164 A CENSUS OF THE SEA [PART II 



individuals of the genus Ghaetoceros. At the same time there 

 were also present some 102 millions of Rhizosolenia per cub. metre, 

 or 102 specimens per c.c. These figures really underestimate the 

 richness of the sea in diatom life, for it is certain that many of 

 these organisms living in the sea were too small to be retained by 

 the silk of the nets used. Speaking quite generally one may 

 estimate the number of diatoms which inhabit the North Sea or 

 Baltic beneath every square metre of surface as anything between 

 one and four hundred millions. 



Peridinians. These protozoa are less abundant than the di- 

 atoms. We see from Apstein's results that Geratium, the commonest 

 of the peridinians, was present in the eastern side of the North Sea 

 to the extent of about 1^ millions in February and about 12 

 millions in November, for every square metre of sea surface. 

 According to Hensen's results Geratium is present in the West 

 Baltic in greater abundance than in the North Sea. The average 

 number was about 1^ millions per cubic metre, or three individuals 

 to every cubic centimetre. The maximum number counted was 

 thirteen per cubic centimetre. Geratium tripos, the common 

 peridinian, is, however, only one species of many that are found 

 in the plankton, and hosts of the smaller forms must pass through 

 the meshes of the nets. One may estimate their abundance in the 

 North Sea and Baltic as anything between one and ten millions 

 per square metre of surface. 



Copepods. These animals are not present in the sea in any- 

 thing like the abundance of the diatoms or peridinians. But they 

 are much larger, and their chemical composition indicates that, they 

 are of ver}'- great importance as a source of food for the larger 

 animals. One single copepod is (on the average, for they differ 

 greatly in size) about equal to 160 Geratia or about 1500 Ghaeto- 

 ceros cells. They are relatively very uniformly distributed 

 throughout the sea and are ubiquitous, for one finds them at all 

 times and places. Apstein's tables shew that their abundance in 

 the North Sea is about one-quarter to one million per square 

 metre of surface, the numbers varying with the season. Hensen 

 found that they were present in the West Baltic to the extent of 



