Frog

Frog,1 a name in zoology, of somewhat wide application, strictly for an animal belonging to the family Ranidae, but also used of some other families of the order Ecaudata or the sub-class Batrachia (q.v.).

Frogs proper are typified by the common British species, Rana temporaria, and its allies, such as the edible frog, R. esculenta, and the American bull-frog R. catesbiana. The genus Rana may be defined as firmisternal Ecaudata with cylindrical transverse processes to the sacral vertebra, teeth in the upper jaw and on the vomer, a protrusible tongue which is free and forked behind, a horizontal pupil and more or less webbed toes. It includes about 200 species, distributed over the whole world with the exception of the greater part of South America and Australia. Some of the species are thoroughly aquatic and have fully webbed toes, others are terrestrial, except during the breeding season, others are adapted for burrowing, by means of the much-enlarged and sharp-edged tubercle at the base of the inner toe, whilst not a few have the tips of the digits dilated into disks by which they are able to climb on trees. In most of the older classifications great importance was attached to these physiological characters, and a number of genera were established which, owing to the numerous annectent forms which have since been discovered, must be abandoned. The arboreal species were thus associated with the true tree-frogs, regardless of their internal structure. We now know that such adaptations are of comparatively small importance, and cannot be utilized for establishing groups higher than genera in a natural or phylogenetic classification. The tree-frogs, Hylidae, with which the arboreal Ranidae were formerly grouped, show in their anatomical structure a close resemblance to the toads, Bufonidae, and are therefore placed far away from the true frogs, however great the superficial resemblance between them.

Some frogs grow to a large size. The bull-frog of the eastern United States and Canada, reaching a length of nearly 8 in. from snout to vent, long regarded as the giant of the genus, has been surpassed by the discovery of Rana guppyi (8½ in.) in the Solomon Islands, and of Rana goliath (10 in.) in South Cameroon.

The family Ranidae embraces a large number of genera, some of which are very remarkable. Among these may be mentioned the hairy frog of West Africa, Trichobatrachus robustus, some specimens of which have the sides of the body and of the hind limbs covered with long villosities, the function of which is unknown, and its ally Gampsosteonyx batesi, in which the last phalanx of the fingers and toes is sharp, claw-like and perforates the skin. To this family also belong the Rhacophorus of eastern Asia, arboreal frogs, some of which are remarkable for the extremely developed webs between the fingers and toes, which are believed to act as a parachute when the frog leaps from the branches of trees (flying-frog of A. R. Wallace), whilst others have been observed to make aerial nests between leaves overhanging water, a habit which is shared by their near allies the Chiromantis of tropical Africa. Dimorphognathus, from West Africa, is the unique example of a sexual dimorphism in the dentition, the males being provided with a series of large sharp teeth in the lower jaw, which in the female, as in most other members of the family, is edentulous. The curious horned frog of the Solomon Islands, Ceratobatrachus guentheri, which can hardly be separated from the Ranidae, has teeth in the lower jaw in both sexes, whilst a few forms, such as Dendrobates and Cardioglossa, which on this account have been placed in a distinct family, have no teeth at all, as in toads. These facts militate strongly against the importance which was once attached to the dentition in the classification of the tailless batrachians.