The Common Frog (Rana temporaria), Fig. 55, is the type of the order Anurous Amphibia or Batrachians, and belongs to the family Ranidae (frogs). It is characterized by having the skin smooth, the hind legs long, and the feet webbed. Thus it swims with great vigour, and on land progresses by a series of leaps. By its smooth soft skin it absorbs fluids rapidly, and thus has a double function both in nutrition and as an aid to respiration. It is an air-breather, but is capable of remaining for a considerable time under water. It retires in winter to the bottom of ponds and marshes, numbers usually congregating and burying themselves in the mud. This hibernation ends in February, and breeding shortly afterwards begins.

The Common Frog and Tadpole.

Fig. 55. - The Common Frog and Tadpole.

In March the spawn is deposited in masses, to which several individuals contribute, each furnishing many hundred eggs as gelatinous masses with blackish globules scattered through them. These globules soon manifest change, and after a time the young escapes as a tadpole with a short body, circular suctorial mouth and long tail, gills projecting on either side of the head, 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 as the lungs become more fully developed, and the tail gradually shrinks and disappears, and the animal, which is first fish-like, closely resembling a Urodele Amphibian (or newt), finally assumes the anourous or adult form. The process is that of metamorphosis, since there is a change not merely of form and proportion, but of internal organs. In its tadpole state the animal was essentially amphibious, but after its change has taken place it is not able to exist under water for any great length of time, but is forced to come to surface to breathe, and be able to live on land.

In the successive stages of its development each resembles the adult form of a lower group of animals; but there has been no passage of one form into another, they have rather descended from common ancestors, and the fish and newt have each reached a stage beyond which the frog has become developed. The frog is most active after rain, when, squatted in the grass, its abdomen rapidily absorbs water. It is found in meadows and other damp places during summer. The food of the frog consists of worms, slugs, woodlice, millipedes, and insects, including their larvae, and it greedily devours wireworms. The prey is captured by means of the tongue, which 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.