GEOGRAPHIC DIVISIONS OF THE PELAGIAL 283 



and pelagic, are known only from the Mediterranean. In this category 

 belong, among others, two species of starfish (Astropecten spinuloses 

 and A. jonstonii) , 54 the barnacle Pachylasma giganteum, 55 21 decapod 

 crustaceans 56 , 2 species of salpas, and 4 species and varieties of Ap- 

 pendicularia. 57 Of the fishes, the anchovies (Engraulis encrasicholus) 

 contain a race distinctive of the Mediterranean; Clupea papalina is 

 the Mediterranean form of the northern sprat (C. sprattus) , 58 and the 

 sardine (C. pilchardus) never attains the length of the oceanic form 

 of this species (maximum of 18 cm. in length compared with 26 cm.) , 59 

 These species may be considered endemic only with reservations; yet 

 the faunae of the French and Spanish coasts of the Atlantic, as well 

 as of the Azores and the Canary Islands, are fairly well known, so 

 that it may be assumed that a large number of those species do not 

 occur in the neighboring seas. 



The Baltic Sea. — In many respects, the Baltic Sea 60 is just the 

 opposite of the Mediterranean. A widespread land area sends its waters 

 into the relatively small and shallow basin, and the great influx of water 

 is accompanied by the smaller loss of water through evaporation in 

 these latitudes. For these reasons, the salt content is small, and from 

 the Baltic relatively large quantities of water flow outward into the 

 ocean, whereas the inflow from the ocean is much smaller. As the out- 

 flowing water is less dense on account of the lower salt content, the 

 outflow occurs on the surface, and the inflow is at deeper levels, the 

 opposite of the Mediterranean situation. The salt content, even of the 

 surface water, is greatest in the western Baltic near the connection with 

 the ocean and distant from the great inflow of fresh water, and de- 

 creases toward the east until a salt content of only 2% c and less is 

 reached in the surface water in the Gulf of Bothnia (Fig. 76) . 



As a result only very euryhaline forms, brackish-water animals, 

 and a few fresh-water animals can live in the Baltic Sea. Pike, perch, 

 and several other fresh-water fishes have been caught in the Strela 

 Sound between the Island of Ruga and the mainland, and the number 

 of fresh-water species rises to 20 in the Gulf of Finland and of Bothnia. 

 The number of species of marine animals decreases regularly from the 

 west toward the east; thus 55 species of decapod crustaceans are 

 found in the Kattegat, 9 in Kiel Bay, 2 in the Baltic Sea basin, and 

 only 1 in the Gulf of Finland; of the annelid genera, Nephthijs and 

 Nereis, the North Sea contains at least 20 species, the western Baltic 

 9, the Baltic basin 5, and only 2 species extend beyond 17°E. longi- 

 tude. 61 Differences also occur in the plankton. The number of copepod 

 species and individuals becomes less toward the east while the clado- 

 cerans Podon and Evadne increase; and to these may be added Bos- 



