ctenidia

The ctenidium (plural ctenidia) is a comb-like respiratory organ found in many molluscs, especially within the groups of gastropods and bivalves, as well as in a few related invertebrates. It consists of a sheet or stack of gill filaments that maximize the surface area available for gas exchange.

In most molluscs the ctenidium is a paired structure housed within the mantle cavity. The gill filaments, or lamellae, are arranged along a central axis and are often covered with mucus. Cilia on the filaments help create water flow and transport trapped particles through the gills toward the mouth or exhalant pathways.

The primary function of the ctenidium is respiration: water passes over the lamellae, allowing oxygen to diffuse into the blood and carbon dioxide to diffuse out. Many ctenidia also assist in feeding by capturing suspended particles; mucus and ciliary action guide these particles toward the mouth and digestive tract.

Variation among molluscs is common. Ctenidia can differ in size, ornamentation, and lamellar arrangement depending on habitat and lifestyle. In aquatic species they are usually well developed for gas exchange and filtration. In terrestrial pulmonate gastropods, the ctenidium is often reduced or transformed into a lung-like pallial cavity as a shift to air breathing occurred.

The term derives from Greek ktenos, meaning comb, reflecting the distinctive comb-like appearance of the gill lamellae.