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PHYLUM MOLLUSCA
PHYLUM MOLLUSCA

gastropods, bivalves,cephelapods
3 GENERAL BODY REGIONS
 1. head- with “brain” or sense organs
 2. visceral mass- contains internal organs
 3. foot- muscular part of body
CLASS GASTROPODA (STOMACH-FOOTED)
Soft body in CaCO3 shell secreted by mantle
 Body is bilaterally symmetrical
 Ventral muscular foot for locomotion
 Radula- rasping tongue, ribbon of small teeth
used to feed on algae
 “breathe” through gills

HI I’M GARY!
I’M
SPONGE
BOB’S
PET
SNAIL!
HEAD-FOOT REGION

The radula is a
rasping, protrusible
feeding structure
found in most
molluscs (not
bivalves).

Ribbon-like
membrane with rows
of tiny teeth.
CLASS GASTROPODA

The shell of a gastropod
is always one piece –
univalve – and may be
coiled or uncoiled.


The apex contains the
oldest and smallest
whorl.
Shells may coil to the
right or left – this is
genetically controlled.
CLASS GASTROPODA

Early gastropods had a
planospiral shell where
each whorl lies outside the
others.


Conispiral shells have each
whorl to the side of the
preceding one.


Bulky
Unbalanced
Shell shifts over for better
weight distribution.
CLASS GASTROPODA

Many snails can
withdraw into the
shell and close it off
with an operculum.
SNAILS
ABALONE
LIMPETS
CONCH
CONE SNAIL (car nivorous)
NUDIBRANCH (sea
slugs) gastropods that
have lost their shell
CLASS BIVALVIA
Clams, oysters, mussels
 Laterally compressed body enclosed in two
shells(valves) joined at a hinge, controlled by
strong muscles
 Gills used for getting 02 and filter feed
 No head or radula

CLAMS- use foot to burrow
in sediment. Uses siphon
to draw in water for O2 and
food.
Largest bivalve- giant
clam up to 3ft in
length!
(Tridacna gigas)
*symbiotic zooxanthella
help its size
MUSSELS- not burrowers, secrete strong
byssal threads to attach to surfaces
SCALLOPSsome can swim
by ejecting
water rapidly
out siphon
OYSTERS- cement their
shells to a hard surface,
sometimes to other
oysters
GEODUCK
PEARL OYSTERS- when
there’s an irritant or
parasite in the mantle,
shiny layers of nacre
coats it to form pearl in
some
* Most pearls we encounter are cultured pearls
CLASS BIVALVIA

Scallops have a row of small blue eyes along the
mantle edge.
CLASS BIVALVIA


Native freshwater clams in
the U.S. are jeopardized.
 Of more than 300
species once present,
12 are extinct, 42 are
endangered and 88
more are of concern.
 Sensitive to water
quality changes,
Zebra mussels are a
serious exotic invader into
the Great Lakes Region.
CLASS CEPHALOPODA (“HEAD-FOOTED”)
Octopuses, squids, cuttlefish
 Active lifestyles, agile swimmers
 Complex nervous system, eyes similar to ours
 Reduction/loss of shell
 Foot is modified into tentacles/arms with
suckers

NAUTILUS
Last living type of externally-shelled cephalopod
 Don’t exhibit many other squid/octopus-like
qualities
 “Swimming snail”


“Jet-propulsion” of water, controlled by flexible
siphon, for quick movement

Fans out body in defense
OCTOPUSES8 long arms, lack shell,
bite prey (crabs,
lobsters, shrimp) with
beak-like jaws
Distract predators by
emitting cloud of dark
fluid produced by ink
sac
SQUID- better swimmers, elongated body, two triangular
fins, 8 arms and two tentacles w/suckers circling mouth,
shell reduced to stiff pen
Colossal squid- largest
invertebrate, hooks on
tentacles
GIANT SQUID: up to 60ft in length,
teeth on suckers.
CUTTLEFISH: have calcified internal shell that helps
in bouyancy and gives its shape (cuttle bone)
Look like squid but have fin running along sides
CAMOUFLAGE
Chromatophores-Red, brown, yellow pigments
under skin controlled by muscles
 Light reflectors under skin
 Muscles able to change texture
