68 PLANT-ANIMALS [OH. 



at the beginning of an " up " phase than it is toward 

 the end of that phase. Thus, if specimens are col- 

 lected as soon as the tide is off the colonies and are 

 brought in a vessel into the laboratory, they swarm 

 up to the surface almost as soon as the vessel ceases 

 to be shaken, whereas animals collected after a long 

 light-exposure and placed in a similar position, may 

 remain down till the next tidal " up " phase is due. 



Thus it is reasonable to conclude that, after some 

 five or six hours of light-stimulation, internal changes 

 are induced which act as stimuli and cause the animal 

 to change the sign of its response to gravity. It 

 becomes positively geotropic and descends beneath 

 the sand. In the darkness of the sand, recovery of the 

 original, normal state takes place gradually, and the 

 animals now respond to the stimulus of gravity by 

 a movement in the opposite sense. They ascend to 

 the surface. In its simplest form, the hypothesis 

 involves the assumption that prolonged light-exposure 

 and prolonged dark-exposure modify the tone or state 

 of nervous irritability of the animals, and that these 

 changed conditions manifest themselves by a changed 

 mode of response to gravitational stimulus. After 

 a prolonged light-exposure, the animals are positively 

 geotropic ; after a corresponding sojourn in the dark, 

 they become negatively geotropic. The reversal of 

 the direction of a tropistic movement is by no means 

 unusual among plants and animals. Thus, in order 



