656 MRS. E. W. SEXTON ON 



35. Some Brackish-water Ampliipoda from the mouths o£ the 

 Weser and the Elbe, and from the Baltic *. By E. W. 

 Sexton, Marine Biological Laboratory, Plymouth t- 



[Received March I, 1912 : ReaJ April 23, 1912.] 



(Plates LXXIII. & LXXIY.t) 



Index. 



Pai^e 



Variation 656 



Systematic: New Species under Gammarus 657 



While engaged in determining the Copepoda of the harbour at 

 Bremei'haven, Herr W. Klie met with certain Gammaridea, some 

 of which were not easily referable to known forms. Through the 

 intervention of a friend these were submitted to me for examina- 

 tion, and the present paper contains the result, with notes on 

 some other collections of brackish-water Gammarus. There 

 proved to be three species in Herr Klie's collection, one of 

 them {Lejyiocheirus pilosics Zaddach) already known, the other 

 two apparently new, which I described. But before the descrip- 

 tion was published I received a paper from Dr. Vanhoffen, which 

 contained the description and figures of one of these species 

 under the name of Corophmm lacustre. I have therefore in- 

 cluded here only a few notes on this species. Dr. YanhofFen 

 kindly allowed me to examine his specimens of Amphipoda from 

 the Frische HafT, and it is interesting to note that the three 

 species, Leptocheirus pilosus Zadd., Corophium, lacustre Yanhoffen, 

 and Gammarus zaddaahi, sp. n., are present in both his and Herr 

 Klie's collections. All three appear to flourish equally well in 

 absolutely fresh water as in the brackish water of river estuaries 

 and harbour basins. 



The classification of the Amphipoda is rendered exceedingly 

 difficult by the changes or modifications resulting from each 

 successive moult. We know practically nothing yet of the 

 factors influencing the development of any given species, except 

 those of groivtli and sex. The modifications caused by these two 

 alone are responsible for an excessive multiplication of synonyms; 

 in some cases, Jassa for instance, almost every moult has been 

 given a different specific and sometimes a diflferent generic 

 name by different observers. But in the species of Gammar^is 



* [Since this paper was commnnicated to the Society, under the title " Some 

 Amphipoda from_ Bremerhaven," the author received some large collections of tlie 

 Gammarus described, which showed remarkable differences between those living in 

 salt and in fresh water. These facts have been included in the text, and the title 

 has been slightly modified.] — Ed. P. Z. S. 



+ Communicated by the Rev. T. R. R. Stkbbtng, M.A., F.R.S., P.Z.S. 



X VoY explanation of the Plates see p. 665. 



