The bones of the extremities, except in the serpents, which have no limbs, correspond with those occurring in the higher vertebrata.
The mouth, except in the chelonians, is usually provided with conical teeth, adapted rather for seizing- and holding prey, than for dividing and masticating food. teeth, like those of fishes, are successional; that is to say, new teeth are being constantly developed, whilst the older ones are regularly shed. In the crocodiles, three, or even four generations of teeth, sheathed one within the other, may often be seen in the same socket. In some instances, the teeth are attached solely to the jaws, while in others they are also attached to the pterygoid or palate bones. In chelonians, the teeth are repNeed by a horny beak, which, according to the habits of the animal, is adapted for bruising as well as cutting, and which in some species constitutes a somewhat formid ab:e weapon.
The digestive organs present less marked differences than the osseous system. With the exception of certain chelonians, all reptiles are carnivorous, and swallow their prey whole. Hence the jaws are adapted, by their mobility and subdivision into segments, to open very widely, and the oesophagus is capable of great dilatation. The tongue is commonly free, elomrated, and bifid, except in the crocodiles, in which it is immovable, whence the popular idea that these animals do not possess this organ. The stomach is sometimes scarcely larger than the cesophagus and intestines (as in serpents), while in other cases it forms a sac of considerable size. In either case, it is capable of great dilatation. A liver, pancreas. and spleen are always present, the two former glands • pouring their secretions into the upper part of the intestine, which is short, wide, and not much twisted, and divided into two portions, corresponding to the small and large intestines of mammals, by a valve. It finally terminates in a wide cloaca, into Which the ducts of the urinary and generative organs usually open. The anal aperture of this • cloaca is transverse in serpents and lizards, and longitudinal in crocodiles and tortoises. These peculiarities in the anal aperture are accomr,anied by remarkable differences in the external generative organs of the male, and seem to divide the class into two great sections.
It is in their circulating and respiratory organs that reptiles present the most marked characteristics. Like birds and mammals they breathe air, but like fishes, they arc cold-blooded. The reason why they are unable to sustain a fixed temperature above and independent of that of the surrounding medium, is due partly to the arrangement of the blood-vessels (see CIRCULATION), and partly to the structure of the lungs. The lungs
are usually of large size; but as they are not subdivided, as in mammals and birds, into innumerable microscopic air-cells, the real aerating surface is comparatively small. In several orders, they are merely capacious bags, whose vascular or aerating surface is but slightly increased by sacculi developed in their cells. Imserpents, the pulmonary arrangement is singular, one lung (usually the right one) being of extraordinary length, while the other remains altogether rudimentary. It is in the tortoises and crocodiles that the lung is most highly developed. This inferiority of the respiratory apparatus of rep tiles is further shown in the absence of those means for the continuous introduction and expulsion of air which are observed in birds, and still more in mammals, and which are described in the article RtSPIRATION.
The cerebral portion of the nervous system iu many respects resembles that of fishes, but the cerebral hemispheres are larger in proportion to the optic lobes, while the cere bellum is usually smaller. The ormus of the senses are better developed than in fishes. The eye is always present in reptiles, and presents no remarkable peculiarity. We here first meet with a special arrangement for the protection of this delicate organ; for while in serpents the skin of the head passes continuously in front of the eyes, merely transparent where it covers the cornea, it is doubled in most other reptiles into two folds, constituting the upper and lower eyelids, which can be drawn together by a sphincter muscle; and we also find a rudiment of a third eyelid, formed by an addi tional fold of membrane at the inner angle, which is so completely developed in croco diles as to form a nictitating membrane, that can be drawn completely across the eye as in birds, by a muscle specially adapted for that purpose."—Carpenter's General and Comparative Physiology, 3d ed. p. 495. The organ of hearing is more highly developed than in fishes or amphibia. There is no external auditory canal, the membrane of the tympanum being covered externally by the integument of the head. The senses of taste and touch are probably obtuse in most animals of this class, and from its structure, the tongue is probably rather an organ of touch than of true taste.