Mollusks and Segmented Worms

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MOLLUSKS

• Section 27 1
• Slugs, snails, and animal that once lived in
  shells in the ocean or on the beach.
• Phylum Mollusca

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• Oysters and mussels live firmly attached to the
  ocean floor or to the bases of docks or wooden
  boats.
• Squid and the octopus swim freely

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• 100 000 species
• Some have shells
• Bilateral symmetry
• True coelom
• Two body openings
• Muscular foot for movement

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How mollusks obtain food

• Radula located within the mouth of a mollusk,
  is a tongue-like organ with rows of teeth.
• The radula is used to drill, scrape, grate or cut
  food.

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Reproduction in Mollusks

• Most have separate sexes and reproduce sexually.
• Eggs and sperm are released at the same time into
  the water where external fertilization takes
  place.

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Larval stages

• Larval stages of all mollusks are similar.

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Nervous Control in Mollusks

• Simple nervous system with a brain and associated
  nerves that coordinate movement and behavior.

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Circulation in Mollusks

• Well-developed circulatory system with a
  three-chambered heart.
• Open circulatory blood is pumped through
  vessels and into open spaces surrounding organs.

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• Some mollusks (such as the octopus) has a closed
  circulatory system.

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Respiration in Mollusks

• Most mollusks have respiratory structures called
  gills.
• Gills increase the surface area through which
  gases can diffuse. They are an extension of the
  mantle.

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Excretion in Mollusks

• Oldest known animals to have evolved excretory
  structures called nephridia
• Nephridia organs that remove metabolic wastes
  from an animals body.

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Diversity of Mollusks

• Seven classes
• Gastropoda
• Bivalvia

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Gastropods

• One-shelled mollusks
• Largest class
• Stomach-footed (named for the way that the large
  foot is positioned under the body)

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• May be found in freshwater, saltwater, or moist
  terrestrial environments.

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Slugs

• No shell
• Body is protected by a thick layer of mucus.
• Nudibranchs colorful sea slugs

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Bivalves

• Two-shelled mollusks
• Clams, oysters and scallops
• Figure 27-7
• Most are marine, but a few a freshwater.
• Range in size from 1 mm to 1.5 meters

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• A ligament like a hinge connects the two shells.
• Filter feeders
• Cilia beat to draw water into an incurrent
  siphon.

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• The cilia also act as a sorting device.

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Cephalopods

• Head-footed mollusks
• All marine

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• The only cephalopod with a shell is the chambered
  nautilus.
• The cuttlefish has a reduced internal shell.
• Very complex structures
• .

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• The foot has evolved into structures with hooks,
  suckers or adhesive structures.
• The tentacles bring the prey to the mouth where
  it is bitten by the beak-like jaw.
• The food is then pulled into the mouth by the
  radula.
• Figure 27-8

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• Possess siphons that expel water.
• By expelling water forcefully, these mollusks can
  move quickly by jet propulsion.
• Squids can attain speeds of 20 meters per second
  with this method.
• They also expel ink to confuse and escape
  predators.

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