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 or 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- 
