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  • flat worms read - Somma Science...Flatworms in the Environment Flatworms live in a variety of environments. Some species of flatworms are free-living organisms that feed on small organisms
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Flat Worms Name: _________________________________________ Class:__________ Date: ____________ Vocabulary Term Definition cephalization Concentration of the sense organs and nervous tissue in a head region. flatworm Invertebrate animals with bilateral symmetry and an elongated, flattened body. gastrovascular cavity Internal space that is the site of digestion and distribution of nutrients. scolex The front end of a tapeworm, bearing suckers and hooks for attachment. tapeworm Parasitic flatworm, which lives in the intestines of vertebrates. Do worms have eyes? You might think that worms can't see. But some worms, such as the above Dugesia flatworm, do have eyespots. These are not exactly like your eyes, however. Eyespots can only detect light in their environment. Flatworms The word "worm" is not very scientific. But it is a word that informally describes animals (usually invertebrates) that have long bodies with no arms or legs. (Snakes are vertebrates, so they are not usually described as worms.) Worms are the first significant group of animals with bilateral symmetry, meaning that the right side of their bodies is a mirror of the left. One type of worm is the flatworm. Worms in the phylum Platyhelminthes are called flatworms because they have flattened bodies. There are more than 18,500 known species of flatworms. Features of Flatworms • The main characteristics of flatworms (Figure below) include: • Flatworms have no true body cavity, but they do have bilateral symmetry. Due to the lack of a body cavity,flatworms are known as acoelomates. • Flatworms have an incomplete digestive system. This means that the digestive tract has only one opening. Digestion takes place in the gastrovascular cavity. • Flatworms do not have a respiratory system. Instead, they have pores that allow oxygen to enter through their body. Oxygen enters the pores by diffusion. • There are no blood vessels in the flatworms. Their gastrovascular cavity helps distribute nutrients throughout the body. • Flatworms have a ladder-like nervous system; two interconnected parallel nerve cords run the length of the body. • Most flatworms have a distinct head region that includes nerve cells and sensory organs, such as eyespots. The development of a head region, called cephalization, evolved at the same time as bilateral symmetry in animals. This process does not occur in cnidarians, which evolved prior to flatworms and have radial symmetry.

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Page 1: flat worms read - Somma Science...Flatworms in the Environment Flatworms live in a variety of environments. Some species of flatworms are free-living organisms that feed on small organisms

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.

Page 2: flat worms read - Somma Science...Flatworms in the Environment Flatworms live in a variety of environments. Some species of flatworms are free-living organisms that feed on small organisms

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?


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