THE BALTIC SEA 



305 



the Baltic Sea — Cyanea capillata and Aurelia aurita — the second penetrates 

 farther to the east and north, reaching the shores of Finland ; it is encountered 

 in areas with 5-75 to 6-0% salinity at the surface and 7% at the bottom. Four 

 actinians penetrate as far as Kiel Bay — Helcampa duodecimcirrata, Urticina 

 felina, Metridium dianthus and Sagartia viduata ; but they do not go farther 

 east than Kiel Bay. 



There is a marked decrease in the number of polychaete species in the Belt ; 

 even in the southern part of the Baltic Sea only 25 species of them are known 

 including: Travisiaforbesi, Syllis armillaris, Nereis pelagica, Fabricia sabella, 

 Arenicola marina, Nephthys ciliata, N. coeca, Scoloplos armiger, Terebellides 



35 33 31 29 27 25 23 21 19 17 15 13 II 9 7 5 3 I 0% 



Salinity 



Fig. 145c. Decrease in number of species from North 



Sea to Baltic compared with the decrease in salinity 



(Zenkevitch). 



stromii, Pygospio elegans, Harmothoe sarsi and Nereis diver sicolor. In the Belt 

 and the Sound about 143 species of polychaetes have been identified (Elias- 

 son, 1920). Pygospio elegans and Terebellides reach the entrance of the Gulfs 

 of Finland and Bothnia (Fig. 146). Nereis diver sicolor and Harmothoe sarsi 

 penetrate into the Gulfs (a little farther into the Gulf of Finland) and there 

 survive a lowering of salinity in the surface layers to 5-25% . 



Among the Gephyrea only Priapulus caudatus penetrates into the Baltic 

 Sea, remaining in the most westerly parts of it, while Halicryptus spinulosus, 

 which thrives in great numbers at the bottom of the Baltic Sea, reaches half- 

 way up the Gulf of Finland and to the Aland Islands and the Quarken of 

 Finland (Fig. 146). 



Bryozoa are represented in the Baltic Sea proper by only four forms ; among 

 these only Membranipora pilosa f. membranacea is still found at a salinity of 

 4% (Fig. 146). 



According to the summary due to Haas (1926), only five of the 87 species 



