Fine structure of mushroom bodies and the brain in Sthenelais boa (Phyllodocida, Annelida)

Abstract Mushroom bodies are known from annelids and arthropods were formerly assumed to argue for a close relationship of these two taxa. Since molecular phylogenies univocally show that both taxa belong different clades in the bilaterian tree, similarity must either result convergent evolution or transformation an ancestral mushroom body. Any morphological differences ultrastructure composition could thus indicate results similar functional constraints. We here study bodies, glomerular neuropil, glia-cells general anatomy nervous system Sthenelais boa . The neuropil is composed densely packed, small diameter neurites lack individual clusterwise glia enwrapping. Neurites other regions brain much more prominent, enwrapped by glia-cell processes can be discriminated bodies. same applies respective neuronal somata. insects region higher synaptic activity spheroid appearance structures. However, while structures sharply delimited surrounding enwrapping insects, this not case Although superficially similar, there anatomical arrangement between annelids. Hence, we suppose observed rather evolved convergently solve constrains than transforming body design.