CHAPTER XXI 



ANIMAL LIFE 



Relation of Plants to Animals. - - The simplest plants 

 are composed of a single cell. The simplest animals 

 also are composed of a single cell. Both plants and 

 animals possess the power of irritability ; both have the 

 power to change their' position ; and both possess the 

 power of reproduction. The elements necessary to the 

 growth of plants and animals 'are all found in the soil, 

 the air, and the water. There is a difference however 

 between plants and animals. Animals do not have the 

 power of making their own foods from the elements found 

 in the soil, air, and water, but are dependent upon plants 

 for certain essential foods. Animals require green plants. 

 The protoplasm of green plants reacting under the in- 

 fluence of sunlight manufactures food which is used by 

 both plants and animals. Whether an animal is herbiv- 

 orous, eating plants only, or carnivorous, eating flesh 

 only, its food can be traced to the same source. Animals 

 that eat both plant and animal flesh are called omnivorous. 



In the scale of animal life we have many gradations 

 from the simplest one-celled animal to man, the highest 

 type of vertebrate. Between these two extremes we have 

 such animals as worms, oysters, snails, lobsters, insects, 

 fishes, reptiles, birds, and animals like the horse. 



One-celled Animals. Amoebae are one-celled animals 

 which may be obtained for purposes of study from vege- 

 tation growing near the surfaces of ponds. Unlike other 



350 



