Phylum PlatyhelminthesThe “flat” “worms”

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Section21.4

PhylumPlatyhelminthes(flatworms):Planarians,flukes,andtapeworms

FlatwormsHaveBilateralSymmetry

Figure21.2

Section21.4 Figure21.12

Keyfeatures

FlatwormsHaveBilateralSymmetry

Section21.4 Figure21.12

Diversity

FlatwormsHaveBilateralSymmetry

Flatworm: ©Leslie Newman & Andrew Flowers/Science Source; planarian: ©Carolina Biological Supply Company/Phototake; fluke: ©Volker Steger/Science Source; tapeworm: ©Biophoto Associates/Science Source

ClassTurbellariaincludes:Planarians

ClassCestodaIncludes:Tapeworms

ClassTrematodaIncludes:Flukes

Section21.4 Figure21.13

Anatomy

FlatwormsHaveBilateralSymmetry,DorsoventrallyFlattened,andlackaskeleton

• Theyhavethreegermlayers:• Ectoderm: Outside• Endoderm: Inside• Mesoderm:Middlelayeroftissuebetweentheectodermandtheendoderm

• ThoughFlatwormshavethreegermlayerstheyareacoelomates andhavenobodycavity(thinkofyour

normalsandwich)

• Flatwormsdohavetrueorgans andprimitiveorgansystems thatareusedfordigestion andexcretion

• Flatworms are also the most primitive organisms to show cephalization.

• The head region of Flatworms contains a concentration of nerve tissue called ganglia (singular=ganglion) that resembles a primitive brain.

Cephalization

¨ Theyhavetwonervecords thatrunfromtheganglia intheheadregionalongtheventral sideofthewormtothetaleregion

¨ IntheheadregionofFlatwormsaretwoeye-spots¨ Theyalsohavelobesonthesideoftheirhead

calledauricles(Primitiveear)

Auricles

Feeding• Flatwormshaveamouth/anus whichisconnectedtothegastrovascularsystem throughalongmuscularpharynx.

• Smallinvertebratesortheremainsofdeadanimalsaretakenintothemouth/anusbythemuscularpharynx

• Thefoodisthendigestedinthehighlybranchedgastrovascularcavity

• Thenutrientsmovesfromthegutintothebodycellsbydiffusion

Respiration

• Respirationoccursbydiffusion duetoacoelomatebodyplan(superflat)

InternalTransport• Nutrients andoxygen inthegutaresimplyabsorbedintothebodycellsbytheprocessofdiffusion

Excretion

• Mostundigestedfoodisreleaseddirectlyoutoftheanus/mouth:Pharynx

• Aflamecell isaspecializedexcretorycell• Flamecellsfunctionlikeakidney,removingwastematerials.• Thebeatingofciliaresemblesaflame,givingthecellitsname.

NerveResponse¨ Flatwormsareabletosenseandrespondtoatleast

threeformsofstimuli:¡ Senseandrespondtolight:Theeye-spots candetectlightandallowtheFlatwormstorespondtoit

¡ Senseandrespondtochemicals: Pits onthesideoftheirheadregionscansensechemicals inthewaterandallowtheFlatwormstorespond(like“smelling”)

¡ Senseandrespondtotouch:Theauricles oneithersideoftheheadregioncansensetouch andallowtheFlatwormstorespond

• Theganglia intheheadregionrelaymessagesfromthesensoryorgansdownthenervecords totherestofthebody.Thenervecords cancontrolmuscles inthebodywhichallowtheFlatwormstomove oreat.

Auricles

Movement

• Theflatwormsmoveacrossasurfaceusingcilia ontheirventralsurface

• Theycanalsomovebycontractingcircular andlongitudinalmusclesthatlayjustbelowtheectoderm.Thesemusclesarecontrolledbythenervecords.

Reproduction

• AsexualReproduction:• Flatwormscanasexuallyreproducethroughaprocesscalledfission.Theanterior andposterior endsholdasurfaceandthemidsectionconstricts.Thisresultsintwonewflatworms,onefromtheanterior endoftheoriginalflatwormandtheotherfromtheposterior endoftheoriginalflatworm.

Reproduction• Flatwormscanalsoregenerate partstheyhavelost.

Regenerationvideo

Planaria Regeneration

• Planarians will spontaneously detach the tail end of their bodies and each half will regenerate into a full size flatworm

• Planarians can be cut either transversally (shown above) or dorsally and most will regenerate into a full size worm

Super Cool Fact: the smallest piece of planarian to ever regenerate in a lab into a new planarian was 1/279th of a planarian! That’s approximately 10,000 cells!

• SexualReproduction:• Flatwormsarehermaphroditic, butmostpracticecrossfertilization

• Aftertwoflatwormshavecopulatedtheyreleasecocoonoffertilizedeggs andattachthemtoasurface

StructurestoKnow/IdentifyinPlanarianEyespotsAuriclesCiliatedEpidermallayer(ventralsideonly)PharynxGastrovascularCavityGangliaVentralNerveCordTransverseNerveCordFlamecellAKAProtonephridiaEggTesteSeminalreceptacleGenitalPoreOviductOvary

ReviewSymbioticRelationships:• Symbiosis:twospecieslivingtogetherwithanotherorganismincloseassociation

• 3Types:

MUTALISM PARASITISM COMMENSALISM

ParasiticTrematoda:TheLiverFluke

LiverFlukeLifeCycle

• Adultsliverflukeslivesintheliverofsheep• Eggsarepassedintointestineandmixw/feces• Eggspassedfromfecesfromsheepandfallintowater.• Eggembryonatesanddevelopslarvaecalledmiracidia• Miracidiainfectswatersnails• Miracidiagoesthroughmultiplestagesbeforedevelopsintocercaria(swimminglarvae).

• Cercarialeavesnailandformacystscalledmetacercariaandgeteatenbycattle• Metacercariaburrowsthroughtissueandtravelstoliverandmatesandcyclerepeats

LifeCycleofLiverFluke

LiverFlukeLifeCycle

Parasitic FlatwormsThe pork tapeworm – Taenia solium

• Infectspigsandhumans• Livesintheintestineofitshostandpasseseggsthroughthefeces

• Highlyadaptedtoconstantinternalenvironments

• Lackssensoryorgans,coordinationformobilityandadigestivesystem

• Haveamodifiedepidermis“tegument”whichprotectsagainstthedigestiveenzymesandtheimmunesystemsofthehost

• Canreach7minlengthinhumans!• Areflatandlongwhichmaximizesabsorptionofnutrientsfromthehost

SupercoolFlatwormFact• thelargesttapewormeverreportedwasinaspermwhaleandwas98feetinlength!

Tapeworm Scolex

ParasiticFlatworms:Tapeworms

• Thetapewormhasananteriorendcalleda scolex withcomplicatedhooks forattachingtotheintestinesofitshost.

• Thetapewormdoesnothaveamouthordigestivesystem.• Insteadtheybathinthepre-digestedfluidsoftheirhostandabsorbnutrientsdirectlyintotheirbodycells

• Why?Evolutionaryadvantage?• Moreroomforreproductivestructures- everyProglottidhascapabilityofbecominganothertapeworm

Proglottids

Hooks

Scolex

Suckers

Rostellum

The tapeworm “up close and personal….”

• Theheadendhasascolexwithfoursuckers(twoseenhere)thathelpitattachtotheintestineofitshost

• Thebodyisseparatedintosectionscalled“proglottids”thathousehighlydevelopedreproductivesystems(darkenedareas)capableofproducinghundredsofthousandsofeggsandsperm

Platyhelminthes:Cestoda(Tapeworm)

GeneralAnatomy

Taenia solium.

LifecycleofaTapeworm

Summarizing Identifying Characteristics of the Phylum Platyhelminthes• Acoelomate• Exhibit bilateral symmetry• Have a bilateral nervous system with cephalization at the

head end. Some species exhibit eyespots sensitive to light• Possess a Gastrovascular Cavity (GVC) and primitive

organ systems for digestion and excretion• Do not have a circulatory or respiratory system but do

take in O2 through their body surface (integumentary exchange)

• Hermaphroditic : can reproduce sexually (do not self fertilize) or asexually by regeneration

• Are motile and utilize an undulation form of motion• Found in marine, freshwater and damp terrestrial habitats