586 EAYMOND PEARL. 



its way along the bottom. This behaviour when the animal 

 is able to reach the stimulating- object is evidently the action 

 Avhicli most frequently occurs in natural environmental con- 

 ditions. 



With reg'ard to the localisation of the stimulus producing" 

 this reaction, I may say that I have been able to produce it 

 by proper stimulation of any part of tlie edge or dorsal 

 surface of the head region under favourable circumstances. 

 It seems to be moi-e certainly produced — that is, in a larger 

 number of cases — by stimulation of the auricles than of any 

 other part of the head, and it may be that in this is to be 

 found an indication of the chief function of these sense-organ 

 bearing structures. At any rate, this is the only indication 

 of a special function for them which I have been able to 

 discover. The positive reaction given in response to light 

 stimulation of the dorsal surface of the head is necessarily 

 somewhat different from the typical reaction which has been 

 described. In this case there can be no turning towards one 

 side, because if this were done the liead would not be directed 

 towards the source of the stimulus. Instead, what takes 

 place is this : the head is sharply raised and twisted, so as 

 to form a part of a spiral in the region posterior to the head. 

 This brings the anterior end into a position pointing towards 

 the source of the stimulus, and at the same time the ventral 

 surface is brought around so as to be, in most instances, the 

 tirst portion of the body to touch the stimulating point. 

 This reaction, following stimulation of the dorsal surface of 

 the head, is not an easy one to obtain. I have succeeded 

 best in producing it in the case of individuals in the hyper- 

 sensitive condition mentioned above. 



With regard to the strength of the stimulus necessary to 

 call forth the positive reaction, only very relative statements 

 may be made. Unfortunately Ave have no method of measur- 

 ing the intensity of such weak mechanical stimuli as are 

 used in work on lower organisms. Our only idea of the 

 strength of the stimulus must come from the reaction of the 

 organism itself. It must suffice to say, regarding the reaction 



