The nuchal organ is a ciliated pit or groove present at the posterior end of the prostomium of annelid

Annelid

The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...

 worms, some cephalopod

Cephalopod

A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

s, 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.