Many aspects of the lives of the tunicates, the closest living relatives of the vertebrates, are poorly understood. It was thought that in ascidian tunicates such as the sea squirt (dona intestinalis), the superficially vertebrate-like central nervous system of the 'tadpole' larva degenerates to be replaced by an adult nervous system developed from scratch. Transgenesis and imaging techniques now show that this is not the case. In fact, the adult nervous system develops from stem-cell-like ependymal cells found in the larva.
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