
Dahl 
zone and to vegetation. No statistical data concerning its seasonal 
distribution are given by any of these authors, though Meissner 
states that it reaches its maximum in May in the Volga. 
In addition to the species of Cyclops here listed for the channel 
plankton of the Illinois, E. B. Forbes (’97) records in May-—Septem- 
ber, 1896, C. varicans Sars as common, and C. fimbriatus var. popper 
Rehberg and C. bicolor Sars as rare. 
Owing to the impossibility of separating with certainty the 
nauplii and young of the various species of Cyclops they were al] 
recorded together under the head of“ nauplii” and “ young Cyclops.”’ 
The former includes also the nauplii of the two species of Diaptomus 
occurring in our plankton. 
Young Cyclops.—Average number, 4,780; in 1897, 16,035; in 
1896, 10,196; in 1895, 21,960; and in 1894, 5,960. With two ex- 
ceptions in January and February they occur in every collection 
examined. Numbers are, however, at a minimum in November— 
March, only 9 instances of more than 1,500 per m.* appearing in our 
records in this season. With the exception of two pulses in the 
autumn of 1897, and two in this season in 1895, all pulses of an 
amplitude exceeding 8,000 per m.* are confined to the interval 
between April 20 and October 1, practically to temperatures above 
70°. They also exhibit relations to hydrographic conditions of the 
same nature as those found in case of the adults of the various 
species of Cyclops, and manifest. likewise the phenomenon of re- 
current pulses (Table I.). The totals of all young Cyclops in 1894— 
1899 are almost five times those of all adults of the genus. This 
ratio gives an index of the extent of the decimation by enemies and 
inimical factors of the environment which exists after the nauplius 
stage has passed and before that of the adult is reached. 
Nauplii of the Copepoda (excluding the Harpacticide).—Average 
number, 36,707; in 1897, 53,786; in 1896, 24,560; in 1895, 88,442; 
and in 1894, 45,648. Nauplii were recorded in all collections ex- 
amined with but two exceptions. As in the case of the adults and 
young, the large numbers are, however, confined to the warmer 
season between April 15 and October 1. During the colder months 
the pulses rarely rise above 20,000 per m.*, and those in excess of 
35,000 during these months are with one exception confined to the 
delayed high temperatures of the stable autumn of 1897. During 
