OBSERVATIONS ON UNICELLULAR FORMS 121 



indicate that it is not in the membranellae or in the ridge 

 from which they project. 



On February 27, 3.30 p.m., a number. of Stentors were 

 put into a one-half-normal glycerine solution. In the course 

 of about two minutes the entire ridge with the membranellae 

 in nearly all the specimens was thrown off, after which the 

 anterior end was rounded and the oral opening was tightly 

 closed. They were then transferred to normal culture fluid, 

 in which they swam about much like normal specimens. 

 They rotated on the long axis, proceeded on a spiral course 

 and responded with the avoiding reaction when they were 

 mechanically stimulated or when suddenly exposed to strong 

 light. The threshold for light reactions was however much 

 greater than normal. At 8.30 p.m. there was no indication 

 that regeneration had begun, but on the following morning 

 nearly all the specimens were normal again. 



d. Localized stimulation. — Are the reactions in Stentor 

 differential responses to localized stimulation ? In Euglena, 

 it will be remembered, we were obliged to answer this ques- 

 tion in the negative. The results described above seem to 

 show that there is a highly sensitive structure in Stentor 

 at' the anterior end near the oral side. It is therefore 

 probable that whenever these animals are stimulated by 

 light they are stimulated in this region regardless of which 

 surface is exposed; and if this is true it is evident that the 

 fact that these organisms may turn toward the illuminated 

 side or toward the shaded side does not prove that they 

 do not give a differential response to localized stimulation, 

 nor does it prove that they do. We have therefore no 

 conclusive evidence bearing on this question with reference 

 to Stentor. 



Summary 



(i) Stentor coeruleus collects in shaded regions either by 

 orienting and swimming directly toward such regions or by 

 wandering into them aimlessly. They remain in the shaded 

 region because whenever they come to the edge of it, the 



