The famous sea squirt, beloved of popular neuroscience lectures, in its larval stage is motile and has a primitive nervous system (called a notochord) so it can navigate the sea – at least, its own very small corner of it. In its adult stage it fastens limpet-like to a rock and feeds passively, simply depending on the influx of seawater through its tubes. It then reabsorbs its nervous system – it is no longer needed since the creature no longer needs to move.
