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 EpistyUs, 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 



