PLANKTON 367 



the fluid, it is necessary to select a tough, unglazed paper 

 (see p. 415). A record which is frequently omitted on 

 labels is the method of preservation. It is by no means 

 an unimportant detail, especially when the specimens 

 are handed over to a specialist for examination. 



III. PRESERVATION. 



Plankton. — The preservation of the whole catch of 

 a tow net, just* as it comes on board, without any 

 sorting of the specimens, is a simple operation. 

 Animals with hard coats, such as Crustacea, stand the 

 treatment very well, but soft-bodied animals, such as 

 jelly-fishes, usually contract and become very much 

 distorted. 



The best method is to pour the Plankton from the 

 can (at the bottom of the tow net) into a glass jar. 

 Stir the Plankton round with a rod, and at the same 

 time pour in a little formalin 5 per cent, or 10 per 

 cent. Take out the larger animals with lifter and 

 pipette. Keep on stirring for about a minute, so as 

 to mix thoroughly the formalin with the sea water ; 

 then allow the Plankton to settle to the bottom. 

 Soon after the Plankton has settled, pour off as much 

 of the fluid as possible, and transfer the Plankton to a 

 bottle. Allow the Plankton to settle, and reduce the 

 quantity of fluid to a minimum. The next step is to 

 fill up the bottle with formalin 5 per cent, or 10 per 

 cent., and occasionally give the contents of the bottle 

 a stir up. A bottle should not be more than half full 

 of Plankton. It is advisable after a few days either 

 to pour off the formalin and add a fresh supply, or to 

 pour in a very small quantity of strong formalin. 

 Opalescence of the fluid is a sign that it needs changing. 



