This is not a planaria with a spine and rudimentary legs. It is an Australian LEECH! Apparently the "spine" is it's developing young, and when they become free swimming, they attach itself to the mother and look like a billion tiny wiggly legs. Totally disgusting and mine was about a fourth of an inch long.
Actually, I come to find a year later, it was some sort of snail leach. Even more disgusting.