_Lateral Line Organs._[52]--Epidermal sense buds are scattered about in the ectoderm of fishes. A special arrangement of these in lines along the sides of the body and on the head region form the highly characteristic sense organs of the lateral line system. In _Lepidosiren_ these organs retain their superficial position; in other fishes they become sunk beneath the surface into a groove, which may remain open (some Selachians), but as a rule becomes closed into a tubular channel with openings at intervals. It has been suggested that the function of this system of sense organs is connected with the perception of vibratory disturbances of comparatively large wave length in the surrounding medium. _Peripheral Nerves._--In the Cyclostomes the dorsal afferent and ventral efferent nerves are still, as in _Amphioxus_, independent, but in the gnathostomatous fishes they are, as in the higher vertebrates, combined together into typical spinal nerves. As regards the cranial nerves the chief peculiarities of fishes relate to (1) the persistence of the branchial clefts and (2) the presence of an elaborate system of cutaneous sense organs supplied by a group of nerves (_lateralis_) connected with a centre in the brain which develops in continuity with that which receives the auditory nerve. These points may be exemplified by the arrangements in Selachians (see fig. 31). I., II., III., IV. and VI. call for no special remark. [Illustration: From Bridge, _Cambridge Natural History_, vol. vii. "Fishes" (by permission of Macmillan & Co., Ltd.). After Wiedersheim, _Grundriss der vergleichenden Anatomie_ (by permission of Gustav Fischer). FIG. 31.--Diagram of Cranial nerves of a Fish. Cranial nerves and branchial clefts are numbered with Roman figures. Trigeminus black; Facialis dotted; Lateralis oblique shading; Glossopharyngeal cross-hatched; Vagus white. bucc, Buccal. c, Commissure between pre- and postauditory parts of lateralis system. d.r, Dorsal roots of spinal nerves. g.g, Gasserian ganglion. gn.g, (Geniculate) ganglion of VII. hy, Hyomandibular. l.n.X, Lateralis vagi. m, Motor branches of hy. md, Mandibular. md.ex, External mandibular. mk.c, Meckel's cartilage. mx, Maxillary. oc, Occipitospinal. ol.o, Olfactory organ. op.p, Ophthalmicus profundus. op.s, Ophthalmicus superficialis. pn, Palatine. pq., Palatopterygo-quadrate cartilage. s, Spiracle. st, Supra-temporal branch of lateralis system. t.a, Lateralis centre in brain. v.n, Visceral nerve. v.r, Ventral roots.]