MODIFIABILITY OF BEHAVIOR 175 



Sometimes the reversal of the current takes place before the turn- 

 ing away described first ; it may then be followed by the turning away. 

 But usually the two reactions are tried in the order we have given. 



If the Stentor does not get rid of the stimulation in either of the 

 ways just described, it contracts into its tube. In this way it of course 

 escapes the stimulation completely, but at the expense of suspending 

 its activity and losing all opportunity to obtain food. The animal 

 usually remains' in the tube about half a minute, then extends. When 

 its body has reached about two-thirds its original length, the cihary 

 disk begins to unfold and the ciha to act, causing ciirrents of water to 

 reach the disk, as before. 



We have now reached a specially interesting point in the experi- 

 ment. Suppose that the water currents again bring the carmine grains. 

 The stimulus and all the external conditions are the same as they were 

 at the beginning? Will the Stentor behave as it did at the beginning? 

 Will it at first not react, then bend to one side, then reverse the current, 

 then contract, passing anew through the whole series of reactions? 

 Or shall we find that it has become changed by the experiences it has 

 passed through, so that it will now contract again into its tube as soon 

 as stimulated? 



We find the latter to be the case. As soon as the carmine again 

 reaches its disk, it at once contracts again. This may be repeated 

 many times, as often as the particles come to the disk, for ten or fifteen 

 minutes. Now the animal after each contraction stays a Httle longer 

 in the tube than it did at first. Finally it ceases to extend, but contracts 

 repeatedly and violently while still enclosed in its tube. In this way 

 the attachment of its foot to the object on which it is situated is broken, 

 and the animal is free. Now it leaves its tube and swims away. In 

 leaving the tube it may swim forward out of the anterior end of the tube ; 

 but if this brings it into the region of the cloud of carmine, it often forces 

 its way backward through the substance of the tube, and thus gains the 

 outside. Here it swims away, to form a new tube elsewhere. 



While swimming freely after leaving its tube, Stentor shows the 

 characteristic behavior of the free-swimming infusoria, such as Para- 

 mecium. Upon this, therefore, we need not dwell, passing at once to 

 the behavior in becoming reattached and forming a new tube. 



On coming to the surface film of the water, or the surface of solid 

 objects, the free-swimming Stentor behaves in a peculiar way. It 

 applies its partially unfolded disk to the surface and creeps rapidly 

 over it, the ventral side of the body being bent over close to the 

 surface. It may thus creep over a heap of debris, following all the 

 irregularities of the surface rapidly and neatly, seeming to explore it 



