Vertical Distribution of Individual Species. 399 



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; 



