Keeping your anus behind your valve and your mouth in front gives a long enough gut to stay roughly the same shape. You take the form of a fat little creeping worm with a capacious shell. You feed on whatever you find, and occasionally, things feed on you. If you hang out near the top of the sea, where there is most food, you tend to find that you are the food. Yet again, you need to protect yourself, or hide away. Often, evolution is caused by the accidental duplication of body segments. That might be an option…