METHODS 



plankton itself. For example, a fish, or a jellyfish, if it is to be preserved 

 in a volume of formalin about equal to its own bulk, would require lO per 

 cent formalin, which would after penetration be 5 per cent. A sample with 

 numerous small organisms should preferably have a free volume of pre- 

 servative of several times its own volume. Colours arc gradually bleached 

 and as formalin is slightly acid, calcareous shells are gradually dissolved, but 

 this can be reduced by 'buffering' with borax, sodium bicarbonate or pre- 

 ferably with an excess of powdered calcium carbonate which neutralizes 

 any acid as it forms. 



The minute organisms are best preserved with Lugol's iodine, as some 

 of them are so fragile that the addition of formahn causes them to 'explode'. 

 The expert histologist in his study of the actual cell structure needs special 

 preservatives for his work, but he needs no advice about his methods from 

 this book. 



With a little forethought and ingenuity, the beginner can make his own 

 nets, using a small hoop or similar ring and some butter muslin, mosquito 

 netting or cheese cloth. Silk is, of course, better and can be obtained from 

 John Staniar & Co., Manchester Wire Works, Manchester, or from Henry 

 Simon Ltd, Cheadle Heath, Stockport. It is usually sold in lengths 40 

 inches wide; grade 3 costs about 30s. and the finest about ^4 per yard. 

 Nylon is cheaper, does not rot if left damp, but the meshes are also less 

 rigid and it is not easy to sew the seams satisfactorily. The complete nets 

 suitable for schools, etc. and for work in fresh water, can be boiight at 

 various scientific instrument suppliers (e.g. Flatters and Garnet Ltd, 309 

 Oxford Road, Manchester, 3, or P. K. Dutt dc Co. Ltd, i Alfred Place, 

 London, W.C.i) and larger nets from the Marine Biological Association, 

 Citadel Hill, Plymouth or from the Freshwater Biological Association, 

 Far Sawry, Ambleside, Westmoreland. 



Two last hints: always label your collections, and include the date, the 

 exact locality and the type of gear used. Write the label in soft pencil on a 

 good quality paper and put it i)iside the jar. The formalin will preserve the 

 paper and it will not get lost. If you wish, of course, use an outside label 

 too, but do not omit the inside one. The second hint concerns magnifica- 

 tions. Never just say 'three times as big' without defining the meaning, 

 'Three times linear' is clear, or ' X 3 diam.', but a piece of paper three times 

 as big, or X 3, could mean three times the area, or three times the length 

 and width which is nine times the area. A ball, X 3, could mean three times 

 the volume or weight, or three times the diameter which is twenty-seven 

 times the volume. A convenient method used with drawings is that used in 

 this book, where a line representing a stated length is drawn to the same 

 scale. 



c 27 



