236 VERTEBRATE ANIMALS. 



the food. Except in the Crocodiles, the teeth are not sunk 

 into distinct sockets, and they are usually replaced as fast as 

 shed. They likewise do not differ from one another sufficient- 

 ly in form or function as to allow of their being divided into 

 different sets, as they can be in the Mammals. Usually the 

 teeth are confined to the jaws proper, but in some cases they 

 are carried by other bones of the mouth. In the Tortoises 

 and Turtles there are no teeth, and the jaws are simply 

 sheathed in horn, so as to constitute a kind of beak, like that 

 of a bird. The integumentary skeleton is in the form of 

 scales, sometimes combined with bony plates. In the Tor- 

 toises and Turtles the integumentary skeleton is so united 

 with the true skeleton as to form a kind of bony case or box, 

 in which the body is enclosed. 



The digestive system presents little worthy of special no- 

 tice, except that the termination of the intestine (rectum) opens 

 into a cavity called the " cloaca," which receives also the ducts 

 of the urinary and generative organs. 



It is, however, in the structure of the circulatory and respir- 

 atory organs that the most important characters of the Reptiles 

 are to be looked for. The heart in all Reptiles may be regarded 

 as being, in function^ three-chambered, being composed of two 

 auricles and a single ventricle, imperfectly divided by an in- 

 complete partition. In the Crocodiles alone the heart is, struct- 

 urally, four-chambered, the ventricle being divided into two 

 by a complete partition. Here, however, the same results are 

 brought about as in the other Reptiles, by means of a commu- 

 nication which subsists between the great vessels which spring 

 from the ventricles thus formed. In the ordinary Reptiles the 

 course of the circulation is as follows (Fig. 117) : The impure 

 or venous blood that has circulated through the body is poured 

 by the great veins into the right auricle (a). The pure or 

 arterial blood that has been submitted to the action of the 

 lungs is poured by the pulmonary veins into the left auricle 

 (a 1 ). Both auricles empty their contents into the ventricle, 

 and, as the partition which divides the ventricle is an incom- 

 plete one, it follows that the venous and arterial streams must 

 mix to a greater or less extent in the ventricle. From the 

 ventricle arise the great vessels which carry the blood to the 

 lungs and to all parts of the body, and it follows, as a matter 

 of necessity, that all these parts are supplied with a mixed 

 fluid, consisting partly of impure or venous blood, and partly 

 of pure or arterial blood. In the Crocodiles, in which there 

 are two ventricles completely separated from each other, the 



