148 
April-May with the possibility of a second on the June rise; and 
prompt and complete disappearance when maximum summer tem- 
peratures are established. Low water in the autumn seems to inter- 
fere with an autumnal pulse. In 1894 there was a well-sustained 
rise in September (Pt. I., Pl. VIII.) and a pulse of A. aculeata. In 
1896, however, no pulse occurred in the high water of the autumn. 
No midwinter occurrences followed the very low water of 1897. It 
is thus in channel waters a vernal planktont, with its temperature 
optimum near 70° but below the summer maximum. Hempel’s 
statement (99) that it is a “winter species’ is borne out by its 
presence from December through the winter, but its numerical 
distribution ranks it at once with the vernal organisms. Lauterborn 
(94) finds it abundantly in winter months in the Rhine, and Ap- 
stein (96) speaks of it as a “Sommerform,”’ absent from Lake Plon 
from November till March, and with maxima from April to July in 
different bodies of water where it continues through the summer 
and till October, and then disappears. Summer temperatures in 
these waters, however, are not recorded by him above 21° C. (69.8° 
F.), which is about the temperature at the time of the vernal maxi- 
mum in the Illinois, and at least 10° F. below that of the summer 
maximum in our waters. Jennings (94, ’96, and ’00) records it as 
abundant in the summer plankton of Lake Erie, Lake Michigan, and 
some inland lakes of Michigan. These waters also are somewhat 
cooler (5°-10° F.) than those of the Illinois River in midsummer. 
Temperature, it seems, must have a decided effect upon the seasonal 
distribution of this organism in our waters, though the chemical 
conditions and food supply may also enter as factors in the summer 
suppression of the species. 
Females carrying usually a single egg appeared in 1898 early in 
April, and were most abundant during the maximum of the pulse. 
On an average, less than a fourth of the females were ovigerous. 
Empty loricze appeared May 10 (4,800) and 17 (3,200) at the crest 
and decline of the spring pulse, and the same phenomenon of deca- 
dence was noted in previous years during this period. Outbreaks 
of parasites were not recorded for the species, and the decline is to 
be attributed to cessation of reproduction and to the death and 
destruction of the individuals by the more usual causes. 
This species is quite variable, but no effort was made to follow 
its seasonal history. The type form is by far the most abundant. 

FA 
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