VERTEBRATA 151 



by the gills alone, so these fishes occasionally come to 

 the surface and take air into the lungs. Lungfishes feed 

 upon the small animals captured among the water plants. 



FIG. 128. Protopterus annectens ; one fourth natural size. African rivers. 



The representatives are Ceratodus from Australia, Pro- 

 topterus from Africa, and Lepidosiren from Brazil. 



CLASS III. Amphibia 



These cold-blooded vertebrates are distinguished by 

 having gills when young, and usually true lungs when 

 adult. They have no fin rays, and the limbs, when 

 present, have the same divisions as those of higher ani- 

 mals. 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 

 (Fig. 284). The heart consists of two auricles and one 

 ventricle (Fig. 273). In the course of development 

 nearly all undergo metamorphosis upon leaving the egg, 

 passing through the "tadpole " state (Fig. 370). They 

 commence as water-breathing larvae, when they resemble 

 fishes in their respiration, circulation, and locomotion. 

 In the lowest forms, the gills are retained through life ; 

 but all others have, when mature, lungs only (Fig. 282), 

 the gills disappearing. The cuticle is frequently shed, 

 the mode varying with the habits of the species. 34 The 

 common frog, the type of this class, stands intermediate 



