Ultrastructural Similarities Between Setae of Brachiopods and Polychaetes

Larvae of the brachiopod Terebratalia transversa Sowerby have 2 bundles of setae on each side of their mantle lobe. Each seta grows out of a follicle of epidermal cells. The cisternae and associated vesicles of the Golgi bodies in lateral follicular cells are filled with electron dense material. The basal follicular cell, or chaetoblast, has apical microvilli projecting into the basal ends of the longitudinal channels of the seta. The channel partitions are composed of fine fibers oriented parallel to the setal axis. The close structural and developmental similarities between polychaete and brachiopod setae are discussed.