282 Birge — The Crustacea of the Plankton. 



In all of the work reported in this paper and done before the 

 11th of July, 1896, a single net was employed. After that 

 date the net was replaced by one of silk bolting cloth, number 

 16, containing about 3600 meshes to the square cm. This net 

 was cut from the same pattern as the old one. In order to 

 compare the two nets they were similarly mounted in the same 

 frame, and a series of comparisons made to determine their 

 relative coefficient. 



To my surprise the two nets showed practically the same co- 

 efficient. The numbers caught necessarily varied considerably, 

 but the average of each of two series of five pairs showed prac- 

 tically the same number of Crustacea; the silk net catching on 

 the whole about 5 per cent, less than the old net. It did not 

 seem necessary therefore to alter the coefficient of the dredge 

 with the change of the net. On the 20th of August the dredge, 

 with all its appurtenances, was lost by the accidental breaking 

 of the line, and the work for the remainder of the year was 

 done with a similar instrument of smaller size, having a square 

 opening of 100 square cm. The coefficient of this net was de- 

 termined by comparing it with the tube, one set of comparisons 

 being made by determining the number of the Crustacea. A 

 second set was made by determining the bulk of the plankton 

 caught by the tube and net when allowed to settle for the same 

 length of time in similar tubes. Two other determinations 

 were made by Hensen's last method. (Hensen, '95, p. 92.) The 

 net was fitted with a cover having an opening of 2.5 square cm. 

 Ten successive hauls of the net were made with the small open- 

 ing and their contents mingled. This was preserved and allowed 

 to settle and compared with the amount of plankton caught with 

 the full opening of the net, the two quantities being similarly 

 preserved and allowed to settle in similar tubes. The result of 

 these three methods of determination of the coefficient of the 

 net was substantially identical, the coefficient varying from 1.81 

 to 2.04. The coefficient 1.9 was selected, and as a result the 

 catch of this net is multiplied by 190 in order to give the num- 

 ber of Crustacea per square meter of surface area. 



An important question has been raised, first by Hensen ('87, 

 p. 12) and especially by Kofoid ('97, p. 11) regarding the vari- 



