Complex One-celled Animals.--In the paramoecium we find a single cell, 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 function seems to be movement; the cilia are located at one end of the cell and serve to create a current of water which will bring food particles 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_. 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 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 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.[26] (See page 217.) Footnote 26: Teachers may find it expedient to take up the study of protozoan diseases at this point. 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, _Lessons in Elementary Biology_. The Macmillan Company. Wilson, _The Cell in Development and Inheritance_. The Macmillan Company.