Flatworms (Platyhelminthes), Roundworms (Nematoda), Segmented worms (Annelida), acorn worms (Hemichordata) all get their own phylums. Why?
In: Biology
Some worms are more different from other worms, than they are from humans. Conversely, simply sharing a new feature like a spine means that humans have way more in common with fish and birds, than two phyla of worms that have had literally billions of years to differentiate.
The definition of “worm” is basically “elongated bilateral invertebrate with no legs”. This category is so loose, that many animals that are not closely related will fall into it.
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Lots of different worm-like things evolved in various directions, most of them still worm-like yet different.
One branch of animal evolved a crude spinal cord, and all chordata evolved from that one branch.
Different worm-like things have evolved from different ancestors and are therefore not classified together, whereas all chordata share a common ancestor.
I believe a lot has to do with how the worms develop in the embryonic stages, these differ a lot compared to how chordata develop. As chordata usually rely on a similar developing patterns early on in the embryonic stages (f.e. at certain stages we would resemble an amphibian embryo).
Whilst worms, i think, have a lot of different ways of organizing within an embryo. Not sure about the differences exactly but i do know i studied this for a semester lol. Different ways of cell structures migrating and developing within the embryo.
Hope this helps a bit 😀
Beinj
Why do some human families have one sibling who has a bazillion children, and two other siblings that have no children at all?
That is what you’re asking here. “Phylum” just refers to one particular layer of the evolutionary tree of life. Some branches fork many many many times, other branches barely fork at all. Some branches seem (to the untrained eye) like they aren’t meaningful, while other branches seem incredibly important.
The phyla you mention all have many traits they do not share with one another. Meanwhile, *all* chordates share a set of five common traits. Hence, despite how different a lionfish looks from a lion, they belong to the same phylum.
it’s in the name: Chordata, basically everything in there has a “spine” or more accurately a notochord among other shared features, and the body plans are all built around it. The lowest common denominator if you will. “worms” on the other hand are such a simple body plan (essentially a tube) that many things can be “worms” without sharing a “lowest common denominator” more specific than being “Animalia”
imagine if you had a large bag full of marbles and other various polished rocks, the marbles are in a huge variety of designs from super simple to very very intricate. Technically everything in the bag is some kind of “rock”, and they are all at least somewhat round and smooth. The next best categorization would probably be what each is made of. This groups all marbles, simple, complex, large, or small into “glass” and all the other polished rocks are grouped into “sandstone” or “granite”, etc… You could make the argument to group them into “glass” and “not glass” but the same could be said for “granite” and “not granite”, it would be pointless. Though on the surface (outside of your classification) you could call out that all of the glass ones are “marbles” and everything else are “stones” much like how so many things fit into “Chordata” but there are many “everything else” with a simpler “worm like” body plant that fit into other categories
Edit: reddit seemed to die right around when I was trying to post this, please ignore if it posts duplicates because reddit is a very well built website that wouldn’t possibly do such a thing lol
They were all called “worms” because people were generally ignorant about such animals. It turned out they are very distant groups.
Btw the same applies to “fish”. There’s no animal division that correspond to “fish”. We are more closely related to some fish than these fish are to sharks.
Despite looking superficially similar to you, each phyla you listed is drastically different from each other.
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