Mollusks Name:_________________________________________Class:__________Date:____________Vocabulary

Term Definition

chitin Toughcarbohydratefoundintheshellsofanimalsandcellwallsoffungi.

mantle Foldoftheouterskin,whichsecretestheexternalshellinsomemollusks.mollusk Invertebratethatusuallyhasahardshell,amantle,andaradula.

radula Feedingorganfoundinmolluskswithteethmadeofchitin.What does this snail have in common with a clam?

Youmightnoticethatbothhaveashell.Thatisonefeatureofthegrouptheybothbelongto,themollusks.Mollusksareaverydiversegroup.Theyincludeanimalsthatliveonlandandintheocean.Withwellover100,000species,therecanbealotofshells.What are Mollusks? Whenyoutakeawalkalongabeach,whatdoyoufindthere?Sand,theocean,lots

ofsunlight.Youmayalsofindshells.TheshellsyoufindaremostlikelyleftbyorganismsinthephylumMollusca.Onthebeach,youcanfindtheshellsofmanydifferentmollusksincludingclams,mussels,scallops,oysters,andsnails.Mollusksareinvertebratesthatusuallyhaveahardshell,amantle,andaradula.Theirglossypearls,motherofpearl,andabaloneshellsarelikepiecesofjewelry.Somemollusks,suchassquidandoctopus,donothaveshells.Features of the Mollusk TheMollusk'sbodyisoftendividedintodifferentparts:

• Aheadwitheyesortentacles.• Inmostspecies,amuscularfoot,whichhelpsthemolluskmove.Somemollusksusethefootfor

burrowingintothesand,andothersuseitforjet-propulsion.• Amantle,orfoldoftheouterskinliningtheshell.Themantleoftenreleasescalciumcarbonate,which

createsanexternalshell,justliketheonesyoufindonthebeach.Theshellismadeofchitin,atough,semitransparentsubstance.

A mass housing the organs. Acompletedigestivetractthatbeginsatthemouthandrunstotheanus.Mostoceanmolluskshaveagillorgillstoabsorboxygenfromthewater.Manyspecieshaveafeedingstructure,theradula,foundonlyinmollusks.Theradulacanbethoughtofasa"tongue-like"structure.Theradulaismademostlyofchitin.Typesofradulaerangefromstructuresusedtoscrapealgaeoffofrockstothebeaksofsquidandoctopuses.

Evolution of Mollusks MollusksareprobablymostcloselyrelatedtoorganismsinthephylumAnnelida,alsoknownassegmentedworms.Thisphylumincludestheearthwormandleech.Scientistsbelievethesetwogroupsarerelatedbecause,whentheyareintheearlystageofdevelopment,theylookverysimilar.Mollusksalsosharefeaturesoftheirorgansystemswithsegmentedworms.Unlikesegmentedworms,however,mollusksdonothavebodysegmentation.Thebasicmolluskbodyshapeisusuallyquitedifferentaswell.What's the world's largest mollusk? Thecolossalsquid,oneofthelargestinvertebrates,measures14feetinlengthhere.Someofthesesquidsareevenlargerandcangrowuptoalmost50feetlong!Thesmallestmollusksaresnailsthataremicroscopicinsize. Types of Mollusks Thereareapproximately160,000livingspeciesandprobably70,000extinctspeciesofmollusks.Theyaretypicallydividedintotenclasses,ofwhichtwoareextinct.Themajorclassesoflivingmollusksincludegastropods,bivalves,andcephalopods(Figurebelow).Gastropods

Gastropodsincludesnailsandslugs.Theyusetheirfoottocrawl.Theyhaveawell-developedhead.Therearemanythousandsofspeciesofseasnailsandseaslugs,aswellasfreshwatersnails,freshwaterlimpets,landsnailsandlandslugs.Gastropodsliveinmanydiversehabitats,fromgardenstodesertsandmountains.Theyalsoliveinrivers,lakesandtheocean.Mostshelledgastropodshaveaone-pieceshellthatistypicallycoiledorspiraled,butnotallgastropodshaveshells.Gastropodshavenosenseofhearing,buttheycanseeandhaveakeensenseofsmell.Inland-basedgastropods,theolfactoryorgans(forsmell)arethemostimportant.Thesearelocatedonthetentacles.Bivalves

Bivalvesincludeclams,scallops,oysters,andmussels.Astheirnameimplies,theyhavetwopartsoftheirshell,whichcanopenandclose.Bivalvesliveinbothmarineandfreshwaterhabitats.Mostbivalveshaveapairoflargegillsthatenablethemtoextractoxygenfromthewater(tobreathe)andtocapturefood.Waterisdrawnintothebivalveandwashesoverthegills.Mucusonthegillshelpscapturefoodandciliatransferthefoodparticlestothemouth.Onceinthemouth,foodpassesintothestomachtobedigested.Bivalveshaveamouth,heart,intestine,gills,andstomach,butnohead.Bivalveshaveamuscularfoot,whichinmanyspeciessuchasclams,isusedtoanchortheirbodytoasurfaceordigdownintothesand.Cephalopods

Cephalopodsincludetheoctopusandsquid.Theyhaveaprominentheadandawell-developedbrain.Typicallythefoothasbeenmodifiedintoasetofarmsortentacles.Membersofthisclasscanchangecolor.Theycanalsochangetextureandbodyshape,and,andifthosecamouflagetechniquesdon'twork,theycanstill"disappear"inacloudofink.Cephalopodshavethreeheartsthatpumpblueblood,they'rejetpoweredbytheirmuscularfoot,andthey'refoundinalloceansoftheworld.Cephalopodsarethoughttobethemostintelligentofinvertebrates.Theyhaveeyesandothersensesthatrivalthoseofhumans.Manycephalopodsareactiveandefficientpredators.Whatfeaturesdoyouthinkallowsforthis?

Summary

• Themolluskbodyoftenhasaheadwithtentacles,amuscularfoot,afeedingorgancalledtheradula,andacompletedigestivetract.

• Molluskshaveamantle,whichoftensecretesanexternalshell.• Mollusksaremostcloselyrelatedtosegmentedworms.• Mollusks are divided into ten living classes, including the familiar gastropods, cephalopods, and bivalves. • Mollusks live in marine and freshwater habitats, as well as on land.

Review

1. Whatisamollusk?2. Describethreemaincharacteristicsofmollusks?3. Whatevidenceshowsthatmollusksandsegmentedwormsarerelated?Howarethey

different?4. Namefiveexamplesofmollusks.5. Whathabitatsdogastropodslivein?6. Whatisthedefiningfeatureofabivalve?7. Whatmolluskisthroughtobeveryintelligent?8. Describethefootofagastropod,bivalve,andcephalopod.