VERTEBRATA. 



317 



long, ribbon-like pectoral and ventral fins, set far apart ; 

 two auricles, and one ventricle ; and, besides gills, a cellu- 

 lar air-bladder, which is used as a lung. 



The representatives are Ceratodus from Australia, Pro- 

 tqpterus from Africa, and Zepidosiren from Brazil. 



Class II.— Amphibia. 



These cold-blooded Vertebrates are distinguished by 

 having gills when young, and true lungs when adult. 

 They have no fin-rays, and the limbs, when present, have 

 the same divisions as those of higher animals. The skin 

 is soft, and generally naked, and the skeleton is ossified. 

 The skull is flat, and articulates with the spinal column 

 by two condyles. There is no distinct neck; and the ribs 

 are usually small or wanting. The heart consists of two 

 auricles and one ventricle. All undergo metamorphosis 

 upon leaving the egg, passing through the " tadpole" state. 

 They commence as water-breathing larvae, when they re- 

 semble Fishes in their respiration, circulation, and locomo- 

 tion. In the lowest forms, the gills are retained through 

 life ; but all others have, when mature, lungs only, the 

 gills disappearing. The cuticle is frequently shed, the 

 mode varying with the habits of the species. 161 The com- 

 mon Frog, the type of this class, stands intermediate be- 

 tween the two extremes of the vertebrate series; no fun- 

 damental part is excessively developed. 



There are about four 

 hundred and fifty liv- 

 ing species, grouped 

 in four orders : 



1. Urodela have a 

 naked skin, a tail, and 

 two or four limbs. 

 Some retain their gills 



through life, as the ^ Head and G\\)&(AMenobranchu8. Cayuga 



