SCHULZE, A*; RICE, M.E.; Smithsonian Marine Station; Smithsonian Marine Station: Neurogenesis and Myogenesis in Sipunculan Worms

According to most recent molecular phylogenetic analyses of protostome relationships, Sipuncula are either closely related to or nested within the Annelida. Morphological characters in the adults, however, hold few cues to the evolutionary origin of the group. We are therefore studying the trochophore and pelagosphera larvae of sipunculan worms to search for additional morphological evidence for phylogenetic affinities. Specifically, we are studying the development of the nervous and muscular systems using fluorescent dyes and immunohistochemistry, in conjunction with confocal laser scanning microscopy. In all examined species, the circular body wall musculature forms in rings along the anterior-posterior axis. In at least one species, we observed a similar arrangement in the nervous system. We are examining whether the ring nerves correspond in number and position with the ring muscles. In neither the nervous nor the muscular system could a terminal growth zone, as present in many annelids, be observed. Furthermore, we found that in several species the ventral nerve cord is initially paired and fuses in the late pelagosphera larva into a single strand. In summary, although we can show certain similarities to annelids, our current findings still leave room for different interpretations.