ORIENTATION IN COLONIAL ORGANISM 6 



positive) never in the opposite direction, as frequently occurs in 

 Euglena, Stentor and the Hke. The turning of the colonies is 

 due to an increase in the effective stroke of the flagella on the 

 shaded side. So much has been definitely established. As to 

 the cause of the increase in the activity of the flagella on this 

 side we are, however, not in a position to speak with so much 

 assurance, but our evidence seems to indicate that it is depend- 

 ent upon the time-rate of change of light intensity on the photo- 

 sensitive tissue in the individual zooids. Let us briefly consider 

 this evidence. 



During the process of locomotion the colonies continuously 

 rotate on the longitudinal axis; consequently, when they are not 

 oriented and opposite sides are unequally illuminated, the zooids 

 are continuously transferred from a region of higher to one of 

 lower light intensity and vice versa. This results in a decrease 

 of intensity on each zooid, when it reaches the shaded side of 

 the colony; but there is another factor involved in producing 

 changes of intensity, in all probability of more significance than 

 this. In unoriented colonies the zooids are not only continuously 

 subjected to a transfer from one intensity of light to another, 

 but during the transfer different surfaces become exposed; for 

 when they are on the more highly illuminated side of the colony 

 the outer surface, and when they are on the opposite side the 

 inner surface is directed toward the source of light. Since the 

 zooids contain numerous translucent and refractive bodies this 

 change in the surface exposed necessarily results in numerous 

 changes of intensity on the different parts of each zooid. The 

 eye-spots are the most prominent of the translucent bodies men- 

 tioned and the}^ are consequently of greatest importance in pro- 

 ducing changes of intensity within the zooids. The orienting 

 stimuli, in my opinion, depend upon the time-rate of change of 

 intensity thus produced. 



This opinion is strongly supported by the reactions to hght 

 observed in Gonium. These reactions are essentially like those 

 in Volvox, but before describing them it will be necessary to re- 

 fer briefly to the structure of the organisms under consideration. 



