Download Mollusks and Segmented Worms

MOLLUSKS:
• Slugs, snails, and animal that once lived in
shells in the ocean or on the beach.
• Phylum Mollusca
• Range from slow-moving slug to jetpropelled squid
• Found in oceans, freshwater and moist
terrestrial habitats
• Oysters and mussels live firmly attached
to the ocean floor or to the bases of docks
or wooden boats.
• Squid and the octopus swim freely
• Slugs and snail crawl slowly over the
forest floor.
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100 000 species
Some have shells
Bilateral symmetry
True coelom
Two body openings
Muscular “foot” for movement
Mantle: thin membrane that surrounds the
internal organs of the mollusk
In mollusks with shells, the mantle secretes the
shell.
How mollusks obtain food:
• Radula: located within the mouth of a mollusk,
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is a tongue-like organ with rows of teeth.
The radula is used to drill, scrape, grate or cut
food.
Some mollusks are grazers, some are predators,
and some are filter feeders.
Bivalves do not have radulas, they are filter
feeders.
Reproduction in Mollusks:
• Most have separate sexes and reproduce
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sexually.
Eggs and sperm are released at the same time
into the water where external fertilization takes
place.
Many gastropods are hermaphrodites.
Having the same individual produce both sperm
and eggs increases the likelihood of fertilization
in slow-moving animals.
Nervous Control in Mollusks:
• Simple nervous system with a brain and
associated nerves that coordinate
movement and behavior.
• Paired eyes that range from simple cups
that detect light to complex eyes in the
octopus that have irises, pupils and retinas
and function as well as the human eye.
Circulation in Mollusks:
• Well-developed circulatory system with a
three-chambered heart.
• Open circulatory: blood is pumped
through vessels and into open spaces
surrounding organs.
• This exposes organs directly to blood
containing oxygen and nutrients and easily
removes wastes.
• Some mollusks (such as the octopus) has
a closed circulatory system.
• Closed circulatory system: blood moves
through the body enclosed in a series of
vessels
Respiration in Mollusks:
• Most mollusks have respiratory structures
called gills.
• Gills increase the surface area through
which gases can diffuse. They are an
extension of the mantle.
• In land snails and slugs, the mantle has
evolved into a primitive lung.
Excretion in Mollusks:
• Oldest known animals to have evolved excretory
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structures called nephridia
Nephridia: organs that remove metabolic
wastes from an animal’s body.
Mollusks have one or two nephridia that collect
wastes from the coelom.
Wastes are discharged into the mantle cavity,
and expelled from the body by the pumping of
the gills.
Diversity of Mollusks:
• Seven classes
• These are the improtant 3
– Gastropoda
– Bivalvia
– Cephalopoda
Gastropods:
• One-shelled mollusks
• Largest class
• Stomach-footed (named for the way that
the large foot is positioned under the
body)
• Slugs have not shell
• Snails, abalones, conches, periwinkles,
whelks, limpets, cowries and cones
• May be found in freshwater, saltwater, or
moist terrestrial environments.
• May be plant eaters, predators or
parasites.
Slugs:
• No shell
• Body is protected by a thick layer of
mucus.
• Nudibranchs: colorful sea slugs
• Nudibranchs feed on jellyfish and
incorporate their nematocysts into their
own bodies which are expelled whenever
a predator tries to feed on the slug.
Bivalves:
• Two-shelled mollusks
• Clams, oysters and scallops
• Most are marine, but a few a freshwater.
• Range in size from 1 mm to 1.5 meters
• No distinct head or radula
• Most use their muscular foot for burrowing
in the mud or sand at the bottom of the
ocean or lake.
• A ligament like a hinge connects the two shells.
• Filter feeders
• Cilia beat to draw water into an incurrent
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siphon.
As water moves over the gills, food and
sediment are trapped in mucus. Cilia that line
the gills push the food particles into the
stomach.
• The cilia also act as a sorting device.
• Any rejected material is transported to the
mantle where it is expelled through the
excurrent siphon.
Cephalopods:
• Head-footed mollusks
• All marine
• Octopus, squid, cuttlefish, and chambered
nautilus
• The only cephalopod with a shell is the
chambered nautilus.
• The cuttlefish has a reduced internal shell.
• Very complex structures
• Predators that swim or walk over the
ocean floor pursuing their prey.
• They grab prey with their tentacles.
• The foot has evolved into structures with
hooks, suckers or adhesive structures.
• The tentacles bring the prey to the mouth
where it is bitten by the beak-like jaw.
• The food is then pulled into the mouth by
the radula.
• Possess siphons that expel water.
• By expelling water forcefully, these
mollusks can move quickly by jet
propulsion.
• Squids can attain speeds of 20 meters per
second with this method.
• They also expel “ink” to confuse and
escape predators.