CHAPTER XXV 

 THE LIFE OF AN ANIMAL 



331. Simple and Complex Organisms. When we speak 

 of organisms as lower and higher, simple and complex, we refer 

 to the structure of the body. The simplest organisms con- 

 sist of one cell, microscopic in size, and averaging about 



FIG. 153. ONE-CELLED ANIMALS, (magnified) 



These minute animals live in stagnant fresh water. Their food con- 

 sists of microscopic plants and animals. The globular part of C consists 

 of grains of sand which the animal gathers upon its surface for protection. 



sinnr of an inch in diameter. Such bodies possess no organs. 

 Food is absorbed in liquid form at any part of the cell. Re- 

 production takes place by increase in the size of the cell, 

 followed by separation into two cells. These simple organ- 

 isms have all the physiological properties of protoplasm, 

 which is their sole constituent. They are themselves food 

 for organisms slightly more complex, which perhaps have 

 one opening that serves both to receive food and "to reject 

 the insoluble residue, and a central cavity where digestion 

 takes place. 



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