Planarian fragments behave as whole animals.

Behavioral responses of freshwater planarians have been studied for over a century. In recent decades, behavior has been used as a readout to study planarian development and regeneration, wound healing, molecular evolution, neurotoxicology, and learning and memory.The planarian nervous system is among the simplest of the bilaterally symmetric animals, with an anterior brain attached to two ventral nerve cords interconnected by multiple commissures.