Flatworms (Platyhelminthes), Roundworms (Nematoda), Segmented worms (Annelida), acorn worms (Hemichordata) all get their own phylums. Why?
In: Biology
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.
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