CHAPTER V 



THE ELASMOBRANCHS 



Simple as the primitive glomenilar-tubular kidney was, 

 it has served the fresh-water fishes for some five hmidred 

 million years, and it appears to have served the ostra- 

 coderms equally weU, since they increased in variety and 

 numbers in the fresh-water rivers and lakes of the Silu- 

 rian and Devonian periods. We have noted that by the 

 Devonian, the ostracoderms had become diversified 

 into at least three major orders: the Anaspida, the 

 Cephalaspida, and the Pteraspida. Whether any of these 

 now extinct forms occasionally invaded the sea we do not 

 know, but in view of the fact that their degenerate 

 descendants— the lamprey, Petromyzon (thought to be 

 a derivative of the cephalaspid-anaspid root), and the 

 hagfish, Myxine (thought to be a derivative of the 

 pteraspids)— can live in salt water, it seems likely that 

 they may have done so, but that breeding or other habits 

 prevented them from establishing themselves perma- 

 nently in this habitat. 



It has been emphasized that evolution has no main- 

 tained direction, but rather many directions sustained 

 only by the opportunistic calculus of probabilities. In the 

 ostracoderms random possibihty took some new direc- 

 tions that were ultimately to prove of great value to the 

 vertebrates. In the cephalaspids the primitive method 

 of laying down the waterproof covering had been trans- 



