58 THE BEHAVIOR OF LOWER ORGANISMS. 



that is, by a turning toward the dorsal side. The simplest case is per- 

 haps that of the reversal of orientation, produced when strong sunlight 

 is allowed to fall from in front upon specimens that are swimming 

 toward a diffusely lighted window. Under these circumstances, as 

 we have seen, the Euglenae turn toward the dorsal side, changing their 

 course. They may turn directly through 180°, in which case they are 

 at once oriented with anterior ends away from the light ; but usually 

 the orientation is less direct than this. The reaction is generally 

 repeated several times. Through its continued swerving toward the 

 dorsal side, combined with the revolution on the long axis, the organism 

 directs its anterior end successively in every direction. When the 

 anterior end has finally come into a position where it points away from 

 the strong light the reaction ceases, and the organism swims forward 

 in the usual way. The details of the orienting reaction will be brought 

 out more fully in the following account of the way in which the anterior 

 end becomes directed toward a source of light of moderate intensity. 

 Let us now take a case in which the change in the direction of the 

 rays of light is not accompanied by a change in the intensity of illumi- 

 nation. Euglense are swimming about at random in a diffuse light 

 when all the light is allowed to fall upon them from one side. They 

 then become oriented, with anterior ends directed toward the source of 

 light. Or, the organisms are swimming toward a source of light when 

 the direction of the light rays is changed or reversed by quickly 

 moving the source from which the light comes. The Euglenae then 

 after a time become reoriented. Under such circumstances there is no 

 sudden, decided reaction, such as occurs when the illumination is 

 suddenly decreased. The organism merely begins to swerve farther 

 toward the dorsal side than usual. Thus the spiral has become wider, 

 and the anterior end comes to be pointed successively in many dif- 

 ferent directions, as illustrated at 1-6 in Fig. 23. In some of these 

 positions the anterior end is directed farther away from the source 

 of light, as at 3 ; in other positions more nearly toward the source 

 of light, as at 6. In the latter case the swinging toward the dorsal 

 side becomes less marked ; hence the succeeding phase of the swing, 

 which carries the anterior end away from the light, is less pronounced ; 



dorsal side, owing to the decrease in the illumination of the anterior end. Thus 

 the spiral becomes wider, a and b showing the limits of the swerving. At 3 the 

 normal amount of swerving is restored, so that the new path is at an angle with 

 the old one. Now the organism swerves at each turn of the spiral a short dis- 

 tance away from the source of light, as at c, e, g^ and a longer distance toward 

 the source of light, as at d, f, k, for the reasons shown in Fig. 23. At h it has 

 in this manner become directed toward the source of light, and there is no fur- 

 ther cause for swerving more to one side than to the other; it therefore swims 

 in a spiral with a straight axis toward the source of light. 



