Abstract
The nuchal organs of Spio cf. filicornis from northern Europe have been studied by scanning and transmission electron microscopy. Spio cf. filicornis is the first species in which metameric nuchal organs are described. The nuchal organs consist of a distinct cephalic nuchal complex followed by metameric structures for a variable number of chaetigers. Their microanatomy corresponds to the general structural plan of nuchal organs: these are ciliated supporting cells and bipolar sensory cells with sensory cilia traversing an olfactory chamber. The organs are overlaid by a secondary paving-stone-like cover and innervated by longitudinally elongated paired nuchal nerves. The findings clearly favour the hypothesis that the paired metameric ciliated structures found in some Spionidae are in fact homologous with the prostomial nuchal organs characteristic of Polychaeta.
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