6G0 RAYMOND PEARL. 



to take up all of tliem in this paper, a few of the specially 

 important ones may be considered. 



One such important general question which arises is the 

 problem of orientation to diffusing chemicals. Do planarians 

 orient themselves along radial lines of diffusion and proceed 

 towards the centre of diffusion ? It would seem that in the 

 case of such a perfectly bilaterally symmetrical organism as 

 PI an aria, if anywhere, Loeb's theory of orientation ought to 

 hold good. This theory accounts for orientation by sup- 

 posing that when an organism is stimulated unilaterally its 

 motor organs are caused to act either more strongly or more 

 weakly, as the case may be, on that side than on the other. 

 This results in bringing the long axis of the body parallel 

 with the lines of action of the stimulus; and then, since 

 symmetrical points on either side of the body must be equally 

 stimulated, the organism moves in a straight line towards or 

 away from the stimulus. Jennings has shown (: 01) that for 

 most stimuli this theory of orientation does not hold in the 

 case of the Infusoria. 



From the account of the reactions of planarians to 

 chemical stimuli which has been given, it will be at once 

 seen that there is in this case, to some degree at least, 

 an orienting reaction. With weak chemical stimuli the 

 head turns towards the stimulus in such a way as to point 

 the anterior end very directly towards the source of stimula- 

 tion. It might be thought that this marked a pure orienta- 

 tion, but it must be remembered that the organisms turn the 

 head just as precisely towards the point from which a weak 

 mechanical stimulus comes. The two reactions are evidently 

 exactly the same thing. However, a single mechanical 

 stimulus can hardly be considered a directive stimulus of 

 the sort which induces an orientation, such as, for example, 

 the electric current. The orientation of unicellular organisms 

 to the constant current is the purest type of an orienting 

 response, however, and the most characteristic thing about it 

 is that the organism, after having the anterior end turned 

 towards one of the poles, keeps the long axis of the 



