[Illustration: FIG. 22.—Section through the eye (“surface-eye”) of a Marine Worm (Neophanta). _i_, integument spreading over the front of the eye _c_; _l_, cuticular lens; _h_, cavity occupied by vitreous body; _p_, retinal cells; _b_, pigment; _o_, optic nerve: _o´_, expansion of optic nerve.]
[Illustration: FIG. 23.—_A._ Vertical section through the head of a very young fish, showing in the centre the cavity of the brain _c_. On each side is a hollow outgrowth (_a_) which will form the retina of the fish’s eye (“cerebral eye”); _b_, will become the optic nerve connecting the brain and the retina; _d_, integument.—_B._ Later condition of the hollow outgrowth (_a_) of _A_. Its outer wall _r_ is pressed against its deeper wall _p_ by an ingrowth (_l_) from the outer skin (ectoderm) _e_; _r_, gives rise to the retinal cells, whilst only _l_, the cellular lens, is derived from the surface of the skin.]
The cases of degeneration which I have up to this point brought forward, are cases which admit of very little dispute or doubt. They are attested by either the history of the individual development of the organisms in question, as in Sacculina, in the Barnacle, and in the Ascidian, or they are cases where the comparison of the degenerate animal, with others like it in structure, but not degenerate, renders the hypothesis of degeneration an unassailable one. Such cases are the Acarus or mite, and the skin-worm (Demodex).
We have seen that degeneration, or the simplification of the general structure of an animal, may be due to the ancestors of that animal having taken to one of two new habits of life, either the parasitic or the immobile. Other new habits of life appear also to be such as to lead to degeneration. Let us suppose a race of animals fitted and accustomed to catch their food, and having a variety of organs to help them in this chase—suppose such animals suddenly to acquire the power of feeding on the carbonic acid dissolved in the water around them just as green plants do. This would lead to a degeneration; they would cease to hunt their food, and would bask in the sunlight, taking food in by the whole surface, as plants do by their leaves. Certain small flat worms, by name Convoluta, of a bright green colour, appear to be in this condition. Their green colour is known to be the same substance as leaf-green; and Mr. Patrick Geddes has recently shown that by the aid of this green substance they feed on carbonic acid, making starch from it as plants do. As a consequence we find that their stomachs and intestines as well as their locomotive organs become simplified, since they are but little wanted. These vegetating animals, as Mr. Geddes calls them, are the exact complement of the carnivorous plants, and show how a degeneration of animal forms may be caused by _vegetative nutrition_.