Bryozoan larvae as mosaics of multifunctional ciliary fields: Ultrastructure of the sensory organs of Bugula solonifera (Cheilostomata: Cellularioidea).

The ultrastructure of ciliated sensory and effector organs has been examined by light microscopy and by scanning and transmission electron microscopy in larvae of the cellularioid cheilostome, Bugula stolonifera. The larval surface is formed by a mosaic of ciliary fields, each distinctive in organization and function. The most extensive ciliary field consists of the motile cilia of the corona, which collectively constitute the primary larval locomotory organ and form most of the larval surface. The corona is innervated basally by a nerve ring that circumscribes the larval equator. A circular field of immotile shorter cilia at the aboral pole arise from bands of radial cells in the apical disc. The radial ciliated cells form chemical synapses with underlying nerves of the paired aboral nerve cords, which extend to an equatorial nerve plexus on the median anterior side of the larva, beneath the pyriform organ. The pyriform organ is a complex of ciliated and glandular regions aligned along the anterior midline of the larva. It consists of, in aboral to oral sequence: the superior glandular field, the vibratile plume, the inferior glandular field, and the ciliated cleft. The vibratile plume is distinctive from the surrounding ciliation by its length and pattern of beating. It consists of four bundles of cilia approximately 64 μm long that beat rhythmically in synchrony into the ciliated cleft. Each bundle of cilia arises from a single cell which is further distinguished by apical pits of stout microvilli. The glandular fields and the vibratile plume are bordered by a variety of previously undescribed ciliated cells, longitudinal myoepithelial cells, and a pair of antero-medial eyespots. Another type of ciliated sensory cell is located between cornal cells. These intercoronal cells form synapses directly with the adjacent coronal cells. On the basis of these ultrastructural observations, the radial ciliated cells of the apical disc and the intercoronal cells are classified as primary sensory cells with different motor pathways. The radial ciliated cells are associated with the various effector organs of the larva via the nervous system. The intercoronal cells may combine sensory and motor functions by synapsing directly with the larval locomotory organ.