tchthyopsida 457 



organ of hearing. The tympanic mechanism serves to increase the 

 efficiency of the ear as an organ of hearing. In this connection, it is 

 significant that most anurans are capable of producing sounds. In some 

 anurans, especially those of burrowing habit, the tympanic apparatus 

 is rudimentary or entirely absent. 



The Anura are much more specialized than the Urodela. The 

 members of the two orders pass through very similar embryonic and 

 larval stages, but the metamorphosis of frogs and toads involves pro- 

 found anatomic changes while, aside from loss of gills and gill-clefts, 

 the metamorphic changes in urodeles are relatively inconspicuous. 

 Further, the Order Anura is anatomically more homogeneous than 

 Urodela — that is, the anatomic differences among the frogs and toads 

 are not so great as those among the urodeles. It is easy to distinguish 

 the common toad from certain equally common frogs (Fig. 354), but, 

 taking the group as a whole, the two types so blend into one another 

 that it is impossible to draw a sharp line between frogs and toads. 



The Anura are much more widely distributed than the Urodela. 

 They are found in all parts of the world except where it is very cold 

 or very dry or where, as on small oceanic islands, there is lack of fresh- 

 water streams and ponds. Their modes of living are most diverse. Most 

 of them are amphibious. Some are wholly aquatic, others wholly ter- 

 restrial. In a few of the latter the eggs, especially large, are laid on 

 land and develop directly into the adult, omitting the larval stage. The 

 terrestrial anurans may live on the surface, or burrow, or spend most 

 of the time in trees. In Java and Borneo there are "flying frogs." By 

 extending all four legs and spreading apart the long digits so that the 

 intervening webs of skin are stretched to the limit, the frog becomes a 

 "'glider plane" capable of making a sailing "flight" of perhaps 20 or 30 

 feet from one tree to another or to the ground. In one of the largest 



Fig. 354. {Left) Frog, Rana sphenucephala. (Right) Toad, Scaphiopus holbrookii. 

 (Courtesy, Dickerson: "The Frog Book," New York, Doubleday & Co.) 



