278 



THE ELASMOBRANCH FISHES 



In addition to what we have said of this system in a typical ray, it may be 

 added that in a sluggish type like Torpedo certain of the cephalic canals may 

 be lacking ventrally while in an active form like Dicerohates {Cephalopoda) 

 tubules may branch off of the canal, unlike the simple tubules of Baja clavata, 

 and form a complex net (fig. 243) . 



The internal structure of the canals may be studied in a transverse section 

 through the lateral sensory canal of the leopard shark, Triakis semifasciatus 

 (see p. 26, fig. 29, ll.c). In such a figure it will be seen that the walls of the 

 canal are unequal in thickness. Both in the lateral line and the dorsal cephalic 





Fig. 244. Longitudinal section of lateral sensory canal, Mustelus canis. (From S. E. 

 Johnson.) 



Grp., neuroniasts; CJvi., nerve; Lat.Cn., lateral sensory canal; HmJ., ramus of lateral 

 nerve; Sn.CL, primary hair cell; Sim., supporting cells; Tub., tubule to exterior. 



canals the lumen is flattened; but in the ventral canals it is rounded. On the 

 median wall of the canal the section passes through a sense organ or neuro- 

 mast, composed of cells derived from the basal layer of epidermis (s.c). These 

 are of two types, one a crescentic supporting cell and the other an elongated 

 club-shaped sense cell. A longitudinal section through the lateral sensory 

 canal of Mustelus canis (fig. 244) by Johnson (1917) shows that the neuro- 

 niasts are much more numerous than are the tubes {Tub.) which open to the 

 surface. Each of these groups {Grp.) is composed of primary hair cells 

 {Sn.Cl.), secondary sense cells at the sides of the primary cells, and under- 

 lying these the supporting cells. 



Innervation of the lateral line in the body is by means of the lateral division 

 of the vagus or tenth cranial nerve. It will be observed from figure 244 that the 

 ramuli {Rml.) reach the canal at about the position of the tubules, but that 

 they break up into numerous fibers, which supply a multitude of neuroniasts 

 {Grp.). In the most anterior part of the lateral line canal, however, a few 

 twigs are received from the ramus dorsalis X {dr.X, fig. 245) and the supra- 

 temporalis X. Other twigs of the supratemporalis X supply the supratemporal 

 canal. The segment of the lateral canal immediately anterior to the supratem- 



