Structure and Development of the Apical Organ in Trochophores of Spirobranchus polycerus, Phyllodoce maculata and Phyllodoce mucosa (Polychaeta)

The apical organ of a typical serpulid trochophore (Spirobranchus) and of the more complex trochophore larva of Phyllodoce are described on the basis of ultrastructural surveys and three-dimensional reconstructions. A plexus-like structure formed by vesicle-filled processes from certain of the apical cells is shown to be an integral part of the apical organ in both genera, persisting in Phyllodoce long after the regression of the external apical tuft. Because of its central position and early development, the plexus may play an important role in the development and functioning of the larval nervous system as a whole. With regard to its developmental role, morphological evidence supports the hypothesis that the plexus serves as an initial target for growing neurites responsible in part for establishing the path along which the cerebral commissure later develops. The constituent cells of the apical organ in Spirobranchus are catalogued as to type and position, including cells associated with the entry of nerves into the apical plexus. Constancy from larva to larva is shown to be greatest for the eight most apical cells, comprising two ciliated cells, one of which is responsible for the apical tuft, three plexus cells, and three capsular cells. There are clear parallels between Spirobranchus and Phyllodoce in the arrangement of apical cells, suggestive of a common plan, and the probable origin of this arrangement in terms of cell lineage is discussed.