258° VERTEBRATE ANIMALS, 
beyond the base of the skull (fig. 138, a). Teeth are generally 
present, but these are used chiefly to hold the prey, and not in 
biting or chewing the food. Except in the Crocodiles, the teeth 
are not sunk into distinct sockets, and they are usually replaced 
as fast as shed. They lhkewise do not differ from one another 
sufficiently 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 Tortoises 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 notice, 
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 respira- 
tory organs that the most important characters of the Reptiles 
are to be looked for. The heart in all Reptiles may be regarded 
as being, 2 funcizon, 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, s¢yzc- 
turally, 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 com- 
munication which subsists between the great vessels which 
spring from the ventricles thus formed. In the ordinary Rep- 
tiles the course of the circulation is as follows (fig. 139): The 
impure or venous blood that has circulated through the body is 
poured by the great veins into the right auricle (a2). The pure 
or arterial blood that has been submitted to the action of the 
lungs is pouredby the pulmonary veins into the left auricle (¢). 
Both auricles empty their contents into the ventricle, and as the 
partition which divides the ventricle is an incomplete 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 
