The Animal Kingdom - 671 



limited or transitional sort of terrestrial 

 adaptation. Except for a few, like the mid- 

 wife toad, which carries its eggs and young 

 embedded in the skin of the female, all 

 amphibians return to the water to lay their 

 eggs and all spend the embryonic and larval 

 stages of their life in the aquatic environ- 

 ment. All have relatively simple lungs; and 

 most carry on a supplemental respiration 

 through the skin, which typically is moist and 

 devoid of scales. On dry land, such a skin 

 permits considerable water to be lost and, 

 except for toads and a few other forms hav- 

 ing a cornified skin, most amphibians fre- 

 quently return to some pond or lake to 

 reabsorb more water. 



Typically, the amphibian possesses two 

 pairs of legs, each with four or five toes 

 (webbed in the hind limb). A pair of nasal 

 passages leads from the outside into the 

 mouth cavity. The skeleton is largely bony 

 and there is little or no residual notochordal 

 material in the adult. The heart displays 

 three chambers (two auricles and one ven- 

 tricle) and usually only one pair of aortic 

 arches is represented in the systemic circula- 

 tion. Typically the mouth has numerous fine 

 teeth and a protrusible tongue. 



The earliest amphibian fossils are found 

 in formations of the Devonian period, a 

 time when periodic droughts prevailed. These 

 ancient Amphibia resembled some of the 

 lobe-finned fishes. These fish had lunglike 

 swim bladders, which could be used for aerial 

 respiration, and limblike fins, which enabled 

 them to clamber overland in search of a 

 deeper pool, when a shallower one dried up. 

 On the other hand, another group of ancient 

 Amphibia had a crocodilelike form, large 

 complexly ridged teeth, and a body armored 

 with hard dermal plates, and probably these 

 Labyrinthodonta represent the stock from 

 which the reptiles (and hence all higher Ver- 

 tebrata) took origin. 



The Reptiles. Modern reptiles, although 

 numerous and varied, occupy a relatively 

 minor position in comparison with other 

 vertebrates. But during the Mesozoic era, 



which often is called the Age of the Rep- 

 tiles, they were the dominant animals. They 

 ranged in size from small lizards to gigantic 

 dinosaurs. For example, Diplodocus (Fig. 29- 

 8) reached a length of 90 feet and weighed 

 about 30 tons. The ancient reptiles, more- 

 over, usurped almost every type of land 

 habitat, from the lowlands, whether swampy 

 or arid, to the dry highlands; and some, 

 even though air-breathing, lived in the ocean; 

 and a few, the pterodactyls, took to the air. 



The earliest reptile fossils, found in late 

 formations of the Carboniferous period, are 

 scarcely distinguishable from Amphibia. By 

 the end of the Triassic period, however, 

 fourteen major reptilian orders had evolved. 

 Ten of these became extinct by the end of 

 the Cretaceous period; but the other four 

 gave rise to the four now living groups. These 

 are as follows: (1) turtles, tortoises, and simi- 

 lar forms (order Chelonia); (2) crocodiles and 

 alligators (order Crocodilia); (3) snakes, 

 lizards, and similar forms (order Squamata); 

 and (4) a very primitive type, represented 

 solely by the lizard-type tuatara of New Zea- 

 land, which has a well-developed median eye 

 and does not have any copulatory organ 

 (order Rhynchocephalia). 



The success of the reptiles, particularly 

 with reference to their ability to occupy dry 

 areas on the land, derives from a combina- 

 tion of several features. The body is pro- 

 tected by a thick cornified skin, superim- 

 posed upon which, usually, there are a 

 number of closely fitting scales, or scutes. 

 Usually there are two pairs of limbs, each 

 equipped with five clawed toes, suitable for 

 running, climbing, or crawling. However, in 

 aquatic forms, the limbs are paddlelike; in 

 some extinct reptiles they were wings; and 

 in snakes and some lizards they are vestigial 

 or absent. The lungs and the pulmonary cir- 

 culation are well developed. The heart is 

 four-chambered, and although the separa- 

 tion of right and left ventricles is not com- 

 plete (except in the Crocodilia), very little 

 mixing of aerated and unaerated blood oc- 

 curs (p. 326). The cranium, vertebral column, 



