1912] Esterly: Copepoda of the San Diego Region 323 
GENERAL CONSIDERATIONS. 
In the preceding study of the distribution of nineteen species 
of copepods in as much detail as the data allow, the fact of 
greater abundance at night in the upper water has been promi- 
nent in the ease of every species. From collections with closing 
nets we are able to locate the day plurimum or center of migra- 
tion with a reasonable degree of accuracy, and this is never at 
the same depth for daylight hauls that it is for collections made 
during the night. The obvious conclusion is that there is an 
upward migration from the region of greatest abundance during 
the day to that during the night. For this reason the term 
“center of migration’’ as first used by Michael (1911) is par- 
ticularly appropriate in referring to the region of the day 
plurimum. 
The possible effect of the lack of as many night as day hauls 
in deep water in modifying this conclusion has been mentioned 
several times in the course of the paper, but it seems impossible 
that the widespread occurrence of the upward movement at 
night can be due to excess of day hauling. It is exceedingly 
desirable, nevertheless, to have more hauls with the closing nets 
at depths of 200 fathoms and below, and during the night 
especially. 
The matter of upward migration at night is most evident in 
table 54 which deals with surface and closing net hauls, all 
separated into day and night and the latter into those above 
100 fathoms or at this level and below. 
