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 Hntomostraca 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 [linois (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 E’ntomostraca 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 larve (principal- 
ly Chironomus, with a few Dixa and Tanypus larve) alone are 
present in larger numbers per cubic meter in Spoon River than 
