Body pyriform, pre-equatorial furrow separating conical anterior portion from globular trunk. 1, possibly 2, macronuclear nodules in mid-body. Two contractile vacuoles pre-equatorial in conical body portion; position in rear end very likely erroneous. Without extrusomes (?). Longitudinal ciliary rows 16-25 in number, short, bipartite, in area of pre-equatorial furrow; anterior portions form 2 pre-equatorial ciliary belts composed of anteriorly and laterally directed bundles of about 8-12 µm long and stiff cilia ("cirri"), bundles bifurcate in distal half, without barbs; posterior portions form equatorial belt (difficult to recognize because cilia closely adjoin pellicle). Dorsal brush absent (?). Cytostome apical, surrounded by up to 8 retractile, bifurcated processes ("tentacles"). Moves in very rapid backward jumps, rests with pre-equatorial cilia rigidly spread. Size in vivo 12-20 x 8-15 µm. Foissner et al. (1995) propose that Mesodinium acarus, which is actually a freshwater species, is possibly a synonym of Mesodinium pulex.
Ref.: Tamar (1987), Foissner et al. (1995).