The phylum Mollusca is the second largest animal phylum. They are soft-bodied organisms that often have an external shell or plates. Their bodies generally have a head, muscular foot, and visceral hump or mass. They have a mantle that secretes the shell and covers the body. Molluscs live in nearly every habitat and have diverse feeding adaptations like the radula. Major classes include gastropods, bivalves, cephalopods, and polyplacophorans.

1. Phylum Mollusca
General characteristics and
classification
Numreen Nazir
Ph.D. Zoology

4. • A large, muscular foot variously modified for locomotion, digging,
attachment, and prey capture.
• Mantle, a highly modified epidermis that covers and protects the
soft body. In most species, the mantle also secretes a shell of
calcium carbonate.
• A visceral mass housing the internal organs.
• A mantle cavity, the space between the mantle and viscera. Gills,
when present, are suspended within this cavity.
• A radula, a protrusible, rasp-like feeding organ present in most, but
not all, species. In herbivorous mollusks (e.g., chitons and snails),
the radula is used for scraping algae from rocks. In carnivores, the
radula can be fang-like and is used for piercing prey (e.g., squids
and octopods), or may be pointed and used for drilling through
shells (e.g., some snails).

6. • This is the second largest animal phylum. Molluscs are
terrestrial or aquatic (marine or fresh water) having an
organ-system level of organization.
• They are bilaterally symmetrical, triploblastic and
coelomate animals.
• Body is covered by a calcareous shell and is unsegmented
with a distinct head, muscular foot and visceral hump. A soft
and spongy layer of skin forms a mantle over the visceral
hump.
• The space between the hump and the mantle is called the
mantle cavity in which feather like gills are present. They
have respiratory and excretory functions. The anterior head
region has sensory tentacles. The mouth contains a file-like
rasping organ for feeding, called radula.

7. • Molluscs are a clade of organisms that all have soft bodies
which typically have a "head" and a "foot" region. Often
their bodies are covered by a hard exoskeleton, as in the
shells of snails and clams or the plates of chitons.
• A part of almost every ecosystem in the world, molluscs are
extremely important members of many ecological
communities.
• They range in distribution from terrestrial mountain tops to
the hot vents and cold seeps of the deep sea, and range in
size from 20-meter-long giant squid to
microscopic aplacophorans, a millimeter or less in length,
that live between sand grains.

9. Morphology
Despite their amazing diversity, all molluscs share some unique
characteristics that define their body plan. The body has a head, a foot
and a visceral mass. This is all covered with a mantle (also known as
a pallium) that typically secretes the shell. In some groups, like slugs
and octopuses, the mantle is secondarily lost, while in others, it is
used for other activities, such as respiration.
The freshwater Sinistral Pond Snail (Physella sp.) scrapes algae from
the glass with its radula, the two "toothy" arcs you can see lining the
mouth.
• The buccal cavity, at the anterior of the mollusc, contains a radula
(lost in bivalves) — a ribbon of teeth supported by an odontophore, a
muscular structure. The radula is generally used for feeding. The
ventral foot is used in locomotion. This foot propels the mollusc by
utilizing muscular waves and/or cilia in combination with mucus.

10. • Typically, at least in the more primitive members of
each group, there are one or more pairs of gills (called
ctenidia) which lie in a posterior cavity (the pallial
cavity) or in a posterolateral groove surrounding the
foot. The pallial cavity typically contains a pair of
sensory osphradia (for smelling) and is the space into
which the kidneys, gonads, and anus open.
• Molluscs are coelomate, although the coelom is
reduced and represented by the kidneys, gonads, and
pericardium, the main body cavity which surrounds the
heart.

11. • Life history and ecology
 Molluscs occur in almost every habitat found on Earth,
where they are often the most conspicuous organisms. While
most are found in the marine environment, extending from
the intertidal to the deepest oceans, several major gastropod
clades live predominantly in freshwater or terrestrial
habitats.
 Remarkably, one study found around 3000 species within a
single locality at a coral reef in New Caledonia. In terrestrial
communities, gastropods can achieve reasonably high
diversity and abundance: as many as 60-70 species may
coexist in a single habitat and abundance in leaf litter can
exceed more than 500 individuals in four liters of litter.

14. • After their initial appearance, molluscan taxonomic
diversity tended to remain low until the Ordovician,
when gastropods, bivalves, and cephalopods show large
increases in diversity.
• For bivalves and gastropods this diversification
increases throughout , with relatively small losses at the
end-Permian and end-Cretaceous extinction events.
• Cephalopod diversity is much more variable through
the Phanerozoic, whereas the remaining groups
(monoplacophorans, rostroconchs, polyplacophorans,
and scaphopods) maintain low diversity over the entire
Phanerozoic or became extinct.

16. • There are four major groups within the phylum
Mollusca:
• Class Polyplacophora consists of chitons, snail-like
molluscs with eight-part overlapping scale shells .
• Class Gastropoda are true snails and slugs. They
represent the most diverse class within phylum Mollusca
with 60,000 to 80,000 extant species in marine,
freshwater, and terrestrial habitats.
• Class Bivalvia are molluscs with hinged two-part shells.
Examples include clams, oysters, mussels, and scallops.
• Class Cephalopoda are molluscs with large heads, large
eyes, and grasping tentacles. Examples include octopus,
squid, cuttlefish, and nautiloids.

17. • The foot is a muscular organ found in all molluscs.
Polyplacophorans and gastropods have a single flat
foot used for crawling.
• Some bivalves, such as clams, have a paddle-shaped
foot adapted for digging into soft sediments.

18. • Because a sea slug’s stomach is in its foot, it is
named Gastropoda, “stomach-foot” (from the Greek
root
words gastro meaning stomach and pod meaning foot;
The foot in octopus and squid is modified into many
tentacles that are attached to the animal’s head. That
feature gave the class its name Cephalopoda (from
the Greek root word cephal- meaning head), or the
“head-foot” molluscs. Octopus and squid use their
tentacles for moving and for grasping and holding the
prey they capture for food.

19. • In most molluscs, the mantle produces a hard protective
shell. The mantle also creates patterns of color on a shell.
The shell is an exoskeleton, even though it is completely
surrounded by soft tissue in some molluscs.
• The shell is continually produced and grows with the
animal. Chitons are in the
class Polyplacophora (poly meaning many; placo meaning
plate or shell; phora meaning bearing).
• A chiton’s mantle produces eight shell-like plates that cover
the body. Joints between the plates allow the chiton to curl
up in a ball and to move flexibly. The class of molluscs
called Bivalvia (from Latin root words bi- meaning two and
-valv meaning folding door) includes clams, oysters,
mussels, and scallops. Bivalves produce two shells that are
hinged at the top

20. • The mouth structures of many molluscs include a
specially adapted rasp-like tongue called a radula.
The radula is a hard ribbon-shaped structure covered in
rows of teeth.
• Herbivorous snails have a mouth with a radula of usually
five to seven complex teeth.
• There is a great diversity of radula forms in the mollusca.
• The snail uses its radula like a file, rasping it back and
forth over the substrate to scrape off small bits of food

21. • As radular teeth wear down or break off, new teeth
are formed to replace them.
• The tooth patterns of snail radulas are distinctive to
species, and scientists can identify snails by looking
at their radulas.
• Some radulas are highly specialized. A group of
gastropods called cone snails are carnivorous (meat-
eating) hunters that produce venom in glands near the
mouth. Their radulas are shaped into long, hollow
teeth, which they thrust one at a time into their prey
like harpoons

22. • Molluscs breathe with gills called ctenidia that sit in
a cavity between the mantle and body mass.
• In some molluscs, most notably bivalves like oysters
and mussels, the ctenidia are also used as filter
feeding apparatus to strain particulate food from the
water. Molluscs have a complete digestive tract
surrounded by a small coelom.

23. • The molluscan circulatory system is composed of a
series of blood sinuses or cavities, rather than closed,
discrete vessels.
• This is referred to as an open circulatory system.
• Molluscs display a large diversity of nervous
systems, from the rudimentary nervous system of the
brainless bivalves to the complex systems of the
cephalopods, who have well-developed brains and are
considered the most intelligent of invertebrates.

24. • Chitons (Polyplacophora) are basal relative to other
extant molluscs .
• Their soft bodies are covered with a series of eight
shell plates. The joints between these shell plates
enable to chitons to roll up for protection.
• Chitons are mobile and contract their muscular foot
in waves to move about.
• The primarily herbivorous chitons have a well-
developed radula.