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.
The nuchal organ, indicated by "no". Source: 
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 in food detection and mating.