108 BIOLOGICAL LECTURES. 



difference was little ; it varied in Spirostoma from different 

 cultures. 



In the other organisms on which the effects of localized 

 mechanical stimuli were tried, particularly Loxodes rostrum, 

 Dileptus anser, Oxytricha fallax, and one or two other Hypo- 

 tricha, the following results were obtained: (i) The side 

 toward which the animal turns after a stimulus is entirely inde- 

 pendent of the side which is touched. In every case the organ- 

 ism turns toward one structurally defined side. If that is the 

 side which is touched, the organism turns continually toward 

 the source of stimulus, no matter how many times the latter is 

 repeated ; if the other side is touched, the creature of course 

 turns away from the source of stimulus. The impression is 

 given that it is physiologically impossible for the organism 

 to turn otherwise than toward this one side. (2) But the for- 

 ward or backward movement of the animals after a stimulus is 

 not thus independent of the localization of the stimulus. If the 

 anterior end is touched, the organism darts backward, turns 

 toward one side, then swims forward. The posterior half of 

 the body is very insensible, so that as a rule there is no response 

 to a mechanical stimulus occurring here. If, howeve-r, a strong 

 stimulus is given here, as by thrusting the tip of the rod 

 strongly against the resting animal, the latter simply swims 

 forward; if already swimming forward, it merely hastens its 

 forward motion when thus stimulated. 



Thus, in the case of mechanical stimuli in these organisms 

 the direction of motion after a stimulus depends, to a certain 

 extent, so far as backward or forward motion is concerned, upon 

 the localization of the stimulus. This introduces a greater com- 

 plexity into the psychology of these creatures than the results on 

 Paramecium alone, or on the reactions to chemical stimuli alone 

 would lead us to judge. The organisms do in certain respects 

 react with reference to the localization of a stimulus affecting 

 them. The differing results gained with chemical stimuli are 

 probably to be interpreted, in view of the facts shown by a study 

 of mechanical stimuli, as follows : When a chemical diffuses 

 from a point lying behind the infusorian, it of course comes 

 first in contact, as a very weak solution, with the posterior end 



