536 



Excepting, then, in seasons of prolonged low water in the 

 autumn, the contributions of Spoon River to the rotiferan 

 plankton of the main stream stream result in its dilution, and 

 aside from a greater proportion of the littoral fauna they add 

 little to its diversification. 



The Entomostraca are numerically a very small factor in 

 the life of Spoon River and form a very small volume of its 

 plankton product, the ratio of the entomostracan population 

 in the two streams being on an average 1 to 28. This ratio is 

 well maintained if the years are considered individually, being 

 1 to 32 in that portion of 1896 represented in our collections, 1 

 to 35 in 1898, and 1 to 23 in 1897. The ratio in this latter year 

 thus shows some effect of the prolonged low water, but not to 

 the degree which is shown in the case of the rotifers. The 

 Entomostraca illustrate most clearly the effect of the time ele- 

 ment in the development of the plankton. Their growth is less 

 rapid than that of any other type of planktonts, and in conse- 

 quence they cannot attain in tributary waters the numbers that 

 they do in the older waters of the main stream and its back- 

 waters. The number of species is also much smaller in Spoon 

 River than it is in the Illinois (13 to 49), and none peculiar to 

 the tributary was noted. Most of the adult Copepoda belonged 

 to a single species, Cyclops serrulatus, and it is probable that 

 most of the immature stages should be referred to the same 

 species. This seems to be a creek species, and to have its center 

 of distribution here rather than in the main stream, where it 

 never attains the numbers that most of the other Copepoda do. 



The contributions of Entomostraca made by the tributary 

 are thus very small at all times, and have only a diluent effect 

 upon the entomostracan plankton of the main stream, adding 

 some diversification in the case of a single species of Cyclops. 

 Spoon River carries no marked contributions of littoral Entomos- 

 traca to the main river. 



Of all the groups of planktonts the insect larvae (principal- 

 ly Chironomus, with a few Dixa and Tanypus larvae) alone are 

 present in larger numbers per cubic meter in Spoon River than 



