What Is a Mollusk?
Mollusks are soft-bodied animals that usually
have an internal or external shell.
Mollusks include snails, slugs, clams, squids,
Many mollusks share similar developmental
Wide size range (slugs – giant squids)
Many shared characteristics among
creatures that appear to be very diverse.
Two body parts head-foot & visceral
-Two body parts head-foot & visceral mass
-Mantle covers visceral mass, secretes shell
-Reduced coelom contains only heart, gonads,
-Most have open circulatory system
have a free-swimming
larval stage called a
also characteristic of
these two groups may
be closely related.
Form and Function in Mollusks
Mollusks have true coeloms surrounded by
They have complex, interrelated organ
systems that function together to maintain the
body as a whole.
What is the basic body plan of mollusks?
The body plan of most mollusks has four
parts: foot, mantle, shell, and visceral
The muscular foot takes many forms
flat structures for crawling
spade-shaped structures for
tentacles for capturing prey
The mantle is a thin layer of tissue that covers
most of the mollusk's body.
The shell is made by glands in the mantle that
secrete calcium carbonate.
Just beneath the mantle is the visceral mass,
which contains the internal organs.
Snails and slugs feed
using a flexible,
as a radula.
Hundreds of tiny teeth
are attached to the
The radula is used to
scrape algae off rocks
or to eat the soft tissues
Clams, oysters, and scallops use gills.
Food is carried by water, which enters
the incurrent siphon.
A siphon is a tube like structure through
which water enters and leaves the body.
The water flows over the gills and leaves by the
Aquatic mollusks breathe using gills inside their
As water passes through the mantle cavity,
oxygen in the water moves into blood flowing
through the gills.
At the same time, carbon dioxide moves in the
Some mollusks have open circulatory systems;
other mollusks have closed circulatory systems.
In an open circulatory system, blood is pumped
through vessels by a simple heart.
Blood leaves the vessels and works its way
Blood passes from the sinuses to the gills, where
oxygen and carbon dioxide are exchanged. Blood
is then pumped back to the heart.
Slow-moving mollusks often have open
Faster-moving mollusks have a closed
A closed circulatory system can transport
blood through an animal’s body much more
quickly than an open circulatory system.
Groups of Mollusks
Gastropods (SNAILS, WELKS)
Bivalves (CLAMS, OYSTERS, MUSSELS)
Cephalopods (OCTOPUS & SQUID)
The three major classes of mollusks are
Gastropods are shell-less or single shelled
mollusks that move by using a muscular foot
located on the ventral side.
Many gastropods have a single shell that
protects their bodies.
When threatened, they can pull completely
into their shells.
Name comes from Greek
– Gaster = “gut”, podos = “foot”
Class Bivalvia (CARD)
Name comes from Latin
– Bis = “twice”; valva = “leaf”
• 2nd largest class of mollusks
• Includes clams, oysters, scallops, mussels
• Many are edible
• Virtually all marine or freshwater
– Eat via filter feeding
• The two halves of the shell are called valves
– Shell forms in one piece during development = single
structure, not a pair of shells
– Shell secreted by mantle mantle secretes extra
protein at the dorsal area that will become the hinge
• Ventral side has “teeth” that fit together and keep
the halves lined up properly
• Umbo = oldest part of shell at anterior end;
• Name comes from Latin & Greek
– Cephalic = “head”; podos = “foot”
• Most complex class of mollusks
• Includes octopuses, squid, cuttlefish, chambered
• Anterior portion of foot modified into tentacles
– Tentacles arranged in a circular pattern
– Used for capturing prey, attachment, movement,
Cephalopods are typically soft-bodied mollusks in which
the head is attached to a single foot. The foot is divided
into tentacles or arms.
Cephalopods have eight or more tentacles equipped with
sucking disks that grab and hold prey.
Most modern cephalopods have only small internal
shells or no shells at all.
The only present-day cephalopods with external
shells are nautiluses.
Cephalopods have complex sense organs
that help them detect and respond to
Cephalopods distinguish shapes by sight
and texture by touch.
The eyes of many cephalopods are
50,000 – 200,000 species (many remain
• Evolved ~500 million years ago.
• Oldest resemble today’s Nautilis.
• Fossilize well – hard shell
• Mollusk shell grows entire life – rings like a tree trunk
• Many land snails can lift 10x’s own weight.
• 1 acre of cultivated mussels can produce
~10,000 pounds of meat/year – 500x’s more that an acreof
pasture can produce beef.