{156}[Illustration: FIG. 59.—_Stentor polymorphus._ I, Young individual attached, extended; II, adult in fission, contracted; _cv_ in I, afferent canal of contractile vacuole; in II, contractile vacuole; N, moniliform meganucleus (micronuclei omitted); _o_, mouth; the fine lines are the myoneme fibrils. (From Verworn.)] Among Peritrichaceae, _Vorticella_ (Fig. 60) and its allies have long been known as Bell-animalcules to every student of pond-life. The body has indeed the form of an inverted bell, closed at its mouth by the "peristome," or oral disc; this is a short, inverted truncate cone set obliquely so that its wide base hardly projects at one side, but is tilted high on the other; the edge of the bell is turned out into a rim or "collar," separated from the disc by a deep gutter. The collar, habitually everted, or even turned down, contracts over the retracted disc when the animal is retracted (E^2), which is brought about by any sort of shock, or when it swims freely backwards. For the latter purpose a posterior ring of cilia (or rather membranellae) is developed round the hinder end of the bell (A, _cr_, E^3). The cilia of the adoral wreath are very strong, united at the base into a continuous membrane, and indeed themselves partake of the composite nature of membranellae. The wreath forms more than one turn of a right-handed spiral, the innermost turn ending abruptly on the disc, the outer leading down into the mouth at the point where the disc is most tilted and the groove deepest.[174] The pharynx (_p_) is long, and contains an undulating membrane (_u.m_) on its inner side projecting out through the mouth, and numerous cilia; it leads deep into the body (_p_). The first part is distinguished as the "vestibule" (_v_), as into it opens the anus, and the contractile vacuole (_c.v._), the latter sometimes opening by a reservoir (_r_). The body contains in the ectoplasm {157}myonema-fibrils which, by their contraction, withdraw the disc, and at the same time circular fibrils close the peristome over it. In the type-genus the pellicle is continued into a long, slender elastic stalk (_s_), of which the longitudinal myoneme fibrils of the ectoplasm converge to the stalk, and are prolonged into it as a spirally winding fibre, sometimes transversely striated.[175] The effect of the contraction of this is to pull the stalk into a helicoid spiral (like a coil-spring), with the line of insertion of the muscle along the inner side of the coils, which is, of course, the shortest path from one end to the other (Fig. 60, B).