

8 MISCELLANEOUS PUBLICATION 955, U.S. DEPT. OF AGRICULTURE 



division occurs so that several daughter cells result. Sexual repro- 

 duction also occurs. In some, gametes fuse to form a single cell or 

 zygote. In others, two individuals conjugate for the purpose of ex- 

 changing nuclear material. 



Protozoa are extremely sensitive to slight changes of temperature, 

 light, acidity, chemical composition of the water, and kinds and 

 amounts of food. Some protozoa apparently cannot live unless their 

 food consists of bacteria or other minute protozoa. If a small amount 

 of hay is placed in a beaker of water and kept in the laboratory, a 

 whole progression of protozoa species will be observed. Those seen 

 active during the first few days will disappear completely in a week 

 or so. 



When one collects protozoa, practically every source of water, soil, 

 and organic material will vary in the kinds and number of species, 

 and these will change radically even in a single collecting location 

 depending on the season of the year. Some flagellates have an absolute 

 symbiotic relationship to other animals. Thus the termite when freed 

 of its intestinal flagellates dies even when its gut is filled with wood 

 chips. When the flagellates are removed from the termite intestine, 

 they likewise die. 



Protozoa offer one advantage over algae in pure culture work — light 

 is not required for growth. Cell for cell, protozoa have more complex 

 cellular structures than any other group of micro-organisms, as seen 

 with the light microscope. Protozoa cells may contain definite points 

 in which food is ingested — contractile vacuoles, food vacuoles, several 

 nuclei, and eye spots. 



Protozoa are often divided into several classes based on the mode 

 of locomotion. The first of these is the Mastigophora, or protozoa 

 that possess one or more flagella. Some of the forms in this class 

 have plantlike characteristics, like chlorophyll, and some can carry 

 out photosynthesis in the presence of light, but when light is with- 

 drawn, they may carry on a saprophytic nutrition as colorless protozoa 

 do. 



The second class is Sarcodina, or amebae that lack a definitive pel- 

 licle or cell covering and can form pseudopodia. Their principal 

 means of classification is based on the types of pseudopodia they form. 



The third class is Sporozoa, in which the protozoa without exception 

 are parasitic and form spores in their development. Their hosts are 

 scattered throughout the animal kingdom. 



The remaining protozoa are forms that possess cilia sometime during 

 their life. In the class Ciliata, cilia are present throughout the entire 

 life cycle. Their classification is based on the nature of the nuclei 

 and on the arrangement of cilia. The second class of protozoa with 

 cilia is the Suctoria. At maturity they do not have cilia. 



Apart from their use for biological studies, protozoa are important 

 in reducing bacteria in soil and sewage. Many protozoa are eaten 

 by microscopic animals and small fish, which in turn become part of 

 the food chain for larger animals. Still other protozoa because of 

 their skeletons are sources of diatomaceous earth. Some of the para- 

 sitic forms cause serious diseases in man and animals, such as amebic 

 dysentery and trypanosomiasis. 



