52 STRUCTURE OF THE VERTEBRATES 



larval condition is called paedogenesis and was discovered in the 

 Axolotl of Mexico. The animals had been classified as a separate 

 genus and family, but when the environment was changed 

 the young metamorphosed into a well known salamander (Am- 

 bly stoma). Many students of the subject today interpret the 

 retention of external gills in other forms as paedogenesis, al- 

 though experiment has so far failed to confirm the theory. 



Order Axura. The tailless amphibia are the frogs and toads, 

 and are distributed throughout the world. They are in many 

 respects the most specialized living amphibia. j\Iost of the genera 

 are capable of living their adult life away from water. This is 

 particularly true of the toads, whose warty skin, containing 

 relatively few mucous glands, adapts them for life in semi-arid 

 regions. But although the adult may live in her burrow on a dry 

 hillside, the eggs are invariably laid in water where they hatch 

 as tadpoles. In the smaller toads the larval life is very short 

 and they will hatch into tiny toads in a few weeks, giving rise 

 to the myth that it rains toads. Other myths relating to their 

 longevity are also without foundation. They may live for long 

 periods without food or water, but to believe that they are found 

 embedded in solid rock demands credulity rarely found in a 

 scientist. 



Evolution of the Amphibia. Almost from the beginning of 

 amphibian history two lines of evolution are apparent. Some 

 families retained rather generalized amphibian characteristics, 

 and probably gave rise to the amphibia of today; other families 

 became highly specialized with enormous heads, or other struc- 

 tures which eventually rendered them unfit for survival in a 

 changing environment; and others assumed characteristics which 

 appear reptilian. Some of these pro-reptilian forms were un- 

 doubted amphibia (Eryops, for example) for the skeletons of 

 gilled larvae have been discovered. 



Embryology of the Frog. The frog has a very generalized 

 development, more primitive than the recent fish. For this reason 

 it is selected for discussion in most texts, as it logically follows 

 the developmental processes of Amphioxus. The Apoda have a 

 development which verges toward the reptilian and, as was men- 



