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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.
