54 



THE BKHAVIOR OF LOWER ORGANISMS. 



cone, or the frustum of a cone, as indicated in Fig. 21. The result, as 

 seen from above, is that all the specimens seem to vibrate from side to 

 side ; in other words, they are taken with a sudden oscillation or trem- 

 bling. This oscillation when the intensity of the light is suddenly 

 changed was observed by Strasburger (1878, 

 pp. 25 and 50) in flagellate swarm-spores ; he 

 speaks of it as " Erschutterung " or " Z it- 

 tern." During this oscillation the anterior end 

 becomes pointed successively in many different 

 directions, as Fig. 21 shows. When, now, the 

 usual forward course is resumed (with only the 

 usual amount of swerving toward the dorsal 

 side), the animal follows one of these directions. 

 Thus its path is changed (Fig. 22). Strasburger 

 (1878, p. 25) noticed that the path followed after 

 the oscillation was oblique to the former path. 

 As a study of Figs. 21 and 22 will show, this is a 

 necessary consequence of the increased swerving 

 toward the dorsal side, to which the oscillation 

 itself is due. All these relations become much 

 clearer if a model of an actual spiral is studied ; 

 it is difficult to represent them upon a plane 

 surface. 



If the stimulus is stronger, as when there is a 

 greater decrease in illumination, the swerving 

 toward the dorsal side is much greater ; the or- 

 ganism wheels far to that side, so that the spiral 

 course seems entirely interrupted. But there is 

 really nothing in this reaction differing in prin- 

 ciple from what is happening in the normal 

 forward swimming. If the swerving toward the 



r dorsal side is long continued the specimen may 



be seen to swing first far to the (observer's) right, 

 then, after it has revolved on the loner axis, far 



f IG 22. 



to the (observer's) left ; in reality it swings an 



equal amount upward and downward and in intermediate directions. 

 It may, however, swing at once so far to the dorsal side that the new 



* FIG. 22. Shows the spiral path of Euglena, illustrating the effect of a 

 slightly marked reaction. At a the illumination is decreased; the organism 

 therefore swerves toward the dorsal side, causing the spiral to become wider. 

 At b the ordinary method of swimming is resumed; since at this point the 

 organism was more inclined to the axis of the spiral than before the reaction, 

 the new course lies at an angle with the previous one. Compare with Fig. 21. 



