The prostomium (sometimes also called the acron) is the first body segment in an annelid worm's body in the anterior end. It is in front of (but does not include) the mouth, being usually a small shelf- or lip-like extension over the dorsal side of the mouth. It sometimes bears antennae and eyes. It often functions like a kind of overlip when the animal is feeding. The shape and composition of prostomium are important for annelid systematics.
The prostomium often also has appendages such as palps and tentacles or cirri. Some polychaete prostomia have a posterior extension or ridge with sensory function, called a caruncle. Another sensory organ, called the nuchal organ (or a variation, the nuchal epaulette), is a ciliated pit or groove at the posterior end of the prostomium.