THE ELASMOBRANCHS 6l 



are very primitive and very old; and the more recent and 

 so-called Tiigher fishes, the Osteichthyes (osteon = 

 bone; ichthys = fish) , which include all the other and 

 so-called *bony* fishes of the seas and fresh waters of the 

 world. It is only with the cartilaginous fishes or elasmo- 

 branchs that we are immediately concerned. 



When the ancestors of the elasmobranchs were driven 

 by the widespread aridity of the Devonian to seek sanc- 

 tuary in the sea, they faced (as did the later bony 

 fishes) a major physiological problem. The sea contained 

 a relatively high concentration of salt (even as it did 

 back in the Cambrian when the protovertebrate had 

 abandoned it to take up residence in the continental 

 fresh waters), and with the invasion of this salt water, 

 the osmotic relations between the organism and its ex- 

 ternal environment were completely reversed. Where 

 once the difficulty had been the influx of excess water 

 into the body, now the difficulty lay in excessive loss; 

 the greater salt content in the sea water, by lowering 

 the diffusion pressure of water below that of the blood, 

 caused water to move out of the body through the gills 

 and oral membranes (if not through the skin, which now, 

 thanks to the impervious scales that had been evolved 

 from the armor of the ostracoderms, was a fairly water- 

 proof structure). Without provision to arrest or offset 

 this constant osmotic loss of water, the animal would die 

 of exsiccation just as certainly as though it were being 

 slowly dried in the air and sun. 



In theory, a fish moving from fresh water into salt 

 water might solve its water-balance problem by drinking 

 sea water and excreting the salt in a urine osmotically 

 more concentrated than the sea, thereby maintaining it- 

 self in salt and water balance. Natural selection, how- 

 ever, does not work along the lines of theoretical blue- 

 prints but on the raw materials randomly supplied by 

 mutation and, as evolution worked out, the capacity to 

 make a highly concentrated urine— or even one more 



