echinoderms read h - somma science€¦ · summary • members of the phylum echinodermata include...

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?