158 STUDIES IN GENERAL PHYSIOLOGY 



and the dark. The fact that the animal assumes a vertical 

 position in every case seems to indicate 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, larvae of Musca, winged 

 ants, etc. a phenomenon which I have described in greater 

 detail in previous publications. 1 



i "The Heliotropism of Animals," p. 1, and also "Further Investigations on the 

 Heliotropism of Animals," p. 89. 



