[Illustration: FIG. 56.—_Trachelius ovum._ A, general view; B, section through sucker; C, section through contractile vacuole and its pore of discharge. _al_, Alveolar layer of ectoplasm; _cil_, cilia; _c.v_, contractile vacuole; _m_, mouth; _N_, meganucleus; _s_, sucker, from which pass inwards retractile myonemes. (After Clara Hamburger.)] {154}[Illustration: FIG. 57.—_Pleuronema chrysalis_ (Aspirotrichaceae). A, Unstimulated, lying quiet; B, stimulated, in the act of springing by the stroke of its cilia. (From Verworn.)] The Heterotrichaceae present very remarkable forms. _Spirostomum_ is nearly cylindrical, and, a very giant, may attain a length of 4 mm. (1/6"). _Stentor_ can attach itself by its hinder end, which is then finely tapered and prolonged into a few pseudopodia; its body is trumpet-shaped, with a spiral peristome forming a coil round its wide end, and leading on the left side into the mouth. Many species when attached secrete a gelatinous sheath or tube. _S. polymorphus_ is often coloured green by _Zoochlorella_ (p. 125); _S. coeruleus_[171] and _S. igneus_ owe their names to the brilliant pigment, blue or scarlet, deposited in granules in lines between the conspicuous longitudinal myonemes. From their large size and elongated meganucleus accompanied by numerous micronuclei, these two genera have frequently been utilised for experiments on regeneration. In _Metopus sigmoides_ the peristomial area forms a dome above its wreath of membranellae; and in _M. pyriformis_ this is so great as to form the larger part of the cell, which is top-shaped, tapering behind to a point. _Caenomorpha_ (Fig. 58) has the same general form, with a peg-like tail, and possesses a girdle of cirrhi.[172] The converse occurs in {155}_Bursaria_; the cell is a half ellipse, something like a common twin tobacco-pouch when closed: a deep depression thus occupies the whole ventral surface, and opens by a wide slit extending along the anterior end. The peristomial area occupies the dorsal side of the pocket so formed, and the mouth is in the hinder left-hand corner. _Blepharisma_ sp. is parasitic in the Heliozoon _Raphidiophrys viridis_ (Fig. 20, 1, p. 74). [Illustration: FIG. 58.—_Caenomorpha uniserialis._ _crh_, Zone of cirrhi; _c.t_, cilia of tail; _c.v_, contractile vacuole; _c.w_, ciliary wreath; _g_, granular aggregate; _m_, zone of membranellae; _N_, meganucleus; _n_, micronucleus; _oe_, pharynx; _t_, tail-spine; _t_^1, accessory spine; _u.m_, undulating membrane; _v_, vacuole; _z_, precaudal process. (After Levander.)] Among Oligotrichaceae, _Halteria_, common among the débris at the bottom of pools in woods containing dead leaves, is remarkable for an equatorial girdle of very long fine setae, and for its rapid erratic darting movements, alternating with a graceful bird-like hover. The Tintinnidae are mostly marine, pelagic, with the general look of a stalkless _Vorticella_; some have a latticed chitinous shell.[173]