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1、智课网TOEFL备考资料托福阅读OG七：游泳机【附完整版】摘要：托福阅读OG对于托福考生来说是非常重要的，托福OG里面给出的题能够告诉你本次托福考试考得题目类型，让你有充足的时间去复习，逐个托福难点，之前我们已经为大家介绍了托福阅读OG1-6，今天我们陆续为大家介绍剩下来的内容。托福阅读OG七：SWIMMINGMACHINES(OG)Tunas,mackerels,andbillfishes(marlins,sailfishes,andswordfish)swimcontinuously.Feeding,courtship,repr
2、oduction,andeven"rest"arecarriedoutwhileinconstantmotion.Asaresult,practicallyeveryaspectofthebodyformandfunctionoftheseswimming"machines"isadaptedtoenhancetheirabilitytoswim.Manyoftheadaptationsofthesefishesservetoreducewaterresistance(drag).Interestinglyenough,severa
3、lofthesehydrodynamicadaptationsresemblefeaturesdesignedtoimprovetheaerodynamicsofhigh-speedaircraft.Thoughhumanengineersarenewtothegame,tunasandtheirrelativesevolvedtheir"high-tech"designslongago.Tunas,mackerels,andbillfisheshavemadestreamliningintoanartform.Theirbodie
4、saresleekandcompact.Thebodyshapesoftunas,infact,arenearlyidealfromanengineeringpointofview.Mostspecieslackscalesovermostofthebody,makingitsmoothandslippery.Theeyeslieflushwiththebodyanddonotprotrudeatall.Theyarealsocoveredwithaslick,transparentlidthatreducesdrag.Thefin
5、sarestiff,smooth,andnarrow,qualitiesthatalsohelpcutdrag.Whennotinuse,thefinsaretuckedintospecialgroovesordepressionssothattheylieflushwiththebodyanddonotbreakupitssmoothcontours.Airplanesretracttheirlandinggearwhileinflightforthesamereason.Tunas,mackerels,andbillfishes
6、haveevenmoresophisticatedadaptationsthanthesetoimprovetheirhydrodynamics.Thelongbillofmarlins,sailfishes,andswordfishprobablyhelpsthemslipthroughthewater.Manysupersonicaircrafthaveasimilarneedleatthenose.Mosttunasandbillfisheshaveaseriesofkeelsandfinletsnearthetail.Alt
7、houghmostoftheirscaleshavebeenlost,tunasandmackerelsretainapatchofcoarsescalesneartheheadcalledthecorselet.Thekeels,finlets,andcorselethelpdirecttheflowofwateroverthebodysurfaceinsuchaswayastoreduceresistance(seethefigure).Again,supersonicjetshavesimilarfeatures.Becaus
8、etheyarealwaysswimming,tunassimplyhavetoopentheirmouthsandwaterisforcedinandovertheirgills.Accordingly,theyhavelostmo
