BEHAVIOR UNDER NATURAL CONDITIONS 185 



there, producing a strong vortex leading to its mouth. This often carries 

 other infusoria, of considerable size, to the mouth. These are then 

 seized and worked gradually back into the internal protoplasm. Some 

 species move about more rapidly and more extensively, while the ciliary 

 vortex is reduced so that it is of little consequence for food getting. On 

 coming in contact with another infusorian the latter is seized by the 

 usually armored mouth ; this is opened widely and the prey is swallowed. 

 In this way such infusoria often feed upon other animals almost or quite 

 as large as themselves, the mouth opening widely and the body becoming 

 greatly distended. 



An excellent example of one of these hunter ciliates is furnished by 

 Didinium. This animal (Fig. 113) is cask-shaped, with a truncate 

 anterior end, bearing in its centre the mouth on 

 a slight elevation. The body bears but two 

 circles of cilia. By the aid of these, Didinium 

 swims about rapidly, revolving to the right on 

 its long axis and frequently changing its direc- 

 tion. On coming in contact with a solid object 

 it stops, pushes forward against the object the 

 conical projection which bears the mouth, and 



. . ,, . , . „, . Fig. 113. — Didinium seiz- 



revolves rapidly on its long axis. The mouth ing p ara mecium. After Bal- 

 is armed with a number of strong ribs ending biani - 

 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. Paramecium often falls a victim to 

 Didinium in this way (Fig. 113). Sometimes the Didinium is smaller 

 than its prey, forming after the feeding process a mere sac over its 

 surface. 



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 sides of the glass vessel, apparently 

 making persevering efforts to pierce the glass. Others "try" water 

 plants, or masses of small algae, 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 



