STOP 6: PHYLUM MOLLUSCA
Dec 24, 2015
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