Regeneration is the process by which animals replace tissue or structures after they are lost. Regeneration abilities vary within the Animal Kingdom. For example, many invertebrates such as planarians and hydra can regenerate all of their bodies even after they have been cut into small pieces. Some vertebrates are also capable of complex regeneration. Salamanders and zebrafish are research model systems which regenerate a plethora of body parts, including limbs and heart. However, mammals have limited regenerative ability and in humans and mice it is limited to regeneration of digits in childhood and liver regeneration.