326 Our Surroundings 



It seems clear, then, that the simplest forms of animals require 

 no organs of nutrition, since the cells that compose their bodies 

 come into immediate contact with food and with water, containing 

 oxygen, so that all substances needed for growth and for energy- 

 making are taken up readily by absorption. More complex forms, 

 however, since they are made up of tissues containing many cells, 

 must have channels of communication through which to convey 

 food and oxygen to all cells in their bodies. These channels are 

 provided by organs. 



SUMMARY 



Protozoa are single-celled animals. Metazoa are composed of 

 many cells. All higher animals and man are metazoa. 



Protozoa, without special organs, perform all the fundamental 

 life processes of higher animals. 



Some protozoa are useful. Other protozoa are harmful, caus- 

 ing disease in man and animals. 



In all metazoa there are special cells, or groups of cells, for 

 different functions. This is called the physiological division of 

 labor. 



The cell is the unit of both structure and function in all 

 metazoa. All tissues are composed of cells, each of which per- 

 forms the same vital functions as the one-celled animal. 



Higher animals require special organs to aid in carrying on the 

 necessary functions. By means of these organs, all the cells of the 

 body receive food and air and get rid of wastes. 



FACT AND THOUGHT QUESTIONS 



1. Define (a) protozoa; (b) metazoa. 



2. How does the paramecium secure food? 



3. How does the amceba secure food? 



4. Mention the life processes of protozoa. 



5. Of what economic importance are protozoa? 



6. How does the amoeba carry on respiration, motion and reproduction? 



7. Compare the cat and the amoeba for certain common functions of life. 



8. In what parts of the kitchen are dangerous protozoa most likely to 

 exist? 



9. Why screen windows in summer? 

 10. Name several common metazoa. 



