Three-dimensional reconstruction of the central nervous system of Macrobiotus hufelandi (Eutardigrada, Parachela): implications for the phylogenetic position of Tardigrada
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The morphology of the central nervous system of the tardigrade species Macrobiotus hufelandi was analysed with anti α-tubulin immunostaining in combination with confocal-laser-scanning-microscopy and computer aided three-dimensional reconstruction. The brain anatomy is unexpectedly complex with distinct tracts and highly intermingled nerve fibres. In contrast to older descriptions, we could not detect a suboesophageal ganglion. Furthermore, we found no evidence for a tripartite/three-segmented brain organisation. The median part of the brain is directly connected to the first pair of trunk ganglia via circumoesophageal connectives. Surprisingly, the four paired ventral ganglia do not show segmental commissures typical for the ladder-like nervous system of arthropods. Our findings constrain the phylogenetic position of Tardigrada. The much simpler organisation of the central nervous system of Tardigrada compared to that of Onychophora and Euarthropoda and some similarities to the nervous system of Cycloneuralia support a phylogenetic position of Tardigrada outside an Onychophora/Euarthropoda clade. This means that Tardigrada might be either the sister group to all other Arthropoda or they are more closely related to Cycloneuralia.