Answer 1:
Salamanders and lizards will often regenerate
their tails. Their tails are used to store extra
nutrients but if they are threatened or possibly
caught by a predator, they will detach their tails
in hopes of the tail being a distraction to the
predator while the rest of the animal escapes.
Salamanders and young crabs will also
regenerate a limb that is lost to a predator or in
a fight.
I do not know of any animals that will
regenerate eyes or internal structures.
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Answer 2:
Generally speaking, the simpler an organism's
body, the more regenerative it is. There are some
exceptions of animals with very complicated bodies
(like some lizards) that can regrow portions of
their bodies, but on the whole the simpler your
body plan, the better you are at regenerating.
Generally speaking, vertebrate animals and insects
cannot regenerate lost appendages. Most others
can.
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Answer 3:
Yes, there are most definitely organisms that
can regenerate - even humans can regenerate to a
certain extent. In humans, the distal tip (portion
of your finger until your first knuckle), can
regenerate until 2 years of age. It can only be
done until you are 2 because that's the age our
immune system gets stronger and our bodies can
fight off disease better then. Also your liver can
regenerate after partial transection, blood
regenerated cells every 120 days, the lining of
intestines turns over every 5 days, muscles are
replaced monthly, and our liver is replaced
annually. We cannot regenerate from amputations
though. Some other organisms in which regeneration
is common are sponges, hydra (Cnidaria),
protosomes (crustaceans, insets annelids,
mollusks), deuterosomes (crabs, sea stars/
cucumbers), chordates, vertebrates (newts,
salamander), planaria (flatworms). These are
organisms that can recover from amputations. They
have stem cells which are "totipotent", meaning
can form any type of cell in the body. Totipotent
cells are present in early embryonic development
in human when the blastocyst starts to form into a
cells with major body systems, like the
cardiovascular system.
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