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. ' 



Fig. 362. — Brain of a dogfish 



The Sense-organs. — The ol fa c- shark, Scyllium catulus, dorsal 

 tory sac (Fig. 362) is characteristi- , view - 2 < pineal stalk; j, olfactory 



. lobe; 4, cerebral hemisphere; 



cally large in elasmobranchs. The 5 , thalamencephalon ; 7, optic 

 ears (Fig. 350) are membranous l ^': Q : cerebellum; 10, roof of 



hind-brain; n, 12, 13, 14, muscles 

 Sacs each with three semicircular that move the eyeball; 15, ninth 



canals; they lie within the auditory nerve; l6 < l6a : bran ches of vagus 



J J nerve; 17, main trunk of vagus 



Capsules. The eyes (Fig. 362) are nerve; II-X, roots of the cranial 



well developed. Along each side J™: (Frora Shipley and Mac * 

 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. 



