METHODS OF PLANKTON RESEARCH. 533 



however, the contents of a vertical water column would 

 have to be calculated from the samples taken at various 

 depths. For this purpose, too, it would be necessary to 

 obtain more water than is brought up by the bottles now 

 in use. The Kriimmel bottle as now used by hydro- 

 graphers has three litre contents, and there should be no 

 difficulty in increasing the size to five litres, which would 

 give a sufficiently large sample. 



Other Plankton Apparatus Used for Qualitative 



Work. 



The apparatus above described is intended for the 

 quantitative estimation of the plankton in volume, 

 chemical constitution, or by the counting method of 

 Hensen. For mere purposes of qualitative investigation 

 the procedure is naturally much more simple. The net 

 described is, in any case, of great use as a vertical net, 

 and would completely supplant the pump and filter, 

 whilst the centrifuge would be used to catch organisms 

 that pass through the net. Under special circumstances, 

 however, other nets are used, which are coarser, and 

 have their special use according as they are for surface 

 or deep work, and for small, large, or very large 

 organisms. Again, it is sometimes desirable to investi- 

 gate the plankton of areas over which the ship is passing 

 at a considerable speed, and for this purpose other devices 

 are necessary. 



For general use in qualitative work there is the 

 ordinary small tow-net, well known at all biological 

 stations. This is constructed out of bolting silk, and 

 has the same conical shape as the vertical net, but does 

 not have a mouth-piece as described for the quantitative 

 nets. No calculations can be made as to the quantity of 

 water it filters. These nets are generally used for 



