molluscs annelida

• Phylum: Molluscs


  Molluscs are a large phylum of invertebrate animals that have a soft, unsegmented body and usually a shell
• Molluscs


  • Inhabit terrestrial and aquatic environments
  • Share a mantle, foot, and visceral mass
  • Are protostomes
• Body structure of molluscs


  Soft bodied, typically have a head and foot region, often covered with hard exoskeleton from a few micrometres up to 25 m
• Body parts of molluscs


  • Mantle with a single cavity used for breathing and excretion
  • Visceral mass – digestive, circulatory, reproductive, respiratory organs
  • Head – sensory organs, mouth + radula
  • Muscular foot – locomotion, attachment
• Molluscs have bilateral body symmetry
• Body covering of molluscs


  • Epidermis with mucus glands
  • Periostracum – outermost layer, organic coating
  • Prismatic layer – consisting of crystals of calcium carbonate + protein
  • Nacreous layer – inner layer, organic-inorganic / nacre (mother of pearls)
• The body support in molluscs is an exoskeleton
• Muscular system in molluscs

  • Muscle fibres, propulsion
• Digestive system in molluscs


  Tubular, intracellular. Food is digested in the cecum
• Molluscs are coelomates
• Respiratory system in molluscs

  • Feathery gills or pallialal lungs
• Gas exchange in molluscs


  Through gills, where oxygen diffuses into the bloodstream from water and CO2 diffuses out
• Circulatory system in molluscs


  • Open, heart: 1 ventricle + 1-4 atria. Haemocyanin + haemolymph
• Haemocoel


  The primary body cavity of most invertebrates, containing circulatory fluid
• Excretory system in molluscs


  • Metanephridia (Organ of Bojanus - excretory glands that serve as kidney)
• Nervous system in molluscs

  • 2 or 3 nerve cords around a number of paired ganglia, brain encircles the oesophagus
• Sensory system in molluscs


  • Eyes and other sensors to detect chemicals (chemoreceptors), vibrations and touch, the foot carries statocysts
• Reproduction in molluscs


  Sexual, usually external fertilization, most hermaphrodites
• Development in molluscs


  Eggs - trochophore larva - complex veliger larvae or miniature adult - adult
• Classes in the phylum Mollusca


  • Chitons
  • Monoplacophora
  • Gastropoda
  • Bivalvia
  • Cephalopoda
  • Scaphopoda
• Class: Gastropoda

  Includes snails and slugs, found in marine, freshwater or terrestrial environments
• Body structure in gastropods


  • Head, muscular foot, visceral mass, shell +/- operculum or epiphragma (larval shell - protonoch). Snails distinguished by torsion (curve). Shell isn't permanently connected
• Operculum


  Lid over the shell
• Digestive system in gastropods

  Radula. No liver or pancreas, but developed hepatopancreas
• Circulatory system in gastropods

  • Open, 1 ventricle + 1 atrium. Haemocyanin + haemolymph
• Excretory system in gastropods

  • Metanephridium
• Sensory system in gastropods


  • Well defined head, 2-4 sensory tentacles with eyes, ventral foot (+propodium). Olfactory organs, eyes, statocysts, mechanoreceptors
• Class: Bivalvia

  Marine, freshwater molluscs with a two part shell enclosing their laterally compressed bodies
• Bivalvia

  • Oysters
  • Cockles
  • Mussels
  • Scallops
• Body structure in Bivalvia

  • Reduced head, NO radula, visceral mass, muscular foot – on the ventral side is the Byssus which produces Byssus threads (filaments) for attaching to the surfaces, 2 calcareous shells (CaCO3) connected by hinge (pánt) and umbo
• Digestive system in Bivalvia

  No mouth, mostly filter feeders through gills, phytoplankton
• Circulatory system in Bivalvia

  • 1 ventricle + 2 atria
• Excretory system in Bivalvia

  • Organ of Bojanus
• Nervous system in Bivalvia


  • Less complex, no brain, nervous network and series of paired ganglia
• Sensory system in Bivalvia

  • Less developed, mechanoreceptors, chemoreceptors, short tentacles, statocysts, mostly no eyes
• Reproduction in Bivalvia

  Gonochoristic (the opposite of hermaphroditic)
• Class: Cephalopoda

  Includes octopuses, squid, cuttlefish, marine animals (only 1 brackish species)
• Body structure in Cephalopoda

  • Bilateral symmetry, large prominent head, visceral mass, arm/tentacles
• Body covering in Cephalopoda

  Epidermis with chromatophores, no exoskeleton
• Body support in Cephalopoda is cartilage (cuttlebone)