212 PROCEEDINGS OF THE AMERICAN ACADEMY. 



lating light rays. Here, then, arises another problem, similar to the 

 one treated of in the first part of this paper, viz., What determines 

 whether a particular slug shall be positively or negatively phototactic ? 

 In the first series of experiments — in fact throughout this whole set — 

 the animals used were about equally divided between large, small, and 

 medium-sized individuals. The two positive animals in series 1 were both 

 of large size. They were very active. The only peculiarity wherein 

 they seemed to differ from other individuals was in the unusually 

 sticky character of the slime. Whether there is any correlation between 

 this fact and the liking for strong light, I am not prepared to say; It 

 is possible — and certain observations seem to indicate that it is highly 

 probable — that the food conditions of the animals have some influence 

 on their responses to light, as they were shown to have on their responses 

 to gravity. The psychic state of the animal is also to some extent, I 

 think, a factor. Freshly caught slugs when put on a glass plate some- 

 times acted as if in great fear. They displayed unusual activity and 

 were very erratic in their movements. If forcibly checked or held, 

 they made strenuous efforts to escape. The great activity of the posi- 

 tive individuals indicates a possible state of fear. One animal in par- 

 ticular seemed highly abnormal. Several times it moved directly toward 

 the circular field of light and even placed its tentacles against the oiled 

 paper which covered the opening. This was the only individual in the 

 whole course of the experiments which exhibited a response like that of 

 moths. No definite set of experiments was planned or carried out in 

 regard to this matter. 



As we run down column 5, we see that the average arithmetical 

 response varies quite strikingly at the different intensities. The first 

 seven series show a gradual decrease in the average response as the 

 strength of the light is diminished. Although not so regular, there is 

 also a gradual decrease in the degree of negative response on the part 

 of these seven groups of animals, as shown by the average algebraic sums 

 of their responses (column 6). 



Owing to the constant dying off and deterioration of the stock, it was 

 found impossible to use the same set of animals in all the different series 

 of experiments. Moreover, this was not desirable, for the reason that 

 an animal which is constantly experimented on gradually loses its sensi- 

 tiveness, and thus its responses become untrustworthy. Not knowing 

 the factors which determine the kind of phototaxis, it was of course 

 impossible to make a uniform selection in this respect. We see, how- 

 ever, that the number of negative animals (column 8) is less at the 



