/ QUEKETT MICROSCOPICAL CLUB. 191 



proposal would be to keep the eye central. A hearty vote of 

 thanks was accorded to Mr. Nelson for his communication. 



The President then called on Mr. D. J. Scourfield to deliver 

 his address on " Nanno plankton and its Collection by means 

 of the Centrifuge." Mr. Scourfield said that he had already 

 brought the subject of collecting very minute pond-life organisms 

 by means of the centrifuge before the Club in a paper read in 

 1911 (see J.Q.M.C. vol. xi. p. 243), but he thought the matter 

 might possibly be worth bringing up again especially in view 

 of the fact that the more minute organisms that float in fresh 

 water had not even yet received their due attention in this 

 country. The word " plankton " was introduced by Hensen 

 in 1887 to describe the small, mostly microscopic, organisms pos- 

 sessing but feeble powers of locomotion, that float in the ocean, 

 lakes, ponds, etc. Not much attention was paid at first to the 

 smallest of these forms, i.e. to those which passed freely through 

 bolting silk of 200 threads to the inch, whose openings are about 

 l/400th to l/500th of an inch across. Lohmann observed, how- 

 ever, that the Appendicularia possessed an apparatus which 

 collected great numbers of extremely small organisms, which 

 were accumulated in a sac before being swallowed, and in 1908 

 he suggested that the centrifuge be used for the concentration 

 of such very small forms. A little later he introduced the term 

 " nannoplankton," i.e. " dwarf plankton," for those organisms 

 which pass readily through the meshes of the finest silk nets 

 and whose upper limit of size is about, say, 11,000th in. Mr. 

 Scourfield then described the method followed. A two-speed 

 hand centrifuge is used, which will give with the lower gear 

 2,000 to 3,000 revolutions per minute with 15-c.c. tubes. If 

 the higher gear is used with tubes of lA^-c.c. capacity, a speed 

 of 10,000 revolutions per minute may be obtained. Even with 

 this speed some very minute forms {e.g. bacteria) may not be 

 thrown down. Unconcentrated material is used, collected in a 

 bottle and centrifugated for two to three minutes. The water 

 is then nearly aU gently pipetted ofi and the sediment examined 

 either in a Rousselet live-box or on a shp with a cover. Mr. 

 Scourfield prefers Rousselet live-boxes with a thin metal ring 

 in the cover, to which a cover- glass 1 /300th in. thick is cemented. 

 Nannoplankton comprises phytoflagellates, peridinieae, bacteria, 

 small volvocaceae, diatoms, desmids and, in the sea, silico- 



