(_a_) Cilia nearly uniform, usually extending over the whole body, without any special adoral wreath of long cilia or membranellae; mouth with one or two undulating membranes at its margin or extending into the short pharynx. Order 2. ASPIROTRICHACEAE. _Paramecium_, Hill; _Colpoda_, O. F. Müll.; _Colpidium_, St.; _Leucophrys_, Ehrb.; _Cyclidium_, Cl. and L.; _Lembadion_, Perty; _Cinetochilum_, Perty; _Pleuronema_, Duj.; _Ancistrum_, Maup.; _Glaucoma_, Ehrb.; _Uronema_, Duj.; _Lembus_, Cohn; _Urocentrum_, Nitzsch; _Icthyophtheirius_, Fouquet. (_b_) Strong cilia or membranellae forming an adoral wreath, and bounding a more or less enclosed area, the "peristome," at one point of which the mouth lies. (i.) Body more or less equally covered with fine cilia; adoral wreath an open spiral Order 3. HETEROTRICHACEAE _Spirostomum_, Ehrb.; _Bursaria_, O. F. Müll.; _Stentor_, Oken; _Folliculina_, Lamk.; _Conchophtheirus_, St.; _Balantidium_, Cl. and L.; _Nyctotherus_, Leidy; _Metopus_, Cl. and L.; _Caenomorpha_, Perty; _Discomorpha_, Levander; _Blepharisma_, Perty. (ii.) Body cilia limited in distribution or absent; peristome anterior, nearly circular, sinistrorse. Order 4. OLIGOTRICHACEAE. _Halteria_, Duj.; _Maryna_, Gruber; _Tintinnus_, Schrank; _Dictyocystis_, Ehrb.; _Strombidium_, Cl. and L. (= _Torquatella_, Lank.). (iii.) Peristome extending backwards along the ventral face, which alone is provided with motile cirrhi, etc.; dorsal cilia fine, motionless. Order 5. HYPOTRICHACEAE. {138}_Stylonychia_, Ehrb.; _Kerona_, O. F. Müll.; _Oxytricha_, Ehrb.; _Euplotes_, Ehrb.; _Stichotricha_, Perty; _Schizotricha_, Gruber. (iv.) Body cilia reduced to a posterior girdle, or temporarily or permanently absent; peristome anterior, nearly circular, edged by the adoral wreath,[153] bounded by a gutter edged by an elevated rim or collar. Order 6. PERITRICHACEAE. _Lichnophora_, Cl.; _Trichodina_, Ehrb.; _Vorticella_, L.; _Zoothamnium_, Bory; _Carchesium_, Ehrb.; _Epistylis_, Ehrb.; _Opercularia_, Lamk.; _Vaginicola_, Lamk.; _Pyxicola_, Kent; _Cothurnia_, Ehrb.; _Scyphidia_, Lachmann; _Ophrydium_, Bory; _Spirochona_, St.