CHAPTER IV. 



THE FROG 



The frog lends itself to laboratory work in biology probably better 

 than any other animal. It is sufficiently common so that it is at least 

 somewhat familiar to the student, and it can be procured 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 statement refers to the animal's inability to live either on land 

 alone or entirely 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 (Latin re=back-f-spiro= 

 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 he leaps into the water when disturbed along the banks. 

 If one observes him while in water that is beyond his depth, it will be 

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

 exposed to the air. Should he 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-|-tendo=stretch) 

 completing the movement which forces him forward. 



Everyone also knows the sound of croaking frogs at night, especially 

 when the atmosphere becomes damp, though it is not so generally 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 by the air from the 

 lungs being 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 almost 

 always laid in water and hatched there. Little tadpoles develop from 

 these eggs and breathe by gills in the larval ( ) condition. 



