154 



surpassed by several other species. Zimmer ('99) finds that this 

 species is the most common winter rotifer in the plankton of the 

 Oder, with a maximum in the spring and a predominance of var. 

 tecta from July to September. Schorler ('00) finds it to be the most 

 common rotifer in the Elbe from April to November ; and Skor- 

 ikow ('97) finds it in the Udy, in Russia, throughout the summer 

 in great numbers, but surpassed by Synchczta, Polyarthra, and 

 Brachionus angularis. The variety tecta greatly exceeds var. 

 stipitata in these waters. Seligo ('00) finds it throughout the year 

 in Prussian lakes near Danzig, with a maximum in May. There are 

 indications, in his data, of recurrent pulses during the summer, but 

 his interval of collection is too great to follow their history. Burck- 

 hardt ('OOa) finds it throughout the year in Swiss waters, with its 

 single maximum in August. Jennings ('94, '96, and '00) reports it 

 in the summer plankton of Lake Michigan and Lake Erie and of 

 inland waters of Michigan. 



Anuraza hypelasma Gosse. Average number of females, 2,390; 

 of eggs, 1,917. This species has a very definite limitation to a 

 period extending from early in June to the first days of November. 

 There are but two records outside of these limits a single female and 

 egg on Jan. 11, 1898, and another upon April 19 of the same year. 

 The probabilities of occurrence in very small numbers at all tempera- 

 tures is thus indicated. The following table gives the data of pulses 

 and temperatures. 



All of the pulses save one occur at temperatures above 70, and 

 with this exception the species declines rapidly and disappears 

 shortly after temperatures pass below 60. It is plainly, in our 

 waters, a summer planktont, with its optimum temperature close 

 to the summer maximum. This species takes no share in the vernal 

 pulse, and there is no satisfactory evidence of any fluctuation 

 corresponding to it at any other season. There are three or four 

 pulses in each summer, and the species is apparently poly cyclic, for 

 winter eggs were found in 1898 either at the maximum of the pulse 

 or the week or fortnight following. Thus 24,000 winter eggs were 

 recorded on Sept. 27, 1898, the date of the maximum of the Septem- 

 ber pulse. The parthenogenetic eggs preponderate during the rise 

 of the pulses in a very marked manner in this species. For example, 

 in this September pulse 55,400 eggs were recorded during its rise 

 to 500 during its decline. . In like manner, in the case of the 



