184 The Animal Mind 



planarian is right side up would of course produce a turn- 

 ing in the opposite direction, a negative reaction. In this 

 case, however, the opposite side does not contract to 

 allow of turning, but maintains the same length. The 

 necessary result is that the body is thrown into a spiral : as 

 soon as the ventral surface of the head comes into contact 

 with the solid, in consequence of the turning, the negative 

 reaction of that end ceases. Thus the righting is pro- 

 gressively accomplished (561). 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 position as readily 

 as any other provided that the ventral surface and not the 

 dorsal is in contact with a support. 



51. Orientation to Gravity: Annelids 



Geotropism, in the marine worm Conwluta 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 



