AMPHIBIA, a class of back-boned creep ing animals comprising the newts, frogs and toads, together with several extinct groups, which is classified between the fishes and the reptiles. The most prominent characteristic is indicated by the name, which refers to the fact that these animals are provided with a respira tory apparatus which enables them to breathe both water and air. It is not meant, however, that the Amphibia are able to breathe in either air or water at the same time, but that the young are provided with gills and live in water up to a certain age, or in rare cases per manently, after which they acquire lungs and thereafter breathe atmospheric air. As these young as a rule are different from their parents and must undergo metamorphosis from the larval into the adult condition, amphibians as a class are usually said to undergo metamor phosis, but this is equally true of some fishes and it is not true of all amphibians. The evidence not only of modern similarity of structure, but that obtained from a study of the fossil forms, makes it plain that the Amphibia are the result of the evolution of a branch from an ancient fishstock, probably by way of the lung-fishes or Dipnoi (q.v.). On the other hand they are related in a not very dif ferent degree to the reptiles. The connection link, according to Gadow, is formed by the Stegocephali; all the recent orders are far too specialized. The line leading from Stegocephali to fossil reptiles is extremely gradual, and the same consideration applies to the line which leads downward to the fishes; but the great gulf within the Vertebrata lies between fishes and amphibians, that is, between absolutely aquatic creatures with internal gills and fins, and terrestrial four-footed creatures with lungs and fingers and toes. No great phylogenetic importance attaches to the possession of ex ternal gills, as it is not unlikely that in the Amphibia these organs owe their origin to en tirely larval requirements.
Although in the Palaeozoic age the great stegocephalous amphibians (more usually called labyrinthodonts, q.v.), flourished as the only terrestrial vertebrates of importance, the class never attained a dominant position. Inter mediate between the aquatic fishes and the gradually rising terrestrial reptiles, the amphib ians were pushed aside in a double way by the struggle of evolution, until now most of them have become extinct. The remainder persist
only because they have found shelter in the nooks and corners of the world to which they have become adapted by small size and aquatic habits; and only one group, the frogs and toads, fortunate in their plasticity, have spread over the whole globe and exhibit some richness in forms.
The class Amphibia is divided into two classes: (1) Stegocephali (q.v.), which is wholly extinct; (2) Lissamphibia, which cludes all of the modern forms, contained in '.-r •c• orders: (1) Apoda, or Gymnophiona, ' of the family Cayilliidce (see C(Ecu.
IAN); (2) Urodela, including the long-tailed, smooth-skinned, aquatic salamanders, newts, mud-puppies and the like; (3) Anura, compris ing the tailless forms, or frogs and toads, of which there are two divisions,— the few Aglossa, which have no tongue, and the tongue bearing Phaneroglossa, which includes the great majority of forms. The existing species num ber about 1,000.
Fossil The modern frogs and salamanders are a small and scanty remnant of the Amphibia of Palmzoic time. During the Carboniferous and Permian periods they were the dominant form of life and of great variety in form, including some of very large size, 12 feet or more in length. All these ancient Amphibia belong to an extinct group, the Labyrinthodonta (sometimes called Steece phalia), or "armored amphibians," distinguished by having the wide flat head completely roofed over with bone, and the body more or less armored with bony plates and scales. The skull has two openings for the eyes, two at the front margin for the nostrils and a single one in the middle for the pineal eye. Like modern amphibians, they breathed by gills when young, but by lungs when adult. All had long tails and most of them short, stout legs. Some were elongated and snake-like, others tadpole-like with large heads shaped like a broad arrow (Diplocaulus) and no limbs: others, and these the largest, heavy-bodied, with flat conical or semi-circular heads, short legs and five-toed feet (Labyrinthodon, Eryops).