32 ZOOLOGY. 



407. The Reptiles are very diverse in form. Perhaps the 

 lizards may be taken as typical, with cylindrical body, more 

 or less distinct head and neck, distinct tail, and usually two 

 pairs of appendages, each possessing five digits armed with 

 claws. They are mostly small animals, though one species is 

 known to attain a length of five feet. The crocodiles and 

 alligators are similar in shape but much larger. The turtles 

 and snakes are most widely different from the type and must 

 be regarded as much specialized, or even degenerate, forms. 

 The turtles have sought protection by means of a bony box, 

 and are ill adapted for motion either on land or water. Snakes, 

 on the other hand, elongated and devoid of appendages, are 

 among the most rapid and graceful of animals in their motions. 

 The long tapering body is a successful prehensile organ. Some 

 of the lizards agree with the snakes in lacking legs. 



408. Covering. The external covering in reptiles is in the 

 form of scales or plates formed by the epidermis, or the dermis, 

 or both. That deposited by the epidermis is horny and that 

 by the dermis, bony. In snakes and many lizards the scales 

 are epidermal and may be periodically shed and renewed. The 

 scales usually differ in shape and size in different parts of 

 the body. In turtles and their allies the horny constituent, 

 which is illustrated by the " tortoise shell " of commerce, is in 

 the form of plates and is reinforced by bony dermal plates 

 beneath. The latter do not, in the adult at least, correspond in 

 number and size with, the former, but are closely associated 

 with the bones of the internal skeleton. In crocodiles the der- 

 mal scales correspond in general with the epidermal. 



409. Internal Skeleton. The vertebral column, except in 

 the snakes and snake-like lizards, shows the customary regions 

 (see 341). In the limbless forms only two regions are 

 recognized, the pre-caudal which bear the ribs, and the 

 caudal or tail vertebrae. The vertebrae are usually concave in 

 front and convex behind, thus making a kind of ball-and- 

 socket joint. In snakes the number of vertebrae is very large. 



