Download BIOL 2015 – Evolution and Diversity

BIOL2015–EvolutionandDiversity
Lab12:Deuterostomia:
Echinodermata,Hemichordata,&Chordata
Introduction
Inourfinaldiversitylabwewillexaminethedeuterostomes,whichincludethefollowingthreephyla:
Echinodermata,Hemichordata,andChordata.Althoughthesephylaseemoutwardlyverydifferent,theyshare
commonfeaturesintheirearlydevelopmentthatdistinguishthemfromotherbilaterallysymmetricalanimals.
Rememberindeuterostomes,theembryonicblastoporegivesrisetotheanus.Intheprotostomes,which
includesannelids,arthropods,andmollusks,theblastoporegivesrisetothemouth.Althoughadultechinoderms
haveradialsymmetry,theirlarvaearebilaterallysymmetrical,thustheyareclassifiedalongwithchordatesas
bilaterallysymmetrical.
Figure1:PhylogenyofAnimalia
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Echinodermata
Thewordechinodermmeansspinyskin.Thesearemarineorganismswithfive-foldradialsymmetry,commonly
calledstarfish(ClassAsteroidea),sanddollarsandseaurchins(ClassEchinoidea),seacucumbers(Class
Holothuroidea),andsealilies(ClassCrinoidea).Theyarecoelomatesandtheyhaveanendoskeletonthatismade
ofcalciumcarbonate,whicharisesfromthemesoderm.Sexesareseparateinechinodermsandfertilizationis
external;thereisalarvalstagewithbilateralsymmetry.Auniquederivedfeatureofthegroupisitswatervascularsystem;seawaterispumpedthroughaseriesofductstoworkthetubefeetandthesuctioncupsattheir
tips.
Asteroidea(Starfish)
Findthepreservedstarfishondisplay.On
the
dorsalsideyoucanseethecentraldisk
and
radiatingarms.Justoffcenteronthe
centraldiskthereisasmallsieve-like
plate,calledthemadreporite,which
servesastheconduitthroughwhich
seawaterentersintothewatervascular
system.Smallfleshyextensionsamong
the
spinesarethesoft,hollowskingillsfor
respiration;theycommunicatewiththe
coelom.Ontheventralsideofthearms
there
areambulacralgroovesthatarefilledwith
the
fleshytubefeet.Tubefeetandskingills
are
Figure2:Starfish
bothusedforgasexchangeandexcretion
of
nitrogenouswaste.Themouthisinthecenteroftheventralside.Echinodermsmovebyalternatingthesuction
andreleaseoftubefeet.Asteroidscanregeneratelostordamagedarms,ifenoughofthecentraldiscisintact.
Anisolatedarmsoondies(thereisanexception,knowninonegenuswhereanarmcanregeneratetherest).
Useadissectingmicroscopetofindthepincer-likepedicellariaethatprojectfromthesurfaceofthedermis.
Examineanopenedstarfishandnotethelargecoelom.Findtheampullaeatthebasesofthetubefeet,andthe
ringandradialcanalsofthewatervascularsystem.
Observethelivingstarfishintheaquariumandnoticethemovementsoftheirtubefeet.
Ophiuroidea(brittlestars)
Findthepreservedbrittlestarsondisplay.Organsareconcentratedinthelargecentraldiskbecausetheyhave
longslenderarms.Brittlestarsprimarilyusetheirarmstomovewithspeedandagility.Somebrittlestarsare
carnivoreswhileothersarefilterfeeders.
Examinethedorsalsurfaceandnoticetheabsenceofamadreporite.Ontheventralsurface,noticethat
ossiclescovertheambulacralgroovesandthatthemouthissurroundedbyfivemoveablejawplates.The
bursalslits(whichradiatearoundthemouth)openintosacsinwhichwaterconstantlycirculates,andgas
exchangeoccurs.
Findthelivingbrittlestarsinthedisplaylargeaquariumandnotewheretheyresidewithintheaquarium.
Echinoidea(SeaUrchins,SandDollars)
Echinoidshaveashellmadefromwell-developeddermalossicles.TheTubefeetlinetheambulacralregions,
buttheambulacralgroovesareclosed.Pedicellariaearepresent,likestarfish.Seaurchinsmoveslowlyacross
thesubstrateconsumingsmallanimalsorscrapingalgaeoffofrockswithpowerful,plate-linedjaws.Observe
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thelivingseaurchinsinRoomthelargetankandnoticehowthespinesmove(musclesatthespine’sbase
controlstheirmovement).
Examinethepreservedseaurchinsondisplay.
Examinethepreservedsanddollarsondisplay.Notehowtheshellisflattened.Beabletoidentifythe
differentechinoids.
Holothuroidea(SeaCucumbers)
Althoughtheyareechinoderms,seacucumbershavesoftskinthanotherechinoderms.Allthatremainsoftheir
calcareousskeletonsaresmallspicules.Theirambulacralregionsextendfrommouthtoanus,andthe
ambulacralgroovesareclosed.Modifiedtubefeetcalledcircumoraltentaclessurroundthemouth.The
pedicellariaeseeninotherechinodermsareabsent.Findthelivingseacucumbersinthemainaquariumin
Room170,andobservethecircumoraltentacles.
Examinethepreservedseacucumbersondisplay.
Hemichordata
Hemichordatesaresmalltomediumsizedmarinewormsandarerepresentedbythreeextantclasses,
Enteropneusta(acornworms),Pterobranchia(small,deepwaterworms),andPlanctosphaeroidea(knownonlyby
larvaofonespecies).Enteropneustaisthemostspeciousofthesethreeclassesandisrepresentedbyabout70
species.ThiswillbetheonlyHemichordatewehaveforyoutoexamineinlab.
Enteropneusta(acornworms)
Ofthechordatecharacteristics,acornworms
haveonlypharyngealslits.Theyfeedon
detritusandburrowinsoftmarinesediments.
Findthepreservedacornwormondisplayand
observe(begentile–theyarefragile)the
proboscis,thecollar,thetrunkandthe
pharyngealslits.
Figure3:AcornWorm
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Chordates
ThePhylumChordatacontainsthreesubphyla,Cephalochordata,Urochordata,andVertebrata(sometimescalled
Craniata).WewilllookattheCephalochordata,thelancelets.AnotherSubphylumistheVertebrata;Fish,
amphibians,reptiles,birds,andmammalsareallvertebrates.Amphioxusismostlikelythedescendantfroman
earlystageinchordateevolution,anditshowsallfiveofthedefiningcharacteristicsofchordates:
1.
2.
3.
4.
5.
notochord
pharyngealslits
endostyleorthyroidgland
dorsalhollownervecord
post-analtail
Allchordatesshowthesecharacteristicsatsomestageintheirlifetimes.Youwillalsonoticefeaturesthatare
commonamongmanyanimalphyla,includingbilateralsymmetry,coelom,andbodysegmentation(seeninthe
seriesofmuscleblockscalledmyomeres).Thecirculatoryanddigestivesystemsareseparate.
Thefirsttwosubphyla,CephalochordataandUrochordata,areoftenreferredtocollectivelyasthe
Protochordates,meaningthefirstChordates.Whileall5synapomorphiesarepresentinthesetwosubphyla,
somepersistthroughouttheentirelifespanoftheorganismwhileothersaretransientandareonlypresentinthe
larvalstate.
Cephalochordata(Amphioxus)
Althoughtheyspendmostoftheirtimeburiedinthe
sandfilterfeedingonsmallfoodparticles,lanceletsare
small,'fish-like'animalsandaredecentswimmers.
FindthepreservedAmphioxusondisplay.Thereis
relativelylittlecephalizationinlancelets,butyoumay
observeasmallenlargementattheanteriorendofthe
dorsalnervechord.Likevertebrates,butunliketunicate
larvae,thetrunkmuscleshavemyomeres(segmented
structures).Theendostyle(aciliatedgrooveinthe
pharynx)isusedtotrapfoodparticlesandtransport
themtothestomach.Alsoobservethenotochordand
theatriopore,whichistheequivalentoftheexcurrent
siphonoftunicates,andtheanusandpostanaltail.
Findapreparedslidelabeled"Amphioxusimmature
w.m."and"Amphioxusimmatureadultw.m."
Figure4:Lancelet
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Findallthechordatefeaturesontheimmatureslide
(youmaynotbeabletoseetheendostyle).Theadult
slideshowsthepharyngealslitsandtentacles(cirri)
better.Youshouldalsobeabletolocatetheforwardpointingcecumofthedigestivetract.Whatdoesthe
cecumdo?
Findapreparedmicroscopeslidelabeled
"AmphioxusPharynxandGonadsec."
Findthenotochord,dorsalnervecord,pharynxwith
gillbarsandgillslits,andthelargemuscles.You
shouldtrytofindtheendostyleatthebottomofthe
pharynx,andtrytodistinguishtheatriumfromthe
coelom.
Figure5:Lancelet
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Urochordata(Tunicates)
Ascidiacea
Althoughurochordatesmaylookabitlikespongesintheiradultlifestage,theyarecurrentlythoughttobethe
sistergroupofvertebrates.WhymightthesebethebettersistergrouptoVertebrates?MostUrochordatesare
hermaphroditicandusecross-fertilizationtoreproduce.Somehaveasingletestisandsingleovary(whenboth
gonadsarecombinedintoasinglemassitisoftencalledtheovotestis)andarelocatedintheloopofthegut.
Somespeciespossessmanygonadsthatareembeddedintheconnectivetissuelayerofthebodywall.Solitary
speciesareoviparousandtheireggshaveverylittleyolk.Eggsarereleasedthroughtheatrial(excurrent)
siphonandarefertilizedexternally.Incontrast,mostcolonialspeciesareviviparousandtheireggshave
significantamountofyolk.Gestationtakesplaceineithertheoviductorwithintheatriumitselfandthe
lecithotrophictadpolelarvaearethenreleasedfromtheatrialsiphon.
Findthepreservedseasquirtsondisplay(CionaandMolgula).Observetheoral(incurrent)andatrial
(excurrent)siphonsinCiona(youdon’tneedtodistinguish
theminMolgula).
Findthelivingseasquirtsinthesmallerdisplayaquarium.
Trytoobservewatermovementaroundthesiphons.
Findapreparedmicroscopeslidelabeled"Ecteinascidia
w.m."
Usethe4Xobjectivetoobservetheseadultanimals.Find
thesiphons,theverylargepharynxwithitsconspicuous
perforations.
TheAscidiantadpolelarvaearelecithotrophicandtherefore
obtainalltheirnutrientsfromtheyolk.So,dependingonthe
amountofyolk,thelarvalstagecanbeextremelyshort
rangingfromafewminutestoabout36hours.Itisduringthis
stagethatallfiveChordatecharactersareevident.Someof
theChordatecharactersarelostasthetadpoleundergoes
metamorphosisandtransformsintothesessilefilter-feeding
Figure6:Ciona
adultthatwearefamiliarwith.
Findapreparedmicroscopeslidelabeled"AscidianTadpolew.m."
Thetatpolesaresmallanditisdifficulttoidentifymanyoftheinternalstructures.Findthepostanaltailand
thenotochord.
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Vertebrata
UnfortunatelywedonothaveenoughtimetoreallyexplorethediversityoftheVertebrates,whichisthemost
speciousgroupwithinDeuterostomia.ThoughwewillnotbegoingintomuchdetailIthoughtitwouldbefunto
atleastmakeasuperficialsweepoverthemajorvertebratetaxonomicalgroups.Fortunately,vertebratesare
usuallymorefamiliartothevastmajorityofpeopleandarethereforemucheasiertolearnandremember.Since
wedonothavetimetogointomuchdetailwiththevertebratesIhavechosen1to2unifyingmorphologicalor
functionalthemesforeachofthemajorvertebrateclassesthatwecanexploreduringourfinallaboratory
session.
Agnatha:Jawlessvertebrates
Myxini(Hagfish)andCephalapidomorphi(Lamprey)
Hagfishareentirelymarineandaremajorscavengersonsunkenvertebratecarcasses.Althoughthey
arehighlyspecializedinmanyrespects,hagfishesarethoughttobethemostprimitiveliving
vertebrates.Theabsenceofboneinthegroupisatrulyprimitivecondition.
Adultlampreysarehighlypredaceousfishesthatattachthemselvestotheexternalsurfaceofother
fishesandscrapethefleshoftheirpreywithamodified“tongue.”Lampreyevolvedfromanimportant
groupoffishesthathadheavybonyplatesintheirskin.Duetothesecondarylossofbone,thereare
almostnofossillampreys.
ExaminethepreservedHagfishandLamprey.Findtheeyes,mouth,andthepharyngealopeningson
bothorganisms.Whatarethemostobviousdifferencesbetweenthesestructuresonthesetwo
primitivevertebrates?Whyarethesestructuressodifferent?
Gnathostomata:Jawedvertebrates
Chondrichthyes-Sharks,rays,skates,ratfish
Mostofthevertebratesinthisgroupwillprobablyfamiliartoyou.Theylackabonyskeletonbutoften
elementsoftheirskeletonwillcalcifyformingcalcifiedcartilage.Itissimilartoboneinthatitis
mineralizedandisthereforestrongincompression,howeveritisdifferentfromboneinmanyways.
Onebigfunctionaldifferenceisthatcalcifiedcartilageisnotdynamic,meaningthatitcannotremodel
itselfwhentheloadingregimeschange–somethingbonedoesexceptionallywell.
Sharksaretheonlyfishthathaveplacoidscales.Placoidscalesareverydifferentthanallotherfish
scales.Theyaremadeofdentin,coveredwithenamel,andhaveapulpcavity,makingthem
morphologicallysimilartoteeth.Infact,developinacontinuouslinewiththeteethandcoverthe
entiresurfaceofthebody.
Findtheslideentitled“Placoidscale”.Trytoidentifytheenamel,dentin,andpulpcavityontheplacoid
scale.Thereisalsoalaminatedphotoyoucanlookatthatshowstheseregions.
Compareandcontrastthefollowingfeaturesforthepreservedchondrichthyanspecimensinlab:
generalbodyform
sizeandshapeoffins
positionofgillslits,spiracleandeyes
Howdothesefeaturesrelatetodifferencesinlifehabit?
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Actinopterygii–Ray-finnedfishes
Actinopterygianfishesareanincrediblydiversegroupofvertebrates.Theyfirstappeared500millionyears
ago.ThemoreprimitiveactinopterygiansarewithinthegroupHolosteiwhilethemorederived
actinopterygiansarewithinthegroupTeleostei.
Holostei
Holosteansarethemostprimitiveoftheray-finnedfishes.Theyhaveverysturdyskullswithteethon
allthebonesformingthejaws.Comparethejawsoftheholosteanswiththoseoftheteleostfishes.
Teleostei
OfalltheordersassociatedwithActinopterygii,theTeleostsarethemostdiverseanddominatethe
fishfaunasofbothfreshandmarinewaters.Theprimaryreasonoftencitedforthedominanceof
Teleostfishistheinnovationofanewtypeofsuctionfeeding,onethatismadepossiblebyachangein
theshapeofthemouthcoupledwithextrememobilityinspecificjawbones.Forclarificationpurposes,
itisthemoreadvancedTeleoststhathavethemorphologicalfeaturesnecessarytosuctionfeed,which
aredescribedintheparagraphbelow.PrimitiveTeleostsdonotsuctionfeedandthereforemust
capturepreywiththeirteeth.
AdvancedTeleostshaveevolvedamoreroundedgapeandtheabilitytoprojecttheirpremaxillae.As
theclosedmouthispushedforwardthevolumeoftheoralcavityincreaseswhilethepressurewithin
theoralcavitydecreases.Whenthemouthisopenedthepreyitemandthewatersurroundingitis
rapidlytransportedintothemouthaswatermovesfromhightolowpressure.Thustheyareableto
capturepreywithoutactuallygrabbingaholdofit.Oneoftheconsequencesofthisnewmodeof
feedingisthatadvancedTeleostsnolongerhaveteethonthemaxillae.Thisallowsthemaxillaetoact
asleverarmsthatcauseprojectionofthepremaxilla.SomehighlyadvancedTeleostsareevenmore
specializedbecausetheyhavelostteethonthedentaryandpremaxillaeaswell.Canyoucomeupwith
anexplanationforwhyteethmightbeproblematicforsuctionfeeding?
Examinethefishskullsandidentifythebonesofthejawjoint.Canyoudeterminewhichisthemost
primitiveandwhichisthemostderived?
Sarcopterygii–Lobe-finnedfishes
Lungfish(Dipnoi)andtheCoelacanth(Actinista)aretheonlylivingrepresentativesoftheSarcopterygian
fishes.Athirdgroupofsarcopterygians,Rhipidistia,isancestraltothefirsttetrapodsandtherefore
representsanimportantgroup.
Rhipidistia
ThefeaturethatlinksRhipidistiatothestemamphibiansisavery
distincttoothmorphologycalledthelabyrinthodonttooth(see
illustrationtotheright).Inadditiontothistoothtype,theRhipidistian
fisheshadmuscularfinsthatcontainedrobustbonyelements
(characteristicofSarcopterygianfishes),whichareverydifferentfrom
theflimsyfinassociatedwiththeActinopterygianfishes.Itisthe
muscularfinandtherobustboneswithinthemthatservedasa
preadaptationfortheabilitytosupportthebodyonland.
ExaminetheCoelacanthphotointheTimeLifebookentitledThe
Fishes.Notethefleshyfinstructure.
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Tetrapoda:Vertebrateswithfourlimbs
Amphibia
TheclassAmphibiaincludesmanygenerathatarenowextinct.Duetolimitedtimewewillnotbe
consideringanyoftheextinctmembersofthisgroup.Instead,wewillfocusourattentionontheclade
knowasLissamphibia(lissos=smooth,amphi=both,bios=life),the“smooth-skinned”amphibians.
Lissamphibia-Salamanders,frogs,andcaecillians
Allmodernlissamphibiansarethoughttostemfromacommonancestrywithinthelabyrinthodonts.
Threedistinctorders:Urodela,Anura,andApodaareincludedwithinthesubclass.Detailedstructural
characteristicsoftheinnerear,teeth,andlimbsaresharedbyalllissamphibiansandservetounitethe
group.
Caudata–Salamandersandnewts
Caudatesaretailedamphibians,whichincludethesalamandersandnewts.Unlikeanurans,thebody
buildofcaudatesisfairlygeneralized(comparelimbandtrunkproportionswiththoseofthefrog).
Caudatesmaybeaquaticorfullyterrestrial.Someofthelargeraquaticsalamandersfailto
metamorphosecompletelyandretainsuchlarvalfeaturesasexternalgillsintoadulthood.
Anura–Frogsandtoads
Frogsandtoadsareamongthemostspecializedtetrapods,especiallyintheirmodesoffeedingand
locomotion.Fusionofthetailvertebraeintoasinglerod-likebone(urostyle),elongationofthehind
limbs,andshorteningofthetrunkareallcharacteristictraitsofanurans.IncontrasttoUrodeles,there
isgenerallyawell-developedeardrum.
Gymnophiona–Caecillians
Caeciliansareoftenmistakenforeelsorearthworms.Theyshownotraceoflimbsorgirdles,hencethe
nameApoda(“nofeet”).Aburrowinglifestyle(possessedbycaecilians)isusuallyaccompaniedbylimb
reductionorloss,aswellasreducedeyesoreyesight.Mostcaecilianshaveverypooreyesight,some
lackeyesentirely,hencethecommonnamecaecilian(fromcaecus,meaningblind).Unlikefrogsand
salamanders,caecilianshaveasolidandcompactskull,usefulforburrowing.Theyarerestrictedto
damp,tropicalhabitats;someareentirelyaquatic.
Examinetheskeletalspecimensofthefrogandsalamander.Whichpartsofthefrogskeletonare
highlyspecializedforitsdistinctivemodeoflocomotion?
Amniota
Reptilia
ThoughtheReptilesareaparaphyleticgroup,itisconvenienttousebecausethenameisfamiliartomost
people.Wewillincludethebirdsinthissectiontomakeitmoreaccurate.Themostimportantinnovation
withinthisgroupofanimalsistheamnioticegg,whichallowedanimalstomakeacompletebreakfrom
thewater.Wewillstartwiththeturtles,whichareoneofthemorebasalgroupsofreptilesaccordingto
molecularevidence.
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Testudines–Turtles
Turtles(Testudinata)aretheonlysurvivingreptileswithanapsidskulls(withoutfenestrations;compare
withcrocodilianskull).Theyarequitespecializedinsuchfeaturesastheirrigidshell(which
incorporatestheribs,vertebrae,andsternalelements),andthecompleteabsenceofteeth.
Squamata–Snakesandlizards
LizardsandsnakescomprisetheorderSquamata.Snakeshaveevolvedfromprimitiveburrowing
lizards.Evidentagain,aburrowinglifestyleoftenleadstolimbreduction,orabsence.Snakesdiffer
fromlizardsinmanydetailsoftheiranatomyotherthanabsenceoflimbs(somelizardsarelikewise
limbless).Forexample,lizardshavemoveableeyelidsandanexternalauditorymeatus(opening),
charactersthatsnakeslack.
Archosauria
Thiscladeincludesthecrocodiliansandthebirds.Thoughthesetwogroupsareonlydistantlyrelated,
molecularevidencesuggeststhattheyareclosertooneanotherthantheyaretothesquamates(lizards
andsnakes)andthetestudines(turtles).
Crocodylia–Crocodilesandalligators
TheorderCrocodylia(sometimesspelledCrocodilia)isrepresentedbycrocodilesandalligators,the
groupalsoincludescaimansandgavials.Thecrocodiliansareoneofthefewgroupsofanimalsthat
readilythinkofhumansasapreyitem.Thecrocodilianskullisnotlikeanyotherlivingreptile.
Examinethecrocodylianandiguanidskulls.Disregardingthesizediscrepanciesbetweenthem,what
featuresaredifferent?Trytocomeupwithanexplanationforthedifferencesyousee.
Aves–Birds
Almostallbirds,fromalbatrossestozebrafinches,allbelongtotheSuperorderNeognathae(meaning
newjaw),definedbyamodifiedandmobilepalatestructure.Allhaveakeeledsternum;referredtoas
acarina(meaningkeeled),thoughnotallareabletofly(e.g.,penguins–thoughtheydo“fly”inthe
water).Inwhatotherwaysdoesthemorphologyofbirdssuggestthattheyarecapableofflying?
Examinethepigeonskeleton.Notethehighlymodifiedforelimb.Whichspecificpartoftheforelimbis
themostderived?Howdobirdscompensatefortheirextremelyspecializedforelimb?
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Mammalia–Mammals
Today,placentalmammalsaredominantonalmostalllandmassesexceptAustralia.LiketheAmphibians,
wedonothavetimetoexploremammalsinanyrealdetail.Instead,wewillfocusonaparticular
mammaliansynapomorphy,uprightlimbposture.Moderninsectivores(e.g.shrewsandtenrecs)are
presentdaymammalsthatarethoughttomostresembleancestralmammals.Oneofthebiggest
innovationsweseeinmammalsisanuprightposture,placingthelimbsunderneaththetrunk.
Comparethemammalianandsalamanderlimbposture.Howaretheysimilar?Howaretheydifferent?
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