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 larvae, 

 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 larvae 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 so on. 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. 



