OGY

The naturalist's dredge is an instrument consisting essentially of a net
or bag attached to a framework of iron which forms the mouth of the net.
When in use as the apparatus is drawn over the sea-bottom mouth
forwards, some part of the framework passes beneath objects which it
meets and so causes them to enter the net. It is intended for the
collection of animals and plants living on or near the sea-bottom, or
sometimes of specimens of the sea-bottom itself, for scientific
purposes.

Until the middle of the 18th century, naturalists who studied the marine
fauna and flora relied for their materials on shore collection and the
examination of the catches of fishing boats. Their knowledge of
creatures living below the level of low spring tides was thus gained
only from specimens cast up in storms, or caught by fishing gear
designed for the capture of certain edible species only. The first
effort made to free marine biology from these limitations was the use of
the dredge, which was built much on the plan of the oyster dredge.

[Illustration: FIG. 13.--Otho Frederick Muller's Dredge (1770).]

_The Oyster Dredge._--At first naturalists made use of the ordinary
oyster dredge, which is constructed as follows. The frame is an iron
triangle, the sides being the round iron "arms" of the dredge, the
base a flat bar called the shere or lip, which is sloped a little, not
perpendicular to the plane of the triangle; an iron bar parallel to
the base joins the arms. The net is fastened to the parallel bars and
the portion of the arms between them, and consists of two parts: that
attached to the shere is of round iron rings linked together by
smaller ones of wire lashings, that attached to the upper bar is of
ordinary network. Where these two portions of the bag meet a wooden
beam is fastened. In use the frame is towed forward by its apex: the
shere passes below oysters, &c., which pass back on to the iron
netting. The length of each side of the triangular frame is about 6
ft., the width of the shere 3 in. and the height of the mouth just
under a foot. The rings vary in size, but are usually some 2-1/2 in.
in diameter. The weight is about 60 lb. This dredge was soon
abandoned: its weight was prohibitive for small boats, from which the
naturalist usually worked, its wide rings allowed precious specimens
to fall through, and its shallow net favoured the washing out of light
objects on haul