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Phylum Mollusca
Polyplacophora
Gastropoda
Bivalvia
Scaphopoda
Cephalopoda
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Phylum Mollusca
Molluscs
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One of the largest of all phyla
Have adapted to a wide variety
of habitats
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Terrestrial, marine, benthic,
and accomplished swimmers.
Phylum Mollusca
The Molluscan Body Plan
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Despite the external differences between snails,
clams, and squids the body plan is similar and
distinct from all other phyla
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The Mollusca body plan includes:
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A large muscular foot
A radula
Mantle and mantle cavity
Usually a small head
Soft unsegmented body
A hard non-living calcareous shell
Phylum Mollusca
The Foot
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Phylum Mollusca
The Radula
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Radula
Phylum Mollusca
The Mantle
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The mantle is the body wall that enclose the body cavity
Phylum Mollusca
The Mantle Cavity
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Houses the visceral mass and comb-like gills
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Gills are respiratory in function and can collect food particles
Mantle cavity also is the site for reproductive,
excretory, and digestive systems
The molluscan coelom is very small; being restricted
to the area surrounding the heart and gonads
Phylum Mollusca
Taxonomic Summary
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Phylum Mollusca
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Class Polyplacophora
Class Gastropoda
Class Bivalvia
Class Scaphopoda
Class Cephalopoda
Phylum Mollusca
Systems
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Skeletal- Mantle may secretes a shell. Use
hydrostatic pressure for ventral muscular
foot.
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Muscles -Ventral muscular foot and other
muscles present.
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Digestive- complete complex with salivary
glands, digestive gland and Rasping tongue
(Radula).
Systems
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Circulatory - Open except for Cephalopoda.
Dorsal heart, usually in a pericardial cavity.
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Respiratory - Ctenidia (gills) in mantle cavity,
respiratory pigment is copper.
Phylum Mollusca
Systems
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Excretory- by nephridia usually connecting to the
pericardial cavity,
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the coelom is usually reduced to the cavities of the nephridia,
gonads and pericardium.
Nervous - Nerve ring with various pairs of ganglia—
two pairs of nerve cords, one innervating the foot, the
other the visceral mass (modified ventral ladder-like
system)
Systems
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Integumentary – Mantle
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Endocrine - nervous systems produces
hormones.
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Reproductive - varied- monoecious,
protandric, or dioecious. Larva in marine =
trochophore and veliger, in freshwater clam
is glochidium.
Phylum Mollusca
Phylum Mollusca
Class Polyplacophora
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Phylum Mollusca
Class Polyplacophora
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Chitons
Phylum Mollusca
Chiton Lifestyles
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Found close to shore
mainly in the intertidal
where they live on hard
substrates
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Strong foot and low profile
help from being swept away
Phylum Mollusca
Ingestion and Digestion
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Radula is used to scrape
algae from the rocks
Mouth is anterior and
anus is posterior; linear
digestive tract
Phylum Mollusca
Phylum Mollusca
Class Gastropoda
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Phylum Mollusca
Class Gastropoda
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Defining characteristics
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Visceral mass and nervous system become twisted
90-180° during embryonic development
Largest and most varied group of the phylum
Mollusca
Phylum Mollusca
Gastropod Morphology and
Movement
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The typical snail
consists of a visceral
mass, which sits atop a
muscular foot
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The visceral mass is
protected by a univalve
shell that is coiled
Phylum Mollusca
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Phylum Mollusca
Prosobranchia Anatomy
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Molluscan gill
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Consists of a series of
flattened, triangular
sheets
Water is drawn into
the animal by gill cilia
Phylum Mollusca
Phylum Mollusca
Class Bivalvia
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Phylum Mollusca
Class Bivalvia
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Defining characteristics
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Two-valved shell
Body flattened laterally
This class contains clams, oysters, mussels, scallops,
and shipworms
Phylum Mollusca
Bivalves and You
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Many species are edible
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Also, an important food item for our
commercially important fish
Important to humans in their ability to strain
harmful bacteria out of polluted waters
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In today’s waters this can cause certain problems
since bivalves are filter feeders
Some can pump up to 40 liters per hour
Phylum Mollusca
Bivalve Locomotion?
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Most bivalves move by expansions and contractions of
the foot
Mussels usually are sessile and attach to substrate
with threads
Phylum Mollusca
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Phylum Mollusca
Bivalve shells
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Protein hinge ligament
stores energy required to
open the shell
Adductor muscles is
responsible for closing
the valves
The umbo is dorsal and
the valves are addressed
as left and right
Siphons are posterior
and the excurrent
siphons are always
dorsal
Phylum Mollusca
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Phylum Mollusca
The Mantle
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Phylum Mollusca
Bivalve Feeding
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Bivalves are filter
feeders
– Ctenidia, gills, are
used for feeding &
respiration and are
enlarged and folded
to increase surface
area
– Cilia on the gills
create a current to
bring food-laden
water into the mantle
cavity
Phylum Mollusca
Bivalve Reproduction
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Dioecious
The male sheds sperm into
the water column
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Sperm can enter the
female or fertilization can
occur in the water column
Phylum Mollusca
Phylum Mollusca
Class Scaphopoda
Class Cephalopoda
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Phylum Mollusca
Class Scaphopoda
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Defining characteristics
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Tusk shaped conical
shells, open at both ends
Development of anterior
thread-like adhesive
feeding tentacles
Elongated tapering
tubular shells that burrow
in sandy bottoms
Phylum Mollusca
Phylum Mollusca
Class Cephalopoda
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Phylum Mollusca
Class Cephalopoda
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Defining characteristics
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Closed circulatory
system
Foot modified to form
flexible arms and
siphons
Ganglia fused to form a
large brain encased in a
cartilaginous cranium
Phylum Mollusca
Cephalopod Characteristics
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The most highly organized of all the molluscs,
the foot is closely associated with the head
Nautilus, octopus, cuttlefish, and squids
Shell size varies from a full shell (nautilus) to a
vestige of a shell in the squid and octopus
Can be as large as 1,000kg. and 18m.
Phylum Mollusca
Squids
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10 tentacles.
Propels by pumping jets of water with the mantle
through an excurrent siphon.
Octopuses
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8 tentacles
They often crawl along the ocean bottom or lie in
wait for prey.
Chambered Nautiluses
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The only existing cephalopod with external shell.
Shell is coiled & divided into chambers.
The body is confined to the outermost chamber.
Defensive Mechanisms
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Since they are
susceptible to predation
they have evolved
chromatophores
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May posses hundreds or
thousands, all controlled
by the brain
An ink sac that is
associated with the
digestive system and
discharged out the anus
Phylum Mollusca
Sensory & Nervous System
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Eyes
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All but the nautilus have
image forming eyes that
are like mammals
Convergent evolution
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Phylum Mollusca
Reproduction
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Sexes are separate
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Males have one modified
arm (hectocotylus) for
sperm transfer
Sperm are enclosed in
packets called
spermatophores
Phylum Mollusca
Squid
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10 sucker bearing arms surround the mouth
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Large bites of food are removed and swallowed quickly
Form large schools and feed on crustaceans, and fishes
The shell is reduced and lies under the mantle, pen
Large folded gills and a circulatory system distribute
oxygen quickly
High oxygen demands are meet by the contraction of the
mantle cavity
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2 longer tentacular arms can shoot forward and capture prey
This method also removes waste products
Phylum Mollusca
Squid Internal Anatomy
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Phylum Mollusca
Squid Locomotion
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The foot forms the funnel
The upper end of the mantle
is extended into a pair of
triangular fins
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Water enters the mantle cavity at
the free end
When the mantle contracts the
edge is tightly sealed and water
is forced out the funnel
Can move very quickly
 5-10 m/s
Phylum Mollusca
Octopods
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Have 8 arms all equal in length
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Rows of suckers from base to tip
Compact body with a greatly reduced shell
The body is extremely flexible and assumes a streamlined shape
when swimming with squid-like jets through the funnel
Phylum Mollusca
Learning and Memory
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Benthic feeders on
bivalves, crabs and
fishes
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Some are pelagic and
spend their entire life
swimming, usually at great
depths
Amazing ability to learn
quickly and remember
for several weeks
Phylum Mollusca