STOP 6: 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 to Arthropoda

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

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

Characteristics

◦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

Characteristics

◦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)

Class Cephalopoda or Amphineura

◦Includes octopus, squid, cuttlefish, & chambered nautilus  

◦All marine

Class Cephalopoda

◦Most intelligent mollusk◦Well developed head◦Active, free swimming predators◦Foot divided into tentacles with suckers◦Use  their radula & beak to feed

Class Cephalopoda

◦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

Squid

* 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

2) Class Polyplacophora

◦All marine

◦Have a shell divided into 8 over-lapping plates

◦Live on rocks along seashore feeding on algae

3) Class Gastropoda

◦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

Snails◦Have open circulatory system* Secrete mucus & use muscular foot to move* Land snails are hermaphrodites* Aquatic snails have separate sexes* Use internal fertilization

Pteropods

◦Called "sea butterflies“◦Marine◦Have a wing-like flap for swimming

Oyster Drills◦Radula modified to drill into oyster shells

Nudibranchs

◦Marine slug◦Lacks shell

Class Bivalvia or Pelecypoda◦Sessile or sedentary

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

◦Filter feeders

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

Class Bivalvia

◦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

Class Bivalvia

◦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

Class Bivalvia

◦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

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