Diaptomus oregonensis. 
451 
while there is nearly ten per cent, difference in favor of the up¬ 
per level in the day and the 6-9 m. level at night. In view of 
the small number of observations this indication of a nocturnal 
descent should not be pressed, especially as the greater part of it 
is due to one of the four sets of observations. In Period II, when 
the lake was exceptionally calm, the day observations, fifteen in 
number, gave 68.4 per cent, in the 0-3 m. level, but only 4.84 
per cent, in the 6-9 m. level. The night, observations gave 51.75 
per cent, and 17.6 per cent, respectively. Not only is this true, 
but every one of the night observations in the 6-9 m. level 
yielded numbers greater than any of those taken by day. One 
can hardly resist the conclusion that there is a tendency to the 
surface by day. The nights in this period were moonlight and 
cloudless. These facts are graphically shown in PI. IX, fig. 2. 
Even when the whole month is taken into account, there 
are in the 6-9 m. level only 8 of the 32 day observations 
which are above the average for the month, while only 4 of the 
26 night observations are below the average of the month. In 
the 0-3 m. level the distribution on each side of the average o r 
the month is more nearly equal, since there are in the day obser¬ 
vations 14 below the average and 19 above it, and in the night 
observations 19 below and 7 above the average for the month. 
All of the exceptionally large catches in the upper level were 
made by day. There were 12 cases, a little over 20 per cent, of 
the total number, where more than 65 per cent, of Diaptomus 
occurred in the upper three meters. All of these were found 
by day. There is thus a clear indication of a rise of Diaptomus 
by day, but it is plain that the tendency is not strong and is 
easily overcome by wind, etc. 
The observations of Period I, 5 day and 4 night, all in the 
early part of the night, show a slight excess in the upper level 
at night,— an excess of 2.2 per cent, of the whole number of 
Diaptomus. Period II shows a day excess of 7.85 per cent in 
the 0-3 m level, and of 10.8 per cent, by night in the 6-9 m. 
level. In Period IV the night shows an excess over the day of 
3.54 per cent in the 0-3 m level. These numbers while not 
decisive of any marked movement, are conclusive against any 
considerable sinking of the Crustacea by day. 
30 
