METHODS OF PLANKTON RESEARCH. 



503 



of three parts: — (1) The filtering net itself; (2) the metal 

 filtering bucket; and (3) the conical mouth piece. The 

 mouth piece is constructed of thick material, which does 

 not allow water to filter through it. It is supported by 

 two brass rings, one of which serves to keep the mouth 

 of the net open, and to this ring are attached the cords, 

 three in number, which support the net. This upper 

 ring, and therefore the mouth of the net, has a diameter 



of 14 centimetres. The lower 

 brass ring is thicker and is sup- 

 ported by the mouth-piece cloth 

 itself and by three cords which are 

 attached to the upper ring and take 

 the place of the three iron rods 

 that separate the two rings in the 

 Hensen net. 



The length of the mouth-piece 

 is 20 cm. down the side. This part 

 of the net serves three purposes : — 

 (1) It prevents mud passing into the 

 net if it be lowered on to very soft 

 ground ; (2) it prevents the catch 

 from being upset in a rough sea ; (3) 

 it performs the function of keeping 

 the net mouth small in comparison 

 Fig. l.-Apstem's middle and with the n i ter ing area of the net. 



small plankton nets. m ° 



It is obvious that the water will 

 be most completely filtered if as little as possible is 

 allowed to enter and the greatest facility is given for it to 

 leave the net. If the mouth-piece was not present, a 

 greater quantity of water would attempt to stream 

 through the large ring than could be filtered by the area 

 of the net, and, therefore, as the net was hauled up, water 

 would remain in the entrance and the water of the vertical 



