255 



The cause of this limitation of Moina to periods of low levels in 

 maximum temperatures appears to lie in the food relations of the 

 species. Moina abounds in waters approaching stagnation. The 

 slackened current, increased sewage contamination, and excessive 

 growth of the smaller algas and chlorophyll-bearing flagellates at 

 such seasons in the channel of the Illinois furnish an environment 

 favorable to the great increase in Moina, such as was recorded in 

 the low water of July- August, 1894, of June- July, 1895, and of 

 September, 1897, exceeding in each instance that of any other 

 species of Entomostraca in the plankton. The relatively smaller 

 numbers of Moina at the same seasons in the less contaminated 

 backwaters lends additional support to the view that these condi- 

 tions approaching stagnation are in a measure responsible for its 

 unusual development in channel plankton. 



Of the total Moina population, over 65 per cent, are young or 

 immature, 7 per cent, are egg-bearing females, embryos are often 

 freed from the parent on application of the preserving fluid, 11 per 

 cent, are males, and the remainder, females without eggs. Males 

 appeared with the maximum or decline of the major pulse for the 

 year in 1894 (August), 1895 (July), 1897 (July and September), and 

 1898 (September), but ephippial females were recorded only in 

 June-July, 1895. 



The seasonal distribution of Moina conforms to the type of a 

 series of recurrent pulses wherever the numbers are considerable 

 and the collections sufficiently frequent to delineate their courses. 

 Even in the small numbers of 1898 (Table I.) there are suggestions 

 of such pulses. 



Moina micrura seems to be a species characteristic of the pota- 

 moplankton. It is not mentioned as a constituent of the plankton 

 or littoral fauna by any of the various investigators quoted else- 

 where in this paper who deal with lakes or ponds in Europe or North 

 America; nor does it appear as a frequent constituent of the 

 potamoplankton elsewhere. Skorikow ('02), indeed, makes the 

 statement, " Bemerkenswert ist fur die Fliisse vollstandiges Fehlen 

 der Gattung Moina." This, however, is hardly the case, for 

 Sowinski ("88) finds it in the plankton of the Teterew, a tributary 

 of the Dnieper, and Fric and Vavra ('01) report it from the Elbe 

 in 0-1 m. strata in July-September, males appearing in the latter 

 month. Meissner ( '02 and '03) also finds it in the Volga at Saratoff, 



