298 A. M. Banta 



was sufficiently obvious and definite, as it also was in all other 

 similiar experiments. For the whole time of the experiment the 

 average number of animals in the illuminated area was 40.5 per 

 cent of the entire number of individuals in the tank, but for the 

 last half hour only 16 per cent stayed in the illuminated portion. 



Table XII gives in condensed form the results of the experi- 

 ments of this series. 



From this table it may be seen that the average length of time 

 from the beginning of the experiment until the majority of the 

 animals began to collect in the dark region of the tank was 17 

 minutes, and the average time until the maximum response had 

 been virtually reached was i hour and 18 minutes. The average 

 per cent remaining in the illuminated region for the whole time 

 of the experiments was 33 — , and the average per cent remaining 

 in the illuminated region at the close of the experiment was 19.2. 

 Hence it seems clear enough that under these conditions of 

 illumination the Caecidotea tend to collect in the dark end of the 

 tank and that once collected and settled, they remain there. 



If it be granted that Caecidotea sometimes reacts to the abrupt 

 change in illumination at the plane of division between the dark 

 and the illuminated regions, then the reactions are due to the 

 effects of unequal stimulation upon the different parts of the ani- 

 mal's body, and to a tendency in the animal to react in a charac- 

 teristic manner when strongly stimulated. This characteristic 

 reaction in Caecidotea consists in a more or less abrupt turning 

 about and starting off in a new direction. This reaction may aid 

 the animal in getting partly within the dark region when once 

 entirely across the plane and in the illuminated region. This 

 occurs so rarely, however, as to be practically negligible. 



Those general reactions to the light which were manifest in an 

 increased activity in the illuminated region were of course photo- 

 kinetic, and the collecting of the animals within the dark region 

 was likewise due to photokinesis. The animals were from the first 

 more active in the illuminated than in the dark region. This 

 alone would soon cause individuals to collect in the dark region, 

 where the activity was less. When the photokenetic effect ceased 

 to be sufficient to cause the animals to recoil from the end of 



