A HISTORY OF VERTEBRATES: AMPHIBIANS AND REPTILES 



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there is the usual joint between the lower jaw and the quadrate, and 

 finally one between the quadrate and the rest oi the skull. Other 

 features which characterize snakes are the absence of movable eyelids, 

 of a tympanic membrane and middle ear cavity, and of legs. There are 

 exceptions to these generalizations, for geckos do not have movable 

 eyelids, glass "snakes" lack legs, and some of the more primitive snakes, 

 such as the python, have vestigial hind legs. 



Snakes doubtless evolved from some primitive lizard group, and 

 very probably from burrowing members of that group. The most primi- 

 tive living snakes are burrowing species, and many details of ophidean 

 anatomy suggest a iossorial ancestry. The structure of their eyes, for 

 example, indicates that the eyes redeveloped from eyes that had under- 

 gone marked retrogressive changes. Their forked tongue, which is often 

 seen darting from the mouth (Fig. 23.13 A), is an organ concerned with 

 touch and smelling. Odorous particles adhere to it, the tongue is with- 

 drawn into the mouth, and the tip is projected into a specialized part 

 of the nasal cavity (jacobson's organ). The great elaboration of such a 

 device would seem to be an adaptation to a burrowing mode of life in 

 which other senses woidd be less useiul. 



In their subsequent evolution some snakes gave up the burrowing 

 habit and developed a method of locomotion that depended upon 

 squirming and the movement of their ventral scales. Their adjustments 

 to epigean life and their unique feeding mechanism enabled them to be 

 a successful group and to undergo an extensive adaptive radiation. 



Among the more interesting adaptations has been the evolution, 

 in several distinct lines, of a poison mechanism that involves specialized 

 oral glands associated with grooved or hollow, hypodermic-like teeth— 

 the fangs. Most of our poisonous North American snakes (rattlesnakes, 

 copperhead, cottonmouth, \\ater moccasion) are pit vipers. They have 

 a pair of large, hollow fangs at the front of the mouth that are articu- 



Figure 23.13. A, A coachwhip snake protruding its tongue; B, "milking" a rattle- 

 snake to get poison for the production of antivenom. The tongue is a tactile and ol- 

 factory organ that is perfectly harmless; it should not be confused with fangs, which 

 are specialized teeth. (A, Courtesy of the New York Zoological Society; B, courtesy of 

 Ross Allen's Reptile Institute.) 



