286 MOTOR RESPONSES 



surface, a short distance from the anterior end, with the convex surface 

 directed outward and backward. When the anterior end, or the abeyespot 

 surface, is directed toward the hght, the enlargement in the eyespot is 

 fully exposed; but when the eyespot surface faces the light, the enlarge- 

 ment is in the shadow cast by the pigmented portion. It is evident, then, 

 that rapid change from illumination of the anterior end, or the abeyespot 

 surface, to illumination of the eyespot surface causes rapid decrease 

 in illumination of the enlargement in the eyespot, and that if the en- 

 largement is photosensitive, change in the direction of the rays or rota- 

 tion on the longitudinal axis has the same effect as decrease in the in- 

 tensity of the light in the field. It is therefore highly probable that the 

 enlargement in the eyespot is photosensitive and that the pigmented por- 

 tion functions in producing changes in intensity of light on it, when the 

 axial position of the organism changes and when it rotates on the longi- 

 tudinal axis in lateral illumination. This contention is supported by the 

 facts that the region of maximum stimulating efficiency in the spectrum 

 is in the blue for Euglena (Mast, 1917) and that blue is absorbed by the 

 yellowish-red pigmented portion of the eyespot (Fig. 102). 



In photonegative specimens the responses to changes in light intensity 

 in the field and to changes in the surfaces illuminated are precisely like 

 those of photopositive specimens, except that the responses are induced 

 by (1) increase rather than decrease in light intensity and (2) by change 

 from illumination of the eyespot surface to illumination of the abeyespot 

 surface. 



The process of orientation in free-swimming specimens is, in prin- 

 ciple, precisely the same as it is in crawling specimens. 



Orientation in Euglena is, then, clearly due to a series of responses 

 dependent upon the rate of change in the intensity of the light on the 

 photosensitive substance, which is probably situated in the concave sur- 

 face of the pigmented portion of the eyespot. The light does not act 

 continuously, and there is no evidence whatever indicating anything in 

 the nature of balanced or antagonistic action of locomotor appendages 

 on opposite sides, in accord with the Ray-Verworn theory. 



The evidence in hand indicates, in short, that the photosensitive sub- 

 stance is confined to the concavity in the pigmented portion of the eye- 

 spot; that rotation on the longitudinal axis results in alternate shading 

 and exposing of this substance, if the organisms are not directed toward 



