I 14 S. O. MAST. 



die Flagelleten zu jagen." From this quotation it must be con- 

 cluded that the flagellates are taken only in the absence of Para- 

 mecium, never if both forms are present in the same culture. 

 Didinia must, therefore, have some method of distinguishing 

 between them. 



The assumption of choice based upon Balbiani's description 

 of course falls to the ground, since it has been shown (p. 98) 

 that his observations were erroneous, and Thon unfortunately 

 did not ascertain the species to which the flagellates mentioned 

 belong. It is therefore impossible to repeat his experiments. In 

 all the forms on which I worked it was, however, found that 

 there is no apparent selection among the different forms in the 

 first group. All of them, excepting Nassula, were at different 

 times seen captured in the presence of Paramecia. In order to 

 emphasize this point, let me quote from my notes written immedi- 

 ately after the observations were made. " On March 7 Didinia 

 were put into a drop of solution on a slide containing numerous 

 Colpoda and some Paramecia. Scarcely any of these creatures 

 were attacked for an hour after the Didinia were added, although 

 there were frequent collisions. Finally one Colpoda was captured 

 and immediately after this, in the same vicinity, two more and 

 also a Paranieciiuii. It is evident then that Colpoda and various 

 other forms are taken in the presence of Paramecia. This fact, 

 however, does not exclude the possibility of choice even among 

 these. It may be that while various organisms are captured in 

 the presence of Paramecia, the number of these taken is relatively 

 small. Whether this is actually true or not can be ascertained 

 only by statistically comparing the number of successful contacts 

 in the various organisms. It is clear that Didinia come in con- 

 tact with their prey by chance. The question is, does the seizing 

 organ adhere to one relatively more often than to another? 

 However this may be, it is certain that Didinia come in contact 

 with all sorts of objects and devour all such to which the seizing 

 organ will adhere. The question then remains, why does this 

 organ adhere to the surface of certain forms and not to that of 

 others ? 



The following quotation presents Jennings' view (1906, p. 

 185) : " On coming in contact with a solid object it (Didinium) 



