After all, internal shells no longer offered protection, so there was no reason to keep lugging around all that extra weight.

毕竟， 内壳不再提供保护，所以没有理由继续拖着所有额外的重量。

In time, the internal shell was streamlined down to a long, chitinous structure, kind of like a backbone, called a gladius.

随着时间的推移，内壳被流线型化为长长的几丁质结构，有点像脊椎骨，被称为短剑。