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