37-1 Mollusks

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37-1 Mollusks

•  Invertebrates like clams, snails, slugs and
  octopuses
•  Phylum mollusca
•  112,000 species. Some are predators, others are
  filter feeders
• Have bilateral symmetry

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Common Features
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Coelomates

•  Have a true coelom (hollow fluid filled cavity)
• Advantage is muscles of body wall are separate
  from those of the gut, so body wall muscles can
  contract without hindering the digestion process.
  Also transports blood

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Trochophore

•  Shared by aquatic mollusks and annelids
  during the larval stage
•  Contains cilia at both ends and in the middle
• Cilia contribute to dispersal of trochophore

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Body Plan of Mollusks

• 2 regions
• 1.Head-foot
• 2.Visceral mass
• 1. Head-foot (fig. 37-2)
• head, mouth, sensory structures,foot for
  locomotion
• 2. Visceral mass (fig. 37-2)
• heart, digestive organs, excretory organs,
  reproductive organs

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 Mantle

• Covers visceral mass, which secretes 1 or more
  hard shells made of calcium carbonate. Found in
  both sexes. Protects entire animal. The
  disadvantage is that the animal cannot exchange
  gases, so they had to evolve gills, which
  exchange gases with water. The gills are
  protected by the mantle cavity.

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Ganglia

•   Located in the head-foot region
• Connected by 2 pairs of nerve cords

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Radula

• A feeding adaptation. It is a flexible,
  tongue-like strip covered with abrasive teeth.
  (fig. 37-3)

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3 Classes of Mollusks

• 1. Gastropoda
•  Snails, abalones, conchs
• 2. Bivalvia
•  Clams, oysters, scallops
• 3. Cephalopoda
• Octopuses, squids, chambered nautilus
• Only mollusk to have closed
  circulatory system

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1. Class Gastropoda

•  Body undergoes torsion during larval stage.
  The visceral mass twists 180 degrees towards
  head, so everything like gills, mantle cavity and
  anus are near head. (fig. 37-4)
•  Have open circulatory system. A circulatory
  fluid called hemolymph passes through body into
  tissue spaces called hemocoels then the hemolymph
  goes back to the heart.

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Class Gastropoda

• Snails (land and aquatic, fresh water and salt
  water)
•  Aquatic snails respire through gills
•  Land snails respire through mantle cavity
  and are hermaphrodites
• Move around for food

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Abalone
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Conch
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2. Class Bivalvia

•  Shell is divided into 2 halves (or valves)
  connected by a hinge
•  Use adductor muscles to close shell
• Has open circulatory system

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Valves

•  Each valve contains 3 layers of cells
  secreted by mantle
• a. Outer layer protects shell against
  acidic conditions
• b. Middle layer is calcium carbonate
• c. Inner layer is smooth and protects soft
  body. This is what forms a pearl.

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Bivalves

• Bivalves are sessile
• They use muscular foot to dig into sand
  and become filter feeders
•  Only mollusk not to have a radula
•  Have 3 pairs of ganglia (nerves)
• near mouth
• near digestive tract
• near foot

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Clams

•  Buried in mud or sand
•  Use siphons
• 1. Incurrent siphon allows food to enter,
  sticks to mucus on gills which enters mouth.
  Gases are also exchanged and sperm also enters
  here.
• 2. Excurrent siphons allow all waste
  materials to exit the clam

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Reproduction

• Reproduction in fresh water clams vs. salt-water
  clams
• 1. Fresh water
•  Sperm enters incurrent siphon and
  fertilization is internal
• 2. Salt-water
• Egg and sperm are released into the water.
  Fertilization is external.

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Cephalopoda

• Chambered Nautilus

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• Show video
• Hand out clam terms and clam anatomy
• Dissect clam and define all terms