Time passes, and you consistently grow one larger valve and one smaller, and lie on one side more than the other.

Your shell becomes more domed, so that you can fit all your body inside, but when you come out to feed, you expand in front and behind to far beyond the shell. As more time passes, you stop growing a valve on one side at all.

A simple matter of plumbing becomes a problem which you must solve. As you are now larger, and eating more, you have to poop more! Previously you would simply open your shells and let the water carry away the waste, but now that you are spreading out from beneath your single shell, you don’t want your bottom underneath, trapping your wastes against you while you poop.

It’ll be better if your anus is outside. Either it needs to be on the part you stretch out in front, or the part you stretch out behind. Either is better than in the middle.