FEEDING REACTIONS IN DILEPTUS GIGAS. 125 



By comparing these results with those on inanimate substances 

 as shown in Tables I., II., and III. it will be noted at once that 

 the living material was ingested by a much greater percentage of 

 the dilepti tested, and also that the average number of vacuoles 

 formed during the twenty minutes of the experiment was almost 

 three times as great in the experiments on living- material as in those 

 on inanimate substances. It will also be noted that when feeding 

 on inanimate substances Dilcptus tends to stop feeding after having 

 formed one vacuole, but that when it is feeding on animate sub- 

 stances it does not. This clearly indicates some power of selection, 

 for Dilcptus refuses to take in useless materials while it ingests 

 nutritive substances in large amounts. 



Having thus observed that Dilcptus can select between animate 

 and inanimate substances, the question naturally arose as to whether 

 there is any choice between different kinds of organisms. Many 

 species of organisms were used in attempting to answer this ques- 

 tion. The same methods were used as described above, but only 

 the results obtained in observations on the actual process of feeding 

 were recorded. 



The results obtained in these observation, in so far as they per- 

 tain to the problem of selection, are briefly summarized in Table 

 VII. By referring to this table it will be seen that Dilcptus does 



TABLE VII. 



DISCRIMINATION BETWEEN DIFFERENT ORGANISMS. 



Table giving results of feeding tests with Dileptus showing selection among 

 living organisms. 



in. 



i. n. Organisms 



Organisms Organisms Never Captured or Injured 



Readily Captured. Captured only Rarely. in Any Way 



Euglena gracilis( ?) Paramecium aurelia Paramecinm candatum 



Trachelmonas Frontonia 1 Frontonia 1 



Amoeba Rotifers Euplotes 



Halteria Stylonyclna Nassiila 



Urocentrum turbo Spirostomum 



Chilomonas paramcciitm 

 Colpidium 

 Colpoda 



Stentor cccruleus 

 Stentor polymorphns 



1 Two races of Frontonia were used, the one being entirely immune to the 

 attacks of Dileptus, and the other only partially. 



