Order 4. HYPOTRICHIDA Stein, 1859

(syns. Hypotricha, Hypotrichina, Hypotrichorida)

Dorsoventrally flattened, oval or elongate, medium-sized forms, occasionally tailed; unique cursorial mode of locomotion; prominent zone of generally numerous adoral (para)membranelles on left-anterior portion of the ventral surface, bordering a broad peristomial field and sometimes continuing over apical end of body onto the dorsal surface; paroral membrane may be multiple (diplo- or polystichomonad condition); somatic ciliature commonly represented by rows or localized groups of cirri, conspicuous on the ventral surface, and rows of widely spaced pairs of short cilia ("sensory bristles") dorsally; a perilemma present in some species; stomatogenesis generally apokinetal, but apparently parakinetal in more primitive forms; macronuclear reorganization bands common; cytoproct, as well as a contractile vacuolar system, always present, and mucocysts plentiful; widely distributed free-living forms in many and diverse habitats, with a few ectocommensalistic on various invertebrates and one inquilinic in an echinoid.

[It is extremely difficult to arrange families, genera, and even species of the hypotrichs into their proper taxonomic groupings until we have available many more data of a comparative nature, especially with respect to the morphogenetics of their binary fission. Thus the classification presented below must, in many instances, be considered provisional.]

Body often elongate, sometimes very drawn out posteriorly; ventral cirri generally small and quite inconspicuous (occasionally as few as 2-3 cilia per cirrus) and typically arranged in 3-12 longitudinal (sometimes spiraled) rows; marginals common, transverse cirri sometimes absent; stomatogenesis parakinetal in the allegedly primitive forms studied to date; several species produce loricae, with a few of these exhibiting colony-formation; found free-living in diverse habitats, with one well-known species (Kerona) an ectocommensal on Hydra.

Suborder (1) Stichotrichina Fauré-Fremiet, 1961

Body sometimes elongate, even tailed, but often oval to elliptical in outline; ventral cirri typically heavy and conspicuous and arranged in specific, localized groups; marginal cirri often absent or else reduced; stomatogenesis apokinetal; found in widely diverse habitats (fresh-water, edaphic, marine, interstitial, etc.); a few species are symbiotic, either as ectocommensals on integument (or in branchial caviry) of several invertebrates or as inquilines of echinoids.

Suborder (2) Sporadotrichina Fauré-Fremiet, 1961