114 Journal of Comparative Neurology and Psychology. 



in the direction of rotation is caused by contact stimuli at all it 

 must be by contact stimuli along the sides of the colonies. 



Volvox colonies were subjected to such stimuli by laying a glass 

 slide into an aquarium containing filtered water about 3 mm. deep, 

 so that the edge of the slide made an angle of about 45 degrees 

 with the rays of light. When the colonies moved toward the 

 source of light and came in contact with the slide, the point of 

 contact was not at the anterior end but some little distance from 

 it. After being thus stimulated they immediately turned from the 

 slide making an angle of about 95 degrees with their previous 

 course. Then they gradually turned toward the source of light 

 again and thus continued along the edge of the slide making a zig- 

 zag path. In following along the edge in this way they frequently 

 came in contact with the slide before they were perfectly oriented 

 and were consequently stimulated at a point further from the 

 anterior end than usual, sometimes about midway between the two 

 ends. In all these reactions the direction of rotation was seldom 

 changed. It is therefore clear that a single contact stimulus on 

 the side of a colony, which does not obstruct forward progress, 

 does not cause reversal in the direction of rotation. In the experi- 

 ment just referred to a small portion of one of the upper corners 

 of the slide was slivered off, making an incline on which the water 

 became gradually more shallow until, at the upper end, it was not 

 deep enough for the larger colonies to swim w^ithout difficulty. 

 As the colonies worked up this incline, they came in close contact 

 with the glass and the direction of rotation was frequently changed. 



It may then be concluded that continuous contact stimulation 

 on the sides causes reversal in the direction of rotation, providing 

 the contact is such that considerable resistance is offered to for- 

 ward motion. 



But why should contact stimuli on the anterior end, which pre- 

 vents forward motion, not cause reversal as well as similar stimuli 

 along the sides ? Considering the structure of the organism in 

 question, it seems probable that rotation is brought about largely 

 by an oblique stroke of the cilia along the side and that those at 

 the ends have little if anything to do with it. Now it seems reason- 

 able to assume that when a certain proportion of these cilia on the 

 sides meet considerable resistance they all strike in the opposite 

 direction and thus produce reversal of rotation. When the ante- 

 rior end is in contact with an object the cilia along the sides are of 



