Most of the glands found in the skin of reptiles are scent glands, which give to these animals their characteristic odour of musk, which has, no doubt, a sexual significance. All these glands are sac-shaped, multicellular structures opening by a pore on the surface, and their secretion is set free by disruption of cells.
The musk glands have the following distribution :—In crocodiles there is a pair which open by longitudinal slits on the inner sides of the lower jaws, and another pair lie within the lips of the cloaca; these are present in both sexes. Crocodiles possess also a row of small sac-like glands without external openings along each side of the back.
In Chelonia there is a pair of inguinal glands opening near the hypoplastra, and sometimes an anterior pair similarly related to the hyoplastra. Sphenodon has a pair of cloacal glands. Lizards have cloacal glands and, in addition, in certain forms there are the so-called femoral pores, which extend along the lower and hinder surface of the thigh to pass on to the belly in front of the cloaca. They are present in both sexes, but best developed in males. Each pore opens in the middle of a scale and leads into a canal which ends in a pocket with many shallow diverticula. The cells of the walls of these become detached, filling up the lumen of the gland and duct, and forming a rod which may pro ject beyond the surface of the skin. It is possible that these structures are of assistance in copulation.
Body Cavity.—The body cavity of reptiles is always more or less completely divided into sacs. A completely closed pericar dium is always present. In lizards, a post-hepatic septum built up by special folds of the mesentery and suspensory ligament of the liver, may reach the ventral surface and bring about an almost complete division of the peritoneal cavity into two. In snakes, similar folds enclose the two lobes of the liver and the stomach in separate sacs. In Chelonia, the lungs lie above a fold of peritoneum which reaches the liver, excluding them from the general cavity. In crocodiles, there are two pleural cavities and a combination of other folds connected with the liver forms a complete transverse partition separating the pericardium, lungs and liver from the rest of the peritoneal cavity. This sheet is
muscular, and probably functions in respiration like the non homologous mammalian diaphragm.
Digestive System: Teeth.—The teeth of reptiles may be found on the pre-maxillae, maxillae, on all the bones of the palate, and on the dentary and coronoid bones of the lower jaw. Indi vidual teeth are generally simple cones with a conical pulp which produces dentine and an enamel cap. They may be set in sockets (thecodont) or fused to their supporting bone (acrodont or pleurodont). In the majority of reptiles they are shed periodi cally, and replaced as often as necessary. In reptiles, the marginal teeth of both jaws appear to belong to two series, whose members alternate with one another, and in primitive forms were func tional alternately.
Thus, in these animals two teeth are usually separated by an empty emplacement in which a new tooth will arise, the original pair being shed together when it has grown to its full size. When the original teeth have been shed a new dental papilla passes outward from the lingual side to the empty socket and there produces a new tooth. In crocodiles this process has already happened before the tooth is shed, so that these new tooth crowns may often be found in the pulp cavity of the original tooth. Sphenodon, and some other recent reptiles with acrodont teeth, exhibit no replacement after maturity has been reached. The mammal-like reptiles in their various orders show all stages in the reduction of tooth change from the primitive unlimited replacement of all teeth to a mammalian condition in Cynodonts, where the incisors, canines and pre-molars are replaced once during the animal's life, and the molars, when once formed, are never shed.
The dentition of reptiles is usually homodont, that is, uniform or regularly varying from front to back of the jaw, but the Theriodont dentition is heterodont and mammal-like. The tooth crown may be elaborated into a crushing mechanism, in Placo donts, some Ichthyosaurs, and very effectively in the Trachodont dinosaurs, where several successive series of teeth are in use at one time, forming a splendid triturating surface, in Cotylo saurs, Theromorpha, and even, though imperfectly, in some lizards. The peculiarly specialized fangs of poisonous snakes are described in the article on these animals.