CLASS CHONDRICHTHYES. CARTILAGINOUS FISHES 



375 



Sting ray 



Hammerhead shark 



Figure 251. Some cartilaginous fishes showing extreme variation in form. The sting ray has 

 pectoral fins so oversized that when the ray is swimming, the fins look like the wings of a bird 

 in flight. 



securing food or for offense and defense. A 

 horse wading through a shallow river where 

 the electric eel lives has received enough 

 shock to cause it to throw its rider. 



RELATIONS OF 



CHONDRICHTHYES TO 



OTHER GROUPS 



The cartilaginous fishes exhibit a number 

 of structural advances over the cyclostomes; 

 they possess paired fins, a lower jaw, gill 

 arches, and placoid scales. Among the peculi- 

 arities which separate the cartilaginous fishes 

 from the bony fishes (Osteichthyes) are the 

 absence of lungs, bones, air bladder, true 

 scales, and the presence of skeletal charac- 

 teristics which are not found in bony fishes. 



RELATIONS OF THE 

 CARTILAGINOUS 

 FISHES TO MAN 



Sharks feed chiefly on crustaceans, squids, 

 fish, other aquatic animals, and not on hu- 

 man beings as one might infer from some 

 newspaper accounts. However, there are reli- 

 able reports of shark attacks on swimmers in 

 all warm seas, but only an occasional attack 

 in temperate waters. Most of these attacks 

 have occurred near beaches or reefs, and a 

 few in Lake Nicaragua by fresh-water sharks 

 living in this lake. In recent years, an ever 

 increasing number of sports enthusiasts, 

 equipped with swimfins, snorkels, and aqua- 

 lungs, have moved out to new underwater 

 frontiers. Some of those who have been too 

 venturesome have been killed by sharks. 



