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Sharks.doc

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用心爱心专心SharksSharksaresurelyoneofthemostfearsomecreaturesonEarth.Buttheyarealsooneofthemostsophisticatedandenduring.Theoldestsharkfossilsdatefrommorethan300millionyearsagobeforetheageofthedinosaurs.Andafewexistingsharkspeciessuchasthehornsharkhavekeptthesamebasicphysicalcharacteristicsformorethan150millionyears.Scientistsattributethisremarkablelongevitytotheshark'ssuperiorphysiologicaldevelopments.Sharksareoutfittedwithseveralspecialcharacteristicsthatmakethemhighlyeffectivebothashuntersandsurvivors.Inthisarticlewe'llexaminetheseuniquequalitiestoseewhythesharkistopdogintheocean.SharkAnatomyWhenpeoplethinkofsharkstheygenerallyimagineananimalliketheoneinthispicture.Thetalldorsalfintorpedo-shapedbodyandgiantteethofthisgreatwhitesharkarefamiliartomosteverybody.Butthereareactuallymorethan400differentsharkspeciesalivetodayandtheyvaryconsiderablyinsizeandappearance.Infactroughly50percentofsharkspeciesarelessthanameterlong.Sowhatmakesasharkashark?Oneofthemostwell-knownsharksthegreatwhiteSharksalongwithraysandchimerasaredistinguishedfromotherfishprimarilybytheirbodycomposition.Mostotherfishhaveskeletonsmadeofbonejustlikemammalsreptilesamphibiansandbirds.Sharksandraysontheotherhandhaveskeletonsmadeentirelyofcartilagethesameflexiblematerialinyournoseandears.Cartilageissturdylikebonebutithasamuchlowerdensity.Thismaterialkeepssharksrelativelylightweightsotheydon'tsinkintheoceanandtheydon'tneedanairbladderlikeotherfish.Sharksalsohaveaveryuniqueskintexture.Theydon'thavethelargeprominentscalesfoundinbonyfish.Insteadthey'recoveredwithsmallertooth-likescalescalleddenticles.Thesetoughprotectivedenticlesarealignedsothattheychannelwaterovertheshark'sbodyminimizing