1 66 PHYLUM CHORD ATA 



ment in which it lives, and may change from time to time, like 

 that of a chameleon, although not to the same extent as in that 

 animal. 



The skin of the frog is without scales, claws, or other hardened 

 integumentary structures, such as are possessed by other verte- 

 brates. It is, however, provided with numerous integumentary 

 glands which secrete a protective slime. The characteristic tri- 

 radiate openings of these glands may be seen with the aid 

 of a hand lens in skin which the frog has shed ; pieces of skin 

 will often be found in the water in which the animals have 

 been kept. 



The body of the frog may be divided into two regions, the 

 head and the trunk. The neck region, which is wanting in 

 fishes and is so characteristic of land vertebrates, is just be- 

 ginning to make its appearance in amphibians. A distinct neck 

 is not present ; there is present, however, one cervical vertebra 

 with which the skull articulates. The caudal region is also want- 

 ing in the adult. In the larval frog and toad a long tail is present, 

 by means of which the animal swims ; it is, however, gradually 

 absorbed as the tadpole passes through its metamorphosis. 



The Head. This body division is triangular in shape. The 

 mouth is large and bordered by skinny lips, which close tightly to- 

 gether like the cover on a box and thus prevent air from escaping 

 during the act of respiration. The eyes are large and protruding. 

 Each is protected by two eyelids, the upper one of which is large 

 and thick and with little power of movement ; the lower one is 

 semitransparent and movable. 



In front of the eyes are the nostrils ; each of these is provided 

 with a valve which can be tightly closed. The nostrils communi- 

 cate directly with the mouth. Probe them with a bristle. Back 

 of each eye is a large, circular area, the tympanic membrane, or 

 ear drum, which is thus on the outer surface of the body. Be- 

 tween the eyes is a small, dark spot which marks the frontal organ ; 

 it is a rudiment of a median eye. In the male frog, of certain 

 species, a pair of large vocal sacs project from the hinder part of 

 the head in the breeding season. Probe them from the mouth, if 

 present, and determine their extent. 



