L.: “wheel-bearers,” sometimes called “wheel animalcules.” They have no wheels.
600px-20090730_020239_Rotifer
(source)
But they get around.  Very few species live in salt water, but pretty much anywhere else will do.  In moss, in ice; many can even be dried out and revived later — they are anhydrobionts.
Rotifers are indicative of a good, stable sludge, as I recall from my old job back in Louisville.

Rotifers are wonderfully weird!

Rotifers in wastewater treatment, with more pics

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