52 University of California Publications in Zoology [Vol.19 



their negative geotropism involves both locomotion and maintenance of 

 position at high levels. If they are in section V when subjected to 

 darkness they do not descend, providing the temperature is low 

 enough. 



When exposed to light from above in water of room temperature 

 the animals descend and remain at the bottom. But in the cold water 

 they ascend toward the light and remain at the top or near it. In 

 light from below, however, there is a marked movement upward at 

 room temperatures; at low temperatures, on the other hand, the 

 animals will descend toward the light and remain in the bottom 

 sections. 



Inspection of the figures in table 20 showing the centers of distri- 

 bution reveals the difference in distribution of the animals as the 

 temperature is higher or lower 



The results obtained with individual animals are confirmative of 

 those with sets of animals, although no tests at low temperature were 

 made in the former case. Table 21 (p. 55) deals with such records and 

 it shows that the dominant response is positive geotropism in all condi- 

 tions except when the light is below. 



The experiments show: (1) That Metridia lucens is negative to 

 light at ordinary temperatures, but positive if the water is cooled to 

 9°C. (2) The animals are positively geotropic at ordinary tempera- 

 tures in darkness and under any condition of illumination except light 

 from below, and these responses are reversed in cold water. 



POSSIBLE BEARING OF THE EXPERIMENTS ON DIURNAL 

 MIGRATION 



It is safe to say that so far as these experiments go they offer a 

 ready explanation of some phases of the vertical migration. Metridia 

 is negative to light and is positively geotropic (except when lighted 

 from below) at temperatures ranging from 16°C to 19°C. Both the 

 geotropism and phototropism are reversed at 9°C, 



Now if the animals in the sea descend to sufficient depths they will 

 encounter temperatures in which the direction of movement is re- 

 versed and it may be expected that they will begin to ascend. The 

 field data show that these copepods are found in largest numbers dur- 



