BRANCH CHORD AT A: CLASS BATRACHIA 295 



stretched across the short windpipe; the vibration of 

 these cords produces the croaking. 



The heart is always three-chambered, consisting of the 

 right and left auricles and a single ventricle. The circu- 

 lation of the more generalized salamanders like the mud- 

 puppies is essentially like that of a fish. In the frogs and 

 toads there is a distinct advance beyond this condition. 

 The red corpuscles of the blood are oval in shape and are 

 the largest found among any of the vertebrates. 



In the nervous system the small size of the hindbrain 

 or cerebellum is noticeable. The sense organs are fairly 

 well developed. The skin of the whole body is provided 

 with tactile nerve-endings. There are special taste organs 

 on the lining membrane of the tongue and mouth-cavity. 

 The eyes have no lids in some of the lower forms ; most 

 of the frogs and toads have an upper lid but no under one, 

 although a thin membrane, called the nictitating mem- 

 brane, arises from the lower margin of the eye and can be 

 drawn up over it. The ears have no external parts, other 

 than the thin tympanic membranes. The nostrils of frogs 

 and toads can be closed by the contraction of certain 

 special muscles. 



Life-history and habits. The sexes are distinct, and in 

 most cases the young hatch from eggs. A few of the sala- 

 manders give birth to free young. The eggs are usually in 

 strings or chains enclosed in a clear gelatinous substance; 

 these chains of eggs are either simply dropped into the 

 water or are fastened to water-plants. The young, called 

 tadpoles (fig. 120), in their earlier larval stages are ex- 

 tremely fish-like in character, long-bodied, tailed, swim- 

 ming freely about by means of the fin-like flattened tail, and 

 breathing by means of external gills. Nor do they show 

 any sign of legs. As the tadpoles grow and develop the 

 legs begin to appear, the hind legs first in the frogs and 

 toads, the fore legs first in the salamanders ; lungs develop 



