988 THE AMERICAN NATURALIST. [Vor. XXXV. 
PHOTOTAXIS. 
The most conspicuous reaction is the reaction to light. The 
animal is negatively phototactic for even moderate light inten- 
sities. The response is very distinct and definite. If a num- 
ber of animals in a dish or jar be brought suddenly into diffused 
daylight, each begins almost immediately to move in the direc- 
tion away from the source of light. Within a few seconds 
every animal in the dish is moving in this manner. Arrived 
at the opposite side of the dish, the animals do not immedi- 
ately come to rest, but move about restlessly, following the 
sides of the dish, or in some cases even turning back toward 
the light for a short distance. The final result of these move- 
ments in a cylindrical jar is the aggregation of the worms in 
the region of least light intensity, not at the point farthest 
from the light but on the two sides at a point midway between 
the points farthest from and nearest to the light, where a large 
part of the light is shut off by the walls of the jar. In this 
respect these animals resemble Planaria (Loeb, '93), with the 
difference, however, that they are more restless and do not 
form such distinct and well-marked aggregations under these 
conditions as does Planaria. 
In jars kept in a dark closet or in diffuse daylight on dark, 
cloudy days or toward night, the worms crawl about freely ; 
i.e., they are not distinctly negative for very low intensities. 
Examination of a jar during the night shows the worms to be 
active. They are thus evidently nocturnal in habit like Planaria. 
Animals from which the whole head, including eyes and 
brain, has been removed, react to light in the same manner as 
do normal animals, but the reactions are slower and less exact 
(Parker and Burnett, '00). 
The question as to whether the reactions of Stichostemma 
are strictly speaking phototactic or photopathic, whether two 
different kinds of reaction occur, is perhaps not easily decided. 
Certainly, so far as experiments already made indicate, there 
is nothing in the reactions of the animals to light that cannot 
be explained as due to the effect of different intensities with- 
out relation to the direction of the rays (Holt and Lee, '01). 
