The planarian Schmidtea mediterranea is rapidly emerging as an important model system for the study of stem cells, regeneration and tissue homeostasis. Left, the central nervous system as visualized by whole-mount in situ hybridization with a probe against Smed-synapsin (syn), the planarian homolog to the neurotransmitter release regulator. Top right, confocal projection of the cephalic ganglia (green) after anti-tubulin staining. Bottom right, the remarkable developmental plasticity of planarians illustrated by the generation of a duplicitas cruciata after surgical manipulation. Images are from Hara Kang, University of Utah; Phillip A. Newmark, University of Illinois; and Andrew Glazer, Massachusetts Institute of Technology, respectively. See Reddien et al., Developmental Dynamics 237:3099–3101. © 2008 Wiley-Liss, Inc.