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 …
Update 13/10/2020 — I recently picked up In Praise of Walking by Shane O’Mara on an unplanned visit to a book store. He mentions the sea squirt as well:
Eventually, as it grows, the squirt transitions to a fixed stage, sticking itself to a convenient rock. There it consumes its own semi-brain, spinal cord and eye, none of which it now needs. (…) The larger lesson is clear: brains have evolved for movement.
O’Mara also mentions that the reverse pattern exists in certain jellyfish. They start out as polyps attached to rocks but then ‘start to develop a nerve net that allows them to engage in patterned movements, to attack prey and to ingest food.’ Couch potatoes can aspire to be more like jellyfish …