2. In the _Anomiidae_, _Arcadae_, _Trigoniidae_, and _Mytilidae_ each gill consists of two plates or rows of much longer filaments, which consequently occupy a much larger space in the mantle cavity (Fig. 76, B). Unable to extend beyond the limits of the mantle, filaments are reflected or doubled back upon one another, those of the external plate being reflected towards the outside, those of the internal plate towards the inside. Each separate filament is not connected with the filament next adjacent, except by surface cilia situated on small projections on the sides of the filaments, and interlocking with the cilia of the adjacent filament. The two superposed plates or leaves of the gill may or may not be united by cords running between the two parts of a filament. (_Filibranchiata._) 3. In the _Pectinidae_, _Aviculidae_, and _Ostreidae_ a further development takes place. The filaments of each gill are reflected in the same way as in the _Filibranchiata_, but the part thus reflected may become completely united or ‘concresce’ with the mantle on the exterior and with the base of the foot on the interior side. The leaves of each gill plate, which have thus become doubled (the gills being apparently two instead of one on each side), are folded or crumpled, and the filaments are modified at the re-entrant angles of the fold. (_Pseudolamellibranchiata._) 4. In all the remaining _Pelecypoda_, except class 5, in other words, in the very large majority of families, the filaments are either reflected, as in (3), or simple; but the process of concrescence is so far advanced that the adjacent filaments are always intimately connected with one another in such a way as to admit the passage of the blood; and the leaves of each gill-plate (Fig. 76, C) are united by cross channels in a similar way. (_Eulamellibranchiata._) 5. In certain of the _Anatinacea_ alone (_Cuspidaria_, _Lyonsiella_, _Poromya_, _Silenia_) the gills are transformed into a more or less muscular partition, extending from one adductor muscle to the other (Fig. 76, D), and separating off the pallial chamber into two distinct divisions, which communicate by means of narrow slits in the partition. (_Septibranchiata._) [Illustration: FIG. 78.--Transverse section of portion of an outer gill plate of _Anodonta_, highly magnified: _il_, inner lamella; _il´_, outer lamella; _ilj_, interlamellar junctions; _v_, large vertical vessels. (After Peck.)]