802 
Birge—The Crustacea of the Plankton. 
bility. Most of the littoral forms of Crustacea also appear oc¬ 
casionally in the plankton, especially after storms, as also do 
Hydrachnids and Ostracoda. 
Of these eleven species, the isolated forms do not contribute 
any appreciable addition to the number of limnetic Crustacea. 
Their combined number is rarely as great as one per cent, of 
the total Crustacea present. They have, therefore, been neg¬ 
lected in determining the total number of Crustacea, and this 
general account will deal with the eight abundant species only. 
The limnetic Crustacea on lake Mendota show a rhythm of de¬ 
velopment quite complex, but recurring in closely similar form 
during the time covered by my observations, July, 1894 — De¬ 
cember, 1896. (Fig. 6.) Observations less numerous have been 
continued to the present date, September, 1897, and show a 
similar development during the present year. The following 
periods can be distinguished: 
Winter minimum. 
Spring maximum. 
Early summer depression 
Mid-summer maximum.. 
Late summer minimum... 
Autumn maximum. 
There are, thus, three maxima and minima which are of un¬ 
equal value. The spring maximum is by far the greatest, the 
Crustacea reaching a maximum number of 3,000,000 per sq. m. 
of surface, and in 1896 reaching an average of nearly 2,500,000 
for the first half of May. This maximum is due almost entirely 
to the rapid development of Cyclops brevispinosus. After the 
maximum has passed, this species rapidly declines in number, 
and the total number of Crustacea sinks with it, so that by tho 
middle or last of June the number is reduced to less than half 
the maximum. This is the early summer depression, which may 
be greatest at any time from the middle of June to the first 
week in July. A rapid, but slight, recovery follows, due chiefly 
to renewed reproductive activity on the part of the species al¬ 
ready present in the lake, leading to the mid-summer maximum, 
in July, Then follows a decline, usually somewhat slow, reach- 
December to April, then increase to the 
In May, followed by a great decline to the 
June or early July, 
July, 
Late July or August, 
September and October, declining to the' 
winter minimum, through late October, 
November and early December. 
