162 



UNDERWATER GUIDE TO MARINE LIFE 



Fig. 71. One of the authors is photograpJi'nig a large soutliern sti>ig ray, Dasyatis 

 americana, four feet in diameter, which was found covered with sand sleeping on 

 the hottorn. 



distinct from the body- We see this evolutionary transition in following the 

 changes in body form from the sawfishes to the guitarfishes to skates and tor- 

 pedoes and finally to the eagle rays. This evolutionary trend is an adaptation to 

 bottom-living. But like almost every group of animals, there are departures from 

 the principal theme, and so the rays have also risen from the bottom to take on 

 the more pelagic life that we see in the eagle rays and mantas. 



Paralleling these evolutionary tendencies are distictive methods of locomo- 

 tion. In the most sharklike and least flattened rays, the sawfishes and guitarfishes, 

 locomotion is accomplished by powerful sculling motions of the tail as in sharks. 

 The pectoral is used little. In the torpedoes also, propulsion is chiefly caudal, 

 but this is probably because the disc is so inflexible, it being the location of 

 the electric organs. The skates move almost entirely with the pectoral fins, un- 

 dulating waves traveling in this fin from front to rear. Sting rays also use 



