156 PLANKTON OF WISCONSIN LAKES 



Cyclops, and Limnocalanus, while the Cladocera comprise Holopedium, 

 Leptodora, and three species of Daphnia. 



The Crustacea and the insects possess chitinous coverings which con- 

 tain a certain amount of nitrogen. Since chitin is non-protein in char- 

 acter it is necessary to make a correction for this part of the total nitro- 

 gen in order to ascertain the amount of nitrogen that belongs to the 

 crude protein. In making this correction the crude fiber has been 

 regarded as chitin and the percentage of nitrogen in the crude fiber 

 has been determined in most of these samples. The percentage of nitro- 

 gen in the fiber varied from a little less than 6.0 per cent to a little 

 more than 7.0 per cent of the dry weight of the crude fiber; thus for 

 the samples on which no determinations were made, an average of 6.5 

 per cent of the crude fiber has been deducted from the total nitrogen as 

 a correction for the nitrogen of the chitin. In the samples on which 

 determinations were made the actual amount of this nitrogen has been 

 deducted from the total. The percentages of nitrogen shown in table 

 49, therefore, represent the amount in the crude protein and they do not 

 include the nitrogen in the chitin. 



Copepoda. The five samples of copepods yielded a high percentage 

 of nitrogen, thus indicating a correspondingly large proportion of 

 crude protein; the sample containing Limnocalanus gave the smallest 

 percentage and that containing Diaptomus the largest. On an ash free 

 basis only three other samples in this series yielded a larger percentage 

 of nitrogen than Diaptomus and these three consisted of animal ma- 

 terial; the maximum for the blue-green algae, however, is only four- 

 tenths of one per cent below Diaptomus. 



All of the copepod samples contained a relatively large amount of 

 ether extract or fat ; Diaptomus gave the smallest percentage and Lim- 

 nocalanus the largest. In the latter the ether extract amounted to 41.6 

 per cent of the organic matter; this large yield is due to fairly large 

 drops of oil which are present in the thoracic region of this animal. 

 The crude protein and the ether extract combined constitute from 73.0 

 per cent to almost 85.0 per cent of the dry w^eight of these samples of 

 copepods ; this means that they are a source of very nutritious food for 

 the organisms which feed on them. 



The Cyclops material yielded the largest percentage of crude fiber 

 or chitin and Limnocalanus the smallest. 



The nitrogen free extract in these five samples of copepods varies 

 from a minimum of 4.89 per cent of the dry organic matter in Cyclops 

 to a maximum of 14.45 per cent in one of the samples containing both 

 Cyclops and Diaptomus. In other words the crude protein, the ether 

 extract, and the crude fiber constitute from 85.0 per cent to more than 

 95.0 per cent of the organic matter in these copepod samples. 



