146 Journal of Comparative Neurology and Psychology. 



to light. If stimulated they turn toward a structurally defined 

 side regardless of the direction of the rays or difference in light 

 intensity on opposite sides of the organisms. If they fail to 

 become oriented by a single motor reaction they repeat the reaction, 

 turning successively in different directions, until they turn in the 

 right direction; this direction they hold and thus become oriented. 

 The process of orienting in these organisms is, therefore, strictly 

 on the trial and error basis. 



In Volvox, taking a colony as a whole, there is no evidence of 

 motor reactions, nor is there any hit or miss method about its 

 orientation. It makes no mistakes in the process. If exposed to 

 light it turns toward the source of light without error. What 

 sort of mechanism has this organism, by means of which it can 

 thus regulate the direction of its motion ? 



Fig. 12. After Holmes, 1903, p, 325. 



A colony of Volvox may be conceived to turn in its course by 

 decreasing or increasing the backward stroke of the flagella on one 

 side or the other, or by using the flagella on either or both ends as 

 rudders, or even by directing the stroke of these flagella in such a 

 way as to turn the organism. But since the organisms orient 

 when either the posterior or the anterior end is missing, and prob- 

 ably also when both ends are missing, it is clear that the flagella 

 on the ends do not function primarily in changing the direction of 

 motion. Such changes must, therefore, be the results of inequality 

 of the strokes of the flagella on opposite sides. What then is it 

 that causes the strokes on opposite sides to become unequal ? 



Holmes ('03, p. 325) after concluding that it cannot be caused 

 by difference in light intensity on opposite sides, suggests the 



