In fact our backbones are not straight .

实际上我们的脊柱不是直的。

Invertebrates are animals without backbones .

无脊椎动物体内没有脊柱的动物.

Creatures without backbones have the hardest shells .

没有脊骨的动物都有坚硬的壳。

They are actually chordates-in other words , part of the same group of animals as humans , even though they do not have backbones .

他们实际上是脊索动物，换句话说，和人类属同门动物，虽然它们没有脊椎。

Strange that creatures without backbones have the hardest shells .

奇怪的是没有脊骨的生物都有最坚硬的壳。