142 LIGHT AND THE BEHAVIOR OF ORGANISMS 



due to difference in light intensity on opposite sides of the 

 colonies, it is brought about in positive specimens by the 

 flagella striking backward with greater effect on the side 

 in lowest light intensity than elsewhere. I suggest the fol- 

 lowing explanation of this: 



" First, it must be remembered that the organism con- 

 stantly rotates on its longitudinal axis. If then a colony 

 is so situated that one side is more highly illuminated than 

 the opposite, it is clear that the zooids will constantly be 

 carried from a region of higher to a region of lower light 

 intensity, and vice versa. They are thus subjected to con- 

 stant changes in strength of illumination. As stated above, 

 the flagella strike backward with greater vigor on the shaded 

 side than on the opposite one and, therefore, it is evident 

 that as the zooids reach the region of lower light intensity, 

 in other words when the light intensity to which they are 

 subjected decreases, they increase the effect of the back- 

 ward stroke of the flagella, i.e., they attempt to turn toward 

 a structurally defined side (the side facing the anterior end 

 of the colony). This is precisely what Euglena does when 

 it passes from a region of higher to one of lower light inten- 

 sity, i.e., it turns toward a structurally defined side, the 

 larger lip. The individuals in a colony then respond with a 

 motor reaction induced by change in light intensity; they 

 react on the same basis as do Euglena, Paramecium, Stentor 

 and other unicellular forms, in their trial and error reactions, 

 ? but owing to the way in which they are interrelated, and 

 to the rotation of the colony on the longitudinal axis, this 

 reaction of the zooids causes orientation in the colony as a 

 whole, without error. 



*' This explanation of orientation in entire colonies holds 

 also for orientation in segments. As previously stated, only 

 those segments orient which have such a form that they 

 can rotate. As they rotate the cut surface constantly faces 

 the center of the spiral, so that if the axis of the spiral is 

 not directed toward the source of light, the outer surface 

 where the zooids are situated is alternately turned toward 



