These are crustaceans that survive by dragging around a cast-off mollusc shell for protection and shelter.

ECONOMIST: Nomads at last

The snail once found on islands in the Pacific Ocean became wiped out after another mollusc was introduced in the 1970s.

BBC: Partula tree snail

We also learn to eat the connective tissue between shell and mollusc, la membrane, an extra little treat like a mini scallop.

BBC: Postcard from Etang de Thau, France