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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. Some aquatic gastropods possess one ctenidium known as monopectinate. And others have a pair of ctenidia known as bipectinate.

A ctenidium is shaped like a comb/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 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

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