ZOOPLANKTON IN THE SOUTHERN OCEAN 197 



unsuitable for measuring the volumes of small samples containing but a few differently shaped 

 organisms. It is also not suitable for use at sea where steady conditions, necessary for settlement, 

 are unusual. A variation of the method was, however, used at sea by Jespersen (1923, 1935) in his 

 extensive studies in the Atlantic and Mediterranean, the samples being allowed to drain and settle in 

 a graduated sieve from which the volume could be read directly. This method, however, is only 

 suitable for large samples. 



The displacement method consists essentially of filtering off the plankton on to a piece of filter 

 paper (Sheard, 1947) or silk (Apstein, 1909; Moore, 1949) and adding it to a known volume of water. 

 The difference in measurements gives the volume of the plankton. Many workers have used this 

 technique, including Leavitt (1935), Bigelow & Sears (1939), Clarke (1940), Redfield (1941), Sheard 

 (1947), Moore (1949) and Gundersen (1951). By some, notably Apstein (1909), Jacobsen & Paulsen 

 (1912), Ussachev (1939), Berardi (1953), Ealey (1953) and Frolander (1954), the method has been 

 elaborated by the design of special apparatus, some of which allow very small volumes to be measured. 



It will be realized that neither the settlement nor the displacement method measures the absolute 

 volume of the plankton since, in both, the moisture contained between the individual animals is in- 

 cluded in the final volume. In the latter method, however, this error is reduced to a minimum 

 (see p. 198), and much smaller samples, as are caught with vertical closing nets, can be measured with 

 some accuracy. Furthermore, the displacement method is relatively quick and admirably suited to use 

 at sea where some of the samples in the 1950-51 commission of R.R.S. 'Discovery II' were 

 measured. 



All volumes used in this work were measured by this method, the particular procedure used being 



as follows: 



The preserved sample, after the large animals had been picked out (see p. 199), was poured into 

 a brass funnel, the stem of which, measuring 1 \ in. diameter, was closed at its lower end by a disk of 

 fine silk held in position by a rubber band. The use of N50 silk, which has 200 meshes compared with 

 74 meshes per linear inch of N70 silk, ensured that even the smallest organisms in the sample were 

 retained on the disk. 



Most of the liquid was allowed to drain off, the process being accelerated by placing the disk on a 

 pad of blotting paper, great care being taken not to injure the plankton by complete drying out. The 

 disk and plankton were then removed from the funnel and placed, care being taken not to enclose air 

 bubbles, into a measured volume of water contained in a graduated cylinder, and the increase in 

 volume noted. The difference in measurements minus the volume of the silk disk (o-i cc.) gave the 

 volume of the plankton. By measuring the volume of plankton and filter disk together, one ensures 

 that the plankton is not damaged, as it might be by the alternative method of scraping the animals off 

 the silk into the measured volume of water. 



The size of the measuring cylinder had naturally to be suited to the size of sample, but in most 

 cases a 25 cc. cylinder graduated in 0-2 cc. was used, and the volumes were measured to the nearest 

 o-i cc 



After measurement, the sample was completely washed off the silk and replaced in 10 per cent 

 neutral formalin. The importance of not losing any of the sample need not be emphasized, and the 

 washing of the silk was repeated a number of times, the time taken being much longer than that for 

 the actual volume measurement. Finally, by a very brief and rough inspection a note was made of 

 the dominant animal in the sample and any other features. 



It will be obvious from the foregoing account that the inherent limitation to the accuracy of the 

 method is the amount of liquid retained by the plankton after drainage. Samples from 70 cm. vertical 

 closing nets, however, are on the whole small— much smaller than samples from towed nets — and, 



