FRANDSEN. — REACTIONS OF LIMAX MAXIMUS. 213 



weaker intensities than at the stronger. When we come to series 8 of 

 the table, we meet with a uew condition of affairs. Instead of a still 

 further decrease in the amount of deviation, there is a sudden slight in- 

 crease, from 9.1° to 13°, and a reversal in phototaxis for the series from 

 an average response of — 6° to + 1.4°. The number of positive indi- 

 viduals has increased from 3 to 7. It was because of this striking: 

 change that it was thought best to repeat this series and the three suc- 

 ceeding ones on another set of animals. The absolute positive or nega- 

 tive migration was this time taken without regard to the position of the 

 body axis. Sei'ies la, 8a, 9a, and 10a are hence taken at the same 

 intensities as 7, 8, 9, and 10 respectively. These repeated sei'ies indi- 

 cate as strongly as the first set that an intensity of .001,69 C. P. very 

 nearly marks the lower limit of negative phototaxis in the slug. Some- 

 where near a candle power of .000, 75-4, lies an intensity which attracts 

 about as many animals as it rej^els and in about the same degree. That 

 is, the average phototaxis (algebraic sum) is zero. Below this intensity, 

 there is more attraction than repulsion, and hence there is an average in- 

 crease of migration toward the light. The table shows that the average 

 positive response increases to some extent correlatively with the diminution 

 of the light intensity, up to a certain point. This point, according to 

 the results here obtained, is the intensity of .000,022 C. P., where 

 the average movement toward the light, in a period of 45 seconds, was 

 through an angle of 22.3°. As we go below this intensity, there is 

 again a falling off in the strength of the positive response, which dimin- 

 ishes, however, with a good deal of irregularity until absolute darkness 

 is reached. These facts will become more apparent from the study of 

 their graphic portrayal in the curve here given. 



The continuous line represents the curve as plotted from the results of 

 Table X., column 6 ; the dotted line, the curve of responses as one may 

 assume theoretically it would have been, could all of the conditions 

 other than intensity of light have been equalized. The abscissae here 

 represent the logarithms of the intensities of light + 10. Beginning 

 with darkness on the left end, there is a constant increase of intensity 

 as we move toward the right. The sines of the angles of response are 

 marked off" on the ordinates. Remembering that the left represents a 

 region of weak intensity and the right a region of strong light, that all 

 points above the line x x' are points of positive response and all points 

 below it of negative response, we can understand the significance of 

 the curve. In the region of strong light, the curve lies far below the 

 line X x', but gradually rises toward and finally crosses it, as the light 



