St. Nicholas/Volume 32/Number 3/Nature and Science/Nature Mends Injuries
Some Curious Methods by which Nature Mends Injuries.
How many weak and timid creatures there are in the world, with neither teeth and claws for their protection, armor for their defense, nor speed with which to escape their enemies! One can hardly understand why they have not all been killed and eaten up long ago. Nature is, however, kinder to these poor animals than she seems; for if she has left them defenseless against attack, she has given them a marvelous power of recovery from injuries.
When a tiny lizard has to scamper for his life in search of a crack in the rock, he often has “so close a call” that his pursuer snaps off his tail just as he whisks into safety. A loss like this would kill most larger animals, but not the little lizard. He simply waits round quietly until a new tail grows, and then is as well off as before, except that the new tail has a flexible rod of cartilage where the old one had a back-bone.
If an earthworm happens to be retiring to his hole when a robin is out looking for breakfast, there is apt to be a lively tug of war between the eater and the breakfast. Nat infrequently the bird gets the tail end of the worm, while the other half crawls away into safety. Not even a lizard could survive such treatment as this, but the earthworm is, in ability to recover from injuries, almost as much superior to the lizard as the lizard is to us. He grows a new half-body to replace the one which has
1. A worm that had grown a rear half anew.
2. When the worm lost its head, nature put a new head on the rear of the severed head instead of a tail.
3. Two heads on one V-like body when the head end was split.
4. A side piece that made two new heads at the side.
5 and 6. Two branches of one body (split apart} growing new heads at the upper portion of each branch.
7. Novel form of two heads on one body.
8. Two worms, half new, held together by the tail end.
9. A worm nearly cut apart grew two half-heads at the union.
There are, besides, some snail-like water-worms which quite outdo the earthworms in bearing up against misfortune. If one of these chances to lose his entire head, in a week or so—sometimes in only four or five days—he grows a new one, brain, eyes, and all, and is as well off as ever. Even if a hungry fish gets two bites at him, so that he loses both head and tail, the worm can patch himself out with new members and go about his business as before. They have even been known to get divided into two pieces about equal in size,and each piece grow a new half-body, so that there were two entire worms in place of one.
After this it will easily be guessed that if the head end of the worm happens to be split halfway down he will grow two new sides and become Y-shaped with two heads. Or if the tail end is split new sides grow and a two-tailed worm is made. Sometimes one or two new heads develop close behind the old one in the angle of the Y. Indeed, the little creature seems to have a sort of mania for making new heads and tails wherever be finds a chance. If, therefore, the worm, after receiving several wounds, manages to escape with his life from the cuts which happen to open forward, little heads growout, and from those opening backward little tails—no doubt greatly to his embarrassment.
But what of the cut-off heads and tails? Do they make new bodies and become whole animals again? Not usually. The severed head seems to become confused, so that it does not know what to do. If it lives it is most apt to produce another head like itself, and change into two heads placed neck to neck so that they look in opposite directions. So, too, the severed tail, equally foolish, doubles itself and becomes two useless tails growing end to end.
But is n’t this really quite impossible? A head or a tail or even a half-body cannot get food. If it cannot eat 1t cannot grow: and that is all there is about it. Well, it is true that a fragment cannot eat. But still it can make the 
An injury upward near the head developed a new tail, downward a new head. Again another injury (at the lower left) developed heel and tail.new part out of its own tissue. So the animal keeps getting smaller as it becomes more nearly complete, until, when the new part is finished, the whole body may be no more than the tenth part of its proper size. The reconstructed animals are therefore forced to begin life over again like young worms. In time, however, they grow up to full size. When a head end makes a new head instead of a tail, or a tail makes a new tail instead of a head the little creatures must necessarily waste away and die.
Note to Science and Nature-study Teachers.— The statements in this article are based on “Regeneration.” by Thomas Hunt Morgan, Professor of Biology in Bryn Mawr College. The editor gratefully acknowledges the permission of his publisher the Macmillan Company, co make the accompanying illustrations from the sketches in this book.