The Tetrapoda: Amphibia, Reptiles, Birds 595 



must have perished during these crucial times, but some crossopterygians 

 must have met this challenge and successfully emerged on land. 



Among the forests of the Carboniferous, the new land form, the 

 amphibians, attained great diversity. These early types are represented 

 by the labyrinthodonts which were still scaled, but possessed the angular 

 limbs of land animals and modifications of other skeletal characters. At 

 best, their descendants, the modern amphibians, have but a tenuous hold 

 on land dwelling: the lungs are inefficient, necessitating skin respira- 

 tion, the weak legs are ill suited for supporting the body, and the larvae 

 are usually aquatic. They have, however, lost the scales typical of 

 their crossopterygian and labyrinthodont ancestors; this loss was es- 

 sential if respiration were to be possible through the skin. 



THE CLASS AMPHIBIA 



When compared to the other vertebrate classes, the modern amphi- 

 bians are small in number and of little economic importance. Although 

 they were the first to emerge upon land and must have at one time been 

 the only terrestrial vertebrates, at the present time they are over- 

 shadowed in importance by their more versatile descendants, the rep- 

 tiles, birds, and mammals. 



The present-day number of amphibians is relatively small when 

 compared to the fossil record. Indicative of this is the fact that modern 

 amphibians are grouped into three orders while the fossil forms require 

 ten orders for their classification. In general, the modern amphibians 

 are small, relatively inconspicuous forms which are forced to live only 

 in moist areas. Two of the three modern groups of amphibians are 

 well known to most people; these are the frogs, toads, and the tailed 

 salamanders. The third order contains a small number of legless 

 wormlike forms, the caecilians, which live only in moist tropical areas. 



Characteristics of the Class Amphibia.^ — Characteristically the 

 modern amphibians have a smooth moist glandular skin. While the 

 ancient amphibians were scaled, only the caecilians among the modern 

 forms possess any scales. These scales, however, are very small and 

 inconspicuous. All possess two nostrils which unlike those of most 

 of the fish are connected directly with the mouth cavity. The majority of 

 the members of this class possesses lungs when adult, although a few 

 have secondarily lost them. The lungs are small, relatively inefficient 

 sacs; so lung respiration must be supplemented by gaseous exchange 

 through the skin. Associated with the development of lungs, a change 



