220 01^ THE BRITISH SPECIES OF ENTOMOSTRiCA 



'/ ephippia," and are usually set free late in the year by tlie 

 death of the parent, remaining in the water over the winter and 

 hatching out on the advent of light and warmth in the spring. 

 The males, therefore, are not always available as helps in the 

 determination of species, and even when they are to be found, 

 it does not seem certain that the characters relied upon by some 

 authors are always quite trustworthy. 



The synonymy of the Daphnice is, as might be expected, very 

 perplexing, and I have scarcely attempted to give more references 

 than are necessary for identification : many species have been 

 proposed which have ultimately been withdrawn by the authors 

 themselves as being of no more than varietal value. Bat in a 

 group where there exist between the recognized species so many 

 intermediate forms, it becomes a matter of convenience and 

 almost of necessity to admit specific and even generic names, 

 which sometimes rest upon very small differences of structure. 

 Such, for instance, is the genus Hijalodaphnia, which differs 

 from Baphnia only in the absence of an "eye-spot" — a struc- 

 ture possibly of not very much importance, and of which we do 

 not know the physiological significance. 



I have received much valuable help from many friends in the 

 preparation of this paper, and to the following my thanks are 

 especially due — to Prof. G. 0. Sars of Christiania ; the Eev. 

 Canon Norman, F.E.S., of Houghton-le-Spriug ; Mr. Thomas 

 Scott, F.L.S., of the Fishery Board for Scotland ; Mr. D. J. 

 Scourfield, F.L.S., of Leytonstone, Essex; Dr. Creighton, of 

 Ballyshannon, Ireland ; and to Dr. H. J. Hansen, of the Copen- 

 hagen Zoological Museum. 



The following list embraces the species and varieties here 

 described or figured : — 



Daphnia pulex, De Geer. 



,, ohttisa, Kurz. 



,, ,, ydir. jyropinqua, G. 0. Sars. 



,, Sclioeclleri, G. 0. Sars. 



,, hamata^ G. S. B. 



,, longispina, 0. F. Miiller. 



,, ,, var. ««««to-, G. 0. Sars. 



