Phylum Mollusca

• ·       Phylum Mollusca includes Gastropods (snails and slugs), Bivalves (oysters and clams) and Cephalopods (octopuses and squid)

  ·       Bilateral symmetry with definite head

  ·       Most are marine, though some inhabit freshwater and terrestrial habitats

  ·       Soft-bodied with a shell for protection
• Body structure of gastropods

  • Major internal organs are in the visceral mass
  • Mantle covers and protects visceral mass, cells in the mantle secrete and produce shell
  • Space between visceral mass and mantle is the mantle cavity
  • Foot is very muscular to give rise to waves of contractions to propel animal forward
  • Mollusc has a distinct head – cephalisation
  • Mouth has radula inside it – made up of many sharp teeth
  • Osphradium contains chemoreceptors to monitor water the snail is going into
  • Operculum is the “door” located on the foot of the snail that closes up the mantle cavity when the snail retracts
• Torsion


  1. Changing position of mantle cavity as visceral mass twists 180 degrees during development

  2. Mantle cavity moves from posterior end to anterior end
• Mantle cavity in larvae


  • Located at the posterior end
• Mantle cavity in adults


  • Located at the anterior end
• Mantle cavity shifts

  1. Water resistance as snail moves forward causes water to rush into mantle cavity, causing it to open wider

  2. Snail can retract its head into the mantle cavity if threatened

  3. Shorter travelling distance of oxygen from gills to sensory neurons in the head

  4. Monitor water quality using osphradia
• Problem of mantle cavity shifting - the waste egested out causes fouling (excreta on mouth and sensory organs)
• How to solve fouling

  • Mantle cavity is asymmetrical, the right side is smaller than the left side
  • Right gill has disappeared, only left gill is left
  • Left gill expands and the size of the left side mantle cavity increases for accommodation of gill
  • Anus shifts towards right side
  • Water flows from left to right, guided by the cilia on the gills
  • Waste doesn't foul the gills
  • Removed right gill ensures no fouling because freshwater flows over left gill before flowing over anus
• Feeding of gastropods

  • Mouth contains radula – protrusible organ that can protract and retract
  • Repeated cycles of protraction and retraction results in a rasping motion that allows the teeth to scrap things
  • Algal grazers scrape algae off the ground
  • Radula can evolve to scrape flesh, get through shell and inject venom
  • Some snails bore through clams
  • Use expanded foot to surround clam to secrete proteases and acid to weaken shell
  • Scrape using radula until they form a hole in the shell
  • Snail then secretes digestive enzymes to partially digest them, then eat it up
  • Three possible ways to suck up flesh from clam shells
  • Suck from hole directly via radula if flesh is close to the hole
  • Open shell by squeezing it tightly
  • Proboscis goes into hole to suck up the flesh
• Cone snail


  A type of snail that has a radula and harpoon (tooth) to catch fast moving fish prey
• Radula of cone snail 


  • Every tooth is elongated
  • Pointed tooth pierces through the flesh of the prey
• Harpoon (tooth)


  Hollow to contain venom
• Harpoon (tooth)

  • Since the snail is a slow-moving predator, to catch fast moving fish prey, it needs to have a fast-acting venom
• Functions of the shell of gastropods is to protect against predators, mechanical injury and dehydration
• Snails can be classified as right-handed or left-handed depending on the coiling of their shell
• Helmet snail has a large shell, more energy required to make the shell, limited locomotion and body shape due to large shell
• General body structure

  • Mantle covers entire body of bivalve
  • Foot of bivalve is for burrowing into sand or mud
  • Edge of foot is like a blade (hatchet)
  • No distinct head
  • Bivalves filter feed using gills
  • Anterior end is where the protruding bit of shell is, ventral end is where foot is
  • Hinge of bivalve holds halves of the shells together
• How bivalves bury themselves into the sand/mud

  o   Clam extends foot to dig into sand/mud

  o   Tip of foot swells up, acting as an anchor to putt body into the sand/mud

  o   Muscles of the feet contract, drawing the clam forward
• Feeding process of bivalves

  • The labial palps inside the shell guide trapped food into the gills
  • Chemoreceptors and mechanical receptors receive signals so that only organic molecules of appropriate size enter the mouth
  • Two siphons – incurrent (ventral) siphon and excurrent (dorsal) siphon
  • Cilia on gills beat to draw water into incurrent siphon
  • Cilia pushes mucus on food groove containing food to go towards and into mouth
  • Incurrent siphon are projections to filter out big food particles
  • Gas exchange and filter feeding can occur due to extension of siphon even when buried
• Bivalves are soft-bodied animals and most are protected by a hard shell

  • The shell is a pair of laterally compressed valves, hinged and held together by hinge ligaments and adductor muscles
  • Adductor muscles are relaxed – shell is open
  • Adductor muscles are contracted – shell is closed
  • Prevent predators from entering, dehydration or mechanical injury
• Shell of molluscs

  • Periostracum is acid-resistant to protect bottom two calcium carbonate layers from acid erosion
  • Prismatic layer is densely packed with calcium carbonate in long prisms arranged parallel to one another and perpendicular to other layers
  • Nacreous layer is shiny and smooth, consists of calcium carbonate crystals arranged in sheets
  • Smooth because the mantle is in contact with it and rubs against it, minimise friction in order to protect mantle from abrasive injury
• Disadvantages of shells

  • Need a lot of energy and resources to synthesise and maintain shell
  • Restricting changes in body shape and type of body form
• Shells consist of a pair of laterally compressed valves which are hinged and held together by hinge ligaments

  • Hinge ligaments are tough protein fibres that connect the two valves
  • Hinge teeth and sockets ensure that valves don’t slide sideways
• Adductor muscles and hinge ligaments help open and close the shell

  • When adductor muscles contract, outer ligament stretches and inner ligament compresses to store elastic potential energy and keep the valves tightly shut
  • When adductor muscles relax, hinge ligaments recoil and release elastic potential energy to open valves
  • Closing of valves is controlled by adductor muscles, opening of valves is controlled by hinge ligaments
• Cephalopods have huge mantle cavity with fins that are modified mantle

  • Mantle contains radial and circular muscles that are antagonistic
  • When circular muscles contract and radial muscles relax, mantle contracts
  • When circular muscles relax and radial muscles contract, mantle relaxes
  • Fins are for steering and balance
• Space between head and mantle is the neck

  • When mantle relaxes, pressure drops and water is taken in via neck
  • When mantle contracts, water is pushed out of excurrent funnel to propel squid forward
• Siphon, arms and tentacles of cephalopods are modified feet
• Mantle of cephalopods forces out water from mantle cavity
• Ink forced out by muscles around the ink sac of cephalopods
• Excurrent funnel of cephalopods is muscular to change its position to propel itself – used for steering and direction
• Body of cephalopod is streamlined for less water resistance
• When cephalopod is alive, the tentacles are contracted inside the body and are extended only to get food

  o   Retracts back in to pull food

  o   Arms hold food in place for mouth to swallow it
• Shell of cephalopods is internalised and reduced

  • Reduces drag produced by external shell
  • Reduced shells means no need of much resources and energy to maintain
  • Pen is internal shell on dorsal side, provides support for streamlined shape of body
  • Body is lighter and more flexible due to internalisation, hence more locomotion possible
• Beak-like jaws of cephalopods are hardened with chitin and sharp to injure prey and tear food into chunks

  • Radula’s rasping motion scrapes off flesh from chunks
  • Brain is doughnut-shaped with oesophagus running through it, so if it accidentally swallows food to big then brain stretches and is damaged, thus it dies