180 PLANKTON OF WISCONSIN LAKES 



per cent. In terms of phosphorus pentoxide these percentages are 1.27 

 per cent and 3.52 per cent respectively. The marine copepod shows a 

 somewhat smaller percentage of phosphorus pentoxide than the two 

 freshwater copepods included in table 50, while the marine shrimp 

 shows the same percentage as the freshwater Cladocera. 



Calcium (CaO). The largest percentage of calcium oxide or lime is 

 recorded for the crayfish material, namely, 17.82 per cent of the dry 

 weight, while the amphipod Hyalella comes second with 14.82 per cent 

 and one sample of Daphnia pulex is third with 9.89 per cent. Calcium 

 is a very important constituent of the shells of these Crustacea. In two 

 other samples of Daphnia pulex and in Leptodora, a much smaller 

 amount of calcium oxide was found, ranging from 2.01 per cent to 3.22 

 per cent. The percentage is still smaller in the copepods and leeches 

 as well as in the insects. 



In the plant material the maximum percentage of calcium oxide is 

 recorded for Myriophyllum, with Cladophora ranking second. The 

 other plant samples show relatively small amounts. 



Magnesium (MgO). Only three of the eighteen samples on which 

 determinations were made yielded more than one per cent of mag- 

 nesium oxide. In the samples of plant material the maximum percent- 

 age was found in Cladophora, while the maximum for the animal sam- 

 ples was found in the insect larva Sialis. These data indicate that 

 magnesium is used only in rather small amounts by the various forms 

 represented in table 50. 



