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The gills, often referred to as ctenidia, are two large, curtain‐like structures that are suspended from the gill axis, which is fused along the dorsal margin of the mantle (Figure 2.8A). Within the gill axis are the branchial nerves and afferent and efferent branchial haemolymph (blood) vessels. Each gill is made up of numerous W‐shaped (or double‐V) ciliated filaments and an internal skeletal rod rich in collagen strengthens each filament. Each V is known as a demibranch and each arm is called a lamella, giving an inner descending and outer ascending lamella (Figure 2.8Bi). In the space between the descending and ascending lamellae is the exhalant chamber, connected to the exhalant area of the mantle edge; the space ventral to the filaments is the inhalant chamber, connected to the inhalant area of the mantle edge (Figure 2.8B). In mussels, the gills follow the curvature of the shell margin, with the maximum possible surface exposed to the inhalant water flow (Figure 2.6).

Figure 2.8 (A) Section of a lamellibranch gill showing the ctenidial axis and four W‐shaped filaments. For greater clarity, the descending and ascending lamellae of each demibranch have been separated. Solid arrows indicate direction of water flow through the filaments from inhalant (INH) to exhalant (EXH) chambers and broken arrows indicate path of particle transport to the food grooves. (B) (i) section of a fillibranch gill in the mussel, Mytilus edulis. Adjacent filaments are joined together by ciliary junctions. (ii) Transverse section through one fillibranch gill filament (shaded in Bi), showing pattern of ciliation. Source: (A) From Barnes et al. (1993), with permission from John Wiley & Sons; (B) From Pechenik (2010). Reproduced with permission from the McGraw–Hill Companies

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When the individual filaments are free or loosely attached to one another through interlocking clumps of cilia, this is known as the fillibranch condition (Figure 2.8Bi); it is seen in mussels and scallops. In more advanced bivalves, neighbouring filaments are joined to each other at regular intervals by tissue connections, leaving narrow openings or ostia between them. This gill type, termed eulamellibranch, is a solid structure and is found in the majority of bivalves. In oysters, tissue connections are less extensive than in most eulamellibranch species, so the gills are often referred to as pseudoeulamellibranch. Also, when filaments are similar, as in mussels, the gill is termed homorhabdic, and when there are different types of filaments, through folding of the gill area, it is called heterorhabdic. The filibranch homorhabdic gill in adult mussels is regarded as the ancestral condition from which the other gill types evolved (Beninger & Dufour 2000).