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 Parameecium 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 Paramcecium 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This organ can be protruded and withdrawn, and may be 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 
