It is a monopectinate gill and is situated on the right side of the branchial chamber hanging from its dorsolateral wall. It consists of a long axis, the ctenidial axis which remains attached to mantle wall.
Ctenidial axis contains two blood vessels an afferent and an efferent. Afferent blood vessel carries deoxygenated blood from the gill to the heart. Axis bears a long series of triangular leaflets called lamella and makes the actual surface of gaseous exchange. Lamellae are highly vascularized and in the middle they are larger but smaller at both the ends. Ctenidium is innervated by nerves from the left pleural and supra-intestinal ganglia. This nerve supply indicates that it is actually an organ of left side but has shifted to the right side during torsion.
Histologically, each lamella is formed of a double layer of epithelium which encloses a narrow cavity. Each epithelial layer consists of (i) non-ciliated columnar cells (ii) ciliated columnar cells and (iii) Glandular cells. 2. Pulmonary sac: It is developed from the mantle wall and hangs into the pulmonary chamber and opens into mantle cavity through pulmonary aperture.
Dorsal wall of the pulmonary chamber is pigmented, while the ventral wall is creamy white. These walls are muscular and highly vascular. 3. Nuchal lobe: These are highly contractile structures of the mantle situated one on either side of the head.