28o 



SYSTEMATIC ZOOLOGY 





ventral side of the body, and the eyes and spiracles to the dorsal 

 side. The dorsal and tail fins are usually but slightly developed. 

 Many of the Selachii are viviparous, the young being developed 

 in an enlargement of the oviduct, which is called the uterus. In 

 most other cases the ova are surrounded by a firm, chitinous cap- 

 sule, which is secreted by the walls of the oviduct; it usually has 



long, threadlike filaments which become 

 entangled in seaweed or other objects, 

 and so anchor the egg during its develop- 

 ment (Fig. 289). The dried empty cap- 

 sules are familiar objects on the seashore, 

 and are known popularly as " mermaids' 

 purses." 



In their general habits of life the sharks 

 and rays differ widely. The sharks of 

 all sorts and their allies, the dogfishes, 

 are active, free-swimming animals, living 

 near the surface, feeding on fishes and 

 other relatively large animals, and pos- 

 sessed of great muscular strength. None 

 are very small, and the largest attain 

 a length of from eleven to more than 

 twelve meters. The maximum length is 

 reached by the basking shark, Cetorhinus, 

 which is the largest living fish known. 

 The rays are sluggish, living for the most 

 part in shallow water on the bottom of the 

 ocean ; they feed chiefly on Crustacea and 

 Mollusca, and sometimes attain a diameter 

 Skates are eaten by man to some extent. 

 One of the most curious rays belongs to the genus Torpedo, 

 the electric ray. Several genera of the Selachii have the power 

 of generating electricity in certain parts of the body, and giving 

 a considerable electric shock; this power is best developed in 

 Torpedo, in which the electric organ, consisting of modified 

 muscles, lies on either side of the head and branchial region. 

 and extends the entire thickness from the dorsal to the ven- 

 tral side. By placing the fingers on the ventral side and the 

 thumb on the dorsal, the shock is readily felt. 



FlG. 289. Egg case of dog- 

 fish. (After Dean, from Parker 

 and Haswell's Manual.) 



of nearly five meters. 



