Nuchal organ

The nuchal organ is a ciliated pit^[1] or groove present at the posterior end of the prostomium of annelid worms,^[1] some cephalopods,^[2] and other invertebrates.^[3]

The nuchal organ, indicated by "no". Source: ^[1]

Annelids only possess one nuchal organ, although the nature of the grooving may make it appear to be a pair of organs.^[4]

It may be involved in light detection,^[2] and may have a role in food detection and mating.^[5]