Reactions of Isopods to Light 269 



influence, but this influence, even though in some cases it may 

 not be asserted at once, ultimately appears, the photokinetic 

 eff^ect meanwhile becommg less pronounced. 



After retention in darkness for a few hours, Asellus is positive 

 to intensities between 2.5 CM. and 80 CM.; but to intensities 

 of I CM. or less it does not respond. To an intensity of 2855 C 

 M. Asellus is generally negative, though sometimes it is positive 

 at first, but in that case it very quickly becomes negative. 



3. CcBcidotea 



The experiments upon Caecidotea were made under precisely 

 the same conditions as in the case of Asellus. Caecidotea was 

 found to be still less responsive to light than Asellus. Even after 

 the conditions of experimentation and observation had been so 

 refined that Asellus was seen to be definitely responsive to hori- 

 zontal illumination, Caecidotea long seemed quite unresponsive, 

 except in a purely photokinetic way. Finally, however, numerous 

 careful experiments showed that Caecidotea was feebly responsive 

 in a directive way to horizontal illumination. This species, like 

 Asellus, after being transferred to the tank, kept up random move- 

 ments for some time, doubtless as the result of thigmo tactic 

 or some other non-directive stimulus, and these random move- 

 ments were even more marked and longer continued — on an 

 average, nearly twice as long — than with Asellus. 



The glass ring was often used to confine the Caecidotea until they 

 should become thoroughly settled in the middle of the tank. 



Because of the general reluctance of the animals to move or to 

 respond to light stimulation after they had once become settled, 

 many of the experiments were begun as soon as the animals were 

 placed in the tank. In such cases directive results from light 

 stimulation ordinarily appeared only after the lapse of some time; 

 for the movements due to thigmotaxis or other non-directive 

 stimuli predominated at first. 



With Caecidotea, as with Asellus, the efi^ects of stimulation by 

 light appeared to be both photokinetic and phototactic. In man\ 

 cases when animals almost or quite settled in the tank were sub- 



