

302 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 December to April, then increase to the 



Spring maximum In May, followed by a great decline to the 



Early summer depression June or early July, 



Mid-summer maximum July, 



Late summer minimum Late July or August, 



Autumn maximum September and October, declining to the 



winter minimum, through late October, 

 November and early December. 



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 the 

 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- 



