A FEW FACTS ABOUT ZOOLOGY. 11 



upper cavity are the brain and the organs of sense; in the 

 lower cavity the organs of digestion, respiration, circulation, 

 and reproduction. Every vertebrate has four locomotive 

 appendages, composed of the same substance that forms the 

 backbone : in birds, they appear as wings and legs ; in quadru- 

 peds, as four legs; in man, as arms and legs ; even where they 

 seem wanting, as in serpents, a minute study of the gradual 

 reduction of the locomotive' appendages in various groups of 

 reptiles will show that they, too, are true to this typical plan. 

 Beginning with the lowest class of vertebrates, we find 

 the fishes; they are cold-blooded, they breathe through gills, 



Fig. 7. 



and they lay eggs. A step higher come the amphibia — 

 amphibious animals — receiving their name from the fact that 

 they are able to exist both on land and in water. They are 

 cold-blooded ; they breathe by gills during some part of 

 their lives, but soon or later possess lungs. All undergo 

 changes after leaving the egg, passing through the tadpole 

 state, in which they resemble fishes. We all know these 

 changes in the frog and toad. Still rising, we come to the 

 reptiles, including snakes, lizards, turtles, crocodiles. The 

 structure of the turtle is so peculiar that one might be 



