Evolution of limbs (Cephs to Mammals?)

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...
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....
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:
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...
cuttlegirl said:
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...

That doesn't sound familiar, but I'm mostly going on memory of reading up on them about 12 years ago, so I could easily be misremembering...

wikipedia ( Echinoderm - Wikipedia ) says:

Echinoderms evolved from bilaterally symmetric creatures. Later forms were lopsided. Echinoderms' larvae are ciliated free-swimming organisms that organize in a bilaterally symmetric fashion that makes them look like embryonic chordates. Later, the left side of the body grows at the expense of the right side, which is eventually absorbed. The left side then grows in a pentaradially symmetric fashion, in which the body is arranged in five parts around a central axis.

a google for "echinoderm homeobox" turned up this:

Radical alterations in the roles of homeobox genes during echinoderm evolution - PubMed

but I'm not sure I should look for the paper tonight (but I may not be able to resist)

Now I really would be curious about Sake-san's opinions...
I can't find my development books right now (must not have unpacked them yet... only moved here 11 months ago...).

My invert text says that 50% of echinoderms have direct development. In the others, the left side of the larvae becomes the oral surface and the right side becomes the aboral. In sea stars, the larvae undergoes metamorphsis. The mouth, esophagus, part of the intestine and anus degenerate and then are formed again in a radial symmetrical fashion in the baby sea star.

Ok, if you are ready to :bugout: then check out this link...http://www.palaeos.com/Invertebrates/Deuterostomia/Deuterostomia.htm

I find it interesting (I had forgotten...) that chordates and echinoderms are deuterostomes.
cuttlegirl said:
Some more thoughts... maybe it helps the octopus go forwards, backwards and sideways. Crabs can walk sideways (they have ten legs).

... but lobsters walk straight...

At first I was going to comment that the octopus only took up the walking around on the bottom lifestyle comparatively recently, but then I realized that most models of how the first shelled cephs developed was that some limpet-like critter was on the bottom and gained the ability to use its shell to become buoyant. One might imagine that as it became lighter, it could move a lot faster by walking on its arms than by using a gastropodish foot, even if it wasn't a free swimmer yet... in fact, maybe it was the walking faster and being a more active predator that came first, and the floating initially was an adaptation to be more effective at that...

And yes, :read: ing that link and the nature paper are very :bugout: :bonk: inducing... definitely puts me back in my place as an official "ignorant computer scientist" rather than a real bio- or paleo- expert!
But maybe since the ancestral cephalopod only moved in two dimensions, it didn't need 8 appendages. Modern cephalopods navigate three dimensional space.

Also, most snails can only crawl forward, they can turn and move another direction, but it is still head forward.
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