GEOTROPISM IN ANIMALS 181 
zontal axis in the aquarium, the animal endeavors unceas- 
ingly to creep upward as often as the plate is turned through 
an angle of 90°. We are not dealing in this case with a 
compensatory motion brought about by centrifugal force; 
while the plate is being turned the animal remains quiet, 
and not until some fifteen or 
twenty minutes later does it 
begin to move upward. 
Nor do we deal in this case B == 
with the effect of daylight 
entering the aquarium from A 
above. If theanimalsare kept 
in an aquarium into which light is allowed to enter by suit- 
able means only from below and without, the animals con- 
tinue to creep up the vertical surfaces, the direction in which 
they move not being influenced in any way by the light. 
One might think that the need of oxygen determines the 
movements of Cucumaria toward the surface of the water, 
but it can be shown that this also is not the case. Ifa large 
beaker, from which the air has been removed by filling it 
with water, is placed in an aquarium upside down—that is 
to say, with the bottom of the beaker directed upward—the 
Cucumarie nevertheless continue to creep up to the bottom of 
the beaker. They do this even when the experiment is made 
as shown in Fig. 86. A bridge BB is suspended in the aqua- 
rium AA so that its horizontal part B,B, is below the sur- 
face of the water n in the aquarium. In the bridge is a 
small circular hole o, over which the inverted beaker abcd, 
in which the air has been displaced by water, is placed. 
Fresh water is supplied through o under slight pressure 
by means of a suitably curved glass tube g. The Cucu- 
mariz nevertheless leave the neighborhood 0, collecting 
either on the bottom of the beaker cd or upon its vertical 
sides near cd. 
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