Figure 110. Diagrammatic representation of the process of orientation in Volvox. 

 A, B, C, D, four zooids at the anterior end of the colony; l-a, longitudinal axis; large 

 arrows, direction of illumination ; small arrows, direction of locomotion ; curved arrows, 

 direction of rotation ; /. flagella ; e, eyes, containing a pigment-cup represented by a 

 heavy black line and photosensitive tissue in the concavity of the cup. Note that when 

 the colony is laterally illuminated, the photosensitive tissue in the eyes on the side 

 facing the light is fully exposed and the flagella on this side beat laterally. Those on the 

 opposite side, shaded by the pigment-cup and the flagella on this side, beat directly 

 backward. The difference in the direction of the beat of the flagella on these two sides is 

 due to alternate decrease and increase in the luminous intensity to which the photo- 

 sensitive tissue in the eyes is exposed, owing to the rotation of the colony on its longi- 

 tudinal axis — an increase causing, in photopositive colonies, a change in the direction 

 of the stroke of the flagella from backward or diagonal to lateral ; and a decrease, a change 

 from lateral or diagonal to backward. In photonegative colonies, precisely the opposite 

 obtains. In photopositive colonies, this results in turning toward and in photonegative 

 colonies turning from the source of light. In both, the turning continues until opposite 

 sides are equally illuminated, when changes of intensity on the photosensitive tissue are 

 no longer produced by rotation and the orienting stimulus ceases. (After Mast, 1926a.) 



