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Comparative
anatomy. Jawed fishes
In
the jawed fishes, including the sharks, the axial musculature
of the trunk and tail (a single block in cyclostomes) differentiates
into dorsal and ventral components, which are separated
by connective tissue. The dorsal block of muscle is known
as the epaxial musculature, and the ventral block, the hypaxial.
The epaxial block runs from the back of the skull to the
end of the tail, while the hypaxial block is not present
any farther forward than the pectoral (shoulder) girdle
(because of the presence of the branchial [gill] apparatus).
The hypaxial musculature in the tail forms a solid block
of muscle, while in the trunk it encloses the body cavity.
Ribs develop in the horizontal septum separating the two
blocks of muscle and usually lie in the myocommata, the
fascial tissue separating each myomere. In fishes, the ribs
primarily serve to improve the leverage of the myomeres
in producing the undulatory movements of swimming. The ribs
are short in sharks but may develop to considerable length
in bony fishes. Unlike the cyclostomes, where the myomeres
form a series of essentially vertical strips of muscle,
the myomeres of all jawed fishes are folded in a complex
fashion. This development is related to the development
of a more powerful swimming ability in the jawed fishes.
The myomeres are folded in a zigzag pattern, projecting
strongly forward halfway down the side of the fish, with
a smaller, backward projection both dorsal and ventral to
this point: the effect is of a W on its side.These projections
become sharper and more cone-shaped deep to the surface
of the fish and thus come both to be overlapped by the folds
of several anterior myomeres and to overlap those of several
more posterior myomeres. The folding and overlapping of
myomeres has the effect that contraction of a single myomere
produces curvature over a considerable part of the body
of the fish. The fishes who swim faster thus tend to have
a greater degree of folding and overlapping. In the tunny,
for example, one myomere may have an overlap with 20 others.
The undulations of the body and caudal (tail) fin produced
by these axial muscles can produce much greater thrust than
is produced by the beating of the appendicular fins. The
latter are mostly used in slow “precision” swimming, as
when a fish is investigating food, while undulations of
the body are used for faster, powerful swimming. The axial
musculature of fishescontributes up to half the weight of
the fish, while the appendicular muscle contributes less
than a fifth of the fish's mass.
In
all higher vertebrates the most anterior element in the
axial musculature is the set of six eye muscles derived
from the three pre-otic somites (those anterior to the ear
region of the embryo). The rectus muscles move the eyes
about the longitudinal axis of the body, that is superiorly
(upward), or inferiorly (downward), or about a vertical
axis, in other words, laterally (backward) or medially (forward),
according to their position relative to the eyeball. They
take appropriate names. The oblique muscles, superior and
inferior, rotate the eyes about a transverse axis.
Jawed
fishes have single midline fins and two sets of paired fins.
The unpaired dorsal and anal fins of teleosts (advanced
bony fishes) have axially derived muscle sheets on either
side, which, when contracted, may change their angle and
even fold the fins. The paired pectoral fins and the weaker
pair of pelvic fins, however, have a mass of musculature
both dorsal and ventral to them that is derived from mesenchymal
cells. The dorsal muscle mass lifts the fin or pulls it
posteriorly; the ventral mass pulls it down or forward.
The two major muscle masses are attached at one end to the
pectoral or pelvic girdle and on the other to the base of
the fin. The amount of downward or upward movement of the
fin versus the amount of backward or forward movement can
be adjusted, in some fishes, by small slips of muscle derived
from the major dorsal and ventral masses, which twist the
fin.
The
hypobranchial muscles of jawed fishes are straplike muscles
running from the pectoral girdle to the structures of the
visceral skeleton, the jaws, and gill bars. Some muscles,
like the coracomandibularis, are specialized as jaw openers,
although most of the work of jaw opening isdone by gravity.
In
bony fishes the gill septum of the hyoid arch is greatly
modified to become a single, movable,bony covering for the
whole gill chamber: the operculum. The individual gill septa
are lost, and there is a great modification of the posterior
branchial muscles, with many of the elements found in sharks
(levators, adductors) becoming reduced or absent. The superficial
constrictor of the hyoid arch in sharks is remodeled in
bony fishes to control the opening and closing of this protective
cover.
Electric
organs appear to have arisen independently in several fishes.
They are modifications ofthe axial musculature of the tail,
as in the electric “eel” Gymnotus, a teleost, or of the
muscles of the pectoral fins, as in the ray Torpedo. In
a few cases electric organs lie superficially to the musculature
and may be derived from modified glandular tissue, as in
the Nile catfish Malapterurus.
