[Illustration: FIG. 26.—Metamorphosis of an Ascidian. A, free-swimming tailed larva; B, the metamorphosis—larva attached; C, tail and nervous system of larva degenerating; D, further degeneration and metamorphosis of larva into E, the young fixed Ascidian. _at_, Atrial invagination; _ch_, notochord; _hy_, hypoblast cells; _i_, intestine; _m_, mouth; _mes_, mesenteron; _n.c_, neural canal; _n.v_, neural vesicle with sense-organs. (Modified from Kowalevsky and others.)] The unpaired optic organ in the cerebral vesicle when fully formed has a retina, pigment layer, lens and cornea; while the ventral median organ is a large, spherical, partially-pigmented otolith attached by delicate hair-like processes to the summit of a hollow "crista acustica" (Fig. 26, A). After a few hours, or at most a day or so, the larva attaches itself {61}by one or more of the three anterior ectodermal glandular papillae (one dorsal and two lateral) to some foreign body, and commences the retrogressive metamorphosis which leads to the adult state. The adhering papillae, having performed their function, begin to atrophy, and their place is taken by the rapidly increasing test. The tail which at first vibrates rapidly is partly withdrawn from the test and absorbed, and partly cast off in shreds (Fig. 26, B, C, D). The notochord, nerve-tube, muscles, etc., are withdrawn into the body, where they break down and are absorbed by phagocytes. The posterior part of the nerve cord and its anterior end with the large sense-organs disappear, and the middle part or trunk-ganglion is reduced to form the relatively small ganglion of the adult, underneath which the hypophysial tube gives rise to the neural gland. While the locomotory, nervous and sensory organs are thus disappearing, or being reduced, the alimentary canal and reproductive viscera are growing largely. The branchial sac enlarges, its walls become penetrated by blood-channels, and grow out to form bars and papillae, and the number of openings greatly increases by the primary gill-slits being broken up into the transverse rows of stigmata. The stomach and intestine, which developed as an outgrowth from the back of the branchial sac at the right side, become longer and curve, so that the end of the intestine acquires an opening into at first the left hand side, and eventually the cloacal or median part of the atrial cavity. The adhering papillae have now disappeared, and are replaced functionally by a growth of the test over neighbouring objects; and at the same time the region of the body between the point of fixation and the mouth (branchial aperture) increases rapidly in extent, so as to cause the body of the Ascidian to rotate through about 180°, and thus the branchial siphon is carried to the opposite end from the area of attachment (see Fig. 26, B, C, D, E). Finally the gonads and their ducts form in the mesoderm between the stomach and intestine. We thus reach