BEHAVIOR 23 



dish away from the window ; two showed no response ; the remain- 

 der showed, fair, weak, or doubtful response. 



That Stentor coeriileus moves to the dark side of an elongated 

 aquarium was reported long ago by Holt and Lee (1901) and 

 interpreted tropistically as due to an orientation in the field of light 

 away from the source of illumination. An extensive study of this 

 subject was made by Mast (1906) with different conclusions. 

 Swimming animals placed in a dish lighted from one side simply 

 showed repeated random avoiding responses of backing up and 

 turning until they found themselves headed away from the light, 

 and then they continued swimming forward to the dark side. This 

 response kept animals confined to the darkened end of an aquarium, 

 as if an invisible wall were present. That the confinement was not 

 due to warming caused by the light was shown by the fact that 

 paramecia which are quite sensitive to heat but not to light swam 

 readily into the irradiated area. Attachment of course prevented 

 the avoiding response of stentors, which did not even lean 

 (tropistically) away from the light; but if the light was strong 

 enough the animals detached and then gave the characteristic 

 response. Mast concluded that the anterior end was most sensitive 

 to light because, when the water was a thin film so that the stentors 

 could not face the light source above and received stimulation 

 only on their sides, collecting at the darker end was slower. This 

 point is confirmed by my observation that decapitated stentors no 

 longer avoid the light (unpublished). 



Contrasting wdth coer ulcus, the yellow S, niger shows a marked 

 attraction to Hght, according to the studies of Tuffrau (1957). He 

 states that all parts of the animal appear to be equally sensitive to 

 the stimulus because there was no orientation in a field of light, yet 

 the head end could be more sensitive if the response is not tropistic 

 but one of trial and error. Although there was some individual 

 variability, most of the animals accumulated rapidly at an illumin- 

 ated opening in the side of a covered tube, and the response was so 

 strong that a spot of light acted like a trap in preventing stentors 

 from leaving it after they entered. The shorter the wave length of 

 light, the stronger the attraction: red elicited almost no response 

 and the aggregations increased as the spectrum shifted to blue, 

 violet, and ultraviolet. Animals dark-adapted for fifteen hours 

 recovered photoresponsiveness in an hour or two. The rapidity of 



