Ctenidium (mollusc)

A live individual of Pleurobranchaea meckelii; the ctenidium is visible as a feather-like structure in this view of the right-hand side of the animal

A ctenidium is a respiratory organ or gill which is found in many mollusks. This structure exists in bivalves, cephalopods, and in many aquatic gastropods, i.e. in some freshwater snails and sea snails and also in some sea slugs. Some aquatic gastropods possess one ctenidium, others have a pair of ctenidia (the plural form of the word).

A ctenidium is shaped like a comb or a feather, with a central part from which many filaments or plate-like structures protrude, lined up in a row. It hangs into the mantle cavity and increases the area available for gas exchange. The word is Latinized but is derived from the Greek ktenidion which means "little comb", being a diminutive of the word kteis meaning comb.

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  Pair of large, grey gills visible inside the mantle cavity of a giant squid