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Phylum Mollusca
Introduction:
• Meaning – soft body
• Examples – clams, oysters, mussels, scallops,
snails, slugs, squid, octopus, limpets
• Habitat – fresh and marine water / slugs and snails
can be terrestrial
• One of the largest of all animal phyla – 50 000 –
110 000 living species (depending on who is
counting)
Unique Characteristics:
• Body that is divided into 3 regions:
1. Head – absent in some
- have a mouth, appendages, and sensory
organs
2. Foot – muscular; in squid and octopus it is
modified into tentacles
3. Mantle – tissue that functions to: surround and
protect internal organs and secretes the shell
Unique Characteristics
Other unique parts:
• Mantle Cavity – a space that houses the molluscan
gills, and is closed off in terrestrial slugs and
snails and made into lungs.
• Radula – teeth like structures in the esophagus that
are used for feeding. (not present in bivalves)
Abalone
Radula
The Radula
Radulas from different mollusks
are adapted for different functions.
Some radulas are adapted for
scraping algae; others are adapted
for scraping flesh.
These are electron micrographs
from different mollusk species.
Gumboot chiton
– showing the
foot of the
chiton
Common Characteristics:
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Bilateral symmetry
3 cell layers
Coelom
Complete digestive system with a digestive
gland surrounding the stomach
Classification:
• Are several classes of mollusks:
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Class Polyplacophora
Class Bivalvia
Class Gastropoda
Class Cephalopoda
Class Polyplacophora: Chitons
• Chitons can be identified by having a shell that is
formed from a series of 7 to 8 separate plates.
Because the shell is multisectioned, the body can
bend to conform to a wide variety of underlying
substrate shapes.
Class Bivalvia
• Includes clams, oysters, mussels, scallops
• Have “2 valves”
Class Gastropoda
• Means “belly foot” or “stomach foot”
• Eg: marine snails, nudibranchs, slugs, and
terrestrial snails, limpets
Olive
Snail
Purple-ring
Top Snail
Class Cephalopoda
• Meaning - “head foot”
• Eg – squid and octopus
• Modified foot into tentacles and/or arms
Giant Squid Beak
Giant Squid radula, found
inside the beak
The suckers of a giant
Pacific Octopus. The
largest octopus on record
was over 23 feet long!
Octopus Beak
You will learn that molluscs have open circulatory
systems. This is NOT true for cephalopods. Cephalopods
have closed circulatory systems. This allows them to be
larger and much faster than all others.
LIFE ACTIVITIES OF
CLASS BIVALVIA – A CLAM
The Anatomy of A Clam
Ingestion, Digestion, and
Elimination
• Incurrent siphon contracts bringing in water
and plankton  through mantle  gills
• Mucus in gills traps food, cilia move the
food (gills have cilia)  mouth
• Palps (‘fleshy-lips’) around mouth sort the
food and pass it onto the mouth
• Food then moves down the esophagus into
the stomach where digestion occurs
Ingestion, Digestion, and
Elimination
• Digestive glands secrete enzymes for
digestion
• Nutrients enter intestine and are absorbed
• Wastes pass into the rectum  through the
anus  excurrent siphon (solid wastes exit)
Excretion
• 2 kidneys filter N-wastes from the body and
the fluid surrounding the heart  outside
mantle cavity  excurrent siphon (liquid
wastes exit)
Respiratory System – Gills are
used for:
• For gas exchange
• To filter and trap food in aquatic molluscs
• Snails and slugs have a highly vascularized
mantle cavity that is otherwise modified to
form a lung
The Valve - AKA Shell
• Made up of CaCO3, produced by the mantle
for protection and camouflage
• The bivalve shell is made up of 2 pieces
Note: Not all mollusks have shells, many mollusks, such as
octopus, slugs and nudibranchs have lost their shells
The Circulatory System
• Open system in all molluscs except for
Cephalopods which have a closed system
• Consists of blood vessels, heart and sinuses
(open spaces), the blood is NOT confined to
vessels
• Nutrients and O2 in blood are pumped by
heart into sinuses where body tissues /
organs are bathed
Reproduction
• Most mollusks are dioecious, with external
fertilization
• Most mollusks have a larval stage
• Depending on species, fertilization may internal
• Slow-moving species like snails may be
hermaphrodites
• Some slugs may exhibit interesting but strange
reproductive behaviour eg. apophallation
Ecological / Economic
Importance
• Food source for marine, freshwater, and terrestrial
animals
• Host symbiotic algae and parasites (eg – sheep
liver fluke has an intermediate host in the snail)
• Feed on plants and animals to clean up their
environment and can also provide habitats /
shelters (eg – hermit crabs live in gastropod shells)
Ecological / Economic
Importance
• Commercially sold for food
• “Ship worms” / burrowing bivalve causes
damage to wooded boats and docks
• Some cause sickness and death due to
bivalves eating dinoflagellates – RED TIDE
• Snails and slugs can cause crop / garden
damage
Evolution
• It is believed that molluscs share an
ancestor with annelid worms.
• Both most likely evolved from a primitive
flatworm.
• The molluscan evolutionary pathway and
how all of the organisms are related is still
relatively unknown.
• DNA sequencing will give further detail