Phylum Mollusca
General Characteristics
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Soft-bodied invertebrates
Internal or external shell
Second largest animal phylum
Include snails, slugs, clams, squids, and
octopi
• Fully-lined coelom
• Show cephalization
General Characteristics
• Free-swimming, larval stage called a
trochophore
• Development very similar to annelids
which shows common ancestry
• Bilaterally symmetrical
• Most have separate sexes that crossfertilize eggs
• Gills between mantle and visceral mass
used for gas exchange
General Characteristics
• Includes four classes:
– Polyplacophora (chitons)
– Gastropoda (snails, slugs, nudibranchs,
and conchs)
– Bilvalvia or Pelecypoda (clams,
oysters, and mussels)
– Cephalopoda (squids, octopus, and
nautilus)
Body Plan of Mollusks
• Four main parts: foot, mantle, shell,
and visceral mass
• Foot: muscular; used for crawling,
burrowing, or capturing prey
• Can be flat (snails), spade-shaped
(clams), or modified into tentacles
(squid and octopus)
Body Plan of Mollusks
• Body covered with protective mantle:
thin layer of tissue that covers most of
the mollusk’s body; may or may not
produce a shell
• Visceral mass: body organs lying below
and protected by the mantle (all
internal organs)
• Shell: made by glands in mantle that
secrete calcium carbonate
Feeding
• Can be herbivores, carnivores, filter
feeders, detritivores, or parasites
• Snails and slugs use radula for feeding
• Radula: flexible, tongue-shaped
structure covered with tiny teeth
• Herbivores use radula to scrape;
carnivores use radula to drill through
shells or flesh
Feeding
• Clams, oysters, and scallops filter feed
using feathery gills
• Mucus produced by gills traps plankton
as water comes in through siphons
• Siphon: tubelike structure through
which water enters and leaves
• Octopi and squid use tentacles and
sharp bird like beak to capture and eat
prey
Respiration
• Aquatic species use gills inside mantle
to exchange gases
• Terrestrial species use mantle cavity
lined with blood vessels to capture air
and exchange gases
• Lining must be kept moist to allow for
diffusion of oxygen and CO2 exchange
Circulation
• Can be open or closed circulatory
system
• Open system: blood is pumped by
simple heart into sinuses (large saclike
space) then to gills, then back to heart;
slower system; found in snails and clams
• Closed system: blood is pumped by
heart through blood vessels; faster
system; found in faster-moving mollusks
Excretion
• Nitrogen-containing wastes
(ammonia) is released from blood
through nephridia
Response/Nervous System
• Can be simple or very complex
• Bivalves have small ganglia, few
nerve cords, and simple sense
organs
• Cephalopods have well-developed
brains, good memory, and complex
eyes; Can be taught to do many
tasks: open jar, perform for reward
Movement
• All use foot for movement
• Snails/slugs use mucus and rippling
motion of foot
• Aquatic species use siphons to suck in
water and shoot it out “jet propulsion”
Reproduction
• Sexual reproduction
• External fertilization
• Release enormous amounts of eggs and
sperm into water
• Free swimming larvae
• In tentacled species, fertilization is
internal and female lays fertilized eggs
Class Polyplacophora
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Common name is chiton
All marine
Shell divided into 8 over-lapping plates
Live on rocks along seashore
Feed on algae
Class Gastropoda
• Head has a pair of retractable tentacles
with eyes located at the ends
• Have a single shell or valve (snails) or
none (slugs)
• Known as univalves
Class Gastropoda
• PTEROPODS
– Called "sea butterflies“
– Marine
– Have a wing-like flap for swimming
Class Gastropoda
• NUDIBRANCHS
– Marine slug
– Lacks shell
– Can be poisonous
Class Bivalvia
• Sessile or sedentary
• Includes marine clams, oysters, &
scallops and freshwater mussels
• Filter feeders
• Have two-part, hinged shell (2 valves)
• Have muscular foot that extends from
shell for movement
• Scallops clap valves together to move
Class Bivalvia
• Shell secreted by mantle & made of 3
layers --- outer horny layer protects
against acids, middle prismatic layer
made of calcium carbonate for strength,
& inner pearly layer next to soft body
• Mantle secretes substance called
"mother of pearl" to surround irritants
like grains of sand
• Oldest, raised part of shell called umbo
Class Bivalvia
• Powerful anterior & posterior adductor
muscles open & close shell
• No distinct head
• Have an incurrent & excurrent siphon
that circulate water over the gills to
remove food & oxygen
• Have heart & open circulatory system
Class Bivalvia
• Nervous system made of 3 pairs of
ganglia, nerve cords, & sensory cells
that detect light, chemicals, & touch
• Separate sexes with external
fertilization of eggs
Class Cephalopoda
• Includes octopus, squid, cuttlefish, &
chambered nautilus
• All marine
• Most intelligent mollusk
• Well developed head
• Active, free swimming predators
• Foot divided into tentacles with
suckers
Class Cephalopoda
• Use their radula & beak to
feed
• Closed circulatory system
• Lack external shell
• Highly developed nervous
system with vertebratelike eyes
• Separate sexes with
internal fertilization
Class Cephalopoda
• SQUID
– Largest invertebrate is the Giant Squid
– Large, complex brain
– Ten tentacles with longest pair to catch
prey
– Use jet propulsion to move by forcing water
out their excurrent siphon
– Chromatophores in the skin can help change
squid color for camouflage
Class Cephalopoda
– Can squirt an inky
substance into
water to
temporarily blind
predators
– Have internal shell
called pen
– Female lays eggs in
jellylike material &
protects them until
hatching
Class Cephalopoda
• OCTOPUS
– Eight
tentacles
– Similar to
squid
– Crawls
along
bottom
looking for
prey
Class Cephalopoda
• CHAMBERED NAUTILUS
– Has an exterior shell
– Lives in the outer chamber of the shell
– Secretes gas into the other chambers to
adjust buoyancy
Economic Importance of Mollusks
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Used by humans for food
Pearls from oysters
Shells used for jewelry
Do crop & garden damage
Serve as intermediate hosts for some
parasites such as flukes