Dakin and Latarche — the Plankton of Lough Neagh. 33 



The plankton is shaken up in this 150 cc. for about one minute until the 

 organisms are uniformly distributed. -j-L cc. is then removed with a 

 Hensen "Stempel" pipette. This amount is placed on a glass-plate ruled 

 with parallel lines, and with -the help of a large mechanical stage the whole 

 drop is passed slowly under the objective. Each organism is enumerated as 

 it passes under the objective. The personal equation in a comparison of 

 catches is eliminated by this method, and such vague and relative terms as 

 ' very rare,' ' rare,' ' common ' are deleted. If the number of any species of 

 organism in -j-'g cc. of the above dilution was too great to enumerate easily, 

 the species was counted in a greater dilution. Similarly, if there were too 

 few individuals in -j-V cc. of 150 cc. dilution, the catch was filtered and used 

 more concentrated. In every case a factor was noted; and the number of 

 individuals in the count multiplied by the factor gave the number of 

 individuals in the catch. A certain error enters owing to the non-uniform 

 distribution of organisms in the shaken-up catch. 



In Hensen's first paper on the method of quantitative plankton investiga- 

 tions instructions were given for a very elaborate counting. There is no 

 wonder that Haeckel, in his " Plankton Studien," remarked, " Wie eine solche 

 arithmetische Danaiden-Arbeit ohne Euin des Geistes und Korpers durchzu- 

 fiihren ist, kann ich nicht begreifen." Still later it was advised that several 

 plates be counted for each dilution. In practice most of these theoretically 

 exact ways have been abandoned even by the Kiel workers themselves. 



We have counted on an average four or five plates for each catch, the 

 dilutions being different according as the species to be counted was frequent 

 or rare. 



In order to determine the probable error in the withdrawal of organisms 

 by the pipettes, an oi'ganism was^adopted which could be easily observed and 

 was not broken up by much shaking. Ceratium liirunclinella was taken for 

 purposes of the test; and a catch containing this species was diluted until 

 about 140 occurred in -^-^ cubic centimetre. A -^ cc. was abstracted from 

 this diluted catch after proper shaking and the Ceratium correctly enume- 

 rated. This process was repeated fifty times, the results being as follows : — 



146 



118 



126 



146 



126 



141 



132 



121 



126 



136 



133 



133 



123 



166 



143 



144 



162 



131 



125 



129 



136 



136 



148 



121 



129 



156 



138 



135 



121 



137 



149 



141 



122 



159 



142 



137 



125 



112 



117 



157 



144 



138 



141 



133 



177 



143 



121 



147 



122 



128 



