With a shell above, and a shell below, you’re safely protected, and can open and close to permit feeding, and to hide. When the shell is open, you filter feed happily.

As ‘babies’ laterally shelled lophotrocozoa float around until they have accreted enough material in their shells. At the floating stage they are called plankton. Once your shells have grown sufficiently, you sink to the bottom. You’re safe from being predated on, but you don’t get much choice about where you live.

This is sufficiently successful that you can grow live longer, and grow larger. However, big mineral outgrowths like shells don’t scale up brilliantly. You simply can’t get any bigger after a while, and all the time the creatures which predate on you are getting larger and larger. You could save some energy by losing one of the ‘valves’ or shells and spending that energy on growing or reproducing. After all, you can always lie on a hard flat surface with the valve above you to keep you safe. Naturally you’ll need to hold on.