Spatially Determined Reactions 185 



negative phase recurs (253) . 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 (61). The geotropism of Con- 

 voluta is dependent on the statocyst (253). 



52. Orientation to Gravity: Mollusks 



Among Mollusks, the slug has had its reactions to gravity 

 carefully observed. When placed in a horizontal position 

 on an inclined glass plate, these animals tend to turn either 

 upward or downward, moving either with or against the 

 force of gravity. Davenport and Perkins found that the 

 same individuals differed at different times in this respect, and 

 concluded that the sense of the geotropism was determined 

 by obscure conditions. They also found that an inclination 

 of only 7.5 on the part of the glass plate, representing only 

 13 of the full force of gravity, is sufficient to make the slugs 

 orient themselves with reference to the pull of the earth, 

 though the precision of such orientation increases as the 

 angle increases (175). Frandsen thought it was the 

 weight of the posterior part of the body that determined 

 whether the movement should be up or down : that the 

 natural tendency of all was to go downward, but that in 

 some individuals the posterior part, which is poorly con- 

 trolled, was heavier than the anterior, and pulled the animal 

 around head upward (236). 



Kanda (392, 393), on the other hand, thinks that in 

 freshwater and marine snails the statoliths are the organs 

 determining orientation to gravity, and that it is not 

 merely passive: he claims to have observed that this 

 orientation is most marked, the less the slope of the surface 



