prostomium

Prostomium is the anterior region of an annelid worm that lies in front of the mouth and is not considered a true body segment. It forms the head region and is usually distinct from the first true segment, the peristomium, though the boundary between them can be variable among taxa. The appearance of the prostomium ranges from a small rounded bump to a prominent hood, shield, or lobe, and its musculature and overlying cuticle reflect the worm’s ecology.

In many polychaetes and some oligochaetes, the prostomium bears sensory structures. Common features include eyes, tentacle-like palps, antennae, and ciliated nuchal organs that participate in chemosensation. These sensory organs help the animal locate prey, navigate, and avoid danger in the benthic environment. The prostomium may also play a role in feeding, either by assisting in prey capture or by shielding the mouth during burrowing.

The prostomium’s presence and morphology vary with lifestyle. Burrowers often have reduced or concealed prostomium, while active predators or surface swimmers may display a prominent, highly sensory anterior region. Developmentally, the prostomium originates from the head region during embryogenesis and is distinguished from the segmental body plan that follows.

Etymology: prostomium derives from Latin pro- 'before' and Greek stoma 'mouth'. In anatomy and systematics, the prostomium is a standard reference point for describing anterior features of annelids.