VERTEBRATA 



155 



some cases each caudal vertebra has a division in the 

 middle, so that the tail, when grasped, breaks off at one 

 of these divisions. The chameleon has a prehensile tail. 

 The iguana is distinguished by a dewlap on the throat 

 and a crest on the back. Except some of the monitors 

 of the Old World, all the lizards are terrestrial. 



f FIG. 133. Lizard (Lacerta). 



The snakes (Ophidia) are characterized by the absence 

 of visible limbs ; 38 by the great number of vertebrae, 

 amounting to over four hundred in the great serpents ; 

 by a corresponding number of ribs, but no sternum ; and 

 no true eyelids, the eyes being covered with a transparent 

 skin. The tongue differs from that of nearly all other 

 reptiles in being bifid and extensile. The mouth is very 

 dilatable. The skin is frequently shed, and always by 

 reversing it. Snakes make their way on land or in water 

 with equal facility. 



As a rule, the venomous snakes, as vipers and rattle- 

 snakes, are distinguished by a triangular head covered 



