TRANSFORMATION OF HELIOTROPIO ANIMALS 291 
have made experiments on the small animals obtained in the 
surface dredgings at Woods Hole, on Copepods, and on the 
larvee of crustaceans, worms, and molluscs, and have thus far 
been able to find no pelagic animal of these classes which 
is not either permanently, or at least at times, positively 
heliotropic.’ 
8. It would be incorrect, however, to assume that heli- 
otropism is the only condition for the depth-distribution of 
animals. Just as in the vegetable kingdom positive heli- 
otropism and negative geotropism often act together toward 
the same end, we must expect similar conditions in the ani- 
mal kingdom. In a paper on geotropism I have already 
shown that certain starfish and Actinians, which always live 
near the surface of the water, are compelled to creep con- 
stantly upward, owing to a peculiar form of irritability, and 
I have made it probable that this irritability is negative 
geotropism.” I have since been able to convince myself that 
in certain animals which would be forced by their positive 
heliotropism alone to go to the surface, other conditions are 
at work which co-operate with heliotropism. This is the 
case, for example, in the freshly hatched larvee of Loligo. 
These animals are constantly positively heliotropic, and, 
besides, live at the surface of the sea. When these animals 
are introduced into a long eudiometer tube filled with sea- 
water which is set up vertically, they move to the surface of 
the sea-water. As long as the effective rays of light fall 
into the tube from above, the positive heliotropism of the 
larvee would compel them to move upward. I found, how- 
ever, that the animals come to the surface of the water also 
in the dark room. Moreover, when the eudiometer tube is 
exposed to the light, with the upper part of the tube covered 
1I have even found a young pelagic fish at Woods Hole which is as pronouncedly 
positively heliotropic as the insect larve described in a preceding paper. I was not 
able to ascertain the species to which it belonged. [1903] 
2 Part I, p. 176. 
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