88 Herbert Eugene Walter 



In all the experiments made upon the effects of change of inten- 

 sity in directive light, more responses were found to occur when the 

 intensity was decreased than when it was increased. This is in 

 agreement with the experiments already described relating to the 

 critical region between two intensities, in which it was found that 

 worms show a greater number of responses when going from a 

 higher into a lower intensity than vice versa. 



Bdelloura is particularly sensitive to changes in intensity. It 

 is necessary to throw a shadow on a moving worm only momen- 

 tarily to cause it to perform vigorous wigwag movements or to 

 change the direction of its course. 



Whitman ('99), writing of Clepsine, suggests that the extreme 

 agitation of this animal when a shadow is thrown upon it may be 

 the result of natural selection, since any sudden shadow cast upon 

 it in its natural environment may be caused by a turtle swimming 

 overhead, to which the leech, if it is quick enough, may become 

 attached. It may be that Bdelloura, which is also an ecto-parasite, 

 has developed this extreme responsiveness to sudden decrease of 

 intensity in a similar way. 



Changes in Direction. The precision with which all the pla- 

 narians in a dish may be made to pass back and forth by shifting 

 a directive light from one side to another is a striking phenomenon, 

 which is sure to impress anyone who sees it. By careful manip- 

 ulation of the light, it is possible even to make an individual 

 planarian follow a predetermined path in the most undeviating 

 manner. For example, when two lights, placed near the ends of an 

 aquarium, are alternately turned on and off, the worm will zigzag 

 across the field, at right angles to the direction of the lights, while 

 under a moving light it may be made to turn around and around, 

 almost as if its posterior end were a pivot, to trace figure 8's and 

 curves of various patterns, or to turn abruptly at right angles an 

 imaginary corner. 



Unlike the changes in intensity previously described the degree 

 of abruptness in any change in the direction of the light made no 

 apparent difference in the quality of the reaction, since any change 

 in direction, however gradual, met with an immediate response 

 on the part of the worm. Indeed it was necessary to abandon an 



