Eli5: Animalia phylum. Why are there so many divisions for worms while mammals, fish, amphibians, birds, and reptiles all get lumped into Chordata?

Question

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

In: Biology

Answer 1

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.