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用心爱心专心 Sharks SharksaresurelyoneofthemostfearsomecreaturesonEarth.Buttheyarealsooneofthemostsophisticatedandenduring.Theoldestsharkfossilsdatefrommorethan300millionyearsago,beforetheageofthedinosaurs.Andafewexistingsharkspecies,suchasthehornshark,havekeptthesamebasicphysicalcharacteristicsformorethan150millionyears. Scientistsattributethisremarkablelongevitytotheshark'ssuperiorphysiologicaldevelopments.Sharksareoutfittedwithseveralspecialcharacteristicsthatmakethemhighlyeffective,bothashuntersandsurvivors.Inthisarticle,we'llexaminetheseuniquequalitiestoseewhythesharkistopdogintheocean. SharkAnatomy Whenpeoplethinkofsharks,theygenerallyimagineananimalliketheoneinthispicture.Thetalldorsalfin,torpedo-shapedbodyandgiantteethofthisgreatwhitesharkarefamiliartomosteverybody.Butthereareactuallymorethan400differentsharkspeciesalivetoday,andtheyvaryconsiderablyinsizeandappearance.Infact,roughly50percentofsharkspeciesarelessthanameterlong.Sowhatmakesasharkashark? Oneofthemostwell-knownsharks,thegreatwhite Sharks,alongwithraysandchimeras,aredistinguishedfromotherfishprimarilybytheirbodycomposition.Mostotherfishhaveskeletonsmadeofbone,justlikemammals,reptiles,amphibiansandbirds.Sharksandrays,ontheotherhand,haveskeletonsmadeentirelyofcartilage,thesameflexiblematerialinyournoseandears.Cartilageissturdylikebone,butithasamuchlowerdensity.Thismaterialkeepssharksrelativelylightweight,sotheydon'tsinkintheoceanandtheydon'tneedanairbladderlikeotherfish. Sharksalsohaveaveryuniqueskintexture.Theydon'thavethelarge,prominentscalesfoundinbonyfish.Insteadthey'recoveredwithsmaller,tooth-likescalescalleddenticles.Thesetough,protectivedenticlesarealignedsothattheychannelwaterovertheshark'sbody,minimizingdragduetofriction. Likebonyfish,sharksbreathebyextractingdissolvedoxygenfromwater.Thewaterentersthemouth,passesthroughthegillsandisexpelledthroughgillslitsbehindthehead.Inbonyfish,theseslitsarecovered,butinmostsharksyoucanseethemclearly.Asthewaterflowsthroughthegillopening,itpassestinygillfi