Order 3. OLIGOTRICHIDA Bütschli, 1887

(syns. Oligotricha, Oligotrichorida)

Body form ovoid to elongate, sometimes tailed, with thickened pellicle; commonly reduced somatic ciliature; buccal paramembranelles extensive and conspicuous, often in two sections one inside buccal cavity proper, other ("somatic" portion) out onto body surface encircling anterior pole of organism; infraciliary base of paroral membrane composed of single, nonzigzag row of kinetosomes (monostichomonad); stomatogenesis apokinetal in a below-surface pouch; a perilemma present external to the cell (plasma) membrane in many species; possibly no cytoproct; macronucleus with reorganization bands; mainly (but not exclusively) marine, pelagic organisms, and free-swimming (even when loricate, as in suborder Tintinnina); all free-living except for two oligotrichine species associated with echinoids.

[Some workers would raise this order to the level of an independent subclass.]

Body typically small, rounded or gently pointed posteriorly (though several species with lengthy tail or caudal cilium); somatic ciliature typically reduced to a few short rows of cirrilike bristles or an equatorial belt of very short "sensory" cilia (but one genus holotrichous); "bipartite" adoral zone of membranelles, with the more conspicuous "somatic" portion ("open" or "closed") used in locomotion; perilemma in some species, not others; pellicle may be strengthened by skeletal elements, especially in posterior half of body; predominantly marine organisms, but several well-known and widely distributed species live solely in fresh-water habitats, and at least one is strongly edaphic.

Suborder (1) Oligotrichina Bütschli, 1887

Body cylindrical or cone-shaped, highly contractile, often with elongate posterior end; all loricate; ciliature dominated by apically located buccal paramembranelles (sometimes with interspersed tentaculoids) in a "closed" arrangement; a perilemma presumably universally present; loricae range 25-1,000 pm in length, up to 3,000 pm if certain aberrant çuestionable fossil material included (but usual size, 100-200 pm); widespread members of marine pelagic and neritic plankton (with fossil evidence for past aeons), but several forms found abundantly in fresh-water habitats; form, size, and composition of the universally possessed lorica most important diagnostically at all infrasubordinal taxonomic levels.

Suborder (2) Tintinnina Kofoid & Campbell, 1929