An order of the Holotrichia which contains many species that often are of small size and fairly uniform ciliation. Primarily, these protozoa are of importance as the first possessors of a definite, though inconspicuous, buccal ciliature. This ciliature consists of an undulating membrane on the right side of the buccal cavity and an adoral zone of membranelles that is primitively composed of three membranelles on the left side. This tetrahymenal, or four-part, buccal ciliary apparatus is considered the fundamental condition from which the oral ciliature of many subsequent higher groups evolved.