1 86 The Animal Mind 



on which the animal crawls. The response of Physa, a 

 freshwater snail, to gravity depends in an interesting way 

 on the animal's physiological condition : when the snail 

 is in need of air it is strongly negative in its geotropism, 

 crawling upward towards the surface of the water and dis- 

 regarding all other stimuli : as soon as its lungs are full of air 

 it is no longer sensitive to gravity (177). Buddenbrock 

 (107, 108, 109) and Baunacke (29-32) have brought evi- 

 dence to support the view that the statocysts in many 

 mollusks are useful not so much in securing orientation to 

 gravity, which is of little importance in such slow-moving 

 animals, but rather in enabling them to right themselves, 

 to direct their movements, and to dig in the sand. The 

 statocyst organs in a cephalopod, Eledone, have been 

 shown to function in maintaining equilibrium (249). 



53. Orientation to Gravity: Echinoderms 



Righting reactions in the starfish have been described by 

 Romanes (641). The tips of two or three rays are twisted 

 around until the suckers in the ventral side have a firm 

 hold of the supporting surface; the twisting is then con- 

 tinued, always in the same direction on the different rays, 

 until the whole body is turned. Jennings (380) enumerates 

 twelve different factors which determine which particular 

 rays shall twist over and attach themselves first, but 

 Moore (501) thinks that the "positive stereo tropism, " 

 that is, the tendency to remain in contact with solids, of the 

 tube feet is a sufficient explanation. It is not clear how a 

 tendency to remain in contact with a mechanical stimulus 

 can explain a tendency to seek such a stimulus when it is 

 absent, and Jennings's view, that the original impulse to 

 turn comes from the general state of unrest in which the 



