280 PROCEEDINGS OF THE ACADEMY OF [1896. 



The methods adopted by Hensen and his followers in estimating 

 the plankton content of any given area of water, are tedious in the 

 extreme, and hold the same relation to practical fish and oyster cul- 

 ture as do the old fashioned methods of counting blood corpuscles 

 and milk globules to the modern use of the hematocrit for the 

 quantitative estimation of blood corpuscles ; or of the various cen- 

 trifugal machines and the Babcock system for the determination of 

 the fat contents of milk. To the use of the pelagic tow-net we are 

 indebted for practically all our present knowledge of minute aquatic 

 organisms, and in so far as concerns the enumeration of the species 

 constituting the plankton of any given region, no improvement can 

 be suggested over the methods now employed. Prof. Haeckel has, 

 however, very clearly pointed out the difficulties connected with 

 Hensen's method of counting the individuals obtained in each haul 

 of the net and that such counting " possesses only an approximate 

 and relative value," and further, that " the only thorough method 

 of determining the yield in planktology is the determination of the 

 useful substance according to mass and weight, and subsequent 

 chemical analysis." Without undervaluing in any way the count- 

 ing methods at present employed by planktologists, I desire here to 

 call attention to an apparatus which I have devised and by means 

 of which one may make a large number of plankton estimations in 

 a single .day, in each case determining the volume and weight, 

 rather than the number of individuals. By means of this apparatus 

 one is enabled to judge of a given area of water at different times of 

 the day, states of the tide, from various depths, in fact of the plank- 

 tonic variations as regards depth, temperature, density, wind, tide, 

 etc. 



The method which I employ is that of the centrifuge, an appara- 

 tus which consists of a series of geared wheels driven by hand or 

 belt, and so arranged as to cause an upright shaft to revolve to 

 a speed of 8,000 revolutions per minute, corresponding to 50 revolu- 

 tions per minute of the crank or pulley wheel. To this upright 

 shaft is fastened an attachment by means of which two funnel- 

 shaped receptacles of 1 litre capacity each may be secured and 

 made to revolve with the shaft. The main portion of each of these 

 receptacles is constructed of spun copjoer, tinned. To this is at- 

 tached the stem of the funnel consisting of a heavy annealed glass 

 tube of 15 mm. in outside diameter with a central bore of 2? to 5 

 mm. These glasses are held in place and protected by a cover, 

 such as is employed in mounting a water-gauge. 



