156 



but none appears in this interval, while hypelasma runs a normal 

 course of recurrent pulses throughout the summer. This August 

 pulse of hypelasma (Table I.) culminates August 16, just a week 

 after the symmetrical and well-defined pulse of chlorophyll-bearing 

 organisms (PI. II.) of August 9. 



With a single exception, all of the pulses of 1896 and 1897, indi- 

 cated in the table, fall a week later than, or coincide with, the pulses 

 of chlorophyll-bearing organisms, as in 1898 t 



This species has not occupied a prominent place in the literature 

 of fresh-water plankton. Weber ('98) finds it rare in Swiss waters 

 in the summer. Lauterborn ('93) classes it with the monocyclic 

 summer forms in the plankton of the Rhine, though he states in a 

 footnote that he had" found winter eggs once in June. It is probably 

 poly cyclic in our waters. Skorikow ('96) finds it in the summer 

 plankton of the river Udy, in Russia, but it is not mentioned by other 

 investigators of the potamoplankton of Europe. Apstein ('96) does 

 not report it from Lake Plon. 



Asplanchna brightwellii Gosse. Average number, of adults 2,079, 

 of eggs, 396; averages in 1897, 16,161 and 2,156. This is a poly- 

 cyclic perennial planktont in our waters. It has been found in 

 every month of the year, but the greater numbers and more con- 

 tinuous occurrences lie between May 1 and October 30. In 1898 

 (Table I.) all but 200 of the 108,120 recorded, lie within these limits, 

 and all but 260 above 60. In previous years approximately the 

 same limits are found. The following table gives the data of pulses 

 and temperatures. 



PULSES OF ASPLANCHNA BRIGHTWELLII. 



