434 



forms. Many of tlio forms derive thrir milrition in lu'.rt from the dead 

 organic matter in the itond, to wiiieli all of the forms contribute. The 

 ultimate food sources ii. 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 iiito the pond, e. g., insects. The formation of 

 nitrates from simple nitrogen comix>unds was established by the well 

 known work of Winogradsky ('S9). 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 alg« and phanero- 

 gams. The inclusion of diatoms and the smaller desmids by Difflugia has 

 been demonstrated by observation. Siinocepliahis is the only animal in the 

 pond tliat is dependent wholly upon algie for food. It jnay 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 Tj-pha 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 Brad.y feeds on the material which 

 forms a slimy laj^er over the Typha stems, sticks, etc. Of course, this 

 laj-er includes some organisms ; however, their inclusion is accidental. I 

 am sure they do not select algse. SimoceijJialus, 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 iii 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 ma.v be reasonably sure that the feeding instincts exhibited were 

 natural. The nymiihs experimented with belonged to the family Libellu- 

 lidse. The preference of the dragon fly nyni)ibs is indicated by the order 



