245 



Daphnia hyalina Leydig. Average number, 417. In channel 

 waters this species has appeared in but two years, in 1895 in 

 April-July, attaining on June 19 a maximum of 166,208 per m. 3 , of 

 which 150,626 were immature. The collections were too infrequent 

 in these months to trace the course of this vernal pulse. D. hyalina 

 did not reappear until the spring of 1898, on May 24, in a single 

 vernal pulse culminating at 1 1,600 per m. 3 on June 7, and disappear- 

 ing a fortnight later. Its occurrences with one exception were all 

 at temperatures above 70. There is no apparent - cause for its 

 absence in later months or in other years. Males and ephippial 

 eggs were not found. 



Daphnia hyalina is an exceedingly variable species, and a large 

 number of forms have been described which belong to the hyaUna 

 group. Burckhardt ('00), for example, recognizes 26 such forms 

 as varieties of this cosmopolitan planktont. This variability and 

 the difficulties attending the resulting synonymy cause any discus- 

 sion of the species in other waters to be attended by much uncer- 

 tainty. I shall therefore not attempt to distinguish in my dis- 

 cussion between the various varieties included by Burckhardt in 

 the hyalina group. 



In lakes of northern Germany, Apstein ( '96) finds that D. hyalina 

 is essentially a winter planktont with a seasonal range of September- 

 July, and with maximum numbers in November- January. The 

 maximum thus appears there at the time of complete extinction in 

 our waters. Stenroos ('97) records it (as D. galeata) in the summer 

 plankton of Karelia, Huitfeldt-Kaas ('98) finds it in Norwegian 

 lakes in July and September in considerable numbers, and Scour- 

 field 's careful studies ( '98) of its seasonal occurrence in waters of 

 Epping Forest reveal an interrupted distribution in April-Novem- 

 ber. vScott ( '99) finds it in numbers in Scottish lochs in the plankton 

 examined at long intervals in March-January. Fuhrmann ('00) 

 reports it as perennial in Neuenbergersee, with a maximum in June 

 followed by a midsummer minimum. Burckhardt ('OOa) finds 

 great diversity in different Swiss lakes and in different years in the 

 relative numbers present. His intervals of collection were too great 

 to detect any pulse-like movement in the production, and it may be 

 that the diversity is due in part to the incompleteness of his records. 

 He concludes that D. hyalina is at a minimum in March-May, 

 increases in numbers slowly (with a preponderance of young indi- 



