ABSTRACT
Despite several unquestionably homologous characters with Annelida, the Echiura have generally been considered to form a distinct taxon due to apparent lack of segmentation: neither in the body cavity nor in any other structures of the adult animals have clear signs of a metameric organization been observed. However, it must be considered that this lack of segmentation could represent a secondary condition and that Echiura are derived from segmented ancestors. An immunohistochemical analysis visualized with confocal laser-scanning microscopy (cLSM) shows the development of serially repeated units in the nervous system of Bonellia viridis. This organization corresponds to the metameric ganglia typical of Annelida. Antibodies against the neurotransmitters serotonin (5-hydroxytryptamine) and FMRFamide label distinct subsets of neurons. Their perikarya are arranged in discrete repetitive units in the ventral nerve cord of different larval stages. Labeling of neurotubuli using antibodies against different tubulin isoforms provides additional support for the metameric character of the nervous system. Contrary to previous descriptions, the peripheral nerves extending from the ventral nerve cord into the body wall musculature are paired and are evenly distributed; their arrangement corresponds to that of the serotoninergic perikarya. Morphological and neurochemical differentiation of the nervous system clearly proceeds from anterior to posterior, indicating the occurrence of a posterior growth zone. The serial ganglionic organization of the nervous system of Articulata is generally regarded as having evolved in conjunction with repetitive muscular units, in particular with the formation of typical annelid segments. Therefore, these results are interpreted as an indication that Echiura are derived from segmented ancestors and thus support the systematic inclusion of the Echiura within the Annelida.
