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