The Ciliata have so complex an organisation that, as with the Metazoa, it is well to begin with the description of a definite type. For this purpose we select _Stylonychia mytilus_, Ehrb. (Fig. 49), a species common in water rich in organic matter, and relatively large (1/75" = ⅓ mm.). It is broadly oval in outline, with the wide end anterior, truncate, and sloping to the left side behind; the back is convex, thinning greatly in front; the belly flat. It moves through the water either by continuous swimming or by jerks, and can either crawl steadily over the surface of a solid or an air surface such as an air bubble, or advance by springs, which recall those of a hunting spider. The boundary is everywhere a thin plasmic pellicle, very tender, and readily undergoing diffluence like the rest of the cell. From the pellicle pass the cilia, which are organically connected with it, though they may be traced a little deeper; they are arranged in slanting longitudinal rows, and are much and variously modified, according to their place and function. On the edge of the dorsal surface they are fine and motionless, probably only sensory (_s.h._); except three, which protrude well over the hinder end (_c.p._), stout, pointed, and frayed out at the ends, and possibly serving as oars or rudders for the darting movements. These are distinguished from simple cilia as "cirrhi." {139}[Illustration: FIG. 49.—Ventral view of _Stylonychia mytilus_. _a.c_, Abdominal cirrhi; _an_, anus discharging the shell of a Diatom; _c.c_, caudal cirrhi; _c.p_, dorsal cirrhi; _cv_, contractile vacuole; _e_, part of its replenishing canal; _f.c_, frontal cirrhi; _f.v_, food vacuoles; _g_, internal undulating membrane; _l_, lip; _m_, mouth or pharynx; _mc_, marginal cirrhi; _N_, _N_, lobes of meganucleus; _n_, _n_, micronuclei; _o_, anterior end; _per_, adoral membranellae; _poc_, preoral cilia; _p.om_, preoral undulating membrane; _s.h_, sense hairs. (Modified from Lang.)]