7 2 



THE HABITAT OF THE EURYPTEEIDA 



A second change which takes place in the Baltic fauna and which 

 may be correlated with the variation in salinity, is that the stenoha- 

 line forms of the marine fauna disappear altogether, while the eury- 

 haline ones become dwarfed. Thus, the common cockle shell, Car- 

 dium 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 

 Konigsberg, with the decreasing salinity, the size reaches that of a 

 haze) nut, whereas at Reval, it is only the size of a pea. In like man- 

 ner, 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 Mobius 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 Riigen and the southern 

 extremity of Sweden" (182, 277). The following table shows the 

 distribution for the invertebrates. The numbers given for the Bal- 

 tic 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. 



Protozoa 



Porifera 



Coelenterata. . . 

 Echinodermata 



Vermes 



Bryozoa 



Crustacea 



Mollusca 



Tunicata 



Total 



WATERS 

 AROUND 

 GREAT 

 BRITAIN 



35 permille 



69 



42 



682 



1040 (+) 



BALTIC 



ASA 

 WHOLE 



7.8 permille 



7 



28 



6 



50 

 68 



5 



243 



BAY OF 

 KIEL 



20 permille 



3 

 24 



S 

 50 



8 

 36 

 64 



4 



194 



BAY OF 

 TRAVE- 



MUNDE 



12 permille 



26 



5 



19 



40 



4 



107 



