
backwards, cutting away the arches of the remaining vertebrae.

Next, using the scissors in the same manner, cut away, bit by bit, the
roof of the skull : two large bones — the fronto-parietals (see Figs. 8
and 9), forming a considerable part of the roof, can be more easily re-
moved by raising them up with the edge of a scalpel. Note the cavity
of the skull and its contained brain.

General Structure of the Limbs- — With a strong knife, cut

across one of the legs at about the middle of the thigh. Compare p. 28,
and notice the thigh-bone, muscles, and skin. Sketch.

Now preserve your specimen in formaline (about 2 percent.), or spirit
(70 per cent.).

CHAPTER III

THE FROG (continued) : THE SKELETON

IF you have followed the description given in the preced-
ing chapter with a frog before you, testing every statement
as you proceeded by reference to the specimen, you will
now have a very fair notion of the general build of the
animal. The next thing to do is to study its various parts
in somewhat greater detail.

As the bones and cartilages form the framework on which
all the other parts are supported, it is convenient to begin
with them. You may study them on a prepared skeleton, but
a far better plan is to make a skeleton for yourself as
directed on p. 53.

Parts of the Skeleton. — The skeleton consists of the
following regions :-—

1. The skull (Figs* Sand 9) : a complex mass of mingled
bone and cartilage, enclosing the brain and the organs of
smell and hearing, and supporting the upper jaw. Connected
loosely with the skull, but really forming part of it, are the
lower jaw and the tongue cartilage.

2. The vertebral column or backbone , consisting of nine
movably united pieces, the vertebra (Fig. 8 v. i — V.Q), and
of a long bony rod, the urostyle (UST), representing a num-
ber of fused vertebrae belonging to the tail-region of the
tadpole.

D 2

36 THE FROG CH. in

3. The shoulder -girdle or pectoral arch, an inverted arch
of bone and cartilage nearly encircling the anterior part of
the trunk and giving attachment to —

4. The bones of the fore-limbs.

5. The hip-girdle or pelvic arch, an apparatus shaped
somewhat like a bird's "merrythought"": it is attached in
front to the ninth vertebra and behind gives attachment to —

6. The bones of the hind-limbs.

The Vertebral Column. — The essential structure of a
vertebra may be best studied by examining any of the nine
from the second to the seventh : the first, eighth, and ninth
present certain peculiarities, and so may be left till last.

The whole vertebra (Fig 8, B) has something the form of
a signet-ring with its sides produced into two outstanding
projections. The part comparable to the stone of the
ring is ventral in position, and is called the body or centrum
(en), the form of which is proccelous, i.e., its anterior face is
concave, its posterior face convex, and both faces are
covered with a thin layer of cartilage. The part corre-
sponding with the circle of the ring is the neural arch
(pd, /;//) : it arches over the spinal cord and is produced
in the middle line above into a blunt projection, the neura\
spine (n. sp). From the arch is given off, on either side, the
large outstanding projection already referred to, the
transverse process (tr. pr\ which is tipped with cartilage in
the second, third, and fourth vertebrae.

The neural arch gives off from its anterior face, just above
the origin of the transverse processes, a pair o'f small shelf-
like projections, the articular processes or zygapophyses (a. zyg).
Each has its upper surface flat and smooth, and covered with
a thin layer- of cartilage. A similar pair of processes spring1
from the posterior face of the arch, but have the smooth,
cartilage-covered surface or facet looking downwards.

FIG. 8. — A, skeleton of Frog from the dorsal aspect ; B, anterior face of the fourth
vertebra. In A the left half of the shoulder-girdle and the left fore- and hind-
limbs are removed, as also are the investing-bones of the left side of the skull.
Cartlilaginous parts are distinguished by dotting. The names of replacing
bones are in thick capital letters, those of investing bones in italic capitals, other
references in small italics.

a.c.hy. anterior horn of hyoid ; actb. acetabulum ; AST. astragalus ; a.zyg. anterior
articular processes, or zygapophysis ; b.hy. basi-hyal ; C. calcar ; CAL. cal-
caneum ;.cn. centrum; EX.OC. exoccipital : FE. femur; fon, fon.' fontan-
elles ; FR. PA. fronto-parietal ; HU. humerus ; IL. ilium; Im. roof of
neural arch ; MX. maxilla ; n. j/. neural spine ; olf. cp. olfactory capsule ;
of. pr. otic process ; /. c. hy. posterior horn of hyoid ; pd. base of neural
arch ; PMX. premaxilla ; PR. OT. pro-otic ; QU.JU, quadrato-jugal ; RA. UL.
radio-ulna; SP. ETH. sphenethmoid ; 6"^. squamosal ; S. SOP. supra-scapula ;
s#.y. suspensorium ; TI. FI. tibio-fibula ; tr. pr. transverse process ; UST. uro-
style ; V. 1, cervical vertebra ; V. 9, sacral vertebra; VO. vomer ; l—V digits.
(From Parker and Haswell's Zoology, after Howes, slightly altered.)

38 THE FROG CHAP.

When two vertebrae are placed in position, the convex
posterior face of the foremost centrum fits into the concave
anterior face of its successor, like a cup and ball, and at
the same time the posterior articular processes of the first
fit over the anterior articular processes of the second. All
the touching surface is, as we have seen, capped with
cartilage, and the vertebras can be moved upon one another,
either up and down or from side to side.

The centra and articular processes are the only parts of
the vertebrae which are in contact when the bones are in their
natural positions. Large gaps or notches are left between
the dorsal portions of the arches (see Fig. 8, A) to allow of
the requisite amount of up and down movement, and there
are similar gaps between the sides of the arches, bounded by
the articular processes above and the centra below. These
are called the intervertebral foramina : through them the
nerves pass from the spinal cord.

The only differences of importance between the vertebrae now under
consideration is in the form of their transverse processes, which are
specially large in the third, short and devoid of cartilaginous tips in the
fifth, sixth, and seventh.

The first vertebra has no transverse processes, and on
either side of the very small " body " its anterior face bears,
instead of the ordinary articular processes, a pair of obliquely
placed, oval, slightly concave surfaces or facets, covered
with cartilage, and serving for the articulation of the
condyles of the skull, presently to be described. The
transverse processes of the ninth or sacral vertebra are very
long and strong, directed backwards, and tipped with
cartilage : to them the arms of the pelvic girdle are articu-
lated.

The eighth vertebra differs from its predecessors in having its body
concave behind as well as in front. Corresponding with this, the ninth

in AXIAL SKELETON. 39

(v. 9) has its centrum convex in front, while behind it presents two
little rounded elevations placed side by side.

It will be seen that the vertebrae are all corresponding
structures, following one another in a regular series from
before backwards. A correspondence of this kind, in
which there is a repetition of similar parts along the body, is
termed a serial homology, and thus not only the vertebrae as
a whole, but also their various parts are serially homologous,
each to each, the correspondence being disturbed only by the
first vertebra, in which the transverse processes are absent
and the anterior face modified for articulation with the skull.

The urostyle (UST) is a long bone with a gradually dim-
inishing ridge along its dorsal surface (see p. 35). Its anterior
face has somewhat the appearance of a small vertebra with
no transverse processes, and has a double concavity for
articulation with the double convexity on the ninth vertebra.
Near its anterior end there is on each side a small aperture,
representing an intervertebral foramen for the last spinal
nerve.

The skull is a very complex structure, consisting partly
of bone, partly of cartilage. It is divided into the following
regions : —

1. The brain-case or cranium, a sort of oblong box con-
taining the brain (Figs. 8 and 9) : it forms the middle portion
of the skull and is a direct forward continuation of the
vertebral column.

2. The auditory capsules (aud. cp\ a pair of outstanding
masses arising, right and left, from the posterior end of the
case. They lodge the organs of hearing.

3. The olfactory capsules (olf. cp}, smaller masses pro-
ceeding from the anterior part of the brain-case and united
with one another in the middle line. They lodge the organs
of smell

40 THE FROG CHAP.

4. The suspensoria (sus), a pair of projections spring-
ing from the outer and upper portions of the auditory
capsules, and directed downwards, outwards, and backwards.
To them the ends of the lower jaw are attached.

5. The upper jaw ', a half-circle of bone and cartilage,
united in front to the olfactory capsules and behind to the
auditory capsules and suspensoria. On either side of the
skull, between the cranium and upper jaw, is a large space,
the orbit, in which the eye is contained,

6. The lower jaw ', a roughly semicircular bar of bone and
cartilage, articulated at each end with the corresponding
suspensorium.

7 . The tongue-cartilage or hyoid (b. hy), a shield-shaped
cartilage connected by delicate curved rods (a. c. hy) with
the auditory capsules.

On the posterior surface of the brain-case is a large hole,
the foramen magnum (fig. 9, c, for. mag), on either side of
the lower edge of which is an oval elevation covered with
cartilage, the occipital condyle (oc. en). The foramen magnum
leads into the cavity in which the brain is contained. If the
first vertebra is placed in its natural position with regard to the
skull it will be seen that the foramen magnum corresponds
with the neural canal of the vertebra, and that the condyles
fit into its articular surfaces. Thus the skull readily moves
up and down upon the vertebra, the condyles acting as'
rockers ; a space between the neural arch and the dorsal
edge of the foramen magnum covered by membrane in
the fresh state, allows of the requisite amount of play.

The discrimination of the separate bones and detailed structure of the
skull is rather difficult, and may very well be omitted by the beginner at
the present stage.

The occipital condyles are borne on a pair of irregular bones
(EX.OC) which bound the sides of the foramen magnum, nearly uniting

SKULL

above and below it, and extending over a considerable portion of the
posterior surface of the auditory capsule. These bones are the exocci-
pitals. In front of each exoccipital is another irregular bone (PR.OT)
forming the front part of the auditory capsule, and called the pro-otic.

g

'

Each pro-otic is separated from the corresponding exoccipital, in young
frogs, by a band of cartilage, but in old specimens the two bones are
more or less completely united.

In the disarticulated skull it can be made out that the exoccipital and

42 THE FROG CHAP.

pro-otic of either side enclose a cavity ; in this the organ of hearing is
contained. The exoccipital is perforated, just in front of the condyle,
by a double aperture (Nv. 9, 10) through which two nerves, the
glossopharyngeal and the vagus, pass on their way from the brain. The
pro-otic is similarly perforated or notched for the trigeminal and facial
nerves (Nv. 5, 7).

The dorsal surface of the brain-case is covered by two longish, flat
bones (FR.PA]. In the young condition each of these consists of two
distinct bones, the front one the frontal, the hinder the parietal. As
the young frog grows the frontal and parietal of either side becomes com-
pletely fused, forming a single fronto-parietal. On the upper surface of
each olfactory capsule is a roughly triangular bone, the nasal (NA], in
front of which is the corresponding nostril.

The ventral surface of the brain-case is covered by a single bone
(PA.SPH] having the shape of a T. The stem extends forwards in
the middle line as far as the olfactory capsules, while the arms stretch
outwards beneath the auditory capsules. This very characteristic bone
is the parasphenoid. On the under surface of the olfactory capsules,
corresponding tc the nasals above, are a pair of irregular bones, the
vomers ( VO}. Their outer edges are notched and help to bound the
internal nostrils ; their posterior ends bear the vomerine teeth.

The anterior end of the brain-case is surrounded by a bone
(SP.ETH) which extends forwards into the region of the olfactory
capsules, and is partly covered by the fronto-parietals above and by the
parasphenoid below. This is the girdle-bone or sphenethmoid. In the
disarticulated skull it is seen to have a very peculiar shape. Its
posterior half encloses a single cavity in which the fore-end of the brain
(Fig. 6, olf. 1} is lodged. Its anterior half encloses two cavities, right
and left, separated from one another by a vertical partition, and serving
to lodge the posterior ends of the olfactory sacs or organs of smell.
Each of these cavities communicates with the single posterior cavity by
a small hole through which the nerve of smell passes.

Between the girdle-bone in front and the pro-otic behind, the side-
walls of the skull are formed of cartilage perforated by a rounded
aperture, the optic foramen (Nv. 2\ for the nerve of sight.

Forming the outer part of the suspensorium is a hammer-shaped bone,
the squamosal (SQ) ; its head is applied to the auditory capsule and
projects forwards into the orbit.

The upper jaw is formed of three bones on either side. In front is the
premaxilla (PJlfX), a short bone, sending off an upward process

in SKULL 43

towards the nostril. Next follows the maxilla (MX), a long, curved
bone, forming the greater part of the upper jaw, and joined at its
posterior end to a small, slender bone, the quadrato-jugal (QU.JU),
which is firmly connected with the lower end of the suspensorium.
Both premaxilla and maxilla are produced below into a prominent edge
from which spring a number of small conical teeth, arranged in a
single row.

Besides these three bones there are two others which seem, as it were,
to brace the upper jaw to the brain-case and suspensorium. The
palatine (PAL) is a narrow, rod-like bone, placed transversely behind
the olfactory capsule. The pterygoid (PTG) is a large, three-rayed
bone ; one ray is directed forward and connected with the outer end of
the palatine and with the inner face of the maxilla; another passes
backwards and inwards and is connected with the auditory capsule ; the
third extends backwards and outwards and forms the inner and ventral
• portion of the suspensorium. The main mass or core of the suspen-
sorium, between the squamosal outside and the pterygoid v/ithin, is a
rod of cartilage (stts), which is continued forwards by a bar (pal. qu]
supporting the pterygoid and palatine.

There is an important distinction to be drawn between the bones of
the 'skull which can be made out only by the exercise of a good deal of
care and patience. By softening the connective -tissue which binds the
bones together, it is possible to remove the majority of them without
injuring the underlying cartilage (compare the right and left sides of the
skull in Figs. 8 and 9, A and C), provided, of course, that the opera-
tion is skilfully performed : these bones are the nasals, vomers, fronto-
parietals, parasphenoid, premaxillae, maxillae, quadrato-jugals, palatines,
pterygoids, and squamosals. A sort of foundation or groundwork (left
side of figure in Figs. 8 and 9, A ; right side in Fig. 9, C) is then left
behind, consisting mainly of cartilage, but containing the exoccipitals,
pro-otics, and girdle-bone. These five bones cannot be removed
without pulling the cartilaginous groundwork or chondro cranium to
pieces. We thus get a distinction between replacing bones (" cartilage-
bones ") which are actually continuous with the cartilage and form part
of the chondrocranium and investing bones (" membrane bones") which
lie outside the chondrocranium, united to it only by connective tissue.

The chondrocranium has a cartilaginous roof, underlying the fronto-
parietals ; it is pierced by one large (Fig. S,fon) and two small (fort')
spaces, oa&z&fontanelles, covered by membrane. It has also a cartila-

44 THE FROG CHAP.

ginous floor (Fig. 9, A) underlaid by the parasphenoid. The olfactory
capsules (olf. cp] also have a cartilaginous roof and floor of irregular form,
with the posterior end of which is united the cartilaginous palato-quadrate
bar (pal. qti], with which the palatine and pterygoid bones are connected.
Posteriorly this bar is continuous with the cartilaginous (quadrate]
groundwork or core of the suspensorium (sus), which unites above with
the auditory capsule by two processes (Fig. 9, C, ot. pr", ped] and below
furnishes an articular surface for the lower jaw.

Notice that in describing the vertebral column no distinction was drawn
between replacing and investing bones. As a matter of fact the vertebrae
and the urostyle are all replacing bones ; each consists, in the tadpole, of
cartilage which subsequently undergoes ossification, i.e., is replaced by
bone in which a deposition of lime salts takes place.

The lower jaw (Fig. 9, B) consists of two halves, or
rami, united with one another in front by ligament. At
its posterior end each half bears on its upper surface a
shallow pit, by which it articulates with the suspensorium,
and a little in advance of this pit is an elevation of the
dorsal edge of the jaw, called the coronary process.

Each half of the lower jaw consists of a cartilaginous core, the
mandibular or MeckeF s cartilage^ which furnishes the articular surface
referred to, and in front is ossified as a small replacing bone, the mento-
meckelian (M. MCK). Outside the cartilage are two investing bones.
One, the angulo-splenial, extends along the inner surface and lower edge
of the jaw and forms the coronary process, while the dentary (DNT)
forms the outer surface of the anterior half of the jaw.

The hyoid is a thin, shield- shaped plate of cartilage
(Figs. 8 and 9, b. hy] produced, both in front and behind,
into a pair of processes or horns, as well as into less impor-
tant offshoots. The anterior horns (Fig. 9, a.c.hy] are long,
delicate, cartilaginous rods which curve backwards and then
upwards, finally joining with the auditory capsules. The
posterior horns (ti.c.hy) are short bony rods which pass
backwards, diverging as they go, one on either side of the
glottis.

Ill

TYMPANIC CAVITY

45

Two apparently insignificant structures connected with
the skull must be- described because of their connection with
the organ of hearing. Behind the suspensorium is a recess,
roofed over by the squamosal, and, in the entire frog,
converted by muscle and other tissues into a chamber, the
tympanic cavity (Fig. 10, tymp. cav\ bounded externally by
the tympanic membrane, and communicating with the mouth

o.sl

FIG. 10. — Transverse section (diagrammatic) through the head of a Frog at the
level of the tympanic cavity. The various parts of the skull shown in section
are black, the muscle, &c., grey, and the skin and mucous membrane white.
an. tymp. tympanic ring ; b. hy. body of hyoid ; hue. cav. cavity of pharynx ;
ch.plx. choroicf plexus ; col. columella ; eus. t. Eustachian tube ; fen. ov. fenestra
ovalis ; med. obi. medulla oblongata ; memb. lab. membranous labyrinth ; mnd.
mandible ; Nv. VIII. auditory nerve ; o. st. omosternum ; ptg. pterygoid ;
gu.j'u. ^quadrato-jugal ; stp. stapes ; tymp. cav. tympanic cavity ; tymp. inemb.
tympanic membrane. (From Parker and Haswell's Zoology.)

by the Eustachian tube. Supporting the tympanic mem-
brane, as the frame of a tambourine supports the parch-
ment, is a cartilaginous ring, the tympanic ring (shown in
section in Fig. 10, an. tymp]. Stretching across the tym-
panic cavity from the outer wall of the auditory capsule to
the tympanic membrane is a small, hammer-shaped rod, the
columella (cot), having a bony handle and a cartilaginous

46 THE FROG CHAP.

head, the latter firmly fixed to the inner face of the tym-
panic membrane. The inner end of the handle is tipped
with cartilage, and is attached to a small cartilaginous nodule,
the stapes (st\ which is inserted into an aperture in the
auditory capsule known as thefenestra ovalis (fen. ov). With
care the columella, in a wet skull, may easily be removed
with small forceps, and examined under a magnifying glass.
The shoulder-girdle has the form of an inverted arch
encircling the anterior region of the trunk, and having its
dorsal ends turned inwards so as partly to cover the second
to the fourth vertebrae (Figs. 8 and u). The dorsal region,

JfC/t

&

FIG. ii. — Diagrammatic transverse section through the shoulder-girdle.
cor. coracoid ; ep. cor. epicoracoid ; gl. glenoid cavity ; hu. humerus ; scp. scapula ;
s. scp. supra-scapula ; v. j, third vertebra.

on either side, is formed by a broad plate, the supra-scapula
(s. scp\ or upper blade-bone. It is mostly formed of bone,
but its free edge consists of cartilage which, when dried, is
seen to be impregnated with a granular deposit of lime-salts.
This rough, brittle tissue is called calcified cartilage, and is
distinguishable from true bone, which has usually a smooth,
enamelled surface.

Connected with the ventral end of the supra-scapula and
passing vertically downwards is a flat bone, broadened at
each end, the scapula or blade-bone (Fig. \\^scp\ Fig. 12, S).

Ill

SHOULDER-GIRDLE

47

From its lower end two bones (Fig. 12, C7, Co-, Fig. n,
cor) pass directly inwards, parallel with one another, to end
In a plate of cartilage (Co1}, which meets with its fellow
of the opposite side in the middle line of the chest (m).
The more anterior of these (Cl) is a narrow bone and
is called the clavicle or collar bone? the posterior one is

FIG. 12. — The shoulder-girdle of the Frog from the ventral aspect.
Co. coracoid ; Co', epicoracoid ; Cl. clavicle : Ep. omo-sternum ; G. glenoid cavity ;
Fe. fenestra between clavicle and coracoid ; KC. cartilage separating scapula
and clavicle ; Kn. xiphisternum ; in. junction of epicoracoids ; 6". scapula ;
St. sternum. (From Wiedersheim's Comparative Anatomy.}

broader and is known as the coracoid (Co). Between the
scapula on the one hand and the clavicle and coracoid on
the other, there is a cartilaginous interval (KC\ the posterior
edge of which is scooped out into a depression, the glenoid
cavity (Fig. 12, G ', Fig. n, gt), for the articulation of the
upper-arm bone.

48 THE FROG CHAP.

Connected with the median ventral portion of the shoulder-
girdle is the sternum, or breast-bone, which consists of two
separate parts, one extending forwards, the other backwards,
in the middle line, and each formed of a flattened bony rod
(Ep, Sf], tipped with a flat plate of cartilage.

The anterior bony rod with its terminal cartilage is called
the omosternum (Ep) ; the posterior bony rod is the stet num (St ), the
bilobed cartilage at its end the xiphisternum (Kn ). The cartilages
uniting the inner or ventral ends of the clavicles and coracoids are dis-
tinguished as the epicoracoids ( Co').

All the bones of the shoulder -girdle and sternum are replacing bones
except the clavicle. This can be removed, and is seen partly to surround
a bar of cartilage, the procoracoid, which stretches between the scapula
and the epicoracoid and is ordinarily completely concealed by the
clavicle.

The Fore-limb. — The upper arm is supported by a single
bone, the humerus (Fig. 8, HU), the first example we have
had of what is conveniently called a long bone. It consists
of a roughly cylindrical shaft, formed of dense bone, and of
two extremities — the proximal of partially calcified cartilage,
the distal of spongy or cancellated bone. The proximal
extremity or head is convex, and fits into the glenoid cavity
of the shoulder-girdle (Fig. n); the distal extremity or
condyle is almost globular, and is articulated with the bone
of the fore-arm.

In a longitudinal section of a humerus which has not been
allowed to dry you will see that the shaft (Fig. 13, sti) is not a
solid rod, but a tube, containing a cavity, the marrow-cavity.
In this way the weight of the bone is diminished without its
strength being impaired. The marrow-cavity contains a
substance called bone-marrow, composed chiefly of connective
tissue and fat, with blood-vessels, The proximal end of the
hollow shaft is, as it were, plugged by the cartilaginous
extremity.

FORE-LIMB

49

The fore-arm is also supported by a single bone, the radio-
ulna (Fig. 8, RA. UL). Its proximal end is concave and
articulates with the almost globular condyle of the humerus :
the outer or posterior edge of the concavity is produced into
a short process, the olecranon or elbow. The distal end is

\hct

FIG. 13. — Longitudinal sections of the principal long bones of the Frog.
A, humerus ; B, radio-ulna ; C, femur ; D, tibio-fibula. en. condyle ; _/! foramen
for artery ; fi. fibula ; hd. head ; m. marrow ; ol. olecranon process ; p. bony par-
tition ; ra. radius ; sh. shaft ; ti. tibia ; ul. ulna.

incompletely divided into two articular surfaces, and between
these is a groove passing for some distance towards the
proximal end of the bone. A section shows that at this end
there are two distinct marrow-cavities, indicating that the bone
is really double. That this is the case is proved by the

PRACT. ZOOL. E

So THE FROG CHAP.

examination of a very young frog, in which the single fore-
arm bone is represented by two distinct cartilages, the radius
on the inner or thumb-side, and the ulna on the outer or
little-finger side. The olecranon is a process of the ulna.

The skeleton of the hand is divisible into three regions :
the carpus or wrist, the metacarpus or mid-hand, and the
phalanges or finger-bones. The carpus consists of six
small irregular bones, arranged in two rows (Fig. 8). The
proximal row articulates with the radio-ulna, while to the
distal row are attached the metacarpals, which together
constitute the metacarpus. Four of these are long, rod-like
bones and support the bases of the four fingers or digits :
to them are attached the phalanges, of which the first or
innermost digit (II) has two, the next two, and the remain-
ing two digits three apiece. A very small metacarpal, with
a single phalanx (I), occurs on the radial side and is con-
cealed by the skin in the entire frog : it corresponds with
our own thumb, so that the apparent first digit of the frog
is really the second or index-finger.

The Hip-girdle — This, as we have seen, has somewhat
the form of a bird's merrythought. It consists of two long
arms (Fig. 8, IL : Fig. 14, //), which are articulated with the
transverse processes of the ninth vertebra, and sweeping
backwards, unite in a disc-shaped mass, having on either
side of it a deep, hemispherical cavity, the acetabulum (Fig. 8,
actb • Fig. 14, G\ for the articulation of the thigh-bone.

Two sutures, or lines of separation, nearly at right angles
to one another, divide the disc-shaped portion into three
parts. One of these, dorsal and .anterior in position, is
continued into one of the arms of the hip-girdle and forms
half of the acetabulum; this is the ilium (Fig. 14, //, P).
The second, posterior in position, is the ischium (Is) ; like
the ilium it is made of true bone. The third, or pubis (Kn\

HIND-LIMB

is ventral, and is formed of calcified cartilage. Originally each
of these elements is paired, i.e., there is an ilium, an ischium,
and a pubis on either side, the three together forming the
innominate- but in the adult the right and left ischia and
pubes become united in the median
plane, the ilia only remaining free.

The Hind-limb.— The thigh, like
the upper arm, is supported by a
single long bone, the femur (Fig. 8,
FE), having a gently curved shaft
and extremities of calcified cartilage.
Its rounded proximal extremity, or
head, fits into the acetabulum : its
distal end articulates with the single
bone of the shank, the tibio-fibula
(TI. FI). This, the longest bone
in the body, also has a shaft and
extremities, and is further distin-
guished by grooves running from
each end towards the middle of the
shaft. Sections show that the grooved portions of the bone
contain a double marrow-cavity (Fig. 13, D), and in the young
animal there are found two shank-bones which afterwards
unite, the tibia on the inner side, the fibula on the outer side.

The foot, like the hand, is divisible into three regions :
the tarsus or ankle, the metatarsus or mid-foot, and the
phalanges or toe-bones. The tarsus, like the carpus,
consists of two rows, but with only two bones in each.
Those of the proximal row (astragalus and calcaneum) are
greatly elongated (AST., CAL.), and furnish an additional
segment to the limb, thus increasing the frog's leaping
powers : those of the distal row are very small.

The metatarsals are five in number : those of the first

E 2

FIG. 14. — The pelvic girdle of
the Frog seen from the
right side.

G. acetabulum ; Kn. pubis ;
//, P. ilium ; Is. ischium.
(From Wiedersheim's Cow
parative Anatomy?) .

52 THE FROG CHAP.

and second digits (I, II) bear two phalanges each, those
of the third and fifth, three each, and that of the
fourth, four. Attached to the inner side of the tarsus
is a little claw-like structure (C) composed of two or three
bones and called the calcar or spur.

Notice the striking correspondence in structure
between the fore- and hind-limbs, a correspondence which
extends also, though less obviously, to the limb-girdles.
The humerus corresponds or is serially homologous (p. 39)
with the femur, the radius with the tibia, the ulna with the
fibula, the carpals with the tarsals, the metacarpals with
the metatarsals, and the phalanges of the fingers with
those of the toes. Then in the limb-girdles the glenoid
cavity corresponds with the acetabulum, the scapula and
supra-scapula (being above the articular cavity) with the
ilium, the procoracoid and clavicle (being ventral and
anterior in position) with the pubis, and the coracoid with
the ischium. Thus not only are the limbs and limb-
girdles serially homologous structures, but their several parts
are also serially homologous, each to each.

Nature of Bone. — It is a mistake to suppose that
bones are made exclusively of hard mineral matter, like
rocks or stones. If one of the long bones, for example,
is put into weak acid, bubbles of gas will rise from the
bone, showing that the phosphate and carbonate of lime,
of which it is partly composed, is being decomposed with
the liberation of carbonic acid. When the liberation of
bubbles is over, the bone will be found to be unaltered
in form, but to be quite flexible instead of hard and rigid.
It can be bent in any direction, and a bone of sufficient
length, such as a sheep's rib, can be tied in a knot. This
shows that the bone contains a large amount of organic
or animal matter. On the other hand, a bone may be

in PRACTICAL DIRECTIONS 53

completely calcined by heating to redness in a closed vessel,
when its animal matter is completely consumed and its
mineral matter left. Under these circumstances it becomes
very brittle, falling to pieces at a touch, and its appearance
is far more altered than by the removal of the mineral
matter.

PRACTICAL DIRECTIONS.

Preparation of the Skeleton.— Kill a frog with chloroform (p. 31),

open the abdomen as directed on p. 32, but without cutting the shoulder-
girdle, and remove the contained organs. Then, the frog being firmly
pinned down, remove the skin and gradually cut away the flesh from the
bones. In the case of the long bones of the limbs, it is best to cut
through the muscles near one end of the bone and then gradually to strip
them back towards the other end until the bone is exposed. The
process is facilitated by dipping the frog occasionally into boiling
water (maceration in cold water requires a considerable time) : this
softens the connective tissue by which the bones and muscles are bound
together, and thus allows them to be more readily separated. While at
work keep Fig. 8 before you, and be particularly careful not to injure
those parts of the skeleton which are made of cartilage (dotted in the
figure), and are therefore easily cut : the most important of these parts
are the hyoid or tongue-cartilage (J).hy\ lying in the floor of the mouth,
the omosternum (Fig. 12, Ep]^ the xiphistemum (fCn], and the supra-
scapula (Ffg. n, s.sc). Great care will also be required in cleaning
the bones of the hands and feet, since the fine cords or tendons which pass
to them from the muscles are very strong, and if pulled upon with much
force are sure to bring away the small toe-bones with them : they should
be separated as far as possible and then cut off, close to the bones, with
scissors.

Keep all the parts of the skeleton together, avoiding separation of the
various bones until the general characteristics of the entire skeleton
have been made out : the only part which cannot be kept in connection
with the rest is the shoulder-girdle, together with the fore-limbs.

Examination of the Skeleton-— You should have two skeletons

to examine — one dried, after it has been thoroughly cleaned, and one
which has been kept from the first in spirit or formaline : the latter is
the more instructive (see also the paraffin method, given in Part II; under

54 THE FROG CHAP, in

Dogfish). An additional skull should be carefully cleaned, and then
boiled until the numerous bones become separated from one another or
disarticulated.

After observing the form and relations of different parts of the
skeleton as described on pp. 35 and 36 (Fig. 8, A), they may be sepa-
rated from one another for more detailed examination. The individual
vertebrae should be strung on a piece of wire or string so as to prevent
their being lost or misplaced.

With the specimen before you, work through the characters of the
axial skeleton (pp. 36 — 46) : if you omit the details given in small type
at the present stage, do not forget to examine them subsequently.
Make sketches of — a. Any one of the vertebrae from the 1st to the 7th,
from the side and from the front or back ; b. the ist vertebra ; c. the
urostyle ; d. the skull from above and from below ; and e, the
hyoid.

It requires considerable skill to make a satisfactory preparation of the
chondrocranium, and it is advisable to examine that of a Dogfish first ;
but if you wish to attempt it, procure a large skull which has not been
dried, and boil it in water. Carefully separate, by means of a scalpel,
most of the investing bones (p. 43) ; the palatines, pterygoids and
quadratojugals, and the dentaries and angulosplenials cannot well be
disarticulated without destroying the soft cartilaginous parts beneath
them.

Make out — I. The brain-case and \\sfontanelles and nerve-apertures
2. The olfactory capsules. 3. The auditory capsules. 4. The palato-
quadrate bar (to which the palatine, pterygoid, and quadratojugal bones
have been left attached). 5. The mandibular or MeckeFs cartilage
(to which the angulosplenial and dentary have been left attached).
6. The cartilage-bones (exoccipitah ', pro-otics, sphtnethmoid, and mento-
meckelians]. 7. The columella, stapes, and fenestra ovalis. Sketch
from above and from below.

Now proceed to examine the appendicular skeleton (pp. 46 — 52),
and sketch the shoulder-girdle and fore-limb, and the hip-girdle and
hind-limb.

Split one of the longer limb-bones (e.g., humerus or femur)
longitudinally with a knife, and note the marrow-cavity in the shaft
(Fig. 13). Place another of the long bones in 10 per cent, hydro-
chloric acid for an hour or two ; wash thoroughly in water and
examine.

CHAPTER IV

THE FROG (continued] : THE JOINTS AND MUSCLES

IN the previous chapter the bones — more than 150 in
number — which together constitute the greater part of the
skeleton of the frog, have been considered as so many
separate parts, fitting into or against one another in certain
ways. We must now see how they are joined together in
the entire animal so as to afford the requisite support, and,
at the same time, to allow of free movement.

The Hip-joint. — Let us begin by a study of the hip-joint

(Fig- 15)-

The acetabulum (actb\ as you have already seen (p. 50),
is a hemispherical depression on the outer surface of the
hip-girdle. It is formed of cartilage, continued into a pro-
jecting rim round the edge of the cavity. The head of the
femur (hd) is also formed of cartilage, and fits accurately
but rather loosely in the acetabulum.

The acetabulum is lined, and the head of the femur is
covered, by a thin layer of connective tissue, the perichon-
drium (chd\ which, in both cases, is continued on to the
adjacent bone, where it receives the name of periosteum
(p. ost}.

Attached all round the rim of the acetabulum is a strong
sheet of connective tissue called the capsular ligament

56 THE FROG CHAP.

(cps. lg\ forming a short, fibrous tube. The other end of
this tube is fixed to the femur, just below the head, the
ligament being continuous, in each case, with the perichon-
drium. There is thus a space between the head of the
thigh bone and the acetabulum, closed all round by the
capsular ligament. This space is filled with a delicate, fibrous,
closed bag, the synovial capsule (sy. cps}, one side of which

med

chct actb

FIG. 15. — Horizontal section of the Frog's hip-joint.

cictb. acetabulum ; chd. perichondrium ; cps. Ig. capsular ligament ; lid. head of
femur ; il. ilium ; med. marrow ; p. ost. periosteum ; pu. pubis ; sh. shaft of
femur ; sy. cps. synovial capsule.

fits closely into the acetabulum, while the other as closely
invests the head of the femur. The capsule is filled with a
watery fluid, the synovia^ and thus forms a buffer or water-
cushion between the adjacent bones. Thus the synovial
capsule keeps the two bones slightly apart and prevents
friction, while the capsular ligament keeps them together
and prevents dislocation.

It is obvious that, in such a joint as this, movement is
possible in all directions. The femur can be inclined either
upwards, downwards, or sideways, and is capable of a certain
amount of rotation. The joint is, in fact, a cup-and-ball

TV JOINTS 57

joint) and is capable of movement in any plane. A similar
biit less perfect cup-and-ball joint is that of the shoulder,
in which the cup is furnished by the glenoid cavity, the ball
by the head of the humerus.

Other Joints. — The elbow- and knee-joints are constructed
on the same general plan, but, owing partly to the form of
the adjacent surfaces, partly to the mode of attach-
ment of the ligaments, they are capable of movement
in one plane only, /.*., up and down, but not from side to
side. They are therefore distinguished as hinge-joints.

The vertebrae are connected with one another in a similar
way. Between the convex hinder face of one centrum and
the concave front face of its successor is a synovial capsule,
and the two centra are bound together by ligament, a
shallow cup-and-ball joint, with a very limited range of
movement, being produced. There are also synovial
capsules between the articular processes, which, being in
contact with one another by flat surfaces and working mainly
from side to side, form gliding-joints. Strong ligaments
connect the neural arches with one another and join the first
vertebra to the skull.

In all cases where free movement is necessary the
joints are formed in the same way ; the bones are bound
together by ligaments, and a synovial capsule is interposed
between their adjacent surfaces. Where little or no move-
ment is required, as between the bones of the shoulder- and
hip-girdles, the union is effected by cartilage or ligament
only, and there is no synovial capsule. Such joints are
therefore distinguished as immovable or imperfect joints.

The Muscles. — We see then that the bones of the skeleton
are attached to or articulated with one another by means
of ligaments, so arranged, in most cases, as to allow of more
or less free movement between the bones. We must now

cc.st

FIG. 16. — The muscles of the Frog from the ventral aspect. On the left side (right
of figure) many of the superficial muscles have been cut and reflected to show
the deep layer.

add, brev. adductor brevis : add. long, adductor longus ; add. mas:, adductor mag-
nus ; del. deltoid ; ext. cr. extensor cruris ; ext. trs. extensor farsi ; FE. femur ;

CHAP, iv MUSCLES 59

hyoid ; pet. pectoralis ; pctn. pectineus ; per. peronaeus ; ret. abd. rectus ab-
dominis ; rect. int. maj. rectus internus major ; red. int. min.^ rectus internus
minor ; sar. sartonus ; sb. mt. submentahs ; sem. ten. semitendinosus ; tib.
ant. tibialis anticus ; tib. post, tibialis posticus ; TI. FI. tibio-fibula ; vast. int.
vastus internus ; x. st. xiphisternum. (From Parker and Haswell's Zoology.}

try to find out how the movements are effected in the
living frog.

It was pointed out in the second chapter that the
flesh is made up of distinct bands or sheets, the muscles ','
some of which came under your notice in your first dissec-
tion. It is quite easy to convince yourself that the whole of
the flesh has this character by skinning a frog and carefully
removing the fascia (p. 18) which covers the muscles and
the more delicate web of connective tissue which forms a sort
of packing substance between them. After noticing some
of the muscles shown in Fig. 16, especially those of the leg,
give your attention to the muscle marked gstr, a prominent
spindle-shaped mass of flesh forming the calf of the leg, and
known as the gastrocnemius (gstr).

The spindle-shaped, fleshy mass or belly of this muscle is
continued at either end into a band of strong, tough con-
nective tissue, the tendon (Fig. 17). The tendon at the
proximal end is flat, and is attached to the distal end of
the femur and to the proximal end of the tibio-fibula, in
each case becoming continuous with the periosteum of the
bone. The tendon at the distal end has the form of a stout
cord, and is distinguished as the tendo Achillis ; it corresponds
with the strong tendon just above the heel of the human
foot. At its distal end it is continued into a broad sheet of
connective tissue, the plantar fascia, which spreads over the
whole sole or plantar surface of the foot.

If the foot is bent upon the shank as in the ordinary
sitting position of the frog, and the gastrocnemius pulled

60 THE FROG CHAP.

upwards or towards the thigh, the foot will instantly be
bent backwards, so as to come into a straight line with
the shank, the action being one of those performed by the
living frog when leaping. It will be seen that the proximal
tendon is attached to a relatively fixed point : it is
distinguished as the tendon of origin^ or the muscle is
said to arise from the femur and tibio-fibula. The distal
tendon is attached to a relatively movable part, the foot,
and is, called the tendon of insertion, the muscle being said
to be inserted into the plantar fascia.

Muscular Contraction. — Obviously, however, there is
nothing to pull upon the muscle from outside in the living
frog. We must, therefore, try to form some idea as to
how the action of bending the foot, roughly imitated in the
dead subject, is performed during life. If the gastro-
cnemius be exposed in a recently killed frog, the foot bent
up as before, and a smart pinch be given to the belly of
the gastrocnemius, the foot will be bent back, although
no pull has been exerted on the muscle. The same thing
will happen if you drop on the gastrocnemius a single drop
of weak acid or of a strong solution of common salt, or if
you touch it with a hot wire, or if you apply to it the
electrodes from an induction coil so as to pass an electric
current through it.

Careful observation shows that what happens under either
of these circumstances is, that the belly of the muscle
decreases in length and at the same time increases in
breadth, so as to become shorter and thicker (Fig. 17).
The result of this must necessarily be to cause its two ends to
approach one another. As the tendon of origin is attached
to the femur, which we suppose to be fixed, it is unable to
move, and the insertion is therefore drawn upwards, bringing
with it the movably articulated foot. In fact exactly the same

MUSCULAR CONTRACTION

61

thing takes place as when we raise our own fore-arm. This
action is performed by means of the biceps muscle which
arises from the scapula and is inserted into the fore-arm.
When the latter is raised we feel a lump rise on the front of
the upper arm due to the thickening of the biceps.

FIG. 17. — Diagram of apparatus for demonstrating the contraction of the gastro-

cnemius muscle.

A, upright, bearing two adjustable horizontal arms. To the upper of these (B) is fixed
by a clamp the femur (fe), having the gastrocnemius (gstr) in connection with
it. To the lower arm (C) is fixed a light lever (L) movable in a vertical plane,
and having the tendon of insertion of the muscle attached to it by a thread.
The dotted lines show the form of the gastrocnemius and the position of the lever
during contraction of the muscle, sc. nv. the sciatic nerve.

This shortening and thickening of the muscle is termed
a contraction. Do not fail to notice that this word is used
in a special sense. When we say that a red-hot bar of iron
contracts on cooling, we mean that it becomes smaller in all
dimensions — undergoes an actual decrease in bulk. But in
muscular contraction there is no alteration in bulk : the

62 THE FROG CHAP.

decrease in length is balanced by an increase in thickness,
as when a stretched piece of india-rubber is relaxed.

The external influence by which a contraction is induced
is called a stimulus. As we have seen, a stimulus may be
produced by actual contact of some external object
(mechanical stimulus}, or by chemical action (chemical
stimulus), or by heat (thermal stimulus}, or by an electrical
current (electrical stimulus).

Relation of Muscle and Nerve. — Evidently, however, we
have by no means got to the bottom of the matter. In the
living frog movements are always going on, and are all due
to the contraction of muscles, and yet no stimuli of the
kind enumerated are applied to any of them. As the
muscles retain the power of contraction for some little time
after the death of the animal, it is easy to make such experi-
ments as that described in the next paragraph.

Running longitudinally between the muscles on the dorsal
side of the thigh is a shining, white cord, the sciatic netve
(Fig. 17, sc.nv\ accompanied by a vein: it gives off
branches to the muscles and skin, and, amongst others, one
to the gastrocnemius. If, when quite fresh, this nerve be
carefully separated as it traverses the thigh and pinched with
the forceps, the gastrocnemius will contract just as if the
stimulus had been applied to it directly, and the same will
happen if a chemical, thermal, or electrical stimulus be
applied.

Thus a stimulus applied to the nerve of a muscle has the
same effect as if applied to the muscle directly : it gives rise
to a nervous impulse, which, travelling along the nerve,
induces contraction of the muscle.

Once more, however, external stimuli are not applied to
the frog's nerves during life, and it is obvious that we must
carry our inquiry a little further. The sciatic nerve if traced
upwards will be found to pass into the trunk (Fig, 51, Sci\

iv MUSCULAR SYSTEM 63

and finally to join the spinal cord, which, as we have seen,
is in connection with the brain. In the living frog nervous
impulses originate in the brain, without the direct interven-
tion of an external stimulus, and are conducted along the
cord and nerves to the muscles. But further consideration
of this subject must be deferred until we have made a
special study of the nervous system.

The Muscular System in General. — All over the body
the muscles, though varying greatly in form — some being
elongated and band-like (Fig. 16, sar\ others spindle-shaped
(gastr), others in the form of broad flat sheets (my. hy, obi.
exf) — have the same general relation to the skeleton as in
the case of the gastrocnemius. Each muscle arises or has
its origin in a relatively fixed part, and is inserted into a
relatively movable part. As each muscle contracts in one
direction only, it follows that the more complex the move-
ments any part is capable of performing, the more numerous
must be its muscles. For instance, the femur, which, as we
have seen, is capable of universal movement, has no fewer
than nine muscles, arising from various parts of the hip-girdle,
inserted into it. Even the minute phalanges of the fingers
and toes all have their little slips of muscle by which the
various movements of grasping and relaxing, approximating
and separating the digits, are effected.

There are certain terms applied to muscles which it is useful to know.
A muscle which raises a part, e.g., the lower jaw, is called a levator,
one which lowers a part a depressor. A muscle which serves to straighten
one part upon another, e.g. , to bring the shank into line with the thigh,
is an extensor ; one which bends one part on another is a flexor. A
muscle which draws a limb towards the trunk is an adductor •, one which
draws it away && abduct or > one which rotates one part upon another, e.g.,
the femur on the pelvis, a rotator.

Thus all the complex and accurately adjusted movements
of the frog are performed by the contraction of its numerous

64 THE FROG

CHAP.

muscles, acting either singly or in concert. The contractions
of these muscles are brought about by nervous impulses sent
from the brain or spinal cord along nerves which branch
out and are distributed to the muscles, thus bringing the
whole of the complex machinery which affects the movements
of the animal under the direct control of its will.

PRACTICAL DIRECTIONS.
The Joints and Ligaments.

1. The hip-joint. Strip off most of the muscles from the thigh and
the adjacent parts of the pelvis ; decalcify in weak acid and then wash
thoroughly. Cut the femur through lengthwise and continue the section
through the pelvic girdle (Fig. 15). The details are more easily made
out if the femur of a larger animal (e.g., rabbit) be used.

Note : a. The cartilage of the acetabulum and head of the femur ;
the perichondrium and periosteum, (b] The capsular hgament and its
relations to the synovial capsule. Observe that the hip-joint is a cup-
and-ball joint. Sketch.

In a prepared wet skeleton —

2. Examine and compare a hinge-joint (e.g., elbow or knee).

3. Examine the cartilaginous union between the bones of the shoulder-
or hip-girdle (immovable or imperfect joints).

4. Examine the joints and ligaments of the vertebral column.

The Muscles.

1. Remove the skin from part of the body and legs of a preserved
frog (the one you have already dissected will do): Then clear away the
fascia here and there and separate some of the muscles by dissecting
away the connective tissue which binds them together. Notice the
different forms of the muscles in different parts (Fig. 16 and p. 63).

2. In the hind-leg carefully dissect away the connective tissue
investing the gastrocnemius muscle (Figs. 16 and 17), and trace it
upwards towards the thigh, and downwards towards the foot, cutting
away any of the other muscles which obscure the view. Notice the
belly i and the tendons of origin and insertion (p. 60). Sketch.

Bend the foot upon the shank as in the ordinary sitting position of the
frog. Hold the thigh firmly with one hand, and with the other take hold

iv PRACTICAL DIRECTIONS 65

of the gastrocnemius and pull it upwards or towards the thigh. Note
the result.

3. In a freshly killed frog (see p. 103) expose the gastrocnemius as
directed above, and with the small forceps give a sharp pinch to the belly
of the muscle. Note the contraction following the stimulus.

Then remove the skin on the dorsal side of the thigh, and separate
the muscles in this region so as to expose the sciatic nerve (Figs. 17 and
51). Trace this towards the shank and notice its branch going to the
gastrocnemius. Carefully separate the nerve as it traverses the thigh
and pinch it with the forceps, noting again the contraction following
the stimulus.

PRACT Zooi

CHAPTER V.
THE FROG (continued} : WASTE AND REPAIR OF SUBSTANCE —

THE DIGESTIVE ORGANS — NUTRITION.

Waste and Repair, — The effects of prolonged muscular
exertion are familiar to every one. Sooner or later sensa-
tions of fatigue, hunger, and thirst are produced, accom-
panied by a loss of weight. Indeed, however little exertion
we make and however often we feed, our weight always goes
down between meals and rises again when we take food.
The loss of substance, of which the diminution in weight
is the index, takes place largely in the form of perspiration,
a fluid consisting of water, with certain organic and inorganic
matters in solution. A further loss is due to the air
breathed out from the lungs ; this is always moist, i.e.,
contains a good deal of water, and is further distinguished
by containing a considerable volume of the gas carbon
dioxide, or carbonic acid (CO2). Besides these two constant
sources of loss, there is an intermittent loss in the urine,
which consists of water, containing certain matters in
solution, the most characteristic of which are two complex
substances called urea (CON2H4) and uric acid (C5H4N4O3).
Both of these, as well as carbon dioxide, act as poisons if

CHAP, v WASTE AND REPAIR 67

allowed to remain in the system. Lastly there is an inter-
mittent source of loss in the waste matters or faeces which
are passed out from the intestine.

These losses are made good in two ways. Firstly, by
breathing, in which process we constantly inhale pure air
and replace the poisonous carbonic acid by oxygen. Secondly
by eating and drinking, by which, at intervals, we make good
the loss of solids and liquids. Just as a clock is constantly
running down and has to be wound up in order to keep
it going, so our bodies run down by loss of substance
between meals, and require to be woun d up by the repair of
substance which results from food and drink.

The same thing is true of the frog. Every one of its
numerous and often vigorous movements is done at the
expense of a certain waste of substance. The various
tissues of the body are constantly undergoing a process of
wear and tear, expressed, not as in machines of human
construction, by a wearing away of surfaces and a loosening
of bolts and screws, but by a slow and almost imperceptible
dwindling, the lost material being carried off principally in
the form of water, carbon dioxide, and urea or some allied
compound containing nitrogen.

Food of the Frog : general characteristics of the Diges-
tive Process. — As we have seen, the food of the frog
consists of worms, slugs, insects, and the like. These it
catches and swallows whole, the stomach often becoming
immensely distended with numbers of captured animals.
After remaining for some time in the stomach the carcases
are found to have undergone a marked change. Their soft
parts become softer and slimy and finally semi-fluid, and in
this way the food undergoes gradual disintegration. The
quantity of food in the stomach decreases, some of it is
passed into the intestine, which it traverses from duodenum

F 2

68 THE FROG CHAP.

to rectum, and certain portions of it are finally ejected from
the vent in the form of faeces.

It is not difficult to assure one's self that the weight of the
faeces passed during a certain time is very much less than
that of the food swallowed during the same time. Obviously
some constituents of the food have disappeared during its
progress through the enteric canal. The character of the
faecal matter, moreover, is very different from that of the food;
the only portions of the swallowed animals discoverable
in the rectum are bits of their hard parts ; for the rest, the
faeces form a pulpy, black mass. That this change is due
to certain definite chemical processes taking place in the
enteric canal may be inferred from the fact that the contents
of the stomach, as well as the walls of that organ, have
an acid reaction, and turn blue litmus paper red. On
the other hand the contents of the small intestine are,
to a greater or less extent, alkaline, restoring reddened
litmus paper to its original blue colour.

It is also obvious that there must be some definite
mechanism for propelling the food from one end of the
enteric canal to the other; its passage through so long,
narrow, and coiled a tube can certainly not be accounted
for by supposing it to be merely pushed onwards as fresh
food is swallowed.

In order to understand the various processes connected
with digestion we must make a renewed and more careful
examination of the organs concerned, after removing them
from the body.

The Digestive Organs. — Arising from the gall-bladder
and passing backwards to the duodenum is an extremely
delicate tube (Fig. 18, DC, Dc^\ the common bile-duct, which
opens into the duodenum. By gently squeezing the gall-
bladder a drop of greenish fluid may be made to ooze out

DIGESTIVE ORGANS

69

of the end of the duct (Dc*) into the intestine ; this fluid is
the bile.

Very careful dissection shows that the common bile-duct

FIG. 18. — Stomach and duodenum of Frog, with liver and pancreas.
DC, Dc± common bile duct ; Dc.'2 its opening into the duodenum ; D. cy. cystic
ducts ; Dh.t Dh.^ hepatic ducts ; Du. duodenum ; G. gall-bladder ; L, L1, L'*, L%,
lobes of liver, turned forwards ; Lhp. duodeno-hepatic omentum, a sheet of
-eritoneum connecting the liver with the duodenum ; M. stomach ; P. pancreas ;
>*, pancreatic duct ; Py. pylorus. (From Wiedersheim's Comparative

Anatomy.)

is joined by several other tubes which are traceable into the
liver and are called hepatic ducts (Dh, Dh1). The tubes
going to the gall-bladder are called cystic ducts (D.cy) ; by

70 THE FROG CHAP.

their union with the hepatic ducts the common bile-duct is
formed.

The bile is manufactured in the liver, and the gall-bladder
is merely a reservoir in which it is stored up, to be
discharged into the intestine when required for digestion.

It requires still more accurate observation to show that
the pancreas also discharges a fluid, \^\Q pancreatic juice, into
the intestine. A very delicate branching tube, the pan-
creatic duct (Pl), joins the bile-duct, into which it discharges
the pancreatic juice, the two fluids entering the intestine
together. Both fluids are digestive juices, i.e., liquids which,
as we shall see, act in a particular way upon the food.

By cutting open the enteric canal and examining its inner
surface under water with a magnifying glass, it is seen that the
wall of the canal consists of two layers, easily separable
from one another. The outer or muscular layer (Fig. 19 A,
muse], is tough and strong, the inner layer or mucous
membrane (m.m), is soft and slimy. Between the two is
very loose connective tissue, the submucosa — really a part of
the mucous membrane — which, being easily torn, allows of
the ready separation of the muscular and mucous layers.

In the stomach the mucous membrane is raised into
longitudinal folds (r\ in order to allow of distension ; in an
empty stomach these are well marked, and give the cavity a
star-like cross-section (Fig. 40) ; in one full of food they are
entirely obliterated and the walls of the organ so stretched
as to be almost transparent. Anteriorly the ridges thin out
and disappear at the cardia or junction between the gullet
and stomach ; posteriorly they converge, as the stomach nar-
rows, towards the pylorus (Fig. 19 A,/JJ>), or junction with the
duodenum. Here the muscular coat is greatly thickened in
a ring-like form, forming the pyloric valve (py.v}, by which
the aperture of communication between the stomach and

DIGESTIVE ORGANS

intestine is greatly narrowed, so that only small particles
can pass through. In the duodenum (dii) the mucous mem-
brane is raised into little tuft-like elevations (r) • in the ileum
the ridges (B, r") become longitudinal again ; in the rectum
(ret) they are absent. Another ring-like muscle, or sphincter,
is present round the vent.

When food is taken into the stomach, a fluid, the gastric
mice, oozes from the mucous membrane. It is this fluid

ret

FIG. 19. — Portions of the enteric canal of the Frog in longitudinal section.
A, stomach and duodenum ; B, part of ileum and rectum, du. duodenum ; il. ileum ;
m. m. mucous membrane J mrisc. muscular layer ; py. pylorus ; py. v. pyloric
valve ; r. longitudinal ridges (rugae) of stomach ; r'. transverse ridges of duo-
denum ; r". longitudinal ridges of ileum ; ret. rectum ; st. stomach.

which reduces the slugs, insects, etc., to the pulpy condition
referred to above : it is, like the bile and pancreatic fluid,
a digestive juice.

General Properties of Food,— We must now devote a
' little attention to the characters of the food itself and to the
precise nature of the changes brought about by the digestive
process.

72 THE FROG

CHAP.

As we have seen, the frog is a carnivorous animal Now
the digestible part of the substance of animals consists
mainly of two classes of chemical compounds, called proteids
and fats. The most familiar example of a proteid is white
of egg : other proteids, of varying composition, are found in
muscle, in blood, and in other animal tissues. All are
composed of the five chemical elements — carbon, oxygen,
hydrogen, nitrogen, and sulphur— the five elements being
combined in the following proportions : —

Carbon - - from 51-5 to 54-5 per cent
Hydrogen „ 6-9 „ 7-3 „

Oxygen - - „ 20-9 „ 23*5 „
Nitrogen „ 15-2 „ 17-0

Sulphur „ 0-3 „ 2-0

Fats differ from proteids in containing no nitrogen or
sulphur : they are formed of carbon, oxygen, and hydrogen,
the number of atoms of hydrogen being always more than
twice as great as the number of atoms of oxygen.

It will be noticed that two important articles of diet are
absent from the above list, namely sugar and starch — the
latter the largest constituent of flour, oatmeal, rice, &c.
The vegetable substances used as food by animals, such as
corn and grass, contain these bodies in varying proportions
in addition to vegetable proteids, and there is no doubt
that the frog must eat a small quantity of such vegetable
food, if only in the stomachs of the herbivorous animals upon
which it preys. Now starch and sugar belong to a group of
compounds called carbohydrates^ composed of carbon,
oxygen, and hydrogen, but differing from fats in that the
number of atoms of hydrogen is always exactly double that
of the atoms of oxygen, as in water. Lastly, the food always
contains a certain quantity of saline or mineral matters, as
well as water.

v FOODS 73

Diffusible and Non-diffusible Foods, — These four classes
of food materials — proteids, fats, carbohydrates, and
minerals — may be arranged in two groups according to a
certain physical peculiarity. If a solution of common salt
is placed in a vessel with a bottom made of bladder, called a
dialyser, which is floated in a larger vessel of pure water, it
is found that, after a certain lapse of time, the water in the
outer vessel has become salt. The sodium chloride has, in
fact, passed by diffusion through the bladder. The same
thing will happen if a solution of sugar is placed in the inner
vessel : salt and sugar are both diffusible substances, capable
of passing through an animal membrane.

On the other hand, if the inner vessel contains white of
egg, or oil, or starch well boiled in water, no diffusion takes
place. Hence proteids, fats, and starch are non-diffusible
foods, and are thus sharply distinguished from salt and
sugar, which are diffusible.

The mucous membrane of the stomach and intestine are
animal membranes having the same physical properties as
bladder. We may consequently infer that any salt or sugar
contained in the enteric canal will diffuse through the mucous
membrane and make its way, as we shall see more particularly
hereafter, into the blood, thus serving to nourish the whole
body, Proteids, fats, and starch, on the other hand, will
be incapable of diffusing, and will, therefore, unless
some change happens to them, be absolutely useless
as nutriment. For, since the enteric canal communi-
cates with the outer world at both ends, the food, para-
doxical as it may sound, is practically outside the body
as long as it remains in the canal : it is only when
it is absorbed into the blood or lymph that it is ac-
tually, in the strict sense, taken into the body. Thus
if proteids, fats, and starch are to be of any use to the

,74 THE FROG CHAP.

frog, they must, in some way, be rendered capable of
being absorbed.

Action of Digestive Juices,— This is exactly what is done
by the digestive juices. If white of egg or any other proteid
is mixed with gastric juice and kept at a suitable tem-
perature, it is converted into a form of proteid called
peptone, which is capable of diffusing through an animal
membrane. The change is effected by means of a substance
called pepsin, contained in the gastric juice, in which there
is also a certain proportion of hydrochloric acid. To this
the acid reaction of the gastric juice already alluded to (p.
68) is due.

By means of the gastric juice the bodies of the animals
swallowed by the frog have their proteids largely converted
into peptones, which, being diffusible, pass through the
mucous membrane as readily as sugar or salt. Hence
the great diminution in the bulk of the food during its
sojourn in the stomach : a large proportion of it is absorbed
there and then, and only a comparatively small quantity
is passed through the pyloric valve into the intestine, where
it becomes alkaline, owing to the action of the fluid which
enters the duodenum through the bile-duct, and which,
as we have seen (p. 70), consists of bile and pancreatic
juice.

Pancreatic juice has a similar effect on proteids, the
change being effected by a substance called trypsin, which,
however, acts in a alkaline solution. It also has the
property of converting starch into sugar, and of splitting
up fats into fatty acids and glycerine, both of which are
diffusible. The substances by which these changes are
effected are called by the general name of ferments :
pepsin and trypsin are proteolytic or proteid-converting
ferments, and the pancreatic juice also contains an amylolytic

v DIGESTION 75,

ferment, which converts starch into sugar, and a fat-
decomposing ferment.

The exact mode of absorption of the fats is not thoroughly
understood. It is usually supposed that only a small pro-
portion of them are decomposed into fatty acids and gly-
cerine, and that the greater part is merely broken up into
particles so small that they can be taken up by the epi-
thelial cells of the intestine. This emulsification of fat is
effected by the combined action of the pancreatic juice and
bile, the fats being reduced to the condition in which they
exist in milk and in the emulsions of cod-liver oil so much
used in place of the natural form of that medicine.

Thus during the passage of the food through the intestine
the remainder of the proteids, the whole of the fats, and any
starch which may be present, are rendered capable of
being absorbed : they pass through the mucous membrane
into the blood, and by the time the rectum is reached all
the nutriment is extracted from the food, and there re-
mains only a small quantity of indigestible matter, which is
passed out in the form of faeces.

Peristaltic Movements. — The passage of the food
through the enteric canal is effected by the contraction of its
muscular layer, which is really double, and which is composed
of muscular fibres (see Chapter VII, and Figs. 39, 40). In the
inner layer these fibres have a transverse direction, encircling
the tube, and by their contraction narrowing it ; the outer
layer consists of longitudinal fibres, which by their contraction
shorten it. By the alternate contraction and relaxation of
the two layers are produced a series of peristaltic movements,
not unlike those by which an earthworm makes its way over
the ground : they can be seen in a freshly-killed frog, and
still better in a rabbit or rat.

Summary of Chapter.— The body is constantly under-

76 THE FROG CHAP.

going waste, and in consequence needs continual repair.
The waste-matters chiefly take the form of carbon dioxide,
water, and urea. Repair is effected partly by breathing (see
Chapter ix), partly by feeding. Food, consisting of proteids,
fats, carbohydrates, inorganic substances, as well as water, is
taken into the enteric canal, where, by the action of the three
digestive juices — gastric juice, pancreatic juice, and bile — it
is converted partly into a solution (peptones, sugar, fatty acids,
glycerine), partly into an emulsion (fats). As it is driven
along the canal, from stomach to rectum, by the action of
the muscular coat, the dissolved or emulsified substances
gradually disappear from the canal, and are absorbed into
the system. Finally, the indigestible constituents are ex-
pelled as faeces.

Our next task must be to learn something of the process
of absorption, and of the means by which the digested food
is conveyed to the various parts of the body, so as to supply
them with the means of repairing the waste they are
constantly undergoing. For this purpose we must now
study what is called the vascular system, i.e. the heart, the
blood-vessels, and the various cavities containing lymph.

PRACTICAL DIRECTIONS.

The Digestive Organs- — Pin down under water, with the ventral
side uppermost, as before, the specimen already dissected, or another in
which the body- cavity has been opened in the same way. Note the
positions of the postcaval' vet 'n (Figs. 3 and 4, //. cv), the hepatic portal
vein (Fig. 3, hp. pt], the aorta (Fig. 4, d. ao], and the splanchnic or
cceliaco-mesenteric arlery (Figs. 3 and 4, cosl. mes). Then, taking care
not to injure the aorta, remove the greater part of the digestive organs,
including the liver, by cutting through the gullet and rectum and severing
the mesentery, cutting through the postcaval also where it enters the liver.
(The cloaca will be examined at a later stage). Pin the organs in the dis-
secting dish as nearly as possible in their natural position. Turn the

v PRACTICAL DIRECTIONS 77

lobes of the liver forwards (i.e., towards the head) and after making out
the relations of the parts already examined in situ (pp. 20 — 23), note —

The common bile-duct, formed by the union of the hepatic and cystic
ducts, and the point at which it enters the duodenum (Fig. 18). Make
a small slit in the duodenum just opposite its entrance, and gently squeeze
the gall-bladder between your finger and thumb, so as to force a drop of
bile into the intestine. (The pancreatic duct and its communication with
the bile-duct cannot easily be made out by dissection). Sketch the whole
dissection.

Now remove the liver, unravel the intestine by tearing through the
mesentery, and lay open the enteric canal by inserting one blade
of the scissors into the gullet, and cutting through the whole tube in a
longitudinal direction. Test the contents of the stomach and intestine
with litmus paper — (for this experiment a freshly killed frog is of course
necessary) — and then pin out your dissection with the inner surface
upwards, wash it under the tap, and examine under water with a
magnifying glass. Make out —

1 . The cardia, pylorus, and pyloric valve.

2. The mucous membrane, and its different appearances in the stomach,
small intestine, and large intestine.

3. The muscular layer, covered externally by the peritoneum.
Make a simple dialyser (p. 73) by tying a piece of wet bladder firmly

over one end of a wide glass tube about six inches long. Into this put a
solution of sugar or salt, and immerse the tube up to the level of the
solution in a rather larger vessel of distilled water, and leave it for a
short time : taste the water in the outer vessel. Then place some white
of egg in the dialyser, and test for albumen by heating some of the water
in the outer vessel over a flame : if albumen is present, it will become
coagulated and form a cloud in the water.

CHAPTER VI.

THE FROG (continued] : THE VASCULAR SYSTEM — THE CIR-
CULATION OF THE BLOOD.

IN our preliminary examination of the frog (Chapter II)
we learned one or two facts about the vascular system. We
found that there is a heart within a pericardium, two sets of
vessels, arteries and veins, containing red blood, and a set
of irregular cavities or sinuses, containing lymph. We must
now try to get some more accurate and detailed information
on these matters.

General Characteristics of Blood and Lymph. — It will
be convenient to begin by studying certain easily verified
characteristics of the blood. Frog's blood may be used, or
as it is as well to have a considerable quantity, that of some
larger, freshly-killed, red-blooded animal, such as a rat or
rabbit.

When first drawn from the heart or vessels the blood will
be seen to be a fluid, nearly as mobile as water or milk ;
it " finds its level," like any other liquid, and can be readily
poured from one vessel to another. In a few minutes,
however, it undergoes a change ; it ceases to be fluid, and
coagulates, or " sets " into a jelly, which if turned out of the
vessel, retains the shape of the latter. Before long a further
change takes place ; the jelly begins to shrink, drops of

CHAP, vi HEART 79

yellowish fluid appear on its surface and gradually run to-
gether into larger and larger drops. The jelly contracts still
further, and finally draws itself away from the walls of the
vessel and floats in the accumulated fluid, still retaining the
form of the vessel, but being greatly reduced in size. The
process of coagulation of the blood is now complete ; the
red, jelly-like substance is called the clot, the yellowish fluid
the serum.

When first drawn from most veins the blood is deep purple
in colour, and the clot retains for a time the same hue.
But before long all parts of it which are fully exposed to the air
take on a bright scarlet colour. We may therefore distinguish
between red, or aerated, and purple, "or non-aerated blood.

Lymph also coagulates on standing, producing a colourless
clot. It is practically blood minus its peculiar red colouring
matter, the properties of which, as well as the real nature of
coagulation, will be discussed in the next chapter.

The Heart : external characters. — Some of the divisions
of the heart have already been noticed (p. 20). The ventricle
(Figs. 3, 4, 7, 20,2 1 and 22, v, vl) is a conical body of a pinkish
colour, having its bluntly-pointed apex directed backwards.
To its broad base is attached the dark-coloured, thin -walled
auricular division, actually consisting of two chambers, the
right and left auricles (r. an, L au\ but appearing single in
the entire heart. Passing obliquely across the auricles is a
cylindrical structure, the conus arteriosus (c. art] ; it starts
from the right side of the base of the ventricle, and passes
forwards and to the left, finally dividing near the anterior
boundary of the auricles into two branches, which extend
respectively right and left.

By lifting up the ventricle, or turning it to one side (Figs.
3 and 4), there is seen in the dorsal part of the pericardial
cavity a thin- walled chamber (Fig. 2i,s.v) of a dark colour, con-

8o THE FROG CHAP, vt

nected with the right side of the auricular division. This is
the sinus venosus.

The Arteries. — The two branches of the conus arteriosus
just referred to soon branch again. Each divides into three
vessels, often spoken of as arterial arches, called respectively
the carotid trunk (Fig. 20, car. tr.) the systemic trunk (syst. tr)
and the pulmo-cutaneous trunk (put. cu. tr). All these con-
form to the definition of an artery given on p. 27, i.e.,
they are stout, elastic vessels, containing little blood after
death, and not collapsing when empty.

The carotid trunk divides immediately into two, a lingual
artery (lg\ which can be traced to the tongue, and a
carotid artery (car), which branches repeatedly, its ultimate
ramifications going to various parts of the head. At the
origin of the carotid is a little rounded mass with a sponge-
like interior, the carotid labyrinth (car. gl).

The systemic or aortic trunk extends outwards, in contact
with the gullet, then sweeps upwards, backwards, and inwards
— /.£., towards the middle line — and finally joins with its
fellow of the opposite side to form a single median vessel,
the dorsal aorta (Figs. 4, 5, and 20, d. ao\ which passes
backwards just beneath the vertebral column and between
the kidneys.

As it sweeps round the gullet, the systemic trunk gives off
a vertebral artery (Fig. 20, vert) to the vertebral column and
part of the head, a subclavian artery (sd\ passing into the
fore-limb as the brachial, and an cesophageal artery (o3S ) to
the gullet.

From the point of union of the two aortic trunks springs
a single splanchnic or cczliaco-mesenteric artery (ca>l. mes) ; it
divides into several branches, which are traceable to the
liver (hp\ stomach (gs\ duodenum (du\ spleen (spl) and
ileum (inf).

The dorsal aorta gives off on either side four renal arteries

sysl.lr
pul.cuZr

sa

FIG

. 20. — The arterial system of the Frog, with the heart, lungs, kidneys and left
spermary, supposed to be removed from the body and viewed from the ventral
aspect.

car. carotid artery ; car.gl. carotid labyrinth ; \c. art. conus arteriosus ; car. tr.
carotid trunk ; ccel. mes. splanchnic or coeliaco-mesenteric artery ; cu. cutaneous
artery ; d. ao. dorsal aorta ; du. duodenal artery ; gs. gastric artery ; hp. hepatic
irtery ; //. iliac artery ; int. intestinal arteries ; kd. kidney ; /. au. left auricle ;

82 THE FROG CHAP, vi

(rn) to the kidneys, and spermatic arteries (spni) in the male, or
ovarian in the female, to the reproductive organs, and at the
posterior 'end of the abdominal cavity divides into the
right and left iliac arteries (it) which go to the hind-limbs.

The pulmo-cutaneous trunk divides into two main
branches, the pulmonary artery (put) which goes to the
lung, and the cutaneous artery (cu) which forms an extensive
. system of branches over the skin.

With proper care all these arteries can be traced into
the various organs to which they are distributed, when they
will be found to branch repeatedly, sending ramifications to
all parts. The iliac artery, for instance, may be followed
along the whole length of the leg, giving off branches to
all the muscles, to the skin, and to the digits, with their
intervening web.

The Veins. — Since every part of the body has its vein
as 'well as its artery, there is a rough correspondence
between the two kinds of blood-vessel. The arrange-
ment of the principal trunks is, however, very different in
the two cases.

On either side of the base of the heart is a large vein
called \he precaval or vena cava anterior (Figs. 3 and 21, pr.
cv) : by turning the ventricle aside, the two precaval veins can
be seen to join the anterior end of the sinus venosus (s.v).
Each precaval is formed by the confluence of several
veins, of which the most important are the external jugu-
lar (ext. ju) from the lower jaw and tongue, the internal jugu-
lar (int. ju] from the brain, eye, etc., and the subclavian (scl),
formed by the union of the brachial, from the fore-limb, and
the musculo-cutaneous (mu. cu\ already noticed, from the
superficial parts of the head and part of the skin and
muscles of the abdomen.

With the posterior end of the sinus venosus is connected
a single large vein, the postcaval or vena cava posterior

pv

FIG. 21. — The venous system of the Frog, with the heart, lungs, liver, kidneys and
right spermary, supposed to be removed from the body and viewed from the dorsal
aspect.

int.ju. internal jugular vein ; kd. kidney ; /. au. left auricle ; Ing. lung ; Ivr.
liver ; MS, cu. musculo-cutaneous vein ; pr. cv. precaval vein ; pt. cv. postcaval
vein ; pul. pulmonary vein ; pj>. pelvic vein ; r. au. right auricle ; rn. renal
veins ; rn. pt. renal portal vein ; sc. sciatic vein ; scl. subclavian vein ; spl.
splenic vein ; spin, spermatic vein ; s. v. sinus venosus ; ts. spermary ; ves. vesical
veins (from bladder). (From Parker and Haswell's Zoology.}

G 2

84 THE FROG CHAP.

(ft. cv\ a wide vessel lying between the kidneys and
extending forwards to the liver (Fig. 4). It runs parallel with
and beneath, i.e., ventral to, the dorsal aorta (Fig. 5), from
which it is at once distinguished by its greater diameter.
Posteriorly it is formed by the confluence of four renal
veins (Fig. 21, rn) from each kidney, and it also receives
in the male, spermatic veins (spni) from the spermaries, and
in the female, ovarian veins from the ovaries. Anteriorly
it perforates -the liver (lvr\ receiving two hepatic veins (lip]
from that organ, and finally enters the sinus venosus.

We have now to consider a striking want of correspondence
between the arterial and venous systems of the frog. As
you will remember, the dorsal aorta, after giving off the
renal arteries, passes backwards and divides into the two
iliac arteries for the legs. You might naturally expect a
somewhat similar arrangement with the veins, especially if
you have studied human physiology and learnt how the
posterior (or inferior) vena cava of man is formed by the
confluence of the veins from the legs, and receives higher
up those from the kidneys. In the frog, as we have just
seen, the postcaval does not reach to the hinder boundary
of the kidneys, and the renal veins are the only vessels
entering it posteriorly.

In the frog, as a matter of fact, the connections of the
veins of the legs are very peculiar. You remember the
abdominal vein seen in our preliminary dissection (Figs. 2,
3 and 4, abd. v). This vessel, if traced backwards, is found
to fork at the posterior end of the abdomen — or more
accurately, the single abdominal vein is formed by the
confluence of two pelvic veins (Fig. 2i,/z;), which can be
traced along the base of the leg (compare Fig. 3). Towards the
front of the thigh is the principal vein of the leg, \hefemoral
vein (Fig. 21, fm), which on emerging from the leg, divides

vi VEINS 85

into two branches. One of these is the pelvic vein already
seen ; it unites with the corresponding vessel of the opposite
side to form the abdominal vein (abd). The other branch of
the femoral is called the renal portal vein (rn.pt) ; it passes
directly forwards, receiving the sciatic vein (sc) from the back
of the leg, and then goes along the outer border of the
kidney, finally branching out in that organ. It also receives
a large vein (ds. Imb) from the muscles of the back.

The abdominal vein receives, near its posterior end,
small veins (ves) from the urinary bladder. It passes
forwards, as already seen, receiving veins from the abdo-
minal muscles, and, quite at its anterior end, a small vein
(cd) from the heart. It then divides into two branches
which enter the liver and branch out in that gland.

The veins from the stomach (gs\ the intestine (du, int\
spleen (spl\ and pancreas run in the mesentery alongside
the corresponding arteries. Near the liver they all unite to
form a large vessel, the hepatic portal vein (hp. pt\ which
enters and branches out into the liver, first sending off a
branch which joins the abdominal.

Thus the veins from all parts of the body, except the
lungs, ultimately discharge into the sinus venosus. The
veins from the lungs are quite singular in their course ; they
unite, in each lung, to form a single pulmonary vein (Figs.
3 and 2 1,/«/), which passes dorsal to the sinus venosus, and
discharges, with its fellow of the opposite side, into the left
auricle.

Character of the Blood in the Arteries and Veins. — There
are certain differences between the arteries and veins in
respect of the blood they contain. As a rule, the veins
contain purple or non-aerated blood, the arteries more or
less thoroughly aerated or scarlet blood. But there are
certain exceptions. As we shall see in a later chapter, the

86 THE FROG CHAP.

blood is aerated in the lungs and skin ; hence the blood
returned from those organs by the pulmonary and musculo-
cutaneous veins is aerated. On the other hand, the blood
in the pulmo cutaneous artery is non-aerated.

Flow of Blood. — We must now try to understand the
function of this complicated blood-system, and the reason
why every part of the body has two vessels, an artery and a
vein. That there is some kind of movement of the blood
has been hinted in the foregoing description, in which
arteries have been described as branching out to various
parts, veins as formed by the confluence of smaller veins
from various parts.

If an artery were cut in a living frog, the blood would
be found to flow out in a series of jerks corresponding with
the -beats of the heart. Moreover the blood would flow
from the side of the cut nearest to the heart, and the flow
might be stopped by tying or compressing the artery on that
side, i.e., between the heart and the cut. Evidently, then,
the blood in the living animal flows from the heart along
the arteries to the various parts of the body, and is propelled
by the pulsation of the heart.

If a vein were cut the result would be very different.
The blood would flow in a comparatively slow stream and
without jerks ; it would flow, moreover, from the side of the
cut furthest from the heart, so that, in order to stop the
bleeding, the vein must be tied or compressed on the far
side of the cut. The blood in the veins flows, therefore,
towards the heart in an even stream, unaffected by the
heart's pulsations.

Thus the blood is driven by the heart to the various parts
of the body through the efferent arteries, and is returned from
the various parts of the body to the heart by the afferent veins.
Two questions thus naturally arise : how is it that the blood

STRUCTURE OF HEART

takes this direction and not the other, and how does it make
its way from the artery of a given organ into the vein ?

Internal Structure of the Heart, — To answer the first
question — why the blood leaves the heart by the arteries

car.gl

syst.tr

C.a,rt

FIG. 22. — The heart of the Frog frc

and returns to it by the veins, and not vice versa — we must
examine the heart itself in some detail.

The ventricle is a hollow structure with thick spongy walls
and a small cavity (Fig. 22, vt\ and there are two perfectly

88 THE FROG CHAP.

distinct auricles, the right (r. au\ considerably larger than the
left (/. au), separated from one another by a vertical partition
(spt. aur).

You have already seen that the conus arises from the
right side — the frog's right, not yours — of the base of the
ventricle. A little to the left of this point there is an
aperture through which a bristle can be passed from the
ventricle into either of the auricles. Both auricles, then,
communicate with the ventricle, by a single auriculo-
ventricular aperture. This is guarded by two little mem-
branous flaps (au. v. v), which spring, one from the dorsal,
one from the ventral edge of the aperture, and hang down
into the ventricle, to the walls of which they are attached
by little tendinous cords, represented in the figure by white
streaks. Thus the flaps have the character of folding doors
or valves opening only one way ; they readily flap backwards,
i.e., into the ventricle, but are prevented from flapping
forwards, or into the auricles, by the tendinous cords
attached to them. The two flaps are the auricula-ventri-
cular valves. Their mode of action is easily understood.
If the auricles, being full of blood, contract and squeeze
themselves together, the pressure will force aside the valves
and allow the blood a free passage into the ventricle. On
the other hand, if the ventricle contracts, the blood, getting
behind the valves, will force them together and close the
aperture, the tendinous cords preventing their being driven
into the auricles by undue pressure.

In the interior of the conus is a longitudinal fold or valve
(/.z>), which traverses it obliquely, attached to its dorsal wall
and free ventrally ; and there are also three little semilunar
or watch-pocket shaped valves (v) guarding the aperture
between the ventricle and the conus : they are arranged with
their edges turned forwards or towards the cavity of the

vi CIRCULATION 89

.conus, so that any pressure of fluid from the side of the
ventricle must force them back and allow ready ingress into
the conus, while pressure in the opposite direction must fill
them, bringing their edges together, and so barring the
passage.

Anteriorly the longitudinal valve projects in the form of
a free flap, and at the same level is a semilunar valve (v'} .
the two together separate the conus proper from a small
chamber, the bulbus aorta, from which the right and left
carotid (a, a) and systemic (<£, b'} trunks arise. The pulmo-
cutaneous trunks (c) spring from the conus by an aperture
(/) situated just posteriorly to the valve v', and itself
guarded by a small valve.

In the dorsal wall of the right auricle is a large transverse
aperture (s. au. ap\ This leads into the sinus venosus : it is
therefore called the sinu- auricular aperture • its two edges
are produced into flaps, the sinu-auricular valves^ which
allow free passage from the sinus to the right auricle, but
prevent any flow in the opposite direction.

Valves of the Veins. — In addition to ihe valves of the
heart, many of the veins contain small watch-pocket valves,
all arranged with their concavities directed towards the
heart, so as to allow of a free passage in that direction. Any
attempt to flow in the opposite direction i.e., from the larger
to the smaller veins, will result in filling the valves, bringing
their edges into contact with the opposite wall of the vein,
and thus effectually blocking the passage.

Circulation of the Blood. — We see then that an inves-
tigation of the structure of the heart shows that fluid can
traverse it in one direction only, viz., from the sinus to the
right auricle, from the auricles to the ventricle, from the
ventricle to the conus, and from the conus to the bulbus
aortae, and so to the arteries. The valves in the veins are so

90 THE FROG CHAP, yi

arranged as to allow the blood in these vessels to flow only,
towards the heart. The experiment of cutting the vessels
shows that the blood in the arteries does actually flow from
the heart, that in the veins towards the heart. We thus
demonstrate that there is not merely a movement but a true
circulation of the blood, the current starting from the heart,
passing by the arteries to all parts of the body, and being
returned to the heart by the veins.

Action of the Heart. — The circulation of the blood is
effected by the pulsation of the heart. This organ is
made of muscle ; each of its cavities is to be considered
as a bag, the walls of which are formed of muscular fibres
crossing one another in various directions and encircling
the cavity. We have seen that when an ordinary spindle-
shaped muscle contracts, its two ends are brought nearer
together. When a hollow muscular bag contracts the effect
will be to squeeze the walls together and so diminish the
cavity. Hence when any chamber of the heart contracts
it must expel a part or the whole of its contained blood.
The contraction of the chambers of the heart takes place in
regular order : first the sinus, then the two auricles together,
then the ventricle, and lastly the conus. The contraction
in each case is visible as a sort of throb and is followed
by a period of rest, during which the chamber regains its
former dimensions.

The course of the blood through the heart will now be
clear. When the sinus (Fig. 23, s.v) contracts, the con-
tained blood, which, coming by the precavals and postcaval,
is non-aerated, is acted upon in all directions and might
therefore be forced either into the three great veins (pr.cv.v,
pt.cv.v) or into the right auricle (r.au). But the veins are
full of blood steadily flowing towards the heart, and any
regurgitation is further prevented by their valves : the right

92 THE FROG CHAP.

or spermary ; Cp. Hd. of head ; Cp. H. L. of hind-limb ; Cp. Kd. of kidney ;
Cp. Lng. of lung ; Cp. Lvr. of liver ; Cp. Pn. of pancreas ; Cp. Sk. of skin ;
Cp. Spl. of spleen ; cu. a. cutaneous artery ; cu. v. cutaneous vein ; Cu. Gl.
cutaneous gland ; d. ao. dorsal aorta ; Ent. C. enteric canal ; Ep. Ent.
epithelium of enteric canal ; Ep. Lng. of lung ; Ep. Sk. of skin ; Ep. Ur. T.
of urinary tubule ; glrn. glomerulus ; il. a. iliac artery ; int. a. artery to
stomach and intestine ; int. v. vein from stomach and intestine ; ju. v.
jugular vein ; hp. a. hepatic artery (to liver) ; hp.pt. v.- hepatic portal vein ; hp. v.
hepatic vein ; /. au. left auricle ; Ln?. lung ; Lvr. C. liver-cells ; ly. cp. lymph
capillaries ; ly. v. lymphatic vessels ; Mlp. Cp. Malpighian capsule ; nst. nephros-
tome ; /. ly. ht. posterior lymph-heart ; Pn. C. cells of pancreas ; Pn. D. pan-
creatic duct ; pr. cv. v. precaval vein ; pt. cv. v. postcaval vein ; pul. a. pul-
- monary artery ; pul. cu. tr. pulmo-cutaneous trunk ; pul. v. pulmonary vein ;
r. au. right auricle; rn.pt. v. renal portal vein; set. a. subclavian artery;
s. c. ly. s. sub-cutaneous lymph-sinus ; scl. v. subclavian vein ; s/>. a. splanchnic
artery ; spl. a. splenic artery ; spl. v. splenic vein ; s. v. sinus venosus ; syst. tr.
systemic trunk ; U. Bl. urinary bladder ; Ur. ureter ; v. valve in vein ; vs. a.
vesical artery (to bladder) ; vs. v. vesical vein ; vl. ventricle.

auricle, on the other hand, has finished its contraction and is
now relaxing ; it is therefore empty. Thus, on the principle
of least resistance, the contraction of the sinus fills the right
auricle with blood from the great veins, and the sinus itself
is refilled from the same source as soon as it begins to relax.

Immediately after the sinus has ceased to contract the
two auricles contract together : the right, as we have seen,
has just been filled ftom the sinus, the left (/. au] is full of
aerated blood brought to it by the pulmonary vein (pul. v).
The presence of the sinu-auricular valves prevents the blood
in the right auricle from being forced back into the sinus :
that in the left auricle is prevented from being forced back
into the pulmonary veins by the steady onward flow in the
latter. On the other hand the ventricle is beginning to
relax and is empty. Consequently the auriculo-ventricular
valves are forced back into the ventricle (vl) and the blood
from both auricles flows into and fills that chamber, the
right half of which becomes filled with non-aerated, the left
with aerated blood, the two taking an appreciable time to
mingle.

The instant it is thus filled, the contraction of the ventricle
begins. As it does so the blood, getting behind the auriculo-

vi CIRCULATION 93

ventricular valves, forces them together, and thus prevents any
backward flow into the auricle. At the same time the
semilunar valves at the entrance of the conus (c. arf) are
pushed aside and the blood flows into that chamber. Since
the conus opens from the right side of the ventricle, the blood
first entering it will be non-aerated ; there will then follow a
certain amount of mixed blood ; and finally, as the ventricle
reaches the limit of its contraction, the -aerated blood
from its left side will be forced into the conus (compare
Fig. 22.)

Last of all the chambers of the heart, the conus begins
its contraction. The semilunar valves are immediately
filled with blood, and, closing together, stop all backward
flow into the ventricle. Two alternative courses are now
open to the blood : it can pass either directly from the
conus into the pulmo-cutaneous trunk (puL cu. tr), or make
its way into the bulbus aortae (b. ao}. As a matter of fact it
takes the former course, owing to the circumstance that
there is little resistance in the limited blood-system of the
lungs, while that in the systemic and carotid trunks is very
great. Hence the blood just received into the conus from
the ventricle, which, as we have seen, is non-aerated, goes
immediately to the lungs and skin to be aerated.

Before long — in a fraction of a second — the flow of
blood into them increases the pressure in the pulmonary
vessels, and at the same time the blood is continually flow-
ing onwards — /.£., away from the heart — in the systemic and
carotid trunks. Consequently the pressure in these vessels
rapidly diminishes, and the blood soon forces aside the
valves between the conus and the bulbus and fills the latter.
Here again the question of pressure comes in. It is easier
for the blood to make its way into the wide systemic trunks
(syst. tr) uniting immediately into the long dorsal aorta

94 THE FROG CHAP.

(d. ao) than into the comparatively narrow carotid trunks
(car. tr], obstructed by the carotid labyrinths. Hence, the
non-aerated blood having been mostly driven into the
pulmo-cutaneous trunk, the mixed blood, from the middle
part of the ventricle, goes into the systemic trunk, and
thence to the various arteries supplying the limbs (scl. a, iL a)
and the viscera (sp. a, etc.). Finally, when the pressure is
sufficiently raised in the systemic trunks the remaining
blood, which, coming from the left side of the heart, is
aerated, is pumped into the carotid trunks (car. tr] and
thence to the head.

Thus, owing to the arrangement of the valves, and to the
varying pressures in different parts of the vascular system,
the non-aerated blood returned from the various parts of
the body to the heart is mostly sent to the lungs and skin to
be aerated. Mixed blood is sent to the trunk, limbs, and
viscera, while for the head with its contained brain — the
directing and controlling organ of the whole animal — a
special supply of pure, aerated blood is reserved.

We see then that the course of the circulation may be
proved, as a simple matter of induction, from the structure
of the heart and its valves, the direct observation of its
beat, and the manner in which the flow from cut vessels
takes place. It was by observation and experiments of this
kind that the circulation of the blood in the higher animals
was demonstrated by William Harvey in the seventeenth
century. But the final and most conclusive proof of the
circulation — from directly observing the flow — became
possible only after the invention of the microscope. This
instrument, by furnishing a sufficiently high magnifying
power, allows us to see for ourselves the actual movement
of the blood in an animal or organ of sufficient transparency ;
and, at the same time, clears up the question, previously

vi CAPILLARIES 9$

insoluble, of how the blood, having reached a given part or
organ by the arteries, finds its way into the veins to begin
its return journey.

The Circulation in the Frog's Web. — There are three
parts in the frog transparent enough to allow of the blood-
flow being seen in them — the web of the foot, the tongue,
and the mesentery. Of these the web is the most con-
venient, and can be examined under the microscope without
any injury to the animal.

The Capillaries.— If you have the makings of a naturalist,
you will acknowledge the sight to be one of the most won-
derful you ever saw. In the thickness of the web is an
irregular network of minute blood vessels, called capillaries
(Fig. 24), and through them the blood is seen to flow with
great rapidity, its course being made especially evident by
the minute particles or corpuscles it contains, the structure
of which we shall study later on. You will also notice much
larger vessels, the smallest arteries and veins. The arteries
(a) are distinguished by the fact that the blood in them
flows in the direction from the leg towards the margin of the
web, while in the veins (v) it takes the opposite direction.
You must remember, however, that under the microscope
everything is reversed ; right appears left and left right, and
a current actually flowing towards the observer appears to
go in the opposite direction.

By careful examination you will see that both arteries and
veins are in connection, by minute branches, with the
capillary network, and will be able to trace the blood from
an artery, through the capillaries, into a vein.

The same thing can be seen in other transparent organs ;
and by injecting the vascular systern with a fluid injection-
mass, such as gelatine suitably coloured, it can be proved
that all parts of the body are permeated with a capillary

96

THE FROG

network into which the blood is passed by the arteries, and
from which it is received into the veins.

FIG. 24. — Blood-vessels of the web of a frog's foot seen under a low magnifying power.

a. small arteries ; v. small veins. The minute tubes joining the arteries to the

veins are the capillaries. The arrows show the direction of the circulation.

In the small portion marked off, the pigment cells, which occur throughout

the web, are also represented. (From Huxley's Physiology.}

Thus by means of the microscope we are able to take the
final step in demonstrating the circulation. The fact that

vi LYMPHATIC SYSTEM 97

the blood can flow in one direction only is proved by the
disposition of the valves of the heart and of the veins, but
the passage of the blood from the smallest arteries to the
smallest vein by a connecting system of minute tubes or
capillaries can be proved only by the employment of con-
siderable magnifying powers. We see that the vascular system
of the frog is a closed system of vessels : the blood is every-
where confined within definite tubes through which it flows
in a definite direction, never escaping, as in some of the
lower animals, into large irregular spaces among the tissues.
The Lymphatic System. — Included in the vascular-
system are certain cavities and vessels containing lymph,
and together constituting the lymphatic system. We have
already noticed the subcutaneous lymph sinuses (p. 18, Fig.
23, s.c. ly. s) and the sub-vertebral lymph sinus (p. 27, Fig. 5,
s.v. fy.s). There are also found in nearly all parts of the body,
delicate, thin-walled, branching tubes, the lymphatic vessels
(Fig. 23, ly. v)% Unlike the blood-vessels, the lymphatics
are all of one kind, there being no distinction into anything
of the nature of arteries and veins. They arise in lymph-
capillaries (ly. cp\ which are, as it were, interwoven with the
blood-capillaries, but have no connection with them. By
the lymph-capillaries the fluid which has exuded from the
blood in its passage through the tissues is taken up and
passed into the lymphatic vessels or sinuses, and these in their
turn finally communicate with certain transparent muscular
organs called lymph-hearts. Of these there are two pairs. The
anterior lymph-hearts (a. ly. hi] lie, one on either side,
beneath the scapula and just behind the transverse process of
the third vertebra : the posterior pair (/. ly. h£) are situated
one on each side of the posterior end of the urostyle.
These organs pulsate regularly, like miniature hearts, and
pump the lymph into the veins, the anterior pair communi-

PRACT, ZOOL. H

98 THE FROG CHAP.

eating with the subclavian, the posterior with the renal portal
vein.

The lymphatics of the enteric canal have an important function to
perform in that they absorb the fatty portions of the food (p. 75). The
fluid they contain has a milky appearance, owing to the presence of
minute suspended fat-globules, and for this reason they receive the name
of lacteals.

The ccelome is really a great lymph-sinus. It communi-
cates with the veins of the kidneys through certain micro-
scopic apertures called nephrostomes (nst) (Fig. 23, ccel).

The spleen (p. 23, and Fig. 3, spt) has important relations
with the blood- and lymph-vessels, and probably acts as a
blood-filter, removing particles in the blood which are no
longer wanted.

PRACTICAL DIRECTIONS.
The Vascular System.

a. Let some blood from a frog — or better from the veins of some larger,
freshly-killed, warm-blooded animal, such as a rat or a rabbit — flow
directly into a white cup or porcelain capsule. Note that it soon
coagulates i and soon afterwards separates into clot and serum. Notice
also the difference in colour between the blood freshly drawn from a
vein (cf. p. 85), and that soon assumed by exposed portions of the clot.

b. Pin a freshly killed frog to the dissecting board, dorsal side
upwards, and cut through the skin of the back along the middle line.
The posterior lymph-hearts (p. 97, Fig. 23) will then be seen. To make
out the anterior lymph-hearts, carefully separate the supra-scapuloe from
the vertebral column. Some of the chief lymph-sinuses have already
been seen : special methods are required to trace the lymph-vessels.

c. Now turn the frog the other way upwards, pin it down in the dissect-
ing dish, and open the body-cavity as before (p. 32), taking great care
not to cut the abdominal, musculo-cutaneous, and other veins. Slit open
the pericardium and remove as much of it as possible, so as to
expose the entire heart. The structure of the heart and the course of
many of the blood-vessels can also be made out in the specimen from

vi INJECTION OF BLOOD-VESSELS 99

which you have already removed the alimentary canal. In the following
dissections, use a dissecting lens whenever necessary.

I. In the heart (Figs. 3, 4, 7, 20 and 21), notice again the ventricle,
and the right and left auricles (appearing single in the entire heart), and
make out also the conns arteriosus, dividing into two distally, and the
sinus venosus (dorsal).

If the heart is still beating, notice the order of contraction of its
different divisions (p. 90).

Injection Of the Arteries.— The tracing of the arteries is greatly
facilitated by filling them with some coloured substance. The operation
requires, therefore, a coloured fluid or injection-mass capable of
traversing the arteries, and some contrivance by which it can be injected
into them.

The most convenient injection-mass is made as follows : —

1. Grind up in a mortar 4 grammes of " French blue" (to be had at
the oilman's) with 4 cubic centimetres of glycerine and the same
quantity of methylated spirit.

2. Grind up 50 grammes of common laundry-starch, with 50 cubic
centimetres of water and 25 of methylated spirit, and add to the
mixture the colour prepared as above. Mix thoroughly and strain
through muslin.

This injection-mass will keep for an indefinite period in a stoppered
bottle, requiring only to be stirred up when used. If it is considered
too troublesome to make, a simpler but less satisfactory mass may be
made by simply stirring up some French blue in water in the proportion
of a teaspoonful to a tumbler.

For injecting the mass into the blood-vessels, the most satisfactory
instrument is a brass injecting syringe, holding about one ounce,
provided with nozzles of various sizes. This is, however expensive,
and an ordinary glass syringe, to be had of any druggist, will answer the
purpose very fairly if provided with a proper nozzle or cannula. This
latter is made by drawing out one end of a piece of glass tubing about
two inches long until it is fine enough to pass into the conus arteriosus :
a piece of india-rubber tubing is then used to connect it with the fixed
nozzle of the syringe. A still simpler injecting apparatus is furnished
by a common ' ' medicine dropper " (see p. 12, and Fig. 25). By alterna-
tive squeezing and releasing the cap, fluid is drawn into or expelled
from the tube.

Having provided these ' requisites proceed as f-Sl'lovvs. ° Open

H 2

THE FROG

c.art

the abdomen of a freshly-killed frog in the usual way, taking
great care not to injure the blood-vessels. Remove the middle
portion of the shoulder-girdle, so as to
expose the heart, lay open the peri-
cardium, and with the scissors make
a snip into the ventricle, allowing the
blood to escape freely. Pass a piece of
thiead (not cotton), about six inches long,
round the heart, at about the junction of
the auricles and ventricle, and give it a
single loose tie, as shown in Fig. 25.
When the bleeding has ceased, fill the
medicine dropper, or syringe, with injec-
tion-mass and pass the narrow end of the
former, or the nozzle of the latter, through
the cut end of the ventricle into the conus
— take care not to push it into one of the
auricles instead — and tighten the thread so
as to keep it in place. Then squeeze the
cap of the medicine dropper, or push in the
piston of the syringe, and if the operation is
successful, you will see the blue injection
pass from the conus into the arterial trunks,
and thence into the various arteries of the
body. The contrast between the arteries,
filled with the blue mass, and the veins,
filled with blood, is then very striking,
particularly in the mesentery. When the
arteries are well-filled, withdraw the nozzle
from the heart and instantly draw the thread
tight and knot it so as to prevent escape of
the injection. Then place the whole frog in
spirit (methylated spirit 3 parts, water I
part), for a few hours, after which time the

injection mass will be found to have set hard enough to allow of the
arteries being conveniently traced.

Injection of the Veins.— The veins are much more difficult to
inject than the arteries, but ;f you wish -to make a double injection on
the -same specimen, colo^iv MieinJL'ctioii-rnaVs'wi'h vermilion. or carmine

FIG. 25. — Sketch showing the
method of injecting the
frog's arteries.

a. glass " medicine-dropper "
with india-rubber cap (£) ;
its pointed end (dotted) is
passed through the cut end
of the ventricle (zi) into
the conus (c. art) ; au.
auricular division of the
heart ; t. thread.

vi PRACTICAL DIRECTIONS 101

in the case of the arteries instead of with French blue, using the latter
for the veins. The operation is best performed by inserting the nozzle
into an incision in the abdominal vein (by directing the nozzle for-
wards, the portal vein will be injected : by directing it backwards
the pelvic and renal portal veins), and also into one of the subclavians.
But for a really satisfactory preparation, it is best to inject from the
heart through the conus, as directed above, with a weak, warm solution
of gelatine (in the proportion of one part of gelatine to two parts of
water), coloured with precipitated carmine. In this case the injection
mass, containing only microscopic particles, passes from the arteries
through the capillaries into the veins, keeping throughout to the
course taken by the blood during life, and therefore unimpeded by
the valves of the veins. A syringe must be used, since the medicine-
dropper will not give sufficient pressure, and the animal should be
placed in warm water during the process.

II. Now pin down under water and make out the course of the chief
veins (p. 82, Fig. 21) : —

1. The two precavals, and the external jugular ^ internal jugular,
subclavian, and musculo-cutaneous .

2. The postcaval, to see which turn the viscera on one side (Figs. 3
and 4). Note the renal, spermatic or ovarian, and hepatic veins.

3 . The hepatic portal vein and its factors.

4. The abdominal vein and pelvic veins.

5. The veins from the hind-legs can be more easily seen at a later
stage, after the alimentary canal is removed, and so their examination is
best left until certain of the arteries have been traced (or use the
specimen you have dissected previously for this purpose). Remove the
skin from the thigh, place the frog on its side, and make out the
femoral, pelvic (already seen), renal portal, and sciatic veins, as well as
a large vein from the muscles of the back.

6. The two pulmonary veins.

Make a sketch of the heart and as many of the veins as you have
followed out up to this point, inserting the others after removing the
alimentary canal (see p. 102).

III. The chief arteries may now be followed out (p. 80, Fig. 20) : —
Note the carotid, the systemic, and the pulmo-cutantoiis trunk, arising

from the conus arteriosus, and then trace each of these out as follows : —

I. The carotid tr tink gives off a lingual artery, and is continued into

the head as the carotid artery, having at its origin the carotid labyrinth.

102 THE FROG CHAP.

2. The systemic trunk unites with its fellow to form the dorsal aorta,
first giving off vertebral, subclavian, and cesophageal arteries. From the
point of union of the two systemic trunks arsies the splanchnic or
cceliaco-mes enteric artery. After following this out to its distribution,
remove the alimentary canal as directed on p. 76, when the following
branches of the dorsal aorta will be more plainly seen : — the renal,
spermatic or ovarian, and iliac arteries.

3. ^}\^ pulmo-cutaneous trunk divides into a pulmonary artery, passing
along the outer side of the corresponding lung, and a ctitaneous artery.

Sketch the heart and chief arteries, and then make out and sketch
the renal portal system (p. 101), if you have not already done so.

IV. 1 Cut out the heart of a frog preserved in formaline, taking great
care not to injure it. Fasten it down in a dissecting-dish with the ventral
surface upwards, by sticking very small pins through the arteries and
veins — not through the heart itself. Pinch up the ventricle with fine
forceps, and with small scissors gradually snip away its ventral wall,
noting that it is a hollow structure with thick, spongy walls and a small
cavity, which will probably be full of clotted blood. Wash this out,
and then proceed to open the auricles in a similar way, and to wash out
the blood they contain. Observe the right and left auricles, separated
by a partition. Slit open the conus arteriosus, and continue the cut
forwards to the origin of the main arteries. Examine with a lens and
make out (p. 87, Fig. 22) :—

1 . The auricula-ventricular aperture and its valves.

2. The longitudinal valve and the small semilunar valves in the
conus arteriosus.

3. The origins of the carotid and systemic trunks from the bulbus
aortce, and the small aperture leading into the pulmo cutaneous
trunks.

4. The sinu-auricular aperture and itsvafves.

5. The aperture in the left auricle leading into the pulmonary veins.
Sketch.

Turn over the heart, so that its dorsal surface is upwards, and cut away
enough of the dorsal wall of the sinus venosus to show the sinu-
auricular aperture from the other side.

1 On account of its small size, the examination of the structure of the
frog's heart is somewhat difficult, and the student is advised to dissect
first the heart of a larger animal, such as a dogfish. (See Part II.)

vi PRACTICAL DIRECTIONS 103

V. Get a piece of thin board — e.g. , the side of a cigar box — about six
inches long by three wide. Towards one end make a round hole about
half an inch in diameter, and opposite this, on either side, make a
notch, or rather slit, with a penknife. This is called a " frog-board."

Next get as light coloured a frog as possible. The web may be
examined in the living animal without hurting it, or the animal
may be first chloroformed as directed on p. 31, but removed from
the influence of the anaesthetic as soon as it is insensible, when
the brain may be destroyed by the operation of pithing, so that
there may be no suspicion of the frog feeling any inconvenience
from this harmless experiment. To pith the frog, feel with the finger
the joint between the skull and first vertebra on the dorsal side,
and with a sharp scalpel make a small cut through the skin and
underlying tissue, so as to expose the spinal cord in this region ;
then rapidly insert a blunt instrument, such as a seeker or a small
piece of wrood, into the cranial cavity, and move it about until
the brain is completely destroyed. Lay the frog on the frog-board
with a piece of wet rag wrapped loosely round the body, and take one
or two turns around both frog and board with a piece of tape —
you must avoid tying it tightly or the circulation will be impeded.
Stretch out one leg, and selecting the most transparent web, tie a piece
of thick soft silk round each of the two toes by which it is bounded.
Adjust the leg so that the web comes just over the hole in the frog-
board, and bring the two pieces of silk through the slits, regulating them
until the web is evenly stretched out over the hole. Lastly, place the
frog-board on the stage of the microscope,1 with the hole over the
aperture in the stage, and either fix it with the clips or rest the opposite
end on some support : adjust the mirror so as to illuminate the web from
beneath, and examine it with the low power. Note the network of
capillaries and the tirculation of the blood through the arteries, capil-
laries and veins (Fig. 24).

1 A brief description of the compound microscope will be given at
the end of the next chapter.

CHAPTER VII

THE FROG (continued) : THE MICROSCOPICAL EXAMINATION

OF THE SIMPLE TISSUES.

BEFORE carrying our enquiries any further into the
anatomy and physiology of the frog it will be necessary to
devote some consideration to its microscopic structure or
histology, since there are many matters in connection with
the various organs which can be further elucidated only by
the examination of the minute structure of the organs as
revealed by the microscope (see p. 119).

Let us, first of all, examine a drop of the frog's blood
under the low power of the microscope. It will at once be
seen that the blood is not a simple homogeneous fluid, but
that it contains a large number of minute solid bodies
flqating in it. These are called by the general name of
blood-corpuscles : the fluid part of the blood in which they
float is called the plasma. At first, owing to currents in the
fluid, the corpuscles will be found to move to and fro, but
after a time they come to rest. Under the high power you
will notice that the corpuscles are of two kinds. The greater
number of them are regularly oval in form (Fig. 26, C), and
of a yellow colour. If the drop of blood is thick enough
in one part for the corpuscles to lie over one another, so

CHAP, vii BLOOD-CORPUSCLES 105

that the light passes through two or three layers of them to
reach the eye, they will appear red : they are hence called red
corpuscles. Frequently they are seen turned on edge (D),
and their appearance in this position shows them to be flat
oval discs with a swelling in the centre. They are about
~th of a millimetre (about -lyVo^ri inch) in long diameter.
Among the red corpuscles are found, in much smaller
numbers, bodies not more than half the long diameter of the
red corpuscles in size, quite colourless, distinctly granular —
so as to have the appearance of ground glass — and with a

1)

FIG. 26. — Blood-corpuscles of the frog, highly magnified.

A, colourless corpuscle ; B, the same in process of division ; C, red corpuscle
surface view ; D, the same, edge view. nu. nucleus. (From Parker's Biology.}

slightly irregular outline (Fig. 26, A). These are the colour-
less corpuscles or leucocytes. They are not flat, like the red
corpuscles, but have the form of irregular lumps.

The plasma, like the leucocytes, is quite colourless, so that
the colour of the blood is seen to be due entirely to the
large number of red corpuscles it contains.

If the drop of blood has been prepared and examined
under the high power with sufficient rapidity, a remarkable
phenomenon can be made out with regard to the colourless
corpuscles. This can be most easily demonstrated by making
a series of outline sketches of the same leucocyte at intervals
of a minute or two. You will then notice that the sketches
all differ from one another : in one there will perhaps be a
little projection going off to the right ; in the next this will
have disappeared and a similar projection will have appeared

io6 THE FROG CHAP.

on the left, and so on. As a matter of fact, as long as the
blood is quite fresh, the leucocytes are in constant movement,
sending out and withdrawing little processes of their substance
called pseudopods or "false feet," by means of which they
can crawl slowly along like independent living things.
These very peculiar and characteristic movements are called
amoeboid movements. Occasionally a leucocyte may be seen
to elongate itself and divide into two (Fig. 26, B) : this is a
case of what is called simple fission. The red corpuscles
neither move nor divide.

If a drop of some dye or staining fluid be run in under
the cover-glass, the corpuscles will be seen to become rather
faint in outline, very transparent, and lightly tinted ; but the
most obvious effect is that in the middle of each is seen a
rounded or oval granular body (nu) deeply stained by the
dye, so as to make a very well-defined coloured area in the
interior of the corpuscle. This body is called the nucleus :
it is present both in the red and the colourless corpuscles.

By adding to a fresh drop of blood, in the same manner,
a drop of weak acetic acid, the nucleus again becomes
distinct, while the body of the corpuscle is rendered very
transparent and almost invisible : indeed it finally disappears
altogether. It is thus proved that the corpuscles, both red
and colourless, consist of a substance which is known as
protoplasm, but slightly affected by dyes, and soluble in
weak acids ; and enclosed in this is a nucleus, stained by
dyes, and unaffected by weak acids. Both nucleus and
protoplasm consist mainly of proteids (p. 72), together with
water and a small proportion of mineral matters.

When distilled water is added to a drop of blood on the
slide, the corpuscles are seen to swell up and become partly
dissolved : the red colouring matter of the red corpuscles
is dissolved out, the plasma becoming tinged with yellow.

vii CHARACTERS OF BLOOD 107

Thus the colouring matter is evidently a distinct substance
from the protoplasm, and is called hemoglobin. It is
characterised, among other things, by a strong attraction for
oxygen : in combination with that gas it assumes a bright
scarlet colour : when deprived of oxygen, it becomes
purple. This affinity for oxygen accounts for the change
undergone by the blood when exposed to the air, as
described on p. 79.

Coagulated blood, as seen under the microscope, is
characterised by the plasma being traversed by extremely
delicate threads, forming a sort of network in which the
corpuscles are entangled. These threads are formed of a
substance called fibrin, which is separated from the plasma
during coagulation, the remaining or fluid portion of the
plasma constituting the serum. We may therefore express
the coagulation of the blood in a diagrammatic form as
follows : —

Fresh Blood. Coagulated Blood.

f Serum

Plasma' • • • \ Fibrin \r,
^ , J- Clot.

Corpuscles J

Having observed the microscopic characters of a drop of
blood, let us examine once more the circulation in the web,
this time under the high power (Fig. 27). The red corpuscles
(1?) can be seen streaming through the vessels, those in the
capillaries in single file, those in the small arteries and veins
two or more abreast : as they pass through narrow capillaries
or round corners, they become bent or squeezed (G, H}.
The leucocytes (/) travel more slowly and often stick to the
sides of the vessels.

Columnar Epithelium. — By carefully teasing out a small
piece of the inner surface of the mucous membrane of the

io8

THE FROG

FIG. 27. — The circulation in the frog's web, under a high power.
A, wall of capillaries ; B, tissue of the web in which the capillaries lie ; C, epiderm-
cells ; JD, their nuclei ; £, pigment-cells ; F, red corpuscles ; G, ff, red corpuscles
being squeezed through a narrow capillary ; K, capillary seen through the
epiderm ; /, colourless corpuscles. (From Huxley's Physiology.)

EPITHELIUM

109

intestine into the smallest possible particles, it will be
found that the process has detached numerous, minute,
conical bodies, about ^ mm. (y^ in.) in length, polygonal
in transverse section, and having
one end flat and the other
more pointed (Fig. 28). These
bodies are called epithelial cells :
in the natural position they lie
closely applied to one another,
like the blocks of a wood-pave-
ment, their flattened ends facing
the cavity of the intestine, while

FIG. 28. — Columnar epithelial cells

from the frog's intestine.
m. droplet of mucus exuding
from cell ; nu. nucleus.

their narrower ends abut against

the submucosa (p. 70). Thus the epithelial cells together
form an epithelium or epithelial layer of the mucous mem-
brane directly bounding the cavity of the enteric canal.

Each cell consists of protoplasm and contains a rounded,
granular nucleus (nu\ which is made very conspicuous by
staining, and in which are one or more small bodies or nucleoli.
Certain of these cells have a space towards
their free ends containing slime or mucus,
and thus have the form of little cups or gob-
lets : they are known as goblet-cells (see right
hand cell in Fig. 28).
FIG. 29.-ciiiated Ciliated Epithelium. — By the same method

epithelial cells /- i , .

from the mu- the^mucous membrane of the mouth is also

cous membrane , .. .. , • •,-,•

of the frog's seen to be lined by an epithelium, but the
(From Parker's cells comprising it (Fig. 29) are shorter in

Biology, after . . .1*1 ., i i i

Howes.) proportion to their length, and each is pro-

duced on its free surface into a number of
delicate, transparent threads of protoplasm called cilia, which,
in the living condition are in constant movement, lashing
backwards and forwards like minute whip-lashes, or, more

i io THE FROG CHAP.

accurately, like the blades of grass in a field when acted
upon by a strong wind. If you happen to get under the
microscope a good-sized bit of mucous membrane with the
cells in position, you will see that the cilia produce a strong
current by which small particles are swept along, while
detached cells swim about, like little independent animals,
by the action of their own cilia. These ciliated epithelial
cells, like the ordinary columnar cells of the intestine, are
made of protoplasm, and each contains a nucleus with one
or two nucleoli clearly brought into view by staining.

The action of the cilia can be demonstrated, on a large
scale, by placing a freshly-killed frog on its back, turning
back or cutting away the lower jaw, and placing a very small
cube of cork on the roof of the mouth near to the projection
due to the eyes. The cork will be slowly swept back
towards the throat.

Squamous or Pavement Epithelium. — By scraping the
outer surface of a piece of skin with a sharp knife, and

examining the scrapings in a
drop of water, after staining
them, the superficial layer of
the skin will be found to be
made up of flattened, roughly
hexagonal plates (Fig. 30 and
Fig. 27, C, D) set closely to-

FIG. 30. — Squamous epithelial cells ,., ., r

from the frog's skin. nu. nuclei. gether, like the tlleS OI a

mosaic pavement. Each plate

has a nucleus, and, from its flattened form, is distinguished
as a squamous or scale-like epithelial cell.

Meaning of the word " Cell." — We see thus that the body
of the frog is partly made up of distinct elements, which,
under a considerable diversity of form, exhibit the same
essential structure. Each consists of a mass of living proto-

UNSTRIPED MUSCLE

plasm, which appears almost clear or more or less granular,
containing in its interior a rounded body, the nucleus,
specially distinguished by the affinity of parts of its substance
for colouring matters. To a body having this essential
structure, whatever its form, the name cell is applied.

Unstriped Muscle. — Examination of a teased preparation
of the muscular coat of the intestine, stomach, or urinary
bladder will show that it is com-
posed of delicate fibres (Fig. 31)
tapering at both ends, and with
a nucleus in the middle. These
are called smooth or unstriped
muscular fibres : they are ob-
viously cells which have under-
gone a great elongation in length.

During the peristaltic move-
ments of the intestine (p. 7 5) each
fibre alternately contracts and
relaxes, becoming shorter and
thicker during the former pro-
cess, like the large muscles of
the body (p. 60). The move-
ments, in this case, however, are
not under the control of the will,
and unstriped muscular tissue is
therefore often spoken of as
involuntary muscle.

Contractility of Protoplasm.
— We have now studied three
different kinds of movement in

cells : — muscular movement in the unstriped muscle-fibres,
ciliary movement in the ciliated epithelial cells, and amoeboid
movement in the colourless blood-corpuscles. Muscular

FIG. 31. — Unstriped muscular
fibres from the frog's intestine.
To the right are shown fibres from
the longitudinal and circular
layers (see Chap. VIII) cross-
ing one another ; to the left
isolated fibres. (After Howes.)

H2 THE FROG CHAP.

movement is due to the fibre undergoing a sudden shorten-
ing in a particular direction and a consequent approxima-
tion of its two ends. Ciliary movement is due to the
alternate bending and straightening of the cilia; and the
bending of a cilium in a particular direction is caused by
the protoplasm of which it is composed shortening or
contracting on the side towards which it bends. Amoeboid
movement is the protrusion and withdrawal of irregular
processes of the cell : this results from the protoplasm
undergoing a contraction or squeezing in a given direction,
as a consequence of which one part of its substance is
drawn in and another pushed out. Hence all three kinds
of movement are movements of contraction ; and con-
tractility, or the power of contraction, may be considered
as a general property of protoplasm.

Striped Muscle. — If a small piece of any of the body-
muscles is carefully teased out with the grain, /.*., in the
direction of the length of the fibres, so as to break away
the connective tissue binding them together, the fibres,
which are much larger than those of smooth muscle, will
readily separate from one another, and they will be seen to
be long and cylindrical. Under the microscope each fibre
shows a delicate transverse striation (Fig. 32), being made
up of alternate bright (b) and dim (d) bands — or more
accurately discs, the fibre being cylindrical — set at right
angles to its length. Hence the ordinary body-muscles or
voluntary muscles are composed of striped muscular fibres?-
In addition to the transverse striation a fainter longitudinal
striation is more or less distinctly visible.

Each fibre is covered by a delicate membrane (s\ called
sarcolemma^ beneath which nuclei (n) occur at intervals.

1 The muscles of the heart, although not under the control of the
will, are transversely striated ; but their structure differs from that of
ordinary striped voluntary muscle.

vii STRIPED MUSCLE 113

It will be seen that striped muscle, unlike the tissues
previously considered, does not appear to be composed
of cells, although the occurrence of nuclei seems to indicate
their presence. In the embryo, however, the muscle is
formed of ordinary nucleated cells, which, as growth goes

FIG. 32. — A, part of a fresh muscular fibre of a frog. B, the same after treatment

with distilled water followed by methyl green.

b. bright bands ; d. dim bands ; n. nuclei ; s, s'. sarcolemrna, rendered visible as a
minute blister (/) by absorption of water and by the rupture of the muscle-
fibre at .?. (A, from Huxley's Physiology.}

on, increase in length while their nuclei multiply by fission,
each enormously elongated cell thus containing a consider-
able number.

Connective-tissue. — We will next examine a piece of the
delicate web of connective-tissue which binds the muscles
together.

Under the high power, connective-tissue is seen to be

FRACT. ZOOL. I

THE FROG

CHAP.

composed of a sort of irregular network of delicate bundles
of wavy fibres called white connective-tissue fibres (Fig. 33, w\
which cross one another in all directions. Amongst them
are found single fibres sweeping across the field in bold

FIG. 33. — Connective-tissue from between muscles of frog's leg.

c. cells ; e. elastic fibres ; iv. white fibres : -all of which are imbedded in a delicate
matrix, not specially indicated in the figure.

curves and called elastic fibres (e) : it is owing to their
elasticity that the tissue cannot be spread out when wet.
Scattered among the fibres are numerous nucleated cells (c) of
very varied and often irregular form : these are the connective
tissue cells. The fibres, as well as the cells, are imbedded
in a soft, homogeneous, ground-substance or matrix.

vii CARTILAGE 115

Thus connective-tissue consists partly of cells, but
between these and forming the main substance of the tissue,
is a matrix or intercellular substance, enclosing fibres. In
the embryo the tissue consists of closely packed cells, but, as
development proceeds, these separate from one another, and
the ground-substance is formed between them.

Cartilage. — The ordinary clear or hyaline variety of this
tissue is conveniently studied by examining a piece of the
thin edge of the omo- or xiphi-sternum, or by taking a thin
section with a razor of the head of the humerus or femur.

Cartilage consists of a tough, elastic, transparent, homo-
geneous matrix (Fig. 34, in) containing numerous cavities

FIG. 34. — Section of cartilage, from the head of the frog's femur.
c. cells ; c'. cells undergoing fission ; c. s. empty cell-space ; m. matrix.

or cell-spaces (c.s\ in each of which is a nucleated cell (c).
The cell-spaces, or lacunce, are in many cases arranged in
groups of two or four, sometimes close together, sometimes
with a narrow space of matrix or intercellular substance
between them. This is due to the fact that cartilage grows
by the cells undergoing binary fission, so that two cells are
formed in one cell-space : the two then gradually separate

I 2

u6 THE FROG CHAP.

from one another and intercellular substance is formed
between them. In the embryo, this tissue consists entirely
of closely packed cells which gradually separate and form a
structureless matrix which is firm and elastic, and which in
some parts (pp. 46 and 48) may become calcified.

Bone. — As we have already seen (p. 52) bone is formed
of two constituents, a basis of animal matter, in which mineral
matter — calcium phosphate and carbonate — is deposited.
In microscopic examination we may therefore investigate

FIG. 35. — Transverse section of dry femur of frog.
c. canaliculi ; Ic. lacunae ; tin. lamellae ; in. marrow cavity.

either the mineral matter by examining dried bone, or the
animal matter by examining decalcified bone.

A thin section of a dried long bone, such as the femur,
shows that it is formed of very numerous thin layers or
lamella (Fig. 35, lm\ surrounding and concentric with the
marrow cavity. The lamellae contain numerous cavities,
the lacuna (lc\ with delicate, branching tubes, the canaliculi

BONE

117

(<:), radiating from them in all directions. Both lacunae and
canaliculi commonly appear black, owing to their being filled
either with air or with bone-dust produced in grinding the
section.

In a section of decalcified bone (Fig. 36) the marrow is
seen to be surrounded by lamellae of a delicate fibrous
substance, arranged in two layers, an outer (<5), having the
periosteum (p) closely investing it, and an inner (^'), in
contact with the marrow. In the fibrous substance of the
lamellae are cell-spaces, corresponding with the lacunae of the

B

WmKpM

FIG. 36. — Transverse section of decalcified frog's femur under a low power. B, portion

of the same under a high power.

b. outer, and b' . inner layer of bone ; b. c, bone-cells ; ni. marrow ; o. layer of
osteoblasts in connection with periosteum ; o' . layer of osteoblasts in connection
with marrow ; p. periosteum. (After Howes.)

dried bone, and each containing a bone-cell (b.c\ which sends
off delicate branched processes of its protoplasm into the
canaliculi. Thus the bone, like connective-tissue and
cartilage, consists of cells with an intercellular substance :
the latter is in the form of concentric layers and is impreg-
nated with lime-salts.

The long bones of the frog grow in two directions.
Between the periosteum and the bone is a layer of cells, the

iiS THE FROG CHAP.

osteoblasts (p\ by which new lamellae of bone are formed on
the outside of those already existing : thus the outer layer of
bone (^) grows from within outwards. Between the marrow
and the inner surface of the bone is another layer of osteo-
blasts (0') which forms new lamellae on the inner side of
the existing bone, so that the inner layer (b') grows from
without inwards.

Summary. — The various simple tissues studied in the
present chapter consist either entirely of cells, or of cells
separated by an intercellular substance. Formed entirely
of cells are the various kinds of epithelium — columnar,
ciliated, and squamous, and unstriped muscle. In striped
muscle the cells have elongated into fibres and their
nuclei have multiplied. Of tissues consisting of cells with
intercellular substance, connective-tissue has the matrix soft
and homogeneous, with fibres imbedded in it; in hyaline
cartilage it is structureless and tough, though elastic ; and
in bone laminated and calcified. In the blood, the plasma
may be looked upon as a kind of liquid intercellular
substance.

Cells, wherever they occur, have the same essential struc-
ture, being formed of protoplasm with a nucleus. In
nearly all cases they increase by binary fission, first the
nucleus and then the protoplasm dividing into two.

The distribution of the various tissues throughout the body
is worth noting. Epithelium always bounds a free surface
— e.g., that covering the outer surface of the body or lining
the inner surface of the enteric canal. Striped muscle forms
the " flesh," unstriped muscle the outer layer of the enteric
canal (p. 70). Bone and cartilage form the framework of
the body, while connective-tissue is the packing between
the other tissues.

THE MICROSCOPE

119

PRACTICAL DIRECTIONS.

General Structure of the Compound Microscope, —The com-
pound microscope, with which you must now become acquainted,
consists of a strong stand (Fig. 37, a)
from which rises a vertical pillar (b}.
To the latter are attached— a horizontal
plate or stage (c), perforated in the cen-
tre with an aperture (d), the size ot
which can be varied by means of a
diaphragm : an adjustable mirror (e\
placed below the stage : and a vertical
tube (f) attached above the stage by a
horizontal arm. Two combinations of
lenses are used : an objective or object-
glass (/$), consisting of a metal tube with
two or more lenses fixed into it, which
screws into the lower end of the tube :
and an ocular or eye-piece (z), consisting
of a metal cylinder with a lens at each
end, which slides into the upper end of
the tube. It is this arrangement of
lenses which forms the essential feature
of the compound microscope : the ob-
ject, placed on the stage, is magnified
by the objective, and the magnified
image, thrown into the interior of the
tube, is further enlarged by the ocular.

The object is brought into focus — i.e.,
placed at such a distance from the
objective that a perfectly clear and well-
defined image is obtained — in one of
two ways. The tube can be raised or
lowered either by sliding it up and
down in an outer tube or collar (£•), or,

in the more expensive instruments, by a rack and pinion : this move-
ment forms the coarse adjustment. In addition, all good microscopes
have a fine adjustment ', usually consisting of a spring concealed in the

FIG. 37. — Diagram of compound

microscope.

a, stand ; b. pillar ; b' '. movable
portion of pillar, raised and
lowered by fine adjustment ;
c. stage ; d. aperture in stage ;
e. mirror ; f. tube ; f . milled
ring for raising and lowering
tube ; g. collar ; h. objective ;
i. ocular ; k. screw of fine
adjustment.

120 THE FROG CHAP.

pillar, and acting upon the horizontal arm which carries the tube : it is
worked by a screw (£), and by means of it the tube can be adjusted to
within TD-oi><rth of an inch.

When the object is transparent — as in most cases with which we shall
have to deal — it is placed over the hole in the stage on a glass slide,
and illuminated from below by adjusting the mirror until a beam of light
from the window or lamp is reflected vertically upwards : a small hole
in the diaphragm should be used with the high power. The object is
thus said to be viewed by transmitted light. In the case of opaque
substances the mirror is not used, and the object is illuminated by the
light falling upon it directly : it is then said to be viewed by reflected
light.

A student's microscope should have two objectives, one — the low
power — magnifying about 80, the other — the high power — about 300 to
400 diameters. One eye-piece is sufficient, and a sliding coarse ad-
justment is nearly as convenient as a rack and pinion, besides being
cheaper and less likely to get out of order. The mistake often made in
choosing a microscope is to get one of elaborate construction, the money
going largely in brass-work. The proper thing is to get the simplest
form of stand consistent with perfect rigidity, fitted with the best
possible fine adjustment and lenses : to save on either of the latter is
false economy.

Requisites for Microscopic Work.— In addition to the micro-
scope, the following will be required before starting work : —

1. A few slides or slips of glass, 3 inches long by I inch wide, which
can be obtained from an optician. They must be thoroughly cleaned
before being used.

2. A supply (about 4 oz. ) of cover-glasses, small pieces of very thin
glass, to be had at the optician's. The most convenient size is f inch
square. They are best cleaned by being soaked for a few minutes in
strong nitric acid, and then thoroughly washed under the tap, after
which they should be dried by being placed flat on a clean surface and
rubbed with a handkerchief : if held between the finger and thumb, they
are very liable to be broken.

3. One or two thin glass rods, about 6 inches long and ^th inch in
diameter ; and one or two dipping ttibes, or pieces of glass tubing about
6 inches long and -^-th inch in diameter. The ends both of rods and
tubes should be smoothed off in the flame of a blow-pipe.

4. Three or four dissecting needles, made by sticking a fine sewing

vii PRACTICAL DIRECTIONS 121

needle into the end of a wooden penholder, allowing the point to project
about half an inch.

5. A few reagent-bottles for holding the various fluids used for applying
what are called micro-chemical tests to the tissues. Special bottles can
be bought for the purpose, but sufficiently convenient ones can be made
from ordinary one-ounce phials, fitted with sound corks. Bore a hole
lengthwise through the cork, and insert into the hole a piece of narrow glass
rod, pointed at the end, just long enough to reach nearly to the bottom
of the bottle when the cork is inserted. This arrangement allows of the
ready application of a single drop of fluid to the object under examination.

6. The following micro-chemical reagents : —

a. Salt solution. Dissolve 0*75 gramme of sodium chloride in 100 c. c.
of distilled water, so as to make a f per cent, solution.

b. Acetic acid, I per cent. One c.c. of strong acetic acid to 99 c.c.
of distilled water.

c. Distilled water.

d. Solutions of one or two aniline dyes. For fresh tissues, dissolve
enough methyl green in distilled water to make a deep bluish-green
solution, and add I per cent, of strong acetic acid. For preserved
(or fresh) tissues, make a saturated solution of magenta or safranin
in strong alcohol, and dilute with an equal bulk of water.

e. Glycerine, 50 per cent. Equal parts of pure glycerine and distilled
water.

Microscopical examination of the simple tissues.

For the following work, a freshly-killed frog must be used.

(If you wish to measure each kind of tissue-element, you must learn to
use a micrometer, which consists of a circular piece of glass, marked at
regular intervals with lines or squares, the distance between which can
be calculated by comparing them with a scale engraved on a slide
known as a stage-micrometer. )

I. The blood. — Have ready a clean dry slide and cover-glass. In a
freshly-killed frog open a vein or make an incision in the heart, and
with a clean glass rod remove a drop of blood to the middle of a slide.
Take hold of the edge of the cover-glass with small forceps, and sup-
porting it with a mounted needle, gently lower it on to the drop of
blood until the latter is spread out into an even, transparent, yellowish
film. This operation of covering the drop of blood requires a little
practice : if not done quickly (unless a drop of salt-solution is added),
there is danger of the blood coagulating before it is covered, in which

122 THE FROG CHAP.

case it will not spread out into a transparent layer : if the cover is
lowered too suddenly, bubbles of air are commonly included.

Examine your preparation, first of all, under the low power, and
learn to recognise the appearance of air-bubbles. Note the colourless
plasma and the numerous minute blood-cells or corpitscles.

Now replace the low by the high power. Bear in mind that the
higher the power, the shorter the focal distance. With the low power
you will probably find that the object is in focus when about half an
inch from the bottom lens of the objective. The high power, on the
other hand, has to be brought to within about -fafa of an inch of the
cover-glass, which is therefore liable to be broken and the lens to be
injured by careless focussing. The safest plan is to lower the tube,
keeping your eye at the level of the stage, until the objective almost
touches the cover-glass : then, looking through the microscope, very
slowly raise the tube by means of the coarse adjustment, until the 'object
comes into view. Note (Fig. 26) :—

a. The numerous flat, oval, red corpuscles, each with a central
swelling in which the nucleus is contained. Sketch.

b. The colourless corpuscles or leucocytes^ much less numerous, having
a granular appearance, and an irregular or rounded outline.

Focus a colourless corpuscle under the high power, and note its
amoeboid movements : sketch its outline rapidly but accurately, and after a
minute or two make another sketch, and then another, until some half-
dozen outline drawings of the corpuscle have been obtained : then
compare your sketches.

Now place on the slide, against one edge of the cover-glass, a drop of
methyl-green, and against the opposite edge a small strip of blotting
paper. If the blood is sufficiently fresh — and if it has coagulated
you must get another drop — the blotting paper will slowly absorb the
blood on one side, and the methyl-green will be drawn in and will
gradually mingle with the blood. When this has taken place, put a
drop of salt solution in the place of the methyl-green, and allow it to
be drawn across so as to remove the superfluous stain : then remove the
blotting paper, and examine the blood once more under the high
power. Notice the nucleus present in each kind of corpuscle, and the
surrounding protoplasm. «Eketch.

To a fresh drop of blood add in the same manner, a drop of I per cent,
acetic acid. Note that the body of each corpuscle becomes transparent,
while the nucleus is rendered distinct.

vii PRACTICAL DIRECTIONS 123

To another drop of blood add distilled water. The corpuscles be-
come swollen up and partly dissolved, and the colouring matter
(hemoglobin} of the red corpuscles is dissolved out into the plasma.

Examine some coagulated blood under the microscope, and note the
threads vl fibrin in which the red corpuscles are entangled, and the
serum.

Examine once more the circulation in the web (p. 103), using the high
power, and follow the course of both red and colourless corpuscles
through the vessels and capillaries. By focussing to the surface of the
web the flattened epithelial cells of the epiderm or outer skin can be
seen, and at a deeper level the black pigment cells (see p. 128, and
compare Figs. 24 and 27).

2. Columnar epithelium. — Take a small piece of frog's intestine,
and place it for 24 hours in a mixture called Ranvier's alcohol, consisting
of one part of methylated spirit and two parts of water. With fine
scissors snip off a very small piece — not larger than a pin's head — from
the inner surface of the mucous membrane, place it on a slide in a drop
of water, and with two dissecting needles tease it out by tearing it into
the smallest possible particles. The operation is best done under a lens.
Then put on a cover-glass, and examine first with the low and then with
the high power. (Remember that a cover-glass must always be used
with the high power. )

Note the minute, more or less conical cells of columnar epithelium
(Fig. 28), each containing a midetis. Observe the goblet-cells amongst
the ordinary columnar cells. Stain with magenta, which is more effective
than methyl-green in specimens previously treated with alcohol, add
a trace of acetic acid, wash with water (salt-solution need only be
employed in the case of fresh tissues), and add glycerine. Sketch.

3- Ciliated epithelium, — Snip off a very small bit of mucous
membrane from the mouth of a recently killed frog, and tease it out in
salt solution (Fig. 29).

Note the form of the cells and their nuclei : they are relatively
shorter than the columnar cells just examined, and each bears a number
. of delicate vibratile cilia at its free end. Observe the movements of
the cilia. Sketch.

Treat with methyl-green, magenta, or acetic acid, when the nucleus
will become more apparent.

Perform experiment described on p. no to show the action of the cilia
as a whole in the entire animal.

124 THE FROG CHAP.

4- Pavement or squamOUS epithelium.— Take a bit of frog's skin
which has been kept for a day or two in Ranvier's alcohol, scrape the
outer surface with a sharp knife, tease out and examine the edges of the
scrapings in a drop of water, afterwards staining with magenta. (Fig. 30.)

Note the flattened cells fitting together like tiles in a pavement, each
one with its nucleus. Sketch.

5- TInstriped Muscle. — Snip off a small piece from an inflated
urinary bladder of a frog which has been preserved in formaline, wash
with water, and mount. Or, snip off a very small piece — not bigger
than a pin's head — from the muscular coat of the intestine or stomach
(or from a urinary bladder) which has been in Ranvier's alcohol for
at least twenty-four hours ; then tease out in a drop of water very
thoroughly. Note the elongated unstriped muscular fibres tapering
at both ends, each containing a nucleus (Fig. 31). Stain with
magenta. Sketch.

6. Striped Muscle. — Snip off a small piece — about |th inch long
— from any of the body-muscles of a freshly-killed frog, put it on a
slide in a drop of salt-solution, and tease it out, with the grain, i.e.,
in the direction of the length of the fibres. The fibres will readily
separate from one another : the teasing process must be stopped as
soon as they are apart, and care must be taken not to tear or crush
the individual fibres, which are large enough to be readily distin-
guishable with a magnifying-glass.

Observe under the low power of the microscope the long cylindrical
fibres (Fig. 32, A), bound together by connective-tissue, and showing a
distinct transverse striation and a less distinct longitudinal striation.
Examine a single fibre under the high power (Fig. 32, B), and make
out the sarcolemma and the numerous nuclei, which will be rendered
more distinct by the addition of acetic acid or by staining. Sketch.

7- Connective-tissue' — Carefully separate two of the muscles of
the leg in a fresh frog, and note the delicate web of connective-tissue
between them : or, note the fine strands of connective-tissue between
the skin and the muscles of the body-wall. With fine forceps lift up
a small shred of this, snip it off with scissors, and place it on a dry
slide. Then, with two needles, spread it out on a thin, even layer,
breathing on it occasionally to prevent drying. Lastly, place a drop of
salt-solution on a cover-glass, and quickly lower it on the preparation.
The reason for this procedure is that if connective-tissue is placed in
fluid, it contracts into a little lump, which is too opaque for examination
and cannot be readily spread out.

vii PRACTICAL DIRECTIONS 125

Examine first with the low and then with the high power, and
note the bundles of white fibres, and the elastic fibres (Fig. 33).
Sketch.

Add acetic acid : the white fibres will be dissolved, the elastic fibres
more readily distinguished, and the connective-tissue cells seen ; the latter
and the delicate ground-substance will be rendered more distinct by
staining. For comparison, mount in salt-solution a piece of one of the
fine tendons of the toes : treat as before, and examine. Sketch.

8. Cartilage, — Snip off the thin edge of the omo- or xiphi-sternum
or supra-scapula and examine it as before in a drop of salt-solution.
Or, cut a thin section of the head of the humerus or femur with a razor.

Note the transparent, homogeneous matrix, containing numerous
lacuna, in each of which is a nucleated cell : observe here and there
the groups of cells formed by binary fission (Fig. 34). Stain, and
sketch.

9- Bone. — For the examination of dried bone, cut a very thin slice of
one of the long bones with a fret-saw : fasten it to a slide with Canada
balsam (p. 136), and, when the balsam has dried quite hard, rub down the
section on a hone until it is thin enough to be quite transparent. Or,
a transverse section of a bone from a larger animal may be prepared in
the same way or bought from a dealer in microscopic objects.

a. Examine first a transverse section of dry frog's bone (e.g. femur or
humerus), and note the marrow-cavity the lamella, and the lacuna and
canaliculi", the two last will probably appear black (p. 117 and
Fig. 35). A section of human bone, such as is usually supplied ready
prepared, or of the bone of some other larger animal, shows a more
complicated structure : instead of a single system of lamellae, the bone
consists of a number of such systems, each surrounding a central canal,
in which blood-vessels and nerves run, and which corresponds to the
marrow-cavity in the simpler frog's bone described above. Sketch.

b. Compare with a section of decalcified frog's bone,1 and notice — the
fibrous lamella arranged in two layers, the outer of which is closely
invested by the periosteum ; the lacuna, containing bone-cells ; and
the outer and inner layers of osteoblasts (Fig. 36). Sketch.

(For the histology of nervous tissue see Chapter X.)

1 The method of preparing sections of this and other tissues will be
described at the end of the next chapter.

CHAPTER VIII.

THE FROG (continued) : THE MICROSCOPIC EXAMINATION

OF THE COMPOUND TISSUES GLANDS — SECRETION AND

ABSORPTION.

WITH the exception of the tissues of the nervous system,
which will be described in Chapter X, we have now
studied the principal simple tissues by the method of dis-
sociation, i.e. by separating their constituent parts. We have
now to consider the way in which these tissues are combined
in the various organs, and for this purpose must adopt some
method of examination by which they are seen in their
natural relations.

The method adopted for this purpose is that of section-
cutting. You know how, by cutting sections, in various
directions, of a bit of twig, the arrangement and natural
relations of its various parts — wood, bark, and pith — can be
ascertained. The same thing applies to the organs of the
frog and other animals, but owing to their soft and non-
resistant texture, it is impossible to cut them into sections
thin enough for microscopic examination without special
preparation. The methods employed are by no means easy
for the beginner, especially without verbal instruction and
the resources of a biological laboratory, but in the event of
your wishing to make the preparations described and

CHAP. VIII

SKIN

127

figured in this chapter for yourself, the directions' given on
PP- I35 — J39 are simple enough to be carried out with very
limited appliances.

FIG. 38. — Vertical section of the frog's skin, Highly magnified.

D. derm, formed of hf. hf . hf". horizontal, and v.f. vertical fibres of connective-
tissue, and containing b. v. blood-vessels, and pg. pigment cells. E, epiderm,
with m. 1. active or Malpighian layer, and h. L horny layer of epithelial
cells ; c. gl. cutaneous gland in section ; c.gl '. in surface view ; d. duct. (After
Howes.)

The Skin. — A vertical section of the skin, i.e. one taken at
right angles to its surface, will be seen to have the following
structure.

128 THE FROG CHAP.

The skin is clearly divisible into two layers, an outer,
the epiderm (Fig. 38, E) and an inner, the derm (D). The
epiderm is built up of several layers of epithelial cells.
These differ greatly in form according to their position,
those in the lower or internal layer (m. /) being columnar,
while those in the upper or external layer (h. /) are squamous,
and have their protoplasm converted into horny matter so
as to furnish a comparatively hard and insensitive covering
to the body.

The horny layer is cast off periodically in shreds, and to
make up for this, the cells of the inner or deep layer
multiply by binary fission, the increase in their number
necessarily resulting in a pushing upwards of the super-
jacent layers. There is thus a constant travelling of cells
from the inner to the outer surface of the epiderm : as
they pass towards the outer surface they become more and
more flattened, and at last squamous and horny. The
whole process takes place in such a way that the multi-
plication of the columnar cells in the lower layer is just
sufficient to make good the loss of the squamous cells in the
superficial layer.

The derm (D) is formed of connective-tissue, the fibres of
which are mostly horizontal (h.f, h.f, h.f"\ or parallel to the
surface of the skin, but at intervals are found bands of
vertical fibres (v.f). The derm also differs from the
epiderm in having an abundant blood-supply (^. v), capillaries
ramifying through it in all directions. It also contains
nerves, the ultimate fibres of which have been traced into
the deeper layers of the epiderm. Imbedded in the derm,
especially in its external portion, are irregular cells (pg), the
protoplasm of which contains pigment, often appearing in-
tensely black. It is to these pigment-cells that the coloured
patches in the frog's skin are due (Figs. 24 and 27).

vni CUTANEOUS GLANDS 129

In this, as well as in the other sections described in the
present chapter, the structure of the nuclei of the various
cells can be more easily made out than in the fresh prepara-
tions you have already examined. Each nucleus will be
seen to be enclosed by a definite nuclear membrane, and to
contain in its interior a number of minute bodies, which
take up the stain more deeply than the rest of the nucleus.
One or more of these bodies may correspond to the nucleoli
already seen (p. 109), but many of them are of a different
nature and can often be seen to form a network : the material
of which these are composed is known as chromatin, and is
surrounded by a semi-fluid substance forming the ground-
work of the nucleus.

Cutaneous Glands — Secretion. — In the superficial part of
the derm are seen numerous rounded spaces (Fig. 38,
c. gl, c.gl'\ each of which can be proved, by taking sections in
various directions, to be a nearly globular cavity, from which
a narrow canal (d), like the neck of a flask, passes through
the epiderm to open on the external surface. Both the
body and the neck of the flask are lined with epithelium,
the cells lining the body being nearly cubical, those of the
neck squamous.

These structures are called cutaneous glands : they per-
form the function of manufacturing the slimy fluid which,
as we have seen, is constantly exuding on the surface of
the skin. The epithelial cells of the gland have the power
of forming minute droplets of the fluid out of the materials
supplied to them by the blood : the droplets escape from
the cells and accumulate in the interior of the gland, whence
the fluid is finally discharged by the duct.

The cells lining the duct are continuous on the one hand
with those of the gland, and on the other with those of the
epiderm. The whole gland with its duct is to be looked

PRACT. ZOOL. K

130

THE FROG

upon as a depression of the skin, lined by pushed-in epiderm
cells.

Epithelial cells having the power of manufacturing and
discharging a specific substance are called gland-cells, and
the process of manufacture is known as secretion. We
have already met with isolated gland-cells in the case of the
goblet-cells of the intestine (p. 109), which secrete mucus;

FIG. 39. — A, transverse section of frog's intestine ; B, small portion of the same

highly magnified.

b. v. blood-vessel ; c. in. circular layer of muscle-fibres ; ep. epithelium ; /. in
longitudinal layer of muscle-fibres ; pr. peritoneum ; m. muscular layer ; s. m.
submucosa. (After Howes.)

but commonly, as in the present instance, gland-cells are
aggregated into a definite organ called a gland.

The Intestine. — A transverse section of the intestine
shows also a very definite and characteristic combination of
simple tissues. The mucous membrane, like the skin, is
composed of two layers, an epithelial layer (Fig. 39, ep\
corresponding to the epiderm, and a connective-tissue layer
(s.m\ corresponding to the derm and called the submucosa.
The epithelium consists of a single layer of cells only (B, ep\
all columnar, and with their long axes at right angles to the

vi n GASTRIC GLANDS Iji

elevations into which, as we have seen (p. 71), the mucous
membrane is thrown. Amongst the ordinary epithelial cells,
numerous mucus-secreting goblet-cells will be recognised.
The submucosa, like the derm, contains blood-vessels, lym-
phatics, and nerves.

The muscular layer (A, ;//) is also divisible into two : an
outer layer of longitudinal fibres (B, /.;//), running parallel
with the long axis of the tube, and an inner, much thicker
layer of circular fibres (c.m) which encircle it, and conse-
quently lie at right angles to the longitudinal fibres. Thus
in a transverse section, such as Fig. 39, the fibres of the
circular layer are cut longitudinally, those of the longitudinal
layer transversely, while the opposite would be the case in a
longitudinal section.

The peritoneum (pr) which, as we have seen (p. 27)
forms an outer covering to the intestine, is formed of a thin
inner layer of connective-tissue and an outer of squamous
epithelium.

The Stomach. — Transverse sections of the stomach (Fig.
40) show it to differ from the intestine not only in the much
greater thickness of all its layers, but also in the fact that
the epithelium, instead of simply forming an even layer over
the ridges of mucous membrane, is sunk into the submucosa
in the form of simple or branched tubes, the gastric glands
(&• gfy These differ from the cutaneous glands in being
not flask-shaped but test-tube shaped, each being a long,
narrow tube, with an extremely small cavity (B and c, c).
They are lined by a single layer of gland-cells, and open
by minute apertures (m) on the surface of the mucous
membrane.

The cells of the gastric glands have the power of forming,
out of the materials supplied to them by the blood, the
gastric juice, by which, as we have learnt (p. 74), proteids
are digested. Thus, while the raw material supplied to both

K 2

THE FROG

CHAP.

cutaneous and gastric glands is the same, the manufactured
article is entirely different in the two cases. Each kind of
gland-cell has the faculty of picking and choosing, the
material supplied being worked up in the one case into the
cutaneous secretion, in the other into gastric juice.

l.7Tt

FIG. 40. — A, part of a transverse section of the frog's stomach ; B, one of the gastric
J glands in longitudinal section, highly magnified ; C, transverse section of a

gastric gland.

b. r'. blood-vessel ; c. cavity of gastric gland ; c. in. circular muscles ; c. in. m.
circular layer of muscularis mucosse ; ep. epithelium ; g. gl. gastric glands ;
/. ///. longitudinal muscles ; /. in. in, longitudinal layer of muscularis mucosae ;
;//. mouth of gastric gland ; mi. nucleus ; pr. peritoneum ; s. in. submucosa.,

The submucosa of the stomach is traversed by a narrow
band of unstriped muscle, the muscularis mucoscc, formed,
like the main muscular layer, of an outer layer of longitudinal
(/.;;/.;;/) and an inner of circular (c.m.m) fibres.1

1 A very thin muscularis mucosoe is also present in the intestine
(compare Fig. 39, B).

VIII

LIVER

133

The Liver.— Sections of the liver show it to be made up of
innumerable large, polyhedral cells (Fig. 41, hp, c\ arranged
in columns, and bounding extremely fine channels or bile-
passages (c.b) b.p]. These are found to open into one

bp
\-nif

FIG 41.— A, portion of a section of the frog's liver ; B, small portion of the same,

showing the origin of a bile-duct.

bl, blood-capillaries ; b.p., c.b, c.b', c.b", bile-passages; c.b"', the same in^ trans-
verse section ; c, hp, liver cells ; d, smallest bile-duct ; nc, nu. nuclei. (A,
after Howes ; B, after Hoffmann.)

another, and finally to discharge into definite tubes (B, d\
lined with epithelium. These, in their turn, unite into larger
and larger tubes, which form the hepatic ducts and ultimately
open into the common bile-duct (p. 68).

'34

THE FROG

The liver-cells are glandular and secrete the bile, which,
as it is formed, drips into the bile-passages and passes into
the hepatic ducts, thence making its way either directly into
the intestine or into the gall-bladder (p. 70). The whole
liver, which is the largest gland in the body, is traversed by
a complex network of capillaries (^/), arising partly from
the hepatic artery, partly from the portal vein ; and from
the blood thus supplied, the liver-cells obtain the materials
necessary to enable them to discharge their function of
secreting the bile.

FIG. 42. — A, small portion or a section 01 the frog's pancreas ; B, diagram showing

the connection between the lobules and ducts.
c. connective-tissue covering the lobules ; d. duct ; /. lobules ; nil. nuclei. ]&fK

The liver-cells have, however, other functions, one of
which is to manufacture a substance called glycogen or
animal-starch. This is stored up in the cells in the
form of minute insoluble granules, which, being afterwards
transformed into soluble sugar, pass into the blood and
so to the tissues.

The Pancreas. — Sections of this gland (Fig. 42) show
it to be made up of microscopic masses or lobules (/), each
of which consists of a cluster of gland-cells enclosing a very

vin NUTRITION 135

narrow central space. The cavities of adjacent lobules com-
municate with one another and open into tubes or ducts (d)
lined with epithelium, which join with one another and
finally discharge into the bile-duct as it traverses the pancreas
(p. 70). The pancreas is distinguished as a racemose or
grape-bunch gland : the duct is comparable to the branched
stalk and the lobules to the grapes.

Connection of the foregoing facts with the physiology
of nutrition. — You will now be able to understand more
clearly the various processes connected with the nutrition
of the frog, hitherto studied without the aid of histology.

When the food enters the enteric canal the various gland-
cells are stimulated into activity, and the gastric juice, bile,
and pancreatic juice are poured out and mingle with the
food, which is digested in the manner already described.
The soluble products of digestion — peptones, sugar, salts,
fatty acids, and glycerine, diffuse through the epithelium of
the enteric canal into the blood-capillaries of the underlying
submucosa, and the blood, now loaded with nutriment, is
carried by the portal vein to the liver and thence by the
hepatic and postcaval veins to the heart (see Fig. 23). At
the same time the fats make their way into the lymph-
capillaries and are finally pumped, by the lymph-hearts,
into the veins. Thus the products of digestion all find
their way ultimately into the blood, and are distributed,
through the circulatory mechanism, to all parts of the
body.

PRACTICAL DIRECTIONS.
Materials required for the Preparation and Sectioning

of Animal Tissues. — In • addition to the requisites mentioned in
chapters I and VII, the following will be required : —

a. Corrosive sublimate : a saturated solution in water. Care should
be taken in using this solution, as it is a very virulent poison.

I36 THE FROG CHAP.

b. Absolute alcohol.

c. Turpentine.

d. Paraffin: "hard" and "soft." Get an ounce or two of each.

e. A solution of Canada balsam in chloroform or turpentine. This
should be kept in a small glass bottle, with a ground glass cap — not a
stopper or cork.

f. A solution of alcoholic borax-carmine. This (like many other useful
staining-solutions, such as Ehrlich's hamatoxyliri) may be bought ready
prepared ; or it may be made as follows : — Grind up in a mortar 2
grammes of carmine and 4 grammes of borax, and dissolve in loo c.c.
of distilled water : to this solution add an equal volume of 70 per cent,
alcohol : allow to stand for a day or two and filter.

g. A water-bath, in which melted paraffin may be kept at a constant
temperature. For a make-shift you can use a saucepan with a flat
piece of tin over it ; fill the saucepan about half full of water, and
heat it over a spirit lamp or a small oil-lamp or gas-burner, regulating
the distance of the flame so as to keep the temperature of the water at
about 55° C. (131° F.).

h. Two or three watch-glasses or other small shallow vessels for con-
taining melted paraffin.

i. A sharp, flat-ground razor.

i. A section-lifter, made by beating out flat about half an inch of the
end of a piece of stout copper wire, about 6 in. long, and bending the
flattened portion at an obtuse angle with the rest.

Preparation of Tissues for Section-Cutting.

a. Fixing^ hardening, and decalcifying.

Sections may be cut from specimens which have been carefully pre-
served in alcohol : — first in 70 per cent. , and after a day or two trans-
ferred to 90 per cent. But certain other reagents are more effective for
the purpose vi fixing the tissues — i.e., quickly killing and coagulating
the protoplasm of the cells with a minimum of shrinkage, — and of these
the one most generally useful is a solution of corrosive sublimate (see a,
above), in which, from a freshly-killed frog, place small pieces of the
various organs and tissues to be examined — e.g., skin, intestine, stomach-
liver, pancreas, kidney, ovary, spermary, and spinal cord, as well as
the inner half of the eyeball. The intestine and stomach should be
first washed out in salt-solution, and then cut into pieces about f inch
long ; the liver should be cut into pieces not more than £ inch cube.
After about half-an-hour to two hours, according to the size of the

vin SECTION-CUTTING 137

piece, place in water under a tap, and wash thoroughly for a quarter
of an hour or more, until the corrosive sublimate is removed.

After washing, transfer to 50 per cent, alcohol for a few hours,
and then to 70 per cent, for twenty-four hours, after which the pieces
may be stained at once (see below), or transferred to strong methylated
spirit (90-93 per cent.), in which they may be kept until wanted. This
completes the process of hardening : it is done gradually, by alcohols of
increasing strength, in order to avoid shrinkage.

In order to decalcify such tissues as bone, from which the lime-salts
must be extracted before cutting into sections, place a small piece for a
few days in 70 per cent, alcohol to which 2 per cent, of strong nitric
acid has been added : then wash thoroughly, transfer to alcohol, and
stain.

b. Staining. — Place the organs, cut into convenient sizes for im-
bedding— i.e. not more than £ inch long, and, in the case of such
organs as the liver, \ inch in thickness — into borax-carmine or hoema-
toxylin for one or two days or even more. They will become stained
throughout, and the difficulty of staining the sections after cutting will
thus be avoided. After staining, place them in weak alcohol (50-70
per cent.), slightly acidulated with hydrochloric acid : if a watch-glass
or some such vessel is used, it is sufficient to dip the end of a glass rod
into acid and stir it round in the alcohol. The effect of the acid is to
remove much of the colour from the protoplasm of the cells, leaving
the nuclei brightly tinted. After half-an-hour or less in the acid alcohol,
place the tissues once more in strong methylated spirit.

c. Dehydrating. — Transfer from the methylated spirit to absolute
alcohol, which must be kept in a stoppered or tightly corked bottle, as
it will otherwise deteriorate by absorption of water from the air. It has
the effect of withdrawing the last traces of water from the tissues, an abso-
lutely necessary step in order that they may be permeated with paraffin.

d. Imbedding. — Transfer the objects from absolute alcohol to turpen-
tine. This fluid acts as an intermediary between alcohol and paraffin,
being freely miscible with both : it gradually replaces the alcohol in the
tissues, rendering them transparent. If they are not transparent in the
course of an hour or so, the process of dehydration has not been com-
plete, and they must be returned to absolute alcohol. In the mean-
time, melt some paraffin over the water-bath, using various mixtures
of hard and soft according to the season : in a cold room in winter soft
paraffin will be hard enough ; in the height of summer hard paraffin

133

THE FROG

alone will be suitable. The temperature of the water-bath must never
be allowed to rise more than a degree or two above the melting point
of the paraffin. Transfer the objects from turpentine to melted paraffin
and keep them in it for some hours — the time varies according to the
size of the piece — until they are thoroughly permeated.

If you wish to cut sections by hand, get some ordinary medium -sized
bottle-corks : around each wind a piece of paper, allowing it to project
about J an inch beyond one end of the cork, and fixing it with a pin,
as in Fig. 43, A. Into the little cylindrical vessel or imbedding box
thus formed pour some melted paraffin, and immediately transfer to it,
by means of a warmed section-lifter or forceps, one of the prepared

pieces, adjusting its position by

A p means of a heated needle. When

& the paraffin is quite cold remove

J) AT^PV ^e PaPer> anc^ vou wiN have

fixed to the cork a solid block
of paraffin containing the object
to be cut.

e. Section- cut ting. — Pare away
the block of paraffin until the
object comes into view : then
trim the block, as in Fig. 43, B,
until its upper surface, with the
object in the middle, is not more
than \ inch square. Hold the
cork firmly in the left hand, with
the wrist resting on the table,
and with a razor cut the thin-
nest possible slices of the paraffin

block, including the imbedded object. The razor must be held firmly
grasped at the junction of blade and handle, and kept with the surface
of the blade parallel with that of the block : use almost the whole length
of the edge for each section. With a little practice you will be able
to cut sections so thin as to be quite transparent under the high power.1

1 If you are working in a properly furnished laboratory you will
probably learn how to cut sections with a microtome, or section-cutting
machine, which gives much better results and is absolutely necessary
when a complete series of sections of the same object is required,

FIG. 43. — A, imbedding box made by wrap-
ping paper round a cork ; B, cork after
removal of the paper, showing the
paraffin pared down to a convenient
size for sectioning. «, object to be
cut.

viii PRACTICAL DIRECTIONS 139

f. Clearing and mounting. — Place the section on a slide and warm
it gently on the water-bath until the paraffin melts, and then add a
large drop of turpentine in order to dissolve the paraffin. Then draw
off the turpentine with blotting-paper and replace it by a fresh drop,
repeating the process until all the paraffin is dissolved : put on a cover-
glass, and examine.

If you wish to be sure that the parts of your sections are not displaced
in mounting, or to mount several sections on your slide, the latter should
first be smeared over with a very thin layer of a mixture of collodion
and oil of cloves, in equal parts: then place the sections on the slide,
warm, and immerse the whole slide in a small vessel of turpentine,
leaving it until all the paraffin is dissolved.

In order to make a permanent preparation, remove the paraffin
with turpentine, as above, draw off the turpentine, place a drop of
Canada balsam on a cover-glass and very gently lower the cover-glass
on the object, spreading out the balsam in a thin, even layer. Before
long, the balsam will have set quite hard, and the sections may be pre-
served for an indefinite period ; the balsam will set more quickly if you
leave your preparations over the water-bath for a short time.

Remember that object, razor, slide, and cover must be kept free
from water, the presence of which, from the stage of dehydration on-
wards, is fatal to success.

Examination of Compound Tissues.

Examine the following sections, prepared as described above, first
with the low, and then with the high power, noting the parts
enumerated in each case, as well as the structure of the nuclei (nuclear
membrane, chromatin, and micleoli].

1. Vertical section of skin (Fig. 38).

a. Epiderm, stratified, divisible into outer (horny] and inner (Mal-
pighian) layers.

b. Derm, connective-tissue fibres, blood-vessels and pigment-cells.

c. Cutaneous glands with their ducts. The apertures of the ducts on
the surface you will probably have noticed already in your preparation
of the epithelial cells of the skin (p. 124).

Sketch.

2. Transverse section of intestine (Fig. 39).

a. Mucous membrane : a superficial epithelial layer of columnar cells,

140 THE FROG CHAP, vin

with goblet-cells amongst them ; and a deeper connective-tissue layer,
the submucosa^ enclosing vessels and nerves.

I). Muscular layer : an external longitudinal, and an internal circular
layer of unstriped muscular fibres.

c\ Peritoneal layer. This is very thin, and a careful examination
of good preparations is required in order to make out its structure

(P- 131).
Sketch.

3- Transverse section of stomach (Fig. 40).— After recognising

the layers as above, note :—

a. The gastric glands ^ and b. the muscnlaris untcosce.
Sketch.

4- Sections of liver (Fig. 41).

a. Polyhedral gland-fells^ arranged in columns ; b. bile-passages and
ducts ; c. blood -capillaries and vessels.
Sketch.

5- Sections of pancreas (Fig. 42).

a. Lobules, separated by connective-tissue, and each consisting of
gland-cells ; b. ducts.
Sketch.

CHAPTER IX.
THE FROG (continued}: RESPIRATION AND EXCRETION.

IN the fifth chapter it was pointed out that a continual
waste of substance goes on in the frog's body, the lost
material taking the form of three chief waste-products or
products of excretion, water, carbon dioxide, and urea. It was
further stated that these substances are got rid of by means
of the lungs, the kidneys, and the skin.

The Organs of Respiration.— At their anterior ends the
two lungs open into, a small, thin- walled chamber (Fig. 44,
/. tr. c\ which, as it corresponds both with the larynx or
organ of voice, and the trachea or windpipe in ourselves, is
called the laryngo-tracheal chamber-, it communicates with
the pharynx through the glottis (gt). The walls of the
chamber and the edges of the glottis are supported by
cartilages (ar).

The structure of the lung is best made out by distending
it with air. and then placing it in formaline or spirit until
thoroughly hardened : its walls contain so much elastic tissue
that if cut when fresh, it contracts immensely, and its struc-
ture is then difficult to see. The inner surface of the lung is
raised. up into a complex network of ridges (A, r. Ing], which
project into the interior and produce the appearance of an

142

THE FROG

irregular honeycomb. All these ridges are abundantly
supplied with blood-vessels fed by the pulmonary artery, the
blood being carried away by the pulmonary vein.

The main substance of the lung is made of connective-
tissue containing elastic fibres and unstriped muscle, and
traversed by a network of capillaries. Its cavity is lined
by a layer of pavement-epithelium, and its outer surface
is covered with peritoneum.

Respiratory Movements. — In breathing, the frog depresses
the floor of the buccal cavity (Fig. 45, A), and, the mouth

A 9l

p.c.hy

ring.

FIG. 44. — The respiratory organs of the frog from the ventral aspect ; B, the laryngo-

tracheal chamber in longitudinal section, with the right lung.

ar. the arytenoid, or principal cartilage of the larynx ; b. hy. body of hyoid ; gl.
glottis ; /. Ing. left lung ; /. tr. c. laryngo-tracheal chamber ; /. c. hv. posterior
horn of hyoid ; r. Ing. right lung, laid open in A to show its internal surface ;
v. cd. vocal cord. (After Howes).

being kept shut, air is drawn in through the nostrils. The
floor of the mouth is then raised (.B} by muscles attached
to "the hyoid. At the same time the anterior end of the
lower jaw presses upon the movable premaxillae (pwx), the
upward processes of which (p. 42, Figs. 8 and 9, PMX) act
upon certain cartilages in connection with the external
nostrils in such a way as to produce closure of these
apertures (Fig. 45, £}. The gullet (gul) is so contracted,
except during the act of swallowing, as to be practically

RESPIRATORY MOVEMENTS

143

closed. Thus when the floor of the mouth is raised, the air
contained in it can escape in one way only, viz., through the
glottis into the lungs.

Thus inspiration, or breathing-in, is produced by the
buccal cavity acting as a force-pump : the lowering of its
floor draws in air through the nostrils, the raising of its floor
forces the imprisoned air into the lungs. Expiration, or

FIG. 45. — Diagrams illustrating the respiratory movements of the frog. In A the
floor of the mouth is depressed and air is being drawn in through the nostrils ;
in B the floor of the mouth is raised, the nostrils are closed, and air is being forced
into the lungs.

e. n. external nostril ; gl. glottis ; gul. gullet ; /. //. internal nostril ; Ing, lung ;
olf. s. olfactory sac ; pmx. premaxilla ; tng. tongue.

breathing-out, is due to a contraction of the elastic lungs,
accompanied by a slight lowering of the tip of the lower
jaw : the latter movement releases the premaxillse and thus
opens the external nostrils.

M4 THE FROG CHAP.

Respiration. — By these alternate movements of inspira-
tion and expiration, fresh air passes into the lungs at regular
intervals, while part of the air already contained in them is
expelled. Now we saw, when studying the blood (p. 79),
that dark purple blood drawn from a vein becomes bright
scarlet when exposed to air, and we subsequently learnt
(p. 107) that this change is due to the absorption of oxygen
by the red corpuscles.

The blood brought to the lungs by the pulmonary artery
is, as we have seen (p. 94), non-aerated, being the impure
blood returned by the three caval veins to the right auricle.
When this blood is pumped into the capillaries of the lungs
it is separated from the air contained in those organs only
by the extremely thin walls of the capillaries themselves
and the equally delicate pavement-epithelium lining the
lungs (p. 142, Fig. 23, Cp. Ing, Ep. Ing). Under these
circumstances an interchange of gases takes place between
the air and the blood : the haemoglobin of the red
corpuscles absorbs oxygen, and the carbon dioxide in the
blood, derived from the waste of the tissues, is given off into
the cavities of the lungs. The blood in the pulmonary
capillaries thus become aerated and is returned as red blood
to the left auricle : at the same time it loses carbon dioxide,
together with a certain amount of water, and these waste
substances are expelled from the body with the expired air,

Voice. — It was mentioned above (p. 141) that the glottis
and laryngo-tracheal chamber are supported by cartilages.
The largest of these are a pair of semilunar arylenoid
cartilages (Fig. 44, ar\ which bound the glottis to right and
left. The mucous membrane on the inner or adjacent faces
of the arytenoids is raised into a pair — right and left — of
horizontal folds, the vocal cords (v.cd). By means of muscles
these folds can be stretched and relaxed, and can be brought

IX

KIDNEYS

into either a parallel or a divergent position. When they
are parallel the air, passing to and from the lungs, sets their
edges in vibration and gives rise to the characteristic croak,
the pitch of which can be slightly altered by stretching or
relaxing the cords.

Structure of the Kidneys. — The form and situation of the
kidneys (Figs. 3, 4, 5, and 7, kd) have already been referred
to. Each is a flattened organ of a deep reddish-brown
colour, its inner edge nearly straight but for one or two
notches, its outer edge curved. Its ventral face is covered

ur.tu

m.cfi

per'

nst

FIG. 46. — Transverse section of frog's kidney.

gl. glomerulus ; ;//. cp. Malpighian capsule ; nst: nephrostome (see p. 98 and Part II) ;
Per. peritoneum covering ventral face of kidney ; per' . fold of peritoneum sup-
porting its outer border ; per", fold supporting its inner border ; Ur. ureter ;
ur. tu. urinary tubules. (After Marshall and Bles.)

by peritoneum (Fig. 46, per\ continued on the one hand
into the parietal layer (Fig. 5, /. per), of that membrane, on
the other into the mesentery (mes) ; its dorsal face is bathed
by the lymph of the sub-vertebral sinus (sv. fy. s). From the
posterior end of its outer edge a delicate tube, the ureter
(Figs. 3, 4, and 7, ur), passes backwards and opens into the
dorsal wall of the cloaca. On its ventral face is a singular
yellowish-white stripe, the adrenal body, an organ of im-
perfectly-known function (Fig. 7, between the lines from
kd and ts\

PKACT. ZOOL. L

146

THE FROG

Of.V

A thin section shows the whole kidney to be made up of
a tangled mass of microscopic tubes (Fig. 46, ur. tu), so

twisted together that
any section cuts them
in various planes, some
transversely, when they
appear as circles, others
longitudinally or ob-
liquely. Amongst these
urinary tubules or ne-
phridia, as they may be
called, are seen globu-
lar sacs, the Malpighian
capsules (;;/. cp), each
having in its interior a
little rounded bunch,
known as the glomerulus
(gl\ Very accurate ex-
amination of numerous
sections, as well as of
teased-out specimens,

shows that each Mal-
'«/-

FIG. ^.-Diagram of a single urinary tubule pighian CapSUk (Fig. 47,

with its blood-vessels to illustrate the /;/> cp\ Js connected with
structure 01 the trog s kidney. f ''

afv. afferent vessel I of glomerulus ; Cp capil- a urinary tubule (ur. tll\
l.irv network nf kirinev ! cf. 71. efferent J

to which it forms a
blind, bulb-like end.
The tubule itself winds
through the substance of
the kidney, is joined by
other tubules, and finally discharges into the ureter (ur).
The tubules are lined with somewhat cubical cells of
glandular epithelium, which, in some parts (ur. tu, ur. tu"\

lary network or kidney; ef.v. enetent
vessel of glomerulus ; gl. glomerulus ;
in. cp. Malpighian capsule, showing epi-
thelium (which in reality also covers the
glomerulus) ; nst. ciliated nephrostome ;
r. a. renal artery; r. pt. v. renal portal
vein ; r. v. renal vein ; ur. ureter ; ur. tu.,
ur. tu'., ur. tu.", ur. tu.'", different por-
tions of urinary tubule, showing epithelium

and cilia.

ix EXCRETION 147

are ciliated. The Malpighian capsules are lined with
flattened cells of pavement epithelium.

The arrangement of the blood vessels is peculiar. Like
other organs, the kidney is permeated by a network of
capillaries (cp) which form a close mesh between the urinary
tubules, so that the cavity of the tubule is separated from
the blood only by the thickness of the gland-cells and of the
capillary wall. The capillary network is supplied partly by
the renal arteries (r. «), partly by the renal portal vein (r.pt. v},
and is drained by the renal veins (r. v). It is in the
behaviour of the renal arteries that the chief peculiarity of
the kidney-circulation lies. On entering the kidneys they
break up into smaller and smaller arteries, but each of the
ultimate branches (af. v\ instead of discharging into the
general capillary network, passes to a Malpighian capsule, in
the interior of which it breaks up into a little bunch of
coiled capillaries (gl), the glomerulus. From this the blood
is carried off by a minute vessel (ef. v) by which it is poured
into the general capillary network and finally discharged
into the renal vein (r. v).

Renal Excretion,— While circulating through the glorner-
ulus, water and certain soluble matters are separated from
the blood and make their way into the Malpighian capsule
and thence into the urinary tubule. As the blood circulates
through the general capillary network, the gland-cells of the
tubules excrete, out of the materials brought to them by
the blood, the nitrogenous waste matter urea, in the formation
of which the liver plays an important part ; it is discharged
from the cells into the cavity of the tubule, where it is dis-
solved in the water separated out of the glomerulus. In this
way the urine is formed. Accumulating in the tubules, it
makes its way into the ureter and thence drop by drop into
the cloaca, whence it is either expelled at once, or stored for
a time in the bladder.

L 2

148 THE FROG

Note that the formation of urine is a process of secretion
of a similar nature to the secretion of gastric juice, bile, &c.
The fluid secreted is, however, of no further use to the
animal, and would, in fact, act as a virulent poison if retained
in the system. . It is therefore got rid of as soon as possible.
Secretions of this kind, consisting not of useful but of
harmful or waste matters, are distinguished as excretions.

Bile is also in part an excretion as it contains pigments due to the
disintegration of haemoglobin, and thus by its means the effete colouring
matters of the blood are passed into the intestine and got rid of.

Pulmonary and cutaneous Excretion. — The lungs, be-
sides being organs of respiration, take their share in excretion,
since they get rid of the important waste product, carbon
dioxide, together with a considerable quantity of water.
Similar functions are discharged by the skin, which is also an
organ both of respiration and of excretion. Interchange of
gases takes place between the outer air. and the blood in the
capillaries of the derm : the carbon dioxide of the non-aerated
blood brought to the skin by the cutaneous artery (p. 93
and Fig. 23) is exchanged for oxygen, and the blood, in the
aerated condition, is returned by the musculo-cutaneous
vein to the heart. Moreover the cells of the cutaneous
glands separate water and other constituents from the blood,
and the fluid thus formed is poured out on the surface of
the body. Here it serves to keep the skin moist, and is
finally lost, either by evaporation or by mingling with the
water in which the frog is immersed. The cutaneous
secretion has also poisonous properties, and so probably
serves as a defence against some of the animal's enemies.

Summary of the processes of Nutrition.— We are now
in a position to understand the general features of the
whole complicated series of processes which have to do with
the nutrition of the frog, which are collectively spoken

ix NUTRITION 149

of as metabolism* These processes are illustrated in the
diagram (Fig, 23, p. 91), which should be constantly
consulted in connection with the following summary.

All parts of the body are placed in communication with
one another by means of the blood-vessels, through which a
constant stream of blood is flowing in a definite direction.

In all parts, waste of substance (destructive metabolism) is
continually going on, and the waste products, water, carbon
dioxide, and some nitrogenous substance which ultimately
takes the form of urea— are passed either directly into the
blood, or first into the lymph and thence into the blood.

At the same time the cells withdraw nutrient materials
from the blood, and assimilate them, i.e., form new living sub-
stance, whereby the waste is made good, and the tissues
adequately nourished (constructive metabolism}. Oxygen is also
withdrawn from the blood ; like the air supplied to a fire, it is
essential to the oxidation or low temperature combustion
with which the waste of the tissues is associated. By the
withdrawal of its oxygen the haemoglobin of the blood alters
its colour from scarlet to purple.

Thus the blood as it passes through the body is constantly
being impoverished by the withdrawal of nutrient matters
and of oxygen, and as constantly being fouled by the
discharge into it of waste products. It reaches the capil-
laries of an organ as bright red, aerated blood, and leaves
it as purple, non-aerated blood.

These changes, by which the blood loses nutrient matters
and oxygen, and gains waste products, takes place all over
the body. The converse processes by which nutrient matters
and oxygen are absorbed and waste products got rid of are
carried on in certain definite portions of the circulatory
system.

In the walls of the enteric canal (Fig. 23, Ent. C), the

ISO THE FROG CHAP.

products of digestion pass into the blood (Cp. Ent. C\ or in
the case of fats, first into the lacteals and ultimately into the
blood. In this way the due proportion of nitrogenous and
other food-materials is kept up.

In passing through the capillaries of the lungs (Cp. Lng\
carbon dioxide is exchanged for oxygen and a certain
amount of water is given off. In the capillaries of the skin
(Cf. Sk), a large quantity of water and smaller proportions
of other waste matters are got rid of. In the kidneys
(Cp. Kd\ a considerable quantity of water, together with the
bulk of the urea, are removed from the blood and finally
expelled from the body.

Note that all these changes are produced by the special
activity of particular groups of epithelial cells, which,
however alike they may be in general appearance and
structure, have a marvellous selective faculty 'peculiar to
themselves. Like all other parts of the body, they are
constantly undergoing the usual processes of waste and
repair, withdrawing nutrient matters and oxygen from the
blood, and passing waste matters into it. But, in addition
to the ordinary processes of nutrition, each particular group
of cells has the power of withdrawing a specific substance
from the blood or of passing substances into it. Thus the
epithelial cells of the enteric canal (Ep. Eni) pass in
digested food, those of the skin (Ep. Sk) and Malpighian
capsules (Mlp. Cp} withdraw water, those of the urinary tubules
(Ep. Ur. 7), urea, and so on. Similarly, the various gland-
cells, such as those of the liver (Lvr. C\ pancreas (Pn. C\
gastric and cutaneous (Cu. Gl) glands, withdraw specific
substances, or secretions, which are discharged on the free
surface of the epithelium and serve various purposes.

We see that the blood loses — (i), nutrient matters and
oxygen all over the body ; (2), water in the skin, lungs, and

ix NUTRITION 151

kidneys ; (3), carbon dioxide in the lungs and skin ; (4), urea,
principally in the kidneys ; and (5), various substances in
the glands. It gains (i), waste products all over the body;
(2), nutrient matters in the enteric canal ; (3), liver-sugar
(p. 134) in the liver ; (4), oxygen in the lungs and skin. It
is therefore richest in oxygen and poorest in carbon dioxide
as it leaves the lungs and skin, i.e. in the pulmonary and
musculo-cutaneous veins ; richest in nutriment as it leaves
the enteric canal, i.e. in the portal vein ; poorest in urea as it
leaves the kidneys, i.e. in the renal veins ; poorest in water
as it leaves the skin and kidneys, i.e. in the cutaneous and
renal veins.

In this way a single closed system of pipes not only
supplies all parts of the body with everything necessary for
their sustenance, but serves also as a drainage system to
carry away their various waste products.

Notice that we must distinguish between the nutrition,
respiration, and excretion of the frog as a whole, and of its
various parts. Every one of the thousands of cells, fibres,
&c., in the entire body is nourished, breathes, and excretes,
taking its nourishment and oxygen directly from the brood,
and discharging its waste products into it. What are
called the organs of nutrition and respiration are special
portions of the body set apart for taking in fresh supplies, of
food or of oxygen for the organism as a whole, such supplies
being finally distributed by the blood-system. Similarly,
what are called the organs of excretion are special portions
of the body by which the waste products, collected by the
blood from all parts of the organism, are finally discharged.

Evolution of Heat. — The oxidation of the tissues, like
that of coal or wood in a fire, is accompanied by a rise in
temperature. But in the frog, as in other cold-blooded
animals, the evolution of heat is never sufficient to raise the

152 THE FROG CHAP.

temperature of the body more than very slightly above
that of the surrounding medium. In warm-blooded animals,
such as ourselves, the temperature is regulated, according
to the season, by a greater or less evaporation of water
from the surface of the body. In the frog this is not
the case : the temperature of the animal is always
nearly the same as that of the air or water in which it
lives, and hence the frosts of winter would be fatal to it,
but for the habit of hibernation (p. 8).

Death, and Decomposition. — The decomposition under-
gone by a dead frog (p. n) may be looked upon as an ex-
cessive process of waste unaccompanied by repair. Owing
to the action of certain microscopic plants known as
Bacteria , which will be referred to again in Part II., the
proteids undergo oxidation, amongst the principal products
of which are water, carbon dioxide, ammonia, and certain
gases of evil odour, such as sulphuretted hydrogen and
ammonium sulphide. Most of the gases escape into the air,
while the ammonia is finally converted into nitrous and nitric
acids. These, combining with certain substances in the
soil, give rise to. salts called nitrates and nitrites, which
furnish one of the chief sources of the food of plants.

PRACTICAL DIRECTIONS
The Organs of Respiration and of Voice,— Pin down a frog in

the usual way (pp. 31 and 32), remove the heart, and make out the
precise relation of the lungs, first distending them with air through the
glottis. The specimen already used for the dissection of the vascular
system or alimentary canal will serve the purpose. Harden thoroughly
in formaline or spirit and note (Fig. 44) the laryngo-tracheal chamber,
which communicates on the one hand with the pharynx through the
glottis, and on the other with both lungs. Observe also the posterior
horns of the hyoid which embrace the glottis, and then separate them

ix PRACTICAL. DIRECTIONS 153

from the laryngo-tracheal chamber, so as to remove the latter, together
with the lungs, from the body.

Then dissect off what remains of the mucous membrane of the pharynx
around the glottis, and notice the small laryngeal muscles in connection
with the laryngo-tracheal chamber : remove these, and pin the
respiratory organs down under water, ventral surface uppermost, by
means of a pin through each lung. Cut away the ventral wall of one
lung, so as to expose the cavity and its connection with the laryngo-
tracheal chamber. (You will very probably find some parasites in the
lungs — small worms called Ascaris nigrovenosa, belonging to the group
of Nematode worms.) Note : —

1. The two arytenoid cartilages , and a ring-shaped cartilage sur-
rounding the base of the lungs.

2. The network of ridges on the inner surface of the lungs. Examine
with a lens. Sketch.

3. The vocal cords. Observe these first in their natural position,
and then with the scissors cut through the laryngo-tracheal chamber
along the line of the glottis so as to divide it into right and left halves
and thus expose the vocal cords from their surface. Sketch.

The Kidneys, — a» Examine these organs in situ (Figs. 3, 4, and 7)
and note : —

1. Their form and position, and the relations of the peritoneum,
which covers them on the ventral side only, (See Fig. 5.)

2. The ureters (their openings into the cloaca may be seen at a later
stage).

3. The yellowish adrenals.
Sketch.

b. Examine under the microscope a transverse section of the kidney,
prepared as directed on p. 136, and make out (Fig. 46) :—

1 . The urinary tubules •, cut through in various planes.

2. The Malpighian caps^des and their glomerttli.

3. Blood-capillaries and vessels.

4. The nephrostomes (p. 98, Figs. 23, 46 and 47, and see Part II).
Sketch a portion under the high power. Compare with a section of

kidney in which the blood-vessels have been injected with coloured
gelatine (p. 101).

CHAPTER X.

THE FROG (continued) : THE NERVOUS SYSTEM.

IN a machine of human construction, such as the engine
of a steamer, the proper working of the whole depends, pro-
vided the parts of the machine itself are in good order, upon
two things — the stoking or regulation of the fires, and the
turning of certain cocks and levers by the engineer. In that
very complex machine the frog, we have already studied
what corresponds to stoking, viz., feeding and breathing.
We must now direct our attention to what may be considered
roughly to correspond with the work of the engineer — the
means by which the whole complex machinery is kept under
control, and its various parts made to work together to a
common end.

How does it come about, for instance, that the various
digestive glands begin to secrete actively as soon as food is
taken into the enteric canal ? How is it that a touch on any
part of the body, or even the sight of an enemy, is followed
instantaneously by a series of vigorous muscular movements
so ordered as to facilitate escape from the source of danger ?

In the fourth chapter (p. 62) we got so far as to learn
that muscular contractions are induced by nervous impulses
travelling from the brain or spinal cord, along the nerves, to
the muscles. It may therefore be inferred that the con-

CHAP, x SPINAL CORD 155

trolling apparatus, by which the functions of the body are
regulated, is lodged in either the brain or the spinal cord, or
in both.

Divisions of the Nervous System. — The nervous system
is divisible into (i) the central nervous system, consisting of
the brain and spinal cord (Figs. 6 and 7), and (2) the peri-
pheral, nervous system, Consisting of the nerves which pass
from the central nervous system to the various parts of the
body. The nerves are divisible into (i) cerebral nerves
(Fig. 53), arising from the brain, (2) spinal nerves (Fig. 51),
arising from the spinal cord, and (3) sympathetic nerves.

The Spinal Cord. — In form the spinal cord (Figs. 6 and
7, sp. cd} is irregularly cylindrical. It is continuous in front
with the brain, and tapers off posteriorly into a fine thread-
like portion, the filum terminale (//), while opposite the
fore-limbs, and again just anteriorly to the filum terminale,
it presents an enlargement : these are known respectively
as the brachial and sciatic swellings. Along its dorsal surface
runs a delicate longitudinal line, the dorsal fissure (Fig. 48,
d.f), and a distinct groove, the ventral fissure (v.j\ extends
along its lower surface.

The cord is covered with a delicate pigmented mem-
brane known as the//# mater (p.w) and the neural canal in
which it lies is lined with a stout, tough membrane, the dura
mater (Fig. 52, d.m). Between the two is a space filled with
a lymphatic arachnoid fluid, which, like the pericardial fluid,
preserves the contained organ from shocks.

Examination of a transverse section of the cord under a
low power will show that the dorsal fissure is an extremely
narrow vertical wall formed by an extension inwards of the
pia mater. The ventral fissure is a distinct cleft. Thus the
two fissures divide the cord into paired half-cylinders, right
and left, joined in the middle by a narrow bridge. This

iS6

THE FROG

bridge is traversed from end to end by a very narrow longi-
tudinal canal, the central canal (c.c\ lined by epithelium, so
that the cord is not a solid cylinder, but a tube with an
extremely narrow cavity and excessively thick walls.

The section also shows that the cord is not homogeneous,
but is composed of two different substances. Its outer part
is pure white and shining in the fresh cord, and is hence

t/.r

r IG. 40. — Iransverse section ot spinal cord ot trog.

r. c. central canal ; d.f. dorsal fissure ; d. h. dorsal horn of grey matter ; d. r. fibres
of dorsal root of spinal nerve ; ni>. c. nerve cells of dorsal horn ; nz>. c'. nerve-cells
of ventral horn ; p. ia>. pia mater ; v.f. ventral fissure ; v. h. ventral horn of grey
matter ; v. r. fibres of ventral root of spinal nerve ; iv. m. white matter. (After
Howes.)

called the white matter (w.m). Its internal substance has
a pinkish colour when fresh, and is called the grey matter.
The grey matter has a squarish outline in transverse section.
It surrounds the central canal, and is continued upwards
and downwards, forming what are called the dorsal (d.ti) and
ventral (v.h] horns of the grey matter.

The Brain. — Anteriorly the spinal cord passes insensibly
into the brain (Fig. 49), which is of somewhat greater
diameter than the cord, and is made up of several very dis-

X BUAIN 157

tinct parts or divisions. The hindermost division is called
the bulb or medulla oblongata (Med. obi) ; this appears to be
simply a widening of the spinal cord (Sp. cd\ except that
on its dorsal surface is a triangular body (D, ch. plx*-\ of a
reddish colour in the fresh condition, called the posterior
choroid plexus : it is simply a thickening of the pia mater
containing abundant blood-vessels.

The choroid plexus forms a kind of lid to a triangular
cavity (A and D, £>4) excavated in the dorsal region of the
medulla oblongata, and called the fourth ventricle. The apex
of the cavity, which is directed backwards, opens into the
central canal of the spinal cord (Fig. 50, z>4, c.c\ and the
fourth ventricle is to be looked upon simply as the anterior
part of the central canal which has become widened out
and is covered only by a thickened portion of the pia
mater.

The fourth ventricle is bounded in front by a narrow
ledge of nervous matter (Fig. 49, Cb\ which would be hardly
worthy of being considered as a special division of the
brain but for the fact that the corresponding part in many
animals — e.g.) dogfish, rabbit, man — is a large and important
structure. It is called the cerebellum.

In front of the cerebellum comes a pair of rounded
elevations, the optic lobes (Figs. 49 and 50, Opt. /). Each
contains a cavity, the optic ventricle (Opt. v\ communicating
with a narrow median passage, the iter (/*), which is
continuous behind with the fourth ventricle. The bulb is
continued forwards beneath the optic lobes as the crura
cerebri(Cr. C).

In front of the optic lobes is an unpaired structure, the
diencephalon or 'tween-brain (Di). On its upper surface is
a small rounded vascular body, the an terior choroid plexus
(Fig. 49 D, ch. plx1), formed, like the posterior choroid

Jbin

Mcd.cbi.

cd

/ A^W««SSKH y as 'Sf>

Cfat.ch i,tf /! II vi

J in pd v

Cer.H for. At c/tl ,

n ^^B^^Ccm pm com & /

Olfl <^i 'rs" ' ' ' — Lss=^^- ' ^

Afed. obi

FIG. 49. — Brain of frog. A, from above ; B, from below ; C, from the side ; D, in

longitudinal vertical section.

Cb. cerebellum; Cer.H. cerebral hemispheres; ch.pljc.l- anterior and ch.pljc.~
posterior choroid plexus (removed in A) ; coin, transverse bands of nerve-fibres

BRAIN

159

Clf.v

or commissures connecting the left and right halves of the brain ; Cr. C. crura
cerebri ; Di. diencephalon ; for. M, foramen of Monro ; z. iter, or aqueduct of
Sylvius ; inf. infundibulum ; Med. obi. medulla oblongata ; Olf. I. olfactory
lobe ; opt. ch. optic chiasma ; Opt. I. optic lobe ; opt. v. optic ventricle ; pin. stalk
of pineal body , pit. pituitary body ; Sp. cd. spinal cord ; zfi. third ventricle ;
7'4. fourth ventricle ; I — X, cerebral nerves ; i Sp. sSp., first and second spinal
nerves ; (A — C, after Gaupp ; D, from Wiedersheim's Comp. Anatomy, after
Osborn).

plexus, of a thickening of pia mater, containing numerous

blood-vessels. It helps to roof over a narrow slit-like cavity,

the third ventricle (z/3), the

sides of which are formed by

thickenings of nervous matter,

the optic thalami (Di}. On

the ventral surface of the brain

the diencephalon is continued

into a funnel-like extension,

the infundibulum (Fig. 49, inf),

to which is attached a rounded

structure, the pituitary body

(pit). On the dorsal surface,

just behind the choroid plexus,

is the delicate stalk (pin) of

the pineal body- — the vestige of

a sensory apparatus, part of

which in some lizards, for

example, has the structure of

-an eye situated beneath the

skin on the top of the head, and

which was probably functional

in the ancestors of the frog. We

shall meet with other examples

of such vestigial organs in the

course of our studies.

In front of the 'tween-brain comes a pair of long, oval
bodies, wider behind and narrower in front. These are the
cerebral hemispheres (Cer. If). Each contains a cavity, the

FIG. 50. — Diagrammatic horizontal

section of frog's brain.
c. c. central canal ; Cer. H. cerebral
hemisphere ; Di. diencephalon ;
for. M. foramen of Monro j z'. iter ;
Lat. v. lateral ventricle ; Med.
obi. medulla oblongata ; Nv. /,
olfactory nerve ; Olf. L. olfactory
lobe; Olf.v. olfactory ventricle;
Opt. I. optic lobe ; Opt. v. optic
ventricle ; Sp. cd. spinal cord ;
v. 3, third ventricle ; v. 4, fourth
ventricle. (After Ecker and Wie-
dersheim.)

160 THE FROG CHAP.

lateral ventricle (Lat. v), which communicates with the
third ventricle by a small aperture, the foramen of Monro
(for. M\

Lastly, each cerebral hemisphere is continued forwards by
a rounded olfactory lobe (Olf. /), which is fused with its fellow
of the opposite side, the single mass lying in the posterior
compartment of the girdle-bone. The lateral ventricles are
continued forwards into the olfactory lobes, forming the
small olfactory ventricles (Fig. 50, Olf. v).

The brain, like the spinal cord, is formed of grey and
white matter, but differently arranged. In the olfactory
lobes, cerebral hemispheres, and 'tween-brain the white
matter is internal, and the grey forms a thin outer layer or
cortex. In the optic lobes and medulla the grey matter is
mainly around the ventricles, and the white matter more
external.

Like the spinal cord, the whole brain is covered with pia
mater, densely pigmented in the region of the optic lobes,
and the cranial cavity in which it is contained is lined with
dura mater.

The Spinal Nerves. — The spinal nerves arise symmetri-
cally from the spinal cord on the two sides of the body, and
pass out at the neural canal through the intervertebral
foramina (p. 38).

There are altogether ten pairs of spinal nerves in the adult
frog (Fig. 51, 1—X\ each of which on leaving the neural
canal divides into a smaller dorsal and a larger ventral
branch (Figs. 52 and 53). The first pair leaves the cord
through the intervertebral foramina between the first and
second vertebra. Each passes at first directly outwards,
its large ventral branch, known as the hypoglossal, turning
forwards, and going to the muscles of the tongue (Fig. 51,
/, Fig. 53, i Sp).

The second pair (Fig. 51, //) is very large; it emerges

SPINAL NERVES

161

between the second and
third vertebrae, and each
is soon joined by the small
third nerve (///) which
emerges between the third
and fourth vertebrae, as well
as by a small branch or two
from the first, thus forming
a simple network or plexus
— the brachial plexus (br.
pl\ from which pass off
nerves to the fore-limb,
supplying both skin and
muscles.

The fourth, fifth, and
sixth nerves take a very
similar course.. The fourth
(IV) emerges between the
fourth and fifth vertebra,
the fifth ( V) between the
fifth and sixth, and the
sixth ( VI) between the sixth
and seventh. They all pass
obliquely backwards, and
supply the walls of the
body, being distributed to
both skin and muscles.

The seventh, eighth and
ninth nerves supply the
muscles and skin of the
hind-limbs. The seventh
( VII) leaves the neural
canal between the seventh

PRACT. ZOOL.

firfol

sci.pl

fe

FIG. 51. — The ventral branches of the
spinal nerves and the sympathetic of
the frog, ventral view : shown on the

: irog, vent
ght side only.

spinal nerve:
br. pi. brachial plexus ; C. calcareous

rign
I—X,

spinal nerves ; ^40. systemic arch ;

bodies which surround the spinal
ganglia ; D. Ao. dorsal aorta ; fern,
femoral nerve ; //. A. iliac artery ; sci.
sciatic nerve ; sci. pi. sciatic plexus ;
Sk. skull ; Sp. A . splanchnic artery ;
Sy. sympathetic cord ; Sy. c. communi-
cating branches between the sympa-
thetic and spinal nerves ; Sy. g.
sympathetic ganglia ; Ust. urostyle ;
V* — K9. centra of vertebrae ; Vg. vagus
nerve, with its ganglion. (After
Gaupp. slightly modified.)

M

1 62 THE FROG CHAP.

and eighth vertebra, the eighth ( VIII) between the eighth
and ninth, and the ninth (IX) between the ninth vertebra
and the urostyle. They all pass almost directly backwards,
and are united with one another by oblique cross branches
so as to form the sciatic plexus (sci. pi), from which are
given off, amongst others, two nerves to the leg, the largest
of which, the sciatic nerve (sci) being that already mentioned
in the chapter on the muscular system (p. 62).

The tenth (X ) is a very small nerve. It emerges through
a small aperture in the side of the urostyle (p. 39), and
supplies the cloaca, urinary bladder, and adjacent parts. It
is connected by cross branches with the ninth.

It will be noticed that while the large ventral branch of the
first spinal nerve — the hypoglossal — supplies muscles only,
and is therefore a motor nerve, all the others go to both
muscles and skin, and are therefore both motor and sensory,
or mixed nerves. They all branch out in a complex manner,
and are traceable to the remotest parts of the body.

The Sympathetic Nerves. — On either side of the dorsal
aorta is a very delicate nerve, having at intervals little swel-
lings called ganglia, each of which is connected with a spinal
nerve by a communicating branch (Figs. 51 and 53, Sy, Sy.g,
Sy. c). In front of the point where the dorsal aorta (D. Ao)
is formed by the union of the two systemic trunks (Ao),
these two sympathetic nerves, as they are called, are con-
tinued forward, one on either side of the vertebral column,
towards the head, when they enter the skull and become
connected with certain of the cerebral nerves.

Each sympathetic nerve has altogether nine or ten ganglia,
each connected with one of the spinal nerves, and from
the ganglia, branches are given off which supply the heart
and blood-vessels, the stomach, intestine, liver, kidneys,
reproductive organs, and urinary bladder.

x SPINAL AND CEREBRAL NERVES 163

Origin of the Spinal Nerves. — The mode of origin of the
nerves from the spinal cord is peculiar and characteristic.
Traced towards the cord, each nerve is found, on reaching
the intervertebral foramen from which it emerges, to divide
into two — a dorsal root which springs from the dorsal, and
a ventral root which arises from the ventral region of the
cord (Fig. 52, dr^ vr). The dorsal root is distinguished
from the ventral by being dilated into a ganglion (gn). In
Fig. 51 these ganglia lie hidden within certain calcareous
bodies (C) in this region.

n.sp.

./
w.m ^^^^^

c/t.

FIG. 52. — Transverse section through the vertebral column and spinal cord of the frog,

to show the mode of origin of the spinal nerves.

c. c. central canal ; en. centrum ; d. f. dorsal fissure ; d. m. dura mater ; d. r. dorsal
root ; g. m. grey matter ; gn. ganglion of dorsal root ; n. a. neural arch ; n. sp.
neural spine ; /. in. pia mater (the reference line .should point to the margin of
the cord) ; t. nerve trunk (ventral branch) ; t '. dorsal branch ; Tr. pr. transverse
process ; v. f. ventral fissure ; v. r. ventral root ; w. m. white matter. (After
Howes.)

Cerebral Nerves. — There are ten pairs of cerebral nerves,
some of which are purely sensory, some purely motor, some
mixed.

The first or olfactory nerves (Fig. 49, /) arise from the
olfactory lobes, and pass through the holes in the trans-
verse partition of the girdle-bone. Each is distributed to
the mucous membrane of the nasal sac or organ of smell
of the same side, and is purely sensory.

The second or optic (//) is a large nerve which springs
from the ventral surface of the 'tween-brain. At their

M 2

1 64 THE FROG CHAR

origin the right and left optic nerves have their fibres
intermingled, forming a structure something like a St.
Andrew's Cross and called the optic chiasma (opt. ch),
the other limbs of the cross passing upwards and back-
wards to the optic lobes. The optic nerve makes its
exit from the brain-case through the optic foramen, and is
distributed to the retina, a delicate membrane which
lines the eyeball, and is, as we shall see, the actual organ
of sight. This nerve also is purely sensory.

The third or oculomotor (III) is a small nerve arising
from the crura cerebri beneath the optic lobes. It passes
through a small hole in the side of the skull behind the
optic foramen, and supplies four out of the six muscles
by which the eyeball is moved, and is purely motor.

The fourth QI pathetic (IV) is a very small nerve leaving
the dorsal surface of the brain between the optic lobes and
the cerebellum, and making its exit from the skull above
the third nerve. It is also purely motor, supplying one of
the muscles of the eye — the superior oblique.

The fifth or trigeminal (Figs. 49 and 53, V) is a large
nerve arising from the side of the medulla oblongata. Its
root dilates to form a ganglion, the Gasserian ganglion,
and leaves the skull by the large aperture noticecT in the
pro-otic bone. It owes its name to the fact that it soon
divides into three main branches ; one, the ophthalmic (Fig.
53, F1), going to the skin of the snout; another, the maxillary
( V1), to the upper lip and lower eyelid ; and the third, or
mandibular ( F3), to the muscles and skin of the lower jaw.
The trigeminal is a mixed nerve.

The sixth or abducent (Fig. 49, VI) is a very small motor
nerve arising from the ventral aspect of the bulb, and
supplying one of the muscles of the eyeball called the
posterior rectus.

CEREBRAL NERVES

165

The seventh or facial nerve (Figs. 49 and 53, VII} arises
just behind the fifth and soon joins the Gasserian ganglion.
Both it and the sixth leave the skull by the same aperture

N

FIG. 53. — Dissection of the head and anterior part of the body of the frog from the
left side, to show the distribution of the fifth, seventh, ninth, and tenth cerebral
nerves, as well as of the hypoglossal and part of the sympathetic.
Ao. systemic arch ; br.pl. brachial plexus; co. columella ; D. Ao. dorsal aorta;
ctu. duodenum ; H. heart ; Hy. body of hyoid ; Hy*-. anterior, and Hy*. posterior
horns of hyoid ; L. lung ; N. olfactory sac ; On. orbit ; Pul. pulmonary artery ;
Sp. A. splanchnic artery; s^. stomach ; Sy. sympathetic; //. cut end of optic
nerve ; K1. ophthalmic, I72, maxillary, and V&. mandibular branch of trigeminal
(V)\ VII^. palatine, and Vlfi. hyomandibular branch of facial; IX. glosso-
pharyngeal ; X. vagus ; Xcd. cardiac, Xgas. gastric, Xlar. laryngeal, and
Xpul. pulmonary branch of vagus ; / sp. first spinal nerve (hypoglossal) ;
2sp.—5sp. second to fifth spinal nerves. (After Howes, slightly modified.)

as the fifth. It divides into two branches, one of which,
ti\e palatine (Fig. 53, F//]), supplies the mucous membrane
of the roof of the mouth, and the other, or hyomandibular
(F//2), sends branches to the skin and muscles of the

166 THE FROG CHAP.

lower jaw and to the muscles of the hyoid. It is a mixed
nerve.

The eighth or auditory nerve (Fig. 49, VIII) arises from
the medulla just behind the seventh, passes through an
aperture in the inner wall of the auditory capsule, and is
distributed to the auditory organ or membranous labyrinth
(see Figs. 10 and 59). It is the nerve of .hearing, and
is purely sensory.

The ninth or glossopharyngeal (Figs. 49 and 53, IX) arises
behind the auditory nerve. It sends a branch to join the
facial, and supplies the mucous membrane of the tongue and
pharynx as well as certain small muscles connected with the
hyoid. It is also a mixed nerve.

The tenth or vagus (Figs. 49 and 53, X) is a large nerve
arising in common with the ninth, and dilating, shortly after
leaving the skull, into a vagus ganglion. It supplies the
larynx (Xlar\ the heart (Xcd\ the lungs (Xput), and the
stomach (Xgas\ and is therefore often known as thepneumo-
gastric. It has thus an extraordinarily wide distribution, being
in fact the only cerebral nerve which supplies parts beyond
the head. It is a mixed nerve, and contains many motor
fibres, but its branches — some of which have to do with the
regulation of the heart's contraction and with respiration —
are better described as efferent and afferent than as motor
and sensory : the meaning of these terms will be explained
later on. The ninth and tenth nerves leave the skull close
together through the aperture noticed in the exoccipital bone.

The sympathetic nerve (Sy) extends forwards from its junction with
the first spinal nerve, joins the vagus, and finally ends anteriorly in the
Gasserian ganglion.

Microscopic Structure of Nervous Tissue. — Examination
of a piece of nerve under the microscope shows it to be

x NERVE-FIBRES AND CELLS 167

composed, like striped muscle, of cylindrical fibres, bound
together by connective-tissue. The latter is much more
abundant than in muscle, and in particular forms a thick
sheath round the nerve which must be torn off before the
nerve-fibres are reached.

Each fibre (Fig. 54, A) is a cylindrical cord in which
three parts can be distinguished. Running along the axis
of the fibre is a delicate protoplasmic strand, the neuraxis
or axis-fibre (nx). Around this is a sheath formed of a
fatty substance and known as the medullary sheath (m.s) ; 1
and, finally, investing the whole fibre is a delicate, structure-
less membrane, the neurilemma (ne). At intervals the
medullary sheath is absent, and a node is produced, where
the fibre consists simply of the neuraxis covered by the neuri-
lemma. Directly beneath the neurilemma nuclei are found
at intervals.

In the ganglia are found, not only nerve-fibres, but
nerve-cells (Fig. 54) : these are cells of a relatively large
size, each with a large nucleus and nucleolus. In the
spinal ganglia (B) the cell-body is produced into two pro-
cesses, which may be united at their base. One of these
processes is continuous with the neuraxis of a nerve-fibre ;
the other is also a protoplasmic process which passes into
the spinal cord and sends off branches, each branch finally
ending in a complicated branch-work or arborisation, which
is interlaced, but not actually continuous, with a similar
arborisation arising from a nerve-cell in the spinal cord or
brain (Fig. 55).

The white matter of the brain and spinal cord consists of
nerve-fibres, those in the cord having a longitudinal direction ;
the grey matter contains numerous much-branched ("multi-

1 The medullary sheath may be absent in certain nerve-fibres (*.£., in
the sympathetic and olfactory nerves).

1 68

THE FROG

polar ") nerve-cells (Figs. 48 and 54, A), each continuous
with a neuraxis and enclosed in a tissue formed partly

FIG. 54. — A, nerve-cell from the grey matter of the spinal cord of a frog, and the

nerve-fibre arising from it ; B, cell from the ganglion of a dorsal root.
ne. neurilemma ; nu. nucleus ; {nx. neuraxis ; m. s. medullary sheath. (After
Howes.)

of the axis-fibres of nerves which enter the grey from the
white matter, losing their medullary sheath as they do so,

x REFLEX ACTION 169

and partly of a delicate fibre-cellular tissue called neuroglia^
in which the other elements are imbedded.

Functions of the Nervous System : Reflex Action. — In
the fourth chapter you learned that a muscle may be
made to contract by a stimulus applied either to the muscle
itself or to its nerve. You are now in a position to pursue
the subject of the control of various parts of the body by
the nervous system a little further.

A frog is either decapitated or pithed, i.e., the medulla
oblongata is severed and the brain destroyed (see p. 103) :
there can thus be no question either of sensation or of
voluntary action on the frog's part. It is then hung up
by a hook or string, so that the legs are allowed to hang
freely. If one of the toes is pinched with the forceps,
the foot will be drawn up as if to avoid the pinch; or,
if some very weak acid be applied to a toe, the foot will
again be withdrawn, being raised every time it is touched
with the acid with the regularity of a machine. Again, if
acid be applied to various parts of the body, the foot of the
same side will immediately try to rub off the irritating
substance ; or if that foot be held down, the other will come
into play.

Movements of this kind are called reflex actions : the
stimulus applied to the skin is transmitted by sensory nerve-
fibres to the spinal cord, where it is, as it were, reflected
in another form, and passed along motor fibres to one or
more muscles, causing them to contract (p. 60).

As already stated, the spinal nerve-trunks are mixed, i.e.,
contain both sensory and motor fibres. It has been found
by numerous experiments that as the nerve approaches
the spinal cord these two sets of fibres separate from one
another, the sensory passing into the cord by the dorsal
root, the motor by the ventral root. As a consequence of

1 70 THE FROG CHAP.

this, if the dorsal root be cut and its proximal or central
end — i.e., the end in connection with the cord — stimulated,
muscular contraction will follow just as if the stimulus had
been applied to the skin supplied by the nerve in question.
If, the other cut end — the distal or peripheral end — be
stimulated, there is no result. On the other hand, if the
ventral root be cut and its distal end stimulated, the

FIG. 55. — Diagram illustrating the paths taken by the nervous impulses.
c. c. central canal ; col. collaterals ; c. cort. cell in rind or cortex of the cerebral
hemisphere ; c. g. smaller cerebral cell ; d. c. cells in dorsal horn of grey matter ;
d. ?. dorsal root \g. ganglion of dorsal root ; g. c. cell in ganglion of dorsal root ;
g. in. grey matter ; M. muscle ; m. c. cell in medulla oblongata ; m.J. motor
fibre ; S. skin ; s.f. sensory fibre ; sp. c. spinal cord ; v. c. cells in ventral horn
of grey matter ; v. r. ventral root ; w . in. white matter. The arrows indicate
the direction of the impulses.

muscles supplied by it will contract, while stimulation of the
proximal end produces no result.

Very accurate observations have shown that the connec-
tion between the motor and sensory fibres is as follows (Fig.
55). A motor fibre (in.f) is traceable from the nerve-trunk
through the ventral root (v.r) into the white matter; and
then, its medullary sheath being lost, passes into the ventral
horn of the grey matter, its neuraxis being directly con-

x REFLEX ACTION 171

tinuous with the axis-fibre process of one of the large motor
nerve cells (v.c) : the remaining processes of these cells simply
branch out in the neuroglia. The sensory fibres (s.f) are
traceable into the dorsal root (d.r) ; in passing through the
ganglion of the root (g) they are found to be continuous
with its simple ("bipolar") nerve cells (g.c\ and then pass
into the cord. Instead, however, of entering the grey matter
at once, they pass forwards as well as backwards for some
distance, along the white matter of the cord, giving off
numerous branches, or collaterals (col), which, losing their
medullary sheaths, enter the dorsal horn of the grey matter
and branch out into a complex series of fine fibres which
interlace with the similar arborisations of the nerve-cells

(P. i67).

The path of a nervous impulse will now be obvious. The
stimulus applied to the skin (Fig. 55, s) is conducted by a
sensory fibre to the nerve-trunk and by the dorsal root to
the spinal cord ; it then passes along the white matter of
the cord, enters the grey matter, and is conducted by the
collaterals to the nerve-cells of the ventral horn, either
directly, or after passing through the cells of the dorsal
horn : from one of the cells of the ventral horn it is con-
ducted by an axis-fibre process continuous with the neur-
axis of a nerve fibre, which, leaving the cord by a ventral
root, passes along the nerve-trunk and finally goes to a
muscle (M) as a motor fibre.

It will be noticed that a single stimulus applied to the
skin may result in the contraction of numerous muscles — as,
e.g., when the application of a drop of acid to the toe causes
the lifting of the leg, and that the movements are of such a
nature as to withdraw the part stimulated from the irritating
substance. Moreover, as shown by the experiment of apply-
ing acid to various parts of the body, the movements are

172 THE FROG CHAP.

varied according to circumstances ; if one leg is prevented
from rubbing off the irritating substance, the other imme-
diately comes into play. Obviously, then, a simple stimulus
reaching the spinal cord may be transmitted to numerous
motor cells of the ventral horn, and through these to
numerous motor nerves, the particular nerves affected differing
according to circumstances (compare Fig. 55). The spinal
cord, therefore, is able, in response to a stimulus reaching it
by a sensory nerve, to originate motor impulses causing
complex muscular movements so adjusted as to serve
definite purposes. Without such external stimulus, how-
ever, the spinal cord of a brainless frog is quite inactive, and
the body of the animal will remain without movement until
it dries up or decomposes.

In the uninjured frog, i.e., the frog with its brain intact, the
case is very different. The animal no longer acts like an
unintelligent machine, each stimulus producing certain in-
evitable movements and no others ; but a single stimulus
may produce varied movements, the nature and direction of
which cannot be predicted ; the animal will probably give a
series of leaps, but the number and extent of these vary
according to circumstances.

This is explained by the fact that certain nerve-fibres
of the cord pass forwards to the brain, and that the nerve-
cells in the grey matter of the cord are in communication —
owing to the interlacing of their branching processes with
those of the collaterals — with similar cells in the grey matter
of the brain (Fig. 55, m.c, c.g, c.cort}. In certain of these
brain-cells (c.cort\ voluntary impulses originate and exercise
a controlling effect upon the cells of the spinal cord, so that
these latter do not constitute, as in the brainless frog, a
machine every movement of which can be accurately pre-
dicted.

x REFLEX AND VOLUNTARY ACTION 173

Moreover, it can be shown by experiment that the process
of originating voluntary impulses is not performed by the
whole brain, but is confined to the cerebral hemispheres.
If the hemispheres and optic lobes are removed so as to
leave nothing but the bulb and cerebellum, the frog no
longer lies in any position in which it may be placed, ex-
hibiting no movements- beyond the beating of the heart, as
is the case when the whole brain is removed. It sits up in
the ordinary attitude, breathes, swallows food placed in the
mouth — while making no attempt to feed itself, turns over
and sits up if placed on its back, and swims if placed in
water. If left alone, however, it remains in the sitting
posture until it dies. Hence the bulb and cerebellum
are evidently concerned with the co-ordination of muscular
movements, but have no power of originating impulses. If
the optic lobes as well as the medulla oblongata and cere-
bellum are left, the animal is affected by light, is able to per-
form complex balancing movements, and will even croak
when stroked in a particular way. There is still, however,
no voluntary action ; without the application of stimuli, the
animal sits motionless until it dies.

To sum up in the language of the illustration with which
this chapter was begun, comparing the frog with an engine
of human construction : — the grey matter of the brain may
be compared with the engineer ; much of the work of the
engine may go on without him, certain levers, valves, &c.,
acting automatically ; but it is only by his controlling intel-
ligence that the whole mechanism is adapted to the circum-
stances of the moment.

So far, we have considered the nervous system only in its
relations to the skin or general surface of the body and to
the muscles or organs of movement. The other parts of the
body are, however, under nervous control.

174 THE FROG CHAP.

It has been mentioned that the heart continues to beat in
a frog when the brain has been removed : not only so, but
it pulsates with perfect regularity when removed from the
body. This is due to the fact that the muscles of the
heart, like the cilia of ciliated epithelium, have the power
of contracting rhythmically quite independently of the
nervous system, although the heart contains nerve-cells
which were formerly supposed to serve as a special
nervous system for this organ, originating all its motor
impulses. It is, however, under the control of the central
nervous system. We have seen that it is supplied by a
branch of the vagus ; when this is stimulated, the heart stops
in the dilated state and begins to beat again only after a
certain interval. A feebler stimulus to the vagus will not
actually stop the heart, but will diminish the rate and the
strength of its contractions and consequently the amount of
blood propelled through the body The vagus is accom-
panied by a branch of the sympathetic which has an exactly
opposite effect ; t.e., stimulation of it accelerates the heart's
action. In this way, the general blood-supply of the body
is regulated by the central nervous system.

The blood-supply of the various parts and organs is
regulated by the vaso-motor nerves. These are traceable
through the sympathetic into the spinal cord by the ventral
roots : distally they send branches to the muscular coat of
the arteries. Under ordinary circumstances, a constant
succession of gentle stimuli pass along these from a group of
nerve-cells in the medulla oblongata, and, as a result, the
arteries are ordinarily in a state of slight contraction. By
various circumstances these stimuli may be diminished for any
given artery and at the same time stimuli pass down another
kind of vaso-motor fibres : the artery will then dilate and
the blood-supply of the organ to which it is distributed will

x AFFERENT AND EFFERENT NERVES 175

be temporarily increased. For instance, the presence of
food in the stomach acts, through the central nervous
system, upon the coeliac branch of the splanchnic artery,
causing a dilatation of its capillaries and promoting an in-
creased secretion of gastric juice. The secretion of other
glands is regulated in the same way. In some cases, how-
ever, it has been proved that the nerves of a gland do not
act simply by producing dilatation of the capillaries, but have
a direct effect upon the gland-cells, causing an increased
secretion.

You will thus note that there are nerve-fibres carrying
impulses to the central nervous system which have nothing
to do with sensation, and fibres carrying impulses from the
central nervous system which have nothing to do with
motion, but result in increased secretion or in stoppage of
motion. It is therefore best to use the term afferent (which
includes sensory) for a nerve carrying an impulse to the
brain and spinal cord, and efferent (including motor) for
one carrying an impulse in the other direction.

PRACTICAL DIRECTIONS.

I- The Central NervOUS System (Fig. 6). Lay bare the brain
and spinal cord as directed on p. 33, noting the dura mater and pia
mater : the latter is densely pigmented over parts of the brain. The
specimen in which this operation has already been performed will
do, if the dissection has been done carefully.

Observe the origins of the cerebral and spinal nerves, noting the
\fmgdorsal and ventral roots of the latter (compare Fig. 52) — which pass
backwards for some distance before making their exit from the neural
canal ; and also the ganglia on the dorsal roots, lying just outside the
canal and each hidden in a whitish calcareous body in this region
(Fig. 51, c). (The ganglia, however, can be more easily made out at a
later stage.) Then sever the nerves very carefully from the brain and

i?6 THE FROG CHAP.

spinal cord and remove the whole central nervous system from the neural
canal : it is best examined after hardening in formaline or spirit. Lay
it in a dissecting dish, under water, and make out its several parts as
follows : —

a. The spinal cord.

1. Note its cylindrical form, the brachial and sciatic swellings, the
fihim terminate, and the dorsal and ventral fissures.

2. Examine a transverse section of the spinal cord, prepared as
described on p. 1 36, under the low power of the microscope, and make
out the dorsal and ventral fissures •, the central canal, and the relations
of the grey and white matter (Figs. 48 and 52). Sketch.

b. The brain (Fig. 49).

Beginning from the posterior end of the brain, where it passes into
the spinal cord, make out its several divisions as follows : —

1. The bulb or medulla oblongata, with the posterior choroid plexus
on its dorsal side : remove the latter, and notice that it roofs over the
cavity of the fourth ventricle.

2. The small ledge-like cerebellum f

3. The two rounded optic lobes, and the crura cerebri beneath
them.

4. The diencephalon, formed of a right and left optic thalamus. On
its dorsal side is the anterior choroid plextis, roofing in the third
ventricle ; and on its ventral side the infundibulum, to which the
pituitary body is attached ; and, more anteriorly, the optic chiasma.

5. The cerebral hemispheres, continuous in front, with

6. The olfactory lobes, which are fused in the middle line.

Sketch the whole central nervous system from above, and also the
brain from below and from the side.

With the small scissors or a sharp scalpel, snip off a small piece of
the wall of the hemisphere and optic lobe of one side — say the left, so
as to expose the lateral ventricle and the optic ventricle (Fig. 50). Then
with a sharp scalpel divide the whole brain into two by a longitudinal
vertical cut very slightly to the left of the middle line, so as to reduce
it to a longitudinal section (Fig. 49, n). Examine the cut surface of
the right side under water, and make out as much as possible of the
commissures and of the relations of the ventricles : — viz.., the fourth
ventricle, the iter and optic ventricle, and the third ventricle, which
communicates with the lateral ventricle through the foramen of Monro.
Sketch.

x PRACTICAL DIRECTIONS 177

II. The Peripheral Nervous System.

a. The spinal nerves.

Fasten out a frog with the ventral side uppermost, and remove the
heart, enteric canal, reproductive organs, kidneys, and lungs with great
care, leaving behind most of the systemic trunk and the dorsal aorta
(Fig. 51). (One of the specimens already dissected will probably serve
the purpose if the previous directions have been accurately followed. )
Note the spinal nerves passing outwards from the vertebral column on
either side, and the calcareoiis bodies close to their points of exit,
covering up the ganglia of the dorsal roots (p. 163). If the centra
of the vertebrae are removed, the nerve-roots and their origins from the
spinal cord can be made out : the removal of the centra is rendered
easier if the frog is first decalcified by being placed in 10 per cent, nitric
acid for twenty-four hours and then thoroughly washed in running water.

Confine your attention to the large ventral branches of the ten pairs
of spinal nerves, as described on pp. 160-162.

b. The sympathetic nerves (Figs. 51 and 53).

Examine the systemic trunk and dorsal aorta carefully with a lens.
Closely connected with it will be seen on either side a sympathetic nerve-
cord, covered by pigmented connective-tissue. Carefully dissect the
cord away from the aorta, and note the ganglia and the branches
(rami conununicantes] connecting them with the spinal nerves. Sketch
the spinal nerves and sympathetic.

c. The cerebral nerves (Fig. 53).

The dissection of these in the frog is not an easy task for a beginner,
and it is best to examine those of a larger animal (e.g. dogfish) before
attempting it (see Part II). The origin of some of the nerves from the
brain, and the apertures through which certain of them pass out from
the skull, have already been seen.

in. The Microscopic Structure of Nervous Tissue.

a. Examine your transverse section of the spinal cord (Fig. 48) under
the high power of the microscope, and observe —

1. The nerve-cells, present in the grey matter only (compare Fig. 54,
A). Note their branched form and their nuclei ; the larger motor cells
are seen in the ventral horns of the grey matter. Sketch.

2. The nerve-fibres, in both grey and white matter, cut across trans-
versely as well as in other directions, and each showing a deeply-
stained central neuraxis. Sketch.

PRACT. ZOOL. N

1 78 THE FROG CHAP, x

b. Tease up a fresh spinal (Fig. 54, B) or sympathetic ganglion in salt-
solution, and stain with methyl-green. Compare the form of the nerve-
cells with those in the spinal cord. Sketch.

c. Cut off a very small piece of any fresh nerve (e.g., sciatic), and
tease it out carefully, in a longitudinal direction, in salt solution. Note
that it is made up of cylindrical, unbranched nerves-fibres, bound together
by connective-tissue.

Examine a single fibre under the high power (Fig. 54, A), and make
out the neurilemma, the medullary sheath, and the nodes : at the nodes,
the neuraxis can also be seen. Sketch.

Tease out another piece of fresh nerve in chloroform, so as to partially
dissolve the medullary sheath, and note the central neuraxis. Sketch.

Tease out in glycerine a piece of nerve which has been treated with
a i per cent, solution of osmic acid in water for an hour or two and
then well washed in water. The medullary sheath will appear nearly
black, and the neurilemma, with its underlying nuclei, as well as the
nodes, can be plainly seen. Sketch.

Reflex Action. The experiment described on p. 169 should be

CHAPTER XI

THE FROG (continued} : THE ORGANS OF SPECIAL SENSE

IN the previous chapter you have learnt how the nervous
system controls the various functions of the body, and how
voluntary action is absolutely dependent upon the con-
nection of the brain, through the spinal cord, with the
nerves. Obviously, in order that the power of voluntary
action should be of full use to its possessor, some means of
communication with the external world is not only desirable
but necessary ; the frog, in order to adjust its actions to the
circumstances in which it from time to time finds itself,
must be able to distinguish friends from enemies, suitable
from unsuitable food, darkness from light, heat from cold.

The avenues of communication between the animal and
its surroundings are, as in ourselves, the senses of touch,
taste, smell, sight, and hearing.

The sense of touch, including that of temperature, is
lodged in the whole extent of the skin, which, as you have
already learnt, is abundantly supplied with sensory nerves.
Many of the nerves terminate in connection with what are
known as tactile cells — large flattened cells arranged in groups
just below the epiderm and around which the ultimate fibres
of the sensory nerves are distributed. Stimuli applied to
the skin, either by direct touch or by the heat of the sun,

N 2

i So THE FROG CHAP.

are transmitted to the tactile cells and thence through the
sensory nerves to the brain. Notice that the stimulus is
transmitted to the nerve-ends through the epithelial cells of
the skin ; if the skin be wounded and a stimulus applied
directly to the tactile cells or the nerves, the sensation is
one, not of touch, but of pain.

The sense of taste is lodged in the mucous membrane
of the mouth, especially in the tongue and in the neighbour-
hood of the vomerine teeth, but extending also as far back
as the gullet. Certain of the epithelial cells have an
elongated form and are arranged in groups known as
taste-buds, to which the fibres of the ninth and palatine
branch of the seventh cerebral nerves, or nerves of taste, are
distributed ; on the tongue, these taste-buds are situated on
papilla of the mucous membrane. In this case the stimulus
is supplied, not by direct touch or by alteration of tempera-
ture, but by the contact of sapid or tasty substances. As
before, the stimulus is applied to epithelial cells, and by
them transmitted to the nerves and so to the brain, when
the sensation of taste becomes manifest. Thus, just as
common sensation may be abolished in any part of the
body in three ways — by destruction of the skin, by cutting
the sensory nerve, or by destroying the cerebral hemispheres
— so the sense of taste is lost if either the mucous mem-
brane of the mouth is injured or if the glossopharyngeal
and palatine nerves are cut, or, again, if the cerebral
hemispheres are destroyed.

The sense of smell is lodged in the nasal or olfactory sacs,
which are enclosed in the olfactory capsules of the skull and
separated from one another by a partition, the nasal septum.
Each sac has two apertures, the external nostril, opening
on the surface of the snout, and the internal nostril,
opening into the mouth (p. 17). The sacs are lined by a

NOSE AND EYE

181

delicate mucous membrane, some of the epithelial cells of
which are of the ordinary columnar type, while others are
extremely slender and produced into delicate processes at
their free ends. With these latter the fibres of the olfactory
nerves are connected, and they are distinguished as olfactory
cells (Fig. 56). As the epithelial cells of the skin are af-
fected by direct contact or by heat, so
the olfactory cells are affected by the
minute particles given off from odorous
bodies : the contact of these particles
acts as a stimulus, which, being trans-
mitted by the olfactory nerves to the brain,
gives rise to the sense of smell. Thi
sense can be destroyed, as in the case Of
feeling and taste — either by destruction of
the olfactory mucous membrane or by
cutting the olfactory nerves, or by de-
stroying the brain.

The organ of sight or eye of the frog
is so similar in structure to that of man,
that the reader may be referred for details
both of structure and of function to the
text-books of Physiology, and it will only
be necessary to give a brief outline here.

Each eye (compare Fig. 57) is a nearly
globular organ, and when removed from
the orbit and cleaned by dissecting away
the attached muscles, &c., two regions can be distinguished
in it— an opaque portion of a dark bluish colour which forms
some two-thirds of the entire globe and is hidden within the
orbit in the entire animal ; and a clear, transparent portion
which is directed outwards and freely exposed between the
eyelids in the living frog. The outer coat of the concealed

FIG. 56.— Epithelial
cells" of the olfac-
tory mucous mem-
brane of an Am-
phibian.

E. interstitial
cells ; R. olfactory
cells. (From Wie-
dersheirn's Comp.
Anatomy.)

1 82

THE FROG

portion of the eyeball is the sclerotic (Set), and is formed of
cartilage in the frog ; the dark colour is due to the presence
of a layer of black pigment which forms one of the internal
coats ; this will be referred to hereafter. Entering the
sclerotic on its inner side, i.e., the side next the brain-case,
will be seen the cut end of the optic nerve (O.N). The

ecf

So)

FIG. 57. — Diagrammatic horizontal section of the human eye.

c. cornea ; Ch. choroid (dotted) ; C.P. radiating folds of the choroid, known as the
ciliary processes ; e. c, e. cj. conjunctiva ; /. iris ; L. lens ; O.N. optic nerve ; os.
ora serrata, a wavy line forming the boundary of the visual portion of the
retina ;/. c. R. non-visual portion of retina ; PE. pigmented epithelium (black) ;
•*R. retina ; Scl. sclerotic ; spl. suspensory ligament of lens ; V. H. vitreous
chamber, (From Foster and Shore's Physiology.}

transparent, exposed portion of the eyeball is the cornea (c\
a superficial thin layer of which, or conjunctiva (e.c., e. cf),
is continuous with the lining of the eyelids and thus with
the skin covering the head ; through it can be seen the
coloured part of the eye, or iris (/), with a black spot-
really a hole — in its centre, the pupil.

xi EYE 183

The interior of the globe ( V. H) is filled with a colour-
less, transparent jelly, the vitreous humour -, surrounding
which, everywhere but on its external face, is a thin semi-
transparent membrane, reddish when perfectly fresh, but
becoming grey soon after death ; this is the retina (R)-
Between the retina and the sclerotic is a membrane called
the choroid (Cti), the inner face of which, i.e., that in contact
with the retina, is coloured black. It is this layer of black
pigment which gives the dark tint to the semi-transparent
sclerotic in the entire eye ; strictly speaking, it belongs to
the retina, but actually it adheres to the choroid and
appears like the innermost layer of that coat. The retina is
readily detachable from the choroid, but at the place where
the optic nerve enters (blind spot) it becomes continuous
with the fibres of the latter, which pass through the
sclerotic and choroid. The choroid is made up of
connective-tissue and contains numerous vessels as well as
pigment-cells.

Lying just internal to the pupil is a nearly globular body,
perfectly transparent when fresh, the crystalline lens (L) ; it
is kept in place by a delicate membrane, the capsule of the
lens. The iris, which covers the outer face of the lens
except where it is perforated by the pupil, is covered on its
inner surface with black pigment, and is continuous all
round its outer margin with the choroid. Between the iris
and the cornea is a space, the aqueous chamber of the eye,
which contains a watery fluid, the aqueous humour. The
main cavity of the eyeball, containing the vitreous humour,
is called the vitreous chamber.

The actual relations of these parts in the entire eye are
best grasped in a vertical section, such as is represented in
Fig. 57. The main part of the eyeball forms a chamber,
enclosed by the sclerotic, darkened internally by the choroid

1 84 THE FROG CHAP.

and lined by the retina. Into the outer side of this dark
chamber is let a transparent window, the cornea ; behind
which, and separated from it by a space containing the
aqueous humour, is a vertical curtain, the iris, perforated by
an aperture, the pupil. Behind the iris and in close contact
with it is the lens, and filling the whole of the dark chamber
between the lens and iris and the retina is the vitreous
humour.

The whole eye thus has the structure of a camera ob-
scura. The cornea, aqueous humour, lens, and vitreous
humour are a series of lenses, so arranged that the rays of
light from an external object are refracted and brought to a
focus on the retina, where they form a greatly diminished
and inverted image of the object. The iris is provided with
muscles, by means of which the pupil can be enlarged or
diminished ; it therefore acts as a diaphragm and regulates
the amount of light entering the eye. Attached to the
capsule of the lens are delicate muscles, by means of which
the lens can be made more or less convex ; in this way the
focus of the entire apparatus can be altered according to
whether the object viewed is nearer or farther from the eye.
This arrangement for accommodation is, however, much
less highly developed in the frog than in man and the
higher animals, in which the relatively smaller lens is
flatter and distinctly biconvex in form (Fig. 57). Thus the
various parts of this complicated organ are so adjusted as
to bring the images of external objects to an accurate focus
on the back part of the interior of the eyeball, i.e., on the
retina.

A vertical section of the retina (Fig. 58) reveals a very
complex structure. On its inner face, i.e., the surface in
contact with the vitreous humour, is a layer of nerve-fibres
(«./), formed by the ramifications of the optic nerve, which,

RETINA

185

passing through the sclerotic and choroid, perforate the
retina, and spreads out over its inner surface. Next comes
a layer of nerve-cells (g), and then several layers of fibres
and nuclei (gr, nc) ; and finally, forming the outer surface of
the retina proper, is a layer of delicate,
transparent bodies called, from their
form, the rods (r) and cones (c) ; these
are known from their development to
be modified epithelial cells. The whole
of these structures are supported by
a complex framework of connective-
tissue. In close contact with the outer
or free ends of the rods and cones is
a layer of cells the protoplasm of
which is filled with a dense black
pigment. It is this pigment-layer
(p. ep\ which, as we have seen, is
often counted as part of the choroid.

In spite of its complex structure, the
retina is not much more than ^th mm.
(_i._th inch) thick, and is perfectly
transparent. Hence, when an image
is formed on it, the rays of light easily
penetrate its whole thickness until
they are stopped by the opaque layer
of pigment. The rays can thus
stimulate the rods and cones, and the
stimulus is transmitted through the layers of nuclei and
nerve-cells to the fibres of the optic nerve, along which it
is conveyed to the brain. Thus the actual organ of sight
is not the eye as a whole, but the retina : all the rest is
to be looked upon as an accessory apparatus, for focussing
and for regulating the admission of light.

FIG. 58. — Vertical section

of frog's retina.
c. cones ; g. layer of
nerve - cells ; gr, gr .
outer and inner granu-
lar layers; tic, nc. 'outer
and inner nuclear
layers ; n.f. nerve-fibre
layer ; /. ep. pigment-

epitl
(Aft.

:er Howes.)

1 86 THE FROG CHAP.

As with the other sense-organs, sight may be destroyed
by injury to the retina or actual organ of sight, by cutting
the optic nerve, or by destroying the brain. But unlike the
other sense-organs already considered, that of sight has a
complex accessory or focussing apparatus in connection
with it, and vision may also be rendered impossible by
injury to the cornea or lens.

It is an obvious advantage to an organ of sight such as
the frog's that it should be capable of movement in any
direction, so as to allow the light from any object to enter
the pupil. As a matter of fact, the animal can direct its
gaze through a very wide range by means of eight muscles
connected with the eyeball in the orbit. One of these, the
levator bulbi, raises the whole eye, causing it to project further
on the surface of the head. Another, the retractor bulbi,
withdraws it, causing it to bulge into the mouth. Four
others (compare p. 164 and Fig. 126), the superior, inferior,
anterior, and posterior recti, rotate it respectively upwards,
downwards, forwards, and backwards. And finally, two oblique
muscles, the superior and inferior, produce a rotation along
an axis joining the optic nerve with the middle of the cornea.

The conjunctiva, which covers the outer surface of the eye
and lines the eyelids, is kept moist by the secretion of a
lacrymal gland, known as the Harderian gland, situated
between the eyeball and the orbit in the antero-ventral
region. The excess of this secretion is carried away into
the olfactory chamber by means of a tube, the naso-
lacrymal duct.

Each organ of hearing, like that of sight, consists of an
essential portion and an accessory apparatus. The essential
organ of hearing is a structure called the membranous laby-
rinth, contained within the auditory capsule of the skull
(Fig. 10), and consisting of a kind of bag of very peculiar

EAR

187

and complicated form (Fig. 59). It is made up, in the
first place, of two somewhat ovoid sacs separated by a
constriction : the dorsal one is called the utriculus (u\ the
ventral the saaulus (s), and from the latter a small process,
the cochlea (/), projects backwards, and a narrow canal, the
endolymphatic duct (de, se), upwards. With the utriculus are

se

ass

FIG. 59. — External view of left organ of hearing of a Vertebrate (semi-diagrammatic).
aa. ampulla of anterior canal, ae. of external canal, and ap. of posterior canal ;
ass. apex of utriculus ; ca. anterior semicircular canal ; ce. horizontal canal ; cp.
posterior canal ; cus. constriction between sacculus and utriculus ; de, se. endo-
lymphatic duct ; /. cochlea ; rec, sp, ss, u. utriculus ; s. sacculus. (From
Wiedersheim's Coinp. Anatomy.}

connected three tubes, called, from their form, the semi-
circular canals, each of which opens into the utriculus at
either end. One of them, the anterior canal (ca), is
directed forwards ; another, the posterior canal (cp), back-
wards ; both these are vertical in position and are united
to one another at their adjacent ends. The third, the

1 88

THE FROG

external canal (ce\ is directed outwards and has a hori-
zontal position. Each canal has one end dilated into
a bulb-like swelling or ampulla (aa, ap, ae) ; those of
the anterior and external canals are at their anterior ends,
while that of the posterior canal is at its posterior end.

The whole of this apparatus is rilled with a fluid, the
endolymph, in which are contained calcareous particles, the
otoliths or ear-stones. It is made of connective -tissue and

Fig. 60.— Longitudinal section through an ampulla.

a. e. auditory epithelium ; a. h. auditory hairs ; c, part of semicircular canal ; c.r.
acoustic spot and ridge; c.t. connective-tissue; e. epithelium; n. nerve;
u. junction with utriculus. (From Foster and Shore s Physiology.}

lined with epithelium, the cells of which are cubical for the
most part; but in certain places the wall is thickened,
forming what are called acoustic spots, of which there is
one to each ampulla, situated on a ridge (Fig. 60), while
others occur in the utriculus and sacculus. On these acoustic
spots the epithelial cells are greatly elongated, and produced
at the surface into delicate processes called auditory hairs :
to these cells the fibres of the auditory nerve are distributed.

xi EAR 189

The membranous labyrinth does not fit tightly into the
cavity of the auditory capsule in which it is contained ; a
space being left between it and the surrounding bone and
cartilage (Fig. 10). This space is filled by loose connec-
tive-tissue and a fluid called perilymph, by which tile mem-
branous labyrinth is surrounded and protected from shocks.
As you learnt in studying the skull, the outer wall of the
auditory capsule is perforated by a small aperture, the
fenestra ovalis (Fig. 10, fen. ov\ in which is fixed the stapes
(stp\ a small nodule of cartilage connected with a bony
rod or columella (col\ the cartilaginous hammer-shaped outer
end of which, or extra-columella, is fixed to the inner side
of the tympanic membrane (tymp. memb). The columella lies
in the tympanic cavity (tymp. cav), which is bounded extern-
ally by the tympanic membrane, internally by the auditory
capsule, and at the sides chiefly by muscles and connective-
tissue ; while below it communicates with the pharynx by
the Eustachian tube (eus. t).

When sound-waves impinge on the tympanic membrane,
the vibrations to which they give rise are transmitted by
the columella to the stapes, and so to the perilymph.
Thence they are communicated to the endolymph and act
as stimuli to the auditory cells of the acoustic spots, and
the impulses being carried to the brain by the auditory
nerve, give rise to the sensation of sound. Whether or not
all the acoustic spots are truly auditory in function is not
known : it seems certain that the semicircular canals are
really organs for the maintenance of equilibrium.

The sense of sound can be destroyed by injury to the
membranous labyrinth, by cutting the auditory nerve, by
destroying the brain, or — to a great extent at least — by
injury to the tympanic membrane or columella.

Notice that the general plan of all the sensory organs,

190 THE FROG CHAP.

those of the skin, eye, and ear, is the same. They
consist of certain peculiarly modified epithelial cells,
specially sensitive to impulses of particular kinds, and in
communication, by means of an afferent nerve, ^with nerve-
cells of the brain. The three things — sensory cell, afferent
nerve, and brain — form a chain, every link of which is
necessary for the performance of the sensory function, so
that the particular sense in question may be destroyed, not
only by destruction of the sense-organs in the strict sense,
but also by section of the afferent nerve or by destruction
of the brain.

General Physiology-Summary. — Before going on to the
next chapter it will be as well to take a final glance at the
physiological processes of the frog as a whole (compare
Fig. 23). The enteric canal is the manufactory in which
the raw material of the food is worked up into a form in
which it can be used by the various parts of the body. The
circulatory organs are the communicating system by which
the prepared food is taken to all parts ; and they also form
a drainage system by which waste matters are collected
from all parts and finally ejected by the three main sewers,
the skin, lungs, and kidneys. The skin and lungs, besides
getting rid of waste matters, serve for the supply of oxygen
— a necessary form of gaseous food. The central nervous
system forms a sort of headquarters' staff by which the
entire body is controlled, the means of communication
being the nerves, and the muscles the executive by which
the orders from headquarters are executed. And finally
the sense-organs may be looked upon as the various branches
of an intelligence department by which the headquarters
are informed of what is going on outside.

PRACTICAL DIRECTIONS 191

PRACTICAL DIRECTIONS.

The Organs of Special Sense.

I. Olfactory organ. Notice again the external and internal nostrils.
Then remove the skin covering the snout, dissect off the nasal bones,
and open up the olfactory sacs. Note the pigmented olfactory epithefaim
lining these, and make out the "olfactory nerves and nasal septum.
Compare with a transverse section through the sacs. Sketch.

II. Eye. •

a. Notice again the eyelids, iris, and pupil. Then remove the skin
covering the head so as to expose the nearly globular eyeballs, lying in
the orbits.

In the antero- ventral region of the orbit make out the Harderian
gland, and the eye-muscles passing from the walls of the orbit to
the eyeball. The four recti and two oblique muscles can be
more easily seen on a larger animal, and directions will be given for
their examination in Part II (see Fig. 126) ; but if you make a dissec-
tfon of them in the frog, you should note at the same time the levator
and retractor bulbi, the latter underlying the eyeball, and the former
situated internally to the recti muscles.

b. Remove the eyeball from a freshly-killed specimen, noticing as
you do so the optic nerve, which is surrounded by the recti and retractor
bulbi muscles : dissect away these muscles and note the cartilaginous
sclerotic, the cornea, iris, pupil, and the cut end of the optic nerve.

c. Divide the eyeball into an inner and an outer hemisphere by a rapid
cut with scalpel or scissors taken vertically, midway between the cornea
and the optic nerve, through the vitreous chamber. Place them both in
a watch-glass or small dissecting dish, under water, and examine with a
lens (compare Fig. 57). In the inner .hemisphere note the vitreous
humoitr, retina, pigmented choroid, and blind spot or entrance of the
optic nerve ; and in the outer hemisphere, the crystalline lens and the
margin of the retina, or ora serrata. Sketch. Remove the lens, and
notice the iris — continuous with the choroid, the pupil, and the aqueous
chamber.

d. Examine sections through the wall of the inner hemisphere of the
eyeball, prepared as directed on p. 136, first under the low, and then
under the high power of the microscope. Note :-

i. The cartilaginous sclerotic*

192 THE FROG CHAP, xi

2. The choroid, enclosing pigment-cells and blood-vessels.

3. The retina (Fig. 58), composed of a number of layers : notice
\hz pigment epithelium, the rod- and cone-layer, and the various other
layers of the retina, the innermost of which is composed of nerve-fibres
continuous with the optic nerve.

Sketch.

The anatomy of the eye can be more easily made out by dissecting
that of an ox or sheep, which is essentially similar to that of the frog,
and directions for the examination of which will be given in Part II.

III. Auditory organ.

Notice again the tympanic membrane and tympanic ring, and then
carefully cut away the former so as to expose the tympanic cavity.
Observe the Eustachian tube, the fenestra ovalis, and the relations of
the stapes, columella, and extra-cohimella (Fig. 10).

The essential part of the auditory organ (membranous labyrinth] is
very small in the frog, and can be more satisfactorily studied in a good-
sized fish (e.g., dogfish or cod). Directions for the preparation of the
membranous labyrinth of the former will be given at a later stage,
but if you have sufficient time and patience to dissect it out in the frog,
proceed as follows : —

Place the head of a large frog in nitric acid (about 10 per cent.) until
the bone is dissolved. Wash well in water so as to remove the acid,
and dissect away the muscle, £c., from the auditory capsule until the
latter is thoroughly exposed. Then with a sharp scalpel slice away the
roof of the capsule until the cavity it contains is seen. Proceed now
with great caution, removing the cartilage and decalcified bone, bit by
bit, until the cavity is sufficiently enlarged to bring the membranous
labyrinth into view (compare Figs. 10 and 59). Observe the utriculus,
sacculus, otoliths, and the three semicircular canals with their ampulla.
Sketch.

CHAPTER XII.

THE FROG (continued} : REPRODUCTION AND DEVELOPMENT.

So far we have considered those parts and organs of the
frog which have to do with its welfare as an individual. We
have now to consider the organs which are connected with
the welfare of the frog as a race, that is, with the propagation
of its kind.

The position of the reproductive organs has already been
seen (pp. 23 and 25) : they must now be examined in more
detail. The essential part of these organs in each sex is
a pair of bodies known as gonads, called in the male
spermaries or testes, and in the female ovaries.

Reproductive Organs of the Male. — The spermaries
(Fig. 3, r. spy, Fig. 5, spy, and Fig. 7, ts) are a pair of ovoid
bodies, each attached by a fold of the peritoneum to the
corresponding kidney, and having connected with it a fat-body
(cp. ad\ From the inner margin of each spermary spring a
number of delicate tubes, the efferent ducts (Fig. 61, ^),
which run in the fold of the peritoneum to the kidney.
Entering this organ near its inner edge, they open into a
longitudinal tube (Z) from which transverse tubes pass hori-
zontally across the kidney to open into the ureter (Ur).
The milt, or spermatic fluid (p. 9), is thus carried off by the
same duct as the urine ; the ureter is therefore often called the

PRACT. ZOOL. O

194

THE FROG

urinogenital duct. On the outer side of the ureter, and com-
municating with it by numerous short ducts, is a glandular
body, the seminal vesicle (Figs. 3 and 7, vs. .$•;//), which serves
to store up the spermatic fluid.

The spermary itself contains a narrow, irregular, central
cavity, from which the efferent ducts proceed and into
which open a number of short tubes or crypts (Fig. 62, A).

FIG. 61. — Spermary and kidney of frog showing the relations of the efferent ducts

(sem id i agr am mat ic).

C. transverse tubes in kidney ; Ho. spermary ; L. longitudinal tube ; N. kidney ;
q. efferent ducts of spermary ; Ur. ureter (urinogenital duct). (From Wieder-
sheim's Comp. Anatomy.)

These are lined with epithelium (f.e), the cells of which
divide and subdivide, forming groups of smaller cells. Each
of the latter undergoes a remarkable change, becoming con-
verted into a rod-like body, produced into a long thread,
which performs lashing movements, very much like those of
the cilia in ciliated epithelium. These bodies are called
sperms or spermatozoa (Fig. 62, A, sp, and B) ; in spite of

xn REPRODUCTIVE ORGANS 195

their peculiar form, they are cells, the rod-like portion, or
head) being the nucleus, and the delicate vibratile part, or
tail, the protoplasm. In th*e breeding season the cavities
of the testes are full of sperms floating in a fluid. Thus
the spermatic fluid, like the blood, owes its distinctive
character to the cells floating in it.

FIG. 62. — A, transverse section of a crypt of the spermary. B, stages in the

development of the sperms.

sp. bundles of sperms ; i.e. germinal epithelium. (A, after Blomfield ; B, after
Howes).

Reproductive Organs of the Female. — Each ovary
(Fig. 4, /. ovy] is a greatly folded sac with thin walls and
a large cavity divided up by partitions. It is attached to
the dorsal body- wall by a fold of peritoneum. As we have
seen (p. 23), its surface is studded all over with little
rounded projections, each of which is an ovisac, and contains
an egg. The egg or ovum (Fig. 63) is a large globular cell with
a clear nucleus (nu) containing numerous nucleoli (nu),
and having its protoplasm (pr) full of yolk-granules — grains
of proteid material which serve as nutriment for the growing

o 2

ic;6

THE FROG

embryo. It is covered with a delicate membrane, the vitel-
line membrane. By the time the egg is mature a superficial
deposit of pigment takes place 6ver one hemisphere.

In the young condition all the epithelial cells forming the
walls of the ovary are alike, but as the organ reaches
maturity, certain of them (<?) enlarge, and give rise to the

c.t

Av.

FIG. 63. — Transverse section of frog's ovary.

b. v. blood-vessels ; c. t. connective-tissue ; ep. outer layer, and ep' , inner layer of
epithelium ; ep". outer layer of ovisac, continuous with ep' . ', ep'" . germinal epi-
thelium, derived from ep ', ep"". follicular epithelium, derived from ep ; nu.
nucleus of ovum ; nu'. nucleoli ; o. young ovum ; pr. protoplasm of ovum
containing yolk-granules. (After Marshall.)

ova, while others form an investment or follicle (ep""}
for each ovum.

The oviduct (Fig. 4, /. ovd, r. ovd\ as you have seen, is a
long and greatly convoluted tube lying above or dorsal to
the ovary. Its anterior end narrows considerably, runs
parallel with the gullet, passes to the outside of the root of
the lung, and then opens into the ccelome by a small aper-
ture (r. ovd'). The greater part of the oviduct is about as
wide as the small intestine, and is thick-walled and lined

xii FERTILISATION 197

with gland-cells, which secrete the jelly surrounding the
eggs when laid (p. 9). Posteriorly it suddenly dilates into
a wide, thin- walled chamber (r. ovd ") which opens into the
dorsal wall of the cloaca. Notice that there is thus no
connection between the generative organs and the kidneys
in the female, such us occurs in the male, the ureters serving
as renal ducts only.

In the breeding season the ovisacs burst and set free the
eggs into the coelome, whence by some means or other they
find their way into the small openings of the oviducts, and so
into these tubes, when each becomes surrounded by a
little sphere of jelly secreted by the gland-cells. Passing
down the oviducts the eggs accumulate in the dilated
extremities, which they distend enormously, so that just
before laying, the abdomen of a female frog is nearly filled
by these two great egg-reservoirs ; the ovaries, having lost
so many of their eggs, are correspondingly reduced in size,
and all the other organs are squeezed out of place.

Fertilisation. — The eggs are now laid, and immediately
the spawn is passed from the oviducts of the female into the
water, the male sheds over them a quantity of spermatic fluid
(p. 9). The sperms, swimming actively through the water,
enter the spheres of jelly and come into contact with
the eggs. A single sperm then penetrates the vitelline
membrane of an ovum, loses its tail, and its head coming
into contact with the nucleus of the egg, fuses or conjugates
with it, so that a single nucleus is formed by the union of
the egg-nucleus with the sperm-nucleus. We may speak
of conjugating cells in general as gametes, the sperm in this
case being the male gamete, and the ovum the female
gamete, the body formed by the fusion of two gametes
being known as a zygote.

This process is known as fertilisation or impregnation.

198 THE FROG CHAP, xn

Without it, as we have seen, the egg is incapable of develop-
ment ; after it has taken place, the egg — or more strictly,
the oosperm, since it is now an ovum plus a sperm — is
potentially a young frog, since if left undisturbed in water,
it will in course of time give rise to a tadpole, which in its
turn will change into a frog.

It must be remembered in the first place that the fertilised
egg is a single cell, comparable with a blood-corpuscle or
an epithelial cell. It is, however, peculiar in two respects ;
first in having its nucleus derived in part from a sperm, so
as to contain matter from both the male and the female
parent ; and secondly in having its protoplasm distended
with yolk-granules to such an extent that instead of being a
minute body visible only under the microscope, it is easily
visible by the naked eye. The yolk is not equally distributed :
on one hemisphere it is less abundant than elsewhere,
and it is this more protoplasmic hemisphere which is super-
ficially blackened by a layer of pigment, and which always
floats upwards in the water when the egg is laid.

Segmentation of the Oosperm. — Almost directly after
being laid and fertilised the egg undergoes a remarkable
change. A furrow appears all around it, as if made with
a blunt instrument, and deepening gradually, at last divides
the oosperm into two hemispheres in contact with one
another by their flat faces (Fig. 64, A). The examination
of sections shows that this process is preceded by the
division of the nucleus into two ; its final result is the
separation of the originally one-celled egg into two cells.
Now if you refer to Chapter VIII, you will be reminded
of the fact that the epithelial cells of the skin multiply by a
similar process of simple fission, or division into two ; the
nucleus in each case dividing first and afterwards the
protoplasm.

FIG. 64. — Developmentrof the frog.

A — F, segmentation ; G, overgrowth of ectoderm ; H, I, establishment of germinal
layers ; J, K, assumption of tadpole-form and establishment of nervous system,

. notochord, and enteric canal ; L, newly-hatched tadpole.

l>l. ccel. segmentation cavity ; blp> blp' . blastopore ; br^, br*. gills ; br. cl. branchial
arches ;- e. eye ; ect. ectoderm ; end. endoderm ; ent. archenteron ; f. br. fore
brain ; h. br. hind-brain ; in. br. mid-brain ; md.f. medullary fold ; rnd.gr. medul
lary groove ; nies. mesoderm ; nig. large lower cells ; mi. small upper cells
nek. notochord ; n. e. c. neurenteric canal ; pcdm. proctodaeum ; pty. invagina
tion of ectoderm which will form the pituitary body ; ret. commencement of
rectum ; sk. sucker ; sp. cd. spinal cord ; stdin. stomodaeum ; t. tail ; yk.
yolk-cells;^.//, yolk-plug which fills the blastopore. (From Parker and
Haswell's Zoology , A— D, F— H, and J from Ziegler's models ; E, I, K, and
L after Marshall.)

200 THE FROG CHAP.

The furrow which effects this division of the oosperm
passes through both black and white poles, so that each of
the two cells formed is half black and half white. Soon a
second furrow is formed at right angles to the first, being,
like it, meridional, i.e., passing through the poles (Fig. 64,
B). It divides what we must now call the embryo into four
cells, each half black and half white. A third furrow is
then formed, passing round the equator, but nearer the
black than the white pole (c). It therefore divides the
embryo into eight cells, four upper black and four lower
white, the latter being obviously the larger.

This process of division or segmentation of the oosperm
continues, the black cells dividing more rapidly than the white,
so that before long the embryo consists of a mass of cells, the
polyplast, somewhat resembling a mulberry, and therefore
often called a morula, one hemisphere being composed of
small cells containing much protoplasm and little yolk, and
externally pigmented (D — F, mi\ and the other of larger
cells containing little protoplasm and much yolk, and not
pigmented (mg). As segmentation goes on, a cavity appears
in the interior of the embryo ; it is called the segmentation
cavity (E, bl. cwl\ and is due to the fact that the work of
segmentation produces a waste of substance, which there
is at present no means of* making good, so that for a time,
while the size of the embryo remains the same, its bulk
diminishes, some of its substance being used up ; in other
words, it feeds on itself.

Segmentation now proceeds to such an extent that the
black cells become too small to be seen except with a lens
of tolerably high magnifying power, so that with the amount
of magnification used in Fig. 64, G, the black hemisphere
shows no division into cells. At the same time the black
hemisphere gradually encroaches on the white until only a

xii DEVELOPMENT 201

small circular space — the blastopore, filled by a mass of yolk-
cells — the yolk-filug, is left uncovered (G, H, yk. pl\

Development of the Chief Organs. — In the meantime
a second cavity appears in the interior of the embryo, and
as it increases in size, the segmentation cavity undergoes a
proportional diminution and finally disappears. This new

mes

—enl

FIG. 65. — Transverse section through a frog-embryo during the formation of the

medullary canal.

c. coelome ; ect. ectoderm ; end. endoderm ; ent. archenteron ; mes. mesoderm ;
mes*. its outer (parietal) layer ; mes*. its inner (visceral) layer ; md.f. medullary
fold; md.gr. medullary groove; nch. notochord ; yk. yolk-cells. (After
Marshall.)

cavity, or archenteron (Figs. 64, i and 65, ent\ is the rudiment
of the enteric canal : it begins to be formed as a narrow
slit at the edge of the blastopore through which it at
first communicates with the exterior, but it soon becomes
closed : at present there is neither mouth nor anus.

By this time the cells of which the embryo is composed
have assumed different forms and become arranged in a

202 THE FROG CHAP.

very definite manner. First of all there are several layers
of small pigmented cells, derived from the black cells of
earlier stages, which cover the whole embryo with the excep-
tion of the yolk-plug : these constitute the ectoderm (Fig. 64,
G, H, i, ecf). Forming the roof of the enteric cavity are
other layers of cells derived from the yolk-cells of earlier
stages and forming the endoderm (i, end} : the floor of the
archenteron is at present formed of unaltered yolk-cells.
Between the ectoderm and the yolk-cells on the lower
pole are several layers of small cells which gradually
spread until they form a complete layer between the ecto-
derm and the endoderm : these constitute the mesoderm (i,
mes). The ectoderm, endoderm, and mesoderm are known
as the three embryonic tissues or germinal layers.

While these processes have been proceeding, a longi-
tudinal groove appears on the dorsal surface of the embryo :
this is the medullary or neural groove, and is bounded by
a pair of medullary folds (Figs. 64, H, and 65, md.gr, md.f).
The medullary groove extends backwards to the blastopore,
which now marks its posterior end. As development goes
on, the folds approach one another and unite, so as to
convert the groove into a canal : the cells lining this canal
give rise to the central nervous 'system (Fig. 64, K, and
compare Part II, Chapter XII), and its cavity becomes
the central canal of the spinal cord and the ventricles of the
brain. The anterior end of the hollow medullary cord thus
developed enlarges to form three vesicles, one behind the
other, and known respectively as the fore-brain^ mid-brain^
and hind-brain^ each containing a ventricle. The fore-brain
eventually gives rise to the cerebral hemispheres, produced
in front into the olfactory lobes, and to the diencephalon ;
the mid-brain to the optic lobes and crura cerebri ; and the
hind-brain to the medulla oblongata and cerebellum. It

xii DEVELOPMENT 203

will be seen (Fig. 65) that the medullary groove is lined
by ectoderm and that therefore the whole central nervous
system is ectodermic in origin ; and this is also true of the
nerves.

A longitudinal thickening of the endoderm on the dorsal
side of the archenteron becomes constricted off as a solid
rod of cells, lying between the medullary cord and the
archenteron. This is the notochord (Figs. 64, K, and 65,
ncti) ; it forms the primary axial skeleton around which the
vertebral column is subsequently developed. The meso-
derm is for some time a solid mass occupying all the space
between the ectoderm and endoderm, so that there is no
body-cavity. But at a later stage a cavity appears divid-
ing the mesoderm into two layers, one in contact with the
ectoderm (parietal layer, Fig 65, mesl\ the other with the
endoderm (visceral layer, mes2). The space between them
is the ccelome (c). The dorsal part of the mesoderm on
either side of the medullary cord and notochord becomes
divided transversely, and gives rise to muscle-segments or
myomeres (Fig. 64, L).

The embryo now begins to elongate in a definite direc-
tion (Fig. 64, j) : its dorsal surface, still marked by the
medullary groove, which now soon becomes closed, is slightly
concave, its ventral surface very convex. The blastopore
closes up, the yolk-plug becoming entirely covered by ecto-
derm ; but for a short time the medullary canal and
archenteron still communicate by the neurenteric canal
(K, n.e.c.}. This soon disappears, and at the hinder end
of the embryo a conical outgrowth forms the rudiment
of the tail (£). The opposite end is rounded, and on its
ventral surface appears a little half-moon shaped groove
(which afterwards becomes subdivided into two, Fig. 66,
c), the rudiment of the sucker by which, the tadpole

204 THE FROG CHAP.

attaches itself to weeds (j and L, sti). Just above the
sucker a depression — the mouth-pit or stomod&um (L, stdui)
— makes its appearance, and is the first indication of the
mouth. A similar depression below the tail — the proctodceum
(pcdm) — marks the anus : both at first are mere blind pouches
and have no communication with the enteric cavity. The
head-region is further marked by two pairs of vertical ridges
separated by depressions : the ridges are the branchial or
gill-arches (j, br.cl\ and the depressions the branchial_
clefts.

Further elongation takes place (Fig. 64, L), the head
becomes distinctly marked off, the tail extends considerably
beyond the anus, a thickening appears which marks the
position of the eye (e\ and a depression that of the ear (just
above br 2 in the figure) ; and from each branchial arch arises
a little tuft, the rudiment of one of the external gills (br \
fir'2). In this condition the tadpole is hatched : it is still
unable to feed, the stomodaeum not yet being in com-
munication with the enteric cavity ; it is nourished therefore
entirely by the yolk with which a large portion of the body
is still filled.

Up to the stage shown in Fig. 64, i, the cells of which the
tadpole is composed, although distinguishable into ecto-
derm, endoderm, and mesoderm, are all more or less similar :
there are no muscles, no cartilages, ^bones, or connective-
tissue. But shortly after the stage referred to, the permanent
tissues begin to be formed : the outer ectoderm cells take
on the form of epiderm, the endoderm cells become the
epithelium of the enteric canal, and from offshoots of the
latter — all of course lined by endoderm — are 'formed the
lungs, the liver, the pancreas, and the urinary bladder.

The mesoderm undergoes much more extensive changes,
giving rise to the connective-tissue, cartilage, bone, and

xii DEVELOPMENT 205

muscle, as well as to other parts — i.e., to by far the greater
portion of the permanent tissues. In certain places the em-
bryonic mesoderm cells, hitherto in close contact, separate
from one another and become branched, while between them
appears intercellular substance with fibres crossing one
another in various directions : in this way the connective-
tissue which forms so large a part of the adult body is
produced.

The various parts of the skeleton first arise by the conver-
sion of portions of the mesoderm into cartilage. The cells
retreat from one another and between them a clear sub-
stance, the matrix, makes its appearance. For a considerable
time almost the whole skeleton consists of cartilage, but
subsequently much of this tissue is replaced by bone
developed from a layer of cells on the inner surface of the
perichondrium. The bones thus formed in connection with
cartilage are replacing bones : investing bones arise in con-
nective-tissue which has no direct relation to the cartilage
(compare p. 43).

In the place where voluntary muscles are to appear the
mesoderm cells elongate, their nuclei multiply by fission, and
their protoplasm gradually becomes converted into the
striated substance so characteristic of the adult muscular
tissue.

These examples will suffice to illustrate the fact, which
further study would show to be true, that all the permanent
tissues are either — like epithelium — formed entirely of actual
cells, or are — like connective-tissue and striped muscle — de-
rived from cells. The entire embryo in its earliest condition —
the oosperrri stage — is a single cell, which multiplies repeatedly
by simple fission, forming a group of cells : and these, by
assuming various . forms and undergoing various changes,
give rise to all the complex tissues — differing from one

206 THE FROG CHAP.

another both structurally and functionally — of the adult
animal. So that every cell, fibre, or what not in the frog is
a linear descendant, through repeated fission, of the oosperm ;
and the oosperm itself is the product of the fusion of two
cells, one — the ovum — derived from the female, the other
— the sperm — from the male parent. Thus in passing from
the oosperm to the adult animal there is a gradual structural
or morphological differentiation accompanied by a differen-
tiation of function or division of physiological labour.

The expression " division of physiological labour " was
invented by the great French physiologist, Henri Milne-
Edwards, to express the fact that a sort of rough correspon-
dence exists between lowly and highly organised animals
and plants on the one hand, and lowly and highly organised
human societies on the other. In primitive communities
there is little or no division of labour : every man is his own
butcher, baker, soldier, doctor, &c., there is no distinction
between '• classes " and " masses," and each individual is to
a great extent independent of all the rest. Whereas in
complex civilised communities society is differentiated into
politicians, soldiers, professional men, mechanics, labourers,
and so on, each class being to a great extent dependent on
every other.

Structure and Metamorphosis of the Tadpole —Develop-
ment of other Organs — Metamorphosis. — A sketch of the
further development of the tadpole and of its metamorphosis
has already been given (pp. 9-11, Fig. i), but it is now
necessary to add a few details to those already mentioned.

A third pair of branchial arches appear behind the two
already noticed, and on each a third external gill. The
first two pairs increase greatly in size and all the
gills become branched (Fig. 66, A). The branchial clefts

xii DEVELOPMENT 207

communicate with the pharynx, and a current of water
enters the mouth and passes out by these clefts, thus pro-
viding the gills with a constant supply of aerated water for
purposes of respiration. A thin median fold of the skin, the
tail-fin (Fig. i), arises all round the tail, both above and
below, and the tail now forms a powerful swimming organ.
The stomodaeum (Fig. 66, B — E) and, at a still earlier
period, the proctodaeum (p. 204) open into the enteric cavity,
so that there is now a complete enteric canal : it grows
much faster than the body generally, and becomes coiled like
a watch-spring (D, E). .The mouth is very small, and is
bounded by lips beset with little horny projections or papillae
and provided with a pair of horny jaws (c.— E) with which
the tadpole now browses upon the water-weeds which form
its staple food. It is on account of this diet that the in-
testine is of such great proportionate length : vegetable diet
contains less nourishment, bulk for bulk, than animal, and
a longer intestine is required to extract all the nutriment
from it.

Soon the external gills show signs of shrivelling, and
on the inner portions of the branchial arches internal gills
(Fig. 66, D1) are developed, like those of a fish. A fold
of skin appears on either side in front of the gills and
gradually grows backwards, covering the branchial clefts and
the external gills, which latter soon disappear entirely ;
this is the operculum or gill-cover (B, c) ; it is similar to
that of an ordinary fish, such as a cod or a perch, except
that it is not supported by bone. The current of water
from the pharynx now, of course, makes its final exit by a
wide cleft between the edge of the operculum and the flank.
The operculum gradually extends, and its free edge unites
with the wall of the body posterior to the gills, so that the
originally large aperture through which the water from the

208

THE FROG

pharynx passes is much reduced. After some time, a com-
plete union of the operculum with the flank takes place on
the right side, the opercular aperture becoming closed : on the
left side the spout-like aperture remains open for some time

FIG. 66. — Stages in the later development of the tadpole.
A, stage with external gills ; B, stage showing the formation of the operculum ;

C, later stage, in which the external gills are disappearing ; D, stage with in-
j ternal gills and budding limbs ; D1, dissection of same stage to show the heart,

gills, and lungs ; E, later stage, in which the limbs are further differentiated.

(After Howes.)

longer and the water from both sides passes out through it
(D). All this time the tadpole is to all intents and
purposes a fish ; apart from the possession of gills and a
tail-fin, in the structure of the circulatory organs and various
other parts it is more like a fish than a frog.

xil METAMORPHOSIS 209

The lungs (o1) become functional, and the tadpole is
now truly amphibious, rising periodically to the surface
to breathe air. At a later stage, however, the single
opercular aperture closes, the gills gradually disappear,
and thenceforth respiration is purely aerial.

In the meantime the limbs are developed. The hind-
limbs appear as little rounded buds, one on either side
of the root of the tail (D) : they gradually elongate and
divide into thigh, shank, and foot (E). The fore-limbs
appear beneath the operculum and are therefore hidden
at first (E) : at a later stage each divides into upper arm,
fore-arm, and hand, and emerges from its concealment.
As the limbs increase, the tail undergoes a progressive
shrinking; its tissues become, as it were, digested, and
are carried off by the blood ; so that for a time the
metamorphosing tadpole is nourished largely by its own tail.

The mouth widens, the horny jaws and papillae disappear
and teeth are formed. The suckers vanish, and the intestine
not only begins to grow less rapidly than the body, but even
becomes reduced in length and loses its spiral arrangement,
while vegetable is exchanged for animal diet. The dark
colour of the tadpole gradually gives place to the bright hues
of the frog. The little tailed frog can now leave the water
and hop about on land : its tail is soon absorbed, and the
metamorphosis is complete (Fig. i., 78).

It will be noticed that in the course of development
a substitution or replacement of organs occurs. Thus, for
instance, the notochord is replaced by a vertebral column,
the gills by lungs, and the horny jaws by teeth.

Fate of the Germinal Layers. — In concluding this chapter
we may enumerate in rather more detail the chief parts
and organs which are derived from each of the three
embryonic tissues respectively. * From the ectoderm are

PRACT. ZOOL. P

210 THE FROG CHAP.

formed — the epiderm and the cutaneous glands ; the
whole of the nervous system, central and peripheral, and the
essential parts of the sensory organs, as well as the crystalline
lens of the eye ; and also the epithelium lining the mouth
(stomodaeum) and outer part of the cloaca (proctodaeum).
The endoderm gives rise to the epithelium lining the enteric
canal and its various offshoots, including the lungs, urinary
bladder, gastric glands, bile- and pancreatic-ducts, as well as
the glandular cells of the liver and pancreas ; and to the
notochord. From the mesoderm are developed the various
parts which are situated between the ectoderm and endo-
derm with the exception of the notochord, mz., the connec-
tive-tissue, cartilage, bone, striped and unstriped muscles,
circulatory organs, and peritoneum, as well as the urinary
and reproductive organs and the accessory parts of the
sensory organs.

PRACTICAL DIRECTIONS.

Organs of Reproduction.

Open a frog in the usual way : cut through the gullet, rectum, and
mesentery, and remove all the digestive organs. In the female take
especial care not to injure the roots of ithe lungs in severing the gullet.
One of the specimens you have already dissected and kept in the pre-
servative fluid should still contain the urinogenital organs intact in the
case of the male : in the female, the ovaries and greater part of the
oviducts will have been removed, but the relations of the two ends of
each oviduct can still be made out.

Examine the reproductive organs, which will now be freely exposed,
under water — or in the case of the female, in I per cent, salt solution.

I. Male Organs. (Figs. 3, 5, 7, and6i.)

1. Notice again the spermaries or testes, each supported by a fold
of peritoneum connecting it with the corresponding kidney ; the efferent
ducts ; the branched fat-body \ the ureter (urinogenital duct}', and the
seminal vesicle. Sketch after examining the cloaca (see next page).

2. Tease up a bit of the spermary of a recently-killed frog in salt-

xii PRACTICAL DIRECTIONS 211

solution, and notice the form and movements of the sperms (Yig. 62, B).
Sketch.

3. Examine under the microscope a transverse section of the spermary,
prepared as directed on p. 136, .and note (Fig. 62, A) the numerous
seminal tubes or crypts which open into it. Under the high power,
observe the epithelial cells (germinal epithelium] lining the tubes, and
their subdivision into smaller cells, which eventually give rise to the
sperms, the tails of which project freely into the cavities of § the tubes.
Sketch.

II. Female Organs.

1. Notice again — (a) the ovaries, varying in size and appearance ac-
cording to the time of year, and each suspended by a fold of peritoneum : •
they are studded all over with ovisacs, each of which contains an egg or
oviun, pigmented when ripe ; and (b) the oviducts (Fig. 4). Trace the
convoluted and glandular middle portion of the oviduct forwards, and
make out the anterior thin- walled portion, running parallel to the gullet
and opening into the ccelome by a small aperture at the ' base of the
lung ; then trace the middle portion backwards, and notice the thin-
walled, dilated, posterior portion, which opens into the cloaca. Sketch
after examining the cloaca (see below).

If a female frog is examined in the spring, just: before the eggs are
laid, the ovaries will be seen to be reduced in size, and the posterior
portion of the oviduct filled with eggs, each surrounded with a gela-
tinous coat secreted by the middle portion.

2. Examine under the microscope a section of the. ovary, prepared as
directed on p. 136, first with the low, and then with the high power
(Fig. 63). Note the epithelial cells (including the germinal epithelium]
forming the walls of the ovary, and the ovisacs in different stages of
development. Each ovisac contains an ovum, surrounded \>y follicle'
cells. In the ova, note the protoplasm and its contained yolk-granules,
the vitelline membrane, the nucleus, and the nucleoli. Sketch.

Cloaca,

Carefully cut through the pelvic symphysis in the middle line with a
scalpel and press apart the two innominate bones, so as to expose the
ventral surface of the cloaca. Inflate from the vent, and note that the
large intestine is continuous with the cloaca and that the urinary
bladder opens into it on the ventral side.

The entire urinogenital apparatus, together with the cloaca, may
now be removed from the body, first cutting through the skin round the

P 2

212 THE FROG CHAP.

vent. Pin down underwater, insert the small scissors into the vent,
and slit open the cloaca very slightly to one side of the middle line,
so as not to injure the connection between it and the bladder.
Notice the openings into the bladder and large intestine respectively,
and also : —

In the male, the apertures of the ureters (urinogenital ducts), situated
on two small papilloe, lying close to one another on the dorsal side of
the cloaca,: insert a bristle into the ureter. Sketch.

In the female , the two small apertures of the ureters, and just in front
of these, the two large apertures of the oviducts, all situated on the
dorsal wall of the cloaca. Insert bristles into the ureter and oviduct.
• Sketch.

Impregnation and Development.

In the early spring (end of February or beginning of March), look
about in ponds and ditches for frogs which have taken to the water for
the purpose of laying eggs. This process can be watched more con-
veniently by catching a few frogs, male and female, and putting them
in a large vessel of water or aquarium. If you have been unsuccessful
in procuring frogs or frogs' spawn, toads will do equally well : their
eggs are laid a few weeks later, and are arranged, each surrounded by
its gelatinous envelope, not in clumps, like those of the frog, but in a
string, like the beads on a necklace.

For purposes of observation, the spawn is best kept in a glass vessel,
together with some water-weeds. Put only a small quantity of spawn
in one vessel ; if the water begins to get foul, change it at once.
Examine the eggs every day with a magnifying-glass as development
proceeds, and keep some of the tadpoles alive until metamorphosis
takes -place in June.

If you wish to see some of the stages in the development of the .frog
during other times of the year, you must obtain some preserved eggs
and embryos, and, if possible, you should also examine the series of wax
models, made on an enlarged scale, which are to be seen in most
zoological museums.

The following are some of the more important things to be noticed.
(Sketch a series of stages) : —

Before hatching.

I. The unsegmented oosperm, black above, and white below.

II. Early stages in segmentation, during which the individual cells
can be made out by means of a lens. (Fig. 64 A-F. )

' xii PRACTICAL DIRECTIONS 213

III. Later stages in segmentation, during which the individual cells
cannot be distinguished without the aid of a microscope. Note the
gradual enclosure of the white by the black hemisphere, until only a small
rounded yolk-plug is left, filling in an aperture --the blastopore — in the
black layer or ectoderm, as it is now called. The yolk-plug is continuous
with a mass of yolk-cells, enclosed by the ectoderm, from which the
endoderm and mesoderm are developed. (Fig. 64, G-i.)

IV. The gradual flattening and elongation of the embryo, and the
position of the blastopore at the posterior end of the dorsal region ; the
formation of the medullary folds and groove along the dorsal side, and
the closure of the blastopore. (Fig. 64, H-J.)

V. The closure of the medullary groove, the formation of the head
with its suckers and of the tail with its tail-Jin ; as well as the
appearance of the eyes, ear-sacs, branchial arches, external gills, and
the involutions for the mouth and anus, which, however, do not open
until after the tadpole is hatched. (Fig. 64, K-L.)

After hatching.

VI. The appearance of the horny jaws and papilla, and the further
development of the head, external gills, and tail. (Fig. I, 1-2, and
Fig. 66, A-B.)

VII. The formation of the operculum, the closure of the right
opercular aperture, and the disappearance of the external gills. The
bud-like rudiments of the hind-limbs, and their further development.
The fore-limbs remain for a long time hidden beneath the operculum.
(Fig. i, 3-4 and Fig. i, 66, C-E.)

Metamorphosis.

VIII. The gradual change in the form and colour of the head and
body ; the widening of the mouth and loss of the horny jaws and
suckers ; the appearance of the fore-limbs from beneath the operculum,
and the shrinking of the tail. . (Fig. I, 5-8.)

Dissection of tadpole.

Pin down a full-sized tadpole under water in a small dissecting dish,
with the ventral surface uppermost, inserting small pins through the tail
only. Carefully dissect off the ventral body walls, and note the coiled
intestine, the heart, the internal gills, &c. (Fig. 66, D, D1, E. )
Sketch.

It is rather a difficult task, and requires much time, to prepare sections
of the early stages of the frog-embryo in order to make out the formation
of the ectoderm, endoderm, and mesoderm, the relations of the seg-

214 THE FROG CHAP, xn *

mentation cavity and archenteron, and the development of the central
nervous system and notochord (Figs. 64, I, K, and 65) ; and directions
for the study of the development of the chief organs of the chick will be
given in a subsequent chapter. But if you wish to make the attempt,
proceed as follows : —

Take a few eggs every day from the time they are laid until the
tadpoles are hatched : place these on a tile or piece of glass, and with a
pair of needles dissect off the gelatinous covering. Then carefully transfer
the segmenting eggs or embryos into corrosive sublimate for a quarter
to half an hour, and after washing in running water, put them into
weak and then strong alcohol ; stain, imbed, and cut sections as
directed on p. 136.

CHAPTER XIII.

THE FROG (continued) : MEANING OF THE TERM SPECIES —

THE PRINCIPLES OF CLASSIFICATION — EVOLUTION —
ONTOGENY AND PHYLOGENY — HEREDITY AND VARIA-
TION— STRUGGLE FOR EXISTENCE — SELECTION — ORIGIN
OF SPECIES.

THE frog which you have been studying is the only one
commonly found in Great Britain. Another kind, very
similar to it, is, like the common frog, abundant in Ger-
many and other parts of the European continent, but is rare
in this country, only occurring in parts of the eastern counties ;
and various other frogs, differing in certain minor respects
from these are found in different parts of the world. This
fact is expressed in the language of systematic zoology by
saying that there are various species of frogs, belonging to
the same genus, which are distinguished from one another
by certain definite characteristics as regards form, structure,
and colour.

According to the system of binomial nomenclature intro-
duced by Linnaeus, each kind of animal receives two names —
one, the generic name, common to all the species of the
genus ; the other the specific name, peculiar to the species in
question. Both generic and specific names are Latin in
form, and are commonly Latin or Greek in origin, although

216 THE FROG CHAP.

frequently modern names of persons or places with latinised
terminations are employed. In giving the name of an animal,
the generic name is always placed first, the specific name
following it, and being written as a rule with a small letter.
Thus the common English frog is called Rana temporaries,
and the continental form referred to above, often spoken of
as the edible frog, Rana esculenta.

You will probably have noticed certain differences in
colour and markings in the different individual frogs you
have examined, and it is matter of common observation
that no two individuals of a species are exactly alike. In
the case of human beings and many of the more familiar
animals this is very apparent to every one : in other cases a
more careful examination of the individuals is necessary in
order to tell them apart ; thus, for instance, the individuals in
a flock of sheep appear all alike to the casual observer, but
the shepherd can easily distinguish them from one another.
These differences we designate individual variations, and it
is often difficult to decide whether two kinds of animals
should be considered as distinct species, or as varieties of a
single species, and no universal rule can be given for deter-
mining this point. Among the higher animals, mutual fer-
tility is a fair practical test, the varieties of a species (e.g.,
common pigeon, fowl, dog, horse) usually breeding freely
with one another and producing fertile offspring, while dis-
tinct species usually do not breed together, or else produce
infertile hybrids or mules. Compare, for instance, the fer-
tile mongrels produced by the union of the various breeds of
domestic dog with the infertile mule produced by the union
of the horse and ass. But this rule is not without exception,
and in the case of wild animals is, more often than not, impos-
sible of application ; failing it, the only criterion of a " good
species " is usually the presence of constant differences from

xin MORPHOLOGY AND PHYSIOLOGY 217

allied species, whereas if there is a complete series of grada-
tions between two forms, they will be considered to form a
single variable species.

In the previous chapters it will have been evident that an
animal may be studied from two chief points of view, firstly,
from the point of view of its structure^ and secondly from that
of the functions performed by its various parts and the way
in which these work together for the welfare of the whole.
The branch of zoology dealing with the former is known as
morphology, and with the latter physiology.

It is evident that a knowledge of morphology is necessary
as a preliminary to the study of physiology, and also that, as
animals have to be distinguished from one another largely
by structural characters, the foundations of a scientific zoo-
logy must be laid in morphology, which, as we have seen,
deals not only with the external characters and the anatomy
and histology of the adult animal, but also with the changes
undergone during the development of the egg into the adult
form, i.e., with embryology. Given a sound knowledge of
the anatomy, histology, and embryology of animals, their
classification may be attempted : that is, we may proceed to
arrange them in groups and sub-groups, each capable of
accurate definition. In doing so we must be careful to dis-
tinguish between homologous parts, or those which correspond
structurally (compare p. 39), and analogous parts, which
have merely a similar function, and are of no value for
purposes of classification. Thus the fore-limbs of a frog
are homologous with — i.e., are formed on a similar plan
to — the wings of a bird, although differing from them in
function ; they are only analogous to the limbs of an insect,
for though the function is similar in both, the structure is
widely different. In the same way the wing of the bird
is only analogous to that of an insect.

218 THE FROG CHAP.

The general method of classification employed by zoolo-
gists may be illustrated by reference to the different kinds
of frogs already referred to in explanation of the terms
genus and species.

The common frog (Rana temporarid) is distinguished
from the " edible frog " (R. esculentd) by its smaller size and
brown colour, by the large dark patch in the tympanic
region, and by the rudimentary character of a pair of infla-
table vocal sacs at the sides of the head in the male, which
are very large and highly distensible in R. esculenta. On the
other hand, these two frogs agree with one another and with all
the other species of the genus Rana, in having teeth on the
upper jaw, and in not having the transverse processes of the
sacral vertebra dilated. Comparing all these frogs with our
English toads, of which there are two species, the common
toad and the rarer " natterjack," we find that in them the
skin is comparatively dry and covered with glandular warts,
the hind-limbs are proportionally shorter, there are no
teeth, and the transverse processes of the sacral vertebra are
more or less dilated. These differences are so great as not
only to necessitate placing the toads in another genus, the
genus BufO) but also to relegate the frogs just mentioned
to one family — the Ranidcz, and the toads to another — the
Bufonidce. All frogs and toads, however, agree with one
another in having no tail in the adult, while the trunk
is relatively short and broad, and the hind-limbs are longer
than the fore-limbs. They therefore differ fundamentally
from such animals as the common English newts and the
salamanders, which retain the tail throughout life, and in
which the fore- and hind-limbs are of approximately equal
size. The differences here are obviously far greater than
those between the families mentioned above, and are
emphasised by placing the frogs and toads in the order

xin CLASSIFICATION 219

Anura, the newts and salamanders in the order
Urodela.

The Urodela and Anura, although differing from one
another in many important respects, agree, e.g., in possess-
ing gills during part or the whole of their existence, and
in nearly always possessing lungs. They usually pass
through a metamorphosis, the young being hatched in the form
of gilled larvae ; their skin is soft and glandular, and the toes
are in nearly all cases without claws. These and numerous
other structural characters separate them from reptiles, in
which gills are never developed, and the young do not pass
through a metamorphosis, while the skin is provided with
scales and the toes have claws. The differences here are
considerably more important than those between the orders
referred to above, and are expressed by placing the latter
in the class Amphibia, while reptiles constitute the class
Reptilia. In the same way the fishes, which possess fins
and gills, form the class Pisces, the feathered birds the class
Aves, and the hairy animals which suckle their young the
class Mammalia.

Mammals, Birds, Reptiles, Amphibians, and Fishes all
agree with one another in the possession of red blood and
an internal skeleton, an important part of which in the
embryo is the notochord (p. 203) — which is nearly always
replaced in the adult by a backbone or vertebral column —
and in never having more than two pairs of limbs. They
thus differ in some of the most fundamental features of
their organisation from such animals as Crayfishes, Insects,
Scorpions, and Centipedes, which have colourless blood,
a jointed external skeleton, and numerous limbs. These
differences— far greater than those between classes — are
expressed by placing the back-boned animals in the phylum
or sub-kingdom Vertebrata, the many-legged, armoured forms

220 THE FROG CHAP.

in the phylum Arthropoda. Similarly, soft-bodied animals
with shells, such as mussels and snails, form the phylum
Mollusca ; various worms, such as the earthworm, the phylum
Annulata • polyps and jelly-fishes the phylum Ccelenterata ;
the simplest animals, mostly minute, such as Amazba, the
phylum Protozoa. Finally, the various phyla recognised
by zoologists together constitute the kingdom Animalia.

Thus the animal kingdom is divided into phyla, the
phyla into classes, the classes into orders, the orders into
families, the families into genera, and the genera into
species ; while the species themselves are assemblages of
individual animals agreeing with one another in certain
definite characteristics. It will be seen that the individual
is the only term in the series which has a real existence ;
all the others are mere groups, formed, more or less arbi-
trarily, by man.

Thus the zoological position of the common frog is
expressed as follows : —

Kingdom — ANIMALIA.

Phylum — VERTEBR ATA.
Class — AMPHIBIA.
Order — ANURA.

Family — Ranid<z.
Genus — Rana.

Species — temporaria.

Let us now briefly consider some of the reasons which
have led zoologists to adopt the system of classification now
in general use.

It is obvious that there are various ways in which animals
may be classified; and the question arises — which of these,
if any, is the right one ? Is there any standard by which

xin CLASSIFICATION 221

we can judge of the accuracy of a given classification, or
does the whole thing depend upon the fancy of the classi-
fiers, like the arrangement of books in a library ? In other
words, are all possible classifications of living things more
or less artificial, or is there such a thing as a natural
classification ?

Suppose we were to try and classify all the members of a
given family — parents and grandparents, uncles and aunts,
cousins, second cousins, and so on. Obviously there are a
hundred ways in which it would be possible to arrange
them— into dark and fair, tall and short, curly-haired and
straight-haired, and so on. But. it is equally obvious that
all these methods would be purely artificial, and that the
only natural way, i.e., the only way to show the real con-
nection of the various members of the family with one
another, would be to classify them according to blood-
relationship ; in other words, to let our classification take the
form of a genealogical tree.

There are two theories which attempt to account for the
existence of the innumerable species of living things which
inhabit our earth : the theory of special creation and the
theory of evolution.

According to the theory of creation, all the individuals
of every species existing at the present day — the tens of
thousands of dogs, frogs, oak-trees, and what not, are
derived by a natural process of descent from a single indi-
vidual, or from a pair of individuals — in each case precisely
resembling, in all essential respects, their existing descend-
ants— which came into existence by a process outside the
ordinary course of nature and known as creation. On this
hypothesis each species of frog is derived from a common
ancestral pair which came into existence, independently of

222 THE FROG CHAP.

the progenitors of all the other species, at some previous
period of the earth's history.

Notice that on this theory the various species of frogs are
no more actually related to one another than is either of
them to a newt, or for the matter of that, to man. The
individuals of any one species are truly related since they
all share a common descent, but there is no more relation-
ship between the individuals of any two independently
created species than between any two independently manu-
factured chairs or tables. The words affinity, relationship,
&c., as applied to different species are, on the theory of
creation, purely metaphorical, and mean nothing more than
that a certain likeness or community of structure exists;
just as we might say that an easy chair was more nearly
related to a kitchen chair than either of them to a three-
legged stool.

We see, therefore, that on the hypothesis of creation, the
varying degrees of likeness and unlikeness between the
species receive no explanation, and that we get no absolute
criterion of classification : we may arrange our organisms,
as nearly as our knowledge allows, according to their re-
semblances and differences, but the relative importance of
the characters relied on -becomes a purely subjective
matter.

According to the rival theory— that of Descent or Organic
Evolution, with which the name of Darwin is inseparably
connected — every species existing at the present day is
derived by a natural process of descent from some other
species which lived at a former period of the world's history.
If we could trace back from generation to generation the
individuals of any existing species, we should, on this
hypothesis, find their characters gradually change, until

xiii EVOLUTION 223

finally a period was reached at which the differences were
so considerable as to necessitate the placing of the ancestral
forms in a different species from their descendants at the
present day. And in the same way, if we could trace back
the species of any one genus, we should find them gradually
approach one another in structure until they finally con-
verged in a single species, differing from those now existing,
but standing to all in a true parental relation.

It will be seen that, on this hypothesis, the relative like-
ness and unlikeness of the various species of frogs are
explained as the result of their descent with greater or less
modification or divergence of character from the ancestral
form : and that we get an arrangement or classification in
the form of a genealogical tree, which, on this hypothesis,
is a strictly natural one, since it shows accurately the
relationship of the various species to one another and to
the parent stock. So that on the theory of evolution, a
natural classification of any given group of allied organisms
is simply a genealogical tree.

Now it is evident that the only way in which we could
be perfectly sure of an absolutely natural classification of
the species of any kind of animal — the frog, for example —
would be by obtaining specimens as far back as the distant
period when the genus first came into existence.

Forming part of the solid crust of the earth are a series of
sedimentary or stratified rocks, and the researches of geolo-
gists have shown that these present a general order of
succession, the lowest, when undisturbed, being in every
case older than the more superficial layers. Imbedded
in these rocks are found the remains of various extinct
animals in the form of what are called fossils • and it might
perhaps, on first considering the subject, be supposed that,

224 THE FROG CHAP.

had a process of evolution taken, place, we ought to be
able to find in the rocks belonging to the various geological
formations a complete series of animal remains, represent-
ing all the stages in the evolution of the highest from the
lowest forms. But owing to various causes which we can-
not further consider here, the record of the succession of
life on the globe is very imperfect and incomplete in the
case of any individual species ; but the evidence furnished
by paleontology — as the study of fossils is called — is very
important in supplying proofs that there has been a gradual
evolution of the higher from the lower forms.

Further evidence in the same direction is furnished by
morphology and embryology, as will be more apparent to
you when you have studied a number of other kinds of
animals. You have, however, seen that the frog begins
life as a single cell, and that it is possible to trace a series of
modifications which gradually convert the unicellular oosperm
into a tadpole — which is to all intents and purposes a fish,
and that the tadpole subsequently undergoes metamor-
phosis into the more highly organised frog. According to
the theory of recapitulation, these facts indicate that the
frog repeats, during its single life, the series of changes
passed through by its ancestors in the course of ages. In
other words, ontogeny, or the evolution of the individual*
is, in its main features, a recapitulation of phytogeny, or
the evolution of the race. At the same time you must
bear in mind that it is not always an easy matter to deter-
mine which characters are of phylogenetic significance, and
which have been secondarily acquired owing to various
causes. To take an example : — the horny jaws of the
tadpole might be taken to indicate that frogs were de-
scended from ancestors with horny jaws and without teeth,

xiii ORIGIN OF SPECIES 225

though in all probability these organs have been secondarily
acquired as adaptations in connection with the habits of the
tadpole, and have therefore, unlike the gills, for instance,
no ancestral significance.

It is obvious that the evolution of one species from
another presupposes the occurrence of variations in the
ancestral form. As we have seen, such individual variation
is of universal occurrence. This may be expressed by
saying that heredity, according to which" the offspring tends
to resemble the parent in essentials, is modified by varia-
bility, according to which the offspring tends to differ from
the parent in detail. If, from any cause, any well-marked
individual variation is perpetuated, there is produced what
is known as a variety of the species, and according to the
theory of the origin of species by evolution, such a variety
may, in course of time, become a new species. Thus a
variety is an incipient species, and a species is a (relatively)
permanent variety.

One other important factor in evolution must be briefly
referred to before concluding this chapter. In order that
every species of animal and plant may flourish, certain
conditions are necessary. Thus the frog requires, for
example, a moist place to live in, and water in which to
lay its eggs. For spots presenting the necessary favourable
conditions, there is constantly going on a competition
between individuals of one species and between the mem-
bers of different species. The nature of this struggle is
well seen when a piece of garden ground is allowed to
run to waste. Its surface is soon overgrown by weeds of
many kinds, which kill out nearly all the original garden-
plants by depriving them of light and food. By and by the
more hardy weeds exterminate and replace such weaker

PRACT. ZOOL. O

226 THE FROG CHAP.

forms as may first have obtained a footing, till an entirely
new set of weeds may take the place of those that first
appeared.

A struggle for existence goes on on all sides among
animals. To begin with, before there is any struggle for
existence in the strict sense, there is — particularly in animals
which, like the frog, produce eggs in great numbers annually
— a very great indiscriminate destruction of ova and young
embryos. Only a few of them reach maturity ; a large
proportion are destroyed at one stage or other, some
failing to reach a spot favourable for their further de-
velopment, others becoming the food of other animals.
But such of the young as are less adapted to escape
the various dangers to be encountered, and less fitted to
procure the necessary food, are more likely to be destroyed.
This is one phase — and the most important, perhaps, of all
— of the struggle for existence amongst animals. But there
is also a struggle for existence, not only between individual
animals of the same kind, but between animals of different
kinds. This struggle, in so far as it relates to the competi-
tion for food and shelter, is more severe between nearly
related species ; for in such a case the food and the favour-
able conditions required are the same, or nearly so, in the
two competitors. Again, a struggle for existence of a con-
stant and severe kind also goes on between carnivorous
animals and the animals on which they prey — e.g.9 between
the frog and the insects and worms on which it feeds, and
between the snake and the frog : a struggle in which the
defensive qualities of the prey — such as swiftness, power of
eluding observation, or of resisting attack — are opposed to
the predatory powers of the attacker.

There can be little doubt that, in the long run, such
individuals will survive as are best fitted to cope with the

xin NATURAL SELECTION 227

conditions to which they are subject. According to
Darwin's theory of natural selection, it is assumed that
such surviving individuals would transmit their special
properties to their progeny, and there would thus be a
gradual approximation towards a perfect adaptation of the
species to its surrounding conditions by virtue of this
" survival of the fittest."

Let us suppose the conditions to change. Gradual alter-
ations in climate and other conditions are known to take
place, owing to subsidence or elevation of the land. But
conditions might be changed in many other ways : some
animal or plant previously used as food might become ex-
terminated, or a new enemy might find its way into the
district inhabited by the species. Then such individuals
as presented variations which enabled them better to cope
with the new surroundings would have the advantage over the
others, and would have a much better chance of surviving
and leaving progeny. The useful variations thus produced
and transmitted to the progeny would tend to increase,
generation after generation, until a form sufficiently distinct
to be regarded as a new species had become developed from
the original one.

That very different varieties of animals and plants can,
and have been produced, in a comparatively short time, by
man selecting those forms which tend to vary in a desired
direction, is a well-known fact. All the breeds and varieties
of our domestic animals have been produced by this process
of artificial selection • and " if man can by patience select
variations useful to him, why, under changing and com-
plex conditions of life, should not variations useful to
Nature's living products often arise, and be preserved or
selected ? "

It does not come within the scope of the present work to

Q 2

228 THE FROG CHAP, xm

discuss either the causes of variability or those which deter-
mine the elevation of a variety to the rank of a species :
both questions are far too complex to be adequately treated
except at considerable length, and anything of the nature
of a brief abstract would only be misleading.

PART II

CHAPTER I

AMC.KBA UNICELLULAR AND MULTICELLULAR ANIMALS.

FROM your study of the frog you will have learnt some of
the more important facts with regard to the morphology and
physiology of a comparatively highly-organised animal, and
will have overcome a number of preliminary difficulties in
acquiring a knowledge of zoological terminology and
technique. You will now, therefore, be in a better position
to undertake a systematic and comparative examination of
a number of other animals — some much less complicated,
some more complicated, than the frog — working upwards
from the simple to the complex forms. In doing so, you
must continually bear in mind the deductions in connection
with the theory of evolution referred to in the previous
chapter.

Let us begin with a very instructive animalcule belonging
to the genus Amoeba. Amoebae are often found in the slime
at the bottom of pools of stagnant water, adhering to weeds
and other submerged objects. They are mostly invisible to
the naked eye, rarely exceeding |th of a millimetre (T^th
inch) in diameter, so that it is necessary to examine them
entirely by the aid of the microscope. Though they can be
seen and recognised with the low power, the high power is
necessary for the accurate examination of their structure.

FIG. 67. — A, Amoeba quarta, a living specimen, showing granular endoplasm sur-
rounded by clear ectoplasm, and several pseudopods (psd), some formed of
ectoplasm only, others containing a core of endoplasm. The larger bodies in the
endoplasm are mostly food-particles ( X 300 diameters).

B, the same species, killed and stained with carmine to show the numerous nuclei
(nu) present in this species (compare Fig 73) (X 300).

PT. ii. CH. i AMCEBA 231

C, Amoeba proteus, a living specimen, showing large irregular pseudopods,
nucleus (nu), contractile vacuole (c. vac), and two food vacuoles (f. vac}, each
containing a small infusor (see Chapter III) which has been ingested as food. The
letter a to the right of the figure indicates the place where two pseudopods have
united to enclose the food-vacuole. The contractile vacuole in this figure is
supposed to be seen through a layer of granular protoplasm, whereas in D,
E, and G it is seen in optical section, and therefore appears clear.

D, an encysted Amoeba, showing cell-wall or cyst (cy), nucleus (mi), clear con-
tractile vacuole, and three microscopic plants (diatoms) ingested as food.

E, Amoeba protens, a living specimen, showing several large pseudopods (psd),
single nucleus (mi) and contractile vacuole (c. vac), and numerous food-particles
embedded in the granular endoplasm ( x 330).

F, nucleus of the same after staining, showing a ground substance containing
deeply-stained granules of chromatin, and surrounded by a distinct membrane
(X TOIO).

G, Amoeba verrucosa, living specimen, showing wrinkled surface, nucleus (nu),
large contractile vacuole (c. vac), and several ingested organisms ( x 330).

H, nucleus of the same ; the chromatin aggregated in the centre (x 1010).
I, Amoeba proteus, in the act of multiplying by binary fission (x 500).
(From Parker's Biology : A, B, E, F, G, and H after Gruber ; C and I after
Leidy ; D after Howes.)

Examined under the high power (Fig. 67), the Amoeba
appears like a little shapeless blob of jelly, nearly or quite
colourless, and closely resembling a colourless blood-cor-
puscle or leucocyte of one of the higher animals (p. 105).
The central part of it, or endoplasm, is granular and semi-
transparent — something like ground-glass — while surround-
ing this inner mass is a border of perfectly transparent and
colourless substance — the ectoplasm.

One very noticeable thing about the Amoeba is that, like
the leucocyte, it is never of quite the same form for long
together, owing to the protrusion of pseudopods (psd\
by means of which it creeps along slowly. The occur-
rence of amoeboid movements is alone sufficient to show
that it is an organism, or living thing, and no mere mass
of dead matter. Moreover, it consists of protoplasm,
and encloses a nucleus (C — H, mi) containing chromatin
and rendered more apparent by staining. The Amoeba
is therefore a cell (compare pp. 106 and no).

A very important difference is thus at once seen between
the Amoeba and the frog : the Amoeba is unicellular, i.e. it
consists of a single cell, while the frog is, as we have seen,

232 AMCEBA CHAP.

a multicellular animal, built up of innumerable cells which are
incapable of an independent existence for any length of time.

Besides the nucleus, there is another structure frequently
visible in the living Amoeba and not present in the leuco-
cyte. This is a clear, rounded space in the protoplasm (c. vac\
which periodically disappears with a sudden contraction and
then slowly reappears, its movements reminding one of the
beating of a minute colourless heart. It is called the
contractile vacuole, and consists of a cavity containing a
watery fluid.

We must now study the physiology of our animalcule.
First of all, as we have already seen, it is contractile ;
although it has no muscles, it can move about from place to
place. Its movements, like the voluntary movements of the
frog (pp. 7, 172), may occur without the application of any
external stimulus, i.e., they are spontaneous or automatic \ or
they may be induced by external stimuli — by a sudden
shock or by coming in contact with an object suitable for
food. Movements of this latter kind, like those resulting
from the stimulation of the nerves in a brainless frog, are
the result of the irritability of the protoplasm ; the animal-
cule is therefore both automatic and irritable, although it
possesses neither nerves nor sense-organs.

Under certain circumstances an Amoeba temporarily loses
its power of movement, draws in its pseudopods, and
becomes a globular mass around which is formed a thick,
shell-like coat, called the cyst or cell-wall (Fig. 67, D, cy).
This is formed by the protoplasm by a process of secretion
(p. 130) : its composition is not known ; it is certainly not
protoplasmic, and very probably consists of some nitro-
genous substance allied in composition to horn and to the
chitin (see Chapters VI — VIII) which forms the external
shell of crustaceans, insects, &c.

I NUTRITION 233

The formation of the cyst is probably of great importance
in preserving the animalcule from destruction by drought, so
that should the pool in which it is living dry up, it may still
remain alive, protected by its shell-like case, until the con-
ditions for its active life are once more restored ; it escapes
by the rupture of the cyst, sometimes having first divided
into numerous young Amoebae.

Very often an Amoeba in the course of its wanderings
comes in contact with a still smaller organism of some kind
or other. When this happens the Amoeba may be seen to
extend itself round the lesser organism until the latter
becomes sunk in its protoplasm in much the same way as a
marble might be pressed into a lump of clay (Fig. 67, c, a).
The diatom or other organism becomes in this way com-
pletely enclosed in a cavity or food-vacuole (f. vac), which
also contains a small quantity of water necessarily included
with the prey. The latter is taken in by the Amoeba as
food : so that the Amoeba, like the frog, feeds. It is to
be noted that the reception of food takes place in a
particular way, viz. by ingestion — i.e. it is enclosed entire
by the organism.

When the prey is thus ingested, its protoplasm becomes
digested, any insoluble portions being passed out or egested,
as fgeces (pp. 8 and 75), from the surface of the Amoeba as it
creeps slowly on. Note that all this is done without either
ingestive aperture (mouth), digestive cavity (stomach), or
egestive aperture (anus) : the food is simply taken in by the
flowing round it of protoplasm, digested as it lies enclosed in
the protoplasm, and those portions for which it has no further
use are got rid of by the Amoeba flowing away from them.

We have seen that the frog possesses certain digestive
glands, the function of which is to secrete digestive fluids
which have an important chemical action on the food

234 AMOEBA CHAP.

swallowed, rendering it soluble and diffusible before it
passes through the epithelial cells of the enteric canal into
the blood : the gastric juice, for example, has the power of
converting proteids into peptones by means of the ferment
pepsin (p. 74) ; the digestion here takes place outside the
cells, i.e. is extracellular. There can be little doubt that
the protoplasm of Amoeba is able to render that of its
prey soluble and diffusible by the agency of some sub-
stance analogous to pepsin, and that the dissolved matters
diffuse through the body of the Amoeba until the latter is,
as it were, soaked through and through with them. The
process of digestion in Amoeba thus takes place within a
single cell, i.e. it is intracellular.

It has been proved by experiment that proteids are the
only class of food which Amoeba can make use of: it is
unable to digest either starch or fat (p. 72). Mineral matters
must, however, be taken with the food in the form of a
weak watery solution, since the water in which the ani-
malcule lives is never absolutely pure.

The Amoeba being thus permeated, as it were, with a
nutrient solution, the elements of the solution, hitherto
arranged in the form of peptones, mineral salts, and water,
become rearranged in such a way as to form new particles
of living protoplasm, which are deposited among the pre-
existing particles. In -a word, the food is assimilated, or
converted into the actual living substance of the Amoeba,
which must therefore grow, if nothing happens to counteract
this formation of new protoplasm.

We have seen, however, that work results in a propor-
tional amount of waste (p. 66), and just as in the frog or in
ourselves, every movement of the Amoeba, however slight,
is accompanied by a proportional oxidation or low tempera-
ture combustion of the protoplasm, i.e. the constituents of the

I EXCRETION AND RESPIRATION 235

protoplasm combine with oxygen, forming waste or excretory
matters — carbon dioxide, water, and certain nitrogenous
substances of simpler constitution than proteids, such as
urea. These products of excretion, formed in the case
of Amoeba without the agency of any special excretory
organs (e.g. kidneys), are given off partly from its general
surface, but partly, it would seem, by the agency of the
contractile vacuole, by means of which the water taken in
with the food is also got rid of.

With this breaking down of proteids the vital activities of
all organisms are invariably connected. Just as useful
mechanical work may be done by the fall of a weight from a
given height to the level of the ground, so the work done by
the organism is a result of its complex proteids falling, so to
speak, to the level of simpler substances. In both instances
potential energy or energy of position is converted into
kinetic or actual energy.

The statement just made that the protoplasm of Amoeba
constantly undergoes oxidation presupposes a constant sup-
ply of oxygen. The water in which the animalcule lives
invariably contains that gas in solution, and diffusion takes
place, oxygen passing into the interior of the Amoeba while
carbon dioxide passes out into the water. This is the process
of breathing or respiration (p. 144), and it occurs in Amoeba
without the agency of lungs or other respiratory organs.
Thus the carbon dioxide is got rid of, and at the same time
a supply of oxygen is obtained for further combustion. The
oxidation of the protoplasm of the Amoeba is doubtless
accompanied by an evolution of heat, as in higher animals
(p. 151), although this has never been proved.

We thus see that a very elaborate series of chemical pro-
cesses is constantly going on in the interior of Amoeba, as in
the frog, the whole series of which is spoken of collectively

236 AMGEBA CHAP.

as metabolism— constructive and destructive (p. 149). Living
protoplasm is thus the most unstable of substances ; it is
never precisely the same thing for two consecutive seconds ;
its existence, like that of a waterfall or a fountain, depends
upon the constant flow of matter into it and away from it.

It follows from what has been said that if the income of
an Amoeba, i.e., the total weight of substances taken in (food
plus oxygen plus water), is greater than its expenditure or
the total weight of substances given out (faeces plus excreta
proper plus carbon dioxide), the animalcule will grow : if less
it will dwindle away : if the two are equal it will remain of
the same weight or in a state of physiological equilibrium.

It is evident that Amoeba must also be able to perform
the function of reproduction. You have learnt that the cells
of the frog multiply \$y simple or binary fission (p. 106) : the
nucleus first divides into two, and then the surrounding
protoplasm ; and precisely the same thing occurs in Amoeba,
the reproduction of which therefore takes place by the
simplest method known, without any special repro-
ductive organs. The animalcule simply divides into two
Amoebae, each exactly like itself; and in doing so ceases to
exist as a distinct individual. Instead of the successive
production of offspring from an ultimately dying parent, we
have the simultaneous production of offspring by the division
of the parent, which does not die, but becomes simply
merged in its progeny. There can be no better instance of
the fact that reproduction is discontinuous growth.

From this it seems than an Amoeba, unless suffering a
violent death, is practically " immortal," since it divides into
two completely organised individuals, each of which begins
life with half of the entire body of its parent, there being
therefore nothing left of the latter to die : it therefore

I REPRODUCTION 237

appears certain that " death has no place as a natural re-
current phenomenon " in that organism. In multicellular
form it is only the reproductive cells which are, physically,
potentially immortal.

It is said that occasionally two Amoebae come into contact
and undergo fusion, just as the gametes of the frog (sperm
and ovum) unite in the processes of fertilisation (p. 197).
This process of conjugation — which probably precedes en-
cystment and multiple fission (p. 233) — will be referred to
again in the following chapters, and it is important to bear in
mind that reproduction can take place quite independently of
such a process.

Amoebae may also be propagated artificially. If a speci-
men is cut into pieces, each fragment is capable of develop-
ing into a complete animalcule provided it contains a
portion of nuclear matter, but not otherwise. From this it
is obvious that the nucleus exerts an influence of the utmost
importance over the vital processes of the organism.

If an Amoeba does happen to be killed and to escape
being eaten it will, like a dead frog, undergo gradual decom-
position, becoming converted into various simple substances
of which carbon dioxide, water and ammonia are the chief

(P- I52)-

Death results if the temperature to which an Amceba is
exposed reaches about 40° C., and at freezing point its move-
ments cease entirely and it becomes inert.

We thus see that complex organs, composed of various
tissues, each consisting of cells of characteristic form, are
not necessary in order that the vital functions may be
performed : the only essential is nucleated protoplasm. As
we pass from the unicellular Amceba to the higher multi-
cellular animals we shall find —just as we found in tracing

238 AMCEBA CHAP.

the development of the frog from the unicellular oosperm
(p. 206) — that a differentiation oj structure accompanied by a
division of physiological labour becomes more and more
marked, some cells giving rise to organs of locomotion,
others to organs of reproduction, and so on. But every
function necessary for the life of an animal — or a plant — is
due in the first instance to protoplasm, and a simple cell,
like the Amoeba, can perform them all.

In the next two chapters we shall study certain other
unicellular organisms which show an advance on Amoeba in
possessing a certain amount of morphological and physio-
logical differentiation. But the structural differentiations, as
they are merely parts of one cell, cannot be spoken of as
" organs " in the sense in which we have used the word
hitherto, as they are not composed of numerous cells.
They are, however, organs in the physiological sense, as
they perform different functions.

PRACTICAL DIRECTIONS.1

Amoeba,

Examine a drop of water from the bottom of a pond, with the low
power, first putting on a cover-glass, and look for Amcebre : if the water
does not contain small particles of sand or mud, place a small piece of
paper under the edge of the cover so as to avoid crushing the organisms.
When you have found a specimen, note —

I. The irregular and changing form of the animal, the protoplasm
running out into blunt pseudopods.

1 You should, if possible, try and obtain specimens of Amoebae and
the other fresh-water organisms described in this and the two following
chapters for yourself, by collecting stagnant pond -water, together with
a little of the mud at the bottom and some water- weeds, and letting
it stand for a few days in a glass jar or bottle. If you are unable to
find the organisms you require, they, as well as most of the other
animals described in this book, may be obtained from dealers in
Natural History objects (see e.g. the advertisements in Nature'].

i PRACTICAL DIRECTIONS 239

2. The granular character of the protoplasm, the granules usually
not extending to the periphery, so that a clear ectoplasm can be dis-
tinguished from a granular cndoplasm. The granules render the flowing
movements of the protoplasm visible.

3. The food-vactioles in the protoplasm, containing fluid, and often
also food particles.

4. The contractile vat 'note ', containing fluid, and its rhythmical con-
tractions.

5. The protrusion and retraction of the pseudopods. Sketch a
specimen several times at short intervals, noting the direction in which
the granules flow. Then put on the high power. Go over §§ I — 5 again,
and make a detailed sketch.

6. Look out for specimens undergoing multiplication by binary
fission, and also for encysted individuals.

7. Run a little dry carmine or indigo under the cover-glass, and
note that the particles can be taken in at all parts of the surface.

8. Treat with methyl-green or magenta and acetic acid (seep. 121).
This will kill the animal, and render the nucleus distinct.

9. Permanent preparations, showing the nucleus, may be made on
the slide as follows : —

Place a drop of water containing Amoebae on a slide, and soak up
with blotting-paper as much of the water as is possible without carry-
ing the Amoebae along with it. Fix (see p. 136) with a drop of
absolute alcohol, stain (a staining-fluid called picrocarmine is better
than borax-carmine for this purpose), wash carefully with weak and
then with absolute alcohol, and add a drop of turpentine — or better,
oil of cloves. Soak off the excess of oil of cloves with blotting-paper,
and mount in Canada balsam.

CHAPTER II

SPH^RELLA AND EUGLENA — MONADS AND BACTERIA —
DIFFERENCES BETWEEN ANIMALS AND PLANTS — SAPRO-
PHYTES.

THE rain-water which collects in puddles, open gutters,
&c., is frequently found to have a green or red colour. The
colour is due to the presence of various organisms —
plants or animals — one of the commonest of which is
Sphczrella (or, as it is sometimes called, Hczmatococcus or
Protococcus) pluvialis.

Like Amoeba, Sphaerella is so small as to require a
high power for its examination. Magnified three or four
hundred diameters it has the appearance (Fig. 68, A)
of an ovoidal body, somewhat pointed at one end, and
of a bright green colour, more or less flecked with bright
red.

Like Amoeba, moreover, it is in constant movement,
but the character of the movement is very different in
the two cases. An active Sphaerella is seen to swim about
the field of the microscope in all directions and with
considerable apparent rapidity. We say apparent rapidity
because the rate of progression is magnified to the same
extent as the organism itself, and what appears a racing
speed under the microscope is actually a very slow crawl

CHAP. II

SPH/ERELLA

241

when divided by 300. It has been found that such
organisms as Sphrerella travel at the rate of one foot in
from a quarter of an hour to an hour; or, to express the

FIG. 68. — A, Sphczrella pluviacis, motile phase. Living specimen, showing
protoplasm with chromatophores (chr) and pyrenoids (far), cell-wall (c. iv)
connected to cell-body by protoplasmic filaments, and flagella (J?). The scale
to the left applies to Figs. A — D.

B, resting stage of the same, showing nucleus (nu) with nucleolus (nu'\ and thick
cell-wall (c. w) in contact with the protoplasm.

C, the same, showing division of the cell-body in the resting stage into four
daughter-cells.

D, the same, showing the development of flagella and detached cell-wall by the
daughter cells before their liberation from the enclosing mother-cell-wall.

E, SphcErella lacustris, showing nucleus (nu\ single large pyrenoid (pyr), and
contractile vacuole (c. vac).

F, diagram illustrating the movement of a flagellum ; ab. itsibase ; c, c', c". differ-
ent positions assumed by its apex. (From Parker's Biology : E, after Biitschli.)

fact in another and fairer way, that they travel a distance
equal to two and a half times their own diameter in one
second. In swimming the pointed end is always directed

PRACT. ZOOL. R

242 SPH^:RELLA CHAP.

forwards, and the forward movement is accompanied by a
rotation of the organism upon its longer axis.

Careful watching shows that the outline of a swimming
Sphaerella does not change, so that there is evidently
no protrusion of pseudopods, and at first the cause of
the movement appears rather mysterious. Sooner or later,
however, the little creature is sure to come to rest, and there
can then be seen projecting from the pointed end two exces-
sively delicate colourless threads (Fig. 68, A, fl\ each about
half as long again as the organism itself : these resemble the
cilia on the epithelial cells lining the frog's mouth (p. 109),
except that they are few in number, and do not vibrate
rhythmically; they are therefore usually distinguished as
flagella. In a Sphaerella which has come to rest these
can often be seen gentty waving from side to side : when
this slow movement is exchanged for a rapid one the whole
organism is propelled through the water, the flagella acting
like a pair of extremely fine and flexible fins or paddles,
Thus the movement of Sphaerella is not amoeboid, i.e.,
produced by the protrusion and withdrawal of pseudpods,
but is ciliary, i.e., due to the rapid vibration of cilia or
flagella.

By staining and other tests it is shown that Sphaerella,
like Amoeba, consists of protoplasm, and that the flagella
are simply filamentous processes of the protoplasm.

The green colour of the organism is due to the presence of
a special pigment called chlorophyll, the substance to which
the colour of leaves is due. That this is something quite
distinct from the protoplasm may be seen by treatment
with alcohol, which simply kills and coagulates the proto-
plasm, but completely dissolves out the chlorophyll, pro-
ducing a clear green solution. The solution, although green
by transmitted light, is red under a strong reflected light,

ii CHROMATOPHORES 243

and is hence fluorescent-, when examined through the
spectroscope it has the effect of absorbing the whole of the
blue and violet end of the spectrum as well as a part of the
red. The red colour which occurs in so many individuals,
sometimes entirely replacing the green, is due to a colouring
matter closely allied in its properties to chlorophyll, and
called hccmatocJiroine.

At first sight the chlorophyll appears to be evenly distri-
buted over the whole body, but accurate examination under
a high power showrs it to be lodged in a variable number
of irregular structures called chromatophores (Fig. 68, A, chr\
which together form a layer immediately beneath the
surface. Each chromatophore consists of a protoplasmic
substance impregnated with chlorophyll.

After solution of the chlorophyll with alcohol a nucleus
(B, mi) can be made out ; like the nucleus of Amoeba, it is
rendered more distinct by staining. Other bodies which
might easily be mistaken for nuclei are also visible in the
living organism. These are small ovoidal structures (A, pyr\
with clearly defined outlines, occurring in varying numbers
in the chromatophores. When treated with iodine they
assume a deep, apparently black, but really dark blue colour.
The assumption of a blue colour with iodine is the charac-
teristic test of the carbohydrate starch (p. 72), as can be
seen by letting a few drops of a weak solution of iodine
fall upon some ordinary washing starch. The bodies
in question have been found to consist of a proteid
substance covered with a layer of starch, and are called
pyrenoids.

In SpJmrella pluvialis there is no contractile vacuole,
but in another species, £. lacustris, this structure is present
as a minute space near the anterior or pointed end (Fig. 68,
E, c. vac}.

R 2

244 SPH./ERELLA CHAP.

There is still another characteristic structure to which no
reference has yet been made. This appears at the first view
something like a delicate haze round the red or green body,
but by careful focussing is seen to be really an extremely
thin globular shell (A, c.w), composed of some colourless
transparent material, and separated, by a space containing
water, from the body, to which it is connected by very
delicate radiating strands of protoplasm. It is perforated
by two extremely minute apertures for the passage of the
flagella. Obviously we may consider this shell as a cyst
or cell-wall, differing from that of an encysted Amoeba
(Fig. 67, D) in not being in close contact with the
protoplasm.

A more important difference, however, lies in its chemical
composition. The cyst or cell-wall of Amoeba, as stated in
the preceding chapter (p. 232), is very probably nitrogenous
that of Sphserella, on the other hand, is formed of a
carbohydrate called cellulose, allied in composition to starch,
sugar, and gum, and, like starch, having the formula C6H10O5.
Many vegetable substances, such as cotton, consist of
cellulose, and wood is a modification of the same com-
pound. Cellulose is stained yellow by iodine, but iodine
and sulphuric acid together turn it blue, and a similar
colour is produced by a solution of iodine and potassium
iodide in zinc chloride known as Schulze's solution. These
tests are quite easily applied to Sphaerella : the protoplasm
stains a deep yellowish-brown, around which is seen a
sort of blue cloud, due to the stained and partly-dissolved
cell-wall.

After leading an active existence for a longer or shorter
time, the Sphaerella comes to rest, loses its flagella, and
forms a thick cell-wall of cellulose (Fig. 68, B), thus
becoming encysted. So that, as in Amoeba, there is an

ii NUTRITION 245

alternation of an active or motile with a stationary or resting
condition.

In the matter of nutrition, the differences between
Sphserella and Amoeba are very marked, and indeed funda-
mental. As we have seen, Sphaerella has no pseudopods,
and therefore cannot take in solid food after the manner
of Amoeba; moreover, even in its active condition, it is
usually surrounded by a cell-wall, which of course quite
precludes the possibility of ingestion. As a matter of
observation, also, however long it is watched it is never
seen to feed in the ordinary sense of the word. Never-
theless it must take food in some way or other, or the de-
composition of its protoplasm would soon bring it to an end.

Sphaerella lives in rain-water. This is never pure water,
but always contains certain mineral salts in solution, espe-
cially nitrates, ammonia salts, and often sodium chloride
or common table-salt. These salts can and do diffuse into
the water which is a constituent part of the protoplasm of
the organism, so that we may consider its protoplasm to be
constantly permeated by a very weak saline solution, the
most important elements contained in which are oxygen,
hydrogen, nitrogen, potassium, sodium, calcium, sulphur,
and phosphorus. It must be remarked, however, that the
diffusion of these salts does not take place in the same
uniform manner as it would through parchment or
other dead membrane. The living protoplasm has the
power of determining the extent to which each constituent
of the solution shall be absorbed.

It water, containing a large quantity of Sphserella is
exposed to sunlight, minute bubbles appear in it, and
these bubbles, if collected and properly tested, are found to
consist largely of oxygen. Accurate chemical analysis has

246 SPH^RELLA CHAP.

shown that this oxygen is produced by the decomposition
of the carbon dioxide contained in solution in rain-water,
and indeed in all water exposed to the air ; the gas, which is
always present in small quantities in the atmosphere, being
very soluble in water.

As the carbon dioxide is decomposed in this way, its
oxygen being given off, it is evident that its carbon must be
retained. As a matter of fact it is retained by the organism
but not in the form of carbon ; in all probability a double
decomposition takes place between the carbon dioxide
absorbed and the water contained in its protoplasm, the
result being the liberation of oxygen in the form of gas and
the simultaneous production of some extremely simple
form of carbohydrate, i.e., some compound of carbon,
hydrogen, and oxygen with a comparatively small number
of atoms to the molecule.

The next step seems to be that the carbohydrate thus
formed unites with the ammonia salts or the nitrates absorbed
from the surrounding water, the result being the formation
of some comparatively simple nitrogenous compound.
Then further combinations take place, substances of greater
and greater complexity are produced, sulphur from the
absorbed sulphates enters into combination, and proteids
are formed. From these, finally, fresh living protoplasm
arises.

From the foregoing account, which only aims at giving
the very briefest outline of a subject as yet imperfectly
understood, it will be seen that, as in Amoeba, the final result
of the nutritive process is the manufacture of protoplasm,
and that this result is attained by the formation of various
substances of increasing complexity. But it must be noted
that the steps in this process of constructive metabolism
are widely different in the two cases. In Amoeba we start

ii NUTRITION 247

with living protoplasm — that of the prey — which is killed
and broken up into diffusible proteids, these being after-
wards re-combined to form new molecules of the living pro-
toplasm of Amoeba. So that the food of Amoeba is, to
begin with, as complex as itself, and is first broken down by
digestion into simpler compounds, these being afterwards
re-combined into more complex ones. In Sphaerella, on
the other hand, we start with extremely simple compounds,
such as carbon dioxide, water, nitrates, sulphates, &c.
Nothing which can be properly called digestion, *.<?., a
breaking up and dissolving of the food, takes place, but its
various constituents are combined into substances of
gradually increasing complexity, protoplasm, as before,
being the final result.

To express the matter in another way : Amoeba can only
make protoplasm out of proteids already formed by some
other organism : Sphaerella can form it out of simple liquid
and gaseous inorganic materials.

Speaking generally, it may be said that these two methods
of nutrition are respectively characteristic of the two great
groups of living things. Animals require solid food con-
taining ready-made proteids, and cannot build up their pro-
toplasm out of simpler compounds. Green plants, i.e., all
the ordinary trees, shrubs, weeds, &c., take only liquid and
gaseous food, and build up their protoplasm out of carbon
dioxide, water, and mineral salts. The first of these methods
of nutrition is conveniently distinguished as holozoic^ or
wholly-animal, the second as holophytic, or wholly-vegetal.

It is important to note that only those plants or parts of
plants in which chlorophyll is present are capable of holo-
phytic nutrition. Whatever may be the precise way in which
the process is effected, it is certain that the decomposition
of carbon dioxide which characterises this form of nutrition

24$ SPH^ERELLA CHAP.

is a function of chlorophyll, or to speak more accurately, of
chromatophores, since there is reason for thinking that it
is the protoplasm of these bodies and not the actual green
pigment which is the active agent in the process.

Moreover, it must not be forgotten that the decomposition
of carbon dioxide is carried on only during daylight, so that
organisms in which holophytic nutrition obtains are depend-
ent upon the sun for their very existence. While Amoeba
derives its energy from the breaking down of the proteids
in its food (see p. 235), the food of Sphserella is too
simple to serve as a source of energy, and it is only by the
help of sunlight that the work of constructive metabolism
can be carried on. This may be expressed by saying that
Sphaerella, in common with other organisms containing
chlorophyll, is supplied with kinetic energy (in the form of
light or radiant energy) directly by the sun.

As in Amoeba, destructive metabolism is constantly going
on side by side with constructive. The protoplasm becomes
oxidised, water, carbon dioxide, and nitrogenous waste
matters being formed and finally got rid of. Obviously-
then, absorption of oxygen must take place, or in other
words, respiration must be one of the functions of the pro-
toplasm of Sphaerella as of that of Amoeba. In many green,
i.e., chlorophyll-containing, plants, this has been proved
to be the case : respiration, i.e., the taking in of oxygen and
giving out of carbon dioxide, is constantly going on, but
during daylight is obscured by the converse process — the
taking in of carbon dioxide for nutritive purposes and the
giving out of the oxygen liberated by its decomposition. In
darkness, when this latter process is in abeyance, the occuf-
rence <>l ivspinm'on is more readily ascertained.

Owing to the constant decomposition, during sunlight, of
carbon dioxide, a larger volume of oxygen than of carbon

ii METABOLISM 249

dioxide is evolved ; and if an analysis were made of all
the ingesta of the organism (carbon dioxide plus mineral
salts plus respiratory oxygen) they would be found to con-
tain less oxygen than the egesta (oxgyen from decomposition
of carbon dioxide plus water, excreted carbon dioxide, and
nitrogenous waste) ; so that the nutritive process in
Sphaerella is, as a whole, a process of deoxidation. In
Amoeba, on the other hand, the ingesta (food plus respi-
ratory oxygen) contain more oxygen than the egesta (faeces
plus carbon dioxide, water, and nitrogenous excreta), the
nutritive process being therefore on the whole one of
oxidation. This difference is, speaking broadly, character-
istic of plants and animals generally ; animals, as a rule,
take in more free oxygen than they give out, while green
plants always give out more than they take in.

But destructive metabolism is manifested not only in the
formation of waste-products, but in that of substances
simpler than protoplasm which remain an integral part of
the organism, viz., cellulose and starch. The cell-wall is
probably formed by the conversion of a thin superficial
layer of protoplasm into cellulose, the cyst attaining its final
thickness by frequent repetition of the process. The
starch of the pyrenoids is apparently formed by a similar
process of decomposition or destructive metabolism of pro-
toplasm.

We see then that destructive metabolism may result in
the formation of (a) waste products and (b) plastic products,
the former being got rid of as of no further use, while the
latter remain an integral part of the organism

Let us now turn <mru more to the movements of
Sphserella, and consider in some detail the manner of their
performance.

250 SPH.ERELLA CHAP.

Each flagellum (Fig. 68, K,fl] is a thread of protoplasm
of uniform diameter except at its distal or free end, where
it tapers to a point. The lashing movements are brought
about by the flagellum bending successively in different
directions (F). Thus the ciliary movement of Sphaerella,
like the amoeboid movement of Amoeba, is a phenomenon
of contractility. Imagine an Amoeba to draw in all its
pseudopods but two, and to protrude these two until they
became mere threads ; imagine further these threads to
contract rapidly and more or less regularly instead of slowly
and irregularly ; the result would be the substitution of
pseudopods by flagella, i.e., of temporary slow-moving pro-
toplasmic processes by permanent rapidly-moving ones.

To put the matter in another way : in Amoeba the func-
tion of contractility is performed by the whole organism ;
in Sphserella it is discharged by a small part only, viz.,
the flagella, the rest of the protoplasm being incapable of
movement.

Sphaerella multiplies after becoming quiescent in the
encysted condition (Fig. 68, c, D) ; as in the active Amoeba
(p. 236) its protoplasm undergoes simple or binary fission,
but with the peculiarity that the process is immediately
repeated, so that four daughter-cells are produced within the
single mother-cell-wall. By the rupture of the latter the
daughter-cells are set free as the ordinary motile form,
acquiring their fkgella and detached cell- wall before making
their escape (D).

Under certain circumstances the resting form divides into
numerous smaller daughter-cells, and these when liberated
are found to have no cell-wall. Sphaerella therefore occurs,
in the motile condition, under two distinct forms, i.e., is
dimorphic : the larger or ordinary form with detached cell-
wall is called a mcgazpoid, the smaller form without a cell-
wall a microzooid. The microzooids, which are all similar

ii EUGLENA 251

to one another, are liberated as gametes, and conjugate in
pairs to form zygotes (pp. 197, 237), each of which becomes
surrounded with a cell-wall, and after a time again under-
goes multiplication.

We will now examine another small organism which is
often found in puddles and pools, frequently in such vast
numbers as to give the water a green colour. This organism
is known as Euglena viridis.

Euglena is also microscopic, its length varying from
TTJ- mm. to \ mm. The body is spindle-shaped, wide in the
middle and narrow at both ends (Fig. 69, A — E) : one
extremity is blunter than the other, and from it proceeds
a single long flagellum (fl] by the action of which the
organism swims with great rapidity, the flagellum being,
as in Sphaerella, directed forwards. Besides its rapid
swimming movements, Euglena frequently performs slow
movements of contraction and expansion, something like
those of a short worm, the body becoming broadened out
first at the anterior end, then in the middle, then at the
posterior end, twisting to the right and left, and so on (A — D).
These movements are so characteristic of the genus that the
name euglenoid is applied to them.

The organism consists of protoplasm covered with a very
delicate membrane or cuticle which is often finely striated,
and is to be looked upon as a superficial hardening of the
protoplasm. The colour is due to the presence of chloro-
phyll, which tinges all the central part, the two ends
being colourless. It is difficult to make out whether the
chlorophyll is lodged in one chromatophore or in several.

In Sphrerella we saw that chlorophyll was associated
with starch (p. 243). In Euglena there are, near the
middle of the body, a number of grains of paramylum
(H, /), a carbohydrate of the same composition as starch

252

EUGLENA

(QH10O5), but differing from it in remaining uncoloured
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in the water is decomposed in the presence of chlorophyll.

ii NUTRITION 253

its oxygen evolved, and its carbon combined with the
elements of water and used in nutrition. For a long time
Euglena was thought to be nourished entirely in this way,
but there is reason for thinking that this is not the case.

When the anterior end of a Euglena is very highly
magnified, it is found to have the form shown in Fig. 69, F.
It is produced into a blunt snout-like extremity, at the base
of which is a conical depression (ess) leading into the soft
internal protoplasm : — just the sort of depression one could
make in a clay model of Euglena by thrusting one's finger
or the end of a pencil into the clay. From the bottom of
this tube the flagellum arises, and by its continual move-
ment gives rise to a sort of whirlpool in the neighbourhood.
By the current thus produced, minute solid food-particles
are swept down the tube and forced into the soft internal
protoplasm, where they doubtless become digested in the
same way as the substances ingested by an Amoeba. That
solid particles are so ingested by Euglena has been proved
by diffusing finely powdered carmine in the water, when
the coloured particles were seen to be swallowed in the way
described.

The depression in question serves therefore as a gullet, and
its external aperture or margin (m) as a mouth. Euglena,
like Amoeba, takes in solid food, but instead of ingesting
it at almost any part of the body, it can do so only at one
particular point where there is a special ingestive aperture
or mouth. This is clearly a case of specialisation or differ-
entiation of structure : in virtue of the possession of a mouth
and gullet, Euglena is more highly organised than Amoeba.

It thus appears that in Euglena nutrition is both holozoic .
and holophytic (p. 247) : very probably it is mainly holophytic
during daylight and holozoic in darkness.

Near the centre or somewhat towards the posterior end is

-254 EUGLENA CHAP.

a nucleus (E, mi) with a well-marked nucleolus, and at the
anterior end is a clear space (c. vac), looking very like a
contractile vacuole. It has been shown, however, that this
space is in reality a non-contractile cavity or reservoir (H, r)
into which the true contractile vacuole (c. vac) opens, and
which itself discharges into the gullet.

In close relation with the reservoir is found a little bright
red speck (pg\ called the pigment spot or stigma. It con-
sists of haematochrome (p. 243), and is curiously like an
eye in appearance, so much so that it is often known as the
eye-spot. Recent experiments seem to show that it is
specially sensitive to light, and is therefore a true eye in the
sense of a light-perceiving organ, although having no actual
visual function.

As in Sphserella a resting condition alternates with the
motile phase : the organism loses its flagellum and surrounds
itself with a cyst of cellulose (G, cy, see p. 244), from
which, after a period of rest, it emerges to resume active life.

Reproduction takes place by simple binary fission of the
resting form, the plane of division being always longitudinal
(G). Sometimes each product of division or daughter-cell
divides again : finally the two, or four, or sometimes even
eight daughter-cells emerge from the cyst as active Euglense.
A process of multiple fission (p. 237) has also been de-
scribed, numerous, simple, minute, active bodies or spores
being produced, which gradually assume the ordinary form
and size.

We have seen that typical animal-cells, such as those of
the frog (Part I, Chap. VII) are not provided with a
cellulose cell-wall and do not contain chlorophyll. It is
characteristic, on the other hand, of most plant-cells — which

II ANIMALS AND PLANTS 255

also consist of nucleated protoplasm — that they are sur-
rounded with a cellulose cell-wall, and that, in the case of
green plants, they contain chlorophyll. Speaking generally,
the nutrition of animals is holozoic, and that of green plants
holophytic ; and in correspondence with this difference in
the character of the food, most animals have an ingestive
aperture or mouth for taking in the solid food, and some
kind of digestive cavity, either permanent (stomach), or
temporary (food-vacuole) ; they also have, as a rule, some
kind of excretory apparatus. Moreover, animals are usually
capable of automatic movement, while in most plants the
organism, as a whole, exhibits no automatism, but only the
slow movements of growth.

Let us now apply these definitions to the simple forms
described above and see how far they will help us in placing
those organisms in one or other of the two " kingdoms "
(p. 220) into which living things are divided.

Amoeba has a cell-wall, probably nitrogenous, in the rest-
ing condition : it ingests solid proteids, its nutrition being
therefore holozoic : it has a contractile vacuole : and it per-
orms amoeboid movements. It may therefore be safely
considered as an animal.

Sphaerella has a cellulose wall that contains chlorophyll and
its nutrition is purely holophytic : a contractile vacuole is
present in S. lacustris but absent in S. pluvialis : and its
movements are ciliary.

Euglena has a cellulose wall in the encys-ted state : in
virtue of its chlorophyll it is nourished by the absorption of
carbon dioxide and mineral salts, but it can also ingest solid
food through a special mouth and gullet : it has a contractile
vacuole, and performs both euglenoid and ciliary movements.

In both these organisms we evidently have conflicting

256 MONADS CHAP.

characters : the cellulose wall and holophytic nutrition
would place them both among plants, while from the con-
tractile vacuole and active movements of both genera, and
from the holozoic nutrition of Euglena, we should group
them with animals. That the difficulty is by no means
easily overcome may be seen from the fact that both genera
are claimed at the present day by zoologists and by
botanists.

Another mode of nutrition occurs in certain organisms
which must now be referred to very briefly.

FIG. 70. — A Monad (Hcteromita rostrata), showing nucleus (nit), contractile
vacuole (c. vac}, and two flagella (y?1, jff2). From Parker's Biology, after
Dallinger.

When animal or vegetable matter is placed in water and
allowed to stand at the ordinary temperature, the well-known
process of decomposition or putrefaction (pp. n, 152, and
237) sooner or later sets in, the water becoming turbid and
acquiring a bad smell. A drop of it examined under the
microscope is then found to teem with very minute unicellular
organisms, some of which are known as Monads, much smaller
than Euglena, the form represented in Fig. 70 (Heteromita)
being only about T^y- mm. (^o\r <r incn) m length. Like
Sphserella, the Monad swims about by means of two flagella,
but it contains no chlorophyll. The putrefying infusion in
which it lives contains proteids in solution, in part split up

ii BACTERIA 257

by the process of decomposition into simpler compounds,
some of which are diffusible. As the Monad contains no
chlorophyll, its nutrition is evidently not holophytic, and,
apart from the fact that it possesses neither mouth nor
pseudopods, observation seems to show pretty conclusively
that it is not holozoic.

There remains only one way in which nutrition can take
place, namely, by absorption of the proteids and other
nutrient substances in the solution : the Monad may be
said to live immersed in an immense cauldron of broth which
it is for ever imbibing, not by its mouth, for it has none, but
by the whole surface of its body. This is the saprophytic
mode of nutrition, and resembles that which occurs in certain
plants which contain no chlorophyll — the fungi. It will be
seen that the main difference between saprophytic and
holozoic nutrition is that in the former digestion, i.e., the
process of rendering food-stuffs soluble and diffusible, takes
place outside the body, so that constructive metabolism can
begin at once.

In the Monad, the characters are again conflicting (com-
pare p. 255) : the probable absence of cellulose, the presence
of a contractile vacuole, and the flagella all have an
" animal " look, but the mode of nutrition is that of a fungus.

Decomposition, as already stated (p. 152), is due in the
first instance to the action of certain other minute organisms,
known as Bacteria, which appear in the earlier stages of the
putrefaction of an organic infusion. The nutrition of these
organisms, like that of Monads, is usually saprophytic and
the movements are generally ciliary ; but as they have a
definite cell-wall and no contractile vacuole, they are almost
universally classed amongst plants, while Monads are as
constantly included in the animal kingdom.

We see, then, that while it is quite easy to divide the

PKACT. ZOOL. S

258 ANIMALS AND PLANTS CHAP.

higher organisms into the two distinct groups of plants and
animals, any such separation is by no means easy in the
case of the lowest forms of life. It was in recognition of
this fact that Haeckel proposed, many years ago, to institute
a third " kingdom/' called Protista, to include all unicellular
organisms. Although open to many objections in practice,
there is a great deal to be said for the proposal. From the
strictly scientific point of view it is quite as justifiable to
make three subdivisions of living things as two : the line
between animals and plants is quite as arbitrary as that
between protists and plants or between protists and animals,
and no more so : the chief objection to the change is that it
doubles the difficulties by making two artificial boundaries
instead of one.

The important point for the student to recognise is
that these boundaries are artificial, and that there are no
scientific frontiers in Nature. As in the liquefaction of
gases there is a " critical point " at which the substance
under experiment is neither gaseous nor liquid : as in a
mountainous country it is impossible to say where mountain
ends and valley begins : as in the development of an animal
it is futile to argue about the exact period when, for instance,
the egg becomes a tadpole or the tadpole a frog : so in the
case under discussion. The distinction between the higher
plants and animals is perfectly sharp and obvious, but when
the two groups are traced downwards they are found
gradually to merge, as it were, into an assemblage of organ-
isms which partake of the characters of both kingdoms, and
cannot without a certain violence be either included in or
excluded from either. When any given "protist" has to
be classified the case must be decided on its individual
merits : the organism must be compared in detail with all
those which resemble it closely in structure, physiology, and

ii PRACTICAL DIRECTIONS 259

life-history : and then a balance must be struck and the
doubtful form placed in the kingdom with which it has, on
the whole, most points in common.

It will no doubt occur to the reader that, on the theory
of evolution (p. 222), we may account for the fact of the
animal and vegetable kingdoms being related to one another
like two trees united at the root, by the hypothesis that the
earliest organisms were protists, and that from them animals
and plants were evolved along divergent lines of descent.

PRACTICAL DIRECTIONS.

Sph.8Brella. Examine a drop of water containing Sphserelhe (see
note on p. 238), first with the low power, and then, after putting on
a cover-glass, with high power. Note —

1. Their founded form and green (or red) colour ; the thick cellulose
cell-wall ; the protoplasm, enclosing (a) chroviatophores containing
chlorophyll (the red colour is due to another colouring matter, hczmato-
chrome}, and (b] a central nucleus, seen better later ; and in the active
forms, the \.wflagella. Sketch.

2. Dissolve out the chlorophyll by adding alcohol ; the nucleus will
then be visible, and may be made more distinct by staining with
magenta, or a solution of potassic iodide in water, saturated with
iodine. After treatment with the iodine solution, a bluish coloration
will often be seen around the small starch-containing pyrenoids.

3. Treat some specimens either with strong iodine solution and then
with sulphuric acid (75 per cent.), or with Schulze's solution, which is
prepared as follows : — make about 25 c.c. of a syrupy solution of zinc
chloride, and let it stand for a short time ; then pour off the clear fluid,
saturate it with potassium iodide and add iodine until the solution has
about the colour of sherry. Note the blue coloration of the cell-wall.

4. In the living specimens note also the mode of division into
4 megazooids or into numerous microzooids, and observe their move-
ments when set free. Sketch.

Euglena. Examine a drop of water containing Euglena with the
low power, then cover, and put on the high power. Note —

I. The spindle-shaped form of the body, and its changes of form in
contraction and expansion.

S 2

260 PRACTICAL DIRECTIONS CHAI>. n

2. The long flagellum.

3. The superficial cuticle, and the mouth and conical depression
(guile f) at the anterior end.

4. The central part of the body, which contains chlorophyll, except
at the two ends. Near the middle is a nucleus enclosing a nucleohis,
and near the anterior end a non-contractile space into which a contrac-
tile vacuole opens, close to which a red pigment-spot is situated ; the
colour of this is due to haematochrome. Grains of paramylum may be
recognised near the centre of the body. Sketch before and after
staining as above.

5. Look for specimens in the resting condition, and observe if any
of them are undergoing division. Sketch.

Monads and Bacteria. Examine a drop of water containing some
putrefying animal or vegetable substance. An infusion of hay is con-
venient for this purpose : — pour boiling water on a handful of hay and
strain the fluid through blotting-paper ; shortly afterwards decom-
position will set in.

The first organisms to appear in such a putrefying infusion are Bacteria,
which are so minute as to appear as mere specks under the high power
— some of them being only -^ mm. (T?>57jji inch) in length, or even
less. Careful examination will show globular, rod-like, and spiral
forms, all capable of movement at one phase of their existence. Monads
will appear somewhat later, and may be recognised by their ovoidal
form and their flagella (Fig. 70), which, however, like those of
Bacteria, can only be seen under a very high power.

CHAPTER III

PARAMCECIUM : VORTICELLA AND ITS ALLIES — COLONIAL
ORGANISMS.

WE have now to consider certain organisms in which
differentiation has gone much further than in the uni-
cellular forms already considered : which have, in fact,
acquired many of the characteristics of the higher
animals and plants while remaining unicellular (com-
pare p. 238). The study of several of these more or
less highly differentiated though unicellular forms will
occupy the present chapter.

It was mentioned above that, in the earlier stages of the
putrefaction of an organic infusion, Bacteria only were found,
and that, later, Monads made their .appearance (p. 257).
Still later, organisms much larger than Monads are seen,
generally of an ovoidal form, moving about very quickly,
and seen by the use of a high power to be covered with in-
numerable fine cilia. These are called dilate Infusoria, in
contradistinction to Monads, which are often known as
flagellate Infusoria ; many kinds are common in putrefying
infusions, some occur in the intestines of the higher animals

262 PARAMCECIUM CHAP, in

while others are among the commonest inhabitants of both
fresh and salt water.

A very common ciliate infusor is the beautiful " slipper-
animalcule, " Paramcedum, which from its comparatively large
size and from the ease with which all essential points of its
organisation can be made out is a very convenient and
interesting object of study.

Compared with the majority of the organisms which have
come under our notice it may fairly be considered as gigantic,
being no less than 4 — \ mm. (200 — 260;*) in length : in
fact it is just visible to the naked eye as a minute whitish
speck.

Its form (Fig. 71, A) can be fairly well imitated by making
out of clay or stiff dough an elongated cylinder rounded at
one end and bluntly pointed at the other ; then giving the
broader end a slight twist ; and finally making on the side
rendered somewhat concave by the twist a wide, shallow
groove beginning at the broad end and gradually narrow-
ing to about the middle, where it ends in a tolerably deep
depression.

The groove is called the buccal groove (A & B, buc. gr) :
at the narrow end is a small aperture the mouth (mth\ which,
like the mouth of Euglena (Fig. 69), leads into the soft
internal protoplasm. The surface of the creature on
which the groove is placed is distinguished as the ventral
surface, the opposite surface being upper or dorsal ; the
broad end is anterior, the narrow end posterior, the
former being directed forwards as the animalcule swims.
These descriptive terms being decided upon, it will be
seen that the buccal groove begins on the left side,
and gradually curves over to the middle of the ventral
surface.

FIG. 71. — ParauiceciuJti caudatmn.

A, the living animal from the ventral aspect, showing the covering of cilia, the
buccal groove (to the right) ending posteriorly in the mouth (intti) and gullet
(gul) ', several food vacuoles (_/I vac), and the two contractile vacuoles (c. vac).

264 PARAMCECIUM CHAP.

B, the same in optical sections showing cuticle (cu), cortex (corf), and medulla (med)\
buccal groove (due. gr), mouth, and gullet (gul} ', numerous food vacuoles
(_/". vac) circulating in the direction indicated by the arrows, and containing
particles of indigo, which are finally ejected at an anal spot ; meganucleus (nu),
micronucleus (pa. nu), and trichocysts, some of which (trek) are shown with
their threads ejected. The scale to the right of this figure applies to A and B.

C, a specimen killed with osmic acid, showing the ejection of trichocyst-threads,
which project considerably beyond the cilia.

D, diagram of binary fission ; the micronucleus (pa. nu), has already divided, the
meganucleus (nu) is in the act of dividing. (From Parker's Biology: D, after
Lankester.)

As the creature swims its form is seen to be permanent,
exhibiting no contractions of either an amoeboid or a
euglenoid nature. It is however distinctly flexible, often
being bent in one or other direction when passing between
obstacles, such as entangled masses of weed. This perma-
nence of contour is due to the presence of a tolerably firm
though delicate cuticle (B, cu) which invests the whole surface.

The protoplasm thus enclosed by the cuticle is distinctly
divisible into two portions — an external somewhat dense
layer, the cortical layer or cortex (cort\ and an internal more
fluid material, the medullary substance or medulla (med). It
will be remembered that a somewhat similar distinction of
the protoplasm into two layers is exhibited by Amoeba
(p. 231), the ectoplasm being distinguished from the endo-
plasm simply by the absence of granules. In Paramcecium
the distinction is a far more fundamental one : the cortex is
radially striated and is comparatively firm and dense, while
the medulla is granular and semi-fluid, as may be seen from
the fact that food-particles (f. vac, and see p. 266) move
freely in it, whereas they never pass into the cortex. The
medulla has a reticular structure similar to that of the
protoplasm of the ordinary animal-cell, consisting of a
delicate granular network the meshes of which are filled
with a transparent material. In the cortex the meshes of
the network are closer, and so form a comparatively dense
substance. The cortex also exhibits a superficial oblique
striation, forming what is called the myophan layer.

in STRUCTURE 265

The mouth (mtti) leads into a short funnel-like tube, the
gullet (gut), which is lined by cuticle and passes through the
cortex to end in the soft medulla, thus making a free com-
munication between the latter and the external water.

The cilia with which the body is covered are of approxi-
mately equal size, quite short in relation to the entire animal,
and arranged in longitudinal rows over the whole outer sur-
face. They consist of prolongations of the cortex, and each
passes through a minute perforation in the cuticle. They
are in constant rhythmical movement, like the cilia on the
epithelial cells of the frog's mouth (p. 109), and are thereby
distinguished from the flagella of Sphaerella, Euglena, &c.,
which exhibit more or less intermittent lashing move-
ments.

Near the middle of the body, on the inner boundary of
the cortex, is a large oval nucleus (B, c, nu\ and against
one side of it, in P. caudatum, is a small oval structure (pa.
nu) which is also deeply stained by, e.g., magenta or
carmine. This is the micronucleus : it is to be considered
as a second, smaller nucleus, the larger body being dis-
tinguished as the meganucleus. In the closely allied
P. aurelia, there are two micronuclei.

There are two contractile vacuoles (c. vac) in relation with
the cortex, one situated at about a third of the entire length
from the anterior .end of the body, the other at about the
same distance from the posterior end.

The action of the contractile vacuoles is very beautifully
seen in a Paramcecium at rest : it is particularly striking in a
specimen subjected to slight pressure under a cover glass,
but is perfectly visible in one which has merely temporarily
suspended its active, swimming movements. It is then seen
that during the diastole, or phase of expansion of each
vacuole, a number- — about six to ten — of delicate, radiating,

266 PARAMCECIUM CHAP.

spindle-shaped spaces filled with fluid appear round it, like
the rays of a star (upper vacuole in A & B) : the vacuole
itself contracts or performs its systole, completely disappearing
from view, and immediately afterwards the radiating canals
flow together and refill it, becoming themselves emptied and
therefore invisible for an instant (lower vacuole in A & B)
but rapidly appearing once more. There seems to be no
doubt that the water taken in with the food is collected
into these canals, emptied into the vacuole, and finally dis-
charged to the exterior.

The process of feeding can be very conveniently studied
in Paramcecium by placing in the water some finely-divided
carmine or indigo. When the creature comes into the
neighbourhood of the coloured particles, the latter are swept
about in various directions by the action of the cilia : some
of them, however, are certain to be swept into the neigh-
bourhood of the buccal groove and gullet, the cilia of
which all work downwards, i.e., towards the inner end of the
gullet. The grains of carmine are thus carried into the gullet,
where for an instant they lie surrounded by the water of
which it is full : then, instantaneously, probably by the con-
traction of the tube itself, the animalcule performs a sort of
gulp, and the grains with an enveloping globule of water or
food-vacuole are forced into the medullary protoplasm.
This process is repeated again and again, so that in any '
well-nourished Paramcecium there are to be seen numerous
globular spaces filled with water and containing particles of
food — or in the present instance of carmine or indigo. At
every gulp the newly-formed food-vacuole pushes, as it were,
its predecessor before it : contraction of the medullary pro-
toplasm also takes place in a definite direction, and thus a
circulation of food-vacuoles is produced, as indicated in
Fig, 71, B, by arrows.

in NUTRITION 267

After circulating in this way for some time the water of
the food-vacuoles is gradually absorbed, being ultimately
excreted by the contractile vacuoles, so that the contained
particles come to lie in the medulla itself (refer to figure).
The circulation still continues, until finally the particles are
brought to a spot situated about half way between the mouth
and the posterior end of the body : here if carefully watched
they are seen to approach the surface and then to be suddenly
ejected. The spot in question is therefore to be looked upon
as a potential anus, or aperture for the egestion of faeces or
undigested food-matters. It is a potential and not an actual
anus, because it is not a true aperture but only a soft place
in the cortex through which, by the contractions of the
medulla, solid particles are easily forced.

Of course when Paramcecium ingests, as it usually does,
not carmine but minute living organisms, the latter are
digested as they circulate through the medullary protoplasm,
and only the non-nutritious parts cast out at the anal spot.
It has been found by experiment that this infusor can digest
not only proteids but also starch and perhaps fats. The
nutrition of Paramcecium is therefore characteristically
holozoic.

It was mentioned above that the cortex is radially striated
in optical section. Careful examination with a very high
power shows that this appearance is due to the presence in
it of minute spindle-shaped bodies (A and B) closely arranged
in a single layer and perpendicular to the surface. These
are called trichocysts.

When a Paramcecium is killed, either by the addition of
some poisonous reagent or by simple pressure of the cover
glass, it frequently assumes a remarkable appearance. Long
delicate threads suddenly appear, projecting from its surface
in all directions (c) and looking very much as if the cilia had

268 PARAMCECIUM CHAP.

suddenly protruded to many times their original length. But
these filaments have really nothing to do with the cilia ; they
are contained under ordinary circumstances in the trichocysts,
probably coiled up ; and by the contraction of the cortex
consequent upon any sudden irritation they are projected in
the way indicated. In Fig. 71, B, a few trichocysts (trcK) are
^shown in the exploded condition, i.e., with the threads pro-
truded. Most likely these bodies are weapons of offence like
the very similar structures (nematocysts) found in polypes
(see Chapter V., Fig. 77).

Paramcecium multiplies by simple fission, the division of
the body being always preceded by the elongation and
subsequent division of the mega- and micronucleus
(Fig. 71, D).

Conjugation (pp. 237, 251) also occurs, usually after multi-
plication by fission has gone on for some time. Two Para-
mcecia come into contact by their ventral faces, and in each
of these conjugating individuals or gametes the meganucleus
degenerates, and the micronucleus undergoes a somewhat,
complicated series of changes, the essential part of the pro-
cess being the fusion of two products of the division of the
micronuclei, one from each gamete, each of which then con-
tains a single nuclear body, the conjugation-nucleus, formed
by the union of nuclear matter derived from two distinct
individuals, and therefore comparable to the nucleus of the
oosperm in the higher animals (p. 197) : the other products
of division of the micro-nucleus disappear, and a new mega-
and micro-nucleus arise from the conjugation-nucleus. In
this case, however, the two entire gametes do not unite
into one, but separate after the process is complete and begin
once more to lead an independent existence, when ordinary
transverse fission again takes place,

in VORTICELLA 269

It will be noticed that, in the present instance, conjugation
is not a process of multiplication : it has been ascertained
that during the time two infusors are conjugating each
might have produced a very large number of offspring by
continuing to undergo fission at the usual rate. The im-
portance of the process lies in the exchange of nuclear
material between the two conjugating individuals.

The next organism we have to consider is a ciliate infusor
even commoner than that just described. It is hardly
possible to examine the water of a pond with any care with-
out finding in it, sometimes attached to weeds, some-
times to the legs of water-fleas, sometimes to the sticks and
stones of the bottom, numbers of exquisitely beautiful little
creatures, each like an inverted bell with a very long handle,
or a wine-glass with a very long stem. These are the
well-known " bell-animalcules," the commonest among them
belonging to various species of the genus Vorticella.

The first thing that strikes one about Vorticella (Fig. 72, A)
is the fact that it is permanently fixed, like a plant, the
proximal or near end of the stalk being always firmly fixed
to some aquatic object, while to the distal or far end the
body proper of the animalcule is attached.

But in spite of its peculiar form it presents certain very
obvious points of resemblance to Paramcecium. The
protoplasm is divided into cortex (c, corf] and medulla
(med\ and is invested with a delicate cuticle (cu). There is
a single contractile vacuole (c. vac) the movements of which
are very readily made out owing to the ease with which the
attached organism is kept under observation. There is a
meganucleus (nu) remarkable for its elongated band-like
form, and having in its neighbourhood a minute micro-
nucleus. Cilia are also present, but the way in which

FIG. 72. — Vorticella.

A, living specimen fully expanded, showing stalk (sf) with axial fibre (ax.f),
peristome (per), disc (d), mouth (tntJi), gullet (gull\ and contractile vacuole
(c. vac).

B, the same, bent on its stalk and with the disc turned away from the observer.

C, optical section of the same, showing cuticle (cu\ cortex (cort ), medulla (med),
nucleus (nu), gullet (grill), several food-vacuoles, and anal spot (an), as well as
the other structures shown in A.

D1, a half-retracted and D2 a fully-retracted specimen, showing the coiling of the
stalk and overlapping of the disc by the peristome.

E1, commencement of binary fission ; E2, completion of the process ; E3, the
product of division swimming freely in the direction indicated by the arrow.

F1, a specimen dividing into a megazooid and several microzooids (;«) ; F2,
division into one mega- and one micro-zooid.

G1, G2, two stages in conjugation showing the gradual absorption of the micro-
gamete (;«) into the megagamete.

H1, multiple fission of encysted form, the nucleus dividing into numerous masses ;
H2, spore formed by multiple fission ; H«* — H?, development of the spore — H4 is
undergoing binary fission. (From Parker's Biology : E— H after Saville Kent.)

CHAP, in VORTICELLA 271

they are disposed is very peculiar and characteristic. To
understand it we must study the form of the body a little
more closely.

The conical body is attached by its apex or proximal
end to the stalk : its base or distal end is expanded
so as to form a thickened rim, the peristome (per), within
which is a plate-like body elevated on one side, called
the disc (^)/-<and looking like the partly raised lid of a
chalice. Between the raised side of the disc and the peri-
stome is a depression, the mouth (mth\ leading into a
conical gullet (gull}.

There is reason for thinking that the whole proximal region of
Vorticella answers to the ventral surface of Paramoecium, and its
distal surface with the peristome and disc to the dorsal surface of the
free-swimming genus : the mouth is to the left in both.

A single row of cilia is disposed round the inner border
of. the peristome and continued on the one hand down the
gullet, and on the other round the elevated portion of the
disc; the whole row of cilia thus takes a spiral direction.
The rest of the body is completely bare of cilia.

The movements of the cilia produce a very curious optical
illusion : as one watches a fully-expanded specimen it is
hardly possible to believe that the peristome and disc are
not actually revolving — a state of things which would imply
that they were discontinuous from the rest of the body. As
a matter of fact the appearance is due to the successive
contraction of all the cilia in the same direction, and is
analogous to that produced by a strong wind on a field of
corn or long grass. The bending down of successive
blades of grass produces a series of waves travelling across
the field in the direction of the wind. If instead of a field
we had(a large circle of grass, and if this were acted upon

272 VORTICELLA CHAP.

by a cyclone, the wave would travel round the circle, which
would then appear to revolve.

Naturally the movement of the circlet of cilia produces a
small whirlpool in the neighbourhood of the Vorticella, as
can be seen by introducing finely-powered carmine into the
water. It is through the agency of this whirlpool that food
particles are swept into the mouth, surrounded, as in
Paramcecium, by a globule of water : the food-vacuoles
(c) thus constituted circulate in the medullary protoplasm
and the non-nutritive parts are finally egested at an anal
spot (an) situated near the base of the gullet.

The stalk (A, sf) consists of a very delicate, transparent,
outer substance, which is continuous with the cuticle of the
body and contains a delicate axial fibre (ax.f) running along
it from end to end in a somewhat spiral direction. This
fibre is a prolongation of the cortex of the body (c) : under
a very high power it appears granular or delicately striated,
the striae being continued into the cortex of the proximal
part of the body.

A striking characteristic of Vorticella is its extreme
irritability, i.e., the readiness with which it responds to any
external stimulus. The slightest jar of the microscope, the
contact of some other organism, or even a current of water
produced by some free-swimming form like Paramcecium, is
felt directly by the bell-animalcule, and is followed by an
instantaneous change in the relative position of its parts.
The stalk becomes coiled into a close spiral so as to have
a mere fraction of its original length, and the body from
being bell-shaped becomes globular, the disc being with-
drawn and the peristome closed over it (o1, D2).

The coiling of the stalk leads us to the consideration of
the particular form of contractility called muscular, which is
met with in multicellular animals, e.g., the frog (p. 60). It

in AXIAL FIBRE 273

was mentioned above that while the stalk in its fully
expanded condition is straight, the axial fibre is not straight,
but forms a very open spiral, /.£., it does not lie in the centre
of the stalk, but at any transverse section is nearer the
surface at one spot than elsewhere, and this point as we
ascend the stalk is directed successively to all points of the
compass.

Now suppose that the axial fibre undergoes a sudden
contraction, that is to say, a decrease in length accompanied
by an increase in diameter, since as we have already
seen there is no decrease in volume in protoplasmic
contraction. There will naturally follow a corresponding
shortening of the elastic cuticular substance which forms the
outer layer of the stalk. If the axial fibre were entirely
towards one side of the stalk, the result of the contraction
would be a flexure of the stalk towards that side, but, as its
direction is spiral, the stalk is bent successively in every
direction, that is, is thrown into a close, spiral coil.

The axial fibre is therefore a portion of the protoplasm
which possesses the property of contractility in a special
degree ; in which, moreover, contraction takes place in a
definite direction — the direction of the length of the fibre —
so that its inevitable result is to shorten the fibre and con-
sequently to bring its two ends nearer together. This is the
essential characteristic of a muscular contraction, and the
axial fibre in the stalk of Vorticella is therefore to be looked
upon as the first instance of a clearly differentiated muscle
which has come under our notice amongst unicellular
animals.

There are some interesting features in the reproduction of
Vorticella. It multiplies by binary fission, dividing through
the long axis of the body (Fig. 72, E1, E2). Hence it is
generally said that fission is longitudinal, not transverse, as

PRACT. ZOOL. T

274 VORTICELLA CHAP.

in Paramcecium. But on the theory (p. 271) that the peri-
stome and disc are dorsal and the attached end ventral,
fission is really transverse in this case also.

It will be seen from the figures that the process takes place
by a cleft appearing at the distal end (E1) and gradually
deepening until there are produced two complete and full-
sized individuals upon a single stalk (E2). This state of
things does not last long : one of the two daughter-cells takes
on a nearly cylindrical form, keeps its disc and peristome
retracted, and acquires a new circlet of cilia near its proximal
end (ES) : it then detaches itself from the stalk, which it
leaves in the sole possession of its sister-cell, and swims about
freely for a time. Sooner or later it settles down,, becomes
attached by its proximal end, loses its basal circlet of cilia,
and develops a stalk, which ultimately attains the normal
length.

The object of this arrangement is obvious. If, when a
Vorticella divided, the plane of fission extended down the
stalk until two ordinary fixed forms were produced side by
side, the constant repetition of the process would so increase
the numbers of the species in a given spot that the food-
supply would inevitably run short. This is prevented by
one of the two sister-cells produced by fission leading a free
existence long enough to enable it to emigrate and settle in
a new locality, where the competition with its fellows will be
less keen. The production of these free-swimming zooids
is therefore a means of dispersal : contrivances having
this object in view are a very general characteristic of fixed
organisms.

Conjugation occasionally takes place, and presents certain
peculiarities. A Vorticella divides either into two unequal
halves (r2) or into two equal halves, one of which divides
again into from two to eight daughter-cells (r1). There are

in CONJUGATION 275

thus produced from one to eight microzooids which resemble
the barrel-shaped form (E3) in all but size, and like it become
detached and swim freely by means of a basal circlet of cilia.
After swimming about for a time, one of these microzooids
comes in contact with an ordinary form or megazooid, when
it attaches itself to it near the proximal end (c1), and under-
goes gradual absorption (c2), the mega- and micro-zooids
becoming completely and permanently fused to form a
zygote (y. 251).

Notice that in this case the conjugating bodies or gametes
are not of equal size and similar character; but one, which
is conveniently distinguished as the microgamete ( •= micro-
zooid) is relatively small and active, while the other or
megagamete ( = megazooid, or ordinary individual) is rela-
tively large and passive. As we have seen in the case of
the frog (pp. 195 and 196), this differentiation of the
gametes is precisely what we get in the higher animals,
arid, in fact, in almost all organisms with two sexes : the
microgamete being the male, the megagamete the female
conjugating body (p. 197).

The result of conjugation is somewhat different in the two
cases already studied : in Paramcecium no zygote is formed,
conjugation being a mere temporary union (p. 268) : in
Vorticella the zygote is an actively moving and feeding
body, indistinguishable from an ordinary individual of the
species.

Vorticella sometimes encysts itself (Fig. 72, H1), and the
nucleus of the encysted cell has been observed to break up
into a number of separate masses, each doubtless surrounded
by a layer of protoplasm. After a time the cyst bursts, and
a number of small bodies or spores (H2) emerge from it, each
containing one of the products of division of the nucleus.
These acquire a circlet of cilia (n3), by means of which they

T 2

276 VORTICELLA CHAP.

swim freely, and they are sometimes found to multiply by
simple fission (n4). Finally, they settle down (i*5) by the
end at which the cilia are situated, the attached end begins
to elongate into a stalk (HG), this increases in length, the
basal circlet of cilia is lost, and a ciliated peristome and
disc are formed at the free end (HT). In this way the
ordinary form is assumed by a process of progressive
differentiation or development (p. 9), and, moreover, the
free-swimming young (HS), to which the spores formed by
division of the encysted protoplasm give rise, differ strikingly
in form and habits from the adult. This is expressed by
saying that development is in this case accompanied by a
metamorphosis, this word literally meaning simply a change,
being always used in zoology to express a striking and
fundamental difference in form and habit between the young
and the adult ; as, for instance, between the tadpole and
the frog, or between the caterpillar and the butterfly. It is
obvious that in the present instance metamorphosis is
another means of ensuring dispersal.

In Vorticella, as we have seen, fission results not in the
production of equal and similar daughter-cells, but of one
stalked and one free-swimming form. It is, however, quite
possible to conceive of a Vorticella-like organism in which
the parent cell divides into two equal and similar products,
each retaining its connection with the stalk. If this process
were repeated again and again, and if, further, the plane of
fission were extended downwards so as to include the distal
end of the stalk, the result would be a branched, tree-like
stem with a Vorticella-like body at the end of every
branch.

As a matter of fact, this process takes place, not in Vorti-
cella itself, but in some nearly allied infusors, such as
Carchesium and Epistylis. Each of these forms consists of a

in COLONIES 277

main stem attached by its proximal end and giving off at its
distal end several branches, on each of which numerous
bell-animalcules with short stalks are borne, like foxgloves or
Canterbury-bells on their stem.

We see, then, that Carchesium and Epistylis differ from all
our previous types in being compound organisms. The entire
" tree " is called a colony or stock, and each separate bell-
animalcule borne thereon is an individual or zooid,
morphologically equivalent to a single Vorticella or
Paramcecium. The colony is therefore an individual of a
higher grade than the zooid, and such a multicellular animal
as a frog is an individual of a higher grade still.

As in Vorticella, the stem of Carchesium consists of a
cuticular sheath with an axial muscle-fibre which, at the distal
end of the main stem, branches like the stem itself, a
prolongation of it being traceable to each zooid ; so that
the muscular system is common to the whole colony,
and any shock causes a general contraction of all the
zooids. The stalk of Epistylis, on the other hand, is non-
contractile.

PRACTICAL DIRECTIONS.

ParamCBCillHl. — Spread a little cotton-wool on a slide over a drop
of water containing Paramcecia (see note on p. 238), in order to entangle
them in its meshes, and put on a cover-glass. Examine first with the
low power and then with the high power. Note —

1. The elongated form of the animal ; its anterior (more rounded)
and posterior (more pointed) end ; its flattened dorsal and ventral
surfaces ; and its month, near the middle of the ventral surface.

2. The active movements, due to the cilia covering the body.

3. The marked distinction between cortex and medulla.

4. The characters of the elastic cortex : — (a} the superficial cuticle,
and deeper striated layer ; (b] the cilia arising from the deeper layer,
and projecting through the cuticle ; (c] the trichocysts — small oval sacs,
imbedded in the deeper layer ; (d) the two spherical contractile vacuoles.

278 PRACTICAL DIRECTIONS CHAP.

in close relation with the deeper layer on the dorsal side : note that
canals radiate from them when they contract ; (e) the cilia lining the
buccal groove. (The potential amis behind the mouth can only be
seen at the moment of defecation. ) Compare the mode of feeding with
that of Amoeba.

5. The characters of the medulla : — (a] The food-vactwles and their
circulation ; (b] the meganucleus and micro-nucleus, which can be better
seen when stained. Sketch.

6. Add methyl-green or magenta. Then note again the structure of
cortex and medulla, as well as — (a) the oval meganucleus, near the
middle of the body ; (b] the micronucleus, a smaller body, close to the
meganucleus ; (c) the extruded trichocysts. Sketch.

7. Look out for specimens undergoing transverse fission, and also for
others in process of conjugation. Sketch.

Permanent preparations may be made as directed in the case of
Amceba (p. 239).

Voiticella. — Mount some specimens in a drop of water, and examine
with the low power. The Vorticella will be seen to have the form
of a wine-glass or bell with a long stem. The stem frequently contracts
spirally, the edge of the bell being at the same time turned in, so as to
give the animal a rounded form. Put on the high power and note —

1. The form of the bell, its thickened rim or peristome, and the disc,
which forms a cover to the bell.

2. The mouth and gullet, opening on one side between the peristome
and disc, which is here slightly raised. (The anal spot opens into the
oral depression, but can only be seen at the moment of defsecation. )

3. The single row of cilia round the peristome and extending down
the gullet on the one hand, and on to the raised portion of the disc on
the other. Run in a little finely powdered indigo or carmine under the
cover-glass, and note the currents produced by the cilia : the granules
of pigment will be carried down the gullet.

4. The contractions of the bell and stalk.

5. The structure of the cortex, which is similar to that of Para-
mcecium, except that the cilia have a restricted distribution, and that
there are no trichocysts. Moreover, in the stalk (into which the
medulla does not extend) the deeper layer of the cortex gives rise to a
central contractile axial fibre, by means of which the stalk can contract
spirally. The contractile vacuole is single.

6. The medulla, which circulates and contains food-vactioles, as in

in PRACTICAL DIRECTIONS 279

Paramoecium. In it are situated an elongated and curved meganudeiis
and a minute micromtdeus (not easy to distinguish in the resting
condition). Sketch.

7. Make preparations as directed under Paramceciunl. Sketch.

8. Look out for specimens undergoing fission, noting the different
stages, and the second, aboral ring of cilia on one of the daughter
individuals, which eventually becomes detached. Note these free-
six) imming forms, and also search for conjugating individuals — a small
free Vorticella uniting with a large stationary one. Sketch.

Carchesium Or Epistylis. — You will very likely find specimens of
one of these, or of an allied genus, amongst the Vorticellse. Note
that several individual zooids are borne upon a branched stalk, together
forming a colony.

CHAPTER IV

OPALINA I MONOCYSTIS PARASITES — BIOGENESIS AND ABIO-

GENESIS — CLASSIFICATION OF THE UNICELLULAR
ORGANISM EXAMINED.

AMONGST the Protozoa (see p. 220) are found certain forms
which are parasites. Parasites are organisms which live in
association with other organisms, the ready-digested food of
which they utilise or even nourish themselves from the
tissues of the forms they infest. It will be interesting to
compare Paramcecium with a ciliate infusor which lives
in the intestine of the common frog, and is known as
Opalina ranarum.

' Opalina has a flattened body with an oval outline (Fig. 73,
A, B), and full-sized specimens may be as much as i mm.
in length. The protoplasm is divided into cortex and
medulla, and is covered with a cuticle; the cilia are equal-
sized and uniformly arranged in longitudinal rows over the
whole surface.

On a first examination no nucleus is apparent, but after
staining, a large number of globular nuclei can be seen (B, nu) :
these nuclei multiply within the body of the infusor.

The presence of numerous nuclei in Opalina is a fact
worthy of special notice, The unicellular organisms we have

CHAP. IV

OPALINA

281

studied are uninucleate as well as unicellular (compare,
however, Fig. 67, B) : the higher animals consist of numerous
cells each with a nucleus : Opalina, on the other hand, is

FIG. 73. — Opalina ranarum.

A, living specimen, surface view, showing longitudinal rows of cilia.

B, the same, stained, showing numerous nuclei (nu) in various stages of division.

C, i—6, stages in the division of a nucleus.

D, longitudinal fission.

E, transverse fission.

F, the same in a specimen reduced in size by repeated division.

G, final product of successive divisions.
H, encysted form.

I, uninucleate form produced from cyst.

K, the same after multiplication of the nucleus has begun. (From Parker's Biology :
A— C, after Pfitzner ; D— K, from Saville Kent, after Zeller.)

multinucleate but its protoplasm is undivided, so that it
presents a condition of things intermediate between the uni-
cellular and the multicellular types of structure (see p. 1 13).

282 OPALINA . CHAP.

There is no contractile vacuole, and no trace of either
mouth or gullet, so that the ingestion of solid food is impos-
sible. The creature lives, as already stated, in the intestine
of the frog : it is therefore, like the worms you have probably
noticed in the frog's urinary bladder and lungs (pp. 33 and
153), an internal parasite , or endoparasite^ having the frog
as its host. The intestine contains the partially-digested
food of the frog, and it is by the absorption of this that the
Opalina is nourished. Having no mouth it feeds solely by
imbibition : whether it performs any kind of digestive pro-
cess itself is not certainly known, but the analogy of other
mouthless parasites leads us to expect that it simply absorbs
food ready digested by its host, upon which it is dependent
for a constant supply of soluble and diffusible nutriment.

Thus Opalina, in virtue of its parastic mode of life, is
saved the performance of certain work — the work of digestion,
that work being done for it by its host. This is the essence
of internal parasitism : an organism exchanges a free life,
burdened with the necessity of finding food for itself, for
existence in the interior of another organism, on which, in
one way or another, it levies blackmail.

Note the close analogy between the nutrition of an internal
parasite like Opalina and the saprophytic nutrition of a
monad (p. 257). In both, the organism absorbs proteids
rendered soluble and diffusible, in the one case by the
digestive juices of the host, in the other by the action of
putrefactive Bacteria.

The reproduction of Opalina presents certain points of
interest largely connected with its peculiar mode of life. It
is obvious that if the Opalinae simply went on multiplying,
by fission or otherwise, in the frog's intestine, the population
would soon outgrow the means of subsistence : moreover,

iv REPRODUCTION 283

when the frog died there would be an end of the parasites.
What is wanted in this, as in other internal parasites, is some
mode of multiplication which shall serve as a means of dis-
persal (compare p. 274), or in other words, enable the
progeny of the parasite to find their way into the bodies of
other hosts, and so start new colonies instead of remaining
to improverish the mother-country.

Opalina multiplies by a somewhat peculiar process of
binary fission : an animalcule divides in an oblique direction
(Fig. 73 D), and then each half, instead of growing to the
size of the parent cell, divides again transversely (E). The
process is repeated again and again (F), the plane of division
being alternately oblique and transverse, until finally small
bodies are produced (G), about -^ — -£$ mm. in length, and
containing from two to four nuclei.

If the parent cell had divided simultaneously into a num-
ber of these little bodies the process would have been one of
multiple fission (p. 254) : as it is, it forms an interesting link
between simple and multiple fission.

Opalina ranarum multiplies in this way in the spring — i.e.,
during the frog's breeding season. Each of the small pro-
ducts of division (G) becomes encysted (H), and in this
passive condition is passed out with the frog's excrement,
probably falling on to a water-weed or other aquatic object.
Nothing further takes place unless the cyst is swallowed by
a tadpole, as must frequently happen when these creatures,
produced in immense numbers from the frogs' eggs, browse
upon the water- weeds which form their chief food.

Taken into the tadpole's intestine, the cyst is burst or
dissolved, and its contents emerge as a lanceolate mass of
protoplasm (i), containing a single nucleus and covered with
cilia. Conjugation occurs, and as the young Opalina absorbs
the digested food in the intestine of its host, it grows, and

284 SPOROZOA CHAP.

at the same time its nucleus divides repeatedly (K) in the
way already described, until by the time the animalcule has
attained the maximum size it has also acquired the large
number of nuclei characteristic of the genus.

Here, again, we have an interesting case of differentiation
or development (p. 276) : the organism begins life as a very
small uninucleate mass of protoplasm, and as it increases
in size increases also in complexity by the repeated binary
fission of its nucleus.

In another group of the Protozoa known as the Sporozoa
parasitism occurs without exception, and the relation between
parasites and host is much more intimate than in the case
of Opalina : instead of living within the enteric canal and
merely absorbing the contained products of digestion, these
penetrate into the tissues of the host, even passing into the
interior of its cells, from the contents of which they absorb
fluid nutriment. As is the rule in parasites, they are able to
multiply very rapidly.

Sporozoan parasites occur in most classes of animals : many
seem to be comparatively harmless, so that even when
infested with large numbers the host apparently suffers no
harm. In other cases they, like some other parasitic
Protozoa (e.g., the flagellate Trypanosomd] and certain
Bacteria, may cause dangerous diseases and epidemics
(compare p. 287).

As an example which is easily obtainable we will examine
Monocystis (Fig. 74), one of those Sporozoa commonly
known as Gregarines, which occurs abundantly in earthworms.
When the body-wall of an earthworm is cut open, certain
white, lobed organs known as sperm-sacs are very apparent a
short distance from the anterior end of the animal (compare
Chapter VI.). In these, the sperms of the worm undergo

MONOCYSTIS

285

development, and amongst and within the sperm-mother-
cells Monocystis lives and passes through the various stages
of its life-history. The two commonest species are known
as M. agilis and M. magna.

The encysted stage is the most easily recognised, and a
number of spherical cysts, varying between about \ and T^

FIG. 74. — Monocystis.

A, Trophozoites in different stages of contraction. B, encysted gametocytes.
C,' division of gametocytes into gametes. D, conjugation of gametes to form
zygotes. E, Cyst enclosing ripe spores formed from the zygotes. F, single
spore, drawn to a larger scale, showing the (8) sporozoites in its interior.
G, group of developing sperm-cells of the earthworm, enclosing a sporozoite in the
centre. H, young trophozoites still surrounded with the tails of the degenerated
sperms, nu, nuclei.

mm. in diameter, can usually be seen if a small piece of a
sperm-sac is examined under the microscope. When ripe
each cyst contains a large number of small spindle-shaped or
boat-shaped .spores (&). Before encystment Monocystis has

286 MONOCYSTIS CHAP.

a somewhat elongated form, and is capable of slow movements
amongst the cells of the sperm-sacs, contractions of the or-
ganism as a whole, or constrictions in various parts, causing
its shape to vary considerably at different times (A). There is
a clear cortical layer covered by a cuticle (p. 264), and a
very granular medullary portion, enclosing the spherical
nucleus (nu) in which one or more nucleolus-like bodies can
be distinguished. No pseudopods or cilia are formed, and,
as in Opalina, there is no mouth, gullet, or contractile
vacuole, fluid nutriment being absorbed from the cells of
the host. During this stage the organism feeds and grows,
and is spoken of as a trophozoite.

After a time two trophozoites become applied together,
without fusing, and a common resistant cyst is formed around
them : each is now known as a gametocyte (B). The nucleus
of each gametocyte then undergoes successive divisions (c),
the ultimate products of which, each surrounded by proto-
plasm, constitute small gametes, all of which are similar to
one another.1 These gametes then conjugate in pairs (D),
probably those derived from one gametocyte uniting with
those derived from the other. The result is the formation of
numerous zygotes, each of which becomes spindle-shaped and
surrounded by a tough cell-wall, so that the cyst (E) now
contains a number of the characteristic spores already
referred to, each of which is about -^ — —^ mm. in
length.

The contents of each spore then undergo division (F),
forming eight elongated and nucleated bodies or sporozoites,
which become set free by the bursting of the enclos-
ing membrane : apparently, however, they must enter a

1 During this process part of the protoplasm of the gametocytes
remains undivided and serves as nutrient material for the developing
spores.

iv MALARIA PARASITE 287

fresh host before this takes place. It is probable that earth-
worms become infected by swallowing spores which have
been set free into the earth, either directly owing to injury
to the host, or by means of the excrement of worm-eating
birds. The membrane is probably acted upon by the
digestive juices of the worm, the liberated sickle-shaped and
motile sporozoites passing through the wall of the intestine
and so reaching the sperm-sacs, when they enter some of
its cells (G), absorbing so much of the contents of the latter
that there is not sufficient left to form complete sperms.
They then become set free among the cells of the sperm-
sacs, surrounded by the tails of the degenerated sperms which
look like an investment of stiff cilia (H) ; they continue to
grow until the fully developed trophozoite-stage is again
reached and the life-cycle thus completed.

It will be seen that in Monocystis there is no special method for the
parasite to be regularly transferred from one host to another. In some
Sporozoa, as in many parasites belonging to other and higher groups,
infection is provided for, and the life-history is still further complicated
owing to the fact that two hosts, belonging to different groups, are
necessary for the completion of the cycle. A good example is the
malaria-organism (Laverania\ which lives within the red blood-cor-
puscles of man. This sporozoan multiplies rapidly by multiple fission,
thus producing very numerous merozoites which attack other blood -
corpuscles of the same host. Should a drop of blood, containing
merozoites be sucked up by a mosquito, the parasites undergo >a different
mode of development. From two different kinds of gametocytes, ovum-
like megagametes and sperm -like microgatnetes are respectively formed
(p. 275). After conjugation, the active zygotes penetrate the epithelium
of the mosquito's stomach, become encysted, and give rise to an
enormous number of sporozoites, which reach the salivary ducts of the
mosquito. If the insect then bites another human being, the sporozoites
are injected into the wound and again attack the red corpuscles.

The study of the foregoing living things and especially
of Bacteria, the smallest and probably the simplest of all

288 BIOGENESIS CHAP.

known organisms (p. 257), naturally leads us to the con-
sideration of one of the most important problems of Biology
— the problem of the origin of life.

In all the higher organisms we knowr that each individual
arises in some way or other from a pre-existing individual :
no one doubts that every bird now living arose by a process
of development from an egg formed in the body of a
parent-bird, and that every tree now growing took its origin
either from a seed or from a bud produced by a parent-
plant. But there have always — until quite recently, at any
rate — been upholders of the view that the lower forms of life,
Bacteria, Monads, and the like, may under certain circum-
stances originate independently of pre-existing organisms :
that, for instance, in a flask of some organic infusion
boiled so as to kill any living things present in it, fresh
forms of life may arise de novo — may in fact be created
then and there.

We have therefore two theories of the origin of the lower
organisms, the theory of Biogenesis, according to which each
living thing, however simple, arises by a natural process of
budding, fission, spore-formation, or what not, from a parent
organism : and the theory of Abiogenesis, or as it is some-
times called Spontaneous or Equivocal Generation, accord-
ing to which fully-formed living organisms sometimes
arise from not-living matter.

In former times the occurrence of abiogenesis was uni-
versally believed in. The expression that a piece of meat
has " bred maggots " ; the opinion that parasites such as the
gall-insects of plants .or the tape- worms in the intestines of
animals originate where they are found ; the belief still held
in some rural districts in the occurrence of showers of frogs,
or in the transformation of horse-hairs kept in water into
eels : all indicate a survival of this belief.

iv SPONTANEOUS GENERATION 289

As accurate enquiries into these matters were made, the
number of cases in which equivocal generation was supposed
to occur was rapidly diminished. It was not surprising,
however, considering the rapidity with which Bacteria and
Monads were found to make their appearance in organic
substances and infusions, that many men of science imagined
them to be produced abiogenetically. The rapid multiplica-
tion of these forms means, of course, that a certain amount
of fresh living protoplasm has been formed out of the
constituents of the hay-infusion, through the agency in the
first instance of a single living Bacterium. The question
naturally arises, why may not the formation of protoplasm
take place independently of this insignificant speck of living
matter ?

It must not be thought that this question is in any way
a vain or absurd one. That living protoplasm has at some
period of the world's history originated from not-living
matter seems a necessary corollary of the doctrine of
evolution, and is obviously the very essence of the doctrine
of special creation (p. 221) ; and there is no a priori reason
why it should be impossible to imitate the unknown con-
ditions under which the process took place. But at present
we are quite unable to solve this fundamental problem.

Experiments conducted with proper precautions, however,
all tell the same tale : they prove conclusively that in
putrescible infusions that have been properly sterilised — i.e.,
thoroughly boiled so as to kill any organisms they may
contain — and adequately protected from the entrance of
atmospheric germs, no micro-organisms ever make their
appearance. So that the last argument for abiogenesis has
been proved to be fallacious, and the doctrine of biogenesis
shown, as conclusively as observation and experiment can
show itj to be of universal application as far as existing

PRACT. ZOOL. U

290 IIOMOGENESIS CHAP.

conditions known to us are concerned. It is also
necessary to add that the presence of microbes in consider-
able quantities in our atmosphere has been proved
experimentally.

There is another question intimately connected with that
of biogenesis, although strictly speaking quite independent
of it. It is a matter of common observation that, in both
animals and plants, like produces like : that a cutting from
a willow will never give rise to an oak, nor a snake emerge
from a hen's egg. In other words, ordinary observation
teaches the general truth of the doctrine of Homogenesis.

But there has always been a residuum of belief in the
opposite doctrine of Heter agenesis, according to which the
offspring of a given animal or plant may be something
utterly different from itself, a plant giving rise to an animal
or vice versa, a lowly to a highly organised plant or animal
and so on. Perhaps the most extreme case in which hetero-
genesis was once seriously believed to occur is that of the
" barnacle-geese." Buds of a particular tree growing
near the sea were said to produce barnacles, and these
falling into the water to develop into geese. This sounds
absurd enough, but within the last thirty years two or three
men of science have described, as the result of repeated
observations, the occurrence of quite similar cases among
microscopic organisms. For instance, the blood-corpuscles
of the silkworm have been said to give rise to fungi, Euglense
to thread-worms, and so on.

It is proverbially difficult to prove a negative, and it might
not be easy to demonstrate, what all competent naturalists
must be firmly convinced of, that every one of these sup-
posed cases of heterogenesis is founded either upon errors
of observation or upon faulty inductions from correct
observations.

iv HETEROGENESIS 291

It is obvious that the only way in which a case of hetero-
genesis could be proved would be by actually watching the
transformation, and this no heterogenist has ever done ; at
the most, certain supposed intermediate stages between the
extreme forms have been observed — say, between a Euglena
and a thread-worm — and the rest of the process inferred.
On the other hand, innumerable observations have been,
made on these and other organisms, the result being that
each species investigated has been found to go through a
definite series of changes in the course of its development,
the ultimate result being invariably an organism resembling
in all essential respects that which formed the starting point
of the observations : Euglense always giving rise to Euglenae
and nothing else, Bacteria to Bacteria and nothing else, and
so on.

There are many cases which imperfect knowledge might
class under heterogenesis, such as the origin of frogs from
tadpoles or of jelly-fishes from polypes (see Chapter V),
but in these and many other cases the apparently anomalous
transformations have been found to be part of the normal
and invariable cycle of changes undergone by the organism
in the course of its development : the frog always gives
rise ultimately to a frog, the jelly-fish to a jelly-fish. If
a frog at one time produced a tadpole, at another a trout,
at another a worm : if jelly-fishes gave rise sometimes to
polypes, sometimes to infusorians, sometimes to cuttle-fishes,
and all without any regular sequence — that would be
heterogenesis.

It is perhaps hardly necessary to caution the reader against
the error that there is any connection between the theory of
heterogenesis and that of organic evolution. It might be
said — if, as naturalists tell us, dogs are descended from
wolves and jackals, and birds from reptiles, why should not,

u 2

292 PROTOZOA AND METAZOA CHAP.

for instance, thread-worms spring from Euglenae or Infusoria
from Bacteria ? To this it is sufficient to answer that the
evolution of one form from another takes place by a series
of slow, orderly, progressive changes going on through a
long series of generations (p. 222) ; whereas heterogenesis
presupposes the casual occurrence of sudden transformations
in any direction — i.e., leading to either a less or a more highly
organised form — and in the course of a single generation.1

Each of the organisms which we have studied in this
and the two previous chapters consists of a single cell — or
in the case of Carchesium and Epistylis of a colony of cells
to a large extent independent of one another. They are
therefore placed in the lowest primary division of the animal
kingdom — the phylum Protozoa (p. 220). This phylum is
subdivided into a number of classes, examples of certain of
which we have examined. Those in which, like the Amoeba,
the amoeboid form is predominant constitute the class
Rhizopoda : those in which, like the Monads and Euglense
(Flagellatd), the flagellate form is predominant are often
included with the ciliated forms (Ciliata) — such as Para-
mcecium, Vorticella and Opalina— in a single class, the
Infusoria : and those in which, like Monocystis, the encysted
form is predominant, are known as the Sporozoa.

The animals above the Protozoa are placed, as we have
seen, in a number of different phyla, but as they are all
multicellular they are often spoken of collectively as the
Metazoa, one of the simplest of which we must next
examine.

1 Apart from such continuous variations, others, which may be
described as discontinuous, do sometimes appear with apparent sudden-
ness, but not to the extent which would be required by the theory of
heterogenesis.

PRACTICAL DIRECTIONS 293

PRACTICAL DIRECTIONS

Opallna. — Make an incision in the rectum or small intestine
of a freshly-killed frog, and mount a little of its contents in water on
a slide. Having found an Opalina, note its oval and flattened form,
the uniform covering of cilia, the cuticle, cortex, and medulla, and the
absence of a mouth and contractile vacuole. Stain with methyl-green
or magenta, and make out the numerous miclei. Look out for the
products of division, free and encysted.

MonOCystis. — Kill an earthworm by placing it for a few minutes in
methylated spirit. Then, with the scissors, slit through the body-wall
along the mid-dorsal line through about the anterior third of the animal,
when some white, lobed bodies, the sperm-sacs (see Chapter VI) will
be apparent. Snip off a very small portion of one of these, place
it on a slide in salt-solution and examine with the low power. Amongst
the cells of the sperm-sac you will probably be able to distinguish :

(a) Numerous cysts of Monocystis, many of which will be seen to
contain a number of spindle-shaped spores (Fig. 74, E), and in some
others, two gametocytes (B). (b} Trophozoites (A) in various stages of
growth and contraction.

Put on a high power, and after observing the groups of cells of the
sperm -sacs and sperms in various stages of development, examine
in detail : —

1. The trophozoites (A), noting the ciiticle, cortex, medulla, and
nucleus. (Observe their movements. )

2. The encysted gametocytes, before and after division into
gametes (B, c).

3. Further stages in the formation of gametes and zygotes, and the
fully-developed spores (D, E).

4. The structure of an individual spore, which contains eight sporo-
zoites (F).

5. Sporozoites which have entered a clump of sperm-cells (G) ; and
later stages after they have become free, surrounded by the tails of the
sperms (H).

Stain a preparation with magenta or methyl-green, and go over I — 5
again.

CHAPTER V.

HYDRA : OBELIA — SYMBIOSIS — ALTERNATION OF GENERA-
TIONS—CHARACTERS OF THE PHYLUM CCELENTERATA.

A CAREFUL search in ponds will often result in the
capture of some small organisms known as " fresh-water
polypes," belonging to the genus Hydra.

Although far from uncommon in pond-water, Hydra is not
always easy to find, being rarely abundant and by no means
conspicuous. In looking for it the best plan is to fill either
a clear class bottle or beaker, or a saucer, with weeds and
water from a pond, and to let it remain undisturbed for a
short time. If the gathering is successful there will be seen
adhering to the sides of the glass, the bottom of the saucer,
or the weeds, little white, tawny, or green bodies, about as
thick as fine sewing cotton, and 2 — 6 mm. in length. They
adhere pretty firmly by one end, and examination with a
pocket lens shows that from the free extremity a number of
very delicate filaments, barely visible to the naked eye, are
given off.

Under the low power of a compound microscope a Hydra
(Fig. 75) is seen to have a cylindrical body attached by a
flattened base to a weed or other aquatic object, and bear-
ing at its opposite or distal end a conical structure, the

HYDRA

295

FIG. 75. — Hydra.

A, two living specimens of H. viridis attached to a bit of weed. The larger
specimen is fully expanded, and shows the elongated body ending distally in the
hypostome (hyp), surrounded by tentacles (f), and three buds (&/«!, bd2, bd$) in
different stages of development ; a small water-flea (a) has been captured by one
tentacle. The smaller specimen (to the right and above) is in a state of complete
retraction, the tentacles (t) appearing like papillae.

B, H.fusca, showing the mouth (mth) at the end of the hypostome (hyp), the
circlet of tentacles (/), two spermaries (spy), and an ovary (pvy)

C, a Hydra creeping on a flat surface by looping movements.

D, a specimen crawling on its tentacles. (From Parker's Biology : C and D after
W. Marshall.)

hypostome (hyp], at the apex of which is a circular aperture
the mouth (mtti). At the junction of the hypostome with
the body proper are given off from six to eight long delicate

296 HYDRA CHAP.

tentacles (/) arranged in a circlet or whorl. A longitudinal
section shows that the body is hollow, containing a spacious
cavity, the enter on (Fig. 76, A, ent. cav), which communicates
with the surrounding water by the mouth. The tentacles
are also hollow, their cavities communicating with the
enteron.

Thus it will be seen that the Hydra is not bilaterally
symmetrical, like the frog — i.e., equally divisible into two
lateral halves by a median vertical plane passing through
the axis of the body — but is radially symmetrical, i.e., the
body is divisible into similar parts radiating from a common
central axis.

There are three kinds of Hydra commonly found : one,
H. vulgaris, is colourless or nearly so ; another, H. fusca, is
of a pinkish-yellow or brown colour ; the third, H. viridis, is
bright green. In the two latter it is quite evident, even
under a low power, that the colour is in the inner parts of the
body-wall, the outside of which is formed by a transparent,
colourless layer (Fig. 75).

An examination of the living animal shows, in the first
place, that its form is continually changing. At one time (Fig.
75, A, left-hand figure) it extends itself until its length is
fully fifteen times its diameter and the tentacles appear like
long delicate filaments : at another time (right-hand figure)
it contracts itself into an almost globular mass, the tentacles
then appearing like little blunt knobs.

Besides these movements of contraction and expansion,
Hydra is able to move slowly from place to place. This it
usually does after the manner of a looping caterpillar
(c) : the body is bent round until the distal end touches
the surface : then the base is detached and moved nearer
the distal end, which is again moved forward, and so on.
It has also been observed to crawl like a cuttle-fish (D)

v STRUCTURE 297

by means of its tentacles, the body being kept nearly
vertical.

It is also possible to watch a Hydra feed. It is a very
voracious creature, and to see it catch and devour its prey is
a curious and interesting sight. In the water in which it
lives are always to be found numbers of "water-fleas,"
minute animals of about a millimetre or less in length,
belonging to the class Crustacea (see Chapter VII).

Water-fleas swim very rapidly, and occasionally one may
be seen to come in contact with a Hydra's tentacle. In-
stantly its hitherto active movements stop dead, and it
remains adhering in an apparently mysterious manner to the
tentacle. If the Hydra is not hungry it usually liberates its
prey after a time, and the water-flea may then be seen to
drop through the water like a stone for a short distance, but
finally to expand its limbs and swim off. If, however, the
Hydra has not eaten recently, it gradually contracts the ten-
tacles until the prey is brought near the mouth, the other
tentacles being also used to aid in the process. The water-
flea is thus forced against the apex of the hypostome, the
mouth expands widely and seizes it, and it is finally passed
down into the digestive cavity. Hydrae can often be seen
with their bodies bulged out in one or more places by
recently swallowed water-fleas.

The precise structure of Hydra is best made out by cutting
it into a series of extremely thin sections and examining them
under a high power of the microscope. The appearance
presented by a vertical section through the long axis of the
body is shown in Fig. 76, A.

The whole animal is seen to be built up of cells, each
consisting of protoplasm with a large-nucleus (B — D, nu\ and
with or without vacuoles. As in the case of most animal cells,
there is no cell-wall.

298 HYDRA CHAP, v

The essential feature in the arrangement of the cells is
that they are disposed in two layers round the central diges-
tive cavity or enteron (A, ent. cav) and the cavities of the
tentacles (ent. cav}. So that the wall of the body is formed
throughout of an outer layer of cells, the ectoderm (ect), and
of an inner layer, the endoderm (end\ which bounds the
enteric cavity (compare p. 202). Between the two layers
is a delicate transparent membrane, the mesoglcea, or sup-
porting lamella (msgl). A transverse section (B) shows that
the cells in both layers are arranged radially.

Thus Hydra is a two-layered or diploblastic animal, and
may be compared to a chimney built of two layers of
radially arranged bricks with a space between the layers
filled with mortar or concrete.

Accurate examination of thin sections, and of specimens
teased out or torn into minute fragments with needles,
shows that the structure is really much more complicated
than the foregoing brief description would indicate.

The ectoderm-cells are of two kinds. The first and most
obvious (B, ect, and c) are large cells of a conical form, the
bases of the cones being external, their apices internal.
Spaces are necessarily left between their inner or narrow
ends, and these are filled up with the second kind of cells
(int. <:), small rounded bodies which lie closely packed be-
tween their larger companions and are distinguished as
interstitial cells.

The inner ends of the large ectoderm-cells are continued
into narrow, pointed prolongations (c, m. pr) placed at right
angles to the cells themselves and parallel to the long axis
of the body. There is thus a layer of these longitudinally-
arranged muscle-processes lying immediately external to the
mesoglcea (B, m. pr). They appear to possess, like the axial

site

FIG. 76. — Hydra.

A, verticalfsection of the entire animal, showing the body-wall composed of ecto-
derm (ecf) and "endoderm (end), enclosing an enteric cavity (enl. cav), which,
as well as the two layers, is continued (ent. cav') into the tentacles, and opens

300 HYDRA CHAP.

externally by the mouth (nitti) at the apex of the hypostome (hyj>). Between the
ectoderm and endoderm is the mesoglcea (msgl), represented by a black line.
In the ectoderm are seen large (ntc) and small (ntc1) nematocysts ; some of the
endoderm-cells are putting out pseudopods (flsd), others flagella (JT). Two
buds (&/.1, hdp) in different stages of development are shown'on the left side, and
on the right a spermary (spy) and an ovary (ovy) containing a single ovum (ov).

B, portion of a transverse section more highly magnified, showing the large ecto-
derm-cells (ecf) and interstitial cells (int. c) ; two cnidoblasts (cnbf) enclosing

•«*— nematocysts (ntc) and one of them produced into a cnidocil (cnc) ; the layer
of muscle-processes (m. fir) cut across just external to the mesogloea (msgf) ;
endoderm-cells (end) -with large yacuoles and nuclei (nu), pseudopods (pscT),
and flagella (_/?). The endoderm-cell to the right has ingested a diatom (a),
and all enclose minute black granules.

C, two of the large ectoderm-cells, showing nucleus (nu) and muscle-process (in.pr).

D, an endoderm-cell of H. viridis, showing nucleus (nu), numerous Zoochlorellse
(ckr), and an ingested nematocyst (ntc).

K, one of the larger nematocysts with extruded thread, barbed at the base.

F, one of the smaller nematocysts.

G, a single sperm. (From Parker's Biology : D after Lankester ; F and G after
Howes.)

fibre of Vorticella (p. 273), a high degree of contractility, the
almost instantaneous shortening of the body being due, in
great measure at least, to their rapid and simultaneous
contraction. It is probably correct to say that, while the
ectoderm-cells are both contractile and irritable, a special
degree of contractility is assigned to the muscle-processes,
the cells themselves being eminently irritable, the slightest
stimulus applied to them usually being followed by an
immediate contraction of the whole body.

Imbedded in and between some of the large ectoderm-cells
are found plear, oval sacs (n/c\ with very well-defined walls,
called "thread-cells" or nematocysts. Both in the living
specimen and in sections they ordinarily present the ap-
pearance shown in Figs. 76, B,«and 77, A, but are frequently
met with in the condition shown in Figs. 76, E, and 77, B,
that is, with a short, conical tube protruding from the mouth
of the sac, armed near its distal end with three recurved
barbs besides several similar processes of smaller size, and
giving rise distally to a long, delicate, flexible filament.

Accurate examination of the nematocysts shows that the
structure of these curious bodies is as follows. Each con-
sists of a tough sac (Fig. 77, A), one end of which is turned

NEMATOCYSTS

301

A

FIG. 77. — Hydra,

A, a nematpcyst contained in its cnidoblast (cub), showing its coiled filament
and the cnidocil (cnc),

B, the same after extrusion of the thread, showing the larger and smaller barbs at
the base of the thread ; nu, the nucleus of the cnidoblast.

C, a cnidoblast, with its contained nematocyst, connected with one of the processes
of a nerve-cell (nv. c). (From Parker's Biology '. after Schneider.)

in as a hollow pouch : the free end of the latter is continued
into a hollow, coiled filament, and from its inner surface

302 HYDRA CHAP.

project the barbs. The whole space between the wall of
the sac and the contained pouch and thread is tensely filled
with fluid. When pressure is brought to bear on the outside
of the sac the whole apparatus goes off like a harpoon-gun
(B), the compression of the fluid forcing out first the barbed
pouch and then the filament, until finally both are turned
inside out.

It is by means of the nematocysts — the resemblance of
which to the trichocysts of Paramcecium (p. 267) should be
noted— that the Hydra is enabled to paralyse its prey. Prob-
ably some specific poison is formed and ejected into the
wound with the thread : in the larger members of the group
to which Hydra belongs, such as jelly-fishes, the nematocysts
produce an effect on the human skin quite like the sting of
a nettle.

The nematocysts are formed in special interstitial cells
called cnidoblasts (Figs. 76, B, and 77, cnb\ and are thus
in the first instance at a distance from the surface. But the
cnidoblasts migrate outwards, and so come to lie quite
superficially either in or between the large ectoderm-cells.
On its free surface the cnidoblast is produced into a delicate
pointed process, the cnidocil or " trigger-hair " (cnc). In all
probability the slightest touch of the cnidocil causes con-
traction of the cnidoblast, and the nematocyst, thus com-
pressed, instantly explodes.

Nematocysts are found in the distal part of the body, but
are absent from the foot or proximal end, where also there
are no interstitial cells. They are especially abundant in the
tentacles, on the knob-like elevations of which — due to little
heaps of interstitial cells — they are found in great numbers
Amongst these occur small nematocysts with short threads
and devoid of barbs (Fig. 76, A, ntc and F).

In connection with the cnidoblasts small irregular rrlls

v ENDODERM 303

with large nuclei occur (Fig. 77, c, nv. c) • they are supposed
to be nerve-cells^ and to constitute a rudimentary nervous
system (compare p. 167).

The ectoderm cells of the foot differ from those of the rest
of the body in being very granular (Fig. 76, A). The
granules are probably the material of the adhesive secretion
by which the Hydra fixes itself, and these cells are therefore
glandular (p. 130).

The endoderm consists for the most part of large cells
wrhich exceed in size those of the ectoderm, and are re-
markable for containing one or more vacuoles, sometimes
so large as to reduce the protoplasm to a thin superficial
layer containing the nucleus (Fig. 76, A and B, end}. Then,
again, their form is extremely variable, their free or inner
ends undergoing continual changes of form. This can be
easily made out by cutting transverse sections of a living
Hydra, wrhen the endoderm-cells are seen to send out long
blunt pseudopods (psd) into the digestive cavity, and now
and then to withdraw the pseudopods and send out from
one to three long, delicate flagella (fl}. Thus the endoderm-
cells of Hydra illustrate in a very instructive manner the
essential similarity of flagella and pseudopods already re-
ferred to (p. 250). In the hypostome the endoderm is thrown
into longitudinal folds, so as to allow of the dilatation of
the mouth in swallowing.

Amongst the ordinary endoderm-cells are found long
narrow cells of an extremely granular character. They are
specially abundant in the distal part of the body, beneath
the origins of the tentacles, and in the hypOstome, but are
absent in the tentacles and in the foot. There is no doubt
that they are gland-cells, their secretion being a fluid used
to aid in the digestion of the food.

304 HYDRA CHAP.

In Hydra viridis the endoderm-cells (Fig. 76, D) contain
chromatophore-like bodies (chr) coloured green by chloro-
phyll (p. 242), the function of which we have already
considered (p. 247). It has been proved, however, that
these are not actual parts of the endoderm-cells, but are
distinct Sphaerella-like organisms known as Zoochlorellce,
which are passed on from one generation of the Hydra to
another by entering its developing eggs. Such a living-
together of two organisms is known as symbiosis. It differs
essentially from parasitism (see p. 280), in which one
organism preys upon another, the host deriving no benefit
but only harm from the presence of the parasite. In
symbiosis, on the contrary, the two organisms are in a
condition of mutually beneficial partnership. The carbon
dioxide and nitrogenous waste given off by the cells of the
Hydra serve as a constant food-supply to the Zoochlorella :
at the same time the latter by decomposing the carbon
dioxide provides the Hydra with a constant supply of
oxygen, and also with two important food stuffs — starch
and proteids, which, after solution, diffuse -from the proto-
plasm of the Zoochlorella into that of the endoderm-cells.
The latter may therefore be said to keep the Zoochlorellae
constantly manured, while the Zoochlorellae in return supply
them with oxygen and ready-digested food. In the endo-
derm of H. fusca bodies of an orange or brown colour are
present which are devoid of chlorophyll.

Muscle-processes also exist in connection with the endoderm-cells,
and they are said to take a transverse or circular direction, i.e., at
right angles to the similar processes of the ectoderm cells.

When a water-flea or other minute organism is swallowed
by a Hydra, it undergoes a gradual process of disintegration.
The process is begun by a solution of the soft parts due to

V DIGESTION 305

the action of a digestive fluid secreted by the gland-cells of
the endoderm ; it is apparently completed by the endoderm
cells seizing minute particles with their pseudopods and
engulfing them quite after the manner of Amoebae. It is
often found that the protrusion of pseudopods during
digestion results in the almost complete obliteration of the
enteric cavity.

It would seem, therefore, that in Hydra the process of
digestion or solution of the food is to some extent intra-
cellular, i.e., takes place in the interior of the cells them-
selves, as e.g., in Amoeba or Paramcecium : it is, however,
largely extra-cellular or enteric, i.e., is performed in a special
digestive cavity lined by cells (pp. 67 and 131).

The ectoderm-cells do not take in food directly, but are
nourished entirely by diffusion from the endoderm. Thus
the two layers have different functions : the ectoderm is pro-
tective and sensory ; it forms the external covering of the
animal, and receives impressions from without ; the endoderm,
removed from direct communication with the outer world,
performs a nutrient function, its cells alone having the power
of digesting food.

The essential difference between digestion and assimilation
is here plainly seen : all the cells of Hydra assimilate, all
are constantly undergoing waste, and all must therefore form
new protoplasm to make good the loss. But it is the endo-
derm-cells alone which can make use of raw or undigested
food : the ectoderm has to depend upon various products
of digestion received by diffusion or osmosis from the
endoderm.

It will be evident from the preceding description that
Hydra is comparable to a colony of Amoebae in which par-
ticular functions are made over to particular individuals —

PRACT. ZOOL. X

3o6 HYDRA CHAP.

just as in a civilised community the functions of baking and
butchering are assigned to certain members of the com-
munity, and not performed by all. Hydra is therefore an
example of individuation : morphologically it is equivalent
to an indefinite number of unicellular organisms : but,
these acting in concert, some taking one duty and some
another, form, physiologically speaking, not a colony of
largely independent units (compare p. 277), but a single
multicellular individual.

Hydra has two distinct methods of reproduction, asexual
and sexual.

Asexual multiplication takes place by a process of budding.
A little knob appears on the body (Fig. 75 A, bdl\ and is
found by sections to arise from a group of ectoderm-cells ;
soon, however, it takes on the character of a hollow out-
pushing of the wall containing a prolongation of the enteron,
and made up of ectoderm, mesoglcea, and endoderm. (Fig.
76, A, bdl). In the course of a few hours this prominence
enlarges greatly, and near its distal end six or eight hollow
buds appear arranged in a whorl (Figs. 75, A, and 76, A,
bcfi\ These enlarge and take on the characters of ten-
tacles, and a mouth is formed at the distal end of the bud,
which thus acquires the character of a small Hydra (Fig. 75,
A, bcP). Finally the bud becomes constricted at its base,
separates from the parent, and begins an independent ex-
istence. Sometimes, however, several buds are produced at
one time, and each of these buds again before becoming
detached : in this way temporary colonies are formed. But
the buds always separate sooner or later, although they
frequently begin to feed while still attached.

It is a curious circumstance that Hydra can also be mul-
tiplied by artificial division : the experiment has been tried

v REPRODUCTION 307

of cutting the living animal into pieces, each of which was
found to undergo regeneration into a perfect individual.

The sexual organs or gonads (p. 193) are of two kinds,
spermaries and ovaries. Both are found in the same indi-
vidual, Hydra being hermaphrodite or moncecious.

The spermaries (Figs. 75, B, and 76, A, spy) are white
conical elevations situated near the distal end of the body :
as a rule not more than one or two are present at the same
time, but there may be as many as twenty. They are per-
fectly colourless, even in the green and brown species, being
obviously formed of ectoderm alone.

In the immature condition the spermary consists of a little
heap of interstitial cells covered by an investment of some-
what flattened cells formed by a modification of the ordinary
large cells of the ectoderm. When mature each of the small
internal cells becomes converted into a sperm (p. 194),
consisting of a small ovoid head formed from the nucleus of
the cell, and of a long vibratile tail formed from its proto-
. plasm (Fig. 76, G). By the rupture of the investing cells or
wall of the spermary the sperms are liberated and swim
freely in the water.

The ovaries (Figs. 75, B, and 76, A, ovy) are found
nearer the proximal end of the body, and vary in number
from one to eight. When ripe an ovary is larger than a
spermary, and of a hemispherical form. It begins, like the
spermary, as an aggregation of interstitial cells, so that in
their earlier stages the sex of the gonads is indeterminate.
But while in the spermary each cell is converted into a
sperm, in the ovary one cell (Fig. 76, A, ov\ soon begins
to grow faster than the rest, and becomes amoeboid in
form, sending out pseudopods amongst its companions
and ingesting the fragments into which they become broken
up, thus continually increasing in size at their expense.

X 2

308 HYDROID POLYPES CHAP.

Ultimately the ovary comes to consist of this single amoeboid
ovum and of a layer of superficial cells forming a capsule for
it. As the ovum grows, yolk-granules (p. 195) are formed
in it, and in Hydra viridis it also acquires Zoochlorellae

(P- 304)-

When the ovary is ripe the ovum draws in its pseudopods
and takes on a spherical form : the investing layer then
bursts so as to lay bare the ovum and allow of the free access
to it of the sperms. One of the latter conjugates with the
ovum, producing an oosperm (p. 198) or unicellular embryo.

The oosperm undergoes segmentation, dividing into
a number of cells which constitute a morula or polyplast
(p. 200), the outermost cells of which become changed into
a hard shell or capsule, which eventually bursts and sets
free the embryo. The embryo develops into a Hydra, its
cells becoming differentiated into ectoderm and endoderm,
the enteron and mouth being formed, and the tentacles
budding out around the latter.

It was stated on p. 306 that in a budding Hydra the buds
do not always become detached at once, but may them-
selves bud while still in connection with the parent,
temporary colonies being thus produced.

Suppose the state of things to continue indefinitely : the
result would be a tree-like colony or compound organism
consisting of a stem with numerous branchlets each ending
in a Hydra-like zooid. Such a colony would bear much the
same relation to Hydra as Carchesium or Epistylis bears to
Vorticella.

As a matter of fact this is precisely what happens in a
great number of animals allied to Hydra and known by the
name of Zoophytes or Hydroid polypes.

Every one is familiar with the common Hydroids known

v OBELI A 309

as Sertularians of the sea-coast, often mistaken for sea-
weeds : they are delicate, much-branched, semi-transparent
structures of a horny consistency, the branches beset with
little cups, from each of which, during life, a Hydra-like
body is protruded.

A very convenient genus of Hydroids for our purpose is
Obelia, which occurs in the form of a delicate, whitish or
light-brown, almost fur-like growth on the wooden piles of
piers and wharves. It consists of branched filaments about
the thickness of fine sewing-cotton : of these, some are
closely adherent to the timber, and serve for attachment,
while others are given off at right angles, and present at
intervals short lateral branches, each terminating in a bud-
like enlargement.

The structure is better seen under a low power of the
microscope. The organism (Fig. 78) is a colony, consisting
of a common stem or axis, on which are borne numerous
zooids (compare p. 277). The axis consists of a horizontal
portion, resembling a root or creeping stem, and of vertical
axes, which give off short lateral branches in an alternate
manner, bearing the zooids at their ends. At the proximal
ends of the vertical axes the branching often becomes more
complex : the offshoots of the main stem, instead of ending
at once in a zooid, send off branches of the third order on
which the zooids are borne. In many cases, also, branches
are found to end in simple club-like dilatations (Bd. 1,2):
these are immature zooids.

The large majority of the zooids are little Hydra-like
bodies, the polypes or hydranths, each with a hypostome and
a circlet of about two dozen tentacles. Less numerous,
and found chiefly towards the proximal region of the colony,
are long cylindrical bodies or blastostyles (bls\ each bearing
numerous small lateral offshoots, varying greatly in form

Fig. 78.— Obelia. A, portion of a colony with certain parts shown in longitudinal
section ; B. medusa ; C, the same with reversed umbrella ; D, the same, oral
aspect ; Bd. /, 2, buds ; bis. blastostyle ; ca>. ccenosarc ; ect. ectoderm ; end.
endoderm ; ent. enteric cavity ; g.th. gonotheca ; h.t/i hydrotheca ; /. lithocyst ;
in.bd. medusa-bud ; tnnb. manubrium ; insgl. mesogloea ; inth. mouth ; p. peri-
sarc ; P. /, ,?, j, polypes; rod. c. radial canal; t. tentacle; vl. velum. (From
Parker and Haswell's Zoology, reduced.)

CHAP, v OBELIA 311

according to their stage of development, and known as
medusa-buds (m.bd)^ these will be considered presently.

Examination under a high power, either of an entire
branch or of sections, shows that the polypes have essentially
the structure of a Hydra, consisting of a double layer
of cells — ectoderm (with nematocysts) and endoderm —
separated by a supporting lamella or mesoglcea and • en-
closing a digestive cavity (ent) which opens externally by a
mouth placed at the summit of the hypostome. The mouth
is capable of great dilatation and contraction, and accord-
ingly the hypostome appears now conical, now trumpet-
shaped.

The tentacles, however, differ from those of Hydra in two
important respects. In the first place they are solid : the
endoderm, instead of forming a lining to a prolongation of
the enteron, consists of a single axial row of large cells with
thick cell- walls and vacuolated protoplasm. Then in the
position of the muscle-processes of Hydra there is a layer of
spindle-shaped fibres, many times longer than broad, and pro-
vided each with a nucleus. Such muscle-fibres are obviously
cells greatly extended in length (p. 1 1 i), so that the ectoderm-
cell of Hydra with its continuous muscle-^TW&w is here
represented by an ectoderm-cell with an adjacent muscle-
celL We thus get a partial intermediate layer of cells
between the ectoderm and endoderm, in addition to the
gelatinous mesogloea ; and so, while a hydroid polyp is, like
Hydra, diploblastic (p. 298), it shows a tendency towards the
assumption of a three-layered or triploblastic condition
(compare p. 202).

The part of the stem 'and branches continuous with the
bases of the polypes, which is known as the ccznosarc,
is formed of the same layers and contains a cavity con-
tinuous with those of the hydranths : thus the structure of .

312 OBELI A CHAP.

a hydroid polyp is, so far, simply that of a Hydra in which
the process of budding has gone on to 'an indefinite extent
and without separation of the buds.

There is, however, an additional layer added for protective
and strengthening purposes. It is evident that such a
colony would, if formed only of soft ectodermal and endo-
dermal cells, be so weak as to be hardly able to bear its
own weight even in water. To remedy this a layer of trans-
parent, yellowish substance of horn-like consistency, called
the perisarc, is developed outside the ectoderm of the
ccenosarc, extending on to the branches and continuous
with a glassy, cup-like investment, or hydrotheca^ around
the base of each polype, and with a transparent case, or
gonotheca, enclosing each blastostyle. Each hydrotheca
(h.tti) has the form of a vase or wine-glass, and is perfectly
transparent and colourless. A short distance from its
narrow or proximal end, it is produced inwards into a sort
of circular shelf (sh\ perforated in the centre : upon this
the base of the polype rests, and through the aperture it is
continuous with the common stem. When irritated — by a
touch or by the addition of alcohol or other poison — the
polype undergoes a very marked contraction : it suddenly
withdraws itself more or less completely into the theca, and
the tentacles become greatly shortened and curved over the
manubrium (P. 2). At the base of each zooid or branch
the perisarc presents several annular constrictions, giving it
a ringed appearance : for the most part it is separated by an
interval from the ccenosarc, but processes of the latter
extend outwards to it at irregular intervals, and at first
(Bd. 2) the two layers are in close apposition.

It is this layer which, when the organism dies and decays,
is left as a semi-transparent, branched structure resembling
the living colony in form except that polypes, blastostyles,

v STRUCTURE 313

and medusa-buds are wanting. The perisarc is therefore a
supporting organ or skeleton, not, like our own bones,
formed in the interior of the body (endo skeleton) ^ but like
the shell of a crayfish or lobster, lying altogether outside
the soft parts (exoskeletori).

As to the mode of formation of the perisarc : — we saw
that many organisms, such as Sphserella and Amoeba, are
able to form a cyst or cell-wall, by secreting or separating
from the surface of the protoplasm a succession of layers
either of cellulose or of a transparent horn like substance
(pp. 244 and 232). But Amoeba and Sphserella are uni-
cellular, and are therefore free to form this protective
layer at all parts of their surface. The ectoderm-cells of
Obelia, on the other hand, are in close contact with their
neighbours on all sides and with the mesoglcea at their
inner ends, so that it is not surprising to find the secretion
of skeletal substance taking place only at their outer ends.
As the process takes place simultaneously in adjacent cells,
the result is a continuous layer common to the whole
ectoderm instead of a capsule to each individual cell. It is
to an exoskeletal structure formed in this way, i.e. by the
secretion of successive layers from the free faces of adjacent
cells, that the name cuticle is in strictness applied in multi-
cellular organisms.

In the blastostyles both mouth and tentacles are absent,
the zooid ending distally in a flattened disc : the hydrotheca
of a polype is represented by the gonotheca (g.th} which is
a cylindrical capsule enclosing the whole structure, but
ultimately becoming ruptured at its distal end to allow of
the escape of the medusa-buds. These latter are, in the
young condition, mere hollow offshoots of the blastostyle :
when fully developed they have the appearance of saucers
attached by the middle of the convex surface to the blasto-

314 OBELIA CHAP.

style, produced at the edge into sixteen very short tentacles,
and having a blunt process, the manubrium, projecting from
the centre of the concave surface. They are ultimately set
free through the aperture in the gonotheca as little medusa
or jelly-fish (B — D).

The structure of a medusa must now be described in
some detail. The saucer-shaped " bell " or umbrella (Figs.
78 B — D, 79 and 80) is formed of a gelatinous substance

end.fam

FIG. 79. — Dissection of a medusa with rather more than one quarter of the umbrella
and manubrium cut away (diagrammatic). The ectoderm is dotted, the endoderm
striated, and the mesogloea black, circ. c. circular canal ; end. lam. endoderm-
lamella ; gon. gonad ; /. lithocyst ; mnb. manubrium ; mth. mouth ; rad. c, radial
canal ; vL velum.

(Fig. 80, D, msgt) covered both on its inner surface, or sub-
umbrella, and on its outer surface, or ex-umbrella^ by a thin
layer of delicate cells (ect). The clapper-like manubrium
(mnb) is formed of two layers of cells, precisely resembling
the ectoderm and endoderm of Hydra, and separated by a
thin mesogloea ; it is hollow, its cavity (ent. cav) opening
below, i.e. at its distal or free end, by a four-sided aperture,

v MEDUSA 315

the mouth (mth\ used by the medusa for the ingestion of
food. Very commonly as the medusa swims the umbrella
becomes turned inside out, the sub-umbrella then forming
the convex surface and the manubrium springing from its
apex (Fig. 78, c). At its upper (attached or proximal) end
the cavity of the manubrium is continued into four narrow,
radial canals (Figs. 78, B, D, and 79 rad. c, and Fig. 80, D,
and D', rad) which extend through the gelatinous substance of
the umbrella at equal distances from one another, like four
meridians, and finally open into a circular canal (cir. c) which
runs round the edge of the umbrella. By means of this
system of canals the food, taken in at the mouth and digested
in the manubrium, is distributed to the entire medusa.
The canals are lined by a layer of cells (Fig. 80, D and D',
end] continuous with the inner layer or endoderm of the
manubrium ; and extending from one canal to another in
the gelatinous substance of the umbrella, is a delicate
sheet of cells, the endo derm-lamella (D', end. lam).

The edge of the umbrella is produced into a very narrow
fold or shelf, \hevelum (Fig. 79, vl, Fig. 80, v), and gives off
the tentacles (/), which are sixteen in number in the newly-
born medusa, very numerous in the adult. At the bases of
eight of the tentacles— two in each quadrant — are minute
globular sacs (/), each containing a calcareous particle or
lithite. These are the marginal sense-organs or lithocysts :
they were formerly considered to be organs of hearing, and
are hence frequently called otocysts : in all probability their
function is to guide the medusa by enabling it to judge of
the direction in which it is swimming. The marginal organs
in this case may therefore be looked upon as organs of the
sense of direction or of equilibration, and may be spoken
of as statocysts (compare p. 189). The velum consists of a
middle layer of mesogloea with ectoderm on either side :

A

FIG. 80. — Diagrams illustrating thelderivation of the medusa from the hydranth.
In the whole series of figures the ectoderm (ecf) is dotted, the endoderm (end)
striated, and the mesogloea (insgl) black.

A, longitudinal section of a simple polyp, showing the tubular body, with enteric
cavity (ent. cav), hypostome (hv/), mouth {mtk.\ and tentacles (t).

A', transverse section of the same through the plane a b.

B, the tentacular region is extended into a hollow disc.

C, the tentacular region has been further extended and bent into a bell-like form,

CHAP, v MEDUSA 317

the enteric cavity being continued into the umbrella (ent. cav'} ; the hypostome
now forms a manubrium (innU).

C', transverse section of the same through the plane ab, showing the continuous
cavity (ent. cav' ) in the umbrella.

D, fully formed medusa ; the cavity in the umbrella is reduced to the radial
(rod) and circular (dr. c) canals, the velum [(v) is formed, and a double nerve-
ring («£>, nv') is produced from the ectoderm.

D', transverse section of the same through the plane «<5, showing the four radial
canals (racf) united by the endoderm-lamella (end. lani\ produced by partial
obliteration of the continuous cavity (ent. cav') in C. (From Parker's Biology.)

there is no extension of endoderm into it. The tentacles,
like those of the polype, are formed of a core of endoderm
covered by ectoderm, which encloses numerous stinging-
capsules.

At first sight there appears to be very little resemblance
between a medusa and a hydranth, but it is really quite
easy to derive the one form from the other.

Suppose a simple polype or Hydra-like body with four
tentacles (Fig. 80, A, A') to have the region from which the
tentacles spring pulled out so as to form a hollow, trans-
versely extended disc (B). Next, suppose this disc to become
bent into the form of a cup with its concavity towards the
hypostome, and to undergo a great thickening of its meso-
glcea. A form would be produced like c, t.e., a medusa-like
body with umbrella and manubrium, but with a continuous
cavity (c', ent. cav'] in the thickness of the umbrella instead
of four radial canals. Finally, suppose the inner and outer
walls of this cavity to grow towards one another and meet,
thus obliterating the cavity, except along four narrow radial
areas (D, rad) and a circular area near the edge of the
umbrella (dr. c). This would result in the substitution
for the continuous cavity of four radial canals opening on
the one hand into a circular canal and on the other into
the cavity of the manubrium (ent. cav\ and connected with
one another by a membrane — the endoderm-lamella (D', end.
lam) — indicating the former extension of the cavity.

It follows from this that the inner and outer layers of the

3i8 OBELIA CHAP.

manubrium are respectively endoderm and ectoderm : that
the gelatinous tissue of the umbrella is an immensely
thickened mesogloea : that the layer of cells covering both
inner and outer surfaces of the umbrella is ectodermal : and
that the layer of cells lining the system of canals, together
with the endoderm-lamella, is endodermal.

Thus the medusa and the polype are similarly con-
structed or homologous structures (p. 217), and the hydroid
colony is dimorphic (p. 250), bearing zooids of two kinds.
In some allied forms, this individuation may go still further,
the zooids being of very various forms and performing
diverse functions : such a colony is said to be polymorphic.

Sooner or later the medusae separate from the hydroid
colony and begin a free existence. Under these circum-
stances the rhythmical contraction — /'.£., contraction taking
place at regular intervals — of the muscles of the umbrella
causes an alternate contraction and expansion of the whole
organ, so that water is alternately pumped out of and drawn
into it. The obvious result of this is that the medusa is
propelled through the water by a series of jerks. The
movement is due to the contraction of the muscle-processes
and muscle-fibres of the sub-umbrella and velum, some of
which differ from the similar structures in the polype in
exhibiting a delicate transverse striation.

There is still another important matter in the structure of
the medusa which has not been referred to. At the junction
of the velum with the edge of the umbrella there lies, imme-
diately beneath the ectoderm, a layer of peculiar, branched
cells, containing large nuclei and produced into long fibre-
like processes. These nerve-cells (pp. 167 and 303) are so
disposed as to form a double ring round the margin of the
bell, one ring (Fig. 80, D, nv) being immediately above, the

v MEDUSAE 319

other (nv) immediately below the insertion of the velum. An
irregular network of similar cells and fibres occurs on the
inner or concave face of the bell, between the ectoderm and
the layer of muscle-fibres. The whole constitutes the
nervous system of the medusa ; the double nerve-ring is the
central, the network the peripheral nervous system (p. 155).

Some of the processes of the nerve-cells are connected
with ordinary ectoderm-cells, which thus as it were connect
the nervous system with the external world : others, in some
instances at least, are probably directly connected with
muscle-fibres.

We thus see that while the manubrium of a medusa has
the same simple structure as a polype, the umbrella
has undergone a very remarkable differentiation of its
tissues. Its ordinary ectoderm cells, instead of being large
and eminently contractile, form little more than a thin
cellular skin or epithelium (p. 109) over the gelatinous meso-
glcea : they have largely given up the function of contractility
to the muscle-processes or fibres, and serve merely as a
protective and sensitive layer.

Similarly the function of automatism, possessed by the
whole body of Hydra, is made over to the group of specially
modified ectodermal cells which constitute the central
nervous system. If a Hydra is cut into any number of
pieces each of them is able to perform the ordinary move-
ments of expansion and contraction, but if the nerve-ring
of a medusa is removed by cutting away the edge of the
umbrella, the rhythmical swimming movements, stop dead :
the umbrella is in fact permanently paralysed.

It is not, however, rendered incapable of movement, for
a sharp pinch, i.e., an external stimulus, causes a single con-
traction, showing that the muscles still retain their irritability.
But no movement takes place without such external stimu-

320 OBELIA CHAP.

lus, each stimulus giving rise infallibly to one single con-
traction : the power possessed by the entire animal of
independently originating movement, i.e., of supplying its
own stimuli, is lost with the central nervous system (compare
p. 172).

Another instance of morphological and physiological
differentiation is furnished by the marginal sense-organs
situated at the bases of the tentacles (p. 315).

The polype and medusa are respectively nutritive and
reproductive in function, the reproductive zooids becoming
detached and swimming off to found a new colony else-
where : the polypes are purely nutritive zooids ; the me-
dusae, although capable of feeding, are specially distinguished
as reproductive zooids. Hanging at equal distances from
the sub-umbrella, in immediate relation with the radial canal,
are four ovoid gonads (Fig. 79, gon), each consisting of an
outer layer of ectoderm continuous with that of the sub-
umbrella, an inner layer of endoderm continuous with that
of the radial canal and enclosing a prolongation of the latter,
and of an intermediate mass of cells which have become
differentiated into ova or sperms. As each medusa bears
organs of one sex only (spermaries or ovaries, as the case
may be), the individual medusae are dioecious, and not, like
Hydra, monoecious. It will be noticed that the gonad has
the same general structure as an immature zooid — an out-
pushing of the body-wall consisting of ectoderm and endo-
derm, and containing a prolongation of the enteric cavity.

The medusae, when mature, become detached and swim
away from the hydroid colony. The sperms of the males
are shed into the water and carried to the ovaries of the
females, where they fertilise the ova, converting them, as
usual, into oosperms.

The oosperm undergoes segmentation, forming a poly-

v ALTERNATION OF GENERATIONS 321

plast or morula (p. 200) : ectoderm and endoderm become
differentiated, and the ectoderm cells acquire cilia, by means
of which the embryo now swims freely in the water. An
enteron appears in the endoderm, and in this stage
the embryo, which has an elongated form, is known as a
planula. It then loses its cilia and settles down on a rock,
shell, sea-weed, or other submarine object, assuming a
vertical position with its broader end fixed to the support.

The attached or proximal end widens into a disc of
attachment, a dilatation is formed a short distance from the
free or distal end, and a thin cuticle is secreted from the
whole surface of the ectoderm. From the dilated portion
short buds arise in a circle : these are the rudiments of the
tentacles : the narrow portion beyond their origin becomes
the hypostome. Soon the cuticle covering the distal end
is ruptured so as to set free the growing tentacles : an
aperture, the mouth, is formed at the end of the hypostome,
and the young hydroid has very much the appearance of a
Hydra with a broad disc of attachment, and with a cuticle
covering the greater part of the body. Extensive budding
next takes place, the result being the formation of the
ordinary hydroid colony.

Thus from the oosperm or impregnated egg-cell of the
medusa the hydroid colony arises, while the medusa is
produced by budding from the hydroid colony. We have
what is called an alternation of generations, the asexual genera-
tion or agamobium (hydroid colony) giving rise by budding
to the sexual generation or gamobium (medusa), which in its
turn produces the agamobium by a sexual process, i.e. by
the conjugation of ovum and sperm (compare p. 287).

Hydra and Obelia both belong to the simplest class
—the Hydrozoa — of the phylum Coelenterata : this phylum

PRACT. ZOOL. Y

322 CCELENTERATA CHAP.

includes all the polypes or zoophytes, the jelly-fishes, and
the anemones and corals. In all there is an ectoderm
and an endoderm, separated by a mesoglcea, which may con-
sist, as in Hydra, of a structureless membrane containing
no cells, or may be gelatinous as in the medusa, and may
even contain cells, thus assuming more the character of an
intermediate cell-layer or mesoderm. There is no body-
cavity or ccelome (p. 20) surrounding the digestive cavity
or enteron, and tentacles are present round the mouth.
Organs of offence occur in the form of thread- cells or nema-
tocysts.

In all the higher phyla a definite mesoderm is developed
in the embryo in addition to the ectoderm and endoderm
(triploblastic condition), and in nearly all cases there is a
definite cavity or ccelome present in the mesoderm : hence
all these animals are often included together as the
Ccelomata.

PRACTICAL DIRECTIONS.

Hydra.

Examine some living Hydrae in a vessel of water, with the naked eye
or with a pocket lens, and note the differences in form according to
the degree of contraction. The animal is usually attached to foreign
bodies (weeds, &c.), at one end, and at the other end a number of tenta-
cles (usually six to eight) are given off. In the expanded state the body and
tentacles are greatly elongated and thread-like, while when contracted
the body is more globular, and the tentacles appear like small knobs.
Note the brown colour in H. fusca, and the green colour in H. viridis.
Observe the method of seizing food. Place a specimen on a slide
in a drop of water, together with a small piece of water-weed or
paper to prevent crushing, and then put on a cover-glass. Wait till
the animal is fully expanded, and then examine with the low power.
Note (Fig. 75) : —

i. The body., enclosing the digestive cavity or enteron, which opens

v PRACTICAL DIRECTIONS 323

by the mouth at the distal end of the animal, at the summit of a conical
hypos tome. At the proximal end is the/w/, or disc of attachment.

2. The tentacles, arranged in a single circlet or whorl around the
base of the hypostome. They are hollow, and their cavities communi-
cate proximally with the general digestive cavity of the body. On their
surface are a number of small knobs.

3. The contractions of the animal, and of its tentacles.

4. The structure of the body -wall, which is made up of (a) an outer
layer of colourless cells (ectoderm} ; and (b] an inner layer (brown in
H. fusca and green in H. viridis] of cells (endoderm} lining the diges-
tive cavity. Between these two layers is a thin gelatinus non-cellular
supporting lamella or mesoglcea, not easily seen with the low power.
(The tentacles have a similar structure, the details of which cannot be
made out with the low power. ) Sketch.

Put on the high power and examine a tentacle, focussing on to the
surface as well as deeper, so as to get an optical section (Fig. 76, A).
Note :—

5. The relations of the ectoderm, endoderm, and supporting lamella,
and the nuclei of the ectoderm and endoderm cells.

6. The structure of the ectoderm : — (a) large conical cells with their
broader ends outwards, arranged in a single row, and differing in form
according to the state of contraction. The spaces between the inner
narrower ends of these are filled up with (b) smaller rounded interstitial
cells (absent on the foot) ; (c) thread-cells or nematocysts (Fig. 77) — oval
capsules containing a spirally-wound thread, developed within certain
of the interstitial cells called cnidoblasts, and when fully formed, found
imbedded in or between the large ectoderm- cells : they are much more
numerous on the tentacles than on the body, causing the knobs referred
to above. Each cnidoblast gives rise to a small process — the trigger-
hair or cnidocil, which projects from the surface. Notice the discharged
thread-cells, and observe that each consists of a flask-like base (to which
part of the protoplasm and the nucleus of the burst cnidoblast usually
remains attached) and a long filament, with three large and several
smaller spines or barbs at its proximal end. (Smaller thread-cells,
with thicker threads and no spines, are also present ; some of these
have long, spirally coiled threads, others shorter, straight threads. These
can be seen better later on. )

7. The endoderm, consisting of a single layer of large amoeboid cells,
which in H. viridis contain green Zoochlorellce (p. 304). Note the

\ 2

324 HYDRA CHAP.

currents in the tentacles, which are produced by long vibratile flagella
present on many of the endoderm-cells.

8. The thin transparent supporting lamella. Sketch.

9. Treat a specimen with methyl-green. A slight pressure on the
cover-glass will crush the animal, and render the interstitial cells and
thread-cells especially distinct. Note also other isolated cells of the
ectoderm and endoderm. Sketch.

10. Examine a specimen with buds in different stages of development,
and note as much as possible of the mode of asexual reproduction by
gemmation. Sketch.

11. If none of your specimens bears sexual organs, try to procure a
mounted preparation which shows them, and examine first with the
low, and then the high power. Note — (a) The spermaries — several
conical swellings, near the bases of the tentacles. They are covered
with large ectoderm cells, and contain numerous interstitial cells, each
of which eventually gives rise to a sperm with a "head" and vibratile
"tail." These are discharged at the apex of the cone, which when
ripe may be ruptured by a slight pressure on the cover-glass, (b] The
ovaries (sometimes only one), generally situated near the proximal end
of the body. They are larger than the spermaries and more spherical,
but at first have a similar structure. When ripe a single ovum is found
in each. Sketch.

Place some Hydne in a watch-glass with a very small amount of
water, and when they have expanded, pour quickly over them a warm
saturated solution of corrosive sublimate in alcohol or water. Wash
several times with weak alcohol, stain for a few minutes with borax-
carmine, or haematoxylin, and wash with weak and then with stronger
alcohol. Place in absolute alcohol for a few minutes, and afterwards in
turpentine or oil of cloves ; mount in balsam. Work through §§ 5-8
again, noting especially the characters of the various cells and their
nuclei, as wrell as —

12. The contractile processes coming off from the inner ends of the
large ectoderm-cells (Fig. 76, B). These extend longitudinally, and lie
against the outer surface of the supporting lamella. Sketch.

Examine transverse sections through the body or tentacles (Fig. 76, B)
prepared as directed on p. 136, after killing and fixing the specimens as
above. Work through §§ 6-8 again, noting the various cells and their
nuclei, &c. Observe especially —

v PRACTICAL DIRECTIONS 325

13. (a) The contractile processes of the ectoderm- cells, which will be cut
across transversely, so as to appear as dots just outside the supporting
lamella ; (b} the amoeboid 2a\& vacuolated character of the endoderm-co\\.s.
(Special methods of preparation are necessary in order to show the
flagella.) Sketch.

Obelia.1

If possible, examine first alive, and then kill and stain as directed in
the case of Hydra.

1. Examine under the low power and note : —

(a) The polypes, with their tentacles and hypostome, expanded and
contracted ; and the immature polypes, (b) The blastostyles and me-
dusa-buds, (c} The ccenosarc and the perisarc, hydrothecce , and gono-
thectz (Fig. 78). Sketch.

2. Then stain, put on the high power and make out the minute
structure of the polypes, noting the —

(a) Mouth, (b} enteron, (c) ectoderm, supporting lamella, and endo-
derm (solid in the tentacles). In the blasto-styles examine the medusa-
buds '. (If you wish to make permanent preparations, mounted in
balsam, use the method given for Hydra).

Sketch an optical section of a polype and blastostyle.

Sections of an entire branch, prepared in the usual way (see p. 324)
should also be made. Select for examination those which pass as nearly
as possible through the vertical and transverse axes of a polype, and
compare with your sections of Hydra.

3. Place a medusa (Figs. 78 B — D, and 79), on a slide with the sub-
umbrella surface uppermost, stain, and mount carefully in glycerine.
Note —

(a) The umbrella, (b} manubriuni and mouth, (c) tentacles, (c] radial
and circular canals, (d} velum, (e) gonads, and (f) lithocysts (often
difficult to recognise in preserved specimens). Sketch.

1 Specimens living or preserved, both of the colonial and medusa
stage of Obelia or some other Hydroid (as well as other marine animals
described in this book) can be obtained from any Marine Biological
Laboratory ; or the fresh-water Cordylophora will answer the purpose as
far as the colony is concerned, but it has no medusa-stage.

CHAPTER VI

THE EARTHWORM I NEREIS CHARACTERS OF THE PHYLUM

ANNULATA

THE general form and appearance of an earthworm are
familiar to everyone. In this country there are a number
of different species of earthworms belonging to several
genera, the commonest of which are Lumbricus and Allo-
lobophora • but the differences between these are of minor
importance to the beginner, and any one of the common
forms will serve our purpose.

Earthworms burrow into the soil and live on decaying
leaves and other organic matter, which they swallow to-
gether with a considerable quantity of earth. This earth,
mingled with the undigested portions of the food, is
passed from the body on to the surface of the ground in the
form of the well-known little heaps or " castings " which you
must have noticed in gardens and fields, especially after
rain, when the worms come more frequently to the surface.
In this way, a quantity of finely divided earth, mixed with
the faeces of the worms, is constantly being spread
out on the surface of the soil, and Darwin calculated
that on an average a layer of earth about one-
fifth inch in thickness, or about ten tons an acre, is thus
brought to the surface in the course of a year. Earth-
worms are therefore good friends to the gardener and

CHAP, vi THE EARTHWORM 327

agriculturist, as they are continually ploughing and
manuring the soil, and in doing so, they gradually cover up
stones and other objects lying on the surface.

The body of the earthworm is long and narrow, ap-
proximately cylindrical in shape, and bilaterally symmetrical
(p. 296) : in the common forms it reaches a length of
about six inches. Anteriorly it is bluntly pointed, while
more posteriorly it is somewhat flattened, its greatest
diameter being reached at about a third of the entire
length from the anterior end. In the ordinary creeping
movements of the animal, which are effected by the
alternate contraction and extension of its body, the an-
terior end is directed forwards. The colour is pinkish in
most species, and is paler on the lower or ventral than
on the upper or dorsal side.

The surface of the body is distinctly marked by trans-
verse annular grooves into body-segments or metameres
.(Fig. 82), the number of which is about 150, more or less :
the segments are rather longer towards the anterior end
than they are further back. At the extreme anterior end
is a small finger-shaped head-lobe or prostomivm, which
overhangs the mouth, situated on the antero-ventral surface
of the next segment, which is therefore called the peri-
stomium, and is counted as the first metamere. The anus is
a slit-like aperture on the hinder surface of the last or
anal segment. The earthworm is thus a metamerically seg-
mented animal, and the segments are serially homologous
with one another (p. 39).

In adult worms a prominent glandular swelling is
noticeable on the dorsal and lateral surfaces of the body,
extending through from five to ten segments beginning at
about the thirtieth ; this is known as the ditellum, and,
as we shall see, it is important in the process of im-

328 THE EARTHWORM CHAP.

pregnation and in forming a case or cocoon for the eggs.
On the ventral part of this region are some small
glandular swellings, which are more conspicuous in young
worms before the clitellum is developed.

The whole of the body is invested with a delicate, iri-
descent membrane or cuticle (p. 313), formed as a secretion
of the epiderm or outer epithelial layer of the body (p. 128).
Every segment, except the first and the last, is provided with
eight small cuticular spines or seta (Fig. 81, set) — slightly
curved bodies with tapering ends, composed of a horn-like
substance called chitin (compare p. 232) — each of which is
developed in a small sac formed as an involution of the
epiderm and is provided with muscles by means of which
it can be protruded and retracted. These setae are arranged
in couples, forming two double rows along each latero-
ventral region of the body, and their points can be distinctly
felt on drawing the worm through the fingers : they serve to
prevent the animal from slipping backwards as it moves,
along on the surface of the ground or in its burrows.

We have seen that the earthworm takes in its food,
together with quantities of earth, by the mouth, and after
retaining it for a longer or shorter time in the body
expels it by the anus. It is obvious, therefore, that there
must be some kind of digestive cavity into which the
food passes by the mouth, and from which effete matters
are expelled through the anus. Sections (Figs. 81) show
that this cavity is not a mere space excavated in the interior
of the body, but a definite tube, the enteric or alimentary
canal (p. 23), which passes in a straight line from mouth to
anus, and is separate in its whole extent from the walls of
the body by a wide space, the body-cavity or ccelome (cod), as
in the frog (p. 20). So that the general structure of the
earthworm might be imitated by taking a wide tube,

vi TRANSVERSE SECTION 329

stopping the ends of it with corks, boring a hole in each
cork, and then inserting through the holes a narrow tube of
the same length as the wide one. The outer tube would
represent the body-wall, the inner the enteric canal, and the
cylindrical space between the two the ccelome. The inner
tube would communicate with the exterior by each of its
ends, representing respectively mouth and anus ; the space
between the two tubes, on the other hand, would have no
such communication with the outside. A transverse section
of the body has, therefore, the general character of two
concentric circles.

It will be remembered that a transverse section of Hydra
has the character of two concentric circles, formed re-
spectively of ectoderm and endoderm (Fig. 76, p. 298), the
two layers being, however, only separated by the thin
mesoglcea. At first sight then, it seems as if we might
compare the earthworm to a Hydra in which the ecto-
derm and endoderm, instead of being in contact, were
separated by a wide interval ; we should then compare
the body- wall of the earthworm with the ectoderm of Hydra,
and its enteric canal with the endoderm. But this com-
parison would only express part of the truth.

A thin transverse section (Fig. 81) shows the body-wall of
the earthworm to consist of several distinct layers. Outside is
a thin transparent cuticle (cut) showing no structure beyond
a series of intersecting oblique lines. Next comes a layer
of epithelium, the epiderm or deric epithelium (epid). Within
this is a very thin connective-tissue layer representing the
derm (p. 1 28), and a double layer of muscle-fibres by means of
which the movements of the body are produced — an outer,
in which the fibres extend transversely round the body
(circ. mus\ and a much thicker inner layer consisting of
longitudinal fibres, in section arranged like the barbs of a

330

THE EARTHWORM

feather on a central axis (long. mus). Finally, within the
muscular layer and lining the coelome is a thin peritoneal
membrane (parietal layer, compare p. 26), on the inner

dors, v
typh

neph

ext.neph

neph rost

n.co

set

sub.n.wss

FIG. 81. — Lugubrious, transverse section of the middle region of the'-body.
fire. mns. layer of circular muscular fibres ; ccel. coelome ;jcut. cuticle ; dors. v.
dorsal vessel ; epid. epiderm ; e.rt. neph. nephridropore \+\hep. layer of yellow
cells ; long. mus. longitudinal muscles ; neph. nephridium (shown entire) ;
ncphrost. nephrostome ; n. co. nerve cord ; set. setae ; sub. n. vess. sub-neural
vessel ; typh. typhlosole ; vent. v. ventral (sub-intestinal) vessel. (From Parker
and Haswell's Zoology ', after Marshall and Hurst.)

surface of which is a very thin layer of cells, the cozlomic
epithelium.

A transverse section of the intestine shows an inner
layer of ciliated, columnar enteric epithelium (compare p.
109) , a thin middle layer composed of muscle-fibres and

vi TRANSVERSE SECTION 331

connective- tissue, and an outer layer of large yellow cells,
the function of which is said to be excretory, and which
correspond to a special development of the ccelomic epithe-
lium covering the visceral layer of the peritoneal membrane
which invests the intestine.

We are now in a better position to compare the trans-
verse section of the Hydra and the earthworm. The
epiderm of the earthworm being the outermost cell-layer
is to be compared with the ectoderm of Hydra, and its
cuticle with the layer of the same name which, though
absent in Hydra, is present in the stem of hydroid polypes,
such as Obelia (viz., the perisarc). The enteric epithelium
of the earthworm, bounding as it does the digestive
cavity, is clearly comparable with the endoderm of
Hydra. So that we have the double layer of muscle-
fibres and the two layers of peritoneum not represented
in Hydra, in which their position is occupied merely
by the mesoglcea. The muscle-fibres are not of the striped
kind, like those in the corresponding position in the" frog
(p. 112).

But it will be remembered that in Medusae there is some-
times found a layer of separate muscle-fibres between the
ectoderm and the mesoglcea, and it was pointed out
(p. 311) that such fibres represented a rudimentary inter-
mediate cell-layer or mesoderm. We may therefore con-
sider the muscular layer and the peritoneum of the
earthworm as mesoderm, and we may say that in this
animal, as in the frog (p. 203 and Fig. 65), the mesoderm
is divisible into an outer or parietal layer, and an inner or
visceral layer.

The parietal layer is in contact with the ectoderm or deric
epithelium, and with it forms the body- wall; the visceral
layer is in contact with the endoderm or enteric epithelium,

332

THE EARTHWORM

and with it forms the enteric canal. The ccelome separates
the parietal and visceral layers from one another, and is
lined throughout by ccelomic epithelium.

The relation between the diploblastic polype and the
triploblastic worm may therefore be expressed in a tabular
form as follows —

Earthworm .l
Cuticle.

Deric epithelium or epiderm.
Connective-tissue and muscle-

fibres.

Peritoneum with its ccelomic
epithelium (parietal layer).
r Peritoneum with its ccelomic
epithelium (visceral layer).
Connective-tissue and muscle-

fibres.
Endoderm ...... Enteric epithelium.

Hydroid.
Cuticle
Ectoderm

Mesoderm .
(rudimentary)

Parietal
layer

Visceral
layer

Strictly speaking this comparison does not hold good of
the anterior and posterior ends of the worm : at both mouth
and anus the deric passes insensibly into the enteric epithe-
lium, and the study of development shows that the cells
lining both the anterior and posterior ends of the canal are
ectodermal (compare pp. 204 and 207). For this reason the
terms deric and enteric epithelium are not mere synonyms
of ectoderm and endoderm respectively.

It is important that you should, before reading further,
understand clearly the general composition of a tripoblastic

1 It will be seen that the relations of these layers in the earthworm
and frog are similar, except that in the latter the cuticle is wanting
(compare Figs. 5, 38, and 39).

vi CCELOME 333

animal as typified by the earthworm, which may be sum-
marised as follows. It consists of two tubes formed of
epithelial cells, one within and parallel to the other, the
two being continuous at either end of the body, where the
inner tube (enteric epithelium) is in free communication
with the exterior ; the outer tube (deric epithelium) is lined
by a layer of connective-tissue and muscle-fibres, within
which is a thin peritoneum lined by ccelomic epithelium, the
three together forming the body- wall ; the inner tube
(enteric epithelium) is covered externally by a layer of
muscle-fibres and connective-tissue and a thin peritoneum
covered by ccelomic epithelium, which form with it the
enteric canal ; lastly, the body-wall and enteric canal are
separated by a considerable space, the ccelome.

The enteric canal is not, as might be supposed from the
foregoing description, connected with the body- wall only at
the mouth and anus, but is .supported in a peculiar way.
There is no dorsal mesentery as in the frog (p. 27), but a
series of transverse vertical partitions or septa (Figs. 82 and 83)
extend right across the body-cavity, each being perforated
by the canal. The septa are regularly arranged and cor-
respond in position with the external grooves by which
the body is divided into metameres. Thus the trans-
verse or metameric segmentation affects the ccelome as
well as the body-wall, the former being divided up into a
series of chambers, which, however, communicate with one
another ventrally, where the septa are incomplete (Fig. 83,
n.a). Each septum is composed of a sheet of connective-
tissue and muscle-fibres, and is covered on both sides by
coelomic epithelium. The ccelome communicates with the
exterior by a series of dorsal pores situated in the grooves
between all the segments except about the first ten.

•SHS

CHAP, vi ENTERIC . CANAL 335

The digestive canal is not a simple tube of even calibre
throughout, but is divisible into several portions. The
mouth is bounded by a soft lip and leads into a small
buccal cavity, which communicates with a thick-walled
pharynx (Figs. 82 and 83, ph\ extending through about five
segments and connected with the body-wall by a number of
radially arranged muscle-fibres, the septa being absent in
this region. When the worm feeds, the buccal cavity is
everted, and the muscles serve to draw it and the pharynx
back again, as well as to dilate the pharynx. The latter is
followed by a narrow gullet or oesophagus (oes, #>), extend-
ing through about eight segments, and provided at about
the middle of its course with a pair of lateral pouches (Fig.
83, (z.g), with each of which, in Lumbricus, two yellowish
cesophageal or calciferous glands communicate posteriorly :
these contain a calcareous substance which may neu-
tralise the organic acids present in the food swallowed.
The pouches open into the gullet (Fig. 82, oes. gt), which
pass posteriorly into a dilated, thin-walled receptacle, the
crop (cr\ and this, again, communicates posteriorly with a
large gizzard (giz) with thick and muscular walls, which in
about the 2oth segment communicates with the intestine
(/*/). The intestine has a similar character throughout, and
extends from the gizzard to the anus : its dorsal wall is
folded inwards so as to produce a longitudinal ridge or
typhlosole (Fig. 81, typK), which serves to increase the
absorptive surface and in the interior of which the yellow
cells are very numerous.

Certain of the cells lining the enteric canal, and especially
those along the typhlosole, are very granular, and like the
endoderm cells of the hypostome of Hydra (p. 303), are
to be considered as unicellular glands. They secrete a
digestive juice which — mixing with the various substances

336 THE EARTHWORM CHAP.

containing organic acids taken in by the mouth and said to
be neutralised by the calcareous secretion of the cesophageal
glands — dissolves the proteids and other digestible parts so
as to allow of their absorption. It is very probable that
the process is purely extra-cellular or enteric, the food being
dissolved and rendered diffusible entirely in the cavity of the
canal (p. 305). By the movements of the canal — caused
partly by the general movements of the body and partly
by the contraction of the muscles of the canal and septa,
aided by the action of the cilia — the contents are gradually
forced backwards, and the earth and other indigestible
matters expelled at the anus.

The ccelome is filled with a colourless transparent ccelomic
fluid in which are suspended amoeboid corpuscles or leuco-
cytes like those of the frog's blood and lymph (p. 105). The
function of this ccelomic fluid is probably to distribute the
digested food in the enteric canal to' all parts of the
body. In Hydra, where the lining wall of the digestive
cavity is in direct contact with the simple wall of the
body, the products of digestion can pass at once by
diffusion from endoderm to ectoderm ; but in the present
case a means of communication is wanted between the
enteric epithelium and the comparatively complex and
distant body-wall. The peptones and other products of
digestion diffuse through the enteric epithelium into the
ccelomic fluid, and by the continual movement of the
latter — due to the contractions of the body-wall — are
distributed to all parts. Thus the external epithelium
and the muscles, as well as the nervous system, repro-
ductive and other organs not yet described, are wholly de-
pendent upon the enteric epithelium for their supply of
nutriment.

vi VASCULAR SYSTEM 337

The earthworm, like the frog, possesses a series of blood-
vessels, containing red blood, the, whole of which form a
single closed vascular system, there being no com-
munication between them and any of the other cavities of
the body. The main trunks have a longitudinal direction,
the chief ones being a large dorsal vessel, running along the
dorsal surface of the enteric canal, and a ventral or sub-
intestinal vessel, below the canal (Figs. 81, dors, v, vent, v,
and 83). In addition to these there are three smaller longi-
tudinal trunks in relation to the nerve-cord, which, as we shall
see, extends along the ventral side of the ccelome : these are a
median subneural and two lateral neural vessels (Figs. 81 and
83). All these longitudinal trunks give off branches to the
various parts of the body, and certain of them are connected
with one another by a series of pairs of lateral commissural
vessels : in the region of the gullet there are five (Lumbricus)
or six (Allolobophora) pairs of large vessels connecting the
dorsal and subintestinal trunks ; and the dorsal .and sub-
neural trunks are also connected in each segment all along
the body by a pair of smaller commissural vessels, running
in the inner surface of the body-wall (Fig. 83).

Notice that there is here no distinction into arteries and
veins, as in the frog (p. 27), and also that there is no heart.
The vessels gradually divide up into smaller and smaller
branches in the various parts of the body, and then again
unite to form larger and larger vessels which eventually
open into one or other of the main trunks.

The circulation of the blood is effected by the rhythmical,
peristaltic contraction (p. 75) of certain of the larger
vessels : thus the dorsal trunk contracts from behind for-
wards, and the large commissural vesselsy^often spoken of as
" hearts "—which connect it anteriorly with the subintestinal
trunkj from above downwards, so that the blood passes for-

PKACT. ZOOL. Z

338

THE EARTHWORM

wards in the dorsal, backwards in the ventral vessel. The
series of vessels of the enteric canal are connected with the

1.3

FIG. 83. — Blood-vessels of A llolobophora. Dissections from the left side of A, the
twelve anterior segments, and B, three segments in the intestinal region.
The arrows indicate the course of the circulation. (From Howes's Atlas.'}

be, buccal sac ; c.l, nerve-cord ; cm, commissural vessels connecting the dorsal and
subneural vessels; cm', neural commissural vessels; c.m, mesentery, and ex,
vessel of nephridium ; g.c, cerebral ganglion ; g.n, ganglionic swelling of nerve-
cord ; h, commissural vessels (" hearts") connecting the dorsal and subintestinal
vessels anteriorly ; /'./, lateral intestinal vessels ; z, intestine ; i.s, dorsal (supra-
intestinal) vessel ; m.s, septum ; n.a, ventral perforation of septum ; n.l, lateral
neural vessel ; n.s, subintestinal vessel ; n.s' , subneural vessel ; as, gullet ; ce.g,
oesophageal pouch ; ce.l, lateral cesophageal vessel \ph, pharynx ; sg, nephridium.

dorsal and sub-intestinal trunks, and those of the excretory
organs, to be described presently, with the sub-intestinal

vi RESPIRATION AND EXCRETION 339

trunk and the commissural vessels in the body-wall which
connect the dorsal and subneural trunks. By means of
branches of these parietal vessels the body-wall is plentifully
supplied with blood.

The red colour of the blood is due to hemoglobin (p.
107), wrhich is not, as in the frog, contained in red blood-
corpuscles, but is dissolved in the plasma, in which, how-
ever, minute colourless corpuscles can be recognised. The
function of haemoglobin in the process of respiration has al-
ready been described (p. 144) ; but in the earthworm, as in
many other lower animals, there are no specialised res-
piratory organs (lungs or gills), the necessary exchange of
gases being performed by the entire surface of the body, the
minute branches of the blood-vessels in the body-wall being
only separated from the air by the single layer of epidermic
cells — and even penetrating amongst the latter in the region
of the clitellum : this is an exceptional occurrence, for as
we have seen, capillaries do not, as a general rule, extend
amongst epithelial cells (compare, e.g., Figs. 38 — 40).

In discussing in a previous chapter the differences between
plants and animals (p. 255), we found that in the unicellular
organisms previously studied the presence of an excretory
organ in the form of a contractile vacuole was a characteristic
feature of such undoubted animals as Amceba and many
other Protozoa. But the reader will have noticed that Hydra
and its allies have no specialised excretory organ, waste-
products being apparently discharged from any part of the
surface. In the earthworm we meet once more with an
animal in which excretory-organs are present, although,
in correspondence with the complexity of the animal itself,
they are very different from the simple contractile vacuoles of
Paramcecium or Vorticella, and are more nearly comparable
with those of the frog (p. 146).

z 2

340

THE EARTHWORM

CHAP.

The excretory organs of the earthworm consist of

little tubes called nephridia,
of which each metamere—
except the first three and
the last — possesses a pair,
one on either side (Figs.
81-84). You will remem-
ber that in the frog all
the urinary tubules are
connected together to form
a pair of kidneys, each with
a single duct communi-
cating with the cloaca.
In the earthworm each
nephridium is a long and
extremely delicate tube,
arranged in three main
loops (Fig. 84), opening at
one end into the ccelome
by a nephrostome and at
the other communicating
with the exterior directly
(Fig. Si).1 The tubes are
attached to the posterior
faces of the septa. Each
nephrostome (a) is ciliated,
and projects through the
corresponding septum so
as to communicate with
the segment of the body-
which the main part of

FIG. 84. — A nephridium of Lumbricus,
showing the three main loops into which
the different parts of the tubule are ar-
ranged, as well as the different portions
of the tubule.

«. nephrostome ; b. b^b. slender portion
of the tubule into which the nephrostome
opens ; c. c. second ciliated portion ; d.
glandular portion ; e. muscular portion ;
e'. end of e at which the nephridiopore
opens. (From Gegenbaur.)

cavity next in front of that in

1 In the frog the nephrostomes lose their connection with the ne-
phriclia, and open in the adult into the renal veins (Fig. 47, p. 146).

vi NEPHRIDIA 341

the tubule is situated. The nephrostome opens into a
long and slender, transparent part of the tube, lined with
ciliated cells in part of its course and extending along the
first and second loops (b) • this part is succeeded by a
wider, ciliated portion in the second loop (<:), which com-
municates with a still wider portion (d) lined by granular,
non-ciliated, glandular cells, also lying in the second
loop ; the glandular portion opens into a much wider
muscular part of the tube (e), which constitutes the third
loop and communicates with the exterior by a small pore
— the nephridiopore — near the outer seta of the inner couple.
The muscular part is lined by an epithelium, while the rest
of the nephridium is formed of a single row of hollowed
cells, set end to end, like a series of drain-pipes, so that their
cavity is intraedlular, and not intercellular.

Thus the nephridia, which are abundantly supplied with
blood-vessels, are lined in part by gland-cells and in part
by cilia which work towards the exterior. Water and
nitrogenous waste from all parts of the body pass by diffu-
sion into the blood and are conveyed to the nephridia,
the gland-cells of which withdraw the waste products and
pass them into the cavities of the tubes, whence they are
finally discharged from the body. The granular yellow
ccelomic cells on the wall of the intestine also appear to
contain excretory products,1 which become set free in the
body-cavity and are thence got rid of by means of the
nephridia. It will be noticed that a certain amount of loss
of the ccelomic fluid must take place through the dorsal
pores as well as through the nephridia.

In discussing the hydroid polypes we found that one of
the most important points of difference between the loco-

1 It is probable that in primitive forms the whole ccelomic epithelium
was excretory in function.

342

THE EARTHWORM

motive medusa and the fixed polype was the presence in
the former of a well-developed nervous system (p. 319)
consisting of an arrangement of peculiarly modified cells,
to which automatic action was seen to be due. It is
natural to expect in such an active and otherwise
highly-organised animal as the earthworm a nervous sys-
tem of a considerably
higher degree of com-
plexity than that of a
medusa.

The central ner-
vous system (Figs.
82, 83, and 85) con-
sists of two parts,
the brain and the
ventral nerve - cord.
The brain consists of
a pair of white pear-
shaped swellings or
ganglia situated on
the dorsal side of
the buccal sac where
it is continued into
the pharynx. The
ventral nerve - cord
is a longitudinal band
extending along the whole middle ventral line of the
body, internally to the longitudinal muscular layer, from
the third to the anal segment, and slightly swollen in each
segment. The brain is connected with the anterior end of
the ventral nerve-cord by a pair of nervous bands, the
cesophageal connectives, which pass respectively right and left
of the buccal sac, and thus form a nerve-collar.

FIG. 85. — Anterior portion of nervous system of
Lumbricifs.

cer.gang. cerebral ganglia or brain ; com. oeso-
phageal connectives ; ne. co. ventral nerve-cord ;
prost. prostomium. (From Parker and Has-
well's Zoology, after Leuckart.)

vi NERVOUS SYSTEM 343

It is to be noted that one division of the central nervous
system — the brain — lies altogether above and in front of the
enteric canal, the other division — the ventral nerve-cord —
altogether beneath it ; and that, in virtue of the union of the
two divisions by the cesophageal connectives, the enteric
canal perforates the nervous system. Both brain and cord
are composed of delicate nerve-fibres and nerve-cells, the
latter being situated in the ventral and lateral regions
of the cord along its whole length, so that there is
here hardly any distinction into ganglia and connectives,
although the swellings are often spoken of as ganglia.
Along the dorsal side of the cord are three transparent
tube-like structures, known as giant-fibres, the function of
which is not known (Fig. 81). The whole cord is enclosed
in a sheath consisting of connective-tissue and muscular
fibres.

The peripheral nervous system consists of a number of
nerves, both sensory and motor (p. 162), which arise from
the central nervous system and supply the various parts of the
body. From the brain a number of nerves are given off
to the prostomium, and from each ganglionic enlarge-
ment two pairs of nerves can be traced into the body-
wall, while between these enlargements one pair is given
off which supply mainly the septa.

Comparing the nervous system of the earthworm with
that of a medusa it is important to notice the con-
centration of the central nervous system in the higher type,
and the special concentration at the anterior end of the
body to form a brain. When, again, we compare the central
nervous system of the earthworm with that of the frog (pp. 28
and 155) several important points of difference are noticeable.
In the former it lies freely in the ccelome, and, with the
exception of the brain, is situated on the ventral side of the

344 THE EARTHWORM CHAP.

body ; while in the frog it is enclosed in a neural canal
and is dorsal in position. The brain of the frog is a
complicated structure, and the whole nervous system is
hollow, there being ventricles in the brain and a central
canal in the spinal cord : while in the earthworm the brain
consists merely of a pair of cerebral ganglia, and it and
the ventral cord are solid.

The whole nervous system is capable of originating auto-
matic action. It is a well-known fact that if the body of
an earthworm is cut into several pieces each performs in-
dependent movements ; in other words, the whole body is
not, as in the higher animals, paralysed by removal of the
brain (p. 172). There can, however, be little doubt that
complete co-ordination, i.e., the regulation of the various
movements to a common end, is lost when the brain is
removed.

The earthworm is devoid of organs of sight or hearing.
It exhibits sensitiveness to bright light, which may be due
to direct action on the central parts of the nervous system.
The sense of hearing appears to be absent : but a faculty
analogous to taste or smell, enabling the animal to dis-
tinguish between different kinds of food, is well developed.
Groups of narrow sensory cells in the epiderm, which are
most abundant on the prostomium and peristomium, have
probably to do with this faculty.

There are two matters of general importance in connection
with the structure of the earthworm to which special at-
tention must be drawn.

Notice in the first place how in this type, far more than
in Hydra, we have, as in the frog, certain definite parts
of the body set apart as organs (p. 30) for the performance
of particular functions : it is clear that differentiation of

vi REPRODUCTIVE ORGANS 345

structure and division of physiological labour play a far
more obvious and important part than in any of the
lower organisms described in the five previous chapters.

Notice in the second place the vastly greater complexity
of microscopic structure, the body being divisible into tissues
(p. 1 1 8) each clearly distinguishable from the rest. We have
epithelial tissue with its cuticle, muscular tissue, and nervous
tissue, as well as blood and- ccelomic fluid. One result of
this is, that, to a far greater extent than in Hydra, we can
study the morphology of the earthworm, as we have done
that of the frog, under two distinct heads : anatomy and
histology (p. 104).

Asexual reproduction does not take place normally in
the earthworm, but it frequently happens by accident that
a worm is cut into two or more parts. When this occurs,
each end is able to reproduce the missing portion : this
process is known as regeneration (compare p. 307).

The earthworm, like Hydra, is monoecious or herm-
aphrodite (p. 307), and besides the essential organs of sexual
reproduction — ovaries and spermaries — which are, as in the
frog, developed from certain parts of the ccelomic epi-
thelium, it possesses various accessory organs. The whole
reproductive apparatus is situated in segments 9-15.

The ovaries (Figs. 82 0v, and 86 o) are a pair of minute
bodies about i mm. in length, attached by a short stalk,
one on either side, to the posterior face of the septum
separating segments twelve and thirteen, not far from the
nerve-cord. The proximal end of each ovary, nearest the
stalk, is composed of a mass of undifferentiated cells of
germinal epithelium (compare Figs. 62 and 63) : nearer its
middle, certain of these are seen to increase in size so as
to be recognisable as young ova : while the distal end

346

THE EARTHWORM

contains the ripe ova, arranged in a single row, each en-
closed in a vitelline
membrane and con-
taining a large nucleus
and nucleolus and a
number of granules of
food-yolk (p. 195). The
eggs are discharged
into the ccelome and
are received into the
female gonoducts or
oviducts (Fig. 82 o.d,
and Fig. 86) — two
• short tubes, each with
a wide, ciliated mouth
placed opposite the
corresponding ovary.
The oviduct perforates
the next following sep-
tum (i.e., that between
segments thirteen and
fourteen) to open by
a minute aperture on
the fourteenth seg-
ment, near the ventral
couple of setae. Con-
nected with the mouth
of each oviduct is
a small egg-sac (Fig.
82, r. o, Fig. 86, e. s),
developed as an out-
growth from the same septum and extending back into
the cavity of segment 14.

15

FIG. 86. Diagrammatic longitudinal section of
part of a Lumbricus, showing segments
9 — 15 and the contained generative organs
of one side 1X3. In the body-wall the cuticle
is indicated by a clear space, the circular
muscles by irregular dots, the longitudinal
muscles by dotted longitudinal lines, and the
peritoneal membrane by a thin line.

e. s. egg-sac ; o. ovary ; sp. aperture of anterior
spermotheca — both spermothecae are indicated
by dotted lines ; sp. s. posterior sperm-sac, the
anterior and middle sacs are not lettered ;
ss. sperm-reservoir ; t. anterior spermary — the
posterior is not lettered ; ? . aperture of ovi-
duct ; <J . aperture of spermiduct. The ovi-
duct, spermiduct, and seminal funnels are
indicated by thick lines. (From the Cambridge
Natural History, after Hesse.)

vi REPRODUCTIVE ORGANS 347

Certain globular sacs called spermothecce (Fig. 86) also
belong to the female part of the reproductive apparatus.
Of these there are usually two pairs (sometimes more than
two in Allolobophora) situated in the ninth and tenth
segments, and opening to the exterior between the ninth
and tenth, and tenth and eleventh segments respectively ;
their function will bs mentioned presently.

The earthworm possesses two pairs of very minute sperm-
aries attached to the posterior face of the septum between the
ninth and tenth, and tenth and eleventh segments respectively
(Figs. 82 te, and 86, t\ They have a flattened form and
their free or distal ends are produced into finger-shaped
processes. Behind each spermary, and in the same seg-
ment, is a ciliated seminal funnel opening into the ccelome
and produced backwards through the septum next behind
into an efferent duct, the two ducts of either side communi-
cating with a main spermiduct or vas deferens. Each of these
extends backwards in the ventral body-wall to open by a
tumid lip on the fifteenth segment (Fig. 86), near the inner
couple of setae.

The most prominent portions of the reproductive appar-
atus are certain large whitish bodies — the sperm-sacs or
seminal vesicles (Figs. 82, ves. sem, and 86, sp. s\ which are very
apparent in the adult worm as soon as the ccelome is cut open
in this region. Of these there are three (Lumbricus) or four
(Allolobophora) pairs, situated in segments 9 — 12. They arise
as outgrowths of the septa, and communicate with the ccelome;
but in Lumbricus, the anterior pair and the two posterior pairs
respectively become joined across the middle line so as to
form two median sperm-reservoirs (med. ves. sem, and s.s),
each of which encloses a ccelomic cavity in which one pair of
spermaries and seminal funnels becomes enclosed.

The cells of which the spermaries are composed do not

348 THE EARTHWORM CHAP.

develop into sperms in the testes themselves, but pass into the
sperm-sacs, where they undergo division into rounded masses
of cells {gametoeytes, see p. 286) looking very much like a
egmenting oosperm in the polyplast stage. Each of these
products of division of the testicular cells becomes elongated,
and gradually takes on the form of a sperm with a rod-like
head and a vibratile tail (compare pp. 194 and 307). When
set free, the sperms pass into the spermiducts through the
ciliated funnels.

It is well known that many flowers (the reproductive
organs of higher plants) contain the generative cells of both
sexes, enclosed within the ovules and anthers respectively ;
yet in very many cases self-fertilisation does not occur owing
to contrivances of various kinds for its prevention. It has
been proved in numerous instances that cross-fertilisation —
i.e., the impregnation of the ovum in one individual by the
male cell of another — is of great importance in keeping up
the strength and vigour of the plant from generation to genera-
tion. The same is true amongst animals ; and though in
some monoecious forms, such as the Hydra, there is no special
arrangement for the prevention of self-impregnation if the
male and female gametes of the same individual ripen at
the same time, in others, such as the earthworm, the ova are
always fertilised by the sperms from another individual.

This is effected in the earthworm in the following way.
Two individuals, their anterior ends pointing in opposite
directions, become applied together by their ventral sur-
faces and attached to one another in this position by a
viscid secretion from the clitellum. The sperms are then
passed from the male apertures of one into the spermothecae of
the other individual, and the two worms afterwards separate.
The clitellum then secretes a tough chitinous tube or cocoon,

vi CLASSIFICATION 349

which forms a broad ring round the body in this region, and
which is gradually slipped forwards. As it passes over the
apertures of the oviducts and spermothecae, ova and sperms
(the latter derived from the other individual) are passed into
it, as well as albumen secreted by certain glands present in
this region. When the worm has entirely withdrawn itself
from the cocoon, the latter closes up at the ends in virtue of
its elasticity, and the eggs, after fertilisation, undergo seg-
mentation.

The cells of the polyplast soon become differentiated into
an outer ectoderm, and an inner endoderm enclosing the
archenteron, which communicates with the exterior by the
blastopore (compare p. 201). A mesoderm (pp. 202 and
322) is then developed, and each layer gradually gives rise
to the corresponding parts in the adult animal, much as in
the frog, except that the nephridia are apparently derived
in the first instance from the ectoderm (compare p. 209) ;
the mesoderm undergoes segmentation, the ccelome appear-
ing in it as a cavity (p. 203) — or rather as a series of cavities,
one in each segment. The young worm is then hatched,
and it is to be noticed that it passes through no meta-
morphosis (p. n).

There are a number of different kinds of animals
commonly known as " worms," but many of these (e.g. the
parasitic worms in the lungs and bladder of the frog, liver,
flukes, tapeworms, &c.), are very different from the earth-
worm in structure, and are placed in several different
phyla. The earthworm is a member of the phylum
Annulata, which also includes a number of other worms living
in the sea and in fresh-water, as well as the leeches, &c. In
all these the body is elongated, bilaterally symmetrical, and
divided into metameres ; the integument is soft ; the mouth

350 NEREIS CHAP.

is anterior, and the anus posterior ; there is usually an
extensive coelome ; and the nervous system and nephridia
are essentially similar to those of the earthworm. The class
Chatopoda ("bristle-footed" worms), in which the earth-
worms and their fresh-water and marine allies are included,
receives its name from the fact that all its members are
provided with cuticular setae, which in the order to which
the marine forms belong (Polych&ta) are usually long and of
varied forms, and are much more numerous than in the
earthworms and fresh- water worms, which constitute the
order Oligochceta. This order includes several families,
both Lumbricus and Allolobophora belonging to the family
Lumbricidcz.

The oligochaetous earthworm, is, as we have seen, adapted
to a subterranean mode of life, and feeds on decaying
organic matter. For comparison, it will be instructive to
consider the external form and mode of life of a predacious
marine polychaetous worm, called Nereis, various species of
which are of common occurrence between tide-marks on
the sea-shore, under stones, and among sea-weed in all
parts of the world, sometimes swimming actively through
the water. The worm varies considerably in colour even in
the same species.

In shape (Fig. 87) the body, which may be about 7 or 8
centimetres in length, is long and narrow, approximately
cylindrical, somewhat narrower towards the posterior end.
A very distinct head, bearing eyes and tentacles, is recog-
nisable at the anterior end ; the rest is divided by a series
of ring like narrow grooves into a corresponding series of
metameres, which are about eighty in number altogether ;
and each of these bears laterally a pair of movable muscular
processes called parapods, provided with bundles of setae.

EXTERNAL CHARACTERS

351

The head is constituted by the prostomium and the peris-
tomium (p. 327). The former bears on its dorsal surface four
large, rounded eyes, in front a pair of short cylindrical tentacles,
and further back a pair of somewhat longer, stout appendages
or palps. The peristomium, which
bears some resemblance to the
segments of the body, though
wanting the parapods, bears laterally
four pairs of long, slender, cylindrical
tentacles : on its ventral aspect is
a transversely elongated aperture,
the aperture of the mouth. The
segments of the body differ little in
external characters from one another
throughout the length of the worm,
and there is no clitellum ; each
bears laterally a pair of parapods,
which in the living animal are
usually in active movement, aiding
in creeping or acting as a series of
oars for propelling it through the
water. When one of the parapods
(Fig. 88) is examined more atten-
tively it is found to be biramous, or
to consist of two distinct divisions
— a dorsal, which is termed the
notopod (noto), and a ventral, termed
the neuropod (neuro). Each of these
is further subdivided into several
lobes, and bears a bundle of setae, lodged in a sac formed by
invagination of the epiderm and capable of being protruded
or retracted and turned in various directions by muscular
fibres in the interior of the parapod. In each bundle

FIG. 87. — Nereis dumerilii.
Natural size. (From
Parker and Haswell's
Zoology, after Claparede.)

352 NEREIS CHAP.

there is, in addition to the ordinary setae, a stouter, straight,
dark-coloured seta (ac\ the pointed apex of which projects
only a short distance on the surface ; this is termed the
aciculum. The ordinary setae are exceedingly fine but
stiffish, chitinous rods, of which two principal kinds are
recognisable ; both have a terminal blade articulating with
the main shaft of the seta by a distinct joint. On the
dorsal side of the parapod is a short, cylindrical, tentacle-
like appendage, the dorsal cirrus (Fig. 88, dors, cirr), and

dors, cirr

noio
• neuro

^S&fe,

vent.cirr

FIG. 88. — Nereis dumerilii. A single parapod magnified, ac. aciculum ; dors, cirr,
dorsal cirrus ; neuro, neuropod ; noto, notopod ; vent, cirr, ventral cirrus.
(From Parker and Haswell's Zoology, after Claparede.)

a similar, somewhat shorter, appendage, the ventral cirrus
(vent, cirr}, is situated on its ventral side. The last
segment of the body, the anal segment, bears posteriorly a
small rounded aperture, the anus -, this segment is devoid
of parapodia, but bears a pair of appendages, the anal cirri,
similar in character to the cirri of the ordinary segments,
but considerably longer.

On the ventral surface near the bases of the parapods,
there is in each segment a pair of very fine nephridiopores
(p. 341), through which the sperms probably pass to the
exterior, there being no special genital apertures. Dorsal
pores are also wanting.

PHARYNX

353

The mouth leads into a wide buccal cavity, which is
continued back into a pharynx. In the pharynx are a
number of very small, dark brown, chitinous denticles, which
are very regularly arranged. The posterior part of the
pharnyx has very thick walls composed of bundles of
muscular fibres which are concerned in the movements of
a pair of laterally placed chitinous jaws. The anterior
part of the alimentary canal is capable of being everted, as

sp

FIG. 89. — Trochosphere of the worm Enf>omatus, from the side. an. anus ; md.
mid-gut ; n. larval head-nephridium ; sp. neural plate (brain) ; st. stomodaeum ;
tvk. preoral ciliated ring ; wk\. post -oral ciliated ring. (From Lang's Compara-
tive Anatomy, after Hatschek.)

a proboscis, until the jaws are thrust forth and project freely,
so that they can be brought to bear on some small living
animal or fragment of animal matter, which is thus seized
and swallowed.

In correspondence with its different mode of life, Nereis
is much better provided with sense-organs than is the
earthworm. The tentacles and palps, as well as the cirri,
are probably organs of touch ; and, as we have already seen,

PRACT. ZOOL. A A

354 THE EARTHWORM CHAP.

four eyes are present on the prostomium. Each eye
consists of a doubly-pigmented cup, the retina, formed as an
invagination of the ectoderm, with a small, rounded aperture,
or pupil, and enclosing a mass of gelatinous matter, the
lens. The cuticle of the general surface passes over the eye,
and a continuation of the epiderm, with its cells somewhat
flattened, constitutes a cornea (compare p. 182)

Almost without exception, the Polychseta further differ
from the Oligochseta in being dioecious, and in passing
through a metamorphosis. The segmented oosperm gives
rise to a more or less oval larva known as a trochosphere
(Fig. 89), which swims by means of cilia arranged in
circles round the body, and gradually undergoes metamor-
phosis into the adult.

PRACTICAL DIRECTIONS.

EARTHWORM.

Select a large earthworm, and after noting its movements and
mode of progression, kill by immersion in spirit for a few minutes and
then place in a dish and let the tap run on it for a short time.

A. External Characters :—

1. Note : — a. The form and colour of the body and its division into
nietameres ; b. the anterior end, terminating in the prostomiuiii and fol-
lowed by the peristomium ; c. the clitelhim ; and d. the last or anal
segment.

2. If the worm be drawn through the fingers backwards, the setcz
will be felt : examine with a lens and observe their position and the
number in each segment.

3. Make out the following apertures : a. the mouth ; b. the anus ;
c. the dorsal pores (p. 333) ; d. the two apertures of the spermiducts,
with thickened lips, on the fifteenth segment.

(It requires careful examination to see the other apertures, viz. —
those of the oviducts, spermothecce, and nephridid].
Sketch from below or from the side.

vi PRACTICAL DIRECTIONS 355

B. Dissection-:—

I. Take a freshly-killed worm in the left hand, and carefully insert
the point of the fine scissors into the integument about one-third of the
way along the body, close to the middle dorsal line. Place a drop of
the ccelomic fluid which exudes on a slide, add a drop of salt solution,
and cover. Examine with the low and high powers, and note the
structure and movements of the ama-boid corpuscles. Sketch at
intervals. (The small granules you will notice in the ccelomic fluid are
derived from the broken down yellow cells of the intestine. )

II. Continue the cut forwards to the prostomium, keeping very slightly
to one side of the median dorsal line, and taking care that the point
of the scissors does not penetrate deeper than the integument : note the
iridescent cuticle. Place the animal in a dish with just enough water
to cover it ; and carefully insert a pin between the integument and the
yellow intestine on either side, near the posterior end of the incision,
so as to expose the calome : note the septa connecting the body- wall
with the intestine. Then insert more pins, obliquely, so as to ex-
pose the ccelome and enteric canal up to the anterior end, taking
especial care not to tear the ventral parts of the septa and to stretch
the animal longitudinally as much as possible. Then note : —

1. The sperm- sacs — three or four pairs of large white bodies in
segments IX-XII, and varying greatly in size and form according to
the size of the animal. If your specimen is an adult Lumbricus^ you
will notice that the anterior and the two posterior pairs are respectively
united across the middle line, beneath the enteric canal, to form
the two sperm-reservoirs.

2. The enteric canal and its subdivisions : — a. buccal sac ; b.
pharynx ; c. gullet (largely hidden by the sperm-sacs) ; d. crop :
gizzard ; and e. intestine ', covered with a layer of yellow cells.

3. The dorsal blood-vessel, containing red blood and giving off
branches to the enteric canal ; the large rhythmically contractile
commissural vessels connecting the dorsal with the ventral (sub-intestinal)
vessel : the latter will be seen subsequently.

4. A pair of small, whitish, coiled bodies, the nephridia> attached to
the posterior face of each septum exposed (except the first three), on
either side of the enteric canal. Carefully remove one of these in
the region of the intestine -(take hold of the septum with the fine for-
ceps, and cut around the nephridium with the small scissors) — mount in
salt-solution or water, and examine first with the low power, and then

A A 2

356 THE EARTHWORM CHAP.

with the high power. Note that the nephridium consists of a long
coiled tube, plentifully supplied with blood-vessels, and that long
vibratile cilia can be seen in parts of it. (For details see § VI.)

Add a little methylated spirit to the water in your dissecting dish and
sketch your dissection.

5. The ovaries. Examine segment XIII closely, being very careful
not to injure its contents, and the ovaries may then be seen projecting
backwards into this segment, one on either side, just in front of the
crop. They can easily be recognised by their shape, and by the fact
that they hang freely into the ccelome, as can be seen by touching them
with a seeker. Carefully seize the septum between segments XII and
XIII with the small forceps, and cut around the attachment of an
ovary so as to remove it. Stain with methyl-green or magenta, and
mount in glycerine (or else fix, stain, and mount in balsam, as directed
on p. 136). Note the mass of undifferentiated cells at the proximal, at-
tached end of the ovary, and the gradual development of the ova
towards the distal, narrower end. Examine an ovum, and observe the
nucleus, nucleolus, and granules of food -yolk. Sketch the ovary.

(The oviducts and ovisacs are not easy to make out in dissec-
tions, but the latter may be recognised by their red colour. )

6. The globular spermotheca (usually two pairs) in segments IX
andX.

III. Tease out a small portion of a sperm-sac, stain with magenta,
and mount in glycerine. The following stages in the development of
the sperms can then be made out : — a. the sperm-mother-cells or
gametocytes (developed in the spermary) in different stages of division ;
the products of division, each with a nucleus, and arranged in a
single peripheral row, the central mass of protoplasm remaining un-
divided ; b. the gradual elongation of these small cells ; and c. the
conversion of each into a sperm., the nucleus forming the rod-like
"head," and the protoplasm giving rise to the delicate " tail ; " d. free
sperms (also to be found in the spermotheoe ; some of them should be
examined fresh and their movements noted). Sketch a series of stages.

It is difficult to make out the two pairs of sperniaries and the
spermiducts by dissection, and they can be more easily studied by
examining transverse sections, prepared as directed in § C. (In Lum-
bricus the spermaries and seminal funnels are enclosed within the
median sperm-reservoirs, Figs. 82 and 86. ) The spermiducts are partly
embedded in the body-wall.

vi PRACTICAL DIRECTIONS 357

IV. Remove the sperm-sacs carefully, and make out further details
as regards the enteric canal (see § II. 2). Note the oesophageal pouches
and glands. Cut open part of the intestine along one side, and observe
the thick dorsal fold or typhlosole projecting into it. Sketch.

V. i. Note the small cerebral ganglia or brain on the dorsal side of
the buccal sac, and then cut through the anterior part of the pharynx
just behind the brain. Carefully remove the enteric canal, noting as you
do so the ventral sub-intestinal blood-vessel. The nervotis system will
now be exposed. Observe again the paired cerebral ganglia, from
which arise a pair of connectives^ forming a small nerve-ring or collar
around the buccal sac, and continuous ventrally with the ventral nei~ve-
cordj consisting of fused lateral halves and extending along the whole
length of the ventral body-wall, passing through a perforation in each
septum, and expanding slightly in each segment, so as to form ganglionic
swellings. Three pairs of nerves are given off in each segment.
Sketch.

2. Remove the nerve-ring and a small portion of the ventral cord,
and examine with the low power. Sketch.

3. A lateral neural vessel can be seen close to the ventral cord on
either side. . Remove a portion of the cord, and note the sub-neural
vessel.

VI. Further details as regards the structure of the nephridia are best
made out on a worm which has been preserved in spirit. Very care-
fully remove the enteric canal as directed above, so as not to injure the
septa more than necessary : the nephrostomes can then be seen with a
lens, looking like small, whitish dots. Remove an entire nephridium
carefully as before (§ II. 4), stain, and mount in glycerine or balsam. Note
the three loops, and — a. the ciliated nephrostome ; b. the first, slender
part of the tube with its cilia ; c. the second, wider, ciliated part ;
d. the third, still wider, glandular part ; and e. the fourth, much wider,
muscular part which opens on to the exterior by the nephridiopore.
Sketch.

VII. Remove a small piece of the integument containing seta, and
separate the latter out with needles. Mount in water and examine.
Sketch.

C. Transverse Sections.— For the preparation of these, keep a
worm for a few days in coffee-grounds or small pieces of blotting-paper
moistened with water, in order that the gritty contents of its intestine
may be replaced by a soft substance which will not blunt the razor.

35§ THE EARTHWORM CHAP.

Kill the worm, cut a small piece about £ inch in length from the region
of the intestine, and fix, stain, and cut into transverse sections as
directed on p. 136. Examine a section with the low power, and
note : —

1. a. The thin cuticle ; b. the epiderm, enclosing goblet-cells (unicel-
lular glands) ; c. the very thin derm ; d. the seta, with their sacs and
muscles, if your section passes through one or more of them.

2. The muscles of 'the body-wall, a. The external circular layer ; and
b. the thicker longitudinal layer, appearing feather-like in transverse
section, and broken up into bands in the regions of the dorsal pores and
setne. Note that the muscles are unstriped.

3. The ccelome and peritoneal membrane.

4. The intestine, with its thick dorsal typhlosolt. It is lined by a
single layer of columnar cells (enteric epithelium], outside which is a
thin muscular and connective-tissue layer. Externally to this, again,
are the elongated and granular yellow cells, which are especially
abundant in the typhlosole.

5. The dorsal, ventral, and intestinal blood-vessels.

6. The ventral nerve- cord, just internal to the longitudinal muscular
layer. It is enclosed in a muscular and connective-tissue sheath, im-
bedded in which the sub-neural and lateral neural vessels can be seen.
Along the dorsal side are three clear-looking " giant fibres" Observe
the nerve-cells along the cord ventrally and laterally, the nerves coming
off from the cord, and the symmetrical halves of which the cord is
composed.

7. The mphridia : — these will be seen cut through in various
planes.

The thin septa will be cut through in different directions, and their
relations are therefore not easily seen in sections ; note the circular
and radial muscular fibres in them. Sketch the lateral half of your
section, and then put on the high power and work through §§ 1-7 again.
Notice that the cavities in the sections of the nephridia are intra-
cellular, except in the muscular part. Sketch as many details as
possible.

(If time permits, prepare and examine a series of transverse sections
through the genital region, and observe any important points not
already made out in your dissection. Note especially the spermaries
and spermiducts}.

PRACTICAL DIRECTIONS 359

Note : — I. The head., consisting of (a) prostomium with its tentacles ',
palps, and eyes, and (b) peristomium with its tentacles ; also the proboscis,
month, and lateral chitinous jaws.

2. The metameres of the body, each with a pair of (a] dorsal and
ventral cirri, and (b) lobed par apod s, subdivided into nctopod and
neuropod, each bearing a bundle of chitinous setcc.

3. The anal segment with anal cirri, but no parapods.

Carefully cut out an entire parapod and mount it on a slide (Fig.
88). Note the two bundles of seta, each made up of an aciculum
and of numerous jointed setoe of two different forms. The structure of
the setae will be more easily made out if separated from one another
with needles, or if the entire parapod is treated with a solution made
by dissolving 5 grammes of potassic hydrate in 100 c.c. of water.

Prepare sections, in the usual way, of the prostomium, passing through
the eyes, and note the pigmented optic cup and retina, the pupil, and
lens,

Procure some trochosphere larvae. Mount, and compare with Fig. 89.

CHAPTER VII

THE CRAYFISH — CHARACTERS OF THE PHYLUM
ARTHROPODA.

WE have now to study an animal formed on a very similar
plan of structure to the earthworm as regards segmentation
and arrangement of many of the organs, but which reaches
in every respect a far higher grade of organisation.

The common British fresh-water crayfish is usually known
as Astacus fluiiiatiliS) and is found in many of the streams and
rivers of England and Ireland, hiding under stones, or in
holes, into which it darts very suddenly on the approach of
danger. Its ordinary creeping movements are slow, and are
effected by means of a number of jointed limbs, for which
reason it is included, together with insects, spiders, scorpions,
&c., in the phylum Arthropoda (p. 220).

In colour, the crayfish is greenish-grey, and in form it is
very similar to the marine Lobster, to which the following
description will apply almost equally well.

In addition to the presence of paired limbs or appendages,
one of the most striking points of difference between an
earthworm and a crayfish is the smaller and constant
number of segments or metameres in the latter, as well
as the fact that certain of these are more or less

CHAP, vii THE CRAYFISH 361

firmly united with one another — i.e., have undergone
concrescence. The result of this fusion of the segments is
that two distinct regions can be distinguished in the
body —an anterior cephalothorax and a posterior abdomen
(Fig. 93, cth, ab\

The cephalothorax is unjointed, and is covered by a
cuirass-like structure, the carapace ; and the abdomen is
divided into distinct segments, movable upon one another
in a vertical plane. The cephalothorax is divided into two
regions, an anterior — the head, and a posterior — the thorax,
by a transverse depression, the cervical groove. The cara-
pace is developed from the dorsal and lateral regions of
both head and thorax ; it is free at the sides of the thorax,
where it forms a flap or gill-cover (Fig. 94, kd) on each side,
separated from the actual body-wall by a narrow space in
which the gills are contained.

The limbs spring from the ventral surface. Both trunk
and appendages are covered with a sort of shell, formed of
chitin (p. 328), strongly impregnated with carbonate of lime
in most parts, so as to be hard and but slightly elastic.

The abdomen is made up of seven segments: the first
six of these are to be considered as metameres in the sense
in which the word is used in the case of the earthworm.
Each has a ring-like form, presenting a broad dorsal region or
tergum (Fig. 90, T), a narrow ventral region or sternum (S\
and downwardly directed lateral processes, the pleura (PL).
The seventh division of the abdomen is the telson (Fig. 93, /) :
it is flattened horizontally and divided by a transverse groove
into anterior and posterior portions. All seven segments
are calcified, and are united to one another by uncalcified
articular membranes : the first segment is similarly joined
to the thorax. Thus the exoskeleton of the crayfish is a
continuous cuticular structure, discontinuously calcified so

362

THE CRAYFISH

CHAP.

as to have the character of a hard, jointed armour. Tufts of
minute feather-like cuticular structures, or seta, are present
on various parts of the exoskeleton both of the body and
appendages.

It has been stated that the abdominal segments are
movable upon one another in a vertical plane : i.e., the whole

EM,

FIG. go. — Transverse section of abdomen of Crayfish.

DA. dorsal abdominal artery ; EM. dorsal muscles of the abdomen ; EP. space
between the pleuron and the appendage ; FM. ventral muscles of the abdomen ;
M. muscles of the appendage ; N. endopodite ; NG. nerve-ganglion ; P. protopodite ;
PL. pleuron ; PR. hind-gut ; S. sternum ; T. tergum ; V. ventral abdominal artery ;
X. exopodite. (From Marshall and Hurst's Zoology).

abdomen can be extended or straightened, and flexed or bent
under the cephalothorax ; the segments are incapable of
movement from side to side. This is due to the fact that,
while adjacent segments are connected dorsally and ven-
trally by flexible articular membranes, they present at each
side a joint, placed at the junction of the tergum .and

vii EXOSKELETON 363

pleuron, and formed by a little peg-like process of one
segment fitting into a depression or socket in the other. A
line drawn between the right and left joints constitutes the
axis of articulation, and the only possible movement is in a
plane at right angles to this axis.

Owing to the presence of the carapace the thoracic region
is immovable, and shows no distinction into segments either
on its dorsal (tergal) or lateral (pleural) aspect. But on the
ventral surface the sterna of the thoracic segments are
clearly marked off by transverse grooves, and the hindmost
of them is slightly movable. Altogether eight thoracic
segments can be counted.

The ventral and lateral regions of the thoracic exoskeleton
are produced into the interior of the body in the form of a
segmental series of calcified plates, so arranged as to form
a row of lateral chambers in which lie the muscles of the
limbs and a median tunnel-like passage or sternal canal,
containing the thoracic portion of the nervous system
(Fig. 94). The entire endophragmal system, as this
series of plates is called, constitutes a kind of internal
skeleton.

The head exhibits no segmentation : its sternal region is
formed largely by a shield-shaped plate, the epistoma, nearly
vertical in position. The ventral surface of the head is, in
fact, bent upwards, so as to face forwards instead of down-
wards. The cephalic region of the carapace is produced in
front into a large median spine, the rostrum : immediately
below it is a» plate from which spring two movably
articulated cylindrical bodies, the eye-stalks, bearing the
eyes at their ends.

The appendages have very various forms, and are all, like
the abdomen, jointed or segmented, being divisible into
freely articulated limb-segments or podomeres. You will

364 THE CRAYFISH CHAP, vn

at once notice the long feelers attached to the head, the
five pairs of legs springing from the thorax, and the little
fin-like bodies arising from the sterna of the abdomen. It
will be convenient to begin with the last-named region.

The third, fourth, and fifth segments of the abdomen
bear each a pair of small appendages, the swimming feet or
pleopods (pig. 90, P, N, X). A pleopod (Fig. 91,10) consists
of an axis w protopodite having a very short proximal (pr. i)
and a long distal (pr. 2) podomere, and bearing at its free
end two jointed plates, fringed with setae, the endopodite
(en) and exopodite (ex). These appendages act as fins,
moving backwards and forwards with a regular swing, and
probably aiding in the animal's forward movements.

In the female a similar appendage is borne on the
second abdominal segment, while that of the first is
more or less vestigial (p. 159). In the male the first and
second pleopods (Fig. 91, 9) are modified to form incomplete
tubes which serve to transfer the spermatophores '(p. 382)
to the body of the female. The sixth pair of pleopods (n)
are alike in the two sexes : they are very large, both
endopodite and exopodite having the form of broad,
flat plates : in the natural position of the parts they lie
one on either side of the telson, forming with it a large
five-lobed tail-fin : they are therefore conveniently called
uropods or tail-feet. The telson itself bears no appendages.

The thoracic appendages are very different. The four
posterior segments bear long, slender, jointed legs (8),
with which the animal walks : in front of these is a pair of
very large legs terminating in huge claws or chelce, and
hence called chelipeds (Fig. 93, bf. 4). The three anterior
thoracic segments bear much smaller appendages, more
or less leg-like in form, but serving as jaws : they are dis-
tinguished as maxillipeds or foot-jaws.

5. 2n.d Maxilla 6. Ir Maxillibe d

Q.Copuldfory Organs 10. Swim mm g Fo of

FIG. 91. — Typical appendages of the Fresh-water Crayfish, placed in the same
position, with the protopodite (fir) and epipodite (ep} downwards, the endopodite
(en) to the left, and the exopodite (ex) to the right.

The protopodite is typically formed of two podomeres (pr. T.,pr. 2), the endopodite
of -five (en. i — en. 5) ; a gill (g) may be attached to the epipodite.

The three proximal segments of the antennule are marked, i — 3, its flagella ft. i
and./?. 2 ; the distal end of the endopodite of the antenna is a flagellum (y?).
(The tufts of threads in 7 and 8 are very long setae which extend between the
gills). (From Parker and Haswell's Zoology, after Huxley.)

The structure of these appendages is best understood by
a consideration of the third maxilliped (Fig. 91, 7). The
main portion of the limb is formed of seven podomeres

366 THE CRAYFISH CHAP.

'arranged in a single series, strongly calcined, and — with the
exception of the second and third, which are fused — move-
ably articulated with one another. The second podomere,
counting from the proximal end, bears a many-jointed
feeler-like organ (ex\ and from the first springs a thin, folded
plate (ep) having a plume-like gill (g) attached to it. The
first two segments of the axis form the protopodite (pr. i, 2),
its remaining five segments the endopodite (en. i, 5), the
base of which is toothed ; and the feeler, which is directed
outwards, or away from the median plane, the exopodite
(ex). The folded plate is called the epipodite : in the natural
position of the parts it is directed upwards, and lies in the
gill-cavity between the proper wall of the thorax and the
gill-cover.

The five legs (8) differ from the third maxilliped in their
greater size, and in having no exopodite : in the fifth or last
the epipodite also is absent. The first three of them have
undergone a curious modification, by which their ends are
converted into pincers or chela : the fourth segment of the
endopodite (sixth of the entire limb, en. 4) is produced dis-
tally so as to form a claw-like projection (en. 4'), against
which the terminal segment (en. 5) bites. The first leg is
much stouter than any of the others, and its chela is of
immense size and forms an important weapon of offence
and defence. The second maxilliped resembles the third,
but is considerably smaller: the first (6) has its endopodite
greatly reduced, the two segments of its protopodite large
and leaf-like, and no gill is connected with the epipodite.

The head bears a pair of mandibles and two pairs of
maxillae in relation with the mouth, and in front of that
aperture a pair of antennules and of antenna. The hind-
most appendage of the head is the second maxilla (5),
a leaf-like appendage, its protopodite being cut up into

vii APPENDAGES 367

lobes, while the exopodite is modified into a boomerang-
shaped plate, which, as we shall see, is an important
accessory organ of respiration. The first maxilla (4) is a very
small organ having neither exopodite nor epipodite. The
mandible (3) is a large, strongly calcified body, toothed along
its inner edge, and bearing on its anterior border a little
three-jointed, feeler-like structure, the palp, the two distal
segments of which represent the endopodite, its proximal
segment, together with the mandible proper, the protopodite.

The antenna (2) is of great length, being nearly as long as
the whole body. It consists of an axis of five podomeres,
the fifth or last of which bears a long, flexible, many-jointed
structure, or flagellum (fi\ while from the second segment
springs a scale-like body or squame (ex). It is fairly obvious
that the two proximal segments represent the protopodite,
the remaining three, with the flagellum, the endopodite, and
the squame the exopodite. On the ventral side of the basal
segment of the protopodite is a conical elevation on which
the duct of the excretory organ opens (p. 375).

The antennule (i) has an axis of three podomeres ending
in two many-jointed flagella (fl. i, fl. 2), which are some-
times considered as corresponding to the endopodite and
exopodite. But in all the other limbs, as we have seen,
the exopodite springs from the second segment of the axis,
and the probabilities are that there is no exact corre-
spondence between the parts of the antennule and those of
the remaining appendages.

The eye-stalks, already noticed, arise just above the an-
tennules, and are ' formed each of a small proximal and a
large distal segment. They are sometimes counted as
appendages serially homologous with the antennae and legs,
but are more properly to be looked upon as articulated pro-
cesses of the prostomium. It is probable that the anten-

368 THE CRAYFISH CHAP.

nules are also prostomial and not metameric structures :
assuming this to be the case, it will be seen that the
body of the crayfish consists of a prostomium, eighteen
metameres and a telson, which is probably composed
of an anal segment plus a post-anal extension. The
prostomium bears eye-stalks and antennules : the first four
metameres are fused with the prostomium to form the head,
and bear the antennae, mandibles, first maxillae, and second
maxillae: the next eight metameres (5th — i2th) consti-
tute the thorax, and bear the three pairs of maxillipeds and
the five pairs of legs : the remaining six metameres (i3th —
1 8th), together with the anal segment, constitute the
abdomen, and bear five pairs of pleopods and one of
uropods.

The articulation of the various podomeres of the append-
ages is on the same plan as that of the abdominal segments
(p. 362). The podomeres are, it must be remembered, rigid
tubes : they are connected with one another by flexible
articular membranes (Fig. 92, art. m\ but at two points the
adjacent ends of the tubes come into contact with one
another and are articulated by peg-and-socket joints (/£), the
two joints being at opposite ends of a diameter which forms
the axis of articulation. The two podomeres can therefore
be moved upon one another in a plane at right angles to the
axis of articulation and in no other direction, the joints being
pure hinge-joints (p. 57). As a rule the range of movement
is from the perpendicular to a tolerably extensive flexion on
one side — the articulations are single-jointed, like our own
elbows and knees. The whole limb is, however, capable of
universal movement, owing to the fact that the axes of articu-
lation vary in direction in successive joints : the first
joint of a limb bending, for instance, up and down, the
next obliquely, the next backwards and forwards, and so on.
In some cases — e.g., the pleopods — peg-and-socket joints are

vii EXOSKELETON AND MUSCLES 369

absent, the articulation being formed merely by an annular
articular membrane, movement being therefore possible in
any plane.

Sections show the body- wall to consist of an integument
composed of a layer of deric epithelium or epiderm (p. 329)
secreting a thick cuticle, and a layer of connective-tissue
forming the derm, beneath which is a very thick layer of
large and complicated muscles which fill up a great part of
the interior of the body. Neither on the epiderm nor
elsewhere are there any cilia, the absence of these structures
being generally characteristic of Arthropods.

The cuticle is of great thickness, and except at the joints
between the various segments of the body and limbs, is
impregnated with lime-salts so as to form a hard, jointed
armour. It thus constitutes a cuticular exoskeleton, forming
a continuous investment over the whole body but discon-
tinuously calcined. It is shed entire and renewed
periodically — once a year during adult life — the process
being known as ecdysis^ growth taking place during the
period between ecdysis and renewal while the animal is soft.

The muscular system shows a great advance in complexity
over that of the earthworm : and consists entirely of
transversely striated fibres (compare p. 112). In the abdo-
men the muscles are of great size, and are divisible into a
smaller dorsal and a larger ventral set. The dorsal muscles
(Figs. 90 and 93, em) are paired longitudinal bands, divided
into segments called myomeres (p. 203), and inserted by
connective-tissue into the anterior border of each segment :
in front they are traceable into the thorax, where they arise
from the side-walls of that region. When these muscles
contract they draw the anterior edge of each tergum under
the posterior edge of its predecessor, and thus extend or
straighten the abdomen (compare p. 63).

The ventral muscles (Figs. 90 and 93, f.m) are extraordin-

PUACT. ZOOL. B B

37o THE CRAYFISH CHAP.

arily complex and cannot be described in detail here. They
partly aid the dorsal muscles in extending the abdomen, but
are chiefly important in producing an approximation of the
sterna, and thus in flexing the abdomen. The ventral
muscles are, like the dorsal, traceable into the thorax, where
they arise from the endophragmal system (p. 363). The
flexor muscles are immensely powerful, and produce, when
acting together, a sudden and violent bending of the abdomen
upon the cephalothorax, causing the crayfish to dart back-
wards with great rapidity. There is also a paired rotator
of the abdomen.

It will thus be seen that the body-muscles of the crayfish
cannot be said to form a layer of the body-wall, as in the
earthworm (Fig. 81), but constitute an immense fleshy mass,
filling up the greater part of the body-cavity (see p. 373),
and leaving a very small space around the enteric canal.

In the limbs the essential arrangement of the muscles in
relation with the joints in Arthropods is more easily seen
(Fig. 92) : each podomere is acted upon by two muscles
situated in the next proximal podomere. These muscles are
inserted, by chitinous and often calcified tendons, into the
proximal edge of the segment to be moved, the smaller (ex/)
on the extensor, the larger (ft) on the flexor side, in each
case half-way between the two hinges, so that a line joining
.the two muscular insertions is at right. angles to the axis of
articulation.

The digestive organs are constructed on the same general
plan as those of the earthworm/* but present many striking
differences. The mouth lies in the middle ventral line of
the head and is bounded in front by a shield-shaped process,
the labrum, at the sides by the mandibles, and behind by a
pair of delicate lobes, the paragnatha. It leads by a short
wide gullet (Fig. 93, <K) into a capacious gizzard, usually

ENTERIC CANAL

371

spoken of as the stomach,
which occupies a great part of
the interior of the head, and
is divided into a large ante-
rior division (a), and a smaller
posterior division (ps) : the
latter passes into the intestine,
which consists of a narrow
and very short mid-gut or
mesenteron (md} from which a
somewhat wider hind- gut (/id)
extends to the anus (an),
situated on the ventral surface
of the telson. The gullet and
gizzard together constitute the
fore gut.

The outer layer of the en-
teric canal consists of connec-
tive-tissue containing striped
muscular fibres : within this is
a single layer of columnar
epithelial cells, none of them
glandular. In the fore- and
hind-gut the epithelium se-
cretes a layer of chitin, which
thus constitutes the innermost
layer of their cavities. It is
proved by development that
the mid-gut, which has no
chitinous lining, is the only

FIG. 92. — A leg of the Fresh-water Cray-
fish with part of the exoskeleton
removed to show the muscles.
en. 2— en. 5, segments of endopodite ; h. hinges ; art.m. articular membrane , ext.
extensor muscle ; ft. flexor muscle. (From Parker and Haswell'? Zoology.}

B B 2

372

THE CRAYFISH

ctft

part of the enteric canal deve-
loped from the enteron of the
embryo : the gullet and gizzard
(fore-gut) arise from the stomo-
daeum, the hind-gut from the
proctodaeum (p. 204). Thus
only a very small portion of the
enteric epithelium is endo-
dermal.

In the anterior division of
the gizzard the chitinous
lining is thickened and cal-
cified in certain parts, so as
to form a complex articulated
framework, usually known as
the "gastric mill" on which
are borne a median and
two lateral teeth, strongly
calcified and projecting into
the cavity * of the gizzard.
Two pairs of strong muscles
arise from the carapace, and

FIG. 93.— Dissection of Fresh-water Cray-
fish, made by removing the exo-
skeleton with the appendages, and the
muscles, digestive gland and excretory
organ, of the right side.
««. antennary artery ; ab. abdomen ; an.
anus ; b. d. aperture of right digestive
duct exposed by removal of gland ; bf.
4, cheliped ; bm. ventral nerve cord ;
cs. anterior division of gizzard ; cth.
cephalothorax ; ejn. dorsal muscles ;
fm. ventral muscles; g. brain; //.
heart ; hd. hind-gut ; Ir. left digestive
gland ; vid. mid-gut ; o. right lateral
ostium of heart ; oa. ophthalmic artery.

oaa. dorsal abdominal artery ; oe. gullet ; pi. 1—5, pleopods ; pi. 6, uropod ; ps.

posterior division of gi/^ard ; s. a. sternal artery ; t. (near heart), spermary ; t.
"(below an) telscn ; uaa. ventral abdominal artery; v. d. spermiduct ; vdo.

male genital aperture. (From Lang, after Huxley.)

vii DIGESTION 373

are inserted on the gizzard (Fig. 93) : when they contract
they move the mill in such a way that the three teeth meet
in the middle line and complete the comminution of the food
begun by the jaws. The separation of the teeth is effected
partly by the elasticity of the mill, partly by delicate muscles
in the walls of the gizzard. The posterior division of the
gizzard forms a strainer : its walls are thickened and pro-
duced into numerous setae, which extend quite across the
narrow lumen and prevent the passage of any but finely
divided particles into the intestine. Thus the gizzard has
no digestive function, but is merely a masticating and strain-
ing apparatus — it is therefore not a stomach either in the
embryological or in the physiological sense. On either side
of its anterior division is found, at certain seasons of the
year, a plano-convex mass of calcareous matter, the gastrolith
or " crab's-eye," which possibly merely serves to store up
reserve calcareous material for use after the next ecdysis.

The digestion of the food, and to some extent the absorp-
tion of the digested products, are performed by a pair of
large glands (Fig- 93, lr\ lying one on either side of the
gizzard and anterior end of the intestine. They are formed
of finger-like sacs or c&ca, which discharge into wide ducts
opening into the mid-gut, and are lined with glandular
epithelium derived from the endoderm of the embryo.
The glands are often spoken of as the liver, but as they
have a complex function, and the fluid they secrete is able
to digest proteids, it is better to call them simply digestive
glands. The crayfish is largely carnivorous, its food con-
sisting of decaying animal as well as vegetable matter.

The digestive organs and other viscera are surrounded by
an irregular cavity, which is in free communication with the
blood-vessels and itself contains blood. This cavity is not
lined by epithelium, and is to be looked upon as a large

374

THE CRAYFISH

blood-sinus, and not as a true ccelome : it may therefore be
described as a hcemoccele.

There are well-developed respiratory organs in the form
of gills (Figs. 91, g,
and 94, k\ contained
in a narrow branchial
chamber, bounded
internally by the ^
proper wall of the

t-

cC~

OLDnC

thorax, externally by
the gill-cover or
pleural region of the
carapace. Each gill
consists oT a stem
giving off numerous
branchial filaments,
so that the whole
organ is plume-like.
PThe filaments are
hollow and commu-
nicate with two
parallel canals in the
stem — an external,
the afferent branchial
vein, and an internal,
the efferent branchial
veinj¥\g. 96). The
gill is to be considered
as an out-pushing of
the body- wall specially modified for respiration (compare p.
204), and it contains the same layers — a thin layer of chitin
externally, then a single layer of epithelial cells, and beneath
this connective-tissue, hollowed out for the blood channels.

FIG. 94. — Transverse section of thorax of Crayfish ;

diagrammatic.

abut, ventral abdominal muscles ; bf. leg ; bm.
ventral nerve cord ; d. intestine ; dbm. dorsal
abdominal muscles; ep. wall of thorax ;/z.
heart ; k. gills ; kd. gill-cover ; /. digestive
gland ; ov. ovary ; pc. pericardial sinus ; sa,
sn. sternal artery ; i>s. ventral blood-sinus.
The arrows show the direction of the blood-
current. (From Lang's Comparative Ana-
tomy.)

vii RESPIRATORY AND EXCRETORY ORGANS 375

According to their point of origin the gills are divisible
into three sets— first, podobranchs, or foot-gills, springing
from the epipodites of the thoracic appendages, from which
they are only partially separable ; secondly, arthrobranchs,
or joint-gills, springing from the articular membranes con-
necting the thoracic appendages with the trunk ; and
thirdly, pleurobranchs, or wall-gills, springing from the lateral
walls of the thorax, above the attachment of the append-
ages. The total number of gills is eighteen, besides two
filaments corresponding to vestigial or vanishing gills, which
are represented by functional organs • in some allied
forms.

The water in the branchial chamber is constantly renewed
in the living Crayfish by the action of the plate attached
to the second maxilla (p. 367), the movement to and fro of
which bales out the water in front, and consequently causes
fresh water to flow in behind. Thus a fresh supply of water,
containing air in solution, is continually being passed over
the gills. The fact that the podobranchs are attached to the
bases of the limbs must also result in bringing their surfaces
more easily in contact with the water when the animal uses
its legs in walking.

The excretory organs differ both in position and in form
from those of the earthworm. At the base of each antenna
is an organ of a greenish colour, the antennary or green
gland (Fig. 95), by which the function of renal excretion
is performed. The gland is cushion-shaped, and contains
canals and irregular spaces lined by glandular epithelium :
it discharges its secretion into a thin-walled sac or urinary
bladder (bl\ which opens by a duct on the proximal segment
of the antenna. The green glands are to be looked upon
as corresponding to peculiarly modified nephridia (p. 340).

The circulatory organs are in a high state of development.

376

THE CRAYFISH

The heart (Fig. 93, h\ is situated in the dorsal region of
the thorax, and is a roughly polygonal, muscular organ
pierced by three pairs of apertures or ostia (o), guarded
by valves which open inwards. It is enclosed in a spacious
pericardial sinus (Fig. 94, /<r)~which contains blood. From
the heart spring a number of delicate arteries (compare
P- 337)j which serve to convey the blood to various parts of
the body. At the origin of each artery from the heart are

valves which allow of
the flow of blood in
one direction only,
viz., from the heart
to the artery. [From
the anterior end of
the heart arise five
vessels — the median
ophthalmic artery
(Fig. 93, oa\ which
passes forwards to the
eyes ; paired anten-
nary arteries (a a),
going to the anten-
nules, antennae, green glands, &c., and sending off branches
to the gizzard ; and paired hepatic arteries^ going to the
digestive glands. The posterior end of the heart gives off
two unpaired arteries practically united at their origin— the
dorsal abdominal artery (o a a\ which passes backwards
above the intestine, sending branches to it and to the dorsal
muscles ; land the large sternal artery (Figs. 93 and 94, so),
which extends directly downwards, indifferently to right or
left of the intestine, passing between the connectives uniting
the third and fourth thoracic nerve-ganglia, and then turns
forwards and runs in the sternal canal, immediately beneath

FIG. 95. — Diagram of excretory oigan of Crayfish.
bl. bladder ; c. p. outer or cortical green portion ;
d. duct ; .?. yellowish sac-like portion ; ?c. /.
white tubular portion. (From Parker and
Haswell's Zoology, after Marchal.)

vii CIRCULATORY ORGANS 377

the nerve-cord, as a/ ventral thoracic artery sending off
branches to the legs, jaws, &c. At the point where the
sternal artery turns forward it also gives off the median
ventral abdominal artery (Fig. 93, uaa), which passes back-
wards beneath the nerve-cord, and supplies the ventral
muscles, pleopods, &c.

All these arteries branch extensively in the various organs
they supply, becoming divided into smaller and smaller off-
shoots, which finally end in microscopic capillaries (p. 95).
These latter end by open mouths, which communicate with
the blood-sinuses — spacious cavities lying among the muscles
and viscera, and all communicating, sooner or later, with the
sternal sinus, a great median canal running longitudinally
along the thorax and abdomen, and containing the ventral
nerve-cord and the sternal and ventral abdominal arteries.
Mn the thorax the sternal sinus (Fig. 94, vs, and Fig. 96, st.s),
sends an offshoot to each gill in the form of a well-defined
vessel, which passes up the outer side of the gill, and is called
the afferent branchial vein (of. br. v). Spaces in the gill-
filaments place the afferent in communication with the
efferent branchial vein (ef. br. v\ which ocupies the inner
side of the gill-stem. ] The eighteen efferent branchial veins
open into six branchiocardiac veins (br. c. v) which pass
dorsally in close contact with the lateral wall of the thorax
and open into the pericardial sinus, some of them uniting
before doing so.

The whole of this system of cavities is full of blood, and
the heart is rhythmically contractile. When it contracts the
blood contained in it is prevented from entering the peri-
cardial sinus by the closure of the valves of the ostia, and
therefore takes the only other course open to it, viz., into
the arteries. When the heart relaxes, the blood in the
arteries is prevented from regurgitating by the valves at

378 THE CRAYFISH CHAP.

their origins, and the pressure of blood in the pericardial
sinus forces open the valves of the ostia and so fills the
heart. Thus in virtue of the successive contractions of the
heart and of the disposition of the valves, the blood is kept
constantly moving in one direction — from the heart by
the arteries to the various organs of the body, where it
receives carbon dioxide and other waste matters ; thence by

s

FIG. 96. — Diagram illustrating the course of the circulation of the blood in the
Crayfish. Heart and arteries, red ; veins and sinuses containing non-aerated
blood, blue ; veins and sinuses containing aerated blood, pink. The arrows show
the direction of the flow.

The blood from the pericardial sinus (pcd. s) enters the heart (hf) by a valvular
aperture (z'1.) and is propelled into arteries (a), the orifices of which are guarded
by valves (z/2.) ; the ultimate branches of the arteries discharge the blood into
sinuses (s), and the sinuses in various parts of the body debouch into the sternal
sinus (st. s) ; thence the blood is taken by the afferent branchial veins
(of. br. v) into the gills, where it is purified and is returned by efferent branchial
veins (ef. br. v) into the branchiocardiac veins (br. c. v) which open into the
pericardial sinus. (From Parker and Haswell's Zoology.)

sinuses into the great sternal sinus ; from the sternal sinus
by afferent branchial veins to the gills, where it exchanges
carbon dioxide for oxygen ; from the gills by efferent branchial
veins to the branchiocardiac veins, thence into the peri-
cardial sinus, and so to the heart once more.

It will be seen that the circulatory system of the crayfish,
like that of the frog, consists of three sections — (i) the heart,

vii NERVOUS SYSTEM 379

or organ of propulsion ; (2) a system of out-going channels,
the arteries, which carry the blood from the heart to the
body generally ; and (3) a system of returning channels —
some of them, the sinuses, mere irregular cavities, others,
the veins, with definite walls : these return it from the
various organs back to the heart. j The respiratory organs, it
should be observed, are interposed in the returning current,
so that blood is taken both to and from the gills by veins.

Comparing the blood-vessels of the crayfish with those of
the earthworm (Figs. 81 and 83), it would seem that the
opthalmic artery, heart, and dorsal abdominal artery together
answer to the dorsal vessel, part of which has become en-
larged and muscular, and discharges the whole function of
propelling the blood. The horizontal portion of the sternal
artery, together with the ventral abdominal, represents the
main ventral vessel, while the vertical portion of the sternal
artery is a commissure, developed sometimes on the right,
sometimes on the left side, its fellow being suppressed.

The blood when first drawn is colourless, but after ex-
posure to the air takes on a bluish-grey tint. This is owring
to the presence of a colouring matter called hczmocyanin,
which becomes blue when combined with oxygen ; it is a
respiratory pigment, and serves, like haemoglobin (pp. 107 and
339), as a carrier of oxygen from the external medium to the
tissues. The haemocyanin is contained in the plasma of the
blood : the corpuscles are all leucocytes (pp. 105 and 336).
The nervous system consists, like that of the earthworm,
of a brain (Fig. 93, g) and a ventral nerve-cord (bni), united
by cesophageal connectives. But the ganglia of the ventral
nerve-cord are more distinct, and to them the nerve-cells are
confined, the longitudinal connectives betwreen them consist-
ing of nerve-fibres only. .The brain is complex and supplies
not only the eyes and antennules, but the antennae as well :

380 THE CRAYFISH CHAP.

a study of development shows that the ganglia belonging
to the antennary segment have fused with it. Hence we
have to distinguish between a primary brain or archi-
cerebrum — the ganglion of the prostomium, and a secondary
brain or syn-cerebrum formed by the union of one or more
pairs of ganglia of the ventral cord with the archi-cerebrum.
A further case of concrescence of ganglia is seen in the
ventral nerve-cord, where the ganglia of the last three
cephalic and first three thoracic segments have united to
form a large compound sub-ossophageal ganglion. All the
remaining segments have their own ganglia, with the excep-
tion of the telson, which is supplied from the ganglion of
the preceding segment. There is a visceral system of nerves
supplying the fore-gut and hind-gut, the nerves of the
former originating in part from the brain and in part from
the oesophageal connectives, and those of the latter from
the last abdominal ganglion./

The eyes (Fig. 97) have a very complex structure. The chi-
tinous cuticle covering the distal end of the eye-stalk is trans-
parent, is divided by delicate lines into areas or facets which
are mostly square, and constitutes the cornea. Beneath each
facet of the cornea is an apparatus called an ommatideum,
consisting of an outer segment or vitreous body having a
refractive function, and an inner segment or retinula^ en-
closing a striated body, the rhabdome, and forming the actual
visual portion of the apparatus. The ommatidea are opti-
cally separated from one another by black pigment, so that
each is a distinct organ of sight, with a very limited visual
area, and the entire eye is called a compound eye. The optic
nerve (op.n) dilates to form an optic ganglion (op. gn) in
close connection with the inner ends of the ommatidea ; the
latter are thus turned towards the light, and are not, like
the rods and cones of the vertebrate eye (p. 185), covered

vii SENSORY ORGANS 381

by a layer of nerve-fibres, &c., through which the light must
first penetrate.

Each antennule bears two sensory organs, to which
the functions of smell and hearing have been respectively
assigned. The "olfactory" organ is constituted by a num-
ber of delicate spatulate setae, borne on the external

op.ff'V

FIG. 97. — A, Longitudinal section of eyestalk ; B, a single ommatideum ; a, vitreous
body ; b, retinula ; en, cornea, continuous with cnt, cuticle of eyestalk ;
m, muscles ; <?;/;, ommatidea ; op.gn, optic ganglion ; op, n, optic nerve. (After
Howes.)

flagellum, and supplied by branches of the antennulary
nerve. The " auditory " organ, which is better described as
an organ of equilibration, or statocyst (p. 315), is a sac formed
by invagination of the dorsal surface of the proximal seg-
ment, and is in free communication with the surrounding-
water by a small aperture. The chitinous lining of the sac
is produced into delicate feathered sensory setae, supplied
by branches of the antennulary nerve, and in the water

382 THE CRAYFISH CHAP.

which fills the sac are minute sand-grains, which take the
place of the lithites or otoliths (pp. 188 and 315) found in
most organs of this kind, but which, instead of being formed
by the animal itself, are taken in after each ecdysis when
the lining of the sac is shed. Many of the setae on the
body generally have a definite nerve-supply, and are probably
tactile organs.

The crayfish is dioecious (p. 320), and presents a very
obvious sexual dimorphism or structural difference between
male and female, apart from the actual organs of repro-
duction. The abdomen of the female is much broader
than that of the male : the first and second pleopods of
the male are modified into tubular or rather spout-like
organs (p. 364) ; and the reproductive aperture is situ-
ated in the male on the proximal podomere of the fifth
leg, in the female on that of the third.

The spermary (Fig. 93, /) lies in the thorax, just beneath
the floor of 'the pericardial sinus, and consists of paired
anterior lobes and an unpaired posterior lobe. From each
side goes off a convoluted spermiduct or vas deferens (vd\
which opens on the proximal segment of the last leg (vdo).
The sperms differ greatly from those already considered in
other animals : they are curious, rounded, non-motile bodies
produced into a number of stiff processes : by a secretion
of the vas deferens they become aggregated into vermicelli-
like spermatophores.

The ovary is also a three-lobed body, and is similarly situ-
ated to the spermary : from either side proceeds a thin-walled
oviduct, which passes downwards, without convolutions, to
open on the proximal segment of the third or antipenulti-
mate leg. The eggs are of considerable size and contain a
large amount of yolk.

Both ovary and spermary are hollow organs, discharging

DEVELOPMENT

383

their products internally. Their cavities, lined by germinal
epithelium, represent part of the true ccelome (compare p.
374), and their ducts are organs of the same general nature
as nephridia, opening on the one hand into a coelomic
cavity, and on the other to the exterior. The ova, when
laid, are fastened to the setae on the pleopods of the female
by the sticky secretion of glands occurring both on those
appendages and on the segments themselves : they are
fertilised immediately after being laid, the male depositing
spermatophores on the ventral surface of the female's body
just before oviposition.

The process of segmentation of the oosperm presents
certain striking peculiarities. The nucleus divides repeatedly

FIG. 98. — Three stages in the early development of the Crayfish.
In A the products of division of the nucleus (mt) are seen in the centre of the
yolk ; in B and C the nuclei have become arranged in a peripheral layer, each sur-
rounded by protoplasm, so as to form the blastoderm ; yp. yolk-pyramids.
(From Parker and Haswell's Zoology, after Morin.)

(Fig. 98 A, nu\ but no corresponding division of the pro-
toplasm takes place, with the result that the polyplast-stage
(p. 200), instead of being a heap of cells, is simply a multi-
nucleate body (compare p. 281). Soon the nuclei thus
formed retreat from the centre of the embryo, and arrange
themselves in a single layer close to the surface (B, c) : around
each of these protoplasm accumulates, the central part of
the embryo consisting entirely of yolk-material. We thus
get a superficial segmentation, characterised by a central mass
of yolk and a superficial layer of cells collectively known as

3^4 THE CRAYFISH CHAP.

the blastoderm. From this the ectoderm and endoderm are
derived, the latter enclosing a relatively small archenteron,
formed by an in-pushing or imagination of the blastoderm
into the yolk, and communicating with the exterior through
a blastopore (p. 201).

Very soon the embryo becomes triploblastic, or three-
layered, by the budding off of cells from the endoderm
in the neighbourhood of the blastopore : these accumulate
between the ectoderm and endoderm, and constitute the
mesoderm.

Before long the blastopore closes, and a stomodaeum and
proctodaeum (p. 204) are formed as invaginations of the
ectoderm, which eventually communicate with the enteron,
forming a complete enteric canal (p. 371). On each side of
the mouth or aperture of the stomodaeal depression (Fig. 99)
three elevations appear, the rudiments of the anten-
nules (a1), antennae (a2), and mandibles (m) : in front of
them is another pair of elevations on which the eyes (A)
subsequently appear. An unpaired elevation (TA) behind
the mouth, having the anus or aperture of the procto-
daeal depression at its summit (A\ is the rudiment of
the thorax and abdomen. The embryo is now in the
nauplius stage.

In many allied forms the embryo is hatched at this stage in the form
of a free-swimming nauplius larva (Fig. 100), characterised by the
presence of three pairs of appendages used for swimming (the first
simple, the second and third forked), and becoming the antennules,
antennae, and mandibles of the adult : the eye is median and sessile.
In the crayfish there is no free larva, and the young are hatched
at a much later stage.

The embryo is gradually transformed into the crayfish
by the appearance of fresh appendages, in regular order,
behind the first three ; by the elongation of the rudiment of

VII

DEVELOPMENT

385

FIG. 99. — Early embryo of the Crayfish in the nauplius stage.

A in the upper part of the figure is the eye ; /. the labrum overhanging the mouth,
on either side of which are the rudiments of the antennules (a1.), antennae («2.),
and mandibles (in.) ', behind them is the rudiment of the thorax and abdomen
(TA) with the anus (A). The rudiments of the first three pairs of ganglia
(G, ga~, gm^) are seen through the transparent ectoderm. (From Lang's Comp.
Anatomy, after Reichenbach.)

FIG. TOO. — Nauplius larva of Water-flea (Cyclops). (From Gegenbaur's

Comp. Anatomy.)

a, If, c, the three pairs of swimming appendages, representing the antennules,
antennae, and mandibles of the Crayfish : the median eye is seen between the
bases of the first pair.
PRACT. ZOOL. C C

386 THE CRAYFISH < HAP.

thorax and abdomen ; and by the gradual differentiation of
the appendages. When hatched the young animal agrees in
all essential respects with the adult, but its proportions are
very different, the cephalothorax being nearly globular and the
abdomen small. For some time after hatching the young
crayfishes cling in great numbers to the pleopods of the
mother by means of the peculiarly hooked chelae of the first
pair of legs.

All the members of the phylum Arthropoda (p. 360) are
characterised by each typical segment of the body bearing a
pair of appendages divisible into podomeres ; in addition
to this, there is an almost universal absence of cilia in the
phylum, the sperms are usually non-motile, the muscles are
nearly always of the striped kind, and the body-cavity, which
does not correspond to a true ccelome, is largely represented
by blood-spaces in free communication with the circulatory
system.

The phylum is divided into several classes, which are all
air-breathing except the Crustacea, the class to which the
Crayfish belongs and which also includes the Lobsters,
Crabs, Shrimps, Wood-lice, Barnacles, Water-fleas, &c.: in
these, when special respiratory organs are present, they
have the form of gills.

PRACTICAL DIRECTIONS.

CRAYFISH

In a living specimen note the cephalothorax •, abdomen, jointed
appendages, and exoskeleton, as well as the mode of walking and
swimming. Holding the animal out of water between the thumb and
finger, observe that bubbles are continually being formed on either side
of the lower part of the head (respiratory movements, see § B, i). Kill

vii PRACTICAL DIRECTIONS 3^7

with chloroform (p. 31), and at the end of the day's work, preserve your
dissection in 70 per cent, spirit or 3 per cent, formaline.

A. External characters.

1. Note again the cephalothorax and abdomen, and that the abdomen
consists of seven movable segments or metameres. Examine the ventral
side of the cephalothorax, and note that it also is composed of a number
of segments all fused except the ventral part of the last.

2. Examine the third or fourth abdominal segment closely, and note
that it is connected with the segments in front and behind by a sort of
peg- and socket-joint on either side, and that the chitinous exoskeleton
at the joints is soft and pliable, forming an articular membrane ', while
elsewhere it is calcined. Distinguish between the dorsal convex
tergum, the ventral sternum, and the pleuron projecting downwards on
either side from the tergum.

3. Examine the appendages of the same segment : they are attached to
the sterna, near the pleura, by articular membranes. Each consists of a
basal or proximal portion — the protopodite, to which two distal, many-
jointed parts are attached — an inner endopoditc, and an outer exopodite.
The cuticle covering the segments of the limb, or podonieres, is more or
less calcified, and the distal segments are covered with feathery seta.

4. The second to the sixth abdominal segments are essentially similar
to one another except as regards the appendages of the second in the
male and of the sixth in both sexes (p. 364). The first abdominal
segment is smaller than the others, and its pleura are reduced. The
sixth abdominal appendages are very large, and, together with the anal
segment or telson, form the tail-Jin.

5. The cephalothorax is reckoned as consisting of a prostomium and
12 metameres, which are completely fused dorsally and laterally, form-
ing a large calcified shield — the carapace. Thus the entire number
of segments is 20 (prostomium + 18 metameres + telson). The
sternal region of the head is bent upwards.

6. Note : — a, the transverse cervical groove on the dorsal surface of the
carapace, extending forwards laterally and forming the boundary between
the head (prostomium + 4 metameres) and the thorax (8 metameres);
by the two longitudinal branchio-pericardial grooves on the tergal region
of the thoracic portion of the carapace, about \ inch apart : the part of
the exoskeleton between these covers the heart, and the part below each
groove forms a large plate, the gill-cover ; at the side of the thorax ;
c, the rostrum, movable eye-stalks, and epistoma (p. 363).

c c 2

THE CRAYFISH

7. Note the following apertures : Median — a, the mouth, on the
ventral surface of the head, between the jaws ; /;, the anus, on the
ventral side of the telson : Paired — <:, the auditory, aperttire, on the
dorsal side of the basal joint of the smaller feeler or antennule (it
will be seen better later on) ; d, the renal aperture -, on a conical ventral
elevation of the basal joint of the larger feeler (antenna) ; e, the genital
aperture, in the male on the basal joint of the last thoracic leg, and in
the female on the last thoracic leg but two.

B. Respiratory organs.

1. Carefully cut away the left gill-cover with scissors, and fix the
animal under water on its right side, so as to expose the left gill-
chamber containing the feathery-looking gills. The inner wall of the
chamber is formed by the proper wall of the thorax, and the chamber is
open behind and below. In front of the gills is a groove, in which a
flattened plate (see p. 367) works backwards and forwards during life,
driving the water out in front, and causing the bubbles already noticed.

2. The £?//.$• are 18 in number, and each has the form of a bottle-
brush. The six podobranchs are external to the arthrobranchs and
pleurobranchs (p. 375), and each is attached to a large folded and
corrugated epipodite (p. 366). The gills are related to definite metameres,
as will be seen from the following table, in which ep stands for epipodite,
and v for the vestige of a gill. Note that the first pair of thoracic
limbs bears a simple large epipodite only.

THORACIC
SEGMENTS.

I.

II.

III.

IV.

V.

VI.

VII.

VIII.

TOTAL.

Podobranchs. . .

o+ep

i+ef

!+*/

!+*/

i+&

I+#

i+#

o

6 + 7^

Arthrobranchs

0

i

2

2

2

2

2

0

IT

Pleurobranchs

0

o

0

0

O

V

V

I

I + 2Z>

TOTAL

0 + #

2 + #

3+eJ

3 + */

3+ep

3+v+ep

1+v+ep

I

i8 + 2V+7#

3. Turn down the podobranchs and make out the relations of the
arthrobranchs from the above table. Then turn these down also, or cut
them off, and note the single complete pleurobranch and the two
vestigial ones. Cut off an arthrobranch and examine its structure, noting
the afferent and efferent blood-vessels in its stem. Sketch.

vii PRACTICAL DIRECTIONS 389

4. Note the branchiocardiac veins on the inner side of the thoracic
wall. Blow air or inject French blue (see p. 99) into the cut bases of
the gills removed, and note that the branchiocardiac trunks extend up-
wards to the pericardial-sinus (see below) from the gills.

C. General Dissection.

Holding the animal in your left hand, insert a scalpel carefully
beneath the hinder edge of the carapace on the dorsal side, so as to
separate the exoskeleton from the soft integument, and then with the
large scissors cut along the outer side of each branchio-pericardial
groove, and remove the median portion of the carapace. Note the pig-
mented integument and then remove it, when some of the nearly
colourless blood will ooze out.

1. I. Examine a drop of blood under the microscope, adding salt
solution. Note the nucleated amoeboid corpuscles. Sketch.

2. The pericardial sinus will now be exposed, containing the heart
with three pairs of valvular ostia (only the dorsal ostia can be seen at
present), through which the blood enters the heart from the pericardial
sinus.

Inject some French blue into the heart through one of the ostia,
so as to fill the arteries (tying is unnecessary). Then remove the
dorsal part of the exoskeleton and integument bit by bit, all along
the thorax and abdomen, as well as the pair of longitudinal extensor
muscles lying just beneath the dorsal integument of the abdomen. Pin
down under water, dorsal surface uppermost, and note : —

3. The absence of a continuous muscular layer in the body-wall and
of a true cceloine, and the presence of irregular spaces (blood sinuses)
between the viscera and muscles.

4. The delicate arteries, arising from the anterior and posterior
ends of the heart : — a, the anterior median ophthalmic artery, running
forwards to the eye -stalks ; b, the paired antennary artery, on either
side of a, and passing forwards and downwards to supply the gizzard,
renal organ, feelers, &c. ; c, the hepatic artery (also paired), rather
further back and more ventral, extending into and supplying the diges-
tive gland ; d, the median dorsal abdominal artery, arising from the
posterior end of the heart, and running along the dorsal side of the
intestine, giving off branches in each metamere ; c, the sternal artery,
arising just beneath the anterior end of d, and passing directly ventral-
wards to one side of the intestine (this will be seen better later on :
compare Figs. 93 and 94) : it perforates the ventral nerve-cord, and

390 - THE CRAYFISH CHAP.

divides into (a) a ventral thoracic artery supplying the segments and
appendages of the thorax, and (b) a ventral abdominal artery, sup-
plying the segments and appendages of the abdomen (this artery can
be seen in injected specimens through the transparent cuticle).

5. Note the position of the following parts before dissecting further : —
a, the gizzard, a large sac in the head, with two pairs of muscles passing
to the integument (now cut through) ; b, the adductor muscles of the
mandible, just external to a ; c, the paired, brownish or greenish diges-
tive gland on either side of, and extending further back than the
gizzard ; above it are a7, the gonads, on either side of and behind the
pericardial sinus. In the male, the spermary is small and whitish, and
each spermiduct is a coiled, densely white tube ; in the female, the ovary
is a larger, brownish organ, containing prominent ova. (In both sexes,
the paired character of the gonads is partly lost by fusion : a pair of
anterior lobes, and a single posterior lobe can be seen in each. ) Sketch.
By slightly raising the surrounding parts the gonaducts can be seen to
pass ventralwards to their external apertures (p. 388), the oviducts
being thin -walled and straight. White masses, the spermatophores
(p. 382), will very likely be found stuck on to the sternal region of the
body.

6. Tease up a small portion of the spermary or of a spermatophore ;
stain, and mount in glycerine. Examine under the microscope and note
the rounded and flattened sperms, each with a number of stiff, curved
processes coming off from the periphery. The sperms are non-motile.
Sketch.

Remove the heart and reproductive organs carefully, noting the
sternal artery (see above) as you do so, and taking especial care not to
injure the surrounding parts. Examine the heart under water, and
note the six ostia.

II. The enteric canal. Note — I. The oval month, bounded by the
labruin in front, leading into a short and wide gullet (this will be seen
later on), which dilates to form the large gizzard (Fig. 93), filling up
a considerable portion of the head and extending into the thorax : a
transverse constriction divides it into an anterior and a posterior
portion ; both gullet and gizzard are lined by chitin. The chitinous
cuticle of the gizzard is calcined in places to form the sclerites or
so-called "ossicles" of the "gastric mill." Note the two median
sclerites bounding the transverse constriction in front and behind
respectively : to them the anterior and posterior pairs of muscles
(mentioned in § 5) are attached.

vii PRACTICAL DIRECTIONS 391

2. Following on the gizzard is the short, thin-walled mid-gut, on the
dorsal side of which is a small cacum. It has no chitinous lining, and
the large duct of the digestive gland opens into it on either side. Each
digestive gland is made up of three main lobes, and consists of a number
of small blind tubes.

3. The hind-gut, which runs straight to the anus. Its inner surface
is raised into longitudinal ridges which take a slightly spiral course,
and it is lined by a thin chitinous cuticle.

4. Carefully press the gizzard backwards and note a, the brain, just
behind the bases of the small feelers ; b, the § ullet ; and c, a pair of
white nerve cords (connectives'] coming off from the brain and em-
bracing the gullet. Taking care not to injure these parts of the
nervous system, cut through the gullet, just above the connectives, and
then gently remove the whole enteric canal, together with the digestive
gland, from the body, cutting through the intestine just in front of the
anus. Examine the whole digestive system under water.

5. Note again the mid-gut and the digestive glands and ducts ; then
remove the digestive gland of one side, and sketch the enteric canal
from the same side. Slit up the hind-gut so as to see the ridges and
cuticle.

6. Clean the walls of the gizzard and note the other sclerites of the
gastric mill and the " gastroliths" : —

Articulated to each end respectively of the two median sclerites
already referred to, in each lateral wall of the gizzard, is a lateral
sclerite, the two articulating with one another at their other. ends, so
that these six sclerites together form a sort of hexagonal frame. Two
other median sclerites, arising respectively from the median ones men-
tioned above, extend downwards into the constriction between the two
portions of the gizzard, and these join below at an angle, where they
bear a median tooth. Each of the posterior lateral sclerites bears a
lateral tooth. ,

Cut open the anterior end of the gizzard, and note the strongly calci-
fied, brownish, median tooth, and the two large lateral teeth. Seize
hold of the two median ossicles with two pairs of forceps, one in each
hand, and pull gently backwards and forwards (in the direction in
which the muscles pull). It will then be seen that the median and
lateral teeth come together in the middle line so as to act as a " gastric
mill." Note the slit-like lumen of the part of the gizzard behind this
and the arrangement of the seta which act as strainers. Make sketches
as you proceed.

392 THE CRAYFISH CHAP.

III. i. The chief muscles of the body: — a, the paired and seg-
mented dorsal extensor, arising from the side walls of the thorax, and
extending into the abdomen above the intestine, giving off slips to each
segment of the abdomen (this muscle has already been removed) ; and
by the large and complex ventral muscles, the lateral halves of which
are not separate from one another, the fibres being interwoven, some-
what like those of a rope ; slips are given off to the abdominal sterna.
These act mainly as a flexor of the abdomen (compare p. 370).

2. Muscles pass from the body to the proximal joints of the limb :
those between successive podomeres will be examined at a later
stage (§ D).

3. Note again the paired adductor of the mandible (p. 390), and
trace its calcified tendon downwards to its insertion on to the
mandible.

4. Tease out a small piece of muscle so as to separate its fibres from
one another. Stain, and mount in glycerine. Note the transverse
striations, sarcolemma, and nuclei (compare Fig. 32). Sketch.

Remove the muscles of the body described above, noting the sternal
artery (390), and taking especial care to leave the abdominal nerve-
cord in situ when removing the large ventral muscles. Note that
in the thorax, the nerve-cord passes into a sternal canal, formed by a
series of ingrowths of the exoskeleton — the endophragmal system— from
which the muscles passing to the thoracic limbs arise. Insert the
scissors into the sternal canal, and cut away and remove its roof, bit by
bit. The whole of the central nervous system will then be exposed.

IV. Observe that a more marked distinction into ganglia and
connectives is seen than in the case of the earthworm, and that the
fusion of the two lateral halves of the cord or chain has only affected
the ganglia, the connectives being double all the way along.

1. Note: — a. The brain, or compound supra-ccsophageal ganglia ;
b, the (Ksophageal connectives ; and c, the postoral ventral net ve-cord,
consisting of a large compound sub-cesophageal ganglion and of 1 1
segmental ganglia, united by paired connectives. Beneath the cord, the
ventral thoracic and abdominal arteries (p. 390) will be seen, the main
sternal artery passing between the connectives joining the fourth and
fifth postoral ganglia.

2. The brain gives off nerves to the eyes and the two pairs ot
feelers : the subcesophageal ganglion supplies the mandibles and
five following pairs of appendages and their segments. Each of

vii PRACTICAL DIRECTIONS 393

the other ganglia supplies one segment (with its appendages) only,
except in the case of the last or 6th abdominal ganglion, which
gives off nerves to the telson. (The small anterior visceral nerve^
arising from the brain and cesophageal connectives, and supplying the
fore-gut, will have been removed ; the posterior visceral nerve, supply-
ing the hind-gut, arises from the last abdominal ganglion.) Sketch.

3 Tease up a ganglion in salt-solution, stain, and examine for
nerve-cells.

V. The antennary or green-glands are situated just behind the bases
of the large feelers. Blow through the renal aperture of one side (p. 388)
and note the duct and urinary bladder on the dorsal side of the gland.
Then cut through the duct, remove the whole gland, and examine under
water. Sketch.

D. The Appendages. — Remove the appendages of one side,
beginning with the last, one by one, cutting through the articular
membrane with a scalpel, and then taking hold of the basal joint with
the forceps and pulling the appendage away. Work through the de-
scription on pp. 364-367 and sketch typical appendages from each region.
Note the delicate paragnatha behind the mouth and the labrtim in
front of it (p. 370).

Procure some pond- water containing specimens of the small " water,
flea " known as Cyclops, which will be recognised by its pear-shaped
body, and by the two oval egg-sacs of the female. Examine drops of
this water until you find some nauplius- larva- of Cyclops, noting the
three pairs of appendages and the median eye (p. 384 and Fig. 100).

The arrangement of the joints and muscles of the limbs can be well
seen by examining the large first leg, or cheliped. Note that each joint
works in a different plane, and then cut away the exoskeleton
from one side of the two or three distal podomeres, so as to expose
the muscles (compare Fig. 92). Then remove these, and note their
chitinous tendons. Observe that the adductor muscle and its tendon,
which closes the pincers, is much larger than the abductor muscle.

E. Sensory Organs.

1. Tactile organs. Snip off some setae from the body or appendages,
and examine under the microscope. Sketch.

2. " Olfactory"" organs. Examine the outer flagellum of the
antennule under the low power, and. note the tufts of spatula-like
** olfactory " seta on the ventral surface. Sketch.

3. "Auditory" organ (statocyst}. Carefully cut away the convex

394 THE CRAYFISH . CHAP.

ventral side of the basal segment of the antennule with scissors, so as
to expose the statocyst. Cut this out and place it on a slide, carefully
removing the muscles surrounding it, as well as the setae around its
aperture. Note the contained grains of sand, and then wash them
away. Stain with magenta and mount in glycerine, flattening, the sac
out with a cover-glass. Note that the sac is an involution of the
integument lined by cuticle, and that it contains simple, jointed sensory
seta of various sizes, arranged in rows, and that branches of the
antennulary nerve run up the stem of each seta. Sketch.

4. The Eyes. Remove one of the eye-stalks, and note the apparently
black, uncalcified, oval portion of the cuticle (corned] at its distal end.
Strip this off, and note that it is transparent. Then wash off any
pigment which may have come away with it and mount in water.
Observe the c or neal facets. Then cut the eye-stalk into two longitudinal
halves with a knife, and examine with a lens under water. Note the
optic nerve entering the stalk, and enlarging to form the optic ganglion,
from which a number of bodies (ommatided] radiate outwards to the
corresponding facets of the cornea. The ommatidea are separated
from one another by pigment. Sketch.

5. Examine longitudinal sections of the eye-stalks, decalcified
and prepared as directed on p. 136, and note in detail the above
parts. Each ommatideum lies beneath the corresponding corneal
facet, and is made up of an outer vitreous body or crystalline cone, and
an inner retinula formed of sensory cells and enclosing a transversly-
striated, spindle-shaped, refractive body or rhabdome, and closely con-
nected with the optic ganglion. Note also the pigment between the
ommatidea. Sketch.

F. Structure of the Exoskeleton.

In order to follow this out in greater detail, proceed as follows : —

1. Cut through the thorax and abdomen of a crayfish transversely,
and note the relations of the hard and soft parts. (Compare Figs. 90
and 94. )

2. Dip a crayfish into hot water, so that the soft parts come away
easily. Open up the cephalothorax from the dorsal side, separate
some of the abdominal segments, and clean thoroughly. Examine
the joints and make out the relations of the endophragmal system
(p. 392), looking something like a lattice-girder.

3. Examine your preparation of the eye once more under the micro-
scope and notice the part where the section passes through the outer

vii PRACTICAL DIRECTIONS 395

wall of the eye-stalk, so as to make out the microscopic structure of the
integument and exoskeleton, which apart from the calcification, is similar
in all parts. Notice the epiderm, and the thick, laminated, chitinous
cuticle, the superficial layer of which is uncalcified throughout.

G. Dissection from the Side.

Cut through the carapace on the dorsal side as before, but be careful
to keep your cut very slightly to one side — say the left — of the
middle line, and when the pericardium is exposed, inject the arteries
through the heart as before (p. 389). Then remove the gill-cover of the
same side, and examine the gills once more.

Remove and examine again all the appendages of the same side, and
then continue cutting longitudinally through the exoskeleton, both
dorsally and ventrally, close to the middle line, taking care you do
not injure any median organs. Remove the entire exoskeleton of
this side, as well as the dorsal and ventral muscles, cutting through the
latter in the median line very carefully.

After your dissection has been pinned down under water, the anten-
nary gland, digestive gland, and gonad of the same side should also be
removed, and the gizzard cut open. Then tidy up the dissection,
which will now be reduced to a longitudinal section like that represented
in Fig. 93.

Once more carefully follow out the structure and .relations of all the
organs exposed, and sketch your dissection.

CHAPTER VIII

THE FRESH-WATER MUSSEL — CHARACTERS OF THE PHYLUM

MOLLUSCA ENUMERATION OF THE CHIEF PHYLA OF THE

ANIMAL KINGDOM.

IN the mussel we meet with an entirely new type of
structure : the animal, like the worm and crayfish, is bi-
laterally symmetrical, but there is no trace of metameric
segmentation ; the power of locomotion is greatly restricted,
and food is obtained passively by ciliary action, as in Infu-
soria, not by the active movements of definite seizing organs
— tentacles, limbs, or protrusible mouth — as in most of the
Metazoa (p. 292).

Fresh-water mussels are found in rivers and lakes in
most parts of the world. Anodonta cygnea, the swan-mussel,
is the commonest species in England ; but another kind,
Unio pictorum, occurs in many places, and the pearl-mussel,
Unto margaritifera, is found in mountain streams, while
other species of the same genus are universally distributed.

The mussel is enclosed in a brown, calcified shell, formed of
two separate halves or valves hinged together along one edge.
It lies on the bottom, partly buried in the mud or sand,
with the valves slightly gaping, and in the narrow cleft thus
formed a delicate, semi-transparent substance is seen, the
edge of the mantle or pallium. The mantle really consists
of separate halves or lobes corresponding with the valves of

CHAP, vin EXTERNAL CHARACTERS 397

the shell (compare Fig. 103), but in the position of rest the
two lobes are so closely approximated as to appear simply
like a membrane uniting the valves. At one end, however,
the mantle projects between the valves in the form of two
short tubes, one (Fig. 101, ex. sph) smooth-walled, the
other (in. spJi) beset with delicate processes or tentacles.
By diffusing particles of carmine or indigo in the water it can
be seen that a current is always passing in at the fringed tube
-—hence called the inhalant siphon^ and out at the smooth or
exhalant siphon. Frequently a semi-transparent, tongue-like
body (//) is protruded between the valves at the opposite
side from the hinge and at the end furthest from the siphons :
this is they^tf/, by means of which the animal is able slowly
to plough its way through the sand or mud. When the
mussel is irritated the foot and siphons are withdrawn
and the valves tightly closed. In a dead animal, on the
other hand, the shell always gapes, and it can then be seen
that each valve is lined by the corresponding lobe of the
mantle, and that the exhalant siphon is formed by the union
of the lobes above and below it and is thus an actual tube ;
but that the boundary of the inhalant siphon facing the
gape of the shell is simply formed by the approximation
of the mantle-lobes, so that this tube is a temporary one.

The hinge of the shell is dorsal, the gape ventral, the end
bearing the siphons posterior, the end from which the foot
is protruded anterior : hence the valves and mantle-lobes are
respectively right and left.

In a dead and gaping mussel the general disposition of
the parts of the animal is readily seen. The main part of
the body lies between the dorsal regions of the valves : it is
produced in the middle ventral line into the keel-like foot :
and on either side between the foot and the corresponding
mantle-lobe are two delicate, striated plates, the gills or

398 THE MUSSEL CHAP.

ctenidia, as they are often called. Thus the whole animal
has been compared to a book, the back being represented
by the hinge, the covers by the valves, the fly-leaves by the
mantle-lobes, the two first and the two last pages by the
gills, and the remainder of the leaves by the foot (Fig. 103.)

When the body of the mussel is removed from the shell
the two valves are seen to be united, along a straight hinge-
line^ by a tough, elastic substance, the hinge-ligament (Fig.
103, Ig) passing transversely from valve to valve. It is by
the elasticity of this ligament that the shell is opened ; it is
closed, as we shall see, by muscular action : hence the mere
relaxation of the muscles results in opening the shell. In
Anodonta the only junction between the two valves is
afforded by the ligament, but in Unio each is produced into
strong projections and ridges, the hinge-teeth, separated by
grooves or sockets, and so arranged that the teeth of one
valve fit into the sockets of the other.

The valves are marked externally by a series of concentric
lines parallel with the free edge or gape, and starting from
a swollen knob or elevation, the umbo, situated towards the
anterior edge of the hinge-line. These lines are lines of
growth. The shell is thickest at the umbo, which represents
the part first formed, and new layers are deposited under
and concentrically to this original portion as secretions from
the mantle, the shell being, like the armour of the crayfish, a
cuticular exoskeleton. As the animal grows each layer projects
beyond its predecessor, and in this way successive outcrops
are produced, giving rise to the markings in question. In
the region of the umbo the shell is usually more or less
eroded by the action of the carbonic acid in the water.

The inner surface of the shell also presents characteristic
markings. Parallel with the gape, and at a short distance
from it, is a delicate streak caused by the insertion into the

vin SHELL 399

shell of muscular fibres from the edge of the mantle : the
streak is hence called the pallial line. Beneath the anterior
end of the hinge the pallial line ends in an oval mark, the
anterior adductor impression, into which is inserted one of the
muscles which close the shell. A similar, but larger, posterior
adductor impression lies beneath the posterior end of the
hinge (compare Fig. 101,0. ad, p. ad). Two smaller markings
close behind the anterior adductor impression, dorsal and
ventral respectively, mark the origin of the anterior retractor
and of the protractor muscle : one just anterior to the
posterior adductor impression, that of the posterior retractor.
From all these impressions faint converging lines can be
traced to the umbo : they mark the gradual shifting of the
muscles during the growth of the animal.

The shell consists of three layers, the outer layer, as in
the crayfish, being uncalcified. Outside is a brown horn-
like layer, the periostracum, composed of conchiolin, a
substance allied in composition to chitin. Beneath this is
a prismatic layer formed of minute prisms of calcium car-
bonate, separated by thin layers of conchiolin ; and, lastly,
forming the internal part of the shell is the nacre, or
" mother-of-pearl," formed of alternate layers of carbonate
of lime and conchiolin arranged parallel to the surface.
The periostracum and the prismatic layer are secreted
from the edge of the mantle only, the pearly layer from the
whole of its outer surface. The hinge-ligament is continu-
ous with the periostracum, and is to be looked upon simply as
a median uncalcified portion of the shell, which is therefore,
in strictness, a single continuous structure.

By the removal of the shell the body of the animal is seen
to be elongated from before backwards, narrow from side to
side, produced on either side into a mantle-lobe, and con-
tinued ventrally into a keel-like visceral mass, which passes

400 THE MUSSEL CHAP.

below and in front into the foot (Fig. 101, //). Thus each
valve of the shell is in contact with the dorso-lateral region
of the body of its own side together with the corresponding
mantle-lobe, and it is from the epithelium covering these
parts that the shell is formed as a cuticular secretion.
The whole space between the two mantle-lobes, contain-
ing the gills, visceral mass, and foot, is called the mantle-
cavity.

A single layer of epithelial cells, the deric epithelium or
epiderm, covers the whole external surface — i.e., the body
proper, both surfaces of the mantle, the gills, and foot ;
that of the gills and the inner surface of the mantle is cilia-
ted. Beneath the epiderm come connective and muscular
tissue, which occupy nearly the whole of the interior of the
body not taken up by the viscera, the ccelome being, as we
shall see, much reduced. The muscles are all unstriped,
and are arranged in distinct bands or sheets, many of them
being very large and conspicuous. The largest are the anterior
WbA posterior adductors (¥\gs. 101 and 103, a. ad, p. ad], great
cylindrical muscles which pass transversely across the body
and are inserted at either end into the valves of the shell,
which are approximated by their contraction. Two muscles
of much smaller size pass from the foot to the shell, which
they serve to draw back : they are the anterior and posterior
retractors. A third muscle arises from the shell close to the
anterior adductor, and has its fibres spread fan-wise over the
visceral mass and acting as a protractor. The substance of
the foot itself consists of a complex mass of intrinsic muscles,
which aid in withdrawing the foot : its protrusion is largely
due to vascular turgescence. Lastly, all along the border of
the mantle is a row of delicate pallial muscles, which, by
their insertion into the shell, give rise to the line already
seen.

vni ENTERIC CANAL 401

The ccelome is reduced to a single ovoidal chamber, the
pericardium (Figs. 101 and 103,^), lying in the dorsal region
of the body and containing the heart and part of the intes-
tine : it is lined by ccelomic epithelium, and does not corre-
spond with the pericardial sinus of the crayfish, which is
a blood-space (p. 376). In the remainder of the body the
space between the epiderm and the viscera is filled by
the muscles and connective-tissue.

The mouth (Fig. 101, mtJi) lies in the middle line just
below the anterior adductor. On either side of it are two
triangular flaps, the internal and external labial palps ; the
external palps unite with one another in front of the mouth,
forming an upper lip ; the internal are similarly united
behind the mouth, forming a lower lip : both are ciliated
externally. The mouth leads by a short gullet (gut) into a
large stomach (st\ which receives the ducts of a pair of
irregular, dark-brown, digestive glands (d. gl). The intestine
(tnt)\ which is lined by a ciliated epithelium, goes off from
the posterior end of the stomach, descends into the visceral
mass, where it is coiled upon itself, then ascends parallel to
its. first portion, turns sharply backwards, and proceeds, as
the rectum (rct\ through the pericardium — where it traverses
the ventricle of the heart, and above the posterior adductor,
finally discharging by the anus (a) into the exhalant siphon,
or cloaca. The wall of the rectum is produced into a
longitudinal ridge, or typhlosole (Figs. 101 and 103, ty\ and
two similar ridges begin in the stomach and are continued
into the first portion of the intestine. The stomach con-
tains at certain seasons of the year a gelatinous rod, the
crystalline style.

The gills consist, as we have seen, of two plate-like
bodies on either side between the visceral mass and the
mantle : we have thus a right and a hft outer, and a right

PRACT. ZOOL. D D

402

THE MUSSEL

and a left inner gill (Fig. 103, ext. gl, inf. gl.}. Seen from
the surface (Fig. 101), each gill presents a delicate double
striation, being marked by faint lines running parallel with,

> «« 13

2 .

and by more pronounced lines running at right angles to,
the long axis of the organ. Moreover, each gill is double,
being formed of two* similar plates, the inner and outer

GILLS

403

lamella, united with one another along the anterior, ventral,
and posterior edges of the gill, but free dorsally. The gill
has thus the form of a long and extremely shallow bag, open
above (Figs. 102 and 103) : its cavity is subdivided by vertical
plates of tissue, the inter-lamellar junctions (i. I. /), which
extend between the two lamellae and divide the intervening

IV.t

FIG. 102. — Diagram of the structure of the gill of Anodotita.

The gill is made up of V-shaped gill-filaments (f) arranged in longitudinal series
and bound together by horizontal inter-filamentar junctions (i.f.j) which cross
them at right angles, forming a kind of basket-work with apertures, the ostia
(as) leading from the outside and opening (0s') into the cavity of the gill. The
latter is divided by vertical partitions, the inter-lamellar junctions (/. l,j\ into
compartments or water-tubes (?£>. t\ which open also into the supra-branchial
chamber ; b. v. blood-vessels. (From Parker and Haswell's Zoology.)

space into distinct compartments or water tubes (Figs. 101 and
102, w. /), closed ventrally, but freely open along the dorsal
edge of the gill. The vertical striation of the gill is due to
the fact that each lamella is made up of a number of close-set
gill-filaments (Fig. I02,/): the longitudinal striation to the
circumstance that these filaments are connected by horizontal
bars, the inter-filamentar junctions (i.f.j). At the thin free
or ventral edge of the gill the filaments of the two lamellae

D D 2

404 THE MUSSEL CHAP.

are continuous with one another, so that each gill has actually
a single set of V-shaped filaments, the outer limbs of which
go to form the outer lamella, their inner limbs the inner
lamella. Between the filaments, and bounded above and
below by the inner-filamentar junction are minute apertures
or ostia (0s), which lead from the mantle-cavity through a
more or less irregular series of cavities into the interior of
the water-tubes. The filaments themselves are supported
by chitinous rods, and covered with ciliated epithelium, the
large cilia of which produce a current running from the
exterior through the ostia into the water-tubes, and finally
escaping by the wide dorsal apertures of the latter. The
whole organ is traversed by blood-vessels (b.v).

The mode of attachment of the gills presents certain
features of importance (compare Fig. 103, A, B, c). The
outer lamella of the outer gill is attached along its whole
length to the mantle : the inner lamella of the outer, and
the outer lamella of the inner gill are attached together to
the sides of the visceral mass a little below the origin of the
mantle : the inner lamella of the inner gill is also attached to
the visceral mass in front, but is free further back. The gills
are longer than the visceral mass, and project behind it, below
the posterior adductor (Figs. 101 and 103, c),as far as the pos-
terior edge of the mantle : in this region the inner lamellae of
the inner gills are united with one another, and the dorsal
edges of all four gills constitute a horizontal partition between
the pallial cavity below and the exhalant chamber or cloaca
above. Owing to this arrangement it will be seen that the
water-tubes all open dorsally into a supra-branchial chamber
(Fig. 103, s. br. c)^ continuous posteriorly with the cloaca and
thus opening on the exterior by the exhalant siphon.

The physiological importance of the gills wiil now be
obvious. By the action of their cilia a current is produced

TRANSVERSE SECTIONS

405

rcb

FIG. 103. — Anodonta. Three transverse sections. A, through the anterior part of
the visceral mass B, through he posterior part of the visceral mass ; C, through
the posterior adductor muscle.

au. auricle ; bl. urinary bladder ext.gl. external gill ; ft. foot ; i. l.j. inter
lamellar junction ; int. intestine; int. gt. internal gill; /W. kidney; k. o. peri
cardial gland ; Ig. ligament ; m. mantle ; p. ad. posterior adductor ; pc. pericar
dium ; ret. rectum ; .s. br. c. supra-branchial chamber ; sh. shell ; ty. typhlosole
v. ventricle; v. c. vena cava ; v.gn. visceral ganglion, v. m, visceral mass
(From Parker and Haswell's Zoology, after Howes, slightly altered.)

406 THE MUSSEL CHAP.

which sets in through the inhalant siphon into the pallial
cavity, through the ostia into the water-tubes, thence into the
supra-branchial chamber, and out at the exhalant siphon.
The in-going current carries with it not only oxygen for the
aeration of the blood, but also diatoms, infusoria, and other
microscopic organisms, which are swept into the mouth by
the cilia covering the labial palps. The out-going current
carries with it the various products of excretion and the
faeces passed into the cloaca. The action of the gills in
producing the food current is of more importance than their
respiratory function, which they share with the mantle.

The excretory organs are a single pair of curiously-modified
nephridia, situated one on each side of the body just below
the pericardium. Each nephridium consists of two parts, a
brown, spongy, glandular portion or kidney (Figs, roi and 103,
kd\ and a thin-walled, non-glandular part or bladder (bl).
The two parts lie parallel to one another, the bladder being
placed dorsally and immediately below the floor of the
pericardium : they communicate with one another posteriorly,
while in front the kidney opens into the pericardium (r. p. a\
and the bladder on the exterior by a minute aperture (r. ap]
situated between the inner gill and the visceral mass. Thus
the whole organ, often called, after its discoverer, the organ
of Bojanus, is simply a tube bent upon itself, opening at one
end into the ccelome (see p. 401), and at the other on the
external surface of the body : it has therefore the normal
relations of a nephridium (p. 340). The two bladders com-
municate anteriorly (Fig. 101, x\ and their epithelium is
ciliated, producing an outward current.

It seems probable that an excretory function is also discharged by a
large glandular mass of reddish-brown colour, called the pericardia!
gland or A'eber's organ (Fig. 103, /'.<?). It lies in the anterior region of
the body just in front of the pericardium, into which it discharges.

CIRCULATORY SYSTEM

407

The circulatory system is well developed. The heart lies
in the pericardium, and consists of a single ventricle (Figs.
101 and 103 B, v) and of right and left auricles (au). The
ventricle is a muscular chamber which has the peculiarity
of surrounding the rectum (ret] : the auricles are thin-walled

FIG. 104. — Diagram of the circulatory system of A nodonta.

The blood received from the auricles (au) is pumped by the ventricle (v) into the
aorta (ad) and thence passes to the mantle (arfi.) and to the body generally
(arft.). The blood which has circulated through the mantle is returned directly
to the auricle : that from the body generally is collected into the vena cava (v. c),
passes by nephridial veins (nph. v) to the kidneys, thence by afferent branchial
veins (of. br. v) to the gills, and is returned by efferent branchial veins (ef. br. v)
to the auricles ; pc. pericardium. (From Parker and Haswell's Zoology,)

chambers communicating with the ventricle by valvular
apertures opening towards the latter. From each end of the
ventricle an artery is given off, the anterior aorta (Fig. 101,
a. ao] passing above, the posterior aorta (p. ao] below the
rectum, From the aortae the blood passes into arteries (Fig.

408 THE MUSSEL CHAP.

104, artl,arfi) which ramify all over the body, finally forming
an extensive network of vessels many of which are devoid
of proper walls and have therefore the nature of sinuses.
The returning blood passes into a large longitudinal vein,
the vena cava (Figs. 103 and 104, v. c), placed between the
nephridia, whence it is taken to the kidneys themselves
(Fig. 104, nph. v), thence by afferent branchial veins (af. br. v)
to the gills, and finally returned by efferent branchial veins
(ef. br. v) to the auricles. The mantle has a very extensive
blood supply, and probably acts as the chief respiratory organ
(p. 406) : its blood (art.1) is returned directly to the auricles
without passing through either the kidneys or the gills.
The blood is colourless and contains leucocytes. There is
no communication between the blood-system and the
pericardium.

The nervous system is formed on a type quite different
from anything we have yet met with. On either side of the
gullet is a small cerebro-pleural ganglion (Fig. 101, c.pl. gn\
united with its fellow of the opposite side by a nerve-cord,
the cerebral commissure, passing above the gullet. Each
cerebro-pleural ganglion also gives off a cord, the cerebro-
pedal connective, which passes downwards and backwards to
a pedal ganglion (pd. gn) situated at the junction of the
visceral mass with the foot : the two pedal ganglia are so
closely united as to form a single bilobed mass. From each
cerebro-pleural ganglion there further proceeds a long cerebro-
visceral connective, which passes directly backwards through
the kidney and ends in a visceral ganglion (Figs. 101 and 103,
C, v. gn) situated on the ventral side of the posterior adductor
muscle. The visceral, like the pedal ganglia, are fused with
one another. The cerebro-pleural ganglia supply the labial
palps and the anterior part of the mantle ; the pedal, the
foot and its muscles ; the visceral, the enteric canal, heart,

vin SENSORY AND REPRODUCTIVE ORGANS 409

gills, and posterior portion of the mantle. The nerve-cells
are confined to the ganglia.

It will be seen that the cerebral commissures and cerebro-
pedal connectives, together with the cerebro-pleural and
pedal ganglia, form a nerve-ring which surrounds the gullet :
the cerebro-pleural ganglia may be looked upon as a supra-
cesophageal nerve mass corresponding with the brain of the
earthworm and crayfish, and the pedal ganglia as an infra-
cesophageal mass representing the ventral nerve-cord.

Sensory organs are poorly developed, as might be ex-
pected in an animal of such sedentary habits. In connec-
tion with each visceral ganglion is a patch of sensory
epithelium forming the so-called " olfactory organ " or, better,
osphradium, the function of which is apparently to test the
purity of the water entering by the respiratory current. Close
to the pedal ganglion a minute otocyst or statocyst, con-
taining a calcareous lithite (pp. 315 and 381) is found, the
nerve of which is said to spring from the cerebro-pedal
connective, being probably derived from the cerebral
ganglion. Sensory cells, probably tactile, also occur round
the edge of the mantle, and especially on the tentacles of
the inhalant siphon.

The sexes are separate. The gonads (Fig. 101 gon)
are large, paired, racemose (p. 135) bodies, occupying a con-
siderable portion of the visceral mass amongst the coils of
the intestine : the spermary is white, the ovary reddish. Each
gonad has a short duct which opens (g. ap) on the surface
of the visceral mass, just in front of the renal aperture.

In the breeding season the eggs, extruded from the genital
aperture, pass into the supra- branchial chamber, and so to
the cloaca. There, in all probability, they are impregnated
by sperms introduced with the respiratory current. The
X)osperms are then passed into the cavities of the outer gills,

4io

THE MUSSEL

CHAP.

which they distend enormously. Thus the outer gills act as
brood-pouches, and in them the embryo develops into the
peculiar larval form presently to be described.

As in the frog and earthworm, the cells formed by the
segmentation of the oosperm are of two sizes, small cells
composed entirely of protoplasm, and large cells loaded with
yolk-granules. The large become enclosed by the small
cells, but the enteron thus formed is very small and quite
unimportant during early larval life, the young mussels being

B. .t.

FIG. 105. — A, advanced embryo of Anodonta enclosed in the egg-membrane. B, free

larva or glochidium.

f. byssus ; s. shell ; sk. hooks ; sm. adductor muscle ; so. sensory hairs ; 71'.
ciliated area. (From Korschelt and Heider's Embryology.1)

nourished, after the manner of parasites (p. 280), by a
secretion from the gills of the parent.

The dorsal surface of the embryo is soon marked out by
the appearance of a deep depression, the shell-gland, which
secretes, in the first place, a single, median shell. This is,
however, soon replaced by a bivalved larval shell (Fig. 105,
s) of triangular form, the ventral angles being produced into
hooks (sti). The body at the same time becomes cleft from
below upwards (A), forming the right and left mantle-lobes.
On the ventral surface, between the lobes of the mantle, is

vin DEVELOPMENT 411

formed a glandular pouch, which secretes a long thread,
the byssus (/). The larva is now called a glochidium, the
subsequent history of which furnishes another example of a
means of ensuring dispersal in a sedentary animal (compare
pp. 274, 283, and 320).

The glochidia, entangled together by means of their
byssal threads, escape from the gills of the parent by the
exhalant siphon, and eventually attach themselves by their
hooked valves to the body of a passing fish, such as a
stickleback, becoming enclosed by an overgrowth of the skin
of the fish. Here they live for a time as external parasites,
gradually undergoing metamorphosis, and finally dropping
from the host and assuming the sedentary habits of the
adult.

This mode of development is exceptional amongst bivalves, in most
of which (e.g. oyster) the egg gives rise to a larva resembling the
trochosphere of many annelids (Fig. 89), the prostomial region then
growing out into a thickened rim or velum which bears the circlet of
cilia in front of the mouth, the larva at this stage being distinguished as
a veliger.

The Mussel belongs to the phylum Mollusca, which
includes, in addition to the bivalved " shell-fish " (such as
mussels, oysters, and cockles), the snails, slugs, whelks, peri-
winkles, &c. (most of which possess a univalved shell), as
well as the cuttle-fishes and their allies. These are all
sharply distinguished from the Arthropods by the absence
of segmentation, and by having, as a rule, an exoskeleton in
the form of a shell. The bivalves are included in the class
Pelecypoda or Lamellibranchiata, the essential structure of
which you will have learnt from your study of the Mussel.

You have now examined examples of the following chief divisions
or phyla of the animal kingdom (compare p. 219) :

Protozoa. Ammlata. Mollusca.

Ccelenterata. Arthropoda. Vertebrata,

4i2 THE MUSSEL CHAP.

In addition to these, you will probably have seen, when dissecting the
frog, certain parasites. One of these, Polystomum (p. 33), belongs
to a lowly group of worms of a flattened form, constituting the phylum
Platyhelminthes, which includes the parasitic liver-flukes and tape-
worms as well as certain free-living forms; another, Ascaris (p. 153),
belongs to the phylum Nemathelminthes, in which the parasitic
thread-worms are placed. Apart from certain other smaller groups,
which include such animals as "wheel-animalcules," "sea-mats,"
"lamp-shells," &c. , there only remains one other of the larger phyla of
which you will not have studied an example, viz., the phylum
Echinodermata? which is constituted by the star-fishes, sea-urchins,
sea-cucumbers, &c. : these are all inhabitants of the sea, and are peculiar
in exhibiting in the adult a more or less pronounced radial arrangement
of their parts (p. 296), and in possessing a curious calcareous exo-
skeleton developed within their integument, consisting of small particles
or of definitely-shaped plates. All the phyla with the exclusion of the
Vertebrata are spoken of collectively as the Invertebrata.

PRACTICAL DIRECTIONS.

FRESH-WATER MUSSEL.

Examine a living specimen in a vessel of water with some sand at the
bottom, and note the greenish-brown bivalve shell, the foot, and the
manner in which the animal buries itself, anterior end downwards,
with the pallial openings projecting posteriorly. Observe the currents
of water passing in at the fringed inhalant aperture, and out at the
exhalant aperture : these can be more easily observed if a little finely-
powdered indigo or carmine is placed in the water.

In order to kill the animal in as fully-extended a condition as possible,
place it in a saucepan or beaker of water, and heat over a gas-burner
or spirit-lamp until the water is warmed up to a temperature of about
40° C, when the foot will be protruded. Keep at this temperature
until the mussel is dead. The animal is best preserved from day to day
in 70 per cent, spirit, or 3 per cent, formaline.

A. External Characters.

Compare the shell with the carapace of the crayfish, and observe that
it covers the whole body and is densely calcified except along the dorsal
hinge-line, where it forms an elastic ligavient connecting the two lateral

vin PRACTICAL DIRECTIONS 413

valves of the shell. Note the lines of growth, and the umbo near the
hinge-line. The anterior end is rounded, the posterior end more
pointed. In dead specimens the valves gape, owing to the elasticity of
the ligament and to the relaxation of the adductor muscles (see below),
and they can then be seen to be lined by a fold of the integument, the
mantle ot pallium. By wedging the valves open still further, the anterior
and posterior adductor muscles are seen connecting the two valves ; also
the/00/, visceral mass, and^'//j, between the two mantle folds.

B. Dissection.

I. Carefully insert a scalpel between the left valve and mantle-edge,
and separate them all round the margin of the valve. Then cut through
the anterior and posterior adductor muscles close to the shell, and
remove the left valve, cutting through the ligament.

1. Compare the positions of the cut muscles on the body with the
muscular impressions on the shell : — a. the large anterior adductor near
the dorsal and anterior end ; and close behind it — b. the protractor and
c. the anterior retractor, the latter dorsal to the former ; d. the large
posterior adductor near the dorsal and posterior end, on the upper and
anterior margin of which is e. the posterior retractor. Note also the
thickened ventral edge of the mantle, the corresponding pallial line
on the valve, as well as the lines from the muscular impressions con-
verging towards the umbo, the smooth longitudinal hinge (with hinge-
teeth in Unio), the ligament, and the iridescent inner layer of the shell.
Sketch the inner surface of the valve.

2. Break the shell across, and examine the edge with a lens, noting
the three layers of which it is composed : —a. the thin outer uncalcified
periostracum ; b. the thicker middle prismatic layer ; and c. the inner
pearly or nacreotis layer.

Pin down the animal firmly under water, leaving it in the right valve :
insert the pins through the ligament and muscular part of the foot, and
obliquely against the edges of the shell. Note : —

3. The left mantle-lobe uniting with the right above the middle of the
anterior adductor muscle anteriorly, and behind the posterior adductor
posteriorly. Just behind the latter muscle the margins of the mantle
are much thickened ; and in life, the two approximated mantle-edges
here separate to form the exhalaut and inhalant apertures, the latter
of which is provided with short tentacles.

4. Turn back the left mantle-lobe and note the mantle-cavity and its
contents (Fig. 101) : — a. the foot and visceral mass ; b. the left pair of

414 THE MUSSEL CHAP.

%ills at the sides of the visceral mass ; and anteriorly c. the left pair of
small, triangular, labial palps. (Note that there is no distinct head.)
These parts are situated between the two mantle-folds in the large
ventral mantle-cavity. Note also the position of the pericardiiim on
the dorsal side of the gills, the pericardial gland (p. 406), and the left
excretory organ (nephridiuni], between the pericardium and gills, and
extending back to the posterior adductor muscle.

5. Pass a seeker into the exhalant aperture and note that it enters a
short exhalant siphon or cloaca, separated from the mantle-cavity by
the bases of the gills. Slit this chamber open, and note that it extends
above the posterior adductor muscle, as well as above the gills. The
hinder end of the rectum will be seen just above the posterior adductor
muscle, opening by the amis into the cloaca. Insert a seeker into the
motith, between the anterior adductor muscle and the anterior edge of
the foot.

II. I. Remove the left mantle-lobe, cutting very carefully along the
bases of the palps and gills. Then slit open the pericardium longi-
tudinally, a short distance dorsally to the gills, and note the rectum,
ventricle, and delicate left auricle. Cut away sufficient of the wall of
the pericardium to expose these parts, taking care not to injure the
auricle. Note —

2. The rectum, which runs straight through the pericardium, and in .
its middle is surrounded by the elongated muscular ventricle, which is
bilobed posteriorly. The thin-walled, triangular left auricle can be seen
opening into the ventricle by a valvular aperture : the base of the auricle
is attached along the dorsal border of the gills. (Note the right auricle
on the other side.) Inject French-blue (p. 90) into the left auriculo-
ventricular aperture, and note a. the anterior aorta above the rectum,
and b. \\\z posterior aorta below the rectum. In the middle line of the
floor of the pericardium the vena cava can be seen.

3. Examine the gills. Note the left outer and inner gill, and that in
the female the former is often distended with eggs or larva. (Fig.
105) : save some of these for subsequent examination (§ IV).
Each gill consists of an outer and an inner lamella, forming a kind of
trellis-work, with small meshes or ostia, separated by horizontal and
vertical bars. Cut away a piece of the outer lamella, noting that the two
lamellae of each gill are united at intervals by inlerlaineliar junctions.

4. Make out the mode of attachment of the gills (see p. 404, and
Fig. 103). Sketch your dissection with the gills in their natural
position.

vin PRACTICAL DIRECTIONS 415

5. Pass a seeker forwards from the exhalant siphon along the supra-
branchial passages — two longitudinal canals at the bases of the gills
communicating with the interlamellar spaces or water-tubes. The inner
canal unites with its fellow behind the visceral mass (see Fig. 103, c),
and also communicates with the branchial chamber through the slit
between the visceral mass and the middle part of the lamella of the
inner gill. Cut open the canals carefully, and examine their relations.

6. Turn up the gills, and insert the scissors in the slit just mentioned ;
cut forwards through the inner lamella of the inner gill for a short dis-
tance. The renal aperture or nephridiopore can then be seen opening
into the suprabranchial passage, and just below and in front of it is the
genital aperture. Then note the paired reno-pericardial aperture (ne-
phrostome} in the anterior end of the pericardium, just below the point
at which the rectum enters it : this can be more easily seen if the rectum
is cut through and raised up.

7. Blow through the nephridiopore, and note the thin-walled sac-like
bladder into which it opens : this lies just below the pericardium, and
communicates with its fellow anteriorly. Just beneath and internal
to the bladder is the glandular portion of the kidney, which is dark in
colour and extends further back than the bladder, beyond which it forms
a large mass just in front of the posterior adductor muscle : it communi-
cates with the hinder end of the bladder. Sketch.

III. i. Remove both gills carefully, cutting along their bases ; stain
and mount a small piece of one lamella. Note that the lamella is made
up of close-set vertical bars or gill-filaments ^ connected by numerous
transverse inter-filamenlar jtmctions ; the bars are covered with ciliated
epithelial cells, and each is strengthened by pairs of small chitinous
rods. The whole gill is traversed by blood-vessels. Sketch.

2. A small portion of the gill should be preserved, stained, imbedded,
and cut into sections (see p. 136) passing transversely through the gill-
filaments. Compare with Fig. 102, and sketch.

3. Mount in salt-solution a small piece of the edge of a fresh gill,
and also of the mantle, and observe the movements of the cilia.

IV. If your specimen is a female, and contains ova or larvae in the
outer gill, examine some under the microscope. The ova are provided
with a canal or micropyle perforating the vitelline membrane, for the
entrance of the sperms. Note the form of the larvse or glochidia
(Fig. 105). Sketch.

V. The nervous system consists of three pairs of small orange-coloured
ganglia, with connectives between them.

416 THE MUSSEL CHAP.

1. Cut away the left labial palps very carefully and look for the left
cerebro-pleural ganglion, situated just below the attachment of the
mantle-lobe close to the mouth and directly beneath the epithelium : —
it may already have been exposed by removing the mantle-lobe. The
two cerebro-pleural ganglia are connected by a commissure above the
gullet.

2. On the under surface of the posterior adductor muscle are the two
closely applied visceral ganglia, which will be at once seen when the
membranous covering of this muscle is dissected off.

3. The pedal ganglia are more difficult to find, as they are deeply
situated, at the junction of the foot and visceral mass. Slit up the foot
by a median longitudinal incision, and carefully dissect the two halves
apart until the two closely apposed ganglia are seen ; then remove the
left half of the foot.

4. Carefully dissect out the cerebro-pedal connective, which runs
straight from the cerebro-pleural to the pedal ganglion. Then follow
out the cerebro-visceral connective from behind forwards : — each extends
forwards from the corresponding visceral ganglion along the inner
surface of the renal organ, and then passes slightly downwards across
the visceral mass to the cerebro-pleural ganglia. Sketch.

VI. The sensory organs are not easy to make out. In order to see
the osphradium and tactile-cells, it is necessary to cut microscopic
sections. The statocyst may be found by examining under the microscope
a portion of the tissue just behind the pedal ganglion ; but it is much
more easily observed in the small fresh-water bivalve Cyclas, often to
be found in ponds : it is only necessary to examine the entire animal
in water under a low power after removing the shell in order to see
the two globular statocysts, each lined by epithelial cells and containing
a lithite, which is in constant, trembling motion.

VII. The alimentary organs are imbedded in solid tissue in the visceral
mass, the hinder part of the intestine or rectum alone being surrounded
by a ccelomic cavity (pericardium). The anus has already been seen.

i. Remove the animal entirely from the shell, and pin down
through the adductor muscles and foot under water. Dissect away the
epiderm to the left side of and above the stomach (if this has not already
been done), and note the brown digestive gland in the anterior dorsal
part of the visceral mass. Then insert a seeker into the stomach
through the mouth, to act as a guide, and slit up the short gullet and
stomach. Note the ducts of the digestive gland opening into the
stomach.

vni PRACTICAL -DIRECTIONS 417

2. In the same manner slit up the whole intestine, either from the
stomach backwards or from the rectum forwards, using the seeker as a
guide the whole way, and first examining Fig. 101 to see the direction
which the coils take. Note the ventral typhlosole in the rectum,
beginning at the last coil. Sketch.

VIII. i. The gmads appear very similar in both sexes, and fill up a
large part of the visceral mass between the coils of the intestine : their
ducts open at the genital apertures (§ II, 6).

2. Tease up a bit of the ovary or spermary in salt-solution, stain with
magenta, and examine.

C. Transverse sections (Fig. 103).

With a sharp scalpel or razor cut a specimen hardened in spirit or
formaline into transverse sections about J inch thick, passing obliquely
through —

1. The anterior part of the pericardial cavity, visceral mass, and
foot. Note the relations of the mantle-folds, the gonads, the enteric
canal (including the rectum with its typhlosole], the gills and sitpra-
branchial canals, the anterior aorta and vena cava, and the kidneys
and bladders.

2. The ventricle, auricles, and hinder part of the visceral mass.
Note the relations of the mantle-folds, enteric canal, gills, siipr abranchial
canals, vena cava, kidneys, and bladders.

3. The posterior adductor muscle, hinder part of gills, and rectum.
Note the relations of the various parts. Sketch the three sections.

PRACT. ZOOL. E E

CHAPTER IX

CHARACTERS OF THE PHYLUM VERTEBRATA THE LANCELET

FROM your study of the frog you have learnt something
about a vertebrate animal, and we will now examine
a few more examples of the phylum Vertebrata, which,
as we have seen (p. 219), includes several classes, the Pisces,
Amphibia, Reptilia, Aves, and Mammalia. Leaving aside the
lampreys and their allies, which present certain peculiarities
and are therefore placed in a class by themselves, these all
agree with one another and resemble the frog in the follow-
ing essential characters.

They all possess : — a vertebral column — or at any rate a
notochord (p. 203), which is nearly always replaced by a
vertebral column in the adult — and a skull with upper and
lower jaws ; a dorsal, hollow, nervous system, consisting of
brain and spinal cord ; paired olfactgry organs, eyes, and
auditory organs, which take on a close connection with the
skull ; a pharynx, which, at an early stage at any rate, is
perforated by a small number (never more than seven) of
paired gill-clefts (p. 204) ; a mouth which is ventral and
anterior, and an anus which is ventral and posterior ; kidneys
which are composed of numerous urinary tubules or neph-
ridia; a chambered heart and red blood-corpuscles ; a liver,

GENERAL CHARACTERS

419

and a hepatic portal system ; usually two pairs of limbs, and
never more than two pairs ; and a series of body-muscles
which are divided into segments
or myomeres (p. 203), at any rate in
early stages, and are composed of
striped fibres.

Now in various parts of the
world, occurring only rarely off the
English coasts, certain small fish-
like animals called lancelets are
found, the commonest species of * I
which is known as Amphioxus
lanceolatus (Fig. 106). This animal,
which possesses a median fin like
that of the tadpole (p. 207), is not
more than a couple of inches in
length, and lives in the sea near the
shore, burrowing in the sand ; it is
especially interesting owing to the
fact that it presents certain charac-
teristics indicating a near relation-
ship to the primitive ancestors of
Vertebrates. It possesses a noto-
chord, a dorsal hollow nervous
system, a pharynx perforated by
gill-slits, a hollow outgrowth of the
intestine representing a simple
liver, with a hepatic portal system,

and a series of nephridia. But it differs from all the
higher Vertebrates in the following important points.

The epiderm consists of a single layer of cells. There
is no distinct head and no skull ; the persistent noto-

E E 2

420

THE LANCELET

is 5^ OS 'JS . "" <D

a 2 W) u £ xi "d ^

l-^ rf§§li1-s

1 ^-o^l^

chord (Fig. 107,
ncJi) extends to the
anterior end of the
animal, and there
are no paired limbs.
A brain (br) can
hardly be said to be
present, and there
are no paired olfac-
tory, optic, or audi-
tory organs. The
pharynx is relatively
very large, and is
perforated by very
numerous oblique
gill - slits (br. ct),
which do not open
on the exterior
directly, but are sur-
rounded by a cham-
ber, the atrium
(Figs. 107, atr,
and 109 A, b) ; this
opens externally by
a pore (Figs. 106
and 107, atrp\ and
though differing in
its mode of deve-
lopment from the
branchial chamber
of the tadpole (p.
207), has somewhat
similar relations.
There is no heart

ix EXTERNAL CHARACTERS 421

(Fig. no), and the colourless blood apparently contains no
corpuscles of any kind. The nephridia remain distinct, not
being united into a single kidney on either side : they are
situated anteriorly, in the neighbourhood of the pharynx
(Fig. 107, nph, and 109 A, n) ; the gonads (Figs. 106 and
107, gon, and Fig. 109 A, g) are metamerically arranged,
and have no ducts.

In certain of its characters Amphioxus resembles the members of
a group of animals — the Ttmicata, commonly known as "sea-squirts,"
in which the body is enclosed in a " test " or mantle, consisting largely
of cellulose (p. 244). These, like Vertebrates, possess a notochord and a
dorsal, hollow nervous system in young stages, and in the adult retain
numerous pharyngeal gill-slits ; they are almost certainly degenerate
descendants of primitive animals from which the Vertebrata also arose.

These numerous and marked differences between the
lancelet and the higher Vertebrates make it necessary to
place Amphioxus in a separate division of the Vertebrata,
called — from one important negative character — the
Acrania, while all the other Vertebrates, which possess
skulls, are included in the division Craniata.

The external appearance of Amphioxus is represented in
Fig. 1 06. In addition to the points already referred to, it will
be seen that the mouth is surrounded by a fold, the oral hood
(or. hd\ from which a number of tentacles or cirri (cir) are
given off; and that there is a lateral Q\ metapleuralfold(mtpt)
along either side of the body extending backwards as far as
the atrial pore, in addition to the median fin-fold (dors, f,
cd. /, vent, f) extending round the tail as a caudal fin. In
the young animal the gill-slits open directly to the exterior,
but a median canal is subsequently formed along the
ventral side of the body, and as this extends inwards and its
edges unite to form the atrium (Fig. 108 s/, p\ leaving only
the atriopore open, it gradually surrounds the pharynx at the

422

THE LANCELET

sides, pushing the ccelome before it, so that the latter
becomes greatly reduced in this region (Fig. 109 A, co).

The skeleton is very simple ; besides the notochord there
are rod-like bars of a substance somewhat resembling

S. — Awphioxus lanceolatus. Diagrammatic transverse sections of larvae

in three different stages to show the development of the atrium.
ao. aorta ; c. derm ; ch. notochord ; d. intestine ; f. fascia ; fh. cavity for
dorsal fin-ray ; tn. myomere ; n. spinal cord ; p. atrium ; sf. metapleural folds ;
sfh. metapleural cavity ; si. sub-intestinal vein ; sk. wall of neural canal and
sheath of notochord ; si. sub-atrial ridge ; sp. coelome. (From Korschelt and
Heider's Embryology, after Lankester and Willey.)

cartilage supporting the oral hood and cirri (Fig. 107, sk),
chitinoid rods (br.r) supporting the gill-bars between the
clefts, and short rods of connective-tissue, the fin-rays,
in the dorsal and ventral fins (Figs. 106 and 107, dors. />,
vent. f.r).

In the course of development, each primary gill-slit
becomes divided into two by the growth, from above down-

GILL-BARS

423

wards, of a tongue-like process, the secondary gill-bar or
septum (Fig. 107, br. sep. 2\ so that in the adult the slits
and intervening bars are seen to be arranged in couples,
the supporting rods (br. r. i) of the primary bars (br. sep. i)
A B

air

FIG. 109. — Ainphio,rus lanczolatus. A, transverse section of the pharyngeal region.
«, dorsal aorta; b. atrium; c. notochord ; <co. coelome; e. endostyle ; g. gonad

(ovary) ; kb. branchial bars ; kd. pharynx ; /. " liver" ; my. myomere ; n. nephri-

diu •', shown as if opening into the coelome ; r. spinal cord : sn, sn, dorsal and

ventral spinal nerves.
B, transverse section of the intestinal region, atr. continuation of atrium on right

side of intestine ; ccel. coelome ; d. ao. dorsal aorta ; int. intestine ; myom.

myomere ; nch. notochord ; neu. spinal cord ; s. int. v. sub-intestinal vein.
• (From Parker and Haswell's Zoology : A, from Hertwig, after Lankester and

Boveri ; B, partly after Rolph.)

being forked below. A further complication is produced
by the formation of transverse connections, supported by
skeletal rods, between the gill-bars. The gill-slits are more
numerous than the muscle-segments or myomeres (myam)t
and owing to their obliquity, a large number of them always
appear in a transverse section (Fig. 109, A).

424 THE LANCELET CHAP.

On the ventral wall of the pharynx is a longitudinal groove,
the endostyle (Fig. 109 A, e), lined by ciliated and glandular
cells; and a somewhat similar groove, the epipharyngeal groove,
extends along the dorsal aspect of the pharynx : these serve
as channels along which the particles of food pass into the
intestine. The minute mouth (Fig. 107, mtti) leads from the
cavity of the oral hood into the pharynx, and is surrounded
by a membrane, the velum (vl}, produced at its edge into
a number of tentacles (vl. t).

The intestine is straight (Fig. 107, int) and the hepatic
cacum, or rudimentary liver, arises from its anterior end on
the ventral side, and extends forwards to the right of the
pharynx (Fig. 107, /rand Fig. 109 A, /).

Although no heart is present, the blood-vessels, some of
which are contractile, present certain undoubted homologies
with the more complex vessels of the Craniata, and so a
distinction is made between arteries, and veins. Their
arrangement may be seen from the accompanying diagram
(Fig. no), which should be compared with Fig. 83 and
later on with Fig. 119. Blood occurs in certain spaces
as well as in the vessels.

The numerous nephridial tubes (Fig. 107, npJi) are situated
above the pharynx on either side in relation with the
reduced ccelome in this region. The internal end of each
is provided with bunches of peculiar knob-like cells pro-
jecting from closed nephrostomes : the other end opens into
the atrium (Fig. 109 A, n).

The spinal cord has a dorsal fissure but no ventral fissure
(p. 155). Its central canal widens out in front (Fig. 107,
br) to form a small cerebral ventricle (en. coe) in the front
wall of which is an unpaired pigment-spot, representing a
simple kind of eye (e. sp], and at its anterior end is a
ciliated depression usually known as the olfactory pit (olf. p).

GONADS

425

1'here is no auditory organ. The dorsal and ventral nerves
(Fig. 109 A, sn) remain separate, and do not unite to form
a nerve-trunk (compare p. 163).

The sexes are distinct, and the metamerically arranged
gonads (Figs. 106, 107, and 109 A, gon, g) are situated in
pouches, the cavities of which represent part of the coelome
and which project into the atrium on either side of the
pharynx. When ripe, the eggs and sperms burst through

d.CLO &&r.a. <t

d CLO

e.pporl.1/

FIG. no. — Diagram of the chief vessels of A mphiox u s. The arrows show the course

taken by the blood.

of. br. a, afferent branchial arteries ; br. cl. gill-slits ; cp. intestinal capillaries ;
d. ao. paired dorsal aortae ; d. ao . median dorsal aorta ; ef. br. a efferent
branchial arteries ; hep. port. i>. hepatic portal vein ; hep. v. hepatic vein ; int.
intestine ; Ir. " liver" ; ph. pharynx : .y. int. v. sub-intestinal vein ; v. ao. ventral
aorta, which contracts from behind forwards. (Representatives of the precaval
and cardinal veins of fishes are also said to be present.) (From Parker and
Haswell's Zoology.")

»

into fhe atrium and find their way out through the atriopore.
The oosperm undergoes segmentation, and the embryo
is hatched at a relatively early stage as a simple larva, which
gradually develops into the adult form. The embryology of
Amphioxus, which is very instructive, will be referred to in
greater detail in Chapter XII.

426 THE LANCELET CHAP.

PRACTICAL DIRECTIONS.

AMPHIOXUS.

A, External characters.

Examine an entire specimen and note : — I, the form of the body;
2, the continuous median fins — dorsal, caudal, and ventral ; 3, the
paired lateral fin or mctapleure, extending along the body anteriorly
to the ventral fin : 4, the oral hood, anterior and ventral, with its
cirri ; 5, the anus, a short distance from the posterior end, just on
the left side of the caudal fin ; 6, the median and ventral atriopore,
at the junction of the lateral and ventral fins ; 7, the myomeres.

B. Anatomy.

The dissection of Amphioxus is too difficult a task for the beginner,
and so it is better to proceed as follows : —

I. Place a young specimen for1 a short time in absolute alcohol, and
then transfer to oil of cloves : if the specimen is a small one, it may
with advantage be slightly stained first. Then transfer to a hollow
slide or, if you use an ordinary slide, support the cover-glass on two
small pieces of wood of the thickness of the specimen, and mount in
thick balsam : or the preparation may be examined in oil of cloves.
(Many details are better seen in very young specimens, not more than
half an inch long, which may be purchased ready-mounted. ) Examine,
and note in addition to the above points : —

1. The notochord, extending from the anterior to the posterior end of
the body, rather nearer the dorsal than the ventral side.

2. The neural canal enclosing the spinal cord, lying just above the
notochord, and having pigment in its wails.

3. The oral skeleton, consisting c-f a segmented basal bar along the
sides of the oral hood, with a rod passing down each cirrus.

4. The longitudinal series of connective-tissue compartments, which
are filled with gelatinous substance, forming the fin-rays.

5. The elastic, horny rods supporting the gill-bars, and the arrange-
ment of the gill-slits in pairs : — the primary gill-bars separating the
successive pairs, the rods supporting them being forked ventrally ; and
the secondary gill-bars between the members of a pair, the rods of these
being unsplit ventrally. Note also the horizontal bars connecting the
primary and secondary gill-bars and making the walls of the pharynx
appear like a mesh work.

ix PRACTICAL DIRECTIONS 427

6. The cavity of the oral hood, bounded by lateral folds, the
muscular velum between it and the pharynx, and the minute mouth.

7. The intestine, running straight from the hinder end of the pharynx
to the anus, and giving off near its anterior end the hepatic ccecum
or " liver" extending forward on the right side of the pharynx.

8. The myomeres QJR.& the intermuscular septa between them, arranged
like a series of V's, the apices of which point forwards. The muscular
fibres run longitudinally from septum to septum, and are of the striped
kind.

9. The gonads (ovaries or spermaries), arranged in a single row on
either side of and rather further back than the pharynx, extending as
far as the atriopore. Note in the female the large ova, which, when
ripe, cause a great distension of the body ; and in the male the minute
sperm-cells, the structure of which cannot be made out in entire
specimens. Sketch.

II. Cut a specimen into short pieces, about an eighth of an inch in
length, and select portions from — a, between the hinder end of the
atrium and the anus (Fig. 109, B), or through the latter ; b, just in
front of or through the atriopore ; c, through the anterior part of the
pharynx ; and d, through the posterior part of the pharynx (Fig. 109,
A). Stain and imbed these, and prepare a few sections from each
(p. 136). Examine in the order given above, first with the low, and
then with the high power. Sketch a section from each region.

a. i. ,The oval form of the section, the median dorsal and ventral
fins, and the integument (columnar epiderm and thin derm).

2. The myomeres cut across in various planes, and appearing as
squarish masses separated by the septa,

3. The central notochord, oval in transverse section, with transverse
wavy lines indicating the boundaries of the vacuolated notochordal
cells. It is surrounded by a connective-tissue sheath, continuous with
the connective-tissue investment of the neural canal above and with
the intermuscular septa at the sides : the latter pass into the derm
peripherally. '

4. The spinal cord, lying in the neural canal. It is ovoid in trans-
verse section, and has a dorsal fissure extending downwards nearly to
the central canal, which is nearer the ventral than the dorsal surface.
See if your section happens to pass through a dorsal or a ventral
nerve. The dorsal nerves arise by large single roots from about the
middle of the sides of the cord, while the ventral nerves have numerous

428 THE LANCELET CHAP.

fine roots, and arise from the ventro-lateral angles of the cord. The
dorsal and ventral nerves are not in the same transverse plane (compare
Fig. 52), but have an alternating arrangement.

5. The gelatinous connective-tissue forming the fin-rays.

6. The intestine, with its single layer of long epithelial cells. If the
section passes through the anus, the latter will be seen opening on the
left side of the ventral fin.

7. The ccelome, surrounding the intestine, and also enclosing the
subintestinal veins, which are continuous anteriorly with the hepatic
portal vein (Fig. 1 10).

8. The dorsal aorta, just below the notochord.

b Go over 2, 3, 4, 5, 6, and 8 again — all much as in a. Note —

1. The dorsal and lateral (metapleural} fin-folds, ventral body-
wall, small metapleural canals, and atriopore — if present in your
section.

2. The ccelome, a narrow space round the intestine, in which three or
four subintestinal (portal] veins can be seen.

3. The atrial cavity just outside the ccelome, and separated from it
by the atrial membrane. If the section passes through the atriopore,
it will be seen in the mid-ventral line, putting the atrial cavity in com-
munication with the exterior.

c. Note — I. The triangular form of the body in transverse section ;
the dorsal fin, the ventral surface, with its folded integument and thin
transverse ventral muscles ; the metapleural folds and their contained
large canals. Then go over a. 2, 3, 4, and 5 again, which are all much
as in a and b.

2. The pharynx, lying just below the notochord, and extending nearly
to the ventral body walls. Note — (a) the deep, dorsal epibranchial
groove ; (b) the ventral endostyle projecting into the pharyngeal cavity,
and enclosing a ccelomic canal ; (c] the gill-bars, a great number of
which will be cut through obliquely, larger primary alternating with
smaller secondary ones. Note the skeletal rods near their outer surfaces,
and, in the primary bars, the small ccelomic spaces on the outer sides of
the rods ; (d) the dorsal siispensory folds of the pharynx, which enclose
the two dorsal ccelomic canals.

3. The atrial cavity, surrounding the pharynx, except at the mid-
dorsal line where the pharynx is attached to the sheath of the noto-
chord, and communicating with the cavity of the pharynx through the
gill-slits. It is enclosed by the two atrial folds, which are united in

ix PRACTICAL DIRECTIONS 429

the mid-ventral line : the atrial epithelium is often much folded ven-
trally.

4. The single ventral aorta in the endostylar coelomic canal, and the
paired dorsal aorta, on either side of the epibranchial groove.

d. Note again a. 2, 3, 4, 5 ; c. I, 3, 4, and also : —

1. The pharynx, much as in c 2, but laterally compressed. The en-
dostyle is now converted into a groove.

2. The hepatic cacum, on the right side of the pharynx, and covered
by the atrial epithelium. Note the ccelomic space around it, in which
are the portal veins and the hepatic veins (dorsal side).

3. The gonads (ovaries or spermaries) — large masses on either side of
the pharynx, projecting into the atrial cavity and covered by the atrial
folds. The spaces around them represent portions of the coelome.
Note the small sperm-cells in the male, and in the female, the large ova.

(The nephridia can only be made out satisfactorily in good sections of
very wrell preserved specimens. )

CHAPTER X

CHARACTERS OF THE CLASS PISCES — THE DOGFISH

The class Pisces (see p. 418) includes a number of aquatic
Vertebrates which present a considerable amount of differ-
ence in form and structure, and which are distinguished
from the Amphibia, as a whole, by certain constant charac-
teristics, of which the following are the chief.

The organs of respiration and locomotion are adapted for
life in the water. The former consist, as in the tadpole
(p. 207), of a series of vascular processes, the gifts, attached
to the septa separating the gill-clefts and persisting
throughout life : lungs, and internal nostrils, are also present
in a very few instances (viz. in the small sub-class . consti-
tuting the " Mud-fishes " or Dipnoi}. The pectoral and
pelvic limbs have the form of paddle-like fins, which, like
the median fins (p. 421), are supported by skeletsiljin-rays : —
the median fin is usually subdivided into separate dorsal,
ventral, and caudal portions. In addition to the endo-
skeleton, there is usually an exoskeleton, developed in the
derm and consisting of scales ; and peculiar jutf.

sense-organs \ like those of aquatic and larval Amphibians,
and supplied by special nerves not represented in terrestrial
Vertebrates, are always present;. The cerebellum is rela-

CHAP, x ELASMOBRANCHS 431

tively large ; a tympanic cavity and membrane are not
present, and except in the Dipnoi, there is only one auricle
in the heart and no postcaval vein. A urinary bladder
developed as an outgrowth of the enteric canal (p. 210) is
wanting. There is never such a marked metamorphosis as
in the case of the frog.

The two most important sub-classes of the Pisces are the
Elasmobranchii and the Teleostomi. The Elasmobranchs
are all marine forms, and include the dogfishes, sharks,
rays, and skates : their endoskeleton is composed almost
entirely of cartilage, like that of the tadpole. The Teleo-
stomi, in which the skeleton is mainly or to a large extent
bony, include by far the greater number of fishes — both
marine and fresh-water forms — such as the Salmon, Cod,
Herring, Perch, as well as the Sturgeon and its allies.

The dogfishes are small sharks, of which there are a
number of genera and species. They are all powerful
swimmers, and feed voraciously on other fishes, crustaceans,
&c.

The commonest British forms are the Rough Hound
(Scyllium caniculd), the Lesser Spotted Dogfish (S. catulus\
the Piked Dogfish (Acanthias vulgaris), and the Smooth
Hound (Mustelus vulgaris^. The following description,
though referring mainly to Scy Ilium, will apply, in essential
respects, to any of these.

External Characters and General Structure.— The
dogfish has a spindle-shaped body, ending in front in a~
bluntly-pointed snout and behind tapering off into an up-
turned tail. On the ventral surface of the head is the large,
transversely elongated mouth (Fig. 117), supported by a
pair of jaws which 'work in a vertical, and not, like those of
the crayfish, in a transverse plane : they are, in fact, like

432 THE DOGFISH CHAP.

those of the frog, portions of the skull, having nothing to
do with limbs. They are covered with numerous rows of
teeth which vary in form in the. different species. In front
of the mouth, on the ventral surface of the snout, are the
paired nostrils, each leading into a cup-like nasal or olfactory^
sac, and (in Scyllium) connected with the mouth by a
groove. The eyes are situated one on either side of the
head, above the mouth : they are protected by thick folds of
the skin forming upper and lower eyelids, which can be
partially closed over the eye. Behind the mouth are five
pairs of slit-like apertures arranged in a longitudinal series :
these are the gill-clefts or external branchial apertures
(p. 418). Just behind each eye is a small aperture, the
spiracle : like the gill-clefts, it communicates with the
pharynx, and it is found by development to be actually the
functionless first gill-cleft.

On the ventral surface of the body, about half-way
between its two ends, is the vent or anus, leading into the
cloaca (p. 23), and on either side of it a small pouch into
which usually opens a minute hole, the abdominal pore (Fig.
127) communicating with the coelome, which is therefore
not a completely closed cavity, as in the frog. The region
from the end of the snout to the last gill-cleft is considered
as the head of the fish; that from the last gill-cleft to the
,anus as the trunk • and the rest as the tail.

A number of symmetrically arranged, minute apertures on
the skin of the head, particularly numerous on the snout,
lead into a series of tubes known as sensory canals, which
are situated beneath the skin in this region ; and a single
tube, known as the lateral-line canal^ the position of which
is indicated by a very faint longitudinal line, extends along
either side of the body and tail. The whole apparatus
constitutes an important, but imperfectly understood, in-

FINS AND SKIN 433

tegumentary sense-organ : it is represented in the tadpole,
but disappears at metamorphosis.

Springing from the body are a number of flattened folds,
called the fins, divisible into median and paired. The
median folds are not continuous, as in the tadpole and
lancelet, but are subdivided into several distinct parts, viz.,
into two dorsal fins along the middle line of the back,
a caudal fin lying along the ventral edge of the up-
turned tail, and a ventral fin behind the anus. The paired
folds are the pectoral fins^ situated one on either side of the
trunk just behind the last gill-cleft, and the pelvic fins*
one on each side of the vent : these correspond to the
pectoral and pelvic limbs of the frog. In the male there
is connected with the inner border of each pelvic fin a
grooved, rod-like structure known as the clasfier^ ^ which
serves as a copulatory organ.

It is very possible that the paired fins, like the median fins, are
specialised portions of a primarily continuous fin-fold which extended
along either side of the body, like the lateral or metapleural fold of
Amphioxus.

The fish swims by vigorous strokes of the tail : the
pectoral fins are used chiefly for steering, and the dorsal
and ventral fins serve, like the keel of a boat, to maintain
equilibrium.

The skin or external part of the body-wall consists, as
usual of two layers, an outer layer of deric epithelium or
epiderm (Fig. in, Der. Epthm\ formed, like that of the
frog, of several layers of cells ; and an inner layer of con-
nective-tissue, the derm. In the derm are innumerable
close-set, calcified bodies, each consisting of a little irregular
plate of bone produced into a short spine — composed, like
the teeth present in the frog and most Vertebrates, of a calci-

PRACT. ZOOL. F F

434 THE DOGFISH CHAP.

fied tissue harder than bone known as dentine, capped with
a still harder tissue called enamel — which projects through
the epiderm and gives a rough, sandpaper-like character to
the skin. These placoid scales or dermal teeth together con-
stitute the exoskeleton of the dogfish : it is a discontinuous,
mainly dermal, exoskeleton (p. 445), and not a continuous
cuticular one like that of the crayfish.

Beneath the derm is the muscular layer, which, as in
Amphioxus and in the tail of the tadpole, is metamerically
segmented. The muscles are divided into myomeres, follow-
ing one another from before backwards, and having a zigzag
disposition. The fibres composing them are longitudinal,
and are inserted at either end into fibrous partitions or
myocommas which separate the myomeres from one
another.1 The muscular layer is of great thickness, espe-
cially its dorsal portion. The fibres of all the body-muscles
are, as in the frog and Vertebrates generally, of the striped
kind.

There is a large c&lome (Figs, in and 117), which, as in
other Vertebrates, is confined to the trunk. The cavity is
divisible into two parts : a large abdominal cavity, containing
most of the viscera, and a small anterior and ventral com-
partment, the pericardia I cavity (Fig. \\>], pcd. cav\ contain-
ing the heart and communicating with the abdominal cavity
by a canal which opens on the ventral surface of the gullet.
Both are lined by ccelomic epithelium (Fig, in, Cffl.
Epthm\ underlain bv a layer of connective-tissue, a strong
lining membrane being thus produced, called, as in the
frog, peritoneum in the abdominal, pericardium in the peri-
card ial cavity.

Another very characteristic Vertebrate feature is that the

1 In the adult frog a segmentation can still be seen in the rectus
muscle of the abdomen (Figs. 2 and 16).

ENDOSKELETON

435

dorsal body-wall is tunnelled, from end to end, by a median
longitudinal neural cavity^ in which the central nervous
system is contained. The greater part of the cavity is
narrow and cylindrical, and contains the spinal cord (Figs.
in and 117, sp. cd) : its
anterior or cerebral portion
is dilated, and contains the
brain.

Skeleton. — Imbedded in
the body-wall and extend-
ing into the fins are the
various parts of the endo-
skeleton. This character-
istic supporting framework is
mainly composed, as in the
tadpole and in embryos of
Vertebrates generally, of
cartilage, which may be more
or less impregnated with
lime salts, so as to have, in
part, the appearance of bone,
but differing in structure
from true bone and con-
sisting merely of calcified
cartilage (p. 46).

The entire skeleton con-
sists of separate pieces of
cartilage, calcified or not,
and connected with

one

FIG. in. — Diagrammatic transverse
section through the trunk of a
female dogfish (compare Fig. 5).
The ectoderm is dotted, the endo-
derm radially striated, the meso-
derm evenly shaded, and the coelo-
mic epithelium represented by a
beaded line.

Card. V. cardinal vein ; CceL coelome ;
Cccl. Epthm. parietal, and Coel.
Epthiri ' . visceral layer of coelomic
(peritoneal) epithelium ; D. Ao.
dorsal aorta ; Derm, derm ; Derm.
F. R. dermal fin-ray ; D. F. dorsal
fin ; /. int. V. ventral intra- intes-
tinal vein; Int. intestine; K. kid-
ney ; Lat. V. lateral vein ; M.
myomeres ; N. A . neural arch ; N.
coe. central canal of spinal cord ;
Nph. nephridium ; Ovd. oviduct ;
Ovy. ovary ; Sp. Cd. spinal cord ;
Ur. ureter ; V. Cent, vertebral cen-
trum. (From Parker's Elementary
Biology.')

another by ligaments (p. 57) : as in the frog, it is divisible
into skull, vertebral column, and skeleton of the paired fins,
with their arches or girdles-, in connection with the skull
are certain cartilaginous visceral arches, forming the upper

F F 2

436 THE DOGFISH CHAP, x

and lower jaws and supporting the gills ; and there are also
skeletal parts in the median fins.

The cranium or brain-case (Fig. 112, Cr) is an irregular,
cartilaginous box containing a spacious cavity for the brain,
very similar to the chondrocranium of the frog (p. 43). It
is produced into two pairs of outstanding projections : a
posterior pair, called the auditory capsules (aud. cp\ for the
lodgment of the organs of hearing, ridge-like projections
on which indicate the position of the semicircular canals
(p. 187) ; and, in front of the brain-cavity, an anterior
pair, the olfactory capsules (olf. cp\ for the organs of smell,
open below, and separated from one another by a septum.
Between the olfactory and auditory capsules, on either side,
the cranium is hollowed out into an orbit (or), bounded by
a supra-orbital and a sub-orbital ridge, for the reception of
the eye. In front the brain-case is produced into three carti-
laginous rods forming the rostrum (r) and supporting the
snout. On its posterior face is the foramen magnum
(p. 40), on either side of which is an oval condyle for articu-
lation with the first vertebra. On the roof of the skull,
between and behind the olfactory capsules, there is a fon-
tanelle (p. 43), closed over by connective-tissue only ; and
between the two auditory capsules is a depression into
which open the two endolymphatic ducts of the ears
' (p. 187). The nerve- apertures will be referred to at a later
stage.

In the human and other higher vertebrate skulls the
upper jaw, as we have seen to be the case in the frog (Fig.
9), is firmly united to the cranium, and the lower alone is
free. But in the dogfish both jaws (up. /, /. /) are connected
with the cranium by ligaments (Ig, Ig) only, and each consists
of strong, paired (right and left) moieties, united with one
a other by fibrous tissue. The upper jaw, which corre-

•

/ ^
/ . ^

A

d G 0-2 & be ,, 2i S

^

438 THE DOGFISH CHAP

spends to the palatoquadrate cartilage of the frog (p. 44),
presents at its posterior end a rounded surface against
which fits a corresponding concavity on the lower jaw, so that
a free articulation is produced, the lower jaw, or Meckefs^
cartilage (p. 44) working up and down in the vertical plane.

The upper and lower jaws of either side correspond to
the first of a series of seven pairs of visceral arches, and are
therefore often spoken of as the mandibular arch. The
remaining six pairs have the form of cartilaginous half-hoops,
lying in the walls of the pharynx, and united with one
another below so as to form a basket-like apparatus support-
ing the gills. The second of these arches is distinguished
as the hyoid, and is situated immediately behind the jaws.
It consists of two parts : a strong, rod-like hyomandibular
(Fig. 112, hy. ;//), which articulates above with the auditory
capsule and is connected below by fibrous tissue with the
jaws, thus helping to suspend them to the cranium and
serving as a suspensorium — formed in the frog by the quad-
rate (pp. 40 and 44) ; and a hyoid cornu or horn, which
curves forwards inside the lower jaw, and is connected
with its fellow of the opposite side by a median basi-hyal
plate which supports the tongue (compare Fig. 9).

The remaining five arches (br. a. \ — br. a. 5) are called
the branchial arches. Each is formed of several separate,
pieces, united by fibrous tissue so as to render possible the
distension of the throat during swallowing : the fifth is
connected below with its fellow by a large median basi-
branchial plate, which supports the roof of the pericardial
cavity. Both they and the hyoid give attachment to delicate
cartilaginous branchial rays (br. r, br.r ', Fig. 118, r) which
support the gills.

In the frog-tadpole the gills are similarly supported by carti-
laginous arches, which become greatly reduced and modified at meta-

VERTEBRAL COLUMN

439

morphosis : the tongue skeleton of the adult (p. 44) represents the
hyoid and first branchial arch.

In addition to the parts of the skull described above there are certain
small cartilages of minor importance in relation with the nostrils,
spiracle, mouth, and outer sides of the branchial arches (e.g., ib and
ex. brvn. Fig. 112).

The vertebral column has the general character of a
jointed tube surrounding the spinal portion of the neural

n.s/y

h.a

FIG. 113. — Vertebrae of Scyllium canicula.

A and B from the trunk, C and D from the middle of the tail ; A and C, two
vertebrae in longitudinal section ; B and D, single vertebrae viewed from one^end.

/>. calcified portion of centrum ; c. centrum ; for. foramen for dorsal, and for. for
ventral root of spinal nerve ; h. a. haemal arch ; h. c. haemal canal ; h. sp. haemal
spine ; i. n.p. interneural plate ; n. a. neural arch ; n. c. neural canal ; n.p. neural
plate and process ; n. sp. neural spine ; ntc. intervertebral substance (remains of
notochord) ; tr. pr. transverse process ; r. proximal portion of rib.

canal. Lying beneath this cavity, i.e., between it and the
ecelome, is a longitudinal row of biconcave or amphiccelous
discs, the verte.bral centra (Fig. 113, c; Fig. 117, en): they
are formed of cartilage, but have their anterior and posterior

440 THE DOGFISH CHAP.

faces densely calcified ; calcified bars also extend longitudin-
ally through each centrum from face to face, so as to show
a radial arrangement in transverse section. The biconcave
intervals between the centra are filled with a soft inter-
vertebral substance (Fig. 113, ntc), which is also present
between the first vertebra and the skull and which re-
presents part of the embryonic notochord (see pp. 203
and 418). The centra are united by ligament, so that the
whole vertebral chain is very flexible.

In the frog it will be remembered that the completely ossified centra
are proccelous, and are articulated with one another (pp. 36 and 57).

Connected with the dorsal aspect of the series of centra
is a cartilaginous tunnel, consisting of the neural arches .
(Figs. 1 13 and 117, n. a] enclosing the spinal cord: it is
divided into segments, corresponding with, but usually twice
as numerous as the centra, owing to the presence of inter-
calary pieces. Arising from each centrum on either side is
a neural process which is fused with a neural plate
(Fig. 113, n.p) and perforated posteriorly for the exit of
the ventral root of a spinal nerve (for) ; and fitting between
two consecutive neural plates is an intercalary piece, the
interneural plate (i. n. p), perforated by an aperture for the
dorsal root of a spinal nerve (for). The arch is completed
above by the neural spines (n. sp\ which fit in between the
neural and interneural plates respectively, and are thus, like
the lateral elements of the arch, twice as numerous as the
centra. The first vertebra has facets for articulation
with the condyles of the skull.

In the anterior part of the vertebral column the centra
give off paired, outstanding transverse processes (Fig. 113
B, tr. pr\ to the end of each of which is articulated a
short, cartilaginous rod, the rib (r). Further back ribs
are wanting, the transverse processes are directed down-

VERTEBRAL COLUMN 441

wards, instead of outwards, and in the whole caudal region
they unite below, forming hczmal arches and spines (Fig. 113
i), //. a, h. sp, and Fig. 117, h. a), which together constitute
a kind of inverted tunnel in which lie the artery and vein of
the tail. In the region of the caudal fin the haemal spines
are elongated and act as supports for the fin. A centrum
together with the corresponding neural arch and transverse
processes, or haemal arch, represent a vertebra or single
segment of the vertebral column.

In the frog we have seen that there are no independent ribs, and that
the caudal vertebrae are represented by a single bone, the urostyle (p. 39).

It should be noticed that in the vertebral column we have
another instance of the metameric segmentation of the
vertebrate body. The vertebrae do not, however, correspond
with the myomeres, but alternate with them. The myo-
commas (p. 434) are attached to the middle of the vertebrae,
so that each myomere acts upon two vertebrae and thus
produces the lateral flexion of the body.

In the embryo dogfish, as in the tadpole, before the development of
the vertebral column, an unsegmented, cellular rod with an elastic
sheath, the notochord, resembling that of Amphioxus (p. 419), lies be-
neath the neural cavity in the position occupied in the adult by the line
of centra, by the development of which it is largely replaced. Segment-
ally arranged cartilages appear above and below the notochord, which
on the one hand give rise to the arches, and on the other invade the
notochord and constrict it at regular intervals, so as to replace it
completely in those regions which will form the middle parts of the
vertebral bodies, leaving the vacuolated notochordal cells in the
biconvex spaces between the centra (Fig. 113, ntc}. Thus much of the
notochord persists as the soft inter vertebral substance.

The skeleton of the median fins consists of a series of
parallel cartilaginous rods, the fin-rays or pterygiophores, the
proximal ends of which are more or less fused together to
form basal cartilages or basalia. The free edges of the fins

442

THE DOGFISH

CHAP.

are supported by a double series of delicate horn-like fibres,
the dermal fin-rays.

The paired fins are also supported proximally by cartila-
ginous pterygiophores, fused at the bases to form basal
cartilages which articulate with the corresponding arch or
girdle, and distally by horny, dermal fin-rays (Fig. 115,
d. f. r). The pelvic arch (Fig. 114, BP) is a transverse bar
of cartilage situated just in front of the vent, and repre-
senting the pubic and ischiatic portions of the girdle in the

FIG. 114.— Diagram of the Elasmobranch pelvic arch (B P) and fin.
Bas. basal cartilage ; Fo ' . nerve foramen ; /. iliac process ; Pro. anterior ray
articulating directly with the arch ; Rod. the remaining radial cartilages. (From
Wiedersheim's Comp. Anatomy.}

frog (p. 50), an iliac region, extending dorsally, and coining
into connection with the vertebral column, being hardly
represented (I). On its posterior edge are articular facets
for the pelvic fins, each of which has a single very large
basal cartilage (Bas\ but one or two of the anterior rays
(Pro) may articulate separately with the arch. In the male,
the skeleton of the clasper (p. 433) is connected with the
distal end of the basal cartilage.

The shoulder-girdle (Fig. 115) is a strong, inverted
arch of cartilage situated just behind the last branchial

x PECTORAL FIN 443

arch. On its outer surface it presents three articular
facets on either side for the corresponding pectoral fin ; the
presence vof these allows of the division of each side of the
arch into a narrow, pointed, dorsal portion corresponding
to the scapular region of the frog (pet, g), and a broader

bs
bsz.

FIG. 115. — Ventral view of pectoral arch of Scy Ilium canicula with right pectoral fin.
The pectoral arch is divisible into dorsal or scapular (pet. g), and ventral or cora-
coid (pct.g1) portions, continuous at the articular facets (art.f} for the fin.
The pectoral fin is formed of three basal cartilages (bs. i — 3) and numerous
radials (rod) ; its free edge is supported by dermal rays (d.f. r). (After Marshall
and Hurst, slightly modified.)

ventral portion, answering to the coracoid (pct.g) united
in the middle line with its fellow of the opposite side (com-
pare p. 46) : there is no sternum. The pectoral fin is
formed of pterygiophores (rad\ fused proximally to form
basals which are three in number (bs. i — 3), the third,
like the main basal of the pelvic fin, being the largest and
supporting the greater number of the cartilaginous rays,
which give rise distally to a series of polygonal plates.

444

THE DOGFISH

It will be noticed that while the skeleton of the crayfish is a series of
articulated tubes, with the muscles inside them, that of the dogfish and
of the frog is a series of articulated rods with the muscles outside. The
joints, formed by two rods applied at their ends and bound together by
ligament, are not all confined to movement in one plane, like the
hinge-joints of the crayfish, but may be capable of more or less rotatory
movement.

Digestive organs. — The mouth, as we have seen, is a
transverse aperture bounded by the upper and lower jaws.

FIG. 116. — Diagram of the development of a tooth.

&£•> Kg- mesoderm ; DS, dentine ; EM. epithelium of mouth ; Ma. epithelium of
enamel-organ ; O. odontoblasts ; SK. dental lamina ; ZK. dental papilla. (From
Wiedersheim's Comparative Anatomy.}

In the mucous membrane covering the jaws are im-
bedded large numbers of teeth — conical, calcined bodies,
with enamelled tips, arranged in transverse rows. They
are to be looked upon as special developments of the
placoid scales or dermal teeth (p. 433) enlarged for the pur-

DIGESTIVE ORGANS 445

pose of seizing prey, and are continually renewed on the
inner sides of the jaws as they are worn away on the outer
sides.

The teeth, in Vertebrates generally, are developed in the following
manner. The ectodermal epithelium of the mouth (stomodaum, p. 204)
— or in the case of the dermal teeth of the dogfish that covering the body
generally — grows inwards to form a ridge or dental lamina (Fig. 116,
SR] which projects into the underlying mesodermal connective-tissue and
becomes enlarged distally to form a bell-shaped enamel-organ, into the
base of which a mesodermal dental papilla (ZK] extends : the superficial
part of this papilla forms a layer of cells known as odontoblasts (0).
The dentine (DS) is formed, in successive layers, from the odontoblasts,
and gradually accumulates between them and the epithelium lining the
interior of the enamel organ (Ma), which gives rise, also in successive
layers, to the enamel, or hardest part of the tooth. Around the base
of the papilla more or less bony matter — the cement — is formed. It
will thus be seen that while the teeth are mainly mesodermal structures,
a part of them — the enamel — is ectodermal in origin.

The mouth leads into an oral cavity, on the floor of
which is a rudimentary tongue (Fig. 117, tng) capable of
very little movement, and which passes insensibly into the
throat or pharynx (ph}, distinguished by having its walls
perforated by five pairs of slits, the internal branchial aper-
tures (i. br. a) as well as by the inner opening of the
spiracle (sp). The pharynx is continued by a short gullet
(gul) into a capacious, U-shaped stomach, consisting of a
wide cardiac division (cd. sf] and a narrow pyloric division
(pyl. sf). The pyloric division communicates by a narrow
valvular aperture with the intestine (int\ a wide, nearly
straight tube having its lining membrane produced into
a spiral fold, the spiral valve (sp. vl\ which practically
converts the intestine into a very long, closely-coiled tube,
and greatly increases the absorbent surface. Finally the
intestine opens into a large chamber, the cloaca (cl), which
communicates with the exterior by the vent.

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CHAP, x DIGESTIVE ORGANS 447

From the gullet backwards the enteric canal is contained
in the ccelome, to the dorsal wall of which it is suspended
by a median incomplete mesentery (Figs, in and 117,
mes). The greater part of the canal is developed from the
enteron of the embryo, and is consequently lined by
endoderm ; the oral cavity is formed from the stomodseum,
and the cloaca from the proctodaeum (p. 204). Outside the
enteric epithelium are connective-tissue and muscular layers,
the latter formed of unstriped fibres : it is generally charac-
teristic of vertebrates that the voluntary muscles are striped,
the involuntary unstriped (compare pp. in and 112).

The immense liver, divided into two lobes (Mr, r.lr\ is
situated below the stomach along the whole length of the
abdomen, to the wall of which it is attached by a fold of
peritoneum. It discharges its secretion, the bile, into the
anterior end of the intestine by a tube, the bile-duct, which
gives off a blind offshoot terminating in a gall-bladder

pancreas (pan) lies against the anterior end of the
intestine, with which it communicates independently by the
pancreatic duct. Opening into the hinder part of the in-
testine or rectum is a small finger-like rectal gland (ret. gl\
the function of which is not known. In addition to these
there are, as in all Vertebrates, minute tobul&i gastric glands
sunk in the mucous membrane of the stomach (p. 131).

The spleen (spl] is an irregular, dark-red, gland-like body,
of considerable size, attached by peritoneum to the stomach
(compare pp. 23 and 98).

Other so-called ' ' ductless glands " — which are also represented in
the frog, and the functions of which are not thoroughly understood —
are the thyroid in the throat and the thymits in connection with the
dorsal ends of the branchial arches ; there are also suprarenal and
interrenal bodies in the neighbourhood of the kidneys, corresponding to
the adrenals of the frog (p. 145).

THE DOGFISH

CHAP.

Respiratory organs. — The respiratory organs consist of
five pairs of pouches, each opening by one of the internal
branchial apertures (Fig. 117, /. br. a) into the pharynx, and
by one of the external branchial apertures on the exterior.
The walls of the pouches, or inter-branchial septa, which
form the arches separating the clefts, are supported by the
cartilaginous visceral arches and
branchial rays (Figs. 112, br. r, and
1 1 8, r), and are lined with mucous
membrane raised into horizontal
ridges, the branchial filaments,
which are abundantly supplied with
blood-vessels and are the actual
organs of respiration. As the fish
swims, water enters the mouth and
passes by the internal clefts into the
branchial pouches, and thence out-
wards by the external clefts, a constant
supply of oxygen being thus ensured.
The gill-pouches are developed as
offshoots of the pharynx, and
the respiratory epithelium is there-
fore endodermal, not ectodermal,
as in the crayfish and mussel (com-
pare also pp. 204 and 207).

As already mentioned, the walls of the pharynx are supported by the
cartilaginous visceral arches, which surround it like a series of incom-
plete hoops, each half-arch being imbedded in the inner or pharyngeal side
of an interbranchial septum. Thus the visceral arches alternate with the
gill-pouches, each being related to the posterior set of filaments of one
pouch and the anterior set of the next. A n entire gill or ho lo branch therefore
consists of two half-gills or hemibranchs — the sets of branchial filaments
belonging to the adjacent sides of two consecutive gill-pouches (Fig. 118).
On the other hand, a gill-pouch encloses the posterior hemibranch of one
gill and the anterior hemibranch of its immediate successor.

FIG. 1 1 8. — Transverse section
through a gill of an Elas-
mobranch.

a. afferent branchial artery;
b. branchial arch ; 3/1.
anterior, and bft. pos-
terior hemibranch ; h.
septum ; r. branchial
ray ; v. efferent branchial
arteries. (From R. Hert-
wig's Zoology.)

x HEART 449

The first pouch is situated between the hyoid and the first branchial
arch, and the hyoid thus bears a hemibranch only. The first four
branchial arches bear each a holobranch, and the fifth is without gill-
filaments. There is a vestigial hemibranch, or pseudobranch, on the
anterior wall of the spiracle.

Now it is known that parts which have become useless tend to dis-
appear more or less completely (e.g. pineal body of the frog — p. 159,
and certain of the gills in the crayfish— p. 375). In some cases, how-
ever, such vanishing parts take ori new relations with other organs and
thus once more become useful in other ways, undergoing a change of
function. Thus in higher Vertebrates the spiracle is utilised in con-
nection with the auditory organ, and instead of disappearing entirely,
as do the other gill-clefts, it gives rise to the tympanic cavity and
Eustachian tube (compare p. 45).

Circulatory organs. — The heart is situated in the peri-
cardial cavity or anterior compartment of the ccelome(p. 434),
and is a large muscular organ composed of four chambers.
Posteriorly and dorsally is a small, thin-walled sinus venosus
(Figs. 117 and 119, s. v\ opening in front into a single,
capacious, thin-walled auricle (au) ; two auricles are present
only in those Vertebrates which possess lungs. The auricle
communicates with a very thick-walled ventricle (v\ from
which is given off in front a tubular chamber, also with
thick muscular walls, the conus arteriosus (c. art). There
are valves between the sinus ;and the auricle, and between
the auricle and ventricle, and the ., conus contains three
longitudinal rows of valves : all the valves are arranged so as
to allow of free passage of blood from sinus to auricle,
auricle to ventricle, and ventricle to conus, but to prevent
any flow in the opposite direction (compare pp. 79 and 87).

The conus gives off in front a single blood-vessel (v. ao\
having thick elastic walls composed of connective and elastic
tissue and unstriped muscle. This vessel, the ventral aorta,
passes forwards beneath the gills, and gives off on either
side paired lateral branches, the afferent branchial arteries

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CHAP, x ARTERIES 451

(Fig. 119, a. br. a, and compare Fig. no). Each afferent
artery passes to the corresponding gill, and there branches
out into smaller and smaller arteries, which finally open into
a network of delicate capillaries (p. 95), with which the con-
nective-tissue of the branchial filaments is permeated. The
blood in these respiratory capillaries is therefore brought
into close relation with the surrounding water, and as the
blood flows through them it exchanges its carbon dioxide
for oxygen, obtained from the air dissolved in the water.

From the respiratory capillaries the blood is collected
into minute arteries which join into larger and larger
trunks, and finally unite into efferent branchial arteries (e.
br. a) by which the purified blood is carried from the
gills. The efferent arteries of the right and left sides unite
in a median longitudinal artery, the dorsal aorta (d. ao\
which passes backwards, immediately beneath the vertebral
column, to the end of the tail.

From the efferent branchial arteries and the dorsal aorta
are given off numerous arteries supplying the whole of the
body with blood. The most important of these are paired
Cjarotid arteries (c. a) to the head, and subclavians (scl. a) to
the pectoral fins ; unpaired splanchnic arteries (cl.a, ms.a\ to
the enteric canal, liver, pancreas, and spleen ; numerous
paired renals (r. a) to the kidneys, spermatic (sp. a) or
ovarian arteries to the gonads, and a pair of iliacs (il. a) to
the pelvic fins. The posterior part of the dorsal aorta,
supplying the tail, is contained in the haemal canal of the
caudal vertebrae, and is often spoken of as the caudal artery^
(cd. a).

The arrangement of the arteries in the tadpole is very similar to
that described above, and the diagram (Fig. 119) would serve almost
equally well for a tadpole as for a fish. In the former there are four
pairs of afferent and efferent branchial arteries (corresponding to the

G G 2

452

THE DOGFISH

second to the fifth in Fig. 119) in relation with the corresponding
branchial arches and their gills. The afferent and efferent vessels at
first communicate with one another through the respiratory capillaries,
but later on each afferent becomes directly connected with the corre-
sponding efferent artery ; and at metamorphosis, when the gills gradually
disappear, all the blood thus passes
directly from the ventral to the dorsal
aorta, through the four arterial arches
(Fig. 120). The first arch (l) gives
rise in the adult frog to the carotid
trunk (ca), and loses its connection with
the second at the dorsal end ; the
second (2) forms the systemic trunk :
the third (3) disappears ; and the fourth
(4), losing its connection with the
dorsal aorta, forms the pulmo-cutaneous
trunk (/) (compare p. 80).

Having now traced the main course
and arrangement of the chief arteries,
there are a few minor points of detail
to be noticed in the dogfish (Fig. 121).
The five afferent branchial arteries
(af. br i — 5) of either side do not arise
regularly and symmetrically from the
ventral aorta, as represented in the
diagrammatic Fig. 119. The anterior
end of the ventral aorta divides into
right and left branches, each of which
again subdivides to form the first two
afferent branchial arteries, which supply
respectively the hemibranch of the
hyoid arch and the holobranch of the

FIG. 1 20. — Diagram of the arterial

arches of an Amphibian.
i: — 4, the four arterial arches
which pass up the corre-
sponding branchial arches ;
ao. dorsal aorta ; c. a. carotid
artery ; k. embryonic arterial
arch of the mandibular arch,
and h . of the hyoid ; /. pul-
monary artery ; st. ventral
aorta. (From Wiedersheim's
Coinfi. Anatomy, after Boas.)

first branchial arch. The third afferent
branchial artery arises from about the middle of the ventral aorta, and
supplies the holobranch of the second branchial arch ; a short distance
behind it, the fourth and fifth come off close together, and supply the ^
gills on the third and fourth branchial arches respectively: it will be
rememoered that the fifth branchial arch bears no gill-filaments. After

ARTERIES

453

aeration, the blood from each hemibranch passes into an efferent
branchial artery (ef. br\ which joins with its fellow of the same cleft
(except in the case of ef. br?}, and thus forms a loop surrounding the
cleft, the two halves of adjacent loops being connected in the middle by
a commissural vessel. From the upper ends of each of the four loops
arises an epibranchial artery (ep. br) which is connected with the dorsal

ct.tza

d.c

FIG. i2i.— The heart and branchial arteries of Scy Ilium, from the side.
af. br. ! — 5, afferent branchial arteries ; ati. auricle ; c. a. conus arteriosus ; c/.1 — 5,
branchial clefts ; cor. coronary artery ; d. ao. dorsal aorta ; d. c. dorsal carotid
artery ; ef. br.^ — 9, efferent branchial arteries ; ep. br.^ — 4, epibranchial arteries ;
inn. mandibular artery ; sp. spiracle ; ,y. cl. subciavian artery ; s. v. sinus venosus ;
v. ventricle ; v. ao. ventral aorta ; v. c. ventral carotid artery.

aorta (d. ao}, the blood from the last hemibranch passing into the fourth
loop.

From the dorsal end of the first efferent branchial, a dorsal carotia
artery (d. c} is given off: this passes forwards and inwards, gives off a
branch to the upper jaw and snout, and then runs inwards in a groove on
the skull- floor, which it penetrates in the middle line so as to reach the
cranial cavity. A vessel arises from the middle of the first efferent
branchial, and supplies the pseudobranch, from which the blood is col-
lected by a ventral carotid artery (v. c}, which passes through the orbit
into the cranium, giving off branches to the brain and anastomosing
with the dorsal carotid. From the ventral end of the first efferent
branchial a small mandibular artery (mn) passes to the lower jaw. The

454 THE DOGFISH CHAP, x

dorsal aorta is continued forwards, anteriorly to the first epibranchial
artery, as a slender vessel which soon bifurcates and anastomoses with
the dorsal carotid. From the ventral ends of the efferent branchial loops
small arteries are given off which supply the lower parts of the head,
the branchial region, and the heart (cor}. The subclavian arteries
(s. cl) arise from the aorta just before it is joined by the last epi-
branchials.

In the short-bodied frog we have seen (p. 80) that there is only a
single splanchnic or cceliaco-mesenteric artery, which soon divides into a
cceliac and a mesenteric. In the Dogfish there are four splanchnic
arteries, arising separately from the dorsal aorta, viz. , a cocliac^ supply-
ing the proximal limb of the stomach and the liver ; an anterior mesen-
teric, arising a short distance further back and supplying the intestine,
&c. ; a lieno-gastric^ coming off from the aorta close behind the anterior
mesenteric and going to the spleen and part of the stomach and pan-
creas ; and a small posterior mesenteric supplying the rectal gland.

As in all Vertebrates, the arteries branch and branch again
in the various parts to which they are distributed, their ulti-
mate ramifications opening into a capillary network with
which all the tissues except the cartilages and epithelia are
permeated. From these systemic capillaries the blood is
collected into larger and larger efferent thin-walled trunks or
veins, parts of many of which are greatly dilated to form sinuses.

The blood from the head is brought back by a pair of
jugular veins (Figs. 119, /. -v, and 122, jug. v) : each of these
enters a large precaval sinus ( pr. cv. v, dct. c) which passes
vertically downwards and enters the sinus venosus. The blood
from the tail is returned by a caudal vein (cd. v, caud. v)
lying immediately beneath the caudal artery in the haemal
canal ; this vessel enters the ccelome and then divides into
right and left branches, the renal fiortal veins (r. p. v,
r. port, v), which pass to the kidneys and join with the
capillaries of these organs, the impure blood brought from
the tail mingling with the pure blood of the renal arteries
(Fig. 119, r. a). From the kidneys the blood is returned

r.porb.v

ccuui.v

FIG. 122. — Diagram of the chief veins together with the ventral aorta and afferent

branchial arteries of a dogfish (Cheiloscyllinui).

ali. ^enteric canal ; br. v± — br. v5. afferent branchial arteries ; ca^^d. v. caudal
vein ; dct. c. precaval sinus ; Jit. heart ; h. port. v. hepatic portal vein ; hep. s.
hepatic sinus ; inf. jug. v. inferior jugular vein or sinus ; jug. jugular vein or
sinus ; lat. v. lateral vein ; //?'. liver; I. card. s. left cardinal sinus and vein;
/. port. v. left renal portal vein ; neph. kidney ; r. card. s. right cardinal sinus
and vein ; r. port. v. right renal portal vein. (From Parker and Haswell's
Zoology.}

456 THE DOGFISH CHAP.

into a pair of cardinal veins (crd. v) which pass forwards,
become swollen to form sinuses (Fig. 122, /. card, s, r. card. s\
receive veins from the reproductive organs, muscles, &c., and
finally enter the precaval sinus.

From the stomach, intestine, spleen, and pancreas the
blood is collected by numerous veins, which all join to form
a large hepatic portal vein (Fig. 119, h. p. v, Fig. 122,
h. port. v). This behaves in the same way as the renal
portal : instead of joining a larger vein on its way to the
heart, it passes to the liver and breaks up to connect with
the capillaries of that organ ; its blood, deprived of oxygen
but loaded with nutrient matters from the enteric canal,
mingling with the oxygenated blood brought to the liver by
a branch of the cceliac artery. After circulating through
the capillaries of the liver the blood is taken by a pair of
hepatic sinuses (h. z>, hep. s) to the sinus venosus (compare

p- 85).

The course and arrangement of the veins, like that of the arteries, is
very similar to that existing in the tadpole, in which several important
changes occur at metamorphosis. With the disappearance of the tail
and caudal vein, the renal portal veins receive their blood from the
hind-limbs only. The hinder parts of the two cardinal veins, situated
between the kidneys, fuse into one, and their anterior parts disappear,
a new vessel being developed which conducts the blood from the fused
cardinals to the sinus venosus : the whole of the great vein thus
formed is the postcaval (p. 82, Fig. 21), which is present in all
Vertebrates above the fishes.

The iliac veins of the dogfish (Fig. 119, il. v) pour the
blood from the pelvic fins into the lateral veins (Figs, in,
119, and 122, lat. v) — paired trunks running forwards in the
side walls of the body to the sinus venosus, and receiving at
their anterior ends the subclavian veins (scl. v) from the
pectoral fins. In addition to the dorsally situated jugular
veins, there are paired inferior jugulars (Fig. 122, inf. jug. v\

x CIRCULATORY ORGANS 457

bringing back the blood from the ventral parts of the head,
and each opening into the corresponding precaval.

As we have seen, several of the veins, e. £.,' the precavals,
jugulars, cardinals, and the genital veins, are dilated into
spacious cavities called sinuses (Fig. 122). These are,
however, of a totally different nature from the sinuses of the
crayfish, which are mere spaces among the tissues devoid of
proper walls. In the dogfish, as in the frog and Vertebrates
generally, the blood is confined throughout its course to
definite vessels ; the heart, arteries, capillaries, and veins
invariably forming a closed system of communicating
tubes.

The general course of the circulation will be seen to agree with that
already described in the frog, as well as in the crayfish and mussel, i.e.,
the blood is driven by the contractions of the heart through the
arteries to the various tissues of the body> whence it is returned to the
heart by the veins or sinuses (Fig. 123). But whereas in both cray-
fish and mussel the respiratory organs are interposed in the returning
current — both their afferent and efferent vessels being veins, in the dog-
fish they are interposed in the outgoing current — their afferent and
efferent vessels being arteries. An artery, it must be remembered, is
a vessel taking blood from the heart to the tissues of the body and
having thick walls ; a vein is a thin- walled vessel bringing back the
blood from the tissues to the heart.

Moreover, the circulation in the dogfish is, as in the frog, compli-
cated by the presence of the two portal systems, renal and hepatic. In
both of these we have a vein, renal portal or hepatic portal, which,
instead of joining with larger and larger veins and so returning its
blood directly to the heart, breaks up, after the manner of an artery,
in the kidney or liver, the blood finding its way into the ordinary venous
channels after having traversed the capillaries of the gland in question.

Thus an ordinary artery arises from the heart or from an artery of
higher order and ends in capillaries ; an ordinary vein arises from a
capillary network, and ends in a vein of higher order or in the heart.
But the hepatic and renal portal veins end in capillaries after the manner
of arteries, and the efferent branchial arteries begin in capillaries after
the manner of veins.

458

THE DOGFISH

With regard to the general morphology of the blood-system, the
dorsal aorta with the caudal artery may be considered as a dorsal vessel
(compare Earthworm, p. 337, Crayfish, p. 379, and Amphioxus, Fig.
1 10) ; the caudal vein, hepatic portal vein, and ventral aorta as together
representing a ventral vessel ; the afferent and efferent branchial
arteries as commissural vessels ; and the lateral veins as lateral vessels.
It will be seen that the heart of Vertebrates is a muscular dilatation of
the ventral vessel, as is also shown by a study of its development.

The blood, like that of the frog (pp. 104 — 106), consists
of a colourless plasma containing red corpuscles (the colour

•• hra-

n hra.

FIG. 123. — Diagram illustrating the course of the circulation in the dogfish. Vessels

containing oxygenated blood red ; non-oxygenated blood blue.
B. capillaries of the body generally ; E. of the enteric canal ; G. of the gills ; K. of
the kidneys ; L. of the liver ; T. of the tail. a. br. a. afferent branchial arteries ;
au. auricle ; c. a. conus arteriosus ; d, ao. dorsal aorta ; e. br. a, efferent branchial
arteries ; h. p. v. hepatic portal vein ; h. v. hepatic vein ; Ic. lacteals (p. 98) ; ly.
lymphatics ; pr.* cv. <v. precaval vein ; r. p. v. renal portal veins ; s. v, sinus
venosus ; v. ventricle ; v. ao. ventral aorta. The arrows show the direction of the
current. (From Parker and Haswell's Zoology.)

of which is due to haemoglobin) and leucocytes. It must be
remembered that the ventral aorta and the afferent branchial
arteries (Fig. 123), like the pulmonary artery of the frog,
(p. 144), contain venous blood. As in the frog, there are
in addition to the blood-vessels, a number of lymphatic
vessels.

Nervous System. — The nervous system is constructed on

x BRAIN 459

a similar plan to that of the frog (compare Part I, Chapter X)
and of Vertebrates in general. The central nervous system
is dorsal in position and consists of a brain, contained within
the cranial cavity, and continuous posteriorly with a spinal
cord, contained in the neural canal of the vertebral column :
it consists of grey and white matter, and its cavity or
neuroccele^ lined with epithelium, gives rise to the ventricles
of the brain and to the central canal of the spinal
cord.

In correspondence with the form of the body, the spinal
cord is relatively much longer than in the frog, and it is not
swollen opposite the paired appendages.

In the brain (Figs. 117 and 124) the bulb or medulla
oblongata (NH} broadens out anteriorly to form lateral
swellings, and its contained fourth ventricle (F. rho) is roofed
over by the " pia mater." The cerebellum (HH\ which is
very small in the frog, is here relatively enormous, and its
surface is marked by slight grooves : it overlaps the bulb
behind and the optic lobes in front, and contains a ventricle
communicating with the fourth ventricle. The oval optic
lobes (MH) are hollow, their cavities communicating with
the median ventricle or iter, and ventrally to them are the
crura cerebri. The diencephalon (Zff] is relatively narrower
than in the frog. From its thin roof, which covers over the
tMrd-Wntridei. .is a delicate tube-like structure (Gp), which
extends upwards and forwards and ends in a small knob
attached to the roof of the skull : this is the pineal body.
From the ventral surface of the diencephalon arises the
infundibulum, with an oval swelling on either side, and to it
is attached the pituitary body* with a vascular sac on either
side of it. In front of the infundibulum is the optic chiasma
(compare Figs. 49 and 50).

Apart from the large size of the cerebellum, the most

460

THE DOGFISH

J'

marked difference between the brain of the dogfish and that
of the frog is seen in its anterior portion. In the frog, the

diencephalon is con-
tinuous anteriorly
with the paired cere-
bral hemispheres ; in
the dogfish there is in
this region a relatively
smaller, unpaired por-
tion of the brain,
marked in front by
a slight groove, and
known as prosencep-
halon (VH\ which
represents the cere-
bral hemispheres of
the higher Verte-
brates but which does
—F.rho n°t become subdi-

NH vided externally into

paired lobes. Ante-
riorly it gives off,
right and left, a large,
oval olfactory lobe (L.
<?/) each connected
with the prosen-
cephalon by a short,
stout stalk (Tro) and
applied distally to
the corresponding ol-
factory capsule. The

FIG. 124. — Dorsal view of the brain of Scylliutn

canicula.

The posterior division of the brain is the medulla
oblongata (Ar//), enclosing the fourth ventricle
(F. rho). The large cerebellum (HH) nearly
covers the optic lobes (MH ). The diencephalon
(ZH) shows, in the middle, the third ventricle,
and the place of attachment of the pineal stalk
(Gp). The prosencephalon (VH) gives off the
olfactory lobes (Tro, L.ot). The origins of
the following nerves are shown: — optic (//),
trochlear (7K), trigeminal (F), facial (VII\
auditory (F//7), glossopharyngeal (AY), and
vagus (X). (From Wiede
A nato my.)

dersheim s

prosencephalon con-
tains paired lateral ventricles, which communicate posteriorly

x NERVES 461

with the third ventricle and anteriorly are continued into the
olfactory lobes.

Each spinal nerve arises, as in the frog (p. 163), by two
roots, which, however, are not in the same transverse plane,
the dorsal root being slightly anterior to the corresponding
ventral root, so that successive dorsal and ventral roots of
either side alternate with one another. As already men-
tioned (p. 440); the two roots of each nerve pass out from
the neural canal independently, uniting on. the outside of
the canal to form the spinal nerve. Plexuses in connection
with the nerves supplying the paired appendages are much
simpler than in the frog (pp. 161 and 162). A sympathetic
system is represented.

The origin and distribution of the cerebral nerves is in
the main similar to that already described in the case of the
frog (p. 163), the chief differences, characteristic respectively
of pulmonate and of branchiate Vertebrates in general, being
as follows.

In fishes, there are certain nerves, usually counted as
belonging to the facial and vagus, which supply the sensory
canals of the integument (Fig. 125, VIIop, b, e.m, and X!) :
these organs are not present in terrestrial forms, and their
nerves are consequently also wanting. The vagus, more-
over, gives off a series of branchial branches (br.1 — 4) to
the gills instead of a pulmonary branch, and the glosso-
pharyngeal (IX} is also a branchial nerve.

The olfactory nei-ves (Fig. 125, I) arise from the olfactory lobe,
which is situated in a large aperture in the skull communicating
between the cranial and olfactory cavities. The optic nerve (II) is
continued outwards from the optic chiasma, and passes through a
foramen in about the middle of the orbit, towards the ventral side
(compare Fig. 112). The octdomotor (III), arising from the crura
cerebri, makes its exit from the skull a short distance behind and
slightly above the optic nerve. The pathetic (IV), coming off from the

462

THE DOGFISH

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x CEREBRAL NERVES 463

dorsal side of the front end of the medulla oblongata and supplying the
superior oblique muscle, pierces the cranial wall almost directly above
the optic foramen (compare also Fig. 126). All the other nerves arise
from the ventro-lateral regions of the medulla oblongata, the abducent,
supplying the external rectus muscle, coming off nearer the middle line
than, and anterior to, the others. The abducent (VI). and the main
parts of the trigeminal (V) and facial (VII) nerves pass out through a
single foramen in the skull in the posterior and ventral part of the orbit,
just anterior to the auditory capsule. A short distance above this
foramen are two others, the ventral slightly anterior to the dorsal :
these transmit the ophthalmic branches (see below) of the trigeminal and
facial (V op, VII op] respectively, and from them grooves pass along the
dorsal side of the orbit to an aperture just behind the olfactory capsule,
the nerves emerging again on the dorsal side of the skull. The auditory
nerve passes through a large foramen on the inner side of the auditory
capsule to supply the membranous labyrinth. The glossopharyngeal
(IX) emerges behind the auditory capsule at the posterior end of a
horizontal groove in this region, and the vagus (X) passes out through a
foramen between the glossopharyngeal and the foramen magnum.

The nerves supplying the integumentary sense organs are as
follows : (i) The ophthalmic branch of the facial (VII op} runs, as we
have seen, dorsally to the similarly named branch of the trigeminal,
close under the skin, and supplies the sensory tubes and ampullae (see
p. 464) of the upper part of the snout ; those of the lower part of the
snout are innervated by (2) a buccal branch (VII b), which extends
along the floor of the orbit just above the maxillo-mandibular division
of the trigeminal ; and those in the region of the hyomandibular by a
small (3) external mandibular branch (VII e. m), arising from the large
hyomandibular nerve (see below). The lateral-line canal, extending
along the body and tail, is supplied by (4) the lateral branch of the vagus
(X /), which runs backwards to the inner side of the rest of the nerve
and dorsally to the spinal nerves, along the inner side of the body- wall,
giving off branches which extend outwards between the great lateral
muscles to the lateral canal.

The other branches of the facial are : — a small palatine (Mil p], which
extends along the floor of the orbit, just behind the trigeminal, and sup-
plies the roof of the mouth ; and a large hyomandibular (VII ky] which
passes behind the spiracle (first giving off small prespiracular branches
(VII p. s) to its anterior wall), and extends along the anterior border of

464 THE DOGFISH CHAP.

the auditory capsule and the posterior wall of the orbit, just beneath
the skin, to the anterior side of the hyoid arch : it thus forks over
the spiracular or mandibulo-hyoid cleft.

The glossopharyngeal (IX) forks above the first gill-cleft, thus giving
rise to two brandies, one passing down the posterior side of the hyoid,
and the other down the anterior side of the first branchial arch. The
main part of the vagus extends backwards to the outer side of the
lateral nerve and gives off four branchial nerves (X br. 1-4) forking
over the second to the fifth gill-clefts respectively, and is then con-
tinued into the visceral nerves (X v), which supply the stomach and
heart.

Sensory organs. — The dogfish possesses, as we have seen,
a series of peculiar integumentary sense-organs supplied by
the nerves just described, the function of which is not
known with certainty. They are situated within a number of
epithelial canals, developed from the epiderm, the openings
of which on the head have already been noticed (p. 432).

The tubes are of two kinds, known respectively as sensory and am-
pullary canals : the former, which are present in all Vertebrates with
gills (p. 430), are all continuous with one another and are situated along
certain definite tracts on the head and jaws, a canal extending
along the body and tail as the lateral-line canal. The ampullary
canals, which are peculiar to Elasmobranch fishes, and which contain
a gelatinous material, are not continuous with one another, but run side
by side, converging to form large masses in the snout and at the sides
of the head ; at their blind ends they are swollen to form ampullce^ to
which the nerves are distributed. The sensory cells are arranged in
little conical masses in the lining epithelium of the canals or of the
ampullae, a section of one of which nearly resembles that of an ampulla
of a semicircular canal of the ear (Fig. 60).

The olfactory organs are a pair of cup-like sacs in the
snout, enclosed by the olfactory capsules and opening
externally on the ventral side by the nostrils. Notice that
there are no internal nostrils, as in the frog : these are only
present in Vertebrates which possess lungs. The sacs are
lined by the olfactory epithelium, which is supplied by the

SENSORY ORGANS

465

2,7-

olfactory nerves and is raised up into ridges so as to increase
the surface.

The structure of the eye, as well as of the accessory
apparatus in connection with it, is in all essential re-
spects the same as in the frog (p, 181), except for the
differences in the eyelids (pp. 5 and 432), the absence of
a lacrymal apparatus (p. 186), and for the fact that the
four recti muscles
(Fig. 126) do not en-
sheath the optic nerve,
which emerges into
the orbit a short dis-
tance 'in front of their
point of origin.

The "membranous
labyrinth of the ear
(compare Fig. 59, p.
187) is also very simi-
lar to that of the frog
but being larger, and
the auditory capsules
being composed en-
tirely of cartilage, it
can be dissected out
with comparative ease
a knife.

FIG. 126. — Semidiagrammatic figure of the eye-
muscles and their nerves of an Elasmobranch.
///. oculomotor, IV. pathetic, and VI. abducent
nerve ; e, r. posterior rectus muscle ; i. o. in-
ferior oblique ; in. r. inferior rectus ; i. r.
anterior rectus ; or. wall of orbit ;•$. o. superior
oblique ; s. r. superior rectus. (From Parker
and Haswell's Zoology.')

by slicing away the capsule with

A tube given off from the sacculus, called the endolymphatic duct
(Fig. 59), which in the frog communicates with the lymphatic system,
opens to the exterior on the top of the head in the dogfish, and thus
the endolymph is in free communication with the surrounding sea-
water.

As we have seen, the membranous labyrinth is the essential part of
the ear, and it, together with its enclosing capsule, is often spoken of

PRACT. ZOOL. H H

466 THE DOGFISH CHAP.

as the internal ear. In the frog there is also, an accessory apparatus —
the tympanic cavity and membrane, together with the columella —
which is called the middle ear (compare Fig. 108 and pp. 189 and 449).

TJrinogenital organs. — In order to understand the mor-
phology of the kidneys, and the close relations existing
in most Vertebrates between them and the generative
organs, it is necessary to know something of the develop-
ment of these parts (see Chapter XII). In the embryo,
the kidneys appear in the form of separate, segmen tally
arranged tubes having the general character of nephridia,
opening on the one hand by nephrostomes into the ccelome,
and on the other into a longitudinal duct which discharges
into the cloaca. Thus the primitive structure of the verte-
brate kidney furnishes another example of metamerism,
which can no longer be distinctly recognised in the adult
kidney (compare Figs. 46 and 47.)

At a later stage of development in most vertebrate orders
two longitudinal ducts can be recognised on either side,
which in some cases (e.g. Dogfish) are formed by the
subdivision of the single primary duct. These are known
respectively as the Wolffian and the Mullerian ducts : the
former takes on the function of a spermiduct in the male,
although it may to a greater or less extent (compare p. 193)
retain also its function as a ureter ; the latter gives rise to
the oviduct in the female, and vestiges of it may be re-
cognised in the male (Figs. 117 and 127).

In the dogfish the kidneys (Fig. 127, ef, K) are, long,
narrow, lobulated organs, lying close to the vertebral
column on either side, covered ventrally by the thick
peritonium, and extending primarily along almost the
whole length of the ccelome. But in the course of
development, certain important modifications occur in

x URINOGENITAL ORGANS 467

them and in their ducts (Wolffian ducts). In the
male, the anterior part of the kidney takes on a close
relation with the generative organs, and gives rise to a
glandular body — the epididymis (A, k") — with which the
long, convoluted Wolffian duct (Spd), serving mainly as a
spermiduct, is closely connected ventrally. The middle
part (k'} becomes vestigial : in the female this is the case
as regards both anterior and middle parts and the Wolffian
duct. The hinder part of the embryonic kidney in each
sex is retained in the adult as the renal organ (k\ which is
somewhat swollen posteriorly, and on the surface of which
nephrostomes open. The ureters (ur) are independently
developed tubes, about five in number on either side. In
the female they open separately into the swollen persistent
posterior ends of the Wolffian ducts, which unite together to
form a median urinary sinus (B, u.s), opening by a single
aperture into the cloaca ; while in the male (A, ur) most of
them unite to form a wide main ureter before communicat-
ing with a similar median sinus, which, as it receives the
products both of the spermaries and kidneys, is called the
urinogenital sinus (u.g.s).

The spermaries are a pair of large, elongated, soft
organs united with one another posteriorly, and suspended
to the dorsal body- wall by a fold of peritoneum. From
the anterior end of each (A, ts) arise delicate efferent ducts
(ef. d\ which pass to the epididymis to become connected
with the convoluted spermiduct. The latter dilates pos-
teriorly, where it underlies the functional kidney, forming
an elongated, spindle-shaped seminal vesicle (s.v\ which opens
(s.v') into the base of a thin-walled blind reservoir of about
the same length, the sperm-sac (sp. s) ; just to the inner side
of its aperture are the openings of the ureters (ur). The
sperm-sac is continuous posteriorly with the urinogenital

H H 2

cud"

ab.p

els

FIG. 127. — The urinogenital organs of Scy ilium canicula from the ventral side.
A, male, and B, female. Only the anterior end of the gonad is repi'esented in each
figure, and except that in B both kidneys are shown, the organs of the right

CHAP, x URINOGENITAL ORGANS 469

side only are drawn. In A the seminal vesicle and sperm-sac are dissected away
from the kidneys and displaced outwards, and the ureters inwards.
ab. p. depression into which the abdominal pore opens ; cl. cloaca ; ch. clasper ;
ef. d, efferent ducts of spermary ; k. kidney ; k'. vestigial middle portion of the
kidney ; k" , anterior portion of the kidney in the male, forming the epididymis ;
lr. anterior portion of liver ; vz. d. vestigial Miillerian duct in the male ; a>s.
gullet ; ov. ovary ; ovd. oviduct ; ovd' '. its ccelomic aperture ; ovd" . the common
aperture of the oviducts into the cloaca; r. rectum; sh.gl. shell-gland; spd.
spermiduct ; sp. s. sperm-sac ; s. v. seminal vesicle ; j. v' . its aperture into the
urinogenital sinus ; ts. spermary ; n. g. s. urinogenital sinus ; ur. ureters ; ur' .
their apertures into the urinogenital sinus ; u. s. urinary sinus.

sinus, the opening of which into the cloaca is situated on
a papilla.

The female Scyllium has a single ovary (B,ov), suspended
by a fold of peritoneum. In the adult it is studded all
over with rounded ova in different stages of development,
varying in diameter from 12-14 mm- downwards : in other
Vertebrates which produce large eggs, a similar reduction of
one ovary may take place (e.g. Birds). The oviducts (ovd)
are paired, and extend along the whole length of the dorsal
wall of the ccelome, below the kidneys : anteriorly they
unite. with one another below the gullet and just in front of
the liver, where they communicate with the ccelome by
a common aperture (ovd') ; posteriorly they open together by
a single aperture (ovd") into the cloaca, behind the rectum
(r). About the anterior third of each oviduct is narrow and
thin-walled ; the posterior two-thirds is wide and distensible,
and at the junction of the two parts is a yellowish, glandular
mass, the shell-gland (sh. gl).

Development. — Impregnation is internal, and is effected
through the agency of the claspers of the male (p. 433). The
eggs, when ripe, break loose from the surface of the ovary into
the ccelome, and thence pass, through the common oviducal
aperture, into one or other of the oviducts, where fertilisation
occurs. On reaching into the dilated portion of the oviduct,
the oosperm of Scyllium becomes surrounded first by a
gelatinous substance, and then by a horny egg-shell or

470 THE DOGFISH

" Mermaid's purse " 1 secreted by the shell-gland, and having
the form of a pillow-case produced at each of its four angles
into a long, tendril-like process. The eggs are laid among
sea-weed, to which they become attached by their tendrils.
In Acanthias and Mustelus (p. 431) a mere vestige of the
egg-shell is formed, and the eggs undergo the whole of
their development in the oviducts, the young being even-
tually born alive with the form and proportions of the adult.
The great size of the egg is due to the immense quantity
of yolk it contains : its protoplasm is almost entirely
aggregated at one pole in the form of a small disc. When

FIG. 128. — Section of the upper part of the oosperm of a Dogfish which has undergone
segmentation to form the blastoderm. The blastoderm is formed of a single layer
of ectoderm cells (white), and of several rows of cells (shaded) which subsequently
give rise to endoderm and mesoderm.

sg. segmentation cavity ; below the blastoderm is the unsegmented yolk containing
scattered nuclei («). (From Balfour's Embryology.)

segmentation of the oosperm takes place it affects the
protoplasmic part alone, the inactive yolk taking no part in
the process (compare Crayfish, p. 383). The polyplast stage
consequently consists of a little mass of cells, the blastoderm
(Fig. 128), at one pole of an undivided sphere of yolk.
The cells of the blastoderm become differentiated into the
three embryonic layers — ectoderm, mesoderm, and endo-
derm. At the same time the blastoderm extends in a
peripheral direction so as gradually to cover the yolk, and
its middle part becomes raised up into a ridge-like thickening,
which is moulded, step by step, into the form of the embryo

1 An egg is contained in the oviduct figured (Fig. 127 r>).

DEVP:LOPMENT

471

fish. The head, trunk, and tail acquire distinctness, and
become more and more completely separated off from the
bulk of the egg, the latter taking the form of a yolk-sac
(Fig. 129, A, yk.s) attached by a narrow stalk to the
ventral surface of the embryo.

In this condition the various parts of the adult fish can
be recognised, but the proportions are different and the
head presents several peculiarities. The gill-filaments (pr.f}

FIG. 129. — A, embryo of Scyllitim with yolk-sac (xi£); B, under-side of head,
enlarged. br. f. branchial filaments protruding through gill-clefts ; br. f.
branchial filaments protruding through spiracle ; cd.f. caudal fin ; d.f. dorsal
fins ; e.'eye ', ex. br. ap. external branchial apertures ; with, mouth ; na. nostril ;
pet. f. pectoral fin ; Pv.f. pelvic fin ; st. stalk of yolk-stalk ; v.f. ventral fin ;
yk. s. yolk-sac. (From Parker's Biology, after Balfour, slightly altered.) ;

are so long as to project through the external branchial
apertures and spiracles in the form of long threads
abundantly supplied with blood-vessels, and apparently
serving for the absorption of nutriment — the albumen in the
egg-shell in the case of Scyllium, secretions of the oviduct
in the viviparous forms referred to on p. 470. Besides this
mode of nutrition, the yolk-sac communicates with the in-
testine by a narrow duct (st), through which absorption of

472 THE DOGFISH CHAP.

its contents is constantly going on. By the time the young
fish is ready to be hatched or born, the greater part of the
yolk-sac has been drawn into the coelome, a mere remnant
of it still dangling from the ventral surface of the body.

PRACTICAL DIRECTIONS.

Dogfishes are best preserved in 5 pe'r cent, formaline, which, unlike
spirit, does not coagulate the blood, so that the vessels can be injected
in preserved specimens. They can be obtained, fresh or ready preserved,
from any Marine Biological Station.

A. External Characters: see pp. 431-434. Sketch from the side.
Examine a small piece of the skin under the low power with reflected

light, and note the form and arrangement of the dermal teeth. Isolate
some of these by boiling a small piece of skin in caustic potash (p. 359),
and make out the bony basal plate, and the spine composed of dentine
tipped with enamel. Sketch.

B. Skeleton, (If you are working on a fresh fish, and wish to
dissect the soft parts before preservation, the examination of the
skeleton may be posponed until later. )

It is advisable to have one skeleton prepared entire, and one in
which the parts have been disarticulated. Obtain a common butcher's or
cook's pointed knife (a strong pocket-knife will do) for cutting through
the rough skin and for the coarser work of preparation. Prepare as
directed on p. 53, dipping into hot water occasionally, or macerating in
2 per cent, nitric acid for a day or two. When the greater part of the
muscles has been removed, disarticulate the skull from the vertebral
column, leaving the branchial apparatus attached to it, and also remove
the paired fins and their arches. Disarticulate the hyomandibular
cartilage from the cranium so as to separate the visceral arches, includ-
ing the jaws (compare Fig. 112): these should then be thoroughly
cleaned without further immersion in hot water, as the cartilages of
which they are composed come apart very easily. The other parts may
be dipped into hot water for a few seconds from time to time, but care
should be taken that the more delicate elements do not thereby become
separated. It is useful to prepare a second cranium as well as a few
trunk- and caudal vertebrae, which should be bisected vertically into
right and left halves, When prepared, the skeleton should be kept in

x PRACTICAL DIRECTIONS 473

weak spirit or formaline, and not allowed to dry, or the cartilages will
of course shrink, unless the following method is resorted to : —

Thoroughly clean a skeleton, or typical parts of it (e.g. skull, limb-
skeleton, and a few trunk- and caudal vertebrae), and then transfer from
^ weak into strong methylated spirit for a day or so, and afterwards into
absolute alcohol for a few hours. Place in a vessel filled with turpen-
tine for another day, and then transfer into melted paraffin in the water-
bath until the parts are thoroughly permeated, after which they should
be suspended in the water-bath in order to drain off the superfluous
paraffin, and then allowed to cool. Any superfluous paraffin still
remaining may then be removed with a hot wire.

With the specimens before you, work through pp. 435-443, noting
first of all the relations of the parts in the entire skeleton (viz., cranial
and visceral portions of the skull, trunk- and caudal vertebrae, and the
skeleton of the median and paired fins). When examining the skull,
note the nerve foramina (pp. 461 and 463).

Sketch — (a) the skull (including visceral arches) from the side, and
the cranium in longitudinal section ; (b] trunk- and caudal vertebrae
from the side or in longitudinal section and from the anterior or
posterior face ; (c] the pectoral arch, from the side, with the pectoral
fin attached ; and (d] the pelvic arch and fin.

C. General dissection: Enteric Canal, &c.

I. — Fix the animal down on the dissecting board with the ventral
surface uppermost, by means of strong pins inserted through the
paired fins, and make a median longitudinal cut with a common knife
through the skin and underlying muscular layer which is closely
connected with the skin, from the pectoral to the pelvic arch. At
each end of this incision cut through the body-walls transversely, and
reflect and pin down the two flaps. Cut through the pelvic arch
slightly to one side of the median line, so as not to injure the cloaca.
The abdominal cavity, lined by the peritoneum, will then be exposed.
(In the course of your dissection you will probably find many parasitic
thread-worms belonging to the phylum Nemathelminthes (see p. 412).
Make out (compare Fig. 117) : —

I. The liver, with the gall-bladder partly embedded in it close to the
junction of its two lobes ; the gullet, U-shaped stomach, and the
branches of the vagtis nerve on its walls ; the wide intestine, narrow-
ing into a short rectum posteriorly ; the cloaca ; the pancreas, spleen,

474 THE DOGFISH CHAP.

and retal gland; and the incomplete mesentery. Pass a seeker back-
wards, on one side of the cloaca, through an abdominal pore.

2. In the male, the spermaries, fused posteriorly ; and in the female,
the single ovary > and the oviducts and shell-glands. The peritoneum
covering the kidneys is so thick that at present they can only be
recognised as slightly convex ridges.

II. — Remove the skin from the dorsal surface of the head between and
slightly in front of and behind the eyes, and then slice away part of the
roof of the skull with a knife until the brain is exposed, being careful not
to injure some nerves which you will see close beneath the skin on
either side of the brain-case in front. Then cut off the tail transversely,
a short distance behind the pelvic fins, and on the cut surface note —

1. The integument, in which runs the sensory canal of the lateral
line.

2. The centrum and neural and Juzmal arches of the vertebra, and the
soft intervertebral substance (remains of the notochord] ; the spinal cord \
and the caudal artery and vein.

3. The myomeres and myocomvias ; and if your section passes through
a dorsal fin, the cartilaginous pterygiophores and the horny fat-rays
(compare Fig. in). Sketch.

III. — The dorsal aorta and its branches may now be injected (see
p. 99) through the cut end of the caudal artery, into which a cannula
should be inserted for some distance (tying is unnecessary). Now return
to the examination of the abdominal viscera, and make out : —

1. The bile-duct, opening into the intestine just behind the pylorus.
The pancreatic duct runs in the wall of the intestine, and. careful dis-
section is required to make out its course (see § IV, i).

2. The hepatic portal vein and its factors, entering the liver near the
^median plane. If the blood has escaped from it, try to blow it up
with a blowpipe.

3. The position of the dorsal aorta> which will be seen better at a
later stage, but the chief branches of which should now be traced to
their distribution, as follows : #, the cceliac artery ', extending down-
wards and backwards along the stomach from above the posterior end
of the gullet ; b, the anterior mesenteric artery, arising about \\ inch
behind the cceliac ; <r, the lieno-gastric artery ', arising close behind the
anterior mesenteric ; and d, the small posterior mesenteric artery^ pas-
sing downwards to the rectal gland.

x PRACTICAL DIRECTIONS 475

4. The large hepatic sinus, immediately in front of the liver, below
the gullet : slit it open, and note the veins entering it from the liver.
On either side of the gullet in this region, along the dorsal surface of
the coelome, a capacious cardinal sinus will be seen : make an aperture
in this, and pass a seeker backwards, noting that the sinus narrows into
the cardinal vein, which passes along the inner side of the correspond-
ing kidney and parallel to the aorta (Figs. 119 and 122). The genital
(spermatic or ovarian] sinus communicates with the cardinal.

5. The lateral veins, running on either side of the body just beneath
the peritoneum. Cut through the body-wall on one side, a short dis-
tance behind the pectoral fin ; insert a cannula, directed forwards, into
the cut end of the lateral vein (see Fig. ill), and inject. The vein will
then be seen running forwards as far as the pectoral arch, when it turns
towards the dorsal side.

6. In the female, the united anterior ends of the oviducts, and their
coelomic aperture, ventral to the gullet and just in front of the liver.

IV. — Taking care not to injure the anterior ends of the oviducts and
to leave part of the hepatic sinus in situ, remove the liver, together
with the stomach and intestine, without injuring the bile-duct, cutting
through the stomach at its junction with the gullet and the intestine
just in front of the rectal gland. Wash out the portion of the enteric
canal thus removed under the tap, fill it with water, and place the
whole under water in a dissecting-dish. Cut away portions of the wall
of both stomach and intestine, and make out —

1 . The course of the bile-ducts and pancreatic ducts, and their apertures
into the intestine.

2. The pyloric valve, and the spiral valve of the intestine, which
makes about seven or eight close turns, appearing like a series of cones
one within the other.

3. The characters of the mucous membrane of the stomach and
intestine.

Sketch your dissection.

D. Urinogenital organs.

I.— After noting again the gonads, and in the male the delicate
efferent ducts of the spermaries (Fig. 127, A), remove in the male all but
the anterior ends of the latter, and in the female the entire ovary ; then
carefully dissect away the thick peritoneum covering the kidneys, noting

476 THE DOGFISH CHAP.

as you do so the dorsal aorta and its various branches, and once again
the cardinal veins (C, § III, 4), which may be inflated with air. The
renal portal veins are not easily traced without injection : this may be
done from the cut end of the caudal vein.
Note—

1. The brownish kidney T, and in the male the whitish forward
continuation of each into the anterior sexual part or epididymis.

2. In the male : a, the convoluted spermidnct, indistinguishable from
the epididymis anteriorly and enlarging posteriorly to form the elongated
seminal vesicle; and 3, the grooved, eversible claspers, which are
supported by cartilages and have each a gland at the base of the
groove.

II. — Cut through the skin round the vent, and dissect the
entire cloaca and the kidneys (together with the epididymes in the
male) away from the body, noting as you do so some small paired
and median bodies in close relation to the dorsal aspect of the kidneys ;
these are respectively the supra-renal and inter-renal bodies (p. 447).
Pin your dissection down, ventral side uppermost, under water. Clear
away with great care the connective-tissue which binds the ureters and
generative ducts to the kidneys posteriorly, slit open the cloaca, and
make out —

1. In the male (Fig. 127, A) the aperture of the rectum , and the
urinogenital papilla. Insert the small scissors into the aperture at the
apex of the latter, and slit open the urinogenital sinus t continuing the
cut into the two sperm-sacs ; make out the apertures of the seminal
vesicles and ureters. Pass a seeker or probe into each of these
apertures ^(the main ureter maybe injected), and then dissect out, on
one side— #, the elongated and pointed, thin- walled sperm-sac ; and b, the
delicate ureters, three or four of which unite to form a widish common
tube, situated towards the inner border of the kidney, before opening
into the urinogenital sinus. Sketch.

2. In the female (Fig. 127,8), the. thin-walled, anterior united ends
of the oviducts, their thick-walled posterior portion, \\\e shell-glands, the
apertures of the rectum and oviducts into the cloaca, and the urinary
papilla. Insert the point of the scissors into the aperture on the apex
of the latter, and slit open the urinary sinus, in which several
openings of the ureters will be seen on either side. Cut open the
oviducts, and note, if present, the eggs enclosed in horny egg-cases.
Sketch.

x PRACTICAL DIRECTIONS 477

E. Circulatory1 and Respiratory Organs, &c.

I. — After noting the spinal nerves, exposed by the removal of the
kidneys, the body may be cut through just behind the pectoral arch, and
the posterior portion thrown away. Pin down the head and anterior
portion of the body, ventral side uppermost, make a median longi-
tudinal incision through the skin from the lower jaw to the pectoral arch,
and dissect it away on either side as far as the gill clefts. Then,
without injuring the lateral vein (p. 456), remove the middle portion of
the pectoral arch and expose the pericardial cavity and heart. Insert a
seeker, pointing backwards, along the dorsal side of the heart, through
the pericardia-peritoneal canal which communicates between the peri-
cardial and abdominal cavities (p. 434) : it opens on the ventral side
of the gullet by two apertures. Make out —

1. The form and relations of the chambers of the heart (sinus venosus,
auricle^ ventricle and conus arteriosus}.

2. The ventral aorta, to expose which the muscles in front of the
pericardium must be carefully removed ; but before doing so, it is better
to inject the ventral aorta, cutting a small hole in the ventricle, and in-
serting and tying the cannula into the conus arteriosus : use a blue
injection if you have already used red for the dorsal aorta.

3. The five afferent branchial arteries (compare Fig. 121) : trace
these outwards, and note their distribution. Sketch your dissection.

II. — Cut through the ventral aorta at its junction with the conus
arteriosus, and through both ends of the sinus venosus, carefully separ-
ating the latter from the walls of the pericardium and noting the
entrance of the hepatic sinus (p. 456). Remove the entire heart, pin
it down under water, ventral side uppermost. Cut open the ventricle
and conus arteriosus and note —

1. Their cavities and walls ; the auricula-ventricular aperture and
valves ; the valves in the conus arteriosus, of which there are two sets,
consisting of three in each set. Sketch.

2. The cavity and walls of the attricle and the sinu-auricular aper-
ture are best made out by turning the heart over, with the dorsal side
uppermost, before cutting open the auricle. Sketch.

3. Insert a seeker into one of the cut distal ends (still left in situ] of
the sinus venosus, and slit it up so as to expose the precaval sinus of the
same side ; by means of a seeker find the apertures into it of the

1 See also C. § HI.

478 THE DOGFISH CHAP.

following veins or sinuses (compare Fig. 122) — a, tint jugular ; b, the
inferior jugular ; c, the cardinal ; and d, the lateral vein.

III. — Insert the scissors into the external gill-clefts of one side, one
by one, and extend them by cutting dorsally and ventrally, so as to
expose the gill-pouches, communicating with the pharynx by \.\\Q internal
gill-clefts. Make out^

1. The branchial filaments, and observe that there are four complete
gills on the first four branchial arches, and a half-gill or hemibranch on
the posterior face of the hyoid arch. Note also the pseudobranch on the
anterior side of the spiracle.

2. The structure of the gills. Remove two entire gills; dissect one,
and cut the other across transversely (compare Fig. 118), noting the
relations of the septum , cartilaginous branchial arch and rays,
branchial filaments •, and single afferent and paired efferent branchial
artery. Sketch.

IV. — Cut through the floor of the pharynx and mouth close to the
middle line, just on one side of the ventral aorta, and extend the cut
through the lower jaw. On one side, turn the floor outwards, and pin
it back in this position so as to expose the roof of the mouth and
pharynx and the internal gill-clefts ; dissect away the mucous membrane
lining the roof, and trace out on one side (Fig. 121) —

1. The epibranchial and efferent branchial arteries , and the dorsal
aorta.

2. The carotid, subclavian, coronary arteries, &c.

F. Nervous System and Sense-Organs.

I. — Remove with the knife the rest of the skull-roof and a few of the
anterior neural arches so as to expose the entire brain and the anterior
part of the spinal cord. In doing so, be careful not to injure the con-
tents of the orbit, the nerves referred to on p. 474 § II., or the auditory
capsule of one side. After noting the " pta mater" make out —

1. The subdivisions of the brain (olfactory lobes, prosencephalon,
diencephalon, optic lobes, cerebellum, and medulla oblongata.} Sketch.

2. The origins of the cerebral nerves from the brain and the points
at which they penetrate the walls of the skull (pp. 461-464).

3. The spinal cord, and the alternating dorsal and ventral roots of the
spinal nerves. Then cut through the spinal cord just behind the
medulla oblongata, and through the origins of the cerebral nerves.
Remove the brain and place it in formaline or spirit.

II. — Carefully dissect away the skin covering the head and pharyn-

x PRACTICAL DIRECTIONS 479

geal region on the undissected side, expose the orbit, and remove
the delicate connective-tissue surrounding its contents. Pin down
firmly, and dissect out the following from the side (compare Fig. 125
and pp. 461-464).

1. The ophthalmic division of the facial nerve, and immediately
below it that of the trigeminal : trace them backwards to their
foramina in the skull-wall and forwards, through a canal between the
olfactory capsule and the cranium, to their distribution.

2. The large mass of sensory (ampiillary) canals on the dorsal side of
the snout.

3. The four recti and the two oblique eye muscles (Fig. 126), and the
nerves (III, IV, VI) supplying them.

4. The eye, and the optic nerve — anterior to the recti muscles. The
eye may now be removed by cutting through the muscles and optic
nerve, and dissected as directed on p. 191.

5. The large, flat, maxillo-mandibular division of the trigeminal,
running forwards and outwards along the floor of the orbit, and there
dividing into maxillary and mandibular branches.

6. T\IQ facial nerve, entering the orbit close behind the maxillo-mandi-
bular nerve, and giving off : — behind the spiracle — a large hyornan-
dibular branch, passing along the anterior border of the auditory capsule
and posterior wall of the orbit, and down the anterior side of the hyoid
arch just beneath the skin : and in front of the spiracle — a palatine
and prespiracular branches. Of the branches to the sensory
canals, the ophthalmic has already been seen ; the buccal and external
mandibular require very careful dissection in order to make them out
satisfactorily.

7. ^^Qglossopharyngealz.^ vagus nerves. To expose these, slice away
sufficient of the auditory capsule (noting as you do so the semicircular
canals and the endolymphatic duct, p. 465) to expose the foramina by
which they emerge from the skull, behind the auditory capsule, and
separate the mass of muscles lying alongside the vertebral column from
the branchial apparatus, by dissecting away the connective-tissue.
Trace the glossopharyngeal to its bifurcation over the first gill- cleft,
and in the vagus follow out — a, the four branchial branches, forking
over the remaining gill-clefts ; b, the visceral branch ; and c, the lateral
line branch, above and to the inner side of the branchial branches.

8. Separate some of the ampullary sensory tubes from one another,
and note the ampulla and the nerves supplying them.

9. Carefully slice away the cartilage of the auditory capsule of the

480 THE DOGFISH CHAP.

side you have not already dissected so as to expose the membranous
labyrinth. Examine under water, and make out the vestibule with its
contained otolithic mass, the three semicircular canals with their
ampulla, and the branches of the auditory nerve (compare Fig. 59).

III. Now examine the preserved brain from above, from below, and
from the side, making out, in addition to the parts already noticed

(F, § I, i)-

1. The optic chiasma, infundibuhim (with an oval lobe and a vascular
sac on either side), pituitary body, crura cerebri, and, as far as possible,
the origins of the nerves. Sketch from below.

2. On one side of the brain, cut into the olfactory lobe, prosen-
cephalon, optic lobe, and cerebellumjfrom above, so as to expose the olfac-
tory ventricle, lateral ventricle, optic ventricle, and cerebellar ventricle.
Then bisect the entire brain into right and left halves with a sharp
scalpel, and examine the uninjured half in longitudinal section, noting,
in addition to the parts mentioned above, the third ventricle, foramen of
Monro, iter, and fourth ventricle. Sketch.

G. Transverse Sections. — Cut thick transverse sections of an en-
tire dogfish with a knife through — a. the anterior, and b. the posterior
part of the head (pharyngeal region) ; c. about the middle of the
body ; and d. the tail. Make out the relations of the various parts and
organs, and sketch the lateral half of each section.

A more satisfactory method than this is to obtain a very young dog-
fish, not more than \ inch in diameter, and after cutting it transversely
into pieces about | inch in thickness in the regions named above, stain,
imbed, and mount a few sections from each piece (see p. 136). These
can first be examined with a lens or with the low power of the micro-
scope, and then, by putting on the high power, important points in the
histology can be made out. In addition to the minute structure of the
tissues and organs described in Part I. of this book, the structure of the
notochord (p. 441), integumentary sense-organs (pp. 432 and 464), der-
mal teeth (p. 433 and 445), &c. , should be studied.

H. Side Dissection. — It is very instructive to supplement and
recapitulate your work on the anatomy of the dogfish by dissecting
another specimen from the side (compare Fig. 117), as in the case of the
crayfish. Cut open the abdominal cavity as before, very slightly to the
left side of the middle line. Continue the cut forwards through the

x PRACTICAL DIRECTIONS 481

pectoral arch and backwards through the pelvic arch : the arteries may
be injected at this stage. Then dissect off the left half of each arch
with the corresponding fin, and cut away the left body- wall. Cut
through the skin in the mid-dorsal line ; and then, working from the
left side, remove the left half of the skull and visceral arches,
as well as the left side of the vertebral column, so as to expose
the brain and spinal cord. Remove the left kidney (and left ovi.
duct if your specimen is a female). Pin down under water, and tidy
your dissection so as to reduce it to a neat longitudinal section, in
which all the unpaired soft organs are left intact. Without tearing
the mesentery, pin out the liver, stomach, and intestine beyond the
ventral limits of the body-wall, so that the other abdominal organs are
not hidden. Follow out the relations of the various organs as before,
and sketch your dissection.

It is important at this stage to refresh your memory of the anatomy
of the frog", and to compare it with the dogfish, by making a similar
dissection of it from the side (compare Fig. 7). Sketch.

PRACT. ZOOL. 1 I

CHAPTER XI

CHARACTERS OF THE CLASS MAMMALIA THE RABBIT

BEFORE examining a rabbit, as an example of the highest
class of Vertebrates — the Mammalia, it will be well to re-
capitulate some of the characters of the frog, the organisa-
tion of which is higher than that of a fish.

The frog, taken as an example of the class Amphibia, differs
from a fish in the following points amongst others. Its
paired limbs have not the form of paddle-like fins, but the
fore-limb consists of upper arm, fore-arm, wrist and hand,
and the hind-limb of thigh, shank, ankle, and foot, each
with characteristic skeletal parts ; it has no median fin in
the adult, and that of the tadpole is not supported by fin-
rays ; there is no hard, dermal exoskeletoti ; respiration in the
adult is pulmonary, and internal nostrils are present ; there
are two auricles in the heart, and the cardinal veins are
replaced by a postcaval ; there is a urinary bladder formed
as an outgrowth of the cloaca. Moreover the endo-skeleton,
unlike that of the dogfish, is in the adult composed mainly
of bone.

In all these characters the frog resembles the rabbit. But
the Mammal differs from the Amphibian in many important
respects, some of the chief of which are : — the presence of
an epidermic exoskeleton consisting of hairs ; the high
temperature of the blood, which remains almost uniformly

CHAP. XI MAMMALIA 483

within a few degrees of 100° Fahr., and does not vary to
any appreciable extent with the temperature of the air ; the
absence of nuclei in the red corpuscles of the blood ; the
presence of mammary glands beneath the skin in the female
which secrete milk for nourishing the young ; the subdivision
of the body-cavity into two portions — thorax and abdomen —
by a transverse partition, the diaphragm • the presence of
two ventricles as well as of two auricles in the heart, and of a
single systemic aortic arch (that of the left side) ; the higher
differentiation of the brain, and also of the skeleton ; and
the mode of articulation of the lower jaw. Moreover, in
the large majority of Mammals, the teeth are differentiated
into front-teeth for biting or seizing the food and cheek-
teeth or grinders, and their succession is limited to two
functional sets ; an external ear or pinna is present ; there
is no cloaca, the anus and urinogenital apertures opening
separately on the exterior, while the ureters open directly
into the bladder ; the ova are minute ; the young un-
dergo their early development in the oviduct, where they
are nourished by diffusion from the blood-system of the
parent by means of an organ known as the placenta, and
after birth they are suckled by the mother.

Bearing in mind these essential characters of the high
Mammalia as compared with the Vertebrates previously
studied, we can now proceed to examine the structure of the
rabbit in greater detail.

External characters. The rabbit (Lepus cuniculus) is a
very abundant and widely distributed animal which in the
wild state makes burrows in the earth, and the young are
born in these or in special nests. There are a number of
varieties, the habits and general appearance of which have
been modified by domestication (compare p. 227).

I I 2

ligher

484

THE RABBIT

CHAP. XI

In addition to head, trunk, and short tail, the rabbit
possesses a distinct neck, and the whole animal, including the
limbs and even the soles of the feet, is covered with a soft
fur consisting of hairs (Fig. 130). In the wild rabbit, the
fur is of a brownish colour, lighter below, and white under
the tail : in the many domesticated varieties the colour is
very varied.

The hairs correspond to modified epidermic cells which become
converted into a horny material ; they are developed in tube-like involu-
tions of the epiderm called hair-sacs, into the swollen base of each of

FIG. 130. — Rabbit.

Lateral view of skeleton with outline of body. (From Parker and Haswell's
Zoology.}

which a mesodermal hair-papilla projects, the substance of the hair,
with its cortex and medulla, being formed from the epidermic cells
covering the papilla (Fig. 131). Into the hair -sacs open the ducts of
sebaceous glands (HBD), the secretion of which serves to lubricate the
hairs, which can be erected by means of muscles (AP).

There are five digits in the hand or manus, and four in
the foot or pes, each terminated by a pointed and curved
horny claw, developed, like the hairs, from the epiderm.

FIG. 131.— Longitudinal section through a hair-sac. (Diagrammatic.) (From

Wiedersheim's Comp. Anatomy.')

Ap. muscles for erecting the hair ; Co. derm ; F. outer longitudinal layer, and F^.
inner transverse layer of connective-tissue fibres of hair-sac ; Ft. adipose tissue ;
GH. hyaline layer, which lies between the inner and outer hair-sheaths, i.e.,
between the root-sheath and the hair-sac ; HBD. sebaceous glands ; HP. hair-
papilla, containing vessels; M. medulla; O. cuticle of shaft; R. cortex; Sc.
horny layer of epiderm ; Sch. hair-shaft ; SM. Malpigian layer of epiderm ; WS,
IPS*, external and internal root-sheath.

486 THE RABBIT CHAP.

Along the ventral surface of the body in the female are four
or five pairs of papillae' — the teats, on which open the ducts
of the milk-glands, which correspond to modified integu-
mentary glands. The various parts of the skeleton (Fig. 130)
can be felt through the skin, and it will be noticed that the
anterior part of the trunk, or thorax, is surrounded by ribs,
many of which meet below with a breast-bone or sternum,
and which are absent in the posterior part of the trunk, or
abdomen.

Beneath the anterior end of the snout is the transverse
mouth, which has a narrow gape and is bounded by upper
and lower lips : the upper lip is divided by a longitudinal
cleft which is continuous with the oblique, slit-like external
nostrils. Just inside the lips are the upper and lower front
teeth or incisors, which are chisel-shaped, and behind them the
hairy integument is continued on either side into the cavity
of the mouth. The eyes are protected by movable hairy
upper and lower eyelids, as well as by a hairless third eye-
lid or nictitating membrane (compare p. 5), supported by
cartilage and situated in the anterior corner of the eye, over
which it can be partly drawn : it corresponds to the little
red lump in the inner corner of the human eye. On the
upper lip and above and below the eye are certain very long
and stiff hairs — the " whiskers " or vibrissce, and behind the
eyes are a pair of long and movable external ears or pinna :
these are supported by cartilage and are somewhat spout-
shaped, leading to the external auditory openings.

Below the root of the tail is the anus, and in front of
and below this the urinogenital aperture, the space between
them being known as the perinceum. On either side of these
apertures is a hairless depression of the skin on which open
the ducts of t\\e perinaal glands, the secretion of which has
a strong and characteristic odour. In the female the slit-

xi SKELETON 487

like urinogenital aperture is called the vulva ; in the male
the aperture is smaller and is situated'on the conical apex of
a cylindrical organ, the penis, which can be retracted within
a fold of skin, the foreskin or prepuce. On either side of
the penis is an oval pouch of the skin, the scrotal sac, not
very apparent in young animals, in each of which a spermary
or testis is contained.

Skeleton. — The skeleton of the adult rabbit consists
almost entirely of bone, but it must be remembered that in
addition to certain cartilages described below, all articular
surfaces are covered or lined by a thin layer of cartilage, and
that the various parts of the skeleton are connected together
by ligaments.

In the skull, both replacing and investing bones (p. 43)
are much more numerous than in the frog, and the structure
of the entire skull is far more complicated and highly differ-
entiated. A posterior, relatively large cranial region, in the
side walls of which the auditory capsules are embedded, can
be distinguished from an anterior, somewhat conical facial
region, constituting the skeleton of the snout (Fig. 132).
Just behind the junction of these two regions on either side
is a large orbit, separated from its fellow by a thin interorbital
septum, perforated by a foramen for the optic nerve (opt.fo).

At the sides of the foramen magnum are the two
rounded occipital condyles ; the auditory apertures (aud. me)
are situated at the sides of the posterior part of the cranium,
and the external nostrils open at the anterior end of the
snout. Most of the bones remain more or less distinct
throughout life, and are in contact along lines or sutures, many
of which are wavy or zig-zagged : others, again, become
completely fused in the adult, so that their limits are no
longer distinguishable.

A

FIG. 132. — Skull of rabbit. A, lateral'view with lower jaw; B, ventral view.
ang. proc. angular process of mandible'; as. alisphenoid (external pterygoid pro-
cess) ; aud. me, external auditory aperture in tympanic bone ; b. oc. basiocciptal ;

CHAP, xi SKULL 489

b. sph. basisphenoid ; cond. condyle of lower jaw ; [cor. coronoid process ;
fr. frontal ; int. pa. interparietal ; ju. jugal ; Icr. lacrymal ; m. molars ; max.
maxilla ; nas. nasal ; oc. c. occipital condyle ; opt.fo. optic foramen ; o. sph.
orbitosphenoid ; pa. parietal ; pal. palatine ; pm. premolars ; pal. max. palatine
process of maxilla ; par. oc. paroccipital process of exoccipital ; pal. p. max.
palatine process of premaxilla ; p. max. premaxilla ; peri, periotic ; pt. pterygoid ;
p. t.sq. post-tympanic process of squamosal ; .y. oc. supraoccipital ; sph. f. sphen-
oidal fissure ; sq. squamosal ; ty.bul. tympanic bulla ; vo. vomer ; zyg. max.
zygomatic process of maxilla. (From Parker and Haswell's Zoology.}

The upper jaw forms part of the facial region, which
encloses the olfactory chambers, and the lower jaw, consist-
ing of a single bone on either side, articulates directly with
the sides of the cranium without the intervention of a
hyomandibular as in the dogfish (p. 438) or of a quadrate
cartilage as in the frog (p. 44). The rest of the visceral
portion of the skull, representing the hyoid and first branchial
arch, forms the so-called hyoid bone which is embedded in
the base of the tongue (Fig. 135, hy).

The bones * which form the walls of the brain-case are
arranged in three rings or segments, the middle and pos-
terior of which are separated by the auditory capsules (Figs.
133 and 132 A, peri).

The posterior, or occipital segment, consists of three bones,
which in the adult become completely united with one another.
The lower of these is the basiocdpital (b. oc\ a flattened bone
bounding the foramen magnum below, and forming the
hinder part of the base of the skull and the lower part of
each occipital condyle (oc. c}. The two exoccipital (e. oc)
bound the foramen magnum at the sides, and form the
upper part of the occipital condyles : each is produced
downwards into a paroccipital process (par. oc) which fits
closely against the posterior surface of a swollen bone (ty. but)
to be described presently, which is produced into a tube
surrounding the auditory aperture (aud. me). The occipital

1 In the following description, the investing bones are distinguished
by an asterisk from the replacing bones (cf. p. 43).

490

THE RABBIT

segment is completed above by the supraoccipital (s. oc\
bounding the foramen magnum above ; it has a pitted surface
and is marked externally by a shield-shaped prominence.

The middle, or parietal segment, consists of five bones, — a
basisphenoid (b.spJi) below, an alisphenoid (a.sph,a.s] on either
side, and two parietals* (pa) above. The broad posterior
end of the basisphenoid is connected with the basiocci-

fr

efh

a. sp,
for

FIG. 133. — Skull of rabbit in longitudinal vertical section. The cartilaginous nasal

septum is removed.

sph. alisphenoid ; e. oc. exoccipital ; e. tb. ethmo-turbinal ; eth. ethmoid ; fl. fossa
for flocculus of brain ; z. incisors ; mx. tb. maxillary turbinal ; n. tb. naso-
turbinal ; pal' . palatine portion of the bony palate ; /. sph. presphenoid ; _?. t.
sella turcica, or depression in which the pituitary body lies ; I. point at which
the olfactory nerves leave the skull ; II. optic foramen ; V inn. foramen for
mandibular division of trigeminal ; VII. for facial nerve ; VIII. for auditory
nerve ; IX, X, XI, for glossopharyngeal, vagus, and spinal accessory ; XII.
for hypoglossal. Other letters as in Fig. 132.

pital by a thin plate of cartilage, and tapers in front to a
blunt point : it is perforated at about its middle by an oval
pituitary foramen, and on its upper surface is hollowed out
to form a depression in which the pituitary body lies (s. /).
The alisphenoids are wing-like bones, directed upwards
and outwards, and firmly united with the basisphenoid : each
is produced ventrally into a pterygoid process (as\ consist-

xi SKULL 491

ing of two laminae which converge and unite with one
another anteriorly. The parietals are a pair of thin, slightly
arched bones, forming a considerable part of the roof of the
brain-case, and united with one another by suture along the
middle line; the outer edge of each gives off a thin,
ventral process which is covered by the squamosal (sq),
a bone which will be referred to presently and which
separates the parietal from the alisphenoid. Interposed
between the parietals and the supraoccipital is a small
median interparietal* (int. pa).

The frontal segment also consists of five bones — a
presphenoid, (p. sph} two orbitosphenoids (o. spJi) and two
frontals* (fr). The small presphenoid is laterally com-
pressed and is connected with the basisphenoid by cartilage,
so that in the dry skull there is a considerable interval
between the two bones ; it forms the inferior and anterior
boundary of the optic foramen (opt. fo, //), which puts the
two orbits in communication with one another and both in
communication with the cranial cavity. The orbitosphenoids
are two wing-like laminae directed outwards and slightly
backwards, and completely fused with the presphenoid ;
they surround the rest of the optic foramen. The frontals
form the roof and side-walls of the anterior part of the
brain-case, and are united by suture with one another in the
middle line and with the parietals behind ; below they
meet with one another anteriorly on the floor of the brain-
case and unite with the presphenoid by suture ; the outer
part of each forms a prominent crescentic ridge, the
supra-orbita I process.

The brain-case is closed in anteriorly by a bone riddled
with numerous small holes for the passage of the olfac-
tory nerves : this is the cribriform plate of the ethmoid
(ftK).

492 THE RABBIT CHAP.

It will be remembered that in the frog the occipital region is ossified
by exoccipitals only, the parietals and frontals of either side are fused,
there are no ali- or orbito-sphenoids, the cartilaginous walls of the
anterior part of the cranium are ossified as a sphenethmoid, and that
the floor of the skull is supported by a membrane bone, the para-
sphenoid (pp. 40-44).

The auditory capsules are comparatively loosely wedged
in laterally between the parietal and occipital segments ; in
the embryo each is ossified from three centres, instead of
one (the pro-otic) as in the frog, but these early unite to
form the periotic bone (peri\ as the ossified auditory capsule
is called. The internal or petrous portion of this bone
(Fig. 133) encloses the membranous labyrinth of the ear
and is very dense and hard ; posteriorly it is produced out-
wards as the porous mastoid portion, which is visible on the
outer side of the skull (Fig. 132 A). Closely applied to the
outer surface of each periotic is a bone called the tympanic*
consisting of a tubular portion ibove — the edge of which sur-
rounds the auditory opening (aud. me) to which the cartilage
of the pinna is attached, and of a swollen portion, or tympanic
bulla (ty. bul) below : this encloses the tympanic cavity, and
in it, at the base of the tubular portion, is an incomplete
bony ring to which the tympanic membrane is attached
(Fig. 143). The tympanic is incomplete on its inner side,
where its cavity is closed by the outer wall of the periotic,
and between the two, at the antero-inferior angle of the
former, is the aperture by which the Eustachian tube
leaves the tympanic cavity (compare p. 45). When the
tympanic is removed, two small holes are seen on the outer
wall of the periotic : the anterior of them is \hefenestra ova/is*
and is plugged by the stapes — which, together with two small
bones, the malleus and incus (Fig. 143), forms the chain of
auditory ossicles to be described later in connection with

xi SKULL 493

the organ of hearing ; the posterior aperture is called the
fenestra rotunda. On the internal or cranial surface of the
periotic is a large depression (fl) which lodges the flocculus
of the cerebellum (Fig. 141).

The olfactory capsules are roofed in by two long and
narrow nasal bones* (nas), which meet together in the
middle line and unite by suture with the frontals posteriorly.
Their side walls are formed by the bones which bear the
teeth of the upper jaw — the premaxillcz * ( /. max) and
maxilla * (max\ and in the median line below is a single
long and slender bone, deeply grooved on its upper surface,
and formed by the fusion of the two vomers* (vo). The
two nasal chambers are separated from one another in the
middle line by a median vertical plate of cartilage, the nasal
septum (Fig. 135 n.s), embraced below by the vomer. This
cartilage, together with the cribriform plate and a median
vertical plate of bone (etfi) extending forwards from the
latter into the septum, constitutes the mesethmoid. Within
the nasal chambers certain scroll-like folds of the mucous
membrane (Fig. 135) are present in order to increase
the surface, and in these cartilages are developed. The
cartilages become ossified, and the resulting turbinal bones
unite with certain of the bones enclosing the olfactory organs,
and are named accordingly. The ethmoid turbinals (Fig.
133, e. tb\ or true olfactory scrolls, are two complicated,
folded bones united to the cribriform plate of the ethmoid,
and are covered in the fresh condition by the olfactory
epithelium ; the maxillo-turbinals (nix. tb) are similar but
more complex bones in the antero- ventral part of the nasal
cavities ; and the naso-turbinals (n.tb) are thin, folded
bones, much less complex, and fused with the inferior
surface of the nasals.

In the front wall of each orbit, fitting comparatively loosely

494 THE RABBIT CHAP.

between the frontal and maxilla, is a small bone, the lacrymal*
(Fig. 132 A, lcr\ with a notch near its outer border through
which the naso-lacrymal duct passes (p. 186).

As in the frog, the chief bones of the upper jaw on either
side are ft&premaxilla* (p. max) and the maxilla* (max\
and nearer the middle line are the palatine* (pal)
and ** pterygM" * (pi)', in the embryo the position
of the two last-mentioned bones is taken by
cartilage representing the upper jaw of the dogfish (com-
pare Figs. 112 and 9). The premaxillae, in which the
sockets for the front or incisor teeth are situated, form the
anterior boundary of the snout, and articulate with one
another in the median line and with the maxilla behind :
each gives off a nasal process passing backwards between
the nasal and maxilla to the frontal, and a palatine process
(pal. p. max) extending backwards along the palate in
contact with its fellow of the opposite side. The maxillae
are large and irregular bones, parts of the sides of which are
fenestrated, and in which the cheek-teeth are situated. From
the inner and inferior edge of each, opposite the first two
cheek-teeth, a horizontal palatine process (pal. max) is
given off, which, articulating with its fellow of the opposite
side, forms the anterior part of the bony support of the
hard palate — this is of much less extent in the rabbit than
in most mammals : from its outer side arises a zygomatic
process (zyg. max), which forms the anterior part of the
strong zygomatic arch extending below and externally to the
orbit.

The palatines are thin, nearly vertical, bony la-
minae, internal to the maxillae to which they are at-
tached in front, while above they join the presphenoid
and the pterygoid process of the alisphenoid. They
bound the passage of the internal nostrils, and from the

xi SKULL 495

inner and anterior region of each is given off, op-
posite the third cheek-tooth, a horizontal, inwardly
directed process (pafi\ which, articulating in the middle
line with its fellow of the opposite side and in front
with the palatine process of the maxilla, forms the pos-
terior part of the bony support of the hard palate. The
bones usually known as pterygoids are small irregular
plates attached to the posterior edge of the correspond-
ing palatine and the > pterygoid process of the alisphenoid ;
each ends ventrally in a backwardly-curved process.

The squamosals* (sq) are a pair of plates which overlap
and complete the side-walls of the brain-case (p. 491) in
front of the periotics : they articulate with the frontals,
parietals, orbitosphenoids, and alisphenoids. From the
outer face of each is given off a strong zygomatic process,
which bears on its under surface the articular facet for
the lower jaw, and further back a slender process
(p.t.sq) arises which . is applied to the outer surface
of the periotic.

The zygomatic processes of the squamosal and maxilla
respectively are united by a flat bar of bone, thejugal* (ju\
which in the adult is fused with the latter. All these
three bones therefore take part in forming the zygomatic
arch.

Most of the apertures for the transmission of the cerebral nerves have
so far not been mentioned : the branches of the olfactory nerve, as we
have seen, pass out through the numerous apertures in the cribriform
plate (Fig. 133, etk], and the optic foramen (opt. fo, II) is situated
between the orbitosphenoid and presphenoid. Behind and below the
optic foramen is a vertical aperture — the sphenoidal fissure (sph. /) —
between the basisphenoid and alisphenoid, which transmits the third,
fourth, and sixth nerves, as well as the ophthalmic and maxillary divisions
of the fifth. Between the periotic and alisphenoid is a large space
(Vmn), through the anterior part of which the mandibular division of

496 THE RABBIT CHAP.

the trigeminal leaves the skull.1 Between the mastoid portion of the
periotic and the posterior border of the tympanic, at the junction of the
tubular and bulbous portions of the latter bone, is a small aperture — the
sty lomastoid foramen^ which transmits the seventh nerve : this and the
eighth (VII, VIII) enter the periotic just below the depression for the
flocculus of the cerebellum (fl\ A space (IX, X, XI) between the
occipital condyle and tympanic bulla gives exit to the ninth and tenth,
as well as to the eleventh — which is not represented as a distinct nerve
in the dogfish and frog ; and the hypoglossal (p. 160), which in
Mammals is counted as the twelfth cerebral nerve, passes out through
two small apertures (XII) in the exoccipital, just anterior to the condyle.
Various other apertures will be noticed in the skull and jaws : through
some of these branches of certain of the above-mentioned nerves pass,
while others transmit blood-vessels.

The lower jaw or mandible (Fig. 132 A) consists of two
halves or rami, each corresponding essentially to the dentary
of the frog, which unite with one another in front, at the
symphysis, by a rough surface, while behind they diverge
like the limbs of the letter V. Each ramus is a vertical plate
of bone, broad behind and tapering towards the front, where
it bears the incisor teeth : further back, on its upper margin,
are the sockets for the cheek-teeth, and behind them is an
ascending portion which bears the condyle (cond) for articula-
tion with the facet on the squamosal : in front of the condyle
is a curved coronoid process (cor). The postero-inferior
border, which is rounded and inflected, is known as the
angular process (ang.pro).

The hyoid is a small bone situated at the root of the
tongue, anterior to the larynx (Fig. 135, hy\ It consists of
a stout body or basi-hyal, a pair of small anterior horns,
representing the ventral ends of the hyoid arch of lower

1 In many Mammals (e.g. dog, cat), the maxillary division of the
trigeminal passes out through a separate foramen, behind the
sphenoidal fissure ; and the anterior part of the space referred to above
is separated off as a distinct foramen for the mandibular division.

xi VERTEBRAL COLUMN 497

Vertebrates, and a pair of longer, backwardly-projecting pos-
terior horns or thyro-hyals, attached to the larynx and repre-
senting the lower ends of the first branchial arch.

The vertebral column includes about forty-five bony
vertebrae, each consisting of a centrum, a neural arch, and
various processes (compare pp. 36 — 38), but becoming
simplified towards the end of the tail. The centra have
flat anterior and posterior surfaces, and are not con-
nected by synovial articulations, as in the frog, but inter-
posed between them are elastic intervertebral discs of fibro-
cartilage. In addition to the ossification which gives rise to
the main part of the centrum, a separate flat disc of bone
(Fig. 134, ep] is formed on the anterior and posterior
surface of each. These epiphyses are characteristic of the
vertebrae of all or nearly all mammals : they unite com-
paratively late with the centrum proper, and so in dis-
articulated skeletons of young animals they often come
away from the main mass of the centrum and remain attached
to the intervertebral discs.

In correspondence with the differentiation of the parts
of the body, the vertebral column is divisible into five
regions (Fig. 130) : the cervical 'in the neck, including seven
vertebrae, the first two of which — called respectively the
atlas and axis — are peculiarly modified in order to allow
the skull free movement ; the thoracic in the thorax, twelve
or thirteen in number, and bearing ribs ; six or seven
lumbar in the abdominal region : three or four sacral in the
sacral region : and about fifteen or sixteen caudal in the tail.

Examining one of the anterior thoracic vertebrae first (Fig. 134), we
see that the centrtini (c) is continuous above with the neural arch (n. a),
the lower part of which, on either side, presents an anterior and a
posterior notch (i. v. n)., so that when the vertebrae are in their

PRACT. ZOOL. K K

498

THE RABBIT

natural position, an intervertebral foramen is formed for the passage
of a spinal nerve. The roof of the arch is continued into a long
neural spine (n. sp) projecting upwards and backwards, and just
above the intervertebral notches are a pair of anterior and posterior
articular processes or zygapophyses (pr. z, pt. z), which articulate
synovially with the vertebrae next in front and behind respectively.
The articular surface of each pre-zygapophysis looks upwards and out-
wards, that of the post-zygapophysis downwards and inwards. Arising

laterally from either side of
the arch is an outstanding
transverse process (t. pr), on
the under surface of which is
an articular tubercular facet,
(t. f) with which the upper
fork of the rib (p. 500) arti-
culates. The lower fork or
head of the rib articulates
with a facet (c. f) formed
partly by the anterior edge of
the corresponding centrum
just at the base of the neural

4.— Fifth thoracic vertebra of the arch, and partly by the pcs-
rabbit, from the left side ( x i^). terior edp-e of the centrum

c. centrum ; c.f. capitular half-facet for fifth,

and c.f". for sixth rib ; ep. epiphysis ; next in front, SO that each
i. v. n. intervertebral notch ; n. a. neural , ir

arch ; n.sfi. neural spine ; pr.z. pre-zyga- centrum bears half a capltu-
pophysis ; ft. z post-zygapophysis ; t.f. far facet, &$ it is called, on
tubercular facet for fifth rib; t.pr. trans-
verse process. either side, both anteriorly

and posteriorly (c. f , c. f").

There are no free ribs in the vertebrae of other regions, in which,
however, they are represented in the embryo, but early fuse with the
corresponding transverse processes.

The first cervical vertebra, or atlas, is ring-shaped, and its lower
portion is narrow and unlike the other centra. The neural spine is
small, and the transverse processes are broad horizontal plates, each
perforated at its base by a vertebrarterial canal, through which the
vertebral artery runs. On the anterior face of the lateral parts of the
atlas are two concave articular facets for articulation with the occipital
condyles of the skull, and on its posterior face are two smaller facets for
articulation with the second vertebra. The second cervical vertebra, or

xi VERTEBRAL COLUMN 499

axis, has its centrum produced anteriorly into a conical odontoid process,
which fits into the lower part of the ring of the atlas and is held in its
place by a ligament extending transversely across the latter : it is ossi-
fied from a distinct centre, which really belongs to the centrum OA
the atlas. The neural spine of the axis is elongated and compressed,
and its transverse processes small and perforated each by a vertebrarterial
canal. Zygapophyses are present only on the posterior face of the
arch. In all the other cervical vertebrae, the transverse processes are
also perforated by the vertebrarterial canal, and except in the seventh
or last, are divided into dorsal and ventral lamellae. The zygapophyses
resemble those of the thoracic vertebra described above. The seventh
cervical vertebra has a longer spine than the others, and bears a pair
of half facets on the posterior surface of its centrum with which the first
pair of ribs in part articulate.

A typical thoracic vertebra has already been described. In the tenth,
the neural spine is vertical, and in the remaining two or three, which are
larger than the others, it slopes forwards. In the posterior three or
four there are no tubercular facets, the ribs in this region not being
forked ; the capitular facets are entire, and are situated on the
corresponding centrum only. Additional processes are present above
the pre-zygapophyses from the ninth thoracic vertebra onwards.

The lumbar vertebrae are relatively large, increasing in size from
before backwards, and their various processes are greatly de-
veloped. The neural spines are directed upwards and forwards, the
transverse processes are large and project outwards, downwards, and for-
wards. As in the posterior thoracic vertebrae, there are stout processes
above the pre-zygapophyses, and there is also a pair of more slender
processes below the post -zygapophyses and a median ventral process
projecting downwards from the centrum in the first two.

The sacral vertebrae are fused together to form the sacrum, which sup-
ports the pelvic arch. The first— and to a less extent the second also —
has large, expanded, transverse processes which articulate with the ilia ;
these are the sacral vertebrae proper, the others, which decrease in size
from before backwards, are really the anterior caudal vertebrae which
fuse with the true sacral vertebrae to form a compound sacrum.

The more anterior caudal vertebrae resemble those of the sacral
region, but on passing backwards all the processes are seen to diminish
in size, until nothing but the centra are left at the end of the
tail.

K K 2

5oo THE RABBIT CHAP.

There are twelve or occasionally thirteen pairs of ribs,
which have the form of curved rods, situated in the walls of
the thorax, and articulating with the thoracic vertebrae
above and — in the case of the first seven — with the breast-
bone or sternum below : the remaining ribs do not reach
the sternum (Fig. 130).

Each rib consists of a bony, dorsal, vertebral portion, and of a ventral,
sternal portion consisting of cartilage which is calcified or only
incompletely ossified. The dorsal end — the head or capitulum of the
rib — articulates with the capitular facet on the centra, and the first nine
have also a tubercle, a short distance from the capitulum, which articulates
with the tubercular facet ; just externally to the tubercle is a short,
vertical process (compare pp. 498 and 499).

The sternum, which is developed in the embryo by the
fusion of the ventral ends of the ribs (and therefore has a
different morphological significance to the sternum of
Amphibians, see p. 48), consists of six segments or
sternebrce, the first of which, or manubrium, is larger than the
rest, and has a ventral keel. With the last is connected a
rounded, horizontal, cartilaginous plate, the xiphisternum.
The ribs articulate between the successive sternebrae except
in the case of the first pair, the articulations of which are
on the manubrium.

The chief bone of the pectoral arch is the flat, triangular
scapula, the coracoid portion (compare p. 47) becoming early
fused with it and forming a small, inwardly curved, coracoid
process, situated anteriorly to the glenoid cavity at the lower
end or apex of the scapula : the apex lies over against the
first rib, and the bone inclines upwards and backwards to
its dorsal base, which in the fresh condition consists of a strip
of cartilage, the supra- scapula. On its outer surface is a
prominent ridge or spine, the free ventral edge of which is
called the acromion, from which a process, the metacromion,

xi FORE-LIMB 501

projects backwards. The collar-bone or clavicle is never
strongly developed in Mammals in which the fore-limb only
moves in one plane — forwards and backwards : in the
rabbit it is a small, curved, rod-like bone, attached by
fibrous tissue at one end to the sternum and at the other
to the coracoid process of the scapula, there being small
cartilages at either end of it.

The relative positions of the bones of the f OPG-limb are at first sight
somewhat difficult to understand owing to their having become altered
in the course of development. In your own fore-arm the bones can
be rotated on one another, so that the thumb can be made to point
outwards or inwards ; while in the rabbit the first digit has permanently
the same position, pointing inwards. To understand this, extend your
arm outwards with the thumb pointing away from the ground. The
back of the hand and arm, continuous with the dorsal surface of the
body, or back, is its dorsal surface ; the palm of the hand, and the sur-
face of the arm continuous with the chest, is its ventral surface : the
border of the arm and hand continuous with the thumb is the pre-
axial border ; and that continuous with the little finger the postaxial
border. This position is called the position of supination ; if
the fore-arm and hand be now rotated, so that the thumb points
inwards, the position is that of pronation. While in this position,
bend the elbow at right angles and bring it inwards close to the body ;
the preaxial border of the hand will now be on the inner side, and
an examination of the bones of the fore-arm shows that they cross
one another. It is in this position that the bones of the rabbit's fore-
limb are permanently fixed (Fig. 130, and compare Fig. 8).

The proximal extremity of the humerus bears a rounded
head for articulation with the glenoid cavity, in front of
which is a groove for the tendon of the biceps muscle
(p, 61); certain tuberosities for the attachment of mus-
cles will also be observed. Its distal extremity presents a
large, pulley-like surface or trochlea for the articulation of
the bones of the fore-arm, and a deep depression or fossa,
perforated by a foramen, on its posterior side, for the recep-
tion of the end of the ulna. The radius is the shorter,

502 THE RABBIT CHAP.

inner (preaxial) bone of the fore-arm, and is slightly curved.
Its head presents a large double surface for articulation with
the trochlea of the humerus, and its distal extremity a pair
of slight concavities for the bones of the carpus : the shaft
is flattened where it abuts against the corresponding flat-
tened surface of the ulna. Near the proximal end of the
last-mentioned bone is a cavity for the articulation of the,
humerus, and proximally to this, at the elbow, the ulna is pro-
duced to form a large olecranon process, which is received
into the fossa on the humerus when the limb is extended :
its small distal end articulates with the carpus.

The carpus, as in the frog (p. 50), consists of a proximal
and a distal row of small, nodular bones, which articulate
with one another where they are in contact. The bones of
the proximal row, beginning at the inner (preaxial) side, are
the radiale and intermedium, articulating with the radius,
and the ulnare, articulating with the ulna. In the distal
row are five bones, the middle one of which is distinctly
proximal to the other four, so as really to lie in the middle
of the carpus : this is the centrale, the others constituting a
row of distal carpals. Of these the first three articulate with
the corresponding digits, the fourth, on the outer (postaxial)
side, supporting the fourth and fifth digits and really con-
sisting of two carpals fused with one another.

A small bone, the pisiform, articulating with the ulna and ulnare on
the ventral side, is usually looked upon as a sesamoid bone, i.e., an
ossification in the tendon of a muscle ; but it probably represents the
vestige of a sixth digit.

The hand or manus consists of five digits, each made up
of a metacarpal and of phalanges, articulating with one
another. The innermost (preaxial) digit — the thumb or
pollex—is the shortest, and the third the longest : the former
has two phalanges, the others three each, the distal or
ungual phalanx of all the digits having a conical form, its

xi HIND-LIMB 503

dorsal surface being grooved for the firmer attachment of
the horny claw.

The ends of the long bones in both limbs are separately ossified as
epiphyses (compare p. 497), which eventually unite with the shaft of the
bone in question. Small sesamoid bones are situated on the under or
palmar side of the joints of the digits.

The pelvic arch consists of two lateral halves or innominate
bones, the long axis of which is almost parallel with that of
the vertebral column (Fig. 130), and which are firmly united
anteriorly and internally with the transverse processes of the
sacral vertebrae by a rough surface, while ventrally they are
connected together by cartilage at the pelvic symphysis. On
the outer surface of each innominate bone, at about the
middle of its length, is a deeply concave cup, the acetabulum,
for articulation with the head of the femur : in it, in young
rabbits, a triradiate suture can be seen, marking the
boundaries of the three bones of which the innominate is
composed (p. 51). Of these, the antero-dorsal is the ilium,
which is connected with the sacrum. The postero- ventral
portion of the innominate is perforated by a large aperture —
the obturator foramen, through which a nerve of that name
passes, the bone above and behind it being the ischium, and
that below and in front of it the pubis. Behind the
obturator foramen the ischium has a thickened posterior
edge or tuber osity, and then curves round and becomes
continuous with the pubis, both bones taking part in the
symphysis.

In young rabbits it will be noticed that the part of the pubis which
enters the acetabulum consists of a small, distinct epiphysis.

The hind-limb has undergone rotation forwards (Fig. 130), so as to
be brought, like the fore-limb, into a plane parallel with the median
vertical plane of the body ; but the rotation being forwards, and the
bones of the shank not being crossed, the preaxial border is internal
in the whole limb, and the original dorsal surface looks, on the whole,
forwards..

504 THE RABBIT CHAP.

Close to the proximal end of the femur, on its inner
(preaxial) border, is a rounded, projecting head for articulation
with the acetabulum : the actual end of the bone is formed
by a strong process, the great trochanter, while just distal to
the head is a lesser trochanter, and opposite this, on the
outer (postaxial) side, a third trochanter. The distal end of
the bone bears two large condyles, separated from one
another by a notch, for articulation with the tibia : this
notch is continuous with a groove extending for a short
distance along the anterior (dorsal) surface of the femur in
which a large sesamoid bone (p. 502), the knee-cap or
patella, slides : the patella lies in the tendon of the extensor
muscles of the leg, and is connected by ligament with the
tibia. Two other sesamoid bones, the fabellce, occur on the
opposite side of the knee-joint.

The tibia, or inner (preaxial) bone of the shank, is much
larger than the fibula, the distal half of which in the adult
becomes completely fused with it. The proximal end of the
tibia bears two slightly concave articular surfaces for the
condyles of the femur, and distally it articulates with the
tarsus : a prominent ridge — the cnemial rm-/— extends along
the proximal end of its anterior (dorsal) surface. The
slender fibula is attached proximally to the tibia.

The tarsus consists of six bones arranged in three rows.
In the proximal row (compare p. 51) are two tarsals, of which
the inner (preaxial) or astragalus — probably corresponding
to two bones fused together, the tibiale and intermedium —
has a large pulley-like surface for articulation with the tibia ;
while the outer (postaxial) calcaneum or fibulare articulates
with the fused end of the fibula, and is produced into a
strong heel or calcaneal process. In the middle row is a
single bone, the centrale (navicular) of the tarsus, and the
distal row is made up of three bones, the true first, together

xi MUSCLES 505

with the corresponding digit (hallux\ being absent as a
distinct bone. The second (apparent first) distal tarsal
articulates proximally with the centrale, and distally with
the innermost (preaxial) metatarsal : the third (apparent
second) with the astragalus and the corresponding meta-
tarsal : the fourth (apparent third), which corresponds to
the two fused outer (postaxial) tarsals, with the centrale,
calcaneum, and the remaining two digits.

The foot or pes consists of four metatarsals with their
phalanges, of which there are three to each digit. The
metatarsal of the hallux, together with the corresponding
distal tarsal, is probably represented by a distinct ossification
which in the adult becomes fused with the second (apparent
first) metatarsal, and forms a process on that bone which
articulates with the centrale. The phalanges are similar to
those of the manus, and sesamoid bones are also present on
the under surface of the foot.

Muscles and Body-wall. — It will be remembered that in
the lancelet and dogfish the muscles of the trunk are
divided up into myomeres (pp. 419 and 434), while in the
adult frog the only indication of such a segmentation of the
muscles is seen in the recti of the abdomen. In the rabbit
nearly all trace of a segmentation of the muscles has also
disappeared, and the muscular system, although similar in its
general arrangement to that of the frog (compare Fig. 16)
is more highly differentiated and complicated. We shall
have occasion to notice certain of the muscles in the course
of our examination of other organs.

Immediately beneath the skin, which consists of epiderm
and derm (Fig. 131), the whole ventral region of the trunk
and neck is covered by a thin cutaneous muscle, by means of
which the rabbit is able to twitch its skin. Internally to

506 THE RABBIT CHAP.

this muscle in the female are the mammary glands (p. 483),
which, when secreting, appear as whitish, branched masses,
the ducts of which can be traced to the teats, on the apices
of which they open by numerous small apertures.

A whitish band of connective-tissue passes along the mid-ventral line
of the abdomen from the xiphisternum to the pubis : this separates two
longitudinal bands of muscle, the recti abdominis, from one another ;
and laterally to them, the abdominal wall consists of three thin layers
of muscle with their fibres running in different directions — the external
obliqtie, the internal oblique, and the transversalis, the latter being
lined on its inner surface by the peritoneum. A fibrous cord, known as
Pouparfs ligament, beneath which the blood-vessels and nerves pass
outwards to the leg, extends upwards and forwards from each pubis
to the corresponding ilium. In the thorax the muscles of the body-
wall are broken up into separate portions by the ribs, and thus form a
series of intercostal muscles, which, like the oblique muscles of the
abdomen, are arranged in two layers, external and internal, and are
important in respiration.

Extending from the thorax to the fore-limb of either side
are the \zxgepectoral muscles ; and a number of other muscles
can be seen in the neck, in the ventral middle line of which,
covered by the cutaneous muscle, the windpipe or trachea is
visible (Fig. 135). The trachea is strengthened by a series
of cartilaginous rings and ends in front in the larynx, situated
between the two rami of the mandible ; and just in
front of the larynx is the hyoid bone (p. 496), embedded
in a mass of muscle.

The Coelome and its contents. — On cutting open the
body-cavity, it will be seen to be divided into two main
chambers — the thoracic and abdominal cavities — by means of
the diaphragm (Fig. 135, d). The relatively small thorax —
which is lined by a serous membrane corresponding to the
peritoneum of the abdomen and known as the pleura — con-
tains the lungs, as well as the heart enclosed in a pericardium,

xi DIGESTIVE ORGANS 507

on the ventral surface of which latter is an organ known as
the thymus (see p. 447) : the gullet and main blood-vessels
also pass through the thorax. The abdomen encloses the
greater part of the enteric canal, together with the liver
and pancreas, the spleen, and the urinogenital organs.
The diaphragm is convex on its anterior side, towards
the thorax : it consists of a central, thin, tendinous portion
into which radial muscles are inserted. These arise
from the vertebral column and posterior ribs, and are
especially strong on the dorsal side, where they form two
bands known as the pillars of the diaphragm. When the
muscles contract, the diaphragm is made flatter, and thus
the thoracic cavity is enlarged.

Digestive organs. — The mouth-cavity (Fig. 135) is large,
and the small gape is bounded by upper and lower lips, be-
hind which are the incisor teeth (/). On either side of the
cavity are the borders of the upper and lower jaws from
which the cheek-teeth project : these are separated from
the incisors by a considerable interval or diastema. Close
behind the upper incisors are a pair of very small openings
leading into the naso-palatine canals (n.p.c), which communi-
cate with the nasal cavities but must not be confounded
with the internal nostrils. The roof of the oral cavity is
formed by the palate, the anterior part of which, or hard
palate (h. p\ is transversely ridged and partly supported
by bone (h. p' , p. 494) ; while the posterior part, or soft
palate (s. p] is smooth, its hinder, free edge forming a
pendulous flap, the velum palati, on either side of which
is an organ known as the tonsil, consisting of connective
and lymphoid tissue and resembling the " lymphatic glands "
(compare p. 515) which occur along the lymph-trunks ; it
has the form of a small pit with a broad papilla on its

IK a i
lit

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CHAP, xi TEETH 509

ofEustachian tube;/;/. Fallopian tube; g. b. gall-bladder; //./. hard palate;
h. p' . bones of hard palate ; hy. hyoid ; i. incisors ; il. ileum ; i. n. passage of
internal nostrils ; j. Jacobson's organ ; k. left kidney ; /. au. left auricle ; Ing.
left lung, seen through pleura ; Ir. liver ; lv. 4. fourth lumbar vertebra ; /. vn.
left ventricle ; m. tb. maxillary turbinal ; n. p. c. naso-palatine canal ; n. ph.
naso-pharynx ; n. tb. naso-turbinal ; n. s. nasal septum (middle part cut away) ;
as. gullet ; o 1. I. olfactory lobe ; ov. left ovary ; /. a. pulmonary artery ; pc.
pericardium ; per. gl. perineal gland ; ph. pharynx ; phr. n. phrenic nerve
(origin not shown) ; /. m. a. posterior mesenteric artery ; p. mx. premaxilla ;
pn. pancreas ; pn. d. pancreatic duct ; pr. c. left precaval ; p. sy. pelvic sym-
physis ; pt. c. postcaval ; py. st. pyloric region of stomach ; r. ribs ; ret. rectum ;
r.gl. rectal gland ; r. vn. right ventricle ; sk. f. floor, and sk. r. roof of skull ;
si. gl. sublingual gland ; s. mx.gl. submaxillary gland ; s.p. soft palate, ending
in the] velum palati, on the lower side of which a tonsil is seen ; sp. c. spinal cord ;
sp. n. spinal (lumbar) nerves ; s. r. sacculus rotundus of ileum ; s. v. first sacral
vertebra ; sy. sympathetic (the anterior end shown on the right side, the rest on
the left) ; t. tongue; thr. thyroid ; th. v.Q ninth thoracic vertebra ; thy. thymus ;
tr. trachea ; u. bl. urinary bladder ; ur. ureter ; ut. uterus ; vag. vagina ; vb.
vestibule ; Tjg. vagus (the anterior end shown on the right side, the rest on the
left) ; vul. vulva.

outer margin. The tongue (/) lies on the floor of the mouth
to which it is attached below, its anterior, rounded end
being free : the surface of its posterior part is elevated, and
elsewhere — but more particularly on the tip — its covering
of mucous membrane is produced into minute, finger-
shaped papilla, on which some of the microscopic organs
of taste are situated (compare p. 180). Taste-organs are also
present on a pair of circumvallate papilla on the dorsal side
of the tongue towards its posterior end, and on a pair of
transversely ridged areas — the foliate papillae, situated
laterally, slightly anterior to the former. The main
substance of the tongue is composed of muscles, some
extrinsic, or arising from other parts, and others in-
trinsic, or entirely confined to the organ in question.

The teeth (Figs. 132 and 133), as we have seen, are not all
alike, as in the dogfish and frog : there are incisors and cheek-
teeth or grinders, the latter being divisible into two series —
the premolars and the molars. In most Mammals there is
also a pair of canine teeth, situated between the incisors and
premolars, and these are especially long and pointed in such
carnivorous animals as the dog and cat. In the dogfish
and frog, again, the teeth are continually renewed as

5io

THE RABBIT

they become worn out, but in Mammals there are only
two functional sets, which are known respectively as
the deciduous or " milk "-teeth, and the successional or
permanent teeth : certain of the former may even be
absorbed before birth, as is the case with the incisors of the
rabbit. The incisors and premolars
(and in Mammals in which they
are present the canines also) have
deciduous predecessors, the molars
developing behind the premolars
and having no. predecessors.

All the teeth are embedded in deep
sockets or alveoli of the jaw-bones,
and each contains a pulp-cavity
(Fig. 136, PH) extending into it
from the base and containing blood-
vessels and nerves. In the case of
the rabbit, the aperture of the pulp-
cavity (PH'} remains wide open in
each tooth, and the substance of
the tooth is continually added to
at its base as it wears away at
the other end : in many Mammals,
however (e.g., dog, cat, man), the
aperture becomes narrowed and
growth ceases after a time, the base of the tooth forming one
or more roots or fangs. The main substance of each tooth
is formed of dentine (ZB\ into which the pulp-cavity extends
for a considerable distance and round which the enamel
(ZS) forms an external layer, which may become more or less
folded inwards (as in the cheek-teeth and front upper incisors
of the rabbit), the cement (ZC) extending into the folds
(compare pp. 445 and 511).

FIG. 136. — Longitudinal section
of a mammalian tooth, semi-
diagrammatic.

PH. pulp-cavity ; PH'. open-
ing of same ; ZB. dentine ;
ZC. cement ; ZS. enamel.
(From Wiedersheim's Comp.
Anatomy.)

xi TEETH 511

The number of the various teeth in the jaws is con-
veniently expressed by a dental formula, in which the kind
of tooth (incisor, canine, premolar, or molar) is indicated
by the initial letter /, c, pm, or m ; and the whole formula
has the arrangement of four vulgar fractions, in each of
which the numerator indicates the number of teeth in the
upper, the denominator that of those in the lower jaw, only
those of one side being indicated, since the teeth of the right
and left sides are always the same. Thus the dental formula
of the rabbit is /f, c^, pm\, ;//§ = 28.

The anterior incisors in the upper jaw of the rabbit are long and
' greatly curved. They are surrounded by enamel, which is much
thicker on the anterior surface, where it presents a median groove ;
the posterior upper incisors are much smaller and are situated behind
the anterior ones. In the lower jaw the single pair of long and curved
incisors have no median groove, and they bite between the anterior and
posterior upper incisors : owing to the thick layer of enamel anteriorly,
they, like the large upper incisors, wear away less quickly in this region,
and thus remain sharp, like a chisel, at their biting edges. The pre-
molars and molars, on the other hand, are modified for grinding the food,
to do which satisfactorily it is necessary that they should have broad
crowns with a surface which wears unevenly. This is effected in most
of the cheek-teeth by the enamel becoming involuted along the outer
side in a longitudinal direction, so as to form a groove extending into
the dentine almost to the other side, the groove becoming filled up with
cement. As the enamel is harder than the dentine and cement, it thus
gives rise to ridges as the crown wears. The first upper premolar and
the last upper and lower molars are simpler than the others, and the
first lower premolar presents two grooves.

Connected with the mouth-cavity are several pairs of
salivary glands, not present in the other Vertebrates we have
examined, the secretion of which — saliva — contains a
ferment called ptyalin, which is capable of converting starch
into sugar (compare p. 74). The food taken into the
mouth is ground up or masticated and mixed with the saliva

512 THE RABBIT CHAP, xi

before passing down the gullet, and thus digestion begins in
the mouth.

The infra-orbital salivary gland is a large, lobulated, pinkish mass
situated in the antero-ventral region of the orbit, below and in front of
the eyeball : its duct passes downwards to open into the mouth nearly
opposite and externally to the second premolar. The parotid gland is
a soft, irregular, flattened organ, lying close beneath the skin, just
below and in front of the base of the external ear ; its duct passes for-
ward and opens close to the duct of the infra-orbital gland. The sub-
maxillary gland (Fig. 135, s.mx. £•/), is a reddish, ovoid, compact
body situated inside the angle of the lower jaw and near the middle
line, somewhat anterior to the larynx : its duct runs forward to open
into the mouth a short distance behind the lower incisors. The sub-
lingual gland (si. gl] is an elongated structure situated on the inner
side of the mandible, and having several ducts opening independently
into the mouth.

The oral cavity is continued backwards as the pharynx
(pti) : this begins at the velum palati, above which it ex-
tends forwards as the naso-pharynx (n. pJi) • the latter is con-
tinuous with the passage of the internal nostrils, and into
it open the Eustachian tubes (eus, compare pp. 17 and 45).
On the floor of the pharynx, behind the base of the tongue,
is the glottis, which leads into the larynx and is guarded in
front by an elastic, leaf-like, cartilaginous flap, the epiglottis
(epg) : this projects upwards towards the velum palati and
is capable of being pressed backwards over the glottis during
the passage of food from the mouth to the gullet.

The gullet (Figs. 135 and 137, <zs) is a narrow but dilatable
tube which passes backwards along the neck and through
the thorax, entering the abdomen through an aperture
in the diaphragm and then opening into the stomach
(py. st, cd. st\ a wide, curved sac, elongated transversely
and greatly dilated at the cardiac end, which lies towards the
left side of the body : the pyloric end, from which the duo-

II

FIG. 137. — The stomach, duodenum, posterior portion of rectum, and liver (in outline)
of the rabbit, with their arteries, veins and ducts. 4, the coeliac artery of
another specimen (both x f ). The gullet is cut through and the stomach some-
PRACT. ZOOL. L L

5J4 THE RABBIT CHAP.

what displaced backwards to show the ramifications of the coeliac artery (cce. a) ;
the duodenum is spread out to the right of the subject to show the pancreas {pn) ;
the branches of the bile-duct (c. b. d), portal vein (p. v) and hepatic artery (h. a)
are supposed to be traced some distance into the various lobes of the liver.
a. m. a. anterior mesenteric artery ; cari. caudate lobe of liver with its artery, vein
and bile-duct ; c. b: d. common bile-duct ; cd. st. cardiac portion of stomach ;
c.il.a. common iliac artery; Cie. a. coeliac artery; cy.a. cystic artery; cy.d.
cystic duct ; d. ao. dorsal aorta ; du. proximal, and du'. distal limbs of duo-
denum ; du. a. duodenal artery ; du. h. a. (in A), duodeno-hepatic artery ; g. a.
gastric artery and vein ; g. b. gall bladder ; h. a. hepatic artery ; h. d. left hepatic
duct; I.e. left central lobe of liver, with its artery, vein, and bile-duct; Lg.v.
lieno-gastric vein ; /. /. left lateral lobe of liver, with its artery, vein, and bile-duct;
ms. branch of mesenteric artery and vein to duodenum ; ms. r. mesentery of
the rectum ; m. v. chief mesenteric vein ; ces. gullet ; p. m. a. posterior mesenteric
artery ; p. m. v. posterior mesenteric vein ; pn. pancreas ; pn. d. pancreatic duct ;
p. v. portal vein ; py. st. pyloric portion of stomach ; ret. rectum ; r. c. right
central lobe of liver, with artery, vein, and bile-duct ; spg. Spigelian lobe of
liver with its artery, vein, and bile-duct ; spl. spleen ; sp. a. splenic artery.
(From Parker's Zootomy.)

denum arises, towards the animal's right, is less dilated and
has much thicker muscular walls. The mucous membrane of
the stomach, in which the microscopic gastric glands (p. 131)
are contained, is raised into ridges or ruga, and there
is a circular pyloric valve at the entrance to the intestine.

The duodenum (du) extends backwards along almost the
whole length of the abdomen and then turns forwards again,
forming a slightly coiled, U-shaped loop, and becoming
continuous with the very long and coiled second portion of
the small intestine or ileum (Fig. 135, //), which finally
dilates to form a rounded sac (s.r) opening into the proximal
end of the dark-coloured colon (col), or first portion of the
large intestine. The colon has a much greater diameter
than the small intestine, and presents a series of saccula-
tions arranged in three rows, separated by flat regions
of its wall : it passes insensibly into the second portion of
the large intestine or rectum (rct\ which is of about the
same diameter as the small intestine, and is recognisable by
its rounded swellings containing the pill-like faeces : it passes
into the pelvic cavity to open by the anus (a). At the junction
of the ileum and colon is a relatively enormous blind-gut or
ccecum (ccec) — a structure not met with in either the dogfish

xi • ENTERIC CANAL 515

or the frog, and only reaching such a relatively large size
as in the rabbit in certain other herbivorous Mammals in
which the stomach has a simple form : in those which
possess a complicated stomach (viz., Ruminants) the caecum
is comparatively small. It is continuous with the proximal
end of the colon, which contains an intra-colic valve and
into which the round sac at the distal end of the ileum
opens by a circular aperture provided with an ileo-colic
valve. From this point arises the thin-walled caecum, which
lies coiled on itself amongst the folds of the rest of the
intestine : it is about an inch in diameter, and a spiral con-
striction is seen on the outside marking the attachment, on
the inside, of a spiral valve — like that of the dogfish's in-
testine but narrower — which makes about twenty-four turns
and ends at the base of a blind, finger-shaped process, the
vermiform appendix, which forms the apex of the caecum.
The whole canal is supported by a mesentery (p. 27), the
right and left halves of which are united, and which has a
very complicated arrangement in correspondence with the
numerous folds of the intestine.

It will be noticed that the intestine is much more differentiated as
regards its subdivisions than in the Vertebrates previously examined, and
also that it is relatively much longer, being fifteen or sixteen times as
long as the body.

On cutting open the small intestine, its mucous membrane is seen to
be raised into minute, finger-shaped elevations or villi, and here and
there certain patches present a honeycombed appearance : these portions
are known as Peyer's patches^ and, like the tonsils, thymus, and spleen,
consist of masses of lymphoid follicles composed of a connective-tissue
framework in which numerous leucocytes are imbedded. Other
so-called " lymphatic glands" or adenoids are present in the mesentery
and elsewhere. Peyer's patches also occur in the proximal end of the
colon, close to the ileo-colic aperture ; and the round sac with which the
colon communicates, as well as the vermiform appendix, are lined with

L L 2

516 THE RABBIT CHAP.

similar lymphoid tissue. The mucous membrane of the colon has no
villi, but, like that of the spiral valve, is raised into small papillae ;
while that of the rectum is smooth.

The liver (Figs. 135, Ir, and 137 ) is a large organ, consist-
ing of five lobes. Its anterior surface is convex and is
applied to the diaphragm, its posterior concave surface
fitting against the stomach. A median vertical fold of the
peritoneum attaches it to the diaphragm, and marks the
boundary between the right and left central lobes (Fig. 137
r.c, l.c). Externally to the left central lobe, between it and
the cardiac end of the stomach, is the left lateral lobe (I. /),
and externally to the right central lobe the caudate lobe
(cau\ applied to the pyloric end of the stomach and hollowed
posteriorly, when it fits over the right kidney : a small
Spigelian lobe (spg) fits closely against the concave anterior
surface of the stomach. The bile-duct is made up of several
hepatic ducts (h. d} from the various lobes of the liver, as
well as of a cystic duct (cy. d} from the pear-shaped gall-
bladder (g. b], which is imbedded in the right central lobe
of the liver : the common bile-duct (c.b.d) thus formed opens
into the dorsal side of the duodenum by a prominent aper-
ture a short distance beyond the pylorus.

The pancreas (Figs. 135 and 137 pn) is a diffuse gland,
consisting of a number of small lobules looking not unlike
masses of fat, spread all over the mesentery which connects
the two limbs of the duodenum. The small ducts from the
various lobules run together to form the main pancreatic duct
(pn. d] which opens, independently of the bile-duct, into
the distal limb of the duodenal loop a couple of inches or so
beyond the bend.

The spleen (Fig. 137, spt) is a long, flat body of a dark
red colour, attached to the cardiac end of the stomach by a
sheet of peritoneum.

xi RESPIRATORY ORGANS 517

Organs of respiration and voice, — Owing to the presence
of a neck, the lungs are situated some distance from the
glottis (Fig. 135), and, instead of a short laryngo-tracheal
chamber, as in the frog (p. 141), there is a windpipe or
trachea (tr) extending along the neck (p. 506) just ventrally
to the gullet, its anterior end forming the larynx or organ of
voice, and communicating with the pharynx through the
glottis. The cartilaginous rings of the trachea are incom-
plete dorsally, and the cartilages of the larynx are more
highly differentiated than in the frog, apart from the presence
of an epiglottis (epg, see p. 512). The largest and most anterior
laryngeal cartilage is the thyroid, which, like the epiglottis,
is peculiar to Mammals : it has the form of a broad ring,
incomplete dorsally, and is the part of the larynx which can
be felt externally. The second cartilage is the cricoid,
represented in the frog by a ring-shaped cartilage at the base
of the lungs (p. 153) : its form is somewhat like that of a
signet ring, being broad dorsally — where it lies mainly
between the edges of the thyroid — and narrow ventrally.
A pair of arytenoid cartilages are articulated to the dorsal
and inner surface of the cricoid, and each is produced
into a projecting process situated between the two edges of
the thyroid cartilage. The vocal cords (p. 144) are a pair
of elastic folds extending across the cavity of the larynx
from the thyroid below to the arytenoids above, each
bounded in front by a depression.

In the position of rest, the vocal cords lie at an acute angle to one
another, as in the frog ; they can be brought into parallelism and
regulated by the action of a number of intrinsic and extrinsic muscles,
and are set in vibration by the respiratory current of air.

After entering the thorax, the trachea divides into two
bronchi (Fig. 135, br), one entering each lung and giving off
branches to its different lobes : the bronchi, like the trachea,

518 THE RABBIT CHAP.

are supported by incomplete cartilaginous rings at their
anterior ends, but these gradually disappear after the bronchi
have entered the lungs.

The elastic lungs (Figs. 135 and 138, Ing) are not hollow
sacs, like those of the frog, but are spongy bodies, of a light
pink colour, situated on either side of and above the
heart, and rilling the greater part of the thoracic cavity, but
collapsing as soon as the wall of the thorax is perforated.
Each is subdivided into two main lobes, and the right lung
has in addition two small accessory lobes, an anterior and a
posterior, the latter lying in the median line, behind the
heart, and being closely applied to the gullet.

Each pulmonary artery (Fig. 135, p. a) crosses the main bronchus
anteriorly to the point at which it branches into the various lobes,
except in the case of the anterior accessory lobe, the bronchus to which
comes off in front of the artery and may even arise from the trachea
before its bifurcation. Microscopic examination shows that the bronchi
divide and sub-divide to form a ramifying system of tubes, each ultimate
branch of which opens into a minute chamber or infundibulum, which
in structure closely resembles a frog's lung in miniature.

The parietal layer of the pleura (p. 506) lines the cavity of
the thorax, and is reflected over each lung at the entrance of
the bronchus to form the visceral layer : in the median line
it forms a vertical partition, the mediastinum, with which it
is continuous on the ventral side of the vertebral column
above and beneath the pericardium below (Fig. 138). Thus
each lung (/. Ing, r. Ing) has its own separate pleural cavity
(I. pi, r. pi), separated from its fellow by the right and left
mediastinum, the space between which is called the media-
stinal space. The anterior and dorsal parts of this space
are narrow and enclose the posterior part of the trachea
and the bronchi, as well as the gullet (oes) and main blood-
vessels (aort, az.v, pt.cav) \ its middle part is wide, and

RESPIRATION

519

encloses the heart (/. vent, r. vent\ the mediastinum here
fusing with the visceral layer of the pericardium (vis. per)
and thus obliterating the space ; below this it again narrows
to form the ventral mediastinal space (v. med\ in which the
thymus (p. 507) is situated.

In the entire animal, the air-tight pleural cavities are

cent

FIG. 138. — Diagrammatic transverse section of the rabbit's thorax in the region of
the ventricles, to show the relations of the pleurae and mediastinum (dotted line),
etc. The lungs are contracted.

aort. dorsal aorta ; az. -v. azygos vein ; cent, centrum of thoracic vertebra ; /. Ing.
left lung ; I. pi. left pleural cavity ; /. vent, left ventricle ; my. spinal cord ; oes.

v. med. ventral mediastinal space. (From Parker and Haswell's Zoology.)

completely filled by the lungs, so that the parietal and vis-
ceral layers of the pleurae are practically in contact, there
being only a lubricating serous fluid (lymph) between them.
The pressure of the air in the bronchial cavities of the
lungs is therefore sufficient to keep them distended ; but as

520 THE RABBIT CHAP.

soon as the pleural cavities are perforated, air pasess in and
equalises the pressure : the elasticity of the lungs then
comes into play, causing them to collapse. When the
muscles of the diaphragm contract (p. 507), air is drawn
into the lungs, and this process is aided by the external
intercostal muscles (p. 506) and, in forced respiration, by
other muscles of the body-wall also. The mechanism of
respiration may therefore be compared with a suction-pump,
while that in the frog resembles a force-pump (p. 142).

On either side of the larynx is a soft, vascular, gland-like thyroid body,
consisting of two lateral portions connected ventrally by a median
bridge. Its function is not thoroughly understood : morphologically it
represents a gland developed from the pharynx, but it loses its con-
nection with the latter and thus has no duct. The glandular vesicles of
which it is composed give rise to an albuminous substance containing
iodine, which is passed into the blood and lymph ; if extirpated in
the living animal, various functional disturbances result. We are also
ignorant of the function of the thymus (p. 507), which is largest in
young animals, becoming reduced in size in adults (compare p. 447).

Organs of Circulation. — The heart, as in all Vertebrates,
is enclosed in a pericardium consisting of parietal and
visceral layers (Fig. 138), between which is a serous peri-
cardial fluid. There is a complete separation between the
arterial and venous blood in the heart, for in addition to
an auricular septum, as in the frog (p. 88), the ventricular
portion is divided into right and left chambers by a partition
(Fig. 138), the arterial blood from the lungs entering the
left auricle and thence passing into the left ventricle to be
pumped into the aorta, and the venous blood entering the
right auricle and thence into the right ventricle to pass to
the lungs through the pulmonary artery. A distinct conus
arteriosus and sinus venosus (pp. 79 and 449) can no longer
be recognised, the former having become practically absorbed

xi HEART 521

into the ventricular portion of the heart, and the latter into
the right auricle ; so that the aorta (together with the caro-
tids) and the pulmonary artery now arise directly from
the left and right ventricles respectively, and the precavals
and postcaval enter the right auricle directly (Figs. 135 and
140).

The line of separation between the two ventricles can be
seen externally as an oblique depression extending from the
base of the heart backwards and to the right, but not
reaching the apex, which is formed by the left ventricle only.
The small, irregular cavity of the latter is enclosed by very
thick muscular walls, and is partly surrounded by the right
ventricle, the cavity of which is crescentic in transverse
section (Fig. 138), while its walls are much thinner than those
of the left ventricle, as it has only to pump the blood to the
lungs. The auricles have thin walls : each is produced into
a little flap or appendix which envelops the base of the cor-
responding ventricle, and the walls of which are strengthened
by a network of muscular bands.

In the auricular septum is a thin, oval area, the fossa ovalis (Fig.
I39if- ov}> which in the embryo is perforated and so allows the blood
from the body to pass directly into the right auricle without going to
the lungs, which are not, of course, functional until the animal is born.

The two auriculo-ventricular apertures are guarded by
valves — that of the left side, or mitral valve, consisting of
two membranous flaps, that of the right, or tricuspid valve
(Fig. 139, tri. v) of three flaps : the valves are attached by
their bases to the margins of the apertures, their apices ex-
tending into the corresponding ventricles. Attached to their
edges are tendinous cords arising from conical elevations
of the ventricular walls known as papillary muscles, which
are much larger in the left ventricle than in the right

522 THE RABBIT CHAP.

(m. pap) : these serve to prevent the valves from being
pushed into the auricles when the ventricles contract.

The right ventricle narrows towards its base, on the
ventral side of the heart, to form a conical prolongation
from which arises the pulmonary artery (Figs. 135 and 140,
p.d), its aperture being guarded by three pocket-like, semi-
lunar valves (Fig. 139, sem. v) : the aperture of the aorta from

sem,.?/

FIG. 139. — Heart of the rabbit, seen from the right side, the walls of the right auricle

and right ventricle partly removed so as to expose the cavities.
ao. aorta \f.ov, fossa ovalis ; l.pr.c. opening of left precaval ; m.pap. papillary
muscles ; pt. c. postcaval ; pt. c'. opening of postcaval ; r. pr. c. right pre-
caval ; r. pul. right pulmonary artery ; sem. v. semilunar valves at base of
pulmonary artery ; tri. v. tricuspid valve. (From Parker and Haswell's
Zoology. )

the left ventricle is similarly provided with three semilunar
valves. The two precavals (/. pr. c, r. pr. c) and the post-
caval (pt. c} communicate, as we have seen, directly with
the right auricle, the right precaval opening into it anteriorly,
the left precaval posteriorly, the aperture of the postcaval
being just anterior to that of the left precaval. The
pulmonary veins from each lung unite and open together
into the left auricle.

XT ARTERIES 523

Membranous folds, the Eustachian and Thebesian valves, extend
into the right auricle between the apertures of the postcaval and left
precaval : in the embryo these help to conduct the blood through the
aperture in the auricular septum (p. 521) : they afford another example of
vestigial structures, representing the remains of the sinu-auricular valve.

You will remember that in the frog (p. 80) there are
two systemic trunks, representing the second arterial arch of
the tadpole and fish (p. 451, and Fig. 120), and uniting
above to form the dorsal aorta. In the Mammal, one of
these — the right — disappears in the course of development
and all the blood from the left ventricle passes into the
single left aortic arch (Figs. 135 and 140) from the base of
which both carotid arteries arise, the aortic arch then
curving over the left bronchus to pass into the dorsal aorta
(d, ad).

Close to the origin of the aortic arch, just beyond its
semilunar valves, two small coronary arteries are given off to
the walls of the heart ; and more anteriorly, at the curve of
the arch, arise the vessels which supply the head and fore-
limb. There is a certain amount of variation as to the origin
of these, which is asymmetrical, and is usually as follows.
Springing from the arch of the aorta towards the right side
is an innominate artery (Fig. 140, in\ which gives off close
to its origin the left common carotid (L c. <:), and then, passing
forwards, divides into the right common carotid (r.c.c) and
the right subclavian (s. cl. a\ the left subclavian (br) taking
its origin independently from the left side of the arch.
Each common carotid passes forwards along the neck, close
to the trachea, and at about the level of the larynx divides
into an internal carotid (i. c\ which supplies the brain, .and
an external carotid (e. <:), which goes to the head and face.
Each subclavian forms several branches, the most important
of which are a brachial (br) to the fore-limb, a vertebral

524 THE RABBIT CHAP.

(vr) which passes through the vertebrarterial canal of the
cervical vertebrae (p. 498) and supplies the spinal cord and
brain, and an anterior epigastric or internal mammary (a. epg)
running along the inner side of the ventral wall of the
thorax. The aorta gives off, in the thorax, a series of small
paired intercostal arteries (i. cs) to the body- walls, and then
passes into the abdomen, between the pillars of the
diaphragm.

A short distance behind the diaphragm the cceliac artery
(Figs. 137 and 140,, c<z) arises, and supplies the liver,
stomach, and spleen ; and about half or three-quarters of an
inch further back is the anterior mesenteric artery (a. m. a),
the branches of which pass to the small intestine, pancreas,
caecum, and colon. Close behind the anterior mesenteric is
the right — and rather further back the left — renal artery (Fig.
140, r\ and still more posteriorly, a posterior mesenteric
(p. m) to the rectum, and a pair of spermatic or ovarian arteries
(spm) to the spermaries or ovaries, as the case may be.
A small caudal artery (m. sc\ corresponding to the caudal
continuation of the aorta, arises from the dorsal surface of the
posterior part of the latter just in front of a pair of large
common iliac arteries (c. il. a\ which appear like a bifurcation
of the aorta. These are continued outwards and backwards
towards the hind-limbs, each giving off an ilio-lumbar artery
(/./) to the dorsal body- wall and then dividing into an
internal iliac (i. il. a) passing along the dorsal side of the
pelvic cavity, and an external iliac (e. il. a) which gives off an
artery to the bladder (s. vs), and in the female one to
the part of the oviduct known as the uterus (uf) ; and
then, passing beneath Poupart's ligament (p. 506) to the
hind-limb, becomes the femoral artery (fm. a), from the
proximal end of which a posterior epigastric (p. epg)
runs forwards in the ventral abdominal wall. Small lumbar

xi VEINS 525

arteries are also given off from the aorta to the walls of the
abdomen.

The pulmonary artery (p.d) divides soon after its origin
from the right ventricle into two branches, one supply-
ing each lung. Just before its bifurcation it is connected
by a short cord, known as the ductus arteriosus, with the
aorta : this is the solid vestige of the embryonic connection
between the fourth arterial arch and the aorta (compare p.
452, and Fig. 120).

Each precaval (I. pr. c, r. pr. c) receives — a sub-
clavian (s. d. v) from the fore-limb ; an external jugular
(e. ju) from the head, running along the neck just
beneath the skin ; a small anterior epigastric from the
ventral thoracic wall, as well as small vessels from some
of the anterior intercostal spaces (/. cs) and the anterior
surface of the diaphragm (a. ph) • and a small internal
jugular (i. ju) from the brain, opening into the cor-
responding external jugular nearly opposite the sub-
clavian. An azygos vein (az. v\ representing part of the right
cardinal of the embryo (compare p. 456) and receiving blood
from the posterior intercostal spaces, also opens into the
base of the right precaval.

There is no renal portal system, as in the dogfish and frog
(pp 454 and 85). A pair of internal iliac veins (i. il. v) in the
pelvic cavity unite to join a median vessel (c. il. v\ the
hinder end of the postcaval, which receives on either side
an external iliac (e. il. v) : this is constituted by a femoral
vein (fm. v) from the hind-limb ; a, posterior epigastric (p.epg\
from the ventral walls of the abdomen, entering the femoral
just external to Poupart's ligament ; and by small veins from
the bladder as well as from the uterus in the female.
Slightly in front of the external iliacs the postcaval receives
a pair of large ilio-lumbar veins (i. I) from the body-walls :

i.il.v

FIG. 140.

.— The vascular system of the rabbit from the ventral side. The heart
ewhat displaced towards the left of the subject ; the arteries of the right
e veins of the left side are in great measure removed.

s somewat space towars te eft of the subject ; th
and the veins of the left side are in great measure removed.

CHAP, xi VEINS 527

a. epg. anterior epigastric artery ; a.f. anterior facial vein ; a. m. anterior mesen-
teric artery ; a. ph. anterior phrenic vein ; az. v. azygos vein ; br. right
brachial artery ; c. il. a. common iliac artery ; c. il. v. hinder end of post-
caval ; cae. coeliac artery ; d. ao. dorsal aorta ; e. c. external carotid artery ;
e. il. a. external iliac artery ; e. il. v. external iliac vein ; e.ju. external jugular
vein ; fm. a. femoral artery ',fm. v. femoral vein ; h. v. hepatic veins ; i. c. internal
carotid artery ; i. cs. intercostal vessels ; i.ju. internal jugular vein ; i. I. ilio-
lumbar artery and vein ; in. innominate artery ; /. an. left auricle ; /. c. c. left
common carotid artery ; /. pr. c. left precaval vein ; /. v. left ventricle ; m. sc.
caudal artery ; p. a. pulmonary artery ; /. epg. posterior epigastric artery and
vein ; p.f. posterior facial vein ; p. -in. posterior mesenteric artery ; p. ph.
posterior phrenic veins ; ptc. postcaval vein ; /. v. pulmonary vein ; r. renal

. artery and vein ; r. an. right auricle ; r. c. c. right common carotid artery ;
r.prc. right precaval vein ; r, ?>. right ventricle ; scl. a. right subclavian artery ;
scl. v. right subclavian vein ; spm. spermatic artery and vein ; s. vs. vesical
artery and vein ; ut. uterine artery and vein ; vr. vertebral artery. (From
Parker's Zootomy.)

the left ilio-lumbar sometimes runs forwards to open into the
corresponding renal vein. Rather more anteriorly still are
a pair of spermatic (spm) or ovarian veins, and a large renal
vein (r) enters the postcaval from each kidney. As the
postcaval passes through the dorsal border of the liver, it
receives several large hepatic veins (Figs. 137 and 140, h. v)
from the lobes of that organ. Other small veins from the
body-walls and from the posterior surface of the diaphragm
also open into the postcaval, which then passes through the
central tendon of the diaphragm and runs forward in the
mediastinal space (Fig. \^pt. cav) to open into the right
auricle.

The hepatic portal vein (Fig. 137, /. v) is a large vessel
situated in the mesentery, ventral to the postcaval. An-
teriorly it passes into and divides up in the liver, sending a
branch to each lobe : posteriorly it is constituted by a large
anterior mesenteric vein (m. v) returning the blood from the
small intestine, colon, and caecum, and by smaller veins
from the stomach, spleen, and duodenum, as well as by a
posterior mesenteric vein (p. m. v) from the rectum.

The pulmonary veins have already been described (p. 522). In the
freshly-killed animal a number of the delicate, transparent, lymphatic
vessels (p. 97) can be made out, those from the intestine (lacteals}

528 THE RABBIT CHAP.

running in the mesentery. They come into connection with numerous
adenoids (p. 515) in the mesentery and elsewhere, and most
of them communicate with a main trunk — the thoracic duct — which
extends from the abdomen through the thorax on the left and upper
side of the aorta. The thoracic duct also receives the lymphatics from
the right side of the head and neck and the fore-limb, and opens into
the veins at the junction of the left external jugular and subclavian : the
lymphatics of the right side of the head and neck and right fore-limb
communicate with the corresponding veins of the right side.

Nervous System. — The brain (Figs. 135, 141, and 142)
reaches a much higher development than in the other Verte-
brates we have already studied. The prosencephalon is sub-
divided into two cerebral hemispheres (ch\ of much larger
relative size than those of the frog (Fig. 49) and forming
about two-thirds of the whole brain. They are closely ap-
plied to one another along their flat internal surfaces, are
roughly conical in form, narrower in front (frontal lobes],
broadening out posteriorly (parietal lobes) — where they over-
lap the diencephalon and optic lobes and abut against the
cerebellum, and produced ddwnwards into the prominent
temporal lobes which partly overlap the crura cerebri below.
Their external layer or cortex is formed of grey matter, and
their surface is smooth, except for the presence of slight
fissures between the lobes : in many Mammals the hemi-
spheres are highly convoluted, i.e. raised into numerous
winding elevations or gvri, separated by narrow grooves or
sulci. A broad transverse band of nerve-fibres forms a
commissure connecting the two hemispheres known as the
corpus callosum (Fig. 141 cp. <r, and Fig. 142, cp. d) : this
structure is confined to the Mammalia, and is even wanting
in certain of the lower members of the class. The olfactory
lobes (olf) are club-shaped, and extend backwards along the
ventral surface of the hemispheres in the form of narrow
bands as far as the temporal lobes.

BRAIN

529

The diencephalon consists of a right and left optic thalamus
(o. th.} between which is the slit-like third ventricle (v*)

olf

-olf

FIG. 141. Two dissections of the rabbit's brain, from above (nat. size). In A, the
left hemisphere is dissected down to the level of the corpus callosum ; on the
right side the lateral ventricle is exposed. In B, the hemispheres are dissected
down to a little below the level of the anterior end of the corpus callosum ; only
the frontal lobe of the left hemisphere is retained, of the right a portion of the
temporal lobe also remains ; the choroid plexus arid pineal body are removed, as
, well as the greater part of the body of the fornix and the whole of its left posterior
pillar ; the cerebellum is removed with the exception of a part of its right lateral
lobe with the flocculus

a. co. anterior commissure ; a.fo. anterior pillar of fornix ; a.pn. anterior peduncles
of cerebellum ; b.fo. body of fornix ; cb1. central lobe of cerebellum ; eft. its
lateral lobe; c.gn. elevation on the optic thalamus; c h. cerebral hemisphere;
ch.pl. part of choroid plexus ; cp. cl. corpus callosum ; cp. s. corpus striatum ; c. rs.
and d. p. elevations on the bulb \ ft. flocculus ; hp. m. hippocampus ; m.co. middle
commissure ; o. /*, o. ft. optic lobes ; olf. olfactory lobe ; o. th. optic thalamus ; o.
tr. optic tract (continuation of chiasma) ; p. co. posterior commissure ; p.fo.
posterior pillar of fornix ; pn. pineal body ; pd. pn. its peduncle ; /. pn. posterior
peduncles of cerebellum ; p. va. fibres of pons Varolii forming middle peduncles
of cerebellum ; sp. lu. septum lucidum ; st. 1. line on corpus callosum ; t. s. band
of white matter lying beneath choroid plexus ; v. vn. valve of Vieussens ; v&.
third ventricle ; tA. fourth ventricle. (From Parker's Zootomy.)

roofed over by a thin membrane, continuous with a
vascular choroid plexus (Fig. 142, vl. ip\ and from its

PRACT. ZOOL. \. MM

530 THE RABBIT CHAP.

hinder part ^arises a stalk bearing at the end a small,
rounded pineal body (pn). The floor of the diencephalon
is produced downwards to form the infundibulum (inf),
to which the pituitary body (pty} is attached. In front
of the infundibulum is the optic chiasma (o. ch\ and
behind it a small, rounded lobe (c. ma).

Each optic lobe is divided into two by a transverse furrow,
so that there are four rounded elevations in this region — an
anterior, larger pair (o. 71), and a posterior, smaller pair (o. /2).
Below the optic lobes are the crura cerebri (c. c} — two
strong, diverging bands passing forwards and outwards from
the bulb to the hemispheres.

The bulb or medulla oblongata (m.o) is slightly flattened
dorso-ventrally, and passes behind into the spinal cord, the
dorsal and ventral fissures of which are continued into
it : the fourth ventricle (z>4) which it contains is roofed
over by the thin pia mater only (p. 155). Ventrally its
anterior border is marked by a stout band of nerve-fibres
running transversely, and known as the pons Varolii (p. vd).
The large cerebellum is connected with the dorsal surface of
the brain by three pairs of peduncles (Fig. 141, a. pn, p. va,
p. pn), and consists of a median central lobe (cbl) and of
two lateral lobes (<r<£2), on the outer side of each of which
is a smaller floccular lobe (fi). The grey matter is super-
ficial, and the surface is marked by numerous folds
which in section present a tree-like pattern (arbor vita),
brought about by the arrangement of the grey and white
matter (Fig. 142).

The fourth ventricle is not prolonged into the cerebellum
to any extent : it is continued forwards as the tier, from
which no optic ventricles are given off (compare pp. 151 and
459) and which passes into the narrow but deep third
ventricle in front (Fig. 142) : this is bounded anteriorly by

xi BRAIN 531

a thin wall, the lamina terminalis (/. /), and extends into the
infundibulurn below. At its anterior end are the foramina
of Munro (/. m), leading into the middle of the lateral
ventricles in the hemispheres (Fig. 141). In this region
each lateral ventricle is broad from side to side, but narrow
from above downwards ; it extends forwards into the frontal

7 m
olf.

FIG. 142.— Rabbit. ^ Longitudinal vertical section of the rabbit's brain (nat. size).
Letters as in preceding figure ; in addition — cb. central lobe of cerebellum, showing
arbor vitse ; c. c. crus cerebri ; c. h.^ parietal, and c. h2 temporal lobe ot

choroid plexus ; //. optic nerve. (From Parker's Zootomy.)

lobe, backwards into the parietal lobe, and downwards into
the temporal lobe. The olfactory lobes are solid.

A prominent, convex ridge of white matter — the hippocampus (Fig.
141, hp. m) projects into the inner side and floor of each lateral ven-
tricle where it descends into the temporal lobe, and closely applied to it
is a continuation of the choroid plexus (ch. pi], which passes from the
roof of the third ventricle into the lateral ventricle through the
foramen of Monro. In front of the hippocampus the outer side and
floor of the anterior part of the lateral ventricle is thickened to form an
eminence of grey matter, the corpus striatum (cp. j). Just beneath the
corpus callosum the internal wall of each lateral ventricle is thin, and
is known as the septum lucidum (sp. hi) ; and below it and above the
foramina of Monro is another commissure known as the body of the
fornix (Figs. 141 and 142, b. fo] which is continuous on either side with

• M M 2

532 THE RABBIT CHAP.

two bands — one (posterior pillar) lying along the anterior edge of the
hippocampus, and the other (anterior pillar) passing backwards in the
side walls of the third ventricle. Connecting the two optic thalami
are three tranverse bands of nerve fibres, known respectively as the
anterior (a. c0), middle (m. co} and posterior (p. co] commissures: the
middle commissure, which is much the largest, is not represented in the
lower Vertebrata.

The spinal cord (Fig. 135, sp. c) is similar in structure
to that already described in other Vertebrates (pp. 155 and
459). It extends through the entire neural canal, ends in a
filum terminate, and is swollen opposite the fore- and hind-
limbs, where the nerves arise which form the limb-plexuses
(pp. 161 and 162).

The dorsal and ventral roots of the spinal nerves lie in the same
transverse plane, as in the frog (p. 163), but are relatively shorter than
in that animal ; after uniting to form the nerve-trunks, they pass directly
outwards through the intervertebral foramina. The brachial plexus is
formed from the four posterior cervical and the first thoracic nerves, and
gives off a number of nerves to the shoulder and fore-limb. The sciatu
or lumbo-sacral plexus is constituted by the two or three hindermost
lumbar and the first two or three sacral nerves, and gives off branches to
the pelvic region and hind- limb, the chief of which are a fentoral going
to the extensor muscles, and a large sciatic and an obturator (which
passes through the obturator foramen, p. 503) supplying the flexor
muscles. Arising from the fourth cervical spinal nerve of either side is
a phrenic nerve (Fig. 135, phr. n], which passes backwards, between
the heart and lungs, to supply the muscles of the diaphragm ; and a
large auricular nerve , arising from the third cervical nerve, supplies the
external ear.

In addition to the ten cerebral nerves enumerated in the
frog (p. 163) and dogfish (p. 461), two others — the spinal
accessory and the hypoglossal (represented in the frog
by fibres in connection with the vagus and by the first
spinal nerve respectively, p. 160) emerge from the skull and
are therefore counted as the eleventh and twelfth cerebral
nerves. The former arises from the side of the spinal cord

xi NERVES 533

and bulb by numerous fibres, the posterior of which are op-
posite the fifth spinal nerve, from which point it runs forwards
between the dorsal and ventral roots and leaves the skull
together with the glossopharyngeal and vagus (p. 496), sup-
plying certain muscles of the neck and shoulder. The
hypoglossal arises by a number of fibres from the ventral
surface of the bulb, passes out through the condylar foramen,
and supplies the muscles of the tongue as well as certain
muscles of the neck.

The origin and distribution of the first ten pairs of cerebral nerves
correspond in their main features with those seen in the frog. The facial
is almost entirely a motor nerve and is chiefly important in supplying
the facial muscles, which are very highly developed in Mammals. An
anterior and a posterior or recurrent laryngeal nerve are given off from
the vagus.

The relations of the sympathetic nerves (Fig. 135, sy) are
also essentially similar to those occurring in the frog (p. 162).
Each passes backwards along the neck close to the vagus
(vg) and alongside the carotid artery, enlarging to form an
anterior and a posterior cervical ganglion. In the thorax
it runs just beneath the heads of the ribs, having a ganglion
in each intercostal space : it then passes into the abdomen,
lying close to the centra of the vertebrae and having ganglia
at intervals. From all the sympathetic ganglia branches
are given off connecting them with the spinal nerves, others
going to the blood-vessels : others again, in the thorax and
abdomen, are connected with plexuses from which nerves
pass to the heart and abdominal viscera. In the abdomen
these plexuses can be seen in the mesentery, a large cceliac
plexus being present close to the origin of the cceliac and
mesenteric arteries.

Sensory Organs. — The sense of touch is situated in micro-
scopic tactile organs in the skin, and groups of cells, called

534 THE RABBIT CHAP.

taste-buds, are present on the papillae of the tongue (p. 509)
and on the soft palate (compare pp. 179 and 180).

The organs of smell are situated in the olfactory capsules,
the form of which has already been described (p. 493).
They open externally by the external nostrils, and are pro-
duced backwards above the palate into the passage of the
internal nostrils, which communicate with the naso-pharynx
(Fig. 135 i. n, n. ph, p. 512). The olfactory epithelium,
supplied by the olfactory nerves, is situated on the ethmo-
turbinal (e. tb) : the mucous membrane of the maxillo-tur-
binal (m. tb) probably serves merely to warm the inspired air.

On the ventral side of the nasal septum is a pair of small, tubular
structures known as the vomero-nasal oxjacobsorfs organs (Fig. 135,7),
lined by epithelium and enclosed in cartilages situated just to the inner
side of the palatine processes of the premaxillse (p. 494). Each of
them opens anteriorly into the corresponding naso-palatine canal
(p. 507), and receives a special branch of the olfactory nerve. The
function of these organs is not understood.

The structure of the eye (Fig. 57) is similar to that already
described in other Vertebrates (pp. 181 and 465), except that
the sclerotic is not cartilaginous, but is composed of dense
fibrous tissue, and the lens is relatively smaller than in the
dogfish and frog and is markedly biconvex in form, the outer
surface being rather flatter than the inner : it is capable of
adjustment by means of the ciliary muscles contained in the
radiating ciliary processes into which the choroid is thrown
just externally to the iris (compare p. 184).

The eyelids have already been described (p. 486). The four recti
muscles ensheath the optic nerve, as in the frog (p. 186, compare Fig.
126), but the superior oblique, instead of arising — like the inferior oblique
— in the anterior part of the orbit, takes its origin further back, near the
recti, passes forwards through a fibro-cartilaginous pulley at the anterior
angle of the orbit, and then backwards and outwards to its insertion on
the eyeball.

xi AUDITORY ORGAN 535

Between the wall of the orbit and the eyeball are two
glands, the secretion of which, passing through ducts per-
forating the conjunctiva lining the eyelids, serves to keep
the outer surface of the eye moist, and is then conducted
into the nasal chambers by means of the naso-lacrymal
duct (pp. 1 86 and 494). These two glands correspond to
special differentiations of a primarily continuous structure :
one, the Harderian gland — already met with in the frog — is
situated in the antero-ventral region of the orbit : the other,
or lacrymal gland proper, in its postero-dorsal region.
Besides these, a series of small Meibomian glands are present
on the inner side of the edges of the eyelids, and produce
a fatty secretion.

The essential part of the auditory organ consists, as in
other Vertebrates, of the membranous labyrinth with its
three semicircular canals (pp. 186 and 465) enclosed in the
auditory capsule (periotic bone, p. 492), and constituting the
internal ear. The small outgrowth of the sacculus seen in
the dogfish and frog, and known as the cochlea (Fig. 59, /),
is represented by a relatively larger structure, coiled on itself
in a spiral manner. The part of the periotic bone which
directly surrounds the cavity in which the membranous
labyrinth lies is especially hard, and when the outer portion
of the bone is cut away, is seen to form a sort of cast of the
enclosed organ, the form of which it repeats : this is known
as the bony labyrinth (Fig. 143). Internally it is separated
from the membranous labyrinth by a narrow space all round,
containing the perilymph (p. 189) and only shut off from
the tympanic cavity at the fenestra ovalis and fenestra
rotunda (p. 493) by a membrane which closes them.

The membranous cochlea does not run up the middle of the spiral of
the bony cochlea, but is attached between its outer wall and a spiral
shelf arising from its inner wall. Thus the entire cochlea shows

536 THE RABBIT CHAP.

three cavities in transverse section — a middle, the membranous cochlea
or scala media, and a scala vestibuli and a scala tympani on either side of
it respectively, which communicate with one another at the apex of the
cochlea and with the perilymphatic cavity surrounding the rest of the
membranous labyrinth at its base, where the scala tympani abuts
against the membrane of the fenestra rotunda, and the scala vestibuli
against that of the fenestra ovalis. On the wall of the scala media which
separates it from the scala tympani is a specially modified series of
auditory cells forming what is known as the organ of Corti, which
receives nerve-fibres from a branch of the auditory nerve extending along
the spiral shelf of the cochlea.

The middle ear (p. 466) is constituted by the tympanic
cavity in the tympanic bulla (p. 492), and communicates
with the pharynx by the Eustachian tube (Fig. 143, E).
The tympanic membrane (M), situated obliquely at the
boundary of the bulbous and the tubular portions of the
tympanic bone, separates the middle ear from the external
ear, consisting of the auditory passage (Ex) and the pinna
(p. 486).

The fenestra ovalis is plugged by a small stirrup-shaped
bone, the stapes (Fig. 143. Oj), one of the three auditory
ossicles (p. 492) connecting the internal ear with the tym-
panic membrane, and probably corresponding morpho-
logically to part or the whole of the frog's columella (Fig. 10) :
with it is connected a small stapedius muscle, serving to
keep the membrane of the fenestra ovalis on the stretch.
The middle bone of the chain is the incus (Fig. 133, O2), a
short process of which is articulated to the stapes by the
intermediation of a small bony nodule, while its body
articulates with the outer bone of the series, the malleus
(O3). Arising from the body of the malleus is a handle-like
process or manubrium, which is attached to the tympanic
membrane (M) : this has the form of the roof of a tent, and
is kept on the stretch by a small muscle, the tensor tympani,

URINOGENITAL ORGANS

537

arising from the wall of the tympanic cavity and inserted on
to the manubrium of the malleus.

The study of development indicates that the malleus corresponds to
the articular part of MeckePs cartilage of lower Vertebrates, and the
incus to the quadrate (pp. 44 and 438) : the articulation of the bony

Ex

FIG. 143. — Diagram of the mammalian bony labyrinth, tympanic cavity, and external

auditory passage.

Cch. bony cochlea ; E. Eustachian tube ; Ex. external auditory passage ; L. bony
semicircular canals ; M. tympanic membrane ; N. auditory nerve ; O\. stapes ;
£?2' incus ; O%. malleus. (After Headley.)

mandible with the squamosal in Mammals has rendered these parts
unnecessary for their original purpose, and they have undergone a change
of function, forming an accessory part of the auditory apparatus. '

Urinogenital Organs. — The kidneys (Fig. 135, k] are of a
somewhat compressed, oval shape, with a notch or hilus on
the inner side. They are in close contact with the dorsal
wall of the abdominal cavity, the right being somewhat in
advance of the left. Towards the hilus, the tubules of

538 THE RABBIT CHAP.

which the kidney is composed (p. 146) converge to open
into a wide chamber or pelvis, which forms the dilated com-
mencement of the ureter. When the kidney is cut across,
its substance is seen to be divided into a central mass or
medulla and a peripheral portion or cortex. The former
appears radially striated, owing to the tubules in this region
being straight and converging to open on the surface of a
conical process or pyramid \ which projects into the pelvis:
the cortex contains the coiled portions of the tubules and
the Malpighian bodies, and thus has a dotted appear-
ance. The ureter (Figs. 135 and 144, ur) runs backwards
along the dorsal wall of the abdomen to open into the
urinary bladder (u. bl, /£/), a pyriform sac with elastic walls
which vary in thickness according as the organ is dilated or
contracted. Near the front end of each kidney towards its
inner side is a small yellowish adrenal body (Fig. 135, adr,
compare p. 447).

In the male Rabbit the spermaries are oval bodies which
in the young animal are situated close to the kidneys, on the
dorsal wall of the abdomen, but which pass backwards and
downwards as the animal approaches maturity until they
come to lie each in a scrotal sac (p. 487), situated at the side
of the urinogenital opening. The cavity of each scrotal sac
is in free communication with the cavity of the abdomen by
an opening — the inguinal canal. A convoluted epididymis
(p. 467), closely adherent to the spermary and connected
with the distal end of the scrotal sac, forms the proximal
part of the spermiduct or vas deferens (Fig. 144 A, v. d\
which, together with the blood-vessels and nerves of the
spermary, passes through the inguinal canal : it then loops
round the corresponding ureter, and extends back between
the neck of the bladder and a median sac on the dorsal side
of the latter known as the uterus masculinus (u. m). The

XI

URINOGENITAL ORGANS

539

neck of the bladder is continued backwards, through the
pelvic cavity, as the urinogenital canal or urethra, on the
dorsal side of which, just in front of a rounded elevation, is

VCL

II

3f

FIG. 144. — The urinogenital organs of the rabbit. — A, of male ; B, of female : from
the left side (half nat. size). The kidneys and proximal ends of the ureters, in A
the spermaries, and in B the ovaries, Fallopian tubes, and uteri are not shown.
an. anus ; bl. urinary bladder ; c. c. corpus cavernosum ; c. s. corpus spongiosum ;
c.gl. Cowper's gland ; g. cl. apex of clitoris ; g. p. apex of penis ; p. gL perineal
gland, and p.gl '. aperture of its duct on the perineal space ; pr. anterior, pr
posterior, and pr" . lateral lobes of prostate ; ret. rectum ; r. gl. rectal gland ;
u.g. a. urinogenital aperture ; u. in. uterus masculinus ; ur. ureter ; va. vagina ;
vb. vestibule ; v. d. vas deferens. (From Parker's Zootomy.)

an aperture by means of which the uterus masculinus and
vasa deferentia open into it.

A prostate gland (pr}t consisting of several lobes, is imbedded in the
walls of the uterus masculinus and opens by small ducts on either side
of the elevation just referred to ; and a pair of smaller, ovoid Cowper's
glands communicates with the urinogenital canal further back,

540 THE RABBIT CHAP.

The terminal part of the urethra traverses the copulatory
organ or penis ; that part of its wall which in the retracted
condition is dorsal, is constituted by a soft vascular portion,
the corpus spongiosum (c. s), while the opposite surface is
strengthened by two harder bodies, the corpora cavernosa
(c. t), which are closely applied together through the greater
portion of their length, but diverge proximally and are
attached to the ischia.

In both sexes a pair of perineal glands (p. gl) open on the perineal
spaces (p. 486) at the sides of the penis, and two larger rectal glands
(r. gl} lie at the side of the rectum.

In the female the ovaries (Figs. 135 and 145, ov) are small,
ovoid bodies attached by peritoneum to the dorsal wall of
the abdomen behind the kidneys, the ovarian follicles or
ovisacs (p. 195) forming very small, rounded projections on
their outer surface.

The oviducts, instead of remaining separate along their
whole length, are fused proximally to form a wide, median
portion, the vagina (Figs. 144 B and 145, va\ opening
into the urinogenital canal or vestibule (vb\ with
which the bladder communicates and which opens ex-
ternally at the vulva (Fig. 144 B, u. g. a). Into the other or
distal end of the vagina, the paired, thick-walled uteri (Fig.
145, r. ut, /, ut\ or middle portions of the oviducts, open by
separate thick-walled apertures. The eggs undergo develop-
ment in the uteri, which vary in size according to whether
or not they contain embryos, and according to the stage of
development of these. Each uterus is continued forwards as
a narrow, slightly coiled tube — the anterior section of the
oviduct, or Fallopian tube (fl. /), which communicates with
the ccelome by a small aperture (fl. tf) surrounded by a
wide, membranous funnel with thin walls and folded margins,

xi FCETUS AND PLACENTA 541

which is applied to the outer surface of the corresponding
ovary. On the ventral wall of the hinder or proximal end
of the urinogenital canal is a small, hard, rod-like body, the
clitoris (Fig. 135, cl), corresponding to the penis of the male,
and strengthened by two small corpora cavernosa (Fig. 144 B,
c. c} attached at their proximal ends to the ischia.

Vat'

r.ut

FIG. 145. — The anterior end of the vagina, with the right uterus, Fallopian tube,
and ovary of the rabbit (nat. size). Part of the ventral wall of the vagina is
removed, and the proximal end of the left uterus is shown in longitudinal section.
ft. t. Fallopian tube ; fl. t'. its coelomic aperture ; /. ut, left uterus ; /. ut' ' . aper-
ture of same (os uteri) into vagina ; ov. right ovary ; r. ut. right uterus ; r. ut' .
right os uteri ; s. vaginal septum ; va. vagina. (From Parker's Zootomy.)

The Rabbit is viviparous. The minute ova undergo de-
velopment in the uterus, in which each develops into a
foetus, as the intra-uterine embryo is termed, and is nourished
by means of an organ known as t\\e placenta, which will be
described in the next chapter. The young animal escapes
from the uterus in a condition in which all the parts have
become fully formed except that it is practically hairless ;
the eyelids are at first coherent. As many as eight or ten

542 THE RABBIT CHAP.

young are produced at a birth, and the period of gestation,
i.e., the time elapsing between the fertilisation of the ovum
and the birth of the young animal, is thirty days. Fresh
broods may be born once a month throughout a considerable
part of the year, and, as the young Rabbit may begin breed-
ing at the age of three months, the rate of increase is very
rapid.

The class Mammalia is divided into a number of orders,
that to which the Rabbit belongs being called the Rodentia
and also including rats and mice, squirrels, beavers, porcu-
pines, and many others. All these are vegetable feeders and
are mostly of small size. They possess no canine teeth,
and their incisors, which are adapted for gnawing, are never
more than two in number in the lower jaw, and in most
of them there are only two in the upper jaw also.

PRACTICAL DIRECTIONS

The specimen used should be over three months old. Place it in a
sufficiently large jar or box with a close-fitting lid together with a piece
of cotton-wool well soaked in chloroform, and leave it until a short
time after all movements have ceased. A pair of bone-forceps (p. 12) will
be required.

A. External characters (see pp. 483-487).

Microscopic preparations of the skin of a Mammal should be examined,
and the hairs, and hair- sacs, and sebaceous glands (Fig. 131) noted. (For
mode of preparation, see p. 136).

B. Skeleton. If more convenient, the detailed examination of the
skeleton may be postponed until after the soft parts have been
dissected.

The skeleton of a young rabbit, about six weeks old, as well as that

xi PRACTICAL DIRECTIONS 543

of an adult, should be obtained for examination : the former is the more
important of the two for making out the individual bones, which should
all be separated from one another by prolonged maceration in water, or
by boiling for a short time. In the adult skeleton the bones are best
kept in their natural connection. An additional skull should be
prepared, and a longitudinal vertical section made of it with a fine saw ;
but as it is difficult to do this very accurately and perfectly in the
rabbit, owing to the delicacy of some of the bones, the skull of a young
dog or cat may be used for the purpose.

Work through the account of the skeleton given on pp. 487-505, as
well as of the teeth on pp. 509-5 1 1 , comparing the form, number and
arrangement of the teeth in the herbivorous rabbit and the carnivorous
dog or cat, noting the presence of canines in the two last mentioned.
Sketch typical parts of the skeleton.

C. Superficial dissection, and injection of the arteries.

Fix the animal on its back on the dissecting-board by inserting large
pins through the limbs. If you wish to inject the arteries, cut through
the skin on the inner side of one of the thighs, reflect it, and with the
seeker expose the femoral artery (p. 524) ; or if your specimen is a small
one, expose one of the carotid arteries (p. 523) instead, by carefully
cutting through the skin along the middle ventral line of the neck.
Pass a piece of thread round the artery thus laid bare, and make a small
slit in it with the fine scissors distally to the thread. Insert and tie in
a cannula, and first inject a little strong formaline (i part formaline
and 2 parts water), following this with the coloured starch injection-
mass (p. 99), which will force the formaline into the capillaries and help
to preserve the specimen : this method is of special advantage for
the subsequent examination of the enteric canal, especially if it is not
possible to get to the end of § VI, p. 547, on the first day ; the veins
need not be specially injected, as they will be naturally injected with
blood.

Make a median longitudinal incision through the skin in the sternal
region, and continue the cut backwards to the pelvic symphysis and
forwards to the mandibular symphysis ; then dissect away the skin
from the underlying muscles over the whole ventral surface, being
careful not to injure any of the larger blood-vessels (e.g., the jugular
veins in the neck). Note : —

i. The thin cutaneous muscle, part of which you have very likely

544 THE RABBIT CHAP.

removed together with the skin ; and in the adult female, the mammary
glands (p. 506) — trace the main ducts of some of these to their apertures
on the teats.

2. In the neck : — a, the trachea and larynx (p. 506), to see which
and the following structures clearly the cervical portion of the cutaneous
muscle should be dissected away and the underlying parts carefully
separated with a seeker ; &, the hyoid bone, situated in a mass of muscle
just anterior to the larynx ; c, the submaxillary glands (p. 512) ; and d,
the large external jugular veins (p. 525).

3. In the thorax : — #, the sternum and xiphisternum ; b, the small
clavicles (p. 501) ; £, the pectoral muscles; d, the vertebral and sternal
portions of the ribs and the external intercostal muscles ; and e^ the
blood-vessels and nerves going to the fore-limbs.

4. In the abdomen : a, the muscles of the abdominal wall, separated
in the middle ventral line by a whitish fibrous band ; £, Pottparfs
ligament (p. 506) ; c, the blood-vessels and nerves passing beneath
Poupart's ligament to the hind-limbs.

II. On the same day on which the animal is killed remove the skin
from the dorsal surface of the head and anterior part of the neck,
and also the muscles covering the anterior cervical vertebrae. A
membrane between the skull and first vertebra will then be exposed : cut
through this, and part of the spinal cord will be seen. With the bone-
forceps cut away the arches of the first three or four vertebrae and the roof
of the skull— taking particular care in the auditory region — so as to expose
the brain : note the olfactory lobes, cerebral hemispheres •, cerebellum , and
medulla oblongata, and also the dura mater and pia mater. Then cut
through the spinal cord about a quarter of an inch beyond its junction
with the brain, lever up the latter with the handle of a scalpel, and
carefully cut through all the nerves arising from it and separate it
from the cranial walls, working from behind forwards. Dissect away
the olfactory lobes from their attachments, remove the entire brain, and
place it in 3 per cent, formaline.

The animal may be preserved from day to day by wrapping a cloth
round it soaked in 3 per cent, formaline or strong methylated spirit
and placing it in a closed vessel.

D. Dissection of the abdomen.

I. Once more pin the animal down, ventral side uppermost, and
make an incision along the middle line of the abdominal muscles from

xi PRACTICAL DIRECTIONS 545

the xiphisternum to the pelvic symphysis, so as to open up the
abdominal cavity. From the anterior extremity of the incision make
transverse cuts, and turn back the flaps of muscle.1 Note : —

1. a, The peritoneum ; b, the diaphragm , with its muscles and
central tendon ; c , the pink lungs, seen through the latter : make a
small aperture in the diaphragm on one side of the median line and
note the collapse of the corresponding lung.

2. a, The liver, stomach, small intestine, cczcum, colon, and rectum
(pp. 512-516) ; b, the urinary bladder ; c, the scrotal sacs in the male ;
and — by turning aside the intestines— </, the kidneys ; and e, the ovaries
and oviducts (varying much in size according to age) in the female.

3. a, The characters and relations of the lobes of the liver (compare
Fig. 137), and the folds of peritoneum which attach the liver to the
diaphragm ; b, the gall-bladder ; -e, the gullet, and its entrance into— -f,
the stomach (note its cardiac and pyloric portions) ; g, the spleen.

4. a, The parietal and visceral layers of the peritoneum and the
various subdivisions of the mesentery ; b, the U-shaped duodenum,
passing into the ileum, and the connection of the latter with the
proximal end of — c, the catcum, at the distal end of which is the
vermiform appendix, while proximally it passes into — d, the colon,
continuous with — e, the rectum, which enters the pelvic cavity to open by
the anus.

II. Turn over the stomach to the animal's right side and make
out —

1. a, The postcaval vein, passing from the pelvis forwards, near the
ventral surface of the backbone, through a notch in the liver to the
diaphragm; b, \hedorsalaorta, running partly above, partly alongside, the
postcaval ; c, the cceliac artery, given off from the aorta about an inch
posteriorly to the diaphragm — trace its main branches ; d, the anterior
mesenteric artery, arising about half an inch further back than the
coaliac — trace its main branches (Fig. 137).

2. a, The left kidney, with its artery, vein, and ureter ; b, the
yellowish left adrenal, anterior to the origin of the renal artery and
vein ; c, the left ovary, Fallopian tube, and uterus in the female.

1 You will very probably notice a number of transparent, rounded
vesicles, about £ inch in diameter, in the abdomen. These are tape-
worms (phylum Platyhelminthes, p. 412) in the encysted stage, which
develop into the adult form when swallowed by a dog.

PRACT. ZOOL. N N

546 THE RABBIT CHAP.

3. a, The small posterior mesenteric artery, leaving the aorta a short
distance posteriorly to the left kidney and supplying the rectum ; b, the
posterior mesenteric vein, in the mesentery of the rectum.

III. Turn the intestines over to the animal's left side and spread out
the duodenum, putting its mesentery slightly on the sketch but taking
care not to rupture it, so as to make out —

1. a, The large anterior mesenteric vein, into which the posterior
mesenteric vein opens ; 6, the pancreas and its duct, opening into the
distal limb of the duodenal loop an inch or so beyond the bend.

2. a, The right kidney, partly overlapped by the caudate lobe of the
liver, and its ureter ; b, the' right adrenal.

IV. Replace the intestines in their natural position ; cut through the
gullet close to the diaphragm, draw the stomach backwards, turn for-
wards the lobes of the liver, and dissect out the following structures
(Fig. 137) :—

1. The common bile-duct — made up of cystic and hepatic ducts, and its
entrance into the duodenum.

2. a, The large hepatic portal vein, running in the mesentery ventrally
to the postcaval, made up by the union of the mesenteric, gastric,
and splenic veins, and entering the liver, sending a branch to each lobe ;
b, the transparent lymphatic vessels (lacteals] in the mesentery.

V. Tie two pieces of thread, about half an-inch apart, round the portal
vein just before it enters the liver (the hepatic artery and bile-duct may
be included in the ligature) ; cut through the rectum just anteriorly to the
pelvic cavity, and through the portal vein between the ligatures, as well
as through the mesenteric attachments of the stomach and intestine,
and remove them from the body entire. Unravel the intestine by
cutting or tearing the mesentery — except that part of it in which the
pancreas lies — and spread it out on the dissecting-board or in a large
dissecting- dish, arranging it so that the various subdivisions may easily
be distinguished from one another, and blowing it up with the blow-pipe.
In addition to the relations of these parts, note : —

1. The sacculus rotundus (Fig. 135, s. r), and the characters of the
caecum, which proximally passes insensibly into the colon.

2. The length of the intestine as a whole (about fifteen or sixteen
times that of the animal), and the relative length of its five divisions.

Sketch the entire canal.

VI. Remove and cut open the stomach and parts of the small intes-
tine, colon, and rectum : wash thoroughly, and examine under water.

xi PRACTICAL DIRECTIONS 547

Remove the caecum together with a small portion of the ileum and
colon, wash it out by directing a stream of water through it, distend
with water (it is better to harden it first with formaline) and examine in
a dish of water ; with the scissors cut away a piece of the wall here and
there, between the constrictions and in the appendix. Note : —

1. The mucous membrane ', muscular coat, vn& peritoneum.

2. The longitudinal ridges or ruga in the stomach, and the pyloric
valve.

3. The apertttre of the bile-dtict, and the villi and Peyer's patches
(p. 515) in the small intestine.

4. a, The Peyer's patches and infra-colic valve in the proximal part
of the colon ; b, the thick lymphoid tissue in the walls of the sacculus
rotundus ; c, the ileo-colic aperture and valve.

5. The spiral valve of the caecum, and the lymphoid tissue of the
vermiform appendix.

6. The characters of the mucous membrane of the caecum and large
intestine.

(Microscopic sections of the small intestine — injected and uninjected,
should also be examined, compared with Fig. 39, and the villi and
intestinal glands noted. )

VII. Ligature the postcaval at the points where it enters and leaves
the liver : remove the entire liver, noting as you do so the aorta,
gullet, postcaval, hepatic veins, and phrenic veins. Sketch the liver from
the posterior surface. Then return to the examination of the abdomen,
and make out (Fig. 140) —

1. The renal and spermatic (or ovarian} arteries and veins.

2. The caudal artery, arising from the dorsal side of the aorta.

3. The common iliac arteries, each of which gives off an ilio -lumbar
branch and divides into an external and an internal iliac, the former
becoming continuous, beyond Poupart's ligament, with the femoral.
(The course of these arteries and of the corresponding veins can be
more easily traced after the urinogenital organs have been removed —
see § IX.)

4. The ilio-lumbar veins, and the trifurcation of the postcaval into the
external iliacs and a median vein formed by the fusion of the two inter-
nal iliacs.

Sketch all these vessels later, after the removal of the urinogenital
organs.

VIII. Dissect away the peritoneum and fat from the kidneys, ureters,

N N 2

548 THE RABBIT CHAP.

and generative organs, and trace each ureter from the hilus of the
kidney backwards to the bladder, which, if contracted, may be inflated
from the urinogenital aperture. Then make out —

A. In the male (Fig. 144 A) : —

1. The penis, with the prepuce, and the aperture of the urinogenital
cana^ or urethra, which latter is lined by the soft corpus spongiosum, and
strengthened by the corpora cavernosa, which diverge proximally and
are attached to the ischia.

2. The scrotal sacs, each communicating with the abdominal cavity
by the inguinal canal, through which the corresponding spermiduct and
the spermatic artery and vein pass to the spermary, to see which and
the epididymis the scrotal sac should be slit open along its ventral wall.

3. The course of- the spermiducts.

B. In the female (Figs. 144 B and 145) : —

1. The vulva ; and the clitoris, with its corpora cavernosa, on the
ventral wall of the urinogenital canal.

2. The vagina, uteri, and Fallopian tubes with their funnels.

3. The ovaries, studded with ovisacs.

IX. Dissect away the kidneys, ureters — and in the male the scrotal
sacs, in the female the ovaries — from the surrounding parts : cut through
the pelvic symphysis with a knife, and sever the attachment of the
corpora cavernosa to the ischia. Remove the whole of the urino-
genital organs, together with the posterior end of the rectum, from the
body and after noting § VII, 3 and 4, pin them out in a dissecting-dish,
with the ventral side uppermost, taking care to preserve the natural
relations of the parts. Dissect away all the superfluous connective-
tissue and fat, separate the rectum at its cut end from the urinogenital
canal, and turn it on one side, noting the rectal and perineal glands.
Make out, in addition to the parts referred to in § VIII : —

A. In the male. The uterus masculinus and prostate, and the spermi- '
ducts passing between the former and the bladder.

Sketch the entire dissection. Then slit open the uterus masculinus so
as to see the openings of the spermiducts, remove the rectum and rectal
glands, and make out the relations of the corpus spongiosum. Make a
median incision along the whole length of the penis, beginning at the
apex and cutting through the fibrous septum between the two corpora
cavernosa : continue the incision forwards, so as to open the bladder
along its ventral wall, and note and indicate on your sketch the opening
of the uterers into the bladder, and the crescentic aperture of the uterus

xi PRACTICAL DIRECTIONS 549

masculinus into the urinogenital canal, just at the anterior edge of a
cushion -like fold.

B. In the female. The wide urinogenital canal with its vascular
walls.

Sketch the entire dissection.

Make a median longitudinal incision through the urinogenital canal,
and continue it forwards until the cavity of the bladder is exposed : slit
open the vagina and one of the uteri and Fallopian tubes in the same
way. Note and indicate on your sketch the openings of the ureters into
the bladder, the connection of the neck of the bladder with the urino-
genital canal, the large aperture in the latter leading into the vagina,
and the thick-lipped aperture (ps uteri), communicating between the
vagina and thick-walled uterus, between which and its fellow is a vesti-
gial septum, indicating the primarily paired character of the vagina.
Insert a seeker in the ccelomic aperture of the Fallopian tube. If the
uteri contains embryos they present a series of swellings ; cut each
swelling open from the ventral side, and note the foetus attached to a
discoid placenta (p. 541) ; remove each fcetus, together with its placenta,
and preserve it in corrosive sublimate (p. 135). Microscopic sections of
the ovary should be examined, and the hollow ovisacs noted, each
consisting oifollicular cells enclosing a minute ovum.

With a scalpel or razor cut across one of the kidneys through the
hilus, parallel to the dorsal and ventral faces of the organ, and note the
pelvis •, and its prolongations — the calices, the urinary pyramid, and the
cortical and medullary portions. Sketch. Examine also microscopic
preparations of injected and uninjected kidney (compare p. 146 and 537).

X. Dissect out the lumbo-sacral plexus in the pelvic cavity (p. 532), and
then cut through the body just behind the diaphragm ; the abdomen
will be no longer required, but you should retain one of the hind-limbs
if you wish to examine any of its muscles and joints (compare p. 64).

E. Dissection of the thorax and neck.

I. Dissect away the pectoral muscles, cut through all the vertebral
ribs of the left side, except the last five, at about a quarter of an inch
from their junction with the sternal ribs : from the posterior end of the
incision thus made, cut downwards (i.e., towards the sternum) for about
an inch, and then forwards, through the sternal ribs. Turn forward
the flap thus separated and carefully dissect it away from the underlying
tissues at the anterior end, so as to detach it altogether without injuring

550 THE RABBIT CHAP.

the jugular and brachial veins. Note in the thoracic cavity thus laid
open : —

1. T\\Q pericarditim and heart, and the thymus (Fig. 135).

2. The collapsed lungs : make a small aperture in the trachea and
inflate.

3. The parietal and visceral layers of the pleura, and the media-
stinum (Fig. 138). To make out the relations of these and of the
mediastinal space more accurately, cut away a flap from the right wall
of the thorax, as you have already done on the left side, leaving the
sternum intact.

4. The relations of the pericardium, to see which cut through the
posterior end of the sternum : separate the mediastinum from the dorsal
surface of the bone, turn the latter forwards and remove it : then cut
the pericardium open longitudinally, noting the pericardial fluid.

II. Dissect away the pericardium, the thymus, and any fat about the
base of the heart which may obscure the vessels arising from it. Follow
out these vessels to the head by clearing away the connective-tissue,
fat, &c., by which they are surrounded, but taking care not to injure
any of the nervt s of the neck. Make out : —

1. a, The left and right ventricles and auricles, and the ramifications
of the coronary artery and vein ; b, the two bronchi.

2. a, The ptdmonary artery and its division into left and right trunks :
b, the pulmonary veins (the course of which will be better seen at a
later stage) ; c, the two precaval veins, each formed by the union of a
subclavian, an external jugular, and a smaller internal jugular ; d, the
thoracic portion of the postcaval vein.

3. a, The gullet (cervical and thoracic portions) ; b, the phrenic
nerves (p. 532), and the thoracic portions of the vagi (Fig. 135) ;

j c, the arch of the aorta, continuous with the dorsal aorta, and the short
ligament (vestige of the foetal ductus arteriosus] connecting the former
with the pulmonary artery ; d, the thoracic portions of the sympathetic
nerves and their ganglia, lying on the heads of the ribs ; e, the azygos
vein (p. 525), best seen by turning the heart and lungs over to the left
side of the animal. The thoracic dtict (p. 528) cannot easily be made
out at this stage.

4. a, The innominate artery, giving off the left and right common
carotids and the right subclavian ; b, the left subclavian artery ; c, the
division of each common carotid, at about the level of the anterior end
of the larynx, into an external and an internal carotid.

xi PRACTICAL DIRECTIONS 551

Sketch the heart and origins of the main vessels.

III. Now dissect out certain of the nerves and other structures in the
neck, as follows : —

1. The vagiis (Fig. 135), running to the outer side of each common
carotid : with the seeker carefully separate it from the carotid, and trace
it forwards to the angle between the head and neck — at which point it
enlarges to form a ganglion, and backwards to the thorax. Branches are
given off to the larynx (anterior and posterior or recurrent laryngeal
nerves] and to the heart (depressor] nerves, but the dissection of these
may be omitted by the beginner.

2. The sympathetic (Fig. 135), running on the dorsal side of the
carotid artery. This is most easily distinguished by seizing the carotid
with the small forceps just at the junction of the head and neck and
putting it slightly on the stretch : the vagus ganglion will then be seen
on the outer side of the artery, and the more elongated anterior cervical
ganglion of the sympathetic on its inner side : trace the sympathetic
nerve from this ganglion backwards to the thorax, at the anterior end
of which (close to the subclavian artery) it enlarges to form \\\z posterior
cervical ganglion, and then becomes continuous with the thoracic portion
of the cord.

3. The hypoglossal nerve will be seen at about the level of the vagus
ganglion, passing forwards to the tongue.

4. Note — a, the thyroid gland ; b, the thyroid and cricoid cartilages of
the larynx ; c, the brachial and vertebral branches of the subclavian
artery ; and d, the brachial plexus (p. 532).

IV. Remove the heart and lungs from the body, together with the
posterior end of the trachea and recognisable portions of the aorta,
precaval, and postcaval. Fasten out the organs under water with their
ventral surface uppermost, and after making out the course of the pul-
monary arteries and veins, cut through them close to the lungs and
separate the latter from the heart. Note : —

1. The two main lobes of each lung and the two accessory lobes on the
right side.

2. The cartilages of the trachea and bronchi ; trace the bronchi for a
short distance into the lungs. Sketch.

V. Cut away the outer walls of both auricles so as to expose their
cavities, taking care not to injure the veins which enter them. Wash
out the contained clots of blood, and note —

i. a, The appendix of each auricle, and the network of muscular

552 THE RABBIT CHAP.

bands in its walls ; #, the septum auricularum and fossa ovalis (Fig.
139) — best seen from the left side by holding the heart between your
eyes and the light.

2. a, the auricula-ventricular apertures; b, the apertures of the
precaval and postcaval veins ; c, the Eustachian valve ; a, the aperture
of the coronary vein within the tunnel-like opening of the left precaval ;
<?, the apertures of the pulmonary veins.

Then cut away both auricles so as to expose the bases of the ventricles,
and remove all but about an eighth of an inch of the aorta and pul-
monary artery ; pour water into the ventricles through the auriculo-
ventricular apertures and squeeze the ventricles, noting —

3. a, The bicuspid and trictispid valves, and the semilunar valves at
the origins of the aorta and pulmonary artery respectively ; b, the
apertures of the coronary arteries just distally to the aortic valves.
Sketch.

Now remove the outer walls of both ventricles by making first a
transverse incision along the base of each, and then, from its extremi-
ties, converging incisions nearly to the apex of the heart. Make
out : —

4. The relative thickness of the walls of the right and left ventricles,
the form of the septum ventrictdorum, the cavities of the two ventricles,
and the muscular ridges in their walls.

5. The flaps of the tricuspid and bictispid valves, and their tendinous
cords and papillary imtscles (Fig. 139).

6. The apertures of the aorta and pulmonary artery into the left and
right ventricle respectively.

Sketch.

F. Dissection of the head.

I. Carefully dissect the skin away from one side of the head ; either
side will do, but if you are using the head from which you have already
removed the brain, choose the side on which the auditory region is least
damaged : notice the Meibomian glands (p. 535). Cut away, with bone
forceps, the supraorbital process of the frontal, being careful not to injure
any of the contents of the orbit. Note —

i. a, A large mass of muscle (masseter) covering the posterior half
of the mandible, on which branches of the facial nerve will be seen ;
k, the thin, irregular parotid gland, at the base of the pinna, and the
large infraorbital gland , lying mainly within the orbit below (p. 512).

xi PRACTICAL DIRECTIONS 553

2. The four recti and the two oblique muscles of the eye. Note the
course of the superior oblique through its tendinous loop (p. 534).

3. The lacrymal and Harderian glands, situated in the postero-
superior and antero-inferior regions of the orbit respectively.

II. Remove the eyeball by cutting through the muscles and optic nerve,
noting the retractor bulbi muscle around the latter. (For the dissection
of the eye, see p. 555. ) Cut off the pinna, and clear away the muscles,
&c., covering the tympanic bone: remove the entire tympanic and
periotic bones (p. 492) in one piece, lay open the external auditory
passage, and very carefully and gradually cut away the outer wall of
both tubular and bulbous portions of the tympanic with the bone-
forceps. Note :. —

1. The tympanic membrane and the handle of the malleus ; and after
cutting away the former so as not to injure the latter, and removing
rather more of the bulla, make out (Fig. 143) —

2. The tympanic cavity ; the malleus, incus, and stapes ; the tensor
tympani and stapedius muscles ; and the aperture of the Eustachian
tube. Remove the auditory ossicles, noting as you do so the fenestra
ovalis and fenestra rotunda, and examine them under the low power
of the microscope : by cutting through the periotic with the bone
forceps, the position of the membranous labyrinth, including the cochlea,
may be made out.

III. Dissect off the muscles covering the mandible on the side you
are working ; detach its ascending portion from the muscles inserted
on its inner surface, remove it entire, cutting through the symphysis,
and clear away the underlying muscles. Pass a probe from the cut end
of the gullet forwards into the mouth : lay open the gullet along this,
and pull the tongue downwards, so as to get a view into the interior of
the mouth (Fig. 135). Note : —

I. a, The palate and velum palati ; b, the tongiie and its papilla
(p. 507); c, the pharynx ; d, the glottis, and epiglottis, embraced by
the soft palate ; e, the continuation of the pharynx above the velum
palati (naso-pharynx], with which the internal nostrils communicate ;
f, the positions of the teeth (incisors and grinders) ; g, the apertures of
the naso-palatine canals.

Then remove the nasal, premaxilla, and maxilla of the same side,
noting the olfactory nerves and the turbinals (p. 534) ; and after passing
a probe backwards from the external nostril into the nasal chamber as
far as it will go, remove the turbinals on this side, so as to expose the

554 THE RABBIT CHAP.

cartilaginous nasal septum. Also cut away all the remaining bone on
the same side of the middle line, slit up the naso-pharynx, and reduce
your dissection to a neat longitudinal section, noting —

2. The cranial cavity r, the roof of which will have been cut away if
the brain has been removed.

3. The aper tit-re of the Eustachian tube : pass a probe from it into
the tympanic cavity.

4. The position of Jacobsotfs organ, on the ventral side of the nasal
septum ; and after cutting away the latter, the ethmo-turbinals, maxillo-
turbinals, and naso-turbinals of the other side (p. 493, Fig. 135).

Sketch the entire longitudinal section.

IV. Remove the larynx with the anterior part of the trachea. Make
a longitudinal vertical section of it, keep one half entire, and from the
other dissect away the muscles and mucous membrane so as to see the
cartilages clearly. Examine first the cartilages and then the soft parts,
noting the relations of the thyroid, cricoid, and arytenoid cartilages, the
epiglottis, and the vocal cords (p. 517). Sketch.

G. The Brain.

I. — External characters. Note (Fig. 141): —

1. The medulla oblongata, with its dorsal and ventral fissures and
the pons Varolii ; the convoluted cerebellum, consisting of a central lobe
and two lateral lobes &x\&flocculi.

2. The optic lobes, seen on slightly separating the hemispheres and
cerebellum ; the crura cerebri.

3. The oljactory lobes and cerebral hemispheres (frontal, parietal, and
temporal lobes'] — by gently separating them, the corpus callosum will
be seen, and just behind them the pineal body ; the diencephalon is
covered above by the hemispheres, below it is visible and is continued
into the infundibulum ; the pittiitary body is nearly always left
behind in the brain-case when the brain is removed, and a small
aperture is seen in the centre of the infundibulum where it was
attached ; the optic chiasma.

4. The origins of the nerves from the brain ; these cannot, of course,
be plainly made out unless the nerves have been carefully cut through
when removing the brain. Compare pp. 163-166, and note in addition
the spinal accessory and hypogiossaL

Sketch the brain from above and from below.

II. Carefully remove successive slices from one of the hemispheres

xi PRACTICAL DIRECTIONS 555

nearly parallel to its upper surface down to the level of the corpus
callosum, noting the fibres of the latter and the relations of the grey
and white matter in the hemispheres. Cut through the corpus
callosum longitudinally, near the middle line, and remove sufficient
of the hemisphere already dissected to expose the lateral ventricle
and its extensions into the frontal, parietal, and temporal lobes.
Note : —

1. The hippocampus ; the corpus striatum, the choroid plexus, and
the septum lucidum (p. 531). Then cut away the rest of the same
hemisphere so as to expose —

2. a, The diencephalon and its two lateral halves (optic thalamt)
between which the third ventricle is enclosed ; from its roof the stalk
of the pineal body arises ; b, the two pairs of optic lobes. Cut away the
lateral half of the cerebellum of the same side, and note —

3. The valve of Vieussens, the fourth ventricle, and the tree-like
appearance (arbor vitce] of the cerebellum in section.

III. Now reduce the whole brain to a longitudinal vertical section
(Fig. 142), cutting it through with a sharp knife very slightly to that
side of the median line which you have already dissected, so as to
expose the median ventricles in section. Make out, on the uninjured
half, the relations of the third ventricle, tier, fourth ventricle, foramen
of Monro, infundibulum, optic chiasma, corpus callosum and fornix,
lamina terminalis, anterior, middle, and posterior commissure, crtira
cerebri, valve of Vieussens, pons Varolii, &c. Sketch.

IV. A series of thick transverse sections should be made through
another thoroughly hardened brain, and the relations of the parts
already seen by dissection examined.

H. The Eye,

On account of its larger size, it is better to substitute the eye of an ox
or sheep — which can be- obtained fresh from a butcher — for that of the
rabbit. Two specimens should be obtained, and the fat, muscles,
and any portions of the eyelids remaining attached cleared away.
Dissect under water.

I. After noting the conjunctiva, cornea, sclerotic, optic nerve, iris, and
piipil, insert the scissors into the margin of the cornea and cut round
and remove it, so as to expose the aqueous chamber, noting the aqueous
humour, as well as the iris, lens, and ptipil (compare Fig. 57).
Sketch.

55 THE RABBIT CHAP.

Insert the handle of a scalpel between the iris and sclerotic and
separate off the latter from the choroid around the outer half of the eye-
ball, first making four radial cuts in the sclerotic, beginning at its
margin, at equal distances from one another. Turn back the four flaps
thus made, and insert pins through them so as to fix the eye firmly
down under water with the iris uppermost : note the ciliary muscle around
the outer margin of the iris, the choroid , and the ciliary vessels and
nerves. Then make two radial cuts, a short distance from one another,
through the iris, and turn back the portion between the cuts, noting the
ciliary processes. Sketch.

Cut through the choroid and the parts enclosed by it, horizontally,
around the line up to which it has been already exposed, so as to
separate the eyeball into an inner and outer hemisphere : in doing so,
the gelatinous vitreotis humour must be cut with the scissors. Examine
and sketch both sections, noting in the outer hemisphere — the retina,
stopping short at the outer margin of the ciliary processes, and the lens
with its capstde ; and in the inner hemisphere — the retina with its blood-
vessels, and the blind-spot (or point of entrance of the optic nerve).
Then remove the retina and observe the choroid with its iridescent
tapetum.

2. Cut your second specimen into vertical halves with the scissors,
making the cut pass through the cornea, pupil, and optic nerve. As it
is difficult to cut through the lens without disturbing its relations, it
should be carefully separated from one half, and left entire on the other
half. Examine the relations of the parts once more, compare Fig. 57,
and sketch.

I- Dissection of typical flexor and extensor muscles and
joints of the fore -limb.

1. Expose the biceps (chief flexor of the fore-arm). Its origin is
from the anterior edge of the glenoid cavity on the pectoral arch : it
arises by a single long tendon, working in the bicipital groove of the
humerus. It is spindle-shaped, consisting of a single belly, and is
inserted on the proximal end of the radius.

2. Expose the triceps (chief extensor of the fore-arm). It arises
by three main heads from the pectoral arch and humerus, and is inserted
on the olecranon-process of the ulna.

3. Remove the muscles from the shoulder-joint, and note the
capsular ligament (p. 55). Cut through this so as to open the svnovial

xi PRACTICAL DIRECTIONS 557

capsule, and note the synovial membrane 2^^ fluid and the cartilaginous
articular stirfaces of the glenoid cavity and head of the humerus
respectively : or, prepare as directed on p. 64 in the case of the hip-
joint of the frog.

4. As an example of a muscle with a multiple insertion, dissect out
the extensor communis digitorum (dorsal). It arises from the distal
end of the humerus, and at the distal end of the fore-arm divides into
four tendons, which pass through the annular ligament to digits 2-5
and are inserted on the middle and distal phalanges. Note the
sesamoid bones on the palmar side of the joints of the digits.

J. Side dissection,

Obtain another rabbit, and make a dissection from the side (compare
Fig. 135) as directed in the case of the dogfish and frog (p. 481).
Make out the structure and relations of all the principal organs once
more, and sketch your dissection.

K. Transverse sections.

The examination of transverse sections is most easily and satisfactorily
done by preparing microscopical sections of a foetus not more than
a couple of inches in length, as directed in the case of the young
dogfish (p. 480) : a foetal rat or mouse will answer the purpose equally
well.

With a sharp knife, cut transverse sections, about Jth inch thick, from
the following regions : #, snout ; $, cranial region ; c , neck ; d, thorax ;
and e, abdomen. Stain and imbed, and then cut and mount one or
two sections of each, examining them with a low power and making
sketches.

Then put on the high power, and make out as much as possible of
the histology of the various organs.

[For the blood-corpuscles, examine a drop of blood obtained from a
freshly-killed rabbit or by pricking the tip of your finger (compare
p. 121). Note the circular and slightly biconcave form of the red
corpuscles, their relatively small size (about -g-vjVtf in. or *oo8 mm. in
diameter), and the absence of a nucleus.]

CHAPTER XII

THE MINUTE STRUCTURE OF CELLS : CELL-DIVISION :
STRUCTURE OF THE OVUM : SPERMATOGENESIS AND
OOGENESIS : MATURATION AND FERTILIZA1 ION OF THE
OVUM I SEGMENTATION OF THE OOSPERM : EFFECT OF
FOOD-YOLK ON DEVELOPMENT : FORMATION OF THE
CHIEF ORGANS OF VERTEBRATES, AND OF THE AMNION,
ALLANTOIS AND PLACENTA.

Structure of the Cell. — We have learnt in previous chap-
ters that all organisms are formed essentially of one or more
cells (compare e.g., pp. no and 1 18) ; that the cell consists of
protoplasm, and contains a nucleus, enclosing chromatin, and
usually one or more nucleoli (p. 129) ; and that cells multiply
by a process of binary fission (p. 106). It will now be
necessary to study the structure of a typical animal-cell
and of its mode of division in somewhat greater detail.

There seems to be a good deal of variation in the precise
structure of various animal- and plant-cells, but the more
recent researches show that in the cell-body or protoplasm
two constituents may be distinguished, a clear semi-fluid
substance, traversed by a delicate sponge-work. Now
under the microscope the whole cell is not seen at once
but only an optical section of it — that is, all the parts which

CHAP, xii STRUCTURE OF CELLS 559

are in focus at one time (see e.g. Fig. 71 B) : by altering the
focus we view the object at successive depths, each view
being practically a slice parallel to the lenses of the
instrument. This being this case, protoplasm presents the
microscopic appearance of a clear or slightly granular
matrix traversed by a delicate network. In the epithelial
cells of animals the protoplasm is bounded externally by
a delicate cell-membrane : in amoeboid cells the ectoplasm,
or transparent, non-granular portion of the cell consists
of clear protoplasm only, the granular endoplasm alone
possessing the sponge-work.

It is quite possible that the reticular character of the cell may be
merely the optical expression of an extensive but minute vacuolation,
or may be due to the presence of innumerable minute granules
developed in the protoplasm as products of metabolism.

The nucleus is spherical or oval in form, and is enclosed
in a delicate nuclear membrane (Fig. 146 A). Its contents
consist of a homogeneous semi-fluid substance throughout
which an extremely fine network of threads extend : in this
framework are imbedded granules of chromatin (chr\ which
are distinguished by their strong affinity for aniline and
other dyes (compare e.g. Part I., Chapters VII and VIII).
Frequently, as we have seen, one or more minute globular
structures, the nucleoli (kk\ occur in the nucleus and
are to be looked upon as transient structures and not as
actual living parts of the cell : they also have a strong
affinity for dyes, although often differing considerably from
the chromatin in their micro-chemical reactions.

Close to the nucleus a small globular body, called the
centrosphere (cspti), enclosing a minute particle or centrosome,
is present in the cell ; it is inconspicuous and often difficult
to recognise, but probably occurs in all cells which are
capable of division.

A

FIG. 146 — Diagram of nuclear division.

A, resting-cell, with cell-substances (zk) centrosphere (csfih) containing two
centrosomes, nucleolus (,kk), and chromatin (chr), the last distributed in the
nuclear network ; the whole nucleus is surrounded by a nuclear membrane.
B, the chromatin united in a coiled thread ; the centrosphere divided into two
and giving off rays which unite the halves. C, the nuclear spindle (ksp) formed,
the rays more strongly^ developed, the nuclear membrane (k?ri) in process of
dissolution, the chromatin thread divided into eight similar chromosomes (chrs) ;

tAP. xii CELL-DIVISION 561

the rays are becoming attached to the chromosomes. Z>, perfected nuclear
spindle with the two centrospheres at the poles (cspti) and the eight chromo-
somes (chrs) in the equator of the spindle, all now longitudinally split. E,
daughter chromosomes diverging from one another, but still united by filaments ;
the centrosomes (cs) are already doubled for the next division. F, daughter
chromosomes, quite separated from one another, are already beginning to give
off processes ; the cell-body is beginning to be constricted. G, end of the
process of division ; two daughter-cells (tz) each with nuclear network (tk)
and centrospheres (csph) as in A. (From Weismann's Evolution Theory,
adapted from E. B. Wilson.)

Cell-division. — The precise changes which take place
during the fission of a cell are, like the structure of the cell
itself, subject to considerable variation. We will consider
what may probably be taken as a typical case (Fig. 146).

First of all, the chromatin grains (chr) came together so
as to form a loose coil or skein (B). The centrosome, and
then the centrosphere (csph\ divides, and from the latter
fine protoplasmic filaments are seen to radiate ; the products
of its division gradually separate from one another (c),
ultimately passing to opposite poles of the nucleus (D),
delicate threads extending from one to another in the form
of a spindle (ksp). At the same time the nuclear mem-
brane and the nucleoli usually disappear, and the chromatin
skein divides into a number of separate pieces of equal
length called chromosomes (c, chrs\ the number of which
appears to be constant in any given species of animal or
plant, although it may vary in different species from two
to a hundred and sixty-eight or more.

The chromosomes become arranged in the equatorial
plane of the spindle and each of them splits along its whole
length, so as to form two parallel rods or loops in close
contact with one another (D), and arranged in a radiating
manner so as to present a star-like figure when the cell
is viewed in the direction of the long axis of the spindle : in
this way the number of chromosomes is doubled, each one
being now represented by a pair. Everything is now ready

PRACT. ZOOL. O O

562 CELL-DIVISION CHAP.

for division, to which all the foregoing processes are pre-
paratory.

The two chromosomes of each pair now gradually pass to
opposite poles of the spindle (E), two distinct groups
being thus produced, and each chromosome of each group
being the twin of one in the other group. The mechanism
by means of which this is effected is not definitely known :
possibly the fibres of the spindle are the active agents in the
process, the chromosomes being dragged in opposite
directions by their contraction : on the other hand it is
possible that the movement is due to the contractility of
the chromosomes themselves.

After reaching the poles of the spindle (F), the chromo-
somes of each group unite with one another to form a
network around which a nuclear membrane finally makes
its appearance (G). In this way two nuclei are produced
within a single cell, the chromosomes of the daughter-
nuclei^ as well as their attendant centrosomes, being formed
by the binary fission of those of the mother-nucleus.

But pari passu with the process of nuclear division,
fission of the cell-body is also going on. This takes place
by a simple process of constriction (F, G) — in much the same
way as a lump of clay or dough would divide if a loop of
string were tied round its middle and then tightened.

In comparatively few cases the dividing nucleus, instead of
going through the complicated processes just described,
divides by simple constriction ; but this seems to occur only
in the case of certain highly-differentiated cells or of worn-out
cells. We have therefore to distinguish between ///rar/and
indirect nuclear division.1

In this connection the reader will not fail to note the

1 To the latter very elaborate method the name mitosis or karyo-
kinesis is applied : direct division is then distinguished as amitotic.

[i STRUCTURE OF OVUM 563

extreme complexity of structure revealed in cells and their
nuclei by the highest powers of the microscope. When the
constituent cells of the higher animals and plants were dis-
covered, during the early years of the present century, by
Schleiden and Schwann, they were looked upon as the
ultima Thule of microscopic analysis. Now the demonstra-
tion of the cells themselves is an easy matter, the problem
is to make out their ultimate constitution. What would be
the result if we could get microscopes as superior to those
of to-day as those of to-day are to the primitive instruments
of eighty or ninety years ago, it is impossible even to con-
jecture.

Structure of the Ovum. — The striking general resemblance
between the cells of the higher animals and entire unicellular
organisms has been commented on as a very remarkable
fact : there is another equally significant circumstance to
which we must give our attention.

All the higher animals begin life as an egg, which is either
passed out of the body of the parent, as such, as in earth-
worms, crayfishes, frogs, birds, &c. (oviparous forms\ or
undergoes development within the body of the parent, as
in some dogfishes (p. 470) and nearly all mammals
(vivipa rous forms) .

The structure of an egg is, in essential respects, the same
in all animals from the highest to the lowest (compare
p. 195). It consists (Fig. 147) of a more or less globular
mass of protoplasm, spoken of as the vitellus, in which
are deposited particles of a proteinaceous substance known
as yolk-granules. Within the protoplasm is a large nucleus
containing chromatin as well as one or more nucleoli —
which are often known as germinal spots, the entire nucleus
of the ovum being sometimes called the germinal vesicle.

002

564

STRUCTURE OF OVUM

In other words the egg, as we have already seen,'is*a cell.
An investing cuticular membrane is usually present.

The young or immature ova of all animals present this
structure, but in many cases certain modifications are
undergone before the egg is fully formed. For instance, the
protoplasm may throw out pseudopods, the egg becoming
amoeboid (p. 307) ; or, as
mentioned above, the sur-
face of the protoplasm
may secrete a membrane
or cell- wall often of con-
siderable thickness, and
known as the vitelline
membrane (p. 196 and Fig.
147), which may be per-
forated at one pole by an
aperture, the micropyle (p.
415 and Fig. 150, B, c).
The most extraordinary
modification takes place
in some Vertebrata, such
as dogfishes (p. 470) and
birds. In a hen's egg, for instance (Fig. 148), the yolk-
granules increase immensely, swelling out the microscopic
ovum until it becomes what we know as the " yolk " of the j
egg : around this layers of albumen or " white " are de-
posited by the glands of the oviduct, and finally the shell-
membrane and the shell. Hence we have to distinguish
carefully in eggs of this character between the entire " egg "
in the ordinary acceptation of the term, and the ovum or
egg-cell. But complexities of this sort do not alter the
fundamental fact that all the higher animals begin life as
a single cell ; or in other words, that multicellular animals,

FIG. 147. — Ovum of a Sea-urchin (Toxo-
pneustes lividus\ showing the radially-
striated cell-wall (vitelline membrane),
the protoplasm containing yolk-granules
(vitellus), the large nucleus (germinal
vesicle) with its network of chromatin,
and a large nucleolus (germinal spot).
(From Balfour's Embryology ; after
Hertwig.)

SEX-CELLS

565

however large and complex they may be in their adult
condition, originate as unicellular bodies of microscopic
size ; and the same is the case with plants.

Spermatogenesis and Oogenesis. — In the preceding
chapters it has more than once been stated that sperms

sh.m

alb

CLlb

FIG. 148. — Semi-diagrammatic view of the egg of the fowl at the beginning of

incubation.

a. air-space ; alb. dense layer of albumen ; alb', more fluid albumen ; bl. blasto-
derm ; ch. chalaza, a twisted cord of the dense layer of albumen at either end
of the egg, formed as the latter rotates down the oviduct ; sh. shell ; sh. m. shell-
membrane ; sh.m. /, sk.m. 2, its two layers separated and enclosing air-cavity.
(From Parker and Haswell's Zoology, after Marshall, slightly altered.)

arise from ordinary undifferentiated cells in the spermary,
and that ova are produced by the enlargement of similar
cells in the ovary. Fertilisation has also been described
as the conjugation or fusion of two gametes, the ovum and
sperm (compare p. 197). We have now to consider in
greater detail what is known as to the precise mode of

566 SPERMATOGENESIS CHAP.

development of sperms (spermatogenesis] and of ova
(oogenesis), as well as the exact steps of the process by
which an oosperm or unicellular embryo is formed by the
union of the two sexual elements.

Both ovary and spermary are at first composed of cells of
the ordinary kind, the primitive sex-cells ; and it is only by
the further development of these that the sex is determined.

In the spermary the sex-cells (Fig. 149, A) undergo re-
peated fission, forming what are known as the sperm-mother-
cells (B). These have been found in several instances to be
distinguished by a peculiar condition of the nucleus. We
saw (p. 561) that the number of chromosomes is constant
in any given animal, though varying greatly in different
species. At an earlier stage in the formation of the sperm-
mother-cells from the primitive sex-cells the number
becomes doubled : in the case of the mole-cricket, for
instance, shown in Fig. 149, twelve chromosomes can be
recognised in the ordinary cells of the body, including the
primitive sex-cells, while the sperm-mother-cells contain
twenty-four.

The sperm-mother-cell now divides (c), but instead of its
chromosomes splitting in the ordinary way (p. 561, Fig. 146)
half of their total number — in the present instance twelve —
passes into each daughter cell : thus two cells are pro-
duced having the normal number of chromosomes. The
process of division is immediately repeated in the same pecu-
liar way (D), the result being that each sperm-mother-cell gives
rise to a group of four cells having half the normal number
of chromosomes — in the present instance six: this is there-
fore a reducing division. The four cells thus produced are
the immature sperms (E) : in the majority of cases the
protoplasm of each undergoes a great elongation, being
converted into a long vibratile thread, the tail of the sperm

SPERMATOGENESIS

567

(F, G), while the nucleus constitutes its more or less spindle-
shaped head, and the centrosome is included in a small
middle piece at the junction of head and tail.

Thus the sperm or male gamete is a true cell, specially

FIG. 149. — Spermatogenesis in the Mole-Cricket (Gryllotalpa).

A, primitive sex-cell, just preparatory to division, showing twelve chromosomes
(chr) ; c. the centrosome. B, sperm-mother-cell, formed by the division of A,
and containing twenty-four chromosomes ; the centrosome has divided into two.
C, the sperm-mother-cell has divided into two, each daughter-cell containing
twelve chromosomes. D, each daughter-cell has divided again by a reducing
division, a group of four sperm-cells being produced, each with six chromosomes.
E, a single sperm-cell about to elongate to form a sperm. F, immature sperm ;
the six chromosomes are still visible in the head. G5 fully formed sperm.
(From Parker's Biology ', after vom Rath.)

modified in most cases for active movement. This actively
motile, tailed form is, however, by no means essential : in

568 OOGENESIS CHAP.

some animals (e.g. crayfish, p. 382) the sperms are non-
motile and of peculiar form.

As already stated, the ova arise from primitive sex-cells,
precisely resembling those which give rise to sperms. These
divide and give rise to the egg-mother-cells in which, as in
the sperm-mother-cells, the number of chromosomes has
become doubled. The egg-mother-cells do not immediately
undergo division, but remain passive and increase in size
by the absorption of nutriment from surrounding parts : in
this way each egg-mother-cell becomes an ovum. Sometimes
this nutriment is simply taken in by diffusion or osmosis ;
in other cases the growing ovum actually ingests neighbour-
ing cells after the manner of an Amoeba. Thus in the
developing egg the processes of constructive are vastly in
excess of those of destructive metabolism.

We have seen (p. 249) that the -products of destructive
metabolism may take the form either of waste products
which are got rid of, or of plastic products which are stored
up as an integral part of the organism. In the developing
egg, in addition to increase in the bulk of the protoplasm
itself, a formation of plastic products usually goes on to an
immense extent. In plants the stored-up materials may
take the form of starch, of oil, or of proteid substance : in
animals it consists, as mentioned above, of rounded or
angular grains of proteid material, known as yolk-granules.
These being deposited, like plums in a pudding, in the proto-
plasm, have the effect of rendering the fully-formed egg
opaque, so that its structure can often be made out only
in sections.

Maturation of the Ovum. — The fully-formed ovum as
described on p. 563, is, however, incapable of being
fertilised or of developing into an embryo : before it is ripe

xii MATURATION 569

for conjugation with a sperm or able to undergo the first
stages of segmentation it has to go through a process known
as maturation.

Maturation of the ovum consists essentially in a twice
repeated process of cell-division, and thus resembles the
process just described in the case of the sperms. The
nucleus (Fig. 150, A), which as we have seen contains
double the normal number of chromosomes, loses its mem-
brane, travels to the surface of the egg, and takes on the
form of an ordinary nuclear spindle. Next the protoplasm
grows out into a small projection or bud, into which one end
of the spindle projects. Nuclear division then takes place,
without a splitting of the chromosomes, one of the daughter
nuclei remaining in the bud (pol\ the other in the ovum
itself. Then follows as usual a division of the protoplasm,
and the bud becomes separated as a small cell distinguished
as the first polar cell.

In some cases development from an unfertilised female gamete takes
place, the process — which is not uncommon among insects (e.g. the
common little green plant-louse or Aphis] and crustaceans (e.g. water-
fleas) — being distinguished as parthenogenesis. It has been proved that
in the majority of such cases the egg begins to develop after the forma-
tion of the first polar cell, when the ovum contains the number of
chromosomes normal to the species.

In the majority of cases, development takes place
only after fertilisation, and in these maturation is not
complete until a reducing division (p. 566) has occurred in
the formation. of a second polar cell (B, pol}. The ovum has
now lost a portion of its protoplasm together with three-
fourths of its chromatin, half having passed into the first polar
cell and half of what remained into the second : the remaining
one-fourth of the chromatin becomes enclosed in a nucleus,
which is distinguished as the female pronncleus (B, ^prori).

f>ol

ycent

2 pro ft

FIG. 150. The maturation and impregnation of the animal ovum (diagrammatic).

A, the ovum, surrounded by the vitelline membrane (inem\ in the act of forming
the first polar cell (Pol); 9 cent, centrosome. B, both polar cells (/W) are
formed, the female pronucleus (9 /TWZ.) lies near the centre of the ovum, and
one of the several sperms is shown making its way into the ovum at the micropyle
(microp). C, the head of the sperm has become the male pronucleus (6 pron),
its middle piece the male centrosome (<J cent) : other structures as before.
D, the male and female pronuclei are in the act of conjugation. E, conjugation
is complete, and the segmentation nucleus (seg. nucl) formed. (From Parker
and Haswell's Zoology.)

CHAP, xii FERTILISATION 571

The first polar body has been observed to divide after
separating from the egg, so that the egg-mother-cell or
immature ovum gives rise to a group of four cells — the
mature ovum, and three abortive ova or polar-cells, just as
the sperm-mother-cell gives rise to a group of four cells, all
of which, however, become sperms (Fig. 149).

Fertilisation of the Ovum. — Shortly after maturation, the
ovum is fertilised by the conjugation with it of a single
sperm. As we have found repeatedly, sperms are produced
in vastly greater numbers than ova, and it often happens
that a single egg is seen quite surrounded with sperms, all
apparently about to conjugate with it (Fig. 150, B). It has,
however, been found to be a general rule that only one of
these actually conjugates : the others, like the drones in
a hive, perish without fulfilling the one function they are
fitted to perform.

The successful sperm (B) takes up a position at right
angles to the surface of the egg and gradually passes
through the micropyle (microp) or works its way through
the vitelline membrane until its head lies within the egg-
protoplasm. The tail is then lost, and the head, accom-
panied by the centrosome (see p. 567), penetrating deeper
into the protoplasm, takes the form of a rounded body,
the male pronudeus (c, $pron).

The two pronuclei approach one another (D) and finally
unite to form what is called the segmentation-nucleus (E, seg.
nuct)) the single nucleus of what is not now the ovum but
the oosperm—\}\e impregnated egg or unicellular embryo
(compare pp. 197 and 198).

The fertilising process is thus seen to consist essentially
in the union of two nuclear bodies, one contributed by the
male gamete or sperm, the other by the female gamete or

572 SEGMENTATION OF OVUM CHAP.

ovum. It follows from this that the essential nuclear matter
or chromatin of the oosperm — often spoken of as the germ-
plasm and probably constituting the vehicle of heredity —
is derived in equal proportions from each of the two
parents. Moreover, as both male and female pronuclei con-
tain only half the number of chromosomes found in the ordi-
nary cells of the species, the union of the pronuclei results
in the restoration of the normal number to the oosperm ;
and as the reducing division and conjugation of the germ-cells
thus apparently render possible very varied permutations and
combinations of the hereditary qualities of both sexes, these
processes seem to have a great importance in connec-
tion with the phenomenon of individual variation (p.
216).

Fertilisation being thus effected, the process of segmenta-
tion or division of the oosperm takes place as described in
previous chapters.

Different Types of Ova and their Segmentation. — Before
passing on to consider further details in the process of
development of the oosperm, we must briefly refer to some
differences already noted in the ova of different animals.

We have seen that in all cases the immature egg is a
simple, minute cell, but that owing to the deposition of
yolk-granules in its protoplasm, it may reach a compara-
tively large size (e.g crayfish, dogfish, bird). The pres-
ence of a greater or less amount of yolk in the ovum
results, as we know, in very considerable differences as
regards its mode of segmentation, as well as in its subsequent
development. The minute eggs of the lancelet and
rabbit, for instance, which are each only yg- mm. (about
O^-Q- inch) in diameter, contain so comparatively small an
amount of food-yolk as not to interfere materially with the

xii SEGMENTATION OF OVUM 573

process of segmentation : such ova are called alecithal.
When the quantity of food-yolk is relatively greater, it may
become accumulated towards the centre of the egg, even-
tually leaving a layer of protoplasm comparatively free from
yolk round the periphery (centrolecitkal ova, e.g. crayfish,
Fig. 98) ; or, as in the case of teloledthal ova (Figs. 64,
128, and 148), the yolk-granules may become aggregated
more at the lower than at the upper pole (frog), until
in the most extreme cases there is only a layer of yolkless
protoplasm, the germinal disc (dogfish, bird) — lying at ^.the
upper pole of the egg.

As yolk is an inert substance, the more of it an egg
contains the less actively can the latter divide, and the
quantity may be relatively so great in some parts as to pre-
vent segmentation in these parts altogether. We can there-
fore distinguish between holoblastic oosperms, which undergo
entire segmentation (eg., hydroids, earthworm, mussel,
lancelet, frog, rabbit), and meroblastic oosperms, in which
segmentation is limited to that part of the egg in which the
protoplasm is comparatively free from yolk (e.g. crayfish,
dogfish, bird), this portion, after segmentation, being known
as the blastoderm. In the centrolecithal ovum it is evident
that the segmentation must be superficial or peripheral
(p. 383), and in the meroblastic telolecithal ovum discoid,
or restricted to a small germinal disc at its upper pole
(Figs. 128 and 148). In the case of holoblastic ova the
segmenting cells or blastomeres may be equal, or nearly equal,
in size (e.g. lancelet, rabbit) ; or if the yolk is present
in greater quantities towards the lower pole, unequal (e.g.
earthworm, frog).

The influence of the food-yolk in modifying the early
processes of development is thus evidently very great, and
in order to understand these processes in their simplest form

574 DEVELOPMENT OF THE LANCELET CHAP.

it is necessary to select for our study an alecithal holo-
blastic egg, such as that of the lancelet.

Before reading the rest of this chapter, however, you should once
more glance at the account of the development of the tadpole given on
pp. 198-210, in order to refresh your memory of some of the more
important facts and of the terms used.

Development of the Lancelet. — The oosperm of Am-
phipxus (Fig. 151, A) undergoes binary fission (B), each
of the two resulting cells dividing again into two (c, D).
This process is continued until a globular mass of cells or
blastomeres is produced by the repeated division of the
one cell which forms the starting point of the series.
Owing to there being rather more yolk at the lower than at
the upper pole, the lower cells are slightly larger than the
upper, so that the segmentation is not quite equal (E — K).
The embryo has now arrived at the polyplast or morula
stage, and sections show that it is hollow, the blastomeres
being arranged in a single layer around a central segmenta-
tion-cavity (p. 200) : such a hollow polyplast is often known
as a blastula (K). The lower side of the blastula then
becomes tucked in, or invaginated, the result being that the
single-layered sphere is converted into a double-layered cup
(Fig. 152). This process can be sufficiently well imitated
by pushing in one pole of a hollow india-rubber ball with
the finger. The resulting embryonic stage is known as
the gastrula (c) : its cavity is the primitive enteron
or archenteron (p. 201), and is bounded by the invaginated
cells which now constitute the endoderm, the remaining cells
forming the outer wall of the gastrula being the ectoderm
(p. 202). The two layers are continuous at the aperture
of the cup, the gastrula-mouth or blastopore. Between the

xit DEVELOPMENT OF THE LANCELET $7$

ectoderm and endoderm is at first a space, the greatly dimi-
nished segmentation-cavity, which gradually becomes entirely

FIG. 151. — Stages in the segmentation of the oosperm of Amphioxus.
E, represents the four-celled stage (C) from above; H, vertical section of G ; K,
vertical section of the blastula stage (I). (From Korschelt and Heider's
E)}ibryolo^v, after Hatschek.)

576

DEVELOPMENT OF THE LANCELET

obliterated, so that the ectoderm and endoderm are in con-
tact (A, B). The general resemblance of the gastrula to a
simplified Hydra,1 devoid of tentacles, will at once be ap-
parent, and the stage in the development of the frog's egg re-
presented in Fig. 64, F, though much modified by the quantity
of food-yolk, will be seen to correspond to the gastrula-stage.
As in the frog, the blastopore soon closes, the mouth and

FIG. 152.— Three stages in the formation of the gastrula of Amphioxus. In A
the nuclei of the endoderm-cells are omitted. (From Korschelt and Heider's
Embryology, after Hatschek.)

anus being subsequently formed from the stomodeeum and
the proctodseum respectively (p. 204).

The gastrula becomes elongated, flattened on one side,
and convex on the other. The flattened side corresponds

1 It must, however, be remembered (pp. 308 and 321) that the
ectoderm and endoderm of Hydroids are differentiated before the
mouth is formed, so that the mouth does not correspond to the blasto-
pore of the gastrula.

DEVELOPMENT OF THE LANCELET

577

to the dorsal surface of the adult, and the blastopore
now comes to be situated, as in the frog-embryo (Fig. 64
H, K), at the posterior end of the dorsal surface. A

i- — mk

FIG. 153. — Four stages in the development of the notochord, nervous system, and

mesoderm of A mphioxu s.

ak. ectoderm ; ch notochord ; dh. cavity of archenteron ; hb. ridge of ectoderm
growing over medullary plate ; ik. endoderm ; //z. coelome ; mk. coelomic pouch ;
w/k1. parietal, and ink'*, visceral layer of mesoderm ; mp. medullary plate ; n.
medullary cord; ns. mesodermal segment (' ' protovertebra "). (From Korschelt
and Heider's Embryology, after Hatschek.)

medullary plate and groove (Fig. 153, mp) are then formed,
the central nervous system being developed in a manner
essentially similar to that already described in the

PR ACT. ZOOL. P

578

DEVELOPMENT OF THE LANCELET

case of the tadpole (p. 202), except that the central
canal of the medullary cord (//) is formed after the
plate has become separated from the outer ectoderm.
In the mid-dorsal line a thickening of the endoderm
(cJi) soon becomes constricted off to form the notochord

FIG. 154. — Embryo of Amphioxvs.

A, in vertical section, slightly to the left of the middle line; B, in horizontal
section, ak. ectoderm ; en. neurenteric canal ; dk, ud. archenteron ; ik. endo-
derm ; ink. mesodermal folds ; n. medullary canal ; us. first ccelomic pouch ;
nsh. coelomic cavity ; V. anterior ; H ' . posterior end. (From Korschelt and
Heider's Embryology, after Hatschek.)

(pp. 203, 419 and 441), and on either side of this
a series of hollow endodermic pouches arise, arranged
metamerically (Figs. 153, mk, and 154, us, mfc). The
cavities of these, which subsequently give rise to the body
cavity ccelome (Ih, usJi), are thus at first in free communica-
tion with the archenteron and are known as ehteroc&les ;

xii SEGMENTATION IN VARIOUS TYPES 579

from their walls the mesoderm is derived. Subsequently
the communications between the enteric and enteroccelic
cavities become closed, and the paired pouches gradually
extend between the ectoderm and endoderm, both dorsally
and ventrally (Fig. 153, c, D), their outer walls (parietal or
somatic layer of the mesoderm, mJP) being in contact with
the ectoderm and forming with it the somatopleure or body-
wall, and their inner walls (visceral or splanchnic layer of
the mesoderm, ;;//£2) in contact with the endoderm and
with it forming the splanchnopleure or wall of the enteric
tube (compare p. 203). Thus the body-wall and the enteric
canal are separated by a cavity, the ccelome (D, lh}> which,
much as in the adult earthworm, is divided into a series
of metamerically arranged portions : later on, however, the
adjacent walls of these coelomic sacs disappear, and the
ccelome becomes a continuous cavity.

The embryo lancelet is hatched soon after reaching
the gastrula-stage, when it moves about by means of cilia
developed on the ectoderm cells and has to get its own
living, having by this time used up its small reserve of
yolk. It then passes through a complicated series of
larval stages, gradually leading up to the adult form.

Early development of other types. — The presence of a
greater amount of food-material in the egg renders it-
possible for the embryo to go on developing further than
the gastrula-stage before being hatched, and as a general
rule, the greater the relative quantity of yolk present in the
ovum of an animal, the less clearly can a gastrula-stage be
recognised.

In the earthworm and mussel the segmentation is entire, but unequal,
and the larger, lower cells become invaginated to form the endoderm
and archenteron while the smaller upper cells give rise to the ectoderm.

P P 2

580

GASTRULA-STAGE

In the earthworm the blastopore does not become closed, but gives
rise to the mouth.

In the frog (p. 201) the archenteron arises by a split appearing
amongst the yolk-cells, beginning at the edge of the blastopore
and gradually extending forwards : the process is probably supplemented
by a limited amount of invagination of the ectoderm. The archenteron
is at first a very narrow cleft, but soon widens considerably (Fig. 64,
F, ent] : for some time it does not actually communicate with the ex-
terior, the blastopore (blp] being filled up by a yolk-plug (yk. pi}.

hd

a-r.op

pr..

mes

FIG. 155. — Two stages in the development of the blastoderm of the chick, at about

the twentieth and twenty-fourth hour of incubation respectively ; diagrammatic.
ar. op. area opaca ; ar.pl. area pellucida ; hd. head ; ined.gr. medullary groove ;
mes. mesoderm, indicated by dotted outline and deeper shade ; pr. am. pro-
amnion ; pr. st. primitive streak and groove : pr. v. mesodermal segments or
" protovertebrae." (From Marshall's Embryology, in part after Duval.)

As the archenteron extends forwards, and the relatively small segmen-
tation-cavity (bl. c&l\ gradually disappears, the edges of the lower margin
of the blastopore approach one another, and uniting in the median
plane, give rise to a vertical groove — the primitive groove, as it is
called.

In the centrolecithal egg of the crayfish (Fig. 98) a gastrula-stage is
formed by invagination, but as the centre of the oosperm is filled with
solid yolk in the place of a segmentation-cavity containing fluid, the
invagination only extends a short distance inwards, the archenteron

GASTRULA-STAGE

581

being relatively very small and the ectoderm separated from the
endoderm by the yolk.

The gastrula-stage is much less clearly distinguishable in the
segmenting eggs of the dogfish and bird (pp. 470 and 573), in which
the relatively enormous mass of unsegmented yolk is, as in the crayfish,
sufficient to nourish the embryo until it has reached a stage closely
resembling the adult in almost every essential respect except size. A
blastopore can sometimes be recognised in such cases, but in the

FIG. 156. — Oosperm of rabbit 70-90 hours after impregnation.

fo/. cavity of blastodermic vesicle (yolk-sac) ; ep. outer layer of cells (trophoblast) ;
hy, inner mass of cells of the embryonic area ; Zp. albuminous envelope. (From
Balfour's Embryology, after E. van Beneden.)

embryo of the common fowl it is only represented by a primitive groove
(see p. 580 and Fig. 155, pr. st). The blastoderm soon becomes differ-
entiated into an outer ectoderm and an inner, lower layer of cells
(compare Fig. 128), between which and the yolk the enteric cavity is
formed : a segmentation-cavity is hardly recognisable. As the embryo
develops, it becomes folded off from the yolk, which forms & yolk-sac on
its ventral side (Figs. 129 and 165).

The minute egg of the rabbit and of most other Mammals, although
alecithal and undergoing a holoblastic segmentation, has presumably

582 MESODERM AND CCELOME CHAP.

been derived from a meroblastic type with abundant yolk like that
of the bird, and some Mammals living in Australia at the present day
still possess eggs of this type. In the higher Mammalia the yolk has
disappeared, as it is no longer needed, the embryo, as we have seen,
being nourished by means of a placenta, which will be described pre-
sently. The early processes of development are therefore somewhat
peculiar, and though the segmentation is holoblastic, the subsequent
development is essentially similar to that of the bird, the embryo
beginning to appear in a mass of cells (Fig. 156, hy) attached to

cocl/

FIG. 157. — Transverse section through the trunk of embryo of frog.
ccel. coelome ; coel '. prolongation of coelome into mesodermal segment or

" protovertebra " ; ent. mesenteron (archenteron) ; msd. mesoderm ; nch.

notochord ; pr. z>. " protovertebra " ; sg. d. pronephric duct ; som. somatic layer
j of mesoderm ; sp. c. spinal cord ; spl. splanchnic layer of mesoderm ; yk.

yolk-cells. (From Parker and Haswell's Zoology, after Marshall.)

the upper pole of a large blastodermic vesicle (bv), representing the yolk-
sac of a bird but containing a fluid instead of yolk, and being
surrounded by a layer of cells known as the trophoblast (ep}.

In the lancelet alone amongst the triploblastic animals
described in this book, does the mesoderm arise as a series
of enteroccelic pouches : it is usually at first solid, and may
be budded off from the endoderm, from the lip of the
blastopore or primitive groove at the junction of the ecto-

MESODERM AND CCELOME

583

derm and endoderm, or both endoderm and mesoderm may
be differentiated at the same time from the lower layer-
cells or yolk-cells (e.g. frog) ; or, finally, it may arise in
all these ways (e.g. fowl, rabbit). The ccelome is formed

sp. c

so.

om.

FIG. 158. — Transverse section through the trunk ot a duck-embryo with about
24 mesodermic segments, for comparison with Fig. 157 : the peripheral part of
the blastoderm is not shown.

am. developing amnion ; ao. aorta ; ca.v. cardinal vein ; ch. notochord ; hy.
endoderm, which overlies the unsegmented yolk ; ms. muscle-plate of meso-
dermal segment ; sc. somatopleure, and sp. splanchnopleure, with the coelome
between them ; sp.c. spinal cord ; sp.g. spinal ganglion of neural ridge ; st.
mesonephric tubule ; -wd. mesonephric (Wolffian) duct. (From Balfour's
Embryology.}

by a split taking place in the mesoderm on either side
(Fig. 65, .157, and 158), the split gradually extending with
the extension of the mesoderm between the ectoderm and

584 NERVOUS SYSTEM CHAP, xn

endoderm. Thus the coelome is formed not as an entero-
coele, but as a schizoccde.

In Vertebrates each mesoderm-band becomes differ-
entiated into a dorsal portion, the vertebral plate, which
soon loses its ccelomic space, and a ventral portion, the
lateral plate, which is divided into parietal and visceral
layers by the ccelome (Figs 153 D, 157 and 158). The
vertebral plate undergoes metameric segmentation, becom-
ing divided into a row of squarish masses, the mesodermal
segments or " protovertebra " (pr. v\ from which the muscular
segments or myomeres ^e formed (p. 203), and also the
vertebral column, the segmentation of which alternates with
that of the myomeres.

Outline of the development of the chief organs in the
Craniata (compare pp. 201-210). — The nervous system,
as well as the essential parts of the sensory organs are, as
we have seen, in all cases formed from the ectoderm (pp.
202, 209, and Figs. 64, 65, and 153), and in craniate Verte-
brates the anterior end of the hollow medullary tube becomes
dilated, forming three bulb-like swellings — the fore-brain (Fig.
T59> A>/ ^)> mid-brain (m. b), and hind-brain (h. b). Soon
another hollow swelling grows forwards from the first vesicle
(v,prs. en), and the third gives off a similar hollow out-
growth (cblni) from its dorsal surface. The brain now con-
sists of five divisions : the prosencephalon (prs. en) and
diencephalon (dien) derived from the fore-brain, with the
pineal structures (pn. b, pn. e) and the infundibulum
and pituitary body (inf. pty] ; the mid-brain or mesen-
cephalon (m. b) which gives rise to the optic lobes and crura
cerebri ; and the epencephalon or cerebellum (cblni) and
metencephalon or medulla oblongata (med. obl\ derived from
the hind-brain. The original cavity of the brain becomes

4 5

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rams of the Craniate brain.

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erse section of prosencephalo]
mispheres ; K, sagittal sectioi

c. h. cerebral hemispheres ; c.
ain ; f. m. foramen of Monro ;
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586 NERVOUS SYSTEM CHAP.

correspondingly divided into a series of chambers or
ventricles (compare Figs. 159 and 50), all communicating
with one another, and called respectively the fore-ventricle
or prosocczle, third ventricle or diacozle, mid-ventricle or
mesoccele (iter, and optic ventricles or optocales\ cerebellar
ventricle or epicozle^ and fourth ventricle or metaccele.

In some fishes (e.g. dogfish, Fig. 124) the brain consists
throughout life of these five divisions, but in most cases
(Figs. 49 and 141) the prosencephalon grows out into
paired lobes, the right and left cerebral hemispheres or
parencephala (Figs. 159, I-L, c. ti), each containing a cavity,
the lateral ventricle or paracoele (pa. coe) which communi-
cates with the diaccele (di. coe) by a narrow passage, the
foramen of Monro (f. m). From the prosencephalon or
the hemispheres are given off a pair of anterior prolonga-
tions, the olfactory lobes or rhinencephala (olf. /), each
containing an olfactory ventricle or rhinoc&le (rh. coe).

In the preceding description the brain has been described as if its
parts were in one horizontal plane ; but, as a matter of fact, at a very
early period of development the anterior part becomes bent down over
the end of the notochord, so that the whole organ assumes a retort-
shape, the axis of the fore-brain being strongly inclined to that of the
hind-brain. The bend is known as the cerebral flexure (compare
Fig. 1 66) : it is really permanent, but as the hemispheres grow forwards
parallel to the hind-brain and the floor of the mid-brain and hind-brain
thickens, it becomes obscure and is not noticeable in the adult.

The ganglia of the dorsal roots of the spinal nerves are developed
from a paired neural ridge formed close to the junction of the medullary
plate and outer ectoderm, and the dorsal roots themselves appear as
outgrowths from their ganglia (see Figs. 157, above sp. c, and 158) : the
ventral roots arise as direct outgrowths from the medullary cord. Certain
of the cerebral nerves are developed in an essentially similar manner to
the dorsal roots of the spinal nerves, while others arise as direct ventral
outgrowths from the brain, like the ventral roots.

NOSE AND EYE

587

The olfactory organs arise as sac-like invaginations of the
ectoderm, one on either side of the snout, and become
enclosed by the cartilaginous olfactory capsules, developed,
with the rest of the skeleton, from the mesoderm. The
aperture of invagination gives rise to the external nostril,

inv I — m ':• die

FIG. 160. — Early (A) and later (B) stages in the development of the eye.
dicn. diencephalon ; inv. 1. invagination of ectoderm to form lens ; /. lens ; opt. c.
outer, and opt. d . inner layer of optic cup ; opt. st. optic stalk ; opt. v. optic
vesicle \ph. pharynx \pty. pituitary body. (From Parker and Haswell's Zoology,
altered from Marshall).

the internal nostrils (in pulmonate forms) being developed
subsequently.

The mode of development of the paired eye of Vertebrates
is peculiar and characteristic.

At an early stage of development a hollow outgrowth — the
optic vesicle (Fig. 160, A, opt. v) — is given off from either side
of the fore-brain and extends towards the side of the head,
where it meets with an in-pushing of the ectoderm (inv. 1}
which becomes thickened, and finally, separating from the
ectoderm, forms a closed, spherical sac (B, /) with a very
small cavity and thick walls (compare Fig. 64, i, e). This
body is the rudiment of the lens : as it enlarges it pushes

588 EYE CHAP.

against the optic vesicle which becomes invaginated
(JB\ the single-layered optic vesicle thus being con-
verted into a two-layered optic cup (opt. <:, opt. c\ its cavity,
continuous with the ventricle of the fore-brain, becoming
obliterated. Between the edge of the cup and the lens, on
the ventral side, is a small space which gradually extends
towards the stalk of the cup, and thus gives rise to a slit

in the wall of the latter : this
choroid fissure (Fig. i6i,aus),
as it is called, soon becomes
closed by the union of its
edges. The outer layer of
the optic cup gives rise to the
pigment-layer of the retina
(p. 183) • from its inner layer
the rest of that membrane —

FIG. xoi. — Plastic representation of the

optic cup and lens. including the rods and cones

ab. outer wall of optic cup ; tins.

choroid fissure ; gl. cavity of optic — is formed. The Stalk of the
cup ; h. space between the two

walls, which afterwards disappears ; Optic CUp OCCUpieS, in the
ib. inner wall of optic cup ; /. lens ; .

Sn. stalk of optic cup (rudiment of embryonic eye, the place of

optic nerve), (After Hertwig.)

the optic nerve, but the actual

fibres of the nerve are formed from the nerve-cells of the
retina and grow inwards to the brain.

During the formation of the lens, mesoderm extends in
between the ingrowth "from which it arises and the external
ectoderm ; from this the main substance of the cornea and
its inner or posterior epithelium are formed, the adjacent
ectoderm becoming the external epithelium, i.e. that of
the conjunctiva (p. 182). Mesoderm also makes its way
into the optic cup, through the choroid fissure, and gives
rise to the vitreous humour. Lastly, the mesoderm imme-
diately surrounding the optic cup is differentiated to form
the choroid, the iris, and the sclerotic.

xii EAR 589

Thus the eye of Vertebrates has a threefold origin : the sclerotic,
choroid, iris, vitreous humour, and the greater part of the cornea are
mesodermal ; 1 the lens and external epithelium of the cornea are
derived from the ectoderm of the head : the retina and optic nerve are
developed from a hollow pouch of the brain, and are therefore in their
ultimate origin ectodermal. The sensory cells of the retina — the rods
and cones — although not directly formed from the external ectoderm, as
in Invertebrates, are ultimately traceable into the superficial layer of
ectoderm, since they are developed from the inner layer of the optic
vesicle which is a prolongation of the inner layer of the brain, the latter
being continuous, before the closure of the medullary groove, with the
ectoderm covering the general surface of the body (compare Fig. 160).

The organ of hearing, like that of smell, arises in the
embryo as a paired invagination of the ectoderm in the
region of the hind-brain, a shallow depression being forme*d
(Figs. 64 L and 166, au. s) which deepens and becomes
flask-shaped ; and finally, as a rule (compare p. 465), loses
its connection with the external ectoderm, giving rise to
a closed sac surrounded by mesoderm in which the carti-
laginous auditory capsule is subsequently developed. At
first simple, it soon becomes divided by a constriction into
dorsal and ventral compartments, from the former of which
arise the utriculus and semicircular canals, and from the
latter the sacculus and cochlea.

The mode of development of the enteric canal has
already been dealt with (pp. 204 — 210). The first traces
of the liver and pancreas are seen as simple offshoots of the
mesenteron (archenteron), which gradually become branched
in a complicated manner, the numerous lobules being more
or less closely connected together by mesoderm. The
gill-pouches arise as paired outgrowths of the endoderm
lining the pharynx which come into contact with the

1 It is possible, however, that the ectoderm takes part in the forma-
tion of the iris and vitreous humour.

590 RESPIRATORY AND CIRCULATORY ORGANS CHAP.

ectoderm, the latter becoming perforated to form the
external branchial apertures. Gill-clefts appear in the em-
bryo of reptiles, birds, and mammals — animals in which gills
are never developed (Fig. 166); but they early disappear
with the exception of the first cleft, corresponding with the
spiracle of the dogfish, which gives rise in all Vertebrates
above fishes to the tympano-eustachian passage (p. 449) :
the branchial skeleton, as we have seen, undergoes a cor-
responding reduction or modification (pp. 438 and 496).
In pulmonate Vertebrates the lungs arise as a ventral
outgrowth of the pharynx.

The circulatory organs are developed from the meso-
derm, the heart arising in the visceral layer on the ventral
side of the pharynx. It has at first the form of a straight
tube, which soon becomes twisted into an S-shape and in
which transverse constrictions are formed dividing it into the
different chambers. The auricular and ventricular portions
are each at first single, but from the Amphibia onwards the
former subsequently becomes divided into two by a septum,
and the ventricle is similarly subdivided in birds and mam-
mals. The modification of the arterial arches in the ex-
amples studied has already been described (pp. 452 and 423).

In the meroblastic eggs of the dogfish and bird the
dorsal aorta, in addition to its other branches, gives rise to
paired vitelline arteries : these vessels branch up over the extra-
embryonic part of the blastoderm (p. 596) which spreads
over the yolk, and take an important share in the absorp-
tion of the latter by the embryo. From this area vasculosa
(Fig. 165) the blood is returned by splanchnopleuric
vitelline veins (Fig. 158) which join with a subintestinal
vein (compare Amphioxus, Fig. no) and open into
vessels which eventually give rise to the hepatic portal

xii URINOGENITAL ORGANS 591

veins. The chief somatopleuric veins in all embryonic
Craniates are, as in the dogfish, the jugulars and the cardinals,
but in all those above the fishes, the cardinals became
subsequently more or less entirely replaced functionally by
the postcaval (compare p. 456) : the anterior part of one
or both cardinals may, however, persist as the azygos vein
or veins (e.g. rabbit, p. 525).

Urinogenital organs. — The excretory organ, speaking of
craniate Vertebrates as a whole, consists of three parts, all
paired and situated along the dorsal wall of the coelome :
the fore-kidney or pronephros (Fig. 162, A, /. npti), the mid-
kidney or mesonephros (ins. npJi) and the hind-kidney or meta-
nephros (mt. npli). Each of these is provided with a duct
— \hepro- (sg.d], meso- (msn.d\ and meta-nephric(mt. n.d) duct
respectively, opening into the cloaca. The gonads (gon) lie
in the ccelome suspended to its dorsal wall by a fold of perito-
neum : they are developed as ridges covered by coelomic
epithelium (compare pp. 194 — 196 and 345).

The pronephros is nearly always functionless in the adult
and often even in the embryo, and it usually disappears
altogether : in the young tadpole it acts as the sole excretory
organ for some time. The mesonephros is the functional
kidney in the lower Craniata (Fishes and Amphibians), in
which no metanephros is developed, and the mesonephric
duct acts as a ureter, often in addition carrying off the
seminal fluid of the male (e.g. frog). In the higher forms
the mesonephros is replaced in its excretory function by
the metanephros, the metanephric duct being the ureter.

The development of the kidney reveals a resemblance to
the nephridia of worms which would hardly be suspected
from its adult structure. The pronephros (Fig. 162 A, p.
nph) originates as two or three coiled tubes formed from

592 URINOGENITAL ORGANS CHAP, xn

mesoderm in the body- wall at the anterior end of the
coelome ; these are arranged metamerically and each opens
into the coelome by a ciliated funnel (nst). Obviously
such tubes are nephridia l (compare p. 340) ; their chief
peculiarity is that their outer ends do not open directly on
to the exterior, but into a longitudinal tube, the pronephric
duct (sg.d), which passes backwards and discharges into the
cloaca. It seems probable that this arrangement is to be
explained by supposing that the nephridia originally opened
externally in to' a longitudinal groove, which, by the apposition
of its edges, was converted into a tube. The nephridia
of the pronephros open, by their ciliated funnels, into the
anterior end of the coelome, into which projects a branch
of the aorta on either side ending in a single large glomerulus
(p. 146).

The pronephros soon degenerates, its nephridia losing
their connection with the duct (^), but in the mean-
time fresh nephridia, developed in a somewhat different
manner, appear in the segments posterior to the pronephros
and together constitute the mesonephros or Wolffian body
(ms. npti), from which the permanent kidney is formed in the
lower Craniata. The mesonephric nephridia open at one
end into the duct (sg. d. and Figs. 157 and 158), at the other,
by ciliated funnels (nst\ into the coelome ; a short distance
from the funnel each gives off a blind pouch which dilates
at the end and forms a Malpighian capsule (m. c), and
a branch from the aorta entering it gives rise to a glomer-
ulus.

In some forms (e.g. dogfish, p. 466) the pronephric duct
now becomes divided by a longitudinal partition into two
tubes : one retains its connection with part or the whole of

1 It is doubtful whether these pronephric tubules are to some extent
comparable to the nephridia of Amphioxus (p. 424).

0,71,

FIG. 162.— Diagrams, illustratin the development Jof the urinogenital organs of

the Craniata.

A, pronephros and its duct. B, atrophy of pronephros, development of meso-
nephros ; C, differentiation of Wolffian and Miillerian ducts ; D, development of
metanephros, male type ; E, female type.

al. bl. allantoic bladder ; an. anus ; cl. cloaca ; gon. gonad ; int. intestine ; m.c.
Malpighian capsule ; ms. n. d. mesonephric (Wolffian) duct ; ms. nph. meso-
PRACT. ZOOL. Q Q

594 MUSCLES AND SKELETON CHAP.

nephros ; int. n. d. metanephric duct ; mt. nph. metanephros ; nst. nephrostomes ;
ov. ovary ; p. n. d. Miillerian duct ; /. nph. pronephros ; sg.d. pronephric duct;
t. spermary ; v. c. efferent ducts. (From Parker and Haswell's Zoology).

the mesonephros and is now known as the mesonephric or
Wolffian duct (C, ms. n. d) : the other, or Miillerian duct (p.
n.d\ has no connection with the nephridia, but opens into
the coelome in the region of the vanishing pronephros, and
assumes the functions of ah oviduct in the female. In some
Vertebrates the Miillerian duct appears quite independently of
the Wolffian duct ; the latter is then simply the pronephric
duct after the union with it of the mesonephric tubules.

In the higher Vertebrata (Reptiles, Birds, and Mammals)
a diverticulum (D, E, ////. n. d) is given off from the pos-
terior end of the Wolffian duct, which grows forwards and
becomes connected with a series of posterior nephridia. In
this way is formed a metanephros (mt. npli), which gives rise
to the permanent kidney, and a metanephric duct (mt. n. d}
which becomes the ureter. The whole mesonephros ceases
to discharge a renal function, becomes in the female a
purely vestigial organ, and in the male gives rise to the
epididymis (p. 467), which receives the efferent ducts
from the spermary and from which the Wolffian duct
(spermiduct or vas deferens) arises.

In the specialisation of the hinder part of the mesonephros and in
the development of special ureters the kidney of the dogfish (Fig. 127)
resembles the metanephros of higher Vertebrates, which, however,
possess no nephrostomes.

The majority of the muscles are developed, as we have
seen (pp. 203 and 584), from the mesodermal segments,
others arising from the parietal and visceral layers of the
mesoderm.

The first part of the endo skeleton to be formed is the
endodermic notochord (pp. 203 and 578), in the mesoderm

SKULL

595

surrounding which cartilages appear and give rise to the
vertebrae, the notochord becoming constricted by the in-
growing cartilage, and eventually disappearing more or less
completely (compare pp. 441 and 584) : it at first extends
into the head as far as the pituitary body (Fig. 163, C). The
cranial cartilage does not become segmented, but gives rise

. sora proces .

Tr. trabecula. (From Wiedersheim's Comp. Anatomy.)

to a pair of horizontal bars, the parachordals (PE] : these are
continued forwards, diverging around the pituitary body, as
the trabecula cranii (Tr\ and thus a support is formed for
the developing brain. The two parachordals and trabeculse
then unite respectively with one another, and so form a firm
floor (B) for the future brain-case ; this is gradually
developed by the floor growing up on either side and

Q Q 2

596 EMBRYONIC MEMBRANES CHAP.

eventually meeting to a greater or less extent above the
brain : there is never, however, a complete cartilaginous roof
to the cranium, parts of which are only membranous and
form the fontanelles (pp. 43 and 436). In the meantime
the cartilaginous sense-capsules are developed, the olfactory
and auditory capsules uniting with the brain-case in front
and behind respectively.

The visceral skeleton is formed as a series of cartilaginous
bars within the visceral arches, the first of which forms the
mandibular arch, the second the hyoid, and the others the
branchial arches.

The limbs appear as small buds (Fig. 166) composed
of ectoderm with a core of mesoderm, in which their
skeleton arises by the formation of cartilage extending
inwards to form the arches, and outwards to form the
skeleton of the free portions of the limbs.

As we have seen, the endoskeleton may remain more or
less entirely cartilaginous in the adult (e.g. Dogfish), but in
higher forms extensive processes of ossification set in, certain
bones replacing this cartilage to a greater or less extent, and
others being formed in the surrounding connective-tissue
(compare p. 43).

Development of the Amnion, Allantois and Placenta.—

We must now consider some important and characteristic
structures which are developed in the embryos of Reptiles,
Birds, and Mammals, and known as embryonic membranes.
Taking the chick as a convenient example, these are formed
as follows.

The blastoderm gradually extends peripherally so as to
cover the yolk, and thereby becomes divisible into an
embryonic portion, from which the embryo is formed, and
an extra-embryonic portion which invests the yolk-sac and

xii AMNION AND ALLANTOIS 597

takes no direct share in the formation of the embryo
(Fig. 165). The extension of the ectoderm and endoderm
takes place regularly and symmetrically, but the meso-
derm, while extending equally in the lateral and posterior
regions, grows forwards in the form of paired prolongations
which afterwards unite, so that for a time thesis an area of
the blastoderm in front of the head of the embryo formed
of ectoderm and endoderm only, and called the pro-amnion

(Fig- *$5,pr- am\

Before the embryo has begun to be folded off from the
yolk the rudiment of one of the two embryonic membranes,
the amnion, has appeared. A crescentic amniotic fold (Fig.
164, A, am. f) arises in front of the head-end of the embryo
from the region of the pro-amnion / it consists at first of
ectoderm only, the mesoderm not having yet spread into the
pro-amnion. The fold is soon continued backwards along
the sides of the body (B) and round the tail (A), but in
these regions (am. f) it consists from the first of ectoderm
plus the parietal layer of mesoderm, i.e., it is a fold of what
may be called the embryonic body-wall or somatopleure
(Fig. 158, so). Its cavity is a prolongation of the space
between the parietal and visceral layers of mesoderm,
i.e. is an extension of the extra-embryonic ccelome.

The entire amniotic fold gradually closes in above (c),
forming a double layered dome over the embryo. Its inner
layer, formed of ectoderm internally and mesoderm exter-
nally is the amnion (am\ the cavity of which becomes filled
with a watery amniotic fluid, serving as a protective water-
cushion to the enclosed embryo. Its outer layer, formed of
ectoderm externally and mesoderm internally, is the serous
membrane (sr.m) : this comes to lie just beneath the vitelline
membrane, with which it subsequently fuses.

The second of the embryonic membranes, the allantois,

FIG. 164. — Diagrams illustrating the development of the foetal membranes of a bird.
A, early stage in the formation of the amnion, longitudinal vertical section ; B,

ALLANTOIS

599

slightly later stage, transverse section ; C, stage with completed amnion and
commencing allantois ; D, stage in which the allantois has begun to envelop the
embryo and yolk-sac. The ectoderm is represented by a blue, the endoderm by
a red line ; the mesoderm is grey.

all. allantois ; all', the same growing round the embryo and yolk-sac ; am. amnion ;
am.f., am.fl) amniotic fold; an. anus; br. brain; cceL ccelome ; ccel'. extra-embryonic
coelome ; ht. heart ; ms. ent. mesenteron ; wth. mouth ; nch. notochord ; sp. cd.
spinal cord ; sr. m. serous membrane ; umb. d. umbilical duct ; vt. m. vitelline
membrane ; yk. yolk-sac. (Reduced from Parker and Haswell's Zoology.)

is developed as an outpushing of the ventral wall of the
mesenteron at its posterior end (c, a//), and consists,
therefore, of a layer of visceral mesoderm lined by endo-
derm. It has at first the form of a small, ovoid sac having

FIG. 165. — Egg of fowl, at the sixth day of incubation, with embryo and foetal

appendages.

a. air-space ; all. allantois ; am. amnion ; ar. vase, area vasculosa ; emb. embryo ;
yk. yolk-sac. (From Parker and Haswell's Zoology, after Duval.)

the precise anatomical relations of the urinary bladder of the
frog. It increases rapidly in size (Figs. 166 and 165, all\
and makes its way, backwards and to the right, into the
extra-embryonic ccelome, between the amnion and the serous
membrane (Fig. 164, c, D). Arteries pass to it from the
dorsal aorta, and its veins, joining with those from the yolk-sac,
take the blood through the liver to the heart. Next, the
distal end of the sac spreads itself out and extends all round

6oo

ALLANTOIS

the embryo and yolk-sac (D, all'\ fusing, as it does so,
with the serous and vitelline membranes, and so coming to
lie immediately beneath the shell-membrane. It finally en-
closes the whole embryo and yolk-sac, together with the re-
mains of the albumen, which has, by this time, been largely
absorbed. The allantois serves as the embryonic respiratory

Tit

m.ltr

7t.l

FIG. 166. — Chick at the fifth day of incubation.

all. allantois ; am. cut edge of amnion ; au. s. auditory sac ; f. br. fore-brain ; f L
fore-limb ; h. br. hind-brain ; h. 1. hind-limb ; ht. heart ; hy. hyoid arch ; m. br*
mid-brain ; mn. mandibular arch ; t. tail. (From Parker and Harwell's Zoology,
after Duval.)

organ, gaseous exchange readily taking place through the
porous shell ; its cavity is an embryonic urinary bladder,
excretory products being discharged into it from the
kidneys.

At the end of incubation the embryo breaks the shell by means of a
little horny elevation or caruncle at the end of the beak. By this time

xii PLACENTA 601

the remainder of the yolk-sac has been drawn into the ccelome, and the
ventral body-walls have closed round it. On the shell being broken
the allantois gradually shrivels up, respiratory movements begin, the
aperture in the shell is enlarged, and the young bird is hatched and
begins a free life.

In the higher Mammalia the allantois takes on a further
important function. The mode of development of the
amnion and allantois in the rabbit is similar to that
described above in the case of the bird. But the later
history of the allantois is widely different, owing to the
modifications which it undergoes in order to take part in
the formation of the placenta, the structure by means of
which the foetus receives its nourishment from the walls of
the uterus, with which the blastodermic vesicle (p. 582)
becomes adherent. The fatal part of the placenta arises in
a limited disc-shaped area of the outer layer of the
amnion (serous membrane, Fig. 167, sh\ where the distal
portion of the allantois coalesces with it. The membrane
thus formed (chorion) develops vascular processes — the
chorionic villi (pl\ which are received into depressions —
the uterine crypts, in the thickened mucous membrane of
the dorsal wall of the uterus which constitutes the maternal
portion of the placenta. The placenta with its villi is sup-
plied with blood by the allantoic vessels, and the blood
supply of the uterus is at the same time greatly increased :
the foetal and maternal capillaries and sinuses are thus
brought into intimate relation with one another in the
placenta, and diffusion can take place between them,
nutrient matter and oxygen diffusing from the blood of
the mother into that of the foetus, while excretory sub-
stances pass from the blood of the foetus into that of the
mother.

The discoidal placenta of the Rabbit is of the type termed

6O2

PLACENTA

dectduate, the villi of the placenta being so intimately con-
nected with the uterine mucous membrane that a part of the
latter comes away with it at birth in the decidua, or after-
birth, which is attached to the newly-born young by the
umbilical cord, consisting of the stalks of the allantois (a)
and flattened yolk-sac (ed) twisted together. The cord is

FIG. 167.— Diagrammatic longitudinal section of the foetus and embryonic mem-
branes of a rabbit.

a. (on right) amnion ; a- (on left) stalk of allantois ; aL allantois with blood-vessels
c. embryo ; ds. cavity of the flattened yolk-sac (blastodermic vesicle) ; ed. endo
dermal layer of yolk-sac ; ed' . inner portion, and ed" . outer portion of endoderm
lining the compressed cavity of the yolk-sac ; fd. vascular layer of yolk-sac
//. placental villi ; r. space filled with fluid between the amnion, the allantois
and the yolk-sac ; sh. serous membrane ; st. margin of vascular area of yolk-sac
(From Balfour's Embryology, after Bischoff.)

gnawed through by the parent rabbit, the blood-vessels being
compressed in the process ; and it soon shrivels up and comes
away at the navel or umbilicus, which represents the point
of connection between the foetus and the placenta. The.
intra-abdominal portion of the allantois is represented by a
cord or ligament, the urachus, which connects the navel with

xii PRACTICAL DIRECTIONS 603

the apex of the bladder, so that only a small portion of the
allantoic outgrowth, and not the whole of it as in the frog,
persists in the adult.

PRACTICAL DIRECTIONS.

A. Special methods are required to follow out the details of the
structure and division of nuclei, but if you have not the opportunity
of examining preparations illustrating these, a good deal may be made
out as regards the chromatin and the behaviour of the chromosomes
in the process of nuclear division by the following simple method.

Obtain a young, gilled-larva or tadpole of the common newt (p. 218),
kill, and place it in corrosive sublimate for about half an hour ; wash
thoroughly in water and transfer first into weak, and then into strong
alcohol (p. 137). Stain entire, either with a solution of hatmatoxylin
(p. 136), or of alum-carmine, which you can prepare yourself by dis-
solving 200 grams of ammonia-alum in water, boiling the solution and
adding carmine to excess, filtering, and diluting with three or four times
its bulk of water ; a drop or two of carbolic acid should be added to the
solution.

Strip off small pieces of the skin of the stained preparation, and after
putting them through weak and strong alcohol, transfer to absolute
alcohol, then to turpentine or oil of cloves, and mount in Canada balsam
(p, 139). Examine, comparing Fig. 146 and pp. 558-562, and sketch
as many different stages as possible.

B. Observation of the details concerned in the process of maturation
and fertilization of the ovum is too difficult for the beginner, but polar
cells. may easily be seen in the living, freshly-laid eggs of one of the
common pond-snails (e.g. Limncea stagnalis\ in which various
stages of segmentation can also be observed. Keep some of these snails
in a glass vessel with water-weeds, and notice that the eggs, when laid,
are enclosed, a number together, in a common gelatinous envelope.
Separate the eggs from one another with needle, and examine in water
under the microscope. Note that the small egg is surrounded by fluid
enclosed in a relatively large egg-case, and observe the minute polar
cells at the periphcry.of the ovum.

604 PRACTICAL DIRECTIONS CHAP.

C. A series of models of the development of Amphioxus (which,
as well as of the frog and chick, are to be found in most museums),
should be examined (compare Figs. 151-154).

D. In order to follow out the development of the chief organs in a
Vertebrate animal, it is necessary to make a number of serial transverse
sections. For this purpose chick-embryos (see below) are, on the whole,
the most convenient and satisfactory, but if you also wish to make sec-
tions of embryos of the frog, proceed as directed on p. 214. Serial sec-
tions of embryos in different stages should be mounted on the same slide,
after smearing it with collodion and oil of cloves (p. 139). It is a
matter of some difficulty to make satisfactory sections of the early stages :
the most important stages for the present purpose subsequently to seg-
mentation are from the time when the embryo begins to become elongated
up to hatching.

A number of fresh, impregnated fowl's eggs should be obtained and
placed in an incubator at a temperature of from 37° to 40° C., or under a
"broody" hen, first marking each with the date. One or two should
be examined each day or oftener for the first four or five days of incuba-
tion. To expose the embryo, place the egg in a dish of warm water
(temperature as above), in order that, after the first day the beating
of the heart and the circulation of the blood may be seen. With
the forceps, tap the surface of the egg lying uppermost so as to
break the shell into small pieces, which can then be removed : cut
away sufficient of the shell-membrane with the scissors to expose the
entire embryo and blastoderm. The early stages are difficult to observe,
and the most important of those referred to below are from the end of
the first to the third day of incubation. As the medullary groove
only closes gradually from before backwards in the body-region, sections
showing different stages in the development of the central nervous system
may be obtained from the same embryo at these stages. Prepare as
directed on p. 214.

1. Examine first an 21111 nctibated e§g, as directed above, and make out
its structure (compare Fig. 148). (The blastoderm undergoes seg-
mentation before the egg is laid. )

2. First day of incubation (18-20 hours). Examine with a lens and
compare Fig. 155, A. Then carefully cut round the blastoderm with fine

xii PRACTICAL DIRECTIONS 605

scissors, float it off in the water, preserve, stain, and mount entire in
Canada balsam. (The removal and preparation of the blastoderm at this
stage is rather difficult, and sections of the next stage will illustrate the
chief points equally well).

3. End of first day (aboztt 24 hours]. Examine as before, com-
paring Fig. 155, B. Sketch, and then preserve, stain, and eventually
cut sections. Note the ectoderm endoderm, mesoderm, medullary groove,
and its gradual closure to form the medullary cord ; the neural ridges
(p. 586) ; the notochord', the parietal (somatic] and visceral (splanchnic}
layers of the mesoderm, and the ccelome (compare Figs. 64 and 65).
Sketch one or two typical sections.

4. Second day. Examine, prepare, sketch and cut sections as before,
noting the fore-, mid-, and hind-brain, the optic vesicles, lens, auditory
pits, heart, and vitelline vessels, paired aorta and cardinal veins, meso-
nephric ducts, and amnion, as well as the increase in the number of
mesodermal segments (compare Figs. 157, 158, 159, 160, 161, and
164).

5. Third day. Examine, prepare, sketch, and if time permits, cut
sections, noting, in addition to the points already referred to, the further
gradual folding off of the embryo from the yolk (compare p. 471) and'
extension of the amnion and area vasculosa ; the cerebral flexure ; the
visceral arches and clefts ; the arterial arches ; the mesonephric tubules ;
and the further development of the eye and ear (compare Figs, referred
to in § 4).

6. Fourth to sixth days. Observe the further development of the
parts already seen and also the limb-buds and the allantois (compare
Figs. 164-166). Sketch.

7. Examine and compare a few later stages.

E. Obtain some advanced embryos of the rabbit or rat, which
should be examined in situ in the uterus before they are removed and
preserved (p. 549)' Note the amnion, and examine the placenta and
its connection with the uterine wall and with the foetus. Sketch.

INDEX

(The numbers in italics refer to practical directions.}

Abdomen, abdominal cavity, 20,
361, 434, 483, 506

Abdominal pore, 432

Abiogenesis, 288

Acanthias, 431, 470

Acetabulum, 50, 503

Aciculum, 352

Acoustic spots, 1 88

Acrania, 421

Acromion, 500

Adaptation, 225

Adenoids, 507, 515

Adrenal bodies, 145, 447, 538

Agamobium, 321

Alimentary canal, see Enteric canal

Allantois, 597, 601

ALLOLOBOPHORA, see Earthworm.

Alternation of generations, 321

Alveoli of jaws, 510

Amnion, 597

AMCEBA : occurrence and general
characters, 229 ; movements,
232 ; resting condition, 232 ;
nutrition, 233 ; growth, 234 ;
excretion, 235 ; respiration, 235 ;
metabolism, 236 ; reproduction,
236 : immortality, 236 ; conjuga-
tion, 237 ; death, 237 ; animal or
pla'nt ? 255 ; practical directions,
238

Amoeboid movements, 106, 231

Amphibia, 219, 418, 482

Amphiccelous, 439

AMPHIOXUS, see Lancelet

Ampulla, of semicircular canals, 188

Ampulla, of sensory canals of the
integument, 464

Anal segment, 327, 352

Analogous, analogy, 217

Anatomy, 217

Angular process, 496

Animals and Plants : comparison of
typical forms, 255 ; discussion of
doubtful forms, 255, 257 ; bound-
aries artificial, 258

Ankle, see Tarsus

Annulata, 220, 249

ANODONTA, see Mussel

Anura, 219

Antenna, 366

Antennary gland, 375

Antennule, 366

Anus, 6, 267, 272, 327, 352, 371,
401, 426, 432, 486, 514

Aorta, aortic arches, see Arteries

Aperture, or apertures (see also
Foramen, and under Nephridium,
Kidney, Pores, Reproductive
organs, &c.) auditory, Crayfish,
381 ; Rabbit, 486, 492, 536;
cloacal, see anus ; exhalant and
inhalant, 397

6o8

INDEX

Aphis, 569

Appendages, lateral, and their skele-
ton : Frog, 5, 28, 36, 48, 51 ;
Crayfish, 361, 363 ; Dogfish, 433,
441 ; Rabbit, 501, 503 ; develop-
ment of, in Vertebrates, 596

Aqueous chamber and humour,

183

Arachnoid fluid, 155

Archenteron, 201 „ 384, 574

Archicerebrum, 380

Area vasculosa, 590, 599

Arm, see Fore-limb

Arterial arches, 80, 451, 523

Arteries : Frog, 27, So ; Crayfish,
376 ; Mussel, 407 ; Dogfish, 449 ;
Rabbit, 522 ; Vertebrate embryo,
590

Arthobranch, 375

Arthropoda, 220, 360, 386, 411

Articular membrane, 361, 368

Articular processes, see Zygapo-
physes

Artificial selection, 227

Arytenoid cartilage, 144, 517

Ascaris, 153

Asexual generation, see Agamo-
bium

Asexual reproduction, ' see Fission,
Budding, Spore

Assimilation, 149, 234, 305

ASTACUS, see Crayfish

Astragalus, 51, 504

Atlas vertebra, 498

Atrial pore, 420

Atrium, 420, 421

Atrophy, see Vestige

Auditory capsule, 39, 41, 436, 492

Auditory organ : Fwg, 45, 186 ;
Crayfish, 381 ; Mussel, 409 ; Dog-
fish, 465 ; Rabbit, 535 ; develop-
ment of, in Vertebrates, 589

Auditory ossicles, 492, 536

Auricle, see Heart

Automatism, see Movements

Aves, 219, 418

Axial fibre : of Vorticella, 272 ; of
Carchesium, 277

Axial parts, 4

Axis fibre, see Neuraxis

Axis vertebra, 499

B

JACTERIA, 152; structure, 257;
nutrition, 257 ; animals or plants ?
257 ; rapid multiplication, 289 ;
practical directions, 260

Backbone, see Vertebral column

Basal cartilages of fins, or basalia,
441-443

Bell of medusae, see Umbrella

Bilateral symmetry, 296

Bile, 69

Bile-duct, 68, 447, 516

Bile-passages, 133

Binomial nomenclature, 215

Biogenesis, 288

Biology, I

Bird, development of, 581 et seq.

Birds, see Aves

Bladder, see Gall-bladder and Uri-
nary bladder

Blastoccele, see Segmentation-cavity

Blastoderm, 384, 470, 573

,, embryonic and extra-

embryonic portions, 590,
596

Blastodermic vesicle, 582, 601

Blastomere, 573

Blastopore, 201, 349, 384, 574, 580

Blastostyle, 309, 313

Blastula, 574

Blind spot, 556

Blood : Frog, 20, 78, 85 ; Earth-
worm, 336 ; Crayfish, 379 ;
Mussel, 408 ; Lance let, 424 ; Dog-
fish, 449 ; Rabbit, 483, 557

Blood-corpuscles : colourless, see
Leucocytes ; red, 105, 458, 483,

557

Blood-sinus, see Sinus
Blood-vessels : Frog, 78, 107 ;

Earthworm, 337 ; Crayfish, 375 ;

INDEX

609

Mussel, 407 ; Lancelet, 424 ;
Dogfish, 449 ; Rabbit, 520

Body-cavity, see Ccelome

Body of vertebra, see Vertebra

Body-segments, see Metamere

Bojanus, organ of, 406

Bone : replacing and investing, 43,
489 ; nature of, 52 ; microscopi-
cal structure of, 116

Bones, see Endoskeleton, Skull,
Vertebra, Ribs, Sternum, and
under individual bones of limbs

Botany, I

Brachial plexus, see Nerve-plexus

Brain : Frog, 28, 156, 202 ; Earth-
worm, 342 ; Crayfish, 379 ; Mus-
sel, 408 ; Lancelet, 420, 424 ;
Dogfish, 459 ; Rabbit, 528 ; de-
velopment of, in Vertebrates, 584

Brain-case, see Skull

Branchia, see Gill

Branchial apertures, arches, clefts,
and septa : Tadpole, 204, 206 ;
Lancelet, 420, 422 ; Dogfish, 432,
438, 448 ; development of, 589

Branchial rays, 438, 448

Branchial vessels : Crayfish, 377 ;
Mussel, 408 ; Lancelet, 425 ;
Dogfish, 449 ; Tadpole, 451

Breast-bone, see Sternum

Bronchus, 51?

Brood-pouch, 410

Buccal cavity, 16, 335, 445, 507

Buccal groove, 262

Bud, budding, 306, 309

Bufo, BufonidDe, 218

Bulb, see Medulla oblongata

Bulbus aortae, 89

Bulla, tympanic, 492

Byssus, 411

c

, 514

Caenosarc, 311
Calcaneum, 51, 504
Calcar, 52
Calciferous glands, 335

PKACT. ZOOL.

Canal : central, of spinal cord, 156,
424, 459 ; naso-palatine, 507 ;
neural, see Vertebral column ;
neurenteric, 203 ; radial and circu-
lar of Medusa, 315 ; semicircular of
ear, see Auditory organ and Mem-
branous labyrinth ; sensory, of
Dogfish, 432, 464 ; sternal, of
Crayfish, 363 ; vert ebrarterial, 498

Canaliculi, see Bone

Canine teeth, 509

Capillaries, 95, 377, 451, 454

Capitular facet and capitulum, 498,
500

Carapace, 361

Carbohydrates, 72

Carbon dioxide, 66

Carchesium, 277

Cardiac division, see Stomach.

Carotid arch, see Arteries of Frog,
Dogfish, and Rabbit

Carpus, 50, 502

Cartilage, 20, 35, 115, 431, 435;
calcified, 46, 48, 435, 487

Caruncle, 600

Castings of earthworm, 326

Cell, no, 231, 558; see under
various types

Cell-colony, see Colony

Cell-differentiation, see Differentia-
tion, and under development of
various types

Cell-division, 106, 561, 603

Cell-membrane or wall, 232, 244,
255, 275, 285, 559

Cellulose, 244, 254, 421

Cement, 445, 510

Centrale, 502, 504

Centrosome, 559

Centrosphere, 559

Centrum, see Vertebra

Cephalothorax, 361

Cerebellum: Frog, 157 '•> Dogfish,
459 ; Rabbit, 530 ; development
of, 584

Cerebral flexure, 586

Cerebral ganglion, see Brain

Cerebral hemispheres : Frog, 159 ;
Rabbit, 528 ; development of, 586

R R

6io

INDEX

Cerebral nerves, see Nerves

Cerebral vesicles, 584

Cerebro-pleural ganglion, 408

Cervical groove, 361

Chcetopoda, 350

Chalaza, 565 (Fig. 148)

Change of function, 449, 537

Chela, 364, 366

Cheliped, 364

Chiasma, optic, 164, 459, 530

Chick, see Bird

Chitin, 232, 328, 361

Chlorophyll, 242, 251

Choroid, 183,556

Choroid fissure, 588

Choroid plexus, 157, 529

Chromatin, 129, 231, 558, 561

Chromatophores, 243,

Chromosomes, 561

Cilia, and ciliary movement, 109,
242, 250, 265, 271, 280

Cilia, absence of in Crayfish, 386

Ciliary folds, muscles, nerves, and
vessels, 184, 534

Ciliata, Ciliate Infusoria, 261, 292

Circulation of blood, 86, 89, 337,
375, 407, 425, 449, 520

Circulatory organs, see Blood-
vessels and Lymphatic system

Cirri, 352, 421

Clasper, 433, 442, 469

Class, 219

Classification, 217, 220, 223, 411

Clavicle, 47, 501

Clitellum, 327, 348

Clitoris, 541

Cloaca, 23, 401, 445

Cloacal aperture, see Anus

Cnidoblast, 302

Cnemial crest, 504

Cnidocil, 302

Coagulation of blood, 78, 107

Cochlea, 187, 535

Cocoon, 328, 348

Ccelenterata, 321

Ccelome, 20, 203, 322, 328, 401,
422, 434, 506 ; development of,

203, 579, 584
Ccelomata, 322

Coelomic epithelium, 330, and see
Epithelium

Ccelomic fluid, 336

Collaterals, 171

Colon, 514

Colony, Colonial organisms, 277

Columella, 45, 189

Commissures, see Blood-vessels and
Nervous system ; of Brain, 531

Conchiolin, 399

Concrescence, 361

Condyle, occipital, see Skull

Cones of retina, 185

Conjugation, 197, 237, 251, 268,
274, 283, 286, 287, 308, 571, and
see under development of various
types

Conjunctiva, 182

Connectives, 379, 408

Connective-tissue, 18, 113

Contractility, nature of, 112

Conus arteriosus, 79, 88, 449, 520

Coracoid, 47, 443

Coracoid process, 500

Cordylophora, 325

Cornea, 182, 354, 380,555

Coronoid process, 496

Corpus adiposum, see Fat- body ;
callosum, 528 ; cavernosum and
spongiosum, 540; striatum, 531

Corpuscles, see Blood-corpuscles
and Leucocytes

Cortex of Brain, 160, 528

Cortical layer, 264, 269, 280, 286

Craniata, 421

Cranium, see Skull

CRAYFISH : general characters, 360,
386 ; limited number and con-
crescence of metameres, 360 ;
appendages, 363, 393 ; exo-
skeleton, 361, 363, 387, S94>
muscular system, 369, 390, 392 ,
enteric canal, 370, 390 ; gills,
374, 388', kidney, 375, j?j;
blood-system, 375, 389 ; nervous
system, 379, 392 ; sense-organs,
380, 393 ; reproductive organs,
382, 390 ; development, 383, 580

Creation, 221

INDEX

611

Cribriform plate, 491

Cricoid cartilage, 517

Crop, 335

Cross-fertilization, 348

Crura cerebri, 157, 459, 530, 584

Crustacea, 297, 386

Crystalline style, 401

,, lens, see Lens of Eye
Ctenidia, 398
Cutaneous glands : Frog, 1 29 ;

Rabbit, 484
Cuticle : in unicellular animals, 251.

264, 269, 286; in multicellular

animals, 313, 328, 369
Cyst, see Cell-wall and Encystation
Cystic duct, see Bile-duct

D.

D

/aughter-cells and nuclei, 250,

254? 562
Death, n, 152, 236
Decalcifying, directions for, 137
Decidua, 602
Decomposition, u, 152; and see

Putrefaction

Degeneration, 421 ; and see Vestige
Dehydrating, directions for, 137
Dental formula, 511
Dental lamina and papilla, 445
Denticles (Nereis), 353
Dentine, 445, 510
Deric epithelium, see Epiderm
Derm, 128, 369, 433, 485
Dermal teeth, 434
Descent, doctrine of, see Evolution
Development, meaning of the term,

9, 276, 284. For development

of the various multicellular types,

see under their names. Practical

work, 212-214, 6°4
Dialyser, 73, 77
Diaphragm, 483, 506
Diastema, 507
Diastole of heart, 90 ; of contractile

vacuole, 265

Diencephalon, 157, 459, 529, 584
Differentiation, 204, 206, 238, 261
Diffusion, 73, 568, 60 1

Digestion, 68 ; intra- and extra-
cellular, 305, 336

Digestive glands, 373, 401, and see
Digestion, Enteric canal, Liver,
Glands, Pancreas

Digestive system, see Enteric canal

Digits, 56, 502, 505

Dimorphic, Dimorphism, 250, 318

Dioecious, 320, 382

Diploblastic, 298, 311

Directions for dissecting, 14.', for
drawing, 14 ; for killing, 31, 239,
259* 27$) 293, 324, 354, 386,
412,542 ; for preparing skeletons,
S3, 394, 472, 542 ; for injecting
blood-vessels, 99, 389, 414, 474,
543 ; for microscopic work, 119 ;
for histological and embryological
work, 135, 603

Disc of Vorticelia, 271

Dispersal, 274, 283, 287

Dissecting instruments, &c. , 12

Distal, 6

Distribution of food-materials, 148

Divergence of character, 223

Division of physiological labour,
206, 238

DOGFISH : General characters, 431 ;
exoskeleton, 433, 472-, endo-
skeleton, 435, 472 ; enteric canal,
444, 473 ; gills» 448, 4?8'-> blood-
system, 449, 474, 477 ; nervous
system, 458, 478 ; kidneys, 466,
476 ; reproductive organs, 467,
475', sense-organs, 464, 479',
development, 469, 581

Dorsal, 6

Drum-membrane, see Tympanic
membrane

Ducts, see under names of individual
ducts and glands

Ductus arteriosus, 550

Duodenum, 22, and see Enteric

Dura mater, 155

E

iJ ar, see Auditory organ ; in-
ternal, middle, and external, 466,
492, 535, 5^9

R R 2

6l2

INDEX

EARTHWORM : general characters,

326 ; metameric segmentation,

327 ; ccelome and enteric canal,
328, 333, 335, jtf, 358; cell-
layers, 329, 358; blood-system,
337, 355* 357, 35$ ;' nephridia,
340, 355 ', 357 > 358', nervous
system, 342, jtf, J5<?; differ-
entiation of organs and tissues,
344 ; reproduction and reproduc-
tive organs, 345, jjj, jj6 ; de-
velopment, 349, 579

Ecdysis, 369

Echinodermata, 412

Ectoderm, 202, 209, 298, 311, 314,

349, 384, 470, 574
Ectoplasm, 231
Efferent duct of spermary, 193, 347,

467, 538
Egestion, 233
Egg of fowl, 564, 573, 604
Egg-cell, see Ovum
Egg-sac, 346
Elasmobranchii, 431
Embryo, 9, 200 ; and see under

various types
Embryology, 217
Embryonic membranes, 596
Emulsification of fats, 75
Enamel, 445, 510
Enamel-organ, 445
Encystation, 232, 244, 254, 275,

283, 285
Endoderm, 202, 210, 298, 303, 311,

314, 349, 384, 470, 574
Endoderm-lamella, 315, 317
Endolymph, 188
Endolymphatic duct, 187, 436,

465

Endoparasite, see Parasite
Endoplasm, 231
Endophragmal system, 363
Endopodite, 364
Endoskeleton : Frog, 16, 35, 205 ;

Lance let, 422 ; Dogfish, 435 ;

Rabbit^ 487 ; development of in

Vertebrates, 594
Endostyle, 424
Energy, conversion of potential into

kinetic, 235 ; source of, in
chlorophyll-containing organisms,
248

Enteric canal : Frog, 23, 204 ;
Earthworm, 328, 335 ; Crayfish,
370 ; Musset, 401 ; Lance let,
424 ; Dogfish, 444 ; Rabbit, 507-
516; development of in Verte-
brates, 204-210, 589

Enteric epithelium, see Epithe-
lium

^Enteroccele, 578

Enteron or enteric cavity, 296, 298,
311, 314, 330, 373

Epencephalon, 584

Epiderm : Frog, 128; Earthworm
328, 329 ; Crayfish, 369 ; Mussel,
400 ; Lance let, 419 ; Dogfish,
433; Rabbit, 484

Epididymis, 467, 538, 594

Epiglottis, 512, 517

Epipharyngeal groove, 424

Epiphysis, 497, 503

Epipodite, 364

Epistoma, 363

Epistylis, 277

Epithelial cells : columnar, 107 ;
ciliated, 109; glandular, 130 et
seq. ; squamous, 1 10 ; stratified,
128

Epithelium, 109, 3i9;ccelomic, 330,
and see Peritoneum ; deric — see
Epiderm ; enteric, 330, and see
Endoderm

Equivocal generation, see Abio-
genesis

EUGLENA : occurrence and general
characters, 251 ; movements,
251; structure, 251; nutrition,
252 ; resting stage, 254 ; re-
production, 254 ; animal or
plant ? 255 ; practical directions,

259

Eustachian tube 'or recess, 17, 189,

5", 536

Eustachian valve, 523

Evolution : organic, 221, 292 ; of

animals and plants, 258
Excretion, 148, 150, 235

INDEX

613

Excretory organs, see Kidney and
Nephridium, and compare Con-
tractile vacuole

Exopodite, 364

Exoskeleton : cuticular, 313, 369,
398 ; dermal, 434, 444 ; epider-
mal, 482, 484

Expiration, 143

Extra-cellular digestion, 234, 305

Eye: Frog, 4, 181 ; Nereis, 351,
354 ; Crayfish, 380 ; Lance let,
424; Dogfish, 465; Rabbit, 486 ;
development in Vertebrates, 587

Eye: compound, 380

Eyelids, 5, 432, 486

Eye-muscles, see Muscles of eye

Eye-spots : Euglena, 254 ; Lance-
let, 424

Eye-stalks, 363, 380

abellrc, 504

Faeces, 8

Fallopian tube, 540

P'amily, 218

Fascia, 59 118

Fat-body, 25

Fats, 72

Femur, 51, 504

Fenestra ovalis, 46, 189, 492, 536

Fenestra rotunda, 493, 536

Ferment, fermentation : amylolytic,
74 ; peptonising or proteolytic,
74 ; putrefactive, 256, 257

Fertilization, 197, 571, and see also
Conjugation, and under develop-
ment of various types

Fibrin, 107

Fibula, 51, 504

Filum terminale, 155, 532

Fingers, see Digits

Fin-rays, 422, 430, 441-443;
dermal, 442

Fins : Tadpole, 207 ; Lancelet, 42 1 ;
Dogfish, 433, 441

Fishes, see Pisces

Fission, 106, 198, 236, 250, 268,
273, 283; multiple, 237, 254,
283, 286

Fissures of spinal cord, 155

Fixing, directions for, 136

Flagellata, Flagellate Infusoria,
261, 292

Flagellum, 242, 251, 256, 303; of
antenna and antennule, 367

Flocculus, 493, 530

Fcetus, 541

Follicle, ovarian, see Ovisac

Fontanelle, 43, 436

Foods, 67, 72

Foot, of mussel, 397 ; and see Pes

Foramen : lacrymal, 494 ; inter-
vertebral, 38, 440, 498 ; magnum,
40, 436, 487 ; obturator, 503 ; of
Monro, 160, 530

Foramina for cerebral nerves, see
Skull

Fore-brain, 202, 584

Fore-gut, 371

Fore-limb or fin, see Appendages

Fornix, 531

Fossa ovalis, 521

Fossils, 223

FROG : Preliminary account, 4 ;
mouth cavity, 1 6 ; skin and
muscles, 17, j/ ; abdomen and its
contents, 20, 32 ; neural cavity
and its contents, 27, jj ; struc-
ture of limbs, 28, 34 ; skeleton,
35» S3 '; joints, 55, 64-, muscles,
57, 64 ; enteric canal and" diges-
tion, 67, j6 ; vascular system,
78, 98 ; circulation of blood, 89,
103 ; lymphatic system, 97, 98;
simple tissues, 104, 121 ; com-
pound tissues and glands, 126,
/J9 ; lungs and larynx, 141, 152 ;
kidneys, 145, 153 ; structure and
functions of nervous system, 1 54,
175 '; sense-organs, 179, iqi ;
reproductive organs, 193, 210 ;
fertilization of eggs, 9, 197 ; de-
velopment, 9, 198, 212, 214,
580, 604 ; metamorphosis, II,
206,212 Classification, 215; side
dissection, 481 ; summary of
characters, 482

Function, see Physiology

614

INDEX

G.

Fall-bladder, 22, 69, 447, 516

Gallus, see Bird

Gamete, 197, 251, 268, 275, 286,
287

Gametocyte, 286, 287, 348, 356

Gamobium, 321

Ganglion, 163, 167 ; and see Nerve-
ganglia

Gastric glands, see Glands

Gastric juice, 71, 74, 132

Gastric mill, 372

Gastrolith, 373

Gastrula, 574, 579

Gastrocnemius, 59

Gemmation, see Budding

Generation, asexual, see Aga-
mobium

Generation, sexual, see Gamobium
,, alternation of, see Alter-

nation of generations

Generative organs, see Reproductive
organs

Genus, 215, 218

Germinal disc, 573

Germinal epithelium, 194, 196,

345, 383

Germinal vesicle and spots, 563

Germ plasm, 572

Gestation, 542

Giant-fibres, 343

Gills : Tadpole, 10, 204, 207 ; Cray-
fish, 374 ; Mussel, 397, 401 ;
Dogfish, 448, 471

Gill-arches and clefts, see Branchial
apertures, arches, clefts and septa

Gill-cover, 361, 374

Gill-rays, see Branchial rays.

Gizzard : Earthworm, 335 ; Cray-
fish, 370

Gland-cells : Hydra, 303, 305 ;
Earthworm, 335, 348 ; and see
Glands and Goblet-cells.

Glands : Cowper's, 537 ; cutaneous,
129, 484 ; digestive, see Enteric
canal; gastric, 131, 447, 514;
green, 375 ; Harderian, 186, 535 ;
lacrymal, 535 ; mammary, 483,
486 ; Meibomian, 535 ; ceso-

phageal, 335 ; perineal, 486, 540 ;
prostate, 539; racemose, 135;
rectal, 540; salivary, 511; se-
baceous, 484 ; and see kidney,
liver, pancreas, &c.

Glands, ductless, 447, 507

Glenoid cavity, 47

Glochidium, 411

Glomerulus, 146, 592

Glottis, 17, 512

Glycogen, 134

Goblet-cells, 109, 130, 131

Gonad, 193 : and see Reproductive
organs

Gonaduct, see Reproductive organs

Gonotheca, 312

Gregarines, 284

Grey matter of spinal cord and
brain, 156, 160, 167

Growth, 236

Gryllotalpa, 567 (Fig. 149)

Gullet, 253, 265, 271 ; and see
Enteric canal

H

JL~1 semal arch and spine, 441
Hsematochrome, 243
Hsematococcus, see SPH/ERELLA
Hcemoccele, 374
Hsemocyanin, 379
Haemoglobin, 107, 339
Hairs, 482, 484
Hallux, 505
Hand, see Manus
Hardening, directions for, ij6
Head, 4, 350, 361, 432, 483
Heart, 20, 79, 87, 378, 407, 449,

520 ; development of, 458, 590 ;

pulsation of, 90
Heat, evolution of, 151
Hemibranch, 448
Hepatic ducts, see Bile-duct
Hepatic caecum, 424
Hepatic portal system, see Portal

system

Heredity, 225

Hermaphrodite, see Monoecious
Heterogenesis, 290

INDEX

615

Hibernation, 8

Higher (tripoblastic) animals, uni-
formity in general structure, 332

Hind-brain, 202, 584

Hind-limb or fin, see Append-
ages

Hind-gut, 371

Hinge of lamellibranchiate shell,
396, 398

Hip-girdle, see Pelvic arch

Hippocampus, 531

Histological methods, 120, /J5,
603

Histology, 104

Holobranch, 448

Holophytic nutrition, 247, 253

Holozoic nutrition, 247, 253

Homogenesis, 290

Homology and homologous, 217,

3i8

Homology, serial, 39, 52, 327

Host, 282, 284, 287

Humerus, 48, 501

Hybrids, 216

HYDRA : occurrence and general
characters, 294 ; species, 296 ;
movements, 296 ; mode of feed-
ing, 297 ; microscopic structure,
297 ; digestion, 304 ; asexual,
artificial, and sexual reproduction,
306 ; development, 308 ; practi-
cal directions, 322

Hydranth, 309 b.

Hydroid polypes, 308

Hydrotheca, 312

Hydrozoa, 321

Hyoid, 40, 44, 438, 496

Hyomandibular, 438

Hypobranchial groove, see Endo-
style

Hypostome, 295, 309, 317

Immortality, 236

Impregnation, see Fertilization

Incisors, 509

Income and expenditure, 148, 236,
245

Incubation, 604

Incus, see Auditory ossicles

Individual, 220, 277

Individuation, 277, 306

Infundibulum, of brain, 159, 459 ;
530, of lung, 518

Infusoria. 261, 292

Ingesta and egesta, balance of,
236

Ingestion, 8, 233

Inguinal canal, 538

Injection of blood-vessels, 99, J<&?5
414, 474, 477,543

Innominate bone, 51, 503

Insertion of muscle, 60

Inspiration, 143

Integument, structure of, 127 ; and
see under various types

Integumentary sense-organs, 430,
464

Intercellular substance, 115, 117

Intern eural plate, 440

Interstitial cells, 298

Intervertebral discs, 497

Intervertebral foramina, see Fora-
men

Intervertebral substance, 440, 441

Intracellular digestion, 234, 305

Interrenals, 447

Intestine, 22 ; see its various sub-
divisions and Enteric canal

Invagination, 384, 574

Invertebrata, 412

Iris, 5, 182,555

Irritability, 60, 169, 232, 272

Ischium, 50, 503

I

leum, 22 ; and see Enteric canal
Ilium, 50, 503
Imbedding, directions for, /J7

j

J acobson's organ, 534

Jaws, 17, 353, 364, 367, 436, 489,

496
Joints, 55, 362, 368

6i6

INDEX

K

K.

^aryokinesis, 562

Keber's organ, see Pericardial gland
Kidney : Frog, 26, 145 ; Crayfish,

375 ; Mussel, 408 ; Dogfish, 466 ;

Rabbit, 537 ; development of, in

Vertebrates, 591

LUMBRICUS, see Earthworm
Lungs, 10, 22, 141, 204, 209, 518,

590

Lymnsea, 603
Lymph, 18
Lymph-hearts, 97, 98
Lymphatic glands, see Adenoids
Lymphatic system, 18, 97, 458

L

L.

^abial palp, see Palp

Labrum, 370

Lacrymal glands, 186, 535

Lacunae, see Bone and cartilage

Lamellae of gills (Mussel), 403

Lamellibranchiata, 411

Lamina terminalis, 531

LANCELET : general characters,
419 ;. fins, 419; skeleton, 422;
gill-slits and bars, 420, 422 ;
enteric canal, 424 ; blood-vessels,
424 ; nephridia, 424 ; nervous
system, 424 ; gonads, 421, 425;
development, 425, 574 ; practical
directions, 426

Larva, 9, 354, 384, 411, 425

Laryngo-tracheal chamber, 141

Larynx, 506, 517

Lateral line, 432, 464

Laverania, see Malaria organism

Legs, see Appendages

Lens of eye, 183, 354, 534, 555

LEPUS, see Rabbit

Leucocyte, 105, 336, 379, 458

Life, origin of, 288 ; and see Bio-
genesis

Life-history, 8, 206 ; and see under
various types

Ligaments, 55, 57

Limbs, see Appendages

Lips, 486, 507

Lithite, 315, 409

Lithocyst, 315

Liver, 20, 133, 204, 447, 516, 589

Lobster, 360

Lumbricida.1, 350

M

_ alaria organism, 287
Malleus, see Auditory ossicles
Malpighian capsule, 146, 538,

592

Mammalia, 219, 418, 482
Mammary glands, see Glands
Mandible, 16, 44, 366, 438, 496
Mantle, 396
Mantle-cavity, 400
Manubrium : of Medusa, 314; of

malleus, 536 ; of sternum, 500
Manus, 5, 50, 484, 502
Marginal sense-organs, see Lithocyst
Marrow-cavity, 48
Matrix, see Intercellular substance
Maturation of ovum, 568
Maxilla : Crayfish, 366
Maxilliped, 364
Meckel's cartilage, 44, 438
Mediastinum, 518
Medulla oblongata, 157, 459, 530,

584
Medullary cord, folds, groove, and

plate, 202, 577 ; sheath, 167 ;

substance of Protozoa, 264, 269,

280, 286
Medusa, medusa-buds, 311, 313,

3H

Megagamete, 275, 287
Meganucleus, 265, 269
Megazooid, 250, 275
Membranous labyrinth, 186, 465,

Mesencephalon, see Mid -brain
Mesenteron, 371 ; and see Arch-

enteron
Mesentery, 22, 447, 515

INDEX

617

Mesoderm, 202, 203, 210, 349, 384,
47°» 5795 vertebral and lateral
plates of, 584

Mesodermal segments, 584

Mesogloea, 298, 311, 314, 317

Mesonephros, 591, 592

Metabolism, 149, 236

Metacarpus, 50, 502

Metacromion, 500

Metamere, Metameric segmenta-
tion, 327, 360, 368

Metamorphosis, n, 209, 276, and
see Larva

Metanephros, 591, 594

Metapleural folds, 421

Metatarsus, 51, 505

Metazoa, 292

Metencephalon, see Medulla ob-
longata

Microgamete, 275, 287

Micrometer, 121

Micromillimetre (/*) = iTnnr of a
millimetre, or ^-^p of an inch

Micronucleus, 265, 269

Micropyle, 415, 564, 571

Microtome, 138

Microzooid, 250, 275

Mid -brain, 202, 584

Mid-gut, 371

Milk-glands, see Glands

Milk-teeth, 510

Milt, see Spermatic fluid

Mitosis, 562

Molars, 509

Mollusca, 220, 411

MONADS : occurrence and general
characters, 256 ; movements, 256 ;
nutrition, 256 ; animals or- plants?
257 ; rapid multiplication, 289 ;
practical directions, 260 .

MONOCYSTIS : occurrence, 284 ;
structure, 285 ; parasitic nutri-
tion, 286 ; conjugation and repro-
duction, 286, 287 ; encystation
and spore-formation, 285, 286 ;
means of dispersal, 287 ; practical
directions, 293

Monoecious, 307

Morphology, 217

Morula, see Polyplast

Mother of pearl, see Nacre
Mounting sections, directions for,

139
Mouth, 4, 16, 253, 265, 271, 295,

311, 315, 370, 401, 421, 43I5486
Mouth-cavity, see Buccal cavity
Movement, spontaneous, volun-
tary, and involuntary, 7, n, 112,

172, 232, 319, 334, 433; and

see under various types
Mucous membrane, 17
Mule, see Hybrid
Mlillerian duct, 466, 594
Multinucleate, 281
Muscle-fibres: striped, 112; un-

striped, in, 131, 133; and see

under various types
Muscle-processes, 298, 304, 311,318
Muscles : Frog, 18, 57, 59, 63, 205 ;

Obelia, 311 ; Earthworm, 329;

Crayfish, 369 ; Mussel, 399, 400 ;

Rabbit, 505 ; and see Myomere
Muscles : of eye, 186, 465, 534 ;

of middle ear, 536
Muscles, papillary, 521
Muscular contraction, 160, 272
Muscular impressions on shell,

399
Muscular layers of enteric canal,

70, 75

Muscular system, see under various
types ; development of, in Verte-
brates, 203, 584, 594

Muscularis mucosse, 132

MUSSEL : general characters, 396,
412 ; mantle, shell, and foot, 396.
412 ; food-current, 404, 412 ;
gills, 397, 401, 414 ; muscles,
399, 400, 413 ; enteric canal, 401,
416 ; nephridia, 406, -//jf ; blood -
system, 407, 414 ; nervous system,
408, 415 ; sense-organs, 409,
416 ; gonads, 409, 41^ ; develop-
ment and metamorphosis, 410,

4*5* 579

Mustelus, 431, 470
Myocomma, 434
Myomere, 203, 369, 419, 423,

434
Myophan layer, 264

6i8

INDEX

N,

N

i acre : nacreous layer, 399
Nasal organ, see Olfactory organ
Naso-lacrymal duct, 186, 535
Naso-palatine canals, see Canals
Naso-pharynx, 512
Natural History, 2
Natural selection, 227
Nauplius, 384
Navicular bone, 504
Neck, 484

Nemathelminthes, 412
Nematocyst, 300, 311, 317
Nephridiopore, 341
Nephridium, 146, 340, 406, 421,

424, 466, 592
Nephrostome, 98, 145, 146, 340,

424, 466
NEREIS : occurrence and general

characters, 350; parapods and

setae, 350-352 ;trochosphere larva,

354 ; practical directions, ^59
Nerve-cells, 167, 303, 318
Nerve-collar, 342
Nerve-cord, ventral, 342, 379
Nerve-fibre, 167
Nerve-foramina, 495 ; and see Skull

and Vertebral column
Nerve-ganglia, 162, 163, 164, 167,

342, 379, 408
Nerve-plexus: brachial, 161, 532;

cceliac, 533 ; sciatic or lumbo-

sacral, 162, 532

Nerve-roots, 163, 425, 461, 532
Nerves : afferent and efferent, 166,

175 ; cerebral, 163, 461 532 ;

of lateral line, 463; motor and

sensory, 162, 169 ; phrenic, 532 ;

sciatic, 62, 162, 532 ; spinal,

160, 425, 461, 532 ; sympathetic,

162, 461, 533 ; vaso-motor, 174 ;

visceral, 380

Nervous impulse, 62, 154
Nervous system : Frog, 155; Hydra,

303 ; Obelia, 318 ; Earthworm.,

342 ; Crayfish, 379 ; Mussel ', 408;

Lance let, 424 ; Dogfish, 458 ;

Rabbit, 528 ; development of, in

Vertebrates, 577, 584

Neural arch, spine, and canal, see
Vertebra

Neural plate and process, 440

Neuraxis, 167

Neurenteric canal, see Canal

Neurilemma, 167

Neuroccele, 459 ; and see Canal,
central of spinal cord, and Ven-
tricles of brain

Neuroglia, 169

Neuropod, 351

Newt, 218

Nictitating membrane, 5, 486

Nodes of nerve-fibre, 167

Nose, see Olfactory organ

Nostrils, 5, 17, 180, 432, 464, 487,
534

Notochord, 203, 419, 441, 578, 594

Notopod, 351

Nuclear division, 561, 603 ; in-
direct, 562

Nuclear membrane, 559 ; spindle,
561

Nucleolus, 109^ seq., 558, 559

Nucleus, 106 et seq., 129, 231, 243,
254, 265, 269, 273, 280, 286, 558

Nucleus, conjugation-, 268 ; seg-
mentation-, 571

Nutrition, 67 ; and see under various
types

O

V_>/BELIA : occurrence and ge-
neral characters, 309 ; micro-
scopic structure, 311 ; structure
of medusa, 314 ; nervous system,
318 ; lithocysts, 315 ; reproduc-
tion and development, 320 ;
alternation of generations, 321 ;
practical directions, j^j

Occipital condyle, see Skull

Ocellus, see Eye-spot

Odontoblasts, 445 '

Odontoid process, 499-

CEsophagus, see Gullet

Qisophageal glands, see Calciferous
glands

Olecranon, 49, 502

INDEX

619

Olfactory capsule, 39, 42, 436, 493
,, lobe, 1 60, 460, 528
,, organs : Frog, 180 ; Cray-
fish, 381 ; Mussel^ 409 ; Dogfish,
432, 464 ; Rabbit, 534 ; de-
velopment of, in Vertebrates, 587

Olfactory pit, 424, 587

Oligochseta, 350

Ommatideum, 380

Omosternum, 48

Ontogeny, 224

Oogenesis,i96, 568

Oosperm, 198, 571 ; and see under
various types

Oosperm, holoblastic and mero-
blastic, 573

OPALINA : occurrence and general
characters, 280 ; nuclei, 280 ;
parasitic nutrition, 282 ; repro-
duction, 282 ; means of dispersal,
283 ; development, 283 ; practical
directions, 293

Optical section, 558

Optic chiasma, see Chiasma ; cup,
588; lobe, 157, 459, 530, 584;
thalamus, 159, 529; vesicle, 587

Oral cavity, see Buccal cavity

Oral hood, 421

Orbit, see Skull

Order, 218

Organ, 30, 151, 238, 344

Organism, 231

Origin of muscles, 60
,, of species, 225

Osmosis, see Diffusion

Osphradium, 409

Ossicles : of gizzard (Crayfish], 372,
390 ; auditory (Rabbit], see Audi-
tory ossicles

Ossification, 44

Ostia : of heart, 376 ; of gills, 404

Otocyst, 315, 409

Otoliths, 1 88, 382

Ovary : Frog, 23, 193, 195 ; and see
Reproductive organs

Oviduct, 25, 196, 346, 382, 469,

540, 594
Ovisac, 195, 540
Ovum, 195, 563, 568, 572 ; and see

under various types.

Ovum : alecithal, centrolecithal, and

telolecithal, 573
Oxidation of protoplasm, 149, 248

X abeontology, 224

Palate, 494, 507

Palatoquadrate, 44, 438

Pallial line, 399

Pallium, see Mantle

Palp : Nereis, 351 ; Crayfish, 367 ;

Mussel, 401
Pancreas, 22, 70, 134, 204, 447,

5i6, 589

Pancreatic juice, 70
Papillae of tongue, 1 80, 509
Parachordal, 595
Paragnatha, 370
Paramylum, 251

PARAMCECIUM : structure, 262 ;
mode of feeding, 266 ; reproduc-
tion, 268 ; conjugation, 268 ;
practical directions, 277
Parapod, 350
Parasite, parasitism, 33, 153, 280,

284
Parietal layer of peritoneum, 26,

331 ; see also Mesoderm
Parthenogenesis, 569
Patella, 504

Pectoral arch, 20, 36, 46, 442, 500
,, fin or limb, see Appendages
Pedal ganglion, 408
Pelecypoda, 411
Pelvic arch, 20, 36, 50, 442, 503

,, fin or limb, see Appendages
Pelvis of kidney, 538
Penis, 487, 540
Pepsin, peptone, 74
Pericardial gland, 406
Pericardial sinus, 376
Pericardium, 20, 401, 434, 520
Perichondrium, 55
Perilymph, 189
Perinaeum, 486
Periosteum, 55
Periostracum, 399

620

INDEX

Perisarc, 312

Peristaltic movements, 75

Peristome, 271

Peristomium, 327, 351

Peritoneum, 22, 26, 330, 434, 506

Pes, 5, 51, 484, 505

Peyer's patches, 515

Phalanges, 50, 51, 502, 505

Pharynx, 17, 335, 445, 448, 512

Phylogeny, 224

Phylum, 219

Physiology, I, 217

Pia mater, 155

Pigment-cells, 128

Pigment-layer of retina, 185

Pigment-spot, 254, 424

Pineal body, 159, 459, 530

Pineal eye, 159, 585

Pinna, 483, 486

Pisces, 219; general characters

of, 418

Pisiform bone, 502
Pithing, directions for, loj
Pituitary body, 159, 459, 530
Placenta, 483, 541, 60 1
Placoid scales, 434
Planula, 321
Plasma, 104

Plastic products, 249, 568
Platyhelminthes, 412, 545
Pleopod, 364

Pleura, pleural membrane, 518
Pleurobranch, 375
Pleuron, Crayfish, 361
Podobranch, 375
Podomere, 363
Polar cells, 569, 605
Pollex, 502
Polycheeta, 350

Polymorphic, polymorphism, 318
Polype, 294, 309
Polyplast, 200, 574 ; and see under

various types
Polystomum, 33
Pons Varolii, 530
Pores, dorsal, 333
Portal system: hepatic, 85, 456 ;

renal, 85, 454
Post-axial and pre-axial borders of

limb, 501, 503

Poupart's ligament, 506

Premolars, 509

Prepuce, 487

Preservative fluids, ij

Primitive groove, 581

Prismatic layer, 399

Proamnion, 597

Proboscis, 353

Processes of skull, 489 el seq.

Proccelous, 36

Proctodaeum, 204, 384, 447

Pronation, 501

Pronephric duct, 591

Pronephros, 591

Pronucleus, male and female, 569,

571

Prosencephalon, 460, 584
Prostate, see Gland
Prostomium, 327, 351
Proteids, 72
Protista, 258

Protococcus, see SPH/ERKLLA
Protoplasm, 1065.558 ; and see Cell
Protopodite, 364
Protovertebra, 584
Protozoa, 220, 292
Proximal, 6
Pseudobranch, 449
Pseudopod, 106, 231, 250, 303
Pterygiophores, 441
Ptyalin, 511
Pubis, 50, 503
Pulmonary artery and vein ; see

Arteries and Veins
Pupil, 5, 182, 354
Putrefaction, 256, 257, 261, and see

Decomposition
Pyloric division and valve, of

stomach, see Stomach
Pyrenoid, 243

R

JtvABBlT : general characters, 482,
542 ; skeleton, 487,54^ ; muscles
and body- wall, 505.547, 54^,549,
552> 55b ; ccelome, 506,545,550,
digestive organs, 507, 545~547 \
respiratory and vocal organs, 517,

INDEX

621

544, 557,55^ ; circulatory organs,
520, 544, 545-547, 55o~554'^
nervous system, 528, 544, 549,
554 ; sense-organs, 533, 552-
556 ; urinogenital organs, 537 ,
545,548 ; development, 541, 581,
583, 594, 596, 601, 605

Racemose, 135

Radial canals, see Canals

Radial symmetry, 296

Radio -ulna, 49

Radius, 50, 501

Rana esculenta, 216

RANA TEMPORARIA, see Frog

Ranidae, 218

Reagents, hardening, preserving,
mounting, and staining, 135

Recapitulation, theory of, 224

Rectal gland, 447

Rectum, 23 ; and see Enteric
canal

Reducing division, 566, 569

Reflex action, 169, 319

Regeneration, 307, 345

Renal portal system, see Portal
system

Reproduction, 8, and see under
various types, and also Asexual
reproduction

Reproductive organs, 23, 25, 193,
307, 320, 345, 382, 409, 421,
425, 467, 538

Reptilia, 219, 418

Respiration, 141, 144, 235, 339,

374, 448, 517

Respiratory movements, 7, 142,

375, 520

Retina, 183, 184, 354, 55^

Retinula, 380

Rhabdome, 380

Rhinencephalon, 586

Rhizopoda, 292

Ribs, 486, 497, 500

Rocks, sedimentary and stratified,

223

Rodentia, 542
Rods and cones, 185
Rostrum, 363, 436
Rudiment, often used for Vestige

(q.v.)

s.

_)acculus, 187
Sacculus rotundus, 515, 546.
Sacrum, 38, 499
Salamander, 218
Salivary glands, see Glands
Saprophytic nutrition, 256, 257
Sarcolemma, 112
Scales, 430, 434
Scapula, 46, 443, 500
Schizoccele, 584
Sclerite, 390
Sclerotic, 182, 534,555
Scrotal sac, 487, 538, 548
SCYLLIUM, see Dogfish
Sebaceous glands, see Glands
Section-cutting, directions for, 126,

o138.

Secretion, 130

Segment, see Metamere, Podomere

Segmentation-cavity, 200, 574, 580

Segmentation of oosperm, 198, 200,

212, 308, 320, 349, 383, 410, 470,

572

Segmentation, equal and unequal,
573; discoid, 573
,, superficial, 383, 573

,, metameric, see Me-

tamere

,, -nucleus, 571

Selection, natural and artificial, 226,

227

Self-fertilization, 348
Seminal funnel, 347

„ vesicle, 194, 347, 467
Sense-organs and cells, 179, 315,
344, 351, 380, 409, 424, 432,

464, 533, 587
Septa, of earthworm, 333
Septum : lucidum, 531 ; nasal, 42,

493 ; interorbital, 487
Serous membrane of embryo, 597,

60 1

Sesamoid bones, 502-505
Seta, 328, 350, 352, 362
Sex-cells, primitive, 566
Sexual characters, external, 7, 382,

433, 486
Sexual generation, see Gamobium

622

INDEX

Sexual organs, see Reproductive

organs
Sexual reproduction, see under

various types
Shank, 5

Shell, 396 ; larval, 410
Shell of egg, 469, 564
Shell-gland of mussel-embryo, 410 ;

of dogfish, 469

Shoulder-girdle, see Pectoral arch
Sinus : blood-, 376, 377, 408, 457,

lymph-, 1 8, 27 ; urinary and

urinogenital, 467 ; venosus, 80,

89, 449, 520

Siphon, inhalant and exhalant, 397
Skeleton, see Endo- and Exo-

skeleton

Skin, see Integument
Skull; Frog, 16, 35, 39; Dogfish,

436 ; Rabbit, 487 ; development

of. 595

Smell, organ of, see Olfactory organ
Snout, 4

Somatic layer of mesoderm, 579
Somatopleure, 579
Spawn, 9
Species, 215 et seq. ; origin of, 222,

225
Sperm, or spermatozooid, 194, 382,

566 ; and see under various types
Spermary, 25, 193, 194 ; and see

Reproductive organs
Spermatogenesis, 194, 565
Spermatophore, 382
Spermiduct, 193, 347, 382, 467,

538, 594

Spermotheca, 347

Sperm-reservoir, 347

Sperm-sac, 347, 467

SPH^RELLA : general characters,
240 ; rate of progression, 240 ;
ciliary movements, 242, 250 ;
colouring matter, 242 ; motile
and stationary phases, 244 ; nu-
trition, 245 ; source of energy,
248 ; dimorphism, 250 ; repro-
duction, 250 ; conjugation, 251 ;
animal or plant ? 255 ; . practical
directions, 259

Sphincter, 71

Spinal cord, 28, 155, 424, 459, 532

Spiracle, 432, 449

Spiral valve, 445, 515

Splanchnic layer of mesoderm, 579

Splanchnopleure, 579

Spleen, 23, 98, 447, 516

Spontaneous generation, see Abio-

genesis

Spores, 254, 275, 285
Sporozoa, 284, 292
Sporozoite, 286
Stalk of Vorticella, 269, 272
Stapes, 46, 189, 492, 536
Starch, 72, 243
Statocyst, 315, 381, 409
Sterilised infusions, 289
Sternal canal, 363
Sternebrse, 500

Sternum, 16, 48, 361, 486, 500
Stimulus, various kinds of, 62
Stock, see Colony
Stomach, 22, 70,401, 445, 512
StomodiEum, 204, 384, 445, 447
Struggle for existence, 226
Submucosa, 130
Substitution of organs, 209
Sucker, 203
Supination, 501

Supporting lamella, see Mesoglcea
Suprarenals, 447 ; and see Adrenals
Suspensorum, 40, 438
Sutures, 50, 487
Swimmeret, see Pleopod
Symbiosis, 304
Sympathetic, see Nerves
Symphysis, 503
Syn-cerebrum, 380
Synovial capsule, 56
Systemic arch, 80, 452, 523
Systole : of heart, 90 ; of contractile

vacuole, 266

1 actile organs, 179, 353, 382, 533
Tadpole, 9, 203 et seq.
Tail, 9, 209, 432, 483
Tapetum, 356
Tarsus, 51, 504
Taste-organs, 180, 534

INDEX

623

Teasing, directions for, 123

Teats, 486, 506

Teeth, 17, 444, 509

Teleostomi, 431

Telson, 361

Tendon, 59

Tentacles, 296, 309, 314, 315, 351

Tergum, 361

Testis, see Spermary

Thalamencephalon, see Dience-

phalon
Thigh, 5

Thoracic duct, 528
Thorax, 361, 483, 486, 506
Thread-cell, see Nematocyst
Thymus, 447, 507, 519
Thyroid, 447, 520
Thyroid cartilage, 517
Tibia, 51, 504
Tibio-fibula, 51
Toad, see Bufo
Toes, see Digits
Tissues, enumeration of, 31
Tongue, 8, 17, 445, 509
Tongue-cartilage or bone, see Hyoid
Tonsil, 507
Trachea, 506, 517
Trabeculse cranii, 595
Transverse process, see Vertebra
Trichocyst, 267
Triploblastic 311, 322, 332
Trochanter, 504
Trochlea, 501
Trochosphere, 353, 354
Trophoblast, 582
Trophozoite, 286
Trunk, 4, 432, 483
Trypsin, 74
Tubercle, and Tubercular facet, 498,

500

Tuberosity, 501, 503
Tunicata, 421
Turbinals, 493
Tympanic cavity, membrane, and

ring, 5, 45, 189, 536
Typhlosole, 335, 401

Umbilical cord and umbilicus, 602

Umbo, 398

Umbrella, 314

Unicellular, 231

UNIO, see Mussel

Urachus, 602

Urea, 66, 147

Ureter, 26 ; and see under various

types

Urethra, see Urinogenital canal
Uric acid, 66
Urinary bladder, 23, 375, 406, 538,

603

,, tubules, see Nephridium
Urine, 8, 66, 147
Urinogenital aperture, 486

canal, 539, 540
,, duct, 194

organs, 193, 466, 537
,, organs, development

of, 591

Urodeles, 219
Uropod, 364
Urostyle, 35, 39
Uterine crypts, 601
Uterus, 540

,, masculinus, 538
Utriculus, 187

v.

u

U

Ina, 50, 502

acuole : contractile, 232, 243,

254, 265, 269 ; food-, 233, 266,

272

Vagina, 540
Valve : of Vieussens, 529 ; spiral

445, 515 ; ileo-colic, 515
Valves : of heart, 88, 376, 521, 523 ;

of shell, 396 ; of veins, 89
Variation, 216, 225, 292
Variety, 225
Vascular system, see Blood-vessels,

Arteries, Veins
Vas deferens, see Spermiduct,

Wolffian duct

Vasa efferentia, see efferent ducts
Veins : Frog, 19, 82 ; Crayfish, 377;

Mussel, 408 ; Dogfish, 454 ;

Rabbit, 525 ; embryo Vertebrate,

590

624

INDEX

Veliger, 411

Velum : of Medusa, 315 ; of Lance-
let, 424

Velum palati, 507
Vena cava, see Veins
Vent, see Anus
Ventral, 6
Ventricle, see Heart
Ventricles of brain, 157, 159, 424,

459, 529, 531
Vermiform appendix, 515
Vertebrata, 219 ; general characters

of, 418
Vertebra and vertebral column, 16,

35. 36, 439, 497
Vertebrarterial canal, see Canal
Vessels, see Blood-vessels
Vestibule, see Urinogenital canal
Vestige, vestigial, 159, 364, 521
Vibrissae, 486
Villi : of intestine, 515 ; ofchorion,

601

Viscera, abdominal, 20, 444, 512
Visceral arches and clefts, 438,

448, and see Branchial apertures
Visceral ganglion, 408
Visceral layer of peritoneum, 27,

331

Visceral mass, 399

Vitelline membrane, 196, 564, 597

Vitreous body of compound eye,

380
,, chamber and humour, 183

Vocal cords, 144, 517
,, sacs, 218

Vomero-nasal organ, 534

VORTICELLA : occurrence and
general characters, 269 ; struc-
ture, 269 ; reproduction, 273 ;
conjugation, 274 ; means of dis-
persal, 274 ; encystation, 275 ;

spore-formation, 275 ; meta-
morphosis, 276 ; practical direc-
tions, 2*j8
Vulva, 487, 540

w.

W

aste-products, 8, 66, 249
White matter of brain and spinal

cord, 156, 160
Wolffian body, see Epididymis

,, duct, 466, 594
Work and waste, 66, 148, 234
Worms, 349
Wrist, see Carpus

x

iphisternum, 48, 500
Y

J_ ellow, cells of Earthworm,

331, 335, 34i
Yolk, yolk-granules or spheres, 195,

564, 568 ; and see under various

types

Yolk-cells, 200, 202
Yolk-plug, 201, 580
Yolk-sac, 471, 581

/^oochlorella, 304

Zooid, and see Megazooid and

Microzooid
Zoology, I

Zoophytes, see Hydroid polypes
Zygapophysis, 36, 498
Zygoma, zygomatic arch, 494, 495
Zygote, 197, 251, 275, 286

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