94 FROM FISH TO PmLOSOPHER 



on fish, the larvae of other amphibians, or the giant cock- 

 roaches, dragonflies and other insects that inhabited all 

 the swamps. They reached their greatest expansion in 

 the Pennsylvanian and began to dwindle in numbers 

 from the Permian onward, perhaps in part because of 

 competition with the reptiles that came to their ascend- 

 ancy in the late Mesozoic, in part because of climatic 

 conditions unfavorable to their mode of hfe. After the 

 Triassic only two main lines survived: one that led to 

 the frogs or toads that have secondarily lost the tail, and 

 hence are called the Anura {an = without; oura = tail), 

 and a second line that led to the salamanders or Urodeles 

 (owm = tail; delos = evident) . A third and rare group 

 survives in the Apoda (a = without; poda = feet), small 

 wormhke creatures that Hve in the moist earth. 



To convert a mud-crawhng fish into an amphibious ani- 

 mal required anatomical transformations throughout the 

 body— and something between fifty and one himdred mil- 

 lion years. In a fish, the blood goes directly from the 

 heart to the gills by the ventral aorta, and from the gills 

 it is distributed by the dorsal aorta to all parts of the 

 body. When the lung took precedence over the gills as 

 a means of respiration, the blood had to be diverted from 

 the gills to the lung by reconstruction of the aortic cir- 

 culation and the partial division of the heart into two 

 chambers, a division that was not to be completed until 

 the evolution of the birds and mammals. The pectoral 

 and pelvic arches were strengthened to support the legs, 

 and the vertebral column was transformed into a strong 

 but fiexible arch capable of lifting the abdomen more 

 or less free of the ground. The body for the most part 

 lost the scales of the ancestral fishes except on the 

 belly (which needed protection against rubbing on the 

 ground) and became covered with moist skin that came 

 to play an important part in salt and water exchange 

 and in respiration, and this skin had to be periodically 

 replaced by molting, in order to keep it viable. The eye 



