-ēəm noun
( plural ctenid·ia -ēə)
Etymology: New Latin, from cten- + -idium
1. : the gill of a mollusk consisting typically of a respiratory structure that resembles a comb or feather, has a main stem with lateral lamellae, and is developed from the inner side of the mantle — distinguished from ceras
2. : a structure consisting of a row of spines resembling the teeth of a comb on the head or thorax, or both, of certain fleas