VEKTEBKATA. 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- 

 topterus from Africa, and Lepidosiren 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 

 (Fig. 174). They commence as water -breathing larvae, 

 when they resemble Fishes in their respiration, circula- 

 tion, and locomotion. In the lowest forms, the gills are 

 retained through life; but all others have, when mature, 

 lungs only, the gills disappearing. The cuticle is frequent- 

 ly shed, the mode varying with the habits of the species. 161 

 The common Frog, the type of this class, stands interme- 

 diate between the two extremes of the vertebrate series ; 

 no fundamental part is excessively developed. 



There are about sev- 

 en hundred living spe- 

 cies, grouped in four 

 orders : 



1. Proteida have a 

 naked skin, a tail, and 

 two or four limbs. 

 Some retain their gills 

 through life, as the - 



