318 ZOOLOGY SECT. 



left between them for the head and neck, the tail and the limbs. 

 The neck is long and mobile ; the tail short. The limbs are fully 

 developed though short. In some (land and fresh-water Tortoises) 

 they are provided each with five free digits terminating in curved 

 horny claws ; in the Turtles the digits are closely united together, 

 and the limb assumes the character of a " flipper " or swimming 

 paddle. The cloacal aperture is longitudinal. 



The Crocodilia, the largest of living Reptiles, have the trunk 

 -elongated, and somewhat depressed, so that its breadth is much 

 greater than its height. The snout is prolonged, the neck short, 

 the tail longer than the body and compressed laterally. The 

 limbs are relatively short and powerful, with five digits in the 

 manus and four in the pes, those of the latter being partly or 

 completely united by webs of skin. 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 longi- 

 tudinal slit. The dorsal and ventral surfaces are covered with 

 thick squarish horny plates, often sculptured or ridged, which are 

 supported on bony dermal plates or scutes of corresponding form ; 

 the horny plates of the dorsal surface of the tail are developed 

 into a longitudinal crest. 



Integument and Exoskeleton. Characteristic of the Squa- 

 mata is the development in the epidermis of horny plates which 

 cover the entire surface, overlapping one another in an imbricating 

 manner. These differ considerably in form and arrangement in 

 different groups ; sometimes they are smooth, sometimes sculptured 

 or keeled. Sometimes they are similar in character over all parts 

 of the surface ; usually there are specially developed scales the 

 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 (Chamseleons 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 

 forrii of large tubercles or spines. Underlying the horny epi- 

 dermal scales in some Lizards (Skincoids) are a series of dermal 

 bony plates. In the integument of the Geckos are numerous 

 minute hard bodies which are intermediate in character between 

 cartilage and bone. 



In the Snake-like Amphisbaenians there are no true scales, with 

 the exception of the head shields, but the surface is marked out 

 into annular bands of squarish areas. 



In addition to the modification of the scales, the integument of 

 the Chamaeleons is remarkable for the changes of colour which it 

 undergoes, these changes being due to the presence in the dermis 

 -of pigment cells which contract or expand under the influence of 

 the nervous system, in a way that reminds one of the integument 



