Method of Using the Dredge. 
431 
depth, yielding six hauls. The collecting tubes were provided 
with corks marked in sets from 1 to 6, so that no labels were 
needed while on the water. This is often a convenience, especi¬ 
ally when one is working alone and at night. The tubes are 
shown in PI. VII, figs. 2, 3. 
The dredge was used from a rowboat. In the bow of the 
boat an upright was stepped like a mast, about seven feet in 
length, having at the top a cross arm, to whose end was attached 
the pulley for the dredge. The small size of the boat caused us 
to lose several sets of observations. When the wind was strong, 
the lake was too rough to permit the boat to go out, and in 
several sets of observations the waves ran so high as nearly 
to swamp the boat. 
The observations included in Period I were not made at any 
one place since they were experimental in character. All of 
them, however, were made in the same general region as were 
the observations of the later periods. The water, however, 
was shallower in most of these observations than at the place 
finally selected. As a result, only one of the observations of 
Period I extended below 15 m. 
In the last three periods the observations were made at a buoy, 
which was moored in water something over 18 m. in depth, so 
that the dredge could be raised through six levels of 3 m. each. 
This distance was chosen because it gave us an interval small 
enough to give a fair indication of the vertical distribution 
of the Crustacea, and at the same time large enough 
to bring our observations within a manageable num¬ 
ber. Each interval is known as a level, and is named from 
the depths between which it lies. The upper level is known 
as 0-3 m. level, and so on to the bottom. A series of six hauls 
of the dredge therefore constituted one complete observation 
called a “series” in this paper. 
In beginning an observation the dredge was lowered to the 
depth of 18 m., opened, raised to 15 m. at the rate of ^ m. per 
second, closed, and drawn to the surface. In this way the 
work proceeded, passing regularly from the deeper levels to the 
higher ones. The reason for this order lay in the fact that the 
greater number of Crustacea were in the upper level of the lake, 
