408 
Birge—The Crustacea of the Plankton. 
very greatly. On some occasions the catch of certain species 
from the upper meter was larger than that obtained by a second 
catch from the entire three meters. Such instances were due 
to the presence of very large numbers of young in the upper 
meter, with a somewhat irregular distribution, so that the 
catches varied considerably. Upon the whole, however, the 
average number derived from these twenty observations agreed 
surprisingly in all the species. It was found that the upper 
meter contained an average of 43 per cent, of the entire catch 
of Diaptomus from the upper three meters; 47 per cent, of Cy¬ 
clops; and 50 per cent, of Daphnia hyalina. These catches 
were made during the day and may be taken as fairly indicating 
the relative number of Crustacea in the upper meter during the 
daylight hours. It will be seen that these observations fully 
justify the statement made in my former paper (Birge, ’95, 
p. 479) that “ a general movement of the Crustacea as much as one 
meter would have been detected, ” and indicates that at no time 
is the population of the upper meter of the lake notably de¬ 
ficient. The minimum percentages were very irregularly dis¬ 
tributed and depended more upon the presence or absence of 
young individuals than upon any influence of light, weather, or 
wind. 
These observations also indicate the extent to which the lines 
of Figs. 29 and 30 should be altered in the upper three meters 
in order to express the average distribution within that level. 
During 1897 observations were made with a view of deter¬ 
mining the exact distribution of the Crustacea in the upper 
meter. They were made by two methods: First, a net with an 
opening ten centimeters in diameter was supported so that it 
could be drawn horizontally through the water for a known dis¬ 
tance at an uniform rate of speed. The Crustacea so obtained 
were counted and the number present at a given level was thus 
determined. Second, a pump was taken out in the boat, by 
whose aid the water of the lake was pumped through a hose and 
strained by the plankton net, the mouth of the suction hose be¬ 
ing placed at the successive levels. Water was taken from the 
surface at a depth varying from two or five centimeters in calm 
weather, to ten when the lake was agitated by the wind; at one- 
