Fig. 214.—The Frog. (× 1.)
Food.—The frog’s method of catching its prey is very interesting, and is graphically described by Dr. Hans Gadow[26] as follows:—“The food, which consists chiefly of insects, snails, and worms, must be moving to excite interest; then the frog, whose favourite position is half squatting, half supported by the arms, erects itself and, facing the insect, turns round upon its haunches, adjusts its position anew by a shifting of the legs, and betrays its mental agitation by a few rapid movements of the throat. All this time the prey is watched intently until it moves; then there follows a jump, a flap of the tongue, and the insect is seen no more.” This flap of the tongue is well illustrated in [Fig. 215]. The frog’s tongue is free behind, but is attached, by its anterior end, close to the middle of the lower jaw,—an arrangement which enables it to be flicked out to its full length. Further, it is covered by a glutinous secretion, which sticks tenaciously to the prey.
Fig. 215.—Three Stages of
the Movement of the
Tongue of a Frog. (× 1.)
The frog is fortunate also in the extremely wide gape of its mouth, which stretches, almost literally, from ear to ear. Once in the mouth, the captive is prevented from escaping by the teeth. Of these there are two sets; a row of fine teeth is present along the greater part of the margin of the upper jaw, and, in addition, two small patches of teeth—the vomerine teeth (Vo., [Fig. 216])—occur on the roof of the mouth. Fairly large insects are promptly gulped down into the stomach; those which, owing to their minute size, escape being swallowed in the ordinary manner, are slowly but surely forced down the throat by the incessant lashing of thousands of tiny threads—too small to be seen except by high powers of the microscope—which are carried by the skin of the roof of the mouth. The action of these invisible threads may easily be seen if a frog’s mouth be opened widely and a small cork-shaving be placed near the top of the throat (Expt. 60, 5, b). The shaving steadily travels backwards and is soon lost to sight.
It should be clearly understood that frogs and toads are of incalculable value in keeping down insect pests, and deserve systematic protection for this if for no other reason.
How a frog breathes.—When a resting frog is watched, the floor of the mouth is seen to be raised and lowered alternately. It is a common belief among children that these movements are a sign that the animal is “getting ready to spit.” Frogs do not spit, however, and the action is simply a part of the breathing process, which is performed in the following manner. The mouth being closed, the nostrils are opened, and, by alternate up-and-down movements of the floor of the mouth, the air present in the mouth-cavity is completely replaced by fresh air. The nostrils are then closed, and the slit-like glottis (gl., [Fig. 163]) which leads to the lungs (l. lng., r. lng.) is opened. The foul air is forced out of the lungs and mixed with the pure air in the mouth-cavity. Then, immediately, the floor of the mouth is raised—pumping the mixed air into the lungs—and the glottis is closed. In the lungs an exchange takes place between the oxygen of the refreshed air, and the surplus carbon dioxide in the blood which is circulating in the capillaries ([p. 242]) of the walls of the lungs. In the meantime the nostrils have again been opened, and the first stages of the process are already being repeated.