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老托福阅读100篇-练习阅读最好的材料

2015-04-08 12:00 三立在线 admin

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  Swimming Machines (OG Practice Set 6)

  Tunas, mackerels, and billfishes (marlins, sailfishes, and swordfish) swim continuously. Feeding,courtship, reproduction, and even "rest" are carried out while in constant motion. As a result, practically every aspect of the body form and function of these swimming "machines" is adapted to enhance their ability to swim.

  Many of the adaptations of these fishes serve to reduce water resistance (drag). Interestingly enough,several of these hydrodynamic adaptations resemble features designed to improve the aerodynamics of high-speed aircraft. Though human engineers are new to the game, tunas and their relatives evolved their"high-tech" designs long ago.

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  Tunas, mackerels, and billfishes have made streamlining into an art form. Their bodies are sleek and compact. The body shapes of tunas, in fact, are nearly ideal from an engineering point of view. Most species lack scales over most of the body, making it smooth and slippery. The eyes lie flush with the body

  and do not protrude at all. They are also covered with a slick, transparent lid that reduces drag. The fins

  are stiff, smooth, and narrow, qualities that also help cut drag. When not in use, the fins are tucked into

  special grooves or depressions so that they lie flush with the body and do not break up its smooth

  contours. Airplanes retract their landing gear while in flight for the same reason.

  Tunas, mackerels, and billfishes have even more sophisticated adaptations than these to improve their

  hydrodynamics. The long bill of marlins, sailfishes, and swordfish probably helps them slip through the

  water. Many supersonic aircraft have a similar needle at the nose.

  Most tunas and billfishes have a series of keels and finlets near the tail. Although most of their scales

  have been lost, tunas and mackerels retain a patch of coarse scales near the head called the corselet.

  The keels, finlets, and corselet help direct the flow of water over the body surface in such as way as to

  reduce resistance (see the figure). Again, supersonic jets have similar features.

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