214 CHARLES HARLAN ABBOTT 



As an angle of negativeness of 90° represents the average course 

 of an animal placed at right angles to the hght, if it is indifferent 

 to Hght, the angle for the first day shows that this individual 

 was somewhat positive on that day, because it turned on the 

 average 11.25° toward the light. This accords with the 45 per 

 cent positive response calculated according to the first method. 

 The responses in any record fall into three classes with respect 

 to the angle of negativeness: 



1. When the animal is facing the light (position A), it is pos- 

 sible to measure only the angle of turning away from the light. 



2. When the animal is facing away from the light (position 

 B), it is possible to measure only the angle of turning toward the 

 hght. 



3. When the light strikes the anunal on the side (positions C 

 and D) either the angle of turning away from the light or the 

 angle of turning toward the light can be measured. 



This difference raised the question whether the angle of 

 negativeness should be measured only from the results in positions 

 C and D or from those of the four positions combined. A test 

 series showed that the results are approximately the same, 

 whichever method is used. Of course the smallest angle is ob- 

 tained when the animal is facing the light and the largest when 

 facing away from the light, but these two extremes balance each 

 other. The series which shows this is given in the section on 

 the comparison of Oniscus and Porcellio (table 10, page 228). 

 Compare also the last two columns in table 4. In the follow- 

 ing pages, wherever the angle is given without further expla- 

 nation-, the angle calculated from the positions C and D is 

 given. 



4. Conclusions 



a. The individual isopods which have been considered in the 

 preceding pages were, except in the instance of one individual on 

 a single day, definitely negative to light. This is shown by the 

 calculation of a high percentage of negative responses and by 

 the measurement of the average angles through which these 

 animals turned in their movements away from the light. 



