210 PROCEEDINGS OF THE AMERICAN ACADEMY. 



negative movement of the animal was measured by taking the radius 

 which passed midway between the two tentacles, without regard to the 

 position of the body axis. A comparison of the two methods showed but 

 little difference in the results. The animals only occasionally made these 

 irregular responses, first in a plus and then a minus direction. As a rule, 

 the migration was unequivocal after the head end had oriented itself to 

 the stimulus. Experiments were made with 18 different intensities of 

 light, each constituting a " series." Six successive observations were 

 made on each individual (3 with the right side exposed ; 3 with the left), 

 and from 8 to 14 animals were employed in each "series." i.e., at each 

 intensity of light, making a total of from 48 to 84 observations at each 

 candle power used. A summary of the results for each of 18 such 

 " series " is given in Table X. 



The first column gives the number of the series ; the second, the 

 intensities of light. This intensity is expressed in terms of the standard 

 candle power at a distance of one meter. The next column (3) shows 

 the total positive migration of the (8 to 14) animals experimented with. 

 Column 4 similarly gives the total negative migration. Column 5 repre- 

 sents the average arithmetical angular deviation from the original posi- 

 tion due to phototactic stimuli, effected in a period of 45 seconds by all 

 the slugs, without regard to the positive or negative character of the 

 individual phototaxis. This average was obtained by adding together the 

 average phototactic responses (whether plus or minus) of each individual 

 of the series and dividing the result by the number of animals. The 

 average plus or minus phototactic response (algebraic average) for each 

 series (column 6) was obtained by getting the difference between the 

 sums of all the plus and all the minus movements of each series and 

 dividing this difference by the number of tests (observations) made. 

 Column 7 gives the number of positively phototactic animals in each 

 series; column 8, the number of negative animals; column 9, the num- 

 ber of indifferent animals ; and column 10, the total number of individ- 

 uals employed in each series. The sequence of the series is not the 

 same as that of the experiments, but is based on gradually diminishing 

 light intensities. I did not determine the possible influence of the heat 

 of the candle for each of the series, but in one series of experiments in 

 the dark (186), a candle, covered (to shut out the light) with an opaque 

 paper of the same thickness as the paraffined paper, was left burning at 

 a distance of 30 cm. (intensity .676 C. P.). 



A casual glance at the table at once answers the first of the questions 

 proposed in the statement of the problems (pp. 207-208). All slugs are 



