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Comparative
anatomy. Jawless fishes
The earliest known vertebrates were jawless fishes of the
class Agnatha, and their only living representatives are
the cyclostomes—the lampreys and the hagfishes. The modern
agnathans retain much of the general organization of the
ancestral vertebrates, and therefore, much of their musculature
is relevant to an understanding of the evolution of muscles
in more advanced vertebrates.
The
cyclostomes are free-swimming animals with prominent axial
somatic musculature, which during contraction produces undulating
waves that propagate from head to tail to produce thrust.
The axial muscles form a single segmented mass in which
each embryonic myotome has given rise to a strip of muscle
running vertically down the side of the fish. These muscle
segments, known as myomeres, consist of relatively short
fibres that insert into septa of connective tissue, the
myocommata, between the adjacent myomeres. There is only
a rudimentary axial skeleton and no appendicular skeleton,
so there are no limb muscles. The eyes of cyclostomes are
degenerate structures, and the six axially derived muscles
normally found associated with vertebrate eyes are diminished
or absent. The branchiomeric muscles incyclostomes are represented
by a sheet of constrictors that compresses the gill pouches
and helps the pumping mechanism draw water through the pharynx
to the gills. Other muscles of the branchiomeric series
have been modified for specialized feeding functions.
The
branchiomeric musculature of more primitive jawless fishes
would probably have been similar for each of the gill arches.
The sharks and other cartilaginous fishes (the class Chondrichthyes)
have modified the structure of the first two arches, the
cartilages of the anterior arch forming the mandible and
upper jaw (palatoquadrate), and modifications also having
taken place in the second, hyoid arch. The posterior five
gill arches of more primitive sharks, however, are a good
model for the condition in the ancestral jawless fishes.
Each arch has a visceral skeleton comprising five cartilages
named, from dorsal to ventral, the pharyngobranchial, epibranchial,
ceratobranchial, hypobranchial, and basibranchial. The cartilages
are arranged at angles to each other. Each cartilaginous
arch is provided with a set of branchial muscles, which
receives separate, visceral innervation. Superficially,
a thin sheet comprising dorsal and ventral constrictor muscles
runs in the flap of skin that covers each gill slit and
forms the gill septum. Most fibres attach, dorsally and
ventrally, to connective tissue sheathing the body (fascia).
Some of the deeper fibres attach to the gill bar and may
run between adjacent bars. These thin, broad muscles squeeze
the pharynx closed as part of the pumping action necessary
for gill breathing. Dorsal and deep to this layer, a levator
muscle runs from the sheathing fascia to the pharyngobranchial,
and it can elevate the gill arch. In some sharks, however,
the most posterior sets of levator muscles, whose fibres
run diagonally down and back, may join adjacent levators,
become enlarged, and attach to the pectoral girdle. This
mass is known as the trapezius and evolves into the tetrapod
muscle of the same name. Adductor muscles are positioned
so as to close the angle between the epibranchial and ceratobranchial,
and an interarcual muscle performs the same function for
the angle between the pharyngobranchial and epibranchial
cartilages.
