GRE阅读模拟在线练习附答案二百六十二

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　　The two claws of the mature American lobster aredecidedly different from each other. The crusherclaw is short and stout;the cutter claw is long andslender. Such bilateral asymmetry, in which theright side of the body is,in all other respects, amirror image of the left side,is not unlike handednessin humans. But where the majority of humans areright-handed, in lobsters the crusher claw appearswith equal probability on either the right or left side of the body.

　　Bilateral asymmetry of the claws comes about gradually. In the juvenile fourth and fifth stagesof development, the paired claws are symmetrical and cutter like. Asymmetry begins toappear in the juvenile sixth stage of development, and the paired claws further diverge towardwell-defined cutter and crusher claws during succeeding stages. An intriguing aspect of thisdevelopment was discovered by Victor Emmel. He found that if one of the paired claws isremoved during the fourth or fifth stage, the intact claw invariably becomes a crusher.whilethe regenerated claw becomes a cutter. Removal of a claw during a liter juvenile stage orduring adulthood, when asymmetry is present,does not alter the asymmetry; the intact andthe regenerate claws retain their original structures.

　　These observations indicate that the conditions that trigger differentiation must operate in arandom,manner when the paired claws are intact but in a nonrandom manner when one ofthe claws is lost. One possible explanation is that differential use of the claws determinestheir asymmetry. Perhaps the claw that is used more becomes the crusher.This would explainwhy, when one of the claws is missing during the fourth or fifth stage, the intact claw alwaysbecomes a crusher. With two intact claws, initial use of one claw might prompt the animal touse it more than the other throughout the juvenile fourth and fifth stages, causing it to becomea crusher.

　　To test this hypothesis, researchers raised lobsters in the juvenile fourth and fifth stages ofdevelopment in a laboratory environment in which the lobsters could manipulate oysterchips. (Not coincidentally, at this stage of development lobsters typically change from ahabitat where they drift passively, to the ocean floor where they have the opportunity to bemore active by burrowing in the substrate.Under these conditions, the lobsters developedasymmetric claws, half with crusher claws on the left, and half with crusher claws on the right. In contrast, when juvenile lobsters were reared in a smooth tank without the oyster chips, the majority developed two cutter claws. This unusual configuration of symmetrical cutterclaws did not change when the lobsters were subsequently placed in a manipulatableenvironment or when they lost and regenerated one or both claws.

　　The passage is primarily concerned with

　　A.drawing an analogy between asymmetry in lobsters and handedness in humans

　　B.developing a method for predicting whether crusher claws in lobsters will appear on the left orright side

　　C.explaining differences between lobsters’ crusher claws and cutter claws

　　D.discussing a possible explanation for the way bilateral asymmetry is determined in lobsters

　　E.summarizing the stages of development of the lobster

　　Each of the following statements about the development of a lobster's crusher claw issupported by information in the passage EXCEPT:

　　A.It can be stopped on one side and begun on the other after the juvenile sixth stage.

　　B.It occurs gradually over a number of stages.

　　C.It is initially apparent in the juvenile sixth stage.

　　D.It can occur even when a prospective crusher claw is removed in the juvenile sixth stage.

　　E.It is less likely in the absence of a manipulatable environment.

　　Which of the following experimental results, if observed, would most clearlycontradict the findings of Victor Emmel?

　　A.A left cutterlike claw is removed in the fifth stage and a crusher claw develops on the rightside.

　　B.A left cutterlike claw is removed in the fourth stage and a crusher claw develops on the leftside.

　　C.A left cutterlike claw is removed in the sixth stage and a crusher claw develops on the rightside.

　　D.Both cutterlike claws are removed in the fifth stage and a crusher claw develops on the leftside.

　　E.Both cutterlike claws are removed in the fourth stage and a crusher claw develops on theright side.

　　It can be inferred that of the two laboratory environments mentioned in the passage, the one with oyster chips was designed to

　　A.prove that the presence of oyster chips was not necessary for the development of a crusherclaw

　　B.prove that the relative length of time that the lobsters were exposed to the oyster-chipenvironment had little impact on the development of a crusher claw

　　C.eliminate the environment as a possible influence in the development of a crusher claw

　　D.control on which side the crusher claw develops

　　E.simulate the conditions that lobsters encounter in their natural environment

　　It can be inferred from the passage that one difference between lobsters in theearlier stages of development and those in the juvenile fourth and fifth stages is thatlobsters in the early stages are

　　A.likely to be less active

　　B.likely to be less symmetrical

　　C.more likely to lose a claw

　　D.more likely to replace a crusher claw with a cutter claw

　　E.more likely to regenerate a lost claw

　　Which of the following conditions does the passage suggest is a possible cause for thefailure of a lobster to develop a crusher claw?

　　A.The loss of a claw during the third or earlier stage of development

　　B.The loss of a claw during the fourth or fifth stage of development

　　C.The loss of a claw during the sixth stage of development

　　D.Development in an environment devoid of material that can be manipulated

　　E.Development in an environment that changes frequently throughout the stages ofdevelopment

　　The author regards the idea that differentiation is triggered randomly when pairedclaws remain intact as

　　A.irrefutable considering the authoritative nature of Emmel's observations

　　B.likely in view of present evidence

　　C.contradictory to conventional thinking on lobster-claw differentiation

　　D.purely speculative because it is based on scattered research and experimentation

　　E.unlikely because of apparent inconsistencies with theories on handedness in humans

　　正确答案:D A B E A D B

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