GEOTROPISM IN ANIMALS 183 



animals to live in the upper regions of the sea. If a larva 

 were driven into deep water, it would be compelled to crawl 

 upward without ceasing,* through its negative geotropism, 

 until it had reached the Surface again, or until death brought 

 an end to these efforts. 



2. Other marine animals, which we also know to live only 

 in the upper regions of the sea behave as does Cucumaria 

 cucumis. Actinia mesembryanthemum of the Bay of Naples 

 belongs to this class. If I remember correctly, however, I 

 have never observed this behavior in the Actinian of the East 

 Sea, which has the same name, but differs somewhat in its 

 characteristics, from that found in the Bay of Naples. But 

 certain starfish, such as Asterina gibbosa, are also negatively 

 geotropic. All the experiments which I have made on Cucu- 

 maria are successful also in Asterina gibbosa, with this 

 difference, however, that when the exceedingly voracious 

 Asterina has reached the highest point on a vertical surface 

 and finds no nourishment there, it does not remain in this 

 position permanently, as does Cucumaria, but begins to move 

 or drop down after two days or even less time. Asterina 

 gibbosa also lives only at the surface of the sea. 



Positive heliotropism acts, of course, in a way similar to 

 negative geotropism. Asterina tenuispina lives near the sur- 

 face of the sea, as does Asterina gibbosa. It is, however, not 

 geotropic, but positively heliotropic. I laid a large number of 

 the two species together in a heap in an aquarium into which 

 light fell almost horizontally from one side only. Within a 

 short time the two kinds of animals were clearly separated; 

 the tenuispina3 crawled along the bottom toward the source 

 of light ; the gibbosaB distributed themselves in all directions 

 over the bottom and crawled upward upon the vertical walls 

 without the direction of their movements being influenced 

 by the light. In the ocean, in which only those rays of light 

 which enter vertically are of importance for the spatial 



