MOTOR RESPONSES 



281 



surface, resulting in a spiral course. Its direction of movement is changed 

 by the shifting of the distal end of the flagellum from the surface of the 

 body so as to increase the angle between it and the surface. This in- 

 creases the deflection of the anterior end (Fig. 98). 



Shock reaction and aggregation. — Engelmann (1882) observed that if 

 the intensity of the light in a field in which euglenae are swimming about 

 at random is rapidly decreased, they stop suddenly, then turn and pro- 



—'^nf] 



A B 



Figure 98. A. Diagrams showing the position of the flagellum as seen in a viscid 

 medium; a, when Euglena is swimming forward in a narrow spiral; b, when swerving 

 sharply towards the dorsal side; c, when moving backwards. B. Dotted area, shows the 

 position of the moving India-ink particles, a, when Euglena is swimming forward in a 

 narrow spiral ; b, when swerving toward the dorsal side during a shock-movement. 

 (After Bancroft, 1913.) 



ceed in various directions. He designated the response as a "Schreck- 

 bewegung" (fright movement, or shock reaction), because the re-ori- 

 ented organisms gave the impression of having been frightened. It was 

 found that if the intensity is slowly changed this response does not 

 occur. It is therefore dependent upon the rate of change in intensity. 



He says that if there is a spot of relatively strong light in the field, 

 it acts just like a trap; owing to random movements, the euglenae get 

 into this spot, but as they reach the boundary on the way out, the rapid 

 reduction in intensity induces the shock reaction and consequently pre- 

 vents their exit. 



Under some conditions the euglenae respond to rapid increase in in- 



