Common Bullfrog — Part 5

Rana catesbiana Shaw

If we bring tadpoles from the pond in early July, we shall find that many of them have hind legs. The size of the tadpole is surprisingly large. It may measure six or seven inches from the mouth to the end of the tail, the tail alone being three or four inches long. Let us examine one of these tadpoles closely to see how he is fitted to live his life in the water, surrounded by innumerable enemies. If we take one into our hands, we find it slippery and soft. The skeleton is cartilaginous. The creature has no means of defence, but must depend wholly on hiding and on flight. Fortunately, he is well provided for both. The marbled speckled brown of his moist skin makes him almost invisible on the muddy bottom, or among brown stems and leaves in the pond. The long tail has a broad fin, and is provided with strong muscles. With this equipment the tadpole can swim rapidly and vigorously, and can splash the mud of the bottom so as to obscure his course, and make it possible for him to glide off unseen in some new direction. The hind legs are fairly well developed, and the broad feet are webbed to the ends of the toes, so they give effective help in swimming. The eyes are large and conspicuous; in fact, they seem already to show a slight elevation, as though they were beginning to take on the characteristics of the frog's eyes. The mouth is small. It consists of fleshy lips covered with rows of tiny teeth. (The mouth structure of the bullfrog tadpole is as follows: The upper lip is relatively narrow, and bears only one complete row of teeth; the lower lip is very broad, has three rows of teeth and a border of papillae; the border of the lower lip extends upward to the upper lip, and is doubled in at the corners of the mouth. This structure corresponds almost exactly with that of the tadpoles of three other water-frogs, R. pipiens, R. palustis, and R. clamitans.) The lower lip is especially broad and ruffled, and bears along its lower edge rows of fleshy papillae, which, probably, serve for determining the nature of substances when the tadpole is seeking food. Between the lips, and somewhat within them, is a horny beak, which, though very small, is much like a bird's beak in shape. It consists of an upper and a lower jaw, strong, and very thin and sharp at their edges. The mouth is well fitted to bite off the delicate ends of leaves and stems, or to scrape off the tender green or brown plants from sticks and stones. It is equally adapted for eating animal food. The Bullfrog tadpole (like that of the Green Frog and of the Wood Frog) is especially fond of any animal food available. Thus these tadpoles act as scavengers and dispose of dead fish, or dead tadpoles even, that otherwise would become a menace to the living creatures of the pond.

On the left side of the tadpole is a conspicuous opening: the breathing-pore. The breathing is done by means of internal gills on each side of the throat, in a position like that of a fish's gills. They are concealed, however, by the outer skin. This is the fold of skin which grows back from the sides of the head in the young tadpole and covers the external gills. There are three sets of these internal gills on each side. They are feathery tufts extending into cavities between the walls of the throat and the outer skin of the sides of the head. These cavities, or gill-chambers, are connected with each other by a passage underneath the skin of the throat. Alternating with the tufts of gills, there are three openings in the throat-wall. These openings (gill-slits) connect the throat and mouth with the gill-chambers at the sides of the head. Water is constantly passing in at the mouth and nostrils, through the gill-slits into the gill-chambers, over the gills, and out at the breathing-pore. The water of the gill-chamber on the right passes into the gill-chamber on the left, and thence out of the breathing-pore on the left side.

One important way in which the tadpole is fitted to cope with the difficulties of his life is his power of reproducing the tail, should a hungry fish or other enemy bite it off. When you collect tadpoles for the aquarium, do not collect and put in with them the giant diving-beetles or any other of the tadpole's enemies. If you do so, on the first morning after the aquarium is arranged the beetles will be looking very hearty and the tadpoles will be scared, wobbling, tailless creatures, which demand quick rescue before the beetles complete their destructive work. However, if the beetles are removed, the tadpoles soon recover their assurance, and in a very few weeks their waving filmy tails are nearly as long as they were before the disaster.