Ultrastructure of ganglia in the brachiopod Coptothyris grayi and its phylogenetic significance
Since the study of the neuroarchitecture has phylogenetic value, it is important to describe the nervous system in poor investigated groups.We describe the ultrastructure of the supraenteric and subenteric gangliain the rhynchonelliform brachiopod Coptothyris grayi. We found that the supraenteric ganglion is a neuroepithelium that consists of monociliated neurons among glial cells and a basal neuropil.The subenteric ganglioncontains neurons of two morphologically distinct types and is represented by a stratified neuroepithelium with three layers. The apical layer is formed by the somata of glial cells. The middle layer contains thelarge perikarya of the neurons of the first type. The inner basal layer consists of a neuropil, basal projections of glial cells, and neurons of the second type. The brachiopod supraenteric ganglion exhibits the featuresof the simple neuroepithelium that is probably the plesiomorphic condition in the bilaterian ancestor. The brachiopod subenteric ganglionis a stratified neuroepithelium that has been described in some groups of protostomes and deuterostomes. The next step in the evolution of the invertebrate nervous system was an internalization. In many groups ofprotostomes, the subepidermal ganglia, which consist of perikarya surrounding a central neuropil, are formed by ingression of theneuroepithelium. In deuterostomes, the nerve tube with an inner neural canal is formed by invagination of the stratified neuroepithelium. Thus,the so-called ganglia of brachiopods are neuroepithelia that have retained the plesiomorphic condition of the bilaterian nervous system and that cannot be regarded as true ganglia.