Ctenidium (mollusc)

A ctenidium is a respiratory organ or gill which is found in many mollusks. This structure exists in bivalves, cephalopods, Polyplacophorans (chitons), and in aquatic gastropods such as freshwater snail and marine snails.[1] Some aquatic gastropods possess one ctenidium known as monopectinate and others have a pair of ctenidia known as bipectinate.

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 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.[2] 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

References

1. Ruppert Invertebrate Zoology: A Functional Evolutionary Approach.
2. Respiratory system The apple snail. Retrieved 2012-04-20.