252 



HOW DO WE CLASSIFY ANIMALS? 



heart, 

 lung., 

 gall bladd 

 pancreas 



Spl<sen. I 



cloocca».._ :! ^..j 



_ —gullet. 



stomach. 



kidney 



HacCden 



The heart pumps the blood through a system of closed tubes to 

 all parts of the body. Oxidation must take place in the cells of the 

 body wherever work is done. Food in the blood is taken to the 



muscle cells or other 

 cells of the body and 

 there oxidized. The 

 products of oxidation, 

 chiefly carbon dioxide, 

 and any other organic 

 wastes given off from 

 the tissues must be 

 eliminated from the 

 body. As we know, the 

 carbon dioxide passes 

 off through the lungs 

 and to some extent 

 through the skin of the 

 frog, while the nitrog- 

 enous wastes are elimi- 

 nated by the kidneys. 



Nervous system. 

 The frog has a brain 

 and spinal cord and in general its central nervous system resembles 

 that of man. 



Reproduction and life history. The eggs of the common frog 

 are laid in shallow water in the early spring. Masses of several 

 hundred, which may be found attached to twigs or other supports 

 under water, are deposited at a single laying. Immediately 

 before leaving the body of the female they receive a protective 

 coating of jellylike material, which swells up after the eggs reach 

 the water. The upper side of the egg is dark, the light-colored side 

 being weighted down with a supply of yolk (food). The eggs are 

 fertilized in the water by sperms which are discharged about the 

 same time as the eggs. The fertilized egg soon divides into many 

 cells and in a week or ten days, if the weather is warm, it devel- 

 ops into a tiny oblong body with a wide tail and indistinct head, 

 which wriggles itself free of the inclosing jelly. This form is known 



A dissected frog. Seen from the under side. What 

 systems are represented in this figure? What parts are 

 left out of the drawing or are not labeled? 



