CHAPTER IV 



THE FROG 



THE frog lends itself to laboratory work in Biology probably better 

 than any other animal. It is sufficiently common to be somewhat 

 familiar to the student, and it can be obtained practically at any 

 season of the year. It is a vertebrate (Latin — vertebratus=jointed), 

 which means it has a back bone, and an amphibian (Greek — amphi= 

 both-(-bios=:life), meaning that it lives a double life. This latter state- 

 ment refers to the animal's inability to live either on land alone, or en- 

 tirely submerged in water. This inability to live entirely in the air or in 

 water is well shown by the fact that, if the frog's skin becomes dry, as it 

 does when the animal is away from water and in a dry atmosphere, the 

 animal dies, because the skin is then no longer capable of serving as 

 an organ of respiration (L. re=back-|-spira=breathe). Contrariwise 

 if it be constantly immersed in water, it will also die, because it must 

 breathe air. 



The particular species (Rana pipiens) that we are describing 

 (though any other of the common forms would answer the same pur- 

 pose) is found in or about fresh-water lakes, ponds, or streams. The 

 species is fairly well distributed over the entire North American con- 

 tinent, except the Pacific slope. 



Everyone has noticed the longer and stronger hind legs of the frog, 

 and the squatting position it assumes on land, as well as the rapidity 

 with which it leaps into the water when disturbed along the banks. 

 If one observes if while in water that is beyond its depth, it will be 

 noted that the hind legs hang out straight and the tip of the nose is 

 exposed to the air. Should it be disturbed while in this position, the 

 hind legs are flexed (L. flecto=bend), which throws the body downward. 

 The fore legs are used in arranging the direction in which the animal 

 will go; the hind legs are then extended (L. ex=out-ftendo=stretch), 

 completing the movement which forces it forward. 



Everyone also knows the sound of croaking frogs at night, 

 especially when the atmosphere becomes damp, though it is not so gen- 

 erally known that the frog croaks far more frequently during the 

 breeding season than at other times. The croaking can be accomplished 

 both in and out of the water. The croaking under water is produced 

 when the air from the lungs is forced past the vocal cords into the 

 cavity of the mouth, and then back again into the lungs. 



There is another reason why the frog may be considered as leading 

 two lives, (Fig. 3) beside the fact that it needs both air and water, and 

 that is that it lives a different type of life when young than when grown. 

 This comes about as follows : The eggs of the female frog are prac- 



