The sea squirt. It has a brain until it hasn’t:
… a young sea squirt navigates the great oceans until it finds a rock that is secure, located in water that is just the right temperature, and surrounded by food. Having found a home, it settles down. Sea squirts are in fact sessile animals; once they take up residence, they never move again, come what may. The first thing a sea squirt does after setting up home is to eat its own brain. And why not? It’s possible to live and be a sea squirt without one.
— Giulia Enders in Gut (Scribe Publications, 2015).
Maybe I shouldn’t overthink it. I have a brain so I have to walk …