COLLECTING AND PRESERVING 273 



to be captured by means of the tow- net. This con- 

 sists of a conical bag of muslin, cheese-cloth, or, best 

 of all, silk " bolting-cloth," attached to a galvanized 

 iron ring of one or two feet in diameter, and having 

 a zinc can or a strong glass jar fixed to the narrow 

 end. The ring is attached by three equidistant cords 

 to the towing line, and the net is towed slowly at 

 or near the surface of the sea. When taken on 

 board, the contents of the can are emptied into a 

 jar of sea-water for examination. The tow-net is 

 best used when there is only enough way on the 

 boat to keep the net from sinking ; if towed more 

 rapidly, delicate organisms are apt to be crushed 

 by the pressure of the water, or the net itself may 

 be burst. The use of unnecessarily fine nets should 

 be avoided. A fine-meshed net may not capture a 

 single specimen of the larger Crustacea, even though 

 these may be swarming in the water through which 

 it is drawn. 



By weighting the tow-net it may be used at various 

 depths to capture the floating animals of mid-water. 

 When it is so used, however, it is impossible to tell 

 from what depth any particular specimen may have 

 come, since it may have been captured during the 

 hauling in of the net. For more precise investiga- 

 tions in deep water, '* closing tow-nets " of various 

 types have been devised, which can be opened by 

 a " messenger " sent down the line when the net has 

 reached the desired depth, and closed again by 

 another " messenger " before it is hauled in. 



A simple method that has proved very successful 

 18 



