REVERSIBILITY OF THE REACTIONS OF PLANARIA 

 DOROTOCEPHALA TO A CURRENT OF WATER. 



GEORGE DELWIN ALLEN, 



DEPARTMENT OF ANIMAL BIOLOGY, UNIVERSITY OF MINNESOTA. 



CONTENTS. 



I. Introduction in 



II. Material and Methods 112 



III. Difference of Reaction to Currents of Different Velocities 115 



IV. Reversal of Reaction Induced by Chemical Changes 118 



V. Reversal of Reaction Induced by Changes of Temperature 121 



VI. Variations in Rheotropic Reactions 122 



VII. Rheotropic Reactions in Nature 123 



VIII. Summary 124 



IX. Bibliography 125 



I. INTRODUCTION. 



A reaction of planarians to currents of water has been de- 

 scribed by Pearl ('03) in his careful study of the general features 

 of the behavior of planarians. He has described the reaction 

 and his method of obtaining it as follows: "In the course of 

 the experiments to localize chemical stimuli by the capillary 

 tube method, it was discovered that by means of a tube with 

 a relatively large opening (from .25 to .50 mm. in diameter) 

 and letting the ordinary tap-water in which the animals were 

 flow out of it by its own weight, a current of just the right in- 

 tensity to cause a positive reaction could be produced. The 

 animals would turn very sharply toward the source of such a 

 current, the reaction being evidently the same as that to other 

 weak stimuli (chemical and mechanical). This reaction is 

 localized 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" (p. 698). He states that earlier in 

 his work a large number of experiments were performed with 

 various devices to determine whether these animals would show 

 such a reaction, but without success. Streams of water from a 



in 



