Anterior neural centres in echinoderm bipinnaria and auricularia larvae: cell types and organization

Serial and interval electron micrograph series were used to examine the anterior part of the ciliary band system in the bipinnaria larva of Pisaster ochraceus and the auricularia larva of Stichopus californicus for evidence of ganglion‐like organization. The bipinnaria has paired concentrations of Multipolar with Apical Processes (MAP) cells in this region that correspond in position with previously identified clusters of serotonergic and peptidergic neurones. MAP cells located in the centre of the band have well‐developed apical processes, but no cilium. Those at the sides of the band have fewer processes, but some have recumbent cilia that extend under the glycocalyx, suggesting a sensory function. Comparable cell types are not found elsewhere in the band, a clear indication that the apical parts of the ciliary band system are organized in a distinctive fashion. Two neuronal cell types were identified in the apical region of the auricularia larva, a conventional bipolar neurone that corresponds with previously described serotonergic apical cells, and more numerous MAP cells for which there is no previous record and hence, no known transmitter. Previous immunocytochemical studies are summarized and re‐examined in the light of these results. Relevant evolutionary issues are also discussed, but the data fail to provide strong evidence either for or against Garstang’s hypothesis that the chordate brain and spinal cord derive from larval ciliary bands resembling those of modern echinoderms.