Body 15-55 µm (usually 15-35 µm) long, lanceolate or flask-shaped, anterior narrowing to blunt point, posterior rounded. Apical pole overlapping up towards the uppermost surface. Two spherical macronuclei with a clearer zone in their central region, but in some cells there is a single, elongated, tapered macronucleus. Contractile vacuole terminal. Feeds only on small protistan prey such as flagellates.