48 IOWA STUDIES IN NATURAL HISTOPwY 



viduals of Bosmina were found in the 5 m.-lO m. stratum 

 while a somewhat larger number of Diaphanosoma was 

 noted in the upper 10 m. Of the three species of Daphnia 

 the most abundant was D. retrociirva which reached 17,000 

 individuals per cubic meter in the upper 5 m. Only about 

 a quarter of these were adults. About a third as many were 

 found in the 5 m.-lO m. stratum and the number was rela- 

 tively small in the deeper strata. This corresponds to the 

 usual distribution of this form, which is confined chiefly to 

 the epilimnion. Daphnia pulex prefers the cool water of 

 the hypolimnion and it was most abundant in that stratum 

 of West Okoboji lake. The maximum number was found 

 in the 25 m.-30 m. catch. A few specimens of Daphnia 

 longispina var. hyalina were noted in two catches, namely, 

 those from the 10 m.-15 m. and the 30 m.-32 m. strata. This 

 form is usually distributed throughout the entire depth of 

 a lake, but in this instance it appeared only in the meso- 

 limnion and in the lower part of the hypolimnion. 



The catch which covered the entire depth of the lake 

 (0-32 m.) yielded 211.7 mg. of dry net plankton per cubic 

 meter of water, of which 91,3% (193.2 mg.) was organic 

 matter. 



The average weight of the individuals belonging to the 

 more abundant forms of Crustacea was ascertained. For 

 this purpose specimens were sorted out under a dissecting 

 microscope and these were transferred to small platinum 

 dishes. They were then dried, weighed, and ignited as 

 described above. The results are given in table 24. 



Six different forms of Crustacea were weighed. Immature 

 individuals of Diaptomus clavipes were substantially the 

 same in size as the adults of Diaptomivs siciloides so that it 

 was impossible to separate the two and obtain weights of 

 each. The adults of D. clavipes gave a larger dry weight 

 and also a larger amount of organic matter for each indi- 

 vidual than any of the other Crustacea. The average was 

 almost three times as large as that for immature D. clavipes 

 and adult D. siciloides and about 40%. larger than Daphnia 

 pidex and Daphnia longispina var. hyalina. Cyclops aver- 

 aged somewhat larger than the immature D. clavipes and 



