32 Proceedings of the Royal Irish Academy . 



up carefully through 10 feet, and then the messenger sent down the line. 

 This releases the jaw, the ring falls out, and the net is supported solely by 

 the throttling line, which, of course, throttles the net. It can fish no longer 

 on the way up to the surface. One point which must be mentioned here is 

 that as the net could fall suddenly when the messenger reaches the closing 

 apparatus, the line should be hauled rapidly for a second or two so that the 

 throttling rope remains taut whilst coming into action. Hensen is not 

 strictly accurate in statiug that a loss of plankton will occur, because the 

 plankton is to be found on the silk and not on the non-filtering part. If the 

 net be used carefully in the still water of a lake, there is no chance of any 

 plankton escapiug when the net is released. At the same time it must be 

 observed that the net is an inaccurate method always — practically all our plank- 

 ton methods are — and the only thing we can do is to make sure that the mode 

 of usage is at least unifonn. In Lough Xeagh, where the catches were made 

 always in calm water at the same station, with the same depth and in exactly 

 the same way, the risks of non-xmiform methods of working were reduced 

 considerably. Certaiuly, compared with marine work, there can be no 

 comparison of the accuracy. For the bottom to surface catches, the net was 

 simply lowered open and pulled up to the surface open all the way. For the 

 top 10 feet the net was lowered until the mouth was 10 feet below the surface, 

 and then raised open all the way. In every case the catch was collected in 

 the bucket by washing down the net from the outside. It was then removed, 

 and fixed at once (in the boat) in a 5 per cent, solution of formalin. 



EXAMIS'ATIOS OF CaTCHES. 



A rough comparison of the catches may be made by measui-ing the 

 volumes. This estimation is carried out by allowing the plankton to settle 

 in tubes terminating in a cone. Centrifuge tubes are very satisfactory. The 

 period given shoiUd be about a week. 



For detailed comparison, however, the method of counting the organisms 

 has been adopted. In any comparisons of plankton catches, the authors 

 consider that counting is absolutely necessary. This unfortunately means 

 great fatigue and huge demands on time. There is no need to detail 

 again the irrefutable reasons for enumeration of the organisms. The reader 

 is referred to Apstein (1905). It is, of course, quite impossible to count 

 every indi\'idual in a catch, ilillions may be present. 



The method adopted by the authors has been therefore to dilute the 

 catch down to 150 cc. of 5 per cent, formalin to which a small amount of 

 glycerine had been added. The glycerine tends to keep the organisms from 

 settling when the mixture has been shaken, and also prevents the drop used 

 in enumeration from drying up too quickly. 



