Why are some organisms able to regenerate?
All organisms, including humans, have the ability to regenerate something in the body. But the process is much more developed in lower organisms such as plants, protists -- unicellular organisms such as bacteria, algae, and fungi  and many invertebrate animals such as earthworms and starfish. These organisms can grow new heads, tails, and other body parts when injured.
Scientists don't know why mammals don't have the same ability to grow new limbs. But they think it is because mammals have more complex biological structures; limb regeneration would require sophisticated controls to ensure that limbs and organs don't grow out of control. Humans, for example, are already equipped with safety mechanisms to ensure that individual cells don't grow uncontrollably. But these mechanisms wear down as a person ages and cancer is often the result.
Nevertheless, mammals do regenerate skin, muscle, and blood. Scientists are just beginning to learn about other types of cells, such as those in the brain and blood, that also regenerate. Further study of the phenomenon might lead to a way of growing replacement organs and limbs outside the body.
Not all organisms regenerate in the same way. In plants and organisms like hydra and jellyfish, missing parts are replaced by reorganizing neighboring tissues into whatever parts have been cut off. Animals with more complex bodies usually regenerate parts by producing a specialized bud, or blastema, at the site of amputation. The blastema supplies the tissue necessary for the regenerated part.
The regenerated body part is not always the same as what was lost. Many insects regenerate abnormally small legs from which some segments may be missing.
Tadpole tails, for example, typically grow back to about only half their original length. And regenerated parts often differ in details. A regenerated lizard tail, for example, contains a cartilaginous tube instead of the vertebrae in the original.