Dictionary:

ctenidium

  (tĭ-nĭd'ē-əm)

A comblike structure, such as the respiratory apparatus of a mollusk or a row of spines in some insects.

[New Latin, from Greek kteis, kten-, comb.]

A spine which occurs in rows on the heads of fleas; called also combs; useful for morphological identification.

The noun has one meaning:

Meaning #1: comb-like respiratory structure serving as the gill of certain mollusks