16 ILLINOIS BIOLOGICAL MONOGRAPHS [210 



of eighteen inches. Large patches of mud-covered Nitella seemed to be the 

 only surviving species of plant life, growing all along the shore and out to a 

 depth of ten inches. The water was ' thick ' and black, with only a few animal 

 forms, except in the case of protozoa which seemed to flourish. Stenostoma 

 was the only rhabdocoel present and aU the individuals collected were very 

 long and slender. In this situation absolutely prohibitive for most species, 

 Stenostoma appeared to be almost the only form above the Protozoa which 

 could carry on its existence. 



Another type of pond is that used by the United States Bureau of 

 Fisheries. Such ponds are artificial in their origin and are always 

 under control, thus presenting data interesting in comparison with 

 natural conditions. These ponds are either cement lined or have an 

 earth bottom. They vary in depth and shape. Some are shallow, 

 mere tanks with a smooth bottom, while others are long and narrow 

 with a graded bottom giving half a dozen different depths. Often a 

 number of half-partitions supply lurking places dark and secluded. The 

 unhned ponds are generally open and broad with a depth varying evenly up 

 to eight feet. The small tanks are about three by eight feet, while the largest 

 ponds will cover an acre of ground. The water is supplied from a common 

 reservoir, filled from the river. There is always an intake and outlet thru 

 which is kept up a constant and steady flow. The loss by evaporation is thus 

 supplied, the oxygen content kept nearly standard, and the water constantly 

 free from organic debris. The life conditions are much less complex than is 

 naturally the case, since either fish or clams are the only large species present. 

 The vast number of microscopic forms have found their way accidentally 

 and flourish because the situation is favorable. All of the ponds contain a 

 larger or smaller amount of filamentous algae, water weed, and lower forms 

 of plant life. This helps to furnish a never-ending supply of oxygen and a 

 hiding place for huge communities of entomostracans and the Uke. There is 

 no possibility of a sudden change in the several life factors, i.e., the conditions 

 are constant and steady, a situation which is ideal for aU sorts of types and 

 makes the number of species very large. Even during the winter, down under 

 the ice in the deepest parts, the stream of warm intake makes possible a con- 

 tinuance of almost summer conditions, and consequently there is not the whole- 

 sale extermination of the species that generally follows the faU drop in tem- 

 perature. Thus generation follows generation without a break, right through 

 the coldest months. 



Many other ponds, permanent and seeming to present conditions suitable 

 for large annual communities, are more or less poor in such forms, and may 

 also be said to be dead water and many which contain various other species do 

 not show planarians or rhabdocoels. Perhaps the most influential factor in this 

 poverty of the fauna is the constant agitation of the water, which, when 

 coupled with a variation in the amount, is sufficient to prevent much Hfe. 

 Then, too, many species are so retiring in their habits and spend so much of 



