OBSERVATIONS ON UNICELLULAR FORMS 133 



of intensity there appears to be perfect order, all the speci- 

 mens turning gradually toward the light. 



The fact that these organisms turn in various directions 

 when the intensity is decreased regardless of the side illu- 

 minated indicates that they always turn toward a struc- 

 turally defined side. I was however unable to follow the 

 reactions under these conditions so as to see if there was 

 a definite relation between the location of the eye-spot 

 and the direction of turning. But by carefully studying 

 specimens swimming about slowly in an optimum light 

 intensity, I saw that with very few exceptions they turn 

 toward the side on which the eye-spot is situated. A 

 few specimens which were loosely entangled in debris 

 were seen to turn in the opposite direction. This was 

 probably due to some interference with the movement 

 of the flagella. It may therefore be concluded that 

 Chlamydomonas always turns toward the side contain- 

 ing the eye-spot. 



The fact that positive specimens turn toward the source 

 of light when the ray direction is changed without a change 

 of intensity, and that they turn toward the side containing 

 the eye-spot, indicates that they are most sensitive when 

 the side without the eye-spot is illuminated, for it is a de- 

 crease of intensity which causes a reaction in these organ- 

 isms when they are positive. It therefore follows that a 

 change from a position in which the eye-spot is on the 

 shaded side to one in which it is on the illuminated side has 

 the same effect as a decrease of intensity. Is this decrease 

 due to the shadow cast by the eye-spot? In Euglena it 

 seems likely that it is. In some specimens of Chlamydo- 

 monas, however, this structure is situated so near the 

 posterior end that it is difficult to see how it could 

 function in this way. The long axis of one of the speci- 

 mens represented in Fig. 17 would have to be at an 

 angle of nearly 45° with the direction of the light before 

 the eye-spot would cast any shadow on structures in the 

 body. 



