Ultrastructure of the nuchal organ in the interstitial polychaete Stygocapitella subterranea (Parergodrilidae)

The nuchal organs of Stygocapitella subterranea are paired narrow pits. They are lined by unciliated cells at the opening and by ciliated cells at the basal parts. The primary sensory cells (6–8) are arranged in a single patch at the bottom of the nuchal pit. The nuclei of the sensory cells are located in the posterior portion of the brain. Their dendrites form the nuchal nerve which is sheathed by the ciliated cells. Each sensory cell bears up to 4 modified sensory cilia and several microvilli extending into the olfactory chamber. The sensory cilia show various patterns of axonemal organization and have no rootlets. The olfactory chamber is covered by a cuticular matrix. Another primary sensory cell lies at the opening of the nuchal pit. It bears cilia which penetrate the cuticle but are enveloped by the epicuticle. Retractor muscles insert caudally on the organ. The nuchal organ of S. subterranea shows similarities to those of opheliids but exhibits several features not to be found in other nuchal organs.