120 University of Calif or)iia Publications i)i Zoology. [Vol.8 



Furthermore, the data show two maxima in every depth above 

 20 fathoms. One occurs between 6 and 10 a.m. and the other 

 between 6 and 8 p.m. Excepting the depth of 100-150 fathoms, 

 only one maximum occurs in each depth below 25 fathoms and 

 that between 10 a.m. and 2 p.m. What does this irregular dis- 

 tribution of maxima signify? Is it merely the effect of chance 

 due to the small amount of hauling, or is some other explanation 

 necessary ? 



The small number of hauls may have affected thi.s distribu- 

 tion of maxima in some depths, but it will not explain the 

 presence of two maxima on the surface. In correlating the 

 number of S. hipunctata on the surface with time of day it is 

 evident that the species increases in abundance from midnight 

 till it reaches a morning maximum between 6 and 8 a.m., and 

 after decreasing rapidly to a minimum between 10 a.m. and 4 

 p.m., it increases again till it reaches an evening maximum 

 between 6 and 8 p.m. It then gradually decreases to a midnight 

 minimum.. As most of our collecting was done during June and 

 July, these data teach us that the positions of these maxima 

 approximately correspond to the time of sunrise and sunset. To 

 be more exact, the species attains its morning maximum on the 

 surface within an hour after sunrise, and its evening maximum 

 within an hour after sunset. This indicates that the species 

 migrates toward the surface during twilight and away during 

 bright daylight and darkness. 



If this is true, we should expect a greater abundance of 

 specimens on the surface during evening than during morning 

 twilight, for the light increases in intensity at dawn while it 

 decreases during the evening. Consequently, if the species 

 migrated to the surface during morning twilight its migration 

 would soon be checked owing to the increasing intensity of light, 

 while its migration during the evening would remain un- 

 checked. The fact that the evening maximum is the larger 

 harmonizes entirely with this hypothesis. Why the species 

 should leave the surface so soon after attaining its evening 

 maximum abundance is puzzling. It may be due to the small 

 amount of collecting between 10 p.m. and 4 a.m., to possible 



