THE PHYLA HEMICHORDATA AND CHORDATA 



that snakes are a comparatively recent ofTshoot from four-footed ancestors 

 within this order. There are also a few limbless lizards now living that 

 resemble snakes but have had an independent and more recent origin. The 

 various lizards in the warmer parts of the United States are familiar ex- 

 amples of the Lacertilia. The giant monitor lizard, Varanus komodoensis, of Java, 

 one of the largest of existing reptiles, reaches a length of 10 feet and a weight 

 of 250 pounds (Fig. 18.22). The African chameleon is a bizarre, slow- 

 moving lizard noted for its ability to adjust its color to match that of the 

 background. It feeds chiefly on insects, which it captures by means of a pre- 

 hensile tongue that can be protruded with great rapidity to a length of 

 several inches. 



The snakes are a very specialized though recent reptilian type. In most 

 genera the limbs have disappeared, but in a few, such as the pythons, minute 

 vestiges are visible externally. Modifications of the ribs and vertebrae adapt 

 the body for lateral coiling, and for locomotion by pushing and bracing the 

 epidermal scales against the substrate. The jaws and related bones are modi- 

 fied in such a manner that the mouth can be stretched enormously in feeding. 

 The viscera are also greatly modified in correlation with the elongation of the 

 body. The fangs of poisonous snakes, such as the rattlesnake, are modified 

 teeth associated with glands by which the venom is secreted (Fig. 18.23). In 

 contrast with this means of offense and defense, the non-venomous boas and 

 pythons, which attain great size, overpower their prey by crushing it in their 

 coils. The great majority of our common snakes are quite harmless, and such 

 species as the bull snake render great service to man by their destruction of 

 rodents. Most species of lizards and snakes are oviparous in the typical 

 reptilian manner (Fig. 18.23); a few species, such as the water snake, Natnx, 

 are ovoviviparous. 



Fig. 18.21. Sphenodon punclatus, the onlv existing species representing the reptilian order 

 Rhynchocephalia. (Photograph courtesy New Yortc Zoological Society.) 



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