50 STRUCTURE OF THE VERTEBRATES 



structures developed shows that the class was poorly adapted 

 for land existence, and could not stand the competition with the 

 reptiles which evolved from them. 



Order Stegocephalia. For simplicity these fossil amphibia 

 have been classified as one order. The Stegocephalia vary widely 

 in structure, the earliest known form (Archegosaurus) having a 

 skull with several bones found nowhere else but in the fish. Dur- 

 ing the years when there was no competition on land the am- 

 phibia multiplied and evolved in many directions. They were 

 well encased in an armor of bone, strong enough to protect them 

 from each other and from any predaceous fish; but too heavy to 

 permit of great activity. The evidence indicates that the primi- 

 tive amphibian foot had six toes, which soon became reduced to 

 the typical five of the land vertebrates. Some of the Stegocephalia 

 have a further reduction on the pectoral foot, like that of recent 

 forms. 



Order Apoda. These are legless amphibia. Their burrowing 

 habits are their only defense against more active animals and 

 came with the loss of appendages. In fundamental structure they 

 are the most primitive of living amphibia. Their skull bones are 

 less modified than those of other orders; they have minute 

 dermal scales beneath the skin; and their development is an 

 important link in comparative embryology. The order is rather 

 widely distributed in the sub-tropical regions of the earth. 



Order Urodela. The urodeles, as the name implies, are the 

 tailed amphibia, and in body shape are more nearly like the- 

 Stegocephalia than any other recent order. They may be com- 

 pletely aquatic, mud-living, or intermediate in their habits. None 

 survive in very dry regions, nor in the tropics. 



The largest living amphibian is Cryptobranchus of China and 

 Japan which attains a length of more than six feet. A closely 

 related species of the same genus inhabits the INIississippi valley. 

 Other large forms live in the southeastern part of the United 

 States. These are very specialized, particularly as to gills and 

 appendages. 



]\Iost urodeles are small and lack gills in the adult. These are 

 the newts and salamanders which can be found in streams and 

 muddy places in most parts of the Northern Hemisphere. Some 

 of the genera are highly modified, one or two lacking both gills 



