]\h'tJiO(h ill Plankton fi/vcstiaiatioiis. \\) 



planktons rich in diatoms nor to the silt-laden planktons from 

 Havana, for a large part of the silt is debris of organic origin, 

 and the method above descril)ed does not differentiate the 

 organic material of the plankton from that of the silt. For 

 the quantitative investigation of these silt-laden planktons we 

 are thus practically limited to the enumerative method with 

 such incidental hel}) as may l)e derived from volumetric 

 determination. 



The voliiinctric dctcnin)iatioii has taken two forms, the 

 settling and the centrifugal methods. The former as used by 

 us is the same as that employed by Eeighard ('94) and Ward 

 ('96a). The i)lankton is transferred to graduated tubes and is 

 allowed to stand twenty-four hours, when the amount of the 

 plankton settled at the bottom of the fluid, is read by the 

 graduations upon the tube. The tul)es used are the carbon 

 tubes employed l)y chemists in the Eggert color test for the 

 estimation of carbon in steel. Our tubes in most frequent use 

 contain 25 and 50 cul)ic centimeters respectively, are about 

 twelve millimeters in inside diameter, and are graduated to 

 tenths of a cul)ic centimeter. For very small planktons 

 another tube, containing only ten cubic centimeters and 

 measuring six millimeters in inside diameter, was used. After 

 a series of measurements in the tubes above descrilied it 

 became evident that a considerable error was involved in the 

 method. Piepeated measurements of the same plankton in 

 the same tube, after standing twenty-four hours, revealed a 

 considerable variation in the volume, as high as 30*'„ in some 

 instances. Furthermore, planktons do hot settle to an equal 

 density. Those comi)osed of Rotifcra or small Cladocera (as 

 Cliydonts) pack closely, while others containing filamentous 

 forms, as Oscillaria or Fragillaria, and those in whicli the larger 

 Entomostraca are predominant, settle very loosely. Thus the 

 determination of the volume of the plankton l)y the settling 

 method does not give a uniform test of the amount of plankton 

 present. Furthermore, the process is a tedious one, especially 

 when large numbers of catches are to be handled. 



The centrifugal machine (Plate VII.) was finally hit upon 

 as affording the best . solution of the difticulties presented in 



