Plegepod Protozoa with cilia confined to the bud or fission-product, and to a temporarily assumed motile phase. Fixed as a rule, and provided with prehensile tentacles of one or of two kinds. Rarely naked; a soft cuticle generally present or a re sis tent lorica. The nucleus is single, occasionally ramified. There are one or more contractile vacuoles, sometimes provided with an efferent duct. Reproduction takes place rarely by binary fission or external gemmation, usually by internal. Carnivorous, feeding on other Protozoa. Freshwater and marine.

The genus Sphaerophrya is free, and typically parasitic in various Infusoria. It may assume a complete or partial coat of cilia when wandering from one host to another. It has been observed that Podophrya libera, P.fixa, Acineta mystacina, and Dendrocometes, may retract their tentacles, or arms in the case of the last-named, become ciliate and wander away; and the first-named has been seen to attach itself again. Possibly the phenomenon is not uncommon. The majority of the class are fixed, not infrequently to some animal, and are either sessile or stalked. Trichophrya spreads itself along the surface to which it is attached; and Dendrosoma forms branching colonies, either erect on a narrow base, or springing from a creeping and anastomosing stolon.

The surface of the body is very rarely naked (Sphaerophrya, Podophrya fixa, Podophrya sp. ?). It is provided with a soft, pliable, and distinct cuticle, or with a firm resistent lorica, e. g. in Acineta, but it is extremely probable that all loricate genera have the parts of the body not protected by the lorica covered with a soft cuticle, the part protected being naked. The soft cuticle perishes soon after the death of the Acinetarian; when fission takes place it divides with the rest of the body; in external gemmation it is continuous over the buds, and when they separate from the parent organism it separates with them. The lorica varies much in shape; it may be a shallow plate as in Dendrocometes, a stellate capsule (Acineta stellata), or it is a more or less complete cup inclosing the body of the organism to a greater or less extent. Its aperture may take the form of a slit, simple or toothed, of a circular or elliptical figure; or it may be double. It does not seem certain whether or no the cuticle covering the exposed surface is continuous with its edge. In substance it is firm; in Acineta gelatinosa, however, soft and gelatinous; similarly Podophrya limbata has generally a gelatinous investment.

If a loricate Acinetarian dwindles in size from any cause, e.g. repeated gemmation, it secretes a septum or additional floor to the lorica on which its body rests. Of such septa there may be more than one. The stalk or peduncle when present is seldom a simple continuation of the cuticle (Fraipont), but has generally a firm tubular wall, sometimes striated both transversely and lengthwise, with clear contents. . It is in this case a secreted structure, and when a lorica is also present it is continuous with it.

The body of an Acinetarian is often changeable in shape, especially in Ophryodendron variabile. The protoplasm has generally an external clear thin layer (ectosarc): the rest is more or less granular, and pigmented, when well fed. The pigment may be green, yellow, reddish, brown, and it appears to be a useless product of nutrition. When Dendrocometes assumes a motile phase, it leaves a vesicle containing all its pigment attached to its lorical plate. There is no mouth. The sole organs for catching the living prey upon which an Acinetarian feeds are the tentacles. Of these there are two kinds. One is long, and either pointed or terminated in some cases by a knob; it may be absent, present with the other kind, or, as in the Actinaria, the sole kind present. It is sometimes motile, and its function is to retain the prey, which it does by bending round it, and then by contraction to bring it within the reach of the suctorial tentacles. It may be denominated therefore 'retinaculum' (Fangfaden). In Ephelota Trold, one of the Acinetaria which possess only retinacula, it has been observed to shorten, and finally to draw the prey completely within the body.

The second kind of tentacle is shorter than the first, and terminates either in a trumpet-shaped sucker or in a knob, the latter being probably an extrusion of the axial substance (Maupas). It is said to consist in some cases of a cortical more contractile substance, and a more fluid axis; in others there is an axial tube. It sucks out the protoplasm of the prey by alternate movements of contraction and elongation, and the granular stream may be traced continuing the line of the tentacle into the body of the Acinetarian for some distance before it is diverted from its course and mingles with the protoplasmic currents of the body1. Both kinds of tentacles are eminently contractile, and Podophrya libera, which has only suctorial, has been observed using them for the purpose of creeping about. As they shorten a spiral fold or line twining round them becomes more and more conspicuous; it has been supposed to be muscular. They may be either almost completely or completely retracted, and then there is either no trace visible of them or they are visible as clear lines converging towards the centre of the body 2. They are said to perforate the cuticle, but it is possible that the latter may be excessively attenuated and extend over them.

As to number, Rhynchaeta Cyclopum has one suctorial tentacle of great mobility; Urnula Epistylidis one which may be branched; but there are usually a number, either scattered over the body, confined to the anterior surface, or grouped in two or more bundles. The following are peculiar. Dendrocometes has 4-6 arms radiating from the body, each arm branched terminally, and each branch ending in a number of short points with retractile tips. A tube is said to commence at the apex of each point, to traverse the arm and enter the body. The arms are slowly retracted when the animal assumes a temporary motile state. No trace of the retracted arms is discernible, and when they reappear, they do so as a process ending in one or more tubular points. The Ophryodendridae are characterised by having one or more contractile proboscides, simple in Acinetopsis, with the free end in Ophryodendron beset with numerous flexible and pointed cirri 1.

1 See Maupas' account of the act in Sphaerophyra magna, A. Z. Expt. ix. 1881, p. 302-3.

2 This persistence of the retracted tentacles is difficult to explain: it is well established in some instances, e. g. Hemiophrya gemmipara; but in others, e. g. Sphaerophrya magna, Acineta fetida, it does not occur, and the tentacles are processes of the peripheral zone of protoplasm (Maupas). It is possible that just as the protoplasm of the prey passes into the body in a stream continuing the axis of the tentacle, so the substance of the tentacles themselves may, owing to differentiation, follow the same path. See on the subject, Fraipont, Bull. Acad. Roy. Beige, xiv. 1878, p. 490; and for views for and against the resemblance of the tentacles to pseudopodia, see Id. ibid. p. 489; Maupas, A. Z. Expt. ix. 1881, p. 353-9.