Phylum Mollusca

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Phylum Mollusca
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General Information

• More than 112,000 species!!!!!!!
• Literally means soft body
• However, most have a hard internal or external
  shell.
• Some are sedimentary filter feeders while others
  are predators with complex nervous systems.
• Bilateral symmetry
• Examples
• Snails, conchs, slugs, clams, oysters,
• squid, octopus.

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Mollusks are Coelomates

• A hollow cavity surrounded by mesoderm.
• Body wall muscles are separated from the gut
  muscles.
• This gives mollusks an advantage
• They are able to contract the body wall muscles
  without hindering the movement of food through
  the gut.

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Coelomates Contd.

• This also provides a space for transporting blood
  without any interference from other internal
  organs.

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All Mollusks Share The Feature Of

• Trochophore larva.
• Usually free swimming.
• Cilia on the surface of the larva propel it
  through the water and assist in drawing food in.
• They are carried in ocean currents and tides.
• This contributes to species dispersal.
• Mollusks share the feature of a trochophore larva
  with annelids.

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Trochophore larva
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Mollusk Body Plan

• Consists of two regions
• Head-foot
• Visceral Mass
• Head-Foot
• Head mouth and a variety of sensory organs.
• Foot large, muscular organ used for locomotion.

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Body Plan Contd

• Visceral Mass
• Heart, digestive, excretory, and reproductive
  systems.
• Protected by the mantle (outer layer of
  epidermis).
• This mantle secretes 1 to 2 hard shells of
  calcium carbonate.
• Gills were adapted to overcome this and a mantle
  cavity protects them.
• The mantle cavity is the space between the mantle
  and the visceral mass.

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Nervous System

• Ganglia paired clusters of nerve cells
  connected by 2 pairs of long nerve cords which
  control muscles responsible for movement and
  feeding.
• Found in both the head-foot region and the
  visceral mass.
• They also control sensory cells that respond to
  light, touch, and chemicals.

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The class Cephalopoda has the most advanced
nervous system of any invertebrate.
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Feeding

• Radula
• Feeding adaptation
• Flexible, tongue-like strip of tissue covered
  with tough, abrasive teeth that point backward.
• Terrestrial mollusks use them to scrape leaves.
• Aquatic mollusks use them to scrape algae or
  drill holes in the shells of other mollusks.
• Some marine mollusks use radula like a harpoon
  that inject venom.

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Radula
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Common chiton
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Blue ringed octopus
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Chambered nautilus
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Giant clam
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Yellow nudibranch
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Flame scallop
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Flamingo tongue cowrie
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Atlantic squid
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Snail slug