CLASS ELASMOBRANCHII 341 



tride. A dilated portion of the ventral aorta just in front of the ventricle 

 is known as the bulbus arteriosus, or conus arteriosus. 



The dogfish brain (Fig. 230) is much more highly developed than 

 that of the cyclostomes, and three regions stand out prominently, these 

 being the olfactory lobes, the optic lobes, and the cerebellum. The 

 size of these three lobes is connected, respectively, with the high develop- 

 ment of the sense of smell, the considerable development of the sense 

 of sight, and the very delicate sense of equilibrium. The olfactory 

 sac is correspondingly large and the eyes well-developed. Only the 

 inner ear is present. It lies in the auditory capsule and consists of a 

 utriculus with three semicircular canals and a small simple sacculus. 

 There are lateral line organs and mucous canals on the head, the latter 

 lodging organs similar in structure to the lateral line organs and which 

 probably have a similar function. The kidney is a mesonephros. 



Fertilization is internal and the eggs develop in a portion of the 

 oviduct known as the uterus. The dogfish shark is viviparous, but 

 when born the "pups" still possess a yolk sac. 



363. Other Sharks. — A great variety of other sharks exist, many 

 of which, unlike the dogfish shark, lay eggs. Such eggs are always 

 provided with horny shells which protect them from injury after they 

 are laid. Whale sharks are interesting because they grow to be the 

 largest fish known, being said to reach sometimes a length of 50 feet. 

 Fossil remains indicate the former existence of types still larger. 



364. Skates and Rays. — Skates and rays are distinguished from 

 sharks by the fact that the sides of the body are greatly extended, the 

 whole animal being flattened dorsoventrally. The skates (Fig. 231), 

 viewed from the upper side, have somewhat the shape of a kite with a 

 sharp tail. The electric rays are more nearly circular in shape and have 

 paired electric organs on the dorsal surface which are capable of giving 

 a powerful electric shock. These electric organs are pillars of modified 

 muscle cells, and the energy produced is turned into a difference in 

 electrical potential instead of being expended in movement. The 

 difference in potential between the two ends of such a pillar of cells is 

 considerable, and the shock correspondingly severe. The sting rays 

 have long flexible tails with spines which are capable of producing 

 very serious wounds. Another interesting type is the sawfish. The 

 body of the sawfish, which may attain a length of 20 feet, is not broadened 

 like that of skates and rays, to which it is related, and the snout is 

 greatly prolonged, reaching a length of half the rest of the body. The 

 snout is armed on each side with a row of sharp teeth, which make it a 

 very formidable weapon capable of cutting a gash in the body of even 

 such an animal as the whale. 



365. Extinct Elasmobranchs. — There are many extinct elasmo- 

 branchs, some of which are obviously more primitive than any now living 



