MOVEMENTS, ETC., OF FREST[-WATER PLANARIAXS. 603 



piece A, containing tlie bi'ain, the results are entirely similar 

 to those obtained in case of tlie normal animal. Stron<if 

 unilateral stimulation of the head causes the negative 

 I'eaction, weak stimulation of the same sort the positive 

 reaction. Stimulation at the posterior end causes the crawl- 

 ing movement to appear, and altogether the appearances are 

 essentially the same as in the normal complete specimen. 



The posterior piece B (lacking the brain) behaves in a 

 somewhat different manner. If the anterior end of this piece 

 is given a stimulus of moderate intensity anywhere on the 

 cut surface the piece will usually start crawling straight 

 backwards. This is almost always true for a short time 

 after the operation, and is especially well shown in such 

 specimens as started crawling backwards as a result of the 

 cut. AVhen from twenty-four to forty-eight hours have 

 elapsed after the operation this tendency of posterior pieces 



Fig, 1G.— Operation diagram. Heavy line indicates cut. 



to crawl backward on stimulation of the anterior end begins 

 to grow less marked, and, as regeneration proceeds, finally 

 disappears. In many such posterior pieces I have been able 

 to produce this backward crawling in a very pronounced 

 form, and of comparatively long duration (three or four 

 minutes at a time). The chai*acter of the movement has 

 been described above. If the stimulus is applied to one side 

 or the other of the anterior end of such a posterior piece, 

 instead of squarely against the cut surface, a well-marked 

 negative reaction is produced ; that is, the anterior end 

 turns away from the stimulus just as a whole animal would. 

 The reaction is very definite, and of precisely the same 

 character as the normal negative reaction. The only 

 difference to be observed is that in proportion to the strength 

 of the stimulus the reaction is not so pronounced as in the 



