The Coefficient of the Dredge. 
283 
ation in the coefficient of the net due to the accumulation of the 
plankton within it as the net is drawn through the water. Un¬ 
questionably the stoppage of the openings of the net by the 
.accumulating catch raises the coefficient, and if the net accum¬ 
ulates a sufficient amount of plankton it will wholly cease fil¬ 
tering the water. In plankton-rich lakes, therefore, serious 
error may be introduced from this source. Since lake Mendota 
during the summer and autumn contains very large amounts of 
vegetable plankton, it was quite possible that the stoppage of 
the net should cause errors. In order to determine whether 
these errors existed, I regularly made hauls of the net from the 
bottom of the lake to the surface during the season of 1895 
and compared the number of Crustacea obtained in the hauls 
from the bottom with the sum of those caught in the six suc¬ 
cessive levels of my series. I append a table showing the num¬ 
ber of Cyclops caught in the months from January to July, 
1895, in order to compare the series and the single haul. It will 
be seen that the number of Cyclops varies, often considerably. 
Out of 41 cases prior to July 1, the total haul exceeded the sum of 
the series in 24 cases and fell below it in 17 cases. There was 
thus no decided advantage on the side either of the series or 
the single haul. If the amount of variation in this table be 
compared with the amount shown in the catches of the tube in 
Table I, it will be seen that the differences are of much the same 
crder as those disclosed by the tube. There is therefore no 
evidence that under .these circumstances the net suffered any 
stoppage in passing through the 18 meters of the lake which 
altered its coefficient to any marked degree ove,r that of the net 
used through 3 meters. 
After the first of July Anabaena and similar small plants de¬ 
veloped rapidly in the lake, and the amount of vegetable plank¬ 
ton increased to a great amount. Under these circumstances 
the number of Crustacea caught in the total haul varied widely 
and irregularly from the sum of the series, and soon became 
uniformly lower than the sum. It was found therefore that the 
coefficient of the net has been raised by the amount of algae 
present and the catches made by the total hauls were not em¬ 
ployed in reckoning the number of the Crustacea after the first 
