534 
water-bloom organisms in the tributary which the surface ap- 
pearances would indicate. The greater turbidity, and the more 
sheltered situation and consequently quieter surface of Spoon 
River may tend to bring its quantitatively smaller water-bloom 
to the surface in a greater proportion than is possible in the 
more disturbed channel waters. These organisms form an early 
link in many chains of food relations in the plankton, and it is 
significant that they reach a considerable development in the 
plankton of tributary streams. 
The species of Mastiyophora are less than half as numerous 
in Spoon River as they are in the Illinois (28 to 62), and none 
was found peculiar to the tributary, or having there its center 
of distribution. Thus, excepting in periods of prolonged low 
water, the Mastigophora fauna of the plankton of the tributary 
neither increases nor diversifies that of the main stream. 
The Infusoria in Spoon River are greatly exceeded by those 
of the main stream, the ratio being 1 to 20. The only marked 
exception to this relation occurs during the early part of the 
low-water period of 1897, when the ciliates, principally Codo- 
nella, attained a greater development in Spoon River. This 
excess was soon masked by the usual autumnal increase of 
Carchesium and Epistylis, and of the free ciliates preying upon 
these sessile forms, in the main stream. This autumnal in- 
crease is not shared to any great extent by the tributary, proba- 
bly by reason of its lesser contamination by sewage. The in- 
fusorian fauna was by no means fully determined in the two 
streams, but so far as identifications were made, they indicate 
much less diversity in the infusorial fauna of Spoon River and 
no species peculiar to, or predominant in, the tributary. Thus, 
excepting the period of low water in early fall, the contribu- 
tions of Infusoria from the tributary do not enrich the plank- 
ton of the main stream, and at no time do they tend to diver- 
sify it. 
The Rotatoria, according to the grand average in Table, XIV., 
are almost equally abundant in the two streams (1 to 1.9). 
This equality at once disappears if the low-water period of 1897 
