外殻的英文：
a case or shell; outer covering or casing; a skin; an envelope; a crust

shell是什么意思：
n. 壳；外壳；外形；炮弹
v. 剥壳；剥落

shell collecting
贝壳采集

The snail retreated into its shell.
蜗牛缩进壳里去了。

reactivation of quoted shell company
上市空壳公司恢复业务

outer是什么意思：
adj. 外面的，外部的；客观外界的

bearing outer race
轴承外座圈

He disclosed the outer covering on the bike.
他揭去自行车上的覆盖物。

He heard a voice in the outer room.
他听到外屋有人说话。

covering是什么意思：
n. 遮盖物；遮盖层
v. cover的现在分词

Cover as if with a shroud.
仿佛用覆盖物掩饰着。

To be covered with or as if with fog.
为雾笼罩被或好象被烟雾掩盖

Covering: The fixing of a book cover to the spine and end papers .
上书皮:把书皮粘贴于书和纸上。