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forms. Many of the forms derive their nutrition in part from the dead 
organie matter in the pond, to which all of the forms contribute. The 
ultimate food sources in the pond are (1) water; (2) carbon dioxide in 
solution in the water (derived from the air above the water); (3) nitro- 
gen, free and in simple compounds, such as ammonia; (4) foreign or- 
ganisms accidentally falling into the pend, e. g., insects. The formation of 
nitrates from simple nitrogen compounds was established by the well 
known work of Winogradsky (89). He demonstrated two kinds of bac- 
teria, one forming nitrous acid, another changing this to nitric acid which 
is neutralized by carbonates already present. This process may be as- 
sumed to form the first step in the proteid synthesis in this pond. 
These bacteria and those present in the decaying organic matter of the 
pond are eaten by the flagellates and ciliates. The ciliates also use the 
flagellates for food. The carbohydrates of this group are derived from 
the dead organic matter in the pond. The synthesis of carbon dioxide and 
water into carbohydrates is of course due to chlorophyl bearing plants. 
These plants consist of desmids, diatoms, filamentous alge and phanero- 
gams. The inclusion of diatoms and the smaller desmids by Difflugia has 
been demonstrated by observation. Simocephalus is the only animal in the 
pond that is dependent wholly upon alge for food. It may be able to 
adapt itself to some other food, but in this habitat its alimentary canal 
contains nothing else. It has not been demonstrated that any organism 
eats the living Typha plants except that the snails sometimes eat the more 
tender shoots. Limnodrilus lives among the roots but its alimentary tract 
contains rather finely comminuted material, some of which is Clearly de- 
caying plant stems. Cypridopsis vidua Brady feeds on the material which 
forms a slimy layer over the Typha stems, sticks, etc. Of course, this 
layer includes some organisms; however, their inclusion is accidental.  [ 
am sure they do not select algee. Simocephalus, Limnodrilus and ostracoda 
are eaten by dragon fly nymphs. Naturally this is difficult to observe in 
the pond. In order to eliminate the unnatural instincts that develop in an 
aquarium, a deep soup plate was kept at the pond, into which dragon fly 
nymphs and other forms were introduced immediately on being taken from 
the pond. The white background made observation easy and accurate, 
and one may be reasonably sure that the feeding instincts exhibited were 
natural. The nympbs experimented with belonged to the family Libellu- 
lide. The preference of the dragon fly nymphs is indicated by the order 
