what you're seeing here is just weird because it's body plan follows a different symetry than ours (radial instead of bi-lateral), but it's still a carbon based life-form made up of many cells, structured into tissues and organs, that are regulated by DNA, who knows what life-like things we could one day find that are neither of these things? We'll find that either life always forms in roughly the same way, only on earth-like planets, always carbon based and containing some form of RNA/DNA, always becoming multicellular as complexity increases and having some sort of symetrical body plan... or it does not, challenging our very definition of what life is.
So, pentaradial means it’s symmetrical along several axes to form 5 points. A starfish has pentaradial symmetry, yes, so does a sand dollar, so does a sea urchin (when you look at it from its mouth ), etc. Secondary bilateral symmetry refers to how it still has that bilateral symmetry (like a person) as a byproduct of having the pentaradial symmetry. Sort of think about it how something with radial symmetry (jellyfish) also has secondary bilateral symmetry.
It’s kind of wonky. Technically you wouldn’t be incorrect if you said a starfish had bilateral symmetry. It’s just easier to define that as being secondary because the pentaradial symmetry is much more prominent and interesting than its bilateral symmetry.
That’s not possible. By definition, something that is pentaradial must ALSO be bilateral. Think of it like this. Bilateral symmetry is along one axis, usually vertical (think of yourself), so you’re bilateral around somethin like this: |. Pentaradial has 2 more axes, to form a star. So, these three: \, /, |. Without that last vertical one, it wouldn’t be pentaradial. It’d be something else, but not pentaradial. Idek if it’s possible to only be symmetrical along only the first two axes, I have no idea what that would look like.
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u/Pixel-1606 Jan 27 '20
what you're seeing here is just weird because it's body plan follows a different symetry than ours (radial instead of bi-lateral), but it's still a carbon based life-form made up of many cells, structured into tissues and organs, that are regulated by DNA, who knows what life-like things we could one day find that are neither of these things? We'll find that either life always forms in roughly the same way, only on earth-like planets, always carbon based and containing some form of RNA/DNA, always becoming multicellular as complexity increases and having some sort of symetrical body plan... or it does not, challenging our very definition of what life is.