The Tetrapoda: Amphibia, Reptiles, Birds 601 



suggested that this developmental rate is correlated with the hydrogen 

 ion concentration of the water. In all forms but one, fertilization is 

 external; this exception is the bell toad, Ascaphus truci, of northwestern 

 California, Oregon, and Washington. The male has an external copula- 

 tory organ by which the sperm are transferred to the female. These 

 forms live near rapidly running mountain streams and the larvae are 

 equipped with sucking discs for adhering to stones. 



Apoda. — The members of this relatively rare order, the caecilians, 

 are found in the tropics of Central America, Africa, Australasia, India, 

 and Malaysia. These wormlike forms burrow in moist soil and feed on 

 insects. The eggs are laid in the soil, and in some species, the larval 

 stages are suppressed within the egg while others have aquatic larvae. 



Economic Importance of the Amphibians. — In comparison with 

 other vertebrate groups, the amphibians cannot be considered to be of 

 any great importance. Nevertheless they have some value. Frog legs, 

 for example, are regarded as a great delicacy and are valued as a food 

 in many countries. Toads destroy vast quantities of insects and thus 

 are of value in destroying insect pests. To the biologists, the frogs are 

 a never-ceasing source of study. They are easily available, may be kept 

 alive in the laboratory, and are excellent experimental forms. Both R. 

 pipiens and the salamander Nccturiis are used in dissections in many 

 biology laboratories. 



THE CLASS REPTILIA 



During the upper Carboniferous as the amphibians were diversi- 

 fying to become the dominant group, some labryinthodonts were grad- 

 ually evolving into the stem reptiles. These stem reptiles or cotylo- 

 saurs (Fig. 203) would not have seemed to have much of a future; in 

 contrast to the dominating, progressive amphibians of that period, the 

 cotylosaurs were conservative and had retained many labryinthodont 

 characteristics such as the scales. As so often happens in evolution, 

 however, these more generalized animals had the potentialities of de- 

 veloping structures to meet new environmental requirements. This was 

 the group that was destined to become the first true land vertebrates 

 and to give rise to the many varied reptiles and finally to the birds and 

 mammals. 



Among these cotylosaurs, the genera Seymoiiria, Diadectes, and 

 Capforhinus are the best known from their fossil remains. Seymouria 

 was transitional in many of its structures ; in fact, it has at times been 



