50 Physiology of the Kidney 



Moreover, with most of the body covered by armor, a few- 

 posterior skeletal muscles had to be selected and developed 

 in order to concentrate leverage in a powerful tail; this em- 

 phasis on the posterior segmental muscles, together with com- 

 pression of the middle segments of the body cavity beneath 

 one or a few armor sheets, would tend to obliterate the primi- 

 tive segmental divisions of the coelome and foster the de- 

 velopment of pericardial and splanchnic cavities as they oc- 

 cur in the higher vertebrates. The evolution of an armored 

 body the remote, articulated parts of which had to be moved 

 in a coordinated manner, would foster the evolution of a 

 central nervous ganglion or brain, which stood in close func- 

 tional relationship to the anterior, distance receptors. The 

 development of armor about the head would foster the cran- 

 ial articulation of mouth parts and the evolution of jaws, 

 which, absent in the ostracoderms, are first discoverable in the 

 mailed acanthodians of Silurian time. But these interesting 

 speculations, and they are nothing else, lie apart from our 

 main theme, that it was in seeking protection against fresh 

 water that the first vertebrates to be preserved in the fossil 

 record, the ostracodermns, came to be depressed, bottom-liv- 

 ing armored creatures far removed from the hypothetical 

 dynamic, fast-swimming prototype of classical theory. For 

 even under their best efforts at free swimming the early 

 armored vertebrates found it easier to sink to the bottom and 

 wiggle upon the mud, where indeed most of them remained 

 until the close of the Devonian. If the ostracoderms are 

 viewed as a consequence rather than a beginning of verte- 

 brate evolution they offer less difficulty to the dynamic 

 theory, which is recommended on so many grounds. 



Yet even thus encased in a waterproof covering, the gills, 

 the mouth and the intestinal tract still afforded routes by 



