FRANDSEN. REACTIONS OF LIMAX MAXIMUS. 209 



and quickly thrown open. The window was covered with a thick, black 

 cloth, so that, when closed, external light was almost completely shut off. 

 Unfortunately, it was impossible, owing to the position and nature of the 

 room used, entirely to equalize all conditions. The temperature was not 

 the same from day to day and varied somewhat in different parts of the 

 room. Generally, it was so hot and close that it was necessary to leave 

 an opening between the sashes, and this of course created a slight draft 

 and produced irregularities of temperature. No account was taken of 

 the varying humidity of the atmosphere, a factor which may have some- 

 what influenced the animal's locomotion. Moreover, as the room was 

 not perfectly light-tight, there were feeble light stimuli in addition to the 

 artificial ones used. However, all these imperfections were but slight, 

 and, since they entered more or less into all the experiments, could not 

 greatly alter the relation between the results, which was the main thing 

 sought in the investigation. Other unestimated possible influences were 

 the nutrition of the animal and such slight thigmotactic stimuli as could 

 not well be avoided. 



The strength of the different intensities of light used was measured by 

 moving a piece of paper, the centre of which was oiled, between a light 

 of known intensity and the light whose intensity it was desired to know, 

 until the oiled spot on the paper was not distinguishable from the rest of 

 the paper. The distance from this point to each source of light was then 

 measured. Since the intensity varies inversely as the square of the dis- 

 tances, it is an easy matter to calculate the relative strengths. This 

 method is accurate enough for all ordinary purposes. 



Operations and Results. — In beginning any experiment, the slug, as 

 soon as it had definitely oriented itself, was rotated into such a position 

 that the axis of its body coincided with the base line, and its head was at 

 the centre of the disk. The window was then immediately closed and 

 the time noted. At the expiration of 45 seconds, the window was thrown 

 open and the animal's position instantly noted. The extent of positive 

 or negative migration was at first ascertained by finding the length of the 

 arc stretching from the base line to the radius which was parallel with 

 the axis of the slug's body. Any movement into the half of the circle 

 toward the source of light was called positive ; any movement into the 

 other half, negative. It would occasionally happen that an animal would 

 at first move into the positive half of the circle and then turn away from 

 the light. In this case the axis of orientation made a negative angle 

 with the base line, although the animal itself lay in the positive half of 

 the circle. Later, in the course of the experiments, the positive or 



VOL. XXXVII. — 14 



