outer layer of boundary layer

例句：

1. 1. The outer layer of theskin is made up of dead cells.
   皮肤的外层由死亡细胞组成。
2. 2. Theouter layer of the ocean is covered inseaweed.
   海洋的外层覆盖着海藻。
3. 3. The outer layer of the planetis composed of rocks and minerals.
   行星的外层由岩石和矿物质组成。