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Many commonly known animals' limbs usually number between 0 and 10. For example, a non-exhaustive list:

  • snakes have 0
  • Members of Bipedidae have 2 legs. Birds and humans have 2 legs (but 4 limbs)
  • Most mammals, reptiles, amphibians have 4 legs
  • Echinoderms (e.g., sea stars) typically have 5 legs.
  • Insects typically have 6 legs
  • Octopi and arachnids have 8 legs
  • decapods (e.g., crabs) have 10 legs

....But I can't really think of many examples of animals containing more legs until you reach 30+ legs in centipedes and millipedes. Some millipedes even have as many as 750 legs! The lone example I am aware of, the sunflower sea star, typically has 16-24 (though up to 40) limbs.

So my question is: what are some examples of animals with 12-28 legs?

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    $\begingroup$ Two questions you may want to consider. So what distribution does it seem to follow? How many replicates did you have to build this distribution? All spiders have 8 legs but it do 45,000 replicates, you actually have only one replicate (see the problem of pseudoreplication in phylogenetics). $\endgroup$ – Remi.b Jan 25 '16 at 21:27
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    $\begingroup$ One will need a good study of the distribution of number of legs (and the sample size used to build this distribution) before saying anything. For the moment the presence of a "distributional gap" is an opinion and is not evidence-based. I am voting to close as such. The question is interesting though! $\endgroup$ – Remi.b Jan 25 '16 at 21:28
  • $\begingroup$ @Remi.b: you're right about your points concerning distribution. So instead, I've modified the question to focus on simply: why are there not examples (or more examples) of animals with between 12-28 legs. ...Visualizing the distribution of species and their leg numbers would be very fascinating, though wayyy too much work to do accurately). $\endgroup$ – theforestecologist Jan 25 '16 at 21:32
  • $\begingroup$ Yes, I realize how much work it would be but without that it makes little sense to speculate on why are there not examples (or more examples) of animals with between 12-28 legs unless we know whether or not this is to be expected from the little sample size (taken pseudoreplicaiton into account) that is available. $\endgroup$ – Remi.b Jan 25 '16 at 22:33
  • $\begingroup$ It's simply the result of evolution. If something 'works' with 4 legs, then there is no selective pressure or natural selection acting on it, and so the organism does not develop more limbs. $\endgroup$ – notorious Jan 26 '16 at 0:20
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As a couple of counterexamples, species in the classes Symphyla (Pseudocentipedes) and Pauropoda within Myriapoda have 8-11 and 12 leg pairs respectively, so between 16 to 24 legs (sometimes with one or two leg pair stronlgy reduced in size).

enter image description here
(species in Symphyla, from wikipedia)

Another common and species-rich group with 14 walking legs (7 leg pairs) is Isopoda.

enter image description here
(Isopod, picture from wikipedia)

You also need to define 'legs' for the discussion to be meaningful. As you say, decapods have 10 legs on their thoracic segments (thoracic appendages), but they can also have appendages on their abdomens (Pleopods/swimming legs), which will place many decapods in the 10-20 leg range.

enter image description here
(Decapod abdominal appendages/legs in yellow, from wikipedia)

So overall, in Arthropoda, having 12-28 legs doesn't seem all that uncommon. There are probably other Arthropod groups besides those mentioned here that also have leg counts in this range.

However, for a general account, the most likely answer (if there is indeed a relative lack of 12-28 legged animals) is probably evolutionary contingencies and strongly conservative body plans within organism groups.

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    $\begingroup$ Also velvet worms. $\endgroup$ – WYSIWYG Feb 3 '16 at 5:29
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    $\begingroup$ The now-extinct, Hallucigenia had 7-pairs (or 14) legs. $\endgroup$ – theforestecologist Dec 12 '16 at 17:35

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