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.

The analogy between the sea squirt and the associate professor was first pointed out, I think, by the neuroscientist Rodolfo Llinás.