Annelida

Your body plan, so far, has been simple, like all worms.  Head, containing brain, mouth and sensory organs.  Thorax, containing lungs and other organs.  Abdomen, or tail, containing the gut.  The best muscles are around the thorax.  If you want more of those, there's only one effective way to do it: make another abdomen.  Then another.  And another.

Yes, you make segments of your body duplicate in their entirety: you become a segmented worm.  This turns out to be incredibly good.  You have redundancy in your organs, speed, and a nice big gut to get the most out of what you eat.

It's so successful that you develop into well over twenty thousand separate species (we just can't cover them all) including the earthworm and leech families.  You can swim, by undulating, wriggle through the soil or crawl across the top, using protrusions from the sides of your segments.  You are an enormously important aspect of soil health, and you are found just about everywhere that is not frozen or completely dry on the planet.