•

GIFT
MICHAEL

OF-
REESE

BIOLOGY
LIBRARY

Q

*- - -f

*•

A COURSE OF
ELEMENTARY INSTRUCTION

IN

PRACTICAL BIOLOGY

BY

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

ASSISTED BY

H. N. MARTIN, M.A., M.D., D. Sc., F.R.S,

REVISED EDITION
EXTENDED AND EDITED

BY

G. B. HOWES,

ASSISTANT PROFESSOR OF ZOOLOGY, NORMAL SCHOOL OF SCIENCB
AND ROYAL SCHOOL OF MINES,

AND

D. H. SCOTT, M.A., PH.D.,

ASSISTANT PROFESSOR OF BOTANY, NORMAL SCHOOL OK SCIENCE
AND ROYAL SCHOOL OF MINES.

WITH A PREFACE BY
PROFESSOR HUXLEY, F.R.S.

Hontfon :
MACMILLAN AND CO.

AND NEW YORK.
1889

[All Rights reserved.]

n o 1 7

BIOLOGY
LIBRARY

G

First Printed 1875. Printed and Stereotyped 1876.
Reprinted March and Jitly 1877, February and December 1879,
1882, 1883, 1885, March, September, December, 1886, 1887.
Revised and extended edition, 1888.
Rerinted

PREFACE TO THE REVISED EDITION.

THE first edition of the Course of Practical Instruction in
Elementary Biology appeared twelve years ago, and the
motives which led to its publication are fully explained
in the original preface, which is subjoined. The present
edition has been carefully revised and, where necessary,
enlarged by my colleagues Mr Howes and Dr Scott, assistant
Professors in Zoology and ''Botany' in the Normal School of
Science and Royal School of Mines, and such additions
and improvements are entirely their work. But besides
these changes, the reader who compares the two editions
will observe that the order in which the subjects are pre-
sented is completely changed. In the first edition the
lowest forms of life were first dealt with; the series of
plants followed in ascending order ; and then the series of
animals, from the Bell animalcule upwards to the Frog.

No doubt there is much to be said for the principle of
this arrangement, which leads the student from the study
of simple to that of complex phenomena ; but the experience
of the Lecture-room and the Laboratory taught me that

vi PREFACE TO THE REVISED EDITION.

philosophical as it might be in theory, it had defects in
practice.

All the simplest forms of life, which are easily accessible,
are of very minute size and their study involves the use
of high microscopic powers. The student who begins with
them is therefore not merely introduced suddenly into a
region in which everything is new and strange, but he has
to familiarize himself with the use of unwonted means of
exploration. By taking this road, the teacher (to whom the
world of the microscope is so familiar that he is apt to
forget its strangeness to students) sets himself against one
of the soundest canons of instruction, which is to proceed
from the known to the unknown, and from familiar methods
of learning to those which are strange.

After two or three years' trial of the road from the simple
to the complex, I became so thoroughly convinced that the
way from the known to the unknown was easier for students,
that I reversed my course, and began with such animals as
a Rabbit or a Frog, about which everybody knows something,
while their anatomy and physiology is illustrated by in-
numerable analogies with those of our own bodies. From
this starting point we proceeded further and further into
the unfamiliar regions of invertebrate organisation until we
reached the border region between animals and plants,
whence there was a natural and easy ascent to the most
complicated vegetable organisms.

This order is followed in the present edition; which is
greatly improved by the addition of the Earthworm and the
Snail in the series of animal, and of Spirogyra in the series
of vegetable, types.

PREFACE TO THE REVISED EDITION. vii

I have every reason to believe that our course of in-
struction in Elementary Biology has been found useful by
many learners and teachers. But whatever the value of
our attempt to carry out a certain method of instruction, I
am more than ever convinced that the method itself is one
which will eventually be universally adopted, not only by
teachers of the biological sciences as such, but by the
teachers of so much of those sciences as constitute the
foundation of medicine.

No man can be competent to deal with the greater
problems of biology as they are now presented to us, unless
he has made a survey, at once comprehensive and thorough,
of the whole field of biological investigation. The animal
and the vegetable worlds are only two aspects of the same
fundamental series of phenomena, and each is capable of
throwing a flood of light upon the other. I know of no
way by which such a broad, but not superficial, survey can
be effected except the method adopted in this work.

Again, while to my mind, nothing is more to be
deprecated than the compulsory waste of the invaluable
time of students of medicine, upon topics so remote from
the serious business of their lives as are systematic Zoology
and Botany, there is no preparatory discipline so well
calculated to serve as a practical introduction to the study
of Human Anatomy and Physiology, as that afforded by a
proper laboratory course of Elementary Biology.

Sundry experiments have left no doubt upon my mind
that, by following such a course of three or four months'
duration, the medical neophyte is enabled to enter upon
his proper studies, provided with a practical knowledge of

Vlii PREFACE TO THE REVISED EDITION.

Anatomy, of Histology, and of the Elements of Embry-
ology and of Physiology, such as under the present system
is either not acquired at all, or is gained at the expense of
time and labour which can be ill spared from practical
subjects.

T. H. HUXLEY.

November •, 1887.

PREFACE TO THE FIRST EDITION.

VERY soon after I began to teach Natural History, or what
we now call Biology, at the Royal School of Mines, some
twenty years ago, I arrived at the conviction that the study
of living bodies is really one discipline, which is divided into
Zoology and Botany simply as a matter of convenience ; and
that the scientific Zoologist should no more be ignorant of
the fundamental phenomena of vegetable life, than the
scientific Botanist of those of animal existence.

Moreover, it was obvious that the road to a sound and
thorough knowledge of Zoology and Botany lay through
Morphology and Physiology ; and that, as in the case of all
other physical sciences, so in these, sound and thorough
knowledge was only to be obtained by practical work in
the laboratory.

The thing to be done, therefore, was to organize a course
of practical instruction in Elementary Biology, as a first
step towards the special work of the Zoologist and Botanist.
But this was forbidden, so far as I was concerned, by the
limitations of space in the building in Jermyn Street, which
possessed no room applicable to the purpose of a labora-

X PREFACE TO THE FIRST EDITION.

tory ; and I was obliged to content myself, for many years,
with what seemed the next best thing, namely, as full an
exposition as I could give of the characters of certain plants
and animals, selected as types of vegetable and animal
organization, by way of introduction to systematic Zoology
and Palaeontology.

In 1870, my friend Professor Rolleston, of Oxford, pub-
lished his " Forms of Animal Life" It appears to me that
this exact and thorough book, in conjunction with the
splendid appliances of the University Museum, leaves the
Oxford student of the fundamental facts of Zoology little to
desire. But the Linacre Professor wrote for the student of
Animal life only, and, naturally, with an especial eye to the
conditions which obtain in his own University; so that
there was still room left for a Manual of wider scope, for
the use of learners less happily situated.

In 1872 I was, for the first time, enabled to carry my
own notions on this subject into practice, in the excellent
rooms provided for biological instruction in the New
Buildings at South Kensington. In the short course of
Lectures given to Science Teachers on this occasion, I had
the great advantage of being aided by my friends Dr Foster,
F.R.S., Prof. Rutherford, F.R.S., and Prof. Lankester, F.R.S.,
whose assistance in getting the laboratory work into prac-
tical shape was invaluable.

Since that time, the biological teaching of the Royal
School of Mines having been transferred to South Kensing-
ton, I have been enabled to model my ordinary course of
instruction upon substantially the same plan.

The object of the present book is to serve as a laboratory

PREFACE TO THE FIRST EDITION. xi

guide to those who are inclined to follow upon the same
road. A number of common and readily obtainable plants
and animals have been selected in such a manner as to
exemplify the leading modifications of structure which are
met with in the vegetable and animal worlds. A brief
description of each is given; and the description is followed
by such detailed instructions as, it is hoped, will enable the
student to know, of his own knowledge, the chief facts
mentioned in the account of the animal or plant. The
terms used in Biology will thus be represented by clear and
definite images of the things to which they apply ; a com-
prehensive, and yet not vague, conception of the phenomena
of Life will be obtained ; and a firm foundation upon which
to build up special knowledge will be laid.

The chief labour in drawing up these instructions has
fallen upon Dr Martin. For the general plan used, and
the descriptions of the several plants and animals, I am
responsible ; but I am indebted for many valuable sugges-
tions and criticisms from the botanical side to my friend
Prof. Thiselton Dyer.

T. H. H.

LONDON,

September, 1875.

CONTENTS.

I.

FROG.

General characters — Development — Specific characters of Rana tern-
poraria and R. esculenta — Splanchnic (pleuro-peritoneal) cavity and
the organs of the viscera — Neural cavity and the cerebro-spinal axis —
Comparison of transverse sections across the head and trunk — General
comparison with Lobster and Crayfish — Skeleton — Digestive system —
Lymph and blood vascular systems — Heart — Respiratory organs and
respiration — Urinogenital system — Ductless glands — Nervous system —
Sense organs and integument — Laboratory work . . pp. i — 172.

II.

FRESH-WATER CRAYFISH AND LOBSTER.

Habitat— General structure — Exoskeleton and appendages — Diges-
tive system — Circulatory system and heart — Respiratory system — Ex^re-
tory organs — Nervous system and sense organs — Reproductive organs —
Development — Ecdysis — Laboratory work. . . pp. 173— 239.

III.

EARTHWORM.

Habitat — General external characters — Exoskeleton and setse — Di-
gestive system — General structure of body wall and alimentary canal —
Body cavity and mesenteries — Excretory system — Circulatory system —
Neurosensiferous system — Reproductive organs — Development — Clitel-
lum — Laboratory work. ...... pp. 240 — 271.

XIV CONTENTS.

IV.
COMMON SNAIL.

Specific characters of Helix aspersa and H. hortensis — Habitat —
General characters — Shell or exoskeleton — Digestive system — Pulmo-
nary sac and excretory organ — Heart, compared with that of Crayfish —
Circulatory system — Nervous system and sense organs — Reproductive
system — Development of Pond Snail (Lymnceus stagnate] — Polar
bodies — Laboratory work pp. 272 — 304.

V.

FRESH-WATER MUSSEL.

Habitat — General characters — Branchiae and alimentary canal, in
relation to the branchial chambers and the exterior — Circulatory system
and heart — Excretory organs — Digestive gland and glycogenous tissues
— Nervous system and sense organs — Reproductive organs — Develop-
ment— Laboratory work. . . . . . . pp. 305 — 341.

VI.

FRESH-WATER POLYPES.

Habitat and movements — Mode of feeding — Reproduction and repro-
ductive organs — Histological structure— Specific characters of Hydra
fusca and H. viridis— Comparison with green-plant and Amoeba —
Neurosensiferous apparatus — Comparison with development of higher
animals — Laboratory work. ..... pp. 342 — 358.

VII.

BELL- ANIMALCULE.

Infusoria, compared with other animals— BELL- ANIMALCULE —
Habitat and movements — Histological structure — Physiology — Green
varieties and chlorophyll— Reproduction — Encystation — Infusoria asso-
ciated with Frog — Laboratory work. .... pp. 359 — 368.

CONTENTS. XV

VIII.

PROTEUS ANIMALCULE. COLOURLESS BLOOD CORPUSCLES.

AMCEBA. Habitat — Movements — Histological structure — Physical
constitution — Physiology — Chemical composition — Effects of tempera-
ture and electric shocks. COLOURLESS BLOOD CORPUSCLE. Facts
which show Amoeba to be an animal — Reproduction of Amceba —
Laboratory work. pp. 369 — 376

IX.

YEAST.

General characters — Fermentation — Appearance under the micro-
scope— Structure of cells — Chemical composition — Mode of multipli-
cation— Gemmation — Spores — Physiology — Nutrition — Metabolism —
Laboratory work. pp. 377 — 388.

X.

PROTOCOCCUS.

Habitat — Histological structure — Physiology — Chlorophyll — Depen-
dence on light — Mode of multiplication — Macrozoospores — Microzoo-
spores — Laboratory work pp. 389 — 395.

XL

SPIROGYRA.

Habitat — External characters — Histological structure — Cell-division
— Sexual reproduction by conjugation — Zygospore — Physiology — Labo-
ratory work. ........ pp. 396 — 407.

XII.
BACTERIA.

General characters — Structure — Movements — Zooglcea stage-
Spores — Physiology — Relation to putrefaction — Life-history — Labora-
tory work. pp. 408 — 414.

xvi CONTENTS.

XIII.

MOULDS.

Fungi — Physiology — Spores — PENICILLIUM — Habitat — General
characters — Histological structure — Asexual reproduction — Conidia —
Development — EUROTIUM — Habitat — General characters — Reproduc-
tion by ascospores — MUCOR — Habitat — General characters — Histologi-
cal structure — Asexual reproduction — Development — Conjugation —
Germination of zygospore — Laboratory work. . . pp. 415 — 429.

XIV.

STONEWORTS.

Habitat — External characters — Histological structure — Mode of
growth — Sexual reproduction — Development — Physiology — Laboratory
work — Protoplasmic movements. ..... pp. 430 — 442.

XV.

BRACKEN FERN.

External characters — Histological structure — Stem — Leaf— Root —
Physiology — Asexual reproduction — Spores— Germination — Prothallus
— Sexual reproduction —Embryo — Alternation of generations — Labora-
tory work pp. 443—459.

XVI.

BEAN-PLANT.

External characters — Reproductive organs — Development — Histo-
logical structure — Stem — Leaf — Root — Homology of the reproductive
organs with those of the Fern — Physiology — Laboratory work.

pp. 460—481.

APPENDIX. General — On dissecting — On injecting — Microscope and
Microscopic examination — Preparation and use of reagents and culture
solutions — Sections and section cutting. . . . pp. 482 — 498.

INDEX pp. 499— 512.

THE FROG (Rana temporaria and Rana esculenta],

THE only species of Frog indigenous in Britain is that termed
the 'common' or 'Grass Frog' (Rana temporaria\ while, on
the Continent, there is, in addition to this, another no less
abundant species, the hind-limbs of which are considered a
delicacy, whence it has received the name of the 'Edible
Frog' (Rana esculenta). Unless the contrary be expressly
stated, the description here given applies to both species.
The Edible Frog is usually larger than the other, and is
therefore more convenient for most anatomical and physio-
logical purposes.

In the body of the Frog the head and trunk are readily
distinguishable; but there is no tail and no neck; the con-
tours of the head pass gradually into those of the body, the
fore-limbs being situated immediately behind the former.
There are two pairs of limbs, one anterior and one
posterior.

The whole body is invested by a smooth moist integument.
The yellowish ground-colour of the skin is diversified by
patches of a more or less intense black, brown, greenish, or
reddish-yellow colour, and, in the Grass Frog, there is a
large, deep brown or black patch on each side of the head,
behind the eyes, which is very characteristic of the species.
The coloration of different frogs of the same species differs
widely; and the same frog will be found to change its colour,

M. i

2 ELEMENTARY BIOLOGY. [CHAP.

becoming dark in a dark place, and light if exposed to the
light.

The body of the Frog presents only two median aper-
tures, the wide mouth and the small cloacal aperture. The
latter is situated at the posterior end of the body, but rather
on its upper side than at its actual termination. It is com-
monly termed the anus, but it must be recollected that it
does not exactly correspond with the aperture so termed in
the Mammalia.

The two nostrils, or external nares, are seen at some dis-
tance from one another upon the dorsal aspect of the head,
between the eyes and its anterior contour. The eyes are
large and projecting, with well-developed lids, which shut
over them when they are retracted. Behind the eye, on
each side of the head, there is a broad circular area of
integument, somewhat different in colour and texture from
that which surrounds it; this is the outer layer of the mem-
brane of the tympanum, or drum of the ear.

The fore-legs are very much shorter than the hind-legs.
Each fore-limb is divided into a brachium, antebrachium and
manus, which correspond with the arm, fore-arm and hand in
Man. The manus possesses four visible digits which answer
to the second, third, fourth, and fifth fingers in Man. There
is no web between the digits of the manus.

The hind-legs are similarly marked out into three divi-
sions, femur, crus, and pes, of which the femur answers to
the thigh, the cms to the leg, and the pes to the foot, in
Man. The pes is remarkable not only for its great relative
size as a whole, but for the elongation of the region which
answers to the tarsus in Man. It will be observed, however,
that there is no projecting heel. There are five long and
slender digits, which correspond with the five toes in Man,
and are united together by thin extensions of the integu-

I.] THE FROG. 3

ment constituting the web. The innermost and shortest
answers to the hallux, or great toe, in Man.

At the base of the hallux, the integument of the sole
presents a small horny prominence, which overlies a bony
calcar ; sometimes there is a similar but smaller elevation
on the outer side of the foot : but there are no nails upon
the ends of any of the digits of either the pes or the manus.
Thickenings, or callosities, of the integument, however, occur
beneath the joints of the digits, both in the pes and the
manus.

During the breeding season, the integument on the
palmar surface of the innermost digit of the manus, in the
male, becomes converted into a rough and swollen cushion.
This, in the Grass Frog, acquires a dark-brown or black
colour.

The Frog, when at rest, habitually assumes a sitting pos-
ture much like that of a dog or cat. Under these circum-
stances the back appears humped, the posterior half being
inclined at a sharp angle with the anterior half. The ver-
tebral column, however, will be found to be straight, and the
apparent hump-back arises, not from any bend in the verte-
bral column, but from the manner in which the bones
of the hip-girdle are set on to the same.

The walk of the Frog is slow and awkward, but it leaps
with great force, by the sudden extension of the hind-limbs,
and it is an admirable swimmer.

In a living Frog, the nostrils will be seen to be alter-
nately opened and shut, while the integument covering the
under side of the throat is swollen out and flattened. The
sucking in and circulation of the air needed for the Frog's
respiration is connected with these movements.

The upper eyelid of the Frog is large and covered with
ordinary pigmented integument, and it has very little

I 2

4 ELEMENTARY BIOLOGY. [CHAP.

mobility. That which performs the function of the lower
eyelid in Man, is a fold of the integument little pigmented
and, for the most part, semi-transparent, resembling the
nictitating membrane of a bird rather than an ordinary
lower lid. If the surface of the cornea be touched, the
eyeball is drawn inwards under the upper lid, which de-
scends a little, at the same time as the lower lid
ascends over the ball, to meet the upper lid and close
the eye.

As is well known, Frogs emit a peculiar croaking sound,
their vocal powers being more especially manifested in the
breeding season, when they collect together at the surface of
ponds, pools and sluggish streams, in great numbers. At
this season, which commences in the early spring for the
Grass Frog, but much later on in the year for the Edible
Frog, the male seeks the female and, clasping her body
tightly with his fore-limbs, remains in this position for days
or even weeks, until her ova are discharged, when he
fecundates them by a simultaneous out-pouring of the
seminal fluid. Shortly after the eggs pass into the water,
the thin layer of mucus secreted by the oviduct, with
which each egg is surrounded, swells up by imbibition and,
with that which surrounds the others, it gives rise to a
swollen mass, in which the eggs remain imbedded during the
early stages of their development.

The process of fecundation above alluded to, usually
results in the fusion between each egg and one of the sper-
matozoa contained in the seminal fluid, and it is only
when this has been effected that the egg, which is then said
to be fertilized, is competent to reproduce the species. It
follows that \hzfertilized ovum is a compound of the egg of
the female and a spermatozooid of the male ; and it has been
accordingly termed the oosperm, by way of distinction from

I.] • THE FROG. 5

the unfertilized egg (ovarian ovum) as it leaves the body
of the female.

The development of the eggs is closely dependent upon
temperature, being greatly accelerated by warmth and re-
tarded by cold. The process of yelk-division, or segmen-
tation which commences within a few hours of impreg-
nation, can be readily observed when the eggs are examined
as opaque objects under a lower power of the microscope.
It is ushered in by the appearance, on the surface of the
egg, of a furrow which passes completely round the same,
and, gradually deepening, constricts it into two equal
halves. Each of these again becomes subdivided, and, the
process being repeated, there results a mulberry-like mass
of embryo-cells, from which the formative tissues of the
body are ultimately derived. Segmentation such as this,
in which the first furrow completely cleaves the whole egg
into two, each successive one similarly subdividing that cell
with which it is related, is termed complete or holoblastic.

While still within the mucous investment the embryo
assumes the form of a minute fish, devoid of limbs and with
only rudiments of gills, but provided with two adhesive
discs on the ventral side of the head behind the mouth.

After leaving the egg, the larva acquires three pairs of
external branchice having the form of branched filaments, at-
tached to the sides of the hinder part of the head. Narrow
clefts in the skin at the roots of the branchiae lead into the
back of the throat or pharynx. Water taken in at the
mouth passes out by these branchial clefts. The animal
crops the aquatic plants on which it browses, by means of
the horny plates with which its jaws are provided.

In the Tadpole, as the larval Frog is called, the intestine,
which is relatively longer than in the adult, is coiled up
like a watch-spring in the body cavity. A membranous lip,

6 ELEMENTARY BIOLOGY. [CHAP.

the surface of which is beset with numerous horny papillae,
surrounds the mouth, and the muscular tail acquires a large
relative size. The eyes, the nasal and auditory organs,
become distinct, but no limbs are at first visible.

A fold of the integument in the hyoidean region, called
the opercular membrane, now grows back over the external
gills and unites with the integument covering the trunk,
leaving only a small aperture or spiracle on the left side,
through which the ends of the external gills may, for some
time, be seen to protrude. The external gills atrophy, and are
succeeded functionally by short processes developed from
the opposing faces of the branchial clefts — the internal
branchice. The rudiments of the limbs appear, rapidly elon-
gate and take on their characteristic shape, the hind pair
only being at first visible on account of the anterior pair
being hidden under the opercular membrane. The lungs
are developed and, for a time, the tadpole breathes both by
these and by its internal gills.

As the legs grow the tail shortens and, at last, is re-
presented merely by the pointed end of the body ; the gape
elongates until the angle of the mouth lies behind the eye,
instead of a long way in front of it, as in the tadpole ; the
labial membrane and the horny armature of the mouth
disappear, while teeth are developed in the upper jaw and
on the roof of the mouth ; the intestine becomes less and
less coiled as, not growing at the same rate as the body,
it becomes relatively shorter; and the animal gradually
relinquishes its mixed diet for one of animal matter — the
perfect Frog being mostly insectivorous.

The two species Rana temporaria and Rana esculenta,
are distinguishable by the following external characters. In
Rana temporaria, the interspace between the eyes is flat or
slightly convex, and its breadth is usually greater than, or

I] THE FROG. 7

at least equal to, that of one of the upper eyelids. The
diameter of the tympanic membrane is less than that of the
eye, often much less. The horny elevation on the outer side
of the pes Is small or absent, and that on the inner is
flattened and has a rounded margin. A patch of dark
colour extends from the eye backwards over the tympanic
membrane. The males have the cushion on the radial side
of the manus black, and they are devoid of vocal sacs.

In Rana esculenta, on the other hand, the interspace be-
tween the eyes is usually concave and narrower than the
breadth of one of the eyelids. The diameter of the tym-
panic membrane is as great as that of the eye. The horny
elevation on the inner side of the pes is elongated, com-
pressed and brought to a blunt edge, so as almost to resem-
ble a spur, and a small outer elevation is constantly present.
There is no patch of colour at the sides of the head, such
as exists in Rana temporaria, and the cushion of the inner
digit in the male is not black. The males have a large
pouch on each side of the head, behind the angle of the
jaw, communicating with the cavity of the mouth, and, when
they croak, these pouches, becoming dilated, assume the
form of spherical sacs.

Having thus become acquainted with the general cha-
racters and life-history of the Frog, and with those features
of its organization which are visible to the naked eye and
without dissection, its structure may next be studied in
detail.

If the trunk be laid open, it will be found to enclose
a cavity in which some of the most important viscera — the
stomach and intestine, the liver, the pancreas, the spleen,
the lungs, the kidneys and urinary bladder, and the repro-
ductive organs — are contained. As this cavity answers to

8 ELEMENTARY BIOLOGY. [CHAP.

those of the pleurae and of the peritoneum in the higher
animals, it is termed ti\z pleuroperitoneal cavity ; and the soft
smooth membrane which lines it and covers the contained
viscera is the pleuroperitoneal membrane.

The vertebral column traverses the middle of the roof of
this cavity, and the layer of pleuroperitoneal membrane
which lines each lateral wall of the cavity, passes downwards
on each side of the vertebral column and joins its fellow in
the middle line to form a thin sheet, the mesentery, which
suspends the alimentary canal. In the triangular interval
left between these two layers before they unite, a wide
canal (subvertebral lymph sinus), the dorsal aorta and the
chain of sympathetic ganglia, are situated.

The antero-dorsal moiety of the pleuroperitoneal cavity
is occupied by the gullet, which places the mouth in com-
munication with the stomach. Beneath the gullet the
pleuroperitoneal cavity is separated only by a thin partition
from a chamber, the pericardium, which contains the heart.
The posterior face of the partition is constituted by the
pleuroperitoneum, its anterior face by a membrane of
similar character, the pericardia! membrane, which lines the
pericardium and is reflected on to the heart, in the same
way as the pleuroperitoneum lines the pleuroperitoneal
cavity and is reflected on to the intestine. The exposed
surfaces of both the pericardial and pleuroperitoneal mem-
branes are kept permanently moist by a fluid (serous fluid]
which more or less completely fills the cavities which they
enclose; hence they are termed serous membrajies.

A layer of the muscular fibres which enter into the body-
wall is continued inwards at the anterior boundary of the
pleuroperitoneal cavity and is attached to the sides of the
oesophagus and to those of the pericardium, thus consti-
tuting a so-called diaphragm; which, it will be observed, is

I.] THE FROG. 9

situated in front of the lungs, and not behind them, as in
the higher animals.

Thus, in the trunk, on the ventral side of the vertebral
column, the body presents two cavities, a large posterior
pleuroperitoneal cavity, and a small anterior pericardial
cavity; while neither of these communicates directly with
the exterior, there is in the female an indirect communica-
tion by the oviducts.

On the ventral side of the head, the very wide mouth
opens into a spacious buccal cavity, the roof of which is hard
and firm, while the floor is soft and flexible, except so far as
the middle of it is occupied by a broad, flat, for the most
part gristly plate, the body of the hyoid bone. Within the
lips the upper jaw is beset with numerous sharp small teeth,
and two clusters of similar teeth are to be seen in the fore
part of the roof of the mouth ; the latter, being attached to
the bones termed the vomers, are the vomerine teeth, while
the former, attached to the premaxill<z and maxilla, are
maxillary teeth. The lower jaw or mandible is edentulous.

At the sides of the clusters of vomerine teeth are the
apertures termed posterior nares, by which the nasal chambers
communicate with the mouth. At the sides of the back part
of the throat two wide passages, the Eustachian recesses, lead
into the tympanic cavities, which are closed externally by the
tympanic membranes. In the male Rana esculenta the small
apertures of the vocal sacs are seen on the inner side of each
ramus of the jaw, close to the angle of the gape below and in
front of the Eustachian recesses. In the middle of the back
of the throat is the opening of the oesophagus, closed by the
approximation of its sides except during deglutition, while in
the median line of the hinder part of its floor lies a longitu-
dinal slit, the glottis. A fleshy tongue, bifurcated and free
at its posterior end, is attached anteriorly to the middle part

10 ELEMENTARY BIOLOGY. [CHAP.

of the lower jaw. In a state of rest, therefore, it lies on the
floor of the mouth with its free end turned backwards, and
one point on each side of the glottis.

The gullet, after traversing the so-called diaphragm,
passes into the elongated stomach. At its posterior end this
narrows and joins the slender small intestine. Though short,
this is too long relatively to the length of the pleuroperi-
toneal cavity to lie straight in it. It is, therefore, thrown
into sundry folds which are suspended to the dorsal wall of
that cavity in the manner before described. Finally, the
small intestine enters the dilated short large intestine, and
this opens into a chamber with muscular walls, the cloaca,
the external aperture of which has been already mentioned.

Thus the alimentary canal is a tube which traverses the
body from the oral to the anal apertures ; and the heart, en-
closed in the pericardium, is situated in the middle line on
the ventral side of the same.

Separated from the pleuroperitoneal and buccal cavities by
the bodies of the vertebrse and the hard roof of the buccal
chamber which continues the direction of these forwards, is
an elongated cavity, widest in the head but becoming very
narrow posteriorly, which is closed on all sides by the bony
and other elements of the head and spinal column. This is
the neural cavity and contains the brain and spinal cord,
which together constitute the cerebro-spinal nervous axis. The
neural cavity is lined by a serous membrane resembling the
peritoneum and the pericardium, and this arachnoid mem-
brane is reflected on fo and covers the contained cerebro-
spinal axis, so that the latter is related to it as the heart is to
the pericardial membrane.

The cerebro-spinal nerves which are given off from the
brain and spinal cord pass to their destination through the
boundary walls of the neural cavity.

I.] THE FROG. II

A transverse section of the head in the region of the eyes
will shew, in the middle line, a dorsal cavity in which the
anterior part of the cerebro-spinal axis, the brain, is con-
tained, separated by the solid floor of the skull from a
ventral cavity, the mouth.

A transverse section of the trunk will shew a dorsal
cavity containing the posterior part of the cerebro-spinal
axis, the spinal cord, separated by the solid floor of the
vertebral column from a ventral cavity enclosed by the
alimentary canal and continuous with that of the mouth.
But the backward continuation of the alimentary canal lies
within the large pleuroperitoneal chamber, of which there is
no indication in the head.

If a transverse section of the trunk of the Frog be com-
pared with one across the middle of the body of the Crayfish
or Lobster (Chapter n.) it will be seen that while the chief
nervous centre is on one side of the alimentary canal and
the heart on the opposite side in both cases, the face of the
body on which the nervous centre lies is that on which the
Crayfish or Lobster naturally rests, while in the Frog it is
the reverse. The limbs are turned towards the neural side
in the Crayfish and away from it in the Frog, and the like
difference obtains between all Arthropoda and all Vertebrata.

Using the term skeleton, in its broadest sense, for the
framework which protects, supports and connects the various
parts of the organism, it consists in the Frog of four kinds of
tissue ; the Horny, the Osseous, the Cartilaginous and the
Connective. Moreover, the hard parts are either developed
in the integument, constituting an exoskeleton, or they are
deeper seated and belong to an endoskeleton.

The exoskeleton in the Frog is almost absent, being
represented only by the horny coating of the calcar.

12 ELEMENTARY BIOLOGY. [CHAP.

The endoskeleton, on the contrary, is well developed and,
as in all the higher Vertebrata, may be resolved into an
axial and an appendicular portion.

The adult axial endoskeleton is represented by the spinal
column and the skull.

The appendicular endoskeleton consists of the limb-bones
and the pectoral and pelvic arches to which they are at-
tached.

In the order of development, the endoskeleton is at first
represented by a membranous rod or notochord alone;
secondly, nascent connective tissue and cartilage are super-
added to the notochord ; thirdly, these acquire their special
characters ; fourthly, they become replaced by bone, wholly
or in part.

The processes of conversion and replacement indicated
under the last head are very incomplete, even in the adult
Frog, for remains of the notochord are to be found in the
centres of the vertebrae (intra-vertebral bodies) ; while the
cartilage, of which the greater part of the skeleton was at
one period of larval existence composed, to a great extent
persists.

Such cartilage is found forming the free surfaces of the
bodies of the vertebrae, the extremities of the caudal style
(itrostyle) and the ends of the transverse processes ; and it
enters largely into the girdles. In the skull, nearly all the
bony elements may be removed, leaving behind the primi-
tive cartilaginous skull, or Chondro-cranium, altered only so
far as parts of it have been replaced by bone.

It furnishes a floor, side walls and roof to the brain-case,
interrupted only by three spaces (fontanelles) in its roof
covered in by membrane, and by the foramina for exit
of the cranial nerves. In front it is continued forward

I.] THE FROG. 13

between the two nasal sacs, as a cartilaginous partition
(septum narium), from which are given off, dorsally and
ventrally, transverse alae of cartilage which furnish a roof
and a floor, respectively, to the nasal chambers. These
pass into one another where the chondro-cranium ends an-
teriorly and give rise to a truncated terminal face, the lateral
angles of which are produced outwards and forwards into
two flattened prce-nasal processes; these widen externally
and support the adjacent portions of the premaxillae and
maxillae. From the ventral face, just behind the truncated
anterior end of the chondro-cranium, spring two slender
cartilages, the rhinal processes. Each of these inclines to-
wards the middle line and ends against the middle of the
posterior face of the ascending process of the premaxilla
by a vertically elongated extremity. An oval nodule of
cartilage is attached to the posterior face of the above-
named process, and serves to connect it with the
rhinal process. On the dorsal face of the chondro-cranium,
just above the point of attachment of the rhinal processes,
the external nasal apertures are situated, and the outer and
posterior margins of each of these apertures are surrounded
and supported by a curious curved process of the cartilagi-
nous ala — the alinasal process. Where the sphenoidal and
the ethmoidal portions of the sphenethmoid meet, a stout,
transverse, partly osseous and partly cartilaginous bar is
given off, which is perforated at its origin by the canal for
the orbito-nasal nerve. It then narrows, but becoming
flattened from above downwards, rapidly widens again, and
its axe-head-like extremity abuts against the inner face of
the maxilla. The anterior angle of the axe-head is free;
the posterior angle is continued back into a slender carti-
laginous pterygoid rod which bifurcates posteriorly; this (the
suspensorium) furnishes the articulation for Meckefs carfl-

14 ELEMENTARY BIOLOGY. [CHAP.

lage which forms the core of the ramus of the lower-jaw or
mandible.

The floor of the mouth is supported by a plate of carti-
lage (the hyoid\ this is attached to the skull by a slender
cartilaginous band or cornu which is connected with the
auditory capsule.

The pectoral and pelvic arches (see Laboratory work)
are, in the young state, paired undivided cartilages, and the
development of bone in and upon them does not really
destroy this continuity, the cartilage persisting at the ends
of the bones, and between them in the cavities for articu-
lation of the limb bones.

In like manner, the bones of the limbs consist originally
of cartilaginous models of the perfect bone ; but, as deve-
lopment proceeds, the middle of the cartilaginous model
commonly becomes invested by a sheath of true bone, while
calcareous deposits take place in the cartilage close to its
growing extremities. As the bone grows, the superadded
sheath invades the middle of the cartilage and more or less
replaces it ; while the terminal portions of cartilage continue
to grow and enlarge, and the calcareous deposit within them
increases, without however reaching their surfaces. Thus
one of the larger adult limb bones (say the femur) consists
of a median portion or shaft, and of two terminal caps of
cartilage containing calcined epiphyses overlying, and more
or less enclosed within, the hollow ends of the shaft.

Seeing that the bony elements of the adult skeleton arise
by superaddition to and replacement of a cartilaginous pre-
decessor, it follows that those portions of it which remain
permanently cartilaginous are persistent representatives of
latter or embryonic endoskeleton.

The general disposition of the parts which are seen in
the mouth has already been described.

I.] THE FROG. 15

Teeth are present only in the upper jaw and roof of the
mouth. They are small, with recurved and pointed crowns.
New teeth are constantly being developed in the gum to
replace those which are worn out or broken away, and as
they attain their full size these teeth become ankylosed to
processes of the subjacent bone. Their distribution and
characters are such as to render mastication an impossibility ;
they are prehensile and utilized as hooks by means of which
the animal as it were gets outside its prey. The eversible
tongue is utilized as a means of capture of smaller creatures,
chiefly insects.

The gullet passes without change of diameter into the
stomach, which lies on the left side of the pleuroperitoneal
cavity and is nearly as long as it. The stomach narrows
posteriorly and the almost tubular terminal portion bends
round sharply and passes into the small intestine. A slight
constriction (pylorus] marks their point of junction. The
small intestine runs forwards parallel with the stomach, so
that with the latter it forms a sort of loop; it is continued
on coiled up into a sort of packet which lies to the right
side of the pleuroperitoneal cavity, being held in its place
by a mesenteric fold of the peritoneum. Thence the small
intestine proceeds backwards in the middle line and opens
into the anterior end of the dilated large intestine.

The inner wall of the stomach is raised up into a number
of strong longitudinal folds which project into its cavity
and give it a stellate appearance in transverse section.
Similar but more delicate folds are met with in the small
intestine, the lining-membrane of which is produced into
parallel series of semi-lunar folds whose free edges project
backwards.

The opening of the ileum into the large intestine is val-
vular, its edges projecting backwards into the cavity of the

1 6 ELEMENTARY BIOLOGY. [CHAP.

latter. On the dorsal aspect the large intestine presents a
slight forward dilatation, which may be regarded as a rudi-
ment of a caecum.

The liver is very large, and incompletely divided into
two lobes, the left of which is further subdivided into two.
The gall bladder is attached to the posterior face of
the right lobe. The bile duct opens into the duo-
denum, at some distance behind the pylorus, running
through the pancreas and receiving the duct of that organ
on its way.

The rounded spleen lies in the mesentery, projecting
more to the left than to the right side, near the point at
which the small intestine first becomes coiled.

The apparatus of circulation in the Frog consists of the
blood and lymph vessels and their contents.

The lymph is a colourless fluid containing colourless
nucleated corpuscles which exhibit amoeboid movements:
it is contained partly in large spaces immediately beneath
the integument; in the pleuroperitoneal cavity and pro-
bably in the other serous cavities; and, partly, in capillaries
and larger trunks which are interlaced with and accompany
the blood-vessels. The largest of the trunks is the great
sub-vertebral lymph-sinus, which lies between the layers of
the root of the mesentery and communicates by small pores
with the pleuroperitoneal cavity.

The blood consists of a colourless plasma which contains
colourless corpuscles, similar to those of the lymph, and in
addition a great number of oval nucleated red corpuscles.
It is contained in the blood-vessels, which consist of capil-
laries, arteries and veins, the two "latter being connected on
the one side by the capillaries and, on the other, by the
heart into which they open. The lymphatics and the
blood-vessels are brought into connexion with one another

L] THE FROG. 17

at opposite ends of the body, through the agency of two
pairs of contractile muscular sacs (lymph-hearts), which pump
the lymph contained in the wide lymphatic vessels and in
the pleuroperitoneal cavity into the great veins in their
neighhourhood.

The blood and lymph systems may thus be regarded as
subdivisions of a common circulatory system.

The heart is connected with the walls of the pericardium,
by the vessels which enter and leave it, and by a slender
band which passes from the dorsal face of the base of the
ventricle to the posterior dorsal wall of the pericardial
chamber.

The heart consists of four readily distinguishable seg-
ments, (i) the sinus venosus, (2) the atrium, (3) the ven-
tricle, and (4) the truncus arteriosus, disposed in such a
manner that the sinus venosus, which is the hindermost
division, lies in the middle line on the dorsal aspect
of the heart : the atrium is also median and on the
dorsal side but is in front of the sinus venosus ; the
ventricle is median, ventral and posterior; and the truncus
passes obliquely forwards from the right side of the
ventricle and is ventral and anterior. The heart there-
fore may be compared to a tube divided by constriction
into four portions and bent somewhat into the shape of
an S.

The sinus venosus receives on each side, in front, a large
vein, the vena cava superior; while behind the, usually sin-
gle, vena cava inferior opens into it. It opens by a valvu-
lar aperture into the atrium. The latter shews no signs of
division externally, but internally, it is divided by a deli-
cate partition, the septum of the auricles, into a smaller left
auricle and a larger right auricle. The sinus venosus opens
into the atrium, to the right of the septum and therefore

M. 2

1 8 ELEMENTARY BIOLOGY. [CHAl>.

into the right auricle. Into the left auricle, the common
pulmonary vein, formed by the junction of the veins from
the right and left lungs, opens.

At its posterior end the atrium opens by the auriculo-
ventricular aperture into the ventricle.

A small valve, prevented from flapping back by fine ten-
dinous cords, exists on each side of this aperture, and the
septum of the auricles is continued back upon the faces of
these valves and ends by a free edge between them, thus
dividing the auriculo-ventricular aperture itself into two
openings.

The walls of the sinus and of the atrium are very thin.
Those of the ventricle, on the other hand, are thick and
spongy, only a comparatively small, transversely elongated,
cavity being left at its anterior end or base. At the
right-hand extremity of this is the aperture which leads
into the truncus arteriosus. Three semilunar valves, which
open from the ventricle into the truncus, surround this
opening.

The walls of the truncus arteriosus are thick and mus-
cular, though not nearly so thick as those of the ventricle.
At its anterior end it appears to divide into two trunks,
which diverge and immediately leave the pericardium to
pass on to the sides of the gullet. The elongated undi-
vided part is the pylangium, the terminal part common to
the divergent trunks is the synangium. The former is
divided throughout its length by a sort of fold which is
attached to the dorsal wall while its opposite edge is free.
Three semilunar valves separate the pylangium from the
synangium, in which are the openings, posteriorly, of the
pulmonary arteries, anteriorly of the carotid trunks ; while,
at the sides, the cavity of the synangium opens into those
of the right and left aortic arches. The apparently simple

I.] THE FROG. 19

branches into which the truncus arteriosus divides, are, in
fact, each made up of three separate trunks, the pulmo-
cutaneous trunk behind, the aortic arch in the middle and
the carotid trunk in front.

When the heart is in action, the sinus venosus, the atrium,
the ventricle and the truncus arteriosus contract in the
order in which they have just been named. Each contracts
as a whole, so that the two auricles are emptied simulta-
neously. The blood from each is forced into the correspond-
ing half of the spongy cavity of the ventricle, so that the
right half of the ventricle contains venous blood and the
left arterial blood. When the systole of the ventricle takes
place, the blood which is first driven into the truncus arte-
riosus (the opening of which is, as has been seen, at the
right end of the cavity) is therefore venous. It fills the
conus arteriosus and, finding least resistance in the short
and wide pulmonary vessels, passes along the left side of the
median valve into them. But as they become distended
the next portion of blood, consisting of the venous and
arterial blood which have become mixed in the middle of
the ventricle, passes on the right side of the longitudinal
valve into the aortic arches. And, as the truncus becomes
more and more distended, the longitudinal valve, flapping
over, tends more and more completely to shut off the
openings of the pulmonary arteries and to prevent any
blood from flowing into them.

Finally, the last portion of blood from the ventricle,
representing the completely arterialized blood of the left
auricle which is the last to arrive at the opening of the
truncus, passes into the carotid trunks and is distributed to
the head.

The principal vessels of the Frog are disposed as fol-
lows : —

2 — 3

2O ELEMENTARY BIOLOGY. [CHAP.

A. Arteries. Efferent in relation to the heart, breaking up
into lesser branches.

1. The system of the anterior aortic arch (carotid
trunks), distributing blood to the parts of the head
generally and to the brain.

2. The system of the middle aortic arch (aortic trunk),
distributing blood to the body and limbs, together
with the organs of the viscera other than the lungs.

3. The system of the posterior aortic arch (pulmo-
cutaneous trunks), distributing blood to the lungs
and skin.

B. Veins. Afferent in relation to the heart, formed by the
union of lesser factors (they break up only in the case
of the portal veins).

T. The system of the superior cava, formed on each
side by the union of the veins bringing back the
blood from the head, fore-limbs, and parts adjacent.
Special veins (great cutaneous) are concerned in the
return of the blood from the skin.

2. The system of the inferior cava, formed by the union
of the efferent-renal, genital and hepatic veins.

3. The system of the anterior abdominal and renal-portal
veins ; formed by the bifurcation of the veins coming
in from the hind-limbs. The former receives blood
from the urinary bladder and the body-wall, and
at its anterior end divides into two branches
which go to the corresponding lobes of the liver.
The latter gives off branches (afferent renal veins)
which break up in the substance of the kidney.

4. The system of the vena porta (hepatic-portal vein)
formed by the union of the veins which bring back

1.] THE FROG. 21

the blood from the alimentary-canal and its appended
glands, with the spleen. This vessel enters the liver
on the left side, breaking up within that organ; before
doing so, it enters into a direct anastomosis with the
anterior abdominal vein.

5. The system of the pulmonary vein, formed by the
union of the veins of the right and left lungs.

The slit-like glottis of the Frog is formed by the appo-
sition of two longitudinal folds of the mucous membrane
of the mouth, each of which contains a cartilage of similar
form. These cartilages are the arytenoid cartilages. They
are articulated with an annular cartilage (cricoid) which
supports the wall of the very short laryngeal chamber.
When the two folds of the glottis are divaricated, there are
seen between them two membranous pouches, the free
edges of which meet in the middle line, while anteriorly
and posteriorly they pass into the mucous membrane which
lines the faces of the longitudinal folds. These are the
vocal ligaments , and the slit between them is what answers
to the glottis in Man. It is by their vibration that the
croak of the Frog is produced.

Laterally the laryngeal chamber opens into the lung of
each side. The lung is a transparent oval sac, somewhat
pointed posteriorly, which lies at the side of the oesophagus
in the anterior region of the pleuroperitoneal cavity. It
is covered by a layer of the pleuroperitoneal membrane
which represents the visceral layer of the pleura in the
higher animals. The wall of the pulmonary sac is pro-
duced inwards so as to give rise to a network, which is
much more prominent in the anterior than in the posterior
part of the lung and divides the periphery of the cavity

22 ELEMENTARY BIOLOGY. [CHAP.

into numerous air-cells, on the walls of which the ramifi-
cations of the pulmonary vessels are distributed.

The lungs are elastic, the distended lung collapsing sud-
denly when it is pricked, and they contain abundant mus-
cular fibres.

It is essential to respiration that the mouth should be
shut/and it is said that frogs may be asphyxiated by keeping
their mouths open.

Inspiration is effected by a buccal force-pump. The
mouth being shut and the external nostrils open, the floor
of the mouth is depressed, and the buccal cavity fills with
air. The muscles of the flank next come into play, exer-
cising a pressure upon the surrounding viscera which suffices
to expel the air (expiration).

The nostrils being then shut, the hyoid, and with it the
floor of the mouth, is raised, and, the aperture of the gullet
being at the same time closed, air is forced through the
glottis, distending the lungs.

Expiration is doubtless aided by the contraction of the
intrinsic muscular fibres of the lungs ; and it may be that
those fibres which form the so-called diaphragm also tend
to diminish the capacity of the lungs.

In addition to its principal pulmonary apparatus of re-
spiration, the Frog has a secondary respiratory apparatus in
its moist and delicate skin. A considerable amount of venous
blood is, in fact, constantly supplied to this organ by the
large cutaneous branch of the pulmo-cutaneous artery. It
has been experimentally ascertained that frogs in which the
lungs have been extirpated will continue to live and respire
for a considerable time, especially at a low temperature, by
means of the skin.

The kidneys are elongated and flattened from side to side,
and are kept in position by the continuation of the peri-

I.] THE FROG. 23

toneum over their ventral faces. The ducts of the kidneys
pass along their outer edges and, approaching as they pass
backwards, open by two slit-like apertures in the posterior
wall of the cloaca.

The urinary bladder (iirocysf] is a large bilobed sac,
opening posteriorly, by a wide median aperture, into the
antero-ventral end of the cloaca.

The testes are ovoidal yellowish bodies situated in front
of the kidneys and enveloped in peritoneum, a fold of which,
forming a sort of testicular mesentery or mesorchium^ is de-
rived from that which covers the ventral face of the kidney.
The delicate vasa efferentia of the testes may be seen travers-
ing this fold to enter the substance of the kidney. They
communicate with the urinary tubules, and thus the duct of
the kidney serves not only as the duct of the urinary se-
cretion but as the vas deferens.

The spermatozoa of the Frog are filiform bodies with
cylindrical, more or less linear, nucleus bearing heads.

The ovaries are broad lamellar organs, very large and
much folded and plaited in the breeding season. In-
numerable ovisacs, or follicles, containing dark-coloured
ova, are scattered through the substance of the ovary and
give rise to projections on its surface as they become
fully developed. The ova are finally thrown off by
dehiscence.

The oviducts are long convoluted tubes situated on each
side of the dorsal wall of the abdominal cavity, to which they
are connected by peritoneal folds; each curves over the
outer face of the root of the lung. Their anterior ends are
very slender, and terminate by open mouths at the sides of
the pericardium, between the attachment of the so-called
diaphragm and the lobe of the liver. For the greater part
of their length their walls are thick and glandular, and

24 ELEMENTARY BIOLOGY. [CHAP.

swell up when placed in water. Posteriorly, the oviducts
dilate into capacious thin-walled chambers and end, close
together, by openings which are situated in the dorsal
wall of the cloaca immediately in front of the apertures
of the ureters. Remnants of the oviducts are usually pre-
sent in the male.

Each ovum, when ripe, consists of a protoplasmic yolk-
laden mass Qi'vitellust enclosed in a structureless vitelline
membrane, and containing a germinal vesicle, within which
are several 'germinal spots.' One half of the vitellus is
deeply coloured, the other pale.

The so-called ductless glands other than the spleen
already described, are three in number. The Thymus ; a
small rounded body situated immediately behind the sus-
pensorium, in a position corresponding to the dorsal ends of
the obliterated branchial arches. The Thyroid; represented
by two or more oval bodies, which are found attached to
the bases of the aortic arches. The Adrenals ; yellow
bodies imbedded in the ventral face of the kidney.

The actions of the different parts of the organism of the
Frog are coordinated with one another and brought into
relation with the external world by means of the muscular
and nervous systems and the organs of sense.

The muscles consist partly of striped and partly of un-
striped fibres, the former being confined to the muscles of
the head, trunk and limbs and the heart, while the latter
are found in the viscera and vessels. The former are usually
arranged in sets, the actions of which may be antagonistic
and simple or coordinated and complex. An account of
the disposition of the muscles in the hind-limb will be
found in the Laboratory work.

The nervous system is conveniently divisible into two
parts, the cerebro-spinal and the sympathetic. The cerebro-
spinal nervous system again consists of the brain, or encepha-

I.] THE FROG. 25

Ion, with its nerves, and the spinal cord, or myelon, with its
nerves.

The encephalon lies in the cranial cavity, which it nearly
fills, and is divisible into the hind-brain, the mid-brain and
the fore-brain, which last again comprises three divisions;
the median thalamencephalon, and the paired cerebral
hemispheres and olfactory lobes.

The greater part of the hind-brain is formed by the
medulla oblongata, which is the continuation of the myelon
forwards and presents, when laid open from its dorsal
aspect, a triangular cavity, the apex of which is directed
backwards. The cavity is the fourth ventricle ; it com-
municates behind with the central canal of the myelon,
while, in front, it narrows into a passage which connects the
fourth ventricle with the brain cavities anterior to it. The
thick lateral ridges of nervous substance at the sides of the
fourth ventricle, which represent the restiform bodies, pass,
in front, into the outer extremities of a short broad tongue-
shaped plate, convex ventrally and concave dorsally, which
overhangs the anterior part of the fourth ventricle, and
is the cerebellum.

In front of this, the dorsal moiety of the mid-brain is
formed by two oval bodies, the optic lobes. When laid open,
each is seen to contain a cavity or ventricle which com-
municates with the Her a tertio ad quartum ventriculum,
as the ventricle of the mid-brain is termed. The floor
of this canal is formed by the thick principal mass of
the cerebro-spinal axis. It exhibits a median longitudinal
depression or raphe, and in this region represents the crura
cerebri.

In front of the mid-brain comes the hinder division of
the fore-brain, or thalamencephalon, which is very distinct in
the Frog and contains a median cavity, the third ventricle.

26 ELEMENTARY BIOLOGY. [CHAP.

On each side, the cavity of the third ventricle is bounded
by a thick mass of nervous matter into which the crura
cerebri pass. These are the optic thalami. The roof of
the third ventricle is very thin and easily torn through ; its
fore part is prolonged up to form a delicate process in con-
nexion with an ovate body, which is lodged between the
posterior parts of the cerebral hemispheres and represents
a portion of the pineal gland. The front part of the floor of
the ventricle, on the other hand, is produced into a back-
wardly directed process, the infundibulum. This is con-
nected below with a highly vascular mass, the pituitary
body.

The hemispheres are elongated bodies, broader behind
than in front, where they are marked off only by a slight
constriction from the olfactory lobes.

Anteriorly, the wall of the third ventricle is thickened
to form the lamina terminalis ; on each side, between this
and the -peduncle of the pineal gland, is a small aperture,
the foramen of Monro, which places the median third-
ventricle in communication with the paired lateral ventricle
of the cerebral hemisphere.

The lateral ventricle is continued on either side into the
base of the olfactory lobe to form the olfactory ventricle. In
front these lobes become nerve-like cords, which leave
the skull and spread out on the posterior faces of the
olfactory sacs.

The inner faces of the olfactory lobes are confluent with
each other, and there pass between corresponding parts
of opposite halves of the brain bands of fibres or com-
missures (see Laboratory work) the most important of which
runs through the substance of the lamina terminalis and
represents the corpus callosum.

The entire cerebro-spinal axis is invested in a highly

I.] THE FROG. 27

vascular membrane or pia mater. This becomes immensely
thickened, above the hind-brain and on the inner faces
of the hemispheres, to constitute the chor old plexuses. The
former of these is median and overlies the greater part
of the fourth ventricle. The latter is paired j in its growth
each half forces its way into the lateral ventricle, pushing the
thin inner wall of the same in front of it ; while therefore
it lies, in a sense, within that cavity, it is in reality
outside it.

There are ten pairs of cranial nerves ordinarily so-called,
the first two of which are proved, by their development, to
be lobes of the brain.

1. OlfactorU.

The olfactory lobes. Their nerves are distributed
exclusively to the olfactory sacs.

2. Opt id.

These diverge from the base of the brain in front of
the infundibulum. They are originally outgrowths
of the thalamencephalon which secondarily become
connected with the optic lobes. They are distributed
exclusively to the retina of the eye.

Of the remaining cranial nerves five pairs leave the skull
in front of the auditory capsules, while one pair enters those
capsules and two pairs pass out behind the same.

The Praauditory nerves are the following.

3. Motores oculorum

arise from the front part of the floor of the mid-brain
and are distributed to all the muscles of the eye
except the external rectus, the superior oblique and
the retractor bulbi.

28 ELEMENTARY BIOLOGY. [CHAP.

4. Pathetici

arise from the floor of the mid-brain and pass out,
on the dorsal aspect of the brain, between the cere-
bellum and the optic lobes. They are distributed
to the superior oblique muscles of the eye.

5. Trigemini

take their origin in the front part of the floor of the
hind-brain and, passing out at its sides, each dilates
into a yellow enlargement — the Gasserian ganglion —
which lies in front of the auditory capsule, in the
foramen by which the nerve, after leaving the ganglion,
passes out of the skull.

This ganglion is connected with the trunk of the sixth
and seventh nerves and with the anterior end of the sym-
pathetic (some of the branches which appear to be given
off from it really belong to the sixth and the seventh nerves,
see infra]. Beyond the ganglion, the nerve divides into
two main branches, the orbito-nasal, and the maxillo-mandi-
bular.

i. The orbito nasal (usually termed the first division of
the fifth nerve); passing through the antorbital pro-
cess of the skull, it is finally distributed to the nasal
mucous membrane and to the integument of the nose.
It anastomoses with the fourth nerve, and sends a
branch to the Harderian gland.

ii. The maxillo-mandibular ; forks over the mouth-cavity
into two trunks, usually termed the second and third
divisions of the fifth nerve.

a. Preoral or Maxillary^ passes outside the eye and
is distributed to the integument of the upper jaw.

I.] THE FROG. 29

b. Postoral or Mandibular, passes between the
temporal and pterygoid muscles, over the articu-
lation of the mandible and along the inner face
of the latter, to the symphysis, giving off branches
to the integument, muscles, teeth and tongue.

6. Abducentes

arise from the floor of the hind-brain and leave the
ventral surface of the medulla oblongata close to the
middle line. Each then unites so closely with the
Gasserian ganglion and with the orbito-nasal division
of the fifth as to appear to be only a subdivision
of the latter. Its fibres are distributed to the external
rectus and retractor bulbi muscles of the eye.

7. The Faciales

take their origin from the floor of the hind-brain,
behind the fifth and in common with the eighth;
and, leaving the hind-brain, enter into close con-
nexion with the Gasserian ganglion. Each then
divides into two branches, an anterior and a pos-
terior.

i. The anterior or palatine nerve; distributed to the
roof of the mouth and palate.

This nerve is in anastomosis with the maxillary
branch of the fifth.

ii. The posterior branch. It forks over the tympanic
cavity into two trunks ; a smaller pre-tympanic one,
and a larger post-tympanic or hyoid nerve, which
supplies the muscles of the hyoid and the floor of
the mouth.

8. The Auditorii

arise in common with the foregoing. Each divided

30 ELEMENTARY BIOLOGY. [CHAP.

into two branches which enter the auditory capsule to
reach the organ of hearing.

The Post-auditory nerves are:

9. The Glossopharyngei.

These nerves arise, side by side with the next, from
the medulla oblongata; and the roots of both leave
the skull by an aperture behind the auditory capsule
on each side, and form a common ganglion. Each
passes downwards and forwards to the root of the
tongue, which organ it finally supplies. Moreover, it
gives off muscular branches and forms an anastomo-
sis with the seventh.

10. The Pneumogastrid or Vagi.

Immediately after leaving the ganglia these nerves
separate from the glossopharyngeal and each gives
off a cutaneous branch to the dorsal integument of
the head and trunk : it then divides into two
branches, one of which is distributed to the larynx,
the other to the heart, lungs, and stomach. (Sym-
pathetic fibres are in part bound up with these.)

The myelon or spinal cord is continued back from the
hind-brain as a subcylindrical cord, which lessens somewhat
rapidly towards its apparent end at the level of the seventh
vertebra. It does not really end here, however, but is con-
tinued back as a slender filament, \\\tfilum terminate, to the
commencement of the canal of the urostyle. The diameter
of the cord is somewhat enlarged opposite the origin of the
nerves for the limbs. In transverse sections, the cord is
seen to be not truly cylindrical but to be indented by
two longitudinal grooves, one dorsal and one ventral, which
leave but a small connecting bridge between its two halves.

I.] THE FROG. 31

In the centre of this is a canal, the canalis centralis, the
cavity of which is continued forwards into the axial ven-
tricles of the brain.

Ten symmetrically disposed pairs of nerves come off
from the sides of the cord, each nerve having two roots, one
from the dorsal surface of the lateral half of the cord and
one from the ventral surface. The dorsal root presents a
small ganglionic enlargement, beyond which it is bound up
in a common sheath with the ventral root to form the com-
mon trunk of the spinal nerve. The roots of the hinder
spinal nerves are very long and lie, side by side, in the
spinal canal.

The first spinal nerve leaves the neural canal by the
interspace between the arches of the first and second
vertebrae, so that there is no nerve in the Frog answering to
the suboccipital. It gives a branch to the muscles which
move the head upon the backbone, but the main trunk of it
descends behind the mandible, along with the glosso-
pharyngeal nerve, and is distributed to the muscles of the
tongue. Its distribution therefore answers to that of the
hypoglossal nerve in the higher Vertebrata.

The second and third spinal nerves unite to form a
'brachial plexus, and are distributed chiefly to the fore-
limb.

The fourth, fifth and sixth spinal nerves go to the middle
parietes of the body.

The seventh, eighth and ninth, are large nerves which
unite to form the lumbosacral plexus, whence nerves are
given off to the posterior parietes of the body, and to the
hind-limb.

The tenth spinal nerve leaves the neural canal by the
coccygeal foramen, and is distributed to the adjacent
parts.

32 ELEMENTARY BIOLOGY. [CHAP.

Sympathetic.

The sympathetic system consists of ganglia, usually ten
in number on each side, connected by longitudinal com-
missures, and situated on the ventral face of the vertebral
column ; in the region of the dorsal aorta they come into
close relation with it. Each sympathetic ganglion is joined
by a communicating filament or ramus with one of the
spinal nerves, and the most anterior ganglia are connected,
in the same way, with the ganglia of the ninth and tenth
cerebral nerves. Thence a delicate cord passes into the
cranial cavity on the inner side of the periotic capsule, and
unites with the Gasserian ganglion.

The branches of the sympathetic accompany the vessels,
and large branches are given off to the viscera.

The sheath of the ganglion of each spinal nerve encloses
a milk-white body (periganglwntc gland) whose function is
unknown. Each consists of a tubular gland, lined by a
single-layered epithelium and containing calcareous matter
in a finely crystalline state. When fully formed these glands
force their way through the inter-vertebral foramina, appearing
as a series of concretionary masses lying around the spinal
nerves at their points of exit from the column, and alternating
with the bases of the transverse processes of the vertebrae.

The Olfactory organs occupy all the space between the
mesethmoid cartilage, the antorbital processes, and the
premaxillse and maxillae, and open in front and externally
by the external nares, behind and internally, into the mouth-
cavity, by the posterior nares. Each consists of an essential
part or olfactory sac, the inner face of which is lined by a
peculiar epithelium, which receives the olfactory branches
of the trigeminal nerves, and an accessory part consisting of
a partly cartilaginous, partly bony, capsule.

I.] THE FROG. 33

The Eyeball is lodged in the orbit and protected by the
eyelids, described above. It has four recti muscles which
proceed from the inner wall of the orbit, and are attached
to the circumference of the globe; within these is a retractor
muscle with similar attachments, ensheathing the optic
nerve, while two oblique muscles proceed from the anterior
and inner wall of the orbit and are attached to the dorsal
and ventral faces of the bulb. In addition, a fine tendon
passes from the outer end of the lower eyelid, or nictitating
membrane, and is attached to the fibres of the retractor
bu!bi—t\\Q effect of which is that when the bulb is retracted
the nictitating membrane is raised over the eye. The upper
lid has no muscles. A secretory organ, termed the Harde-
rian gland, is situated in the anterior part of the orbit
beneath the superior oblique muscle.

The essential part of the eye is the inner lining or retina,
which receives the fibres of the optic nerve ; to this there
are superadded a vascular pigmented choroid and a car-
tilaginous sclerotic, which together constitute an accessory
capsule. The lens is nearly spherical.

The Ear consists of an essential part — the membranous
labyrinth — receiving the fibres of the auditory nerve, lodged
in an accessory partly osseous, partly cartilaginous, periotic-
capsule; to the latter are superadded the columella auris
and the tympanic membrane.

The labyrinth consists of three semicircular canals which
open into a vestibule divided into utriculus and sacculus.
The latter, especially, contains a great quantity of white
crystalline calcareous otoliths.

On the outer side of the vestibule is a small dilatation
which is possibly a rudimentary cochlea.

The membranous labyrinth is filled with a fluid (endolymph),

34 ELEMENTARY BIOLOGY. [CHAP.

and contained, as has been stated, in the periotic capsule
into which it fits but loosely; the interval between the two
being filled with a fluid, the perilymph. In the outer face
of the periotic capsule is an opening, ihefenestra ovalis, into
which the end of the columella aitris fits. This columella is
shaped like a pestle, the end of the handle of which is fitted
with a cross-piece. The rounded inner end of the pestle,
which is fixed by fibrous tissue into the fenestra ovalis, is
cartilaginous. The middle of the handle is ensheathed in
bone, while the outer part is cartilaginous. The cross-piece
is fixed into the inner face of the membrana tympani, which
is covered externally by the integument, and lined internally
by the mucous membrane of the tympanic cavity, which is
continuous with that of the mouth through the Eustachian
recess, and wraps round the ventral face of the columella.
A sound wave, impinging upon the drum of the ear, is trans-
mitted through the agency of the columella to the perilymph
and endolymph, auditory epithelium and nerve, to the brain.

The Tongue. This organ, as has been seen, is fixed only
in front to the mandible, and by the anterior half of its ven-
tral aspect to the floor of the mouth; the posterior half
being free and bifid at the extremity. Narrow-ended and
broad-ended papillae (papilla filiformes and fungiformes) are
scattered over the whole dorsal aspect of the tongue and are
largest in front ; small glands lie between these papillae.

The fungiform papillae contain the ultimate ramifications
of the glossopharyngeal nerve, and the epithelium covering
their summits is peculiarly modified.

The Integument. No special organs of touch have been
observed, but the integument is remarkable for the immense
number of close-set simple glandular caeca (cutaneous glands}
which open upon its surface. In the swollen integument

J.] THE FROG. 35

which covers the base of the inner digit in the males, large
papillae with interposed glands are developed.

A singular spheroidal sac-like body known as the browspot
or inter-ocular gland, occurs in the integument of the fronto-
parietal region of the head. In the young animal it is in
connection with the pineal gland, and represents the rudi-
ment of the median or pineal eye — a structure which is
more fully developed in some Fishes and Lizards.

Cells containing pigment abound in the integument and
undergo remarkable changes of form, the pigment being
sometimes drawn together into a spheroidal mass — at other
times distributed in a radiating fashion.

LABORATORY WORK.

Frogs may best be killed with chloroform. Place the
animal under an inverted tumbler together with a square
inch of cotton-wool saturated with chloroform ; cover with
a cloth and leave for 10 — 15 m.
A. General external characters.

1. Its division into head^ trunk, two pairs of limbs or
appendages.

2. Its anterior or head end; pointed and bearing the
mouth, eyes, and other organs of the higher senses.
Its posterior or tail end. Its dorsal or back region j
clad in a darkly coloured integument. Its ventral
or belly region; clad in a whitish integument. Its
lateral or side area ; bearing the two pairs of limbs.
(Work over the specific characters given on pp. 6, 7.)

a. The head.

Somewhat triangular, with the blunted apex
turned forwards and passing, without any neck-
constriction, into the trunk.

3—2

36 ELEMENTARY BIOLOGY. [CHAP.

a. The prominent eyes with their eyelids.

j3. The membrana tympani^ a thin part of the in-
tegument stretched over a hard ring, on each
side, behind and somewhat below the eyes.

y. The anterior nares ; two small apertures near
the middle line at the end of the snout.

S. The mouth-opening; note its boundaries and
extent.

e. The browspot ; lying in the mid dorsal integu-
ment of the head on a level with the front of the
eyes. Clearly visible under a hand lens.

£. Open the mouth widely; the fleshy tongue will
be seen, with its bifurcated free end turned
backwards ; teeth are present only in the upper
jaw and on the roof of the mouth.

b. The trunk.

Tapering towards the hinder end, and allowing
the hard parts of the skeleton to be felt beneath
the soft integument on the dorsal side, and in
the anterior half of the ventral aspect. Note the
cloacal aperture near the dorsal surface of its
posterior end.

f. The limbs.

a. The anterior pair; their three subdivisions,
brachium, antebrachium, and manus ; the four
digits.

fi. The posterior pair; their length as compared
with that of the anterior; their subdivision into
femur, cms, and pes: the five long digits; the
well-developed web; the horny prominence on
the inner side.

I.] THE FROG. 37

B. Preliminary, and general disposition of the viscera,
i. Place the animal on its back and pin down high and
dry, inserting the pins through the limbs and point of
snout. Raise the ventral integument with forceps and
slit it up with scissors along the middle line from the
lower jaw to the origin of the hind limbs; pin back
the two flaps and note : —

a. The skin; it is but loosely connected with the sub-
jacent muscular wall of the trunk, the two being
separated by spacious chambers (lymph cavities, cf.
Sect. I. a).

b. A large vein (great-cutaneous) on the under-surface
of each flap of skin.

c. Some of the muscles of the body wall ; covered by
a thin sheath or aponeurosis, through which they
can be seen —

a. The rectus abdominis; running from pelvis to
sternum along the middle line, and divided into
a number of segments by transverse tendinous
intersections.

A large vein (anterior-abdominal") will be seen,
running along the ventral middle line, immediately
under cover of this.

ft. "£}\£. pectoralis ; an immense tract of tissue arising
from the ventral wall of the trunk externally, and
in front ventrally, to a. It is subdivided into a
series of lesser muscles which converge towards
the base of the fore-limb. (Note that it is
confluent posteriorly with a, and, in front, sub-
divided up by tendinous intersections.)

y. The mylohyoid or sub-maxillaris ; a sheet of
muscle, tendinous in the middle line, whose

38 ELEMENTARY BIOLOGY. [CHAP.

fibres pass transversely across the floor of the
mouth. It is subdivided into a larger fasciculus
which supports the floor of the mouth, and a
smaller ribbon-shaped one which passes back on
either side to the angle of the lower jaw.

8. The external oblique; underlying and external to
/?; forming an investment for the body wall
laterally. Its fibres run obliquely upwards and
forwards.

e. The internal oblique ; seen on removing a portion
of 8. Its fibres pass obliquely downwards and
backwards.

£. The transver sails; seen on removing a portion of
e. It forms the inner layer of the muscular
constituent of the body- wall; its fibres run
transversely, so as to encircle the trunk ventro-
laterally.

2. Raise the tissues of the body-wall with a pair of
forceps and carefully divide them a little to the right
of the median line, so as to open the underlying body-
cavity without injuring its contents; prolong the
incision from the pelvis to the posterior end of the
breast-bone. Make a transverse incision close to the
pelvis and throw back the flap on each side : on the
under side of the left flap will be seen the anterior ab-
dominal vein (cf. i c. a).

Seize the posterior border of the sternum with a
pair of forceps and raise it up ; then, with a strong pair
of scissors, cut through the hard parts a little to one
side of the middle line, being very careful not to
injure the organs beneath them. Raise each half of

I.] THE FROG. 39

the shoulder girdle thus liberated and remove it, with
care, together with the rest of the ventral portion
of the body- wall.

A spacious cavity (pleuro-peritoneal cavity) will be
laid bare. Note the general characters and mutual
relations of the undermentioned organs of the viscera
which lie within it, as seen in the undisturbed state.
They will be found severally connected with the body-
wall and each other by a delicate membrane or
mesentery.

a. The liver: a great brown two-lobed mass, its left
lobe the larger and subdivided into two, lying in
the anterior end of the cavity.

b. The lungs: the posterior ends of these may be seen
as sacculated pouches, one on each side of the liver,
but they are frequently not visible until the latter
organ has been displaced.

c. The stomach: a small portion of this is seen pro-
jecting beyond the lower left border of the liver.
Note that it lies exclusively to the left side.

d. The intestine: a convoluted tube, continuous with
the stomach, and occupying the middle third of the
cavity. It can be resolved into —

a. The small intestine; a yellowish-white tube, con-
voluted and situated wholly to the right side.

(3. The large intestine; a short straight greyish-green
tube, of greater calibre than a, passing obliquely
backwards in the middle line.

e. The urinary bladder (urocyst], a thin-walled bilobed
sac (which may or may not be distended) lying just
in front of the pelvis, immediately above (ventral
to)/?.

4O ELEMENTARY BIOLOGY. [CHAP.

In R. temporaria, the urinary bladder is larger
proportionately and more deeply lobed than in R.
csculenta.

f. The spleen: a small red body lying in the middle
line, immediately in front and to your right of the
large intestine.

g. The kidneys; two dark red lentil-shaped bodies,
seen, on displacing the intestine, lying close to-
gether in the middle line.

h. The genital glands; immediately adjacent (ventral

and external) to g.

a. In the male. The testes: a pair of yellowish
bodies lying near the anterior ends of the
kidneys.

P. In the female. The ovaries; a pair of blackish
yellow bodies, coincident in position with, but
much more extensive than, a. Each is folded,
and seen to be composed of an immense number
of spherical ova.

i. The fat masses (corpora adiposd] ; long filiform
deep-yellow processes attached to the anterior ends
of the genital glands. They are much the larger in
the male.

j. The genital ducts; obvious in the female as highly
convoluted dead-white tubes (oviducts) lying imme-
diately beneath /?. (Those of the male may be
better studied later on.)

k. The heart; seen lying within a delicate sac (the
pericardium) immediately in front of the liver.

Slit open the pericardium. It will be found to
contain a fluid (serous fluid) which bathes the en-
closed viscus (heart). Examine the pleuro-peri-

I.] THE FROG. 41

toneal cavity; a similar fluid will be found to be
present, but to a lesser degree, imparting a moist-
ness to the organs therein contained.

C. The study of transverse sections. Especially with a
view to ascertaining the limits and mutual relationships
of the larger cavities and the body-wall, together with
the mode of suspension of the viscera.

Obtain a frog (preferably one which has been for
24 hours or longer in spirit) and make transverse
sections across its body, as directed below. Examine
under water.

i. Across the mid region of the trunk; to pass through
the kidneys, genital glands, and large intestine.

a. The body-wall ' ; especially its muscular constituent,
which is thickened dorsally in the region of the
back.

b. The splanchnic (pleuro-peritoneal'] cavity ; ventral
and spacious ; it lodges the organs of the viscera,

c. The neural cavity; relatively small, situated in the
dorsal middle line and enclosed on all sides by
the bony vertebral column. It lodges the central
nervous system, seen, in section, as a delicate white
cord.

d. The alimentary canal; cut across at various points.
Remove those portions of it which do not happen
to be connected by mesentery with the body-wall
or other parts. The large intestine will be seen in
section in the ventral middle line ; take note of its
position and relationships.

e. The kidneys ; lying close together, dorsally to all
the other organs and obliquely disposed.

42 ELEMENTARY BIOLOGY. [CHAP.

f. The genital glands; immediately external and
ventral to e. (cf. Sect. B. 2. h.)

g. The pleu ro-peritoneal membrane. This will be most
readily seen stretching from the dorsal border of
the kidney to the upper angle of the body-wall;
having found it, proceed as under.

a. Start from one kidney and follow it upwards ; on
reaching the body-wall it becomes closely applied
thereto, lining it, in the form of a dense pig-
mented membrane (parietal-layer we peritoneum).

ft. Follow it downwards on the same side. It passes
over the outer face of the kidney and then gives
rise to a fold which suspends the genital gland
(mesorchium ^ , mesoarium f) • the folds of
opposite sides then meet in the middle line and
pass down as a double membrane (mesentery)
which suspends and enwraps the intestine.

Little difficulty will be found with the above, if,
starting from the Sidney, the membrane be care-
fully torn away as followed.

y. The cisterna magna (sub-vertebral lymph sinus) -}
a spacious cavity into which the kidneys project,
enclosed between the folds of the mesentery
dorsal to the alimentary canal. The cut ends of
a large blood-vessel (dorsal aorta) and of certain
small nerves will be seen lying within it.

2. Obliquely forwards, across the anterior end of the
body cavity ; to pass through the heart, the anterior
border of the liver and the roots of the lungs. Cf.
generally with i. and note in addition.

THE FROG. 43

a. The (esophagus; a thick walled tube lying at or near
the middle of the whole section.

b. The lungs; thin walled spongy sacs (tubular in sec-
tion) right and left of a.

c. The liver; occupying the greater portion of the
pleuro-peritoneal cavity and lying immediately below
a. and b.

d. The heart; situated in the middle ventral line.

e. The neural canal and central nervous system; rela-
tively much larger than in Sect. i.

f. The body-wall; in part inflected, giving rise to a
sheet of muscle (so-called diaphragm} which is
attached, on opposite sides, to the oesophagus.

g. The aorta; here paired (aortic arches).

h. The pericardium; a double-walled sac enclosing
the heart, and related to it as is the pleuro-peri-
toneal membrane to the body-wall and its con-
tained viscera.

/. The pleuro-peritoneal membrane. (Cf. generally
with previous section.) Note that having sus-
pended the organs above-named, it passes round
the pericardium, giving rise to a fold (falciform
ligament} which is reflected on to the body-wall
below, there being thus formed a septum which
completely subdivides the pleuro-peritoneal cavity,
in this region, into two.
Its special folds are :
a. The ligamentum latum; suspending the lung to

the oesophagus.

(3. The lesser omentum; passing between the oeso-
phagus and liver in the middle line.

44 ELEMENTARY BIOLOGY. [CHAP.

y. The falciform ligament (suspensory ligament of
the liver). Cf. supra. If the section has passed
through the anterior border of the liver, this will
also be seen to pass between its outer edges and
the body-wall.

j. The sub-cutaneous lymph-spaces ; seen (in this and
the preceding section) as a series of four spacious
chambers, lying between the body-wall and in-
tegument, and separated from each other by mem-
branous partitions.

k. The anterior abdominal vein; this will probably be
seen, enclosed within the folds of the mesentery
immediately below the liver.

3 Across the head ; to pass through the mouth-cavity
and eyes.

a. Note the absence of body-cavity and the relatively
great size of the neural canal and central nervous
system (brain).

b. The mouth cavity; spacious, and enclosed on all
sides in a soft mucous membrane.

c. The tongue; a median up-growth of the floor of the
mouth, supported at its base by a cartilaginous
plate, the hyoid.

d. The eyes ; spherical sacs with firmly resistant cap-
sules, closely applied to the outer integument.
Note, in connection with them,

a. The eye-muscles; passing backwards and down-
wards, between the eye-ball and cranial wall.

ft. The eye-lids ; folds of integument above and
below the eyes, continuous at their bases with
a delicate fold of skin (conjunctiva) which invests

I.] THE FROG. 45

the outer face of the eye-ball. The upper lid is
thick and fleshy; the lower (nictitating membrane]
is thin and transparent.

e. The sub-cutaneous lymph spaces ; present only above
and below.

D. The alimentary system.

i. Obtain a male frog, and pin down upon its back
under water. Remove the whole of the ventral
integument and body-wall and dissect away the
heart. Next remove the dorso-lateral portion of the
body-wall, between the back of the head and the
level of the hinder third of the large intestine, and
with it the kidneys and testes. Carefully dissect
away the mesenteric folds which support the liver
and bladder, and wash clean.

a. The general disposition of the parts. Cf. Sect. B.

2«-/

b. The pancreas : a Y-shaped compact yellow mass
lying in the mesentery between the stomach and
head of the intestine.

c. The gall-bladder: a small green sac, lying between
the right lobe of the liver and the head of the
pancreas.

d. The small intestine; of uniform calibre through-
out. It may be resolved into

a. The duodenum; a short straight segment running
parallel with the stomach and related to the
pancreas.

(3. The ileum; the terminal coiled portion, desti-
tute of connection with the pancreas.

46 ELEMENTARY BIOLOGY. [CHAP.

€. The large intestine; cf. Sect. B. 2. d.

f. The lungs ; apparent as sac-like diverticula of the

gullet, immediately in front of the liver (leave

them in place).

2. Lay open the interior of the stomach and duodenum.
Wash carefully and examine in water under a hand
lens.

a. The stomach: its lining membrane (mucous mem-
brane) is smooth and thrown (if the viscus be not
distended with food) into a number of irregular
longitudinal folds or rugce.

Note the nature of its contents ; consisting of
worms, insects, or other small animals in a partially
digested state.

/>. The duodenum. Note the shaggy nature of its
lining membrane ; it is sharply marked off from
that of the stomach at

c. the pylorus; a valve-like fold, obvious as a con-
striction at the point of junction between a. and b.

d. The ileum; slit it open and examine under a hand
lens.

a. Its contents; food material in a highly emulsified
assimilable condition.

/3. Its lining membrane; beset by numerous semi-
lunar folds (intestinal valves] arranged in parallel
series. These may be ill defined.

e. Open up, in like manner, the large intestine.

a. Its contents; food material in a dry state, little
assimilable and green (faces). Note that the
colour of the same is identical with that of the
bile seen in the gall-bladder.

I.] THE FROG. 47

(3. Its lining membrane; smooth and comparatively
thin.

3. The liver, bile and pancreatic ducts. Cf. Sect. B. 2. a.
Gently squeeze the gall-bladder between the finger
and thumb of the left hand, keeping your attention
fixed on the duodenum and pancreas. The bile-
duct will be thereby injected and the bile will be
seen to enter the duodenum by a small orifice
situated at about its middle.

a. The common bile-duct; its terminal third is plainly
visible, as a delicate tube (now injected green)
emerging from the pancreas to enter the duode-
num. Follow it up; it runs through the pancreas
towards the gall-bladder.

b. The hepatic ducts ; variable in number. Seen, on
moving the gall-bladder about, to pass from the
liver and unite to form the main duct a.

c. The gall bladder; a blind sac, set on at the ex-
treme end of the above system of ducts. Open it,
and note that it communicates with the liver
through the agency of the hepatic ducts alone.

d. The pancreatic ducts ; one or more in number;
seen, on careful examination, to enter the bile-
duct as it courses through the pancreas.

4. The cloaca and bladder. Cut away the front of the
pelvis with a stout pair of scissors, taking care not to
injure the bladder: remove the front wall of the
latter, together with a portion of that of the large
intestine. Pin the bladder aside and examine under
water.

a. The cloaca; the terminal portion of the alimentary

48 ELEMENTARY BIOLOGY. [CHAP.

canal, at that point at which it receives the urino-
genital ducts.

Examine its lining membrane ; it is for the most
part identical with that of the large intestine ; that
of its terminal third has the characters of the inte-
gument.

b. The ureters (urinogenital ducts $ . Cf. Sect. F. 3);
their openings will be seen as two minute orifices,
on the dorsal wall of a., situated close together and
surrounded by tumid lips.

c. The bladder ; a median ventral diverticulum of a.,
having no direct communication with the ureters.

E. The cavity of the mouth and the respiratory organs.

i. Use the specimen dissected above. Cut the ali-
mentary canal across immediately in front of the
liver, and retain the anterior portion. Pin down
under water ventral surface uppermost • enlarge the
mouth opening by cutting through the sides of the
buccal cavity with a pair of scissors : pull down the
lower jaw and pin it back.

a. The two internal openings of the nasal cavities
(posterior nares); near the anterior end of the roof
of the mouth.

b. The openings of the Eustachian recesses; situated
far back near the angles of the jaw.

c. The teeth; confined to the upper jaw and roof of
the mouth.

a. The maxillary teeth; forming a parallel series,
set along the inner face of the whole upper jaw.

I.] THE FROG.

ERSITY

(3. The vomerine teeth ; two oval patches situated
close together near the middle line, immediately
internal to the posterior nares.

d. The tongue; heart-shaped and projecting freely
backwards; its free end is prolonged back into
two fleshy cornua. Note its mode of attachment.

Examine its surface under a hand lens; it is
beset by a number of elevations (papillce).

e. The glottis ; a median longitudinal slit, situated im-
mediately behind the tongue, on the summit of an
elevation of the floor of the mouth (larynx). Pass
a probe down it and note that it enters the lungs.

Occasionally, the mucous membrane at its an-
terior end is prolonged into two short papillae,
which project forwards and probably represent rudi-
ments of the epiglottis.

Close the mouth ; dissect away the mylohyoid muscle
and examine from beneath.

a. The larynx: forming a hard prominence in the
middle line, between and in front of the lungs.

b. A series of small muscles will be seen arising on
all sides and converging immediately in front of a.
Dissect them away on one side ; there will thus be
laid bare the hyoid (cf. Sect. C. 3. c.\ Examine
this, in relation to the larynx.

a. Its body ; a plate of cartilage closely applied to
the floor of the mouth, immediately in front of
the larynx.

/?. Its posterior cornua. That of the exposed side
will be seen as a bony rod (thyro-hyat} in close
apposition with the outer border of the larynx,

M. 4

50 ELEMENTARY BIOLOGY. [CHAP.

immediately under cover of a small muscle (con-
strictor laryngis).

c. Examine the above-named muscles.

a. The geniohyoid; a band of tissue arising from
the body and cornu of the hyoid and passing
forwards to be inserted into the lower jaw.

J3. The hyoglossus ; arising from the posterior cornu
and passing forwards in the middle line be-
neath a. to enter the substance of the tongue.

y. The sterno-hyoid ; its cut end will be seen passing
up from behind to be attached along the outer
border of the body and cornu of the hyoid.

8. The omohyoid ; its cut end will probably be
found attached to the outer anterior border of
the body of the hyoid (it arises from the shoulder
girdle).

e. The petrohyoides ; a series of small slips lying
immediately beneath 8. and passing between
the hyoid and hind region of the skull.

3. Remove the ventral face of the right lung, and ex-
amine its interior under a hand lens.

a. The lung sac ; thin-walled and distensible; highly
vascular and thickened around the larger vessels
which ramify within it.

b. The bronchi ; short membranous tubes placing
the lungs in communication with the laryngo-
tracheal cavity.

4. Remove the respiratory organs together with the
floor of the mouth, and pin the whole down on its
side, left lung uppermost. Dissect away the floor of

I.] THE FROG. 51

the mouth to the level of the larynx and examine the
laryngeal cartilages.

a. The cricoid cartilage ; an annular tract, embracing
the base of the larynx. It sends a spur down
towards the base of each lung.

b. The arytenoids. That of the left side will be seen
as a hood-shaped mass, surmounting a. and lying
within the wall of the larynx.

Muscular fibres (constrictors] will be seen to pass
between a. and b.

5. Turn the specimen over and remove all that remains
of the right half, cutting to the level of the middle
line.

rt. The laryngeal cavity ; spacious above and lined
by a soft mucous membrane.

b. The vocal cords. That of the left side will be seen
as a tense fold of the lining membrane, running
antero-posteriorly. Note its intimate connection
with the arytenoid cartilage.

c. Examine the relations of the laryngeal cartilages,
as seen by following their cut edges.

d. Strip off the mucous membrane with care, so as
fully to expose the cartilages for detailed ex-
amination.

These, and similar cartilaginous structures, may
be isolated with ease by maceration in \ per cent,
nitric acid solution, the excess of acid being after-
wards removed by repeated washings in water.

F. The Urinogenital organs.

Dissect from the ventral aspect, proceeding as di-
rected in Sect. B. Having laid bare the viscera, re-

4—2

52 ELEMENTARY BIOLOGY. [CHAP.

move the alimentary canal between the base of the
oesophagus and posterior third of the large intestine.
Next open up the latter, as directed in Sect. D. 4 ; wash
and examine under water.

1. In both sexes.

a. The kidneys ; symmetrically disposed on opposite
sides; relatively largest in the male. Each is
convex dorsally (outer border), incompletely lobed
on its inner border.

b. The adrenal body ; a band of yellow tissue running
along the ventral face of each kidney.

c. The duct — ureter (female) or genito-urinary canal
(male) — running from the outer side of the pos-
terior part of each kidney to the cloaca. Open
the cloaca and pass a bristle into one of their
openings.

In the male R. esculenta each duct is somewhat
dilated after leaving the kidney : it then narrows
again and opens on the dorsal wall of the cloaca
by an oblique slit with sharply defined edges.
In R. temporaria the duct does not dilate, or only
very slightly ; but on its outer side lies a glandular
mass (vesicula seminalis\ from the inner side of
which a number of minute ducts open into the
genito-urinary canal. The aperture of the latter
in the cloaca is round and has tumid edges. In
the female of both species the ureters are very
slender.

d. The bladder; (cf. Sect. D. 4. c.)

2. In the female.

a. The genital glands (ovaries] (cf. Sect. B. 2. //. /?.) ;
varying much in size with the season of the year.

I.] THE FROG. 53

They overlie the kidneys as seen, each being
suspended by a fold of mesentery (mesoarium).

b. The genital ducts (oviducts) ; convoluted tubes, not
continuous with the ovaries, and running back to
open into the cloaca. Each can be resolved into
three segments.

a. A middle or glandular segment ; forming the
conspicuous coiled portion of the tube. It will
be found to swell rapidly under imbibition of
water.

/3. A terminal or uterine segment ; thin- walled and
lying immediately below and external to the
kidney. Examine it in relation to the cloaca;
it will be found to open on the apex of a pa-
pilla, lying side by side with its fellow, im-
mediately in front of the apertures of the
ureters.
Inflate it ; it is highly distensible.

If the Frog be killed during the breeding season,
this segment will be seen to act as a receptacle
for the eggs prior to oviposition.

y. An anterior or thin-walled segment ; short and
straight; it passes behind (dorsal to) the lung
and courses over the base of the same to open
into the pleuro-peritoneal cavity antero-ventrally
by a wide funnel-shaped mouth.

3. In the male.

a. The genital glands (testes], (cf. Sect. B. 2. //. a);
varying in size with the season of the year, and
each suspended by a fold of mesentery (mesor-
chiuui).

54 ELEMENTARY BIOLOGY. [CHAP.

b. The genital ducts (vasa efferentid}; delicate tubes
lying within the mesorchium and placing each
testis in communication with the inner border of
the kidney of the same side.

c. The genito-urinary canal (ureter]. Cf. Sect. i. c.

Examine the seminal vesicle (R. temporaria)
under a hand lens ; it will appear to be continued
forwards as an excessively delicate filament which
can be traced, within the mesentery, as far as the
base of the lung. Try to isolate this and follow it
back ; it will be found to skirt the outer border of
the vesicula seminalis, becoming lost in the base
of the ureter.

Not unfrequently one or more watery vesicles or
cysts are set along the course of the above-named
filament. More rarely it may be wholly or in part
well denned and tubular, having the appearance of
an immature oviduct, a vestige of which it really
represents.

d. The fat body (cf. Sect. B. 2. /'.). This is, in the
male, very large. Rarely its basal portion may be
swollen and pigmented, giving rise to an organ
(Bidder's organ] having much the appearance of
the ovary of the female and confluent with the
head of the testis.

4. Remove a kidney and pin down under water, ventral
face uppermost. Carefully dissect off the mesentery
and examine under a lens ; its whole surface will be
seen to be studded by an immense number of mi-
nute orifices (tiephrostomes\ placing the interior of
the kidney in communication with the pleuro-peri-
toneal cavity.

I.] THE FROG. 55

G. The skeleton.

The skeleton of a Frog may be prepared for exami-
nation by removing the viscera from the body, and
roughly dissecting away the muscles and other soft
parts. Then place the remainder in water and let
it macerate for about a week ; afterwards carefully
pick away the soft parts, with forceps, from the bones
and cartilages.

Obtain two such skeletons; allow one to dry slowly;
preserve the other in weak spirit.

1. Its general arrangement.

a. The axial skeleton; consisting of the vertebral
column and skull t both of which lie in the same
antero-posterior plane.

b. The appendicular skeleton ; lateral parts (limbs and
limb-girdles], supported, directly or indirectly, by
the axis.

a. The fore-limbs : their supporting shoulder-girdle
or pectoral arch, not directly attached to the
axial column; the limb proper ; its main di-
visions, humerus, radius and ulna (the two
latter ankylosed), carpus and digits.

b. The hind-limbs: their supporting hip-girdle or
pelvic arch, directly attached to the vertebral
column ; the limb proper ; its main divisions, os

femoris, tibia and fibula (ankylosed), tarsus,
digits.

2. The vertebral column; dried preparation.

It consists of an anterior segmented portion (each
segment being a vertebra] and of a posterior unseg-
mented portion (the urostyle).

56 ELEMENTARY BIOLOGY. [CHAP.

. a. Examine carefully and draw various aspects of a

detached vertebra, say the third,
a. Its solid flattened ventral part (centrum), with
an anterior concave and a posterior convex
surface (proccdous).

/?. The neural arch : an arch of bone arising from
the centrum dorso-laterally. It furnishes the
undermentioned.

The transverse process : a bony bar arising on
each side from the base of the arch and passing
outwards and a little downwards.

The articular processes (zygapophyses) ; an an-
terior and posterior pair, springing from the
sides of the arch; the anterior (pre-zygapophyses)
having their smooth articular surfaces directed
upwards ; the posterior (post-zygapophyses) with
similar surfaces directed downwards.

The short spinous process ; springing from the
dorsal aspect of the arch and directed back-
wards.

y. The neural canal ; bounded, above and at the
sides by the arch, below by the centrum.

b. Examine the remaining vertebrae.

a. The first vertebra : its body, produced forwards
into a wedge-shaped process (odontoid process)
which lies between. the occipital condyles of
the skull : its arch, usually incompletely ossified
in the region of the spinous process, which is
rudimentary; posterior zygapophyses alone pre-
sent : the large concave anterior facets (condylar
facets), partly on the arch and partly on the
centrum, for articulation with the skull.

I.] THE FROG. 57

/?. The 2nd, 4th, 5th, 6th and yth vertebrae :
closely resembling the third, the chief differ-
ences being found in the varying size and
direction of the transverse processes, all of
which are smaller than those of the third
vertebra.

y. The 8th vertebra : concave at each end of its
centrum (amphiccelous),

S. The Qth vertebra (sacrum] : its centrum ; convex
in front and with two convex tubercles behind :
its powerful transverse processes directed up-
wards and backwards and expanded for ar-
ticulation of the hip-girdle.

The characters of y. and d. are liable to varia-
tion.

Occasionally the vertebrae may be 8 or 10 in
number.

c. Examine any two vertebrae from the side, in their
natural positions.

a. The zygapophy ses ; cf. supra; note their modes
of articulation.

/?. The neural spines ; short and backwardly di-
rected.

y. The transverse processes ; sloping upwards and
backwards.

8. The inter-vertebral foramina (for exit of spinal
nerves); interspaces left between each pair of
neural arches, immediately below the zyga-
pophyses.

Note that there is no corresponding passage
between the skull and first vertebra.

58 ELEMENTARY BIOLOGY. [CHAP.

d. The unsegmented portion of the column (uro-
style).

a. Its anterior end, enlarged and bearing two ar-
ticular concavities.

This region is liable to variation.

/?. Its posterior end; tubular in the dried skeleton,

but in the fresh state filled with cartilage which

projects beyond it.
y. The prominent arch along its dorsal surface;

getting ridge-like and disappearing posteriorly.
8. The small neural canal, enclosed by y.
e. The two minute foramina (for exit of the tenth

pair of spinal nerves) at its extreme anterior

end.

3. The vertebral column, wet preparation.

Cf. supra, and note in addition.

a. The great mobility of the segments upon each
other. Parts in articulation will be found to be
united by sheets (capsules] or bands (ligaments) of
glistening white fibrous tissue.

b. Make a longitudinal vertical section of the entire
vertebral column, and examine the cut surface.

a. The vertebral bodies (centra). Note the shapes
and mode of articulation of their applied ends
(articular surfaces); each will be seen to be
capped in cartilage, the apposed faces being
kept moist during life.

fi. The infra-vertebral bodies; small gelatinous
masses lying within the substance of a. They
are most marked in young specimens.

I.] THE FROG. 59

y. The urostyle; its body is cylindrical in section
and largely filled with a gristly core, identical
with ft, which is continued back as a free termi-
nation to the whole axial skeleton.

3. The neural canal; note especially its termina-
tion in a pointed extremity, lying within the
arch of the urostyle.

c. Isolate the third vertebra, and examine —

a. Its zygapophyses ; cf. Sect. 2. c. Each is capped

in cartilage, in common with the other articular

faces.
ft. Its transverse processes ; each terminating in a

backwardly prolonged cartilaginous expansion.

4. The skull ; dried preparation.

a. General. Examine from the dorsal aspect; it
embraces

a. The cranium; median and enclosing the brain,
sometimes termed the brain-case.

ft. ^}\£. facial apparatus ; the system of outstanding
bars, conspicuous as the jaw-apparatus.

y. The sense capsules. The auditory capsules,
paired postero-lateral expansions; the olfactory
capsules, similar but less obvious antero-lateral
expansions (they are better seen in the wet state.
See infra).

b. Examine the posterior end of the skull.

a. The large aperture (foramen magnum) in the
middle line, leading into the cranial cavity.

ft. The convex surface (occipital condyle) on each
side of the foramen magnum, for articulation

6O ELEMENTARY BIOLOGY. [CHAP.

upon the corresponding concave facet on the
front of the first vertebra.

y. The exoccipitals ; bearing the condyle on each
side, and bounding the foramen magnum
laterally.

8. The pro-otics; lying immediately in front of y.
and running outwards on each side, forming
the roof and front wall of the auditory capsule.

e. The squamosal; a hammer-shaped bone, ex-
tending from the outer edge of 8. downwards
and backwards towards the articulation for the
lower jaw.

£. The columella auris ; seen, at this stage, as a
bony rod which underlies the pro-otic and
stands out immediately behind the head of the
squamosal. (It is best studied later on.)

c. The roof of the skull.

a. The exoccipitals, pro-otics and squamosal. Cf.
supra.

ft. The fronto-parietals ; two long bones united
by suture in the middle line, roofing in the
cranium.

y. The nasals ; two triangular bones immediately
in front of /?. set transversely and a little ob-
liquely.

8. The pre-maxillcz ; two small bones in front of y.,
meeting in the middle line and bounding the
front of the gape. Each sends up an ascending
process which is directed towards the inner end
of the nasal bone.

I.] THE FROG. 6 1

e. The maxilla; on either side a long bone
bounding the greater part of the gape, and
abutting against 8. in front. Each maxilla
sends up an ascending process, which abuts
against the outer end of the nasal.

I The qnadrato-jugal (jugali); a delicate bone, on
either side, bounding the hinder third of the gape
and passing between the maxilla and squamosal.

d. The base of the skull

a. The exoccipitals ', pro-otics, pre-maxill<z, maxilla,
and quadrato-jugals. Cf. supra.

Note that each pre-maxilla sends back a short
palatine-process.

/?. ThQparasphenoid; a dagger-shaped bone running
along the greater part of the floor of the cranial
cavity.

y. The sphtnethmoid or girdle-lone; a large bone
underlying the anterior portion of ft. and en-
closing the anterior third of the brain-case.

8. The palatines; slender bones, one on each side,
passing out transversely from near the anterior
end of the parasphenoid to the body of the
maxilla. Each underlies the corresponding
nasal bone.

e. The vomers; two broad irregular shaped bones,
immediately in front of 8. These, in common
with the maxillae and pre-maxillse, bear teeth.
(Cf. Sect. E.)

£. The pterygoids ; large triradiate bones, one on
either side. Each abuts against the antero-
ventral border of the auditory-capsule internally;

62 ELEMENTARY BIOLOGY. [CHAP.

anteriorly it is prolonged forwards, flanking the
maxilla, to meet the outer end of the palatine;
posteriorly it runs downwards and backwards,
parallel with the squamosal.

e. The side of the skull.

a. Compare generally with the foregoing; note
especially the backward rotation of the squa-
mosal, and the mutual relationships of the max-
illo-jugal bones.

p. The mandible or lower jaw; it consists of two
distinct halves, or rami, which meet in the
middle line in front.

In each ramus three pieces may be made out
— a main piece, which runs nearly to the middle
line in front (angulo spleniaf}\ a thin scale- like
piece (dentary) flanking the outer anterior face
of the former; and a small nodular one (mento-
Meckelian) meeting with its fellow in the sym-
physial line.

5. The skull; wet preparation.

Remove the lower jaw and hyoid, and carefully
dissect off from the rest of the skull, under water,
the undermentioned bones. This may be done with
comparative ease, if, holding the whole down with
the left hand the bones of one side be removed,
either by inserting the point of a scalpel beneath
them or by the aid of forceps.

The bones to be removed are — dorsal series, fronto-
parietals (one or both), nasal, squamosal, maxilla,
pre-maxilla; ventral series, parasphenoid, vomer, pala-
tine, pterygoid ; care must be taken that none of the
underlying cartilage is brought away with the latter.

I.] THE FROG. 63

There will thus be laid bare the chondro-cranium and
its related bones.

a. Dorsal aspect.

a. The chondro-cranium; a cartilaginous mass en-
closing the brain and olfactory and auditory
sense organs, and bearing, laterally, an out-
standing cartilage which is flanked by the pala-
tine and pterygoid bones (palato-quadrate car-
tilage or sub-ocular arch).

/?. The sphenethmoid, pro-otics and exoccipitals ; seen
to be formed as replacements of the chondro-
cranium in bone; they are inseparable from it,
being termed therefore cartilage bones, by way of
distinction from those superadded ones which
have been stripped off (membrane bones).

Examine in detail.

y. The cranium ; its roof will be seen to be in-
completely cartilaginous, bearing three mem-
branous areas or fontanelles, all of which un-
derlie the fronto-parietals. They are — a larger
median one in front, extending forwards to the
posterior border of the sphenethmoid ; smaller
paired ones behind, extending outwards to the
inner border of the pro-otics.

3. The sub-ocular arch; confluent, in front with
the olfactory capsule, behind with the auditory
capsule, \\spalatinebar; standing out at right
angles to the long axis, and expanded ex-
ternally to form a plate (orbital process) for sup-
port of the maxilla (cf. opposite side). Its
pterygoid bar ; produced backwards, outwards

64 ELEMENTARY BIOLOGY. [CHAP.

and downwards, into a stout rod or suspensorium,
which lies between the pterygoid and squa-
mosal and furnishes the articulation for the
lower jaw.

e. The olfactory capsules ; confluent with each other
and the cranium, and incompletely separated
above by a median longitudinal furrow. The
dorsal wall of each will be seen to be scroll-
shaped and produced — in front, into a blunted
pre-nasal process, which abuts against the inner
face of the maxilla (cf. opposite side) ; behind,
into a laminate all-nasal process, which encircles
the outer and hinder margins of the anterior
nostril.

If the preparation be made with great care, the
front wall of the narial passage will be seen to
lodge a couple of minute labial cartilages, which
overlie the base of the pre-nasal process.

b. Ventral aspect,

a. The cranium; completely cartilaginous below
and at the sides. Note the limits of the car-
tilage bones.

Certain apertures (exits for cranial nerves)
will be seen ; they are best studied later.

/3. The sub-ocular arch; in front, confluent with
the expanded floor of the olfactory capsule;
behind, in articulation with the ventral outer
border of the auditory capsule.

y. The rhinal processes. That of the dissected side
will be seen as a small hammer-shaped cartilage
lying near the middle-line in front, ventral to

I.] THE FROG. 65

the olfactory capsule. It projects freely for-
wards and inwards.

c. Lateral aspect.

Compare generally with a. and b. and note es-
pecially.

a. The suspensorium; its backward rotation; its
rounded free end, for articulation of the lower
jaw.

(3. The quadrate bone; represented by an insig-
nificant nodule lying within the substance of a.
near its articular end. It may or may not be
confluent with the quadrato-jugal.

d. Posterior aspect.

a. The eccocdpitals ; separated above and below
by cartilage (i.e. there are no median occipital
bones).

/?. The columella auris. Remove this on the dis-
sected side ; and note that its inner end plugs a
large aperture (fenestra ovalis) in the outer wall
of the periotic capsule.

y. The suspensorium; now seen, on the same side,
in its entirety. Note ; its pedicle, the main bar,
prolonged down to give articulation to the lower
jaw; its dorsal cross, confluent with the outer
wall of the auditory capsule, and expanded, to
form a ledge (tegmen tympani] which overlies
the columella; its ventral cross (cf. b. /?.) rounded,
and in definite articulation with the antero-
ventral border of the auditory capsule.

e. Inner aspect.

Remove all that remains of the chondro-cranium
M. c

66 ELEMENTARY BIOLOGY. [CHAP.

on the side which has been dissected, cutting a
little to one side of the middle line, thus reducing
the whole to a condition of vertical longitudinal
section. Examine from within.

a. The cranial and nasal cavities ; note their limits
and mutual relationships.

(3. The septum nasi ; a median plate of cartilage,
separating the two nasal chambers in the middle-
line (examine from the front).

y. The sphenethmoid. Seen to consist of a pos-
terior cranial portion which embraces the an-
terior third of the brain-case, and an anterior
paired portion, which is similarly related to the
bases of the olfactory capsules.

8. The exoccipital and pro-otic. Examine these in
relation, as seen from within (cf. § 4 b.\

e. The fronto-paridal, nasal, vomer, and para-
sphenoid (if in place). Examine, in relation to
the chondro-cranium.

£. The rhinal process (cf. § b. y.); seen to form a
support for the pre-maxilla, abutting against its
inner face.

vj. The foramina for exit of the cranial nerves — as
under, on one side.

i. The olfactory foramen (I. Cranial) ; a perfora-
tion of the partition between the two portions
of the sphenethmoid.

ii. The optic foramen (II. Cranial); obvious as a
perforation of an oval membraneous area of
the side wall a short distance behind the
sphenethmoid.

I.] THE FROG. 6/

iii. The foramen of exit for V. VI. VII. ; a large
aperture, below and in front of the pro-otic.

iv. The foramen for IX. and X.; perforating the
body of the exoccipital.

v. The auditory foramen (VIII.); a small per-
foration of the cartilaginous wall of the audi-
tory capsule just behind iii.

vi. The foramen for III.; small and situated im-
mediately in front of iii.

vii. The foramen for IV. («. patheticus) very
small, above and a little in front of the optic
foramen.

/. The mandible. See 4. e. /?.

Strip off the dentary with care; there will be
found, in close apposition with the outer face of
the angulo-splenial,

a. Meckel' s cartilage; a gristly rod, forming a core
for the lower jaw. Examine its posterior end ;
it is alone concerned in articulation.

p. The mento-Meckelian bone ; a small nodule aris-
ing as an ossification of the symphysial end
of a.

6. The hyoid.

a. Its broad somewhat tetragonal central part (body),
bearing a number of outstanding processes, for
attachment of muscles (cf. Sect. E. 2. c.). This
is usually cartilaginous, being ossified only in old
individuals.

b. Its posterior cornua^ or thyro-hyals ; short bony
rods, sloping obliquely backwards and outwards.

5—2

68 ELEMENTARY BIOLOGY. [CHAP.

f. Its anterior cormia, slender cartilaginous rods,
arising from the front of the body on either side :
each is long and curved, running at first forwards,
then backwards and outwards, and finally for-
wards and upwards, to become attached to the
periotic capsule below the fenestra ovalis.

7. The limbs and limb-girdles.

These are best studied in the wet state. In
dealing with the carpus and tarsus, no difficulty will
be found if treated as follows. Isolate sufficient of
the limb-skeleton to embrace the carpus or tarsus
and pick away any fragments of the soft parts which
may remain; immerse in spirit for a few hours;
transfer to absolute alcohol for i hour, thence into
oil of cloves. The latter medium will thoroughly
clarify the preparation, and it may be kept in it for
an indefinite period.

The shoulder-girdle and its related structures.

a. Their general arrangement: they form an incom-
plete ring round the fore part of the trunk, com-
posed partly of bone, partly of cartilage. Note
the cavity (glenoid fossa) with which the fore-limb
articulates.

Examine from the ventral aspect ; the whole is
seen to consist of

a. Paired elements] symmetrical on opposite sides
and meeting each other in the mid-ventral
line.

/?. Median elements; in corresponding relationship
anteriorly and posteriorly with the applied ven-
tral ends of a.

I.] THE FROG. 69

b. The paired elements. Flatten out the whole on one
side, so as to obtain a lateral view. In the region
of the glenoid fossa there will be seen a sharp line
of demarcation between a dorsal scapula segment;
and a ventral coracoid segment, the middle area of
which is fenestrate (coracoid fontanelle).

a. The scapula segment; subdivided into a thin ex-
panded partially ossified dorsal portion (supra
scapula) and a stouter bony piece (the scapula)
which furnishes the dorsal half of the glenoid
fossa.

ft. The coracoid segment; largely ossified, and com-
posed of a smaller anterior clavicle, and a
stouter posterior coracoid, the latter furnishing
the ventral half of the glenoid fossa.

c. The median elements.

a. The xiphisternum ; a median cartilage, en-
sheathed in bone, abutting against the applied
ends of the coracoids. It expands posteriorly
to form a heart-shaped plate, notched from
behind (xiphoid process].

(3. The omosternum, a slender cartilage ensheathed
in bone, similarly related in front It also ends
in a cartilaginous expansion.

It is doubtful if the above really represent
sternal elements, especially in the case of the
anterior one.

The skeleton of the fore-limb. (Cf. Sect. A. 2. c.)
a. The arm-bone (humerus).

a. A somewhat cylindrical bone, composed of a
middle shaft with an articular expansion at each
end.

70 ELEMENTARY BIOLOGY. [CHAP.

J3. The great ridge (deltoid crest} on its antero-
internal surface, to which a muscle (deltoid)
was attached.

The development of this crest is greater in the
male than in the female.

b. The bone of 'the forearm.

a. Excavated above to receive the lower end of
the humerus.

ft. Shewing below two articular heads separated by
a notch, which is the last trace of division be-
tween the two bones of which it is made up;
viz. the radius and the ulna.

y. Pull the limb out straight, palmar surface down-
wards, so that it comes to be situated at right
angles to the long axis of the body. The radius
and pollex lie on the anterior or pre-axial side,
the ulna and little finger on the posterior or
post-axial side, of a line drawn through the axis
of the whole limb.

c. The digits.

Five in number, the first (radial one) rudimentary:
beginning at the outer or ulna side, we find —

a. The fifth digit : it presents a cylindrical proximal
bone (metacarpal} followed by three others
(phalanges'), each shorter than its predecessor.

ft. The fourth digit: a metacarpal bone and three

phalanges,
y. The third digit: a metacarpal bone with two

phalanges.
8. The second digit: a metacarpal bone with two

phalanges.

I.] THE FROG. 71

e. The first digit (pollex) supported only by a small
metacarpal bone.

d. The carpus. Proceed as directed at the outset,
and examine under a low power. Its elements
can be resolved into three sets.

a. Proximal elements ; two in number, articulating,
side by side, upon the radius and ulna respec-
tively.

/?. Distal elements; three in number. The outer
one gives articulation to the three outermost
digits ; and is mainly connected with the outer
element of the proximal series. The two inner
ones give articulation to the pollex and second
metacarpal respectively.

y. The central element (centrale); pre-axial in posi-
tion, and wedged in between the two innermost
elements of the distal series and the correspond-
ing proximal piece.

The hip girdle.

a. Its general form : V-shaped, with the apex directed
backwards. It consists of paired elements which
unite in the middle line and are in direct articula-
tion with the vertebral column (sacrum).

b. The fossa (acetabuluni) on each side, for articula-
tion of the hind-limb.

c. Examine from the side. The whole will be seen
to be subdivided, by a triradiate fissure running
through the acetabulum, into three pieces, viz. —

a. The pubis; a small triangular wedge-shaped
piece (cartilaginous except in old frogs) ; ventral

72 ELEMENTARY BIOLOGY. [CHAP.

in position and furnishing the middle third of
the acetabulum.

{3. An anterior elongated piece (ilium}; subcylindri-
cal in front, behind, laterally compressed (crista
ilii) ; it furnishes rather more than the anterior
third of the acetabulum. Its articulation with the
sacrum is effected through the agency of a car-
tilaginous supra-ilium ; related to the ilium, as is
the supra-scapula to the scapula in the shoulder-
girdle (cf. p. 69).

y. A posterior, laterally compressed piece (ischiuvi) ;
united with its fellow in the middle line. It
meets the ilium above, below it is separated
from it by the pubis; it furnishes rather less
than the posterior third of the acetabulum.

The skeleton of the hind limb.

a. The thigh-bone (os femoris) ; its cylindrical shaft
and rounded articular ends.

b. The leg-bone (os cruris).

a. A long cylindrical bone, expanded at its articular
extremities. Its upper end; excavated for ar-
ticulation upon the femur. Its lower end;
rounded and notched to give passage to the
tendon of the tibialis posticus muscle (Sect. H.
7. a. /?.).

fi. The longitudinal grooves on it ; indicating that
it is really made up of two united bones, the
tibia and \hzfibula.

y. The nutritive foramen (for passage of nerves and
blood-vessels into its interior) ; a small slit-like
perforation of the middle of the shaft.

I] THE FROG. 73

Look for similar perforations in the other bones
examined.

8. Pull the limb out straight, plantar surface down-
wards, as directed for the anterior appendage.
The tibia and hallux are pre-axial; the fibula
and little-toe post-axial.

c. The tarsus.

Its proximal elements are seen as two elongated
bones (separate in the middle but confluent at
their cartilaginous extremities) in articulation with
the ankylosed tibia and fibula; the pre-axial, or
tibial, of these is the astragalus ; the post-axial or
fibular, the calcaneum.

d. The digits. Five in number ; the internal one the
shortest, the fourth the longest. Their compo-
sition—

a. The first, or hallux (the most internal) ; a me-
tatarsal bone, followed by two phalanges.

ft. The second : same as a, but longer.

y. The third: a metatarsal bone with three pha-
langes.

S. The fourth : a metatarsal bone and four pha-
langes.

f. The fifth : like the third, but a little shorter.

e. The tarsus and calcar.

Remove the middle portion of the foot-skeleton,
to include the apposed ends of the astragalus and
calcaneum and the metatarsals ; treat with alcohol
and oil of cloves as previously directed, and ex-
amine under a low power.

74 ELEMENTARY BIOLOGY. [CHAP.

a. The distal tarsal series ; two in number in the
adult and imperfectly ossified ; viz. — a larger
compressed post-axial element, giving articu-
lation to the second and third digits ; a smaller
nodular pre-axial cartilage (hallux tarsal\ lying
in the interspace between the heads of meta-
tarsalia i and 2 (it may be absent).

(3. The central element (navicnlare) ; triangular and
preaxial, interposed between the astragalus, and
hallux metatarsal.

y. The calcar (pre-hallux) ; seen to be composed,
except in very old frogs, of three pieces, identical,
in their characters and relationships, with the
component parts of a digit. It is carried by ft.
In very young specimens a fourth segment may
be detected.
H. Myology; as illustrated in the hind limb.

(For the following dissection it is desirable to have
a frog which has been lying some time in spirit.)

Pin the animal down on its back, and remove the
skin from the hind limbs.

i. General.

In dissecting the muscles, separate them gently
from one another, tearing through the connective
tissue which unites them.

a. Each is chiefly made up of a main mass, or belly.

I). At both ends, in most cases, the belly is replaced
by dense shiny tissue forming a tendon.

c. The tendons are fixed directly or indirectly to
some of the neighbouring bones, the less move-
able attachment is termed the origin of a given
muscle ; the more moveable its insertion.

I.] THE FROG. 75

2. The superficial muscles on the front of the thigh.

a. The sartorius : a thin ribbon-like muscle running
down the middle ; it arises from the symphysis
pubis and is inserted into a tendinous expansion
(aponeurosis] on the inner side of the knee-joint.

b. The rectus internus major: a large muscle running
along the whole inner side of the thigh; arises
from the symphysis pubis below the sartorius and
is inserted into the same aponeurosis as that
muscle.

c. The rectus internus minor: a thin muscle lying in-
side and rather behind the rectus internus major.
It arises from the pelvis close to the anus and is
inserted into the aponeurosis about the knee-joint.

d. The triceps femoris : a powerful muscle on the outer
side of the thigh (it may be best studied later).

3. The deep muscles of the front of the thigh. Cut
across the belly of the sartorius, and turn its ends
out of the way ; there will thus be laid bare —

a. The adductor magnus: a powerful muscle lying im-
mediately above the rectus internus; it arises from
the pelvis, between the origin of the sartorius and
that of the rectus internus major. Its fibres are
inserted directly (i.e. without the intervention of a
specialised tendon) into the inner side of the distal
half of the femur.

b. The adductor longus: a thin band immediately in-
ternal to the sartorius; it arises from the anterior
border of the symphysis pubis and joins the ad-
ductor magnus at its insertion.

76 ELEMENTARY BIOLOGY. [CHAP.

Force the adductors longus and magnus well apart
(noting the insertion of the latter) and pin them back
if needful; there will thus be exposed —

c. The pectineus: a small muscle, arising from the
front of the pelvis, close to the symphysis, and
inserted into the anterior surface of the distal half
of the femur.

d. The adductor brevis: a small muscle lying on the
inner side of the pectineus, close to which it arises
and is inserted.

Sever the rectus internus major and r.i. minor and
turn their cut ends back; thrust the adductor magnus
forwards; there will thus be laid bare — •

e. The semitendinosus ; this is a long slender muscle
bifurcated at its upper end: its two heads, thus
formed, arise, one (anterior head) from the pelvis
between the ischial symphysis and the aceta-
bulum ; the other (posterior head} from the ischial
symphysis: the muscle terminates below in a
rounded tendon which is inserted along with the
sartorius.

Its anterior head enters into a close relationship
with the adductor magnus.

4. The superficial muscles on the back of the thigh.

Turn the frog over on to its belly, and remove the
skin from the back of the limb.

a. The triceps femoris: a very large muscle on the
outer side, divided above into three heads, which
are often regarded as separate muscles, viz.

a. The vastus extcrmts: its outer head; it arises

THE FROG. 77

from the hinder edge of the iliac bone. Cut it
across and reflect the two halves.

ft. The vastus intsrnus: its inner head; a very
large muscle on the outer anterior aspect of the
thigh, arising from the pelvis close to the hip-
joint; it unites with a. to form a common
tendon of insertion into the aponeurosis on the
front of the os cruris.

y. The rectus femoris anticus: its middle head; a
small muscle arising from the postero-ventral
border of the iliac bone and inserted along with
the above.

b. The biceps femoris: a long thin muscle, lying
along the inner side of the vastus externus; it
arises from the iliac bone above the acetabulum;
below it divides into two slips, one of which is
inserted into the middle of the shaft of the femur,
while the other ends in a rounded tendon which is
inserted into the back of the distal end of the same
bone.

c. The semimembranosus: a large muscle immediately
below the biceps; it arises from the upper pos-
terior part of the iliac symphysis and is inserted
into the aponeurosis round the knee-joint.

The deep muscles of the back of the thigh.

Divide and reflect the biceps femoris and semi-
membranosus ; there will thus be exposed : —

a. The ghitczus : a short thick muscle arising from
the hinder two-thirds of the external border of
the ilium ; it runs down between the vastus ex-
ternus and the rectus anticus to be inserted, by a

78 ELEMENTARY BIOLOGY. [CHAP.

powerful tendon, into the back of the head of the
femur.

b. The ileo-psoas : it arises from the internal surface
of the posterior end of the ilium and is inserted
into the upper and outer border of the shaft of
the femur, immediately above the lesser slip of
the biceps.

c. The pyriformis. This arises from the hinder end
of the urostyle and, passing inside the vastus ex-
ternus and biceps femoris, is inserted into the
shaft of the femur.

d. The adductor brevis (cf. § 3. d); a small muscle
lying immediately beneath c.

e. The adductor magnus (cf. § 3. #.); a fleshy mass
lying immediately beneath the semimembran-
osus.

6. The rotator muscles of the femur.

Pin the whole down on one side, and remove all
the muscles hitherto examined with the exception of
the gluteus (5. a.\ leaving their cut ends. Turn
these well back, noting their relations; there will
thus be laid bare : —

a. The gluteus (cf. supra)} its whole course can now
be followed.

b. The quadratus femoris, arising from the postero-
dorsal border of the ilium and inserted into the
posterior upper border of the shaft of the femur,
side by side with the pyriformis (5. c.).

c. The obturatorius : this is best seen on reflection
of b. It arises from the postero-ventral area of
the hip-girdle to which it is closely applied; its

I.] THE FROG. 79

tendon of insertion passes obliquely forwards and
downwards immediately beneath that of the glu-
tens, to reach the head of the femur.

7. The muscles of the leg.

a. Turn the frog on its back, and remove the skin
from the foot: fasten the same down, dorsal sur-
face upwards. The os cruris will now be seen
running down the middle of the leg. Lying on
its inner side are two muscles, viz. : —

a. The gastrocnemius : a muscle with a great fleshy
belly; it arises above by two tendons; one
(much the larger) starts in the region of the
knee-joint partly from the femur, partly from
the os cruris; the other arises from the apo-
neurosis adjacent. Below, the muscle ends in
a great tendon (tendo Achillis), which termi-
nates in an aponeurosis on the plantar surface
of the foot.

/3. The tibialis fosticus : a slender muscle covered
in the main by the gastrocnemius ; it arises
from the posterior surface of the os cruris and,
passing to the inner side of and through the
ankle-joint (cf. p. 72, leg-bone), is inserted into
the astragalus.

b. a. and /3. act as flexors of the foot; on the
opposite side of the leg lie the following ex-
tensors.

a. The peroneus : the largest and most external;
it arises from the outer side of the distal ar-
ticular end of the femur and, running across
the ankle-joint, is inserted into the calcaneum.

80 ELEMENTARY BIOLOGY. [CHAP.

This muscle is best seen from the dorsal
aspect.

ft. The tibialis anticus : a small muscle beneath
and dorsal to the peroneus ; it arises from the
front of the lower end of the femur and from
the capsule of the knee-joint; below it divides
into two slips which are inserted into the dorsal
surface of the astragalus and calcaneum re-
spectively.

y. The extensor cruris brevis : this lies internal to
the upper part of the last muscle; it arises
from the front of the distal articular end of
the femur and is inserted into the middle third
of the os cruris.

S. ^^o. flexor tarsi anterior; a small muscle, best
seen on reflecting the tibialis anticus ; it arises
where the extensor cruris ends, and is inserted
into the dorsal face of the astragalus.

I. The blood- vascular system.

The dissection of the blood-vessels maybe facilitated by
previous injection, as directed in the Appendix C. If
a permanent preparation be desired gelatine or plaister
of Paris may be preferably employed. For ordinary
working purposes, there is nothing more convenient
than a mixture of French blue, injected wherever de-
sirable with a medicine dropper (see Appendix).

i. The heart.

Obtain a freshly-chloroformed Frog and pin down
on its back; slit up the ventral integument along
the middle line as directed in Section B, and pin
back the two flaps, avoiding stretching. Next raise

I.] THE FROG. 8 1

the xiphisternum with forceps, and remove the entire
ventral portion of the shoulder-girdle, being very
careful not to injure the underlying blood-vessels.
Raise the pericardium in like manner and remove
sufficient of it to expose the heart.

a. External anatomy of the heart.

a. Its posterior conical thick-walled portion (ven-
tricle) with the apex directed backwards.

/?. The truncus arteriosus ; a sub-cylindrical part,
arising from the right side of the base of the
ventricle and dividing anteriorly into two (aortfa
arches).

y. The atrium; thin-walled, rounded, lies on the
dorsal aspect of the truncus, in front of the
ventricle. Its subdivision into the two auricles
is not visible from this aspect.

8. Carefully raise the ventricle : lying beneath it
(that is, on its dorsal side) will be seen the sinus
venosus ; an elongated thin-walled sac, seen to
be in immediate relationship to the atrium and
in connexion with the great veins, which enter
it through the dorsal wall of the pericardium.

b. The pulsation of the heart.

a. Watch the movement carefully ; it is a regularly
alternating series of contractions and dilatations.

ft. It will be seen that the two auricles contract
together ; immediately after them, the ventricle ;
and then, instantly, the truncus arteriosus.

y. Raise the ventricle so as to see the sinus ve-
nosus } it will be found to contract immediately
before the auricles.
M. 6

82 ELEMENTARY BIOLOGY. [CHAP.

2. The anterior abdominal vein. (Cf. Sect. B. i. c. a.)

This is seen to receive a series of paired epigastric
veins, which run in the tendinous intersections of
the ventral muscles.

Carefully dissect the body-wall away from it, until
liberated for its whole course. Note —

a. Its origin ; by the fusion in the middle line of a
couple of short veins {pelvic veins), lying at the
base of the pleuro-peritoneal cavity and bringing
back blood from the hind-limbs.

b. Its distribution. Trace it forwards ; it runs be-
neath (dorsal to) the ventricle : on raising the
latter it will be found to divide into two branches,
one of which goes to each lobe of the liver (afferent
hepatic veins).

c. Its lesser factors.

a. The epigastric veins ; described above.

/?. The vesical vein ; entering its base, and bringing
in the blood from the urinary bladder and in-
testinal wall immediately adjacent.

y. The cystic vein ; a small vessel, usually entering
the right afferent hepatic branch, bringing back
the blood from the gall-bladder.

8. The cardiac vein ; a minute vessel, seen, on
displacing the ventricle, bound up in a special
fold of the pericardium. It brings back the
blood from the wall of the truncus arteriosus.

3. The hepatic portal vein.

Raise the left lobe of the liver ; the main trunk of
the above will be seen running through the head of
the pancreas to enter the same; it communicates

,1.] THE FROG. 83

with the anterior abdominal vein by a large vessel
(systemico-portal anastomosis] .

Note that it is formed by the union of those vessels
bringing back the blood from the alimentary canal
and its appended glands (its factors may be better
studied in detail later on).

Frequently, but not invariably, some of the veins
(gastric 'veins] which carry back the blood from the
stomach, enter the liver independently of the rest

4. The renal portal veins.

Remove sufficient of the alimentary canal to fully
expose the kidneys. The above-named veins will
be seen skirting the outer borders of the same ; that
of one side may be examined in detail.

a. Its origin; when traced back it is seen to arise,
side by side with the pelvic vein (i. a.) from the
bifurcation of a vessel which skirts the outer
border of the thigh (femoral vein).

b. Its distribution; on reaching the kidney it is seen
to break up into a number of afferent renal veins,
for distribution to that organ.

c. Its remaining factors.

a. The sciatic vein; a large vessel seen, on
carefully dissecting away the hip-girdle, to bring
in the blood from the deeper parts of the hind-
limb.

P. The dorso-lumbar vein; a well-defined vessel
joining it anteriorly on a level with the kidney;
it brings back the blood from the body-wall
adjacent.

y. The oviducal veins ; two or three vessels entering
it (in the female) immediately behind (3.

6—2

84 ELEMENTARY BIOLOGY. [CHAP.

5. The inferior vena cava.

Remove the stomach and intestine, being careful
not to injure the liver. The above-named vein will
be obvious as a large vessel lying between the
kidneys ; follow it forwards ; it will be seen to pass
behind (dorsal to) the liver, to enter the sinus
venosus. Its factors are —

a. The efferent renal veins; a series of vessels arising
from the ventral faces of the kidneys.

b. The genital veins; variable in number and detailed
relationships, bringing in the blood from the
genital glands.

c. The hepatic veins (efferent hepatic vessels] ; two or
more in number, seen, on turning the liver over,
to enter the vena cava just before it reaches the
heart.

6. The veins of the superior caval system.

Little trouble will be found in following these if
the ventral portion of the shoulder-girdle and its re-
lated pectoral muscles be carefully removed.

a. The superior vence cavcz; short vessels seen to
enter the sinus venosus on either side, im-
mediately beneath the atrium. Each is formed
by the union of the three following factors.

a. The external jugular vein, running down the
under side of the throat and bringing back
blood from the superficial parts of the head. It
is formed by the confluence of the lingual vein
from the tongue, and the inferior maxillary vein
which skirts the outer side of the lower jaw.

I.] THE FROG. 85

ft. The innominate vein; formed by the union of
the internal jugular vein, seen just behind the
angle of the jaw, and bringing back the blood
from the brain, spinal cord, and deep-seated
parts of the head, with the subscapular vein
returning the blood from the muscles of the
shoulder.

y. The axillary vein; formed by the union of the
subdavian vein which returns the blood from the
arm, with a large vessel (great cutaneous] bring-
ing back the blood from the integument.

7. The pulmonary veins.

These are exceedingly conspicuous in spite of
their small size, on account of the black densely
pigmented nature of their walls. On raising the
ventricle they are seen arising along the inner faces
of the lungs and uniting just before entering the left
side of the atrium (1. auricle); the right one is, of
necessity, the longer of the two.

8. The system of the aortic arches.

These may be dissected in the same animal; there
are three sets of vessels on each side, arising from
the truncus arteriosus. They may be readily distin-
guished from the great veins by their pinkish colour,
due to the presence of a small quantity of blood, and
by the thickness of their walls, the latter being very
obvious in the case of any one of the larger arteries
as whitish contour lines.

a. The anterior or carotid arch, concerned with the
blood supply to the head; it arises on each side
as a short trunk (common carotid) which early sub-
divides into —

86 ELEMENTARY BIOLOGY. [CHAP.

a. The external carotid (lingual artery) to the
tongue and floor of the mouth; it and the com-
mon carotid together accompany the external
jugular vein.

ft. The internal carotid; passing outwards and up-
wards to reach the deep-seated parts of the
head. Its branches accompany most of the
factors of the internal jugular vein.

y. The carotid gland '; a pigmented enlargement at
the base of ft.

b. The middle or aortic arch: this is the largest of the
three : it runs round the throat towards the verte-
bral column, and unites with its fellow to form the
dorsal aorta (this may best be followed in detail
later). It gives off, on either side, at the level of
the arm.

a. The snbdavian artery ; mainly distributed to the
fore-limb.

ft. The vertebral artery; arising immediately in the
front of a, and distributed to the vertebral
column and the back, together with the super-
ficial parts of the head and face (occipital
branch).

c. The posterior or pulmonary arch; it runs to the
root of the lung, giving off on its way a cutaneous
branch which passes out to the integument of the
trunk, and supplies most of the blood which is
carried back by the great cutaneous vein.

9. The dorsal aorta and its branches.

Follow the aorta back; it will be found to run along
the middle line beneath the vertebral column giving
off in order

L] THE FROG. 87

a. The cceliaco-mesenteric artery, to the alimentary
organs; the cut end of this will be seen arising
either at the point of union of the two aortic
arches or, more generally, from the base of the
left arch.

Very rarely this is represented by distinct cceliac
and mesenteric trunks.

b. The renal arteries, to the kidneys; variable in
number, arising from the aorta as it passes between
the kidneys.

c. The genital arteries^ to the reproductive glands;
usually arising from the anterior renal vessels.

d. The inferior mesenteric artery; a small vessel
arising far back and supplying the base of the
large intestine.

e. The common iliac arteries; arising as an apparent
bifurcation of the dorsal aorta.

Very rarely, the aorta is continued back, beyond
the point of origin of the above, as a delicate
median sacral or caudal artery.

a. The hypogastric arteries; arising, one on each
side, from the iliacs, just before they leave the
body-cavity, and distributing branches to the
urinary bladder and body-wall adjacent.
i o. The hepatic-portal system in detail.

Pin a newly-killed Frog down upon its left side,
and remove the body-wall and arm of the right one.
Reflect the skin of the back and remove the genera-
tive organs; dissect off the pericardium and liberate
the right lung. Displace the alimentary canal
turning the stomach and liver forwards and the
large intestine well back; and pin down underwater.

88 ELEMENTARY BIOLOGY. [CHAP.

i. The hepatic portal vein will now be seen emerging
from the head of the pancreas to enter the left
lobe of the liver (cf. § 3); it is formed by the
union of the following factors.

a. The gastric vein; bringing back the blood from
the stomach.

b. The superior mesenteric vein ; a very short vessel
running through the head of the pancreas and
receiving

a. The duodenal veins; bringing in the blood
from the duodenum and pancreas.

(3. The lieno intestinal vein; formed by the union
of the Heal from the ileum, the hamorrhoidal
from the large intestine, and the splenic or
lienal from the spleen.

ii. The cceliaco-mesenteric artery may now be traced to
its ultimate distribution. Its origin has been
described at § 9 a; it breaks up near the head of
the pancreas into a and b.

a. The cceliac artery; distributing branches to the
stomach, liver and pancreas,

b. The superior mesenteric artery ; supplying the
intestine and spleen. The branches of this
vessel accompany the factors of the superior
mesenteric vein.

c. The inferior mesenteric artery ; arising from the
dorsal aorta far back and supplying the base of
the large intestine (cf. § 9 d}.

iii. Remove the hinder half of the hip-girdle and the
head of the femur. Scrape away the liver to the

I.] THE FROG. 89

level of the vena cava inferior, and dissect away
the veins of the head.

The dissection, as it now stands, presents a
general view of the blood-vascular system;
especially noticeable are the following.

a. The renal portal system (§ 4). The femoral and
sciatic veins will be seen to be united by an
anastomosing trunk, which encircles the outer
side of the thigh and receives veins from the
adjacent integument.

b. The whole course of the vena cava inferior
(§5)-

c. The aortic arches (§ 8). Examine these with
care; the middle one (aorta) may or may not
be connected with the other two, one or both
but most generally with the carotid arch, by a
longitudinal cord-like ductus Botalli.

That portion of the ductus which connects the
aortic and pulmonary arches, and which is least
constant in the Frog, is also known as the ductus
arteriosus.

ii. The anatomy of the heart.

The examination of the heart requires a good
deal of care and the use of a lens of low magnifying
power, and it may be greatly facilitated by working
as follows. In a chloroformed Frog the heart is dis-
tended with blood when it ceases to beat. When all
signs of contractility have disappeared, the distended
heart should be removed from the body together
with the lungs and sufficient of the adjacent parts to
leave the terminations of the great veins and the
origins of the aortic trunks intact. The whole should

90 ELEMENTARY BIOLOGY. [CHAP.

then be placed in tolerably strong spirit until tho-
roughly set.

i. When sufficiently hardened, remove the lungs and
adjacent parts, so as to isolate the heart together
with the cut ends of the great vessels. Examine as
under.

a. Ventral aspect. Work over the external charac-
ters described in § i.

b. Dorsal aspect. Note especially —

a. The sinus venosus ; spacious and elongated,
receiving the three caval veins.

ft. The atrium ; if the heart be well distended a
slight constriction will be seen subdividing
this into a larger right auricle and a smaller
left auricle.

y. The pulmonary veins (cf. § 7) ; seen to con-
verge immediately in front of the sinus ve-
nosus, to enter the inner border of the left
auricle.

3. The sinu-auricular aperture ; seen on opening
up the sinus venosus and removing its con-
tents. It communicates with the cavity of
the right auricle and is guarded by two mem-
branous flaps (sinu-auricular valves].

ii. General dissection from the front.

Pin down ventral surface uppermost, and dissect
with great care, under water. Remove the front
wall of the ventricle, atrium and truncus arteriosus,
and lay open the bases of the aortic arches ; care-
fully remove the coagulated blood which fills them
and wash clean.

I.] THE FROG. QI

a. The ventricle ; its single ventricular cavity and
thick spongy wall.

b. The auriculo -ventricular aperture; situated to
the extreme left (right of the preparation) of the
ventricle, and bounded by two auriculo-ven-
tricular valves. Raise the nearest of these, if
not already removed, and dissect it away.

c. The atrium; its thin wall; its spacious cavity
completely subdivided into two chambers (r. and
1. auricles) by a longitudinal inter-auricular sep-
tum, lying well to the animal's left side. Ex-
amine the relations of this to the auriculo-ven-
tricular aperture ; it is prolonged down on to
the valves which guard it, subdividing it into
two.

a. The sinu-auricidar aperture; large and situ-
ated near the middle of the right auricle im-
mediately adjacent to the inter-auricular sep-
tum. Note its valvular margins.

ft. The aperture of the pulmonary veins ; small
and rounded, opening near the top of the
left auricle, close to the septum.

y. The auricula-ventricular valve; note its cha-
racters and mode of attachment.

d. The truncus arteriosus ; arising from the ex-
treme right-hand corner of the ventricle and
largely subdivided into two by a longitudinal
valve (cf. p. 1 8). Examine the aortic arches
in relation, passing bristles into them ; they
arise as under

92 ELEMENTARY BIOLOGY. [CHAP.

a. The aortic and carotid arches ; from its an-
terior end, immediately in front of the longi-
tudinal septum, the calibre of the aortic arch
being much the greater of the two.

y8. The pulmonary arches ; the pair arise by a
single aperture lying immediately to the
animal's left of the anterior end of the longi-
tudinal valve.

iii. A companion dissection may be profitably made
from the left side, for general comparison with the
foregoing, especially with respect to the relations
of the auriculo-ventricular valves and inter- auricular
septum.

12. The circulation of the blood in the web of the foot.

i. Get a piece of thin board, about 5 inches long and
2\ broad ; in the middle of one end of it cut a
V-shaped notch about the size of the expanded
web : place the frog on the board, belly down-
wards, and fix it by passing round it two or three
turns of tape : next tie threads round the toes of
one foot, and by means of them spread out the
web over the notch, taking great care that it is
only very lightly stretched. The animal should be
kept moist by spreading a piece of wet blotting-
paper over its back.

ii. Examine the web with i inch obj.: Note —

a. The black pigment- cells in the skin ; sometimes
irregularly branched ; sometimes more compact.

b. The close network of blood-vessels, lying deeper
than the pigment-bearing layer.

I.] THE FROG. 93

a. The arteries, running mainly towards the free
edge of the web, and constantly diminishing
in size as they break up into branches ; the
blood-flow, from larger to smaller branches.

/2. The capillaries, in which the arterial branches
end: small vessels forming a close network
and frequently branching or anastomosing
without much alteration in calibre.

y. The veins, formed by the ultimate union of
the capillaries, and increasing in size as they
unite with one another ; the blood-flow, from
smaller to larger factors.

c. The nature of the blood-flow ; the current is
marked by the solid bodies (corpuscles) carried
along in the blood-fluid : it is most rapid in the
arteries ; slowest, and most uniform, in the ca-
pillaries.

iii. Place a small drop of water on a bit of thin mica
or of a thin cover-slip, and place the same, water
downwards, gently on the web : then examine with
\ or i obj. ; note —

a. The walls of arteries, capillaries, and veins.

a. The arterial walls, tolerably thick, seen as a
clear well-defined line on each side of the
blood-stream.

(3. The capillary walls ; difficult to see ; apparent
as thin slightly transparent boundary lines.

y. The venous walls ; much like the arterial.

b. The blood-flow in the small arteries.

a. The rapid stream in the middle, containing
most of the red corpuscles.

94 ELEMENTARY BIOLOGY. [CHAP.

yS. The slower stream along the edge (inert
layer), containing many granular-looking co-
lourless corpuscles.

c. Thefaw in the capillaries ; much slower than in
the arteries ; the frequent distortion of the red
corpuscles in the capillaries under pressure;
the elasticity of the corpuscles, as indicated by
the readiness with which they recover their
shape when the cause of distortion is removed ;
the manner in which the white corpuscles creep
along, and their tendency to stick to the capil-
lary wall.

J. The lymphatic system. The full study of this system
is beyond the scope of this work ; the undermentioned
may however be made out with comparative ease.

a. The sub-cutaneous lymph-spaces ; cf. Sect. C. z.j.

b. The cisterna magna ; cf. Sect. C. i. g. y.

c. The circum-cesophageal lymph sinus ; a loose fold
of mesentery embracing the base of the gullet,
opalescent in appearance. It becomes exceed-
ingly conspicuous if inflated, by introducing the
point of a blow-pipe beneath the adjacent peri-
toneum.

d. The lymph-hearts ; small thin-walled sacs, paired
and pulsatile ; to be sought for while still beat-
ing, immediately on the death of the Frog.
They are, on either side —

a. The posterior lymph-heart ; to be seen on re-
moving the skin of the back, lying a short
distance in advance of the cloacal aperture

I.] THE FROG. 95

and immediately in front of the pyriformis
muscle (Sect. H. 5. c).

fi. The anterior lymph-heart ; lying in an inter-
space between the small muscles (intertrans-
versi) which pass between the transverse pro-
cesses of the third and fourth vertebrae. Best
seen on slitting open the body- wall ventro-
laterally and cutting into the cisterna magna.

The cartilaginous free end of the transverse
process of the third vertebra is expanded (cf.
Sect. G. 3*:) for protection of the above.

K. The nervous system.

i. The cerebro-spinal axis and neural canal. Take a
recently-killed frog and divide the skin along the
dorsal middle line and reflect it on each side ; re-
move the muscles which overlie the arches of the
vertebrae, and open the neural canal as follows. Di-
vide the membrane (vertebra-occipital ligament} which
passes between the ist vertebra and occiput; then
introduce one blade of a small, but strong, pair of
scissors into the cranial cavity, and cut away bit by
bit the bones which form the roof of the skull,
taking care that the point of the scissors does not
injure the brain. Next remove, in a similar manner,
the tops of the vertebral arches and the head of the
urostyle. In all probability a chalky fluid will be
met with clouding the water and concealing the un-
derlying organs (secretion of the periganglionic glands
(cf. p. 32); if so, wash carefully until clean. There
will thus be exposed

a. The cerebro-spinal axis ; filling the greater part
of the neural canal ; it can be resolved into

96 ELEMENTARY BIOLOGY. [CHAP,

a. The encephalon or brain ; that portion which
lies within the cranium, its constituents in
part paired.

ft. The myelon or spinal cord ; axial and lying
within the neural canal of the vertebral
column.

Its form : wide in front, but narrowing ra-
pidly about the 5th or 6th vertebra, and
thence continued back as a slender tapering
filum terminate.

The groove (dorsal fissure) running along
its middle line.

b. The investing membranes arid the origin of the
spinal nerves.

a. The//0 mater • a delicate densely pigmented
membrane, investing the whole nervous axis.
It is thickened and highly vascular above the
hind portion of the brain (choroid plexus).

y8. The dura mater ; a similar but denser mem-
brane lining the wall of the neural canal.

y. The roots of the spinal nerves ; conspicuous in
the case of the second and eighth and ninth
pairs of nerves. The former are very stout,
and pass out from the myelon at right angles
to the axis, to reach the arm : the latter are
long and delicate, passing back on opposite
sides of the hinder half of the myelon.

c. Obtain a second Frog (preferably one hardened
in alcohol) and make a transverse section across
the entire nervous axis and neural canal im-

I.] THE FROG. 97

mediately behind the above-named second spinal
nerve. Examine in water, under a hand lens.

a. The myelon ; subdivided, dorsally and vent-
rally, by median longitudinal fissures ; en-
closing a small central canal.

/3. The nerve roots ; arising dorsally and ventrally,
and passing outwards and downwards through
the inter-vertebral foramina to form, on each
side, the trunk of the nerve. The dorsal
root bears an enlargement or ganglion of the
root, immediately on leaving the neural canal.

y. The investing membranes (cf. supra) ; seen to
merge into each other and the periosteum, in
the vicinity of the inter- vertebral foramina.

2. The anatomy of the brain.

i. Expose from the dorsal aspect ', as directed in § i,
preferably in a frog which has been previously
hardened in spirit. Examine in situ.

The anterior half of the brain consists of two
elongated masses, each marked off by a slight
transverse depression into a smaller anterior and
a larger posterior portion.

a. The cerebral hemispheres (prosencephalori); the
posterior of the above-named, separated by a
deep cleft.

b. The olfactory lobes (rhinencephalon) ; the anterior
of the above-named, confluent with each other
at their bases. Each passes into a rounded
trunk (commonly termed the olfactory nerve\
which leaves the skull, and ramifies in the
lining membrane of the nose.

M. 7

98 ELEMENTARY BIOLOGY. [CHAP.

c. The thalamencephalon : a thickened mass imme-
diately posterior to the cerebral hemispheres.

Note, in connection with it,

a. The thalami optici ; its lateral walls, consti-
tuting its main portion as here seen.

/?. Its thin roof; transparent and very readily
torn.

y. The pineal gland ; represented by a small
pinkish body, lying in the bay enclosed by
the hinder ends of the hemispheres.

d. The optic lobes (mesencephalon) : a pair of
rounded eminences lying behind the thala-
mencephalon.

e. The cerebellum (inetencephaloti) : a narrow trans-
verse band next in order of succession.

f. The medulla oblongata (myeloncephaloti) : the por-
tion of the brain lying behind the cerebellum
and passing into the spinal cord. Its roof is
covered by the choroid plexus (cf. i. b. a).

ii. Divide the olfactory lobes, and raise the front end
of the brain ; turning it back gradually, divide with
a sharp scalpel any nerves that may be observed
passing from it to the cranial walls : most of these,
being small, will probably be torn across unob-
served, but the large optic nerves will at any
rate be seen. Remove the brain, together with
a small portion of the spinal cord, and examine
from the ventral aspect.

a. The hemispheres and olfactory lobes (cf. supra).
The outer walls of the latter are thickened

I.] THE FROG. 99

ventrally to form two cord-like tracts (ventral
roots of the olf. lobes).

b. The lamina terminalis ; a median bilobed emi-
nence, interposed between the bases of the
hemispheres.

c. The optic lobes ; seen to project laterally, be-
hind the hemispheres.

d. The optic tracts; band-like masses arising in
connection with c\ that of the left side giving
off the optic nerve to the right eye and vice
versa. The interlacing of the two at the point
of decussation, to form the optic chiasma.

e. Lying in the ventral middle line immediately
behind the optic chiasma there will be seen, in
the order named

a. The tuber cinereum ; a large tongue-shaped

mass, bilobed posteriorly.
P. The infundibulum ; a small stalk-like body

giving attachment to
y. the pituitary body ; a rounded pinkish mass

(highly vascular in the fresh state).

iii. Examine from one side (lateral aspect), and com-
pare generally with i. and ii.

iv. Slice away, with a small scalpel, the dorsal walls
of the cerebral hemispheres and olfactory and optic
lobes, and remove in like manner the greater por-
tion of the cerebellum and roof of the thalamen-
cephalon. The central cavities or ventricles will
thus be laid bare ; they can be resolved into two
sets, as under :

7—2

IOO ELEMENTARY BIOLOGY. [CHAP.

a. Median or axial (forward continuations of the
central canal of the spinal cord); in order of
succession from behind forwards —

a. The fourth ventricle ; tongue-shaped and ex-
panded in front ; enclosed by the medulla
and cerebellum.

(3. The iter or aqueduct of Sylvius ; between and
below the optic lobes.

y. The third ventricle; lying within the thala-
mencephalon and bounded in front by the
lamina terminalis (front wall of the median
portion of the cerebro-spinal axis).

b. Paired.

a. The optic ventricles ; lying within the optic
lobes and derivative of the iter.

ft. The lateral ventricles (central cavities of the
hemispheres) in communication, on either
side, with the third ventricle through a wide
passage (foramen of Monrd). Each is seen
to lodge a thickened fold of pia-mater (cho-
roid plexus) which enters it immediately in
front of the above-named foramen. (Cf. p. 27.)

y. The olfactory ventricles ; forward prolongations
of /?, extending into the bases of the olfactory
lobes.

v. Longitudinal vertical section. Obtain the brain of
the largest frog accessible, and, when fully hard-
ened in spirit, remove one half cutting to the near
side of the middle line.

Examine in water under a hand lens, and note
the following

I.] THE FROG. 101

a. The median ventricles (§ iv. a) ; note their limits
and their relationships to the several constituents
of the brain. Take especial note of the third
ventricle; its walls are modified as follows

a. Front wall; greatly thickened to form the

lamina terminalis (iv. a. y).
ft. Roof; thin and delicate, prolonged upwards

and forwards to give attachment to the "pineal

body."
y. Its floor; for the most part thin prolonged

downwards and backwards as the infundibu-

lum (ii. e. ft) to meet the pituitary body;

thickened at its middle around the optic

chiasma (ii. d).
8. The foramen of Monro ; lying immediately

above the lamina terminalis and large enough

to admit a good-sized bristle.

/;. The commissures ; small but definite tracts passing
between the opposite halves of the brain ; rela-
tively whitish in colour. Look for their cut
edges with a powerful hand lens, as under —

a. The posterior commissure ; lying in the sub-
stance of the roof, immediately in front of
the optic lobe.

ft. The corpus callosum ; represented by a well-
defined tract traversing the head of the lamina
terminalis, immediately below and behind
the foramen of Monro.

y. The anterior commissure; smaller than the
above and running through the middle of the
lamina terminalis.

102 ELEMENTARY BIOLOGY. [CHAP.

3. The spinal nerves.

Pin a frog down upon its back and remove the
ventral integument and body-wall, together with one-
half of the floor of the mouth. Carefully dissect
away the viscera and dorsal aorta and remove the
centra of the vertebrae, cutting well down to the level
of the roots of the spinal nerves. Wash, and exa-
mine under water,

a. The spinal cord ; in form cylindrical; enlarged
at the points of origin of the nerves for the
limbs (brachial and lumbo-sacral swellings).

b. The spinal nerves arising from it. Ten on each
side ; the first one passing out between the
first and second vertebrae, the last through the
head of the urostyle (cf. Sect. G. 2. d. e). Each
leaves the neural canal behind that vertebra
with which it corresponds.

a. Their origin ; each arising by two roots (an-
terior and posterior) as is most easily seen in
the yth, 8th and Qth nerves, in which they are
much the longer. Sometimes the fibres of the
anterior roots are bound up into two or more
fasciculi.

ft. The direction of their roots: outwards in the
anterior nerves; obliquely backwards in the
4th, 5th and 6th; almost directly backwards,
and running for a considerable distance in
the neural canal, in the yth, 8th, pth and
loth.

y. The ganglia of their dorsal roots (cf. § i. c. /?);
very conspicuous in the yth, 8th, and pth;
lying, for the most part, outside the spinal

I.] THE FROG. 103

canal and largely surrounded by the perigan-
glionic glands (cf. p. 32).

c. Distribution of the spinal nerves. Follow this
on one side.

a. The ist ; examine it on that side upon which
the floor of the mouth is retained. It passes
at first outwards and then forwards and in-
wards, beneath the mylohyoid muscle, to be
distributed to the muscles of the tongue
(hypoglossal nerve],

(3. The 2nd and 3rd; meeting, on a level with
the end of the transverse process of the third
vertebra, to form the brachial nen>e. Follow
it to the arm.

y. The 4th, 5th, and 6th; each forming a long
trunk which passes back for a considerable
distance, for distribution to the body wall
and integument.

8. The 7th, 8th, and 9th ; running parallel with
each other and the urostyle (lumbo-sacral
plexus), and finally becoming bound together
to form the iliac nerve for distribution mainly
to the hind limb. Follow this to the leg.
This plexus is liable to variation.

€. The loth, a small nerve lying immediately
internal to 8 and close against the urostyle.
It enters into connection with the lumbo-
* sacral plexus, and is distributed to some of
the lesser muscles of the posterior extremity
and parts adjacent.

£. Examine one of the middle spinal nerves
with care, under a hand lens if necessary.

104 ELEMENTARY BIOLOGY. [CHAP.

There will be found to arise immediately
external to its ganglion a small dorsal ramus ;
note that this is distributed to the muscles
and other parts of the body dorsally, much as
is the main trunk or ventral ramus ventrally.

4. The cranial nerves.

The study of these may be greatly facilitated by
placing the animal when dead in J per cent, chromic
acid solution, the same to be frequently changed
until decalcification is complete. This done; wash
well under running water and preserve in methylated
spirit, slightly dilute.

i. Dissect from the ventral aspect as directed at § 3,

leaving the floor of the mouth intact.
a. The gth or glossopharyngeal nerve. Its trunk ;
seen, on removing the middle portion of the
mylohyoid muscle (Sect. B. c. y) lying immedi-
ately external to that of the hypoglossal nerve
(§ 3- £ a) ) its branches, seen, on dissecting the
genio-hyoid, to be distributed to the tongue and
some of its muscles.

ii. Remove the floor of the mouth together with the
heart and gullet, leaving the mandible and a small
portion of the mylohyoid muscle on one side.
Raise the mucous membrane of the roof of the
mouth with forceps and dissect it away with the
utmost care, being especially cautious to avoid re-
moving certain nerves which lie in close appo-
sition with it. Next slice away the floor of the
cranium and auditory capsules so as to expose
their internal cavities; wash clean and examine
in water under a hand lens.

THE FROG. 105

a. The ist or olfactory nerve ; passing forwards in
front (cf. § 2. b}. Remove the vomer of one
side and cut down to its level, following it out ;
it will be seen to ramify on the base of the ol-
factory sac.

b. The 2nd or optic nerve ; arising from the optic
chiasma (§ 2. d] ; it runs forwards and upwards
to enter the eye-ball from below.

c. The $rd.or oculo-motor nerve (described, in com-
mon with the 4th and 6th, in connection with
the eye-muscles. Sect. L. 3. p. 116).

d. The 5/// or trigeminal nerve. Remove the eye-
ball on one side ; lying internally to it, deeply
seated and in close apposition with the cranial
wall, will be seen the ophthalmic or orbito-nasal
nerve (ist division of the 5th) ; trace it back, it
will be found to enter, immediately internal to
the front end of the auditory capsule —

a. the Gasserian ganglion ; a comparatively large
oval pigmented body, seen to be in con-
nection with other nerves also.

J3. The maxillo-mandibttlar nerve ; arising from
a and passing outwards immediately be-
hind the eye. It early subdivides into the
two following; the mandibular nerve (3rd
division of the 5th) to the lower jaw and its
integument (follow it on that side in which
these still remain) ; the maxillary nerve (2nd
division of the 5th) seen, on removing the
hinder third of the upper jaw, to be somewhat
similarly distributed to the maxillary region.

106 ELEMENTARY BIOLOGY. [CHAP.

y. The ophthalmic or orbito-nasal ; its distribu-
tion may now be studied by turning the animal
over and examining from the dorsal aspect.
Remove the nasal and sphenethmoid bones
to its level; and note its branches to the
antero-dorsal integument of the head.

e. The ^th or facial ; intimately connected with
the Gasserian ganglion and the trigeminal. Ex-
amine it, from the ventral aspect, on that side
at which the eye remains.

a. Its root ; arising, side by side with that of the
5th, from the side wall of the medulla ; the
two entering the Gasserian ganglion.

y8. Its anterior branch (palatine nerve) ; leaving
the Gasserian ganglion immediately external
to the ophthalmic branch of the 5th, and
passing forwards, perforating the vomer, to be
distributed to the mucous membrane of the
roof of the mouth.

y. The maxillo-palatine commissure ; connecting
the palatine and maxillary nerves; on a level
with the front end of the eye-ball.

S. The posterior branch of the 7th ; leaving the
outer side of the Gasserian ganglion, im-
mediately behind the maxillo-mandibular
nerve, and passing outwards and backwards
dorsal to the columella auris (beneath it as
here seen). Its distribution may best be
studied later.

/ The ()t/i and io//i (post-auditory nerves)-, arising
close together from the medulla posteriorly to

THE FROG. lO/

the 5th and yth, and passing behind the auditory
capsule, much as do the 5th and 7th in front of
it.

a. The loth or pneumogastric (vagus) \ its cut
end will alone be seen. Soon after leaving the
skull it bears a large ganglion (g. trunci vagi).
/3. The gth or glossopharyngeal ; it arises im-
mediately in front of a, and is largely con-
fluent with the above-named ganglion. It
is commissurally connected with the posterior
branch of the 7th (commissura ad facialem).
Its distribution has been studied (§ i. a).

g. The Wi or auditory ; arising from the medulla
along with the 7th, part of which it appears
to be. Dissect to its level and trace it into the
auditory capsule.

The sympathetic system.

Dissect from the ventral aspect as before, but
leave the aorta, the cut end of the coeliaco-me-
senteric artery and the base of the large intestine
with the bladder, in place. On raising the aorta
the sympathetic will be found immediately beneath
(dorsal to) it.

a. The main trunk of the sympathetic ; paired and
longitudinal, accompanying the arches and trunk
of the aorta.

b. The splanchnic nerve ; arising from a at the point
of junction of the aortic arches and accompanying
the cceliaco-mesenteric artery.

c. The sympathetic ganglia; enlargements on #, vari-
able in size and number; usually one for each

IO8 ELEMENTARY BIOLOGY. [CHAP.

spinal nerve. Best seen on turning the aorta to
one side.

d. The rami communicantes ; relatively long com-
missures between c and the trunks of the spinal
nerves.

e. The distribution of the sympathetic ; best studied
in the pelvic region, and rendered the more
conspicuous if a little acetic acid be added to
the water. Vaso-motor nerves ; arising from the
ganglia and passing to the aorta. Viscero-motor
nen>es ; seen to arise from both sympathetic and
lumbo-sacral plexus for distribution to the pelvic
viscera.

/ The cranial sympathetic. Remove the aortic arches
with care and follow the sympathetic forwards,
dissecting as at § 4. ii. With a little care the above
can be traced to the Gasserian ganglion ; passing
internally to the auditory capsule and entering
into connection, en route, with the post-auditory
nerves.

6. The further course of the facial and vagus nerves
(yth and loth cranial).

Pin a frog down upon its side and remove the
integument of the head, together with the arm and
shoulder-girdle and body-wall adjacent. Pull down
the stomach and pin it back, so as to put the me-
sentery on the stretch and fully to expose the near
lung and the heart.

a. Note the course of the •niandibular nerve (§ 4.
//. ft). It will be seen to run backwards and
downwards immediately in front of the squamosal

I.] THE FROG. 109

and under the jugal, to reach the outer side of
the mandible.

/>. The facial nerve. Its hyoid branch ; seen on dis-
secting away the membrana tympani and outer
end of the columella auris. It leaves the skull
immediately behind the squamosal and passes
downwards and backwards, giving off —
a. A small branch to the integument of the cheek

(in close apposition with the jugal bone).
/?. A mandibulo-hyoid branch ; continued along the
inner face of the mandible, and giving off nerves
to the muscles and skin adjacent. (Be careful
not to confuse these with the branches of the
mandibular nerve.)

c. The Qth and i2th (cf. § 4); the former will be
seen to pass internally, the latter externally, to
the anterior cornu of the hyoid. Remove the
hypoglossal with care, thus laying bare —

d. The loth (vagus). Seen to pass downwards and
backwards, externally to the posterior cornu of
the hyoid. At the side of the gullet it breaks up
into the under-mentioned

a. The gastric rami ; two or more in number, dis-
tributed to the stomach. They pass through
the so-called diaphragm.

(3. The pulmonary ramus, to the lung; accom-
panying the pulmonary artery.

y. The cardiac ramus, to the heart; arising in
front of /? and coursing along the dorsal wall
of the atrium. (Sympathetic fibres are bound
up with this, if not with a and /? also.)

MO ELEMENTARY BIOLOGY. . [CHAE

8. The recurrent laryngeal ; a long nerve arising in
front of and above a — y and coursing round the
base of the pulmonary aortic arch to reach the
larynx.

e. The dorsal ramus ; arising from the main trunk
immediately after it leaves the skull, and dis-
tributed to the adjacent dorsal integument.

e. Incidental,

a. The course of the 5th and 7th (cf. § 4. ii. d. c.
et seq.). Remove the eye and the squamosal
and quadrate- jugal bones and follow the course
of the above nerves, as seen from the side.

Note especially the relations of the 5th to the
mouth-cavity and of the yth to the tympano-
Eustachian passage (Eustachian recess). See
pp. 28, 29.

(3. The sympathetic. Note especially the further
course of the splanchnic nerve (§ 5. ft). It
bears a large ganglion in close proximity with
the cceliaco-mesenteric artery; add a little acetic
acid ; offshoots will be found to proceed from it
to the alimentary canal, accompanying, at their
outset, the branches of the artery.

L. The anatomy of the sense organs,
i. The gustatory organ.

The shape and arrangement of the tongue have
already been described (Sect. E. § i d.). Snip off a
bit of mucous membrane from the upper surface of
the tongue of a recently killed frog, mount in salt
solution and cover in plenty of the fluid with a large
coverslip : examine with one inch obj.

I.] THE FROG. Ill

a. On the surface of the fragment and especially
around its edges numerous minute elevations will
be seen : these are the papilla : some (filiform
papilla] are pointed at the free end and others
(fungiform papilla) are flattened. Note the loops
which the blood capillaries make in many of them.

b. Examine one of the thinner bits of the specimen
with a high power : a flickering motion, due to
the presence of cilia, will be observed. Some of
the papillae however will be seen to have no cilia
except a narrow belt around the somewhat trun-
cated apex : it is on these that the gustatory discs
are placed, and in fortunate specimens nerve-
fibres can be seen entering them.

2. The olfactory organ.

The general relations of this have already been
described (Sects. A and E).

a. Take a frog which has been preserved in spirit;
insert the point of a small pair of scissors into the
external nostril of one side and cut away the roof
of the nasal cavity. A chamber is thus exposed
which has a somewhat triangular form, its apex
being anterior.

a. The walls of the cavity are slightly folded, and
there is a well-marked hemispherical eminence
on its floor which overlies the vomer.

(3. The posterior nostril ; situated some distance in
front of the hinder boundary of the nasal sac
(i. e. the sac is prolonged back behind it).

b. Open the other nasal cavity in a similar way :

a. The septum narium; a median longitudinal par-

112 ELEMENTARY BIOLOGY. [CHAP.

tition separating the apposed sacs of opposite
sides.

y3. The boundaries of the olfactory organ. It lies
within an area enclosed by the premaxilla and
front end of the maxilla and the palatine bones,
being wholly in front of the latter.

c. Obtain a second frog, and lay open the olfactory
sac from the side, cutting obliquely from the
anterior nostril across the outer anterior border of
the eyeball, in order that the line of section may
pass through both anterior and posterior nares.
a. General. Cf. supra.

/?. The anterior nostril ; its upper lip, relatively
small and constricted; its lower lip, produced
antero-laterally into a deep fold around the ali-
nasal cartilage (cf. Sect. G. 5. a. e).

3. The visual organ.

i. General.

Take an uninjured frog and examine its eye. It will
be found to project considerably above the top of
the head, but if touched it is withdrawn into a sort
of socket. If the animal's mouth be opened, a de-
pression, caused by the eye-ball, will be seen on its
roof, and this is more prominent when the eye is re-
tracted. When the eye is open observe from without.

a. The eyelids ; folds of skin developed round the
margin of the eye-ball. The upper lid is thick
and swollen, the lower lid thin and transparent.

Gently touch the eye and observe that it is
closed, by the pulling over it of the lower eye-
lid. The upper lid is hardly moveable.

THE FROG. I I 3

b. When the eye is open, observe the following —

a. The cornea; the transparent covering for its
exposed surface.

j3. The inst a membrane, seen through #, coloured
by brown and golden pigment, the latter form-
ing a very brilliant ring around its inner
margin. Below it is interrupted by a faint
dark line which can be traced downwards
through the whole iris.

y. The pupil ; elliptical and lying within the
iris, its long axis directed antero-posteriorly.

ii. The eye-muscles and their nerves. These can be
most satisfactorily made out, notwithstanding their
small size, in a frog preserved in chromic acid as
directed in Sect K. 4. Examine under a hand
lens.

a. Dorsal aspect. Remove the head from the body,
cutting well back so as to include the first two
vertebrae, and pin down in as small a dish as
may be convenient. Dissect off the integument
of the top of the head together with the upper
eyelids, but be very careful to leave the lower
lids uninjured.

a. The eyeball ; completely filling the orbit.
P. The superior oblique muscle ; ribbon-shaped
and passing obliquely forwards between the
antero-dorsal face of a. and the orbit.
y. The superior rectus ; a trifle larger than ft.
and arising from the dorsal face of the eye-
ball. It passes backwards and inwards, to be
attached to the cranial wall.

M. 8

114 ELEMENTARY BIOLOGY. [CHAP.

8. The ^th cranial nerve (n. patheticus] ; seen, on
slightly pulling the eye-ball outwards, skirting
the inner wall of the orbit to reach the su-
perior oblique muscle.

e. The $rd cranial nerve (oculomotor}. A branch
of this may be seen breaking up in the
superior rectus muscle, if that be cut across
near its point of attachment to the eye-ball
and turned back.

£. The lower eyelid • follow it forwards, it will be
found to be continuous with a strong tendon
which passes through a loop immediately ex-
ternal to the superior oblique muscle.

b. Ventral aspect. Turn the head over and pin
down ventral surface uppermost ; remove the
entire lower jaw and floor of the mouth, and
dissect off the mucous membrane of its roof
with care.

a. The levator bulbi muscle ; a sheet-like mass
underlying (overlying as seen) the whole eye-
ball; its fibres pass obliquely outwards and
backwards.

Compare a transverse section such as is de-
scribed at Sect. C. 3. The fibres will be found
to pass between the upper jaw and the in-
ternal dorsal wall of the orbit.

/3. The inferior oblique ; a small muscle passing
upwards and inwards between the antero-
internal face of the eye-ball and the orbit.

y. The Harderian gland ; a nodular mass im-
mediately behind ft, interposed between the
eye-ball and orbit.

THE FROG. 115

S. The recti muscles other than the superior
one ; seen on removal of the levator bulbi,
to converge postero-internally. The inferior
rectus, arising from the ventral face of the eye-
ball and passing obliquely backwards; the
external rectus, arising from its posterior face
and passing inwards and upwards ; the in-
ternal rectus, arising in front under cover of
the inferior oblique, and passing backwards
along the inner face of the eye-ball.

€. The $rd (oculomotor) nerve ; seen to break up,
immediately in front of the superior rectus,
for distribution to /:?. and 8. with the excep-
tion of the external rectus.

£. The 6th cranial nerve (n. abducens). A small
muscle (pterygoid} will be seen lying in the
posterior region of the orbit ; remove this and
lay bare the Gasserian ganglion (Sect. K.p. 105);
the 6th nerve will be found passing from the
outer face of the latter to the postero-ventral
border of the external rectus. It is very
short and may best be seen by gently drawing
the muscle forwards.

if]. The retractor bulbi muscle; a considerable
mass of tissue, seen, on removing ft. and 8.
It surrounds the optic nerve.

9. The tendon of the lower lid (Cf. a. £.) ; shining
and thread-like, passing round and intimately
connected with the outer edge of 17. It is
inserted behind into the adjacent integument.

The origins of the yd, ^th and 6th cranial nerves.

8—2

Il6 ELEMENTARY BIOLOGY. [CHAP.

Lay bare the cerebro-spinal axis from the ventral
aspect, as directed in Sect. K. § 3, and expose
the Gasserian ganglion with the utmost care.

a. The $rd (oculomotor) nerve ; arising, on each
side, from the ventral surface of the brain on
a level with the pituitary body. It passes up-
wards and outwards to leave the skull in
front of the 5th nerve (cf. p. 67, vi.).

{3. The 6th nerves (n. abducentes) ; extremely
delicate, arising close together in the middle
line from the floor of the medulla, on a level
with the 9th and loth. Each passes upwards
and outwards along with the trunk of the
5th to enter the Gasserian ganglion.

y. The 4//fc (n. patheticus) ; best seen on re-
moving one optic nerve and turning the brain
a little to one side. It leaves the dorsal sur-
face of the brain in front of the cerebellum ;
and passes upwards and forwards to leave the
skull above and in front of the optic nerve
(cf. p. 67, vil).

iii. The structure of the eye-ball. Obtain a freshly-
killed frog and remove the head from the trunk ;
bisect the former longitudinally and examine the
eye-ball while still in position, as under.

Make two sections; that of one side (a.) to
bisect the eye-ball transversely to the long axis of
the body (to pass through the crystalline lens if
possible); that of the other (b.) to bisect it equa-
' torially, at right angles to a. Examine under a
hand-lens.

I.] THE FROG. II/

Section a.

i. Examine high and dry and note —

a. The eye-ball ; firm and resistant, enclosing a

central cavity.
ft. The crystalline lens; a relatively large transparent

body filling a considerable portion of a, globular,

its outer face somewhat flattened. It is held

firmly in position,
y. The vitreous or inner chamber ; that portion of

the cavity of the eye-ball internal to the lens.

It is filled with a dense gelatinous vitreous

humour, which may be raised en masse with a

forceps.
8. The aqueous or outer chamber ; relatively much

smaller than y. and external to the lens. It

lodges a more fluid aqueous humour.

ii. Dissect the inner portion, under water, to the level
of the optic nerve, so as to get the same into
longitudinal section. Examine the coats of the
eye, following their cut edges.

a. The sclerotic ; the outermost coat, dense and
cartilaginous ; it serves to give consistency to
the whole and to furnish attachment for the
muscles. Externally it is continued over the
outer face of the eye-ball as the thin and trans-
parent cornea ; internally it forms the sheath of
the optic nerve.

(3. The choroid ; internal to a, vascular and spongy
and blackened by pigment. It is prolonged
forwards in front of the lens to form the iris
(cf. § i. b).

Il8 ELEMENTARY BIOLOGY. [CHAP.

y. The retina ; a thin filmy layer, greyish and
transparent, internal to /?. It stops short at
the base of the iris (or a serratd) ; internally it
passes into the optic nerve.

If the vitreous humour be much disturbed this
layer will be puckered and torn, or otherwise
displaced.

8. The conjunctiva and eyelids (cf. Sect. C. § 3. d.).
The former can only be distinguished from the
cornea with difficulty.

Section b.

i. Examine the inner half from within, under water.

a. The retina. Gently raise it ; it will be found to
adhere firmly at the point of entrance of the
optic nerve (blind spot). Note the position of
same.

/3. The choroid ; strip off the whole retina and ex-
amine this, noting its texture and great vas-
cularity.

y. The sclerotic; examine this in like manner,
after having removed (3.

ii. Examine the outer half from within, and note
especially the relations of the lens and iris.

4. The auditory organ.

a. Examine the tympanic membrane from the outside
(cf. Sect. A. #./?.). Dissect off its outer or ' tegu-
mentary layer'; there will thus be brought into
view:

a. The annulus tympanicus ; a cartilaginous ring
supporting the edge of the membrane.

THE FROG. 119

P. The columella auris ; its head will be seen
as an opaque white patch in the middle of the
deeper layers of the tympanic membrane, now
laid bare. Cut away the latter around it and
note its relations to the Eustachian recess (Sect.
E.). Follow the cut edge of the mucous mem-
brane; it will be found to wrap round the
columella, suspending it much as the mesentery
suspends the alimentary canal (i.e. the columella
is outside it).

b. The internal ear. Remove the skin from the top
of the head of a large frog which has been pre-
served in alcohol, and scrape the roof of the
auditory capsules quite clean. Isolate the capsule
of one side and hold it between the finger and
thumb of the left hand. If examined with care, a
couple of greyish streaks will be seen on the inner
side of its roof, diverging outwards. Carefully
slice away the bone, being especially cautious to
cut quite superficially along these streaks ; there
will thus be laid bare :

a. The cavity of the internal ear ; enclosed on all
sides by the auditory capsule and filled with a
fluid (perilympJi).

p. The membranous labyrinth ; a portion of this is
now visible as two delicate pigmented canals,
coincident in position with the divergent streaks
afore-named.

c. Still holding the capsule as before, slice away,
bit by bit, its outer wall and the remainder of its
roof. A third membranous canal will be seen on
the outer side ; when this is liberated, transfer the

120 ELEMENTARY BIOLOGY. [CHAP.

whole to a watch-glass or small vessel filled with
water (preferably one having a white bottom). The
membranous labyrinth will probably float out; if
not, it may be liberated with a camel's hair brush.
It consists of a greyish pigmented structure of the
size of a small pea : Note

a. The vestibule; sac-like and constricted into
two — an upper portion or utriculus, and a lower
one or sacculus. It is largely filled during life
by a milk-white otolithic mass.

j3. The semicircular canals ; three in number and
connected with the utriculus. The anterior
and posterior vertical canals, the two divergent
ones referred to at b -} their inner ends unite
before entering the utriculus. The horizontal
canal ; situated externally, on a level with the
bases of the other two.

y. The ampulla ; enlargements of the bases of the
canals. Those of the anterior and horizontal
canals are situated close together at the anterior
end of the utriculus.

8. The ultimate ramifications of the auditory nerve ;
visible on the inner face of the labyrinth.

d. General dissection of the whole auditory organ.
Obtain the head of a frog which has been pre-
served in spirit, and dissect from behind.

Scrape the occipital region of the skull quite
clean and then pin the whole down under water.
Carefully remove the wall of the auditory capsule
— externally, to the level of the columella auris —
internally, to that of the auditory nerve ; cut away

THE FROG. 121

the hinder half of the tympanic membrane, to the
level of the head of the columella.

Work over the mutual relations of the following,
all of which have been previously described.

a. The columella auris; to the tympanic membrane
and fenestra ovalis.

/?. The membranous labyrinth ; to the periotic cap-
sule and its enclosed cavity.

y. The auditory nerve; to the membranous laby-
rinth and brain.

8. The Eustachian recess; to the mouth cavity,
and that of its lining membrane to the colu-
mella.

e. The annulus tympanicus ; to the membrana
tympani.

Histology.

In the undermentioned directions, the reagents which
may preferably be employed in each case are alone
enumerated; details as to preparation and methods
of treatment will be found in the Appendix.

The blood.

a. Freshly drawn blood. Obtain a drop of the same
from a freshly-killed frog, and examine with a
low power, under a cover-glass,
a. The plasma ; thin, watery and colourless.
/3. The corpuscles; exceedingly numerous solid
bodies, freely suspended in a. Two kinds will
be seen: larger and more numerous red cor-
puscles; smaller and less numerous white cor-
puscles, irregular in shape, greyish in colour, and

122 ELEMENTARY BIOLOGY. [CHAP.

about J the size of the red ones. Examine
under a high power.

b. The red corpuscles.

a. Their form • oval when seen en face ; almost
linear in profile but slightly swollen at the
centre.

/?. Their size ; their length, breadth, and thickness ;
measure.

y. Their colour ; pale yellow, when seen indivi-
dually ; redder when seen in the aggregate.

8. Their structure ; homogeneous for the most part,
but possessed of a central oval or rounded nucleus.

€. Treat with water; they swell up and become
more spherical ; their colouring matter (hczmo-
globiri) is gradually dissolved out, leaving be-
hind a colourless protoplasm. The nucleus is
rendered very evident, and ultimately all the
rest of the corpuscle disappears.

£. Treat with dilute acetic acid; results same as
with water, but produced more rapidly.

77. Treat with alcohol and borax-carmine suc-
cessively. The nucleus stains with great in-
tensity, the rest of the corpuscle remaining un-
affected.

c. The white corpuscles.

Obtain a drop of fresh blood and examine as be-
fore under your highest power.

a. Their form ; very irregular, the surface being
produced out into a number of blunted pro-
cesses or pseudopodia.

NIVERSITX 11

*

I.] THE FROG.

ft. Their movements ; best seen if the slide be
gently warmed, by contact with a lighted match
or other heating agent. They creep about in a
sluggish manner through the agency of the
above-named pseudopodia (amoeboid movement}.

y. Their size; cf. generally with the red corpuscles.

8. Their structure ; granular centrally, clear and
transparent peripherally: usually lodging one
or more clear round nuclei.

c. Treat with acetic acid and magenta ; the nucleus
alone will be stained. It will be found to lodge
one or more small granular bodies (nucleoli).

d. The microcytes. Very small bodies, for the most
part colourless, freely suspended in the plasma :
in shape variable; generally fusiform or ovoidal,
more rarely irregular.

e. Coagulating blood. Allow a drop of blood to
coagulate upon a glass slide, taking care that it
does not dry up. Examine under a high power.

a. ^^Q plasma ; transformed into a colourless fluid
(serum] which is permeated by well-defined and
coagulate fibrin filaments. Note the course of
the latter ; they radiate from numerous foci and
anatomose irregularly.

ft. The corpuscles; the red ones show a marked
tendency to arrange themselves along the lines
of coagulation ; the white ones are largely to be
found, together with microcytes, in the foci of
the fibrin filaments.
2. Epithelia.

An epithelium consists of a layer of cells which

lines or invests a free surface : the epidermis cover-

124 ELEMENTARY BIOLOGY. [CHAP.

ing the skin and the epithelium of the alimentary
canal, with which it is continuous at the buccal and
cloacal orifices, may be cited as examples. There
are several main types of epithelium, viz. —

a. Ciliated epithelium. Gently scrape the mucous
membrane of the tongue or roof of the mouth of
a recently-killed frog and transfer that which is
obtained to a slide ; mount in salt solution, avoiding
pressure, and examine under a high power.

a. The cells ; occurring singly or in aggregates. A
shimmering appearance (ciliary action) will be
seen along their free edges, produced by the
rapidly moving cilia.

As the cilia die their movements slacken.
Watch an individual cilium as this happens ; it
moves in a definite direction from a point of
rest and does not oscillate.

Place a freshly-killed frog upon its back and
open the mouth to its utmost. If a fragment of
pith or cork be placed on the roof of the mouth,
it will be carried back to the gullet by the action
of the cilia.

(3. The cell protoplasm. Stain with eosin or ma-
genta, and examine the individual cells. Shape;
flattened at the free surface, rounded or elongated
at the base. Structure ; granular for the most
part (endoplasni) ; clear and transparent peri-
pherally (ectoplasm), especially so at the free
border. Nucleus ; usually central and rounded,
containing one or more nucleoli.

b. Columnar epithelium. Scrape gently the inner
surface of the mucous membrane of the intestine

THE FROG. 125

of a frog, and mount the detached fragments in
water. Examine with a high power.

Numerous elongated cells will be seen, some-
what resembling those of the ciliated epithelium
in shape, but more uniform in size. Each is
flat at one end and somewhat pointed at the
other, and has a well-marked oval nucleus.
Not unfrequently aggregates of them may be
seen : look for such, and note that the cells
are closely applied and arranged in a single
layer.

Goblet cells ; scattered among a; characterised
by the presence of a watery-looking globule
(mucus drop) which distends the free end or
greater part of the cell body. Note the position
of the nucleus.

Scaly or tesselated epithelium. Open the body
cavity of a recently-killed frog ; carefully remove
the viscera and lay bare the cisterna magna. Cut
away its thin wall as carefully as possible, taking
great care not to drag or pull it. Place the frag-
ment in 0-5$- solution of silver nitrate for three
minutes or longer : then remove, wash well in
distilled water, and finally leave the specimen in
the same fully exposed to the sunlight. When it
has assumed a well-marked brown colour stain
with eosin or hsematoxylin and examine with a
high power.

. The matrix (intercellular cementing substance);
rendered highly conspicuous as a network of
black lines, irregularly disposed on the free
surfaces of the mesentery.

126 ELEMENTARY BIOLOGY. [CHAP.

/?. The cells ; flattened and close fitting ; irregular

in outline, as may be seen on following a.
y. The cell structure ; protoplasm differentiated into

ecto- and endoplasm (cf. supra) ; nucleus central,

oval or rounded.
8. Look for lymph stomata ; perforations of the

membrane, each surrounded by small more

deeply stained cells.
d. Stratified epithelium ; see epidermis § 15. iii.

3. Connective tissue.

Of these there are two main varieties, a. and b.

a. White fibrous tissue. This occurs typically in
tendons, but is widely distributed throughout the
body, mixed with other tissues. Tease out a bit of
fresh tendon in water : examine with a high power.

a. It is chiefly made up of very fine wavy fibres
which, in the aggregate, impart a glistening
white colour to the tissue {(white fibres) ; they
run in bundles parallel to one another and do
not branch.

/?. Treat with dilute acetic acid. Most of the above
disappear, but a few well-defined curled fibres
(yellow elastic fibres) remain. Besides these
some small elongated and granular proto-
plasmic masses are brought into view (con-
nective-tissue corpuscles).

b. Areolar tissue. Lay bare the muscles of the hind-
limb ; sheets of areolar tissue will be seen passing
between them and the integument. Remove one
of these, being careful to avoid undue stretching
and transfer to a slide: examine in water.

THE FROG. 127

a. White fibres ; more or less numerous and dis-
posed in wavy bundles.

p. The matrix ; a transparent imbedding mass, of
sufficient density to resist the pressure of the
cover-slip.

y. 1\& yellow elastic fibres (a. ft.). Treat with acetic
acid; the white fibres disappear, the yellow
ones remaining. Note that they occur singly
and anastomose, taking a very irregular course.

8. Stain with magenta. The yellow fibres stain
slowly but intensely; their torn ends will fre-
quently be seen rolled into a spiral or other-
wise contorted, as the result of their elasticity.

The connective-tissue corpuscles; small nu-
cleated cells, variable in size and shape, scat-
tered throughout the whole.

Submit a piece of areolar tissue to the action of
silver nitrate solution as directed for the tesselated
epithelium (§ 2. c.). When it has assumed a deep
brown colour, examine in weak glycerine under a
high power.

a. The matrix ; stained a rich brown, having
reduced the silver salt, as did the cementing sub-
stance of the epithelium.

p. The connective-tissue corpuscles ; little if at all
stained, appearing as a series of irregular white
patches (cell spaces). Examine these with care ;
each is an irregular branching corpuscle, generally
in organic continuity with one or more of its
fellows (i.e. the whole tissue is permeated by
protoplasmic matter).

128 ELEMENTARY BIOLOGY. [CHAP.

4. Hyaline cartilage.

Dissect out the xiphisternal cartilage of a recently-
killed frog and remove its membranous investment
(perickondrium) ; mount in salt solution and examine
under a high power.

a. The matrix ; dense, structureless or finely granu-
lated.

b. The cartilage corpuscles; large cells occurring singly,
or in sets of two's to four's, their apposed faces
being flattened. Examine the individual cells.

a. Cell-protoplasm; finely granulated and usually
containing one or many minute refractive par-
ticles (fat drops}.

fi. Nucleus ; round and sharply defined, containing
a variable number of nucleoli. Two nuclei may
not unfrequently be present.

y. If the preparation be made and examined soon
after death each cell will completely fill the
cavity of the matrix in which it lies : but if it be
kept some time or be treated with distilled water
the cells contract; thereupon the cavities (cell
spaces) become obvious as transparent halos
around the individual cells, or groups of cells if
recently formed.

c. Cell division. The initial phases in this process
can be nowhere more favourably made out than
here. The undermentioned may generally be
found on carefully searching the field ; note that
in all, division of the nucleus precedes that of the
cell.

a. Single cells (those filling an entire cell space);

I.] THE FROG. 129

one nucleus may be present (resting cell) or two
(dividing cell).

ft. A two-celled nest ; one or both of the cells may
lodge two nuclei.

y. A three-celled nest. Usually one cell will be
found to be the larger of the three ; its nucleus
may or may not show traces of division.

5. Bone.

a. Cleave the femur of a recently-killed frog longi-
tudinally in two, and examine while still fresh.

a. The bone ; its central cylindroidal shaft and
terminal epiphyses (cf. p. 14).

ft. The marrow ; a fatty vascular mass, filling the
central cavity.

y. The periosteum ; a tough vascular connective-
tissue sheath, closely investing the shaft.

B. The nutritive foramina. Carefully remove the
marrow and examine from within; the torn
ends of the periosteum will be seen passing in
through the above, for communication with the
marrow.

b. Decalcify the middle third of the femur of a freshly-
killed frog in \ per cent, chromic acid solution,
stain with borax-carmine and cut into transverse
sections as directed in the Appendix. Mount in
Canada balsam, and examine under a low power.

a. The bony shaft ; seen to be divided, by a highly
refractive middle lamella^ into an outer more
deeply stained portion and an inner thinner and
less deeply stained one.

M. 9

130 ELEMENTARY BIOLOGY. [CHAP.

/?. The periosteum ; a thin layer closely applied to
the outer face of a., and staining with great in-
tensity.

y. The marrow ; composed of larger vacuolated
(fat-laden) cells, and smaller rounded deeply-
stained ones (red marrow cells].

8 The nutrient vessels; in section as follows: — a
larger ovoidal and thin-walled vein, and a smaller
cylindroidal and thick-walled artery, lying close
together near the middle of the marrow.

e. Search your sections for any which may have
passed through a nutritive foramen ; if present,
note the relations of the periosteum to the
marrow.

c. Examine your thinnest section under a high
power.

a. The bony shaft • seen to consist of a number of
concentric layers or lamella, rendered distinct
under the action of the reagent.

Find the middle lamella (b. a); it marks the
boundary line between an inner, more lightly
stained series (concentric or peri-medullary lamella)
and an outer, more deeply stained series (circum-
ferential or sub-periosteal lamella}.

ft. The bone corpuscles ; minute fusiform or branch-
ing cells, set along the lines of the above la-
mellae. Each clear and little stained, with a
deeply stained nucleus. (Cf. connective tissue
corpuscle.)

y. The periosteum ; if the section be a good one,
its innermost layer will be seen to consist of a
row of small flattened cells (osteoblasts).

THE FROG. 131

8. The marrow; note the aggregation of small
cells on the inner face of the shaft.

d. Make a transverse section of the dried shaft of the
femur, by grinding it down upon a hone as di-
rected in the Appendix (E.). Mount in Canada
balsam and examine under a high power.

a. The lamella : cf. supra.

(3. The lacuna; oval spots between the lamellae;
black, as they become filled with dirt in grind-
ing. Each originally lodged a bone-corpuscle.

y. The canaliculi; minute black lines radiating
from the lacunae. Those of adjacent lacunae
frequently anastomose.

8. The Haversian canals; present only in the
neighbourhood of the nutritive foramina. Ob-
vious as spaces in the substance of the shaft,
each surrounded with its own lamellae.

e. Examine a longitudinal section of the dried
femur, for comparison with the above. The
Haversian canals are seen to be channels run-
ning for the most part longitudinally, and com-
municating with one another by cross branches.
The lacuna and canaliculi appear much as in
the transverse section.

The study of the dried bone may advanta-
geously be supplemented by that of the long
bone of a mammal.

6. Adipose tissue.

Tease up portions of the corpus adiposum, as under.

a. Fresh, in salt solution. Examine under a low
power ; the following will be met with.

9—2

132 ELEMENTARY BIOLOGY. [CHAP.

a. Connective tissue elements.

(3. Ripe fat cells ; obvious as aggregates of large

highly refractive globules (fat globules] of which

the tissue is chiefly composed.

Note their optical characters, under different

levels of focus.
y. Immature fat cells ; smaller irregular or rounded

cells, each lodging one or more fat drops.

b. Preserve in spirit ; stain with haematoxylin. Ex-
amine the ripe cells under a high power; each
consists of —

a. The fat globule ; apparent as a clear space,
which may or may not contain crystals.

(3. The cell membrane ; deeply stained and more or
less granular, forming a thin envelope for a.

y. The nucleus • oval and compressed, buried up
in the cell membrane and usually surrounded
by granular protoplasm.

8. Immature fat cells; look for those showing
stages in the formation of the fat globule.

7. Unstriped muscle.

a. Place a small piece of the muscular coat of the
intestine in M tiller's fluid for 2 — 3 days. Tease
up in haematoxylin solution and examine under a
high power.

a. The muscle-cells; elongated and fusiform, in
close apposition with each other; protoplasm
granular.

fi. Nucleus ; fusiform or ovoidal, generally situated
near the middle of the cell.

I.] THE FROG. 133

b. Obtain a similar but thin piece from the intestine
of a recently-killed frog. Treat with silver
nitrate as directed at § 2. c.

a. The cementing substance; obvious as a series
of dark lines which cross each other at long
intervals.

/?. Look for fragments showing two sets of lines
which cross at right angles (i.e. cell-contours of
longitudinal and circular layers).

8. Striped muscle.

a. Preserve the middle third of the belly of a limb-
muscle (say the gastrocnemius) in spirit ; stain with
borax-carmine and cut into transverse sections.
Mount in Canada balsam and examine under a
low power.

a. The muscle ; composed of a great number of
large fibres, each oval, or angulated and irre-
gular as the result of mutual compression. Note
their transverse diameter, variable in proportion
as the section passes through their middle
or extremities.

/?. The perimysium (fascia) ; a deeply staining
sheath for the whole muscle, carrying blood-
vessels whose cut ends will be seen (cf. peri-
osteum, § 5.). It will be found to dip into the
interior of the muscle as a delicate black pig-
mented membrane, subdividing it into a number
of muscle bundles or fasciculi,

y. The muscular fibres. Examine the thinnest parts
of your sections with a high power ; each fibre
consists for the most part of a granular pro-

134 ELEMENTARY BIOLOGY. [CHAP.

toplasm lodging a number of fusiform deeply-
staining nucleate bodies (muscle corpuscles],
b. The muscular fibre j structural analysis.

i. Tease out gently a bit of muscle from a freshly-
killed animal, and examine in salt solution with
i inch obj.

u. Composed of elongated fibres, which exhibit,
unless the fibre be quite fresh, a tendency to
split up into finer filaments (so-called fibrillee).

/?. The sarcolemma; visible as a superficial clear
layer of the fibre ; expansions of it may be
seen at points where the continuity of the
fibre has been interrupted by pressure, or
other cause of rupture.

ii. Examine with a high power.

a. The alternate lighter and darker bands placed
transversely to the long axis of the fibre
(transverse striatiori).

ft. The sarcolemma; enveloping the fibre; seen
here and there as a delicate film, where the
fibre is twisted or bruised.

y. The muscle corpuscles ; visible as elongated
and slightly refractive fusiform bodies, dis-
posed longitudinally.

iii. Treat with acetic acid ; the striation is rendered
very indistinct, the sarcolemma and muscle cor-
puscles becoming very conspicuous. Add a
drop of magenta ; the nuclei stain deeply.

iv. Transfer a fragment of fresh muscle fibre to a
slide bearing a drop of salt solution, and tear it
into small shreds with a couple of needles.

THE FROG. 135

Examine the thinnest pieces under your highest
power.

a. The fibre is seen to be marked out into a
close-set series of alternately light and dark
transverse strice, the former being in all pro-
bability the narrower.

/3. Keep your eye steadily fixed on a given piece
while you alter the focus — parts which were
originally dark now become light and vice
versa.

y. Examine a fragment in which the cross striae
are very distinct, still more minutely. When
in exact focus the following will be seen.

The septal zones ; obvious as the afore-
named clear striae.

The interseptal zones ; alternating with and
of greater thickness than a. Each is longi-
tudinally striated.

The septal lines ; dark lines of great deli-
cacy, one in the middle of each septal zone ;
seen, on close examination, to appear as a
parallel series of minute dots..

Very generally the fibre is constricted at each
septal line, having thus a beaded appearance.

8. Repeat the operation described at /?. and note
the different optical effects which are pro-
duced at different levels of focus.

Melland's method. Place some fragments of
fresh muscle in i per cent, acetic acid solu. for
10 seconds; transfer to i per cent, gold chloride
solu. for i hour and then to 25 per cent, formic
acid solu. for 48 hours or longer (to be kept in

136 ELEMENTARY BIOLOGY. [CHAP.

the dark). Finally tear up with a couple of
needles in glycerine.

a. Longitudinal stria; parallel lines, differentiated
under the above treatment, which can be
traced throughout the entire length of the
fibre, traversing clear and dark zones alike.

ft. Transverse networks ; seen, in the regions of
the septal lines, traversing the fibre at right
angles to a and thickened at their points of
intersection with the same. When looked
at en face they present a honeycomb-like
appearance.

§ iv. and v. may be profitably repeated upon
the muscle fibre of an insect.

9. Nerve.

a. Preserve a portion of the iliac nerve in spirit of
increasing strengths; stain with haematoxylin,
imbed and cut transverse sections. Mount in
Canada balsam and examine with a low power.

a. The nerve trunk; composed of an immense
number of small rounded nerve fibres.

ft. The perineurium ; a deeply staining sheath,
seen to carry blood-vessels (cf. perimysium and
periosteum). It may be in part fat laden.

y. The nerve bundles; lesser divisions of the trunk,
each bound up in an ingrowth of the perineu-
rium (cf. muscle bundles).

b. Examine the thinnest portion of a nerve bundle
under a high power ; it will be found to contain —

a. Medullated nerve fibres ; in section round, each
with a central deep-stained portion (axis fibre]

THE FROG. 137

and a peripheral clear portion (medullary sheath)-,
the whole invested in a delicate primitive sheath^
obvious as a darkly stained outline.
/?. Non-medullated nerve fibres ; irregularly dispersed
among a. ; each consisting of an axis-fibre alone.

The medullated nerve-fibre ; structural analysis.

i. Tease out a bit of a fresh spinal nerve in salt
solution. Examine with a low power.

a. Composed of well-defined fibres (medullated
nerve-fibres] mixed with white fibrous tissue.

p. The appearance of the nerve-fibres ; each has
a double contour and a highly refractive
border. Some fibres are regular in outline,
others contorted and irregular.

ii. Examine, under a high power, selecting the least
contorted fibres.

a. The primitive sheath; look for it on bruised

nerve-fibres or at the ends of torn ones.
(3. The highly refractive border (medullary sheath)

within the primitive sheath,
y. The central homogeneous axis (axis fibre};

look for it projecting beyond the medullary

sheath of torn fibres.
8. Treat with chloroform ; the medullary sheath

will be dissolved out, and the axis fibre

plainly seen.

e. RanvieSs nodes; apparent breaks (constric-
tions) in the medullary sheath, seen at com-
paratively long intervals along the course of
the individual fibre.

138 ELEMENTARY BIOLOGY. [CHAP.

£. Schmidt's nodes. Examine the medullary
sheath of the least distorted fibres with great
care ; the above will be found as interruptions
of the same at short and fairly regular in-
tervals, the sheath appearing to consist of a
series of imbricated segments.

rj. Compare a fibre with irregular contour. Note
that the irregularity is due to shrinking of
the medullary sheath under coagulation. The
coagulate masses may often be seen to cor-
respond with the above-named segments.

iii. Tease up a piece of the trunk of a spinal nerve
in a drop of i per cent, osmic acid solu. After
3 — 4 hours immersion in the same, wash well
and mount in weak glycerine. Examine under
a high power.

a. The medullary sheath; rendered blackish and
highly conspicuous by the above method.
Examine it with care, noting especially the
nodes of Ranvier and Schmidt.

ft. The axis fibre ; greyish and well defined,
seen on focussing through a.

y. The nerve corpuscles ; apparent as thickenings
of the sheath which bulge inwards, usually
of a reddish brown colour under treatment
with the above acid. They correspond in
number with the nodes of Ranvier, and each
consists of an oval refractive nucleus, em-
bedded in a small amount of granular proto-
plasm.

I.] THE FROG. 139

10. Nerve-cells.

a. Unipolar cells.

i. Take a spinal ganglion from a recently-killed
frog and tease up under a low power in eosin
solution. Among the pigment cells and other
small cells present, the above-named will be
seen as large round or oval pale granular cells,
each with a conspicuous large round nucleus.
Examine under a high power.

a. The cell-body; granular, and connected at its
base with a single nerve-fibre (unipolar-cell).

/?. The nucleus; generally central in position;
invariably containing one large nucleolus.

y. The cell capsule; a delicate nucleated invest-
ment which may or may not be torn away.

ii. Tease up a second spinal ganglion, and treat
with osmic acid and glycerine, as directed at
9. iii. Examine under a high power,
a. General. Cf. supra.

/?. The polar plate; consisting of two or more
clear cells, situated at the base of the unipolar
cell and in direct connection with the ulti-
mate termination of the nerve fibre.

b. Bipolar cells.

Take a sympathetic ganglion from a recently-
killed frog and tease it up in eosin as directed
above. Examine under a low power. Among
the pigment and other cells present, will be
seen numerous large pale granular cells some-
what like the unipolar ones. Examine under a
high power.

140 ELEMENTARY BIOLOGY. [CHAP.

a. The cell-body; composed of granular proto-
plasm, with a large round or oval nucleus.
Cf. generally with that of the unipolar cell.

ft. The cell capsule; like that of the unipolar cell
but less conspicuously nucleated.

y. The related nerve fibres ', of these there are two
(bipolar cell): one passes straight from the
cell base like that of the unipolar cell; the
other (processus spiralis] leaves the cell at a
higher level, and, in passing downwards, winds
round the base of the cell and the straight
fibre.

c. Multipolar cells. See infra § 1 1 .
ii. The spinal cord.

Place some pieces of the fresh cord in potassium
bichromate solution (2 to 3 weeks), wash well and
afterwards harden in alcohol of increasing strengths.
Stain with borax-carmine and mount in Canada
balsam.

i. Examine under a low power.

a. rT\LQCord; bilaterally symmetrical; composed of
a superficial lighter portion (white matter] and a
deeper more deeply staining portion (grey
matter).

b. Nerve roots; seen as bundles of fibres running
out dorsally and ventrally from prolongations
(cornua) of the grey matter.

c. The canalis centralis ; oval, lined by a deep
staining columnar epithelium.

d. The fissures; median dorsal and ventral, cleaving
the white matter only.

THE FROG. 141

mater; obvious as a darkly stained in-
vestment for the whole. It dips in, especially
at the fissures, to form a supporting framework
for the cord. Prolongations of it into the
substance of the cord may be readily seen, in
the vicinity of the ventral fissure.

ii. Examine under a high power.

a. The white matter; chiefly composed of medul-
lated fibres, cut across and lying within a sup-
porting meshwork (iieuroglia). Non-medullated
fibres may be detected here and there.

b. The grey matter.

a. Composed, for the most part, of non-medul-
lated fibres lodged in a granular matrix.

Note the course of the fibres, especially
of those which pass from side to side.

/8. Multipolar nerve-cells. A cluster of these will
be seen in each ventral grey cornu, as large
irregular cells, deeply staining and each with
a round nucleus. Nerve fibres will be seen
to enter them at several points.

y. Ventral nerve root. Examine with care; con-
tinuity may often be traced between its indi-
vidual fibres and the multipolar nerve cells.

12. The retina.

Highly satisfactory preparations of this organ can
be obtained as follows. Take perfectly fresh eyes
from a frog ; remove a small portion of each, and lay
the eyes for an hour in corrosive sublimate, or for

142 ELEMENTARY BIOLOGY. [CHAP.

3 to 4 hrs. in picric acid; wash and transfer them
to alcohol of increasing strengths.

Finally, stain with borax-carmine, imbed and cut
into longitudinal vertical sections. Mount in Canada
balsam.

i. Examine with a low power, the whole eyeball will
be seen in section.

a. The sclerotic; a cartilaginous outer capsule.

b. The choroid; recognizable by the straggling
nature of its pigment cells, which form a sort
of loose network.

c. The retina; seen to be composed of a number
of layers, differentiated under the action of the
staining reagent. Note the pigment layer ; red-
dish brown and regularly disposed, immediately
internal to the choroid, one-third the thickness
of the whole.

ii. Examine your thinnest section under a high power.
Work from within outwards ; the following will be
seen, in order of enumeration.

a. The internal limiting membrane, a thin structure-
less layer.

b. The nerve-fibre layer ; thin and granular, with
delicate fibres.

[a. and ft. are sometimes difficult to make out.]

c. The nerve-cell layer; composed mainly of large
irregular cells with round nuclei. From some,
branches can be traced into the next layer.

d. The inner molecular layer; this is thicker than
any of the preceding, and has a finely

THE FROG. 143

punctated appearance. It remains unstained

under the above method of treatment.

e. The inner nuclear layer; nearly as thick as d.,

and deeply stained. It is made up of a number

of nuclei, around each of which is collected a

very small amount of protoplasm ; and of fine

fibres, some of which can be traced into the same.

/ The outer molecular (fenestrated) layer. Narrow

and unstained, somewhat resembling d.
g. The outer nuclear layer. Much thinner than
the inner nuclear layer and more closely packed.
It is composed of distinct fibres (rod- and cone-
fibres}, each of which swells out and has a
nucleus developed in the enlargement.
//. The external limiting membrane. A thin homo-
geneous layer like a, obvious as a hard line
which not unfrequently overhangs the outer
nuclear layer.
/. The rod- and cone-layer. Usually stained a faint

pink colour under the above treatment,
a. The rods; parallel and ending in blunted free
ends; each subdivided transversely into two
segments.

/?. The cones; few in number and shorter than
the rods; each ending in a pointed free end.
Look for them among the bases of the rods.
k. '^\VQ pigment layer; seen to consist of a close set
series of elongated cells (pigment epithelium)
forming a cap-like investment for the free ends
of the rods and cones.

a. The cell bases; closely applied, each contain-
ing a round or oval clear nucleus.

144 ELEMENTARY BIOLOGY. [CHAP.

/3. Their free ends ; fimbriated and prolonged

down among the rods and cones.
/. The fibres of Mutter ; highly refractive sup-
porting fibres,*often traceable with ease from
the internal Irl^ting membrane to the fenes-
trated layer. ^^^

iii. Obtain, if possible, stipBl sections which shall
pass through the entrance of the optic nerve.
Examine under a high power, and follow the
course and relations of the nerve-fibres,
iv. Take a fresh frog's eye : prick its cornea and col-
lect the aqueous humour on a slide. Then open
the eye, remove a bit of the retina and tease it
out in the aqueous humour, mount and examine
with a high power.

a. Numerous rods will be seen floating about,
many broken but some intact and showing the
boundary line between their two segments very
plainly. At first both segments are homoge-
neous, but very soon they begin to alter; the
outer layer frequently then gets transversely
striated and shows a tendency to split up into
several pieces : gradually these rods curl up,
swell out, and entirely disintegrate.
13. The olfactory epithelium.

Open the nasal sac of a frog ; remove a portion of
the epithelial lining and transfer to i p. c. osmic
acid solution for 2 — 3 hours; tease up in weak
glycerine and examine with your highest power.

Numerous mutilated cells will be found, and,
among them, more or less perfect representatives
of the following.

THE FROG. 145

a. Epithelial cells; columnar and elongated, each
with an oval nucleus, an unbranched peripheral
process and a branched basal one.

b. Sensiferous cells ; more slender and often some-
what shorter than a. : generally to be recognized
by their swollen bases (enlarged around an en-
closed nucleus). Each terminates at its free end
in a cluster of delicate, hair-like, stiff processes,
and receives at its base a thread-like nerve-fibre.

14. The auditory epithelium.

Place sufficient of the membranous labyrinth to
embrace one of the ampullae (Sect. L. 4. c. y) in i.
p. c. osmic acid solution for 3 — 4 hours ; transfer to
weak glycerine and tease up its epithelium with a
couple of fine needles. Examine under your highest
power, and look for —

a. Hair cells (sensiferous cells of auditory epithelium).
Each is pear-shaped and composed of a granular
protoplasm with a large round or oval nucleus :
from its flattened free end there stands out the
auditory hair ; a long pointed structure (from 3
'to 4 times the length of the cell-body) seen, on
careful examination, to consist of several delicate
fibrils bound together.

15. The skin.

i. Cut out a piece of skin from the back of the thigh
of a recently killed frog : spread it out in water,
cover, and examine with a low power : note —

a. The pigment-cells ; black irregular patches ;

some compact, others more or less branched.
M. 10

146 ELEMENTARY BIOLOGY. [CHAP.

b. The mouths of the cutaneous glands ; seen as
clear round spots, although their openings are
really triradiate : their number.

ii. Preserve a piece of skin in alcohol of increasing
strengths, and stain with borax-carmine; imbed
and cut sections perpendicular to its surface :
mount in Canada balsam. Examine under a low
power.

a. The epidermis ; the superficial thinner and deep
stained portion.

/;. The dermis ; the deeper main mass, for the
most part lightly stained.

c. Pigment; most marked as a conspicuous ir-
regular black layer, in the superficial stratum
of the dermis.

d. Cutaneous glands ; seen in section as numerous
large oval, rounded, or flask-shaped spaces, lying
within the dermis. Many can be seen to open
on the free surface by a narrow neck.

iii. Examine under a high power.

a. The epidermis (stratified epithelium). Work
from within outwards; there will be found in
order :

a. The Malpighian layer ; composed of a usu-
ally single basal row of close-fitting columnar
cells, each with an oval nucleus ; and of a
superficial main mass, consisting of several
rows of rounded or oval cells whose long
axes are parallel with the free surface. Small
pigment granules are frequently aggregated
around the nuclei of the latter.

I.] THE FROG. 147

(3. The horny layer ; usually of a yellowish
colour; made up of flattened cells whose
boundary lines are rarely distinguishable.
Nuclei for the most part absent; pigment
granules present at intervals.

b. The dermis ; work inwards from the epidermis ;
the following elements will be seen.

a. Connective tissue ; forming the main mass, its
cellular elements well marked, fibres for the
most part in bundles.

/?. Unstriped muscle; most marked in the deeper
portion where it forms a thick muscular stra-
tum^ lying beneath the cutaneous glands.

y. Pigment ; deposited for the most part in two
well-defined strata of irregular branching cells ;
rarely diffused throughout the connective-
tissue mass.

The above-named strata are : a superficial
one lying beneath the epidermis (cf. ii. <:.),
generally black and highly conspicuous; a
deeper one, immediately internal to the mus-
cular stratum, of somewhat lighter colour
than the other.

8. Pat ; rarely present : when it is deposited,
like the pigment, within individual cells of
the connective-tissue mass.

e. Blood vessels; inconspicuous in uninjected
preparations. Their cut ends may however
be seen among the cells of the inner pigment
layer ; thence vessels may sometimes be traced
passing up, in a pigmented sheath, towards
the bases of the cutaneous glands.

10 — 2

148 ELEMENTARY BIOLOGY. [CHAP.

c. The cutaneous glands. Look for thin sections

which pass through their necks,
a. The gland; usually lined by a single layer of
flattened epithelial cells (more rarely by two
diversely differentiated ones) occasionally sur-
rounded by pigment. Its secretory product
may often be seen within it, as a pale finely
granular mass.

(3. The duct ; subdivided into two segments : an
inner enlarged one, lined by a single layer of
cells which graduate into the deeper layer of
the epidermis ; an outer much constricted
one, the epithelium of which graduates into
the horny layer of the epidermis.

y. Look for sections showing the glands en face.
Note the boundaries and characters of their
epithelial cells.

1 6. The intestine.

i. Preserve some pieces of the small intestine, first
washing out the contents with care, in ^ per cent,
solution of chromic acid; transfer to alcohol of
increasing strengths; stain with borax-carmine,
imbed and cut transverse sections. Mount in
Canada balsam and examine with a low power.

a. The intestinal wall ; smooth externally, much
folded internally. Its outer layer; uniformly
thick (muscular layer) : its inner layer ; much
folded and more deeply stained (epithelium).

l>. Examine the thinnest portion under a higher
power. Work from without inwards ; there will
be found in the order of enumeration —
a. Serous layer (peritoneum) ; obvious as a faint

THE FROG. 149

contour line. Look for places where it may
have been separated in preparation.

j3. Longitudinal muscular layer ; thin and deeply
stained; seen to consist of the cut ends of
close-set cells.

y. Circular muscular layer; relatively thicker
than ft ; cells close-set and parallel with the
free surface (cf. § 7. b. /?).

8 Submucous layer ; faint and composed of vas-
cular connective tissue. Large blood-vessels
will be seen ramifying within it.

c. Mucous membrane ; composed of three layers.
A muscular layer, forming a thin stratum im-
mediately internal to 8; a lymphoidal layer,
somewhat like the submucous layer, but con-
taining numerous rounded or irregular cells
(lymphoidal cells) ; the epithelium, composed
of close-set columnar cells (cf. § 2. b.) whose
inner ends for the most part stain very deeply.

c. Compare similar sections across an intestine the
blood-vessels of which have been injected with
gelatine (see Appendix C). Note the course
of the vessels, and especially the relations of
their ultimate ramifications.

17. The pancreas.

Preserve small portions in alcohol of increasing
strengths ; stain with borax-carmine ; imbed and cut
transverse sections ; mount in Canada balsam.

a. Examine under a low power.

a. The gland • composed of groups of cells (acini)

150 ELEMENTARY BIOLOGY. [CHAP.

diffused throughout a connective-tissue mesh-
work (stroma).

/?. The tunic; a dense connective-tissue investment
for the whole, continuous with the stroma. Look
for the cut ends of blood-vessels (cf. perimysium,
perineurium, periosteum).

b. Examine under a high power.

a. The acini ; each composed of a single layer of
large epithelial cells, set around a small ir-
regular central cavity.

fB. The ducts ; packed among a, characterized by
the large size and regularity of their central
cavities ; in section rounded or oval, each lined
by an epithelium of squarish cells.

Look for portions showing the transition of
gland into duct.

1 8. The liver.

Preserve some small pieces of fresh liver in alcohol,
and prepare sections as directed for the pancreas.

a. Examine under a low power.

The gland cells will be found to be arranged in
groups, around a ramifying system of clear spaces
with granular-looking contents (blood-vessels).

b. Examine under a high power.

a. The gland cells ; for the most part squarish or
hexagonal, each lodging a large round nucleus
with one larger and several smaller nucleoli.

/3. Look for sections of longitudinal series cells;
note their arrangement in parallel rows.

y. Blood capillaries ; cf. supra. The granular ap-
pearance seen under the low power will be found

THE FROG. 151

to be due to the presence of red corpuscles
whose nuclei alone stain with great intensity,
the bodies of the corpuscles remaining faint and
yellow.

8. Bile capillaries ; apparent as a system of minute
inter-cellular passages. Look for them in your
thinnest sections ; their cut ends appear some-
times as round or oval spaces, at others as deeply
stained dots, between applied cells, much smaller
than the nuclei of the same.

They may often be demonstrated if, prior to
removal of the liver from the body, the bile-duct
be ligatured and the bile injected into them by
gently squeezing the gall-bladder.

c. Compare sections of injected liver, prepared as
directed for the intestine § 1 6. c.

d. Tease up a fragment of fresh liver in salt solution
and examine with \ objective.

a. The hepatic cells ; polygonal and granular, often
containing oil-drops (cf. osmic acid reaction).

/?. Treat with acetic acid : a nucleus, or sometimes
two, will be rendered apparent in each of the
cells.

e. Tease up a similar fragment in iodine solution.
Many of the cells will be seen to contain a product
(glycogeti) which stains a deep reddish brown.

19. The kidney.

Preserve a frog's kidney in alcohol of increasing
strengths ; stain with haematoxylin or borax-carmine ;
imbed it and cut into longitudinal sections parallel
with its flat surface. Mount in Canada balsam.

152 ELEMENTARY BIOLOGY. [CHAP.

a. Examine with a low power.

a. Note the numerous uriniferous tiibutes of which
the organ is mainly composed ; they twist about
in several directions, and are consequently cut,
some transversely, some obliquely, and others
more or less longitudinally. The latter are most
conspicuous at the outer (convex) border.

ft. The clear round spaces, scattered about ; these
are sections of Malpighian capsules. Some may
be seen to lodge a granular mass (glomeruJus).

b. Examine with a higher power —

a. The epithelium of the tubules ; composed for the
most part of a single layer of large squarish
cells, each with a large round deep-staining
nucleus.

p. The epithelium of the capsules ; flattened and
squamous, its nuclei well denned, seen to be
reflected over the glomeruli (i.e. the latter are
outside it). Look for sections showing the con-
nection of tubules with capsules.

y. Blood capillaries ; scattered among the tubules,
conspicuous in borax-carmine preparations by
the refractive deep-staining nuclei of the red
corpuscles (cf. § 18. b. y).

c. Compare sections of the fresh kidney treated with
silver nitrate, as directed at § 2. c. Examine under
a high power.

a. The capsules. The boundary lines of their epi-
thelial cells will be distinctly seen ; they are
those of a tesselated epithelium.

THE FROG. 153

/3. The smaller blood-vessels. In those immediately
related to the glomeruli, longitudinal and trans-
verse striations are generally obvious under the
above treatment. Examine with care; the longi-
tudinal striae will be seen to be the deeper
(boundary lines of epithelial cells); the trans-
verse ones are more superficial (boundary lines
of unstriped muscle cells).

d. Compare sections of injected kidney. Examine
under a low power ; two sets of capillary vessels
will be seen —

a. Around and among the tubules.
/?. The glomeruli ; vascular tufts indenting the
Malpighian capsules (cf. b. ft). Examine them
in detail.

20. The testis.

Imbed a testis which has been hardened in alcohol
of increasing strengths and stained with borax-car-
mine. Cut into transverse sections and mount in
Canada balsam.

a. Examine with a low power.

a. The organ is chiefly made up of large tubules,
radially disposed at the periphery, more tortuous
internally.

Spermatozoa may or may not be present;
if so, they will probably be aggregated into
bundles with their heads (deeply stained) in
close relation with the walls of the tubules.

b. Examine with a high power.

a. Note the epithelium (germinal epithelium) lining
the tubules : it varies with the season of the

154 ELEMENTARY BIOLOGY. [CHAP.

year, and may be one or more layers of cells
deep.

c. The spermatozoa.

i. Tease up a small piece of the fresh testis in
magenta solution. If spermatozoids are present,
each will be seen to consist of
a. Head; rod-like, generally straight, more rarely
arched, mainly composed of a deep-staining
body (nucleus].

ft. Tail ; elongated and flagelliform, stains but
feebly, if at all. In favourable specimens it
may be seen to be continuous with a small
amount of clear protoplasm which surrounds
the "head."

ii. Compare the aggregates of spermatozoa seen in
the prepared section. The applied heads of
many of them will be seen to be capped by a
large irregular nucleated cell (last remnant of
parent cells of the group).

iii. Compare the living spermatozoa, if accessible,
obtained from either the testis or vas deferens.
Note the nature of their movements.

21. The ovary.

Prepare transverse sections of an ovary : — alcohol,
borax-carmine, Canada balsam.

i. Examine under a low power.

a. The ova (ovisacs) j variable in size : smaller
ones, seen to consist of granular protoplasm
with a large germinal vesicle (nucleus) : larger
ones, pigmented peripherally; germinal vesicle

THE FROG. 155

rarely seen, as it only occasionally lies in the
plane of section.

ii. Examine under a high power.

a. The smaller ovisacs.

a. The ovum ; the central large cell ; its proto-
plasm, uniformly finely granular and deep-
staining.

ft. Germinal vesicle; often irregular in outline;
germinal spots diffused or arranged peri-
pherally in a single row.

y. Ovarian follicle ; a single-layered epithelial
investment for the ovum.

b. Compare the larger ovisacs.

a. Protoplasm of ovum. Peripherally, black and
pigmented, otherwise converted, for the most
part, into an immense aggregate of small ellip-
soidal or irregular refractive yolk granules,
ft. Germinal vesicle. Look for specimens in
which this is visible ; the germinal spots are
mostly, if not wholly, aggregated in the middle
of the same.

y. Ovarian follicle ; still recognizable, but gene-
rally flattened.

8. Vitelline membrane; obvious as a thin faint
line, interposed between the ovum and the
follicle. Look for it in ova whose investing
structures may have been ruptured.
c. The germinal epithelium', a cellular mass imme-
diately beneath the investing membrane of the
ovary. Look for young ovisacs still in connection
with it.

156 ELEMENTARY BIOLOGY. [CHAP.

N. The physiological properties of muscle and nerve.

Place a frog under a beaker, with a drop or two of
chloroform : take it out immediately it becomes un-
conscious, which will probably be in a few seconds.
Now feel with a finger-nail for the depression beneath
the skin at the back of the animal's head, which in-
dicates the point of articulation of skull and spinal
column : it lies in a line joining the posterior borders
of the two tympanic membranes. Divide the skin
and muscles at this point until the neural canal is
laid open, and then pass a stout wire into the
cranium and down the neural canal of the vertebral
column. By this process (kn5wn as pithing) the frog
is rendered totally incapable of further consciousness,
though most of its tissues will retain their vitality for
some time.

a. Remove the skin from one leg, so as to lay bare the
muscles : send an interrupted electric current through
any one of them (or tap the muscle sharply with the
back of a scalpel) : it will immediately contract, or
alter its form in a definite way ; it becomes shorter
and thicker, gaining in breadth just so much as it loses
in length; in so doing it moves the bones to which it
is attached.

/>. Very carefully lay bare the sciatic nerve, taking care
not to crush or drag it : divide it as high up as pos-
sible, and, seizing it with a pair of forceps close to its
cut end, lay it over the electrodes of an induction-
coil. Probably when the nerve is cut the muscles of
the limb will contract: whether or not, they will
contract violently while the interrupted current is
going through the nerve.

I.] THE FROG. 157

[If an induction-coil is not at hand a bit of clean
copper wire twisted round a strip of zinc, with the
points of contact moistened with dilute acetic acid,
may be used to stimulate the nerve ; smart tapping or
pinching with a pair of forceps will also excite it, but
by such means the nerve is soon killed.]

The above experiments show :

c. That the muscle is irritable and contractile: certain
external agencies (stimuli] excite some change in it,
the result of which is a muscular contraction.

d. The nerve is irritable: certain external agencies ex-
cite some change in it, which in this particular case
manifests itself by a contraction of the muscles con-
nected with the nerve.

e. The nerve possesses conductivity : although it is stimu-
lated at some distance from the muscles, the change
excited by the stimulus travels along it to them.

O. Development.

Place some freshly-deposited frog's spawn (taken di-
rectly after extrusion by the female) in a table aquarium
or other glass vessel filled with water. Examine and
preserve the eggs and embryos as directed below.

It is necessary for both observation and preservation
that the mucus investment which surrounds the egg
should be removed; this may best be done with a
couple of fine needles, under water in a small saucer, or
on any convenient white surface. When liberated,
float the objects into a shallow watchmaker's glass, and
examine under a low power as opaque objects, on a
white ground unless otherwise directed. Preserve in
Kleinenberg's picric acid solution, and transfer to
alcohol of increasing strengths.

158 ELEMENTARY BIOLOGY. [CHAP.

i. Segmentation of the fertilized ovum (oosperm) and
larval metamorphosis.

The rate of both segmentation and development
generally is liable to vary, in accordance with cir-
cumstances. The periods enumerated below are
approximately those best suited for observation.

The specimens should be examined both in the
fresh state and when fully preserved.

a. One hour after fertilization.

a. General characters ; shape and size ; the black
pigmented upper pole ; the yellowish white non-
pigmented lower pole.

/?. The first cleavage furrow ; an annular constric-
tion passing right round the egg, longitudinally
with respect to the afore-named poles. It con-
stricts the whole into two equal halves (embryo
cells or blastomeres).

b. Two hours after fertilization.

a. The second cleavage furrow ; longitudinal and at
right angles to the first one, subdividing each
blastomere into two. There are now four cells
of equal size.

c. The same at 3 — 4 hours.

a. The third furrow ; at right angles to the two
former and excentric, lying within the pig-
mented pole. It subdivides the whole into four
smaller pigmented upper layer cells^ and four
larger non-pigmented lower layer cells. Ex-
amine from several aspects.

d. The same at 6 — 8 hours.

A mulberry-like mass conspicuous by the irregu-
larity in size of the blastomeres.

THE FROG. 159

a. The lower layer cells ; forming the lower half of
the whole, their boundary lines regular.

ft. The upper layer cells ; forming the upper half,

smaller and more numerous than those of the

lower layer, their boundary lines in part irregular.

i.e. the upper layer cells are dividing more

rapidly than the lower ones.

e. The same at 24 hours.

Recognizable by the preponderance of the pig-
mented area.

a. The lower layer cells ; forming now the lower
third or thereabouts ; their boundary lines can
only be made out with difficulty.

ft. The upper layer cells ; forming the upper two-
thirds at least ; their boundary lines no longer
clearly definable under a low power, the pig-
mented portion having a granulated appear-
ance.

i.e. the upper layer cells are growing round
the lower ones.

f. The same at 4 — 5 days.

The lower layer cells apparent only as a small
white spot (blastopore}.

a. The blastomeres ; boundary lines no longer

recognizable.
ft. The blastopore. Examine this with care ; its lip,

horse-shoe shaped, well defined above and at

the sides, ill-defined below.

g. The same at 6 — 7 days.

Signs of the embryo are now beginning to appear,

160 ELEMENTARY BIOLOGY. [CHAP.

the whole having undergone a slight increase in
total capacity and lost its spherical shape,
a. The blastopore ; still visible but small and

excentric (displaced towards the flattened or

dorsal surface).

/?. The neural plate ; obvious as a lyre-shaped flat-
tening, in front of and in a line with the blasto-
pore ; its edges thickened and raised up (neural
folds), its mid region depressed (neural groove).

The neural folds will in all probability be seen
to unite in front, and die away behind, at the sides
of the blastopore. A more or less marked ap-
proximation of their hinder halves will be obvious.

h. The same at 12 — 14 days.

The embryo may now be definitely recognized as
a pear-shaped body still enclosed within the mu-
cous investment. Remove and examine it.
a. The body ; head and trunk, very obvious if seen

from the dorsal aspect.
/?. The neural folds ; seen, if examined from the

dorsal aspect, to be uniting in the middle line,
y. The blastopore ; no longer recognizable.
B. The suckers', two oval thick-lipped depressions
on the under side of the head (mouth not yet
recognizable).

e. The visceral arches ; generally to be seen at this
stage, as a couple of oblique ridges on either
side, above 8.

i. The embryo at 15 — 18 days.

Still enclosed within the mucus mass; recogniz-

THE FROG. l6l

able by the presence of the tail, the lashing move-
ments of which are very conspicuous (when at
rest it is usually bent round to the right side).
Liberate the embryo, and in doing so note that
the mucous mass is undergoing disintegration.

a. The body; now differentiated into distinct head,
trunk and tail.

Examine from the side.

p. The visceral arches; four in number on each
side, the two hindermost bearing papillate out-
growths (external branchitz).

y. The eye ; obvious as a rounded eminence above
the interspace between the first two visceral
arches.

8. The auditory organ; obvious as a small pit
(auditory pit) behind and above y.

Examine from beneath.

e. The suckers ; now at their maximum of de-
velopment and probably confluent posteriorly.

£. The mouth ; a deep oval pit in the middle line
immediately in front of e.

fj. The olfactory organs ; apparent as two shallow
pits (olfactory pits) in front of the mouth.

9. The anus (proctodczum) ; a minute pore at the
base of the tail.

k. The free swimming larva. Fish-like and bilaterally
symmetrical, tail and external branchiae much
elongated. Introduce some duck-weed or other
small plant into the water, and note the move-
ments and habits of the larvae.

M. H

1 62 ELEMENTARY BIOLOGY. [CHAP.

Examine from the side.

a. The head ; well marked and possessed of func-
tional sense organs.

/?. The external branchice ; long and pectinated ;
three in number on either side, a small third-
pair having appeared.

y. The tail ; elongated and marked out into a
number of well-defined segments (myomeres).

8. The median fin ; a thin fold of integument en-
circling the tail and continued forwards — dor-
sally to the middle of the back — ventrally, for
a short distance in front of the anus (cloacal
orifice).
Examine from beneath.

e. The mouth; transversely enlarged and sur-
rounded by fleshy lips, within which can be seen
the jaws, beset by small horny teeth.

/. The larva, at a later period. Look for larvae in
which the external branchiae of the right side are
no longer visible. Examine from beneath,
a. Head, trunk and tail. Cf. generally with the

foregoing stages, as to relative proportions.
/?. The external -branchial ; visible only on the left
side, projecting out from beneath a fold of the
cephalic integument (operculum). A similar fold
is seen on the opposite side.

y. The mouth; transversely oval and very large;
the lips, now papillate ; horny teeth, more
marked.

8. The suckers ; disappearing, and reduced to the
condition of a couple of small tubercles.

I.] THE FROG. 163

Examine from the side.

e. The mouth; surrounded by a protractile bell-
shaped suctorial lip, and utilized for purposes of
adhesion, in place of the suckers which are now
disappearing.

Cf. the movements of the living larva.
m. The development of the operculum. Look for larvae
intermediate between k and /. Examine from the
side.

The operculum will be seen arising, on either
side, immediately in front of the external gills, as a
backwardly-directed fold of the second (hyoid)
visceral arch.

n. The larva at a later stage than I ; conspicuous by
the great length of its tail and the absence of
external branchiae. Examine from beneath,
a. The body generally ; clad in a thin transparent
integument, elegantly pigmented in black and
gold.

ft. The mouth; still increasing in size, the cir-
cum-oral papillae becoming more marked.

7. The suckers ; still further reduced and repre-
sented by two small vestiges.

S. The intestine; visible through the body-wall as
a greyish coiled tube of large calibre.

e. The anus (cloacal orifice)-, median and ventral,
situated at the base of the tail on the summit of
the pointed posterior extremity of the trunk
(cf. side view).

£ The hind-limbs ; minute papillate outgrowths of
the body- wall, at the sides and a little in front of e.

II — 2

1 64 ELEMENTARY BIOLOGY. [CHAP.

rj. The branchial pore (spiracle] ; asymmetrical and
. on the left side (external branchiae are no longer
present). Remove the ventral portion of the
adjacent integument (coalesced opercula) ; a
spacious branchial chamber will be exposed.
Note its limits.

0. The internal branchice ; paired vascular folds of
the sides of the head, four sets on each side,
lying within the branchial chamber. Try and
make out their relations. They are borne upon
outgrowths of the wall of the pharynx (branchial
visceral arches), which alternate with perfora-
tions of the same (visceral clefts) putting the
pharyngeal cavity in communication with the
branchial chamber.

i. The lungs; seen, on opening up the body-
cavity, as small diverticula of the alimentary
canal not yet distended with air.

o. The larva on the appearance of hind-limbs.
Examine from beneath.

a. The body generally ; note the proportions of its
several constituents.

ft. The mouth; its large size; papillate lips and
horny teeth well marked.

y. The absence of suckers and branchial pores.

8. The hind-limbs; small, but fully differentiated;
one-third the length of the body and separated
in the ventral middle line by the terminal por-
tion of the alimentary canal.

e. The fore limbs ; differentiated, but covered by
(visible through) the opercular membrane.

] THE FROG. 165

£ The tail; in all probability beginning to shrink.

/. The young frog shortly after the moult (shedding
of the larval skin): characterized by the coex-
istence of both pairs of limbs and the tail.

Note its frog-like characters and especially —
a. The mouth ; gape-wide, sucking lips gone, horny

teeth replaced by true teeth.
ft. The tail ; absolutely shorter than in the late

tadpole. Note its relations to the trunk,
y. The cloacal orifice. Examine this with care;
although displaced, owing to the increase in
size of the hind-limbs and their approximation
in the ventral middle line, it still lies at the
base of the tail (i.e. it is ventral in position.
Cf. the adult).

ii. The formation of the embryonic layers and certain
of the more important organs.

Imbed and cut sections of the preserved oosperm
or larvse, as directed below. Staining is unnecessary;
hsematoxylin may be used, if desired.

a. At the first cleavage ; longitudinal and at right
angles to the cleavage furrow.

a. The blastomeres ; equal in size and separated by
the cleavage furrow. Protoplasm ; densely pig-
mented above, little so below, for the most
part laden with yolk granules.

j3. Nuclei. These may or may not be visible;
each is clear and transparent and generally sur-
rounded by pigment. (If visible, look for indi-
cations of division.)

1 66 ELEMENTARY BIOLOGY. [CHAP.

b. On the formation of the third furrow (cf. i. c],

Median longitudinal, as at a.
a. The blastomeres, four in number as seen in

section; two smaller upper ones, densely pig-

mented ; two larger lower ones, pigmented only

at the periphery and yolk laden.
ft. The cleavage cavity (segmentation cavity] \ small

and central, in a line with the transverse furrow.

c. At 6 — 8 hours. (Cf. i. d.) Longitudinal vertical.
a. The upper layer cells ; small and pigmented,

a single layer deep, nuclei generally visible.

ft. The lower layer cells ; large cells with little or
no pigment, nuclei rarely visible.

y. The yolk granules ; aggregated in and largely
confined to the lower cells (vegetative pole of
the oosperm).

S. The cleavage cavity ; large and irregular, inter-
posed between a and ft.

d. At 30 — 38 hours. Longitudinal vertical, to pass
through the first trace of the blastopore.

a. The cleavage cavity ; large and excentric, inter-
posed between the cells of the upper and lower
layers.

ft. The lower layer cells ; several rows deep, nu-
cleated and yolk laden; forming the main mass.

y. The upper layer cells; now two or three rows
deep (epiblast or outer germinal layer) and
differentiated into two layers.

Follow the course of the epiblast; it largely
encloses the lower layer cells and is probably
invaginated at the blastopore.

THE FROG. 167

e. At 4 — 5 days (typical blastopore stage. Cf. i. f).
Longitudinal vertical, to pass through the blasto-
pore.

a. The lower layer cells ; now practically destitute
of pigment and almost completely enclosed in
epiblast; exposed only in the region of the
blastopore.

(3. The archenteron (primitive alimentary canal); a
spacious cavity occupying much the position of
the cleavage cavity in Section d\ in communi-
cation with the exterior at the blastopore.

y. The cleavage cavity ; small and irregular, situated
below (3 at the end opposite the blastopore.

8. The hypoblast (inner germinal layer or digestive
epithelium); a single layer of closely packed
cells forming the roof of the archenteron, con-
tinuous with the epiblast at the upper lip of
the blastopore (i. e. formed as the result of in-
vagination of the latter layer).

e. The mesoblast (middle germinal layer) ; obvious
in median longitudinal section as two series of
cells somewhat irregular and more loosely scat-
tered than the rest ; an upper series lying in the
interspace between the roof of the archenteron
and adjacent epiblast; a lower series somewhat
similarly interposed below between the lower
layer cells and adjacent epiblast, and most
numerous at the blastopore.

/. The same at 12 — 14 days (cf. i. h). Transverse
across the middle of the neural plate. Examine
under a high power.

1 68 ELEMENTARY BIOLOGY. [CHAP.

a. The neural plate; a thickened medio-dorsal
sheet of epiblast, partly invaginated and en-
closing a deep neural groove; its raised edges
(neural folds).

(3. The archenteron ; a wide cavity situated nearer
the upper than the lower pole.

y. The hypoblast; a single layer of cells forming
the roof and side walls of /?: its cells, somewhat
flattened above, graduating into the lower layer
cells laterally.

8. The notochord; a median oval mass of small
cells, immediately beneath the neural plate,
not yet completely constricted off from the
hypoblast.

e. The mesoblast; interposed between the epiblast,
and hypoblast with the lower layer cells; most
marked dorso-laterally and symmetrical on op-
posite sides.

g. The same at 15 — 18 days (cf. i. /"). Transverse
across the middle of the trunk, for comparison
with/

a. The epiblast; well-defined and pigmented.
Examine under a high power, and note the
order of succession and characters of its layers.

ft. The hypoblast and lower layer cells. Cf. f. y.

y. The neural tube (cerebro-spinal axis) pigmented
and medio-dorsal, enclosing a central canal '; it
no longer shows any trace of connection with
epiblast. (Cf. i. h.)

8. The notochord; immediately beneath y, large
and rounded, composed of large vacuolated cells.

THE FROG. 169

The mesoblast; forming a denser mass (meso-
blastic somite) on either side of the neural tube
and notochord; below that point subdivided
into two layers — an outer one (somatic layer)
which applies itself to the epiblast, and an inner
one (splanchnic layer) which invests the hypo-
blast and lower layer cells.

The body cavity ; the interspace between the
somatic and splanchnic mesoblast; more or
less obvious, in accordance with the degree of
shrinkage of the preparation.
The leading blood-vessels, as under. The dorsal
aorta, median and cylindroidal underlying the
sub-notochordal rod; the posterior cardinal
veins, large vessels, ovoidal or irregular in
section, right and left of the aorta. Blood
corpuscles will in all probability be seen within
them.

Examine under a high power. The above
vessels will be seen to be formed as excava-
tions of the mesoblast, each bounded by a
single layer of modified cells.
The pro-renal (segmental) duct • a conspicuous
thick-walled tube seen, on either side, lying
within the somatic mesoblast immediately be-
neath the posterior cardinal vein.

The larva at 12 — 14 days (cf. i. k). Median lon-
gitudinal vertical, to pass through the blastopore
(anus). Examine under a low power.

The body; elongated and enclosing two
cavities: — a ventral or archenteric canal, in
communication with the exterior at the blasto-

I/O ELEMENTARY BIOLOGY. [CHAP.

pore, and a dorsal or neural canal, at this period
in communication posteriorly with the archen-
teron and blastopore by a short narrow passage
(neurenteric canal).

ft. The notochord; a median rod of slightly vacuo-
lated cells, lying in the interspace between the
archenteric and neural tubes.

y. The neural tube; enlarged in front (brain) and
overhanging the notochord.

8. The archenteron; its extent; its roof, formed by
the hypoblast cells; its floor, still bounded by
undifferentiated lower layer cells, now compara-
tively few in number.

e. The mesoblast ; composed of irregular scattered
cells and more extensive than in the earlier
stages examined.

/. At 15 — 1 8 days (cf. i. /); transversely oblique, to
pass through the visceral arches and external
branchiae.

a. The brain ; a large vesicle, situated at the an-
terior end.

ft. The mesenteron (archenteron) ; a large central
cavity behind and in a line with a, wide in front,
constricted behind.

y. The visceral pouches ; paired diverticula of the
hypoblast, 4 or 5 in number on either side, their
blind ends abutting against the epidermis (if the
section be much shrunk their walls may be in
close apposition).

8. The visceral arches ; mesoblastic aggregates
alternating with y; each lying behind its cor-

[.] THE FROG. i;i

responding pouch, and usually lodging a large
blood-vessel (aortic arch) seen in section.

e. The external branchiae. These will be seen in
some of the sections as filiform outgrowths (from
i to 3 in number on either side) of the visceral
arches.

Each consists of a mesoblastic core invested in
epiblast. If the five visceral pouches are seen,
the first gill will be found to arise behind the
second one (first branchial).

k. Examine the lower sections of the above series
and look for the formation of the mouth cavity.

a. The mesenteron ; cf. supra; it ends blindly in
front and is lined by a non-pigmented epithelium
(yellow if prepared as directed and not stained).

j8. The stomodceum ; a sac-like involution of the epi-
dermis abutting against a, spacious, and well
marked on account of its dense pigmentation.
Its enclosed cavity is the mouth cavity.

/. Compare similar sections through the head of an
older larva, in which the internal branchiae are
present.

a. The visceral pouches ; those of the branchial

series are now in open communication with the

exterior (visceral clefts].
/?. The internal branchitz ; vascular pectinations of

the opposite faces of a. Note their number and

arrangement.

y. The operculum ; a backwardly-directed fold
arising behind the first pouch (from the hyoid
arch ; cf. i. m).

ELEMENTARY BIOLOGY. [CHAP. I.

///. Across the head of /'. (15 — 18 days); transversely
to the long axis, to pass through the eyes.

a. The epidermis ; forming a continuous investment
for the whole ; thickened below and produced
into two inverted cup-shaped folds (suckers).

/3. The brain; dorsal and elongated, pigmented
and enclosing a spacious central cavity (3rd
ventricle).

y. The notochord and archenteron ; occurring in
order of succession below ft.

8. The developing retina; on either side a cup-
shaped outgrowth of the brain (optic cup\ con-
nected with the same by a narrow neck (optic
stalk) visible only in a few sections. Examine
the cup and note its thick outer wall ; its thin
inner wall; its central cavity, continuous with
that of the 3rd ventricle.

e. The lens ; visible either as a thickening or invo-
lution of the epiblast, immediately external to 8.

n. Compare similar sections through the eye of a
more advanced larva. Examine under a high
power.

a. The optic cup ; its central cavity becoming ob-
literated, its inner wall densely pigmented (pig-
ment epithelium ; cf. Sect. M. 1 2 k).

fi. The lens ; completely constricted off from the
epidermis and enclosing a central cavity; its
thin outer wall ; its thick inner wall.

y. The remaining constituents of the eye; seen,
for the most part, in course of differentiation
from the surrounding mesoblast.

IT.

THE FRESH- WATER CRAYFISH (Astacus fluviatilis)
AND THE LOBSTER (Homarus vulgaris).

THE Crayfish and the Lobster are inhabitants of the water,
the former occurring in many of our rivers and the latter
abounding on the rocky parts of the coasts of the European
seas. They are bilaterally symmetrical animals, provided
with many pairs of limbs, among which the large prehensile
'claws' are conspicuous. They are very active, walking
and swimming with equal ease and sometimes propelling
themselves backwards or forwards, with great swiftness, by
strokes of the broad fin which terminates the body. They
have conspicuous eyes, mounted upon moveable stalks, at
the anterior end of the head ; and two pairs of feelers, one
pair of which are as long as the body, while the other pair
are much shorter.

The body and limbs are invested by a strong jointed
shell, or exoskeleton, which is a product of the subjacent
epidermis, and consists of layers of membrane which remain
soft and flexible in the interspaces between the segments of
the body and limbs, but are rendered hard and dense else-
where by the deposit of calcareous salts; the exoskeleton
is deeply tinged with a colouring matter which turns red
when exposed to the action of boiling water. The body
presents an anterior division — the cephalothorax — covered

1/4 ELEMENTARY BIOLOGY. [CHAP.

by a large continuous shield, or carapace ; and a posterior
division — the abdomen — divided into a series of segments
which are moveable upon one another in the direction of
the vertical median plane, so that the abdomen can be
straightened out, in extension ; or bent into a sharp curve, in
flexion. Of these segments there are seven. The anterior
six are the somites of the abdomen, and each of them has a
pair of appendages attached to its ventral wall. The seventh
bears no appendages and is termed the telson—it is sub-
divided into two pieces in the Crayfish. The anus is
situated on the ventral aspect, beneath the telson and behind
the last somite.

A groove on the surface of the carapace, which is termed
the cervical suture, separates an anterior division, which
is termed the head or cephalon, from a posterior division
or thorax; and the thoracic division of the carapace fur-
ther presents wide lateral prolongations, which pass down-
wards and cover the sides of the thorax, their free ventral
edges being applied against the bases of the thoracic
limbs. These are the branchiostegites. Each roofs over a
wide chamber in which the gills are contained and which
communicates with the exterior, below and behind, by the
narrow interval between the edge of the branchiostegite and
the limbs. Anteriorly and inferiorly, the branchial chamber
is prolonged into a canal, which opens in front and below
at the junction of the head with the thorax, immediately
behind the cervical suture. In this canal there lies a flat
oval plate — the scaphognathite — which is attached to the
second pair of maxillae and which plays a very important
part in the performance of the function of respiration. Of
the thoracic limbs themselves there are eight pairs, and,
on the ventral face of the body, the lines of demarcation
between the eight somites to which these limbs belong

II.] THE CRAYFISH AND LOBSTER. 1/5

may be observed. There is no trace of any corresponding
divisions in the cephalothorax of the Lobster \ but, in the
Crayfish, the last thoracic somite is incompletely united
with those which precede it. The four posterior pairs of
thoracic limbs are those by which the animal walks and are
termed the ambulatory legs. The next pair in front is formed
by the great claws or chela. The anterior three pairs are
bent up alongside the mouth and are moved to and from
the median line so as to play the part of jaws, whence they
are termed foot-jaws or maxillipedes. The external or third
pair of these maxillipedes are much stouter and more like
the ambulatory limbs than the rest, and the inner edges of
their principal joints are toothed. The innermost or first
pair of maxillipedes are broad, foliaceous and soft. When
these foot-jaws are taken away, two pairs of soft foliaceous
appendages come into view. They are attached to the
hinder part of the cephalon and are the jaws or maxilla.
The second, or outermost, is produced, externally, into the
scaphognathite, which will be seen to lie in a groove sepa-
rating the head from the thorax laterally and known as
the cervical groove.

Anterior to these maxillae lie the two very stout mandibles.
Between their inner toothed ends is the wide aperture of
the mouth, bounded, in front, by a soft shield-shaped plate,
the labrum ; and behind, by another soft plate, divided by a
median fissure into two lobes which simulate appendages —
it is termed the metastoma. Thus far, the surfaces of the
somites to which the appendages are attached look down-
wards, when the body is straightened out and the carapace
is directed upwards. But, in front of the mouth, the wall of
the body to which the appendages are attached is bent up,
at right angles to its former direction, and consequently
looks forwards. This bend of the ventral wall of the body

I?6 ELEMENTARY BIOLOGY. [CHAP.

is the cephalic flexure. In correspondence with this change of
position of the surface to which they are attached, the three
pairs of appendages of the somites which lie in front of the
mouth are directed either forwards, or forwards and upwards.
The posterior pair consists of the long feelers or antenna: the
next, of the short feelers or antennules ; and the most anterior
is formed by the short subcylindrical stalks (pphthalmites],
on the ends of which the eyes are situated.

This enumeration shews that the Lobster and Crayfish
have six pairs of abdominal appendages — the swimmerets
and " tail-fin"; eight pairs of thoracic appendages (four pairs
of ambulatory limbs, one pair of chelate prehensile limbs,
three pairs of maxillipeds), and six pairs of cephalic ap-
pendages (two pairs of maxillae, one pair of mandibles,
one pair of antennae, one pair of antennules, one pair of
eyestalks), making in all twenty pairs of appendages. It
may or may not be that the eyestalks are modified ap-
pendages ; should they have that value, the body in
correspondence with the number of appendages will con-
sist of twenty somites (the telson excepted): of these six
remain moveable upon one another to form the abdomen,
while the other fourteen are, with the exception of a portion
of the last thoracic one in the Crayfish completely united
to form the cephalothorax.

The branchiostegite is an outgrowth of the dorso-lateral
region of the confluent thoracic somites. The serrated
rostrum which ends the carapace is a fixed median pro-
longation of the dorsal wall of the anterior cephalic somites;
while the telson is a moveable median prolongation of the
dorsal wall of the sixth abdominal somite. The labrum and
the metastoma are median growths of the sterna of the
prae-oral and post-oral somites.

Thus the whole skeleton in these animals may be con-

II.] THE CRAYFISH AND LOBSTER. 177

sidered as a twentyfold repetition of the ring-like somite
with its pair of appendages, which is seen in its simplest
form in one of the abdominal somites. Moreover, not-
withstanding the great variety of functions allotted to the
various appendages, the study of the details of their struc-
ture (see Laboratory work) will shew that they are all re-
ducible to modifications of a fundamental form, consisting
of a basal-portion (profopodtit) with two terminal divisions
(endopodite and exopodite). A third division or epipodite is
superadded to the appendages concerned in respiration.

Of the twenty pairs of appendages, the three anterior
are concerned with sensation and the six posterior with
swimming; the mandibles, which bound the mouth, are
most efficient in mastication. Of the ten pairs which re-
main, five are modified to form foot-jaws, the others being
functional for walking or climbing; three of the latter
however, in that they are chelate and often used to capture
prey or to tear up food material, combine the functions of
locomotor feet and foot-jaws.

Each of the larger appendages is composed of a number
of segments, each movable upon its fellow in a single
plane; the various segments are so articulated that the
limb, as a whole, is capable of considerable rotation.

As has been already said, the Lobster and Crayfish are
bilaterally symmetrical; that is to say, a median vertical
plane passing through the mouth and anus divides them into
two similar halves. This symmetry is exhibited not merely
by the exterior of the body and the correspondence of the
paired limbs, but extends to the internal organs ; the alimen-
tary canal and its appendages, the heart, the nervous system,
the muscles and the reproductive organs, being disposed so
as to be symmetrical in relation to the median vertical plane
of the body.

M. 12

1/8 ELEMENTARY BIOLOGY. [CHAP.

The wide gullet leads almost vertically into the spacious
stomach, and both are lined by a chitinous continuation of
the exoskeleton. The stomach is divided by a transverse
constriction into a spacious cardiac, and a much smaller
pyloric division, from which latter the intestine passes. The
walls of the anterior half of the cardiac sac are thin and
membranous, but, in the posterior half, they become calci-
ned so as to give rise to a gastric skeleton of considerable
complexity. The chief part of this skeleton consists of a
median dorsal T-shaped 'cardiac' ossicle, the cross-piece
of which forms a transverse arch, while its long median
process extends backwards in the middle line. The ends of
the transverse arch are articulated obliquely with two small
' antero-lateral ' pieces, the extremities of which again are
articulated with postero-lateral pieces, and these with a
cross-piece, the 'pyloric' ossicle, which arches over the
roof of the pyloric division of the stomach. In this manner
a sort of hexagonal frame with moveable joints is formed,
and the median process projects backwards so far, as to end
below the pyloric piece. It is connected with this, however,
by a short ' pre-pyloric ' ossicle which ascends obliquely
forwards and is articulated with the anterior edge of the
pyloric piece. The lower extremity of this is produced into
the strong median ' uro-cardiac ' tooth ; while the postero-
lateral pieces are flanged inwards, and, becoming greatly
thickened and ridged, form the large ' lateral cardiac ' teeth.
Two powerful muscles are attached to the cardiac ossicle,
and ascend obliquely forwards to be inserted into the
under face of the carapace. Two other similar muscular
bundles arise from the pyloric ossicle, and, passing obliquely
upwards and backwards, are also inserted into the under
face of the carapace, in the region of the cervical groove.
The disposition of all these parts is such that when these

II.] THE CRAYFISH AND LOBSTER. 179

muscles contract, the dorsal ossicles are divaricated, the uro-
cardiac tooth is thereby thrust forwards and downwards, while
the lateral teeth move inwards downwards and backwards,
and the three meet in the middle line. The working of these
muscles can be readily imitated by seizing the anterior and
posterior cross-pieces with forceps and pulling them in the
direction in which the muscles act. The three teeth will
then be seen to come together with a clash. Thus the
food which has been torn by the jaws is submitted to
further crushing in this gastric mill. The walls of the
pyloric division of the stomach are thick, and project like
cushions into its interior, thereby reducing its cavity to a
narrow passage. The cushion-like surfaces of the pyloric
walls are provided with long hairs which stretch across this
narrow passage, and thus convert it into a strainer, which
allows of the passage of only very finely divided matter
from the gastric sac to the thin and delicate intestine.
The intestine, in both Lobster and Crayfish, is made up of
an anterior thin-walled segment whose roof is prolonged up
-into a coecal process, and a posterior segment which, like
the stomach, is lined by a chitinous continuation of the
exoskeleton. The latter is spirally folded and papillose in
the Crayfish. The anus is bounded by two valve-like
thickenings of the exoskeleton, which are connected with
the adjacent intestinal wall by a series of small muscles.
The alimentary-canal is thus to be resolved into a straight
tube of three segments ; an anterior fore-gut and a posterior
hind-gut, each lined by an involution of the cuticular exo-
skeleton, and a non-cuticular mid-gut which bears the above-
named coecal process and receives the ducts of the digestive
gland The digestive gland itself is the seat of the forma-
tion of a combustible carbo-hydrate oily material which is
as it were held in reserve in its constituent cells, as well as

12 — 2

ELEMENTARY BIOLOGY. [CHAP.

of a digestive ferment ; as these physiological activities are
frequently relegated respectively to separate liver and
pancreas, the single gland which here performs the two
functions has been appropriately termed the hepato-pancreas.

The heart is a short, thick, somewhat hexagonal, symme-
trical organ lodged in the pericardiac sinus, to the walls of
which it is attached by fibrous bands. In its anterior half
three pairs of apertures are visible, two being placed upon
the upper face, two at the sides, and two on the under face.
The lateral apertures are the most posterior, the dorsal, the
most anterior in position. Each aperture begins in a funnel-
shaped depression of the outer face of the organ, which leads
obliquely inwards and terminates by a valvular slit in the
cavity of the heart. This cavity is very much reduced by
the encroachment of the muscular bands which constitute
the walls of the heart, so that a transverse or longitudinal
section shews only a small median cavity surrounded by a
thick and spongy wall.

During life, the heart beats vigorously, the whole of its
parietes contracting together. From the dorsal part of its
anterior extremity three arteries are given off, one median
and two lateral, to the cephalon and its contents, and from
the ventral aspect of this end of the heart an hepatic artery
is given off, on each side, mainly to the liver. At its posterior
end, the heart ends in a median dilatation from which two
great arterial trunks are given off: one, the superior ab-
dominal artery, runs along the dorsal face of the intes-
tine, giving off transverse branches as it goes, in each
somite; the other, the sternal artery, immediately on leaving
the pericardial-sinus distributes branches to the genital
gland; it then passes ventrally, to the interspace between
the penultimate and antepenultimate thoracic ganglia,
passes between their commissures and divides into two

II.] THE CRAYFISH AND LOBSTER. l8l

branches, which run, backwards and forwards, between the
ganglionic chain and the exoskeleton.

These arteries divide and subdivide and end in what, in
some parts of the body at any rate, e.g. the liver, is a true
capillary system. The veins are irregular channels, or
sinuses, which lie between the several muscles and viscera.
One of the largest of these is situated in the median ventral
line, and can be readily laid open by piercing the soft inte-
gument which lies between any two of the abdominal sterna.
The blood flows out of the aperture with great rapidity, and
the quantity shed shews the size of the sinus and its free
communication with the rest of the vascular system. By
cutting across any one of the limbs and inserting a blow-
pipe into the place whence the blood wells forth, this ventral
sinus can be readily injected with air. A large and irregular
sinus is also to be found in the median dorsal region of the
abdomen and is freely connected with the median ventral
sinus. The stem of each branchia contains two canals, one
running along its outer and the other along its inner face.
The outer canal communicates, at its origin, with the thoracic
portion of the median ventral sinus. The inner canal opens
into a passage which ascends in the lateral wall of the
thorax and opens, after meeting with other l branchio-cardiac^
canals, opposite the lateral aperture of the heart. As the
valvular lips of this and the other apertures of the heart
open inwards, the blood, when the systole takes place, is
driven out of the heart through the various arteries, and a
considerable part of the blood thus propelled into the
capillaries is collected by the median ventral sinus and
thence, passing through the gills, eventually returns to the
heart. It is customary to speak of a heart such as this,
which propels aerated blood, as systemic, by way of dis-
tinction from a branchial heart, which propels impure venous

1 82 ELEMENTARY BIOLOGY. [CHAP.

blood to the organs of respiration. But whether the whole
of the venous blood takes the same course, or whether
some of it returns from the dorsal sinuses directly to the
pericardium, is a question which is not decided. Nor is it
certain whether the so-called pericardium is to be regarded
as one cavity, or whether the fibrous bands, which connect
the heart with its walls, may not subdivide it into com-
partments in immediate communication with certain of the
cardiac apertures, and not with the rest.

In the Lobster, from which the blood is readily obtained
in quantity, it is a nearly colourless fluid, which usually has
a faint neutral tint. It readily coagulates, a tolerably firm
clot separating from the serum. It contains nucleated cor-
puscles, devoid of any noticeable colour, which throw out
very long pseudopodial prolongations, and thereby take an
irregular stellate form.

It has been seen that the respiratory organs, or branchiae,
are lodged in a chamber situated between the branchiostegite
externally, the lateral walls of the thoracic somites internally,
and the bases of the thoracic limbs below ; and that there
is a narrow interspace between the free edge of the bran-
chiostegite and the latter. At the anterior end of the cham-
ber, a funnel-shaped passage leads to the anterior opening
mentioned above, and, in this passage, the scaphognathite
lies like a swing door.

During life, the scaphognathite is in incessant movement
forwards and backwards, scooping out the water in the bran-
chial chamber through its anterior aperture at every forward
motion. This bailing out of the water results in the in-
ducing of a current which flows in by the inferior and
posterior cleft beneath the free edge of the branchiostegite,
and thus a constant circulation over the gills is secured.

II.] • THE CRAYFISH AND LOBSTER. 183

Each branchia is somewhat like a bottle-brush, having a
stem beset with numerous filaments; and the blood con-
tained in the vessels of the latter, being separated by only a
very thin membrane from the air contained in the water,
loses carbonic anhydride and gains a corresponding amount
of oxygen in its course through the branchiae.

The branchiae are exclusively thoracic, being attached
partly to the inter-articular membranes between the appen-
dages and the body-wall and partly to the proximal ends of
the limbs themselves. The last thoracic appendage is gill-less,
and the branchia present in its vicinity in the Crayfish differs
from the rest in being attached to the epimeral wall of the
thorax; the Lobster has, in addition, three such gills on
either side fully developed and functional : all or most of
these are represented in the Crayfish by short filamentous
rudiments, no longer functional as branchiae. The epipo-
dites of the limbs ascend between the sets of branchiae which
belong to each somite, and separate them. The branchiae
which are attached to the limbs must necessarily be stirred by
the movement of the latter, and hence the exchange of gases
between the blood which they contain and the water must
be, to a certain extent, increased, in proportion to the
muscular contractions which give rise to the movements
of the limbs and the consequently increased formation of
carbonic anhydride.

The excretion of nitrogenous waste goes on in the two
large green glands which lie in the cephalon, close to the
bases of the antennae. Each gland encircles the neck of a
large thin-walled muscular sac which opens by a short
canal upon the ventral face of the basal joint of the antenna.
The gland itself consists of a coiled tube, lined by a
large-celled epithelium and abundantly supplied with blood
vessels.

1 84 ELEMENTARY BIOLOGY. [CHAP.

V The nervous system consists of a chain of thirteen gan-
glia— united by longitudinal commissures — lodged in the
median line of the ventral aspect of the body, from which
nerves are given to the organs of sense, to the muscles
of the trunk and limbs, and to the integument; and of a
visceral nervous system, developed chiefly upon the stomach
and hinder segment of the intestine.

The ganglia are centres of aggregation of the nerve
cells; of the thirteen seen in the adult the most an-
terior lies in the cephalon, close to the attachments of
the three anterior pair of appendages, and gives branches
to them and to the visceral nervous system. It is usually
termed the brain or the supracesophageal ganglion. It is
connected by two commissural cords, which pass on each
side of the gullet, with a larger ganglionic mass, which is
called the subxsophageal ganglion. This occupies the
region of the hinder part of the cephalon and the anterior
part of the thorax, and gives off nerves to the mandible,
maxillae and the three pairs of maxillipeds. Five other
ganglia lie in the five somites which bear the chelae and the
ambulatory limbs, and there is one for each abdominal
somite, the last of these being the largest of the six.

The longitudinal commissures are double, and the
ganglia themselves shew more or less evident indications
of being double also. There is reason to believe that these
thirteen apparent ganglia really represent twenty pairs of
primitive ganglia, one pair for each somite ; the three an-
terior pairs having coalesced preorally to form the brain;
and the six which follow the mouth having united into the
subcesophageal mass.

The only organs of special sense which are recognizable
in the Lobster and Crayfish are eyes and auditory organs,
and a series of specially modified setose appendages which

II.] THE CRAYFISH AND LOBSTER, 185

fringe the exopodite of the antennule, and are thought to
perform an olfactory function.

The eyes are situated at the extremities of the eyestalks,
or ophthalmites, which represent the first pair of appendages
of the head. The rounded end of the eyestalk presents a
clear smooth area of somewhat crescentic form, divided into
a great number of small mostly four-sided facets. This area
corresponds with the cornea, which is simply the ordinary
chitinous layer of the integument become transparent.
The inner face of each facet of the cornea corresponds with
the outer end of an elongated transparent slightly conical
body — the crystalline cone — the inner end of which passes into
a relatively long and slender connective rod, by which it is
united with a spindle-shaped transversely striated body —
the striated spindle. The inner extremity of this again is
connected by a nerve fibre with the optic bulb, the
dilated gangliform termination of the optic nerve. The
respective striated spindles, connective rods and crystalline
cones, thus radiate from the outer surface of the terminal
ganglion to the inner surface of the cornea, and each is
separated from its neighbour by a nucleated sheath, parts
of which are deeply pigmented. Nothing is accurately
known as to the manner in which the function of vision is
performed by the so-called compound eye which has just been
described. The inner and outer faces of the corneal facets
are flat and parallel. They therefore cannot play the part
of lenses; and, if they could, there is no trace of nerve
endings so disposed as to be affected by the points of light
gathered together in the foci of such lenses. Morpho-
logically, the striated spindles and their nerve fibres and
probably the optic bulb itself wholly or in part, are in
many ways analogous to those elements of the retina of
the Vertebrata which make up the layers of rods and cones

1 86 ELEMENTARY BIOLOGY. [CHAP.

and the granular layers. These structures are properly
modifications of the epidermis; inasmuch as the cerebral
vesicle, of which the retinal elements are outgrowths, is an
involution of the epidermis of the embryo, and, morpholo-
gically speaking, the free ends of the rods and cones of the
vertebrate eye are, as in the crustacean, turned outwards.
There is good reason for believing the crystalline cones to
be derivatives of the investing epidermis.

The auditory organ of the Lobster and Crayfish is situated
in the basal joint of the antennule, on the dorsal surface of
which its small slit-like opening, protected by numerous seta?,
is to be seen. The chitinous layer of the integument is
invaginated at the opening, and thus gives rise to a small
flattened sac lodged in the interior of the antennule. One
side of this sac is in-folded so as to produce a ridge, which
projects into the cavity of the sac, and is beset with very fine
and delicate hair-like setae. The auditory nerve enters the
fold, and its ultimate filaments pass into the setae at their
bases. The sac contains water in which minute particles of
sand are suspended in the manner of otoliths.

The sexes are distinct in the Lobster and Crayfish. The
external characters of the males and females and the form
of the reproductive organs are described in detail in the
Laboratory work.

The ovary is median and saccular, and its investing mem-
brane is prolonged backwards to form a paired oviduct
whose walls are glandular. Each ovicell is invested, during
its maturation, in an epitheloid follicle of a single layer of
cells; by the rupture of this the ripe ovum is liberated, and
thrown off thus into the interior of the ovary it makes its
way down the oviduct and so to the exterior. The impreg-
nated ova are attached in great numbers, to the hairs of the
swimmerets where they undergo their development, by the
viscid secretion of a scries of "cement glands" borne upon

II.] THE CRAYFISH AND LOBSTER. l8/

the ventral surfaces of the abdominal somites and bases of
the last pair of swimmerets of the female. A Lobster with
eggs thus attached, is said by the fishermen to be 'in berry.'
The segmenting egg of the Crayfish differs in some im-
portant respects from that of the Frog. The yolk material
is aggregated in the central protoplasm, and not at one pole
as in the case of the latter animal, and segmentation is
restricted to the superficial least yolk-laden protoplasm;
the egg is at no period completely cleft into two parts,
segmentation is from first to last partial or meroblastic, a
cellular investment being as it were formed around a central
yolk-bearing mass. The investing cells become invaginated
at one point to form a small sac, which remains for a time
in open communication with the exterior. The embryo at
this stage is to be resolved into a double-walled sac or gas-
trula, the interspace between the two walls being filled with
yolk. The outer layer or epiblast eventually gives rise to
the epidermis and its derivatives — the nervous system and
sensiferous epithelia; the inner one or hypoblast forms the
lining membrane of the mid-gut, and from it the digestive
gland is formed later as a paired outgrowth— it is from first
to last the true digestive epithelium. The cavity enclosed
by the hypoblast in its simple sac-like condition is the
primitive alimentary canal or archenteron, its aperture of
invagination being termed the blastopore. The remaining
constituents of the body are derived from cells which are
budded off from one or both of the above layers. Early
traces of the embryo are obvious in the development of the
cephalo-thoracic appendages, which arise as paired out-
growths of a relatively small area of the surface of the egg
known as the germinal area ; there appear at the same time,
at opposite ends of this, two median papillate outgrowths —
an anterior one which gives rise to the labrum, and a pos-
terior one which, by subsequent elongation and segmenta-

1 88 ELEMENTARY BIOLOGY. [CHAP.

tion is transformed into the abdomen, for which reason
it is termed the abdominal papilla. The blastopore mean-
while becomes closed, the archenteric sac being no longer
in communication with the exterior; to meet this latter there
are instituted, immediately under cover of the labrum and
abdominal papilla, median ingrowths of the epidermis
which give rise to the lining membranes of the fore and
hind gut respectively, their apertures of invagination per-
sisting as the adult mouth and anus.

The yolk does not enter conspicuously into any of the
above-mentioned outgrowths; it is enclosed within the ce-
phalo-thoracic region, which becomes thereby much dis-
tended. The gills and branchiostegites appear late; the
former as simple outgrowths of the body-wall and appen-
dages, the latter as lateral folds of the body wall. The
embryo which results from these developmental processes
passes through all the stages which are needed to bring it
very near to the form of the adult before it leaves the egg :
but, in the Lobster, the young, when hatched, are larvae
extremely unlike the parent, which undergo a series of me-
tamorphoses in order to attain their adult condition. The
larvae may frequently be obtained by opening the eggs of a
* hen-lobster ' in ' berry.' They have when young a rounded
carapace, two large eyes, a jointed abdomen devoid of appen-
dages; and the thoracic limbs are all provided with long
exopodites. During the later metamorphoses the abdominal
appendages appear, as that region of the body elongates
and increases in importance; the growth of the exopodites
of the thoracic appendages is at the same time arrested,
those of the chela and ambulatory legs vanishing entirely
as these appendages become specialised for locomotion.

The ordinary growth, no less than the metamorphoses of
the Lobster and Crayfish, are accompanied by periodical

II.] THE CRAYFISH AND LOBSTER. 189

castings of the outer, chitinous, layer of the integument, or
whole exoskeleton if calcified. The shedding of this is
preceded by a process of disintegration along certain
definite lines, such as the edge of the chela and the inter-
articular membrane between the cephalo-thorax and abdo-
men; these become, during the period at which the animal
is freeing itself, so many points of least resistance and a
consequent rupture of them ensues. After each ecdysis, the
body is soft, being invested in a continuous chitinous cuticle,
and the animal retires into shelter until, by calcification of
this, the ' shell ' is reproduced.

As the hard parts of the exoskeleton are the result of re-
placement in earthy matter of portions of an originally con-
tinuous chitinous cuticle, it follows that the uncalcified areas,
which remain soft and flexible, are but persistent portions of
the cuticular predecessor of the whole; they stand related
to the calcified portions of the exoskeleton, as do the arti-
cular surfaces of the long bones of the Frog's endoskeleton
to their ossified shafts. Special thickenings or ingrowths
of the exoskeleton take place at all points, for purposes
of furnishing the surfaces requisite for the insertion of
muscles: these ingrowths or tendons are, for the most part,
mere tongue-shaped involutions, densest where resistance is
greatest. The maximum of specialization is reached in the
thoracic region where, from metameric ingrowths of the
sternal and epimeral walls, a complicated endophragmal
system is formed. This is fully described in the Laboratory
work.

I QO ELEMENTARY BIOLOGY. [CHAP.

LABORATORY WORK.

Crayfish are best killed under chloroform, proceeding as
with the Frog. (p. 35). They may be best kept alive in a
moist atmosphere near running water, fed upon sopped
bread. A sink, covered with a sheet of glass, meets all
requirements.

A. General external characters.

The animal is covered by a continuous exoskeleton which
is for the most part calcified, the following parts are
readily recognised : —

a. The body proper :

a. Its anterior unsegmented portion (cephalotho-
rax)\ the great shield-like plate (carapace)
covering the back and sides of the cephalotho-
rax; the groove across the carapace (cervical
suture) marking out the line of junction of head
and thorax: the anterior prolongation of the
carapace to form the rostrum or frontal spine.

ft. Its posterior segmented portion (abdomen]: its
seven divisions; the anterior six much like one
another; the most posterior (telson) different
from the rest (Cf. p. 174).

b. The jointed limbs (appendages) attached to the
ventral aspect of the body : their varying characters
in different regions.

c. The external apertures of the body.

a. The mouth; seen on turning aside the append-
ages beneath the head.

II.] THE CRAYFISH AND LOBSTER. 19 1

ft. The anus; a longitudinal slit beneath the
telson. It is bounded by two flap-like folds of
the adjacent integument (anal valves].

y. The paired genital apertures ': in the male, on the
basal joints of the last pair of appendages of the
thorax: in the female, on those of the ante-
penultimate pair.

The two sexes may be at once distinguished
from each other, apart from these apertures, by
the following characters. The transverse diameter
of the abdomen of the J. exceeds, at its widest
part, that of the cephalothorax ; it is the reverse
for the (£. The first two pairs of abdominal ap-
pendages are especially modified for purposes
accessory to reproduction, in the 5 ; in the .f the
second pair are normal, the first are either very
small or absent.

3 The apertures of the excretory organs or green
glands; each on the summit of a tubercle, borne
upon the under surface of the basal joint of the
antenna.

«. The auditory apertures; on the flattened upper
surfaces of the basal joints of the antennules.

These can be better examined when the ap-
pendages on which they are situated have been
removed.

B. The exoskeleton and appendages.

Note that the exoskeleton forms a continuous invest-
ment for the whole body (axial portion) and its ap-
pendages (appendicular portion] \ it is for the most part
calcified, but it remains soft and flexible where freedom
of motion is required.

ELEMENTARY BIOLOGY. [CHAP.

The typical abdominal segment.

Isolate, by carefully dissecting it away from its
fellows, the third abdominal segment and its ap-
pendages. Pick away its contents and examine —

a. The segment proper: arched above; flattened
below. It may be subdivided into

a. a dorsal tergum, the anterior smooth portion of
which is overlapped by the preceding segment,
in extension of the abdomen.

(3. The sternum; that portion of the ventral surface
which lies between the points of attachment of
the appendages.

y. The epimeron: the portion of the ventral surface
which lies on each side immediately external to
the attachment of the appendage.
This region is very short and passes almost
directly into the inner wall of

8. \h.& pleuron; a free wing-like downgrowth of each
lateral wall of the somite, seen, in section, to
be formed by both tergum and epimeron. Note
the smooth anterior half of the pleuron; it is
overlapped by the one in front during flexion of
the abdomen, much as is the tergum during
extension.

e. The articular facets; small tubercles developed,
one on either side, from the anterior edge of the
base of the tergum, immediately above the
pleuron. They fit, during life, into correspond-
ing recesses of the posterior border of the tergum
in front.

b. The appendages or swimmerets, one on each side:
the structure of each —

IT] THE CRAYFISH AND LOBSTER. 193

a. The short two- jointed basal portion (protopo-

dite\ consisting of a shorter proximal and a

longer distal piece.
(3. The flattened lamellae attached to the distal

joint of the protopodite, an inner (endopodite)

and outer (exopodite).

2. The structure of the cephalothorax.

a. Note again the carapace, with its frontal spine and
cervical suture.

b. Turn the animal over and note the narrow sterna
of the thorax; they are laterally compressed, as the
result of the great development of the basal joints
of the appendages.

The sternum of the last thoracic somite is not
completely ankylosed with the one in front in the
Crayfish. In the Lobster it is.

c. Raise with a pair of forceps the free edge of the
lateral expansion of the carapace which overlies the
bases of the thoracic appendages: note that it is
formed by the large united pleura of the thoracic
segments, and overlaps a chamber in which the
gills lie. It is termed the Iranchiostegite ac-
cordingly; its ventral edge is smooth in the Cray-
fish, it is notched metamerically in the Lobster.

d. Make a transverse section across the thorax, im-
mediately behind one of the larger appendages;
boil for five minutes in weak solution of Caustic
Potash and pick away the soft parts, until quite
clean. Examine under water, and note, as com-
pared with the corresponding section across the
abdominal segment —

M. 13

194 ELEMENTARY BIOLOGY. [CHAP.

a. The sternum; its middle portion is laterally
compressed (cf.^.). It gives off (for each seg-
ment) two ingrowths or endosternites, which arise
close together, immediately internal to the bases
of the appendages.

ft. The tergum; arched above, prolonged down on
either side to form the outer limb of the
branchiostegite.

If examined carefully, there will be seen
arising from the cervical groove, close together
near the middle line, two small tubercles or
endotergites. They are most fully developed in
the Lobster.

y. The epimera. Each is vertically elongated —
pushed up as it were upon itself — to form the
inner wall of a great chamber (branchial
chamber) lying under cover of the branchio-
stegite. From it the inner limb of the branchio-
stegite is derived.

Note the endopleurites ; ingrowths of the op-
posite epimeral walls, which abut against the
upper and outer borders of the endosternites.
These, with the endosternites and endotergites
above mentioned, are, one and all, ingrowths
of the exoskeleton, developed in connection
with muscular attachment. Cf. Sect. C. 2.
8. The branchiostegite; its outer limb is dense and
calcified, its inner one is very delicate, largely
chitinous and hirsute.

3. The telson.

Its whole tergal area is densely calcined, its sternal
one is largely chitinous. It bears, in both Lobster

II.] THE CRAYFISH AND LOBSTER. IQ5

and Crayfish, two lateral spines; in the latter animal
it is segmented, at the point of origin of these, into
two portions — an anterior one, the tergal surface of
which remains largely chitinous, and a terminal post-
anal one, which is completely calcified.

The perianal area is calcified on opposite sides, to
form a couple of anal plates; they are most marked
in the Lobster.

Note the delicate seta, which fringe the free border
of the telson.

4. The appendages.

Remove the entire set from one side of the body in
the order enumerated below, cutting through the
inter-articular membranes close to their points of
attachment to the axial skeleton.

Examine under water, posterior face upper-
most.

a. The third abdominal appendage. Its general charac-
ters have been described (i. A); examine in
detail —

a. Its protopodite; made up of two segments — a
shorter proximal hip-segment or coxopodite, and
a long distal one (basipodite) which forms a base
of articulation for the exopodite and endo-
podite.

y8. Its endopodite (the longer of the two terminal
portions). It is subdivided into two segments
of equal length — the basal one is continuously
calcified, the terminal one is multi-articulate.

y. Its exopodite; relatively shorter than (3. It also
is subdivided into a single basal and a terminal

13—2

196 ELEMENTARY BIOLOGY. [CHAP.

multi-articulate segment ; the former however is
but a fourth the length of the whole.

8. Examine under a low power, and note that the
calcifications of the multi-articulate segments
are incomplete; they are restricted to their
outer borders. Both exopodite and endopodite
will be found to be fringed with delicate setae.

The above description applies equally to both
Lobster and Crayfish, except that in the former
animal the whole appendage is more foliaceous
and paddle-like.

b. The second maxillipede (second appendage in
front of the great chela), as compared with a.

a. Its protopodite; its two segments are short and
equal in size. The distal one furnishes the base
of articulation for both exopodite and endopo-
dite; the proximal one is prolonged out into a
delicate lamella or epipodite (not represented in
a) which bears a well-defined gill.

fi. Its endopodite. This now forms the main por-
tion of the whole limb, it is subdivided into
five segments; a basal ischiopodite ; an elongated
laterally compressed meropodite; a small carpo-
podite; an expanded propodite, and a short
terminal dactylopodite. The inner edge of the
dactylopodite is beset by a series of sharp
spines ; the rest of the endopodite is partly
fringed in setag.

y. Its exopodite, long and filamentous; its structu-
ral features recall most nearly those of the
endopodite of the abdominal appendage.

II.] THE CRAYFISH AND LOBSTER. 197

The above description applies equally to both
Lobster and Crayfish, except that in the former
animal the epipodite and gill are in no way con-
fluent.

c. The third maxillipede. Its protopodite ; much as in
£, except that its distal segment is ankylosed with
the basal one (ischiopodite) of the endopodite.
Its endopodite; greatly increased in size and im-
portance, as compared with that of b. The five
segments enumerated for b can be recognized ; the
ischiopodite is the longer of the series, the others
become relatively shorter in proportion as the free
end of the appendage is approached. The inner
edge of the ischiopodite is beset by a single series
of crushing teeth. Its exopodite; structurally identi-
cal with that of £, but considerably shortened up.

Note the presence of a tuft of long setae (coxo-
poditic seta) arising from the base of the epipodite.

In the Lobster, the ischiopodite bears two rows of
teeth and the meropodite one row.

d. The great chela ; much larger and more powerful
than the last appendage, but resembling it in
general structure, and in the ankylosis of its ischio-
podite and basipodite ; it also carries a gill.

The exopodite is entirely suppressed. The
ischiopodite is relatively short, the meropodite and
propodite being, as in b, the longer of the series of
segments. The propodite; greatly enlarged and
prolonged outwardly to form, with the dactylopo-
dite, an opposable forceps (chela).

e. The four posterior thoracic appendages (ambulatory
appendages].

198 ELEMENTARY BIOLOGY. [CHAP.

All are, like d, destitute of exopodite. All are
elongated, the proportionate lengths of the several
segments being identical with those of d. The first
and second pairs are chelate, and the first three
bear both a gill and an'epipodite.
The fourth; destitute of both gill and epipodite:
when at rest it is backwardly directed.

In the Lobster, the gill borne by each of the above
appendages is, like that of b, distinct from the epi-
podite.

The genital orifices are borne upon these ap-
pendages. They have been described at A. c. y.

/. The abdominal appendages, other than the third pair
(described at a).

a. The fourth and fifth pairs : closely resembling
the third, functional as swimmerets.

ft. The sixth pair, modified to form, together with
the telson, a tail-fin. The protopodite : repre-
sented by a single short strong segment. (In
the lobster there is a second incomplete basal
segment.) The exopodite and endopodite :
wide plates fringed with setae. The exopodite ;
divided into two portions by a transverse joint :
the free edges of its proximal portion are markedly
serrated. The endopodite ; continuously calci-
fied; its proximal internal surface is beset, in
the Crayfish, by a patch of short setae, which
play, during life, upon a corresponding hirsute
area of the under surface of the telson.

y. The second pair. Cf. Sect. A. c. y.

Closely resembling the third in the female. In
the male the endopodite is much modified; its

II.] THE CRAYFISH AND LOBSTER. 199

terminal multi-articulate segment being shortened
up, in proportion as its basal one is elongated.
The latter is here at its maximum for the whole
abdominal series; it is produced up into an
accessory piece, which is segmented off and
modified to form a plate, rolled upon itself so
as to enclose a demicanal, concave inwardly.

In the Lobster, this accessory process of the
basal segment of the endopodite is still more
marked.

8. The first pair; in the female rudimentary, ex-
ceedingly variable in size and not unfrequently
absent altogether : except in very rare cases the
exopodite is suppressed. In the male the exo-
podite is invariably absent, the protopodite and
endopodite become ankylosed and terminate in
a plate rolled upon itself. In the Lobster the
terminal division differs slightly from that of the
Crayfish.

g. The first maxillipede. Its protopodite, flattened
and foliaceous, its two segments well defined; its
exopodite, substantially identical with that of a;
its endopodite, reduced to a small two-jointed
structure, lying under cover of the basal joint of
the exopodite; its epipodite, fully developed but
destitute of a gill.

In the Lobster, the reduction of the endopodite is
far less marked.

h. The second maxilla. Its protopodite, foliaceous
like that of g, but pectinated internally ; its endo-
podite, elongated and filamentous, its free end is
recurved to form a hook-shaped process which,

200 ELEMENTARY BIOLOGY. [CHAP.

during life, is received into a recess of the mandible.
Its epipodite; well developed, and united in front
with a lamina, which probably represents the exopo-
dite, to form a wide oval plate (scaphognathite) which
lies at the anterior end of the gill-chamber (Cf.
Sect. H. c. and p. 182).

If the above appendage be examined in the living
animal, it will be found that with every movement
of the scaphognathite there is a corresponding pull
upon its filamentous endopodite.

i. The first maxilla. Epipodite and exopodite un-
developed. The endopodite is reduced to the
condition of a small squame, its basal segment
(ischiopodite) being enlarged and foliaceous1 con-
stituting the main mass of the whole appendage.
Its protopodite is foliaceous and single jointed
(pasipodite), blade-like and recurved, with its free
end inserted, during life, into the oral aperture.
(Cf. Sect. E. 6 d.)

In the Lobster, the endopodite is large and seg-
mented.

k. The mandible. Its strong toothed basal-joint (cox-
opodite) bearing a small appendage (the palp) which
represents the basipodite and endopodite1, epipo-
dite and exopodite unrepresented.

/. The antenna. Its two-jointed basal-portion (pro-
topodite] bearing a flattened protective squame (the
modified exopodite) and a long multi-articulate
filament (the endopodite), the two basal segments of
which are greatly enlarged and modified for pur-
poses of articulation. Note the opening of the
green gland (Cf. Sect. A. c. 8).

1 As seen on comparison with the same in certain of the lower
Crustacea.

THE CRAYFISH AND LOBSTER. 2OI

The distal segment of the protopodite (basipo-
dite) is subdivided into two ; the outer segment is
squamous in the Lobster.

;;/. The antennule. Its large three-jointed basal seg-
ment (protopodite), bearing a pair of multi-articulate
filaments (endopodite and exopodite) : the opening of
the auditory organ, in the midst of a tuft of setae
on the upper surface of the basal joint. The exo-
podite, the longer of the two jointed filaments, is
carried erect during life (it bears sensory setae.
Cf. Sects. A. c. «. and K. 2).

n. The ophthalmite or eyestalk. A short two-jointed
structure which appears to represent the protopo-
dite of an appendage. (Cf. Sect. K. 3 c.)

Remove that half of the body from which the ap-
pendages have been dissected, thus reducing the
whole to the condition of longitudinal vertical section;
boil for a few minutes in weak solution of Caustic
Potash, pick away the soft parts and examine from
within.

a. The inter-articular membranes ; flexible, persistently
uncalcified, portions of the exoskeleton. Note that
(in the Crayfish) between the two last thoracic
sterna. (Cf. Sect. B. 2 b.)

b. The endophragmal system. Each set of endoster-
mites and endotergites of which it is composed (cf.
Sect. B. 2d.) are seen to arise in the same plane;
each endosternite slopes forwardly, carrying with it
the anterior limb of its corresponding endotergite,
the posterior limb of the one in front passing back to
meet it. The points of apposition of the successive
sets of elements alternate with those of their origin.

202 ELEMENTARY BIOLOGY. [CHAP.

c. The pro-cephalic process; a tongue-shaped ingrowth
of the exoskeleton, at the base of the rostrum.

d. The thoraco-abdominal link work; a somewhat com-
plicated system of bars, passing between the last
two thoracic sterna and a special process of that of
the first abdominal segment.

e. The aperture of the mouth; bounded by the man-
dible— note the natural relations of the latter. The
upper-lip or labrum and the lower-lip or metastoma
will be seen, as uncalcified prolongations of the
sternal skeleton, situated, respectively, in front of
and behind the mouth. Compare the same as
looked at from beneath, in an uninjured specimen —

a. The labrum ; a shield-shaped plate calcified mar-
ginally.

/?. The metastoma; composed of a median fold, pro-
duced on either side into a blade-like process,
which simulates the basal joint of the first maxilla.

/ Note the limits of the several segments of the axial
skeleton, and the mode of articulation of each upon
its fellow and of the appendages upon each.

6. The histology of the exoskeleton.

a. Make a thin longitudinal-vertical section of any of
the inter-articular membranes, and examine in water
under a high power. It consists of a highly elastic
membrane, traversed longitudinally by regularly
disposed clear striae; if pressure be applied to the
cover-slip, the layers obvious as the above-named
striae will be found to be denser and more resistant
than the intervening ones, i.e. the whole is composed

THE CRAYFISH AND LOBSTER. 203

of chitinous material, deposited in layers, alternately
denser and less dense.

Compare a similar section of any calcined portion
of one of the appendages, ground down on a hone
(see Appendix E.). A similar longitudinal striation
is obvious.

Examine the peripheral area; the striae — but a
tenth the diameter of those seen in the main mass —
are closely aggregated ; the free surface is invested
in an uncalcified epiostracum. The whole is per-
meated by a close set series of wavy pore-canals^
usually filled with air in the process of manipula-
tion, and rendered thereby highly refractive.

Compare tangential sections, made in a similar
manner. The pore-canals will be obvious as minute
black-dots, closely (in places irregularly) aggregated
together.

An aggregation of the stride, identical with that
described above, is frequently met with on the inner
side also.

Cut a longitudinal vertical section of a piece of the
exoskeleton, together with the underlying in-
tegument, which has been decalcified by treatment
with i§ chromic acid solution. Note, in addition
to the structural features already described —

x. The seta; each is an uncalcified outgrowth,
usually arising at the base of a shallow pit.
They vary greatly in size and detailed structure in
different parts of the body ; the commoner forms
end in a pointed extremity, fringed with lateral
hairlike filaments.

204 ELEMENTARY BIOLOGY. [CHAP.

P. The integument ; largely composed of branched
nucleated granular cells : the outermost giving
off a large number of short processes which pene-
trate a short way into the exoskeleton.

Prolongations of it extend into the setae and
into all skeletal outgrowths.

The ischiopodite of the third maxillipede may
be preferably utilized, as, in it, all gradations from
seta to spine and from spine to tooth can be
readily followed.

C. The muscular system.

1. Detach one of the great chelae from the body and lay
bare its interior. The enlarged base of the propodite
will be found to lodge two muscles, which pass be-
tween it and the dactylopodite ; they are —

a. A larger adductor, arising from the outer side.

b. A smaller abductor, arising from the inner side.

2. Remove the superficial portions of the two muscles ;
there will be found lying within each a central tendon —
a plate-like ingrowth of the exoskeleton, towards which
the muscular fasciculi converge. Remove all the soft
parts and examine —

a. The tendons in relation to the dactylopodite;
they are ingrowths of the opposite sides of its
base.

fi. The articular facets and interaxticular membranes;
so arranged as only to admit of motion in one
plane (abduction and adduction.).

3. Examine the remaining segments of the appendage.
Note that—

DIVERSITY
II.] THE CRAYFISH AND LOBSTER. 2O$

a. The relations between any two of them are identical
with those described above, motion being possible
in only one plane.

b. The individual segments are incapable of rotation
upon each other. The limb as a whole can be
rotated; owing to the variation in the angles of
inclination of its several joints.

4. Make a median longitudinal section of the whole
body, similar to that described in Sect. B. 5; pick
away the viscera under water, wash and examine

a. The flexor abdominis muscle ; a powerful mass, ex-
tending from the anterior border of the thoracic
epimeron and endophragmal system to the base of
the telson. It arises by a series of slips, which
wind round each other in a highly characteristic
manner and are inserted into the abdominal sterna.

b. The extensor abdominis; a much less powerful
muscle, arising immediately above a. Its fasciculi
are inserted into the abdominal terga.

c. The levator abdominis ; arising, immediately above
the extensor, from the antero-dorsal border of the
thoracic epimeron. It passes obliquely backwards,
to be inserted, laterally, into the first abdominal
segment.

This muscle is continuous with a series of fibres
which skirt the entire anterior border of the thoracic
epimeron and attach it to the cervical groove.

Note that the parts of the axis are, like those of
the appendages, capable of motion only in one plane.

d. The adductor muscle of the mandible ; a powerful
fan-shaped mass, passing obliquely upwards from

2O6 ELEMENTARY BIOLOGY. [CHAP.

the hinder border of the mandible, to be attached
to the cephalic shield, slightly in front of the cervical
groove. Note its powerful tendon.

Small muscles will be seen, passing from the man-
dible and antennary organs to the adjacent exo-
skeleton.

e. Lay bare the basal portion of the great chela, and
note the relations of its muscles. They arise from
the endophragmal system, and are attached to
tendinous ingrowths of the appendage itself similarly
to those described in § 2.

D. The general disposition of the viscera.

i. Place the animal on its ventral side and pin down
under water, inserting the pins through the tail fin
and bases of the chelae j remove the whole tergal
skeleton and with it the extensor abdominis muscle.
Note in order —

a. The alimentary canal ; a straight tube, running the
whole length of the body in the middle line. It is
greatly enlarged in front to form the so-called
stomach, which fills the greater portion of the
cephalon.

b. The heart ; a slightly yellowish pentagonal organ of
delicate texture, lying above a in the middle thoracic
region.

c. The genital gland. In the female, a yellowish brown
mass ; in the male, a whitish tongue-shaped mass ;
lying immediately beneath b.

It varies in size and character with the season of
the year. (Cf. Sect. G.)

d. The genital duct. In the male; a highly convoluted

II.] THE CRAYFISH AND LOBSTER. 2O/

dead-white tube, immediately external to c. In the
female; a short membranous tube, only visible at
this stage with difficulty.

e. The digestive gland; a yellow pulpy-looking mass,
filling up the interspaces in the cephalothorax ; it
is most obvious in front of the genital apparatus.

/ The excretory organ (green-gland) ; a paired delicate
green structure, lying, at a low level, in the extreme
anterior end of the cephalic cavity. It can be at
once seen on slightly displacing the stomach.

g. Note, incidentally.

a. The adductor mandibuli muscle; a large oval

fleshy mass, lying immediately external to the

stomach (cf. Sect. C. 4^.).
P. The extensor abdominis muscle ; its cut ends will

be found, attached to the thoracic epimera.
y. The gastric muscles ; passing from the stomach to

the adjacent exoskeleton (for details see Sect. E).
8. The integument (hypodermis) ; a delicate dark red

layer, lying immediately under cover of the

exoskeleton.
€. The body-cavity; obvious as an ill-defined chamber,

within which the various organs of the viscera are

contained.
£. The respiratory organs ; seen lying within the

branchial chamber ; they are exclusively thoracic.

Follow the cut edge of the branchiostegite.

E. The alimentary organs.

i. Dissect from the tergal aspect, as directed for Sect. D.,
and remove the heart and reproductive apparatus.
Examine —

208 ELEMENTARY BIOLOGY. [CHAP.

a. The stomach ; it is marked off by a transverse con-
striction into a large cardiac chamber and a small,
posterior, pyloric one.

b. The intestine, a straight tube leading back to the
anus; its wall is thrown into a series of shallow
folds, which take a longitudinal and slightly spiral
course.

c. The ccecum ; a median dorsal upgrowth, immediately
behind the stomach. It is directed obliquely for-
wards.

In the Lobster, this is situated far back, near the
anus.

2. Some small muscles will be seen attached to the roof
of the stomach. Remove these. In doing so, note
that there comes away with them the soft cellular wall
of the viscus, under cover of which there is seen a
chitinous lining. Examine the latter with care ; it will
be found to be calcified to form a series of ossicles,
related as under.

a. The antero-dorsal (cardiac) ossicle ; a transverse bar,
extending across the roof of the cardiac chamber.

b. The postero-dorsal (pyloric) ; similarly related to the
roof of the pyloric chamber as is a to that of the
cardiac one.

c. The antero-lateral (lateral-cardiac) ossicles; two
small hammer-shaped pieces, abutting, one on either
side, against a.

d. The postero-lateral (lateral-pyloric) ; two long bars,
similarly related on either side to b as are c to a.

They extend forwards to meet the antero-lateral
ossicles.

TH£ CRAYFISH AND LOBSTER. 2OQ

e. The antero- median (urocardiac) ossicle; a long
tongue-shaped bar, passing back, from the antero-
dorsal one, in the middle line of the roof of the
cardiac chamber.

/ The postero-median (prepyloric) ; a similar but
shorter bar, arising from the postero-dorsal piece.
When the parts are at rest, it passes obliquely down-
wards and backwards, meeting the antero-median
under cover of the postero-dorsal.

It will be observed that the above-named parts
form a repetitional series ; the roof and side walls
of each chamber are calcified to form a dorsal trans-
verse, a median, and a pair of lateral ossicles, the
two latter sets articulating with each other.-

g. The digestive gland; a paired mass, roughly trilobed,
on either side. It is an aggregation of short
cceca.

Carefully dissect away its upper half on one side,
until a spacious central duct is reached, around
which the glandular cceca are seen to be arranged.
Wash until quite clean, and look for their orifices.

Follow the duct inwardly ; it enters the head of
the intestine by a wide aperture, immediately in
front of the crecum (i. c).

3. Place the animal on its side and remove the exposed
half of the body and the digestive gland, so as fully
to lay bare the entire alimentary canal.

The cellular wall and certain muscles will be seen
as before; remove these, and examine the chitinous
lining and its associated structures. Note, in addition
to the parts described above, the following.

M. 14

2 10 ELEMENTARY BIOLOGY. [CHAP.

a. The gastrolith ; a discoidal stony-mass, interposed
between the cellular and cuticular layers of the
anterior cardiac wall.

This is developed only in the summer season ; an
underlying thickening of the chitinous cuticle is in-
variably present to mark its position.

b. The postero-ventral ossicles ; delicate vertically elon-
gated bars in the hind wall of the cardiac sac.

c. The accessory lateral ossicle; a small bar, extending
backwards and downwards from the point of junc-
tion between the two main lateral ones.

d. The intestine, the orifice of the digestive duct and
the coecum. (Cf. supra.)

4. Reduce the whole gut to the condition of median
longitudinal section, by carefully removing the exposed
half with scissors ; wash until quite clean and examine
under water.

a. The gullet ; a spacious tube placing the stomach in
communication with the exterior. Note its re-
lations to the mouth.

b. The median cardio-pyloric valve ; seen, in section,
as an upgrowth constricting the passage from the
cardiac to the pyloric chambers.

c. Follow the cut edge of the chitinous lining of the
stomach; note

a. That it is continuous at the oral aperture with
the exoskeleton.

ft. That it ends abruptly, in front of the orifice of
the digestive duct; terminating in a series of
valve-like processes, one of which can be very

II.] THE CRAYFISH AND LOBSTER. 2lt

readily seen to project backwards and down-
wards immediately in front of the intestinal
ccecum.

d. Examine, in like manner, the cuticular lining of the

intestine,
a. It is continuous with the exoskeleton, at the anal

orifice.
/?. It terminates abruptly, in a thickened border,

immediately posterior to the ccecum.
y. Examine it under a lens ; it is folded longitudinally
and slightly spirally and beset by a number of
short papillae.

€. Examine the middle segment of the alimentary canal
(head of the intestine) : it is destitute of chitinous
lining; it bears the ccecal appendage and receives
the digestive ducts.

f. Examine the interior of the stomach and note the
characters and relations of the gastric teeth.

a. The median tooth ; a. triangular red-brown struc-
ture, set on at the point of apposition of the two
median ossicles.

It will probably be seen in section.

ft. The lateral tooth ; larger but similar in character,
and situated (when at rest) in front of and below
a. It is carried by the postero-lateral ossicle.

y. The accessory lateral tooth; a small denticle, borne
upon the posterior end of the accessory lateral
ossicle (§ 3. c).

g. The lateral cardio-pyloric valve; a thickening of
the side wall of the stomach, immediately below

14—3

ELEMENTARY BIOLOGY. [CHAP.

the lateral tooth. Examine it under a powerful
hand lens ; it is beset by short stiff setae.

5. Carefully remove the alimentary canal, cutting the
gullet through close to the stomach; slit open longi-
tudinally from beneath, and examine the interior under
water.

a. The crushing surfaces of the lateral teeth ; note that
they approximate anteriorly, being obliquely set.

b. Seize the opposite ends of the stomach between two
pairs of forceps and pull them apart. Note that
with the divarication of the cardiac and pyloric
ossicles the median tooth is elevated (depressed in
life) to meet the lateral ones, which are at the
same time rotated inwardly and slightly backwardly.

c. Examine the free border of the chitinous lining;
the pyloric valves (4. c, fi) can be well seen from
this aspect.

6. Isolate the stomach of a fresh specimen, together with
the adjacent mouth organs. Dissect from the front and
remove the anterior half, cutting through the gullet
below and the cardiac ossicle above. Wash out the
food contents and examine under water. Note —

a. The median and lateral cardio-pyloric valves
(§ 4. b and g) ; they form an efficient straining ap-
paratus.

b. The gastric teeth. Note that they meet, when in
action, wholly in front of a.

c. The lesser cardio-pyloric valves ; three small setose
eminences, on either side, adjacent to the lateral
teeth.

II.] THE CRAYFISH AND LOBSTER. 213

d. Note incidentally —

a. The metastoma ; its lateral lobes are seen to be

outwardly directed.

/?. The basal joints of the first maxillae; two blade-
like structures, curving round the bases of a to
reach the gullet (cf. Sect. B. 4. i).
y. The relations of the gastrolith, if present.

7. The Gastric muscles. All are paired, those of one
side are alone described. Dissect the cephalo-thoracic
region from the side, removing the exoskeleton and
body-wall to the level of the pro-cephalic process
(p. 202). Examine, in the order given, the extrinsic mus-
cles. A spirit specimen may be used with advantage.

a. The anterior and posterior gastric muscles (divari-
cators of the gastric skeleton). Those of the ex-
posed side will be seen — the anterior ; arising from
the roof of the cardiac chamber (cardiac ossicle)
and inserted into the pro-cephalic process : the
posterior ; arising similarly from the roof of the
pyloric chamber (pyloric ossicle) and inserted into
the carapace, immediately in front of the cervical
groove.

b. The kvator muscles.

a. A conical mass, arising from the roof of the py-
loric chamber, immediately in front of the ccecum.
It is attached to the cervical groove (endotergite,
Sect. B. 2. d. ft).

ft. Two thin strips, arising from the side wall of the
cardiac sac, and inserted above into the mid-
cephalic region of the carapace. They cross
each other on nearing their insertions.

214 ELEMENTARY BIOLOGY. [CHAP.

c. The depressor muscles. Three sets are obvious —

a. Two delicate strips, arising from the cardiac sac,
immediately above the gullet. They pass ob-
liquely forwards and downwards, converging as
they do so, to be attached to the sternal skeleton
immediately in front of the labrum.

/?. A small slip, arising from the extreme posterior
wall of the cardiac sac, and passing downwards
and inwards to unite with its fellow of the oppo-
site side, prior to its attachment to the anterior
end of the endophragmal system.

y. A long slip, arising from the ventral wall of
the pyloric sac, and passing between /? obliquely
forwards and outwards. It is inserted close to
the base of the gullet.

d. The dilator muscles ; small fan-shaped tracts, arising
from the gullet and attached —

a. the anterior ones, to the adjacent exoskeleton ;
(3. the posterior, to the endophragmal system.

The upper anterior one may receive a depressor
slip.

The intrinsic muscles. Carefully dissect off the le-
vator and depressor fibres described above and ex-
amine—

e. The great constrictor ; a large sheet investing the
postero-ventral half of the cardiac sac (a seeker can
be readily inserted beneath it). Its fibres are seen
to be interrupted by the intervention of the postero-
ventral ossicles (3. b} to which they are attached.

f. The lesser constrictors ; of these there are two sets.

II.] THE CRAYFISH AND LOBSTER. 21$

a. Two tracts of fibres, arising from the head of the
postero-ventral ossicle and attached — the slender
ventral one to the base of the antero-lateral
ossicle ; the fan-shaped dorsal one to the postero-
lateral bar.

fi. A sheet of fibres, investing the side wall of the
pyloric chamber and usually broken up into three
fasciculi.

The above-named constrictor muscles exercise
a direct control over the cardio-pyloric and other
" valves " of the straining apparatus.

y. The cardio-pyloric muscles ; two delicate sheets,
seen, on carefully removing the cellular roof of the
stomach, to pass obliquely forwards and inwards
from the lateral-pyloric to the cardiac ossicles.

g. The anal muscles ; extremely delicate tracts, passing
between the anal valves and the immediately ad-
jacent wall of the intestine.

8. The digestive gland. Its general characters have
been previously described.

a. Tease out a bit in water ; it is made up of ccecal
tubes, which are seen to be lined by a large celled
epithelium.

b. Submit a small piece to the action of i °/0 solution
of Osmic Acid until well blackened. Examine in
water under a high power ; many of the cells will
be found to contain a fatty product, deposited in
globules, blackened under the action of the reagent.

c. Prepare (as directed in the Appendix) transverse
sections of the frozen gland ; transfer them, as cut,

2l6 ELEMENTARY BIOLOGY. [CHAP.

to a glass slide bearing a drop of Osmic Acid solu-
tion; wash, when sufficiently stained, and examine
under a high power.

Note that the cells are arranged in aggregates
and flattened, as the result of mutual compression.
Look for —
a. Hepatic cells ; characterized by the presence of

fatty globules, differentiated as at b. Their nuclei;

rounded or oval, each frequently surrounded by

a clear refractive halo.

ft. Ferment cells ; each containing a single large
secretory globule, which does not decompose the
Osmic Acid. Their nuclei ; larger than those of
a and oval, they usually contain a single nu-
cleolus.

The larger of these cells (the older of the series)
will be found in or near the middle of each group.

d. Compare similar sections of material, hardened in
alcohol and stained with borax-carmine. The nuclei
of the hepatic cells stain with great intensity. Look
for stages in the elaboration and discharge of the
secretory product of the ferment cell.

F. The excretory-organ or green gland.

i. Dissect from the left-side, and remove the greater part
of the exposed cephalic shield. The entire organ lies
in the extreme front part of the cephalo-thoracic
cavity, immediately in front of the gullet (cf. Sect. D./).
It consists of —

a. The glandular segment ; a soft greenish mass, po-
sition as above.

b. The muscular segment ; a distensible sac, overlying

II.] THE CRAYFISH AND LOBSTER. 2 1/

a. It may be rendered the more obvious by in-
flation from the excretory orifice (Sect. B. 4. /.).

Having inflated in, lay open the basal joint of
the antenna ; on entering this, the neck of the sac
becomes constricted and duct-like.

2. Tease up a small portion of the glandular segment in
eosin solution; examine under a high-power. It is
largely composed of a coiled tube, lined by a square-
celled epithelium.

G. The reproductive organs.

These differ in detail, in the Crayfish and the Lobster.
They lie partly ventral to the heart; their general re-
lationships have been described (Sects. A. and D.).
Dissect the cephalo-thoracic region from the tergal
aspect, and remove the heart. Examine the genital
apparatus in situ.

i . The male organs.

a. The testis. A median greyish-white mass, bilobed
anteriorly.

b. The two vasa deferentia, arising where the posterior
lobe of the testis meets the two anterior. Each is
narrow near the gland; its calibre increases as it
proceeds back from it, and, becoming extremely
convoluted, it finally ends at the genital opening on
its own side (Sect. A. c. y).

These ducts generally contain a dense milk-white
product, secreted by their glandular lining (sper-
matophoral glands). (Cf. Sect. L. i. b.)

In the Lobster, the testes are two long tubes which
extend back into the abdomen. Their posterior
portions meet in the middle line, but in front they

2l8 ELEMENTARY BIOLOGY. [CHAP.

diverge, and at about one fourth the length of each
from its anterior end a short transverse branch unites
the two. The vas deferens arises a little in front of
the middle of each testis and passes without con-
volution towards the genital opening. Its distal half
is dilated.

c. Tease out a bit of the testis in water, and examine
with J- obj. : it will be seen to be composed of
glandular tubes terminating in grape-like saccula-
tions. In it or in the vas deferens ripe spermatozoa
may be found : they are motionless and have the
form of discoidal cells provided with long radiating
processes. Stain with eosin, each will be seen to
contain

a. The annulate corpuscle; a transparent ring, lying

within the middle of cell body.
/3. The oval corpuscle ; a granular mass, lying to one

side of a.

In the Lobster the spermatozoa are also motion-
less ; each consists of an elongated cell, from one
end of which three rigid pointed processes radiate.

2. rY\\Q female organs.

a. The ovary ; in shape very similar to the testis of
the male, being bilobed anteriorly.

b. The oviducts ; two short ducts which, like the vasa
deferentia of the male, are directly continuous with
the genital gland. They pass directly downwards
to the genital openings, and can be at once seen on
displacing the anti-penultimate walking legs to one
side (Sect. A. c. y).

II.] THE CRAYFISH AND LOBSTER. 219

In the Lobster the ovaries are elongated and pro-
longed into the abdomen. Each is a dark green
mass, and near their anterior ends the two meet in
the middle line and remain confluent for a short
distance. An oviduct arises from each ovary a little
in front of its middle, and passes directly to the
genital opening of its own side.

3. The genital organs, having been first examined in
situ as above directed, may be profitably compared
side by side, after removal from the body, together
with their related appendages and thoracic sterna.

A highly instructive view of them may be obtained
from the side, dissecting as directed for Sect. I. 3.

4. Lay open the ovary from the dorsal aspect and wash
carefully until quite clean. Examine under water

a. Its wall ; directly continuous with that of the ovi-
duct.

b. Its central cavity ; similarly continuous with the
lumen of the oviduct.

c. The ovisacs ; spherical masses developed within the
wall, they are sometimes irregular in shape owing
to mutual compression.

Examine under a hand-lens. They vary in size,
the younger being very minute white bodies irregu-
larly scattered in groups throughout the mass ; re-
move a few of these, and transfer to a glass slide.
Stain with magenta or eosin and examine under a
low power, avoiding pressure.

a. The ovum ; a single large cell, filling the entire
ovisac. Note its highly granular vitellus ; its
large round germinal vesicle containing a number
of germinal spots.

220 . ELEMENTARY BIOLOGY. [CHAP.

Examine the peripheral portion of the youngest

ovisac under a high power. Look for —
/?. The vitelline membrane; transparent and well

defined, closely investing the ovum,
y. The follicular epithelium ; a single layer of small

clear cells investing the whole ; they are slightly

flattened.

8. The membrana propria ; a delicate cuticle in-
vesting y.

Look for very young ovisacs. Note the rela-
tively small size and simple structure of the
ovum, as compared with the older egg-cell;
there is greater uniformity in size between it and
the cells of the follicular epithelium.

H. The respiratory organs. Remove the branchiostegite
of one side and examine the gills : the functional pnes
are 18 in number, arranged in three sets.

1. Examine the branchial- chamber; it is bounded in front
by the cervical-groove, foremost among its contents
are —

a. The epipodites of the thoracic appendages ; seven in
number, all upwardly directed. The most anterior
one (that of the first maxillipede) is gill-less ; the
others bear each a gill (podobranchia).

b. The scaphognathite (Sect. B. 4. ti) lying in the lower
anterior region of the branchial chamber, immedi-
ately in front of a. Examine its base of attach-
ment ; compare the living animal.

2. Cut each of the gill-bearing epipodites across at its
base and remove thus the podobranchice as a set. Pin
down under water and examine any one in detail.

II.] THE CRAYFISH AND LOBSTER. 221

a. The epipodite ; a delicate chitinous lamina, folded
upon itself and pleated posteriorly.

b. The podobranchia ; made up of a series of gili-
filaments set upon a central stem.

i. Examine the same in transverse section. The lamina
of the epipodite and the stem of the gill are con-
tinuous.

Cf. the Lobster : the gill is free of the epipodite,
and the gill-filaments, which are very numerous, are
arranged in close set parallel series.

3. Examine the remaining 12 branchiae in situ, each con-
sists of a central stem giving off a number of delicate
filaments. There is nothing answering to the epipodite.
They are arranged in two sets —

a. The arthrobranchice ; attached to the inter- articular
membranes of the thoracic appendages, with the
exception of the first maxillipede and the last am-
bulatory leg. There are eleven of them, the odd
one being related to the second maxillipede.

b. The pleurobranchia —

a. Functional ; a solitary gill, attached to the inner
wall of the branchial chamber, immediately above
the last ambulatory appendage.

(3. Vestigial ; two to four delicate filaments (best
seen after removal of the arthrobranchiae) at-
attached anteriorly to and in a line with a.

In the Lobster the above are all functional.
There are, in that animal, 20 gills on each side ;
six podobranchiae and four pleurobranchias at-
tached as in the Crayfish, but only ten arthro-
branchise — there being no such gill related to the
second maxillipede.

222 ELEMENTARY BIOLOGY. [CHAP.

I. The circulatory organs.

1. Dissect away the greater portion of one branchiostegite,
so as to expose the branchial chamber and gills un-
injured. Place the animal on its side and drill a
small hole in the roof of the carapace near the heart ;
insert the point of a cannula into the same, inject
and examine under water. Upon displacing the
branchiae there will be seen —

a. The branchio-cardiac canals ; six well-defined chan-
nels, running parallel with each other immediately
beneath the inner wall of the branchial chamber.

b. Remove the podobranchiae and turn back the gills
remaining in situ. The stem of each will be seen
to lodge a vessel (efferent branchial vessel] — those of
a given set of branchiae uniting to form one branchio-
cardiac trunk.

c. Cut all the gills short and follow the branchio-
cardiac canals upwards. They converge towards the
region of the heart, to enter the pericardial sinus — a
spacious chamber now largely filled with injection.

As a rule, the trunks related to the ambulatory
appendages 2, 3, and 4 enter the pericardium in-
dependently : the last of them receives the posterior
trunk of the series, which brings in the blood from
the pleurobranchia alone ; the upper ends of the
three are connected by short anastomoses. The an-
terior trunk is formed by the union of those related
to the second and third maxillipedes and the chela.

2. Dissect a specimen which has been at least 24 hours
in alcohol, under water, from the tergal aspect, cutting
away carefully with a pair of scissors the mid-thoracic
portion of the carapace.

II.] THE CRAYFISH AND LOBSTER. 223

A chamber (the pericardial sinus) is thus laid bare
in which lies the heart (D. i. b\

a. The alary muscles ; delicate strands passing from
the angles of the heart to the adjacent pericardial
wall.

b. The two dorsal cardiac apertures; oblique openings
in the cardiac roof, guarded by valves.

c. Carefully dissect off the roof of the heart, and
note

a. Its wall ; thick and spongy, the muscular fibres
being arranged in irregular bundles.

ft. Its cavity; central and single ; i.e. there are no
auricles.

y. The ventral apertures and valves; identical in
their characters with b ; they are situated at the
anterior end, transversely to the long axis.

8. The lateral apertures ; similar perforations of the
middle region of the side walls. Dissect to the
level of one and examine its valves; they are
pocket-shaped, and so arranged as only to admit
of a passage of the blood from the pericardial
sinus into the heart.

d. Compare the heart of a similar specimen, after
removal from the body.

3. The great arteries. Obtain a fresh uninjured specimen
and pin down under water on its side; lay bare the
pericardial sinus with extreme care, and inject from
the lateral cardiac aperture. Work leisurely and
apply a steady gentle pressure.

Cut along the post-mandibular portion of the ex-
oskeleton to the level of the middle line (with the

224 ELEMENTARY BIOLOGY. [CHAP.

exception of the ambulatory appendage bearing the
genital orifice ; leave that in situ] and remove the
skeleton together with its related muscles, being careful
not to injure the digestive or reproductive organs.

Cut along the dorsal middle line of the cephalic
skeleton, and continue the incision downwards, clear
of the green-gland. Remove the hard parts, but leave
the adductor muscle of the mandible in situ. Dissect
away any extraneous tissue and wash carefully.

a. The superior abdominal artery ; a backward con-
tinuation of the heart, immediately above the
intestine. It distributes branches metamerically
to the intestine (hind-gut) and adjacent parts.

b. The ophthalmic artery; a similar forward continua-
tion of the anterior end of the heart. It runs over
the stomach and distributes branches to the eye
stalks and parts immediately adjacent.

t. The hepatic artery. Turn the exposed digestive
gland downwards ; branches of the above vessel
will be seen ramifying in it. Cut its main trunk
across and remove the gland, noting the branches
to the mid-gut ; the main trunk can now be traced
beneath the genital gland, to its origin, from the
antero-ventral region of the heart.

d. The sternal artery ; a large trunk, arising from the
heart at the base of a. It runs vertically down-
wards towards the ventral surface, passing to one
side of the intestine, and perforating the nervous
system (obvious as a delicate greyish median-ven-
tral cord. Cf. Sect. J.). It subdivides into
a. The antero-ventral artery ; a median trunk, ex-
tending forwards to the mouth region, and dis-

IT.] THE CRAYFISH AND LOBSTER. 225

tributing branches metamerically to the appen-
dages, muscles and parts adjacent.

/3. The postero-ventral (inferior-abdominat) artery ;
a median trunk extending back to the telson. It
also distributes branches metamerically to the
adjacent parts.

The above trunks furnish the main vessels to
the reproductive organs. They are, on either
side — a dorsal one, arising from the sternal artery
soon after it leaves the heart and distributed
to the genital gland and duct ; a ventral one,
arising from the base of ft, and distributed to the
terminal segment of the duct alone.

The sternal trunk may pass to either side of the
alimentary canal. If the injection be successful,
it will be found that the vessels distributed to the
ends of the intestine and genital ducts debouching
on to the exterior, terminate in special capillary
networks.

e. The antennary artery ; arising from the antero-
dorsal region of the heart, immediately external to
b. Its main trunk runs downwards and forwards,
passing round (external to) the adductor mandibuli
muscle and immediately under the carapace, to
terminate anteriorly in the antenna. It gives off
on its course —

a. The gastric artery ; arising close behind the pos-
terior gastric muscle, and distributed to the
stomach (fore-gut).

ft. Branches to the gastric muscles, several en route.
y. A rostro-antennary branch ; arising from the main

M. I

226 ELEMENTARY BIOLOGY. [CHAP.

trunk at the base of eye-stalk, and distributed to
the antennule and rostrum.

8. The renal arteries ; two or more branches break-
ing up on the anterior face of the green gland.

e. Other minor branches, which are variable.

4. Puncture any one inter-sternal membrane of a freshly-
killed animal ; blood will escape freely (a blood space
has been entered). Treat the inter-articular mem-
brane between the two last thoracic sterna similarly ;
insert the point of a cannula and inject.

a. Remove the branchiostegite and examine the
branchiae. The injection will have filled some of
the afferent branchial vessels ; large trunks running
(one for each gill) along the outer face of the stem
externally to the efferent vessel (§ I. b.}.

b. Make a transverse section across the thoracic region.
With a little care the afferent branchial vessels can
be traced to a great ventral sinus (sternal sinus)
now largely injected. It is in free communication
with the adjacent blood-spaces.

5. The blood. Obtain a drop of blood, from a freshly-
killed Crayfish and examine under a high power.

a. The plasma ; colourless.

b. The corpuscles ; colourless and amoeboid, each with
a large round nucleus. Their pseudopodia; fili-
form and frequently very numerous.

Watch their movements; they exhibit a great
tendency to run together into aggregates prior to
disintegration, upon exposure, after removal from
the body.

II.] THE CRAYFISH AND LOBSTER. 22/

J. The nervous system.

i. Pin the animal down tergal surface uppermost and lay
bare the whole body cavity. Remove the heart,
reproductive and renal organs and the digestive gland ;
sever the alimentary canal across its middle and turn
the two halves to one side; cut short the tendon of
the adductor mandibuli muscle, and remove the flexor
abdominis, together with the endophragmal system.
The whole nervous axis will thus be laid bare.

a. The supraozsophageal ganglia; two considerable
masses, confluent in the middle line, situated at
the base of the rostrum immediately behind the

eye-stalks.

b. The circumcesophageal commissures passing back
from a.

c. The longitudinal ventral commissures; backward
continuations of #, extending to the base of the
telson. They are widely separated in the mid-
thoracic region, where they enclose the sternal
artery (seen in transverse section. Cf. Sect. I. 3 d.)-t
elsewhere they are closely applied.

d. The abdominal ganglia ; six paired masses, borne
upon c in the abdominal region. They are equi-
distant, there being one pair for each somite. The
posterior pair are larger than the rest, and distribute
fibres to the last somite and the telson. Note, in
relation —

a. The ganglionic nerves ; paired trunks arising from
the ganglia.

(3. The inter- ganglionic nerves; arising from the
longitudinal commissures, alternately with a.

228 ELEMENTARY BIOLOGY. [CHAP.

e. The suboesophageal ganglion ; a large oval mass,
situated immediately behind the gullet. It dis-
tributes nerves to the mandibular and five following
somites.

/. The thoracic ganglia; five pairs, each somewhat
larger than the abdominal, distributing nerves to
the five posterior thoracic somites,
a. The posterior pair ; situated in front of their cor-
responding somite, and consequently far removed
from the first abdominal pair. This is seen to
be the result of approximation towards the pair
in front, the two lying close together immediately
behind the passage for the sternal artery.
ft. The second and third pairs; similarly but less
conspicuously approximated, in front of the
artery.

y. The anterior pair ; equidistant between ft and
the suboesophageal ganglion.

2. The nerves of the supracesophageal ganglion. Carefully
remove one half of the rostrum and lay open the
eye-stalk of the same side. Note, passing into the
latter —

a. The optic nerve ; a thick tract, arising from the
anterior end of the ganglion and expanding within
the eye-stalk to form the optic bulb.

b. The antennulary nerve; a delicate trunk, arising
from the outer border of the ganglion ; it passes
outwards and forwards to enter the base of the an-
tennule.

c. The antennary nerve ; a similar trunk, arising from
the hinder region of the ganglion ; it passes back-

IT.] THE CRAYFISH AND LOBSTER. 22Q

wards and downwards to reach the base of the
antenna.

3. Examine the circumcesophageal commissures under a
lens ; they will be seen to be connected by —

a. The postazsophageal commissure; a delicate trans-
verse band, slightly in front of the suboesophageal
ganglion.

b. The antozsophageal commissure; a similar structure
to a, arising at the sides and passing round the front
of the gullet.

c. The median-ventral commissure; a very delicate
trunk, passing back from the supraoesophageal
ganglia in the middle line, to meet b.

d. The anterior visceral nerve ; a median nerve arising
from the point of union between b and c; it passes
upwards and forwards to reach the roof of the
stomach, upon which it subdivides for distribution
to its walls.

e. The posterior visceral nerves — arising, one on either
side, from the last abdominal ganglia. They pass
slightly backwards to reach the base of the intes-
tine, the side walls of which they skirt.

4. A companion dissection may profitably be made, by
bisecting the whole body a little to one side of the
middle line.

The real double nature of the entire nervous axis
may best be appreciated by removing it and examin-
ing it on a blackened surface, under water.

5. a. Tease out a bit of perfectly fresh nerve-cord in

water and stain with magenta or eosin.

230 ELEMENTARY BIOLOGY. [CHAP.

Composed of slender fibres of variable size, each
consisting of a structureless outer wall, surrounding
a finely granular or obscurely fibrillated central
axis. Nuclei ; seen at intervals.

b. Tease up in water, a ganglion which has been
treated with osmic acid.

Composed of large oval branched cells, each
consisting of a granular mass in which lies a clear
round nucleus, containing a nucleolus.

K. The sense organs.

i. The auditory organ. This lies in the basal joint of
the antennule and is best examined in the Lobster.
The upper surface of this basal joint is flat posteriorly
and arched in front. It bears several tufts of setae :
one of these is very small and lies at the inner side of
the flattened surface, just at the angle where it meets
the arched part ; among these setae is the opening into
the auditory sac, through which a bristle can easily be
passed.

a. Take a fresh antennule from a Lobster and cut
away the under surface of its basal joint, with a
scalpel. A transparent chitinous sac will readily be
found in it, among the muscles ; this is the auditory
sac and is about \ of an inch long. Carefully dissect
it out.

b. If this sac be held up to the light a little patch of
gritty matter will be seen on its under surface near
the external aperture. Behind this can be seen a
curved opaque line; behind this, and concentric
with it, a shorter brownish streak. Cut out carefully
the part of the sac which bears these marks : mount

II.] THE CRAYFISH AND LOBSTER. 231

in sea-water or salt solution and examine with one-
inch objective.

a. The gritty matter will be seen to consist of a
number of irregular earthy particles, largely, if
not wholly, foreign to the organism; they are
functional as otoliths. Carefully remove them
with a camel-hair brush.

/?. The white line will be seen to answer to a ridge,
on the summit of which is a row of large setae
(auditory setce or hairs), and both on the brown
patch and on the opposite side of the main row
will be seen scattered groups of smaller setae.

f. Examine with ^ obj.

a. Each auditory seta is now seen to be covered over
its whole surface with numerous very delicate
secondary setae ; these are shortest near the base
of the primary one. Towards its base each of the
primary setae is constricted and then dilates into
a bulbous enlargement for articulation upon the
wall of the sac.

($. The brown patch is seen to owe its colour to
a single layer of polygonal epithelial cells con-
taining pigment granules.

y. By focussing through this epithelial layer a num-
ber of parallel fibres will be seen passing up, one
to the base of each seta of the main row.

8. If a perfectly fresh auditory sac be put in i per
cent, solution of osmic acid for half an hour, and
be then well washed in distilled water and ex-
amined, each of the fibres mentioned above (ul-
timate ramifications of the auditory nerve) will be

232 ELEMENTARY BIOLOGY. [CHAP.

seen to terminate, near the free end of the seta,
in an elongated end organ.

The auditory sac in the Crayfish is very similar
to that in the Lobster, and may be examined in a
similar way. It is, on the whole, not so good,
chiefly on account of its smaller size.

2. Examine, in the Crayfish, the exopodite of the an-
tennule; it exceeds the endopodite in calibre and
length, and is carried, during life, upwardly directed.
(Cf. Sect. B. 4 m.) Remove it from the rest of the
appendage and examine in water, under a low power.

a. Its seta; short filaments, developed from the an-
terior borders of the several joints.

b. Its sensory setce; developed in tufts, from the under
face of the filament. There are usually two sets
for each segment and they slope obliquely down-
wards and forwards.

c. Examine under \ or \ obj. Each seta is seen to be
subdivided into two segments; a shorter basal one
and an elongated terminal one: in the ordinary
setae the latter is pointed, in the sensory ones it
is expanded and spatulate.

More or fewer of the terminal segments of the
sensory setae are, not unfrequently, globular and
abnormal.

3. The visual organ. Place the eye of a Crayfish for
four or five days in 0*5 per cent, solution of chromic
acid and then for twenty-four hours or more in
alcohol.

a. Strip off a portion of the cornea and examine under
a J- obj. It is marked out into a great number of

II.] THE CRAYFISH AND LOBSTER. 233

minute facets; mostly square, more rarely hexagonal
or irregular.

b. Tease up in weak glycerine a portion of the con-
tents which underlie the cornea. Look for —

a. Crystalline cones ; elongated angular transparent
bodies, each of which may or may not be in con-
tinuity with

ft. A striated spindle ; fusiform and about a third
the length of a. It is beset by blackish transverse
striae.

c. Imbed an eye similarly prepared and cut a number
of longitudinal sections from it : mount in glycerine
and examine under one inch objective. Look for
the sections which have passed through its middle;
they will be seen to present a central mass (optic
bulb. Cf. Sect. J. 2 a.) from which a number of lines
appear to radiate to the facets on the surface.
These radiating lines (which are obscured here and
there by concentric pigmented layers) are indica-
tions of the striated spindles and crystalline cones.

a. The exoskeleton ; calcified to form two segments,
a smaller basal annular one and a larger terminal
dome-shaped one, united by an inter-articular
membrane (Cf. Sect. B. 4 «.). That of the ter-
minal segment is thick and dense, except where
it overlies the crystalline cones ; there it is thin
and transparent, forming the cornea.

ft. The muscles; seen on either side, immediately
beneath a, they pass between the two segments,
as between those of any one appendage. (Cf.
Sect. C.)

234 ELEMENTARY BIOLOGY. [CHAP.

y. The intermediate mass; a plug of vacuolated con-
nective tissue, filling up the entire interspace be-
tween the optic bulb and the muscles.

d. Examine your thinnest section with a high power.
Beginning at the exterior make out successively —

a. The corneal facet ; its flat outer and slightly ex-
cavated inner surface.

ft. The crystalline cones. Cf. supra. Each is in
close apposition with the inner face of a corneal
facet.

y. The striated spindles ; cf. supra. They are dis-
posed lineally with ft.

8. The connective rod. Each is widest in front
where it joins the cone, but narrows posteriorly
where it is continuous with the striated spindle.
If fresh eyes be treated with osmic acid and then
teased out, each of these rods can be split up
into four fibres.

e. The sheath of the above ; a nucleated investment
for each set of constituents. Pigment is de-
posited therein, being densest around the outer
ends of the cones and the greater portion of the
spindles, whence the appearance of the two black
zones seen under the low power.

£ The layer of nerve fibres ; passing between y
and the optic bulb. Look for them in places
where the pigment may have fallen away.

17. The optic bulb; mainly composed of delicate
fibres and small cells with large round nuclei,
partly scattered, partly aggregated into two oval

II.] THE CRAYFISH AND LOBSTER. 235

central masses (gangliform bodies) which stain
deeply.

A zone of pigment is deposited at its outer
border.
L. Development.

1. Examine the under surface of the body of a living
female "in berry."

a. The fertilized ova ; each is attached to the setae
which fringe the swimmerets, by a delicate peduncle.
Make an incision into one : the ovum will be
liberated; the peduncle will be seen to be deriva-
tive of an egg-capsule which remains behind (a secre-
tory product of the abdominal wall and swimmerets
(cement glands of) cf. p. 186.

b. Examine the thoracic sterna ; aggregates of filiform
whitish bodies (spermatophores) will in all proba-
bility be found adherent to the hinder ones, having
been deposited by the male.

Tease one up ; it consists of a bundle of sperma-
tozoa, enclosed in a dense milk-white sperm-
capsule (a secretory product of the vas deferens.
Cf. Sect. G. i b.).

Spermatophores may frequently be found project-
ing from the reproductive orifices of the male, if
killed during the breeding season.

2. Remove the developing ova from time to time from
the body of the parent, and dissect off their egg-cases
with a couple of needles. Transfer to picric acid
solution and harden in alcohol. Look for —

a. The segmenting ovum (oosperm). Examined under a

low power, its exterior appears slightly granulated.

Imbed and cut sections; stain with carmine. Note —

a. The food yolk ; aggregated centrally, and chiefly

236 ELEMENTARY BIOLOGY. [CHAP.

deposited in conical masses whose bases look
outwardly.

. /?. The clear protoplasm; forming a superficial in-
vestment for the whole. Nuclei are regularly ar-
ranged within it.

y. The blastomeres ; segmentations of /?, around the
nuclei. The furrows which mark them off do not
extend into the yolk-laden protoplasm.

b. The same at the gastrula stage. Easily recognisable
by the large horse-shoe shaped blastopore.

Make sections of the area of invagination,
through the long axis of the blastopore. Note —

a. The ectoderm; now established as a superficial
layer investing the whole.

/?. The endoderm; a small sac, derived from a by in-
vagination, completely enclosing the archenteron.

c. The same, at a slightly later stage. Look for speci-
mens having on the surface a shoe- shaped white
patch (blastoderm], immediately in front of the
blastopore now greatly reduced in size. Preserve
as before, stain and examine en face.

a. The blastoderm; shoe-shaped or heart-shaped,
thickened and expanded in front to form two
rounded procephalic lobes.

ft. The blastopore; a small oval aperture, at the
extreme hinder end of a.

y. The abdominal papilla; a median outgrowth of
a, immediately in front of the blastopore.

d. The same, at a later stage. Look for specimens in

which the blastopore has disappeared. Treat as atr.

II.] THE CRAYFISH AND LOBSTER. 237

a. The appendages; paired differentiations of the
blastodermal area, separated by a median space
(more or fewer will be seen).

/?. The labrum; an oval median outgrowth, situated
about on a level with the first pair of appendages.

y. The abdominal papilla ; now elongating.

If the specimen be a young one the involution
to form the mouth may be seen immediately
behind the labrum; that to form the anus, under
cover of the abdominal papilla. If it be an
advanced one, segmentation of the papilla will be
very distinct.

8. The optic pits; two small round differentiations
within the procephalic lobes (c. a.).

€. The carapace; now appearing as a marginal thick-
ening of the blastoderm, immediately external to
the appendages. It is at first paired, the two
folds meeting behind the abdominal papilla.

t. The hatched embryos. To be found in numbers,
fixed to the swimmerets of the mother.

In general they resemble broadly the parent or-
ganism. Examine —

a. The cephalo-thorax ; more arched than that of the
adult, owing to distension by the food-yolk, not
yet completely absorbed. The rostrum is bent
downwards and the cervical groove is, at best, ill
defined.

ft. The cephalo-thoracic appendages; differing but
little from those of the adult. Remove and
examine the free-end of the chela ; its pincers

238 ELEMENTARY BIOLOGY. [CHAP.

apparatus is spinous and recurved at the tips,
for purposes of attachment to the parent.

•y. The abdomen ; fully segmented and terminating
in the telson. Four pairs of appendages are
alone free, the first and last pairs being buried
beneath the larval integument.

e. Look for and examine stages intermediate between
c and d.

f. The larval Lobster, when about half an inch in
length. Obtain if possible a specimen in which
the abdominal appendages are first appearing.
Compare generally with the Crayfish larva and note
especially —

a. The exoskeleton ; like that of the Crayfish larva, a
continuous uncalcified cuticle.

P. The carapace; terminating in front in the ros-
trum, which assumes the form of an immense
protective spine.

y. The abdomen; fully segmented and bearing several
smaller protective spines.

S. The eye; its great size at this stage.

e. The antennary organs ; both are short and com-
pletely under protection of the rostrum.

£. The thoracic appendages. The third maxillipede,
great chela and ambulatory appendages, are more
nearly uniform in size than in the adult ; they
all bear well developed exopodites.

g. Remove the second ambulatory appendage and
the swimmeret of the third abdominal somite.
Examine them side by side, and note in both —

IT.] THE CRAYFISH AND LOBSTER. 239

a. The great relative size of the protopodite.

ft. The increase in length of the endopodite over the
exopodite ; little marked but obvious in the case
of the abdominal appendage. (Cf. the cor-
responding abdominal appendage of the adult
Crayfish.)

in,

THE EARTHWORM (Lumbricns terrestris).

THE Earthworm is to be found wherever damp earth is
accessible, no matter how hard or stony the surface; the
presence of moisture is indispensable to its existence. Into
this earth it burrows, excavating a tubular habitation to
which it repairs during the day time, emerging at night to
seek food and to work, or at early morning to reproduce its
species. It remains within the burrow throughout both the
winter and the dry summer seasons.

The interior of the burrow is smooth and frequently lined
with minute stones ; its mouth is often surrounded with
"castings" and plugged with leaves drawn in by the animal
itself, or covered, as with a lid, by stones sometimes of
relatively great weight and size.

The body of the worm is fairly uniform in dimensions
throughout; it is bilaterally symmetrical, both mouth and
anus being situated at opposite ends. The metameric
symmetry such as was seen in the abdomen of the Crayfish,
is here common to the whole body, which can no longer be
subdivided into well-marked regions. It is constricted
externally into a number of repetitional segments or somites,
of which there may be, in a sexually mature animal, as few
as 68 or considerably more than 200. For each of these
somites there is a similar repetitional series of certain of the
internal organs. Each somite is subdivided externally into

III.] THE EARTHWORM. 241

at least two lesser divisions or zonites, these however are but
skin deep and in no way correlated with the internal parts.
The body is invested in a continuous uncalcified exoskeleton
in the form of a delicate iridescent cuticle. No limbs of
any sort are present, and locomotion is effected by means of
four longitudinal series of bristle-like seta, which project
freely from all but some few of the anterior segments ; a
powerful muscular apparatus is developed in connection
with each of these, and the chitinous setae themselves are
largely buried in tegumental sacs lined by cuticular invo-
lutions. The setae are replaced when cast off.

The posterior terminal segments of the body are, during
life, flattened from above downwards, and when the animal
on removal from its burrow is observed crawling, a charac-
teristic spatulate appearance is imparted to that region, at
the upturned end of which the anus is situated. Under
ordinary circumstances the worm, on coming to the surface,
retains its hold on its burrow by means of this expanded
extremity.

The Earthworm is omnivorous, living mainly upon leaves
(for certain kinds of which it has a decided preference), and
less conspicuously upon both animal and vegetable organ-
isms ingested with the earth passed through its alimentary
canal, in burrowing or otherwise. The alimentary canal is
a straight tube, running the entire length of the body. The
mouth leads into a thin-walled eversible buccal sac, which
opens into a spacious muscular suctorial pharynx ; this, in
turn, passes into a long tubular oesophagus, the terminal
fifth of which is enlarged to form a distensible crop. The
crop is, in the common earthworm, succeeded by a whitish
thick-walled gizzard, so called as it performs a mechanical
crushing action. This finally opens into a long sacculated
intestine, which is continued on with but slight modification

M. 16

242 ELEMENTARY BIOLOGY. [CHAP.

to the hind end of the body, where it communicates with
the exterior by a valvular laterally compressed anus. The
absorptive area of the intestine is increased by the develop-
ment of a conspicuous intestinal-valve or typhlosole which
lies along its whole roof, and not by a coiling of the whole
viscus as in the case of the Frog.

The intestinal wall is invested in a yellowish-brown
mass sometimes termed a "liver." This tissue has no sort
of connection with the lumen of the gut; it is intimately
associated with the walls of the great blood-vessels, many
of which it completely surrounds, and there is reason for
regarding it as functional in the formation of an excretory
product which is discharged into the body-cavity, if not in
that of some constituent of the blood also.

The intestinal juice of the Earthworm contains a di-
gestive ferment, and there open into the mouth-cavity a
number of small so-called "salivary" glands. Digestion
is nevertheless not wholly intra-intestinal, for the animal
when feeding vomits a digestive fluid ; this is allowed to
act upon the raw food material for some time, prior to its
ingestion.

Digestive glands other than those just named arc un-
known, and such diverticula of the alimentary canal as
remain for consideration are somewhat remarkable, and
little understood physiologically. There are usually three
pairs of cesophageal pouches, lying midway between the
crop and pharynx ; they are smallest in winter, and it not
unfrequently happens that one or more of them may be
absent. Occasionally, their connections with the lumen of
the oesophagus may become obliterated, and in rare cases
no trace of them is to be found. These cesophageal diver-
ticula are highly vascular, and contain a milk-white product
consisting of carbonate of lime either in a finely divided

III.] THE EARTHWORM. 243

state or in concretionary masses sometimes reaching a
diameter of i millim. Such being the case they are termed
caldferous glands, but the purpose served by them is as yet
not fully understood.

The body-wall is invested in a single layer of elongated
epidermal cells, many of which become converted into
small unicellular cutaneous glands. This investing epi-
thelium of the body and the cuticle which it secretes (vide
supra] are continuous respectively, at the mouth and anus,
with the epithelial lining of the alimentary canal and its
cuticular product. The greater mass of the body-wall is
however composed of two sets of muscular fibres, an ex-
ternal circular set and an internal longitudinal one; these
also pass, at opposite ends of the body, into an internal
circular and an external longitudinal series of fibres entering
into the composition of the wall of the digestive tube. The
two walls are widely separated by a spacious body-cavity,
the alimentary canal being suspended by a series of trans-
verse mesenteric septa, instead of by a median longitudinal
fold as in the Frog. These septa are metamerically ar-
ranged, one for each somitic constriction; circular muscular
fibres are developed within them and radial ones pass across
them from body-wall to wall of alimentary canal. The
body-wall is lined internally by a peritoneum, clad in
tessellated epithelium, which can be traced on to both the
mesenteric septa and the free surfaces of the various organs
contained within the body-cavity. It is from this layer that
the yellow brown tissue aforenamed takes its origin. The
walls of the body and alimentary canal are thus seen both
to consist of at least three clearly defined Jayersj a pro-
tective cuticular, an epitheloid, and a jnechanical mus-
cular one respectively. These layers are continuous at
both mouth and anus, and the order of succession is the

1 6 -2

244 ELEMENTARY BIOLOGY. [CHAP.

same for each except that that which is external in the one
wall is internal in the other and vice versa.

The body-cavity is subdivided by the mesenteric septa
into a series of somitic compartments, and the metameric
symmetry thus established extends to the excretory organs,
there being one pair of these attached to each septum, the
first three somites excepted. These organs, being thus
segmentally arranged, are termed segmental organs or
nephridia. Each consists of a tortuous tube which can be
resolved into three segments; a middle glandular one,
abundantly supplied with blood-vessels, passing — internally,
into a delicate thin-walled loop, which perforates the
mesentery and opens into the segment in front by means of
a ciliated funnel or nephrostome — externally, into a vesicular
muscular segment, which communicates with the exterior in
the vicinity of the ventral pair of setae. The lining mem-
brane of these excretory tubules is profusely ciliated, and a
current is thus induced from within outwardly. In addition
to the indirect communication established, through the
agency of the nephridia, between the body-cavity and the
exterior, a direct one is instituted by means of a metameri-
cally disposed series of median dorsal or peritoneal pores.
The precise function of these is as yet not fully understood.

The mesenteric septa themselves are incomplete ventrally;
they do not subdivide the body-cavity into a series of closed
chambers. As this is so, it follows that that cavity is a con-
tinuous one in open communication with the surrounding
medium. It contains during life a colourless perivisceral
fluid, in which there are present immense numbers of
nucleated amoeboid corpuscles.

The true red-blood fluid circulates in a system of vessels,
having, so far as is known, no direct communication with
the body-cavity. The larger trunks of this circulatory

Ill ] THE EARTHWORM. 245

system are six in number; median longitudinal supra and
subintestinal and supra and subneural vessels respectively,
and two small lateral neural ones. These are connected
metamerically in a manner described in the Laboratory
work (p. 261), and highly efficient capillary systems are
established in connection with them. In the segments
numbering six to twelve, there exist two sets of vessels not
met with elsewhere. These are, firstly, six pairs of enlarged
drcumossophageal vessels, connecting the supra-intestinal
and supra- neural trunks; secondly, a pair of lateral cesopha-
geal trunks, which are connected with the supra-intestinal
vessel in the twelfth segment alone. The latter vessels are
specially concerned with the blood supply to the anterior
portion of the oesophagus and its calciferous glands. A con-
dition somewhat exceptional in the animal kingdom is met
with in the blood vessels of the clitellum, and less con-
spicuously of the body-wall generally ; where the superficial
capillaries pass up and ramify among the actual epidermal
cells themselves, giving rise to an epidermal blood plexus.

The exact seat of respiration in the Earthworm is not fully
determined, but there can be little doubt that the red-blood
fluid is directly concerned in the process. This fluid con-
sists of a watery plasma in which are suspended exceedingly
minute transparent non-nucleated corpuscles. These are
somewhat variable in shape and size, being usually about
the soVtfth of an inch in length; their structural features
agree closely with those of the nuclei of the epithelial lining
of the vessels in which they circulate.

The feature which most clearly distinguishes the red-
blood fluid of the worm from that of the Frog, is that its
colouring matter is diffused through the plasma and in no
way related to the corpuscles.

246 ELEMENTARY BIOLOGY. [CHAP.

The nervous system is in an exceedingly simple and in-
teresting condition. It consists of two pear-shaped supra-
(esophageal ganglia which abut together in the middle line
in front; from these there arise two commissures which,
like those of the Crayfish, run longitudinally side by side
for the whole ventral surface. These commissures are
somewhat enlarged for each somite, and there arise from
them a metamerically repeated series of nerve trunks. The
supra-cesophageal ganglia distribute fibres to the buccal
sac and to the anterior end of the body; the latter termi-
nate in modified sensiferous cells borne upon the first seg-
ment, which constitute all that the worm has in any way
representative of sense organs. One of the most important
facts concerning the nervous system of this animal, is the
disposition of the nerve-cells. These, instead of being re-
stricted to the ganglia, as they are in the Crayfish, are
regularly diffused throughout the entire axis — gangliform
enlargements and longitudinal commissures alike. The
sheath of the nervous axis is remarkable in being muscular,
and especially as concerns the existence of a dorsal neuro-
chord made up of three longitudinally disposed "tubular
fibres" These fibres are highly elastic, and the muscular
sheath forms a most efficient protective apparatus.

The proper reproductive organs are restricted to seven of
the anterior segments, and will be found described on p. 268.
They are somewhat complicated, chiefly owing to the con-
ditions of maturation of the seminal vesicles ; but the actual
genital glands, though exceedingly small, are well defined.
The ripe ovum consists of a round nucleated cell contain-
ing a moderate food-yolk, and invested in a vitelline mem-
brane. The sperm-producing cells undergo changes which
result in the formation of a number of filiform spermatozoa,
each with an elongated nucleus-bearing " head ;" and the

III.] THE EARTHWORM. 247

conditions are such as to render observation upon the ma-
turation of these exceedingly easy and instructive.

The worm is hermaphrodite but not self-impregnating.
During copulation — which usually takes place at early
morning — the bodies of two individuals are brought into
apposition, and a transfer of ripe spermatozoa takes place.
These are passed into definite seminal receptacles^ there to
await final deposition. During the interval which follows
there is secreted by the clitellum an egg capsule or cocoon,
within which functionally mature ova and spermatozoa are
ultimately deposited. Segmentation of the fertilized ovum
is holoblastic, and there result two layers of cells — a more
rapidly dividing one, which differs from that described for
the Frog mainly in the absence of pigment, and a less
rapidly dividing yolk-laden one. The smaller cells over-
grow the larger ones very rapidly, and there results a simple
two-layered sac or gastrula which becomes ciliated ex-
ternally. The embryo early assumes a bilaterally sym-
metrical form, and as the body elongates there are de-
veloped, mainly if not entirely from the archenteric wall,
a series of paired cellular masses which become metameri-
cally arranged. The segmentation of the body receives its
initiative in the appearance of these mesoblastic somites; from
the central cavities ultimately developed within them the
body-cavity is derived, while their walls, coming into apposi-
tion antero-posteriorly, give rise to the mesenteric septa.

The nervous system arises as a thickening of the investing
epiblastic layer, and the supra-intestinal vessel when first
formed is paired. The larva continues to elongate by the
addition of fresh segments at its hinder end, but the sim-
plicity and uniformity of structure so characteristic of it, is
retained with but slight modifications by the adult animal.

The clitellum, to which reference has been made, must

248 ELEMENTARY BIOLOGY. [CHAP.

not be regarded as in any way distinctive of the adult worm.
It undergoes a periodic enlargement, and may be present in
young worms but two inches in length or absent in fully
formed ones of six or eight.

LABORATORY WORK.

The unnatural displacement of the organs of this animal,
resultant upon contraction of the muscles during death
under chloroform, may be entirely obviated by killing the
worm in alcohol. Let it be immersed in methylated spirit
for two minutes, and allowed to remain in running water for
half an hour.

A. General external characters.

1. Examine the living worm and note —

a. Its shape, elongated. Rounded and pigmented
dorsally — flattened and whitish ventro-laterally.
Anteriorly it tapers off to a point, posteriorly it is
flattened and spatulate.

2. Compare the body of the dead animal, killed as
above. Note — •

a. Its subdivision by a number of constrictions into a
recurring series of body segments or somites. Each
of these is again subdivided by a lesser constriction
into two zonites.

The zonites of the genital segments (9 to 15) are
frequently more numerous, though they never exceed
four in number.

b The clitellum or cingulum (if present, vide supra),
a whitish saddle-shaped enlargement usually re-

III.] THE EARTHWORM. 249

stricted to segments 29 to 35 (rarely 30 — 36 or
29—36).

This is best studied in a chloroformed specimen.

The capacity of the whole body is greatest at this
point. Posteriorly to the clitellum it is uniform in
calibre, except for the postero-terminal region (cf.
i. a), where its transverse diameter is greatest. An-
teriorly, it is enlarged in the genital region (segments
8 to 12) tapering off to a point in front.

c. The locomotor organs or seta, four double rows
(two lateral and two ventral) of bristle-like ap-
pendages, which project freely from all but the
extreme anterior and posterior segments.

The body is flattened ventrally between the two
sets of ventral setae, and laterally between the
lateral and ventral ones. The setae project out
at the angles formed by these flattened areas, the
lateral ones arising within the limits of the pig-
mented portion of the body-wall.

d. The apertures.

a. The mouth) antero-terminal. It perforates the
first segment, which is subdivided into a dorsal
prostomium which overhangs the mouth, and a
circumoral peristomium. Examine the latter with
a lens ; its free edge is delicately serrated.

Compare the living animal. The movements of
the body clearly show that the first segment is
highly sensitive.

/?. The anus, postero-terminal, and laterally com-
pressed. It perforates the last segment.
Note the valve-like nature of its lips.

250 ELEMENTARY BIOLOGY. [CHAP.

y. The dorsal or peritoneal pores ; small median per-
forations of the body-wall, lying at the bases of
the somitic constrictions between all but some
few of the anterior segments.' They are best
seen in specimens which have been preserved
in spirit. If examined in freshly killed worms,
their presence can be made manifest by gently
squeezing the body, whereupon there will exude
from each a drop of perivisceral fluid.

S. The sexual apertures. Examine the worm from
beneath, and note the presence of two glandular
enlargements immediately external to the ventral
setae of the i5th segment. Each surrounds the
slit-like orifice of the vas deferens of its own side.
Opening, in a line with the above, on to the
surface of the i4th segment, there are the smaller
apertures of the oviducts.

The ventral integument of segments 9 to n
(more rarely 8 to 12) is generally swollen, like
that of the lip of the vas deferens or the clitellum,
owing to the presence of the so-called capsulo-
genous glands.

The exact function of these is at present un-
known.

Examine the somitic constrictions in this
region from the side. There will be found,
opening upon those which subdivide segments
9 — 10 and ic — n in a line with the lateral setae,
the small apertures of the spermatheccz.

Not unfrequently one or other of these may
open by two orifices, and their presence may
easily be demonstrated by gently squeezing the
body.

III.] THE EARTHWORM. 251

«. The orifices of the excretory or segmental organs,
too minute to be examined here. They are best
seen in section. (Cf. Sect. H. i. c.)

B. The setae and exoskeleton.

i. Remove one of the setae from a post-genital segment
with a small pair of forceps, and examine in water
under a low power. Note —

a. Its form, curved and needle-shaped, blunted inter-
nally and terminating in a point.

b. Its structure. Examine under a high power. It
consists of a chitinous rod, a portion of the central
axis of which is soft and granular.

The setae vary somewhat in shape and calibre in
different regions of the body, especially so in the
clitellar and genital segments.

c. The exoskeleton, a delicate iridescent cuticle, in-
vesting the whole body. Take a freshly killed
worm and strip off its cuticle under water; in doing
so note that it passes in to line the alimentary
canal at both mouth and anus. Transfer a small
portion of it to a slide, mount in water and examine
under a high power. Note —

a. It consists of a colourless transparent membrane,
traversed by delicate lines or stricz crossing each
other at an angle of 45°.

(3. Examine the same under a low power and note
the cuticular sacs of the seta, finger-shaped pro-
longations of the cuticle which occur in pairs.
They embrace, during life, the middle of the setae.

It not unfrequently happens that the setae are
dragged away with the cuticle or it with them, upon

252 ELEMENTARY BIOLOGY. [CPTAP.

removal from the body. Examination of a specimen
in which this is the case shows that but one-third
of the seta projects beyond the body-wall when at
rest (Cf. Sect. H. 3).

C. The body-cavity and general disposition of the viscera.

Pin a large worm down under water dorsal side upper-
most, and slit up the body-wall along the dorsal middle line.
Pin back the two flaps and note —

a. The presence of a spacious body or perivisceral
cavity.

b. The alimentary canal, a straight tube running the
whole length of the body.

The yellowish-brown tissue investing its walls, ren-
ders it especially conspicuous for all but the anterior
20 segments.

c. The mesenteric septa, a series of delicate transverse
membranous partitions, coincident with the somitic
constrictions (Cf. Sect. A. 2. a]. They attach
the alimentary canal to the body- wall.

d. The segmental organs or nephridia, whitish fluffy-
looking appendages, one pair* to each mesenteric
septum. Examine under a lens, and note that each
consists of a delicate convoluted tube, often of a
dead-white colour owing to the presence of ex-
cretory matter.

e. The reproductive organs. Conspicuous among these
will now be seen the seminal vesicles ; three pairs of
large white bodies, overlying the alimentary canal
in segments 9 to 12.

The relative size of these is variable. (Cf. p.
246.) The reproductive organs are restricted to this

ITT.] THE EARTHWORM. 253

region of the body, but further details concerning
them may be left till later.

f. The perivisceral fluid. Take a freshly killed worm
and remove a small portion of the dorsal body-
wall. Insert the point of a capillary tube and draw
off a little of the perivisceral fluid. Examine at
once under a high power, and note —

a. The watery serum, in which float colourless
vacuolated amoeboid corpuscles.

ft. Watch the corpuscles. Aggregates of them fre-
quently fuse on exposure, prior to disintegration.

y. Take a second drop of the fluid and treat with
acetic acid and magenta. The nuclei of the
corpuscles alone stain deeply.

U. The alimentary organs.

i. Pin the worm down as before and open it up dor-
sally. Remove the seminal vesicles and work out in
order —

a. The buccal sac, a thin-walled sac lying within the
first 2 — 3 segments. Muscular fibres pass from its
anterior end to the body-wall. The aperture of
the mouth can be seen through its transparent
roof.

b. ThtfAarynx, a spacious thick- walled structure, ex-
tending back to the sixth segment. It is tied
down to the body-wall by a fan-shaped series of
muscular fibres.

The cut ends of those fibres which attach it to the
dorsal surface of the body-wall (removed in dis-
secting), give it a roughened appearance when
viewed from above.

254 ELEMENTARY BIOLOGY. [CHAP.

The pharynx is marked off from the buccal sac
by a deep recess, in which the pear-shaped supra-
cesophageal ganglia are seen to lie.

. c. The (esophagus, a tube of relatively small calibre,
continued back to the i5th segment, where it en-
larges to form

d. the crop, a spacious thin-walled sac, lying within
segments 15 and 16.

e. The oesophageal (caldferous] glands, three pairs of
lateral cesophageal diverticula, conspicuous by their
light yellow colour. The two anterior pairs arise
in segment n and the posterior one in segment
12. When fully formed, the anterior pair project
into segment 10.

Puncture one of the glands — a milk-white fluid
will escape. Examine this under a low power, and
note the presence of carbonate of lime. (Cf. p. 243.)

If any difficulty is experienced in following out the
oesophagus, it can be overcome by starting from the
crop and working forwards.

f. The gizzard, a pearly-white thick-walled sac, follow-
ing immediately upon the crop. It usually oc-
cupies segments 17 to 19.

g. The intestine, a spacious yellow thin-walled tube,
extending back to the extreme end of the body.
The sacculations of its walls become less con-
spicuous as its hinder end is reached.

h. Open up the crop, gizzard, and a small portion of
the intestine from above, wash out the contents
and note —

a. The crop. Its walls are thin and membranous.

III.] THE EARTHWORM. 255

There is sometimes present a well-marked lateral
thickening of a highly vascular nature.

j3. The gizzard. Note the great thickness of the
muscular wall of its anterior half. The cuticular
lining of the alimentary canal here attains its
maximum of development.

y. The intestine. Note the lateral ingrowths of its
walls, developed metamerically at the points of
attachment of the mesenteric septa. In trans-
verse section they are seen to be refolded and
complex.

2. Take the body of a second worm killed as directed at
the outset, but which has been subsequently pre-
served for 1 8 — 24 hours in alcohol. Pin it down
and dissect from the side.

Make a median longitudinal section of the ali-
mentary canal, wash out its contents and examine —

a. The buccal sac and pharynx. The floor of the
former and roof and side walls of the latter are
both thickened and muscular.

(3. The oesophagus. Its metameric constrictions

are plainly visible. Look for the orifices of the

calciferous glands. (Cf. p. 242.)
y. The intestine. Nofe the typhlosole, an immense

median dorsal fold of the lining membrane of its

roof; it is plicated laterally.

Cf. the same in transverse section.

Examine the lateral intestinal sacculations as
seen from within (Cf. i. h. y).

S. Work over the relations of the whole to the

256 ELEMENTARY BIOLOGY. [CHAP.

mesenteric septa. Those of the crop-gizzard
region are liable to considerable variation.

E. The excretory or segmental organs.

1. Take the largest worm you can find and open up its
anterior third along the dorsal middle line. Pin down
the two flaps of the body-wall and remove the ali-
mentary canal, being especially careful to avoid
stretching the mesenteric septa more than is neces-
sary. Wash until quite clean and examine in water
under a lens. With a little care there will be seen —

a. The segmental organ or nephridium, a delicate tube,
coiled upon itself and suspended to the mesenteric
septum by a special fold or mesentery of its own.

The whole organ is thrown into a series of loops.
At its inner end, close to the nervous axis, a very
conspicuous internal loop will be seen; it projects
backwardly and outwardly.

b. Examine the region of this internal loop with care.
The nephridium will be seen to perforate the mes-
enteric septum with which it is connected, and to
open into the somite in front by an enlarged
pendant extremity or nephrostome.

2. Take a similar but freshly killed worm, and open up
as before. Remove as much as possible of a mesen-
teric septum (preferably from somite 15 or there-
abouts, where the calibre of the alimentary tube is
least and the reproductive organs do not overcrowd)
with one of its related segmental organs. Examine
under a low power, and restrict your attention to that
portion of the tube in which ciliary action is going on.
Follow it along until a point is reached at which it

III.] THE EARTHWORM. 257

undergoes a bending upon itself and a sudden in-
crease in calibre; substitute a high power and fix
your attention on this last-named area. Note —

a. The thick glandular walls of the tubule in which
ciliary action is going on (glandular segment). Care-
ful examination will reveal the presence of large
oval transparent nuclei.

b. The thin-walled segment, a delicate transparent
tube lying immediately adjacent to the lesser limb
of a. It also is bent upon itself and ciliated.

Its walls appear as mere hard outlines, and the
transverse diameter of its two limbs collectively is
about equal to that of the lesser one of a.

c. The muscular segment. Examine under a low
power. The position in which this segment will
be found varies, as it is generally displaced in the
process of manipulation. Like both a and b it is
bent upon itself, and can be at once recognised by
its great transverse diameter as compared with the
rest of the organ. Follow it along — it will be
found to become suddenly constricted at the point
at which the glandular segment enters it.

Its aperture of communication with the exterior
is best seen in section. (Cf. Sect. H. i c.)

d. Look for the internal loop (i a). Note that both
glandular and thin-walled segments enter into its
composition.

e. Look for the nephrostome (i b\ Having found it,
examine under a high power. It is lined by a
columnar ciliated epithelium, the individual cells of
which are very large and clearly defined. Follow

M. i

258 ELEMENTARY BIOLOGY. [CHAP.

it back ; it is continuous with the thin-walled seg-
ment.

The actual course taken by the coils of the seg-
mental organ can only be made out with very con-
siderable trouble. Sufficient is here described to
render it clear that three segments are present — an
internal thin-walled, a middle glandular and an
external muscular one respectively ("internal" and
"external" having reference to the fact that these
segments communicate respectively with the body-
cavity and the exterior). The only source of diffi-
culty which will be found in attempting to unravel
the whole, is the internal loop (d) formed by a
secondary folding of both thin-walled and glandular
segments. Eliminate that, and the whole resolves
itself into a tube of three segments — each bent upon
itself.

/ The nephridial blood-plexus. Remove, as directed
above, the segmental organ of a worm which has
been at least two days in alcohol. Examine in
water under a high power. Its sheath carries a
complicated series of blood-vessels, conspicuous
by their yellow colour. Note —

a. The main nephridial vessels; two thin-walled
tubes running side by side, parallel with the long
loop of the nephridium. They are connected
by an excretory plexus, the smaller vessels of
which form a bold series of loops on the surface
of the organ.

ft. The appendages of the excretory vessels ; a series
of relatively large globular dilatations, filled with
blood and generally crowded with minute colour-
less non-nucleated corpuscles.

III.] THE EARTHWORM. 259

These structures are sometimes absent, and
they are not confined exclusively to the nephridial
plexus.

F. The circulatory system.

Examination of this system is materially facilitated by
killing the animal as directed at the outset, and allowing it
to remain undisturbed for six or eight hours, in order that
the muscular walls of the larger vessels, no less than those
of the body itself, may be rigid and well-set prior to dis-
section.

1. Pin down, at opposite ends, the body of a worm
treated as above, and open it up along the dorsal
middle line. Reflect the two flaps of the body wall,
and note —

a. The supra-intestinal vessel ; a large median dorsal
vessel, running along the whole length of the roof
of the alimentary canal, to which it distributes
branches.

Large circumcesophageal vessels will be seen aris-
ing from this in segments 6 — 12. Their detailed
connections are described at § 2 d.

b. Dissect away some few of the mesenteric septa of
one side, and displace a portion of the alimentary
tube thus liberated. Note, overlying the nervous
axis, the median longitudinal supra-neural vessel.

The smaller lateral-neural vessels may sometimes
be observed at this stage. (Cf. Sect. G. e. y.)

2. Take a second worm and pin it down on its side,
being careful not to injure >.the anterior twenty seg-
ments. Make an incision with the small scissors
along its dorsal middle line, raising the body wall

17—2

260 ELEMENTARY BIOLOGY. [CHAP.

with the instrument as you cut, in order to avoid the
possibility of injuring the underlying vessels. Care-
fully remove the lateral body wall of the above seg-
ments to the level of the middle line, and without
dissecting further, note —

a. The supra-intestinal and supra-neural vessels, to-
gether with such of their branches as have been
already mentioned.

Look out for the lateral-neural vessels.

b*> The sub-neural vessel, a delicate median-ventral
trunk, seen on raising the nervous axis, to which it
is closely applied.

c. The sub-intestinal vessel, a small median longi-
tudinal trunk buried up in the ventral wall of the
alimentary canal. It may best be seen on gently
displacing the gizzard, the walls of which are
white. Examine under a lens ; it will be found
to be connected with the supra-neural trunk for
each segment, by two or three delicate commissures.

d. The circumcesophageal vessels ; six in number and
usually greatly distended. (Cf. i a.) They lie in
segments 6 to n, and connect the supra-intestinal
and supra-neural trunks.

e. The lateral-cesophageal vessel, a conspicuous usually
highly distended trunk, closely applied to the wall
of the oesophagus on either side. Large branches
of it are seen on the pharynx and cesophageal
glands. It runs beneath the latter, and communi-
cates with the supra-intestinal trunk in segment 12.

d. and e. are the most conspicuous vessels in the
whole system, and their general relations can be
well seen if dissected from beneath.

III.] THE EARTHWORM. 26 1

3. Still working from the side, carefully cut (with scissors)
into any three or four somitic constrictions of the
intestinal region, and without dissecting further ma-
nipulate the parts with forceps under water. Note
the relations of the mesenteric septa and nephridia,
as seen in the undisturbed condition (cf. Sect. E.)
and next examine in order —

a. The circular commissural vessels; one on either side
for each segment. They lie close under the body-
wall (to which they give branches) midway between
adjacent mesenteric septa, and connect the supra-
intestinal and sub-neural trunks.

b. The excretory plexus. The posterior of the two
vessels from which this is derived springs from the
sub-neural trunk immediately adjacent to the ne-
phridium itself. The anterior one arises from the
supra-neural in the segment in front, and perforates
the mesenteric septum ventro-laterally.

c. Remove a portion of the body-wall of this region,
and gently scrape away as much as possible of the
yellow-brown tunic of the intestine. There will
thus be brought into view the lateral intestinal
vessels ; two in number on either side in each seg-
ment. They arise from the supra-intestinal trunk
and break up into an alimentary plexus within the
walls of the digestive canal. A series of small
vessels will be seen to pass up towards these from
the sub-intestinal trunk.

The blood-vessels described above can all be made
out by careful dissection. No mention has been
made of the delicate lateral-neural commissures, or
the smaller branches of the majority of the great

262 ELEMENTARY BIOLOGY. [CHAP,

vessels ; for the study of these recourse must be had
to transverse sections. (Cf. Sect. G. y.)

d. The blood fluid. Lay open, high and dry, the
body cavity of a freshly-killed worm, and introduce
a fine capillary tube into one of the larger vessels.
Transfer the drop of red blood thus obtained to a
slide, cover and examine at once under a high
power. Note —

a. The blood serum, a watery yellowish-red fluid in
which float

/?. minute colourless corpuscles. In shape, they are
somewhat irregular (rarely oval); each usually
contains a central dark granule.

If unsuccessful by the above method, remove
a small portion of one of the larger vessels en
masse between two pairs of forceps ; transfer to
a slide and allow the fluid contents to escape.

G. The nervous system.

Dissect an entire worm from above, removing the dorsal
portion of the body wall, and the alimentary canal from the
pharynx backwards. Pin down, wash until quite clean, and
dissect under water. Examine under a lens and note —

a. The supra-oesophageal ganglia; two pear-shaped
masses lying in the third segment in a depression
between the buccal-sac and pharynx. (Cf. Sect.
D. i. b.} Fibres can be traced from them to the
circum-oral integument.

/>. The circumcesophageal commissures ; seen, on dis-
placing the pharynx to one side, to arise from a.
The nerves arising from them are distributed —
externally, to the first two segments — internally, to

III.] THE EARTHWORM. 263

the pharynx (visceral nerves}. On reaching the
ventral surface the commissures become firmly
united and pass on, to the extreme end of the
body, as longitudinal ventral commissures. Note
the metameric gangliform enlargements formed upon
the latter.

c. The ganglionic nerves ; two pairs for each somite.
They arise from the ganglionic swellings — trace
them to the body wall.

d. The interganglionic nerves ; one pair for each somite,
arising from the longitudinal commissures at the
bases of the mesenteric septa, to which they are
largely distributed.

e. The histology of the nervous axis and its associated
structures (for method of preparation see Sect. H.).

Examine transverse sections under a high power,
. note —

a. The sheath of the nervous system; a deeply
staining investment, mainly composed of the cut
ends of muscular fibres.

^. The neurochord. It consists of three transparent
thick-walled " tubular-fibres " (a larger central
and two smaller lateral ones) buried up in the
roof of a.

y. The cut ends of the sub- and lateral neural blood-
vessels^ together with their anastomoses, all of
which are buried up in a.

8. The ventral nervous axis. Note the close appo-
sition of the two commissures. Each is com-
posed of a transparent matrix in which are seen

264 ELEMENTARY BIOLOGY. [CHAP.

the cut ends of delicate nerve-fibres ; and of a
denser peripheral ventro-lateral portion, in which
large nerve-cells are lodged.

Compare sections through the longitudinal
commissures and ganglionic enlargements.
Nerve-cells are present in both, and except
so far as their relative proportions are con-
cerned, there is no important structural differ-
ence between them. In sections through the
latter, look for the origins of the ganglionic
nerves, and the small blood-vessels arising from
the lateral neural trunks which accompany
them.

H. The study of transverse sections.

i. Wash out the contents of the alimentary canal of a
freshly killed worm with \ p. c. chromic acid solu-
tion. When all the earthy matter has been removed,
cut up into segments of an inch in length and trans-
.fer to solutions of chromic acid and alcohol of in-
creasing strength (see appendix E). Stain with
borax-carmine and mount in the usual manner in
Canada balsam.

Examine the thickest sections under a low power,
and note —

a. The body wall. It is mainly composed of a thick
muscular layer, external to which is a thin epidermis
clad in a delicate cuticle.

b. The intestinal wall ; its thick tunic of yellow-
brown tissue. Internally to this there is a thin
muscular layer^ and the whole is lined by a well-
defined epithelium. Examine the minute structure
of the typhlosole.

III.] THE EARTHWORM. 265

c. The mesenteric septum. A thin fibrous sheet pass-
ing from a to b. Note its circular and radial
muscular fibres.

It is perforated ventrally (circum-neural arcade)
to give passage to the nervous axis and its four
accompanying blood-vessels.

Look for the nephridia — portions of them will
certainly be visible in the thickest sections. Note
especially their communication with the exterior,
each by a narrow duct passing externally to the
ventral setae.

Look for the dorsal pore, for sections of the great
blood-vessels and traces of their branches (see
Sect. F).

2. Examine a thin portion of the body-wall of the above,
under a high power. Note —

a. The epidermis ; composed of a layer of elongated
cells. They stain very lightly, and their nuclei
are very small.

b. The unicellular cutaneous glands, large ovoidal
structures buried up in a. Each opens on to
the exterior by a short neck. They are rendered
very conspicuous by the affinity of their granular
contents for the staining reagent.

c. The cuticular exoskeleton, a delicate investment for
a. It is structureless, and stains uniformly and
with moderate intensity.

It is frequently puckered or otherwise displaced
in the process of manipulation.

d. The circular muscular layer. It lies immediately
beneath, and is two or three times the transverse
diameter of, a.

266 ELEMENTARY BIOLOGY. [CHAP.

Pigment granules will be found scattered among
its fibres.

e. The longitudinal muscular layer, somewhat thicker
than d. Each of its several tracts is subdivided
into a great number of fasciculi by uniformly
arranged radial septa, within which small blood-
vessels are carried. The muscular fibres are
arranged in close set parallel lamellae, disposed
at right angles to the septa.

/. The peritoneum, a delicate membrane immediately
internal to e.

3. The seta, and their associated structures. When at
rest they project inwardly to the level of the perito-
neum. Examine in order —

a. The integumentary sheath of the seta. Trace its
origin from the epidermis — it can be followed to
the inner end of the seta.

b. The cuticular sheath, carried in with a. It can be
traced some distance up the side of the seta.
(Cf. Sect. B.)

c. The muscles of the setce. Examine any one pair
of setae and note, passing obliquely downwards
from their approximated inner ends, the fan-shaped
(in section) protractor fibres. They pass into the
circular layer of the body- wall.

Two sets of retractor fibres will be seen arising
from each seta at about its middle. By their
union there are formed two ribbon-shaped bands
which pass right and left of the setae, upwards
and inwards, between the longitudinal fibres of the
body-wall and the peritoneum.

III.] THE EARTHWORM. 267

d. The follicle of the seta ; a small granular proto-
plasmic mass internal to each pair of functional
setae. It lies close under the peritoneum, and
usually bears two conical immature setse.

4. The wall of the alimentary canal. This will be found
to vary slightly in detail in different regions, — the in-
testine is here described.

a. The epithelium, a single layer of elongated cells,
lined internally by a delicate cuticular mem-
brane.

b. The muscular layer — feebly developed. It consists
of an internal circular and an external longitudinal
series of fibres.

The thickening of the wall of the gizzard is due to
an excessive development of the circular fibres.

c. The sub-epithelial tissue; a vascular layer interposed
between a and b. Look for the cut ends of its
small vessels.

d. The yellow-brown tissue; a thick investment for the
whole, immediately external to b (outside the wall
of the whole viscus). Its component cells are
elongated and club-shaped, each containing a
single oval nucleus and crowded with minute
highly refractive globules.

Examine the muscular fibres of the mesenteric
septum. An aggregation of circular fibres is fre-
quently met with immediately external to the ali-
mentary canal. Many of its radial fibres can be
traced into the body-wall.

5. The central nervous system, described in Sect. G. e.

268 ELEMENTARY BIOLOGY. [CHAP.

I. The reproductive organs. (Cf. Sect. C. <?.)

i. Obtain the body of a sexually immature worm (i.e.
one in which there is no trace of clitellum) which has
been well hardened in alcohol. Open up the an-
terior twenty segments along the dorsal middle line
and remove the alimentary canal with care, leaving
the mesenteric septa undisturbed. Look for —

a. The spermathecce^ two pairs of small yellowish-
white globular bodies, attached to septa 9 — 10
and 10 — ii.

They are developed within the substance of the
septa referred to, and may project forwardly into
segments 9 and 10 or backwardly into 10 — u.

Their points of exit are invariable, and they may
conveniently be used as landmarks in dealing with
the remaining organs of the reproductive system.

b. The seminal vesicles, three pairs of soft whitish organs
attached — in the sexually immature worm — to septa
9 — 10, 10 — u and n — 12. • They are internal to a.

In the sexually mature animal they unite with each
other and their fellows of the opposite side, below
the alimentary canal, so as to form two masses ; an
anterior bilobed one on either side, the body of
which lies in segment 10, and a posterior unilobed
one for segment 11.

c. Remove the seminal vesicles of one side and ex-
amine the mouths of the vasa deferentia (ciliated
rosettes). They are dead-white glairy structures,
lying near the middle line in segments 10 and 11 ;
when fully formed exceedingly conspicuous and
chalky looking.

d. Trace back the ducts of <:, on one side. Each per-
forates the septum behind it and becomes im-

III.] THE EARTHWORM. 269

mediately convoluted, uniting in segment 12 with
its fellow to form the vas deferens. Follow this
back, it runs longitudinally midway between the
nervous axis and the exterior, passing out on seg-
ment 15.

e. The testes (visible only in sexually immature speci-
mens), two minute white bodies truncated pos-
teriorly; attached, on either side, to septa 9 — 10
and 10 — ii close to the middle line. They pro-
ject into segments 10 and n.

In sexually mature worms these and the ciliated
rosettes are both embraced by the enlarged seminal
vesicles.

/. The ovaries, two small conical bodies, attached by
their bases to septum 12 — 13 in a line with e.
Their free ends are not unfrequently recurved.
They project into segment 13.

g. The oviducts. Remove the segmental organs from
septum 13 — 14, seize the latter with forceps and
turn it backwards — the mouths of the oviducts will
be seen opening into segment 13 in a line with/
The ducts themselves are very short and closely
bound down to the hinder face of the mesenteric
septum. They pass outwards and backwards,
opening on to the exterior in a line with d.

h. The receptacula ovorum; tuft-like appendages of the
hind-walls of the oviducts. They project freely
into segment 15, and are rendered highly con-
spicuous by their great vascularity.

/. Remove the entire ovary and transfer it to a slide,
stain with eosin and examine under a high power.
Commence at its free end, and note —

2/0 ELEMENTARY BIOLOGY. [CHAP.

a. The ripe ova, large round bodies completely
filling the pointed extremity of the saccular
ovary. A granular vitellus, a large round ger-
minal vesicle and a vitelline membrane will be
observed for each.

{$. The immature ova. Work towards the base of
the ovary — the ova decrease in size and become
more numerous. The younger of them are
destitute of a vitelline membrane. Work back
towards the attached end, and note graduating
into /3 as (3 does into a —

y. the germinal epithelium, a solid mass of small
undifferentiated cells filling the base of the
ovary.

/ Remove a small portion of a testis of the youngest
worm obtainable, crush and examine in eosin under
a high power. Note the cells of the germinal epi-
thelium, irregular in shape, each with a large central
nucleus and a single nucleolus.

k. Tease up in a similar manner a small portion of
one of the seminal vesicles of an older animal.
Treat with acetic acid and magenta — spermatozoa
in all stages of development will be seen. Look
for the following —

a. Cells of the germinal epithelium identical with
j. They are sometimes to be found only with
difficulty.

/?. The same during the early maturation stages —
conspicuous as the predominant mulberry-like
masses. Focus to the surface of one of these
and work through it. It consists of a large pro-
toplasmic body, differentiated — peripherally into

III.] THE EARTHWORM. 2/1

a series of transparent nucleated protuberances —
centrally into a large non-nucleated mass.

y. The same during the later maturation stages.
Nuclear division has gone on with rapidity, and
the central mass is now surrounded by a great
number of small nucleate bodies. Examine care-
fully the protoplasmic investments of these — each
is prolonged out into a short filament.

8. The ripe spermatozoa; aggregates of filiform struc-
tures resulting from the nucleate bodies above
described. Each is enlarged at its attached end
to form a 'head,' within which the elongated
nucleus is contained. Focus to the centre — the
non-nucleated mass has remained passive and
unaltered.

e. Look for free spermatozoa.

IV.
THE COMMON SNAIL (Helix aspersd],

THE "Common Snail", H. aspersa, and the "Garden
Snail", H. hortensis, are to be found in abundance in our
gardens and hedgerows, and the descriptions here given
apply equally to either with the exception of the shell.
This, in the ' Garden Snail ', can be at once recognized by
its delicate texture and predominant whitish-yellow colour,
as compared with the rough-surfaced brown -banded shell of
its ally. During the summer months the 'Common Snail'
is to be met with, leading an active independent existence
upon or in the immediate neighbourhood of fruit-bearing
shrubs. It is very susceptible to cold, and retires during
the later autumn to some recess in a wall or tree, where it
usually remains dormant and hibernating until the following
spring. It not unfrequently buries itself for the same pur-
pose, and even in the warm season may be induced to
hibernate temporarily if starved or submitted to a reduced
temperature. During such a period, or during normal hi-
bernation, the body is completely retracted within the shell,
the mouth of the latter being sealed by a film of mucus
secreted by the animal, which hardens on exposure to the
atmosphere ; this is perforated to allow of the passage of
air during respiration and is best termed for obvious reasons
the hybernaculum. During the hibernating period the animals
are frequently to be encountered huddled together in as-

CHAP. IV.] THE COMMON SNAIL. 2/3

semblages ; hence it is no uncommon thing to find, during
the warm season, individuals, to the exterior of whose shells
there adhere one or more (often a great number) of these
hybernacula, cast off by their fellows on emerging from the
dormant state.

The body of the snail is soft and unsegmented, and,
unlike that of any other animal dealt with in this work,
asymmetrical — inasmuch as the anal, respiratory, excretory
and genital orifices all open to the right side, the latter
being situated far forwards near the mouth. The ventral
surface of the body is thick and fleshy, giving rise to a
locomotor foot, by the wave-like contractions of which the
sluggish movements of the animal are performed. So deli-
cately adjusted are these, that the creature can crawl with
ease and comfort over a knife-edged surface. The anterior
end of the body is differentiated into a well-marked head
segment bearing two pairs of tentacles — a shorter labial pair
adjacent to the mouth and a longer ocular pair situated
above and behind these. The integument covering the apex
of each tentacle is especially modified in connection with a
nerve supply derived from a large underlying ganglion,
whence it follows that the eye-bearing tentacle performs a
double function. It may be that the labial one is either
tactile or olfactory, but the exact functions of these sensi-
ferous areas have yet to be fully elucidated.

. Between the head-segment and the free anterior end of
the pedal-disc there is a cleft, at the base of which opens a
large mucus secreting pedal- gland which extends far back
to the hind end of the body.

The dorsal surface of the body is produced into a spirally
coiled hump, within which the whole digestive gland and
portions of the alimentary and reproductive viscera are
lodged — it is hence termed the visceral sac. This sac, to-

M. 18

2/4 ELEMENTARY BIOLOGY. [CHAP.

gather with the wall of the pulmonary chamber which over-
lies it in front, is invested in the spirally coiled shell, the
apex of which lies altogether to the animal's right side.
The mouth or peris tome of the shell overlies the thickened
anterior border of the pulmonary sac, from which a constant
addition of shelly matter is secreted during the growth of
the animal, as is also the hybernaculum during repose.

The aperture of the mouth is bounded by soft fleshy lips,
and it leads into a spacious buccal cavity the walls of which
are excessively thick and muscular. A denticulate horny
upper jaw or beak is present, and the floor of the mouth is
raised up into a cushion-shaped odontophore or tongue,
which is in turn surmounted by a dentigerous lingual-ribbon
or radula. This is thrown into a licking rasp-like motion
during feeding, by the activity of an underlying musculo-
skeletal apparatus, the odontophoral cartilages connected
with which are worthy of note as composing an endoskeleton.
The mouth itself leads into a long tubular oesophagus,
which passes straight back and, on entering the visceral
sac, opens into a small stomach which receives the secretion
of the digestive gland. The stomach in turn gives origin
to a coiled intestine which, on nearing the exterior, skirts
the lower right-hand border of the pulmonary sac, termi-
nating in an anus which lies to the right of the respiratory
orifice. The middle segment of the oesophagus is enlarged
to form a distensible crop, applied to which there are a
pair of salivary glands, confluent above and pouring their
secretion into the mouth by means of two elongated
ducts.

The digestive gland is a paired structure ; its lobes are
asymmetrical — the smaller right one lying altogether within
the top whorls of the shell. Microchemical examination
shows that it performs a complex function, serving both as a

IV.] THE COMMON SNAIL. 275

storehouse of combustible fatty carbo-hydrate material and
as a centre for secretion of a digestive ferment.

The pulmonary sac or mantle arises as a fold of the
body wall, in which pulmonary vessels appear during de-
velopment. At the hinder end of the enclosed pulmonary
chamber there are situated, side by side, the heart and
kidney. The heart is enclosed in a definite pericardium,
the floor of which is in open communication with the ex-
cretory organ by means of a short ciliated reno-pericardial
duct. The excretory organ itself lies altogether to the right
side of the body and debouches on to the exterior by a
long duct, running parallel with the rectum.

The heart consists of a single auricle and ventricle, the
valves between them being so disposed as only to admit of a
current passing from the lung sac to the body. It therefore
transmits only aerated blood, and as it is in no way con-
cerned with the propulsion of the blood to the respiratory
organs it is termed — like that of the Crayfish already con-
sidered— a systemic heart. The ventricle gives origin to a
single aorta which, on entering the body-cavity, subdivides
into two branches. The anterior of these supplies all parts
of the body which lie in front of the heart, and the posterior
is restricted to . the visceral sac and its contents. These
arterial trunks break up into minute ramifications, which
pass either into capillary systems or lacunar spaces, all of
which converge, directly or indirectly, towards a great sinus
which lies at the base of the pulmonary sac. From this,
afferent pulmonary vessels arise on all sides ; the branches
of these, reuniting in the substance of the lung-sac, form
a system of efferent pulmonary vessels, which unite to
form a large pulmonary vein which enters the heart. The
efferent pulmonary vessels of the right side pass, on their
way to the heart, through the excretory organ, in the sub-

18— 2

2/6 ELEMENTARY BIOLOGY. [CHAP.

stance of which they break up into a second (renal)
capillary network.

The blood contains amoeboid corpuscles, which float in
an opalescent serum ; it assumes a bluish tinge on exposure
to the atmosphere.

The central nervous system is enclosed in a membranous
circum-oesophageal sheath. It consists of three yellowish
ganglionic masses ; the supra-asophageal or cephalic lying
above the gullet and giving off nerves to the head segment
and related parts ; the pedal which supplies the foot and
body-wall; and the parieto-splanchnic which distributes
fibres to the body-wall and viscera, and all parts lying behind
its point of origin, irrespective of the foot. These ganglionic
centres are connected together by lateral commissures ; and
from the cerebral mass there arise a system of buccal nerves
in relation with the buccal mass and its odontophore, and
others distributed to the sense organs. The latter are, a
pair of small auditory vesicles to be hereafter described (see
Sect. J. 3) and visual and tegumental sense organs borne
by the tentacles, to which reference has already been made.

The snail is hermaphrodite and the sex-organs are highly
complicated. With the exception of the hermaphrodite
gland or ovotestis, a portion of the duct of the same and its
appended albumen-secreting gland — all of which are lodged
in the visceral sac, they fill the greater part of the spacious
body cavity and can be at once recognized by their dead-
white colour. As the hermaphrodite duct approaches the
exterior it suddenly divides into distinct oviduct and vas
deferens ; the base of the latter is enlarged to form a swollen
eversible intromittent organ or penis, which opens, side by
side with the oviduct, into an integumental pit or genital
cloaca. Appended to the whole apparatus there are several
accessory glands and diverticula. Chief among these is a

IV.] THE COMMON SNAIL.

ccecal diverticulum of the base of the penis which secretes
a mucilaginous investment for the spermatozoa; the sper-
matophores or packets of spermatoza thus formed are trans-
ferred, during copulation, to a corresponding ccecal diver-
ticulum of the oviduct known as the receptaculum seminis.
Fleshy valves are developed within the lips of the genital
cloaca and at the orifices of the genital ducts which open
into it, and the whole condition of the organs is such as to
obviate the possibility of self-fertilization.

In the spiculum amoris, an accessory to the female portion
of the apparatus (see Sect. G. 3 /) we have a structure,
almost without parallel in the whole animal kingdom. It
reaches maturity during the breeding season, and is forcibly
ejected from individual to individual during the amorous
overtures, which last for a period of some hours.

The spermatozoa are long filiform bodies, each with
an enlarged nucleus-bearing "head." The ova are chiefly
noteworthy on account of the absence of a distinct vitelline
membrane ; they are comparatively large and are provided
with a nutritive food-yolk.

The reproductive elements of opposite sexes ripen alter-
nately, the maturation of a given batch of spermatozoa pre-
ceding that of the ova they are destined to fertilize, and in
view of the facts above related it follows that the sperma-
tozoa must be transferred, during copulation, to the body of
the second individual — there to await the descent of the ova.

Fertilization takes place as the ova leave the body, and
there are to be found in the haunts of these animals during
the summer months, usually beneath some stone or de-
caying wood or more rarely buried in the earth itself,
aggregates of 100 or more eggs, each invested in an albu-
minous envelope, within which the early developmental
phenomena are undergone.

278 ELEMENTARY BIOLOGY. [CHAP.

The conditions under which embryonic development is
passed through in the " Pond Snail " (Lymnceus stagnalis) are
much more favourable to observation than in the case of the
"Common Snail," and they are accordingly, here dealt with.

The "Pond Snail" is a sluggish carnivorous animal which
may be found in abundance during the summer months in
ponds and stagnant waters ; it is moreover an indispensable
acquisition to the aquarium. If confined in a hand-glass or
table aquarium and well fed, the animals will very generally
deposit their eggs upon the vessel, and they will be found
in aggregates, firmly adherent to each other and to the. sur-
face by means of the albuminous secretion aforementioned.
The important developmental changes are undergone while
still in this investment.

The transverse diameter of the fertilized ovum is about
the 2-^-o'th of an inch. The segmentation is holoblastic and
unequal, and the changes undergone during the early de-
velopmental period are substantially such as have been
already described for the Earthworm (cf. p. 247), resulting
in the formation of a simple two-layered gastrula.

During segmentation there appear on the surface of the
dividing mass, some two or more minute protuberances,
which finally become constricted off and lost. A portion of
the nucleus of the cell whence each arises is carried away
with it. Various interpretations have been put upon these
polar bodies; they are of fairly general occurrence in the
animal kingdom, but further discussion concerning them is
beyond the scope of this work. They are alluded to here,
as the conditions of observation are exceedingly favourable.

After the gastrula phase is passed, the embryo assumes a
somewhat spherical shape, during which period the mouth
is formed as a median involution of the epidermis. There
now appears on the surface, immediately in front of the

IV.] THE COMMON SNAIL. 2/9

mouth, a thickened zone incomplete ventrally. The sur-
face of the body becomes ciliated and especially so this
zone, whereupon there results a rapid rotation of the embryo
within its albuminous investment. This being so, the zone
in question is termed the trochal ridge, the larva possessed
of it being said to be in the trochosphere stage. This, in
turn, gives place to a more advanced veliger stage, so called
on account of the changes undergone by the trochal ridge,
which now becomes more marked, being produced out into
a hood-shaped pre-oral lobe or velum. During this stage
the mantle arises as a fold of the body-wall, which, as age
advances, takes on the characters of a lung sac.

During the final stages of larval metamorphosis the left
side of the body grows much more rapidly than the right
one. Thus it is that the originally bilaterally symmetri-
cal larva becomes converted into an asymmetrical adult,
a fact which renders clear the displacement of the orifices
(other than those of the mouth and pedal gland) and of
the organs connected therewith, the suppression of the ex-
cretory organ of one side, and the enormous increase in
size of the left lobe of the digestive gland, as compared
with the smaller right one.

Very early in the history of the larva the locomotor foot
arises, as a median ventral outgrowth between the mouth
and anus ; and as the importance of this structure becomes
more marked, the cilia of the velum undergo a reduction.
Consequent upon these changes the rotatory movements of
the animal, so characteristic of the trochosphere, give place
to a sluggish creeping motion. The velum itself does not
entirely vanish in Lymnaeus, but persists throughout life
as a couple of so-called subtentacular lobes which lie im-
mediately above the mouth. These are wanting in the
" Common Snail."

280 ELEMENTARY BIOLOGY. [CHAP.

LABORATORY WORK.

If the Snail be killed, by immersion in water heated to a
temperature such as the hand can comfortably bear, the
shell will readily part company with the muscles to which it
gives attachment. This being the case, no difficulty will be
experienced in removing the animal, if, holding it in the left
hand, the shell be twisted off by the finger and thumb of
the right.

In dissecting the internal organs it is advisable to re-
move the visceral sac in starting. This may best be
done by cutting away its thickened edge with a pair of
scissors, and tearing it off with a couple of pairs of strong
forceps.

A. General external characters,
i. In the living animal observe :

a. The body ; produced ventrally into an expanded
locomotor disc or foot ; and dorsally into a spirally
coiled visceral-sac, invested in the single coiled
shell or exoskeleton.

b. The head segment ; a freely projecting anterior lobe
of the body, which overhangs the foot. It bears
two pairs of retractile tentacles; a smaller lower-
most labial pair, situated at the sides of the mouth,
and a longer dorsally placed ocular pair, at the
summit of each of which there is borne, when fully
extended, a minute black eye.

c. The apertures. Examine the animal en face, and
note :

IV.] THE COMMON SNAIL. 28 1

a. The mouth ; surrounded by a thick circular lip,
externally to which there is, on either side, a
well-marked lateral lip. Observe that the mouth
is bounded above by a denticulate horny beak,
lying within the circular lip.

/8. 'Ite pedal gland ; opening by a wide aperture, at
the base of a depression between the head seg-
ment and the foot. Insert a seeker into it — it
can be readily introduced for a distance of more
than an inch.

y. With the animal still in this position, note that
the shell (and visceral sac which underlies it)
is carried altogether to the creature's right side.

Place the snail right side uppermost, and examine :

8. The genital orifice, situated a short distance be-
hind the bases of the tentacles of the right side.

e. Gently raise the shell, and note, underlying its
free edge, the thickened glandular border of the
pulmonary sac. Enclosed by valve-like folds of
this there lies the large respiratory orifice. Situ-
ated side by side with, and a little to the right
of, this, is the anus. (The excretory orifice also
opens at this point, but within the lip of the
respiratory one. It is not visible without dis-
section. Cf. Sect. C. r d.)

£. The genital furrow, a feebly defined integumental
groove, extending from the base of the pulmonary
sac to the genital orifice.

B. The shell or exoskeleton.

a. It forms a continuous investment. Its free edge
is produced into a whitish porcellanous reflected

282 ELEMENTARY BIOLOGY. [CHAP.

border or peristome, It is thrown into a spiral of
four whorls.

b. Note its texture and colour (cf. p. 272). As the
apex (first formed part) is approached, a smooth
friable texture and a pearly lustre are assumed.

c. The columella. If, with the peritreme directed to-
wards you, the shell be carefully opened up with
scissors, the columella will be seen as a central
pillar or axis. Cut into this with care ; it will be
found to be hollow, and closed in below by an
overgrowth of the peritreme — the shell is thus a
spirally coiled tube.

In young shells the columellar cavity opens freely
below, by an aperture or umbilicus.

d. The columellar muscles. Remnants of the glistening
white tendons of these are often to be found, at-
tached to the upper end of the shell axis. (Cf.
Sect. E. i a.}

e. The hybernaculum or epiphragm. Examine a dor-
mant specimen, and note that the mouth of the
shell is completely closed by this. Remove it and
examine under a low power ; note the perforation
of its central area.

C The pulmonary sac and its associated structures.

i. Examine the pulmonary sac from above. It consists
of a membranous expansion of the body-wall, which
overlies the entire antero-dorsal region of the visceral
hump. Large blood-vessels are developed within its
walls; and there is visible through it, on the right
side, the yellow excretory organ. Insert a scissors

IV.] THE COMMON SNAIL. 283

into the respiratory orifice and make an incision along
the right-hand border of the sac, cutting clear of
the excretory organ. Carry a second incision along
the thickened base of the sac towards its left side, and
reflect the whole. Note :

a. The floor of the pulmonary chamber; thin and
transparent, there being seen through it the dead-
white reproductive organs.

b. The rectum, a thin walled tube coursing along the
floor of the pulmonary chamber at its extreme right
hand border.

Follow it to the anus. It opens, at the base of
a groove-like depression in the lip of the respira-
tory sac, below and to the right of the respiratory
orifice.

c. The excretory gland, a considerable yellowish mass,
lying in the posterior right hand end of the pul-
monary sac. Make an incision into it and wash
out its contents ; note the thickened spongy texture
of its glandular lining, which is thrown into a series
of folds by the underlying blood-vessels.

d. The excretory duct, a yellow thin-walled tube,
running to the right of and parallel with the rectum.
Its orifice lies a little above and to the right of the
anus. Insert a blowpipe into this and inflate the
whole ; note that the duct is continued along the
right hand border of the gland to its summit.

If examined minutely, it will be seen that the
excretory duct is continued on beyond the above-
named orifice as a well marked excretory groove,
whose walls are contractile. This courses over the
base of the rectum dorsally to the anus, and, passing

284 ELEMENTARY BIOLOGY. [CHAP.

backwards and downwards, terminates to the left
of the respiratory orifice.

e. The pericardium, a small sac lying in a recess of
the left hand border of the excretory gland. Re-
move its front wall and examine the heart / it is
subdivided into a single auricle and ventricle.

2. Cut away the upper part of both the excretory organ,
and pericardium, and remove so much of them as
remains, together with a portion of the adjacent
visceral sac. Wash carefully until clear of sediment,
and examine in water under a low power.

A short ciliated duct will be found, passing from
the base of the pericardium to the excretory gland,
into which it opens by a reno-pericardial aperture.

1). The alimentary organs.

i. Place the animal on its left side, and pin it down
through the muscular foot. Remove the lung-sac and
liberate the rectum from its surroundings. Next dis-
sect away the right half of the body wall and visceral
sac.

There will be seen filling up the greater part of the
body cavity the dead-white generative organs; re-
move these en masse, whereupon there will be clearly
visible —

a. The crop ; a large sac-like organ, filling a con-
siderable portion of the"body cavity". It is generally
rendered the more conspicuous on account of the
yellow colour of its contents. Its lining membrane
is thrown into a series of longitudinal folds ; these
are visible through its thin walls, giving it an
apparent longitudinal striation.

IV.] THE COMMON SNAIL. 285

b. Follow the crop forwards, it will be found to arise
from the roof of the thick-walled muscular buccal
mass, as a simple oesophageal tube.

c. The sac of the radula, a small backwardly directed

diverticulum of the floor of the buccal mass.

d. The salivary glands, two irregular whitish masses
closely applied to the sides of the crop. They are
confluent postero-dorsally, and each is connected
with the roof of the mouth by an elongated
salivary duct. Follow one of these forwards, its
base becomes enlarged as it enters the mouth, at
the re-entering angle between the oesophagus and
buccal mass.

e. Follow the crop backwards, it will be found to pass
into the visceral sac under cover of the right lobe
of the digestive gland (a yellowish brown mass,
filling the apical whorls of the visceral hump).
Turn this gland downwards and forwards, and
note, lying beneath it, the small sac-like stomach,
into the hinder end of which the oesophagus is seen
to pass.

f. The digestive gland. Buried up in its small right

lobe will be found the white ovotestis (remove this
with care). Its left lobe is much the larger of the
two; it is subdivided into three lobules which adapt
themselves to the coils of the intestine. Arising
from these are seen well-defined ducts; follow
them and note that they unite to form a single
short digestive duct.

g. The stomach. Open this from the side, being
careful not to injure the right lobe of the digestive
gland, and wash out its contents. It is a simple

286 ELEMENTARY BIOLOGY. [CHAP.

sac, slightly subdivided into two by a longitudinal
fold. Four apertures will be found in its walls;
they are —
a. The cesophageal orifice, opening into its hinder

end.
J3. The origin of the intestine, lying below and to

the right of a.

y. The apertures of the digestive ducts ; opening —
that of the left side just above the intestinal
orifice — that of the right into its apex (this duct
skirts the free edge of its gland).

//. The intestine. After leaving the stomach it passes
upwards and forwards, altogether to the left side.
It then makes two turns upon itself, approaching
the right side as it does so, and finally courses
along the right border of the pulmonary sac. (Cf.
Sect. C. § i b.)
E. The buccal mass and odontophore.

i. Examine the buccal mass from the side and make out
its muscles. The constrictor fibres have already been
referred to, as constituting the muscular wall of the
whole structure. Of those which remain the more
important are—

a. The retractor ; a large sheet, arising from its floor
and side walls below the sac of the radula, and
passing back, side by side with the immense
retractor pedis fibres arising from the foot, to be
inserted into the columella or shell axis. (Cf.
Sect. B. d.)

b. The protractors ; delicate muscles arising from its
side walls, and passing downwards and forwards,
to be inserted into the cephalic integument.

IV.] THE COMMON SNAIL. 'Anfafy

c. The hvators ; delicate muscles arising just above

the protractors, and passing upwards to be in-
serted into the cephalic wall, near the bases of the
smaller tentacles.

d. The depressors, small muscles underlying the pro-
tractors, and passing obliquely backwards.

2. Carefully remove one half of the buccal mass, cutting
to the near side of the middle line. Examine under
water and note —

a. The odontophore, a cushion- shaped elevation of
the floor of the mouth, completely covered in
mucous membrane. There overlies it a yellowish
ribbon-shaped band — the lingual ribbon or radula;
follow this back into its sac (see Sect. D. i. c)
and note that as that is approached it assumes a
whitish colour and membranous texture.

b. The radula. Remove this bodily, with a pair of
forceps • transfer it to a glass slide, cover in water
and examine under a low power. Note the
presence of an immense number (between twelve
and thirteen thousand) of chitinous teeth. Those
which are functional can be readily distinguished
by their yellow colour and sharp pointed cusps.
Examine that portion of the radula which lay
within the sac. Note the transparent immature
teeth, becoming simpler and more papilla-like as
the hindermost border is reached. Examine in
like manner the front end. It is beset by teeth
whose cusps are worn down, and reduced, in many
cases, to the condition of mere functionless rudi-
ments.

288 ELEMENTARY BIOLOGY. [CHAP.

c. Examine the functional region under a \ or \
objective. Note that the bases of the two sets of
lateral teeth (uncini), by which the radula is for
the most part beset, are concave externally. Follow
those of one side inwardly — a single median longi-
tudinal row (rachis) will be reached, the bases of
whose teeth are concave on either side.

The rasping surface of each median tooth is pro-
duced into a pointed cusp. In the lateral teeth there
appears, on the inner side of this, a second smaller
one which increases in size relatively, as the free
edge of the radula is approached. There appears, at
the same time, a smaller external accessory cusp,
which is not represented in the median teeth.

d. The odontophoral cartilages ; two gristly masses to
whose presence the elevation of the floor of the
mouth is due. There are attached to their bases a
series of small intrinsic muscles^ arising from the
side walls of the buccal mass.

e. The horny beak ; seen to be formed in relation with
a special fold of the lining membrane of the roof of
the mouth. It lies wholly within the circular lip,
seen in section to be thick and fleshy.

/ The salivary duct ; opening into the roof of the
mouth by a minute orifice situated just above the
odontophore.

F. The Pedal Gland.

This is best examined at this stage, by removing the foot
to one side of the middle line. It has the appearance of a
white fluffy-looking mass, lying immediately above the
pedal disc, and extending back for two-thirds the length of
the same. Open it up with care — it consists of a coecal

IV.] THE COMMON SNAIL. 289

diverticulum of the integument, opening below the mouth
by an expanded orifice." (Cf. Sect. A. i. c. p.)

Compare the same, as seen in a transverse section across
the foot. Note its thick glandular walls and central lumen.
It is accompanied on either side by a well-defined lateral
pedal blood sinus.

G. The Reproductive Organs.

1. Pin the animal down as directed in Sect. D., and re-
move the right half of the body-wall posterior to the
genital orifice, together with the rectum. Dissect off
the visceral sac and examine in order —

a. The ovotestis or hermaphrodite gland; a small white
mass, buried up in a fossa of the right lobe of the
digestive gland. Remove sufficient of the latter
to fully expose it.

b. The duct of the ovotestis or hermaphrodite duct; a
short highly convoluted glistening white duct,
passing upwards from the ovotestis towards the
main mass of the reproductive apparatus.

2. Remove sufficient of the right lobe of the digestive
gland to liberate the ovotestis and hermaphrodite
duct; note —

a. The albumen gland; a large greyish white structure
lying, apex downwards, to the immediate left of the
duct of the ovotestis.

This gland swells up very rapidly under water, in
the manner of the glandular segment of the Frog's
oviduct already described (p. 53). Dissection may
conveniently be carried on under weak alcohol.
M. 19

2QO ELEMENTARY BIOLOGY. [CHAP.

In the vicinity of the base of this gland, the
genital organs come into close apposition with the
intestine and aortic vessels. Carefully liberate
them en masse from their surroundings and pin
down under water — the albumen gland with its
flattened face uppermost. Now follow in order —

b. The further course of the hermaphrodite duct. On
reaching the albumen gland it becomes suddenly
constricted, to form an exceedingly delicate thread-
like tube. As this is usually of a dead- white colour
it can be seen through the more transparent albu-
men gland, in the superficial portion of which it lies
buried. It passes for a short distance towards the
apex of this gland, and then, turning sharply upon
itself, is continued straight back towards the base
of the same ; it here becomes suddenly enlarged
to form a convoluted duct with sacculated walls,
which is continued on towards the genital orifice.

The transition between the two segments of the
hemaphrodite duct here described, is so sudden as
to make it appear that they are distinct structures ;
hence it is that the sacculated segment is sometimes
spoken of as an oviditct.

c. Its walls are beset externally by a continuously
straight series of glandular follicles, making up an
accessory gland (so-called prostate].

d. Open up the albumen gland and note its spacious
central duct. It will be found to enter the enlarged
"head" of the sacculated portion of the hermaphro-
dite duct. Follow the hermaphrodite duct onwards
— its sacculations gradually disappear, and it
suddenly divides into two — oviduct and vas deferens.

TV.] THE COMMON SNAIL. 2QI

e. The oviduct; a very short thick-walled tube, entering
into immediate relationship with a large white sac
— the dart sac (§ 3 e).

f. The vas defer ens; a thin-walled tube, arising side by
side with g, but of lesser calibre than it. Its inner
end is tied down to the female duct ; leaving this
it passes under the retractor muscle of the right
tentacles and becomes enlarged to form

g. the penis, apparent now as the elongated swollen
end of the vas deferens. It passes to the exterior
side by side with the female duct.

It is attached to the body-wall of the left side by
a ribbon-shaped retractor muscle. Cut this across
and displace the whole organ.

h. ^tespermatophoral gland mflagellum; an elongated
thread-like diverticulum of the base of the penis.
It is much coiled.

3. Remove the entire reproductive system from the body,
together with a portion of the body-wall immediately
adjacent to the sexual orifice. Pin the whole down
carefully under water, and open up the larger duct's
and their appendages, as they stand related to the
exterior. Make out in order :

a. The genital cloaca or vestibule; a shallow integu-
mental pit, opening on to the exterior by the single
genital orifice.

b. The penis. Remove a portion of its enlarged
eversible base. It opens into the front wall of the
vestibule ; its lining membrane is produced into a
fold or prepuce, above which there is usually an
eminence.

19—2

ELEMENTARY BIOLOGY. [CHAP.

c. The oviduct. Follow this down ; its first segment
already described (§ 2 c) is sometimes termed
the vagina. This opens by a tumid lip into a thin-
walled terminal segment, which enters the vesti-
bule side by side with the penis.

d. The spermatheca or receptaculum seminis (receptacle
of the spermatophores) ; an elongated diverticulum
of the above-named terminal segment of the ovi-
duct, bound down during life to the hermaphrodite
duct (§ 2 b). Liberate it from this, and note that
it arises by a short neck which subdivides into
two ccecal diverticula — a longer and stouter coiled
one, and a shorter one which terminates in a globular
enlargement (this lies, during life, tucked away in
the bend of the first coil of the intestine. Cf.

e. The dart-sac ; an immense melon-shaped diverti-
culum of the base of the oviduct, in life backwardly
directed. Open it up and note its powerful muscular
walls ; lying within it will be seen

f. the dart or spiculum amoris, a four-bladed calcareous
projectile. Its head overlies a papillate ingrowth
of the lining membrane of the sac.

g. The accessory mucus or digitate glands; two
branching masses opening side by side into the
basal segment of the oviduct, immediately above e.

H. The circulatory system.

Examine an animal freshly removed from its shell, before
it is in any way dissected. The pulmonary vein will be seen
running in the wall of the lung-sac, in front of and in a line
with the excretory organ. Make an incision in this and

IV.] THE COMMON SNAIL. 293

introduce a small syringe or medicine-dropper filled with
injecting material. Seize the vessel and the inserted nozzle
between the finger and thumb of the left hand, and inject
If this simple operation be carefully performed, all the
leading vessels which carry aerated blood will be injected.

1. Lay open the pulmonary-sac, and note :

a. The efferent pulmonary vessels ; a uniformly arranged
series of trunks bringing in the blood from the
lung-sac ; they unite to form a large pulmonary
vein which enters the auricle.

b. The efferent renal vessels ; two or three small trunks
conveying the blood from the excretory organ to
the pulmonary vein.

During the process of injecting these become very
rapidly filled, and in the exercise of the pressure
necessary to inject the whole arterial system their
walls are not unfrequently ruptured.

2. Open the heart (cf. Sect. C. i e). Note the thick
muscular wall of the ventricle, and the auriculo-ven-
tricular valves — so disposed as only to admit of a
passage of the blood from the auricle to the ventricle.

3. The arterial system. Remove the right half of the
body-wall and visceral sac, and together with them
the rectum and greater portion of the genital ap-
paratus— cutting the sacculated hermaphrodite-duct
across at about its middle.

There will thus be exposed the great arteries. Work
them over in the following order :

a. The aortic trunk, a very short vessel, arising from
the base of the- ventricle. It lies to the left of the
head of the first coil of the intestine, immediately

294 ELEMENTARY BIOLOGY. [CHAP.

below which it suddenly subdivides into an ante-
rior and a posterior branch.

/>. The anterior aorta; a well-defined vessel running
parallel with the sacculated portion of the her-
maphrodite-duct. On entering the body-cavity it
passes beneath the first coil of the intestine and
the adjacent globular head of the spermatheca
(G. § 3 d] ; it then travels to the right side of the
body and runs on, beneath the crop, to the anterior
extremity. Branches are given off along its course
to the genital apparatus, foot, crop, salivary glands
and tentacles, with the adjacent integument; it
finally perforates the sheath of the nervous system,
ending in relation with the floor of the buccal mass.
Special branches accompany the rectum and pedal
gland.

c. like posterior aorta; a considerable vessel which ac-
companies, at its outset, the first limb of the in-
testine. It descends into the visceral sac, and is
mainly distributed to those portions of the ali-
mentary canal and its appended glands contained
within the same.

4. The venous system. With the exception of certain
well-defined sinuses and vessels detailed below, the
venous channels consist mainly of a series of lacunar
spaces, in free communication with the"body-cavity."
Reference has already been made (Sect. F.) to the
lateral pedal sinus, and if, in an uninjured Snail, the
point of the injecting apparatus be carefully inserted
into this, no difficulty will be found in demonstrating
the existence of the vessels here described.

Having injected the animal, reflect the lung-sac,

IV.] THE COMMON SNAIL. 295

cutting close alongside the rectum, in order that that
portion of it bearing the smaller pulmonary vessels
may be reflected intact.

Note in order :

a. The circulus venosus pulmonis ; a great sinus sur-
rounding the base of the pulmonary sac.

b. The marginal sinus of the visceral sac; a spacious
channel continuous with a. posteriorly and running
within the thickened edge of the visceral sac

On exposing the interior of the pedal sinus in an
uninjected specimen, there will be seen opening
into it the orifices of adjacent lacunae. During the
process of injection from the above sinus the fluid
introduced passes through these — partly into the
body-wall, partly into the "body-cavity— in either
case finally reaching the pulmonary circulus, to-
wards which all those channels in which venous
blood circulates eventually converge.

c. The afferent pulmonary vessels ; arising from the pul-
monary circulus on all sides. They alternate with
striking regularity with the efferent pulmonary
vessels, the two sets of trunks being connected by
a well-differentiated respiratory plexus.

The afferent pulmonary vessels of the left side
and front wall of the lung-sac are elongated.
Those of its right side are very short, but in so far
as their relations to the lung-sac are concerned
they differ from their fellows in no other respect ;
after leaving that to reach the heart however, they
enter the excretory organ and break up within it
into a renal plexus^ from which the efferent renal
vessels previously described (Sec. H. i b) arise.

296 ELEMENTARY BIOLOGY. [CHAP.

The walls of the vessels concerned in respiration
are frequently pirmented. The entire pulmonary
system can therefore be generally followed in an
uninjured specimen, without the aid of injection, by
examining with care the wall of the lung-sac from
the outside.

I. The nervous system.

i. Pin the animal down right side uppermost and open
up the lung-sac, cutting along the edge of the rectum;
divide the latter, together with the adjacent excretory-
duct and lung-sac, across its middle transversely, and
reflect the left hand half. Next remove the right
half of the wall of the body and visceral-sac, together
with the tentacles of the same side and the greater
portion of the genital apparatus. Remove the foot
to the level of the pedal gland, and with it the right
half of the retractor pedis muscle. Follow in the
order given :

a. The central nervous system. It consists of a cir-
cum-oesophageal mass, buried up in a loose con-
nective-tissue sheath. Carefully dissect away the
superficial portion of the sheath, until the nerve-
ganglia are exposed (looked at thus from the side,
but one set will be seen) ; they are —

b. The supra- (Ksophageal ganglia; two considerable
masses lying above the gullet, connected together
by a short band-like commissure.

c. The pedal ganglia ; two large masses lying im-
mediately above the pedal-disc. They distribute a
considerable series of fibres ventrally to the foot
and pedal gland, and a smaller set laterally to the
adjacent body-wall. Each pedal ganglion is united

IV.] THE COMMON SNAIL.

to the supra-cesophageal of its own side by a
cerebro-pedal commissure.

d. The parieto-splanchnic ganglia; a series of gangli-
onic masses lying in close apposition, immediately
behind and a little above c. They give off fibres
(on the right side three sets and on the left two),
which enter the body wall at the base of the
visceral sac.

The anterior fibres (pallial nerves) enter the
thickened border of the lung sac, within which they
anastomose.

The ganglia of either side are connected with
the supra-oesophageal by a cerebro-splanchnic com-
missure, which runs parallel with but is somewhat
shorter than the cerebro-pedal one.

e. The posterior-visceral nerve; a single nerve arising
from d. altogether to the left side. It passes back
immediately beneath the anterior aorta (which it
accompanies) to enter the visceral sac. On doing
so, it gives off branches as under —

a. A main one, running on to reach the left side of
the visceral sac.

p. A right one, whose fibres can be traced to the
albumen-gland and duct of the ovotestis.

y. A dorsal one, arising from the main trunk, in
the immediate vicinity of the enlarged head of
the spermatheca. It is distributed to the excre-
tory-organ and heart.

2. Remove sufficient of the cephalic wall of the right
side to expose the whole buccal mass. Look for —

298 ELEMENTARY BIOLOGY. [CHAP.

a. The buccal nerves. That of the right side will
alone be seen ; it arises from the supra-cesophageal
ganglion, and passes downwards and forwards to-
wards the buccal mass. Trace it with care — it
passes beneath the dorsal protractor muscle (see
Sect. E. i b), and terminates in a small buccal
ganglion, lying just behind the enlarged base of the
salivary-duct.

Just before it reaches the muscle referred to, it
usually gives off two or three branches to the
antero-lateral wall of the buccal-mass.

b. The anterior visceral nerves ; a system of fibres
arising from the above ganglion. The following
can be seen with ease.

a. A ventral branch, distributed to the oesophagus.

/?. A dorsal one, to the salivary gland, the duct
of which it accompanies.

y. An anterior one, to the roof of the buccal-
mass.

3. Remove the whole nervous system, together with the
buccal-mass and tentacle-bearing portion of the
cephalic integument. Pin down under water with
its posterior end uppermost, and dissect off as much
as possible of the sheath of the nerve-collar. Note,
in addition to the nerves already dissected —

a. The buccal commissure; a delicate tract of nerve-
fibres, seen, on raising the cut end of the oeso-
phagus, to connect the buccal ganglia of opposite
sides. It passes beneath the gullet.

IV.] THE COMMON SNAIL. 299

b. The anterior aorta. Its cut end is clearly visible
at this stage. (Cf. Sect. H. 3 b.} It perforates
the nerve-collar ventrally, running beneath the
parieto-splanchnic and above the pedal ganglia.

c. The tentacular nerves ; arising, together with the
labial nerves, from the sides of the supra-ceso-
phageal ganglia. The nerve to the optic tentacle
is connected with a special ganglionic lobe.

J. The Sense-Organs.

i. Obtain a specimen in which the tentacles are fully
extended. Remove the cephalic integument and
tentacles, together with the nervous system, en masse;
pin down very carefully under water and dissect
under a lens. Open up the tentacles and note —

a. The retractor muscles; two ribbon-shaped bands
arising from the extreme anterior ends of the
tentacles ; they pass back to be inserted into the
shell axis, side by side with the larger retractors
already described (Sect. E. i a).

The muscles of the smaller pair of tentacles re-
ceive slips from the adjacent cephalic integument.

b. The labial tentacle. Note that the upper end of its
retractor muscle is darkly pigmented. Trace up
the tentacular nerve which accompanies the muscle;
it ends in a large pear-shaped ganglion, from which
fibres are distributed to the integument investing
the free end of the tentacle.

The nerve sends a branch to the adjacent cephalic
integument.

c. The ocular tentacle and eye. Its retractor muscle
and nerve repeat the conditions described for b ; a

300 ELEMENTARY BIOLOGY. [CHAP.

large terminal ganglion is present, in relation to
the integument investing its free end.

The visual organ is perceptible as a minute
black spot, lying to the outer side and a little in
front of the ganglion. A delicate optic nerve can
be traced back from it to the tentacular nerve,
which it enters a short distance behind the above-
named ganglion.

2. Remove the eye together with a small portion of its
related integument, and examine in water under a
low power. Note —

a. The choroid; a pigmented investment for the eye,
under cover of which the optic nerve passes to
reach the retina.

b. The crystalline lens; seen to project freely beyond
a. in front. It lies under cover of a delicate
cornea.

3. The auditory organ. Pin one half of the nerve-collar
down under water, and, having dissected off the
superficial stratum of its sheath, examine with a lens.
Note—

a. The auditory nerve ; a delicate undulating trunk,
lying in the interspace between the cerebro-pedal
and cerebro-splanchnic commissures. Follow it
up — it arises from the supra-cesophageal ganglion.
Below, it enters the otic vesicle, now seen as
a minute white speck, overlying the point of
apposition of the pedal and parieto-splanchnic
ganglia.

b. The otic vesicle. Remove this, together with a
small portion of its surrounding tissues, and

IV.] THE COMMON SNAIL. 30 1

transfer in toto to a glass slide. Cover and ex-
amine in water under a low power. It appears as
a conspicuous round vesicle, lodging a refractive
black-looking granular contents.

Examine under a high power. It is seen to
consist of a small sac (otocyst) closed on all sides
and in no way connected with the exterior. It is
filled with a fluid contents, in which are suspended
a large number of small oval calcareous corpuscles
or otoliths. Trace the passage of the auditory
nerve into the vesicle.

K. The reproductive elements.

Remove a small piece of the ovotestis, and mount in
aqueous solution of Eosin. Examine under a low power
and note —

a. The hermaphrodite follicles ; small coecal diverticula
of the ovotestis, from the epithelial lining of which
the sexual products are derived. They are arranged
in grape like aggregates.

The appearances met with will be found to vary
with the season of the year. If spermatogenesis is
going on there will be found, floating in the central
cavity of each follicle, spermatozoa in all stages of
development. If the ova are undergoing maturation,
they will be found, in all stages, still adherent to
the germinal epithelium.

Crush under the weight of a cover-glass and examine
under a high power.

a. The ripe spermatozoa. Each is an elongated fili-
form body, bearing a small head-like enlargement
at one end. They are commonly met with in
aggregates.

302 ELEMENTARY BIOLOGY. [CHAP.

Immature spermatozoa are at times present in
•abundance. They can be recognised by the relatively
large size of the 'head', which is oval and can be
distinctly seen to lodge a small nucleus. (Cf. Lum-
bricus and Rana.}

b. The ripe ovum. Examine under a high power.
a. Its shape; usually round, more rarely oval.
y8. The absence of a distinct vitelline membrane,
y. rF&z protoplasm ; consisting of a central uniformly

granular vitellus, which graduates peripherally

into a superficial clear layer.
3. The germinal vesicle ; a large round structure,

usually containing one immense ger?ninal spot.

L. Development. (The Pond Snail, Lymnaus stagnalis.)

See p. 278.

Examine the developing eggs from time to time, and look
especially for —

a. The fertilized ovum (posperm) during segmentation.
It divides up into two sets of cells — a smaller
rapidly dividing clearer set, and a larger slowly
dividing yolk-laden set (the presence of the yolk
gives these an opaque coarsely granular appear-
ance). Look out for polar bodies (see p. 278).

b. The same at the close of segmentation. Looked at
from the outside the larger cells appear as a dark
mass, lying within the smaller more transparent
ones, which have now completely overgrown them.

c. The gastrula phase, immediately following upon b.
Examine from beneath and note

a. the blastopore; a small slit-like orifice on the
flattened under surface.

TV.] THE COMMON SNAIL. 303

Examine from the side in optical section.
/3. The archenteron; a sac-like pit opening externally

by the blastopore, its walls are formed of the

invaginate yolk-bearing endoderm cells,
y. The ectoderm ; a single layer of investing cells, the

product of the more transparent rapidly dividing

ones referred to above.
8. The cleavage-cavity or blastoccele ; a spacious cavity,

enclosed between the investing and invaginate

layers.

d. The Trochosphere larva; recognizable by its egg-
shaped contour and rotatory movements,
a. The mouth-, a small median orifice situated at the

enlarged end.

/3. The trochal ridge ; a saddle-like band encircling
the dorso-lateral area pre-orally. Look for its
cilia.

Examine in optical section and note —

y. The J vof (it first appears at this stage); a median
ventral outgrowth of the body wall just behind
the mouth.

3. The stomodceum ; a blind sac-like involution of
the integument, its aperture giving rise to the
mouth.

e. The archenteron; now partially surrounded by~a
conspicuous large-celled granular mass — the
digestive gland, arising as an outgrowth of its
wall.

Note the bilateral symmetry of the larva at this
period. The blastopore appears shifted back,
as the result of elongation of the embryo and

304 ELEMENTARY BIOLOGY. [CHAP. IV.

displacement, upon the development of the foot.
An integumental pit known as the shell-gland,
appears at the hind end of the body during this
stage.

e. The Veliger larva. A head-segment is now be-
coming differentiated, and the rotatory movements
of the animal are less marked. Bilateral symmetry
is becoming disturbed. Note especially —

a. The velum; a ciliated pre-oral fold, occupying
the position of the original trochal-ridge.

ft. The foot ; now greatly increased in size and
utilized for purposes of locomotion.

y. The eye-spots; two black masses at the sides of
the head segment.

f. The advanced larva. Tentacles are now appearing,
as paired outgrowths of the cephalic wall (the eyes
are not carried up by them as in Helix). The
original bilateral symmetry no longer obtains.
Note the beat of the heart ; the presence of the
visceral sac and of its related shell — at this stage
a transparent chitinous cap. The velum is under-
going a marked reduction.

If any difficulty is experienced in examining the
embryos in the living state, they should be liberated
from the surrounding albuminous investment by
means of a couple of needles, and submitted, for 20
minutes, to the action of \ p.c. Osmic acid solution.
They may then be transferred to alcohol of in-
creasing strength, and preserved for future ex-
amination.

V.

THE FRESH-WATER MUSSEL

(Anodonta Cygncea).

UNDER the name of 'Fresh -water Mussel' two distinct kinds
of animals, which are not unfrequently abundant in our ponds
and rivers, are included; namely, the Anodonta and two or
three kinds of Unto. The Anodonta is chosen for special
study here, but what is said about it applies very well to
nearly all parts of Unto except the shell.

The animal is enclosed in a shell composed of two pieces
or valves, which are lateral, or right and left, in relation to
the median plane of the body. The more rounded and
broader end is anterior, the more tapering, posterior. If
placed in a vessel of water, at the bottom of which there is a
tolerably thick layer of soft mud or sand, and left quite un-
disturbed, the Anodonta will partially bury itself with its an-
terior end directed obliquely downwards; and the valves will
separate at their ventral edges for a short distance. At the
edges of this 'gape' of the shell the thickened margins of a
part of the contained body which is called the mantle, be-
come visible, and between them a large, whitish, fleshy,
tongue-shaped structure — the foot — not unfrequently pro-
trudes, and is used to perform the sluggish movements of
which the Anodonta is capable. Mud 'tracks^ are left
behind the animal when in motion, as can be seen on ob-
serving its natural habit in slow running or still shallow

M. 20

306 ELEMENTARY BIOLOGY. [CHAP.

waters. If some finely dividing colouring matter, such as
indigo, Is dropped into the water, so as to fall towards the
gape, it will be seen to be sucked in; while, after a short
time, a current of the same substance will flow out from an
opening between the two edges of the mantle on the dorsal
side of the posterior end of the body; and these 'inhalent'
and 'exhalent' currents go on, so long as the animal is
alive and the valves are open. Any disturbance, however,
causes the foot, if it was previously protruded, to be re-
tracted, while the edges of the mantle are drawn in and the
two valves shut with great force. This adduction results
from the contraction of two thick bundles of muscular
fibres, which pass from the inner face of one valve to that
of the other, one at the anterior and the other at the
posterior end of the body, and are called the anterior and
posterior adductors. The valves of the dead Anodonta
always gape, and if they are forcibly shut they spring open
again. The reason of this is the presence of an elastic
band or ligament, which unites the dorsal margins of the
two valves, for some distance, and is put upon the stretch
when the valves are forcibly brought together. The natural
tendency to divarication of the valves resulting from this, is
held in check during life by the contractions of the afore-
named adductor muscles. These respond to a nervous
stimulus, and, on the death of the animal, that being with-
drawn, the full force of the elasticity comes into play, the
,valves becoming divaricated to their utmost extent.

The animal can be extracted from the shell without
damage, only by cutting through these muscles close to their
attachments. It is bilaterally symmetrical, the foot pro-
ceeding from the middle of its ventral surface. There is a
total absence of any distinct head segment such as is seen
in the Snail. The mouth is median and situate between

V.] THE FRESH-WATER MUSSEL. 307

a projection, which answers to the under surface of the
anterior adductor muscle, and the superior attachment of
the foot. On each side of the mouth are two triangular
flaps with free pointed ends — the labial palpi — and behind
these, on each side, two broad plate-like organs, with ver-
tically striated outer surfaces, are visible. These are the
gills or branchice. In the dorsal region, the integument is
soft and smooth; on each side, it is produced into large
folds, the lobes of the mantle or palliitm> which closely
adhere to the inner surface of the valves of the shell, and
end, ventrally, in the thickened muscular and glandular
margins already mentioned. They pass into one another
in front of the mouth; at the sides, they are united with
the dorsal edges of the outer gill-plates ; and, behind,
they extend upwards and on to the dorsal face of the
body, finally passing into one another above and in front
of the anus, which is small, tubular, prominent and
median. Thus the anus is inclosed in a part of the cavity
bounded by the two mantle-lobes which lies above the gills,
it is relatively small and shallow, and is termed the supra-
branchial or cloacal chamber; while the gills, the foot, and
the palps, hang down into the relatively large infra-branchial
chamber which occupies the space between the mantle-
lobes for the rest of their extent. During life the posterior
margins of the mantle lobes are prolonged for a short dis-
tance behind the free edge of the valves, and they come
into apposition at the point of attachment of the gills
in such a way as to give rise to two tubes or siphons — a
dorsal supra-branchial one which receives the anus, and a
ventral one communicating with the infra-branchial chamber
alone. The dorsal siphon is the channel through which
the exhalent currents pass; the ventral, that for the inhalent
currents.

308 ELEMENTARY BIOLOGY. [CHAP.

The currents are produced and kept up by the action of
the cilia, which abound upon the gills, labial palps and
inner face of the mantle. The gills are perforated by in-
numerable small apertures, and the cavities contained
between the two lamellae of which each is formed, are
in communication, above, with the cloacal chamber. The
cilia work in such a way as to drive the water in which the
animal lives from the outer surface of each gill towards its
interior. Hence, to a large* extent, the current which sets
from the infra-branchial to the cloacal chamber.

The current of water which is thus continually drawn into
the infra-branchial chamber carries with it minute organ-
isms, Infusoria^ Diatoms and the like, and many of these
are swept to the fore part of the chamber, where they enter
the mouth, and are propelled by the cilia which line its
cavity into the alimentary canal. The latter presents a
short and wide gullet, a stomach surrounded by a digestive
gland, a long intestine coiled upon itself, in a somewhat
complicated manner, and, finally, a rectum, which lies in
the middle line of the dorsal aspect of the body, traverses
the pericardium and the heart which lies therein, and ends
in the anus.

The subdivision of the mantle-cavity into supra and
infra branchial chambers is, in the long run, the result of
confluence of adjacent gill-laminae with each other and
with the pallial lobes. The gill-perforations, the groove en-
closed by the labial palps and the aperture of the mouth
towards which it leads, all open into the lower or inha-
lent chamber; while the upper or exhalent one receives
the anus, and both genital and excretory orifices. During
life the thickened margins of the pallial lobes are in close
apposition; consequently, the infra-branchial chamber is
closed below and in front, both it and the supra-branchial

V.] THE FRESH-WATER MUSSEL. 309

one communicating with the surrounding medium through
the agency of the siphonal prolongations of the mantle
border alone. The walls of the two chambers and the
surfaces of all the organs contained within them, together
with the whole lining membrane of the alimentary canal,
are ciliated, and the currents thereby induced set in, as
before stated, by the lower and out by the upper siphon.
One of three courses is open to the inhalent current with its
suspended food material — it may pass either through the
gills, through an interspace between the gills and the body
wall, or into the mouth. Take whichever course it may, it
finally reaches the supra-branchial chamber. It follows that
the insetting current is both a nutritive and a respiratory
one, while the outsetting one serves to carry away the
waste products of respiration and digestion, together with
the products of the excretory and genital organs.

The clue to the real meaning of the more important
structural features in the organization of this animal, is to
be sought in an understanding of the above facts.

Digestion, that is solution of the proteinaceous and other
nutritive matters contained in food, is effected in the sto-
mach and intestine; and the nutritious fluid, thus formed,
transudes through the walls of the alimentary cavity and
passes into the blood contained in the blood-vessels which
surround it. This blood is thence carried into a large sinus,
which occupies the middle line of the body under the peri-
cardium and between the organs of Bojanus (see Laboratory
Work E), and receives the greater part of the blood return-
ing from all parts of the body. From this median vena cava,
branches are given off to the gills and open into the exten-
sive vascular network which those organs contain. From
this, again, trunks lead towards the pericardium and open
into one or other of the two auricles of the heart, which

310 ELEMENTARY BIOLOGY. [CHAP.

communicate by valvular apertures with the ventricle. The
ventricle gives off two aortic trunks, one of which, the ante-
rior, runs forwards in the middle line, above the rectum,
while the other runs backwards, below the rectum. From
these two aortse branches are given off which divide into
smaller ramifications for the different regions of the body,
and for the viscera, and finally terminate in channels which
answer to the capillaries of the higher animals.

The pericardial cavity, in which the heart is lodged, is
situated in the posterior half of the dorsal region of the
body. Through its thin dorsal wall, and, still better, when
it is carefully laid open, the heart can be seen beating. The
auricles contract, and, after them, the ventricle; the wave-
like contraction of the latter being much the more easily
visible. The lips of the auriculo-ventricular apertures are so
disposed that the blood is impeded from flowing back into
the auricles, when the ventricle contracts, and is forced out,
either forwards or backwards, through the two aortse. From
these it finds its way to the capillaries, and returns from them
to the vena cava; whence it is carried, through the organs of
Bojanus, to the branchiae. Here it becomes purified of car-
bonic anhydride, and receives oxygen from the water in
which the branchiae are plunged; and it is finally brought
back in an arterialized condition to the heart.

The heart is therefore systemic and propels aerated
blood. As the heart, in most animals, first appears either
in close proximity to, or in the substance of, the wall of the
embryonic alimentary canal, there is no real anomaly in its
position in the Anodonta.

The blood of the Anodonta is colourless, and contains
colourless corpuscles, which resemble those of Man in struc-
ture and present the same Amoebiform movements.

The majority of the vessels which convey the blood from

V.] THE FRESH-WATER MUSSEL. 311

the vena cava to the branchiae, traverse the walls of the
dark-coloured organs — the organs of Bojanus — which have
already been mentioned; and they here part with their
nitrogenous waste matters — the organ of Bojanus playing
the part of a kidney. The cavity of the organ of Bojanus
communicates, on the one hand, with the pericardium ; and,
on the other, with the exterior, by an aperture to which
reference has already been made. Thus the cavity of the
pericardium communicates directly with the exterior, though
by a roundabout way.

The organ of Bojanus consists of a pair of modified tubes,
each of which can, like the nephridium of the worm, be
resolved into three segments — an internal non-glandular, a
middle glandular, and a terminal muscular one. The rela-
tions of the whole to the pericardium in this animal, are as
those of any one pair of segmental organs to the body-cavity
in the worm; and if so be that the pericardium of Anodonta
is, like that of the Frog, a direct derivative of the body-cavity,
the excretory organ is in no way anomalous in its rela-
tionships.

The digestive gland of this animal is mainly concerned
in the elaboration of a digestive fluid. The production
of combustible carbo-hydrate material — a function by no
means confined to the liver in other animals — goes on to a
considerable extent in other far removed parts of the body,
glycogen being formed. The connective-tissues and espe-
cially the mantle lobes are remarkable for its presence, it
being elaborated in the form of large intra-cellular vesicles,
as the product of activity of certain of the connective-tissue
corpuscles1.

The nervous system of the Anodonta consists mainly of

1 Similar glycogenous cells are met with in the walls of the lacunar
spaces and on the ' mesenteries ' of the Snail.

312 ELEMENTARY BIOLOGY. [CHAP.

three pairs of yellow ganglia; the cephalic, situated at the
sides of the mouth; the pedal, placed in the foot; and the
parieto-splanchnic, on the under face of the posterior adductor
muscle. They are united by commissural cords which con-
nect the cephalic ganglia with one another, and with the
pedal and parieto-splanchnic ganglia, respectively. Traces
of a small fourth pair of ganglia may be obvious on the
cerebro-splanchnic commissures, near the anterior end of
the pericardium. As the parieto-splanchnic ganglia are
immediately connected with a patch of sensiferous epi-
thelium in the roof of the inhalent siphon, they are sometimes
regarded as olfactory. The only other sense organs which
have been discovered, are a pair of auditory vesicles, con-
nected by nervous cords with the pedal ganglia.

The sexes are distinct. The testes and ovaria are similar
in character, being racemose glands, which, in the breeding
season, occupy a great part of the interior of the body.
There is one gland on each side, opening by a minute aper-
ture close to that of the organ of Bojanus.

The spermatozoa have minute, short, rod-like bodies, to
which a long, filamentous, active cilium is attached; they
are thrown off in enormous numbers, and make their way
out with the exhalent currents.

The ova are spherical, and the vitelline membrane is defi-
cient at one point, leaving a terminal aperture or micropyle,
through which, in all probability, the spermatozoon makes its
entrance. When fully formed, multitudes of these ova pass
out of the oviducal apertures and become lodged in the
chambers of the gills, particularly the external ones, which
during autumn and winter are completely distended by
them and the embryos to which they give rise.

Segmentation is holoblastic, and the early developmental
phases resemble, in their main features, those already de-

V.] THE FRESH-WATER MUSSEL. 313

scribed for the Snail — the original bilateral symmetry is
however never disturbed.

The embryos when hatched, are so wholly unlike the
parent Anodonta, that they were formerly thought to be
parasites, and received the name of Glochidium. Each is
provided with a bivalve shell, and each valve has the form
of an equilateral triangle united by its base with its fellow,
by means of an elastic hinge, which tends to keep the two
wide open. The apex of the triangle is sharply incurved,
and is produced into a strong serrated tooth, so that when
the valves approach, these teeth are directed towards one
another. The mantle is very thin, and the inner surface of
each of its lobes presents three papillae, terminated by fine
pencils of hair-like filaments. The oral aperture is wide,
and its margins are richly ciliated. There is a single ad-
ductor muscle and a rudimentary foot, from which one or
two long structureless filaments, representing the byssus of
the sea-mussel, proceed. These byssal filaments become
entangled with one another and tend to keep the ' Glochi-
dia ' in their places.

The gill-laminae of Anodonta will accommodate some
three millions or more of these Glochidia. If the animal is
living in company with fish it will eject them, whereupon
they attach themselves to floating bodies — very commonly
to the tails, fins or gills, of fishes — by digging the incurved
points of their valves into the integument in the latter case,
and holding on by them as if they were pincers. In this situa-
tion they become encysted in an epidermal overgrowth of
the host's body, within which they undergo a final metamor-
phosis.

The alimentary canal and foot become more marked with
the increased development of the whole body \ the gills
appear, in the form of filamentous outgrowths of the body-

314 ELEMENTARY BIOLOGY. [CHAP.

wall, which become plumose and subsequently unite to
form the adult gill-lamellae; the byssus is thrown off, and
the larval shell and adductor muscle are replaced by newly-
formed valves provided with two adductors. The young
Anodonta at length drops off and falls into its ordinary
habitation in the mud, a bilaterally symmetrical animal,
differing from its parent only in the absence of functional
reproductive organs.

LABORATORY WORK.

The contraction of the foot and consequent displace-
ment of the viscera, resultant upon death under chloroform,
may be overcome by killing the animal in water sufficient to
cover it — heat slowly to about 40° C.

In an animal freshly removed from the water only the
shell or exoskeleton is visible, but if killed as above directed
this will be slightly open, the foot will be protruded as
in life and the edge of the membrane lining it (the mantle]
will be visible. Raise one valve of the shell, by separating
the mantle from it with the handle of a scalpel, and then
cut through two strong bodies (the adductor muscles), one
at each end of the animal, which pass from valve to valve
and prevent their separation. The two valves will now be
united only by their ligament, as the result of whose elas-
ticity they gape ; cut through this, thus liberating one
valve and work over —

A. General external characters.

i. In the animal now laid bare may be distinguished —
a, A dorsal border turned towards the hinge of the

V.] THE FRESH-WATER MUSSEL. 315

shell, and nearly straight. A curved ventral border^
opposite the dorsal.

b. A wider anterior end. A narrower posterior end.

c. A right and left side.

2. Pin the animal down under water without removing
its remaining valve (insert the pins obliquely, so that
they embrace, and exercise a downward pressure
upon, the edges of the valve). Examine in order — •

a. The mantle m pallium ; a bilobed semitransparent
membrane, one lobe lining each valve of the shell.
Examine its ventral border, it is yellowish in colour
and thickened to form &palliaZ muscle.

b. The adductor muscles of the valves ; two immense
masses whose cut edges are visible at opposite ends
of the body.

c. The retractor pedis muscles; smaller masses, one at
the upper end of each adductor. The anterior one
is posterior, and the posterior one anterior, to its
corresponding adductor.

d. The protractor pedis muscle; uniform in diameter
with c. It lies a little below the anterior re-
tractor.

e. The lesser retractors ; insignificant bundles of fibres
arising from the extreme dorsal surface of the body
wall.

3. Turn back the parallel lobe, and note under-
lying it —

a. The ventral portion of the body. It projects for-
wards and downwards, immediately behind the
anterior adductor muscle : it is thickened ventrally

316 ELEMENTARY BIOLOGY. [CHAP.

to form the locomotor foot; a large, yellowish,
somewhat ploughshare-shaped mass, whose apex
projects freely between the mantle-lobes.

b. The gills or branchice; two lamellar organs on each
side of the body, extending to its posterior end.

c. The labial palps ; a pair of membranous folds on
each side, in front of the gills and immediately
below the protractor pedis muscle. Note the
structural similarity between them and the gills.

B. The pallial-lobes, in relation to adjacent structures and
the exterior. The branchial chambers and siphons.

Remove the animal completely from its shell, by detach-
ing the other mantle-lobe from the valve to which it is fixed
and cutting through the attachments of the muscles to the
same.

Pin down under water (the pins should preferably be
thrust through the adductor muscles). Raise the pallial
lobe nearest you with forceps and remove it, cutting along
its line of confluence with the underlying organs and the
body wall. Examine carefully —

a. The infra-branchial chamber; bounded above and
externally by the pallial lobe, a — c of Section A. 3
lie within it.

Follow the cut edge of the mantle, working from
behind forwards. It courses along the dorsal
border of the external gill lamella, whence it passes
downwards and backwards to reach the labial
palps j it skirts these in a similar manner, and is
continued on beneath the anterior adductor
muscle.

V.] THE FRESH-WATER MUSSEL. 317

b. The mouth; a wide aperture, lying immediately
behind and below the anterior adductor muscle.
It is encircled by the labial palps.

c. Examine the thickened posterior border of the
pallial lobe. It is prolonged backwards for a
short distance as a pigmented tentacle-bearing lip
— the wall of the ventral siphon. Compare the
living animal, as it lies with its anterior end buried
in the sand, and note —

a. The projection of the ventral siphon beyond the
free edges of the valves.

fi. The close apposition of the thickened ventral
margins of the pallial lobes.

y. The siphonal tentacles. A series of small con-
tractile finger-like lobes, the larger of which
project freely across the siphonal passage in
the path of the insetting current.

Touch one of them with a bristle and note
the immediate closure of the siphonal aperture
— the tentacles are highly sensitive. Examine
one of the longer ones under a lens ; its base is
swollen and it terminates in a slightly expanded
and flattened extremity.

8. Examine the relations of the gill laminae. The
outer one is confluent with the pallial lobe
"externally and with the inner lamina internally,
for its whole length : the inner one is confluent
with the body wall for a short distance in front
and with its fellow of the opposite side behind ;
there is a spacious cleft between it and the body
wall.

318 ELEMENTARY BIOLOGY. [CHAP.

d. Insert the point of a scissors into the pallial lobe
immediately above the gill laminae, and make an
incision which shall pass under the posterior
adductor muscle and immediately over the line of
attachment of the outer gill-lamina. A large
opening will be found in the dorsal middle line, in
the region of the posterior adductor muscle; insert
the scissors blade into this and cut obliquely
backwards. There will now be laid bare the
supra-branchial or doacal chamber.

a. Follow its boundaries. It ends blindly in front,
immediately above the most anterior point ot
attachment of the gills. It opens posteriorly by
two orifices; a dorsal median one referred to
above, and a posterior one, whose lips are
produced into a short smooth-walled dorsal
siphon. It is walled in dorsally and externally
by the pallial lobes and ventrally by the gill-
laminae.

ft. The suspensory ligament of the gills can now be
seen ; it is a membranous fold arising immediate-
ly beneath the posterior adductor muscle. In-
sert a seeker from below into the supra-branchial
chamber, through the cleft between the body
wall and inner gill-lamina. It passes internally
to this ligament, which therefore subdivides the
greater part of the supra-branchial chamber into
two portions, each of which is in open commu-
nication with one gill-cavity.

y. Remove the anterior portion of ft and note,
opening into the inner subdivision of the supra-
branchial chamber, the genital and excretory

V.] THE FRESH-WATER MUSSEL. 319

orifices; two small apertures lying one above the
other and embraced postero-ventrally by a
common lip. (For further details see Sects. E.
and L. a. £.).

8. The rectum; a yellowish thin- walled tube cours-
ing over the hind face of the posterior adductor
muscle. It opens by a terminal wide-mouthed
anus.

Introduce into the immediate vicinity of the
ventral siphon of the living animal some finely
divided colouring matter. It will be drawn into
this with the inhalent current induced by the
ciliary action at work, and ejected, a little later,
through the dorsal siphon with the exhalent one.
(Cf. pp. 308—9).

e. Note the position and general relations of the
pericardia! cavity. It lies in the middle line
immediately above the supra-branchial chamber,
wholly in front of the posterior adductor muscle.
Examine in relation to it

a. the organ of Keber (pericardial gland} ; a red-
brown thickening of the adjacent body-wall
and pallial-lobe; it is most conspicuous ante-
riorly, above the gills.

/?. Lay open the cavity of the pericardium and note
the rectum; a yellowish tube, passing through it
on its way to the supra-branchial chamber. It
is surrounded at its middle by the yellow thick-
walled ventricle. (For further details see Sects.
F. and L.)

Keber's organ can be advantageously seen
at this stage, if looked at from within.

320 ELEMENTARY BIOLOGY. [CHAP.

C. The alimentary organs.

1 . Remove the valve, mantle-lobe, gills and labial-palps
of the left side, and open up the pericardium.

Pin down under water (preferably without remov-
ing the right valve) and dissect away the body-wall;
the coils of the intestine will come into view, two
which lie parallel to one another at the posterior
end of the body, being probably those first seen.
Note, lying within the body —

a. The digestive gland; a large greenish-brown mass,
filling the interspace between the anterior adductor
muscle and the pericardium.

b. The reproductive gland ; a yellowish-white mass
lying around the coils of the intestine, and filling
the greater portion of the body-cavity.

2. Remove one half of the digestive gland piecemeal
and pick away the reproductive gland, until as much
as possible of the course of the intestine is exposed.
Work out the whole alimentary tube in detail, com-
mencing at the mouth; examine in order —

a. The gullet ; a spacious tube lying immediately
behind the anterior adductor muscle; it runs
upwards and backwards to enter

b. the stomach, a large irregular sac lying immediately
in front of the pericardium.

Both a. and b. are buried up in the substance of
the digestive gland ; the ducts of this will be seen
opening into b. by a series of very large orifices.
Examine the relationship of one or more of these
to the lobes of the gland itself. Cf. Sec. L. d.

c. The intestine. It arises by a wide aperture from
the floor of the stomach; insert a scissors blade

V.] THE FRESH-WATER MUSSEL. 321

into it and lay bare its entire lumen — its course
is as follows. It first passes to the left side down-
wards and backwards, and, on nearing the hinder
end of the body, turns upwards towards the peri-
cardium; it then bends sharply upon itself and
passes downwards and backwards, in a line with
the free posterior edge of the body-wall. On
reaching the middle ventral region it is again bent
sharply upon itself, passing upwards and back-
wards altogether to the right side; at about the
middle of the reproductive gland it becomes
suddenly greatly enlarged, and passes (parallel
with the first segment) upwards and forwards
towards the anterior end of the pericardium. It
enters this at its base, and, running upwards and
backwards, leaves it postero-dorsally, finally pass-
ing over the posterior adductor muscle. (Cf.
Sect. B. d. 8.)

d. Having laid bare the interior of the intestine, re-
duce its walls to the condition of median longi-
tudinal section and examine the intestinal valve or
typhlosole; a thick rich yellow ingrowth of its lining
membrane. It is very conspicuous in the first and
the enlarged terminal segments.

e. The labial-palps^ in relation to the mouth. Obtain an
undissected specimen, and, after removal from the
shell, cut away the anterior end of the body, to
the level of the labial-palps ; pin down and examine
from beneath. The aperture of the mouth is oval.
The labial-palps diminish in size as the mouth is
reached and they embrace it in a lip-like fashion;
they are confluent on either side and their free

M. 21

322 ELEMENTARY BIOLOGY. [CHAP.

edges enclose a ciliated groove which leads directly
to the oral aperture.

f. Scrape off a little of the epithelium of the intestine ;
mount in eosin and examine under a high power.
Note the presence of cilia.

Should difficulty be experienced in following the
coils of the intestine, recourse may be had to injec-
tion. Mix equal parts, by bulk, of plaster of Paris
and water, stir well and strain through fine muslin ;
inject with a small syringe per anum.

D. The gills or branchiae.

a. Cut out one of the gills and examine it under
water; it will be found to consist of two lamellae
united by their ventral edges and enclosing a
central cavity, which opens into the epibranchial
chamber above. The gill-cavity is subdivided by
partitions, which pass from one lamella to the
other, and carry the larger blood-vessels of the
respiratory plexus.

b. Carefully isolate a small piece of one lamella ;
mount in water and examine with i inch obj.
It will be seen to be traversed by a great number
of small perforations, whose walls are supported
laterally by short chitinous rods; the substance of
the gill is permeated by a meshwork of large
vessels.

c. Examine under a higher power : the margins of
each cleft are lined by a ciliated epithelium.

E. The excretory organ (organ of Bojanus).

Remove the animal from its shell and pin down
through the adductor muscles. Dissect off the pallial

V.] THE FRESH-WATER MUSSEL. 323

lobe as directed for Sect. B. ; lay open the pericardial
chamber and remove the auricle. The excretory organ
will now be visible as a blackish mass underlying the
floor of the pericardium; it becomes more marked
posteriorly, and extends behind the pericardium, termi-
nating immediately in front of the posterior adductor
muscle.

Remove the anterior two-thirds of the outer gill-
lamina leaving its cut edge, and then dissect away
carefully a portion of the anterior third of the outer
wall of the inner lamina; there will thus be laid bare

a. The excretory orifice (renal aperture); a small pore
situated immediately below and behind the anterior
end of the pericardium; it opens into the supra-
branchial chamber, immediately above the genera-
tive orifice. (Cf. Sect. B. d. y.) Insert a seeker
into it and wash the whole under a gentle current
until quite clean; the seeker will be found to
project into

b. the excretory vestibule (non-glandular portion of
Bojanus); a thin-walled muscular sac, extending
back to the hinder end of the pericardium. Open
it up and gently raise its cut edge ; at its extreme
anterior end it will be seen to be in communica-
tion with its fellow of the opposite side, by an
immense oval inter-renal aperture.

c. Underlying b. and visible through its thin wall,
there will now be seen the glandular segment of the
organ. Open up the posterior portion of this, it is
tubular and its walls are plicated.

Examine the cut edges of the whole with care;
note the transition from the glandular to the mus-

21 2

324 ELEMENTARY BIOLOGY. [CHAP.

cular segments, as seen in the cut edge of the roof
of the hinder portion of b.

d. Follow the glandular segment forwards; it be-
comes constricted anteriorly (in the region of the
inter-renal aperture) to give rise to the thin-walled
segment; a short whitish tube, which opens into
the extreme anterior end of the pericardial
chamber by a small reno-pericardial aperture.
(Cf. Sect. F. 2. a.)

F. The heart.

i. Dissect an Anodonta from its shell, and remove the
whole ventral portion of the body, well below the
pericardium. Pin the upper portion down under
water dorsal surface uppermost, and carefully lay
bare the pericardial cavity.

The heart will now be exposed; it is a yellowish
transparent sac, probably exhibiting regular contrac-
tions, composed of a median and two lateral chambers,
these are

a. the ventricle; a median pear-shaped body, its thick
end directed backwards; it embraces the hind
segment of the alimentary canal. All parts of the
wall of the ventricle do not contract together; but
a wave of contraction passes, from one end of it to
the other, like the peristaltic contraction of the
intestine in one of the higher animals.

b. The auricles; one of these will be seen on each
side, if the ventricle be gently pushed out of the
way: each is a somewhat pyramidal sac, continuous
with the ventricle at the apex of the pyramid.

c. Remove the dorsal wall of the ventricle and of a
portion of one auricle. Note the thick spongy

V.] THE FRESH-WATER MUSSEL. 325

nature of the former : at its point of communication
with the auricle will be seen the auriculo-ventricular
valves; long pocket-shaped flaps, so disposed as to
admit only of a flow of the blood from the auricle
to the ventricle.

2. Remove all but the bases of the auricles, and cut the
rectum across at its extreme anterior end; turn it
and the heart backwards, so as to lay bare the floor
of the pericardium. Running along the middle line
of this will be seen a large blood- sinus, the vena cava:
it lies between the two excretory-organs.

a. At the extreme front end of the pericardiac floor,
immediately under the point at which the intestine
enters the cavity, will be found the reno-pericardial
apertures ; pass a bristle, or seeker, into one of
them and open up the excretory vestibule of the
same side. The glandular segment will now be
seen — the seeker projecting into it. Examine the
general relations of the excretory organs, and
their apertures of communication with the supra-
branchial chamber. (Cf. Sect. E.)

b. Examine the floor of one of the auricles under
a lens, first having washed the specimen quite
clean. Note the large orifices of the efferent bran-
chial vessels which open into it.

c. The organ of Keber. Follow its cut edge; it em-
braces the antero-lateral region of the pericardium.
(Cf. Sect. B. e. a.}

G. The circulatory system.

i. The arterial system. Lay bare the pericardial cavity
and insert the point of a medicine-dropper, filled with

326 ELEMENTARY BIOLOGY. [CHAP.

injecting material, into the ventricle. Inject slowly, and dis-
sect under water, right side uppermost. Remove the gills
of the exposed side, together with the hinder three-fourths
of the pallial-lobe ; leave the labial-palps, but remove the
entire body-wall. Work from the ventricle and follow in
order —

a. The anterior aorta; a spacious trunk embracing the
rectum dorso-laterally. It runs across the digestive-
gland to the right side, distributing branches to it
as it does so, and enters the body immediately
behind the anterior retractor muscle. Pick away
the digestive and reproductive glands, and follow
its course; it subdivides into visceral and pedal
branches, asunder.

a. The visceral artery. Its main branch accom-
panies the first coil of the intestine; it distributes
blood to the alimentary and reproductive organs.

ft. "IhQ pedal artery. Follow its further course; its
main trunk skirts the upper surface of the foot,
and distributes a number of branches to the
substance of the same. Soon after leaving the
anterior aorta, it gives off a labial branch to the
palps, and a pallial one to the anterior adductor
muscle and pallial-lobe.

b. The posterior aorta ; a much shorter trunk than a.
It passes beneath the rectum and is mainly distri-
buted to the posterior adductor muscle, the pallial
lobes and body-wall.

The two pallial arteries of either side anastomose
within the substance of the thickened border of the
mantle-lobe.

V.] THE FRESH-WATER MUSSEL. 327

2. Make a perforation in the roof of the ventricle, and
pass the injecting apparatus through it into one of the
auriculo-ventricular apertures; inject under a steady pres-
sure, wash and examine the undissected animal under
water. There will be seen —

a. The pallial sinuses ; a series of irregular channels
permeating the substance of the pallial lobe.

b. The efferent pallial vessel; a circular trunk, running
along the upper surface of the thickened mantle-
edge; it receives branches from the pallium at all
points.

Trace it upwards — it follows the line of attachment
of the pallial-lobe to the underlying organs (see
Sect. B.) ; it is coincident, along the line of attach-
ment of the gills, with the efferent-branchial sinus

(d.).

c. Remove the greater portion of the pallial-lobe, and
examine the efferent-branchial vessels — a series of
short parallel trunks, lying in the walls of the gill-
lamellae; each is formed by the confluence of
lesser vessels, coming in from the gill substance.
They open into

d. the efferent branchial sinus, a spacious chamber
overlying the gills ; in the pericardial region it
becomes expanded to form the auricle (Cf. Sect.
F. 2. b.).

3. Remove the rest of the mantle-lobe and open up the
underlying external gill-lamina, along its line of junction
with the. same. There will be seen running along the top
of the gill, at the base of the suspensory ligament (Sect. B.
d. (3.), the afferent-branchial sinus — a well-defined longitu-
dinal blood-space; make a perforation in this and inject,

328 ELEMENTARY BIOLOGY. [CHAP.

backwardly and forwardly. Wash carefully, and remove a
good portion of the outer wall of the external lamina; there
will be seen

a. the afferent branchial vessels, a series of short
parallel trunks, conveying the blood from the
above-named sinus to the gills.

Examine the afferent branchial sinus and note that
the injection has passed from it into the ventro-
external portion of the excretory organ, having
filled the efferent renal vessels.

b. Open up the pericardium, and remove the auricle,
thereupon exposed. Make a small hole in the
middle of the vena cava (Sect. F. 2) and inject both
backwardly and forwardly, exercising a gentle
pressure. Wash carefully, and note that the ex-
cretory organ is now completely injected, the
colouring matter having passed from the vena cava
Into the afferent renal vessels.

c. Remove the anterior half of the gill-laminae, and
dissect to the level of the vena cava. Follow this
into the body; it emerges immediately behind the
rectum, and is seen to be formed by the confluence
of a number of venous channels — some of the more
important of which accompany the intestinal coils.
Follow it backwards ; it can be traced to the under
surface of the posterior adductor muscle.

The vena cava can be dissected from above, as
directed in Sect. F, with comparative ease, in an
uninjected specimen; its lumen will admit of the
passage of a seeker.

H. The pallial lobe.

a. Tease up a portion of the substance of the pallial

V.] THE FRESH-WATER MUSSEL. 329

lobe of an animal which has been killed under
chloroform. Conspicuous among the cells thus
isolated there will be found large oval or rounded
ones, each lodging a central refractive globule.
b. Remove the mantle-lobe from a mussel which has
been killed as- above and subsequently preserved
in alcohol, and split it into two. Transfer a
portion of one half to a glass slide, torn surface
upwards, and treat with iodine solution; the
whole will be studded with small brown spots.
Examine under a high power ; the spots are seen
to be identical with the globules observed in
a. (their chemical reactions are those of gly-
cogen).

I. The nervous system.

i. Pin the animal down so as to get the body absolutely
rigid ; remove the mantle-lobe and gills of one side
and slit open the organ of Bojanus.

Wash until quite clean and examine under water,

a. Find the cerebro-splanchnic commissures; two parallel
white cords traversing the excretory organ. Trace
the near one backwards; it passes round the
posterior retractor tendon towards the under side
of the posterior adductor muscle. Turn this latter
over, so as the better to display its under surface,
and note

b. the parieto-splanchnic ganglia; two elongated yellow
masses confluent in the middle line, seen on
removing the membranous investment from the
ventral surface of the above-named muscle. They
distribute branches to the same, to the gills and
pallial lobe.

33O ELEMENTARY BIOLOGY. [CHAP.

c. Carefully snip away the membranous tissue at the
base of the labial palps ; there will thus be laid
bare the cerebral ganglia.

Each is about the size of a pin's head, and
somewhat triangular in form; it lies immediately
in front of the protractor pedis tendon, and dis-
tributes fibres to the anterior adductor muscle,
labial-palps and pallial lobe. Examine, in connec-
tion with the one exposed
a. the inter-cerebral commissure.

A short cord uniting the two cerebral ganglia
across the middle-line, above the gullet.
/?. The further course of the cerebro-splanchnic com-
missure.

Dissect off the fibrous tissue between the
anterior retractor and protractor tendons, and
remove the body-wall, between these and the
anterior end of the pericardium. The commis-
sure will be exposed, on carefully picking away
the superficial portions of the digestive and
reproductive glands along the same line.
tL Remove a small portion of the body-wall along the
point of origin of the muscular foot, and pick away
the genital gland until the pedal ganglia are reached;
they are a pair of deep-orange-coloured oval
bodies, each rather larger than a big pin's head,
applied to one another in the middle line, near the
point of junction between the body and the mus-
cular foot.

Branches are given off to the muscles of the
foot and to the auditory organ. See infra.

e. The cerebro-pedal commissure ; a cord which runs

V.] THE FRESH- WATER MUSSEL. 33!

upwards and forwards, from the pedal to the cere-
bral ganglion of either side.
(Cf. Sect. L./)

The cerebro-splanchnic commissures run close
together and parallel with each other, in their
course from behind forwards ; on leaving the peri-
cardialarea, they suddenly diverge and pass down-
wards and forwards to meet the cerebral ganglia.
If one of the commissures be removed and
examined under a low power, there may occa-
sionally be found, at the point of divergence, an
enlargement (visceral ganglion}.

J. The auditory organ.

a. This is rather difficult to dissect out in Auodon :
it is a small sac which may be found by tracing
back the posterior cords given off from the pedal
ganglion, to a branch of one of which it is attached.
There is a vesicle connected with each pedal
ganglion.

Failing the above, the otocyst can best be found
by removing a portion of the reproductive gland
lying immediately behind the pedal ganglion, and
examining under a low power.

b. If a fresh Cyclas1 be obtained, and its foot re-
moved, mounted in water, and examined with
i inch obj., the auditory sac can readily be seen
with a constantly-trembling particle, the otolith,
in it.

K. The shell or exoskeleton.

a. Its two hardened lateral pieces or valves ; each
with a straight dorsal and a curved ventral edge,
1 Cydas cornea — a small fresh- water lamellibranchiate mollusk.

332 ELEMENTARY BIOLOGY. [CHAP.

and an anterior larger and posterior smaller end,
the latter compressed dorso-laterally — note the soft
chitinous ventral edge of each valve.

b. The umbo; a small blunt eminence on the
dorsal border of each valve near its anterior
end; its apex is directed forwards.

Note the delicate texture of this area.

c. The ligament ; an elastic uncalcified part of the
exoskeleton behind the umbones, uniting the two
valves and tending to divaricate their ventral edges.

d. External markings. The outside of the shell is
greenish brown, and on it are seen a number of
concentric lines of growth, running parallel to the
margin of the shell and more numerous towards
its ventral edge.

e. Detach one valve by cutting through the ligament
with a scissors; before removing the animal
examine under water, and note the relations of the
chitinous ventral edge (a.). It extends along the
straight dorsal border of the valve in front of the
ligament ; and from that point in front, as from
the posterior edge of the ligament behind, it is
sharply inflected — passing inwardly to be reflected
on to the free border of the pallium. (Cf. Sect.
L. a. y.)

/ Internal markings. The interior of the valve is
white and iridescent : on it are seen, near the dorsal
border, two oval scars, the anterior and posterior
adductor impressions.

Joining the two adductor impressions is a double
curved line, the pallial impression, which marks the
point of attachment of the pallial muscle.

THE FRESH-WATER MUSSEL. 333

In front of the posterior adductor impression, is
seen a small scar of attachment of the posterior
retractor muscle.

Behind the anterior adductor impression are two
others, one opposite its upper, the other opposite
its lower end: the former indicates the point of
attachment of anterior retractor, the latter of the
protractor pedis muscle.

The adductor and retractor scars are not unfre-
quently confluent, and there extends from each
into the umbo a fainter tapering impression, in-
dicative of the growth of the muscles themselves
as the animal has increased in size.

g. Prepare (as directed for the Crayfish on p. 203)
a dried section across the middle of one valve, cut
at right angles to its long axis. Mount in Canada
balsam and examine under a low power, note —
a. The epiostracum; a thin uncalcified superficial
layer; it is greenish-yellow in colour, and very
frequently torn away in the process of grinding.
ft. The prismatic layer; composed of elongated
columns of calcified substance running parallel
with each other.

y. The nacreous or pearly layer; immediately in-
ternal to /?. Its relative thickness is proportion-
ate to the age of the animal from which the
valve was taken.

Examine under a high power — it is finely
granulated and traversed by delicate longi-
tudinal striae.

8. Make a tangential section across the prismatic
layer and examine under a low power. The

334 ELEMENTARY BIOLOGY. [CHAP.

prisms are mostly hexagonal or pentagonal in
contour; transverse diameter variable, often in
proportion as the section passes through their
inner ends.

e. Dissolve out the earthy matter from a piece of
a valve, by treatment with weak hydrochloric
acid. An organic basis remains behind, conform-
able in shape to the fully formed structure.

L. The study of transverse sections.

Remove one valve from an animal which has been dead
some 6 — 8 hours, and make sections as directed below —
cutting through the soft parts with a razor or sharp scalpel
and through the remaining valve with a bone-forceps.
Examine under water, and if some of the more minute parts
to which attention is directed do not fall in the plane of
section, dissect until they are reached.

Sections a, b, c, e, and / to be transverse to the long axis
of the body, d to be oblique.

a. Through the anterior pericardial region, immedi-
ately behind the excretory and genital orifices.
Work over —

a. The body ; median, laterally compressed below.
Note the muscular nature of the body-wall; it is
thickened ventrally to form the locomotor foot —
above the gills it is expanded dorso-laterally and
modified to form the organ of Keber.

/3. T\iz pallial lobes ; membranous outgrowths of the
thickened dorsal region of the body- wall; they
are in close apposition with the valves. Note
the thickening of their free borders to form the
pallial muscles.

THE FRESH-WATER MUSSEL. 33$

The exoskeleton. Observe that this forms, during
life, a continuous investment for all the exposed
surfaces of the body, with the exception of the
foot. It consists of a pair of calcareous valves,
which pass — dorsally into a cornified ligament —
ventrally into a chitinous free-border, which is in-
flected on to the edges of the pallial muscles.

The branchice; two pairs of lamellate organs,
confluent with each other and the body-wall
internally and with the pallial-lobes externally.
Examine their central cavities and attachments,
noting especially the subdivision of the supra-
branchial chamber into two, by the suspensory
ligament of the gills. (Cf. Sect. B. d, p.)

The body-cavity. This is almost obliterated by
the great development of the reproductive gland;
note the large pericardial chamber, situated in the
dorsal middle line.

The viscera. The cut edges of the intestine will
be seen, lying within the body and traversing the
pericardial chamber.

The excretory organ; median and paired, lying
immediately beneath the pericardial chamber.
The glandular segments of opposite sides are
seen to be approximated in the middle line;
note the reno-pericardial apertures lying imme-
diately above and internal to the attachments of
the inner gill lamellae. The muscular segments
are here confluent in the middle line, enclosing
a spacious cavity; dissect to the level of the
excretory orifice of one side, and note that it is a
perforation of the floor of this segment, opening

33^ ELEMENTARY BIOLOGY. [CHAP.

into the supra-branchial chamber, internally to
the suspensory ligament of the gills.

The cerebro-splanchnic nerve commissures will be
found, closely applied to the ventro-internal walls
of the glandular segments of the above. The
-vena cava here lies in the middle line immediately
below them.

The reproductive gland; a yellowish mass,
filling up all the available space in the infra-
branchial portion of the body-cavity.

Dissect it away on one side to near the level of
the excretory orifice; there will thus be laid bare
the genital duct. Insert a bristle into this and
open it up; it communicates with the supra-
branchial chamber immediately beneath and in-
ternal to the excretory orifice (Cf. Sect. B. d,
y). Follow the duct down and note the orifices
of the smaller ducts which open into it, and by
whose confluence it is formed.

A good notion of the ramifying nature of this
duct may be obtained by means of injection.

>/. Pick away the reproductive gland and note the
transverse muscles ; delicate tracts of tissue pass-
ing between the body-wall of opposite sides.

Examination of the above section will render it
clear that the pallial lobes are paired outgrowths
of the dorso-lateral portion of the body-wall. The
cavity which they bound during life, is sub-
divided— as the result of the confluence and
attachments of the gills — into infra and supra
branchial chambers ; the latter being partitioned
off into two portions, the inner of which receives
the excretory and genital ducts.

V.] THE FRESH-WATER MUSSEL. 337

b. Through the middle pericardial region, across the
heart and auriculo-ventricular valves. Compare
generally with a, and work over, in addition.

a. The gill-lamina ; the inner one is free internally,
the supra and infra-branchial chambers being
thus in open communication.

/8. The excretory organ. Note the increase in
calibre of the glandular segment, as its posterior
end is n eared; the vena cava is here interposed
between the muscular segments of opposite sides.
The cerebro-splanchnic commissures lie close
together beneath the vena cava. Cf. a. £.

y. The pericardial chamber and heart. The former
is here at its maximum of development. Observe
the fleshy wall of the ventricle and the thin auri-
cles; the auriculo-ventricular valves are pocket-
shaped in section, their mouths being directed
towards the ventricle.

Note the course of the rectum; it traverses
the ventricle, but is in no way bound down to it.

c. Through the middle of the posterior -adductor
muscle.

a. The attachments of the adductor muscle; it tra-
verses the body and pallial lobes, and is inserted
directly into the substance of the valves of oppo-
site sides.

fi. The rectum; lying in the dorsal middle line
immediately above the adductor muscle.

y. The supra-branchial chamber; completely shut
off, by the confluence and attachments of the
gills, from communication with the infra-bran-

M. 22

338 ELEMENTARY BIOLOGY. [CHAP.

chial one, except through the gill-slits. Its dorsal
prolongation (cf. Sect. B. d\ seen, in section, as
a small cavity overlying the rectum.
8. The parieto-splanchnic ganglia; two yellow bo-
dies confluent in the midddle line, buried up in
a connective tissue sheath, immediately beneath
the posterior adductor muscle.

d. Obliquely backwards, through the stomach and
first coil of the intestine. Examine from behind.

a. The stomach; a -spacious sac, passing into the
intestine below. The orifice of the gullet is
seen as a wide transverse aperture, situated near
its roof. Immediately below this there is a cor-
responding depression, which receives the main
ducts of the digestive gland — two enormous tubes
admitting the seeker with ease; note the pre-
sence of a series of lesser ducts.

y8. The intestine; passing straight down in the middle
line. Note the typhlosole; a median ingrowth of
the lining membrane of its front wall.

y. The labial palps. These, like the gills, are con-
fluent— externally with the pallial-lobes, inter-
nally with each other and the body wall; thei-r
free edges enclose a ventral ciliated groove.

c. Through the anterior adductor muscle, immediately
in front of the mouth.

a. The adductor muscle. Cf..r. a.

J3. The mouth; median, transversely elongated. It
lies immediately beneath the adductor muscle,
and is embraced on all sides by the labial palps,
which are confluent round it. Note that they

V.] THE FRESH-WATER MUSSEL. 339

are so disposed that the ciliated groove (cf. supra)
leads directly to the mouth aperture,
y. The foot; a fleshy mass, projecting forwardly,
immediately below the mouth.

/ Dissect away the adductor muscle and labial palps
on one side, and note the relations of the retractor
and protractor pedis muscles. Their fibres are largely
derived from the muscular layer of the body-wall;
follow them to their attachments. (Cf. Sect. A.)

Find the cerebral ganglion of the same side (see
Sect. I. c] and trace the course of the inter-
cerebral commissure; it runs over the gullet, in
the substance of the labial-palps. Follow in like
manner the cerebro-pedal commissures, until the
pedal ganglia are reached ; note that these — like
the parieto- splanchnic ones — are confluent in the
middle line.

M. The reproductive elements and the larva.

a. The animals are dioecious, but the reproductive
organs are similarly constructed in both sexes :
they vary much in size with the season, being
large in winter and spring, but small at other times.

b. Tease up a small portion of the ovary in eosin
solution, and examine under a high power. The
ripe ova are large, rounded, rarely ovoidal bodies,
and their special features are —

a. The mtelline membrane; usually separated from
the ovum by a cavity filled with a coagulate
albuminous fluid.

/?. The micropyle ; a perforation, or frequently a
short neck-shaped prolongation of a.

22 — 2

34O ELEMENTARY BIOLOGY. [CHAP.

Look for specimens In which the ovum Is pro-
longed into a stalk passing through the above
— the remnant of an original pedicle of attach-
ment to the germinal epithelium.

y. The germinal vesicle; very large and transparent;
it is rounded and usually contains two germinal
spots — a larger and a smaller one in close appo-
sition.

c. The Glochidium larva. If the outer gill appear to
be thick and distended, it will be found full of the
above. Note, while living, the characters of their
shells, the entangled byssus filaments, with which
they are provided, and the spasmodic contractions
of the adductor muscle.

Preserve a portion of the larva-laden gill (taken
from an animal killed under chloroform) in spirit;
when well hardened stain with magenta and exa-
mine under a low power. Look for individuals
whose valves gape, and note —

a. The larval exoskeleton ; composed of two trans-
parent valves, united dorsally by an elastic hinge.

(3. The shell-teeth ; formed on either side as a spur-
like inflection of a. Each terminates in a sharp
upwardly directed spike, and its exposed surface
is produced into a number of parallel serrated
ridges.

y. The adductor muscle; a powerful transverse band
passing between the upper parts of the two
valves.

8. The mantle. This consists, at this stage, of very
large cells which project freely into the enclosed

V.] THE FRESH-WATER MUSSEL. 341

cavity. Three sets of bristle-like appendages are
developed in connection therewith; an upper
one — passing obliquely downwards immediately
beneath the adductor muscle, and two shorter
ones passing upwards, immediately above the
shell-teeth.

e. The byssus; an elongated, transparent, much-
tangled filament, projecting beyond the free edges
of the valves.

d. Examine a similar larva from the side. The valves
are quite transparent, and there can be seen through
them

a. the adductor muscle; appearing as a median dark

mass.
(3. The visceral mass; a somewhat less conspicuous

mass lying in one of the top corners, altogether

to one side,
y. The byssus organ; a conspicuous transparent

tube, arising from /?. and coiled upon itself. It is

unpaired, being present on one side of the body

only.
8. Note the shape of the shelly valve; it tapers off

below to form the shell-tooth. Focus to the

level of this, and examine its serrations. The

valve is seen to be dotted all over by minute

perforations— pore-canals.

VI.

THE FRESH-WATER POLYPES (Hydra viridis
and H. fusca).

IF a waterweed, such as duckweed, from a pond, is placed
in a glass and allowed to remain undisturbed for a short time,
minute gelatinous-looking bodies of a brownish or green
colour may frequently be found attached to it, or to the sides
of the glass. They have a length of from \ to J of an inch,
rarely more, and are cylindrical or slightly conical in form.
From the free end delicate filaments, which are often much
longer than the body, proceed and spread out with a more
or less downward curve, in the water. These threads, which
are the tentacles, may vary in number1; if touched they
rapidly shorten and together with the body shrink into a
rounded mass. After a while, the contracted body and the
tentacles elongate and resume their previous form. These
are Polypes, the brown ones belonging to the species usually
termed Hydra fusca, the green to that called H. viridis.
The polypes generally remain attached to one spot for a
long time, but they are capable of crawling about by a
motion similar to that of the looping caterpillar; and,
sometimes, they detach themselves and float passively in
the water.

1 In H. Hexactinella, an Australian species, their number is re-
ported to be invariably six.

CHAP. VI.] THE FRESH-WATER POLYPES. 343

When any small animal, such as a water-flea, swimming
through the water comes in contact with the tentacles, it is
grasped, and conveyed by their contraction to the aperture
of the wide mouth, which is situated on the summit of a
cone (hypostome) in the middle of the circle formed by the
bases of the tentacles. It is then taken into a cavity which
occupies the whole interior of the body; the nutritive
matters which it contains are dissolved out and absorbed
by the substance of the Hydra; and the innutritious
residuum is eventually cast out by the way it entered.
Small pieces of meat, brought within reach of the tentacles,
are seized, swallowed and digested in the same manner.

If a Hydra is well fed, bud-like projections make their
appearance upon the outer surface of the body. These
gradually elongate and become pear-shaped. At the free
end a mouth appears; and around it minute processes are
developed and grow into tentacles ; and thus a young Hydra
is formed by gemmation from the parent. This young Hydra
becomes detached sooner or later, and leads an independent
existence; but, not unfrequently, new buds are developed
from other parts of the parent before the first is detached,
and the progeny may themselves begin to bud before they
attain independence. In this manner, temporarily compound
organisms may be formed. Experiments have shewn that
these animals may be cut into halves or quarters and that
each portion will repair its losses, and grow up into a perfect
Hydra; and there is reason to believe that this process
of fission sometimes occurs naturally.

The Hydra multiplies by budding through the greater
part of the year; but in the summer sexual organs appear
in the form of projections of the surface of the body.
These, when ripe, may be resolved into a larger and a
smaller set. The latter may appear on any part of the

344 ELEMENTARY BIOLOGY. [CHAP.

body, but they are not unfrequently restricted to the free
end, at or near the bases of the tentacles. Within them
(testes) great numbers of minute spermatozoa, each moved
by a vibratile cilium, are developed and eventually set free.

The former are large globular bodies, from one to eight
in number, usually formed near the attached end of the
polype. Each becomes much larger than the testis, and is
the ovary. Within it is developed a single large egg-cell, or
ovum. This ovum, which is a huge nucleated cell, is im-
pregnated by the spermatozoa and undergoes division into
two parts. Each of these again divides into two; and so
on, until the ovum is broken up into a number of small
embryo-cells. The mass of embryo-cells thus formed be-
comes surrounded with a thick, usually tuberculated or
spinous, case; and, detaching itself from the body, forms
the 'egg,' from which a new Hydra is developed.

Microscopic examination shews that the body of the
Hydra is a sac, the wall of which is composed of two
membranes, a transparent outer (ectoderm), and a coloured
inner (endoderm). The tentacles are tubular processes of
the sac, and therefore are formed externally by the ectoderm
and lined internally by the endoderm. Both the endoderm
and the ectoderm are made up of nucleated cells; the inner
ends of certain of these are prolonged into delicate fibres,
those of the ectoderm, which are most marked, running
parallel with the long axis of the body. The green colour
of the Hydra viridis results from the presence of chloro-
phyll grains imbedded in the protoplasm of the endoderm
cells.

The cells of the ectoderm, and especially that of the
tentacles, contain very singular bodies, — the so-called
Urticating capsules, thread cells, or nematocysts. These are
oval bags, with thick and elastic walls, containing a spirally

VI.] THE FRESH-WATER POLYPES. 345

coiled or looped filament which can be unrolled, present-
ing the appearance of a long filament attached to the
capsule, and often provided with recurved spines near its
base. When the nematocyst is ejected these spines or barbs
lie in close apposition, with their pointed ends directed
forwards and their bases under tension ; on reaching the
attacked prey they undergo a forcible displacement, fixing
the body of the nematocyst and clearing the way for the in-
troduction of the more delicate thread. These thread-cells
appear to exert a noxious influence upon the animal's prey.
Very rarely, nematocysts are to be found in individual cells
of the endoderm ; there is reason to believe that they are
introduced with the captured prey, but argument from
analogy to allied hydroids renders it probable that they may
be developed in situ.

The chlorophyll granules contained in the endoderm of
the green Hydra are doubtless functional in the manner of
those of the plant-cell, but none but faint traces of an
'assimilation product' have yet been observed. The brown
or orange-coloured particles predominant in the endoderm
of the other species, and rarely present in that of H.
viridis, are probably identical with the chlorophyll bodies
(see Laboratory work) l.

The larger endoderm cells of Hydra are throughout life
amoeboid, and the like is partly true of the ectoderm in at
least the young state of one variety (H. viridis var. Baker •/).
The Hydra, then, may be compared to an aggregate of
Amoeba, which are arranged in the form of a double walled
sac and have undergone an amount of metamorphosis.

The cavity of the body alone represents a stomach and

1 It has been assumed, upon this, that the green and brown species
are mere varieties of one and the same. On the other hand, structural
differences in the nematocysts and their parent-cells have been claimed,
as sufficient to justify a subdivision into three species.

34-6 ELEMENTARY BIOLOGY. [CHAP.

intestine; there are no organs of circulation, respiration or
urinary secretion; the products of digestion are doubtless
transmitted, by imbibition, from cell to cell, and those of
the waste of the cells exuded directly into the surrounding
water. While the Hydra has none of the special appara-
tuses which are termed glands, definite secretory cells are
nevertheless present. Among the more important of these
are those developed in the ectoderm of the foot which are
utilized for purposes of adhesion, and the secretory cells of
the endoderm, most numerous in the hypostome, which is
eversible.

Nematocysts are generally, but not invariably, ejected, if
any portion of the body which bears them be touched.
Certain of the ectoderm cells, usually, if not always, lodging
nematocysts, bear each a stiff filament or cnidocil such as
can readily be seen in life projecting beyond the free surface
of the tentacles. Continuity has been traced between these
cells or cnidoblasts and certain small nerve-cells sparsely dif-
fused in the deeper layer of the ectoderm; the whole con-
stituting an elementary neuro-sensiferous apparatus, through
the agency of which control of at least the cnidoblasts and
their contained urticating capsules is exercised.

The fully formed Hydra may further be compared to
those animals previously dealt with, at that stage in their
development when the body consists of a double-walled sac
(cf. especially the gastrula stage of the Snail and its repre-
sentative in the Crayfish). The inner layer gives origin,
in the latter, to the digestive epithelium and its appended
glands; in Hydra it forms the digestive layer. The ecto-
derm is, in all, protective, and from it such neuro-sensifer-
ous organs as are formed, exclusively arise. Between the
applied surfaces of the ectoderm and endoderm there is
interposed a gelatinous middle layer, which, under the

VI.] THE FRESH-WATER POLYPES. 347

action of reagents, appears as a structureless membrane.
This furnishes attachment for the cells themselves and sup-
port for the body generally; its optical characters in the
prepared state are expressed in the term supporting lamella,
its real nature more nearly in that of mesoglaia.

It is possible that the longitudinal fibres connected with
the cells of the* ectoderm may be specially contractile, and
represent muscles; but, however this may be, each cell has
its own independent contractility. Similar fibres, disposed
transversely, arise from the bases of the endoderm cells.
These have been met with only in the body, and they ap-
pear to be in connection with the fibres of the ectoderm
cells, by means of delicate fibrils which perforate the above-
named mesoglcea.

LABORATORY WORK.

1. Put into a beaker some water containing bodies to
which Hydrae are attached, and place the beaker in a
window not exposed to direct sunlight : in the course of
some hours many Hydrae will be found attached to that
side of the glass which is turned towards the light. Note
their size, form, colour, mode of attachment and move-
ments.

2. Transfer a Hydra, by means of a pipette, on to a
slide with plenty of water. In order to avoid crushing,
place the animal between two coverslips, so arranged as
to support the edges of a third one which shall overlie it.
Examine with a low power.

348 ELEMENTARY BIOLOGY. [CHAP.

a. Form.

a. The body proper ; cylindrical, varying much in length
and diameter with the state of extension of the
animal j its conical free end (hypostome) with an
opening (mouth} at its summit.

It is often difficult to observe the mouth, especially
in the green species. It may be readily seen however
if a Hydra, placed in a drop of water without a
coverslip, be watched under an inch objective, until
it turns its anterior end up towards the observer.
Occasionally, under these circumstances, the hypo-
stome may be everted.

ft. The tentacles : ranged round the mouth ; their num-
ber and shape ; their varying length and diameter ;
the knob-like eminences on them.

Look for developing tentacles, shorter and more
rigid than the rest. Very rarely, one or more may
branch.

y. The base (so called foot) : a flattened disc ; narrower
or wider than the body, according to the state of
extension of the latter. It is functional as an organ
of adhesion. If the animal be attached, its se-
cretory product can often be seen, as a transparent
laminated mass, interposed between the body and
the surface of attachment ; if the animal be free, the
disintegrating remains of the same are often to be
found adherent to the 'foot'; if it be forcibly de-
tached, rupture of the base of the polype may ensue,
the animal appearing to possess that which has
been erroneously termed an 'anus'.

& The buds; young Hydrae, of various sizes and stages
in development, attached to the sides of the parent.
There may be one or more of them.

VI,] THE FRESH-WATER POLYPES. 349

e. The sex-organs, if present; colourless prominences,
variable in number and position (cf. Sect. 8).

S, e may be one or more absent, or all may coexist,
in individual specimens.

b. Structure.

a. The animal evidently composed of two layers, an
outer transparent, ectoderm., and inner, endoderm; the
latter alone containing chlorophyll in the green
species, or such colouring matter as is present in
the brown one. The ectoderm is marked out into
areas, and may with care be seen to be composed
of distinct cells, though this is a little difficult to
make out in fresh specimens.

/?. The body-cavity ; least obvious in the green species,
frequently visible in the brown ones as a darker
central area with which the mouth-opening is con-
tinuous. Note that it extends into the tentacles;
corpuscles can, with care, be seen floating within
these, when extended.

c. Movements.

a. The general contractility of the animal; it is con-
stantly either extending or shortening its body and
tentacles, and so altering its form and place.

ft. Its irritability ; slight pressure or other stimulus im-
mediately causes it to contract.

3. The tentacles and nematocysts. Attention has al-
ready been called to the knob-like eminences of the former.
Note that these are widely separated in extension ; closely
applied in contraction. Examine one of them under a high
power, with the tentacle fully extended.

ELEMENTARY BIOLOGY. [CHAP.

a. The cnidocils ; well defined stiff processes, each
pointed and standing out from the free surface.

b. The nematocysts or thread-cells; highly refractive
bodies embedded in the common mass. They can
be resolved into

a. Smaller and more numerous ones ; ovoidal, and
situated almost invariably at the bases of the
longer cnidocils.

/?. Larger and less numerous ones (very often one
only) lying near the middle of the mass ; globular
when seen en face, flask-shaped when looked at
from the side.

c. Run in a drop of acid magenta solu. and watch, under
a low power, the ejection of the threads of the ne-
matocysts. Note from what parts of the body they
are thrown out. Examine, under a high power, a
portion of a tentacle beset by them. There will be
found :

a. Short, deeply staining, stout threads, usually thrown

into a spiral. Note that these are related to the

smaller nematocysts.
/?. Delicate whip-lash filaments, from 8 to 10 times

the length of a. Trace them to their origin from

the larger nematocysts.

d. Should opportunity offer, examine any small or-
ganism or other prey that may have been seized
by the tentacles. When about to be swallowed it
will be found to be studded by nematocysts, espe-
cially of the smaller kind.

4. The asexual buds. (Cf. Sect. 3 a. 8) Examine in
relation to the parent, under a high power.

VI.] THE FRESH-WATER POLYPES. 351

a. The body-cavity ; in young buds continuous with that
of the parent; in buds ripe for dehiscence com-
pletely constricted off therefrom. Note that the
mouth is formed as a secondary perforation.

b. The tentacles ; variable in number. A solitary one
may appear first, the second and third being sym-
metrical with respect to each other. Compare the
fully-formed animal.

The rate of development of the tentacles, indi-
vidually or collectively, varies under change of tem-
perature, and their order of appearance is somewhat
inconstant. There are usually in the fully-formed
animal (8 — 12 hrs.), 6 for H. fusca, 8 for H. vt-
ridis.

5. The study of prepared sections. Place one or more
Hydras, preferably in full diet, in Kleinenberg's picric
acid solution; after two hours' immersion therein, transfer to
alcohol of increasing strengths. When fully hardened, stain
with borax-carmine, imbed in paraffin and cut into trans-
verse sections. Mount in Canada balsam and examine
under a low power.

a. The ectoderm; of uniform thickness and composed,
for the most part, of squarish-looking cells which stain
lightly.

b. The endoderm; composing the inner two-thirds or
more of the body-wall, extremely variable in thick-
ness. Its component cells are highly vacuolated.

c. The body-cavity (enteron) ; enclosed by b. Variable
in capacity in different sections, in accordance with
the state of contraction of the body-wall.

d. a and b will be seen to be separated by a hard line,

352 ELEMENTARY BIOLOGY. [CHAP.

which stains deeply. Examine this under a high
power and look for

a. the mesoglcea or supporting lamella; visible as the
above-named line.

/?. Kleineribergs fibres (contractile-processes of the
ectoderm-cells) ; obvious, if the section be a good
one, as a parallel series of deeply-stained dots im-
mediately external to a.

s. Examine the ectoderm under a high power, selecting

your thinnest sections. Look for

a. The larger ectoderm cells ; conical, their bases being
external, and fairly uniform in contour. Each is
composed of a lightly staining protoplasm, nu-
cleated and at times vacuolated. Note the strong
granulation of their free outer borders.

/?. The interstitial tissue ; composed of aggregates of
smaller cells, at the bases of #; rendered con-
spicuous by its affinity for the stain and by the
great development of nematocysts.

Cf. sections across a tentacle. The knob-like pro-
tuberances previously seen (Sect. 2 a. /?) will be found
to consist of aggregates of these cells which have
reached the surface. Similar eminences, less regu-
larly disposed, will be found in the body-wall.

/ Examine, in like manner, the endoderm. Its larger
cells will show —

a. Shape, irregular ; size and contour, variable.

ft. The cell-protoplasm; largely replaced in one or
more clear vacuoles. It forms peripherally a cell
membrane, and is generally densest at the base,
where it is aggregated to form the so-called 'foot'.

VI.] THE FRESH-WATER POLYPES. 353

Occasionally some of the vacuoles may be seen to
contain groups of minute needle-shaped crystals.

y. The secretory cells of the endoderm ; smaller deeply-
staining pear-shaped cells, interposed between the
bases of the larger ones. Numerous and well-de-
fined in sections across the hypostome, fewer in
those across the body. They may be recognized
by their tapering inner ends, the highly-granular
nature of their cell-protoplasm, and by their small
deep-staining nucleus.

g. Place a Hydra in i per cent, osmic acid solution for
24 hours, and then tease up a portion of the en-
doderm in weak glycerine, under a low power. Ex-
amine, under your highest objective, and look for the
flagella; long whip-lash filaments, one to six being
borne upon a single cell.

As these structures are capable of withdrawal, cells
will be present which do not bear them. Look for
specimens showing stages in their elongation.

6. Structural analysis of the individual cells. Pre-
serve some Hydras in Miiller's fluid (2 — 3 days). Transfer
to alcohol of increasing strengths, and finally tease up under
a low power in eosin or haematoxylin solution. Alter-
natively, preserve for a similar period in i per cent, solu-
tion of ammonia bichromate, transfer to alcohol and tease
up in carmine solution. Select the best preserved cells and
examine, under your highest power, in order —

a. The larger ectoderm cells ; nucleated, with a flattened
base and a rounded free end (cf. Sect. 5. e). Their
nuclei ; frequently containing two nucleoli. Their pro-
toplasm ; often differentiated into a superficial cuticle-
like product. Look especially for Kleinenberg1 s fibres

M. 23

354 ELEMENTARY BIOLOGY. [CHAP.

(Sect. 5. d. ft) ; one or more of these may be con-
nected with an individual cell. They arise as out-
growths of the constricted cell-base, the whole fre-
quently having the appearance of an inverted T with
a thick stem (cell-body) and greatly elongated arms
(the fibres).

b. The cells of the interstitial tissue (cf. Sect. 5. e. ft) much
smaller than a, and often separable from one another
only with difficulty. Numbers of them will be found
to contain two nuclei (evidence of active division).

c. The nematocysts ; highly refractive bodies to be found
in both a and b — in the latter in abundance and in
all stages of development. Look for the under-
mentioned (cf. Sect. 3).

a. The larger nematocysts ; each consists of a body,
ovoidal and truncated, with a strongly-marked
double contour due to the thickness of its wall ; a
neck, inverted and^ beset by three or four powerful
spines ; a filament, lying within the base of the
body, coiled into a spiral the edges of which may
occasionally be seen (cf. Sect. 3).

Compare a large nematocyst in the everted state ;
the summit of the neck is beset by a series of ex-
cessively delicate spines.

ft. The smaller nematocysts; rarely present in the larger
cells. Each is ovoidal and much smaller than a,
having a similar double contour. The filament ;
but i to jV the length of, and very much stouter
than, that of a; at rest looped, in eversion fre-
quently spiral (cf. Sect. 3).

VI.] THE FRESH-WATER POLYPES. 355

y. Examine, in detail, a nematocyst in relation to its
parent-cell The cell-protoplasm will be ' seen to
form a delicate envelope, thickened only around
the nucleus, which stains deeply and is to be found
in close apposition with the nematocyst.

8. The cnidoblasts ; recognizable by the presence of a
stiff spinous process or cnidocil, usually projecting
from one corner of the cell-body.

Note the variable size of the cnidocils (cf. Sect.
3). Nematocysts may or may not be present;
where they are it will generally be found that the
longer cnidocils are associated with the smaller
thread-cells and vice-versa.

e. Look for nerve-cells; small, stellate, and with a
deeply-staining round nucleus. They are rare and
only to be found with difficulty.

£. Immature nematocysts ; occasionally to be met with
in the isolated state. Bottle-shaped and bearing a
short blunted filament, which subsequently becomes
invaginated (be careful not to confound this with
the cnidocil).

d. The larger cells of the endoderm. Examine cells freshly
isolated from specimens killed with osmic acid vapour.
Note, in addition to the characters already observed
(Sect. 5).

a. The cell-base ; not unfrequently expanded and pro-
longed out to form contractile processes, identical
with, but shorter and less marked than, those of
the ectoderm cells.

/?. The nucleus ; variable in position with the state of
distension of the cell ; generally marginal.

23—2

356 ELEMENTARY BIOLOGY. [CHAP.

y. Superadded to the cell-protoplasm. In H. viridis
the chloroplastids ; small round bodies fairly uniform
in size, for the most part aggregated at the cell-
base. They stain very feebly, and may or may
not contain chlorophyll : note the manner of its
deposition.

In H. fusca. The sooty corpuscles ; irregular and
transparent, rarely rounded ; sooty particles may or
may not be present. Look for the presence of
chlorophyll ; occasionally to be found in individual
cells.

8. Compare the living cells, as obtained by crushing a
Hydra. They are highly amoeboid. Watch the
nature of their movements.

e. The mesogl&a. Search under a low power for stray
pieces of this ; obvious as transparent shreds of mem-
brane with parallel striae. There will be found in
connection therewith —

a. Nuclei and fragments of cells, mostly of the in-
terstitial tissue.

/?. Contractile processes ; firmly adherent to the meso-
glcea, the above-named striation being due to their
presence.

Look for fragments in which the two sets of
striae cross each other (cf. p. 347).

y. Compare a piece of the same, obtained from the
freshly-killed animal. If forthcoming, press gently
upon the cover-glass with the point of a needle;
it will be found to be gelatinous and compressible.

7. Food material and digestion. Work through all your
sections under a low power, and look for ingested prey.

VI.] THE FRESH-WATER POLYPES. 357

a. Small organisms; especially Entomostraca and other
small Crustacea. When present one such will gene-
rally be found, tightly embraced by the endoderm.

Examine under a high power and look for evidences
of digestion (infra-enteric) and assimilation.

b. Smaller organisms; especially Infusoria, Diatoms, and
the like. Frequently to be met with in the central
cavity; they may however be found, unchanged or in
process of digestion (mtra-cellular\ within the indivi-
dual cells of the endoderm.

8. The reproductive organs. (Cf. Sect. 2. a. c).

a. The testes; small conical colourless eminences below
the point of attachment of the tentacles, more rarely
irregularly scattered over the whole body.

Gently flatten out a testis in eosin or magenta by
pressure on the coverslip, and examine with a high
power. According to its state of maturity the follow-
ing contents will be found in it —

a. A collection of the smaller ectoderm (interstitial)
cells of variable shape.

j3. The same, but having become ovoidal and hyaline.

y. Cells otherwise like ft, but with a long filament
proceeding from them.

B. Ripe spermatozoa; bodies consisting of a very small
oval nucleated head to which a very delicate
flagellum is attached, by the movements of which
they swim about in the water. They may fre-
quently be seen in motion within the imruptured
testis.

b. The ovaries; one or more in number, larger than a,
and situated near the base of the polype.

358 ELEMENTARY BIOLOGY. [CHAP. VI.

When young, each appears as a hill-shaped enlarge-
ment of the ectoderm ; when ripe, it becomes rounded
and very prominent.

c. Press out an ovary : according to its stage of develop-
ment there will be found in it —

a. Ectoderm cells with a marked preponderance of

the smaller form (interstitial tissue).
/2. Imbedded among a, one (ovicelt} which has become

larger and clearer than the rest, and possesses a

distinct central germinal vesicle.

d. The ripe ovum. Conspicuous in H. viridis by its
green colour. It consists of a great irregularly
branched (amoeboid) mass of protoplasm (vitellus), in
which is a clear space (germinal vesicle) containing one
larger and a number of smaller germinal spots.

Examine under a high power, and note —
a. The absence of a vitelline membrane.
/?. The yolk granules; exceedingly large and modified to
form the so-called 'pseudo-cells'. Each is rounded
or oval, thickened on one side to form a plug-
shaped ingrowth and filled with a fluid contents,
y. The chloroplastids ; present in H. viridis; identical
with those found in the endoderm (Sect. 6. d. y).

e. The segmented ovum (oosperni): composed of a large
number of small cells. Its thick horny capsule,
rough on its external surface.

VII.
THE BELL-ANIMALCULE (Vorticella).

THE bell-animalcule is one of a very large group of animals
called the Infusoria, on account of the fact that many mem-
bers thereof make their appearance in infusions of certain
animal and vegetable substances.

The higher multicellular animals begin their existence as
simple nucleated cells, and the single nucleated cell which
constitutes the whole animal in its primitive condition
divides and subdivides until an aggregation of similar cells
is formed. And it is by the differentiation and metamor-
phosis of these primitively similar histological elements that
the organs and tissues of the body are built up. In the
Infusoria, the protoplasmic mass which constitutes the germ
does not undergo this process of preliminary subdivision,
but such structure as the adult animal possesses is the
result of the direct metamorphosis of parts of its proto-
plasmic substance. Hence, morphologically, the bodies of
these animals are the equivalents of a single cell; while,
physiologically, they may attain a considerable amount of
complexity.

The Infusoria abound in fresh and salt waters, and many
make their appearance, as before stated, in organic in-
fusions, their germs either being contained in the substances
infused, or being wafted through the air. Their diffusion is
greatly facilitated by the fact that many of them retain their

360 ELEMENTARY BIOLOGY. [CHAP.

vitality when dried, and reduced to the condition of an ex-
cessively light dust; while their rapid propagation is, in
the main, due to their power of multiplying by division,
with extraordinary rapidity, when duly supplied with nou-
rishment. The majority are free and provided with nu-
merous cilia by which they are incessantly and actively pro-
pelled through the medium in which they live; but some
attach themselves to stones, plants, or even the bodies of
other animals. A few are parasitic, and the bladder and
intestines of the Frog are usually inhabited by several spe-
cies of large size.

The Bell-animalcules are Infusoria which are fixed, usu-
ally by long stalks, to water plants, or, not unfrequently,
to the limbs of aquatic Crustacea. They are barely visible
to the unaided vision. The body has the shape of a wine-
glass with a very long and slender stem, provided with a
flattened disc-like cover. What answers to the rim of the
wine-glass is thickened, somewhat everted, and richly
ciliated, and the edges of the disc are similarly thickened
and ciliated. Between the thickened edge of the cover, or
peristome, and the edge of the disc, is a groove, which, at
one point, deepens and passes into a wide depression, the
vestibulum. From this a narrow tube, the oesophagus, leads
into the central substance of the body, and terminates ab-
ruptly therein; and when faecal matters are discharged, they
make their way out by an aperture which is temporarily
formed in the floor of this vestibule. The outermost layer
of the substance of the body is denser and more transparent
than the rest, forming a cuticula. Immediately beneath the
cuticle it is tolerably firm and slightly granular, and this
part is distinguished as the cortical layer or cctosarc; it
passes into the central substance or endosarc, which is still
softer and more fluid.

VII.] THE BELL-ANIMALCULE. 361

In the undisturbed condition of the Bell-animalcule,
the stem is completely straightened out; the peristome is
everted, and the edges of the disc separated from the peri-
stome; the vestibule gaping widely and the cilia working
vigorously. But the least shock causes the disc to be re-
tracted, and the edge of the peristome to be curved in and
shut against it, so as to give the body a more globular form.
At the same time, the stem is thrown into a spiral, and the
body is thus drawn back towards the point of attachment.
If the disturbing influence be continued, this state of retrac-
tion persists; but if it be withdrawn, the spirally coiled stem
slowly straightens, the peristome expands, and the cilia
resume their activity.

In the interior of the body, immediately below the disc,
a space, occupied by a clear watery fluid, is seen to make
its appearance at regular intervals — slowly enlarging until
it attains its full size and then suddenly and rapidly dis-
appearing by the approximation of its walls. This is the
contractile vesicle or vacuole. It communicates with the ex-
terior at the moment of contraction, and in all probability
performs an excretory function. If the Bell-animalcule is
well fed, one or more watery vesicles of a spheroidal form,
each containing a certain portion of the ingested food, will
be seen in the soft central mass of the body. And by
mixing a small quantity of finely divided carmine or indigo
with the water in which the Vorticellce live, the manner in
which these food-vesicles are formed may be observed.
The coloured particles are driven into the vestibule by the
action of the cilia of the peristome and the adjacent parts,
and gradually accumulate at the inner end of the gullet.
After a time the mass here heaped together projects into the
central substance of the body, surrounded by an envelope
of the accompanying water; and then suddenly breaks off,

362 ELEMENTARY BIOLOGY. [CHAP.

as a spheroidal drop, henceforward free in the soft central
substance. Such being the mode of formation of the vesicles,
they have been termed vesicles of ingestion; in some Bell-
animalcules, they are carried round with the deeper layer of
protoplasm or endosarc in a movement of circulation,
passing up one side of the body, then crossing over below
the disc and descending on the other side. Sooner or later
the contents of these vesicles are digested, and the refuse is
thrown out of the body, surrounded as a rule by a watery
vacuole or vesicle of egestion. This process takes place by
an aperture leading into the vestibule, which exists only at
the moment of extrusion of the faeces, and is indistinguish-
able at any other time.

A portion of the substance of the body, which is slightly
different in transparency and in its reactions to colouring
substances from the rest, is called the nucleus or endoplast.
It is elongated and bent upon itself into a crescentic or
horseshoe shape.

The numerous species and varieties of bell-animalcules
are, for the most part, colourless. Green varieties are how-
ever occasionally to be met with, the green colour being
due to a deposit of chlorophyll within the endosarc, com-
parable to that seen within the endoderm of the green-
hydra. This colouring matter may be restricted to small
bodies or chloroplastids identical with those of the polype
referred to, or diffused throughout the cell-protoplasm (en-
dosarc) in a manner such as, there is good reason to believe,
is never the case in the vegetable organism.

The Bell-animalcules multiply in two ways ; partly by
longitudinal fission, when a bell becomes cloven down the
middle, each half acquiring the structure previously pos-
sessed by the whole ; and partly by gemmation from the
cndoplast, in which latter case the endoplast divides into a

VII.] THE BELL-ANIMALCULE. 363

number of rounded spore-like masses, which are ultimately
set free as locomotive germs.

Sometimes a rounded body, encircled by a basal circlet
of cilia but having otherwise the characters of a Vorticella
bell, is seen to be attached to the base of the bell of an
ordinary Vorticella. It was formerly supposed that these
were buds, but it is now known that they are independent
individuals, formed as the result of repeated longitudinal
fission, which have attached themselves to that to which
they adhere and are gradually becoming fused with it, so
that the two will form one indistinguishable whole. There
is here a process of "conjugation" sexual in nature, and it
is, moreover, preliminary to the budding of the endoplast
and the subsequent formation of germs.

Under certain circumstances a Vorticella may become
encysted. The peristome closes and the bell becomes con-
verted into a spheroidal body, in which only the nucleus
and the contractile vesicle remain distinguishable. This
surrounds itself with a structureless envelope or cyst, the
whole process sometimes following that of conjugation and
preceding that of germ formation. It does not appear
however that this encystment is invariably associated with
reproduction; for the encysted animal after remaining for
a longer or shorter time in a temporary condition of rest,
may emerge and resume its former state of existence.

The two genera of Infusoria which most commonly occur
in the Frog are Nyctotherus and Balantidium. Both are
free and actively locomotive, and the former is particularly
remarkable for its relatively large size and semilunar con-
tour, and for the length and distinctness of its curved oeso-
phagus. Balantidium is pyriform, and has a very short
cesophageal depression.

364 ELEMENTARY BIOLOGY. [CHAP.

LABORATORY WORK.

A. Examine duckweed roots, confervas, &c., with J inch
objective avoiding pressure ; having found a group of
Vorticella note the following points with a higher
power.

i. In the extended state of the animal,
a. The body.

a. Its size (measure).

b. Form ; broadly speaking, that of an inverted
bell : note —

a. The prominent everted rim (peris tome).

j3. The flattened central disc projecting above

the peristome.

y. The cilia fringing the disc.
8. The depression between the peristome and

disc.

e. The vestibulum ; a chamber in the hollow
between the peristome and disc.

c. Structure.

a. The thin, transparent, homogeneous external

layer (cuticle),
ft. The granular layer (ectosare) inside the cuticle,

longitudinally striate at the base of the bell

(inyophan layer).
y. The central more fluid part (endosarc) not

sharply marked off from /?.

VII.] THE BELL-ANIMALCULE. 365

The various clear spaces (food vacuoles) in
it, containing foreign (ingested) bodies (Dia-
toms, Protococcus, &c.).

8. The contractile vesicle; its position, in the
cortical layer just beneath the disc ; its systole
and diastole.

e. The nucleus ; an elongated curved body in
the deeper layer; sometimes nearly homo-
geneous, sometimes more distinctly granular.
The nucleus is usually indistinguishable until
after treatment with iodine, or acetic acid and
magenta (cf. § 5).

£. The gullet ; sometimes seen in optical trans-
verse section as a clear round space ; some-
times seen sideways as a canal opening above
into the vestibulum, and ending abruptly be-
low in the body-substance.

rj. The anus ; look for the egestion of solid par-
ticles.
b. The stalk.

a. Its length and diameter (measure).
y8. Its structure ; the external homogeneous layer
(sheatti) continuous with the cuticle; the highly
refractive centre (axis or contractile filament}
continuous with the cortical layer of the bell.
2. In the retracted state,
a. The body.

a. Its form; pear-shaped; rounded off above;

neither disc nor peristome visible.
/?. The clear transverse space near the top, indi-
cating the interval between the retracted disc

366 ELEMENTARY BIOLOGY. [CHAP.

and the rolled-in peristome. In this space the
cilia can frequently be seen moving.
y. Structure; as in i. a. c.
b. The stalk ; thrown into corkscrew-like folds.

3. The movements of Vorticella.

a. The ciliary movement.

a. Examine the cilia carefully; delicate homo-
geneous processes; their length, diameter and
form ; their position.

/?. The continuity of the cilia with the cortical
layer.

y. The function of the cilia; their rapid move-
ments, alternately bending and straightening :
the co-ordination of these movements; they
work in a definite order; note the currents
produced in the neighbouring water (if neces-
sary introduce a few particles of carmine under
the coverslip) ; the sweeping of small bodies
down the gullet.

b. The moveme?zts of the contractile vesicle. Tolerably
regular rhythmic distension and collapse (diastole
and systole).

c. The currents in the endosarc carrying round the in-
gested bodies.

d. The movements of tJie animal as a whole. (Ex-
amine under \ inch or \ inch obj.)

a. Its extreme irritability; it contracts on the
slightest stimulation, often without any appa-
rent cause.

J3. The movements which occur in contraction;

Of THE

VII.] THE BELL-ANIMALCULE. 367

the coiling up of the stalk ; the rolling in of the
disc. The rapidity of these movements.

y. The mode of re-expansion ; the stalk straightens
first ; then the peristome is everted ; finally the
disc and its cilia are protruded.

4. Stain with iodine or magenta ; the cuticle uncoloured
— the rest stained ; the nucleus especially becomes
deeply coloured.

5. Treat with acetic acid ; the contents soon disappear
(except perhaps some swallowed bodies) — the cuticle
later or not at all.

6. Look for the following in various specimens —

a. Multiplication by fission^ early stage ; a bell par-

tially divided into two by a vertical fissure start-

ing from the disc.
/?. The same, late stage ; two complete bells on one

stalk; the result of completion of the fission.

The development of a basal circlet of cilia by

one or both of these bells.
y. Free swimming unstalked bells (detached bells

from ft).
8. Conjugation; the attachment of a small free

swimming bell to the side of a stalked one.
e. Ency station ; the body contracted into a ball

and surrounded by a thickened structureless

layer, the contractile vesicle being persistently

dilated.

B. Other forms closely allied to Vorticella which may be
met with, and which will do nearly as well for exami-
nation, are ; —

368 ELEMENTARY BIOLOGY. [CHAP. VII.

a. Epistylis. Bell-shaped animals growing on a
branched non-contractile stalk.

b. Carchesium. A form very like Vorticella but
borne on a branched contractile stalk.

c. Cothurnia. An almost sessile form, provided
with a cup or envelope into which the bell can
be retracted.

[The activity of the movements of the free Infusoria interferes
with the complete examination of the living animal. It is well
therefore to add a little osmic acid solution to the drop of water
under examination. This kills such Infusoria as Paramcecium,
Nyctotherus and Balantidium instantly, without destroying the
essential features of their organization.]

VIII.

THE PROTEUS ANIMALCULE (Amoeba) AND THE
COLOURLESS BLOOD CORPUSCLE.

Amoeba are minute organisms of very variable size which
occur in stagnant water, in mud, in damp earth and else-
where, and are frequently to be obtained by infusing any
animal matter in water and allowing it to evaporate while
exposed to direct sunlight. An Amoeba1 has also been
found living, apparently as a parasite, in the diseased skin
of the sheep.

The Amoeba has the appearance of a particle of jelly,
which is more or less granular and fluid in its central
parts, but usually becomes clear and transparent, and of
a firmer consistency, towards its periphery. Sometimes
Amcebce are found having a spherical form and encased
in a structureless sac, and in this encysted state they ex-
hibit no movements. More commonly, they present in-
cessant and frequently rapid changes of form, whence the
name of "Proteus Animalcule" given to them by the older
observers ; and these changes of form are usually accom-
panied by a shifting of position, the Amoeba creeping about
with considerable activity and, in most cases, with no con-
stancy of direction.

The changes of form, and the movements, are effected

1 A . parasitica. New S. Wales.
M. 24

3/0 ELEMENTARY BIOLOGY. [CHAP.

by the thrusting out of lobe-like prolongations of the
peripheral part of the body, which are termed pseudo-
podia^ sometimes from one region and sometimes from
another. Occasionally, a particular region of the body is
constantly free from pseudopodia, and therefore forms its
hindmost part when it moves. Each pseudopodium is evi-
dently, at first, an extension of the denser clear substance
(ectosarc) only; but as it enlarges, the central, granular,
more fluid endosarc flows into its interior, often with a
sudden rush.

The protoplasm is in some Amcebce crowded with small
watery vacuoles. In most there is present a clear space
which makes its appearance at intervals, in a particular
region of the ectosarc, and then disappears by the rapid
approximation of its walls. After a while, a small clear
speck or a line appears at the same spot and slowly dilates
until it attains its full size, when it again rapidly disappears
as before. Sometimes two or three small clear spots arise
close together, and run into one another to form the single
large cavity. The structure thus described is termed the
contractile vesicle or vacuole, and its rhythmical systole and
diastole often succeed one another with great regularity.
Nothing is certainly known respecting its function; but
there is reason to think that it may be excretory like the
corresponding structure in Vorticella.

Very generally one part of the Amoeba exhibits a rounded
or oval nucleus. This structure is liable to considerable
variation, and not unfrequently more than one nucleus may
be present.

The gelatinous body of the Amoeba is not as a rule
bounded by anything that can be properly termed a
cuticular membrane; all that can be said is, that its ex-
ternal or limitary layer is, in most cases, of a somewhat

VIII.] THE PROTEUS ANIMALCULE. 371

different constitution from the rest, so that it acquires a
certain appearance of distinctness when it is acted upon
by such reagents as acetic acid or iodine, or when the
animal is killed by raising the temperature to 45° C.
Physically, the ectosarc might be compared to the wall
of a soap-bubble, which, though fluid, has a certain vis-
cosity, which not only enables its particles to hold together
and form a continuous sheet, but permits a rod to be passed
into or through the bubble without bursting it ; the walls
closing together, and recovering their continuity, as soon as
the rod is drawn away.

It is this property of the ectosarc of the Amoeba which
enables us to understand the way in which these animals
take in and throw out again solid matter, though they have
neither mouth, anus, nor alimentary canal. The solid body
passes through the ectosarc, which immediately closes up
and repairs the rent formed by its passage. In this manner,
the Amoeba take in the small, usually vegetable, organisms,
which serve them for food, and subsequently get rid of the
undigested solid parts.

Ingested matter is invariably taken in, in conjunction
with a watery vacuole of ingestion such as has been de-
scribed for Vorticella (pp. 361 — 2). If the matters thus
introduced be nutritive and digestible the vacuole persists,
during at any rate the earlier stages of digestion ; later, the
unassimilable innutritious portions of the same are cast out
surrounded by a vacuole of egestwn. Experiment has led
to the belief that Amozbce will not digest fat globules or
starch grains. In the case of the latter the grains have
been seen to be thrown out 6 — 7 days after ingestion,
unchanged in their optical characters and in their behaviour
towards certain reagents and unaccompanied by the vesicle
of egestion.

24—2

372 ELEMENTARY BIOLOGY. [CHAP.

It is of interest to note that in the case of small vege-
table organisms the cellulose coats have been at times
observed to be thrown out in a distorted, if not a digested
and disintegrating condition.

The chemical composition of the bodies of the Amctbce.
has not been accurately ascertained, but they undoubtedly
consist, in great measure, of water containing a protein com-
pound, and are similar to other forms of protoplasm. They
absorb oxygen and give out carbonic acid, and the presence
of free oxygen is necessary to their existence. When the
medium in which they live is cooled down to the freezing
point their movements are arrested, but they recover when
the temperature is raised. At a temperature of about 35° C.
their movements are arrested, and they pass into a condition
of "heat-stiffening," from which they recover if that tem-
perature is not continued too long; at 40° to 45° C. they
are killed.

Crystals are generally to be met with in the cell proto-
plasm, but of their origin and significance nothing is at
present known.

Electric shocks of moderate strength cause Amoeba at
once to assume a spherical still form, but they recover after
a while. Strong shocks kill them and, at the same time,
bring the nucleus distinctly into view.

If Amoeba are not to be found, their nature may be
understood by the examination of the colourless corpuscles of
the blood. (Cf. ch. I. pp. 121 — 123.)

The colourless corpuscles of the blood of some of the
cold-blooded vertebrates, such as Frogs and Newts, may be
kept alive for many weeks in serum properly protected from
evaporation; and if finely divided colouring matter, such
as indigo, is supplied to them, either in the body or out of
it, they take it into their interior in the same way as true

VIII.] THE PROTEUS ANIMALCULE. 373

Amoeba would. In the earliest condition of the embryo,
the whole body is largely composed of such nucleated cells
as the colourless corpuscles of the blood; and the colourless
corpuscles must be regarded as simply the progeny of such
cells, which have not become metamorphosed, and have
retained the characteristics of the lowest and most rudi-
mentary forms of animal life.

The Amoeba is an animal, not because of its contractility
or power of locomotion, but chiefly because it is devoid
of the power of manufacturing protein from bodies of a
comparatively simple chemical composition. The Amoeba
has to obtain its protein ready made, in which respect
it resembles all true animals, and therefore is, like them,
in the long run, dependent for its existence upon some
form or other of vegetable life .

Amcebtz multiply by fission in a manner similar to that
described for the bell-animalcule; the nucleus first divides,
the cell becoming subsequently cleft in two. Occasionally
an Amoeba has been seen to engulph another of smaller size
than itself; and there is reason to believe that this process,
originally thought to have been one of cannibalism, may
probably be one of conjugation of dissimilar individuals,
for reproduction, such as is seen in the bell-animalcule.

LABORATORY WORK.

A. Place a drop of water containing Amcebce on a slide,
cover with a cover glass, avoiding pressure, and search
over with J inch obj. : having found an Amoeba, ex-
amine with a higher power.

374 ELEMENTARY BIOLOGY. [CHAP.

1. Size; differing considerably in different specimens.
Measure.

2. Outline; irregular, produced into a number of thick
rounded eminences (pseudopodia) which are con-
stantly undergoing changes : sketch it at intervals of
five seconds.

3. Structure.

a. Outer hyaline layer (ectosarc), tolerably sharply
marked off: inner granular layer (endosarc) be-
coming more fluid centrally.

b. Nucleus (indistinguishable in some specimens);
a roundish more solid-looking body, often highly
granular. There may be one, two, or many.

f. Contractile vesicle; (if present) a roundish clear
space in the ectosarc which disappears peri-
odically, and after a short time reappears; its slow
diastole — rapid systole.

d. Ingested foreign bodies; Diatom cases, Des-
midice, &c.

4. Movements.

a. Watch the process of formation of a pseudopo-
dium. A hyaline ectosarcal lobe first appears;
then, as it increases in size, the granular endosarc
flows into it.

//. Locomotion ; watch the process, — a pseudopodium
is thrown out, then the rest of the body is drawn
up to it, and the process is repeated.

c. If the opportunity presents itself, watch the pro-
cesses of the ingestion and egestion of solid
matters.

VIII.] THE PROTEUS ANIMALCULE. 3/5

d. Observe the movements on the hot stage; warmth
at first accelerates the movements, but as the tem-
perature approaches 40° C. they cease, and the
whole mass remains as a motionless sphere.

e. Effects of electrical shocks on the movements.

5. Mechanical Analysis; crush. The whole collapses,
except sometimes the nucleus, and even that after a
time disappears: there is no trace of a distinct
resisting outer sac.

6. CJiemical Analysis; Treat with magenta and iodine.
The whole stains, and there is no unstained envelop-
ing sac. Iodine produces no blue colouration unless
starch has been swallowed; if so, blue specks become
visible.

7. Look for encysted specimens; and for specimens
which are undergoing fission.

8. Another form of Amoeba is not unfrequently found
which differs from that just described in being much
less coarsely granular, and in having ill-defined
ectosarc and endosarc, together with much longer,
more slender and pointed pseudopodia. Another
common form progresses rapidly with a slug-like
movement, only throwing out pseudopodia at its an-
terior end.

B. White Blood-Corpuscles (human).

Prick your finger and press out a drop of blood:
spread out on a slide under a coverslip, avoiding
pressure, and surround the margin of the coverglass
with vaseline or oil. Neglect the red corpuscles, and
examine the larger and much less numerous colourless
ones.

3/6 ELEMENTARY BIOLOGY. [CHAP. VIII.

Note

1. Size; (measure).

2. form; changing much like that of the Amoeba, but
less actively. Draw at intervals of ten seconds.

3. Structure; Some more and some less granular;
ectosarc and endosarc less distinct than in the
Amoeba. Nucleus rarely visible in the fresh state.
No contractile vesicle.

4. Treat with dilute acetic acid: the whole is clarified
and a nucleus is brought into view in a more or less
central position. Occasionally two nuclei may be
seen.

5. Stain with magenta; the whole becomes coloured,
the nucleus most intensely.

6. Place a fresh preparation on the hot stage, and
gradually warm up to 50° C. The movements are
at first rendered more active, but ultimately cease,
the pseudopodia-like processes being all retracted
and the whole forming a motionless sphere.

IX.
YEAST (Torula or Saccharomyces Cerevisice).

YEAST is a substance which has been long known on ac-
count of the power which it possesses of exciting the process
termed fermentation in substances which contain sugar.

If strained through a coarse filter, it appears to the naked
eye as a brownish fluid in which no solid particles can be
discerned. When some of this fluid is added to a solution
of sugar and kept warm, the mixture soon begins to dis-
engage bubbles of gas and become frothy ; its sweetness
gradually disappears ; it acquires a spirituous flavour and
intoxicating qualities ; and it yields by distillation a light
fluid — alcohol (or spirits of wine) which readily burns.

When dried slowly and at a low temperature, yeast is
reduced to a powdery mass, which retains its power of
exciting fermentation in a saccharine fluid for a considerable
period. If yeast is heated to the temperature of boiling
water, before it is added to the saccharine fluid, no ferment-
ation takes place ; and fermentation which has commenced
is stopped by boiling the saccharine liquid.

A saccharine solution will not ferment spontaneously. If
it begins to ferment, yeast has undoubtedly got into it in
some way or other.

If the yeast is not added directly to the saccharine fluid,
but is separated from it by a very fine filter, such as porous
earthenware, the saccharine fluid will not ferment, although

3/8 ELEMENTARY BIOLOGY. [CHAP.

the filter allows the fluid part of the yeast to pass through
into the solution of sugar.

If the saccharine fluid is boiled, so as to destroy the
efficiency of any yeast it may accidentally contain, and then
allowed to come in contact only with such air as has been
passed through cotton wool, it will never ferment. But if
it is exposed freely to the air, it is almost sure to ferment
sooner or later, and the probability of its so doing is greatly
increased if there is yeast anywhere in the vicinity.

These experiments afford evidence (i) that there is some-
thing in yeast which provokes fermentation, (2) that this
something may have its efficiency destroyed by a high tem-
perature, (3) that this something consists of particles which
may be separated from the fluid which contains them by a
fine filter, (4) that these particles may be contained in the
air; and that they may be strained off from the air by
causing it to pass through cotton wool.

Microscopic examination of a drop of yeast shews what
the particles in question are.

Even with a hand-glass, the drop no longer appears
homogeneous, as it does to the naked eye, but looks as if
fine grains of sand were scattered through it ; but a con-
siderable magnifying power (5 — 600 diameters) is necessary
to shew the form and structure of the little granules which
are thus made visible. Under this power, each granule
(which is termed a Torula) is seen to be a round, or oval,
transparent body, varying in diameter from -g-g^lh to
T oVuth of an inch (on the average about ^oVotn)-

The Torultz are either single, or associated in heaps or
strings. Each consists of a thin-walled sac, or bag, contain-
ing a semi-fluid matter, in the centre of which there is often
a space full of a more clear and watery fluid than the rest,
which is termed a 'vacuole.' The sac is comparatively

IX.] YEAST. 379

tough, but it may be easily burst, when it gives exit to its
contents, which readily diffuse themselves through the sur-
rounding fluid. The whole structure is called a ' cell ; ' the
sac being the ' cell-wall,' and the more solid portion of the
contents the 'protoplasm.' It appears that a nucleus is
also present, but on this point there is still some difference
of opinion.

When yeast is dried and burned in the open air it gives
rise to the same kind of smell as burning animal matter,
and a certain quantity of mineral ash is left behind. Ana-
lysed into its chemical elements, yeast is found to contain
Carbon, Hydrogen, Oxygen, Nitrogen, Sulphur, Phosphorus,
Potassium, Magnesium and Calcium ; the last four in very
small quantities.

These elements are combined in different ways, so as to
form the chief proximate constituents of the Torula, which
are (i) a Protein compound, analogous to Casein, (2) Cellu-
lose, (3) Fat, and (4) Water. The cell-wall contains all the
Cellulose and a small proportion of the mineral matters.
The protoplasm contains the Protein compound and the Fat
with the larger proportion of the mineral salts.

These Torultz are the ' particles ' in the yeast which have
the power of provoking fermentation in sugar; it is they
which are filtered off from the yeast when it loses its effi-
ciency by being strained through porous earthenware; it
is they which form the fine powder to which yeast is reduced
by drying, and which, from their extreme minuteness, are
readily diffused through the air in the form of invisible
dust.

That the Torula are living bodies is proved by the manner
in which they grow and multiply. If a small quantity of
yeast is added to a large quantity of clear saccharine fluid
so as hardly to disturb its transparency, and the whole is

380 ELEMENTARY BIOLOGY. [CHAP.

kept in a warm place, it will gradually become more and
more turbid, and, after a time, a scum of yeast will collect,
which may be many thousand, or million, times greater in
weight than that which was originally added. If the Torulce
are examined as this process of multiplication is going on, it
will be found that they are giving rise to minute buds, which
rapidly grow, assume the size of the parent Torula, and
eventually become detached; though, generally, not until
they have developed other buds, and these yet others. The
Torulce thus produced by gemmation, one from the other,
are apt long to adhe-re together, and thus the heaps and
strings mentioned, as ordinarily occurring in yeast, are pro-
duced. No Torula arises except as the progeny of another;
but, under certain circumstances, multiplication may take
place in another way. The Torula does not throw out a bud,
but its protoplasm divides into (usually) four masses, each
of which surrounds itself with a cell-wall, and the whole are
set free by the dissolution of the cell-wall of the parent.
These endogenous reproductive bodies are termed spores,
and retain their power of germination for a long time.

As each of the many millions of Torulce. which may thus
be produced from one Torula has the same composition as
the original progenitor, it follows that a quantity of Protein,
Cellulose and Fat proportional to the number of Torulce
thus generated, must have been produced in the course of
the operation. Now these products have been manu-
factured by the Torulce out of the substances contained in
the fluid in which they float and which constitutes their
food.

To prove this it is necessary that this fluid should have
a definite composition. Several fluids will answer the pur-
pose, but one of the simplest (Pasteur's solution) is the
following :

IX.] YEAST. 381

Water (H2O).

Sugar (C12H280U).

Ammonium Tartrate (C4H4(NH4)2O6).

Potassium Phosphate (KH2PO4).

Calcium Phosphate (Ca3P2O8).

Magnesium Sulphate (MgSO4).

In this fluid the Torulce will grow and multiply, But it
will be observed that the fluid contains neither Protein nor
Cellulose, nor Fat, though it does contain the elements of
these bodies arranged in a different manner. It follows that
the Torula must absorb the various substances contained in
the water and arrange their elements anew, building them
up into the complex molecules of its own body. This is a
property peculiar to living things.

If, on the other hand, some Torula cells be added to a
quantity of pure distilled water, it will be found not only
that no growth or multiplication of the cells takes place, but
that the amount of their protoplasm actually undergoes a
perceptible diminution. Hence we see that there are
processes going on in the cells which tend to diminish their
substance.

The chemical changes going on in a living cell, such as
that of yeast, may conveniently be spoken of collectively as
its metabolism. Among these changes those which result in
the building up of more complex chemical bodies, and thus
in an increase of the organic substance of the cell, constitute
its constructive metabolism or anabolism, while to those
which bring about a diminution of organic substance, with
evolution of simpler compounds, such as carbonic anhydride,
the term destructive metabolism or catabolism has been
applied.

It must be pointed out that the anabolic processes of the

382 ELEMENTARY BIOLOGY. [CHAP.

organism not only result in the building up of its organic
substances, but also involve an accumulation of potential
energy. If a quantity of dried yeast is burned a definite
amount of heat is given off, due to the conversion of this
potential energy into the kinetic form. Conversely, the
catabolic processes involve a change of potential into kinetic
energy. The evolution of heat by actively growing yeast is
due to changes connected with its destructive metabolism.

The Torula being alive, the question arises whether it is
an animal or a plant. Although no sharp line of demarca-
tion can be drawn between the lowest form of animal and of
vegetable life, yet Torula is an indubitable plant, for two
reasons. In the first place, its protoplasm is invested by a
cellulose coat, and thus has the distinctive character of a
vegetable cell. Secondly, it possesses the power of con-
structing Protein out of such a compound as Ammonium
Tartrate, and this power of manufacturing Protein is dis-
tinctively a vegetable peculiarity. Torula then is a plant,
but it contains neither starch nor chlorophyll, and it cannot
obtain the whole of its food from inorganic compounds, thus
differing widely from the green plants. On the other hand,
it is, in these respects, at one with the great group of Fungi.
Like many of the latter, its life is wholly independent of
light, and in this respect, again, it differs from the green
plants.

Whether Torula is connected with any other form of
Fungi v$> a question which must be left open for the present.
It is sufficient to mention the fact that under certain circum-
stances some Fungi (e.g. Mucor) may give rise to a kind of
Torula different from common yeast.

The fermentation of the sugar is in some way connected
with the living condition of the Torula, and is arrested by all
those conditions which destroy its life. At the same time

IX.] YEAST. 383

it appears that the conditions which are most favourable to
the growth and multiplication of the Torula are unfavourable
to the process of fermentation. Thus the latter goes on
most vigorously in the absence of free oxygen, while its
presence is favourable to the increase of the yeast-cells.
The greater part of the sugar is resolved into Carbonic
anhydride and Alcohol, the elements of which, taken to-
gether, equal in weight those of the sugar. A small part
(4 — 5 per cent.) breaks up into Glycerine and Succinic acid,
and one or two per cent, is not yet accounted for, but is
perhaps assimilated by the Torulce.

This is the more probable as Torulcz will grow and multiply
actively in a solution in which sugar and Ammonium Nitrate
replace the Ammonium Tartrate of the former solution, in
which case the carbon of the Protein, Cellulose and Fat
manufactured must be obtained from the sugar. Moreover,
though oxygen is essential to the life of the Torula, it can,
as already mentioned, live in saccharine solutions which
contain no free oxygen, appearing, under these circum-
stances, to carry on its life by the substitution of the ab-
normal catabolic process of fermentation for the normal
respiration.

It has further been ascertained that Torula flourish re-
markably in solutions in which sugar and pepsin replace
the Ammonium Tartrate. In this case, the nitrogen of their
protein compounds must be derived from the pepsin; and
it would seem that the mode of nutrition of such Torulcz
approaches that of animals.

384 ELEMENTARY BIOLOGY. [CHAP.

LABORATORY WORK.

Sow some fresh baker's yeast in Pasteur's fluid1 with
sugar and keep it in a warm place: as soon as the mixture
begins to froth up, and the yeast is manifestly increasing in
quantity, it is ready for examination.

A. Morphology.

1. Spread a little out, on a slide, in a drop of the fluid,
and examine it with a low power (J inch objective)
without a cover-glass. Note the varying size of the
cells, and their union into groups.

2. Cover a similar specimen with a thin glass and
examine it under a high power (J. objective).

a. Note the size (measure), shape, surface and mode
of union of the cells.

b. Their structure : cell-wall, protoplasm, vacuole.
a. Cell-wall ; homogeneous, transparent.

/?. Protoplasm,- less transparent; often with a few

clear shining dots in it.
y. Vacuole; sometimes absent; size, position.

1 See Appendix E, p. 494.

IX.] YEAST. 385

8. The relative proportion of cell- wall, protoplasm,
and vacuole in various cells.

Draw a few cells carefully to scale.

3. Run in magenta solution under the cover-glass. (This
is readily done by placing a drop of magenta so-
lution in contact with one side of the cover-glass,
and a small strip of blotting-paper at the opposite
side.)

a. Note what cells stain soonest and most deeply,
and what part of each cell it is that stains : the cell-
wall is unaffected; the protoplasm stained; the
vacuole unstained, though it frequently appears
pinkish, being seen through a coloured layer of
protoplasm.

4. Burst the stained cells by placing a few folds of
blotting-paper on the surface of the cover-glass and
pressing smartly with the handle of a mounted
needle : note the torn empty and colourless, but
solid and uncrushed transparent cell-walls ; the soft
crushed stained protoplasm.

5. Repeat observation 3, running in iodine solution
instead of magenta. The protoplasm stains brown ;
the rest of the cell remains unstained. Note the
absence of any blue coloration; starch is therefore
not present.

6. Treat another specimen with potash solution, running
it in as before : this reagent dissolves out the proto-
plasm, leaving the cell-wall unaltered.

M. 25

386 ELEMENTARY BIOLOGY. [CHAP.

7. After treatment with ether and alcohol, and staining
with haematoxylin solution, a single deeply-stained body,
probably of a nuclear nature, may be distinguished in
each cell.

8. Sow a few yeast-cells in Pasteur's solution in a moist
chamber and keep them under observation from day to
day ; watch their growth and multiplication.

9. Spore-formation : take some dry German yeast ; sus-
pend it in water and shake so as to wash it. Let the
mixture stand for half-an-hour : pour off the super-
natant fluid, and, with a camel's hair pencil, spread out
the creamy deposit in a thin layer on fresh cut potato
slices or on a plate of plaster of Paris, and place with
wet blotting-paper under a bell-jar : examine from day
to day with a very high power (800 diam.) for spores •,
which will probably be found on the eighth or ninth
day.

B. PHYSIOLOGY.

(Conditions and results of the vital activity of Torula)
i. Sow a fair-sized drop of yeast in —

a. Distilled water.

b. 10 per cent, solution of sugar in water.

c. Pasteur's fluid without the sugar.

d. Pasteur's fluid with sugar.
e. Mayer's pepsin solution1.

Keep all at about 35° C., and compare the growth of the
yeast, as measured by the increase of the turbidity of the
fluid, in each case, "a" will hardly grow at all, "£" better,
"c" better still, "</" well, and "e" best of all. Note that
bubbles of gas are plentifully evolved from both the so-
lutions which contain sugar.

1 See Appendix E, p. 494.

IX.] YEAST. 387

That any growth at all takes place, in the case of
experiments a and b, is due to the fact that the drop of
yeast added contains nutritious material sufficient to pro-
vide for that amount of growth.

2. Prepare two more specimens of "d" and keep one
in a cold — the other in a warm (35° C.) place, but
otherwise under like conditions. Compare the growth
of the yeast in the two cases ; it is much greater in
the specimen kept warm.

3. Prepare two more specimens of "*/"; keep both
warm, but one in darkness, the other exposed to the
light: that in the dark will grow as well as the other;
sunlight is therefore not essential to the growth of
Torula.

4. Sow some yeast-cells in Pasteur's solution in a flask,
the neck of which is closed by a plug of cotton
wool, and boil for five minutes ; then set it aside ;
no signs of vitality will afterwards be manifested by
the yeast in the flask ; it is killed by exposure to this
temperature.

5. [Take two test tubes; in one place some yeast, with
Pasteur's solution containing sugar; in the other place
baryta water, and then connect the two test tubes by
tightly fitting perforated corks and a bent tube passing
from above the surface of the fluid in the first tube to
the bottom of the baryta water in the second ; pass a
narrow bent tube, open at both ends, through the cork
of the baryta water tube, so that its outer end dips just
below the surface of some solution of potash1. All gas
formed in the first tube will now bubble through the
baryta water in the second, and, from thence, any that

1 The object of the potash is to shield the baryta water from any
carbonic anhydride that may be in the atmosphere.

388 ELEMENTARY BIOLOGY. [CHAP. IX.

is not absorbed will pass out through the potash into
the air. An abundant precipitate of barytic carbonate
will be formed which can be collected and tested. The
fermenting fluid, therefore, evolves carbonic anhydride.]

6. [Grow some yeast in Pasteur's solution (with sugar), in a
nearly closed vessel (say a bottle with a cork through
which a long narrow open tube passes) : as soon as the
evolution of gas seems to have ceased, distil the fluid in
a water bath and condense and collect the first fifth
that comes over: redistill this after saturation with
potassic carbonate, and test the distillate for alcohol by
its odour and inflammability.]

7. [Determine that heat is evolved by a fluid in which
active alcoholic fermentation is going on. Place 200 cc.
of fresh yeast in a flask, and add I litre of Pasteur's
fluid with sugar : put another litre of the fluid alone in
a similar flask, cover each flask with a cloth and place
the two side by side in a place protected from draughts.
When gas begins to be actively evolved from the yeast-
containing solution, take the temperature of the fluid in
each flask with a good thermometer ; the temperature of
the one in which fermentation is going on will be found
the higher.]

X.
PROTOCOCCUS (Protococcus pluvialis ').

IF the mud which accumulates in roof-gutters, water-butts,
and shallow pools be collected, it will be found to contain,
among many other organisms, specimens of Protococcus.
In its vegetative condition Protococcus is a spheroidal body
^w to WOTJ °f an mcn m diameter, composed, like Torula,
of a structureless tough transparent wall, inclosing viscid and
granular protoplasm. The chief solid constituent of the
cell-wall is cellulose. The protoplasm contains a nitrogenous
substance, doubtless of a proteinaceous nature, though its
exact composition has not been determined, and some
small starch-grains are sometimes to be found in it. Certain
definite portions of the protoplasm, termed chromatophores,
contain a red or green colouring matter. The latter is
called cJdorophylL These chromatophores occupy a large
part of the cavity of the cell, so that under a low power the
whole contents appear to be coloured.

Individual Protococci may be either green or red ; or half
green and half red ; or the red and green colours may co-
exist in any other proportion.

In the middle of the cell is a distinct nucleus, containing
a nucleolus.

1 The names Hsematococcus, and Chlamydococcus pluvialis have
also been applied to this form, but it must not be confused with the
more abundant Pleurococcus which everywhere forms a green powdery
layer on trees, palings, &c.

390 ELEMENTARY BIOLOGY. [CHAP.

The influence of sunlight is an essential condition of the
growth and multiplication of Protococcus ; under that influ-
ence, it decomposes carbonic anhydride, appropriates the
carbon, and sets oxygen free. The energy thus derived
from the sun's rays is stored up in the potential form, to be
again partly expended in the growth of the plant, and in the
movements of its spores. It is this power of obtaining the
carbon which it needs from carbonic anhydride, which is
the most important distinction of Protococcus, as of all
plants which contain chlorophyll, from Torula and the
other fungi.

As Protococcus flourishes in rain-water, and rain-water con-
tains nothing but carbonic anhydride, which it absorbs
along with other constituents of the atmosphere, ammonium
salts (usually ammonium nitrate, also derived from the air)
and minute portions of earthy salts which drift into it as
dust, it follows that it must possess the power of con-
structing protein by rearrangement of the elements supplied
to it by their compounds. Torula, on the other hand, is
unable to construct protein matter out of such materials as
these.

Another difference between Torula and Protococcus is
only apparent : Torula absorbs oxygen and gives out car-
bonic anhydride; while Protococcus, on the contrary, absorbs
carbonic anhydride and gives out oxygen. But this is true
only so long as the Protococcus is exposed to sunlight. In
the dark, Protococcus, like all other living things, undergoes
oxidation and gives off carbonic anhydride; and there is
every reason to believe that the same process of oxidation
and evolution of carbonic anhydride goes on in the light,
but that the loss of oxygen is far more than covered by the
quantity set free by the carbon-fixing apparatus, of which
the chlorophyll forms an essential part.

X.] PROTOCOCCUS. 391

The multiplication of Protococcus takes place by means of
actively locomotive spores, termed zoospores. These are of
two kinds. The larger, or macrozoospores, are produced by
the division of the contents of one of the ordinary cells into
two or four portions. These portions assume a pear-like
shape and become free by the resorption of the wall of the
mother- cell. When they first escape the zoospores are
naked protoplasmic bodies (primordial cells) destitute of any
cell-wall. They swim actively through the water by means
of two delicate processes of the protoplasm, termed ciliat
which are inserted at the pointed end of the zoospore. The
movement is of a double kind, the progressive motion, in
which the pointed ciliated end goes foremost, being accom-
panied by a rotation of the zoospore about its long axis.
The macrozoospore soon acquires a thin cell-wall, through
which the cilia protrude. The wall does not remain in
immediate contact with the protoplasm of the primordial
cell, but becomes separated from it by a space containing
clear cell-sap. This space is however traversed by fine
radiating protoplasmic threads. The internal structure is
similar to that of the vegetative cell, each zoospore contain-
ing a nucleus and chromatophores. The ciliated end con-
sists of colourless protoplasm only. The movement of the
cilia is so rapid, and their substance is so transparent and
delicate, that they are invisible until they begin to move
slowly, or are treated with reagents, such as iodine, which
colour them, and arrest their movements.

The smaller, or microzoospores, are produced by the division
of the vegetative mother-cell into a larger number of por-
tions, the number being always some power of two. They
differ from the macrozoospores, apart from their size, in
never acquiring a cell-wall so long as their movement con-
tinues. In other respects the two kinds of zoospores agree.

392 ELEMENTARY BIOLOGY. [CHAP.

Both eventually come to rest, assume a globular shape,
lose their cilia and form a thick cell-wall around their pro-
toplasm, thus reproducing the vegetative condition of the
plant.

Both kinds of zoospores are sensitive to light, swimming
towards the source of light, when its intensity is moderate,
and away from it when its intensity is increased.

For reasons similar to those which prove the vegetable
nature of Torula, Protococcus is a plant, although its zoo-
spores are curiously similar to the Monads among the lowest
forms of animal life. But it is now known that many of the
lower plants, especially in the group of Algce, to which Pro-
tococcus belongs, also give rise to locomotive spores propelled
by cilia, like those of Protococcus, so that there is nothing
anomalous in the case of the latter.

Like the yeast-plant, Protococcus retains its vitality after it
has been dried. It has been preserved for as long as two
years in the dry condition, and at the end of that time has
resumed its full activity when placed in water. The wide
distribution of Protococcus on the tops of houses and else-
where, is thus readily accounted for by the transport of the
dry Protococci by winds.

LABORATORY WORK.

A. Morphology.

a. Vegetative stage.

i. Spread out in water some mud from a gutter or
similar locality, and put on a cover-glass. Look for
the red or green Protococcus cells with a low power.
Having found some, put on a high power and make
out the following points.

x.] PROTOCOCCUS. 393

Size; (measure) — very variable.

Form; more or less spheroidal, with individual
variations.

Structure; cell-wall, protoplasm, chromatophores, some-
times a vacuole, always a nucleus containing a
nucleolus.

Colour; generally green — sometimes red — sometimes
half and half — sometimes centre red, periphery
green — the colouring matter always in the proto-
plasm of the chromatophores only.

Draw carefully to scale.

2. Apply the methods of mechanical and chemical
analysis detailed for Torula. (I. A. 3. 4. 5. 6.) The
application of iodine is especially useful for bringing
out the limits of the chromatophores, and also the
nucleus and nucleolus. The same reagent will also
often show the presence of some small starch-grains.
Treat a specimen with strong iodine solution and
then with sulphuric acid (75 per cent.): the cell-wall
will become stained blue.

3. Look out for cells the contents of which are dividing
up to form zoospores.

b. Zoospores.

a. Mount a drop of water containing zoospores of
Protococcus, and examine with a high power.
Note the actively locomotive green bodies, of
which two varieties can be distinguished.
a. Cells of relatively large size derived from
the vegetative cells by division of their con-
tents into 2 or 4 portions. These are the
macrozoospores. Observe the thin colour-

394 ELEMENTARY BIOLOGY. [CHAP.

less cell-wall, surrounding the protoplasm,
but separated from it by a clear space.
Note in various specimens — The two cilia
prolonged from the protoplasm through aper-
tures in the cell-wall; their motionless part
within the wall; their vibratile portion out-
side it. The colourless thin external layer
of the protoplasm collected into a little heap
at the point from whence the cilia arise.
The delicate colourless processes radiating
from the outer protoplasmic layer to the in-
terior of the cell-wall. The colour — usually
green, but frequently one bright red spot is
present.

Here, as in the vegetative stage, the colour-
ing matter is limited to the relatively large
chromatophores, and here also a nucleus is
present.

/?. Cells of smaller size but much like the above
if the cellulose sac were removed, and the
radiating processes extending to it from the
protoplasm withdrawn. These are the micro-
zoospores.

b. Try to find specimens in which the movements
are becoming sluggish, and see the cilia in
motion.

f. Stain with iodine ; this kills the cells, and stops
their movements, and frequently renders the
cilia very distinct.

d. Try to find zoospores which have come to rest,
and are beginning to form a new cell-wall.

x.] PROTOCOCCUS. 395

[B. PHYSIOLOGY.

1. Get some water that is quite green from containing
a large quantity of Protococcus ; introduce some of it
into two tubes filled with and inverted over mercury,
and pass a small quantity of carbonic anhydride into
each : keep one tube in the dark and place the other
in bright sunlight for some hours. Then measure the
gas in each tube and afterwards introduce a fragment
of caustic potash into each ; the gas from the specimen
kept in the dark will be more or less completely ab-
sorbed (= carbonic anhydride), that from the other
will not be absorbed by the potash alone, but will be
absorbed on the further introduction of a few drops of
solution of pyrogallic acid (= oxygen). Protococcus,
therefore, in the sunlight, takes up carbonic anhydride
and evolves oxygen. A comparative experiment may
be made with a third tube containing water but no
Protococcus.

2. Place some water containing numerous zoospores of
Protococcus near a window, exposed to bright sun-
light. Observe that the majority of the zoospores
accumulate on the side away from the source of
light.

Next remove them to some distance from the window,
or otherwise diminish the intensity of the light acting
on them. Observe that now the zoospores accumulate
on the side towards the light.

The zoospores of Protococcus are thus seen to be
photometric^ i.e. they seek or avoid the light according
to the degree of its intensity.]

XL
SPIROGYRA.

IN ponds and tanks, the water of which is tolerably pure,
we constantly find in summer floating masses, of a light
green colour, which feel slippery when handled. A very
superficial examination is sufficient to show that these
masses consist of a vast number of very long and fine green
threads. The organisms in question may belong to various
genera of a group of plants called the Conferuoid Algae:
one of the genera, representatives of which are most com-
monly found, is Spirogyra.

On microscopical examination we find that each thread
of Spirogyra consists of a single, unbranched, row of
cylindrical cells, of very variable length, but of constant
diameter throughout the filament. There is no distinction
of apex and base ; both ends of the filament are quite
alike, and neither is attached to any substratum. We have
to do here with a plant which is morphologically multi-
cellular, for each thread consists of many cells. All these
cells, however, are uniform in structure, and each performs
for itself, so far as can be ascertained, all the functions of
the plant. Hence Spirogyra may be spoken of as phy-
siologically a unicellular plant. In each of the cells there
are one or more (sometimes as many as 10) bright green
spiral bands, and by these the genus may at once be
recognized. Their number varies according to the species,
which are very numerous, and in many cases hard to

CHAP. XL] SPIROGYRA. 397

distinguish. The most favourable for investigation are of
course the larger ones, and among these the most con-
venient forms are those in which the spiral bands have
somewhat lax windings, so that they do not interfere with
the view into the interior of the cell.

Each cell of Spirogyra is bounded on the outside by a
cellulose wall. This consists of a number of layers, which
however can only be made out distinctly by the use of
reagents such as potash. In many species the wall is sur-
rounded by a thick gelatinous sheath. Immediately inside
the cell-wall and closely applied to it is a layer of proto-
plasm, to which the name primordial utricle is applied. In
Spirogyra this is so thin that it is difficult to make out under
natural conditions except with a very high power. It can
however be easily demonstrated by the use of what are
termed plasmolysing re-agents, such as solution of common
salt, which withdraw water from the interior of the cell, and
thus cause its contents to contract. The contracted mass
will then be seen to be bounded on all sides by a thin
membrane, which is the primordial utricle. The latter is of
precisely the same nature as the protoplasm in the cells of
Yeast or Protococcus. Its occurrence in the form of a
relatively thin parietal layer, while the greater part of the
cavity of the cell is occupied by a large vacuole, is character-
istic of most vegetable cells in their mature condition.

The green spiral bands correspond to the chromatophores
in Protococcus, and bear the same name, each band being
spoken of as a single chromatophore. Each cell may there-
fore contain one chromatophore, or several, according to
the species. In these bodies all the chlorophyll of the
plant is contained. They themselves consist of protoplasm
and retain their form unaltered when the chlorophyll is
extracted by alcohol or other solvents, and they are in im-
mediate contact with the primordial utricle throughout their

398 ELEMENTARY BIOLOGY. [CHAP.

whole course. At intervals along each chromatophore
round bodies will be observed which appear green like
the rest of the band, but on careful examination are
each found to contain a small colourless mass of proteid
substance, termed the pyrenoid. This pyrenoid is of a
•crystalline form, usually appearing hexagonal in optical
section, and can be brought out more clearly by the use of
staining reagents. Bodies of this nature are of very general
occurrence in the chromatophores of the Algae. It is only
around the pyrenoids that starch is formed, and if the plant
has been exposed to the light before examination, the
pyrenoid will be found to be surrounded by a layer of small
starch-grains often present in so great a mass that the band
is distinctly swollen at the places where they occur.

As already mentioned the greater part of the interior of
the cell is occupied by a single large vacuole, containing
cell-sap, i.e. water in which certain inorganic salts, and
certain organic substances, such as sugar and acids, are
dissolved.

In addition to the structures already described each cell of
Spirogyra contains a conspicuous nucleus. Its position varies
somewhat in different species, for in some cases it is placed
in the middle of the cell, while in others it is nearer to
the external wall. Forms with a central nucleus are more
convenient for observation. The shape of the nucleus also
shows certain specific differences. A very common form is
that of a biconvex lens, the edge of the lens being turned
towards the lateral walls of the cell, and the circular faces
towards its ends. The nucleus is surrounded by a layer of
protoplasm, from which long processes radiate which
traverse the vacuole and are attached at their further ends
to those parts of the chromatophores in which the pyrenoids
are situated. Both in these processes and in the primordial
utricle delicate protoplasmic currents can be detected,
flowing in all directions (cf. Tradescantia p. 441).

XL] SPIROGYRA. 399

The nucleus contains a relatively large nucleolus, the
substance of which seems to be homogeneous, and some-
what resembles that of the pyrenoids in its reactions. The
nucleus itself usually appears to be finely granular. Its real
structure is however fibrous, but for the study of these
details the nucleus of Spirogyra is not favourable.

The growth of Spirogyra in length is accompanied by the
increase in number of its cells. This increase takes place
by the repeated division of each cell into two equal parts,
by means of a transverse wall; the process is not localized
in any special region of the filament; all its cells are alike
capable of division. In this fact Spirogyra differs from all
the more perfect plants to be subsequently considered, and
here also we have an indication of the physiological equiva-
lence of the constituent cells of Spirogyra.

The process of cell-division takes place, under normal
conditions, in the night, the products of assimilation which
have been accumulated in the daytime being used up during
the darkness to form new cell-wall and protoplasm. The
division is however dependent on temperature, and by keep-
ing the plants cold during the night it is possible to make
them postpone their cell-division till the morning, when it
can be conveniently observed. The nucleus divides into
two, the daughter nuclei being at first connected by fine
protoplasmic threads. In the meantime a transverse ring of
cellulose is formed by the protoplasm around the cell, mid-
way between its two ends. This ring is continuous, at its
outer edge, with the lateral wall on which it abuts. By
further additions from the protoplasm it gradually grows in-
wards until its inner edges meet, so that a complete disc of
cellulose is formed across the cavity of the cell. The con-
tents necessarily become constricted by the inward growth
of the new cell-wall, and are eventually severed into two

4OO ELEMENTARY BIOLOGY. [CHAP.

distinct halves, each containing one of the daughter nuclei.
The whole process, especially as regards the division of the
nucleus, is a complicated one, but it is not necessary to enter
into the details here. Cell-division, of which this may be
taken as a type, is the general mode of formation of cells
throughout the vegetable kingdom. In the higher plants
however it is usual for the new wall to be formed simul-
taneously over its whole area, instead of by progressive
growth from without inwards.

The Reproduction of Spirogyra takes place in quite a
different way from that of the plants hitherto considered,
and affords one of the simplest examples of a sexual process.
In the form in which it here occurs this process is termed
conjugation, which may be defined as the union of two ex-
ternally similar cells to form a single reproductive cell.
Conjugating specimens may be distinguished from vege-
tative ones by their more tangled filaments, and duller
colour, and by feeling rougher when handled. When under
cultivation Spirogyra can usually be induced to conjugate
by allowing the water in which it is growing to evaporate
very gradually. In the ordinary form of conjugation two
filaments lying side by side send out short lateral protrusions,
one from each cell. The protrusions from the one filament
grow to meet those from the other, their contiguous walls
become absorbed, and in this way the two filaments become
connected by a number of transverse tubes, arranged like the
rungs of a ladder. Changes now begin to take place in the
contents of the cells, and it is at this stage that a difference
first shows itself in the behaviour of the two filaments. In
the one filament the contents of each cell contract away
from the wall, so as to lie freely in the cavity of the cell,
as a distinct sac bounded by the primordial utricle. In
the other filament these changes take place somewhat later.

XI.] SPIROGYRA. 401

After a time the contents of the cells which first under-
went contraction begin to move into the connecting tube.
The entire contents pass over and fuse completely with
those of the cell which receives them. Recent researches
have shown that the corresponding constituents of the
contents of each cell unite one with another, primordial
utricle fusing with primordial utricle, chromatophore with
chromatophore, and, which is the most important point,
nucleus with nucleus.

The cell formed by conjugation is termed the zygospore.
It is at first spherical in shape and destitute of any cell-wall
of its own. It soon increases in size, by taking up water,
assumes an elliptical outline, and becomes surrounded by a
cell-wall. The latter consists, in its mature condition, of
three layers, the middle one of which is of a brown colour,
while those on either side of it are colourless. The wall is
cuticularized, that is to say, it has undergone a chemical
change which renders it almost wholly impermeable to
water. Meanwhile changes go on in the contents of the
zygospore, the most important being the appearance of oil
in place of starch. In this condition the zygospore passes
through a long stage of rest, during which it can withstand
great extremes of temperature, and is not injured by periods
of drought. On germination each zygospore bursts its cuti-
cularized wall and grows out directly into a new plant.

We have seen that in the normal case of conjugation just
described a distinct differentiation of the filaments into
male and female individuals is shown, as all the cells of
each of the conjugating filaments behave in the same way,
either giving up their own contents or receiving those of the
other thread. This sexual difference however has not yet
become completely fixed, for in certain cases adjoining cells
of one and the same filament conjugate with each othei
M, 26

402 ELEMENTARY BIOLOGY. [CHAP.

This process, to which the name of monoecious conjugation
may be applied, occurs in the same species which are usually
reproduced by the normal ladder-like or dioecious con-
jugation.

Some allies of Spirogyra show no demonstrable difference
of sex between the conjugating cells, for the zygospore is
formed in the connecting passage, each cell thus appearing
to take an exactly equal part in the process.

The mode of reproduction in Spirogyra and its allies, simple
as it is, may serve as the type of all sexual reproduction.
Among the innumerable modifications under which the
sexual process takes place, the one point which is found to
be constant is the fusion of the nuclei of two distinct cells.

A few words on the Physiology of Spirogyra may be added.
Its nutrition takes place in all important respects in the
same way as that of Protococcus. The carbonaceous food
of the plant is obtained by decomposition of the carbon
dioxide dissolved in the water, and this of course only goes
on under the influence of light. Starch is the first easily
demonstrable product of this process, though it is certainly
not the first product actually formed. It is easy to show
that the formation of the starch-grains is dependent on the
action of light. If the plant be kept for some hours in the
dark all the starch will be found to have disappeared. A
very short exposure to daylight is sufficient to induce its
renewed formation. Under the action of direct sunlight a
demonstrable amount of starch may be developed within so
short a time as five minutes. It will be remembered that
the starch-grains are formed around the pyrenoids. As re-
gards the part played by the latter in the process we have
no definite knowledge at present, but similar proteid crys-
talloids are found elsewhere in connection with starch-
forming corpuscles.

XL] SPIROGYRA. 403

Spirogyra will not of course grow in distilled water any
more than will Yeast or Protococcus. If the water be sup-
plied with the following salts the Spirogyra will b€ able to
obtain all the food it requires : Potassium Nitrate, Calcium
Sulphate, Magnesium Sulphate, Calcium Phosphate, Sodium
Chloride, and Ferric Chloride. The sodium chloride is only
of indirect service as neither of its elements is essential as a
food-constituent. As regards the other elements required
by the plant, we already know that Carbon, Hydrogen,
and Oxygen are the elements of starch and cellulose, while
the protoplasm and nucleus are built up of these elements
with the addition of Nitrogen, Sulphur and Phosphorus.
Iron is necessary for the formation of chlorophyll, while
potassium plays an essential, though obscure, part in the
process of assimilation. Calcium and Magnesium have
been shown by experiment to be necessary for the nutrition
of green plants, but their exact function is still doubtful.

LABORATORY WORK.

A. MORPHOLOGY.

I. Vegetative condition.

1. Naked-eye characters.

Observe that the masses of Spirogyra consist of
long, delicate, bright green, unbranched filaments,
which in the vegetative condition are quite smooth
and glossy.

2. Microscopical characters.

Mount one or two filaments of Spirogyra in water
and observe first with the low, and then with the
high power.

26—2

404 ELEMENTARY BIOLOGY. [CHAP.

a. Note that each filament is composed of a series
of cylindrical cells, that it is unbranched, and that
all its cells are alike (apart from slight differences
of length). Observe especially that there is no
difference of structure between the two ends of
the filaments.

b. In each cell observe

a. The cell-wall, completely surrounding the cell.
It consists chiefly of cellulose and gives the
same reactions as the cell-wall of Protococcus
(p. 393). On treatment with potash solution,
the stratification of the cell-wall can be made
out with a high power.

y8. The protoplasmic primordial utricle, completely
lining the cell-wall on the inside. This is
colourless and appears finely granular under
a very high power. In order to see the
primordial utricle better, plasmolyse the cell by
running in 10 p. c. salt solution. The prim-
ordial utricle will now contract away from the
cell-wall and be easily seen.

y. The bright green spiral chromatophores. These
are from i to 10 in number, according to
the species. Observe the roundish bodies
occurring at intervals in each chromatophore.
These are the pyrenoids, each of which is
surrounded by a layer of starch-grains if the
plant has been exposed to light. Run in Iodine
solution to demonstrate the presence of starch.

B. The large vacuole occupying the greater part of
the interior of the cell.

c. The nucleus in the middle of the cell. In most

SPIROGYRA. 405

species its form is that of a biconvex lens, as
described in the text. Inside it is the nucleolus.
The nucleus is surrounded by a layer of pro-
toplasm. Observe that strands of protoplasm
radiate from the latter, crossing the cavity of
the cell, and attached at their outer ends to
those parts of the chromatophores where the
pyrenoids occur.

The minute structure will be better made out by
means of staining. Place some Spirogyra in satu-
rated solution of Picric acid1 for not less than
12 hours. Then wash thoroughly in distilled
water. The chlorophyll will now have entirely
disappeared. Stain the filaments thus prepared
with Borax carmine solution2 for an hour or more.
Then mount in dilute glycerine3 and examine with
the highest power available. The cell-wall will
be unstained. The protoplasm of the primordial
utricle, chromatophores, and radial strands will
have taken the stain very lightly. On the other
hand the nucleus will be decidedly, and the nu-
cleolus very deeply, coloured. The pyrenoids
stain deeply, and their crystalline form can now
be clearly made out. The layer of starch-grains
surrounding each pyrenoid remains quite colour-
less. It is especially in material prepared in this
way that the relation of the radial protoplasmic
strands to the pyrenoids can be traced.

The process of cell-division, as described in the
text, can be best observed by keeping the Spiro-
gyra at a low temperature during the night, and

1 Appendix, E.

2 Appendix, E.

3 The best plan is to mount in water and gradually run in glycerine
under the cover-slip.

406 ELEMENTARY BIOLOGY. [CHAP.

examining in a warm room the following morning.
In cold weather it is sufficient to put the vessel
containing the Spirogyra out-of-doors ; in summer
it may be surrounded with ice.

In mounting the filaments to observe the di-
vision it is necessary to support the cover-slip by
means of two bristles or fragments of thin glass, so
that it may not press on the object. The higher
the power used the better.
II. Reproduction.

1. Look out for masses of Spirogyra in which the fila-
ments are tangled, and of a dull-green colour, as
described in the text. These will generally be found
late in the summer, or at any time when the water
in which the plant is growing is getting low.

2. Examine such filaments first with a low power, in
order to find the various stages of conjugation.
The details at each stage are then to be made out
with the high power. Observe

a. Pairs of parallel filaments which have begun to
produce the lateral protrusion. Note that the
protrusions in the filaments are formed opposite
one another.

b. Similar pairs in which the protrusions have al-
ready become continuous by absorption of their
walls at the point of contact, so that they now
form transverse tubes connecting the filaments.
Note that at this stage the cells of the one fila-
ment show a contraction of their contents, while
those of the other are unaltered.

c. Stages in which the cell-contents of the one fila-
ment are passing over through the connecting
tubes into the cells of the other.

XL] SPIROGYRA. 407

The details of the fusion of the contents are best
made out in material prepared and stained as de-
scribed above (I. 2. b. £.)• Without some such
treatment the nuclei cannot be seen at this stage
at all.

d. Stages of development of the Zygospore, formed
from the united contents of the two cells. Ob-
serve its form, at first spherical, then oval, and the
gradual thickening of its wall, which ultimately
consists of 3 layers. Treat a ripe zygospore with
Sulphuric acid. Observe that its wall is not dis-
solved, showing that it has become cuticularized.
Observe the abundant oil-drops in the contents of
the Zygospore.

B. PHYSIOLOGY.

1. Keep some filaments of Spirogyra in distilled water.
Observe that although they may remain alive for some
time, growth soon ceases entirely.

Place other filaments in a food-solution1 containing
the salts enumerated in the text. Under proper con-
ditions of light and temperature active growth of the
filaments and division of their cells will now take place.

2. Place a vessel containing Spirogyra in the dark for
12 hours. Then mount some filaments, test with
Iodine, and observe that the starch has entirely dis-
appeared.

Expose some of the filaments to as bright a light as
possible. Observe again after a short interval (varying
from 5 minutes to an hour or more according to the
intensity of the illumination). Test with Iodine as
before, and observe that starch-grains have again made
their appearance around the pyrenoids.

1 Appendix, E. For Spirogyra the solution should be considerably
diluted.

XII.
BACTERIA (Schizomycetes).

UNDER these names a considerable variety of organisms,
for the most part of extreme minuteness, are included.

They may be defined as globular, oblong, rod-like or
spirally coiled masses of protoplasmic matter enclosed in a
more or less distinct structureless membrane, in most cases
devoid of chlorophyll and multiplying by transverse division.
The smallest are not more than s-ol^o^h °f an mcn m dia-
meter, so that under the best microscopes they appear as
little more than mere specks, and even the largest have a
thickness of little more than TQ^00th of an inch, though they
may be very long in proportion. Many of them have two
conditions — a still and an active state. In their still condi-
tion, however, they very generally exhibit that Brownian
movement which is common to almost all very finely divided
solids suspended in a fluid. But this motion is merely oscil-
latory, and is readily distinguishable from the rapid transla-
tion from place to place which is effected by the really active
Bacteria.

In a large number of forms the presence of cilia has been
detected. In Spirillum volutans, for example, one of the
largest species, there is a cilium at each end of the spirally
coiled body. In some cases it is probable that these cilia
consist of protoplasm, and are really the organs by which

CHAP. XII.] BACTERIA. 409

the movement is effected. In other Bacteria however,
there is reason to believe that the so-called cilia are simply
prolongations of the cell-membrane, and destitute of any
power of independent movement. In the latter case the
movement of the organism must be due to some other
cause, such as the contractility of the protoplasmic body as
a whole. Many forms, such as the Vibriones, so common in
putrefying matters, appear obviously to have a wriggling or
serpentiform motion, but this is an optical illusion. In this
Bacterium the body does not rapidly change its form ; but
its joints are bent zigzag- wise, and the rotation of the zig-zag
upon its axis, as it moves, gives rise to the appearance of
undulatory contraction. A cork-screw turned round, while
its point rests against the finger, gives rise to just the same
appearance.

In other forms, however, such as Spirillum, true contrac-
tile curvatures have been observed.

A nucleus has not yet been detected in any of these
plants. The delicate membrane of the Bacteria is in all
cases surrounded on the outside by a gelatinous layer, often
difficult of detection. In the still state, however, this gela-
tinous substance is often developed to a very large extent,
forming a continuous mass of considerable size, in which
the individual cells appear embedded. This is termed the
Zooglcea form of the Bacteria. The membrane of the Bac-
teria, with its gelatinous layers, corresponds to the cell-wall
of Protococcus or yeast. Its reactions, however, vary in
different cases, sometimes agreeing with those of cellulose,
and sometimes indicating an albuminous composition.

All forms of Bacteria, of which the life-history has been
fully investigated, are found to produce resting spores.
These bodies make their appearance when the supply of
food has become exhausted or the conditions have otherwise

410 ELEMENTARY BIOLOGY. [CHAP.

grown unfavourable. Either the spore may arise as a small
cell in the interior of certain of the vegetative cells, or the
latter may themselves become directly converted into
spores. In either case the spore is characterized by its
more granular protoplasm and firmer cell-wall. On germi-
nation, which may be delayed until after a long period
of rest, the hard membrane is thrown off, and the pro-
toplasm of the spore resumes the ordinary vegetative deve-
lopment.

Bacteria grow and multiply in Pasteur's solution (without
sugar) with extreme rapidity, and, as they increase in
number, they render the fluid milky and opaque. Their
vital actions are arrested at the freezing point, but no tem-
perature has yet been reached low enough to kill them.
They thrive best in a temperature of about 30° C. but, in
most fluids, they are killed by a temperature of 60° C.
(140° F.). This, however, only applies to the vegetative
condition, for the spores can in many cases resist a tempera-
ture very considerably over the boiling point of water.

In many points of their physiology Bacteria closely re-
semble Torulcz; and a further resemblance lies in the cir-
cumstance that many of them excite specific fermentative
changes in substances contained in the fluid in which they
live, just as yeast excites such changes in sugar.

All the forms of putrefaction which are undergone by
animal and vegetable matters are fermentations set up by
Bacteria of different kinds. Organic matters freely exposed
to the air are, in themselves, nowise unstable bodies, and
if due precautions have been taken to exclude Bacteria
they do not putrefy, so that, as has been well remarked,
" putrefaction is a concomitant not of death, but of life."

Bacteria, like Torulce and Protococci, are not killed by
drying up, and from their excessive minuteness they must

XII.] BACTERIA. 41 I

be carried about still more easily than Torulcz are. In
fact there is reason to believe that they are very widely
diffused through the air, and that they exist in abundance
in all ordinary water and on the surface of all vessels that
are not chemically clean. They may be readily filtered off
from the air, however, by causing it to pass through cotton-
wool.

The Bacteria have no near relationship to any of the
other plants described in this book, and are introduced
chiefly on account of the great importance of the various
fermentative changes which they produce.

^he life-history of the Bacteria is only completely known
in comparatively few species. Some of them have been
found to be highly pleomorphic, that is to say, the same species
may appear under very different forms at different periods
of its development. In other cases the life-history is com-
paratively simple. Hence the various forms mentioned in
the Laboratory work must not be regarded as necessarily
representing so many distinct species.

LABORATORY WORK.

Infuse some hay in warm water for half-an-hour —
filter, and set aside the filtrate: note the changes
which go on in it — at first clear, in 24 or 36 hours it
becomes turbid ; later on, a scum forms on the sur-
face and the infusion acquires a putrefactive odour.
Rub some gamboge down in water and examine a
drop of the mixture with a high power: avoid all
currents in the fluid and watch the Brownian move-
ments; note that they are simply oscillatory — not
translative.

412 ELEMENTARY BIOLOGY. [CHAP.

3. Take a drop of fluid from a turbid hay infusion —
and examine it, using the highest power you have;
in it will be found multitudes of

Moving Bacteria. Note their —

a. Form; elliptic or rodlike — sometimes forming
short (2 — 8) jointed rows.

b. Size; breadth, very small but pretty constant;
length, varying, but several times greater than
their breadth : measure.

c. Structure; an outer more transparent layer en-
veloping less transparent matter: in the com-
pound forms the envelope appears only where
two joints come in contact, so that the rod looks
as if made up of alternating transparent and more
opaque substances.

d. Movements; some vital, and some purely physical
(Brownian). The former various but progressive :
the latter a rotatory movement round a stationary
centre; study it in a drop of boiled infusion in
which the Bacteria are all dead.

4. Treat with iodine — only the more opaque parts

stain; probably then we have to do with proto-
plasm, enveloped in nonprotoplasmic matter.

5. Resting Bacteria. (Zooglcea-stage.)

a. Examine the scum from the surface of a hay
infusion; it exhibits myriads of motionless Bac-
teria, embedded in gelatinous material.

b. Treat with iodine ; the Bacteria stain as before :
the gelatinous uniting material remains un-
stained.

XII.] BACTERIA. 413

6. Besides the form hitherto described, which would
be known as Arthrobacterium, many other Schi-
zomycetes will certainly be found, both in the
pellicle and the fluid beneath. The following may
be especially mentioned :

a. Micrococcus. Bodies much like Arthrobacterium
but short and rounded, and occurring singly, or
in bead-like rows. They may be found free or
in a Zooglcea stage.

b. Bacillus. Threads composed of straight cylin-
drical joints much longer than those of Arthro-
bacterium, but of similar structure: they are al-
ways free-swimming.

c. Vibrio. Like Bacillus, but with bent joints.

d. Spirillum. Elongated unjointed threads rolled
up into a more or less perfect spiral : frequently
two spirals interwine. In some of the largest
forms a vibratile cilium can be made out on
each end of the thread.

e. Spirochsete. Much like Spirillum, but longer
and with a much more closely rolled spiral. A
very actively motile but not common form.

7. Examine various putrefying substances for different
forms of Bacteria. Successful cultures may be
made on hard-boiled white of egg, or slices of
potato, kept moist under a bell-glass. Various
brilliantly coloured micrococci including the blood-
red " Micrococcus prodigiosus " will make their
appearance on such cultures. An infusion of pea-
flour, filtered, is especially favourable for Spiro-
chaete.

414 ELEMENTARY BIOLOGY. [CHAP. XII.

8. Place some fresh-made hay infusion in three flasks;
boil two of them for three or four minutes, and while
one is boiling briskly stop its neck with a plug of
cotton-wool and continue to boil for a minute or
two: leave the necks of the other two flasks un-
closed, and put all three away in a warm place.

a. In a day or two abundant Bacteria will be found
in the unboiled flask.

b. In the boiled but unclosed flask Bacteria will
also appear, but perhaps not quite so soon as
in a.

c. In the flask which has been boiled and kept
closed Bacteria will not appear, if the experi-
ment has been properly performed, even if it be
kept for many months.

XIII.
MOULDS (Penicillium, Eurotium and Mucor).

Torula, Protococcus and Amoeba are extremely simple con-
ditions of the two great kinds of living matter which are
known as Plants and Animals. No plants, except perhaps
the Bacteria, are simpler in structure than Torula and Pro-
tococcus, and the only animals which are simpler than Amoeba,
are essentially Amoeba devoid of a nucleus and contractile
vesicle. Moreover, however complicated in structure one
of the higher plants may be in its adult state, when it com-
mences its existence it is as simple as Torula or Protococcus;
and the whole plant is built up by the fissive multiplication
of the simple cell in which it takes its origin, and by the
subsequent growth and metamorphosis of the cells thus
produced. We have already seen that the like is true of all
the higher animals. They commence as nucleated cells,
essentially similar to Amoeba and colourless blood-corpuscles,
and their bodies are constructed by aggregations of meta-
morphosed cells, produced by division from the primary
cell. It has been seen that Torula and Protococcus, similar
as they are in structure, are distinguished by certain im-
portant physiological peculiarities j and the more compli-
cated plants are divisible into two series, one produced by
the growth and modification of cells which have the physio-
logical peculiarities of Torula and contain no chlorophyll,
while the other, and far larger, series contains chlorophyll,

416 ELEMENTARY BIOLOGY. [CHAP.

and has the physiological peculiarities of Protococcus. The
former series comprises the Fungi, the latter all other plants,
only a few parasitic forms among these being devoid of
chlorophyll.

The Fungi take their origin in spores t a kind of cells,
which, however much they may vary in the details of their
structure, are essentially similar to Tortda. Indirectly or
directly, the spore gives rise to a long tubular filament,
which is termed a hypha, and out of these hyphae the
Fungus is built up.

One of the commonest Moulds, the Penicillium glaucum^
which is familiar to every one from its forming sage-green
crusts upon bread, jam, old boots, &c. affords an excellent
and easily studied example of a Fungus. When examined
with a magnifying glass, the green appearance is seen to be
due, in great measure, to a very fine powder which is de-
tached from the surface of the mould by the slightest touch.
Beneath this lies a felt-work of delicate tubular filaments,
the hyphae, forming a crust like so much blotting-paper,
which is the mycelium* From the free surface of the crust
innumerable hyphae project into the air and bear the green
powder. These are the aerial hyphce. On the other hand,
the attached surface gives rise to a like multitude of longer
branched hyphae, which project into the fluid in which the
crust is growing, like so many roots, and may be called the
submerged hyphce. If the patch of Penicillium has but a
small extent relatively to the surface on which it lies, mul-
titudes of silvery hyphae will be seen radiating from its
periphery and giving off many submerged, but few or no
vertical, or subaerial branches. Submitted to microscopic
examination, a hypha is seen to be composed of a transpa-
rent wall (which has the same characters as the cell-wall of
Torula] and protoplasmic contents, which fill the tube

XIII.] MOULDS. 417

formed by the wall, and present large central clear spaces,
or vacuoles. At intervals, transverse partitions, continuous
with the walls of the tube, divide it into elongated cells,
each of which contains a correspondingly elongated proto-
plasmic sac, or primordial utricle. Each cell contains a
number of small nuclei, the presence of which can only be
detected by the use of staining reagents. Thus in Peni-
rillium we have our first example in the vegetable kingdom
of multinudeate cells. The hyphae frequently branch, and,
in the crust, they are inextricably entangled with one
another ; but every hypha, with its branches, is quite dis-
tinct from every other. Those aerial hyphae which are
nearest the periphery of the crust end in simple rounded
extremities; but the others terminate in brushes of short
branches, and each of these branches, as it grows and
elongates, becomes divided by transverse constrictions into
a series of rounded spores arranged like a row of beads.
The spores formed in this manner are termed conidia, and
the hyphae bearing them are termed conidiophores. At the
free end of each filament of the brush the conidia become
very loosely adherent, and constitute the green powdery
matter to which reference has been made. Examined
separately, a conidium is seen to be a spherical body, com-
posed of a transparent sac, enclosing a minute mass of pro-
toplasm and a nucleus, in all essential respects similar to a
Torula. If sown in an appropriate medium, as for example
Pasteur's solution, with or without sugar, the conidium
germinates. Upon from one to four points of its surface an
elevation or bulging of the cell-wall and of its contained
protoplasm appears. This rapidly increases in length, and,
continually growing at its free end, gives rise to a hypha, so
that the young Penidllium assumes the form of a star, each
ray being a hypha. The hyphae elongate, while side branches

M. 27

4T8 ELEMENTARY BIOLOGY. [CHAP.

are developed from them by outgrowths of their walls ; and
this process is repeated by the branches, until the hyphae
proceeding from a single conidium may cover a wide circular
area, as a patch of mycelium. When, as is usually the case,
many conidia germinate close together, their hyphae cross
one another, interlace, and give rise to a papyraceous crust.
After the hyphae have attained a certain length, the pro-
toplasm divides at intervals, and transverse septa are formed
between the masses thus divided off from one another. But
neither in this, nor in any other Fungus, are septa formed in
the direction of the length of the hypha.

Very early in the course of the development of the
mycelium, branches of the hyphae extend downwards into
the medium on which the mycelium grows ; while, as soon
as the patch has attained a certain size, the hyphae in its
centre give off vertical aerial branches, and the develop-
ment of these goes on, extending from the centre to the
periphery. The outgrowth of pencil-like bunches of branches
at the end of these takes place in the same order ; and
these branches, becoming transversely constricted as fast as
they are formed, break up into conidia, which are ready to
go through the same course of development.

The conidia may be kept for a very long time in the dry
state, without their readiness to germinate being in any way
impaired, and their extreme minuteness and levity enable
them to be dispersed and carried about by the slightest
currents of air. The persistence of their vitality is subject
to nearly the same conditions of temperature as that of
yeast. Pullulating cells, resembling Torulse, are not un-
frequently derived from the conidia of Penicillium and many
other of the lower Fungi, but they must not be confounded
with true Yeast.

In Penicillium we have for the first time a differentiation

XIII.] MOULDS. 419

of the body of the plant into distinct organs. On the one
hand we have the hyphae, constituting the vegetative part of
the organism, on the other the conidta, the function of
which is purely reproductive.

The hyphae again, are either submerged or aerial. The
former may be described from a purely physiological point
of view as the root, being characterized by the two func-
tions of attaching the plant to the substratum, and of taking
up food-material from it. The aerial hyphae, on the other
hand, in so far as they absorb oxygen from the air, and
bear the reproductive organs, are analogous to the shoot, or
leafy stem of the higher plants.

Besides the purely asexual organs of reproduction above
described, Penicillium also occasionally produces structures
which appear to represent sexual organs, though their func-
tional activity is in this case more than doubtful. A com-
plex truffle-like fruit is produced from them, but these
phenomena will be more easily studied in another mould,
the Eurotium Aspergtllus glaucus. Though less abundant
than Penicillium, this plant is of very common occurrence,
especially on such substances as preserved fruits. Apricot
jam, for example, may be used for growing the Eurotium
upon. In the vegetative structure and the asexual repro-
duction there is little difference from Penicillium. All the
hyphae are stouter, and the conidiophores are unbranched.
Each head of conidia can be distinguished separately with
the naked eye, which is not the case with the more minute
Penicillium. After the fungus has been growing for some
weeks the sexual fruits will begin to make their appear-
ance. They may be recognized as small round yellow
bodies, easily distinguished from the heads of conidia by
their colour and their sessile position. The development
of these fruits is somewhat complicated. A branch grows

27 2

420 ELEMENTARY BIOLOGY. [CHAP.

out from one of the hyphae, and becomes spirally coiled,
like a cork-screw. This branch is called the ascogonium. A
second branch (sometimes termed the pollinodiuni) is formed
in the immediate neighbourhood of the first. This applies
itself closely to the ascogonium, and it is alleged that fusion
takes place at the point of contact. Subsequently the as-
cogonium increases greatly in size, and produces a number
of clavate branches called asci, in the interior of each of
which eight spores are formed. In the meantime closely
interwoven hyphae, arising from below the ascogonium, form
the dense envelope of the fruit. The production of spores
in asci is characteristic of a very large group of Fungi, but
it is only in exceptional cases that the asci owe their origin
to an act of fertilization and the occurrence of such an act
is open to question even in Eurotium. The spores formed
in the asci are termed ascospores, and reproduce the ordinary
form of the Fungus.

If some bread be placed in a jar and kept very wet and
moderately warm, its surface will, in two or three days, be
covered with white cottony filaments, many of which rise
vertically into the air, and end in rounded heads, so that
they somewhat resemble long pins. The organism thus
produced is another of the Fungi — the mould termed Mucor
stolonifer.

Each rounded head is a sporangium ; the stalk on which
it is supported rises from one of the filaments which ramify
in the substance of the bread, and are the hyphcz. In this
species the hyphae send out branches (the so-called "sto-
lons") which grow out from the bread, and on reaching
any substratum, such as the plate on which the bread rests,
may produce a new crop of sporangia. Each hypha is,
as in Penicillium, a tube provided with a tough thickish
structureless wall, which is composed of a form of cellulose,

XIII.] MOULDS. 421

and is filled by a vacuolated protoplasm. In old speci-
mens, transverse partitions, continuous with the walls of
the hyphae, may divide them into chambers or cells, but
the plant is as a rule unicellular, or rather non-cellular,
as regards its vegetative organs. The hyphae contain very
numerous nuclei. The stalk of the sporangium is a hypha
of the same structure as the others. The wall of the
sporangium is beset with minute asperities composed of
oxalate of lime, and it contains a great number of minute
oval bodies, the spores, held together by a transparent in-
termediate substance. When the sporangium is ripe, the
slightest pressure causes its thin and brittle coat to give
way, and the spores are separated by the expansion of
the intermediate substance, which readily swells up and
finally dissolves, in water. The greater part of the wall
of the sporangium then disappears, but a little collar,
representing the remains of its basal part, frequently ad-
heres to the stalk. The cavity of the stalk does not com-
municate with that of the sporangium, but is separated
from it by a partition, which bulges into the cavity of
the sporangium, forming a central pillar or projection.
This is termed the columella and stands conspicuously
above the collar, when the sporangium has burst and the
spores are evacuated.

The spores are oval and consist of a sac, having nearly
the same composition as the wall of the hypha, which
encloses a mass of protoplasm and a nucleus. When they
are sown in an appropriate medium, as for example in
Pasteur's solution, they enlarge, become spheroidal, and
then send out several thick prolongations. Each of these
elongates, by constant growth at its free end, and becomes
a hypha, from which branches are given off, which grow
and ramify in the same way. As all the ramifying hyphae

422 ELEMENTARY BIOLOGY. [CHAP.

proceed from the spore as a centre, their development
gives rise, as in Penicillium, to a delicate stellate mycelium.
At first, no septa are developed in the hyphse, so that the
whole mycelium may be regarded as a single cell with
long and ramified processes, or as an organism which does
not show a cellular structure at all. The distinction be-
tween this condition and the multicellular structure of
Penicillium is not so sharp as it appears at first sight, for
it has been shown that even in plants which consist of
many cells the protoplasm is very generally continuous,
through minute perforations in their walls. From near
the centre of the mycelium a branch is given off from a
hypha, rises vertically, and after attaining a certain length
ceases to elongate. Its free end dilates into a rounded
head, which gradually increases in size, until it attains
the dimensions of a full-grown sporangium; and, at the
same time, the protoplasm contained in this head be-
comes separated from that in the stalk by a septum,
which is curved towards the cavity of the sporangium, and
constitutes the columella. The wall of the sporangium,
thus formed, becomes covered externally with a coat of
oxalate of lime spines. As the sporangium increases in
size, its protoplasmic contents become marked out into a
large number of small oval masses, which are close together,
but not in actual contact. Each of these masses next
becomes completely separate from the rest, surrounds itself
with a cellulose coat, and becomes a spore; while the
protoplasm not thus used up in the formation of spores,
appears to give rise to the gelatinous intermediate sub-
stance, which swells up in water, referred to above. The
walls of the spores become coloured, and that of the spo-
rangium gradually thins, until it is reduced to little more
than the outer crust of oxalate of lime. The sporangium

XIII.] MOULDS. 423

now readily bursts, and the spores are separated by the
swelling and eventual dissolution of the gelatinous inter-
mediate matter.

There appears to be no limit to the extent to which the
Mucor may be reproduced by this process of asexual de-
velopment of spores, by the fission of the contents* of the
sporangium ; nor does any other mode of multiplication
become apparent, if the mould be grown in a fluid medium
and abundantly supplied with nourishment.

But when growing in nature, a method of reproduction
is set up which represents the sexual process in its simplest
form, such as we have already seen in Spirogyra. Adjacent
hyphae, or parts of the same hypha, give off short branches,
which become dilated at their free ends, and approach one
another, until these ends are applied together. The proto-
plasm in each of the dilated ends becomes separated by a
septum from that of the rest of the branch ; the two cells
thus formed open into one another by their applied faces,
and their protoplasmic contents becoming mixed together,
form one spheroidal mass, to the shape of which the coa-
lesced cell-membranes adapt themselves. Here, as in Spi-
rogyra, the product of conjugation is termed a zygospore.
Its cellulose coat becomes separated into an outer layer
of a dark blackish hue and an inner colourless layer.
The outer coat is raised into irregular elevations, to
which corresponding elevations of the inner coat cor-
respond.

Placed in suitable circumstances, the zygospore does not
immediately germinate ; but, after a longer or shorter period
of rest, the walls burst, and a bud-like process is thrown
out, which grows out into a mycelium. Under favourable
conditions this may at once assume the normal develop-
ment, but if the supply of food be scanty, the mycelium

424 ELEMENTARY BIOLOGY. [CHAP.

remains rudimentary, and immediately gives rise to a
sporangium, in which the asexual conidia are produced.

When Mucor is allowed to grow freely on such a sub-
stratum as bread, or at the surface of a saccharine liquid, it
takes on no other form than that described; but, if it be
submefged in the same liquid, the mode of development of
the younger hyphee becomes changed. They break up, by
a process of constriction, into short lengths, which separate,
acquire rounded forms, and at the same time multiply by
budding after the manner of Torulcc. While these changes
are in progress, an active fermentation is excited in the
fluid, so that this u Mucor-Torula" functionally as well as
morphologically, bears a resemblance to the yeast-plant,
from which however its life-history shows it to be quite
distinct.

If the Mucor-Tornla is filtered off from the saccharine
solution, washed, and left to itself in moist air, the " Torulcz"
give off very short aerial hyphae, which terminate in minute
sporangia. In these a very small number of ordinary
Mucor spores is developed, but, in essential structure, both
the sporangia and the spores resemble those of normal
Mucor.

LABORATORY WORK.

A. PENICILLIUM.

Prepare some Pasteur's fluid, and leave it exposed to
the air in saucers in a warm place ; if Penicillium spores are
at hand add a few to the fluid in each saucer: if spores
cannot be obtained, the fluid, if simply left to itself, will
probably be covered with Penicillium in ten days or a

XIII.] MOULDS. 425

fortnight. Sometimes, however, the fluid will overrun with
Bacteria, to the exclusion of everything else. And very
frequently other moulds, such as Eurotium, or Mucor,
may appear instead of or along with Penicillium.

1. NAKED-EYE CHARACTERS. Note the powdery-looking
upper surface, white in young specimens, pale
greenish in older, and later still becoming dark sage-
green : the smooth pale under surface : the dense
tough character of the mycelium.

2. HlSTOLOGICAL STRUCTURE.

a. The mycelium.

a. Tease a bit out in water, and examine first with
low, and then with a high power : it is chiefly
made up of interlaced threads or tubes — the

a. Hyp/ice. Note their diameter (measure)—
form — subdivisions (cells)- — mode of branch-
ing— and structure : the external homogeneous
cell-wall; the granular less transparent proto-
plasm ; the small round vacuoles, the nuclei
(only to be seen on treatment with alcohol, and
staining with haematoxylin). Draw.

/5. The intermixed " Torulce" Note their size and
number.

b. Hold a bit of the mycelium between two pieces
of carrot, and cut a thin vertical section with a
sharp razor: mount in water and examine with
low and high power.

b. The submerged hyphse.

Small branched threads hanging down from the under
surface of the mycelium: repeat the observations
2. a. a. a.

426 ELEMENTARY BIOLOGY. [CHAP.

c. The aerial hyphse and conidiophores.

Tease out in water a bit from the surface of one of
the greenish patches; observe the difficulty with
which water wets it. Examine with low and high
power.
Note;—

a. The primary erect hypha.

/?. Its division into a number of branches.

7. The division of the terminal branches by con-
strictions into a chain of conidia. Draw.

d. The conidia.

a. Their Size (measure).

Form; spherical.

Structure; cell -wall, protoplasm, vacuole,

and nucleus.

b. Stain different specimens with magenta, hasma-
toxylin, and iodine, to bring out the above
points of structure.

c. Treat another specimen with potash. The cell-
contents gradually disappear.

e. The germination of the Conidia, and building up
of the Mycelium.

a. Sow some conidia in Pasteur's fluid in a watch glass;
protect from evaporation, and watch the development
of the mycelium (examine the surface with a low
power) ; then the formation of aerial hyphae ; finally
the production of new conidia.

b. Sow Conidia in Pasteur's fluid in a moist chamber, and
watch from day to day ; note the formation of eminences
at one or more points on a conidium ; the elongation of
these eminences to form hyphas; the branching and
interlacement of the hyphas.

XIIL] MOULDS. 427

B. EUROTIUM ASPERGILLUS-GLAUCUS.

1. Place some apricot jam under a bell-jar and keep
moist and warm. In a few days the Eurotium will
probably make its appearance. A crop of conidia
will first be produced. Neglect these, and look out,
after some weeks, for the sexual fruits, as described
in the text.

2. Mount some of the mycelium and fruits in water,
and observe first with a low and then with a high
power. Examine

a. The mature fruits. These are approximately
spherical in form. Observe the outside of the
fruit formed of dense cellular tissue. Burst some
of the fruits by pressure on the coverslip. Observe
the asci in the interior, each containing eight asco-
spores.

b. The development of the fruit. Make out the
spirally coiled branch forming the ascogonium,
and the"pollinodium"m contact with it. Observe
the outgrowth of hyphae from below the asco-
gonium, to form the envelope of the fruit

C. MUCOR STOLONIFER.

1. Place some bread under a bell-jar and keep very
moist and warm; in from 24 to 48 hours its surface
will nearly always be covered by a crop of erect
aerial mucor-hyphae, each ending in a minute en-
largement (sporangium} just visible with the unas-
sisted eye.

2. Snip off a few of the hyphae with a pair of scissors,
mount in water, and examine with i inch obj.

428 ELEMENTARY BIOLOGY. [CHAP.

a. Large unbranched hyphae, each ending in a
spherical enlargement (sporangium).

3. Examine with J obj.

a. The hyphae.

a. Their size; they greatly exceed the hyphae of
Penidllium both in length and diameter.

^. Their structure; homogeneous cell-wall, gran-
ular protoplasm, vacuoles; septa absent ex-
cept close to the sporangium.

y. Treat with iodine and magenta or haema-
toxylin; the protoplasm is stained and nuclei
may be made out.

8. Treat another specimen with Schulze's solu-
tion; the cell-wall is stained violet.

b. The sporangia.
Examine with \ obj,

a. Their size and form.

b. Their structure.

a. The homogeneous enveloping cell-wall, cover-
ed by irregular masses of calcic oxalate.

j3. The granular protoplasmic contents : un-
segmented in some ; divided into a great
number of distinct oval masses (spores) in
others.

y. The projection into the sporangial cavity of
the convex septum (columella) which separates
the hypha from the sporangium.

3. The collar projecting around the base of the
columella of burst sporangia.

XIII.] MOULDS. 429

c. Stain some with iodine; others with Schulze's
solution.

c. The spores or conidia.

a. Crush some ripe sporangia by gentle pressure
upon the cover-glass. Examine with -| obj.

a. The size of the spores (measure}.

ft. Their form ; cylindrical and elongated.

y. Their structure.

8. Stain with iodine and magenta or haema-
toxylin.

XIV.
STONE WORTS (Char a and Nitella}.

THESE water-weeds are not uncommonly found in ponds
and rivers, growing in tangled masses of a dull green colour.
Each plant is hardly thicker than a stout needle, but may
attain a length of three or four feet. One end of the stem
is fixed in the mud at the bottom, by slender thread-like
roots, the other floats at the surface. At intervals, append-
ages^ consisting of leaves, branches, root-filaments, and repro-
ductive organs, are disposed in circles, or whorls. In the
middle and lower parts of the plant these whorls are dis-
posed at considerable and nearly equal distances; but,
towards the free upper end, the intervals between the whorls
diminish, and the whorled appendages themselves become
shorter, until, at the very summit, they are all crowded
together into a terminal bud, which requires the aid of the
microscope for its analysis.

The parts of the stern, or axis, from which the append-
ages spring are termed nodes ; the intervening parts being
internodes. When viewed with a hand-magnifier the inter-
nodes exhibit a spiral striation.

In Chara, each internode consists of a single, much-
elongated cell, which extends throughout its whole length,
invested by a cortical layer, composed of many cells, the
spiral arrangement of which gives rise to the superficial
marking which has been noted. And this multicellular

CHAP. XIV.] STONEWORTS. 431

structure is continued from the cortical layer, across the
stem, at each node. The stem therefore consists of a series
of long, axial cells, contained in as many closed chambers
formed by the small cortical cells. The nodes are the mul-
ticellular partitions between these chambers. The branches
are altogether similar in structure to the main stem. The
leaves are also similar to the stem, so far as they consist of
axial and cortical cells, but they differ in the form and
proportions of these cells, as well as in the fact that the
summit, or free end, of the leaf is always a much-elongated
pointed cell. The most important distinction, however,
consists in the fact that the growth of the leaves is limited
while that of the axis is unlimited. The branches spring
from the acute angle between the stem and the leaf, which
is termed the axil of the leaf; and, in the same position,
at the fruiting season of the plant are found the repro-
ductive organs. These are of two kinds, the one large
and oval, the oogonia, the other smaller and globular, the
antheridia. Both, when ripe, have an orange-red colour,
and are seated upon a short stalk.

If a growing plant be watched, it will be found that it con-
stantly increases in length in two ways. New nodes, inter-
nodes, and whorls of appendages are constantly becoming
obvious at the base of the terminal bud; and these append-
ages increase in size and become more and more widely
separated, until they are as large and as far apart as in the
oldest parts of the plant. The appendages at first consist
exclusively of leaves and root-filaments (rhizotds), and it is
only when these have attained their full size, that branches,
oogonia and antheridia are developed in their axils.
Sometimes rounded cellular masses appear in the axils of
the leaves, and, becoming detached, grow into new plants.
These are comparable to the bulbs of higher plants.

432 ELEMENTARY BIOLOGY. [CHAP.

If the innermost part of the terminal bud, which con-
stitutes the free end of the axis, or stem, be examined, it
will be found to be formed by a single nucleated cell,
separated by a transverse septum from another. The former
of these is the apical cell, from the divisions of which all the
other cells are derived ; the latter is the last segment cut off
from it. Beneath this last follows another cell, which has
already undergone division into several smaller cells by the
development of longitudinal septa. This is the most newly-
formed node. Below this again is a single cell, which is
both longer and broader than those at the apex, and is an
internodal cell. Each nodal cell is the sister- cell of the
internodal cell next below it, both having arisen by the
transverse division of a single segment. Next follows an-
other node, composed of more numerous small cells than
the first. Some of the peripheral cells of this node are
undergoing growth and division, and thus give rise to
cellular prominences, which are rudiments of the first whorl
of leaves. In the still lower parts of the stem the inter-
nodal cells get longer and longer, but they never divide.
The nodal cells, on the other hand, multiply by division,
but do not greatly elongate. From the first, the nodal cells
overlap the internodal cell, so as to meet round its equator,
and thus completely invest it externally. And, as the in-
ternodal cell grows and elongates, the overlapping parts of
the nodes increase in length and become divided into
internodal and nodal cells, which take on a spiral arrange-
ment, and thus give rise to the cortical layer.

Thus the whole plant is composed of an aggregation of
simple cells, just as is the case with Hydra among animals ;
and, while it lives, new nodes and internodes are continually
being added at its summit, or growing point. The inter-
nodal cells which give rise to the middle of the stem un-
dergo no important change, except great increase of size,
after they are once formed. The nodal cells, on the con-

XIV.] STONEWORTS. 433

trary, undergo division with comparatively little increase in
size. And out of them, the nodes, the cortical layer, and
all the appendages, are developed.

In all the young cells of Chara a nucleus of relatively
large size is to be seen imbedded in the centre of the pro-
toplasm, and the latter is enclosed in a structureless cell-wall,
composed of cellulose. As the cell grows larger, the centre
of the protoplasm becomes occupied by a watery fluid, and
its thick periphery, which remains applied against the cell-
wall, constitutes a sac, or primordial utricle, in which the
nucleus is imbedded. In the larger cells the primordial
utricle is readily detached and made to shrivel up into the
middle of the cell by treatment with strong alcohol. In the
older cells the nucleus divides up into a number of portions.

Numerous small green bodies — chlorophyll grains — are
imbedded in the outer, or superficial, part of the primordial
utricle, and they increase in number by division, as the cell
enlarges. These chlorophyll grains are composed of proto-
plasmic matter, which frequently contains starch granules,
and is impregnated with the green colouring substance.

During life, the layer of the primordial utricle which lies
next to the watery contents of all the larger cells is in a state
of incessant rotatory motion, while the outermost layer
which contains the chlorophyll grains is quite still. In the
large cells, so long as the nucleus is discernible, it is carried
round with the rotating stream.

The antheridium is a globular spheroidal body with a
thick wall, made up of eight pieces, which are united by
interlocking edges. The four pieces which make up the
hemisphere to which the stalk of the antheridium is at-
tached, are foursided, the other four are triangular. From
the centre of the inner, concave face of each piece a
short process, the handle or manubrium, projects into the
cavity of the hollow sphere. At the free end of the manu-
M. 28

434 ELEMENTARY BIOLOGY. [CHAP.

brium is a rounded body, the capitulum^ which bears six
smaller secondary capitula ; and each secondary capitulum
gives attachment to four long filaments divided by trans-
verse partitions into a multitude (100 to 200) of small
chambers. Thus, there may be as many as 20,000 to
40,000 chambers in each antheridium (8 x 6 x 4 x 100 or
x 200). The several pieces of which the wall of the an-
theridium is composed, the manubria, the capitula, the
secondary capitula and the chambers of the filaments are
all more or less modified cells, as may be proved by tracing
the antheridia from their earliest condition, as small pro-
cesses of the nodal region, to their complete form. The
cells of the filaments are, at first, like any other cells ; but,
by degrees, the nucleus of each becomes changed into a
thread-like body, thicker at one end than at the other, and
coiled spirally like a corkscrew. From the thin end two
long cilia proceed ; and, when the cells have burst, and the
spermatozoids are set free, they are propelled rapidly, with
the small end forwards, by the vibration of the cilia. These
spermatozoids answer to the spermatozoa of animals, and
represent the male element of Chara.

The oogonia are borne upon short stalks, the end of
which supports a large oval central cell or ovum; five
spirally-disposed sets of cells invest this, an aperture being
left between the investing cells at the apex of the oogonium.
When the antheridia attain maturity they burst, the sper-
matozoids are set free, and swarm about in the water. Some
of them enter the aperture of the oogonium, and, in all
probability, pierce the free summit of the oval central cell,
and enter its protoplasm ; but all the steps of this process
of impregnation have not been worked out. The result,
however, is, that the contents of the impregnated central
cell now called the oospore become full of starchy and oily

XIV.] STONEWORTS. 435

matter ; the spiral cells forming its coat acquire a dark
colour and hard texture, and the oogonium, detaching itself,
falls into the mud.

After a time the oospore germinates ; a tubular process,
like a hypha, protrudes from it through the aperture between
the investing cells and almost immediately gives off a branch,
which is the first root. The tube elongates, and becomes
divided transversely into cells, the protoplasm of which
developes chlorophyll. Very soon, the further growth of
this pro-embryo is arrested. But one of the cells, which lies
at some distance below the free end of the pro-embryo,
undergoes budding, and gives rise to a set of leaves (which
are not arranged in a whorl), amidst which a bud appears,
which has the structure of the terminal bud of the adult
Chara stem, and grows up into a new C/fora-plant.

We have then, in Chara, a plant which is acrogenous (or
grows at its summit), and which becomes segmented by the
development of appendages, at intervals, along an axis;
which multiplies, asexually by bulb-like buds, and also
multiplies sexually by means of the spermatozoids (male
elements) and central cells of the oogonia (female elements) ;
in which the first product of the germination of the oospore
is a simple filament, from which the young Chara is de-
veloped by the germination and growth of one cell ; so that
there is a sort of alternation of generations, though the
alternating forms are not absolutely distinct from one
another.

Chara flourishes in pond-water under the influence of
sunlight, and by the aid of its chlorophyll CO2 is decom-
posed, so that its nutritive processes must be the same as
those of Protococcus. From its complete immersion, and the
absence of any duct-like, or vascular tissues, it is probable
that all parts absorb and assimilate the nutriment contained

28—2

436 ELEMENTARY BIOLOGY. [CHAP.

in the water; and that, except so far as the reproductive
organs are concerned, there is a morphological differentia-
tion of organs, unaccompanied by a corresponding phy-
siological differentiation.

Nitella is a rarer plant than C/iara, and is simpler in
structure, its axis being devoid of the cortical layer. In
other respects, however, it is very similar to Chara, and its
structure is more easily made out.

[The Characece, or plants belonging to the genera Chara and
Nitella, are found in all parts of the world, and belong to the
class of Algcz, which also includes the sea-weeds.]

LABORATORY WORK.

A. NAKED-EYE CHARACTERS.

Note the slender elongated axis (stem); the whorled
appendages (leaves); the nodes and internodes ; the shortening
of the latter towards the apex of the stem ; the rhizoids.

a. The roots ; small ; serving chiefly for attachment,
the plant getting most of its nutrition, through
other parts, from matters dissolved in the water.

b. The leaves ; their sub-divisions (leaflets] ; their
form, size, &c.

c. The oogonia and antheridia ; their position,
size, form, colour.

Draw a portion including two or three internodes.

B. HlSTOLOGlCAL STRUCTURE.

a. The stem.

i. Examine the outside of a fresh internode with a low
power, or pocket lens, to see the spirally-arranged
cortical cells.

XIV.] STONEWORTS. 437

2. Hold a bit of fresh stem between two pieces of elder-
pith or imbed it in paraffin, and, with a sharp razor,
cut thin transverse and longitudinal slices through
nodes and internodes. Note the cavity of the large
central internodal cell ; the cortical cells, round the
internodal cell ; the nodal cells, and the interruption
of the central cavity at the nodes.

3. Examine similar sections in specimens treated with
spirit, and also preparations made by teasing or press-
ing out in glycerine bits of stem from chromic acid
(0-2 per cent.) preparations: make out in these, —

a. The nodal, internodal, and cortical cells.

/3. The wall, protoplasmic layer (primordial
utricle), nucleus, and vacuole of each cell.
(The nucleus will have undergone fragmenta-
tion in the older cells.)

4. Examine sections from the fresh stem to make out
the points detailed in B. a. 3. p. The protoplasm
and nucleus are difficult to see. Note the chloro-
phyll-granules. (See B. b. y.)

5. Stain sections of the fresh stem with iodine, and with
magenta : note the results.

"b. The leaves.

Examine fresh and chromic acid specimens,
a. The large uncovered terminal cell.
ft. Then a series of internodal cells, separated
from one another, and covered-in, by nodal
cells : the cell-wall, protoplasm, nucleus, and
vacuole of each.

y. The oval chlorophyll-granules^ arranged so as
to leave an oblique uncoloured band round

438 ELEMENTARY BIOLOGY. [CHAP.

each cell; the position of these granules, in
the more superficial layer of the protoplasm.

8. The protoplasmic movements (see C. a.).

c. The terminal bud.

Dissect out chromic acid specimens as far as pos-
sible with needles, and then press gently out in
glycerine. Note in different specimens —

a. The terminal or apical cell:

a. Us form: hemispherical, the rounded surface
free; the flat surface attached to the cell
below it.

ft. Structure: cell- wall, protoplasm, nucleus; no
vacuole present.

y. Sometimes two nuclei, preliminary to division.

8. Its mode of division; across the long axis of
the stem, giving rise to two superimposed
nucleated cells.

b. The further fate of the new cells derived from
the segments of the terminal cell; work back in
your specimens from the terminal cell.

a. The new cells are successively nodal and inter-
nodal t one nodal and one internodal cell being
formed from each segment; the internodal
cells enlarge, develope a large vacuole, and ul-
timately form the medullary cells of the inter-
nodes ; they never divide.

ft. The nodal cells divide freely, and do not
increase much in size; they give origin to the
nodes and the cortical cells.

c. The development of leaves : by the multiplication
and outgrowth of nodal cells.

XIV.] STONEWORTS. 439

d. Their growth at the base, the terminal leaf-cell
soon attaining its full size and not dividing.

e. The development of branches ; from nodal cells
in leaf-axils, which take on the character of ter-
minal cells.

d. The oogonia.

Examine fresh, under a low power.

a. Made up externally of five twisted cells, bearing
at their apices five smaller, not twisted cells.

fi. Cut sections from embedded specimens, and
examine with a high power : make out the
large central nucleated cell or ovum; the fatty
and starchy matters contained in it ; stain
with iodine.

y. Press out chromic acid specimens in glycerine;
make out the above points (d. a, /3).

8. Examine chromic acid specimens for young
oogonia, and press them out in glycerine:
make out in the youngest the five roundish
cells surrounding a central one; then in older
specimens the elongation, and twisting of the
external cells, and the separation of their apices
as five distinct cells.

e. The antheridia.

a. Examine, with a low power, a ripe (orange-
coloured) one.

a. Make out its external dentated cells.

P. Tease out a ripe antheridium in water ; and
examine with a high power ; note the flat,
dentated, nucleated external cells ; the cylin-
drical cell (manubriwri) springing perpendicu-

44° ELEMENTARY BIOLOGY. [CHAP.

larly from the inner surface of each; the
roundish cell (capitulum) on the inner end of
the manubrium ; the six secondary capitula
attached to the capitulum ; the thread-like
filaments (usually four) proceeding from each
of the secondary capitula.

y. The structure of these threads ; each consists
of a single row of cells, containing in unripe
specimens nucleated protoplasm; in older
specimens each contains a coiled-up spermato-
zoid.

b. The spermatozoids (or antherozoids).

a. Their form and structure ; thickened at one
end and granular ; tapering off gradually to-
wards the other end, which is hyaline and
has two long cilia attached to it.

fi. The movements in water of ripe spermato-
zoids.

[Sometimes Chara cannot be obtained, when Ni-
tella, another genus of the same Natural Order, and
of similar habit and structure, can. Nearly all the
points above described for Chara can be made out
in Nitella, with the following differences : the cortical
cells of the stem and leaves are absent, and, in the
commoner species, the plant is not hardened by cal-
careous deposit ; the branches arise, not one from a
whorl of leaves, but two; and the five twisted cells of
the spore-fruit are each capped by two small cells,
instead of one.]

C. PROTOPLASMIC MOVEMENTS IN VEGETABLE CELLS.
a. Chara. Take a vigorous -looking fresh Chara or
Nitella cell (say the terminal cell of a leaf), and

XIV.] STONEWORTS. 441

examine it in water with a high power. Note
the superficial layer of protoplasm in which the
chlorophyll lies ; it is stationary : focus through
this layer and examine the deeper one; note
the currents in it, marked by the granules they
carry along: their direction; in the long axis
of the cell, up one side and down the other,
the boundary of the two currents being marked
by the colourless band, in which no movements
occur. Try to find the nucleus; it has usu-
ally undergone fragmentation in cells in which
currents have commenced. The original nu-
cleus or the products of its division are passively
carried along by the currents. Sometimes it
is very difficult, on account of the incrustation
of the leaf-cells of Chara, to make out the proto-
plasmic movements in them; if this is found to
be the case, the manubrial cells from an anthe-
ridium should be used instead.

b. Tradescantia. Examine in water, with a high
power, the hairs which grow upon the stamens :
they consist of a row of large roundish cells, each
with cell-wall, protoplasm, nucleus, and vacuolar
spaces. Note the protoplasm; partly forming
a layer (primordial utricle) lining the wall and
heaped up round the nucleus, and partly form-
ing bridles running across the cell in various
directions from the neighbourhood of the nu-
cleus, and from one part of the protoplasm to
another; observe the currents in these bridles;
from the nucleus in some, towards it in
others. Sometimes there are two currents in
opposite directions in the same strand.

442 ELEMENTARY BIOLOGY. [CHAP. XIV.

Observe also the various currents in the proto-
plasm of the primordial utricle.

c. Vallisneria. Take a leaf beginning to look old;
split it into two layers with a sharp knife and
mount a bit in water; examine with a high
power. Note the larger rectangular cells, be-
longing to the deeper layers, with well-marked
currents in them, which carry the chlorophyll
granules round and round inside the cell-wall.

If no currents are seen at first, gently warm
the leaf by immersing it for a short time in
water heated to a temperature between 30° and
35° C.

d. Anacharis*. Take a yellowish-looking leaf:
mount in water and examine with a high power;
the phenomena observed are like those in Val-
lisneria. They are best observed in the single
layer of cells at the margin of the leaf.

e. Nettle-hair. Mount an uninjured hair in water
with the bit of leaf to which it is attached
(it is essential that the terminal recurved part
of the large cell forming the hair be not broken
off); examine with the highest available power:
currents carrying along very fine granules will
be seen in the cell, their general direction being
that of its long axis.

* Also called EloJea.

XV.
THE BRACKEN FERN (Pleris aquilina).

THE conspicuous parts of this plant are the large green
leaves, or fronds, which rise above the ground, sometimes
to the height of five or six feet, and consist of a stem-like
axis or rachis^ from which transversely disposed offshoots
proceed, these ultimately subdividing into flattened leaflets,
\hepinnules. The rachis of each frond may be followed for
some distance into the ground. Its imbedded portion ac-
quires a brown colour, and eventually passes into an irre-
gularly branched body, also of a dark-brown colour, which
is commonly called the root of the fern, but is, in reality, a
creeping underground stem, or rhizome. From the surface
of this, numerous filamentous true roots are given off.
Traced in one direction from the attachment of the frond,
the rhizome exhibits the withered bases of fronds, developed
in former years, which have died down; while, in the
opposite direction, it ends, sooner or later, by a rounded
extremity beset with numerous fine hairs, which is the apex,
or growing extremity, of the stem. Between the free end
and the fully formed frond one or more processes, the rudi-
ments of fronds, which will attain their full development in
following years, are usually found.

The stem branches in two ways; ist by apparent dicho-
tomy of the terminal growing point, and 2nd by the forma-

444 ELEMENTARY BIOLOGY. [CHAP.

tion of adventitious buds. The latter are produced singly
on the dorsal side of the leaf-stalks, near the base.

The attachments of the fronds are nodes, the spaces
between two such successive attachments, internodes. It
will be observed that the internodes do not become crowded
towards the free end, and there is nothing comparable to
the terminal bud of Chara with its numerous rudimentary
appendages.

When the fronds have attained their full size, the edges
of the pinnules will be observed to be turned in towards
the underside, and to be bordered by a fringed membrane
called the indusium, which roofs over the groove enclosed
by the incurved edge. At the bottom of the groove brown
granules are aggregated in large numbers, so as to form a
streak along each side of the pinnule. The granules are the
sporangia, and the streaks formed by their aggregation, the
son.

Examined with a magnifying glass, each sporangium is
seen to be pouch-shaped, like two watch-glasses united by
a thick rim. When ripe, it has a brown colour, readily
bursts, and gives exit to a number of minute bodies which
are the spores.

The plant now described is made up of a multitude of
cells, having the same morphological value as those of
Chara, and each consisting, at least when young, of a pro-
toplasmic mass, a nucleus and a cellulose wall. These cells,
however, become very much modified in form and structure
in different regions of the body of the plant, and give rise
to groups of structures called tissues, in each of which the
cells have undergone special modifications. These tissues
are, to a certain extent, recognizable by the naked eye.
Thus, a transverse section of the rhizome shews a circum-
ferential zone of the same dark-brown colour as the external

XV.] THE BRACKEN FERN. 445

epidermis, enclosing a white ground-substance, interrupted by
variously disposed bands, patches, and dots, some of which
are of the same dark-brown hue as the external zone, while
others are of a pale yellowish-brown.

The dark-brown dots are scattered irregularly, but the
greater part of the dark-brown colour is gathered into two
narrow bands, which lie midway between the centre and
the circumference. Sometimes the ends of these bands are
united. Enclosed between these narrow, dark-brown bands
are, usually, two elongated, oval, yellowish-brown bands;
and, outside them, lie a number of similarly coloured
patches, one of which is usually considerably longer than
the others.

A longitudinal section shews that each of these patches
of colour answers to the transverse section of a band of
similar substance, which extends throughout the whole
length of the stem ; sometimes remaining distinct, some-
times giving off branches which run into adjacent bands,
and sometimes uniting altogether with them.

At a short distance below the apex of the stem, however,
the colour of all the bands fades away, and they are trace-
able into mere streaks, which finally disappear altogether in
the semi-transparent soft tissue which forms the growing
end of the stem. Submitted to microscopic examination,
the white ground-substance is seen to consist of large poly-
gonal cells, containing numerous starch granules ; this tissue
is called the ground-parenchyma. The circumferential zone
is formed of somewhat elongated cells, containing little or no
starch, the thick walls of which have acquired a dark-brown
colour. These cells constitute the epidermis and hypoderma.
The dark-brown bands, on the other hand, consist of cells
which are so much elongated as almost to deserve the name
of fibres. Their walls are very thick and of a deep-brown

446 ELEMENTARY BIOLOGY. [CHAP.

colour ; but the thickening has taken place unequally, so as
to leave short, obliquely directed, thin places, which look
like clefts and are called pits. This tissue is termed scleren-
chyma. The yellow bands, lastly, are vascular bundles.
Each bundle is surrounded on the outside by a layer of
rather thick-walled, elongated, parallel-sided cells, con-
stituting the bundle- sheath. The bundle itself consists of
two main parts ; a central portion, constituting the xylem or
wood, and a peripheral portion which is the phloem or bast.
Bundles of this structure, in which the xylem is surrounded
by the phloem, are termed concentric. The xylem consists
chiefly of vessels, many of them of relatively large size. They
are derived from cells, the transverse walls between which
have been partially broken down. In the mature vessels
the protoplasmic contents have entirely disappeared ; they
only contain water or air. Their walls are greatly thickened,
the thickening having taken place along equidistant trans-
verse lines, the thin spaces left between them being the
pits. The vessels have become flattened against one another,
by mutual pressure, so that they are five- or six-sided ; and,
as the markings of their flattened walls simulate the rounds
of a ladder, they have been termed scalariform ducts or
vessels. The cavities of these scalariform ducts are divided
at intervals, in correspondence with the lengths of the cells
of which they are made up, by oblique, often perforated,
partitions. Among the smaller vessels, a few will be found,
in which the thickening forms a closely wound spiral.
These are spiral vessels. They usually occur in two groups,
and as they are the first elements of the xylem to be differ-
entiated, they are said to constitute the protoxylem of the
bundle. Among the vessels a few parenchymatous cells
containing starch are scattered. The phloem is at once dis-
tinguished from the xylem by the smaller average si/e of its

XV.] THE BRACKEN FERN. 447

cells, and their thinner walls. The most conspicuous elements
of the phloem are the sieve-tubes, which consist of long cells,
the lateral walls of which show groups of minute perforations.
The outer layer of the phloem is formed of long narrow cells,
with comparatively thick walls. These cells, are developed
before the rest, and are hence termed the protophloem. Be-
tween the phloem and the bundle-sheath is a layer of paren-
chymatous cells containing starch, termed the phloem-
sheath.

The rachis of a frond, so far as it projects above the
surface of the ground, is of a bright green colour ; and, in
transverse section, it presents a green ground-substance,
interrupted by irregular paler markings, which are the trans-
verse sections of longitudinal bands of a similar colour.
There are no brown spots or bands. Examined micro-
scopically, the ground-substance is found to be composed
of polygonal cells containing chlorophyll. These are in-
vested superficially by an epidermis, composed of elongated
cells. The pale bands are vascular bundles, of similar
structure to those in the stem, with which they are con-
tinuous.

The vascular bundles, the green parenchyma, and the
epidermis are continued into each pinnule of the frond.
The epidermis retains its ordinary character on the upper
side of the pinnule, except that the contours of its com-
ponent cells become somewhat more irregular. On the
under side, many hairs are developed from it, and the
cells become singularly modified in form, their walls being
thrown out into lobes, which interlock with those of adja-
cent cells.

Between certain epidermal cells on the under surface an
oval space is left, forming a channel of communication
between the interior of the frond and the exterior. The

448 ELEMENTARY BIOLOGY. [CHAP.

opening of this space is surrounded by two kidney-shaped
cells, the concavities of which are turned towards one
another, while their ends are in contact. The opening left
between the applied concave faces is a stoma, and the two
cells are the guard-cells, and, as the stomata are present in
immense numbers, there is a free communication between the
outer air and the intercellular passages which exist in the sub-
stance of the frond. Those cells of the green parenchyma
of the frond which form the inferior half of its thickness, in
fact, are irregularly elongated, and frequently produced into
several processes, or stellate. They come into contact with
adjacent cells only by comparatively small parts of their sur-
faces, or by the ends of these processes. They thus bound
passages between the cells, intercellular passages, which are full
of air, and are in communication with similar, but narrower,
passages, which extend throughout the substance of the plant.

The vascular bundles break up in the pinnules, and
follow the course of the so-called veins, which are visible
upon its surface j ducts being continued into their ultimate
ramifications.

The growing point of the stem terminates in a single
apical cell, by the divisions and subdivisions of which all the
tissues of the stem and leaves are formed.

Each root presents an outer coat of epidermis, bearing a
number of unicellular root-hairs, and enclosing parenchy-
matous and sclerenchymatous tissues traversed by a central
vascular bundle. The latter contains the same elements as
are found in the bundles of the stem, but the xylem and
phloem have a different arrangement. The roots, like the
stem, develop by means of an apical cell at the growing
point, but this is not situated at the extreme end of the
organ, as the growing point of the rhizome is, but is covered
by a root-cap of protective cells.

XV.] THE BRACKEN FERN. 449

The nutrition of the Fern takes place essentially as in
the other chlorophyll-containing plants previously con-
sidered. The root-hairs constitute the organs for the ab-
sorption of water and dissolved salts. As regards the
vascular bundles, the xylem has been found to be the tissue
in which the water is conducted to the leaves, while the
phloem serves for the transference of assimilated food,
especially proteids, from the leaves to all parts of the plant.

Passing on to the reproductive organs, the sporangia have
first to be considered. The study of their development has
shown that each sporangium arises from the growth and
subdivision of a single epidermal cell of the leaf. The
sporangium consists of the pouch-shaped head above de-
scribed, which is borne on a short stalk. The interior of
the former part is occupied by the spores, usually 64 in
number, which are formed by the division into four of six-
teen mother-cells. The thick rim of the sporangium (called
the annulus) consists of a row of thick-walled cells ex-
tending from the stalk over the top of the sporangium. The
annulus contracts in dry weather and thus tears the spo-
rangium open, setting the spores free. Each of the latter is
a single cell, and the outer layer of its wall is strongly cuti-
cularized.

When the spores, are sown upon damp earth, or a tile, or
a slip of glass, and kept thoroughly moist and warm, they
germinate. Each spore bursts its outer wall and gives rise
to a tubular filamentous prolongation, the protoplasm of
which contains chlorophyll-grains. At the base of this a
similar but colourless process, the first root-hair, is de-
veloped. The green filament at first undergoes transverse
division, so that it becomes converted into a series of cells.
Then, the cells at its free end divide longitudinally, as well
as transversely, and thus give rise to a flat expansion, which
M. 29

450 ELEMENTARY BIOLOGY. [CHAP.

gradually assumes a bilobed form, and becomes thickened
in its middle part by division of its cells in a direction
parallel to its surface. The thickened portion is termed
the cushion. Numerous colourless, unicellular root-hairs
are given off from the under surface of the little plant,
which is called a prothallus or prothallium, and attach it to
the surface on which it grows.

The prothallus attains no higher development than this,
and does not directly grow into a fern such as that in which
the spores took their origin ; but, after a time, rounded or
ovoidal elevations are developed, by the outgrowth and
division of the cells which form its under surface. These
bodies are of two kinds, the antheridia and archegonia. In
the former, which may be developed on any part of the
under side of the prothallus, the nucleus of each of the
cells contained in their interior is converted into a sperma-
tozoid somewhat similar to that of Chara, but provided
with many more cilia. The antheridium bursts, and the
spermatozoids set free from their containing cells are pro-
pelled through the moisture on the under surface of the
prothallus by their cilia.

The processes of the second kind, the archegonia, acquire
a more cylindrical form. The outside of the organ is formed
of a single layer of cells, which are persistent, but those
which are situated in the axis of the archegonitim all dis-
appear with the exception of that which lies at the bottom
of its cavity. This is the ovum or oosphere, and when the
archegonium is fully formed, a canal leads from its summit
to this cell. The spermatozoids enter by this canal, and
impregnate the ovum.

The ovum now begins to divide, and becomes converted
into eight cells, which give rise to the stem, leaf, root and
foot, of the fern-plant, the foot being the organ by which

XV.] THE BRACKEN FERN. 451

the embryo is attached to the prothallus and derives food-
supplies from it.

As the rhizome grows, and developes its fronds, it rapidly
attains a size vastly superior to that of the prothallus, which
at length ceases to have any functional importance, and dis-
appears.

Thus Pteris presents a remarkable case of the alternation
of generations. The large and complicated organism com-
monly known as the ' Fern ' is the product of the impreg-
nation of the ovum by the spermatozoid. This 'Fern,'
when it attains its adult condition, developes sporangia;
and the inner cells of these sporangia give rise, by a perfectly
asexual fissive process, to the spores. The spores when set
free germinate; the product of that germination is the in-
conspicuous and simply cellular prothallus ; an independent
organism, which nourishes itself and grows, and on which,
eventually, the essential organs of the sexual process — the
archegonia and antheridia — are developed.

Each impregnated ovum produces only a single 'fern,'
but each 'fern' may give rise to innumerable prothalli,
seeing that every one of the numerous spores developed in
the immense multitude of sporangia to which the frond gives
rise, may germinate.

LABORATORY WORK.

A. THE FERN-PLANT ; ASEXUAL GENERATION.
a. External characters.

a. The brown underground stem or rhizome, with
a lighter band (the lateral line) running along
each side of it : its nodes and internodes.

29-2

452 ELEMENTARY BIOLOGY. [CHAP.

b. The roots springing from the rhizome.

c. The leaves or fronds arising from the rhizome at
intervals, along the lateral lines.

a. The great amount of subdivision of the frond :
its main axis (rachis)\ the primary divisions or
pinna ; the ultimate divisions or pinnules.

/?. The son; small brown patches along the
margin of the under surface of some of the
pinnules.

y. The indusium or membrane covering the
sorus.

d. The nodes and internodes of the rhizome. The
naked growing point at its extremity.

t. The rhizome.

1. Cut it across and draw the section as seen with the
naked eye.

a. The outer brownish layer (epidermis and hypo-
derma) ; the latter thins aways somewhat oppo-
site the lateral lines.

b. The yellowish-white substance (ground-substance
or parenchyma) forming most of the thickness of
the section.

c. The internal incomplete brown ring (sclerenchyma)
imbedded in the parenchyma.

d. The small patches of sclerenchyma scattered
about in the parenchyma outside the main
sclerenchymatous ring.

e. The yellowish tissue (vascular bundles} lying in-
side and outside the ring of sclerenchyma.

2. Cut a longitudinal section of the rhizome ; make out
on the cut surface b. i. a, b, c, d, e.

XV.] THE BRACKEN FERN. 453

3. Cut a thin transverse section of the rhizome, mount
in water and examine with i inch obj.

a. The single layer of much thickened epidermal
cells.

b. The small opaque angular contours of the
hypodermal cells (external sclerenchyma).

c. The large polyhedral more transparent paren-
chymatous cells.

d. The small opaque angular contours of the cells
of the internal sclerenchyma.

e. The great openings of the vessels in the fibro-
vascular bundles.

Draw the section.

4. Examine with \ obj.

a. The epidermis: its thick- walled cells.

b. The parenchyma : its large thin-walled cells: their
wall, protoplasm and nucleus : the great number
of starch granules in them.

6. The various patches of sclerenchyma^ made up of
thick-walled angular cells.

d. The vascular bundles. Note in each :—

a. Outside, a single layer of cells with brownish
walls, containing no starch granules (bundle
sheatJi).

(3. Within the bundle sheath a layer of small
parenchymatous cells containing starch (inner
Gf phloem sheath).

y. Within the last layer comes the bast of the
bundle (phloem) consisting of — externally, two
or more layers of small rectangular cells with

454 ELEMENTARY BIOLOGY. [CHAP.

thickened walls (protophloeni) and then a
single row of large thin-walled cells (sieve-
tubes or bast vessels] between which lie smaller
thin-walled cells containing starch granules
(bast parenchyma}.

8. The whole of the space within the phloem
is occupied by the xylem or wood. Note the
greatly thickened walls of its large vessels, and
their central cavity containing no protoplasm ;
the groups (usually two in number) of much
smaller vessels (protoocylem) \ scattered here
and there, in the spaces between the angles
of the vessels, note the small parenchymatous
cells (wood parenchyma) containing starch
granules.

e. Treat with iodine : observe the protoplasm
stained brown ; the starch granules deep blue,
rendering some of the cells quite opaque and
almost black-looking; the cell-walls of the
xylem light brown.

5. Cut a thin longitudinal section of the stem and
examine with i inch and then with \ obj. Make
out the various tissues described in 3 and 4.

a. The epidermis, hypoderma and parenchyma, much
as in the transverse section, except that the
hypodermal cells are longer.

b. The sclerenchyma is seen to be made up of greatly
elongated cells, tapering towards each end.

c. The vascular bundles ; note in them —

a. The cells of the bundle sheath and of the
phloem sheath much as in the transverse

XV.] THE BRACKEN FERN. 455

section but longer; the protophloem cells,
elongated, with thickened walls ; the cells of
the bast parenchyma somewhat elongated;
the relatively large sieve tubes, of great length
with oblique ends ; their lateral walls marked
by irregular groups of minute pores.

fi. The vessels of the xylem: elongated tubes
presenting oblique perforated partitions, at
long intervals. Two forms of vessels will be
seen, viz. scalariform vessels, with regular
transverse thickenings on their walls and
smaller spiral vessels (protoxylem), less nume-
rous than the last form, with a continuous
spiral thickening on their walls. Among the
vessels the square-ended cells of the xylem-
parenchyma will be recognised.

6. By macerating in Schulze's mixture (see Appendix E)
the various cells and vessels may be isolated one from
another, and their form better observed.

7. By making a series of transverse sections from the
growing end of a stem the gradual development of the
various forms of tissue from the originally uniform
parenchyma (meristem] of the growing point, may be
traced. If a section through the actual apex be obtained
it will be possible to make out the relatively large,
wedge-shaped apical cell.

c. The leaf.

i. Hold a small piece of a barren leaflet in pith or imbed
it in paraffin and cut a thin vertical section at right
angles to one of the veins.
Observe with low and high power

a. The epidermis of the upper and lower surface.

456 ELEMENTARY BIOLOGY. [CHAP.

b. The mesopkyll, the cells of which contain chloro-
phyll-grains in large numbers.

c. The transverse sections of the vascular bundles.

2. Slice off the epidermis from the under side of a leaflet.
Mount in water and observe with a high power

a. the ordinary epidermal cells of irregular form,

b. the hairs,

c. the stomata, each stoma with its two kidney-shaped
guard-cells.

d. The root.

1. Examine the tip of an uninjured root with a lens ; note
the relatively large root-cap covering the end of the
root.

2. Make a transverse section of a mature root held in
elder-pith, or imbedded in paraffin. Note, under a low
power,

a. the epidermis with its root-hairs,

b. the cortex consisting of parenchyma towards the
outside, and of very thick-walled sclerenchyma
nearer the centre,

c. the central vascular bundle.

e. The reproductive organs.

1. Examine a sorus with a low power without a cover-
glass. It is composed of a great number of minute
oval bodies, the sporangia, roofed in by the indusiuni.

2. Scrape off some sporangia and mount in water : ex-
amine with i inch obj.

a. Their form : they are oval biconvex bodies
borne on a short stalk.

b. Their structure: composed of brownish cells, one
row of which has very thick walls, and forms a

XV.] THE BRACKEN FERN. 457

marked ring (annulus) round the edge of the
sporangium.

c. Their mode of dehiscence (look out for one that
has opened) : by a cleft running transversely
across the sporangium.

3. Burst open some sporangia by pressing on the cover-
glass : examine, with J- obj., the spores which are set
free.

a. Their size : measure.

b. Their form: somewhat triangular.

\c. Their structure : a thick outer coat, a thin inner
coat, protoplasm, and a nucleus : crush some by
pressure on the cover-glass.]

B. THE PROTHALLUS ; SEXUAL GENERATION.

Prothalli may be obtained by sowing some spores on
peat and keeping them warm and very moist for about three
months. They are small deep green leaf-like bodies.

a. The Prothallus.

i. Transfer a prothallus to a slide, and mount it in water
with its under surface uppermost. Examine with i
inch obj.

a. Its form : a thin kidney-shaped expansion from
which, especially towards its convex border, a
number of slender filaments (root-hairs] arise.

b. Its structure.

a. The leafy expansion : it consists throughout
most of its extent of a single layer of polyhe-
dral chlorophyll-containing cells, but at a part
(the cushion) a little behind the depression
marking the growing point it is several cells
thick.

458 ELEMENTARY BIOLOGY. [CHAP.

ft. The root-hairs : each composed of a single
cell which contains no chlorophyll.

c. The antheridia and archegonia : the former can
just be seen with an inch objective as minute
eminences on the under surface of the older
parts of the prothallus especially among the
root-hairs; the latter are partly imbedded in
the cushion.

b. The reproductive organs.

These are to be found by examining the under surface of
the prothallus with \ obj.

1. The antheridia. Most numerous near and among the
root-hairs.

a. Their form : small hemispherical eminences.

b. Their structure: made up of an outer layer
of cells containing a few chlorophyll-granules,
through which can be seen, according to the
stage of development, either a single central cell,
or a number of smaller cells (mother- cells of
spermatozoids] resulting from its division : in the
latter cells, in ripe antheridia, spirally coiled
bodies (ipermatozoids] can be indistinctly seen.

2. The spermatozoids.

Some of these are sure to be found swimming about
in the water if a number of ripe prothalli are examined.

a. Small bodies, coiled like a corkscrew, thick at
one end, and tapering towards the other, which
has a number of cilia attached to it. To the
thicker end of the spermatozoid is often attached
a rounded mass containing colourless granules.

XV.] THE BRACKEN FERN. 459

b. Treat with iodine ; this stains them and stops
their movements, so that their form can be more
distinctly seen.

3. The archegonia. Make vertical sections of the pro-
thallus passing through the cushion ; this is best
done while holding it between two pieces of pith.
Note in the archegonia —

a. Their form: chimney-shaped eminences with a
small aperture at the apex.

b. Their structure. Each is composed of a layer
of transparent cells containing no chlorophyll,
arranged in four rows, and surrounding a central
cavity which extends into the cushion formed by
the thickened part of the prothallus (a. i. b. a).
In this cavity lies, in young specimens, a large
nucleated granular central cell, with two or three
smaller granular cells (neck canal-cells) above it
in the narrow upper part of the cavity ; in older
specimens this upper part is empty, forming a
canal leading down to the central cell. The
latter has now divided into two ; the upper
smaller cell resulting from its division forming
the ventral canal-cell, which becomes disorganized,
while the larger lower cell is the ovum or oosphere.

4. Examine a young fern-plant in connection with its
prothallus, to the lower side of which it is attached
by means of the _/#<?/.

XVI.
THE BEAN-PLANT (Vida Faba}.

IN this, which is selected as a convenient example of a
Flowering Plant, the same parts are to be distinguished as
in the Fern; but the axis is erect and consists of a root im-
bedded in the earth and a stem which rises into the air. The
appendages of the stem are leaves, developed from ..the op-
posite sides of successive nodes; and the internodes become
shorter and shorter towards the summit of the stem, which
ends in a terminal bud. Buds are also developed in the
axils of the leaves, and some of them grow into branches,
which repeat the characters of the stem; but others, when
the plant attains its full development, grow into stalks which
support the flowers ; each of which consists of a calyx, a
corolla, ten stamens and a central pistil; the latter is ter-
minated by a style, the free end of which is the stigma.

The flower-stalks are modified branches, and the flower
itself consists of several whorls of modified leaves.

The stamens form a tube which ends in ten filaments,
four of which are rather shorter than the rest ; and the fila-
ments bear oval bodies, the anthers, which, when ripe, give
exit to a fine powder, made up of minute pollen grains, each
of which is a single cell. The pistil is hollow; and, attached
by short stalks along the ventral side of it, or that turned
towards the axis, is a longitudinal series of minute bodies,
the ovules. Each ovule consists of a central conical nucellus,

CHAP. XVI.] THE BEAN PLANT. 461

invested by two coats, an outer and an inner. Opposite
the summit of the nucellus, these coats are perforated by
a canal, the micropyle, which leads down to the nucellus.
One of the cells of the nucellus is very much larger than
the rest ; this is called the embryo-sac. In its interior several
smaller cells are developed, the most important of which
is the ovum or oosphere, which lies at the end of the embryo-
sac towards the micropyle. When the pollen is deposited
on the stigma the grains germinate. Each sends out a long
filament, the pollen-tube, which elongates, passes down the
style, and eventually reaches the micropyle of an ovule.
Traversing the micropyle, the end of the pollen tube pene-
trates the nucellus, and comes into close contact with the
embryo sac. The original nucleus of the pollen-grain has
in the mean time divided into two, and one of the daughter
nuclei passes down the tube, reaches the embryo-sac, and
then fuses with the nucleus of the ovum. This is the
process of impregnation, and the result of it is that the
ovum divides and gives rise to a cellular embryo. This
becomes a minute Bean-plant, consisting of a radicle or
primary root ; of two, relatively large, primary leaves, the
cotyledons ; and of a short stem, the plumule, on which
rudimentary leaves soon appear. The cotyledons now in-
crease in size, out of all proportion to the rest of the em-
bryonic plant ; and the cells of which they are composed
become filled with starch and other nutritious matter. The
nucellus and coats of the ovule grow to accommodate the
enlarging embryo, but, at the same time, become merged
into an envelope which constitutes the coat of the seed.
The pistil enlarges and becomes the pod; this, when it has
attained its full size, dries and readily bursts along its edges,
or decays, setting the seeds free. Each seed, when placed
in proper conditions of warmth and moisture, then germinates.

462 THE BEAN PLANT. [CHAP.

The cotyledons of the contained embryo swell, and burst
the seed coat, but in this species remain underground, and
do not become green. The nutritious matters which they
contain are absorbed by the plumule and radicle, the latter
of which descends into the earth and becomes the root, while
the former ascends and becomes the stem of the young Bean-
plant. The apex of the stem retains, throughout life, the
simply cellular structure which is, at first, characteristic of the
whole embryo ; and the growth in length of the stern, so far
as it depends on the addition of new cells, takes place chiefly,
if not exclusively, in this part. The growing point does not
terminate in a single apical cell, as in the Fern, but con-
sists of a number of small, actively dividing cells, termed
collectively the apical meristem. The root likewise develops
its tissues from an apical meristem, but this, as in the Fern,
is protected by a root-cap.

The leaves cease to grow by cell multiplication at their
apices, when these are once formed, the addition of new
cells taking place at their bases. Each leaf is compound,
the common petiole bearing from four to six leaflets.

The tissues which compose the body of the Bean-plant
are similar, in their general characters, to those found in the
Fern, but they differ in the manner of their arrangement.
The surface is bounded by a layer of epidermal cells, among
which are stomata similar to those described in the Fern.
"Within the epidermis is a broad zone of tissue, termed
the cortex. The greater part of this zone consists of paren-
chymatous cells of the usual structure. At the four pro-
jecting corners of the stem however, the cortical tissue
has a somewhat different structure, consisting of cells which
have their walls much thickened at the points of junction.
Within the cortex comes a ring of vascular bundles, and
within this is the parenchymatous pith extending to the very

XVI.] THE BEAN PLANT. 463

centre in the younger parts of the stem, while, in the older
parts, the centre is occupied by a more or less consider-
able cavity, full of air. This cavity results from the central
parenchyma becoming torn asunder, after it has ceased to
grow, by the enlargement of the peripheral parts of the
stem.

The arrangement of the vascular bundles in the Bean is
not quite regular. Most of them, as already stated, form
a ring between cortex and pith, but besides these there are
two bundles, which are situated outside the ring. Each
of these occurs opposite one of the projecting corners of the
stem. Each vascular bundle consists of two halves; the
xylem or wood, which is turned towards the centre of the
stem, and the phloem or bast, which is turned towards its
periphery. Bundles with this arrangement of xylem and
phloem are termed collateral. The wedge-shaped bundles
of the ring are separated from one another by narrow bands
of parenchymatous tissue, which extend from the paren-
chyma within the circle of woody and vascular tissue
(medulla or pith) to that which lies outside it. These are
the medullary rays. In each of the bundles the xylem
and phloem are separated by a thin layer of small, and very
thin-walled cells, termed the cambium layer. In the older
parts of the stem this layer extends across the medullary
rays between the bundles so as to form a continuous ring
all round the stem. The tissues inside this layer are the
wood and pith, while those outside it are the bast, cortex,
and epidermis.

The great morphological distinction between the axis of
the Bean and that of the Fern lies in the presence of this
cambium layer. The cells composing it, in fact, retain
their power of multiplication, and divide by septa parallel
with the length of the stem, or root. Thus new cells are

464 THE BEAN PLANT. [CHAP.

continually being added, on the inner side of the cambium
layer, to the thickness of the wood, and on the outer side
of it, to the thickness of the bast \ and the axis of the plant
continually increases in diameter, so long as this process
goes on. This constant addition to the outer face of the
wood and the inner face of the bast is characteristic of the
Dicotyledons and Gymnosperms, to which two groups all our
forest trees belong. In the Bean this process of secondary
thickening only goes on to a comparatively small extent.

At the apex of the stem, and at that of the root, the
cambium layer is continuous with the cells of the apical
meristem which retain the capacity of dividing in these
localities. As the plant is thickest at the junction of the
stem and root, and diminishes thence to the free ends, or
apices, of these two structures, the layer of cambium and
meristem may be said to have the form of a double cone.
And it is the special peculiarity of the groups of plants
above-mentioned to possess this doubly conical layer of
constantly dividing cells, the upper end of which is free,
at the growing point of the terminal bud of the stem,
while its lower end is covered by the root-cap of the ultimate
termination of the principal root.

The most characteristic tissues of the wood are pitted and
spiral vessels, the spiral vessels being particularly abundant
close to the pith. They are the first elements of the xylem
to be formed. The outer part of the bast consists of elon-
gated bast fibres, while the inner, or soft bast, contains
sieve-tubes, the transverse walls of which are perforated.

Stomata are absent in the epidermis of the root : they
are to be found, here and there, in the epidermis of all the
green parts of the stem and its appendages, but, as in the
Fern, they are most abundant in the epidermis of the under
side of the leaves. As in the Fern, they communicate with

XVI.] THE BEAN PLANT. 465

intercellular passages, which are widest in the leaves, but
extend thence throughout the whole plant.

The blade of the leaf is traversed by the branched vascular
bundles, the xylem being turned towards the upper, and the
phloem towards the lower surface. The parenchyma of the
leaf, or mesophyll, is of two. kinds; towards the upper surface
the cells are closely packed, and elongated at right angles
to the surface, forming the palisade parenchyma. Towards
the lower surface the cells are of more irregular shape and
very loosely arranged, and are termed the spongy paren-
chyma. Both kinds of cells contain chlorophyll-grains, but
they are most abundant in the palisade cells.

The root has an epidermis, bearing unicellular root-hairs.
Within this is a wide cortex of parenchyma, while the centre
of the root is traversed by a single vascular bundle, of
radial structure, usually containing four groups of xylem,
and four of phloem, which alternate one with another. The
lateral roots arise endogenously, immediately outside the vas-
cular bundle, opposite the xylem groups. They thus have
to force their way through the whole of the cortex before
reaching the surface. There are in typical cases four rows
of lateral roots, corresponding to the four xylem-groups
opposite which they originate.

The difference between a flowering plant, such as the
Bean, and a flowerless plant, such as the Fern, at first sight
appears very striking, but it has been proved that the two
are but the extreme terms of one series of modifications.
The anther, for example, is strictly comparable to a leaf
bearing sporangia, the sacs in which the pollen is contained
answering to the sporangia themselves. The pollen grains
exactly resemble spores in their mode of development and
answer to the small spores of those flowerless plants in which
the spores are of two kinds — some spores giving rise to
M. 33

466 THE BEAN PLANT. [CHAP.

prothallia which develope only antheridia, and others to
prothallia which develope only archegonia ; instead of the
same prothallia producing the organs of both sexes, as in
Pteris. And the pollen tube may be compared to the first fila-
mentous process of the spore. But, in the flowering plants,
the protoplasm of the pollen tube does not undergo division
and conversion into a prothallus, from which antheridia are
developed, giving rise to detached fertilizing bodies or
spermatozoids, — but exerts its fertilizing influence without
any such previous differentiation, other than the division of
its nucleus. The connecting links between these two ex-
treme modifications are furnished, on the one hand, by the
Conifers, in which the protoplasm of the pollen tube be-
comes divided into cells, from which, however, no sperma-
tozoids are developed; and by Selaginella, in which the
protoplasm of the smaller spores ( = pollen grains) divides
into cells which form no prothallus, but give rise directly to
spermatozoids.

On the other hand the embryo-sac is the equivalent of
the large spore which gives rise to a prothallus bearing
female organs. The ovum of the flowering plant cor-
responds to the ovum contained in the archegonium of the
prothallus. There are other cells produced from the pro-
toplasm of the embryo-sac, which probably answer to the
cells of a prothallus. Here again the intermediate stages
are presented by the Conifers and Selaginella. For, in
the Conifers, the protoplasm of the embryo-sac gives rise
to a solid prothallus-like endosperm, in which bodies called
corpuscula, which answer to the archegonia, are formed,
and in each of these an ovum is produced; while, in Selagi-
nella the prothallus developed in the large spores does not
leave the cavity of the spore, but remains in it like an
endosperm.

xvi.] THE BEAN PLANT. 467

The physiological processes which go on in the higher
green plants, such as the Fern and the Bean, resemble,
in the gross, those which take place in Protococcus and
Cham. For such plants grow and flourish if their roots
are immersed in water containing a due proportion of
certain saline matters, while their stem and leaves are
exposed to the air, and receive the influence of the sun's
rays.

A Bean-plant, for instance, may be grown, if supplied
through its roots with a dilute watery solution of potassium
and calcium nitrate, potassium phosphate and iron sulphate,
and magnesium sulphate. While growing it absorbs the
solution, the greater part of the water of which evaporates
from the extensive surface of the plant. In sunshine, it
rapidly decomposes carbonic anhydride, fixing the carbon,
and setting free the oxygen ; at night, it slowly absorbs
oxygen, and gives off carbonic acid ; and it manufactures a
large quantity of protein compounds, cellulose, starch, sugar
and the like, from the raw materials supplied to it.

It is further clear that, as the decomposition of carbonic
anhydride can take place only under the combined in-
fluences of chlorophyll and sunlight, that operation must
be confined, in all ordinary plants, to the tissue imme-
diately beneath the epidermis in the stem, and to the
leaves. And it can be proved, experimentally, that fresh
green leaves possess this power to a remarkable extent.
The decomposition of carbonic anhydride and of water
appears to go on simultaneously, and as the result of the
process, various carbohydrates, such as grape-sugar, make
their appearance.

On the other hand, it is clear that, when a plant is grown
under the conditions described, the nitrogenous and mineral
constituents of its food can reach the leaves only by passing

30 2

468 THE BEAN PLANT. [CHAP.

from the roots, where they are absorbed, through the stem
to the leaves. And, at whatever parts of the plant the nitro-
genous and mineral constituents derived from the roots
are combined with the carbohydrates produced in the leaves,
the resulting compound must be diffused thence, in order to
reach the deep-seated cells, such for instance as those of
the cambium layer and those of the roots, which are
growing and multiplying, and yet have no power of ex-
tracting carbon directly from carbonic anhydride. In fact,
those cells which contain no chlorophyll, and are out of
the reach of light, must live after the fashion of Torula ;
and manufacture their protein out of the nitrates and salts
of ammonia taken up by the root, in combination with such
bodies as grape-sugar, already formed in the leaves. Thus,
the higher plant combines within itself the two, physiologi-
cally distinct, lower types of the Fungus and the Alga.

That some sort of circulation of fluids must take place
in the body of a plant, therefore, appears to be certain, but
the details of the process are by no means clear. There is
evidence to show that the ascent of fluid from the root to
the leaves takes place, to a great extent, through the vessels
of the wood, which in the higher plants have their transverse
walls broken down so as to form very fine capillary tubes
traversing both stem and root.

The mechanism by which this ascent is effected is of two
kinds ; there is a pull from above, and there is a push from
below. The pull from above is the evaporation which takes
place at the surface of the plant, and especially in the air-
passages of the leaves, where the thin-walled cells of the
parenchyma are surrounded, on almost all sides, with air,
which communicates directly with the atmosphere through
the stomata. The push from below is due in the first
instance to the absorptive action of the root hairs. The

XVI.] THE BEAN PLANT. 469

water they take up passes to the parenchymatous cells of
the cortex of the root, and is thence pressed into the vessels.
In a vine, for example, before its leaves have grown in the
spring, this process, called " root-pressure," causes a rapid
ascent of fluid (sap) absorbed from the soil. A certain
portion of the fluid thus pumped up from the roots to the
surface of the plant doubtless exudes, laterally, through the
walls of the vessels (the thin places which give rise to the
pits on the walls of these structures especially favouring this
process), and passing from cell to cell, eventually reaches
those which contain chlorophyll. The distribution of the
carbohydrates formed in the chlorophyll-bearing cells, pro-
bably takes place by slow diffusion from cell to cell. The
proteid compounds are in all probability conveyed through
the sieve-tubes of the vascular bundles.

There is no doubt that all the living protoplasm of the
plant undergoes slow oxidation, with evolution of carbonic
anhydride. In the green parts, and in daylight, this process
of respiration is disguised by the more conspicuous one of
assimilation, in which carbonic anhydride is decomposed
and oxygen given off. In the deeper seated cells, and in
all parts of the plant when light is absent, respiration alone
goes on. The supply of oxygen needful for this purpose
is sufficiently provided for, by the air-passages which are to
be found between the cells in all parenchymatous tissues.
The replacement of the oxygen of the air thus absorbed,
and the removal of the carbonic anhydride formed, will be
sufficiently provided for by gaseous diffusion.

From what has been said, it results that, in an ordinary
plant, growing in damp earth and exposed to the sunshine,
a current of fluid is setting from the root towards the surface
exposed to the air, where its watery part is for the most
part evaporated ; while gaseous diffusion takes place, in the

4/0 THE BEAN PLANT. [CHAP.

contrary direction, from the surface exposed to the air,
through the air-passages which extend from the stomata
to the radicles; the balance of exchange being in favour
of oxygen, in all the chlorophyll-bearing parts of the plant
which are reached by the sunlight, and in favour of carbonic
anhydride, in its colourless and hidden regions. At night,
the evaporation diminishing with the lowering of the tem-
perature, the ascent of liquid becomes very slow, or stops,
and the balance of exchange in the air-passages is entirely
in favour of carbonic anhydride ; even the chlorophyll-
bearing parts oxydizing, while no carbonic anhydride is
decomposed.

LABORATORY WORK.

a. General characters.

a. The erect central main axis (root and stem}.

b. The branches of the stem; some, mere repetitions
of the main axis; others, modified and bearing
flowers.

c. The nodes and internodcs.

d. The appendages.
a. Foliage leaves.
ft. Floral leaves.

b. The root.

a. Its main central portion (axis).

b. The rootlets attached to the axis in four rows.

c. The root-hairs, only found on the younger parts
of the root.

XVI.] THE BEAN PLANT. 4/1

d. The root-cap, covering the tip of each rootlet :
this is difficult to get whole out of the ground in
the bean, but is readily seen by examining the
roots of duckweed (Lemna) with i inch obj. In
the latter plant it consists of several layers of
cells forming a cap on the end of the root, and
ending abruptly with a prominent rim some way
up it. In the bean the root-cap can be well
seen by making a longitudinal section of the
radicle of the seed. See below, f. i. c.

e. Make a transverse section of the main root of
a seedling about an inch below its junction with
the stem. Note that the whole root is destitute
of chlorophyll. Observe with a low power

a. The epidermis.
j3. The wide cortex.

•y. The central vascular bundle. Note the four
xylem - groups alternating with the same
number of phloem groups.

8. In a section passing through the insertion of
a lateral root, observe that this arises endo-
genously and immediately outside one of the
groups of xylem.

c. The stem.

1. Erect, green, four-cornered, with a ridge at each
angle; not woody; the gradual shortening of the
internodes towards its apex.

2. Cut a thin transverse section of the stem through an
internode; note its central cavity, and the whitish ring
of fibro-vascular bundles in it, which is harder to

472 THE BEAN PLANT. [CHAP.

cut than the rest : mount in water and examine with
i inch obj. : note —

a. The medullary or pith-cavity in the centre of the
section.

b. The pith-cells, around the central cavity : large
and more or less rounded (parenchyma] \ some-
times with dotted walls from spots of local thin-
ness on them (pits).

c. The fibro-vascular bundles arranged in a ring im-
mediately outside the pith. Two of them how-
ever will be found separate from the ring in two
opposite corners of the stem. Commencing at
the side nearest the pith, note in each bundle —

a. The small openings formed by the transverse
sections of the spiral vessels (protoxylem).

ft. The larger cavities of the pitted vessels.

y. The small thick-walled wood-cells, wedged in
between the vessels. These three (a, ft and y)
form the wood or xylem of the bundle.

8. The cambium zone : granular-looking, and com-
posed of small angular thin-walled cells, ranged
in regular radial rows.

e. The bast or phloem. It presents internally
thin-walled cells of various sizes, the bast
parenchyma and bast vessels or sieve tubes.
Externally it appears in cross section to be
composed of rounded cells with thickened walls;
the bast fibres or sclerenchyma. Draw the section.

d. The cortex, consisting of several layers of large
rounded cells containing chlorophyll. Note that
at the four corners of the stem the walls of these

XVI.] THE BEAN PLANT. 473

cells are much thickened at the angles (collen-
chymatous).

e. The medullary rays: radiating rows of paren-
chymatous cells passing between the bundles and
uniting b and d ; not quite continuous, being in-
terrupted by the cambium zone (c. 8.).

f. The epidermis: composed of a single layer of
somewhat squarish looking cells, containing no
chlorophyll. Note the stomata, their two small
guard-cells being seen in section.

3. Cut a transverse section through a node, and com-
pare it with that through the internode. Observe
the bundles passing out from the stem into the leaf.

4. Cut a thin longitudinal section through part of an
internode (if necessary the bit of stem may be im-
bedded in paraffin first), and mount it in water;
working from the medullary cavity outwards, note
the following layers, using at first a low power : —

a. The pith-cells : much as in the transverse section.

b. The fibro-vascular bundles presenting —

a. The spiral vessels : elongated tubes with a spiral
thickening on their walls.

ft. The wood-cells: elongated and with much
thickened walls.

y. The pitted vessels: much like a, but with their
walls pitted instead of spirally thickened.

8. The cambium zone : made up of small, prismatic,
thin-walled cells, containing abundant proto-
plasm.

e. The bast parenchyma: thin-walled elongated
cells.

4/4 THE BEAN PLANT. [CHAP.

£. The bast vessels: larger elongated cells with

oblique perforated septa (sieve-tubes],
rj. The bast fibres, fusiform and thick-walled.

c. More parenchymatous cells, constituting the cortex.

d. Epidermis: composed apparently of cubical colour-
less cells : here and there the opening of a stoma
(d. 2. d. ft.) may be seen.

Draw the section.

5. Compare the transverse and longitudinal sections
together, making out the corresponding parts in each.

6. Put on a high power, and examine each of the
above-mentioned tissues carefully.

7. Stain with iodine : note the cell- walls; the protoplasm
— its presence or absence, and relative quantity in
the various tissues ; the nuclei of the cells ; starch
granules in some, stained deep blue by the iodine.

d. The leaves.

1. Their form and composition.

a. Each leaf consists of a number of different parts,

viz. : —

a. The stalk or petiole.
ft. The four to six oval leaflets attached laterally to

the stalk,
y. The pair of small leaf-like expansions (stipules]

at the base of the petiole.
8. The rudimentary tendril terminating the petiole.

2. The histo logical structure of a leaflet.

a. Imbed a leaflet in paraffin or hold it between two
bits of elder pith and cut a thin section from it,

XVI.] THE BEAN PLANT. 4/5

perpendicular to its surfaces. Let the section lie
in alcohol ' a few minutes to drive the air out of its
intercellular spaces, and then mount it in water,
and examine with i inch objective.
I). Begin at the upper surface (marked out by its
more closely packed cells), and work through to
the lower. Note —

a. The colourless epidermal layer — consisting of a
single row of cells ; the openings here and there
in it (stomatd).

p. Beneath the upper epidermis come elongated
chlorophyll-containing cells, set on perpendicu-
larly to the surface, forming the palisade paren-
chyma.

y. Then come irregularly branched cells forming
the lower half of the leaf-substance ; these also
contain chlorophyll. They constitute the spongy
parenchyma.

8. The epidermal layer of the lower surface ; like a.

c. The intercellular spaces^ through the whole thick-
ness of the leaf: the direct communication of
some of them with stomata.

£. Here and there sections of ribs or veins : make

out in them the same elements as in c. 2. c.
Draw.

c. Treat with iodine : make out the wall, protoplasm
(primordial utricle], nucleus and vacuole of the
cells : the chlorophyll grains, the starch granules.

d. Peel off a strip of epidermis from a leaf and ex-
amine with a low power : note —

1 This will extract the colouring matter from the chlorophyll grains.

476 THE BEAN PLANT. [CHAP.

a. The large close-fitting cells, with irregularly
wavy margins and no chlorophyll, which chiefly
make up the epidermis.

/?. The openings here and there in it (stomata) ;
the two curved, chlorophyll-containing guard-
cells bounding each stoma.

e. Gently pull a midrib in two across its long axis ;
note the fine threads uniting the two broken ends;
cut them off with a sharp pair of scissors, mount
in water and examine with \ or | objective : they
will be found to consist of partially unrolled spiral
vessels.

c. The flower.

1 . Its general structure.

a. Borne on a short stalk (peduncle).

b. Composed of four rows or whorls of organs.
a. The external green cup-like calyx.

(3. Inside the calyx the corolla: the most con-
spicuous part of the flower,
y. Inside the corolla the stamens.
8. Within the stamens the pistil.

2 . The calyx.

A cup terminated at its free edge by five prominent
points, two dorsal, and three ventral : the five small
midribs running along it (one to the end of each of
the points) represent the free ends of five sepals,
which are united below.

3. The corolla.

a. Composed of five pieces or petals.

a. On the dorsal side, a single large piece (vexil-

XVI.] THE BEAN PLANT. 477

luni) expanded at its free end and folded over
the rest.

/?. On the sides, two oval pieces (ala\ each
attached by a distinct narrowed stalk (unguis).

y. The inferior part of the corolla (carina), com-
posed of two oval pieces united along their
lower edge but readily tearing apart.

4. The stamens.

a. Ten in number, each consisting of a stalk-like
part, the filament, terminated by a small knob,
the anther.

b. The union of nine of the filaments for three-fourths
of their length to form the staminal-tube, the tenth
being free : the sharp bend of the filaments to-
wards the upper side at the point where they sepa-
rate from one another.

c. Tease out an anther in water and examine with
\ obj. : there will be found numerous —

a. Pollen-grains : small oval cells, with projections
on the cell-wall in the equatorial region.

d. The anther of a bean is so small that sections
cannot be made of it without considerable skill:
the structure of an anther can however be easily
made out by imbedding one from a tiger-lily in
paraffin or holding it between two bits of elder-
pith, cutting transverse sections, mounting in water
and examining with i inch. obj.

a. It contains four chambers, tJie pollen-sacs, two
on each side of the continuation of the fila-
ment, and in each chamber lie numerous pollen-
grains.

478 THE BEAN PLANT. [CHAP.

{3. By making careful transverse sections of young
flower buds the stages of development of the anther
and pollen may be made out. Observe especially
the origin of the pollen-grains by the successive
division of each of the numerous mother-cells, into
two and then into four. Each of the four daughter-
cells forms a wall of its own and becomes a
pollen-grain.

5. The pistil.

a. It is found by tearing open the stamen-tube : it
is a long green tapering body, somewhat flattened
laterally and ending in a point (the style) which
bears a tuft of strong hairs.

b. Slit it open carefully: its central cavity contains
a number of small oval bodies, the ovules, attached
along its ventral side by short pedicles.

f. It is difficult to get a section of a bean-ovule,
but its essential structure may be readily made
out by making thin transverse sections of the
ovary of a large lily (where the ovules are closely
surrounded by the tissue of the pistil) and examin-
ing with i inch obj.

a. The central cellular portion of the ovule (nucel-
lus) made up of a large number of cells.

/?. Its two coats, an inner (primine) and outer
(secundine).

y. The small passage (micropyle) leading through

the coats down to the nucellus.
8. In median sections of the ovule, a very large

cell (the embryo-sac) will be seen in the nucellus

just opposite the micropyle.

XVI.] THE BEAN PLANT. 479

€. The contents of the embryo-sac are best studied
in material which has been preserved, when quite
fresh, in absolute alcohol, and then transferred to
a mixture of alcohol and glycerine. The alcohol
is allowed to evaporate, and the sections are then
made, and mounted in glycerine. With a high
power the following structures may be made out
in the embryo- sac :

aa. At the micropylar end three cells forming collec-
tively the " egg- apparatus" The most deeply in-
serted of the three is the ovum itself, the other
two are the synergidae and undergo no further
development.

/?/?. About the middle of the sac the large nucleus. After
fertilization this divides, repeatedly, to form the
nuclei of the endosperm-cells.

77. At the end opposite the micropyle the three " anti-
podal cells."

f. The seeds.

i. Soak some dried beans in water for twenty-four
hours; they will slightly swell up and be more readily
examined than when dry.

a. Note the black patch (the hilum) on one end of
the bean, marking where the stalk (funiculus) which
fixed it in the pod was attached to it.

b. Having wiped all moisture off the bean gently
press it while observing that part of the black
patch which is next its broader end : close to
the patch a minute drop of fluid will be observed
to be pressed out through a small opening, the
micropyle.

c. Carefully peel off the outer coat (testa) of the seed :

480 THE BEAN PLANT. [CHAP.

the two large fleshy cotyledons of the embryo will
be laid bare. Joining the cotyledons together
will be found the rest of the embryo : it consists
of a conical part (the radicle) lying outside the
cotyledons, with its apex directed towards the
point where the micropyle was : and of the rudi-
ments of the stem and leaves (plumule) lying
between the cotyledons.

g. The process of fertilization.

This is difficult to follow in the bean; but by using
different plants for the observation of its various stages
it is fairly easy to observe all its more important steps.

1. A plant well adapted for seeing the penetration of
the pollen tube into the stigma and style is the
Evening Primrose (CEnothera biennis).

Detach the style from the flower and hold the
club-shaped stigma between the finger and thumb
of the left hand. Moisten it with a drop of water
and then make with a wetted razor several successive
cuts through it. This will divide the stigma into
several slices. Spread these out on a glass slide
with a needle in water and examine the thinnest,
after putting on a covering-glass.

The triangular grains of pollen will be seen send-
ing out from one angle a tube into the stigmatic
tissue, which is easily seen from its slight difference
in colour.

2. The entrance of the pollen-tube into the micropyle
can be readily made out in some species of Veronica.
The common V. serpyllifolia — often to be found
in shady places on lawns — is well adapted for the
purpose. A flower should be taken from which the

XVI.] THE BEAN PLANT. 481

corolla has just dropped. Dissect out the minute
ovary and, using the dissecting microscope, open
with a needle one of its two cells in a drop of water ;
remove the mass of ovules and gently tease them
apart. Then put on a covering-glass and examine
with a low power till an ovule is found which shews
the entry of the pollen-tube. The addition of dilute
glycerine will make the ovule more transparent, so
that after some time the embryo-sac can be seen,
and the progress of the pollen-tube into the ovule
followed.

3. In favourable median sections of the ovule of the lily
prepared as directed in e. 5. c. e., the pollen-tube may be
traced through the micropyle and between the cells
of the nucellus to the embryo-sac.

4. Among other plants favourable for the study of the
details of fertilization may be mentioned Caltha (marsh-
marigold), Helleborus, and Campanula (Canterbury
Bells). In each case thin transverse sections of the
fruit are to be made, and the material is best prepared
as above described.

M. 31

APPENDIX.

A. GENERAL.

The animals and plants employed in this work should be
obtained alive, and if possible by the student himself. Direc-
tions for killing and preserving them will be found under each
head. Living specimens should be kept ready at hand, in order
that their habits and movements may be studied side by side
with their structure.

Apparatus required.

i. Dissecting instruments, as under

Two or three scalpels of variable sizes, with straight

blades.
Two pairs of scissors with straight points (one large and

one small).
Two pairs of forceps (one large and one small) with

straight points, the inner faces of which shall be ground

or notched.

A seeker with a tapering point.
A German-silver blow-pipe.
Two razors (for section-cutting).
A section-lifter. This may most readily be made by

beating out the terminal jth of a piece of copper-wire,

4 inches in length.
3 or 4 camel's hair brushes.
6 needles mounted in long wooden handles.

APPENDIX. 483

2. Dissecting dishes.

A small pie-dish 6 or 8 inches in length, and an ordinary
salve-pot will meet all requirements. They should be
half filled with a mixture of paraffin and lamp-black, put
in hot, and weighted down with a piece of sheet lead.

3. Injecting apparatus.

Two or three ^-oz. medicine-droppers, with their points
drawn out to varying degrees of fineness.

A 1-oz. syringe, preferably of metal, for histological
work.

4. A compound microscope, preferably with a short body ;
fitted with low power (i inch) and high power (^th inch)
objectives. A shallow eye-piece of low magnifying power
will suffice, but a higher one may conveniently be to
hand.

5. A hand-lens : preferably of the pocket or watchmaker's
type.

A simple dissecting microscope may conveniently be
added.

6. Sundries.

Drawing material. Unlined paper; pencils H and
HB, and a box of moist-colours. Paints may be em-
ployed in preference to chalks, and it is advisable to
use corresponding colours in representing corresponding
organs of different organisms, or corresponding parts of
organs themselves.

In drawing, accurate representation in outline should
be aimed at, and it will generally suffice to colour in light
flat washes.

Microscopic slides and cover-slips.

Of the former, ^ a gross, with rough or ground edges at

will.

Of the latter, 2 oz., preferably £ square thin.

Glass dipping-tubes of various calibres and lengths ;

two or three to be drawn out to a point

31—2

484 APPENDIX.

Three or four small thin glass beakers.

Half a dozen watch-glasses.

Three or four glass salve-pots with lids.

B. ON DISSECTING.

Unless otherwise directed, all the dissections embraced by
this work should be performed under water.

The undermentioned precautions are indispensable to success.

1. The animal or organ under dissection should be firmly
pinned down, the pins being thrust through those more
solid parts of the same, furthest removed from the point
of operation.

2. Displacement of parts of organs, prior to removal or
otherwise, should be effected by means of forceps, the
larger ones being employed wherever possible (in order
that the smaller ones may be the more fit for delicate
work) ; the direct use of the fingers should be dis-
couraged.

3. Never dissect under dirty water. Should the water be-
come clouded, as it may do from numerous causes,
change it at once and, in doing so, wash the dissection
clean under a gentle stream.

4. In dissecting a given system remove nothing unneces-
sarily. Dissect away only such parts as may interfere
with the immediate purpose of inquiry, and do this only
after full deliberation.

5. Dissecting instruments should always be wiped perfectly
clean and dry after use. The hinges should be occa-
sionally oiled by means of a camel's hair-brush ; and if
put away for a lengthy period after use, all parts subject
to immersion in water should be similarly treated.

APPENDIX. 485

C. ON INJECTING.

Precautions, indispensable to success under all circum-
stances.

a. Injection should always be performed as soon after
death as possible; and, in the case of the Frog,
it is advisable to first remove the apex of the ven-
tricle, in order to allow of the escape of as much
blood as possible.

b. In filling the syringe (or its equivalent) with injecting
material care should be taken to first expel the con-
tained air : when fully charged it should be held
nozzle uppermost, while, by displacement of the
piston, all trace of air is dispelled.

c. In opening a blood-vessel prior to injection the
exposed wall should be slit longitudinally with small
scissors. Escape of as much blood as possible
should be permitted. Before inserting the cannula
or its equivalent, the whole should be well washed
and the incision examined to make sure of the ab-
sence of a blood-clot or other obstructive agent.

To this end the introduction of the cannula may
be advantageously preceded by that of the seeker.

d. If the cannula is to be tied in place (as must always
be the case when injecting for histological purposes)
the thread (preferably one of surgical silk) should be
passed round the vessel and loosely knotted, prior to
making the incision in its wall. The incision should
be made a short distance from the thread, in a
direction away from that in which it is intended to
inject.

Under such circumstances the knotted thread will
be found to serve as a landmark, which experience
will show to be occasionally necessitated, in the
course of the work.

486 APPENDIX.

e. Upon removal of the injecting apparatus the cut end
of the blood-vessel or other organ through which it
was inserted should be immediately ligatured or
clamped, in order to guard against reflux.

2. Coarse injection, for anatomical purposes.
Complete satisfaction may be obtained by using a mix-
ture of French blue and water in the proportion of a tea-
spoonful to half a tumbler. The mixture should be well
stirred immediately before use, and it may be introduced
under water or otherwise as occasion demands. Most
satisfactory results are to be obtained with this mixture
by injecting piecemeal from such of the larger vessels as
may be desirable.

For permanent anatomical preparations plaster of Paris
may be preferably employed. Mix with two-thirds its
bulk in water and colour with French blue or vermilion ;
stir thoroughly and strain through two thicknesses of
fine muslin. The mixture thus prepared will remain for
8 — 10 minutes sufficiently fluid for all practical purposes.

3. Fine injection, for histological purposes.

Allow a given quantity of gelatine to stand for 3 — 4 hours
in twice its bulk of cold water; heat slowly until quite
fluid and colour to taste with Berlin blue or carmine.
Inject when lukewarm, the animal being immersed in
water at the same temperature.

In the above process the following precautions should
be taken.

a. The point of the cannula should bear a slight con-
tusion near its tip, round which the thread may bite
when ligatured.

b. The cannula should be as short as possible, and
there should be attached to its base an inch or so of
india-rubber tubing to receive the nozzle of the
syringe.

c. The cannula should not be inserted until all bleed-
ing has ceased.

APPENDIX. 487

d. While injecting, a steady pressure should be main-
tained. Should resistance to this be offered, the
operation must be interrupted until the cause of
obstruction shall have been ascertained and, if pos-
sible, removed.

Obstruction generally arises from one or more of
the undermentioned causes.

i. The presence of clotted blood in the smaller
vessels. This source of difficulty is usually
fatal to success.

ii. Resistance offered in the capillary systems,
often resulting from the forcing back of the
blood upon the great vessels. This may ge-
nerally be overcome by puncturing a large
vessel as far removed from the point of opera-
tion as possible (i. e. if injecting the arteries
puncture one of the larger veins, or vice versa).

iii. A too rapid cooling of the gelatine within the
smaller vessels and capillary systems. To over-
come this, increase the temperature of the water
in which the animal is immersed.

e. Complete injection of a given capillary system ul-
timately results in visible distension of the organ
concerned ; should enlargement such as this become
suddenly obvious the operation must be terminated,
otherwise rupture and extravasation will ensue.

f. On withdrawal of the syringe the end of the india-
rubber tube attached to the cannula should be either
plugged with a glass-rod or other convenient stopper,
or clamped ; and the whole should be placed in cold
water until the gelatine shall have set.

g. Animals or tissues thus injected should never be
placed at once into strong alcohol, but into spirit
of increasing strength, from 50 per cent, upwards as
directed at Appendix E.

488 APPENDIX.

D. MICROSCOPE AND MICROSCOPIC
EXAMINATION.

1. In using the microscope the first thing to be considered
is the illumination. A position facing a window should
be selected, and if the window be a north one there will
be an advantage in the absence of direct sunlight, which
should never be employed for microscopic work. The
light from a south window is however equally good, if a
white blind be used to exclude bright sunshine.

The mirror must be so placed that the field of the
microscope appears quite bright. The admission of
light from the mirror to the object is regulated by the
diaphragms. The size of the opening to be used should
depend on the magnifying power. With a low power a
large opening is necessary, or the whole field will not be
illuminated ; with a high power a smaller opening gives
a better definition, though it diminishes the intensity of
the illumination. Most modern microscopes are pro-
vided with a double mirror : the flat one should be used
low down in illuminating with a low power ; with a high
power on the other hand, the concave one should be
used obliquely and high up.

2. The microscope has two adjustments of focus. The
coarse adjustment is effected by sliding the body of the
microscope up and down in its tube, or in the larger in-
struments by a rack and pinion movement. The fine
adjustment is worked by a screw. In all observations
the low power should be used first, and then the high, if
necessary. The focal distance of the low power is of
course relatively long, usually either an inch or half an
inch. Hence there is no difficulty in focussing and no
danger of crushing the object. With the high power,
which may have a focal distance of £ in., J in., or
less, great care is necessary, lest the objective should
touch the cover-slip. It is best to slide the body of the

APPENDIX. 489

microscope very carefully down, until the object is just
visible, and then to focus accurately with the fine ad-
justment.

3. Never observe with a high power until the object is
covered with a cover-slip. If the object appears in-
distinct, this may be due to dirt or condensed vapour,
on the cover-slip, the objective, or the eye-piece. If the
first is in fault the distinctness will vary as the slide is
moved. The presence of any foreign particles on the
eye-piece can be easily detected by turning it round. A
general indistinctness, which is not affected either by
moving the slide, or by rotating the eye-piece, must be
due to the objective having become dirty, or injured.

4. Should the objective be dirty, it must be cleaned with a
soft linen rag, or with a piece of wash-leather, never
with a coarse cloth. The greatest care must be taken
that the objective does not become dirty, for even the
most careful cleaning is likely to injure the lens. In
order to keep the objective clean, it is essential that no
more fluid should be used in mounting, whether tem-
porarily or permanently, than will exactly fill the space
under the cover-slip.

5. During observation the focussing should constantly be
slightly varied, by means of the fine adjustment, as this
greatly aids in getting a clear idea of the object.

6. It is best to cultivate a habit of using the microscope in
the vertical position, so that the stage is horizontal. An
inclined position of the instrument is only admissible in
examining permanent preparations.

7. The body of the microscope should always fit accurately,
but not tightly, in its tube. It should be oiled if it does
not work up and down quite smoothly.

8. A micrometer of some kind is indispensable, and the
purpose to be aimed at in its use is the knowledge of the
size of objects under examination. An eye-piece mi-

490 APPENDIX.

crometer ruled into squares answers most purposes and
the squares and objects delineated should be drawn to
the proportions observed. Having thus a record of ob-
jects, drawn in proportion relative to the said squares,
it remains but to ascertain the value of the latter. This
is best done by using, in conjunction with the eye-piece
micrometer above named, a stage micrometer ruled to
intervals of known value ; there will thus be superposed
lines of absolute and relative measurement. The actual
value of the squares of the eye-piece micrometer may by
this means be once for all calculated, and a record of the
same should be kept for each lens combination.

E. PREPARATION AND USE OF REAGENTS
AND CULTURE SOLUTIONS.

The reagents employed in microscope work are best put up
for use in 5 oz. bottles. Those marked thus * should be kept in
glass bottles with ground necks and stoppers, their contents
being removed by means of clean capillary tubes. The re-
ceptacles of the remaining ones should be corked, each cork to
carry a thin glass rod long enough to reach near the bottom of
the bottle.

1. Acetic acid, Dilute.

Mix i cub. centimetre of glacial acetic acid with 99 cub.
cent, of distilled water.

2. Alcohol.

Methylated spirit should be kept ready to hand in stock
bottles, diluted with water to various strengths, viz. —
50 per cent., 75 per cent., 90 per cent. Upon placing
any specimen, organ, or tissue in the same, at least 3 — 4
times its bulk in fluid should be employed. Immersion
in the weaker solutions should not exceed 6 — 8 hours in
the case of whole organs, or 2 — 3 hours in those of
tissues in course of hardening for histological work.

N
APPENDIX. 49

The stronger solutions employed in the final stages of
hardening or preservation should be replaced at intervals.
In the case of preparations which have been transferred
from acid solutions, the spirit must be repeatedly changed,
until all excess of acid is removed.

Preparations of nervous tissues or sense-organs, if
treated with alcohol, should be put at once, when quite
fresh, into strong spirit.

3. Ammonic bichromate, Solution of.

Dissolve 10 grammes of crystallized ammonic bichro-
mate in a litre of distilled water.

4. Canada balsam*.

A chloroform or turpentine solution of fairly fluid con-
sistency should be employed. It must be kept in a well-
stopped bottle with a wide neck.

5. Carmine, Solution of.

Carmine 2 grammes.

Strong solution of ammonia 4 cub. cent

Distilled water 48 cub. cent.

Dissolve the carmine in the ammonia and water ; leave
in an unstoppered bottle until nearly all smell of ammo-
nia has gone. Afterwards keep in a well- closed bottle.
Dilute a small quantity with fifteen or twenty times its
bulk of water, when required for use.

6. Carmine, Borax, Solution of.

Carmine i gramme.

Borax 4 grammes.

Distilled water 56 cub. cent.

To this solution add twice its volume of absolute alcohol.
Filter.

7. Chromic acid, Solution of.

Dissolve 10 grammes of crystals of chromic acid in one
litre of water. This gives a i per cent, solution, from

492 APPENDIX.

which weaker ones can readily be prepared when re-
quired.

Preparations should be placed in -i per cent, solution
for the first 24 hours, that being ultimately replaced for a
similar period by one of '5 per cent, which should be
changed if necessary. When well hardened they should
be transferred to 75 per cent, alcohol.

When used for purposes of decalcifkation the maxi-
mum quantity should be employed, and to it a few drops
of nitric acid may be added.

8. Corrosive sublimate, Solution of.

To a saturated aqueous solution add a few drops of
acetic acid.

A relatively large quantity of the above must be em-
, ployed, and the preparation, after at most an hour's
immersion therein, should be well washed under running
water before being transferred to alcohol.

9. Eosin, Solution of.

Aqueous and alcoholic solutions should be prepared ; —
the former for use with fresh material, the latter with
that which has been previously hardened.

10. Glycerine.

By 'weak glycerine,' referred to in the text, is meant a
solution composed of equal parts in bulk of glycerine
and distilled water.

1 1. Gold chloride, Solution of*

A i per cent, solution is customarily employed.

Tissues submitted to the action of this reagent should
be kept free of contact with metal.

1 2. Hsematoxylin, Solution of.

a. Prepare a saturated solution of crystallized calcic
chloride in 70 per cent, alcohol ; then add alum to
saturation.

APPENDIX. 493

b. Prepare a saturated solution of alum in 70 per cent,
alcohol. Add I volume of a to 8 of b.

c. To the mixture of a and b add a few drops of a
saturated solution of pure hcematoxylin in absolute
alcohol.

d. Filter.

This reagent stains with great intensity. Overstain-
ed preparations may be clarified by immersion for
a longer or shorter period in acidulated alcohol
(70 per cent, solution plus '25 per cent, solution of
nitric acid).

13. Hsematoxylin-ammonia, Solution of.

Expose some crystals of Haematoxylin under a bell-glass
to the action of the Ammonia gas given off from a strong
solution. The crystals can then be dissolved in distilled
water. This reagent must be prepared fresh when re-
quired.

14. Iodine, Solution of.

Prepare a saturated solution of potassic iodide in dis-
tilled water; saturate this solution with iodine. Filter.
Dilute to a brown sherry colour.

It sometimes happens that the iodine and sulphuric
acid tests for starch and cellulose fail, when dealing with
fresh material. Under such circumstances a check ex-
periment should be performed, as follows, viz. : — preserve
in alcohol, transfer to weak solution of caustic potash
for 6 — 8 hours, neutralize with dilute acetic acid and
finally stain, as in ordinary, with iodine and strong sul-
phuric acid.

The presence of starch or cellulose should never be
denied before both the above-named tests have failed.

1 5. Magenta, Solution of.

Dissolve i decigr. of crystallized magenta (roseine) in
160 cubic centimetres of distilled water: add i cub. cent,
of absolute alcohol. Keep in a well-closed bottle.

494 APPENDIX.

16. Mayer's Solution.

1 5 per cent, solution of sugar-candy 20 cc.

Dihydropotassic phosphate o- i grm.

Calcic phosphate o- i grm.

Magnesic sulphate o* i grm.

Pepsin 0-23 grm.

17. Muller's Fluid.

Bichromate of potash 25 grammes.

Sodic sulphate 10 grammes.

Distilled water i litre.

1 8. Osmic Acid, Solution of.

Best bought ready made in the form of i per cent, solu-
tion.

The crystals are supplied in small tubes each containing
i gramme. Such a tube should be broken up in 100 c. c.
of distilled water, great care being taken to avoid contact
with the resulting vapour.

This reagent should be kept in a black stoppered
bottle, perfectly in the dark.

19. Paraffin.

See Appendix F.

20. Pasteur's Solution.

Potassium Phosph 20 parts.

Calcium Phosph 2 „

Magnesium Sulphate 2 „

Ammonium Tartrate 100 „

[Cane Sugar 1500 „ ]

Water 8376 „

10,000 parts.

The sugar is to be omitted when Pasteur's fluid "without
sugar" is ordered. Pasteur himself used actual yeast
ash ; the above constituents give an imitation ash, which,
with the ammonium salt and sugar, answers all practical
purposes.

APPENDIX. 495

21. Picric Acid, Solution of.

Make a saturated solution in distilled water.

22. Picric Acid, Kleinenberg's solution of.

To a cold saturated solution of the acid add 2 parts of
nitric or concentrated sulphuric acid.

Filter, and add to the filtrate three times its bulk of
water.

Immersion of from 3 — 5 hours will suffice for most
preparations.

23. Potash Solution.*

Dissolve 5 grammes of potassic hydrate in 100 cubic
cent, of water.

24. Sachs's Food-solution for green plants.

Distilled water 1000 cub. cent.

Potassium nitrate I gramme.

Sodium chloride 0*5 gramme.

Calcium sulphate o'5 gramme.

Magnesium sulphate o-5 gramme.

Calcium phosphate 0*5 gramme.

To this solution add a trace of a weak solution of ferric
chloride. The calcium phosphate is only slightly soluble
in water.

25. Schulze's Macerating mixture.

Dissolve i gramme of potassium chlorate in 50 cub. cent,
of nitric acid. Immerse the tissue in this solution, and
heat it. The tissue will then readily break up into its
constituent cells, their middle lamellae being dissolved.

Cold maceration often gives better results, but takes
longer.

Take care that the gases given off from the mixture do
not injure the microscope.

26. Schulze's Solution.

Dissolve some zinc in hydrochloric acid ; permit the
solution to evaporate, in contact with metallic zinc, until
it has attained a syrupy consistence. Saturate the syrup

496 APPENDIX.

with potassic iodide, and then add enough iodine to
make a dark sherry-coloured solution. The object to be
stained must be placed in a little water, and then some
of the above solution added.

27. Silver Nitrate, Solution of.*

Dissolve 0-5 grammes of silver nitrate in 100 cubic cent,
of distilled water. Keep in an opaque stoppered bottle.

28. Sodic Chloride, Solution of. (Normal saline Solution.
Salt solution.}

Dissolve 7*5 grammes of sodic chloride in i litre of
distilled water.

F. SECTIONS AND SECTION CUTTING.

Directions for preparing sections of the vegetable tissues will
be found incorporated in the text ; those which follow apply to
the animal series alone.

1. Imbedding.

All tissues or embryos about to be imbedded, whether
stained or unstained should have been first well hardened
in 90 p.c. alcohol.

For imbedding, a mixture of hard and soft paraffins is
most serviceable, such as shall melt at from 50° to 60°.

The preparation to be imbedded, if stained, should be
first soaked in turpentine to saturation; if unstained, it
may be transferred direct from the alcohol. In either
case it must be placed in melted paraffin (the temperature
of which must not exceed that of its melting point) until
thoroughly permeated thereby. When ready for imbed-
ding, take of the solid paraffin a piece of about the
calibre of a candle and excavate at one end a pit, large
enough to fully accommodate the preparation ; then
transfer the latter and fill the pit with melted paraffin.
Put the whole aside, until quite cool and firmly set.

2. Cutting.

For this purpose an ordinary razor will suffice, so far as

APPENDIX. 497

the requirements of this volume are concerned. The
edge should be kept permanently sharp.

Before cutting, pare away the imbedding material, so
as to reduce that which surrounds the preparation to the
minimum.

Mounting.

Transfer the sections as cut, paraffin and all, to micro-
scopic slides previously prepared by one of the two
undermentioned methods, and proceed as directed in
either case.

a. White of egg method.

Smear the surface of the slide with a thin but uniform
film of freshly drawn white of egg and deposit the
sections in order of cutting. Gently heat the slide
until the paraffin begins to melt and then put it aside.
When set, put the whole bodily into turpentine and
leave it until all the paraffin is dissolved out ; upon ex-
amination the sections will be found to be firmly at-
tached to the glass slide by means of the coagulated
albumen.

Immersion in turpentine for an indefinite period
will do no harm.

b. Kreasote and shellac method.

Smear the surface of the slide with a heated solution
of white shellac in Kreasote. Submit the whole to the
temperature of the melting point of the paraffin until
the kreasote is evaporated, whereupon the sections will
become firmly adherent to the glass by means of the
shellac. Next immerse the whole in turpentine, and
leave it at rest until the excess of imbedding material
is dissolved out.

This method is best applicable to preparations which
have been previously stained and clarified. The white
of egg process is not only the simpler of the two, but
the more advantageous, as the sections may be cut and

M. 32

498 APPENDIX.

mounted unstained, that process and the subsequent
clarifying being permissible after fixture to the slide.

4. Final mounting.

Allow sufficient of the Canada balsam to drop upon the
sections to fully cover them. When thoroughly diffused
among them, smear the under face of a cover- slip with
balsam and place one edge of it upon the slide support-
ing its body upon the point of a needle ; by gradually
withdrawing the latter the cover-slip will descend ob-
liquely expelling the enclosed air.

Finished preparations should be examined from time
to time, in order that loss of the mounting medium by
evaporation may be made good.

5. To prepare ground sections of shells, bones, or other
hard parts.

A small piece of the structure to be manipulated should
be first isolated and then cemented in the desired posi-
tion to a piece of plate glass, by means of Canada balsam.
When firmly set it should be ground down upon a hone
or rough surface to the required thinness, and finally dis-
lodged for mounting, by immersing the whole in benzole
or chloroform. It may then be put up in Canada balsam
in the manner described above.

6. Frozen sections.

Material for this purpose should be preserved in weak
glycerine. For the preparation of these sections an ether-
spray freezing microtome is desirable ; good preparations
may however be made as follows.

Obtain a metal rod of the calibre of a candle and 2 — 3
inches in length : place the preparation (which, unless
quite fresh, should be previously soaked in gum-water)
on one end of it and 'add 6 — 8 times its bulk in ordinary
fluid gum. Freeze with pounded ice and salt. Cut.

Mount in weak glycerine.

INDEX.

A.

ABDOMEN, 174, 190, 192, 238
Abdominal appendages, 195, 198;

papilla, 188, 236
Acetabulum, 71

Adipose tissue, histology of, 131
Adrenal, 24, 52
Adventitious buds, 444.
Afferent branchial vessels, Cray-
fish, 226 ; Mussel, 328
Abe, 477

Albumen gland, 289; duct of, 290
Alcoholic fermentation, 377, 383,

388

Alga, 392, 396, 436
Alimentary canal, Crayfish, 178,

207; Earthworm, 241, 253,267;

Frog, 10, 15, 45; Mussel, 308,

320 ; Snail, 274, 284
Alinasal process, 13, 64
Alternation of generations, Chara,

435; Fern, 451

Ambulatory appendages, 175, 197
Amoeba, 369 — 376
Amoeboid movement, 123
Anabolism, 381
Anacharis, 442
Angulo-splenial, 62
Annulus, Fern, 449, 457 ; tym-

panicus, 118

Anodonta cygnea, 305 — 341
Antenna, 176, 200
Antennule, 176, 201
Anther, 460, 465, 477
Antheridium, Chara, 431, 433,

439 ; Fern, 450, 458, 466

Antherozoid (see Spermatozoid)

Antipodal cells, 479

Anus, Crayfish, 179; Earthworm,
241, 249 ; Mussel, 307 ; Snail,
281, 283

Aorta, Frog, 19, 20, 86; Snail,
293; anterior, Mussel, 326;
Snail, 294, 299; posterior,
Mussel, 326; Snail, 294

Aortic arches, 20, 85, 89, 92

Apical cell, Chara, 432, 438;
Fern, 448, 455

Appendages, Chara, 430 ; Cray-
fish and Lobster, 174, 176, 177,
192, 195 ; development of, 237,
238

Aqueduct of Sylvius (see Iter)

Aqueous humour, 117

Arachnoid, 10

Archegonium, 450, 459, 466

Archenteron, Crayfish, 187, 236;
Frog, 167, 170; Snail, 303

Artery, antennary, 225; antero-
ventral, 224; carotid, 18, 20,
85 ; cceliac, 88 ; ccelicao-mesen-
teric, 87, 88 ; common iliac, 87;
gastric, 225; genital, 87; he-
patic, 180, 224; hypogastric,
87 ; inferior abdominal (see pos-
tero- ventral) ; inferior mesen-
teric, 87, 88 ; ophthalmic, 224;
pedal, Mussel, 326 ; postero-
ventral, 225; pulmo-cutaneous,
19, 20, 86; renal, Crayfish, 2 26;
Frog, 87 ; rostro-antennary,225;
splenic, 88; sternal, 180, 224;
subclavian, 86 ; superior ab-

32—2

5oo

INDEX.

dominal, 180, 224; superior me-

senteric, 88 ; vertebral, 86 ;

visceral, Mussel, 326
Arthrobacterium, 413
Arthrobranchiae, 221
Arytenoid, 21, 51
Ascent of sap, 468
Ascogonium, 420, 427
Ascospore, 420, 427
Ascus, 420, 427
Asexual generation : Fern, 45 1
Astacus fluviatilus, 173 — 239
Astragalus, 73
Atrium, 17, 81, 90
Auditory epithelium, Frog, 145 ;

Lobster, 232
Auditory capsule, 59 ; organ :

Crayfish, 186, 230; Cyclas, 331 ;

Frog, 33, 118 ; development

of, 161 ; Mussel, 331 ; Snail,

276, 300
Auricle, Frog, 17, 90; Mussel,

324; Snail, 275, 284
Auricular septum, 17, 91
Axil, Bean, 460 ; Chara, 431

Bacillus, 413

BACTERIA, 408—414

Balantidium^ 363

Basipodite, 195

Bast, Bean, 464, 472 ; Fern, 446,

453

Bast fibres, 464, 474

Bast parenchyma, 454, 473

Bast vessels, 454, 473

BEAN-PLANT, 460 — 481

BELL- ANIMALCULE, 359—368

Bidder's organ, 54

Bile ducts, 47

Bipolar nerve-cell, 139

Blastoderm, 236

Blastopore, Crayfish, 187, 236 ;
Frog, 159, 1 60; Snail, 302

Blood, Crayfish, 226; Earthworm,
245, 262; Frog, 16, 12 1 ; Lob-
ster, 182 ; Mussel, 310; Snail,
276

Body - cavity, Earthworm, 247
252; Frog, dev. of, 169; Hydra,
349' 35i ; Mussel, 335

Body-wall, Earthworm, 243, his-
tology of, 264; Frog, 41, 43;
Hydra, 344, 349

Bone, histology of, 1 29

Brachial plexus, 103

BRACKEN FERN, 443—459

Brain, n, 24, 96, 170

Branch, 430, 443, 460

Branchiae, Crayfish, 182, 220;
Mussel, 307, 316, 317, 322,
335> 33^ ; suspensory ligament
of, 3l8> 335? Tadpole, 5, 6, 161,
162, 164, 171

Branchial clefts, 5

Branchio-cardiac canals, 181, 222

Branchiostegite, 174, 176, 193,
194

Bronchi, 50

Brownian movement, 408, 411

Brow-spot, 35, 36

Buccal-mass, 285, 286; sac, 241,

253» 255
Bud, Bean, 460 ; Chara, 432, 438 ;

Hydra, 350

Bundle-sheath, 446, 453
Byssus, 313, 341 ; organ, 341

C.

CAECUM, Crayfish, 208; Frog, 16 ;

Lobster, 208
Calcaneum, 73
Calcar, 3, 73

Calciferous glands, 243, 254
Calcium, 379, 403
Call ha, 481
Calyx, 460, 476
Cambium, 463, 473
Campanula, 481
Canal cells, 459
Canalis centralis, 31
Capillaries, 93
Capitulum, 434
Capsulogenous glands, 250
Carapace, 174, 190; development

of, 237

INDEX.

501

Carbo-hydrate, 467

Carbon, 379, 390, 403

Carbonic anhydride, 381, 390, 402,
467

Carchesium, 368

Carina, 477

Carotid gland, 86

Carpopodite, 196

Carpus, 71

Cartilage, histology of, 1 29 ; re-
placement in bone, 14

Cartilage bones, 63 .

Catabolism, 381

Cell-division, hyaline cartilage,
128; Spirogyra, 399, 405; wall,

379

Cellulose, 379

Cement glands, 186, 235

Centrum, 56, 58

Cephalon, 174

Cephalothorax, 173, 190, 193, 237

Cerebellum, 25, 98

Cerebral hemisphere, 97

Cerebro-spinal axis, 10, 95 ; de-
velopment of, 1 60, 1 68; neive>,io

Cervical groove, 174, 175

"Chara, 430 — 441

Chela, 175, 197, 237

Chlamydococcus, 389

Chlorophyll, Bean, 465, 467 ;
Chara, 433 ; Fern, 447 ; Hy-
dra, 344, 345, 356 ; Protococ-
cus, 389 ; Spirogyra, 397 ; Vor-
ticella, 362

Chloroplastid, Hydra, 356 (see
also Chromatophore)

Chondro-cranium, 12, 63

Choroid, coat, 33, 117, 300;
plexus, 27, 100

Chromatophore, Protococus, 389,
391, 393; Spirogyra,^ 397, 404

Cilia, Bacteria, 408 ; Chara, 434 ;
Fern, 450; Protococcus, 391,

.394
Ciliary action, Frog, 124; Vorti-

cella, 366

Ciliated groove, 322, 338
Circulation, Crayfish, 182 ; Frog,

19,92

Circulatory system, Crayfish, 180,
222; Earthworm, 244, 259;
Frog, 16, 80; Mussel, 309, 325;
Snail, 275, 292

Circulus venosus pulmonis, 295

Circum-neural arcade, 265

Circum-cesophageal vessels, 245,
259, 260

Clavicle, 69

Clitellum, 245, 248

Cloaca, Frog, 2, 10, 47 ; orifice of
in Tadpole, 163, 165

Cnidoblast, 346, 355

Cnidocil, 346, 355

Cocoon, 247

Cochlea, 33

Collateral bundle, 463

COLOURLESS BLOOD CORPUSCLE,
122, 369, 372, 375

Columella, Mucor, 421 ; Snail,
282 ; auris, 33, 34, 60, 65, 119

Commissures, ant, cesophageal,
229; buccal, 298; cerebro-
pedal, Mussel, 330, 339; Snail,
297 ; cerebro-splanchnic, Mus-
s?li 329» 330. 336'» Snail, 297;
circum - cesophageal, Crayfish,
227; Earthworm, 262 ; of brain,
Frog, 26, 101 ; inter-cerebral,
Mussel, 330, 339 ; lateral neural,
26 r ; longitudinal ventral, Cray-
fish, 184, 227; Earthworm,
263; maxillo - palatine, 106;
median ventral, 229 ; post-ceso-
phageal, 229

Concentric bundle, 446

Condylar facet, 56

Confervoid algae, 396

Conidium, 417, 419, 426

Conidiophore, 417, 419, 426

Conifers, 466

Conjugation, Amoeba, 373 ; Mu-
cor, 423 ; Spirogyra, 400, 406;
Vorticella, 363

Conjunctiva, 44

Connective rod, 185, 234

Connective tissue, histology of,
126

Coracoid, 69

502

INDEX.

Cornea, 117, 185, 300; facets of,

233» 234

Corolla, 460, 476

Corpora adiposa, 40, 54

Corpus callosum, 26, 101

Corpuscula, 466

Cortex, 462, 465, 472

Cortical layer, Chara, 430, 432

Cothurnia.) 368

Cotyledon, 461, 480

Coxopodite, 195

Cranium, 63

CRAYFISH, 173 — 239

Cricoid, 21, 51

Crop, Earthworm, 241, 254;
Snail, 274, 284

Crystalline cone, 185, 233, 234,
lens, 117; Snail, 300, develop-
ment of, in Frog, 172

Crystals, Amoeba, 372, Hydra, 353

Cutaneous glands, 34, 146, 148;
Earthworm, 243, 265

Cuticle, Crayfish, 189; Earth-
worm, 241, 251 ; Hydra, 353 ;
Vorticella, 360, 364

Cuticularized cell-wall, 401, 407

Cyclas, 331

D.

DACTYLOPODITE, 196

Dart, 277, 292 ; sac, 291, 292

Dentary, 62

Dermis, 146, 147

Development, Crayfish, 187, 235 ;

Earthworm, 247 ; Frog, 5, 157;

Hydra, 344; Lobster, 238;

Mussel, 312, 340; Snail, 278,

302

Diaphragm, 43
Dicotyledons, 464
Digestion, Amoeba, 371 : Cray-
fish, 179; Earthworm, 242;

Hydra, 343, 357
Digestive gland, Crayfish, 179,

207, 209, 215; Earthworm, 242 ;

Mussel, 311, 320; Snail, 285;

development of, 303
Digitate gland, 292
Disc, 364

Ductus arteriosus, 89; Botalli, 89
Duodenum, 45, 46
Dura Mater, 96

E.

EAR (see Auditory organ)

EARTHWORM, 240—271

Ecdysis, 189

Ectoderm, 236, 303; Hydra, 344;
^ histology of, 351, 352, 353

Ectoplasm, 124

Ectosarc, Amoeba, 370, 374; Vor-
ticella, 360, 364

Efferent branchial vessels, Cray-
fish, 222; Mussel, 327

Egg- apparatus, 479

Egg-capsule, 235

Elodea, 442

Embryo, Bean, 461, 480; Fern,

Embryo-cells, 5, 158

Embryonic layers, 165

Embryo-sac, 461, 466, 478

Encephalon, (see Brain)

Encystment, Amoeba, 369 ; Vorti-
cella, 363

Endoderm, 236, 303; Hydra, 344
histology of, 351, 352, 353, 355

Endolymph, 33

Endophragmal system, 189, 201

Endoplasm, 124

Endoplast (see Nucleus)

Endopleurite, 194

Endopodite, 177, 193, 239

Endosarc, Amoeba, 370, 374;
Vorticella, 360, 364

Endoskeleton, Frog, 11,55; Snail,
274

Endosperm, 466, 479

Endosternite, 194

Endotergite, 194

Epiblast, 1 66, 187

Epidermis, Bean, 462, 473 ; Earth-
worm, 245, 265; blood plexus
of, 245; Fern, 445, 447, 453;
Frog, 121, 146

Epiglottis, 49

Epimeron, 192, 194

INDEX.

503

Epiostracum, 203, 333

Epiphragm (see Hybernaculum)

Epipodite, 177, 183, 221

Epistylis, 368

Epithelium, 124

Eurotium, 419

Eustachian recess, 9, 48

Excretory organ, Crayfish, 183;
Earthworm, 244; Frog (see
Kidney); Mussel (see Organ of
Bojanus); Snail, 275, 283; ori-
fice, Mussel, 323, 335

Exoccipital, 60, 63, 65

Exopodite, 177, 193, 238

Exoskeleton, Crayfish, 173, 189,
191; Earthworm (see Cuticle);
Frog, ii ; Mussel, 331, 335;
Snail, 274, 281 ; histology of,
Crayfish, 202; Mussel, 333

External characters, Crayfish, 173,
190; Earthworm, 240, 248;
Frog, i, 6, 35; Hydra, 342,
348; Mussel, 305, 314; Snail,
272, 280

Eye, Crayfish, 184, 233; Frog, 2,
36, 44, 112; Snail, 273, 299

Eyelids, 2, 3, 36, 44, 112

Eyespot, 304

Eye- stalk (see Ophthalmite)

F.

FALCIFORM ligament, 43, 44
Fat, 379

Fat bodies (see Corpora adiposa)
Fenestra ovalis, 34, 65
Fermentation, Bacteria, 410; Mu-

cor, 424; Yeast, 377, 382, 388
Fern, 443—459
Fertilization, Bean, 480; Chara,

Fern (see Impregnation) ; Frog,

4; Mussel, 312; Snail, 277
Fibula, 72
Filament, 460, 477
Filum terminale, 30, 96
Flagellum (see Spermatophoral

gland)

Flower, 460, 476
Flowering plant, 460 — 481.

Follicular epithelium, Crayfish,

220, Frog, 155
Fontanelles, 12—63
Food yolk, Crayfish and Frog, 187,

235
Foot, Fern, 450, 459; Hydra, 348;

histology of, 346 ; Mussel, 334 ;

Snail, 273, 280; development

of, 304
Foramen magnum, 59 ; of Munro,

26, 100, 101
Foramina for exit of cranial nerves,

66

Fourth ventricle, 100
Fragmentation of nucleus, 437
FRESHWATER POLYPES, 342 — 358
FROG, i — 172
Fronto-parietal, 60
Fungi, 382, 416
Funiculus, 479

G.

GALL bladder, 45, 47

Ganglion, abdominal, 227; cere-
bral, Mussel, 330, 339; Gas-
serian, 28, 105; parieto-splanch-
nic, Mussel, 312, 329,338; Snail,
276, 297; pedal, Mussel, 312,
339; Snail, 276, 296; spinal,
102; subcesophageal, Crayfish,
184, 228; supra cesophageal,
Crayfish, 184, 227; Earthworm,
246, 262 ; Snail, 276, 296;
sympathetic, 107; thoracic, 228;
visceral, 331

Gastrolith, 210

Gastrula, 187, 236, 247, 278, 302

Gemmation, 362, 380

Genital cloaca, 276, 291

Genital duct, Crayfish, 206; 250;
Frog, 40, 53; Mussel, 336;
Snail, 276

Genital furrow, 281

Genital gland, Crayfish, 206; Frog,
40, 42, 53; Mussel, 336; Snail,
276, 301

Germinal epithelium, Frog, 155;
Earthworm, 270; spots, Cray-

504

INDEX.

fish, 219; Mussel, 340; vesicle,

Crayfish, 219; Earthworm, 270;

Frog, 24, 155; Mussel, 340;

Snail, 302
Germination, Bean, 461 ; Chara,

435; Fern, 449; Mucor, 423;

Penicillium, 417, 426; Spirogyra,

401

Gills (see Branchiae)
Girdle-bone (see Sphenethmoid)
Gizzard, 241, 255
Glenoid fossa, 68
Glochidium, 313, 340
.Glottis, 9, 21, 49
Glycerine, 383
Glycogen, 151, 311 , 329
Green gland, 183, 207, 216
Ground parenchyma, 445
Growing point, Bean, 462; Chara,

432; Fern, 448
Guard-cells, 448, 473
Gullet, Crayfish, 210; Mussel, 320
Gustatory organ, no
Gymnosperms, 464

H.

Hamatococcus, 389

Harderian gland, 33, 114

Head segment, 273

Heart, Crayfish, 180, 181, 206,

223; Frog, 17, 40, 43, 80, 89;

Mussel, 310, 324, 337; Snail,

275, 284
Helix aspersa, H. hortensis, 272 —

304

Helleborus, 481
Hepato-pancreas, 180
Hermaphrodite duct, 276, 289;

gland (see Ovotestis)
Hilum, 479
Hip-girdle, 71

Homarus vulgaris, 173 — 239
Humerus, 69
Hybernaculum, 272, 282
Hydra fusca, viridis, 342 — 358,

hexactinella, 342
Hydrogen, 379, 403

Hyoid, 9, 14, 49, 67

Hyphre, 416, 419, 420

Hypoblast, 167, 187

Hypoderma, 445

Hypodermis, Worm, (see Epider-
mis)

Hypostome, 343, 348; histology
of, 346, 353

ILEUM, 45, 46

Ilium, 72

Impregnation, Bean, 461 ; Chara,
434; Crayfish, 235; Earth-
worm, 247; Fern, 450; Hydra,

344

Indusium, 444, 456
Infra-branchial chamber, 316
Infundibulum, 26, 99, 101
Infusoria, 359
Integument, Crayfish, 204; Frog,

34 (see also Skin)
Inter-articular membrane, 201
Intercellular passages, 448, 475
Internal ear, 119
Internodal cell, Chara, 432
Internode, Bean, 460; Chara, 430;

Fern, 444

Inter-ocular gland (see Brow-
spot)
Interstitial tissue, 352; histology

of, 354

Inter-vertebral foramina, 57
Intestinal valve, Earthworm, 242,

255; Mussel, 321; Frog, 15, 46
Intestine, Crayfish, 179, 208;

Earthworm, 242, 254; Frog, 39;

Mussel, 320; Snail, 274, 286;

Tadpole, 163 ; histology of,

Earthworm, 264; Frog, 148
Intra- vertebral bodies, 12, 58
Iris, 113
Iron, 403
Ischiopodite, 196
Ischium, 72
Iter, 100

INDEX.

505

K.

KEBER'S organ, 319, 334
Kidney, 22, 40, 41 ; histology of,

151

Kleinenberg's fibres, 352, 353

LABIAL cartilages, 64
Labial palpi, 307, 316, 321, 338
Labrum, 175, 176, 202; develop-
ment of, 237

Lamina terminalis, 99, 100
Large intestine, 10, 15, 39, 41
Larynx, 21, 49, 5 l
Lateral intestinal vessels, 261
Lateral neural vessels, 259
Lateral oesophageal trunks, 245,

260

Lateral roots, 465, 471
Lateral ventricle, 100
Leaf, Bean, 460, 462, 465, 474;
Chara, 430, 431, 437; Fern,

443» 455

Leaflets, Bean, 462, 474; Chara,
436; Fern, 443

Lemna, 471

Lesser omentum, 43

Ligament, of shell, 306, 332, 335

Ligamentum latum, 43

Lily, 477, 478, 481

Limb-girdle, 14, 55, 68

Limbs, Frog, 2, 36; development
of 163, 164; Crayfish, 175

Lingual ribbon (see Radula)

Liver, 16, 39, 43, 47 ; histology of,
150

LOBSTER, 173 — 239

Lower layer cells, 158, 159, 166

Lumbo-sacral plexus, 103

Lumbricus terrestris, 240 — 271

Lungs, 21, 39, 43, 46, 50; deve-
lopment of, 164

Lymph, 16

Lymph-cavities, subcutaneous, 37,

44» 45
Lymph-hearts, 17, 94

Lymph sinus, circum-cesophageal,

94; sub- vertebral, 8, 16, 42
Lymph stomata, 126

M.

MACROZOOSPORE, 391, 393

Magnesium, 379, 403

Mandible, Frog, 9, 62; Crayfish,
200

Mantle, 305, 307, 316, 328, 334;
development of, 340

Manubrium, 433

Manus, 2

Marginal sinus, 295

Maxilla, Crayfish, 175, 199, 213;
Frog, 9, 6 1

Maxillipede, 175, 196, 197, 199

Mayer's solution, 386

Meckel's cartilage, 13, 67

Medulla oblongata, 98

Medullary rays, 463, 473

Membrana propria, 220

Membrane bones, 63

Membranous labyrinth, 33, 119,
1 20

Mento-Meckelian bone, 62, 67

Meristem, 455, 462, 464

Meropodite, 196

Mesencephalon, 98

Mesenteric septa, 243, 252, 265

Mesenteron (see Archenteron)

Mesentery, 8, 39, 42

Mesoarium, 42

Mesoblast, Earthworm, 247; Frog,
167, 169, 170

Mesoglcea, 347, 352, 356

Mesophyll, 456, 465

Mesorchium, 42

Metabolism, 381

Metacarpals, 70

Metastoma, 175, 176, 202, 213

Metatarsals, 73

Metencephalon, 98

Micrococcus, 413

Micropyle, Bean, 461, 478; Mus-
sel, 312, 339

Microzoospore, 391, 394

Monoecious conjugation, 401

INDEX.

MOULDS, 415—429

Mouth, Crayfish, 175, 202 ; Earth-
worm, 249; Frog, 9, 44, 48;
Hydra, 343, 348; Mussel, 306,
3T7' 32i> 338J Snail, 274, 281;
Tadpole, 161, 163.

Mucor, 420; Torula of, 424

Multinucleate cells, 417

Multipolar nerve-cell, 141

Muscles, abdominal, Crayfish, 205,
207; adductor, Mussel, 306,
3*5. 337; anal» 2I55 ®f body
wall, Earthworm, 265 ; of buc-
cal mass, 286 : columellar, 282 ;
constrictor laryngis, 50 ; of eye,
33, 113; of eyestalk, 233; of
hind limb, Frog, 74 — 80 ; of
hyoid, 50; limb, Crayfish, 204;
mandibular, 205, 207; mylo-
hyoid, 37; oblique of trunk, 38;
pallial, 315; pectoral, 37; pe-
dal, Mussel, 315, 339; of seta,
Earthworm, 266; of stomach,
Crayfish, 178, 213; transver-
salis, 38

Muscle, histology, striped, 133;
unstriped, 132; physiology, 156

Muscular system, Crayfish, 204;
Frog, 24

MUSSEL, 305—341

Mycelium, 416, 422, 425

Myelon (see Spinal cord)

Myophan layer, 364

N.

NARES, 2, 9, 48, i n

Nasal, 60

Nematocysts, 344, 346, 350; his-
tology and development of, 354

Nephridium (see Segmental organ),
blood plexus of, 258

Nephrostome, Earthworm, 244,
256, 257 ; Frog, 54

Nerve, histology of, Crayfish, 229 ;
Earthworm, 264; Frog, 136;
physiology, 156

Nerve-cells, Crayfish, 230; Earth-

worm, 246, 264, Frog, 139;
Hydra, 346, 355

Nerves, ist Cranial (olfactory) 27,
105; 2nd Cranial (optic) 27,
105 ; 3rd Cranial (oculomotor)
27, 105, 114, 116; 4th Cranial
(pathetic), 28, 114, 116; sth
Cranial (trigeminal), 28, 105 ;
6th Cranial (abducent) 29, 115,
1 16; 7th Cranial (facial), 29, 106,
109; 8th Cranial (auditory), 29,
107 ; 9th Cranial (glossopharyn-
geal),30, 104, 106; loth Cranial
(vagus or pneumogastric), 30, 1 06,
108 ; antennary, 228; anten-
nulary, 228; buccal, 276, 298;
ganglionic, Earthworm, 263,
Crayfish, 227; hypoglossal, 31,
103; interganglionic, Earth-
worm, 263 ; Crayfish, 227; la-
bial, Snail, 299; maxillo-man-
dibular, 105; ophthalmic, 105;
optic, Crayfish, 228; Snail, 300;
pallial, Mussel, 329 ; Snail, 297 ;
palatine, 106; recurrent laryn-
geal, no; spinal, 31, 96, 102;
splanchnic, 107, no; sympa-
thetic, 32, 107 ; tentacular,
299; vaso- motor, 108; visceral,
Crayfish, 229; Earthworm, 263 ;
Snail, 297, 298 ; viscero-motor,
1 08

Nervous system, Crayfish, 184,
227; Earthworm, 246, 262;
Frog, 10, 24, 95 ; Mussel, 312;
Snail, 276, 296; sheath of,
Earthworm, 263; Snail, 296

Nettle, 442

Neural arch, 56; canal, 56, 59;
cavity, 10, 41 ; folds and plate,
160, 1 68

Neurochord, 246, 263

Neurosensiferous apparatus, Hy-
dra, 346

Nit el la, 436, 440

Nitrogen, 379, 403

Nodal cell, 432

Node, Bean, 460, 473 ; Chara,
430 ; Fern, 444

INDEX.

SO/

Notochord, 12, 168, 170

Nucellus, 460, 478

Nucleolus, 123, 389, 393, 399

Nucleus, 122 ; Amoeba, 370 ; Bean,
461; Chara, 433, 434; Fern,
444; Mucor, 421 ; Penicillium,
417; Protococcus, 389, 391,
393 ; Spirogyra, 398, 404 ; Vor-
ticella, 362, 365 ; Yeast, 379

Nyctotherus, 368

O.

OCCIPITAL condyle, 59

Odontoid process, 56

Odontophore, 274^287, 288

(Enot/iera, 480

OZsophageal glands (see Calcifer-
ous glands)

OZsophagus, Earthworm, 254, 255;
Frog, 43; Snail, 274, 285 (see
also Gullet)

Oil, 407

Olfactory capsule, 64 ; epithelium,
144; lobe, 97; organ, Crayfish,
184, 232, Frog, 32, in; ven-
tricle, 100

Omosternum, 69

Oogonium, Chara, 431, 434, 439

Oosperm, 4, 158, 235, 302, 358

Oosphere,45O,46i (see also Ovum)

Oospore, 434

Opercular membrane, 6, 162 ; de-
velopment of, 163, 171

Ophthalmite, 176, 201

Optic bulb, 233, 234 ; chiasma,
99; cup, 172; lobes, 25, 98;
pit, 237; stalk, 172; thalami,
26, 98 ; tract, 99; ventricle, roo

Organ of Bojanus, 311, 322, 335,

337

Os cruris, 72

Ossicles, Crayfish stomach, 178,
208, 210

Otocyst, 300, 331

Otolith, Crayfish, 231; Cyclas,
331 ; Frog, 120; Snail, 300

Ovary, Crayfish, 186, 218; Earth-
worm, 269 ; Flowering plant,

478; Frog, 23, 40, 52, 154;
Hydra, 344, 357; Lobster, 219

Oviduct, Crayfish, 218; Earth-
worm, 269; Frog, 23, 40, 53,
54 ; Snail, 290, 292

Ovisac, 219

Ovotestis, 276, 285, 289

Ovule, 460, 478

Ovum, Bean, 46 r, 466, 479;
Chara,434; Crayfish,2t9; Earth-
worm, 246, 270 ; Fern, 450,
459; Frog, 5, 24, 154; Hydra,
344,' 357J Mussel, 312, 339;
Snail, 277, 302

Oxygen, 379, 390, 403

P.

PALATINE, 61

Palato-quadrate cartilage, 63

Palisade parenchyma, 465, 475

Pallium (see Mantle)

Pancreas, 45 ; ducts of, 47 ; his-
tology of, 149

Papillae, of tongue, 34, m

Parasphenoid, 61

Parenchyma, 453, 454

Pasteur's solution, 380, 384

Pectoral arch (see Shoulder girdle)

Pedal gland, 273, 281, 288; sinus,
289, 294

Peduncle, 476

Pelvic arch (see Hip girdle)

Penicillium^ 416

Pericardium, Crayfish, 182, 222;
Frog, 8, 40, 43; Mussel, 310,
3i9. 335, 337J Snail, 275,
284

Periganglionic glands, 32, 103

Perilymph, 34, 119

Periotic capsule (see Auditory
capsule)

Peristome, 274, 282, 360, 364

Peristomium, 249

Peritoneal pores, 244, 250

Peritoneum, Frog, 42 ; Earth-
worm, 243, 266

Perivisceral fluid, 244, 253

Pes, 3

5o8

Petal, 476

Petiole, 461, 474

Phalanges, mantis, 70; pes, 73

Pharynx, Earthworm, 741, 253,

*55 » Fr°g« 5

Phloem, Bean, 463, 471; Fern,
446, 448, 449, 453

Phloem-sheath, 447, 453

Phosphorus, 379, 403

Photometric Zoospores, 395

Pia mater, 77, 96

Pineal gland, 16

Pinnule, 443, 447

Pistil, 460, 478

Pith, 467, 471

Fits, 446, 472

Pitted vessels, 464, 473

Pituitary body, 76, 99

Hasmofyw, 397

Pleotnorphic Bacteria, 411

Pleurobranchue, 721

PLEUEOCOCCUS, 389

Pleuron, 192

Plenroperitoneal cavity, 7, 39, 41;
membrane, 8, 47, 43

Plexus, brachial, 31 ; epidermal,
245; excretory, 761 ; him bo-
sacral, 31, 103

Plumule, 461, 480

Pod, 461

Podobranchiae, 720

Polar plate, 130

Pollen, 460, 465, 477

Pollen-sac, 465, 477; tube, 461,
480

Pollinodium, 420

Pore-canals, Crayfish, 203 ; Mus-
sel. £4 1

Posterior commissure, 101

Potassium, 379

Pre-hallux (see Calcar)

Pre-maxilla, 9, 60

Pre-nasal process, 13, 64

Primine, 478

Primordial cell, 391

Primordial utricle, 397, 404, 417,

433 ,

Procephahc process, 202
Processus spiralis, 140

Pro-embryo, 435

Pro-otic, 60

Propodite, 196

Pro-renal duct (see Segmental
duct)

Prosencephalon, 97

Prostate, 290

Prostomiom, 249

Protein, 379, 390, 467

Proteus animalcule, 369 — 376

Prothallus, 450, 457, 466

PROTOCOCCUS, 389—395

Protophloem, 447, 454

Protoplasm, 3/9

Protoplasmic movement, Anacha-
ris, 442; Chara, 440; Nettle-
hair, 442; Tradescantia, 441;
Valhsneria, 442

Protopodite, 177, 193, 239

Protoxylem, 446.454. 47*

Pseudopodia, 122, 370, 374

JVeris, 443—459

Pterygoid, 61

Pubis, 71

Pulmonary sac, 274, 282

Putrefaction, 410

Pylangium, 18

Pylorus, 15, 46

Pyrenoid, 398, 404

Q-

I

Quadrato-jugal, 61

R.

R AC HIS, 288

Radial bundle, 465

Radicle, 461, 480

Radius TO

Radula, 287 ; sac of, 285

Rana csculcnta — temporaria, i —

172

Receptaculum ovorum, 269
Receptaculum seminis, 292
Rectum, Mussel, 337 ; Snail, 283

INDKX.

509

Renal organs (see Excretory organ

ami Kidney)
Renal vessels, Snail, 195

Urn.) ji.-iii-.ii.li.il aprilinc; MuS-

sel, 3M> 335J d«ct, Snail, 275,

*4

Urproduction, Aimrlu, 373 ; Cray-
iish, iS;; Karthworm, 347; Frog,

5 : Hvdi.i, .;.(.; ; Minor, 413;
MII-. . I, ;, i .• ; Snail, .•;•/ ; Spno
l-.Via, 400, 406; Vorti«-lla, ;6-

Ui -p i.i.hu live organs, Bean, 465;

( 'li.n.i, .( ; ; ; < 'i..\ !r,li, 1 86, 717 ;

Earth worm, 146, »68;Etirotluin,

-»">: I "',, ,,,,, 456, 458; Frog,
33, 5' ; 1 1 v- 1 1. 1. 349, .157 ; MU-

1 "i . i • i , t •'.; ; Mu ..-I, .in, 310 ;

Prim illimn, 4 n) ; Snail, 776,189
Ui-.s|iiiati.'ii, r.t-.in, .|UM ; < 'rayli .h,
183 ; Kailliwoim; .'.(. ,
• • ; MII , ,rl, joH ; Snail, 175 ;

Yeast, 383

U<- .piialoi v c.i ;ni ., ( 'i.iylish, iHj,
207, 22 I ; I''H>K, -21, 4>s ; M 11 isi-1,
.507, 333 ; Snail, 375, 281

Uc.lil.iiin body, 15

Ui-tina, .5 ;, i iS ; dc\ clopinrnt <>!,

i ; .- ; lir.tology of, 141
Ulnnal pioi-c-,-,, i _;, o.(

Uliinciu.-phalon, 97

Uhi/oid, 431

Uoot, P.< -an, .joo, .(05, .jyo; l''i-in,

448, 452, 4y>

Root-cap, 448, 461, 471
Root-filaments, Chara, 430

U....I I...,. ,, ., |\ , ,,, |. , ,'..

Uool |)n-ssinc, .,09
Uo.,tiuni, M;O, '2%}H

SAC (MT Cell-wall)
Sait/i.iriiHiyccs, 377—38B
SftO "hi ., .1.5, HO
Sariiim, 57

S:ilivaiy duct, Sllllil, 285, 288;
jdand, l''.ai I hwoi m, /.( i ; Sn.ul,

Scalariform vessels, 446, 455

Si-apho-MialliiU-, 174, iSi

Scapula, 69

Schizomycctesy 408 — 414
Sclerenchyma, 446, 453, 454, 471
Sclerotic, 33, 117

Sivondary thirknnn.;, .»o^
Sivundinr, 478

Seed, 479

Siy.im-nl.il duel, if>y ; orj;an, -|-|.

«5'. »5*» »5<5

Segmentation, ( a\ny, 166 j of

OVUm, C.'i.iyli .h, 187, 135;

Earthworm, 147 ; Frog, «;, 158,
165; Mussel, 319; Snail, 178,

Sclajpnella, 466

Seminal iccrplai'lf (MT Spciina
tlicca) ; vcsii-lc, 946, ^68

Sen .c oij-aiis, I'.ailliuoim, >.\<> ;
I lydia ffeC < nidocil)

Sepal, 476

Sfptinn naiinni, i %^, 66, i i i
Si-la, Ciaylish, ^0.5 ; K.-nlliworm,

a4i, 749, 151, 166; sac of, 351,

Sex-organs (see Reproductive
organs)

SI-MI. il };<-nrialion, 1'rin, 457
ScMial irpiodnclion, .joo, 402,

Slul'l9(sJeExoskeleton)
Shell-teeth, 340

Shoul.U-i-j-irdlc, 6S

Su-vc tul.rs, P.can, 464, 469, 47j;

Fern, 447, 454, 455

Sinn, vrnoMis, 17, Hi, 90
Siphonal tcnl.ulrs, ^17
Siphon^ ;,07, Ml
Sl;rl«-toii,< ravli ,h, 173, 189; I i

II. 55

Skin. 1' io)', In .toloj-y of, 145

SUull, 12

Small inleslinr, 10, 15, 39, 45

SNAIL, 177—304

Soinitf, 174, 177, 240, 248

Sooly corpuscles, 356

Sorus, 444

S|H-iinalht.-c;i, 250, 268, 39,1

5io

INDEX.

Spermatophoral gland, 217, 291
Spermatophore, Crayfish, 235 j

Snail, 277
Spermatozoid, Chara, 434, 440 ;

Crayfish, 218; Earthworm, 246,

Fern, 450, 458; Frog, 23, 154;

Hydra, 344, 357; Mussel, 312;

Snail, 277, 301 ; development

of, 270

Sperm -capsule, 235
Sphenethmoid, 61, 63, 66
Spiculum amoris (see Dart)
Spinal cord, n, 25, 30, 96 ; histo-
logy of, 140
Spinous process, 56
Spiral vessel, 446, 455, 464, 473
Spirillum, 408, 413
Spirochcete, 413
SPIROGYRA, 396 — 407
Splanchnic cavity (see Pleuro-

peritoneal cavity)
Spleen, 16, 40

Spongy parenchyma, 465, 475
Sporangium, Fern, 444, 449, 456 ;

Mucor, 420, 427
Spore, Bacteria, 409 ; Fern, 444,

449>. 457 J Mucor, 421, 428;

Penicillium, 417 ; Yeast, 380,

386

Squamosal, 60
Stamen, 460, 477
Starch, 389, 398, 402, 433, 445
Stem, Bean, 460, 47 1 ; Chara, 430,

436; Fern, 443
Sternal sinus, 226
Sternum, 192, 194
Stigma, 460
Stipule, 474
Stoma, Bean, 462, 464, 473 ; Fern,

448, 456
Stomach, Crayfish, 1 78, 208 ;

Frog, 15, 39, 46 ; Mussel, 320,

338 ; Snail, 285
Stomodaeum, 171, 303
STONEWORTS, 430 — 441
Stratification, 404
Striated spindle, 185, 233, 234
Style, 460, 478
Sub-intestinal vessel, 245, 260

Sub-neural vessel, 245, 260

Sub -ocular arch, 63, 64

Sub-tentacular lobe, 279

Succinic acid, 383

Suckers, 160, 162

Sulphur, 379, 403

Supporting lamella (see Meso-

gloea)
Supra- branchial chamber, 307,31 8,

o 33? ,

Supra-mum, 72

Supra-intestinal vessel, 245, 259,

260

Supra-neural vessel, 245, 259
Supra-scapula, 69
Suspensorium, 13, 64, 65
Swimmerets, 192
Synangium, 18
Synergidse, 479

T,

TADPOLE, 5, 162

Tail, 162, 165

Tail fin, 162

Tarsus, 73

Teeth, Cray fish stomach, 178, 211,
212; Frog, 9, 15, 48

Telson, 174, 190, 194

Tendon, 74, 189

Tendril, 474

Tentacles, Hydra, 342, 348; de-
velopment of, 351 ; histology of,
349, 352 ; Snail, 273, 280, 299

Tergum, 192, 194

Testa, 479

Testis, Crayfish, 217 ; Earthworm,
269 ; Frog, 23, 40, 53 ; histology,
153; Hydra, 357; Mussel, 312

Thalamencephalon, 25, 98

Thoraco- abdominal linkwork, 202

Thorax, 174

Thread cells (see Nematocysts)

Thymus, 24

Thyroid, 24

Tibia, 72

Tongue, 9, 34, 44, 49

Torula, 377—388

Tradescantia, 441

INDEX.

Transverse process, 56
Trochal ridge, 279, 303
Trochosphere, 279, 303
Truncus arteriosus, 17, 19, 81, 91
Tuber cinereum, 99
Tubular fibres (see Neurochord)
Tympanic cavity, 9, 33 ; mem-
brane, 2, p, 33
Typhlosole (see Intestinal valve)

U.

ULNA, 70

Umbilicus, 282

Umbo, 332

Uncini, 288

Unguis, 477

Unio, 305

Unipolar nerve-cell, 139

Upper layer cells, 158, 159, 166

Ureter, 48

Urinary bladder (see Urocyst)

Urino-genital duct, 48, 52, 54 ;

organs, 51
Urocyst, 23, 39
Urostyle, 12, 55, 58, 59
Utriculus, 33, 120

VACUOLE, Chara, 437,438 ; Mucor,
421, 428 ; Penicillium, 417, 425,
426; Protococcus, 393; Spiro-
gyra, 397, 39^, 4°4 '•> Trades-
cantia, 441 ; Yeast, 378, 384

Vallisneria, 442

Valves, of heart, Crayfish, 223 ;
Frog, 18; Mussel, 325, 337;
Snail, 293

Valves, of stomach, Crayfish, 211,
212 ; of shell (see Exoskeleton)

Vasa efferentia, 23, 54

Vascular bundle, Bean, 463 ; Fern,
4*6, 453

Vasdeferens, Crayfish, 217; Earth-
worm, 268; Frog, 23; Snail, 291

Vein, afferent hepatic, 82 ; afferent
renal, 20, 83 ; anterior abdo-
minal, 20, 37, 38, 44, 82 ; ax-

illary, 85 ; cystic, 82 ; dorso-
lumbar, 83 ; duodenal, 88 ;
efferent hepatic, 20, 84 ; effer-
ent renal, 20, 84 ; epigastric, 82 ;
external jugular, 84 ; femoral,
83 ; gastric, 88 ; genital, 20, 84 ;
great cutaneous, 20, 37, 85 ;
haemorrhoidal, 88 ; hepatic-por-
tal, 20, 82, 87 ; inferior cava,

17, 20; innominate, 85; in-
ternal jugular, 85 ; lieno-intesti-
nal, 88 ; lingual, 84 ; oviducal,
83 ; pelvic, 82 ; posterior car-
dinal, 169 ; pulmonary, Frog,

18, 21, 85; Snail, 292; renal
portal, 20, 83, 89 ; sciatic, 83 ;
subclavian, 85 : superior cava,
17, 20; superior mesenteric, 88;
vesical, 82

Veliger, 279, 304

Velum, 279, 304

Venous system, Crayfish, 181 ;
Frog, 20; Mussel, 328; Snail,
294

Ventricle, Frog, 17, 8r, 91 ; Mus-
sel, 324 ; Snail, 275

Ventricles of brain, 26, 100

Veronica, 480

Vertebral body (see Centrum)

Vertebral column, n, 55

Vesicle, contractile, Amoeba, 370,
374; Vorticella, 361, 365; of
egestion, Amoeba, 371 ; Vorti-
cella, 362; ofingestion, Amoeba,
371; Vorticella, 362

Vesicula seminalis, 52

Vessel, Bean, 464 ; Fern, 446, 455

Vestibulum, 360, 364

Vexillum, 476

Vibrio, 409, 413

Vicia, 460 — 481

Visceral arch, 160, 161, 164, 170;
cleft, 164, 171 ; pouch, 170, 171;
sac, 273

Visual organ (see Eye)

Vitellus, 24, 270

Vitelline membrane, 155, 220, 270,

,T-339

Vitreous humour, 117

512 INDEX.

Vocal sac, 9, 51 ; ligament, 21
Vorticella, 359, 368

WOOD, 446, 454, 463. 472
Wood-cells, 473

X.

XlPHISTERNUM, 69
Xylem, Bean, 463, 464,
Fern, 446, 448, 449, 454
Xylem-parenchyma, 455

473

Y.

YEAST, 377—388

Yellow-brown tissue, 242, 243, 267

ZONITES, 241, 248

Zoogloea, 409, 412
Zoospores, 391, 393
Zygapophyses, 56
Zygospore, Mucor, 423 ; Spiro-
gyra, 4or, 407

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