52 ELEMENTS OF COMPARATIVE ZOOLOGY. 



system, and this persists to some extent in the adult of 

 the aquatic salamanders, though disappearing in all other 

 forms. 



The vertebral column varies greatly in length, and in all 

 except the footless forms it can be divided into neck (cer- 

 vical), breast (thoracic), sacral, and caudal or tail regions, 

 the sacral being that which connects with the pelvic girdle. 

 In some the bodies of the vertebrae are amphicoelous (p. 

 14) ; in most salamanders they are opisthoccelous (rounded in 

 front, hollow behind), while in the frogs and toads they are 

 proccelous (hollow in front). The transverse processes of 

 the vertebrae are different from anything in fishes in that they 

 arise from the neural arch and not from the centrum. In 

 some forms the ends of these processes are jointed, and 

 from this and other facts they must be regarded as in part 

 equivalent to ribs. It is to be noticed that these ribs never 

 reach the sternum (p. 45), which, by the way, is a struc- 

 ture lacking in all fishes. 



A noticeable feature in the Batrachia is the metamor- 

 phosis during growth, the chief features of which have 

 already been mentioned, the result being that the adult 

 differs very considerably from the young. 



All living Amphibia live either in fresh water or on the 

 land ; none occur in salt water. The existing forms are 

 comparatively small, the largest being the giant salamander 

 of Japan, which may be three to four feet in length. 

 Existing Batrachia are conveniently divided into three 

 groups or orders. 



ORDER I. URODELA (Salamanders, etc.). 



These forms retain the tail throughout life, and haye the 

 extremities weakly developed, fitted for creeping rather than 

 jumping. Some live in the water throughout life, while 



