128 journal of Comparative Neurology and Psychology, 



the directive force ot the hght and the tendency to swim in curves. 

 This would necessarily result in movement at an angle with the 

 light rays. The size of this angle would depend upon the relative 

 efficiency of the directive force of the light and the tendency to 

 swim in circles. If the organisms are strongly positive, the direc- 

 tive force of the light is strong compared with the tendency to move 

 in curves and the angle becomes small. But if they are not 

 strongly positive, the directive force of the light is relatively weak 

 and the angle becomes large. The theoretic results thus formu- 

 lated are in accord with the experimental results described in the 

 preceding pages. 



J. Effect of Contact Stimulation and Rctation on Orientation. 



When colonies of Volvox come in contact with the side of the 

 aquarium nearest the light and the rays are perpendicular to this 

 side, many of them soon begin to drift to the right along the glass 

 wall, and in a short time a large majority are found in the right 

 hand corner of the aquarium nearest the source of light. This 

 movement to the right takes place in a field of graded light as well 

 as in light of uniform intensity, and it is apparently as marked 

 if the intense end of the field is to the left as it is if this end is to the 

 right. Thus the organisms were frequently seen to move along 

 the wall toward the right, on the one hand, into regions gradually 

 decreasing in intensity until they passed into darkness and, on the 

 other, into regions gradually increasing in intensity until they 

 became negative. The movement to the right along the wall 

 takes place, with much greater regularity, however, in specimens 

 containing large daughter-colonies or spores than in young colo- 

 nies. Indeed it is doubtful whether more of the young colonies 

 turn to the right than to the left after they reach the wall of the 

 aquarium. At any rate shortly after the introduction of a group 

 containing both large and small colonies, practically all the large 

 colonies, together with some small ones, have gathered in the right 

 hand corner, some small ones have collected in the left hand corner, 

 and a few of both kinds usually remain scattered along the entire 

 side. What is the cause of this movement to the right along the 

 wall .? 



After reaching the wall the colonies ordinarily remain with the 

 anterior end in contact with it for some little time, but sooner or 



