Suborder (1) Oligotrichina Bütschli, 1887

(syn. Strombidiina)

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.

[Third family is controversially included here.]

Small, active forms; circle of apical membranelles "open"; somatic ciliature present, though generally reduced to a few bristles; predominantly fresh-water forms, but also a few are edaphic; marine forms poorly known.	Family HALTERIIDAE Claparède & Lachmann, 1858
Free-swimming (rarely loricate), pelagic forms; circlet of apical membranelles "open"; somatic ciliature greatly reduced or even absent altogether; pellicle, especially in posterior half of body, made rigid by presence of both "trichites" (in a band) and polysaccharide plaques (over a broad area); perilemma present in some species; predominantly in marine habitats, with one species an ectosymbiont of echinoids, though several very common planktonic fresh-water forms.	Family STROMBIDIIDAE Fauré-Fremiet, 1970
Peristomial field entirely apical, with uniquely "closed" circlet of apical membranelles; somatic ciliature, though not conspicuous, present (e.g., in slightly spiraled, widely spaced longitudinal rows in some species); found in both fresh-water and marine habitats, with one species an inquiline of echinoids.	Family STROBILIDIIDAE Kahl in Doflein & Reichenow, 1929