1:j6 quantitative plankton investigations [part II 



closure of the meshes of the net the filtering surface of the 

 latter was made as large as possible when it was intended to be 

 used in fresh water. The fresh-water net used by Apstein had an 

 area which was 48-times that of the area of the opening through 

 which the water entered. In such parts of the sea as were free 

 from sediment and contained comparatively little plankton the 

 surface of the net was only 28-times that of the opening. Xow 

 Kofoid used the smaller net, which was designed for sea fishing, 

 and he used it in a mass of water which was rich in plankton, and 

 over a bottom consisting of a soft sediment which was stirred up 

 on the slightest movement. His first objection has therefore little 

 force. 



But the second objection is fully justified ^ There are indeed 

 many organisms which are either not taken by the tow-nets 

 usually employed, or are taken only in very inadequate proportion. 

 One is often struck with the absence from the catches of the tow- 

 nets of hosts of organisms which he suspects must be present in 

 the sea. Where, for instance, are the multitudes of the younger 

 fish larvae which must surely be present in the sea at certain times 

 of the year ; or the naked protozoa or smaller protophyta ; or the 

 ciliated embryos of the hosts of trematodes which infest the 

 entrails of fishes, and which pass through a free-swimming stage ; 

 or the ciliated trochospheres of molluscs which ought to be more 

 numerous in the sea than is indicated by the catches of the 

 tow-nets ? These are either not taken by the nets, being so small 

 that they are able to pass through the meshes of the fabric, or 

 they are crushed out of all recognition by the pressure of water 

 against the threads of the cloth ; or they are so susceptible to 

 injury that they are destroyed by the preservative fluid employed 

 for the fixation of the catch. In the paper last cited Lohmann 

 tells us that he obtained a large quantity of water from the Baltic 

 and filtered it through the ordinary MllUergaze (No. 20, area of 

 mesh = 00025 mml) and then counted the organisms thus caught. 

 He then refiltered the water passing through the silk cloth by 

 passing it through a hardened filter paper and again counted the 

 organisms retained by this second filtration. The results were 

 very noteworthy : most of the metazoa were retained by the 

 1 It had, however, already been anticipated by the Kiel planktologists. 



