In Pelecypoda the branchiae are placed on each side of the body, between the mantle and the visceral mass. They lie in a chamber known as the _branchial cavity_. Leading into this cavity, and behind it, are, as a rule, two tubes or siphons, one of which conducts water to the branchiae, while the other carries it away after it has passed over them. The lower is known as the _branchial_ or _afferent_ siphon, the upper as the _anal_ or _efferent_ siphon (see Figs. 72 and 73). The action of these siphons can readily be observed by placing a little carmine in water, near to the siphonal apertures of an _Anodonta_ or _Unio_. In many cases (_e.g._ _Psammobia_, _Tellina_, _Mya_, genera which burrow deeply in sand) both the siphons are exceedingly long, sometimes considerably longer than the whole shell. In some cases the two tubes are free throughout their entire length, in others they become fused together before their entrance within the shell (Fig. 74). In other genera, which do not burrow (_e.g._ _Ostrea_, _Pecten_, _Arca_, _Mytilus_), the siphons are rudimentary or altogether absent (Fig. 75). [Illustration: FIG. 74.--_Solecurtus strigillatus_ L., Naples: _s.af_, afferent siphon; _s.ef_, efferent siphon, the two uniting in _SS_ externally to the shell, × ½.] [Illustration: FIG. 75.--_Mytilus edulis_ L., attached by its byssus (=By=) to a piece of wood: =F=, foot; =S=, anal siphon, the branchial siphon being below it and not closed. (After Möbius.)] The number and arrangement of the branchiae varies considerably. It appears probable that the different degrees of complication of the gill indicate degrees of specialisation in the different groups of Pelecypoda, in other words, assuming that a simpler form of gill precedes, in point of development, a more complicated form, the nature of the gill may be taken as indicating different degrees of removal from the primitive form of bivalve. 1. The simplest form of gill (_Nucula_, _Leda_, _Solenomya_, etc.) is that which consists (Fig. 76, A, compare Fig. 100, p. 201) of two rows of very short, broad, not reflected filaments, the rows being placed in such a way that they incline at right angles to one another from a common longitudinal axis. The filaments are not connected with one another, nor are the two leaves of each gill united at any point. (_Protobranchiata._) [Illustration: FIG. 76.--Morphology of the branchiae of Pelecypoda, seen diagrammatically in section: =A=, _Protobranchiata_; =B=, _Filibranchiata_; =C=, _Eulamellibranchiata_; =D=, _Septibranchiata_; _e_, _e_, external row of filaments; _i_, _i_, internal row of filaments; _e´_, external row or plate folded back; _i´_, internal row folded back; _f_, foot; _m_, mantle; _s_, septum; _v_, visceral mass. (From A. Lang.)] [Illustration: FIG. 77.--Four gill filaments of _Mytilus_, highly magnified; _cj_, ciliary junctions; _f_, filament. (After Peck.)]