1) Branchia 2) Gill 
[mollusc] 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 ...
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Series of stout, peg-like spines on the head (genal ctenidium) and first thoracic tergite (pronotal ctenidium) of many fleas.
[n] - comb-like respiratory structure serving as the gill of certain mollusks
noun comb-like respiratory structure serving as the gill of certain mollusks
any of various comblike or featherlike structures, as the row of stiff bristles on the legs of a psocid.