prostomium
noun
pro·sto·mi·um prō-ˈstō-mē-əm 
plural prostomia prō-ˈstō-mē-ə
: the portion of the head of an annelid worm (such as an earthworm) that is situated in front of the mouth
prostomial
prō-ˈstō-mē-əl
adjective 
Word History
Etymology
New Latin, from Greek pro- + stoma mouth — more at stomach entry 1
First Known Use
1870, in the meaning defined above