400 



LIFE ON THE EARTH 



the aid of the sun's energy, free the oxygen and store up 

 the carbon. Thus the cycle goes on, over and over, the 

 plants freeing oxygen and taking up carbon dioxide, and 

 the animals freeing carbon dioxide and taking up oxygen. 

 The cells of plants which feed upon the food prepared by 

 the chlorophyll of the leaves use oxygen and give out carbon 

 dioxide just as the animal cells do ; so also do other plants 

 to some extent. 



Classification of Animals. For convenience of study 



the animal kingdom has been divided into two great classes 



the invertebrates (without backbone) and the vertebrates 



(with backbone). The invertebrate is the much more 



numerous class as it contains the worms, shellfish, insects, 



and those almost countless 

 forms of animal life which 

 have no internal bony skele- 

 ton and backbone. The 

 higher animals, like fishes, 

 amphibia, reptiles, birds, and 

 mammals, belong to the 

 class of vertebrates. Man 

 himself is the highest of the 

 vertebrates, and his struc- 

 ture will be studied later. 



GLOBIGERINA (Greatly magnified) 



Invertebrates : Protozoa. 



The shells of these minute animals _ The very lowest f orms of 

 cover much of the ocean floor. 



animal life, the protozoa, 



are single-celled animals. In some species they are very 

 difficult to distinguish from plants of the lowest orders. 

 They are microscopic in size and most of them live in water. 

 Some of these tiny protozoa living in the sea are covered 



