Some worms regenerate when cut in half laterally, but what would happen if they were cut in half longitudinally?
Thomas Barker
Cambridge, UK
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It seems this depends heavily on which phylum of worm you are bisecting. Most worms in the Annelida phylum can regenerate lost segments to some extent, but many will die if you cut them in half laterally. The tail end of a common earthworm is sometimes able to regenerate in this case, but the head end will never survive. Marine bristleworms, however, can often fully regrow a worm from both pieces if halved laterally.
Regardless of which species you pick, though, a longitudinal cut will almost certainly spell doom for an annelid. They have a series of concentric ringed pallial blood vessels running down their bodies. A cut down the length of the worm will inevitably sever all of these. Even if the worm somehow survives this, to recover it would then have to regrow half of its body after losing much blood and with a non-functional digestive system.
A 2011 article reported a flatworm growing two identical heads after its original head was cut in half down the middle
The story is very different if you have a planarian flatworm of the Platyhelminthes phylum under the scalpel. These are already something of a New Scientist favourite thanks to their neoblasts, clumps of adult stem cells that give them incredible regenerative abilities. One flatworm in an was able to regrow from just 1 per cent of its original body. Meanwhile, a 2011 New Scientist article reported another flatworm growing two identical heads after its original was cut in half down the middle. Surviving a longitudinal cut is well within the capabilities of these worms.
Kevin Lumney
Sunbury, Ohio, US
Among bilateral animals, and worms particularly, planaria are well known for their proficiency at regenerating missing portions of their bodies. My daughter, for a middle school science project, explored the limits of this phenomenon when adults of such worms were sectioned laterally (dividing the head from the tail) at varying distances from the middle of the body.
She did find there was a point at which the smaller portion of the worm resulting from her cut was incapable of regenerating the missing part of its body (though of course the larger portion easily regenerated its lost part). If by a 鈥渓ongitudinal cut鈥� the questioner means sectioning a planarian (for example) worm鈥檚 body lengthwise at varying distances from its sagittal plane (the plane dividing it into equivalent right and left mirror-image halves), I would be willing to bet a similar limit exists, where a portion is just too small to entirely regenerate.
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