178 PLANKTON OF WISCONSIN LAKES 



listed as being present in such food materials as beef, mutton, and fish, 

 except in the heart and the liver, but they are found in some of the 

 aquatic organisms that are used as human food in percentages that are 

 comparable to the nitrogen free extract in these samples of animal ma- 

 terial. In oysters, for example, the carbohydrates constitute 28.2 per 

 cent of the dry weight of the animal, exclusive of the shell, while mus- 

 sels contain 26.25 per cent and scallops 17.26 per cent ; the edible por- 

 tion of the crayfish yields 4.35 per cent of carbohydrates. 



Brandt^* states that the carbohydrates may constitute from 20.0 per 

 cent to 25.0 per cent of the dry weight of copepods, depending upon the 

 amount of plant food that may be present in the alimentary canal of 

 the specimens. The percentage of nitrogen free extract given for the 

 various samples of plankton Crustacea in table 49 exceeds Brandt 's min- 

 imum in only five samples and only one exceeds his maximum. 



While part of the carbohydrate material in some of the animal sam- 

 ples given in table 49 may be derived from plants that have been con- 

 sumed as food, yet it does not seem probable that any large amount 

 comes from this source because the food contained in the alimentary 

 canal would constitute a relatively small part of the weight of the entire 

 animal. In four of the Daphnia samples the nitrogen free extract con- 

 stitutes from a quarter to nearly a third of the dry weight of the or- 

 ganic matter and but little more than one-third of the organic matter in 

 the plankton algae consists of nitrogen free extract or carbohydrates. 

 The fact that the nitrogen free extract is nearly as large in the four 

 Daphnia samples as in the plankton algae indicates that a large part of 

 the carbohydrate material in this extract is derived from the animals 

 themselves rather than from plant food in the alimentary canal. 



In the three samples of Leptodora, the nitrogen free extract consti- 

 tutes from 9.25 per cent to 15.65 per cent of the organic matter and no 

 part of the carbohydrate material in this extract comes from plants 

 because this animal is predaceous, feeding upon other Crustacea. In 

 the leeches, also, which are not plant feeders, the nitrogen free extract 

 constitutes 14.44 per cent of the organic matter and no part of the car- 

 bohydrates therein is of direct plant origin. 



The results obtained in these analyses show that most of the forms 

 represented in table 49 are excellent sources of food for other organisms 

 bcause the major portion of most samples consisted of crude protein and 

 ether extract. The smallest percentages of these two substances were 

 found in the large aquatic plants. The plankton algae, on the other 

 hand, yielded substantially as large a percentage of crude protein as 

 the animal material, but the percentage of ether extract in the former 



" Wissensch. Meeresuntersuch., Abt. Kiel, N. F., Bd. 3, 1898, p. 77. 



