CLASS REPTILIA. 317 



The lieptilia are essentially scaly tetrapodous, pentadactyle 

 animals, but great modification in form and habit are met with 

 in the group. Functional gills are not developed at any time of 

 life, and they usually lay large yolked eggs which develop outside 

 the body of the mother. 



The integument is scaly in all livmg reptiles. The scales are 

 horny epidermal structures usually placed on dermal papillae. 

 In some cases {Chelonia, some Crocodilia, and Lacertilia), 

 osteoderms may be present m the cutis in some parts of the body. 

 It is possible that in some of the extuict forms e.g. Ichthyosauria, 

 scales may have been absent. 



An upper and lower eyelid is generally present, and frequently 

 a third eyelid, the nictitating membrane. A tympanic mem- 

 brane is also usually present. But in snakes there is no tym- 

 panic membrane, and the eyelids are transparent and fused over 

 the eye, so that they appear to be absent.* There is a cloaca 

 which receives the openings of the urinogenital ducts, and opens 

 to the exterior by the anus. The tail is usually of considerable 

 length, and the limbs are sometimes capable of supporting the 

 weight of the body ; but more often they serve merely to push 

 on the body which glides along the ground on its belly. In snakes 

 and some lizards limbs are absent. 



Cutaneous glands are confined to certain places and are not 

 generally distributed. The skin is usually diversely coloured, 

 owing to the presence of pigment in the dermis and sometimes 

 in the deeper layers of the epidermis. 



The phenomenon of colour-change is met with in lizards and 

 some snakes : it is particularly developed in the chameleons. 



The endoskeleton is well ossified, and the membrane bones 

 are so closely incorporated with the skeleton that they cannot be 

 peeled oft". 



The vertebral column is usually divided into cervical, thoracic, 

 sacral and caudal regions. The faces of the centra vary con- 

 siderably : they are frequently hollow in front and convex be- 

 hind (procoelous), but thej' may be flat, or hollow at both ends 

 (amphicoelous), or even hollow behind (opisthocoelous). The 

 amphicoelous condition is found hi some extinct forms and in 

 Sphenodon and the Geckonidce among the living. In this case 



* It is possible that they are absent and that the transparent membrane 

 stretched across the eye is the nictitating membrane. 



