616 RAYMOND PEARL. 



The reason why the specimen in this last expei'iment does 

 not turn towards the stimulus when stimulated on the side x, 

 is apparently because the regeneration has proceeded only far 

 enough to produce just enough new tissue to form the 

 beginning of a new side to the body. This new side receives 

 the stimulus and is sufficiently potent to determine the re- 

 action of the whole (the straight longitudinal contraction), 

 but is lacking in the mechanism necessai-y to produce its own 

 proper reaction, the negative reaction. On the other hand, 

 in the case of the individual with the split anterior end, each 

 piece turns towards the stimulus after stimulation of the cut 

 edge because here only one half the organism is present 

 either to be stimulated or to react; there is not even the 

 beginning of the formation of a new side along the cut edge. 



Putting all the evidence together, I think it must be re- 

 garded as demonstrated that the turning away from the 

 stimulus in the negative reaction to mechanical stimuli is due 

 to an extension of the side of the body stimulated. This 

 extension is brought about by the contraction of the circular 

 and dorso -ventral muscle-fibres — probably also assisted by 

 the transverse and oblique systems of fibres — in the region 

 stimulated. This reaction is a simple reflex act involving 

 only the side stimulated. The normal organism, so far as this 

 response is concerned, is to be considered as composed of two 

 identical, but in a certain sense independent longitudinal 

 halves. Thus, representing these halves diagrammatically, 

 as in Pig. 23, a, the evidence presented indicates that stimula- 

 tion of one side of the worm, as A, causes a reaction in that 

 side, and, so far as essential features of the directive reactions 

 go, only in that side. The movements of half A after its 

 stimulation determine and, in fact, cause the reaction of the 

 whole animal. Furthermore, these longitudinal halves retain 

 their individuality as halves if they are isolated from each 

 other. A separated half-worm (longitudinal) reacts as a 

 half-worm, just as it did when in connection with the other 

 half in the body, and not, as might perhaps be expected on 

 a priori grounds, as a whole worm. It reacts as a whole 



