Enteropneusta

As you crawl along, you run the edge of your mouth along the bottom, taking in sand and dirt, and digesting anything useful which goes in with it.  Your long gut becomes longer and longer, to extract as much as possible from the detritus you find on the ocean floor.  Eventually, some of you become more than two metres long, all comprised of gut behind a muscular 'head' section.

For safety, you tend to burrow into the sea bottom, and then up again, making a U shape.  You stick your head out to feed, or stick your tail out to defecate.  When you live at the sea's edge, this makes makes characteristic 'casts' of sand at low tide.

The muscular head gives you a characteristic shape, which looks familiar and gives you your common name.  You are an acorn worm.