The Annual Distribution of the Crustacea. 



305 



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 1896. 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. 



Diaptomus 



Cyclops 



Daphnia hyalina . . . 

 Daphnia pulicaria 



Chydorus 



Total 



1895. 



24,500 

 52,100 

 46,200 



122,800 



1896. 



34,800 



120,900 



22,700 



48,400 



7.9C0 



244,500 



20 



