An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata (fish, amphibians, reptiles, birds, and mammals).
Invertebrates form a paraphyletic group. Given a common multicellular, eukaryotic ancestor, all contained phyla are invertebrates along with two of the three subphyla in Phylum 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.