Frog (amphibian)

The FROG is an aquatic amphibian of the tailless order Anura and the family Ranidae, distinguished from its fellow anurans, the toads, by having teeth in the upper jaw, the lower bones of the shoulder girdle' firmly conjoined, a smooth skin and the tympana visible, and toes entirely webbed. It differs from the tree frogs in having the toes free from viscid diskpads and in other features. Frogs occur in most parts of the world having a moderate climate, and make their home more or less continuously in and about stagnant water. Some never leave it for more than a few hours, while others resort to the water only in the egg-laying season, after which (as in the case of the American buff-and-black wood frog, Rana sylvatica) they betake themselves to the woods and remain there, searching among the fallen leaves for insects and worms, until the next spring.
Other species wander more or less, while some inhabit the water continually, feeding upon anything and everything edible which they are able to catch above or beneath the surface. During the winter the thoroughly aquatic species conceal themselves in the mud, going into a cold torpor, where the climate is severe, until revived in the spring; others bore into the soil, or beneath the fallen leaves, or into rotting slumps, etc., and exist quiet and lethargic. The early spring brings them out and all species haslen to the water, where they croak loudly (in some species only the males, in others both sexes joining in the chorus) during the process of mating and egg production. At this season the males may be distinguished by seasonal marks, as brighter colors, a swollen forefinger, or some other unusual feature. The eggs are laid in a long, stringlike tube of gelatinous matter, which forms an irregular mass; the male, sitting on the back of the female, fertilizes them as they exude from her. The parents pay no attention to the eggs after they have been laid but leave them glued to or entangled in the swamp vegetation.

In about a fortnight's time the larvae or "tadpoles" wriggle from the jelly envelope into the surrounding water. They are fishlike, without limbs, with external gills and a swimming tail. At first very inactive, these tadpoles soon begin to swim about freely, feeding on vegetable food or on refuse of various kinds; they soon lose their external gills but acquire internal ones like those of a fish. Henceforth growth is rapid; hind limbs appear and gradually increase in size, and the tadpole, especially if the water is foul, comes to the surface to breathe, showing that lungs are developing. About three months after hatching, the forelimbs, which have been concealed within the gill chambers, are suddenly revealed, the tadpole ceases to feed, its tail gradually diminishes in size, and finally, the metamorphosis complete, it hops on shore as a tiny frog. After quitting the water the little frogs spend the remainder of the warm season on land, before beginning hibernation in the autumn.

Frogs feed chiefly on worms and insects, which they capture by means of a sticky tongue. This organ is peculiar in that it is attached at the front of the mouth, the tip of the tongue lying posteriorly, and in order to be extended, must be flipped over. Once prey is in the mouth it is held fast by the vomerine and maxillary teeth. The food is then passed whole to the muscular esophagus where digestive fluid is added. In the stomach this fluid is activated by acid, enzymes are released, and digestion begins, aided by the muscular contractions of the stomach. Digestion is completed and absorption takes place in the small intestine, the food being moved along the intestinal tract by peristalsis (waves of mus­cular contraction). Undigested food is stored in the large intestine until it is eliminated through the anus as feces.