CH. VIIl] A CENSUS OF THE SEA 165 



about 80,000 per cubic metre. If we suppose that this represents 

 the average copepod population of the sea over large areas, and the 

 fairly regular distribution of these animals justifies this assumption 

 to some extent, then one square mile of the Baltic contained from 

 80 to 100 billion of copepods. Or putting it in another way, every 

 €ubic inch of the waters of that sea contained one copepod. The 

 estimates of the abundance of copepods furnished by quantitative 

 plankton investigations are more reliable than those of diatoms or 

 protozoa, for the crustaceans are more evenly distributed through- 

 out the sea, and they are so large that all are retained by the 

 meshes of the silk nets employed. 



The micro-plankton. Thus even the comparatively large 

 organisms, like the larger diatoms and protozoa, and the copepods, 

 may be present in the sea in very great abundance. But there 

 are also hosts of smaller organisms, such as the bacteria, the 

 •smaller diatoms, protozoa like the flagellates, and other minute 

 unicellular animals and plants which are not taken in the quanti- 

 tative plankton nets in the proportion in which they exist in the 

 sea. I have already referred to the investigations of Lohmann on 

 the density, and the means of collection, of the micro-plankton 

 which usually escape capture in the nets made from Mlillergaze. 

 By using hardened filter-paper and thick tafieta silk as a filtering 

 medium, and still more by utilising the contents of the filtering 

 apparatus of appendicularians as a measure of the numbers of the 

 •smaller planktonic organisms of the sea, Lohmann shewed that 

 there was a wealth of life in the sea not hitherto disclosed by the 

 older plankton fishing apparatus. In the following table ^ I give 

 some of Lohmann's results. The numbers of the organisms grouped 

 under the term " microplankton " present in one cubic metre of the 

 Avater of the Mediterranean off Syracuse are given. Those in the 

 lirst column were taken by means of silk bolting cloth No. 20, and 

 those in the second column by all other means (hardened filter- 

 paper, taffeta silk, and by the filtering apparatus of Oikopleura). 

 The numbers in the third column represent the numbers of the 

 •organisms actually present in the volume of water dealt with. 



^ The table is slightly rearranged from Lohmann's Table XIII., Neue Unter- 

 ■suchungen, &c., loc. cit. p. 72. 



