The fringed spines of Triarthridae are jointed appendages moved by powerful muscles; in _Triarthra_ one is median and ventral, the others being attached to the shoulders. In _Polyarthra_, there are twelve flattened and serrated spines, a bunch of three being attached to the dorsal and ventral faces of either shoulder. An easy transition leads to the hollow appendages of Scirtopoda, which end in a fringe of bristly hairs, themselves feathered with finer hairs (Fig. 117). These processes are in _Pedalion_ six in number, two median (respectively dorsal and ventral), two antero-lateral, and two postero-lateral. As they contain proper muscles, {202}and the postero-lateral pair contain part of the nephridia and bear the lateral antennae, they are true outgrowths of the body, and are not homologous with the spines of Triarthridae. [Illustration: FIG. 108.—Diagrammatic views of disc of Rotifers. Cingulum represented by a black line, groove shaded; trochus dotted; the black spot represents the mouth. 1, Simple disc of _Microcodon_; 2, Bdelloid disc of _Rotifer_ or _Callidina_, the star represents the ciliated proboscis; 3, disc of _Hydatina_, groove represented by lobes bearing ciliated styles; 4, disc of _Melicerta_, the star represents the ciliated ventral cup with openings into it from the groove; 5, disc of _Conochilus_; 6, disc of _Stephanoceros_, cingulum (?) of setose lobes, trochus horseshoe-shaped, mouth central.] The front of the body constitutes the HEAD, which is scarcely distinct, though usually separated by a slight neck-like constriction. The DISC, which terminates the head, varies greatly in shape and in the arrangement of its parts. Imagine a circular funnel, finely ciliated within, and with the mouth at the bottom, the prominent rim bearing two zones of cilia, the inner or anterior being the coarser, and termed the "trochus" or _hoop_; the outer finer, and termed the "cingulum" or _girdle_, while a very finely ciliated groove lies between the two zones. Either or both of these zones may be interrupted on the dorsal or ventral median line, or both; and the funnel-shaped mouth may be shifted—usually ventrally, so that it forms only a dilatation of the ciliated groove. Again, the wreath as a whole may be festooned or lobed; or the lobing may be confined to the area between the cingulum and trochus, as in most Ploima (Figs. 106 and 108, 3). Very frequently on these lobes adjacent cilia are fused together during life, producing "vibratile styles," whose true nature is only revealed after death. In Microcodonidae the structure of the disc (Fig. 108, 1) nearly conforms to the primitive type; but the ciliated groove is absent, and the "trochus" is in two separate half-elliptical bands. In the Flosculariaceae (Fig. 108, 6) the mouth is also central, the disc is funnel-shaped, {203}and the trochus is a horseshoe-shaped ridge, with its ends dorsal and raised into prominent knobs. The margin of the funnel is in Flosculariidae (Fig.