432 



the water was tlisii oovered with a mixture of paraffine and beeswax. The 

 animals in rhe jar where rhe stems protruded through tlie seal invariably 

 lived lo]jger. The periods for the V)eetles were aboid 1 and 3 days re- 

 spectively. 



Light. — The pond is fairly well lighted throughout its entire depth 

 during the day except when it is covered with snow. The light is reduced 

 considerably by the growth of Typha. Kofoid ('04) found that, with the 

 development of phanerogams in one of the back\vater ponds tributary to 

 the Illinois River, there was a marked reduction in the plankton. Some 

 comparative observations were made on a pond about five miles west of 

 this one. It has about the same area and depith but there is no Typha 

 growing in it. Although no quantitative methods were applied, cladocera, 

 copepoda, and chlorophyceie were mucli more in evidence in it during Sep- 

 tember, "09, than in the pond under discussion. It seems probable that the 

 reduction of the light by the Tyi^ha growth has resulted in fewer species 

 and individuals developing in this ix)nd. 



On Jan. 11, 1909, the ice was partially melted. Openings had formed 

 in the ic-e around the Tji^ha stems and about 2i to 3 inches of water stood 

 above the remaining ice sheet. Cyclops was quite abundant in this upper 

 layer of water which was certainly due to their being phototactic. It was 

 the only organism detected. A lowering of the temperature under such 

 conditions would certainly destroy many individuals. Thus an adaptation 

 presumed to be beneficial under one condition becomes destructive under 

 certain other conditions. 



Food Relations. — Regarding the nutrition of aquatic organisms there 

 are two theories, which, although not mutually exclusive, are essentially 

 different. 



The older one is that the ultimate source of food is chlorophyl bearing; 

 plants and the various forms of bacteria which produce nitrates and ni- 

 trites. The materials thus elaborated or their derivatives are ingested int-i 

 food vacuoles, gastrovaseular spaces, or alimentary tracts of animals, where 

 they are acted on by secretions of the animal, reduced to a solution and 

 absorbed. This theory has been assumed by most zoologists in their dis- 

 cussions of food relations, and it is the most fundamental assumption in 

 the investigations now being prosecuted by the Internatioual Fishery Or- 

 ganization. 



