Growing a pedicle allows you to attach yourself to a convenient location when you find yourself in one, and become ‘sessile’ or permanently stuck to one place.

You can stay there, opening your shell and filter feeding when it’s convenient, and closing it again to hide inside when there is a predator around. You release many, many tiny copies of yourself, which float around with the currents until, in their turn, they find a good place and stick themselves there, where they grow thicker and thicker shells and make more copies.

The Permian-Triassic extinction event didn’t wipe you out, but you haven’t grown back as rapidly as a closely related species, the bivalve molluscs. As a result, they appear to have dominated the upper, richer, waters, and you are mostly found at deeper locations.

It’s a simple, relatively low cost way to make efficient use of free resources, and you are found in deep seas all over the world.