PHOTOTAXIS 



45 



region of higher light intensity it is apt to reverse its move- 

 ment and change its course. In this way it may avoid 

 regions of strong illumination. In weak light, however, 

 Euglena is positively phototactic. It swims in a spiral, 

 but in a fairly direct path toward the light. 

 If during its progress the light is carried to 

 one side Euglena gradually turns until it is 

 oriented to the direction of the rays. Al- 

 though Jennings attempts to explain the 

 orientation as a result of a modified motor 

 reflex, I cannot see but that the method em- 

 ployed is one of direct orientation. Depar- 

 tures from the line are corrected, not as in 

 the earthworm, by a lot of undirected move- 

 ments until the right one is hit upon, but by 

 an appropriate turn in the right direction. 



Among a very large number or organisms 

 in various phyla of the animal kingdom we 

 find that there is a fairly definite and direct 

 orientation to the rays of light. Deviations 

 from the path to or from the light are checked 

 by a movement which brings the animal into 

 line again. In lower forms this movement 

 is doubtless an involuntary one based upon 

 the property of responding to a localized 

 light stimulus by a direct reflex. Depar- 

 ture from the line of orientation subjects 

 the animal to unequal stimulation on the two sides and 

 the unequal motor activity thus produced brings the animal 

 back into line again so that both sides are equally stimulated 

 and locomotion takes place either toward or away from the 

 light in the direction of the rays. The course of the animal is, 

 as it were, automatically regulated. The mechanics of the 



FIG. 4. Euglena 

 viridis. 



