Your question is interesting and it needs some
explanation. Here it is....
is the process by which some organisms replace
lost body parts and a number of regenerative
mechanisms have been evolved by different
species. Regeneration is most common in
invertebrates, occurring in almost all
coelenterates and planarians, most annelids
(segmented worms), and many insects.
spontaneous loss and replacement of a body part,
occurs in many insects and crustaceans, and
enables them to shed a crippled leg or claw. The
new part can be an exact replica of the lost
structure, or can be functionally similar but
anatomically different from the lost part.
Also, sponges have remarkable powers of
regeneration. Even if large parts of a sponge's
body are lost or damaged, they may be replaced or
Many starfish can drop off arms as a
defensive reaction. They can then regenerate new
arms to replace the old ones. If a starfish is cut
in two, each of the pieces may regenerate into a
Among vertebrates, no mammals
have the ability to re-grow lost limbs or
tails, but some species can regenerate other
peripheral appendages, (e.g. a deer's antlers) or
internal organs (e.g. the human
In most fishes and salamanders, limited
regeneration of limbs occurs, and tail
regeneration takes place in larval frogs and toads
(but not adults). The whole limb of a Salamander
or a Triton will grow again and again after
amputation. In reptiles, Chelonians, crocodiles
and snakes are unable to regenerate lost parts.
But many (not all) kinds of lizards, geckos and
Iguanas possess regeneration capacity in a high
degree. Usually, it involves dropping a section of
their tail and regenerating it as part of a
defense mechanism. While escaping a predator, if
the predator catches the tail, it will disconnect.
The tail lays flopping in the predator's mouth or
on the ground. While the predator is occupied or
distracted by the wriggling tail, the reptile runs
away. The skin, muscles, blood supply, nerves and
bone separate at almost any place along the length
of the tail (below the reproductive organs).
Later, when growing a new tail, it will not include
all of the tissues and structures of the original
one. Instead of the segmented vertebrae, a long
tapering cartilaginous tube develops within which
the spinal cord is located and outside of which
are segmented muscles. The spinal cord is replaced
by an epithelial tube, which gives off no nerves.
Very frequently super-regeneration occurs, the
amputated limb or tail being replaced by double or
multiple new structures.
While the loss of
the tail may be natural, and may save a lizards
life, it isn't without cost. It is stressful to the
lizard, especially if that lizard stores critical
fat deposits in the tail, such as leopard geckos.
Should a lizard be attacked twice, it is
advantageous to have a re-grown appendage. However,
tail regeneration is energetically expensive and
can also result in lowered social status. Still,
it is better than being someones dinner. The less
stress the lizard has to deal with, the faster the
stump will heal and, if the tail is going to
regenerate (they do not always do so), it will do
so fairly rapidly.
Most lizards can
regenerate more than once. The glass lizard is
only able to shed and regenerate the tail once in
its lifetime. When the new tail grows back, it
will be smaller than the original. Unlike other
lizards the monitor's tail does not break off and
Regenerating tissues apparently
follow a strict polarity, growing back in the
proper orientation to the rest of the body. Since
the most commonly lost structures are limbs and
tails, the pattern of growth is usually outward
from the body, suggesting that tissues more
proximal than the injury contain all the necessary
information to replace the lost part, but not
those closer to the main trunk of the body. In
some cases, however, as in fish fins, regeneration
may occur in both directions. Regeneration is part
of developmental biology and involves many
unresolved problems. Scientists are trying hard to
understand the molecular basis of regeneration.
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