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Phylum Mollusca

  • Soft-bodied invertebrate covered with protective mantle that may or may not form a hard, calcium carbonate shell
  • Includes chitons, snails, slugs, clams, oysters, squid, octopus, & nautilus
  • Second largest animal phylum
  • Have a muscular foot for movement which is modified into tentacles for squid & octopus
  • Complete, one-way digestive tract with a mouth & anus
  • Have a fully-lined coelom
  • Cephalization – have a distinct head with sense organs & brain
  • Have a scraping, mouth-like structure called the radula
  • Go through free-swimming larval stage called trochophore

Trochophore Larva

  • Body organs called visceral mass lie below mantle
  • Have circulatory, respiratory, digestive, excretory, nervous, & reproductive systems
  • Bilaterally symmetrical
  • Most have separate sexes that cross-fertilize eggs
  • Gills between the mantle & visceral mass are used for gas exchange
  • Includes 4 classes — Polyplacophora (chitons), Gastropoda (snails, slugs, nudibranchs, conchs & abalone), Pelecypoda or Bivalvia (clams, oysters, & mussels), & Cephalopoda (squid, octopus, & nautilus)


Class Polyplacophora

  • All marine
  • Have a shell divided into 8 over-lapping plates
  • Live on rocks along seashore feeding on algae


Class Gastropoda

  • Head has a pair of retractable tentacles with eyes located at the ends
  • Have a single shell or valve (snails) or none (slugs)
  • Known as univalves
  • Snails
    * May be marine, freshwater, or terrestrial
    * Aquatic snails breathe through gills & use their radula to scrape algae for food
    * Terrestrial snails use their mantle cavity as a modified lung & saw off leaves
    * Retreat into shell in dry periods & seals opening with mucus
    * Have open circulatory system
    * Secrete mucus & use muscular foot to move
    * Land snails are hermaphrodites
    * Aquatic snails have separate sexes
    * Use internal fertilization

  • Slugs
    * Live in moist terrestrial areas
    * Lack a shell


  • Pteropods
    * Called “sea butterflies”
    * Marine
    * Have a wing-like flap for swimming


  • Oyster Drills
    * Radula modified to drill into oyster shells


  • Nudibranch
    * Marine slug
    * Lacks shell


Class Bivalvia or Pelecypoda

  • Sessile or sedentary
  • Includes marine clams, oysters, shipworms, & scallops and freshwater mussels
  • Filter feeders
  • Have two-part, hinged shell (2 valves)
  • Have muscular foot that extends from shell for movement
  • Scallops clap valves together to move

  • Shell secreted by mantle & made of 3 layers — outer horny layer protects against acids, middle prismatic layer made of calcium carbonate for strength, & inner pearly layer next to soft body
  • Mantle secretes substance called “mother of pearl” to surround irritants like grains of sand
  • Oldest, raised part of shell called umbo
  • Powerful anterior & posterior adductor muscles open & close shell
  • Lack a distinct head
  • Have an incurrent & excurrent siphon that circulate water over the gills to remove food & oxygen


  • Have heart & open circulatory system
  • Nervous system made of 3 pairs of ganglia, nerve cords, & sensory cells that detect light, chemicals, & touch
  • Separate sexes with external fertilization of eggs

Class Cephalopoda or Amphineura

  •  Includes octopus, squid, cuttlefish, & chambered nautilus  
  • All marine  




  • Most intelligent mollusk
  • Well developed head
  • Active, free swimming predators
  • Foot divided into tentacles with suckers
  • Use  their radula & beak to feed
  • Closed circulatory system
  • Lack an external shell
  • Highly developed nervous system with vertebrate-like eyes
  • Separate sexes with internal fertilization

  • Squid
    * Largest invertebrate is the Giant Squid
    * Large, complex brain
    * Ten tentacles with longest pair to catch prey
    * Use jet propulsion to move by forcing water out their excurrent siphon
    * Chromatophores in the skin can help change squid color for camouflage
    * Can squirt an inky substance into water to temporarily blind predators
    * Have internal shell called pen
    * Female lays eggs in jellylike material & protects them until hatching


  • Octopus
    * Eight tentacles
    * Similar to squid
    * Crawls along bottom looking for prey


  • Chambered Nautilus
    * Has an exterior shell
    * Lives in the outer chamber of the shell
    * Secretes gas into the other chambers to adjust buoyancy


Economic Importance of Mollusks

  • Used  by humans for food
  • Pearls from oysters
  • Shells used for jewelry
  • Do crop & garden damage
  • Serve as intermediate hosts for some parasites such as flukes