4.8 Mollusca

• Mollusks

  • Soft bodied animals that typically have an external or internal shell
  • Examples include snails, slugs, squids, cuttlefish and octopi
  • Coelomates
  • Bilateral symmetry and cephalization
  • Share similar developmental stages, many have a larval stage called trochophore
  • The trochophore larva has the characteristics of annelids which suggests a common ancestor (Embryological evidence)
• Body plan of most mollusks

  • Foot: Can help with crawling, burrowing, and catching prey (tentacles)
  • Mantle: Thin layer of tissue that covers the body (like a cloak)
  • Shell: Glands of the mantle that secrete calcium carbonate
  • Visceral mass: Consists of internal organs
• Feeding & Digestion of mollusks

  • Herbivores (Some snails and slugs): Use their radula to scrape algae off rocks or eat plants
  • Carnivores (Octopi or some slugs): Use radula to drill through shells of other animals, some have sharp jaws (octopi)
  • Filter feeders (Clams, Oysters): Have feathery gills that water passes through, water enters through the Incurrent siphon, the gills have sticky mucus that trap plankton
  • Parasites (Some snails)
• Radula


  Used by mollusks to scrape algae off rocks or drill through shells of other animals
• Open circulatory system

  Mollusks like snails and clams have open circulatory systems and sinuses
• Closed circulatory system

  Mollusks like octopi and squids have closed circulatory systems
• Sinus

  A cavity or space in the body
• Respiration in aquatic mollusks

  Typically breathe using gills in their mantle cavity
• Respiration in land mollusks

  Do not have gills, respire using a mantle cavity with a large surface area lined with blood vessels, this lining must be kept moist for diffusion
• Respiration in land mollusks


  Affects where they live
• Excretion in mollusks

  Cells release waste into the blood as ammonia, tube-shaped nephridia remove ammonia and release it outside the body
• Mollusks have nephridia, which is familiar from another phylum
• Nervous system complexity in mollusks

  • Clams and Oysters have simple nervous system, small ganglia near the mouth, a few nerve cords, simple sense organs (eyespots, chemical receptors)
  • Octopi have well developed brains and are capable of complex tasks
  • Squids have chromatophores that enable them to camouflage
• Complex tasks Octopi can do

  Not specified
• Movement in mollusks


  • Some move using their foot and secreting mucus to slide on
  • Some use jet propulsion, such as octopi
• Reproduction in mollusks


  • Snails and clams reproduce sexually using external fertilization, releasing eggs and sperm into the water, then developing into larvae
  • Tentacled mollusks reproduce sexually using internal fertilization
  • Some are hermaphrodites
• Groups of mollusks

  • Gastropods: Examples - Pond snails, land slugs, sea butterflies, shell-less or single shelled, move using a muscular foot, some can protect themselves by pulling completely into their shells
  • Bivalves: Examples - Clams, oysters, mussels, scallops, have two shells held together by powerful muscles, mostly stay in one place or burrow, are filter feeders
  • Cephalopods: Examples - Octopi, squids, cuttlefishes and nautiluses, soft bodied mollusks with their head attached to a single foot which then divides into tentacles or arms, often have 8 or more tentacles with sucking disks that capture prey, have complex sense organs and can distinguish shapes and texture