This group is a mess.
But one thing you should really know, as often mentioned in Power, Sex, Suicide by Nick Lane (2006): they are all eukaryotes.
Because prokaryotes are fundamentally unable to do phagocytosis, because they have a rigid cell wall. Changing cell shape at will requires a cytoskeleton.
Vertebrates minus tetrapods.
This paraphyletic subgroup is easy to form the "acquatic only" (fishes) vs "things that come out of water" (tetrapods). Though mudfish make that distinction harder.
Which kind of makes sense, why would you want for limbs unless you are going to stay out of water!