CHAPTER XVI 



THE REPTILES 



175. General characteristics. In all the reptiles the gen- 

 eral shape of the body, and to some extent the internal 

 plan, is not materially different from that seen among the 

 amphibians. In spite of external resemblance the actual 

 relationship is not very close. It appears to be true that 

 ages ago the ancestors of the modern reptiles were aquatic 

 animals, possibly somewhat similar to some of the sala- 

 manders; but they have become greatly changed, and 

 are now, strictly speaking, land animals. At no time in 

 their development after leaving the egg do we find them 

 living in the water and breathing by gills. Some species, 

 such as the turtles, lead aquatic or semiaquatic lives, but 

 the modifications which fit them for such an existence 

 render them only slightly different from their land-inhabit- 

 ing relatives. The skin bears overlapping scales or horny 

 plates, united edge to edge, as in the turtles, enabling them 

 to withstand the attacks of enemies and the effects of heat 

 and dryness. Indeed, it is when heat is greatest that rep- 

 tiles are most active. In no other class of vertebrates, and 

 very few invertebrates, do normal activities of the body 

 appear to be so directly dependent upon external warmth. 

 In the presence of cold they rapidly grow sluggish, and 

 sink into a dormant state. 



As in the case of all animals, habits depend upon 

 structure, and accordingly among the reptiles we find 

 many remarkable modifications, enabling them to lead 

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