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 upper 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 



