Reactions of Isopods to Light 279 



1 Caecidotea is not very responsive to horizontal illumination. 



2 The thigmotactic and other influences due to the transfer- 

 ence of the animals to the tank are more potent with Caecidotea 

 than with Asellus, and the effect continues longer. 



3 Qecidotea does not respond to intensities of illumination 

 less than about 80 CM. 



4 Caecidotea responds negatively to intensities from 80 CM. 

 to 2855 CM. 



5 This response is not direct, i. e., it is not a direct phototactic 

 response. 



6 Light produces with Caecidotea a very decided photokinetic 

 effect, which continues for some time. As this effect decreases, 

 the animals gradually settle at or near the negative end of the 

 tank, i. e., they gradually become acclimated to the lower inten- 

 sity of illumination, and when acclimated to the conditions of 

 illumination at the negative end of the tank they accumulate 

 there. 



7 With horizontal illumination both Asellus and Caecidotea 

 respond in a negative way to moderate mtensities as well as to all 

 stronger intensities; but below certain ranges of intensity both 

 are indifferent. Caecidotea is indifferent to light intensities below 

 80 CM., Asellus to intensities below 2.5 CM. Hence Caecidotea 

 is indifferent to a considerable range of intensities (2.5 CM. to 

 near 80 CM.) to which Asellus is more or less responsive. 



8 However, after having been m the dark for several hours, 

 Asellus is positive to intensities of light between 2.5 CM. and 

 about 80 CM.; and sometimes it is positive for a short time to an 

 intensity of 2855 CM., but in that case it very quickly becomes 

 negative. After considerable exposure to any intensity to which 

 it responds at all, Asellus is positive. 



9 Although Asellus is not very responsive to horizontal 

 illumination, it is decidedly more responsive than Caecidotea to 

 those intensities to which it responds at all. This difference 

 appears in the directness and the relative promptness of the re- 

 sponse given by Asellus as compared with the irregular and tardy 

 response of Caecidotea. 



10 This difference in the reactions of Asellus and Caecidotea 



