flat worms read - somma science...flatworms in the environment flatworms live in a variety of...

Flat Worms Name:_________________________________________Class:__________Date:____________Vocabulary

Term Definition

cephalization Concentrationofthesenseorgansandnervoustissueinaheadregion.

flatworm Invertebrateanimalswithbilateralsymmetryandanelongated,flattenedbody.gastrovascular cavity Internalspacethatisthesiteofdigestionanddistributionofnutrients.

scolex Thefrontendofatapeworm,bearingsuckersandhooksforattachment.

tapeworm

Parasitic flatworm, which lives in the intestines of vertebrates. Do worms have eyes?

Youmightthinkthatwormscan'tsee.Butsomeworms,suchastheaboveDugesiaflatworm,dohaveeyespots.Thesearenotexactlylikeyoureyes,however.Eyespotscanonlydetectlightintheirenvironment.Flatworms Theword"worm"isnotveryscientific.Butitisawordthatinformallydescribesanimals(usuallyinvertebrates)thathavelongbodieswithnoarmsorlegs.(Snakesarevertebrates,sotheyarenotusuallydescribedasworms.)Wormsarethefirstsignificantgroupofanimalswithbilateralsymmetry,meaningthattherightsideoftheirbodiesisamirroroftheleft.Onetypeofwormistheflatworm.Wormsinthephylum

Platyhelminthesarecalledflatwormsbecausetheyhaveflattenedbodies.Therearemorethan18,500knownspeciesofflatworms.Features of Flatworms

• Themaincharacteristicsofflatworms(Figurebelow)include:• Flatwormshavenotruebodycavity,buttheydohavebilateralsymmetry.Duetothelackofabody

cavity,flatwormsareknownasacoelomates.• Flatwormshaveanincompletedigestivesystem.Thismeansthatthedigestivetracthasonlyone

opening.Digestiontakesplaceinthegastrovascularcavity.• Flatwormsdonothavearespiratorysystem.Instead,theyhaveporesthatallowoxygentoenter

throughtheirbody.Oxygenenterstheporesbydiffusion.• Therearenobloodvesselsintheflatworms.Theirgastrovascularcavityhelpsdistributenutrients

throughoutthebody.• Flatwormshavealadder-likenervoussystem;twointerconnectedparallelnervecordsrunthelengthof

thebody.• Mostflatwormshaveadistinctheadregionthatincludesnervecellsandsensoryorgans,suchas

eyespots.Thedevelopmentofaheadregion,calledcephalization,evolvedatthesametimeasbilateralsymmetryinanimals.Thisprocessdoesnotoccurincnidarians,whichevolvedpriortoflatwormsandhaveradialsymmetry.

Flatworms in the Environment Flatwormsliveinavarietyofenvironments.Somespeciesofflatwormsarefree-livingorganismsthatfeedonsmallorganismsandrottingmatter.Thesetypesofflatwormsincludemarineflatwormsandfreshwaterflatworms,suchasDugesia.Othertypesofflatwormsareparasitic.Thatmeanstheyliveinsideanotherorganism,calledahost,inordertogetthefoodandenergytheyneed.Forexample,tapewormshaveahead-likeareawithtinyhooksandsuckers(knownasthescolex)thathelpthewormattachtotheintestinesofananimalhost(Figurebelow).Thereareover11,000speciesofparasiticflatworms.Structure and Function of Flatworms Flatwormsrangeinlengthfromabout1millimeter(0.04inches)tomorethan20meters(66feet).Theyhaveaflatbodybecausetheydonothaveacoelomorevenapseudocoelom.Theyalsolackarespiratorysystem.Instead,theircellsexchangegasesbydiffusiondirectlywiththeenvironment.Theyhaveanincompletedigestivesystem.Flatwormsreflectseveralmajorevolutionaryadvancesininvertebrates.Theyhavethreeembryoniccelllayers,includingmesoderm.Themesodermlayerallowsthemtodeveloporgansystems.Forexample,theyhavemuscularandexcretorysystems.Themuscularsystemallowsthemtomovefromplacetoplaceoversolidsurfaces.Theexcretorysystemletsthemmaintainaproperbalanceofwaterandsalts.Flatwormsalsoshowcephalizationandbilateralsymmetry.Flatworm Reproduction Flatwormsreproducesexually.Inmostspecies,thesameindividualsproducebotheggsandsperm.Afterfertilizationoccurs,thefertilizedeggspassoutoftheadult’sbodyandhatchintolarvae.Theremaybeseveraldifferentlarvalstages.Thefinallarvalstagedevelopsintotheadultform,andthelifecyclerepeats.Ecology of Flatworms Bothflukesandtapewormsareparasiteswithvertebratehosts,includinghumanhosts.Flukesliveinthehost’scirculatorysystemorliver.Tapewormsliveinthehost’sdigestivesystem.Usually,morethanonetypeofhostisrequiredtocompletetheparasite’slifecycle.Lookatthelifecycleoftheliverfluke.Asanadult,theflukehasavertebratehost.Asalarva,ithasaninvertebratehost.Ifyoufollowthelifecycle,youcanseehoweachhostbecomesinfectedsotheflukecancontinueitslifecycle.Notallflatwormsareparasites.Somearefree-livingcarnivores.Theyeatothersmallinvertebratesanddecayinganimals.Mostofthefree-livingspeciesliveinaquatichabitats,butsomeliveinmoistsoil.Summary

• Flatwormshavenotruebodycavityandnobloodvessels.• Flatwormscanbefree-livingorparasitic.Tapewormsareparasiticflatworms.

Review

1. Whatisaflatworm?2. Whatiscephalization?3. Howdoflatwormstransportoxygenandnutrients?4. Describetheflatwormnervoussystem.5. Whatisoneexampleofaflatworm?