XII 



PHYLUM CHORDATA 



439 



The eyes are very small ; the nostrils placed close to the 

 end of the snout and capable of being closed by a sphincter 

 muscle. The cloacal aperture is a longitudinal slit. 



Characteristic of the Squamata is the development in the 

 epidermis of horny plates, the scales, which cover the entire 

 surface, overlapping one another in an imbricating manner. 

 Sometimes they are similar in character over all parts of the 

 surface ; usually there are specially developed scales the 



FIG. 263. Grecian tortoise (Testudo graeca). (After Brehm.) 



head shields covering the upper surface of the head. In 

 the majority of snakes the ventral surface is covered with a 

 row of large transversely elongated scales, the ventral shields. 

 In some lizards (chamaeleons and geckos) the scales are 

 reduced and modified into the form of minute tubercles or 

 granules. In some lizards special developments of the 

 scales occur in the form of large tubercles or spines. 

 In the snake-like Amphisbaenians there are no true scales, 



