GEOTROPISM IN ANIMALS 185 
therefore not the cause of the movement of the starfish. 
Finally, when Preyer believes that parasites compel the 
animals to move upward, it is impossible to see why the 
parasites do not compel the animals which are fastened upon 
the vertical wall to leave it, as the stimuli caused by the 
parasites must now seem to the animals to originate from the 
vertical wall. Yet in reality Asterina gibbosa (and much 
more Cucumaria cucumis) remains upon the highest point 
of the vertical wall. I believe it much more rational to take 
into consideration the effect of gravitation, which acts in a 
vertical direction, in explaining the upward movements of 
certain starfish. 
Ill. THE GEOTROPISM OF HIGHER ANIMALS AND ITS 
DEPENDENCE UPON THE INNER EAR 
The higher animals are also compelled, within certain 
limits, to assume a definite orientation toward the center of 
the earth. It is easily seen in many fishes that they orient 
themselves toward the center of the earth, both while swim- 
ming and while at rest, and that they direct their bellies, 
but never their backs, downward. If we try to compel such a 
fish to lie upon its back, it endeavors and succeeds, as soon as 
liberated, in reassuming its usual orientation. It is not 
physically impossible that such a fish should swim with its 
back downward, and only physiological factors are present 
which compel the fish to direct its belly toward the center 
of the earth. Even we are compelled to assume a certain 
position with reference to the center of the earth; we dis- 
cover it when we bend our heads so that the top of the head 
is downward. The force which compels us to assume a defi- 
nite orientation toward the vertical is naturally not very 
great, yet it has a definite intensity, and it requires an 
external stimulus of a certain intensity, or a definite effort of 
the will, to act against this force in order to overcome it. 
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