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Evolution of limbs (Cephs to Mammals?)

Sep 16, 2005
I always wondered why four was such a "magic" number... Vertebrates either have two (birds, although I guess wings count as appendages...) or four limbs (the rest of them...), unless you count snakes (which ancestrally had four...). Other invertebrates usually either have many (centipedes, millipedes) or 6 (insects). Why not 8? Why is it that cephs and arachnids are the only ones that have 8? Hmm... must go do some reading...


Colossal Squid
Staff member
Oct 19, 2003
Three is better, but Steven Spielberg patented those for War of the (barf) Worlds, shame... Echinoderms are (manifolds of) 5, think about it....

When it comes to cooking, I sometimes wish I WERE tako....


TONMO Supporter
Staff member
Mar 8, 2004
cuttlegirl said:
But they have radial symmetry instead of bilateral...I have often thought, as a mom, that a third arm would be useful.

Yeah, but they cheat; developmentally, they're not true radiates, they're bilats who grab their tails with their heads and fuse.

I think the basic animal body plan controlled by HOX genes and the like strongly favors bilateral symmetry and has for a long time; and even recently, it looks like true radiates like cniderians are even based on the same body patterns. So really it's only really primitive things like sponges that aren't based on this. So pretty much anything ambulatory is likely to have things in symmetric pairs... I would be very curious if anyone's studied the genetics of echinoderm development enough to understand what signals the major morphological differences in "inside side" vs "outside side" when they take their adult forms. Other animals have some degree of violation of the bilaterality, though, so it may just be a more dramatic manifestation of the same mechanism.

Anyway, from studying a bit about animal and robot locomotion, I can point out that for things on feet, if you have 4 legs, you can always keep your center of gravity over a triangle of 3 legs while you move the 4th, so it's a lot easier to be stable and not fall over as you walk. Us bipeds have to consistently be in danger of falling over; in fact, walking is really more like continually falling forward and catching yourself, which is not true of quadropeds. Sometimes there is a slight gain in going to 6 legs in this regard, since you don't have to shift your center of mass so much to take the weight off a leg to step; there's a "hexapod" robot someone made to take advantage of this. I'm not sure why there's an advantage in some animals to having more legs, though... roach walking has been studied a fair bit, and they seem to just get more speed out of being able to have more legs in action at a given time. I'm not aware of any theories on why 8 or 10 limbs in cephs, or more for nautilus, or why it's an advantage for spiders or milipedes to have more than 4...

I'm hoping Fujisawa Sake-san will chime in here, it seems like this is right up his alley... however, he's been quiet lately, too quiet... :goofysca:
Sep 16, 2005
Great, now I am going to have to dig out my development books... I know that echinoderm larva have bilateral symmetry, but doesn't the adult stage develop inside of the larvae? Off to do some reading...

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