96 



LIGHT AND THE BEHAVIOR OF ORGANISMS 



was when the organism was in this position before, as 

 represented in Fig. 13. This reaction is repeated during 

 each complete rotation. Every time the eye-spot becomes 



$ ^ I * t ». 



'■' i'/ 



i/ <' 



o 



•<= ^« 



-«^ 



-^< 



-^0< 



-^i^^ 



-^S 



-«a 



-<^ 



■<r- 



-<«s 



^ 



n 



-«*. 



Fig. 13. Euglena sp. (?) in crawling state, showing details in process of orien- 

 tation; V, contractile vacuole; es, eye-spot; n, o, direction of light; a-c, positions 

 of Euglena with light from n 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 ^-eaction 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 straight- 

 ens to g, and rotates to h, when the eye-spot again faces the light and the organism 

 is again stimulated and bends to i, from which it proceeds to j, etc., to m, where 

 it is practically oriented. 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. (Compare with orientation in Stentor, 

 Fig. 14.) 



