Phylum Molluska

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Phylum Molluska

• Over 50,000 species
• Coelomate, Protostomes, Bilaterally symmetrical
• Mostly marine, although some live in fresh water
  or land.
• Generally soft-bodied with a hard shell
• However, squid octopi have internalized,
  reduced or lost their shells.
• Body Plan
• Although phenotypically diverse, mollusks share 3
  common features
• A muscular foot - usually for movement
• A visceral mass - including internal organs
• A mantle - envelopes visceral mass and secretes a
  shell

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Molusk General Anatomy
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Class Polyplacophora (chitons)

• Marine animals - shells divided into 8 plates
• Move using a muscular foot and graze using a
  scraping mouthpart called a radula

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Chiton Anatomy
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Chitons
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Class Bivalvia

• Include clams, oysters and mussels
• Have shells divided into 2 halves, connected by
  strong muscles
• Has hatchet shaped foot for digging and anchoring
• Are typically filter feeders drawing in water end
  expelling it through a siphon tube
• Generally sessile, however, some species can
  quickly move by rapidly closing it's shell

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Bivalve Anatomy
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Bivalves
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Class Gastropoda

• Include snails, abalone, and slugs
• Largest class of mollusks 40,000 species
• Exhibit embryonic process of torsion
• Visceral mass twists 1800 during development
• Most species are herbivores, but some have
  modified radullas for boring into other mollusks
• Some species have successfully colonized land

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Gastropods
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Giant African Land Snail
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Class Cephalopoda

• Unlike other mollusks, these are quick predators
• Have beak-like jaws and poison to catch
  immobilize prey
• Beak is located _at_ the center of several long
  tentacles
• Foot has been modified to form a siphon which
  allows most species to move quickly using a jet
  of water.
• Only mollusks with a closed circulatory system
  (blood is always contained in vessels)
• Very highly developed nervous system
• Can learn complex behaviors
• Octopi have learned to open jars to retrieve prey
• May have rudimentary communication
• Cuttlefish and octopi may communicate by skin
  coloration

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Cephalopod Anatomy
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Octopus
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Squid
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Cuttlefish
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Cambered Nautilus