TRANSFORMATION OF HELIOTROPIO ANIMALS 293 
Of course, negative geotropism is not so closely linked 
with positive heliotropism in all pelagic animals. If, for 
example, the Copepods mentioned in this paper are intro- 
duced into the eudiometer tube, and the upper end is 
covered with the opaque capsule, the animals also rise in 
the tube when they are positively heliotropic. They do not, 
however, go under the opaque cap, as do the Loligo larva, 
but remain at the highest uncovered portion of the tube. 
They therefore move upward entirely, or at least mainly, 
through their positive heliotropism. 
4. The effect of temperature upon the depth-migration 
and depth-distribution of marine animals must yet be 
studied experimentally. We saw that the larve of Poly- 
gordius are negatively heliotropic and positively geotropic 
at high temperatures, but become positively heliotropic and 
negatively geotropic at lower temperatures. The same trans- 
formation of heliotropism occurs in Copepods. It can 
scarcely be assumed that other animals will not show the 
same phenomenon. In water whose surface reaches a very 
high temperature in summer such animals must disappear 
during that period from the surface, as the high tempera- 
ture makes them negatively heliotropic, and perhaps also 
positively geotropic. The negative heliotropism and posi- 
tive geotropism drive these animals into the depths. As 
soon, however, as they reach cooler layers of water below 
the surface, they again become positively heliotropic and 
negatively geotropic; they must then rise again until they 
have reached the warmer layers of water. Here they soon 
again become negatively heliotropic, and perhaps positively 
geotropic, when they must again sink; and soon. In this 
way such animals are kept floating at a certain distance 
under the surface of the water during the summer. When, 
however, the temperature becomes sufficiently low in the 
winter, the animals may rise to the surface without becoming 
negatively heliotropic or positively geotropic. 
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