REACTIONS OF DIDINIUM NASUTUM. I I 5 



stops, pushes forward against the object the conical projection 

 which bears the mouth, and revolves rapidly on its long axis. The 

 mouth is armed with a number of strong ribs ending in points, 

 which apparently project a little from the cone bearing the 

 mouth. When pushed forward against a soft organism, these 

 points apparently pierce and hold it. The revolution on the 

 long axis has the appearance of a process of boring into the 

 body. The mouth now opens widely and swallows the prey. . . . 

 The point which interests us at present is that Didinium reacts 

 in the way described not merely to objects which may serve as 

 food, but also to all sorts of solid bodies. In other words, the 

 process is one of the trial of all sorts of conditions. On coming 

 in contact with a solid, Didinium ' tries ' to pierce and swallow 

 it. If this succeeds, well and good; if it does not, something 

 else is ' tried.' In a culture containing many specimens of 

 Didinium, the author has seen dozens of individuals reacting in 

 this way to the bottom and the sides of the glass vessel, appar- 

 ently making persevering efforts to pierce the glass. Others 

 ' try ' water plants, or masses of small algse, about which many 

 specimens gather at times. Of course they get no food in this 

 way. On coming in contact with each other, the animals react 

 in the same way, often becoming attached to each other, and 

 sometimes forming chains of four or five. But they never 

 succeed in swallowing one another. They often try rotifers in 

 the same way but the outer integument of these organisms is so 

 tough that Didinium does not succeed in piercing it, and the 

 rotifer escapes. S tent or and Spirostomum are often fastened 

 upon, but usually escape, owing to their large size, great activity 

 and rather tough outer covering. The reason why Paramecium 

 is usually employed as food rather than other organisms is 

 clearly due to the fact that when the Didinia try these, they 

 usually succeed in piercing and swallowing them while with most 

 other objects they fail." 



It will thus be seen that Jennings emphasizes three factors 

 in discussing the process of fastening onto the prey : ( i ) the rota- 

 tion of Didinium on its long axis; (2) the size and activity of 

 the organism attacked; (3) the toughness of the outer covering 

 of these organisms. 



