161 



ASPLANCHNA PRIODONTA. 



years, leads us to infer that the species may be poly cyclic in our 

 waters. 



This limnetic rotifer figures largely in the fresh-water plankton 

 of other localities, attaining a relative development greatly surpass- 

 ing that thus far found in the Illinois River. Apstein ('96) reports 

 it of irregular occurrence in the smaller lakes of Holstein, and Seligo 

 ('00) finds it perennial in Prussian lakes, with maxima in April and 

 September. Wesenberg-Lund ('00) also finds it perennial in Danish 

 waters, with sexual cycles in May and September. Marsson ('00), in 

 waters about Berlin finds a great variation in the seasonal occurrence, 

 but the intervals of his collection four to six weeks were too great . 

 to follow seasonal distribution satisfactorily. Zacharias ('98b) finds 

 it in the summer and autumn plankton in a number of German 

 lakes and streams. Zimmer ('99) traces its appearance in the Oder 

 from February to a maximum in May, from which time until the end 

 of July it is "einer der haufigsten Planktonorganismen" (!). It 

 then declines, but returns in small numbers in November. Schorler 

 ('00) records it in the Elbe from April to October, with maxima in 

 April-June and September. Burckhardt ('OOa) finds, on the other 

 hand, that in Swiss waters it reaches its greatest development from 

 December to March with a maximum in January-February. There 

 are also secondary maxima in May- June and in August. Lauter- 

 born ('93) finds it to be a dicyclic perennial planktont in the Rhine, 

 with maxima in April and September-October. A part of the great 

 variation in the seasonal distribution of this species which is ap- 

 parent in this survey of the literature may be due to insufficient 

 collections or too great an interval between collections. The species 



