Here we show that in I. pulchra, piwi is also expressed in a subpopulation of somatic neoblasts. Next, we report on the morphology of stem cells, their distribution and differentiation capacity in this acoel species. Furthermore, we studied the function of the stem cell system during homeostasis, development, regeneration, hydroxyurea treatment, starvation and after irradiation using histology, electron microscopy, BrdU labelling, in situ hybridization and RNA interference. To summarize, these data provide new insides how stem cell systems might have been developed during animal evolution.