Spatially Determined Reactions 159 



consequence of the turning, the negative reaction of that 

 end ceases. Thus the righting is progressively accomplished 

 (316). The whole response can hardly be classed under 

 the head of geotropism. Like that of Hydra, it is not made 

 as the result of the pull of gravity, but is a reaction to contact 

 stimulation; the animal will crawl in an upside-down posi- 

 tion as readily as any other provided that the ventral sur- 

 face and not the dorsal is in contact with a support. 



59. Orientation to Gravity: Annelids 

 Geotropism, in the marine worm Convoluta roscoffensis, 

 has been found to fluctuate with the rise and fall of the tides, 

 even when the animal is removed to an aquarium. In 

 normal life the worms burrow in the sands at rising water, 

 and come to the surface when the tide retreats. Prolonged 

 exposure to air, or increase in the intensity of the light, 

 causes them to move down the slope of the shore to moist 

 places. These movements in the normal environment are 

 represented by upward and downward movements of the 

 animal when confined in a glass tube. Keeble and Gamble 

 thought these oscillations in geotropism did not occur in 

 darkness, and that the stimulus bringing them about was 

 photic. When the summation of light stimuli passes a cer- 

 tain amount, they maintained, positive geotropism appears ; 

 when the after effect of light stimulation is dissipated, the 

 negative phase recurs (140). Bohn, however, finds that the 

 oscillations do persist in darkness, and that their primary 

 cause is the mechanical shock of the waves, as is further 

 indicated by the observation that shaking the tube will 

 cause the worms to descend (35). The geotropism of Con- 

 voluta is dependent on the statocyst (140). 



