BEHAVIOR OF UNICELLULAR ORGANISMS. 107 



backward under any circumstances) the organisms reacted to 

 the chemical stimulus by swimming backward, turning toward 

 the usual structurally defined side, then swimming forward. 

 The swimming backward, of course, sometimes carried the 

 creature away from the densest part of the solution (when the 

 chemical was held in front); at other times, directly toward 

 and into the densest part (when the same chemical was held 

 behind). In the latter case the organisms were frequently 

 killed by swimming into the dense solution. Thus, in chemical 

 stimuli, without exception, the direction of motion after stimu- 

 lation has no relation to the localization of the stimulus. 



In several of the organisms it was possible to use also very 

 precisely localized mechanical stimuli ; and the results so gained 

 tend to modify in some particulars the general conclusions that 

 might be drawn from a study of the action of localized chemical 

 stimuli. Localized mechanical stimuli were produced by touch- 

 ing under a powerful lens any desired part of the body of the 

 organism with a glass rod drawn to the finest hair in a flame. 

 For Paramecium itself this method of experimentation was not 

 satisfactory, owing to the minute size of the cell body. One 

 point of importance was brought out in Paramecium, however. 

 The anterior tip of the body was shown to be incomparably 

 more sensitive than any other part. On bringing the glass hair 

 near the anterior tip, Paramecium leaps backward almost before 

 the hair is seen to have reached it, giving the entire typical 

 reaction already described. Any other part of the body was 

 so insensible that it was not possible to cause a reaction of any 

 sort by touching it with the hair. Paramecium could be pushed 

 about and made to alter its direction of movement mechanically, 

 of course, but there was no active response of any sort when 

 it was touched at any point except the anterior end. 



In Spirostomum ambigiium essentially the same results were 

 reached with mechanical as with chemical stimuli. If any part 

 of the body was touched, whether anterior end, posterior end, 

 or side, the infusorian gave the typical reaction — swimming 

 backward, turning toward the aboral side, then swimming for- 

 ward. A slightly greater percentage of cases of the typical 

 reaction was obtained by touching the anterior end, but the 



