A tortoise has a shell on his back.

乌龟背上有硬壳。

《新英汉大辞典》

He has a shell on his head.

他的头上有个痹忧。

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This strange Malaysian gastropod has a shell that defies the standard laws of shell twisting.

这种奇怪的马来群岛的腹足动物对外壳扭曲的标准法则提出了挑战。

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So remember, that when we talked about Lewis structure, the organizing principle behind Lewis structures is the idea that within the molecule the atoms are going to arrange their valence electrons, such that each atom within the molecule has a complete octet or full outer shell.

记得，当我们讨论路易斯结构的时候，路易斯结构所遵循的组织原则是这样一种思路,那就是分子中的原子都倾向于,重新安排它们的价电子,使得分子有一个完整的,“八隅体“或者排满的外壳层。

麻省理工公开课 - 化学原理课程节选

While the Senate passed a bill in March in a bipartisan 72-24 vote, the House has only a shell bill to offer, plus its "non-negotiable" insistence on mandatory approval of Keystone XL. The Senate majority has its own bazooka: a presidential veto if Keystone XL is in the bill.

CNN: STORY HIGHLIGHTS

The company, after all, was a true pioneer in mobile phones, even as it has become a shell of its former self.

FORBES: Just What Did Google Buy?

"How the sound reflects off this reflecting surface with this composite object on it - which is pretty big and has a cloaking shell on it - really reflects... just like a flat surface does, " Dr Cummer said.

BBC: Acoustic 'cloaking device' shields objects from sound