694 PHYLUM CHORDATA : CLASS REPTILIA 



posterior region of the body passes gradually into the 

 long tail, which is often mutilated in captured specimens. 

 Both fore- and hind-limbs are present, and both are fur- 

 nished with five clawed digits. Of the apertures of the 

 body, the large mouth is terminal, the external nares are 

 close to the end of the snout, and the cloacal aperture is 

 a considerable transverse opening placed at the root of the 

 tail. There is no external ear, but the tympanic membrane 

 at either side is slightly depressed below the level of the 

 skin of the head. The eyes are furnished with both upper 

 and lower eyelids, and also with a nictitating membrane. 



Skin. — As contrasted with that of the frog, the skin has 

 a distinct exoskeleton of epiderm.ic scales, the external 

 covering of which is shed from time to time. In the head 

 region these exhibit a definite arrangement characteristic 

 of the species. With the presence of an exoskeleton we 

 must associate the absence of the numerous cutaneous 

 glands of the frog. Peculiar tubular ingrowths of epidermis 

 form a row of so-called " femoral glands," which open by 

 pores on the ventral surface of the thigh. Their product 

 (debris of epidermic cells) is most obvious in the male at 

 pairing time. The structure of the skin is very similar to 

 that of the frog. Pigment is deposited here also in two 

 layers, of which the outer is greenish, the inner black. 

 Over the parietal foramen on the top of the skull the black 

 pigment is absent, the green only feebly represented ; in 

 this region, therefore, the skin is almost transparent. In 

 moulting — which means casting off the outermost layer of 

 the epidermis — there is a distension of the blood vessels 

 and a great increase of blood pressure. 



Many lizards, such as the Chamaeleons, exhibit in a remarkable degree 

 the power of rapidly changing the colour of their skin. This is due to 

 the fact that the protoplasm of the pigment cells contracts or expands 

 under nervous control. The change of colour is sometimes advan- 

 tageously protective, but it seems often to be merely a reflex symptom 

 of the nervous condition of the animals. 



In many cases, e.g. in some of the skinks, in Anguis, Helnderma, 

 there are minute dermal ossifications beneath the scales. 



Skeleton. — The backbone consists of a variable number 

 of vertebrae, and is divisible into cervical, dorsal, lumbar, 

 sacral, and caudal regions. Except the atlas and the last 



