Elasmobranch i i (elasmobrangkii)

pl. [NL., IMG:content-0275.png:[merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small] IMG:content-0276.jpg:[graphic] w a V Brain of Skate ( Raia bat is ), an elasmobranch i ate fish. A, from above: s, olfactory bulbs; a, cerebral hemispheres, united in the middle line; 6, thalamencephalon; c, mesencephalon; d, cerebellum; aa, plaited bands formed by the restiform bodies; I, II, IV, V, first (olfactory), second (optic), fourth, and fifth pairs of cerebral nerves; 1, medulla oblong at a; w, a blood-ve el. B, from below, in part enlarged: ch, optic chiasm, pituitary body; and v, ve els connected with h, k, saccus vasculosus; β, pyramids of medulla oblong at a; a, I, II, V, same as in A. or on the under surface of the head; the gill-pouches and -slits usually 5, sometimes 6 or 7, generally with an equal number of external apertures, but in the Holocephali with only one on each side; the optic nerves chiasmal; the intestine with a spiral valve, and the arterial cone with pluriserial valves; and the skin either naked, or with placoid scales, forming shagreen or other armor. The division of the group varies; it is now usually divided into two subcla es, Holocephali and Plagiostomi, the latter including the sharks and the rays.