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BIOL2015–EvolutionandDiversityLab09:Ecdysozoa

IntroductionEcdysozoaisagrouporiginallysupportedbyribosomalRNAgenesanditisnowwidelyacceptedasamonophyleticclade(seeFigure1).Ecdysozoacomprisesthemostdiverseandsuccessfulgroupofanimalsontheplanet.Itincludesnematode,arthropoda,andtardigrada.EcdysozoaandLophotrochozoaarethetwolargesubdivisionsthatcomposethecladeProtostomia,agroupthatsharesthesynapomorphyofthemouthdevelopingfromanembryonicstructurecalledtheblastopore.InturnProtostomiabelongstoalargergroupwithinthekingdomAnimaliacalledBilateriabecausetheseanimalsarebilaterallysymmetrical.

Allecdysozoansbuildacuticleanouterlayeroforganicmaterialthatfunctionsasanexternalskeleton(calledexoskeleton).Thecuticleisgenerallynotmineralizedandthismeansthattheskeletoncanbebuiltthinnerandlighterthaninotheranimalsanddoesnotrequireasourceofmineralsforitsconstruction.Alsoamineralskeletonrequiresjointstoallowflexibilitywhereasanorganiccoveringcanbeflexibleifitisthinenough.

ThenameEcdysozoareferstothefactthatmanymembersofthisgroupregularlyshedtheircuticleinaprocesscalledecdysis(Greek:ek=out,off+duien=toput).Ecdysisishormonallycontrolledbyaclassofsteroidsappropriatelycalledecdysteroids.Ifyouhaveeverseenaninsectcrawloutofitsoldskinorabutterflyleavingitschrysalisthenyouhaveseenecdysistakeplace.Theabilitytoshedthecuticlehasopenedupoptionsforecdysozoansthatarenotavailabletoanimalswithskeletons.Inanimalswithamineralizedskeleton,growthcanonlyoccurbyaddingmineraltotheexistingskeletonlimitingtheanimal'sformasitgrows.Whilemanyecdysozoansmaintaintheirbasicbodyformthroughouttheirentirelife,ecdysisremovesthislimitation.Forexample,manyinsectshavetakenadvantageofthisprocessandundergopartialorcompletemetamorphosisbeforereachingtheadultstage.allowingthelarvatolookcompletelydifferentfromadultand,insome,casesthisallowstheadulttoliveinacompletelydifferentenvironmentthanthelarva.

Inbasalanimalsandmanydeuterostomessexualreproductionisaccomplishedbyreleasingmassquantitiesofgametesintotheenvironmentandleavingfertilizationtochance.Inmostspeciesofecdysozoansthespermiseitherdelivereddirectlyintothefemale'sbodyordepositedontotheeggsastheyarereleased.Thesestrategiesgreatlyincreasethelikelihoodofsuccessfulfertilization.Wewillseethatthesesamestrategiesareusedbythevertebratesbuthaveevolvedindependently.

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Figure1.AphylogenetictreebasedonDNAdata.Dunnetal.2008.

EcdysozoaTrochozoa

Lophophorata

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Nematoda

Nematodes have amore complex body plan than that of flatworms but simpler than annelids. Free-living (non-parasitic)nematodesareextremelycommon inmostkindsofhabitatsbut theyareusuallytoosmalltosee.Parasiticnematodesarealsoverycommonandareoftenmuchlarger.

Nematodefeatures:• Three tissue layers inembryo.Thepresenceofmesoderminadditiontoectodermandendodermisatraitthatisbasaltoallbilaterians.

• Pseudocoelomate:The inside of a nematode's body ismostly filledwith empty space called thepseudocoel. Thisspacemightatfirstappeartobethesameasthecoelominan earthworm but it isn't. The pseudocoel develops in adifferentwaythanthecoelomofacoelomateand,unlikeatrue coelom, there are no mesodermally-derived tissuesand there are no mesodermlly-derived structuressurroundingtheendodermofthedevelopinggut.

• Complete digestive tract:Comparable to that of annelidsbutnematodes show littleorno specializationofdifferentgutregions.

• No circulatory system: Nematodes don't have hearts bloodvesselsorblood.Thefluidinthepseudocoelpassivelycirculatesnutrientsandothersubstancesthroughoutthebody.

• Cuticle:a tough flexible acellular layer on the outside of thebody.Becausethecuticleisn'tstretchynematodesmustmoltinorder to grow. Nematodes have this feature in common witharthropods.

Find the preserved specimens of Ascaris lumbricoides the giantroundworm of humans on display and find the mouth anus andlaterallineswhichrundownthesidesasveryshallowgroovesinthecuticle.Thelaterallinesareasensoryorgan.

Findapreparedslidelabeled"Ascarislumbricoidesesophagusc.s.".

Noticethecuticle,thelargepseudocoelandthemuscularintestine(esophagus)inthecenteroftheanimal.Oneachsideofthebodythehypodermisthickensandprojectsintothebodythelateralline.

Figure2.Ascarisfemaleslide.

Figure3.Ascarismaleslide.

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Eachcordcontainsanexcretorycanal(atleastinthissection)andalateralnervecord.Findthemuscletissuethatlinestherestofthebodywallandfindthesmalldorsalandventralnervecords.Nematodesareuniqueinthatthemusclecellsareindirectcontactwiththenervecords.

Findapreparedslidelabeled"Ascarismale&femalec.s.".

Notethefollowingstructuresonthefemalesection:OvaryandUterus.Onthemalesectionnotethetestis-alongtubeloopingbackandforththroughoutthebody,incross-sectionthetestisappearstobemultipletubes.Spermareproducedwithinthetestis.Thevasdeferens(alsocalledductusdeferens)isanotherpartofthesametubularmalereproductivetract;itisresponsibleforstoringsperm.ComparetoFigures2and3.

TrichinellaspiralisThediseasetrichinosisiscausedbyTrichinellaspiralis,whichresultsfromeatingundercookedmeat.Thecontaminatedmeatcontainsencapsulatedcystsofcontaininglarvae.Thehost'sdigestiveenzymesbreakdowntheprotectivecapsuleofthecystthusreleasingtheenclosedlarva.Itmaturesintoanadultworminthedigestivetractandformscystsinmuscletissue(amongothers).

Findapreparedslidelabeled"Trichinellaspiralismusclesec."toobserveacyst.

Arthropoda

Thephylumarthropodaisbyfarthemostdiverseofalltheanimalphylaintermsofnumberofspecieshabitatsandwaysofliving.Thereareatleastamillionarthropodspeciesaccountingforover80%ofknownanimalspecies.Arthropodshaveadaptedtomanydifferentenvironments(marinefreshwaterandterrestrial);fewotheranimalphylaareinsuchadiverserangeofplaces.Arthropodshaveanumberofdistinctivefeaturesintheirbodyplan:

• Segmentedbodywithspecializedandfusedsegments.Arthropodsareclearlysegmentedandthedifferentsegmentsareverydifferentfromoneanotherinformandfunction.Alsoarthropodbodiesaremadeofseveralgroupsoffusedsegments;thefusedsegmentsarecalledtagmata(singular:tagma)andtheyactlikeindividualsuper-segments.Eachgroupofarthropodshasacharacteristicsetoftagmata;forexampleinsectshavethreemaintagmatawhilespidershaveonlytwo.Ineachgroupthesesegmentshavetheirownuniquenames.

• Cuticle.Arthropodbodiesarecoveredwithacuticlemadeofchitinforminganexoskeleton.Whereelsehaveweseenchitinousstructuresthissemester?Foreachstructureyoucomeupwithtrytoidentifythephylum,class,andorderoftheorganism.Insomearthropods(suchascrabs)theexoskeletonismademorerigidwithcalciumdeposition.Thefirstarthropodslivedintheocean;theirexoskeletonsprotectedthemfromattackandprovidedplacesfortheirmusclestoattach.Whenlatergroupsofarthropodsmovedontothelanditturnedoutthattheexoskeletonhappened

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tobeveryfunctionalinpreventingthebodyfromdryingout.Thisisagoodexampleofhowacharacteristicthatevolvedinoneenvironmentcanbecomeimportantinanorganism’sabilitytoadapttoanotherenvironment.

• Jointedappendages.Mostarthropodshavemultiplejointedlegs,antennae,mouthparts,andotherbodypartsaspartoftheexoskeleton.

• Metamorphosis.Manyanimalscansimplygrowcontinuouslythroughouttheirlives,calledindeterminategrowth.Arthropodshoweverareconfinedintheirrigidcuticle.Inordertogrowtheymustmolt.Thentheyquicklygrowbigger(byabsorbingwaterorair)beforetheyformanewhardcuticle.Theirdevelopmentdoesn’talwaysproceedinaseriesofgradualchanges.Instead,theyoftengothroughadistinctmetamorphosisinwhichtheychangetheirbodyformdramaticallyastheymoltandformanewexoskeleton.Theclassicexampleofthisisthemetamorphosisofacaterpillarintoabutterfly.

• GasExchange&Osmoregulation.Confinedintheirexoskeletonsarthropodsneedspecialstructuresforgasexchange,osmoregulation,andexcretion.Thesespecializedstructuresalsoseemtocreateopportunitiesforsomearthropods.Withtheirtrachealsystemforgasexchangeandtheirtubulesforosmoregulation,insectsareabletoliveindryconditionsthatwouldkillmostinvertebrates.

Chelicerata–Spiders,Ticks,Mites,andtheHorseshoeCrabTheCheliceratesaremostlyterrestrialandalmostentirelypredatory(exceptmitesandtickswhichcanbeparasitic).Thehaveuniramusappendagesthatiseachappendagecomesoutsinglynotinpairsasincrustaceans.Theirbodyisdividedintotwotagmata,theprosoma(alsocalledthecephalothorax),andtheopisthosoma(orabdomen).Thereareover75,000extantspeciesinthesubphylumchelicerata.Theprosomahas6pairsofappendages:chelicerae(whichformthemouthparts),pedipalps(whichhavedifferentfunctionsindifferentchelicerategroups),and4pairsofwalkinglegs.Theyhavenomandiblesorantennaeandhavesimpleeyes.

Figure4.Theexternalanatomyofchelicerates.Thebodyisdividedintoacephalothorax(prosoma)andanabdomen(opisthosoma).

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MerostomataThemerostomataisaclassofmarinecheliceratescommonuntilthegreatPermianextinctionbutwithonly4speciestoday.TheyliveincoastalwatersoftheEasternUSandinSoutheastAsia.Theyusecyanoglobintobindoxygen(containingcopperinsteadofiron)makingtheirbloodblue.Theypreyonmollusksandpolychaetes.FindthepreservedspecimensofLimuluspolyphemustheAtlantichorseshoecrab.Identifythecarapace,theabdomen,andthetelson(tail).Youshouldbeabletofindthesimplemedianeyeandlateralcompoundeyes.Notetheunfusedbodysegmentsontheventralsurface.Findthemouth,anus,chelicerae,andpedipalps.Noticethattheseappendagesareverysimilartothewalkinglegs.Gentlylifttheoperculatoobservethegills.

ArachnidaArachnidscomprise98%ofthechelicerates,withmostbeingwelladaptedtoterrestrialenvironmentsandpossessingadaptationsthatenablethemtofeedoninsects.Thisclassincludesspiders,scorpions,ticks,andmites.Italsoincludesfascinatinggroupssuchasamblypygids,opiliones,andpseudoscorpions.Thecheliceraeareoftenmodifiedasfangsinthisgroup(Figure6)andmaybeassociatedpoisonglands.Becausetheylackjawsarachnidscan'tingestsolidfood.Insteadtheysuckthebodyfluidsfromtheirprey.Inthisclassgaseousexchangeisaccomplishedwithbooklungs.SpidersFindthepreservedspiderTarantulaondisplay.Carefullyexaminethepreservedtarantulatolocatethefangs(chelicerae),pedipalps,cephalothorax,andabdomen.Lookforthesmallsimpleeyes.Alsoexaminethelivingtarantula.(Pleasedon'ttrytoremoveitfromtheterrarium!)Theselargespidersrelyheavilyondetectinggroundvibrationsthrough

Figure6.Externalmorphologyofaspider.Observethecephalothoraxandtheabdomen(oropistosoma).

Figure5.ExternalanatomyofHorseshoeCrabs.

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theirlegsandbodiestosensepreyandenemies.Manyofthedensehairsontheirbodiesaresensillaeortactilereceptors.ScorpionsFindthepreservedscorpionsondisplay(handlethemverycarefully).Scorpionsaredistinguishedbyasubdividedabdomentoproduceatail-likeextensiontippedbyastinger(Figure4).Theirpedipalpsaremodifiedaspincers.Examinetheventralsurfacetofindtheslit-likeopeningsthatleadtothebooklungsandthefringedsensorystructurescalledpectines.TicksandMitesThesearachnidshavecompletelyfusedcephalothoraxesandabdomens.TicksfeedonvertebratebloodandmaytransmitdiseasesincludingRockyMountainSpottedFever,Tularemia,andLymeDiseasefromanimalsrangingfromwhalestobirds.Mitesfeedonvertebrateskinandskinsecretions.

Findapreparedslidelabeled"TickandMite(wm)".Besureyoucandistinguishbetweenticksandmites(andnotjustonthebasisofsize!).

CrustaceaCrustaceaisaverylargegroupofarthropods.Itincludescrabs,lobsters,crayfish,shrimp,krill,andbarnacles(approximately40,000species).Likeotherarthropodscrustaceanshaveanexoskeleton,whichtheymolttogrow.Theyaredistinguishedfromothergroupsofarthropodssuchasinsectsmyriapodsandcheliceratesbythepossessionofbiramous(two-parted)limbs.Theyshowawidedegreeofmorphologicaldiversitythereisnosuchthingasatypicalcrustacean.Themajorityofcrustaceansaremarine,althoughsomehaveadaptedtofreshwaterenvironmentswithafewbecomingcompletelyterrestrial(pillbugs).Findthepreservedspecimensofcrayfishondisplay.Findthecephalothoraxandabdomen;5pairsofwalkinglegsthefirstpairenlargedaschelipeds(orpincers);2pairsofantennae(thesecondpairshorterandbifurcatedaretheantennules);theabdominalappendagesthatarespecializedasswimmerets.Observetheendopodandexopodregionsoftheseappendages.Carefullylifttheedgeof

Figure7.Externalanatomyofacrustacean.

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thecarapacetoobservethegillsattachedtoeachofthewalkinglegs(howdoesthisrelatestothebiramouscondition).Finally,youshouldexaminethetelsonanduropods.Thetelsonisamodifiedbodysegmentandtheuropodsaremodifiedappendages.BarnaclesFindthepreservedspecimensofgoosebarnacleswehaveondisplay.Thesefilterfeedingcrustaceansaresessileasadults.Foodistrappedbytheelaboratecirri(orfeedinglegs)whichprojectandwavefromthecalcareousshellthattheseanimalssecretearoundthemselves.Barnaclesmatewiththeirsessileneighborswiththelongest(relativetobodysize)malereproductivestructuresintheanimalkingdom.

MyriapodaThesearthropodshavemanyuniramouslegsasthenamessuggests.Theybreathethroughspiraclesthatconnecttoatrachealsystem.Centipedesarenocturnalfastandactivepredators.Theyhaveapairofforcipulesfromwhichtheyinjectvenomtotheprey.Findthepreservedcentipedesondisplay.Notethatmostbodysegmentshaveonlyonepairoflegs.Millipedesareslowmovingandfeedonplantmaterialthathasbeguntodecompose.Findthepreservedmillipedesondisplay.Notethatmostbodysegmentshavetwopairsoflegs.

HexapodaWithapproximately1.4to1.8millionspeciesandcountingtherearemorespeciesofinsectsthanallotheranimalgroupscombined.Manymillionsmorearelikelytobediscovered.Insectsarekeyplayersinthevastmajorityofterrestrialenvironments.Theyinventedflyinglongbeforereptilesandevencultivation(leaf-cuttingantscultivateafunguswithcollectedleaves)andfarming(manyantspeciesherdaphidsfromplanttoplant).Theyarepollinators,herbivores,parasites,diseasevectors,andpredators.

Figure9.Insectexternalanatomy.

Figure8.AnatomyofMillipedesandcentipedes.

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ThemostdiversegroupofinsectsistheColeoptera(thebeetles).ForexampleErwinandScott(1980)foggedtropicaltreesinPanamawithinsecticidesandestimatedthateachtreespecieshas170uniquespeciesofbeetlesadaptedtolivingonit.Insectsuccessprobablycomesfromavarietyoffactors.Theyarewelladaptedtolandbyhavingawaxycuticlethatpreventsdesiccation.Theyaretheonlyinvertebratesthatcanfly,whichallowsgreaterdispersion,theabilitytolocatefarawayfoodsourcesand/ormates,andescapefrompredators.SomeinsectsaremigratoryliketheMonarchbutterfly.Anotherlikelyreasonfortheirsuccessismetamorphosis,whichallowstheyoungtospecializeinfeeding(forexamplethelarvaeofbutterflies)whileadultsareusuallyspecializedfordispersionandreproduction.Canyouthinkofanyotherexplanationfortheirsuccess?Theinsectbodyhasthreepartstheheadthethorax(withthreeleg-bearingsegments)andtheabdomen.Theheadhasantennaeamouthwithmandiblesandcompoundeyes.

Findthepreservedlubbergrasshoppers.Locatethethreeregionsofthebody;thehead,thorax,andabdomen(Figure10).Ontheheadfindthelargecompoundeyesandtheantennae.Findthelabrumandliftittoobservethemandiblesandthetwopairsofmaxillaewithpalps(Figure11).Onthethoraxfindthethreesubdivisionsthateachbearasetofwalkinglegs.

Noticethatthetwopairsofwingsalsooriginatefromthethorax.Eachabdominalsegmenthasapairofopeningscalledspiracles,whichadmitairtothetrachealsystem.Findtheinsectcollectionsondisplay.Observethedegreeofdiversitybodysizechangesinstructureandsoforth.

Figure11.Grasshopperexternalanatomy.

Figure10.Mouthpartsofagrasshopper.

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TardigradaUsuallytardigradesare1millimeter(0.039in)longwhentheyarefullygrown.Theyareshortandplumpwith4pairsoflegseachwith4-8clawsalsoknownasdisks.Tardigradesareprevalentinmossandlichen.Technicallytheyarenotextremophilesbecausetheydonotpossessstructuresthatarespecificallyadaptedtoanextremeenvironments,theyaresimplyabletowithstandmanyextremeenvironments(desiccation,heat,cold,pressure,etc…).Forexample,theycansurvivetemperaturesof-272°C(oneaboveabsolutezero)forafewminutes.Theycansurvivethevacuumofspaceandpressuresofupto1200atmosphereswhichisthepressureofwaterat36,000feet(thesamepressureatthebottomofoceantrenches).Mostarephytophagous(planteaters).Beabletoidentifyawaterbear.

Figure12.Ascanningelectronmicrographofawaterbear.