50 THE BEHAVIOR OF LOWER ORGANISMS. 



the typical motor reaction described in my previous paper as a response 

 to other classes of stimuli (Jennings, 1900, p. 235). That is, they 

 turn at once toward the dorsal side (that bearing the larger lip and the 

 eye). This is very easily seen when the Euglenae are mounted in the 

 ordinary manner in a thin layer of water on a glass slide and observed 

 with the microscope in the neighborhood of a window. If the hand 

 is interposed between the slide and the window all the Euglense react 

 in the way just described. 



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

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

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

 the slide the regularity is destroyed ; every Euglena turns strongly and 

 may seem to oscillate from side to side in the manner described later. 



The turning is often preceded by a slight movement backward. 

 This was not observed in the reactions to other stimuli (Jennings, 1900, 

 p. 235), though it agrees with what we find in most other ciliates and 

 flagellates. In Euglena the reaction to variations in the intensity of 

 light seems more sharply defined than to most other stimuli. The fact 

 that the turning is always toward the dorsal side is observable with the 

 greatest ease. It is particularly evident when the organisms are con- 

 fined to a thin layer of water, so that they cannot swerve up or down, 

 but only to the right or left. 



The reaction occurs whenever the light is suddenly decreased in any 

 way. Certain different conditions under which it occurs deserve special 

 mention, (i) As we have seen, the reaction occurs when a screen is 

 brought between the organisms and the source of light toward which 

 they are swimming. (2) It also occurs when the illumination is de- 

 creased by cutting off' light from some other source than that toward 

 which they are swimming. Thus the organisms on the stage of the 

 microscope may be lighted from below, by the substage mirror, and at 

 the same time may receive light from the window at one side of the 

 preparation. They swim toward the window, since the light from that 

 quarter is much stronger than that from below. If now the light from 

 below is suddenly decreased by closing the iris diaphragm, the Euglenae 

 react as usual by turning strongly. This is notwithstanding the fact 

 that the proportion of light coming from the window, to which they 

 were oriented, is now greater than before, so that it might be supposed 

 that they would remain more strongly oriented than ever. For the rest, 

 the disturbed orientation is soon restored. (3) The reaction occurs 

 when the decrease in illumination is due to the movements of the 

 Euglenae ; that is, when the swimming organisms come to the edge of 

 a lighted region where they would, if the course were continued, pass 

 into the darkness. As a result of the reaction they return into the light. 



