THE DOGFISH 



37 1 



efferent or mixed. The spinal nerves of the dogfish are 

 more numerous than those of the frog, but in their general 

 structure and arrangement resemble them. The dorsal 

 and ventral roots by which each arises from the spinal cord 

 pass through the wall of the neural canal by small notches 

 in the hinder edges of the intercalary pieces and neural 

 arches respectively. The sympathetic system is irregular 



Fig. 265. — The head of a dogfish, seen from above with the right orbit 

 opened. 



6 ., Eyeball ; o.i., o.s., inferior and superior oblique muscles ; r.e., r.i., r.int., r.s., 

 external, inferior, internal, and superior recti [muscles ; sp., spiracle ; //., optic 

 nerve; IV., fourth nerve. 



and difficult of dissection in the dogfish, but in the main 

 outlines of its plan it resembles that of the frog. 



Each of the olfactory organs of the dogfish (Figs. 262 and 

 258) is a sac enclosed in the olfactory capsule 



Sense Organs. Qf ^ gide Qf the body It opens externa Hy by 



the nostril, but has no internal opening into the mouth. 

 Its walls are thrown into vertical folds covered with an 

 epithelium which contains sense cells. The eyes resemble 

 in all important respects those of the frog, and need not here 



