584 RAYMOND PEARL. 



ical sfciraulation of tlie head. Stimuli strongs enong'h to be far 

 above the usual upper limiual value for this reaction will call 

 it forth. Such specimens show the reaction in a much more 

 pronounced type than is usually the case. After a stimulus 

 lias been given they will turn towards it, and if the source is 

 not touched immediately they will remain in the same spot 

 waving the head about the region from which the stimulus 

 came, at the same time stretcliing the anterior end of the 

 body far out in all directions, precisely as if in search of the 

 stimulating body. Usually this hypei'sensitive condition 

 passes off in a short time, and the animals behave again in a 

 more normal fashion. It was thought that possibly this 

 condition was due to hunger, but experiments^ devised to 

 test this question indicated that this was not the case. 

 We can only say that it is due to some intimate physio- 

 logical condition, the exact nature of which we do not know. 

 Another fact which may be mentioned in this connection is 

 that sometimes a specimen in normal condition will give the 

 positive reactioti in response to a certain strength of stimulus 

 only a part of the time. Other trials I'esult in entire indiffer- 

 ence on the part of the organism. Of course, it is not 

 possible to give mechanical stimuli always of the same 

 strength, yet with the closest possible approximation to this 

 by an experienced operator, some of the trials will not affect 

 the animal in any way except to cause a slight local con- 

 traction at the point on the head stimulated. The worm 

 glides along without any change in rate or direction. 

 Altogether we must conchide that the reaction is one which 

 is very closely dependent on the existence of certain definite 

 internal conditions as well as the external ones. 



The typical course of the reaction is, as has been described, 

 first a momentary pause, followed by a turning of the head 

 towards the stimulus, accompanied by a raising of the ante- 

 rior part of the body. From this typical form of the reaction 

 there are many variations. The raising of the anterior end 

 from the bottom just before and during the time it is being 

 ' See section on " Reactions to Fooii and Chemicals." 



