Mollusks
Niño A.Crisolo
II-BSE

What Is a Mollusk?
Mollusks are soft-bodied animals that usually
have an internal or external shell.
Mollusks include snails, slugs, clams, squids,
and octopi.
Many mollusks share similar developmental
stages.

Wide size range (slugs – giant squids)
Many shared characteristics among
creatures that appear to be very diverse.
Two body parts head-foot & visceral
mass
-Two body parts head-foot & visceral mass
-Mantle covers visceral mass, secretes shell
-Bilateral symmetry
-Reduced coelom contains only heart, gonads,
& nephridia
-Most have open circulatory system

Many aquatic
mollusks
have a free-swimming
larval stage called a
trochophore.
The trochophore
larva is
also characteristic of
annelids, indicating
that
these two groups may
be closely related.

Form and Function in Mollusks
Mollusks have true coeloms surrounded by
mesoderm tissue.
They have complex, interrelated organ
systems that function together to maintain the
body as a whole.

What is the basic body plan of mollusks?
Body Plan
The body plan of most mollusks has four
parts: foot, mantle, shell, and visceral
mass.

The muscular foot takes many forms
flat structures for crawling
spade-shaped structures for
burrowing
tentacles for capturing prey
Snail
Clam
Squid

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.

Feeding
Mollusks can be herbivores, carnivores, filter
feeders, detritivores, or parasites.

Snails and slugs feed
using a flexible,
tongueshaped
structure known
as a radula.
Hundreds of tiny teeth
are attached to the
radula.
The radula is used to
scrape algae off rocks
or to eat the soft tissues
of plants.

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
excurrent siphon.
Respiration
Aquatic mollusks breathe using gills inside their
mantle cavity.
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
opposite direction.

Circulation
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
through
different sinuses.
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
circulatory systems.
Faster-moving mollusks have a closed
circulatory system.
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
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”

Gastropods include
pond snails
land slugs
sea butterflies
sea hares
Limpets
nudibranchs

G
A
S
T
R
O
P
O
D
A

Bivalves
Common bivalves include:
• clams
• oysters
• mussels
• scallops
Bivalves have two shells that are held together by
one or two powerful muscles.

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

Bivalve Shell
• 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
ligament
• Ventral side has “teeth” that fit together and keep
the halves lined up properly
• Umbo = oldest part of shell at anterior end;
swollen appearance

Cephalopods
• Name comes from Latin & Greek
– Cephalic = “head”; podos = “foot”
• Most complex class of mollusks
• Includes octopuses, squid, cuttlefish, chambered
nautilis
• Anterior portion of foot modified into tentacles
– Tentacles arranged in a circular pattern
– Used for capturing prey, attachment, movement,
copulation

Cephalopods
 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
external stimuli.
Cephalopods distinguish shapes by sight
and texture by touch.
The eyes of many cephalopods are
complex.

CEPHALOPODS
• SQUID
• OCTOPUS

General Facts
50,000 – 200,000 species (many remain
undiscovered)
• 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.