Mast, Light Reactions in Loiuer Organisms. II5 



course free, and if it is the action of these cilia which causes rota- 

 tion we should not expect a change in the direction of rotation 

 when the anterior end is stimulated. 



As stated above, we find reversal in the direction of rotation 

 frequent in water containing numerous small particles. What is 

 the cause of this ^ This is probably due to particles becoming 

 entangled in the cilia and obstructing their free movement, thus 

 causing a change in the direction of rotation. 



While we have thus found that reversal of rotation is largely 

 caused by external agents, it is unquestionably true that it depends 

 to some extent upon the condition of the organism itself, for under 

 similar external conditions difference in the frequency of reversal 

 was repeatedly noted. 



6. ORIENTATION — GENERAL DISCUSSION. 



It is well known that if Volvox is subjected to light of moderate 

 intensity, it swims toward the source of light; but if the light inten- 

 sity is high, as e. g.y direct sunlight, it travels in the opposite direc- 

 tion. In casually studying such movements it appears as if the 

 course of the colonies in either direction were nearly parallel with 

 the light rays, and investigators have, in general, assumed this 

 to be true. Holmes ('03, p. 320), writes: "It is easy to deter- 

 mine that Volvox orients itself, and that very accurately, to the 

 direction of the rays of light. If specimens of Volvox are taken 

 into a dark room and exposed to the light from an arc lamp they 

 travel towards the light in almost a straight course, swerving 

 remarkably little to the one side or the other. They will often 

 travel a foot without deviating as much as a quarter of an inch 

 from a perfectly straight course." 



In studying the effect on the direction of movement, of difference 

 in light intensity on opposite sides of a Volvox colony, I accident- 

 ally discovered that, contrary to Holmes' conclusion, Volvox 

 very seldom orients "accurately to the direction of the rays." The 

 colonies do, of course, swim toward or from the source of light 

 in a general way; but movement parallel with the rays is quite the 

 exception. In swimming toward a source of light the colonies 

 may deflect not only to the right or left but also up or down. De- 

 flection up or down will be discussed under the effect of gravi- 

 tation on orientation (p. 122); deflection to either side will be taken 

 up in connection with the description of the following experiments. 



