700 RAYMOND l'EAl{Ti. 



ditioiis. As a result of the action of some one or all nf these 

 abo\e-iiieiitioiied factors^ collections or o-rou])s of ])lanai-ians 

 are frequently fomned. 



Planarians which have been injured by operative procedure 

 move comparatively little during the course of regeneration, 

 thus showing a sort of regulation or correlation between 

 behaviour and morphogenetic processes (pp. 573, 574). 



2. There are two principal qualitatively different reactions 

 to stimuli, the positive and negative reactions. 



The negative reaction is given in response to strong 

 unilateral stimulation of the anterior portion of the body. 

 It consists essentially in a turning of the head away from 

 the side stimulated. It is brought about by the extension of 

 the body on the side stimulated. This extension is pi'oduced 

 by a contraction of the circular, dorso-ventral, and transverse 

 systems of muscle-fibres. The purpose of the negative 

 reaction is evidently to get the organism away from harmful 

 stimuli. 



The positive reaction is given only in response to weak uni- 

 lateral stimulation of the anterior portion of the bod}'. It is 

 essentially a turning of the head towards the source of the 

 stimulus. This reaction is one of considerable precision, 

 bringing the anterior end into such a position that it points in 

 most cases exactly towards the source of the stimulus. The 

 turning is brought about by the contraction of the longi- 

 tudinal muscle-fibres of the side stimulated. The evident 

 purpose of the positive reaction is to get the animal into 

 regions of beneficial stimuli. 



3. Whether the negative or the positive reaction shall b(> 

 given in response to a particular stinndus de])ends ])rimarily 

 on the intensity of the stinndus, and secondarily on its loca- 

 tion. Neither reaction is given unless some part of the body 

 in front of the pharyngeal region is stimulated. The negative 

 reaction is given only in response to stimuli above a 

 certain intensity (strong stimuli). This relation between 

 intensity of stimulus and foi'iii (tf reaction holds for both 

 mechanical and chemical stinndi. 



