ONE-CELLED ANIMALS— PHYLUM PROTOZOA 87 



of the cells in your own body move by this method. These are the white 

 blood cells which aid in the continual battle against the germs of infec- 

 tion. Some of these cells move through the tissues of your body in this 

 manner ready to engulf any disease germs which may gain entrance. 



Pseudopodia function in food getting as well as in locomotion. The 

 amoeba obtains its nourishment from small plants and animals that share 

 its habitat as well as from decaying bits of organic matter. The food 

 is ingested by the interesting process of engulfing. Sometimes this can 

 be seen under the microscope. The amoeba will approach the food, 

 throw out a pseudopodium on either side of it, and then just flow around 

 the food until it is inside the body. There is nothing like a mouth, or 

 any certain place where the food must enter — it can be taken in at any 

 point. 



Although the food is now in the cell, the amoeba's problems of nour- 

 ishment are by no means solved. We learned in Chapter 5 that food 



Fig. 7.2. Method of ingestion and the formation of a food vacuole in an amoeba. 



must be digested before it is in a form which can be used. After the 

 food of an amoeba is ingested it floats around in the cytoplasm sur- 

 rounded by a thin membrane. This is known as a food vacuole, and it 

 might be thought of as a temporary stomach in which digestion occurs. 

 Enzymes diffuse into this vacuole and break the food down into soluble 

 substances which can be utilized by the cell. After digestion there will 

 remain an indigestible residue which must be eliminated. This is done 

 by egestion, which is like ingestion in reverse. When digestion is com- 

 pleted the vacuole will move to the outer edge of the cell and burst 

 through the plasma membrane, carrying the waste out. 



Digested food can be used to build protoplasm within the amoeba 

 and thus bring about growth through the anabolic phase of metabolism. 

 The life processes of the organism require energy, and some of the food 

 must be oxidized in the catabolic phase of metabolism to provide this 

 energy. The oxygen which is necessary for this reaction diffuses 

 through the plasma membrane from the surrounding water. There 

 will be some dissolved oxygen in the water which supplies this need. 

 (Boiling drives dissolved oxygen from water and amoebae will die rather 



