CH. Vl] QUANTITATIVE PLANKTON INVESTIGATIONS 141 



Thus the Mtillergaze of the Hensen quantitative nets is a very 

 imperfect filtering apparatus. If we knew nothing of the nature 

 and abundance of the plankton in the sea except what is revealed 

 to us by the fishing of these apparatus we should possess a very 

 distorted picture of life in the sea. Generally speaking about 

 half the number of the organisms which enter the mouth of the 

 vertical net pass out again through the meshes of the silk cloth ; 

 others may be destroyed by the pressure of the water against the 

 threads of the silk (though in a properly used quantitative net 

 this loss is probably inappreciable in amount) and others again are 

 destroyed by the action of the preservative liquid employed. 

 This filtration loss too is not constant but depends on the nature 

 of the organisms making up the catch and on the stage of develop- 

 ment of these. Sometimes the quantitative net may only catch 

 about 2*^/0 of the number of the organisms entering into it^. 



Nevertheless the mass of these smaller organisms which 

 enter the net and again pass out of it is often so small that it 

 may be neglected for many purposes. All the fish eggs, practically 

 all the copepoda and other Crustacea, and generally speaking all 

 the adult metazoa are caught. The larger and more abundant 

 diatoms are also caught, and so are many of the larger protozoa. 

 Tintinus cavtpa7iula, Ceratium t7^ipos, Peridinium divergens are 

 caught, but hosts of the smaller ones, such as Skeletonema, 

 Synedra and Frorocentrum, are lost, and so are very many of the 

 smaller diatoms and protophyta. Still for many of the economic 

 problems, such as questions which can be solved by a knowledge of 

 the distribution of the fish eggs or Crustacea, the Hensen net is 

 an efficient instrument. But we shall see later that there are other 

 questions of theoretical importance, for the discussion of which a 

 knowledge of the abundance of the smaller organisms which are 

 not caught by the net cannot properly be dispensed with. 



^ The reader must refer to Lobmann's paper for tables which give the detailed 

 comparison between the catches made by the Mtillergaze, the taffeta silk and 

 filter paper, and the houses of appeudiculariaus. These have all been counted 

 and the results are compared in a most careful manner. 



