Mollusca Study Guide

Mollusca

  • Clams, snails, slugs, octopus
  • 2nd most variety

Structure/advancement

General Structure
  • Bilateral symmetry
  • Cephalized
  • Triploblast
  • Coelomates: Fluid filled body cavity surrounded by mesoderm
    • Added movement
    • Protection of organ structure
      • Organs can move independently from muscles
    • Circulatory system
  • Alimentary canal
  • Trochophore: Larval stage of development
    • Sometimes hatches egg and becomes free-forming
    • Cillia at the ends and middle propel the trochophore and bring food to its mouth
Anatomy
  1. Head foot
    1. Head
      1. Mouth, sensory structures
    2. Foot
      1. Muscular organ used for movement
  2. Visceral mass
    1. Is located above the head foot
    2. Heart, digestive organs, excretion, reproduction
  • Mantle: layer of epidermis that covers the visceral mass
    • Secretes one or more hard shells made of calcium
      • These shells protect the body, but also reduce the amount of surface area for gas exchange
        • So, to increase the amount of gas exchange, they adapted Gills: Provide large surface area, and have lots of blood, which helps with exchanges of gas
          • Located in the Mantle cavity: the space between the mantle and visceral mass
    • OR is skin with chromatophores
      • Camo
  • Radula: The flexible tonguelike strip of tissue that is covered with backward teeth
    • Has many functions
  1. Terrestrial snails
    1. Use it to cut leaves
  2. Aquatic snails
    1. Scrape algae
    2. Drill holes into other organisms shells
  3. Core shell
    1. Harpoon-shaped radula
    2. Captures fish and injects venom

Hunting

  • Filter feeders/active hunters
  • Use radula to hunt (see above)

Digestion

Reproduction

Circulatory

  • Being a coelomate helps the circulatory system
    • A coelom creates a space where circulatory fluid can be transported without the interference from organs
  • Open circulatory system
  • Cannot pressurize circulatory fluid
  • Hemolymph: Their circulatory fluid
  • Hemocoel: The space where the hemolymph exits the tubes

Respiratory

  • Use gills

Nervous system

  • Ganglia: Nerve cell clusters
  • In head/foot and visceral mass, connected by two pairs of long nerve cords

Types

Gastropoda
  • Largest
  • Snails, abulones, conch (all have single shells)
  • Slugs, nudibranch (have no shells)
Structure
  • Torsion: Stage during larval development where the visceral mass twists 180 degrees
    • The mantle cavity, gills, and anus move to the front
      • Allow the head to go inwards
Circulatory system
  • Have open circulatory system
    • Circulatory fluid does not remain within vessels
  • Hemolymph: Circulatory fluid
  • Hemocoel: The cavity where hemolymph is stored, spaces between the tissues
  • Collects hemolymph from the the gills/lungs, pump through the heart, released into the hemocoel, goes back to heart with gills
Snails
  • On land, freshwater, ocean
Structure
  • Eyes on tentacles
    • These retract when they sense danger
Types
  • Aquatic snails: gills in the mantle cavity
  • Terrestrial snails: the mantle cavity acts as a modified lung
    • Mantle cavity: acts as a modified lung
      • Can only do this if the mantles membrane is moist
  • Slugs: Terrestrial snails without shells
  • Nudibranchs: Aquatic snails without shells
    • Covered with fingerlike extensions for more surface area
  • Pteropods: Flap their foot in a winglike motion to swim
Bivilia
  • Ex. Aquatic mollusks: clams, oysters, scallops
Structure
  • 2 halves (valves), with 3 layers each (secreted by mantle)
    • Connected by a hinge
      • Adductor muscles
    • 3 layers
      • Thin outer layer for protection
      • Thick middle layer to strengthen (calcium)
      • Smooth, shiny, inner layer to protect body
  • Most are sessile filter feeders
    • Do not radula
  • Extend foot into the sand and fill with hemolymph
  • No distinct head
  • Nervous system = 3 ganglia pairs connected with nerve cords
    • The nerve cells work with sensory cells on the edge of the mantle
      • Can cause it to open and close
  1. Mouth
  2. Digestive
  3. Foot
  • Some have eyes on mantle edge
Clams
  • Live buried in mud/sand
  • Sealed mantle - BESIDES siphons: hollow fleshy tubes
  • Cilia on the gills beat, drawing water in through the incurrent siphons, and leaving through the excurrent siphon
    • Making water circulate within the clam
    • Circulating water goes through gills and filter feeds
    • Circulating water exchanges gasses
  • Shed sperm and egg into water, and fertilize externally
    • Becomes a trochophore and then develops to an adult
Others
  1. Oysters: stick to hard surface
  2. Scallops: open and close with a snap to move
  3. Shipworm: drill into the wood of a ship, decomposes with the symbiotic bacteria in the intestine
Cephalopoda
  • Ex. octopus, squid, cuttlefish, chambered nautiluses
Structure
  • Tentacles with suction cups
  • Jaws that resemble a parrots beak
  • Chromatophores: Cells used to camouflage that are on the outer mantle
Hunting
  • Use ink to defend selves and camouflage with chromatophores
Reproduction
Separate sexes
  • the male uses a tentacle to transfer sperm from their mantle cavity to the females mantel cavity
  • fertilization happens in the females mantle cavity
  • She lays eggs and guards them till they hatch
  • SKIP THE TROCHOPHORE stage
Circulatory
  • Closed circulatory system
    • Can transport fluid rapidly
Nervous
  • Largest brain among the invertebrates
    • Divided into several lobes