182 STUDIES IN GENEKAL PHYSIOLOGY 



Yet the hydrostatic pressure does not compel the animals 

 to move upward to the surface of the water, for when the 

 animals are put into a flat dish containing only 1-2 cm. of 

 water, so that the animals are just under the surface of the 

 water, they also move to the vertical wall of the vessel and 

 attach themselves to it, even though they are no nearer the 

 surface of the water in this position than anywhere else in 

 the aquarium. The nature of the conditions of the experi- 

 ment shuts out the possibility of rheotropism and hydrot- 

 ropism. 



Experiments with the centrifugal machine yielded no 

 results, as the animals do not move at all during the rotation 

 of the machine. The. one condition which compels the 

 animals to seek vertical surfaces and crawl upward is gravi- 

 tation. I imagine that the way in which gravity compels 

 the animals to move upward is similar to that which compels 

 insects, such as the butterflies which have just left their pupa 

 case, to crawl upward. Immediately after hatching, the 

 wings of the butterfly are not yet unfolded, and the animal 

 runs about in a restless way until it reaches a vertical sur- 

 face upon which it creeps and remains, with its head directed 

 upward, for a relatively long time, until the unfolding of the 

 wings or other conditions cause the animal to become rest- 

 less again. In cockroaches this dependence of rest and 

 unrest upon gravitation is still more apparent. They remain 

 perfectly quiet only when the weight of their bodies exerts 

 a pull upon their legs, but not when the weight of their bodies 

 presses upon their legs. Cucumaria cucumis seems to behave 

 in a similar way. The direction of the pull, however, influ- 

 ences in addition the direction of the progressive movements 

 upward. Yet it is possible that these reactions in Cucumarise 

 and insects are dependent upon specific organs, as in the case 

 of vertebrates. 



This relation of Cucumarise to gravitation compels the 



