From Consciousness Explained by Daniel C. Dennett (Little, Brown and
Company, Boston, 1991):
The juvenile sea squirt wanders through the sea searching for a suitable rock or hunk of coral to cling to and make its home for life. For this task it has a rudimentary nervous system. When it finds its spot and takes root, it doesn’t need its brain any more so it eats it. It’s rather like getting tenure.
((This was something I ran across on-line, so I can’t vouch for its veracity. I find it nevertheless highly amusing.))