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 of 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- yy¢,4.—Kuglena 
ture from the line of orientation subjects ee 
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 therays. The course of the animal is, 
as it were, automatically regulated. The mechanics of the 

