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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

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.

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