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.
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.
Because they are always swimming, tunas simply have to open their mouths and water is forced in and over their gills. Accordingly, they have lost most of the muscles that other fishes use to suck in water and push it past the gills. In fact, tunas must swim to breathe. They must also keep swimming to keep from sinking, since most have largely or completely lost the swim bladder, the gas-filled sac that helps most other fish remain buoyant.
One potential problem is that opening the mouth to breathe detracts from the streamlining of these fishes and tends to slow them down. Some species of tuna have specialized grooves in their tongue. It is thought that these grooves help to channel water through the mouth and out the gill slits, thereby reducing water resistance.
There are adaptations that increase the amount of forward thrust as well as those that reduce drag. Again, these fishes are the envy of engineers. Their high, narrow tails with swept-back tips are almost perfectly adapted to provide propulsion with the least possible effort. Perhaps most important of all to these and other fast swimmers is their ability to sense and make use of swirls and eddies (circular currents) in the water. They can glide past eddies that would slow them down and then gain extra thrust by "pushing off" the eddies. Scientists and engineers are beginning to study this ability of fishes in the hope of designing more efficient propulsion systems for ships.
The muscles of these fishes and the mechanism that maintains a warm body temperature are also highly efficient. A bluefin tuna in water of 7°C (45°F) can maintain a core temperature of over 25°C (77°F). This warm body temperature may help not only the muscles to work better, but also the brain and the eyes. The billfishes have gone one step further. They have evolved special "heaters" of modified muscle tissue that warm the eyes and brain, maintaining peak performance of these critical organs.
原題
Paragraph 8: There are adaptations that increase the amount of forward thrust as well as those that reduce drag. Again, these fishes are the envy of engineers. Their high, narrow tails with swept-back tips are almost perfectly adapted to provide propulsion with the least possible effort. Perhaps most important of all to these and other fast swimmers is their ability to sense and make use of swirls and eddies (circular currents) in the water. They can glide past eddies that would slow them down and then gain extra thrust by "pushing off" the eddies. Scientists and engineers are beginning to study this ability of fishes in the hope of designing more efficient propulsion systems for ships.
9. According to the passage, one of the adaptations of fast-swimming fishes that might be used to improve the performance of ships is these fishes' ability to
○swim directly through eddies
○make efficient use of water currents
○cover great distances without stopping
○gain speed by forcing water past their gills
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答案
9. ○ 2
This is a Factual Information question asking for specific information that can be found in the passage. The correct answer is choice 2, "make efficient use of water currents." Paragraph 8 explicitly states: "Perhaps most important of all to these and other fast swimmers is their ability to sense and make use of swirls and eddies (circular currents) in the water. They can glide past eddies that would slow them down and then gain extra thrust by "pushing off" the eddies. Scientists and engineers are beginning to study this ability of fishes in the hope of designing more efficient propulsion systems for ships." The other choices are not mentioned in connection with the performance of ships.
金槍魚(yú)，鯖魚(yú)，和長(zhǎng)嘴魚(yú)（或者說(shuō)成是槍魚(yú)、旗魚(yú)和箭魚(yú)）的游動(dòng)從不停止。它們的進(jìn)食，求偶，繁殖，甚至“休息”都在不斷的運(yùn)動(dòng)中進(jìn)行。事實(shí)上，這些游泳“機(jī)器”身體結(jié)構(gòu)的每個(gè)部位及其功能都有利于它們更好地游行。
為了減少在水中前行的阻力，這類(lèi)魚(yú)身上產(chǎn)生了很多適應(yīng)性變化。非常有趣的是，人類(lèi)為了降低空氣阻力加快高速飛機(jī)運(yùn)行速度所進(jìn)行的設(shè)計(jì)和這些魚(yú)的適應(yīng)性變化非常相似。這種設(shè)計(jì)只是人類(lèi)工程師的初步嘗試，但金槍魚(yú)和他們的同類(lèi)們已經(jīng)擁有這種“高科技”設(shè)計(jì)很久很久。
金槍魚(yú)、鯖魚(yú)和長(zhǎng)嘴魚(yú)的流線(xiàn)體型簡(jiǎn)直就是一件工藝品。他們的身體光滑而堅(jiān)實(shí)。從工程師的角度來(lái)看，金槍魚(yú)的體型近乎完美。很多魚(yú)類(lèi)的絕大多數(shù)皮膚上是沒(méi)有魚(yú)鱗的，特別光滑。它們的眼睛和身體處于同一平面，根本不會(huì)凸顯出來(lái)。身體表面還覆蓋著一層光滑透明的外衣，魚(yú)鰭部分堅(jiān)硬、平穩(wěn)而狹窄，這些特征都有助于降低前行中的阻力。當(dāng)魚(yú)兒們不使用魚(yú)鰭時(shí)，會(huì)將它們折回到特殊的溝槽或者凹陷的地方，與身體保持同一平面，以維持它們平滑的外形。飛機(jī)收回起落裝置，和這是同樣的道理。
和上述特征相比，金槍魚(yú)、鯖魚(yú)和長(zhǎng)嘴魚(yú)們擁有更加精明的手段來(lái)增加它們?cè)谒械倪m應(yīng)性，比如他們的大長(zhǎng)嘴。很多超音速飛機(jī)的頭部就有類(lèi)似的針狀設(shè)計(jì)。
大多數(shù)金槍魚(yú)和長(zhǎng)嘴魚(yú)的尾巴附近會(huì)長(zhǎng)有一串脊骨和小鰭。雖然它們身上大部分地方是無(wú)鱗的，但在頭部附近還保留著一塊較粗的鱗片，叫做（魚(yú)的）胸甲。脊骨、小鰭和胸甲有助于水直接流經(jīng)魚(yú)體表面，降低阻力（見(jiàn)附圖）。同樣，超音速飛機(jī)的噴頭也有類(lèi)似的特征。
因?yàn)榻饦岕~(yú)的游動(dòng)從不停止，它們必須張著嘴使水流經(jīng)它們的腮。而其他魚(yú)類(lèi)的嘴里都會(huì)有一塊肌肉，用于吸水和從腮里排水，金槍魚(yú)的這塊肌肉已經(jīng)退化。實(shí)際上，它們必須通過(guò)游泳來(lái)呼吸。大部分金槍魚(yú)很大程度上已經(jīng)喪失了其他魚(yú)類(lèi)用于保持漂浮狀態(tài)的魚(yú)鰾，或者說(shuō)已經(jīng)完全喪失，因此，它們必須保持持續(xù)游泳的狀態(tài)。
一個(gè)可能存在的問(wèn)題在于，金槍魚(yú)張嘴呼吸破壞了它們的流線(xiàn)型體型，有可能會(huì)降低它們的游泳速度。為此有的金槍魚(yú)會(huì)在舌頭上長(zhǎng)有特殊的凹槽，以便引導(dǎo)水流通過(guò)嘴巴從腮縫流出，從而減少了阻力。
和降低阻力一樣，金槍魚(yú)們?cè)谟斡緞?dòng)力的加強(qiáng)上也有產(chǎn)生適應(yīng)性變化。人類(lèi)工程師在他們面前不得不自嘆不如。向后傾斜并且長(zhǎng)而狹窄的尾部非常有利于它們用最省力的方式前行。對(duì)這些魚(yú)兒以及其他的魚(yú)類(lèi)游泳健將們來(lái)說(shuō)，要保持在水里快速前行，最重要的可能就是對(duì)漩渦和逆流感知及利用的能力。漩渦會(huì)降低它們的速度，但它們?cè)诹鹘?jīng)漩渦時(shí)不僅可以輕而易舉地滑過(guò)而且會(huì)通過(guò)“推動(dòng)”漩渦獲得額外的動(dòng)力。科學(xué)家和工程師們正在研究魚(yú)的這種能力，以期設(shè)計(jì)出更高效的輪船推進(jìn)系統(tǒng)。
這些魚(yú)類(lèi)的肌肉組織和保溫機(jī)制也非常高效。一只藍(lán)鰭金槍魚(yú)在7°C (45°F)的溫度下可以保持25°C (77°F)以上的體溫。溫暖的體溫可以使得肌肉、大腦和眼睛更好地運(yùn)轉(zhuǎn)。長(zhǎng)嘴魚(yú)更厲害。它們有專(zhuān)門(mén)改善肌肉組織的加熱器，可以使眼睛和大腦保持一定溫度，從而保證自己的重要的器官保持在最好的運(yùn)行狀態(tài)中。
