FROG. This familiar animal is the type of the anurous Amphibia (order Amara). The family Ranide, to which it belongs, is char acterized by having the skin smooth, the hind legs long, and the feet usually completely webbed; teeth are present in the upper jaw and palate, seldom in the lower jaw. The tympanic membrane is situated behind the eyes, and is not concealed. The nostrils are placed at the extremity of the rounded muzzle just above its margin, and open directly into the mouth. When the mouth is filled with air the nostrils are closed, and the animal swallows the bolus of air into the sacculate lungs, there being, in the absence of ribs, no provision for such re spiratory movements as take place in the chest of tnammals. Frogs are thus air breathers, but they are capable of remaining for a consider able time under water. They swim with great vigor, and on land progress by a series of vio lent leaps, the long hind limbs being powerful levers. Their food is chiefly insects, which they capture by means of the tongue: this organ is covered with a viscid secretion and is attached in front, its free border being behind; it is rapidly projected from the mouth, the insect adheres to it, and is at once swallowed. The frog does not drink, but its soft skin absorbs fluids rapidly, and thus has a double function both of nutrition and as an aid to respiration. As the frog grows the old outer skin cracks from time to time, and is pulled off and swal lowed. The animal retires in winter to the bottom of ponds, from which clusters of frogs may be drawn buried in mud. This hibernation, which is associated with low vital energy, ends in February; in March the spawn is deposited in gelatinous masses of many hundreds of eggs, the males riding for a long period at that season on the backs of the females, and fertilizing the eggs as fast as they are extruded. The eggs soon manifest change, and after a time the young escapes as a a larval animal with short body, circular suctorial mouth, and long tail, compressed from side to side. Gills project on either side of the head from a cleft which answers in position to the gill opening of fishes. The hind limbs first appear as buds, later the fore limbs project, the gills disappear, the lungs becoming more fully developed; the tail gradually shrinks and disappears, and the animal, which was at first fish-like, then closely resembled a newt (or urodele amphibian), finally assumes the adult or anurous form. This is a true process of metamorphosis as com plete as that of the butterfly; since there is a change not merely of form and proportion, but also of internal organs. The frog is highest among Amphibia, and the successive stages of its development resemble each the adult form of a lower group in its line of ancestry.
Frogs, themselves useful in clearing gardens of slugs and insects, are in turn the prey of birds, especially herons and aquatic bards, of serpents, and fish, the latter destroying large quantities of the spawn. Though exposed to droughts, they can bury themselves in the moist soil and thus live after the ponds are dried up. Though thus tenacious of life, the stories of frogs being found in stone and in trees are for the most part founded on imperfectly noted facts, though it is possible that a frog may now and then get closed into a cavity for which, after entering, it had grown too large; but an aperture must always be present by which water can get access to them. Their fossil his tory goes back to the early Tertiary days, and probably will be found to extend farther, as Eocene examples differ little from modern forms.
It is by no means easy to define the word "frog" in classification, as distinguished from "toad" (q.v.), and the safest method here will be to deal only with the aquatic family Ranide, already defined, except as the most highly de veloped of amphibians. It contains about 280 species and is represented in every part of the world not too cold except southern South America and Australia, where all the so-called frogs belong to a related family, the Cystig nathidce, whose members, especially of the sub family Cystignathinte, may be said to represent the Ranide in Notogwa. °Some of them," says Gadow, "can be distinguished from the true, typical frogs solely by the arciferous type of the shoulder-girdle and sternum." The type-genus Rana contains more than half the known species, and is scattered all over the northern hemisphere, but is absent from the southern. It is to this genus that the common frogs of Europe and the United States be long — the bull-frog, spring-frog, European grass-frog, etc. The American bull-frog (R. catesbiana) is the largest of the whole tribe, occasionally reaching a length of eigfit inches; and its muffled grunting cry may be heard a mile or more over the water. It is greenish bright upon the head and mottled elsewhere, while the legs are distinctly blotched. This species abounds in all sluggish waters from Kansas eastward, laying its eggs in long strings, and .its tadpoles require two years to reach maturity. It is bold and voracious, catching fish, salamanders, other frogs and even duck lings. Its size and the chicken-like daintiness of the flesh in its hind legs, or "saddle,'" make it the favorite frog for market, and great quantities are eaten in all parts of the country. In the springs, swamps and ditches lives the green frog (R. clamata), not half as big, but very similar in color except that it is yellowish or white below. Another green aquatic frog, still smaller, is the leopard frog (R. virescens), whose bright coat is marked with irregular blotches of black edged with whitish, in two rows along the back, and the legs are barred. This species is numerous everywhere as far west as the Sierra Nevada. Another checkered frog, confined to the Eastern States, is the pickerel frog (R. pctlustris), which is light brown with two rows of large oblong square blotches of dark brown on the back, and one or two on the sides. The head is short, and a dark line extends from the nostril to the eye, while the upper jaw is white, spotted with black spots. Another well-known little kind is the wood-frog (R. sylvatica), which goes to the water to breed in early spring, but during most of the year lives in the dry woods. It is a variable reddish brown, with the side of the head marked with a dark-brown band. Several other less conspicuous species of frog inhabit North America, including a few representatives of another family (Engystomida), besides the tree-frogs, elsewhere described.
For frogs generally consult Gadow's 'Am phibia and Reptiles> (1901) ; for those of the United States the writings of Holbrook, C. C. Abbott, 0. P. Hap, A. W. Butler, and especially 'North American Batrachia,> by E. D. Cope; Dickerson, The Frog Book> (New York 1906) ; Mivart, The Common Frog' (London 1874) ; Marshall, The Frog' (11th ed., New York 1912). See also AMPHIBIA ; METACHROSIS.