Plegepod Protozoa with a stable outline and a complete or partial investment of cilia. Compound cilia, pecti7iellae, and vibratile membranes are not uncommon. There is usually a special adoral band of cilia. The protoplasm may or may not be differentiated into an exo- and endo-plasm, and it may or may not be protected by cuticular structures. There is as a rule an oral tube or cytostome, and an anal spot or cytopyge constant in position, rarely an anal tube. The nucleus is single or multiple, and is accompanied by one or more paranuclei, except in a few multinucleate forms. A contractile vacuole is seldom absent, usually single, occasionally midtiple, always constant in position. Conjugation is temporary or permanent: the conjugating individuals similar or dissimilar. Fission is in most instances binary, sometimes repeated rapidly, causing a diminution in the size of the organism. Gemmation is not common.

The majority of Infusoria are free-swimming: a few lead a sedentary but free life within a gelatinous tube, which may be simple (Stichotricha)i or dichotomously branched (Maryna, Schizosiphon). Some species of Stentor are able to attach themselves at will by pseudopodial processes; others inhabit a gelatinous tube, to which they cling by means of retractile setae: so too Tintinnidium fluviatile and Stylocola. Some genera, such as Spirochona and its allies, are permanently attached by a suckerlike disc or by a cement. Of the attached forms, some are simple, and then naked, stalked, e.g. Vorticella, or tubicolous, the tube or lorica being sessile or stalked; others are colonial, either naked and stalked, e. g. Epistylis, or protected by a gelatinous investment as in Ophrydium.

The Infusoria present great varieties of external shape: their body is sometimes eminently contractile, therefore changeable in outline. The Hypotricha, however, have a flattened locomotor or ventral surface, and a convex dorsal; and the fixed Peritricha are more or less bell-shaped. The surface of the body is rarely completely naked, but is invested with a cuticular layer of varying thickness and consistence1. It is elastic, generally hyaline, seldom granular, or marked by vertical lines (? rods or spaces); and is often scarcely distinguishable from the underlying protoplasm. There is sometimes, however, a very distinct cuticle, either a dorsal thickened cuirass (Euplotidae among Hypotricha), a bivalved cuirass (some Dysteridae in the same order), or rings of plates, as in the Holo-trichan Coleps hirtus. The coat of needles in Tiarina (= Coleps fusus), or the similar structures covering the pedicles of some Vorticellae, are probably cuticular: so too the simple or denticulate ring which supports the sucker of all the Peritrichan Urceolarina save Licnophora and the corneous band or teeth of the Opalinid Hoplitophrya. A number of Infusoria secrete a distinct tube or lorica, usually fixed, sometimes free as in Tintinnodea, which may be, as in some species of Stentor, only a temporary structure.

It may be gelatinous, or though at first soft and colourless, becomes hardened, dark in hue, and resistent to reagents. In the Tintinnodean Codonella, particles of silica are imbedded in its substance, and in its congener Dictyocysta, it is perforate and Radiolarian in appearance. Its chemical composition is unknown, save in the case of the hyaline investment of the colonial Ophrydium versatile, where it is composed of a cellulose akin to Tunicin (p. 107), and though apparently of a uniform structure, is really divisible into a number of coherent sheaths, one to each individual.

The cilia, which are so characteristic of the class, are extensions of the outer layer of protoplasm, and are retractile under certain conditions. They are sometimes used as organs of adhesion, and vary in character. The cilia proper are slender, of the same size from base to apex, vibratile throughout: the cirri, the elongated triangular membranellae s. pectinellae, the membranes, are composed of agglutinated cilia, and may be wholly or partially vibratile; the setae are slender, rarely compound, non-vibratile2. The modes in which the cilia are disposed afford a basis for classification. In the Holotricha they are arranged in parallel lines and are of uniform size, but there may be a band of larger adoral cilia. The Heterotricha have an investment of fine cilia covering the body, but a band, straight or curved, of membranellae borders a naked peristome. Its posterior end, or left-hand end, enters the mouth with a spiral twist if the band is curved. In shape, extent, and position the peristome varies a good deal. It is much depressed in the Tintinnodea, and in some of them a row of fine paroral cilia lie to the inner side of the membranellae. Some species of Stentor possess fine setae, apparently pro- and re-tractile, mingled with the cilia of the body.