TADPOLES AND FROGS. 183 
heart, too, like that of a fish, has only two chambers, 
an auricle and a ventricle. 
After a time lungs develop, and the tadpole, while 
still using his gills to some extent, finds it necessary 
to come to the surface of the water occasionally for air. 
Finally the gills disappear and the frog breathes only 
Fic. 137.—Under Side of Tadpole Showing Coiled Intestine and Internal 
Gills. 
by lungs. While these changes are going on, the 
heart develops another auricle on the left side for the 
reception of the purified blood from the lungs. The 
right auricle receives the impure blood returning from 
the circuit of the body. 
Not only does the frog, at different times, breathe in 
the three ways just mentioned, but at all times it 
Fic. 138.—Heart of Fic. 139. — Blood-cells 
Adult Frog. a, of a Frog. a, red cor- 
auricles ; v, ven- puscle; 4, colorless cor- 
tricle. puscle. 
breathes by the entire surface of the body, the skin 
being especially rich in blood-vessels. 
