Since there’s been some discussion on new body plans, here is my understanding of how they come about. Note that I am not a biologist. I’m sure someone will correct me if I am wrong. Most of this has to do with how branching processes work.
To explore how new body plans arise, we are going to travel back in time with my trusty time machine. I obviously don’t have a time machine, which is why this is only a thought experiment. As I travel back in time, I take along a passenger, whom I shall call “William”. We want to find out where the octopuses, with an obviously different body plan, arose.
We start our journey in the present, looking at a person — perhaps at William himself. Then we go back in time to look at the ancestors. Well go fast, or this would take for ever.
As we go back in time examining ancestors, we see earlier vertebrates, such as fish. We keep on going, and get to the early chordates. They had a neural bundle, but no vertebrae. We have to go further back to find the ancestors of the octopus. As we go back, we see ancestors without even a neural bundle. Eventually, we get back to the point where the ancestors of the octopus branched off.
At this point, we see two sibling organisms. One of those is an early ancestor of us, and the other is an early ancestor of the modern octopus.
I hand William a microscope, and ask him to examine both as closely as possible, and to describe the body plan that he can see in each. “What body plan” says William. “I don’t see any body plan.”
That’s where the phylum branching occurred. The body plans came later. The whole idea of “macro-evolution”, as that term is used by creationists and ID proponents, is just a complete misunderstanding.