Structure and organization of the nervous system in the trochophore larva of Spirobranchus

The trochophore larva of the polychaete Spirobranchus polycerus is described, based on ultrastructural surveys and three dimensional reconstructions, with emphasis on the structure and organization of the nervous system. A complete and detailed description is provided of the larval parts of the nervous system at the cellular level for the 48 h stage, by which time the larval system is fully developed in most respects. The adult nervous system, whose rudiments form a largely separate system of nerves and nerve cells, appears progressively during later development. Its principal structures, the brain, commissures and ventral cords, are briefly described based on an examination of the metatrochophore. The larval nervous system is entirely presegmental and is divisible into two parts: (1) a system of pretrochal cells and nerves arising from them that innervates the prototroch, linking it to the apical organ and the single larval eye, and (2) a system of intratrochal and intraepithelial nerves supplying the feeding apparatus of the larva. The latter consists of two nerves that encircle the pharynx and join basally beneath the cluster of cells that make up the basal pharyngeal complex. The pharyngeal nerves are then linked by means of a suboral complex of four sensory cells and their nerves to the nerves supplying the metatroch and neurotroch. The two parts of the larval system are anatomically separate and develop separately, each in association with its own organizational centres. These are: the apical organ and its central plexus in the case of the pretrochal system, and the suboral and pharyngeal complexes in the case of the oral and pharyngeal nerves. Like the larva itself, the larval nervous system is specialized and highly reduced. There are comparatively few cells, but a number of distinctive cell types. At 48 h, the larval system comprises 36 cells, including among these between 16 and 18 recognizably different types of sensory and non-sensory nerve cells and non-neural accessory cells. The majority of the cells are individually identifiable by morphology, ultrastructure and location, and are invariant or nearly so from larva to larva. The development of the system as a whole involves production of fibres by certain of these followed by fibre growth either along preestablished pathways, for example along the trochal bands or cells derived from these, or towards identifiable targets, for example, the apical plexus or pharyngeal complex. The resulting sys