284 



MARINE ANIMALS 



mina maritima and further in the northeast a large number of rotifers. 

 The salt content of 3.7% seems to set limits which even euryhaline sea 

 animals pass with difficulty; only the barnacle Balanus improvisus 

 and the bryozoan Membranipora pilosa extend beyond it. 



Fig. 76. — Map of the Baltic Sea. The salt content per thousand in vertical figures, 

 the depth in meters in oblique figures. After Brandt. 



Many immigrants from the North Sea are not able to propagate in 

 the Baltic and restore their numbers only by means of ever-renewing 

 reinforcements, e.g., the copepod Paracalanus -parvus. 62 Of 11 species 

 of marine fishes, which are common to the Gulf of Finland and the 

 North Sea, 9 are here unfertile; only the sprat (Clupea sprattus) and 



