1912] Esterly: Copepoda of the San Diego Region 305 
TABLE 31 
Summary of table 30 
Specimens Success. 
per fathom Total hauls hauls Frequency 
Day Night Day Night Day Night Day Night 
Between 0 and 40 fathoms 0.5 10.1 106 100 1) 26" 059) 326 
Between 40 and 200 fathoms 23.2 13.1 51 41 20 24 40 58 
The almost total absence of Metridia during the day above 
40 fathoms is evident (table 30, columns A—K, lines 1-7). The 
daylight plurimum is at 200-150 fathoms, which checks roughly 
with the results from the horizontal nets (table 28, section A, 
column 6, line 6). As in the other species, a detailed com- 
parison of vertical with horizontal closing nets is not possible 
because of the different times at which collecting was done in 
the two cases. 
It is plain, however, from table 31 that Metridia is much 
more abundant by night than by day above 40 fathoms, and 
also more frequent; and that between 40 and 200 fathoms it 
is more abundant but not so frequent by day. These facts 
suggest that there is an upward migration at night, which was 
indicated from hauls with the surface and horizontal closing 
nets. A puzzling question arises in connection with table 31 
as to why the catches by day below 40 fathoms should be 
larger than those at night though there are more successful 
hauls at night. There is no evidence available now to show 
why that condition should prevail. It is possibly due to the 
times at which the vertical closing hauls were made, but such 
results were not obtained in the case of Calanus (table 15) or 
of Eucalanus (table 23). In the ease of Metridia the night 
collections below 40 fathoms are larger than the day collections 
above that level (table 31), but this does not hold for the 
other two species. These considerations suggest that we have 
to deal with a peculiarity of the species in table 31, but the 
matter should be left open until more collections are at hand. 
There seems to be little room for doubt that Metridia as 
a whole is a good deal more abundant at night in the upper 
water than it is by day. 
