460 Birge — Vertical Distribution oj Pelagic Crustacea. 
find that the upper level contained somewhat more Cyclops by 
day than by night, the percentage found in the 0-3 m. level 
being 44.75 per cent, by day and 39.64 per cent, by night. 
The percentage found in the 3-6 m. level was almost the same 
in the two cases, being 30.81 per cent, by day and 30.05 per 
cent, by night. The 6-9 m. level showed, of course, a smaller 
number by day, 15.08 per cent., and a larger number at night, 
18.55 per cent. These numbers are, however, not large enough 
to show decisively any migration of the Crustacea. In the 
different periods much' the same differences in diurnal distribu¬ 
tion are noticed as for Diaptomus. In Period I, the number 
found by night in the 0—3 m. level is slightly greater than by 
day, but the difference is so small as to come well within the 
limits of error. In Period II, the day observations show a very 
considerable excess, having 52 per cent, of the total catch, while 
the night observations show only 44.5 per cent. A difference 
still greater was found in Period III, where the day observa¬ 
tions gave 42.75 per cent, in the upper level, and 33.09 per cent, 
by night. This last difference is greater than the difference of 
Diaptomus for the same period. In Period IV, the night 
catches show a slight excess over the day catches, the upper 
level containing 39.44 per cent, by day and 43.7 per cent, by 
night. It is clear that here are no indications of a descent of 
the Crustacea by day and also that they do not indicate an as¬ 
cent by day as plainly as does Diaptomus. If there is any 
diurnal migration of the Crustacea, Cyclops would probably not 
show it as distinctly as Diaptomus , since its power of loco¬ 
motion are so much smaller. In Period I, while the number in 
the 0-3 m. level at night was somewhat larger than that by 
day, the number in the 9-12 m. level was also considerably 
larger by night than by day. The number of specimens and 
the number of observations in this period were both so small 
that chance would play a considerable part in the apparent dis¬ 
tribution of the Crustacea, and the night observations were 
all made earlier than 10 p. m. 
Center of distribution .—The level above and below w T hich lay 
50 per cent, of the Cyclops varied somewhat during the month. 
In Period I it lay at 4.54 m. below the surface. In Period II it 
