Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/464

Rh 440 REPTILES [ANATOM-S pterygia. They were, on the contrary, extremely divergent in certain Dinosauria, and still more in the Ornithosauria. Every Reptile has a body composed of organs of nutrition, circulation, respiration, secretion, reproduction, sensation, and motion, supported internally by a solid framework, the internal skeleton or endoskeleton, and enclosed in a firm investment, constituting the external skeleton or exoskeleton. The External Skeleton. The external investment of the body consists of two layers. The outer of these, the epidermal layer, or epidermis, is of an epithelial horny nature and never becomes bony. The deeper or dermal layer, the dermis, is a fibrous structure which may become bony. Neither hairs nor feathers are developed. The exoskeleton is characterized in this class of animals by being composed of distinct superficial thickenings placed side by side and separated one from another by thinner interspaces. According to the size and form of these thickenings they are known by different names. If they are very small and rounded they are called "tubercles," and such we see on the body of the Chamseleons and Geckos. If they are large, flat, and not overlapping, they are called shields, and such we see, e.g., on the head of the true Lizards. If the hinder part of each thickening is more or less prolonged over the anterior part of one or more next behind it, structures of this kind are called scales, such as we find dorsally clothing the bodies of most Lizards and many Serpents. If the median part of each such scale is still more thickened longitudinally, then such a scale is said to be carinate. In most of these cases the epidermis itself is also more or less thickened locally, as well as the subjacent dermis, remaining thin in the folds or interspaces. The dermis may be ossified, forming dense bony plates, or scutes, beneath the epidermal investment. The whole body may be thus clothed, as in the Lizard Cyclodus and others, or the bony plates may be confined to parts of the back, as in some Crocodilians. The development of the exo- skeleton is carried to the highest degree in Chelonians, when large osseous plates form, in the Land Tortoises and Terrapins, a complete and continuous bony case for the body, invested externally by a rich corneous epidermis, and becoming internally anchylosed with the endoskeleton itself. xoskele- In most Ophidians the body is clothed with scales above and with large transversely elongated shields beneath (single or double beneath the tail), though the body may be entirely invested by small scales except the head, as in Typhlops, or by tubercles, as in Acrochordus. Sometimes, as in Cerastes and the River Jack, two horny appendages are erectly developed over the nose and sometimes, as in Herpeton tentaculatum, cutaneous appendages project from the snout, or the snout may be exceptionally produced. The skin of the side of the body just behind the head may form a distensible fold, as in Naja, and there may be, as in Python, a claw on each side of the vent, which claw is a rudiment of the pelvic limb, as will be explained in describing the appendicular skeleton. A peculiar cutaneous depression exists between the nostril and the eye in Crotalus and the other Pit-Vipers. For full details on these subjects see the separate articles on the orders. Lacer- In the Lacertilia we find a number of different ians; cutaneous conditions which have been already noticed in describing the various groups systematically. It will then suffice here to remind the reader that the Amphis- baenians have quadrate shields over the entire body except the head, arranged in transverse rings, and that in such forms as Chalcis the scales are also verticillate, while in m of phi- ans : the Scincoidea and Anguidse, they are imbricate, and on the Chamseleons granulate. The head may be covered with large shields, as in the Lacertidy, or with small scales like those of the body, as in the Monitors, and there are often long sharp spines on the head and body, as in Grammatophora, Phrynosoma, and Moloch. The in- tegument may also extend out from the body along the back and tail, as in Basiliscus and Loplmra, or from beneath the throat, as in Iguana, or largely across the neck, as in Chlamydosaurus, or on cither side of the tail, as in Ptychozoon, or from each side of the body so as to be distensible and to serve as a parachute, as in Draco ; while in the Ornithosauria it passed from the elongated hand to the body and leg as in the wing of the existing Bats. It has also been before stated that in certain cases different genera are distinguished by pores, i.e., the apertures of small cutaneous sacs placed on the inner side of each thigh or in front of the cloacal aperture. Folds of skin may invest the digits and constitute webbe ( d feet, as in some Geckos, as well as in Crocodiles and Terrapins, or may bind the digits in two opposite bundles, as in each ex- tremity of the Chamseleons. Normally each digit is ter- minated by a claw. In Crocodilia the epidermal thickenings do not overlap of Cro as in some Lizards, but form conspicuous prominences in the dorsal region, which serve as specific characters, and which have bony plates beneath, which plates may, as in Alligators, exist on the belly as well as the back, and may form continuous rings round the tail, those of each anterior row overlapping those of the row next behind. Not all the digits are provided with claws. The feet are webbed. The Chelonia present considerable differences as to the of Ch exoskeleton. The Mud Tortoises, Trionyx, have a soft loniai epidermis, and Sphargis has a coriaceous outer layer of Fio. 3. A portion of the osseous plates of the carapace of Sphargis eoriacea, showing three large keeled plates of one of the longitudinal ridges of the carapace, with a number of the small irregular plates on either side of them. (From nature.) integument covering a carapace divided into small sub- hexagonal shields. In the other Chelonians there are large epidermal shields, which may overlap, as in the Tortoise- shell Turtle (C. imbricata) and others, or may be conter- minous, as in Testudo and Emys. These shields form a mid-dorsal series and two lateral rows on either side, while there are two longitudinal series on the ventral side of the body. Beneath the epidermal layer of the integument osseous plates are found, which plates do not coincide either in