FEEDING REACTIONS IN DILEPTUS GIGAS. 



119 



In preparation for the experiments described in the following 

 pages, a number of the organisms, usually about fifteen, were 

 transferred with a capillary pipette from the stock-culture to small 

 dishes containing about 5 c.c. of spring water. They were left in 

 this water without food for from 2 to 3 days. During this starva- 

 tion process about ten per cent, usually encysted and the rest 

 diminished considerably in size, became very hungry, and were 

 consequently in excellent condition for observation on the selection 

 of food and the process of feeding. When kept longer, without 

 food, they continued to diminish in size until in the course of a 

 week they were reduced to less than one tenth of their former 

 length and probably to less than one one-hundredth of their former 

 volume, after this they disintegrated unless this was prevented by 

 opportune feeding. 



All the experiments on the selection of food were conducted as 

 follows : A designated number of starved dilepti, usually fifteen, 

 were placed in a small watch-glass with about 3 c.c. of spring water. 

 A drop of a concentrated suspension of the substance to be tested 

 was then added and the whole thoroughly mixed. The feeding 

 reactions of some few of the specimens were carefully observed 

 for longer or shorter periods of time. At the end of twenty min- 

 utes all the dilepti were taken out of this medium by means of a 

 small pipette and introduced into a like volume of spring water. 

 They were then observed, either individually or two at a time, on 

 a slide under a magnification of about 300 diameters, and the num- 

 ber of vacuoles in each was carefully counted. In this way various 

 living organisms and numerous inanimate particles were tested. 



The protoplasm of Dilcptus is quite opaque under ordinary con- 

 ditions of culture. Occasionally a culture was obtained in which 

 the organisms were relatively transparent and it was only with such 

 material that the feeding experiments were made, for even under 

 optimum conditions there was some difficulty in seeing precisely 

 what passed into the body unless it was specifically colored as, e.g., 

 carmine or india ink. Another factor which favored observation 

 on the amount of food ingested was the characteristic large size 

 of the first vacuoles formed, whenever an organism was introduced 

 into a new medium, especially after a period of starvation. Such 

 large vacuoles are very specific. 



