In the Pelecypoda the mantle cavity is equally developed on each side, enclosing the two sets of branchiae. The mantle may thus be regarded as consisting of two equal portions, which form a sort of lining to the two valves. The lower or ventral portion of the mantle edges may be simple, or provided with ocelli (_Pecten_, _Arca_), tentacles, cilia (_Lima_, _Lepton_), or doubled folds. The two portions of the mantle touch one another along the whole line of the edge of the two valves, and, although thus in contact, may remain completely separate from one another, or else become permanently united at one or more points. This fusion of the mantle edges corresponds to important changes in the organisation of the animal as a whole. The anal and branchial siphons are no more than prolongations of the mantle edges on the posterior side into a tubular form. These ‘siphons’ exhibit the siphonal form more distinctly according as the adjacent portions of the mantle become more definitely fused together. [Illustration: FIG. 80.--Diagram illustrating the various stages in the closing of the mantle in Pelecypoda: =A=, mantle completely open; =B=, rudiments of siphons, mantle still completely open; =C=, mantle closed at one point; =D=, mantle closed at two points, with complete formation of siphonal apertures; =E=, development of siphons, ventral closure more extended; =F=, mantle closed at three points, with fourth orifice: _f_, foot; _s.a_, _s.b_, anal and branchial siphons; 1, 2, 3, first, second, and third points of closure of mantle. (After A. Lang.)]