THE AMPHIBIANS 181 



more or less coiled in the shorter species. In some cases 

 no well-marked stomach exists, but ordinarily the different 

 portions, as they are shown in Fig. 109, are well defined. 



As noted above, the circulation in the tadpole is the 

 same as in fishes, then lungs arise, and for a time respi- 

 ration is effected both by gills and lungs, and the cir- 

 culation resembles in its essential points that of the 

 lung-fishes. This may continue throughout life, but more 

 frequently the gills and their vessels disappear, and the 

 circulation approaches that of the reptiles. In such forms 

 the heart consists of two auricles and one ventricle. Into 

 the left auricle pours the pure blood from the lungs ; into 

 the right the impure blood from the body. To some 

 extent these mix as they are forced into the general cir- 

 culation by the single ventricle. The amount of oxygen 

 carried is therefore smaller than in the higher air-breathers, 

 the amount of energy is proportionately less, and hence it 

 is that all are cold-blooded and of comparatively sluggish 

 habits. 



In some species of salamanders the lungs may also dis- 

 appear, and breathing is carried on by the skin, as it is to 

 a certain extent in all amphibians. In the frogs and toads 

 lungs are invariably present, and vocal organs are situated 

 at the opening of the windpipe in the throat. These pro- 

 duce the characteristic croaking and shrilling, which in 

 many species are intensified through the agency of one or 

 two large sacs communicating with the mouth-cavity. 



Although the brain is small in the amphibians, it is 

 more complex in several respects than it is in fishes. 

 The eyes are also usually well developed, but in some of 

 the cave and burrowing salamanders they are concealed 

 beneath the skin, and are rudimentary. The ear varies 

 considerably in complexity in the different species, but in 

 the possession of semicircular canals and labyrinth resem- 

 bles that of the fishes. In the frogs and toads, as one may 

 readily discover, the drum or tympanum is external, ap- 



