BATRACHIA 



133 



Is the skin of a frog tight or loose ? Does it have any 

 appendages corresponding to scales, feathers, or hair of 

 other vertebrates ? Is the skin rough or smooth ? The 

 toad is furnished with glands in the skin which are some- 

 times swollen ; they form a bitter secretion, and may be, 

 to some extent, a protection. Yet birds and snakes do not 

 hesitate to swallow toads whole. Show how both upper 

 and under surfaces of frog illustrate protective colouration. 



All batrachians have large and numerous blood vessels 

 in the skin by which gases are exchanged with the air, 

 the skin being almost equal to a third lung. That the 

 skin may function in this way, it 

 must not become dry. Using this 

 fact, account for certain habits of 

 toads as well as frogs. 



If a frog is kept in the dark or 

 on a dark surface, its skin zvill be- 

 come darker than if kept in the light 

 or on a white dish. Try this experi- 

 ment, comparing two frogs. This 

 power of changing colour is believed 

 to be due to the diminution in size 

 of certain pigment cells by contrac- 

 tion, and enlargement from relaxation. 

 This power is possessed to a certain 

 degree not only by batrachians but 

 also by many fishes and reptiles. 

 The chameleon, or green lizard, 

 surpasses all other animals in this 

 respect (Fig. 280). What advan- 

 tage from this power? 



Fig. 255. — Digestive 

 Canal of Frog. 



Mh, mouth; Z, tongue pulled 

 outward; 5", opening to 

 larynx; Oe, gullet; M, stom- 

 ach; D, intestine; P, pan- 

 creas; L, liver; G, gall 

 bladder; R, rectum; Hb, 

 bladder; CI, cloaca; A, 

 vent. 



Digestive System. — The large mouth cavity is connected 

 by a short throat with the gullet, or oesophagus (Fig. 255). 



