[Illustration: FIG. 18.—Ascidian Tadpole with a part only of the tail _C_. _N_, nervous system with the enlarged brain in front and the narrow spinal cord behind _n_; _N´_, is placed in the cavity of the brain: _O_, the single cerebral eye lying in the brain; _a_, similarly placed auditory organ; _K_, pharynx; _d_, intestine; _o_, rudiment of the mouth; _ch_, notochord or primitive backbone. (From Gegenbaur’s “Elements of Comparative Anatomy.”)] 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. [Illustration: 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.] [Illustration: 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.] [Illustration: 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.)]