Mollusca III: Bivalves

• Bivalves


  • Larger mantle cavity
  • No cephalization
  • No radula or osphradium
  • Most are marine, burrowing, suspension feeders (pulling in a flow of water hoping there's food in water)

  
• Ctenidium


  Basis of traditional classification, always gas exchange, sometimes feeding
• Siphons


  • Water flow in and out
• Foot


  • Locomotion, burrowing (shovel like function)
• Palps


  • Touch every article that comes into bivalve and figure out if it's food or not food, then they sort the particles, food transport to mouth
• Mantle


  • Secrets shell (umbo is oldest, ventral edge is newest)
• Circulatory system

  • Heart but no vessels
• Protobranch:

  rounded sheets or long-thin filaments, adjacent sheets attached by non-motile cilia
• Particle Sorting and Filtering

  1. Particles landed on gill filaments are covered with cilia and cilia transports particles to ventral and dorsal food grooves (heavy to ventral, light/food to dorsal)

  2. Particles moved along grooves to palps for more sorting

  3. Palps sort by size, also food vs sediment (food quality)

  4. Food passed to mouth and gut, rejects deposited in mantle and periodically expelled (pseudofeces)
• Style and gastric shield

  • Mortar and pestle to grind food
• Style sac


  • Secretes style crystals, secretes digestive enzymes, turns style via cilia
• Diverticula


  • Intestine-like for digestive efficiency
• Midgut and intestine


  • Nutrient absorption
• Mantle

  • Only surface for respiration (capillaries)
• Septibranch


  • Highly modified gills, septa with ciliated openings, motion of septa pumps water through siphons, traps small organisms, stomach with cheating grinds food, carnivorous
• Digestion of wood requires cellulose - digesting bacteria (and protists) in specialized sac
• • Gill sheet is W-shaped, blood vessels and food groove, some ciliary junction, some tissue junctions
  • Eulamellibranch: gill sheet is W-shaped, blood vessels and food groove, all tissue junctions