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 (9) 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. The olfac- 

 tory sac (Fig. 362) is characteristi- 

 cally large in elasmobranchs. The 

 ears (Fig. 350) are membranous 

 sacs each with three semicircular 

 canals ; they lie within the auditory 

 capsules. The eyes (Fig. 362) are 

 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. 



FIG. 362. Brain of a dogfish 

 shark, Scyllium catulus, dorsal 

 view. 2, pineal stalk; 3, olfactory 

 lobe ; 4, cerebral hemisphere ; 

 5, thalamencephalon ; 7, optic 

 lobes; 9, cerebellum; 10, roof of 

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

 that move the eyeball; 15, ninth 

 nerve; 16, ida, branches of vagus 

 nerve; 77, main trunk of vagus 

 nerve ; II-X, roots of the cranial 

 nerves. (From Shipley and Mac- 

 Bride.) 



