LIZARDS. 571 



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 fur- 

 nished with both upper and lower eyelids, and also with a 

 nictitating membrane. 



Skin. 



As contrasted with that of the frog, the skin is remarkable 

 as possessing a distinct exoskeleton of epidermic scales. In 

 the head region these exhibit a definite arrangement char- 

 acteristic of the species. With the presence of an exo- 

 skeleton we must associate the absence of the numerous 

 cutaneous glands of the frog ; these are here represented 

 only by a row of "femoral glands," which open by pores on 

 the ventral surface of the thigh. Their secretion is most 

 obvious in the male at pairing time. The histological com- 

 position of the skin is very similar to that of the frog's skin. 

 Pigment is deposited here also in two layers, of which the 

 outer is greenish, the inner black. It is of special interest 

 to notice that over the parietal foramen (see Skull) the black 

 pigment is absent, the green only feebly represented ; in this 

 region, therefore, the skin is almost transparent. 



Many Lizards, such as the Chameleons, 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 advantageously pro- 

 tective, but it seems often to be merely a reflex symptom of the nervous 

 condition of the animals. 



In a few cases, e.g., some of the skinks, 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 caudal, all the 

 vertebrae are procoelous. 



The atlas consists of three separate pieces, its centrum ossifies as usual 

 as the odontoid process of the axis. There are two sacral vertebrre with 

 large expanded sacral ribs. To the ventral surfaces of many of the 

 caudal vertebrae Y-shaped "chevron" bones are attached. Across the 

 centre of the caudal vertebrae there extends a median unossified zone, it 

 is in this region that separation takes place when a startled lizard loses 

 its tail. 



