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A fish is unmistakable. There
are the telltale pairs of fins, gills and the scales. A
skeleton of backbone, ribs and a skull support the body of
a fish. The brain of the fish has a main nerve cord that
passes through its backbone. From the sides of the nerve
cord, nerves branch out leading to sensitive "pits"
in the
body wall. These pits form rows along each side of the
body, which appear as a dark line, called the lateral
line.
Except in the case of
lungfish, a fish does not use its nostrils to breathe.
They are used for smelling. Fish use their gills to
breathe the oxygen dissolved in the water. On each
arch of the gill, there is a lining of delicate skin full
of blood vessels. When the water passes through the gill
slits, these vessels pick up oxygen that dissolves in the
blood. Carbon dioxide is given off in the water. In the
case of bony fish, a shield called an operculum covers the
gills.
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The scales of different
species of fish vary a lot. While sharks and rays are
covered with hard scales that resemble minute enamel teeth
and that render the skin rough and painful to touch, other
fish have scales in regular rows. Carp-like fish had round
and bony scales. Catfish do not have scales, but are
covered with a leathery skin. The sturgeon has fewer rows
of large, bony scales.
Generally, a fish has two
pairs of fins, the pectorals in the front and the pelvis
behind. Normally, there is a single dorsal fin on the
back, but the perch, the largest group of fish, has two.
The anal fin is located at the base of the tail.
Plenty of fish have teeth
either on the jawbones or on bones further down the
throat. Hunting fish have sharp teeth, while others have
more flattened teeth. Food passes into the stomach and
intestine and is absorbed into the blood.
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A fish can hear with its
lateral line, by picking up vibrations in the water. This
is probably why a goldfish does not hit the sides of a
fish tank. As the fish moves, it sends out pressure waves
that bounce back off the glass like an echo, giving the
fish an indication of the presence of an obstacle ahead,
even though it cannot see the glass.
Instead
of lungs, bony fish have an air sac or swim bladder. The
bladder contains gas and can change pressure to match the
pressure of the water outside. This effectively means that
a fish can become weightless and "hang" in the water, at
different depths. In other words, a fish can stop swimming
when it wants to and take a break from routine. However,
in the case of sharks and rays, they have no swim bladder
and will sink if they stop swimming. It is their body that
gives them the lift. The pectoral fins act like an
aircraft's wings, while the large upper lobe of its tail
gives the fish the lift that it requires. |
