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. Jt 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 
184. 
