REACTIONS OF DIDINIUM NASUTUM. 99 



begins to rotate in harmony with the Didinimn, is carried off and 

 on casual observation it appears to be dead or at least thoroughly 

 paralyzed. If, however, the Paramecimn is very much larger 

 than the Didinimn the result is quite different. 



During the first part of my work on this subject, I had a race 

 of Paramecia, none of which were much more than twice as 

 large as the Didinia. After studying the food reactions with 

 these specimens and seeing many Paramecia captured and swal- 

 lowed without any apparent struggle and several, which escaped, 

 die immediately, I was of the opinion that Thon (1905, p. 296) 

 is correct in his statement that the Paramecia are poisoned and 

 suddenly killed. 



Later, however, there appeared in one of the cultures a race of 

 Paramecia which were several times as large as those previously 

 worked on (see Fig. 6, p. 97), and by selecting Didinia which had 

 been without food for a few days, it was a simple matter to 

 find specimens not more than one sixth to one tenth as large as 

 these Paramecia. These minute Didinia attack the huge Para- 

 mecia as vigorously as they do the smaller ones. In such an 

 attack, however, the scene of action and the result are quite 

 different from that described above. The Paramecimn attacked 

 does not become quiet, it makes a vigorous struggle and fre- 

 quently succeeds in throwing off the enemy by breaking the 

 attachment of the seizing organ. I shall discuss the significance 

 of this process later (p. 103). The Paramecium thus almost in- 

 variably escapes after the first attack and I have frequently seen 

 two and sometimes three Didinia successively thrown off before 

 one finally remained attached. And even after one does succeed 

 in this, it merely momentarily checks the movement of the prey. 

 There is no indication of paralysis. The Paranieciitin attacked 

 continues to swim about, carrying the Didinimn with it. A large 

 specimen studied was thus seen to swim about with a small 

 Didinimn fastened to its sides for over four minutes, when it was 

 attacked by a second individual, badly wounded and killed. The 

 first individual had swallowed about one fifth of the Paramecium 

 before it died, and the second had drawn in a small portion. 

 Both then continued the swallowing process until they had sur- 

 rounded a large proportion of the body, when the ectosarc gave 



