458 Birge — Vertical Distribution of Pelagic Crustacea. 



Crustacea remained the same the density in different portions of 

 the water was quite different. 



The average number of Cyclops caught in the different peri- 

 ods greatly increases in the later periods over the earlier ones. 

 The catch was as follows : 



Period I... 

 Period II . . 

 Period III. 

 Period IV . 



Average . 



Minimum. 



Maximum 



626 



176 



1006 



1611 



836 



3087 



3147 



1800 



5460 



2531 



1336 



4683 



It is thus evident that the number of Cyclops was increasing 

 during the month of July. The percentage of Cyclops in the 

 total number of Crustacea caught during the month was 30.5. 

 In Period I, Cyclops averaged 13 per cent, of the total Crustacea; 

 in Period II, 26.2 per cent.; in Period III, 27.4, and in Period 

 IV, 40.5. This conclusion is confirmed by observations made 

 later in the year. Three catches made in the latter part of 

 August gave an average of more than 3000, and a maximum 

 of more than 6000 Cyclops was reached in the latter part of. 

 September and the first part of October. Observations made 

 in the upper levels of the water on the 23d of July, for the pur- 

 pose of testing the horizontal distribution of the Crustacea, 

 showed a large increase of Cyclops above the observations of 

 the 19th, indicating apparently the development of a new gen- 

 eration of Cyclops at that time. The falling off in number of 

 Period IV, as compared with Period III, is probably not sig- 

 nificant of any real change in the average number of the species. 



Vertical distribution. — In general the distribution of Cyclops 

 agreed closely with that of Diaptomus, as may be seen on the 

 accompanying plates and tables. The percentage of Cyclops in 

 the upper level is not so great as that of Diaptomus, and the 

 number found in the lower levels is somewhat greater. Yet the 

 distribution of the two species agrees quite closely, as PI. IX 

 shows at once. The upper 9 m. of the lake contain on an 

 average from 78 per cent, to 93 per cent, of the total number. 



