184 NATURE STUDY REVIEW [9:6— Sept., 1913 



Results. — Isopods and amphipods will collect at the bottom, 

 that is they are positive to the stimiili acting. Eubranchipus 

 will be at the top, hence, they give a negative reaction. 



Discussion. — This experiment as it stands is simply a double 

 experiment without a control. It may be varied by admitting 

 light at the top of one cylinder and leaving the other covered. If 

 this is done the fairy shrimps (Eubranchipus) will leave the top 

 and collect at the bottom of the cylinder, because they are said to 

 be positive to gravity when light is acting. Another variation 

 is to illuminate strongly the bottom inch of one cylinder while 

 the other is left dark as a control. Under these conditions isopods 

 and amphipods will move up out of the light thus disregarding 

 their reaction to gravity in order to react light. 



Any small water animal may be tested in this device. 



In a darkened cylinder such as was first described, it must not 

 be thought that gravity is the only stimulus acting. For one 

 thing the deeper the water, the greater the amount of pressure, 

 so that there is a water pressure gradient which is lowest at the 

 surface and increases rapidly with increasing depth. Then, too, 

 the gaseous content of the water is different at different depths. 

 As an instance, in a ditch near Hammond, Indiana, with the water 

 about one foot deep and the bottom covered with leaves to a 

 depth of nine inches the following differences were found: 



Oxygen in Carbon dioxide in 

 Position of collection. cc. per liter cc. per liter. 



Surface 3.87 13.5 



Mid depth 2.79 6.0 



Top of leaves 2.79 6.0 



Top of leaves 2.79 6.0 



Middle of leaves 2.49 8.5 



Bottom of leaves 1.95 9.0 



In open lakes greater differences are shown, thus : 



Oxygen in 

 Position of collection. cc. per liter. 



Surface 4.6 



20 in. below surface 13.8 



40 in. below surface 9.8 



Small fishes will react negatively to the difference of carbon 



dioxide shown above, while both fish and isopods will react to the 



oxygen gradient given in the lake. Obviously then animals 



may react to a gas gradient in the conditions described for the 



experiment on gravity and they are always subjected to differences 



in water pressure under the given conditions. Thus it is apparent 



that what has been usually classed as a response to gravity may 



have a much more complex cause. 



