LIFE HISTORY OF DIDINIUM 227 



out, there is no evidence whatsoever of choice of food nor any 

 evidence of chemiotactic guidance of captor to prey. The en- 

 tire process is apparently fortuitous, some one of hundreds of 

 jabs is successful and a Paramecium, once hit, rarely gets away 

 (fig. 1). The victim is partially or wholly paralyzed and is 

 speedily swallowed, the walls of the Didinium being stretched 

 around the prey like a rubber bag. If the Paramecium is seized 

 at or near one end, this end goes in first (fig. 2) until it reaches 

 the extremity of the captor, (fig. 3). It is then doubled on it- 

 self until it lies like a U completely ingested (fig. 4) . The Para- 

 mecium protoplasm becomes highly vacuolated, broken up into 

 small pieces and is quickly digested (fig. 6). If the Paramecium 

 is seized in the middle, this part goes in first and the two ends 

 last, so that the victim is swallowed in the U form. Not only 

 can a small Didinium thus capture and engulf a Paramecium six 

 times its size, as shown by Mast, but it will swallow a dividing 

 Paramecium, and I have frequently watched one attack and 

 swallow a pair of conjugating Paramecium. My imagination 

 has pictured the surprise which such a Didinium might feel 

 when, having completed its usual task, it found itself com- 

 pelled to swallow another equally large meal. In such cases 

 one of the free ends of the two victims is usuallj^ seized; this 

 individual is ingested and the process is continued until the 

 second individual is completely engulfed. It means a little 

 more tension on the part of the elastic walls of the captor but, 

 usually, he is equal to it. Such stuffed individuals are subject, 

 however, to diffluence, especially if transferred shortly after 

 feeding to fresh water, and I have watched more than one 

 individual explode, victims of their gluttony. 



Structure of the proboscis arid seizing organ 



The proboscis of Didinium is a conical projection in the center 

 of the anterior end. It is supported by a dense layer of trichites 

 which are anchored deep in the protoplasm. These are evidently 

 strengthening organs and probably play a part in preventing 

 rupture when a large food body is swallowed, in much the same 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 19, NO. 2 



