Nematodes are a large and diverse monophyletic taxon with about 20,000 known species. They are part of the taxon Ecdysozoa, some of which possess a characteristic type of brain called the cycloneuralian brain. Because animals with a cycloneuralian brain likely are not a monophyletic group, it is important to describe and understand the structure of cyloneuralian brains and associated nervous system structures. With respect to nematodes, most studies focus on model organisms, such as Caenorhabditis elegans and immunohistochemical data in general, are rare. To expand the knowledge, we investigated two species of the family Plectida, Plectus acuminatus and Plectus aquatilis by immunohistochemical experiments combined with confocal laser-scanning microscopy. In comparison with C. elegans and further nematodes, our results reveal a conserved architecture of the nervous system and several structures can be homologized with known counterparts. Nevertheless, in detail, some striking differences can be demonstrated, which become visible in the arrangement of longitudinal and transverse neurites as well as in specialized neurons, such as the neurosecretory motor neurons within the pharynx. Compared to C. elegans, the branching pattern of these neurons in Plectus spp. is more complex. Overall, there are indications that the nervous system among nematodes is more diverse in structure than originally expected and further studies on taxonomically basal, free-living nematodes needs to be in focus on future investigations. |
