Diaphanosoma brachyurum. 
347 
Zacharias (’96, p. 53), gives August and September as the 
maximum, and also says nothing about males. The species 
was only occasionally present in the winter. Apstein (’96, p. 
170), gives August as the date of the maximum for all species 
of Daphnia. He does not mention a sexual period, though he 
gives no especial attention to the subject. Had there been 
.such a period as is shown by D. retrocurva it could not have 
been missed. 
Diaphanosoma brachyurum Sars. 
Figure 19.—Table I, Appendix. 
Table XXI. — Average number per square meter of surface , . 
thousands and ten ths. 
stated in 
1894. 
1895. 
1896. 
•July 1-15. 
s 
s 
-July 16-31. 
0.8 
6.9 
s 
August 1-15. 
6.3 
31.5 
8.9 
August 16-31. 
18.0 
32.2 
147.4 
September 1-15. 
No obs. 
27.1 
108.3 
•September 16-30. 
19.6 
17.2 
32.9 
October 1-15. 
5.2 
3.4 
0.4 
October 16-31 ... 
3.0 
0.0 
0 0 
This species is the least numerous of the limnetic Crustacea 
which appear in large numbers, and has the shortest season. 
Scattering individuals may be seen as early as the middle of 
May, but they do not become a regular constituent of the plank¬ 
ton catch before the middle of July or the earlier part of Au¬ 
gust. They disappear in October, and are greatly reduced in 
number by the cold storms which usually come in late Septem¬ 
ber. Males appear about the middle of September, and the win¬ 
ter eggs are then produced. The species was far more abundant 
in 1896 than in either of the two preceding years, which agree 
with each other fairly well. For this difference I can assign no 
reason. The numbers were constantly greater in 1896, so that 
the increased number was not the result of a few large 
