FROGS AND THEIR RELATIVES 



111 



of two kinds of cells (the red and white corpuscles) which 

 are floating in a liquid known as plasma. The plasma con- 

 sists largely of water and the digested foods that have been 

 absorbed from the alimentary canal (83). 



As in the fish, the circulatory system consists of the heart 

 and three kinds of blood vessels ; namely, arteries, veins, and 



FIG. 80. Internal organs of the frog. 



capillaries. The heart is located in the body cavity just back 

 of the head and consists of two auricles and a ventricle 

 (Fig. 81), instead of a single auricle and ventricle as in the fish 

 (Fig. 100). As might be expected, this makes necessary other 

 differences in the circulatory system of the frog. In the fish 

 we shall see that there is only one stream of blood flowing into 

 the heart, while in the frog there are two. One stream 

 enters the heart from the various organs of the body which 

 the blood has supplied with food and oxygen, and from which 

 the blood has received carbon dioxid. The second blood 



