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Listen to part of a lecture in a biology class. Professor: Today, we're going to talk about a fascinating adaptation in the animal kingdom: the ability of some creatures to regenerate lost body parts. While many people are familiar with the salamander's ability to regrow a tail or limb, fewer know about the planarian, a type of flatworm. Planarians have an extraordinary capacity to regenerate not only limbs but also their entire bodies from tiny fragments. This is possible because of a population of adult stem cells called neoblasts. These cells are distributed throughout the planarian's body and are activated when the worm is injured. When a planarian is cut into pieces, the neoblasts in each piece divide and differentiate to form all the missing tissues, including a new brain. Interestingly, the regeneration process is not always perfect; sometimes, the new body part can be slightly different in size or shape. But this ability has made planarians a key model organism in stem cell research. Scientists hope that by studying these animals, we can unlock secrets that may one day lead to new medical therapies for humans.