MOTOR RESPONSES 



?85 



to rotation, disappear. The organism is then oriented (Fig. 99). The 

 response induced by changing the direction of the rays, or by rotation in 

 lateral illumination of uniform intensity, is precisely the same as the 

 response induced by a decrease in the intensity of the light in the field 



a.'s. 



- -iC.U. 



Figure 100. Side view of anterior end of Euglena viridis. e, pigmented portion of 

 eyespot; /, ilagellum; e.f, enlargement in flagelium; c.v, contractile vacuole; e.s. eye- 

 spot surface of the organism; a.s. abeyespot surface of the organism. (After Wager, 

 1900.) 



without a change in the surface illuminated. The change from illumina- 

 tion of the anterior end or the abeyespot surface to illumination of the 

 eyespot surface therefore must, in some way, result in a rapid decrease 

 in the illumination of the photosensitive substance. How is this brought 

 about? 



Wager (1900) demonstrated that the eyespot in Euglena consists 

 of a spoon-shaped portion containing red pigment and a small globular 

 enlargement of one of the roots of the flagelium in the concavity of the 

 pigmented portion (Fig. 100). The eyespot is situated near the eyespot 



