GERMAN PLANKTON INVESTIGATIONS. 289 



net is first hauled up a large quantity of the catch remains 

 lodged just above the lower ring (Fig. 1, d). The washing 

 is best accomplished by directing a stream of sea water 

 on to the outside of the net by means of a hose, or if that 

 is impracticable by throwing buckets of water on to the 

 net, care being taken that nothing enters the net through 

 the ring. 



The Horizontal Net. 



In addition to the vertical net a horizontal net has been 

 devised, that is to say, a net which is towed along 

 through the water in a horizontal direction, with 

 the opening of the net forward. This net can only 

 be used in the upper layers of the water, and when the 

 ship is passing over a certain definite course, the length 

 of which is given by the log. The construction and use 

 of this net also presents many difficulties, the chief being 

 the strong pull brought to bear on the net owing to the 

 variable speed with which it is drawn through the water. 

 An alternative method to the use of this net is to pump 

 up the water on deck by means of a steam pump, and 

 there filter it. 



Preservation of the Catch. 

 The catch is first of all freed from sea water in the 

 filtrator, in which similar silk bolting cloth to that of the 

 net is used. Several kinds of preservative fluids have been 

 recommended. Hensen first of all tried picrosulphuric 

 acid, but this has proved unsatisfactory. Carbonate of 

 lime is dissolved out by this reagent, and the shells of 

 Foraminifera are entirely destroyed. In addition the 

 exoskeletons of the Copepoda also contain calcium car- 

 bonate, which is dissolved out. Under certain conditions 

 Flemming's solution (chromo-aceto-osmic acid) may be 



