Methods in Plankton Investigations. 3 



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disturbance mingles with the water, rendering it impossible to 

 take a clean plankton collection. The soiling of the net and 

 the fouling of the water consequent upon dropping a large 

 plankton net upon the bottom further preclude the vertical 

 haul in the plankton work at Havana. 



I. THE OBLIQUE-HAUL METHOD. (PLATES I.-III.) 



The oblique haul was at first adopted as the method best 

 suited to the situation. This is accomplished by suspend- 

 ing the net to a carriage which runs upon a rope stretched 

 obliquely from the bottom to the surface of the water. By 

 this means the column of water traversed by the net is 

 increased to an adequate length. It also permits the employ- 

 ment of a net small enough to be easily operated from a row- 

 boat. A short description of this method of plankton collection 

 has been given by Professor Forbes ('94). The parts of the 

 apparatus used in making the oblique haul will now be de- 

 scribed. 



1. The Quantitative Net (Plates I. and II.). — The net used 

 by us is the modification of the Hensen net suggested by 

 Apstein ('91 and '92) for fresh- water work, and more fully 

 described by him ('96) as the smaller model quantitative net. 

 It consists of three parts : (a) the filtering net proper, (/>) the 

 detachable bucket, and (r) the head-piece of the net. 



a. The filtering net has the form of an inverted truncated 

 cone, whose base has a diameter of 25 cm. and whose side is 

 40 cm. The truncated apex, to which the bucket is attached, 

 is 4 cm. in outside diameter. This net is made of No. 20 

 silk bolting cloth, which can be obtained from wholesale 

 dealers in supplies for flouring mills. That used by us bears 

 the brand, " Especially for milling purposes. Dufour & Com- 

 pany." This is the same brand of cloth as that which was 

 used by Pieighard ('94) and Ward ('96a) in their plankton 

 work upon the Great Lakes. It is stated by the manufact- 

 urers to contain 29,929 meshes to the English square inch. 

 Examination under the microscope shows that this cloth 

 contains 32477 (— 172.75x188) meshes to the square 

 inch after sponging and pressing with a hot iron four times. 



