10 



Marine Invertebrates 



Fig. ii. — The box dredge. 



viding additional weight and pushing the material scraped by the blade 

 into the bag. The usual dimensions are as follows: arms, 2% feet; cross 

 bar, 2 feet; blade, 2% feet long and 2 inches wide. For operating on 

 rough bottoms the blade is set level or even with a slight upward incline, 

 so the dredge will slide over the bottom. A dredge of this type may be 

 made of any size and is very useful for general collecting. For small 



organisms the bags must be 

 made of a fine twine netting 

 or of some other strong, 

 coarse material of the de- 

 sired mesh. 



Instead of a cutting blade 

 the dredge may be provided 

 with a set of teeth. This 

 type is commonly used for dredging oysters. 



The box dredge. The box dredge (Fig. n) consists of a rectangular 

 iron framework 27 x 12 inches, with two folding arms and two cutting 

 blades, one on each side of the dredge. A bag of coarse netting is at- 

 tached to the blades. When in operation the two arms are tied together 

 by a piece of string and the drag line is fastened only to one arm. If the 

 dredge is caught under rocks the string breaks and the instrument may 

 be saved by dragging it sidewise by one arm. This small dredge is very 

 useful for general collecting. 



The triangular dredge. The triangular dredge (Fig. 12) has some ad- 

 vantages over the other types because 

 no matter which side rests on the 

 bottom one of the blades will cut into 

 the ground when the instrument is 

 dragged. 



To facilitate the finding of a 

 dredge in case the drag line snaps, a 

 tail buoy is attached with a length of 

 rope slightly greater than the depth 

 of the water. When the instrument is being dragged slight shocks caused 

 by the impact of the frame with rocks or other objects are conveyed 

 along the rope and are easily noticed by the operator holding it in his 

 hands or only touching it. He can easily feel the change in vibration 

 when the dredge slides over the bottom without cutting into it. In this 

 case more rope should be given out or the speed of the boat reduced. 



The material collected in the dredge must be washed free of mud and 

 sorted. For this purpose it is convenient to have a set of sieves with 

 various meshes into which the contents of a dredge is dumped and washed 

 by dipping into the sea. (See p. 534 for drill trap dredge.) 



Fig. 12. — The triangular dredge. 

 After Hagmeier. 



