Planarians are flatworms with complex internal anatomy (Figure 1A). They possess a branched intestine that connects to a centrally located pharynx (the only opening of the animal); a nervous system with two cephalic ganglia, two ventral nerve cords, sensory neurons including those of the eyes, and many peripheral neurons; an excretory system (a network of ciliated ducts); and body wall muscle fibers. Because a small planarian tissue fragment can regenerate an entire animal, regeneration requires mechanisms for the specification of the many distinct cell types of adult planarians.