OBSERVATIONS ON UNICELLULAR FORMS 1 15 



surface illuminated as indicated by the currents in the 

 water. 



(2) A number of attached Stentors in a small rectangular 

 glass aquarium were repeatedly suddenly exposed in light 

 intensity of 8000 ca. m. produced by a Nernst lamp. When 

 they were thus exposed they were directed toward various 

 points of the compass, so that various parts of the surface 

 faced the source of light in different individuals. A few 

 always contracted immediately after each exposure, others 

 began to swing about the point of attachment, some clock- 

 wise and others counter-clockwise, but all turned toward 

 the ventral surface. The cilia must consequently beat the 

 same in all individuals no matter which surface is exposed 

 to the light. There is therefore no evidence in these results 

 that light acts constantly as a directive stimulus. 



(3) After Stentors had oriented in light from a single 

 Nernst glower, the glower was slightly moved to one side 

 so as to change the direction of the rays slightly, and the 

 method of reorienting was observed. It was found that 

 under such conditions the Stentors merely swerve farther 

 away from the source of light each time after the oral side 

 is directed toward it in the process of rotation. Thus they 

 soon become oriented again. There is no definite avoiding 

 reaction in this process of orientation. The organisms 

 never increase the swerving toward the source of light ; they 

 always increase it in a direction which tends to turn the 

 anterior end from the light. Does light act as a constantly 

 directing stimulation in this process of orientation or does 

 it act by causing repeated successive stimulations due to 

 changes of intensity on some part of the surface of the 

 organism as in Euglena? Is Stentor heliotropic according 

 to Loeb's definition or is it unterschiedsempfindlich? The 

 following experimental observations will furnish answers to 

 these questions. 



Two Nernst glowers were arranged and screened so as 

 to produce two small beams of light which crossed at right 

 angles in a small aquarium containing numerous Stentors. 



