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salt water of the Baltic. Organisms from the Skagerak and Kattegat continue 

 through the Belts and the Belt Sea into the true Baltic, likewise in the lower 

 layers. The outflowing water from the Baltic, on the other hand, carries with 

 it brackish water forms into the surface layers of Kattegat and Skagerak. 



One of the clearest cases of such wandering with the current has been 

 closely treated. In the autumn of 1903, a Diatom, Biddulphia sinensis, not hi- 

 therto known from our area, nor indeed from the whole of the Atlantic, made 

 its appearance in great quantities in the North Sea in the German Bay (outside 

 the mouth of the Elbe), extending thence northward along the coast of Jutland 

 into the Skagerak and Kattegat, continuing also along the south and southwest 

 coasts of Norway, always following the current, and thus serving to distinguish 

 it. Since then it has continually appeared in the North Sea and the Skagerak, 

 and has moved southwards along the east-coast of Great Britain, as far as the 

 southernmost part of the North Sea, and further over into the Irish Sea. It can 

 therefore now no longer be taken as a guide to the direction of the currents, but 

 its first appearance furnished a good opportunity of so doing, and even of cal- 

 culating a minimum value for the rate of flow of the current. 



The instance here briefly referred to is, however, unusually fortunate, 

 and it is owing to the international co-operation that it was possible to thus 

 explain it in detail; as a general rule, the application of plankton organisms to 

 the study of ocean currents must naturally be made with care, taking into con- 

 sideration the possibility of inaccuracy in the sources of information employed. 



In the foregoing, an attempt has been made to give a brief and easily in- 

 telligible view of the plankton work carried out by the International Investi- 

 gation of the Sea, and the problems dealt with or now requiring to be investi- 

 gated, care being taken to avoid as far as possible the burdening of the descrip- 

 tion with names of organisms. It will appear, from the statements made, that 

 much yet remains to be done. The study of plankton is moreover, especially 

 difficult in precisely that area with which the International Investigations have 

 to deal, the region being extraordinary complicated, both hydrographically 

 and biologically. 



