WORMS TO VERTEBRATES: SKELETON AND HEAD
In tracing the genealogy of any American family it is often
difficult or impossible to say whether a certain branch is descended
from John Oldworthy or his cousin or second cousin. In the latter
cases to find the common ancestor we must go back to the grandfather
or great-grandfather. The same difficulty, but greatly enhanced,
meets us when we try to make a genealogical tree of the animal
kingdom. Thus it seems altogether probable that all higher forms are
descended from an ancestor of the same general structure and grade
of organization as the turbellaria, although probably free swimming,
and hence with somewhat different form and development, especially
of the muscular system. It seems to me altogether probable that all,
except possibly Mollusca, are descended from a common ancestor
closely resembling the schematic worm last described. Some would,
however, maintain that they diverged rather earlier than even the
turbellaria; others after the schematic worm, if such ever existed.
As far as our argument is concerned it makes little difference which
of these views we adopt.
From our turbellaria, or possibly from some even more primitive
ancestor, many lines diverged. And this was to be expected. The
coelenterata, as we saw in hydra, had developed rude digestive and
reproductive systems. The higher groups of this kingdom had
developed all, or nearly all, the tissues used in building the
bodies of higher animals--muscular, reproductive, connectile,
glandular, nervous, etc. But these are mostly very diffuse. The
muscular fibrils of a jelly-fish are mostly isolated or parallel in
bands, rarely in compact well-defined bundles. The tissues have
generally not yet been moulded into compact masses of definite form.
There are as yet very few structures to which we can give the name
of organs. To form organs and group them in a body of compact
definite form was the work pre-eminently of worms. The material for
the building was ready, but the architecture of the bilateral animal
was not even sketched. And different worms were their own
architects, untrammelled by convention or heredity, hence they built
very different, sometimes almost fantastic, structures.
We must remember, too, the great age of this group. They are present
in highly modified forms in the very oldest palæozoic strata, and
probably therefore came into existence as the first traces of
continental areas were beginning to rise above the primeval ocean.
They are literally "older than the hills." They were exposed to a
host of rapidly changing conditions, very different in different
areas. This prepares us for the fact that the worms represent a
stage in animal life corresponding fairly well to the Tower of Babel
in biblical history. The animal kingdom seems almost to explode into
a host of fragments. Our genealogical tree fairly bristles with
branches, but the branches do not seem to form any regular whorls or
spirals. Few of them have developed into more than feeble growths.
They now contain generally but few species. Many of them are
largely or entirely parasitic, and in connection with this mode of
life have undergone modifications and degeneration which make it
exceedingly difficult to decipher their descent or relationships.
Four of these branches have reached great prominence in numbers and
importance. One or two others were formerly equally numerous and
have since become almost extinct; so the brachiopoda, which have
been almost entirely replaced by mollusks. The same may very
possibly be true of others. For of the amount of extinction of
larger groups we have generally but an exceedingly faint conception.
Indeed in this respect the worms have been well compared to the
relics which fill the shelves of one of our grandmother's
china-closets.
The four great branches are the echinoderms, mollusks, articulates,
and vertebrates. The echinoderms, including starfishes, sea-urchins,
and others straggled early from the great army. We know as yet
almost nothing of their history; when deciphered it will be as
strange as any romance. The vertebrates are of course the most
important line, as including the ancestors of man. But we must take
a little glance at mollusks, including our clams, snails, and
cuttle-fishes; and at the articulates, including annelids and
culminating in insects. The molluscan and articulate lines, though
divergent, are of great importance to us as throwing a certain
amount of light on vertebrate development; and still more as showing
how a certain line of development may seem, and at first really be,
advantageous, and still lead to degeneration, or at best to but
partial success.
When we compare the forms which represent fairly well the direction
of development of these three lines, a snail or a clam with an
insect and a fish, we find clearly, I think, that the fundamental
anatomical difference lies in the skeleton; and that this resulted
from, and almost irrevocably fixed, certain habits of life.
We may picture to ourselves the primitive ancestor of mollusks as a
worm having the short and broad form of the turbellaria, but much
thicker or deeper vertically. A fuller description can be found in
the "Encyclopædia Britannica," Art., Mollusca. It was hemi-ovoid in
form. It had apparently the perivisceral cavity and nephridia of the
schematic worm, and a circulatory system. In this latter respect it
stood higher than any form which we have yet studied. Its nervous
system also was rather more advanced. It had apparently already
taken to a creeping mode of life and the muscles of its ventral
surface were strongly developed, while its exposed and far less
muscular dorsal surface was protected by a cap-like shell covering
the most important internal organs. But the integument of the whole
dorsal surface was, as is not uncommon in invertebrates, hardening
by the deposition of carbonate of lime in the integument. And this
in time increased to such an extent as to replace the primitive,
probably horny, shell.
Into the anatomy of this animal or of its descendants we have no
time to enter, for here we must be very brief. We have already
noticed that the most important viscera were lodged safely under the
shell. And as these increased in size or were crowded upward by the
muscles of the creeping disk, their portion of the body grew upward
in the form of a "visceral hump." Apparently the animal could not
increase much in length and retain the advantage of the protection
of the shell; and the shell was the dominating structure. It had
entered upon a defensive campaign. Motion, slow at the outset,
became more difficult, and the protection of the shell therefore all
the more necessary. The shell increased in size and weight and
motion became almost impossible. The snail represents the average
result of the experiment. It can crawl, but that is about all; it is
neither swift nor energetic. Even the earthworm can outcrawl it. It
has feelers and eyes, and is thus better provided with sense-organs
than almost any worm. It has a supra-oesophageal ganglion of fair
size.
The clams and oysters show even more clearly what we might call the
logical results of molluscan structure. They increased the shell
until it formed two heavy "valves" hanging down on each side of the
body and completely enclosing it. They became almost sessile, living
generally buried in the mud and gaining their food, consisting
mostly of minute particles of organic matter, by means of currents
created by cilia covering the large curtain-like gills. Their
muscular system disappeared except in the ploughshare-shaped "foot"
used mostly for burrowing, and in the muscles for closing the shell.
That portion of the body which corresponds to the head of the snail
practically aborted with nearly all the sense-organs. The nervous
system degenerated and became reduced to a rudiment. They had given
up locomotion, had withdrawn, so to speak, from the world; all the
sense they needed was just enough to distinguish the particles of
food as they swept past the mouth in the current of water. They have
an abundance of food, and "wax fat." The clam is so completely
protected by his shell and the mud that he has little to fear from
enemies. They have increased and multiplied and filled the mud.
"Requiescat in pace."
But zoölogy has its tragedies as well as human history. Let us turn
to the development of a third molluscan line terminating in the
cuttle-fishes. The ancestors of these cephalopods, although still
possessed of a shell and a high visceral hump, regained the swimming
life. First, apparently, by means of fins, and then by a simple but
very effective use of a current of water, they acquired an often
rapid locomotion. The highest forms gave up the purely defensive
campaign, developed a powerful beak, led a life like that of the old
Norse pirates, and were for a time the rulers and terrors of the
sea. With their more rapid locomotion the supra-oesophageal
ganglion reached a higher degree of development, and it was served
by sense-organs of great efficiency. They reduced the external
shell, and succeeded, in the highest forms, of almost ridding
themselves of this burden and encumbrance. Traces of it remain in
the squids, but transformed into an internal quill-like, supporting,
not defensive, skeleton. They have retraced the downward steps of
their ancestors as far as they could. And the high development of
their supra-oesophageal ganglion and sense-organs, and their
powerful jaws and arms, or tentacles, show to what good purpose they
have struggled. But the struggle was in vain, as far as the
supremacy of the animal kingdom was concerned. Their ancestors had
taken a course which rendered it impossible for their descendants to
reach the goal. Their progress became ever slower. They were
entirely and hopelessly beaten by the vertebrates. They struggled
hard, but too late.
The history of mollusks is full of interest. They show clearly how
intimately nervous development is connected with the use of the
locomotive organs. The snail crept, and slightly increased its
nervous system and sense-organs. The clam almost lost them in
connection with its stationary life. The cephalopods were
exceedingly active, developed, therefore, keen sense-organs and a
very large and complicated supra-oesophagal ganglion, which we
might almost call a brain.
The articulate series consists of two groups of animals. The higher
group includes the crabs, spiders, thousand-legs, and finally the
insects, and forms the kingdom of arthropoda. The lower members are
still usually reckoned as worms, and are included under the
annelids. Of these our common earthworm is a good example, and near
them belong the leeches. But the marine annelids, of which nereis,
or a clam-worm, is a good example, are more typical. They are often
quite large, a foot or even more in length. They are composed of
many, often several hundred, rings or segments. Between these the
body-wall is thin, so that the segments move easily upon each other,
and thus the animal can creep or writhe.
These segments are very much alike except the first two and the
last. If we examine one from the middle of the body we shall find
its structure very much like that of our schematic worm. Outside we
find a very thin, horny cuticle, secreted by the layer of cells just
beneath it, the hypodermis. Beneath the skin we find a thin layer of
transverse muscles, and then four heavy bands of longitudinal
muscles. These latter have been grouped in the four quadrants, a
much more effective arrangement than the cylindrical layer of the
schematic worm. Furthermore, the animal has on each segment a pair
of fin-like projections, stiffened with bristles, the parapodia.
These are moved by special muscles and form effective organs of
creeping.
[Illustration: 7. EUNICE LIMOSA (ANNELID). LANG, FROM EHLERS.
Front and hind end seen from dorsal surface.
_fa, fp, fc_, feelers; _a_, eye; _k_, gill;
_p_, parapodia; _ac_, anal cirri.]
Within the muscles is the perivisceral cavity, and in its central
axis the intestine, segmented like the body-wall. The reproductive
organs are formed from patches of the lining of the perivisceral
cavity, and the reproductive elements, when fully developed, fall
into the perivisceral fluid and are carried out by nephridia, just
such as we found in the schematic worm. Beside the perivisceral
cavity and its fluid there is a special circulatory system. This
consists mainly of one long tube above the intestine and a second
below, with often several smaller parallel tubes. Transverse
vessels run from these to all parts of the body. The dorsal tube
pulsates and thus acts as a heart. The surface of the body no longer
suffices to gather oxygen, hence we find special feathery gills on
the parapodia. But these gills are merely expanded portions of the
body wall, arranged so as to offer the greatest possible amount of
surface where the capillaries of the blood system can be almost
immediately in contact with the surrounding water.
[Illustration: 8. CROSS-SECTION OF BODY SEGMENT OF ANNELID. LANG.
_dp_ and _vp_, dorsal and ventral halves of parapodia; _b_ and _ac_,
bristles; _k_, gill; _dc_ and _vc_, feelers; _rm_, lateral muscles;
_lm_, longitudinal muscles; _vd_, dorsal blood-vessel; _vo_, ventral
blood-vessel; _bm_, ventral ganglion; _ov_, ovary; _tr_, opening of
nephridium in the perivisceral cavity; _np_, tubular portion of
nephridium. The circles containing dots represent eggs floating in
the perivisceral fluid.]
The nervous system consists of a large supra-oesophageal ganglion
in the first segment; then of a chain of ganglia, one to each
segment, on the ventral side of the body. With one ganglion in each
segment there is far more controlling, perceptive, ganglionic
material than in lower worms. Furthermore the supra-oesophageal
ganglion is relieved of a large part of the direct control of the
muscles of each segment, and is becoming more a centre of control
and perception for the body as a whole. It is more like our brain,
commander-in-chief, the other ganglia constituting its staff. The
sense-organs have improved greatly. There are tentacles and otolith
vesicles as very delicate organs of feeling, or possibly of hearing
also.
But the annelids were probably the first animals to develop an eye
capable of forming an image of external objects. The importance of
this organ in the pursuit of food or the escape from enemies can
scarcely be over-estimated. The lining of the mouth and pharynx can
be protruded as a proboscis, and drawn back by powerful muscles, and
is armed with two or more horny claws. Eyes and claws gave them a
great advantage over their not quite blind but really visionless and
comparatively defenceless neighbors, and they must have wrought
terrible extinction of lower and older forms. But while we cannot
over-estimate the importance of these eyes, we can easily exaggerate
their perfectness. They were of short range, fitted for seeing
objects only a few inches distant, and the image was very imperfect
in detail. But the plan or fundamental scheme of these eyes is
correct and capable of indefinitely greater development than the
organs of touch or smell, perhaps greater even than the otolith
vesicle.
And the reflex influence of the eye on the brain was the greatest
advantage of all. Hitherto with feeble muscles and sense-organs it
has hardly paid the animal to devote more material to building a
larger brain. It was better to build more muscle. But now with
stronger muscles at its command, and better sense-organs to report
to it, every grain of added brain material is beginning to be worth
ten devoted to muscle. The muscular system will still continue to
develop, but the brain has begun an almost endless march of
progress. The eye becomes of continually increasing advantage and
importance because it has a capable brain to use it; and brain is a
more and more profitable investment, because it is served by an
ever-improving eye.
[Illustration: 9. MYRMELEO FORMICARIUS. ANT-LION. HERTWIG, FROM
SCHMARDA.
1, adult; 2, larva; 3, cocoon.]
The annelid had hit upon a most advantageous line of development,
which led ultimately to the insect. The study of the insect will
show us clearly the advantages and defects of the annelid plan.
First of all, the insect, like the mollusk, has an external
skeleton. But the skeleton of the mollusk was purely protective, a
hindrance to locomotion. That of the insect is still somewhat
protective, but is mainly, almost purely, locomotive. It is never
allowed to become so heavy as to interfere with locomotion. In the
second place, the insect has three body regions, having each its own
special functions or work. And one of these is a head. The annelid
had two anterior segments differing from those of the rest of the
body; these may, perhaps, be considered as the foreshadowings of a
structure not yet realized; they can only by courtesy be called a
head. Thirdly, the insect has legs. The annelid had fin-like
parapodia, approaching the legs of insects about as closely as the
fins of a fish approach the legs of a mammal. The reproductive and
digestive systems, while somewhat improved, are not very markedly
higher than those of annelids. The excretory system has more work to
perform and reaches a rather higher development.
But in these organs there is no great or striking change; the time
for marked and rapid development of the digestive and reproductive
systems has gone by. Material can be more profitably invested in
brain or muscle. Air is carried to all parts of the body by a
special system of air-sacks and tubes. This is a very advantageous
structure for small animals with an external skeleton. In very large
animals, or where the skeleton is internal, it would hardly be
practicable; the risk of compression of the tubes at some point, and
of thus cutting off the air-supply of some portion of the body,
would be altogether too great.
The circulatory system is very poor. It consists practically only of
a heart, which drives the blood in an irregular circulation between
the other organs of the body much as with a syringe you might keep
up a system of currents in a bowl of water. But the rapidity of the
flow of the blood in our bodies is mainly to furnish a supply of
oxygen to the organs. A tea-spoonful of blood can carry a fair
amount of dissolved solid nutriment like sugar, it can carry at each
round but a very little gas like oxygen. Hence the blood must make
its rounds rapidly, carrying but a little oxygen at each circuit.
But in the insect the blood conveys only the dissolved solid
nutriment, the food; hence a comparatively irregular circulation
answers all purposes.
The skeleton is a thickening of the horny cuticle of the annelid on
the surface of each segment. The horny cylinder surrounding each
segment is composed of several pieces, and on the abdomen these are
united by flexible, infolded membranes. This allows the increase in
the size of the segment corresponding to the varying size of the
digestive and reproductive systems. In this part of the body the
skeletal ring of each segment is joined to that of the segments
before and behind it in the same manner. But in other parts of the
body we shall find the skeletal pieces of each segment and the rings
of successive segments fused in one plate of mail. The legs are the
parapodia of annelids carried to a vastly higher development. They
are slender and jointed, and yet often very powerful. A large
portion of the muscular system of the body is attached to these
appendages.
But the insect has also jaws. The annelid had teeth or claws
attached to the proboscis. But true jaws are something quite
different. They always develop by modifying some other organ. In the
insect they are modified legs. This is shown first by their
embryonic development. But the king- or horseshoe-crab has still no
true jaws, but uses the upper joints of its legs for chewing. There
are primitively three pairs of jaws of various forms for the
different kinds of food of different species or higher groups. But
some of them may disappear and the others be greatly modified into
awls for piercing, or a tube for sucking honey. Into the wonderful
transformations of these modified legs we cannot enter.
The muscles are no longer arranged to form a sack as in annelids.
Transverse muscles, running parallel to the unyielding plates of
chitin or horn could accomplish nothing. They have largely
disappeared. The work of locomotion has been transferred from the
trunk to the legs.
The abdomen of the insect is as clearly composed of distinct
segments as the body of the annelid. Of these there are perhaps
typically eleven. The thorax is composed of three segments, distinct
in the lowest forms, fused in the highest. This fusion of segments
in the thorax of the highest forms furnishes a very firm framework
for the attachment of wings and muscles. These wings are a new
development, and how they arose is still a question. But they give
the insect the capability of exceedingly rapid locomotion.
The three pairs of jaws, modified legs, in the rear half of the head
show that this portion is composed of three segments. For only one
pair of legs is ever developed on a single segment. Embryology has
shown that the portion of the head in front of the mouth is also
composed of three segments. Possibly between the præ- and post-oral
portions still another segment should be included, making a total of
seven in the head. The head has thus been formed by drawing forward
segments from the trunk, and fusing them successively with the first
or primitive head segment. This is difficult to conceive of in the
fully developed insect, where the boundary between head and thorax
is very sharp. But the ancestors of insects looked more like
thousand-legs or centipedes, and here head and thorax are much less
distinct. But in the annelid the mouth is on the second segment;
here it is on the fourth. It has evidently travelled backward. That
the mouth of an animal can migrate seems at first impossible, but if
we had time to examine the embryology of annelids and insects, it
would no longer appear inconceivable or improbable. And its backward
migration brought it among the legs which were grasping and chewing
the food. And in vertebrates the mouth has changed its position,
though not in exactly the same way. Our present mouth is probably
not at all the mouth of the primitive ancestor of vertebrates. Thus
in the insect three segments have fused around the mouth, and three,
possibly four, in front of it. This makes a head worthy of the name.
The ganglia of the three post-oral segments, which bear the jaws,
have fused in one compound ganglion innervating the mouth and jaws.
Those of the three præ-oral segments have fused to form a brain.
Eyes are well developed, giving images sometimes accurate in detail,
sometimes very rude. Ears are not uncommon. The sense of smell is
often keen.
Perhaps the greatest advance of the insect is its adaptation to land
life. This gives it a larger supply of oxygen than any aquatic
animal could ever obtain. This itself stimulates every function, and
all the work of the body goes on more energetically. Then the heat
produced is conducted off far less rapidly than in aquatic forms.
Water is a good conductor of heat, and nearly all aquatic animals
are cold-blooded. The few which are warm-blooded are protected by a
thick layer of non-conducting fat. In all land animals, even when
cold-blooded, the work of the different systems is aided by the
longer retention of the heat in the body.
Let us recapitulate. The schematic worm had a body composed of two
concentric tubes. The outer was composed of the muscles of the body
covered by the protective integument. The inner tube was the
alimentary canal with its special muscles. Between these two was the
perivisceral cavity, filled with nutritive fluid, lymph, and
furnishing a safe lodging-place for the more delicate viscera. It
represented fairly the trunk of higher animals.
The annelid added segmentation, and thus greater freedom of motion
by the parapodia. But the segments were still practically alike. In
the insect division of labor took place, that is, each group of
segments was allotted its own special work; and these groups of
segments were modified in structure to best suit the performance of
this part of the work of the body. The abdomen was least modified
and its eleven segments were devoted to digestion, reproduction, and
excretion--the old vegetative functions. Three segments were united
in the thorax; all their energy was turned to locomotion, and the
insect became thus an exceedingly active, swift animal. The third
body-region, the head, includes six segments, of which three
surrounded the mouth and furnished the jaws, while two more were
crowded or drawn forward in order that their ganglia might be added
to the old supraoesophageal ganglion and form a brain. It is
interesting to note that a form, peripatus, still exists which
stands almost midway between annelids and insects and has only four
segments in the head. The formation of the head was thus a gradual
process, one segment being added after another.
In the turbellaria the dominant functions were digestion and
reproduction, and their organs composed almost the whole body. Here
only eleven segments at most are devoted to these functions, and
nine in head and thorax to locomotion and brain. Head and thorax
have increased steadily in importance, while the abdomen has
decreased as steadily in number of segments. And the brain is
increasing thus rapidly because there are now muscles and
sense-organs of sufficient power to make such a brain of value. And
this brain perceives not only objects and qualities, but invisible
relations between these, and this is an advance amounting to a
revolution. It remembers, and uses its recollections. It is capable
of learning a little by experience and observation. The A, B, C of
thinking was probably learned long before the insect's time, and the
bee shows a fair amount of intelligence.
The line of development which the insect followed was comparatively
easy and its course probably rapid. Certain crustacea, aquatic
arthropoda, are among the oldest fossils, and it is possible that
insects lived on the land before the first fish swam in the sea.
They had fine structure and powers; and yet during the later
geologic periods they have scarcely advanced a step, and are now
apparently at a standstill. They ran splendidly for a time, and then
fell out of the race. What hindered and stopped them?
One vital defect in their whole plan of organization is evident. The
external skeleton is admirably suited to animals of small size, but
only to these. In larger animals living on land it would have to be
made so heavy as to be unwieldy and no longer economical. Their mode
of breathing also is fitted only for animals of small size having
an external skeleton. Whatever may be our explanation the fact
remains that insects are always small. This is in itself a
disadvantage. Very small animals cannot keep up a constant high
temperature unless the surrounding air is warm, for their radiating
surface is too large in comparison with their heat-producing mass.
At the first approach of even cool weather they become chilled and
sluggish, and must hibernate or die. They are conformed to but a
limited range of environment in temperature.
But small size is, as a rule, accompanied by an even greater
disadvantage. It seems to be almost always correlated with short
life. Why this is so, or how, we do not know. There are exceptions;
a crow lives as long as a man; or would, if allowed to. But, as a
rule, the length of an animal's days is roughly proportional to the
size of its body. And the insect is, as a rule, very short-lived. It
lives for a few days or weeks, or even months, but rarely outlasts
the year. It has time to learn but little by experience. The same
experience must be passed, the same emergency arise and be met, over
and over again during the lifetime of the same individual if the
animal is to learn thereby. And intelligence is based upon
experience. Hence insects can and do possess but a low grade of
intelligence. But instinct is in many cases habit fixed by heredity
and improved by selection. The rapid recurrence of successive
generations was exceedingly favorable to the development of
instincts, but very unfavorable to intelligence. Insects are
instinctive, the highest vertebrates intelligent. The future can
never belong to a tiny animal governed by instincts. Mollusks and
insects have both failed to reach the goal; another plan of
structure than theirs must be sought if the animal kingdom is to
have a future.
The future belonged to the vertebrate. To begin with less
characteristic organs the digestive system is much like that of the
annelid or schematic worm, but with greatly increased glandular and
absorptive surfaces. The present mouth of nearly all vertebrates is
probably not primitive. It is almost certainly one of the gill-slits
of some old ancestor of fish, such as now are used to discharge the
water which is used for respiration. The jaws are modified branchial
arches or the cartilaginous or bony rods which in our present fish
support the fringe of gills. These have formed a pair of exceedingly
effective and powerful jaws. The reproductive system holds still to
the old type and shows little if any improvement. The excretory
organs, kidneys, are composed primitively of nephridial tubes like
those of the schematic worm or annelid, but immensely increased in
number, modified, and improved in certain very important
particulars. The muscles in simplest forms are composed of heavy
longitudinal bands, especially developed toward the dorsal surface
of the body to the right and left of the axial skeleton. Locomotion
was produced by lashing the tail right and left, as still in fish.
There is improvement in all these organs, except perhaps the
reproductive, but nothing very new or striking. The great
improvement from this time on was not to be sought in the vegetative
organs, or even directly to any great extent in muscles.
The new and characteristic organ was not the vertebral column, or
series of vertebræ, or backbone, from which the kingdom has derived
its name. This was a later production. The primitive skeleton was
the notochord, still appearing in the embryos of all vertebrates and
persisting throughout life in fish. This is an elastic rod of
cartilage, lying just beneath the spinal marrow or nerve-cord, which
runs backward from the brain. The nerve-centres are therefore here
all dorsal, and the notochord or skeleton lies between these and the
digestive or alimentary canal. The skeleton of the clam or snail is
purely protective and a hindrance to locomotion. That of the insect
is almost purely locomotive, but external, that of the vertebrate
purely locomotive and internal. It does not lie outside even of the
nervous system, although this system especially required, and was
worthy of, protection. It does not protect even the brain; the skull
of vertebrates is an after-thought. It is almost the deepest seated
of all organs. But lying in the central axis of the body it
furnishes the very best possible attachment for muscles. Around this
primitive notochord was a layer of connectile tissue which later
gave rise to the vertebræ forming our backbone.
[Illustration: 10. CROSS-SECTION OF AXIAL SKELETON OF PETROMYZON.
HERTWIG, FROM HIEDERSHEIM.
_SS_, skeletogenous layer; _Ob_, _Ub_, dorsal and ventral processes
of _SS_; _C_, notochord; _Cs_, sheath of notochord; _Ee_, elastic
external layer of sheath; _F_, fatty tissue; _M_, spinal marrow;
_P_, sheath of _M_.]
The nervous system on the dorsal surface of the notochord consists
of the brain in the head and the spinal marrow running down the
back. The brain of all except the very lowest vertebrates consists
of four portions: 1. The cerebrum, or cerebral lobes, or simply
"forebrain," the seat of consciousness, thought, and will, and from
which no nerves proceed. Whether the primitive vertebrate had any
cerebrum is still uncertain. 2. The mid-brain, which sends nerves to
the eyes, and in this respect reminds us of the brain of insects.
Its anterior portion appears from embryology to be very primitive.
3. The small brain, or cerebellum, which in all higher forms is the
centre for co-ordination of the motions of the body. 4. The medulla,
which controls especially the internal organs. The spinal marrow, or
that portion of the nervous system which lies outside of the head,
is at the same time a great nerve-trunk and a centre for reflex
action of the muscles of the body. But the development of these
distinct portions and the division of labor between them must have
been a long and gradual process.
We have every reason to believe that here, as in insects, the head
has been formed by annexation of segments from the rump and the
fusion of their nervous matter with that of the brain. But here,
instead of only three segments, from nine to fourteen have been
fused in the head to furnish the material for the brain. Notochord
and backbone may be the most striking and apparent characteristic of
vertebrates, but their predominant characteristic is brain. On this
system they lavished material, giving it from three to four times as
much as any lower or earlier group had done. They very early set
apart the cerebral lobes to be the commander-in-chief and centre of
control for all other nerve-centres. To this all report, and from it
all directly or indirectly receive orders. It can say to every
other organ in the body, "Starve that I may live." It is the seat of
thought and will. The other portions of the brain report to it what
they have gathered of vision or sound; it explains the vision or
song or parable. It is relieved as far as possible from all lower
and routine work that it may think and remember and govern. The
vertebrate built for mind, not neglecting the body.
Every trait of vertebrates is a promise of a great future. Its
internal skeleton gives it the possibility of large size. This gave
it in time the victory in the struggle with its competitors, as to
whether it should eat or be eaten. It is vigorous and powerful, for
all its organs are at the best. It gives the possibility of later,
on land, becoming warm-blooded, _i.e._, of maintaining a constant
high temperature. It is thus resistant to climate and hardship. In
time its descendants will face the arctic winter as well as the heat
of the tropics.
But it has started on the road which leads to mind. The greater size
is correlated with longer life. The lessons of experience come to it
over and over again, and it can and must learn them. It is the
intelligent, remembering, thinking type. The insect had begun to
peer into the world of invisible and intangible relations, the
vertebrate will some day see them. This much is prophecied in his
very structure. He must be heir to an indefinite future.
* * * * *
You have probably noticed that the vertebrate differs greatly from
all his predecessors. The gulf between him and them is indeed wide
and deep. His origin and ancestry are yet far from certain. But an
attempt to decipher his past history, though it may lead to no sure
conclusions, will yet be of use to us. Practically all aquatic
vertebrates lead a swimming life, neither sessile nor creeping. The
embryonic development of our appendages leads to the same
conclusion. We must never forget that the embryonic development of
the individual recapitulates briefly the history of the development
of the race. Now the legs and arms, or fore- and hind-legs, of
higher vertebrates and the corresponding paired fins of fish develop
in the embryo as portions of a long ridge extending from front to
rear of the side of the body.
This justifies the inference that the primitive vertebrate ancestor
had a pair of long fins running along the sides of the body, but
bending slightly downward toward the rear so as to meet one another
and continue as a single caudal fin behind the anal opening. Such
fins, like the feathers of an arrow, could be useful only to keep
the animal "on an even keel" as it was forced through the water by
the lateral sweeps of the tail. They would have been useless for
creeping.
But there is another piece of evidence that he was a free swimming
form. All vertebrates breathe by gills or lungs, and these are
modified portions of the digestive system, of the walls of the
oesophagus, from which even the lung is an embryonic outgrowth.
Now practically all invertebrates breathe through modified portions
of the integument or outer surface of the body, and their gills are
merely expansions of this. In the annelid they are projections of
the parapodia, in the mollusk expansions of the skin, where the foot
or creeping sole joins the body. Why did the vertebrate take a new
and strange, and, at first sight, disadvantageous mode of
breathing? There must have been some good reason for this. The most
natural explanation would seem to be that he had no projections on
his outer surface which could develop into gills, and farther, that
he could not afford to have any. Now projections on the lower
portion of the sides of the body would be an advantage in creeping,
but a hindrance in any such mode of swimming as we have described,
or indeed in any mode of writhing through the water.
Furthermore, if he lived, not a creeping life on the bottom, but
swimming in the water above, he would have to live almost entirely
on microscopic animals and embryos; and these would be most easily
captured by a current of water brought in at the mouth. The whole
branchial apparatus in its simplest forms would seem to be an
apparatus for sifting out the microscopic particles of food and only
later a purely respiratory apparatus. Moreover, we have seen that
the parapodia of annelids naturally point to the development of an
external skeleton, for their muscles are already a part of the
external body-wall and attached to the already existing horny
cuticle. The logical goal of their development was the insect.
Now I do not wish to conceal from you that many good zoölogists
believe that the vertebrate is descended from annelids; but for this
and other reasons such a descent appears to me very improbable. It
would seem far more natural to derive the vertebrate from some free
swimming form like the schematic worm, whose largest nerve-cord lay
on the dorsal surface because its branches ran to heavy muscles much
used in swimming. Later the other nerve-cords degenerated, for such
a degeneration of nerve-cords is not at all impossible or
improbable. "No thoroughfare" is often written across paths
previously followed by blood or nervous impulses, when other paths
have been found more economical or effective.
But where did the notochord come from? I do not know. It always
forms in the embryo out of the entoderm or layer which becomes the
lining of the intestine. Now this is a very peculiar origin for
cartilage, and the notochord is a very strange cartilage even if we
have not made a mistake in calling it cartilage at all. My best
guess would be that it is simply a thickened portion of the upper
median surface of the intestine to keep the "balls" of digesting
nutriment or other hard particles in the intestine from "grinding"
against the nerve-cord as they are crowded along in the process of
digestion. Once started its elasticity would be a great aid in
swimming.
Professor Brooks has called attention to the fact that the higher a
group stands in development, the longer its ancestors have
maintained a swimming life. Thus we have noticed that the sponges
were the first to settle; then a little later the mass of the
coelenterates followed their example. But the etenophora, the
nearest relatives of bilateral animals, have remained free swimming.
Then the flat worms and mollusks took to a creeping mode of life,
while the annelids and vertebrates still swam. Then the annelids
settled to the bottom and crept, and all their descendants remained
creeping forms. The vertebrates alone remained swimming, and
probably neither they nor their descendants ever crept until they
emerged on the land, or as amphibia were preparing for land
life. If this be true, it is a fact worthy of our most careful
consideration. The swimming life would appear to be neither as easy
nor as economical as the creeping. It is certainly hard to believe
that food would not have been obtained with less effort and in
greater abundance at the bottom than in the water above. The
swimming life gave rise to higher and stronger forms; but did its
maintenance give immediate advantage in the struggle for existence?
This is an exceedingly interesting and important question, and
demands most careful consideration. But we shall be better prepared
to answer it in a future lecture.
The period of development of mollusks, articulates, and vertebrates,
is really one. They developed to a certain extent contemporaneously.
The development of vertebrates was slow, and they were the last to
appear on the stage of geological history.
You must all have noticed that development, during this period,
takes on a much more hopeful form than during that described in the
last chapter. Then digestion and reproduction were dominant. Now
muscle is of the greatest importance. If this fails of development,
as in mollusks, the group is doomed to degeneration or at best
stagnation. But we have seen the dawn of a still higher function. In
insects and vertebrates the brain is becoming of importance, and
absorbing more and more material. This is the promise of something
vastly higher and better. Better sense-organs are appearing, fitted
to aid in a wider perception of more distant objects. The vertebrate
has discovered the right path; though a long journey still lies
before it. The night is far spent, the day is at hand.