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Mollusks
Section 29.1
Soft-bodied Animals
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Snails, slugs, oysters, clams, scallops,
octopuses, squid
A larval stage called a trochophore
Second largest phylum of animals
Found in almost all marine, freshwater
and land ecosystems
Three major classes:
Gastropods
Bivalves
Cephalopods
Key Characteristics
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Body cavity – a true coelom; in most species it is a
small area around the heart
Symmetry – bilateral
Three-part body plan – a.) visceral mass: central
section with organs, b.) mantle: heavy outer layer of
the body, c.) foot: muscular organ used for
movement
Organ systems – excretion, circulation, respiration,
digestion, reproduction
Shell – many have an exoskeleton of 1 or 2 shells
made of protein and calcium carbonate
Radula – most have a tongue-like organ in the
mouth with rows of pointed teeth to scrape food off
rocks
Organ Systems
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Excretion:
Body waste collects in the coelom
Cilia moves the fluid into nephridia
Fluid is filtered – useful molecules
returned to body & fluid wastes
leave
Organ Systems, continued…
Circulation:
 Most have a three-chambered heart
and an open circulatory system
 Octopuses and squid have a closed
circulatory system
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Organ Systems, continued…
Respiration:
 Most have gills located in the mantle
cavity
 Land snails have no gills – but the
mantle cavity acts as a primitive lung
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Organ Systems, continued…
Reproduction
 Most have males and females – some
snails and slugs are hermaphrodites
 Some species can change back and
forth between sexes
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Gastropods
“Stomach-foot”
 Snails and slugs
 Mainly marine, some in freshwater or
on land
 Most are herbivores
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Gastropod means ‘stomach-foot’
Gastropods have a radula
Bivalves
“Two shells”
 Clams, oysters, mussels, scallops
 Two shells are connected to adductor
muscle which can close shell and may
also help in swimming
 Most are filter feeders which use
tubes called siphons to remove food
from water
 Many of these are important food
sources for people
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Bivalve means ‘two-shells’
The sea scallop –
a typical bivalve
Bivalves have a muscular
foot for movement
Cephalopods
“Head-foot”
 Squids, octopuses, cuttlefish,
nautiluses
 Large head attached to tentacles
 Tentacles have suction cups or hooks
 The most intelligent of all
invertebrates
 Well-developed eyes similar to
vertebrates
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Cephalopods – ‘head-foot’
squid, chambered nautilus,
octopus
Cephalopods have welldeveloped eyes
Cephalopod have tentacles
with suckers