Shelled Lophotrochozoa

A shell is a useful addition.  There's far too many things out there bigger than you willing to gobble you up.  The way you make yours is fascinating.... You develop a special set of cells on your outside called the mantle.  These contain proteins which extract calcium carbonate from sea water, then express it on their outer surfaces (with a little bit of protein to provide a scaffold).  This makes a hard surface just outside your body, which makes you a little less attractive as food.

As you grow older, this layer grows thicker and thicker.  However, it's a bit of a restriction, as you can't actually flex this stuff - it's just too hard.  Over many generations, a pattern emerges which gives you the best of both worlds - protection and movement.  It is a simple hinge, which sits between two separate shell structures.

The hinge allows you to open the two parts of your shell to feed, and close it when there is danger.  Perfect!

There are two effective ways to make the hinge.  Your two shell cases can be:

•  on your left and right sides
•  on your top and bottom sides