Evaluating neurophylogenetic patterns in the larval nervous systems of brachiopods and their evolutionary significance to other bilaterian phyla.

Recent structural analyses of invertebrate nervous systems have supported hypotheses stating that specific developmental and cytological aspects of larval and adult brains are conserved among bilaterian animals. Opposing views argue that structural similarities in larval nervous systems may be the result of convergent evolution and that the developmental diversity of adult brains is more indicative of several independent origins. Here, I use various cytological probes, confocal microscopy, and reconstruction techniques to investigate the cellular diversity within the larval nervous systems of Glottidia pyramidata and Terebratalia transversa (Brachiopoda). Neuronal cell types are compared among the rhynchonelliform, linguliform, and craniiform brachiopods as well as the phoronids. Although the respective larval types of the previously mentioned systematic groups clearly diverge in the neuroarchitecture of their larval apical organs (and nervous systems in general), a ground plan is proposed based on shared, centrally-located, peptidergic neuronal cell types that can be compared with similar cell types in other lophotrochozoan phyla (bryozoans and spiralians). Assessing hierarchal levels of homology within and among the nervous systems of morphologically disparate phyla is challenging in that many phyla share early developmental signals that induce the specification of the neural ectoderm, clouding our ability to discern divergent larval and juvenile brain structure. Solving these problems will require a combined effort involving both traditional and more recent cytological techniques with a diversity of molecular probes that will better map the neuronal complexity of diverse invertebrate nervous systems.