ON E-CELLED ANIMALS— PHYLUM PROTOZOA 85 



turn, may be eaten by still larger forms and so on. At the top of such a 

 food chain we find many fish and other water animals that are an important 

 source of food for man. Also, we have obtained much valuable information 

 through research with the protozoa. There is one in particular, Tetra- 

 hymena gcleii, which has come into prominent use in recent years. Many 

 of the nutritional requirements of this protozoan are the same as those of 

 human beings and it is valuable, therefore, in nutrition research. The role 

 of various foods, the effects of poisons, etc., have been worked out on Tetra- 

 hymena. In Chapter 1 we showed how it has been of value in cancer re- 

 search. Not all the protozoans are of benefit to man, however. On the 

 debit side of the economic ledger we find that some of the protozoans have 

 accommodated themselves to a parasitic existence in man and other animals 

 and cause tremendous destruction of life and health. Some of the most 

 serious human diseases, such as malaria and amoebic dysentery, result from 

 protozoan infections, and the control of such diseases is one of our great- 

 est medical problems today. 



Protozoa with False Feet — The Sarcodina 



Protozoans that fall in the class Sarcodina are characterized by the 

 presence of false feet, pseudopodia, which are projections that are pushed 

 from the body when needed and withdrawn after they have served their 

 function. Any part of the body may be extruded to become a foot and, 

 when withdrawn, it loses its identity as a foot. There is a genus known 

 as Amoeba which serves as a good example of this group of protozoans. 

 Amoeba proteus is the species most commonly studied in the laboratory, 

 but our descriptions apply generally for the genus as a whole. Amoebae 

 may be found most abundantly at the bottom of fresh-water ponds and 

 streams among the debris formed there by decaying vegetation that set- 

 tles from the water. 



When placed under the microscope, we can see that an amoeba is a 

 naked bit of protoplasm surrounded only by its thin plasma membrane. 

 It has no definite shape and is constantly changing as it adjusts to its 

 surroundings. It moves by pushing out a slender pseudopodium in the 

 direction in which it is going. Then the rest of the body flows into the 

 pseudopodium. This movement is made possible by changes in the sol- 

 gel relationship of the protoplasm in different parts of the cell. The 

 inner portion of the protoplasm is more fluid than the outer portion, and 

 a flow occurs when a weakness develops in the outer portion. To illus- 

 trate, let us think of an amoeba at rest; all pseudopodia are withdrawn 

 and the animal assumes a spherical shape. Then a weakness develops 

 in the thicker outer protoplasm, thinner protoplasm from the center flows 



