436 MANUAL OF ZOOLOGY sect. 



In some groups of lizards the tail is comparatively short 

 and thick; and in others it is depressed and expanded into 

 a leaf-like form. In the chameleons the long and tapering 

 tail is used as a prehensile organ, the coiling of which 

 round branches of the trees in which the animal lives, aids 

 in maintaining the balance of the body in climbing from 

 branch to branch. 



In the limbs there is likewise a considerable amount of 

 variation in the different groups of the Lacertilia. Moder- 

 ately long pentadactyle limbs, like those of Lacerta, are the 

 rule. In the chameleons both fore- and hind-limbs become 

 prehensile by a special modification in the arrangement and 

 mode of articulation of the digits. In these remarkable 

 arboreal reptiles the three innermost digits of the manus are 

 joined together throughout their length by a web of skin, 

 and the two outer digits are similarly united ; the two sets 

 of digits are so articulated that they can be brought against 

 one another with a grasping movement much analogous to 

 the grasping movements of a parrot's foot or the hand of 

 man. A similar arrangement prevails in the pes, the only 

 difference being that the two innermost and three outermost 

 digits are united. In some groups of Lacertilia, on the 

 other hand, such as the blind-worms (A/iguis), limbs are 

 entirely absent, or are represented only by mere vestiges ; 

 and numerous intermediate gradations exist between these 

 and forms, such as Lacerta, with well-developed limbs. The 

 limbless lizards, such as the glass-snake, and Pygopus (Fig. 

 261), bear a very close resemblance to the snakes, not only 

 in the absence of the limbs, but also in the general form of 

 the body and the mode of locomotion. 



The body of a snake is elongated, narrow, and cylindrical, 

 usually tapering towards the posterior end, sometimes with, 

 more usually without, a constriction behind the head. In 



