136 BEHAVIOR OF THE LOWER ORGANISMS 



a circle. In other words, the direction of the path has been changed 

 (Fig. 92). The whole action may be expressed as follows: when the 

 light is suddenly decreased, the organism tries successively many 

 different directions, finally following one of these. 



The reaction is a very sharp and striking one, and produces a most 

 peculiar impression. At first all the Euglenae are swimming in parallel 

 lines toward the window. As soon as the shadow of the hand falls upon 

 the preparation, the regularity is destroyed ; every Euglena turns strongly 

 and may appear to oscillate from side to side. This apparent oscilla- 

 tion is due to the swerving toward the dorsal side, combined with the 

 revolution on the long axis. The organism swings thus first to the 

 right, then upward, then to the left, then down, etc. (see Fig. 91). 



This reaction occurs whenever the light is decreased in any way. 

 Thus, in place of cutting off the light coming from the window, that 

 coming from the mirror of the microscope may be decreased by closing 

 the iris diaphragm. The Euglenas react in the manner above de- 

 scribed, though they soon resume their movements toward the window. 

 Again, if the light from the window is decreased only slightly, the Eu- 

 glena? react in the manner described, thus changing their direction of 

 movement ; very soon, however, they swim again toward the window. 

 The same reaction occurs in Euglenae that are for any reason not swim- 

 ming toward the source of light. Even if a specimen is swimming away 

 from the window, it gives the avoiding reaction in the usual way when 

 the light from the window is decreased. 



It is clear that the reaction is due to the decrease in the intensity of 

 light, not to a change in the direction of the light rays. In the first and 

 second experiments mentioned in the preceding paragraph, the Euglenae 

 are, some time after the light is decreased, swimming in the same direc- 

 tion as they were before, though at the moment of decrease there is a 

 reaction. 



Engelmann (1882) tried shading parts of the body of Euglena. He 

 found that a shadow which is cast on the body of the organism without 

 affecting the anterior one-third produces no effect whatever. On the 

 other hand, a shadow affecting only the anterior tip — if even only the 

 part in front of the eye spot — causes the same reaction as shading the 

 entire body. Thus it is clear that the anterior end is more sensitive to 

 light than the remainder of the body. These results of Engelmann are 

 of much importance for understanding the remainder of the reaction to 

 light. 



If Euglenae are placed on a slide and a certain spot is lighted from 

 below by the mirror of the microscope, a dense collection is in the course 

 of time formed in the lighted region. Observations show that the Eu- 



