104 FROM FISH TO PHILOSOPHER 



turn, supplied a considerable part of the knowledge 

 through which there were developed some of the im- 

 portant methods for evaluating renal function in man. 

 The marine fishes also supphed the key ideas which 

 made possible the reinterpretation of the evolutionary 

 history of the kidney. Hence a brief exciursus into the 

 bhnd alley of the sea can scarcely be avoided. 



The higher or bony fishes, which populate the seas, 

 oceans, rivers, and lakes of the world today, are known 

 as the Teleostei (teleos = complete, perfect; osteon = 

 bone) because of the complete calcification of the verte- 

 brae. These bony fishes are represented by some thirty 

 thousand living species and subspecies classified imder 

 several thousand genera and six hundred-odd families. 

 No accurate count by families is available, but the vast 

 majority are marine. 



Just when the marine teleosts, as represented by sur- 

 viving forms, became established in that habitat remains 

 a matter of conjecture, though it was certainly long after 

 the elasmobranchs entered the sea. If the predecessors 

 of the recent marine forms invaded salt water in the late 

 Carboniferous, an interval of nearly a hundred million 

 years separated their migration from that of the elas- 

 mobranchs; if the marine teleosts have been inhabitants 

 of that medium only during the Cenozoic, this interval 

 is nearer three hundred million years. In any case, the 

 marine elasmobranchs and the marine teleosts are very 

 difi^erent from each other in many ways, and particularly 

 in respect to the mechanism by which they maintain 

 water-balance. 



Where the elasmobranchs used the urea-retention de- 

 vice to extract water from sea water, the teleosts, evolved 

 from fresh-water fishes, independently and at a much 

 later time, lacked this device. Nevertlieless, they had to 

 solve the same problem as did the elasmobranchs: they 

 had to separate water from sea water— which is two and 

 a half times as concentrated as their blood— in order to 

 make a urine no more concentrated than the blood. They 



