CHOICE OF FOOD IN AMEBA 249 



gestible material (carmine) are refused to a much greater extent 

 when food is present than when absent. Large particles of 

 glass, sulphur, starch, are consistently refused. 



The experimental evidence makes it highly probable that 

 selection is based upon the physical (tactual) properties of the 

 substances (Schaeffer, '10). 



As compared with Paramecium it is true that stentor is 

 capable of much nicer discrimination in the laboratory tests, 

 but it is probable that in natural surroundings the methods of 

 Paramecium in food selection are quite as well fitted as stentor' s 

 for successful living, if not more so. 



Amoeba proteus, A. dubia, A. discoides. Ameba feeds on large 

 solid particles, living or dead; protozoa, protophyta, rotifers, 

 zoospores, masses of zooglea, dead masses of protoplasm, etc. 

 Whether ameba takes in liquid food is not known. 



Usually only one or two particles are eaten at a time. Several 

 particles of such forms as urocentrum, Paramecium, desmids, 

 may be sufficient for a day's feeding; or if the organisms are 

 small, such as chilomonas, several hundred are eaten. There 

 is therefore sufficient time for selection in every case. 



Ameba eats normally only digestible matter. The degree of 

 precision of selection in ameba compares favorably with that 

 in stentor. 



Ameba still possesses the power of selecting its food. 



Selection is based, in many cases, upon the physical properties 

 of objects such as, for example, movement. 



Ameba rejects an undesirable object by moving away after 

 coming into contact with it; or by not moving toward it if the 

 stimulus, such as very violent movement, is too intense. 



Comparing the processes of choice in food in these three 

 protozoans, we observe that in Paramecium an enormous number 

 of particles must be eaten daily to sustain life, so many particles 

 in fact that a process of selection could not be applied to each 

 individual particle. And experiment shows that selection of 

 food is almost completely suspended in this organism. Stentor 

 feeds on large particles as well as on small ones, such as bacteria. 

 Stentor can live readily on a few large particles eaten daily, 

 and there is abundant time for careful discrimination. Experi- 

 ment shows that stentor is capable of very nice discrimination. 

 Ameba eats only large particles and in this organism also very 

 nice discrimination is exercised. In all these protozoans selection 



