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Muscle
in soft animals
Slugs,
worms, and many other invertebrate animals have no skeleton,
and thus movement is not produced by lever action. Even
vertebrates have parts of the body that have muscles but
no skeletal component (for example, the tongue). Many soft-bodied
animals have muscle systems based on the principle illustrated
in Figure 5, which shows a simple wormlike animal. The longitudinal
muscle fibres run lengthwise along the body and the circular
fibres encircle it. The body contents are liquids or tissues
that can be deformed into different shapes, but they maintain
a constant volume. If longitudinal muscles contract and
the body shortens, it must widen to accommodate its volume;
if the circular muscles contract and the body thins, it
must lengthen. Thus the longitudinal and circular muscles
are antagonistic, and shortening of either extends the other.
Further, if the length of a circular muscle remains constant
while the longitudinal muscle of one side of the body shortens,
the body bends and the longitudinal muscle of theother side
is stretched. Thus the longitudinal muscles of the left
and right sides can be antagonistic toward each other. In
worms the body fluids render muscles antagonistic through
hydrostatic forces. The principle involved is sometimes
called the principle of the hydrostatic skeleton.
The
principle of the hydrostatic skeleton can apply to individual
muscles as well if their fibres run in several directions.
For example, in a muscle that has some fibres running longitudinally
and others running circularly and/or radially, when the
longitudinal fibres shorten, the muscle becomes shorter
and fatter; and when the circular and radial fibres shorten,
it becomes longer and thinner. There are many examples of
muscle structure like this in the mollusks. One such isthe
shell muscle of the abalone Haliotis, which connects the
domed shell of the animal to its adhesive foot. When the
muscle shortens, with the foot attached to a rock, the shell
is pulled down over the animal to protect it. When the muscle
lengthens (by contraction of circular and radial fibres)
the shell is raised from the rock, allowing respiratory
water currents to circulate.
