Mollusks

~ 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 larvae

~ 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)

...................................................................... Clam and chitons

............................................................................. Squid

.............................................................................Snail

Class Polyplacophora

Characteristics
~ All marine
~ Have a shell divided into 8 over-lapping plates
~ Live on rocks along seashore feeding on algae

.................................................................. Chitons

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 fertilizationSlug

~ life in moist terrestrial areas
~ lack a shell

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

Oyster Drills
* Radula modified to drill into oyster shells

Nudibranch
* Marine slug
* Lacks shell
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
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

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

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

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