Above is a type of rotifer, belonging to the genus euchlanis. This species are usually around 0.1-0.5 mm in length, and is common in freshwater environments. (See citation below.) They seem to be able to swim quite well, using their tails to aid them.

Thorp, James H.. Ecology and Classification of North American Fresh Water Invertabrae. Academic Press, San Diego, 1987, pg.190

Below is a picture of the aforementioned parimecium: