You hunt. You grow. You crawl forward over the bottom of the sea, looking for smaller creatures and eating them. Around your mouth, the frilly section which enabled you to pull in tiny fragments of food become longer, and more powerful, until they form a ring of tentacles around a tough, muscular mouth.

The rich diet allows you to grow larger and more muscular. During your life your body can outgrow your shell easily, which makes you vulnerable. In a similar way to snakes shedding skins, or insects shedding exoskeletons, you need to find a way to grow a new, larger shell.

Evolution provides…

As you grow, your shell is extended on one side, and begins to curve. You are attached to the bottom of it, so you can pull yourself back into it when threatened. However, eventually, you will become too big to continue, as you would become very long and thin. When this happens, you move your body up, and seal off the old redundant section with a little wall. To avoid having to make too much new shell, the shape of the shell spirals around itself so that one side of the old section is re-used as the inner side of the new section.

Your tentacles aren’t powerful enough to drag you along, although they are easily stronger than the little shrimps and other small sea creatures that you eat. To move, you develop a small sac from which you can squirt water to drive you along by jet propulsion.

Eventually you become large and powerful enough to move through open water instead of crawling along the bottom. At this point the shell might be useful for buoyancy control, or just something to drag you down.