CLASS ELASMOBRANCHII 



427 



by gill-rays. Water entering the mouth passes between the 

 branchial arches and out through the gill-slits (Fig. 359, GS), 

 thus bathing the gills and supply- 

 ing oxygen to the branchial blood- 

 vessels. 



The Nervous System. - - The 

 brain (Fig. 362) is more highly 

 developed than that of the cyclo- 

 stomes. It possesses two remark- 

 ably large olfactory lobes (j), a 

 cerebrum of two hemispheres (4), 

 a pair of optic lobes (7), and a 

 cerebellum (g) which projects 

 backward over the medulla oblon- 

 gata (10). There are ten pairs of 

 cranial nerves (Fig. 362 and Table 

 XIV). The spinal cord is a dorso- 

 ventrally flattened tube with a 

 narrow central canal; it is pro- 

 tected by the vertebral column. 

 Spinal nerves arise from its sides 

 in pairs. 



The Sense-organs. 

 tory sac (Fig. 362) is characteristi- 

 cally large in elasmobranchs. The 

 ears (Fig. 350) are membranous 



V VII 

 VIII 



FIG. 362. Brain of a dogfish 

 The OJfaC- shark, .SYv///Mz i al it! us, dorsal 

 view. 2, pineal stalk; 3, olfactory 

 lobe ; 4, cerebral hemisphere ; 

 5, thalamencephalon ; 7, optic 

 Q, cerebellum; 10, roof of 

 _-brain; //, 12, ij, 14, muscles 

 SaCS each with three Semicircular that move the eyeball; 15, ninth 



16, i6a, branches of vagus 



17, main trunk of vagus 

 II-X, roots of the cranial 



(From Shipley and Mac- 



canals; they lie within the auditory nerve 



J nerve 



Capsules. The eves (Fig. 362) are nerve; 



well developed. ' Along each side 



of the head and body is a longi- 



tudinal groove, called the lateral line (Fig. 359, LL), and on 



the head are also mucous canals which open on the dorsal and 



ventral surfaces and end in ampullae at the anterior end of the 



snout. These structures are supposed to be sensory in function. 



