The Reactions of Planarians to Light 91 



had been all day in the light. The average at the bottom of the 

 table indicates, first, that an approximately equal percentage of 

 worms was found on the sides of the aquarium at both times of day, 

 which may therefore be left out of the reckoning, and, secondly, 

 the occurrence of a significant migration during the interval be- 

 tween 8 a. m. and 4 p. m., demonstrated by the distribution of the 

 worms at the top and bottom of the jar respectively. Accord- 

 ing to the data obtained, at least 30 per cent of the worms in the 

 top group must have become positively geotactic and gone to the 

 bottom during the day. 



A later set of experiments in which an aquarium was kept 

 swathed in black cloth during the day showed less migration. 

 The conclusion naturally follows that geotaxis is more likely to 

 occur in the presence of light than in its absence. Whether there 

 is a regular diurnal vertical migration among planarians in nature, 

 as Birge ('97) and Schouteden ('02) found to be true for fresh- 

 water entomostraca, and various authors 4 for different forms in 

 marine plankton, remains unknown. It is probable, however, 

 that planarians ordinarily remain quiescent on the under sides of 

 stones or in other shaded places for considerable intervals of time, 

 coming under the influence of light only when started into activity 

 through some other stimulus. 



A worm placed in an aquarium with square sides and left free 

 to travel undisturbed on the bottom or the sides occupies the 

 sides more frequently than the bottom. 



In a trial to test this point, an aquarium was used, the bottom 

 area of which measured approximately five times that of the sides. 

 The course pursued in this aquarium by one worm (P. gonoceph- 

 ala) in directive light and covering 1340 cm., was plotted and the 

 percentage of distance traveled on the sides was found to be prac- 

 tically equal to that traveled on the bottom, notwithstanding the 

 fact that the animal was started in the middle of the bottom, where 

 it had five times as much available territory to travel over as on 

 the sides. Other things being equal, therefore, this worm showed 

 itself five times as ready to travel on the sides of the aquarium as on 

 the bottom. 



* Groom and Loeb ('90), Loeb ('93a), and Parker ('02). 



