Page "Invertebrate" Paragraph 2

Invertebrates accordingly form a massively paraphyletic group.

It is generally accepted that the phyla comprising modern Eukaryota share a common multicellular ancestor, but with the sole exception of one subphylum of the phylum Chordata, all those phyla are classified as invertebrates along with two of the three subphyla in the Chordata: Tunicata and Cephalochordata.

These two, plus all the other known invertebrates, have only one cluster of Hox genes, while the vertebrates have duplicated their original cluster more than once.

Within palaeozoology and palaeobiology, invertebrates are often studied within the fossil discipline called invertebrate palaeontology.