Source : Webster's Revised Unabridged Dictionary (1913)

Prostomium \Pro*sto"mi*um\, n.; pl. {Prostomia}. [NL., fr. Gr. ?
   before + ?, ?, mouth.] (Zo["o]l.)
   That portion of the head of an annelid situated in front of
   the mouth. -- {Pro*sto"mi*al}, a.