Page:Natural History, Reptiles.djvu/243

Rh similar apparatus; it is in general moist or clammy, and in many cases is provided with numerous glands which pour out a viscid secretion. The outer skin, or epidermis, is sloughed at irregular intervals like that of the Lizards and Serpents. The form is generally more or less lizard-like, but in one Order, that of the Frogs, this form is lost at an early period of life. They possess, in general, four limbs; but, like the Lizards, they shew a tendency to merge into a serpentiform condition, by the lengthening of the body, and loss of the hinder extremities in the Sirens, and by the total absence of feet, and snake-like contour of the Cæciliæ.

Most of the Amphibia undergo a metamorphosis, or change of form, which is connected with a change in the character of their respiration and in the medium in which they live. Thus the Frog is, in its first stage of active existence, a fish-like Tadpole, breathing the water by means of gills, but afterwards undergoes a great change of form, gradually acquires four well-developed limbs, throws off its tail, and at the same time loses its external gills, and becomes an air-breathing animal, possessed of internal lungs, and capable of crawling on the land as well as swimming in water. Some of the genera, indeed, possess both series of respiratory organs during their whole life, and are consequently Amphihia in a more strict and literal sense, possessing a capacity of living and breathing in two elements.

The reproduction is effected by means of eggs, which are numerous, more or less globular in figure, pellucid, and destitute of a calcareous