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 sometimes 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 coloration.
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 will become darker than if kept in the light or on a white dish. Try this experiment, comparing two frogs. This power of changing color is believed to be due to the diminution in size of certain pigment cells by contraction, 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 of the Gulf states, surpasses all other animals in this respect (Fig. 280). What advantage from this power?
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Fig. 255.—Digestive Canal of Frog.
Mh, mouth; Z, tongue pulled outward; S, opening to larynx; Oe, gullet; M, stomach; D, intestine; P, pancreas; L, liver; G, gall bladder; R, rectum; Hb, bladder; Cl, cloaca; A, vent.
Digestive System.—The large mouth cavity is connected by a short throat with the gullet, or esophagus (Fig. 255).
