The Vertical Distribution of the Crustacea. 377 



frequently during the winter. But as the observations were few 

 in number in comparison with those made at the regular point 

 of observation, they have not been used in the preparation of 

 the tables. 



During the last half of the year 1894, 75 serial observations 

 were made, 127 during 1895, and 131 during 1896. These were 

 most numerous during the summer months. In general it may 

 be said that on every day on which observations were made as 

 stated in Table A of the appendix, a series was taken, and on 

 some occasions more than one. The general distribution, of 

 the observations, however, can be ascertained from the table. 

 At least five were made in each two week period from the mid- 

 dle of April to the middle of November. During the winter of 

 1895, some observations were made by six meter intervals in 

 the lower water of the lake, and the result of these observations 

 was equally divided between the two levels covered by them. 



In Table B, accompanying this part of the report, the popu- 

 lation of each level is given in thousands per cubic meter, the 

 total population of the level being divided by three on the as- 

 sumption that the Crustacea are equally distributed throughout 

 the level. Under some circumstances this assumption is incor- 

 rect. In the 0-3 m. level, the upper meter contains more than 

 one-third of the Crustacea, especially when there are large 

 numbers of young. It may contain twice as many as any meter 

 below. On the other hand, on bright calm days, when few 

 young Crustacea are present, the upper meter may contain less 

 than one-third of the total catch from the upper level. 



In the level which includes the region of the thermocline the 

 population of the single meters varies greatly, as will be shown 

 later in this paper; the Crustacea being found in considerable 

 numbers above this stratum and practically absent below it. A 

 third error arises at times when large numbers of Crustacea are 

 settling to the bottom and dying. This occurs with Cyclops 

 during the winter and spring, and with Daphnia hyalina in the 

 early part of June. At such times the lower meter of the lower 

 level would contain more than one-third of the Crustacea present 

 in that level. These variations from an approximately uniform dis- 

 tribution are however so varying themselves that it has not been 



