448 VERTEBKATE LIFE AND ORGANIZATION 



the Triassic. However, the group is important for it included not 

 only the ancestors ol modern amphibians, but also those of reptiles and 

 hence ol all higher tetrapods. Studies of the details of vertebral develop- 

 ment suggest that frogs and toads (order Anura) are fairly dnect de- 

 scendants of labvrinthodonts, whereas the salamanders (order Urodela) 

 and ilic legless, burrowing caecilians of the tropics (order Apoda) appear 

 to have followed a different course of evolution from some early lab- 

 yrinthodont stock. 



In the course of evolution, amphibians lost many fishlike character- 

 istics, such as bony scales, the lateral line sensory system (present in 

 larval amphibians but not the adults), and gills. The loss of gills made 

 possible a more efficient circulatory system, for blood returning to the 

 heart from the lungs can be distributed directly to the tissues of the body 

 without the loss of pressure entailed in passing through gill capillaries. 

 Ami)hibians also evolved such terrestrial features as the five-toed, 

 tetrapod ajjpendage, a vertebral column with interlocking vertebrae that 

 provides greater support for the body, a tongue with which food is 

 manipulated within the mouth, eyelids and tear glands that protect and 

 cleanse the eye, and a mechanism with which ground or air-borne vibra- 

 tions can be detected. 



However, the terrestrial adaptation of amphibians is deficient in 

 several respects. First, most are unable to prevent a large loss of body 

 water when on land and must stay close to fresh water. Second, all are 

 cold-blooded, or poikilothermic, as are fishes; their body temperature 

 is close to that of the environment and fluctuates with it. They cannot 

 maintain a constant and rather high body temperature. Since the rate 

 of metabolic processes fluctuates with temperature changes, they cannot 

 be active at low temperatures. The terrestrial poikilotherms living in 

 temperate regions must move during the winter to areas that do not 

 freeze, and enter a dormant state known as hibernation. Amphibians 

 bury themselves in the mud at the bottom of ponds, or burrow into soft 

 ground below the frost line. During hibernation metabolic activities are 

 at a minimum. The only food utilized is that stored within the body; 

 respiration and circulation are very slow. Some tropical amphibians 

 during the hottest and driest parts of the year go into a comparable 

 dormant state known as aestivation. 



Finally, amphibians are unable to reproduce under truly terrestrial 

 conditions. Like the common leopard frog (Rana pipiens), most of them 

 must return to the water to lay their eggs. Even the terrestrial toad 

 returns to this medium, for it has no means of internal fertilization and 

 sperm cannot be sprayed over eggs upon the land. Neither has it sup- 

 pressed the free larval stage in development, and these larvae cannot 

 withstand the rigors of the terrestrial environment. 



204. Amphibian Adaptations 



Sofamanders. Salamanders are not such familiar amphibians as 

 frogs and toads, for most have secretive habits. They may be found 

 beneath stones and logs in damp woods or beneath stones along the side 



