BATRATHIA (from Gr. Latrachos, a frog), in zoology, nearly synonymous with amphibia, the name of what is now generally regarded as a distinct class of the sub kingdom rolebrata, intermediate in ninny respects between fishes and _reptiles (q.v.). The B. used often to be treated as one of four orders into which the reptiles were divided. The most important difference between the B. and the reptiles is that the young B. undergo metamorphoses, and breathe by gills alone, in the earlier part of t4ieir life; whilst in their adult state they either breathe by lungs alone, or possess at once both lungs and gills. The body is also covered with a soft nakedskin, through which water is imbibed, and through which the aeration of the blood appears to be in part carried on. The B. are all oviperous; their eggs are not covered with a hard shell, but merely with a soft membrane. Fecundation commonly takes place after the eggs have been deposited. It is sometimes given as a distinctive character of B., that, in their adult state, they have limbs, but in some genera these are very rudimentary, and they are altogether wanting in Cecilia (q.v.), a genus which is now decidedly referred to this order, because it has been found to undergo the metamorphosis from a gill-breathing to a lung breathing state, and which Cuvier, with hesitation, placed serpents, because the fact of its metamorphosis had not then been ascertained, The ordinary number of limbs is four, but in the sired (q.v.) there are only two.—Another character frequently given as distinctive of the B., that their feet are destitute of claws, is in like manner only general, and not universal.
In the earlier period of life, the form of the B. is fish-like, of which the common tadpole, the young of the frog, is a familiar example; and this form some of them retain with comparatively little modification, whilst some of them ultimately acquire a form resembling that of lizards, with which the newts were indeed ranked by Lin/mils as species of the same genus; and others, as frogs and toads, assume a peculiar quadruped form, the tail entirely disappearing, except in the elongated coccygeal bone which repre sents it to the anatomist.
In their anatomy, the adult B. present some important points of resemblance to fishes; in some important points, they differ both from fishes and from other reptiles. The skull resembles that of fishes in its general form, although rather agreeing with other reptiles in the parts of which it is composed. Teeth are often entirely wanting, some times they are present only in one jaw; when present, they are generally small and numerous, either in a single row or aggregated. In some of the fossil genera, however, which are referred to this order, the teeth are of large size.—The B. have either no ribs, or they have mere rudimentary ribs. They have, however, a breast-bone, often in great part cartilaginous, to which some of the most important muscles are attached. They breathe air by a sort of gulping.—The heart of the B. was long believed to have only one auricle and one ventricle, but the apparently single auricle is now known to consist of two divisions. As in the class reptiles, only a part of the blood received from the circu
lating system is sent to the organs of respiration, and another part returns immediately into the circulation. See REPTILES.
• In the wonderful transformations which the B. undergo. the circulation of the. blood •is changed in accordance with the change in the organs of respiration. These, in the earliest stage. are external gills, which appear as long colored fringes, hanging loosely upon each side of the neck. In some 13., these external gills, which resemble those of the aquatic molluscs, remain till the lungs are sufficiently developed for respiration; in seine, as the axolotl (q.v.), they are permanent duriug the whole of life. In the greater part of the B. however, the external gills soon disappear, and are replaced by internal gills, when the tadpole exhibits its most perfectly fish-like form, its mode of progression also corresponding with that of fishes. Its respiration is carried on essentially as in fishes, water entering the cavity of the mouth, and being forced out through (lie gill open ngs, so as to come in contact with the minute filaments of the gills. The gills are attached, as in fishes, to arches connected with the hyoid bone. In this stage of existence, the large arterial trunk which proceeds from the ventricle of the heart, sends forth, from a bulb ous enlargement which it forms, as in fishes, an artery to each of the gills, and the blood after being Orated in them, is collected into an aorta, and proceeds into the general cir culation. But an artery is also provided on each side for the conveyance of blood to the lungs, both the lungs and their arteries being at first but increasing, whilst the gills, on the contrary, diminish along with the blood vessels connected with them; and the gill-breathing is gradually transformed into a lung-breathing animal, no longer perfectly aquatic, as at first, or capable of existence only in water, but amphibious, or almost entirely terrestrial, and incapable of remaining long under water without coming to the surface to breathe.—Whilst these changes take place, others no less extraordinary are also going on. The tadpole which subsisted on vegetable food, and possessed a mouth adapted to the purpose of feeding on it—a small horny acquires a mouth fitted for seizing and swallowing small insects, slugs, etc., upon which the adult B. chiefly or exclusively feed, and his habits change accordingly. The moutlr of the siren, however, always retains a character somewhat similar to that of the tad• pole.—In the course of transformation, a pelvis is formed, and limbs sprout forth, which in some B., as frogs, become very perfect and powerful. Whilst the limbs grow, with all their bones, joints, muscles, blood-vessels, and nerves, the vertebrae, in many B., diminish in number, and the tail gradually shortens and disappears.