370 Biology in America 



up' the surface of the hitter, wliieh is caught by the bag, the 

 size of ()I)j('('ts retained de])ending upon its size of mesh. 

 The grai)i)le-dre(lge or scoop may also be employed for this 

 purpose, with tlie advantage of deeper penetration and con- 

 sequent capture of burrowing animals which might escape 

 the ordinary diedge. The trawl is constructed on much the 

 same plan as the dredge except that it is designed to skim 

 over the bottom taking those animals which live on, rather 

 than in the latter. This is effected by supporting the bag, 

 made of fish netting in the trawl, on runners like those of a 

 sled, whicli slide over, rather than penetrate into the bottom. 

 The smaller dredges in shallow water are operated by hand, 

 but for the larger apparatus, which may bring up tons of 

 material at one haul, heavy lines and steam winches, operated 

 on the deck of a large vessel are essential. 



For collecting the organisms present in the different levels 

 of the sea a large variety of appliances have been devised, 

 the details of which are too many and too intricate to describe 

 here. For merely qualitative work, that is, for determining 

 the kinds of organisms present at different depths, the appli- 

 ances are relatively simple. Surface collections are made with 

 tow-nets, of materials of various degrees of fineness, from 

 coarse fish netting down to number 20 silk bolting cloth, 

 with spaces only 1/400 of an inch in diameter. These are 

 usually of conical shape, with a large metal hoop at the 

 opening and a small metal cup or bucket at the lower end, 

 for holding the catch as the net is towed through the water. 

 For collections at lower levels similar nets may be lowered 

 to any desired depth and held horizontally while towed, by 

 a weight attached to the tow-line .iust in front of the net. 

 Or several of them may be attached to one main line at any 

 desired points by means of side lines, the main line being 

 carried down by a weight attached to its lower end. The 

 kinds of organisms present at different levels may also be 

 roughly determined by lowering tow nets to different depths 

 and drawing them vertically to the surface and then com- 

 paring the catches taken in each. If, for example, certain 

 organisms are present in the deeper hauls, which are absent 

 in the shallower ones, it may safely be inferred that these 

 occurred only in the lower levels. The relative abundance 

 of organisms at different depths may also be roughly deter- 

 mined in this way. 



Both of these methods however are open to the objection 

 that the nets catch not only the organisms present at the 

 particular depth whicli it is desired to study, but also at 

 all depths intermediate between it and the surface, as the 

 nets are draAvn in. To overcome this difficulty various types 



