Why do worms grow back when you cut them in two?
(Lansing State Journal, May 24, 1995)
One of the most obvious characteristics about the earthworm is that its body is segmented. Often we refer to the location of internal organs in a worm by the number of segments from the mouth they are located. For instance, the pharynx is located three to five segments from the mouth. After about the 21st segment from the mouth many of the internal organs of the earthworm repeat. These organs repeat about every twenty-one segments. The longer a worm is, the more sets of the same organs it can have.
As long as a worm is cut so that it has at least one set of these repeating organs, the pieces of the worm can heal and survive.