Nuchal organs are known to be important sensory organs of many polycha
ete annelid species. This study describes the fine structure of the nu
chal organs and associated glands of Parapionosyllis manca. In each nu
chal organ, the ciliated epithelium of the nuchal cleft encloses an ol
factory chamber through which sensory cilia and microvillus-like proce
sses radiate. These sensory components originate from bulbs of sensory
cells that are closely associated to form a sensory bundle. Each bund
le of cells is encased in a palisade of support cells bearing motile c
ilia. Numerous secretory vesicles occur in the cytoplasm of the suppor
t cells and beneath the modified cuticle of the nuchal organ. The supp
ort cells of the olfactory region extend posteriorly to form the nucha
l lobes, or glands. The cells are elongate and taper to a thin neck ju
st before leaving the sensory area of the nuchal organ. There are esse
ntially no nuclei in the sensory region as these and most of the other
cellular organelles are concentrated in the lobes that extend into th
e body cavity but are contained by the epidermal basement matrix. The
nuchal organ is innervated by the nuchal nerve, which extends posterio
rly from the hind portion of the brain.