292 A. M. Banta 



5 This characteristic reaction sometimes brings the animal's 

 eyes back to the plane, in which case the animal seldom fails to 

 return into the dark region. 



6 Animals which meet the plane at a sharp angle usually enter 

 the dark regions whatever may be the direction in which they are 

 headed. 



7 Individuals which, in going into the illuminated region, 

 pass the plane without being stopped by the sudden influence of 

 the light, sometimes show increased activity almost instantly. 

 At other times the increase comes less quickly. 



8 Individuals entering the dark region from the illuminated 

 one very generally stop on crossing the plane separating the two, 

 but in cases where the individual does not stop a decrease in 

 activity occurs. 



9 The eyes are the effective light receptive organs of this ani- 

 mal, as is shown by the actions of the animal at the plane between 

 the dark and light regions. 



10 Unsymmetrical stimulation of the two eyes governs the 

 animal's movements at the plane. 



1 1 The photokinetic effect keeps the animals moving for some 

 time. It causes them to recoil from the dark end of the tank 

 and then to continue to go back into the illuminated area. 



12 More than half of the animals collect within the dark region 

 on the average within 4.3 minutes after the experiment starts. 

 Virtually all the animals ultimately settle down and remain in 

 the dark region of the tank, this occuring on the average 44 min- 

 utes after the experiment starts. 



3. Coecidotea 



The experiments upon Caecidotea with vertical illumination 

 were made under exactly the same conditions as were those upon 

 Asellus. However, two preliminary experiments were made upon 

 Caecidotea with intensities lower than 6983 CM. In one of these 

 experiments (the bottom of) one end of the tank was illuminated 

 with an intensityof 97 C. M. ( 60 c.p. incandescent at 75 cm. from 

 surface of water); in the otherthe intensity was 172 CM. (49c. p. 



