AMPHIBIA, a term in Zoology bestowed on different groups of animals, and with a considerable variety of signification. When rigorously interpreted, it applies to very few animals, as, with rare exceptions, naturalists know of no species which are strictly amphibious; that is to say, possessed of the power of performing the vital act of respiration either in air or water. When intended, however, merely to express the habit of existing alternately for a time in either of these elements, the term amphibious applies to various groups, which have little in common with each other. Hence the artificial mode of arrangement adopted by Gesner and the older naturalists, who, taking into consideration chiefly the habitations of animals, bestowed the name of amphibia on beavers, otters, frogs, and other incongruous genera. Linnaeus applied the name, in his earlier works, to a class composed of reptiles and chondropterygeous fishes; but he afterwards restricted it to reptiles alone possessed of cold red blood and a simple circulation. The amphibia of Baron Cuvier, again, are such of the mammalia or mammiferous quadrupeds as are enabled by their motive organs to dwell in water as well as on the land; for example, seals, sea lions, the walrus, &c. In his system they occupy an intermediate station between the feline and marsupial or pouched animals.· They pass the greater portion of their time at sea, and seldom come ashore except to suckle their young, or dry their moistened furs by exposure to sun and air. Their power of sustaining without injury a long-continued submersion does not, however, result from a continuance of the foramen ovale, by which it was supposed that the blood of these creatures passed from one chamber of the heart to another, without the necessity of being propelled through the lungs. Seals do not differ in that respect from other animals.

Buffon was of opinion that the newly born young of all mammiferous quadrupeds might be submerged in a nourishing fluid without depriving them of life, because their circulation at that early period was supposed to resemble that of amphibious reptiles. Experiments, however, on this subject would no doubt have accorded perfectly with those of Le Gallois, instituted with a view to ascertain the possibility of dividing the spinal marrow without the production of death. It is singular that Daubenton, his skilful and laborious coadjutor, did not remind Buffon that the foetus, while suspended in the liquid of the amnios, receives oxygenated blood from the mother, whilst after birth a necessity ensues for the vital act of respiration being performed by the individual itself. It is also known that the foramen ovale does not exist at the period of birth, except in such species as are destined to enjoy a continuance of that peculiarity of structure; from which we may infer that a primordial disposition of organs is of greater importance, and less easily alterable, than many modern physiologists are accustomed to suppose.

The principal characteristic of reptiles in general consists in this, that only a portion of the blood is transmitted through the lungs, the remainder being projected by the heart directly to the other parts of the body, without being specially subjected to the influence of the respiratory organs; whereas in the higher classes, such as man, the rest of the mammalia, and birds, the whole of the blood must pass by the lungs before it is retransmitted to the more distant parts of the circulating system. The respiration of animals, or the process by which the blood is oxygenated, becomes weaker and less frequent in proportion to the diminution which takes place in the quantity of blood transmitted to the lungs, compared with that which passes directly from the heart; and as it is respiration which warms the blood, and produces in the fibres their susceptibility of nervous irritation, it follows, as observed by Cuvier, that the blood of reptiles is cold, and their muscular strength much less than that of birds or quadrupeds. The seat of their sensations is also much less centralized than in the last named classes; and hence many of them exhibit life and motion long after their heads have been severed from their bodies.

A truly amphibious animal, according to the proper meaning of the term, ought to possess the power of breathing under water like a fish, and of respiring the atmospheric air like a land animal. According to this interpretation, neither seals nor beavers, nor even whales, are truly amphibious; for they cannot sustain their existence under water except by aid of a certain portion of air which they have inspired at the surface. In like manner, neither the frog nor the tadpole is amphibious (unless it may be for a short time at a certain intermediate period of life); for the former only seeks the water as a place of temporary resort, in which, however, it cannot breathe; and the latter is entirely aquatic, being unprovided with lungs, and consequently unable to respire, except through the medium of water. A frog can therefore only be said to be amphibious in as far as it possesses, at two distinct periods of its life, the faculty of living first in the water and then on the land; but its habit of life is by no means optional, and the change of structure, both in the respiratory and digestive system, as it passes from the immature to the adult state, is among the most extraordinary in the whole range of the animal economy. Born with gills, and destitute of external members, its form and functions are rather those of a fish than of a reptile; but as it advances in growth, the four limbs become developed, the tail decreases, and finally disappears, the jaws are formed, the gills are absorbed, and their uses are supplied by lungs. It can no longer breathe under water.

Among the many wonderful anomalies with which the kingdom of nature presents us, there exist, however, certain truly amphibious animals, classed under the genera oĩ Proteus and Siren, both of which are provided at one and the same time with the gills of a fish and the lungs of a terrestrial creature; but their habits and propensities[7:2:667] are decidedly aquatic. The former inhabit the subterranean waters of Carniola; the latter (of which there are several species) occur among the marshes and ricefields of South Carolina. For an account of these singular animals see the article Reptilia. (t.)