Frog (Rana temporaria, Linn.).

With the Common Frog, popularly classed as a Reptile, we commence acquaintance with the zoological class Batrachia, creatures that begin life at a much lower stage of development and have to pass through a fish-like larval form before attaining to any likeness to their parents. The Reptiles get through these developmental stages whilst they are still in the egg; they never have water-breathing organs. The Batrachians or Amphibians are clothed with soft skin which is not protected by armour plates or scales as seen in the Lizards and Snakes, but through which they are able to oxygenate the blood. The Frogs, Toads, and Newts constitute a class intermediate in structure and development between the Fishes and the Reptiles. Our native species represent the two orders—Ecaudata (tailless), including the Frogs and Toads; and Caudata (tailed) comprising the Newts.

Everybody knows the Frog as well as they know any of the backboned animals, and every youngster even is familiar with the main facts of its development, from the jelly masses of eggs in the pond early in spring, through the tadpole stage to the attainment of four legs and wonderful leaping powers. It is common knowledge that he has a moist, smooth skin (the supersensitive erroneously say "slimy") of yellowish ground colour overlaid with streaks and spots of brown. There is a big patch of brown behind each eye, and the long hind legs have cross-bars of the same colour. The ground tint of the Frog varies in different individuals according to the situation in which we may find them; for the pigment cells of the skin expand and contract under the influence of varying intensities of light reflected from the surroundings, causing colour changes much after the manner of those of the Chameleon, though less marked.

Skeleton of Frog.

[View Larger Image Here.]

The Frog's forelimbs are very short compared with the hind pair, and the four moderate-sized fingers are not connected by webs; whereas, the hind limbs have their several bones lengthened, and the abnormal lengthening of those of the ankle gives the legs the appearance of having a supplementary joint. The leg is one and a half times the length of head and body. The foot has five long toes connected for half their length by a "web" of skin which constitutes a very efficient paddle when the Frog is in the water. Of these hind toes the fourth is considerably longer than the long third and fifth.

The Frog's head is as broad as it is long, the muzzle rounded, and the horizontal gape of the mouth extends back beyond the eye. The prominent eyes are perched up on the forehead, and have a fine golden iris and a horizontal pupil. The Frog differs from the Snakes and agrees with the Lizards in having eyelids; he has also, like the Birds, an additional lid—the nictitating membrane. There is a row of delicate teeth along the upper jaw, but none on the lower; there are others on the palate. The deeply notched tongue is attached by its base to the front part of the mouth, the tip far in towards the throat; in use it has to be suddenly turned over so that the tip is projected far beyond the muzzle. The large circular depression behind and below the eye is the drum of the Frog's ear.