A ctenidium is a respiratory organ or gill which is found in many molluscs. This structure exists in bivalves, cephalopods, polyplacophorans (chitons), and in aquatic gastropods such as freshwater snails and marine snails.^[1] Certain molluscs, such as the bivalves,^[2] possess paired ctenidia, but others, such as members of the Ampullariidae,^[3] bear a single ctenidium.^[4][5]

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. Some aquatic gastropods possess a single row of filaments on their ctenidium, known as the monopectinate condition,^[3] and others have a pair of filament rows, known as the bipectinate or aspidobranch condition.^[6] The ctenidium hangs into the mantle cavity and increases the area available for gas exchange.^[7] 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