182 STUDIES IN GENERAL 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 Cucumarie 
and insects are dependent upon specific organs, as in the case 
of vertebrates. 
This relation of Cucumarie to gravitation compels the 
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