出现在奥陶纪，距今5亿年前的鹦鹉螺，跟章鱼、乌贼是远亲。在进化发展中，它的那些远亲身体都发生了很大变化。外壳有的转入身体里面；有的仅仅留下一层胶质的薄膜；还有的壳已经完全消失了，如章鱼。可惟有鹦鹉螺的壳，一直没有多大变化。鹦鹉螺在4.4亿年前极其兴旺。现在已 Occurs in the Ordovician period. The Nautilus 500 million years ago was a distant relative to the octopus and squid. In the course of evolution, its distant relatives have undergone major changes. Some of the shells were transferred into the body; some left only a thin layer of glial film; others had completely disappeared, such as octopuses. The shell of Nautilus is the only one that has remained unchanged. Nautilus flourished during 440 million years ago. Now