The abdomen also presents on its under surface, usually towards the posterior end or apex, a group of finger-like mammillae or spinnerets. They are normally six in number, two superior (or posterior), two median, and two inferior (or anterior). The number is reduced, in most of the Theraphosae, to four, while a few spiders possess only a single pair of spinnerets. These organs are described more fully on p. 325. A small papilla, the “colulus” (Fig. 174, _c_), is often observable, projecting between the anterior spinnerets. The “anal tubercle” (Fig. 174, _a.t_), on which the vent is situated, terminates the abdomen, and is generally in close juxtaposition with the posterior spinnerets. =Appendages.=—The cephalothoracic appendages are the chelicerae, the pedipalpi, and the four pairs of ambulatory legs. Those of the abdomen are the mammillae or spinnerets. =Chelicerae.=—These are two-jointed appendages, articulated immediately below or in front of the clypeus. They are the “mandibles” of many authors, but there is good reason for believing that they are not homologous with the mandibles of Insects. There is little agreement, moreover, with regard to the names given to the two joints of which they consist. The term “falx,” often applied to the basal joint, is much more appropriate to the sickle-like distal joint. Base and fang are tolerably satisfactory, or we may avoid ambiguity by adopting the terms “paturon” and “unguis” suggested by Lyonnet.[257] The paturon is a stout joint of more or less cylindrical or conical shape. The unguis (the “crochet” of Simon) is hook-like, and can generally be folded back upon the paturon, which often presents a groove for its reception. The Theraphosid spiders are distinguished from all others by the fact that the plane of action of the chelicerae is vertical and longitudinal. The paturon projects forward in a line parallel with the axis of the body, and its distal end can be raised or depressed, but not moved laterally; while the unguis in action has the point directed downwards, and, at rest, is applied to the under surface of the paturon. In other spiders the patura hang more or less vertically, and while to some extent mobile in all directions, their principal motion is _lateral_, and the ungues have their points directed towards each other in action, and are applied to the inner surfaces of the patura in repose. The plane of action in this case is also more or less vertical, but _transverse_. [Illustration: FIG. 177.—Front view of _Textrix denticulata_. × about 10. 1, Caput; 2, eyes; 3, paturon; and 4, unguis of chelicera. ]