Reactions of I so pods to Light 295 



TABLE XI Continued TABLE XI Continued 



plane was established in the middle of the illuminated region for 

 comparison with the boundary between the light and dark regions. 

 The hood around the illuminated part of the tank was then opened 

 somewhat to permit observations from above, and the actions of 

 the animals when they passed through the arbitrary plane and like- 

 wise when they passed the boundary between the dark and illumi- 

 nated regions were carefully watched and compared. 



It was found, after observing Caecidotea passing through these 

 planes repeatedly, that the reactions at the bounding plane between 

 light and dark were not very marked. However, out of a total of 

 2126 observations at the two planes, 50 cases were noted in which 

 the animal after starting across the boundmg plane into the illum- 

 inated region turned about and remained within the dark re- 

 gion whereas only 30 cases of similar sharp turning about of ani- 

 mals moving away from the dark half of the tank occurred at the 

 arbitrary plane. Conversely, only 16 cases were noted in which the 

 animals upon entermg the dark region from the illuminated one 

 turned back sharply and remained within the illuminated region, 

 whereas 23 cases were noted in which a similar turning about of 

 animals moving toward the dark region occurred at the arbitrary 

 plane. There were 208 cases in which the animal on entering the 

 dark from the illuminated regions stopped when crossing the 



