METHODS OF PLANKTON RESEARCH. 529 



methods here described will alone give an accurate 

 sample of all the forms present in the plankton. 



Lohmann has used with success a centrifuge which 

 carries four glass tubes and which gave easily 1,300 

 revolutions per minute when turned by hand. He found 

 that 9,000 revolutions, in seven minutes, were usually 

 sufficient. 



For the investigation of the living forms, samples 

 of only 5 to 15 c.cm. are taken. The tubes for containing 

 the sample, on the centrifuge, are small cylindrical 

 vessels, with the point drawn out slightly to form a cone- 

 shaped end, in which the material will form a well- 

 defined sediment. 



After the completion of centrifugation, most of the 

 water can be poured away, and the sediment remains 

 undisturbed with the water that fills the conical end. By 

 means of a pipette, the sediment, through repeated 

 sucking up and forcing out, is finely distributed in the 

 water remaining, and is finally completely sucked up 

 and transferred to the glass numbering plate used with 

 a specially constructed microscope stage. This is much 

 smaller than the numbering stage for the large net 

 catches, is comparatively cheap, and can be fitted to any 

 microscope.* 



The conical end of the tube is now washed out with 

 a very little water (some of that originally poured off), 

 and this is added to the main part of the catch on the 

 glass plate. The whole catch should form only a single 

 drop, such as can be covered with an ordinary 12 mm. 

 cover glass. 



If the water contains many flagellates and ciliates, 

 the counting of such rapidly moving organisms is 

 impossible. The cover glass should then be held over 



* Zwickert, Optician, Kiel, is the maker of this stage. 



