Deer grow new antlers annually; sea stars are experts at growing back rays; and flatworms can regrow all sorts of body parts. The axolotl, an aquatic salamander, can keep regenerating lost parts throughout its life. Of the many creatures that do grow back body parts, humans, despite being the rulers of Earth, cannot regenerate lost appendages. It seems that the more advanced the species, the less able they are to regrow legs or heads.

1 of 9 Skinks James Gerholdt / Getty Images Skinks can't walk upright, but they can release their tail at will. If a predator tries to attack from behind, the tail detaches and keeps wiggling to distract the predator while the skink scurries away. The skink can grow a new tail in three to four months, but it is more vulnerable during that period.

2 of 9 Sea Star Stephen Frink / Getty Images When accidents happen, sea stars have the ability to grow back their arms (known as rays) and tube feet. Also called starfish, most sea stars have five arms, but some have up to 40. Some sea stars can regenerate entire bodies, or a new sea star just from a portion of a severed limb, in part because most of their vital organs are in their arms.

3 of 9 Worms up close with nature / Getty Images Researchers around the world are fascinated by the impressive regeneration ability of flatworms. The majority of planarians are able to grow back all kinds of body parts, including their heads, using stem cells. Freshwater flatworms have been doing this for quite some time. These asexual creatures reproduce by tearing themselves in two. And it only takes about a week for these two pieces to become two new worms.

4 of 9 Conch Reinhard Dirscherl / Getty Images Conch (pronounced "conk") are slow-moving marine gastropods. If you see a conch on the move, you may notice that the eyes of this creature are positioned at the ends of long stalks. What you may not know, however, is that conchs can regenerate a lost eye. Compared with other gastropods, eye regeneration in conchs is fast — it takes only a few weeks.

5 of 9 Deer Boris SV / Getty Images When it comes to mammals, deer antlers are the only organ that can fully regenerate, and it occurs annually. The regeneration of antlers, which is initiated and maintained by neural-crest-derived stem cells, is being used by scientists to study and model organ regeneration in other mammals. With the exception of caribou (also known as reindeer), only male deer have antlers. Males grow antlers to compete with other males for mates and to find food in the snow. The growth rate for antlers is extremely fast — a quarter-inch per day.

6 of 9 Crayfish Ragnar Schmuck / Getty Images Crayfish can regrow their claws, just like other arthropods. The claw regeneration usually takes one molt to complete. It can grow back even faster if the crayfish is younger, warmer, and well fed. But research of the crayfish brain has uncovered something even more exciting. Scientists have found a link between the immune system and the regeneration of neurons in crayfish. This same process resembles the human production of white blood cells, which leads to the human immune system.



7 of 9 Zebrafish NERYX / Getty Images The zebrafish can keep its stripes and its tail. If the fish's caudal fin gets bitten off by, say, another hungry fish, the zebrafish can grow a new tail in about two to four weeks. Because the zebrafish are such experts at regeneration, researchers have been using them as a model for complex tissue regeneration.

8 of 9 Axolotl Paul Starosta / Getty Images The axolotl is an aquatic salamander that is able to regenerate not just its limbs, but also its spinal cord, heart, eyes, and parts of its brain. Unlike other vertebrates, the axolotl is able to keep regenerating throughout its life. By sequencing an axolotl's genome, scientists hope to discover how the species uses stem cells to regenerate tissue. Found only in Mexico, the axolotl is critically endangered in the wild.