154 yournal of Comparative Neurology and Psychology. 



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 backward stroke of the 

 flagella, /. e., they attempt to turn toward a structurally defined 

 side (the side facing the anterior end of the colony). This is pre- 

 cisely what Euglena does when it passes from a region of higher 

 to one of lower light intensity, i. ^.,-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 inter-related, and to the rota- 

 tion 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 seg- 

 ments 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 the light and away from it. Thus the zooids are 

 carried from regions of higher to regions of lower light intensity 

 and vice versa, and the motor reaction is induced just as it is in 

 entire colonies. 



Orientation in negative colonies can be explained in precisely 

 the same way as that in positive ones, assuming merely that in 

 this condition the zooids respond with the motor reaction when 

 they pass from lower to higher light intensity instead of when they 

 pass from higher to lower (as is true when the organisms are posi- 

 tive). The backward stroke then becomes most effective on the 

 side most highly illuminated. 



II. REACTION OF NEGATIVE COLONIES. 



Volvox becomes negative when exposed to light of a certain 

 intensity. The intensity, however, varies greatly in different colo- 

 nies and in the same colony under different conditions. Radl 

 ('03, p. 103) concludes his discussion on the difference between 

 positive and negative phototropism with the following paragraph : 

 "Ich glaube nun, dass der Unterschied zwischen positivem und 

 negativem Phototropismus ahnlich wie beim Menschen nicht ein 



