The nuchal organ is a ciliated pit or groove present at the posterior end of the prostomium of annelid worms, some cephalopods, and other invertebrates.
Annelids only possess one nuchal organ, although the nature of the grooving may make it appear to be a pair of organs.
It may be involved in light detection, and may have a role on food detection and mating.
References
Nuchal organ Wikipedia(Text) CC BY-SA