MOTOR RESPONSES 



299 



greatly. On the other hand, if the intensity is rapidly increased, the for- 

 ward movement stops and the rate of rotation increases. If the colonies 

 are swimming away from the light, the reverse occurs, i.e., forward move- 

 ment decreases if the intensity is increased, and increases if it is de- 

 creased. If the colonies are neutral, there are no such responses to changes 

 of intensity. These responses consist chiefly, if not entirely, of rapid 

 changes in the direction of the stroke of the flagella. In other words, a 

 rapid decrease in the illumination of photopositive colonies changes the 



Figure 109. Sketches showing the structure of the eyespot in Volvox and its action 

 on light entering the pigment-cup at different angles, p, pigment-cup; /, lens; y, yel- 

 low focal spot; b, bluish green focal spot; ss, photosensitive substance; large arrows, 

 incident rays of light. Note that the longer waves of the incident light are brought to 

 focus in the wall of the pigment-cup and that the shorter waves are brought to focus in 

 the cup, after being reflected from the inner surface, and then continue in the form 

 of a concentrated beam of bluish-green light. Note also that the more obliquely the 

 incident light enters the pigment-cup, the nearer the edge of the cup the yellow focal 

 spot is located. (After Mast, 1927.) 



stroke of the flagella from diagonally backward to straight backward. 

 An increase in the illumination causes it to change from diagonally back- 

 ward to sidewise. In photonegative colonies precisely the reverse obtains. 

 These responses continue for only a few seconds, although if the luminous 

 intensity is slowly changed they do not occur at all. They are therefore 

 dependent upon the rate of change in intensity, i.e., they are shock- 

 reactions which are somewhat similar to those observed in Euglena. 



Kinetic responses. — If Volvox is kept in weak illumination or in dark- 

 ness for several hours, it becomes inactive; but if the illumination is 

 afterwards increased, it gradually becomes active again. These responses 

 consist chiefly, if not entirely, in changes in the rate or the efficiency of 



