IV. VERTEBRATA: PISCES. 



563 



base of the fin, so that the dorsal part is reduced, the ventral greatly 

 enlarged, the result being extremely asymmetrical, as seen from 

 the exterior. The homocercal fin is symmetrical externally, but 

 in reality is extremely asymmetrical. The end of the vertebral 

 column, the unossified notochord, is bent abruptly upwards, and 

 hence the fin is almost entirely formed of the ventral portion, 

 which is usually divided by a terminal notch into upper and lower 

 halves. The homocercal fin begins with a diphycercal and passes 

 through a heterocercal stage in development. 



In correspondence with the simple motions the musculature is simple 

 and consists largely of longitudinal muscles divided into myotomes, which 

 are conical with the apex in front, and are 

 so inserted in each other that a cross-sec- 

 tion gives concentric circles. In a section 

 there are at least two such systems, the 

 muscles being divided by a lateral in- 

 cision into dorsal and ventral halves. 

 There are also smaller groups of muscles 

 related to fins, gill arches, jaws, etc., but 

 of much smaller size, derivatives from the 

 larger mass. Electric and pseudelectric 

 organs, which occur in different fishes, 

 sometimes in the trunk, at others in the 

 tail, are formed by the modification of mus- 

 cles. Each organ consists of numerous 

 closely packed vertical or horizontal col- 

 umns, each column, like a Voltaic pile, 

 consisting of layers of gelatinous plates 

 (equivalents of muscle bundles) in which 

 the nerves, with special end plates, termi- 

 nate. The discharge is electronegative. 



The brain shows the low position of the class in the slight development 

 of the cerebrum (fig. 591). This is especially true of the teleosts, in which, 

 in place of a cortex, there is only a thin epithelial layer (Pall), what was 

 formerly called cerebrum being only the corpora striata. The independent 

 olfactory lobes lie either close to the cerebrum (most teleosts, Lol) or are 

 separated from it by an olfactory tract (fig. 592, h). The optic thalami 

 are small (d), but below them are enlargements characteristic of fishes, the 

 lobi inferiores, and between them the sacculus vasculosus. Both optic 

 lobes and cerebellum are greatly developed. 



The nose consists of two preoral pits, the opening being divided by 

 a bridge of skin into anterior and posterior nostrils. In many selachians 

 the nostrils are connected with the mouth by a groove covered by a fold of 

 skin, and in the Dipnoi there is a choana. The eye has several peculiari- 

 ties. The lens is very convex, almost conical, due to the slight refraction 

 caused by the passage of light from the water into the cornea. Further, 



EP 



FIG. 590. Driagrammatic section 

 of electrical apparatus. (From 

 Wiedersheim.) The arrow points 

 dorsally or anteriorly. BG, con- 

 nective-tissue framework; EP^ 

 electrical plates; G, gelatin- 

 ous tissue; N, nerves entering 

 through the septa; NN, nerve 

 terminations. 



