General Distribution of the Crustacea. 
441 
The Crustacea were far more numerous in the upper part of 
the water than in the deeper levels. Nearly one-half of the entire 
catch was found in the upper 3 m. The average for the month 
from this level was 48.6 per cent., ranging from 41.75 percent, 
to 57.6 in the different periods. The 3-6 m. level contained on 
an average about 30 per cent, of the total number of Crustacea, 
and the 6-9 m. level about half as many, 15.3 per cent. On 
an average, then, the upppr 9 m. of the lake contained nearly 
94 per cent, of the total number of Crustacea. This distribu¬ 
tion was maintained with great regularity. Very few observa¬ 
tions showed any great number of Crustacea below 9 m., and 
these were almost wholly confined to the 9-12 m. level. The 
number found in this level varied very greatly, ranging in the 
case of Diaptomus from 2 to 570 in different observations. Most 
of these Crustacea were probably in the upper part of the level, 
and everything indicates that the densely populated portion of 
the lake is of somewhat varying thickness, as would be expected. 
The Crustacea apparently stopped rather abruptly either some¬ 
what above or somewhat below the 10 m. level. On the average 
during the month 5.4 per cent, of all of the Crustacea were 
found between 9-12 m., the averages for the different periods 
ranging from 3.7 per cent, in Period II, when there was almost 
no wind, to 7 per cent, in Period IV, when the lake was very 
greatly disturbed. Below 12 m. practically no Crustacea were 
present. Only .8 of 1 per cent, of the total number was found 
between 12 m. and 15 m., and only .1 of 1 per cent, between 
15 m. and 18 m. Observations in the deeper parts of the lake, 
down to 22 m., made for purposes of control, showed similar 
conditions. We may therefore conclude that practically all of 
the Crustacea during July are contained in the upper 12 m. of 
the water, and that more than 90 per cent, of them will be 
tound above the 9 m. level. This distribution apparently con¬ 
tinues throughout August and the early part of September. 
Later in the year, as the temperature of the lake falls and be¬ 
comes uniform, the distribution changes, and the Crustacea be¬ 
come pretty uniformly distributed throughout the whole depth. 
In general, this new distribution continues, though more or 
less irregularly, throughout the winter and into the spring, but 
