242 



600 per m. 3 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. 3 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 in 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 

 Entomostraca 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. 



