Vertical Distribution of the Crustacea — Winter. 379 



toer of Daphnia pulicaria present in that winter. The 15-18 

 m. level was the second in population, except in the early part 

 of January, owing again to the accumulation of Cyclops in that 

 region. The middle strata of the lake were the poorest in popu- 

 lation in both years. 



Some illustrations ma} r be added showing the concentration of 

 the two species in question in the lower and upper water of the lake 

 respectively. On February 15th, 1895, out of 870 Cyclops taken 

 by the net, 570 were below 12 meters; on the 19th 880 out of 

 .1,130. On March 9th, 1,017 were found below 15 meters, out 

 of a total of 1,650; on March 12th, 485 out of 710. This aggre- 

 gation at the bottom was not seen in January, and some few 

 catches of later date did not display it. 



In 1896 the same tendency was shown, and began as early 

 as January. On the 7th of that month 1,250 Cyclops out of 

 2,070 were below 12 meters, and similar catches were made 

 through January and February. In March the old Cyclops were 

 greatly reduced in number, aggregated only about 640 indi- 

 viduals for the whole depth, and showed no tendency to col- 

 lect at the bottom. At this time the young Cyclops were pres- 

 ent, averaging over 2,000 to the catch, and the 0-3 m. level 

 contained about twice as many as any other. 



Daphnia pulicaria was absent in 1895 but was numerous in 

 1896. During January and until the middle of February there 

 were at least five times as many in the 0-3 m. level as in any 

 lower one. As the numbers declined in February they fell off 

 chiefly where they were the greatest and the 0-3 m. level be- 

 came about twice as populous as any below. 



Thus the tables of distribution in winter for 1895 and 1896 

 show resemblances and differences. In 1895 the 0-3 m. level 

 shows no noteworthy excess over those below, while in 1896 it 

 is about twice as populous. Between 65 and 70 per cent, of the 

 population of this level in 1896 are due to Daphnia pulicaria. 

 In both years the bottom water is more populous than that at the 

 middle of the lake, due to the settling of Cyclops. This species 

 furnished from 75 to 85 per cent, of the population of the bot- 

 tom level in both years. The average population per cubic 

 meter is much greater in 1896 than in 1895, especially so in 



