132 LIGHT AND THE BEHAVIOR OF ORGANISMS 



function of the eye-spot, since in this species it is located 

 well toward the posterior end of the body. 



These creatures react very definitely to light. They are 

 positive in weak and negative in strong light, and swim on 

 a spiral course. The intensity however in which they are 

 positive or negative varies greatly in different individuals 

 and in the same individual under different conditions. It 

 is very difficult to follow their movements since they are 

 so small and swim so rapidly. Jennings (1904, p. 64) found 

 that they "react to a decrease in illumination by a sudden 

 turn to one side, by an increase in the width of the 

 spiral, and by a change in the course just as happens 

 in Euglena and Cryptomonas." But he was '' unable to 

 determine the relation of its structure to the spiral path 

 and to the direction of turning in the reaction." My 

 observations confirm the conclusions of Jennings as stated 

 above. 



The orienting reactions in these organisms were studied 

 just as they were in Euglena. They were alternately 

 exposed in each of two beams of light which crossed on the 

 stage of the microscope at right angles. If the light from 

 the two beams is equal and the organisms are not very 

 sensitive, all turn toward the source of light, none in the 

 opposite direction, when the direction of the rays is sud- 

 denly changed. This they do by swerving farther in one 

 direction than in the opposite as they proceed on their 

 spiral course, just as do Euglenae and Stentors under similar 

 conditions. But if the light from one source is more 

 intense^ than that from the other, and the intensity is 

 increased at the same time that the direction of the rays is 

 changed, many of them stop and turn sharply, some toward 

 the light, others away from it. This reaction is very strik- 

 ing; there is apparent confusion for some time after the ray 

 direction and the intensity are simultaneously changed, 

 whereas if the ray direction is changed without a change 



^ The light from one source was 100 ca. m.; that from the other 160 ca. m. 

 in these experiments. 



