Amphibia, animals which frequent both land and water. There is probably no truly amphibious animal, as that would imply the possibility of living and breathing equally well in air and in water. The old naturalists gave the name to beavers, otters, frogs, and other animals from all the orders of vertebrata; Lin-naeus restricted the term to reptiles with cold blood and simple circulation; Cuvier called amphibia such mammals as can dwell on land or in the water, like the seal, the walrus, and the dudong, occupying a position intermediate between the feline and the marsupial animals. There are animals which have both gills and rudimentary lungs, as the proteus, the siren, and the menobranchus, but they are decidedly aquatic, and perish sooner or later in the air. The amphibia constitute an order of reptiles (the batrachia of later herpetologists), and may be characterized as vertebrated animals, with cold blood and naked skin, reproducing by means of eggs, and most of them undergoing a metamorphosis or change of condition, having relation to a transition from an aquatic respiration by gills to an atmospheric respiration by lungs, and a consequent alteration in general structure and mode of life.

These characters have led some writers to consider the amphibia as a distinct class, instead of a mere order of reptilia. No arrangement proposed by naturalists presents a perfect division of the different forms; the following by Mr. Thomas Bell of London, founded on the absence or presence and duration of the gills, seems the most natural: Class Amphibia Order 1. Amphipneura. Body elongated formed for swimming; feet either four, or two anterior only; tail compressed, persistert; respiration aquatic throughout life, by exter nal persistent gills, coexisting with rudiment ary lungs; the eyes with lids. Genera, pro tens, siren, siredon, menobranchus, pseudo-branchus. - Order 2. Anoura. Body short and broad; feet during the tadpole state wanting, afterward four, the hinder ones long and formed for leaping; tail before the metamorphosis long and compressed, afterward wanting; ribs wanting; vertebrae few and anchy-losed together; respiration at first aquatic by gills, afterward atmospheric by lungs; gills at first external, but withdrawn into the chest before the metamorphosis; impregnation effected externally during the passage of the eggs.

Genera, rana, hyla, ceratophrys, bufo, rhinella, otilopha, dactylethra, bombinator, breviceps. - Order 3. Urodela. Body long and slender; feet always four; tail long and persistent; ribs very short; respiration at first aquatic by external gills, afterward atmospheric by cellular lungs; vertebrae numerous and movable; impregnation internal. Genera, salamandrina, salamandra, molge. -Order 4. Abranchia. Body long and formed for swimming; feet four; cranium solid; tail compressed; respiration by lungs only; gills absent; no metamorphosis known. Genera, menopoma, amphiuma. - Order 5. Apoda. Body elongated, slender, and snake-like; feet none; tail very short, almost wanting; lungs, one larger than the other; ribs very short; breast bone wanting; impregnation unknown, probably internal. The existence of gills at any period of life is unknown. Genus, caecilia. - In the adult amphibia the skeleton varies considerably. In the frog, which moves extensively on land, we find the hind legs greatly developed to enable it to take the enormous leaps by which it pursues its prey and escapes from danger; hence it has no useless tail, the body is contracted longitudinally into a short space, and the vertebraae are few, united into a single immovable piece unprovided with ribs.

On the contrary, the water salamanders or newts have a long tail, a slender flexible body, and all their organs fitted for aquatic life. The structure of the bones is more compact and calcareous, and less transparent and flexible, than in fishes. The bones of the skull do not overlap each other, but have their margins in contact and occasionally united; the bones of the face are more closely united to those of the skull than in the fishes; the intermaxillary and jaw bones are much developed transversely, expanding the general form of the skull without involving any enlargement of the brain cavity, which is very small. The hyoid bone experiences most remarkable changes in those genera which undergo metamorphosis. In the highest amphibia, the bones of the spine are very few; in the frog there are 9, in the pipa only 8, unprovided with ribs, but having long transverse processes; the anterior articular surfaces of the bodies are concave, and the posterior convex. In the tadpole, and in the genera which retain their gills through life (siren, menobranchus, etc, hence called peren-nibranchiate), the substance between the vertebrae is soft, and contained in cup-like hollows formed by the concave articular surfaces of contiguous bones, precisely as in fishes; another proof of the intermediate condition of these lower forms of the class.

In the salamanders the vertebras are more numerous, especially those of the tail; in the siren the trunk has 43, and the tail as many more. The limbs are arranged on very different plans in the different groups; in the frogs and toads they are well developed, and suited for active leaping and swimming; in the elongated aquatic species the limbs are small and feeble; in caecilia, there are not even the rudiments of limbs; in these the imperfect development of the extremities is compensated by the extent of the spine of the body and tail, by the curves and flexures of which they swim with considerable rapidity. In the fish-like tadpole, the limbs are at first scarcely perceptible, and in their gradual development acquire a considerable size and well-defined form while yet imprisoned beneath the skin; the hind legs are first seen; the tail is much developed, gradually disappearing by absorption as the limbs acquire their full size. All the amphibia have teeth on the palate; the salamanders have them also in both the upper and lower jaws, the frogs in the upper only, and the toads in neither; the jaw teeth are always slender, sharp-pointed, and closely set; the frog has about 40 on each side of the upper jaw; the salamander has about 60 above and below; the palatine teeth are generally arranged transversely, parallel to the jaw teeth.