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BIOL2015–EvolutionandDiversityLab11:Deuterostomia:

Echinodermata,Hemichordata,&ChordataIntroduction

Inourfinaldiversitylabwewillexaminethedeuterostomes,whichincludethefollowingthreephyla:Echinodermata,Hemichordata,andChordata.Althoughthesephylaseemoutwardlyverydifferent,theysharecommonfeaturesintheirearlydevelopmentthatdistinguishthemfromotherbilaterallysymmetricalanimals.Rememberindeuterostomes,theembryonicblastoporegivesrisetotheanus.Intheprotostomes,whichincludesannelids,arthropods,andmollusks,theblastoporegivesrisetothemouth.Althoughadultechinodermshaveradialsymmetry,theirlarvaearebilaterallysymmetrical,thustheyareclassifiedalongwithchordatesasbilaterallysymmetrical.

Figure1:PhylogenyofAnimalia

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Echinodermata

Thewordechinodermmeansspinyskin.Thesearemarineorganismswithfive-foldradialsymmetry,commonlycalledstarfish(ClassAsteroidea),sanddollarsandseaurchins(ClassEchinoidea),seacucumbers(ClassHolothuroidea),andsealilies(ClassCrinoidea).Theyarecoelomatesandtheyhaveanendoskeletonthatismadeofcalciumcarbonate,whicharisesfromthemesoderm.Sexesareseparateinechinodermsandfertilizationisexternal;thereisalarvalstagewithbilateralsymmetry.Auniquederivedfeatureofthegroupisitswater-vascularsystem;seawaterispumpedthroughaseriesofductstoworkthetubefeetandthesuctioncupsattheirtips.

Asteroidea(Starfish)Findthepreservedstarfishondisplay.On thedorsalsideyoucanseethecentraldisk andradiatingarms.Justoffcenteronthecentraldiskthereisasmallsieve-likeplate,calledthemadreporite,whichservesastheconduitthroughwhichseawaterentersintothewatervascularsystem.Smallfleshyextensionsamong thespinesarethesoft,hollowskingillsforrespiration;theycommunicatewiththecoelom.Ontheventralsideofthearms thereareambulacralgroovesthatarefilledwith thefleshytubefeet.Tubefeetandskingills arebothusedforgasexchangeandexcretion ofnitrogenouswaste.Themouthisinthecenteroftheventralside.Echinodermsmovebyalternatingthesuctionandreleaseoftubefeet.Asteroidscanregeneratelostordamagedarms,ifenoughofthecentraldiscisintact.Anisolatedarmsoondies(thereisanexception,knowninonegenuswhereanarmcanregeneratetherest).Useadissectingmicroscopetofindthepincer-likepedicellariaethatprojectfromthesurfaceofthedermis.Examineanopenedstarfishandnotethelargecoelom.Findtheampullaeatthebasesofthetubefeet,andtheringandradialcanalsofthewatervascularsystem.Observethelivingstarfishintheaquariumandnoticethemovementsoftheirtubefeet.Ophiuroidea(brittlestars)Findthepreservedbrittlestarsondisplay.Organsareconcentratedinthelargecentraldiskbecausetheyhavelongslenderarms.Brittlestarsprimarilyusetheirarmstomovewithspeedandagility.Somebrittlestarsarecarnivoreswhileothersarefilterfeeders.Examinethedorsalsurfaceandnoticetheabsenceofamadreporite.Ontheventralsurface,noticethatossiclescovertheambulacralgroovesandthatthemouthissurroundedbyfivemoveablejawplates.Thebursalslits(whichradiatearoundthemouth)openintosacsinwhichwaterconstantlycirculates,andgasexchangeoccurs.Findthelivingbrittlestarsinthedisplaylargeaquariumandnotewheretheyresidewithintheaquarium.

Echinoidea(SeaUrchins,SandDollars)Echinoidshaveashellmadefromwell-developeddermalossicles.TheTubefeetlinetheambulacralregions,buttheambulacralgroovesareclosed.Pedicellariaearepresent,likestarfish.Seaurchinsmoveslowlyacrossthesubstrateconsumingsmallanimalsorscrapingalgaeoffofrockswithpowerful,plate-linedjaws.Observe

Figure2:Starfish

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thelivingseaurchinsinRoomthelargetankandnoticehowthespinesmove(musclesatthespine’sbasecontrolstheirmovement).Examinethepreservedseaurchinsondisplay.Examinethepreservedsanddollarsondisplay.Notehowtheshellisflattened.Beabletoidentifythedifferentechinoids.

Holothuroidea(SeaCucumbers)Althoughtheyareechinoderms,seacucumbershavesoftskinthanotherechinoderms.Allthatremainsoftheircalcareousskeletonsaresmallspicules.Theirambulacralregionsextendfrommouthtoanus,andtheambulacralgroovesareclosed.Modifiedtubefeetcalledcircumoraltentaclessurroundthemouth.Thepedicellariaeseeninotherechinodermsareabsent.FindthelivingseacucumbersinthemainaquariuminRoom170,andobservethecircumoraltentacles.Examinethepreservedseacucumbersondisplay.

HemichordataHemichordatesaresmalltomediumsizedmarinewormsandarerepresentedbythreeextantclasses,Enteropneusta(acornworms),Pterobranchia(small,deepwaterworms),andPlanctosphaeroidea(knownonlybylarvaofonespecies).Enteropneustaisthemostspeciousofthesethreeclassesandisrepresentedbyabout70species.ThiswillbetheonlyHemichordatewehaveforyoutoexamineinlab.Enteropneusta(acornworms)Ofthechordatecharacteristics,acornwormshaveonlypharyngealslits.Theyfeedondetritusandburrowinsoftmarinesediments.Findthepreservedacornwormondisplayandobserve(begentile–theyarefragile)theproboscis,thecollar,thetrunkandthepharyngealslits.

Figure3:AcornWorm

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Chordates

ThePhylumChordatacontainsthreesubphyla,Cephalochordata,Urochordata,andVertebrata(sometimescalledCraniata).WewilllookattheCephalochordata,thelancelets.AnotherSubphylumistheVertebrata;Fish,amphibians,reptiles,birds,andmammalsareallvertebrates.Amphioxusismostlikelythedescendantfromanearlystageinchordateevolution,anditshowsallfiveofthedefiningcharacteristicsofchordates:

1. notochord2. pharyngealslits3. endostyleorthyroidgland4. dorsalhollownervecord5. post-analtail

Allchordatesshowthesecharacteristicsatsomestageintheirlifetimes.Youwillalsonoticefeaturesthatarecommonamongmanyanimalphyla,includingbilateralsymmetry,coelom,andbodysegmentation(seenintheseriesofmuscleblockscalledmyomeres).Thecirculatoryanddigestivesystemsareseparate.

Thefirsttwosubphyla,CephalochordataandUrochordata,areoftenreferredtocollectivelyastheProtochordates,meaningthefirstChordates.Whileall5synapomorphiesarepresentinthesetwosubphyla,somepersistthroughouttheentirelifespanoftheorganismwhileothersaretransientandareonlypresentinthelarvalstate.Cephalochordata(Amphioxus)Althoughtheyspendmostoftheirtimeburiedinthesandfilterfeedingonsmallfoodparticles,lanceletsaresmall,'fish-like'animalsandaredecentswimmers.FindthepreservedAmphioxusondisplay.Thereisrelativelylittlecephalizationinlancelets,butyoumayobserveasmallenlargementattheanteriorendofthedorsalnervechord.Likevertebrates,butunliketunicatelarvae,thetrunkmuscleshavemyomeres(segmentedstructures).Theendostyle(aciliatedgrooveinthepharynx)isusedtotrapfoodparticlesandtransportthemtothestomach.Alsoobservethenotochordandtheatriopore,whichistheequivalentoftheexcurrentsiphonoftunicates,andtheanusandpostanaltail.

Findapreparedslidelabeled"Amphioxusimmaturew.m."and"Amphioxusimmatureadultw.m."

Figure4:Lancelet

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Findallthechordatefeaturesontheimmatureslide(youmaynotbeabletoseetheendostyle).Theadultslideshowsthepharyngealslitsandtentacles(cirri)better.Youshouldalsobeabletolocatetheforward-pointingcecumofthedigestivetract.Whatdoesthececumdo?Findapreparedmicroscopeslidelabeled"AmphioxusPharynxandGonadsec."Findthenotochord,dorsalnervecord,pharynxwithgillbarsandgillslits,andthelargemuscles.Youshouldtrytofindtheendostyleatthebottomofthepharynx,andtrytodistinguishtheatriumfromthecoelom.

Figure5:Lancelet

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Urochordata(Tunicates)AscidiaceaAlthoughurochordatesmaylookabitlikespongesintheiradultlifestage,theyarecurrentlythoughttobethesistergroupofvertebrates.WhymightthesebethebettersistergrouptoVertebrates?MostUrochordatesarehermaphroditicandusecross-fertilizationtoreproduce.Somehaveasingletestisandsingleovary(whenbothgonadsarecombinedintoasinglemassitisoftencalledtheovotestis)andarelocatedintheloopofthegut.Somespeciespossessmanygonadsthatareembeddedintheconnectivetissuelayerofthebodywall.Solitaryspeciesareoviparousandtheireggshaveverylittleyolk.Eggsarereleasedthroughtheatrial(excurrent)siphonandarefertilizedexternally.Incontrast,mostcolonialspeciesareviviparousandtheireggshavesignificantamountofyolk.Gestationtakesplaceineithertheoviductorwithintheatriumitselfandthelecithotrophictadpolelarvaearethenreleasedfromtheatrialsiphon.Findthepreservedseasquirtsondisplay(CionaandMolgula).Observetheoral(incurrent)andatrial(excurrent)siphonsinCiona(youdon’tneedtodistinguishtheminMolgula).Findapreparedmicroscopeslidelabeled"Ecteinascidiaw.m."Usethe4Xobjectivetoobservetheseadultanimals.Findthesiphons,theverylargepharynxwithitsconspicuousperforations.TheAscidiantadpolelarvaearelecithotrophicandthereforeobtainalltheirnutrientsfromtheyolk.So,dependingontheamountofyolk,thelarvalstagecanbeextremelyshortrangingfromafewminutestoabout36hours.ItisduringthisstagethatallfiveChordatecharactersareevident.SomeoftheChordatecharactersarelostasthetadpoleundergoesmetamorphosisandtransformsintothesessilefilter-feedingadultthatwearefamiliarwith.Findapreparedmicroscopeslidelabeled"AscidianTadpolew.m."Thetatpolesaresmallanditisdifficulttoidentifymanyoftheinternalstructures.Findthepostanaltailandthenotochord.

Figure6:Ciona

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VertebrataUnfortunatelywedonothaveenoughtimetoreallyexplorethediversityoftheVertebrates,whichisthemostspeciousgroupwithinDeuterostomia.ThoughwewillnotbegoingintomuchdetailIthoughtitwouldbefuntoatleastmakeasuperficialsweepoverthemajorvertebratetaxonomicalgroups.Fortunately,vertebratesareusuallymorefamiliartothevastmajorityofpeopleandarethereforemucheasiertolearnandremember.SincewedonothavetimetogointomuchdetailwiththevertebratesIhavechosen1to2unifyingmorphologicalorfunctionalthemesforeachofthemajorvertebrateclassesthatwecanexploreduringourfinallaboratorysession.

Agnatha:Jawlessvertebrates

Myxini(Hagfish)andCephalapidomorphi(Lamprey)Hagfishareentirelymarineandaremajorscavengersonsunkenvertebratecarcasses.Althoughtheyarehighlyspecializedinmanyrespects,hagfishesarethoughttobethemostprimitivelivingvertebrates.Theabsenceofboneinthegroupisatrulyprimitivecondition.Adultlampreysarehighlypredaceousfishesthatattachthemselvestotheexternalsurfaceofotherfishesandscrapethefleshoftheirpreywithamodified“tongue.”Lampreyevolvedfromanimportantgroupoffishesthathadheavybonyplatesintheirskin.Duetothesecondarylossofbone,therearealmostnofossillampreys.ExaminethepreservedHagfishandLamprey.Findtheeyes,mouth,andthepharyngealopeningsonbothorganisms.Whatarethemostobviousdifferencesbetweenthesestructuresonthesetwoprimitivevertebrates?Whyarethesestructuressodifferent?

Gnathostomata:Jawedvertebrates

Chondrichthyes-Sharks,rays,skates,ratfishMostofthevertebratesinthisgroupwillprobablybefamiliartoyou.Theylackabonyskeletonbutoftenelementsoftheirskeletonwillcalcifyformingcalcifiedcartilage.Itissimilartoboneinthatitismineralizedandisthereforestrongincompression,howeveritisdifferentfromboneinmanyways.Onebigfunctionaldifferenceisthatcalcifiedcartilageisnotdynamic,meaningthatitcannotremodelitselfwhentheloadingregimeschange–somethingbonedoesexceptionallywell.

Sharksaretheonlyfishthathaveplacoidscales.Placoidscalesareverydifferentthanallotherfishscales.Theyaremadeofdentin,coveredwithenamel,andhaveapulpcavity,makingthemmorphologicallysimilartoteeth.Infact,theydevelopinacontinuouslinewiththeteethandcovertheentiresurfaceofthebody.

Findtheslideentitled“Placoidscale”.Trytoidentifytheenamel,dentin,andpulpcavityontheplacoidscale.Thereisalsoalaminatedphotoyoucanlookatthatshowstheseregions.

Compareandcontrastthefollowingfeaturesforthepreservedchondrichthyanspecimensinlab:

generalbodyformsizeandshapeoffinspositionofgillslits,spiracleandeyes

Howdothesefeaturesrelatetodifferencesinlifehabit?

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Actinopterygii–Ray-finnedfishesActinopterygianfishesareanincrediblydiversegroupofvertebrates.Theyfirstappeared500millionyearsago.ThemoreprimitiveactinopterygiansarewithinthegroupHolosteiwhilethemorederivedactinopterygiansarewithinthegroupTeleostei.

HolosteiHolosteansarethemostprimitiveoftheray-finnedfishes.Theyhaveverysturdyskullswithteethonallthebonesformingthejaws.Comparethejawsoftheholosteanswiththoseoftheteleostfishes.TeleosteiOfalltheordersassociatedwithActinopterygii,theTeleostsarethemostdiverseanddominatethefishfaunasofbothfreshandmarinewaters.TheprimaryreasonoftencitedforthedominanceofTeleostfishistheinnovationofanewtypeofsuctionfeeding,onethatismadepossiblebyachangeintheshapeofthemouthcoupledwithextrememobilityinspecificjawbones.Forclarificationpurposes,itisthemoreadvancedTeleoststhathavethemorphologicalfeaturesnecessarytosuctionfeed,whicharedescribedintheparagraphbelow.PrimitiveTeleostsdonotsuctionfeedandthereforemustcapturepreywiththeirteeth.AdvancedTeleostshaveevolvedamoreroundedgapeandtheabilitytoprojecttheirpremaxillae.Astheclosedmouthispushedforwardthevolumeoftheoralcavityincreaseswhilethepressurewithintheoralcavitydecreases.Whenthemouthisopenedthepreyitemandthewatersurroundingitisrapidlytransportedintothemouthaswatermovesfromhightolowpressure.Thustheyareabletocapturepreywithoutactuallygrabbingaholdofit.OneoftheconsequencesofthisnewmodeoffeedingisthatadvancedTeleostsnolongerhaveteethonthemaxillae.Thisallowsthemaxillaetoactasleverarmsthatcauseprojectionofthepremaxilla.SomehighlyadvancedTeleostsareevenmorespecializedbecausetheyhavelostteethonthedentaryandpremaxillaeaswell.Canyoucomeupwithanexplanationforwhyteethmightbeproblematicforsuctionfeeding?

Examinethefishskullsandidentifythebonesofthejawjoint.Canyoudeterminewhichisthemostprimitiveandwhichisthemostderived?

Sarcopterygii–Lobe-finnedfishes

Lungfish(Dipnoi)andtheCoelacanth(Actinista)aretheonlylivingrepresentativesoftheSarcopterygianfishes.Athirdgroupofsarcopterygians,Rhipidistia,isancestraltothefirsttetrapodsandthereforerepresentsanimportantgroup.

RhipidistiaThefeaturethatlinksRhipidistiatothestemamphibiansisaverydistincttoothmorphologycalledthelabyrinthodonttooth(seeillustrationtotheright).Inadditiontothistoothtype,theRhipidistianfisheshadmuscularfinsthatcontainedrobustbonyelements(characteristicofSarcopterygianfishes),whichareverydifferentfromtheflimsyfinassociatedwiththeActinopterygianfishes.Itisthemuscularfinandtherobustboneswithinthemthatservedasapreadaptationfortheabilitytosupportthebodyonland.ExaminetheCoelacanthphotointheTimeLifebookentitledTheFishes.Notethefleshyfinstructure.

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Tetrapoda:Vertebrateswithfourlimbs

AmphibiaTheclassAmphibiaincludesmanygenerathatarenowextinct.Duetolimitedtimewewillnotbeconsideringanyoftheextinctmembersofthisgroup.Instead,wewillfocusourattentiononthecladeknowasLissamphibia(lissos=smooth,amphi=both,bios=life),the“smooth-skinned”amphibians.

Lissamphibia-Salamanders,frogs,andcaecilliansAllmodernlissamphibiansarethoughttostemfromacommonancestrywithinthelabyrinthodonts.Threedistinctorders:Urodela,Anura,andApodaareincludedwithinthesubclass.Detailedstructuralcharacteristicsoftheinnerear,teeth,andlimbsaresharedbyalllissamphibiansandservetounitethegroup.

Caudata–SalamandersandnewtsCaudatesaretailedamphibians,whichincludethesalamandersandnewts.Unlikeanurans,thebodybuildofcaudatesisfairlygeneralized(comparelimbandtrunkproportionswiththoseofthefrog).Caudatesmaybeaquaticorfullyterrestrial.Someofthelargeraquaticsalamandersfailtometamorphosecompletelyandretainsuchlarvalfeaturesasexternalgillsintoadulthood.Anura–FrogsandtoadsFrogsandtoadsareamongthemostspecializedtetrapods,especiallyintheirmodesoffeedingandlocomotion.Fusionofthetailvertebraeintoasinglerod-likebone(urostyle),elongationofthehindlimbs,andshorteningofthetrunkareallcharacteristictraitsofanurans.IncontrasttoUrodeles,thereisgenerallyawell-developedeardrum.Gymnophiona–CaecilliansCaeciliansareoftenmistakenforeelsorearthworms.Theyshownotraceoflimbsorgirdles,hencethenameApoda(“nofeet”).Aburrowinglifestyle(possessedbycaecilians)isusuallyaccompaniedbylimbreductionorloss,aswellasreducedeyesoreyesight.Mostcaecilianshaveverypooreyesight,somelackeyesentirely,hencethecommonnamecaecilian(fromcaecus,meaningblind).Unlikefrogsandsalamanders,caecilianshaveasolidandcompactskull,usefulforburrowing.Theyarerestrictedtodamp,tropicalhabitats;someareentirelyaquatic.Examinetheskeletalspecimensofthefrogandsalamander.Whichpartsofthefrogskeletonarehighlyspecializedforitsdistinctivemodeoflocomotion?

Amniota

ReptiliaThoughtheReptilesareaparaphyleticgroup,itisconvenienttousebecausethenameisfamiliartomostpeople.Wewillincludethebirdsinthissectiontomakeitmoreaccurate.Themostimportantinnovationwithinthisgroupofanimalsistheamnioticegg,whichallowedanimalstomakeacompletebreakfromthewater.Wewillstartwiththeturtles,whichareoneofthemorebasalgroupsofreptilesaccordingtomolecularevidence.

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Testudines–TurtlesTurtles(Testudinata)aretheonlysurvivingreptileswithanapsidskulls(withoutfenestrations;comparewithcrocodilianskull).Theyarequitespecializedinsuchfeaturesastheirrigidshell(whichincorporatestheribs,vertebrae,andsternalelements),andthecompleteabsenceofteeth.

Squamata–SnakesandlizardsLizardsandsnakescomprisetheorderSquamata.Snakeshaveevolvedfromprimitiveburrowinglizards.Evidentagain,aburrowinglifestyleoftenleadstolimbreduction,orabsence.Snakesdifferfromlizardsinmanydetailsoftheiranatomyotherthanabsenceoflimbs(somelizardsarelikewiselimbless).Forexample,lizardshavemoveableeyelidsandanexternalauditorymeatus(opening),charactersthatsnakeslack.

ArchosauriaThiscladeincludesthecrocodiliansandisrelatedtobirds.Thoughthesetwogroupsareonlydistantlyrelated,molecularevidencesuggeststhattheyareclosertooneanotherthantheyaretothesquamates(lizardsandsnakes)andthetestudines(turtles).

Crocodylia–CrocodilesandalligatorsTheorderCrocodylia(sometimesspelledCrocodilia)isrepresentedbycrocodilesandalligators,thegroupalsoincludescaimansandgavials.Thecrocodiliansareoneofthefewgroupsofanimalsthatreadilythinkofhumansasapreyitem.Thecrocodilianskullisnotlikeanyotherlivingreptile.

Examinethecrocodilian,testudinianandsquamateskulls.Disregardingthesize,whatfeaturesaredifferent?Trytocomeupwithanexplanationforthedifferencesyousee.

Aves–BirdsAlmostallbirds,fromalbatrossestozebrafinches,allbelongtotheSuperorderNeognathae(meaningnewjaw),definedbyamodifiedandmobilepalatestructure.Allhaveakeeledsternum;referredtoasacarina(meaningkeeled),thoughnotallareabletofly(e.g.,penguins–thoughtheydo“fly”inthewater).Inwhatotherwaysdoesthemorphologyofbirdssuggestthattheyarecapableofflying?

Examinethepigeonskeleton.Notethehighlymodifiedforelimb.Whichspecificregionoftheforelimbisthemostderived?Howdobirdscompensatefortheirextremelyspecializedforelimb?

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Mammalia–MammalsToday,placentalmammalsaredominantonalmostalllandmassesexceptAustralia.LiketheAmphibians,wedonothavetimetoexploremammalsinanyrealdetail.Instead,wewillfocusonaparticularmammaliansynapomorphy,uprightlimbposture.Moderninsectivores(e.g.shrewsandtenrecs)arepresentdaymammalsthatarethoughttomostresembleancestralmammals.Oneofthebiggestinnovationsweseeinmammalsisanuprightposture,placingthelimbsunderneaththetrunk.Comparethemammalianandsalamanderlimbposture.Howaretheysimilar?Howaretheydifferent?Anothermammaliansynapomorphyisamuscleexclusivelyforventilation,thediaphragm.Withadiphragm,mammalswereabletouncouplemovementfromventilation.Withthesetwouncoupled,weseetheriseofspecializedlimbposturesforenhancedlocomotiveability,illustratedtotheright.Evolutionarymodificationofthedistalendofthelimbincreasesstridelength,whichinturnincreasesspeed(speed=stridelengthxstriderate).Inadditiontotheincreaseinlimblengththereisalsomodificationtothenumberofbonesinthemiddleanddistallimbsegments.