THE PHYLA HEMICHORDATA AND CHORDATA 



and the land was at that time occupied by no other large animals. Perhaps 

 inability to maintain a constant body temperature, and brains smaller than the 

 lumbar enlargements of their spinal cords, were important in the decline of 

 reptiles when the Age of Mammals began. 



Among living reptiles the members of the order Chelonia, or turtles and 

 tortoises, represent the most primitive type, although superficially they may 

 seem most specialized (Fig. 18.20). The box tortoise, Terrapene, and the 

 painted tortoise, Chrysemys, are familiar examples. Chrysemys, and more 

 particularly the soft-shelled tortoise, Amyda, illustrate forms that have 

 shifted from land to water. This change of habitat has occurred in many 

 types of reptiles since their original adaptation for terrestrial life. The 

 Chelonia are fundamentally air-breathing land animals, but some of them, 

 both fresh-water and marine species, have come to live partly in the water. 

 These forms still breathe air and still come to land for their egg-laying. 

 The extinct plesiosaurs and mosasaurs were reptiles that evidently lived 

 in a similar fashion and presumably laid their eggs on land, as the dinosaurs 

 seem to have done. There is evidence, however, that the ancient marine 

 ichthyosaurs were ovoviviparous and hence more completely adapted to 

 aquatic life. Among existing chelonians, an extreme development of size 

 is seen in the giant tortoise of the Galapagos Islands and in the large marine 

 turtles. These are dwarfs, however, in comparison with some of the ancient 

 turtles. 



A lone survivor of another primitive type, the order Rhynchocephalia, is the 

 genus Sphenodon (Fig. 18.2L), now found only in New Zealand. Sphenodon, 

 the "tuatera," is of great interest to the comparative anatomist because it 

 represents a very generalized and primitive type of reptile. It is more closely 

 related to the lizards and snakes than to other familiar forms. 



The lizards (Lacertilia) and the snakes (Ophidia) are so closely related that 

 they are placed together in the order Squamata. The fossil record indicates 



i. f ^ '. -^ 



Fig. 18.16. Representative Salientia, or leaping amphibians. A, a toad, Bufo americanus. 

 B, a spring peeper, Hyla crucifer, calling, its vocal sac distended with air. C, tadpoles 

 of the green frog, Rana clarmtans, some ready for metamorphosis to adulthood. {A, courtesy 

 General Biological Supply House, Inc.; others, courtesy New York Zoological Society.) 



567 



