So all vertebrate critters on Earth now have four limbs.
Were there ever any six-limbed critters, other than insects?
Why no Green Martian-types? or angels with legs, arms and wings?
No idea, but I was just reminded to not Google for “sextaped”.
I note that insects are the most well known of a subphylum called Hexapoda, which is named after their characteristic possession of six limbs.
Mostly, because vertebrates happened to evolve with only four limbs in the first place. Evolution is very chance-and-history driven like that. If by chance vertebrates had evolved with six limbs, then it’s quite possible we’d have four arms.
Angels have aerodynamic and mass problems, though. They are poorly shaped and heavy for a flying creature. A smaller flyer would be much better; a humanoid flyer would be more of an imp or cherub than an angel, to keep up the mythology theme.
Spiders and ticks have 8 legs. Lobsters and crabs have 10.
I think 6 legs is very efficient because you can have a very simple walking algorithm that always leaves 3 on the ground. Maybe that’s one reason there are so many insects.
Terrestrial vertebrates have four limbs because they are descended from bony fish that had paired pectoral and pelvic fins. No known vertebrate has had more than four limbs. (The earliest amphibians, though, had as many as eight toes, this number subsequently being reduced to five.)
However, vertebrates have a segmented body plan (although not as much so as arthropods) so it would not have been inconceivable for ancestral fish, and their descendants, to have more than two pairs of limbs.
The ancestral arthropod probably had many pairs of legs like a millipede. The various numbers found in crustaceans, spiders, and insects are reductions from a greater number of legs. Some kinds of butterflies are functionally four-legged, having the first pair vestigial.
Are there no fish with more than four fins, or are you considering fins to not be limbs?
No fish, even the most primitive ones, have ever had more than two sets of paired fins, which are the kind of fins from which the limbs of terrestrial vertebrates evolved.
Fish have medial fins along the midline of the body, usually one or two dorsal fins, a tail fin, and an anal fin, but these are unpaired and have not evolved into limbs.
In some vertebrates, such as some monkeys and other climbing animals and kangaroos, the tail has functionally evolved into a fifth limb, but that’s as far as it goes.
Ignorance fought, thanks!
Since you’ve answered that, what about cephalopods (octopus, nautilus, etc)-- is it possible any of them had exactly six tentacles/arms?
Since octopuses don’t fossilize well, it’s possible some species might have in the past. Today none have six arms aside from very rare mutants.
That is certainly far enough, thank you very much.
I thought some kind proto-nautiloid might be more likely, though at that point, one might argue about whether the feeding tentacles are ‘limbs’ or not. Of course, their soft parts didn’t fossilize very well, either, so ‘possible’ is probably about all we’ll ever get.
This is how we know “intelligent design” is false. Anyone caring for small children knows we would be better off with at least 2 more hands each.
I was very glad to see that they used “octopodes” as the plural in that article. “Octopi” is a pet peeve of mine since it’s pseudo-Latin instead of real Latin. I would have said either octopuses or octopods, but octopodes is at least correct a correct plural.
And “algar” a single species of “algae”.
“The octopodes were concealed in the agar.”
Actually, the singular of algae is “alga.” And although agar is made from algae, it would take an awful lot of it to conceal several octopods. 
Although it actually has 12 limbs, there is also the Human Centipede. I dare you to search for that.
I shall take issue with a fellow who wrote about alga(r).
You sure it isn’t another Latin thing? :dubious:
I was thinking of macroalga(r) which has things that look and act like leaves, roots and stems, but are not. It is not a vascula(r) plant.
It all comes down to the fact that everything that lives in the ocean hates us, and wants to kill us, yes, even non-vascula(r) plants.
I have never seen algar used as the singular as algae, nor for that matter in any other way. It doesn’t appear in Merriam-Webster.
Where exactly did you see this? It simply seems to be a mistake.
An article on the web or a book on algae. I probably remember it incorrectly. 
And don’t forget, even a few where the nose has evolved into a fifth limb.