METHODS OF PLANKTON RESEAECH. 537 



(fig. 6, /. /.) in order to keep tliem vertical in tlie water. 

 Each pole is attached by two strong ropes, fixed to the 

 upper and lower ends respectively, to the " otter " boards 

 (fig. 6, d.e.). The ropes are about 12 feet long, and 

 represent the " foot rope " and " head line " of the otter 

 trawl. The otter boards are strong wooden structures 

 bound with iron, and measure 4 feet by 2 feet. When 

 the net is lowered it sinks, owing to its weight, and the 

 pressure of the water forces the two otter boards out- 

 wards, thus pulling the two vertical poles as far apart as 

 possible, and in this way the mouth of the net is kept 

 open. This net can be used satisfactorily at very con- 

 siderable depths. 



There remains to be described a very convenient and 

 simple little instrument by which catches can be made 

 whilst a vessel is travelling at a considerable speed, and, 

 consequently, any changes in the nature of the plankton 

 between two stations can be followed without interfering 

 with the progress of the steamer. Several instruments 

 have been invented for this purpose, but it will only be 

 necessary to mention here the " Plankton Rolire," which 

 was invented by Apstein and has not yet been described. 

 It has the great advantage of being simple, and so small 

 that it can be very easily carried about with one, so that 

 plankton catches may be made on a sea voyage other than 

 a scientific expedition. Fig. 7 shows the external appearance 

 of the instrument. The Plankton Ilohre consists simply 

 of a brass tube 25 cm. long, one end of which, however, 

 is not of the same diameter as the rest of the tube, but 

 forms a truncated cone, making the mouth opening of the 

 tube very narrow. The diameter of the cylindrical 

 section of the tube is 3*5 cm., and the length 22*5 cm. 

 The conical mouth-part is 2' 6 cm. in length, and the 

 opening is only 1 cm. in diameter. This narrow opening 



