398 
Birge—The Crustacea of the Plankton. 
species declines in numbers, as it sometimes does in August,, 
the percentage in the lower levels may be nearly, or quite, as; 
great as in the 0-3 m. level. (See August, 1896.) At the time of 
the fall maximum great numbers of young often appear at once. 
At this time the brood sacs of the females contain from five to nine' 
eggs. There are very few half-grown animals, and the eggs 
may all hatch in the course of a week. At such a time it is not 
difficult to determine the difference in distribution of the young 
and old, and the following tables show these relations in the 
latter part of October, 1896: 
Table XXXIV. — Daphnia hyalina , per cubic meter. 
Depth. 
October 26, Noon. 
October 27, 8 A. M. 
Young. 
Adult. 
Young. 
Adult. 
0-3m. 
122,200 
0 
30,400 
1,200 
3-6... 
27,500 
250 
13,300 
760 
6-9. 
15,800 
380 
1,900 
6,300 
9-12. 
1,600 
4,100 
2,500 
3,800 
12-15... 
0 
2,500 
2,500 
8,900 
15-18. 
950 
1,300 
19,000 
After the production of the young in late October or early 
November, the old females die off rapidly; some few remaining 
as late as the first of January. In the latter part of May, or 
the early part of June, according to the progress of the sea¬ 
son, those individuals that have lived over winter become weak, 
are attacked by various diseases, caused by fungi, bacteria, and 
microsporidia, settle toward the bottom of the lake and die. 
This downward movement of the older and weaker individuals 
causes an increase of the number in the lower part of the lake, 
which was quite conspicuous in June, 1895, and in the latter' 
part of May, 1896. 
Shortly after this date the Crustacea begin to disappear en¬ 
tirely from the lower water, and during the remainder of the 
summer the life of the species goes on, like that of the other 
Crustacea, in the region above the thermocline. 
The vertical distribution of this species does not appear to 
have been carefully studied by other authors. 
