yVuiA ■ J 



A Revision of the British Species of Fresh-water Cyclopidcc 

 and Calanidce. 



Nowhere amongst the Entomostraca is there a group whose 

 members offer greater difficulties as to specific discrimination 

 than the genus Cyclops. In the first place, the changes under- 

 gone by each animal in the course of individual development are 

 very extensive and have been as yet only imperfectly investi- 

 gated. There can be no doubt that many so-called species have 

 been founded upon forms which represent only transitory evolu- 

 tionary phases, and as, not only amongst Crustacea but in many 

 other groups of animals, the function of reproduction is certainly 

 by no means confined to fully-developed adults, it is certain that 

 we cannot entirely rely on the presence of ovisacs or other 

 reproductive organs as conclusive evidence of the morphological 

 maturity of the animals in which they occur. Nor is there in 

 most cases any salient feature, such as an easily distinguished 

 peculiarity of colour or form, which can enable one at a glance, 

 by the help of an ordinary hand lens, to distinguish between 

 nearly related species. It therefore becomes a necessity to 

 dissect and examine under high microscopic powers very large 

 numbers of specimens ; and even when this is done, variations 

 dependent upon race, habitat and other circumstances are so 

 abundant that it often becomes a work of the greatest difficulty 

 to decide as to the species to which any particular specimen 

 may belong. 



No wonder then that much confusion prevails in the nomen- 

 clature of these species : it is, indeed, rather wonderful that the 

 confusion is not still greater, more especially when it is remem- 

 bered that many copious authors have furnished only written 

 descriptions of their species — drawings being absolutely essential 

 to a proper understanding of minute points of specific difference. 

 I cannot pretend to have cleared up all the doubtful points even 

 amongst the small number of species noticed in this paper, nor 

 do I suppose that no new or unrecorded species have been missed 



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