Download Chapter 13.1

Chapter 13.1
Mollusks
Characteristics of Mollusks
These are soft-bodied
invertebrates, have bilateral
symmetry, and usually 1 or 2 shells.
Most mollusks live in water, but a
few live on land.
Examples are: snails, clams, and
squid.
Body Plan
All mollusks have a mantle – this is a thin
layer of tissue that covers the body organs.
They also usually have gills – organs that
exchange carbon dioxide for oxygen in water.
The mantle is also important for making the
shell or protecting the body.
Mollusks have an open circulatory system –
this is where the heart moves blood out into
the open spaces around the body organs.
(they do not have veins)
Most mollusks also have a well-developed
head with a mouth and sensory organs.
Most mollusks also have a foot used for
movement.
Classification of Mollusks
The first thing when classifying
mollusks are whether or not they
have a shell. Mollusks that have a
shell are then classified by the kind
of shell and the kind of foot they
have.
The 3 most common groups are
gastropods, bivalves, and
cephalopods.
Gastropods
Gastropods include snails and slugs, as
well as, conchs, abalones, whelks, and
sea slugs.
If a gastropod has a shell it only has 1.
Gastropodss use a radula – a
tonguelike organ with rows of teeth –
to get food.
The radula can scrape and tear food.
Snails and slugs live on land and
usually will be active at night. They eat
plants.
Radula
Bivalves
These mollusks have 2 shells.
Clams, oysters, and scallops are in
this group.
They have powerful muscles that
can close their shells.
They have a muscular foot for
movement. They dig into the soil
and stay there – most of these
animals are filter feeders.
Cephalopods
Squid, octopuses, cuttlefish, and
chambered nautiluses are all in this group.
Cephalopod means head-footed
All cephalopods have well developed
heads, they do not have a foot, instead
they have tentacles. This group of mollusks
are predators that feed on fish,
crustaceans, worms, and other mollusks.
Squid and octopus have well developed
nervous systems and large eyes.
They have a closed circulatory system –
this means that blood moves through
closed blood vessels.
Cephalopod Propulsion
Cephalopods have the ability to
move water out through an
opening in near their head. This
water propels the cephalopod
backwards and allows them to get
away quickly.
Value of Mollusks
They are food for fish, sea stars, birds,
and humans.
Some animals, like hermit crabs, use
empty mollusk shells as a house.
Many mollusk shells are used for
jewelry or decoration. Pearls,
produced by oysters also are used as
jewelry.
Land slugs and snails damage plants.
Some snails are hosts of parasites that
infect humans. Shipworms can cause
damage to boats.