Chapter 27-2
PHYLUM MOLLUSKA
Mollusks
THE WORD MOLLUSK MEANS
“SOFT BODY”
MOLLUSKS ARE SOFT BODIED INVERTEBRATES.
But these are hard shells.!!!!***
Where’s the “beef”???
ANIMAL
CLASSIFICATION
CHARACTERISTICS OF
MOLLUSKS
1. SOFT BODY COVERED BY A FOLD OF
TISSUE CALLED A MANTLE
2. HEAD AND A MUSCULAR FOOT FOR
MOVEMENT
4 Characteristics of Mollusks
All Mollusks have:
1. Muscular foot - used for locomotion
2. Mantle – thin, delicate tissue that covers the
body of the mollusk, like a cloak
3. Shell – usually CaCO3, and secreted by
mantle
4. Visceral mass – just below mantle,
containing the internal organs
Foot of a Clam
Foot, Shell, Mantle, Visceral Mass
are always Present in Mollusks
MOLLUSKS ARE GROUPED
INTO 3 CLASSES ACCORDING TO
THE TYPE OF FOOT
SHELL / NO SHELL
KIND OF SHELL
2 CLASSES WITH SHELLS
OUTSIDE THE BODY
UNIVALVE
(ONE SHELLED MOLLUSK)
BIVALVE
(TWO SHELLED MOLLUSK)
3RD CLASS HAS NO SHELL
•INSIDE
•OR
•OUT
OCTOPUS
SQUID
The All Important COELOM is alive and well
in the Mollusks.
Here’s why it’s important…
The development of a coelom allows for the
formation of more complex tissues and
organs.
It also allowed for a wide variety of body
architectures; which allows for coelomates to
grow larger.
The primary means of locomotion is via a
muscular FOOT.
Sometimes the foot is modified, but it is
always present.
Mollusks have a definite “head” region that
contains organs for chemo-sensory
functions.
All mollusks, except bivalves, possess a
rasping tongue organ called the RADULA.
Anatomy of the Rasping Tongue, or
Radula of a Snail
Function of the Radula
• A layer of flexible skin, with hundred of
sandpapery teeth, used to scrape algae off
of rocks
• In full-on carnivore mode, the radula acts
like a drill to pierce shells. These things
even have poison glands to make things
nastier Octopi and certain sea slugs do
this.
Bivalves are Usually Filter Feeders
• Think “Clam” when
Bleecker says Bivalve.
• Come on, say it out loud
now 
• Feathery Gills are
extended from Sessile,
or virtually motionless
bivalves to screen the
water for
microorganisms and
algae to munch on
Bleecker Swam up to one of these in
Hanauma Bay, on Oahu (Hawaii, people!)
Respiration
• Gills are used, and have many capillaries
to permit diffusion of O2 into blood stream
• Gills on the Nudibranch are particularly
cool 
Circulatory System
• Open Circulatory System – the blood does
not always travel in blood vessels, and
often deposits into spaces or sinuses to
drain back once organs have been bathed
with blood
• This works for clams and snails, but not
highly motile octopi or squid, which have
Closed Circulatory Systems
Excretion- Waste Management 101
• 2 openings in the
digestive tract
• Ammonia wastes
from protein digestion
can be toxic, and are
eliminated via
Nephridia, simple
tube shaped organs
leading out of the
body, via the skin.
Reproduction
• In a watery environment, egg and sperm are
often released in mass quantities in hopes of
fertilization. Larvae then swim to a spot on the
bottom and grow into bivalves, etc.
• In tentacled mollusks, modified tentacles are
used in Internal Fertilization.
• Spontaneous sex changes do occur, if there are
not enough members of a given sex. Holy cross
dressing Batman!
Classifying Mollusks Further
1. Class Gastropoda – stomach foot
– Broad muscular foot on stomach side
– Snails, Slugs, Nudibranchs
– Some shelled, others not
– Snail Shell shows RADIAL TORSION, creating the
distinct spiral shape
Two Shelled Mollusks – Class
Bivalvia
Oysters, Scallops,
Clams, Mussels
Free swimming
larvae
Mantle
manufactures the
shell
Pearls occur
when a sand grain
or foreign particle
is covered in
“mother of pearl”,
a material used to
keep the inside of
the shell smooth
The Pearl Emerges – Black is
Particularly Rare 
Classifying Mollusks Further
2. Class Cephalopoda – “head foot”
– High degree of CEPHALIZATION
– Octopi, Squid, Nautilus, Cuttlefish
– Tentacled and highly motile, with excellent senses,
particularly the eyes
– Originated 500 million years ago, in Cambrian Period
– No shell, but internal shell is present
– Jet propulsion
– Ink defense mechanism, Poisonous radula – ewww!
Bleecker-Vision!
The Nautilus
Bioluminescence! Complicated
Chemical Reactions!
Ecological Role
1.
2.
3.
4.
5.
Food source – clams, oysters, squid, octopi
Filter feeders remove pollutants
Indicators of pollution
Ships worn down by bivalves
Recyclers of waste or detritus