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]
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]