158 STUDIES IN GENERAL PHYSIOLOGY 
and the dark. The fact that the animal assumes a vertical 
position in every case seems toindicate that gravity is the deter- 
mining factor. I have tried to see whether the animal would 
assume upon the centrifugal machine the position of one of the 
radii, that is, with its foot directed toward the periphery 
and its head toward the center of the rotating disc. But 
the animal had always to be kept in a vessel of water in 
these experiments, and the currents set up in the water by 
the rotation interfered with the movements of the exceedingly 
soft animal. Even when the animal was fastened to the 
wall of the vessel by a needle, its free ends were always set 
in motion by the water. Nothing remained, therefore, but 
to introduce the animal into a long test-tube which was fas- 
tened radially upon the revolving table, and to observe 
whether the animal directed its foot or its head toward the 
center of the revolving table. The experiments which have 
been performed thus far have not given a uniform result. 
9. The animal retains a vertical position permanently 
only when at the same time contact stimuli act constantly 
upon its entire surface. The animal retains a vertical position 
permanently in the sand, but only for a few days at the best 
upon a wire screen. Iwas also able to keep the animal perma- 
nently in a horizontal position in a closely fitting test-tube. 
The head which projected beyond the lips of the test-tube 
was directed vertically upward. 
How strongly these animals are compelled to bring as much 
as possible of their bodies in contact with other solid bodies is 
evidenced by the fact that they crowded themselves forcibly 
under lead blocks and lead plates which I had laid upon the 
bottom of the aquarium. This is the same form of contact- 
irritability that is found in Forficula, larvee of Musca, winged 
ants, etc.—a phenomenon which I have described in greater 
detail in previous publications.’ 
1“The Heliotropism of Animals,” p. 1, and also “ Further Investigations on the 
Heliotropism of Animals,” p. 89. 
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