I have handled four nautilus shells in thirty years and I regret three of them.

The one I kept is on the shelf behind the desk where I write invoices, and I have never once described it to a client, because a client would hear a description and understand a spiral. It is not a spiral. It is a sequence of rooms.

The animal builds a chamber, lives in it, outgrows it, builds the next one forward, and seals the one behind with a wall of nacre. It does this thirty times, forty. It never re-enters any of them. What it gets instead is buoyancy — the sealed rooms hold gas, and the whole structure floats because of the parts that are closed. So the animal is carried, exactly and permanently, by everything it can no longer get into.

Cut the shell in half and people say beautiful. What they are looking at is a record of departures.