222 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the right, for, if this were so, we should expect an average positive 

 response of the right side as much greater than that of the left side, as the 

 average negative response of the left is greater than that of the right side, 

 for both these would mean a greater movement to the right. These 

 facts curiously suggest that the right and left sides are attuned to slightly 

 different intensities of light. Is this possibly due to ancestral habits of 

 life in which environment, acting unequally on the two sides, produced 

 this difference ? 



The results obtained for the right and left sides from the experiments 

 in darkness (series 18a) are rather puzzling. If the responses are due 

 to some uncontrolled directive stimuli of the kind already suggested, it 

 would seem that the two sides had given opposite responses. As these 

 experiments represent two series taken at different periods, it is the 

 more surprising that they should both show this peculiarity. Again, in 

 the responses to weak candle heat (series 186) the left seems to have 

 been positively, and the right side negatively affected. So far as is known, 

 there was no unequal operation of stimuli on the two sides. 



Related to this matter is the question, — Is there any tendency on the 

 part of all slugs to move either to the right or to the left? Individuals 

 were noticed which seemed to have a marked tendency to continue 

 moving toward the right, and there were others which seemed to be as 

 strongly biassed toward the left. Not many seemed entirely indifferent. 

 The total movement of all the slugs in the region of negative response 

 (series 1-8, Table XV.) toward the right side was 8786° (col. 9), and to 

 the left 6471° (col 10). In the positive region (series 8-18, Table XV.), 

 the total migration toward the right side was 8540° (col. 9), and to- 

 ward the left 8568° (col. 10). Thus, there seems to have been con- 

 siderably less migration toward the left in the range of negative 

 responses, but only a slightly greater movement toward the left in 

 the region of positive response. In all the 17 series, there was a mi- 

 gration towards the right of 17,326°, and towards the left of 15,039°, 

 That is, there appears on the whole to have been a slightly greater 

 average movement for all slugs toward the right than there has been 

 toward the left. What do we find to be the case with the animals experi- 

 mented on in the dark? Out of the 120 determinations made on 20 

 animals in the dark (series 18«), the amount of right-hand movement 

 was 2270° and the left-hand movement only 460°. That is, there was 

 nearly five times more migration toward the right than there was toward 

 the left. In series I8b, however, there seems to have been a marked pre- 

 ponderance of movement toward the left. From the foregoing exjieri- 



