64 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



found that a single arm containing only the peripheral 

 arm-nerve rights itself when laid on its back. Hence 

 the central nerve-ring acts only as a conductor and 

 not as a " centre " for this reaction (i). 



2. In analysing this righting reflex of the starfish, 

 there are two possibilities to be considered. Either 

 gravity forces the starfish to turn the ventral side 

 toward the centre of the earth, or contact-irritability, 

 i. e., stereotropism, forces the animal to bring the vent- 

 ral side in contact with solid bodies. The fact that 

 the animals leave the horizontal bottom of an aqua- 

 rium and attach themselves to the vertical sides shows 

 that gravity is not the cause. Preyer made an ex- 

 periment from which he concluded that the righting 

 of the starfish is due to their being forced to have 

 the ventral side down. He suspended a starfish in 

 the middle of the aquarium by fastening each of its 

 arms by threads to a cork that floated on the surface 

 of the aquarium. If suspended with its back down, 

 Preyer noticed that the starfish turned over. This 

 might suggest the idea that the righting of the star- 

 fish is a geotropic phenomenon. I have repeated 

 Preyer's experiment and confirmed his observation (2). 

 At the same time, however, I made a control experi- 

 ment which Preyer omitted. In the beginning I 

 fastened the starfish to the cork-plate in such a way 

 that the ventral side was turned toward the bottom. 

 But the starfish even then turned over. This shows 

 that the suspension makes it restless and causes it to 

 perform all sorts of turning movements. I believe 



