168 PLANKTON OF WISCONSIN LAKES 



Brandt also records an analysis of a sample of marine copepods which 

 yielded substantially the same percentage of nitrogen as his sample of 

 freshwater copepods, but the ether extract and ash were somewhat 

 larger in the marine copepods. 



Cladocera. Eight samples of Daphnia pulex var. pulicaria were 

 collected from three different Wisconsin lakes. Those from Lake Mo- 

 nona were secured in different years, while different years as well as 

 different seasons of the same year are represented in the material from 

 Devils Lake. These samples thus afford a good opportunity to study the 

 variations in the chemical composition of this form. (Table 49.) 



The nitrogen ranged from a minimum of 5.82 per cent to a maximum 

 of 8.63 per cent of the dry weight of the material, the latter being 

 almost one and a half times as large as the former. The largest per- 

 centage was found in material from Devils Lake and the smallest in a 

 sample from Lake Monona ; the difference is nearly as great, however, 

 if only the samples from the former lake are considered because a maxi- 

 mum of 8.27 per cent is recorded for one of the Devils Lake samples. 

 The mean percentage of nitrogen in the five samples from Devils Lake 

 is 7.13 per cent of the dry material, which is more than one per cent 

 below the mean of the two samples of Daphnia pulex from Lake Mo- 

 nona, and it is smaller also than the percentage of nitrogen found in the 

 sample from Lake Waubesa. 



On an ash free basis the nitrogen varied from a minimum of 7.22 per 

 cent to a maximum of 10.98 per cent. With the exception of one sam- 

 ple, considerably more than half of the organic matter in these eight 

 catches of Daphnia pulex consisted of crude protein. 



Similar variations were noted in the ether extract or fat, the range 

 being from 2.82 per cent of the dry sample to 27.9 per cent. The sam- 

 ple of Daphnia pulex obtained from Lake Waubesa yielded the maxi- 

 mum percentage of extract and one of those from Devils Lake the mini- 

 mum. On an ash free basis the maximum amount of ether extract con- 

 stituted 31.75 per cent of the organic matter. The physical and chem- 

 ical constants of the ether extract obtained from the Monona sample 

 which was collected on April 4, 1914, were determined by Schuette 11 . 

 The index of refraction at 25° C. is 1.4810, the iodine number 172.88, 

 the saponification number 208.56, the Reichert-Meisel number 0.94, and 

 the Polenske number 1.22. The odor from the extract was decidedly 

 like that of a fish oil and it was noted also that this extract solidified 

 upon exposure to the air. 



The variations in the ether extract bear a fairly close relation to the 

 age and the condition of the Daphnias. The developing embryos are 

 well supplied with fat and the material will yield a high percentage of 



Trans. Wis. Acad. Sci., Arts, and Let., Vol. 19, 1918, p. 599. 



