376 Birge — The Crustacea of the Plankton. 



dredge was lowered to the bottom of the level from which speci- 

 mens were to be taken, raised through the proper space, and 

 then closed by means of a messenger sent down the line. It 

 was then drawn to the surface, washed out, and the collection 

 preserved for future study. 



My observations show so much variation in catches made at 

 the same place and in succession that I have little confidence 

 in the differential method of determining vertical distribu- 

 tion; unless a very large number of observations is made and 

 averaged, so as to eliminate the chance of variation in the sin- 

 gle observation. See p. 281. 



The distance employed in all of my collections was three 

 meters. This interval was selected because it divided the 

 lake at the point of observation into six levels of uniform 

 thickness, and also because of the close correspondence be- 

 tween three meters and ten feet. Experience has shown that 

 the distance was fortunately chosen as the number of Crus- 

 tacea begins to decline rapidly between 2 and 3 m. from the sur- 

 face. The place of regular observation is about 850 me- 

 ters from shore, where the water is about 18.5 meters in depth 

 or somewhat more when the water is highest in the spring of 

 the year. The greatest depth observed in the lake is between 

 23 and 24 meters. The slope of the bottom in the deeper water 

 is very gradual, and a depth substantially greater than 18 

 meters is only reached at a considerably greater distance from 

 the shore. If observations had been made in the deepest part 

 of the lake, the distribution as shown in thousands per cubic 

 meter would not vary from the facts as shown in the tables, nor 

 would the summer percentile distribution be altered, since dur- 

 ing the summer the deeper parts of the lake contain no Crustacea. 

 During the fall and winter months the distribution is nearly uni- 

 form in the lower water. The average percentile distribution 

 would, of course, be changed by the addition of one or more 

 levels during winter, and the aggregations of Crustacea, espe- 

 cially Cyclops, which are found in the bottom levels, would 

 of course, be moved from the 15-18 m. level to those lying be- 

 low. Observations were made occasionally in the deeper water, 

 as often as once a week during the summer and fall months; less 



