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. 





October 26, Noon. 



October 27, 8 A. M. 



Depth. 



Young. 



Adult. 



Young. 



Adult. 



0-3m 



122,200 

 27,500 

 15,800 



1,600 

 







250 



380 



4,100 



2,500 



950 



30,400 

 13,300 

 1,900 

 2,500 

 2,500 

 1,300 



1,200 



3-6 



760 



6-9 



6,300 



9-12 



3,800 



12-15 



8,900 



15-18 



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. 



