86 Herbert Eugene Walter 



had been previously covered with black cloth to exclude most of 

 the light, this species was found in the morning in the light area, 

 a behavior exactly the reverse of that shown by fresh-water pla- 

 narians. Another peculiarity of this species is that individuals in 

 coming to rest arrange themselves in compact rosettes with the 

 anterior end of the body pointed toward the circumference of the 

 rosette, while the sucker-like posterior end remains attached near 

 the center of the group. They are so delicately responsive to me- 

 chanical stimuli that any slight disturbance of one member of such 

 a rosette is sufficient to throw the whole group into activity. The 

 advantage to the individual worm of such a habit of arrangement 

 in coming to rest, is evident. 



Finally, Bdelloura was repeatedly seen on taking the resting 

 position to point directly toward the light with the anterior end of 

 the body raised and the posterior end flattened out into a sucker- 

 like expansion. 



Summary. Fresh-water planarians (Dendrocoelum, Planaria 

 and Phagocata) are negatively phototactic while Bdelloura is posi- 

 tively phototactic. 



Negative planarians deviate most from the direction in which 

 they are started if pointed toward the light and least if pointed 

 away from the light, an intermediate deviation occurring when 

 they are pointed at right angles to the light. 



The rate of locomotion is greater when worms are headed toward 

 the light than when they are headed away from it. 



During successive trials the rate of locomotion decreases. 



Negative planarians frequently take an apparently positive 

 course because the impulse to move in any direction is greater than 

 the phototactic impulse. 



The normal negative phototaxis of a worm may change tem- 

 porarily to positive by reason of some physiological state which is 

 not obviously referable to external stimuli. 



The greater the intensity of the light the less worms wander in 

 their course. When they are headed away from the source of 

 light, there is less error in the precision of their orientation than 

 when they are started toward it. 



Planarians frequently travel away from the source of light diagon- 

 ally instead of directly. 



