318 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



little difficulty. Two of the most important genera of 

 the salt water Diatom flora are Chcetoceros and Rhizoso- 

 lenia, both with numerous species. They appear in 

 enormous quantities in the Baltic, but do not seem to be 

 present so abundantly at some seasons of the year, and the 

 number also varies considerably in different years. 



The species of Chcetoceros attain their maximum in 

 March, with an average of 457 million per cubic metre, 

 or 457 in every ccm. of water. The principal species of 

 Rhizosolenia that occur in the Baltic are Rhizosolenia 

 (data, which reaches its maximum in May with a total of 

 85*7 per ccm., and Rhizosolenia semispinal in March with 

 an average of not less than 102 - 4 per ccm. Since a cubic 

 centimetre of water contains on the average about 30 

 drops, it is no exaggeration to say that every drop of sea 

 water in the Baltic is inhabited by Diatoms. 



The North Sea, and especially the open ocean, contain 

 very much smaller numbers of Diatoms than the Baltic. 

 The Copepoda of the North Sea, on the other hand, show 

 no diminution. In the North Sea, in spite of the much 

 smaller quantity of the total catch than in the Baltic, the 

 meshes of the net were much sooner obstructed, and the 

 net itself took on a yellowish-green colour. Hensen con- 

 cluded from this that there exists in the plankton still 

 smaller unknown organisms which escaped through the 

 meshes of the net. Subsequent research has shown this 

 surmise to be justified. If the number of Diatoms in a 

 drop of water is occasionally so high, what must be the 

 case with regard to these still smaller organisms? 



All animal life in the sea is ultimately dependent on 

 vegetable life. This latter is in turn dependent on sun- 

 light, and is therefore only able to exist down to depths to 

 which the sun is able to penetrate, that is, a maximum of 

 400 metres from the surface. Thus by far the greater 



