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

Comparative Number of Species of Invertebrates in the Baltic, etc.

PHYLA	WATERS AROUND GREAT BRITAIN	BALTIC AS A WHOLE	BAY OF KIEL	BAY OF TRAVE- MUNDE
	35 permille	7.8 permille	20 permille	12 permille
Protozoa	69
Porifera	42	7	3	3
Coelenterata	98	28	24	8
Echinodermata	48	6	5	2
Vermes	101	68	50	26
Bryozoa		11	8	5
Crustacea		50	36	19
Mollusca	682	68	64	40
Tunicata		5	4	4
Total	1040 (+)	243	194	107