Taxonomy

Good question Jack,

I assume you meant 'why are vertebrates all grouped in one phylum'. The short answer is that a phylum, by definition is a monophyletic group, which basically means all of the descendants of a common ancestor. All of the vertebrates share a common ancestor and all of the descendants of that common ancestor can be classified as vertebrates. All of the invertebrates share a common ancestor also, but not all of the descendants of that ancestor are invertebrates. Therefore the invertebrates do not form a monophyletic group and cannot be classified together in one phylum.