9 2 VERTEBRATE ZOOLOGY 



numerous integumentary glands which secrete a protective slime. 

 The characteristic triradiate 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 beginning to 

 make its appearance in amphibians. A distinct neck is not pres- 

 ent ; there is present, however, one cervical vertebra, with which 

 the skull articulates. The caudal region is also wanting 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 together 

 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 eyelid is semi- 

 transparent and movable. 



In front of the eyes are the nostrils, or external nares ; each of 

 these is provided with a valve which can be tightly closed. The 

 nostrils communicate 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. Between the eyes is a small dark spot which marks 

 the frontal organ ; it is a rudiment of a median eye. In the male 

 frog, in 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. 



The Trunk. This body division is short and shows externally 

 no marks of segmentation ; it bears the appendages. In the middle 

 of the back will be noticed a prominent hump, which indicates the 

 position of the sacrum, where the hinder appendages articulate with 

 the spinal column. At the posterior end of the trunk and slightly 

 dorsal in position is the small opening of the cloaca, the anus. 



