Frog, a batrachian reptile of the anourous or tailless order, embracing the group phanero-glosses (Dum. and Bib.), with the families rani-dcB or common frogs, and hyladoe or tree frogs. The general characters of the class and of the order have been sufficiently given in the article Amphibia, so that the principal families, genera, and species will only be mentioned here. The family of frogs or ranidoe include those genera the free extremities of whose fingers and toes are not dilated into disks, and whose upper jaw is provided with teeth; among these are many whose thick and clumsy bodies resemble those of toads (bufonidoe) rather than of frogs; in addition to maxillary teeth, most have also teeth on the palate and vomer, whose groupings, together with the form of the tongue and the visibility of the tympanum, are characters distinctive of genera and species. Almost all have, in the males, the vocal vesicles in the throat, communicating with the mouth, by the entrance of air into which their remarkable and loud sounds are produced; the nostrils open laterally, near the end of the snout; they have four non-palmated fingers, with the rudiment of a thumb, and five webbed toes; the back is generally irregularly roughened by glandular and other eminences, while the under surface is smooth.

Frogs pass most of their time in the water, being excellent swimmers: the length of their hind limbs enables them to make considerable leaps, and to travel over land in this way long distances in search of water: they are unable to climb trees, like the family hyladoe or tree frogs. Some species prefer moist localities and damp woods, where they hide in the grass and under leaves; others dwell in subterranean hollows which they dig on the borders of marshes, coming forth at evening or on rainy days. All the species when adult are decidedly carnivorous, even the smaller eating mollusks, insects, and worms, and all are characterized by great voracity. The frog family is found throughout the globe, though most abundantly in America; indeed five of the eight genera admitted by Dumeril and Bibron are peculiar to the new world; after America come Asia, Europe, Africa, and Polynesia, in the order of abundance of species. Of the numerous genera described, the genus rana (Linn.), which includes the common frogs, is the best known and the most interesting.

The principal characters of the skeleton of the frog are the small number of vertebra?, the absence of true ribs, the development of the transverse processes of the sacrum, the mobility of the iliac bones, the length of the coccyx, the presence of occipital condyles and an arch of scapular bones constituting a shoulder, and the elongation of the bones of the lower extremities. The muscles of the thigh and leg resemble considerably those of man and mammals. When a frog is at rest, the articulations of the pelvis, thigh, leg, and foot form four great folds or levers, by the sudden opening of which at the same time its remarkable leaps are effected; the swimming of the frog, which has erroneously been taken as a model for man in this respect, consists in a series of horizontal leaps, the body being sustained by the water, and its general form offering little resistance, and the anterior limits being folded against the trunk instead of acting as aids to the legs in locomotion; walking of course must be difficult and slow where there is such disparity in the length of the arms and legs.

The skin is smooth, made up of the usual layers, and in many parts of the body separated from the muscles to such an extent that it may be considerably distended at the will of the animal: the thin epidermis is frequently renewed; in the pigment layer are seated various colors, especially bright in the season of fecundation. The sense of smell is very imperfect; the tongue is not an organ of taste but of prehension, soft and covered with a viscid mucus, its base attached to the concavity of the lower jaw, its bifurcated point extending backward, and the whole organ capable of being projected from the mouth in a reversed position for the seizure of its insect prey; the organ of hearing has a tympanum, and an aerial cavity under it communicating with the throat. The mouth is very widely cleft, and some of the larger species have been known to swallow small mammals and birds; like other amphibians they cannot drink. The structure of the heart, gills, and lungs, and the phenomena of the circulation in the tadpole and adults, and of the branchial, pulmonary, and cutaneous respirations, have been described in the article Amphibia. The well known voice of the frog varies so much in intensity and tone as to render it difficult from the sound to ascertain the distance of the animal, far surpassing in this respect the efforts of the most skilful ventriloquist; it can make a dull sound even under water.

Among the many authors who have attempted to imitate in words the sounds of the frog, one of the most successful is Aristophanes, in whose comedy of the "Frogs" a frequent verse in the chorus is brekekekex koax koax, whose night-long repetition in spring and summer sometimes renders sleep impossible to those unaccustomed to it. By their power of retarding or accelerating the respiratory movements, and of aerating the blood through the vessels distributed to the skin, frogs are able to resist considerable changes of cold and heat, and to sustain life during their winter torpidity; the absorption and exhalation performed through the skin explain their occurrence and prolonged existence under circumstances where ordinary animals would soon perish, as under water and in air-tight places. The sexes are separate, and the reproductive functions are performed in the same mechanical and passionless manner as in most fishes; the ova are fecundated at the moment of their exclusion. As the eggs are expelled they are enveloped in a glairy mass, in which the embryos are seen distributed like black dots; the development is very rapid under favorable circumstances of temperature, the head and tail becoming perceptible in the course of the second day, the gills on the third, and the tadpole at the temperature of 80° F. (as in Rusconi's experiments) may leave the egg on the fourth or fifth day; but in the ordinary seasons of temperate Europe and America, the young are not hatched until about a month after the deposit of the eggs.