Phylum Mollusca

Phylum Mollusca Mrs. Adora G. Ilac,RN,MSBioMollusksMost are marine, some inhabit freshwaterMore than 150,000 known speciesSoft-bodied animals (from L. molluscus, soft) but most are protected by a hard shell made of calcium carbonateSome have reduced shells (slugs, squids, and octopuses) most of which are internal, or they have lost their shells completely during their evolution

VISCERAL MASSCoelomHeartReproductiveorgansDigestivetractShellRADULAMouthNervecordsKidneyMANTLEMantlecavityAnusGillFOOTMollusks

All have similar body plan3 main partsMuscular footVisceral massMantle

VISCERAL MASSCoelomHeartReproductiveorgansDigestivetractShellRADULAMouthNervecordsKidneyMANTLEMantlecavityAnusGillFOOT3 main partsMuscular foot- For movementVisceral mass- Containing most of the internal organsMantleA fold of tissue that drapes over the visceral massSecretes a shell (if one is present)

MollusksIn many, the mantle extends beyond the visceral mass, producing a water-filled chamber, the mantle cavityMany feed by using a straplike rasping organ called radula to scrape up foodHave separate sexes, with gonads (ovaries or testes) located in the visceral massSnails are hermaphroditesLife cycle (marine mollusks) includes a ciliated larva (trochophore)Major Classes of Phylum MolluscaClass and ExamplesMain CharacteristicsPolyplacophora (chitons)Marine; shells with eight plates; foot used for locomotion; head reducedGastropoda (snails, slugs)Marine, freshwater, or terrestrial; asymmetric body, usually with a coiled shell; shell reduced or absent in some; foot for locomotion; radula presentBivalvia (clams, mussels, scallops, oysters)Marine and freshwater, flattened shell with two valves; head reduced; paired gills; most are filter-feeders; mantle forms siphonsCephalalopoda (squids, octupuses, chambered nautiluses)Marine; head surrounded by grasping tentacles, usually with suckers; shell external, internal, or absent; mouth with or without radula; locomotion by jet propulsion using siphon made from mantleClass PolyplacophoraChitonsMarine animals with oval shapes Shells divided into eight dorsal platesCling to rocks along the shore during low tideThe foot acts as a suction cup, grips the rockGrazers that use radula to cut and ingest algae

Class GastropodaThe largest of the molluscan classesHas more than 40,000 living speciesMarine, but there are freshwater speciesMost distinctive characteristics: torsionTorsionDuring embryonic development, an asymmetrical muscle forms, and contraction of the muscle and uneven growth causes the visceral mass to rotate up to 1800, so that the anus and mantle cavity are places above the head in the adultAdvantage is to place the visceral mass and heavy shell more centrally over the snails body

Class GastropodaAre protected by single, spiraled shells into which the animals can retreat when threatenedShell is often conical, but abalones are limpets have somewhat flattened shellsMany have distinct heads with eyes at the tips of tentaclesMost used their radula to graze on algae or plant materialsSome are predators, and radula is modified for boring holes in the shells of other mollusks or tearing apart tough animal tissueClass GastropodaIn one group, the cone snails, the teeth of the radula form separate poison darts, which penetrate prey, including fishesAre among the few invertebrate groups to have successfully populated the landTerrestrial snails lack the gills, and instead the lining of mantle cavity functions as a lung, exchanging respiratory gases with the airClass BivalviaShells divided into two halvesTwo parts of the shell are hinged at the mid-dorsal line, and a powerful adductor muscles draw the two halves tightly together to protect the soft-bodied animalWhen the shell is open, the bivalve may extend its hatchet-shaped foot for digging or anchoring

Class BivalviaMantle cavity contain gills that are used for feeding as well as gas exchangeMost are suspension-feedersNo distinct head, and the radula is lost

RadulaMouthClass CephalopodaAre built for speed, an adaptations that fits their carnivorous dietUse beaklike jaws to bite their prey; then they inject poison to immobilize the victimMouth is at the center of several long tentaclesMantle covers the visceral mass, but the shell is reduced and internal (squids) or missing altogether (many octopuses)Shelled cephalopods: chambered nautilusesClass CephalopodaThe only mollusks with a closed circulatory systemHave well-developed nervous system with a complex brainHave well-developed sense organsShelled cephalopods called ammonites, some of them as large as truck tires, were the dominant invertebrate predators of the seas for hundreds of million years until their disappearance during mass extinction at the end of the Cretaceous period.