Order 3.—Heterotrichaceae; body covered with fine uniform cilia, usually circular in transverse section.
Order 4.—Oligotrichaceae; body covering partial or wholly absent; transverse section usually circular.
Order 5.—Hypotrichaceae; body flattened; body cilia represented chiefly by stiff cirrhi in ventral rows, and fine motionless dorsal sensory hairs.
Order 6.—§§ ii.—Peritrichaceae. Peristomial ciliary wreath, spiral, of cilia united at the base; posterior wreath circular of long membranelles; body circular in section, cylindrical, taper, or bell-shaped.
Illustrative Genera (selected).
1. Gymnostomaceae. (a) Ciliation general or not confined to one surface. Coleps Ehr., with pellicle locally hardened into mailed plates; Trachelocerca Ehr.; Prorodon Ehr. (fig. i. 6, 7); Trachelius Ehr., with branching endosarc (fig. i. 8); Lacrymaria Ehr. (fig. i. 5), body produced into a long neck with terminal mouth surrounded by offensive trichocysts; Dileptus Duj., of similar form, but anterior process, blind, preoral; Ichthyophthirius Fouquet (fig. i. 9-12), cilia represented by two girdles of membranellae; Didinium St. (fig. i. 13), cilia in tufts, surface with numerous tentacles each with a strong terminal trichocyst; Actinobolus Stein, body with one adoral tentacle; Ileonema Stokes. (b) Cilia confined to dorsal surface. Chilodon Ehr.; Loxodes Ehr., body flattened, ciliated on one side only, endosarc as in Trachelius; Dysteria Huxley, with the dorsal surface hardened and hinged along the median line into a bivalve shell, ciliated only on ventral surface, with a protrusible foot-like process, and a complex pharyngeal armature. (c) Cilia restricted to a single equatorial girdle, strong (probably membranelles); Mesodinium, mouth 4-lobed.
2. Aspirotrichaceae. Paramecium Hill (fig. ii. 1-3); Ophryoglena Ehr.; Colpoda O. F. Müller; Colpidium St.; Lembus Cohn, with posterior strong cilium for springing; Leucophrys St.; Urocentrum Nitsch, bare, with polar and equatorial zones and a posterior tuft of long cilia; Opalinopsis Foetlinger (fig. i. 1, 2); Anoplophyra St. (fig. i. 3, 4). (The last two parasitic mouthless genera are placed here doubtfully.)
3. Heterotrichaceae. (a) Wreath spiral; Stentor Oken. (fig. iii. 2), oval when free, trumpet-shaped when attached by pseudopods at apex, and then often secreting a gelatinous tube; Blepharisma Perty, sometimes parasitic in Heliozoa; Spirostomum Ehr., cylindrical, up to 1″ in length; (b) Wreath straight, often oblique; Nyctotherus Leidy, parasitic anus always visible; Balantidium Cl. and L., parasitic (B. coli in man); Bursaria, O.F.M., hollowed into an oval pouch, with the wreath inside.
4. Oligotrichaeceae. Tintinnus Schranck (fig. iii. 3); Trichodinopsis Cl. and L.; Codonella Haeck. (fig. iii. 5); Strombidium Cl. and L. (fig. iii. 4), including Torquatella Lank. (fig. iii. 6, 7), according to Bütschli; Halteria Duj., with an equatorial girdle of stiff bristle-like cilia; Caenomorpha Perty (fig. iii. 23, 24); Ophryoscolex St., with straight digestive cavity, and visible anus, parasitic in Ruminants.
5. Hypotrichaceae. Stylonychia Ehr.; Oxytricha Ehr.; Euplotes Ehr. (fig. i. 14, 15); Kerona Ehr. (epizoic on Hydra).