Page:The Habitat of the Eurypterida.djvu/78

 A second change which takes place in the Baltic fauna and which may be correlated with the variation in salinity, is that the stenohaline forms of the marine fauna disappear altogether, while the euryhaline ones become dwarfed. Thus, the common cockle shell, Cardium edule, in the North Sea, of normal marine salinity, is the size of a small apple, at Stockholm, where the salinity is below 10 permille, the shell in the deeper, more saline water is only as large as a walnut and is even smaller along shore where the water is fresher. At Königsberg, with the decreasing salinity, the size reaches that of a hazel nut, whereas at Reval, it is only the size of a pea. In like manner, Mytilus edulis, which is 8 to 9 cm. long at Kiel is only 3 to 4 cm. long at Gotland. The fish and worms also show dwarfing.

A third point to be noted is that the fauna decreases very rapidly in the number of species which occur. Karl Möbius in his report on the faunal survey in the Baltic Sea made by the Pommerania, states that:

"The total number of observed invertebrate animals amounts to about 200 species, not including the infusoria and crinoids.

"We have found scarcely one-fifth of these in the great eastern basin of the Baltic which begins between Rügen and the southern extremity of Sweden" (182, 277). The following table shows the distribution for the invertebrates. The numbers given for the Baltic as a whole should be compared with those given for Kiel. In this latter bay the conditions are not so different from those in the open