CHAPTER XVI 



ANIMALS 



332. What is an Animal? We have already studied 

 something of the relation between animals and plants. 

 Plants (those containing chlorophyll; cf. 310) change 

 carbon dioxide, water, and the minerals of the soil into 

 living material, which serves as food for the animal world. 

 In taking carbon dioxide from the air, plants remove a 

 waste product of animals, and restore oxygen in its place. 

 Thus they keep the composition of the air the same, year 

 after year. It was once supposed that plants did not 

 produce carbon dioxide. This is a mistake. Plants give 

 off carbon dioxide in their respiration, just as animals 

 do; but green plants take up much more than they give 

 off. Hence they seem not to produce any. 



In general, we may say that an animal is an organism 

 that cannot build up its food out of mineral matter, while a 

 plant can. Plants having no chlorophyll (fungi) are excep- 

 tions to this statement (cf. 324). The lowest forms of 

 animals and plants are really so much alike that it is often 

 hard to say to which class a given creature belongs. The 

 fact that the higher animals can move about freely is not a 

 real difference; for many animals are attached to some 

 fixed place, while some plants have very rapid movements. 



333. One-Celled Animals. In stagnant water or in 

 mud there is often found a bit of jelly-like substance that 



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