Every collection needs someone ordinary, and this is the one.
Paramecium is the protist of the school microscope, the one on the worksheet, the one whose name people half-remember from a classroom they did not enjoy. It is shaped like a slipper, it is covered head to foot in beating hairs, and it spends its entire life doing one thing: moving forward until that stops working, then trying a slightly different forward.
The mechanism is worth stating plainly, because it is so much simpler than it looks from outside. Run into something, and the impact changes the voltage across the cell’s membrane. Calcium comes in. Every cilium on the body reverses its beat at once, and the animal reverses out. Then the voltage settles, the cilia resume, and it sets off again on a slightly rotated heading. If that also fails, it does the same thing again. It will do it as many times as it takes.
There is no map and no memory of the obstacle. There is a rule — hit something, back up, turn a bit, go — that has been running unchanged for a very long time, and it is sufficient. Watching a Paramecium work its way out of a dead end is watching an algorithm that predates nervous systems solve a problem by being willing to fail at it repeatedly.
Most of the animals in this zoo are here because they do something startling. This one is here because it doesn’t, and it is still going.