170 PLANKTON OF WISCONSIN LAKES 



11.06 per cent of the dry weight of full grown females with eggs and to 

 14.15 per cent in medium sized individuals. Similar differences in ash 

 corresponding to differences in age have been noted in specimens of 

 Daphnia hyalina. 



The various analyses of blue-green algae and of Daphnia pulex show 

 clearly the necessity of analyzing a number of samples of each form if 

 one wishes to obtain a general idea of the chemical composition. The 

 number of analyses may be reduced, however, by making a composite 

 sample : that is, one which contains material that has been collected at 

 different periods in the development of the form in question. 



The composite sample containing Daphnia pulex and D. hyalina col- 

 lected in Lake Monona on August 5, 1913, and that of D. hyalina and 

 D. retrocurva secured on Lake Mendota on August 28, 1917, yielded 

 results similar to some of the samples of pure D. pulex. (Table 49.) It 

 seems probable, therefore, that a series of catches of these two forms 

 would give substantially the same results as those obtained for the 

 various samples of D. pulex. 



Another cladoceran, Holopedium gibberum, also has percentages 

 which agree closely with some of those obtained for the Daphnia ma- 

 terial, with the exception of the pentosans. The percentage of nitrogen, 

 for example, is the same as that in the composite sample containing D. 

 pulex and D. hyalina, and it is very close to that of three other samples 

 of Daphnia. The ether extract, crude fiber, and ash are also within the 

 range of the percentages noted for the Daphnia material ; the pentosans 

 constitute a much larger percentage of the dry weight of Holopedium 

 than of any of the other animal material. 



Quantitative determinations of the pentosans were made on fourteen 

 samples of animal material as shown in table 49. With one exception 

 the percentages are comparatively small in such samples and this excep- 

 tion is Holopedium. The latter sample is the only one of the fourteen 

 which yielded more than two per cent of the dry weight in pentosans 

 and eight of them contained less than one per cent, the smallest amount 

 being found in the leeches. 



The first determination of the pentosans in Holopedium was made on 

 a sample collected in Kawaguesaga Lake at Minocqua, Wisconsin, in 

 1913 ; the pentosans in this material amounted to 6.17 per cent of the 

 dry weight, which was regarded as surprisingly high. A second sample, 

 therefore, was secured from this lake in 1917 and this gave a still larger 

 percentage of pentosans, namely 9.62 per cent. This percentage is not 

 only much larger than that in any of the other animal material included 

 in this series of samples, but it is exceeded by the percentages in only 

 two samples of plant material. In 1918 a third sample of Holopedium 

 was secured from a neighboring lake and it yielded 4.35 per cent of 



