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

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.