1. Bilateral symmetry.
This must be advantageous. It evolved for the bilaterians. Then again, secondarily, for sea cucumbers.
2. Cephalization.
Yup. Sense organs clustered around the mouth end. Anterior ganglia enlarging and fusing to become "brains." You see it in vertebrates and in mollusks (cephalopods) and in arthropods.
3. Funny, I can't think of anything else. Maybe:
Closed circulatory system, allows the animal to grow larger and thicker.
You see it in annelids, vertebrates, cephalopod mollusks.