Following neural injury, the complex sequence of events leading to neural regeneration will not be undertaken unless survival in the short term is assured. For most species to survive injury to the central nervous system (CNS), neural control of homeostatic functions must remain intact, although some very simple animals, such as flatworms, ribbon worms, or sea cucumbers, can sustain extreme injury and require only that enough neural tissue be present to provide a stimulus for regeneration. The nervous system apparently plays an organizing role in the regeneration of many animals (Chap. 2).