CLASSIFICATION OF ANIMALS 217 



sunshine and air, wholesome food, sufficient rest and a 

 tranquil mind. With these aids, the body is usually pre- 

 pared in itself to kill the germs that come into it. Every 

 day each person probably receives into his system thousands 

 of disease germs and usually it is only when the vitality 

 of the body is low that these germs are able to establish 

 themselves. Right living is the great disease preventer. 



As has already been stated, however, disease bacteria 

 are only a small portion of the bacteria group of plants 

 and the usefulness of the other members of this group is 

 far greater than their harmfulness. Science each year is 

 becoming more and more able to fight the disease germs, 

 but it is entirely unable to supply the necessary aid given 

 by the useful bacteria to animals and plants and, through 

 them, to man. 



100. Animals. Animals do not take their energy directly 

 from the sunlight, but indirectly from the latent energy 

 stored up in the foods prepared by green plants. These 

 foods may be eaten as stored by the plants, or they may 

 have passed through the medium of other plants and 

 animals. The energy thus stored up is liberated by com- 

 bining the carbon with oxygen. Carbon dioxide is freed. 



The green plants use this carbon dioxide again and, by 

 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, but this is in small quantities. 



101. Classification of Animals. For convenience of study 

 the animal kingdom has been divided into two great 

 classes the invertebrates (without backbone) and the 



