PRANDSEN. — REACTIONS OF UMAX MAXIMUS. 215 



diminishes in strength. Then there is a gradual increase in positive 

 reaction, which reaches its height in a response of +22. °3 at a .000,022 

 C. P., and then falls toward the zero line as we approach darkness. 

 There is some irregularity in the negative region, but on the whole the 

 rise is gradual. In the region of positive response, there is a consider- 

 able lack of regularity, especially marked by the interpolation of one 

 series (12) of very low response between the two series of greatest 

 response. These series intermediate between Nos. 11 and 16 represent 

 later experiments than the two series bearing those numbers. Having 

 obtained such a marked positive response at two widely separated in- 

 tensities of light, it was thought desirable to get other intermediate 

 series. Hence, the order of the series as arranged in the table, on the 

 basis of gradually diminishing light intensities, does not, as already stated, 

 represent the order in which the series were obtained in my experiments. 

 While the slugs, thus far, had, on the whole, been in good active condi- 

 tion, they were not so in these intermediate series. Although a fresh 

 supply was obtained, all the animals seemed much more stupid and 

 irresponsive than usual. Some of them refused to move, when put on 

 a plate, and many of those that did, responded in a very half-hearted 

 way. The cause of this unusual lack of activity, I could not discover. 

 It may be that a slight change in the food of the animals, which I made 

 at this time, was partly responsible. At any rate, instead of obtaining 

 responses intermediate in amount between those of series 11 and 16 as 

 might have been expected, the results were as have beeu given. Series 

 12 was the last one taken. In this, the animals were noticeably more 

 stupid and irresponsive than in any of the preceding experiments. It is 

 very evident from these results, I think, that the precision of response 

 will vary to some slight extent from day to day. The negative responses — 

 those to strong intensities of light — will not be as variable at different 

 times as the positive responses — those to weaker stimuli — as the curve 

 shows. The varying thermal conditions of the room, already mentioned, 

 may have been in part a cause of this irregularity. Furthermore, an 

 animal that has had plenty of food is likely to be stupid and slow in 

 movement and is more apt than a hungry one to seek darkness and 

 concealment. On the other hand, a hungry, active slug will probably ex- 

 hibit positive phototaxis in a most marked and sometimes abnormal degree, 

 as was the case occasionally with the positive animals at the strongest 

 light intensities. Besides this individual variation, there is, I think, a 

 general variation for all slugs from time to time, for reasons imperfectly 

 known, which will find its expression in curves of different heights. 



