Regeneration of Lost Parts in Animals
Nature Bulletin No. 751 April 11, 1964
Forest Preserve District of Cook County
Seymour Simon, President
David H. Thompson, Senior Naturalist
REGENERATION OF LOST PARTS IN ANIMALS
ages, mankind has been fascinated with the idea that lost parts of
animals can be regrown. According to Greek legend, one of the twelve
"labors" of Hercules was the destruction of the Hydra, a gigantic
monster with nine serpents' heads. Finding that as soon as one head
was cut off two new ones grew in its place, at last he burned out their
roots with firebrands.
All animals have the power of regeneration to a greater or lesser
degree. In man and higher animals it is quite limited. We see it most
often in the healing of wounds and the mending of bones. A lost
fingernail can be replaced but not a lost finger. Lower animals have a
much greater ability to replace parts. For instance, the little half-inch
flatworm, Planaria, that lives under rocks in clean creeks can be cut
into as many as 32 pieces and each fragment is able to rebuild a
miniature flatworm complete with head, tail, eyes, mouth and internal
One of the most striking examples of regeneration is found among the
common crayfishes of our streams and lakes. An individual with
unequal claws or pincers, or with one of the eight walking legs smaller
than its mate, means that one has been lost and is being replaced.
The entire process of regeneration can be watched in the schoolroom
or laboratory. Select a very small crayfish because young ones grow
rapidly and molt their shells often. Remove a leg or a pincer. Keep in a
gallon jar with a half-inch of clean water and feed small bits of raw
meat. With each molt the lost part grows larger and soon reaches
The crayfish has an unusual "breaking joint" near the base of each
claw and leg which is a safety device. When grabbed by a fish,
snapping turtle, bird or other enemy, it merely twitches a special
muscle, the joint breaks and the crayfish escapes. Some lizards
(including the famous "glass snake" which is really a legless lizard)
also have a breaking joint which allows the tail to drop off when it is
seized. A new tail is regenerated but it lacks the backbone of the
The common earthworm or nightcrawler of our lawns, gardens and
bait cans, has a body made up of a series of 100 or more segments
marked off by shallow grooves. If the worm is cut in half, the head end
can grow a new tail. The tail end, if it lives at all, grows another tail
instead of a head and eventually starves to death. If only 15 or 20
segments of the head end are cut off, they are replaced by a new head
with but five segments.
A fish has a sort of autobiography recorded in its scales. Each lies in a
pocket in the skin and grows as the fish grows. From the markings on
the scale's surface fish biologists are able to read its age, seasons of
good growth or of famine, and other items of its life history. However,
it is often necessary to examine several scales in order to find one with
a complete record. This is because scales are frequently lost and
regenerated leaving a blank page in its history.
Embryos and young animals regenerate lost parts much more readily
than adults. For example, the rudimentary hind limb of a frog tadpole
can be replaced while the leg of an adult frog cannot..
Theories explaining regeneration have been a battleground among
zoologists and physiologists for more than a century.
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Update: June 2012