The Probert Encyclopaedia of Nature

FROG

Frog is the common English name of a number of animals belonging to the class Amphibia, having four legs with four toes on the fore feet and five on the hind, more or less webbed, a naked body, no ribs, and no tail. Owing to the last peculiarity frogs belong to the order of amphibians known as Anura or tailless Amphibia. The tongue is fleshy, and is attached in front to the jaw, but is free behind, so that the hinder extremity of the tongue can be protruded.

Frogs are remarkable for the transformations they undergo before arriving at maturity. In the spring the spawn is deposited in ponds and other stagnant waters in large masses of gelatinous matter. These masses, with black globules scattered through them, soon manifest change, and after a time the young escapes as a tadpole, as an 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 tailed Amphibian), finally assumes the adult or tailless form.

The mature frog breathes by lungs, and cannot exist in water without coming to the surface for air. The only British species is the common frog (Rana temporaria), but the tribe is very numerous, other varieties being the edible frog (Rana esculenta) of the south of Europe, eaten in France and South Germany, the hind quarters being the part chiefly used; the bull-frog of America (Rana pipiens), 8 to 12 inches long, so named from its voice resembling the lowing of a bull; the blacksmith frog of Janeiro; the Argus frog of America, etc. Of the tree-frogs most belong to the genus Hyla. Frogs swim with rapidity, and move by long bounds, being able from the power of the muscles of their hind-legs to leap many times their own length.
Research Frog