Reactions of Isopods to Light ■ 307 



entered the dark region, and less often returned to the illu- 

 minated one. 



A general discussion of these results is reserved for the second 

 part of this paper. 



IV. SUMMARY OF REACTIONS TO LIGHT 

 /, With horizontal illumination 



1 Asellus communis exposed to horizontal illumination is not 

 responsive to intensities of light of I CM. or less. 



2 It is very decidedly affected in a photokinetic way by those 

 hght intensities to which it responds. 



3 It is also affected in a phototactic way by horizontal illum- 

 ination. Following exposure to such hght, it is negative to an in- 

 tensity of 2.5 CM. or more. It is neutral to an intensity of i CM. 

 or less. After retention in darkness for a few hours, it is positive 

 to such intensities as call forth any response (2.5 CM. or more); 

 but to an intensity of 2855 CM. the positive response is only 

 momentary. 



4 Its response appears to be direct, being produced by the 

 effects of unsymmetrical stimulation of the two eyes of the animal. 



5 Caecidotea stygia is not responsive to light intensities below 

 about 80 CM. It is negative to such intensities as it responds to 

 at all — 80 CM. or more. 



6 This response of Caecidotea is photokinetic in its nature, the 

 random movements causing the animals ultimately to settle in the 

 negative end of the tank, where the intensity of illumination is 

 least; to this intensity the animals soonest become acchmated. 



7 After considerable exposure to strong light both Caecidotea 

 and Aselliis become less reactive to it. Conversely, following 

 retention to darkness they are both apparently somewhat more 

 responsive to light. 



II. 



With Vertical Illumination 



With one half of the tank illuminated by light of 6983 CM. 

 intensity falling vertically upon it, and with the other half as 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 6, NO. 3. 



