112 GEORGE DELWIN ALLEN. 



pipette and similar devices caused only a stopping, longitudinal 

 contraction, and gripping of the bottom without any turning 

 either toward or away from the source of the stimulus. 



That the rheotropic reactions of planarians were found so 

 difficult to demonstrate, it seems probable, must have been 

 due to the experimental methods employed. Using the methods 

 described below, it has been found that rheotropic reactions 

 of these animals can be demonstrated very easily, not only in a 

 "current of just the right intensity" but in currents of a large 

 range of intensities. Under the conditions of these methods a 

 worm is entirely surrounded by the flowing water on all surfaces 

 except the ventral surface which is attached to the substratum, 

 and the conditions of stimulation are more typically rheotropic 

 than when the stream of water is directed as a small jet against 

 a localized part of the body. A negative reaction, i. e., a turning 

 away from the side stimulated, was not described by Pearl but 

 Dr. C. M. Child, who suggested this study, has observed a 

 negative reaction as well as a positive reaction in currents of 

 water in his laboratory stocks of worms used in studies in re- 

 generation. It has been found that these reactions are rever- 

 sible experimentally. The study of their reversibility which is 

 reported in the present paper was preliminary to a more detailed 

 study of the rheotropism of these animals which is in progress 

 at the present time. 



The work reported in the present paper was done some time 

 ago in the zoological laboratory of the University of Chicago. 

 My acknowledgments are due Dr. C. M. Child and Dr. V. E. 

 Shelford for helpful suggestions. 



II. MATERIAL AND METHODS. 



Planaria dorotocephala has been used exclusively for this 

 study. Specimens were collected from a spring-fed marsh at 

 the margin of the Fox River near Chicago, Illinois. These 

 animals are very easy to keep in the laboratory without special 

 care. Since this study was in the nature of a testing of the re- 

 action possibilities of the worms toward currents of water, rather 

 than an effort to determine the normal habits, no efforts 

 were made to duplicate the normal conditions of existence in 



