兩棲動物百科全書AmphibiansEvolutionofAmphibians

"Amphibian"comesfromtheGreekmeaning"bothlife".Amphibianscanliveonwaterandonland.Scientistinferthatamphibiansevolvedfromlobe-finnedfishescalledcrossopterygians.CrossopterygianBiologistconcludethatamphibiansappearedduringthelateDevonianperiod,about345millionyearsago.Crossopterygianshadno

gillsbuttheyhadinternalnostrilsandaprimitivelungthatmayhaveenabledthemthentorespireforperiodsoftimeonland.CharacteristicsofAmphibiansAmphibiansarecold-blooded,whichmeanstheirbloodtemperaturerisesandfallswiththatofthesurroundingenvironment

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Theyusegills,lungs,skin,andmouthcavityinrespiration.

Theyhavemoist,smooth,thinskinwithnoscales.Feetarewebbedandthetoeslackclaws.Amphibianshavemanyprominentcharacteristicsthatareadaptationstoalifespentbothonlandandinwater:

Theychangefromanaquaticlarvalstagetoaterrestrialadult

form.Thistransformationiscalledmetamorphosis

Amphibiansenterastateofdormancyortorporwhenconditionsareunfavorable.

Theyoftenburythemselvesinmudorleaves,emergingwhenconditionsarebetter.Suchstatesofinactivityareknownasknownas:

HibernationwhenitoccursinthewinterEstivationwhenitoccursinthesummerLarvaehavetwo-chamberedhearts;adultshavethree-chamberedheartsandwell-developedcirculation.Eggslackmulticellularmembranesorshells.Theyareusuallylaidinwaterorinamoistenvironmentandfertilizedexternally.

ClassificationofAmphibiansBiologistshaveidentifiedabout2,375livingspeciesofamphibiansandhaveclassifiedthemintofourorders

Anura–Frogs&ToadsUrodela–SalamandersTrachystoma–MudeelsApoda-CaecilliansFrogsandtoadsmakeuptheOrderAnura("withoutatail").SalamandersandotheramphibianswithlegsandtailsmakeuptheOrderUrodela("visibletail")..Someaquaticamphibians(mudeels&sirens)belongtotheOrderTrachystoma("roughmouth")SirenApodaincludescaecilians,atropical,burrowingworm-likeamphibiansthatisoften

Legless

AnuranCharacteristicsFrogsandtoadscomprisetheorderAnura

Thereareabout3,500knownspeciesoffrogsand300kindsoftoads

TheyarefoundoneverycontinentexceptAntarcticaSometypesspendtheirentirelifeinornearwater,butotherslivemainlyonlandandcometothewateronlytomateSomefrogsandtoadsareclimbersthatdwellintreesorburrowersthatliveunderground.

Toadsandfrogshavemanysimilaritiesinthewaytheylook.Somebasicdifferencesbetweenthemare:toadshavedry,wartyskin,whilefrogshavesmooth,wetskin.Bothfrogsandtoadsreturntowatertoreproduce.Innearlyallspecieseggsarefertilizedexternally.ThefertilizedeggshatchintoswimminglarvalformscalledtadpolesUrodelanCharacteristicsSalamanders,typicalmembersoftheOrderUrodela,haveelongatedbodies,longtails,andsmooth,moistskinComparedtotheanurans,salamandersarelessabletoremainondryland,althoughsomecanliveindryareasbyremaininginactiveduringthedaySalamanderlaytheireggsinwaterandlikeanuranstheyhatchintoswimminglarvaSalamandereggsOtherspeciescanreproduceindamplandenvironments.EggslaidonlandhatchintominiatureadultsalamandersMarbledSalamanderApodanCharacteristicsCaecilians,membersoftheOrderApoda,composeahighlyspecializedgroupoftropicalburrowingamphibians

Theseleglesswormlikecreaturesaverage30cmlong,buttheycanbeupto1.3mlong.Theyhaveverysmalleyesandareoftenblind.TheyeatwormsandotherinvertebratesThecaecilianmaledepositsspermdirectlyintothefemale,andthefemalebearsliveyoungTrachystomaCharacteristics

TheOrderTrachystomacontainsthreelivingspeciesofmudeels,orsirens.SirensliveintheeasternUnitedStatesandnorth-easternMexico.FrogExternalAnatomyThefrog'spowerfulhindlegsareequallyeffectiveinjumpingorswimming.Onlandfrogssitwiththeirhindlegsfoldedagainstthebody,poisedtojumpatthefirstsignofdanger.MostfrogscanmakeleapsmanytimestheirbodylengthFrog'seyesalsoworkequallywellinoroutofwater.Becausetheeyesbulgeoutfromthehead,thefrogcanstaysubmergedwhileliterally"keepinganeyeout"forpredatorsEyelidsthatcanblinkprotectthefrog'seyesfromdustanddehydrationInadditiontoupperandlowereyelids,athird,transparenteyelidcalledanictitatingmembranecoverseacheyeballandjoinsthelowereyelidThismembranekeepstheeyelidmoistandprotectsitwhenitisunderwaterFrogshaveeardrums,ortympanicmembranes,whicharecircularstructureslocatedbehindeacheyeTympanicmembraneThefrog'sthick,moistskinservestwoimportantfunctions—respirationandprotectionGlandssecretemucustokeepitfromdryingupSomeglandssecretefoul-tastingorpoisonoussubstancesthatprotectthefrogfromenemies

Somefrogs,suchasHylaversicolor,canchangecolorinordertoblendwiththeenvironment.

Countthenumberoffrogsinthepicture?FrogInternalAnatomySkeletalSystemThefrog'sspinehasninevertebraeThecervicalvertebraattheanteriorendofthespineallowsneckmovementthathelpsfrogscatchpreySkeletalSystemPosteriortothisareseventrunkvertebrae,andthenasinglesacralvertebrathatsupportsthehindlegsAlong,slimbonecalledtheurostyleextendsfromthesacralvertebraSkeletalSystemBonesofthepectoralgirdle,whichformtheshoulders,connecttothefrontlegsTheyalsoprovidetheprimaryprotectiontotheinternalorgans,sincethefroghasnoribsThepelvicgirdleconnectstothehindlegsDigestiveSystemDigestiveSystemMostfrogsfeedoninsects,andtheirdigestivesystemisadaptedtotheirdietAfrog'stongueisanexcellentinsectcatcher.Thefrogsimplyflicksoutitslongstickytongue,curlsitarounditsprey,andpullstheinsectbackintoitsmouth.Thenthefrogsnapsitsmouthshutandswallows.DigestiveSystemFrogshavetwotypesofteeththatholdontoprey.Arowofmaxillaryteethlinetheperimeteroftheupperjaw.Twopatchesofvomerineteethprojectfrombonesintheroofofthemouth.DigestiveSystemSometimesthefrogblinks,pullsitseyesinward,andpressesthemagainsttheroofofitsmouth.ThisactionhelpspushthefooddownitsthroatDigestioninfrogstakesplaceinthealimentarycanal,whichincludestheesophagus,stomach,smallintestine,largeintestine,andcloaca

CirculatorySystemAnadaptationtothegreateroxygenneedsoflandanimalsisamoreefficientcirculatorysystemthanthefish'stwo-chamberedheart.Theamphibian'sthree-chamberedheartpartiallymixesoxygenatedwithdeoxygenatedbloodandpumpsthebloodthroughoutthebodyathigherpressurethandoesthefish'sheartTheleftatriumreceivesoxygenatedbloodfromthelungs,andtherightatriumreceivesdeoxygenatedbloodfromthebody.Boththeatriaemptyintotheventricle,themainpumpingchamberoftheheart.Intheventricleoxygenatedanddeoxygenatedbloodmixpartiallyandarepumpedtothelungsandtherestofthebody.Bloodtobody

Deoxygenatedbloodfrombody

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Oxygenatedbloodfromlungs

Fromtherightatriumthebloodentersthesingleventricle.Theventriclethencontracts,pumpingsomebloodtothelungstoreceiveoxygenandsometotherestofthebody.Thebloodgoingtothebodyleavestheventriclethroughtheconusarteriosus,alargevesselthatliesagainstthefrontsideoftheheartThisvesseldividesintoarightandalefttruncusarteriosus,whichimmediatelybranchagainintothreearchesthatcarrybloodtovariouspartsofthebody.Deoxygenatedbloodtravelsinveinsbacktotherightatriumfromthevariousregionsofthebody.OxygenatedbloodreturnsfromthelungstotheleftatriumviathepulmonaryveinsCopyrightCmassengaleRespiratorySystemTadpolesrespire,orexchangecarbondioxideandoxygen,throughgillsRespiratorySystemAdultfrogslosethegillsbutcanrespireinthreeways:throughthelungs,throughtheskin,andthroughthemouth.Respirationthroughthelungsiscalledpulmonaryrespiration.AfrogbreathesbychangingthevolumeandpressureofairinitsmouthwhileeitheropeningorclosingitsnostrilsRespiratorySystemAirmovesfromthethroattothelungsthroughaslit-likepassagecalledtheglottis.Becausethefrog'slungsaresmall,cutaneousrespiration,orrespirationthroughtheskininbothairandwater,isveryimportant,especiallyduringestivationorhibernation.Oxygencandiffuseacrosstheliningofthemouthandintotheblood.Frogsusemouthbreathingforonlyarelativelysmallamountoftheirrespiration.

ExcretorySystemUrineandwastesfromthedigestivesystemareeliminatedthroughtheanus.Whenafrogisinwater,itspermeableskinallowsthewatertoenteritsbody.Frogsthatliveprimarilyinwaterridthemselvesofexcesswaterbyexcretingalargevolumeofverydiluteurine.Frogsthatlivemainlyonlandconservewaterbyproducingasmallvolumeofmoreconcentratedurine.ExcretorySystem

Amphibianseliminatetwoprimarytypesofmetabolicwasteproducts—carbondioxidefromrespirationandwastecom-poundsfromthebreakdownoffoods.Aretheprimaryexcretoryorgansandlieoneithersideofthespineagainstthedorsalbodywall.ThekidneysfilternitrogenouswastesfromthebloodTheNervousSystemThefrogbrainismorecomplexthanthefishbrain,enablingthefrogtocontendwithamorevariedenvironment.Theopticlobes,whichcontrolvision,liebehindthecerebrum.Thecerebellum,asmallbandoftissuelyingatrightanglestothelongaxisofthebrain,isthecenterofbalanceandcoordination.NervousSystemThemedullaoblongataliesatthebackofthebrainandjoinsthespinalcord.Itcontrolsorganfunctions.Tenpairsofcranialnervesextendoutdirectlyfromthebrain.Thespinalcordtransmitssignalsfromallpartsofthebodytothebrainandfromthebrainbacktothebody.Thespinalnervesbranchfromthespinalcordtovariouspartsofthebody.ReproductiveSystemBothmaleandfemalefrogshaveinternalsexorgansThemalefrog'sfore-legmusclesandfirstfingersswellTheseswellingshelpthemalemaintainhisgrasponthefemaleThereproductivesystemofthemalefrogincludestwobean-shapedcreamywhiteoryellowishtesteslocatednearthekidneysSpermcellsdevelopinthetestesandpassthroughtubestothekidneysandurinaryductsMaleSystemReproductiveSystem

Femalefrogsapairoflarge,lobedovariescontainingthou-sandsoftinyimmatureeggslienearthe