GEOTROPISM IN ANIMALS 188 
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. Actiniamesembryanthemum 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 tenuispine crawled along the bottom toward the source 
of light; the gibbose 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 
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