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Mollusks
Phylum Mollusca
Mollusks
• Mollusk: soft-bodied
organisms that have
either an internal or
external shell
– Belong to the Phylum
Mollusca which means
“soft.”
– Found living in both
aquatic and terrestrial
environments
– Are invertebrates
Development
• Many mollusks share similar
developmental stages.
•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.
Body Plan
• The body plan of most
mollusks has four parts:
foot, mantle, shell, and
visceral mass
• The muscular foot takes
many forms for
crawling, burrowing or
capturing prey
Feeding
• Mollusks can be herbivores,
carnivores, filter feeders,
detritivores, or parasites.
• Have a structure called a
radula that is used to obtain
food
– Can be used to scrape
food off of the groundsnails and slugs
– Can be used to
penetrate and rip flesh
like octopus and squid
Respiration
• Aquatic mollusks breathe
using gills inside their
mantle cavity.
• However in some
freshwater snails their gills
are reduced, instead leaving
the snail the thin wall of the
mantle cavity to assimilate
oxygen through. This organ
is called the snails' lung
Gills
Circulation and Excretion
• In an open circulatory system,
blood is pumped through vessels
by a simple heart.
Heart
• Nephridia remove ammonia from
the blood and release it outside
the body.
Nephridia
Mollusks
• Groups of Mollusks
– A) Gastropods
• Shell-less or single shelled mollusks that move by a muscular foot
located on the ventral (bottom) side
– Ex: snails and slugs
– B) Bivalves
• Have two shells that are held together by
powerful muscles
– Ex: clams, scallops, and oysters
– C) Cephalopods
• Mollusk in which the head is attached to a
single foot that is usually divided into
tentacles
– Ex: octopus and squid