EXPERIMENTS ON ECHINODERMS 69 



cumarire are of necessity inhabitants of the surface- 

 regions of the ocean. If a larva were carried clown 

 to a great depth, its negative geotropism would force 

 it to migrate upward until the highest point was 

 reached or until death put an end to its upward 

 journey. 



Certain starfish - - for instance Astcrina gibbosa, 

 which also lives near the surface of the water behave 

 like Cucumaria. All the experiments I have made 

 on Cucumaria can likewise be successfully performed 

 on Asterina gibbosa, but with the difference that the 

 exceptionally voracious Asterina does not remain per- 

 manently at the highest point of the vertical surface. 

 In two days, or sometimes even sooner, it begins to 

 move or drops down. 



Positive heliotropism naturally has the same effect 

 as negative geotropism. Asterina tenuispina, like As- 

 terina gibbosa, lives at the surface of the sea. It is 

 not, however, geotropically irritable ; but it is posi- 

 tively heliotropic. I put a large number of specimens 

 of both species in a heap in an aquarium, into which 

 rays of light from one side only fell nearly horizon- 

 tally. In a short time the two species had parted, 

 the Tenuispinse crawling off on the floor toward the 

 source of light. The Gibbosae, scattered about on 

 the bottom of the aquarium in every direction, 

 crawled up the vertical sides without being influ- 

 enced at all by the light in their movements. In the 

 ocean, where the vertical rays of daylight are chiefly 

 concerned in the orientation of animals, positive 



