Rudi Loesel

Background: The now thriving field of neurophylogeny that links the morphology of the nervous system to early evolutionary events relies heavily on detailed descriptions of the neuronal architecture of taxa under scrutiny. While recent accounts on the nervous system of a number of animal clades such as arthropods, annelids, and molluscs are abundant, in depth studies of the neuroanatomy of nemerteans are still wanting. In this study, we used different staining techniques and confocal laser scanning microscopy to reveal the architecture of the nervous system of Lineus viridis with high anatomical resolution. Results: In L. viridis, the peripheral nervous system comprises four distinct but interconnected nerve plexus. The central nervous system consists of a pair of medullary cords and a brain. The brain surrounds the proboscis and is subdivided into four voluminous lobes and a ring of commissural tracts. The brain is well developed and contains thousands of neurons. It does not reveal compartmentalized neuropils found in other animal groups with elaborate cerebral ganglia. Conclusions: The detailed analysis of the nemertean nervous system presented in this study does not support any hypothesis on the phylogenetic position of Nemertea within Lophotrochozoa. Neuroanatomical characters that are described here are either common in other lophotrochozoan taxa or are seemingly restricted to nemerteans. Since detailed descriptions of the nervous system of adults in other nemertean species have not been available so far, this study may serve as a basis for future studies that might add data to the unsettled question of the nemertean ground pattern and the position of this taxon within the phylogenetic tree.

Background: Paired mushroom bodies, an unpaired central complex, and bilaterally arranged clusters of olfactory glomeruli are among the most distinctive components of arthropod neuroarchitecture. Mushroom body neuropils, unpaired midline neuropils, and olfactory glomeruli also occur in the brains of some polychaete annelids, showing varying degrees of morphological similarity to their arthropod counterparts. Attempts to elucidate the evolutionary origin of these neuropils and to deduce an ancestral ground pattern of annelid cerebral complexity are impeded by the incomplete knowledge of annelid phylogeny and by a lack of comparative neuroanatomical data for this group. The present account aims to provide new morphological data for a broad range of annelid taxa in order to trace the occurrence and variability of higher brain centers in segmented worms.Results: Immunohistochemically stained preparations provide comparative neuroanatomical data for representatives from 22 annelid species. The most prominent neuropil structures to be encountered in the annelid brain are the paired mushroom bodies that occur in a number of polychaete taxa. Mushroom bodies can in some cases be demonstrated to be closely associated with clusters of spheroid neuropils reminiscent of arthropod olfactory glomeruli. Less distinctive subcompartments of the annelid brain are unpaired midline neuropils that bear a remote resemblance to similar components in the arthropod brain. The occurrence of higher brain centers such as mushroom bodies, olfactory glomeruli, and unpaired midline neuropils seems to be restricted to errant polychaetes.Conclusions: The implications of an assumed homology between annelid and arthropod mushroom bodies are discussed in light of the 'new animal phylogeny'. It is concluded that the apparent homology of mushroom bodies in distantly related groups has to be interpreted as a plesiomorphy, pointing towards a considerably complex neuroarchitecture inherited from the last common ancestor, Urbilateria. Within the annelid radiation, the lack of mushroom bodies in certain groups is explained by widespread secondary reductions owing to selective pressures unfavorable for the differentiation of elaborate brains. Evolutionary pathways of mushroom body neuropils in errant polychaetes remain enigmatic.

Invertebrate nervous systems are highly disparate between different taxa. This is reflected in the terminology used to describe them, which is very rich and often confusing. Even very general terms such as 'brain', 'nerve', and 'eye' have been used in various ways in the different animal groups, but no consensus on the exact meaning exists. This impedes our understanding of the architecture of the invertebrate nervous system in general and of evolutionary transformations of nervous system characters between different taxa.

The conference ‘Celebrating Darwin: From the Origin of Species to Deep Metazoan Phylogeny’ was held at the Humboldt University in Berlin, from 3 to 6 March 2009. Specialists from the fields of bioinformatics, molecular biology, developmental biology, comparative morphology and paleontology joined forces to present and discuss novel approaches in reconstructing the still unresolved early branching patterns of the metazoan tree of life.

Comparative analyses of neuroanatomical characters can make valuable contributions to the inference of phylogenetic relationships. Whereas investigations in this field are numerous for arthropods, in-depth studies on other protostomes are sparse. Here, we provide a survey of the internal neuroarchitecture of the brain of the aciculate ragworm Nereis diversicolor (Polychaeta, Annelida). Descriptions are based on confocal laser scanning microscope analyses of brain sections labeled with the nuclear marker DAPI and antibodies raised against FMRF-amide, serotonin, and histamine. Autofluorescence of the nervous tissue has been utilized to further elucidate the anatomical structures of the brain. The architecture of two major brain compartments, i.e., the paired mushroom bodies and the central optic neuropil, is described in detail. The findings are compared with existent literature on polychaete neuroanatomy and on arthropod neuroanatomy, and possible phylogenetic implications are outlined.