82 Herbert Eugene Walter 



light was flashed upon it vibrated as vigorously after the experiment 

 had been repeated 410 times in close succession as it did at first. 



An attempt was made with Planaria maculata to see how long 

 activity would continue in a succession of trials in directive light. 

 The worm was started on the middle of an aquarium floor and 

 allowed to glide in any direction. As soon as it stopped and 

 assumed the relaxed contour of the resting worm, the time required 

 for the journey being noted, it was immediately returned to the 

 starting point. Subjected to this treatment, the worm made 39 

 trips, which in general occupied an ever decreasing length of time, 

 ranging from 18 minutes to if minutes, or an average of 5 minutes 

 and 53 seconds each. When returned to the starting point the 

 fortieth time the worm refused to start. Although in this experi- 

 ment, which lasted 4^ hours, the worm became gradually less 

 responsive to the mechanical stimulus of the brush by means of 

 which it was transferred to the starting point, its fatigue did not 

 materially affect the negative character of its response to light. 



Time Required to Leave a Unit Circle. In obtaining the data 

 on this point, the apparatus and method already described (p. 73) 

 were employed. It was found that when worms of different 

 species were subjected to three different intensities in immediate 

 succession the degree of intensity did not prove to be as important 

 a factor as fatigue in determining the average number of seconds 

 necessary for the worm's exit from a circle 10 cm. in diameter. 



During the series of experiments upon this point care was exer- 

 cised so to vary the succession of intensities that the effect obtained 

 could not be attributed to any cumulative increase or decrease of 

 intensity. Thus, on one day the order of intensities was 1, 2, 3, 

 on the next 2, 3, 1, and on the third, 3, 1, 2. In Table XVII the 

 data obtained are arranged on the left with reference to the actual 

 intensities employed and on the right with reference to the suc- 

 cession of trials made upon the various species which are desig- 

 nated in the middle column. The averages in the table are each 

 made up of four records. 



It will be noted that Phagocata gracilis and Planaria gono- 

 cephala are, according to these figures, less subject to fatigue than 

 Dendroccelum lacteum or Planaria maculata. 



