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BIOL2015–EvolutionandDiversityLab8:Lophotrochozoa–PartI:

Platyhelminthes,Rotifera,Bryozoa,andBrachiopoda

Introduction

Lastweekwestudiedtheporiferaandthecnideria.Spongeslacktruetissueandanysymmetricalorganizationofthebody,whilecnideriahastruetissue(withtwogermlayers)andradialsymmetry.Todaywebeginthestudyoftheanimalswithbilateralsymmetry(Bilateria).Allbilateralanimalshavethreegermlayers(endoderm,mesodermandectoderm).Thebilateriaaredividedintotwomajorgroups(seeFigure1),theprotostomesandthedeuterostomes(thesenamesrefertowherethemouthformsintheearlyembryo.SeeFigure32.7ofyour2010textbook).

Theanimalphylogenyhasbeenrevolutionizedinthelast15yearsbymolecularphylogeneticstudies,whichhavechangedmanyoftheacceptedrelationshipsthatwerebasedsolelyonmorphologicaldata.OnesuchgroupistheLophotrochozoa(Figure1),whicharealargegroupofanimalswithintheprotostomesandwillbeourfocusforthenexttwolabs.Figure2showsamoredetailedphylogenyofthemajorgroupsintheLophotrochozoa–theyarediverse!ThenameLophotrochozoacomesfromthenamesofthetwomajoranimalgroupsincluded:theLophophorataandtheTrochozoa.

Trochozoa:Manyofthemembersareworm-like,thoughnotallofthemarefamiliarorcommon.ThetwolargestgroupsoftrochozoansaretheMollusca(mollusks)andtheAnnelida(segmentedworms).

Itmightseemstrangeatfirsttogroupearthwormsandsquidstogether.Theycertainlydon'tlookmuchalike,butthatisonlytruewhenlookingattheadultform;theyshareafundamentalfeatureoftheirlife

Figure1.Asimplifiedphylogenetictreeofanimals.

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Figure2.AphylogenetictreebasedonDNAdata.Dunnetal.2008.

Trochozoa

Lophophorata

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history.Manyannelidsandmolluskssharepatternsofdevelopmentinearlyembryonicstages.Whentheirlarvaehatch,eachisamicroscopicswimmerknownasatrochophorelarva.Thelarvahastwobandsofciliaaroundthemiddlethatareusedforswimmingandforgatheringfood,andatthe"top"isaclusteroflongerflagellae.Sothelarvaeofthesegroupsarenearlyidentical,eventhoughtheymatureintoverydifferentadultforms.Untilveryrecently,theArthropoda(insects&crustaceans)wereconsideredcloserelativesoftheAnnelida,basedonthefactthatbothgroupsaresegmented,butnoarthropodhasatrochophorelarvaandnomolecularstudiessupportacloserelationshipbetweenthem.

Lophophorata:ThisgroupincludesthephylaPhoronida,Entoprocta(bothsmallgroups),Bryozoa(formerlycalledEctoprocta)andBrachiopoda,withthelattertwohavinganextensivefossilrecord.Thefeaturesharedbythisgroupisthelophophore,anunusualfeedingappendagebearinghollowtentacles.

WhiletheLophophorataareawell-recognizedgroup,phylogeneticstudiesdonotyetagreeontheidentityoftheirclosestrelatives.TheseanimalswereonceincludedinthePseudocoelomata,becausetheydonothaveadistinctinternalbodycavityliketheTrochozoa,butthisgroupingdoesnotholdtogetherinmodernstudies.

Platyhelminthes(Flatworms)

Flatwormsareanatomicallysimplecomparedtonematodesandannelids.Theylackcirculatoryandrespiratorysystems.Manymembersofthisphylumareparasitic(e.g.,flukesandtapeworms)buttherearealsoanumberofspeciesthatarefree-living,liketheplanarianswewillhaveinlabthisweek.Thesenon-parasiticPlatyhelminthesarefoundinfreshwater,marineandinsometerrestrialenvironmentsandarefuntowatch.

Flatwormfeatures:

• Threetissuelayersinembryo.Almostallanimalssharethisbasicfeature;thespongesandcnidariansareexceptions.

• Acoelomate:Flatwormsdon'thaveanykindofcoelomorpseudocoelom;theirbodiesarebasicallysolid.ThissimplebodystructureledbiologiststobelievethatthephylumPlatyhelminthesbranchedofffromtherestoftheanimalsbeforetheevolutionofthecoelom.However,newphylogeneticstudieshaveledsomeresearcherstoconcludethatflatwormsdescendedfromanancestorthathadacoelom,andlaterlostthecoelom.

• Gastrovascularcavity:Thedigestivetracthasonlyoneopening,andbranchesthroughoutthebody.Flatwormshaveextracellulardigestion,likemostanimals(butunlikesponges).

• Pharynx:amusculartubethroughwhichtheflatwormcansuckfoodintoitsgastrovascularcavity.Theopeningintothepharynxcouldbeconsideredthemouth,butsincethisanimalhasatwo-waygut,thatopeningalsomustfunctionastheanus.

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Trepaxonemata

Thereareapproximately4,500speciesofplanarians.Mostarefree-livingandmarine,althoughyoucanalsofindtheminfreshwaterandonland.Manyarepredators,theyfeedonprotozoans,smallinvertebratesordeadanimals.Thecanclonethemselvesbylongitudinalortransversalpartitioning;theyarealsohermaphrodites.

Dugesia(Planarians)

ObservelivingDugesiaatadissectingmicroscope.Noticetheflattenedbodiesoftheseanimals,andtheirgracefulmovements.Athickmucouscoatinganddenselyciliatedepidermisontheirventralsideallowsthemtoglidealongsurfaces.Alsonotethattheyhavea“head-end”wherephotosensitiveeyespotsorocelliarelocated(lendingtheDugesiaa"cross-eyed"appearance),whichhelpthemavoidbrightlight.

Findthefollowingslides:

• Findapreparedslidelabeled"Planariaw.m.".• Findapreparedslidelabeled"Planariacombinationw.m.".• Findapreparedslidelabeled"Planaria

c.s.".

Thisspecimenisstainedtohighlightthegastrovascularcavity,showingthesmallbranchescalleddiverticula.Notethatthereisonemainbranchofthegastrovascularcavityintheanteriorpartofthebody,buttwomainbranchesposteriortothepharynx.

Theeyespotsaresimpleanddon'tformanimage;that'swhytheyarecalledeyespotsinsteadofeyes.However,theyareslightlycup-shapedandfacetowardthesides.Withthisarrangement,theflatwormcantelllightfromdarkandmovetowardthedark.Cup-shaped

Figure3.AwholemountviewofaPlanaria.

Figure4.Cross-sectionviewsofPlanaria.

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eyesarejustonestepinthetransitionfromphotoreceptivecellstoeyeballs.

Planariacrosssections:ThismicroscopeslidehasseveraldifferentcrosssectionsofaPlanaria,showingdifferentregionsofthebody.Ontheventralsurfacesyoucanobserveciliausedforlocomotion.

Anteriorregion:Thissectionanteriorto(infrontof)thepharynx.Thebodyismoreorlesssolid;theonlyopeningsarethediverticulaofthegastrovascularcavity.

Pharynxregion:Thiscrosssectionshowsthepharynx,whichisretractedintothepharyngealcavity(theemptywhitespacesurroundingthepharynx).Thepharynxitselfisathick,musculartube;whenPlanariaeats,itevertsthepharynx,stickingitoutofthebodytosuckupbitsoffood.

Posteriorregion:Thegastrovascularcavityisdividedintotwomainbranches,withsmallerdiverticula.Theverticalpinklinesaredorsoventralmuscles.

Trematoda(Flukes)

TheflatwormsoftheclassTrematoda,alsocalledflukes,areinternalparasitesofmollusksandvertebrates.Thereareapproximately18,000to24,000species.Theyhaveoralsuckers,sometimessupplementedbyhooks,withwhichtheyattachtotheirvertebratehosts.Trematodeshaveretainedthesamebodyformanddigestivecavityastheturbellarians.However,practicallytheentireinteriorisoccupiedbythereproductivesystem;theseorganismsarecapableofproducinghugenumbersofoffspring.TrematodesoftheorderDigeneahavecomplexlifecyclesinvolvingtwoormorehosts.Thelarvalwormsoccupysmallanimals,typicallysnailsandfish,andtheadultwormsareinternalparasitesofvertebrates.Manyspecies,suchastheliverflukeClonorchissinensisandthebloodfluke(Schistosoma),causeseriousdiseasesinhumans.

Clonorchissinensis(ChineseLiverFluke)

Findapreparedslidelabeled"Clonorchissinensisw.m.".

Figure5.Cross-sectionoftheposteriorregionofPlanaria.

Figure6.AwholemountviewoftheChineseliverfluke.

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Someoftheseslidescontainlargeranimals,somesmallerones–makesuretolookatboth. Findtheoralsucker;theacetabulumorventralsucker;andthepairedandelongateintestines(labeled“gut”inFigure6).Notethatithasreduceddigestivesystemandalso,howmuchofthespacewithintheadultflukeisdevotedtoreproduction.

NoticealsothatTrematodeshavebothmaleandfemalereproductiveorgans(theyarehermaphrodites).Thelargeuterus(nothomologouswithouruterus)isfilledwitheggsandislocatedanteriortothetestes,whichproducesperm.Whydoyouthinkthattheorganisminvestssomuchenergyonreproduction?

Fasciolahepatica(CommonLiverFluke)

Thecommonliverflukeorsheepliverflukeisgloballydistributed,andinfectstheliversofvariousmammals,includinghumans.Fasciolahasacomplexlifecyclewithmultipleintermediatestagesandhosts(Figure7).Thediseasecausedbytheflukeiscalledfascioliasisandcausesgreateconomiclossesinsheepandcattle.ObservethepreservedspecimensofFasciolahepaticaondisplayandbeabletoidentifyit.

Findapreparedslidelabeled"Fasciolahepaticamiracidiaw.m.".

Fasciolatakeondifferentformsdependingonthestageoftheirlifecycle.Miracidiaarethefree-livingmotileform,coveredwithcilia,andsettleintoamolluskhost.Theyaresmallandvariableinform,soobservemorethanone.

Findapreparedslidelabeled"Fasciolahepaticarediaew.m."and"Fasciolahepaticametacercariaw.m."toseewhatformthisparasitetakesinitsintermediatehosts(Figure7).

Figure7.Fasciolalifecycle.

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Schistosomamansoni-BloodFluke

Thisflukecausesthediseaseschistosomiasis,whichaffectsmorethan20millionpeopleayear.Schistosomesareatypicaltrematodesinthattheadultstageshavetwosexes(dioecious)andarelocatedinbloodvesselsofthedefinitivehost(mostothertrematodesarehermaphroditicandarefoundintheintestinaltractorinorgans,suchastheliver).Schistosomeshavetwohosts:adefinitivehost(i.e.human)wheretheparasiteundergoessexualreproduction,andasingleintermediatesnailhostwherethereareanumberofasexualreproductivestages.

Findapreparedslide(demonstration)labeled"Schistosomamansonimaleandfemalew.m.".Youwillobserveapairofmatingflukes.Duringcopulation,thefemaleisheldinaspecialchannel(thegynecophoralgroove).

Cestoda(Tapeworms)

Cestodaisthenamegiventoaclassofparasiticflatworms,commonlycalledtapeworms.Asadultstheyliveinthedigestivetractofvertebrates,andofteninthebodiesofvariousanimalsasjuveniles.Overathousandspecieshavebeendescribed,andallvertebratespeciescanbeparasitizedbyatleastonespeciesoftapeworm!Infectioninhumansoccursbecauseofconsumptionofimproperlycookedorprocessedmeat(pork,beef,orfish,forexample).Taeniasaginata,thebeeftapeworm,cangrowupto20minthegutofitshost,butthewhaletapeworm,Polygonoporusgiganticus,cangrowtoover30m!Cestodeeggshaveevenbeendiscoveredinfossilsharkfeces(coprolites)datingtothemid-tolatePermian,some270mya.

Figure8.LifecycleofSchistosomamansoni.

Figure9.ThisisanadultTaeniasaginatatapeworm.Whichsideisthehead?

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Tapewormslackadigestivetrack.Allnutrientsareabsorbeddirectlythroughthebodysurface.Thehost’sdigestiveenzymesdoallthework.Theheadregion,orscolex,ismodifiedforattachmenttotheintestinalwall.Thescolexhassuckers,andoftenhasaringofhooksaswell.

Mostofthetapeworm’sbodyconsistsofrepeatedsegmentscalledproglottids(althoughit’snotanexampleoftruesegmentation).Eachproglottidisaself-containedpacketcontainingmostlyreproductiveorgans.Newproglottidsareproducednearthescolex.Themostmatureproglottidsareattheoppositeendoftheanimal.

Findthepreparedslideslabeled"Taeniapisiformisscolexw.m."and"Taeniapisiformismatureandgravidw.m.".Ontheformerobservethesuckersandhooks,onthelatterobservehowmuchofthebodyisdevotedtothereproductivestructures.Alsonotehowthe“segments”calledproglottidschangeshapeoverthelengthoftheanimal.

Rotifera

BdelloideaRotiferameans“wheelbearer”.Theyaregenerallymicroscopic(barelyseenwiththenakedeye),withtwomaindistinguishingfeatures.Thefirstisananteriorregioncalledthecoronathatisusedinfeedingandinlocomotion.Thesecondisamuscularpharynx(calledamastax)thatpossessescomplexjawsknownastrophi.RotifersarepseudocelomatesExaminethelivespecimensandthepreparedslidesofarotifertofamiliarizeyourselfwiththeirfunctionalmorphology.Notethe3generalregions:head,bodyandfoot(andtoes).Observealivespecimenatincreasinglyhighermagnification,ultimatelyaimingtoobservethepatternofciliationonthecorona.Asinmanyfreshwaterorganisms,manyrotiferspossess

Figure7:Rotifer

Figure10.StainedadultTaeniapisiformis.

Figure11.Aschematicdrawingofarotifer.

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remarkableadaptationstowithstandperiodsofdroughtand/orcoldtemperatures.Themostnotableofthesearetheirabilitytoundergoparthenogenesis(growthanddevelopmentofeggswithoutfertilization)andproducerestingeggs.Manyspecieshavetheabilitytoundergoanhydrobioisis,arestingstagethatoccursduringdryconditionsinwhichtheanimalcansurvivecompletelydesiccatedforlongperiodsoftime.Findapreparedslidelabeled"Rotifersw.m.".Findthesamestructuresasabove,butalsotrytofindthestomachandpseudocoel.

Bryozoa

PhylactolaemataBryozoa(formerlycalledectoprocta),means"mossanimals”).Theyareageologicallyimportantgroupofsmallanimals.ThevastmajorityofBryozoansarecolonialandmostaremarine.Superficiallytheycanresemblecorals.Bryozoansaremostabundantintemperate-tropicalwatersthatarenottooturbid.Theyrequireafirmsubstrateontowhichtheyattachorencrust,andclearagitatedwaterfromwhichtheyobtaintheirsuspendedfood.Enclosedwithinaskeletonofcalcite,Bryozoanshaveasac-likecoelomatebodywithawell-definedmouth,anus,andotherspecializedorgans.Onesuchorganisthelophophore(aciliatedstructureusedinfoodgathering)thatisattachedtotentaclesthatsurroundthemouth.Findapreparedslidelabeled"BryozoaPectinatellaw.m.".Beabletoidentifytheorganismandthelophophores.

Figure12.Aclose-upofaBryozoancolony.Thehorseshoe-shapedlophophoresareclearlyvisible.

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Brachiopoda

LingulataWhilerecentbrachiopodsarearatherrareandinsignificantgroup,theirlongfossilhistoryshowsthattheywereattimesthemostprominentanimalsintheseas.Consequently,brachiopodsreceiveonlypassinginterestfromzoologists,butagreatdealofattentionfrompaleontologists.Thephylumisquiteimportantforbiostratigraphy,paleoecology,andevolutionarystudiesbecauseitshowsagreatvarietyofchangesinformandfunctionthroughtime.Brachiopodsresembleclams,buttheirshellsaredorsal-ventral(thatis,there’satopshellandabottomshell),whereasclamshavelateralshells(there’saleftandarightshell).BrachiopodsfeedusingalophophorelikeEcoproctans,whichcreatesawatercurrentthatenablesthemtofilterfoodparticlesoutofthewater.FindpreservedspecimensofLingulaondisplay.ThegenusLingulafirstevolvedover400millionyearago,makingittheoldestlivinganimalgenusthatstillcontainsextantspecies!IdentifytheinternalstructureslabeledinFigure13onthepreservedspecimensinthelab.

Figure13.Adissectedbrachiopodshowingtheinternalstructure.