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EVOLUTION OF ANIMALS 



tp dig iP 



Part V 



Fig. 35.10. One side of the head of a poisonous snake, with the skin and cheek 

 muscle removed to show the duct connecting the poison gland with the tubular 

 fang. When the jaws are opened the fangs drop downward; when they clutch the 

 contraction of cheek muscles pushes poison into the fangs, ta, dig, tp, ta, muscles; 

 pg, poison gland; d, duct; g, sensory groove; n, nostril. (Courtesy, Gadow: "Am- 

 phibia and Reptiles," in Cambridge Natural History, vol. 3. London, The Mac- 

 millan Co., 1909.) 



with the type of snake. Venoms produce two main eflfects. In one, the venom 

 breaks up the blood cells and injures the linings of blood vessels. In the other, 

 it attacks the nerve centers especially those of the respiratory system. Anti- 

 venins are available in certain countries but are in no wise so accessible as they 

 should be. First aid treatment, however, is described in almost all recent books 

 about snakes. Anti-venins are prepared by immunizing horses against a par- 

 ticular poison by gradually increasing injected doses of the venom. The clear 

 serum of the horse's blood with its antitoxins is then ready to be used to 

 inoculate patients. Snake venom is one of the most complex poisons produced 

 by animals and it has not been possible to prepare a general antitoxin for it. 

 In some cases, an antitoxin works against the venom of only one species, in 

 others against those of two or more. Snakes of the United States that produce 

 the most serious poisons are: the western diamond-backed rattlesnake, eastern 

 diamond-backed rattlesnake, prairie and Pacific rattlesnake, timber rattlesnake, 

 and water moccasin. If frequency of the bite, not strength of poison and danger, 

 is considered the copperheads would top this list. 



Hibernation. Large numbers of snakes commonly of one species, some- 

 times of two or three, hibernate in one locality, in various protected holes in the 

 ground where the temperature stays above freezing. They congregate in autumn, 

 always in warm places, mate and finally retire for the winter, sometimes dozens 

 intertwined in clumps in which heat and moisture are conserved. 



Order Crocodilia — Alligators and Crocodiles 



Crocodilians are the giants among reptiles. They are ponderous, lizardlike 

 and clothed with exceedingly tough skin and an armor of bony plates overlaid 



