UNICELLULAR AND MULTICELLULAR ANIMALS
We must now turn to the main project of this book, which is to attempt
to trace out the lines along which animal Evolution has proceeded,
with special reference to that particular line which leads up to man.
Indeed, we shall have to stick somewhat closely to this one main
highway, and can but barely pause to glance along the numerous branch
roads, interesting though the travelling there might be.
It is perhaps necessary to say, at the outset, that the history of the
Evolution of man cannot be written as a plain, matter-of-fact tale.
Many portions of this history are tolerably well understood, but there
are other periods, in some of which notable steps of progress were
made, of which no record has ever been discovered. We must therefore
expect occasionally to be reduced to speculation, and here and there to
meet with controversy and with opposing theories.
It is not proposed here to enter into any full discussion as to the
origin of life. It may shortly be said that in the existing state
of knowledge, no very definite theory is possible. We know that
life is associated with a jelly-like or semi-fluid substance called
_protoplasm_, which consists of a very complex mixture of albuminoids.
These albuminoids are continually undergoing changes and interactions
of a complex kind, the sum total of which constitutes life. Many of
these reactions have been reproduced, or imitated, artificially, and
have been shown to be purely chemical or physical. The chemical nature
of the albuminoids is indeed so complex that some considerable time
must yet elapse before it can be completely investigated; and until
such time it is obvious that we cannot hope for any very definite
conceptions as to the nature of life. Broadly, however, the majority
of physiologists regard life as a highly intricate series of purely
physical and chemical processes, and if such a view be accepted,
there is no insuperable objection to a general theory of the origin
of living from non-living matter. By this it is not intended to imply
that the manufacture of living matter is an immediate possibility;
for even according to such a theory as we have indicated, it would
be supposed that living substance came into being by a very slow
process of Evolution, which it is hardly conceivable could ever be
repeated in the laboratory. Knowing, as we do, that there was a time
when no life existed upon the earth, and believing, as there is good
reason to believe, that there is no fundamental distinction between
living processes and ordinary chemical and physical reactions, we may
logically regard life itself as a product of a natural process of
Evolution.
[Illustration: FIG. 16.--A typical cell (greatly magnified).
(_k_) Nucleus; (_p_) cell protoplasm.]
[Illustration: FIG. 17.--The process of cell division.
_c_, The centrosome, the body which divides first, and which controls
the division of the nucleus.]
To begin at the beginning of our tale, we may ask ourselves what are
the lowest, simplest, living things that are known. The question does
not admit of any very definite answer. For as we look around among a
number of the most simple forms, we find ourselves handicapped in our
attempt to judge between them, by a lack of knowledge of their nature.
We come upon organisms so small that they appear, even under the most
powerful microscope, only as the tiniest specks; whose size is to be
measured in hundredths of thousandths of an inch. We even find good
evidence that living things exist which we are unable, in any manner
whatsoever, to see. Among the smallest known forms, and also among
some of the larger, we find organisms that we can only describe as
practically structureless, that appear as specks of almost homogeneous
protoplasm; but it seems reasonable to suppose that this appearance is
due rather to our imperfect observation than to an actual absence of
differentiation.
It is certain, however, that the lowest of the great groups is that of
the one-celled organisms. As all the higher types are built up of large
numbers of cells, essentially similar to those which constitute the
unicellular forms, it is important that we should know something of the
nature of this organic unit. A typical cell is illustrated in Fig. 16.
It consists of a mass of protoplasm, with a distinctly differentiated
portion called the nucleus. The function of the nucleus is that of
directing and controlling the activities of the cell; if it is removed,
the remaining portion of the cell soon dies; while, on the other hand,
a small portion of the cell, if it contains the nucleus, may frequently
live, and build up new protoplasm to replace what was lost. Cells are
formed only from previously existing cells, by a process of division,
which is usually simply one of halving. This process is begun in the
nucleus; it undergoes a complex rearrangement of its parts, the object
of which appears to be to insure an absolute equality in the halves,
and finally divides in two. The bulk of the protoplasm then separates
into two portions, a portion remaining round each of the nuclei. The
process of cell division is illustrated in Fig. 17.
Now it is a somewhat remarkable fact that we do not know whether or
not all the humbler forms of life possess a nucleus. It was formerly
believed that a considerable number of one-celled organisms were devoid
of the body in question, but in most of such it has been shown that
nuclear matter is present, though it may be distributed, in small
portions, throughout the cell. If organisms do exist which consist of a
cell without a nucleus, we must regard them as the simplest of living
things. In any case, the formation of a nucleus, a process by which a
kind of central government was formed, was probably one of the great
early steps of Evolution.
[Illustration:
FIG. 18.--Organism of sleeping sickness in blood. The round bodies are
red blood corpuscles.
Photo: F. Martin Duncan.]
The life-history of an ordinary one-celled organism may be briefly
summed up. It absorbs nourishment and energy, adds to its substance
until it reaches a certain fairly definite size, and then divides in
two, the halves separating, and going each its own way. In the world
of one-celled organisms there is no 'death from natural causes.' The
individual is potentially immortal, except in so far as we may regard
the individual life as ceasing when division takes place. Death occurs
only, as we say, accidentally--for example, from starvation or from the
attacks of enemies. A number of simple unicellular organisms are shown
in Figs. 18, 19, and 20.
The reader will have observed that we have referred to the group under
consideration in general terms, and without endeavouring to classify
its members as plants or animals. And indeed it is impossible to carry
this great distinction down to the lowest group of the organic world.
This stands below the first great forking of the tree of life; its
members remain in what has been described as a condition of 'chronic
indecision,' neither clearly vegetable nor definitely animal. But very
soon, in the march of progress, the forking of the roads was reached,
and whosoever was bent on journeying farther had perforce to make the
choice. We must here briefly consider what this choice was, and wherein
the fundamental distinction between a plant and an animal consists;
for, strange as the statement may seem, the basis of this distinction
is by no means generally appreciated.
[Illustration: FIG. 19.--The bacillus of bubonic plague (× 1000).
Photo: F. Martin Duncan.]
The typical plant lives by absorbing carbon dioxide gas, water, and
mineral salts from the surrounding media. These substances, by means of
energy which it gathers from the rays of the sun, the plant builds up
into organic substances, to be used in the maintenance of life, and for
growth and reproduction. This process of chemical construction occurs
only in the green, exposed parts of the plant, and indeed can occur
only in the presence of chlorophyll, the green colouring matter of the
leaves.
[Illustration: FIG. 20.--The bacillus of typhoid (× 2500 diameters).
Photo: F. Martin Duncan.]
The animal, on the other hand, lives by appropriating, either directly
or indirectly, what the plant has produced. All flesh is indeed grass,
in a different sense from that originally intended by the statement.
It is this essential difference which lies at the root of all the
plain and obvious distinctions between animals and plants. The plant
has neither the necessity to go forth in search of its food materials,
which nature brings to it, nor has it to spare of its painfully
collected energy for the labour of locomotion. Hence it remains
stationary. The animal must of necessity go to seek its more elaborate
fare, therefore it moves. Moreover, to be successful in its search,
the animal obviously requires a nervous system to direct and control
its movements, which system, except in the simplest and crudest forms,
is absent from the plant. In the main, then, the plant builds up and
saves, the animal breaks down and spends. The plant is the producer,
the animal the consumer.
[Illustration: FIG. 21.--Amœba.
_K_, Nucleus; _V_, contractile vacuole.]
Turning now to those of the lower organisms that are somewhat more
definitely animal in nature, we may describe the common Amœba.
Microscopic in size, this creature consists of a speck of semi-liquid
protoplasm, which is irregular and ever-changing in shape. It is
continually pushing out finger-like projections from various parts of
its surface, feeling, in a dim, vague way, for its food. It moves, if
but slowly, by withdrawing its substance in one direction and pouring
it forth in another. It indulges in such fare as bacteria or particles
of dead organic matter and feeds by the simple method of surrounding
the food particle with its protoplasm, and gradually digesting and
absorbing whatever it contains of nutriment. Undigested portions are
simply left behind as the creature moves on. The waste products are
drained into a simple cavity in the protoplasm called the contractile
vacuole, which empties itself periodically to the outside. The Amœba
reproduces by the ordinary process of simple fission, illustrated, with
the creature in its ordinary condition, in Figs. 21 and 22.
[Illustration: Fig. 22--Stages in division of Amœba.
K, nucleus.]
[Illustration: FIG. 23.--Paramœcium.
_EC_, Denser outer layer; _EN_, inner protoplasm; _N_, nucleus; _PV_,
contractile vacuole; _M_, mouth; _X_, cilia.
From Marshall and Hurst's _Practical Zoology_ (Smith, Elder & Co.).]
Somewhat higher than the Amœba, and apparently along the main line of
progress, stands the group which includes the slipper animalcule,
Paramœcium, shown in Fig. 23. This creature, barely visible to the
naked eye, is found in pools of water, or, for example, in drops of
rain or dew on plants, and it can generally be obtained in great
numbers by soaking a little hay in water for a day or two. It has,
as may be seen from the illustration, an elongated shape, with a
depression, the mouth, about the middle of one side. The progress made
good from the stage of the Amœba has been largely in the direction
of a more efficient method of locomotion. Instead of crawling, with
painful slowness, the Paramœcium swims freely and rapidly by means of
the numerous whip-like projections or _cilia_ which cover it, and with
which it lashes the water. An advance is also to be recognised in the
fact that the organism is surrounded by a dense outer wall; and that
its shape is consequently fixed. Hence also the Paramœcium cannot take
in food at any part of its surface, as the Amœba can, but only through
the special depression already mentioned. Excretion is carried on in
the same manner as in the Amœba. The Paramœcium is a water animal, yet
it can resist drying, and remain alive in the absence of water, for a
long period. This it accomplishes by becoming encysted, that is, by
contracting into a ball and surrounding itself with a resistant shell,
from which it can emerge when suitable conditions for active life
return. It is worth passing notice that there exist a number of forms
occupying a position intermediate between the two types which we have
described, and indicating that the second has, in all probability, been
derived from the first. One of these is shown on Fig. 24.
[Illustration:
FIG. 24.--Cercomonas, a form intermediate between the crawling Amœba
type and the free-swimming Paramœcium type.]
There is another interesting fact in connection with Paramœcium.
Under natural conditions, division and redivision continue in the
ordinary way for a large and indefinite number of generations. But
very occasionally, a process known as _conjugation_ occurs. Two
individuals lay themselves side by side, and partially unite; they
exchange portions of their nuclear substance, and finally separate
again, simple division afterwards proceeding as before. Conjugation,
although distinctly different from the ordinary process of sexual
reproduction, appears to serve the same purpose. Until quite lately its
meaning, and that of the process of sexual reproduction in general,
seemed to bid fair to remain a perpetual puzzle to biologists. But
at last we seem to be approaching the solution. The characters of a
species are determined, it is tolerably certain, by the constitution of
the cell nucleus, and accordingly as this varies from one individual
to another, so the characters of the individuals will vary. Now, if
simple division were to continue indefinitely, successive generations
would be produced on the same plan, and the racial characters would
in the main remain constant. But conditions of life vary from time to
time and from place to place, and the particular type which succeeds
best under one set of circumstances may be ill adapted for another. It
is therefore an advantage to a race to be capable of variation. And
the process of sexual reproduction, by continually bringing about a
mixture of the nuclear substance, ensures the regular production of a
variety of types. Of these various combinations of characters the few
that are suited to the prevailing conditions will, for the time being,
constitute the dominant types. When conditions change, fresh types will
be available to replace them. The process of conjugation is illustrated
in Fig. 25.
[Illustration: FIG. 25.--Stages in conjugation of Paramœcium.
_meg._, The meganucleus; _mic._, the micronucleus, which divides,
and half of which is exchanged; _p.b._, Polar bodies, which the
micronucleus throws off, and which disappear.
From Dendy's _Outlines of Evolutionary Biology_ (Constable).]
There are many groups of one-celled animals other than those typified
in the Amœba and the Paramœcium, but they do not appear to have any
significance so far as the descent of the higher animals is concerned,
and they therefore do not immediately concern us.
We have already mentioned that water is the life medium of the slipper
animalcule. It was destined to remain the natural element, both of
animals and of plants, throughout many subsequent stages of progress.
The reason of this is not far to seek. Active protoplasm consists to
the extent of about three-fourths of water, and a plentiful supply
of this is one of the essentials for the continuance of active life.
Therefore, before the conquest of the dry land could be accomplished,
devices had to be evolved both for maintaining and for conserving
the water supply--roots in the plant; in the animal, some method of
locomotion by land or air, so that water could be frequently reached;
protection against evaporation, in the form of a skin, in both; and
numerous other special devices. Add to this the fact that locomotion on
land presents much greater difficulties than that in water, and it will
hardly occasion surprise that vast ages were yet to be required before
the Evolution process could produce a land animal.
A striking analogy may be drawn between animal Evolution, from this
point onwards, and social Evolution. In the latter case we begin
with men, brought by a slow process of Evolution to a high state of
individual perfection, living in a state of savage individualism.
Each thinks and acts for himself, provides his own food, raiment, and
dwelling; constitutes his own standing army and police. From this
condition of affairs there has gradually been developed the modern
social arrangement, by which each individual helps to carry out some
distinctly special part of work for the community--be it wheat-growing,
cloth-weaving, bricklaying, or the arresting of burglars--and trusts
to the community for his requirements in all other directions. These
requirements themselves have so multiplied during the course of social
Evolution that innumerable forms of activity have sprung up between
those occupations which provide the original necessities of life. The
essence of the whole process has been co-operation and the division of
labour.
In the story of animal Evolution we have reached a point where a
highly perfected individual cell has been produced, which carries out
for itself, and for itself alone, all the activities of life. From
now onwards, co-operation and specialisation are the watchwords of
progress. There is a clubbing together, first of a few cells, then of
hundreds, and finally of millions upon millions, to form the bodies
corporate which we recognise as individual higher animals. Division
and distribution, subdivision and further distribution of the life
activities proceed at the same time, until we reach the condition
prevailing in the higher animals, where the degree of specialisation
almost passes conception. In such there is, to begin with, a vast
frontier army of skin cells, occupied in securing peace, as far as
possible, for the industries that go on within. There are directors
and controllers of these industries--the brain cells--with a myriad
of workers under their guidance, and a great and complex telegraph
system between. The workers themselves are of all descriptions--common
labourers like the cells of the muscles; transport workers like those
of the circulatory system; skilled factory hands like those of the
glands; even scavengers in the shape of the sweat gland and kidney
cells. Nay, there is even a numerous police force, of white blood
corpuscles, which patrol everywhere, arresting intruders and disposing
of them by the summary method of swallowing them whole.
[Illustration: FIG 26.--Spondylomorum, a small colony of flagellates.]
[Illustration: FIG. 27.--Magosphæra, a colonial flagellate.]
Our information regarding the early history of this co-operative
movement is fragmentary and incomplete, for only an odd species or
so seems to survive of the group which we regard as the earliest of
multicellular animals. In certain forms which are still essentially
unicellular, such as the Spondylomorum shown in Fig. 26, there is a
tendency to form smaller or larger cell colonies. When the individual
cell divides, the two daughter cells do not separate, but remain
somewhat loosely attached to each other, and the process of division
without separation continues until a considerable group is produced.
From this colony occasional individuals break away and proceed to form
new colonies. From such a type it is a comparatively easy step to
the Magosphæra described by Haeckel and illustrated in Fig. 27. This
consists of a simple ball of ciliated cells, which reproduces by the
occasional breaking away of an individual member, which divides and
redivides until a new sphere is produced. Unfortunately this animal
has only once been discovered, and many hold that it has not been
sufficiently investigated. No other of the same type is known.
If we turn to the plant kingdom, however, we find a comparatively
common organism of somewhat similar form. This is the Volvox, a plant
which consists of some thousands of cells, and reaches a size of about
a pin-head. It has the form of a hollow sphere, the wall of which
is one cell thick, and the cavity of which contains only water. The
cells bear whip-like cilia on their outer surface, by whose means
the organism is able to move, swimming by a rotary motion round a
definite axis. The individual cells are separated by layers of a
gelatinous substance, through which, however, pass connecting strands
of protoplasm. The cells, of course, contain the green colouring matter
common to plants in general. Distributed among the ordinary cells occur
a few that are distinguished by their larger size, and by the fact that
they lack cilia. These are the special reproductive members of the
colony. When the Volvox reaches maturity, these cells begin to divide,
and form new growths which take the form of hollow sacs, which project
into the cavity of the parent sphere. Later they separate from the
wall of the parent, and begin to move about, in the internal cavity,
by means of the cilia which they have developed. Finally the parent
breaks up and dies, and the progeny are set free to commence life for
themselves.
[Illustration: FIG. 28.--Volvox. A female, showing egg cells.]
[Illustration: FIG. 29.--Volvox. Male, showing packets of sperm cells.]
The fundamental importance of this type is that we have already
a division of the life activities. The majority of the cells are
concerned in the nutrition of the individual as a whole. These
ultimately perish. A minority, however, are fed and protected by them,
and these in return secure the perpetuation of the race. This division
into a mortal 'body' portion and an immortal reproductive portion is
the first and most important division of the life activities, whether
in the animal or in the plant kingdoms. The body cells, modified in
various directions for their special purposes, could not, and do not,
reproduce complete new individuals. Therefore a generalised type of
cell is maintained for the express purpose of the propagation of the
race. It is to be observed, now, that the process of reproduction
in Volvox is not always such as we have described. Sometimes the
reproductive cells are of two kinds. The one type divides into a great
number of small ciliated cells, which escape separately and directly
to the _outside_ of the sphere, and swim away. These free-swimming
individuals do not form new colonies, but seek out the reproductive
cells of the other type, which latter still form part of the organism
which has produced them. One of the free-swimming cells enters each
of those of the other kind, and the nuclei of the two merge into
one. The cell so produced, after a longer or shorter rest period,
commences to divide and redivide in the manner already described,
forming a new colony. The process that we have described is that of
sexual reproduction, and its essential features are the same as in
Volvox throughout the whole animal kingdom. The small free-swimming
cells are the male reproductive bodies or sperms, the others are the
female or egg cells. The union of the two produces the fertilised egg,
and the process of union is termed fertilisation. In Volvox, the male
and female elements are sometimes produced by the same individual, at
other times by different ones. Separation of the sexes is no necessary
accompaniment of the process of sexual reproduction, and indeed it is
only in the higher groups of animals that separate sexes are the rule.
The various conditions in Volvox are illustrated in Figs. 28, 29, and
30.
[Illustration: FIG. 30.--Volvox. Portion of a hermaphrodite individual,
showing egg cells (_O, O_{1}_), and sperms (_S_{1} S_{2} S_{3}_).]
The next great groups of animals are, on the one hand, that of the
sponges, and, on the other, that which includes the sea-anemones,
jelly-fishes, corals, etc. At first sight their structure seems vastly
different to that of the Volvox, from some form similar to which they
have probably been derived. The evidence obtained from the study of
their individual development, however, strongly suggests a process by
which we suppose that they evolved from Volvox-like ancestors. We shall
therefore briefly describe the earlier stages of the development of a
coral. The sexually produced individual starts life as a single cell,
the fertilised egg. This divides and redivides until a hollow ball of
cells is produced, which cells, like those of the Volvox, bear cilia.
Although simply spherical in shape, the creature moves by rotating
round a definite axis, like a planet. Moreover, nutriment is absorbed
not by any or every part of the surface, but only by a small area
round the lower pole. Now as development proceeds, the cells at this
pole divide more rapidly than the rest, with the natural result that
the ball begins to get out of shape. The distended portion, however,
develops to the inside, so that one part of the sphere is, as it were,
pushed into the other. When this process has been completed, the
original internal cavity is almost entirely eliminated, and a form is
produced which resembles a double-walled flask or vase. Such a form may
be taken as the fundamental architectural type of the groups that we
are now to consider. The meaning of this further step of Evolution is
again specialisation. The inner layer of cells takes on the functions
of digestion and absorption of food, there having been evolved, in
fact, the simplest possible form of mouth and stomach. Such other
functions as those of locomotion, protection, and support are exercised
by the outer layer. This process is illustrated in Fig. 31.
[Illustration: FIG. 31.--Process of gastrulation in a coral.
_A_, _B_, Blastula, or simple hollow ball; _C_, _D_, intermediate
condition; _E_, _F_, gastrula, or double-walled flask condition.]
But there is no known type of animal which, in its adult form, shows
quite the simple structure that we have described. Perhaps the
nearest approach is to be found in the lower sponges, in which two
modifications of the original plan have already been introduced.
In the first place, the creature is sedentary, being fixed, in an
inverted position, to some solid basis. It has, so to speak, ceased to
be a hunter, and is become a fisher. Secondly, its wall is pierced
in many places, so as to permit of a freer circulation, through the
digestive cavity, of the water which contains the food material. The
water passes in through these numerous perforations, and out through
the main central opening or 'mouth.' The sponges do not appear to
represent a stage in the main line of Evolution, but lead us almost
immediately into a cul-de-sac. We therefore cannot pause to describe
fully the many peculiar and interesting developments which occurred in
the group. An ordinary 'sponge,' by the way, bears the same relation to
the creature which produces it as does a 'coral' to the coral animal.
It represents, that is to say, the skeletons of a large colony of
individuals. The structure of a sponge is shown in Fig. 32.
[Illustration: FIG. 32.--Diagrammatic section of lower sponge.
_e_, inner cell layer.
_m_, middle jelly-like layer.
_z_, outer cell layer.
_a_, digestive cavity.
_i_, perforations in the wall.]
The other great group of primitive multicellular animals is that of the
Cœlenterata, and as an example of the most primitive of these we may
take the common freshwater Hydra. The Hydra reaches a length of nearly
half an inch, and is to be found attached to water-weed and the like
in streams. It consists of a hollow tube-shaped body which is fixed by
the so-called 'foot.' Two layers of cells form the wall of this tube,
these being separated by a thin membrane of gelatinous material. At
the upper end is the mouth, which leads immediately into the internal
cavity or stomach. The mouth is surrounded by a ring of from six to
eight tentacles, which are outgrowths of both cell layers. The cells
of the inner layer are large, and bear cilia that protrude into the
internal cavity. Their functions are those of digestion and absorption.
Part of the protoplasm of the outer cells is modified into a fibrous,
contractile substance, which represents the beginnings of muscle
tissue. The outer layer also forms a protective skin-like covering.
In the outer layer also occur a large number of stinging cells, each
of which has a complex mechanism for injecting a fluid poison into
any creature which should happen to come in contact with them. These
'nettle cells' occur in much greater numbers in the tentacles than
elsewhere, and here they are brought into play against the animals,
such as minute Crustaceans, which form the Hydra's prey. Coming in
contact with the tentacles, such creatures are caught, paralysed by
means of the stinging cells, and are gradually transferred into the
mouth by a slow contraction of the tentacles. The Hydra reproduces,
for the most part, by a simple process of budding. Small lateral
outgrowths are formed, which gradually develop mouth and tentacles
of their own. Ultimately these separate and are carried off by the
water, later to settle down and become attached to some fixed object.
Sometimes, however, sexual reproduction occurs. The reproductive
cells are produced, male and female on the same individual, among the
ordinary cells of the outer layer. These are set free, fertilisation
occurs in the water, and the egg develops in the same manner as that
of the coral. The Hydra is able, by means of the fibrous protoplasm of
its outer cells, to show well-marked movements. It can bend its body in
this direction or that, can contract its whole body into a small oval
mass, and is even able, by performing a number of slow somersaults, to
change its position. The structure and the methods of reproduction in
Hydra will be readily understood from the illustrations of the creature
on Figs. 33 and 34.
[Illustration: FIG. 33.--Specimens of Hydra on green water-weed.
_A_, Contracted; _B_, extended; _C_, specimen with vegetative buds;
_D_, specimen with sex cells; _sp_, sperm cells; _e_, egg.]
[Illustration: FIG. 34.--Diagrammatic section of Hydra.
_en_, Inner cell layer; _ec_, outer cell layer; _c_, nettle cell.]
If now we make a brief general survey of the group to which the Hydra
belongs, we find in it two somewhat strikingly different types. On
the one hand are sedentary forms that resemble, in a general way, the
Hydra; that consist of a tube-shaped body, with the mouth, surrounded
by a ring of tentacles, at the upper end. The sea-anemones and
corals are examples of this type, in which, however, the structure
shows various complexities as compared with that of the Hydra, which
complexities we cannot here pause to describe. On the other hand is the
well-known Medusa form, of which the common jelly-fish is a typical
example. This creature, as is well known, is mushroom shaped, with
tentacles round the edge. The mouth is in the middle of the lower
aspect, at the end of a short 'stalk.' This type is very different in
general appearance from the Hydra or sea-anemone, yet the one form
may be somewhat easily derived from the other; we have only to imagine
that a Hydra is turned upside down, that it is squashed, vertically,
until the internal cavity is greatly reduced, and the circumference,
especially in the region of the tentacles, greatly increased, and we
should have something resembling a Medusa. That the two types are
actually closely related is shown by the fact that there is in the
life-history of one group of Cœlenterates a regular alternation between
the one and the other.
[Illustration: FIG. 35.--Diagram of Medusa.
_rad_, Radial canals, with reproductive bodies, _o_; _r_, ring canal;
_t_, tentacle canal.]
If the above general conception of the structure of the Medusa be
borne in mind, its details will be easily understood. The internal
cavity, instead of being simple, has become complicated, through
the obliteration of certain parts of it, where the upper and lower
walls come in contact. What is left is a comparatively small cavity
immediately above the mouth, a number of symmetrically arranged canals
radiating out from this, and a ring canal connecting the ends of these
with each other. Another special characteristic is that there is a
great mass of gelatinous substance between the outer and inner cell
layers. The reproductive cells, as in the sea-anemones, are produced by
the inner cell layer, and escape by the mouth.
[Illustration: FIG. 36.--Diagrammatic section of Medusa.]
[Illustration: FIG. 37.--Group of Cœlenterates--Medusæ, Sea-anemones,
and Corals.]
Something remains to be said regarding the specialisation of tissues
in this group. We have already mentioned the stinging cells, and
the beginnings of muscular tissue, in Hydra. The former are a
constant feature of the Cœlenterates, while the latter reaches a very
considerable development in the higher forms, as may be judged from
the surprising rapidity with which the Medusa can swim, or from the
strength with which the sea-anemone can retract its tentacles and draw
itself together. Important, further, is the nerve tissue. This consists
of cells whose business it is to receive and transmit stimuli. They
have long fibrous projections connecting them with each other, so that
there is a network of communication throughout the whole animal. In the
Medusa, where co-ordinated movements of various portions is necessary,
there is a concentration of nerve cells into a double ring near the
edge. Here also there are special organs, probably of sight and of the
sense of balance; but as these cannot be regarded as the forerunners of
the analogous organs in higher animals, we need not pause to describe
them. The anatomy of the Cœlenterates will be better understood if the
reader will study the diagrams in Figs. 35 and 36, while some idea of
the beauty and variety met with in the group may be obtained from Fig.
37.
[Illustration: FIG. 38.--Diagram of Ctenophore.
_f_, Tentacle; _fs_, tentacle sac; _t_, central cavity; _tg_,
upper canal; _rud_, plate bearing cilia; _g_, radial canal; _r_,
longitudinal canal; _si_, sense organ.]
There is another group of jellyfish-like marine animals which have been
given the name of Ctenophora. By some they are regarded as a divergent
sub-class of the Cœlenterates, by others as a distinct main group;
in any case they appear to be important from our point of view. The
structure of a typical member is shown in Fig. 38, and a few other
forms are illustrated in Fig. 39. Our typical example is pear-shaped,
with the mouth at the lower pole. The internal cavity is complex, but
is on a different plan from that of the Medusa. There is a central
cavity communicating with the outside not only by the mouth but also by
two canals opening near the upper pole. There are two radial canals,
each of which divides into four, the branches of which lead at right
angles into other canals, running from pole to pole and blind at both
ends. There are two tentacles, as shown, which can be withdrawn into
special sacs. At the opposite end from the mouth are sense organs,
seemingly of smell and balance respectively. On the outer surface,
above each of the longitudinal canals, is a row of small plates bearing
cilia. It is by the movement of these cilia, like a multitude of minute
oars, that the animal swims--a method of locomotion which does not
occur in the true Cœlenterates. An additional feature is the formation,
at an early stage of development, of a definite third layer of cells
between the outer and the inner. This layer ultimately forms the
greater part of the jelly-like mass of the body.
[Illustration: FIG. 39.--Group of Ctenophora.]
Regarding the interrelationships of the various types that we have
described, and their respective importance with reference to the
descent of man, opinions are somewhat divided. Some believe the
Ctenophora to have been derived from the Medusa form, but the more
probable view seems to be that they have evolved separately from
some earlier and more primitive type than any existing Cœlenterate,
and that their ancestors have all been free-swimming and ciliated.
Now the Ctenophora are considered, on good grounds, to be somewhat
nearly akin to the lowest worms, and thus to stand fairly close to
the main line of Evolution. If this view be correct, the whole group
of existing Cœlenterates forms a side branch of the Evolution tree.
This fact, however, does not take away the importance of the group in
relation to the theory of the descent of the higher animals, for the
Cœlenterates have certainly retained many of the characters which were
possessed by the direct ancestors of man, such, for instance, as the
simple digestive cavity, the primitive type of body, consisting of
two cell layers, the diffuse and elementary nervous system, and the
radial arrangement of parts. Moreover, the course of Evolution in the
group, leading from the Hydra to the sea-anemone and the Medusa, has
probably been in many respects parallel to that which started from some
primitive extinct form, and led up to the Ctenophora. Therefore the
study of the group has thrown much light on the earlier history of the
animal world. Regarding the age of the group, it may be mentioned that
fossil corals, etc., are found, along with Crustaceans and Molluscs, in
the earliest known fossil-bearing beds, belonging to the Cambrian age.