At the right hand of the frontal area there begins, just within the dorsal edge, a row of strong cilium-like organs (Fig. 49, _per_); these, on careful examination, prove to be transverse triangular plates, which after death may fray into cilia.[154] They are the "adoral membranellae." This row passes to the left blunt angle, and there crosses over the edge of the body to the ventral aspect, and then curves inwards towards the median line, which it reaches about half-way back, where it passes into the pharynx (_m_). It forms the front and left-hand boundary of a wedge-shaped depression, the "peristomial area," the right-hand boundary being the "preoral ridge" or lip (_l_), which runs nearly on the median line, projecting downward and over the depression. This ridge bears on its inner and upper side a row of fine "preoral cilia" (_poc_) and a wide "preoral undulating membrane" (_p.om_), which extends horizontally across, below the peristomial area. The roof of this area bears along its right-hand edge an "internal undulating membrane" (_g_), and then, as we pass across to the left, first an "endoral membrane" and then an "endoral" row of cilia. In some allied genera (not in _Stylonychia_), at the base and on the inner side of each adoral membranella, is a "paroral" cilium. {140}All these motile organs, with the exception of the preoral cilia, pass into the pharynx; but the adoral membranellae soon stop short for want of room. There are some seventy membranellae in the adoral wreath. The rest of the ventral surface is marked by longitudinal lines, along which the remaining appendages are disposed. On either side is a row of "marginal cirrhi" (_mc._), which, like the membranellae, may fray out into cilia, but are habitually stiff spine-like, and straight in these rows; these are the chief swimming organs. Other cirrhi, also arranged along longitudinal rows, with so many blank spaces that the arrangement has to be carefully looked for, occur in groups along the ventral surface. On the right of the peristome are a group which are all curved—the "frontal cirrhi" (_f.c._). Behind the mouth is a second group—the "abdominal cirrhi" (_a.c._), also curved hooks; and behind these again the straight spine-like "caudal" or "anal" cirrhi (_c.c_), which point backwards. These three sets of ventral cirrhi are the organs by which the animal executes its crawling and darting movements. Besides the mouth there are two other openings, both indistinguishable save at the very moment of discharge; the anus (_an_) which is dorsal, and the pore of the contractile vacuole, which is ventral.