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 winte'r, 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 loricae 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. 



