THE BEHAVIOR OF PROTOZOA 77 



distance, then settles against the paper and remains. After 

 remaining a few seconds, it may move to another position, 

 still remaining in contact with the paper. Then it may 

 leave the paper and go on its way. All this may happen 

 without the slightest evident change in the outer conditions. 

 So far as can be seen, the Paramcecium first responds to the 

 solid by the avoiding reaction, later by the positive contact 

 reaction, and still later suspends the contact reaction, all with- 

 out any change in external conditions. The changes inducing 

 the change in reaction must then be within the animal." 



The behavior of Paramoecium is quite typical for infusor- 

 ians in general, but different forms present some interesting 

 modifications. What Maupas has called the ' 'hunter ciliates" 

 show a more highly developed behavior in taking food, as 

 they not only exhibit a power of selection of certain kinds of 

 food, but have a remarkable power of engulfing large objects. 

 One of those whose habits are the best known is Didinium 

 nasutum (Stein). The body is in the shape of a barrel; 

 at the middle of the anterior end is a small projection where 

 the mouth is located. The mouth, which is usually kept 

 closed, leads to a pharynx lined with chitinous rods which 

 can act as a sort of "seizing organ," in the capture of prey. 

 This organ can be protruded and withdrawn, and may be 

 opened out to a remarkable degree. 



When Didinium in the course of its swimming comes in 

 contact with a Paramcecium or other small organism, the 

 seizing organ is shot forth into the body of the prey. The 

 seizing organ is then drawn in and the mouth spreads open 

 to receive the prey which is gradually pulled into the body 

 of the Didinium. 



The swallowing capacity of Didinium is almost incredible. 

 Mast observed one of these infusorians which swallowed a 

 Paramcecium ten times as large as its captor, the latter 



