1 1 Tr ~^?~ 



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 

 with the effect of ' daylight 

 entering the aquarium from 

 above. If the animals are 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. If a 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 

 Cucumariae nevertheless continue to creep up to the bottom of 

 the beaker. They do this even when the experiment is made 

 as shown in Fig. 36. A bridge BB is suspended in the aqua- 

 rium AA so that its horizontal part BBi 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 abed, 

 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- 

 marige nevertheless leave the neighborhood o, collecting 

 either on the bottom of the beaker cd or upon its vertical 

 sides near cd. 



