582 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



the plankton, either qualitatively or quantitatively, one 

 must use all the three above methods side by side. If 

 only a definite part of the plankton is to be studied, then 

 the method must be chosen to suit the case. For large 

 Crustacea, fish eggs, and medusae and other large 

 plankton forms occurring but seldom in the water, in 

 comparison to the Copepoda and smaller forms generally, 

 the large Hensen net described by Jenkins should be 

 used, to work through much larger quantities of water. 

 For the main constituents of the plankton, the Copepods, 

 Ceratium, and, in fact, for general use, the Middle 

 Plankton Net of Apstein is to be preferred. This has 

 been the chief instrument used in German investigations, 

 and holds its place because of the ease of working and 

 the general applicability. It must be borne in mind, 

 however, that when these nets are used there is a con- 

 siderable loss, as has been shown above, and, therefore, 

 when possible, the use of the net should be replaced by 

 the pump, tube and filter. In fact, in water of moderate 

 depths, and shallow water, the pump method is " the " 

 method for plankton investigation, and the net and 

 centrifuge should be used only to complete the results 

 when the greatest possible accuracy is required and the 

 complete constitution of the plankton is to be discovered. 

 The points against the pump method are the difficulties 

 encountered in deep water or when there is a strong 

 current, together with the greater time that is required 

 for pumping and filtering. 



There is another possible method for arriving at 

 these results, which could be applied to the greatest 

 depths, and in comparatively stormy weather. It is to 

 use a water bottle for collecting a volume sufficiently 

 large to allow of its being filtered and examined in the 

 same way as the water from the pump. In this way, 



