Two groups of soft-bodied worms, the platyhelminths and the acoels, possess an unusual stem cell system. (a) The current view of animal phylogeny indicates that Platyhelminthes and Acoela represent distinct evolutionary lineages, with acoels representing the outgroup to the rest of the Bilateria (namely, the Deuterostomia, Lophotrochozoa, and Ecdysozoa). Whether their similar stem-cell systems are homologous or convergent remains an open question. The biology of these stem cells is currently being investigated in the planarians Schmidtea and Dugesia, the macrostomid Macrostomum, and the acoel Isodiametra. (b) Somatic stem cells called neoblasts (green dots) are distributed in the parenchyma throughout much of the body of planarians, macrostomids, and acoels. (c) Neoblasts can self-renew, produce differentiated somatic cell types, and produce germ-line cells. Neoblasts are morphologically characterized by a large nucleus-to-cytoplasm ratio relative to differentiated cells (nuclei represented by gray shading).