12 



to luy observations in »Hauchs Togter«, p. 462, and to m_y treatisc »Om de 

 skalbærende Molluskers Udbredniugsforhold«, 1888, p. 44 seqq., on tlie occur- 

 rence of arctic animals in the Baltic Sea. I do not see that it is more diffi- 

 cult to understand the occurrence of the arctic diatoraaceæ 4han that of 

 the auimals. It may even be explained in various ways; it is not necessary 

 to presume that there are yearly currents connecting the arctic seas with the 

 Baltic. 



To be sure, it is possible that certaiu diatomaceæ are fitter than others 

 to follow the currents, but I dåre say that, as a rule, they can but with great 

 caution by used as »floaters«, i. e. objects thi-own into the water in order to 

 follow the course of the current through long distances and under changing 

 conditious. They can shoot up and die away so suddenly that I must compare 

 it to the formation of a cloud which appears at every high mountain-top near 

 the sliore when it is met by the damp air from the sea. The cloud stands still 

 in the air-current, certainly, but the single drops of water are coustantly re- 

 newed, fresh ones being continually coudensed on oue side while others are dis- 

 solved into invisible mist on the other side of the cloud.*) — 



As soon as it had been decided that the Biological Station in 1897 should 

 commence investigations in more open seas, i. e. outside our uarrow fjords, 

 I set to work to construct au apparatus which could fish plankton in the 

 deejjer layers of the water without getting at the same time plankton from the 

 surface water mixed into it. I tried to make a closing-apparatus to Hensen's 

 ordinary »quantitative« plankton-net, so that the opeuing of the net might be 

 closed at any depth by a falliug weight, while the bag, as usual, is sunk open 

 to the depth from which one wants to fish, exactly, consequently, as when it 

 is used in the ordinary way. I succeeded at last in makiug a closing-apparatus 

 which we could use, which closes the opening of the net with two plane and 

 horizontal brass piates that stand perpcndicular when the net is open. I shall 

 put up the further descriptiou of this apparatus, as I should like to make it 

 more perfect in its details before I describe its construction ; as it is, however, 

 it is certainly quite tit for use and, particularly, it closes perfectly tight while it 

 is hauled up. As the first experimeuts I have made with it have really giveu 



*) It would be interesting more closely to investigate the occurrence of the young 

 oyster in the Limfjord, and to follow its development during the time when it appears 

 in masses in the plankton. It would for instance be possible, I think, approximately to 

 calculate how many grown-up cysters there live in the fjord, when we counted how many 

 young ones they brought forth. It would certainly be much easier to undcrtake this inve- 

 stigation than Hensen's attempt at counting the plaicc in the Baltic Sea. 



