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Mark Wells, age 11, of Calgary, Alberta, Canada, for his question:

Can an earthworm really survive being cut in half?

This is somewhat like asking whether the human body can survive a leg amputation. The answer depends upon who performs the surgery, how and just where the severing operation is done. An earthworm can survive being cut into two if he is cut into the right two pieces.

The smooth body of the earthworm is a neat row of about 150 circular, stretchable segments. He may be pulled into two pieces by a determined robin. The piece swallowed by the bird has no chance of surviving, naturally. But the remaining half may survive and live to become a complete worm again. The outcome of the experience depends entirely upon where the worm was severed. And to arrive at the safe severing point we must count those ringed segments that circle his body from end to end.

Begin counting from his head. This is the direction in which he crawls. If he refuses to budge, you may make your deduction by examining both his ends. The head end is somewhat more rounded and tapered. The tail end tends to be flattened and somewhat thicker. Count twelve sections back from his head. For best results, the cut should be made somewhere between the 12th and 18th segments. And the results may be doubly successful for the work.

After the operation, the earthworm will start to grow new tissues at the cut areas on both pieces. The piece that was the tail end will sprout up to five new segments to serve as a head. The piece with the old head will grow a larger number of new segments to serve as a new tail. A few weeks after the doubly successful operation there are two living earthworms instead of one.

The operation will be only half as successful if the worm is sliced behind the18th segment. The severed head end will grow a new tail and the patient will become one complete earthworm again. The severed tail end will do its best but it is doomed to failure. New segments sprout from the wound but they are unable to recon¬struct a head. They grow into another tail. We now have a worm with a tail at each end of his body and no head. This means he has no mouth. He can crawl around, but he cannot eat and the poor fellow soon dies of starvation. Certainly a worm can survive being cut into two pieces and sometimes the division produces two complete worms. But the magic ingredients that can re-grow a new head are in front and not behind the 18th segment.

Regrowth of this kind is called regeneration. A human being cannot regenerate a lost limb nor can cats and dogs or other so called higher animals. But regenera¬tion is common in many of the simple animals such as the earthworm. A lizard can regenerate a new tail. A starfish can regenerate a lost arm. If a starfish loses four or even all five of his arms, he can replace them. And if severed apart into the proper sections, he can duplicate his body and become two starfishes.


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