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Echinoderms Name: _________________________________________ Class:__________ Date: ____________ Vocabulary Term Definition echinoderm Invertebrate, such as a sea star or a sand dollar, that is characterized by a spiny endoskeleton, radial symmetry as adults, and a water vascular system. nerve net Interconnected neurons with no central brain. radial symmetry Symmetry of a body plan in which there are no head or rear ends, so the body can be divided into two identical halves at any point, like a pie. sea urchin Echinoderm with a round, soft body enclosed in a shell covered with long spines. Believe it or not, this is an animal. See the mouth and arms? It is a sea lily, a crinoid echinoderm. Crinoids are essentially a mouth on the top surface that is surrounded by feeding arms. Although the basic echinoderm pattern of fivefold symmetry can be recognized, most crinoids have many more than five arms. Crinoids usually have a stem used to attach themselves to a surface, but many become free- swimming as adults. Characteristics of Echinoderms Echinoderms are marine organisms that make up the phylum Echinodermata. Members of the phylum include sea stars (starfish), sea urchins, sea cucumbers, sand dollars, brittle stars, and feather stars. There are 7000 different living species of echinoderms and 13,000 identified fossil species. Although the number of echinoderm species is very small compared to the number of species within the arthropod phylum, Echinodermata is the largest phylum to lack freshwater and terrestrial species. Another distinction is that echinoderms are among the most distinct organisms within the animal kingdom. You learned in the Evolution concepts that evolution is not a linear series of steps and does not necessarily lead to increasing levels of complexity over time. It is based on the response of populations of species to the adaptive pressures of their environments and favors those that have the greatest reproductive success. The phylum Echinodermata provides an excellent example of the non-linear, bush-like trajectory of evolutionary change. Echinoderms are the most closely related phylum to the phylum Chordata, which includes many complex organisms such as humans. Their shared common ancestor was likely a bilaterally symmetrical organism with a cephalized (centralized in a head region) nervous system. Yet, as you will learn in this lesson, many details of echinoderm structure and function are simpler than those of this likely predecessor. The organisms within this phylum evolved these structures as adaptations to the pressures placed on them by their environments, just as the various groups within the phylum Chordata did. You will see in this chapter that the results of these two evolutionary paths are very different, despite having a fairly recent common origin. Let’s take a look at some of the distinguishing features of echinoderms, and then we will examine their structures and how they function in more detail. The word Echinodermata means “spiny skin.” If you have ever seen a sea urchin then this name will certainly seem fitting. Although these spines may look like components of an exoskeleton at first glance, echinoderms do not have an exoskeleton. Instead, the spines are extensions of an internal endoskeleton. The endoskeleton is composed of calcium carbonate plates and spines that are actually covered by a thin layer of epidermis (skin). These plates and spines are held together by mesodermal tissue.

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Page 1: echinoderms read H - Somma Science€¦ · Summary • Members of the phylum Echinodermata include sea stars (starfish), sea urchins, sea cucumbers, sand dollars, brittle stars, and

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.

Page 2: echinoderms read H - Somma Science€¦ · Summary • Members of the phylum Echinodermata include sea stars (starfish), sea urchins, sea cucumbers, sand dollars, brittle stars, and

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.

Page 3: echinoderms read H - Somma Science€¦ · Summary • Members of the phylum Echinodermata include sea stars (starfish), sea urchins, sea cucumbers, sand dollars, brittle stars, and

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?


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