spatially Determined Reactions 187 



ammal is thrown by its position, seems plausible. But 

 what is the stimulus inducing the unrest? Not contact 

 of the back with a solid object, for a starfish is not disturbed 

 if its back is touched when it is crawling in the ordinary 

 position ; and not merely having its back- directed down- 

 ward, for it will crawl upside down on the under surface 

 of rocks. Something abnormal about the stimulation 

 of the tube feet when they are in contact, not with a 

 soUd support, but with the water flowing over them, 

 must present the condition for the internal state of 

 instability which occasions the twisting movements of 

 the rays. 



The sea-urchin, "a rigid, non-muscular, and globular 

 mass, " as Romanes calls it, with relatively feeble suckers, 

 has a much harder time to right itself, and does not succeed 

 in pulhng itself over unless it is perfectly fresh and vigorous. 

 It occasionally rests for some time when it has reached a 

 position of stability halfway over, before continuing the 

 process (641). 



Lyon has observed marked negative geotropism in the 

 larvae of the sea urchin. He was unable to test Davenport's 

 theory of the nature of the geotropic response by putting the 

 animals in a solution of the same density as their own bodies, 

 for the reason that such a fluid was too dense and sticky 

 (being made of gum arable and sea water) for them to 

 swim in. That the response was merely a passive one 

 he thinks improbable, because the larvEe from eggs that 

 have been rapidly rotated, or "centrifuged," as it is 

 called, have all the pigment on one side of their bodies 

 and may therefore be supposed to have their ordinary 

 balance disturbed; yet they rise to the surface just like 

 the rest (450). 



