In the adult amphibia the gullet is wide and short; the stomach is a simple sac, elongated in the aquatic species; the intestine is but slightly convoluted, and terminates in a cloaca, or pouch, which also receives the openings of the genital and urinary organs. The vent in the frogs and toads opens on the hinder part of the back; in the other forms it is beneath the commencement of the tail, as in ordinary reptiles. The liver, pancreas, and spleen are found in all the class. The lymphatic and lacteal systems are extremely developed; in the course of the lymphatics are found pulsating cavities or ventricles which propel their contents toward the veins, but their pulsations do not coincide with those of the heart, nor those of one side with those of the other. In the circulating system we find the most convincing proof of the intermediate position of the amphibia; they begin life with the single heart and gills of fishes, but as their metamorphosis goes on, the heart assumes the compound character necessary for the pulmonary respiration of the reptiles.

The heart in the early stage of these animals consists of a single auricle which receives all the blood after its circulation, and a single ventricle which sends it through a third cavity, the bulbus arteriosus, to the gills or branchiae; in the gills the blood is purified by the oxygen of the water, and returned by their veins to the aorta; from the last branchial artery a branch is given off on each side, which goes to the rudimentary lungs and afterward becomes the pulmonary artery; between the branchial arteries and veins are small communicating branches, which, as the gills are absorbed, gradually enlarge until they form continuous trunks, and the original branchial artery becomes the root of the two descending aorta?, at the base giving off the pulmonary artery; the two veins which return the blood from the rudimentary lungs enlarge as these organs are developed, and become the pulmonary veins, which last, being distended at their point of union with the heart, ultimately form the second auricle.

The circulation, then, of the adult amphibia assumes the character which we find in the reptiles generally, being performed by a single ventricle and two auricles; the pure blood in the pulmonic auricle and the impure blood in the systemic auricle are sent together and mixed in the single ventricle, so that a half-purified blood is distributed to the organs. The gills, whether external or internal, are expanded in a leaf-like form, considerably divided, though much less so than in fishes; in the earliest stages they are always external, and remain so permanently in the order amphipneura, but in the higher orders they become very soon internal; they are supported by cartilaginous or bony arches connected with the hyoid bone, which changes its form in accordance with the development of the respiratory organ. On the leaflets of the gills the minute blood vessels run, bringing the venous blood into contact with oxygen, and sending it back purified to the heart. While some amphibia retain their gills through life, with coexistent rudimentary lungs, others lose them entirely as the lungs are developed, so that there probably is a brief period in the life of some of these animals in which they may truly be called amphibious.

In the caduci-branchiate genera (or those which lose their gills), the early condition of the lungs is that of a mere sac without any appearance of the cells or pulmonary structure afterward developed; it becomes, therefore, an interesting question whether the similar rudimentary lungs of the perennibranchiate genera can serve any of the purposes of respiration. From the mechanism of respiration in the higher genera, and the anatomical deficiencies in the accompanying apparatus in the lower, it would be at once seen that these simple sacs could hardly aid the gills, and much less could perform their office in aerating the blood. Rusconi con-eludes that in the proteus, at least, these sacs do not assist in respiration; and experiments alluded to in the "Proceedings of the Boston Society of Natural History," vol. vi., p. 153, show that the menobranchus perishes in about four hours when removed from the water. In the higher orders, the reception of air into the lungs is effected by a simple act of swallowing; the air enters the mouth through the nostrils, and, the gullet being closed, is forced into the lungs by the action of the muscles of the hyoid bone.

The fish-like structure of the nostrils in the lower genera, and the imperfect condition of their glottis and windpipe, with the perfect development of the gills, show that, like the air bag of fishes, these simple sacs, though they represent rudimentary lungs, are not for the purposes of respiration proper. The brain offers the same gradations, from the fish-like simplicity of that of the tadpole and lower genera to that of the reptile, in which the hemispheres become enlarged laterally, and the spinal cord shorter and thicker at the origins of the nerves of the limbs. In the frogs the eyes are large and prominent, in the salamanders comparatively small, and in the caecilia scarcely visible; in all they resemble those of fishes in the flattened anterior surface, the small quantity of the aqueous humor, and the depth of the crystalline lens; the eyes of the frog are fully developed, and provided with lids. In the lower genera and in the tadpole state, the organ of hearing is very imperfect, consisting of a hollow in the temporal bone, at the bottom of which is the sac containing the cretaceous body on which the nerve is spread; there is no drum nor tympanic cavity; the organ is covered by the skin, without any external communication.

In the frog the ear is more complicated, having the drum with its cavity and bones, and communicating with the mouth by a Eustachian tube. The sense of smell must be imperfect in the amphibia; in the lower forms the nostrils are mere cavities in the front of the head, with no communication with the mouth, as in fishes; in the higher orders the nose communicates with the mouth, but in them the apparatus for smelling is probably less sensitive than in the lower forms, the nose being more connected with the act of breathing. The sense of taste is probably also very obtuse; in the frogs and toads the tongue is developed to an extraordinary degree, being long, with the anterior half free, doubled back on itself, and capable of being thrown forward and retracted with great quickness, serving for the seizure of the insects on which they feed, and for conveying them instantly to the back part of the mouth. The amphibia are distinguished from reptiles by the absence of all scaly covering; a naked skin characterizes all, from the snake-like caecilia to the fish-like axolotl.

The skin of the aquatic genera is soft, smooth, and kept constantly moist by the cutaneous secretions; in the land genera, as frogs and toads, the glands of the skin secrete a thick whitish fluid, which may become acrid and irritating, giving rise to the opinion that these secretions are poisonous; in the salamander the fluid thus poured out is so abundant, and so suddenly secreted when the animal is irritated, that it is quite probable that even the effects of fire may be for a short time arrested by it - doubtless the origin of the well known fable in regard to this animal. The cuticle is frequently shed; that of the aquatic genera being washed away in shreds, while that of the toads is divided along the median line, removed by the contortions of the animal, and finally swallowed at a single gulp. The amphibia breathe by the skin as well as by the lungs and gills; a frog will live from five to six weeks in a state of complete strangulation, and even after total excision of the lungs; indeed, the pulmonary respiration is insufficient to support life in these animals without the assistance of the skin.

The medium through which the blood is oxygenated, whether lungs, gills, or skin, whether in air or in water, is always a modification of the cutaneous surface; and, as in frogs the same surface is capable of performing both aquatic and atmospheric respiration, it is natural to infer that lungs and gills are identical in structure. The amphibia, like many of the invertebrate animals, have the power of reproducing parts lost by accident or design; the common water salamander will invariably restore the limbs which have been cut off for experiment, and even repeatedly; the common land salamander also reproduces its tail, which it so easily separates in case of need. Impregnation is effected without sexual congress, except in the land salamander; in the order anoura, the male embraces the female with great tenacity, and impregnates the eggs, several hundred in number, as they are ejected under his pressure; in the tailed aquatic species, impregnation takes place during the instantaneous apposition of the cloacal openings of the two sexes. The testes and ovaries are double, and symmetrically arranged; they are much enlarged at the breeding season. When the young are ready for aquatic life, they burst the thin membrane of the egg, and come out in the fish-like form.

In the terrestrial salamander, whose habits do not permit this mode of development, the young undergo their metamorphosis in the oviduct, in which they pass their tadpole state, and appear without gills when they are ready for terrestrial existence. Like the viper, the salamander is ovo-viviparous. In the pipa, or Surinam toad, the eggs, instead of being dropped into the water, are placed by the male on the back of the female; here they are attached by a thick mucus, and the skin gradually thickens in the intervals so as to form a cell around each egg; in these cells the young undergo their metamorphosis, and come from them perfect animals. After the young are hatched out, the cells of the female disappear. Like many of the saurian and chelonian reptiles, some of the amphibia are used as food. The delicacy of the frog's thigh is well known to the epicure. The Mexicans consider the siredon or axolotl very good eating; and it is quite probable that, like the reptile iguana, many of the hideous amphibia are wholesome articles of food.