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The molluscs are a
hardy and successful
group of
animals. There
are about 128,000
known species of
molluscs. For
many of them, their life
strategy is to
protect themselves
from predators by
building protective shells of
calcium carbonate, a
mineral that is
available in sea
water. Some molluscs,
however, have
evolved away from
exterior shells,
reducing the size of
the shell or
eliminating it
entirely.
Some of the earliest
fossils are made of
"little shelly
bits",
fragments of the shells
of what may have
been early
molluscs.
Shells are well
preserved as
fossils, but the
soft tissues are
not, so if all you
have are bits of
shells it is
challenging to try
to understand what
animals made the
shells.
If
you imagine a
primitive animal as
essentially a
digestive tract, you
wind up with a tiny
worm with an inside
and an
outside. Now
there's a tempting
morsel!!! Toughening the
outside of the worm with a
little calcium
carbonate could
provide some protection
against predators,
and experimentation
with the shell would allow the
development of the
molluscs that we see
today.
The molluscs are one
of the only three
phyla that includes
members who
colonized the
land. Molluscs
can be found in
fresh water and all
depths of the
oceans. They
are unsegmented
animals and most
have heads and eyes.
Mollluscs that are
familiar to us
include:
Snails
and Slugs
Snails
and slugs can be
found on land and in
both fresh and
ocean water.
The underside of
these animals has
developed into a
strong
"foot"
which they use for
locomotion.
Both snails and
slugs have a strap-
like structure
called a radula which
they use for
scraping algae off
rocks .
Most of these
animals are
herbivores, but a
few are carnivores,
eating other
molluscs and other
small animals.
The
animals will have
shapes suited to
their life
strategies. Many
simple animals are
little more than tubes with
openings at each end
so that food can
pass through and be
digested. Such
animals, though, are
tender and fragile,
a tasty morsel for a
passing predator.
Some of them will
learn to hide in the
mud or will grow
various kinds of
protective shells.
Shells may limit
movement, ho wever.
On
earth we see many
sea animals that
have simple shells
with no moving
parts. Some mollusks have
only a single shell,
some have shells
like little tubes,
some have coiled
shells, snail
shells.
This
picture is of the
Chambered Nautilus,
a mollusk that can
swim in the ocean.
It is noted for its
large, beautiful,
spiral shell.
As the nautilus
grows, it gets too
large to live in the
smaller part of the
shell. By
filling that space
with gas, the
nautilus gains
buoyancy so that it
can float in the
water and use its
energy for swimming
to wherever it wants
to go. This is
an interesting
adaptation!
Some
mollusks have two
shells that can be
opened and closed.
Clams and mussels
are examples of
bivalve mollusks
Bivalves
Bivalves are
molluscs with two
shells, for example,
clams and
oysters. The
two shells are
joined by a hinge,
and can be closed
and held closed by a
powerful
muscle. In
this way the
shellfish can
protect themselves
from enemies
mussels
and oysters
clams
Cephalopods
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