GEOTROPISM IN ANIMALS 



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. 



III. 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. 



