ANALYSIS OF OEGANISMS 159 



of leeches. The last ten items in the table represent aquatic insects, all 

 except the last two being larvae; the sample of Gyrinids consisted of 

 adults, while that of the Hemiptera contained both young and adults. 

 The Corethra larvae are limnetic in habit and they are the only insect 

 constituents of the plankton. 



In part I of table 49 the results of the analyses are stated in percent- 

 ages of the dry weight of the sample when the ash is included ; in part 

 II the percentages are given on an ash free basis. The percentage of 

 ash varied so much in the different samples that it seemed worth while 

 to state the results on an ash free basis also, so that the different com- 

 ponents of the organic matter in the various samples might be more 

 readily compared. 



The chemical study of these samples included the usual determina- 

 tions that are made in a food analysis, namely, a quantitative deter- 

 mination of the nitrogen, the ether extract, the crude fiber, and the 

 ash. The nitrogen multiplied by the factor 6.25 gives the crude pro- 

 tein. These items do not account for all of the organic matter in a 

 sample and the remainder is usually designated as the nitrogen free 

 extract ; that is, the sum of the percentages of crude protein, ether ex- 

 tract, crude fiber, and ash deducted from 100 gives the percentage of 

 nitrogen free extract. This extract consists principally, if not entirely, 

 of carbohydrate material, but in some instances small amounts of other 

 substances may be present; it may contain, for example, a certain 

 amount of fats which are not completely removed by ether from some 

 of the compounds in which they occur. The pentosans are the only 

 carbohydrates which have been studied in these samples and they 

 were determined quantitatively in thirty-one samples as indicated in 

 table 49. The other carbohydrate constituents of these samples remain 

 as a problem for future chemical investigation. 



Plants 



Myxophyceae. Ten samples containing material derived from four 

 genera of blue-green algae, namely, Microcystis, Anabaena, Aphanizo- 

 menon, and Lyngbya, are shown in table 49. With the exception of one 

 sample they yielded a high percentage of nitrogen ; a sample of Micro- 

 cystis gave only 6.32 per cent of nitrogen when the ash is included in 

 the dry weight of the sample, but in the other nine the nitrogen ranged 

 from 8.21 per cent to 9.94 per cent. This high percentage of nitrogen 

 represents a correspondingly large amount of crude protein ; the latter, 

 in fact, falls below 50.0 per cent of the dry weight in only one sample, 

 while it exceeds 60.0 per cent in one instance. This large percentage 

 of crude protein is not surprising in view of the fact that these forms 

 contain only a relatively small amount of material besides the proto- 



