gp:ographical distribution. 381 



Salinity, in its relation to Distribution. 



We know very little about salinity as a factor determining the distribution 

 of Siphonophores, except in the most general way. We can say that Siphono- 

 phores are absent or at least uncommon in regions of very low salinity. Thus 

 none have ever been recorded from the Black Sea or from the Baltic, although 

 the Ctenophore Pleurobrachia pileus is found in both. The North Sea and the 

 Baltic are especially instructive because the plankton of these regions has been 

 so thoroughly explored. 



There is no dearth of Siphonophores in those parts of the Atlantic from 

 which they might enter the North Sea. Thus various species are common in the 

 Bay of Biscay (Bigelow, : lib) ; at least eight genera are known from Irish waters 

 (Stephens, :05), and the yearly invasion of the Norwegian Sea by Physophora 

 and Agalmidae has been noted (p. 371). In the English Channel, too, Muggiaea 

 atlantica is an important member of the plankton, Agalma often appears from 

 there in the lists of the International committee, and Galeolaria is likewise 

 recorded. But according to Gough M. atlantica does not penetrate the Channel 

 beyond the Isle of Wight, though the temperatures are no barrier to it in summer 

 (p. 376); and though. Physophora hydrostatica and Diphyes arctica are known 

 from the northern, and various Siphonophores have been recorded from the 

 southern part of the North Sea, I can not learn that any Siphonophore ever 

 attains any faunal prominence there. D. arctica extends as far as the Skag- 

 gerak, where it is known from the intermediate waters, but further east, i. e. 

 in the Baltic, the Gulf of Finland, and the Gulf of Bothnia, Siphonophores are 

 unknown. Comparing these phenomena of distribution with the salinity charts 

 of the International committee, it appears that Siphonophores, in the region in 

 question, are almost a negligible factor in the plankton in waters with a salinity 

 less than 35 "o", and that they are entirely absent where the salinity is below 

 about 30%°. 



On the east coast of the United States the coast water is comparatively 

 fresh; and here, too, Siphonophores never form a conspicuous or constant con- 

 stituent of the plankton, though various Medusae and Ctenophores are often 

 extraordinarily abundant. For example, Stephanomia cara appears only sporadi- 

 cally in Massachusetts Bay or the Bay of Fundy, while the large collections 

 from Labrador and Newfoundland which I have studied did not contain so much 

 as a fragment of it. Agalma elegans enters Narragansett Bay only when the 

 surface waters of the Gulf Stream are driven on shore by southerly winds, and 



