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Animals with four, six and eight legs are widespread while the concept of seven legs (fictitious Heptapods) are the makings of amazing scifi short stories and blockbuster movies.

Are starfish the only types of animals currently (or ever) with five mostly equal sized appendages and substantial five-fold symmetry? Are there theories why five is unusual on land but common in the sea?

update: There is an interesting subtlety here. This answer points out that a sea star's legs are the little things under the five appendages, so I've adjusted the question to address instead five-fold symmetry and five appendages.


below: "Ophiopetra lithographica from the Lower Hienheim Beds (Lower Tithonian,Upper Jurassic) near Regensburg, Germany" from here.

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below: "A green brittle star - Ophiarachna incrassata" from here.

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below: "Une ophiure superbe (Ophiolepis superba) à la Réunion." from here.

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  • $\begingroup$ I suggest that you will find it very interesting if you knew that actually there is a land animal with five limbs. If you are interested check here . I didn't want to post it as an answer because I thought it does not fully answers your question ;) . I forgot to mention that the land-animal I mentioned doesn't show symmetry to the fifth "limb". $\endgroup$ – The_Mad_Fish Jun 19 '17 at 12:33
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    $\begingroup$ @The_Mad_Fish I was a little hesitant to click your link until I saw it was the NewScientist. I clicked and then a big Aha! yep, well done :) $\endgroup$ – uhoh Jun 19 '17 at 12:45
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    $\begingroup$ hahaha I hope that I covered you lol ;) $\endgroup$ – The_Mad_Fish Jun 19 '17 at 12:48
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Many (if not most) of the Echinoderms (to which starfish belong as well) have five-fold symmetry, such as many sea liliy (Crinoidea) and sea urchin (Echinodea) species.

Five-fold symmetrical sea lilly:

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Sand dollar (type of sea urchin):

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    $\begingroup$ Maybe you could add that they are, however, *Bilaterians (as opposed to Radiata), as their larvae, representing the ancestral state in the group, have bilateral symmetry. $\endgroup$ – AlexDeLarge Jun 19 '17 at 10:17
  • $\begingroup$ I should have recalled sand dollars, thanks. OK so there are diverse members of an entire phylum, that's great! Echinoderms are also the largest phylum that has no freshwater or terrestrial (land-based) representatives so that settles it. $\endgroup$ – uhoh Jun 19 '17 at 10:17
  • $\begingroup$ @AlexDeLarge that's an interesting feat. $\endgroup$ – uhoh Jun 19 '17 at 10:19

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