CLASSES OF ANIMALS 457 



times, however, there were considerably larger forms. The limb- 

 less batrachians comprise only a few species, confined for the 

 most part to tropical regions. The other two groups are widely 

 scattered over the earth, but no batrachians live in salt water. 

 They show a wide range of adaptation in organs of locomotion, 

 in color, in feeding habits, and in resistance to extreme heat or 

 extreme cold ; for they will curl up and sleep in the mud when 

 it gets too warm, and curl up and sleep in the bottom of a pond 

 or a stream when it gets too cold. 



^lost batrachians are harmless and are of practical value to 

 us in their greedy feeding upon insects, worms, small crus- 

 taceans, and fish. In Europe the legs of frogs are widely used 

 for food, while in this country they are considered a delicacy. 



338. Reptiles. These animals are all air-breathers. Although 

 they resemble the amphibians in some respects (especially in 

 the similarity of form between lizards and salamanders), they 

 undergo no metamorphosis after leaving the egg, and they al- 

 ways have skeletal structures in the skin, which batrachians 

 never have. They vary in form and size from small, wormlike 

 snakes, only a few inches long, to the giant turtles weighing half 

 a ton, and the alligators and crocodiles, which may reach a 

 length of thirty feet. In past ages there were many more rep- 

 tiles, and they attained much greater size. Reptiles also show 

 a great range in degree of development and differentiation. 

 Some of the snakes have a large number of substantially similar 

 ribs and vertebrae (about four hundred of the latter in some 

 species), whereas in the turtles and crocodiles the number is 

 definite and each is distinct in its structure and in its relations 

 to the body as a whole. While most reptiles are tropical, they 

 range pretty far into the temperate regions ; they occupy salt 

 water as well as fresh water ; most of them live underground, 

 though a few live in trees. Reptiles are cold-blooded, and most 

 of them have a heart resembling that of the batrachians ; among 

 the crocodiles there is at last a four-chambered heart. 



In certain respects, which appear most strikingly in the skele- 

 tons, and especially in the fossil skeletons, reptiles resemble 



