Corliss, 1974
Nuclear apparatus dual, but macronuclei diploid, nondividing, forming by division of micronuclei. Body fragile, often elongate, highly thigmotactic. Oral area apical or ventral. Contractile vacuole probably often lacking. Many obligate interstitial forms, all except one genus marine. Recently elevated to class rank (Small and Lynn, 1985).
Family Trachelocercidae
Family Protocruziidae