STRUCTURE OF THE VERTEBRATES 63 



mental conditions. The mechanism is similar to that of the tree 

 frogs, depending upon the shrinkage or expansion of the pigment 

 cells. 



In size the lizards range from a few inches to several feet; a 

 species recently captured in the Malayan Islands measuring more 

 than twelve feet. They are carnivorous in diet, the smaller ones 

 depending upon insects, the larger eating small mammals or birds, 

 particularly the eggs of the latter. 



Order Ophidia. The snakes are legless reptiles. Most genera 

 have even lost the girdles, although the boas and pythons have 

 minute vestiges of hind legs. They are all elongated in body 

 shape, and their skeleton and internal organs are highly spe- 

 cialized for the condition. The long ribs are attached to the 

 ventral plates. Locomotion is attained by the motion of the ribs 

 and by lateral coiling, bracing, and pushing forward. There are 

 no records of snakes moving by dorso-ventral coiling. 



The skull is the most specialized among the reptiles. The bones 

 of the mouth and skull are so arranged that the jaws can be 

 stretched downward and laterally to enormous limits. A small 

 boa, with a transverse head measurement of a centimeter, is able 

 to swallow a grown mouse. The soft anatomy is almost as modi- 

 fied. Usually there is a single, highly elongated lung, and the 

 digestive glands and tract are built on the same long lines. The 

 skin has few glands, but most snakes have anal glands which 

 secrete fluids of such distinctive odors that a number of genera 

 or families can be identified by the scent. Like the lizards, the 

 snakes are both oviparous and ovo-viviparous, the latter having 

 given rise to the myth that snakes swallow their young in times 

 of danger. 



The poison glands are highly developed in a number of snakes. 

 In correlation with this there are modifications of certain teeth, 

 the fangs. These may have simply a groove down one face, or the 

 edges of the groove may grow over to form a hollow tube. The 

 glands lie in close contact, and usually open into the cavity of 

 the tooth. The result is a natural hypodermic syringe, the poison 

 being forced under the skin. There is no sure rule for telling 

 poisonous from non-poisonous snakes except by close inspection, 

 and this is often inconvenient in the field. In the United States 

 the only poisonous snakes of any importance are the rattle- 



