Asked by: Martin Harrison, by email
To a biologist, ‘worm’ refers to many, quite different organisms. The ability to re-grow body parts differs enormously between them, although tails are generally easier to re-grow. If you mean earthworms, their tails usually die when severed, while for many other species the head portion (down to the ‘saddle’) can re-grow a tail – although often a stunted one. Some species can also regenerate the very front of the head.
My guess is certain earthworms could fully regenerate from one specific quarter of their body, although it’s not really known and cruel to try and find out – most would end up dead or stunted.
Some non-British earthworms can even repeatedly re-grow lost tails, although other segmented worms beat that: many reproduce asexually by severing themselves repeatedly, and parchment worms can entirely regenerate from just their 13th body segment.
However, the distantly related planarian flatworms have more extraordinary regenerative powers. Some Dugesia species can fully re-grow from random body fragments of about 10,000 cells: approximately 1/20th of the adult body size.