172 PLANKTON OF WISCONSIN LAKES 



data for comparison with the results obtained on the plankton Crus- 

 tacea. These crayfishes ranged in age from about six months to two 

 years and the whole animal was used for the sample. The crayfish ma- 

 terial yielded a smaller percentage of nitrogen and of ether extract 

 than most of the samples of plankton Crustacea; only two samples of 

 Daphnia gave a lower percentage of nitrogen than the crayfishes and all 

 of the copepod material contained a larger amount. On an ash free 

 basis, however, the percentage of nitrogen in the crayfishes is larger 

 than it is in two samples of Copepoda and also larger than in eleven of 

 the fourteen samples of Cladocera. 



The ether extract in the crayfishes is smaller than in seven of the ten 

 samples of Daphnia and very much smaller than in all of the copepod 

 material; this statement holds true for the percentages which include 

 the ash as well as for those which do not. 



The percentage of crude fiber is slightly larger in one sample of 

 Daphnia than in Cambarus, while the sample of Cyclops shows nearly 

 as large a percentage as the latter ; in all of the other samples of Cope- 

 poda and Cladocera the percentage of crude fiber or chitin is appre- 

 ciably smaller than in the crayfish material with the ash included. On 

 an ash free basis, the crude fiber is larger in Cambarus than in any of 

 the samples of Copepoda or Cladocera in table 49. 



"With the ash included, the nitrogen free extract is smaller in the 

 crayfish material than in all except two of the fourteen samples of 

 Cladocera and it is also smaller than two of the five samples of Cope- 

 poda. On an ash free basis the percentage of nitrogen free extract in 

 the crayfish sample is below those of nine samples of Cladocera, but it is 

 larger than those in all of the copepod samples. 



The percentage of ash is much larger in the crayfishes than in the 

 various samples of Cladocera and Copepoda, being one-third larger than 

 the maximum for Daphnia and from five to eight times as large as the 

 percentage of ash in the five copepod samples. 



Hyalella. Including the ash the sample composed of the amphipod 

 Hyalella knickerhockeri yielded a larger percentage of nitrogen than 

 the crayfish material, but on an ash free basis the two are very nearly 

 the same. (Table 49.) The percentages of ether extract and of nitro- 

 gen free extract are larger in the former than in the latter sample, but 

 the reverse is true of the crude fiber and of the ash. 



In comparison with the plankton Crustacea this sample of Hyalella, 

 with the ash included, yielded a somewhat larger percentage of nitrogen 

 than the minimum of the copepod samples, namely, that in Limno- 

 calanus, but on an ash free basis the percentage in the former is much 

 larger than that in the latter. In fact the organic matter of Hyalella 

 contained a larger proportion of nitrogen than three of the copepod 



