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 it's spot and takes root, it doesn't need its brain any more so it eats it. It's rather like getting tenure.
In a footnote, Dennett writes:
The analogy between the sea squirt and the associate professor was first pointed out, I think, by the neuroscientist Rodolfo Llinás.

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