284 



MOTOR RESPONSES 



faces the light, when they again respond. The gradual straightening 

 during rotation results in greater deflection of the anterior end toward, 



■^^ 



Figure 99. Euglena sp. in a crawling state, showing details in the process of orienta- 

 tion; p, contractile vacuole; es, eyespot; n, o. direction of light; a-c, positions of 

 Euglena with light from n is intercepted ; c-m, positions after light from n is turned 

 on and that from o cut off, so as to change the direction of the rays. If the ray direction 

 is changed when the Euglena is in position c, there is no reaction until it reaches d. 

 Then it suddenly reacts by bending away from the source of light to e. after which 

 it continues to rotate and reaches position /, where it gradually straightens to g, and 

 rotates to h, when the eyespot again faces the light and the organism is again stimulated 

 and bends to /, from which it proceeds to /, and so forth. If the ray direction is changed 

 when the Euglena is at d, it responds at once and orients as described above. If the 

 intensity from n is lower than that from o the organism may respond at once when the 

 ray direction is changed, no matter in which position it is. (After Mast, 1911.) 



rather than away from the light. Thus the anterior end becomes directed 

 more and more nearly toward the light source, until an axial position 

 is reached in which changes in illumination of the eyespot surface, owing 



