The ususal reaction I get from my students when they see this creature in the microscope is, "Ewwww, a worm!" This is an annelid, or an aquatic earthworm. In a clearer photo the rings would be visible. It also has hair-like projections called setae (just barely visible on its sides), they help the worm move more easily through the water. The worm's digestive tract is easily seen in this photo taken at 10X.

Green algae and diatoms