518 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



sary. This 50 c.cm. with the catch is placed in a shaking 

 flask, and, by means of the plankton pipettes, O'l c.cra. 

 of the fluid is withdrawn after carefully distributing the 

 organisms by thorough shaking. 



It is necessary here to emphasise the use of these 

 special pipettes devised by Eensen (1 and 5), since no 

 other apparatus will allow of the accurate abstraction of 

 such small quantities. 



First, O'l c.cm. is taken and removed to the counting 

 plate under the microscope, and the organisms counted. 

 A sheet of paper is used with the names of the species 

 to be counted, and, as each form is passed over, a stroke 

 is placed opposite the name on the paper. Since O'l c.cm. 

 is 5i?o of the 50 c.cm. to which the catch was diluted, 

 the numbers must be multiplied by 500 to give the full 

 number for the catch, and then from this the number 

 per cubic metre is calculated. In general use only one 

 plate is counted with O'l c.cm., and then a pipette 

 abstracting 0'2 c.cm. is used in the same way, but only 

 those organisms occurring in very small numbers, or 

 doubtful in the first plate, are counted in the second, so 

 that whereas 50 species may be counted in the 0*1 c.cm., 

 this number may be reduced to 12 in the 0'2 c.cm. 

 Following these two plates, 0'5 c.cm. and then 1 c.cm. 

 are taken, and finally the rest of the catch for the 

 larger forms and for rare forms is counted, making 

 a total of five plates. When greater accuracy is required, 

 more plates are counted for the same pipette until the 

 difference between the number of organisms on the last 

 counted and the average number for the previous plates 

 is less than 5 per cent. If an organism is required for 

 a preparation or for further observation, it can be 

 removed from the numbering plates by very small 

 capillary pipettes about two inches long. 



