136 



Herbert Eugene Walter 



was delicately mounted upon a turntable, such as is used in "ring- 

 ing" microscopic slides, a very slight rotation was sufficient to 

 bring to a halt momentarily a gliding worm in this aquarium. 

 It was possible to control this momentary response to such a 

 point of nicety that the anterior end of the worm could be made 

 to halt for an instant without interfering with the onward loco- 

 motion of the posterior end. If this slight rotation was repeated 

 at intervals of a second it was found that the worm under obser- 

 vation halted with less and less certainty, until after a dozen or 

 more trials it continued to glide on without halting at all. In 

 ordinary phraseology the worm had learned by experience not 

 to be alarmed by a sudden mechanical shock. The lesson, how- 

 ever, was always very soon forgotten, for after an interval of less 

 than a minute, during which the aquarium remained stationary, 

 the worm responded exactly as it did at first, whenever a slight 

 rotation was made. In a similar way the skioptic response of 

 Bdelloura Candida became less pronounced upon repetition, until 

 it was possible to throw a shadow upon the animal without 

 obtaining any response at all. 



Again, when worms were placed in afield of non-directive light, 

 parts of which were of two different intensities, the number of 

 wigwag responses made at the critical line separating the two 

 intensities grew less after the animals had repeatedly crossed the 

 line. At first the new condition of sharply contrasted light 

 intensities in the worm's field of locomotion called out a large 

 percentage of wigwag responses. Later, however, by repeated 

 experiences the worm became familiar with this feature of its 

 environment and made fewer wigwag motions. A definite 

 instance of such a decrease in response is given in Table XXXVII. 



TABLE XXXVII 



Responses of Planaria gonocephala on crossing the line separating two intensities of non-directive light 



First 25 crossings. . 

 Second 25 crossings 

 Third 25 crossings. 

 Fourth 25 crossings 



Wigwag movements 



n) 



No response 



Percentage of response 



84 

 76 

 48 

 3 2 



