Parapodial glandular organs (PGOs) of Spiophanes (Polychaeta: Spionidae) were studied using light and electron microscopy. These organs are found in parapodia of the mid body region, starting on chaetiger 5 and terminating with the appearance of neuropodial hooks (chaetiger 14 or 15 in adult individuals). Large PGOs in anterior chaetigers display different species-specific types of openings whereas small PGOs in posterior parapodia of the mid body region always open in a simple vertical slit. Each PGO is composed of three main complexes: (1) the glandular sac with several distinct epithelia of secretory cells and secretory cell complexes and the reservoir filled with fibrous material, (2) the gland-associated chaetal complex (including the region of chaetoblasts and follicle cells, follicular canals, two chaetal collector canals, the combined conducting canal, the chaetal spreader including the opening of the glandular organ with associated type-1 secretory cells, and the gland-associated chaetae), and (3) a bilayered musculature surrounding the gland. A considerable number of different cell types are involved in the secretory activity, in the guidance of the gland-associated chaetae, and in the final expulsion of the fibrous secretion at the opening slit. Among these different cell types the type-5 secretory cells of the proximal glandular complex with their cup-shaped microvilli emanating thick microfibrils into the lumen of the glandular sac are most conspicuous. Secretory cells with cup-shaped microvilli being involved in the production of beta- chitin microfibrils have so far only been reported from some representatives of the deep-sea inhabiting Siboglinidae (Polychaeta). We suggest that the gland-associated chaetae emerging from inside the PGOs of Spiophanes are typical annelid chaetae formed by chaetoblasts and follicle cells. Functional morphology implies the crucial role of PGOs in tube construction. Furthermore, the PGOs are discussed in consideration of phylogenetic aspects.
