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 of ten 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 genera] 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 j 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, 



