244 D. J. SCOURFIELD ON THE USE OF THE 



usual way. He therefore sought for some new method of con- 

 centrating these organisms and kickily liit upon the centrifuge 

 as the best apphance for the purpose. Lohmann undoubtedly 

 deserves the credit for this demonstration of the real value 

 of the centrifuge in plankton work, namely, for the concen- 

 tration in an uninjured state of the smallest organisms ; and since 

 the issue of his paper the instrument has been coming more 

 and more into use in investigations on the minute life of the 

 sea and fresh waters. As regards the latter it may be men- 

 tioned that immediately after the publication of Lohmann's 

 paper, Woltereck and Ruttner took up the matter at the Lunz 

 Fresh-water Biological Station, and have recorded some pre- 

 liminary but extremely interesting results in the Iiiternatioiiale 

 Hevue der gesamten Hydrohiologie und Hydrograjjhie (10 and 11). 

 Some suggestive observations on the periodicity of the " centri- 

 fuge plankton " in relation to the " net plankton " have also 

 been published in the same periodical by Brehm (1), 



Coming now to the actual methods of work with the centrifuge, 

 it is most important to note first of all that the samples of water 

 to be tested should be taken directly from the pond or other piece 

 of water into the collecting-bottle without the intervention of any 

 net or other filtering appliance. If this is not done, a wholly 

 false impression of the relative abundance of the minutest forms 

 in comparison with the other microscopic organisms will be 

 obtained, owing to the fact that the vast majority of the former 

 will either pass through the meshes of the filtering material or 

 else be damaged beyond recognition on the filtering surface. 

 Neither should any preservative be added to the water, as it is 

 practically certain that many of the very minute animal forms 

 at any rate, are extremely sensitive to even small traces of 

 alcohol, formalin, etc., and are apt to disintegrate upon the 

 slightest provocation. It follows, therefore, that to get reliable 

 results nothing but the plain pond water in its natural condition 

 should be used, and, needless to say, it should be centrifuged 

 without delay while all the organisms are still alive. 



As regards the amount of water necessary, Lohmann showed 

 that quite small quantities were sufiicient for obtaining a fairly 

 accurate idea not only of the various kinds of smaller organisms 

 present but also of their relative numbers. He seems to have 

 worked usually with tubes containing 15 c.c. (= about J oz.), but 



