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Phylum Mollusca
Mollusk General Characteristics
• Are a very diverse phylum
– Most have a specific type of larvae
• Are all soft-bodied with an internal or
external shell
Body Plan
• Have a coelom (true body cavity), with a one
way digestive system
Are bilaterally symmetrical with 4 basic body
Foot- usually contains mouth and feeding parts
Mantle- tissue layer that covers most of the body
Shell- may be internal or external
Visceral Mass- contains the internal organs
Mollusk Body Plan
• Many contain a tongue shaped structure called a
radula that has hundreds of teeth
• Some have jaws surrounding radula
• Others use gills to filter-feed (eg bivalves)
• Aquatic mollusks use gills (ctenidia)
• Terrestrial mollusks use mantle cavity
(adapted to resemble a lung)
Internal Transport
• Many have an open circulatory system
• Faster moving mollusks have a closed circulatory
Elimination and Excretion
• Solid waste leaves through anus as feces
• Nitrogen waste is excreted by nephridia
• Greatly varies within the phylum
– Many have simple nervous systems (eg.
– Some have highly developed nervous systems
with a well developed brain (eg. octopuses)
• Octopuses have well developed eyes
• Sexes are separate in most
• Fertilization is external in most
Major Classes
1. Gastropods- have stomach next to foot
Move via a muscular foot on ventral side
Have one shell, a reduced shell, or no shell
Snails and slugs
Major Classes
2.Bivalves- have two shells
• Held together by one or two powerful muscles
• Most are sessile
• Clams, oysters, mussels, scallops
3.Cephalopods- have head next to foot
• Foot is divided into tentacles
• Most have small internal shells or no shells
• Octopuses, squids, cuttlefish, nautiluses
4. Polyplacophora- chitons
- bear a number of plates
27-1 Section Review
1. Mollusks are soft bodied animals with an
internal or external shell.
• Characterized by having a trochophore
larvae during life cycle
2. Effect of Mollusks on Humans
• Mollusks as food- snails, clams, oysters, scallops,
squid, etc (can cause illness during red tides).
• Mollusks as pests- snails and slugs damage
crops, shipworms drill through wood
• Used in biological research- eg. Cancer research
3. Uses of radulla
• Herbivores use radula to scrape algae off
rocks and twigs, or to eat plants
• Carnivores use radula to drill through the
shell of other animals
• Cone shells use radula as poisonous darts
4. Protection
• Some burrow or hide
• Some have well developed brains and rely on senses
and intelligence to avoid predation
• Some are fast swimmers e.g. sea butterfly or flap their
shells eg. scallops
• Some rely on tasting bad (or poisons) to predators eg.
Snails and slugs
• Some release ink eg. Squid
• Some are able to change colours to blend into
surroundings eg. Octopi