REACTIONS OF LA*ND ISOPODS TO LIGHT 211 



reaction when the animal is illuminated on the side shows the 

 true degree of negative reaction, and, moreover, that the re- 

 sults in positions A and B balance each other, so that the aver- 

 age in all four positions is a suitable control for the correctness 

 of the results from stimulation on the side. 



The reaction of Oniscus A 12 is given in figure 7. These 

 curves show more positive responses than occurred in A 10, due 

 to the many instances in which this isopod went toward the 

 light during the first four days. From the average for the 

 entire period of the experiment, it appears that A 12 became 

 the more negative of the two animals, because, in spite of the 

 degree of positiveness at first, the average in figure 7 shows the 

 greatest number of responses in classes 7 and 8 on the abscissa, 

 while in figure 6 classes 5 and 6 are the largest. In figure 7 the 

 correspondence between the average reaction and the reaction in 

 positions C and D is not quite so close as in figure 6, but the 

 average is much nearer to these than to the reactions in posi- 

 tions A and B. 



The graphical method shows more definitely what is involved 

 in the negative reaction than was shown by a simple enumera- 

 tion of positive and negative responses. The animals regularly 

 turned away from the light in whatever position they were 

 originally placed, except that when they were already headed 

 away from the light they nearly alwaj^s traveled in the direction 

 in which they were headed. From this method of analysis it 

 may be concluded that Oniscus is definitely oriented by the 

 light and that the response involves something more than 

 moving at random into a lower intensity. 



3. Angle of negativeness — third tnethod 



This method was devised to express the degree of negativeness 

 of any one individual isopod or group of isopods by a single 

 figure, which might be used for comparison with the results ob- 

 tained in succeeding experiments. For this purpose the angle 

 of negativeness was chosen. This term will be applied to the 

 average angle of the course taken by the animal with respect to 

 the position of the source of light. 



