160 The Animal Mind 



60. 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 geotaxis 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 (95). 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 in- 

 dividuals the posterior part, which is poorly controlled, 

 was heavier than the anterior, and pulled the animal around 

 head upward (135). The statocyst organs in a cephalopod, 

 Eledone, have been shown to function in maintaining 

 equilibrium (137). 



61. Orientation to Gravity: Echinoderms 



Very interesting righting reactions, in the starfish and sea 

 urchin, are described by Romanes. The starfish rights 

 itself by twisting around the tips of two or three of its rays 

 until the suckers in the ventral side have a firm hold of the 

 supporting surface, and then continuing the twisting, always 

 in the same direction on the different rays, until the whole 

 body is turned. The sea urchin, "a rigid, non-muscular and 

 globular mass," with relatively feeble suckers, has a much 



