535 



be excluded from the averages. The ratio in 1898 is 1 to 36, 

 and as this is a year of normal hydrograph it may represent 

 more nearly the true proportion in the production of rotifers 

 in the two streams. The data in the remainder of 1897, in 

 1896, and in 1899, so far as they go, sustain this higher ratio. 

 This ratio is somewhat greater than that of the algae, rhizopods, 

 and infusorians, which form earlier links in the chain of food 

 relations. The rotifers, on the other hand, depend upon these 

 groups for food, and become later links in the chain, and conse- 

 quently do not, for lack of time, attain the development in 

 tributary waters that they do in the main stream. The diatoms 

 and MastigopJiora also form considerable elements in the food 

 of the rotifers, and since these groups are proportionally less 

 frequent in Spoon River than in the main stream, the deficiency 

 in this food element may be one cause of the lesser develop- 

 ment of rotifers in the tributary. Most rotifers, however, do 

 not exhibit such limitations in diet, and the effect of this de- 

 ficiency in food must be largely quantitative. 



The number of species of rotifers found in Spoon River is 

 much less than that in the main stream (44 to 107 — see p. 519), 

 and none peculiar to the tributary was noted. The Bdelloida, 

 which are principally shore-loving and bottom forms, constitute 

 a relatively greater proportion of the species and individuals in 

 the tributary, though their absolute numbers per cubic meter 

 of water are rarely in excess. 



In the low-water period of 1897 the limnetic species com- 

 mon in the main stream and in the residual backwaters reached 

 an unusual development in Spoon River, even in excess of that 

 in the main stream. The conditions then prevalent, the higher 

 temperatures of a late autumn, stagnating water with little or 

 no current, and the absence of the usual autumnal flushings, 

 combined to favor the unusual phenomenon. During this sea- 

 son the contributions of Spoon River to the rotiferan fauna of 

 the main stream would increase the amount of rotiferan plank- 

 ton therein, though not diversifying it — as will be seen on com- 

 parison of the relative number of species in the two streams at 

 that season. 



