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residues from Lake Mendota and other lakes. Therefore, there is every reason to 
believe that the nitrogen compounds of the residues from evaporated lake water are 
suited for animal nutrition. 
Lake water contains organic matter in quantities very large in comparison to 
those found in the water of the open sea. The question of the possible use of this 
material as food, therefore, presents a different aspect. 
Only a small fraction of this organic matter can be removed by net, by centri- 
fuge, or by Berkefeld filter, and the relatively large amounts left behind by the 
centrifuge may be called “dissolved,” as contrasted with the particles that make up 
the plankton. They certainly contain crude proteins of the same kinds and in much 
the same proportions as are found in the plankton; the small amount of fat appar- 
ently is of the same kind, and there is no reason to believe that there is any essential 
difference between plankton and residues in the miscellaneous assortment of sub- 
stances grouped under the name of “nitrogen-free extract.” 
In the study of the plankton of Lake Mendota an attempt was made to estimate 
the proportion of eaters and food (plankton report, p. 155). Numerous counts were 
made, which gave the number of Crustacea and rotifers, and the average dry weight 
of their organic matter also was determined. The result showed that the dry organic 
matter of the two main groups of eaters constitutes about one-third of the organic 
matter of the net plankton, in which are found virtually all rotifers and Crustacea, 
including Nauplii. The average net plankton is a little more than one-sixth (17.4 
per cent) of the total plankton; in the particulate matter removable by the cen- 
trifuge, therefore, there is from 15 to 20 times as much organic matter as exists in 
Crustacea and rotifers. This would seem to be in itself a sufficient quantity of food, 
but not all of it is practically available for all of the eaters. Many of the plankton 
algffi are too large to serve as food for most rotifers, and some algae, like the larger 
forms of Lyngbya, probably are too large to be eaten by any of the plankton animals. 
Gloiotrichia is perhaps in the same situation, and though Cyclops sometimes browses 
on the ends of its filaments it probably contributes much more to the weight of 
organic matter than to available food. 
By far the larger part of the organic matter of the plankton is found in Naumann’s 
groups (1918, p. 44) of the smaller organisms and the “larger particles of peritripton. ” 
This material should be available as food for all of the larger eaters in the plankton, 
and its weight is so many times that of Crustacea and rotifers that there would 
seem to be in the lake a sufficient supply of particulate food, so far as organic sub- 
stance is concerned. 
How far these algse, even though of moderate size, can be utilized as food by 
plankton animals is another question. Naumann (1921) has shown that food 
remains in the intestine of most of the limnetic Cladocera only for a very short 
time (15 to 30 minutes), and that almost all of the algse pass through the intestine 
without being affected by the digestive process. These Crustacea seem to get 
their nourishment from the minuter organisms and from similar particles of debris. 
Copepods and rotifers (Naumann, 1923) seem able to make a more complete use 
of the food material ingested. 
The facts presented in the present paper offer a new basis for the investigation 
of the nutrition of plankton animals. Looking at the proteins alone, there is dissolved 
