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A dredge took samples along the surface of the seafloor while being dragged across the bottom. The Challenger dredges were between 3 and 5 feet in width and were just metal nets shaped into bags. A piece of tightly woven cloth in the bottom of the bag trapped small animals and samples of mud. The swabs, or "tangles," attached to the bottom of the dredge were standard Navy-issue mops that were very good at sweeping up smaller organisms.

Trawls were rugged metal nets used at a variety of depths to collect larger animals. The Challenger crew found it easier to use trawls, rather than the heavier and more awkward dredges, when collecting life from the bottom.

Nets of different size and mesh were the primary tool aboard Challenger for collecting living samples from all depths of the ocean. A tow net was attached at various depths on the sounding line, and collected both floating and slow-swimming life forms.

December 30th, off Vigo
Sighted the coast of Spain this morning but are still 150 miles from Lisbon. At 10 oclock this morning the first cast of the dredge was made and bottom obtained at 1500 fathoms but in hauling up the dredge & 100 fathoms of line was lost over board. The dredge was again cast but came up bottom upwards but on another attempt being made they succeeded in bringing up mud and several species of Fish from a bottom of 1125 fathoms, or nearly 1 1/2 miles; on the mud being analysed numerous insects were found in it--the fish are preserved in bottles. The Dredge is of Iron & not unlike a big trough with a net over it & weighs with the weights attached several hundred weight. The strain on the line is very great as it reaches the bottom & to ease it, several gutta percha ropes are spliced to it which will stretch when strained. The Dredging line is about the thickness of a man's two fingers. When dredging the Ship is hove to, & the Dredge is let down from the main yard & hauled up by a small steam engine & the line coiled away on the upper deck to dry; 1200 fathoms of line will take one hour in reaching the bottom & 3 hours in hauling up.