236 



Aquatic Organisms 



The Amphibians are the smallest of the five great 

 groups of vertebrates. They are represented in our 

 fauna mainly by frogs and salamanders. A few of the 

 more primitive salamanders (Urodela) , such as Necturus, 

 breathe throughout life by means of gills, and are 

 strictly aquatic. A few are terrestrial, but most are 

 truly amphibious. They develop as aquatic larvae 

 (tadpoles), having gills for breathing and a fish-like 

 circulation: they transform to air-breathing, more or 

 less terrestrial adult forms; and they return to the 

 water to lay their eggs in the primeval environment. 



Fig. 142. A leopard frog. Rana pipiens. 



The period of larval life varies from less than two 

 months in the toad to more than two years in the bull- 

 frog. 



The eggs of amphibians are, for the most part, de- 

 posited in shallow water, often in masses in copious 

 gelatinous envelopes (see fig. 201 on p. 342). In some 

 cases the egg masses are large and conspicuous and well 

 known. Examples are the long egg-strings of the toad 

 that lie trailing across the weeds and the bottom; or 

 the half -floating masses of innumerable eggs laid by the 

 larger frogs. The eggs of the smaller frogs are less 

 often seen, those of the peeper being attached singly 

 to plant stems. Dr. A. H. Wright (14) has shown that 

 the eggs of all our species of frogs are distinguishable 

 by size, color, gelatinous envelopes and character of 

 cluster. 



