[Illustration: FIG. 60.—_Vorticella._ A, expanded; B, stalk in contraction; _c_, eversible collar below peristome; _cr_, line of posterior ciliary ring; _c.v_, contractile vacuole; _m_, muscle of stalk; _N_, meganucleus; _n_, micronucleus; _p_, pharynx; _r_, reservoir of contractile vacuole; _s_, tubular stalk; _u.m_, undulating membrane in vestibule; _v_, hinder end of vestibule. E^1, E^2, two stages in binary fission; E^3, free zooid, with posterior wreath; F^1, F^2 division into mega- and micro-zooids (_m_); G^1, G^2, conjugation; _m_, microzooid. (Modified from Bütschli, from Parker and Haswell.)] The members of the Vorticellidae are very commonly attached to weeds or to various aquatic Metazoa, each species being more or less restricted in its haunts. _Vorticella_, the type, is singly {158}attached to a contractile stalk; fission takes place in the vertical plane, and one of the two so formed retains the original stalk, while the other swims off (Fig. 60, E^1-E^3), often to settle close by, so that the individuals are found in large social aggregates, side by side, fringing water-weeds with a halo visible to the naked eye, which disappears on agitation by the sudden contraction of all the stalks. _Carchesium_ and _Zoothamnium_ differ from _Vorticella_ in the fact that the one daughter-cell remains attached by a stalk coming off a little below the body of the other, so as to give rise to large branching colonies. In _Carchesium_ (Fig. 51) the muscular threads of each cell are separate, while in _Zoothamnium_ they are continuous throughout the colony. _Epistylis_ has a solid, rigid stalk, and may give rise to branching colonies, which often infest the body of the Water-Fleas (Copepoda) of the genus _Cyclops_. _Opercularia_ is characterised by the depth of the gutter, the height of the collar, and the tapering downward of the elongated disc. _Vaginicola_, _Pyxicola_, _Cothurnia_, _Scyphidia_, all inhabit tubes, some of extreme elegance. _Ophrydium_ is a colonial form, found in ponds and ditches, resembling _Opercularia_, but inhabiting tubes of jelly[176] that coalesce by their outer walls into a large floating sphere; it usually contains the green symbiotic Flagellate _Zoochlorella_. _Trichodina_ is free, short, and cylindrical, with both wreaths permanently exposed, and is provided with a circlet of hooks within the aboral wreath. It is often parasitic, or perhaps rather epizoic, on the surface of _Hydra_ (see p. 254), gliding over its body[177] with a graceful waltzing movement; it occurs also in the bladder and genito-urinary passages of Newts, and even in their body-cavity and kidneys. II. SUCTORIA = TENTACULIFERA _Infusoria with cilia only in the young state,[178] without mouth or anus, but absorbing food (usually living Ciliates) by one or more tentacles, perforated at the apex; mostly attached, frequently epizoic, rarely parasitic in the interior of other Protozoa._