首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
The two claws of the mature American lobster are decidedly different from each other. The crusher claw is short and stout; the c
The two claws of the mature American lobster are decidedly different from each other. The crusher claw is short and stout; the c
admin
2011-02-26
69
问题
The two claws of the mature American lobster are decidedly different from each other. The crusher claw is short and stout; the cutter claw is long and slender. Such bilateral asymmetry, in which the right side of the body is, in all other respects, a mirror image of the left side, is not unlike handedness in humans. But where the majority of humans are right-handed, in lobsters the crusher claw appears with equal probability on either the right side or left side of the body.
Bilateral asymmetry of the claws comes about gradually. In the juvenile fourth and fifth stages of development, the paired claws are symmetrical and cutterlike. Asymmetry begins to appear in the juvenile sixth stage of development, and the paired claws further diverge toward well-defined cutter and crusher claws during succeeding stages. An intriguing aspect of this development was discovered by Victor Emmel. He found that if one of the paired claws is removed during the fourth or fifth stage, the intact claw invariably becomes a crusher, while the regenerated claw becomes a cutter. Removal of a claw during a later juvenile stage or during adulthood, when asymmetry is present, does not alter the asymmetry; the intact and regenerate claws retain their original structures.
These observations indicate that the conditions that trigger differentiation must operate in a random manner when the paired claws are intact but in a nonrandom manner when one of the claws is lost. One possible explanation is that differential use of the claws determines their asymmetry. Perhaps the claw that is used more becomes the crusher. This would explain why, when one of the claws is missing during the fourth or fifth stage, the intact claw always becomes a crusher. With two intact claws, initial use of one claw might prompt the animal to use it more than the other throughout the juvenile fourth and fifth stages, causing it to become a crusher.
To test this hypothesis, researchers raised lobsters in the juvenile fourth and fifth stages of development in a laboratory environment in which the lobsters could manipulate oyster chips. (Not coincidentally, at this stage of development lobsters typically change from a habitat where they drift passively, to the ocean floor where they have the opportunity to be more active by borrowing in the substrate.) Under these conditions, the lobsters developed asymmetric claws, half with cutter 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 cutter claws did not change when the lobsters were subsequently placed in a manipulatable environment or when they lost and regenerated one or both claws.
The passage is primarily concerned with ______.
选项
A、drawing an analogy between asymmetry in lobsters and handed in humans.
B、developing a method for predicating whether crusher claws in lobster will appear on the left or right 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
答案
D
解析
转载请注明原文地址:https://kaotiyun.com/show/1jBO777K
0
考博英语
相关试题推荐
•Lookatthechartbelow.Itshowstheaveragesharepricesofeachmonthofthreecompaniesduringaneight-monthperiod.•
Thesectoraverageroseintheearlypartoftheweek,thensankback,althoughthiscompany’ssharesremainedfirmafteranear
•Readtheextractbelowaboutcustomsprocedure.•Choosethecorrectwordtofilleachgap,fromA,B,orContheopposite
•Readtheextractbelowaboutcustomsprocedure.•Choosethecorrectwordtofilleachgap,fromA,B,orContheopposite
Readthenewspaperarticlebelowaboutapartnershipbetweenindustryandschools.Aresentences16-22belowthearticle"Ri
Surprisinglyenough,modernhistorianshaverarelyinterestedthemselvesinthehistoryoftheAmericanSouthintheperiodbefo
ManypeopleinvestinthestockmarkethopingtofindthenextMicrosoftandDell.However,Iknow【21】personalexperiencehowdi
It’saresultmoreof______thananythingelsetowinthetopprizeinalottery.
Improvementsinlaborproductivity【C1】______increasewagesandsalaries.Wagesandsalaries【C2】______about75percentofall
随机试题
幼儿园环境的设计应尽量促进幼儿的探究活动的开展。()
患者女性,40岁,以“双手遇冷发白、疼痛3年伴乏力3个月”来诊,不考虑的检查是
头颅后前位摄影听眦线与胶片平面的角度关系是
46岁,女性,白带增多一年,偶而呈粉红色,近3个月有性交出血,月经尚规则,妇科检查见宫颈中度糜烂,子宫正常大小,质软、双侧附件无异常所见。阴道镜下活检有癌细胞已突破基底膜5mm,有淋巴管被侵袭及病灶融合现象。该患者的临床分期应是
下行为构成抢劫罪的是哪些?
根据《建筑法》的规定,下列关于建筑安全生产管理制度的说法中,正确的是()。
如果在散装货的交易合同中,没有规定溢短装数量,则与使用“约,,,≠的含义一样,允许交货数量有5%的机动幅度。()
分析下面的谱例。要求:简要说明乐曲的特点。
TheplayDeathofaSalesmanwaswrittenby______.
第二代数据库系统是指
最新回复
(
0
)