Download Mollusks and Segmented Worms

MOLLUSKS:
• Section 27 – 1
• Slugs, snails, and animal that once lived in
shells in the ocean or on the beach.
• Phylum Mollusca
• 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
• 100 000 species
• Some have shells
• Bilateral symmetry
• True coelom
• Two body openings
• Muscular “foot” for movement
How mollusks obtain food:
• Radula: located within the mouth of a
mollusk, is a tongue-like organ with rows
of teeth.
• The radula is used to drill, scrape, grate or
cut food.
Reproduction in Mollusks:
• Most have separate sexes and reproduce
sexually.
• Eggs and sperm are released at the same
time into the water where external
fertilization takes place.
Larval stages:
• Larval stages of all mollusks are similar.
Nervous Control in Mollusks:
• Simple nervous system with a brain and
associated nerves that coordinate
movement and behavior.
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.
• Some mollusks (such as the octopus) has
a closed circulatory system.
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.
Excretion in Mollusks:
• Oldest known animals to have evolved
excretory structures called nephridia
• Nephridia: organs that remove metabolic
wastes from an animal’s body.
Diversity of Mollusks:
• Seven classes
– Gastropoda
– Bivalvia
Gastropods:
• One-shelled mollusks
• Largest class
• Stomach-footed (named for the way that
the large foot is positioned under the
body)
• May be found in freshwater, saltwater, or
moist terrestrial environments.
Slugs:
• No shell
• Body is protected by a thick layer of
mucus.
• Nudibranchs: colorful sea slugs
Bivalves:
• Two-shelled mollusks
• Clams, oysters and scallops
• Figure 27-7
• Most are marine, but a few a freshwater.
• Range in size from 1 mm to 1.5 meters
• A ligament like a hinge connects the two
shells.
• Filter feeders
• Cilia beat to draw water into an incurrent
siphon.
• The cilia also act as a sorting device.
Cephalopods:
• Head-footed mollusks
• All marine
• The only cephalopod with a shell is the
chambered nautilus.
• The cuttlefish has a reduced internal shell.
• Very complex structures
•.
• 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.
• Figure 27-8
• 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.