191"! Esierhj: Occurrence of Rhi/thinx in Geotropisni 399 



are in tlie lowest section at any time, but tubes standiu": before the 

 windows show most animals in the upper sections. 



The bearing of the experimental data given liere on an expla- 

 nation of the diurnal depth migrations in the sea will be considered 

 when the tield collections and observations, that are being made now, 

 can be presented. 



Summary 



1. The marine copepods Acartia toiisa and A. clausi both show a 

 marked tendency to stay toward the top of a column of water in 

 diffuse light. 



2. Both species, if kept in the dark, show larger numbers on the 

 whole toward the bottom. 



3. But there is a marked increase in relative numbers in the upper 

 parts of the column, as compared with the lower portions, from 6 to 

 S P.M., and not at other times of the da}'. This may be repeated on 

 the second day although the animals have been in darkness all the 

 time. 



4. The descent begins, apparently, between 8 and 9 p.m. At any 

 rate, in experiments that extend into a second day, the animals are 

 in the lower section for the most part, in the morning; they ascend 

 again in the evening. 



5. Temperatures below 16° C have the effect in the case of tonsa 

 of causing more animals, relatively, to remain toward the top, though 

 the periodic ascent is still evident. 



Transmitted January S9, 1917. 



ScMPPS Institution, 

 La Jolla, Calif. 



