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I have often heard the story that if you cut a worm in half in the middle, both halves of the worm will grow the other part back, and you will eventually end up with two worms.

Is this true?

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This is a common claim. When I searched for an example of the claim to show notability, I found many but they all contained answers, making them tricky to use as examples of the claim. (This also makes the question kind of pointless; anyone who finds us will have found dozens of authorative sources...) –  Oddthinking Apr 15 '12 at 1:42
    
I've cut open an earthworn and it certainly doesn't look like the head and the tail are identical. The organs are clearly visible and they are not distributed equally. From that it seems unlikely that the tail alone would be able to survive. –  Fabian Apr 15 '12 at 18:17
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the currently accepted answer by @Martin is restricted to earthworms but the question has no such restriction. Therefore, the answer is not a correct answer to your question. The answer by @Kevin is the correct answer, and I think it would be a more appropriate selection. Another option would be to add the earthworm qualifier to the question if that was the original intent and keep the answer by Martin. This is not an ideal solution but would at least provide a compatible q/a pair. –  David Apr 17 '12 at 4:52
    
I agree with @David in the point that it would be beneficial to edit the question to be more specific. At the moment is says "a worm" meaning basically "if you cat any worm, both halves will survive", which is a very general claim. My answer addresses the most common type of worm encountered by normal people (In Germany it's basically "the worm"). Therefore it is a valid answer against this common misconception. On the other hand, if the question would say "Is there a worm species for which this is really true?" than Kevin answer would indeed spot on. I don't mind which one is accepted. –  Martin Scharrer Apr 17 '12 at 6:57
    
I've gone ahead and created a Meta question where this can be discussed further. –  Kevin Cathcart Apr 17 '12 at 14:47
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3 Answers 3

I know this claim from my childhood about earthworms, not any kind of worms.

This story is untrue according to the Worm Farm Guide. The two half do not survive independently. However, the head part can regrow some fraction at his back end if it got lost:

You may have heard that you can cut an earthworm in half and both halves will grow back. That's not true. If an earthworm is cut in half, it will usually die. However, it is true that if a third or less of the posterior portion of the worm gets chopped off, the earthworm can usually regrow that portion - but the chopped off posterior will not grow a new head

This is also stated by the following sources:

http://compost.css.cornell.edu/worms/faq.html:

What happens if you cut a worm in half?

Almost everyone wants to know the answer to this question. Some species of worms can regenerate, or re-grow, a new tail, if their tail is cut off. However, a worm cut too closely to its' head will have difficulty growing a new tail. Most worms will not regenerate a head.

Generally, we tell students that if you cut a worm in half, you will most likely end up with two dead pieces of worms. However, if you are lucky, the piece with the head may grow a new tail, so you will have one alive worm and one piece of dead worm.

Some worms have a natural reflex, in which they will eject their tail when the tail is pulled. For example, when a bird catches the tail end of a worm, the worm would eject or sever its' tail from the rest of its' body. Thus, the worm remains alive and safe, while the bird gets only part of the worm.

http://www.animalcorner.co.uk/insects/worms/worm_anatomy.html:

There is a common myth that has been around for a long time now that if you cut a worm in half, the two halves will grow into worms - making 2 worms out of 1. This is very untrue. It is true that if a worm loses part of its body it will survive, but if you cut a worm into 2 pieces, one half will surely die. The half with the saddle (the fatter, pink part) will burrow itself into the soil and survive. It is not a good idea to cut worms in half or any other creature for that matter - it is very cruel - so please do not be taken in by this myth and leave the worms whole.

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your answer is correct for earthworms, but the question does not specify earthworms. Therefore, your answer is not a correct answer to the original question. –  David Apr 17 '12 at 4:41
    
@David: The question does only say "a worm" and is therefore not specific. The earthworm is the most common worm encountered by normal people (at least where I'm from) and therefore this claim is most likely about the earthworm. I at least specifically heard the same claim about it. Also showing that there is at least one worm species which can't be cut this way shows that the general claim ("cut a worm" which means basically "cut any worm") is not true. Therefore my answer is a valid answer to the original (vague) question. –  Martin Scharrer Apr 17 '12 at 6:51
    
But the claim is often also made, correctly, about planaria. Also, earthworms may be the charismatic megafauna among worms, but people encounter, and indeed host vastly more diverse and abundant non-earthworm worms on our bodies. –  David Apr 17 '12 at 8:27
    
@David: I don't have an issue with Kevin's answer being accepted instead of mine. He has some scientific sources after all. It's perfectly fine to have two answers which address the question from two different sides. –  Martin Scharrer Apr 17 '12 at 8:33
    
I fully agree that your answer is valid and useful even though it requires a reframing of the question –  David Apr 17 '12 at 8:43
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There are some worms with extraordinary regenerative capabilities.

For example many species of flatworm categorized as "Planarians" which are well known for being able to regenerate into two individuals when cut in half.

The best general purpose reference on this I can find is a page written by Dr. Wolfgang Seifarth.

The diagram on that page show some of the regenerations possible for that species. It can also regenerate if cut laterally.

For a more rigorous reference, Various research into the causes of this effect has been summarized in a paper published in the Annual Review of Cell and Developmental Biology. An illustrative quote from that paper:

When a planarian is amputated transversely, two fragments are generated and are capable of regenerating (Pallas 1774; Johnson 1822, 1825). The term polarity has been used to describe the fact that an anterior-facing wound will regenerate a head and a posterior-facing wound will regenerate a tail (Morgan 1901). “Something in the piece itself determines that a head shall develop at the anterior cut surface and a tail at the posterior cut surface. This “something” is what we call polarity”.

Indeed not only can it regenerate a head, but in a few cases, it can accidentally produce one with a head on each end!

Second, some small fragments, for example, from Dugesia tigrina animals, occasionally regenerate two-headed animals (Janus-heads) (Morgan 1898, 1904a) or, for example from D. lugubris animals, two-tailed animals (Morgan 1904d)

As for what they look like, perhaps this image from Wikipedia showing one of the two headed ones will give you an idea. Two Headed Planarian

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That paper doesn't seem, to me, to support your claim. For example, it mentions that flatworms have a "head". Would the tail develop a head? Is this mentioned by this article? Can you add a verbatim blockquote? –  Sklivvz Apr 15 '12 at 16:33
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@Sklivvz: Does my edit address your concerns? –  Kevin Cathcart Apr 16 '12 at 16:22
    
This is a really great answer! I found this video which describes a Planaria regeneration lab and it tells you how new growth looks different based on color. Feel free to add it into your answer if you'd like. –  Sam I Am Apr 17 '12 at 3:23
    
I really like the historical references; would be great to link directly, although the review is sufficient. –  David Apr 17 '12 at 8:31
    
@kev thank you, up voted! –  Sklivvz Apr 18 '12 at 17:06
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up vote 4 down vote accepted

(The information in this answer was taken and fused from Kevin Cathcart's Answer, and Martin Scharrer's Answer. )


Most people would immediately interpret this question as asking about the most familiar type of worm, the Earthworm. Interpreted like that it is an untrue urban legend. However the legend is based in fact, in that some other "less traditional" types of worms do actually regenerate. Therefore, I will address both.


Earthworms:

Most commonly, this myth is about earthworms, not any kind of worms.

This story is untrue according to the Worm Farm Guide. The two half do not survive independently. However, the head part can regrow some fraction at his back end if it got lost:

You may have heard that you can cut an earthworm in half and both halves will grow back. That's not true. If an earthworm is cut in half, it will usually die. However, it is true that if a third or less of the posterior portion of the worm gets chopped off, the earthworm can usually regrow that portion - but the chopped off posterior will not grow a new head

This is also stated by the following sources:

http://compost.css.cornell.edu/worms/faq.html:

What happens if you cut a worm in half?

Almost everyone wants to know the answer to this question. Some species of worms can regenerate, or re-grow, a new tail, if their tail is cut off. However, a worm cut too closely to its' head will have difficulty growing a new tail. Most worms will not regenerate a head.

Generally, we tell students that if you cut a worm in half, you will most likely end up with two dead pieces of worms. However, if you are lucky, the piece with the head may grow a new tail, so you will have one alive worm and one piece of dead worm.

Some worms have a natural reflex, in which they will eject their tail when the tail is pulled. For example, when a bird catches the tail end of a worm, the worm would eject or sever its' tail from the rest of its' body. Thus, the worm remains alive and safe, while the bird gets only part of the worm.

http://www.animalcorner.co.uk/insects/worms/worm_anatomy.html:

There is a common myth that has been around for a long time now that if you cut a worm in half, the two halves will grow into worms - making 2 worms out of 1. This is very untrue. It is true that if a worm loses part of its body it will survive, but if you cut a worm into 2 pieces, one half will surely die. The half with the saddle (the fatter, pink part) will burrow itself into the soil and survive. It is not a good idea to cut worms in half or any other creature for that matter - it is very cruel - so please do not be taken in by this myth and leave the worms whole.


Some... "Less Traditional" Worms:

There are some worms with extraordinary regenerative capabilities.

For example many species of flatworm categorized as "Planarians" which are well known for being able to regenerate into two individuals when cut in half.

The best general purpose reference on this I can find is a page written by Dr. Wolfgang Seifarth.

The diagram on that page show some of the regenerations possible for that species. It can also regenerate if cut laterally.

For a more rigorous reference, Various research into the causes of this effect has been summarized in a paper published in the Annual Review of Cell and Developmental Biology. An illustrative quote from that paper:

When a planarian is amputated transversely, two fragments are generated and are capable of regenerating (Pallas 1774; Johnson 1822, 1825). The term polarity has been used to describe the fact that an anterior-facing wound will regenerate a head and a posterior-facing wound will regenerate a tail (Morgan 1901). “Something in the piece itself determines that a head shall develop at the anterior cut surface and a tail at the posterior cut surface. This “something” is what we call polarity”.

Indeed not only can it regenerate a head, but in a few cases, it can accidentally produce one with a head on each end!

Second, some small fragments, for example, from Dugesia tigrina animals, occasionally regenerate two-headed animals (Janus-heads) (Morgan 1898, 1904a) or, for example from D. lugubris animals, two-tailed animals (Morgan 1904d)

As for what they look like, perhaps this image from Wikipedia showing one of the two headed ones will give you an idea. Two Headed Planarian

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protected by Sklivvz Apr 18 '12 at 17:10

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