172 SINGLE-CELLED ANIMALS AS ORGANISMS 



Habitat of Protozoa. — Protozoa are found almost everywhere 

 in shallow water, especially close to the surface. They appear 

 to be attracted near to the surface by the supply of oxygen. 

 Every fresh-water lake swarms with them; the ocean contains 

 countless mjTiads of many different forms. 



Use as Food. — They are so numerous in lakes, rivers, and the 

 ocean as to form the food for many animals higher in the scale of 

 life. Almost all fish that do not take the hook and that travel 

 in schools, or companies, migrating from one place to another, 

 live partly on such food. Many feed on slightly larger animals, 

 which in turn eat the Protozoa. Such fish have on each side of the 

 mouth attached to the gills a series of small structures looking like 

 tiny rakes. These are called the gill rakers, and aid in collecting 

 tiny organisms from the water as it passes over the gills. The 

 whale, the largest of all mammals, strains protozoans and other 

 small animals and plants out of the water by means of hanging 

 plates of whalebone or baleen, the slender filaments of which form 

 a sieve from the top to the bottom of the mouth. 



Protozoa cause Disease. — Protozoa of certain kinds play an 

 important part in causing malaria, yellow fever, and other diseases, 

 as we shall see later.^ (See page 217.) 



Reference Books 



elementary 



Hunter, Laboratory Problems in Civic Biology. American Book Company. 

 Davison, Human Body and Health. American Book Company. 

 Jordan, Kellogg and Heath, Animal Studies. D. Appleton and Company. 

 Sharpe, Laboratory Manual, pp. 140-143. American Book Company. 



ADVANCED 



Calkins, The Protozoa. Macmillan Company. 



Jennings, Study of the Lower Organisms. Carnegie Institution Report. 



Parker, LessoTis in Elementary Biology. The Macmillan Company. 



Wilson, The Cell in Development and Inheritance. The Macmillan Company. 



1 Teachers may find it expedient to take up the study of protozoan diseases at 

 this point. 



