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Multilayered
animals. Mollusks
The
phylum Mollusca includes the gastropods (snails, slugs,
periwinkles), the bivalves (clams, oysters, mussels, and
scallops), the cephalopods (octopods and squids), and other
smaller classes. All mollusks, except the cephalopods, have
a highly muscular organ called the foot, through which muscle
fibres run in all directions. The foot of a gastropod is
a flat structure usedfor crawling. Waves of muscular contraction
travel along its length, moving the animal slowly over the
ground. The foot of a bivalve mollusk is a bulbous or tonguelike
organ that is used for burrowing in sand or mud. The foot
pushes down into the substrate, then swells to anchor itself
and pulls the rest of the animal down behind it.
In addition to the muscles of the foot, gastropod and bivalve
mollusks have large muscles attached to their shells. The
columellar (shell) muscles of gastropods pull the foot and
other parts of the body into the shell. The adductor muscles
of bivalves (Figure 4) shorten to close the shell or relax
to allow the shell to spring open, enabling the mollusk
to extend its foot or to feed. The adductor muscle can shorten
rapidly and close the shell quickly. The muscle is also
capable of maintaining the tension needed to hold the shell
shut against the spring action of the hinge ligament without
using much metabolic energy. Economy of energy is particularly
important if the shell hasto be kept closed for long periods;
for example, for several hours while the mollusk is exposed
on the beach at low tide. Fast muscles can shorten rapidly
because their cross bridges detach and reattach quickly;
however, they use much energy while maintaining tension
because there is an energy cost every time a cross bridge
detaches and reattaches. Muscles that are economical in
their energy usage are generally slow. Accordingly, most
bivalve mollusks have two parts to their adductor muscles,
a translucent part, which is fast, and an opaque part, whichis
slow but economical.
Squids
and other cephalopod mollusks also swim by jet propulsion.
They draw water into the mantle cavity (the cavity that
houses the gills) and expel it rapidly. Vigorous movements
of thiskind provide jet propulsion, but gentler ones serve
for breathing by circulating water, and thus oxygen, through
the mantle and gills. Fast-swimming squid have mantle cavities
whose muscular walls make up as much as 35 percent of the
mass of the body.
These
walls mainly consist of circular muscle fibres that squeeze
water out of the mantle cavity when they contract. Other
fibres run radially through the thickness of the wall. These
fibres make the wall thinner when they contract, stretching
the circular muscle and enlarging the cavity again. Cephalopods
do not have longitudinal muscle fibres; however, layers
of collagen fibres on the outer and inner surfaces of the
muscle prevent the animal from lengthening when the muscles
contract. Thus the circular and radial muscle fibres are
antagonistic. Enlargement of the cavity, however, is not
solely due to the radial muscle fibres: the cavity tends
to expand by elastic recoil of the tissues, when the circular
muscles relax.
Though
many mollusks have shells, most molluscan muscle systems
depend on the principle of the hydrostatic skeleton. In
some cases body fluids are involved, for example, in the
feet of clams, which are extended and inflated by inflow
of blood. In other cases the muscle itself serves as the
incompressible element that must thicken as it shortens
or become slender as it elongates, to maintain constant
volume. Examples include the shell muscle of the abalone
and the tentacles of squid, which are shortened by contraction
of longitudinal muscle fibres and lengthened by circular
and transverse ones.
