Hymenostomatida

(invertebrate zoology) An order of ciliated protozoans in the subclass Holotrichia having fairly uniform ciliation and a definite buccal ciliature.

An order of the Holotrichia which contains many species that often are of small size and fairly uniform ciliation. Primarily, these protozoans 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. See also Ciliophora.

The majority of hymenostomes are free-living fresh-water forms. Paramecium (see illustration) is the best-known genus of ciliates. It is a good-sized, widely distributed ciliate, and is a much-studied form. Tetrahymena, beginning to rival Paramecium as a favorite ciliate in much experimental work, owes its scientific popularity primarily to its ability to grow axenically, that is, free from all other organisms, in a chemically defined medium. See also Protozoa.

A representative Paramecium.