Mollusks

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Phylum Mollusca
Characteristics

• Soft-bodied invertebrate covered with protective mantle that may or may not form a hard, calcium carbonate shell

• Includes chitons, snails, slugs, clams, oysters, squid, octopus, & nautilus

• Second largest animal phylum

• Have a muscular foot for movement which is modified into tentacles for squid & octopus

• Complete, one-way digestive tract with a mouth & anus

• Have a fully-lined coelom

• Cephalization - have a distinct head with sense organs & brain

• Have a scraping, mouth-like structure called the radula

• Go through free-swimming larval stage called trochophore

Trochophore Larva

• Body organs called visceral mass lie below mantle

• Have circulatory, respiratory, digestive, excretory, nervous, & reproductive systems

• Bilaterally symmetrical

• Most have separate sexes that cross-fertilize eggs

• Gills between the mantle & visceral mass are used for gas exchange

• Includes 4 classes --- Polyplacophora (chitons), Gastropoda (snails, slugs, nudibranchs, conchs & abalone), Pelecypoda or Bivalvia (clams, oysters, & mussels), & Cephalopoda (squid, octopus, & nautilus)

SNAIL, CLAM, CHITON, & SQUID

Class Polyplacophora
Characteristics

• All marine

• Have a shell divided into 8 over-lapping plates

• Live on rocks along seashore feeding on algae

CHITON

Class Gastropoda
Characteristics

• Head has a pair of retractable tentacles with eyes located at the ends

• Have a single shell or valve (snails) or none (slugs)

• Known as univalves

• Snails 
  * May be marine, freshwater, or terrestrial
  * Aquatic snails breathe through gills & use their radula to scrape algae for food 
  * Terrestrial snails use their mantle cavity as a modified lung & saw off leaves
  * Retreat into shell in dry periods & seals opening with mucus
  * Have open circulatory system
  * Secrete mucus & use muscular foot to move
  * Land snails are hermaphrodites
  * Aquatic snails have separate sexes
  * Use internal fertilization

• Slugs
  * Live in moist terrestrial areas
  * Lack a shell

SLUG

• Pteropods
  * Called "sea butterflies"
  * Marine
  * Have a wing-like flap for swimming

"SEA BUTTERFLY"

• Oyster Drills
  * Radula modified to drill into oyster shells

OYSTER DRILL

• Nudibranch
  * Marine slug
  * Lacks shell

NUDIBRANCH

Class Bivalvia or Pelecypoda
Characteristics

• Sessile or sedentary

• Includes marine clams, oysters, shipworms, & scallops and freshwater mussels

• Filter feeders

• Have two-part, hinged shell (2 valves)

• Have muscular foot that extends from shell for movement

• Scallops clap valves together to move

• Shell secreted by mantle & made of 3 layers --- outer horny layer protects against acids, middle prismatic layer made of calcium carbonate for strength, & inner pearly layer next to soft body

• Mantle secretes substance called "mother of pearl" to surround irritants like grains of sand

• Oldest, raised part of shell called umbo

• Powerful anterior & posterior adductor muscles open & close shell

• Lack a distinct head

• Have an incurrent & excurrent siphon that circulate water over the gills to remove food & oxygen

INTERNAL CLAM ANATOMY

• Have heart & open circulatory system

• Nervous system made of 3 pairs of ganglia, nerve cords, & sensory cells that detect light, chemicals, & touch

• Separate sexes with external fertilization of eggs

Class Cephalopoda or Amphineura
Characteristics

•  Includes octopus, squid, cuttlefish, & chambered nautilus  

• All marine  

• Most intelligent mollusk

• Well developed head

• Active, free swimming predators

• Foot divided into tentacles with suckers

• Use  their radula & beak to feed

• Closed circulatory system

• Lack an external shell

• Highly developed nervous system with vertebrate-like eyes

• Separate sexes with internal fertilization

• Squid
  * Largest invertebrate is the Giant Squid
  * Large, complex brain
  * Ten tentacles with longest pair to catch prey
  * Use jet propulsion to move by forcing water out their excurrent siphon
  * Chromatophores in the skin can help change squid color for camouflage
  * Can squirt an inky substance into water to temporarily blind predators
  * Have internal shell called pen
  * Female lays eggs in jellylike material & protects them until hatching

GIANT SQUID

• Octopus
  * Eight tentacles
  * Similar to squid
  * Crawls along bottom looking for prey

OCTOPUS

• Chambered Nautilus
  * Has an exterior shell
  * Lives in the outer chamber of the shell
  * Secretes gas into the other chambers to adjust buoyancy

NAUTILUS

Economic Importance of Mollusks

• Used  by humans for food

• Pearls from oysters

• Shells used for jewelry

• Do crop & garden damage

• Serve as intermediate hosts for some parasites such as flukes