Mollusks are second only to arthropods in numbers of living species

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Phylum MolluscaCh 12 Molluscan Success

• Mollusks are second only to arthropods in numbers
  of living species

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Phylum Mollusca

• Mollusca Soft Bodied (Latin)

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• 555 Million Years Ago

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Molluscan Characteristics

• Range in size and body shape
• Largest ? 1000 lbs, 18m long (giant squid)
• 80 less than 5 cm

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Molluscan Characteristics

• Body of three regions head-foot, visceral mass,
  and mantle
• Mantle cavity functions in excretion, gas
  exchange and reproduction
• Bilateral symmetry
• Protostome development
• True coelomates (eucoelomates), reduced size
• Open circulatory system in all but cephalopods
• Radula present and used for scraping food

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Mollusk Body

• Three main regions
• 1. Head-foot
• Head (nervous structures) and foot (locomotion)
• 2. Visceral Mass
• Contain organs
• 3. Mantle

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Mollusk Body

• Mantle
• Usually attaches to the visceral mass and enfolds
  most of the body. May secrete the shell.
• Mantle Cavity
• Between the mantle and foot. Opens to the outside
  and functions in gas exchange, excretion and
  reproduction.
• Radula
• A rasping structure found in the mouth. Toothed
  tongue

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Class Gastropoda

• Stomach foot
• Largest and most varied class of mollusks
• 35,000 living species
• Examples snails, slugs, conch, and limpets
• Marine, freshwater, and terrestrial habitats

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Class Gastropoda

• Shell coiling whorl, apex
• Flattened foot for locomotion
• Cilia
• Muscular contractions
• Use radula for scraping food

• Most have shells calcium carbonate
• Torsion - 180 twisting of the visceral mass,
  mantle, and mantle cavity. Used to bring head
  into shell.
• Operculum closes shell opening.

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Class Gastropoda

• Maintenance Functions

• Gas exchange occurs in mantle cavity
• Open circulatory system
• 3 chambered heart
• Hydraulic skeleton fluid under pressure in body
• Some dioecious, some monoecious

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Class Bivalvia

• Two valves
• Second largest molluscan class 30,000 species
• Includes clams, oysters, mussels, and scallops
• Many edible, some form pearls

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Bivalve Characteristics

• Marine and freshwater
• No head or radula
• Most are filter feeders
• Adductor muscles used for defense
• Mantle attaches to shell around adductor muscles
• Pearl forms when sand lands between mantle
  adductor

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Bivalve Characteristics

• Foot projects from front end of animal, through
  the valves
• Foot used for burrowing
• Siphon neck used for intake of water and food
  and release of waste.
• Complete digestive tract
• Reduced nervous system

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Bivalve Shell and Shell Structures

• Two convex halves of the shell valves
• Umbo oldest part of shell near anterior end

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Class Bivalvia

• Maintenance Function

• Reproduction

• Cilia covered gills
• Cillia move water in and out of mantle
• Incurrent siphon
• Excurrent siphon
• Gills in mantle cavity for gas exchange
• Food trapping cilia move food toward mouth

• Most are dioecious, some monoecious
• Gonads in visceral mass
• External fertilization (most)
• Trocophore larvae

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Class Cephalopoda

• Head Foot
• Includes octopuses, squid, cuttlefish, and
  nautiluses
• Most complex mollusks/invertebrates
• Foot modified into a circle of tentacles and a
  siphon
• Shell reduced or absent in most
• Head in line with elongate visceral mass

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Cephalapod Shell

• Shell reduced or absent in all but nautilus
• Cuttlefish
• Shell is internal. Cuttlebone is used to make
  polishing powder and bird treats (for calcium)
• Squid
• Pen internal, chitinous structure
• Cartilaginous plates in mantle wall, head, neck
• Octopus
• Absent

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Class Cephalopoda

• Locomotion

• Maintenance Functions

• Predators
• Jet-propulsion system
• Muscle contractions
• Forces water out through narrow funnel
• Cuttlefish and squid very motile
• Octopus more sedentary

• Adhesive cups used for capture.
• Beaklike jaws and radula
• Complete digestive tract
• Closed circulatory system
• Complex nervous system
• Brain and Eye
• Chromatophores for defense (color change)
• Discharge of ink

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Cephalopod Reproduction

• Cephalopods are dioecious
• One tentacle of male is modified for
  spermatophore transfer
• Tentacles intertwine during copulation.
  Spermatophores burst open and eggs are released
  through the oviduct.
• Fertilized eggs attach to substrate

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Class Polyplacophora

• many plates
• Chitons
• Shallow marine waters
• Reduced head, flattened foot
• Shell divides into eight dorsal valves
• Crawl similar to gastropods

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Class Scaphopoda

• Boat foot
• Tooth shells or Tusk shells
• Burrowing marine animals
• Conical shell that is open at both ends
• Head and foot project from wider end

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Class Monoplacophora

• One plate
• Undivided, arched shell
• Broad, flat foot
• Repeated pairs of gills and foot-retractor
  muscles
• Thought to be extinct until 1952

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Class Aplacophora

• Without plate
• Lack a shell and crawl on ventral surface
• Body and nervous system similar to flatworms
• Most are surface dwellers on corals