REACTIONS OF DIDINIUM NASUTUM. 113 



At different times during the progress of this work, Didinia 

 were tested as to their ability to capture various organisms. 

 These organisms may be divided into two groups: (i) those 

 which were captured or attacked and (2) those which were not. 



Group I. Paramccinin caudaiitm. P. aureleus, Colpoda, various sp., Col- 

 pidium, various sp., I'orticella, sp. (?), Frontonia, sp. (?), Nassula, sp. (?). 



Group .'. Stcntor ctrrulens. Euglena, various sp., Spirostomum, various 

 sp., rotifers, various sp., Loxophyllum sp. (?), Paramecium bursaria, Oxytricha, 

 various sp.. Stylonychia, various sp., numerous small flagellates, some identified 

 and some not, Paramccia which had been killed by rapid heating. 



Of those in the first group all but Nassula were at different 

 times seen captured and swallowed by Didinia. Nassula was 

 vigorously attacked, but every time the seizing organ became 

 attached the connection was immediately broken by the discharge 

 of trichocysts. I did not, however, work very much with these 

 organisms. It may be that Didinia can master them under con- 

 ditions different from those to which they were subjected during 

 my observations. It will thus be seen that Didinia feed on 

 various forms. Paramccia, however, seem to be essential to their 

 well-being. I have never been able to get rapid multiplication in 

 their absence. 



CHOICE OF FOOD. 



If organisms from both groups mentioned above are present 

 in a culture at the same time, only those belonging to the first 

 group will be taken. Under such conditions it appears as though 

 the Did in in in had the power of selecting its food, and because 

 of this apparent power of selection Didiiiiitin is frequently re- 

 ferred to in discussions on the choice of food in protozoa. 



In the account of the process of feeding as given by Balbiani 

 (see above, p. 97) it must be assumed that the Didinia can in some 

 way distinguish different protozoa at a distance, for he says that 

 these hunters discharge their darts even before they come in con- 

 tact with the victim. Thon's description (1905, p. 201) also im- 

 plies the ability to distinguish different infusoria, although not at 

 a distance. " Sehr wichtig sind die Beobachtungen cles Zustandes, 

 wenn die Didinien aus Mangel an Paramaecien, gezwungen sind 



