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are some species that appear to have no scales at all but are instead covered in
what is termed an enameloid tissue. Many fish have bodies covered in protective
mucus; some, such as eels, catfish, and bream, have large amounts of this mucus.
In the case of eels this outer coating makes the scales all but undetectable.
Fish have a lateral line that runs along the length of the fish and is part of
their central nervous system, allowing them to detect movement, vibrations,
and changes in current. The lateral line is also instrumental in assisting fish to
locate prey and in schooling behavior.
Muscle, as in other forms of animals, provides the power for locomotion in
fish. The muscles that move the pectoral fins that do much to assist with
maneuverability are located at the base of the fin; the muscles that provide
the power for swimming are located along the flanks of the fish. These, known
as myotomal muscles, are the source of the majority of power behind the
undulating action in swimming. The myotomal muscles are placed in various
triangular configurations along the flanks of the body and consist of two
different types of muscle: slow-muscle fiber and fast-muscle fiber, though
these remain distinct and are not intermingled. The greater mass of myotomal
muscle is made of red fast-muscle fiber that allows for short periods of high-
speed swimming. The rest is made up of white slow-muscle fiber, used in
sustained swimming at slower speeds.
FIG 4.79 Myotomal muscle is
arranged in a zigzag configuration
down the flank of the fish.
A number of fish do not fall comfortably into the simple classifications I have
set out in this chapter. Though they may share some of the common features
FIG 4.80 Some fish have unusual
placing of the fins. (a) The seahorse
and (b) the sea dragon swim not with
the undulating action of a cordal fin
on the rear of the fish but by a gentle
and constant wafting of a fin located
on the back of the fish.
(a)
(b)
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