Difflugia pyriformis (sketch)
Mainly 150-250 micrometres
Freshwater, especially in pools around submerged moss and filamentous algae
Omnivorous, feeding on small algae and other protists
Difflugia and relatives have dark shells formed by cementing together sand, diatom shells, or other debris.
Kingdom Protozoa / Protista
Also called Difflugia oblonga, a name which would have priority except it was likely some smaller species.