1 62 Journal of Comparative Neurology and Psychology. 



to 29° C. ; Massart ('9 1 , p. 1 64), who found Chromulina, a flagellate, 

 to be positive at 20°, and negative at 5; and Strasburger ('78, 

 p. 605), who states that swarm-spores, positive to a given light 

 intensity at 16 to 18° C. are negative to the same intensity at 40. 

 It seems strange that organisms so nearly alike as Chromulina, 

 VoJvox and swarm-spores should be affected so differently by 

 change in temperature. 



14. EFFECT OF MECHANICAL STIMULI ON THE CHANGE IN 

 SENSE OF REACTION TO LIGHT. 



In working on the light reactions of Temora longicornis, a cope- 

 pod, LoEB ('93, p. 96) noticed that the animals, ordinarily nega- 

 tive, were frequently positive immediately after being caught. 

 This change m the sense of reaction was due probably to mechani- 

 cal agitation. Miss Towle ('00) found that the light reaction 

 of Cypridopsis could be changed from positive to negative by tak- 

 ing the animals up in a pipette or by making them pass through 

 a maze constructed with needles. Holmes ('01) thinks that the 

 fact that Orchestia gracilis is positive in air and negative in water, 

 may be due to the contact stimulus of the water. It was demon- 

 strated by Parker ('02, p. 117) that certain forms of tactual stim- 

 ulation cause the light reactions in the copepod, Labidocera, to 

 change from positive to negative. 



The effect of stimulation by light on Volvox can readily be over- 

 come momentarily by mechanical stimulation, but it was found 

 impossible to change the sense of reaction by such stimuli. In 

 attempting to do this various methods were used, as, for example, 

 shaking the organisms violently, lifting them in a pipette and 

 squirting them into water, and making them swim toward the 

 source of light among numerous large sand grains with which they 

 came in contact. 



Whatever the cause of reversal in the sense of light reaction in 

 Volvox may be, it is clear that such reversal is of primary impor- 

 tance in the life of the organism. While continuous exposure to 

 very intense light is fatal to Volvox colonies, they must have a cer- 

 tain amount of light, since they depend upon photosynthesis in 

 the process of feeding. It is therefore evident that it is of great 

 advantage to them to be able to move into regions of comparatively 

 high intensity during dark, cloudy days, early in the morning, and 

 late in the evening, and into shaded places when the light becomes 

 very intense. 



