It is clear then that the Ascidians must be admitted to be Vertebrates, and must be classified in that great sub-kingdom or branch of the animal pedigree. The Ascidian tadpole is very unlike its parent the Ascidian, and has to go through a process of degeneration in order to arrive at the adult structure. The diagrams which are reproduced in Figs. 19 and 20, show how this degeneration proceeds. It will be observed, that in somewhat the same manner as the young barnacle, the young Ascidian fixes itself to a stone by its head: then the tail with its notochord and nerve-chord atrophies. The body grows and gradually changes its shape, whilst the cloacal chamber forms. The brain remains quite small and undeveloped, and the remarkable myelonic eye (the eye in the brain) disappears. The number of gill-slits increases as the animal grows in size and its outer skin becomes tough and leather-like.
Fig. 19.—Degeneration of Ascidian Tadpole to form the adult. The black pieces represent the rock or stone to which the Tadpole has fixed its head.
Fig. 20.—Very young Ascidian with only two gill-slits. Compare with Fig 15; which is, however, seen from the other side, so that left there corresponds to right here.
Fig. 21.—Section through the eye (“surface-eye”) of a Water-beetle’s larva. All the cells are seen to be in a row continuous with h, the cells of the outermost skin or ectoderm. p, pigmented cells; r, retinal cells connected at o with the optic nerve; g, transparent cells (forming a kind of “vitreous body”); l, cuticular lens. (From Gegenbaur’s “Elements of Comparative Anatomy,” after Grenacher.)
Before saying anything further on the subject of degeneration, it seems desirable once more to direct attention to the myelonic or cerebral eye which the Ascidian tadpole possesses in common with all Vertebrates. All other animals which have eyes develop the retina or sensitive part of the eye from their outer skin (See Figs. 21 and 22, and explanation.) It is easy to understand that an organ which is to be affected by the light should form on the surface of the body where the light falls. It has long been known as a very puzzling and unaccountable peculiarity of Vertebrates, that the retina or sensitive part of the eye grows out in the embryo as a bud or vesicle of the brain, and thus forms deeply below the surface and away from the light (see Fig. 23, and explanation). The Ascidian tadpole helps us to understand this, for it is perfectly transparent and has its eye actually inside its brain. The light passes through the transparent tissues and acts on the pigmented eye, lying deep in the brain. We are thus led to the conclusion—and I believe this inference to be now for the first time put into so many words—that the original Vertebrate must have been a transparent animal, and had an eye or pair of eyes inside its brain, like that of the Ascidian tadpole. As the tissues of this ancestral Vertebrate grew denser and more opaque, the eye-bearing part of the brain was forced by natural selection to grow outwards towards the surface, in order that it might still be in a position to receive the influence of the sun’s rays. Thus the very peculiar mode of development of the Vertebrate eye from two parts, a brain-vesicle (Fig. 23, A a, and B p r), and a skin-vesicle (Fig. 23, B e, l), is accounted for.
