322 University of California Puhlications in Zoology. [Vol. S 



the net and permit the spreading effect of the resistance of the 

 filtering surface. It is probable that the chances of such an 

 intermittent gaping under normal operation of the net are re- 

 mote, but one would be more certain of the accuracy of the 

 results if this remote possibility were removed. 



The Cori net is criticized by Burckhardt (1900), who used a 

 model constructed by Ilofer (1896), double the size of the orig- 

 inal Cori pattern, on the ground that the closure is not complete 

 on account of the lack of attachment of the bag at the six hinges 

 and the insufficiency of the vertical closing plate across the closed 

 mouth of the net. He also found the net failing to function 

 because the net or the wire became entangled in the trips, pos- 

 sibly due to lack of experience in handling the apparatus. He 

 also expresses uncertainty as to the control of the opening of 

 the net and of the level at which it operates, and finally calls 

 attention to the practical difficulty of determining the distance 

 traversed by the net. Burckhardt is a proponent of the vertical 

 net and has quite fully stated the difficulties of the horizontal 

 net. They can, however, be obviated in the main: the closure 

 perfected by slight changes in construction, the net bag con- 

 trolled by a supporting arm, the messengers adjusted in weight 

 to give certain action on the trips, and the horizontal course of 

 the net improved by increase in weight of the frame and adjust- 

 ment of the speed of towing. The determination of the length 

 of the haul presents the greatest practical difficulty of the defects 

 he enumerates. 



The Burckhardt (1900) vertical closing-net has a contracted 

 conical head after the Hensen model Avith circular lid hinged 

 at one side, opened and closed by weights released by a trip of 

 the Cori pattern. The trip is operated by messengers. Burck- 

 hardt experimented with the net, sinking it open, immediately 

 closing it before raising it, and found that the net "ganz leer 

 blieb." He therefore rearranged the connections of the net and 

 trips so that the first trip closed the net and the second released 

 the head of the net so that it fell to one side as in the Nansen 

 net. This decreased the resistance in hauling in and hastened 

 the operation of collecting. The objections to a net of this pat- 

 tern, for work at sea, are the dangling weights, and the resist- 



