242 






Date | Females eee Young At otal tS oouee 
SOs A omannoagauds 160 320 640 We ALAX0) 57 
DO Eh cps yer aaah ees 7,520 4,000 12,800 24,320 52 
Octane cea Ske 3700 10,800 58,400 72, 60 82 
12 oraeaeieeey tee cas 1,600 7,600 9,200 83 
OS ae cparins chro peters 560 840 4,440 5,840 76 




600 per m.*are found after the temperatures pass 70°, with the’single 
exception of the decline of the October pulse and the rise of the 
November pulse to 2,040 per m.? at 47°, following the high tempera- 
tures in the late autumn and stable conditions of 1897. From the 
depression in numbers during the period of maximum heat 1n mid- 
summer and the occurrence of the major vernal and autumnal pulses 
before and after its reign it appears that the temperature optimum 
for D. cucullata in channel waters lies. below this level, that is, 
below 80°. 
D. cucullata is evidently very easily affected by the changes in 
hydrographic conditions. Thus, in July-December, 1897 and 
1898, the total movement in river levels was 12.4 and 31.4 ft., 
respectively, while the total cucullata population for these months 
was 186,420 and 1,140—164-fold greater in the more stable year. 
D. cucullata thus exhibits the maximum sensitiveness among the 
Fntomostraca to these environmental factors. 
The D. cucullata population in the plankton consists almost 
entirely of parthenogenetic females and young. The immature 
stages form about 60 per cent. and the egg-bearing females 16 per 
cent. of the total individuals. Dead, parasitized,or fungused indi- 
viduals were found at times of the maxima or shortly thereafter, 
but never in very large numbers. Males were found once in 
December, 1896, and ephippial females also but once, on October 
19, 1897, during the decline of the maximum pulse in our records. 
Daphnia cucullata var. apicata Kurz, in well-developed condi- 
tion, was found in relatively small numbers during the vernal pulses 
of 1895 and 1896 and the autumnal pulse of the former year. 
a “ew 
a a 
aH Ay eos OT rh GED Ort eta mH f 
