Reactions of I so pods to Light 257 



even more, of the entire number experimented upon stopped in 

 the extreme negative section of the tank. 



Hence, sometimes the activity of Asellus in responding to the 

 hght was itself the real cause of the apparent tardiness of the direc- 

 tive response, as indicated by the mean average position of the 

 animals. The directive (phototactic) effect of the light in con- 

 junction with a vigorous photokinetic effect, served to direct the 

 animals to the negative end of the tank, from which owing to the 

 relatively stronger photokinetic influence, they recoiled and wan- 

 dered about sufficiently to be pretty generally scattered. As the 

 photokinetic effect became less pronounced, however, the photo- 

 tactic effect became relatively more effective and negative 

 phototaxis caused the animals to congregate toward the end of 

 the tank farther from the source of illumination. 



In some cases the photokinetic influence was a very important 

 factor during the first part of the experiment. The length of the 

 tank (51 cm.) was suflicient to reduce this factor somewhat. At 

 any rate, the ultimate response could not be affected in cases where 

 the phototactic response was not altered by the length of exposure. 

 In the present series of experiments the phototactic response did 

 not change with long exposure to light. The photokinetic influence 

 in its disturbing effect upon the phototactic responses will be seen 

 to have been similar to the thigmotactic and other influences 

 mentioned before, which kept the animals intermittently on the 

 move for some time after they were introduced into the tank. It 

 was only when all these non-directive mfluences had become sub- 

 sidiary in their effect that the phototactic responses were recogniz- 

 able and decisive. 



The photokinetic effect naturally varied with the intensity, but 

 the negative phototaxis also varied with the intensity, so that a 

 directive response occurred as quickly with high as with the lower 

 intensities. 



Two experiments, in which Asellus after previous exposure to 

 diffuse daylight was subjected to intensities of 3 CM. (5 c.p. 

 incandescent at 1.3 m. from middle of tank) and 2855 CM. (772 

 c.p. 6-glower Nernst lamp at 0.52 m. from middle of tank) are 

 given in detail in Tables II and III. 



