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 (Pl. IT.) 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. 
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 1s probably 
polycyclic 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 Pl6n. 
Asplanchna brightwellit 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. 
PuLseés oF ASPLANCHNA BRIGHTWELLII. 
Year Date | Temp.| No. Date |Tenip.| No: 
1 See BsReh Oe S eich ora sae Usa cee mon S | eS | 
SOS ic ee ere teas fetes ates ee June 19; 80° 6,678 
INCOSE Bee ae eee athe OME Ane Re See May 1 70° 1,788 | June 27} 80° 1,600 
LOU Ae e iate diet Sith tonie ern See ——— | | | re 
USO ROE ee aerernrse cao hAn May 5 | 60° 120,800.) June 2m iy 1,100 
