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Mollusks Name: _________________________________________ Class:__________ Date: ____________ Vocabulary Term Definition chitin Tough carbohydrate found in the shells of animals and cell walls of fungi. mantle Fold of the outer skin, which secretes the external shell in some mollusks. mollusk Invertebrate that usually has a hard shell, a mantle, and a radula. radula Feeding organ found in mollusks with teeth made of chitin. What does this snail have in common with a clam? You might notice that both have a shell. That is one feature of the group they both belong to, the mollusks. Mollusks are a very diverse group. They include animals that live on land and in the ocean. With well over 100,000 species, there can be a lot of shells. What are Mollusks? When you take a walk along a beach, what do you find there? Sand, the ocean, lots of sunlight. You may also find shells. The shells you find are most likely left by organisms in the phylum Mollusca. On the beach, you can find the shells of many different mollusks including clams, mussels, scallops, oysters, and snails. Mollusks are invertebrates that usually have a hard shell, a mantle, and a radula. Their glossy pearls, mother of pearl, and abalone shells are like pieces of jewelry. Some mollusks, such as squid and octopus, do not have shells. Features of the Mollusk The Mollusk's body is often divided into different parts: • A head with eyes or tentacles. • In most species, a muscular foot, which helps the mollusk move. Some mollusks use the foot for burrowing into the sand, and others use it for jet-propulsion. • A mantle, or fold of the outer skin lining the shell. The mantle often releases calcium carbonate, which creates an external shell, just like the ones you find on the beach. The shell is made of chitin, a tough, semitransparent substance. A mass housing the organs. A complete digestive tract that begins at the mouth and runs to the anus. Most ocean mollusks have a gill or gills to absorb oxygen from the water. Many species have a feeding structure, the radula, found only in mollusks. The radula can be thought of as a "tongue-like" structure. The radula is made mostly of chitin. Types of radulae range from structures used to scrape algae off of rocks to the beaks of squid and octopuses.

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Page 1: Mollusks - Somma Science...they're found in all oceans of the world. Cephalopods are thought to be the most intelligent of invertebrates. They have eyes and other senses that rival

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.

Page 2: Mollusks - Somma Science...they're found in all oceans of the world. Cephalopods are thought to be the most intelligent of invertebrates. They have eyes and other senses that rival

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?

Page 3: Mollusks - Somma Science...they're found in all oceans of the world. Cephalopods are thought to be the most intelligent of invertebrates. They have eyes and other senses that rival

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.


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