Abstract
Recently, a number of neurodevelopmental studies of hitherto neglected taxa have become available, contributing to questions relating to the evolution of the nervous system of Lophotrochozoa (Spiralia + Lophophorata). As an example, neurogenesis of echiurans showed that these worm-shaped spiralians, which as adults do not exhibit any signs of segmentation, do show such traits during ontogeny, e.g. by segmentally arranged perikarya and commissures. Similarly, sipunculan worms, which have a single ventral nerve cord in the adult stage, develop this nerve cord by gradual fusion of a paired larval nerve during metamorphosis, and show transitional stages of segmentation. These findings indicate that echiurans, annelids and sipunculans stem from a segmented ancestor. By contrast, no traces of body segmentation are present during neurogenesis of basal molluscs. However, a tetraneurous condition (i.e. one pair of ventral and one pair of lateral nerve cords), as is typical for Mollusca, and a serotonergic larval apical organ that matches the complexity of polyplacophoran apical organs, were found in larval entoprocts, thus strongly supporting a mollusc-entoproct clade. Within the Lophophorata (Ectoprocta + Phoronida + Brachiopoda), data on nervous system development for any of the 3 lophophorate phyla are as of yet too scarce for profound phylogenetic inferences. Taking into account the most recent advances in molecular phylogenetics and developmental neurobiology, a scenario emerges that proposes a clade comprising Sipuncula + Annelida (including Echiura) on the one hand and a monophyletic assemblage of Entoprocta + Mollusca on the other.
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Wanninger, A. Comparative Lophotrochozoan Neurogenesis and Larval Neuroanatomy: Recent Advances from Previously Neglected Taxa. BIOLOGIA FUTURA 59 (Suppl 2), 127–136 (2008). https://doi.org/10.1556/ABiol.59.2008.Suppl.21
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DOI: https://doi.org/10.1556/ABiol.59.2008.Suppl.21