374 METAZOAN PHYLA 



respiration and the functioning of the lungs the skin becomes very 

 vascular and respiration is carried on through it. Thus metamorphosis 

 takes place gradually and changes the tadpole into a frog differing from 

 the adult only in size. 



401. Neoteny and Pedogenesis. — Pedogenesis has already been 

 defined as the production of young by an immature animal. In the 

 tailed amphibians cases are known in which the larval characters are 

 retained until after sexual maturity. These animals may either be 

 looked upon as adults which, having not metamorphosed, retain certain 

 larval characteristics, or they may be considered as being larvae in 

 which the reproductive organs are precociously developed. This 

 prolongation of larval characteristics into advanced age has been termed 

 neoteny, and reproduction by these animals may be termed pedogenesis. 

 A classical example of these is seen in the larvae of species oiAmby stoma. 

 Under certain circumstances these salamanders do not metamorphose 

 but retain their gills and their aquatic life and yet become sexually 

 mature. In this condition they are known as axolotls (Fig. 2475). 

 It has been found possible under experimental conditions to control 

 metamorphosis and to produce the axolotl type at will. In nature these 

 are particularly abundant in alkaline lakes and ponds throughout the 

 semiarid regions of the West, and south into Mexico. 



402. Regeneration. — Amphibians possess greater powers of regenera- 

 tion than any other vertebrates. Limbs and tails of larvae when cut 

 off readily regenerate. This of course is an advantage when mutilation 

 occurs as the result of seizure by enemies. 



403. Hibernation. — The power of hibernation is sometimes considered 

 as an additional adaptation to terrestrial life. During the winter and 

 frequently during seasons when bodies of water become dry, amphibians 

 will bury themselves in the mud at the bottom and remain there in a 

 dormant condition until spring or until the water is restored. During 

 this period of dormancy the lungs are not used in breathing, and respira- 

 tion must take place through the skin. The temperature of the hiber- 

 nating animal remains slightly above that of the earth about it, only a 

 small amount of physiological activity is maintained, and the organism 

 lives on food stored in its body. In some tropical countries amphibians 

 exhibit a similar dormant condition during the heat of the summer. 

 This phenomenon is known as estivation. 



404. Economic Importance. — Amphibians are almost without excep- 

 tion beneficial, and some, particularly the toads, are of considerable 

 importance as destroyers of noxious insects. Frogs are used as food, 

 the hind legs only being eaten, and frog farms are now being operated 

 in Wisconsin, California, and a number of other states. Frogs are 

 also used very extensively in laboratory experimentation and as fish 

 bait. 



