Planarian fragments behave as whole animals

Behavioral responses of freshwater planarians have been studied for over a century.1 In recent decades, behavior has been used as a readout to study planarian development and regeneration,2-6 wound healing,7,8 molecular evolution,4,9,10 neurotoxicology,11-13 and learning and memory.14-17The planarian nervous system is among the simplest of the bilaterally symmetric animals,18 with an anterior brain attached to two ventral nerve cords...