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. The Frog has no neck, the base of his skull coming close to the collar-bones, and there are only a few pairs of very short apologies for ribs between the shoulders and the long pelvis which produces that steep incline at the rear of his back. He is clothed entirely with a smooth, soft skin, which is kept moist by the action of minute mucous glands distributed all over the body. A row of these glands of larger size forms a pale line running back from the eye on either side. The skin plays an important part in the oxygenation of the Frog's blood; and the experimental physiologists have shown that a Frog deprived of its lungs can carry on its respiration for a lengthened period through the skin alone. Owing to the absence of ribs he has to fill his lungs by swallowing air. The male is less portly than the female, and he is further distinguished by having two pads on the first finger which in the breeding season become large rough cushions enabling him to hold his mate. In his throat there is a pair of vocal sacs enabling him to produce his love songs, and when these are in use their inflation causes a distension of the skin of the throat; but without these adjuncts the female manages to give answering croakings. When these duets are sung under water they produce some curious effects.