78 STUDIES IN GENERAL PHYSIOLOGY 
side of the vessel, the oral piece moved toward the room 
side, but not the aboral piece. When the oral piece moved 
from the room side toward the window, it soon turned about. 
Under similar conditions the aboral piece continued to creep 
until it reached the window. When the vessel containing 
the animals was carefully reversed, the aboral animal was 
not affected, but the oral animal immediately moved toward 
the room side. 
It can easily be shown that in leeches the head, which 
contains the eyes, reacts more energetically toward light 
than the aboral pole. If some small stones are lying on the 
bottom of a beaker which contains such animals, and the 
vessel is suddenly illuminated, the animals push their heads 
under the stones, while the aboral pole remains at rest even 
though exposed to the light. It is astonishing to notice 
how long after the illumination the reaction appears. It is 
not unusual for thirty to seventy seconds to elapse between 
the illumination and the beginning of the movement. Hoff- 
meister observed a still longer latent period in the case of 
the earthworm. It would be unnecessary to show that in 
animals which possess eyes the oral pole is more sensitive 
toward light than the aboral. We may therefore accept it 
as certain that the oral pole of an animal is more sensitive 
toward light than the aboral, whether the animal does or 
does not possess eyes. 
In consequence of this fact, it is difficult for an animal to 
move perpendicularly or obliquely to rays of light emanating 
from a sufficiently intense source, for, as the oral pole is 
more sensitive than the aboral, the former must turn more 
energetically toward or away from the source of light 
(depending upon whether the animal is positively or nega- 
tively heliotropic) than the aboral. 
2. The heliotropic irritability is also different on the 
ventral and dorsal surfaces of a dorsiventral animal. To 
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