698 I^AYMOND rEARL. 



more firmly to the substrate. Weaker currents caused no 

 effect wliatever. I was inclined to believe that the lono-jtu- 

 dinal contraction and the gripping" of the bottom Avere tlie 

 only rheotactic reactions which the organism exhibited. It 

 was found later, however, that there was a very precise 

 rheotactic reaction of a different character. In the course 

 of the experiments on reactions to localised chemical stimuli 

 by the capillary tube method, it was discovered that by using 

 a tube with a relatively large opening (from j to 4 mm. in 

 diameter) and letting the ordinarj^ tap-water in which the 

 animals were flow out of it, by its own weight, a current of 

 just the right intensity to cause a positive reaction could be 

 produced. The animals would turn very sharply towards the 

 source of such a current, the reaction being evidently the 

 same as that given to other weak stimuli (chemical and 

 mechanical). This reaction is localised in the same way as 

 the usual positive reaction. It is given only when the current 

 is directed against the head or anterior part of the body. 



It is thus seen that the planarian is positively rheotactic to 

 very weak currents, the form of the reaction being precisely 

 the same as that given to other weak stimuli. It seems very 

 doubtful if this reaction is of any impoi'tance in the normal 

 activity of the animal. 



G. General Summary and Discussion of Results. 



As was stated earlier in the paper, the problem witli which 

 this study deals is the analysis of the beliaviour of the 

 common fresh-water planarian. The movements and reac- 

 tions to all the more important stimuli, with the exception of 

 lio-ht and heat, have been described and analysed into their 

 component factors in the body of the paper. It is believed 

 that it is of the greatest importance to have as complete and 

 detailed an account of the various activities as possible, and 

 as a consequence full details have been given in the case of 

 each subject treated. Since this method of treatment 

 necessarily makes the account of considerable length, it has 

 a tendency to obscure the general and significant results in a 



