The Annual Distribution of the Crustacea. 805 
and its rise and fall seem measurably independent of the condi¬ 
tions to which the other species respond. 
A third fact concerns Daphnia pulicaria. This species had a 
biennial period of development about thirteen months long, ex¬ 
tending from July to August of the following year, and a period 
of rest, in which it was almost entirely wanting in the plankton, 
extending from late August to the following July. In 1894 a 
few representatives of this species were found in July, and it 
wholly disappeared in August. In 1895 they were an important 
constituent of the crustacean life from. July on, increased greatly 
in late fall and early winter, and continued numerous through¬ 
out the winter. In April and May, they increased enormously, 
producing males and sexually mature females, and then declined, 
practically disappearing in September. This species was there¬ 
fore a constant and important factor in the number of the Crus¬ 
tacea during the last half of 1895, the following winter, and the 
spring and early summer of 1898. It was absent during the 
latter half of 1894 and the spring and early summer of 1895. 
I will now pass to a brief discussion of the general crustacean 
life as it appears in the different seasons. I shall reserve most 
of the discussion of the causes and conditions affecting the num¬ 
ber of Crustacea to a later chapter. 
The Crustacea in Winter. 
All of the perennial Crustacea are, of course, constituents of 
the winter plankton, and their numbers are not very unequal. 
The number is by no means small, averaging about 125,000 per 
sq. m. from January to the middle of April, 1895, and about 
235,000 from January to April 1st, 1896. The following list 
shows the species present during the two winters in question. 
Table III. — Species, with average number of each per square meter. 
1895. 
1896. 
Diaptomus .......................... 
24,500 
52,100 
46,200 
84,800 
120,900 
22,700 
48,400 
7,900 
244,500 
Cyclops.... .. 
Daphnia hyalina........... 
Daphnia pulicaria ....... 
Chydorus.... 
Total....... 
122,800 
20 
