152 'Journal of Comparative Neurology and Psychology. 



under such conditions they appear to be hanging in the water 

 motionless. They are, however, rotating on their longitudinal 

 axis. If now the light intensity, to which these apparently motion- 

 less organisms are exposed, is increased they soon begin to turn 

 toward its source; but in so doing they swim upward, as repre- 

 sented in the accompanying diagram. 



In thus swimming upward and horizontally toward the source 

 of light, it is clear that the effect of the backward stroke of the 

 flagella increases both on the shaded side and on the illuminated 

 side, for both sides move forward. But the shaded side moves 

 farther than the illuminated side, consequently the increase in the 

 effect of the backward stroke must be greater on the former than 

 on the latter. The difference in the effect of the stroke of the 



Fig. 13. Diagram representing the reaction of a Volvox colony when the light intensity is suddenly 

 changed, a, outline of colony; h, longitudinal axis; c, light rays; d, point in the course where the light 

 is suddenly decreased; «, point where it is suddenly increased; /, course taken by colony. In continuing 

 from e, the side of the colony facing the source of light travels over a shorter distance than the shaded 

 side. Consequently the backward stroke of the flagella on the latter side must be more effective than 

 that of those on the former. 



flagella on opposite sides which results in orientation of positive 

 Volvox colonies is, therefore, due to a greater increase in the back- 

 ward stroke of the flagella on the shaded side than of those on the 

 illuminated side. 



If the light thrown upon apparently motionless colonies is quite 

 intense, they frequently may be seen to sink 4 or 5 mm. immedi- 

 ately after the light is turned on, but while they are sinking this 

 short distance, they apparently become acclimated and soon turn 

 toward the light, and at the same time swim upward, just as 

 described above. During the time in which these colonies sink 

 they continue to rotate in the same direction as before. The 



