THE SARCODINA 159 



their selection of food. A wide variety of microscopic organisms 

 may be utilized, and even manj^-celled animals and plants, if they 

 are small enough for the amoeba to lay hold upon, may serve as its 

 food. Under ordinary laboratory conditions, unicellular and 

 other minute green plants among the fresh-water algse are com- 

 mon food for certain species of amoebas and are often seen within 

 the cell in various stages of digestion. Am(Eha proteus normally 

 feeds upon such green plant cells and also upon motile prey such 

 as ciliate or flagellate protozoa and the minute multicellular animals 

 known as rotifers (Fig. 88). As with amoeboid movement, 

 investigators have made repeated attempts to reduce the process of 

 ingestion to a basis that can be imitated by non-living bodies, as 

 when a drop of chloroform is placed in a watchglass of water 

 and made to '' ingest " bits of shellac or paraffin. The majority of 

 observations, however, indicate that the feeding reactions are com- 

 plex and variable according to the nature of the prey and the state 

 of the amoeba. It is found that food bodies that are motionless, 

 like many of the green plant cells, call forth reactions that differ 

 from those induced by active prey, like cilates and flagellates. 

 Moreover, the two types of reaction are not fixed, but each varies 

 with the particular conditions. The reactions, like those of higher 

 animals, tend to be " qualitative " and " in the interests of " the 

 reacting amoeba in a way that does not seem to occur in non-living 

 bodies. Amoebas have been carefully studied with reference to 

 other forms of behavior, such as their responses to light, to con- 

 tact, and to chemicals. These reactions cannot be discussed here, 

 but it may be said in general that the behavior is not so simple as 

 might be supposed. 



Metabolic Processes. — In common with other animal bodies, 

 the unicellular amoeba carries on the activities related to waste, 

 repair, and growth in a living organism. Food is ingested, digested, 

 and assimilated. Waste products resulting from dissimilation are 

 excreted. At first glance, these metabolic processes in the protozoa 

 seem much simpler than the complex series of events that have 

 been described under metabolism in a vertebrate animal (p. 71). 

 In reaUty, the metabolism of the amoeba is fundamentally the same 

 as that of the many-celled organism when its essential features are 

 alone considered. The amoeba ingests the smaller organisms which 

 serve as food, it digests this food within its cytoplasm, and then 

 assimilates the fluid products of digestion in the same manner that 



