THE PROTOZOA 



195 



the waste material. The single cell is, in fact, an organism able 

 to carry on the business of living as effectually as a very complex 

 animal. 



Complex One-celled Animals. — Iq the paramoecium we find a 

 single ceU, but we find certain parts of the cell having certain definite 

 functions : the cilia are used for locomotion ; a definite part of the cell takes 

 in food, while the waste passes 

 out at another definite spot. 

 In another one-celled animal 

 called vorticella, part of the 

 cell has become elongated 

 and is contractile. By this 

 stalk the little animal is 

 fastened to a water plant 

 or other object. The stalk 

 may be said to act like a 

 muscle fiber, as its sole func- 

 tion seems to be movement ; 

 the ciUa are located at one 

 end of the cell and serve to 

 create a current of water 

 which will bring food par- 

 ticles to the mouth. Here 

 we have several parts of the 

 cell each doing a different 

 kind of work. This is known 

 as physiological division of 

 labor. 



Photograph of a living bell animalcule (vorti- 

 cella) enlarged two hundred diameters. Notice 

 the contractile stalk and the circle of cilia 

 about the mouth. 



Habitat of Protozoa. — Protozoa are found almost everywhere 

 in shallow water, seemingly never at any great depth. They ap- 

 pear to be attracted near to the surface by light and the supply of 

 oxygen. Every fresh-water lake swarms v.ith them ; the ocean con- 

 tains countless myriads 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 fisli have on cacli 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 



