METHODS OF COLLECTING AND PHOTOGRAPHING 77 



the bucket removed, and the water allowed to drain from it. 

 When only so much water remains as fills the conical bottom 

 of the bucket, the stopper is drawn and the contents allowed to 

 fall into a suitable container. Organisms adhering to the inside 

 of the bucket are then rinsed into the container with a little filtered 

 or distilled water from a wash bottle. If the contents are to be 

 preserved they may be allowed to fall directly into a bottle 

 which contains the preservative or fixing fluid, so concentrated 

 that the addition of the plankton brings it to its normal consti- 

 tution. Ninety-five per cent alcohol may be used and in that 

 case the plankton may be allowed to fall from the bucket into 

 about three times its own volume of alcohol, so that it is preserved 

 in alcohol of about 70 per cent strength. 



If it is desired to use a fixing fluid before preservation in alcohol, 

 the stronger picrosulphuric acid may be diluted with two volumes 

 of water and three volumes of this may be used to one of plankton, 

 so that the latter is fixed in Kleinenberg's solution. Other fluids 

 may be used in like manner, adapted either to the plankton as a 

 whole, or to special groups of plankton organisms. The plankton 

 is then best caught in a strainer made by removing the bottom of 

 a short eight-dram homeopathic vial and tying bolting cloth over 

 the neck (Fig. 18). The plankton may be kept in this strainer 

 by tying bolting cloth over the bottom, and the strainer may 

 then be passed through fixing fluids and grades of alcohol. The 

 fluids may be made to enter the strainer by withdrawing the air 

 by means of a pipette held against the bolting cloth (Reighard, 

 1894). 



Plankton nets may be made closable and various devices have 

 been used for this purpose (e.g., by Marsh, 1897). Such a net may 

 be lowered, drawn upward any desired distance, then closed and 

 drawn to the surface. It thus filters only that part of the column 

 of water through which it is drawn while open, and aids the inves- 

 tigator to determine what forms occur at various depths. 



Although the plankton net may seem to filter a vertical column 

 of water, the base of which is equal in area to the net opening, 

 it does not in practice do this. The resistance of the net gauze 

 causes a certain part of this column to be pushed aside. The part 



