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

For 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 
organs.

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 
normal size.

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 
original tail.

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.