they’re often called the paper nautilus
The Argonaut is the only species of modern octopus that has kept up the tradition of having a shell. While they are distantly related to nautiluses, there are a few big differences between them. Unlike nautiluses, Argonauts are not born with their shells, only female Argonauts grow them. The shells are not attached to them, but rather they wear them like a backpack. This is because Argonauts use their shells to carry their eggs around, since they don’t live on the sea floor like most other octopuses. This is why they are often called the paper nautilus.