DIGESTION OF THE AMCEBA 



147 



While the methods of food taking of one-celled animals 

 differs greatly, some engulfing the food like the amoeba, 

 others having a distinct opening or mouth for the purpose, 

 and rows of ciUa to direct the water currents toward the 

 mouth, the digestion proper is practically similar in method 

 in all. The amoeba will serve for a type. 



Digestion of the amoeba. — With the ingested food is 

 always taken in some water which collects about the food 

 and forms a food bubble (food vacuole) in the cytoplasm. 

 Soon the cytoplasm around the food vacuole begins to 

 pour into it a secretion which contains hydrochloric acid 

 and probably an enzyme. This secretion is therefore a 

 sort of gastric juice. At the same time the vacuole is 

 kept in motion by the streaming of the cytoplasm. Under 

 the combined actions of this secretion and movement, the 

 solid particle is broken up and dissolved and is then ready 

 to be assimilated directly by the cytoplasm. In this way 

 new protoplasm is built up, and the animal increases in 

 size and strength. 



In certain one-celled forms the food vacuoles move in a per- 

 fectly definite path (see 

 Fig. 49) through the cy- /cy- 



toplasm, and this move- 

 ment probably serves 

 to distribute the digested 

 food uniformly, as well as 

 to break it up. In such 

 cases the resemblance to 

 a primitive circulatory 

 system is evident. 



Another way in which these one-celled forms differ is 

 in the different nutrients which they ar-e able to dissolve. 



Fig. 49 — Path of food particles in a. para- 

 moecium ; TV, nucleus ; jl/, mouth ; I'. K., 

 food vacuoles. Note the decrease in size 

 of the food vacuoles as the food is digested. 



