Marine Biological Laboratory

Rp^^.VpH August 21, 1958

Accession No..

73939

Given By
Place

Oxford University Press

New York Gitv

BORRADAILE'S

MANUAL OF

ELEMENTARY ZOOLOGY

i
J

BORRADAILE'S

MANUAL OF

ELEMENTARY
ZOOLOGY

THIRTEENTH EDITION
Revised by

W. B. YAPP, M.A.

Lecturer in Zoology, Uni'uersity of Birmingham

LONDON

OXFORD UNIVERSITY PRESS

NEW YORK TORONTO

1958

Oxjord University Press, Amen House, London E.G. 4

GLASGOW NEW YORK TORONTO MELBOURNE WELLINGTON

BOMBAY CALCUTTA MADRAS KARACHI KUALA LUMPUR

CAPE TOWN IBADAN NAIROBI ACCRA

@ Oxford University Press, ig^S

First Edition 191 2

Second Edition 1918

Third Edition 1920

Fourth Edition 1923

Fifth Edition 1926

Sixth Edition 1928

Seventh Edition 1932

Eighth Edition i935

Ninth Edition 1938

Tenth Edition 1941

Eleventh Edition i945

Twelfth Edition 1955

Thirteenth Edition 1958

PRINTED IN GREAT BRITAIN BY
MORRISON AND GIBB LIMITED, LONDON AND EDINBURGH

*v*

PREFACE

The twelfth edition of Borradaile, the first major revision since
the book was pubhshed forty-five years ago, has on th(.' whole
been well received both by reviewers and by readers. Complaint
was commonly made of the illustrations, both old and new, and
with some justification. The necessity for a new edition less than
two years after the edition was published has allowed the replace-
ment of some forty-three of these, and I hope that in future
editions more will be treated in the same way. I am grateful to
all those who have pointed out misprints or lapsus calami, all of
which have been corrected. I thank also those who have reported
errors; if a reviewer finds that any of these remain uncorrected
it is because, after consulting the best authorities available to me
(including, in two instances, the animal itself) I have come to the
conclusion that what I originally wrote was nearer to the truth.
I have also tried to remove a few ambiguities, and statements
throughout the book have been corrected to accord with new
knowledge.

The interest that the book has aroused is very pleasing, but
there is one respect in which I cannot agree with some of my
critics, who would have liked to see the old morphological basis
entirely abandoned. While I agree completely with them on the
importance of physiology and ecology, I believe that there is
still good reason for not trying to force on students a fusion of
the morphological, physiological and ecological aspects of the
subject, which, at the level for which this book is written, few of
them will understand. I have retained the type-system not, as
one reviewer said, because I have never known any other, but
because I have seen, as a teacher, as an examiner, and as an
attender at scientific meetings, the bad results of attempts to be
'synthetic' or 'comparative' before the student knows the basic
facts of morphology. They simply do not work. The sections in
this book on parasitism (now put as a separate chapter) and on
the coelomate body, and those on vertebrates at the end, read after
those on the relevant types, will give the student of sixteen to
nineteen or so, enough on which to exercise his mind (I hope he
will do so critically). I originally intended to include a general

vi MANUAL OF ELEMENTARY ZOOLOGY

chapter on physiology, and may still do so in a later edition.
But, with some experience of reducing the whole range of
physiology to small compass, I am not sure that such extreme
compression can usefully be done.

W. B. Y.

Stourbridge,
August 1957.

CONTENTS

1. Introductory : The Animal Organism

2. Amceba .....

3. Flagellate Protozoa

4. monocystis ....

5. CiLiATE Protozoa

6. The Protozoa as Parasites of Man

7. The Classification of Protozoa .

8. Sponges .....

9. Hydra and Obelia .

10. Flatworms ....

11. Roundworms ....

12. Parasitism ....

13. Earthworms and other Annelids

14. The Ccelomate Body.

15. The Crayfish and other Arthropods

16. Cockroaches

17. Insects .

18. Molluscs

19. The Starfish

20. The Lancelet

21. The Dogfish

22. The Frog

23. The Pigeon

PACE

I

26

43
46

62

78

82

89
118

134

14;

157
187

195
225

240

264

286

299

311
343
392

7S939

VIU

MANUAL OF ELEMENTARY ZOOLOGY

PAGE

24. The Rabbit ...... 417

25. Mammals ...... 474

26. The Cell and Mammalian Histology . . 498

27. The Classification and Structure of Verte-

brates . . . . . . 542

28. Embryology . . . . . . 620

29. Heredity and Cell Division . . . 680

30. The Origin of Species .... 706
Index ....... 727

1

INTRODUCTORY: THE ANIMAL ORGANISM

BIOLOGY

Zoology, the science of animals, is a branch of biology, the
science of living beings. Of such beings there are various kinds
besides the animals ; but all the kinds have important features
in common. Rightly to understand any animal is therefore to
comprehend properties which it shares with non-animal living
beings as well as its purely animal characters. Thus at the outset
of our study of zoology it is desirable that we should spend a
little time in considering the nature in general of living beings.

LIVING AND LIFELESS

Out of the multitude of material objects that surround us,
we distinguish some, as alive, from the rest, which are lifeless
— that is, either are dead or have never been alive. It would be
helpful if we could set down in precise terms the properties on
which we base this distinction.

The commonsense view is that a living being, such as a cow
or a tree, grows and reproduces, while a non-living thing, such as
a stone or a piece of wood, does not, but a little reflection will
show that no precise distinction can be made in these terras. A
cow may be capable of reproduction, but for a variety of reasons,
such as absence of a bull or infection by a parasite, may not
in fact produce offspring ; an ox can never reproduce, but is
nevertheless alive, and if it be objected that an ox is in an artificial
state through the interference of man, there is the parallel
example of the worker bee, which is sterile by nature. It is a
matter of common observation that growth, in the ordinary sense
of the word, ceases in man relatively early in hfe, and in old age
it becomes negative, if the expression may be permitted. In
some of the lower animals, too, ' degrowth ' is known ; thus, if
a pond fiatworm is starved it decreases in size and appears to
reverse its normal trend.

No better definition of life can be made from any of the other
properties which have from time to time been proposed for the

THE ANIMAL ORGANISM

purpose. Neither irritability, nor self-preservation, nor respira-
tion is at all times characteristic of things which are called
living, and one or more of these properties (as well as growth and
reproduction) may at times be found in things which are called
non-living. In the limit, it remains a matter of opinion w^hether
the viruses (small particles which cause a number of diseases
such as smallpox and measles) should be considered as living or

non-living.

In spite of the impossibihty of making a definition, there are
certain properties which are generally found in things which men
tacitly agree to call living, and which are seldom found in things
which they do not so call. We w^ill now briefly consider the more
important of such properties.

GROWTH

Growth means the increase in quantity of material in a body,
and is usually measured, for biological purposes, as increase in
dry weight (this word being used incorrectly for mass), that is,
weight after free water has been driven off at 105° C. It thus
excludes mere addition of water, which may be so rapid and tem-
porary as to be meaningless, and differs fundamentally from
increase in size, which may occur by absorption of water or even
air. It depends on an uptake of matter from the surroundings,
and on this more is said below.

REPRODUCTION

An animal or plant may, by itself or in co-operation with
another, produce a new living thing, a process known as repro-
duction. In a sense reproduction is a consequence of growth, for
growth without reproduction would lead to an impossible expan-
sion in size ; this connection is seen at its clearest in an animal
Hke Amceha which simply divides into two when it reaches a
certain size, but in the higher animals the connection is obscured.
The division of a cell of a higher organism (p. 688) though it may
take place by a similar mechanism, is philosophically distinct
from the reproduction of Amceha.

Reproduction always includes, though it may be much more
than, the fission or division of an existing body. Whatever may
have been its origin, all the evidence suggests that under the

REPRODUCTION 3

conditions which now exist hfe never starts anew, but is always
passed on from one Hving being to another which arises from it.
A living being which divides to produce others is a parent ;
those which it forms are offspring. These are always at first unlike
the parent. There are certain creatures, like Amoeba, mentioned
above, in v^hich the only evident difference between the offspring
and the individual by whose division they arose is the necessary

Fig. I. — Human ovum from the uterus. X 480. — From Hamilton, Boyd and
Mossman, Human Embryology, 1945. Heffer and Sons, Cambridge.

one of size, but in the great majority of cases there is also an obvious
difference in form, the offspring being at first very unlike the parent
in structure. This difference is obscured in the case of man and
some other animals, where the offspring (Fig. i) undergoes
changes in the womb before birth, but it is seen unmistakably in
animals which are born in the condition of an egg. In their
immature condition the offspring are known as reproductive

bodies.

In spite of this unlikeness at starting, the oftspring become in
time Hke the parent or parents from which they arose, though

THE ANIMAL ORGANISM

they never resemble them in every detail. The succession of
changes which brings this about is called development, and is
sometimes straightforward, or direct, sometimes, as in the well-
known case of the butterfly, very roundabout, or indirect. In
reproduction by budding (Chap. 9) development may take
place partly or mainly before fission. Thus the life of an animal
or plant is a cycle, in which it passes through a series of stages,
beginning with the small and simple reproductive body, and
ending with the larger and usually more complex adult, ready
to undergo fission again. Every individual goes through the same
cycle of changes as its parent, resembling in each stage a similar
stage passed through by the latter, till it reaches the likeness
of the individual that produced it, that is, it shows the property
known as heredity. Thus, in the strict sense of the word, repro-
duction includes the whole life cycle and consists of two distinct
processes — fission, and the development of the reproductive
body into the adult — for until this cycle has been completed
the parent is not reproduced. From this point of view, growth is
that part of the process of development by which the reproductive

body reaches the size of the adult. At the
same time, usually, and perhaps always, the
growing individual is undergoing the changes
in structure to which we have alluded.

SYNGAMY

Here must be mentioned a process which
is an essential part of reproduction in many
organisms and in all the higher animals. In
such organisms the reproductive bodies are
of two sorts, each produced only by one of
the sexes, and neither sort can develop
except after fusion with one of the other
sort. That fusion is an example of the
process known as a syngamy, union of two
distinct living bodies, which occurs from
time to time in nearly all species of animals
and plants. The bodies which unite are
known as gametes, and that which results
from their fusion as a zygote. In some of
the smallest living beings (Fig. 2) syngamy

Fig. 2. — Copromonas, a
minute inhabitant of
dung. — After Dobell.

a, Adult individual ; b, the same
in fission ; c, two adult in-
dividuals in syngamy;
d, the zygote, enclosed in a
cyst.
.v.. Food vacuole;^?., flagellum ;
g., gullet; nu., nucleus; res.,
reservoir of contractile
vacuole (see p. 41),

SYNGAMY

is the union of fully-grown adults, but in other such creatures
(Fig. 8), and in all large and complex animals and plants,
syngamy takes place only between the reproductive bodies,
which are generally unable to develop without it, so that it
becomes a necessary part of the reproductive process. In these

creatures the reproductive
bodies are of a kind known
as germ cells, distinguished
from other reproductive
bodies (free buds, etc.) by
their small size and the sim-
plicity of their structure.
The germ cells of such
creatures are usually of two
sizes which unite larger with
smaller (Fig. 8C). In all
large and complex animals

Fig. 3. — Heads of human spermatozoa, in side and face view. Electron photo-
micrographs. The scale is one micron. — From Friedlander Proc. roy. Soc. B.,
1952, 140.

(and in some of the smallest) the gametes differ in form and
behaviour as well as in size (Figs, i and 3). One is larger and
passive, and is called the female gamete, or, in large animals, the
egg or ovum. The other is smaller and active, and known as
the male gamete or spermatozoon or sperm ; it has usually a tail
(flagellum) with which it swims in the fluid in which it is borne,
and thus it moves to the e^g and enters the latter (Fig. 4). This
process is known as the fertihsation of the ovum. After it the
fertilised ovum proceeds to develop. Ova and spermatozoa are
usually formed by different adults, known respectively as female

THE ANIMAL ORGANISM

O.M

and male, but in some animals both kinds are formed by one
individual, which is then known as a hermaphrodite. If sperms
are formed before ova, a hermaphrodite is said to be protandrous ;
if the ova are formed first, it is protogynous. In some aquatic
animals the gametes are set free, and syngamy takes place out-
side the body of the parent. In others, however, and in all land
animals, the ova are kept within the body of the mother, and the
male gametes are transferred in the seminal fluid by the male
to the body of the female and there fertilise the ova. This trans-
ference is known as coition or copulation. Reproduction in which

syngamy is necessary before the
reproductive bodies can develop
is known as sexual reproduction ;
that in which the reproductive
bodies are not gametes is asexual.
In some animals there is a
kind of reproduction (partheno-
genesis) in which a female germ
cell (ovum) develops without
syngamy. This kind is best re-
garded as an aberrant form of
sexual reproduction.

The terminology of these pro-
cesses is in some confusion.
Syngamy is the fusion of two
cells (p. 498), nucleus with
nucleus and cytoplasm with cyto-
plasm, though one of the two
may have little cytoplasm and possibly sometimes has none. The
union of nuclei — which is by far more important than that of
the cytoplasms — is karyogamy ; the union of cytoplasm is plas-
mogamy. The term conjugation has been used as a synonym
for syngamy but is best restricted to the peculiar procedure by
which syngamy is accomplished in the Ciliata (Chap. 5).

Fig. 4. — The"^ovum of a bat, after
the entry of the spermatozoon. —
From F. H. A. Marshall, after
van der Stricht.

o.n., nucleus of the ovum ; p.b., ' polar
bodies ' (see p. 620) ; spn., spermatozoon.

ACTIVITY

Most familiar animals, and some plants, may be said to be
active in a way in which a stone is not, although machines have
a certain type of activity which simulates that of living things.
The activity may be the result of the receipt of a stimulus, that

ACTIVITY 7

is a change in the external environment, or it may occur without
apparent cause. The first, examples of which are the shaking of
the head of a dog when the hairs in its ear are touched and the
folding and falling of the leaf of a sensitive plant when it is
knocked, has been called irritabihty, and the second, which ma\-
be illustrated by the beating of the heart, automatism. In man\-
cases a distinction is difficult. Irritabihty differs from mere
mechanical change produced by external circumstances, such as
the melting of ice, in that the magnitude of the response, as
measured by the energy involved, bears no relation to the
magnitude of the stimulus. It follows from this, and from the
first law of thermodynamics, that the energy for the response
must come from within the organism.

ABSORPTION AND ASSIMILATION

Growth requires the intake of new material, and response
needs energy, which depends on the breakdown or conversion of
chemical substances. The incorporation of food is therefore
characteristic of living matter. Two distinct processes may be
recognised in incorporation — absorption and assimilation. Before
it can be absorbed the food of animals has generally to undergo
a preliminary process of digestion, whereby solid or indiffusible
nutriment which it contains is made soluble and diffusible. The
food must always contain the following materials : (i) water,
which is of the highest importance both as an essential con-
stituent of the living matter (protoplasm) and also because it is
used in the body for transporting substances in solution, as in
the blood and urine, (2) certain inorganic ions, such as chloride
and phosphate and those of sodium, potassium, and calcium,
(3) the very complex compounds known as proteins. A protein
is a colloid substance consisting of carbon, hydrogen, oxygen,
and nitrogen, with sometimes small quantities of sulphur and
phosphorus. A familiar example is the albumen which, mixed
with water, forms white of egg. Proteins are very complex
linkages of amino-acids, that is, compounds which contain both
the basic radicle — NHg and the acid group -COOH. A simple
example is aminoacetic acid or glycine, CHo.NHg.COOH. Thus
in the complicated chemistry of the body proteins are able to
exercise the power, which amino-acids have, of uniting cither
with acids or with bases ; and on final disintegration they always

8 THE ANIMAL ORGANISM

yield their nitrogen in a form related to ammonia. The proteins
of the body are man}^ and even those of similar parts in different
animals are slightly different. That the food does not consist of
proteins identical with those of the body it is entering does not
matter, since in digestion proteins are resolved into the amino-
acids of which they are composed and the animal so recombines
these as to meet its own needs. The food must, however, supply
the right amino-acids in sufficient quantities. It is found, for
instance, that mice fed upon a diet in which the only protein
present is zein, the protein of maize, which does not contain the
important constituent tryptophane, are unable to support life.
Proteins are important in the food of all animals, because,
while, like other substances that we shall mention below, they
can be oxidised to provide energy, it is normally they alone that
can make good the protein matter that every living body contains
and loses by wear and tear and also that can provide such material
for growth. When they are to be used for fuel the nitrogen is
discharged from their molecules as ammonia. This is deamina-
tion ; it is the ultimate source of most of the nitrogenous com-
pounds which are mentioned below as forming part of the excreta.

Besides these substances the food usually contains (4) carbo-
hydrates (sugars, starches, and related substances), (5) fats. It is
chiefly these two classes of substances that are oxidised to provide
energy. Both contain carbon, hydrogen, and oxygen. In carbo-
hydrates the oxygen is present in exactly the proportions to
oxidise the hydrogen, as in cane sugar and malt sugar or maltose,
which both have the formula C12H22O11, grape sugar or glucose,
CgH^gOg, and starch (CeHioOg);^. In fats there is relatively less
oxygen ; therefore they require for complete combustion more
of that element than is needed to oxidise the carbon, and their
potential energy is greater than that of carbohydrates. In
digestion, insoluble carbohydrates, such as starch, are dissolved
by conversion into glucose or other simple sugars, and fats are
partially split into soluble components — fatty acids and glycerol.

Both these processes, and also the digestion of protein, are
hydrolyses — decompositions into smaller molecules with the
aid of water taken up. Thus :

(Starch) (Maltose)

C12H22O11 + H2O =2C6Hi206

(Maltose) (Glucose)

ABSORPTION AND ASSIMILATION

and again :

(Ci7H35COO)3C3H5+3H20=3Ci7H35COOH+C3H5(OH)

(the fat Stearin)

(Stearic acid)

(Glycerol)

They are all initiated by organic substances called enzymes
(p. 444), which take part in the reaction but are restored at the
end. Enzymes are named by the addition of the suffix -ase to the
name of the substrate on which they act.

Since proteins, carbohydrates, and fats are among the com-
pounds known as ' organic ', which, in nature, are found only in
the bodies of plants and animals and
in their remains, such bodies are a ^°
necessary part, and the chief part,
of the food of all animals. From the
same source animals must also
obtain (6) other organic substances
needed in small quantities. These
include the vitamins. These sub-
stances, originally manufactured by
plants, are transmitted to herbi-
vorous animals, and so to the
carnivores, and though needed in
very small quantities, are essential
to life. When, for instance, young
rats are fed upon an artificial liquid 30
containing the protein, sugar, and
fat of milk in the usual proportions,
they fail to grow, but the addition

70

50

/

s.

20

^J

Fig. 5. — Curves showing the effect
of vitamins on the growth of
rats. — From Hopkins.

to their diet of a very small quantity Lower curve (white circles), rats fed on

r r -I Mi/i-i J • 1 artificial milk alone. Upper curve (black

01 iresn milk (Wnicn contains the circles), rats fed on artificial milk and 2

• . , . , c.c. of cow's milk daily. Average weight

vitamins) causes them to grow in a in grams, vertical. Time in days, hori-

normal manner (Fig. 5). The

structure and mode of action of many vitamins are now known.

They are nearly all required to enable some particular reaction to

go on ; thus both B^ and B2 (riboflavin) are concerned in cell

oxidations.

The digested materials undergo absorption into the substance
of the body, leaving the indigestible matter to be cast away as
the dung or faeces. Incorporation, however, is not brought about
simply by the absorption of digested matter. Neither before nor
after digestion is the food of the same composition as the substance

10 THE ANIMAL ORGANISM

to which it is to be added. The flesh of a dead ox or sheep differs
considerably in composition from that of a hving man, and the
difference is increased by its digestion. In the course of incor-
poration the food has therefore to undergo chemical changes
by which it is converted into the substances which compose the
body, and these changes it undergoes by the activity of the living
matter itself. That is to say, the living substance has the power
of making, out of unlike materials, additional matter of its own
composition. The process by which this is done is known as
assimilation. Both absorption and assimilation are processes in
which work is done, and therefore involve the use of energy,
but their net result is to add to the amount of material composed
of complex molecules, and therefore to the amount of energy,
in the body.

PROVISION OF ENERGY

The mode in which the living body avails itself of energy
contained in its own substance depends upon the following facts.
When atoms unite to form molecules, energy is generally set
free, and the stabler the molecules formed, the greater, almost
invariably, is the amount of energy liberated at their formation.
The same amount of energy must be used to break up a molecule
as was set free when it was formed. The molecules that compose
the substances from which the body obtains its energy contain
carbon, hydrogen, oxygen, and sometimes nitrogen and other
elements, and are complex and relatively unstable and rich in
energy. The body breaks down these molecules so as to form
smaller and more stable molecules. The energy which is freed
in the formation of the stabler molecules is so much greater than
that which is required to break down those that are less stable
that a large balance of energy is set free, and becomes available
for the work of life. Usually the breaking-down process is con-
tinued until the carbon and hydrogen atoms are in the very
small and stable molecules of carbon dioxide and water — that
is to say, it is a complete oxidation. But it is not always so. The
substances which are broken down never contain enough oxygen
to combine with all the carbon and hydrogen in their molecules,
and therefore many animals and plants which live in surroundings
from which they cannot obtain additional oxygen are unable
to complete the process of disintegration. Thus the fungus known
as yeast, living in solutions which contain no dissolved oxygen,

PROVISION OF ENERGY jj

breaks down the sugar glucose according to the following equation :

C6Hi206=2C2H60+2C02

(Glucose) (Alcohol)

leaving, as an unoxidised residue, ethyl alcohol, which is produced
in this way in the brewing of beer and other fermented drinks.
Similarly, many animal tissues which are temporarily deprived of
oxygen, and perhaps some internal parasites which are perman-
ently short of this element, break down the substance glycogen
(which is related to starch) so as to form lactic acid, according to
the equation :

(C^'R^fi,)n -{-nll.fi =2nC^Rfi^

(Glycogen) (Lactic acid)

Organisms or tissues which carry out such processes as these
are said to be anaerobic. Their mode of obtaining energy, since
it leaves a residue containing energy of which they have not
availed themselves, is wasteful as compared with that of the
majority of living beings, which are aerobic, that is, draw from
their surroundings — air or water (see p. 12) — free oxygen, and
with it complete the oxidation of the substances from which
they obtain their energy. The obtaining of free energy by the
disintegration of complex substances is familiar to us in various
processes employed by man. Thus the energy imparted to a bullet
by an explosive is liberated, like the energy of anaerobic animals
and plants, by a decomposition without importing oxygen, while
the energy of a petrol or steam engine or the light of a candle
is obtained by the use of oxygen from the air in combustion,
like the energy of aerobic beings.

It is not difficult to prove that this disintegration is taking
place in the body. The large molecules break down, as we have
seen, to produce carbon dioxide and water. Since many of the
disintegrated molecules contain nitrogen, there are formed also
certain fairly simple nitrogenous compounds, such as urea,
CO(NH2)2- The intake of oxygen and loss of carbon dioxide
during life are easily demonstrated. Men or animals enclosed in
a vessel to which air has not access are unable to live for more
than a short time. The animals are stifled, just as a lire or the
flame of a candle may be stifled, by want of air, and subsequent
examination of the gases in the vessel will show that the oxygen

12 THE ANIMAL ORGANISM

has been depleted and replaced by carbon dioxide, just as it
would be if a candle had been burnt in it. This loss of carbon
dioxide and the intake of oxygen which usually accompanies it
are characteristic of living animals. This gaseous exchange is
sometimes, especially in medical works, called respiration,
though this word is also used for the whole series of processes,
including the chemical reactions, by which energy is obtained.
In man and animals like him, gas exchange takes place through
the lungs, in breathing. If the breath be tested, it will be found
to have undergone the same changes as the air in a vessel in
which an animal has been stifled. Fishes and other aquatic
animals use the oxygen which is held in solution in the water
in which they Hve. They usually respire by means of structures
known as gills, which offer to the water a large surface upon
which gases can be exchanged ; of these we shall consider examples
when we study the crayfish and dogfish. The necessity for
renewing by aeration the dissolved oxygen in the water of an
aquarium is due to the respiration of the inhabitants. The nitro-
geneous w^aste matters may be identified by chemical analysis
in excreta such as the urine of man. The formation of water is
less easily demonstrated, because the bulk of the water lost to
the body has been taken in as such through the mouth to perform
certain indispensable functions, one of which is the washing out
of nitrogenous waste substances, which are harmful, but a careful
comparison of the quantities of water which enter and leave the
body shows that more goes out than has entered.

COMPLICATIONS I V^EAR AND TEAR

While the chemical processes by which energy is liberated in the
body are all of the general character which we have just outlined,
they are nevertheless varied in detail and extremely complicated.
Oxidation takes place, not by single reactions between oxygen and
the substances which are ultimately oxidised, but by chains of
reactions. These cannot here be described more fully. It must,
however, be mentioned that, besides those processes in which by
the disintegration of certain substances energy is liberated, there
is involved a considerable loss of material by wear and tear of
the more permanent part of the living matter in which the oxidised
substances are contained.

13

APPEARANCE OF THE LIBERATED ENERGY IN

VARIOUS FORMS

The energy freed in the disintegration of the body-substance
appears, as we have seen, in various processes. The most char-
acteristic and important of these are contraction, chemical work,
excretion, secretion, and the conduction of impulses. Contraction
is the process by which mechanical movements are carried out.
In it a portion of the living substance changes in shape but
not in size, growing shorter in one direction but thicker in others.
This may easily be felt in the working of any of the great muscles
of the human body, as when the well-known biceps, in short-
ening to pull up the forearm, grows at the same time thicker.
It should be noted that the opposite of contraction, as used in
this sense, is not expansion but relaxation. Instances of chemical
activity are seen in the formation of the constituents of the
many juices which are used for various purposes in the body.
Thus the gastric juice, by which food is digested and dis-
infected in the stomach, contains among other substances hydro-
chloric acid, whose formation in face of the alkalinity of the
blood involves very considerable chemical work. Other examples
of liquids formed for special purposes are the spittle or saliva
which helps in the swallowing and digestion of food, tears which
wash clean the surface of the eyes, and so forth. The regions
in which materials are thus formed are known as glands. Again,
a part of the energy liberated in the body is used in the discharge
of materials from the substance of the body. We have seen that
in the process of disintegration there arise waste products of
which the body gets rid ; with these it casts out poisonous or
excessive materials absorbed from the food. We have just seen
also that certain activities of the body consist in the chemical
manufacture of materials which are not purely waste but have
their uses to the body. The casting out from the substance of
the glands of the materials of these two classes, and of the
water in which they are dissolved, is a necessary part of the
working of the bodily machine. The harmful or excessive products
are got rid of because they are injurious, and the products of
chemical manufacture are removed in order to be of use else-
where. Both kinds of material are accordingly shed, sometimes
upon the surface of the body, but usually into tubes known as
ducts, in which they flow to the required locality. This shedding

14 THE ANIMAL ORGANISM

out is a distinct process, carried out by an exercise of the activity
of the Hving substance of the body. No real distinction can be
drawn between the two cases, but the process is called excretion
when the substances cast out are purely waste, as in the urine,
and secretion when they are of some further use to the body, as
in the gastric juice. Finally, an expenditure of energy is involved
in the conveyance of impulses which bring about events in the
body, from the localities where the impulses are started by
stimuli (p. 6) to the localities in which the events take place.
Thus, when a drop of water which has fallen upon the skin is
to be brushed off, an impulse is started in the skin and conveyed
along those tracts of the body which we know as nerves till it
causes such movements of the muscles of the arms as are necessary
to brush off the drop. This property in living matter of conveying
impulses is known as conductivity, and it involves the evolution
of energy by disintegration in the conducting substance.

It should be noted that the forms in which the energy of the
body is used in these and other processes are very different.
Besides mechanical movement, the exhibition of molar energy,
it may bring about chemical changes, or become heat, as is shown
by its warming the human body, or light, as in the glow-worm,
or electricity, as in the well-known electric eel, and less conspicu-
ously in many events in the human and other living bodies ;
and there are other processes, such as secretion, its action in
which has not yet been certainly compared with any event in
the lifeless world.

METABOLISM

It will be seen that disintegration and its complementary
assimilation constitute a series of chemical changes, continually
taking place in the body, whereby there is kept up a continual
evolution of energy. These changes, regarded as a whole, are
known as metabolism, the disintegrative changes being known as
katabolism and the assimilative as anabolism.

THE STRUCTURE OF LIVING MATTER

The characteristics of living matter with which we have dealt
so far have been functional, that is, they have been concerned
with processes or actions, but there are also others which are
structural, or concerned with the form which living matter takes.

STRUCTURE OF LIVING MATTER

15

The first is that Hving bodies always contain the substance
known as protoplasm ; in fact most of the processes characteristic
of life go on only in the protoplasmic parts, the others being by
comparison Hfeless. Protoplasm will be considered in more detail
later, but it may be said here that it is an aqueous solution in
which protein is the most important constituent. The other parts
of the body are formed material, made by the protoplasm ;
an example is the ground substance of bone (Fig. 403), consisting
largely of salts of Hme, to which it owes its hardness.

.^.s.

ABC

Fig. 6. — Portions of animal tissues, highly magnified, to show cells.

A, The lining of an artery ; B, muscular tissue from the wall of the intestine ; C, the lining of the intestine.

A and B are shown in surface view, C in section.
c, Cells ; g.s., ground or intercellular substance, traversed by threads of protoplasm from cell to cell.

The second structural feature of living matter is that it possesses
a considerable degree of organisation. In many animals, as we
shall see in a later chapter, the protoplasm is not continuous,
but is arranged in a number of minute units known as cells
(Fig. 6). In each cell a small protoplasmic body, the nucleus,
acts as a regulative centre, and on the surface the protoplasm is
modified to form a cell wall. If the protoplasm is not divided
into cells, nuclei are still present.

ORGANS

Apart from the microscopic division of its protoplasm, the
living body consists of a number of parts each of which does a

l6 THE ANIMAL ORGANISM

particular portion of the work of the whole. Such parts are called
organs. Thus animals may have sense organs, such as the eyes and
ears, for the reception of stimuli ; nervous organs, forming a
nervous system (usually provided with a central station such as
the brain), for the conduction of impulses set up by these and
other stimuli, to the organs which carry out the main part of
the reaction ; locomotive organs, such as legs and wings and fins,
to carry the body towards food or from danger ; organs of offence
and defence, such as teeth and claws, for procuring food and
resisting attack ; organs of digestion, such as the stomach and
bowels ; organs of circulation, such as the heart and blood vessels,
which distribute digested food, carry waste matters to the
excretory organs, such as the kidneys, and gases to and from
organs of respiration, such as lungs and gills, and transport
materials in general ; and organs of reproduction. An organ may
consist of subsidiary organs. Thus the leg is supported by skeletal
organs known as bones, moved by muscles, and served by blood
vessels and nerves. A complex of parts which work together is
known as an organism, and this name is often applied to animals
and to plants, for plants also are provided with organs, and are
alive. The provision of separate organs for particular functions
is called organisation or differentiation ; the assignment of par-
ticular functions to separate organs is called, by analogy with the
similar separation of functions in modern industry, the division
of physiological labour. This exists to a very various extent
among animals, and of two animals that which has the larger
number of different organs is said to be the more highly organised
or more highly differentiated, or simply the higher, though this
last word is also used in a slightly different sense in connection
with the theory of evolution (Chap. 30). The higher the organism,
the greater is its efficiency in coping with its surroundings, the
greater the vicissitudes in them which it can survive. There are
also great differences in form between the organs of animals of
the same grade of organisation. Thus a butterfly is as highly
organised as a fish, but its organs are utterly different in form.
The differences in structure between animals may correspond to
differences in their modes of life. Many animals which live in
water have, for instance, very different organs of locomotion
and respiration from those which live on land ; the sense organs
of an internal parasite are much less highly differentiated than
those of an animal which has to seek food and avoid enemies

ORGANS j^

from hour to hour ; and a carnivorous animal has organs for
seizing and eating its food which are different from those of one
whose diet is vegetarian. This correspondence between organisa-
tion and mode of Hfe is known as adaptation.

TISSUES

Organisation involves more than the mere localising of functions
— more, that is, than the existence in the body of regions where
special functions are performed. It involves also a specialisation
of each of these regions to fit it for its special functions. This
speciahsation is found partly in the shape of each organ, but also
largely in its texture and composition. The substance of the body
is not alike throughout, but different portions of it have differences
in texture and chemical composition which confer upon them
different properties. Thus the outer layer of the skin is firm and
hard to penetrate, bone is rigid, blood is fluid, the substance
known as connective tissue is tough and binds other tissues
together, nerve has the power of conduction highly developed,
and muscle that of contraction, and so forth. Such a portion of
the body-substance with particular properties, due to a particular
texture and composition, is known as a tissue. An organ may
consist of one tissue throughout, but is usually built up of several,
upon the nature and arrangement of which its powers depend.
Thus a muscle contains, besides muscular tissue, connective
tissue to bind it together and nervous tissue to conduct through
it the impulses which cause it to contract.

CO-ORDINATION

Many of the processes which go on in living organisms lead to
action upon the outer world, but others are directed only to
keeping the machine in condition. The needs of the several
organs in the way of food, oxygen, and the removal of waste,
are very different, and vary from time to time with the activity
of the organ. Often, too, the activity of one organ must be
accompanied by an increase or depression of that of some other
organ, as when heavy work by muscles calls for a release by the
liver of fuel in the form of sugar, or in an active gland or muscle
the walls of the blood vessels relax their contraction and so
allow a better flow of blood through the working tissue. Again, in

l8 THE ANIMAL ORGANISM

growth the formation of the various parts of the body needs very
strict adjustment. In all such respects the processes of the body
are subject to co-ordination. This is effected in animals by the two
systems of communication within the body — the blood vessels
or other transporting system, and the nervous system.

Substances secreted into the blood by various organs affect the
working of other organs which they reach in the course of the
circulation. Some of these substances are not produced ad hoc.
Thus the carbon dioxide passed into the blood by active organs
as a result of the oxidation going on within them alters the degree
of acidity or alkalinity of the blood, and this regulates the
quantity and quality of man's blood supply, the acidity causing
small local blood vessels to dilate, so that the active organs
are flushed with the blood they need, and stimulating the part
of the brain which governs respiration, so that rapid breathing
oxygenates the blood and removes the excess of carbon dioxide.
But the most remarkable instances of regulation of this kind
are effected by the secretion in small quantities of very powerful
special agents known as hormones. Various organs despatch
these, but the most conspicuous examples of their formation are
afforded by the ductless glands. About them we shall have more
to say later on (see p. 367), and one example of their functioning
must suffice here. The adrenal bodies, little glands which lie
near the kidneys of backboned animals, are, in moments of anger,
fear, or other emotions which forerun violent exertion, caused,
by stimuli received through the nerves, to discharge into the
blood small quantities of the substance adrenaline. This is carried
round in the circulation and tunes up the body for the crisis.
It increases the flow of blood in the muscles and brain by quicken-
ing the heart beat and constricting the blood vessels of the viscera,
augments the supply of fuel for muscular action by causing the
liver to pour sugar into the blood, and in other ways prepares the
animal for action. The passing of secreta into the blood instead
of into tubes (ducts) to be led to their destination is known as
internal secretion. The other conducting system, the nervous
system, is set into regulative action sometimes by the action of the
blood upon the central nervous organ, as in the case of breathing
mentioned above ; but more often messages sent in along nerves
from organs are translated at the centre into outgoing messages
to other organs, whose action they regulate appropriately. By
them the contraction of muscles, the secretion of glands, the

CO-ORDINATION jg

narrowing or dilatation of blood vessels, the beating of the heart,
to which the pressure of the blood is due, are all affected, and thus
the necessary co-ordination is brought about.

DIFFERENCES BETWEEN ANIMALS AND PLANTS

We have now to observe what are the differences between
animals and the members of the other principal division of living
beings, the plants. There is no fundamental difference in the
composition of the protoplasm which is the essential part of all
living things, nor do they differ in the essentials of their life.
This will be seen if we compare instances of the activities of plants
with those which in the foregoing paragraphs we have drawn
from the lives of animals. That the protoplasm of plants is irritable
we see in such cases as the turning of a sunflower towards the
sun, or the stimulation by gravity of the stem to grow upward
and root downward. That it is automatic appears in such facts
as the slow turning of the tendrils of climbing plants till they
meet with objects to which they can cling. That it has conductivity
can be seen when a stimulus given to the leaf of a mimosa causes
distant leaflets to fold. That it can execute movements may in
many cases be seen under the microscope, when it will be found
to stream round the cell. That it makes substances by chemical
activity and secretes them is illustrated by the long list of drugs
and other substances obtained from plants. That it grows and
reproduces need not be argued. In the sexual reproduction of
the higher (or flowering) plants, the part of the sperm is played
by bodies produced from the pollen, that of the ova by 'egg-
cells ' which are contained in the flowers, in organs known as
carpels.

For all this agreement in essentials, however, there are
between most animals and most plants distinctions which are
both far-reaching and obvious. We may take our start from
familiar notions on the subject. Anyone who tried to state in
words the ideas which he had unconsciously formed of animals
and plants would probably find them to be somewhat as follows :
An animal is a being that moves and feeds ; a plant is a green
thing that grows in the earth. Let us examine these notions.
It will be best to base our analysis upon our definition of a plant.
We find that the information it implicitly contains is : (i) That
the plant is green, (2) that it does not swallow food, but draws

20

THE ANIMAL ORGANISM

nourishment from the earth (the fact that it also obtains food from
the air is less generally known), (3) that it is fixed in one place
and does not move about — usually, indeed, does not move at
aU.

I. The green colour of plants is due to the presence of the
substance known as chlorophyll. This is contained in protoplasmic
structures known as chloroplasts, which
in the green cells of the higher plants
are usually numerous and lens-shaped
(Fig. 7). Chlorophyll is a mixture of
four or more complex compounds of
carbon, hydrogen, oxygen, and nitrogen,
some of which contain in the molecule
an atom of magnesium. It is only found
in those parts of plants which are
exposed to sunlight, and is never found

,cur.

-^ j\ ^t)

Fig. 7. — Plant cells.

A, A small portion of green tissue from a plant. B, Part of a section through a leaf. — From Godwin.
a.s., Air spaces between the cells ; ch., chloroplasts ; c.k'., cell wall ; cu., cuticle ; ep., epidermis surface

layer of cells ; nw., nucleus ; ppm., protoplasm ; s/., stoma (opening through which air enters) ; vac.

vacuole (space containing fluid). The arrows show the paths of diffusion of carbon dioxide.

in multi-cellular animals, except in certain cases where minute
green plants live embedded in the transparent protoplasm of
animal bodies, as in the green hydra (p. 89). At the same time it
must be remembered that certain plants, such as the Fungi, have
no chlorophyll, while many of the simplest animals, the Protozoa,
do have it (Chap. 3).

2. More important than the mere presence of chlorophyll is
its function in the body, which is connected with the nutrition

ANIMALS AND PLANTS! DIFFERENCES 21

of the plant. This function is the obtaining of carbon from carbon
dioxide by means of the energy of the sun's rays, and the use of
it in the manufacture of complex organic substances. Absorbing
certain rays of light, the chlorophyll enables the protoplasm to
use the energy of the rays to reduce (in the chemical sense)
molecules of water and carbon dioxide so that the products
can combine to form carbohydrates. This process is known as
photosynthesis, and is accompanied by the liberation of oxygen.
This can easily be shown in the case of water plants, from whose
leaves in sunlight a stream of fine bubbles of oxygen may be
seen to ascend. The carbohydrates are used for the formation
of various organic substances present in the protoplasm of plants,
and in particular of proteins. The nitrogen, sulphur, and phos-
phorus for this purpose are obtained by the plants as salts in
solution in the water which is taken in by their roots, or sometimes,
as in seaweeds, by the whole surface of the body. From this
peculiarity of nutrition arise several other features peculiar
to the life of plants, (i) We have here the reason for the well-
known fact that green plants cannot live indefinitely in the dark,
(ii) While animals, as we have seen, are always taking in oxygen
and giving out carbon dioxide, green plants in the light are
continually taking in carbon dioxide and giving out oxygen.
Yet it must be remembered that the protoplasm of plants
undergoes continually respiration like that of animals although
this is obscured by the reverse process taking place to a greater
extent during daylight, and that some animal tissues can assimi-
late carbon dioxide, though not by photosynthesis, (iii) Though
the material included in the protoplasm is similar in the two kinds
of organisms, plants manufacture its organic components from
simple substances, whereas animals obtain them from other
organisms or their products. Therefore, while the food of animals
consists of complex organic substances, usually in the state of
a solid or liquid protoplasm, and has to be swallowed through an
opening, the materials taken in by green plants are simple
inorganic substances which can be absorbed as gases or liquids
through the surface of the body. It must be noticed, however,
that plants which have no chlorophyll, such as Fungi, and some
animals which live as parasites or in decaying matter, absorb
their nourishment through the surface of the body, but take it
in the form of organic substances, more or less complex, from
the living or dead bodies of other organisms.

22

THE ANIMAL ORGANISM

3. I'Yom the mode of nutrition of plants there follows the
tliird character which we have marked in them. In the great
majority of animals food must be either sought by locomotion
or at least seized by other active movements, as it is, for instance,
in a sea-anemone or Hydra (Chap. g). In plants, on the other
hand, not only is this necessity absent, but, since it is desirable
that they should expose as great a surface as possible to air

.-Cv.

A

B

Fig. 8. — Chlamydomonas, a minute, motile plant.

.A. Ordinary individual. B. B', Two stages in the conjugation of gamrtesof equal sire (isogamy) ; C, C,

Two stages in the conjugation of gametes of diflereut sizes (anisogauiy). The conjugation is ' head on'

in each case.
cv.. Contractile vacuoles ; ck., chloroplast ; cm,, cuticle of cellulose, e, ty^ spot -.ft., flagelluui ; t%u., nucleus:

HI*'., nuclei ol two gametes about to fuse ; f>yr., p\Tenoid (a protoplasmic lK)dy which is ooncexned in

starch formation) ; s.g., starch grains..

and water for absorption — as they do, for example, in leaves and
roots — the shape of their bodies is necessarily such as to be an
actual hindrance to motion. Thus in most plants active motion
is restricted or absent, and muscular and nervous tissues are not
found in plant bodies. Certain microscopic aquatic organisms,
however, chielly unicellular Algie, are exceptions to the rule
that locomotion accompanies the animal mode of nutrition only.
Though they have one or more chloroplasts and nourish them-

ANIMALS AKD PLANTS: DIFFERENCES -y-y

selves like plants, their body is compact, shaped like an egg or
a spindle, and possesses one or more hne lashes of protoplasm
(flagella), by the working of which it is rowed or dra^^-n t" :h
the water. Many of them, including the exa: CkUi nas

sho^^-n in Fig. 8, have a pigment spot which is a sense organ for
the necessary- appreciation of hght, whose rays it absorbs. While
some of these unicellular creatures are undoubtedly plants, and
others, in spite of their chlorophyll, are best regarded as animals,
some, such as Euglena (p. 39;, are difficult to place. The line
between animals and plants, like that between li\-ing and non-
living, can only be arbitrarily drawn.

4. The necessity for a large surface leads to a fourth character in
plants. An extensive surface needs strong support. In correspon-
dence \^ith this need we find in plants a massive skeleton which
forms a strong wall to each cell, so that the protoplasm is upheld
by an intricate framework of compartments whose walls are
thickest in the most woody parts of the body. Owiug, no doubt,
to the ample supply of starch at the command of the plant, this
skeleton usually consists of a modified form of starch kno^/^-n as
cellulose. Some groups of plants use other substances, and though
cellulose is rare amongst animals it is present in some Protozoa
and in tunicates (p. 310).

THE BALANCE OF NATURE

The difference in nutrition between animals and plants has the
important result that in their action upon the inorganic world
these two kinds of organisms bring about precisely opposite
changes, and do so in such a way that each sets up conditions
favourable to the activity of the other. The plant, absorbing
the energy of the sun's rays, builds up complex organic compounds
from simple inorganic substances and in so doing stores chemical
potential energy*. Though it destroys some organic substances
in respiration, the net result of its activity is to increase the stock
of them in the world. At the same time it sets free oxygen. The
animal, on the other hand, uses as food the substances manufac-
tured by plants, taking them either directly from plant bodies
or after thev have been incorporated in a somewhat altered
form into the protoplasm of other animals, A rabbit feeding on
grass and a stoat feeding on the rabbit and a parasite feeding on
the stoat are equally dependent on the plant for their organic

24 THE ANIMAL ORGANISM

food. Such series of dependencies are called food chains. In the
protoplasm of the animal these organic substances undergo
destruction, in consequence of which there are set free carbon
dioxide and simple nitrogen compounds. Thus plants provide
food and oxygen for animals, while animals, destroying this food,
provide simple nitrogen compounds and carbon dioxide for the
use of plants. Since the nitrogen compounds actually produced
by animals can mostly not be used by plants, the presence of
bacteria is necessary to change them to nitrate, the form in which
nitrogen is generally absorbed.

ZOOLOGY : PLAN OF STUDY

Biology comprises botany, which deals with plants, and zoology
which deals with animals. Now an organism may be regarded
from two points of view according as attention is concentrated
upon its structure or its functions, though of course these two
are so closely connected that it is impossible to study structure
intelligently or function at all without reference to the sister
topic. The sciences of zoology and botany are correspondingly
divided each into two subordinate sciences, anatomy or
morphology, which deals with the structure of the bodies of
organisms, and physiology, which deals with their functions.
Anatomy is sometimes used as a synonym for morphology, but
is often, especially by botanists, taken in a slightly more restricted
sense to mean the detailed study of structures, morphology
being restricted to general form.

In this book we shall approach zoology chiefly from the
anatomical side, partly because our knowledge of the physiology
of animals in general is still fragmentary, but chiefly because
knowledge of physiology must of necessity be grounded in
previous knowledge of anatomy. Wherever possible we shall con-
sider the functions of the structures we find, but we shall avoid
imputing imaginary functions to structures which have not
been adequately studied. We shall begin with some of the
structurally simplest animals, and proceed through the animal
kingdom in what is now generally considered to be an order of
increasing complexity. In so doing we shall be following what,
according to the hypothesis of organic evolution, is roughly the
line which animal life has taken in its temporal development
through the ages. Before we consider the evidence for evolution in

zoology: plan of study 2S

Chapter 30, we may accept it as a useful working hypothesis,
which makes, as it were, a thread on which to hang the discrete
beads of our knowledge of particular animals. Of necessity we
shall describe and illustrate examples or types, but at the end
of each chapter shall consider the more general characters of
groups of animals. When we reach the rabbit we shall consider
physiology, and also minute anatomy, in rather more detail,
and those who prefer may begin with that chapter. Finally,
we shall discuss certain topics such as evolution and heredity
which concern animals in general.

Proper consideration of the classification of animals must be
postponed to a later chapter (p. 706), but something must be said
here to explain the terms used in the following pages. Common
sense early recognised that animals are not all the same, but that
some are alike enough to each other to be included under a
common name. Thus all cats are recognisably cats, and are clearly
not dogs ; further, cats produce kittens, which in time become
cats, while dogs produce puppies which in turn become dogs.
To a set of animals distinct enough from all the rest to be called
by a separate name, but like enough to each other to be not
worth subdividing, the zoologist gives the name species. A set
of species with many similarities makes a genus. Genera are
collected into families, these into orders, these again into classes,
and classes into some dozen or so phyla, which comprise the
whole animal kingdom. Occasionally a species is so different
from all other species that it has to occupy a genus, a family, or
even a class by itself. For convenience in the handling of groups
with very large numbers of species, divisions such as super-
families and subclasses are interpolated. Families and higher
groups are given names which in form are always Latin plurals.
Every genus has a name which is a singidar Latin noun, and its
species are particularised by adding an adjective, also Latin,
which agrees in gender with its noun and is called the trivial
name, the two together being the specific name. All these except
the trivial names are written with capital initial letters, and
generic and trivial names are usually italicised. F'amily names
are usually derived by adding — idee to the root of the name of
one of the constituent genera.

M.z. — 2

AMCEBA

The Protozoa are structurally simpler than other animals in
that their bodies are not divided into cells ; each individual con-
sists of a mass of protoplasm with some sort of boundary layer and
containing a single nucleus, or occasionally two nuclei or more.
We shall begin with the genus Amceha, not because it is really
the simplest of Protozoa but because it is easy to observe. There
are several species, which agree in a number of points. The
protoplasm is divided into a clear outer ectoplasm and an inner
granular endoplasm ; the surface is specialised as a thin
plasmalemma, of dough-like consistency, but there is no secreted
cell -wall ; and from the surface there are put out from time to
time one or more blunt finger-like processes called pseudopodia.
One of the largest species is Amceha proteus, which is about one-
hundredth of an inch across and lives in the mud of ponds,
though it is not very common. A. lescherce (Fig. 9) is larger and
A. discoides (Fig. 11) smaller, but they are otherwise difficult for
the student to distinguish. The following account applies, unless
otherwise stated, to all three species. A. proteus produces
about three or four pseudopodia at a time, all of which are of
relatively large size and sub-cylindrical in shape. There is normally
one nucleus, which is more or less centrally placed, but about
five per cent, of individuals have two, three, or occasionally four
nuclei. The nucleus is only just visible in living specimens, but
when the animal is killed and stained with certain dyes, the
nucleus is conspicuous, since it takes up the colour more deeply
than the cytoplasm, and can be seen to be lens-shaped. Just
behind the nucleus is a clear spherical space, the contractile
vacuole. Besides the granules, the endoplasm contains several
small animals or plants which have been taken in as food.

MOVEMENTS

It is common, but incorrect, to describe Amoeba as shapeless ;
in fact each species has a characteristic form, by which it can be
recognised, though it is true that there is more variation in that

a6

•^/"^ ^

x\

k' -o-"-*

.-«■- ,

-y

p/

1 - »» ■"'.■

I . ^- - ,

C HROWN KBl-^Y

Fig. 9. — Amceba lescherce. Drawing of a living individual. — From Taylor and

Hayes, 1944. Quart. J. micr. Set. 84, 295.

C, crystals ; C.V., contractile vacuole ; F.V., food vacuole ; N, nucleus ; N.S., nutritive sphere, x c. 200.

28

AMGEBA. PHYLUM PROTOZOA

form than is usual in animals. A pseudopodium begins as an out-
flowing of the ectoplasm, into which the endoplasm presently
flows. The projection grows, and in time so much of the body has
moved into it that protoplasm has to be drawn up from behind
to take its place, and locomotion has occurred ; this is known as
amoeboid movement (Fig. lo). Subsidiary pseudopodia are formed
at the side of the main one, but these are eventually withdrawn.
It appears that whatever happens to the amoeba, it is always the

Fig. io. — Successive changes in shape of an individual of Amoeba proteus,

drawn at intervals of two minutes.

same end which leads, so that in spite of its apparent irregularity,
the animal really has right and left sides. In another species
{A. Umax) which forms only one pseudopodium and is therefore
easier to observe, it has been shown that the formation of pseudo-
podia takes place in the following manner. The ectoplasm and the
outer part of the endoplasm together form a firm coat, the plas-
magel, around the fluid inner endoplasm or plasmasol. Where
a pseudopodium is to be thrust out the plasmagel softens, and
the contraction of the rest of that layer then presses the plasmasol
towards this spot, which bulges. As the bulge grows, a covering
of plasmagel for its flanks is provided by conversion of plasmasol.

MOVEMENTS 2Q

The plasmalemma, which is sticky, adheres to the ground where
it is in contact, so that the effect of the forward thrusting of the
protoplasm within is to roll it along, as an india-rubber bag
filled with water may be rolled over a surface, and thus the
animal travels in the direction of the thrust. Those pseudopodia
of A. protetis which do not touch the ground merely protrude
without causing locomotion, but the creature may place their
tips upon the ground and thus walk upon them. When it is
floating freely it puts out slender, finger-like pseudopodia and
appears to be searching with them for foothold. During the
movements the contents of the endoplasm — nucleus, food
particles, etc. — are carried about freely from place to place in the
body, but the contractile vacuole adheres to the inner surface of
the ectoplasm and moves with it.

NUTRITION

Amceba feeds on small organisms, which it ingests by surround-
ing them, together with a drop of water, with outgrowths of its
protoplasm and so engulfing them. The space in the body which
the prey comes to fill would thus be lined with ectoplasm, but the
ectoplasm here becomes absorbed into the surrounding endoplasm,
so that it is clear that there is no essential difference between
the m^aterials which compose these layers. There are then secreted
around the food particle substances which kill it and digest its
nourishing part. The space containing the digestive juice is known
as a food vacuole, and its reaction while digestion is going on is
acid. The chief food of Amceba is protein, but A. proteus and other
species can digest fat. Their ability to deal with carbohydrate is
doubtful. The dissolved substances are incorporated, and the
undigested parts are egested by the simple process of being left
behind as the animal flows along. Different species have different
food preferences and Amceba proteus does not feed on diatoms but
can survive indefinitely on the small ciliate Colpidium, which is
found in infusions in association with Paramecmm (p. 46).

IRRITABILITY, AUTOMATISM AND CONDUCTIVITY

The protoplasm of Amoeba is irritable, automatic, and con-
ductive. Its irritability is not, as in higher animals, localised in
sense organs, but that this property exists in it is shown in

30 AMCEBA. PHYLUM PROTOZOA

various ways. If Amoeba be stimulated by slight contact or by
meeting very dilute solutions of various chemical substances
it will form a pseudopodium on the side towards the stimulus. If
it be pricked with the end of a fine thread of glass, or come into
contact with stronger solutions of chemical substances, it will draw
back and flow away. In this case the formation of a pseudopodium
in a region of the body other than that which has been stimulated
shows that the protoplasm has the property of conductivity.
Again, it does not swallow every particle it comes across, but
chooses those that either contain nourishing substances or are in
motion (in which case they are probably alive and therefore fit for
food). By an unkind deception of this ' sporting instinct ', it may
be induced to capture and swallow moving particles of glass. Its
mode of seizing food is not fixed, but adjusted with an uncanny
appearance of intelligence to the nature and behaviour of the prey
of the moment, which it dogs with perseverance and resourceful
changes of method. It wiU move away from strong light, but does
not appear to perceive a particle of food better in the light than in
the dark. All this shows that it receives from foreign bodies various
stimuli, and discriminates between them. In contrast to these
instances, many of its actions cannot be traced to any stimulus,
and must therefore be classed as automatic in the sense in which
that word is used in biology. In much of its activity it appears to
be exploring its surroundings and to continue on a course until
it receives some stimulus which repels it, but sometimes, as in
capturing food, it appears to be attracted in the direction from
which a stimulus comes.

RESPIRATION AND EXCRETION

Since the peUicle of Amoeba is thin it is probable that liquid
and gaseous substances other than those with very large molecules
can pass in and out with some ease, and what evidence there is
suggests that this is so. Amoeba is known to absorb oxygen, and
since there are no special organs for the purpose it must be
presumed that the gas diffuses in all over the surface. In the same
way, since it is virtually certain that no animal can feed on
protein without producing nitrogenous waste products, it must
be assumed that these diffuse out. In A. leschercB masses of
crystals have been observed to collect in a large vacuole and,
after being violently swirled round, to be shot out. These have

RESPIRATION AND EXCRETION

been assi;
unknown.

31

been assumed to be excretory, but their chemical nature is

THE CONTRACTILE VACUOLE

The contractile vacuole gradually increases in size up to a
maximum, and then bursts, shedding its clear, watery contents
to the exterior ; a new vacuole then grows again in the same spot
and the process is repeated. There is no true contraction, but
from analogy with the contraction and swelling of the vertebrate
heart the small phase is sometimes called systole and the large,
diastole. The vacuole removes water, and this is its only known
function. The pellicle of Amoeba has differential permeability for
water and for dissolved substances, so, since the osmotic pressure
of the cytoplasm is higher than that of fresh water, water must
enter osmotically. If this were not eliminated the creature
would swell. Marine amoebae, living in an environment of osmotic
pressure approximately the same as that of protoplasm, have no
contractile vacuole. The passage of water from the cytoplasm into
the contractile vacuole is against the direction of osmotic flow,
and must mean that work is being done by the animal. In fact,
in an amoeba deprived of oxygen, swelling of the contractile
vacuole ceases.

THE NUCLEUS

It is possible, in various ways, to remove the nucleus from
Amceha proteus. Individuals thus amputated continue living for
some days, and then die. They can move, but their pseudopodia
are fewer and unusual in shape ; they can in jest and kill small
prey, but cannot digest it ; they give normal responses to
stimuli, but their oxygen consumption is greatly reduced. All
this suggests that the nucleus is in some way responsible for the
formation or activity of enzymes.

DEPRESSION

Unfavourable conditions of life may bring about a condition
known as depression, in which the nucleus of the amoeba is
enlarged and the various functions become deranged. This
condition, however, is more famihar and has been more closely

Fig. II. — Amceha discoides. A-C, stills from a cinematograph film of a dividing
animal ; the arrows indicate the tail, which always remains posterior in

locomotion. D, a photomicrograph of another individual showing the tail
more prominently, x c. 400. (Photographed by Dr K. J. Goldacre.)

34 AMCEBA. PHYLUM PROTOZOA

investigated in some other minute organisms, as, for instance, in
Paramecium (p. 55).

ENCYSTMENT

In certain circumstances Amoeba withdraws its pseudopodia
and becomes a rounded mass which secretes about itself a tough
case or cyst. In this it lies dormant and can survive the drying
or freezing of the pond in which it lives or be transferred in mud
to other ponds. We have here an instance of a widespread pheno-
menon known as suspended vitality, which is found, for instance,
in seeds and in frozen tissues. The exact condition of the
protoplasm in such cases is a mystery, but no vital processes
can be detected, and it has been shown by experiments on
seeds that, if they be kept perfectly dry, not even respiration
takes place. We must conclude that life, regarded as a process,
has slowed down and, at least in some cases, ceased, but that the
protoplasm retains the power of resuming it in certain circum-
stances. At death, on the other hand, the protoplasm passes into
a condition in which it will indeed remain intact in suitable
circumstances (as when it is frozen) but has lost the power of
resuming life.

REPRODUCTION

Amoeba reproduces by the process known as binary fission, in
which first the nucleus and then the cytoplasm parts asunder into
two halves, each of which appears, at all events, to differ from the
parent in nothing but size. The division of the nucleus is a peculiar
kind of mitosis (p. 688) in which the place of centrosomes is
taken by a mass of clear protoplasm at each end of the nucleus.
These masses are known as pole plates and arise within the
nuclear membrane, which does not break up during division as
in ordinary mitosis. After the division of the nucleus the cyto-
plasm flows apart into two bodies, each of which contains one
of the daughter nuclei. The new bodies are in some species at
first connected by a bridge of protoplasm, but this becomes
narrower until it breaks through and two new individuals come
into being, the whole process having taken about one hour. The
part where the break occurs becomes the ' tail ' of each daughter

REPRODUCTION

amoeba, and is always posterior as the animal moves. Another

kind of fission, known as multiple fission or spore formation,

takes place at times. The nucleus divides till a very large number

of small nuclei has been formed. These pass to the surface of the

cytoplasm, which gathers into a little mass around each of them

to make spores, which break free, feed

for a week, and then encyst. After a

week or more they emerge, feed and

grow, and then encyst again for ap to a

month. This process is repeated, and

after three months the amoebae, which

are now seven times the diameter of the

original spores, emerge from the cysts

and grow rapidly to the adult size. In

other species the spores are formed in a

cyst (Fig. 12). The young form is of

various types, but, contrary to what

is sometimes stated, is never, so far as

is known, flagellate.

Syngamy has not yet been proved to
occur in Amceha proteus. The animal
does, however, occasionally undergo a
process known as plastogamy, in which
the cytoplasm of several individuals
fuses, forming a single mass which
contains several nuclei. Such a mass is
known as a plasmodium. Quite another
kind of multi-nucleate body is found ^^' ln~Aniaeba^ —^Ait^T

Scheel.

A,

in certain species of Amceha and in the
amoeba-like animals known as Pelomyxa,
where two or more nuclei are formed
by the division of a single nucleus,
giving a coenocyte or symplast. Plas-
modia and symplasts differ only in their
mode of formation and are collectively known as syncytia.

Amaba encysted ; B, section of
a cyst in which numerous nuclei
have been formed, more highly
magnified ; C, surface view of a
ripe cyst in which the spores
are beginning to separate and
the cyst wall to break up ; D,
a single spore highly magnified.

AMCEBA AND ITS SURROUNDINGS

We have studied Amceba as an example of apparent simpHcity
in organisation. That is not to say that it is primitive in the
sense in which that term is used by zoologists ; it is unhkely that

o6 AMCEBA. PHYLUM PROTOZOA

this creature is a survivor from among the earUest living beings.
Indeed it is more probable that if we could trace back the descent
of the amoeba we should come to ancestors not unhke the
organisms which will be described in the next chapter. But
Amceha does contrive to carry on its life with less apparatus
than almost any other creature, possessing as it does no obvious
permanent organs except the cytoplasm, nucleus, and contractile
vacuole, and besides these only the temporary organs known as
plasmalemma, plasmasol, plasmagel, and pseudopodia.

FLAGELLATE PROTOZOA

POLYTOMA

Water in which organic matter is decaying always contains
numerous small organisms of various kinds. Among these, when
decomposition is well advanced, there can be found with the aid
of the microscope minute, colourless organisms of a species known
as Polytoma uvella (F g. 13), which feed by absorbing from the
water through the surface of their bodies substances n solution
derived from the decaying matter. The body of a Polytoma
is an egg-shaped mass of protoplasm without any internal skeleton.
A pair of long protoplasmic lashes or flagella project from one
end ; by a backward lashing of
these it swims with a somewhat
jerky course, the end at which the
flagella are placed being forward.
The permanent shape of the body
is due to a thin cuticle ; that is,
not to a surface layer of the pro- ^^•
toplasm, but to a protective cover-
ing formed by secretion. It is
pierced by two pores for the
flagella. Two contractile vacuoles lie

c. V.

cu.

s.g.

Fig.

13. — Polytoma iivella : three

stages in ordinary fission.

C.V., Contractile vacuole ; cu., cuticle
nucleus ; s.g., starch grains.

nu.

close behind the flagella and con-
tract alternately. There is one nucleus, placed somewhat behind
the middle, and there is sometimes a spot of red pigment situated
in the front part of the body. The hinder region contains
numerous starch granules. These must be formed by the proto-
plasm from substances absorbed in the food : they serve as a
reserve of nutriment, and are used up during starvation. Their
presence is interesting, for starch, though it is common in plants,
is rare in the protoplasm of animals, which, if they store carbo-
hydrates, usually do so in the form of glycogen. Together with the
spot of red pigment — which is an organ that enables small,
motile, green plants to find the sunlight which is necessary to their
mode Oi nutrition— the starch granules betray the fact that
Polytoma is at least closely related to plants. It is, to all intents
and purposes, a colourless Chlamydomonas, an organism which

37

38 FLAGELLATE PROTOZOA

is clearly a unicellular alga. Like animals, Polytoma uvella needs
an organic source of carbon, but like plants it can exist on
inorganic nitrogen. Polytoma can encyst, and in the encysted
state is carried about in dust, etc., to germinate in favourable
circumstances elsewhere.

REPRODUCTION

Reproduction is usually brought about by a process known
as repeated fission, in which binary fission is repeated so as to
form four daughters before the young separate, but sometimes
there are only two offspring. Fission takes place within the cuticle,
this being carried about during the process by the action of the
fiagella, which remain attached to one of the daughters. The
nucleus divides by a kind of mitosis. The first division is nearly
transverse, the second at right angles to it. The fiagella are then
withdrawn, each daughter forms two small fiagella, and the
cuticle of the parent is dissolved. At intervals of a few days
syngamy takes place. Two ordinary individuals come together
and fuse, their nuclei joining and their cytoplasm flowing into
one mass, which then encysts. After a resting period the zygote
divides by repeated fission into eight, each of the daughters
grows two fiagella, and the cyst is dissolved. In regard to this
process we must notice : (i) that syngamy can occur at any time in
the life of the individual, and does not take place between special
germ cells which cannot develop without it : in most animals,
on the other hand, syngamy is obviously impossible in the adult
and can only take place between the germ cells before they develop
the rest of the body ; (2) that the gametes are alike, and not, as in
most animals, of two kinds, a passive kind, which bears the bulk
of the cytoplasm, and an active kind, by which is carried out the
locomotion which the process involves. Both gametes in Polytoma
are fairly well supplied with cytoplasm and both are motile.
Only when one is older than the other is there sometimes a
difference in size.

In Chlamydomonas (Fig. 8) syngamy takes place, not, as might
seem possible, between ordinary individuals, but between special
small forms which arise by repeated fission of the ordinary forms.
These special gametes, however, are like the ordinary individuals
in all but size. In some species of Chlamydomonas they are
themselves of two sizes, which conjugate large with small, so

POLYTOMA

39

that, as sometimes in Polytoma, there is a difference in size,
though not in any other respect, between the gametes. The syn-
gamy of two hke gametes, whether they be ordinary or special
individuals, is known as isogamy ; syngamy of unlike gametes
is anisogamy ; if, as in most animals, they differ not only in size
but also in that the larger is passive and the smaller active,
the process is known as oogamy.

EUGLENA

Euglena viridis (Fig. 14), often so
common in puddles as to give them a
green colour, is a flagellate organism of
a rather higher grade than Chlamydomonas
or Polytoma. It is a minute, spindle-
shaped creature, which may reach a length
of 0.17 mm. The front end is blunt and
bears one fiagellum rooted at the base of a
funnel-shaped pit, which is known as the
* gullet ' but probably never used as such.
The base of the fiagellum appears to ^^
bifurcate but it is possible that there is
really a second short fiagellum since some
related species have two, of which one is
shorter than the other. On the other
hand, some workers think that more
probably two flagella have fused. The
electron microscope shows that the
fiagellum bears a row of filaments, some
five or six times its diameter in length.
There is a strong pellicle, a distinct
ectoplasm, and a central, spherical nucleus.
Band-shaped, green chloroplasts (p. 20)
radiate from a point in front of the nucleus, where granules of
the starch-like substance paramylum accumulate. Waves of
contraction pass along the body (Fig. 15), but contractile
strands (myonemes, p. 43) are lacking. There is a large
contractile vacuole which discharges into the expanded base
of the gullet. Just before the collapse a number of small
vacuoles appear near the old one, and these coalesce to
form the new vacuole (Fig. 16). The system is, therefore, similar

Fig. 14. — Euglena viridis,
highly magnified.

av.. New contractile vacuole
beginning to form ; cv., main
contractile vacuole ; chp.,
one of the chloroplasts ; cu.,
pellicle ; e.s., eye-spot ; ec,
ectoplasm ; fl., fiagellum ;
g., gullet ; nu., nucleus ;
p.g., paramylum granules ;
/).g'., protoplasmic mass, with
paramylum granules, from
which chloroplasts radiate ;
res., reservoir.

40

FLAGELLATE PROTOZOA

to that in other Protozoa and is not, as it has been described,
specially comphcated. A red pigment spot or stigma Hes
against the front side of the reservoir and enables the working
of the flagellum to be regulated by the amount of light in the

Fig. 15. — Euglena viridis.
A, B, C, Three postures of the body.

surroundings. Reproduction is by binary fission, beginning at
the front end, the nucleus undergoing a peculiar mitosis. It may
take place in free individuals after the loss of the flagellum, or
in a gelatinous cyst, within which it may be repeated several
times. The occurrence of syngamy is extremely doubtful. Our
knowledge of the nutrition of Euglena is too confused and compli-

flagellum

gullet

eyes pot

contractile vacuole

Fig. 16. — stages in the formation and discharge of the contractile vacuole in
Phacus, a flagellate related to Euglena. — Modified from Hyman. The
Invertebrates: Protozoa through Ctenophora, by kind permission of McGraw-
Hill Book Company.

cated to be summarised in an elementary book. It is doubtful
if E. viridis can feed in an entirely plant-like manner, though
E. gracilis can. All species can grow if they are in the light,
provided that a suitable source of organic nitrogen be present,
and many can also grow in the dark if organic carbon and nitrogen
be available ; under these circumstances the chloroplasts lose
their green colour.

41

COPROMONA S

1

Copromonas (or Scytomonas, Fig. 2) is a flagellate which lives
in the moisture of dung. It is related to Englena but colourless,
as Polytoma is a colourless Chlamydomonas. It nourishes itself,
however, not as Polytoma does by absorbing through its surface
the products of decomposition amid which it lives, but by
swallowing through its gullet the bacteria which live in the same
solution. Its syngamy is performed solely by fully-grown ordinary
individuals. Usually the syngamy takes place when the dung is
becoming uninhabitable for the Copromonas, and the zygote
becomes encysted. From this condition it only emerges in fresh
dung, to reach which it must be swallowed in contaminated food
by a frog, and passed intact with the faeces.

PERANEMA

Peranema, common in stagnant water, is another colourless
relation of Etiglena. It is larger than Copromonas, pear-shaped
at rest but very active in changing its shape, has one flagellum,
rooted in a reservoir which opens in front of the gullet, and feeds
by swallowing smaller organisms into the gullet, the wall of
which is strengthened by stiff rods. Probably Peranema is also
saprophytic (see below).

MODES OF NUTRITION

The organisms which we have been discussing in this chapter
exhibit all the three types of nutrition practised by animals and
plants. In Chlamydomonas simple inorganic substances are
absorbed through the surface, and from them complex substances
are manufactured by means of the energy of the sun's rays.
Such organisms are said to be holophytic. In Copromonas and
Peranema complex organic substances are taken in through a
miouth, after the manner of animals. Such organisms are said
to be holozoic. In Polytoma, organic substances are absorbed in
solution through the surface of the body. Such organisms are
said to be saprophytic if they are plants, or saprozoic if they are
animals. The substances which form the food of various sapro-
phytic organisms differ a great deal. In Polytoma they are

42 FLAGELLATE PROTOZOA

relatively simple (acetates, etc.), but many parasites in the ali-
mentary canals of animals nourish themselves saprozoically on
the digested food of their holozoic hosts.

FLAGELLATA

In later chapters (pp. 65, 82), there will be found descriptions
of the organisms known as Trypanosoma and the Choanoflagellata,
which resemble those described in this chapter in the possession
of flagella and in certain other respects, and with them are classed
by zoologists as Flagellata or Mastigophora.

MONOCYSTIS

Among the organs of reproduction of an earthworm are certain
sacs, known as the seminal vesicles, in which the sperms ripen.
Here are generally to be found specimens of the parasites known
as MonocysHs (Fig. 17), which live by absorbing, through the
surface of their body, the fluid in the vesicles which is intended
for the nourishment of the spermatozoa. Two kinds of these
creatures may be present, differing in size
and in certain other particulars. The larger
kind, M. magna, is easily visible to the
naked eye as white threads, hanging by
one end from the funnels of the vasa
eferentia (see p. 175). The smaller, known
as M. lumbrici, is more often found
free in the fluid among the developing
spermatozoa. The body of a full-grown
MonocysHs is long and narrow, and consists
of a soft, granular endoplasm and a firm,
clear ectoplasm. The endoplasm contains
numerous granules, many of which consist of
the carbohydrate substance paraglycogen,
and the ectoplasm is covered with a stout
cuticle and has in its deeper layer a net-
work of contractile threads, the myonemes.
While the cuticle makes it impossible for
the protoplasm to flow out into pseudo-
podia, the myonemes enable the animal
to change its shape by squeezing the fluid
endoplasm from one part of the body to
another. Slow waves of contraction of this
kind are constantly passing along the
body. In the endoplasm there is a large
nucleus, but there is no contractile vacuole.
At one end of the body an indefinite knob
enables it to adhere to one of the cells of
the funnel.

Fig. 17. — MonocysHs.

A, M. tnagfhi ; B, M. lumbnci.
The latter is covered with
the tails of spermatozoa, the
offspring of the sperm mother
cell in which it was embedded.

4?

44

MONOCYSTIS. PHYLUM PROTOZOA

REPRODUCTION

In the stage which we have just described, the animals are
known as trophozoites. When they are full grown, two of them
come together and form themselves into a rounded mass without
fusing. Around this mass a two-walled cyst is secreted (Fig. i8).
Each individual now divides by multiple fission, in which the

spc

Fig. 1 8. — The life-history of Monocystis. — After BiitschU.

1. Young individual (f) lying within a sperm mother cell of an earthworm.

2. Association of two individuals within a cyst, ready to form gametes.

3. Numerous spore-cases [sp.c, pseudonavicellae) within a cyst.

4. A spore-case with eight spores (sp.) and a residual core (rb.).

YiG, 19. Part of a cyst of Monocystis lumbrici show-ing the two kinds of gametes

'and the residual protoplasm of one of the parents. — After Hoffmann.

mitosis resembles that of higher animals in that the centrosome
appears outside the nucleus and the nuclear membrane disappears.
There arise thus, as in the spore formation of Amceba, a number
of small germ cells, a certain amount of residual protoplasm being
left, which is absorbed by the germ during their development.
The germ cells unite in pairs, in which one member is probably
derived from each parent. Thus, although the parents are to all
appearance exactly alike, there happens here what is known as
cross-fertihsation, such as is found in the vast majority of cases
throughout the animal kingdom.

It is said that in M. magna the germ cells from the two parents

REPRODUCTION .c

45

are alike, but in M. lumhrici those of one parent, the ' female '
are rounded, and those of the other, the ' male ', pear-shaped
(Fig. 19). Each zygote is known as a sporont ; it now secretes a
boat-shaped, horny case, and is known as a pseudonavicella. It
was given this odd name because of its resemblance to the diatom
(a small plant) called Navicella ; this word means a little boat,
and refers to the shape of the organism. Within the case the
pseudonavicella divides by repeated fission into eight sickle-
shaped sporozoites. There are thus two generations of spores in
the life-history of Monocystis.'^ The cysts fall into the cavity of
the body (the coelom, see p. 160) and accumulate in the hinder
segments, but no further development takes place until the
pseudonavicellae get free from the worm. This, presumably,
sometimes occurs through the last few segments being broken
off by autotomy (the fracture of a body or limb by its own con-
tractions), or the parasite may have to await the death of the
worm. In either case the cysts might later be swallowed by
another worm with the earth from which it obtains its food,
but proof of this is lacking. The spore-cyst is dissolved in the
intestine of the worm, and the sporozoites come out and bore
their way through the wall of the gut and other tissues till they
reach the vesiculae seminales. Possibly, occasionally, the sporo-
zoites pass directly from one worm to another during coition, but
as young worms are free from parasites this is unlikely. In the
vesiculae seminales each enters a sperm-mother-cell, where it
grows by absorbing the protoplasm which is meant to serve for
the nourishment of the spermatozoa (see p. 173). The latter are
formed, but wither, their tails only remaining attached to the
young Monocystis, which looks as though it had a coat of cilia.
Finally they disappear, w^hile the Monocystis continues to grow.
Thus the sporozoites become trophozoites by development.

It has recently been claimed that some species of Monocystis
ingest cells of their hosts and form food vacuoles as do other
Protozoa. If this observation is confirmed they would be less
modified for a parasitic mode of life than at first appears.

^ A spore is a small reproductive body formed by multiple or repeated fission.
It may or may not be a gamete. If it be enclosed in a case it is known as a
chlamydospore (e.g. pseudonavicellae), if it be naked, as a gymnospore (e.g. spores
of the Amceba shown in Fig. 12). Amoeboid spores are known as ama-buiae or
pseudopodiospores, flagellate spores as fiagellulae or flagellispores.

CILIATE PROTOZOA
PARAMECIUM

The Paramecium caudatum, slipper animalcule (Fig. 20), is
a minute animal found in water in which dead leaves or other
remains of organisms are decaying. The decay is brought about
by bacteria, and upon these the slipper animalcules feed. A
rich culture of Paramecium may be obtained by steeping hay
in water, allowing it to decay, and adding to the infusion thus

C.V.

ec.

c.v.

/.

:: /?=i.

i i

A^^'

.vV

w^

^^-

f.v.

u.m. g.

\

an

Fig. 20. — Paramecium, caudatum,.

A, An individual seen from the left side, highly magnified; B, a diagrammatic view o an individual
from the ventral side, less highly magnified.

an.. Position of temporary anus ; c.v., contractile vacuole ; ec, ectoplasm with trichocysts ; f.v., food
vacuoles ; g., gullet ; meg., meganucleus ; mi., micronucleus pst., peristome ; u.tn., undulating
membrane.

made mud or weeds from a freshwater pond which contains

Paramecium. The animals may easily be seen with the naked

eye as minute, greyish white, oblong creatures, moving slowly

about in the water. The body of Paramecium is spindle-shaped,

somewhat flattened on one side, and with one end blunter than

the other. The fiat side is called ' ventral ' and the blunt end is

anterior. This end appears as though it had been twisted, so that

a groove which it bears is spiral, starting in front on the left

and curving round to the ventral side, where it is continued back

in the middle line to within about a third of the length of the

body from its hinder end. The groove is known as the peristome :

from its hinder end there passes backwards into the body a

46

PARAMECIUM ^y

funnel-shaped gullet or vestibule, the opening from vestibule to
endoplasm being known as the mouth. The whole body is covered
with fine protoplasmic threads of the kind known as cilia (Fig. 21)
by whose lashing the animal swims and gathers its food. The cilia
are set at equal distances in rows, which run lengthwise in the
hinder part of the body, but follow the spiral twist in front :
they also line the gullet, where two or three rows of them beat

Fig. 21. — ^Electron photomicrographs of a ciUum (on the left) and tips of dis-
charged trichocysts of Paramecium. The cihum can be seen to consist of 1 1
separate threads. The measure in each photograph is i micron. — From.
Jakus and Hall.

together to form an apparent undulating membrane which
hangs from the roof. The ciHa work regularly in waves, lashing
backwards and driving the blunt end of the animal forwards,
with a rotating movement hke that of a rifle bullet owing to its
spiral shape. The animal can encyst.

ECTOPLASM AND ENDOPLASM

Paramecium has a soft, granular endoplasm and an ectoplasm
which is firm and gives the body its shape, but elastic, so that the
an mal can bend and squeeze through narrow gaps. The outermost
layer of the ectoplasm is a tough pellicle. Below the pellicle comes
the cortex, a thicker, clear layer of ectoplasm in which are

Basal granule.
Connectinq fibril .
Trlchocyst granule.

Accessory
basal granule.

.3 CILIATE PROTOZOA

embedded peculiar structures known as trichocysts (Fig. 21).
These are spindle-shaped bodies with a fine point, and consist of
some semi-liquid substance. They are placed at right angles to
the surface, with the point in the outer part of the layer. If the

animal be stimulated by
impact or by a solution of
some irritating substance,
they suddenly elongate and
project from the body as
threads, of which the points
are sticky while the rest has
hardened. The trichocysts
are organs of adhesion by
Fig. 22. — Paramecium which the animal anchors.

multimicronucleatutn. ^-u -^ breaks free the

Diagrammatic drawing of vvnen It oreaKS iree ine

a portion of the dorsal threads are lost and the

_,^ surface, showing a Hne of trichocysts replenished. The

/ 4^\ hexagonal pits, the basal ,,. , ,^. s . i j

"<!>>" granules, and the fibrils pelhcle (Fig. 22) IS marked
connecting them, x 3700. by rows of rectangular or
~~ er un . hexagonal pits, in each of

which a cilium arises, or sometimes a pair, while the trichocysts
lie under the transverse ridges between the pits. Each cilium
consists of a sheath surrounding a ring of nine double threads
and a central pair. Below the cilium a fibre is continued in-
wards into the cortex, within which it bears a swelling known as
the basal granule. The basal granules are united by a system of
threads known as neuronemes which are possibly conductile. The
endoplasm contains numerous granules, some of which appear to
consist of waste matter ready for excretion, while others may
be stored nutriment. Glycogen is diffused through the endoplasm.

NUCLEI

Paramecium caudatiim has two nuclei, one of which is large and
is known as the meganucleus, while the other, the micronucleus,
is small and is situated in a groove on the meganucleus. The
nuclei lie in the endoplasm above the gullet ; Paramecium aurelia
has two micronuclei.

There are two contractile vacuoles, which lie in the cortex of
the dorsal side, one towards each end. At its full size each is a
large spherical space surrounded by from six to ten pear-shaped

PARAMECIUM: NUCLEI

49

radiating canals, whose wide ends lie under it (Fig. 23). These are
the formative vacuoles. Systole affects only the central vacuole.
After it has taken place, the formative vacuoles flow together at
their inner ends and thus form the beginning of a new contractile
vacuole, round which new canals appear, starting as mere slits and
swelling to a pear shape by
the enlargement of their
inner ends. Over each con-
tractile vacuole there is a
minute gap in the pellicle,
through which the contents
of the vacuole are discharged. K ^ /^ M

NUTRITION

O

>o<

Successive stages of the contractile
vacuole of Paraynecium.

The food consists of
bacteria and other minute
organisms. These are drawn
towards the mouth by the
current set up by the cilia Fig. 23
of the peristome and driven
down the gullet by the

working of the cilia. Particles appear to be forcibly sucked in by the
mouth. A drop of water containing the food particles is now pinched
off by a contraction of the endoplasm and becomes a food vacuole,
which is carried by a streaming of the endoplasm around the
body, passing first backward along the ventral side, then forward
nearly to the middle of the body, then through several turns of

a short circuit in this region of the
body, and finally forward to the
front end and back so as to com-
plete the circuit of the body
(Fig. 24). During these wanderings
the food is digested. The un-
digested remains are then expelled
at a spot just behind the end of
the gullet, where a passage through the ectoplasm, known
as the temporary anus, is formed when it is required. As in
Amceha, digestion takes place in an acid vacuole and protein is
the chief food. The formation of the vacuoles and their acidity
are easily demonstrated if a suspension of i g. of yeast and 10

«. c.

Fig. 24. — A diagram of the course
of the circulation of the food
vacuoles in Paramecium.

I.e., Long circuit ; s.c, short circuit.

50

CILIATE PROTOZOA

mg. of Congo Red is boiled for ten minutes with a few ml. of
water and then fed to the ciHates. The indicator turns blue at
pH 3. The animal is able to some extent to distinguish between
particles which are good for food, and those such as carmine
which are inert. Moreover, having been deceived experimentally
by carmine it learns to avoid it in the future, and can remember
to do so for two or three days.

EFFECT OF STIMULI

Like all other organisms, Paramecitim shows automatism. Its
incessant activity is spontaneous, but is continually modified

a

, - '

> » '

10'-

25-

FiG. 25. — The reaction of Paramecia to heat

and cold. — From Jennings, after

Mendelssohn.

At a., the Paramecia are placed in a trough both ends of
which have a temperature of 19° C. They are equally
scattered. At b, the temperature of one end of the
trough is raised to 38° C, while the other is only 26° C.
The Paramecia collect at the end which has the lower
temperature. At c. one end has a temperature of 25° C.
while the other i ; lowered to 10° C. The animalcules
now collect at the end which has the higher
temperature.

»

\

/

'0

*

-

•

•

N

•.

/

•

V

*

7

^

/"

/

*

»

Fig. 26. — Paramecia collecting in a drop of 0-02
per cent, acetic acid. — From Jennings

PARAMECIUM: EFFECT OF STIMULI rj

by external stimuli. The movements of Paramecium arc much
more active and definite than those of Amceba, and it is corre-
spondingly easier to observe the effect of various stimuH upon
the animal. These effects are of two kinds, of which the first is
merely an alteration in rate of movement. Many acids, alkalis,
salts, and other substances in dilute solutions cause an increase
in the rate of motion owing to a more rapid working of the cilia.
Increase of temperature up to about 35° C. has the same effect.
On the other hand, dilute solutions of narcotics, such as alcohol,
ether, or chloroform, cause the cilia to work more slowly. All
these reactions may be merely the products of the direct effects
which such changes in the environment are known to have upon
protoplasm. Many of the above also have another effect, although
this may only be produced at a critical point. They cause the
animal to give a particular response known as the avoiding reaction
or phobotaxis ; the creature stops, may swim a short distance
backwards, and then moves forwards again at an angle to its
previous path. By these means it can avoid a solid obstacle, hot
water, or an acid, and if it is placed in a situation where there is a
continuous gradient, e.g. of temperature, it may appear to be
attracted, for example, to the cool end ; in reality, as can easily
be seen with a low-powered microscope, it is repelled from the hot,
(Figs. 25-26). In the same way it may become trapped in a drop
of acid. Paramecium has two other types of response ; under
some circumstances contact with a solid body, especially on two
sides at once, as when it swims into a comer, causes protrusion
of the trichocysts, and this can also be induced by chemicals such
as tannic acid ; lastly, when two Paramecia meet they ma}'
adhere in conjugation (see below). These two responses suppress
the avoiding reaction which is normally produced by the stimuhis
of contact.

REPRODUCTION

Paramecium reproduces by binary transverse fission. The
meganucleus divides amitotically, that is, without division
of the constituent chromosomes (p. 688) or separation of their
pairs, the micronucleus by a mitosis in which, as in that of
Amceba, the nuclear membrane does not break up, and the place
of centrosomes is taken by pole plates. Meanwhile a groove appears
round the middle of the body and deepens till the cytoplasm

52 CILIATE PROTOZOA

is Split into two, each half containing a daughter nucleus of
each kind and one of the contractile vacuoles. The two bodies
formed by this fission are, like those of Amoeba, asexually pro-
duced young, analogous to the buds of certain higher animals
of which we shall speak in a later chapter (p. loi). Their develop-
ment involves not only growth but also the remodelling of the
body, since each of them lacks half the outward organs of the
parent, while those which it has are too large for it. In a well-fed
culture, division takes place two or three times a day, but if
the animals be ill-nourished it is much less frequent, and if they
be starved thev cease to divide.

CONJUGATION

The conjugation of Paramecium is a remarkable process, of
a kind found only in this creature and in the other members of
its class (Figs. 27-29). As a rule, the process begins during
the late hours of the night and lasts till the next afternoon.
The details differ in different species, but the following is the
usual course of events in P. caudatum. Two individuals, which
we will call conjugants, come together as those of Monocystis do,
but without encysting, and lie with their ventral sides touching,
the endoplasms becoming continuous in the region of the gullets,
which degenerate, We may compare this with coition. The micro-
nucleus of each conjugant leaves its normal position, lies free in
the cytoplasm, and grows larger. It then divides twice, and three
of its four products degenerate. During these divisions the
number of chromosomes is halved, as it is in the gametogenesis
of higher animals, though the details of the process differ in the
two cases. The remaining micronucleus divides again, this time
somewhat unequally, the smaller product being the male pro-
nucleus, the larger the female pronucleus. At this stage we may
regard each conjugant as containing two gametes, the male,
represented by the pronucleus, the female by the pronucleus plus
the cytoplasm of the conjugant. These are analogous to a sperma-
tozoon and an ovum, so that the animal may be said to be
hermaphrodite. The true syngamy now takes place. The m^ale
pronucleus of each conjugant passes over into the other and fuses
with the female pronucleus of the latter and there is also some
mixture of cytoplasm. The body which belonged to each conjugant
comes thus to contain a micronucleus of mixed origin. It is, in

PARAMECIUM: CONJUGATION

53

fact, a zygote. The zygotes separate and are known as exconju-
gants. During conjugation the meganucleus degenerates, sphtting
up into shreds, which disappear. After separation the zygote

B

Fig. 27. — Conjugation in Ciliata.

A, Vorticella ; B, Paramecium,
c, Pseudo-female conjugant ; c'., pseudo-male ; me., meg., meganuclei ; mtrg'., disintegrating fragments of
meganucleus; wi., micronuclei ; ;«»'., abortive microauclei.

nucleus of the exconjugant undergoes a development whereby
nuclei of both kinds are provided. It divides three times success-
ively, so that the body contains eight nuclei. After an interval
the body divides into two, each half containing four nuclei, and

54

CILIATE PROTOZOA

tT?vtt9T fj.f<f

after a further interval these halves divide, so that there are
four indivitluals, each with two nuclei, one of which becomes
a nieganucleus and the other a micronucleus.

The conditions under which conjugation takes place in
raramccium have been, and are still, the subject of much investi-
gation. Many points still remain to be cleared up, but certain
results have now been reached. Conjugation generally occurs at
the beginning of a falling off in the supply of food after a period of

exceptional plenty that has brought
about rapid multiplication. Thus it
will often take place in an infusion
in which the bacteria, having used up
the nourishment provided by the
plant-remains, are falling off in
numbers, and thus the animals, after
a plentiful supply of food, are begin-
ning to experience dearth. It has been
said that descendants of the same
exconjugant will not conjugate, and
that individuals from another stock
must be introduced, but this has been
disproved ; generally there seem to
be two mating types (they cannot be
called sexes, since both are herma-
phrodite) which will conjugate with
each other but not amongst them-
selves. There are some races in which
it is difficult to bring about conjuga-
tion, others in which it has never
been seen, and yet others in which it takes place at short intervals
without apparent cause. It seems that each clone, that is, the
population derived by asexual reproduction from a single parent,
has its own life-history. At first it is sexually immature, and
this phase lasts in P. bursaria for several months. Then, for
years there is sexual maturity, and conjugation readily takes
place. Finally the stock seems to age, and may die out. The
species is apparently maintained by occasional very vigorous
products of conjugation. Many exconjugants die at once, or after
a few divisions.

Paramecium also has a type of pseudo-sexual process known as
endomixis. In this much the same nuclear changes go on as in

Fig. 28. — A diagram of the
behaviour of the micronuclei
during the conjugation of
Paramecium caxidatitm. The
white circles represent the
portions which degenerate.

PARAMECIUM: CONJUGATION

55

conjugation, but in a single individual. The meganucleus dis-
appears, the micronucleus divides a number of times, and new
individuals, with new nuclear outfits, are formed by fission.
According to some workers the last surviving pair of micronuclei
immediately fuse with each other. If this is so, the process is one
of autogamy, or the fusion of gametes derived from a single
parent. Endomixis seems to have some of the same good effect
on a culture as does conjugation. In P. aiirelia the two processes
alternate.

Laboratory stocks of Paramecium often show a condition called

---jmeg

TTLL

Fig. 29. — Semidiagrammatic views of individuals of
Paramecium caudatum.

Ay In depression ; B, in conjugation ; C, in fission.
g.. Gullet ; tneg., meganucleus ; mi., micronucleus.

depression (Fig. 29/I). The meganucleus is enlarged and there
are changes in the shape of the body, and death follows. Various
changes in the environment can alleviate depression, and
it is probable that the cause lies in unnatural conditions of
culture.

Genetical experiments have shown that the characters of an
individual Paramecium are determined by its meganucleus, and
this is in agreement with the fact that ciliates without micronuclei
can carry out all functions except those of conjugation and
endomixis, while loss of the meganucleus leads to death within
two days. It is obvious that transmission of hereditary characters
can take place onty through the micronucleus, since the other is
destroyed.

56

CILIATE PROTOZOA

\,u,„„ llu- most boautiful forms of pond life are the bell-
animalcuies, many of whicl>arc of the genus Vortudla (Hgs. 30,31)^
Various species may be found as mmute, ^o ourless bod^s
fastened .0 weeds by stalks which contract at the shghtest
disturbance of the water. Some of them also appear m infusion

The body of a Vorttcella is
outwardly shaped Hke a bell,
but has no hollow within, the
bell being filled with a mass
of protoplasm. In the place
of the handle is a long stalk,
by which the animal is
fastened to some sohd object.
Animals which are thus fixed
are said to be sessile. The
bell can be bent upon the
stalk. The wide end of the
bell has a thickened rim,
within which is a groove, the
peristome. On one side there
FiG. 30.— A group of individuals of Vorti- passes from the peristome,

cella in various phases of the life- (JqWU into the maSS that fills

a.. Ordm^'ldividual ; b., the same contracted ; c, tfic bcU, a tubC Which iS the
ordinary fission ; <i., a later stage of the same ; c, crnl1p>+ Thp fir«;f nart of thm
•rec-swimming individual produced by ordinary gUliei. 1 ilC iilbL pd.it Ul Liiis

tis&ion ; /.. /'. two modes of fission to form a con- • ,,j\Af:,r fViorj +ViP rp^f ur\(\
jugant ; g.. conjugation. 1^ WlUCi tllctll tiic icsL, a.ii\x

the name vestibule is some-
times restricted to it. The part of the upper surface which
is encircled by the peristome is known as the disc. It is not
level, but slopes, being raised on the side where the gullet lies.
The disc can be retracted, and the rim of the peristome drawn
inward over it. Around the edge of the disc and down into the
vestibule two rows of cilia wind spirally counter-clockwise, the
inner long and upright, the outer short and slanting outwards.
In the vestibule the members of the outer row beat together to
form an apparent undulating membrane. There are no cilia
elsewhere upon the body.

VORTICELLA

57

-t.Cl.

an

ECTOPLASM AND ENDOPLASM

The general character of the ectoplasm and endoplasm is the
same in Vorticella as in Paramecium, but the pellicle of the
bell-animalcule is sculptured in various ways according to the
species, and below it is a distinct alveolar layer so called because
the protoplasm appears to be full of bubble-like spaces or alveoli.
Just under the alveolar layer, in the walls of its bubbles, is a layer
of very fine contractile fibres
or myonemes. Near the stalk
the ectoplasm is much thickened
and the myonemes pass inwards
through it to join in the middle,
where they form a central con-
tractile fibre which, with a cover-
ing of ectoplasm, makes up the
stalk. This is enclosed in a
cuticular tube formed by secre-
tion. The contractile fibre is not
quite straight, but lies in a very
open spiral, so that when it con-
tracts it draws the stalk into a

close coil. There are no tricho- Fig. 31. — Vorticella. highly magnified.

cysts. The endoplasm is granular. an., Position of temporary anus ; c.f.. contractile
"^ JT o filament ; c.v., contractile vacuole ; cut.st.,

cuticle of the stalk; dsc, disc; ec. ecto-
plasm ; f.v., food vacuoles ; g., narrower
part of gullet ; i.ci., inner row of cilia ; meg.,
meganucleus ; mi., micronucleus ; myn.,
myonemes ; o.ci., outer row of cilia ; pst.,
peristome ; res., reservoir of contractile
vacuole; rim; u.m., undulating membrane;
v., vestibule.

meg.-

INTERNAL ORGANS

A meganucleus and a micro-
nucleus are present, the former
a long, curved band, the latter small and placed beside the
meganucleus, usually in the upper part of the body. There is a
contractile vacuole, which has no canals. It lies in the upper
region of the body and communicates with the vestibule through
a reservoir, which has a narrow permanent opening. The con-
tractile vacuole contracts sharply at intervals, discharging into
the reservoir. The latter then contracts slowly, driving its contents
into the vestibule, but not itself disappearing. Feeding and diges-
tion take place much as in Paramecium. The little organisms
which serve as food are collected and driven into the gullet by
the action of the cilia. The food vacuoles follow a definite, winding
M.z. — 3

o CILIATE PROTOZOA

course in the bodv, passing througli stages similar to those m
rara,ma,.m. The "faeces are discharged into the vestibule by an
anus, Nvliich in some species is a permanent openmg through the
ectoplasm.

REPRODUCTION

The reproduction of Vorticella takes place by binary fission,
which is of two kinds— ordinary fission, and that which forms
conjugants. In ordinary fission, the rim closes in over the disc,
the body becomes shorter and wider, and the meganucleus
contracts and lies across the body, which then divides into two,
the plane of fission being in line with the stalk. The nuclei behave
as in Paramecium. One of the daughters remains upon the stalk ;
the other grows a circlet of ciha in the hinder region, at the level
at which the ectoplasm thickens, breaks off, and swims away by
means of its cilia, to settle down elsewhere by the end which was
attached to the stalk of the parent. It grows a new stalk for itself.
In this form of reproduction the offspring are equal in bulk. In
the fission which forms conjugants the parent gives rise to one
large individual and one or more of a smaller size. The small
individuals may arise by unequal binary fission, sometimes called
budding, or by equal fission, followed by division of one product
into four by repeated fission. ^ The small individuals, by which-
ever method they are formed, resemble the free product of
ordinary fission in aU but size.

CONJUGATION

The small individuals thus formed swim away, and each
attaches itself by its hinder end to the lower part of the body of
one of the stalked individuals. Most of the organs of the small
individual now disappear, and the ectoplasm between the two
conjugants is absorbed into their endoplasm, which becomes
continuous. The meganucleus in each begins to break up and
disappear. Meanwhile the micron ucleus of the small conjugant
has divided into two. Now the micronuclei of both conjugants
divide twice, so that the larger contains four and the smaller

' The various kinds of fission of Atnceba, Vorticella, and animals related to them
(i*rotozoa, p. 78) may be classed as : (i) equal binary fission (p. 34), (2) budding,
'3) repeated fission (p. jii), {4) multiple fission (p. 35).

VORTICELLA I CONJUGATION 59

eight micronuclei. In each conjugant all but one of these perish and
the survivor divides into two, which correspond to the male and
female pronuclei of Paramecium. This division takes place while
the two micronuclei are lying in the region where the endoplasm
of the conjugants became continuous. One half of each micro-
nucleus passes into the larger conjugant, where the two fuse as
male and female pronuclei. The other half of each passes into the
smaller conjugant, but these halves, instead of fusing, degenerate
and disappear. The endoplasm of the small exconjugant is now
drawn into the larger, the ectopasm shrivelling up and falling
off. It will be seen that the conjugation of Vorticella takes place
in the same way as that of Paramecium, but that one of the two
exconjugants perishes and is partly absorbed by the other. ^

OTHER STALKED CILIATES

Carchesium is a small freshwater animal whose body consists
of a number of members, each of which has the structure of a
whole Vorticella. It arises from a Vorticella-like body, by divisions
like those which take place in the ordinary reproduction of
Vorticella, save that the division passes some way down the stem
and then stops, leaving the bells joined by their stalks. Thus
the body is increased by the addition of new members which
repeat the structure of the old. The whole body of a Carchesium is
said to be a colony, and its members are zooids. Reproduction is
brought about by the complete fission from the body of certain
zooids, which thus become asexually produced young (buds).
Each of these swims off, settles down, and forms by growth and
nuclear division a new colonial individual. Conjugation like that
of Vorticella also takes place. Each bell of Carchesium has its own
myoneme. and contracts independently of its neighbours ;
Zoothaminium is much like Carchesium, but there is one con-
tinuous branched myoneme, and the whole colony contracts
together. Epistylis is colonial but non-contractile. Many species of
Epistylis and some of Carchesium are epizootic on freshwater
Crustacea and other animals.

^ The student should beware of comparing the smaller conjugant of Vorticella
with a spermatozoon and the larger with an ovum. Ova and spermatozoa are
gametes of unlike kinds. The conjugants of Vorticella are unUke, hermaphrodite
parents, each of which forms two unlike gametes.

6o

CILIATE PROTOZOA

CILIATES OF THE FROG

The rectum of the frog contains an interesting population of
ciUates. which hve chiefly in the hghter-coloured contents of its

an., Anus

Pig. 32. — Ciliata from the rectum of the frog.

A Ot>alina ranarum ; B, Balantidium mtozoon ; C, Nyctotherus cordiformis.
; C.V.. contractile vacuoles ; ec, ectoplasm ; en., endoplasm ; g., gullet ; meg., meganucleus ;

mi., micronucleus ; nu., nuclei ; pst., peristome.

Fig. 33. — Opalina ranarum.

A , Ordinary individual in longitudinal fission ; B, the same in transverse fission ; C, small encysted
individual (distributive phase) ; D, gamete ; E, encysted zygote.

foremost region (Figs. 32, 33). Balantidium entozoon differs
from B. coli (p. 77) in having four contractile vacuoles and a
longer peristome. Nyctotherus cordiformis resembles the Balantidia
in its general features, but is bean-shaped, with a long gullet
placed in the middle of the hollow side, an undulating membrane,
one contractile vacuole in the hinder part of the body, and a
remarkable permanent anus, lined with ectoplasm, at the hind
end.

CILIATES OF THE FROG 6l

More numerous and conspicuous than either of these is Opalina
rananim, a flat, oval, pale-straw-coloured ciliate of very large
size (i mm. long), uniformly covered with equal cilia, and without
mouth, peristome, contractile vacuole, or trichocysts. It has many
nuclei, unlike those of other Ciliata in being all of one kind. The
life-history is also very unlike that of other Ciliata. Nuclei and
cytoplasm divide independently (the latter alternately in a
longitudinal and a transverse direction), and during the greater
part of the year keep pace with one another and with growth, so
that the appearance of the mature animals remains the same ;
but in the spring the division of the cytoplasm gains, so that
small individuals with 3-6 nuclei result. It is said that at this
time a portion of the chromatin of the nuclei passes in granular
or ' chromidial ' form into the cytoplasm, where it perishes. The
little individuals now encyst. The cysts are passed by the frog
into the water and there swallowed by tadpoles, in which they
hatch, and their cytoplasm divides to form uninuclear gametes,
the nuclei meanwhile undergoing a reducing division. The gametes
conjugate, and the zygote encysts. Probably it always at this
stage passes out of the host and enters another, where it hatches.
From the cyst emerges a uninuclear ciliate which grows into the
adult. Daring the whole process cilia are lost only in the zygote
cyst.

THE PROTOZOA AS PARASITES OF

MAN

The interest which the study of the Protozoa has for mankind
is not merely theoretical, in virtue of the remarkable pecuHarities
of their organisation, but is very near and practical, by reason
of the fact that a number of them live in the bodies of man,
and that there they sometimes cause serious diseases. In this
chapter we shall study briefly examples, drawn from all the four

Fig. 34. — Entamoeba, x c. 1000. — After Fantham.

A, E. coli ; B, E. histolytica.
b.c, Ingested red blood corpuscle ; f.v., food vacuole ; nu., nucleus

ps., pseudopodium.

classes of the group, of which man is a host — that is, which he har-
bours as parasites. In so doing, our attention must be given both
to facts which, directly or indirectly, are of medical importance,
and to others which have wider biological significance.

ENTAMCEBA

The several kinds of Entamoeba (Fig. 34) differ from Amoeba
in that they have no contractile vacuole.^ They have one or two
large blunt pscudopodia, chiefly composed of ectoplasm, and they
are all parasites, usually in the alimentary canal of one of the
vertebrate animals. E. coli is about 20-30 /x in diameter, and lives
in the upper part of the large intestine of man, feeding upon
the bacteria which infest that region, and also upon the remains

' A contractile vacuole has been found in one organism which has been classed
with the Entamcebts.

6a

ENTAMCEBA

63

of the food of its host. It never attacks the tissues of its host and
is harmless, and possibly sometimes even beneficial by keeping
down the bacteria. Its life-history differs considerably from that
of Amceha proteus. In the intestine it reproduces by binary fission,

Fig. 35. — Entamceba histolytica.

a and b, Amoebse as seen in fresh stools, showing blunt ectoplasmic
pseudopodia, non-contractile vacuoles, ingested red corpuscles,
and in a, nucleus ; c, an amoeba as seen in a fixed preparation ;
d section of wall of liver abscess, showing an amoeba of spherical
form. The rounded amoebae on this plate must not be confused
with the encysted form.

and some of the small products of fission become rounded, expel
all their contained food, and encyst. In the cyst there is at first
a single nucleus, but after proceedings in which some of its
chromatin is lost, while a large glycogen-filled vacuole temporarily
appears in the cytoplasm, this divides to form eight. The ordinary
EntamcehcE die in the faeces. So do the cysts if the faeces dry.

64

THE PROTOZOA AS PARASITES OF MAN

/

\

t Z'

but if tlu V remain moist until they reach water or human food
and are swallowed by a man the cysts germinate in the intestine

of the new host ; the protoplasm escapes
from the cyst as a syncytium with eight
nuclei, but the cytoplasm divides so that eight
small amoeba are formed. By these the cycle
is re-started.

Entamcvha histolytica, sometimes known as
E. dysenteries, also inhabits the human large
intestine (Figs. 35, 36). It varies much in size
but reaches greater dimensions than E. coli,
from which it also differs in being more active
(it moves by means of a single large pseudo-
podium), in having a distinct ectoplasm over
the whole surface of the body, in taking up strongly, while still
alive, the stain known as ' neutral red ', and in that the principal

Fig. 36. — Entamaba
histolytica in tlu;
encystc<l condition.
— After Tantham.

/

@e)5?©?§©9

Fig. 37. — The life-cycle of Entamoeba histolytica.

l-n I. Binary fission ; a-/, encystnient and multiple fission.
in whirh th.- nucleus has divided ; c. cyst in which the second division has takei

,,.„,,. , , , ''^"''^'/''■'"^'''V' '•'■■'■*" ^'""'^'^'^''"'^'^'^'"^•^■'th four nuclei which lie close together-
/, emiii auiufbul* formed from r by a complicated process of division. logeiner ,

ENTAMCEBA 65

chromatic body or ' karyosome ' of the nucleus is centrally
placed. Unlike E. coli it may attack the mucous membrane of the
intestine, probably by the secretion of an enzyme, so forming
ulcers. Some individuals penetrate the blood vessels in the same
way, and are carried by the circulation to the liver, where they
may set up abscesses. When living free in the gut it feeds on
faecal particles just as E. coli does, but when it invades the tissues
its chief food is red blood cells ; it can also absorb liquid food.
E. histolytica is found all over the world, but only about ten
per cent, of those human beings who are infected show clinical
symptoms of its presence in the form of dysentery. Its life-cycle
(Fig. 37) appears to differ from that of E. coli chiefly in the
number of the cyst nuclei, of which there are only four, though
after emergence these divide with the cytoplasm to form eight
little amoebae. The cysts are the only infective forms, and are
acquired from carriers who do not show the disease, either
through food or water, or after having been ingested by flies or
cockroaches.

TRYPANOSOMA

Flagellate Protozoa of the genus Trypanosoma are responsible
for various very dangerous diseases of man and animals in warm
countries. They are parasitic in the blood and other fluids of
backboned animals, but at one stage in the life-history live in
invertebrates which suck the blood of the vertebrate hosts. The
body (Figs. 38, 39) is worm-like, about one-thousandth of an
inch in length, tapering towards the ends, but more pointed in
front than behind. The shape of the body is maintained by a
strong pellicle. A single flagellum stands at the front end, and
from its base an undulating membrane or sheet of contractile
protoplasm runs along one side nearly to the hind end. The flag-
ellum is continued as a strongly-staining thread along the free
edge of the membrane, and terminates behind in a minute ' basal
granule ' or blepharoplast, embedded in the cytoplasm. By the
working of the undulating membrane and flagellum the animal
swims rapidly with a graceful wavy movement, either forwards
or backwards. There is no contractile vacuole. Near the middle
of the body is an egg-shaped nucleus, and a smaller mass, which
stains like the nucleus, stands close to the blepharoplast. It is
known as the parabasal body, and was formerly called the kineto-
nucleus, but though its functions are unknown it seems to have

66 THE PROTOZOA AS PARASITES OF MAN

nothing to do with either nucleus or movement. Trypanosoma
has no mouth, but nourishes itself, like Monocystis, by absorbmg
through the surface of its body substances obtamed from the

juices of its host. . x v,- v, •+

In spite of the immense amount of investigation of which its
medical importance has caused it to be the subject, the life-
histor>' of Trypanosoma is not yet thoroughly understood. In
the case of T. gambiense, the cause of the terrible sleeping sick-
ness of West and Central Africa, the following facts have been
established. In the body of an infected man the parasites live at

t'.JVUi

Fig. 38. — Trypanosoma gambiense.

A, B,C, Slender, intermediate, and stumpy forms from man ; D, ' latent body ' ; E, slender form from gTit
of fly ; F, crithidial form from salivarj' gland of fly ; G, ripe form from proboscis of fly.

bpt., blepharoplast ; ft., flagellum ; k.nu., parabasal body (kinetonucleus) ; tr.nu., trophonucleus ;
u.m., undulating membranes.

first in the blood, but presently make their way into the lymphatic
glands, and thence into the fluid of the spinal canal and cavities
of the brain. While they are in the blood alone the man suffers
from ' Gambia fever ', but when they reach the central nervous
system the drowsiness which is characteristic of sleeping sickness
comes on, and increases, and is followed by a wasting of the body,
till death almost inevitably results. The individuals found in the
human host are not all alike, some being long and slender, some
short and stumpy, and some intermediate in shape. The thin
forms are the youngest, the animals growing stouter as they
mature, and becoming stumpy in succeeding generations. There
are also differences in size, due to age and to the fact that the

TRYPANOSOMA 67

binary longitudinal fission by which reproduction takes place
is sometimes unequal. In fission, first the blepharoplast divides,
then the parabasal body, and finally the nucleus, while the flag-
ellum and membrane are doubled, but probably not by division.
During the progress of the infection some of the trypanosomes
pass into certain of the internal organs of their host, especially
into the spleen and lungs. There they lose their flagella and
become an oval shape. In this condition they show resemblances

Fig. 39. — Trypanosoma gamhiense,
A stained preparation of the blood of an infected guinea-pig, showing blood corpuscles and parasites.

to the predominant phase of the organism known as Leishmania,
which is the cause of the kala-azar disease and of Delhi boil.
True Leishmania stages, which presently revert to the flagellate
condition, do occur in the life-cycles of other trypanosomes. It
has been supposed that these phases of T. gamhiense are of a
similar nature and that they revert and thus make good the loss
of flagellates in the blood when, as happens between the fits
of the fever, the flagellates are reduced in number by the secretion
of ' antibodies ' (p. 151) by the host. On this theory they have
been called ' latent bodies '. It is more probable, however, that
thev are individuals in a state of degeneration.

68

THK

PROTOZOA AS PARASITES OF MAN

The invertebrates responsible for the spreading of Trypanosoma
gamhunse are the tsetse flies. Glossina palfalis and G. tachinotdes
(Fig 40) These are similar in size and general habits to the
clegs of the English countryside. They suck the blood of various
backboned animals-cattle, antelopes, birds, reptiles, and so
forth as well as man— and thus take into their stomachs such
parasites as ma>' infest the blood vessels of its victims. When the
object of the attack of Glossina is infected with the trypanosome
of sleeping sickness, the insect becomes capable of inoculating
a new host in the course of its feeding. The power is soon lost, but
is regained after about twenty days. It seems probable that the
first inoculations are made with trypanosomes which are still

Fig. 40. — The tsetse fly Glossina palpalis. — From Thomson.

fresh in the proboscis of the insect, but the later ones with
individuals which arise from the stumpy forms after passing
through a course of development in the insect's alimentary
canal and salivary glands. During this development the stumpy
forms become first long and slender, then, attached to the wall
of the salivary gland, they pass through a ' crithidial phase ' in
which the membrane starts in front of the nucleus ; finally, as
stout-bodied, mature individuals, they are injected with the
saliva when a new victim is bitten.

Besides gambiense there are known a number of other trypano-
somes : T. rhodesiense which causes a sleeping sickness in the
southern part of Central Africa, and is transmitted by G. morsitans,
G. swynnertoni and G. pallidipes ; T. brucei, the cause of a disease
of horses and cattle in South Africa ; T. equinum which causes a

TRYPANOSOMA 69

horse disease in South America ; T. cruzi, the cause of a disease in
children in the same continent, and so forth. The same species has
sometimes been described from different parts of the world and
given more than one name, so that there is much confusion. Not
all trypanosomes are carried by tsetse flies. Many, perhaps all,
have a wild host in which they are harmless, though in the un-
accustomed bodies of men or domestic animals they are highly
dangerous. Formerly no treatment was of any avail against them ;
recent research has produced several synthetic drugs, mainly
organic compounds of arsenic and antimony, which can cope with
at least the African species, but the best way to combat them is to
avoid the attacks of the insects which transmit them. Thus the
clearing around places frequented by human beings of the bush
which is the haunt of Glossina has led to a decrease in the number
of cases of sleeping sickness.

PLASMODIUM

MALARIA PARASITES

A much more widespread though less dangerous type of
disease than sleeping sickness is malarial fever or ague. This is
brought about by a minute protozoan parasite known as Plas-
modium,'^ belonging, like Monocystis, to the Sporozoa. The
dangerous stage of the parasite corresponds to the trophozoite
of Monocystis. It lives in the red blood corpuscles, and is at first
a round body with the appearance of a ring (Figs. 41, 42), owing
to the presence of a large (non-contractile) vacuole in its middle.
It has a single nucleus and no mouth, and must absorb food from
its surroundings through the surface of its body. As it grows, it
loses the ring-like appearance and forms in its cytoplasm granules
of pigment, which is no doubt derived from the haemoglobin of
its host. At the same time P. vivax puts out pseudopodia. When
the parasite is ready to reproduce, it is known as a schizont. Its
reproduction, called schizogony, takes place by multiple fission.
The pseudopodia are withdrawn and the nucleus divides repeatedly
tiU there are present some sixteen smaller nuclei. These lie in the
outer part of the body, and most of the cytoplasm now gathers

1 It is unfortunate that this name is also in use to denote a type of relation
of nuclei to cytoplasm — namely, that in which a syncytium is formed by the
fusion of free cells (p. 499) — which, as it happens, is not found in the malarial
parasite.

7^>

MALARIA PARASITES

Fig. 41. -Plasmodium vivax, the tertian ague parasite. — From Muir and Ritchie.

A, Several younK ring shaped trophozoites within the red corpuscles, one of the latter enlarged and showing
a dott. ,! ,i,irf- .r .T>. .. • B, a larger trophozoite, containing pigment granules ; C, two large trophozoites,
*•"• it variation in form ; D, large trophozoite assuming the spherical form and showing

isol..;- .1 .■ .iKiir ii^ uf chromatin, preparatory to schizogony ; E, schizont which has produced eighteen
racrozoites (schizozoites), each of which contains a small collection of chromatin ; F, merozoites
>et free. ( y 1000.)

MALARIA PARASITES

71

. C

Fig. 42. — Plasmodium falciparum, the pernicious malaria parasite. — From Muir

and Ritchie.

A Two small ring-shaped trophozoites within the corpuscles ; B, a ' crescent ' or gamont, showing the
' envelope of the red corpuscle. Figs. C-F illustrate the changes in form undergone by the gamonts
outside the body ; Fig. F shows a male gamont that has -mdergone ' e.xtlagellation, or the formation
of microgametes, which are seen attached to it. { x 1000.)

_, iHF PROTOZOA AS PARASITES OF MAN

rouiui them so as to form a rosette of little, uninucleate individuals
— thf morozoites or schizozoites— which surround some ' residual
protophisni ' containing the pigment granules. Next the sheU of
the red corpuscle breaks up, setting free the merozoites mto the
plasma, where each of them proceeds to infect a new corpuscle,
into which it bores its way with a pointed end.

The time which is required to repeat this cycle of asexual
reproduction varies with the species of parasite. Thus of the four
species of Plasmodium which infest man, P. vivax and P. ovale set
free a generation of merozoites in forty-eight hours, P. malaricB

in seventy-two hours, and P. falciparum
in thirty-six to forty-eight hours or at
irregular intervals. The attacks of fever
occur when the corpuscles break up,
probably because there are then set free
substances formed during the metaboHsm
of the parasite which prove poisonous to
the host. So it comes about that the fever
caused by P. vivax returns every third day,
and is known as ' benign tertian ague ',
and that caused by P. malarice (quartan
ague) returns every fourth day, while P.
falciparum causes malignant tertian ague,
or irregular (quotidian) fevers which are
more or less continuous. These latter are
the ' pernicious malaria ' of the tropics.
Many generations of merozoites may succeed one another during
the course of the illness, but eventually the resisting powers
of the host begin to get the better of the infesting organisms,
or, on the other hand, the patient may be about to die. In
either case it behoves the parasite to arrange for the con-
tinuance of its race elsewhere. This is done by the provision
of a fresh kind of individual, adapted to transmission by gnats to
new human hosts. These stages, because they give rise in the insect
to gametes, are known as gamonts, or gametocytes (Fig. 43),
though the latter name more properly belongs to cells of similar
function in the bodies of Metazoa (p. 79). In the parasites of
tertian and quartan fevers they are rounded, in that of pernicious
malaria crescent-shaped. They are larger than the schizonts and
have more of the dark pigment.

It is said that the gamonts have no ring-stage in their develop-

FiG. 43.— Gamonts of
Plasmodium falciparum.

a. Before taking on the sausage
shape; 6', male gamont in
&ausai;e stage ; b^, female
gamont in the same stage.
The outline is that of the
red corpuscle.

MALARIA PARASITES

73

<ES

ment. They remain in the corpuscle where they arose from a
merozoite, and undergo no change unless they be sucked in by a
gnat ; but in that case, whereas all other forms of the parasite
die and are digested by the gnat, the gamonts, becoming free
by the breaking up of their corpuscles, proceed to develop
gametes. They are of two kinds, male and female, the former with
a larger nucleus and more lightly staining cytoplasm than the
latter. In the male gamont the nucleus breaks rapidly into some
half-dozen fragments, leaving a residual mass in the central
cytoplasm. The daughter nuclei come to the surface, and grow
out, with a suddenness which is almost explosive, into fine threads
of nucleoplasm, projecting from
the body in scarcely perceptible
sheaths of cytoplasm. These are
the microgametes. They lash
violently, dragging about the
residue of the gamont body, till
they break free. The remains of
the gamont perish. The female
gamont, by a process in which
the nucleus loses a part of its
contents, becomes a single macro-
gamete. It is now ripe for fertilisa-
tion by a microgamete which
penetrates the body, and the nuclei

(male and female pronuclei) fuse. The zygote changes from a
rounded to a worm-Hke creature called an ookinete, which glides
about by contractions of its body, pierces the epithelium of the
insect's stomach with one end, which is pointed for the purpose,
and comes to rest in the sub-epitheHal tissue, where it rounds
itself off and forms a structure called an oocyst. It is also known as
the sporont on account of its further history, which is as follows.
Through its thin cyst-wall the parasite continues to absorb
nutriment, and grows in size, bulging out the wall of the stomach
into the body cavity so as to form a kind of blister (Fig. 44).
As it grows, its nucleus multiphes by binary fission and cytoplasm
becomes concentrated round each nucleus to form a body known
as a sporoblast. Now the nucleus of each sporoblast divides
repeatedly and the surface of the body grows out into slender
processes, into each of which one of the daughter nuclei passes.
Finally the processes break off, and so the cyst contains hundreds

Fig. 44. — Part of the alimentary
canal of a mosquito infested with
Plasmodium. — From Lankester's
Zoology, after Ross.

cy., Cysts of the parasite ; int., intestine;
M.t., Malpighian tubes; ces., oesophagus;
St., stomach.

74 THE PROTOZOA AS PARASITES OF MAN

of needle-like sporozoites together with some residual protoplasm.
The ripe cysts burst and scatter their contents into the body
cavity of the gnat, from which the sporozoites pass into the
salivary glands. In these the gnat secretes a Hquid which is
injected into its prey when it bites ; this saliva contains an anti-
coagulant, which prevents the clotting of the blood. When next
it feeds, the little parasites pass with the saliva along the proboscis
into the blood of the man on whom the gnat is feeding.

It has been known for a long time that for about ten days
after infection there is a latent period, during which there is
no fever and no parasites can be found in the blood. It has now
been shown that in P. vivax and P. falciparum they spend part
of this time in the liver. No sporozoites can be found in the blood
half an hour after the gnat bites, and by the fourth day they are
recognisable in the cells of the liver. Here they grow as schizonts
to four times their original diameter, and on the seventh or eighth
day from the bite divide to form merozoites, about a thousand
coming from each schizont. Some of the merozoites invade the
red cells of the blood and begin the cycle of fever which we have
already described in infections of P. vivax, others remain in the
liver and repeat the cycle there, but in P. falciparum this organ
is emptied. Neither species causes any pathogenic reaction in
the liver.

Long after apparent recovery from an attack of malaria caused
by P. vivax or P.. malarice, a patient may suffer a recurrence of
the disease. This is probably due to persistence of a few^ of the
trophozoites in the liver, where they continue to carry out the
exoerythrocytic cycle, i.e. that which goes on outside the red cells.
Plasmodium vivax may live in man for as long as ten years,
though it usually disappears after two. P. falciparum does not
usually persist for more than twelve months.

Malaria is very widely distributed ; it is found in almost all
tropical and sub-tropical lands, and to a varying degree in the
temperate zones. In England it was formerly well known under
the name of ague, but fortunately this was nearly always of the
benign sort caused by P. vivax or P. malaricB. The reason for this
is that P. falciparmn needs a relatively high average temperature
— about 21° C. in summer and 8° C. in winter — to carry out
the insect part of its life-cycle, so that although the disease can
continue when a traveller brings it here from the tropics, and may
infect a few gnats and so a few more people, long-continued

75

Fig. 45. — -Diagram of the life-cycle of Plasmodium cynomolgi ; that of P. vivax
is now known to be similar. — From Shortt. Trans. Roy. Soc. trap. Med. Hyg.,
1948. 42, 227.

1, Sporozoites from salivary glands of mosquito enter liver cells.

2. Liver cell containing early stage of pre-erythrocytic parasite.

3 and 4. Stages in development of the pre-erythrocytic schizont in liver cells.

5. Fully developed pre-erythrocytic schizont rupturing and releasing pre-erythrocytic merozoites.

6. Liver cell containing merozoite of exo-erythrocytic cycle of schizogony.

7-9. Remaining stages in exo-erythrocytic schizogony ending in second generation of merozoites.
10. Red cell of circulating blood.

11-14. Stages in erythrocytic schizogony in circulating blood.
15, Fully developed erythrocytic schizont rupturing and releasing erythrocytic merozoites and

gamonts.
16-20. Repetition of erythrocytic schizogonv.
21-22. Development of male gamont in circulating blood.
23-24. Development of female gamont in circulating blood.

25. Wall of stomach of mosquito.

26. Exflagellating male gamont producing microgametes in stomach of mosquito.

27. Female gamont extruding polar bodies and so becoming a macrogamete.

28. Microgamete free in stomach o J mosquito and seeking macrogamete.

29. Zygote.

30. Ookinete about to penetrate epithelium of stomach.

31. Oocyst lying under elastic membrane on outer surface of stomach.
32-33. Stages in development of oocyst with production of sporozoites.

34. Rupture of mature oocyst with dispersion of sporozoites.

35. Salivary gland of mosquito containing mature sporozoites.

76

THE PROTOZOA AS PARASITES OF MAN

epidemics are impossible. In the tropics P. falciparum is endemic,
and all children become infected by the age of eighteen months,
but after ten years of age they develop a partial immunity. The
chief symptom shown by such patients is a considerable lethargy.
The insect which carries the parasite is of the genus Anopheles
(Fig. 46), different species being important in different parts of

Anopheles

CULEX

Fig. 46. — A comparison of the various stages in the hfe-history of Anopheles (left)

with those of Culex (right). — From Shipley.

Note how the larvce and pupae hang from the surface film of the water (represented by a thin line). The
organs by which they are suspended contain air tubes, and if these be prevented by a film of paraffin
from functioning the insect is drowned. Note also that the eggs of Culex cling together as a raft.

e., two views of an egg, magnified.

the world. There is no justification for trying to restrict the
word mosquito, which is simply the Spanish for little fly, or gnat,
to this one genus, while using gnat only for the commoner Culex ;
there are many other genera of related flies, and not all species
of Anopheles are natural carriers of malaria. All gnats lay their
eggs in water, though there are great variations in their ecological
requirements, and the easiest way to reduce malaria is to prevent
the breeding of the insects (p. 257). For long the only direct attack
on the parasites was by quininC; a drug discovered empirically by

MALARIA PARASITES

n

the South American Indians. Now other and more active synthetic
substances are available, especially mepacrine, paludrine and
chloroquine.

TERMINOLOGY

The following summary of the Ufe-history of the malaria parasite
gives in the left-hand column the terms generally used in zoology, and
in the right-hand column those used in medicine. Sohd lines represent
a reproductive process ; dotted ones development without reproduction.

Zoological Terminology

Medical Terminology

►•TROPHOZOITE<

(Ring-stage, amoebulae)

SCHIZONT

(schizogony)

SCHIZOZOITES-

* ^TROPHOZOITE^

I

Development

in man or

other vertebrate

SCHIZONT

(schizogony)

i

•MEROZOITES

GAMONTS

I

MALE
GAMETES

FEMALE
GAMETES

Development
in insect

L

GAMETOCYTES

Exflagellation

MICROGAMETES MACROGAMETES

ZYGOTE-

(Gnat)

(Mosquito)

ZYGOTE-

J

OOKINETE

SPORONT

OOCYST

I

— SPOROZOITES

i

SPOROZOITES

BALANTIDIUM CO LI

The Ciliata are not commonly parasites of man, but Balantidium
coli, which closely resembles B. entozoon (p. 60), may cause serious
disease, and even death. It normally lives in the large intestine
of the pig, which it appears not to harm, but if the cysts which
it forms are swallowed by man, it may penetrate the epithelium
of the gut and cause dysentery. It may occasionally be carried in
the blood stream to other parts of the body and there cause
abscesses. It occurs in all parts of the world.

THE CLASSIFICATION OF PROTOZOA

The one characteristic which the examples that we have discussed
have in common is that, whether they have one nucleus or several,
no nucleus is separated from another by a cell wall ; or, in
physiological rather than morphological terms, no nucleus ever
exerts its controlling influence on a part only of the cytoplasm.
This is true of all the animals which are commonly called Protozoa
with the exception of a small group called Cnidosporidia, which
have peculiar structures called pole capsules which seem to have
their own nuclei. As we have seen, a formal distinction between
animal and plant flagellates is impossible, but if we begin by
saying that the Protozoa are animals, the criterion of absence of
cell walls makes a workable, if not entirely adequate, definition.
It avoids the necessity of defining the cell, and so the necessity
of deciding whether the Protozoa are all non-cellular or not. It
is clear that they are not all unicellular, for all definitions of a
cell agree that it has but one nucleus.

At some point, however, we shall have to consider the mieaning
of the word cell, and it may be said here that the two views
which have been current differ rather in their metaphysical
outlook than in their morphological terminology. According to
one view, a cell is a part of an organism, and as such it can have
no fundamental similarity to a protozoan such as Trypanosoma.
The other view, which looks on uninucleate Protozoa as being
single cells, is historically connected with the evolutionary view
that all other animals are derived from Protozoa by a series of
fissions after which the daughter units remained in contact, as they
do in the embryology of higher forms (Chap. 28). How^ever
probable on general grounds one may choose to consider this
hypothesis, there is not a single piece of evidence for it, and it is
therefore unwise to make much of it. As was first stressed by
Dobell, a protozoan carries out all the ordinary activities of the
so-called higher animals, and differs from them only in the organs
used ; our increased knowledge now makes this even clearer.
For some reason or other Amoeba is generally venerated as the
first ancestor of all animals, but for this view also there

7«

THE CLASSIFICATION OF PROTOZOA 79

is no evidence. As Dobell wrote in 191 1 of this universal
parent :

* Concerning this fabulous " amoeba " we know nothing,
save that its correct systematic position is probably in the
group which contains the centaur, the phoenix, and the
hippogriff.'

If the Protozoa are indeed the first animals, one can be fairly
confident that the primitive form was flagellate rather than
amoeboid, for it is the flagellate forms which connect most closely
with plants.

The group Protozoa is a phylum, but it is sometimes also raised
to the rank of a sub-kingdom, the only other sub-kingdoms being
the Parazoa or sponges (p. S8), and the Metazoa, including all
other animals. All modern writers are agreed on the main divisions
of the Protozoa, but there is some variation in the names used.

CLASS L—MASTIGOPHORA

Protozoa which are predominantly flagellate although they
may form pseudopodia. It is best to avoid the name Flagellata as
a title for this group, since the word is used by botanists with a
different and wider connotation. There are two sub-classes —
Phytomastigina, which possess chlorophyll or closely resemble
forms which do; and Zoomastigina, which neither possess chloro-
phyll themselves, nor resemble forms which do. Most of the
Phytomastigina, such as Polytoma and Eiiglena, are on the border
line between the animal and plant kingdoms ; others, such as
Chlamydomonas, have no claim to be called animals, and are only
included, by some zoologists, as Mastigophora because of their
structural similarity to colourless forms. The biochemistry of the
sub-class is diverse, which suggests that they are not closely
related to each other, and there is some evidence that they are
degenerate Algae. The Zoomastigina, on the other hand, are
entirely animal in nature, and have little or nothing in common
with the Phytomastigina except flagella. In only one genus is any
trace of a syngamous process known. Many have a somewhat
compHcated internal structure of nuclei, skeletal rods, conducting
threads and so on, and many also have several flagella. Trypano-
soma is an example with one flagellum. Trichomonas, parasitic in
the gut, has several.

8o

THE CLASSIFICATION OF PROTOZOA

CLASS II.—RHIZOPODA

Protozoa which have pseudopodia for most of their life histories,
and flagella, if at all, in young stages only. Besides forms like

shell

gas
vacuole

nucleus

ventral
opening
of shell

B

Fig. 47. — Testacea.

A. Difflugia ; living individual, showing shell of sand grains and one pseudopodium ; B., Arcella; stained
individual with eight nuclei, dorsal view ; the opening of the shell is seen through the cytoplasm.

Amceha (Order Amcebina) there are others with blunt pseudopodia
and shells (Testacea, Fig. 47), with shells and reticulate pseudo-
podia (Foraminifera), and with fine radial pseudopodia (Radio-
laria). The order Heliozoa has pseudopodia which have a stiff
supporting rod. Flagellate gametes are common throughout the
sub-class.

CLASS IIL.—SPOROZOA

This group is rather a waste-paper basket for parasites which
have no organs of locomotion except in young or distributive
stages, and includes forms of no particularly close relationship.
Many are intracellular in habitat at some stage in their life-history.
Examples are Monocystis and Plasmodium. The Cnidosporidia,
the only Protozoa with cells, are put here, though whether they
belong here is another matter.

CLASS IV.—CILLOPHORA

Protozoa with nuclei of two types, one concerned with somatic
matters and the other with syngamy, and cilia at some stage of
the life-history. There are two sub-classes — Suctoria, in which in
the adult the cilia are replaced by tentacle-hke structures (Fig.
48), and Ciliata, which bear cilia throughout life. The latter are

CILIOPHORA

8l

further subdivided by the arrangement of the ciha. Opalina has a
number of nuclei which are all alike, but it is claimed that each
contains two sets of chromosomes (p. 688), one representing the
micronucleus of other species, and one the meganucleus. Opalina,
does not, however, undergo the typical ciliate conjugation, which
we have seen in Paramecium and Vorticella. The cilia of Ciliata

Fig. 48. — A suctorian (Podophrya) feeding on the ciliate Colpidium —
From Kitching, /. exp. Biol., 1952, 29, 255.

have a similar structure to those of Metazoa (p. 503), but can
reverse their direction of beat, which those of Metazoa other
than coelenterates never do.

On account of the difference in the nuclei some protozoologists
feel that the Ciliophora should be raised to the rank of a sub-
phylum, to which the name Heterokaryota has been given.
When this is done the other three classes may be united as
Subphylum Homokaryota.

8

SPONGES

CHOANOFLAGELLATA

That the objects of everyday use known as sponges are the
skeletons of marine animals is tolerably common knowledge ;
but what may be the nature of the soft parts of such animals
is less well known. A hving sponge, however, is not only a very
remarkable, but even a fairly highly organised creature. Its
nearest relations, outside its own sub-kingdom, are to be found
among the flagellate Protozoa. Certain of these, known as the
Choanoflagellata, with a shape of body not unUke that of Poly-
toma, but having Hke Trypanosoma a single fiagellum, are fastened
by a stalk at one end, and have at the other end the fiagellum,
and around it a delicate collar of protoplasm. Fig. 49, a shows
one of these organisms. The Choanoflagellata often secrete a
cup or case which houses and protects the body (Fig. 49, h) and
sometimes the cups form a kind of colony, being joined together
to give a shape Hke that of a filamentous alga (Fig. 49, c) ; but in
one of them, known as Proterospongia (Fig. 50), the substance
secreted takes the form of a mass of jelly, in which a number of
separate individuals are lodged. Some stand on the outside, half-
imbedded, with the collared end projecting ; others pass into the
jelly and there become amoeboid, and some of these divide into
minute spores, which may be gametes. The simplest sponge is a
multicellular organism, the elements of whose body suggest a
rearrangement of those of the colony of Proterospongia.

A SIMPLE SPONGE

This simple sponge is a little creature, known as the Olynthus,
which is found only as a fleeting stage in the development of
a few members of the group ; but the bodies of all may be regarded
as ideally derived from it, even though it may not appear as a
stage in their life-history. It is a hollow vase, perforated by many
pores, and having at the summit a single large opening, the
osculum. Through the pores water constantly enters it, to pass out

through the osculum. Herein it and its kind differ from all other

82

SIMPLE SPONGES 83

animals, using the principal opening not for intaking — as a mouth
— but for casting out. The wall of the vase consists of two layers,
a gastral layer, composed of collared
flagellate cells, or choanocytes, standing
side by side but not touching, which
lines the internal cavity or paragaster
except for a short distance within the
rim ; and a dermal layer, which makes
up the greater part of the thickness of
the wall and is turned in a little way at
the rim. This layer again consists of two
parts, a covering layer of flattened cells,
known as pinacocytes, rather like those
of a pavement epithelium, but with the
power of changing their shape, and the
skeletogenous layer, between the cover-
ing layer and the gastral layer. The
skeletogenous layer consists of scattered
cells, embedded in a jelly of collagen. ^^ ^ ., .

^, , -V A, n Fig. 49.— Choanoflagellata.

The most numerous of these cells are ^^ ^^^^^,^^^ t, saipin,.ca;

engaged in secreting spicules of calcium cPoiyaca.

carbonate by which the wall is supported.

They wander from the covering layer into the jelly, and then

each divides into two, and the resulting pair secrete in their

protoplasm, which is continuous,
a needle-like spicule which pre-
sently outgrows them. Most often
the original spicule cells come
together in threes before this
process, so that the three spicules
which they secrete become the
rays of a three-rayed compound
spicule. This lies in the wall with
two rays towards the osculum
and one away from it. Some-
times a fourth cell joins the
others later, and forms a fourth
ray which projects inwards to-
wards the paragaster. Often there

are simple spicules which project from the surface of the sponge.

Other cells, known as porocytes, of a conical shape, extend

Fig. 50. — Proterospongia.

a, Nucleus of a collared member of the colony ;
b, contractile vacuole ; c, amoeboid member ;
d, division of amoeboid member ; e, flagellate
member with collar contracted ; /, jelly ;
g, member forming spores.

^4

SPONGES. PHYLUM PORIFERA

,ozc

through the jelly, having their base in the covering layer while
their apex reaches the paragaster between the choanocytes. Each
is pierced from base to apex by a tube, which is one of the pores.
Besides these cells of the dermal layer there are in the jelly
wandering amcrboid cells which appear, in some cases at least,
to belong neither to the gastral nor to the dermal layer, but to be

descended independently from blasto-
meres of the embryo. Some of them
become ova ; others, it is believed, give
rise to male gametes ; the rest are
occupied in transporting nutriment and
excreta about the sponge. The current
which flows through the body is set up
by the working of the fiagella of the
choanocytes. It carries with it various
minute organisms which serve the sponge
for food, being swallowed, in some way
which is still in dispute, by the collar
cells. These digest the food, rejecting the
indigestible parts into the space within
the collar, and passing on the digested
food to amoebocytes, which visit them to
obtain it.

-sp

COMPLEX SPONGE BODIES

Fig. 51. — The Olynthus
stage of a calcareous
sponge, from which a
portion of the wall has
been removed to expose
the paragaster.

Osculum ; p., pores ; sp.
spicule in wall.

osc

No sponge remains at this simple stage
throughout its life. At the least the body
branches and thus complicates its shape,
and then often new oscula appear at the
ends of the branches. A higher grade is
reached when, as in the calcareous sponge
Sycon, the greater part of the vase is covered with blind, thimble-
shaped outgrowths, regularly arranged, and touching in places, but
leaving between them channels, known as inhalant canals, whose
openings on the surface of the sponge are often narrowed and are
known as ostia. The thimble-shaped chambers are known as
flagellated chambers, and are lined by choanocytes, but these
are now lacking from the paragaster, where they are replaced
by pinacocytes. Water enters by the ostia, passes along the
inhalant canals and through the pores, now known as prosopyles,

COMPLEX SPONGE BODIES

85

into the excurrent canals, leaves these through the openings,
known as apopyles, by which they communicate with the para-
gaster, and flows outwards through the osculum. A third grade
is found in sponges such as the calcareous sponge Leucilla, where

a

y-

ch^

Fig. 52. — Part of a longitudinal section of the wall of an Olynthus, including a

portion of the rim of the osculum.

a., Amoeboid cell ; ch., choanocyte ; e, flat covering cells (pinacocytes) of dermal layer ; e'., similar cells lining
the rim of the osculum ; j, jelly ; p, pore ; pc, young porocyte ; pc'., fully developed porocyte ; sp.,
spicule ; spc, spicule cell.

the wall of the paragaster is folded a second time, so that the
flagellated chambers, instead of opening direct into the paragaster,
communicate with it by exhalant canals lined with pinacocytes.

The three grades of sponge
structure, in which successively
the choanocytes line the whole
paragaster, are restricted to flag-
ellated chambers, or are still
further removed by the presence
of exhalant canals, are known
as the ' Ascon ', ' Sycon ', and
' Leucon ' grades. In many of the
sponges whose canal systems are
of the third grade, the flagellated
chambers are no longer thimble-
shaped, but small and round. As
the canal system has grown more
intricate, complication has taken
place also in the skeletogenous
layer. It has grown thicker, form-
ing outside the flagellated chambers a layer known as the cortex,
in which the inhalant canals ramify ; and there appear in it
branched connective tissue cells which can change their shape.
The effect of these advances in complexity is (a) to increase
the rate and precision of the currents of water through the

Fig. 53. — A branched calcareous
sponge of the first grade. — From
Sedgwick.

86

SPONGES. PHYLUM PORIFERA

sponge, (b) by strengthening the wall, to make it possible for
sponges to grow to a greater size.

SILICEOUS AND HORNY SPONGES

The sponges of which we have so far spoken have calcareous
skeletons. A vast number of others have a skeleton of siliceous

osc

a

I

E

Fig. 54. — Diagrams of the canal system of sponges. — Partly after Minchin.

exhx., Exhalant canal; inh.c, inhalant canal; fi.c. flagellated chamber; osc, osculum ; osL, ostium;

p., pore ; par., paragaster.

(flinty) spicules. In all these the canal systems are of one of the
more complicated types, and usually they are made still more
intricate by ramifyings of the paragaster, and the appearance
of numerous oscula, which put it into communication with the
water at many points. The sponges of domestic use belong to a
comparatively small group in which the skeleton is not spicular
but a network of horny fibres, usually strengthened by sand
grains embedded in the fibres. Their canal system is of the type
which has small, round chambers, and in most of them these

SILICEOUS AND HORNY SPONGES 87

communicate with the exhalant canals by narrow canals called
aphodae, as in the majority of the siliceous sponges. When the
sponge is prepared for human use, the soft parts are allowed to die
and rot, leaving the horny skeleton, which is then cleaned. The
large holes on the upper part of the dried skeleton mark the
position of the oscula ; in its interstices formerly lay the ramifica-
tions of the canal system. The softness and wearing qualities of the
sponge depend upon the fineness of the meshwork of its skeleton,

Fig. 55. — A diagram of the structure of a bath sponge.

exh.c, Exhalant canal ; inh.c, inhalant canal ; fl.c, flagellated chamber ; osc, osculum ; ost.. ostium ;
sd.c, subdermal cavity ; sk., one of the principal pillars of the skeleton, containing embedded sand
grains ; sk'., minor fibres of the skeleton.

and upon the amount of the sand particles which are embedded
in it. The true Bath Sponge {Euspongia officinalis) has very few
foreign particles. It is gathered in 10 to 15 fathoms of water, the
finest varieties from the Adriatic, coarser ones from elsewhere in
the Mediterranean, the West Indies, and Australian seas. Various
species of Hippospongia yield coarse kinds of sponge.

Sponges have free larvae, of several different kinds, but all are
covered with flagellate cells, by which they swim. The remarkable
feature of the metamorphoses by which these larvae become the
fixed adults is that the flagellated cells pass into the interior,
develop coUars, and become the choanocytes.

g3 sponges. phylum porifera

porifera: general features

The sponges are known in zoology as Porifera. In that their
bodies consist of many ' cells ', they might seem to be Metazoa.
But thev differ from all members of that group in several
important respects. In no metazoan are choanocytes found. In
none is the principal opening exhalant. In none is there during
development an inversion whereby a flagellated outer covering
becomes internal. Lastly, and perhaps most significantly, in a
sponge the ' cells ' are far less speciaHsed and dependent upon
one another than the cells of a metazoan. Many of them can
assume various forms, becoming amoeboid, collared, etc. Many
are isolated in the jelly, and when they touch there is no cyto-
plasmic continuity. There is no nervous system. Even the
choanocytes, though their efforts together produce a current, do
not keep time in their working. In short, the Porifera are practi-
cally colonies of Protozoa. For these reasons it is best that, in
a classification of animals, they should be given the same rank as
the Protozoa and Metazoa : when this is done, they are known as
phylum Porifera, sub-kingdom Parazoa.

HYDRA AND OBELIA

HYDRA

If a handful of weeds gathered from a freshwater pond be placed
in a beaker of water and allowed to stand for a while, there will
often be found hanging from the sides of the beaker or from the
weeds some short threads of a green, brown, or whitish colour.
By one end each thread
sticks to the glass. At the
other it bears about half a
dozen finer threads, which
hang down in the water if
they be left undisturbed. A
touch will cause these to be
withdrawn and take on a
shorter and thicker shape, and
a stronger blow will cause
the whole structure to con-
tract into a subspherical knob,
surmounted by a crown of
smaller knobs. It is clear that
these objects are living beings :
in point of fact each of them
is a specimen of the animal
known as Hydra. According
to their colour and other
characters they have been
divided into a number of
species and subgenera. Chloro-
hydra viridissima {=H . viridis)
is green. Hydra oligactis [=H.

fusca) is brown and has a stalk, and tentacles up to 25 cm. long ;
and H. attenuata, also brown, has no stalk, and the tentacles
are only twice the length of the body. The following account
applies in general to all of them.

Fig

6. — Two specimens of Chlorohydra
viridissima x c. 6, one contracted, the
other in a state of moderate expansion,
the latter bearing two buds in different
stages.

»»., Mouth ; or.c, oral cone.

M.Z. 4

89

90

HYDRA. PHYLUM CCELENTERATA

SHAPE

The body of Hydra is a hollow cylinder, with a ring of hollow
outgrowths or tentacles surrounding an opening or mouth at

one end, and the other end closed
by a flat basal disc or foot. The
mouth is raised upon an oral cone
or hypostome ; it leads into the
cavity of the cylinder, w^th
which the hollows of the tentacles
are continuous. This space is the
enteron. The cylinder is rather
wider in the middle than near
the ends. The wall of the body
is composed of two protoplasmic
layers (Fig. 57), the outer known
as the ectoderm and the inner
as the endoderm, with a structure-
less lamella or mesogloea between
them, consisting of a gelatinous
substance which they secrete.
Such a body as this is know^n
as a polyp, because of its super-
ficial resemblance to an octopus
(Latin polypus, having many feet) .

or."

ECTODERM

The ectoderm consists of several

kinds of cells (Fig. 59), of which

the most conspicuous are those

known as musculo- or myo-

^ epithelial cells. These have broad

Fig. 57 —a diagrammatic, longitud- outer ends, which meet and form
inai section of Hydra, magnified, the surface of the body, Standing

— From Shipley and MacBride. . ... , . , °

bat., Battery of nematocysts. Only a few of OU SCVCral pillarS WhlCh rCach

'^l%^il^erT'en^,7.'Si:L^^^ and cxpaud upou the mesogloea,

^;;JX^':^lX^S^S^^^^ where each cell is generally con-

■' ^*^'^"' sidered to be drawn out into one

or more contractile processes. The processes, each containing a

fibre, run along the cylinder and tentacles, at right angles to the

ECTODERM QI

cell, forming a distinct layer on the outer side of the structureless
lamella. It has, however, been claimed recently that these ap-
parent fibres are mere crystalhsations in the mesogloea, and have
no connection with the cells. Over the greater part of the body the
surface layer of the protoplasm is a firm pellicle, but in the disc this
is absent. The cells in this region are also peculiar in containing
granules of a substance secreted by the protoplasm which is
used to fix the animal to the surface it hangs from. Each musculo-
epithelial cell has a large oval nucleus in one of its pillars. In the
tentacles these cells are less tall than elsewhere. Between the
pillars are spaces which contain small, rounded interstitial cells.
These form a reserve from which, in
various circumstances, any of the other
cells of the body can arise and they
thus retain the undifferentiated nature
of the germ cells. Between the pillars
stand also peculiar cells known as
cnidoblasts, which project through the
surface protoplasm. These are very
numerous in the tentacles, where they
lie in groups or batteries (Fig. 57), but
absent from the basal disc. Each of
them has a pear-shaped body with the
narrow end at the surface of the
animal, where there projects from it a
short process known as the cnidocil. On
this side the cell contains a pear-shaped
sac, called the nematocyst, with a lid,

the operculum. The narrow outer end of the sac is tucked in and
produced into a long, hollow thread, which hes coiled up in the
sac. The space between the thread and the wall of the sac contains
a fluid. On stimulation the thread is expelled, being turned inside
out in the process, but Httle is known of how this is brought about.
There is no nervous control and there is no evidence for the
common statement that the cnidocil is a sense organ. The
nematocysts are discharged by tactile stimuli preceded by a
chemical stimulus, the latter facihtating the effect of the former,
and a strong mechanical stimulus is effective by itself. Bursting
of the operculum is probably caused by the absorption of water.
The nematocysts are of four kinds— a large kind with a straight
thread provided with barbs at the base, a small kind with a

Fig. 58. — A transverse section
of Hydra, stained and seen
under the low power of the
microscope X c. 120.

ect.

Ectoderm; end., endoderm ; st.l.,
structureless lamella.

92

HYDRA. PHYLUM CCELENTERATA

spiral thread and reduced spines, and two other small kinds with a
straight thread and a narrower sac than the others, one of which
has spines while the other has not. The broad end of each cnido-
blast is anchored into the body by a process which runs inward
to the structureless lamella. The tentacles are covered with a

©0

m.s.c

n.c.

Fig. 59. — The histology of Hydra.

The figures are diagrammatic and not drawn to scale.

A, Musculo-epithelial cells ; B, a nerve cell ; C, a cnidoblast ; D, nematocysts o f three

kinds ; E, zoochlorellae ; F, a sense cell.

cnb, Cnidoblast ; cnb', interstitial cell which will become a cnidoblast, with vacuole for nematocyst ;
cnc., cnidocil ; fib., fibre ; int.c, interstitial cell ; m.prs., contractile process ; m.s.c, musculo-epithelial
cell ; n.c, nerve cell ; ntc, nematocvst ; nu., nucleus ; prs., basal process of the cnidoblast; s.c, sense
cell.

number of wart-like knobs, each consisting of a large musculo-
epithelial cell, in which is sunk a battery of cnidoblasts consisting
of one or two of the large kind with several of the smaller kinds
around them. Each of the kinds of nematocysts has a function of
its own. The large, barbed nematocysts are weapons of offence and
perhaps also of defence. When they are discharged, their barbs
emerge first and make a wound in the tissues of the prey, into

ECTODERM

93

which the thread is driven (Fig. 60). In piercing the chitin of
arthropods, upon which the Hydra feeds, they are assisted
by the corrosive action of a fluid which they contain, either
in the hollow of the tucked-in thread or in that of the sac. This
fluid also temporarily numbs the prey, but the main function
of the mematocysts is not to kill but to hold the prey until it
is swallowed. In this the spiral kind possibly assist by coiling
round bristles upon the body of the prey, while the other nemato-

Chibin
Terminal bubc

oLubion oF chibin
by capsular
conbenbs

PcrForabion dus
bo mechanical
action oF sty Ubs

Fig. 60. — Nematocyst of Hydra with the tube perforating the chitin of an
arthropod. — From Yapp, after Toppe. Yapp, An Introduction to Animal
Physiology, 1939. Clarendon Press, Oxford.

cysts are perhaps of use in attaching the tentacles of the hydra,
either to its prey or to other objects, by the stickiness of their
threads. The cnidoblasts arise from the interstitial cells by
the formation of a vacuole and its gradual modification into a
nematocyst. They are formed in the upper region of the cylinder
and migrate thence to various parts of the body, where they take
up their position in the outer layer. The germ cells also arise
in the ectoderm from the interstitial cells by a process which we
shall describe later. Lastly, among the bases of the ectoderm
pillars lies a mesh-work of branching nerve cells which is joined
by rootlets from tall, narrow sense cells that, like the cnidoblasts

94

HYDRA. PHYLUM CCELENTERATA

pierce the surface protoplasm. How these cells are connected is
not certain ; probably it is not by continuity of their processes
but, as in the synapses of higher animals (p. 512), by contiguity.
The function of a nervous system is to conduct impulses
started in it by sense organs, and thus to co-ordinate the move-
ments and reactions of the body. Fundamentally all nervous
systems work in the same way, but that of Hydra differs in some

'^

'i:-.^

Fig. 61. — A small portion
of a transverse section
of Hydra X c. 500.

ect.. Ectoderm ; end., endoderm ;
f.p., food particle, ingested
by an endoderm cell ; int.c,
interstitial cells ; tn.e.c,
musculo-epithelial cell ; m^c,
nematocyst ; st.L, structure-
less lamella ; vac, vacuoles in
endoderm cells ; vac'., vacu-
oles in ectoderm cells.

Fig. 62. — A diagram of the
nervous system of Hydra. —
After Hadzi.

respects from those of most other animals. The nerve cells form
a network, in which it is impossible to distinguish either nerves
or a central nervous system, such as vertebrates possess. Impulses
travel in the network equally well in all directions, and it is
therefore difficult to see how variation in behaviour is possible,
but there is in fact some. Most of our knowledge of the physiology
of the nerve net is based on that of the sea anemones (p. 116),
but there is no reason to think that Hydra is greatly different. A
stimulus, such as a touch, sets up in a sense cell a train or volley of
impulses, and the stronger the stimulus the greater their number.
Both the nerve-muscle junction, and to a lesser extent the synapse,

ECTODERM

95

are such that the arrival of an impulse which is not strong enough
to jump across makes it easier for the next impulse to do so, a
phenomenon known as facilitation. The result is that a strong
stimulus affects more muscles than does a weak one, and in
particular it affects muscles at a greater distance, as is very
obvious when a tentacle is touched. A further result of facilitation
is that a repeated weak stimulus may have the same effect as a
single stronger one. Both the sense cells and the muscles vary in
their sensitivity from time to time, so that the animal does not
always react in the same way to the same stimulus. A well-fed

Sensory cell

Nucleus of musculo - epitheUal cell

Fig. 63. — Sensory cell protruding through an epitheliomuscular cell of
Hydra oligactis. X 600. — Redrawn from McConnell.

hydra, for instance, cannot easily be induced to give the feeding
reaction (p. 97), and repeated stimulation induces fatigue. This
primitive type of nervous system allows of much less precise
movements than that in which a particular sense organ is pre-
ferentially connected with a particular muscle by means of a
nerve. So far as is known, the sense cells are not specialised to
serve particular senses, but this, as we shall see later, is largely true
even of the sense cells (as distinct from the sense organs) of man.
A description of the behaviour of Hydra condensed for a text-
book necessarily gives the impression that it is a somewhat rigid
matter of responses to stimuh, but a little observation of the
creature under the microscope will show that there is both much
spontaneity and much individual variation. Indeed, a great

g5 HYDRA. PHYLUM CCELENTERATA

difficulty in experimental work with Hydra is that one can never
be quite certain whether the animal is responding in a particular
way to the stimulus one has just given it, or merely doing so
because of an internal situation of which one knows nothing.
Besides the contraction already described, and the locomotion
and feeding mentioned below, there is another reaction in which
a tentacle kinks suddenly through a right angle or more, and then
slowly straightens.

ENDODERM

In the endoderm the cells are tall and columnar. Some of them,
especially numerous in the oral cone and absent from the tentacles,
are glandular. They have a narrow stem and a wide end, turned
towards the enteron and containing granules of a substance
which they secrete. The most numerous and conspicuous cells are
nutritive. They are stout, and have their bases produced into
contractile fibres, which are shorter than those of the musculo-
epithelial cells and run around the body, not along it. Their
protoplasm contains large vacuoles, and also, in the green hydra,
a number of round bodies of a green colour, each of which con-
sists of a central mass of protoplasm with a covering containing
the green substance known as chlorophyll, to which the colour of
plants is due. These bodies multiply by division. In the other
species there are no green bodies, but there are present some
yellowish bodies of similar shape, in which, however, no structure
can be made out. The ends of the cells which abut on the enteron
bear fiagella, which can be replaced by pseudopodia. There are
some sense cells and a few isolated nerve cells.

The green bodies, known as zoochlorellae, are non-flagellate
individuals of a minute plant related to Chlamydomonas, of the
genus Carteria. Like other green plants they nourish themselves
by building up complex organic compounds from simple inorganic
ones (carbon dioxide, water, salts, etc., see p. 20). They pre-
sumably obtain these simple substances as waste products of
the metabolism of the hydra. It may be that the hydra absorbs
from them in return the excess of carbohydrates which they
form ; and this would account for the absence from them of
starch, which is so constantly found in plants. Thus there would
be between the two. organisms a partnership, in which the animal
benefited by the removal of waste products and the supply of

ENDODERM 97

oxygen and possibly of carbohydrates, and the plant benefited
by the rich supply of nitrogeneous material and carbon dioxide.
Such a partnership is known as symbiosis and is in contrast with
parasitism, in which one of the partners benefits at the expense
of the other.

MOVEMENTS AND REACTIONS

The movements of Hydra are carried out mainly by the
muscular processes of the cells, though the surface of the basal
disc can put forth pseudopodia, and it is possible that by means
of these the animal can slowly change its position. The muscular
processes of the ectoderm cells, when they contract, make the body
shorter and wider ; those of the endoderm make it narrower and
longer. The position of rest is one of moderate extension. The
green hydra does not remain passive in the absence of stimuli,
but, after standing for some time extended in readiness for prey,
it automatically contracts either the whole body or the tentacles
only, and then extends in a new direction. Thus it explores the
whole of its surroundings. From time to time it changes its
position. This is done by extending the body and bending it,
so that the tentacles touch some neighbouring object and adhere
to it by means of the nematocysts with sticky threads. The basal
disk is then either withdrawn altogether from the spot to which
it was fixed and put down in a new spot close to the tentacles,
or caused to ghde up to the tentacles. If the base is put down
beyond the tentacles the animal may be said to somersault
(Fig. 64). A hydra responds to every stimulus, except that of
food, by contracting. A stimulus applied to one side of the body
a number of times causes it presently to move away in some other
direction. Hydra avoids both too feeble and too strong a Hght.

NUTRITION AND EXCRETION

The food of Hydra consists of small animals, which are caught
by the tentacles, and carried by them to the mouth, which then
opens and swallows the prey. It is not pushed in by the tentacles.
Hydra is seldom able to kill water-fleas, though it can swallow
them once they are killed, and it can live indefinitely in water
in which there are no other animals large enough to be seen

98

HYDRA. PHYLUM CCELENTERATA

with the naked eye. The brown hydra, when starved is able to
feed on nmd. The animal will not feed unless it be hungry. If
it be well fed, creatures which swim against the tentacles are
allowed to escape, but. if food has been scarce, as soon as the prey
has become temporarily attached by the nematocysts to one ten-
tacle the others bend over towards it and help to secure it and
push it towards the mouth. If the animal be starvmg the mere
smell of food in the neighbourhood is enough to set the tentacles
working, but usually they are not put into action till the food
has been both smelt and touched. It is not possible to deceive
the hydra into swallowing substances, such as pieces of blottmg-

YiG. 64. — Hydra in the attitudes which it assumes successively in
two of its modes of locomotion.

A , ' Looping ' ; B,' somersaulting'.

paper, which do not smell like food, but blotting-paper soaked
in beef-tea is swallowed when it touches the tentacles. Once
swallowed, the food is passed deep into the enteron and there
softened by a juice which the gland cells secrete, broken up by
the churning which it gets as the body expands and contracts,
and swept about by the fiagella. Part of the food is dissolved
in the enteron and absorbed in solution, part of it taken up by
pseudopodia of the endoderm cells and digested within their
protoplasm. Presumably the ectoderm is nourished by substances
passed on from the endoderm, either by diffusion through the
structureless lamella or along the fine threads of protoplasm
which put the two layers into connection across it. The undigested
remains of the food are driven out of the mouth by a sudden
contraction of the wall of the body. In unnatural conditions of
culture the animals become liable to depression much like that

NUTRITION AND EXCRETION

99

of Paramecium, in which the powers of movement, feeding, and
fission are affected and death ensues. Respiration and excretion
probably take place from the surface of the ectoderm and endo-
derm ; there is no special organ for either process.

REPRODUCTION

The species of Hydra reproduce themselves both sexually and
asexually. The green form is hermaphrodite, often protandrous,

'^"K ■ V . A ■- "1 •»»>*! •■■ -

^ afe

• a

X

B

Fig. 65.— Reproductive organs of the green hydra.

In each case a testis is shown above, to the left, and an ovary below, to the right. In A the ovum is unripe,
in B it is ripe, has burst its covering of ectoderm cells, and hangs by a stalk. The large round spots
in the ovum are zoochlorellae.

but the two British brown species have the sexes separate.
H. oligactis usually reproduces sexually in the autumn, the other
two in spring and summer. The generative organs are ectodermal
structures developed when sexual reproduction is about to take
place (Fig. 65). The ovaries, of which there is generally only
one in each individual, are found in the lower part of the body ;
the testes, of which there are several, are in the upper part. In
the early stages of both organs the interstitial cells multiply
and push out the musculo-epithehal cells so as to form a swelling.

100

HYDRA. PHYLUM CCELENTERATA

In the ovary one of the interstitial cells becomes an oocyte.
This increases in size and begins to throw out pseudopodia, by
which it swallows the rest of the interstitial cells contained in
the swelling. At the same time it lays up in its protoplasm
nunieious dark, spherical granules of yolk. As the swelling
increases, the musculo-epithelial cells are stretched, their conical
bodies forming long stalks, which are pushed apart by the oocyte,
their outer layer forming a thin covering for the latter. When
the oocyte has swallowed all the surrounding cells it withdraws
its pseudopodia and becomes a large rounded body, about which a
gelatinous coat is secreted. Polar bodies (p. 620) are now formed,
the covering of musculo-epithehal cells parts and shrinks back so
that the ovum is exposed save for the gelatinous coat, and fertilisa-
tion is effected by one of the spermatozoa which are present in
the surrounding water. In the formation of a testis the multi-
plication of the interstitial cells stretches the musculo-epithelial
cells as in the ovary. The interstitial cells become spermatocytes,
which lie among the stalks of the musculo-epithelial cells and
undergo two divisions, the resulting cells developing into sperma-
tozoa with a conical head, a neck, and a tail. By the breaking of
the covering layer the spermatozoa are set free and swim in the
water, where they perish unless they find a ripe ovum. Since in
the green hydra the testis generally ripens first, cross-fertiHsation
will usually take place, but it does not appear that self-fertilisation
is always impossible in this species.

DEVELOPMENT

After fertilisation the egg undergoes cleavage into blasto-
meres (p. 621), which as they increase in numbers form at first
a hollow sphere known as the blastula, whose wall consists of
a single layer of cells. Some of these migrate into the hollow which
they fill. The outer layer now represents ectoderm and the inner
mass endoderm. The cells of the ectoderm become smaller than
those of the endoderm and lose their yolk granules. A thick,
spiny covering of a horny substance is now secreted by the ecto-
derm, and the round, prickly body thus formed falls away from
the parent and rests for several weeks, during which it may be
carried about by currents, in mud on the feet of water animals,
etc. After a time the ectoderm differentiates into musculo-
epithelial and interstitial cells, the mesogloea is secreted, the shell

DEVELOPM ENT

lOI

cracks, and the embryo projects. A split in the endoderm forms
the enteron, tentacles grow out, a mouth is formed, and finally
the young hydra frees itself from the remains of the shell, moves
away, and begins to feed and grow (Fig. 66).

Asexual reproduction also begins with the formation of a
swelling of ectoderm by the multiplication of the interstitial
cells, which afterwards become converted into musculo-epithelial
and endoderm cells, passing through the structureless lamella in
the latter case. The result of this is an increase in the extent of the

Bad

\:^- End V^:^

Fig. 66. — The development of Hydra. — After Brauer.

1. sp.. Spermatozoa.

2. AmcEboid ovum ; g.v., germinal vesicle or nucleus ; y.s., yolk spherules.

3. Ovum protruding ; n., its nucleus ; ect., the ruptured ectoderm of the parent ; end., the endoderm.

4. Prickly envelope (sh.) of embryo liberated from parent.

5. Section of blastula — Ect., ectoderm ; End., endoderm arising ; it will fill the blastula cavity (blastocoele).

6. Section of young Hydra leaving shell. Ect., ectoderm ; End., endoderm ; g.c, enteron ; sh., ruptured

envelopes.

ectoderm and endoderm which leads to a bulging of the body
wall. The knob or bud thus formed becomes larger, about four
or five tentacles grow out around its free end, a mouth is formed,
and finally the base narrows. Then the bud's tentacles attach
themselves to a soHd object, and pull the young creature apart
from its parent. It grows in size and adds a few more tentacles.
The buds arise in the middle or lower part of the body of the
parent. Several may be formed at the same time, and a bud may
form secondary buds before it is set free. While it is still on the
parent, the bud is wholly a part of the body of the latter. Each of
the layers of the parent is continuous with the corresponding layer
of the bud, a suitable stimulus is transmitted by the nervous
system from one to the other, and the entera are in free

HYDRA. PHYLUM CCELENTERATA
102 n I A^xv

comnninication, so that food obtained by either is available for the
other In spite of this, bud and parent may attempt to swallow the
same prey while the parent may snatch food away from the young.
In starved individuals buds may be withdrawn. Occasionally a
hvdra will reproduce by fission, the whole body dividmg, either
lengthwise or transversely, into two. In this event, as m the fission
of Paramecium, structural development as well as the growth of
each product of fission must take place after separation, whereas
in the bud, as we have seen, the structural development takes
place before fission.

REGENERATION

A property akin to asexual reproduction is that of regeneration
or the replacement of lost parts, which is possessed by Hydra
in a very high degree. To some extent all organisms have this
power, but as a rule the higher the animal the less is its faculty
for regeneration. In man it is little more than the power of healing
wounds. Not only will Hydra grow anew any part, such as a
tentacle, which is cut off, but any fragment of the body, provided
it be not too small and contain portions of both layers, will grow
into an entire animal. This ability is connected with the presence
of large numbers of the unspecialized interstitial cells.

OBELIA

HYDROID COLONIES

We must now look at the budding of Hydra from a somewhat
different point of view. By the outgrowth of buds, the animal
increases the size of its body in precisely the same way as
Carchesiiim ; that is to say, by the addition of new members, each
of which repeats the whole structure of the body as it existed
at first. In Hydra the process is carried further by the fission of
the repeated part from the parent body, so that an act of repro-
duction takes place, but it is easy to imagine that this might not
happen. The result would be the permanent conversion of the
body of the Hydra into a colony, of which the buds would be the
zooids. Now there are a number of animals related to Hydra in
which this actually takes place. Such animals are known as
hydroids, and nearly all of them are marine. A common example

HYDROID COLONIES

103

is Ohelia geniculata, which is found growing upon seaweeds near
low watermark on the British coast (Fig. 67).

ANATOMY OF THE POLYP

Certain comparatively unimportant differences distinguish the
polyps of Ohelia from those of Hydra. The tentacles are more
numerous and, instead of being hollow, have a solid core of large
endoderm cells, with very stout walls of intercellular substance

-end.t.

Fig. 67.—
Obelia
lens.

-Part of a colony of
seen under a hand

Fig. 68. — A longitudinal section of a
hydranth of Obelia, highly magnified.

ect.. Ectoderm ; end., endoderm ; end.t., endoderm of
the tentacles ; hyth., hydrotheca ; or.c, oral cone ;
St. I., structureless lamella.

and highly vacuolated contents. In the ectoderm the muscular
fibres are independent cells with nuclei of their own, lying below
the epithelium. The oral cone is very large and forms a chamber
above the rest of the enteron. From the middle of the basal disc
of each polyp the body- wall is continued as a narrow tube,
which joins the tubes from other polyps so as to form a branching
structure like the body of a flowering plant (Figs. 68, 69). This
is continuous at its base with a root-Hke arrangement of tubes
known as the hydrorhiza, on the surface of the seaweed. The tubes
of the whole structure are known as the coenosarc, and the

104

OBELIA. PHYLUM CCELENTERATA

polyp heads as hydranths. The whole colony is enclosed in a
perisarc of chitin (p. 159), which is secreted by the ectoderm and

Fig. 69. — Part of a colony of Obelia, magnified.

bL, Blastostyle ; ect., ectoderm ; end., endoderm ; gth., gonotheca ; hyth., hydrotheca;
med., medusa bud ; p.b., polyp bud ; perith., perisarc'.

follows closely the outline of the body, but is separated from it
by a small space, bridged by processes from some of the ectoderm
cells. At the base of each hydranth the perisarc expands into a
cup or hydrotheca into which the hydranth can be withdrawn.

ANATOMY OF THE POLYP I05

THE MEDUSA

The generative organs are not borne by the polyps, but by
special bodies, which originate as members of the colony, are
set free by breaking away as the buds of Hydra are, and carry
out sexual reproduction as independent individuals. These differ
widely from the polyps, and are, indeed, so unlike them that their
origin from the colony would never have been guessed unless it
had been seen to take place, but they are fundamentally similar
in structure (see p. 112). They are small jelly-fish or medusae (Figs.
70, 71). Each has the shape of a mushroom with a short, thick
stalk and a fringe of tentacles around the edge. The convex upper
side is called the exumbrella, the concave lower side the sub-
umbrella, the whole disc-shaped upper part the umbrella, and the
stalk the manubrium. Around the edge of the subumbrella a low

Fig. 70. — A medusa of Obelia, magnified.

ridge projects inwards. This is the velum and represents a much
larger structure in the same region of many other medusae. At the
end of the manubrium is the mouth, which leads by a tubular
gullet along the manubrium to a stomach in the middle of the
body. From this four radial canals run outwards to a ring canal at
the edge of the umbrella. The lining of all these internal spaces
consists of endoderm, and the radial canals lie in a sheet of
endoderm, known as the endoderm lamella. In fact we may regard
the internal cavities of a medusa as corresponding to the enteron
of a polyp in which the walls have come together over a large area,
leaving certain spaces which form the gullet, stomach, and canals.
The whole outside of the body and tentacles is covered with ecto-
derm. Between the ectoderm and the endoderm is a layer of jelly,
which is very thick, especially on the exumbrella side. The medusa
may be compared to a polyp which is greatly widened and short-
ened, the walls of the wide, fiat enteron coming together in places,
as we have seen, and the structureless lamella increasing in

lob

HYDRA

AND OBELIA. PHYLUM COELENTERATA

thickness to form the jelly. The manubrium represents the oral cone
and the tentacles stand around it at a greater distance owing to the
widening of the body. The arrangement of the organs of a medusa
is an excellent example of what is known as radial symmetry. In
bilateral symmetry (p. i88) the parts of the body are arranged
on each side (right and left) of a plane, in such a way that no
other plane will divide the body into two halves which are alike.
In radial symmetry the parts of the body are arranged about a

point in such a way that many
planes divide the body into like
halves. Polyps also are radially
symmetrical.

...5

Fig. 71. — The medusa of Ohelia,
seen from the subumbrella
side. — From Shipley and
MacBride.

I, Mouth, at end of manubrium ; 2, ten-
tacle ; 3, gonad ; 4, radial canal ; 5,
statocyst.

MOVEMENTS OF THE MEDUSA

The medusa floats in the sea with
the manubrium downwards and the
tentacles hanging like the snaky
locks of its classical namesake. It
swims by contractions of the plentiful
musculature of the subumbrella side,
which drive water out of the um-
brella and send the animal in the
opposite direction. The contractions
are started by impulses which originate in the nerve net at the
umbrella margin. There nervous transmission is facilitated by the
nerve-rings — two specially well-developed circular tracts of the
net — and there is provision for keeping balance by means of
eight sense organs, known as statocysts, situated each at the base
of one of the tentacles. These are small hollow vesicles each
containing a calcareous body which hangs in a single cell that
secreted it. The swaying of the calcareous bodies against fine
processes on sense-cells which line the outer side of the vesicle
gives rise to impulses by which the movements of the animal are
regulated through the nervous system, stronger contractions
being caused on the side which is at the lower level.

REPRODUCTION

Each medusa is of one sex only. The generative organs, which
are not fully developed till after the animal is set free, are four in

REPRODUCTION

107

ex.u.

can.r.

; can.c.

Fig. 72. — A diagram of a vertical section
of the medusa of Obelia. The section is
supposed to pass on one side along a
radial canal, and on the other across
the endoderm lamella. In reality this
would not be possible, since the canals
are opposite one another.

can.c. Circular canal ; can.r., radial canal ; en.L,
endoderm lamella ; ex.u., exumbrella surface ; g.,
gonad ; j., jelly ; m., mouth ; mb., manubrium ;
n.r., nerve ring ; oes., oesophagus ; s.u., subum-
brella surface; st., stomach; stc., statocyst;
<^n., tentacle ; rm., velum.

number and lie on the subumbrella below the radial canals.

Each consists of a knob of ectoderm, into which passes a short

branch from the radial canal. The germ-mother-cells originate

in the ectoderm of the

manubrium, migrate into the J «^-

endoderm, and pass along the

radial canals to the generative

organs, where they migrate

into the ectoderm again.

When the ova or sperms are

ripe, they are shed by the

rupture of the ectoderm into

the water, where fertilisation

takes place. As in Hydra,

segmentation leads to the

formation of a hollow

blastula. From this, by immi-
gration of cells at one spot,

there is reached a stage with

a solid mass of endoderm

such as that found in Hydra.

The animal at this stage is of a lengthened egg-shape and has

a ciliated ectoderm, by which it swims freely for a while. It is

known as a planula (Fig. 73). The
planula then settles down by its broader
end, an enteron is formed by a split
in the endoderm, tentacles and a
mouth form at the other end, and thus
there develops a polyp, from which a
colony arises by budding. When the
colony has reached a certain size there
appear, in the angles between the stem

Fig. 73. — A, Planula larva; ^\ , , , i • v -u xu

B, the young polyp into and the branches which bear the
which the planula grows hydranths, tubular outgrowths known

as blastostyles, each enclosed m a vase
of perisarc known as a gonotheca. A blastostyle and its gonotheca
are together known as a gonangium. The blastostyle is probably
an incomplete zooid. On its sides are formed a number of buds
which develop into little medusae and escape through the opening
at the top of the gonotheca (Fig. 74).

B

108 HYDRA AND OBELIA. PHYLUM CCELENTERATA

ALTERATION OF GENERATIONS
It will be seen that Obelia, like Hydra, reproduces itself both
sexually and asexually. Sexual reproduction is carried out by
the medusa and leads to the formation of polyps. The asexual
reproduction consists in the budding off of medusae from the polyp
stock. But whereas in Hydra, the two processes go on side by
side sometimes in the same individual, and succeed one another
quite irregularly, in Obelia there are two different types of
individual— the polyp stock and the medusa— which follow
one another regularly and are each confined to one method of
reproduction. Thus we have a definite alternation of generations,

a sexual and an asexual form suc-
ceeding one another. It will be
remembered that such generations
also alternate in Monocystis and
that the malarial parasite has a
more complicated life-history of the
same kind. The asexual generation
of Ohelia is relatively inactive,
gathering much nourishment and
spending little : the sexual genera-
tion is active, spending its sub-
stance freely in locomotion, which
ensures the distribution of the
species and thus opens up fresh
food supplies and increases the
chances of escape from local dangers. The gist of the story is the
distribution of labour among individuals of different kinds.

su

Fig. 74. — A diagram to show the
development of medusae as
buds on a blastostyle.

bl., blastostyle ; s.u.c. subumbrellar cavity ;
1-6, successive stages in the develop-
ment of a medusa.

METAGENESIS

The designation ' alternation of generations ' has been applied
to a number of different types of life-history which have in com-
mon only the fact that reproduction is accomplished differently
in successive phases of the reproducing organism. It is a useful
' omnibus ' term but should not be taken to imply more than
superficial resemblance between the processes it covers. The type
met with in hydroids is known as metagenesis. We shall observe
a quite different process, which does not involve asexual repro-
duction, in the Uver fluke and again in a nematode worm. Botanists
restrict the term to life-histories, such as those of mosses and

METAGENESIS lOQ

ferns, where a stage with only a single set of chromosomes (p. 694)
alternates with one containing a double set. This condition is found
in animals only in the Sporozoa. In view of this diversity of usage,
many biologists prefer not to speak of the life-history of Obelia as an
alternation of generations, but to use for it the term metagenesis.

REPRODUCTION AND COLONY-BUILDING

The above account of the reproduction of hydroids differs in
one respect from that which is sometimes given. On the analogy
of the budding of Hydra, some authors regard the formation of
a hydroid colony by budding as a kind of asexual reproduction
in which there are formed numerous ' individuals ' which do not
separate. If that is so the alternation of generations contains an
indefinite number of acts of asexual reproduction between two
sexual acts. We have preferred to treat the polyp stock as one
individual containing a number of semi-independent parts — the
hydranths — each of which repeats the structure of the whole
body as it was at first, and having certain other parts — the
blastostyles and most of the coenosarc — which are differently
constructed, serve the entire body, and are wholly dependent
upon it. This view involves the following considerations. The
development of the individual and its reproduction are essentially
the same process — morphogenesis, which is also at work in
regeneration. Any part of an organism, from the smallest organ
to the whole body, is Uable to be repeated, with or without
differences between the repeated parts. This phenomenon has
been called merism : we have seen it in ciHa, trichocysts, contractile
vacuoles, cells, limbs, zooids, etc. Sometimes, as in cilia, cells,
or zooids of the same kind, it has not involved differences.
Sometimes, as in cells or limbs or zooids of different kinds, it has
involved differences between the repeated parts. Sometimes the
parts are present in their full number from the first ; sometimes
they increase in number as growth goes on. From time to time
every organism produces a part which not only repeats its whole
structure, but also separates from it by an act of fission. This
process is reproduction. In some types of reproduction, as in the
budding of Hydra, the repetition of structure takes place before
separation. In others, as in the formation of germ ceUs,i the part

1 Here there may be the additional compUcation that two such separated parts
so develop as to produce but one body after fusion.

no HYDRA AND OBELIA. PHYLUM COELENTERATA

which separates is simple in structure, but has the power of
repeating the structure of the parent body after fission.

INDIVIDUALITY

The term individual, whose application was in question in
the foregoing paragraph, has been used in zoology with very
different meanings, which are well illustrated by the life-history
of the hydroids. An individual is a single complete living being.
Since the medusa carries out all the functions of life in itself,
it seems natural to assert that it is a complete living being,
and since, as we have seen, its structure is essentially that of a
polyp, we might assume that each polyp is also an individual.
On the other hand, the whole polyp stock is a unit, and we
might consider it to be one individual, of which the separate
polyps are members, still regarding the medusa as an individual.
Of these alternatives the first is open to the objection that it
ignores the fact that the stock with all its polyps may be regarded
as a whole since it has common nourishment and reproductive
organs (the gonangia), and obliges us to regard as individuals the
blastostyles, which are morphologically equivalent only to parts
of other individuals. The second alternative is that which we
have adopted above. It may be stated as follows : ' Every con-
tinuous mass of living matter which arises normally by fission
is an individual'. That view has the advantage that it does
not force us to create artificial units of any kind. According
to it, the act which makes an individual is the act of fission
by which it becomes independent of its parent, and fertilisation
is the blending of two undeveloped individuals into one, while
the polyp stock is an individual which contains a number of
units meristically repeated, and the medusa is an individual
consisting of one unit of the same type as those which exist in
the polyp stock. All the products which one individual produces
by fission, such as the medusae of a single hydroid colony or the
whole family produced by the repeated fission of a Paramecium,
are collectively known as a clone.

CCELENTERATA

The phylum Coelenterata, to which Hydra and Obelia belong,
differs from all other groups in that its members have a body-wall
composed of two layers of cells only, the endoderm and ectoderm ;

SUBPHYLUM I : CNIDARIA III

they are therefore called Diploblastica. The rest of the Metazoa,
called collectively Triploblastica, not only have generally bulkier
bodies many cells thick, but, after their embryos have reached a
stage where the two layers of endoderm and ectoderm are present,
they develop a third and usually detached layer between them ;
it is called the mesoderm. In place of this the coelenterates have
the secreted jelly-like mesogloea, which has little or no structure,
although cells sometimes wander into it. Other structural features
of the group are the radial symmetry, the nerve-net, and the
cavity or enteron in the endoderm with a single opening which
serves as both mouth and anus. The first and second of these
they share with the echinoderms, in the third they resemble the
fiatworms. Except for Hydra and a few other aberrant genera,
all coelenterates are marine.

SUBPHYLUM I— CNIDARIA

Most of the coelenterates, including both Hydra and Ohelia,
belong to this group. They move by muscle-cells, and possess
nematocysts. The body is organised on a plan which can be
reduced either to a polyp or to a medusa, and often the two forms
alternate in one life-history. There is, in fact, little difference,
except in shape, between the two, as can be shown by drawing
a polyp in section upside down alongside a similar drawing of
a medusa (Fig. 75). The larva is typically a planula. There are
three classes.

CLASS I—HYDROZOA

There is generally an alternation of polyp and medusa, the
former being colonial ; the tentacles of the polyp are usually
soHd ; there are no vertical partitions in the enteron ; the medusa
has a velum and a nerve ring ; the gonads are formed in the
ectoderm ; and there is usually an external skeleton. Obelia is a
typical member of the class. In some genera, especially those
living in the tidal zone (e.g. Tuhidaria), the medusa remains
attached to the polyp and the egg develops in situ into a peculiar
creeping larva, the actinula. In still other genera the medusa is
completely suppressed, and Hydra may be looked on as an extreme
example of such suppression, having also other peculiarities,
such as the hollow tentacles and sohtary habit. In some

PHYLUM CCELENTERATA
112

orders on the other hand, the polyp has disappeared. In the order
Siphonophora, including Physalia, the Portuguese man-of-war.
the colonics float freely on the surface of the sea, and the polyps
are of many different forms with specialised functions.

A

B

C

D

Fig. 75. — Diagrammatic vertical sections of polyps and medusae. All are shown
with the mouth below, whatever the natural position. Endoderm close-hatched ;
ectoderm open-hatched ; gonads in solid black.

A, Hydrozoan polyp ; B, hydrozoan medusa ; C, hydridan polyp ; D, actinozoan polyp ; E, scyphozoan

medusa.

CLASS II—SCYPHOMEDUSM

The medusa is the predominant phase, but there is also a polyp
which forms the medusae by transverse fission. The tentacles are
solid, but otherwise the characters are the opposite of those of the
Hydrozoa, that is to say, the enteron has vertical partitions ;
there is no velum or nerve ring ; the gonads are formed in the
endoderm ; and there is no skeleton. Beneath the gonads are
sub-genital pits in the ectoderm of the subumbrella.

A typical member is the common jellyfish, Aurelia aurita,

SUBPHYLUM i: CNIDARIA

113

shown in Fig. 76 and in section in Fig. 77. From the main
digestive cavity a series of radial canals, some branched, run out

Fig. 76. — A small specimen of the common jelly-tish, Aurelia aurita, natural size.

Note the horseshoe-shaped gonads, showing through the transparent tissues ; the radial canals, alternately
branched and unbranched ; the little sense tentacles in notches each opposite the middle branch of a
canal ; the marginal tentacles ; and the arms of the manubrium, each folded and fringed.

Water circulates from the stomach by the unbranched (adradial) canals to the circular canal, and
back by the branched (per- and inter-radial) canals.

to a circular canal running round the edge of the disc. Through
these a circulation of sea water is kept up by ciHa, the stream

ex.H,

cau.r'

sien.

Fig. 77. — A diagram of a vertical section through one of the large jelly-fishes,
such as Aurelia. The section is divided by a dotted line into two halves, m
one of which it is supposed to pass through a radial canal, and m the other
through the endoderm lamella.

canx. Circular canal ; can.r., radial canal ; en.L, endoderm lamella ; ex.u., exumbrella ectoderm ; g.. gonad ;
g./., gastral hlament ; hd., hood covering sense tentacle; ;., jelly or mesogoea ; m6. manubnum ;
CBS., oesophagus ; s.g.p., subgenital pit ; s.ten., sense tentacle ; st., stomach ; ten., tentacle.

going out by the eight unbranched canals and returning by the
alternating branched ones. Partially digested particles of food
are carried in the stream, and are ingested by any endodermal

114

PHYLUM CCELENTERATA

cell which needs them. Possibly also the current helps in taking
oxygen to the cells, for many jelly-fish are of large size (C^/anea
atmta may be six feet in diameter) and considerable thickness^
The bulk of the body is. however, mesogloea. The eggs are liberated
mto the enteron, are fertilised there, and develop into planute.
These escape, and settle on a rock or weed, and each develops
into a polyp known as a scyphistoma. This may divide by budding
into more polyps like itself, but its chief function is to produce

.1 «• H.

Fig. 78. — The life-history of Aurelia.
A, Planula ; B-H, stages in the development of the scyphistoma ; L, ephyra.

medusa by a series of horizontal fissions, the whole process being
called strobilisation (Fig. 78). The structure looks much Hke a
pile of saucers, of which each in turn, known as an ephyra, as
it becomes the top one floats off, turns over, and grows to be a
new jellyfish. Strobilisation occurs only in winter and when the
animal is well fed. There is much variation in the development
of the species of this class, and even in individuals of a species.
Some genera, including Haliclystus and Lucernaria of British
coasts, have only one stage in the life-history, which is a sessile
polyp.

SUBPHYLUM I : CNIDARIA

115

CLASS III—ACTINOZOA

A medusa is never present ; the tentacles are hollow ; the
enteron has vertical partitions (mesenteries) ; the gonads are formed
from the endoderm ; and there may or may not be a skeleton.

The typical members of this class are the sea-anemones,
illustrated in Figs. 79, 80. These are solitary forms, usually
capable of many muscular movements both of feeding and
locomotion. They have a ciliated ectoderm, and in the plankton
feeders, such as Metridium, this is important in carrying food
to the mouth. Others, such as the
common Actinia equina, kill and eat
relatively large animals which ac-
cidentally touch the tentacles, and
experimentally can swallow pieces of
flesh almost as large as themselves.
The larva is a planula, which develops

\

i^^y^^^

\-T7UiA

pOTi.

Tnu»!

Fig. 79. — External appearance of
Tealia felina, a sea anemone. —
From Thomson.

Fig. 80. — A diagram of a vertical
section of a sea-anemone.

ect., ectoderm of tentacle ; ect.g., ectoderm
lining gullet : end., endoderm ; ent.,
enteron ; g., gullet ; gon., gonad ; w./.,
mesenterial filament ; mus., longitud-
inal or ' retractor ' muscle ; mus'.,
oblique or ' parietal ' muscle ; st.l.,
mesoglcea ; ten., tentacle ; i°mes.,
primary mesentery.

directly into a new polyp. Many Actinozoa are colonial, and
amongst such forms are most of the corals. These include the red
coral, which forms its calcareous skeleton in the mesogloea, and the
reef-building species in which it is external. The coral reef, which
may be thousands of feet thick, consists of a large mass of calcium
carbonate built up by past generations of animals. The living
individuals protrude from tubes of this material only in the
narrow zone from the surface to a depth of twenty or thirty
fathoms.

'PV

TTUZOw

l°m^A

d^TTves.

spg

Fig. 8i. — Diagrams of transverse sections of a
sea-anemone.

A., Through the gullet ; B, below the gullet.

d.mes., ' directive ' mesentery ; ect., ectoderm ; ed.g.,
ectoderm of gullet ; end., endoderm ; ent., enteron ;
gon., gonad ; g., gullet ; m./., mesenterial filament ; nius.,
muscle ; spg., siphonoglyphs ; st.L, mesogloea ; i°mes.,
2°mes., 3'^mes., primary, secondary, and tertiary
mesenteries.

SUBPHYLUM II : CTENOPHORA

117

SUBPHYLUM II— CTENOPHORA

These differ from other coelenterates in having neither polyp
nor medusa, in being without nematocysts, and in swimming
by cilia instead of by muscular cells. They are all marine, and

m.

Fig. 82. — Hormiphoraplumosa. X c. 6. — After Chun, The Invertebrata, 2nd edition,

1935. Cambridge University Press.

ab.p., aboval pole ; ab.fu., aboval funnel ; tn., tentacle ; tn.po., tentacular pouch ; ca. and 8, one of the
rows of cilia ; gel., gelatinous material ; pa.can., paragastric canal; tn., mouth.

are known as sea-gooseberries, a name which roughly describes
their appearance. The ciha are arranged in eight conspicuous
rows, and there may be two long tentacles. HormipJwra plumosa
(Fig. 82) is a common British form.

10

FLATWORMS

Many of the lower animals are popularly known as ' worms '.
They have little in common save that their bodies are longer
than they are broad and have bilateral symmetry, and that their
organisation is rather simple. The lowliest of such creatures are
known as the tlatworms or Platyhelminthes. As their name
implies the bodies of these worms are flat. They have no anus
and no blood vessels or body cavity, being constructed internally
of a spongy mass of tissue (parenchyma, p. 119), containing
muscle fibres, and, embedded in this, a gut (except in the tape-
worms), a nervous system with a rudimentary brain, an excretory
system formed of branched tubes ending internally in cihated
' flame cells ' (p. 121), and a complicated, nearly always hermaph-
rodite, generative system. The gut has only one opening. The most
characteristic flatworms are the relatively simple free-living
Turbellaria (p. 131), but more important to man are the two
classes that are parasitic — Trematoda or flukes, and Cestoda or
tapeworms. These are more complicated in structure, and
especially in their reproductive organs and life-histories, than
the others.

CLASS TREMATODA

FASCIOLA

Sheep which are fed in damp meadows are liable to a serious

and usually fatal disease known as ' liver rot ', in which the wool

falls oft", dropsical swellings appear, and the animal wastes away.

This has been found to be caused by a parasite known as the

liver fluke, Fasciola hepatica [=Distomuni hepatictim), which

lives in the bile ducts of the sheep and sometimes of other animals,

including cattle, and more rarely a wide variety of mammals

from kangaroos to man. It is a flat, brownish worm (Fig. 83),

about one inch long by half an inch broad, shaped like a leaf

with a blunt triangular projection at the broader end. At the

tip of this projection lies the mouth, in the midst of an anterior

sucker, and just behind the projection an imperforate posterior

or ventral sucker is placed in the middle of the ventral side ;

when the worm is kept in the laboratory it moves by looping,

with alternate attachments of the suckers. Nearly midway

118

THE LIVER FLUKE

119

between the suckers is a smaller genital opening, at the hind end
of the body is a minute excretory pore, and on the dorsal surface
at about a third of the length of the animal from the front end
lies the opening of the Laurer-Stieda canal presently to be men-
tioned. The body is covered with
a cuticle, in which little back-
ward-pointing spinules are em-
bedded.

GUT

Mouth .

Ventral sucker.

Genital openinq.

The mouth leads into an ovoid,
muscular pharynx, from which a
short oesophagus passes back-
wards to divide before the
posterior sucker into right and
left branches or intestines, which
run on either side of the middle
line to the hind end of the body,
giving off on either side many
offsets, which in turn are much
branched. There is no anus. The worm feeds largely on the blood of
its host, which is sucked from the wall of the duct, but probably
also on pieces of tissue rasped off by the sucker. It can also
absorb sugars, especially monosaccharides such as glucose and
fructose, through the general body surface.

Fig. 83. — The Liver Fluke.

LAYERS OF THE BODY

The ectoderm cells have sunk inward after secreting the cuticle.
Below the cuticle lie successively circular, longitudinal, and
diagonal layers of muscle fibres, with the epidermal cells among
the longitudinal fibres (Fig. 84). Between these and the endo-
derm, which is a columnar epithelium hning the gut, lies the
parenchyma, a meshwork of protoplasm with nuclei at the
nodes and oval cells in the meshes. Muscle fibres pass across the
parenchyma from the dorsal to the ventral side of the body.
There are no blood vessels or coelom. It will be noticed that in
the fluke a mass of tissue hes between the ectoderm and endoderm
in place of the structureless lamella of Hydra. This is known as
the mesoderm. We shall allude to it in more detail in describing
the earthworm.

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THE LIVER FLUKE

121

^-e.v

RESPIRATION

There is no evidence that the Hver fluke is anaerobic (p. ii).
Experimentally a reduced oxygen tension shortens the life of the
worms, and measurements show
that bile is nearly saturated with
oxygen.

EXCRETORY SYSTEM

The excretory system (Fig. 85)
lies in the parenchyma. It consists
of a meshwork of tubules joining
into a main duct which lies in the
middle line, from a point about a
quarter of the length of the body
behind its front end to the ex-
cretory pore at the hind end.
The ultimate branches of the
tubules are very fine and end, at
least in the larva, in little struc-
tures known as flame cells or
solenocytes (Fig. 86). These are
minute vesicles containing a few
long cilia which keep up a flicker-
ing movement like that of a flame
and so perhaps drive towards the
main duct the fluid secreted into
the vesicle by its walls. Each
vesicle has a nucleus and may
be regarded as a hollow cell. It is
connected with its fellows by fine
protoplasmic processes which are
said to be hollow. The flame
cells are said to disappear in the

Fig. 85. — The structure of a liver
fluke. — After Sommer. From the
ventral surface. The branched
gut {g.) and the lateral nerve
[l.n.) are shown to the left of the
figure, the branches of the
excretory vessel {e.v.) to the
right.

Position of cirrus sac ; eg., lateral head
ganglion ; m., mouth ; ph., pharyn.\ ; v.s.,
ventral sucker. An arrow indicates the
excretory aperture.

c.s.

cercaria stage (p. 126) and to be

invisible in the adult. The whole system is derived from the

ectoderm and is called a protonephridium.

The fluid in the tubules contains much fat, and is almost
continuously squeezed out through the excretory pore by con-
tractions of the body. Fat is an unusual excretory product, but

M.Z.— 5

J22 TREMATODES. PHYLUM PLATYHELMINTHES

its occurrence is perhaps connected with the nature of the food ;
it is present also in the urine of the highly carnivorous and largely
blood-feeding cats. The chief nitrogenous product is ammonia.

NERVOUS SYSTEM

The nervous system includes a brain which consists of a collar
around the pharynx with a mid-ventral swelling and a pair of
lateral sweUings. From these swellings or gangha nerves are
given off to the forepart of the body, and from each lateral

ganglion arises a large
lateral nerve cord which
runs backwards below the
gut to the hinder end of
the body, giving off
branches on the way. The
nerve cords contain nerve
cells as well as fibres.

GENERATIVE ORGANS

The liver fluke is
hermaphrodite, and has
very complex generative
organs (Fig. ^y). The
testes are two much-
branched tubes lying one
behind the other in the middle part of the body. The branches
of each are gathered into a vas deferens, and the two vasa
deferentia run forwards side by side to join, above the
posterior sucker, a large, pear-shaped seminal vesicle. From
this a fine, somewhat twisted tube, the ejaculatory duct,
passes forwards to enter a stout, muscular penis or cirrus, which
opens at the generative pore. Normally the penis lies in a cirrus
sac, but it can be turned inside out and thus thrust out of the
pore. The ovary is a branched tubular structure on the right side
in front of the testes. Its branches join to form the oviduct, which
passes towards the middle line and there joins the yolk duct.
This is formed by the union of two transverse ducts, which lead
each from a longitudinal duct at the side of the body. The yolk
glands are very numerous, small, round vesicles lying along the

Fig. 86. — Two flame cells, highly magnified.

THE LIVER FLUKE : REPRODUCTION

123

sides of the body, and communicating by short ducts with the
longitudinal ducts. The Laurer-Stieda canal is a short tube
leading from the union of oviduct and yolk duct to a pore on the
back ; it is probably functionless, but in other species it serves
for copulation. The oviduct and yolk duct are surrounded where

,m

^^

\ ^ 7xj 7

g

'^^-£mt^^

^s.v.

T

Fig. 87. — The reproductive organs of a liver fluke, from the ventral side. — After

Sommer.

c,s., Cirrus sac ; /,, female aperture ; g., anterior lobes of gut ; m., mouth ; ov., ovary (dark) ; p., penis;
S.V., seminal vesicle ; sh.g., shell gland ; T., testes (anterior) ; ut., uterus ; v.d., vas deferens ; y.gl.
diffuse yolk glands.

they join by a rounded mass, which is known as the shell gland
though it does not form the egg shell, which is secreted by the
yolk glands, which are numerous, minute, and unicellular.
The shell glands may harden the shell, or their secretion may
activate the sperms. From this point the joined ducts proceed
forwards as a wide, twisted tube, the uterus, to the generative

J24 TREMATODES. PHYLUM PL AT YHELMINTHES

Fig. 88.— The life-histoiy of a liver fluke.— After Thomas.

I DcvcloninK embryo in egg-case ; 2. free-swimming ciliated embryo ;3. sporocyst ; 3a. sheU of LimncBa
■ <r«SS£ ;T ivision of sporocyst which sometimes occurs ; 5. sporocyst with redi^ formmg withm it

6. redia wi'th'cercariae forming within it ; 7- tailed cercaria ; 8. young fluke.

from the yolk glands, and spermatozoa. The material of the shell
is a quinone-tanned protein similar to the sclerotin of insects
(p. 227). The animal is probably as a rule self-fertilised.

THE LIVER FLUKE 125

LIFE-HISTORY

The life-history of the Hver fluke is a remarkable and interest-
ing process (Fig. 88). The eggs, up to 3,500 a day, are laid in
the bile ducts of the sheep. So long as they remain within the
body of the latter they do not develop, but when they have
been carried by the bile to the intestine, and thence passed to
the exterior with the droppings, they will develop in damp
spots if the weather be warm, and the pH of the water be less
than 7.5, the optimum being about 6.5. In a few weeks a larva
known as the miracidium emerges. It is rather more than a tenth
of a milhmetre long, conical, covered with a layer formed by
five rings of big, ciliated cells, provided with two eye-spots, a
small gut, and two flame cells, and filled by a mass of cells.
It swims by means of the cilia, with the broad end forwards. At
this end there is a knob which can be thrust out as a conical spike.
If it can find a member of a particular species of water snail
known as LimncBU tnmcatula ^ it works its way into the tissues
of the snail, thrusting out its spike and rotating by means of its
ciha so as to bore in. Within the snail the ciliated cells are lost
and the larva increases in size and grows into a hollow sac or
sporocyst. Sometimes this multipHes by dividing transversely.
Within the sporocyst some of the cells lining the cavity behave
Kke fertihsed ova, dividing to form a blastula, which invaginates
to give rise to a two-layered sac or gastrula. These cells, however,
have not undergone fertilisation, and their development is an
example of parthenogenesis, the development of unfertilised ova.
The gastrula grows into another form of larva, the redia, which
bursts out of the sporocyst and migrates, usually into the liver
of the snail. The rediae devour the tissues of the snail and finally
kill it. Each redia has an elongated body with an anterior mouth, a
muscular pharynx, and a short, sac-hke gut. A little way behind
the pharynx the body-wall is thickened to form a muscular collar,
and not far from the hind end are two blunt conical processes
on one side. Posteriorly there is a large body cavity hned by an
epithelium like that of the cavity in the sporocyst. Cells derived
from the wall of the body cavity develop in much the same
way as in the sporocyst and give rise to daughter rediae, which
escape from the parent by an opening behind the collar. Several

1 Other species and genera of water snail are sometimes infected in foreign coun-
tries, and occasionally L. stagnalis and others may be infected in Britain.

126 TREMATODES. PHYLUM PL ATYHELMINTHES

generations of redia usually succeed one another, but eventuaUy
they cease to produce daughters of their own kind, and give
birth instead to creatures known as cercariae, with a flat, heart-
shaped body, two suckers, a forked gut, and a tail. The cercaria,
which is about 0.3 mm. long, with a tail of twice that length,
emerges from the redia, works its way out of the snail, and swims
by means of the tail. Soon it settles upon a wet blade of grass,
loses its tail, secretes around itself a cyst by means of special
cystogenous cells of the ectoderm, and waits till the grass is
eaten by a sheep ; during this interval the larva, which is now
known as a metacercaria, undergoes some slight structural
changes. The larva may remain ahve inside the cyst for twelve
months, but is killed by sunshine and drying. In the gut of the
sheep the cyst is digested and the metacercaria pierces the
intestinal wall so that it enters the body cavity, and in two or

I - qeneration
2"-'generorion
3"^ qe'^e''«.rior\

Egg ».Miracidium >.Sporocysr

I

— Redia -.„...^^ Sporocyst

Cercaria—* Metacercaria— ♦Adulr. Redia

Fig. 89. — A diagram of the life-history of the liver fluke.

three days bores into the liver. Later it enters the bile ducts and
there grows into an adult fluke. When the gonads are fully
developed, which is ten to twelve weeks after infection, the worms
begin to lay their eggs, and migrate to the duodenum of the host.
Flukes may live for eleven years, or may be lost with the faeces,
and if the sheep survives till this happens it will usually recover,
though, owing to permanent damage to the liver, the recovery is
never complete.

It will be seen that in this life-history we have an example of
alternation of generations far more complicated than that of
Obelia, and differing from the latter also in that not sexual and
truly asexual, but sexual and parthenogenetic generations succeed
one another. The former kind of alternation of generations is known
as metagenesis, the latter as heterogamy. It should also be noticed
that there are three kinds of individual involved in the cycle.
The life-history of the liver fluke is shown by a diagram in Fig. 89.

The internally parasitic flukes usually have a life-history of
the same general pattern, with the adult in a vertebrate and the

THE LIVER FLUKE I LIFE-HISTORY

127

larvae in a mollusc, although there are variations in detail and

some species do not have an intermediate host. Another important

example is Schistosoma {=Bilharzia), an important parasite of

man of which various species are found

in Africa, Asia, and America. The adults

live in the veins, and are peculiar in being

dioecious. The female lives for most of the

time in a groove, formed by the rolling up

of the male (Fig. 90), but she can leave

this and does so to lay her eggs in the

capillaries. It is the eggs which cause the

disease schistosomiasis by penetrating the

walls of the capillaries with their spines

and by secreting an enzyme and causing

haemorrhage. Eventually the eggs get into

the bladder and leave the body in the

urine. When this is diluted they hatch into

miracidia which have a life of about

thirty hours, during which they must

infect a snail. Here two generations of

sporocysts, and eventually cercariae, are

formed, but there are no rediae. The

cercariae are set free, and can penetrate

any part of the skin of man. In countries where the parasite is

common, any contact with fresh water is therefore dangerous, as

a mere splash may bring the cercariae on to the skin. The

parasite is very successful and in some areas is present in all the

human population.

Fig. go. — Schistosoma
hcBmatohinm. — From
Sedgwick.

<J, male ; $, female ; S, sucker.

CLASS CESTODA

TMNIA

TcBuia solium (Fig. 91), found in man, is an example of a tape-
worm. There is seldom more than one worm in the host (hence
the trivial name) but it may reach a length of twenty feet. It
lives in the intestine, in the wall of which the head or scolex is
usually buried. On the scolex are four lateral suckers, and a
crown of 22-32 hooks which are borne in two rows on a projection
or rostellum. Behind the scolex is a narrow neck, which is con-
tinually forming new sections of the body, called proglottides,
of which there may be as many as 1000. Each proglottis (there

J28 CESTODES. PHYLUM PLAT YHELMINTHES

is no excuse for the illiterate form, proglottid) is pushed away
from the neck as the next is formed, so that a cham is produced
with the youngest links most anterior. Such a cham is called a
strobila The proglottides thus differ in their formation and

Fig. 91. — The life-history of Tcsnia solium. — After Leuckart.
I. Six-hooked embryo in egg-case ; 2. proscolex or bladder-worm stage, with invaginated head ; 3. bladder-
worm with evaginated head ; 4. enlarged head of adult showing suckers and hooks ; 5. general view
of the tapeworm, from small head and thin neck to the ripe joints ; 6. a ripe joint or proglottis with
branched uterus ; all other organs are now lost.

arrangement from the segments of annelids, anthropods or
chordates. When a proglottis is formed it possesses nervous tissue
and execretory canals continuous with those of the head ; as it
grows older it becomes larger, and reproductive organs develop
in it. Young proglottides are broader than they are long, but these
proportions are reversed in those which are sexually mature.

The body is covered with a cuticle, under which lies a la^^er of
circular muscle fibres and then one of very deep epidermal cells

T^NIA SOLIUM

129

^■te*

with longitudinal muscle fibres between them and a transverse
layer of muscle below them. Inside this is a mass of parenchyma
like that of the fluke, in which are embedded the excretory, genera-
tive, and nervous systems. There is no alimentary canal, nutriment
being absorbed through the surface of the body. There is a
flame-cell system of the same type as that of the fluke, with
a larger and a smaller main duct on each side, connected by
a transverse vessel on the hinder side of each proglottis. In the
last proglottis these vessels open by a median pore. The nervous
system consists of a ring in the head, small forward nerves, two
lateral nerve cords and branches.

REPRODUCTION

The reproductive organs have
the same general structure as in
the liver fluke : they are shown in
Figs. 92 and 93. Each proglottis
contains a complete set of them.
Since no other worm is usually
present, it must be fertihsed
either by another proglottis or
by itself. From time to time the
last proglottis breaks off, singly
or with others, and passes out
of the anus of the host. It is not
able to crawl about, although
that of the related beef tape-
worm T. saginata can do so,
discharging eggs as it goes. The

eggs, to the number of 850,000 or more, are set free by the rupture
of the proglottis wall. If, as may happen in various circumstances,
they are now swallowed by a pig, or, as occasionally, by man, their
shells are dissolved in the ahmentary canal and a httle spherical
six-hooked embryo or oncosphere is set free from each. This
bores its way from the intestine of the host into his blood vessels
and is carried to the muscles and other organs, where it loses its
hooks, increases in size, and becomes a bladder-worm or cysticercus.
The wall of this becomes tucked in at one spot, forming a pouch,
on the inner wall of which the suckers and hooks of the future
head appear. This happens ten weeks after the e^g has been eaten

Fig. 92.^ — A proglottis of Tcsnia solium
with the reproductive organs at the
stage of complete development.

C.S., Cirrus sac ; excr. excretory canals ; g.o., genital
opening ; n.c, nerve cord ; ov., ovary ; sh.g.,
shell gland ; t., testes ; v.d., vas deferens ; ut.
uterus ; vag., vagina ; y.g., yolk gland.

130

CESTODES. PHYLUM PL ATYHELMINTHES

by the pig. No further change takes place unless the flesh of a pig
infested with such bladder-worms (known as ' measly ' pork) be
eaten raw or ' under-done ' by man. When this happens, the
stimulus of the new surroundings causes the pouch to be turned
inside out. the effective agent being the bile salts (p. 444) acting
in a neutral or alkaline medium. This evagination cannot therefore
occur until the intestine is reached. The bladder is digested, but
the head fixes itself and begins to bud off proglottides. The life-
history of Tccnia solium may be summed up as follows : —

Egg_>.Oncosphere^-Cy sticercus— >-Scolex->Adult .

It will be seen that only one generation is involved, unless each
proglottis be regarded as a complete individual, and not merely

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'^v$mmmsM

m^^
1'.

Fig. 93. — A transverse section through a proglottis of TcBuia in which the
reproductive organs are well developed.— From Shipley and MacBride.

I, Cuticle : 2, long-necked cells of ectoderm ; 3, longitudinal muscle fibres cut across ; 4, layer of
circular muscles ; 5, split in the parenchyma which lodges a calcareous corpuscle ; 6, ovary ; 7, testis
with masses of male germ-mother-cells forming spermatozoa ; 8, longitudinal excretory canal ;
9, longitudinal nerve cord; 10, uterus ; 11, oviduct.

as a part of the parent body broken off to carry the eggs. Although
the pig is the normal intermediate host, the eggs will develop
in man, and heavy human infestations with bladder- worms are
known. The adult worm may live in man for more than twenty-
five years.

PLATYHELMINTHES

The members of the phylum Platyhelminthes are adequately
diagnosed as being triploblastic acoelomate Metozoa with no
anus. In addition they are bilaterally symmetrical and dorso-
ventrally flattened ; they have a flame-cell or protonephridial
system (a feature which they share with several other groups) ;
and have characteristic complicated reproductive organs.

131

Cf

CLASS I—TURBELLARIA

These are almost all free-living, and retain the gut, though
this may be reduced and solid. The outer layer of the body
consists of cells covered with cilia, by which the animals move
on surfaces, although many of them also use muscles for this
purpose and for swimming. In or just below the ectoderm are
usually scattered cells containing peculiar rod-shaped bodies or
rhabdites, of unknown function. There are sense cells of various
sorts, mostly in the anterior part of the body, and
usually eyes, which have the retina well-marked,
but no lens. The nervous system (Fig. 94) is
interesting and shows the beginning of a brain.
There is a pair of ganglia in the head, united by
a commissure and giving off longitudinal nerves
to the body. These supply a nerve net under the
epidermis and another deeper in the body. The
function of this brain is merely to relay and distri-
bute the impulses from the important sense organs
on the head. Unlike Hydra, where no part of the
body permanently dominates the rest, these
creatures, moving as they do always with the same
end forwards, have that end organised for
perception and the constant stimulation of the
rest of the body. This permanent organisation
of a dominant region of the body unifies the
reaction of the body as a whole to changes in its
surroundings. But in the Turbellaria the brain does
not co-ordinate the activities of different regions of
the body. It sets them in action : that each plays
its proper part is due to local organisation.

The Turbellaria are mostly carrion feeders, and protrude a
long muscular pharynx, into which the food is sucked (Fig. 95).
Two interesting features of them are their ability to regenerate
when cut into pieces, and the fact that when starved they degrow.
Not only do they decrease in size, but the internal organs are
reduced and disappear in order, first the eggs and reproductive
organs, and then the gut and muscles, the nervous system alone
being unreduced. When given food these starved animals
regenerate their organs and return to their normal size.

The commonest British species of Turbellaria all belong to the

Fig. 94. — A
diagram of the
nervous system
of a turbellarian,

e.g., brain ; e., eye;
/.«., longitudinal
nerve cord ; o.,
opening through
which the pharynx
is protruded ; ph.,
pharynx ; ph.s.,
pharynx-sheath.

PHYLUM PLATYHELMINTHES
132

Order Tricladida. in which the gut has three main branches.
Demlrocaliun lackiim, Planaria lugnbris and Polycehs nigra are
all found HI ponds and slow streams ; the first is white, and the
other two black, but Poly eel is has an anterior rmg of eyes by

'y- 9.0.

ph.s.

Fig. 95.— a turbcUarian {Planaria polychroa) .—From Shipley and MacBride.
csl CiliAted sensory slit at side of head ; eye ; g.o.. genital opening ; m., rnouth. at end of outstretched
CM., cuiaiea sen^ > pharynx ; ph.s., sheath into which pharynx can be withdrawn.

which it may be recognised. Planaria alpina, also black, is found
in mountain streams. Procerodes litoralis {■=Gunda hIvcb) is marine
and is found in rock pools. Some species are terrestrial.

CLASS II—TREMATODA

These are parasitic, or rarely epizoic. They have a thick cuticle,
and suckers, and the adult has no external ciha. Fasciola is
characteristic of the class, though its genital organs are more
complicated than those of most species.

CLASS III—CESTODA

The tapeworms are entirely endoparasitic. There is no gut,
and the ectoderm cells have sunk into the parenchyma, leaving a
thick cuticle at the surface. CiUa are absent except for the flame-
cells. The characteristic form of the body of the adult is that with
scolex and proglottides as shown by Tcenia solium.

Other common tapeworms are : Tcenia saginata, without hooks,
found in man, with the bladder-worm stage in the ox ; it may be
up to forty feet, or occasionally eighty feet long, and may have
2000 proglottides ; T. serrata in the dog, with a bladder-worm in
rabbits, hares, and mice ; T. coenurus in the dog, with the bladder-
worm known as Ccenurus cerebralis in the brain of sheep and
other hoofed animals, where it causes ' staggers ' ; and T.
echinococcus, which has only three proglottides, in the dog, with
the bladder-worm Echinococcus in sheep, oxen, pigs, and some-

CLASSIFICATION

133

times in man. The latter two species produce numerous heads in
the bladder-worm stage. Since only one of these can be regarded
as continuing the individuality of the bladder-worm, the others

2.

Daughter cyst.
Brood capsule.

Fig. 96. — Diagrams of bladder-worms.

I. The ordinary Cysticercus type, with one head; 2. The Coenurus type, with many heads; 3. The
Echinococcus type, with secondary cysts or brood capsuJes producing many heads. Hooks and
suckers are not shown in 2 and 3.

must be looked upon as buds from it, so that there is here a
metagenesis. The bladder produced by T. echinococcus is known as
a ' hydatid cyst ' ; it is very large, containing sometimes as much
as a gallon of fluid, and its wall buds off secondary cysts into the
cavity.

11

ROUND W^ O R M S

Besides the flukes and tapeworms there is another group of
parasitic worms, even more important medically and economi-
cally, and of at least equal biological interest. This is the group

Nematoda, or round-
worms, so called because
in contrast to the flat-
worms their body is
circular in cross-section,
and so resembles a thread.

ASCARIS

Some of the largest
roundworms are those
belonging to the genus
Ascaris. The species suillce
is frequently present in the
small intestine of pigs,
and lumhricoides (Fig. 97)
is not uncommon in man,
especially in children ; the
male may be as large as 17
centimetres long and 0-2
centimetres in diameter,
while the female may be
25 centimetres by 0-5.
A. megalocephala, which lives in the horse, is even larger. The
following description is intended primarily for the human form,
but will apply almost equally well to the others, which are indeed,
according to some workers, merely physiological races of the
human species.

The general shape is that of a cylinder with pointed ends,
and the surface is smooth and yellowish-white in colour. Along
the middle of the back and of the ventral side run white lines,
and there is a brownish line along the middle of each flank. At

134

Fig. 97. — Ascaris lumbricoides

A, Male; B, Female.

a. Anterior end ; an., anus ; e., 'excretory' pore;
genital opening ; p., posterior end p.s., penial setae

g.o.

ASCARIS

135

the front end is the mouth, guarded by three Ups, one above and
one at each side below. The dorsal lip bears at its base two papillae
and the ventro-lateral lips one each. The edges of the lips bear
many small teeth, those of suillcB being smaller than those of
lumhricoides . Median on the under side, about two millimetres
from the mouth, is a so-called ' excretory ' pore. The female
bears a median genital pore at about one-third of the length
of the body from the front end, on the ventral side of a region
which is slightly narrowed. The tail is curved downwards,
slightly in the female and strongly in the male. The anus lies
below, about a couple of millimetres from the hind end. In the

Fig. 98. — Ascaris lumhricoides. — From Sedgwick, after Leuckart.

a, Hind end of male ; b, head, from above ; c, head, from below ; d, egg, in shell ; p, ' excretory ' pore ;

Sp, penial setae.

male this opening serves also as a genital pore, and there project
from it a pair of penial setae.

Internally, a spacious perivisceral cavity separates a straight,
simple gut from a simple body-wall. The cavity is traversed by
numerous delicate strands of a remarkable connective tissue,
which is composed of processes of a few cells, notably of one very
large cell placed on the dorsal side just behind the nerve ring.
Over the gut and the muscle fibres of the body-wall the strands
join a thin covering layer which lines the cavity. Thus the body
cavity may be regarded as intracellular, and is thus unlike
that of other animals, which is either coelomic or haemocoelic
(p. 188).

The body-wall is made up of three layers (Fig. 99) : a stout,
smooth cuticle, made of the protein collagen with some keratin,
which consists of several layers and is shed four times, an ecto-
derm (' hypodermis ') which is without cell-limits and must
therefore be classed as a syncytium (p. 35), and a single layer of
peculiar muscle fibres. The body-wall contains a peculiar form

136

ROUNDWORMS. PHYLUM NEMATODA

of the red pigment haemoglobin (p. 190), with a high affinity for
oxvgen which is presumably used in respiration. Haemoglobins
are also present in other parts of the worm. The nuclei of the
ectoderm, except at the hinder end, are collected along the mid-

cLn- dJ-

Int-

ex.c.

-vc-L.

\:rv

Fig. 99. — A transverse section through the middle of the body of a female

Ascaris lumbricoides.

cu.. Cuticle ; d.l., dorsal line ; d.n., dorsal nerve ; ect., ectoderm ; egg ; eni., endoderm ; ex.c, excretory
' canal; int., intestine: /./.. lateral line; m.f., muscle fibre; ov., ovary; p.c, perivisceral cavity;
ut., uterus ; v.L, ventral line ; r.n., ventral nerve.

dorsal, mid-ventral, and lateral lines. Along these lines the
protoplasm bulges towards the body cavity. A nerve cord is
embedded in the dorsal and ventral lines, and a canal in each
lateral line. The canals have no internal openings ; they unite
in front to open by the excretory pore ; in the free-living Rhabditis
they have been shown to expel a fluid to the exterior. The two
have but one nucleus, which is very large, and lies in the wall of the
left-hand canal, near its front end. Thus they may be said to be
hollowed in the body of one immense cell. Two more nuclei lie in
the wall of the median duct to the exterior. Four very large,
hr^nrhofi cells lie upon the lateral lines near the anterior end of the

ASCARIS

137

Lateral nerve

Submedian
nerve.

Sublateral
nerve .

Ventral
nerve .

Dorsal
nerve.

body and have the power of taking up particles from the body
cavity. They are known from this function as the phagocytic
cells. The nerve cords
(Fig. 100) are connected
by transverse commis-
sures in the ectoderm,
and in front join a ring Lateral
a little way behind the ^^'^^^"^'^'

mouth. From this ring ^^^^^^i

four other cords run gangUon
back at the sides, and
six forward. The nerve
ring is slightly thickened
above and rather more
below, and contains
some nerve cells. The
only other ganglia are
placed at the sides of
the nerve ring and at
the hinder end of the
ventral cord. A few
cells are scattered
among the fibrils of
which the cords are
composed, but there is
no sign of segmental
arrangement in these or
any other organ of the
body. The number of
cells which compose the
nervous system is small
and remarkably con-
stant, each cell being
recognisable in the same

position in every individual. Each muscle fibre (Fig. loi) consists
of an outer part which has fibrils running longitudinally and an
inner part of undifferentiated cytoplasm containing a nucleus.
Strands of protoplasm stretch from the inner parts of the muscle
fibres to the dorsal and ventral nerves. The alimentary canal con-
sists of three parts : a short stomodaeum, or fore-gut, known as
the pharynx, a long mid-gut, and a short proctodaeum, or hind-gut,

Anal __
ganglion

Fig. 100.

Anus

A diagram of the nervous system of
a nematode.

138

ROUNDWORMS. PHYLUM NEMATODA

known as the rect.nn. Ihe fore- and hind-guts are lined bj' in-
turned ectodonn, with a prolongation of the outer cuticle, which
is shrd with the latter. They have in their walls muscular fibres.
The mid-gut is composed solely of a layer of columnar epithehum,
with a basement membrane outside it. The food consists of solids
and liquids taken up from the contents of the intestine of the host.
There is neither vascular nor respiratory system.

REPRODUCTION

Fig. ioi.— Muscle fibres of Ascaris. — From Parker and Hasvvell, after Leuckart.

4 A sincle fibre ■ B several fibres in traDSverse section, with a portion of the ectoderm (below) ;
" ' c. Contractile 'part of the fibre ; /, processes ; nu, nucleus ; p, undifferentiated protoplasmic part ot
tile fibre.

The genital organs (Figs. 102 and 103) are of a type peculiar
to the Nematoda. They are paired in the female and unpaired
in the male, and lie free in the body cavity. The male apparatus
is composed of (a) the testis, a long, coiled thread consisting
in its anterior part of a solid mass of immature sex-cells,
and in its hinder part containing a cord or ' rachis ' in the
middle with riper sex-cells attached to it, {b) the vas deferens
of much the same width as the testis, (c) the vesicula seminalis,
a wider tube, (d) a short, narrow, muscular ejaculatory duct.
The spermatozoa are simple rounded cells, which become
amceboid when they have been transferred to the female. The
"lie organs correspond with those of the male. Each ovary

139

Fig. 1 02. — A male Ascaris
lumbricoides dissected
from above.

Fig. 103. — A female Ascaris
lumbricoides dissected from
above.

d.e., Ductus ejaculatorius ; p.s.,
sacs of the penial setae ; /.,
testis ; v.d., vas deferens ; v.s.,
vesicula seminalis.

int., Intestine ; /./., lateral lines ; m.j.,
muscle fibres; od., oviduct: ov.,
ovary ■,ph., phar>Tix ; rm., rectum ;

M/., uterus ;
ventral line.

vas.

vagina ; v.l.;

j.^) ROUNDWORMS. PHYLUM NEMATODA

has the same general structure as the testis, a hollow region
which may be called the oviduct connects it with the wide uterus,
and the two uteri unite in a short, muscular vagina. The eggs
are produced in inunense numbers— up to 200,000 a day — fer-
tilised in the upper part of the uterus, enclosed in a chitinoid
shell, passed into the gut of the host, and by it voided with the
fiEces. Before they can bring about a new infection, they must
pass through a period of ripening, which normally lasts thirty
to forty days, and needs moisture, a temperature above 60° F.,
abundant oxygen, and the absence of putrefaction, and therefore
cannot take place except in the outer world. The eggs survive
poorly in heat or on drying, and in sandy soils they are washed
to the surface and damaged. The most favourable situation for
development is therefore a clay soil, in which they are protected,
and, by being swallowed with food or water, may reach a new
host. Usually they hatch in his intestine, but in warm, damp
places may do so in soil. In the egg occurs the first of the four
moults which Ascaris, like other nematodes, undergoes in the
course of its hfe. The worms hatch as infective larvae which in
the new host do not at once become intestinal parasites but under-
take first a remarkable journey. Freeing themselves from the
remains of the second cuticle and piercing the wall of the intestine,
they enter venules and lymphatics (p. 454) and are carried through
the Uver and heart to the lungs, where they cause congestion and
haemorrhage and are thus discharged into the alveoli (p. 446).
Thence they travel along the bronchi and trachea into the gullet
and descend the alimentary canal to reach the intestine once
more. In the lungs they undergo two more moults, and acquire as
swimming-organs lateral membranes, which they afterwards lose,
and grow from 0.28 mm. to 2 mm. in length. The final moult
occurs in the duodenum, and a mature adult is formed four to
five weeks after infection. The adult may live for a year.

Ascaris lumbricoides is found throughout the world, and can
only be avoided by care taken in regard to the cleanness of raw
foods and drinking water. It may cause little trouble to the host,
or be the source of diarrhoea, anaemia and other complaints,
the latter apparently through an enzyme formed b}^ it which
interferes with digestion. In severe infections with the eggs,
temporary bronchitis may occur during the passage of the larvae
through the lungs. Santonin, thymol, and other vermifuges are
1 against the worm.

141

OTHER ROUND WORMS

Though many nematode worms are known, none of them
has been found to differ anatomically from Ascaris in any import-
ant respect : this is remarkable, because while some are parasitic
in vertebrates, others live in invertebrates, some parasitise plants
and yet others are free-living, so that one type of structure serves
for a wide variety of habitats. Their life-histories, however, are
as diverse as they are remarkable, probably because it is only
by strange and various devices that they can obtain entry to their
several hosts. The following are brief outlines of examples of the
principal t^^pes of nematode life-history : —

1. Free-living throughout life. — One of the best-known examples
is Rhahditis aherrans (Fig. 104), which is about one mm. long and
is common in soil. The adult is easily observed as a transparent
object under the microscope, when it can be seen that the oeso-
phagus shows active muscular pumping movements while the
contents of the mid-gut are moved only by the general contractions
and bendings of the body in locomotion. The general anatomy does
not differ much from that of Ascaris, but the genital organs are
simpler. Copulation having taken place, fertihsation occurs in
the upper part of the uterus, and the eggs are laid. They develop
into larvse which undergo the four moults usual in the group.
After the second of these the larva may remain within the shed
skin, in an apparently encysted condition. It retains the power
of movement, but can resist desiccation and so serves as a dis-
tributive and dormant stage. Larvae in this state are attracted
by any strong concentration of decaying organic matter such as
a piece of meat placed in the soil, and on feeding on this they
rapidly develop into sexually mature adults. In some species of
Rhahditis the males are few and sexually degenerate, while the
females have developed into protandrous hermaphrodites.

Another free-hving species is Anguillula aceti, found in vinegar.

2. Free as larvce, parasitic on plants as adults. — The Cockle
Worm, Anguillulina {^Tylenchits) tritici (Fig. 105) causes
ear-cockles in wheat. A pair, living in a single flower of the
plant, become mature in late summer, and produce hundreds
of larvae. The plant tissues react by forming a gall (the
cockle) instead of a proper fruit, and in this the larvae may
survive for at least twenty years. In damp earth, however, they
become active and escape. In the spring the larvae enter the

142

ROUNDWORMS. PHYLUM NEMATODA

h.cav.

Fi

(.. lyt^.—Rhabditis.'

-From Borradaile and Potts, The Invertehrata, 2nd edition,
1935. Cambridge University Press.

Mature female ; B, mature male ; C, ventral view of hind end of male, slightly turned to one side ;
D, side view of hind end of male ; E, encysted larva ; an., anus ; b.cav., buccal cavity ; c.b., copulatory

cl., cloaca ; cut., stretched skin of the last larval moult ; ex.a.,
/.?., fore-Rut ; jg/.c, gland cells ; h.g., hind-gut ; ni.g., mid-gut ; n.c, nerve collar;
■^'s fertihsed and in 2-cell stage ; ov.. ovary ; p.v., pharynx ; rec.
; ut., uterus ; va., vagina ; vd., vas deferens.

bur
ext
0..

- *■ , ropulatory spicule
'jrture

LIFE-HISTORIES

143

stomata of a young wheat plant, or pierce the cell-wall with a
projection from the buccal lining, and crawl up through the
tissues to the flowers, where the life-cycle begins again.

Heterodera by contrast is an ectoparasite. The female larva
attaches herself to a root of tomato,
cucumber, beet, or other plant,
and sucks the sap. She is fertilised
by visiting males.

3. Free as larvcs, parasitic in
animals as adults. — Ascaris is one
example of this type ; another is
Ancylostoma duodenale, the hook-
worm (Fig. 106), which is pink,
and lives in the small intestine
of man. The male is 8-1 1 mm. long,
the female 10-18 mm. It browses

on the tissues, and so causes a considerable anaemia. Copulation
occurs, and the eggs are laid and are passed to the exterior
having divided to the four- or eight-cell stage. Further develop-
ment requires air, moisture, a moderately high temperature.

Fig. 105. — The Corn-cockle ^^'o^m.
From Theobald.

A, Cockle gall ; C, larvae ; D, gaU cut open ;
E, larvae magnified.

Fig. 106. — The Hookworm {Ancylostoma duodenale). — Fromi Parker and

Has well, after Leuckart.

A, Male and female in coitu ; B, anterior end ; C , mouth, with spines ; D, hinder end of male, with e.xpansion

known as bursa ; cv.gl., cervical glands ; ph., pharynx.

and preferably darkness. In temperate climates they can there-
fore only grow in such places as mines and tunnels, which has
led to the animal being known in this country as the miners'
worm, and the disease which it causes as miners' anaemia. In
all tropical and sub-tropical countries, however, the worm is
widespread, as the larvae can develop in the open. The third-
stage larva, which as is usual in nematodes is the infective or

j,^ ROUNDWORMS. PHYLUM NEMATODA

waiuloring stage, is reached in about a fortnight. It remains
witliin the second larval skin, and can penetrate through the skin
of an>' part of the body of man. The foot is a common site in
countries where shoes are not worn, and larvae on salads or in
drinking water may enter through the membrane of the mouth.
Having pierced the skin, the larva enters a venule or a lymphatic,
and goes by the veins to the right side of the heart and to the
lung. It is too large to enter the capillaries, and so pierces the
lung walls and passes into a bronchiole, whence it crawls up
the trachea and is swallowed. It acquires a temporary capsule
in the stomach, and after two more moults is adult. The hooks
by which it attaches itself to the intestinal wall are acquired
at the third moult. The period from infection to the appearance
of eggs in the fseces is about eight weeks.

4. Larva: parasitic, adults free. — This is a reversal of the common
state of affairs. Mermis nigrescens, the Rain Worm, also illustrates
the Nematoda parasitic solely in invertebrates, although there
are others in which the life-cycle is more normal. The adults
live deep in soil and pair there ; the larvae penetrate the skin of
insect larvae, and develop in the body cavity until they are
sexually mature, when they return to the soil. The worm gets
its English name from the peculiar habit of the adults of climbing
the stems of plants after summer rain in such numbers as to give
rise to the legend of ' showers of worms '.

5. LarvcB and adults parasitic in different animals, with a free
stage. — An example is the Guinea Worm, Dracunculus medinensis,
the female of which may be three or four feet long. The posterior
part of the gut is lost and there is no anus, and almost the whole
body is occupied by an expanded uterus containing larvae. These
escape, either through a female genital opening or vulva just
behind the mouth, or through the mouth itself, while the host is
immersed in water. The larvae must penetrate a crustacean of the
genus Cyclops (page 221), and here they develop for three to five
weeks, with two moults. If a man drinks water containing infected
Cyclops the larvae are released, and bore their way to the sub-
cutaneous tissue. As the worm grows it goes towards the extrem-
ities, especially the ankle, and here the head comes near the surface
and is exposed in a small ulcer. It is the contact of cold water
with this which stimulates the liberation of the eggs. The male is
shorter than the female and has rarely been seen. The worm

ts man in many tropical countries.

LIFE-HISTORIES

145

6. LarvcB and adults parasitic in different animals, with no free
stage. — The best-known of these is Wuchereria [Filaria) hancrofti,
the cause of elephantiasis, which is found in all tropical countries
and as far north as southern Spain. The females are about four
inches long, the males half this size, and both live in the lymphatics
of man. The female is ' ovo viviparous ', that is, the egg-case
contains a completely formed larva. This is set free and goes to
the blood, where it lives for some time and is known as a Micro-
filaria. The larvae congregate in the peripheral blood vessels at
night, retreating to the capillaries of the lungs and to neigh-
bouring vessels during the day. In due course the host is bitten by
a gnat, and some of the microfilariae

are sucked up. They develop for about
a fortnight, and may then be passed
to a new host when the gnat next
bites. They are carried by several species
and genera of gnats, including the com-
mon Culex pipiens. Other species of
Filaria have somewhat similar life-
histories, with different invertebrate
vectors ; F. ( =Loa) loa is carried by the
tabanid fly Chrysops, and F. perstans
by a leech.

7. Adult and larvcB parasitic, without
free stage, in the same or a closely
related species of host. — Trichinella
spiralis lives as an adult in the small
intestine of man, rat, and pig, and some

other mammals , and experimentally even salamanders may be
infected. The males are a millimetre long, the females three or four
millimetres. The male dies soon after copulation, but the female
bores into the lymphatics and produces many larvae, which are
carried in the general circulation to the muscles and there
encyst (Fig. 107). A new host is infected if it eats meat containing
the cyst. The commonest natural cycles are rat/rat, rat/pig, and
pig/rat, but man may be infected if he eats raw pork (as in German
sausages) or even that which is underdone. The worms in the
intestine cause general intestinal symptoms resembling those of
typhoid, and the encysted larvae produce general weakness and
often death.

8. Parasitic in vertebrates, no larval stage. — Trichocephalus

Fig. 107. — Trichinella spiralis,
young encysted in muscle
of host. — From Thomson,
after Leuckart.

146

ROUNDWORMS. PHYLUM NEMATODA

iinimi

CUV. .

mi-

nt.

Sho.

yrtgt,

{=Trichnyis) and Enterohius (=Oxyuris) (Fig. 108) are medically
i]H)rtant inhabitants of the human gut, the former about an

inch long, the latter half an inch in the
female and an eighth in the male. Eggs
pass out with the host's faeces, and may be
swallowed with raw vegetables. Those of
Enterohius are ripe as soon as they reach
the exterior, so that reinfection by scratch-
ing often takes place.

9. A free bisexual generation alternates
with a parasitic hermaphrodite. — The her-
maphrodite stage of Rhabdias bufonis
{=Rhabdonema nigrovenosum) lives in the
lungs of the frog. It is protandrous.
Embryos escape by the glottis and cloaca,
and become sexual adults. The young
produced by these wander, and may be
swallowed by another frog.

The nematodes are an isolated and uni-
form group. They show certain superficial
resemblances to arthropods (ecdysis, absence
of cilia, absence of coelom, unflagellated

Fig. los.-Oxyuns, some- sperms), but these are not enough to justify
what diagrammatic, to the Suggestion that the parasitic forms

show arrangement of , . . , , t ji r

are degenerate arthropods and the free-
living species descended from the parasites.
There are many important differences, and
it seems best to regard the nematodes as
fairly primitive forms which never pos-
sessed a coelom. Their success at their
own level is obvious.

afu

organs.

A, Male ; B, Female.

an.. Anus ; g.o., genital opening ;
int., intestine ; oes.b., bulb
of oesophagus ; ov., ovary ;
ph., pharynx ; /., testis ; ut.,
uterus; v.d., vas deferens;
vag. vagina.

12
PARASITISM

Parasitism is a very strange mode of life, and as the nematodes
include some of the most successful, as well as most important,
parasites in the animal kingdom, it is convenient to discuss here
some of the generalisations that can be made on the subject.

A parasite is generally formally defined as an organism which
lives in or on the body of another to the detriment of the latter, but
any such definition brings one into trouble at once. It is best to
make definitions, if such must be made, of the relationships rather
than of the relatives. It is clear as soon as one studies the animal
kingdom that most species of animal stand in some sort of special
relation to one or more other species, and what we are trying to
do is to set limits and give names to these relations. (A similar
argument can be apphed to plants, but it is not relevant here.)

The simplest relationship is that where one animal merely
uses another as a base on which to stand, as barnacles often grow
on the shells of Hmpets, and the ciHate Kerona runs about on
Hydra. So far as is known neither has any effect on the other.
The nomenclature of this sort of relationship is imperfect, but
on the analogy of epiphytism it might be called epizoonism,
and the animal which is supported an epizoon. A stage beyond
this is commensalism, where two animals live in close physical
proximity so as to be mutually helpful ; since the partnership
is equal, each member may be called a commensal. The small
hermit crab Eupagiirus pridemtxi often carries on its mollusc shell
an anemone, Adamsiapalliata, which finally dissolves the shell and
takes its place, so saving the crab from having to move house ; it
also protects the crab by its nematocysts. In return it acquires the
advantages of locomotion and receives as food scraps which fall
from the mouth of the crab. The syrphid flies which scavenge in the
nests of bees, and the aphides tended by ants, are other examples.

A partnership for mutual benefit where the advantages and
association are intimately physiological is called symbiosis, and
the partners are symbionts. It is often probable that the benefits
are rather one-sided. Though the relation between Chlorohydra
viridissima or the corals and their green Algae is generally called
symbiosis, it is not known what advantage the coelenterates
derive from the plants. They seem not to use the carbohydrate

147

o PARASITISM

14S

producocMn- i^liotosvnthesis. and indeed starved corals expel their
guests. The plant coils presumably acquire carbon dioxide and per-
haps other end-products from the animals, and it may be an ad-
vantage for these to be quickly removed. A clearer example of
symbiosis is given bvthe flagellates which live in the gut of termites,
and so enable those insects to Uve on wood. For their part, the
termites do a preliminary breaking down and softening of the food.
Where one partner gets all the benefit, and the other almost
inevitably suffers some disadvantage, we have parasitism. It is
generally recognised that the line between commensalism and
symbiosis on the one hand, and parasitism on the other, is often
blurred, but it is not so often reahsed that it is equally difficult
to distinguish a parasite from a carnivore. Although we seldom
put it into our definitions, the assumption that a parasite is
smaller than its host is implicit in our thinking ; we call a flea
or a leech a parasite, a spider, which also feeds on the juice of prey
which is at first living, a carnivore. Parasites do not in general
kill their hosts, or at least not very quickly, but many do, and
there is at least one carnivore which does not. Grebes on the Lake
of Tiberias feed on the eyes of large fish, which they bite out
of the head, while the fish, though blinded, go on living. The
association of parasite with host is usually longer in time than
that of carnivore with prey, but there is in fact a complete series
from parasites which take only an occasional meal to those which
cannot five away from the host (or hosts) at all. For the Insecta,
for example, Keilin has drawn up the following list :

1. Aedes, a gnat which needs no blood but will take it if it
gets the chance,

2. Anopheles, a gnat of which the female needs occasional
blood meals before it lays eggs.

3. Stegomyia, a gnat which needs frequent blood meals.

4. Cimex, the bed bug, which feeds on nothing but blood, drops
off the host after a meal, but remains near him.

5. Piilex, the flea, does not often leave the host, and dies if
kept away for long.

6. Pedicnhts, the louse, remains on the surface but moves about.

7. Phihirus, the crab-louse, moves very little.

8. Sarcopsylla, a flea of which the female burrows into the
skin and lives permanently in a tumour.

9. The larvse of Hypoderma, the warble fly, and of some other
Diptera, are completely internal. It is impossible to complete

CHARACTERISTICS 149

the series within the Insecta, but we have already seen various
degrees of independent hfe in the nematodes, and have met
examples of this group and of Protozoa where there is no free-
living stage in the life-history at all.

If we are to have a definition of a parasite which has any
practical value, we must extend the usual one by several clauses.
A parasite may be defined, with some approach to accuracy,
as an animal which, for an appreciable time, lives in or on the
body of another, considerably larger, animal, called the host,
to the detriment of the latter, but without causing its death
until reproduction of the parasite has occurred. This leaves the
zoologist free to put his own meaning on * appreciable time '
and ' considerably larger '. Having digested this definition you
may remember that ornithologists agree in calHng the cuckoo
a parasite, although it lives neither in nor on the body of the
host, and so far from feeding on the host, is voluntarily fed by it.

Fortunately, although few biological terms can be rigidly
defined, the general meaning is often clear. This is true of the
term parasite, and it is the typical parasites that we must now
consider. In the first place, it is convenient to divide them into
ectoparasites which Uve on their hosts, and endoparasites which
live in them, although, as we have seen, there are intermediate
forms. The modifications in both form and physiology which are
necessary in parasites differ in these two groups, as do their effects
on their hosts.

EFFECTS ON HOSTS

The effects of parasites on their hosts make up much of the
subject-matter of pathology, and it is impossible in a textbook
of zoology to go far in their study, but some generalisations are
possible. It is often said that the successful parasite does not kill
its host, and even that the really successful parasite does not
even harm it, but there does not seem to be any justification
for this view. Judged by their ubiquity, malarial parasites and
Trichinella are highly successful, but they often cause death ;
the ichneumon flies which live in the caterpillars of Lepidoptera
also appear to be highly successful, but they always completely
consume their host. It is true that there must be a limit to the
power of the parasites to kill, for if all hosts were killed there could
be no parasites, but in fact the reproduction of the hosts seems
to keep pace with the deaths, and a low density of population

PARASITISM

inevitably means a smaller chance of infection, which makes the
host-parasite ratio to some extent self-regulatmg. It is this
which may account for the violent iiuctuations m numbers of
many animals, especially rodents. The numbers of voles, for
instance, increase steadily until a certain degree of overcrowding
is reached, and then an epidemic caused by a parasite spreads
rapidly and reduces the numbers.

Apart from death, the effects on the hosts vary with the
parasite and the place where it lives. Ectoparasites seldom cause
more than irritation, although secondary infection with another
parasite, such as the plasmodium carried by the gnat, may lead
to serious results. Endoparasites which Hve in the gut (which,
strictly, is outside the body) may do nothing more than deprive
the host of some food ; such is the case with a small number of
Ascaris or tapeworms ; larger numbers may take so much food
that the host cannot supply enough, with general effects on health,
and symptoms such as diarrhoea and vomiting are common.
Hookworms may cause enough loss of blood to produce anaemia.
Parasites hving in the fluid tissues (the blood and lymph) may
merely steal some food, as does Filaria perstans, which produces
no known pathological effect ; they may mechanically block the
vessels, like Filaria bancrofti ; or, like Trypanosoma, and
Plasmodium, they may manufacture toxins which cause a
violent reaction in the host. Those which live in the soHd tissues,
such as the Guinea worm and Trichinella, always cause irritation,
and may have more serious effects. A special case is made by
those parasites which preferentially attack the gonads, causing
parasitic castration. Examples are known in the Protozoa,
trematodes and insects, but the most familiar is the cirripede
crustacean Sacctdina, parasitic on crabs, in which the castration
is accompanied by changes in the secondary sexual characters.
Parasites which attack the brain, such as Trypanosoma in its
later stages, and the cysticercus of Tcenia cosnurus in sheep,
have profound behavioural effects. According to the situation
and degree of toxicity of the parasite, various local reactions,
from inflammation to tumours, may occur in the host's tissues.

IMMUNITY

A matter which is of great importance both to the parasite
its efforts to establish itself in a new host and to man in his

IMMUNITY 151

efforts to combat disease, is the degree of immunity which the
host shows to the parasite. The word ' immunity ' is used in a
wide sense to cover a number of different modes of reaction. At
the one extreme, host restriction and other difficulties in infection
may be determined simply by the fact that the ordinary conditions
of the environment are not suitable for the development of the
parasite. Very often the egg-shells or cyst-cases of intestinal
parasites must be digested so that the embryo is liberated ;
we have seen that this is so in tapeworms, and it is true also of
many Protozoa, such as the sporozoan genus Eimeria, various
species of which are common parasites of domestic animals,
with no very great degree of host restriction. The cysts of Eimeria
from fish, however, when ingested by man, are unaffected by his
digestive enzymes, and pass right through and remain alive in the
faeces. Live proglottides of tapeworms cannot infect man, because
the egg-shells need the successive action of acid and bicarbonate,
and unless the proglottis is first killed, the eggs are not liberated,
and so exposed to the digestive juices, until the intestine is
reached and the acid of the stomach has been left behind.

At the other extreme, there may be an active warfare of the
host against the invading parasite, and it is this that we most
commonly think of as immunity. It takes two chief forms. In
the first, which may be called the cellular mechanism, amoeboid
cells of the host, called phagocytes, actively ingest and digest the
parasites. Phagocytes are found chiefly in the blood (the poly-
morphonuclear leucocytes ; see pp. 524-25) and in connective
tissue (macrophages; see p. 515). In the second, the chemical
or humoral mechanism, the presence of the parasite causes the
production of substances which have a specific action on the things
which bring them into being. This is a particular example of the
general effect of the introduction of a foreign protein or carbo-
hydrate of large molecule into the tissues. Such a substance, called
an antigen, induces the formation of a specific antibody. This may
act in any of three main ways ; it may merely neutrahse the objec-
tionable effect of the antigen (the antitoxic effect), it may render
the parasite more vulnerable to the attack of phagocytes
(opsonification), or it may itself bring about the death and
destruction of the invader (lysis). Antibodies acting in these ways
are called antitoxins, opsonins, and lysins respectively.

There are also substances which act in the same way called
natural antibodies, because they are naturally present, and they

PARASITISM

could obviously assist ui host restriction (p. 156), but they are less
interesting than those which only develop in the presence of the
parasite and so confer an acquired immunity. It seems to be on
the whole rare for a host to acquire complete resistance or
immunity to an animal parasite, that is, to develop such a
chemical reaction that it cannot subsequently be infected by
the same species. Cattle may, however, acquire a resistance to
Trypanosoma hrucei and others, and man to Plasmodium vivax.
l^iVtial resistance to the cystercus stage of tapeworms is common,
but resistanre to the adults is probably never acquired. A pecuHar
form of resistance, called premunition, does, however, prevent
a second infection in a man who harbours a single beef or pork
tapeworm, and premunition also prevents subsequent infection
with Protozoa in a number of animals and diseases, including malaria
and trypanosomiasis, so long as any individuals derived from the
first infection are present ; these are themselves also kept in
check. Many bacteria confer a much stronger immunity, so that
all the original invaders are killed, and reinfection is impossible for
long afterwards.

EFFECTS ON PARASITES

In attempting to determine the effects which parasitism has
had on parasites one must compare them with what appear to
be their nearest free-living relatives. This is easy with insects like
the warble-fly or the ichneumons, where parasitic larvae grow into
normal adults, or the crustacean Sacculina, w^here the parasitic
adult is formed from a normal nauplius larva, but in the more
typically parasitic groups it is more difficult. Neither the Sporozoa
nor the Trematoda nor the Cestoda are at all closely related to
other members of their phyla, and though there are free-living
nematodes they seem to differ not at all from the parasites.
The ' adaptations to parasitism ' of elementary textbooks and
examination papers are, in fact, mostly imaginary ; there is no
evidence that any parasite possesses any physiological mechanism
or structural peculiarity that is not possessed or paralleled by free-
living animals. What is true is that parasites show an unusual
development of features which are known elsewhere, and that the
few parasitic members of the ' higher ' groups, chiefly the Crustacea
and Insecta, have lost almost all their characteristic features.

Ectoparasites generally possess piercing and sucking mouth-
parts, for they typically feed on blood ; the Mallophaga or bird

EFFECTS ON PARASITES 153

lice are exceptional in feeding on feathers which they seize with
biting mouthparts. Often ectoparasites also possess some means
of attachment, such as the posterior sucker of leeches and the
well-developed claws of lice. Gut-living endoparasites also often
possess means of attachment, such as the hooks and suckers of
tapeworms and the jaws and sucking mouth of the hookworm ;
most nematodes, however, have none. Many parasites possess at
some stage of their life-history considerable powers of locomotion,
as the frequent occurrence of the sentence ' bores its way into the
tissues ' in the accounts of life-histories shows. Once in place they
may not move, but they have seldom lost any organs of locomo-
tion other than those specially developed for the use of the
larvae. The flatworms show a progressive loss of cilia. The uniform
environment in which many parasites live, at least as adults,
presumably means that they receive few stimuli, and one might
therefore expect there to be few sense organs. This is found to be
true, but at the level of body-organisation of the three major
parasitic groups there would be few sense organs anyway. The
abihty of the Guinea worm to get its head to the right place
at the right time suggests a co-ordinated response of a high order.
Most parasites are well supplied with food, and often this is
of a simple form ; in the small intestine and in the blood stream
there are hexoses and amino-acids in place of the polysaccharides
and proteins which are all that most free-living forms can get.
Parasites in such situations therefore need few if any digestive
enzymes, and can begin synthesis or oxidation straight away ;
it is tempting to connect the absence of the gut from the cestodes
with this, but it must be remembered that the free-living
Platyhelminthes show a progressive degeneration of the gut,
and that some of the turbellarians have it replaced by a solid
mass of cells. It seems, therefore, that parasitism need not have
caused the absence of the gut from the tapeworms, but that the
parasitic mode of life allowed an innate tendency to go to an
extreme. Most nematodes have a well-developed gut wherever
they live. Parasites such as the hookworms, which eat the tissues,
are in no different case from any animals to whom food comes
easily. It is possibly the large food supply that has induced the
large reproductive capacity of parasites, for it is a general
observation that much food means many eggs. Pelagic fish, the
queen honey bee, and the barndoor hen are non-parasitic
examples. Parasites have as much need to excrete as have any
M.z. — 6

j£-. PARASITISM

Other animals, for it cannot be expected that the amino-acids of
their food will exactly fit their own tissues. Fasciola, TcBuia, and
Ascaris have all been shown to produce much ammonia, which
is the normal nitrogenous excretory product of aquatic species.

Contrary to common belief parasites do not generally live in
an atmosphere devoid of free oxygen. It is obvious that those
living in arterial blood have plenty, and even in venous blood
and in the tissues there is no mean amount. The partial pressure
of oxygen in vertebrate tissue ranges from lo to 45 mm. of
mercury (that of the atmosphere is about 150 mm.). As this is
largely produced by the dissociation of oxyhaemoglobin, which
acts as an oxygen store, it represents a quantity of oxygen far
greater than would be available in the same volume of most
natural waters. There is no reason to expect that parasites would
respire in a different manner from other simple animals or the
tissues of the host, and experiments have confirmed the similarity.
Parasites use oxygen when they can get it, and most of them
have plenty of opportunity of doing so ; when it is not available
they respire anaerobically, just as does vertebrate muscle, by a
breakdown of carbohydrate, especially glycogen. Lactic acid is often
produced, valeric (=valerianic) acid (CH3CH2CH2CH2COOH) is
even commoner, and Trichinella rather oddly produces carbon
dioxide anaerobically without any organic acids at all. This
worm is also peculiar in that the adult has no glycogen. The
difficulties of culturing most parasites outside their hosts are
great, but it appears from a good many experiments that absence
of oxygen lowers activity and reduces survival time. The lumen
of the gut differs from all other parasitic habitats in sometimes
having a very low oxygen concentration, though it is no lower
than is often found in other natural habitats, such as the mud of
ponds. It is here, if anywhere, that we should expect to find
parasites that can live without oxygen, and Ascaris, for example,
seems to need and use only a small quantity ; the Protozoa
which live in the rectum of many vertebrates probably get very
little oxygen, and probably come as near as any animals to being
anarobic. There is, however, no reason to think that they are
completely without oxygen, or that they cannot use what they
do get ; since so aerobic a tissue as mammahan skin epithelium
can survive under strictly anaerobic conditions for a week, there
is no need to postulate any pecuhar type of Hfe for the parasites.

A feature of their life which parasites share with the inhabitants

EFFECTS ON PARASITES 155

of small ponds is that their ecological niche is discontinuous and
short-lived. Like pond animals therefore they need a distributive
phase. We have seen many instances where this is an egg or an
encysted larva, but most interesting are those where a second host
acts as an intermediary. This may itself be an ectoparasite, like the
gnat which carries Plasmodium or Filaria bancrofti, or it may
be the food of a carnivorous host, like the Trichinella-iniected rat
eaten by a pig. In strict usage the host which harbours the sexual
phase is called primary, the other secondary, but where there is
no sexual phase, as in trypanosomes, or often for medical reasons,
man is considered as the primary host. Parasites generally
produce many eggs, which may, as has been said above, be merely
the result of their good food supply. When the large numbers of
parasitic nematodes, almost all of which are bisexual, are con-
sidered, it is very doubtful if hermaphroditism is any commoner
amongst parasites than in the animal kingdom as a whole. It is
obviously not necessary for success, and it is difficult to see how,
without self-fertilisation, it could be of any advantage. Even
hermaphrodite parasites such as the liver fluke and tapeworm
have well-developed copulatory organs, suggesting that cross-
fertilisation is either still practised or has only recently been given
up. It should be noticed that hermaphroditism is much commoner
in free-living nematodes than in the parasitic species.

There remains to be considered the chemical relationship
between parasites and their hosts. This has two aspects. In the
first place gut parasites, and to a lesser extent those living else-
where, are exposed to enzymes which might at first thought be
expected to attack the parasites and dissolve their cells ; this
would seem to be especially likely with worms in the small
intestine exposed to trypsin. It is clear that the worms survive,
and it can be shown that neither trypsin nor pepsin has any effect
on them so long as they are alive. The same is, however, true of
living earthworms, arthropods, fish, and Protozoa, so that it seems
that the living cell surface of all animals is not attacked by, and
is impermeable to, proteolytic enzymes. Over and above this,
however, some parasites produce anti-enzymes, which diffuse
into the medium and neutralise the enzymes there present. The
evidence for tapeworms is conflicting, but it seems certain that
Ascaris produces, or causes the production of, a substance
which combines with and neutralises trypsin (and pepsin). It is
a polypeptide, and it so closely resembles an anti-enzyme extracted

jc() PARASITISM

from beef pancreas that it has been suggested that it is not formed
by the worm, but that it is produced by the host under stimulation.
However that may be, the production of anti-enzymes is not
pecuHar to the parasite, and at the most, as was said at the
beginning of this discussion, it is only making special use of a
general property of living tissue.

The second aspect of the chemical relationship is that of the
reaction of the host to the parasite, which leads, it is to be pre-
sumed, to the host-restriction of the parasite. Sometimes only a
single species of host seems possible ; thus all attempts to infect
chimpanzees with Plasmodium malaricE have failed ; human P.
vivax transferred to chimpanzees disappeared and apparently gave
no infection, but that it was present was proved because another
man could be infected from the ape. At the other extreme adults
of Trichinella spiralis have been found naturally in man, pig,
fox, cat, and both species of rat, and experimentally all mammals
that have been tried have taken the infection, and although
birds are not susceptible, salamanders are if they are kept at 30° C.
Some degree of restriction between these extremes is the common
state. It is often found that, while transference from one host
to another is impossible, parasites of morphologically identical
form occur in the second host. Thus the three species of Plasmodium
found in man are all paralleled in the higher apes ; Entamoeba
histolytica of man and E. ranarum of frogs are morphologically
indistinguishable, and, except for the size of the labial teeth, the
same is probably true of Ascaris lumbricoides and A. suillce.
Whether such forms should be classed as separate species is a
question beyond the scope of this chapter, but it is well to remem-
ber that man, living in a visual world, is apt to overvalue visual
stimuli. He cannot consciously distinguish E. histolytica and E.
ranarum, because he tries to do so by his eyes alone ; the epithelium
of his intestine can, however, tell the difference between them.
Other animals can make a similar distinction between the morpho-
logically identical Trypanosoma equiperdum living in the con-
nective tissue of horses and transmitted during coitus, and T.
evansi living in blood and carried by a tabanid vector. We may
safely assume that the differences between all these forms,
whether we call them biological races or distinct species, are
fundamentally chemical. Such biological races, though commoner
amongst parasites, are not confined to them, being also found, for
'■^^.tance, amongst phytophagous insects.

13

EARTHWORMS AND OTHER ANNELIDS

LUMBRICUS TERRESTRIS

Almost everywhere in England earthworms are found, but most
are absent from the very acid types of soil which botanists call
mor. They live usually in the upper layers of the soil in burrows,
the sides of which are lined with a slime secreted by unicellular
glands in the skin, and if the opening be not protected by a worm
cast it is usually closed by leaves or small stones. Such leaves
may often be seen sticking up from the ground, and will be
found to have been pulled into the burrow skilfully, with the
narrowest part foremost. At night, if the weather be warm and
not too dry, the worms will stretch themselves out of their holes,
keeping the hinder end of the body fixed in the opening, so that
they can pull themselves back at once if danger threatens. In
dry weather or hard frost they burrow deep and retire to a small
chamber, which they line with little stones. In wet weather they
are sometimes flooded out, but they rarely leave their burrows
in other circumstances, except when they are about to die owing
to the attacks of parasitic maggots which are the young of
certain flies. The food of earthworms consists of the organic
matter in the soil, which they swallow, and of leaves both fresh
and decaying. They will also eat animal matter, and are said
to be very fond of fat. Charles Darwin showed the remarkable
effects which these insignificant creatures have upon the surface
of the earth. By making the soil more porous they expose the
underlying rocks to the disintegrating action of water, by solution
owing to the presence of carbon dioxide and other acids of the
soil, and by frost ; and the small stones which eventually result
from this action are made still smaller by friction and solution
within their bodies. Thus they help in the formation of the soil.
At the same time they are aiding in its removal. Their castings
dry and crumble, and are blown about by the wind or else are
washed down by the rain. On sloping ground this fine material
tends to be carried away downwards, and thus the denudation
of hills is largely due to the action of earthworms. On the other
hand, their work is highly beneficial to the farmer. The soil is
by them thoroughly mixed, submitted to the action of the air,

157

i:^S EARTHWORMS. PHYLUM ANNELIDA

and constantly supplied with a fine ' top dressing ', which may
be as much as 25 tons per acre per annum. Organic matter is
converted into a useful form and amalgamated with the earth,
and the latter is made easier of penetration by the roots of plants.

ProstomLum.^
penstomium

A.

Opening of
Oviduct.

Opening of
vos deferens

Spermatic
groove ■

Mouth

Frost omium
Peristomium

Clite/lum

Lateral chaetae.

'

X

-3Z

^^^

-26

Fig. 109. — A, The first three segments of an earthworm, Ltimbricus terrestris,
ventral ; B, the same, dorsal showing the epilobous condition ; C, the
first three segments of Allolobophora, dorsal (tanylobous) ; D, Lumbricus
terrestris, ventrolateral ; E (after Grove), worms' in coition. In B-E the
numbers refer to the segments.

The only species which form worm casts in this country are
Allolobophora longa and A. nocturna ; the others void their faeces
below the surtace and so are of less importance in agriculture.

EXTERNAL FEATURES

One of the commoner English earthworms is Lumbricus terrestris
(Fig. 109). The body of this animal is roughly cylindrical, but

LUMBRICUS TERRESTRIS

159

pointed in front and flattened behind. It reaches a length of seven
inches. There is no distinct head, but a lobe known as the pro-
stomium overhangs the mouth, which is a crescentic opening
on the lower side of the front end. The body is divided into a
series of rings, the segments, and at the hinder end is the terminal
anus. The first segment is the peristomium and the mouth lies
between it and the prostomium. On the dorsal side, the latter
projects across the peristomium.
There are about 150 segments,
but the number probably increases
slightly throughout life. (In the
related Allolohophora the pros-
tomium reaches only half-way
across the peristomium, and the
common A. chlorotica has about
no segments.) At about one-third
of the length of the body from its
front end, in segments 32-37 in-
clusive, a glandular thickening of
the epidermis lies athwart the back
like a saddle ; it is known as the
clitellum. The colour of the worm
ranges from brown to purplish,
but is somewhat paler below. The
skin is covered with an iridescent
cuticle of collagen secreted by the
underlying cells. In every somite
except the first and the last there
are eight bristles, the chaetas or
setae (Fig. no) in two pairs on each
side, a lateral pair, slightly above

the middle of the side, and a ventral pair between the lateral
and the mid-ventral line. The chaetae can be felt with the
fingers ; they are made of chitin, which is effectively an amino-
cellulose, and are embedded in sacs of the epidermis, by which
they are secreted, and to these sacs are attached muscles, by which
they can be moved. The chaetae, as we shall see later, assist in
locomotion. The ventral chaetae of the clitellum, of the twenty-
sixth, and of the tenth to the fifteenth segments, are straighter
and more slender than those of other segments, which are stout
and somewhat bent. This modification is in connexion with the use

Fig. 1 10. — A diagram of a chaeta of
the earthworm and the struc-
tures connected with it. — From
Potts, after Stephenson.

cm.. Circular muscle of body-wall ; ch.
chaeta; cm., cuticle; ect., ectoderm;
foL, follicle; fm.c, formative cell of
chaeta ; per., peritoneum ; pr.nt., pro-
tractor, and rt.m., retractor muscles of
chaeta.

j5^^ earthworms, phylum ANNELIDA

of the chc-Eta of the twenty-sixth segment during coition, and of the
other straight chietae during the formation of the cocoon in which
the eggs are laid.

EXTERNAL OPENINGS

A number of internal organs open separately upon the surface
of the body. We have already mentioned the mouth and anus.
The openings of the vasa deferentia are a pair of slits with swollen
lips found on the under side of the body in segment 15. In front of
them, in somite 14, are the two small openings of the oviducts.
The spermathecal pores are two pairs of small, round openings
in the grooves between segments 9-10 and lo-ii at the level
of the lateral chaetae, but Allolobophora may have three pairs.
The nephridiopores are openings which lead from the excretory
tubes or nephridia. They are found, as a pair of minute pores
in front of the ventral chaetse, in each somite except the tirst
three and the last. The dorsal pores are small, round openings
on the mid-dorsal line in the grooves between the segments. The
first is behind the eighth segment, and there is one in each subse-
quent groove. They open into the body cavity, the fluid in which
oozes out through them and moistens the surface of the body,
mingling with the slime secreted by the unicellular glands of
the skin. As this fluid contains amoeboid cells which attack
bacteria and other small parasites, it is a valuable defence to
the worm against such enemies, which are numerous in the soil.

BODY-WALL

The body of the worm may be said to consist of two tubes,
one within the other (Fig. iii). The inner tube is the gut, the
outer the body-wall. Between the two lies the coelom or body
cavity, divided into compartments by a series of septa, which
reach from the gut to the body-wall, where they are attached
opposite the grooves on the surface of the body. The compartments
communicate by numerous openings in the septa. The coelom
contains a fluid, and in this float the leucocytes already mentioned,
by which small parasites are surrounded and destroyed, both
within and without the body. The body-wall is covered by a
cuticle. Under this lies the epidermis, an epithelium consisting
of columnar cells, many of which are glandular or sensory, with
small cells between their bases. The cuticle is composed of
hardened protein and is perforated by a pore above each gland

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j52 earthworms, phylum ANNELIDA

cell. Ihe epidermis of the clitellum consists of several layers
of gland cells. Below the epidermis is a circular layer of muscle,
consisting of unstriped fibres running around the body, and
below this again lies a much thicker longitudinal layer of muscle,
composed of similar fibres running along the body and placed
in rows which stand at right angles to the surface, supported
by connective tissue. On the inner side of the longitudinal muscle
is the coelomic epithelium, which is here a layer of pavement cells
lining the body cavity.

NUTRITION

The ahmentary canal is straight. It begins with a short, wide,
thin-walled mouth or buccal cavity in the first three somites,
which leads to a muscular region known as the pharynx. This lies
in front of the septum between the fifth and sixth somites, but
pushes that septum backwards as far as the seventh. When the
worm is swallowing soil the pharynx is everted to a length of a
few millimetres. Its dorsal wall is thickened by the presence of
a number of glands, whose secretion, containing mucin and a
ferment which digests proteins, is poured over vegetable tissues
while the animal is feeding upon them. Numerous strands of
muscle run from it to the body- wall. Behind it lies the oesophagus,
a straight, narrow, thin-walled tube, which extends to the
fourteenth segment. In the eleventh segment it bears at the sides
a pair of oesophageal pouches, and in the twelfth two pairs of
oesophageal or lime glands. These contain large cells which secrete
calcium carbonate and pass it through the pouches into the
oesophagus. In the fifteenth and sixteenth segments the oesophagus
expands into a large, thin-walled crop, which in turn communi-
cates behind with the gizzard, another swelling, with thick
muscular walls and a chitinous lining, in segments 17 and 18. From
the gizzard to the anus runs a wide, thin-walled tube known as
the intestine. The intestine is narrowed w^here it passes through
the septa, and its dorsal wall is infolded to form a longitudinal
ridge known as the typhlosole. The gut is lined with a layer of
columnar epithelium, outside which are thin longitudinal and
circular muscular layers, covered by the coelomic epithelium,
which here consists of the chloragogenous cells. These cells,
which also fill the typhlosole, are large and contain yellow
granules of a substance which is possibly a phospholipid. They
'^f into the coelomic fluid, and there break up and set free

LUMBRICUS TERRESTRIS

163

their granules, which are taken up by amcebocytes. It is said that
these may go to the exterior, but as the granules contain only
four per cent, of nitrogen they cannot be an important excretory

Wkm^-:.

^-ei

gi-.c.

Fig. 112. — Histology of the earthworm.

A , The end of the first hank of the nephridium ; A' , part of a section of the same ; B, part of a transvers
section of the body.

br.t., Brown, ciliated tube ; b.v., blood vessel ; c.c, chloragogenous cells ; c.m.b., circular muscle of body
wall ; c.m.g., circular muscle of gut ; cu., cuticle; ep., epidermis ; end., endoderm ; g.f., giant fibres;
gl.c, gland cell in the epidermis ; l.m.b.^ longitudinal muscle of body-wall ; l.m.g., longitudinal muscle of
gut ; l.n.v., lateral neural vessel ; n., nerves ; n.c, nerve cord ; n.c.t., glandular, non-ciliated tube ;
n.f., nerve fibres; n.t., narrow tube, ciliated in parts; p.e.b., peritoneal epithelium of body-wall;
s.i.v., subintestinal blood-vessel ; s.n.v., subneural blood vessel ; ves.tiss., connective tissue with
vesicular cells and blood vessels.

product. There are also amceboid yellow cells which take up ex-
creta in the blood, pass into the gut, and are voided with the faeces.
Food swallowed in the course of burrowing is passed along the
oesophagus, stored in the crop, ground up in the gizzard with
the aid of small stones which have been swallowed, and in the
intestine first digested by juices secreted from the epithelium,
and then absorbed, for which processes the surface is increased

l54 EARTHWORMS. PHYLUM ANNELIDA

by the presence of the typhlosole. The contractions which cause
the passage of the food are alternately caused through the nerves
to the pharynx and inhibited through the plexuses in the septa.
From the anus faeces and undigested soil are passed out as the
familiar worm casts (in Allolobophora) or in the burrows below
the surface. The function of the oesophageal glands is probably
the excreting of the calcareous matter which is very plentiful
in the dead leaves of which the food is largely composed. Possibly
their secretion is also of importance in removing carbon dioxide
in the form of calcium carbonate. They are characteristic only
of those species of worm which are large and Hve in a relatively
dry environment.

EXCRETION

Besides the yellow cells of the intestine, the earthworm
has excretory organs which, like those of vertebrates, consist
of tubes with walls that are glandular and excretory and richly
supplied with blood vessels ; but the tubes, instead of being
collected into compact kidneys, are distributed along the body,
one pair to each segment, except the first three and the last which
have none. Each tube or nephridium is thrown into loops, bound
together by connective tissue containing blood vessels. The
nephridium (Figs. 113, 114) begins as a flattened, kidney-shaped
funnel or nephridiostome hanging from the front side of a septum
near the nerve cord. The nephridiostome has an overhanging
lip which consists of a large crescentic central cell with a row
of marginal cells around it. This lip is ciliated. The lower lip is
not ciliated. From the funnel theie leads a narrow tube, ciliated
on its sides. This passes through the septum to the main part of
the nephridium, which lies behind the septum, in the coelom
of the next somite, opening to the exterior by the nephridiopore
in that somite. The narrow part of the tube is long and winding
and loses its ciha in places. It is followed by a wider, short,
brown region, cihated throughout, this by a still wider tube
which is not ciliated, and finally a short, very wide, muscular
tube leads to the nephridiopore. The whole tube, except the
muscular region, is formed of hollow cells shaped like drain-pipes
and lying end to end. The middle part of the nephridium stores
excretory granules probably throughout hfe, and a fluid is also
driven to the exterior, being liberated by the opening of the
nephridiopore once every three days. The chief excretory con-

i65

net.

,,sep

hr.U 'n. C.U

Fig. 113. — A diagram of a nephridium of the earthworm.

bt.U, Brow, ciliated tube; m.t, muscular tube; n.ci., glandular, non-ciliated tube ; nt.. narrow tube
ciliated in parts; «s^., nephrostome ; se^., septum ; ves.hss., connective tissue with vesicular ceUs
and blood vessels ; i, 2, 3, the three hanks of the tube.

. 1H.4S,

deb^

mc-

pe.—m

6

Fig 114— The nephridiostome or funnel of a nephridmm of the earthworm.
A, seen from in front as a transparent object; B, in side view, opaque, semi-
diagrammatic, and without its ciha.

r,*. r Central ceU • deb debris of ccelomic corpuscles and excretory granules vyhich is probably not able
to enter thefuAnel"'/ /lower Up of opening ; m.c, marginal cells ; p.e., superhcial layer of the peritoneal
SitS£um;T^'.! thickened deeper layer of the same ; x., point at which the marginal cells ]om the
lining of the tube, which turns over round the opemng.

1 66

EARTHWORMS. PHYLUM ANNELIDA

stituents of this urine are urea and ammonia. These are probably
derived from both blood and coelomic fluid, and there appears to
be tilt rat ion, reabsorption and secretion, much as in the vertebrate
kidney (p. 379)-

BLOOD VESSELS

Earthworms have no special respiratory organs, but an inter-
change of gases between the air and the blood takes place in the
skin, which is richly supplied with vessels.

hi.

int.

oes.

b.w. - - jmiiimimmiiiiimiDiiiiiii,

Fig. 115.

e/^i>.it:v pMr.v. a//n.v.

-A diagram of the blood- vascular system of the earthworm.

aff.i.v., Afiferent vessels of the intestine ; aff.n.v., afferent vessels of the nephridia: b.w., body-wall;
d.b.v., dorsal blood vessel; .s.v., dorso-suDneural vessel ; ejj.b.w.v., efierent vessel from body-wall ;
eff.t.v., efierent vessel from intestinal wall ; hi., pseudo-hearts ; int., intestine ; oes., oesophagus ;
par.v., parietal vessel ; s.i.v., subintestinad vessel ; s.n.v., subneural vessel ; v.n.c, ventral nerve cord.

A simpler form of this diagram will be found below.

The blood of an earthworm is red owing to the presence in it
of a substance generally called haemoglobin, although it is different
in composition from the pigment of the same name in vertebrates.
This haemoglobin is in solution, not in corpuscles. Colourless

cLb.y. ht.

\SUp.pil.^
ph.

dJ.v.

S.V»7

xrui

K7UC~

i-^

Fig. 116. — A diagram of the principal blood vessels of the earthworm.

d.b.v., dorsal blood vessel; d.s.v., dorso-subneural vessel; ht., one of the 'hearts'; int., intestine;
m., mouth ; as., oesophagus ; ph., pharynx ; sup.ph.g., suprapharyngeal ganglion ; s.i.v., subintestinal
vessel ; s.n.v., subneural vessel ; v.n.c, ventral nerve cord.

corpuscles are also present. The blood- vascular system is very
complicated. Its main outlines are as follows (Figs. 115-117). A
large dorsal vessel runs the whole length of the body from the
hinder end to the pharynx. It is contractile and its walls contain
muscle fibres, and in it the blood is driven forwards. It receives
blood by many small vessels from the intestine and by two larger

LUMBRICUS TERRESTRIS

167

vessels in the tenth segment from the oesophagus, and ends in
front by breaking up into branches which supply the pharynx.
In each of the segments 7-1 1 it gives off a pair of large contractile
vessels or pseudo-hearts. These encircle the oesophagus and join a
ventral or subintestinal vessel which hangs by a mesentery
below the gut. In the pseudo-hearts the blood flows downwards
from the dorsal to the ventral vessel, and in the latter it flows
backwards and forwards from the region of the hearts. From the
ventral vessel the blood passes by a series of small vessels to the
intestine, and by parietal vessels to the nephridia and to the body-
wall. From these organs it is returned along various paths to
the dorsal vessel. Among
the subsidiary vessels are
a subneural and two lateral
neural vessels, in which the
blood flows backwards, and
dorso-subneural vessels, a
pair in each segment of the
intestinal region of the
body, which carry blood
to the dorsal vessel from
the subneural vessel, the
nephridia, and the body-
wall. The main blood vessels
of the earthworm cannot
be distinguished into art-
eries and veins, but their
ends are joined by capil-
laries. The dorsal vessel
and the pseudo-hearts are

provided with valves which keep the blood flowing in the proper
direction.

NERVOUS SYSTEM

The earthworm has a well-developed central nervous system
(Fig. 118), which consists of (i) a pair of suprapharyngeal ganglia,
rounded bodies lying above the mouth, and sometimes known
together as the brain, (2) two slender circumpharyngeal com-
missures running from these round the pharynx, and (3) a ventral
nerve cord which starts from the commissures between the
third and fourth somites and runs the whole length of the body

s.n^.v s.i.v.

Fig. 117. — A diagram of a transverse section
of the earthworm in the intestinal region
to show the arrangement of the blood

vessels.

ejf.n.v., efferent vessel from nephridium ; l.n.v., lateral
neural vessel ; nph., nephridium ; other lettering as
in Fig. 115.

1 68

EARTHWORMS. PHYLUM ANNELIDA

sup ph.g

in the ccElom below the gut, swelHng into a ganglion in each
somite The first of these ganglia is bilobed and is known as the
subpharyngeal ganglion. Nerves are given off to the prostomium
from the suprapharyngeal gangha, and to the first two somites
from the commissures, and the ventral cord gives off in each
somite three pairs of nerves which run upwards as girdles in the
body-wall, giving off branches as they go. The ahmentary canal
receives nerves from the circumpharyngeal commissures and
fibres from plexuses in the septa. Though the ventral cord
appears to be single, it is really double, and can be seen in trans-
verse sections to be rather imperfectly
divided into right and left halves by
connective tissue. Transverse sections
also show that the middle and upper
parts of the cord consist of fine,
chiefly longitudinal, nerve fibres, and
the lower and outer parts contain
nerve cells. Above the mass of fine
fibres are three longitudinal bundles
of such fibres, each bundle being
enclosed in a sheath and known as a
giant fibre. Nerve cells are more
numerous in, but not confined to,
the ganglia. The nerves consist of
afferent fibres, which start from sense
cells in the epidermis and muscles

c.<»/i.c., Circumpharvngeal commissure ; n., ,-^. ^ -, • i x xt_

nerves; ph., pharynx cut through; (Fig. Iig) aud Carry impulSCS tO thC
Sep., septa ; subph.g., subpharyngeal . , , • i_ • t_ a.i^

ganglia ; sup.ph.g., suprapharyngeal Central uervous system, lu which tuey
f^^'^mi't£;^^"^''^°^''^"^^^''°'' ' end as bunches of efferent fibres,

which start from nerve cells in the
ganglia and end against muscle and other cells, to which they
convey impulses and also of fibres which join nerve nets in the
skin, muscles, and septa.

MOVEMENT

On the surface, worms move by means of a peristaltic con-
traction of the muscles of the body-wall. Two series of waves of
contraction, one of the longitudinal and one of the circular muscles,
pass along the body from the anterior end ; the waves of the
two series are out of phase with each other, contraction in one
set of muscles in a particular segment being accompanied by

Fig. 1 1 8. — A diagram of the
forepart of the nervous sys-
tem of the earthworm.

e.n.a.

e.fi^

"^

<P

LUMBRICUS TERRESTRIS 169

relaxation of the other. If a worm which is lying quiescent, in a
moderately contracted state, starts moving, the following things
happen in order. The circular muscles of the first segment con-
tract, making it longer and nar-
rower ; then the second segment
behaves similarly, and the third
and fourth, and so on, so that the
anterior part of the body is
elongated. When this contraction
wave has reached about a third
of the way along the body, the
longitudinal muscles of the first
segment begin to contract ; they
are followed by those of the
following segments, and the second ^^
type of waves has begun. As
each wave dies out it is succeeded
by another. These waves by
themselves would not lead to
locomotion at all, but merely to
successive extensions and with-
drawals of the two ends of the
body, the mid-point or centre of
gravity remaining stationary ;
something like this may some-
times be seen when a worm tries
to move on a wet glass plate. To
translate the waves into pro-
gression there must be some
greater resistance to movement
backwards than there is to move-
ment forwards. This differential
resistance is achieved in three
ways. It can often be seen that
the part of the body which

is moving forward is raised off the ground ; this eliminates
surface friction. In ordinary slow crawling the head is lifted and
thrust forward ; the contraction wave dies out about half-way
along the body, and when the anterior end shortens the tail part
is raised slightly and dragged forwards. Secondly, the chaetae play
some, though probably a minor, part in movement. When the

c.n.i

Fig. 119. — A diagram showing the
mode of ending of the sensory
nerve fibres in the epidermis of the
earthworm and the relation of this
type to that which is found in
some other animals.

See also Fig. 418.

A, The arrangement found in the earthworm :
B, that of the worm Nereis ; C, that of a
fish ; D, that of a frog or man.

c.n.s., Ending of the neuron in the central nervous
system ; ep. , ending in the epidermis.

jyQ EARTHWORMS. PHYLUM ANNELIDA

longitudinal muscles contract the chsetae are generally simultane-
ously extended. As those of the first dozen or so segments point
backwards they will resist backward movement more than
forward ; those of the middle part of the body stand out radially,
or slightly irregularly, so that they will resist forward and
backward movement equally. Those of the posterior segments
point strongly forwards, and if erected would tend to make the
worm move backwards ; as has been said, the contraction waves
do not normally reach this part of the body, and so the chsetae are
not extended. Finally, and probably most important, the anterior
end of the body acts as a sucker. The prostomium and peris-
tomium are pressed against the surface, and the prestomium
is then withdrawn ; when this happens the anterior end is
anchored, and any shortening of the body must pull the animal's
centre of gravity forward. By this means a worm can crawi up a
vertical plate of glass or metal.

Burrowing takes place by two methods. If, when the anterior
end of the body is elongated, it comes into any small crevice,
instead of contracting, as it normally would do on contact, it
continues to press in. Then when the wave of longitudinal con-
traction starts, the anterior segments expand radially and press
against the sides of the crevice so that they cannot be withdrawn.
By this means, which is akin to the technique of chimney-
climbing known as backing-up, a worm can move through a
glass tube, and it is probable that much of its burrowing is done
in this way. In loose soil a more or less cylindrical burrow will
be formed by the pressure of the segments against the side of
the crack into which the worm puts its snout. Sometimes on
contact with soil the worm extends its pharynx and sucks in
a mouthful ; repeated sucking creates a hollow in which the
backing-up process can be used. A little soil is sometimes swallowed
without protrusion of the pharynx. It is probable that burrowing
in firm soil is impossible unless the worm eats its way through.

When a worm is strongly stimulated it may move backwards
by a reversal of the normal waves. It is probable that this
reaction seldom occurs on the surface in nature, but it is important
in preventing the worm from being withdrawn from its burrow.
The longitudinal muscles of the posterior segments are strongly
contracted, with extrusion of their chaetae. As has been said, these
project forwards, and if this part of the worm is below ground
thev will be driven into the soil. If the anterior end is seized

LUMBRICUS TERRESTRIS

171

and pulled, as it often is under these circumstances by a thrush,
either the chaetae, or the soil, or the worm, must break, and
usually it is the worm.

The co-ordination of the contraction waves is carried out in
part mechanically because the stretching of the muscles causes
them to contract, and in part by the nervous system. The con-
traction of circular or longitudinal muscles in a somite
causes, through an impulse sent along the ventral cord, the
contraction of the corresponding muscles in the somite behind.
Further, in each somite the contraction of the circular muscu-
lature sends, by afferent and efferent fibres, through the

^p.v.s:

I
int.

1Z^11\ 20 19 18; 17 1615^1413 12/11 /lO 9 ,' 8 7

giz.

cr.

p.v.s.

a. V. s.

oes.

ph.

•\

3 \ 2

v.n.c'.

Fig. 120.— a diagram of a longitudinal section of an earthworm,

a.v.s.. Anterior vesicula seminalis; a.v.s'., posterior lateral horn of the same overhanging the oesophagus;
cr., crop; giz.. gizzard : int., intestine; m., mouth ; oes., oesophagus ; p.v.s., posterior vesicula seminalis;
p.v.s'.. horn of the same overhanging the oesophagus ; per., peristomium ; ph., pharynx ; pr., prosto-
mium ; sup.ph.g., suprapharyngeal ganglion ; ty., typhlosole ; v.n.c, ventral nerve cord ; 1-23,
segments. The blood vessels are omitted.

ganglion, impulses which relax the longitudinal musculature ;
and similarly the longitudinal muscles in contracting relax the
circular. Rapid contraction is co-ordinated by nervous impulses
which travel in the giant fibres, forwards in the laterals, and from
head to tail in the median fibre.

SENSE ORGANS

An earthworm has no well-developed organs of sense, but
certain of the columnar cells of the epidermis are rod-shaped
and prolonged at their inner ends into fibres, which run in the
nervous system. These are sense cells, and in the forepart of the
body some of them are collected into groups, which are rudimentary
sense organs. There are also sense cells which contain a refractive

EARTHWORMS. PHYLUM ANNELIDA

body and are probably affected by light. Experiment shows that
the worn>s are sensitive to light and to vibrations of the ground,
and can smeU, but gives no evidence of a sense of hearing.

REPRODUCTION

Earthworms are hermaphrodite, every individual having a com-
plete set of organs of each sex (Figs. 121, 131). The female organs

C.V.S'.r-.-

P

.v.s:~z

Fig. 121. — An earthworm (L. terrestris), dissected from above.

avs' Horns of the anterior vesicula seminalis ; cr., crop; d.b.v. dorsal blood vessel; giz., gizzard;
itt. heart-. ; irtL, intestine ; m., mouth ; nph., nephridia ; oss., oesophagus ; as.g, oesophageal glands ;
as.p., « >1 pouch ; p.v.s'., horns of the posterior vesicula seminalis ; ph., pharynx ; sep., septa ;

V* =.-'.,-■ ■; .?H^./'/'.e., suprapharyngeal ganglia.

LUMBRICUS TERRESTRIS

^73

include the ovaries, oviducts, and spermathecse. The ovaries
are two small, pear-shaped bodies hanging into the coelom of
the thirteenth segment from the septum in front of it. Each ovary
is a local thickening of the coelomic epithelium, and is just visible
to the naked eye, as a whitish spot (Fig. 122). The broad end of
the pear is attached to the septum and contains a fused mass
of unripe ova. Ova fall from the stalk into the coelom and are
taken up by the oviducts, which lead by wide funnels from the
coelom in the thirteenth somite, pass through
the septum behind, and open to the exterior
in the fourteenth. In the latter somite, each
bears a swelling, the receptaculum ovorum
or egg sac, in which the eggs are stored and
maturation divisions take place. The sperma-
thecse are two pairs of small, round sacs
which lie in the ninth and tenth somites
and open in the grooves behind them. Their
function is to receive sperm from another
worm. The male organs consist of testes,
vesicul3eseminales(seminal vesicles), and vasa
deferentia. These testes are two pairs of
small, flat, finger-lobed bodies attached to the
hinder side of the septa in front of segments 10
and II. Like the ovaries, to which they cor-
respond in position, they are local thicken-
ings of the coelomic epithelium. The testes
bud off cells known as sperm-mother-cells,
which give rise to spermatozoa in the
seminal vesicles. The latter are large sacs,
formed by the walling-off of parts of the coelom, which enclose
the testes. Each consists of a median part and lateral horns.
The anterior seminal vesicle, in segment 10, has four lateral
horns, two in front and two behind, which push out the septa
and bulge into the ninth and eleventh segments. The posterior
seminal vesicle in segment 11, has only two such horns, which
project into the twelfth segment. Each sperm-mother-cell forms
by multiple fission, in the course of which the usual reduction
division takes place, a mulberry-hke mass (Fig. 123), consisting
of Httle cells attached to a central mass of residual protoplasm
known as the cytophore, by which they are nourished. The little
cells become pear-shaped, with the broad ends on the cytophore,

Fig. 122. — One of the
ovaries of an earth-
worm.

174

EARTHWORMS. PHYLUM ANNELIDA

gradually increase in length, and change their shape till the mul-
berry has become a tuft of threads, each thread being a sperma-
tozoon with a very slender head. Finally the spermatozoa break
loose. In the median part of each seminal vesicle, directly behind

Fig. 123. — The development of the spermatozoa of the earthworm,

^.Stages from the vesicula seminalis of a young worm ; B, from that of an older worm.
1, bperm rnothor-r.f>ll ; 2-7. stages in the division to form spermatozoa ; 7-11, shaping of the spermatozoa,
^' t to the mass of residual protoplasm (cytophore) ; 12. a ripe spermatozoon,

_^ "'' . 12 is represented rather too broad.

The «iaik uudics cuc Itic nuclei, stained.

LUMBRICUS TERRESTRIS I75

the testes, is a pair of large ciliated funnels with folded walls,
known as sperm rosettes. These funnels lead into the vasa effer-
entia, of which the two on each side join and pass back as a vas
deferens to open on somite 15. The cilia of the rosettes draw the
ripe sperm into the ducts.

Copulation (Fig. 109) takes place at any time from spring to
autumn in warm, damp weather, with a maximum frequency in
the hot weather of summer. Two worms stretch themselves out
of their burrows and place their ventral sides together with the
heads pointing in opposite directions, their bodies being held
together by a substance secreted from the clitella, and by the
chaetae, which stab into the body-wall of the partner. They remain
like this for two or three hours, and sperms are passed from the
vas deferens of each worm, along a temporary seminal groove,
into the spermathecae of the other. Some time after the worms
have separated the eggs are laid in a cocoon, which, secreted by
the clitellum as a broad band round the body, is passed forwards
over the head. The cocoon contains a nutrient fluid. While it is
still on the clitellum eggs are passed back to it along a temporary
groove from the oviducal opening, and as it passes the sperm a-
thecal openings, semen received from another worm is squeezed
into it and fertilisation takes place immediately. In passing over
the head the ends of the elastic cocoon close, and it becomes a
small, lemon-shaped body, which is left in the earth. Each cocoon
contains eight to sixteen ova, which are fertilised in it, but usually
only one completes development, a process which takes several
weeks. The development of the earthworm is referred to on p. 670.

REGENERATION

Many earthworms have an extensive power of regeneration,
which depends on a hormone secreted by the central nervous
system. If the body be cut in half, the head end will grow a new
tail, and the tail end, though more slowly, a new head. It appears
that, of the two common species described above, Allolohophora
chlorotica does regenerate, but Litmbriciis terrestris does not.

NEREIS CULTRIFERA

The earthworm has a burrowing habit and a vegetarian diet.
Many marine worms, however, while they resemble the earthworm

j„^, MARINE WORMS. PHYLUM ANNELIDA

in most respects, lead a free and predacious existence. Of
these the genus Nereis, of which several species are found on our
coasts, is a good example ; Nereis cultrifera (Fig. 124) is common
under stones on the south coast of England, where it is known
as the red cat and is used as bait. The body of this worm is
about six inches in length, of a greenish colour, with red on the
Hmbs and where the dorsal blood vessel shows through, roughly
cylindrical, tapering towards the hinder end, and divided into

Fig. 124. — Nereis cultrifera. — From Thomson.
a., Anus ; c, tentacular cirri ; e., eyes ; p., palp ; pe., peristomium ; t., tentacles.

about eighty segments. Like the earthworm, it is covered with a
thin cuticle, but instead of having a small number of short chaetse
protruding directly from the body-wall, it has many longer
ones borne on movable lobes called parapodia, of v/hich a pair
is found on each somite. A parapodium (Fig. 125) is a fiat, hollow
vertical process of the body-wall, standing out at the side of
its segment. It is cleft into two principal lobes, a dorsal noto-
podium and a ventral neuropodium. Each of these is again
divided into smaller lobes and bears at its base a slender process
known as a cirrus. A stout, deeply embedded chseta or aciculum,
which does not project to the exterior, supports the notopodium
and another the neuropodium, and each of these bears a tuft
of other chaetae. In the sexually mature stage of the worm known
as Heteronereis these are oar-shaped and there are additional
complications in the parapodia. The front end of the body is
modified to form a definite head (Fig. 126). This consists of the

NEREIS CULTRIFERA

177

prostomium and the peristomium. On the prostomium are situated
dorsally a pair of prostomial tentacles and two pairs of eyes,

Fig. 125. — A transverse section through Nereis cultrifera, shghtly simphfied. The
parapodia are shown in perspective. Magnified.— After Shipley and MacBnde,
with modifications.

I, Cuticle; 2, epidermis; 3, circular muscles; 4, longitudinal muscles; 5, obHque muscles forming a
partition ; 6. somatic layer of oeritoneal epithelium ; 7, ccelom ; 8, splanchnic layer of epithehum ;
9 cavitv of intestine; io, dorsal blood vessel; 11, ventral blood vessel; 12, ventral nerve cord;
13, nephridium in section ; 14, ova ; 15, notopodium ; 16, neuropodmm ; 17, dorsal cunrus; 18 ventral
cirrus ; 19, cha3t« ; 20, aciculum ; 21, muscles which protrude the acicula and thus the noto- and
neuropodium ; 22, ciliated organ (vestige of coelomoduct).

Fig. 126 —The head of Nereis, with the pharynx protruded.

Eves • i jaw • p., palp ; pe., peristomium (first two segments fused) ; ph., pharynx ; pp., first ordinary
parapodium ; pr., prostomium ; t., accessory teeth ; tc, tentacular cirri ; te., tentacle.

each of which is a pit Uned by pigmented cells and enclosing a
gelatinous mass which serves as a lens. Ventrally the prostomium
bears a pair of stout palps. The peristomium carries on each side

178

~-ap.c.

stm.-

^pr.

-^ines.

B

\Vi*l,

ap.

.mu.sc.

MARINE WORMS. PHYLUM ANNELIDA

two pairs of long, slender tentacular cirri, and probably corre-
ponds to two fused somites. Behind the last segment is a conica
region without parapodia which bears a pair of slender anal
^ cirri and the terminal anus.

The musculature of Nereis
is more complicated than that
of the earthworm, the longi-
tudinal muscle fibres being
grouped into four powerful
longitudinal bundles, two dorsal
and two ventral, while there
are obhque muscles to move
the parapodia. As might be
expected from the better pro-
vision of sense organs on the
head, the brain also is more
complex. The alimentary canal
is simpler than that of the
earthworm ; the pharynx can
be caused to protrude by being
turned inside out, and is lined
with cuticle, thickened in
places to form numerous small
teeth and a pair of strong jaws
with which the prey is seized.
The sexes are separate. The
reproductive organs are very
simple, consisting of temporary
masses of cells, which arise from
the coelomic epithelium. The
sexually mature heteronereid
forms differ from the less active
asexual stage not only in the
chctta,' but in having larger eyes and in certain other respects. They
swarm near the surface of the sea, and the ova and sperm probably
escape through ruptures of the body-wall ; fertilisation takes place
in the water. The free young are at first very unlike the parents,
being minute, globular creatures, known as trochospheresor trocho-
phores (Fig. 127, A), which swim by means of a girdle of cilia in
front of the mouth and have an apical tuft at the upper pole. They
'— rVrgo a gradual change into the adult, becoming oval and

seg.

TTies.

aTV.Cr

CLTt.

Fig. 127. — A, The trochosphere of
Nereis. — Modified, after Wilson.

B.

an.

.\ typical trochosphere in an early
stage of the transformation into
the adult.

, Anus ; an.c, anal tuft of cilia ; ap.c, apical
tuft of cilia ; eye ; m., opening of mouth ;
nus., mesoderm ; muse, larval muscles ;
nph., larval nephridium ; pr., preoral ring of
cilia ; pt., postoral ring ; seg.mes., segments
beginning to form in the mesoderm ; sttn.,
stomodspum (the pouch of ectoderm which
forms the mouth and gullet).

NEREIS CULTRIFERA 179

then lengthening and segmenting. Their mesoderm is formed
as two ventro-lateral bands, each thrown off by the continual
division of a pole cell at the hinder end, and spreads round the
gut, between ectoderm and endoderm, the coelom appearing in
it. The larva of Nereis is not in all respects a typical trochosphere.
In Fig. 127, B a more typical example is shown, in a later stage
of development than that represented in Fig. 127, A. It has
between ectoderm and endoderm a large space (blastocoele)
which is not present in the trochosphere of Nereis.

HIRUDO MEDICINALIS

THE MEDICINAL LEECH

The leeches are another group of segmented worms related to
the earthworms. Hirudo medicinalis, the medicinal leech, a
dweller in freshwater pools, marshes, and sluggish streams, was
becoming rare, as readers of Wordsworth will remember, a
century and a half ago, but it is still found in the Lake District,
in the New Forest, and in some other places in Great Britain. It
is commoner on the Continent, where, when it was more used in
medicine than at present, it was bred in large numbers in special
ponds. It lives normally by sucking the blood of frogs and fishes,
but when it is full grown it also takes that of warm-blooded
animals, and it will feed on man, though to induce it to do so his
skin may have to be moistened with blood or milk or pierced by
a small cut. An active specimen will draw three or four cubic
centimetres of blood. The body of the leech varies in length,
according to the state of contraction ; at its maximum it is about
six inches and somew^hat flattened, and it is provided at each
end with a downward-facing sucker. It is encircled by 95 minute
rings or annuh (Fig. 128), and brightly marked in various shades
of green, yellow, and black, paler below than above. The annuh
do not indicate the true segmentation. In the greater part of the
body five of these lesser rings go to a segment, but towards the
ends there are fewer, and in the head or region of the anterior
sucker (prostomium and first five segments) there are eleven
annuh, while the posterior sucker represents seven segments fused
without annulation. Unlike that of the earthworm or Nereis, the
number of segments is a definite one, amounting in all to 32,
including those of the head and hinder sucker. The mouth lies

i8o

LEECHES. PHYLUM ANNELIDA

^.S.

Fig 128.— The medicinal leech [Hirudo medicinalis) , and details of its anatomy.

A Vrntral view • B dorsal view of hinder end ; C, dorsal view of head and succeeding region ; D, female
' i.".,t.,l orL'.it'is ; E, anterior view of nephridium ; F, nervous and genital organs (male organs of right

as .,, i^'-- "^'' ■' albumen gland ; an., anus ; bl., bladder ; e.g.. cerebral ganglion ; e., eyes;

■ " ep ad ganglion of ventral cord ; j., jaws ; m., mouth ; np., nephndiopore, by

,i,, .„.. iii ; od.. oviduct ; ov., ovary ; p., penis; pr., prostate ; p.s., posterior sucker;

. ; s.ph.g., subpharyngeal ganglion ; /., testes ; t.l., lobe of nephridium which ends in

lUi , L.U.. vas deferens ; v.n.c, ventral nerve cord ; vag.. vagina ; <J male opening ; ? female

; 1-26, somites.

HIRUDO MEDICINALIS

l8l

in the midst of the anterior sucker, and the anus is a minute
opening above the base of the hinder sucker. The male and female
genital openings are median on the second annuh of the loth and
nth segments respectively. On the last annulus of each segment
from the sixth to the twenty-second, are the openings of a pair
of nephridia. On the first annulus of each segment is a transverse
row of minute sense-papillae. On the head a pair of these in each
segment are transformed into minute eyes, recognisable by their

Fig. 129. — A transverse section of the medicinal leech. — After Bourne.

b.t., Botryoidal tissue ; cm., circular muscles ; cap., capillary ; cr., crop ; cr'., a caecum of the same ; cu ;
cuticle ; d.m., dorsoventral muscle ; d.s., dorsal sinus ; ep., epidermis ; g.c, gland cells ; l.m., longitudinal
muscles ; l.v., lateral vessel ; n.c, nerve cord ; t., testis ; t.s., testis sinus, with end of nephridium.,
v.d., vas deferens ; v.s., ventral sinus.

pigment as dark spots. There are no chaetae. The worm can walk
by looping Hke a ' looper ' caterpillar, and swim by undulation

of its body.

The body is covered by a thin cuticle (Fig. 129), which is shed
from time to time. Under this Hes an epidermis, between the bases
of whose cells run blood capillaries, so that the skin is a respiratory
organ, in which the blood is exposed to the surrounding water.
Below are circular and longitudinal muscle-layers, and within
these a layer of botryoidal tissue, composed of branched canals,
whose walls are laden with black pigment, while their cavity is
full of blood. This tissue takes the place of a perivisceral cavity
and embeds the gut. The mouth is provided with three jaws,
which are compressed cushions of muscle covered with a finely

l82

LEECHES. PHYLUM ANNELIDA

-ph

crt

sis

UU.\}

■cr.u.

toothed layer of cuticle : by these the skin of the prey is pierced
with a cliaracteristic tri-radiate wound. A muscular pharynx
(Fig. 130) succeeds the mouth ; it sucks up the juices of the prey,
and into it open numerous unicellular glands whose secretion
prevents blood from clotting so that the leech's food remains
fluid while it is being taken and passed backward to be digested.
It is owing to this secretion that bleeding continues for some

time after the leech is removed from its
wound, and an extract of the heads of these
worms is sometimes used in physiological
experiments to prevent clotting. After the
pharynx comes the very large crop (segments
8-18) with eleven pairs of lateral caeca, of
which the last is much larger than the rest
and extends backwards on each side of the
remainder of the alimentary canal. This
consists of the stomach — a narrow tube, with
an enlargement at its start and a spiral fold
of its inner wall — the intestine, narrower
than the stomach, and the rectum, somewhat
wider. The blood sucked for food is stored
in the crop, whose caeca are more or less
dilated according to the amount of their con-
tents, and passed drop by drop into the
Fig. 130.— a diagram- stomach, where it at once turns green and then

matic view of the ali- • j- -. j a x n 1 -n i j. xi, • i

mentary canal of the IS digested. A lull meal Will last the animal
medicinal leech, as for several months or a year. Seventeen

pairs of nephridia lie in segments 6-22. Each

is a mass of glandular tissue traversed by a

system of intracellular ductules. There is

no internal opening, but those which lie

in the testis somites have a swollen end

lying in the capsule of the testis, and bearing a number of ciliated

funnels which do not communicate v/ith the ductules. There are

two systems of tubes which contain a fluid like blood — a red

plasma with a few colourless corpuscles. One of these systems is

the true blood-vascular system : its principal vessels are two

lateral trunks, which unite before and behind, and are connected

also by a network of capillaries. There are no hearts, but the

lateral vessels are contractile. The other vascular system consists

of n dorsal and a ventral longitudinal sinus, which are also

rm.-

seen when it is not
gorged with blood.

cr.i., cr.ii., Caeca of the
crop; int., intestine;
ph., pharynx ; rm., rec-
tum ; St., stomach.

HIRUDO MEDICINALIS 183

connected before and behind and by means of a capillary system.
The walls of this system are thinner than those of the true blood
vessels. They represent the coelom, as may be seen from the fact
that the ventral sinus encloses the nerve cord and communicates
with capsules around the ovaries and testes. The botryoidal
tubes are also in communication with the sinus system. It is said
that the capillaries of the true blood system are connected with
it. The nervous system is of the same type as those of the earth-
worm and Nereis. A small brain, above and before the pharynx,
is connected by a pair of very short circumpharyngeal commissures
with a ventral cord which carries at wide intervals twenty-three
ganglia. Of these the first or subpharyngeal and the last each
represent several fused. Nerves are given off from the brain
and the ganglia. The commissures which unite the ganglia of the
ventral cord are seen in section to be double, with a slender
median strand.

The animal is hermaphrodite. There are nine pairs of testes,
enclosed in spherical capsules in the I2th-20th somites, with on
each side a common vas deferens, which is coiled as an ' epi-
didymis ' in the loth somite, where the vasa deferentia join to
open on a muscular, protrusible penis, surrounded at its base by
a ' prostate ' gland. The single pair of ovaries lies in the nth
somite, in which its short oviducts open by a common vagina.
The eggs are laid in cocoons secreted by ' clitellar ' glands in the
skin of the ioth-i2th segments and placed in holes made in the
banks, above water. The young resemble the parents and feed
at first on the juices of water insects and the like.

ANNELIDA

The phylum AnneUda, to which the creatures described in
this chapter belong, is very difhcult to define, since its most
distinctive features are not possessed by all its members. Annelids
are segmented animals with one preoral segment, the prostomium,
but all their other universal characteristics they share with other
groups ; they are triploblastic and coelomate ; the nervous
system consists of a double ventral cord, usually with a pair of
gangha in each segment, and of a pair of commissures surrounding
the gut and leading to a pair of preoral gangha ; they have
nephridia and ccelomoducts ; the larva if present is a trochosphere.
Negative characters of importance are the absence of any

jg, PHYLUM ANNELIDA

well-formed appendages except in connection with the mouth (the
parapodia do not qualify as such) and the absence of a continuous
thick chitinous exoskeleton. Typical members of the phylum
possess chcTtiE. Some of the above characters are discussed more
fully in the next chapter.

CLASS I—CHMTOPODA

Ch«Tt£e are present and the coelom is well developed. There are
two Orders.

ORDER I — POLYCHyETA

The chaetae are many and parapodia are characteristic ; the
head is comparatively well developed, with cirri, tentacles, and
jaw apparatus ; gills are frequent ; the sexes are usually separate,
fertilisation is external, and the larva is a trochosphere.

The polychaetes are marine, and are divided ecologically into
the errant forms, which are typically free swimming, though they
sometimes live in temporary burrows, and the sedentary forms,
which are subdivided into those which are tubicolous, that is,
live in formed tubes which they seldom or never leave, and those
which burrow in soft sand. The body, especially the head region,
is generally modified to suit the mode of life. Nereis is a typical
errant genus, Serpida lives in little tubes of calcium carbonate
attached to rocks, and Arenicola, the lug worm, is a burrowing
form living between tide-marks.

ORDER II — OLIGOCH^TA

The characters are in general the reverse of those of the poly-
chsetes. A clitellum is present. They are terrestrial or freshwater
in habitat, Lumhricus being an example of the first, and Nais,
common in ponds, of the second.

CLASS II—HIRUDINEA

These are the leeches. They have 32 segments and two ventral
suckers. Chatae are absent except in some primitive forms ;
the ccelom is reduced ; they are hermaphrodite, with a chtellum
and a rudimentary cocoon and direct development. Hirudo is

i85

S.V.—

'° V s^f. "

15

.-NT J

spth,2zA

v.d. 0. od,

lines are septa. ' ^^'^t^^^- ^^e numbers are those of the segments, the vertical

vesicle : sptH., spermathec'a ;".', Jest'^fs^-'tesTScf.Z vas dS^ ''"™ "' ^ ^"- ^^™'"^^

M.Z. — 7

o, PHYLUM ANNELIDA

Ic50

an example and another is Aulostomum ( =HcBmopis), the smaller
horse-leech,' which is common in ponds. In the tropics there are
terrestrial species.

Other classes of annehds of lesser importance are the Archi-
annelida and the Gephyrea.

14

THE CCELOMATE BODY

With the annelids we meet the type of body structure which is
found in all the remaining groups of the animal kingdom. It
seems to have been adequate for all the developments required
for different types of habit and habitat, and although there are
many modifications and some regressions there are no changes
in the basic plan. In this chapter the chief characters of the
coelomate body, as it is called, are briefly described ; for details
the chapters dealing with the various types should be consulted.
The basic plan is an arrangement of three layers of tissue, with,
in the middle one, a series of spaces. Generally speaking the inner
and outer layers are formed in the embryo simultaneously, and
before the middle one ; the inner one is called the hypoblast,
and the tissues to which it gives rise are known collectively
as endoderm. This forms the lining of the gut and little
else. The outer layer is similarly called the epiblast, and the
tissues which it forms are the ectoderm. This is chiefly the
outer parts of the skin (and outgrowths from this such as
hair and feathers) but it also includes the nervous system.
Between the hypoblast and the epiblast, and generally formed
a little later, often from the hypoblast, is the mesoblast. It
gives rise to the mesoderm, which includes almost all the other
tissues of the body : muscles, gonads, often the excretory organs,
and in the vertebrates the skeleton. The three germ layers,
as hypoblast, mesoblast, and epiblast are called, are not layers
of cells in the sense in which one speaks of layers of chocolates
in a box, but in the sense in which the term is applied to a
sandwich, to describe plates of material, of different origin,
separated from their neighbours ; that is to say, any of the
germ layers may be more than one cell thick. In addition to the
annelids and those groups which we have not yet described, the
flatworms and roundworms have three germ layers ; all these
animals are known collectivel}) as Triploblastica, in distinction
from the coelenterates, which have no mesoderm and are Diplo-
blastica. The gonads of the coelenterates are in the ectoderm or
endoderm according to the class, but they have little other
specialisation of tissue. The mesogloea is non-cellular, and corre-
sponds to the mesoblast in little else besides position.

187

1 88

THE CGELOMATE BODY

BILATERAL SYMMETRY

All triploblastic animals early acquire bilateral symmetry, so
that there is a median plane on either side of which are two
halves of the body which are roughly mirror images of each
other. In the annehds there is Httle difference between the two
sides, but in the molluscs and chordates there is a greater or lesser
degree of secondary internal asymmetry, especially noticeable in
the coihng of the gut, but often showing itself in blood vessels,
gonads or other organs. The external appearance of similar right
and left sides is usually maintained much more closely, although
differences are sometimes apparent, as in the pincers of the cray-
fish and the scrotum of many mammals. The echinoderms com-
pletely lose their bilateral symmetry early in embryonic life, and
acquire a secondary radial symmetry.

The existence of bilateral symmetry enables us to distinguish
between anterior and posterior ends, between right and left sides,
and between dorsal and ventral surfaces. A section cut parallel
to the anteroposterior axis and in a vertical plane is called
longitudinal, and if it is in the middle line it is sagittal. A hori-
zontal section is called frontal, and one which is at right angles
to the axis is transverse. The surface of a structure nearer to the
axis of symmetry is called medial, that further away is lateral,
but these terms are not often used outside human anatomy. The
end of a structure such as a limb which is nearer to the body is
called proximal, that further away is distal.

THE CCELOM

The flatworms and roundworms are solid, except for the gut
and excretory and genital canals. All the other triploblastic
groups have two large series of spaces in the mesoderm, although
sometimes these are reduced. The most obvious of these is the
ccelom, often called simply the body cavity. It is sometimes
formed from pouches which grow out from the gut while the
mesoblast is being formed, sometimes by a separation of meso-
derm cells to form a space. Its simplest shape is that of a trough-
Uke cavity (in vertical transverse section like a U) partly
surrounding the gut, but it is often divided up in various ways.
The coelom contains a fluid, in which there may be cells, but it is
doubtful if the cavity itself can truthfully be said to have any

THE CCELOM 189

function. The fluid no doubt acts as a shock-absorber for the
viscera, which hang in it, and the cells may be excretory, as they
are in the earthworm. Its walls generally give rise to the ova
and spermatozoa, and may help in excretion, as they do in the
vertebrate kidney (p. 615). The coelom always has openings to
the exterior. In Annelida there are dorsal pores, and in the dogfish
abdominal pores ; these seem to be holes of no well-defined
homology or purpose, and perhaps act merely as safety-valves,
allowing coelomic fluid to escape if the pressure becomes too great.
More important are coelomoducts, which are paired tubes, often
segmental, which grow out laterally from the walls of the coelom
so that they are made entirely of mesoderm. They are the route
by which the gametes escape, and may become highly speciahsed
genital ducts, and in the vertebrates they form the excretory
tubes. In many annelids, such as the earthworm, the nephridia,
which are ingrowths from the ectoderm, may pierce the wall of
the coelom and form a third type of communication with the
exterior. In many polychaetes each nephridium becomes intimately
attached to a coelomoduct to form a composite structure called a
nephromixium.

THE HiEMOCCELE

The other space in the mesoderm is the haemocoele, or the cavity
of the blood-vascular system. This is sometimes, as in the earth-
worm, directly derived from the spaces between the cells of the
early embryo ; in such animals it is a part of the external environ-
ment which has come to be surrounded by cells, and into which
fluid is secreted and cells wander. Sometimes, however, as in the
vertebrates, it is formed by the separation of mesoderm cells along
definite lines, some of the cells being left as the blood corpuscles.
Whatever its origin it is always developed as a branching system
of vessels with definite walls, which in places are muscular and
contractile. In the arthropods and molluscs a part of the blood-
vascular system is also expanded to surround the other organs
much as the coelom does in a vertebrate ; such creatures have the
coelom reduced, and although formally coelomate are very
different from other such animals. The term haemocoele is
sometimes restricted to these organ-containing expansions of the
cavity of the blood system. The haemocoele is sometimes called
the primary body cavity, on account of its derivation from the

jgQ THE CCELOMATE BODY

chinks between the cells of the embryo, and the coelom the secon-
dary body cavity. Since in many embryos this temporal dis-
tinction cannot be made, and since neither is ever present in
an adult animal without the other and they add nothing in
meaning, these terms are best abandoned.

FUNCTIONS OF BLOOD

The properties of the blood, the fluid and cells present in the
blood-vascular system, are usually listed as many, but all the
important ones can be summed up in one word : transport. The
mesoderm enables an animal to become large ; bulk prevents
easy access to the cells by simple diffusion across adjacent cells,
and so a transport system becomes necessary. The things which
are carried to the cells are oxygen, either in solution or in chemical
combination with a substance such as haemoglobin ; food, chiefly
as amino-acids and hexoses, and hormones (p. i8). These last are
known in vertebrates, cephalopods, insects, crustaceans and
annelids, that is, in all the chief phyla with blood. From the cells
the blood takes excretory products, including carbon dioxide, and
heat. In large animals the latter is very important, for a cell in
the middle of a big mass of muscle is well insulated, and if there
were no cooling system it would overheat. The fact that in insects,
where oxygen is carried by different means, the blood system is
far less well developed than it is in other arthropods, suggests that
oxygen-transport is the most important function of the blood.
Besides carrying substances from one place to another, the blood
is also important in forming a relatively stable internal environ-
ment for the active body cells. This environment is an aqueous
solution in which the main constituents are kept at an approxi-
mately constant composition ; the hydrogen ion concentration of
vertebrate blood does not greatly fluctuate, even though large
quantities of lactic acid and carbon dioxide are put into it, because
it is well buffered with phosphate. Other ions, and substances
such as glucose, are kept constant by the co-operation of the
kidney, which eliminates them if they are present in excess and
holds them back if there is not enough of them. Other excretory
organs, such as the nephridia of the earthworm and the green
glands of the crayfish, act in a similar, but probably less elaborate,
way.

These are the fundamental functions of blood, which would

FUNCTIONS OF BLOOD I9I

have to be carried out somehow even if the animal had no blood
system ; it is difficult to imagine that any of them, with the
partial exception of oxygen transport, which in small animals
may be done by a system of air-containing tubes, could be
properly carried out except by a circulating liquid. Various types
of animal have also taken advantage of the existence of blood
to use it for other and minor purposes, which can equally well
be fulfilled in other ways. It is an accident that the oxygen-
carrying substances contained in most bloods are pigments, and
in a number of animals they are visible from the outside, possibly
or probably to the advantage of the species. The most familiar
example is man himself, for in the white races the colour of the
haemoglobin is presumably important in sexual selection, but
its most striking use is seen in some monkeys, where the chest
or buttocks blush vividly in excitement. As is well known, a
fluid can transmit a pressure and so be used to increase a volume,
and these properties have more than once been used by animals.
The swan mussel moves by pushing its foot forward through
the mud and then inflating it with blood. When the muscles
which withdraw the foot are contracted, the latter is stuck
fast, and so the whole animal is pulled forward ; the blood is
then withdrawn and the process repeated. The swelling of the
labium of blood-sucking insects, the eversion of the insect-
catching tongue of the chamaeleon, and the increase in size
and stiffness of the mammalian penis necessary for copulation,
are brought about in a similar way. The method used in all
these instances is largely the same ; the veins are constricted
so that the blood which is pumped in by the heart can only escape
slowly and is retained under pressure. The resulting turgidity
is comparable to that w^hich is normal in herbaceous plants,
although here the fluid is enclosed and its pressure is osmotic,
not hydrostatic, in origin. In both the Amphibia and the insects
the phagocytes in the blood assist in metamorphosis by ingesting
unwanted material, such as the tadpole's tail, and transferring
it elsewhere. Finally, in mammals and birds, the blood-vascular
system, rather than the blood itself, is used in temperature
regulation. Although the blood takes no direct part in temperature
control, its presence and circulation are necessary to the efficient
working of such control, just as a stirrer increases the efficiency
of a thermostat. In large animals without temperature regulation
the blood is probably important in preventing local rises of

THE CCELOMATE BODY

temperature in active organs by conveying heat away to the sur-
face or to less active regions of the body. This was probably
very important in the large extinct reptiles (unless, as some people
think, these creatures must have had a proper regulating

mechanism).

Although blood seems to be necessary for a large animal, its
possession brings dangers. A fluid under pressure is easily lost
if the vessel which contains it is pierced, and a medium which
takes food and oxygen to the body cells is an ideal environment
for parasites, so that it is not surprising that devices have been
developed to overcome these dangers ; they are not functions of
blood, since if blood did not exist they would be unnecessary,
but they are important properties of it. It is not the function of
a motor car to have brakes, but they are necessary in order that
it can safely perform its function of transporting people from
place to place. Excessive loss of blood by bleeding is prevented
because on exposure to air blood forms a clot ; parasites are
attacked by phagocytes, and the poisons which they produce
are neutralised by antitoxins (p. 151).

Many of the functions of blood may be carried out, often in
a less efhcient way, by the coelomic fluid. It must maintain
something of a constant environment ; it carries excretory
products and contains phagocytes ; and in the earthworm it
transmits the pressures produced by the muscles so that burrowing
is possible.

SEGMENTATION

A division into parts, or segments in a loose and general sense,
is common in animals. The egg is sometimes said to segment when
it divides into cells or blastomeres ; the limbs of arthropods are
divided into jointed parts or podomeres ; and the tapeworm
grows a linear succession of proglottides, the oldest being farthest
from the growing point. In many coelomates there is a different
type of repetition of parts, in which the growing point remains
posterior. This is known as metameric segmentation, or often
simply as segmentation ; and the repeated parts are called
segments or metameres. It seems to be dependent on the meso-
derm, for the endoderm and ectoderm are never fully divided,
although they may secondarily have to follow the pattern imposed
on them by the mesoderm, the segmentally arranged embryonic
blocks of which are called somites. In an extreme case muscles.

SEGMENTATION I93

glands, gonads, coelomoducts, nephridia, ganglia, and nerves may
be repeated in almost indistinguishable units down the body.
Such a state is found in many polychaetes. The earthworm has
fairly complete segmentation, but the gonads are confined to one
or two segments and the distribution of the accessory sexual
organs such as seminal vesicles makes many of the anterior seg-
ments distinct. The segmentation of the crayfish and of insects is
more superficial, and there is little visible internally, but the
continuous exoskeleton makes external jointing functionally
important. The embryos of vertebrates have a conspicuous and
fundamental segmentation ; in the fishes much of this persists in
the adult, but in the other classes practically no traces of it are
left. Even the vertebrae, which appear to be obviously segmental,
do not correspond to the segments of the embryo.

Many segmented animals have the first few segments much
modified to form a head. This process is known as cephalisation ;
it occurs to some extent in Nereis, and is conspicuous in the
insects and vertebrates. The changes generally include the
development of special limbs or other structures around or in the
mouth ; the specialisation of sense cells and their association with
other types of cell to form conspicuous organs such as eyes ; and
internally the concentration of nervous tissue into a brain.
Cephalisation is generally regarded as an inevitable result of
always moving forwards, and it certainly seems reasonable that
that part of the body which first meets food and first learns about
the outside world should be specialised to eat and see. It is only,
however, in segmented animals that cephalisation is highly
developed ; the most highly organised unsegmented animals, the
molluscs, do indeed have something of a head, but it is remarkable
that in many cephalopods, where it is best developed, it is pretty
well in the middle of the body, and that these creatures move
backwards much more rapidly than forwards. The octopus, alone
amongst animals, bears any resemblance to Shakespeare's ' Men
whose heads, Do grow beneath their shoulders '.

OSMOTIC REGULATION

It is a general property of living membranes that the}' are
semipermeable, that is, allow some substances, especially water, to
pass through them much more rapidly than do others. A result of
this property is that when they separate two aqueous solutions

THE CCELOMATE BODY
194

of different strengths, water passes across them from the weaker
to the stronger solution (a phenomenon known as osmosis),
with consequent sweUing and increase of pressure. For most
animals living in the sea the semipermeability of livmg matter is
of little consequence, for their protoplasm has much the same
concentration of solutes as has sea water, so that there is little
movement of water. In fresh water things are different, and the
prevention of osmosis is important for animals living there ; it is
especially so in animals with a body cavity, for here not only
do the cell walls act as semipermeable membranes, but so do
the tissues themselves, causing water to be attracted into the
ccelomic fluid or blood. The most awkward situation of all is
that of estuaries, where there is a large daily change in the
external salt concentration. According to their osmotic relations
animals may be classified into two or three groups. First are most
of those which live in the sea, including the majority of inverte-
brates and the cartilaginous fishes ; their environment does not
normally change, and they have no means of coping with any
osmotic changes that do occur ; they are called stenohaline. If
put into fresh water they absorb it. Secondly, some invertebrates,
such as the shore crab and Nereis diversicolor, live where sea
water and fresh water mix, so that the salt concentration is
continually changing, and many bony fishes, such as the salmon,
pass from the sea into rivers and back again. They are able, to
a greater or lesser extent, to prevent the entry of water, or to
expel it after it has entered, and are called euryhaline, as the
salt concentration in their body fluids remains approximately
constant. Although living in a constant environment the marine
bony fish also have to regulate and are euryhaline ; their blood
has a lower salt concentration than sea water, so that water
tends to leave them. The animals which live permanently in
fresh water are also able to regulate their salt concentration in
the sense that they can cope with a tendency of water to enter,
but they may for various reasons be unable to survive in sea
water, so that while osmotically they are euryhaline ecologically
they make a third class. Osmotic regulation may be carried out
by a special organ, such as the contractile vacuole of Protozoa,
but more often use is made of an organ which has other functions
as well. The shore crab and fish use their gills, and most eury-
haline coelomates their excretory organ. In vertebrates the relative
impermeability of the skin is important.

15

THE CRAYFISH AND OTHER
ARTHROPODS

Crayfishes are found in many English rivers, especially in those
which rise in chalk or limestone hills. They are little, lobster-like
creatures, which make burrows in the river banks. They dislike
strong light and during the daytime generally remain in their
holes with only their pincers and long feelers projecting. When
they come out they crawl stealthily about, searching constantly
for their food, which consists of organic matter of any kind,
plant or animal, dead or alive, that they are able to seize and
break up with their pincers. If danger threatens, they dart
backward suddenly and swiftly. They are used for food, especially
for garnishing salads, and were formerly caught in large numbers
in this country by means of wicker crayfish-pots, but in 1887
their numbers were greatly reduced by a disease. The name is
also used for a number of related genera in various parts of the
world.

EXTERNAL FEATURES

The English crayfish, Astacus pallipes (Fig. 132), is about
three inches long, and of a dull, greenish colour, which harmonises
well with the surroundings in which it lives. A number of other
species are found on the Continent. The body of a crayfish is
armoured with a thick cuticle. It is segmented, each segment
bearing a pair of jointed limbs, but in the front part the segments
are fused to form a fore-body or cephalo thorax, where the only
conspicuous sign of their existence is the presence of several
pairs of limbs, though parts of the armour and certain internal
organs are also segment ally arranged. The rest of the body,
known as the hind body or abdomen, is more completely seg-
mented. At the end of the abdomen is a flat piece known as
the telson, on the under side of which the anus opens. The telson
bears no limbs, and is divided by an imperfect transverse joint.
The armour of each segment of the abdomen consists of a broad
back-piece or tergum and a narrow belly-plate or sternum, with
a pair of V-shaped prolongations, known as the pleura, joining
them at the sides. There are no pleura on the first abdominal

195

196

THE CRAYFISH. PHYLUM ARTHROPODA

somite. The tergum, sternum, and pleura of each somite form a
continuous ring. The hmbs are jointed to the hinder side of the
sternum near its outer ends, and the part of the sternum between
each Hmb and the adjoining pleuron is sometimes called an
epimeron. The terga overlap one another from before backwards
and slide over one another as the abdomen is straightened and
bent, the armour of each somite being joined to that of the next
by thin cuticle, which allows of movement. In the cephalothorax
the terga are fused to form a shield or carapace. This is prolonged

in front into a beak-hke rostrum and
is crossed by a furrow, which is called
the cervical groove because it is sup-
posed to mark the separation of two
regions known as the head and thorax.
At each side of the body a fold of the
carapace overhangs as a lean-to roof,
the gill cover or branchiostegite, w^hich
encloses between itself and the side
of the bod}^ a chamber in which the
gills lie. Behind the cervical groove a
branchiocardiac groove on each side
marks off the branchiostegite from a
median cardiac region which roofs the
thorax. The cuticle of the inside of the
branchiostegite and part of the side of

Fig. 1^2. — View of a crayfish ,, , , ■, ii -x j.v^- r\

from above.— After Huxley, the body underneath it are thm. On

Note cervical groove, cardiac region of thc VCUtral Sidc of thc CCphalothoraX
carapace, rostrum, and tail fan. •,,•-■ r -i • -i i j_i

the limbs of each pair are close together,
but small sterna lie between them. The head, is of course, the
region which contains the mouth and the principal sense organs.
The mouth is placed on the ventral surface at some distance
from the front end, and in front of it the sternal surface slopes
upwards to the rostrum. At the sides of the latter, upon a pair
of short, movable stalks, are placed the eyes, and below these
stand two pairs of feelers or antennae.

LIMBS

The limbs or appendages number nineteen pairs, without
counting the eyes, which are by some authorities reckoned as
limbs. We shall not take this view, but as there is evidence in

197

Fig. 133. — The anterior appendages of the left side of a crayfish.

A External view of the antennule of the left side ; B, ventral view of the antenna of the right side ;

C, mandible in ventral view ; C, the same in dorsal view, somewhat enlarged, showing molar process. ;

D, maxillule (first maxilla) ; E, maxilla (second maxilla) ; F, first maxilliped ; G, second maxilliped;
H, third maxilliped.

bp., Basipodite ; cp., coxopodite ; crp., carpopodite ; dtp., dactylopodite ; en., endopodite ; ep., epipodite;
ex., exopodite ; gill; isp., ischiopodite ; ntrp., meropodite ; o.g., opening of the green gland; pr.,
protopodite ; prp., propodite ; sb., setobranch or tuft of coxopodite setae ; 1-5, joints of endopodite.

198

THE CRAYFISH. PHYLUM ARTHROPODA

the development of the crayfish and of related animals that the
foremost region of the head corresponds to a somite, we shall
regard the body as containing twenty segments, of which the fore-
most bears no Hmbs. The telson is not a segment. Of the twenty
segments, the first six form the head, the next eight the thorax,
and the last six, with the telson, the abdomen. The parts of which
a complete limb consists are best seen in the hmbs known as
the third pair of maxillipeds (Fig. 133, H), which lie immediately
in front of the great pincers. This iUustrates fairly weU the type of
crustacean appendage called the biramous limb or stenopodium,
which has a base or protopodite, two branches called endopodite
and exopodite, and sometimes extra processes or epipodites. The

Fig. 133 H. — Third maxilliped ; the first walking leg of a crayfish.

other limbs are built upon the same general plan as the third
maxilhped, but they have modifications suited to their functions,
and parts are often missing. They are shown in Fig. 133 F and G
and Table L More primitive Crustacea have a flattened type of
limb called the phyllopodium.

CUTICLE AND EPIDERMIS

The ectoderm or epidermis of the crayfish consists of a layer
of protoplasm with nuclei, which in many parts is not divided
into cells and is therefore a syncytium (p. 35), though in places
it forms a columnar epithelium. Outside it lies a cuticle which
it secretes, which contains chitin (p. 159), and is for the most
part thick and hardened with salts of lime, but remains thin and
flexible in certain places so as to form joints which allow the
parts of the body to move upon one another, and also in the
gill chambers. In places it bears bristles (setae) of various shapes.

199

TABLE I

The Limbs of the Crayfish

Segment

Head-<

I

2

5
6

7
8

Thorax-*

lO

II

12

13

Abdomen-:

15

i6

i7|

18

IQl

20

Name

No appendage
Antennule
(ist antenna)

Antenna
(2nd antenna)

Mandible

Maxillule

(ist maxilla)
Maxilla

(2nd maxilla)

ist maxilliped
2nd maxilliped
3rd maxilliped

Pincers

(forceps,

cheliped)
ist walking leg
2nd walking leg

3rd walking leg

4th walking leg

Swimmerets

Swimmerets

Swimmerets

Paddles

Function

Sensory

Sensorj'

Feeding

Feeding
Respiratory

Respiratory

and feeding
Respiratory

and feeding
Respiratory

and leeding
Fighting,

holding

Walking
Walking and

sexual
Walking

Walking and
sexual

Sexual

Swimming
and sexual

Swimmmg
and sexual

Swimming

Structure

Three basal segments.
Two branches, probably
not homologous with
endopodite and exopo-
dite. Bears statocyst.

Long endopodite.
Bears opening of green
gland.

Toothed biting base and
short 2-segmented endo-
podite making the palp.

Three flat plates.

Several lobes. The exo-
podite forms the scapho-
gnathite or baler, which
maintains a current of
water over the gills.

Large exopodite and epi-
podite.
Similar to ist maxilliped.

Typical structure.

No exopodite. Very
large endopodite with
grasping chelate end.

As last, but much smaller.

As last. Bears genital
opening in female.

As last, but not chelate.
No genital opening.

As last, but no epipodite.
Pears sexual opening in
male.

Form canal for seminal
fluid in male ; vestigial in
female.

Modified in male ; flat-
tened endopodite and
exopodite in female.

Flattened endopodite and
exopodite ; carry eggs in
female.

Large flattened endo-
podite and exopodite
form, with telson, the
tail fan.

200 THE CRAYFISH. PHYLUM ARTHROPODA

These are quite different from the chsets of the earthworm ;
thev are hollow, and the epidermis is continued mto them and
is here often connected with nerve fibres, so that the bristles

Fig. 134. — A ventral view of a female crayfish, with the limbs of the right side
removed and the branchiostegite of that side partly cut away. — Partly
after Howes.

ab.l.ifi, First and sixth abdominal limbs ; at.i, antennule ; at.2, antenna ; an., anus ; br., branchiostegite ;

e.g., cervical groove ; ch., cheliped ; en., endopodite ; en.sk., endophragmal skeleton ; epm., epimeron ;

ex. exopodite ; g.c, gill chamber ; g.op., openings of oviduct ; Ibr., labrum ; m.l., limbs adjoining the

■, mxp.2, third maxilliped ; pi., pleuron ; pr., protopodite ; rst., rostrum ; st.ab., abdominal

z'.h., sternal region of body in part of mouth st.th., thoracic sterna ; t., telson ; t.g., tubercle

-'•.. ■ ..L^ green gland opens ; w.l.i, first walking leg.

CUTICLE AND EPIDERMIS 201

serve as sense organs of various kinds. From time to time the
cuticle is shed and a new one secreted ; this allows of growth.
Moulting takes place frequently while the animal is young,
but the old male sheds its cuticle only twice a year, and the
female only once. As the time for moulting draws near, a new
cuticle begins to form under the old one, which is loosened
from the epidermis, and the crayfish goes into hiding for some
days while the new cuticle is soft, and the animal is helpless
while it is hardening. The shell then splits across the back and
along the limbs, and the crayfish, lying on its side, draws itself
out of the old cuticle.

SKELETON, MUSCLES, AND LOCOMOTION

There is in the crayfish no continuous muscular body-wall,
but numerous muscles, composed of striped fibres, move the
various parts of its body, being attached to the inside of the pieces
of the armour. Thus the skeleton is external, not, like that of a
vertebrate, internal. Its pieces, known as sclerites, usually abut
upon one another across the soft jointing membranes by hard
knobs which serve as hinges. In the thorax ingrowths of the cuticle
provide a kind of false internal skeleton. This has the form of a
complicated scaffolding along the ventral side of the animal
and is known as the endophragmal skeleton. In the limbs, as in
those of vertebrates, opposing muscles (flexors and exten-
sors) bend and straighten each joint. Ingrowths of the cuticle
serve as tendons for them. The abdomen also is moved by two sets
of muscles (Figs. 136, 138). A dorsal set of extensors starts from
the inside of the carapace and is inserted into the terga of the
abdominal somites. When they contract, these muscles draw
forward the terga and thus straighten the abdomen. Ventrally,
powerful and complicated flexors connect the sterna with one
another and with the endophragmal skeleton (Fig. 138). The
flexors, when they contract, draw closer the sterna and thus
bend the abdomen. By this movement, spreading at the same
time its tail fan, the crayfish can suddenly jump backwards to
escape from its enemies. Its gentle forward movements are
carried out by the walking legs, aided by a paddhng of the
abdominal Hmbs. The legs of the first three pairs pull and
those of the last pair push, and their movements are carried
out in such a way that the animal is always standing upon

202 THE CRAYFISH. PHYLUM ARTHROPODA

six legs while two— which are on opposite sides and of different
pairs — are in motion.

Fig. 135. — View of a crayfish from the right side.

ab.l/j, Sixth abdominal limb ; o/.i, antennule ; at. 2, antenna ; br., branchiostegite ; e.g., cervical groove;
ck., cbeliped ; crd., cardiac region of carapace ; en., endopodite ; ex., exopodite ; mxp.-i, third maxilliped;
yi., pieuron ; pr., protopodite ; rsL, rostrum ; sea., scale of antenna ; /., telson ; tg., terga ; w.l.^, last
walking leg.

203

d.ah.a.

ext.tn.

REGENERATION AND AUTOTOMY

The power of regeneration, though it is less in the crayfish
than in earthworms and much less than in Hydra, is still con-
siderable. A whole limb which is injured can be grown again.
The injured leg is first cast off by a spasmodic contraction of
some of its muscles which
causes it to break through at
the basipodite, the internal
cavity — which, as we shall
see, is a blood space — being
here crossed by a partition
which leaves only a small
opening, through which the
nerves and blood vessels pass.
When the limb is cast off this
opening is quickly closed by a
blood clot, after which the
cuticle grows across the
wound. Beneath the scar the
new limb is formed as a bud
and gradually takes shape.
At the next moult it becomes
free, though it is still small,
and it increases in size at
each moult, until a normal

limb has been provided. This power of casting off limbs is known
as autotomy. It is sometimes used as a means of escape from
enemies which have seized one of the limbs, but this is not so
common in the crayfish as in some animals that are related to it.

v.n

v.ah.a.

Fig. 136. — A semi-diagrammatic drawing
of a transverse section of the abdomen
of the crayfish.

bp.

Basipodite ; cp., coxopodite ; d.ab.a., dorsal
abdominal artery ; en., endopodite ; ex., exo-
podite ; ext.m., extensor muscles ; ji.m., flexor
muscles ; h.g., hind-gut ; pi., pleuron ; pr.,
protopodite ; tg., tergum ; st., sternum ; v.ab.a.,
ventral abdominal artery ; v.n.c, ventral nerv'e
cord.

PERIVISCERAL CAVITY AND ALIMENTARY SYSTEM

The body of the crayfish contains a spacious perivisceral
cavity, in which the internal organs lie. This is not a coelom, but
an enlarged portion of the haemocoele (p. 189), and communicates
with the blood vessels. The alimentary canal fills the greater part
of this cavity. The mouth is an elongated opening below the head
between the mandibles. It has in front a wide upper lip or labrum,
and behind it is a pair of lobes (paragnatha) known together as
the lower lip or metastome. A short, wide gullet leads upwards

204

THE CRAYFISH. PHYLUM ARTHROPODA

into the large proventriculus, often called the stomach (Fig. 137).
This consists of two chambers, a large forepart or mill-chamber,

.t.ear. •P''"--
1 u, ca r. , car.

B

-^.c.

-iOes.

U£ar.

"")!

-car.

~? \)

-p. car.

f-'-lJI--u.car.
. /Ji--z.car.

W'-p-py-

h-cm.

'j-mg.

fiar. Q

Fig. 137. — The proventriculus ot the crayfish.

A The whole organ from above ; B, the same from the right side ; C, the left half from within, the muscles
being relaxed ; £>, the ossicles of the mill in median section, the anterior and posterior gastric muscles
being contracted ; £, the mill in plan. All the figures are semi-diagrammatic, much detail being
omitted.

bri.. Bristles for filtering ; car., cardiac ossicle ; cm., caecum ; j.c, pyloric or filter-chamber ; gUh., position
of gastroUth ; h.g., hind-gut ; l.p., lateral pouch ; l.t., lateral tooth ; m.c, mill-chamber ; m.g., mid-
gut ; m.t., median tooth ; o.h., opening of bile duct ; as., oesophagus ; p. car., pterocardiac ossicle;
p.py., prepyloric ossicle ; py., pyloric ossicle ; u.car., urocardiac ossicle ; v., the several pieces of an
arrangement of valves which directs the solid residue of the food into the hind-gut, there to become
the faeces ; z.car., zygocardiac ossicle.

often known as the cardiac division of the stomach, and a smaller
hind part or filter-chamber, often known as the pyloric division
of the stomach, separated from the mill-chamber by a pit in
the roof. From the filter-chamber the short mid-gut or mesenteron
leads backwards to the long hind-gut, sometimes called the
' intestine'. The epidermis and cuticle turn inwards at the mouth
and line the gullet and proventriculus, which are together known
as the fore-gut. The mid-gut is lined with soft endoderm, and the
hind-gut is again lined with epidermis and cuticle. The cuticle
in the gut is for the most part thin, but in places in the

PERIVISCERAL CAVITY AND ALIMENTARY SYSTEM 205

proventriculus it forms stout plates or ossicles, certain of which
bear strong teeth which project into the forepart of the organ. By
the action of muscles these can be brought together to crush the
food. The whole apparatus is known as the gastric mill. Into the
mid-gut opens on each side a large, lobed, yellow gland, often
called liver or hepatopancreas, consisting of numerous short
tubes joined by ducts which finally communicate with the mid-
gut by an opening on each side. It is, however, much more than
a gland, as the finer particles from the fore-gut are directed into
it by the filter chamber and are digested in it ; it is best known
by the collective name digestive diverticula. The roof of the mid-
gut is prolonged into a short blind gut or caecum. Food is either
raked up by the third maxillipeds or seized by the chelipeds and
torn up by them and the smaller pincers. It is passed forwards
by the jaws to the mouth, where pieces are cut from it by the
mandibles and thrust by the mandibular palps and the maxillules
into the mouth. It is chewed in the proventriculus, and partially
digested by a protease sent forward from the diverticula, before
being passed into the diverticula themselves for further digestion
and absorption. The cuticle of the gut is shed with that of the
body. Shortly before a moult two fiat calcareous bodies, known
as * crabs' eyes ' or gastroliths, are laid down in the forepart of
the proventriculus. They are ground up before the moult takes
place. It is uncertain whether they consist of matter removed
from the armour of the body to weaken it in preparation for the
moult or are a store of material for the strengthening of the new
cuticle. Possibly they serve both purposes.

BLOOD VESSELS

The heart (Fig. 139) is a hollow organ with thick, muscular
walls. It is roughly hexagonal in outline, as seen from above,
and lies in the thorax, above the hind-gut and immediately
below the cardiac region of the carapace, in a space, known as
the pericardial sinus, with membranous walls, to which the heart
is connected by six fibrous bands called the alae cordis. Three
pairs of valved openings or ostia admit blood from the pericardial
sinus to the heart : one pair is dorsal, another lateral, and the
third ventral. From the front end of the heart arise three vessels
— a median ophthalmic artery, which runs straight forwards
over the proventriculus to supply the eyes and other organs of

2()6

THF, CRAYFISH. PHYLUM ARTHROPODA

the head, and a pair of antennary arteries, which start one on
each side of the ophthalmic, run forwards and outwards, and divide
each into two branches, one gastric and the other to the antennae
and green gland. Behind and below the antennaries arise a pair
of hepatic arteries, which supply the liver, and from the hinder
angle of the heart there is given off a vessel that at once divides
into a dorsal abdominal artery, which runs backwards above

hi

od.

d.atp-S. extm.

heg.Q

Fig. 138. — The internal organs of a female crayfish in situ. Slightly diagrammatic.

a.g.m.. Anterior gastric muscle; an., anus; ant.a., antennary artery; at.i, antennule ; at.2, antenna;
bl.g., bladder of the green gland ; e.g., cervical groove ; c.oes., circumoesophageal commissure ; cer.,
cerebral ganglion ; ch., cheliped ; cm., caecum ; d.ab.a., dorsal abdominal artery ; en.sk., endophragmal
skeleton ; eye ; ext.m., extensor muscles ; fl.m., flexor muscles, looping from one sternum to another
over v.l.m. ; g.gl., green gland ; h.g., hind-gut ; hep.a., hepatic artery ; ht., heart ; Ir., liver ; Ibr., labrum ;
md., mandible ; m.p., mandibular palp; o.b., opening of bile duct ; od., oviduct ; ces., oesophagus ;
op.a., ophthalmic artery ; os., ostia ; ov., ovary ; p.g.m., posterior gastric muscle ; pern., pericardium :
prv., proventriculus ; rst., rostrum ; st.a., sternal artery ; st.ab., abdominal sterna ; st.h., sternal region
of the body in front of the mouth ; st.th., thoracic sterna ; t.g., tubercle for green gland ; tg., terga ;
v.l.m., ventral longitudinal muscles ; v.n.c, ventral nerve cord ; w.l.^, last walking leg.

the intestine and supplies it and the muscles of the abdomen,
and a sternal artery. This passes downwards, through an opening
in the ventral nerve cord, and divides into a backward-running
ventral abdominal and a forward-running ventral thoracic artery,
by which the limbs are supplied. Each of the arteries branches
many times, till it finally gives rise to minute vessels in the organs
it supplies, but there are no capillaries.

From these vessels the blood passes into great sinuses which
surround the organs. The largest of these is the perivisceral
cavity, but there are also blood spaces in the limbs and elsewhere.
The blood from the limbs and a great part of that from the
perivisceral cavity is gathered up into a sternal sinus, which lies

;i tunnel formed by the endophragmal skeleton and contains

BLOOD VESSELS

207

the ventral nerve cord and ventral thoracic artery. From this a
series of afferent branchial sinuses carries the blood to the gills,
where it is oxygenated. From the gills it passes by efferent
branchial sinuses to the pericardial sinus. Part of the blood from

Fig. 139. — A male crayfish dissected from the dorsal side, after injection of

the arteries.

a.c, ala cordis ; a.g.m., anterior gastric muscle ; ant.a., antennary artery ; c.p.m., cardiopyloric muscle;
car., cardiac ossicle ; d.ab.a., dorsal abdominal artery ; f.c, part of filter chamber, blue coloured in
fresh specimens ; fl.m., flexor muscles of abdomen ; g.a., gastric artery ; h.g., hind-gut ; ht., heart ;
Ir., liver; md.tn., muscle of mandible; op.a., ophthalmic artery; 05., ostium; p.g.m., posterioi
gastric muscle; prv., proven triculus ('cardiac' division) ; py., pyloric ossicle; ts., anterior lobf
of testis ; ts', posterior lobe of the same : v.d., vas deferens.

208

THE CRAYFISH. PHYLUM ARTHROPODA

around the stomach, however, passes on each side into the
space between the two sides of the fold of carapace which forms
the branchiostegite, and thence to the pericardial sinus by a vessel
which follows the hinder edge of the branchiostegite. It will be
noted that the pericardial cavity of the crayfish is a part of the
hitmocctle and contains blood, unhke that of the vertebrates,
which is a separate part of the ccelom. A blood-vascular system

.hi.

.OS. ,'pcrn.

ext.m.

,eff.hY,s.

ov. -_

' jLm.

v.n.c.'^'^^-^- '*'•'•

'w.l.

Fig. 140. — A diagram of a transverse section through the thorax of a crayfish.

urb., Arthrobranch ; hr ., outer layer of branchiostegite ; br"., inner layer of the same ; eff.br.s., efferent
branchial sinus ; en.sk., endophragmal skeleton ; ext.m., extensor muscle of abdomen ; fl.nt., flexor
muscles of abdomen ; g.c, gill-chamber ; h.g., hind-gut ; ht., heart ; Ir., liver ; os., ostia ; ov., ovary ;
Pcm., pericardium ; pbr., podobranch ; plb., pleurobranch ; st.s., sternal sinus ; v.n.c, ventral nerve
cord ; v.th.a., ventral thoracic artery ; w.l., walking leg.

Small arrows in the sinuses on the right-hand side show the course of the circulation of the blood.

in which, as in the crayfish, the blood on leaving the arteries
bathes the organs of the body is said to be open. One in which,
as in the worm and the vertebrates, it is carried through the
organs in capillaries which lead direct to veins is said to be
closed. The blood of the crayfish is a clear fluid, which contains
white corpuscles and clots readily — an obvious advantage to an
animal whose open vascular system causes it to bleed freely
from any wound. A respiratory pigment known as haemocyanin,
which is an organic compound of copper, is dissolved in it, and
plays the same part as haemoglobin, taking up oxygen in the

BLOOD VESSELS 2O9

respiratory organs and parting with it to the tissues. In the
oxidised condition it is of a blue colour and tinges the blood.

RESPIRATORY ORGANS

The respiratory apparatus of the crayfish is contained in the
gill-chambers (Fig. 141). The gills (Fig. 142) are branched, thin-
walled structures, standing upon the coxopodites of the thoracic
limbs and the inner wall of the gill-chamber. In them the blood
circulates and exchanges its carbon dioxide for the oxygen which
is dissolved in the water that is kept flowing through the chamber

ah.

arb.

scg.

Fig. 141. — The forepart of the body of a crayfish, viewed from the right-hand
side, with the legs and the branchiostegite cut away and the gills displayed.

arb., Arthrobranchiae ; ep., epipodite of the first maxilliped ; pbr., podobranchice ; plb., pleurobranchia ;

scg., scaphognathite.

by the action of the second maxilla. This limb is held firm by
the curved end of its endopodite, which fits into a groove upon
the mandible at the base of the palp, while the exopodite or
scaphognathite, flapping at the rate of sixty strokes a minute,
bales water forwards, out of the gill-chamber and under the
opening upon the antenna of the green gland, whose excreta it
thus sweeps away with the foul water from the gills. By this action
fresh water is drawn into the chamber between the bases of the
legs, and when the oxygen concentration in the water is reduced,
the baler flaps faster, so that the supply of oxygen is kept up.
No doubt the blood in the branchiostegite is oxygenated through
the thin inner wall of that organ.

The gills receive different names according to their position.
Those which are attached to the epipodites of the limbs are known

210

THE CRAYFISH. PHYLUM ARTHROPODA

TABLE II

Maxillipeds.

Legs.

Total.

I.

II.

III.

I.

II.

III.

IV.

V.

Podobranchiae.
Anterior arthro-

branchiaj
Posterior arthro-

branchiae
Pleurobranchiae

Ep

o

o
o

I

I

o
o

I

I
I

o

I

I

I
o

I

I

I
R

I

I

I
R

I

I

I
R

o
o

o

I

6+Ep

}■

1 + 3R

Total

Ep

2

3

3

3 + R

3+R

3 + R

I

i8+3R-fEp

Ep=epipodite without a gill.

R=abortive rudiment.

(a)>t.

as podobranchicE. Others stand upon the membranes which join
the hmbs to the body, and are known as arthrobranchiae, and a

few stand upon the inner wall of
the gill-chamber (the side wall of
the thorax) above the legs, and are
known as pleurobranchiae, the three
names being often anghcised by omission
of the terminations. The distribution of
the gills is shown in Table II.

Each arthrobranch and pleurobranch
has a tree-like structure, consisting of a
trunk or axis arising from the body by
one end, with numerous short branches
or filaments. In the podobranch the axis
is fused to the epipodite along the
greater part of its length, so that the
filaments appear to arise from the
epipodite. The tip of the gill, however,
stands free. The epipodite is folded
along its length, so that a groove is
formed, into which fits the gill of the limb next behind.

oase

Fig. 142. — A podobranch
of the crayfish, seen
from behind.

base ; cp., coxopodite ; gill ; lam.,
lamina ; sb., setobranch or
tuft of coxopoditic setae ;
stm., stem.

OSMOTIC REGULATION AND EXCRETION

In the head is a pair of coelomoducts (p. 189) called antennary
or green glands, situated immediately behind the antennae, upon
who^e basal joints they open. Each consists (Fig. 143) of a

OSMOTIC REGULATION AND EXCRETION

211

glandular mass and above it a thin-walled bladder from which a
short duct leads to the opening. In the centre of the mass is a small,
brownish sac, known as the end-sac. The cavity of this is a vestige
of the coelom, which otherwise is in the crayfish represented only
by the hollow of the gonad. Partitions project into it from its wall,
and it communicates by a small opening with the rest of the mass,
known as the labyrinth, which is essentially a winding and much
complicated tube leading from the end-sac to the bladder. Its
first section, which forms the
outer part of the gland, known
as the cortex, is greenish in
colour and broken into a mesh-
work of channels. The rest, the
medulla of the gland, is a
whitish, coiled tube, simple for
a short distance and then made
spongy by ridges of its wall. The
gland behaves as an osmotic
regulator in much the same way
as the vertebrate kidney ; a
filtrate from the blood is first
formed, containing its crystalloids
but not its proteins, and lower
down there is absorption of most
ions and secretion of some others.
The resulting fluid is of lower
concentration than the blood, so
that the body gets rid of water.
Some nitrogen is lost from the gland but the chief excretory organ
is the digestive diverticula. Certain gland cells found on the gills
are possibly also excretory. The principal nitrogenous excreta
are ammonia and amino compounds.

trrr^^ '^^

e.s:

CDK -

Fig. 143. — A diagram of the structure
of the green gland of a crayfish.
Above, the whole gland is seen in
longitudinal section ; below, the
end sac and cortex are seen as
dissected out and viewed from the
surface.

hi.

Bladder ; cor., cortex ; e.s., end-sac ; med.,
medulla ; o., opening on antenna.

NERVOUS SYSTEM

In its general plan the nervous system of the crayfish resembles
that of the earthworm. In the front part of the head, between
the green glands, Hes a supra-oesophageal or cerebral ganglion,
or brain (Fig. 144), which corresponds in position to the supra-
pharyngeal gangha of the worm. It gives nerves to the eyes,
antennules, and antennae, and from it two long circumoesophageal

n. m

$t.

212 THE CRAYFISH. PHYLUM ARTHROPODA

commissures pass backwards to join behind the oesophagus in the
subcesophagcal ganghon. This gives nerves to the Hmbs as far
as the second maxilhpeds, inclusive, and immediately behind

it lies the first thoracic
"^///f ganglion, which supplies the

/^ ^ third maxillipeds. In each of

the remaining segments of
the thorax lies an indistinctly
double ganglion which sup-
plies by several nerves the
limbs and other organs of its
segment. These ganglia are
set at some distance apart and
are connected by double com-
missures, forming thus a ven-
tral cord. Between the fourth
and fifth ganglia the com-
missures part widely to allow
the sternal artery to pass
between them. In the ab-
domen the cord is continued
and consists of a ganglion in
each somite united to its
fellows by longitudinal com-
missures, which are really
double, but appear at first
sight to be single. The last
ganglion supplies the telson
as well as its own somite.
The commissures contain no
nerve cell bodies. The brain
is more complex than those
of annelids and exercises more
control over the rest of the ner-

A semi-diagrammatic view of VOUS Systcm. Giant fibres run

from cells in it along the whole
length of the cord and enable
it to bring about sudden move-
ments which involve distant
parts of the body, such as the
backward escape movement.

Fig. 144.

central nervous system of a crayfish.

ab.i, ab.b, Thf first and sixth abdominal ganglia ; cer.,
cerebral ganglion ; c.oes., circumoesophageal
commissure ; I.e., longitudinal commissures of
ventral cord ; n.ab.l., nerves to abdominal limbs ;
n.a.ti, nerve to antennule ; n.at.2, nerve to antenna';
n.ch., nerve to cheliped ; n.w., nerves to limbs
adjoining the mouth ; o.n., optic nerve ; s.ces.
''"bo ,1 ganglion; st.a., sternal artery;

'*i| ■ l and sixth thoracic ganglia; v.n.,

nerve to proventriculus ; v.n'., nerve to hind-gut!

NERVOUS SYSTEM 213

A transverse commissure immediately behind the oesophagus
joins the two circumoesophageal commissures. It contains fibres
which take this roundabout course between the portions of the
brain which supply the antennae, thus indicating that these limbs
belong to the same series as those behind the mouth. That is
probably also true of the antennules, and the fact that the
antennules and antennae are innervated from the supraoesophageal
gangha must be connected with the position of the mouth, which,
as a result of cephahsation (p. 193) to a high degree is farther
back than in the earthworm, where it Hes in front of the first
somite. The alimentary canal is suppHed by two visceral nerves.
The first has a three-fold origin, being formed by the junction
of a nerve from the cerebral ganglion with two nerves which arise
each from a small ganghon on the course of the circumoesophageal
commissure. The second arises from the last abdominal ganghon.

SENSE ORGANS

The eyes of the crayfish are compound, containing a number
of elements, known as ommatidia, each of which is capable of
forming a separate image. The whole eye is black, owing to the
presence of pigment in some of its cells, and is covered with a
colourless portion of the cuticle known as the cornea, divided
into a number of square facets, each of which corresponds to an
ommatidium. The structure of an ommatidium is shown in Fig.
145. The inner ends of the visual cells are continued into fibres
which pass into an optic ganglion in the eyestalk, and from this
arises the optic nerve.

In strong light the pigment is spread through the cells so that
each ommatidium is isolated, and its corneal facet and refractive
bodies combine to form a small image of a portion of the field of
view. The separate images are combined to form a mosaic image,
which is necessarily erect, of the whole field. In weak hght the
pigment is retracted, and a single diffuse image is formed by the
whole eye. In prawns, and presumably in the crayfish, the move-
ment is controlled by hormones. When separate images are
formed a compound eye presumably gives very accurate directional
vision, especially to a small point of light. It is this which helps
the fixation by which the moth flies towards the candle, turning
always so that the same ommatidium is brightly illuminated.

The statocysts (Fig. 146) are pair of sacs, situated in the basal

214

THE CRAYFISH. PHYLUM ARTHROPODA

joints of the antennules and provided with nerves. Each has a
cuticular lining beset with hairs, with which the nerve fibres

'^-^''^- c2r/.

B

omm.

f^P-g-

-mus.

op.n.

cnc—

P9—

pr---

D

)r,tt-

iy^rh.

n. f.

Fig. 145. — The eye of the crayfish.

lA, The left eye removed ; B, a portion of the cornea magnified to show the facets ; C, a longitudinal section
of the eye under low magnification ; D, a single omraatidium highly magnified. — D after Parker.

cr.c, Outer refractive body or crystalline cone ; cu.f., cuticular facet ; epid., epidermis (hypodermis) ;
mus., muscles which move the eye ; n.f., nerve fibres ; omm., ommatidia ; op.g.. optic ganglion ;
op.n., optic nerve ; p.g., outer pigment cells ; p.g'., inner pigment cells ; ret., retinula cells (the sense
cells) — these cells contain pigment ; rh., inner refractive body or rhabdome secreted by the retinulae ;
vt., vitrellx or cells which secrete the crystaUine cone.

are in communication. Within it are grains of sand, which are

scattered over the opening of the sac by the pincers and fall into

it. It is probable that the principal function of the organ is inform-

the animal of its position by the movements of the sand grains

SENSE ORGANS

215

against the hairs, and thus enabUng it to keep its equiUbrium. If
the statocysts be removed, the crayfish loses its sense of position
and will often swim upside down. Presumably the sand grains
falling to the bottom of the sac by gravity stimulate nerve-

,ex.

8

9rn.

Fig. 146. — The statocyst of the crayfish.

A , The right antermule, seen from the median side with the basal joint opened and the flagella cut short ;
B, basal joint of the left antennule from above ; C, two hairs from the statocyst. — C partly after
Howes.

en., Inner flagellum ; ex., outer flagellum ; grn., sand grains ; n., nerve of the statocyst ; n.f., nerve fibres ;
0., opening of the statocyst ; stc. statocyst.

endings, so that the necessary muscular movements for the
maintenance of position are produced. It has been possible to
replace the sand grains in the statocysts of a prawn by iron
filings, and the animal can then be induced, by a magnet held
above it, to turn over on its back. The antennules bear on their
outer flagella bristles which are sensitive to chemical substances.
Various of the setae, especially those of the antennae, are organs
of touch.

2l6

THE CRAYFISH. PHLYUM ARTHROPODA

REPRODUCTION

The sexes of the crayfish are separate. The generative organs
He in the thorax, above the gut and below the pericardium. They

Pig 1^7, — The reproductive organs of a female crayfish. — After Suckow.

od Oviduct ; ov., ovaries ; ov'., fused posterior part (median lobe) ; vu., female aperture on the second
"' ' walking leg (p^).

have the same general shape in the two sexes, consisting of three
lobes, two anterior and one posterior, with a pair of ducts, which

start from the junction of the anterior
and posterior lobes and run to openings
on walking legs. The ovary (Fig. 147)
is larger and broader than the testis,
and has an internal cavity into which
the eggs are shed. The oviducts are short,
straight, and wide ; they open upon the
coxopodites of the second pair of walk-
ing legs. The testes (Fig. 148) consist of
a number of branching ducts which end
in small alveoli, in which the sperma-
tozoa are formed. The vasa deferentia
are narrow and much coiled ; their
first part is very slender and translucent,
the second part, which forms most of
the duct, is wider and glandular, and
a short terminal region has muscular
walls which force out the sperm. The
spermatozoa (Fig. 149) are relatively large discs about 15 microns
in diameter, with stiff, pointed processes round the edge. The
nurlf^iis is a round capsule and to one side of this is a small,

Fig. 148. — The reproductive
organs of a male crayfish.
— After Huxley.

t. Testes ; vd., vas deferens ; vd', open-
ing of vas deferens on last walking
leg.

REPRODUCTION

217

oval body. Pairing takes place in September and October. The
male seizes the female, throws her upon her back, and passes
semen through the tubular limbs of his first abdominal segment
on to the parts in the neighbourhood of her oviducts, the limbs
of his second abdominal pair aiding the process by working
to and fro on the hollows of the first. The semen consists of a
sticky substance, secreted by the vasa deferentia, carrying the

■ B

Fig. 149. — Spermatozoa of a crayfish.

A, Whole spermatozoon from above ; B, part, enlarged, in section.
cps., capsule ; pr., stiff processes.

spermatozoa, and forms white masses on the sterna of the female.
The eggs, which are large and yolky, are laid in November. The
processes of the spermatozoa adhere to them, and by a sudden
expansion of the contents of the capsule the rest of the body is
forced into the ovum. Each egg is attached to one of the hairs
on the abdominal Hmbs by a stalked shell formed of a substance
secreted by certain glands on the sterna, and is thus under the
protection of the mother during its development. By the division
of the nucleus of the fertilised ovum a syncytium is formed
which does not divide into cells until a number of nuclei have
arisen. The young are hatched at the beginning of the next
M.z. — 8

2l8

THE

CRAYFISH. PHYLUM ARTHROPODA

summer. They do not differ greatly from the adult but have
curved tips to the pincers, by which they chng for a time to the
empty shell or the abdominal Umbs of the mother (Fig. 150),
and are thus protected from enemies and kept from being swept
away by currents and so eventually reaching the sea, where they
would perish.

Fig 150.— a, Two recently hatched crayfish holding on to one of the swimmerets
of the mother ; B, pincers of the young more highly magnified.— From Huxley.

e.c. Ruptured egg cases ; en., endopodite ; ex., exopodite ; pr., protopodite.

ARTHROPODA

The phylum Arthropoda, to which the crayfish belongs, is
not easy to characterise. Its members resemble the Annelida
in being bilaterally symmetrical triploblastic segmented Metazoa,
in the general plan of the nervous system, and in the possession
of coelomoducts. They differ in the great reduction of the coelom
and parallel development of a perivisceral hsemocoele, in the
absence of nephridia, and in the absence (except for Peripatus)
of cilia. As positive features they possess paired jointed limbs.

CLASSIFICATION OF ARTHROPODS 219

of which at least one pair serves as jaws, and a continuous
thickened chitinous cuticle. The classification of the phylum is
also difficult, on account of the vast number of species which
it includes. It is convenient, in order to get manageable units
at the lower end, to begin by a division into subphyla, although
these are probably more nearly the equivalent of classes in other
groups.

SUBPHYLUM I— ONYCHOPHORA

There are two genera, Peripatus (Fig. 151) and Peripatopsis.
They are aberrant forms, possibly not strictly arthropods at all,
possessing tracheae (p. 234) and cilia, and without either thick
cuticle or jointed limbs.

Fig. 151. — Peripatus capensis, slightly enlarged. — From Borradaile, after
Sedgwick. Borradaile and Potts, The Invertebrata, 2nd edition, 1935.
Cambridge University Press.

SUBPHYLUM II— TRILOBITA

These are extinct forms, from the Palaeozoic era (p. 712),
which bear some resemblance to the next subphylum.

SUBPHYLUM III— CRUSTACEA

These are typical arthropods, nearly all aquatic in habit, so
that respiratory organs, if present at all, are usually gills. There
are two pairs of antennae, on the second and third somites, and
on the fourth somite there is a pair of mandibles. The limbs, of
which there is usually a fairly complete series, are reducible to
a type with two rami. There are not more than two pairs of
coelomoducts, both in the head.

The typical larva is the characteristic nauplius, unsegmented,
but with three pairs of appendages and a median eye (Fig. 152).
The nauplius is however often absent or modified, while there
may be other quite different types of larvae.

220

Fig. 152. — Development of Sac-
culina. — After Delage. (Not
drawn to scale.)

A, Free-swimming nauplius, with three
pairs of appendages ; B, pupa stage ; C,
adult protruding from the abdomen of a
crab.

Fig. 153. — Daphnia,

Eye; A'^, second antenna; A^, first antenna; dg.,
digestive caeca ; s.g.^ shell gland ; go., gonad ; h., heart
in pericardium ; 0., ovum ; B.p., brood-pouch ; sp.,
spine ; /., furca ; s., setae ; Ab., rudimentary abdomen ;
t., caudal fork ;g., gut ; 1-5, thoracic limbs.

Fig. 154. — Cypris, side view,
after removal of one valve.
— -After Zenker.

e.. Eye \ Ai, first antennae ; A2, second
antenna ; MN, mandibles ; mxi,
first maxilla ; mx2, second maxilla;
/i, /2, thoracic legs ; Ab, rudi-
mentary abdomen.

SUBPHYLUM CRUSTACEA

221

The crayfish, which has already been described as a type,
belongs to the most highly developed class, the Malacostraca,
the most prominent features of which are the stalked compound
eyes and the large carapace covering the thorax. This class
includes the lobsters, crabs, prawns, and shrimps, and also the
best-known terrestrial Crustacea, the woodlice of the Order
Isopoda. Other classes are the Branchiopoda, characterised

Fig. 155. — Cyclops.

abA, First abdominal segment ; a^.i, antennule ; at 2, antenna ; c.f., caudal fork ; cph., cephalothorax (fused
head and first two thoracic segments) ; e.s., egg sac ; eye (single and median) ; g., ahmentary canal ;
gen.op., genital opening ; t., telson ; th.^, thfi, third and sixth thoracic segments.

In comparing this crustacean with the crayfish, note the absence of proventriculus, paired eyes,
vuropods, and carapace, the presence of median eye and caudal fork, and the difference in the
number of segments.

by fiat abdominal appendages (phyllopodia) , e.g. Daphnia,
the water flea ; the Ostracoda, with a bivalve carapace, e.g. the
freshwater Cypris ; the Copepoda, with no abdominal appendages,
which include the free-living freshwater Cyclops, and a number
of interesting parasites such as Argulus, the carp louse ; and the
Cirripedia, which are sedentary when adult and include the
barnacles such as Lepas and Balanus, and extreme parasites
such as Sacculina, which, living partly attached to the underside
of the intestine of a crab, and partly beneath its abdomen,
sends suckers which penetrate throughout the body of its host.

222 PHYLUM ARTHROPODA

The Cirripedia are scarcely recognisable as Crustacea apart
from their larval forms, which are more or less normal.

SUBPHYLUM IV— MYRIAPODA

These are terrestrial, and possess tracheae (p. 234) ; there is a
head with one pair of antennae, and many segments and legs.
They include two classes, which are not very closely related ;
the Chilopoda or centipedes, which are carnivorous, with a
flattened body and one pair of legs on each segment, and the
Diplopoda or millepedes, which are herbivorous with a cylindrical
body and two pairs of legs on each apparent segment, which
has been formed by the fusion of two somites.

SUBPHYLUM V— INSECTA
This is discussed more fully below (p. 240).

SUBPHYLUM VI— ARACHNIDA

The body is divided into a prosoma of six somites, with
appendages of which four pairs are legs and none is an antenna
or mandible, and an opisthosoma of thirteen or fewer somites
which are often without appendages. The eighth segment bears
the genital opening, and the anterior part of the opisthosoma
bears respiratory openings of various types, including sometimes
the openings of tracheae. Traces of coelomoducts are present,
sometimes functional as excretory organs or genital ducts, as
in the scorpions. The development is nearly always direct.

Most modern arachnids are terrestrial, and the best-known
forms in Britain are the class Araneida or spiders (Fig. 156). Here
the opisthosoma is large and sharply marked off from the prosoma
by a waist ; it has lost its segmentation and bears spinnerets,
which are transformed appendages, for guiding the silk which
is prepared by various glands. The first appendages are sub-
chelate chelicerae, and the second or pedipalps are modified in the
male to take up the semen which he has discharged on to
a leaf and convey it into the genital opening of the female.
Respiration is by lung-books, vascular plates situated in pockets
on the opisthosoma, and tracheae (Fig. 157). Apart from their
web-spinning, the most interesting thing about the spiders is

SUBPHYLUM ARACHNIDA

223

their strongly proteolytic external digestion ; all but the chitin
of a fly is dissolved by saliva which is injected into its body,
and only liquid is taken into the spider's mouth.

UNGUIS
PAT U RON

TARSUS
TIBIA
PATELLA

FEMUR

TROCHANTER
COXA

PEDICLE

LABIUM
MAXILLA

FEMUR
PATELLA
TIBIA

■ -METATARSUS

— TARSUS
CLAWS

LUNG

EPICYNUM

SPINNERETTES

Fig. 156. — Dorsal (A) and ventral (B) surfaces of spider. The ' maxilla ', or
gnathobase of the pedipalp, is not homologous with the maxilla of other
arthropods. — From Savory, British Spiders, 1926. Oxford.

Other arachnid classes are the Scor-
pionidea or scorpions, with the
opisthosoma divided into meso- and
meta-soma ; the Acarina, or mites
and ticks, and the Phalangida or
harvestmen, with greatly elongated
legs. Many of the Acarina are blood-
sucking ectoparasites of man or his
domestic animals, important because
of the diseases which they carry ;
the ticks Ornithodorus mouhata on
man and Boophilus annulatus on cattle
carry relapsing fever and Texas fever
respectively, both diseases being

::itrT

vs

Fig. 157. — A diagram of a
vertical, longitudinal sec-
tion through a lung-book.

U.S., Air space; /., anterior end;
h., hinder end ; //., ' leaves ' of
book in which the blood flows ;
o., opening, on v.s., ventral surface
of body.

224

PHYLUM ARTHROPODA

caused by spirochaetes. The mites Demodex folliculorum (Fig. 159)
which lives in the sebaceous glands, and Sarcoptes scahiei
(Fig. 160), causing the itch, are ectoparasites of man which may
produce nothing more than irritation.

Fig. 158. — The sheep tick {Ixodes ricinus) female, ventral surface on left, dorsal

on right.

A , anus ; G, genital aperture ; H, hypostome ; L.I, L.IV, first and fourth legs ; P, palp ; R, rostrum

SH, oval shield ; ST, stigma.— After Wheler.

'-^v^-'

MM

..' I.

■"■/

• iV::V

mm

».:--.«..:^
^--'-.^

t--. ■ yi

V. :.-.•>

"«";.:;■•'

Fig. i59._The Follicle
Mite (Demodex follicu-
lorum), in ventral view.
— From Thomson.

Fig. 160.— The Itch Mite (Sarcoptes
scahiei), in dorsal view. The two
hinder pairs of legs are hidden
under the body. — From Thomson.

16
COCKROACHES

The first cockroaches were brought to England, perhaps from
the East, in the sixteenth centur}^ ; at first they spread slowly,
but later became ubiquitous ; they were of the species Blatta
orientalis, now known as the common cockroach. In the nineteenth
century two other species, Blattella germanica, the ' German '
cockroach (not a native of the country
from which it takes its name), and Peri-
planeta americana, the American cock-
roach, perhaps from tropical America,
were introduced. The first two are found in
houses ; the third generally only in the
larger buildings, such as warehouses and
hotels ; P. australasice also occurs, chiefly
in glasshouses. The use of DDT as an
insecticide has now removed cockroaches
from many buildings, and they are difficult
to obtain. All cockroaches are nocturnal,
and if a light is suddenly switched on
they run back to shelter ; occasionally
one will freeze instead, and is easily
caught. They prefer warm places, and are
omnivorous, eating not only human food,
but paper, hair, and leather. They find
their food by smell.

w.g..^

aJb.^

~ah. 7

, — can.

'0&. 10

Fig. i6i. — A female of
the common cockroach.
The body is somewhat
compressed so as to
show the membranes
between the abdominal
terga. The legs have
been removed.

The simplest way of telling the adults a6.i-a*.io, Abdominal terga, a/.,
. , , ' • -i •x^ • antenna ; can., anal cerci ;

of the three common species is by tneir h., head; th.i, prothoradc

^ ,, . . 7->7 // • • t, 1 tergum ; th.2, mesothoracic

wmgs and their size. Biatta is an men long, tergum; th.^, metathoracic

T^, 77 i-,,i ,^ ^ ^£ • i_ J tergum; w.g., vestige of fore

Blatella a little more than half an inch, and wing.

Periplaneta an inch and a half. The wings

of Blatella and Periplaneta (which are not Hkely to be confused)

cover the abdomen ; those of the male Blatta leave three or

four segments exposed, and those of the female are vestigial.

In the general structure they are ahke.

2Z-

226

COCKROACHES. PHYLUM ARTHROPODA

EXTERNAL FEATURES

In its main lines the anatomy of a cockroach resembles that
of a crayfish. The animal is segmented, the segments (somites)
being nnlike and grouped into three regions known as head,
thorax, and abdomen, but these do not correspond with the
parts similarly named in the crayfish. There is a thick cuticle,
not moulted by the adult, and some somites bear jointed limbs.
The thorax bears also two pairs of wings. At the sides of the head

Fig. 162. — Insect cuticle. — From Wigglesworth, Biol. Rev., 1948. 23, 408.

A , ideal section of the integument ; B, schematic section of the epicuticle.

a, endocuticle ; b, exocuticle ; c, epicuticle ; d, bristle ; e, pore canals ; /, duct of dermal gland ; g, basement

membrane ; h, epidermal (hypodermal) cell ; i, k, and I, special cells of epidermis ; m, blood cell ;

n, dermal gland ; o, cement layer ; p, wax layer ; q, polyphenol layer ; r, cuticuUn layer ; 5, pore canal.

lie a pair of large, unstalked, compound eyes. The coelom, of
which traces are found in development, disappears in the adult,
but there is a hsemocoelic perivisceral cavity containing blood.

The insect cuticle is much more than a mere covering of
chitin. There are three layers (Fig. 162). The innermost and
thickest is the endocuticle, which is made of chitin and a protein
called arthropodin, intimately associated and perhaps chemically
combined. Outside this is the exocuticle ; it has basically the same
structure as the endocuticle, but the protein has been tanned,
a process which involves both oxidation and the introduction
of aromatic groups to the protein molecule. The result is a

EXTERNAL FEATURES 227

substance called sclerotin, which is much harder and less
permeable to water than arthropodin, its predecessor. It is
sclerotin which has made possible the rigid arthropod exo-
skeleton, and so, as a special development, insect flight. Outside
the exocuticle is a very thin layer, a few microns only, called the
epicuticle. It has three main layers but contains no chitin ;
on the inside a tanned protein called cuticulin, then wax, and on
the outside a protective cement which is probably also a tanned
protein. Almost all the waterproofing of the insect cuticle is done
by the waxy layer of the epicuticle, yet it is never more than a
fraction of a micron in thickness. The total thickness of the cuticle
in the cockroach is about 40 microns ; in many other insects it is
much less, and in some, such as the larva of the water beetle
Dytisciis, much more.

The whole of the cuticle is secreted by the cells of the epidermis
(hypodermis) ; cuticulin first, then the exocuticle, and then the
endocuticle. The endo- and exocuticle, and probably the cuticulin
of the epicuticle, are at first traversed by vertical pore canals,
which contain cytoplasm. When the cuticulin and exocuticle have
been formed, a somewhat temporary layer, the polyphenol layer,
is spread over the surface, and the waxy layer is next spread over
this ; presumably both these come through the pore canals.
At this point the old skin is shed in ecdysis. The cement layer is
almost immediately spread over the surface from the dermal
glands, and the hardening of the exocuticle takes place. The endo-
cuticle is continuous with the exocuticle, but is laid down after
ecdysis. While the first part of the new cuticle is being secreted,
and before ecdysis, a liquid called moulting fluid appears between
the new epicuticle and the old endocuticle. It contains a protease
and probably a chitinase, and gradually dissolves the old endo-
cuticle, the products being absorbed, through the new cuticle,
into the epidermal cells. The exocuticle and epicuticle are not
attacked, but as there are lines where no exocuticle has been
laid down, the cuticle here becomes reduced to the epicuticle,
and so is very thin. It is along these lines that it splits in ecdysis,
so that the insect can crawl out of its own skin.

In some insects the pore canals retain their cytoplasm through-
out life, and the cuticle is therefore living ; in others they become
occluded. The various stages of the formation of cuticle,
and so the ecdysis, are controlled by hormones secreted by
structures called corpora allata, situated just behind the brain.

228 COCKROACHES. PHYLUM ARTHROPODA

There is some evidence that the secretion of the hormone
is itself determined by events in the outer world, acting via

the sense organs and nervous

t!cr. sut.
/\ '.' J'en.

system.

'uiL

\--et/^

THE HEAD

mil. '''A.^

^

_7X-"^^^

^-

^JA U mx.'p.

lb

m.

lb. p.

Fig. 163. — The head of a cockroach
seen from in front.

at.. Antenna; dp., clypeus ; ecr., epicranium;
eye ; fen., fenestra ; fr., frons ; gen.,
gena ; Ibm., part of the labium ; lb. p.,
labial palp ; Ibr., labrum ; md., mandible,
mx., part of the maxilla; mx.p., maxillary
palp ; sui., sutures.

and are shown in Fig. 164

The head (Fig. 163) is short
and deep. Seen from in front it
has a pear-shaped outHne, with
the narrow end downwards. Its
armour consists of several pieces —
two epicranial plates side by side
above, two genae at the sides
below the eyes, a frons and clypeus
in front. A labrum is hinged on
to the clypeus below ; its lining
is known as the epipharjmx. The
appendages of the head are paired,
and in Table III.

TABLE III
Head Appendages of a Cockroach

Segment
I

Appendages
None

Principal Features

2

Antennae

Long, unbranched, many-jointed.

3

None

4

Mandibles

Strong, toothed ; no palps.

.5

Maxillae

Two lobes, and a long palp.

6

Labium

Basal portion of the appendages

fused ;

attached to this on each side

are two

lobes and a palp.

THORAX

The head is joined by a soft neck to the thorax. This consists
of three segments — the prothorax, mesothorax, and metathorax.
Each has a tergum or notum above and a sternum below, joined
to one another at the sides by membrane in which lie small
sclerites — the pleura — which are really basal podomeres of the
legs. The pronotum is the largest and projects in front so as to
hide the neck. Each sternum bears a pair of legs. The shape of

THORAX

229

these legs and the names of their podomeres are shown in Fig. 165.
The mesothorax and metathorax bear each a pair of wings jointed
to the anterior corners of the notum. The wings are membranous
folds of the skin, in which the epidermis has practically disappeared
and the two layers of cuticle have come together. Branched
ridges known as ' veins ' or nervures strengthen the wings. The
veins are hollow and each contains a trachea (p. 234) and a nerve.

Fig. 164. — The mouth-parts of a cockroach. — From Imms.

I, Mandibles; ab.m., ad.m., abductor and adductor muscles; 2. maxilla; c, cardo; g., galea; /.,
lacinia ; mx.p., maxillary palp ; s., stipes ; 3, labium ; gl., glossa ; l.p., labial palp ; w., men turn ;
pg.,. paraglossa ; pgr., palpiger ; pm., prementum ; sm., submentum ; 4, hypopharynx ; sL, left
vestigial superlingua.

The first pair of wings are dark-coloured and tough (the tegmina)
and form a cover for the second, which, when they are at rest,
are folded lengthwise and laid along the back. In the female of
B. orientalis the wings are very small. \Mngs are not appendages
of the same kind as the Umbs, but movable expansions of the terga.

ABDOMEN

The abdomen consists of ten somites, each with a tergum and
a sternum, joined at the sides by soft cuticle. The hinder somites
are telescoped, so that the eighth and ninth are hidden, except

230

COCKROACHES. PHYLUM ARTHROPODA

that in the male B. orientalis, portions of the terga remain un-
covered. The first sternum is rudimentary, and the tenth tergum
projects backwards as a plate with a deep notch in its hinder
edge. A pair of many-jointed, spindle-shaped anal cerci, which
may represent limbs, are attached under this plate, and below
it is the anus, between two podical plates or paraprocts, which
may represent the sternum of an eleventh somite. In the female
the seventh sternum is produced backwards into a large boat-
shaped process, which forms the floor of a genital pouch, and in
the male the ninth sternum bears a pair of limbs in the form of

15

14

Fig. 165. — A male of the Common Cockroach (P. orientalis) in side view. — From

Shipley and MacBride.

I, Antenna ; 2, head ; 3, prothorax ; 4, fore wing ; 5, soft skin between terga and sterna ; 6, sixth abdominal
tergum ; 7, split portion of tenth abdominal tergum ; 8, anal cerci ; 9, styles ; 10, coxa of third leg ;
II, trochanter ; 12, femur ; 13, tibia ; 14, tarsus ; 15, claws.

slender, unjointed styles. The genital opening is placed below
the anus and is surrounded by a complicated set of processes
known as gonapophyses. A pair of stink glands, deterrent to most
enemies, opens on the membrane between the fifth and sixth terga.
Segments one to eight are limbless.

LOCOMOTION

While the insect is walking, three legs are in contact with the
ground at one time. On one side the first leg pulls and the third
pushes while on the opposite side the second leg acts as a prop.
Meanwhile the other three legs are being moved forwards to
repeat the process.

In flight, the hind wings beat in a complicated figure which
both supports the body and drives it forwards. They are moved
by two sets of muscles — an indirect set, consisting of vertical
and longitudinal muscles of the thorax which by alternately

LOCOMOTION

231

lowering and raising the tergum, to which the wings are attached,
lever the wings up and down upon the side plates (pleura) upon
which they rest (Fig. 167), and a direct set attached to the base
of each wing, which they can both rotate upon its axis and
extend from the body or retract. The hind-wings are beaten
down and up, and at each downstroke the strong front edge
(costa) is by muscular action rotated downwards and forwards
so that the somewhat concave lower surface faces obliquely
downwards and backwards. This process is assisted by the

Fig. 166. — The ventral aspect of a male American cockroach with the wings
extended. An imaginary median line has been inserted. — From Thomson.

A, Antennae ; C, cercus ; Co, coxa, the breadth of which makes it look, in its present position, like a
ventral plate on the body ; E, eye ; F, femur ; P.T, prothorax ; St, style ; Ta, tarsus ; Ti, tibia ; Tr,
trochanter; W^, first pair of wings ; W^, second pair of wings.

resistance of the air below bending the thin hinder part of the
wing upward. As a result, during the beat the wing exerts pressure
both downwards and backwards while a region of decreased
pressure is created above and in front of it. Thus the insect is
pressed and drawn upwards and forwards. The fore wings are held
at right angles to the body, but do not beat.

ALIMENTARY SYSTEM

The alimentary canal (Fig. 168) has long, ectodermal fore- and
hind-guts, lined with cuticle as in the crayfish. The fore-gut
comprises (i) the mouth, with a tongue-like ridge (hypopharynx)
which bears on its under surface the duct of the salivary glands

232

COCKROACHES. PHYLUM ARTHROPODA

and at its sides a pair of minute structures, the superlinguse,
which may represent the paragnatha of the crayfish (p. 204) ;
(ii) the narrow gullet, lying in the neck ; (iii) the swollen crop ;
(iv) the proventriculus or gizzard, which has muscular walls

F"iG. 167. — A diagram to show how the wings of an insect are lowered and raised

in flight.

.4 , The downstroke : the tergum (t) is raised, owing to being arched fore and aft by the contraction of
the longitudinal muscles (l.m.) ; this forces the wing {w.) down, pivoting over a point on the pleuron
(pL). B, the upstroke : the tergum is lowered by contraction of the dorso- ventral muscles (dv.m.) ; this
levers the wing up.

and contains six hard, cuticular teeth and some pads covered with
bristles which form a strainer. Two diffuse labial or salivary
glands lie on each side of the crop, and between each pair lies a

^' ling.

mx.n.

Fig. 168. — A semi-diagrammatic drawing of the head and thorax of a cockroach,

dissected from the left side.

cer.. Cerebral ganglion; cr., crop; fr.g.. frontal ganglion giz., gizzard hp.c. hepatic caeca l.v.n., left
visceral nerve leaving the brain ; Ibm., labium ; ling., hypopharynx ; Ibr., labrum ; m.g., mesenteron,
md., mandible ; mx.n., maxillary nerve ; nk., neck ; oes., oesophagus ; s.as., suboesophageal ganglion ;
sal.g., sahvary gland ; sal.r., sahvary receptacle ; th.i, th.2, th.^, segments of the thorax ; v.g., visceral
ganglion ; v.n., visceral nerve.

salivary bladder or receptacle. The ducts of the two glands of
each side join ; the common ducts of the two sides then unite to
form a median tube, and this is joined by another median tube
formed by the union of the ducts of the receptacles. The final
opening is on the hypopharynx. The mid-gut or mesenteron,
lined by soft endoderm, is short and narrow and bears at its

ALIMENTARY SYSTEM

233

beginning seven or eight club-shaped pyloric caeca. The gizzard
projects funnel-wise into the mid-gut. The hind-gut is coiled
and divided into a narrow ileum, a wider colon, and a wide
rectum, which has six internal ridges. At the beginning of the
hind-gut are attached a number of long, fine Malpighian tubules
the epithelium of which is excretory.

DIGESTION AND EXCRETION

The food is cut up by the mandibles
and maxillae, moistened with saliva, and
pushed by maxillae and labium into the
mouth ; it is held up for a time in the
crop, where it is acted upon by the saliva,
which digests only starch, and by the
mid-gut secretion which leaks forward
and digests fat. Most of the crop digestion
is by yeasts and bacteria which are
subsequently themselves digested by
their host. The food is then admitted,
little by little, into the gizzard and there
broken up fine by the teeth and strained
by the bristles as it passes into the mid-
gut. The juice secreted here digests all
classes of food stuffs : it is secreted by
the break-up of epithelial cells, which are
replaced from reserve cells. The delicate,
uncuticlate epithelium is protected from hard particles not,
like that of backboned animals, by the secretion of mucus
(p. 509), but by a very delicate chitinous envelope, the peri-
trophic membrane, which is secreted by the epithehum but
adheres to it only around the entrance from the gizzard. This
membrane is permeable both to digestive enzymes and to digested
food. It is in the mid-gut that absorption mainly takes place.
The pyloric caeca are mere extensions of the mid-gut and do not
differ from it in function. In the hind-gut water is absorbed both
from the f^ces and from the urine excreted by the Malpighian
tubules. Nitrogen is excreted as uric acid. In most insects some
is got rid of by the Malpighian tubules and some laid up in the
fat body (see p. 235), but the cockroach appears not to eliminate
nitrogen in the urine.

Fig. 169. — The heart and
neighbouring structures
of a cockroach ; some-
what diagrammatic.

a.tn., Areas marked by dotted lines
to show the position of alary
muscles below the fatty body ;
f.b., fatty body ; ht., heart
tr., tracheae.

234

COCKROACHES. PHYLUM ARTHROPODA

RESPIRATORY ORGANS

The respiratory system consists of branching tubes or trachese
(Fie 1 69) of ectodermal origin with a spirally thickened
lining of 'cuticle, which arise from ten pairs of opemngs or
stigmata at the sides of the body. There are two large stigmata
on each side of the thorax, one between prothorax and meso-
thorax, one between mesothorax and metathorax, and m each
of the first eight abdominal somites a stigma is placed on each

r-fex,"^"-

^.p/)W.

cu.

Fig. 170.— A portion of the tracheal tissue of a cockroach, highly magnified.

Only parts of the tubes are in focus.

cu Cuticular lining with spiral thickening ; nu., nuclei of the protoplasmic layer; ppm., protoplasmic
' layer continuous with the epidermis (hypodermis) of the surface of the body.

side between the tergum and the sternum. Air is pumped in and
out of the larger tracheae by contraction and expansion of the
abdomen, and by diffusion renews the gases in the fine branches
of the tracheal system (tracheoles) , which have no cuticular
lining, ramify in the tissues, and end upon or actually in the cells.
(Figs. 170, 171).

When the insect is at rest the ends of the tracheoles are full
of fluid. When the muscles are active, products of their metabolism
raise the osmotic pressure in the tissues, and this withdraws the
fluid so that air extends more deeply into the tracheoles and

RESPIRATORY ORGANS

235

reaches their cells (Fig. 171). While some carbon dioxide is lost
through the tracheal system most diffuses directly through the
skin.

BLOOD VESSELS

The direct supply of air to the tissues is no doubt the reason
for the simple condition of the blood-vascular system, which
consists of a long heart (Fig. 169), lying along the mid-dorsal
line of the abdomen and thorax, an anterior aorta, and a system
of ill-defined sinuses, of which the principal is the perivisceral
cavity. The heart is enclosed in a pericardial space and is divided

Fig. 171. — Tracheoles running to a muscle fibre. — From Imms, after Wigglesworth.

A, Muscle at rest ; the terminal parts of the tracheoles (shown dotted) contain fluid ; B, muscle fatigued;

air extends far into the tracheoles.

into thirteen chambers corresponding to the segments. Each
chamber communicates by a pair of ostia at its sides with the
pericardial space. Blood from outlying parts of the body flows
to the perivisceral cavity, thence into the pericardial cavity
through openings in the floor of the latter, and so through the
ostia into the heart, which, contracting from behind forwards,
drives it through the aorta into the sinus system, by way of the
sinuses of the head. Paired triangular alary muscles, whose outer
ends are attached to the terga, move the pericardial floor, and
thus cause the flow of blood from the perivisceral cavity into the
pericardial. Both these cavities are hsemoccelic (p. 189). They
contain a white tissue known as the fat body (Fig. 172), some of
whose cells hold reserves of fats, carbohydrate, and proteins,
others at least temporarily retain nitrogenous excreta as ucic
acid, and others harbour micro-organisms (bacteroids) which are

236

COCKROACHES. PHYLUM ARTHROPODA

probably in some sort of symbiotic relationship with the insect.
The blood resembles that of the crayfish but, as might be expected
in view of the mode of respiration, contains no respiratory
pigment.

NERVOUS SYSTEM AND SENSE ORGANS

The nervous system (Fig. 173) is on the same general plan as
that of the crayfish. It comprises a pair of supra-oesophageal
ganglia, which receive optic and antennar}^ nerves, a pair of
short, wide circumoesophageal commissures, a suboesophageal

ganglion, and a double ventral cord
with a ganglion in each of the first
nine segments behind the head. In many
insects, especially the Hymenoptera
(p. 260), the supra-oesophageal ganglia,
which represent three pairs of ganglia
fused together, are highly developed and
deserve the name brain. The alimentary
'/-y canal is supplied by a visceral nervous
system which receives nerves from the
circumoesophageal commissures and
the brain. Its principal ganglion lies
on the upper side of the crop. The
sense organs include the large compound
eyes, which resemble those of the
crayfish in structure, the antennae, which
are tactile and olfactory, the labial and
maxillary palps, which are gustatory and olfactory, and anal
cerci, which are tactile and also sensitive to sound vibrations,
various sensory bristles, and possibly a pair of oval white patches
which are found above the bases of the antennae and are known
as the fenestrae.

ORGANS OF REPRODUCTION

The sexes are separate. The testes (Fig. 174) are small, paired
organs, embedded in the fat body below the fifth and sixth
abdominal terga. In the adult the testes are no longer functional.
Two vasa deferentia lead backwards and downwards from them
to the seminal vesicles, which are beset with short finger-like
processes and lie side by side to form the so-called mushroom-

Fig. 172. — A section
through a lobe of the
fat body of a cock-
roach X 650. — From
Imms.

e. Excretory cell with concre-
tions ; /., fat cell ; m.. cell
containing bactetia.

ORGANS OF REPRODUCTION

237

shaped gland. The seminal vesicles join behind to form a mus-
cular tube, the ejaculatory duct, which opens by a median pore
between the ninth and tenth abdominal sterna. A gland of

Cerebral /.atrum .
gan^Ua.

Sate vary
receptacle.

Visceral
nerves

Hepatic
caeca

Mesenteron)
Ileum

MalpLghtan
Cubes.

Colon.'

Head.

Antenna.

SaUvary
gla.'^c/s.

Vent rat
nerve cord.

Ovasij.

Colic tenat
gland.

HecCum\

Fig. 173. — A female cockroach, dissected from above. -

and MacBride.

Oviduct.

sSpermathecat

Analcerci

Genital pouch.

-Adapted from Shipley

doubtful function, known as the conglobate gland, lies below
the ejaculatory duct and opens with it. The ovaries (Fig. 175)
are paired organs in the hinder part of the abdomen, each con-
sisting of eight tapering tubes, which show swelhngs correspond-
ing to ova. There is a single, short, wide oviduct which opens on

238

COCKROACHES. PHYLUM ARTHROPODA

the eighth abdominal sternum. On the ninth sternum a pair of
branched colleterial glands pour out by two openings a secretion
which forms the cases of the egg-capsules. There is an unequal

S 8 Terqa 7 6 f\

Ana/cerci

Rectum.

Testis.

(jonapophys ts.

Ninth Sternum
Ductus ejaculatonus

I/as deferens.

Mushroom Shaped
gland.

on gL abate gland.

Pig 174 A semi-diagrammatic view of the hinder part of the body of a male

P. americana dissected from the right side to show the generative organs.

96 7

Analcerci

Anus
Rectum.

I crga -

CoUtterLai
'gland.

Spermathecae

Gonapophysts

Sterna

Ovary.

Oviduct.

p-jG. 175.— A semi-diagrammatic view of the hinder part of the body of a
female P. americana dissected from the right side to show the generative
organs.

pair of spermathecae, which open between the eighth and ninth
abdominal sterna and store spermatozoa received from a male
in copulation. The eggs are produced alternately by the two
ovaries, and as they pass down the oviduct are enclosed in a single
capsule or ootheca. As more eggs are formed this protrudes from

ORGANS OF REPRODUCTION 239

the genital opening, and is finally deposited. One female lays
several capsules, and breeding goes on throughout the year.
The number of eggs in a capsule varies ; in Blatta orientalis it is
about 16 ; in Blatella germanica about 40 ; and in Periplaneta
americana about 20. Parthenogenesis occasionally occurs. When
the eggs hatch the capsule splits and the young emerge. The time
taken to reach sexual maturity varies with the species, and
depends much on external factors such as food and temperature ;
an average time from e.^^ to imago in the common cockroach is
a year or a little less. This species has seven moults, but in no
cockroach is there a true metamorphosis (p. 247).

17
INSECTS

The number of different species of insect is enormous, probably
not less than 500,000 and equal to that of all other animals put
together. All insects resemble the cockroach in the main features
of their anatomy, but many of them depart widely from it in
detail, the differences affecting principally the mouth-parts, the
wings and the hfe-history.

MOUTH-PARTS

The full set of mouth-parts consists of three pairs of appendages
and three median structures. Segment four bears mandibles,
segment five maxillae, and segment six the labium. The median
structures above the mouth are the labrum, which is the most
anterior dorsal chitinous plate of the head and is capable of
some movement, and the epipharynx, which is the membranous
roof of the buccal cavity, often produced outwards and associated
with the labrum. Below the mouth is the hypopharynx or lingua,
which is a median process from the floor of the buccal cavity,
and bears the opening of the common salivary duct. The cock-
roach mouth-parts, which have already been described, have all
these parts, and their only important development is the fusion
of the basal portions of the labium to form the mentum, which
is common to all existing insects which possess the appendage
at all. The submentum is probably derived from the wall of the
head. Mouth-parts like these, with strong crushing mandibles,
are called mandibulate, and are characteristic of the more
primitive orders of insects ; they are also found in some members
of more advanced groups, especially in the larval stages. Besides
the cockroach and its relatives, other important insects which
possess them are the dragonfiies, beetles, and sawfiies.

From this full and primitive set of mouth-parts adaptive radia-
tion (p. 474) has gone on in several directions to produce structures
suitable for different types of food. Often the same end is reached
by more than one means ; there are, for example, several different
modifications for sucking hquid food. On the whole, where the

mandibles are well developed the maxillae are primitive, and parts

240

MOUTH - PARTS

241

of the maxillae which are large have small counterparts in the
labium, and vice versa. One of the commonest types is that
suited for piercing and sucking, that is, for making a hole in the
epidermis of a plant or animal and then drinking the sap or

c

— -r

Fig. 176. — Diagram of the mouth-parts and adjacent region of the head of a
hemipterous insect. On the left are transverse sections (not all to the same
scale) at the levels shown by dotted hnes of the same lettering.— From Imms,
A General Textbook of Entomology, 3rd edition, i934- Methuen, London.

cl clypeus : ec, ejection canal with salivary- duct ; /, labnim ; w, mandibles ; mx, maxilla ■ p pharynx
pd, pharN-ngeal duct ; r, labium ; sd, salivar>' duct ; sc, suction canal with phar>-ngeal duct.

blood. In the aphis, which feeds on plant juices, the labium is
rolled to form an incomplete tube, in the hollow of which are
two pairs of stylets, which are the mandibles and maxillae
(Fig. 176). The two maxillae fit together in such a way as to make
two tubes, the smaller of which is used for conveying saliva
into the wound, and the larger for taking the food into the mouth.
The labium supports the stylets while they make a hole, often
deep into the plant tissues, in which process they are aided by

242

INSECTS. PHYLUM ARTHROPODA

nw.

Fig. 177. — Mouth-parts of Diptera. — From Borradaile and Potts, The Invertebrata,
2nd edition. 1935. Cambridge University Press. A-D after Patton and Cragg.

.-1, Culex pipiens i\ B, Glossina submorsitans ; C, transverse section through proboscis of Culex; D,

transverse section through proboscis of a muscid fly ; E, proboscis of a muscid fly, extended ; F, the

same, hulf folded.
an .antenna ; e., eye ;/.c., food channel ; hyp., hypopharynx ; Ibm., labium ; Ibl., labellum ; Ibr.ep., labrum

epipharynx ; md., mandible ; tnx., maxilla ; mxp., maxillary palp ; ph., pharynx ; ph.p., pharyngeal

pump ; pstra., pseudotracheae ; sd., salivary duct.

MOUTH-PARTS

243

the solvent action of the saUva. The sucking is done by the
pharyngeal muscles. The mouth-parts of the gnats, some of which
also feed on plant juices, and others on blood, are more complicated
(Fig. 177, A and C). The labium again supports the other parts,
and the puncture is made, as before, by the mandibles and maxillae

Fig. 178. — Head and proboscis of a moth. — -From Borradaile and Potts, The
Invertebrata, 2nd edition, 1935. Cambridge University Press. A and B after
Metcalf and Flint, C after Eltringham.

A, Front view ; B, side view ; C, transverse section of proboscis.

dp., clypeus ; dm., diagonal muscles ; e., eye ; ep., epipharynx ; gal., galea ; Ibr., labruin ; /./«., locking
hooks ; Ip., labial palp ; md., mandible ; mxp., maxillary palp ; »., nerve ; tra., trachea.

acting as stylets, but the food canal is made by the almost com-
pletely rolled-up labrum-epipharynx, and the salivary duct
runs down an elongated hypopharynx. The end of the labium is
expanded into a pair of soft lobes, the labella ; the labium and
the structures which it contains are collectively known as the
proboscis. Outside this is a pair of maxillary palps. Sucking is
carried out by the muscles of the pharynx. Mandibles are absent
from the male, and the maxillae are represented only by the palps ;
he is further distinguished by having longer bristles on the

244

INSECTS. PHYLUM ARTHROPODA

ant.niKc that, has the female. The houseflies and blowflies are
incapable of piercing, and can only suck at a surface which is
already liciuid, or one which can easily be made so. The general
p-ittorn of the mouth-parts is similar to that m the gnat, but the

an.

l'"iG. 179. — Head and mouth-parts of a honey bee. — From Borradaile and Potts,
The Invertebrata, 2nd edition, 1935. Cambridge University Press, after Cheshire.

an., antenna ; gal., galea ; gs., glossa ; Ibr., labrum ; Ip., labial palp ; md., mandible ; mxp., maxillary

palp ; oc, ocellus; pg., paraglossa.

mandibles and maxillae (except the palps) are missing, and the
labella are greatly enlarged and covered with grooves called
pseudotracheai, which act like a sponge (Fig. 177 F). Solid food is
liquefied by the regurgitation of fluid from the gut, and small teeth
on the labella can scrape the surface to assist the enzyme action.
The butterflies and moths also have mouth-parts suitable for
drinking only , their food being the nectar from flowers. The proboscis

MOUTH -PARTS 345

is formed of the interlocked galese of the maxillae, and is carried
at rest coiled up under the head. All the other parts are reduced
or absent except for a three-jointed labial palp (Fig. 178).

The bees have peculiar mouth-parts best known as licking and
sucking (Fig. 179) ; they collect pollen as well as feeding on
nectar, and also use their mouth-parts for building. The proboscis
is formed by the elongated and fused glossae of the labium, and
food is sucked up a groove on the dorsal surface of this. The well-
developed labial palp and galeae surround the glossa and probably
help to form the tube. The mandibles are used for building.

WINGS

Although insects are typically flying creatures, wings are
absent from the most primitive living orders, and have also been
lost by a number of other groups, particularly parasites such as
fleas, lice, and bed bugs, but also by some free-living forms.
The simplest condition is for both pairs of wings to be functional,
membranous, and alike. When both pairs are used they are
often locked together, so that they beat as one ; the bees have
a series of hooks, and the butterflies have interlocking bristles.
The beetles have the front pair of wings modified, even more
strongly than in the cockroach, to form hard wing-cases, known
as elytra or shards. In the butterflies and moths both pairs are
covered with small scales, which also clothe the rest of the body,
and come off as a powder when the insect is handled. The flies
in the strict sense (Diptera, p. 256) have lost the hind pair,
which are represented only by small knobs, the halteres (sing.,
halter) or balancers, which assist in maintaining the animal's
balance in flight.

LIFE-HISTORIES

The different orders of insects have various types of life-
history, according to their habitat and evolutionary level, but
it is very common for there to be a larva, by which should be
meant an immature form, living a life independent of the parents,
and possessing structures which are not present in the adult. Since
insects are arthropods they undergo the characteristic moults or
ecdyses which are necessary to allow the inelastic cuticle to be
replaced. The period between two ecdyses is a stadium, and the
form of a larva in a particular stadium is an instar. The last instar
which comes after the last ecdysis, is the adult or imago (pi..

2,(^ INSECTS. PHYLUM ARTHROPODA

imagines). The insects may be divided into a number of groups
according to their Hfe-histories ; these groups correspond only
partially to the subclasses of the formal classification.

rhe Ametabola are those which have no larva and no meta-
morphosis ; the form hatched from the egg is not mature, but
differs from the adult in hardly anything but the reproductive
system, which gradually develops. This group includes the
primitive wingless insects such as silverfish and springtails, and
a luimber of wingless members of other groups, such as the lice
and the workers of termites.

In the Paurometabola the young instars differ from the adults
not only in having undeveloped gonads but in not possessing
wings ; they have, however, no positive characters of their own,
and are not strictly larvae although often called so. They are
known as nymphs, which, since the word means, in origin,
maidens fit for marriage, is a somewhat inappropriate term for
sexually immature animals. Development consists in the growth
of the reproductive organs, and in the gradual formation of
wings. In this group comes the cockroach, and most of the orders
of insects which in the formal classification are called Exoptery-
gota. There is no metamorphosis, although entomologists often
speak as if there were.

The Hemimetabola are similar to the Paurometabola, but
differ in that the young stages live in water and have peculiar
features, suited to the habitat, which are absent from the adult ;
they are, therefore, true larvae. They are often called nymphs,
but a better term is naiads, which points out both their similarity
to and difference from the nymphs of the Paurometabola. (In
Greek mythology the naiades were water-nymphs.) There is
necessarily a metamorphosis, which takes place as the animal
emerges from the water. In the mayflies, for instance, the naiad
crawls up a plant stem into the air, and goes through an ecdysis
which produces a winged instar. After a short time a final moult
produces the imago. There is here foreshadowed the connection
of metamorphosis with moulting so notable in the higher insects.
The Hemimetabola include only the stoneflies, dragonflies, and
mayflies. The sole larval characters of the first are the gills, but
the naiads of the last two possess also mouth-parts different
from those of the adult.

The Paurometabola and Hemimetabola are usually grouped
together as Heterometabola, an assemblage of orders almost

LIFE-HISTORIES 247

equivalent to the subclass Exopterygota, but this connection ob-
scures the important formal difference between nymph and naiad.

The remaining insects are Holometabola. There is a true larva
in every sense of the word, and so a true metamorphosis. The
features in which the larva differs from the adult are its general
form ; the immaturity of its gonads ; the presence of internal
wing buds instead of wings ; the mouth-parts ; and the possession
of small lateral simple eyes or ocelli in place of compound eyes.
In addition aquatic larvae possess respiratory adaptations. The
Holometabola comprise all the typical members of the subclass
Endopterygota. There are several different types of larvae, such
as those which, possessing only thoracic legs, resemble the primi-
tive wingless insects and are called oligopod or campodeiform
(e.g. the water-beetle Dytiscus and other beetles), the caterpillars
of butterflies and others, which have abdominal legs as well as
thoracic, and are called polypod or cruciform, and the apodous
or limbless grubs or maggots of flies. In several insects more
than one type of larva is found in a single life-history, when
hypermetamorphosis is said to occur.

Metamorphosis is concentrated into a restricted stage of the
life-history, that of the penultimate instar, which is called a pupa.
In most Holometabola the wings and legs have no secondary
attachment to the body and may be capable of some movement ;
the pupa has a form roughly between that of the larva and the
adult, and is caUed exarate. In butterflies and moths, some
beetles, and most two-winged flies, the limbs, though just visible
externally, are closely glued down to the body by the moulting
fluid of the previous ecdysis, such a condition being called obtect.
In the houseflies and their relatives the pupa is coarctate, that is,
enclosed in the last larval skin, called the puparium, and no limbs
are visible. The typical pupa is inactive, quiescent, and incapable
of feeding. Inside its skin a great deal of breakdown of the body
goes on, largely by phagocytosis (p. 192), and the form of the
imago is built up by the division and growth of persistent em-
bryonic cell masses called imaginal discs. The gut, limbs, many
muscles and many tracheae are often completely replaced in this
way. The nervous system persists throughout metamorphosis,
although there may be much growth and alteration, especially of
the brain ; the heart beats throughout pupation and undergoes
only slight changes. The gonads grow throughout life, and in a
few species even become functional in the larva. Finally the pupal

g INSECTS. PHYLUM ARTHROPODA

skin splits and the imago emerges. Pupation, like ecdysis, seems
to be generally under the control of hormones. An imagmal
hormone, secreted in a part of the brain called the pars mter-
cerobralis, activates the thoracic gland (cells of the fat-body of
the pro- and meso-thorax) to produce another hormone which
stinuilates epidermal cell-division and the secretion of a new
cuticle ; it therefore induces moulting. In most insects the
thoracic glands atrophy in the adult and so moulting does not
continue. A third hormone, sometimes called the juvenility
factor, is secreted in the corpora allata, a part of the nervous
system near the heart. It stimulates the growth of the nymph,
but suppresses imaginal characters; metamorphosis can only
occur when production of the juvenility factor is reduced. Whether
this is gradual, or in two well-marked stages, determines the
absence or presence of a pupa.

The instar before the pupa is often slightly specialised, and
is known as the prepupa. The rudiments of the limbs and wings,
which in the larva are held in sacs of the body-wall, become
everted, and the animal's behaviour changes. It is the prepupa
of butterflies which seeks shelter and spins the cocoon or mat.

CLASSIFICATION

The classification of insects is based primarily on the presence
or absence of wings, on the way in which these develop, and on
the degree of metamorphosis. Some of the wingless insects,
however, resemble so closely those which possess wings that they
are generally considered to have lost wings which their ancestors
once possessed, and are grouped with the winged orders. It is
for this reason that the names of the groups, derived from the
condition of the wings, are sometimes unfortunate ; by no means
all wingless insects are Apterygota, although that word means
' creatures without wings '. It is very common now to use the
terms descriptive of the type of metamorphosis (Ametabola, etc.)
as alternative taxonomic proper names to those ending in
-pterygota, which refer to the wings. This introduces a second
set of illogicahties, and it is simplest to use the terms ending in
-metabola only as descriptions of the actual degree of meta-
morphosis undergone.

The insects are divided into two classes, or sometimes the
two subclasses of the second class are raised to the rank of classes.

CLASSIFICATION 249

CLASS I—APTERYGOTA

These are primitively wingless and the abdomen bears appen-
dages of various sorts in addition to the external genitalia and
cerci. There are four orders : (i) the Diplura, (2) the Thysanura
or bristle tails, (3) the Collembola or springtails, and (4) the
Protura. The first, second, and third have biting mouth-parts, the
fourth piercing. The commonest example is the silverfish, Lepisma
saccharina, a thysanuran, which is a frequent inhabitant of
cupboards, bookcases and cracks in the floorboards of houses,
and the largest is the shore-living Petrobius
maritimus (Fig. 180), also a thysanuran. \

CLASS II—PTERYGOTA

These are insects which possess wings, or,
if they do not, closely resemble in other
respects species which do.

sub-class I — exopterygota
The winefs develop outside the body. There ^^^- ^^o.—Petrobtus

c> ir J maritimus X i. —

are several orders mostly with biting mouth- From Bandars, An
parts. There is no larva in the strict sense ex- ^pocket^''''^ ^""^ ^^'
cept in those orders which have aquatic naiads.

Order 5. Orthoptera. This includes the cockroaches, crickets,
grasshoppers and stick insects, and its main features, such as
biting mouth-parts and hardened forewing, have been seen in
the previous chapter. Many of the jumping forms are notable
not only for their long legs but also for the way in which they
chirp or sing by rubbing wings or legs against some part of the
body, and for the presence of ears by which the noise made
by another individual is heard. The song is largely sexual in
character.

Order 6. Dermaptera. These are the earwigs, which closely
resemble the Orthoptera but are always recognisable by the
gripping forceps, which is a modified pair of anal cerci, at the
posterior end of the body. The mouth-parts are biting. The
common earwig is Forficula auricular ia.

Order 7. Isoptera. The termites or white ants of the tropics
are remarkable for their social organisation, which closely
M.z. — 9

250

INSECTS. PHYLUM ARTHROPODA

parallels that of the true ants, to which they are not nearly
related. Most of the individuals are sterile and wingless workers
and soldiers of both sexes. Each colony is founded by a royal pair,
the king and queen, which at first have wings, but they lose these
when they settle down in matrimony. Many termites feed solely
on wood." but they can only do so with the help of symbiotic
flagellate Protozoa which inhabit the gut and break down the
lignin. rixperimentally termites can Hve on the purest cellulose
known, and the source from which their symbionts obtain nitrogen
remains a mystery. The mouth-parts are biting.

Order 8. Plecoptera. The stoneflies have biting mouth-parts
and aquatic larvae (naiads) with tracheal gills— expansions of

^Uppc- lip labrum
• Antennae

'Compound Eyes

Winq Cases

Mask
Fig. 1 8 1. — Dragonfly larva. — From Sandars.

the body- wall richly supplied with tracheae. Perla carlukiana
( = marginata) is common by English streams.

Orders 9, Embioptera, and 10, Psocoptera, are small orders of
small insects, with many apterous forms. Trogiiim (=Atropas)
pulsatorium, the book-louse (Psocoptera) is notorious for living
on the paste of book-bindings, but it is not confined to this diet.

Order 11. Odonata. The dragonflies are large insects with
biting mouth-parts, predacious in both larval and adult stages ;
the eyes are big and the antennae reduced. The larvae are naiads
with tracheal gills either at the posterior end of the body or in the
rectum, and a characteristically modified labium known as the
mask ; this is normally carried folded under the head, but can be
suddenly shot out to seize an animal such as a tadpole.

Order 12. Hemiptera or Rhynchota. These are generally known
as bugs, although they include many species, such as the water
scorpion (Nepa) and water boatman [Notonectd], to which the
term is seldom applied. All have piercing and sucking mouth-parts
of the same general type as those of the aphis, which has been

HEMIPTERA

251

Pro^horojc

M£so hhorax

rvvncf

(caUi

described above (p. 241), but while most feed on plant juices,
some, such as the bed-bugs [Cimex, Fig. 182) suck blood. Many
of the herbivorous forms are of great economic importance both
because of the direct damage which they do and because of the
virus diseases which they carry.
Many species, such as the bed-
bugs, are wingless, while others,
such as the Aphididse or greenflies
(Fig. 183) have wingless forms.
This last family is also of interest
for its peculiar and complicated
life-cycles, which are generally of
the following pattern. An egg
which has survived the winter
hatches in spring to form a wing-
less female. She feeds, and rapidly
produces about forty young ones ;
these are all wingless females,
have been formed by parthenogenesis, that is, from eggs
which have not been fertilised, and are hatched within the
body of their mother, so that they are said to be produced
viviparously. Each in its turn reproduces in the same way, and
similar generations occur throughout the summer. There is thus

Fig. 182. — The Bed Bug, Acanthia
[=^ Cimex) lectidaria x c. 12. —
From Murray, after Butler.

Fig. 183. — The Turnip-leaf Plantlouse {Aphis rapcB). — After Curtis.
2 and 4 winged and wingless parthenogenetic females ; i and 3 natural size of the same.

very rapid multiplication but Httle spread of the animal. Even-
tually, in late summer, a winged brood appears, and some of the
bugs in this are males. The winged forms fly to neighbouring
plants and copulate. Each female then lays one fertihsed egg,
which may survive the winter and start a new cycle in the spring.
There are variations on this theme, mostly in the direction of
greater complexity. Dor alls {= Aphis) rumicis lays the fertilised

252

INSECTS. PHYLUM ARTHROPODA

oo the spindle tree. Euonymus europaea ; after a few wingless
generatioiis have been prodnced in the early summer, winged,
bat still parthenogenetic. females a^jear, and migrate to beans
(VicU) and other plants, where more parthenogoietic broods,
some winged and some win^ess, are produced. When the winged
sexnal individoals are formed, both s-x-s migrate back to the

Ori£r 13- Ephemeroptera. The most strikinz f-ature of
the ma\-flies, and that which gives its name to the order, is

Fic. 184. — ^Marfly lar%-a.
Frcan TboDQaoD, aiter

E^tOCL.

FiG- 185.

Tbe body loose in dorsal
c. yj. — Afttrr NuttaJL

the brief life-span of the imago, sometimes a mere day, and
never more than a few. The imagfrn^ do not feed, and have
vestigial mouth-parts, but those of the larv a: are biting. The larvae
are aquatic naiads, which, except in the first instar, possess tracheal
giDs, Kk*? small i»ings, attadied to the abdominal segments
(Fig. I _. Both imago and larva have long cerci and a long
caudal filament, giving a characteristic triple tail which makes
tbem ea5>' to recognise.

Ori^ 14. Malloph.^ga- The bird hce are wingless and have

bitmg mouth-p>art5 uith whidi they feed on particles of feather

are parasitic on mammals, not birds; hair. The chief

MALLOPHAGA

253

zoological interest of the MaBopfaaga lies m tbe attempt

been made to use diem in avian taxoooonr. Since tibe UmiSrs of

lice are in general restricted to particular groaps ai birds^ it hsas
been suggested that wben the afcntjps of tbe Ike are dearer tkaa
those of the birds, the classi&caticm of the birds may be based
on that of the Hce.

Ord^ 15. Anopleura. The sadoDg hce. or smply hce, are
wingless ectoparasites of ntammals and have pier : r : - _ - - -parts,
Two species are found on man. Pedicidus s (Fi^. 1S51

LZ-i Pktkirus pmbis (Fig. i^c . The latter, the crab or pubic
louse, is found chiefly, though not solely, on the hairs of the pobtc
region, and is of no great
medical importance. P.
k%mumus occurs in. two
forms, the r : iy louse,
P. hun cotpowis,

found only on the body
and its clothing, and the
head louse, P. hmwumus
capitis y found on tbe
bead and occasionally
elsewhere. These were
formerly amsidered to
be different species. Both
transmit the parasites
which cause t\'phus.
trench fever, and re-
lapsing fever, the first two being organisms of
Rickettsia, and the third the bacterium Trefomfma [ =5^w»cW«t>
recurrentis. Lice are common in almost all crowded alienations
of man, whether they be primitive tribes, urban schools, or prisons.
An\-thing which leaves the hce to a peaceful life, such as Tin-
changeddothes. or long hair (and especially p^maneiLt waves*,
will encourage them, but the indi\-idual infestation is seidoaaMOie
than twenty. Since neither the insects nor their eggs can h\^
long away from man, the chief method by which the paraates
spread is personal contact. Infection may. however, take place
through towels or combs or stray hairs, especially when these
bear the eggs or nits, and since the Hce can move at about nine
inches per "minute, propinquity to an infected person is almost as
dangerous as contact. Lice may be completely remo\-ed from tbe

Fig.

1S6, — TTie crab lo«ase (Pifctfctr»s pmkik^. —
From Sedgwick, aiter LaxL-dois.

5t, Sdgsnsa ; Tr^ tracaea.

INSECTS. PHYLUM ARTHROPODA

body and hair by disinfectants, and from clothes by very moderate
temperatures (53.5° C. for five minutes kills both insects and eggs).
Persons who wash, and change their clothes with some frequency,
are never likely to become more than temporarily and lightly

infected. , ^-^ . n • .

Order 16. Thysanoptera. These are the thrips, small msects

with sucking mouth-parts, mostly feeding on plants, of some
economic importance.

SUB-CLASS II— ENDOPTERYGOTA

The wings develop in sacs which are pushed inward from the
surface of the body. There is a true larva, and at metamorphosis,
which is restricted in time, the wings are everted. Some orders
have biting mouth-parts, but the majority have specialised
mouth-parts of various kinds.

Order 17. Neuroptera. This is the rump of a much larger
Linnaan order, and as now defined it includes insects with biting
mouth-parts and a characteristic ladder-Uke venation on the
fore edge of the wings. The alderfly, Sialis lutaria, has an aquatic
larva with long tracheal gills, and biting mouth-parts, and the
lacewings, Chrysopa, have terrestrial larvae with pecuhar sucking
mouth-parts. All neuropteran larvae are carnivorous and cam-
podeiform (p. 247).

Order 18. Mecoptera. This is a small order with biting mouth-
parts, eruciform larvae (p. 247) and external male genitaUa which
superficially resemble the tail of a scorpion. An example is Panorpa
communis, the scorpion fly.

Order 19. Trichoptera. The caddis flies are well known for
their aquatic caterpillars which build themselves cases of sticks
or stones from which only the head and thorax protrude. The
mouth-parts of the larvae are biting, but in the adults the
mandibles are vestigial, and if they eat anything it can only
be liquid. The bodies are covered with hair-like processes. The
larvee have tracheal gills.

Order 20. Lepidoptera. The butterflies and moths have larvae
which are caterpillars and have biting mouth-parts, and adults
with characteristic sucking mouth-parts which have been
described on p. 244. The body and wings are covered with scales.
The pupa is often enclosed in a cocoon spun of silk secreted by the
larva through spinnerets on the head. A diagram of a typical

LEPIDOPTERA

255

larva is shown in Fig. 187. The prolegs, though not fully formed,

are rehcs of true abdominal appendages. The eggs of a butterfly

are laid on a food plant which is characteristic of the species ; they

hatch into larvae of the first instar. These begin eating and grow

rapidly, and about four

ecdyses (the number is y\EAO

constant for the species)

take place. The larval

instars generally differ

slightly in appearance, as

well as in size. The last

larval instar, or prepupa,

becomes nomadic, and

goes in search of a place suitable for pupation. It then

spins silk, which is used either as a mat, from which the

larva hangs itself by the claspers, or a girdle, by which the

larva binds itself to a plant stem, or a cocoon like that of the

silkworm. Next the larval skin is spht and shed, and a new pupal

Jaws

Antennae Legs
Spinnerets

Pro- legs

Fig.

187. — A caterpillar. — From Sandars,
An Insect Book for the Pocket.

SkuLL

Forehead

CLipeus
Upper Up

6 Eyes

Antenna
Jaw

.Max. paLp
Lab. paLp

1^^ Segment

Spiracle

Fore leg

6 Eyes
"Antenna
MaxiLlaand M.paLp
Lower Lip and Lab. palp.
Spinnerets

Fig. 188. — Head of caterpillar. — From Sandars, A Butterfly Book for the Pocket.

skin is formed which completely encases the body and hmbs ;
there are no breaks in this skin except for the spiracles. Such a
completely encased pupa is called obtect. After an apparent rest,
during which metamorphosis has been going on, the pupal skin
sphts down the middle of the back, and the butterfly crawls out.
The wings are at first hmp and small. They are steadily expanded,
largely by the inflation of their tracheae with air, until they have
about nine times their original area. In the process the wing has

2r6 INSECTS. PHYLUM ARTHROPODA

changed from a hollow bag to a flat double membrane. There may
be one, two, or three generations in the year, according to the
species.' The' winter is passed in a quiescent state, sometimes as
an egg, sometimes as a larva, sometimes as a pupa and sometimes
as an adult, but in British species, with one exception, there is
only one method of wintering in each. The large white, Pieris
brassiccp, lavs its eggs in batches of six to a hundred on leaves of
plants of the family Cruciferae, such as cabbages and turnips.
The black and yellow larvae hatch in from four to seventeen
days, and are gregarious, feeding together on a mat of silk which
they' have spun over the leaf. There are four moults, and the
pupa is formed after about a month. It is fastened to a support
both by a girdle of silk round the body and by a terminal pad
in which the claspers are embedded. There are two or three
generations in a summer, and while the earlier caterpillars may
pupate on the food plant, the later ones usually carry out a
vertical migration and anchor themselves on the under side of a
ledge of a wall or fence. Metamorphosis takes about a fortnight,
but the late summer pupae have their development inhibited by
the cold weather, and may survive the winter. If they do, the
imago emerges in April. Butterflies of the first generation are
on the wing from April until June, and those of the second from
June until the end of August. In favourable weather there may
be a third generation which flies until October. A few imagines
survive the winter, but generally an individual does not live for
much over three weeks, and feeds on the nectar of several flowers,
especially beans, clover, and lucerne. These are Leguminosae, a
different family from that to which the food plants of the larvae
belong. Examples of butterflies which winter in other ways are
the brown and purple hairstreaks, Thecla hetulce and T. quercus
(eggs), the meadow brown, Maniola jurtina (larvae), and the
small tortoiseshell, Aglais urticcB, and brimstone, Gonepteryx
rhamni (imagines). It is natural that most of the earliest butterflies
of spring should be those which have survived the winter in the
adult state.

Order 21. Diptera. These are ' flies ' in the narrow sense, two-
winged insects with the hind pair replaced by knobbed balancers.
The mouth-parts are sucking but may also be piercing, either
primarily by means of mandibles, as in the gnat described on
p. 243, or secondarily by stiffening of the labella, as in the tsetse
fly Glossina and the stable fly Stomoxys. The three thoracic

DIPTERA 257

segments are fused. The larva is apodous (Fig. 191), and the pupa
may be obtect (though often capable of movement) or coarctate.
The gnats or mosquitoes lay their eggs on the surface of water,
different species requiring different ecological conditions of size
of pond, temperature, hydrogen ion concentration, salinity, and
so on. The larvae, although legless, are very active, and swim
beneath the surface by a wriggling movement of the whole body ;
they feed on minute particles in the water by means of brush-like
structures which continually sweep a current of water into the

Fig. 189. — A ventral view of the head of a fully grown larva of the mosquito
Anopheles maculipennis. — From Nuttall and Shipley.

b., Brush with which fcod is swept from the surface film of the water (Culex larvas, hanging with the head
down, collect from a lower stratum); c, antenna: d., palp of maxilla ; ;'., stout hairs which arrange
the brush ; k., teeth of mandible ; m., hooked hairs at edge of maxilla ; />., a median tuft of hairs ;
q., a median structure known as the metastoma ; r., rim of head.

mouth and are combed by the mandibles and maxillae (Fig. 189),
and breathe air which they obtain at the surface. Most of the
spiracles are closed, but the two main tracheal tubes open on a
projection of the eighth segment of the abdomen called the respir-
atory siphon. This is closed with flaps when the creature is under
the water, and when it comes to the surface they spread out on the
surface film, expose the tracheal openings to the air, and hang the
insect from the film. Water, which has high surface tension, will
not enter the narrow tube, but oil, with low surface tension, will-
as can easily be observed under the microscope ; this is the
principle of the method of attacking the carriers of malaria
(p. 76) by spraying paraffin on the water. After four months

2^8 INSECTS. PHYLUM ARTHROPODA

the larva becomes a pupa, which is shaped Hke a comma. It
can swim, in much the same way as the larva, but instead of

Fig. 190. — A mosquito. Anopheles, sucking blood. — After Nuttall and Shipley.

The curved line under the head is the labium.

having one respiratory siphon on the abdomen it has two just
behind the head, which are used in the same way. The pupa does
not feed, and after a few days the adult gnat emerges into the

Fig. 191.— The life-history of the house fly {Musca domesiica).— From Theobald.

a, Mandible of larva with adjacent structures ; b, larva ; c, anterior spiracle of the same ; d, eggs ; e, pupa

case ; /, remnants of spiracle on the same.

air through a dorsal split, standing on the floating pupal skin
until its own has hardened. There are several generations in the
course of the year.

DIPTERA 259

The mouth-parts of the housefly [Mnsca domestica) have been
described on p. 244. The eggs are laid in any rotting organic
matter, particularly stable manure, and hatch, in eight to
seventy- two hours according to the weather, into maggots or
gentles. These are soft, white and legless, and shaped like an
ice-cream cone. There are twelve segments, and the head at the
pointed end can be withdrawn under the first segment and carries
a pair of hook-like mandibles and a pair of minute antennae.
The mandibles help to draw the animal forward ; it moves
rapidly away from light so that it tends to burrow into organic
matter, on which it feeds. Food is liquefied by a sahvary secretion
and then sucked in. The second and last segments bear a pair of
spiracles each, and the fifth and following somites have each a
spiny pad below. The last larval instar buries itself, often away
from its food, to a depth of an inch or so, and pupates, the pupa
being enclosed in the last larval skin. The imago, which flies
towards light, i.e. is positively phototactic, escapes by the
inflation of a pecuHar bladder or ptilinum on its head. This is
filled with blood under pressure, and used to break successively
the pupal skin and puparium, and then to make a way to the
open air. The length of life of the larva is from two days to eight
weeks, and of the pupa from three days to four weeks or more,
according to the temperature, and the imagines may be capable
of laying eggs a fortnight after emergence ; a complete generation
may therefore be accomplished in three weeks. The first house-
flies of the season appear in Great Britain in June and, except
for a few which linger indoors, the last are seen on the wing in
October or November. Where flies go in the winter time is still
unknown. A few may remain as dormant larvae, pupae or imagines,
but it is also possible that all die and that those which appear
the next summer are immigrants from warmer countries where
breeding continues throughout the year. Many imagines are
killed at the onset of winter by a fungus, Empitsa mtisccs, which
continues to live saprophytically after the fly has died, and may
surround its corpse with a grey halo of hyphae. Other related
species hibernate as adults.

The blowflies or bluebottles (Calliphora) lay their eggs on
flesh and have a similar Ufe-history to the housefly. Other
Diptera are the warble flies and bot flies (Oestridae) with larvae
parasitic on hoofed mammals, the hover flies (Syrphidae) which
superficially resemble bees, the gadflies or clegs (Tabanidae),

25o INSECTS. PHYLUM ARTHROPODA

midges (Chironomids), and daddy-long-legs or crane flies
(Tipulidje), the larvae of which are leather- jackets.

The above five orders of the Endopterygota are sometimes
regarded as being much more closely related to each other than
are the remaining three ; they may have been derived from a
common ancestor which was not unUke a member of the

Mecoptera.

Order 22. Aphaniptera. The fleas, such as Pulex irritans, the
human flea (Fig. 192), are ectoparasites of mammals and have
piercing and sucking mouth-parts. There are no wings, and the
body is strongly laterally compressed. The adults, which alone
suck blood, can live for a short time away from their host, and

c

B.

A.

Fig. 192. — ^The Common Flea {Pulex irritans) x c. 12.
A, Larva ; B, pupa ; C, adult.

leave him to lay their eggs. The larvse have biting mouth-parts,
and feed on organic matter in dust. Pulex irritans pupates in a
cocoon after a larval life of about twelve days. The rat flea,
Xenopsylla cheopis, which occasionally sucks the blood of man,
carries the germ which causes bubonic plague.

Order 23. Coleoptera. The beetles (Fig. 193) are a very large
order of insects of a characteristic form, easily recognised by the
straight longitudinal line made by the meeting of the wing cases.
The mouth-parts are biting, and the fore wings are modified
as hard elytra. The larvse are of various types, and hypermeta-
morphosis sometimes occurs.

Order 24. Strepsiptera. These insects are small, with degenerate
biting mouth-parts, and are perhaps derived from the beetles.
The larvse and usually the adult females are endoparasites of
other insects, and the males have the fore wings reduced to knobs.

Order 25. Hymenoptera. This is an important order, including
ants, sawflies, wasps, and bees. The mouth-parts are primarily

HYMENOPTERA

261

biting, but may secondarily be developed for sucking, as in the
honey bee described on p. 245. The hind wings are smaller than
the fore wings, to which they are attached by hooks. There is
usually a distinct waist, which is between the first and second
abdominal segments, the former being fused to the metathorax.
The abdominal appendages include an ovipositor, which may
be modified as an instrument for stinging or boring. The larvae
are either legless or caterpillar-like, and the pupa is exarate.
Some families, such as the Ichneumonidse, have larvae which are
endoparasites of other insects.

The most striking thing about the Hymenoptera is the social

Fig. 193. — The Turnip Flea beetle {Haltica nemorum). — From Theobald.

I, Adult, magnified ; 2, true length and wing expanse ; 3, adult feeding on leaf ; 4, egg, natural size ; 5, the
same magnified ; 6, 7, tunnel made by larva in leaf ; 8, 9, larva, natural size and magnified ; 10, 11,
natural size and magnified view 01 pupa, which lies in soil.

This very destructive insect feeds, as larva and adult, on the leaves of turnips, cabbages, broccoli,
and other Cruciferae. It has many broods in the year, the last hibernating under stones, etc. Its worst
damage is done to seedlings. Paraffin, derris powder, and a mixture of soot and lime are remedies.

organisation, which has been developed in several groups, and
which is paralleled elsewhere only in the termites (p. 249). Many
bees are solitary, but the hive or honey bee, Apis mellifica, has
one of the most elaborate social organisations in the order. It is
impossible to describe the life-history in the usual way, for new
colonies start, not with one individual, but with a large group,
and we must therefore begin by considering the structure of an
established community. This will consist of a fertile female or
queen, bearing in her spermatheca a supply of sperms, a number
of males or drones, and many thousands of sterile females or
workers. The chief external differences between these three castes
are shown in Fig. 194. The drones have no stings. The workers
have structures on the third legs called pollen baskets, and their

INSECTS. PHYLUM ARTHROPODA

stines are powerful and poisonous. The workers spend the day,
or at least the shining hours, gathering pollen as well as honey
or more strictly, nectar, from every suitable flower. When they
reiurn to the hive they regurgitate the nectar as honey, and
store both this and pollen in special compartments or cells.
Thev also build the combs and carry out various chores. The
hexagonal cells are made of wax which comes from between
the abdominal segments, and is chewed and placed in position
bv the iaws. The cells are of regular shape, hexagonal m cross-
section, but not all of the same size. The larv^ and queen are
fed, dead bodies removed, and the younger workers fan with
their wings to ventilate and cool the hive.

The fecundated queen lays eggs, placing each carefully ma
cell Not all the eggs are fertilised ; if a sperm is released m the

A B C

Fig. 194. — The Honey Bee. — From Shipley and MacBride.
A, Drone ; B, queen ; C, worker.

act of laying, syngamy follows and the larva produced is female ;

if no sperm is released, the resulting parthenogenetic larva will

become a drone. The drone eggs are also placed in larger cells

than those of the workers. All larvae are fed by the workers,

at first on ' royal jelly ', which is secreted by their pharyngeal

glands. Drone and worker larvse are changed to a diet of honey

and predigested pollen on the fourth day. A few female larvae,

which come from eggs which have been laid in larger cells, are

kept on royal jelly throughout ; they develop into queens.

Such big cells are made only after the colony has been growing

for some time, and there are normally only a few of them. Before

any of the new queens emerge the old queen will have left the

hive, attended by about half the workers, the whole mass forming

a swarm which will settle and start a new colony. The first of

the new queens to emerge stings the other pupal queens to death,

and then, after short flights to learn the neighbourhood of the

hive, she soars towards the sun in a nuptial flight, followed by

HYMENOPTERA 263

all the drones in the district. The first to reach her copulates in
the air. All his sperms are passed into the queen, and in separating
from her he is so damaged that he dies. The other drones return
to the hive, but will after a time be killed or denied admittance
by the workers. The queen also returns, and from then on, until
it is her turn to swarm, does nothing but go from cell to cell
laying eggs. Her total life may be three or four years.

It is probable that the workers go through a regular series of
duties in their life of less than two months, at first fanning the
hive and feeding the larvae, then building, and relieving other
workers of their honey and pollen, and lastly, going foraging.
It has recently been shown that when a foraging bee returns it
may perform a dance, which indicates to the other bees the
direction (relative to the sun) and distance away of the flowers
from which they have obtained the food.

The social life of the ants, such as Formica nifa, the wood ant,
which builds large nests of pine needles or twigs, differs in many
respects from that of the bees. A nuptial flight occurs, but many
queens and males go together ; the female in copulation gets
a supply of sperms for life, and the male thereafter dies. The queen
sheds her wings by rubbing them off ; she may return to her
own nest, or go to another of the same species, where she joins the
existing community, or she may make a hole in the ground, lay
eggs, and wait for them to hatch. She feeds them by regurgitation
of material obtained from her own stores of fat, and in due course
they become sterile female workers. They build the nest, fetch
food, and establish a new colony in which the queen does nothing
but lay eggs, which she does every ten minutes for some six
years. The general duties in the nest — feeding the larvae and
queens, building, and so on — parallel those of the worker bees,
but differs in detail. The food is largely, but not entirely, vegetable
matter, and many ants are fond of the Hquid which exudes from
the anus of aphides. Because of the acquisition of new queens
the life of the community is indefinite. A new community is
sometimes founded by a group of members of an old colony
marching out to build a new nest. The workers of many species
of ant, including Formica rufa, are of more than one type ;
morphological differences correspond to differences in duties.
It is probable, but not proven, that, as in the honey bee, the cause
which determines whether a female e^g shall become a worker
or a queen is the nutrition which it receives.

18
MOLLUSCS

SNAILS

Several species of snail are found in Britain, some living
amongst land vegetation and others in fresh water. The two most
frequently studied are the garden snail, Helix aspersa, and the
larger Roman or edible snail, H. pomatia. The following description
applies, except for certain points, to both. H. aspersa is wide-
spread in thick vegetation, although it is rare in or absent from
many districts in which the soil is deficient in lime ; H. pomatia
is much more local in its distribution, and although it was
previously thought to have been introduced by the Romans,
who are known to have cultivated it for food, its shells have now
been found in pre-Roman deposits. Both species are in fact edible,
and eaten ; aspersa was especially sought after in the glass-
blowing districts, where it was considered good for the wind,
while pomatia is the escargot of French menus.

When a snail is moving (Fig. 195) there are three obvious
divisions of the body ; an anterior head, not sharply marked off,
but bearing the mouth and two pairs of tentacles, of which the
posterior pair are longer and bear eyes ; a long muscular foot,
on which the animal moves ; and the dorsal shell, inside which is
the visceral mass or hump. Below the edge of the shell is a thick,
fleshy rim called the collar ; this is the edge of the mantle, which
is referred to below. A number of openings can be seen. Below
the shell on the right side is a conspicuous pulmonary aperture,
which opens not into the body but into a space called the mantle
cavity, within the shell. Just inside the pulmonary aperture is
the opening of the excretory duct, and just behind this is the
anus. On the right side of the front part of the foot a groove runs
forward to the common genital aperture just behind the second
tentacle. The foot consists of longitudinal muscle fibres, and if
a snail be watched crawHng on a glass plate, waves of contraction
can be seen running from tail to head along its length. The
contact between the foot and the surface on which it moves is
lubricated by mucus discharged by a pedal gland which opens
just below the mouth.

264

265

Fig. 195— Anatomy of the edible snail, Helix pomatia

the'end';!fthfch"li:s1 KSk e^^^ ^^d.T^f n^LV^cT ^T^^'^^^ = 5, postenor "tentacle, at
9. common genUal opening To'm^ntle caWt'; of lu^x^ dCr sa 1 lLl?'o?Cv'rSLl""f i '' f""-
?4 ureTe7 irk^v^''^^ of pulmonary vessels froiS Which puTl'n'i^ie'i^'coKs^l.'Te^^-^^

45. pedal gland, which secretes the slime of the snail's track ^ "^^ = '♦'♦' ^^^ ^

256 SNAILS. PHYLUM MOLLUSCA

THE SHELL

The shell is secreted by the mantle and consists of an outer
periostracum, made of an organic material called conchiolin,
and two layers largely made of calcium carbonate. The first of
these, whicii is in contact with the periostracum, is the prismatic
layer, and inside this is the nacreous layer. The last consists of
layers of thin plates of translucent material, set at a slight angle
to the surface of the shell, and it is to the interference caused by the
reflection of light from these plates that the colours of the inside of
the shell are due. They are much more brilliant in some lameUi-
branchs (p. 284). The shell is a three-dimensional spiral, built
round a central axis or columella ; to this is attached the columella
muscle, contraction of which pulls the whole animal inside the
shell. Most shells are right-handed (dextral) spirals, but about one
in a thousand is left-handed (sinistral) ; in these aberrant forms
the asymmetry of the body is reversed. In winter, snails become
inactive and torpid, often collecting in masses in holes in walls.
The opening of the shell is then closed by the epiphragm, a
temporary sheet of mucus containing some lime. In many water
snails there is a permanent hinged operculum for the same
purpose.

TORSION

When the shell is removed, which may sometimes be done
merely by unscrewing the animal, although other individuals
need to be cut up the shell along the spiral, the mantle and
visceral hump are exposed. The latter is covered with a thin skin,
from which hangs down the mantle, a thicker extension of skin
which is normally raised away from the hump and closely applied
to the shell which it has secreted. The lower edge of the mantle,
the collar, is fused to the body-'Wall, so that between the two there
is a chamber, the pulmonary cavity, which although inside the
shell is outside the body. At the pulmonary aperture the mantle
and body- wall are not fused. The main parts of the internal
organs have undergone both torsion and coiling, which makes
their understanding and dissection difficult. The embryo is at
first bilaterally symmetrical, with the anus posterior ; suddenly
the body twists, so that the gut, nerve cords and many other
structures^ are swung ta the right and forwards through nearly

TORSION 267

180°, and the anus comes to lie just behind the mouth. This is
torsion ; the dorsal portion then becomes spirally coiled to fit
the shell. Torsion is a characteristic and peculiar feature of the
Gastropoda (the class to which snails belong) and is developed to
different degrees in the various orders.

NUTRITION

Snails feed on leaves, out of which they cut pieces by means
of a toothed chitinous tongue or radula (Fig. 196), which works

tentac/es

jaw

mouth

radula

cartilage supporting radu/a

radula sac

oesophagus

=5 retractor muscle
of pharynx

pedal gland— —Y^'"'^-^^ ~
foot ^•'""^ =-"

L 10 L20

^^ /4

Fig. ig6.— Helix pomatia. — From Thomson.

A, Diagrammatic section of head and buccal mass, showing position of radula and jaw ; B, portion of
four rows of teeth from radula ( x 40), each horizontal row contains about 160 teeth and the radula
has approximately 160 horizontal rows ; c, central tooth of row; C, three radula teeth (x 170) ;
c, central, l 10, L 20, tenth and twentieth lateral.

against a crescentic jawplate on the roof of the mouth. The radula
is formed in a radula sac, from which it grows as its front part is
worn away. On the dorsal surface of the buccal cavity open two
large buccal glands, the secretion of which is a lubricant. From
the buccal cavity a short gullet leads back and expands into a
large crop, on the dorsal surface of which lie the buccal glands.
After the crop comes a short stomach, and then a longer intestine,
which is coiled in the dorsal hump and then swings forward on
the right side as the rectum to open at the anus behind the mouth.
Surrounding the stomach and intestine, and occupying much of
the visceral hump, is a structure often called liver, but more
accurately known as the digestive diverticula. There are two
lobes, each with its own duct opening into the stomach. Particles
of food are carried into the diverticula, and in them most of the

258 SNAILS. PHYLUM MOLLUSCA

digestion as well as most of the absorption takes place, but the
secretions are carried forward to the crop, where digestion
presumably begins. The finer particles are ingested by the cells
of the diverticula, and most, if not all, of the digestion of proteins
takes place intracellularly. The snail is one of the few animals to
possess an enzyme which can break down cellulose ; what is more
remarkable is that although it is entirely vegetarian it can also
digest chitin.

THE KIDNEY AND CIRCULATION

The ureter, which runs just above the rectum, leads up to a
greyish kidney, which produces a variety of nitrogenous products.
Its cavity has a minute opening, the renopericardial canal, into
the pericardium ; the kidney is in fact a coelomoduct (p. 189).
The kidneys should be paired, but the torsion which gastropods
undergo has caused the loss of that on the left side.

The pericardium, which, with the renal cavity, is all that is
left of the coelom, lies against the kidney. In it is the heart, con-
sisting of an auricle and a ventricle, which drives blood into
an aorta. This divides into an anterior branch to the head and
foot, and a posterior to the visceral hump. The finer branches
of these arteries open into a system of sinuses, constituting a
hsmocoele ; this is well seen in dissection, when the dorsal part
of the foot is opened, as a large cavity containing the crop and
reproductive organs. From the hsemocoele the blood goes to a
system of vessels on the roof of the mantle cavity, and so by a
pulmonary vein to the auricle. Like the left kidney, the left
auricle has been lost. The plasma of the blood contains hsemo-
cyanin, a copper-containing protein which, like the haemoglobin
of vertebrates (p. 524), assists in the transport of oxygen. The
floor of the pulmonary cavity is rhythmically raised and lowered
by muscles, so that air is drawn in and out.

NERVOUS SYSTEM

All the larger nerves and ganglia are concentrated in the head
into a nerve collar which surrounds the gullet. There is a pair
of dorsal cerebral ganglia, from which two pairs of nerves run
round the gut to a ventral suboesophageal ganglion ; this repre-
sents three pairs of ganglia — pedal, pleural, and visceral — which

NERVOUS SYSTEM 269

have fused, although in the swan mussel (p. 280) they are separate.
Nerves from the cerebral ganglia go to the mouth and anterior
sense organs, and from the other ganglia to the parts of the body
suggested by their names. The eye, at the end of the longer
tentacle, has a lens-like structure and a retina ; it is especially
good at detecting quick movements, as is easily shown if a hand
is moved across its field of view, but the range of vision is small.
There is a pair of statocysts near the pedal ganglia, and the
tentacles are highly sensitive to smell.

REPRODUCTION

The snail is hermaphrodite, and is peculiar in producing both
types of gamete in the same gland, which is therefore called an
ovotestis. It is a whitish lobed structure at the top of the visceral
hump. From it there leads a short coiled hermaphrodite duct,
and this passes into a longer common duct, which runs forward.
It is incomxpletely divided into male and female channels. At
the junction of hermaphrodite and common ducts is a large
albumen gland. The common duct finally divides into a left
vas deferens and a right oviduct or vagina. The vas deferens
leads into a muscular penis ; just before it does so there is given
off a blind diverticulum, the flagellum, which runs back alongside
the common duct. The oviduct also gives off a long blind diver-
ticulum or spermatheca, which may be recognised by the sub-
spherical expansion at its end. (This is sometimes called the
receptaculum seminis, but this name is better reserved for another
structure to be mentioned below.) In H. aspersa, but only rarely
in H. pomatia, the spermatheca itself has a diverticulum, which
in dissection is easily confused with the flagellum. Below the
spermatheca there opens into the oviduct a lobed mucous gland
and below this again the dart sac. Oviduct and penis open together
at the common genital aperture. Sperms are produced during
most of the year, and are bound together in packets <K sperma-
tophores by the secretion of the flagellum. When copulation is
about to occur two snails cruise round each other in decreasing
circles until, when they are nearly touching, the dart sacs
violently contract nearly simultaneously, and from each a
calcareous dart is shot into the other snail with enough speed to
penetrate deep into the internal organs. There is a pause after this,
and after a time the snails come into close contact, and the penis

270 SNAILS. PHYLUM MOLLUSCA

of each is protruded and inserted into the vagina of the other.
The spermatophores are ejected and stored in the spermatheca,
where their covering is dissolved and the sperms set free. At
some time about June the ovotestis ceases forming sperms and
makes ova instead. These are fertiUsed at the base of the
hermaphrodite duct in a small chamber, the receptaculum
seminis, by sperms which have swum up the common duct from
the spermatheca. The eggs are then surrounded by albumen
secreted by the albumen gland, and after passing down the female
portion of the common duct, are laid all together in holes in the
ground. This happens in July and August. Development is
direct, and small snails hatch after about twenty-five days.

SWAN MUSSELS

Freshwater mussels may be found in streams, canals, and large
ponds in most parts of Britain, though they are often overlooked
on account of their habit of burying themselves in the mud
with at most a small part of the body projecting. The commonest
of them are the swan mussels, Anodonta cygnea and A. anatina,
which differ only in small details of the shape of the shell. When
one is removed from the mud it is seen to be enclosed in a fiat,
dark-green shell, four to six inches long and roughly oval in out-
line, with one end (the front) rounded and the other more pointed.
The shell consists of two similar pieces, known as valves, which lie
one on each side of the animal joined by a hinge above the back,
where their edges are almost straight. On being disturbed the
mussel holds the valves tightly together, but when it is at rest
in the water they gape somewhat, and at the hind end, which
projects slightly from the mud, there may be seen between them
two fleshy lobes enclosing an opening shaped like a figure of 8,
through one of whose limbs a current flows into the shell, while
through the other, the upper of the two, the water is driven out.
At times the animal moves about, thrusting out a yellowish,
ploughshare-shaped organ known as the foot, with which it
ploughs its way through the mud at the rate of about a mile a
year. Freshwater mussels are not unfit for food and are some-
times eaten. They are preyed upon by water-fowl and other
animals, and in places are fished for on account of the pearls

S WA N MUSSELS

271

which the}^ contain, which may be of considerable value. They
are not killed by the freezing of the water even if they themselves
be frozen solid, but can only survive a few hours of drought.

SHELL AND MANTLE

The shell has the same layers as in the snail. Lines of growth

iib.

.^

d.s

v.s..--

,.. 1.9.

f.

B

t.p.r.

i.p.ad.

Fig. 197. — The Swan Mussel.

A , The shell with the animal, from the right side ; B, the left valve of the shell, from within.

d.s. Dorsal siphon ; /., foot ; i., impressions of muscles ; i.a.ad., of anterior adductor ; i.a.r., of anterior
retractor ; i.p.ad., of posterior adductor ; i.p.r., of posterior retractor ; i.pro., of protractor ; l.g., lines
of growth ; pal., pallial line ; ub., umbo ; v.s., ventral siphon.

parallel with its edge mark the outside of the shell (Figs. 197, 198),
centring upon a point about a quarter of its length from the
front end. This point is known as the umbo and shows the position
of the first shell of the young mussel. On the inside of the shell
may be seen the marks of attachment of the adductor, retractor,
and protractor muscles presently to be mentioned, and parallel
with its edge is a mark known as the pallial line, where the fold
of the body-wall known as the mantle is attached. Above the

272

SWAN MUSSELS. PHYLUM MOLLUSCA

hinge the two valves are joined by an elastic ligament, which
pulls them together and thus causes them to gape below w^hen

the adductor muscles are relaxed. To
open the shell of a living mussel the
handle of a metal scalpel is passed
between the valves and they are prised
apart ; they are then held open, for
example by turning the handle at
right angles, and the muscles can be
cut close to the shell on one side. The
body of the animal is then found to
be soft, without a cuticle, and pro-
vided with a flap of tissue which
hangs down on each side and covers
the other organs. This is the mantle
(Figs. 200, 201). It has a thick edge
which secretes the two outer layers of
the shell, while the pearly layer is laid
down by the whole outer surface of the mantle and skin of the
back. Pearls are formed in the same way in pockets of the mantle

Fig. 198.— Part of the shell of
a swan mussel, seen from
above.

l.g., Lines of growth ; lig., ligament;
ub., umbo.

^r. mV.

Fig. 199.— a swan mussel removed from its shell and lying on its right side with

the greater part of the left lobe of the mantle cut away.
a.ad Anterior adductor muscle a.r., anterior retractor ; d.s., dorsal siphon ;/., foot ; l.i.g., left inner giU ;

i'SuX"mu'c^e i r^^no 'J'^^' 'y'"f ^^'^ ' ^■'■^- ^'^' °"^^^ ^"^ ^ /./>. labial ps.lps;'p.ad., posterior'
pH««f fv, ' ^ •• posterior retractor; pro., protractor ; r.w/., right mantle lobe; r.m/'., thickened

edge of the same ; v.s., ventral siphon with papilla. ' - & . '•"»» •, imcKeneu

surface around foreign bodies which have intruded between
mantle and shell. The Hne of attachment of the mantle from the
side of the body is not straight but higher in the middle than near

SHELL AND MANTLE

273

the two ends, though at the extreme ends it turns upwards to
the hinge Hne. At the hind end each mantle edge is fused for some
distance with its fellow ; it then separates widely from it twice,
so as to form the figure of 8 already mentioned, and lies against
its fellow for the rest of its length. The upper opening is known as
the dorsal siphon, the lower as the ventral siphon. The lips of
the latter bear a fringe of small tentacles. The space enclosed
by the two mantle lobes is known as the mantle cavity.

EXTERNAL FEATURES I LOCOMOTION AND FEEDING

It will have been noticed that the shell and mantle of the mussel

gen.
I. p. ; k.o. l-ml.

a.ad.

Fig. 200. — A swan mussel removed from its shell and lying on its right side with
the left mantle lobe and left gills turned back. A portion of the inner lamella
of the left inner gill has been cut away to show the openings of the kidney
and gonad.

a.ad.. Anterior adductor muscle ; d.s., dorsal siphon ; /., foot ; gen., opening of the duct of the gonad ;
k.o., opening of the kidney ; l.i.g., left inner gill ; l.ml., left mantle lobe ; l.o.g., left outer gill ; l.p., labial
palps ; r.i.g., right inner gill ; r.ml., right mantle lobe ; r.ml'., thickened edge of the same ; r.o.g., right
outer gill ; v.s., ventral siphon.

are bilaterally symmetrical. The same symmetry is found in all
the other organs of the body, both internal and external, except
for the slightly coiled gut. Above the attachment of the mantle,
at its lowest point near each end, may be seen on each side the
cut surface of the great adductor muscles, anterior and posterior,
which pass through the body from side to side and draw together
the valves of the shell. To the upper and inner sides of these lie
the anterior and posterior retractor muscles, which draw the
body forwards upon the foot when the latter has been thrust
out. Behind the lower end of each anterior adductor is a protractor

274

SWAN MUSSELS. PHYLUM MOLLUSCA

muscle, which draws the body backward upon the foot. If the
mantle' is turned back the rest of the external organs are laid

bare.

At the front and in the middle is a wedge-shaped organ called
the foot ; its lower part is muscular, and the upper part contains
the genital organs and intestine. Blood can be forced into sinuses
which it contains and is prevented from returning by sphincter
muscles round the veins ; the foot is thus caused to protrude
between the valves and to swell, in much the same manner
as does the mammahan penis when it is erected. In this state it
is wedged into the mud or between the stones at the bottom of the
water, and when the retractor muscles contract the body is

cl.c.

ax.g.

Fig. 20I.

A B C D

-Diagrams of transverse sections through the swan musseL

A passes through the middle of the foot and shows the inner lamella of the inner gill attached to the side
of the foot ; B passes through the hinder part of the foot and shows the inner lamella of the inner
gill free ; C is taken behind the foot and shows the inner lamella of the inner gills joining in the middle
line ; D is further back and shows the axes of the gills free.

ax.g.. Axes of the gills ; cl.c, cloacal chamber ; ep.sp., epibranchial space ; /., foot ; i.g.i, inner lamella of
inner gill ; i.g.2, outer lamella of inner gill ; il.sp., interlaraellar space ; o.g.i, inner lamella of outer
gill ; o.g.2, outer lamella of outer gill ; ml., mantle lobe ; tnl.c, mantle cavity.

pulled forwards. The foot is withdrawn by the contraction of its
own muscles, which empty the sinuses. Between the foot and the
mantle on each side, and extending to the posterior end, are two
double flaps called gills, although they have little function as
such. At the front end of the foot and also inside the mantle, is
another double pair of flaps, the labial palps, which do not in
the least resemble the structures of that name in insects. Between
the palps lies the mouth ; its upper lip joins the outer palps in
front of the mantle, and its lower lip joins the inner palps behind.
The structure of the gills is complicated and is shown in
Figs. 201, 202. The general structure of each gill is like that of
two sheets of trellis work, joined continuously along the bottom
edge, and at intervals elsewhere. Posteriorly the inner lamella
of the outer gill, and the whole of the inner gill, become free
from the body, although the inner lamellae of the two inner gills

LOCOMOTION AND FEEDING

275

are joined to each other ; the result of this is a large space between
the gills and the body, which, since the anus opens into it, is
called the cloacal space ; it opens to the exterior by the dorsal
siphon. The mantle, gills, and palps are all covered with cilia
of various lengths and direction of beat. These maintain a current
of water which comes in at the ventral or inhalent siphon ;
particles in the water are sorted by size, some being taken to
the mouth, and others rejected through the inhalent siphon.
Cilia in the dorsal or exhalant siphon drive out faeces and excreta.
The muscular closing of the valves ejects water rapidly from both

o. fc.

Fig. 202. — A, A horizontal section through a gill of the swan mussel, under low
magnification ; B, a single filament of the same, more highly magnified.

af.c, Abfrontal cilia ; bl.sp., blood spaces ; f.c, frontal cilia; HI., filaments ; i., side of filament towards
interlamellar space ; i.f.j., interfilamentar junction ; i.l.j., interlamellar junction ; i.l.sp. interlamellar
space ; Lc, lateral cilia ; l.f.c, laterofrontal cilia ; o., outer side of filament ; sk.r., sections of the
chitinous skeletal rods which support each filament.

siphons, and takes place both when the animal is disturbed and
when noxious water enters the mantle cavity. The major part
of the gas exchange takes place through the surface of the mantle.

GENERAL ANATOMY AND ALIMENTARY SYSTEM

The swan mussel is a coelomate animal, intermediate between
the earthworm and the crayfish in respect to its coelom and
haemocoele. It has a perivisceral coelom, situated in the back,
enclosing the heart and rectum and communicating with the
exterior by an excretory tube on each side. This space is the peri-
cardial cavity (Fig. 203). In the rest of the body the organs are
separated by blood sinuses, the circulation being an open one.
The cavity of the gonads represents a part of the coelom. ]\Iost
of the viscera lie in the upper part of the body, known as the

276

SWAN MUSSELS. PHYLUM MOLLUSCA

-po.

azL.

rnu

— p-r.

— p.CLd^

visceral hump, but the gonads and intestine lie in the soft region
of the foot. The mcnith leads into a gullet, which passes upwards
into a moderate-sized stomach situated behind the anterior

adductor muscle. Into the
stomach opens by several ducts
a series of digestive diverticula,
often miscalled a liver ; small
particles are circulated through
these by cilia, taken up by the
cells, and digested intracellu-
larly. The hinder end of the
stomach communicates on the
right side with a closed groove
of the intestine, the caecum,
which contains a transparent,
gelatinous rod, known as the
crystalline style (Fig. 204).
This is composed of a protein
substance and projects into
the stomach, where it rotates
and dissolves. It liberates an
amylase which digests carbo-
hydrates ; this is the only
extracellular digestive enzyme
produced by the mussel. The
intestine (Figs. 205, 206) starts
from the lower side of the
stomach, takes several coils in
the soft upper part of the
foot, turns upwards, and runs
straight backwards in the
middle line of the upper part of the body to the anus. The
straight part of the intestine is known as the rectum. It lies
in the pericardial cavity surrounded by the ventricle of the heart.
The ventral wall of the rectum is folded to form a longitudinal
ridge or typhlosole. Almost the whole movement of the food
in the gut is carried out by cilia.

^ ---c?.^

Fig. 203. — Part of the dorsal side of a
swan mussel in which the peri-
cardial cavity has been opened.

ail.. Auricle ; d.s., margin of dorsal siphon ; g.,
hinder tips of gills, fused to form floor of
cloacal chamber ; p. ad., posterior adductor
muscle ; pr., posterior retractor muscle ; rm.,
rectum ; rp.o., renopericardial opening ; v.,
ventricle ; v.s., margin of ventral siphon
(opened out by spreading the mantle).

Note between the posterior adductor muscles the
fusion of the mantle edges for a short distance,
roofing in the cloacal chamber just above the
dorsal siphon.

277

Fig. 204. — Sections of part of the alimentary canal of Donax. — After Barrois.
Yapp, An Introduction to Animal Physiology, 1939. Clarendon Press, Oxford.

A , Longitudinal section ; B, transverse section of caecum.

cc.m , caecum ; cil., ciliated epithelium ; est., crystalline style ; g.s., gastric shield ; int., intestine;

M., mouth; 0^., oesophagus ; si., stomach.

Fig. 205. — The structure of Anodonta. — After Rankin.

a.a.. Anterior adductor; c.p.g., cerebral or cerebropleural ganglia; st., stomach; t-., ventricle, with an
auricle opening into it ; k., kidney, above which is the posterior retractor of the foot ; r., rectum ending
above posterior adductor ; v.g., visceral gangha with connectives (in black) from ccrebropleurals ;
g., gut coiling in foot ; p.g., pedal ganglia in foot, where also are seen branches of the anterior aorta
and the reproductive organs ; l.p., labial palps behind mouth. \i the posterior end the exhalant
(upper) and inhalant (lower) siphons are seen.

27^

SWAN MUSSELS. PHYLUM MOLLUSCA

EXCRETORY ORGANS AND GONADS

The kidneys or organs of Bojanus (Fig. 207) are two in number
and lie side by side below the pericardium. Each is a wide tube,
doubled on itself, with one Hmb above the other, and the two ends
close together in front. The lower limb is glandular and opens

Fig. 206.

-A semi-diagrammatic drawing of a transverse section of the swan
mussel in the region of the hinder part of the foot.

au.. Auricle ; B, glandular limb of kidney ; B', non-glandular limb of the same ; com., commissure,
between cerebral an