Fig. 253.—Skeleton of Frog.
Fig. 254.—Leg Muscles of Frog.
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 colouration.
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.
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 colour 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, surpasses all other animals in this respect (Fig. [280]). What advantage from this power?
Digestive System.—The large mouth cavity is connected by a short throat with the gullet, or œsophagus (Fig. [255]). A slit called the glottis opens from the throat into the lungs (Fig. [255]). Is the gullet long or short? Broad or narrow? Is the stomach short or elongated? Is the division distinct between stomach and gullet, and stomach and intestine? Is the liver large or small? Is it simple or lobed? The pancreas lies between the stomach and the first bend of the intestines (Fig. [255]). What is its shape? A bile duct connects the liver with the small intestine (Dc, Fig. [255]). It passes through the pancreas, from which it receives several pancreatic ducts. After many turns, the small intestine joins the large intestine. The last part of the large intestine is called the rectum (Latin, straight). The last part of the rectum is called the cloaca (Latin, a drain), and into it the ducts from the kidneys and the reproductive glands also open. The kidneys are large, elongated, and flat. They lie under the dorsal wall. The urinary bladder is also large. Does the salamander have a similar digestive system? (Fig. [256]) Why are the liver and the lungs (Fig. [256]) longer in a salamander than in a frog?