Watch the stalk, not the bell.
Vorticella has arranged its life around not going anywhere. It fixes itself to a surface, opens the wide end of its body into a rim of beating cilia, and spins a small tornado of water down into its own mouth. Whatever the water was carrying is now lunch. It is a perfectly good living, and it means the animal spends its days at the end of a tether, exposed.
So the tether is the weapon. Running down the inside of the stalk is a helical thread — the spasmoneme — that will collapse when calcium reaches it. It does not need ATP to do this. It does not need muscle, because there is no muscle: the whole contraction is a protein spring letting go. The stalk coils, the bell is snatched backwards out of danger, and the entire event is over in a couple of thousandths of a second.
Then comes the slow part. Re-extending the stalk is the step that actually costs energy, and it takes the better part of a second — an eternity, at this scale. The animal is fast in exactly one direction: away.
You can see the thread in the footage. Whoever filmed it knew what they were pointing at.