368 PRESERVATION OF MARINE ORGANISMS 



Another method is to first kill the Plankton by 

 pouring some picric acid solution into the glass jar, 

 and then add a little formalin 5 per cent, or 10 per 

 cent. Leave the Plankton in this very dilute picro- 

 formol solution for an hour or two, occasionally stirring 

 up. Finally transfer the Plankton to a bottle, and 

 fill it up with formalin 5 per cent, or 10 per cent. The 

 fluid will remain yellow, but the small quantity of 

 picric present may be neglected. It is important to 

 use plenty of formalin, for an extra strong dose does 

 no harm, and to remember occasionally to stir up the 

 contents of the bottle. 



Corrosive sublimate must never be used instead of 

 picric acid for this wholesale method of quickly pre- 

 serving Plankton. The presence of corrosive sublimate 

 in formalin leads to the formation of a precipitate, 

 which adheres firmly to all the specimens. 



Protozoa may be preserved in weak osmic acid. If 

 they are numerous, the osmic acid may be added to 

 the water containing them (2 or 3 drops of \ per cent, 

 osmic to 10 c.c. of water) ; the Protozoa are allowed to 

 settle, or, better, separated by means of a centrifuge. 

 They should be stained with dilute picro-carmine, and 

 brought gradually into 70 per cent, alcohol. 



If one is dealing with a few specimens only in a drop 

 of water on a slide, the drop may either be exposed 

 to osmic acid vapour, or some solution may be run 

 in under a cover-glass. 



Radiolaria should be preserved with dilute osmic 

 acid, or with corrosive sublimate. Acanthometra killed 

 with corrosive sublimate (about one minute in saturated 

 solution) keep better than those preserved in osmic acid. 



Porifera. One simple and ready method of pre- 

 serving sponges satisfactorily is to hang them up in 

 strong alcohol (the strongest obtainable, absolute 



