277 



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. poppei 

 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 all 

 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. 3 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. 3 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 Harpacticidce) . 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 1 5 and October 1 . During the colder months 

 the pulses rarely rise above 20,000 per m. 3 , 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 



