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nets
nephelometer instrument for measuring the cloudiness ( transparency) of water
which is chiefly related to the concentration and particle size of the suspended material. Measures that are used for cloudiness are the light transmission (light attenuation) through the water or the scattering of light by
the suspended particles.
nephridium excretory organ present in many Invertebrata and in Cephalochordata,
serves to convey the excretory and reproductive products from the coelom to
the exterior. Consists of a tube of ectodermal origin opening to the exterior
end. The other end may be closed with flame cells opening into it; or may open
into the coelom.
neritic originally defined by E. Haeckel (1834-1919) as the aquatic environment
overlying the sublittoral zone (0 to 200 m), i.e., the water over the continental shelves, as a complement to the oceanic environment of the waters over
the deep sea. Paleontologists have also applied the word "neritic" to the environment of the bottom itself.
Nero Expedition (1897) this US ship surveyed a route for a telegraph cable
between the Sandwich Islands and the Philippines, making many soundings in
very deep water (deepest sounding 5269 fathoms near Guam). Deep-sea sediments brought up with the sounding lead indicated the deep-sea sediments
to be radiolarian ooze with numerous large diatom skeletons of Coscinodiscus
rex.
net primary production -7production.
nets fishing gear of a wide variety of designs and meshed materials, developed
by man since prehistoric times to collect fish and other food animals. A division can be made into active and passive nets. Active nets are trawls and
seines. Trawls are nets towed through the water or over the bottom of the sea
(-7trawl). A seine is a large vertical net with weights and floats, that can be
used to encircle a shoal of fish. A purse seine can be closed with a ground
rope. Passive nets are vertical screens of netting floating in the water (drift
nets as, e.g., used in the herring fishery) or attached to poles standing in
shallow water (gill nets). The main idea is that migrating fish swim through
the meshes with the head, and cannot move back because of the gill covers.
Other passive nets are fykes and different types of fish traps, in which
migrating fish are trapped. In marine science, nets for the collection of
organisms from water or seabed show developments based on the same
principles. Plankton is collected with fine-meshed nets (mesh size as small as
50 or even 20 11m), mostly conical in shape with a steel ring in the opening.
For the collection of the larger zooplankton, such nets are mounted inside funnel-shaped metal bodies which can be towed at speeds of several knots. When
nets
nephelometer instrument for measuring the cloudiness ( transparency) of water
which is chiefly related to the concentration and particle size of the suspended material. Measures that are used for cloudiness are the light transmission (light attenuation) through the water or the scattering of light by
the suspended particles.
nephridium excretory organ present in many Invertebrata and in Cephalochordata,
serves to convey the excretory and reproductive products from the coelom to
the exterior. Consists of a tube of ectodermal origin opening to the exterior
end. The other end may be closed with flame cells opening into it; or may open
into the coelom.
neritic originally defined by E. Haeckel (1834-1919) as the aquatic environment
overlying the sublittoral zone (0 to 200 m), i.e., the water over the continental shelves, as a complement to the oceanic environment of the waters over
the deep sea. Paleontologists have also applied the word "neritic" to the environment of the bottom itself.
Nero Expedition (1897) this US ship surveyed a route for a telegraph cable
between the Sandwich Islands and the Philippines, making many soundings in
very deep water (deepest sounding 5269 fathoms near Guam). Deep-sea sediments brought up with the sounding lead indicated the deep-sea sediments
to be radiolarian ooze with numerous large diatom skeletons of Coscinodiscus
rex.
net primary production -7production.
nets fishing gear of a wide variety of designs and meshed materials, developed
by man since prehistoric times to collect fish and other food animals. A division can be made into active and passive nets. Active nets are trawls and
seines. Trawls are nets towed through the water or over the bottom of the sea
(-7trawl). A seine is a large vertical net with weights and floats, that can be
used to encircle a shoal of fish. A purse seine can be closed with a ground
rope. Passive nets are vertical screens of netting floating in the water (drift
nets as, e.g., used in the herring fishery) or attached to poles standing in
shallow water (gill nets). The main idea is that migrating fish swim through
the meshes with the head, and cannot move back because of the gill covers.
Other passive nets are fykes and different types of fish traps, in which
migrating fish are trapped. In marine science, nets for the collection of
organisms from water or seabed show developments based on the same
principles. Plankton is collected with fine-meshed nets (mesh size as small as
50 or even 20 11m), mostly conical in shape with a steel ring in the opening.
For the collection of the larger zooplankton, such nets are mounted inside funnel-shaped metal bodies which can be towed at speeds of several knots. When