TKANSFOBMATION or HELIOTROPIC ANIMALS 291 



have made experiments on the small animals obtained in the 

 surface dredgings at Woods Hole, on Copepods, and on the 

 larvaa 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. 1 



3. 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. 2 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 larvae 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 

 larvae 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 



1 1 have even found a young pelagic fish at Woods Hole which is as pronouncedly 

 positively heliotropic as the insect larvse described in a preceding paper. I was not 

 able to ascertain the species to which it belonged. [1903] 



2 Part I, p. 176. 



