
255 
The cause of this limitation of Morna to periods of low levels in 
maximum temperatures appears to lie in the food relations of the 
species. Moima abounds in waters approaching stagnation. The 
slackened current, increased sewage contamination, and excessive 
growth of the smaller alge and chlorophyll-bearing flagellates at 
such seasons 1n 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 Momma 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 Moma 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 Mota 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. 
Moima 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 1n Europe or North 
America; nor does it appear as a frequent constituent of the 
potamoplankton elsewhere. Skorikow (’02), indeed, makes the 
statement, “ Bemerkenswert ist ftir die Flusse vollstandiges Fehlen 
der Gattung Mona.” ‘This, however, is hardly the case, for 
Sowinski (’88) finds it in the plankton of the Tetérew, a tributary 
of the Dnieper, and Fri¢ and Vavra (’01) report it from the Elbe 
in O-1 m. strata in July-September, males appearing in the latter 
month. Meissner (’02 and ’03) also finds it in the Volga at Saratoff, 
