COLLECTING AND PRESERVING 275 



For preserving Crustacea the best medium to use 

 is 70 per cent, alcohol. Strong spirit diluted with 

 a little less than one-third its bulk of water gives 

 about the required strength. If too strong spirit 

 is used, the specimens tend to be hard and brittle, 

 and delicate organisms become shrivelled. Methy- 

 lated spirit as sold in the shops in this country 

 contains mineral naphtha, and turns milky when 

 water is added, so that it is unsuitable for preserving 

 specimens. Methylated alcohol without naphtha 

 can be bought, by permission of the Inland Revenue 

 authorities, but only in considerable quantities at a 

 time. 



Formalin is very cheap and readily obtained, but 

 it is much less suitable than spirit for most Crustacea, 

 as it tends to make them stiff and fragile, and small 

 forms containing much lime, such as Cumacea, may 

 become decalcified. For Crustacea collected by the 

 tow-net, however, formalin gives good results. A 

 few drops of strong formalin, added to the water 

 into which the tow-net has been washed, kills the 

 animals in a few minutes. After they have sunk 

 to the bottom, the liquid may be poured off and 

 replaced by formalin diluted with sea-water (for 

 marine plankton), or by a mixture of formalin and 

 spirit. The most suitable strength of formalin varies 

 with different organisms, but 5 per cent, (i.e., i part 

 of commercial formalin to 19 parts of water) is 

 perhaps most generally useful. 



Crabs, Prawns, and the like, if put alive into 

 strong spirit, may throw off some of their limbs, 



