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Echinoderms Name:_________________________________________Class:__________Date:____________Vocabulary
Term Definition
echinoderm Invertebrate,suchasaseastarorasanddollar,thatischaracterizedbyaspinyendoskeleton,radialsymmetryasadults,andawatervascularsystem.
nerve net Interconnectedneuronswithnocentralbrain.
radial symmetry
Symmetryofabodyplaninwhichtherearenoheadorrearends,sothebodycanbedividedintotwoidenticalhalvesatanypoint,likeapie.
sea urchin Echinodermwitharound,softbodyenclosedinashellcoveredwithlongspines. Believe it or not, this is an animal. See the mouth and arms?
Itisasealily,acrinoidechinoderm.Crinoidsareessentiallyamouthonthetopsurfacethatissurroundedbyfeedingarms.Althoughthebasicechinodermpatternoffivefoldsymmetrycanberecognized,mostcrinoidshavemanymorethanfivearms.Crinoidsusuallyhaveastemusedtoattachthemselvestoasurface,butmanybecomefree-swimmingasadults. Characteristics of Echinoderms
EchinodermsaremarineorganismsthatmakeupthephylumEchinodermata.Membersofthephylumincludeseastars(starfish),seaurchins,seacucumbers,sanddollars,brittlestars,andfeatherstars.Thereare7000differentlivingspeciesofechinodermsand13,000identifiedfossilspecies.Althoughthenumberofechinodermspeciesisverysmallcomparedtothenumberofspecieswithinthearthropodphylum,Echinodermataisthelargestphylumtolackfreshwaterandterrestrialspecies.Anotherdistinctionisthatechinodermsareamongthemostdistinctorganismswithintheanimalkingdom.YoulearnedintheEvolutionconceptsthatevolutionisnotalinearseriesofstepsanddoesnotnecessarilyleadtoincreasinglevelsofcomplexityovertime.Itisbasedontheresponseofpopulationsofspeciestotheadaptivepressuresoftheirenvironmentsandfavorsthosethathavethegreatestreproductivesuccess.ThephylumEchinodermataprovidesanexcellentexampleofthenon-linear,bush-liketrajectoryofevolutionarychange.EchinodermsarethemostcloselyrelatedphylumtothephylumChordata,whichincludesmanycomplexorganismssuchashumans.Theirsharedcommonancestorwaslikelyabilaterallysymmetricalorganismwithacephalized(centralizedinaheadregion)nervoussystem.Yet,asyouwilllearninthislesson,manydetailsofechinodermstructureandfunctionaresimplerthanthoseofthislikelypredecessor.Theorganismswithinthisphylumevolvedthesestructuresasadaptationstothepressuresplacedonthembytheirenvironments,justasthevariousgroupswithinthephylumChordatadid.Youwillseeinthischapterthattheresultsofthesetwoevolutionarypathsareverydifferent,despitehavingafairlyrecentcommonorigin.Let’stakealookatsomeofthedistinguishingfeaturesofechinoderms,andthenwewillexaminetheirstructuresandhowtheyfunctioninmoredetail.ThewordEchinodermatameans“spinyskin.”Ifyouhaveeverseenaseaurchinthenthisnamewillcertainlyseemfitting.Althoughthesespinesmaylooklikecomponentsofanexoskeletonatfirstglance,echinodermsdonothaveanexoskeleton.Instead,thespinesareextensionsofaninternalendoskeleton.Theendoskeletoniscomposedofcalciumcarbonateplatesandspinesthatareactuallycoveredbyathinlayerofepidermis(skin).Theseplatesandspinesareheldtogetherbymesodermaltissue.
Asmentionedearlier,echinodermspecieslookverydifferentfrommostotherinvertebrates.Thisispartlyduetotheirsymmetryandtheirspinyendoskeleton.
Mostofthephylawehavediscussedsofar(exceptspongesandcnidarians)exhibitbilateralsymmetry,meaningthattheycanonlybedividedintotwoequalhalvesbyasinglecutalongthemiddleoftheanterior-posterior(front-back)axis.Althoughechinodermsevolvedfromabilateralancestor,adultechinodermsexhibitfive-foldradialsymmetrycalledpentameralradialsymmetry.Thismeansthattheirbodieshavefivesimilarregionspositionedaroundacentralaxis.Unliketheadults,echinodermlarvaearealwaysbilaterallysymmetrical,reflectingthefactthat
radialsymmetryevolvedsecondarilyfromabilaterallysymmetricalancestor.Onehighlyuniquefeatureofechinodermsisanorganizednetworkofcanalsthroughoutthebodycalledthewatervascularsystemthatfunctionsinlocomotion,feeding,excretion,andrespiration.Thewatervascularsystemuseswaterpressuregeneratedthroughmusclecontractionstomovetheorganism.Wewilllookatthishydraulicsysteminmoredetailinthenextsection.Unlikemollusks,annelids,andarthropods,echinodermsarenotprotostomes.ThespeciesofthephylumEchinodermataandthephylumChordataarealldeuterostomes.IntheIntroductiontoAnimalsandInvertebrateschapteryoulearnedthatthereareanumberofdifferencesbetweenprotostomesanddeuterostomes,allhavingtodowiththeirpatternsofembryonicdevelopment.Thesearewheredifferencesbetweenthetwolie:
• Patternofthefirstseveralcelldivisions.• Whenthefateofembryoniccellsbecomesfixed.• Formationofthecoelom.• Formationofthemouthandanus.
Indeuterostomes,thefirstseveralcelldivisionsaredescribedasradialcleavagebecausetheytakeplaceinparalleltoorperpendiculartotheaxisoftheembryo.Anotherfeatureoftheseearlycellsisthattheydonothaveasetfate.Forexample,ifonecellisremovedfromtheembryoatthisstage(8-cellstage),itisstillcapableofdividingtoformallofthecellsofanentirelyneworganism.Thiscontraststheearlycelldivisionsinprotostomesthatproducecellswithafixedcellfate.Iftheyareremovedfromtheembryo,theywillonlybeabletodivideanddifferentiateintoafewdifferentcelltypes,sotheyarenotcapableofproducinganeworganism.Anotherdifferencebetweenprotostomesanddeuterostomesistheoriginofthemouth.In
protostomes,theblastoporebecomesthemouth.Indeuterostomes,theblastoporebecomestheanus,andanotheropeninggeneratedlaterindevelopmentproducesthemouth.Theblastoporeisthefirstopeningofthedigestivecavitythatisformedduringthegastrulationstageofembryonicdevelopment.Finally,protostomesanddeuterostomeseach
havedifferentwaysofformingacoelom,orbodycavity.Inprotostomes,themesoderm(middletissuelayer)splitsintotwosheetsoftissue,andthespacebetweenthesesheetsbecomesthefluid-filledbodycavity.Deuterostomesformacoelombyinvaginatingpocketsofendodermal(internaltissuelayer)tissuefromthedevelopinggutcavityformedduringgastrulationThetissuesurroundingthepocketsdifferentiatesintomesodermaltissue,andthepocketsbecomethecoelom.Theechinodermcoelomiswell-developed,anditformstwomajorcavitiesduringdevelopment.Onecavitybecomestheactualcoelomthathousesinternalorgans,whiletheotherdevelopsintothewatervascularsystem.Inthenextsectionwewilltakeacloserlookatthewatervascularsystem,theendoskeleton,andotherstructuralfeaturesofechinoderms.
Summary
• MembersofthephylumEchinodermataincludeseastars(starfish),seaurchins,seacucumbers,sanddollars,brittlestars,andfeatherstars.
• EchinodermsarethemostcloselyrelatedphylumtothephylumChordata,whichincludesmanycomplexorganismssuchashumans.
• Echinodermsaredeuterostomesthatexhibitpentameralradialsymmetry.• Thewatervascularsystem,usedforlocomotion,isuniquetoechinoderms.
Review
1. Althoughtherearenotasmanyechinodermspeciesastherearearthropods,theechinodermsarestillthelargestphyluminwhichenvironment?
2. EchinodermsarecloselyrelatedtothephylumChordata,whichincludeshumans,eventhoughtheylookmorecloselyrelatedtocnidarians.Whatisonecluethatsupportthetheorythatechinodermsarerelatedtochordates?
3. Howdoesearlycelldevelopmentdifferbetweenprotostomesanddeuterostomes?4. Howdoesthecoelomdevelopindeuterostomes?5. Unlikeotheranimals,thecoelomofechinodermsformstwocavities.Whatarethesetwo
cavitiesfor?