Vertical Distribution of Individual Species. 
899 
Daphnia pulicaria. 
Figures 30-32. — Table G, Appendix. 
The vertical distribution of this species is so peculiar that it 
demands a somewhat more detailed account than has been given 
to the other species. The history of the species begins or¬ 
dinarily in the early part of July of the odd numbered years. 
During the first part of July it has been present only in very 
small numbers, but in the second part of July, 1895, its numbers 
were so large that it appears in the lists. At that time more 
than 50 per cent, of the species was found between 6 and 9 
meters, in the region of the thermocline, and nearly all of the 
remainder was found between 9 and 15 meters. In August the 
species moved downward, following the downward movement of 
the thermocline, and continued in this position until the coming 
on of the autumnal homothermous period in late September and 
October. During October the species was distributed with 
approximate uniformity through the water of the lake. In 
November, as the lake cooled, the animals began to move toward 
the surface, and in late November and December a period of 
active reproduction began. The young animals were found in 
the upper level of the lake, most numerously in the upper meter, 
and as the result of this distribution, the numbers in the upper 
level were far greater than those in any other portion of the 
lake. This relation continued throughout the winter of 1895-96, 
during which time reproduction also continued, although more 
slowly, until in March and the early part of April reproduction 
nearly ceased and the numbers of the species declined somewhat 
rapidly. At this time the distribution was uniform, or such 
irregularities as were present seemed to be accidental. In the 
latter part of April the spring period of reproduction began 
and an enormous number of young were produced in the upper 
water. At this time as many as 80-85 per cent, of the species 
were found in the upper level; a larger proportion than has 
been found there of any other species except Chydorus. In the 
early part of May a reproductive pause occurred, during which 
the animals were pretty evenly distributed through the water; 
