ON THE FEEDING HABITS OF AMEBA 547 



As a rule food cups continue forming as long as the stimulus 

 acts; when the stimulus ceases to act the food cup is withdrawn. 

 A temporarily immobile Phacus triqueter was agitated with a 

 glass needle near a raptorial ameba. A food cup was at once 

 formed about it, but when the needle was removed and agita- 

 tion ceased, the food cup was retracted slowly, although it was 

 about three-fourths completed. This experiment was repeated 

 twice with the same result. It frequently happens that when a 

 food cup is partly formed over an organism, the organism sud- 

 denly escapes. In such cases the formation of the food cup is 

 arrested. But in two or three cases where a raptorial ameba 

 was Hickled' with oscillaria threads and a food cup formed and 

 nearly closed, the alga threads were then quickly removed and 

 the food cup closed up completely. 



On the other hand, even though the stimulus remains exactly 

 the same, food cups formed over the inciting object are occa- 

 sionally not completed; they may be arrested at almost any 

 stage. In these instances a period of quiet of a few seconds 

 usually precedes the withdrawal of the partly formed food cup. 



The amount of water enclosed in a food cup with the food ob- 

 ject varies greatly. The type and condition of the ameba and 

 the nature of the stimulating object seem to be the essential fac- 

 tors concerned. In general raptorial amebas include more water 

 in the food cups than the granular amebas ; hungry amebas and 

 those not having eaten for some time form larger food cups over 

 objects of the same size than those replete with food; actively 

 moving objects, such as organisms, and mechanically agitated 

 particles are eaten with more water than quiet objects. Some 

 objects such as isolated proteins are sometimes ingested appar- 

 ently without any water; sometimes with just a slight amount of 

 water. On other occasions again, the same kinds of objects are 

 ingested in food cups with larger amounts of water. 



All the instances of feeding that have been observed can be 

 arranged in a uniformly graded series, at one end of which would 

 be placed those cases of feeding where very little water is taken 

 in, and at the other end those in which a large amount of water is 

 ingested with the food. In those cases which stand at the middle 



