Comparative electron microscopic investigation of the nuchal organs in Protodriloides, Protodrilus, and Saccocirrus (Annelida, Polychaeta)

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Abstract
The nuchal organs of the interstitial polychaetes Protodriloides chaetifer, Protodriloides symbioticus, Protodrilus ciliatus, Protodrilus adhaerens, Saccocirrus krusadensis, Saccocirrus papillocercus were investigated by scanning and transmission electron microscopy. These organs vary from spherical to elongated ciliary brushes and usually lie in shallow pits. In P. symbioticus only a reduced nuchal organ exists, whereas the other species all have well-developed nuchal organs of similar structure consisting of ciliated supportive cells and bipolar primary sensory cells. The perikarya of the sensory cells form the nuchal ganglia, which lie behind the brain. Difference retractor muscle cells are attached to the ciliated cells. The number of sensory cells varies from 4 to about 90 according to the size of the nuchal organs. Each sensory cell gives rise to a distal process (dendrite), and 4-25 processes at a time unite to form bundles that penetrate between the ciliated cells. Apically the dendrites terminate in small sensory bulbs, each bearing several microvilli and a modified cilium. The sensory cilia usually branch, lose their axonemes, and extend as microvillus-like structures into the olfactory chamber representing an extracellular space below the reduced cuticle. Specific microvillar processes of the ciliated cells form a dense cover above the cuticle which is only penetrated by the motile cilia of these cells. The ciliated cells are highly pinocytic. The nuchal organs of the species investigated show striking similarities to those of spionids.