Didinia are rounded, oval or barrel-shaped and range in length from 50 to 150 micrometres.[3] The cell body is encircled by two ciliary bands, or pectinelles. This distinguishes them from the related genus Monodinium, which have only a single band, except during cell division.[4] The pectinelles are used to move Didinium through water by rotating the cell around its axis.[5] At the anterior end, a cone-shaped structure protrudes, supported by a palisade of stiff microtubular rods (nematodesmata). This cone encloses the cytostome or "mouth" opening, as in other haptorian ciliates. The dimensions of this protuberance vary among the different species.