16 REPTILE GALLERY. 



tion, but each has an action independent of the other one eye 

 may be looking forwards, whilst an object behind the animal is 

 examined with the other. The faculty of changing colour, which 

 they have in common with many other Lizards, is partly dependent 

 on the degree in which the lungs are filled with air, and different 

 layers of chromatophores* are pressed towards the outer surface of 

 the skin. The adult males of some of the species possess long 

 horns or other excrescences on the head. The largest species 

 attain a length of 18 and 20 inches. About 75 species are known. 



Order IV. OPHIDIA, or SNAKES. 



The Snakes, or Ophidians, are scaly Eeptiles, with exceedingly 

 elongate, limbless body, without sternum, without, or with only 

 rudiments of, a pelvis, with the mandibles united in front by an 

 elastic ligament. The ribs are articulated movably with the verte- 

 bral column. The jaws are armed with sharp, fang-like teeth, 

 which are ankylosed to the bone. The peculiar mobility of the 

 jaw-bones enables these animals to extend the gape in an extra- 

 ordinary degree, and to work their prey (which generally is much 

 thicker than the Snake itself and always swallowed whole) through 

 the throat into the stomach. The tongue is narrow, retractile into 

 a basal sheath, and terminates in two long thread-like points ; 

 it is frequently and rapidly exserted when the animal is excited or 

 wants to touch an object. Snakes have no eyelids ; but the part 

 of the epidermis which covers the eye is transparent, convex, and 

 has the shape of a watch-glass, behind which the eye moves. There 

 is no ear-opening. The scales are not isolated formations, as in 

 fishes^, but merely folds of the outer skin, which is cast off in a 

 single piece several times every year. The head is generally covered 

 with large, symmetrical, juxtaposed plates (see figs. 15&16),and the 

 belly with large transverse shields. The organs of locomotion for 

 the exceedingly elongate body of the Snakes are the ribs, the 

 number of which is very great, nearly corresponding to that of 

 the vertebras of the trunk. Although the motions of Snakes arc 

 in general very quick, and may be adapted to every variation of 



* Cells in the skin in which the colouring-pip-ment is deposited. 



