Tetrahymena  

Small to medium ciliate, ovoid to pyriform with anterior end narrowed. Oral aperture small in anterior body third, pyriform in outline with its axis parallel to that of the major body axis. There is an undulating membrane on the right and three small inconspicuous membranelles on the left. In some species the formation of macrostomes with great enlargement of the buccal cavity leads to the splitting of these membranelles such that five or six membranelles may be found; similar splitting of the membranelles has been noted in some microstomes (Kaczanowski, 1975). Macrostome formation allows the cell to lead a carnivorous way of life. Somatic ciliation complete with a straight pre-oral suture. Some rarely with a caudal cilium. Single terminal contractile vacuole. Macronucleus spherical centrally positioned.