32 THE BEHAVIOR OF LOWER ORGANISMS. 



the Stentors are soon found in the shaded half. S. cceruleus thus shows 

 the phenomenon usually called negative phototaxis. 



It is to be noted that not all the Stentors are to be found on the side 

 away from the light, or in the shaded half of the vessel. On the con- 

 trary, a considerable fraction of the whole number will usually be found 

 swimming about in all parts of the dish, or at rest in the lighted portion. 

 The light reaction is thus somewhat inconstant, and varies among 

 different individuals. It varies considerably with Stentors of different 

 cultures ; from some cultures almost all the individuals show it, while 

 from others it is barely noticeable. This variability and inconstancy 

 run through all manifestations of the light reaction in Stentor. 



A word further needs to be said as to the behavior of individuals 

 which are not free-swimming, but are fixed by the posterior end. Such 

 individuals do not react at all to light. When light is thrown on them 

 they remain in the positions in which they are found at the beginning, 

 neither contracting nor in any way changing their position. No matter 

 whether the light is weak or strong, and without regard to the direction 

 from which it comes, fixed Stentors give no reaction and show no 

 orientation with reference to light. The contact reaction apparently 

 inhibits the light reaction completely. We shall therefore omit the 

 fixed individuals from consideration in the remainder of the account, 

 confining attention to the free-swimming specimens. 



The typical motor reaction of Stentor, by which it responds to most 

 stimuli, is as follows: The Stentor stops or swims backward a short 

 distance, then turns toward the right aboral side, and resumes its for- 

 ward motion. This is the reaction which is produced by strong 

 mechanical stimuli, by heat, and by chemical stimuli, acting upon the 

 anterior end or upon the body as a whole. 



How is the reaction to light brought about? To answer this ques- 

 tion it is best to arrange experiments in such a way as to distinguish 

 as far as possible the effect due to unequal illumination of different 

 areas from the effect due to the direction from which the light is coming. 



In order to produce strong differences in illumination in different 

 areas of the space in which the Stentors are found, a flat-bottomed 

 glass vessel containing many Stentors in a shallow layer of water was 

 placed on the stage of the microscope, in a dark room. From beneath 

 strong light was sent upward through the opening of the diaphragm, 

 by throwing the light from the projection lantern (using the electric 

 arc light) on the substage mirror. By this the light was directed up- 

 ward through the vessel containing the Stentors. Thus a small, 

 definitely bounded circular area was illuminated, while the rest of the 

 vessel remained in darkness. A black screen was usually placed over 

 the diaphragm opening of the microscope in such a way as to shade one- 



