THE BEHAVIOR OF OTHER INFUSORIA 117 



2. REACTION TO MECHANICAL STIMULI 



In the responses of infusoria to contact with solid objects we may- 

 distinguish the same two reaction types that we found in Paramecium. 

 The animal may react in what might be called a "negative" way, avoid- 

 ing the object, or it may react "positively," placing itself against the 

 soKd body. 



The negative response to contact with sohd bodies is the typical 

 "avoiding reaction." The animal moves backward, turns toward a cer- 

 tain definite side, then swims forward again. In other words, it tries 

 a new direction. If this leads again against the obstacle, the animal 

 again reacts in the same way, and this is repeated, till through frequent 

 trials the obstacle is avoided. 



There are certain important points regarding the relation of the direc- 

 tion of movement in this avoiding reaction to the part of the body that is 

 stimulated. Since the animal usually swims forward under natural con- 

 ditions, it will as a rule come in contact with large solid objects at its 

 anterior end. Further, small objects may be carried by the ciliary cur- 

 rents to the oral side. Thus the movement backward and the turning 

 toward the aboral side in the avoiding reaction remove the animal from 

 the source of stimulation. But experimentally other parts of the body 

 can be stimulated. Thus in Oxytricha (Fig. 79), we may with the tip 

 of a fine glass rod stimulate either the left (oral), or the right (aboral), 

 side. In either case the animal backs and turns to the right. If the 

 right side is repeatedly stimulated, the animal continually wheels toward 

 the stimulated side; if the left side is touched, it wheels continually 

 away from the stimulated side. Thus the direction of movement in the 

 reaction is not determined by the side stimulated, but by the structural 

 relations of the organism. On the other hand, if we stimulate the 

 posterior end sharply, the animal does not respond by the typical avoid- 

 ing reaction, but simply runs forward. The direction of movement is in 

 this case determined by the part stimulated. These results have been 

 found to hold also in many other infusoria. 



Experiments of the kind just described have shown that the anterior 

 end is as a rule much more sensitive than the remainder of the body 

 surface. A light touch, having no effect at the posterior end, produces 

 a strong reaction when applied to the anterior end. 



It is a general rule that unlocalized mechanical stimuli, such as are 

 produced by jarring the vessel containing the animals, have the same 

 effect as stimuU applied to the anterior end; they induce the avoiding 

 reaction. 



In the positive contact reaction, the animal places itself in contact 



