In all reptiles the blood is conveyed from the ventricular part of the heart, really or apparently, by a single trunk. In Lepidosiren the veins from the lung-like air-bladders traverse the auricle which opens directly into the ventricle. In some the vein dilates before communicating with the ventricle into a small auricle, which is not outwardly distinct from the much larger auricle receiving the veins of the body. In Proteus the auricular system is incomplete. In Amphiuma the auricle is smaller and less fringed than in the Sirens, the ventricle being connected to the pericardium by the apex as well as the artery. This forms a half spiral turn at its origin, and dilates into a broader and shorter bulb than in the Sirens.
"The pulmonic auricle," continues the learned Professor, "thus augments in size with the more exclusive share taken by the lungs in respiration; but the auricular part of the heart shows hardly any outward sign of its diversion in the Batrachians. It is small and smooth, and situated on the left, and in advance of the ventricle in Newts and Salamanders. In Frogs and Toads the auricle is applied to the base of the ventricle, and to the back and side of the aorta and its bulb."
In the lower members of the order, the single artery from the ventricle sends, as in fishes, the whole of the blood primarily to the branchial organs, during life, and in all Batrachians at the earlier aquatic periods of existence. In the Newt three pairs of external gills are developed at first as simple filaments, each with its capillary loop, but speedily expanding, lengthening, and branching into lateral processes, with corresponding looplets; those blood-channels intercommunicating by a capillary network. The gill is covered by ciliated scales, which change into non-ciliated cuticle shortly before the gills are absorbed. In the Proteus anguinus, three parts only of branchial and vascular arches are developed, corresponding with the number of external gills. In Siren lacertina the gills are in three pairs of branchial arches, the first and fourth fixed, the second and third free, increasing in size according to their condition.
The Amphibia, then, have all, at some stage of their existence, both gills and lungs co-existent: respiring by means of branchiæ or gills while in the water, and by lungs on emerging into the open air.
All these creatures seem to have been well known to the ancients. The monuments of the Egyptians abound in representations of Frogs, Toads, Tortoises, and Serpents. Aristotle was well acquainted with their form, structure, and habits, even to their reproduction. Pliny's description presents his usual amount of error and exaggeration. Darkness envelops their history during the middle ages, from which it gradually emerges in the early part of the sixteenth century, when Belon and Rondiletius in France, Salviani in Italy, and Conrad Gesner in Switzerland, devoted themselves to the study of Natural History with great success. In the latter part of the same century Aldrovandi appeared. During fifty years he was engaged in collecting objects and making drawings, which were published after his death, in 1640, edited by Professor Ambrossini, of Bologna, the Reptiles forming two volumes. In these volumes, twenty-two chapters are occupied by the Serpents. But the first arrangement which can be called systematic was that produced by John Ray. This system was based upon the mode of respiration, the volume of the eggs, and their colour.
Numerous systems have since appeared in France, Germany, and England; but we shall best consult our readers' interest by briefly describing the classification adopted by Professor Owen, the learned Principal of the British Museum, in his great work on the Vertebrata.
The two great classes Batrachians and Reptiles, include a number of animals which are neither clothed with hair, like the Mammalia, covered with feathers like the birds, nor furnished with swimming fins like fishes. The essential character of reptiles is, that they are either entirely or partially covered with scales. Some of them—for instance, Serpents—move along the ground with a gliding motion, produced by the simple contact and adhesion of the ventral scales with the ground. Others, such as the Tortoises, the Crocodiles, and the Lizards, move by means of their feet; but these, again, are so short, that the animals almost appear to crawl on the ground—however swiftly, in some instances. The locomotive organs in Serpents are the vertebral column, with its muscles, and the stiff epidermal scutes crossing the under surface of the body. "A Serpent may, however, be seen to progress," says Professor Owen, "without any inflection, gliding slowly and with a ghost-like movement in a straight line, and if the observer have the nerve to lay his hand flat in the reptile's course, he will feel, as the body glides over the palm, the surface pressed as it were by the edges of a close-set series of paper knives, successively falling flat after each application." Others of the class, such as the Tortoises, Crocodiles, and Lizards, move by the help of feet, which are generally small and feeble—in a few species being limited to the pectoral region, while in most both pairs are present. In some, as in various Lizards, the limbs acquire considerable strength.
There is one genus of small Lizards, known as the Dragons, Draco, whose movements present an exception to the general rule. Besides their four feet, these animals are furnished with a delicate membranous parachute, formed by a prolongation of the skin on the flanks and sustained by the long slender ribs, which permits of their dropping from a considerable height upon their prey.
Batrachians, again, differ from most other Reptilia by being naked: moreover, most of them undergo certain metamorphoses; in the first stage of their existence they lead a purely aquatic life, and breathe by means of gills, after the manner of fishes. Young Frogs, Toads, and Salamanders, which are then called tadpoles, have, in short, no resemblance whatever to their parents in the first stage of their existence. They are little creatures with slender, elongated bodies, destitute of feet and fins, but with large heads, which may be seen swimming about in great numbers in stagnant ponds, where they live and breathe after the manner of fishes. By degrees, however, they are transformed: their limbs and air-breathing lungs are gradually developed, then they slowly disappear, and a day arrives when they find themselves conveniently organized for another kind of existence; they burst from their humid retreat, and betake themselves to dry land. "The tadpole meanwhile being subject to a series of changes in every system of organs concerned in the daily needs of the coming aërial and terrestrial existence, still passes more or less time in water, and supplements the early attempt at respiration by pullulating loops and looplets of capillaries from the branchial vessels." (Owen.)
Nevertheless, they do not altogether forget their native element; thanks to their webbed feet, they can still traverse the waters which sheltered their infancy; and when alarmed by any unusual noise, they rush into the water as a place of safety, where they swim about in apparent enjoyment. In some of them, as Proteus and the amphibious Sirens, where the limbs are confined to the pectoral region, swimming seems to be the state most natural to them. They are truly amphibious, and they owe this double existence to the persistence of their gills; for in these perennibranchiate Batrachians, arteries are developed from the last pair of branchial arches which convey blood to the lungs: while, in those having external deciduous gills, the office being discharged, they lose their ciliate and vascular structure and disappear altogether. The skull in Reptiles generally consists of the same parts as in the Mammalia, though the proportions are different. The skull is flat, and the cerebral cavity, small as it is, is not filled with brain. The vertebral column commences at the posterior part of the head, two condyles occupying each side of the vertebral hole ([Fig. 2]). The anterior limbs are mostly shorter than the posterior, as might be expected of animals whose progression is effected by leaps. Ribs there are none. The sternum is highly developed, and a large portion of it is cartilaginous; it moves in its mesial portions the two clavicles and two coracoid bones, which fit on to the scapula, the whole making a sort of hand which supports the anterior extremities, and an elongated disk which supports the throat, and assists in deglutition and respiration. The bone of the arm (humerus) is single, and long in proportion to the fore arm. In the Frogs (Rana), the ilic bone is much elongated, and is articulated in a movable manner on the sacrum, so that the two heads of the thigh bones seem to be in contact. The femur, or thigh, is much lengthened and slightly curved, and the bones of the leg so soldered together as to form a single much elongated bone.