Mel hods in PI a nk ton Investigations. 17 



<b ' 



removal of all water from the cylinders when the pump is not 

 in use. 



4. Advantages of the Pumping Method. — As compared with 

 methods dependent upon hauling the plankton net through 

 the water, several points of advantage are to be found in 

 the pumping method. It is more accurate, since the actual 

 volume of water strained can he determined, and the changes 

 in the coefficient of the net due to seasonal and local varia- 

 tions in the quantity and composition of the plankton and to 

 alterations in the silk of the net with use are to a very large 

 degree eliminated. The method is also widely applicable : as 

 water may be drawn from any desired level, it may be applied 

 to the problem of vertical distribution ; and it may be used in 

 very shallow water, in the midst of vegetation, in creeks, in 

 strong currents, under the ice — in fact, in a wide variety of 

 situations from which the vertical or oblique hauls are wholly 

 excluded or to which they are with difficulty applied. Again, 

 no matter how poor the water may be in plankton, it is always 

 possible to strain an amount sufficient to furnish enough plank- 

 ton for measurement. The method is also a comparatively rapid 

 one, requiring for a plankton collection only about one third of 

 the time consumed in making the oblique haul. 



The pumping method is thus admirably adapted to the 

 situation with which we deal at Havana, i. e., shallow water 

 and an abundance of vegetation. It is not, however, limited 

 in its applicability to such situations, but with the help of a 

 steam-vessel and a steam-pump it is capable of application to 

 larger and deeper bodies of water. 



III. PRESERVATION AND EXAMINATION OF THE PLANKTON. 



1. Preservation. — The living plankton is transferred di- 

 rectly from the bucket of the net to a wide-mouth two-ounce 

 bottle, and the sides of the bucket are rinsed down thoroughly 

 with a spray of 1% formalin to insure the complete removal 

 of all of the catch. Enough strong alcohol is then added to 

 the bottle to make a grade of about 75%". Surface and bot- 

 tom collections are usually preserved in 1% formalin, or in 

 75% alcohol after killing inpicro-sulphuric acid. The bottles 

 are all labeled with a gummed slip bearing the accessions' 



