PROTOZOA TO WORMS: CELLS, TISSUES, AND ORGANS
The first and lowest form in our ancestral series is the amoeba, a
little fresh-water animal from 1/500 to 1/1000 of an inch in
diameter. Under the microscope it looks like a little drop of
mucilage. This semifluid, mucilaginous substance is the Protoplasm.
Its outer portion is clear and transparent, its inner more granular.
In the inner portion is a little spheroidal body, the nucleus. This
is certainly of great importance in the life of the animal; but just
what it does, or what is its relation to the surrounding protoplasm
we do not yet know. There is also a little cavity around which the
protoplasm has drawn back, and on which it will soon close in again,
so that it pulsates like a heart. It is continually taking in water
from the body, or the outside, and driving it out again, and thus
aids in respiration and excretion. The animal has no organs in the
proper sense of the word, and yet it has the rudiments of all the
functions which we possess.
A little projection of the outer, clearer layer of protoplasm, a
pseudopodium, appears; into this the whole animal may flow and thus
advance a step, or the projection may be withdrawn. And this power
of change of form is a lower grade of the contractility of our
muscular cells. Prick it with a needle and it contracts. It
recognizes its food even at a microscopic distance; it appears
therefore to feel and perceive. Perhaps we might say that it has a
mind and will of its own. It is safer to say that it is irritable,
that is, it reacts to stimuli too feeble to be regarded as the cause
of its reaction. It engulfs microscopic plants, and digests them in
the internal protoplasm by the aid of an acid secretion. It breathes
oxygen, and excretes carbonic acid and urea, through its whole body
surface. Its mode of gaining the energy which it manifests is
therefore apparently like our own, by combustion of food material.
[Illustration: 1. AMOEBA PROTEUS. HERTWIG, FROM LEIDY.
_ek_, ectosarc; _en_, endosarc; _N_, food particles;
_n_, nucleus; _cv_, contractile vesicle.]
It grows and reaches a certain size, then constricts itself in the
middle and divides into two. The old amoeba has divided into two
young ones, and there is no parent left to die, and death, except by
violence, does not occur. But this absence of death in other rather
distant relatives of the amoeba, and probably in the amoeba
itself, holds true only provided that, after a series of
self-divisions, reproduction takes place after another mode. Two
rather small and weak individuals fuse together in one animal of
renewed vigor, which soon divides into two larger and stronger
descendants. We have here evidently a process corresponding to the
fertilization of the egg in higher animals; yet there is no egg,
spermatozoon, or sex.
It is a little mass of protoplasm containing a nucleus, and
corresponds, therefore, to one of the cells, most closely to the
egg-cell or spermatozoon of higher animals. If every living being is
descended from a single cell, the fertilized egg, it is not hard to
believe that all higher animals are descended from an ancestor
having the general structure or lack of structure of the amoeba.
But is the amoeba really structureless? Probably it has an
exceedingly complex structure, but our microscopes and technique are
still too imperfect to show more than traces of it. Says Hertwig:
"Protoplasm is not a single chemical substance, however complicated,
but a mixture of many substances, which we must picture to ourselves
as finest particles united in a wonderfully complicated structure."
Truly protoplasm is, to borrow Mephistopheles' expression concerning
blood, a "quite peculiar juice." And the complexity of the nucleus
is far more evident than that of the protoplasm. Is protoplasm
itself the result of a long development? If so, out of what and how
did it develop? We cannot even guess. But the beginning of life may,
apparently must, have been indefinitely farther back than the
simplest now existing form. The study of the amoeba cannot fail to
raise a host of questions in the mind of any thoughtful man.
As we have here the animal reduced, so to speak, to lowest terms, it
may be well to examine a little more closely into its physiology and
compare it briefly with our own.
The amoeba eats food as we do, but the food is digested directly
in the internal protoplasm instead of in a stomach; and once
digested it diffuses to all parts of the cell; here it is built up
into compounds of a more complex structure, and forms an integral
part of the animal body. The dead food particle has been transformed
into living protoplasm, the continually repeated miracle of life.
But it does not remain long in this condition. In contact with the
oxygen from the air it is soon oxidized, burned up to furnish the
energy necessary for the motion and irritability of the body. We are
all of us low-temperature engines. The digestive function exists in
all animals merely to bring the food into a soluble, diffusible
form, so that it can pass to all parts of the body and be used for
fuel or growth. In our body a circulatory system is necessary to
carry food and oxygen to the cells and to remove their waste. For
most of our cells lie at a distance from the stomach, lungs, and
kidney. But in a small animal the circulatory system is often
unnecessary and fails. Breathing and excretion take place through
the whole surface of the body. The body of the frog is devoid of
scales, so that the blood is separated from the surrounding water
only by a thin membrane, and it breathes and excretes to a certain
extent in the same way.
But another factor has to be considered. If we double each dimension
of our amoeba, we shall increase its surface four times, its mass
eight-fold. Now the power of absorbing oxygen and excreting waste is
evidently proportional to the excretory and respiratory surface, and
much the same is true of digestion. But the amount of oxygen
required, and of waste to be removed is proportional to the mass;
for every particle of protoplasm requires food and oxygen, and
produces waste. The particles of protoplasm in our new, larger
amoeba can therefore receive only half as much oxygen as before,
and rid themselves of their waste only half as fast. There is
danger of what in our bodies would be called suffocation and
blood-poisoning. The amoeba having attained a certain size meets
this emergency by dividing into two small individuals, the division
is a physical adaptation. But the many-celled animal cannot do this;
it must keep its cells together. It gains the additional surface by
folding and plaiting. And the complicated internal structure of
higher animals is in its last analysis such a folding and plaiting
in order to maintain the proper ratio between the exposed surface of
the cells and their mass. And each cell in our bodies lives in one
sense its own individual life, only bathed in the lymph and
receiving from it its food and oxygen instead of taking it from the
water.
But in another sense the cells of our body live an entirely
different life, for they form a community. Division of labor has
taken place between them, they are interdependent, correlated with
one another, subject therefore to the laws of the whole community or
organism. There are many respects in which it is impossible to
compare Robinson Crusoe with a workman in a huge watch factory; yet
they are both men.
Both the amoeba and we live in the closest relation to our
environment, and conformity to it is evidently necessary: life has
been defined as the adjustment of internal relations to external
conditions. We continually take food, use it for energy and growth,
and return the simpler waste compounds. We are all of us, as
Professor Huxley has said, "whirlpools on the surface of Nature;"
when the whirl of exchange of particles ceases we die. We have seen
that the fusion of two amoebæ results in a new rejuvenated
individual. Why is a mixture of two protoplasms better than one? We
can frame hypotheses; we know nothing about it. What of the mind of
the amoeba? A host of questions throng upon us and we can answer
no one of them. All the great questions concerning life confront us
here in the lowest term of the animal series, and appear as
insoluble as in the highest.
Our second ancestral form is also a fresh-water animal, the hydra.
This is a little, vase-shaped animal, which usually lives attached
to grass-stems or sticks, but has the power to free itself and hang
on the surface of the water or to slowly creep on the bottom. The
mouth is at the top of the vase, and the simple, undivided cavity
within the vase is the digestive cavity. Around the mouth is a ring
of from four to ten hollow tentacles, whose cavities communicate
freely underneath with the digestive cavity. Not only is food taken
in at the mouth, but indigestible material is thrown out here. The
animal may thus be compared to a nearly cylindrical sack with a
circle of tubes attached to it above. The body consists of two
layers of cells, the ectoderm on the outside and the entoderm lining
the digestive cavity. Between these two is a structureless, elastic
membrane, which tends to keep the body moderately expanded.
The food is captured by the tentacles; but digestion takes place
only partially in the digestive cavity, for each surrounding cell
engulfs small particles of food and digests them within itself. The
entodermal cells behave in this respect much like a colony of
amoebæ. The cells of both layers have at their bases long muscular
fibrils, those of the ectodermal cells running longitudinally, those
of the entoderm transversely. The animal can thus contract its body
in both directions, or, if the body contain water and the transverse
muscles are contracted, the pressure of the water lengthens the body
and tends to extend the tentacles.
On the outside of the elastic membrane, just beneath the ectoderm,
is a plexus or cobweb of nervous cells and fibrils. As in every
nervous system, three elements are here to be found. 1. An afferent
or sensory nerve-fibril, which under adequate stimulus is set in
vibration by some cell of the epidermis or ectoderm, which is
therefore called a sensory cell. 2. A central or ganglion
cell, which receives the sensory impulse, translates it into
consciousness, and is the seat of whatever powers of perception,
thought, or will the animal possesses. This also gives rise to the
efferent or motor impulses, which are conveyed by (3) a motor fibril
to the corresponding muscle, exciting its contraction. But there are
also nerve-fibrils connecting the different ganglion cells, so that
they may act in unison. In the higher animals we shall find these
central or ganglion cells condensed in one or a few masses or
ganglia. But here they are scattered over the whole surface of the
elastic supporting membrane.
The reproductive organs for the production of eggs and spermatozoa
form little protuberances on the outside of the body below the
tentacles. But hydra reproduces mostly by budding; new individuals
growing out of the side of the old one, like branches from the trunk
of a tree, but afterward breaking free and leading an independent
life. There are special forms of cells besides those described;
nettle cells for capturing food, interstitial cells, etc., but these
do not concern us.
The distance from the single-celled amoeba to hydra is vast,
probably really greater than that between any other successive terms
of our series. It may therefore be useful to consider one or two
intermediate forms and the parallel embryonic stages of higher
animals, and to see how the higher many-celled animal originates
from the unicellular stage.
The amoeba is an illustration of a great kingdom of similar,
practically unicellular forms, which have played no unimportant part
in the geological history of the globe. These are the protozoa. They
include, first of all, the foraminifera, which usually have shells
composed of carbonate of lime. These shells, settling to the bottom
of the ocean, have accumulated in vast beds, and when compacted and
raised above the surface, form chalk, limestone, or marble,
according to the degree and mode of their hardening.
The protozoa include also the flagellata, a great, very poorly
defined mass of forms occupying the boundary between the plant and
animal kingdoms. They are usually unicellular, and their protoplasm
is surrounded by a thin, structureless membrane. This prevents their
putting out pseudopodia as organs of motion. Instead of these they
have at one end of the ovoid or pear-shaped body a long,
whiplash-like process or thread, a flagellum, and by swinging this
they propel themselves through the water. These flagellata seem to
have a rather marked tendency to form colonies. The first individual
gives rise to others by division. But the division is not complete;
the new individuals remain connected by the undivided rear end of
the body. And such a colony may come to contain a large number of
individuals.
[Illustration: 2. MAGOSPHÆRA PLANULA. LANG, FROM HAECKEL.]
Such a colony is represented by magosphæra. This is a microscopic
globular form, discovered by Professor Haeckel on the coast of
Norway. It consists of a large number of conical or pear-shaped
individual cells, whose apices are turned toward the centre of the
sphere. The cells are cemented together by a mucilaginous substance.
Around their exposed larger ends, which form the surface of the
sphere, are rows of flagella, by whose united action the colony
rolls through the water. After a time each individual absorbs its
flagella, the colony is broken up, the different individuals settle
to the bottom, and each gives rise by division to a new colony. This
group of cells may be considered as a colony or as an individual.
Each term is defensible.
Volvox is also a spheroidal organism, composed often of a very large
number of flagellated cells. But it differs from magosphæra in
certain important respects. In the first place its cells have
chlorophyl, the green coloring matter of plants. It lives therefore
on unorganized fluid nourishment, carbon dioxide, nitrates, etc. It
is a plant. But certain characteristics render it probable that it
once lived on solid food and was therefore an animal. For where
almost the sole difference between plants and animals is in the
fluid or solid character of their food, a change from the one form
into the other is not as difficult or improbable as one might
naturally think. And plants and animals are here so near together,
and travelling by roads so nearly parallel, that, even if volvox
never was an animal, it might still serve very well to illustrate a
stage through which animals must have passed.
The cells of volvox do not form a solid mass, but have arranged
themselves in a single layer on the outer surface of the sphere. For
a time, under favorable circumstances, volvox reproduces very much
like magosphæra, and each cell can give rise to a new, many-celled
individual. But after a time, especially under unfavorable
circumstances, a new mode of reproduction appears. Certain cells
withdraw from the outer layer into the interior of the colony. Here
they are nourished by the other cells and develop into true
reproductive elements, eggs and spermatozoa. Fertilization, that is,
the union of egg and spermatozoon, or mainly of their nuclei, takes
place; and the fertilized egg develops into a new organism. But the
other cells, which have been all the time nourishing these, seem now
to lack nutriment, strength, or vitality to give rise to a new
colony. They die.
We find thus in volvox division of labor and corresponding
difference of structure or differentiation; certain cells retain the
power of fusing with other corresponding cells, and thus of
rejuvenescence and of giving rise to a new organism. And these
cells, forming a series through all generations, are evidently
immortal like the protozoa. Natural death cannot touch them. These
are the reproductive cells. The other cells nourish and transport
them and carry on the work of excretion and respiration. These
latter correspond practically to our whole body. We call them
somatic cells. In volvox they are entirely subservient to, and exist
for, the reproductive cells, and die when they have completed their
service of these. The body is here only a vehicle for ova.
Furthermore, in volvox there has arisen such an interdependence of
cells that we can no longer speak of it as a colony. The colony has
become an individual by division of labor and the resulting
differentiation in structure.
But hydra gives us but a poor idea of the coelenterata, to which
kingdom it belongs. The higher coelenterata have nearly or quite
all the tissues of higher animals--muscular, connective, glandular,
etc. And by tissues we mean groups of cells modified in form and
structure for the performance of a special work or function. The
protozoa developed the cell for all time to come, the coelenterata
developed the tissues which still compose our bodies. But they had
them mainly in a diffuse form. A sort of digestive and reproductive
system they did possess. But the work of arranging these tissues and
condensing them into compact organs was to be done by the next
higher group, the worms.
Let us now take a glance at certain stages of embryonic development
which correspond to these earliest ancestral forms. We should expect
some such correspondence from the fact already stated that the
embryonic development of the individual is a brief recapitulation of
the ancestral development of the species or larger group. The egg of
the lowest vertebrate, amphioxus, shows these changes in a simple
and apparently primitive form.
[Illustration: 3. IMMATURE EGG-SHELL FROM OVARY OF ECHINODERM.
HATSCHEK, FROM HERTWIG.]
The fertilized egg of any animal consists of a single cell, a little
mass of protoplasm containing a nucleus and surrounded by a
structureless membrane. The egg is globular. The nucleus undergoes
certain very peculiar, still but little understood, changes and
divides into two. The protoplasm also soon divides into two masses
clustering each around its own nucleus. The plane of division will
be marked around the outside by a circular furrow, but the cells
will still remain united by a large part of the membrane which
bounds their adjacent, newly formed, internal faces.
Let us suppose that the egg lay so that the first plane of division
was vertical and extending north and south. Each cell or half of the
egg will divide into two precisely as before. The new plane of
division will be vertical, but extending east and west. Each plane
passes through the centre of the egg, and the four cells are of the
same form and size, like much-rounded quarters of an orange. The
third plane will lie horizontal or equatorial, and will divide each
of these quarters into an upper and lower octant. The cells keep on
dividing rapidly, the eight form sixteen, then thirty-two, etc. The
sharp angle by which the cells met at the centre has become rounded
off, and has left a little space, the segmentation cavity, filled
with fluid in the middle of the embryo. The cells continue to press
or be crowded away from the centre and form a layer one cell deep on
the surface of the sphere.
This embryo, resembling a hollow rubber ball filled with fluid, is
called a blastosphere. It corresponds in structure with the fully
developed volvox, except, of course, in lacking reproductive cells.
[Illustration: 4. GASTRULA. HATSCHEK, FROM HERTWIG.
Outer layer is the ectoderm; inner layer, the entoderm; internal
cavity, the archenteron; mouth of cavity, blastopore.]
If the rubber ball has a hole in it so that I can squeeze out the
water, I can thrust the one-half into the other, and change the ball
into a double-walled cup. A similar change takes place in the
embryo. The cells of the lower half of the blastosphere are slightly
larger than those of the upper half. This lower hemisphere flattens
and then thrusts itself, or is invaginated, into the upper
hemisphere of smaller cells and forms its lining. This cup-shaped
embryo is called the gastrula. The cup deepens somewhat and becomes
ovoid. Take a boiled egg, make a hole in the smaller end and remove
the yolk, and you have a passable model of a gastrula. The shell
corresponds to the ectoderm or outer layer of smaller cells; the
layer of "white" represents the entoderm or lining of larger cells.
The space occupied by the yolk corresponds to the archenteron or
primitive digestive cavity; and the opening at the end to the
primitive mouth or blastopore. Ectoderm and entoderm unite around
the mouth. Both the blastosphere and gastrula often swim freely by
flagella.
You can hardly have failed to notice how closely the gastrula
corresponds to a hydra, and many facts lead us to believe that the
still earlier ancestor of the hydra was free swimming, and that the
tentacles are a later development correlated with its adult sessile
life. Yet we must not forget that the hydra is even now not quite
sessile, it moves somewhat. And our ancestor was almost certainly a
free swimming gastræa, or hypothetical form corresponding in form
and structure to the gastrula. The ancestor of man never settled
down lazily into a sessile life.
But how is an adult worm or vertebrate formed out of such a
gastrula? To answer this would require a course of lectures on
embryology. But certain changes interest us. Between the ectoderm
and entoderm of the gastrula, in the space occupied by the
supporting membrane of hydra, a new layer of cells, the mesoderm,
appears. This has been produced by the rapid growth and reproduction
of certain cells of the entoderm which have migrated, so to speak,
into this new position. In higher forms it becomes of continually
greater importance, until finally nearly all the organs of the body
develop from it. In our bodies only the lining of the mid-intestine
and of its glands has arisen from the entoderm. And only the
epidermis, or outer layer of our skin, and the nervous system and
parts of our sense-organs have arisen from the ectoderm. But our
mid-intestine is still the greatly elongated archenteron of the
gastrula.
We may therefore compare the hydra or gastrula to a little portion
of the lining of the human mid-intestine covered with a little flake
of epidermis. This much the hydra has attained. But our bones and
muscles and blood-vessels all come from the mesoderm by folding,
plaiting, and channelling, and division of labor resulting in
differentiation of structure. Of all true mesodermal structures the
hydra has actually none, but in the ectodermal and entodermal cells
he has the potentiality of them all. We must now try to discover how
these potentialities became actualities in higher forms.
The third stage in our ancestral series is the turbellarian. This is
a little, flat, oval worm, varying greatly in size in different
species, and found both in fresh and salt water. Some would deny
that this worm belonged in our series at all. But, while doubtless
considerably modified, it has still retained many characteristics
almost certainly possessed by our primitive bilateral ancestor. The
different parts of hydra were arranged like those of most flowers,
around one main vertical axis; it was thus radiate in structure,
having neither front nor rear, right nor left side. But our little
turbellaria, while still without a head, has one end which goes
first and can be called the front end. The upper or dorsal surface
is usually more colored with pigment cells than the lower or ventral
surface, on which is the mouth. It has also a right and left side.
It is thus bilateral.
The gastræa swam by cilia, little eyelash-like processes which urge
the animal forward like a myriad of microscopic oars. In our bodies
they are sometimes used to keep up a current, _e.g._, to remove
foreign particles from the lungs. The turbellaria is still covered
with cilia, probably an inheritance from the gastræa; for, while in
smaller forms they may still be the principal means of locomotion,
in larger ones the muscles are beginning to assume this function and
the animal moves by writhing. The bilateral symmetry has arisen in
connection with this mode of locomotion and is thus a mark of
important progress.
In the turbellaria we find for the first time a true body-wall
distinct from underlying organs. The outer layer of this is a
ciliated epithelium or layer of cells. Under this an elastic
membrane may occur. Then come true body muscles, running
transversely, longitudinally and dorso-ventrally. Between the
external transverse and the internal longitudinal layers we often
find two muscular layers whose fibres run diagonally. The body is
well provided with muscles, but their arrangement is still far from
economical or effective.
Within the body-wall is the parenchym. This is a spongy mass of
connectile tissue in which the other organs are embedded. The mouth
lies in the middle, or near the front of the ventral surface. The
intestine varies in form, but is provided with its own layers of
longitudinal and transverse muscles, and usually has paired pouches
extending out from it into the body parenchym. These seem to
distribute the dissolved nutriment; hence the whole cavity is still
often called a gastro-vascular cavity as serving both digestion and
circulation. There is no anal opening, but indigestible material is
still cast out through the mouth.
The animal can gain sufficient oxygen to supply its muscles and
nerves, which are the principal seats of combustion, through the
external surface. It has, therefore, no special respiratory organs.
But the waste matter of the muscles cannot escape so easily, for
these are becoming deeper seated. Hence we find an excretory system
consisting of two tubes with many branches in the parenchym, and
discharging at the rear end of the body. This again is a sign that
the muscles are becoming more important, for the excretory system is
needed mainly to remove their waste. These tubes maybe only greatly
enlarged glands of the skin.
[Illustration: 5. TURBELLARIAN. LANG.
_va_ and _ha_, front and rear branches of gastro-vascular cavity;
_ph_, pharynx. The dark oval with fine branches represents the
nervous system.]
The nervous system consists of a plexus of fibres and cells, the
cells originating impulses and the fibres conveying them. But this
much was present in hydra also. Here the front end of the body goes
foremost and is continually coming in contact with new conditions.
Here the lookout for food and danger must be kept. Hence, as a
result of constant exercise, or selection, or both, the
nerve-plexus has thickened at this point into a little compact mass
of cells and fibres called a ganglion. And because this ganglion
throughout higher forms usually lies over the oesophagus, it is
called the supra-oesophogeal ganglion. This is the first faint and
dim prophecy of a brain, and it sends its nerves to the front end of
the body. But there run from it to the rear end of the body four to
eight nerve-cords, consisting of bundles of nerve-threads like our
nerves, but overlaid with a coating of ganglion cells capable of
originating impulses. These cords are, therefore, like the plexus
from which they have condensed, both nerves and centres;
differentiation has not gone so far as at the front of the body.
Sense organs are still very rudimentary. Special cells of the skin
have been modified into neuro-epithelial cells, having sensory hairs
protruding from them and nerve-fibrils running from their bases.
[Illustration: 6. CROSS-SECTION OF TURBELLARIAN. HATSCHEK, FROM
JIJIMA.
_e_, external skin; _rm_, lateral muscles; _la_ and _li_,
longitudinal muscles; _mdv_, dorso-ventral muscles; _pa_,
parenchyma; _h_, testicle; _ov_, oviduct; _dt_, yolk-gland; _n_,
ventral nerve; _i_, gastro-vascular cavity.]
In a very few turbellaria we find otolith vesicles. These are
little sacks in the skin, lined with neuro-epithelial cells and
having in the middle a little concretion of carbonate of lime hung
on rather a stiffer hair, like a clapper in a bell. Such organs
serve in higher animals as organs of hearing, for the sensory hairs
are set in vibration by the sound-waves. It is quite as probable
that they here serve as organs for feeling the slightest vibrations
in the surrounding water, and thus giving warning of approaching
food or danger. The animal has also eyes, and these may be very
numerous. They are not able to form images of external objects, but
only of perceiving light and the direction of its source. A little
group of these eyes lies directly over the brain, near the front end
of the body; the others are distributed around the front or nearly
the whole margin of the body.
The turbellaria, doubtless, have the sense of smell, although we can
discover no special olfactory organ. This sense would seem to be as
old as protoplasm itself.
This distribution of the eyes around a large portion of the margin,
and certain other characteristics of the adult structure and of the
embryonic development, are very interesting, as giving hints of the
development of the turbellaria from some radiate ancestor. The mouth
is in a most unfavorable position, in or near the middle of the
body, rarely at the front end, as the animal has to swim over its
food before it can grasp it. The animal only slowly rids itself of
old disadvantageous form and structure and adapts itself completely
to a higher mode of life.
By far the most highly developed system in the body is the
reproductive. It is doubtful whether any animal, except, perhaps,
the mollusk, has as complicated and highly developed reproductive
organs. By markedly higher forms they certainly grow simpler.
And here we must notice certain general considerations. We found
that reproduction in the amoeba could be defined as growth beyond
the limit normal to the individual. This form of growth benefits
especially the species. The needs and expenses of the individual
will therefore first be met and then the balance be devoted to
reproduction. Now the income of the animal is proportional to its
surface, its expense to its mass, and activity. And the ratio of
surface to mass is most favorable in the smallest animals.[A] Hence,
smaller animals, as a rule, increase faster than larger ones; and
this is only one illustration of the fact that great size in an
animal is anything but an unmixed advantage to its possessor. But
muscles and nerves are the most expensive systems; here most of the
food is burned up. Hence energetic animals have a small balance
remaining. Now the turbellarian is small and sluggish, with a fair
digestive system. With a great amount of nutriment at its disposal
the reproductive system came rapidly to a high development, and
relatively to other organs stands higher than it almost ever will
again.
[Footnote A: Cf. p. 35.]
It is only fair to state that good authorities hold that so
primitive an animal could not originally have had so highly
developed a system, and that this characteristic must be acquired,
not ancestral.
That certain portions of it may be later developments may be not
only possible but probable. But anyone who has carefully studied the
different groups of worms, will, I think, readily grant that in the
stage of these flat worms reproduction was the dominant function,
which had most nearly attained its possible height of development.
From this time on the muscular and nervous systems were to claim an
ever-increasing share of the nutriment, and the balance for
reproduction is to grow smaller.
At the close of this lecture I wish to describe very briefly a
hypothetical form. It no longer exists; perhaps it never did. But
many facts of embryology and comparative anatomy point to such a
form as a very possible ancestor of all forms higher than flat
worms, viz., mollusks, arthropods, and vertebrates.
It was probably rather long and cylindrical, resembling a small
and short earthworm in shape. The skin may have been much like
that of turbellaria. Within this the muscles run in only
two-directions--longitudinally and transversely. Between these and
the intestine is a cavity--the perivisceral cavity--like that of our
own bodies, but filled with a nutritive fluid like our lymph. This
cavity seems to have developed by the expansion and cutting off of
the paired lateral outgrowths of the digestive system of some old
flat worm. But other modes of development are quite possible. The
intestine has now an anal opening at or near the rear end of the
body. The food moves only from front to rear, and reaches each part
always in a certain condition. Digestion proper and absorption have
been distributed to different cells, and the work is better done.
Three portions can be readily distinguished: fore-intestine with the
mouth, mid-intestine, as the seat of digestion and absorption, and
hind-intestine, or rectum, with the anal opening. The front and
hind-intestine are lined with infolded outer skin.
The nervous system consists of a supra-oesophageal ganglion with
four posterior nerve-cords--one dorsal, two lateral, and one (or
perhaps two) ventral. There were probably also remains of the old
plexus, but this is fast disappearing. The excretory system consists
of a pair of tubes discharging through the sides of the body-wall,
and having each a ciliated, funnel-shaped opening in the
perivisceral cavity. These have received the name of nephridia.
Through these also the eggs and spermatozoa are discharged. The
reproductive organs are modified patches of the peritoneum, or
lining of the perivisceral cavity.
The number of muscles or muscular layers has been reduced in this
animal. But such a reduction in the number of like parts in any
animal is a sign of progress. And the longitudinal muscles have
increased in size and strength, and the animal moves by writhing.
Such a worm has the general plan of the body of the higher forms
fairly well, though rudely, sketched. Many improvements will come,
and details be added. But the rudiments of the trunk of even our own
bodies are already visible. Head, in any proper sense of the term,
and skeleton are still lacking; they remain to be developed.
And yet, taking the most hopeful view possible concerning the animal
kingdom, its prospects of attaining anything very lofty seem at this
point poor. Its highest representative is a headless trunk, without
skeleton or legs. It has no brain in any proper sense of the word,
its sense-organs are feeble; it moves by writhing. Its life is
devoted to digestion and reproduction. Whatever higher organs it has
are subsidiary to these lower functions. And yet it has taken ages
on ages to develop this much. If _this_ is the highest visible
result of ages on ages of development, what hope is there for the
future? Can such a thing be the ancestor of a thinking, moral,
religious person, like man? "That is not first which is spiritual,
but that which is natural (animal, sensuous); and afterward that
which is spiritual." First, in order of time, must come the body,
and then the mind and spirit shall be enthroned in it. The little
knot of nervous material which forms the supra-oesophageal
ganglion is so small that it might easily escape our notice; but it
is the promise of an infinite future. The atom of nervous power
shall increase until it subdues and dominates the whole mass.