CH. VIIl] 



A CENSUS OF THE SEA 



161 



square metre in cross area. The worm Sagitta, and some kind of 

 coelenterate (usually the ctenophore Pleurohrachia) were always 

 present, but the larvae of the metazoan animals occurred less 

 constantly. The table shews that pretty much the same groups of 

 organisms occurred during each of the four months ; and that they 

 were present in much the same relative proportions. But there is 

 a seasonal change in progress during the year, and this is better 

 seen when the detailed tables are consulted. 



If we consider all the stations investigated during each cruise 

 we may obtain an average catch, which is in some respects more 

 instructive than the individual catches. It would require too much 

 space to give these average catches for all the organisms considered, 

 so I have grouped together all the species of each of the common 

 diatoms Biddulphia, Rhizosolenia, and Chaetoceros, of the peri- 

 dinian Ceratium, and of all the copepods, and give these average 

 catches in the following table. 



Average numbers of Jive kifids of 2)ic(''ikionic organisms contained in 

 a colum7i of water of 1 square metre of cross area. Middle of 

 North Sea. 



Biddulphia , , 

 Coscinodiscus 

 Chaetoceros .. 



Ceratium 



Copepods 



February, 

 3 hauls 



128,000 



149,000 



1,303,200 



1,402,000 



215,440 



May, 

 15 hauls 



9,070 



131,200 



13,300,000 



2,973,400 



560,000 



August, 

 13 hauls 



1230 



26,600 

 2,660,000 

 8,413,600 

 1,028,000 



November, 

 9 hauls 



10,461,260 



415,000 



140,409,000 



12,172,000 

 497,800 



Variation of density of plankton with locality. In the 

 Appendix I give the numbers of organisms belonging to each of 

 the above four genera, and the numbers of all the copepods, taken in 

 each of the individual hauls. Now the reader should consult these 

 in order to see whether or not the above averages have any value 

 as representing the contents of the whole sea area studied. The 

 distribution of the copepods is pretty uniform, and I think that 

 the average catch possesses some degree of reliability. The occur- 

 rence of the peridinian Ceratium is not so irregular as to preclude 



J. F. 11 



