pro·sto·mi·um 
(prō-stōmē-əm)
(prō-stōmē-əm)n. pl. pro·sto·mi·a (-mē-ə)
The anterior portion of the head of an annelid worm, situated in front of the mouth.
[New Latin, from Greek prostomion, mouth, lips : pro-, in front of; see PRO-2 + stoma, mouth.]
pro·stomi·al (-əl) adj.