You will probably miss it the first time.
Spirostomum spends most of its life as a slow pale ribbon, drifting, unhurried, so long and thin that it looks less like an animal than like something an animal shed. It is one cell. A big one — several millimetres of it in the larger species, which puts it among the largest single cells that move under their own power.
And then something disturbs it, and it is simply somewhere else, shorter, and you did not see the transition because there was nothing to see. The contraction takes a few thousandths of a second. On a screen at ordinary speed it is one frame of ribbon and the next frame of stub.
What makes it strange is what is not doing the work. This is not muscle. There is no actin sliding past myosin, no ATP burned at the moment of contraction. Instead calcium floods a meshwork of protein filaments running the length of the cell, the meshwork collapses, and the cell folds with it — a spring released rather than an engine fired. Recharging it is the slow part, which is why the stretch back out takes a second and the escape takes none.
An organism with no nerves, no muscles, and no brain has arrived at one of the fastest escapes in biology by keeping a wound spring in its own skin.