A PHYSIOLOGICAL CHARACTER. 25 



the side of the tank in the direction of its former orientation, though 

 sometimes it took a diagonal course away from the side of the tank, 

 but in the general direction (as regards Hght) in which it was formerly 

 oriented. The individuals which were slow in reacting either moved 

 very slowly, scarcely more than executing ''place-maintaining move- 

 ments," or they wandered about independently of the directive 

 light stimulation for a time and later attained and maintained orien- 

 tation with reference to the light. Some individuals wandered about 

 so long or swam so Httle that they did not reach either end of the 

 tank in the 15 minutes of the experiment. These were the ''over- 

 time" individuals which were arbitrarily assigned a reaction-time of 

 900 seconds. D. pulex seldom attached to the surface film or to the 

 sides of the tank. Negatively reacting individuals were not especially 

 common with D. pulex. 



The statements regarding the behavior of D. pulex in the experi- 

 mental tank apply to D. longispina; this species sometimes became 

 caught in the surface film and was unable to free itself. Such indi- 

 viduals were discarded and excluded from the record. 



S. exspinosus, on being released within the experimental tank, 

 settled to the bottom at once or swam a few strokes without orien- 

 tation and then settled to the bottom. Occasionally some individuals 

 just after their release swam upward and attached to the surface 

 film. In many broods, some individuals, after setthng to the bottom 

 or rising to the surface, made no further movements during the 15 

 minutes of the test, while with some broods there was no further 

 movement on the part of any individual. The majority of individuals, 

 however, began moving again within a few seconds, or at most within 

 2 or 3 minutes, assumed their orientation, and swam toward the 

 light. Most individuals, after orienting, paused for a time, settled 

 to the bottom, and afterward continued their movement toward the 

 Hght; others, after the second pause, did not move again. With S. 

 exspinosus there was less random movement after the first pause than 

 with the two species of Daphnia. Occasionally individuals, in their 

 movement toward the light, swam near the surface, and when they 

 paused attached to the surface film. Such individuals frequently did 

 not move again. As with Daphnia, the orientation often was not 

 perfect and some individuals reached the sides of the tank, fre- 

 quently attaching there, and making no further movements; but more 

 generally they followed along the side of the tank with the same 

 general orientation as before. Orientation in a negative way was 

 uncommon with this species. 



The frequency of occurrence of large numbers of non-reactive 

 individuals of S. exspinosus was a distinct difficulty to satisfactory 

 experimental conditions. Because of this apparent difficulty, some 

 of the earlier lines of S. exspinosus with which selection was attempted 



