Regeneration, the regrowing of a lost or damaged limb, does occur in some animals. However, the ability to regenerate body parts declines progressively with the increasing complexity of animal species. Among primitive invertebrates (animals lacking a backbone), regeneration frequently occurs. For example, a planarium (flatworm) can grow a whole new body from just a strip of tissue. In humans, regeneration merely takes the form of scar tissue growing over a wound.
Regeneration of limbs occurs in some higher invertebrates, such as starfish, some insects, crabs, crayfish, and lobsters. For example, if a crayfish loses a claw, the missing claw will grow back during the next molt (when an animal molts, it sheds its hard outer skin or shell and replaces it with a new one). Sometimes the crayfish's regenerated claw does not achieve the size of the missing claw. However, after every molt, which occurs two to three times a year, the claw grows. In this way, the claw eventually becomes nearly as large as the original claw.
Some amphibians and reptiles can replace a lost leg or tail. For instance, a lizard that is being pursued by a predator can make its own tail fall off and later grow a new one.
Sources: Gardiner, Mary S. The Biology of Invertebrates, pp. 872-74; Huxley, Thomas H. The Crayfish, pp. 38-39; Magill, Frank N. Magill's Survey of Science: Life Science Series, vol. 5, p. 2311; "Regeneration." Encyclopaedia Britannica CD 97.