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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 respir
atory 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 ph becoming more important, pt for the excretory system is needed mainly to remove their waste. These tubes
may be only greatly enlarged glands of the skin.
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. food and danger must be kept. constant exercise, or selection,
Here the lookout for Hence, as a result of 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-oesophageal 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.
In a very few turbellaria we find otolith vesicles.
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.
These are little sacks in the skin, lined with neuroepithelial 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 compli
cated 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.* 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.
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
* Cf. p. 35.
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 fluidlike 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 hindintestine, or rectum, with the anal opening. The front and hind-intestine are lined with infolded outer skin.